2021 |
Telychko, Mykola; Li, Guangwu; Mutombo, Pingo; Soler-Polo, Diego; Peng, Xinnan; Su, Jie; Song, Shaotang; Koh, Ming Joo; Edmonds, Mark; Jelinek, Pavel; Wu, Jishan; Lu, Jiong Ultrahigh-yield on-surface synthesis and assembly of circumcoronene into a chiral electronic Kagome-honeycomb lattice Journal Article SCIENCE ADVANCES, 7 (3), 2021, ISSN: 2375-2548. @article{ISI:000608481000043, title = {Ultrahigh-yield on-surface synthesis and assembly of circumcoronene into a chiral electronic Kagome-honeycomb lattice}, author = {Mykola Telychko and Guangwu Li and Pingo Mutombo and Diego Soler-Polo and Xinnan Peng and Jie Su and Shaotang Song and Ming Joo Koh and Mark Edmonds and Pavel Jelinek and Jishan Wu and Jiong Lu}, doi = {10.1126/sciadv.abf0269}, issn = {2375-2548}, year = {2021}, date = {2021-01-01}, journal = {SCIENCE ADVANCES}, volume = {7}, number = {3}, publisher = {AMER ASSOC ADVANCEMENT SCIENCE}, address = {1200 NEW YORK AVE, NW, WASHINGTON, DC 20005 USA}, abstract = {On-surface synthesis has revealed remarkable potential in the fabrication of atomically precise nanographenes. However, surface-assisted synthesis often involves multiple-step cascade reactions with competing pathways, leading to a limited yield of target nanographene products. Here, we devise a strategy for the ultrahigh-yield synthesis of circumcoronene molecules on Cu(111) via surface-assisted intramolecular dehydrogenation of the rationally designed precursor, followed by methyl radical-radical coupling and aromatization. An elegant electrostatic interaction between circumcoronenes and metallic surface drives their self-organization into an extended superlattice, as revealed by bond-resolved scanning probe microscopy measurements. Density functional theory and tight-binding calculations reveal that unique hexagonal zigzag topology of circumcoronenes, along with their periodic electrostatic landscape, confines two-dimensional electron gas in Cu(111) into a chiral electronic Kagome-honeycomb lattice with two emergent electronic flat bands. Our findings open up a new route for the high-yield fabrication of elusive nanographenes with zigzag topologies and their superlattices with possible nontrivial electronic properties.}, keywords = {}, pubstate = {published}, tppubtype = {article} } On-surface synthesis has revealed remarkable potential in the fabrication of atomically precise nanographenes. However, surface-assisted synthesis often involves multiple-step cascade reactions with competing pathways, leading to a limited yield of target nanographene products. Here, we devise a strategy for the ultrahigh-yield synthesis of circumcoronene molecules on Cu(111) via surface-assisted intramolecular dehydrogenation of the rationally designed precursor, followed by methyl radical-radical coupling and aromatization. An elegant electrostatic interaction between circumcoronenes and metallic surface drives their self-organization into an extended superlattice, as revealed by bond-resolved scanning probe microscopy measurements. Density functional theory and tight-binding calculations reveal that unique hexagonal zigzag topology of circumcoronenes, along with their periodic electrostatic landscape, confines two-dimensional electron gas in Cu(111) into a chiral electronic Kagome-honeycomb lattice with two emergent electronic flat bands. Our findings open up a new route for the high-yield fabrication of elusive nanographenes with zigzag topologies and their superlattices with possible nontrivial electronic properties. |
Li, Sifan; Li, Bochang; Feng, Xuewei; Chen, Li; Li, Yesheng; Huang, Li; Fong, Xuanyao; Ang, Kah-Wee Electron-beam-irradiated rhenium disulfide memristors with low variability for neuromorphic computing Journal Article NPJ 2D MATERIALS AND APPLICATIONS, 5 (1), 2021. @article{ISI:000607110700001, title = {Electron-beam-irradiated rhenium disulfide memristors with low variability for neuromorphic computing}, author = {Sifan Li and Bochang Li and Xuewei Feng and Li Chen and Yesheng Li and Li Huang and Xuanyao Fong and Kah-Wee Ang}, doi = {10.1038/s41699-020-00190-0}, year = {2021}, date = {2021-01-01}, journal = {NPJ 2D MATERIALS AND APPLICATIONS}, volume = {5}, number = {1}, publisher = {NATURE RESEARCH}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {State-of-the-art memristors are mostly formed by vertical metal-insulator-metal (MIM) structure, which rely on the formation of conductive filaments for resistive switching (RS). However, owing to the stochastic formation of filament, the set/reset voltage of vertical MIM memristors is difficult to control, which results in poor temporal and spatial switching uniformity. Here, a two-terminal lateral memristor based on electron-beam-irradiated rhenium disulfide (ReS2) is realized, which unveils a resistive switching mechanism based on Schottky barrier height (SBH) modulation. The devices exhibit a forming-free, stable gradual RS characteristic, and simultaneously achieve a small transition voltage variation during positive and negative sweeps (6.3%/5.3%). The RS is attributed to the motion of sulfur vacancies induced by voltage bias in the device, which modulates the ReS2/metal SBH. The gradual SBH modulation stabilizes the temporal variation in contrast to the abrupt RS in MIM-based memristors. Moreover, the emulation of long-term synaptic plasticity of biological synapses is demonstrated using the device, manifesting its potential as artificial synapse for energy-efficient neuromorphic computing applications.}, keywords = {}, pubstate = {published}, tppubtype = {article} } State-of-the-art memristors are mostly formed by vertical metal-insulator-metal (MIM) structure, which rely on the formation of conductive filaments for resistive switching (RS). However, owing to the stochastic formation of filament, the set/reset voltage of vertical MIM memristors is difficult to control, which results in poor temporal and spatial switching uniformity. Here, a two-terminal lateral memristor based on electron-beam-irradiated rhenium disulfide (ReS2) is realized, which unveils a resistive switching mechanism based on Schottky barrier height (SBH) modulation. The devices exhibit a forming-free, stable gradual RS characteristic, and simultaneously achieve a small transition voltage variation during positive and negative sweeps (6.3%/5.3%). The RS is attributed to the motion of sulfur vacancies induced by voltage bias in the device, which modulates the ReS2/metal SBH. The gradual SBH modulation stabilizes the temporal variation in contrast to the abrupt RS in MIM-based memristors. Moreover, the emulation of long-term synaptic plasticity of biological synapses is demonstrated using the device, manifesting its potential as artificial synapse for energy-efficient neuromorphic computing applications. |
Qiu, Zhizhan; Holwill, Matthew; Olsen, Thomas; Lyu, Pin; Li, Jing; Fang, Hanyan; Yang, Huimin; Kashchenko, Mikhail; Novoselov, Kostya S; Lu, Jiong Visualizing atomic structure and magnetism of 2D magnetic insulators via tunneling through graphene Journal Article NATURE COMMUNICATIONS, 12 (1), 2021, ISSN: 2041-1723. @article{ISI:000623143000001, title = {Visualizing atomic structure and magnetism of 2D magnetic insulators via tunneling through graphene}, author = {Zhizhan Qiu and Matthew Holwill and Thomas Olsen and Pin Lyu and Jing Li and Hanyan Fang and Huimin Yang and Mikhail Kashchenko and Kostya S Novoselov and Jiong Lu}, doi = {10.1038/s41467-020-20376-w}, issn = {2041-1723}, year = {2021}, date = {2021-01-01}, journal = {NATURE COMMUNICATIONS}, volume = {12}, number = {1}, publisher = {NATURE RESEARCH}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {The discovery of two-dimensional (2D) magnetism combined with van der Waals (vdW) heterostructure engineering offers unprecedented opportunities for creating artificial magnetic structures with non-trivial magnetic textures. Further progress hinges on deep understanding of electronic and magnetic properties of 2D magnets at the atomic scale. Although local electronic properties can be probed by scanning tunneling microscopy/spectroscopy (STM/STS), its application to investigate 2D magnetic insulators remains elusive due to absence of a conducting path and their extreme air sensitivity. Here we demonstrate that few-layer CrI3 (FL-CrI3) covered by graphene can be characterized electronically and magnetically via STM by exploiting the transparency of graphene to tunneling electrons. STS reveals electronic structures of FL-CrI3 including flat bands responsible for its magnetic state. AFM-to-FM transition of FL-CrI3 can be visualized through the magnetic field dependent moire contrast in the dI/dV maps due to a change of the electronic hybridization between graphene and spin-polarised CrI3 bands with different interlayer magnetic coupling. Our findings provide a general route to probe atomic-scale electronic and magnetic properties of 2D magnetic insulators for future spintronics and quantum technology applications.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The discovery of two-dimensional (2D) magnetism combined with van der Waals (vdW) heterostructure engineering offers unprecedented opportunities for creating artificial magnetic structures with non-trivial magnetic textures. Further progress hinges on deep understanding of electronic and magnetic properties of 2D magnets at the atomic scale. Although local electronic properties can be probed by scanning tunneling microscopy/spectroscopy (STM/STS), its application to investigate 2D magnetic insulators remains elusive due to absence of a conducting path and their extreme air sensitivity. Here we demonstrate that few-layer CrI3 (FL-CrI3) covered by graphene can be characterized electronically and magnetically via STM by exploiting the transparency of graphene to tunneling electrons. STS reveals electronic structures of FL-CrI3 including flat bands responsible for its magnetic state. AFM-to-FM transition of FL-CrI3 can be visualized through the magnetic field dependent moire contrast in the dI/dV maps due to a change of the electronic hybridization between graphene and spin-polarised CrI3 bands with different interlayer magnetic coupling. Our findings provide a general route to probe atomic-scale electronic and magnetic properties of 2D magnetic insulators for future spintronics and quantum technology applications. |
Woods, C R; Ares, P; Nevison-Andrews, H; Holwill, M J; Fabregas, R; Guinea, F; Geim, A K; Novoselov, K S; Walet, N R; Fumagalli, L Charge-polarized interfacial superlattices in marginally twisted hexagonal boron nitride Journal Article NATURE COMMUNICATIONS, 12 (1), 2021, ISSN: 2041-1723. @article{ISI:000662813400002, title = {Charge-polarized interfacial superlattices in marginally twisted hexagonal boron nitride}, author = {C R Woods and P Ares and H Nevison-Andrews and M J Holwill and R Fabregas and F Guinea and A K Geim and K S Novoselov and N R Walet and L Fumagalli}, doi = {10.1038/s41467-020-20667-2}, issn = {2041-1723}, year = {2021}, date = {2021-01-01}, journal = {NATURE COMMUNICATIONS}, volume = {12}, number = {1}, publisher = {NATURE RESEARCH}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {When two-dimensional crystals are brought into close proximity, their interaction results in reconstruction of electronic spectrum and crystal structure. Such reconstruction strongly depends on the twist angle between the crystals, which has received growing attention due to interesting electronic and optical properties that arise in graphene and transitional metal dichalcogenides. Here we study two insulating crystals of hexagonal boron nitride stacked at small twist angle. Using electrostatic force microscopy, we observe ferroelectric-like domains arranged in triangular superlattices with a large surface potential. The observation is attributed to interfacial elastic deformations that result in out-of-plane dipoles formed by pairs of boron and nitrogen atoms belonging to opposite interfacial surfaces. This creates a bilayer-thick ferroelectric with oppositely polarized (BN and NB) dipoles in neighbouring domains, in agreement with our modeling. These findings open up possibilities for designing van der Waals heterostructures and offer an alternative probe to study moire-superlattice electrostatic potentials.}, keywords = {}, pubstate = {published}, tppubtype = {article} } When two-dimensional crystals are brought into close proximity, their interaction results in reconstruction of electronic spectrum and crystal structure. Such reconstruction strongly depends on the twist angle between the crystals, which has received growing attention due to interesting electronic and optical properties that arise in graphene and transitional metal dichalcogenides. Here we study two insulating crystals of hexagonal boron nitride stacked at small twist angle. Using electrostatic force microscopy, we observe ferroelectric-like domains arranged in triangular superlattices with a large surface potential. The observation is attributed to interfacial elastic deformations that result in out-of-plane dipoles formed by pairs of boron and nitrogen atoms belonging to opposite interfacial surfaces. This creates a bilayer-thick ferroelectric with oppositely polarized (BN and NB) dipoles in neighbouring domains, in agreement with our modeling. These findings open up possibilities for designing van der Waals heterostructures and offer an alternative probe to study moire-superlattice electrostatic potentials. |
Ni, Kun; Du, Jinxiang; Yang, Jin; Xu, Shujuan; Cong, Xin; Shu, Na; Zhang, Kai; Wang, Aolei; Wang, Fei; Ge, Liangbing; Zhao, Jin; Qu, Yan; Novoselov, Kostya S; Tan, Pingheng; Su, Fuhai; Zhu, Yanwu Stronger Interlayer Interactions Contribute to Faster Hot Carrier Cooling of Bilayer Graphene under Pressure Journal Article PHYSICAL REVIEW LETTERS, 126 (2), 2021, ISSN: 0031-9007. @article{ISI:000607525700018, title = {Stronger Interlayer Interactions Contribute to Faster Hot Carrier Cooling of Bilayer Graphene under Pressure}, author = {Kun Ni and Jinxiang Du and Jin Yang and Shujuan Xu and Xin Cong and Na Shu and Kai Zhang and Aolei Wang and Fei Wang and Liangbing Ge and Jin Zhao and Yan Qu and Kostya S Novoselov and Pingheng Tan and Fuhai Su and Yanwu Zhu}, doi = {10.1103/PhysRevLett.126.027402}, issn = {0031-9007}, year = {2021}, date = {2021-01-01}, journal = {PHYSICAL REVIEW LETTERS}, volume = {126}, number = {2}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {We perform femtosecond pump-probe spectroscopy to in situ investigate the ultrafast photocarrier dynamics in bilayer graphene and observe an acceleration of energy relaxation under pressure. In combination with in situ Raman spectroscopy and ab initio molecular dynamics simulations, we reveal that interlayer shear and breathing modes have significant contributions to the faster hot-carrier relaxations by coupling with the in-plane vibration modes under pressure. Our work suggests that further understanding the effect of interlayer interaction on the behaviors of electrons and phonons would be critical to tailor the photocarrier dynamic properties of bilayer graphene.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We perform femtosecond pump-probe spectroscopy to in situ investigate the ultrafast photocarrier dynamics in bilayer graphene and observe an acceleration of energy relaxation under pressure. In combination with in situ Raman spectroscopy and ab initio molecular dynamics simulations, we reveal that interlayer shear and breathing modes have significant contributions to the faster hot-carrier relaxations by coupling with the in-plane vibration modes under pressure. Our work suggests that further understanding the effect of interlayer interaction on the behaviors of electrons and phonons would be critical to tailor the photocarrier dynamic properties of bilayer graphene. |
Su, Jie; Fan, Wei; Mutombo, Pingo; Peng, Xinnan; Song, Shaotang; Ondracek, Martin; Golub, Pavlo; Brabec, Jiri; Veis, Libor; Telychko, Mykola; Jelinek, Pavel; Wu, Jishan; Lu, Jiong On-Surface Synthesis and Characterization of [7]Triangulene Quantum Ring Journal Article NANO LETTERS, 21 (1), pp. 861-867, 2021, ISSN: 1530-6984. @article{ISI:000611082000116, title = {On-Surface Synthesis and Characterization of [7]Triangulene Quantum Ring}, author = {Jie Su and Wei Fan and Pingo Mutombo and Xinnan Peng and Shaotang Song and Martin Ondracek and Pavlo Golub and Jiri Brabec and Libor Veis and Mykola Telychko and Pavel Jelinek and Jishan Wu and Jiong Lu}, doi = {10.1021/acs.nanolett.0c04627}, issn = {1530-6984}, year = {2021}, date = {2021-01-01}, journal = {NANO LETTERS}, volume = {21}, number = {1}, pages = {861-867}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {The ability to engineer geometrically well-defined antidots in large triangulene homologues allows for creating an entire family of triangulene quantum rings (TQRs) with tunable high-spin ground state, crucial for next-generation molecular spintronic devices. Herein, we report the synthesis of an open-shell [7]triangulene quantum ring ([7]TQR) molecule on Au(111) through the surface-assisted cyclodehydrogenation of a rationally designed kekulene derivative. Bond-resolved scanning tunneling microscopy (BR-STM) unambiguously imaged the molecular backbone of a single [7]TQR with a triangular zigzag edge topology, which can be viewed as [7]triangulene decorated with a coronene-like antidot in the center. Additionally, dI/dV mapping reveals that both inner and outer zigzag edges contribute to the edge-localized and spin-polarized electronic states of [7]TQR. Both experimental results and spin-polarized density functional theory calculations indicate that [7]TQR retains its open-shell septuple ground state (S = 3) on Au(111). This work demonstrates a new route for the design of high-spin graphene quantum rings for future quantum devices.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The ability to engineer geometrically well-defined antidots in large triangulene homologues allows for creating an entire family of triangulene quantum rings (TQRs) with tunable high-spin ground state, crucial for next-generation molecular spintronic devices. Herein, we report the synthesis of an open-shell [7]triangulene quantum ring ([7]TQR) molecule on Au(111) through the surface-assisted cyclodehydrogenation of a rationally designed kekulene derivative. Bond-resolved scanning tunneling microscopy (BR-STM) unambiguously imaged the molecular backbone of a single [7]TQR with a triangular zigzag edge topology, which can be viewed as [7]triangulene decorated with a coronene-like antidot in the center. Additionally, dI/dV mapping reveals that both inner and outer zigzag edges contribute to the edge-localized and spin-polarized electronic states of [7]TQR. Both experimental results and spin-polarized density functional theory calculations indicate that [7]TQR retains its open-shell septuple ground state (S = 3) on Au(111). This work demonstrates a new route for the design of high-spin graphene quantum rings for future quantum devices. |
Zambare, Rahul S; Nemade, Parag R Ionic liquid-modified graphene oxide sponge for hexavalent chromium removal from water Journal Article COLLOIDS AND SURFACES A-PHYSICOCHEMICAL AND ENGINEERING ASPECTS, 609 , 2021, ISSN: 0927-7757. @article{ISI:000598932500009, title = {Ionic liquid-modified graphene oxide sponge for hexavalent chromium removal from water}, author = {Rahul S Zambare and Parag R Nemade}, doi = {10.1016/j.colsurfa.2020.125657}, issn = {0927-7757}, year = {2021}, date = {2021-01-01}, journal = {COLLOIDS AND SURFACES A-PHYSICOCHEMICAL AND ENGINEERING ASPECTS}, volume = {609}, publisher = {ELSEVIER}, address = {RADARWEG 29, 1043 NX AMSTERDAM, NETHERLANDS}, abstract = {Chromium pollution in groundwater is a major cause of concern due to significant health risks of hexavalent chromium (Cr(VI)) ions. Graphene oxide (GO) covalently modified with 1-aminopropyl-3-methylimidazolium bromide (mimNH(2)) and freeze-dried to give 1-aminopropyl-3-methylimidazolium bromide functionalized graphene oxide sponge (mimGO sponge). mimGO sponge effectively adsorbed hexavalent chromium (Cr(VI), toxic) by electrostatic interactions and reduced Cr(VI) to trivalent chromium (Cr(III), less toxic) by an indirect mechanism with the contribution of delocalized pi-electrons on the six-membered aromatic ring of mimGO sponge. The change in contact time, adsorbent loading, initial Cr(VI) solution concentration, and different types of adsorbents were investigated. The experimental data were fitted to the pseudo-second order kinetic model, which showed hydro-chromate adsorption on mimGO sponge was surface adsorption with chemical interaction. The adsorption process was well-defined by the Langmuir isotherm model, and equilibrium adsorption capacity was 208.3 mg/g at 23 degrees C in an acidic environment. The electrostatic interaction between the chromate ions and mimGO sponge also caused a reduction of Cr(VI) to less benign Cr(III) on the adsorbent surface. Thus, mimGO sponge has a green adsorbent that showed the potential for Cr(VI) removal from water.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Chromium pollution in groundwater is a major cause of concern due to significant health risks of hexavalent chromium (Cr(VI)) ions. Graphene oxide (GO) covalently modified with 1-aminopropyl-3-methylimidazolium bromide (mimNH(2)) and freeze-dried to give 1-aminopropyl-3-methylimidazolium bromide functionalized graphene oxide sponge (mimGO sponge). mimGO sponge effectively adsorbed hexavalent chromium (Cr(VI), toxic) by electrostatic interactions and reduced Cr(VI) to trivalent chromium (Cr(III), less toxic) by an indirect mechanism with the contribution of delocalized pi-electrons on the six-membered aromatic ring of mimGO sponge. The change in contact time, adsorbent loading, initial Cr(VI) solution concentration, and different types of adsorbents were investigated. The experimental data were fitted to the pseudo-second order kinetic model, which showed hydro-chromate adsorption on mimGO sponge was surface adsorption with chemical interaction. The adsorption process was well-defined by the Langmuir isotherm model, and equilibrium adsorption capacity was 208.3 mg/g at 23 degrees C in an acidic environment. The electrostatic interaction between the chromate ions and mimGO sponge also caused a reduction of Cr(VI) to less benign Cr(III) on the adsorbent surface. Thus, mimGO sponge has a green adsorbent that showed the potential for Cr(VI) removal from water. |
2020 |
Lian, Xu; Ma, Zhirui; Zhang, Zhonghan; Yang, Jinlin; Sun, Shuo; Gu, Chengding; Liu, Yuan; Ding, Honghe; Hu, Jun; Cao, Xu; Zhu, Junfa; Li, Shuzhou; Chen, Wei An in-situ spectroscopy investigation of alkali metal interaction mechanism with the imide functional group Journal Article NANO RESEARCH, 13 (12), pp. 3224-3229, 2020, ISSN: 1998-0124. @article{ISI:000559369900001, title = {An in-situ spectroscopy investigation of alkali metal interaction mechanism with the imide functional group}, author = {Xu Lian and Zhirui Ma and Zhonghan Zhang and Jinlin Yang and Shuo Sun and Chengding Gu and Yuan Liu and Honghe Ding and Jun Hu and Xu Cao and Junfa Zhu and Shuzhou Li and Wei Chen}, doi = {10.1007/s12274-020-2991-6, Early Access Date = AUG 2020}, issn = {1998-0124}, year = {2020}, date = {2020-12-01}, journal = {NANO RESEARCH}, volume = {13}, number = {12}, pages = {3224-3229}, publisher = {TSINGHUA UNIV PRESS}, address = {B605D, XUE YAN BUILDING, BEIJING, 100084, PEOPLES R CHINA}, abstract = {Organic anode materials have attracted considerable interest owing to their high tunability by adopting various active functional groups. However, the interaction mechanisms between the alkali metals and the active functional groups in host materials have been rarely studied systematically. Here, a widely used organic semiconductor of perylene-3,4,9,10-tetracarboxylic diimide (PTCDI) was selected as a model system to investigate how alkali metals interact with imide functional groups and induce changes in chemical and electronic structures of PTCDI. The interaction at the alkali/PTCDI interface was probed byin-situX-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), synchrotron-based near edge X-ray absorption fine structure (NEXAFS), and corroborated by density functional theory (DFT) calculations. Our results indicate that the alkali metal replaces the hydrogen atoms in the imide group and interact with the imide nitrogen of PTCDI. Electron transfer induced gap states and downward band-bending like effects are identified on the alkali-deposited PTCDI surface. It was found that Na shows a stronger electron transfer effect than Li. Such a model study of alkali insertion/intercalation in PTCDI gives insights for the exploration of the potential host materials for alkali storage applications.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Organic anode materials have attracted considerable interest owing to their high tunability by adopting various active functional groups. However, the interaction mechanisms between the alkali metals and the active functional groups in host materials have been rarely studied systematically. Here, a widely used organic semiconductor of perylene-3,4,9,10-tetracarboxylic diimide (PTCDI) was selected as a model system to investigate how alkali metals interact with imide functional groups and induce changes in chemical and electronic structures of PTCDI. The interaction at the alkali/PTCDI interface was probed byin-situX-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), synchrotron-based near edge X-ray absorption fine structure (NEXAFS), and corroborated by density functional theory (DFT) calculations. Our results indicate that the alkali metal replaces the hydrogen atoms in the imide group and interact with the imide nitrogen of PTCDI. Electron transfer induced gap states and downward band-bending like effects are identified on the alkali-deposited PTCDI surface. It was found that Na shows a stronger electron transfer effect than Li. Such a model study of alkali insertion/intercalation in PTCDI gives insights for the exploration of the potential host materials for alkali storage applications. |
Liang, Qijie; Gou, Jian; Arramel, ; Zhang, Qian; Zhang, Wenjing; Wee, Andrew Thye Shen Oxygen-induced controllable p-type doping in 2D semiconductor transition metal dichalcogenides Journal Article NANO RESEARCH, 13 (12), pp. 3439-3444, 2020, ISSN: 1998-0124. @article{ISI:000567368300001, title = {Oxygen-induced controllable p-type doping in 2D semiconductor transition metal dichalcogenides}, author = {Qijie Liang and Jian Gou and Arramel and Qian Zhang and Wenjing Zhang and Andrew Thye Shen Wee}, doi = {10.1007/s12274-020-3038-8, Early Access Date = SEP 2020}, issn = {1998-0124}, year = {2020}, date = {2020-12-01}, journal = {NANO RESEARCH}, volume = {13}, number = {12}, pages = {3439-3444}, publisher = {TSINGHUA UNIV PRESS}, address = {B605D, XUE YAN BUILDING, BEIJING, 100084, PEOPLES R CHINA}, abstract = {Exposure to oxygen alters the physical and chemical properties of two-dimensional (2D) transition metal dichalcogenides (TMDs). In particular, oxygen in the ambient may influence the device stability of 2D TMDs over time. Engineering the doping of 2D TMDs, especially hole doping is highly desirable towards their device function. Herein, controllable oxygen-induced p-type doping in a range of hexagonal (MoTe2, WSe2, MoSe(2)and PtSe2) and pentagonal (PdSe2) 2D TMDs are demonstrated. Scanning tunneling microscopy, electrical transport and X-ray photoelectron spectroscopy are used to probe the origin of oxygen-derived hole doping. Three mechanisms are postulated that contribute to the hole doping in 2D TMDs, namely charge transfer from absorbed oxygen molecules, surface oxides, and chalcogen atom substitution. This work provides insights into the doping effects of oxygen, enabling the engineering of 2D TMDs properties for nanoelectronic applications.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Exposure to oxygen alters the physical and chemical properties of two-dimensional (2D) transition metal dichalcogenides (TMDs). In particular, oxygen in the ambient may influence the device stability of 2D TMDs over time. Engineering the doping of 2D TMDs, especially hole doping is highly desirable towards their device function. Herein, controllable oxygen-induced p-type doping in a range of hexagonal (MoTe2, WSe2, MoSe(2)and PtSe2) and pentagonal (PdSe2) 2D TMDs are demonstrated. Scanning tunneling microscopy, electrical transport and X-ray photoelectron spectroscopy are used to probe the origin of oxygen-derived hole doping. Three mechanisms are postulated that contribute to the hole doping in 2D TMDs, namely charge transfer from absorbed oxygen molecules, surface oxides, and chalcogen atom substitution. This work provides insights into the doping effects of oxygen, enabling the engineering of 2D TMDs properties for nanoelectronic applications. |
Li, Xing; Xu, Hai-Sen; Leng, Kai; Chee, See Wee; Zhao, Xiaoxu; Jain, Noopur; Xu, Hai; Qiao, Jingsi; Gao, Qiang; Park, In-Hyeok; Quek, Su Ying; Mirsaidov, Utkur; Loh, Kian Ping Partitioning the interlayer space of covalent organic frameworks by embedding pseudorotaxanes in their backbones Journal Article NATURE CHEMISTRY, 12 (12), pp. 1115+, 2020, ISSN: 1755-4330. @article{ISI:000584339900001, title = {Partitioning the interlayer space of covalent organic frameworks by embedding pseudorotaxanes in their backbones}, author = {Xing Li and Hai-Sen Xu and Kai Leng and See Wee Chee and Xiaoxu Zhao and Noopur Jain and Hai Xu and Jingsi Qiao and Qiang Gao and In-Hyeok Park and Su Ying Quek and Utkur Mirsaidov and Kian Ping Loh}, doi = {10.1038/s41557-020-00562-5, Early Access Date = NOV 2020}, issn = {1755-4330}, year = {2020}, date = {2020-12-01}, journal = {NATURE CHEMISTRY}, volume = {12}, number = {12}, pages = {1115+}, publisher = {NATURE RESEARCH}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {Mono- or few-layer sheets of covalent organic frameworks (COFs) represent an attractive platform of two-dimensional materials that hold promise for tailor-made functionality and pores, through judicious design of the COF building blocks. But although a wide variety of layered COFs have been synthesized, cleaving their interlayer stacking to obtain COF sheets of uniform thickness has remained challenging. Here, we have partitioned the interlayer space in COFs by incorporating pseudorotaxane units into their backbones. Macrocyclic hosts based on crown ethers were embedded into either a ditopic or a tetratopic acylhydrazide building block. Reaction with a tritopic aldehyde linker led to the formation of acylhydrazone-based layered COFs in which one basal plane is composed of either one layer, in the case of the ditopic macrocyclic component, or two adjacent layers covalently held together by its tetratopic counterpart. When a viologen threading unit is introduced, the formation of a host-guest complex facilitates the self-exfoliation of the COFs into crystalline monolayers or bilayers, respectively.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Mono- or few-layer sheets of covalent organic frameworks (COFs) represent an attractive platform of two-dimensional materials that hold promise for tailor-made functionality and pores, through judicious design of the COF building blocks. But although a wide variety of layered COFs have been synthesized, cleaving their interlayer stacking to obtain COF sheets of uniform thickness has remained challenging. Here, we have partitioned the interlayer space in COFs by incorporating pseudorotaxane units into their backbones. Macrocyclic hosts based on crown ethers were embedded into either a ditopic or a tetratopic acylhydrazide building block. Reaction with a tritopic aldehyde linker led to the formation of acylhydrazone-based layered COFs in which one basal plane is composed of either one layer, in the case of the ditopic macrocyclic component, or two adjacent layers covalently held together by its tetratopic counterpart. When a viologen threading unit is introduced, the formation of a host-guest complex facilitates the self-exfoliation of the COFs into crystalline monolayers or bilayers, respectively. |
Zhang, Zheye; Zhao, Xiaoxu; Xi, Shibo; Zhang, Lili; Chen, Zhongxin; Zeng, Zhiping; Huang, Ming; Yang, Hongbin; Liu, Bin; Pennycook, Stephen J; Chen, Peng ADVANCED ENERGY MATERIALS, 10 (48), 2020, ISSN: 1614-6832. @article{ISI:000587349700001, title = {Atomically Dispersed Cobalt Trifunctional Electrocatalysts with Tailored Coordination Environment for Flexible Rechargeable Zn-Air Battery and Self-Driven Water Splitting}, author = {Zheye Zhang and Xiaoxu Zhao and Shibo Xi and Lili Zhang and Zhongxin Chen and Zhiping Zeng and Ming Huang and Hongbin Yang and Bin Liu and Stephen J Pennycook and Peng Chen}, doi = {10.1002/aenm.202002896, Early Access Date = NOV 2020}, issn = {1614-6832}, year = {2020}, date = {2020-12-01}, journal = {ADVANCED ENERGY MATERIALS}, volume = {10}, number = {48}, publisher = {WILEY-V C H VERLAG GMBH}, address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY}, abstract = {Designing multifunctional catalysts with high activity, stability, and low-cost for energy storage and conversion is a significant challenge. Herein, a trifunctional electrocatalyst is synthesized by anchoring individually dispersed Co atoms on N and S codoped hollow carbon spheres (CoSA/N,S-HCS), which exhibits outstanding catalytic activity and stability for the oxygen reduction reaction, oxygen evolution reaction, and hydrogen evolution reaction. When equipped in liquid or flexible solid-state rechargeable Zn-air batteries, CoSA/N,S-HCS endows them with high power and energy density as well as excellent long-term cycling stability, outperforming benchmark batteries based on a commercial Pt/C + RuO2 dual catalyst system. Furthermore, a self-driven water splitting system powered by flexible Zn-air batteries is demonstrated using CoSA/N,S-HCS as the sole catalyst, giving a high H-2 evolution rate of 184 mmol h(-1). The state-of-art experimental characterizations and theoretical calculations reveal synergistic cooperation between atomically dispersed Co-N-4 active sites, nearby electron-donating S dopants, and the unique carbon support to single-atom catalysts (SACs). This work demonstrates a general strategy to design various multifunctional SAC systems with a tailored coordination environment.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Designing multifunctional catalysts with high activity, stability, and low-cost for energy storage and conversion is a significant challenge. Herein, a trifunctional electrocatalyst is synthesized by anchoring individually dispersed Co atoms on N and S codoped hollow carbon spheres (CoSA/N,S-HCS), which exhibits outstanding catalytic activity and stability for the oxygen reduction reaction, oxygen evolution reaction, and hydrogen evolution reaction. When equipped in liquid or flexible solid-state rechargeable Zn-air batteries, CoSA/N,S-HCS endows them with high power and energy density as well as excellent long-term cycling stability, outperforming benchmark batteries based on a commercial Pt/C + RuO2 dual catalyst system. Furthermore, a self-driven water splitting system powered by flexible Zn-air batteries is demonstrated using CoSA/N,S-HCS as the sole catalyst, giving a high H-2 evolution rate of 184 mmol h(-1). The state-of-art experimental characterizations and theoretical calculations reveal synergistic cooperation between atomically dispersed Co-N-4 active sites, nearby electron-donating S dopants, and the unique carbon support to single-atom catalysts (SACs). This work demonstrates a general strategy to design various multifunctional SAC systems with a tailored coordination environment. |
Li, Bochang; Li, Sifan; Wang, Han; Chen, Li; Liu, Liang; Feng, Xuewei; Li, Yesheng; Chen, Jingsheng; Gong, X; Ang, Kah-Wee An Electronic Synapse Based on 2D Ferroelectric CuInP2S6 Journal Article ADVANCED ELECTRONIC MATERIALS, 6 (12), 2020, ISSN: 2199-160X. @article{ISI:000587430400001, title = {An Electronic Synapse Based on 2D Ferroelectric CuInP2S6}, author = {Bochang Li and Sifan Li and Han Wang and Li Chen and Liang Liu and Xuewei Feng and Yesheng Li and Jingsheng Chen and X Gong and Kah-Wee Ang}, doi = {10.1002/aelm.202000760, Early Access Date = NOV 2020}, issn = {2199-160X}, year = {2020}, date = {2020-12-01}, journal = {ADVANCED ELECTRONIC MATERIALS}, volume = {6}, number = {12}, publisher = {WILEY}, address = {111 RIVER ST, HOBOKEN 07030-5774, NJ USA}, abstract = {Memristors with biological synaptic behaviors and functions have been intensively studied as an important component for neuromorphic computing system, which hold promise to address the power consumption issue in modern computers based on von Neumann architecture. However, the resistive switching mechanism that relies on the stochastic formation of conductive filaments leads to poor cycle-to-cycle (temporal) and cell-to-cell (spatial) variations for filamentary memristors. The emergence of memristors based on 2D ferroelectric materials can potentially avoid these issues. Here, a vertical Au/CuInP2S6 (CIPS)/Ti diode is demonstrated using exfoliated ferroelectric CIPS flake. Through ferroelectric switching, the CIPS diode realizes resistive switching with a ratio larger than 6 x 10(3). The endurance measurement shows a small set and reset voltage variation of 5.3% and 9.1%, respectively. Key synaptic behaviors including spike-time-dependent plasticity, paired-pulse-facilitation, and paired-pulse-depression are successfully mimicked, manifesting the potential application of CIPS diode in a neuromorphic computing system. Moreover, pattern learning and memory behaviors are emulated using a 3 x 3 CIPS crossbar array.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Memristors with biological synaptic behaviors and functions have been intensively studied as an important component for neuromorphic computing system, which hold promise to address the power consumption issue in modern computers based on von Neumann architecture. However, the resistive switching mechanism that relies on the stochastic formation of conductive filaments leads to poor cycle-to-cycle (temporal) and cell-to-cell (spatial) variations for filamentary memristors. The emergence of memristors based on 2D ferroelectric materials can potentially avoid these issues. Here, a vertical Au/CuInP2S6 (CIPS)/Ti diode is demonstrated using exfoliated ferroelectric CIPS flake. Through ferroelectric switching, the CIPS diode realizes resistive switching with a ratio larger than 6 x 10(3). The endurance measurement shows a small set and reset voltage variation of 5.3% and 9.1%, respectively. Key synaptic behaviors including spike-time-dependent plasticity, paired-pulse-facilitation, and paired-pulse-depression are successfully mimicked, manifesting the potential application of CIPS diode in a neuromorphic computing system. Moreover, pattern learning and memory behaviors are emulated using a 3 x 3 CIPS crossbar array. |
Latychevskaia, Tatiana; Zou, Yichao; Woods, Colin Robert; Wang, Yi Bo; Holwill, Matthew; Prestat, Eric; Haigh, Sarah J; Novoselov, Kostya S ULTRAMICROSCOPY, 219 , 2020, ISSN: 0304-3991. @article{ISI:000594768500009, title = {Holographic reconstruction of the interlayer distance of bilayer two-dimensional crystal samples from their convergent beam electron diffraction patterns}, author = {Tatiana Latychevskaia and Yichao Zou and Colin Robert Woods and Yi Bo Wang and Matthew Holwill and Eric Prestat and Sarah J Haigh and Kostya S Novoselov}, doi = {10.1016/j.ultramic.2020.113020}, issn = {0304-3991}, year = {2020}, date = {2020-12-01}, journal = {ULTRAMICROSCOPY}, volume = {219}, publisher = {ELSEVIER}, address = {RADARWEG 29, 1043 NX AMSTERDAM, NETHERLANDS}, abstract = {The convergent beam electron diffraction (CBED) patterns of twisted bilayer samples exhibit interference patterns in their CBED spots. Such interference patterns can be treated as off-axis holograms and the phase of the scattered waves, meaning the interlayer distance can be reconstructed. A detailed protocol of the reconstruction procedure is provided in this study. In addition, we derive an exact formula for reconstructing the interlayer distance from the recovered phase distribution, which takes into account the different chemical compositions of the individual monolayers. It is shown that one interference fringe in a CBED spot is sufficient to reconstruct the distance between the layers, which can be practical for imaging samples with a relatively small twist angle or when probing small sample regions. The quality of the reconstructed interlayer distance is studied as a function of the twist angle. At smaller twist angles, the reconstructed interlayer distance distribution is more precise and artefact free. At larger twist angles, artefacts due to the moire structure appear in the reconstruction. A method for the reconstruction of the average interlayer distance is presented. As for resolution, the interlayer distance can be reconstructed by the holographic approach at an accuracy of +/- 0.5 angstrom, which is a few hundred times better than the intrinsic z-resolution of diffraction limited resolution, as expressed through the spread of the measured k-values. Moreover, we show that holographic CBED imaging can detect variations as small as 0.1 angstrom in the interlayer distance, though the quantitative reconstruction of such variations suffers from large errors.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The convergent beam electron diffraction (CBED) patterns of twisted bilayer samples exhibit interference patterns in their CBED spots. Such interference patterns can be treated as off-axis holograms and the phase of the scattered waves, meaning the interlayer distance can be reconstructed. A detailed protocol of the reconstruction procedure is provided in this study. In addition, we derive an exact formula for reconstructing the interlayer distance from the recovered phase distribution, which takes into account the different chemical compositions of the individual monolayers. It is shown that one interference fringe in a CBED spot is sufficient to reconstruct the distance between the layers, which can be practical for imaging samples with a relatively small twist angle or when probing small sample regions. The quality of the reconstructed interlayer distance is studied as a function of the twist angle. At smaller twist angles, the reconstructed interlayer distance distribution is more precise and artefact free. At larger twist angles, artefacts due to the moire structure appear in the reconstruction. A method for the reconstruction of the average interlayer distance is presented. As for resolution, the interlayer distance can be reconstructed by the holographic approach at an accuracy of +/- 0.5 angstrom, which is a few hundred times better than the intrinsic z-resolution of diffraction limited resolution, as expressed through the spread of the measured k-values. Moreover, we show that holographic CBED imaging can detect variations as small as 0.1 angstrom in the interlayer distance, though the quantitative reconstruction of such variations suffers from large errors. |
Yang, Yaping; Li, Jidong; Yin, Jun; Xu, Shuigang; Mullan, Ciaran; Taniguchi, Takashi; Watanabe, Kenji; Geim, Andre K; Novoselov, Konstantin S; Mishchenko, Artem In situ manipulation of van der Waals heterostructures for twistronics Journal Article SCIENCE ADVANCES, 6 (49), 2020, ISSN: 2375-2548. @article{ISI:000596477400035, title = {In situ manipulation of van der Waals heterostructures for twistronics}, author = {Yaping Yang and Jidong Li and Jun Yin and Shuigang Xu and Ciaran Mullan and Takashi Taniguchi and Kenji Watanabe and Andre K Geim and Konstantin S Novoselov and Artem Mishchenko}, doi = {10.1126/sciadv.abd3655}, issn = {2375-2548}, year = {2020}, date = {2020-12-01}, journal = {SCIENCE ADVANCES}, volume = {6}, number = {49}, publisher = {AMER ASSOC ADVANCEMENT SCIENCE}, address = {1200 NEW YORK AVE, NW, WASHINGTON, DC 20005 USA}, abstract = {In van der Waals heterostructures, electronic bands of two-dimensional (2D) materials, their nontrivial topology, and electron-electron interactions can be markedly changed by a moire pattern induced by twist angles between different layers. This process is referred to as twistronics, where the tuning of twist angle can be realized through mechanical manipulation of 2D materials. Here, we demonstrate an experimental technique that can achieve in situ dynamical rotation and manipulation of 2D materials in van der Waals heterostructures. Using this technique, we fabricated heterostructures where graphene is perfectly aligned with both top and bottom encapsulating layers of hexagonal boron nitride. Our technique enables twisted 2D material systems in one single stack with dynamically tunable optical, mechanical, and electronic properties.}, keywords = {}, pubstate = {published}, tppubtype = {article} } In van der Waals heterostructures, electronic bands of two-dimensional (2D) materials, their nontrivial topology, and electron-electron interactions can be markedly changed by a moire pattern induced by twist angles between different layers. This process is referred to as twistronics, where the tuning of twist angle can be realized through mechanical manipulation of 2D materials. Here, we demonstrate an experimental technique that can achieve in situ dynamical rotation and manipulation of 2D materials in van der Waals heterostructures. Using this technique, we fabricated heterostructures where graphene is perfectly aligned with both top and bottom encapsulating layers of hexagonal boron nitride. Our technique enables twisted 2D material systems in one single stack with dynamically tunable optical, mechanical, and electronic properties. |
Ridolfi, Emilia; Trevisanutto, Paolo E; Pereira, Vitor M Expeditious computation of nonlinear optical properties of arbitrary order with native electronic interactions in the time domain Journal Article PHYSICAL REVIEW B, 102 (24), 2020, ISSN: 2469-9950. @article{ISI:000596083800004, title = {Expeditious computation of nonlinear optical properties of arbitrary order with native electronic interactions in the time domain}, author = {Emilia Ridolfi and Paolo E Trevisanutto and Vitor M Pereira}, doi = {10.1103/PhysRevB.102.245110}, issn = {2469-9950}, year = {2020}, date = {2020-12-01}, journal = {PHYSICAL REVIEW B}, volume = {102}, number = {24}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {We adapted a recently proposed framework to characterize the optical response of interacting electrons in solids in order to expedite its computation without compromise in accuracy at the microscopic level. Our formulation is based on reliable parametrizations of Hamiltonians and Coulomb interactions, which allows economy and flexibility in obtaining response functions. It is suited to computing the optical response to fields of arbitrary temporal shape and strength, to arbitrary order in the field, and natively accounts for excitonic effects. We demonstrate the approach by computing the frequency-dependent susceptibilities of MoS2 and hexagonal BN monolayers up to the third harmonic. Grounded on a generic nonequilibrium many-body perturbation theory, this framework allows extensions to handle generic interaction models or to describe electronic processes taking place at ultrafast time scales.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We adapted a recently proposed framework to characterize the optical response of interacting electrons in solids in order to expedite its computation without compromise in accuracy at the microscopic level. Our formulation is based on reliable parametrizations of Hamiltonians and Coulomb interactions, which allows economy and flexibility in obtaining response functions. It is suited to computing the optical response to fields of arbitrary temporal shape and strength, to arbitrary order in the field, and natively accounts for excitonic effects. We demonstrate the approach by computing the frequency-dependent susceptibilities of MoS2 and hexagonal BN monolayers up to the third harmonic. Grounded on a generic nonequilibrium many-body perturbation theory, this framework allows extensions to handle generic interaction models or to describe electronic processes taking place at ultrafast time scales. |
Han, Yingmei; Maglione, Maria Serena; Cabanes, Valentin Diez; Casado-Montenegro, Javier; Yu, Xiaojiang; Karuppannan, Senthil Kumar; Zhang, Ziyu; Crivillers, Nuria; Mas-Torrent, Marta; Rovira, Concepcio; Cornil, Jerome; Veciana, Jaume; Nijhuis, Christian A Reversal of the Direction of Rectification Induced by Fermi Level Pinning at Molecule-Electrode Interfaces in Redox-Active Tunneling Junctions Journal Article ACS APPLIED MATERIALS & INTERFACES, 12 (49), pp. 55044-55055, 2020, ISSN: 1944-8244. @article{ISI:000599505200077, title = {Reversal of the Direction of Rectification Induced by Fermi Level Pinning at Molecule-Electrode Interfaces in Redox-Active Tunneling Junctions}, author = {Yingmei Han and Maria Serena Maglione and Valentin Diez Cabanes and Javier Casado-Montenegro and Xiaojiang Yu and Senthil Kumar Karuppannan and Ziyu Zhang and Nuria Crivillers and Marta Mas-Torrent and Concepcio Rovira and Jerome Cornil and Jaume Veciana and Christian A Nijhuis}, doi = {10.1021/acsami.0c15435}, issn = {1944-8244}, year = {2020}, date = {2020-12-01}, journal = {ACS APPLIED MATERIALS & INTERFACES}, volume = {12}, number = {49}, pages = {55044-55055}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Control over the energy level alignment in molecular junctions is notoriously difficult, making it challenging to control basic electronic functions such as the direction of rectification. Therefore, alternative approaches to control electronic functions in molecular junctions are needed. This paper describes switching of the direction of rectification by changing the bottom electrode material M = Ag, Au, or Pt in M-S(CH2)(11)S-BTTF//EGaIn junctions based on self-assembled monolayers incorporating benzotetrathiafulvalene (BTTF) with EGaIn (eutectic alloy of Ga and In) as the top electrode. The stability of the junctions is determined by the choice of the bottom electrode, which, in turn, determines the maximum applied bias window, and the mechanism of rectification is dominated by the energy levels centered on the BTTF units. The energy level alignments of the three junctions are similar because of Fermi level pinning induced by charge transfer at the metal-thiolate interface and by a varying degree of additional charge transfer between BTTF and the metal. Density functional theory calculations show that the amount of electron transfer from M to the lowest unoccupied molecular orbital (LUMO) of BTTF follows the order Ag > Au > Pt. Junctions with Ag electrodes are the least stable and can only withstand an applied bias of +/- 1.0 V. As a result, no molecular orbitals can fall in the applied bias window, and the junctions do not rectify. The junction stability increases for M = Au, and the highest occupied molecular orbital (HOMO) dominates charge transport at a positive bias resulting in a positive rectification ratio of 83 at +/- 1.5 V. The junctions are very stable for M = Pt, but now the LUMO dominates charge transport at a negative bias resulting in a negative rectification ratio of 912 at +/- 2.5 V. Thus, the limitations of Fermi level pinning can be bypassed by a judicious choice of the bottom electrode material, making it possible to access selectively HOMO- or LUMO-based charge transport and, as shown here, associated reversal of rectification.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Control over the energy level alignment in molecular junctions is notoriously difficult, making it challenging to control basic electronic functions such as the direction of rectification. Therefore, alternative approaches to control electronic functions in molecular junctions are needed. This paper describes switching of the direction of rectification by changing the bottom electrode material M = Ag, Au, or Pt in M-S(CH2)(11)S-BTTF//EGaIn junctions based on self-assembled monolayers incorporating benzotetrathiafulvalene (BTTF) with EGaIn (eutectic alloy of Ga and In) as the top electrode. The stability of the junctions is determined by the choice of the bottom electrode, which, in turn, determines the maximum applied bias window, and the mechanism of rectification is dominated by the energy levels centered on the BTTF units. The energy level alignments of the three junctions are similar because of Fermi level pinning induced by charge transfer at the metal-thiolate interface and by a varying degree of additional charge transfer between BTTF and the metal. Density functional theory calculations show that the amount of electron transfer from M to the lowest unoccupied molecular orbital (LUMO) of BTTF follows the order Ag > Au > Pt. Junctions with Ag electrodes are the least stable and can only withstand an applied bias of +/- 1.0 V. As a result, no molecular orbitals can fall in the applied bias window, and the junctions do not rectify. The junction stability increases for M = Au, and the highest occupied molecular orbital (HOMO) dominates charge transport at a positive bias resulting in a positive rectification ratio of 83 at +/- 1.5 V. The junctions are very stable for M = Pt, but now the LUMO dominates charge transport at a negative bias resulting in a negative rectification ratio of 912 at +/- 2.5 V. Thus, the limitations of Fermi level pinning can be bypassed by a judicious choice of the bottom electrode material, making it possible to access selectively HOMO- or LUMO-based charge transport and, as shown here, associated reversal of rectification. |
Wong, Hon Fai; Ng, Sheung Mei; Zhang, Wen; Liu, Yu Kuai; Wong, Ping Kwan Johnny; Tang, Chi Sin; Lam, Ka Kin; Zhao, Xu Wen; Meng, Zhen Gong; Fei, Lin Feng; Cheng, Wang Fai; von Nordheim, Danny; Wong, Wai Yeung; Wang, Zong Rong; Ploss, Bernd; Dai, Ji-Yan; Mak, Chee Leung; Wee, Andrew Thye Shen; Leung, Chi Wah Modulating Magnetism in Ferroelectric Polymer-Gated Perovskite Manganite Films with Moderate Gate Pulse Chains Journal Article ACS APPLIED MATERIALS & INTERFACES, 12 (50), pp. 56541-56548, 2020, ISSN: 1944-8244. @article{ISI:000600202300097, title = {Modulating Magnetism in Ferroelectric Polymer-Gated Perovskite Manganite Films with Moderate Gate Pulse Chains}, author = {Hon Fai Wong and Sheung Mei Ng and Wen Zhang and Yu Kuai Liu and Ping Kwan Johnny Wong and Chi Sin Tang and Ka Kin Lam and Xu Wen Zhao and Zhen Gong Meng and Lin Feng Fei and Wang Fai Cheng and Danny von Nordheim and Wai Yeung Wong and Zong Rong Wang and Bernd Ploss and Ji-Yan Dai and Chee Leung Mak and Andrew Thye Shen Wee and Chi Wah Leung}, doi = {10.1021/acsami.0c14172}, issn = {1944-8244}, year = {2020}, date = {2020-12-01}, journal = {ACS APPLIED MATERIALS & INTERFACES}, volume = {12}, number = {50}, pages = {56541-56548}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Most previous attempts on achieving electric-field manipulation of ferromagnetism in complex oxides, such as La0.66Sr0.33MnO3 (LSMO), are based on electrostatically induced charge carrier changes through high-k dielectrics or ferroelectrics. Here, the use of a ferroelectric copolymer, polyvinylidene fluoride with trifluoroethylene [P(VDF-TrFE)], as a gate dielectric to successfully modulate the ferromagnetism of the LSMO thin film in a field-effect device geometry is demonstrated. Specifically, through the application of low-voltage pulse chains inadequate to switch the electric dipoles of the copolymer, enhanced tunability of the oxide magnetic response is obtained, compared to that induced by ferroelectric polarization. Such observations have been attributed to electric field-induced oxygen vacancy accumulation/ depletion in the LSMO layer upon the application of pulse chains, which is supported by surface-sensitive-characterization techniques, including X-ray photoelectron spectroscopy and X-ray magnetic circular dichroism. These techniques not only unveil the electrochemical nature of the mechanism but also establish a direct correlation between the oxygen vacancies created and subsequent changes to the valence states of Mn ions in LSMO. These demonstrations based on the pulsing strategy can be a viable route equally applicable to other functional oxides for the construction of electric field-controlled magnetic devices.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Most previous attempts on achieving electric-field manipulation of ferromagnetism in complex oxides, such as La0.66Sr0.33MnO3 (LSMO), are based on electrostatically induced charge carrier changes through high-k dielectrics or ferroelectrics. Here, the use of a ferroelectric copolymer, polyvinylidene fluoride with trifluoroethylene [P(VDF-TrFE)], as a gate dielectric to successfully modulate the ferromagnetism of the LSMO thin film in a field-effect device geometry is demonstrated. Specifically, through the application of low-voltage pulse chains inadequate to switch the electric dipoles of the copolymer, enhanced tunability of the oxide magnetic response is obtained, compared to that induced by ferroelectric polarization. Such observations have been attributed to electric field-induced oxygen vacancy accumulation/ depletion in the LSMO layer upon the application of pulse chains, which is supported by surface-sensitive-characterization techniques, including X-ray photoelectron spectroscopy and X-ray magnetic circular dichroism. These techniques not only unveil the electrochemical nature of the mechanism but also establish a direct correlation between the oxygen vacancies created and subsequent changes to the valence states of Mn ions in LSMO. These demonstrations based on the pulsing strategy can be a viable route equally applicable to other functional oxides for the construction of electric field-controlled magnetic devices. |
Garcia, Jose H; Vila, Marc; Hsu, Chuang-Han; Waintal, Xavier; Pereira, Vitor M; Roche, Stephan Canted Persistent Spin Texture and Quantum Spin Hall Effect in WTe2 Journal Article PHYSICAL REVIEW LETTERS, 125 (25), 2020, ISSN: 0031-9007. @article{ISI:000600286000017, title = {Canted Persistent Spin Texture and Quantum Spin Hall Effect in WTe2}, author = {Jose H Garcia and Marc Vila and Chuang-Han Hsu and Xavier Waintal and Vitor M Pereira and Stephan Roche}, doi = {10.1103/PhysRevLett.125.256603}, issn = {0031-9007}, year = {2020}, date = {2020-12-01}, journal = {PHYSICAL REVIEW LETTERS}, volume = {125}, number = {25}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {We report an unconventional quantum spin Hall phase in the monolayer WTe2, which exhibits hitherto unknown features in other topological materials. The low symmetry of the structure induces a canted spin texture in the yz plane, which dictates the spin polarization of topologically protected boundary states. Additionally, the spin Hall conductivity gets quantized (2e(2)/h) with a spin quantization axis parallel to the canting direction. These findings are based on large-scale quantum simulations of the spin Hall conductivity tensor and nonlocal resistances in multiprobe geometries using a realistic tight-binding model elaborated from first-principle methods. The observation of this canted quantum spin Hall effect, related to the formation of topological edge states with nontrivial spin polarization, demands for specific experimental design and suggests interesting alternatives for manipulating spin information in topological materials.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We report an unconventional quantum spin Hall phase in the monolayer WTe2, which exhibits hitherto unknown features in other topological materials. The low symmetry of the structure induces a canted spin texture in the yz plane, which dictates the spin polarization of topologically protected boundary states. Additionally, the spin Hall conductivity gets quantized (2e(2)/h) with a spin quantization axis parallel to the canting direction. These findings are based on large-scale quantum simulations of the spin Hall conductivity tensor and nonlocal resistances in multiprobe geometries using a realistic tight-binding model elaborated from first-principle methods. The observation of this canted quantum spin Hall effect, related to the formation of topological edge states with nontrivial spin polarization, demands for specific experimental design and suggests interesting alternatives for manipulating spin information in topological materials. |
Saini, Mahesh; Singh, Ranveer; Sooraj, K P; Basu, Tanmoy; Roy, Abhijit; Satpati, Biswarup; Srivastava, Sanjeev Kumar; Ranjan, Mukesh; Som, Tapobrata Cold cathode electron emission with ultralow turn-on fields from Au-nanoparticle-decorated self-organized Si nanofacets Journal Article JOURNAL OF MATERIALS CHEMISTRY C, 8 (47), pp. 16880-16895, 2020, ISSN: 2050-7526. @article{ISI:000600128000024, title = {Cold cathode electron emission with ultralow turn-on fields from Au-nanoparticle-decorated self-organized Si nanofacets}, author = {Mahesh Saini and Ranveer Singh and K P Sooraj and Tanmoy Basu and Abhijit Roy and Biswarup Satpati and Sanjeev Kumar Srivastava and Mukesh Ranjan and Tapobrata Som}, doi = {10.1039/d0tc03862h}, issn = {2050-7526}, year = {2020}, date = {2020-12-01}, journal = {JOURNAL OF MATERIALS CHEMISTRY C}, volume = {8}, number = {47}, pages = {16880-16895}, publisher = {ROYAL SOC CHEMISTRY}, address = {THOMAS GRAHAM HOUSE, SCIENCE PARK, MILTON RD, CAMBRIDGE CB4 0WF, CAMBS, ENGLAND}, abstract = {Fabrication of highly dense conical nanostructures and their subsequent controlled metallization make them ideal candidates for enhancing cold cathode electron emission efficiency. For instance, hierarchical growth of self-assembled noble metal nanoparticles on self-organized nanostructured materials offers the advantage of fabricating low threshold cold cathode electron emitters. However, the fabrication of stable Si nanostructure-based cold cathode electron emitters with ultra-low threshold fields is a challenging task. This report presents cold cathode electron emission from Au-NP-decorated ensembles of self-organized silicon nanofacets (Si-NFs) having fascinating ultralow turn-on fields (as low as 0.27 V mu m(-1)) and remarkably low threshold electric fields (as low as 0.37 V mu m(-1)) with outstanding stability. It is interesting to note that even as-prepared Si-NFs offer hitherto unseen low turn-on fields (as low as 0.58 V mu m(-1)) and threshold electric fields (0.66 V mu m(-1)) - so far as silicon-based nanostructures are concerned. Kelvin probe force microscopy studies reveal that tunability in the work function of Au-NP-decorated Si-NF samples depends on the dimension and growth-angle of gold nanoparticles (Au-NPs). In addition, in-depth dual pass tunnelling current microscopy measurements demonstrate that the Au-NPs on the apexes and sidewalls of Si-NFs act as cold cathode electron emission sites which help to improve the turn-on and threshold fields for Au-NP-decorated Si-NFs in comparison to their as-prepared counterparts where electron emission takes place mostly from their sidewalls and valleys. Furthermore, finite element electrostatic field-based simulations reinforce the experimental observations. The present investigation paves the pathway to the fabrication of self-organized Si nanostructure-based highly stable cold cathode electron emitting devices having fascinating low turn-on and threshold fields along with extremely high field enhancement factors and high stability for use in nanoscale electronic devices.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Fabrication of highly dense conical nanostructures and their subsequent controlled metallization make them ideal candidates for enhancing cold cathode electron emission efficiency. For instance, hierarchical growth of self-assembled noble metal nanoparticles on self-organized nanostructured materials offers the advantage of fabricating low threshold cold cathode electron emitters. However, the fabrication of stable Si nanostructure-based cold cathode electron emitters with ultra-low threshold fields is a challenging task. This report presents cold cathode electron emission from Au-NP-decorated ensembles of self-organized silicon nanofacets (Si-NFs) having fascinating ultralow turn-on fields (as low as 0.27 V mu m(-1)) and remarkably low threshold electric fields (as low as 0.37 V mu m(-1)) with outstanding stability. It is interesting to note that even as-prepared Si-NFs offer hitherto unseen low turn-on fields (as low as 0.58 V mu m(-1)) and threshold electric fields (0.66 V mu m(-1)) - so far as silicon-based nanostructures are concerned. Kelvin probe force microscopy studies reveal that tunability in the work function of Au-NP-decorated Si-NF samples depends on the dimension and growth-angle of gold nanoparticles (Au-NPs). In addition, in-depth dual pass tunnelling current microscopy measurements demonstrate that the Au-NPs on the apexes and sidewalls of Si-NFs act as cold cathode electron emission sites which help to improve the turn-on and threshold fields for Au-NP-decorated Si-NFs in comparison to their as-prepared counterparts where electron emission takes place mostly from their sidewalls and valleys. Furthermore, finite element electrostatic field-based simulations reinforce the experimental observations. The present investigation paves the pathway to the fabrication of self-organized Si nanostructure-based highly stable cold cathode electron emitting devices having fascinating low turn-on and threshold fields along with extremely high field enhancement factors and high stability for use in nanoscale electronic devices. |
Lyons, T P; Gillard, D; Molina-Sanchez, A; Misra, A; Withers, F; Keatley, P S; Kozikov, A; Taniguchi, T; Watanabe, K; Novoselov, K S; Fernandez-Rossier, J; Tartakovskii, A I Interplay between spin proximity effect and charge-dependent exciton dynamics in MoSe2/CrBr3 van der Waals heterostructures Journal Article NATURE COMMUNICATIONS, 11 (1), 2020, ISSN: 2041-1723. @article{ISI:000596017800003, title = {Interplay between spin proximity effect and charge-dependent exciton dynamics in MoSe2/CrBr3 van der Waals heterostructures}, author = {T P Lyons and D Gillard and A Molina-Sanchez and A Misra and F Withers and P S Keatley and A Kozikov and T Taniguchi and K Watanabe and K S Novoselov and J Fernandez-Rossier and A I Tartakovskii}, doi = {10.1038/s41467-020-19816-4}, issn = {2041-1723}, year = {2020}, date = {2020-12-01}, journal = {NATURE COMMUNICATIONS}, volume = {11}, number = {1}, publisher = {NATURE RESEARCH}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {Semiconducting ferromagnet-nonmagnet interfaces in van der Waals heterostructures present a unique opportunity to investigate magnetic proximity interactions dependent upon a multitude of phenomena including valley and layer pseudospins, moire periodicity, or exceptionally strong Coulomb binding. Here, we report a charge-state dependency of the magnetic proximity effects between MoSe2 and CrBr3 in photoluminescence, whereby the valley polarization of the MoSe2 trion state conforms closely to the local CrBr3 magnetization, while the neutral exciton state remains insensitive to the ferromagnet. We attribute this to spin-dependent interlayer charge transfer occurring on timescales between the exciton and trion radiative lifetimes. Going further, we uncover by both the magneto-optical Kerr effect and photoluminescence a domain-like spatial topography of contrasting valley polarization, which we infer to be labyrinthine or otherwise highly intricate, with features smaller than 400 nm corresponding to our optical resolution. Our findings offer a unique insight into the interplay between short-lived valley excitons and spin-dependent interlayer tunneling, while also highlighting MoSe2 as a promising candidate to optically interface with exotic spin textures in van der Waals structures. One advantage of van der Waals materials is the ability to combine different materials in layers to form new heterostructures. Here, the authors investigate heterostructures of CrBr3 and MoSe2, and find that the ferromagnetism of CrBr3 enhances the valley dependent optical response of the MoSe2.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Semiconducting ferromagnet-nonmagnet interfaces in van der Waals heterostructures present a unique opportunity to investigate magnetic proximity interactions dependent upon a multitude of phenomena including valley and layer pseudospins, moire periodicity, or exceptionally strong Coulomb binding. Here, we report a charge-state dependency of the magnetic proximity effects between MoSe2 and CrBr3 in photoluminescence, whereby the valley polarization of the MoSe2 trion state conforms closely to the local CrBr3 magnetization, while the neutral exciton state remains insensitive to the ferromagnet. We attribute this to spin-dependent interlayer charge transfer occurring on timescales between the exciton and trion radiative lifetimes. Going further, we uncover by both the magneto-optical Kerr effect and photoluminescence a domain-like spatial topography of contrasting valley polarization, which we infer to be labyrinthine or otherwise highly intricate, with features smaller than 400 nm corresponding to our optical resolution. Our findings offer a unique insight into the interplay between short-lived valley excitons and spin-dependent interlayer tunneling, while also highlighting MoSe2 as a promising candidate to optically interface with exotic spin textures in van der Waals structures. One advantage of van der Waals materials is the ability to combine different materials in layers to form new heterostructures. Here, the authors investigate heterostructures of CrBr3 and MoSe2, and find that the ferromagnetism of CrBr3 enhances the valley dependent optical response of the MoSe2. |
Wang, Lin; Wang, Xiaojie; Zhang, Yishu; Li, Runlai; Ma, Teng; Leng, Kai; Chen, Zhi; Abdelwahab, Ibrahim; Loh, Kian Ping Exploring Ferroelectric Switching in alpha-In(2)Se(3)for Neuromorphic Computing Journal Article ADVANCED FUNCTIONAL MATERIALS, 30 (45), 2020, ISSN: 1616-301X. @article{ISI:000567403700001, title = {Exploring Ferroelectric Switching in alpha-In(2)Se(3)for Neuromorphic Computing}, author = {Lin Wang and Xiaojie Wang and Yishu Zhang and Runlai Li and Teng Ma and Kai Leng and Zhi Chen and Ibrahim Abdelwahab and Kian Ping Loh}, doi = {10.1002/adfm.202004609, Early Access Date = SEP 2020}, issn = {1616-301X}, year = {2020}, date = {2020-11-01}, journal = {ADVANCED FUNCTIONAL MATERIALS}, volume = {30}, number = {45}, publisher = {WILEY-V C H VERLAG GMBH}, address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY}, abstract = {Recently, 2D ferroelectrics have attracted extensive interest as a competitive platform for implementing future generation functional electronics, including digital memory and brain-inspired computing circuits. Fulfilling their potential requires achieving the interplay between ferroelectricity and electronic characteristics on the device operation level, which is currently lacking since most studies are focused on the verification of ferroelectricity from different 2D materials. Here, by leveraging the ferroelectricity and semiconducting properties of alpha-In2Se3, ferroelectric semiconductor field-effect transistors (FeSFETs) are fabricated and their potential as artificial synapses is demonstrated. Multiple conductance states can be induced in alpha-In2Se3-based FeSFETs by controlling the out-of-plane polarization, which enables the device to faithfully mimic biosynaptic behaviors. In comparison with charge-trapping-based three-terminal synaptic devices, the electronic synapses based on alpha-In(2)Se(3)have the advantages of good controllability, fast learning, and easy integration of gate dielectric, rendering them promising for neuromorphic computing. In addition, an abnormal resistive switching phenomenon in alpha-In(2)Se(3)is reported when operated in the in-plane ferroelectric switching mode. The findings pave the way forward for alpha-In2Se3-based FeSFETs for developing neuromorphic devices in brain-inspired intelligent systems.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Recently, 2D ferroelectrics have attracted extensive interest as a competitive platform for implementing future generation functional electronics, including digital memory and brain-inspired computing circuits. Fulfilling their potential requires achieving the interplay between ferroelectricity and electronic characteristics on the device operation level, which is currently lacking since most studies are focused on the verification of ferroelectricity from different 2D materials. Here, by leveraging the ferroelectricity and semiconducting properties of alpha-In2Se3, ferroelectric semiconductor field-effect transistors (FeSFETs) are fabricated and their potential as artificial synapses is demonstrated. Multiple conductance states can be induced in alpha-In2Se3-based FeSFETs by controlling the out-of-plane polarization, which enables the device to faithfully mimic biosynaptic behaviors. In comparison with charge-trapping-based three-terminal synaptic devices, the electronic synapses based on alpha-In(2)Se(3)have the advantages of good controllability, fast learning, and easy integration of gate dielectric, rendering them promising for neuromorphic computing. In addition, an abnormal resistive switching phenomenon in alpha-In(2)Se(3)is reported when operated in the in-plane ferroelectric switching mode. The findings pave the way forward for alpha-In2Se3-based FeSFETs for developing neuromorphic devices in brain-inspired intelligent systems. |
Han, Yingmei; Nijhuis, Christian A Functional Redox-Active Molecular Tunnel Junctions Journal Article CHEMISTRY-AN ASIAN JOURNAL, 15 (22), pp. 3752-3770, 2020, ISSN: 1861-4728. @article{ISI:000585269000001, title = {Functional Redox-Active Molecular Tunnel Junctions}, author = {Yingmei Han and Christian A Nijhuis}, doi = {10.1002/asia.202000932, Early Access Date = OCT 2020}, issn = {1861-4728}, year = {2020}, date = {2020-11-01}, journal = {CHEMISTRY-AN ASIAN JOURNAL}, volume = {15}, number = {22}, pages = {3752-3770}, publisher = {WILEY-V C H VERLAG GMBH}, address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY}, abstract = {Redox-active molecular junctions have attracted considerable attention because redox-active molecules provide accessible energy levels enabling electronic function at the molecular length scales, such as, rectification, conductance switching, or molecular transistors. Unlike charge transfer in wet electrochemical environments, it is still challenging to understand how redox-processes proceed in solid-state molecular junctions which lack counterions and solvent molecules to stabilize the charge on the molecules. In this minireview, we first introduce molecular junctions based on redox-active molecules and discuss their properties from both a chemistry and nanoelectronics point of view, and then discuss briefly the mechanisms of charge transport in solid-state redox-junctions followed by examples where redox-molecules generate new electronic function. We conclude with challenges that need to be addressed and interesting future directions from a chemical engineering and molecular design perspectives.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Redox-active molecular junctions have attracted considerable attention because redox-active molecules provide accessible energy levels enabling electronic function at the molecular length scales, such as, rectification, conductance switching, or molecular transistors. Unlike charge transfer in wet electrochemical environments, it is still challenging to understand how redox-processes proceed in solid-state molecular junctions which lack counterions and solvent molecules to stabilize the charge on the molecules. In this minireview, we first introduce molecular junctions based on redox-active molecules and discuss their properties from both a chemistry and nanoelectronics point of view, and then discuss briefly the mechanisms of charge transport in solid-state redox-junctions followed by examples where redox-molecules generate new electronic function. We conclude with challenges that need to be addressed and interesting future directions from a chemical engineering and molecular design perspectives. |
Wang, Feifei; Li, Jing; Zhao, Juan; Yang, Yixiao; Su, Chenliang; Zhong, Yu Lin; Yang, Quan-Hong; Lu, Jiong Single-Atom Electrocatalysts for Lithium Sulfur Batteries: Progress, Opportunities, and Challenges Journal Article ACS MATERIALS LETTERS, 2 (11), pp. 1450-1463, 2020. @article{ISI:000588743300009, title = {Single-Atom Electrocatalysts for Lithium Sulfur Batteries: Progress, Opportunities, and Challenges}, author = {Feifei Wang and Jing Li and Juan Zhao and Yixiao Yang and Chenliang Su and Yu Lin Zhong and Quan-Hong Yang and Jiong Lu}, doi = {10.1021/acsmaterialslett.0c00396}, year = {2020}, date = {2020-11-01}, journal = {ACS MATERIALS LETTERS}, volume = {2}, number = {11}, pages = {1450-1463}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Lithium sulfur (Li-S) battery is considered as one of the most promising energy storage devices, because of its low cost, high energy density, and environmental friendliness. However, the practical applications of Li-S batteries have been hindered by a low utilization efficiency of sulfur arising from complicated chemical conversion of polysulfides and the corrosion of Li metal electrode during charge/discharge processes. Single atom catalysts (SACs) consisting of atomically-dispersed metal sites have been recently exploited as high-performance electrocatalytic materials in various energy storage devices, including Li-S batteries, because of their unique catalytic properties and maximized atom efficiency. In this mini-review, we first describe the major roadblocks and opportunities for the development of commercial Li-S batteries. Following that, we will highlight the specific roles of SAC materials, which are used as cathodes, separators, interlayers, electrolytes, and anodes in Li-S batteries. The detailed catalytic conversion mechanism of polysulfides and nucleation process of Li ions over single-atom active sites are also discussed. Finally, we highlight major challenges to be addressed in this field and provide our perspectives in the rational design and synthesis of superior SACs to accelerate their application in Li-S batteries.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Lithium sulfur (Li-S) battery is considered as one of the most promising energy storage devices, because of its low cost, high energy density, and environmental friendliness. However, the practical applications of Li-S batteries have been hindered by a low utilization efficiency of sulfur arising from complicated chemical conversion of polysulfides and the corrosion of Li metal electrode during charge/discharge processes. Single atom catalysts (SACs) consisting of atomically-dispersed metal sites have been recently exploited as high-performance electrocatalytic materials in various energy storage devices, including Li-S batteries, because of their unique catalytic properties and maximized atom efficiency. In this mini-review, we first describe the major roadblocks and opportunities for the development of commercial Li-S batteries. Following that, we will highlight the specific roles of SAC materials, which are used as cathodes, separators, interlayers, electrolytes, and anodes in Li-S batteries. The detailed catalytic conversion mechanism of polysulfides and nucleation process of Li ions over single-atom active sites are also discussed. Finally, we highlight major challenges to be addressed in this field and provide our perspectives in the rational design and synthesis of superior SACs to accelerate their application in Li-S batteries. |
Xu, Runrun; Xuan, Fengyuan; Quek, Su Ying Spin-Dependent Tunneling Barriers in CoPc/VSe2 from Many-Body Interactions Journal Article JOURNAL OF PHYSICAL CHEMISTRY LETTERS, 11 (21), pp. 9358-9363, 2020, ISSN: 1948-7185. @article{ISI:000589920000062, title = {Spin-Dependent Tunneling Barriers in CoPc/VSe2 from Many-Body Interactions}, author = {Runrun Xu and Fengyuan Xuan and Su Ying Quek}, doi = {10.1021/acs.jpclett.0c02944}, issn = {1948-7185}, year = {2020}, date = {2020-11-01}, journal = {JOURNAL OF PHYSICAL CHEMISTRY LETTERS}, volume = {11}, number = {21}, pages = {9358-9363}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Mixed-dimensional magnetic heterostructures are intriguing, newly available platforms to explore quantum physics and its applications. Using state-of-the-art many-body perturbation theory, we predict the energy level alignment for a self-assembled monolayer of cobalt phthalocyanine (CoPc) molecules on magnetic VSe2 monolayers. The predicted projected density of states on CoPc agrees with experimental scanning tunneling spectra. Consistent with experiment, we predict a shoulder in the unoccupied region of the spectra that is absent from mean-field calculations. Unlike the nearly spin-degenerate gas-phase frontier molecular orbitals, the tunneling barriers at the interface are spin-dependent, a finding of interest for quantum information and spintronics applications. Both the experimentally observed shoulder and the predicted spin-dependent tunneling barriers originate from many-body interactions in the interface-hybridized states. Our results showcase the intricate many-body physics that governs the properties of these mixed-dimensional magnetic heterostructures and suggests the possibility of manipulating the spin-dependent tunneling barriers through modifications of interface coupling.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Mixed-dimensional magnetic heterostructures are intriguing, newly available platforms to explore quantum physics and its applications. Using state-of-the-art many-body perturbation theory, we predict the energy level alignment for a self-assembled monolayer of cobalt phthalocyanine (CoPc) molecules on magnetic VSe2 monolayers. The predicted projected density of states on CoPc agrees with experimental scanning tunneling spectra. Consistent with experiment, we predict a shoulder in the unoccupied region of the spectra that is absent from mean-field calculations. Unlike the nearly spin-degenerate gas-phase frontier molecular orbitals, the tunneling barriers at the interface are spin-dependent, a finding of interest for quantum information and spintronics applications. Both the experimentally observed shoulder and the predicted spin-dependent tunneling barriers originate from many-body interactions in the interface-hybridized states. Our results showcase the intricate many-body physics that governs the properties of these mixed-dimensional magnetic heterostructures and suggests the possibility of manipulating the spin-dependent tunneling barriers through modifications of interface coupling. |
Asmara, Teguh Citra; Lichtenberg, Frank; Biebl, Florian; Zhu, Tao; Das, Pranab Kumar; Naradipa, Muhammad Avicenna; Fauzi, Angga Dito; Diao, Caozheng; Yang, Ping; Lenzen, Philipp; Buchenau, Soeren; Grimm-Lebsanft, Benjamin; Wan, Dongyang; Trevisanutto, Paolo E; Breese, Mark B H; Venkatesan, T; Ruebhausen, Michael; Rusydi, Andrivo Photoinduced metastable dd-exciton-driven metal-insulator transitions in quasi-one-dimensional transition metal oxides Journal Article COMMUNICATIONS PHYSICS, 3 (1), 2020, ISSN: 2399-3650. @article{ISI:000594381200001, title = {Photoinduced metastable dd-exciton-driven metal-insulator transitions in quasi-one-dimensional transition metal oxides}, author = {Teguh Citra Asmara and Frank Lichtenberg and Florian Biebl and Tao Zhu and Pranab Kumar Das and Muhammad Avicenna Naradipa and Angga Dito Fauzi and Caozheng Diao and Ping Yang and Philipp Lenzen and Soeren Buchenau and Benjamin Grimm-Lebsanft and Dongyang Wan and Paolo E Trevisanutto and Mark B H Breese and T Venkatesan and Michael Ruebhausen and Andrivo Rusydi}, doi = {10.1038/s42005-020-00451-w}, issn = {2399-3650}, year = {2020}, date = {2020-11-01}, journal = {COMMUNICATIONS PHYSICS}, volume = {3}, number = {1}, publisher = {NATURE RESEARCH}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {Due to recent developments in steady-state and ultrafast spectroscopic techniques there are more readily available methods to excite and observe anisotropic low-dimensional materials in transient states using light-matter interactions. Here, the authors observe photoinduced metal-insulator transitions in a quasi-one-dimensional metal, and analyse the role of d-d excitonic excitations in the transient state. Photoinduced phase transitions in matters have gained tremendous attention over the past few years. However, their ultrashort lifetime makes their study and possible control very challenging. Here, we report on highly anisotropic d-d excitonic excitations yielding photoinduced metal-insulator transitions (MITs) in quasi-one-dimensional metals Sr1-yNbOx using Mueller-Matrix spectroscopic ellipsometry, transient ultraviolet Raman spectroscopy, transient mid-infrared reflectivity and angular-resolved photoemission spectroscopy supported with density functional theory. Interestingly, the MITs are driven by photo-pumping of d-d excitons, causing the metallic a-axis to become insulating while the insulating b- and c-axis concomitantly become a correlated metal. We assign these effects to an interplay between the melting of charge and lattice orderings along the different anisotropic optical axes and Bose-Einstein-like condensation of the photoinduced excitons. The long lifetime in the order of several seconds of the metastable MITs gives greater flexibility to study and manipulate the transient excitonic state for potential applications in exciton-based optoelectronic devices.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Due to recent developments in steady-state and ultrafast spectroscopic techniques there are more readily available methods to excite and observe anisotropic low-dimensional materials in transient states using light-matter interactions. Here, the authors observe photoinduced metal-insulator transitions in a quasi-one-dimensional metal, and analyse the role of d-d excitonic excitations in the transient state. Photoinduced phase transitions in matters have gained tremendous attention over the past few years. However, their ultrashort lifetime makes their study and possible control very challenging. Here, we report on highly anisotropic d-d excitonic excitations yielding photoinduced metal-insulator transitions (MITs) in quasi-one-dimensional metals Sr1-yNbOx using Mueller-Matrix spectroscopic ellipsometry, transient ultraviolet Raman spectroscopy, transient mid-infrared reflectivity and angular-resolved photoemission spectroscopy supported with density functional theory. Interestingly, the MITs are driven by photo-pumping of d-d excitons, causing the metallic a-axis to become insulating while the insulating b- and c-axis concomitantly become a correlated metal. We assign these effects to an interplay between the melting of charge and lattice orderings along the different anisotropic optical axes and Bose-Einstein-like condensation of the photoinduced excitons. The long lifetime in the order of several seconds of the metastable MITs gives greater flexibility to study and manipulate the transient excitonic state for potential applications in exciton-based optoelectronic devices. |
Noori, Keian; Quek, Su Ying; Rodin, Aleksandr Hydrogen adatoms on graphene: The role of hybridization and lattice distortion Journal Article PHYSICAL REVIEW B, 102 (19), 2020, ISSN: 2469-9950. @article{ISI:000588226400003, title = {Hydrogen adatoms on graphene: The role of hybridization and lattice distortion}, author = {Keian Noori and Su Ying Quek and Aleksandr Rodin}, doi = {10.1103/PhysRevB.102.195416}, issn = {2469-9950}, year = {2020}, date = {2020-11-01}, journal = {PHYSICAL REVIEW B}, volume = {102}, number = {19}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {By performing a detailed study of hydrogen adsorbates on graphene using density functional theory (DFT), we propose a general tight-binding (TB) formalism for a simultaneous treatment of multiple impurities of arbitrary species. To elucidate the details of the hydrogen-graphene bonding, we systematically examine the effects of hybridization and deformation on the band structure and the spectral function. An enhanced understanding of the binding mechanisms leads to a TB model whose predicted spectral function compares favorably with the DFT calculations on the scale of the supercell, as well as the individual adsorbates and carbon atoms. The computational load of our model scales with the number of impurities, not their separation, making it especially useful for experimentally relevant clustered impurity configurations that are too computationally expensive for DFT. The formalism described here allows for the treatment of Anderson impurities and impurities that bind to multiple carbon atoms.}, keywords = {}, pubstate = {published}, tppubtype = {article} } By performing a detailed study of hydrogen adsorbates on graphene using density functional theory (DFT), we propose a general tight-binding (TB) formalism for a simultaneous treatment of multiple impurities of arbitrary species. To elucidate the details of the hydrogen-graphene bonding, we systematically examine the effects of hybridization and deformation on the band structure and the spectral function. An enhanced understanding of the binding mechanisms leads to a TB model whose predicted spectral function compares favorably with the DFT calculations on the scale of the supercell, as well as the individual adsorbates and carbon atoms. The computational load of our model scales with the number of impurities, not their separation, making it especially useful for experimentally relevant clustered impurity configurations that are too computationally expensive for DFT. The formalism described here allows for the treatment of Anderson impurities and impurities that bind to multiple carbon atoms. |
Zhou, Jun; Feng, Yuan Ping; Shen, Lei Atomic-orbital-free intrinsic ferromagnetism in electrenes Journal Article PHYSICAL REVIEW B, 102 (18), 2020, ISSN: 2469-9950. @article{ISI:000589902700001, title = {Atomic-orbital-free intrinsic ferromagnetism in electrenes}, author = {Jun Zhou and Yuan Ping Feng and Lei Shen}, doi = {10.1103/PhysRevB.102.180407}, issn = {2469-9950}, year = {2020}, date = {2020-11-01}, journal = {PHYSICAL REVIEW B}, volume = {102}, number = {18}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {Atomic orbitals play fundamental roles in the modern theory of magnetism, not only providing local moments via their partial occupation, but also offering exchange interactions through their direct or indirect hybridization. Here, we report atomic-orbital-free intrinsic ferromagnetism in monolayer electrides or electrenes, in which excess electrons act as anions. Taking the honeycomb LaBr2(La3+Br2-.e(-)) as an example, our first-principles calculations, in combination with a low-energy effective model in the basis of the Wannier function and Anderson's superexchange theory, reveal that the excess electron is localized at the center of the hexagon, which leads to the spontaneous formation of a local moment due to the strong Stoner instability of the associated state near the Fermi level (E-f). The large off-site Coulomb interaction and extended tails of both wave and Wannier functions indicate a significant spatial extension of the anionic electron state in LaBr2. The overlap of the long tails mediates an extended ferromagnetic direct exchange to second-nearest neighbors (up to 7-8 angstrom), in sharp contrast to the conventional direct exchange which is short ranged due to the overlap of atomic orbitals with limited spatial extension. The dual nature, localization and extension, of the anionic electron state results in a unique magnetic mechanism in such atomic-orbital-free intrinsic two-dimensional ferromagnets.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Atomic orbitals play fundamental roles in the modern theory of magnetism, not only providing local moments via their partial occupation, but also offering exchange interactions through their direct or indirect hybridization. Here, we report atomic-orbital-free intrinsic ferromagnetism in monolayer electrides or electrenes, in which excess electrons act as anions. Taking the honeycomb LaBr2(La3+Br2-.e(-)) as an example, our first-principles calculations, in combination with a low-energy effective model in the basis of the Wannier function and Anderson's superexchange theory, reveal that the excess electron is localized at the center of the hexagon, which leads to the spontaneous formation of a local moment due to the strong Stoner instability of the associated state near the Fermi level (E-f). The large off-site Coulomb interaction and extended tails of both wave and Wannier functions indicate a significant spatial extension of the anionic electron state in LaBr2. The overlap of the long tails mediates an extended ferromagnetic direct exchange to second-nearest neighbors (up to 7-8 angstrom), in sharp contrast to the conventional direct exchange which is short ranged due to the overlap of atomic orbitals with limited spatial extension. The dual nature, localization and extension, of the anionic electron state results in a unique magnetic mechanism in such atomic-orbital-free intrinsic two-dimensional ferromagnets. |
Ma, Zhirui; Yang, Ke; Lian, Xu; Sun, Shuo; Gu, Chengding; Zhang, Jia Lin; West, Damien; Zhang, Shengbai; Liu, Lei; Yuan, Kaidi; Sun, Yi-Yang; Li, Hexing; Chen, Wei Polarity- and Pressure-Dependent Hydrogen Dynamics on ZnO Polar Surfaces Revealed by Near-Ambient-Pressure X-ray Photoelectron Spectroscopy Journal Article JOURNAL OF PHYSICAL CHEMISTRY C, 124 (46), pp. 25431-25436, 2020, ISSN: 1932-7447. @article{ISI:000592958800029, title = {Polarity- and Pressure-Dependent Hydrogen Dynamics on ZnO Polar Surfaces Revealed by Near-Ambient-Pressure X-ray Photoelectron Spectroscopy}, author = {Zhirui Ma and Ke Yang and Xu Lian and Shuo Sun and Chengding Gu and Jia Lin Zhang and Damien West and Shengbai Zhang and Lei Liu and Kaidi Yuan and Yi-Yang Sun and Hexing Li and Wei Chen}, doi = {10.1021/acs.jpcc.0c08881}, issn = {1932-7447}, year = {2020}, date = {2020-11-01}, journal = {JOURNAL OF PHYSICAL CHEMISTRY C}, volume = {124}, number = {46}, pages = {25431-25436}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {ZnO-based catalysts have been widely used in industrial reactions involving syngas conversions. Hydrogen dynamics on the surface of the catalyst is an essential process for understanding the mechanisms of such reactions. As a polar material, however, the role of ZnO surface polarity on hydrogen dynamics has not been studied under operando conditions. Here, we investigate the behavior of polar (0001) and (000 (1) over bar) surfaces of single-crystal ZnO under different H-2 pressures using near-ambient-pressure X-ray photoelectron spectroscopy. We found that the (000 (1) over bar) surface shows a monotonic and irreversible band bending with increasing H-2 pressure. In contrast, the (0001) surface shows two opposite responses depending on H-2 pressure, which are reversible in pressure cycles. This polarity-and pressure-dependent hydrogen dynamics is clearly understood with the assistance from first-principles calculations. In particular, the amphoteric behavior of atomic H is identified to play a key role.}, keywords = {}, pubstate = {published}, tppubtype = {article} } ZnO-based catalysts have been widely used in industrial reactions involving syngas conversions. Hydrogen dynamics on the surface of the catalyst is an essential process for understanding the mechanisms of such reactions. As a polar material, however, the role of ZnO surface polarity on hydrogen dynamics has not been studied under operando conditions. Here, we investigate the behavior of polar (0001) and (000 (1) over bar) surfaces of single-crystal ZnO under different H-2 pressures using near-ambient-pressure X-ray photoelectron spectroscopy. We found that the (000 (1) over bar) surface shows a monotonic and irreversible band bending with increasing H-2 pressure. In contrast, the (0001) surface shows two opposite responses depending on H-2 pressure, which are reversible in pressure cycles. This polarity-and pressure-dependent hydrogen dynamics is clearly understood with the assistance from first-principles calculations. In particular, the amphoteric behavior of atomic H is identified to play a key role. |
Zhang, Panpan; Wang, Lin; Ang, Kah-Wee; Fong, Xuanyao Transition from trap-mediated to band-like transport in polycrystalline monolayer molybdenum disulfide memtransistors Journal Article APPLIED PHYSICS LETTERS, 117 (22), 2020, ISSN: 0003-6951. @article{ISI:000596026700001, title = {Transition from trap-mediated to band-like transport in polycrystalline monolayer molybdenum disulfide memtransistors}, author = {Panpan Zhang and Lin Wang and Kah-Wee Ang and Xuanyao Fong}, doi = {10.1063/5.0031799}, issn = {0003-6951}, year = {2020}, date = {2020-11-01}, journal = {APPLIED PHYSICS LETTERS}, volume = {117}, number = {22}, publisher = {AMER INST PHYSICS}, address = {1305 WALT WHITMAN RD, STE 300, MELVILLE, NY 11747-4501 USA}, abstract = {Multi-terminal memtransistors using polycrystalline monolayer molybdenum disulfide (MoS2) have recently emerged as novel synaptic devices. Due to the coexistence of disorder and strong Coulomb carrier-carrier interactions in MoS2, localization and delocalization of carriers can come into play successively upon the relative strength of disorder and interactions, which can be tuned by the Fermi level ( E F). In this work, we show that the transition from trap-mediated to band-like transport leads to the resistive switching behavior in MoS2 memtransistors, which is driven by the E F shift arising from defect profile redistribution that is facilitated by grain boundaries. In the high resistance state, field-driven hopping conduction can be clearly observed in the high-field region ( E > 0.05MV/cm), whereas the linear dependence of l n ( I / E ) on the square root of the electric field, E 1 / 2, suggests Poole-Frenkel emission in the low-field region ( E <= 0.05MV/cm). In the low resistance state, strong interactions prevailed and a substantial amount of thermally activated electrons are excited into the conduction band, leading to band-like transport.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Multi-terminal memtransistors using polycrystalline monolayer molybdenum disulfide (MoS2) have recently emerged as novel synaptic devices. Due to the coexistence of disorder and strong Coulomb carrier-carrier interactions in MoS2, localization and delocalization of carriers can come into play successively upon the relative strength of disorder and interactions, which can be tuned by the Fermi level ( E F). In this work, we show that the transition from trap-mediated to band-like transport leads to the resistive switching behavior in MoS2 memtransistors, which is driven by the E F shift arising from defect profile redistribution that is facilitated by grain boundaries. In the high resistance state, field-driven hopping conduction can be clearly observed in the high-field region ( E > 0.05MV/cm), whereas the linear dependence of l n ( I / E ) on the square root of the electric field, E 1 / 2, suggests Poole-Frenkel emission in the low-field region ( E <= 0.05MV/cm). In the low resistance state, strong interactions prevailed and a substantial amount of thermally activated electrons are excited into the conduction band, leading to band-like transport. |
Rodin, Aleksandr; Trushin, Maxim; Carvalho, Alexandra; Neto, Castro A H Collective excitations in 2D materials Journal Article NATURE REVIEWS PHYSICS, 2 (10), pp. 524-537, 2020. @article{ISI:000569258700001, title = {Collective excitations in 2D materials}, author = {Aleksandr Rodin and Maxim Trushin and Alexandra Carvalho and A H Castro Neto}, doi = {10.1038/s42254-020-0214-4, Early Access Date = SEP 2020}, year = {2020}, date = {2020-10-01}, journal = {NATURE REVIEWS PHYSICS}, volume = {2}, number = {10}, pages = {524-537}, publisher = {SPRINGERNATURE}, address = {CAMPUS, 4 CRINAN ST, LONDON, N1 9XW, ENGLAND}, abstract = {Research on 2D materials has been one of the fastest-growing fields in condensed matter and materials science research in the past 10 years. The low dimensionality and strong correlations of 2D systems give rise to electronic and structural properties, in the form of collective excitations, that do not have counterparts in ordinary 3D materials used in modern technology. These 2D materials present extraordinary opportunities for new technologies, such as in flexible electronics. In this Review, we focus on plasmons, excitons, phonons and magnons in 2D materials. We discuss the theoretical formalism of these collective excitations and elucidate how they differ from their 3D counterparts. 2D materials host various collective excitations, which either mutate or cease to exist in the bulk. In this Review, we select the most striking properties of 2D plasmons, excitons, phonons and magnons, contrasting them with the bulk versions.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Research on 2D materials has been one of the fastest-growing fields in condensed matter and materials science research in the past 10 years. The low dimensionality and strong correlations of 2D systems give rise to electronic and structural properties, in the form of collective excitations, that do not have counterparts in ordinary 3D materials used in modern technology. These 2D materials present extraordinary opportunities for new technologies, such as in flexible electronics. In this Review, we focus on plasmons, excitons, phonons and magnons in 2D materials. We discuss the theoretical formalism of these collective excitations and elucidate how they differ from their 3D counterparts. 2D materials host various collective excitations, which either mutate or cease to exist in the bulk. In this Review, we select the most striking properties of 2D plasmons, excitons, phonons and magnons, contrasting them with the bulk versions. |
Tsai, Hsin-Zon; Lischner, Johannes; Omrani, Arash A; Liou, Franklin; Aikawa, Andrew S; Karrasch, Christoph; Wickenburg, Sebastian; Riss, Alexander; Natividad, Kyler C; Chen, Jin; Choi, Won-Woo; Watanabe, Kenji; Taniguchi, Takashi; Su, Chenliang; Louie, Steven G; Zettl, Alex; Lu, Jiong; Crommie, Michael F A molecular shift register made using tunable charge patterns in one-dimensional molecular arrays on graphene Journal Article NATURE ELECTRONICS, 3 (10), pp. 598-603, 2020, ISSN: 2520-1131. @article{ISI:000573485500001, title = {A molecular shift register made using tunable charge patterns in one-dimensional molecular arrays on graphene}, author = {Hsin-Zon Tsai and Johannes Lischner and Arash A Omrani and Franklin Liou and Andrew S Aikawa and Christoph Karrasch and Sebastian Wickenburg and Alexander Riss and Kyler C Natividad and Jin Chen and Won-Woo Choi and Kenji Watanabe and Takashi Taniguchi and Chenliang Su and Steven G Louie and Alex Zettl and Jiong Lu and Michael F Crommie}, doi = {10.1038/s41928-020-00479-4, Early Access Date = SEP 2020}, issn = {2520-1131}, year = {2020}, date = {2020-10-01}, journal = {NATURE ELECTRONICS}, volume = {3}, number = {10}, pages = {598-603}, publisher = {NATURE RESEARCH}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {The ability to tune the electronic properties of molecular arrays is an important step in the development of molecule-scale electronic devices. However, control over internal device charge distributions by tuning interactions between molecules has proved challenging. Here, we show that gate-tunable charge patterning can occur in one-dimensional molecular arrays on graphene field-effect transistors. One-dimensional molecular arrays are fabricated using an edge-templated self-assembly process that allows organic molecules (F(4)TCNQ) to be precisely positioned on graphene devices. The charge configurations of the molecular arrays can be reversibly switched between different collective charge states by tuning the graphene Fermi level via a back-gate electrode. Charge pinning at the ends of the molecular arrays allows the charge state of the entire array to be controlled by adding or removing an edge molecule and changing the total number of molecules in an array between odd and even integers. Charge patterns altered in this way propagate down the array in a cascade effect, allowing the array to function as a charge-based molecular shift register. An extended multi-site Anderson impurity model is used to quantitatively explain this behaviour. One-dimensional molecular arrays on graphene field-effect transistors can be reversibly switched between different periodic charge states by tuning the graphene Fermi level via a back-gate electrode and by manipulating individual molecules, allowing them to function as a nanoscale shift register.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The ability to tune the electronic properties of molecular arrays is an important step in the development of molecule-scale electronic devices. However, control over internal device charge distributions by tuning interactions between molecules has proved challenging. Here, we show that gate-tunable charge patterning can occur in one-dimensional molecular arrays on graphene field-effect transistors. One-dimensional molecular arrays are fabricated using an edge-templated self-assembly process that allows organic molecules (F(4)TCNQ) to be precisely positioned on graphene devices. The charge configurations of the molecular arrays can be reversibly switched between different collective charge states by tuning the graphene Fermi level via a back-gate electrode. Charge pinning at the ends of the molecular arrays allows the charge state of the entire array to be controlled by adding or removing an edge molecule and changing the total number of molecules in an array between odd and even integers. Charge patterns altered in this way propagate down the array in a cascade effect, allowing the array to function as a charge-based molecular shift register. An extended multi-site Anderson impurity model is used to quantitatively explain this behaviour. One-dimensional molecular arrays on graphene field-effect transistors can be reversibly switched between different periodic charge states by tuning the graphene Fermi level via a back-gate electrode and by manipulating individual molecules, allowing them to function as a nanoscale shift register. |
Chen, Li; Yu, Zhi Gen; Liang, Dan; Li, Sifan; Tan, Wee Chong; Zhang, Yong-Wei; Ang, Kah-Wee Ultrasensitive and robust two-dimensional indium selenide flexible electronics and sensors for human motion detection Journal Article NANO ENERGY, 76 , 2020, ISSN: 2211-2855. @article{ISI:000571043100005, title = {Ultrasensitive and robust two-dimensional indium selenide flexible electronics and sensors for human motion detection}, author = {Li Chen and Zhi Gen Yu and Dan Liang and Sifan Li and Wee Chong Tan and Yong-Wei Zhang and Kah-Wee Ang}, doi = {10.1016/j.nanoen.2020.105020}, issn = {2211-2855}, year = {2020}, date = {2020-10-01}, journal = {NANO ENERGY}, volume = {76}, publisher = {ELSEVIER}, address = {RADARWEG 29, 1043 NX AMSTERDAM, NETHERLANDS}, abstract = {To accurately detect human motion, a sensing material/device must be flexible, ultrasensitive to strain, and electrically and mechanically robust. However, existing electronics and sensors, which are either not flexible enough, or not ultrasensitive to strain, or suffering from poor on/off ratio and low charge mobility, do not meet these stringent requirements. Here, we demonstrate a flexible and ultrasensitive three-terminal strain sensor based on two-dimensional (2D) InSe for detecting human-motion activities. The 2D InSe exhibits a tunable bandgap and a large piezoresistive effect via strain engineering. Through electrostatic gating effect, the gauge factor of the sensor can be enhanced by 8-fold and 7-fold for a low tensile and compressive strain of only +/- 0.25%, respectively. Remarkably, the fabricated 2D InSe-based transistor achieves both a record high on/off ratio of similar to 10(8) and an electron mobility of similar to 383 cm(2)/V s, superior to all existing ones. Furthermore, flexible InSe logic inverters with a high voltage gain of similar to 10 and a large noise margin of similar to 0.7 x V-DD (supply voltage) are realized under various strain conditions. This work paves the way to enable simultaneous integration of high-performance flexible sensors and electronics based on a common 2D InSe material platform towards achieving a fully integrated sensing system.}, keywords = {}, pubstate = {published}, tppubtype = {article} } To accurately detect human motion, a sensing material/device must be flexible, ultrasensitive to strain, and electrically and mechanically robust. However, existing electronics and sensors, which are either not flexible enough, or not ultrasensitive to strain, or suffering from poor on/off ratio and low charge mobility, do not meet these stringent requirements. Here, we demonstrate a flexible and ultrasensitive three-terminal strain sensor based on two-dimensional (2D) InSe for detecting human-motion activities. The 2D InSe exhibits a tunable bandgap and a large piezoresistive effect via strain engineering. Through electrostatic gating effect, the gauge factor of the sensor can be enhanced by 8-fold and 7-fold for a low tensile and compressive strain of only +/- 0.25%, respectively. Remarkably, the fabricated 2D InSe-based transistor achieves both a record high on/off ratio of similar to 10(8) and an electron mobility of similar to 383 cm(2)/V s, superior to all existing ones. Furthermore, flexible InSe logic inverters with a high voltage gain of similar to 10 and a large noise margin of similar to 0.7 x V-DD (supply voltage) are realized under various strain conditions. This work paves the way to enable simultaneous integration of high-performance flexible sensors and electronics based on a common 2D InSe material platform towards achieving a fully integrated sensing system. |
Kokkoniemi, R; Girard, J -P; Hazra, D; Laitinen, A; Govenius, J; Lake, R E; Sallinen, I; Vesterinen, V; Partanen, M; Tan, J Y; Chan, K W; Tan, K Y; Hakonen, P; Mottonen, M Bolometer operating at the threshold for circuit quantum electrodynamics Journal Article NATURE, 586 (7827), pp. 47+, 2020, ISSN: 0028-0836. @article{ISI:000574283500008, title = {Bolometer operating at the threshold for circuit quantum electrodynamics}, author = {R Kokkoniemi and J -P Girard and D Hazra and A Laitinen and J Govenius and R E Lake and I Sallinen and V Vesterinen and M Partanen and J Y Tan and K W Chan and K Y Tan and P Hakonen and M Mottonen}, doi = {10.1038/s41586-020-2753-3}, issn = {0028-0836}, year = {2020}, date = {2020-10-01}, journal = {NATURE}, volume = {586}, number = {7827}, pages = {47+}, publisher = {NATURE RESEARCH}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {Radiation sensors based on the heating effect of absorbed radiation are typically simple to operate and flexible in terms of input frequency, so they are widely used in gas detection(1), security(2), terahertz imaging(3), astrophysical observations(4)and medical applications(5). Several important applications are currently emerging from quantum technology and especially from electrical circuits that behave quantum mechanically, that is, circuit quantum electrodynamics(6). This field has given rise to single-photon microwave detectors(7-9)and a quantum computer that is superior to classical supercomputers for certain tasks(10). Thermal sensors hold potential for enhancing such devices because they do not add quantum noise and they are smaller, simpler and consume about six orders of magnitude less power than the frequently used travelling-wave parametric amplifiers(11). However, despite great progress in the speed(12)and noise levels(13)of thermal sensors, no bolometer has previously met the threshold for circuit quantum electrodynamics, which lies at a time constant of a few hundred nanoseconds and a simultaneous energy resolution of the order of 10hgigahertz (where his the Planck constant). Here we experimentally demonstrate a bolometer that operates at this threshold, with a noise-equivalent power of 30 zeptowatts per square-root hertz, comparable to the lowest value reported so far(13), at a thermal time constant two orders of magnitude shorter, at 500 nanoseconds. Both of these values are measured directly on the same device, giving an accurate estimation of 30h gigahertz for the calorimetric energy resolution. These improvements stem from the use of a graphene monolayer with extremely low specific heat(14)as the active material. The minimum observed time constant of 200 nanoseconds is well below the dephasing times of roughly 100 microseconds reported for superconducting qubits(15)and matches the timescales of currently used readout schemes(16,17), thus enabling circuit quantum electrodynamics applications for bolometers.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Radiation sensors based on the heating effect of absorbed radiation are typically simple to operate and flexible in terms of input frequency, so they are widely used in gas detection(1), security(2), terahertz imaging(3), astrophysical observations(4)and medical applications(5). Several important applications are currently emerging from quantum technology and especially from electrical circuits that behave quantum mechanically, that is, circuit quantum electrodynamics(6). This field has given rise to single-photon microwave detectors(7-9)and a quantum computer that is superior to classical supercomputers for certain tasks(10). Thermal sensors hold potential for enhancing such devices because they do not add quantum noise and they are smaller, simpler and consume about six orders of magnitude less power than the frequently used travelling-wave parametric amplifiers(11). However, despite great progress in the speed(12)and noise levels(13)of thermal sensors, no bolometer has previously met the threshold for circuit quantum electrodynamics, which lies at a time constant of a few hundred nanoseconds and a simultaneous energy resolution of the order of 10hgigahertz (where his the Planck constant). Here we experimentally demonstrate a bolometer that operates at this threshold, with a noise-equivalent power of 30 zeptowatts per square-root hertz, comparable to the lowest value reported so far(13), at a thermal time constant two orders of magnitude shorter, at 500 nanoseconds. Both of these values are measured directly on the same device, giving an accurate estimation of 30h gigahertz for the calorimetric energy resolution. These improvements stem from the use of a graphene monolayer with extremely low specific heat(14)as the active material. The minimum observed time constant of 200 nanoseconds is well below the dephasing times of roughly 100 microseconds reported for superconducting qubits(15)and matches the timescales of currently used readout schemes(16,17), thus enabling circuit quantum electrodynamics applications for bolometers. |
Zeng, Shengwei; Tang, Chi Sin; Yin, Xinmao; Li, Changjian; Li, Mengsha; Huang, Zhen; Hu, Junxiong; Liu, Wei; Omar, Ganesh Ji; Jani, Hariom; Lim, Zhi Shiuh; Han, Kun; Wan, Dongyang; Yang, Ping; Pennycook, Stephen John; Wee, Andrew T S; Ariando, Ariando Phase Diagram and Superconducting Dome of Infinite-Layer Nd1-xSxNiO2 Thin Films Journal Article PHYSICAL REVIEW LETTERS, 125 (14), 2020, ISSN: 0031-9007. @article{ISI:000574781200009, title = {Phase Diagram and Superconducting Dome of Infinite-Layer Nd1-xSxNiO2 Thin Films}, author = {Shengwei Zeng and Chi Sin Tang and Xinmao Yin and Changjian Li and Mengsha Li and Zhen Huang and Junxiong Hu and Wei Liu and Ganesh Ji Omar and Hariom Jani and Zhi Shiuh Lim and Kun Han and Dongyang Wan and Ping Yang and Stephen John Pennycook and Andrew T S Wee and Ariando Ariando}, doi = {10.1103/PhysRevLett.125.147003}, issn = {0031-9007}, year = {2020}, date = {2020-10-01}, journal = {PHYSICAL REVIEW LETTERS}, volume = {125}, number = {14}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {Infinite-layer Nd1-xSxNiO2 thin films with Sr doping level x from 0.08 to 0.3 are synthesized and investigated. We find a superconducting dome x between 0.12 and 0.235 accompanied by a weakly insulating behavior in both under- and overdoped regimes. The dome is akin to that in the electron-doped 214-type and infinite-layer cuprate superconductors. For x >= 0.18, the normal state Hall coefficient (RH) changes the sign from negative to positive as the temperature decreases. The temperature of the sign changes decreases monotonically with decreasing x from the overdoped side and approaches the superconducting dome at the midpoint, suggesting a reconstruction of the Fermi surface with the dopant concentration across the dome.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Infinite-layer Nd1-xSxNiO2 thin films with Sr doping level x from 0.08 to 0.3 are synthesized and investigated. We find a superconducting dome x between 0.12 and 0.235 accompanied by a weakly insulating behavior in both under- and overdoped regimes. The dome is akin to that in the electron-doped 214-type and infinite-layer cuprate superconductors. For x >= 0.18, the normal state Hall coefficient (RH) changes the sign from negative to positive as the temperature decreases. The temperature of the sign changes decreases monotonically with decreasing x from the overdoped side and approaches the superconducting dome at the midpoint, suggesting a reconstruction of the Fermi surface with the dopant concentration across the dome. |
Wang, Qixing; Zhang, Qi; Luo, Xin; Wang, Junyong; Zhu, Rui; Liang, Qijie; Zhang, Lei; Yong, Justin Zhou; Wong, Calvin Pei Yu; Eda, Goki; Smet, Jurgen H; Wee, Andrew T S Optoelectronic Properties of a van der Waals WS2 Monolayer/2D Perovskite Vertical Heterostructure Journal Article ACS APPLIED MATERIALS & INTERFACES, 12 (40), pp. 45235-45242, 2020, ISSN: 1944-8244. @article{ISI:000579956100085, title = {Optoelectronic Properties of a van der Waals WS2 Monolayer/2D Perovskite Vertical Heterostructure}, author = {Qixing Wang and Qi Zhang and Xin Luo and Junyong Wang and Rui Zhu and Qijie Liang and Lei Zhang and Justin Zhou Yong and Calvin Pei Yu Wong and Goki Eda and Jurgen H Smet and Andrew T S Wee}, doi = {10.1021/acsami.0c14398}, issn = {1944-8244}, year = {2020}, date = {2020-10-01}, journal = {ACS APPLIED MATERIALS & INTERFACES}, volume = {12}, number = {40}, pages = {45235-45242}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Two-dimensional (2D) Ruddlesden-Popper perovskites have been demonstrated to possess great potential for optical and optoelectronic devices. Because they exhibit better ambient stability than three-dimensional (3D) perovskites, they have been considered as potential substitutes for 3D perovskites as light absorbing layers to improve the photoresponsivity of monolayer transition metal dichalcogenide (TMDC)-based photodetectors. Investigation of the optoelectronic properties of TMDC monolayer/2D perovskite vertical heterostructures is however at an early stage. Here, we address the photovoltaic effect and the photodetection performance in tungsten disulfide (WS2) monolayer/2D perovskite (C6H5C2H4NH3)(2)PbI4 (PEPI) vertical heterostructures. A vertical device geometry with separate graphene contacts to both heterointerface constituents acted as a photovoltaic device and self-driven photodetector. The photovoltaic device exhibited an open circuit voltage of-0.57 V and a short circuit current of 41.6 nA. A photoresponsivity of 0.13 mA/W at the WS2/PEPI heterointerface was achieved, which was signified by a factor of 5 compared to that from the individual WS2 region. The current on/off ratio of the self-driven photodetector was approximately 1500. The photoresponsivity and external quantum efficiency of the self-driven photodetector were estimated to be 24.2 mu A/W and 5.7 x 10(5), respectively. This work corroborates that 2D perovskites are promising light absorbing layers in optoelectronic devices with a TMDC-based heterointerface.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Two-dimensional (2D) Ruddlesden-Popper perovskites have been demonstrated to possess great potential for optical and optoelectronic devices. Because they exhibit better ambient stability than three-dimensional (3D) perovskites, they have been considered as potential substitutes for 3D perovskites as light absorbing layers to improve the photoresponsivity of monolayer transition metal dichalcogenide (TMDC)-based photodetectors. Investigation of the optoelectronic properties of TMDC monolayer/2D perovskite vertical heterostructures is however at an early stage. Here, we address the photovoltaic effect and the photodetection performance in tungsten disulfide (WS2) monolayer/2D perovskite (C6H5C2H4NH3)(2)PbI4 (PEPI) vertical heterostructures. A vertical device geometry with separate graphene contacts to both heterointerface constituents acted as a photovoltaic device and self-driven photodetector. The photovoltaic device exhibited an open circuit voltage of-0.57 V and a short circuit current of 41.6 nA. A photoresponsivity of 0.13 mA/W at the WS2/PEPI heterointerface was achieved, which was signified by a factor of 5 compared to that from the individual WS2 region. The current on/off ratio of the self-driven photodetector was approximately 1500. The photoresponsivity and external quantum efficiency of the self-driven photodetector were estimated to be 24.2 mu A/W and 5.7 x 10(5), respectively. This work corroborates that 2D perovskites are promising light absorbing layers in optoelectronic devices with a TMDC-based heterointerface. |
Chen, Xiaoping; Salim, Teddy; Zhang, Ziyu; Yu, Xiaojiang; Volkova, Ira; Nijhuis, Christian A Large Increase in the Dielectric Constant and Partial Loss of Coherence Increases Tunneling Rates across Molecular Wires Journal Article ACS APPLIED MATERIALS & INTERFACES, 12 (40), pp. 45111-45121, 2020, ISSN: 1944-8244. @article{ISI:000579956100073, title = {Large Increase in the Dielectric Constant and Partial Loss of Coherence Increases Tunneling Rates across Molecular Wires}, author = {Xiaoping Chen and Teddy Salim and Ziyu Zhang and Xiaojiang Yu and Ira Volkova and Christian A Nijhuis}, doi = {10.1021/acsami.0c11106}, issn = {1944-8244}, year = {2020}, date = {2020-10-01}, journal = {ACS APPLIED MATERIALS & INTERFACES}, volume = {12}, number = {40}, pages = {45111-45121}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Although the dielectric behavior of monolayers is imPortant in a large range of applications, its role in charge transport studies involving molecular junctions is largely ignored. This paper describes a large increase in the relative static dielectric constant (er) by simply increasing the thickness of well-organized rnonolayers of oligoglycine and oligo(ethylene glycol) from 7 up to 14. The resulting large capacitance of 3.5-5.1 mu F/cm(2) is thicknessindependent, which is highly attractive for field-effect transistor applications. This increase of er results in a linear increase of the them-Jai activation energy by a factor of 6, which suggests that the mechanism of charge transport gradually changes from coherent to (partially) incoherent tunneling. The comparisons of oligoglycine (which readily forms hydrogen bonds with neighboring molecules) and methyl terminated oligo(ethylerie glycol) (which lacks hydrogen bond donors) monolayers, kinetic isotope effects, and relative htimidit-y-dependent measurements all indicate the importance of strong hydrogen bonds involving ionic species and strongly bonded water in the unusual dielectric behavior and the incoherent tunneling mechanism. This partial loss of coherence of the charge carriers can explain the unusually small tunneling decay coefficients across long molecular wires, and the length-dependent increase of epsilon(r) of monolayers opens up interesting new applications.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Although the dielectric behavior of monolayers is imPortant in a large range of applications, its role in charge transport studies involving molecular junctions is largely ignored. This paper describes a large increase in the relative static dielectric constant (er) by simply increasing the thickness of well-organized rnonolayers of oligoglycine and oligo(ethylene glycol) from 7 up to 14. The resulting large capacitance of 3.5-5.1 mu F/cm(2) is thicknessindependent, which is highly attractive for field-effect transistor applications. This increase of er results in a linear increase of the them-Jai activation energy by a factor of 6, which suggests that the mechanism of charge transport gradually changes from coherent to (partially) incoherent tunneling. The comparisons of oligoglycine (which readily forms hydrogen bonds with neighboring molecules) and methyl terminated oligo(ethylerie glycol) (which lacks hydrogen bond donors) monolayers, kinetic isotope effects, and relative htimidit-y-dependent measurements all indicate the importance of strong hydrogen bonds involving ionic species and strongly bonded water in the unusual dielectric behavior and the incoherent tunneling mechanism. This partial loss of coherence of the charge carriers can explain the unusually small tunneling decay coefficients across long molecular wires, and the length-dependent increase of epsilon(r) of monolayers opens up interesting new applications. |
Yadav, Dinesh; Trushin, Maxim; Pauly, Fabian PHYSICAL REVIEW RESEARCH, 2 (4), 2020. @article{ISI:000605392100002, title = {Thermalization of photoexcited carriers in two-dimensional transition metal dichalcogenides and internal quantum efficiency of van derWaals heterostructures}, author = {Dinesh Yadav and Maxim Trushin and Fabian Pauly}, doi = {10.1103/PhysRevResearch.2.043051}, year = {2020}, date = {2020-10-01}, journal = {PHYSICAL REVIEW RESEARCH}, volume = {2}, number = {4}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {Van der Waals semiconductor heterostructures could be a platform to harness hot photoexcited carriers in the next generation of optoelectronic and photovoltaic devices. The internal quantum efficiency of hot-carrier devices is determined by the relation between photocarrier extraction and thermalization rates. Using ab initio methods we show that the photocarrier thermalization time in single-layer transition metal dichalcogenides strongly depends on the peculiarities of the phonon spectrum and the electronic spin-orbit coupling. In detail, the lifted spin degeneracy in the valence band suppresses the hole scattering on acoustic phonons, slowing down the thermalization of holes by one order of magnitude as compared with electrons. Moreover, the hole thermalization time behaves differently in MoS2 and WSe2 because spin-orbit interactions differ in these seemingly similar materials. We predict that the internal quantum efficiency of a tunneling van der Waals semiconductor heterostructure depends qualitatively on whether MoS2 or WSe2 is used.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Van der Waals semiconductor heterostructures could be a platform to harness hot photoexcited carriers in the next generation of optoelectronic and photovoltaic devices. The internal quantum efficiency of hot-carrier devices is determined by the relation between photocarrier extraction and thermalization rates. Using ab initio methods we show that the photocarrier thermalization time in single-layer transition metal dichalcogenides strongly depends on the peculiarities of the phonon spectrum and the electronic spin-orbit coupling. In detail, the lifted spin degeneracy in the valence band suppresses the hole scattering on acoustic phonons, slowing down the thermalization of holes by one order of magnitude as compared with electrons. Moreover, the hole thermalization time behaves differently in MoS2 and WSe2 because spin-orbit interactions differ in these seemingly similar materials. We predict that the internal quantum efficiency of a tunneling van der Waals semiconductor heterostructure depends qualitatively on whether MoS2 or WSe2 is used. |
Duffin, Thorin J; Kalathingal, Vijith; Radulescu, Andreea; Li, Changjian; Pennycook, Stephen J; Nijhuis, Christian A Cavity Plasmonics in Tunnel Junctions: Outcoupling and the Role of Surface Roughness Journal Article PHYSICAL REVIEW APPLIED, 14 (4), 2020, ISSN: 2331-7019. @article{ISI:000579525200001, title = {Cavity Plasmonics in Tunnel Junctions: Outcoupling and the Role of Surface Roughness}, author = {Thorin J Duffin and Vijith Kalathingal and Andreea Radulescu and Changjian Li and Stephen J Pennycook and Christian A Nijhuis}, doi = {10.1103/PhysRevApplied.14.044021}, issn = {2331-7019}, year = {2020}, date = {2020-10-01}, journal = {PHYSICAL REVIEW APPLIED}, volume = {14}, number = {4}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {Electrical excitation of surface plasmons via metal-insulator-metal tunneling junctions (M-I-M TJs) has recently been demonstrated to have experimental conversion efficiencies of 1% [W. Du et al. Nat. Photonics 11, 623 (2017)], 5-6 orders of magnitude higher than theoretically predicted by Parzefall and Novotny [ACS Photonics 5, 4195 (2018)]. In this work we resolve this discrepancy between theory and experiment, and report a rigorous analytical and experimental study on the role of surface roughness in the near-field coupling of the initially excited M-I-M TJ cavity surface plasmon polariton (M-I-M SPP) to the daughter radiative and nonradiative modes. We find that varying the roughness profile of the M-I-M TJ significantly (which is determined with atomic force microscopy and cross-section scanning transmission electron microscopy) improves the near-field outcoupling efficiency for two reasons: the effective thickness of the electrodes reduces with increasing roughness, and roughness provides momentum matching for mode overlap. The role of surface roughness in near-field outcoupling is analysed quantitatively by incorporating a conformal random roughness profile in the finite-element electromagnetic modeling of a plasmonically active M-I-M TJ. We show that the outcoupling efficiency can be enhanced up to 15% for the M-I-M SPP coupling to daughter modes, and demonstrate an overall electron-to-plasmon conversion efficiency of 1.5% (based on an inelastic tunneling efficiently of 10%), supporting recent experimental findings. This work provides an explanation for the high observed experimental efficiencies and bridges the knowledge gap between prior theoretical works and experiments by including the role of roughness in the electromagnetic near-field coupling in M-I-M TJs.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Electrical excitation of surface plasmons via metal-insulator-metal tunneling junctions (M-I-M TJs) has recently been demonstrated to have experimental conversion efficiencies of 1% [W. Du et al. Nat. Photonics 11, 623 (2017)], 5-6 orders of magnitude higher than theoretically predicted by Parzefall and Novotny [ACS Photonics 5, 4195 (2018)]. In this work we resolve this discrepancy between theory and experiment, and report a rigorous analytical and experimental study on the role of surface roughness in the near-field coupling of the initially excited M-I-M TJ cavity surface plasmon polariton (M-I-M SPP) to the daughter radiative and nonradiative modes. We find that varying the roughness profile of the M-I-M TJ significantly (which is determined with atomic force microscopy and cross-section scanning transmission electron microscopy) improves the near-field outcoupling efficiency for two reasons: the effective thickness of the electrodes reduces with increasing roughness, and roughness provides momentum matching for mode overlap. The role of surface roughness in near-field outcoupling is analysed quantitatively by incorporating a conformal random roughness profile in the finite-element electromagnetic modeling of a plasmonically active M-I-M TJ. We show that the outcoupling efficiency can be enhanced up to 15% for the M-I-M SPP coupling to daughter modes, and demonstrate an overall electron-to-plasmon conversion efficiency of 1.5% (based on an inelastic tunneling efficiently of 10%), supporting recent experimental findings. This work provides an explanation for the high observed experimental efficiencies and bridges the knowledge gap between prior theoretical works and experiments by including the role of roughness in the electromagnetic near-field coupling in M-I-M TJs. |
Lin, Fanrong; Qiao, Jiabin; Huang, Junye; Liu, Jiawei; Fu, Deyi; Mayorov, Alexander S; Chen, Hao; Mukherjee, Paromita; Qu, Tingyu; Sow, Chorng-Haur; Watanabe, Kenji; Taniguchi, Takashi; Ozyilmaz, Barbaros Heteromoire Engineering on Magnetic Bloch Transport in Twisted Graphene Superlattices Journal Article NANO LETTERS, 20 (10), pp. 7572-7579, 2020, ISSN: 1530-6984. @article{ISI:000613073900006, title = {Heteromoire Engineering on Magnetic Bloch Transport in Twisted Graphene Superlattices}, author = {Fanrong Lin and Jiabin Qiao and Junye Huang and Jiawei Liu and Deyi Fu and Alexander S Mayorov and Hao Chen and Paromita Mukherjee and Tingyu Qu and Chorng-Haur Sow and Kenji Watanabe and Takashi Taniguchi and Barbaros Ozyilmaz}, doi = {10.1021/acs.nanolett.0c03062}, issn = {1530-6984}, year = {2020}, date = {2020-10-01}, journal = {NANO LETTERS}, volume = {20}, number = {10}, pages = {7572-7579}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Localized electrons subject to applied magnetic fields can restart to propagate freely through the lattice in delocalized magnetic Bloch states (MBSs) when the lattice periodicity is commensurate with the magnetic length. Twisted graphene superlattices with moire wavelength tunability enable experimental access to the unique delocalization in a controllable fashion. Here, we report the observation and characterization of high-temperature Brown-Zak (BZ) oscillations which come in two types, 1/B and B periodicity, originating from the generation of integer and fractional MBSs, in the twisted bilayer and trilayer graphene superlattices, respectively. Coexisting periodic-in-1/B oscillations assigned to different moire wavelengths are dramatically observed in small-angle twisted bilayer graphene, which may arise from angledisorder-induced in-plane heteromoire superlattices. Moreover, the vertical stacking of heteromoire supercells in double-twisted trilayer graphene results in a mega-sized superlattice. The exotic superlattice contributes to the periodic-in-B oscillation and dominates the magnetic Bloch transport.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Localized electrons subject to applied magnetic fields can restart to propagate freely through the lattice in delocalized magnetic Bloch states (MBSs) when the lattice periodicity is commensurate with the magnetic length. Twisted graphene superlattices with moire wavelength tunability enable experimental access to the unique delocalization in a controllable fashion. Here, we report the observation and characterization of high-temperature Brown-Zak (BZ) oscillations which come in two types, 1/B and B periodicity, originating from the generation of integer and fractional MBSs, in the twisted bilayer and trilayer graphene superlattices, respectively. Coexisting periodic-in-1/B oscillations assigned to different moire wavelengths are dramatically observed in small-angle twisted bilayer graphene, which may arise from angledisorder-induced in-plane heteromoire superlattices. Moreover, the vertical stacking of heteromoire supercells in double-twisted trilayer graphene results in a mega-sized superlattice. The exotic superlattice contributes to the periodic-in-B oscillation and dominates the magnetic Bloch transport. |
Wang, Juefan; Quek, Su Ying Isolated flat bands and physics of mixed dimensions in a 2D covalent organic framework Journal Article NANOSCALE, 12 (39), pp. 20279-20286, 2020, ISSN: 2040-3364. @article{ISI:000579877200016, title = {Isolated flat bands and physics of mixed dimensions in a 2D covalent organic framework}, author = {Juefan Wang and Su Ying Quek}, doi = {10.1039/d0nr04428h}, issn = {2040-3364}, year = {2020}, date = {2020-10-01}, journal = {NANOSCALE}, volume = {12}, number = {39}, pages = {20279-20286}, publisher = {ROYAL SOC CHEMISTRY}, address = {THOMAS GRAHAM HOUSE, SCIENCE PARK, MILTON RD, CAMBRIDGE CB4 0WF, CAMBS, ENGLAND}, abstract = {We demonstrate that it is possible to rationally incorporate both an isolated flat band and the physics of zero dimensions (0D), one dimension (1D), and two dimensions (2D) in a single 2D material. Such unique electronic properties are present in a recently synthesized 2D covalent organic framework (COF), where ``I''-shaped building blocks and ``T''-shaped connectors result in quasi-1D chains that are linked by quasi-0D bridge units arranged in a stable 2D lattice. The lowest unoccupied conduction band is an isolated flat band, and electron-doping gives rise to novel quantum phenomena, such as magnetism and Mott insulating phases. The highest occupied valence band arises from wave functions in the quasi-1D chains. Examples of mixed dimensional physics are illustrated in this system. The strong electron-hole asymmetry in this material results in a large Seebeck coefficient, while the quasi-1D nature of the chains leads to linear dichroism, in conjunction with strongly bound 2D excitons. We elucidate strategies to design and optimize 2D COFs to host both isolated flat bands and quantum-confined 1D subsystems. The properties of the 2D COF discussed here provide a taste of the intriguing possibilities in this open research field.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We demonstrate that it is possible to rationally incorporate both an isolated flat band and the physics of zero dimensions (0D), one dimension (1D), and two dimensions (2D) in a single 2D material. Such unique electronic properties are present in a recently synthesized 2D covalent organic framework (COF), where ``I''-shaped building blocks and ``T''-shaped connectors result in quasi-1D chains that are linked by quasi-0D bridge units arranged in a stable 2D lattice. The lowest unoccupied conduction band is an isolated flat band, and electron-doping gives rise to novel quantum phenomena, such as magnetism and Mott insulating phases. The highest occupied valence band arises from wave functions in the quasi-1D chains. Examples of mixed dimensional physics are illustrated in this system. The strong electron-hole asymmetry in this material results in a large Seebeck coefficient, while the quasi-1D nature of the chains leads to linear dichroism, in conjunction with strongly bound 2D excitons. We elucidate strategies to design and optimize 2D COFs to host both isolated flat bands and quantum-confined 1D subsystems. The properties of the 2D COF discussed here provide a taste of the intriguing possibilities in this open research field. |
Qian, Cheng; Zhou, Weiqiang; Qiao, Jingsi; Wang, Dongdong; Li, Xing; Teo, Wei Liang; Shi, Xiangyan; Wu, Hongwei; Di, Jun; Wang, Hou; Liu, Guofeng; Gu, Long; Liu, Jiawei; Feng, Lili; Liu, Yuchuan; Quek, Su Ying; Loh, Kian Ping; Zhao, Yanli JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 142 (42), pp. 18138-18149, 2020, ISSN: 0002-7863. @article{ISI:000580559000038, title = {Linkage Engineering by Harnessing Supramolecular Interactions to Fabricate 2D Hydrazone-Linked Covalent Organic Framework Platforms toward Advanced Catalysis}, author = {Cheng Qian and Weiqiang Zhou and Jingsi Qiao and Dongdong Wang and Xing Li and Wei Liang Teo and Xiangyan Shi and Hongwei Wu and Jun Di and Hou Wang and Guofeng Liu and Long Gu and Jiawei Liu and Lili Feng and Yuchuan Liu and Su Ying Quek and Kian Ping Loh and Yanli Zhao}, doi = {10.1021/jacs.0c08436}, issn = {0002-7863}, year = {2020}, date = {2020-10-01}, journal = {JOURNAL OF THE AMERICAN CHEMICAL SOCIETY}, volume = {142}, number = {42}, pages = {18138-18149}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Covalent organic frameworks (COFs) are an emerging class of crystalline porous polymers with tailor-made structures and functionalities. To facilitate their utilization for advanced applications, it is crucial to develop a systematic approach to control the properties of COFs, including the crystallinity, stability, and functionalities. However, such an integrated design is challenging to achieve. Herein, we report supramolecular strategy-based linkage engineering to fabricate a versatile 2D hydrazone-linked COF platform for the coordination of different transition metal ions. Intra- and intermolecular hydrogen bonding as well as electrostatic interactions in the antiparallel stacking mode were first utilized to obtain two isoreticular COFs, namely COF-DB and COF-DT. On account of suitable nitrogen sites in COF-DB, the further metalation of COF-DB was accomplished upon the complexation with seven divalent transition metal ions M(II) (M = Mn, Co, Ni, Cu, Zn, Pd, and Cd) under mild conditions. The resultant M/COF-DB exhibited extended p-conjugation, improved crystallinity, enhanced stability, and additional functionalities as compared to the parent COF-DB. Furthermore, the dynamic nature of the coordination bonding in M/COF-DB allows for the easy replacement of metal ions through a postsynthetic exchange. In particular, the coordination mode in Pd/COF-DB endows it with excellent catalytic activity and cyclic stability as a heterogeneous catalyst for the Suzuki-Miyaura cross-coupling reaction, outperforming its amorphous counterparts and Pd/COF-DT. This strategy provides an opportunity for the construction of 2D COFs with designable functions and opens an avenue to create COFs as multifunctional systems.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Covalent organic frameworks (COFs) are an emerging class of crystalline porous polymers with tailor-made structures and functionalities. To facilitate their utilization for advanced applications, it is crucial to develop a systematic approach to control the properties of COFs, including the crystallinity, stability, and functionalities. However, such an integrated design is challenging to achieve. Herein, we report supramolecular strategy-based linkage engineering to fabricate a versatile 2D hydrazone-linked COF platform for the coordination of different transition metal ions. Intra- and intermolecular hydrogen bonding as well as electrostatic interactions in the antiparallel stacking mode were first utilized to obtain two isoreticular COFs, namely COF-DB and COF-DT. On account of suitable nitrogen sites in COF-DB, the further metalation of COF-DB was accomplished upon the complexation with seven divalent transition metal ions M(II) (M = Mn, Co, Ni, Cu, Zn, Pd, and Cd) under mild conditions. The resultant M/COF-DB exhibited extended p-conjugation, improved crystallinity, enhanced stability, and additional functionalities as compared to the parent COF-DB. Furthermore, the dynamic nature of the coordination bonding in M/COF-DB allows for the easy replacement of metal ions through a postsynthetic exchange. In particular, the coordination mode in Pd/COF-DB endows it with excellent catalytic activity and cyclic stability as a heterogeneous catalyst for the Suzuki-Miyaura cross-coupling reaction, outperforming its amorphous counterparts and Pd/COF-DT. This strategy provides an opportunity for the construction of 2D COFs with designable functions and opens an avenue to create COFs as multifunctional systems. |
Wang, Dingguan; Wang, Zishen; Liu, Wei; Arramel, ; Zhou, Jun; Feng, Yuan Ping; Loh, Kian Ping; Wu, Jishan; Wee, Andrew T S Atomic-Level Electronic Properties of Carbon Nitride Monolayers Journal Article ACS NANO, 14 (10), pp. 14008-14016, 2020, ISSN: 1936-0851. @article{ISI:000586793400147, title = {Atomic-Level Electronic Properties of Carbon Nitride Monolayers}, author = {Dingguan Wang and Zishen Wang and Wei Liu and Arramel and Jun Zhou and Yuan Ping Feng and Kian Ping Loh and Jishan Wu and Andrew T S Wee}, doi = {10.1021/acsnano.0c06535}, issn = {1936-0851}, year = {2020}, date = {2020-10-01}, journal = {ACS NANO}, volume = {14}, number = {10}, pages = {14008-14016}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Heteroatom-doped carbon-based materials are of significance for clean energy conversion and storage because of their fascinating electronic properties, low cost, high durability, and environmental friendliness. Atomically precise fabrication of carbon-based materials with well-defined heteroatom-dopant positions and atomic-scale understanding of their atomic-level electronic properties is a challenge. Herein, we demonstrate the bottom-up on-surface synthesis of 1D and 2D monolayer carbon nitride nanostructures with precise control of the nitrogen-atom doping sites and pore sizes. We also observe an electronic band offset at the C-N heterojunction. Using high-resolution scanning tunneling microscopy, the atomic structure of the as-prepared carbon nitride nanoporous monolayers are revealed, indicating successful and precise control of the structures and N atom doping sites. Furthermore, corroborated by theoretical calculations, scanning tunneling spectroscopy measurements reveal a valence band shift of 140 meV that results in an electric field of 2.9 x 10(8) V m(-1) at the C-N heterojunction, indicating efficient separation of the electron-hole pair at the N doping site. Our finding offers direct atomic-level insights into the local electronic structure of the heteroatom-doped carbon-based materials.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Heteroatom-doped carbon-based materials are of significance for clean energy conversion and storage because of their fascinating electronic properties, low cost, high durability, and environmental friendliness. Atomically precise fabrication of carbon-based materials with well-defined heteroatom-dopant positions and atomic-scale understanding of their atomic-level electronic properties is a challenge. Herein, we demonstrate the bottom-up on-surface synthesis of 1D and 2D monolayer carbon nitride nanostructures with precise control of the nitrogen-atom doping sites and pore sizes. We also observe an electronic band offset at the C-N heterojunction. Using high-resolution scanning tunneling microscopy, the atomic structure of the as-prepared carbon nitride nanoporous monolayers are revealed, indicating successful and precise control of the structures and N atom doping sites. Furthermore, corroborated by theoretical calculations, scanning tunneling spectroscopy measurements reveal a valence band shift of 140 meV that results in an electric field of 2.9 x 10(8) V m(-1) at the C-N heterojunction, indicating efficient separation of the electron-hole pair at the N doping site. Our finding offers direct atomic-level insights into the local electronic structure of the heteroatom-doped carbon-based materials. |
Leng, Kai; Wang, Lin; Shao, Yan; Abdelwahab, Ibrahim; Grinblat, Gustavo; Verzhbitskiy, Ivan; Li, Runlai; Cai, Yongqing; Chi, Xiao; Fu, Wei; Song, Peng; Rusydi, Andrivo; Eda, Goki; Maier, Stefan A; Loh, Kian Ping Electron tunneling at the molecularly thin 2D perovskite and graphene van der Waals interface Journal Article NATURE COMMUNICATIONS, 11 (1), 2020, ISSN: 2041-1723. @article{ISI:000588063600006, title = {Electron tunneling at the molecularly thin 2D perovskite and graphene van der Waals interface}, author = {Kai Leng and Lin Wang and Yan Shao and Ibrahim Abdelwahab and Gustavo Grinblat and Ivan Verzhbitskiy and Runlai Li and Yongqing Cai and Xiao Chi and Wei Fu and Peng Song and Andrivo Rusydi and Goki Eda and Stefan A Maier and Kian Ping Loh}, doi = {10.1038/s41467-020-19331-6}, issn = {2041-1723}, year = {2020}, date = {2020-10-01}, journal = {NATURE COMMUNICATIONS}, volume = {11}, number = {1}, publisher = {NATURE RESEARCH}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {Quasi-two-dimensional perovskites have emerged as a new material platform for optoelectronics on account of its intrinsic stability. A major bottleneck to device performance is the high charge injection barrier caused by organic molecular layers on its basal plane, thus the best performing device currently relies on edge contact. Herein, by leveraging on van der Waals coupling and energy level matching between two-dimensional Ruddlesden-Popper perovskite and graphene, we show that the plane-contacted perovskite and graphene interface presents a lower barrier than gold for charge injection. Electron tunneling across the interface occurs via a gate-tunable, direct tunneling-to-field emission mechanism with increasing bias, and photoinduced charge transfer occurs at femtosecond timescale (similar to 50fs). Field effect transistors fabricated on molecularly thin Ruddlesden-Popper perovskite using graphene contact exhibit electron mobilities ranging from 0.1 to 0.018 cm(2)V(-1)s(-1) between 1.7 to 200K. Scanning tunneling spectroscopy studies reveal layer-dependent tunneling barrier and domain size on few-layered Ruddlesden-Popper perovskite.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Quasi-two-dimensional perovskites have emerged as a new material platform for optoelectronics on account of its intrinsic stability. A major bottleneck to device performance is the high charge injection barrier caused by organic molecular layers on its basal plane, thus the best performing device currently relies on edge contact. Herein, by leveraging on van der Waals coupling and energy level matching between two-dimensional Ruddlesden-Popper perovskite and graphene, we show that the plane-contacted perovskite and graphene interface presents a lower barrier than gold for charge injection. Electron tunneling across the interface occurs via a gate-tunable, direct tunneling-to-field emission mechanism with increasing bias, and photoinduced charge transfer occurs at femtosecond timescale (similar to 50fs). Field effect transistors fabricated on molecularly thin Ruddlesden-Popper perovskite using graphene contact exhibit electron mobilities ranging from 0.1 to 0.018 cm(2)V(-1)s(-1) between 1.7 to 200K. Scanning tunneling spectroscopy studies reveal layer-dependent tunneling barrier and domain size on few-layered Ruddlesden-Popper perovskite. |
Xiong, Ting; Zhang, Yaoxin; Lee, Wee Siang Vincent; Xue, Junmin Defect Engineering in Manganese-Based Oxides for Aqueous Rechargeable Zinc-Ion Batteries: A Review Journal Article ADVANCED ENERGY MATERIALS, 10 (34), 2020, ISSN: 1614-6832. @article{ISI:000553341000001, title = {Defect Engineering in Manganese-Based Oxides for Aqueous Rechargeable Zinc-Ion Batteries: A Review}, author = {Ting Xiong and Yaoxin Zhang and Wee Siang Vincent Lee and Junmin Xue}, doi = {10.1002/aenm.202001769, Early Access Date = JUL 2020}, issn = {1614-6832}, year = {2020}, date = {2020-09-01}, journal = {ADVANCED ENERGY MATERIALS}, volume = {10}, number = {34}, publisher = {WILEY-V C H VERLAG GMBH}, address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY}, abstract = {The development of advanced cathode materials for aqueous the zinc ion battery (ZIB) represents a crucial step toward building future large-scale green energy conversion and storage systems. Recently, significant progress has been achieved in the development of manganese-based oxides for ZIB via defect engineering, whereby the intrinsic capacity and energy density have been enhanced. In this review, an overview of the recent progress in the defect engineering of manganese-based oxides for aqueous ZIBs is summarized in the following order: 1) the structures and properties of the commonly used manganese-based oxides, 2) the classification of the various types of defect engineering commonly reported, 3) the various strategies used to create defects in materials, and 4) the effects of the various types of defect engineering on the electrochemical performance of manganese-based oxides. Finally, a perspective on the defect engineering of manganese-based oxides is proposed to further enhance their electrochemical performance as a ZIB cathode.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The development of advanced cathode materials for aqueous the zinc ion battery (ZIB) represents a crucial step toward building future large-scale green energy conversion and storage systems. Recently, significant progress has been achieved in the development of manganese-based oxides for ZIB via defect engineering, whereby the intrinsic capacity and energy density have been enhanced. In this review, an overview of the recent progress in the defect engineering of manganese-based oxides for aqueous ZIBs is summarized in the following order: 1) the structures and properties of the commonly used manganese-based oxides, 2) the classification of the various types of defect engineering commonly reported, 3) the various strategies used to create defects in materials, and 4) the effects of the various types of defect engineering on the electrochemical performance of manganese-based oxides. Finally, a perspective on the defect engineering of manganese-based oxides is proposed to further enhance their electrochemical performance as a ZIB cathode. |
Liang, Shiheng; Shi, Shuyuan; Hsu, Chuang-Han; Cai, Kaiming; Wang, Yi; He, Pan; Wu, Yang; Pereira, Vitor M; Yang, Hyunsoo Spin-Orbit Torque Magnetization Switching in MoTe2/Permalloy Heterostructures Journal Article ADVANCED MATERIALS, 32 (37), 2020, ISSN: 0935-9648. @article{ISI:000554472200001, title = {Spin-Orbit Torque Magnetization Switching in MoTe2/Permalloy Heterostructures}, author = {Shiheng Liang and Shuyuan Shi and Chuang-Han Hsu and Kaiming Cai and Yi Wang and Pan He and Yang Wu and Vitor M Pereira and Hyunsoo Yang}, doi = {10.1002/adma.202002799, Early Access Date = AUG 2020}, issn = {0935-9648}, year = {2020}, date = {2020-09-01}, journal = {ADVANCED MATERIALS}, volume = {32}, number = {37}, publisher = {WILEY-V C H VERLAG GMBH}, address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY}, abstract = {The ability to switch magnetic elements by spin-orbit-induced torques has recently attracted much attention for a path toward high-performance, nonvolatile memories with low power consumption. Realizing efficient spin-orbit-based switching requires the harnessing of both new materials and novel physics to obtain high charge-to-spin conversion efficiencies, thus making the choice of spin source crucial. Here, the observation of spin-orbit torque switching in bilayers consisting of a semimetallic film of 1T `-MoTe(2)adjacent to permalloy is reported. Deterministic switching is achieved without external magnetic fields at room temperature, and the switching occurs with currents one order of magnitude smaller than those typical in devices using the best-performing heavy metals. The thickness-dependence can be understood if the interfacial spin-orbit contribution is considered in addition to the bulk spin Hall effect. Further threefold reduction in the switching current is demonstrated with resort to dumbbell-shaped magnetic elements. These findings foretell exciting prospects of using MoTe(2)for low-power semimetal-material-based spin devices.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The ability to switch magnetic elements by spin-orbit-induced torques has recently attracted much attention for a path toward high-performance, nonvolatile memories with low power consumption. Realizing efficient spin-orbit-based switching requires the harnessing of both new materials and novel physics to obtain high charge-to-spin conversion efficiencies, thus making the choice of spin source crucial. Here, the observation of spin-orbit torque switching in bilayers consisting of a semimetallic film of 1T `-MoTe(2)adjacent to permalloy is reported. Deterministic switching is achieved without external magnetic fields at room temperature, and the switching occurs with currents one order of magnitude smaller than those typical in devices using the best-performing heavy metals. The thickness-dependence can be understood if the interfacial spin-orbit contribution is considered in addition to the bulk spin Hall effect. Further threefold reduction in the switching current is demonstrated with resort to dumbbell-shaped magnetic elements. These findings foretell exciting prospects of using MoTe(2)for low-power semimetal-material-based spin devices. |
Hu, Junxiong; Gou, Jian; Yang, Ming; Omar, Ganesh Ji; Tan, Junyou; Zeng, Shengwei; Liu, Yanpeng; Han, Kun; Lim, Zhishiuh; Huang, Zhen; Wee, Andrew Thye Shen; Ariando, Ariando Room-Temperature Colossal Magnetoresistance in Terraced Single-Layer Graphene Journal Article ADVANCED MATERIALS, 32 (37), 2020, ISSN: 0935-9648. @article{ISI:000554474600001, title = {Room-Temperature Colossal Magnetoresistance in Terraced Single-Layer Graphene}, author = {Junxiong Hu and Jian Gou and Ming Yang and Ganesh Ji Omar and Junyou Tan and Shengwei Zeng and Yanpeng Liu and Kun Han and Zhishiuh Lim and Zhen Huang and Andrew Thye Shen Wee and Ariando Ariando}, doi = {10.1002/adma.202002201, Early Access Date = AUG 2020}, issn = {0935-9648}, year = {2020}, date = {2020-09-01}, journal = {ADVANCED MATERIALS}, volume = {32}, number = {37}, publisher = {WILEY-V C H VERLAG GMBH}, address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY}, abstract = {Disorder-induced magnetoresistance (MR) effect is quadratic at low perpendicular magnetic fields and linear at high fields. This effect is technologically appealing, especially in 2D materials such as graphene, since it offers potential applications in magnetic sensors with nanoscale spatial resolution. However, it is a great challenge to realize a graphene magnetic sensor based on this effect because of the difficulty in controlling the spatial distribution of disorder and enhancing the MR sensitivity in the single-layer regime. Here, a room-temperature colossal MR of up to 5000% at 9 T is reported in terraced single-layer graphene. By laminating single-layer graphene on a terraced substrate, such as TiO2-terminated SrTiO3, a universal one order of magnitude enhancement in the MR compared to conventional single-layer graphene devices is demonstrated. Strikingly, a colossal MR of >1000% is also achieved in the terraced graphene even at a high carrier density of approximate to 10(12)cm(-2). Systematic studies of the MR of single-layer graphene on various oxide- and non-oxide-based terraced surfaces demonstrate that the terraced structure is the dominant factor driving the MR enhancement. The results open a new route for tailoring the physical property of 2D materials by engineering the strain through a terraced substrate.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Disorder-induced magnetoresistance (MR) effect is quadratic at low perpendicular magnetic fields and linear at high fields. This effect is technologically appealing, especially in 2D materials such as graphene, since it offers potential applications in magnetic sensors with nanoscale spatial resolution. However, it is a great challenge to realize a graphene magnetic sensor based on this effect because of the difficulty in controlling the spatial distribution of disorder and enhancing the MR sensitivity in the single-layer regime. Here, a room-temperature colossal MR of up to 5000% at 9 T is reported in terraced single-layer graphene. By laminating single-layer graphene on a terraced substrate, such as TiO2-terminated SrTiO3, a universal one order of magnitude enhancement in the MR compared to conventional single-layer graphene devices is demonstrated. Strikingly, a colossal MR of >1000% is also achieved in the terraced graphene even at a high carrier density of approximate to 10(12)cm(-2). Systematic studies of the MR of single-layer graphene on various oxide- and non-oxide-based terraced surfaces demonstrate that the terraced structure is the dominant factor driving the MR enhancement. The results open a new route for tailoring the physical property of 2D materials by engineering the strain through a terraced substrate. |
Yao, Chuanhao; Guo, Na; Xi, Shibo; Xu, Cong-Qiao; Liu, Wei; Zhao, Xiaoxu; Li, Jing; Fang, Hanyan; Su, Jie; Chen, Zhongxin; Yan, Huan; Qiu, Zhizhan; Lyu, Pin; Chen, Cheng; Xu, Haomin; Peng, Xinnan; Li, Xinzhe; Liu, Bin; Su, Chenliang; Pennycook, Stephen J; Sun, Cheng-Jun; Li, Jun; Zhang, Chun; Du, Yonghua; Lu, Jiong Atomically-precise dopant-controlled single cluster catalysis for electrochemical nitrogen reduction Journal Article NATURE COMMUNICATIONS, 11 (1), 2020, ISSN: 2041-1723. @article{ISI:000569891500016, title = {Atomically-precise dopant-controlled single cluster catalysis for electrochemical nitrogen reduction}, author = {Chuanhao Yao and Na Guo and Shibo Xi and Cong-Qiao Xu and Wei Liu and Xiaoxu Zhao and Jing Li and Hanyan Fang and Jie Su and Zhongxin Chen and Huan Yan and Zhizhan Qiu and Pin Lyu and Cheng Chen and Haomin Xu and Xinnan Peng and Xinzhe Li and Bin Liu and Chenliang Su and Stephen J Pennycook and Cheng-Jun Sun and Jun Li and Chun Zhang and Yonghua Du and Jiong Lu}, doi = {10.1038/s41467-020-18080-w}, issn = {2041-1723}, year = {2020}, date = {2020-09-01}, journal = {NATURE COMMUNICATIONS}, volume = {11}, number = {1}, publisher = {NATURE PUBLISHING GROUP}, address = {MACMILLAN BUILDING, 4 CRINAN ST, LONDON N1 9XW, ENGLAND}, abstract = {The ability to precisely engineer the doping of sub-nanometer bimetallic clusters offers exciting opportunities for tailoring their catalytic performance with atomic accuracy. However, the fabrication of singly dispersed bimetallic cluster catalysts with atomic-level control of dopants has been a long-standing challenge. Herein, we report a strategy for the controllable synthesis of a precisely doped single cluster catalyst consisting of partially ligand-enveloped Au4Pt2 clusters supported on defective graphene. This creates a bimetal single cluster catalyst (Au4Pt2/G) with exceptional activity for electrochemical nitrogen (N-2) reduction. Our mechanistic study reveals that each N-2 molecule is activated in the confined region between cluster and graphene. The heteroatom dopant plays an indispensable role in the activation of N-2 via an enhanced back donation of electrons to the N-2 LUMO. Moreover, besides the heteroatom Pt, the catalytic performance of single cluster catalyst can be further tuned by using Pd in place of Pt as the dopant. The fabrication of singly dispersed metal cluster catalysts with atomic-level control of dopants is a long-standing challenge. Here, the authors report a strategy for the synthesis of a precisely doped single cluster catalyst which shows exceptional activity for electrochemical dinitrogen reduction.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The ability to precisely engineer the doping of sub-nanometer bimetallic clusters offers exciting opportunities for tailoring their catalytic performance with atomic accuracy. However, the fabrication of singly dispersed bimetallic cluster catalysts with atomic-level control of dopants has been a long-standing challenge. Herein, we report a strategy for the controllable synthesis of a precisely doped single cluster catalyst consisting of partially ligand-enveloped Au4Pt2 clusters supported on defective graphene. This creates a bimetal single cluster catalyst (Au4Pt2/G) with exceptional activity for electrochemical nitrogen (N-2) reduction. Our mechanistic study reveals that each N-2 molecule is activated in the confined region between cluster and graphene. The heteroatom dopant plays an indispensable role in the activation of N-2 via an enhanced back donation of electrons to the N-2 LUMO. Moreover, besides the heteroatom Pt, the catalytic performance of single cluster catalyst can be further tuned by using Pd in place of Pt as the dopant. The fabrication of singly dispersed metal cluster catalysts with atomic-level control of dopants is a long-standing challenge. Here, the authors report a strategy for the synthesis of a precisely doped single cluster catalyst which shows exceptional activity for electrochemical dinitrogen reduction. |
Lim, Sharon Xiaodai; Zhang, Zheng; Koon, Gavin Kok Wai; Sow, Chorng-Haur Unlocking the potential of carbon incorporated silver-silver molybdate nanowire with light Journal Article APPLIED MATERIALS TODAY, 20 , 2020, ISSN: 2352-9407. @article{ISI:000598818600003, title = {Unlocking the potential of carbon incorporated silver-silver molybdate nanowire with light}, author = {Sharon Xiaodai Lim and Zheng Zhang and Gavin Kok Wai Koon and Chorng-Haur Sow}, doi = {10.1016/j.apmt.2020.100670}, issn = {2352-9407}, year = {2020}, date = {2020-09-01}, journal = {APPLIED MATERIALS TODAY}, volume = {20}, publisher = {ELSEVIER}, address = {RADARWEG 29, 1043 NX AMSTERDAM, NETHERLANDS}, abstract = {We present a novel form of Ag2MoO4-based hybrid nanowire (NW) with a few remarkable attributes. Firstly, the NW is embedded and decorated with Ag NPs. Secondly, carbon atoms are intentionally incorporated within the matrix of the NW. Thirdly the hybrid nanowires are created via a facile process. Namely, focused laser micropatterning of Ag NPs on GO film as seeding sites and subsequent formation of the hybrid NWs by placing the patterned GO films on heated Mo foil on a hotplate. This unique process resulted in the production of hybrid Ag/Ag2MoO4 NWs that emit unique red fluorescence emission. And finally remarkable photodoping effect is observed from a single strand of optically tuned carbon-doped silver nanoparticles embedded silver molybdate nanowire. We demonstrate applications of these hybrid NWs as micro-display and time limiting, logic components for secure transmission of messages. (C) 2020 Elsevier Ltd. All rights reserved.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We present a novel form of Ag2MoO4-based hybrid nanowire (NW) with a few remarkable attributes. Firstly, the NW is embedded and decorated with Ag NPs. Secondly, carbon atoms are intentionally incorporated within the matrix of the NW. Thirdly the hybrid nanowires are created via a facile process. Namely, focused laser micropatterning of Ag NPs on GO film as seeding sites and subsequent formation of the hybrid NWs by placing the patterned GO films on heated Mo foil on a hotplate. This unique process resulted in the production of hybrid Ag/Ag2MoO4 NWs that emit unique red fluorescence emission. And finally remarkable photodoping effect is observed from a single strand of optically tuned carbon-doped silver nanoparticles embedded silver molybdate nanowire. We demonstrate applications of these hybrid NWs as micro-display and time limiting, logic components for secure transmission of messages. (C) 2020 Elsevier Ltd. All rights reserved. |
Xian, Jing-Jing; Chen, Li; Liu, Xin; Zhang, Wen-Hao; Peng, Lang; Li, Rui; Cai, Min; Qiao, Jingsi; Fu, Ying-Shuang Valley dependent superconducting proximity effect in a twisted van der Waals heterojunction Journal Article PHYSICAL REVIEW RESEARCH, 2 (3), 2020. @article{ISI:000604173300004, title = {Valley dependent superconducting proximity effect in a twisted van der Waals heterojunction}, author = {Jing-Jing Xian and Li Chen and Xin Liu and Wen-Hao Zhang and Lang Peng and Rui Li and Min Cai and Jingsi Qiao and Ying-Shuang Fu}, doi = {10.1103/PhysRevResearch.2.033360}, year = {2020}, date = {2020-09-01}, journal = {PHYSICAL REVIEW RESEARCH}, volume = {2}, number = {3}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {Leakage of Cooper pairs through heterointerfaces leads to the superconducting proximity effect, which has been utilized extensively in building functional quantum devices and inducing novel superconductivity. While an atomically sharp interface in real-space is known to be crucial for effective Cooper pair proximity transfer, the influence of electronic valleys in the momentum space has not been sufficiently investigated. Here, we report the observation of valley dependent superconducting proximity effect in a heterostructure with twisted overlapping. The heterostructure is realized by growth of multidomain Bi(111) films on a single-crystal NbSe2 substrate with molecular beam epitaxy. With spectroscopic imaging scanning tunneling spectroscopy, we identified different types of atomic overlapping in the Bi films, and measured drastic changes of proximity-induced superconducting gap sizes on the differently oriented Bi domains. Based on our theoretical model calculation, this phenomenon can be interpreted as valley dependent superconducting proximity coupling between the Bi film and NbSe2. We also investigated the lateral proximity effect between two adjacent Bi domains, which determines a significant reduction of the mean free path of electrons, associated with interfacial scattering. Our study expands the scope of tunable physical properties with the valley degree of freedom.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Leakage of Cooper pairs through heterointerfaces leads to the superconducting proximity effect, which has been utilized extensively in building functional quantum devices and inducing novel superconductivity. While an atomically sharp interface in real-space is known to be crucial for effective Cooper pair proximity transfer, the influence of electronic valleys in the momentum space has not been sufficiently investigated. Here, we report the observation of valley dependent superconducting proximity effect in a heterostructure with twisted overlapping. The heterostructure is realized by growth of multidomain Bi(111) films on a single-crystal NbSe2 substrate with molecular beam epitaxy. With spectroscopic imaging scanning tunneling spectroscopy, we identified different types of atomic overlapping in the Bi films, and measured drastic changes of proximity-induced superconducting gap sizes on the differently oriented Bi domains. Based on our theoretical model calculation, this phenomenon can be interpreted as valley dependent superconducting proximity coupling between the Bi film and NbSe2. We also investigated the lateral proximity effect between two adjacent Bi domains, which determines a significant reduction of the mean free path of electrons, associated with interfacial scattering. Our study expands the scope of tunable physical properties with the valley degree of freedom. |
Xu, Haomin; Xi, Shibo; Li, Jing; Liu, Shikai; Lyu, Pin; Yu, Wei; Sun, Tao; Qi, Dong-Chen; He, Qian; Xiao, Hai; Lin, Ming; Wu, Jishan; Zhang, Jia; Lu, Jiong Chemical design and synthesis of superior single-atom electrocatalystsvia in situpolymerization Journal Article JOURNAL OF MATERIALS CHEMISTRY A, 8 (34), pp. 17683-17690, 2020, ISSN: 2050-7488. @article{ISI:000566092600031, title = {Chemical design and synthesis of superior single-atom electrocatalystsvia in situpolymerization}, author = {Haomin Xu and Shibo Xi and Jing Li and Shikai Liu and Pin Lyu and Wei Yu and Tao Sun and Dong-Chen Qi and Qian He and Hai Xiao and Ming Lin and Jishan Wu and Jia Zhang and Jiong Lu}, doi = {10.1039/d0ta05130f}, issn = {2050-7488}, year = {2020}, date = {2020-09-01}, journal = {JOURNAL OF MATERIALS CHEMISTRY A}, volume = {8}, number = {34}, pages = {17683-17690}, publisher = {ROYAL SOC CHEMISTRY}, address = {THOMAS GRAHAM HOUSE, SCIENCE PARK, MILTON RD, CAMBRIDGE CB4 0WF, CAMBS, ENGLAND}, abstract = {Molecule-like electrocatalysts with FeN(4)motifs have been demonstrated to be excellent candidates for various renewable energy conversions. The ability to further tune the electronic properties of molecular FeN(4)motifs and integrate them onto conductive supports represents a key step towards the synthesis of highly robust and efficient single-atom catalysts (SACs) for practical applications. Here, we developed a new route for the synthesis of a well-defined single-atom FeN(4)electrocatalystvia in situpolymerization of four amino groups functionalized iron phthalocyanine (NH2-FePc) molecules on conductive carbon nanotubes. The intermolecular oxidative dimerization between the amino groups of NH2-FePc creates the desired electron-withdrawing pyrazine linker between FeN(4)motifs, which can significantly optimize their electrocatalytic performances. As a result, the FeN4-SAC exhibits both outstanding ORR activity (a half-wave potential of 0.88 Vvs.RHE) and excellent performance in Zn-oxygen batteries, outperforming the commercial Pt/C and pristine iron phthalocyanine (FePc) catalysts. Our theoretical calculations reveal that the presence of electron-withdrawing linkers shifts the occupied antibonding states towards lower energies and thus weakens the Fe-O bond, which is primarily responsible for the enhancement of ORR activity.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Molecule-like electrocatalysts with FeN(4)motifs have been demonstrated to be excellent candidates for various renewable energy conversions. The ability to further tune the electronic properties of molecular FeN(4)motifs and integrate them onto conductive supports represents a key step towards the synthesis of highly robust and efficient single-atom catalysts (SACs) for practical applications. Here, we developed a new route for the synthesis of a well-defined single-atom FeN(4)electrocatalystvia in situpolymerization of four amino groups functionalized iron phthalocyanine (NH2-FePc) molecules on conductive carbon nanotubes. The intermolecular oxidative dimerization between the amino groups of NH2-FePc creates the desired electron-withdrawing pyrazine linker between FeN(4)motifs, which can significantly optimize their electrocatalytic performances. As a result, the FeN4-SAC exhibits both outstanding ORR activity (a half-wave potential of 0.88 Vvs.RHE) and excellent performance in Zn-oxygen batteries, outperforming the commercial Pt/C and pristine iron phthalocyanine (FePc) catalysts. Our theoretical calculations reveal that the presence of electron-withdrawing linkers shifts the occupied antibonding states towards lower energies and thus weakens the Fe-O bond, which is primarily responsible for the enhancement of ORR activity. |