Liu Wei
Position: Grad Students
Affiliation: NUS Graduate School for Integrative Sciences and Engineering
Research Type: Experiment
Email: a0095681@nus.edu.sg
CA2DM Publications:
2025 |
Fu, Deyi; Liu, Jiawei; Hou, Fuchen; Chang, Xiao; Qu, Tingyu; Felisaz, Johan; Krishnaswamy, Gunasheel Kauwtilyaa; Grebenchuk, Sergey; Jie, Yuang; Watanabe, Kenji; Taniguchi, Takashi; Pereira, Vitor M; Novoselov, Kostya S; Koperski, Maciej; Yakovlev, Nikolai L; Soumyanarayanan, Anjan; Avsar, Ahmet; Yazyev, Oleg V; Lin, Junhao; Ozyilmaz, Barbaros Electric field-tunable ferromagnetism in a van der Waals semiconductor up to room temperature Journal Article NATURE COMMUNICATIONS, 16 (1), 2025. @article{ISI:001620530800035, title = {Electric field-tunable ferromagnetism in a van der Waals semiconductor up to room temperature}, author = {Deyi Fu and Jiawei Liu and Fuchen Hou and Xiao Chang and Tingyu Qu and Johan Felisaz and Gunasheel Kauwtilyaa Krishnaswamy and Sergey Grebenchuk and Yuang Jie and Kenji Watanabe and Takashi Taniguchi and Vitor M Pereira and Kostya S Novoselov and Maciej Koperski and Nikolai L Yakovlev and Anjan Soumyanarayanan and Ahmet Avsar and Oleg V Yazyev and Junhao Lin and Barbaros Ozyilmaz}, doi = {10.1038/s41467-025-59961-2}, times_cited = {0}, year = {2025}, date = {2025-11-20}, journal = {NATURE COMMUNICATIONS}, volume = {16}, number = {1}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {Ferromagnetic semiconductors, coupling charge transport and magnetism via electrical means, show great promise for spin-based logic devices. Despite decades of efforts to achieve such co-functionality, maintaining ferromagnetic order at room temperature remains elusive. Here, we address this long-standing challenge by implanting dilute Co atoms into few-layer black phosphorus through atomically-thin boron nitride diffusion barrier. Our Co-doped black phosphorus-based devices exhibit ferromagnetism up to room temperature while preserving its high mobility (similar to 1000cm(2)V(-1)s(-1)) and semiconducting characteristics. By incorporating ferromagnetic Co-doped black phosphorus into magnetic tunnel junction devices, we demonstrate a large tunnelling magnetoresistance that extends up to room temperature. This study presents a new approach to engineering ferromagnetic ordering in otherwise nonmagnetic materials, thereby expanding the repertoire and applications of magnetic semiconductors envisioned thus far.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Ferromagnetic semiconductors, coupling charge transport and magnetism via electrical means, show great promise for spin-based logic devices. Despite decades of efforts to achieve such co-functionality, maintaining ferromagnetic order at room temperature remains elusive. Here, we address this long-standing challenge by implanting dilute Co atoms into few-layer black phosphorus through atomically-thin boron nitride diffusion barrier. Our Co-doped black phosphorus-based devices exhibit ferromagnetism up to room temperature while preserving its high mobility (similar to 1000cm(2)V(-1)s(-1)) and semiconducting characteristics. By incorporating ferromagnetic Co-doped black phosphorus into magnetic tunnel junction devices, we demonstrate a large tunnelling magnetoresistance that extends up to room temperature. This study presents a new approach to engineering ferromagnetic ordering in otherwise nonmagnetic materials, thereby expanding the repertoire and applications of magnetic semiconductors envisioned thus far. |
2024 |
Wang, Zhe; Kalathingal, Vijith; Trushin, Maxim; Liu, Jiawei; Wang, Junyong; Guo, Yongxin; Ozyilmaz, Barbaros; Nijhuis, Christian A; Eda, Goki Upconversion electroluminescence in 2D semiconductors integrated with plasmonic tunnel junctions Journal Article 11 NATURE NANOTECHNOLOGY, 19 (7), 2024, ISSN: 1748-3387. @article{ISI:001205711600001, title = {Upconversion electroluminescence in 2D semiconductors integrated with plasmonic tunnel junctions}, author = {Zhe Wang and Vijith Kalathingal and Maxim Trushin and Jiawei Liu and Junyong Wang and Yongxin Guo and Barbaros Ozyilmaz and Christian A Nijhuis and Goki Eda}, doi = {10.1038/s41565-024-01650-0}, times_cited = {11}, issn = {1748-3387}, year = {2024}, date = {2024-04-19}, journal = {NATURE NANOTECHNOLOGY}, volume = {19}, number = {7}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {Plasmonic tunnel junctions are a unique electroluminescent system in which light emission occurs via an interplay between tunnelling electrons and plasmonic fields instead of electron-hole recombination as in conventional light-emitting diodes. It was previously shown that placing luminescent molecules in the tunneling pathway of nanoscopic tunnel junctions results in peculiar upconversion electroluminescence where the energy of emitted photons exceeds that of excitation electrons. Here we report the observation of upconversion electroluminescence in macroscopic van der Waals plasmonic tunnel junctions comprising gold and few-layer graphene electrodes separated by a similar to 2-nm-thick hexagonal boron nitride tunnel barrier and a monolayer semiconductor. We find that the semiconductor ground exciton emission is triggered at excitation electron energies lower than the semiconductor optical gap. Interestingly, this upconversion is reached in devices operating at a low conductance (<10(-6) S) and low power density regime (<10(2) W cm(-2)), defying explanation through existing proposed mechanisms. By examining the scaling relationship between plasmonic and excitonic emission intensities, we elucidate the role of inelastic electron tunnelling dipoles that induce optically forbidden transitions in the few-layer graphene electrode and ultrafast hot carrier transfer across the van der Waals interface.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Plasmonic tunnel junctions are a unique electroluminescent system in which light emission occurs via an interplay between tunnelling electrons and plasmonic fields instead of electron-hole recombination as in conventional light-emitting diodes. It was previously shown that placing luminescent molecules in the tunneling pathway of nanoscopic tunnel junctions results in peculiar upconversion electroluminescence where the energy of emitted photons exceeds that of excitation electrons. Here we report the observation of upconversion electroluminescence in macroscopic van der Waals plasmonic tunnel junctions comprising gold and few-layer graphene electrodes separated by a similar to 2-nm-thick hexagonal boron nitride tunnel barrier and a monolayer semiconductor. We find that the semiconductor ground exciton emission is triggered at excitation electron energies lower than the semiconductor optical gap. Interestingly, this upconversion is reached in devices operating at a low conductance (<10(-6) S) and low power density regime (<10(2) W cm(-2)), defying explanation through existing proposed mechanisms. By examining the scaling relationship between plasmonic and excitonic emission intensities, we elucidate the role of inelastic electron tunnelling dipoles that induce optically forbidden transitions in the few-layer graphene electrode and ultrafast hot carrier transfer across the van der Waals interface. |
Lin, Fanrong; Liu, Jiawei; Lu, Huan; Liu, Xin; Liu, Ying; Hu, Zhili; Lyu, Pin; Zhang, Zhuhua; Martin, Jens; Guo, Wanlin; Liu, Yanpeng Evolution of Graphene Dirac Fermions in Electric Double-Layer Transistors with a Soft Barrier Journal Article ADVANCED FUNCTIONAL MATERIALS, 34 (34), 2024, ISSN: 1616-301X. @article{ISI:001195156600001, title = {Evolution of Graphene Dirac Fermions in Electric Double-Layer Transistors with a Soft Barrier}, author = {Fanrong Lin and Jiawei Liu and Huan Lu and Xin Liu and Ying Liu and Zhili Hu and Pin Lyu and Zhuhua Zhang and Jens Martin and Wanlin Guo and Yanpeng Liu}, doi = {10.1002/adfm.202400553}, times_cited = {7}, issn = {1616-301X}, year = {2024}, date = {2024-04-02}, journal = {ADVANCED FUNCTIONAL MATERIALS}, volume = {34}, number = {34}, publisher = {WILEY-V C H VERLAG GMBH}, address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY}, abstract = {The interface and dielectric environment of graphene transistors are of great importance to commercial circuit integrations. The tangling bond in oxide-based dielectric severely lagged the carrier mobility while the 2D dielectric layer (for instance, hexagonal boron nitride) unavoidably hastened complicated condensed physics even at room temperature. Herein, multilayer black phosphorus (BP) a versatile and widely-tunable dielectric candidate for manifesting graphene fermions is demonstrated. Because of hetero-interfacial charge redistributions, a vertical electric double-layer between the bottom BP layer and top graphene spontaneously forms with the central BP layer as a soft barrier. Under dual-gate modulation, abnormal step-like evolution of Dirac fermions and charge-transfer quantum Hall effect arises while the intrinsic Dirac behavior of graphene is preserved, ascribing to the gate-tunable charge redistributions of dielectric BP layer. Moreover, the electric double-layer transistors apply equally well to bilayer graphene with similar Dirac behavior but an enhanced interfacial charge interference. The findings not only create a new avenue to manipulate the fermions by assembling graphene with narrow-gapped 2D layered materials but also promote electric double-layer transistors as a new build block to design multifunctional devices.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The interface and dielectric environment of graphene transistors are of great importance to commercial circuit integrations. The tangling bond in oxide-based dielectric severely lagged the carrier mobility while the 2D dielectric layer (for instance, hexagonal boron nitride) unavoidably hastened complicated condensed physics even at room temperature. Herein, multilayer black phosphorus (BP) a versatile and widely-tunable dielectric candidate for manifesting graphene fermions is demonstrated. Because of hetero-interfacial charge redistributions, a vertical electric double-layer between the bottom BP layer and top graphene spontaneously forms with the central BP layer as a soft barrier. Under dual-gate modulation, abnormal step-like evolution of Dirac fermions and charge-transfer quantum Hall effect arises while the intrinsic Dirac behavior of graphene is preserved, ascribing to the gate-tunable charge redistributions of dielectric BP layer. Moreover, the electric double-layer transistors apply equally well to bilayer graphene with similar Dirac behavior but an enhanced interfacial charge interference. The findings not only create a new avenue to manipulate the fermions by assembling graphene with narrow-gapped 2D layered materials but also promote electric double-layer transistors as a new build block to design multifunctional devices. |
Cording, Luke; Liu, Jiawei; Tan, Jun You; Watanabe, Kenji; Taniguchi, Takashi; Avsar, Ahmet; Ozyilmaz, Barbaros Highly anisotropic spin transport in ultrathin black phosphorus Journal Article 30 NATURE MATERIALS, 23 (4), 2024, ISSN: 1476-1122. @article{ISI:001142010100002, title = {Highly anisotropic spin transport in ultrathin black phosphorus}, author = {Luke Cording and Jiawei Liu and Jun You Tan and Kenji Watanabe and Takashi Taniguchi and Ahmet Avsar and Barbaros Ozyilmaz}, doi = {10.1038/s41563-023-01779-8}, times_cited = {30}, issn = {1476-1122}, year = {2024}, date = {2024-01-12}, journal = {NATURE MATERIALS}, volume = {23}, number = {4}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {In anisotropic crystals, the direction-dependent effective mass of carriers can have a profound impact on spin transport dynamics. The puckered crystal structure of black phosphorus leads to direction-dependent charge transport and optical response, suggesting that it is an ideal system for studying anisotropic spin transport. To this end, we fabricate and characterize high-mobility encapsulated ultrathin black-phosphorus-based spin valves in a four-terminal geometry. Our measurements show that in-plane spin lifetimes are strongly gate tunable and exceed one nanosecond. Through high out-of-plane magnetic fields, we observe a fivefold enhancement in the out-of-plane spin signal case compared to in-plane and estimate a colossal spin-lifetime anisotropy of similar to 6. This finding is further confirmed by oblique Hanle measurements. Additionally, we estimate an in-plane spin-lifetime anisotropy ratio of up to 1.8. Our observation of strongly anisotropic spin transport along three orthogonal axes in this pristine material could be exploited to realize directionally tunable spin transport.}, keywords = {}, pubstate = {published}, tppubtype = {article} } In anisotropic crystals, the direction-dependent effective mass of carriers can have a profound impact on spin transport dynamics. The puckered crystal structure of black phosphorus leads to direction-dependent charge transport and optical response, suggesting that it is an ideal system for studying anisotropic spin transport. To this end, we fabricate and characterize high-mobility encapsulated ultrathin black-phosphorus-based spin valves in a four-terminal geometry. Our measurements show that in-plane spin lifetimes are strongly gate tunable and exceed one nanosecond. Through high out-of-plane magnetic fields, we observe a fivefold enhancement in the out-of-plane spin signal case compared to in-plane and estimate a colossal spin-lifetime anisotropy of similar to 6. This finding is further confirmed by oblique Hanle measurements. Additionally, we estimate an in-plane spin-lifetime anisotropy ratio of up to 1.8. Our observation of strongly anisotropic spin transport along three orthogonal axes in this pristine material could be exploited to realize directionally tunable spin transport. |
2023 |
Lin, Fanrong; Cao, Zhonghan; Xiao, Feiping; Liu, Jiawei; Qiao, Jiabin; Xue, Minmin; Hu, Zhili; Liu, Ying; Lu, Huan; Zhang, Zhuhua; Martin, Jens; Tong, Qingjun; Guo, Wanlin; Liu, Yanpeng Graphene binding on black phosphorus enables high on/off ratios and mobility Journal Article NATIONAL SCIENCE REVIEW, 11 (2), 2023, ISSN: 2095-5138. @article{ISI:001121789900001, title = {Graphene binding on black phosphorus enables high on/off ratios and mobility}, author = {Fanrong Lin and Zhonghan Cao and Feiping Xiao and Jiawei Liu and Jiabin Qiao and Minmin Xue and Zhili Hu and Ying Liu and Huan Lu and Zhuhua Zhang and Jens Martin and Qingjun Tong and Wanlin Guo and Yanpeng Liu}, doi = {10.1093/nsr/nwad279}, times_cited = {2}, issn = {2095-5138}, year = {2023}, date = {2023-12-11}, journal = {NATIONAL SCIENCE REVIEW}, volume = {11}, number = {2}, publisher = {OXFORD UNIV PRESS}, address = {GREAT CLARENDON ST, OXFORD OX2 6DP, ENGLAND}, abstract = {Graphene is one of the most promising candidates for integrated circuits due to its robustness against short-channel effects, inherent high carrier mobility and desired gapless nature for Ohmic contact, but it is difficult to achieve satisfactory on/off ratios even at the expense of its carrier mobility, limiting its device applications. Here, we present a strategy to realize high back-gate switching ratios in a graphene monolayer with well-maintained high mobility by forming a vertical heterostructure with a black phosphorus multi-layer. By local current annealing, strain is introduced within an established area of the graphene, which forms a reflective interface with the rest of the strain-free area and thus generates a robust off-state via local current depletion. Applying a positive back-gate voltage to the heterostructure can keep the black phosphorus insulating, while a negative back-gate voltage changes the black phosphorus to be conductive because of hole accumulation. Then, a parallel channel is activated within the strain-free graphene area by edge-contacted electrodes, thereby largely inheriting the intrinsic carrier mobility of graphene in the on-state. As a result, the device can provide an on/off voltage ratio of >103 as well as a mobility of similar to 8000 cm(2) V-1 s(-1) at room temperature, meeting the low-power criterion suggested by the International Roadmap for Devices and Systems.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Graphene is one of the most promising candidates for integrated circuits due to its robustness against short-channel effects, inherent high carrier mobility and desired gapless nature for Ohmic contact, but it is difficult to achieve satisfactory on/off ratios even at the expense of its carrier mobility, limiting its device applications. Here, we present a strategy to realize high back-gate switching ratios in a graphene monolayer with well-maintained high mobility by forming a vertical heterostructure with a black phosphorus multi-layer. By local current annealing, strain is introduced within an established area of the graphene, which forms a reflective interface with the rest of the strain-free area and thus generates a robust off-state via local current depletion. Applying a positive back-gate voltage to the heterostructure can keep the black phosphorus insulating, while a negative back-gate voltage changes the black phosphorus to be conductive because of hole accumulation. Then, a parallel channel is activated within the strain-free graphene area by edge-contacted electrodes, thereby largely inheriting the intrinsic carrier mobility of graphene in the on-state. As a result, the device can provide an on/off voltage ratio of >103 as well as a mobility of similar to 8000 cm(2) V-1 s(-1) at room temperature, meeting the low-power criterion suggested by the International Roadmap for Devices and Systems. |
2020 |
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 Linkage Engineering by Harnessing Supramolecular Interactions to Fabricate 2D Hydrazone-Linked Covalent Organic Framework Platforms toward Advanced Catalysis Journal Article 122 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}, times_cited = {122}, issn = {0002-7863}, year = {2020}, date = {2020-10-21}, 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. |
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}, times_cited = {9}, issn = {1530-6984}, year = {2020}, date = {2020-10-14}, 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. |
Chen, Hao; Zhou, Pinjia; Liu, Jiawei; Qiao, Jiabin; Oezyilmaz, Barbaros; Martin, Jens Gate controlled valley polarizer in bilayer graphene Journal Article 28 NATURE COMMUNICATIONS, 11 (1), 2020, ISSN: 2041-1723. @article{ISI:000543997700008, title = {Gate controlled valley polarizer in bilayer graphene}, author = {Hao Chen and Pinjia Zhou and Jiawei Liu and Jiabin Qiao and Barbaros Oezyilmaz and Jens Martin}, doi = {10.1038/s41467-020-15117-y}, times_cited = {28}, issn = {2041-1723}, year = {2020}, date = {2020-03-05}, journal = {NATURE COMMUNICATIONS}, volume = {11}, number = {1}, publisher = {NATURE PUBLISHING GROUP}, address = {MACMILLAN BUILDING, 4 CRINAN ST, LONDON N1 9XW, ENGLAND}, abstract = {Sign reversal of Berry curvature across two oppositely gated regions in bilayer graphene can give rise to counter-propagating 1D channels with opposite valley indices. Considering spin and sub-lattice degeneracy, there are four quantized conduction channels in each direction. Previous experimental work on gate-controlled valley polarizer achieved good contrast only in the presence of an external magnetic field. Yet, with increasing magnetic field the ungated regions of bilayer graphene will transit into the quantum Hall regime, limiting the applications of valley-polarized electrons. Here we present improved performance of a gate-controlled valley polarizer through optimized device geometry and stacking method. Electrical measurements show up to two orders of magnitude difference in conductance between the valley-polarized state and gapped states. The valley-polarized state displays conductance of nearly 4e(2)/h and produces contrast in a subsequent valley analyzer configuration. These results pave the way to further experiments on valley-polarized electrons in zero magnetic field. Here, the authors present a gate-controlled valley-polarizer based on bilayer graphene, and through optimized device geometry and stacking method they obtain two orders of magnitude difference in conductance between the valley-polarized state and gapped states without the application of external magnetic fields.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Sign reversal of Berry curvature across two oppositely gated regions in bilayer graphene can give rise to counter-propagating 1D channels with opposite valley indices. Considering spin and sub-lattice degeneracy, there are four quantized conduction channels in each direction. Previous experimental work on gate-controlled valley polarizer achieved good contrast only in the presence of an external magnetic field. Yet, with increasing magnetic field the ungated regions of bilayer graphene will transit into the quantum Hall regime, limiting the applications of valley-polarized electrons. Here we present improved performance of a gate-controlled valley polarizer through optimized device geometry and stacking method. Electrical measurements show up to two orders of magnitude difference in conductance between the valley-polarized state and gapped states. The valley-polarized state displays conductance of nearly 4e(2)/h and produces contrast in a subsequent valley analyzer configuration. These results pave the way to further experiments on valley-polarized electrons in zero magnetic field. Here, the authors present a gate-controlled valley-polarizer based on bilayer graphene, and through optimized device geometry and stacking method they obtain two orders of magnitude difference in conductance between the valley-polarized state and gapped states without the application of external magnetic fields. |
Song, Peng; Hsu, Chuang-Han; Vignale, Giovanni; Zhao, Meng; Liu, Jiawei; Deng, Yujun; Fu, Wei; Liu, Yanpeng; Zhang, Yuanbo; Lin, Hsin; Pereira, Vitor M; Loh, Kian Ping Coexistence of large conventional and planar spin Hall effect with long spin diffusion length in a low-symmetry semimetal at room temperature Journal Article 86 NATURE MATERIALS, 19 (3), pp. 292-+, 2020, ISSN: 1476-1122. @article{ISI:000510823100005, title = {Coexistence of large conventional and planar spin Hall effect with long spin diffusion length in a low-symmetry semimetal at room temperature}, author = {Peng Song and Chuang-Han Hsu and Giovanni Vignale and Meng Zhao and Jiawei Liu and Yujun Deng and Wei Fu and Yanpeng Liu and Yuanbo Zhang and Hsin Lin and Vitor M Pereira and Kian Ping Loh}, doi = {10.1038/s41563-019-0600-4}, times_cited = {86}, issn = {1476-1122}, year = {2020}, date = {2020-02-03}, journal = {NATURE MATERIALS}, volume = {19}, number = {3}, pages = {292-+}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {A large spin Hall angle and long spin diffusion length are found in the low-symmetry, few-layer semimetal MoTe2 at room temperature, thus identifying this material as an excellent candidate for simultaneous spin generation, transport and detection.}, keywords = {}, pubstate = {published}, tppubtype = {article} } A large spin Hall angle and long spin diffusion length are found in the low-symmetry, few-layer semimetal MoTe2 at room temperature, thus identifying this material as an excellent candidate for simultaneous spin generation, transport and detection. |
2017 |
Avsar, Ahmet; Unuchek, Dmitrii; Liu, Jiawei; Sanchez, Oriol Lopez; Watanabe, Kenji; Taniguch, Takashi; Ozyilmaz, Barbaros; Kis, Andras Optospintronics in Graphene textitvia Proximity Coupling Journal Article 79 ACS NANO, 11 (11), pp. 11678-11686, 2017, ISSN: 1936-0851. @article{ISI:000416878100115, title = {Optospintronics in Graphene textitvia Proximity Coupling}, author = {Ahmet Avsar and Dmitrii Unuchek and Jiawei Liu and Oriol Lopez Sanchez and Kenji Watanabe and Takashi Taniguch and Barbaros Ozyilmaz and Andras Kis}, doi = {10.1021/acsnano.7b06800}, times_cited = {79}, issn = {1936-0851}, year = {2017}, date = {2017-11-01}, journal = {ACS NANO}, volume = {11}, number = {11}, pages = {11678-11686}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {The observation of micrometer size spin relaxation makes graphene a promising material for applications in spintronics requiring long-distance spin communication. However, spin dependent scatterings at the contact/graphene interfaces affect the spin injection efficiencies and hence prevent the material from achieving its full potential. While this major issue could be eliminated by nondestructive direct optical spin injection schemes, graphene's intrinsically low spin-orbit coupling strength and optical absorption place an obstacle in their realisation. We overcome this challenge by creating sharp artificial interfaces between graphene and WSe2 monolayers. Application of circularly polarized light activates the spin-polarized charge carriers in the WSe2 layer due to its spin-coupled valley-selective absorption. These carriers diffuse into the superjacent graphene layer, transport over a 3.5 mu m distance, and are finally detected electrically using Co/h-BN contacts in a nonlocal geometry. Polarization-dependent measurements confirm the spin origin of the nonlocal signal. We also demonstrate that such signal is absent if graphene is contacted to bilayer WSe2 where the inversion symmetry is restored.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The observation of micrometer size spin relaxation makes graphene a promising material for applications in spintronics requiring long-distance spin communication. However, spin dependent scatterings at the contact/graphene interfaces affect the spin injection efficiencies and hence prevent the material from achieving its full potential. While this major issue could be eliminated by nondestructive direct optical spin injection schemes, graphene's intrinsically low spin-orbit coupling strength and optical absorption place an obstacle in their realisation. We overcome this challenge by creating sharp artificial interfaces between graphene and WSe2 monolayers. Application of circularly polarized light activates the spin-polarized charge carriers in the WSe2 layer due to its spin-coupled valley-selective absorption. These carriers diffuse into the superjacent graphene layer, transport over a 3.5 mu m distance, and are finally detected electrically using Co/h-BN contacts in a nonlocal geometry. Polarization-dependent measurements confirm the spin origin of the nonlocal signal. We also demonstrate that such signal is absent if graphene is contacted to bilayer WSe2 where the inversion symmetry is restored. |