Barbaros Özyilmaz
Degree: PhD
Affiliation: NUS – Department of Materials Science and Engineering
Research Type: Experiment
Office: EA-03-09
Email: msehead@nus.edu.sg
Contact: (65) 6601 5351
CA2DM Publications:
2024 |
Lai, Wenhui; Lee, Jong Hak; Shi, Lu; Liu, Yuqing; Pu, Yanhui; Ong, Yong Kang; Limpo, Carlos; Xiong, Ting; Rao, Yifan; Sow, Chorng Haur; Ozyilmaz, Barbaros High mechanical strength Si anode synthesis with interlayer bonded expanded graphite structure for lithium-ion batteries Journal Article JOURNAL OF ENERGY CHEMISTRY, 93 , pp. 253-263, 2024, ISSN: 2095-4956. @article{ISI:001203104900001, title = {High mechanical strength Si anode synthesis with interlayer bonded expanded graphite structure for lithium-ion batteries}, author = {Wenhui Lai and Jong Hak Lee and Lu Shi and Yuqing Liu and Yanhui Pu and Yong Kang Ong and Carlos Limpo and Ting Xiong and Yifan Rao and Chorng Haur Sow and Barbaros Ozyilmaz}, doi = {10.1016/j.jechem.2024.02.021}, times_cited = {0}, issn = {2095-4956}, year = {2024}, date = {2024-06-01}, journal = {JOURNAL OF ENERGY CHEMISTRY}, volume = {93}, pages = {253-263}, publisher = {ELSEVIER}, address = {RADARWEG 29, 1043 NX AMSTERDAM, NETHERLANDS}, abstract = {Despite advancements in silicon -based anodes for high -capacity lithium -ion batteries, their widespread commercial adoption is still hindered by significant volume expansion during cycling, especially at high active mass loadings crucial for practical use. The root of these challenges lies in the mechanical instability of the material, which subsequently leads to the structural failure of the electrode. Here, we present a novel synthesis of a composite combining expanded graphite and silicon nanoparticles. This composite features a unique interlayer-bonded graphite structure, achieved through the application of a modified spark plasma sintering method. Notably, this innovative structure not only facilitates efficient ion and electron transport but also provides exceptional mechanical strength (Vickers hardness: up to 658 MPa, Young's modulus: 11.6 GPa). This strength effectively accommodates silicon expansion, resulting in an impressive areal capacity of 2.9 mA h cm -2 (736 mA h g-1) and a steady cycle life (93% after 100 cycles). Such outstanding performance is paired with features appropriate for large-scale industrial production of silicon batteries, such as active mass loading of at least 3.9 mg cm -2, a high -tap density electrode material of 1.68 g cm -3 (secondary clusters: 1.12 g cm -3), and a production yield of up to 1 kg per day. (c) 2024 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by ELSEVIER B.V. and Science Press. All rights reserved.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Despite advancements in silicon -based anodes for high -capacity lithium -ion batteries, their widespread commercial adoption is still hindered by significant volume expansion during cycling, especially at high active mass loadings crucial for practical use. The root of these challenges lies in the mechanical instability of the material, which subsequently leads to the structural failure of the electrode. Here, we present a novel synthesis of a composite combining expanded graphite and silicon nanoparticles. This composite features a unique interlayer-bonded graphite structure, achieved through the application of a modified spark plasma sintering method. Notably, this innovative structure not only facilitates efficient ion and electron transport but also provides exceptional mechanical strength (Vickers hardness: up to 658 MPa, Young's modulus: 11.6 GPa). This strength effectively accommodates silicon expansion, resulting in an impressive areal capacity of 2.9 mA h cm -2 (736 mA h g-1) and a steady cycle life (93% after 100 cycles). Such outstanding performance is paired with features appropriate for large-scale industrial production of silicon batteries, such as active mass loading of at least 3.9 mg cm -2, a high -tap density electrode material of 1.68 g cm -3 (secondary clusters: 1.12 g cm -3), and a production yield of up to 1 kg per day. (c) 2024 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by ELSEVIER B.V. and Science Press. All rights reserved. |
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 NATURE NANOTECHNOLOGY, 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 = {0}, issn = {1748-3387}, year = {2024}, date = {2024-04-19}, journal = {NATURE NANOTECHNOLOGY}, 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. |
Xiong, Ting; Zhang, Deqiang; Yeo, Jing Ying; Zhan, Yufeng; Ong, Yong Kang; Limpo, Carlos Maria Alava; Shi, Lu; Rao, Yifan; Pu, Yanhui; Lai, Wenhui; Lee, Jonghak; Lee, Wee Siang Vincent; Ozyilmaz, Barbaros Interfacial design towards stable zinc metal-free zinc-ion batteries with high energy density Journal Article JOURNAL OF MATERIALS CHEMISTRY A, 12 (9), pp. 5499-5507, 2024, ISSN: 2050-7488. @article{ISI:001156660000001, title = {Interfacial design towards stable zinc metal-free zinc-ion batteries with high energy density}, author = {Ting Xiong and Deqiang Zhang and Jing Ying Yeo and Yufeng Zhan and Yong Kang Ong and Carlos Maria Alava Limpo and Lu Shi and Yifan Rao and Yanhui Pu and Wenhui Lai and Jonghak Lee and Wee Siang Vincent Lee and Barbaros Ozyilmaz}, doi = {10.1039/d3ta07674a}, times_cited = {0}, issn = {2050-7488}, year = {2024}, date = {2024-02-02}, journal = {JOURNAL OF MATERIALS CHEMISTRY A}, volume = {12}, number = {9}, pages = {5499-5507}, publisher = {ROYAL SOC CHEMISTRY}, address = {THOMAS GRAHAM HOUSE, SCIENCE PARK, MILTON RD, CAMBRIDGE CB4 0WF, CAMBS, ENGLAND}, abstract = {Zinc metal-free zinc-ion batteries hold promise for achieving higher energy densities by eliminating the need for dense zinc foil as the anode. However, the direct use of substrates like copper foil in these batteries results in poor cyclic stability due to dendrite growth. Herein, we propose a strategy to modulate the nucleation sites and growth dynamics of Zn. This is achieved by introducing a graphene coating on the copper substrate, which directs the initial nucleation of Zn to form hexagonal plates. Subsequently, the incorporation of positively polarized poly(vinylidene fluoride-trifluoroethylene) promotes growth along these hexagonal plates, resulting in uniform crystalline plates. As a result, the half-cell demonstrated a significant improvement in the cyclic life of 3000 cycles at a high current density of 10 mA cm-2 and capacity of 1 mA h cm-2. When paired with Zn-inserted MnO2 cathode, the full cell exhibited high cyclic stability (retaining 83% capacity after 500 cycles at 1 mA cm-2) and energy density of 378 W h kg-1 at 0.5 mA cm-2. This is notably higher than the conventional Zn ion battery based on a Zn anode (136 W h kg-1). To showcase its potential, we prepared a pouch cell that successfully powered the electric fan and LED lights, suggesting its promising application in high-performance Zn ion batteries.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Zinc metal-free zinc-ion batteries hold promise for achieving higher energy densities by eliminating the need for dense zinc foil as the anode. However, the direct use of substrates like copper foil in these batteries results in poor cyclic stability due to dendrite growth. Herein, we propose a strategy to modulate the nucleation sites and growth dynamics of Zn. This is achieved by introducing a graphene coating on the copper substrate, which directs the initial nucleation of Zn to form hexagonal plates. Subsequently, the incorporation of positively polarized poly(vinylidene fluoride-trifluoroethylene) promotes growth along these hexagonal plates, resulting in uniform crystalline plates. As a result, the half-cell demonstrated a significant improvement in the cyclic life of 3000 cycles at a high current density of 10 mA cm-2 and capacity of 1 mA h cm-2. When paired with Zn-inserted MnO2 cathode, the full cell exhibited high cyclic stability (retaining 83% capacity after 500 cycles at 1 mA cm-2) and energy density of 378 W h kg-1 at 0.5 mA cm-2. This is notably higher than the conventional Zn ion battery based on a Zn anode (136 W h kg-1). To showcase its potential, we prepared a pouch cell that successfully powered the electric fan and LED lights, suggesting its promising application in high-performance Zn ion batteries. |
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 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 = {0}, 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. |
2020 |
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 = {0}, 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. |
Griffin, Eoin; Mogg, Lucas; Hao, Guang-Ping; Kalon, Gopinadhan; Bacaksiz, Cihan; Lopez-Polin, Guillermo; Zhou, T Y; Guarochico, Victor; Cai, Junhao; Neumann, Christof; Winter, Andreas; Mohn, Michael; Lee, Jong Hak; Lin, Junhao; Kaiser, Ute; Grigorieva, Irina; Suenaga, Kazu; Ozyilmaz, Barbaros; Cheng, Hui-Min; Ren, Wencai; Turchanin, Andrey; Peeters, Francois M; Geim, Andre K; Lozada-Hidalgo, Marcelo Proton and Li-Ion Permeation through Graphene with Eight-Atom-Ring Defects Journal Article ACS NANO, 14 (6), pp. 7280-7286, 2020, ISSN: 1936-0851. @article{ISI:000543744100086, title = {Proton and Li-Ion Permeation through Graphene with Eight-Atom-Ring Defects}, author = {Eoin Griffin and Lucas Mogg and Guang-Ping Hao and Gopinadhan Kalon and Cihan Bacaksiz and Guillermo Lopez-Polin and T Y Zhou and Victor Guarochico and Junhao Cai and Christof Neumann and Andreas Winter and Michael Mohn and Jong Hak Lee and Junhao Lin and Ute Kaiser and Irina Grigorieva and Kazu Suenaga and Barbaros Ozyilmaz and Hui-Min Cheng and Wencai Ren and Andrey Turchanin and Francois M Peeters and Andre K Geim and Marcelo Lozada-Hidalgo}, doi = {10.1021/acsnano.0c02496}, times_cited = {0}, issn = {1936-0851}, year = {2020}, date = {2020-06-23}, journal = {ACS NANO}, volume = {14}, number = {6}, pages = {7280-7286}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Defect-free graphene is impermeable to gases and liquids but highly permeable to thermal protons. Atomic-scale defects such as vacancies, grain boundaries, and Stone-Wales defects are predicted to enhance graphene's proton permeability and may even allow small ions through, whereas larger species such as gas molecules should remain blocked. These expectations have so far remained untested in experiment. Here, we show that atomically thin carbon films with a high density of atomic-scale defects continue blocking all molecular transport, but their proton permeability becomes similar to 1000 times higher than that of defect-free graphene. Lithium ions can also permeate through such disordered graphene. The enhanced proton and ion permeability is attributed to a high density of eight-carbon-atom rings. The latter pose approximately twice lower energy barriers for incoming protons compared to that of the six-atom rings of graphene and a relatively low barrier of similar to 0.6 eV for Li ions. Our findings suggest that disordered graphene could be of interest as membranes and protective barriers in various Li-ion and hydrogen technologies.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Defect-free graphene is impermeable to gases and liquids but highly permeable to thermal protons. Atomic-scale defects such as vacancies, grain boundaries, and Stone-Wales defects are predicted to enhance graphene's proton permeability and may even allow small ions through, whereas larger species such as gas molecules should remain blocked. These expectations have so far remained untested in experiment. Here, we show that atomically thin carbon films with a high density of atomic-scale defects continue blocking all molecular transport, but their proton permeability becomes similar to 1000 times higher than that of defect-free graphene. Lithium ions can also permeate through such disordered graphene. The enhanced proton and ion permeability is attributed to a high density of eight-carbon-atom rings. The latter pose approximately twice lower energy barriers for incoming protons compared to that of the six-atom rings of graphene and a relatively low barrier of similar to 0.6 eV for Li ions. Our findings suggest that disordered graphene could be of interest as membranes and protective barriers in various Li-ion and hydrogen technologies. |
Avsar, A; Ochoa, H; Guinea, F; Ozyilmaz, B; Wees, Van B J; Vera-Marun, I J Colloquium: Spintronics in graphene and other two-dimensional materials Journal Article REVIEWS OF MODERN PHYSICS, 92 (2), 2020, ISSN: 0034-6861. @article{ISI:000539243200001, title = {\textit{Colloquium}: Spintronics in graphene and other two-dimensional materials}, author = {A Avsar and H Ochoa and F Guinea and B Ozyilmaz and Van B J Wees and I J Vera-Marun}, doi = {10.1103/RevModPhys.92.021003}, times_cited = {7}, issn = {0034-6861}, year = {2020}, date = {2020-06-02}, journal = {REVIEWS OF MODERN PHYSICS}, volume = {92}, number = {2}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {After the first unequivocal demonstration of spin transport in graphene [Tombros et al., Nature (London) 448, 571-574 (2007)], surprisingly at room temperature, it was quickly realized that this novel material was relevant for both fundamental spintronics and future applications. In the decade since, exciting results have made the field of graphene spintronics blossom, and a second generation of studies has extended to new two-dimensional (2D) compounds. This Colloquium reviews recent theoretical and experimental advances on electronic spin transport in graphene and related 2D materials, focusing on emergent phenomena in van der Waals heterostructures and the new perspectives provided by them. These phenomena include proximity-enabled spin-orbit effects, the coupling of electronic spin to light, electrical tunability, and 2D magnetism.}, keywords = {}, pubstate = {published}, tppubtype = {article} } After the first unequivocal demonstration of spin transport in graphene [Tombros et al., Nature (London) 448, 571-574 (2007)], surprisingly at room temperature, it was quickly realized that this novel material was relevant for both fundamental spintronics and future applications. In the decade since, exciting results have made the field of graphene spintronics blossom, and a second generation of studies has extended to new two-dimensional (2D) compounds. This Colloquium reviews recent theoretical and experimental advances on electronic spin transport in graphene and related 2D materials, focusing on emergent phenomena in van der Waals heterostructures and the new perspectives provided by them. These phenomena include proximity-enabled spin-orbit effects, the coupling of electronic spin to light, electrical tunability, and 2D magnetism. |
Chen, Hao; Zhou, Pinjia; Liu, Jiawei; Qiao, Jiabin; Oezyilmaz, Barbaros; Martin, Jens Gate controlled valley polarizer in bilayer graphene Journal Article 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 = {0}, 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. |
Toh, Chee-Tat; Zhang, Hongji; Lin, Junhao; Mayorov, Alexander S; Wang, Yun-Peng; Orofeo, Carlo M; Ferry, Darim Badur; Andersen, Henrik; Kakenov, Nurbek; Guo, Zenglong; Abidi, Irfan Haider; Sims, Hunter; Suenaga, Kazu; Pantelides, Sokrates T; Ozyilmaz, Barbaros Synthesis and properties of free-standing monolayer amorphous carbon Journal Article NATURE, 577 (7789), pp. 199-+, 2020, ISSN: 0028-0836. @article{ISI:000506682500033, title = {Synthesis and properties of free-standing monolayer amorphous carbon}, author = {Chee-Tat Toh and Hongji Zhang and Junhao Lin and Alexander S Mayorov and Yun-Peng Wang and Carlo M Orofeo and Darim Badur Ferry and Henrik Andersen and Nurbek Kakenov and Zenglong Guo and Irfan Haider Abidi and Hunter Sims and Kazu Suenaga and Sokrates T Pantelides and Barbaros Ozyilmaz}, doi = {10.1038/s41586-019-1871-2}, times_cited = {0}, issn = {0028-0836}, year = {2020}, date = {2020-01-09}, journal = {NATURE}, volume = {577}, number = {7789}, pages = {199-+}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {Bulk amorphous materials have been studied extensively and are widely used, yet their atomic arrangement remains an open issue. Although they are generally believed to be Zachariasen continuous random networks(1), recent experimental evidence favours the competing crystallite model in the case of amorphous silicon(2-4). In two-dimensional materials, however, the corresponding questions remain unanswered. Here we report the synthesis, by laser-assisted chemical vapour deposition(5), of centimetre-scale, free-standing, continuous and stable monolayer amorphous carbon, topologically distinct from disordered graphene. Unlike in bulk materials, the structure of monolayer amorphous carbon can be determined by atomic-resolution imaging. Extensive characterization by Raman and X-ray spectroscopy and transmission electron microscopy reveals the complete absence of long-range periodicity and a threefold-coordinated structure with a wide distribution of bond lengths, bond angles, and five-, six-, seven- and eight-member rings. The ring distribution is not a Zachariasen continuous random network, but resembles the competing (nano)crystallite model(6). We construct a corresponding model that enables density-functional-theory calculations of the properties of monolayer amorphous carbon, in accordance with observations. Direct measurements confirm that it is insulating, with resistivity values similar to those of boron nitride grown by chemical vapour deposition. Free-standing monolayer amorphous carbon is surprisingly stable and deforms to a high breaking strength, without crack propagation from the point of fracture. The excellent physical properties of this stable, free-standing monolayer amorphous carbon could prove useful for permeation and diffusion barriers in applications such as magnetic recording devices and flexible electronics.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Bulk amorphous materials have been studied extensively and are widely used, yet their atomic arrangement remains an open issue. Although they are generally believed to be Zachariasen continuous random networks(1), recent experimental evidence favours the competing crystallite model in the case of amorphous silicon(2-4). In two-dimensional materials, however, the corresponding questions remain unanswered. Here we report the synthesis, by laser-assisted chemical vapour deposition(5), of centimetre-scale, free-standing, continuous and stable monolayer amorphous carbon, topologically distinct from disordered graphene. Unlike in bulk materials, the structure of monolayer amorphous carbon can be determined by atomic-resolution imaging. Extensive characterization by Raman and X-ray spectroscopy and transmission electron microscopy reveals the complete absence of long-range periodicity and a threefold-coordinated structure with a wide distribution of bond lengths, bond angles, and five-, six-, seven- and eight-member rings. The ring distribution is not a Zachariasen continuous random network, but resembles the competing (nano)crystallite model(6). We construct a corresponding model that enables density-functional-theory calculations of the properties of monolayer amorphous carbon, in accordance with observations. Direct measurements confirm that it is insulating, with resistivity values similar to those of boron nitride grown by chemical vapour deposition. Free-standing monolayer amorphous carbon is surprisingly stable and deforms to a high breaking strength, without crack propagation from the point of fracture. The excellent physical properties of this stable, free-standing monolayer amorphous carbon could prove useful for permeation and diffusion barriers in applications such as magnetic recording devices and flexible electronics. |
2019 |
Abidi, Irfan H; Mendelson, Noah; Tran, Toan Trong; Tyagi, Abhishek; Zhuang, Minghao; Weng, Lu-Tao; Ozyilmaz, Barbaros; Aharonovich, Igor; Toth, Milos; Luo, Zhengtang Selective Defect Formation in Hexagonal Boron Nitride Journal Article ADVANCED OPTICAL MATERIALS, 7 (13), 2019, ISSN: 2195-1071. @article{ISI:000474809500016, title = {Selective Defect Formation in Hexagonal Boron Nitride}, author = {Irfan H Abidi and Noah Mendelson and Toan Trong Tran and Abhishek Tyagi and Minghao Zhuang and Lu-Tao Weng and Barbaros Ozyilmaz and Igor Aharonovich and Milos Toth and Zhengtang Luo}, doi = {10.1002/adom.201900397}, times_cited = {0}, issn = {2195-1071}, year = {2019}, date = {2019-07-01}, journal = {ADVANCED OPTICAL MATERIALS}, volume = {7}, number = {13}, publisher = {WILEY-V C H VERLAG GMBH}, address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY}, abstract = {Luminescent defects in hexagonal boron nitride (hBN) have emerged as promising single photon emitters (SPEs) due to their high brightness and robust operation at room temperature. The ability to create such emitters with well-defined optical properties is a cornerstone toward their integration into on-chip photonic architectures. Here, an effective approach is reported to fabricate hBN SPEs with desired emission properties in distinct spectral regions via the manipulation of boron diffusion through copper during atmospheric pressure chemical vapor deposition (CVD)-a process termed gettering. Using the gettering technique the resulting zero-phonon line is deterministically placed between the regions 550 and 600 nm or from 600 to 650 nm, paving the way for hBN SPEs with tailored emission properties. Additionally, rational control over the observed SPE density in the resulting films is demonstrated. The ability to control defect formation during hBN growth provides a cost effective means to improve the crystallinity of CVD hBN films, and lower defect density making it applicable to hBN growth for a wide-range of applications. The results are important to understand defect formation of quantum emitters in hBN and deploy them for scalable photonic technologies.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Luminescent defects in hexagonal boron nitride (hBN) have emerged as promising single photon emitters (SPEs) due to their high brightness and robust operation at room temperature. The ability to create such emitters with well-defined optical properties is a cornerstone toward their integration into on-chip photonic architectures. Here, an effective approach is reported to fabricate hBN SPEs with desired emission properties in distinct spectral regions via the manipulation of boron diffusion through copper during atmospheric pressure chemical vapor deposition (CVD)-a process termed gettering. Using the gettering technique the resulting zero-phonon line is deterministically placed between the regions 550 and 600 nm or from 600 to 650 nm, paving the way for hBN SPEs with tailored emission properties. Additionally, rational control over the observed SPE density in the resulting films is demonstrated. The ability to control defect formation during hBN growth provides a cost effective means to improve the crystallinity of CVD hBN films, and lower defect density making it applicable to hBN growth for a wide-range of applications. The results are important to understand defect formation of quantum emitters in hBN and deploy them for scalable photonic technologies. |
2017 |
Avsar, Ahmet; Unuchek, Dmitrii; Liu, Jiawei; Sanchez, Oriol Lopez; Watanabe, Kenji; Taniguch, Takashi; Ozyilmaz, Barbaros; Kis, Andras Optospintronics in Graphene via Proximity Coupling Journal Article ACS NANO, 11 (11), pp. 11678-11686, 2017, ISSN: 1936-0851. @article{ISI:000416878100115, title = {Optospintronics in Graphene \textit{via} 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 = {0}, 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. |
Avsar, Ahmet; Tan, Jun Y; Luo, Xin; Khoo, Khoong Hong; Yeo, Yuting; Watanabe, Kenji; Taniguchi, Takashi; Quek, Su Ying; Ozyilmaz, Barbaros van der Waals Bonded Co/h-BN Contacts to Ultrathin Black Phosphorus Devices Journal Article NANO LETTERS, 17 (9), pp. 5361-5367, 2017, ISSN: 1530-6984. @article{ISI:000411043500027, title = {van der Waals Bonded Co/h-BN Contacts to Ultrathin Black Phosphorus Devices}, author = {Ahmet Avsar and Jun Y Tan and Xin Luo and Khoong Hong Khoo and Yuting Yeo and Kenji Watanabe and Takashi Taniguchi and Su Ying Quek and Barbaros Ozyilmaz}, doi = {10.1021/acs.nanolett.7b01817}, times_cited = {0}, issn = {1530-6984}, year = {2017}, date = {2017-09-01}, journal = {NANO LETTERS}, volume = {17}, number = {9}, pages = {5361-5367}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Because of the chemical inertness of two dimensional (2D) hexagonal-boron nitride (h-BN), few atomic-layer hBN is often used to encapsulate air-sensitive 2D crystals such as black phosphorus (BP). However, the effects of h-BN on Schottky barrier height, doping, and contact resistance are not well-known. Here, we investigate these effects by fabricating h-BN encapsulated BP transistors with cobalt (Co) contacts. In sharp contrast to directly Co contacted p-type BP devices, we observe strong n-type conduction upon insertion of the h-BN at the Co/BP interface. First-principles calculations show that this difference arises from the much larger interface dipole at the Co/h-BN interface compared to the Co/BP interface, which reduces the work function of the Co/h-BN contact. The Co/h-BN contacts exhibit low contact resistances (similar to 4.5 k Omega) and are Schottky barrier-free. This allows us to probe high electron mobilities (4,200 cm(2)/(Vs)) and observe insulator metal transitions even under two-terminal measurement geometry.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Because of the chemical inertness of two dimensional (2D) hexagonal-boron nitride (h-BN), few atomic-layer hBN is often used to encapsulate air-sensitive 2D crystals such as black phosphorus (BP). However, the effects of h-BN on Schottky barrier height, doping, and contact resistance are not well-known. Here, we investigate these effects by fabricating h-BN encapsulated BP transistors with cobalt (Co) contacts. In sharp contrast to directly Co contacted p-type BP devices, we observe strong n-type conduction upon insertion of the h-BN at the Co/BP interface. First-principles calculations show that this difference arises from the much larger interface dipole at the Co/h-BN interface compared to the Co/BP interface, which reduces the work function of the Co/h-BN contact. The Co/h-BN contacts exhibit low contact resistances (similar to 4.5 k Omega) and are Schottky barrier-free. This allows us to probe high electron mobilities (4,200 cm(2)/(Vs)) and observe insulator metal transitions even under two-terminal measurement geometry. |
Avsar, Ahmet; Tan, Jun Y; Kurpas, Marcin; Gmitra, Martin; Watanabe, Kenji; Taniguchi, Takashi; Fabian, Jaroslav; Ozyilmaz, Barbaros Gate-tunable black phosphorus spin valve with nanosecond spin lifetimes Journal Article NATURE PHYSICS, 13 (9), pp. 888-+, 2017, ISSN: 1745-2473. @article{ISI:000409235100023, title = {Gate-tunable black phosphorus spin valve with nanosecond spin lifetimes}, author = {Ahmet Avsar and Jun Y Tan and Marcin Kurpas and Martin Gmitra and Kenji Watanabe and Takashi Taniguchi and Jaroslav Fabian and Barbaros Ozyilmaz}, doi = {10.1038/NPHYS4141}, times_cited = {0}, issn = {1745-2473}, year = {2017}, date = {2017-09-01}, journal = {NATURE PHYSICS}, volume = {13}, number = {9}, pages = {888-+}, publisher = {NATURE PUBLISHING GROUP}, address = {MACMILLAN BUILDING, 4 CRINAN ST, LONDON N1 9XW, ENGLAND}, abstract = {Two-dimensional materials offer new opportunities for both fundamental science and technological applications, by exploiting the electron's spin. Although graphene is very promising for spin communication due to its extraordinary electron mobility, the lack of a bandgap restricts its prospects for semiconducting spin devices such as spin diodes and bipolar spin transistors. The recent emergence of two-dimensional semiconductors could help overcome this basic challenge. In this letter we report an important step towards making two-dimensional semiconductor spin devices. We have fabricated a spin valve based on ultrathin (similar to 5 nm) semiconducting black phosphorus (bP), and established fundamental spin properties of this spin channel material, which supports all electrical spin injection, transport, precession and detection up to room temperature. In the non-local spin valve geometry we measure Hanle spin precession and observe spin relaxation times as high as 4 ns, with spin relaxation lengths exceeding 6 mu m. Our experimental results are in a very good agreement with first-principles calculations and demonstrate that the Elliott-Yafet spin relaxation mechanism is dominant. We also show that spin transport in ultrathin bP depends strongly on the charge carrier concentration, and can be manipulated by the electric field effect.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Two-dimensional materials offer new opportunities for both fundamental science and technological applications, by exploiting the electron's spin. Although graphene is very promising for spin communication due to its extraordinary electron mobility, the lack of a bandgap restricts its prospects for semiconducting spin devices such as spin diodes and bipolar spin transistors. The recent emergence of two-dimensional semiconductors could help overcome this basic challenge. In this letter we report an important step towards making two-dimensional semiconductor spin devices. We have fabricated a spin valve based on ultrathin (similar to 5 nm) semiconducting black phosphorus (bP), and established fundamental spin properties of this spin channel material, which supports all electrical spin injection, transport, precession and detection up to room temperature. In the non-local spin valve geometry we measure Hanle spin precession and observe spin relaxation times as high as 4 ns, with spin relaxation lengths exceeding 6 mu m. Our experimental results are in a very good agreement with first-principles calculations and demonstrate that the Elliott-Yafet spin relaxation mechanism is dominant. We also show that spin transport in ultrathin bP depends strongly on the charge carrier concentration, and can be manipulated by the electric field effect. |
2016 |
O'Farrell, E C T; Tan, J Y; Yeo, Y; Koon, G K W; Ozyilmaz, B; Watanabe, K; Taniguchi, T Rashba Interaction and Local Magnetic Moments in a Graphene-BN Heterostructure Intercalated with Au Journal Article PHYSICAL REVIEW LETTERS, 117 (7), 2016, ISSN: 0031-9007. @article{ISI:000381478800006, title = {Rashba Interaction and Local Magnetic Moments in a Graphene-BN Heterostructure Intercalated with Au}, author = {E C T O'Farrell and J Y Tan and Y Yeo and G K W Koon and B Ozyilmaz and K Watanabe and T Taniguchi}, doi = {10.1103/PhysRevLett.117.076603}, times_cited = {0}, issn = {0031-9007}, year = {2016}, date = {2016-08-12}, journal = {PHYSICAL REVIEW LETTERS}, volume = {117}, number = {7}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {We intercalate a van der Waals heterostructure of graphene and hexagonal boron nitride with Au, by encapsulation, and show that the Au at the interface is two dimensional. Charge transfer upon current annealing indicates the redistribution of the Au and induces splitting of the graphene band structure. The effect of an in-plane magnetic field confirms that the splitting is due to spin splitting and that the spin polarization is in the plane, characteristic of a Rashba interaction with a magnitude of approximately 25 meV. Consistent with the presence of an intrinsic interfacial electric field we show that the splitting can be enhanced by an applied displacement field in dual gated samples. A giant negative magnetoresistance, up to 75%, and a field induced anomalous Hall effect at magnetic fields < 1 T are observed. These demonstrate that the hybridized Au has a magnetic moment and suggests the proximity to the formation of a collective magnetic phase. These effects persist close to room temperature.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We intercalate a van der Waals heterostructure of graphene and hexagonal boron nitride with Au, by encapsulation, and show that the Au at the interface is two dimensional. Charge transfer upon current annealing indicates the redistribution of the Au and induces splitting of the graphene band structure. The effect of an in-plane magnetic field confirms that the splitting is due to spin splitting and that the spin polarization is in the plane, characteristic of a Rashba interaction with a magnitude of approximately 25 meV. Consistent with the presence of an intrinsic interfacial electric field we show that the splitting can be enhanced by an applied displacement field in dual gated samples. A giant negative magnetoresistance, up to 75%, and a field induced anomalous Hall effect at magnetic fields < 1 T are observed. These demonstrate that the hybridized Au has a magnetic moment and suggests the proximity to the formation of a collective magnetic phase. These effects persist close to room temperature. |
Avsar, Ahmet; Vera-Marun, Ivan Jesus; Tan, Jun You; Koon, Gavin Kok Wai; Watanabe, Kenji; Taniguchi, Takashi; Adam, Shaffique; Ozyilmaz, Barbaros Electronic spin transport in dual-gated bilayer graphene Journal Article NPG ASIA MATERIALS, 8 , 2016, ISSN: 1884-4049. @article{ISI:000379759800001, title = {Electronic spin transport in dual-gated bilayer graphene}, author = {Ahmet Avsar and Ivan Jesus Vera-Marun and Jun You Tan and Gavin Kok Wai Koon and Kenji Watanabe and Takashi Taniguchi and Shaffique Adam and Barbaros Ozyilmaz}, doi = {10.1038/am.2016.65}, times_cited = {0}, issn = {1884-4049}, year = {2016}, date = {2016-06-01}, journal = {NPG ASIA MATERIALS}, volume = {8}, publisher = {NATURE PUBLISHING GROUP}, address = {75 VARICK ST, 9TH FLR, NEW YORK, NY 10013-1917 USA}, abstract = {The elimination of extrinsic sources of spin relaxation is key to realizing the exceptional intrinsic spin transport performance of graphene. Toward this, we study charge and spin transport in bilayer graphene-based spin valve devices fabricated in a new device architecture that allows us to make a comparative study by separately investigating the roles of the substrate and polymer residues on spin relaxation. First, the comparison between spin valves fabricated on SiO2 and BN substrates suggests that substrate-related charged impurities, phonons and roughness do not limit the spin transport in current devices. Next, the observation of a fivefold enhancement in the spin-relaxation time of the encapsulated device highlights the significance of polymer residues on spin relaxation. We observe a spin-relaxation length of similar to 10 mu m in the encapsulated bilayer, with a charge mobility of 24 000 cm(2) Vs(-1). The carrier density dependence on the spin-relaxation time has two distinct regimes; n<4 x 10(12) cm(-2), where the spin-relaxation time decreases monotonically as the carrier concentration increases, and n >= 4 x 10(12) cm(-2), where the spin-relaxation time exhibits a sudden increase. The sudden increase in the spin-relaxation time with no corresponding signature in the charge transport suggests the presence of a magnetic resonance close to the charge neutrality point. We also demonstrate, for the first time, spin transport across bipolar p-n junctions in our dual-gated device architecture that fully integrates a sequence of encapsulated regions in its design. At low temperatures, strong suppression of the spin signal was observed while a transport gap was induced, which is interpreted as a novel manifestation of the impedance mismatch within the spin channel.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The elimination of extrinsic sources of spin relaxation is key to realizing the exceptional intrinsic spin transport performance of graphene. Toward this, we study charge and spin transport in bilayer graphene-based spin valve devices fabricated in a new device architecture that allows us to make a comparative study by separately investigating the roles of the substrate and polymer residues on spin relaxation. First, the comparison between spin valves fabricated on SiO2 and BN substrates suggests that substrate-related charged impurities, phonons and roughness do not limit the spin transport in current devices. Next, the observation of a fivefold enhancement in the spin-relaxation time of the encapsulated device highlights the significance of polymer residues on spin relaxation. We observe a spin-relaxation length of similar to 10 mu m in the encapsulated bilayer, with a charge mobility of 24 000 cm(2) Vs(-1). The carrier density dependence on the spin-relaxation time has two distinct regimes; n<4 x 10(12) cm(-2), where the spin-relaxation time decreases monotonically as the carrier concentration increases, and n >= 4 x 10(12) cm(-2), where the spin-relaxation time exhibits a sudden increase. The sudden increase in the spin-relaxation time with no corresponding signature in the charge transport suggests the presence of a magnetic resonance close to the charge neutrality point. We also demonstrate, for the first time, spin transport across bipolar p-n junctions in our dual-gated device architecture that fully integrates a sequence of encapsulated regions in its design. At low temperatures, strong suppression of the spin signal was observed while a transport gap was induced, which is interpreted as a novel manifestation of the impedance mismatch within the spin channel. |