Browse through over scientific publications by CA2DM staff, students, collaborators, and partners.
2011 |
Rodrigues, J N B; Goncalves, P A D; Rodrigues, N F G; Ribeiro, R M; dos Santos, Lopes J M B; Peres, N M R Zigzag graphene nanoribbon edge reconstruction with Stone-Wales defects Journal Article PHYSICAL REVIEW B, 84 (15), 2011, ISSN: 1098-0121. @article{ISI:000296290000014, title = {Zigzag graphene nanoribbon edge reconstruction with Stone-Wales defects}, author = {J N B Rodrigues and P A D Goncalves and N F G Rodrigues and R M Ribeiro and J M B Lopes dos Santos and N M R Peres}, doi = {10.1103/PhysRevB.84.155435}, issn = {1098-0121}, year = {2011}, date = {2011-10-01}, journal = {PHYSICAL REVIEW B}, volume = {84}, number = {15}, abstract = {In this paper, we study zigzag graphene nanoribbons with edges reconstructed with Stone-Wales defects, by means of an empirical (first-neighbor) tight-binding method, with parameters determined by ab initio calculations of very narrow ribbons. We explore the characteristics of the electronic band structure with a focus on the nature of edge states. Edge reconstruction allows the appearance of a new type of edge states. They are dispersive, with nonzero amplitudes in both sublattices; furthermore, the amplitudes have two components that decrease with different decay lengths with the distance from the edge; at the Dirac points one of these lengths diverges, whereas the other remains finite, of the order of the lattice parameter. We trace this curious effect to the doubling of the unit cell along the edge, brought about by the edge reconstruction. In the presence of a magnetic field, the zero-energy Landau level is no longer degenerate with edge states as in the case of the pristine zigzag ribbon.}, keywords = {}, pubstate = {published}, tppubtype = {article} } In this paper, we study zigzag graphene nanoribbons with edges reconstructed with Stone-Wales defects, by means of an empirical (first-neighbor) tight-binding method, with parameters determined by ab initio calculations of very narrow ribbons. We explore the characteristics of the electronic band structure with a focus on the nature of edge states. Edge reconstruction allows the appearance of a new type of edge states. They are dispersive, with nonzero amplitudes in both sublattices; furthermore, the amplitudes have two components that decrease with different decay lengths with the distance from the edge; at the Dirac points one of these lengths diverges, whereas the other remains finite, of the order of the lattice parameter. We trace this curious effect to the doubling of the unit cell along the edge, brought about by the edge reconstruction. In the presence of a magnetic field, the zero-energy Landau level is no longer degenerate with edge states as in the case of the pristine zigzag ribbon. |
Rappoport, T G; Godoy, M; Uchoa, B; Santos, Dos R R; Neto, Castro A H Magnetic exchange mechanism for electronic gap opening in graphene Journal Article EPL, 96 (2), 2011, ISSN: 0295-5075. @article{ISI:000295974600033, title = {Magnetic exchange mechanism for electronic gap opening in graphene}, author = {T G Rappoport and M Godoy and B Uchoa and R R Dos Santos and A H Castro Neto}, doi = {10.1209/0295-5075/96/27010}, issn = {0295-5075}, year = {2011}, date = {2011-10-01}, journal = {EPL}, volume = {96}, number = {2}, abstract = {We show within a local self-consistent mean-field treatment that a random distribution of magnetic adatoms can open a robust gap in the electronic spectrum of graphene. The electronic gap results from the localization of the charge carriers that arises from the interplay between the graphene sublattice structure and the exchange interaction between the adatoms. The size of the gap depends on the strength of the exchange interaction between carriers and localized spins and can be controlled by both temperature and external magnetic field. Furthermore, we show that an external magnetic field creates an imbalance of spin-up and spin-down carriers at the Fermi level, making doped graphene suitable for spin injection and other spintronic applications. Copyright (C) EPLA, 2011}, keywords = {}, pubstate = {published}, tppubtype = {article} } We show within a local self-consistent mean-field treatment that a random distribution of magnetic adatoms can open a robust gap in the electronic spectrum of graphene. The electronic gap results from the localization of the charge carriers that arises from the interplay between the graphene sublattice structure and the exchange interaction between the adatoms. The size of the gap depends on the strength of the exchange interaction between carriers and localized spins and can be controlled by both temperature and external magnetic field. Furthermore, we show that an external magnetic field creates an imbalance of spin-up and spin-down carriers at the Fermi level, making doped graphene suitable for spin injection and other spintronic applications. Copyright (C) EPLA, 2011 |
Santos, Jaime E; Peres, Nuno M R; dos Santos, Joao Lopes M B; Neto, Antonio Castro H Electronic doping of graphene by deposited transition metal atoms Journal Article PHYSICAL REVIEW B, 84 (8), 2011, ISSN: 2469-9950. @article{ISI:000294325400014, title = {Electronic doping of graphene by deposited transition metal atoms}, author = {Jaime E Santos and Nuno M R Peres and Joao M B Lopes dos Santos and Antonio H Castro Neto}, doi = {10.1103/PhysRevB.84.085430}, issn = {2469-9950}, year = {2011}, date = {2011-08-01}, journal = {PHYSICAL REVIEW B}, volume = {84}, number = {8}, abstract = {We perform a phenomenological analysis of the problem of the electronic doping of a graphene sheet by deposited transition metal atoms, which aggregate in clusters. The sample is placed in a capacitor device such that the electronic doping of graphene can be varied by the application of a gate voltage and such that transport measurements can be performed via the application of a (much smaller) voltage along the graphene sample, as reported in the work of Pi et al. [Phys. Rev. B 80, 075406 (2009)]. The analysis allows us to explain the thermodynamic properties of the device, such as the level of doping of graphene and the ionization potential of the metal clusters, in terms of the chemical interaction between graphene and the clusters. We are also able, by modeling the metallic clusters as perfectly conducting spheres, to determine the scattering potential due to these clusters on the electronic carriers of graphene and hence the contribution of these clusters to the resistivity of the sample. The model presented is able to explain the measurements performed by Pi et al. on Pt-covered graphene samples at the lowest metallic coverages measured, and we also present a theoretical argument based on the above model that explains why significant deviations from such a theory are observed at higher levels of coverage.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We perform a phenomenological analysis of the problem of the electronic doping of a graphene sheet by deposited transition metal atoms, which aggregate in clusters. The sample is placed in a capacitor device such that the electronic doping of graphene can be varied by the application of a gate voltage and such that transport measurements can be performed via the application of a (much smaller) voltage along the graphene sample, as reported in the work of Pi et al. [Phys. Rev. B 80, 075406 (2009)]. The analysis allows us to explain the thermodynamic properties of the device, such as the level of doping of graphene and the ionization potential of the metal clusters, in terms of the chemical interaction between graphene and the clusters. We are also able, by modeling the metallic clusters as perfectly conducting spheres, to determine the scattering potential due to these clusters on the electronic carriers of graphene and hence the contribution of these clusters to the resistivity of the sample. The model presented is able to explain the measurements performed by Pi et al. on Pt-covered graphene samples at the lowest metallic coverages measured, and we also present a theoretical argument based on the above model that explains why significant deviations from such a theory are observed at higher levels of coverage. |
Neto, Castro A H; Novoselov, K New directions in science and technology: two-dimensional crystals Journal Article REPORTS ON PROGRESS IN PHYSICS, 74 (8), 2011, ISSN: 0034-4885. @article{ISI:000293225900001, title = {New directions in science and technology: two-dimensional crystals}, author = {A H Castro Neto and K Novoselov}, doi = {10.1088/0034-4885/74/8/082501}, issn = {0034-4885}, year = {2011}, date = {2011-08-01}, journal = {REPORTS ON PROGRESS IN PHYSICS}, volume = {74}, number = {8}, abstract = {Graphene is possibly one of the largest and fastest growing fields in condensed matter research. However, graphene is only one example in a large class of two-dimensional crystals with unusual properties. In this paper we briefly review the properties of graphene and look at the exciting possibilities that lie ahead.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Graphene is possibly one of the largest and fastest growing fields in condensed matter research. However, graphene is only one example in a large class of two-dimensional crystals with unusual properties. In this paper we briefly review the properties of graphene and look at the exciting possibilities that lie ahead. |
de Gail, R; Goerbig, M O; Guinea, F; Montambaux, G; Neto, Castro A H Topologically protected zero modes in twisted bilayer graphene Journal Article PHYSICAL REVIEW B, 84 (4), 2011, ISSN: 1098-0121. @article{ISI:000292825100016, title = {Topologically protected zero modes in twisted bilayer graphene}, author = {R de Gail and M O Goerbig and F Guinea and G Montambaux and A H Castro Neto}, doi = {10.1103/PhysRevB.84.045436}, issn = {1098-0121}, year = {2011}, date = {2011-07-01}, journal = {PHYSICAL REVIEW B}, volume = {84}, number = {4}, abstract = {We show that a twisted graphene bilayer can reveal unusual topological properties at low energies, as a consequence of a Dirac-point splitting. These features rely on a symmetry analysis of the electron hopping between the two layers of graphene and we derive a simplified effective low-energy Hamiltonian which captures the essential topological properties of a twisted graphene bilayer. The corresponding Landau levels peculiarly reveal a degenerate zero-energy mode which cannot be lifted by strong magnetic fields.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We show that a twisted graphene bilayer can reveal unusual topological properties at low energies, as a consequence of a Dirac-point splitting. These features rely on a symmetry analysis of the electron hopping between the two layers of graphene and we derive a simplified effective low-energy Hamiltonian which captures the essential topological properties of a twisted graphene bilayer. The corresponding Landau levels peculiarly reveal a degenerate zero-energy mode which cannot be lifted by strong magnetic fields. |
Peres, N M R; dos Santos, Lopes J M B; Neto, Castro A H Coulomb drag and high-resistivity behavior in double-layer graphene Journal Article EPL, 95 (1, SI), 2011, ISSN: 0295-5075. @article{ISI:000291990600030, title = {Coulomb drag and high-resistivity behavior in double-layer graphene}, author = {N M R Peres and J M B Lopes dos Santos and A H Castro Neto}, doi = {10.1209/0295-5075/95/18001}, issn = {0295-5075}, year = {2011}, date = {2011-07-01}, journal = {EPL}, volume = {95}, number = {1, SI}, abstract = {We show that Coulomb drag in ultra-clean graphene double layers can be used for controlling the on-and-off ratio for current flow by tuning the external gate voltage. Hence, although graphene remains semi-metallic, the double-layer graphene system can be tuned from conductive to a highly resistive state. We show that our results explain previous data of Coulomb drag in double-layer graphene samples in disordered SiO2 substrates. Copyright (C) EPLA, 2011}, keywords = {}, pubstate = {published}, tppubtype = {article} } We show that Coulomb drag in ultra-clean graphene double layers can be used for controlling the on-and-off ratio for current flow by tuning the external gate voltage. Hence, although graphene remains semi-metallic, the double-layer graphene system can be tuned from conductive to a highly resistive state. We show that our results explain previous data of Coulomb drag in double-layer graphene samples in disordered SiO2 substrates. Copyright (C) EPLA, 2011 |
Nayak, Tapas R; Andersen, Henrik; Makam, Venkata S; Khaw, Clement; Bae, Sukang; Xu, Xiangfan; Ee, Pui-Lai R; Ahn, Jong-Hyun; Hong, Byung Hee; Pastorin, Giorgia; Oezyilmaz, Barbaros Graphene for Controlled and Accelerated Osteogenic Differentiation of Human Mesenchymal Stem Cells Journal Article ACS NANO, 5 (6), pp. 4670-4678, 2011, ISSN: 1936-0851. @article{ISI:000292055200049, title = {Graphene for Controlled and Accelerated Osteogenic Differentiation of Human Mesenchymal Stem Cells}, author = {Tapas R Nayak and Henrik Andersen and Venkata S Makam and Clement Khaw and Sukang Bae and Xiangfan Xu and Pui-Lai R Ee and Jong-Hyun Ahn and Byung Hee Hong and Giorgia Pastorin and Barbaros Oezyilmaz}, doi = {10.1021/nn200500h}, issn = {1936-0851}, year = {2011}, date = {2011-06-01}, journal = {ACS NANO}, volume = {5}, number = {6}, pages = {4670-4678}, abstract = {Current tissue engineering approaches combine different scaffold materials with living cells to provide biological substitutes that can repair and eventually improve tissue functions. Both natural and synthetic materials have been fabricated for transplantation of stem cells and their specific differentiation Into muscles, bones, and cartilages. One of the key objectives for bone regeneration therapy to be successful Is to direct stem cells' proliferation and to accelerate their differentiation in a controlled manner through the use of growth factors and osteogenic inducers. Here we show that graphene provides a promising biocompatible scaffold that does not hamper the proliferation of human mesenchymal stem cells (hMSCs) and accelerates their specific differentiation into bone cells. The differentiation rate Is comparable to the one achieved with common growth factors, demonstrating graphene's potential for stem cell research.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Current tissue engineering approaches combine different scaffold materials with living cells to provide biological substitutes that can repair and eventually improve tissue functions. Both natural and synthetic materials have been fabricated for transplantation of stem cells and their specific differentiation Into muscles, bones, and cartilages. One of the key objectives for bone regeneration therapy to be successful Is to direct stem cells' proliferation and to accelerate their differentiation in a controlled manner through the use of growth factors and osteogenic inducers. Here we show that graphene provides a promising biocompatible scaffold that does not hamper the proliferation of human mesenchymal stem cells (hMSCs) and accelerates their specific differentiation into bone cells. The differentiation rate Is comparable to the one achieved with common growth factors, demonstrating graphene's potential for stem cell research. |
Avsar, Ahmet; Yang, Tsung-Yeh; Bae, Sukang; Balakrishnan, Jayakumar; Volmer, Frank; Jaiswal, Manu; Yi, Zheng; Ali, Syed Rizwan; Guentherodt, Gernot; Hong, Byung Hee; Beschoten, Bernd; Oezyilmaz, Barbaros Toward Wafer Scale Fabrication of Graphene Based Spin Valve Devices Journal Article NANO LETTERS, 11 (6), pp. 2363-2368, 2011, ISSN: 1530-6984. @article{ISI:000291322600029, title = {Toward Wafer Scale Fabrication of Graphene Based Spin Valve Devices}, author = {Ahmet Avsar and Tsung-Yeh Yang and Sukang Bae and Jayakumar Balakrishnan and Frank Volmer and Manu Jaiswal and Zheng Yi and Syed Rizwan Ali and Gernot Guentherodt and Byung Hee Hong and Bernd Beschoten and Barbaros Oezyilmaz}, doi = {10.1021/nl200714q}, issn = {1530-6984}, year = {2011}, date = {2011-06-01}, journal = {NANO LETTERS}, volume = {11}, number = {6}, pages = {2363-2368}, abstract = {We demonstrate injection, transport, and detection of spins in spin valve arrays patterned in both copper based chemical vapor deposition (Cu-CVD) synthesized wafer scale single layer and bilayer graphene. We observe spin relaxation times comparable to those reported for exfoliated graphene samples demonstrating that chemical vapor deposition specific structural differences such as nanoripples do not limit spin transport in the present samples. Our observations make Cu-CVD graphene a promising material of choice for large scale spintronic applications.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We demonstrate injection, transport, and detection of spins in spin valve arrays patterned in both copper based chemical vapor deposition (Cu-CVD) synthesized wafer scale single layer and bilayer graphene. We observe spin relaxation times comparable to those reported for exfoliated graphene samples demonstrating that chemical vapor deposition specific structural differences such as nanoripples do not limit spin transport in the present samples. Our observations make Cu-CVD graphene a promising material of choice for large scale spintronic applications. |
Neto, Antonio Castro H Another Spin on Graphene Journal Article SCIENCE, 332 (6027), pp. 315-316, 2011, ISSN: 0036-8075. @article{ISI:000289516600033, title = {Another Spin on Graphene}, author = {Antonio H Castro Neto}, doi = {10.1126/science.1204496}, issn = {0036-8075}, year = {2011}, date = {2011-04-01}, journal = {SCIENCE}, volume = {332}, number = {6027}, pages = {315-316}, keywords = {}, pubstate = {published}, tppubtype = {article} } |
Neto, Castro A H; Novoselov, K Two-Dimensional Crystals: Beyond Graphene Journal Article MATERIALS EXPRESS, 1 (1), pp. 10-17, 2011, ISSN: 2158-5849. @article{ISI:000311866900002, title = {Two-Dimensional Crystals: Beyond Graphene}, author = {A H Castro Neto and K Novoselov}, doi = {10.1166/mex.2011.1002}, issn = {2158-5849}, year = {2011}, date = {2011-03-01}, journal = {MATERIALS EXPRESS}, volume = {1}, number = {1}, pages = {10-17}, abstract = {Human progress and development has always been marked by breakthroughs in the control of materials. Since pre-historic times, through the stone, bronze, and iron ages, humans have exploited their environment for materials that can be either used directly or can be modified for their benefit, to make their life more comfortable, productive, or to give them military advantage. One age replaces another when the material that is the basis for its sustainability runs its course and is replaced by another material which presents more qualities. Multi-tasking, speed, versatility, and flexibility are at the heart of modern technology. In recent years a new class of materials that can fulfill these needs have emerged: two-dimensional (2D) crystals. Graphene is probably the most famous example, but there are numerous other examples with amazing electronic and structural properties. In this paper we look into the possible routes for exploration of this new field that presents new venues in basic science as well as in applications.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Human progress and development has always been marked by breakthroughs in the control of materials. Since pre-historic times, through the stone, bronze, and iron ages, humans have exploited their environment for materials that can be either used directly or can be modified for their benefit, to make their life more comfortable, productive, or to give them military advantage. One age replaces another when the material that is the basis for its sustainability runs its course and is replaced by another material which presents more qualities. Multi-tasking, speed, versatility, and flexibility are at the heart of modern technology. In recent years a new class of materials that can fulfill these needs have emerged: two-dimensional (2D) crystals. Graphene is probably the most famous example, but there are numerous other examples with amazing electronic and structural properties. In this paper we look into the possible routes for exploration of this new field that presents new venues in basic science as well as in applications. |
2010 |
Pereira, Vitor M; Ribeiro, R M; Peres, N M R; Neto, Castro A H Optical properties of strained graphene Journal Article EPL, 92 (6), 2010, ISSN: 0295-5075. @article{ISI:000287023900021, title = {Optical properties of strained graphene}, author = {Vitor M Pereira and R M Ribeiro and N M R Peres and A H Castro Neto}, doi = {10.1209/0295-5075/92/67001}, issn = {0295-5075}, year = {2010}, date = {2010-12-01}, journal = {EPL}, volume = {92}, number = {6}, abstract = {The optical conductivity of graphene strained uniaxially is studied within the Kubo-Greenwood formalism. Focusing on inter-band absorption, we analyze and quantify the breakdown of universal transparency in the visible region of the spectrum, and analytically characterize the transparency as a function of strain and polarization. Measuring transmittance as a function of incident polarization directly reflects the magnitude and direction of strain. Moreover, direction-dependent selection rules permit the identification of the lattice orientation by monitoring the van Hove transitions. These photoelastic effects in graphene can be explored towards atomically thin, broadband optical elements. Copyright (C) EPLA, 2010}, keywords = {}, pubstate = {published}, tppubtype = {article} } The optical conductivity of graphene strained uniaxially is studied within the Kubo-Greenwood formalism. Focusing on inter-band absorption, we analyze and quantify the breakdown of universal transparency in the visible region of the spectrum, and analytically characterize the transparency as a function of strain and polarization. Measuring transmittance as a function of incident polarization directly reflects the magnitude and direction of strain. Moreover, direction-dependent selection rules permit the identification of the lattice orientation by monitoring the van Hove transitions. These photoelastic effects in graphene can be explored towards atomically thin, broadband optical elements. Copyright (C) EPLA, 2010 |
0000 |
Li, Zejun; Zhang, Xiuying; Zhao, Xiaoxu; Li, Jing; Herng, Tun Seng; Xu, Haomin; Lin, Fanrong; Lyu, Pin; Peng, Xinnan; Yu, Wei; Hai, Xiao; Chen, Cheng; Yang, Huimin; Martin, Jens; Lu, Jing; Luo, Xin; Neto, Castro A H; Pennycook, Stephen J; Ding, Jun; Feng, Yuanping; Lu, Jiong Imprinting Ferromagnetism and Superconductivity in Single Atomic Layers of Molecular Superlattices Journal Article ADVANCED MATERIALS, 0000, ISSN: 0935-9648. @article{ISI:000533211300001, title = {Imprinting Ferromagnetism and Superconductivity in Single Atomic Layers of Molecular Superlattices}, author = {Zejun Li and Xiuying Zhang and Xiaoxu Zhao and Jing Li and Tun Seng Herng and Haomin Xu and Fanrong Lin and Pin Lyu and Xinnan Peng and Wei Yu and Xiao Hai and Cheng Chen and Huimin Yang and Jens Martin and Jing Lu and Xin Luo and A H Castro Neto and Stephen J Pennycook and Jun Ding and Yuanping Feng and Jiong Lu}, doi = {10.1002/adma.201907645, Early Access Date = MAY 2020}, issn = {0935-9648}, journal = {ADVANCED MATERIALS}, abstract = {Ferromagnetism and superconductivity are two antagonistic phenomena since ferromagnetic exchange fields tend to destroy singlet Cooper pairs. Reconciliation of these two competing phases has been achieved in vertically stacked heterostructures where these two orders are confined in different layers. However, controllable integration of these two phases in one atomic layer is a longstanding challenge. Here, an interlayer-space-confined chemical design (ICCD) is reported for the synthesis of dilute single-atom-doped TaS2 molecular superlattice, whereby ferromagnetism is observed in the superconducting TaS2 layers. The intercalation of 2H-TaS2 crystal with bulky organic ammonium molecule expands its van der Waals gap for single-atom doping via co-intercalated cobalt ions, resulting in the formation of quasi-monolayer Co-doped TaS2 superlattices. Isolated Co atoms are decorated in the basal plane of the TaS2 via substituting the Ta atom or anchoring at a hollow site, wherein the orbital-selected p-d hybridization between Co and neighboring Ta and S atoms induces local magnetic moments with strong ferromagnetic coupling. This ICCD approach can be applied to various metal ions, enabling the synthesis of a series of crystal-size TaS2 molecular superlattices.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Ferromagnetism and superconductivity are two antagonistic phenomena since ferromagnetic exchange fields tend to destroy singlet Cooper pairs. Reconciliation of these two competing phases has been achieved in vertically stacked heterostructures where these two orders are confined in different layers. However, controllable integration of these two phases in one atomic layer is a longstanding challenge. Here, an interlayer-space-confined chemical design (ICCD) is reported for the synthesis of dilute single-atom-doped TaS2 molecular superlattice, whereby ferromagnetism is observed in the superconducting TaS2 layers. The intercalation of 2H-TaS2 crystal with bulky organic ammonium molecule expands its van der Waals gap for single-atom doping via co-intercalated cobalt ions, resulting in the formation of quasi-monolayer Co-doped TaS2 superlattices. Isolated Co atoms are decorated in the basal plane of the TaS2 via substituting the Ta atom or anchoring at a hollow site, wherein the orbital-selected p-d hybridization between Co and neighboring Ta and S atoms induces local magnetic moments with strong ferromagnetic coupling. This ICCD approach can be applied to various metal ions, enabling the synthesis of a series of crystal-size TaS2 molecular superlattices. |
Chua, Rebekah; Yang, Jing; He, Xiaoyue; Yu, Xiaojiang; Yu, Wei; Bussolotti, Fabio; Wong, Ping Kwan Johnny; Loh, Kian Ping; Breese, Mark B H; Goh, Kuan Eng Johnson; Huang, Yu Li; Wee, Andrew T S Can Reconstructed Se-Deficient Line Defects in Monolayer VSe2 Induce Magnetism? Journal Article ADVANCED MATERIALS, 0000, ISSN: 0935-9648. @article{ISI:000530864700001, title = {Can Reconstructed Se-Deficient Line Defects in Monolayer VSe2 Induce Magnetism?}, author = {Rebekah Chua and Jing Yang and Xiaoyue He and Xiaojiang Yu and Wei Yu and Fabio Bussolotti and Ping Kwan Johnny Wong and Kian Ping Loh and Mark B H Breese and Kuan Eng Johnson Goh and Yu Li Huang and Andrew T S Wee}, doi = {10.1002/adma.202000693, Early Access Date = MAY 2020}, issn = {0935-9648}, journal = {ADVANCED MATERIALS}, abstract = {There have been several recent conflicting reports on the ferromagnetism of clean monolayer VSe2. Herein, the controllable formation of 1D defect line patterns in vanadium diselenide (VSe2) monolayers initiated by thermal annealing is presented. Using scanning tunneling microscopy and q-plus atomic force microscopy techniques, the 1D line features are determined to be 8-member-ring arrays, formed via a Se deficient reconstruction process. The reconstructed VSe2 monolayer with Se-deficient line defects displays room-temperature ferromagnetism under X-ray magnetic circular dichroism and magnetic force microscopy, consistent with the density functional theory calculations. This study possibly resolves the controversy on whether ferromagnetism is intrinsic in monolayer VSe2, and highlights the importance of controlling and understanding the atomic structures of surface defects in 2D crystals, which could play key roles in the material properties and hence potential device applications.}, keywords = {}, pubstate = {published}, tppubtype = {article} } There have been several recent conflicting reports on the ferromagnetism of clean monolayer VSe2. Herein, the controllable formation of 1D defect line patterns in vanadium diselenide (VSe2) monolayers initiated by thermal annealing is presented. Using scanning tunneling microscopy and q-plus atomic force microscopy techniques, the 1D line features are determined to be 8-member-ring arrays, formed via a Se deficient reconstruction process. The reconstructed VSe2 monolayer with Se-deficient line defects displays room-temperature ferromagnetism under X-ray magnetic circular dichroism and magnetic force microscopy, consistent with the density functional theory calculations. This study possibly resolves the controversy on whether ferromagnetism is intrinsic in monolayer VSe2, and highlights the importance of controlling and understanding the atomic structures of surface defects in 2D crystals, which could play key roles in the material properties and hence potential device applications. |
Leng, Kai; Fu, Wei; Liu, Yanpeng; Chhowalla, Manish; Loh, Kian Ping From bulk to molecularly thin hybrid perovskites Journal Article NATURE REVIEWS MATERIALS, 0000, ISSN: 2058-8437. @article{ISI:000522380500001, title = {From bulk to molecularly thin hybrid perovskites}, author = {Kai Leng and Wei Fu and Yanpeng Liu and Manish Chhowalla and Kian Ping Loh}, doi = {10.1038/s41578-020-0185-1, Early Access Date = MAR 2020}, issn = {2058-8437}, journal = {NATURE REVIEWS MATERIALS}, abstract = {Owing to their layered structure, the properties of 2D perovskites can be controlled by tuning their thickness. This Review surveys how fine-tuning the thickness of 2D perovskites from the sub-micrometre to the molecularly thin regime helps to optimize their electrical and optical properties for use in different applications. Organic-inorganic hybrid perovskites have been intensively researched in the past decade for their optoelectronic properties. The emergence of Ruddlesden-Popper perovskites, which have mixed dimensionality, has heralded new opportunities for tailor-made semiconductors that combine enhanced stability with useful properties between those of 2D and 3D systems. Inspired by advances in 2D materials research, there is growing interest in molecularly thin versions of these hybrid perovskites, owing to their ease of incorporation into electronic devices. There is, thus, a need to understand thickness-dependent electrical, excitonic and phononic properties that go beyond quantum-confinement effects. Recent studies have shown that, apart from tuning the dimensionality of the system, fine-tuning its thickness also helps to optimize performance in different applications, ranging from third-harmonic generation to photodetectors and spintronics.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Owing to their layered structure, the properties of 2D perovskites can be controlled by tuning their thickness. This Review surveys how fine-tuning the thickness of 2D perovskites from the sub-micrometre to the molecularly thin regime helps to optimize their electrical and optical properties for use in different applications. Organic-inorganic hybrid perovskites have been intensively researched in the past decade for their optoelectronic properties. The emergence of Ruddlesden-Popper perovskites, which have mixed dimensionality, has heralded new opportunities for tailor-made semiconductors that combine enhanced stability with useful properties between those of 2D and 3D systems. Inspired by advances in 2D materials research, there is growing interest in molecularly thin versions of these hybrid perovskites, owing to their ease of incorporation into electronic devices. There is, thus, a need to understand thickness-dependent electrical, excitonic and phononic properties that go beyond quantum-confinement effects. Recent studies have shown that, apart from tuning the dimensionality of the system, fine-tuning its thickness also helps to optimize performance in different applications, ranging from third-harmonic generation to photodetectors and spintronics. |
Xiong, Ting; Zhu, Mingke; Zhang, Yaoxin; Lee, Wee Siang Vincent; Yu, Zhi Gen; Xue, Junmin Interlayer Engineering of MnO2 with High Charge Density Bi3+ for High Rate and Stable Aqueous Supercapacitor Journal Article BATTERIES & SUPERCAPS, 0000. @article{ISI:000526099200001, title = {Interlayer Engineering of MnO2 with High Charge Density Bi3+ for High Rate and Stable Aqueous Supercapacitor}, author = {Ting Xiong and Mingke Zhu and Yaoxin Zhang and Wee Siang Vincent Lee and Zhi Gen Yu and Junmin Xue}, doi = {10.1002/batt.202000007, Early Access Date = FEB 2020}, journal = {BATTERIES & SUPERCAPS}, abstract = {Two-dimensional delta-MnO2 has attracted considerable attention as supercapacitor electrode due to its ability to incorporate foreign ions into its interlayer spacing which suggests the possibility of capacitance enhancement through interlayer engineering. Incorporating high charge density cation such as Bi3+ into the MnO2 interlayer becomes an attractive strategy that not only serves to stabilize the layer structure, it is also an important prelude towards the weakening of the chemical bonding between the O from MnO2 and the intercalating electrolyte species. This in turn leads to higher reversibility while ensuring excellent structural stability that is portrayed in enhanced cyclic stability and rate performance. Herein, the Bi3+-modified delta-MnO2 delivered a reversible specific capacity of 421 F g(-1) at 1 A g(-1), which is higher than that of the previously reported mono/divalent cation-intercalant modified MnO2 systems. Using the N-doped carbon nanosheets as the negative electrode, the assembled asymmetric supercapacitor achieved a high capacitance of 77 F g(-1) at 1 Ag-1. Furthermore, the high energy density of 20 Wh kg(-1) was recorded at a high-power density of 39 kW kg(-1), while simultaneously demonstrating excellent cycle performance of 82 % capacitance retention after 40 000 cycles. This work has shown that interlayer engineering of layered MnO2 could potentially provide new insight into the development of high-performance supercapacitor electrode.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Two-dimensional delta-MnO2 has attracted considerable attention as supercapacitor electrode due to its ability to incorporate foreign ions into its interlayer spacing which suggests the possibility of capacitance enhancement through interlayer engineering. Incorporating high charge density cation such as Bi3+ into the MnO2 interlayer becomes an attractive strategy that not only serves to stabilize the layer structure, it is also an important prelude towards the weakening of the chemical bonding between the O from MnO2 and the intercalating electrolyte species. This in turn leads to higher reversibility while ensuring excellent structural stability that is portrayed in enhanced cyclic stability and rate performance. Herein, the Bi3+-modified delta-MnO2 delivered a reversible specific capacity of 421 F g(-1) at 1 A g(-1), which is higher than that of the previously reported mono/divalent cation-intercalant modified MnO2 systems. Using the N-doped carbon nanosheets as the negative electrode, the assembled asymmetric supercapacitor achieved a high capacitance of 77 F g(-1) at 1 Ag-1. Furthermore, the high energy density of 20 Wh kg(-1) was recorded at a high-power density of 39 kW kg(-1), while simultaneously demonstrating excellent cycle performance of 82 % capacitance retention after 40 000 cycles. This work has shown that interlayer engineering of layered MnO2 could potentially provide new insight into the development of high-performance supercapacitor electrode. |
Feng, Xuewei; Li, Yida; Wang, Lin; Chen, Shuai; Yu, Zhi Gen; Tan, Wee Chong; Macadam, Nasiruddin; Hu, Guohua; Huang, Li; Chen, Li; Gong, Xiao; Chi, Dongzhi; Hasan, Tawfique; Thean, Aaron Voon-Yew; Zhang, Yong-Wei; Ang, Koh-Wee A Fully Printed Flexible MoS2 Memristive Artificial Synapse with Femtojoule Switching Energy Journal Article ADVANCED ELECTRONIC MATERIALS, 0000, ISSN: 2199-160X. @article{ISI:000486813700001, title = {A Fully Printed Flexible MoS2 Memristive Artificial Synapse with Femtojoule Switching Energy}, author = {Xuewei Feng and Yida Li and Lin Wang and Shuai Chen and Zhi Gen Yu and Wee Chong Tan and Nasiruddin Macadam and Guohua Hu and Li Huang and Li Chen and Xiao Gong and Dongzhi Chi and Tawfique Hasan and Aaron Voon-Yew Thean and Yong-Wei Zhang and Koh-Wee Ang}, doi = {10.1002/aelm.201900740, Early Access Date = SEP 2019}, issn = {2199-160X}, journal = {ADVANCED ELECTRONIC MATERIALS}, abstract = {Realization of memristors capable of storing and processing data on flexible substrates is a key enabling technology toward ``system-on-plastics''. Recent advancements in printing techniques show enormous potential to overcome the major challenges of the current manufacturing processes that require high temperature and planar topography, which may radically change the system integration approach on flexible substrates. However, fully printed memristors are yet to be successfully demonstrated due to the lack of a robust printable switching medium and a reliable printing process. An aerosol-jet-printed Ag/MoS2/Ag memristor is realized in a cross-bar structure by developing a scalable and low temperature printing technique utilizing a functional molybdenum disulfide (MoS2) ink platform. The fully printed devices exhibit an ultra-low switching voltage (0.18 V), a high switching ratio (10(7)), a wide range of tuneable resistance states (10-10(10) omega) for multi-bit data storage, and a low standby power consumption of 1 fW and a switching energy of 4.5 fJ per transition set. Moreover, the MoS2 memristor exhibits both volatile and non-volatile resistive switching behavior by controlling the current compliance levels, which efficiently mimic the short-term and long-term plasticity of biological synapses, demonstrating its potential to enable energy-efficient artificial neuromorphic computing.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Realization of memristors capable of storing and processing data on flexible substrates is a key enabling technology toward ``system-on-plastics''. Recent advancements in printing techniques show enormous potential to overcome the major challenges of the current manufacturing processes that require high temperature and planar topography, which may radically change the system integration approach on flexible substrates. However, fully printed memristors are yet to be successfully demonstrated due to the lack of a robust printable switching medium and a reliable printing process. An aerosol-jet-printed Ag/MoS2/Ag memristor is realized in a cross-bar structure by developing a scalable and low temperature printing technique utilizing a functional molybdenum disulfide (MoS2) ink platform. The fully printed devices exhibit an ultra-low switching voltage (0.18 V), a high switching ratio (10(7)), a wide range of tuneable resistance states (10-10(10) omega) for multi-bit data storage, and a low standby power consumption of 1 fW and a switching energy of 4.5 fJ per transition set. Moreover, the MoS2 memristor exhibits both volatile and non-volatile resistive switching behavior by controlling the current compliance levels, which efficiently mimic the short-term and long-term plasticity of biological synapses, demonstrating its potential to enable energy-efficient artificial neuromorphic computing. |