Gavin Koon Kok Wai
Degree: PhD
Position: Senior Research Fellow
Office: S16-06-12
Email: c2dgkkw@nus.edu.sg
Contact: (65) 6601 3273
CA2DM Publications:
2024 |
He, Pan; Isobe, Hiroki; Koon, Gavin Kok Wai; Tan, Jun You; Hu, Junxiong; Li, Jingru; Nagaosa, Naoto; Shen, Jian Third-order nonlinear Hall effect in a quantum Hall system Journal Article NATURE NANOTECHNOLOGY, 19 (10), 2024, ISSN: 1748-3387. @article{ISI:001276091000001, title = {Third-order nonlinear Hall effect in a quantum Hall system}, author = {Pan He and Hiroki Isobe and Gavin Kok Wai Koon and Jun You Tan and Junxiong Hu and Jingru Li and Naoto Nagaosa and Jian Shen}, doi = {10.1038/s41565-024-01730-1}, times_cited = {2}, issn = {1748-3387}, year = {2024}, date = {2024-07-24}, journal = {NATURE NANOTECHNOLOGY}, volume = {19}, number = {10}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {In two-dimensional systems, perpendicular magnetic fields can induce a bulk band gap and chiral edge states, which gives rise to the quantum Hall effect. The quantum Hall effect is characterized by zero longitudinal resistance (R-xx) and Hall resistance (R-xy) plateaus quantized to h/(upsilon e(2)) in the linear response regime, where upsilon is the Landau level filling factor, e is the elementary charge and h is Planck's constant. Here we explore the nonlinear response of monolayer graphene when tuned to a quantum Hall state. We observe a third-order Hall effect that exhibits a nonzero voltage plateau scaling cubically with the probe current. By contrast, the third-order longitudinal voltage remains zero. The magnitude of the third-order response is insensitive to variations in magnetic field (down to similar to 5 T) and in temperature (up to similar to 60 K). Moreover, the third-order response emerges in graphene devices with a variety of geometries, different substrates and stacking configurations. We term the effect third-order nonlinear response of the quantum Hall state and propose that electron-electron interaction between the quantum Hall edge states is the origin of the nonlinear response of the quantum Hall state.}, keywords = {}, pubstate = {published}, tppubtype = {article} } In two-dimensional systems, perpendicular magnetic fields can induce a bulk band gap and chiral edge states, which gives rise to the quantum Hall effect. The quantum Hall effect is characterized by zero longitudinal resistance (R-xx) and Hall resistance (R-xy) plateaus quantized to h/(upsilon e(2)) in the linear response regime, where upsilon is the Landau level filling factor, e is the elementary charge and h is Planck's constant. Here we explore the nonlinear response of monolayer graphene when tuned to a quantum Hall state. We observe a third-order Hall effect that exhibits a nonzero voltage plateau scaling cubically with the probe current. By contrast, the third-order longitudinal voltage remains zero. The magnitude of the third-order response is insensitive to variations in magnetic field (down to similar to 5 T) and in temperature (up to similar to 60 K). Moreover, the third-order response emerges in graphene devices with a variety of geometries, different substrates and stacking configurations. We term the effect third-order nonlinear response of the quantum Hall state and propose that electron-electron interaction between the quantum Hall edge states is the origin of the nonlinear response of the quantum Hall state. |
2022 |
He, Pan; Koon, Gavin Kok Wai; Isobe, Hiroki; Tan, Jun You; Hu, Junxiong; Neto, Antonio Castro H; Fu, Liang; Yang, Hyunsoo Graphene moire superlattices with giant quantum nonlinearity of chiral Bloch electrons Journal Article 55 NATURE NANOTECHNOLOGY, 17 (4), pp. 378-+, 2022, ISSN: 1748-3387. @article{ISI:000751676800001, title = {Graphene moire superlattices with giant quantum nonlinearity of chiral Bloch electrons}, author = {Pan He and Gavin Kok Wai Koon and Hiroki Isobe and Jun You Tan and Junxiong Hu and Antonio Castro H Neto and Liang Fu and Hyunsoo Yang}, doi = {10.1038/s41565-021-01060-6}, times_cited = {55}, issn = {1748-3387}, year = {2022}, date = {2022-02-03}, journal = {NATURE NANOTECHNOLOGY}, volume = {17}, number = {4}, pages = {378-+}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {Graphene-based samples have shown a plethora of exotic characteristics and these properties may help the realization of a new generation of fast electronic devices. However, graphene's centrosymmetry prohibits second-order electronic transport. Here, we show giant second-order nonlinear transports in graphene moire superlattices at zero magnetic field, both longitudinal and transverse to the applied current direction. High carrier mobility and inversion symmetry breaking by hexagonal boron nitride lead to nonlinear conductivities five orders of magnitude larger than those in WTe2. The nonlinear conductivity strongly depends on the gate voltage as well as on the stacking configuration, with a giant enhancement originating from the moire bands. Longitudinal nonlinear conductivity cannot originate from Berry curvature dipoles. Our theoretical modelling highlights skew scattering of chiral Bloch electrons as the physical origin. With these results, we demonstrate nonlinear charge transport due to valley-contrasting chirality, which constitutes an alternative means to induce second-order transports in van der Waals heterostructures. Our approach is promising for applications in frequency-doubling and energy harvesting via rectification.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Graphene-based samples have shown a plethora of exotic characteristics and these properties may help the realization of a new generation of fast electronic devices. However, graphene's centrosymmetry prohibits second-order electronic transport. Here, we show giant second-order nonlinear transports in graphene moire superlattices at zero magnetic field, both longitudinal and transverse to the applied current direction. High carrier mobility and inversion symmetry breaking by hexagonal boron nitride lead to nonlinear conductivities five orders of magnitude larger than those in WTe2. The nonlinear conductivity strongly depends on the gate voltage as well as on the stacking configuration, with a giant enhancement originating from the moire bands. Longitudinal nonlinear conductivity cannot originate from Berry curvature dipoles. Our theoretical modelling highlights skew scattering of chiral Bloch electrons as the physical origin. With these results, we demonstrate nonlinear charge transport due to valley-contrasting chirality, which constitutes an alternative means to induce second-order transports in van der Waals heterostructures. Our approach is promising for applications in frequency-doubling and energy harvesting via rectification. |
2020 |
Lim, Sharon Xiaodai; Zhang, Zheng; Koon, Gavin Kok Wai; Sow, Chorng-Haur Unlocking the potential of carbon incorporated silver-silver molybdate nanowire with light Journal Article APPLIED MATERIALS TODAY, 20 , 2020, ISSN: 2352-9407. @article{ISI:000598818600003, title = {Unlocking the potential of carbon incorporated silver-silver molybdate nanowire with light}, author = {Sharon Xiaodai Lim and Zheng Zhang and Gavin Kok Wai Koon and Chorng-Haur Sow}, doi = {10.1016/j.apmt.2020.100670}, times_cited = {3}, issn = {2352-9407}, year = {2020}, date = {2020-09-01}, journal = {APPLIED MATERIALS TODAY}, volume = {20}, publisher = {ELSEVIER}, address = {RADARWEG 29, 1043 NX AMSTERDAM, NETHERLANDS}, abstract = {We present a novel form of Ag2MoO4-based hybrid nanowire (NW) with a few remarkable attributes. Firstly, the NW is embedded and decorated with Ag NPs. Secondly, carbon atoms are intentionally incorporated within the matrix of the NW. Thirdly the hybrid nanowires are created via a facile process. Namely, focused laser micropatterning of Ag NPs on GO film as seeding sites and subsequent formation of the hybrid NWs by placing the patterned GO films on heated Mo foil on a hotplate. This unique process resulted in the production of hybrid Ag/Ag2MoO4 NWs that emit unique red fluorescence emission. And finally remarkable photodoping effect is observed from a single strand of optically tuned carbon-doped silver nanoparticles embedded silver molybdate nanowire. We demonstrate applications of these hybrid NWs as micro-display and time limiting, logic components for secure transmission of messages. (C) 2020 Elsevier Ltd. All rights reserved.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We present a novel form of Ag2MoO4-based hybrid nanowire (NW) with a few remarkable attributes. Firstly, the NW is embedded and decorated with Ag NPs. Secondly, carbon atoms are intentionally incorporated within the matrix of the NW. Thirdly the hybrid nanowires are created via a facile process. Namely, focused laser micropatterning of Ag NPs on GO film as seeding sites and subsequent formation of the hybrid NWs by placing the patterned GO films on heated Mo foil on a hotplate. This unique process resulted in the production of hybrid Ag/Ag2MoO4 NWs that emit unique red fluorescence emission. And finally remarkable photodoping effect is observed from a single strand of optically tuned carbon-doped silver nanoparticles embedded silver molybdate nanowire. We demonstrate applications of these hybrid NWs as micro-display and time limiting, logic components for secure transmission of messages. (C) 2020 Elsevier Ltd. All rights reserved. |
2018 |
Lim, Sharon Xiaodai; Koon, Gavin Kok Wai; Zhang, Zheng; Neto, Antonio Castro H; Tok, Eng Soon; Sow, Chorng-Haur Laser assisted blending of Ag nanoparticles in an alumina veil: a highly fluorescent hybrid Journal Article NANOSCALE, 10 (38), pp. 18145-18152, 2018, ISSN: 2040-3364. @article{ISI:000450820400037, title = {Laser assisted blending of Ag nanoparticles in an alumina veil: a highly fluorescent hybrid}, author = {Sharon Xiaodai Lim and Gavin Kok Wai Koon and Zheng Zhang and Antonio Castro H Neto and Eng Soon Tok and Chorng-Haur Sow}, doi = {10.1039/c8nr03245a}, times_cited = {4}, issn = {2040-3364}, year = {2018}, date = {2018-10-14}, journal = {NANOSCALE}, volume = {10}, number = {38}, pages = {18145-18152}, publisher = {ROYAL SOC CHEMISTRY}, address = {THOMAS GRAHAM HOUSE, SCIENCE PARK, MILTON RD, CAMBRIDGE CB4 0WF, CAMBS, ENGLAND}, abstract = {We report a functional hybrid made of silver nanoparticles (AgNPs) embedded in an amorphous aluminium oxide (alumina) film. This laser-initiated process allows formation of AgNPs and amorphous alumina in localized regions defined by the scanning laser beam. Due to metal enhanced fluorescence, this hybrid exhibits strong blue fluorescence emission under ultraviolet excitation. Upon irradiating with electrons at dosages of 1 to 20 mC cm(-2), AgNPs become more metallic while the Al film is further oxidised. As a result, the fluorescing property is intensified. Using a hybrid irradiated with 10 mC cm(-2), the electronic conductivity of the sample is improved by 11.5 times compared to that of the as-synthesized hybrid film. Excitation by UV light on the sample results in an increase in the detected current of nearly 29 times. Given that the electron beam patterned message is selectively visible only under UV or blue light irradiation, this hybrid film is thus a possible platform for steganographic transmission.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We report a functional hybrid made of silver nanoparticles (AgNPs) embedded in an amorphous aluminium oxide (alumina) film. This laser-initiated process allows formation of AgNPs and amorphous alumina in localized regions defined by the scanning laser beam. Due to metal enhanced fluorescence, this hybrid exhibits strong blue fluorescence emission under ultraviolet excitation. Upon irradiating with electrons at dosages of 1 to 20 mC cm(-2), AgNPs become more metallic while the Al film is further oxidised. As a result, the fluorescing property is intensified. Using a hybrid irradiated with 10 mC cm(-2), the electronic conductivity of the sample is improved by 11.5 times compared to that of the as-synthesized hybrid film. Excitation by UV light on the sample results in an increase in the detected current of nearly 29 times. Given that the electron beam patterned message is selectively visible only under UV or blue light irradiation, this hybrid film is thus a possible platform for steganographic transmission. |
2016 |
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 41 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 = {41}, 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. |
2015 |
Avsar, Ahmet; Lee, Jong Hak; Koon, Gavin Kok Wai; Oezyilmaz, Barbaros Enhanced spin-orbit coupling in dilute fluorinated graphene Journal Article 35 2D MATERIALS, 2 (4), 2015, ISSN: 2053-1583. @article{ISI:000368936600012, title = {Enhanced spin-orbit coupling in dilute fluorinated graphene}, author = {Ahmet Avsar and Jong Hak Lee and Gavin Kok Wai Koon and Barbaros Oezyilmaz}, doi = {10.1088/2053-1583/2/4/044009}, times_cited = {35}, issn = {2053-1583}, year = {2015}, date = {2015-12-01}, journal = {2D MATERIALS}, volume = {2}, number = {4}, publisher = {IOP PUBLISHING LTD}, address = {TEMPLE CIRCUS, TEMPLE WAY, BRISTOL BS1 6BE, ENGLAND}, abstract = {The preservation and manipulation of a spin state mainly depends on the strength of the spin-orbit interaction. For pristine graphene, the intrinsic spin-orbit coupling (SOC) is only in the order of few mu eV, which makes it almost impossible to be used as an active element in future electric field controlled spintronics devices. This stimulates the development of a systematic method for extrinsically enhancing the SOC of graphene. In this letter, we study the strength of SOC in weakly fluorinated graphene devices. We observe high non-local signals even without applying any external magnetic field. The magnitude of the signal increases with increasing fluorine adatom coverage. From the length dependence of the non-local transport measurements, we obtain SOC values of similar to 5.1 meV and similar to 9.1 meV for the devices with similar to 0.005% and similar to 0.06% fluorination, respectively. Such a large enhancement, together with the high charge mobility of fluorinated samples (mu similar to 4300 cm(2) V-1 s(-1)-2700 cm(2) V-1 s(-1)), enables the detection of the spin Hall effect even at room temperature.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The preservation and manipulation of a spin state mainly depends on the strength of the spin-orbit interaction. For pristine graphene, the intrinsic spin-orbit coupling (SOC) is only in the order of few mu eV, which makes it almost impossible to be used as an active element in future electric field controlled spintronics devices. This stimulates the development of a systematic method for extrinsically enhancing the SOC of graphene. In this letter, we study the strength of SOC in weakly fluorinated graphene devices. We observe high non-local signals even without applying any external magnetic field. The magnitude of the signal increases with increasing fluorine adatom coverage. From the length dependence of the non-local transport measurements, we obtain SOC values of similar to 5.1 meV and similar to 9.1 meV for the devices with similar to 0.005% and similar to 0.06% fluorination, respectively. Such a large enhancement, together with the high charge mobility of fluorinated samples (mu similar to 4300 cm(2) V-1 s(-1)-2700 cm(2) V-1 s(-1)), enables the detection of the spin Hall effect even at room temperature. |
Wu, Jing; Koon, Gavin Kok Wai; Xiang, Du; Han, Cheng; Toh, Chee Tat; Kulkarni, Eeshan S; Verzhbitskiy, Ivan; Carvalho, Alexandra; Rodin, Aleksandr S; Koenig, Steven P; Eda, Goki; Chen, Wei; Neto, Castro A H; Oezyilmaz, Barbaros Colossal Ultraviolet Photoresponsivity of Few-Layer Black Phosphorus Journal Article 227 ACS NANO, 9 (8), pp. 8070-8077, 2015, ISSN: 1936-0851. @article{ISI:000360323300035, title = {Colossal Ultraviolet Photoresponsivity of Few-Layer Black Phosphorus}, author = {Jing Wu and Gavin Kok Wai Koon and Du Xiang and Cheng Han and Chee Tat Toh and Eeshan S Kulkarni and Ivan Verzhbitskiy and Alexandra Carvalho and Aleksandr S Rodin and Steven P Koenig and Goki Eda and Wei Chen and Castro A H Neto and Barbaros Oezyilmaz}, doi = {10.1021/acsnano.5b01922}, times_cited = {227}, issn = {1936-0851}, year = {2015}, date = {2015-08-01}, journal = {ACS NANO}, volume = {9}, number = {8}, pages = {8070-8077}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Black phosphorus has an orthorhombic layered structure with a layer-dependent direct band gap from monolayer to bulk, making this material an emerging material for photodetection. Inspired by this and the recent excitement over this material, we studied the optoelectronics characteristics of high-quality, few-layer black phosphorus-based photodetectors over a wide spectrum ranging from near-ultraviolet (UV) to near-infrared (NIR). It is demonstrated for the first time that black phosphorus can be configured as an excellent UV photodetector with a specific detectivity similar to 3 x 10(13) Jones. More critically, we found that the UV photoresponsivity can be significantly enhanced to similar to 9 x 10(4) A W-1 by applying a source-drain bias (V-SD) of 3 V, which is the highest ever measured in any 2D material and 10(7) times higher than the previously reported value for black phosphorus. We attribute such a colossal UV photoresponsivity to the resonant-interband transition between two specially nested valence and conduction bands. These nested bands provide an unusually high density of states for highly efficient UV absorption due to the singularity of their nature.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Black phosphorus has an orthorhombic layered structure with a layer-dependent direct band gap from monolayer to bulk, making this material an emerging material for photodetection. Inspired by this and the recent excitement over this material, we studied the optoelectronics characteristics of high-quality, few-layer black phosphorus-based photodetectors over a wide spectrum ranging from near-ultraviolet (UV) to near-infrared (NIR). It is demonstrated for the first time that black phosphorus can be configured as an excellent UV photodetector with a specific detectivity similar to 3 x 10(13) Jones. More critically, we found that the UV photoresponsivity can be significantly enhanced to similar to 9 x 10(4) A W-1 by applying a source-drain bias (V-SD) of 3 V, which is the highest ever measured in any 2D material and 10(7) times higher than the previously reported value for black phosphorus. We attribute such a colossal UV photoresponsivity to the resonant-interband transition between two specially nested valence and conduction bands. These nested bands provide an unusually high density of states for highly efficient UV absorption due to the singularity of their nature. |
Luo, Xin; Lu, Xin; Koon, Gavin Kok Wai; Neto, Antonio Castro H; Oezyilmaz, Barbaros; Xiong, Qihua; Quek, Su Ying Large Frequency Change with Thickness in Interlayer Breathing Mode-Significant Interlayer Interactions in Few Layer Black Phosphorus Journal Article 103 NANO LETTERS, 15 (6), pp. 3931-3938, 2015, ISSN: 1530-6984. @article{ISI:000356316900041, title = {Large Frequency Change with Thickness in Interlayer Breathing Mode-Significant Interlayer Interactions in Few Layer Black Phosphorus}, author = {Xin Luo and Xin Lu and Gavin Kok Wai Koon and Antonio Castro H Neto and Barbaros Oezyilmaz and Qihua Xiong and Su Ying Quek}, doi = {10.1021/acs.nanolett.5b00775}, times_cited = {103}, issn = {1530-6984}, year = {2015}, date = {2015-06-01}, journal = {NANO LETTERS}, volume = {15}, number = {6}, pages = {3931-3938}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Bulk black phosphorus (BP) consists of puckered layers of phosphorus atoms. Few-layer BP, obtained from bulk BP by exfoliation, is an emerging candidate as a channel material in post-silicon electronics, A deep understanding of its physical properties and its full range of applications are still being uncovered. In this paper, we present a theoretical and experimental investigation of phonon properties in few-layer BP, focusing on the low-frequency regime corresponding to interlayer vibrational modes, We show that the interlayer breathing mode A(g)(3) shows a large redshift with increasing thickness; the experimental and theoretical results agree well. This thickness dependence is two times larger than that in the chalcogenide materials, such as few-layer MoS2 and WSe2, because of the significantly larger interlayer force constant and smaller atomic mass in BP. The derived interlayer out-of-plane force constant is about 50% larger than that of graphene and MoS2. We show that this large interlayer force constant arises from the sizable covalent interaction between phosphorus atoms in adjacent layers and that interlayer interactions are not Merely of the weak van der Waals type. These significant interlayer interactions are consistent with the known surface reactivity of BP and have been shown to be important for electric-field induced formation of Dirac cones in thin film BP.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Bulk black phosphorus (BP) consists of puckered layers of phosphorus atoms. Few-layer BP, obtained from bulk BP by exfoliation, is an emerging candidate as a channel material in post-silicon electronics, A deep understanding of its physical properties and its full range of applications are still being uncovered. In this paper, we present a theoretical and experimental investigation of phonon properties in few-layer BP, focusing on the low-frequency regime corresponding to interlayer vibrational modes, We show that the interlayer breathing mode A(g)(3) shows a large redshift with increasing thickness; the experimental and theoretical results agree well. This thickness dependence is two times larger than that in the chalcogenide materials, such as few-layer MoS2 and WSe2, because of the significantly larger interlayer force constant and smaller atomic mass in BP. The derived interlayer out-of-plane force constant is about 50% larger than that of graphene and MoS2. We show that this large interlayer force constant arises from the sizable covalent interaction between phosphorus atoms in adjacent layers and that interlayer interactions are not Merely of the weak van der Waals type. These significant interlayer interactions are consistent with the known surface reactivity of BP and have been shown to be important for electric-field induced formation of Dirac cones in thin film BP. |
2014 |
Balakrishnan, Jayakumar; Koon, Gavin Kok Wai; Avsar, Ahmet; Ho, Yuda; Lee, Jong Hak; Jaiswal, Manu; Baeck, Seung-Jae; Ahn, Jong-Hyun; Ferreira, Aires; Cazalilla, Miguel A; Neto, Antonio Castro H; Oezyilmaz, Barbaros Giant spin Hall effect in graphene grown by chemical vapour deposition Journal Article 178 NATURE COMMUNICATIONS, 5 , 2014, ISSN: 2041-1723. @article{ISI:000342841200001, title = {Giant spin Hall effect in graphene grown by chemical vapour deposition}, author = {Jayakumar Balakrishnan and Gavin Kok Wai Koon and Ahmet Avsar and Yuda Ho and Jong Hak Lee and Manu Jaiswal and Seung-Jae Baeck and Jong-Hyun Ahn and Aires Ferreira and Miguel A Cazalilla and Antonio Castro H Neto and Barbaros Oezyilmaz}, doi = {10.1038/ncomms5748}, times_cited = {178}, issn = {2041-1723}, year = {2014}, date = {2014-09-01}, journal = {NATURE COMMUNICATIONS}, volume = {5}, publisher = {NATURE PUBLISHING GROUP}, address = {MACMILLAN BUILDING, 4 CRINAN ST, LONDON N1 9XW, ENGLAND}, abstract = {Advances in large-area graphene synthesis via chemical vapour deposition on metals like copper were instrumental in the demonstration of graphene-based novel, wafer-scale electronic circuits and proof-of-concept applications such as flexible touch panels. Here, we show that graphene grown by chemical vapour deposition on copper is equally promising for spintronics applications. In contrast to natural graphene, our experiments demonstrate that chemically synthesized graphene has a strong spin-orbit coupling as high as 20 meV giving rise to a giant spin Hall effect. The exceptionally large spin Hall angle similar to 0.2 provides an important step towards graphene-based spintronics devices within existing complementary metal-oxide-semiconductor technology. Our microscopic model shows that unavoidable residual copper adatom clusters act as local spin-orbit scatterers and, in the resonant scattering limit, induce transverse spin currents with enhanced skew-scattering contribution. Our findings are confirmed independently by introducing metallic adatoms-copper, silver and gold on exfoliated graphene samples.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Advances in large-area graphene synthesis via chemical vapour deposition on metals like copper were instrumental in the demonstration of graphene-based novel, wafer-scale electronic circuits and proof-of-concept applications such as flexible touch panels. Here, we show that graphene grown by chemical vapour deposition on copper is equally promising for spintronics applications. In contrast to natural graphene, our experiments demonstrate that chemically synthesized graphene has a strong spin-orbit coupling as high as 20 meV giving rise to a giant spin Hall effect. The exceptionally large spin Hall angle similar to 0.2 provides an important step towards graphene-based spintronics devices within existing complementary metal-oxide-semiconductor technology. Our microscopic model shows that unavoidable residual copper adatom clusters act as local spin-orbit scatterers and, in the resonant scattering limit, induce transverse spin currents with enhanced skew-scattering contribution. Our findings are confirmed independently by introducing metallic adatoms-copper, silver and gold on exfoliated graphene samples. |
2013 |
Lee, Jong Hak; Koon, Gavin Kok Wai; Shin, Dong Wook; Fedorov, V E; Choi, Jae-Young; Yoo, Ji-Beom; Oezyilmaz, Barbaros Property Control of Graphene by Employing "Semi-Ionic" Liquid Fluorination Journal Article 47 ADVANCED FUNCTIONAL MATERIALS, 23 (26), pp. 3329-3334, 2013, ISSN: 1616-301X. @article{ISI:000327489200010, title = {Property Control of Graphene by Employing "Semi-Ionic" Liquid Fluorination}, author = {Jong Hak Lee and Gavin Kok Wai Koon and Dong Wook Shin and V E Fedorov and Jae-Young Choi and Ji-Beom Yoo and Barbaros Oezyilmaz}, doi = {10.1002/adfm.201202822}, times_cited = {47}, issn = {1616-301X}, year = {2013}, date = {2013-07-12}, journal = {ADVANCED FUNCTIONAL MATERIALS}, volume = {23}, number = {26}, pages = {3329-3334}, publisher = {WILEY-V C H VERLAG GMBH}, address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY}, abstract = {Semi-ionically fluorinated graphene (s-FG) is synthesized with a one step liquid fluorination treatment. The s-FG consists of two different types of bonds, namely a covalent C-F bond and an ionic C-F bond. Control is achieved over the properties of s-FG by selectively eliminating ionic C-F bonds from the as prepared s-FG film which is highly insulating (current < 10(-13) A at 1 V). After selective elimination of ionic C-F bonds by acetone treatment, s-FG recovers the highly conductive property of graphene. A 10(9) times increase in current from 10(-13) to 10(-4)A at 1 V is achieved, which indicates that s-FG recovers its conducting property. The properties of reduced s-FG vary according to the number of layers and the single layer reduced s-FG has mobility of more than 6000 cm(2) V-1 s(-1). The mobility drastically decreases with increasing number of layers. The bi-layered s-FG has a mobility of 141cm(2) V-1 s(-1) and multi-layered s-FG film showed highly p-type doped electrical property without Dirac point. The reduction via acetone proceeds as 2C(2)F((semi-ionic)) + CH3C(O)CH3(l) HF + 2C((s)) + C2F(covalent) + CH3C(O)CH2(l). The fluorination and reduction processes permit the safe and facile non-destructive property control of the s-FG film.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Semi-ionically fluorinated graphene (s-FG) is synthesized with a one step liquid fluorination treatment. The s-FG consists of two different types of bonds, namely a covalent C-F bond and an ionic C-F bond. Control is achieved over the properties of s-FG by selectively eliminating ionic C-F bonds from the as prepared s-FG film which is highly insulating (current < 10(-13) A at 1 V). After selective elimination of ionic C-F bonds by acetone treatment, s-FG recovers the highly conductive property of graphene. A 10(9) times increase in current from 10(-13) to 10(-4)A at 1 V is achieved, which indicates that s-FG recovers its conducting property. The properties of reduced s-FG vary according to the number of layers and the single layer reduced s-FG has mobility of more than 6000 cm(2) V-1 s(-1). The mobility drastically decreases with increasing number of layers. The bi-layered s-FG has a mobility of 141cm(2) V-1 s(-1) and multi-layered s-FG film showed highly p-type doped electrical property without Dirac point. The reduction via acetone proceeds as 2C(2)F((semi-ionic)) + CH3C(O)CH3(l) HF + 2C((s)) + C2F(covalent) + CH3C(O)CH2(l). The fluorination and reduction processes permit the safe and facile non-destructive property control of the s-FG film. |
Balakrishnan, Jayakumar; Koon, Gavin Kok Wai; Jaiswal, Manu; Neto, Castro A H; Oezyilmaz, Barbaros Colossal enhancement of spin-orbit coupling in weakly hydrogenated graphene Journal Article 382 NATURE PHYSICS, 9 (5), pp. 284-287, 2013, ISSN: 1745-2473. @article{ISI:000318550200018, title = {Colossal enhancement of spin-orbit coupling in weakly hydrogenated graphene}, author = {Jayakumar Balakrishnan and Gavin Kok Wai Koon and Manu Jaiswal and Castro A H Neto and Barbaros Oezyilmaz}, doi = {10.1038/nphys2576}, times_cited = {382}, issn = {1745-2473}, year = {2013}, date = {2013-05-01}, journal = {NATURE PHYSICS}, volume = {9}, number = {5}, pages = {284-287}, publisher = {NATURE PUBLISHING GROUP}, address = {MACMILLAN BUILDING, 4 CRINAN ST, LONDON N1 9XW, ENGLAND}, abstract = {Graphene's extremely small intrinsic spin-orbit (SO) interaction(1) makes the realization of many interesting phenomena such as topological/quantum spin Hall states(2,3) and the spin Hall effect(4) (SHE) practically impossible. Recently, it was predicted(1,5-7) that the introduction of adatoms in graphene would enhance the SO interaction by the conversion of sp(2) to sp(3) bonds. However, introducing adatoms and yet keeping graphene metallic, that is, without creating electronic (Anderson) localization(8), is experimentally challenging. Here, we show that the controlled addition of small amounts of covalently bonded hydrogen atoms is sufficient to induce a colossal enhancement of the SO interaction by three orders of magnitude. This results in a SHE at zero external magnetic fields at room temperature, with non-local spin signals up to 100 Omega; orders of magnitude larger than in metals(9). The non-local SHE is, further, directly confirmed by Larmor spin-precession measurements. From this and the length dependence of the non-local signal we extract a spin relaxation length of similar to 1 mu m, a spin relaxation time of similar to 90 ps and a SO strength of 2.5 meV.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Graphene's extremely small intrinsic spin-orbit (SO) interaction(1) makes the realization of many interesting phenomena such as topological/quantum spin Hall states(2,3) and the spin Hall effect(4) (SHE) practically impossible. Recently, it was predicted(1,5-7) that the introduction of adatoms in graphene would enhance the SO interaction by the conversion of sp(2) to sp(3) bonds. However, introducing adatoms and yet keeping graphene metallic, that is, without creating electronic (Anderson) localization(8), is experimentally challenging. Here, we show that the controlled addition of small amounts of covalently bonded hydrogen atoms is sufficient to induce a colossal enhancement of the SO interaction by three orders of magnitude. This results in a SHE at zero external magnetic fields at room temperature, with non-local spin signals up to 100 Omega; orders of magnitude larger than in metals(9). The non-local SHE is, further, directly confirmed by Larmor spin-precession measurements. From this and the length dependence of the non-local signal we extract a spin relaxation length of similar to 1 mu m, a spin relaxation time of similar to 90 ps and a SO strength of 2.5 meV. |
Huang, Yuan; Wu, Jing; Xu, Xiangfan; Ho, Yuda; Ni, Guangxin; Zou, Qiang; Koon, Gavin Kok Wai; Zhao, Weijie; Neto, Castro A H; Eda, Goki; Shen, Chengmin; Oezyilmaz, Barbaros An innovative way of etching MoS2: Characterization and mechanistic investigation Journal Article 167 NANO RESEARCH, 6 (3), pp. 200-207, 2013, ISSN: 1998-0124. @article{ISI:000316087300006, title = {An innovative way of etching MoS_{2}: Characterization and mechanistic investigation}, author = {Yuan Huang and Jing Wu and Xiangfan Xu and Yuda Ho and Guangxin Ni and Qiang Zou and Gavin Kok Wai Koon and Weijie Zhao and Castro A H Neto and Goki Eda and Chengmin Shen and Barbaros Oezyilmaz}, doi = {10.1007/s12274-013-0296-8}, times_cited = {167}, issn = {1998-0124}, year = {2013}, date = {2013-03-01}, journal = {NANO RESEARCH}, volume = {6}, number = {3}, pages = {200-207}, publisher = {TSINGHUA UNIV PRESS}, address = {B605D, XUE YAN BUILDING, BEIJING, 100084, PEOPLES R CHINA}, abstract = {We report a systematic study of the etching of MoS2 crystals by using XeF2 as a gaseous reactant. By controlling the etching process, monolayer MoS2 with uniform morphology can be obtained. The Raman and photoluminescence spectra of the resulting material were similar to those of exfoliated MoS2. Utilizing this strategy, different patterns such as a Hall bar structure and a hexagonal array can be realized. Furthermore, the etching mechanism was studied by introducing graphene as an etching mask. We believe our technique opens an easy and controllable way of etching MoS2, which can be used to fabricate complex nanostructures, such as nanoribbons, quantum dots, and transistor structures. This etching process using XeF2 can also be extended to other interesting two-dimensional crystals.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We report a systematic study of the etching of MoS2 crystals by using XeF2 as a gaseous reactant. By controlling the etching process, monolayer MoS2 with uniform morphology can be obtained. The Raman and photoluminescence spectra of the resulting material were similar to those of exfoliated MoS2. Utilizing this strategy, different patterns such as a Hall bar structure and a hexagonal array can be realized. Furthermore, the etching mechanism was studied by introducing graphene as an etching mask. We believe our technique opens an easy and controllable way of etching MoS2, which can be used to fabricate complex nanostructures, such as nanoribbons, quantum dots, and transistor structures. This etching process using XeF2 can also be extended to other interesting two-dimensional crystals. |
2012 |
Lim, Sharon Xiaodai; Koon, Gavin Kok Wai; Zhan, Da; Shen, Zexiang; Oezyilmaz, Barbaros; Sow, Chornghaur Assembly of suspended graphene on carbon nanotube scaffolds with improved functionalities Journal Article NANO RESEARCH, 5 (11), pp. 783-795, 2012, ISSN: 1998-0124. @article{ISI:000311397700004, title = {Assembly of suspended graphene on carbon nanotube scaffolds with improved functionalities}, author = {Sharon Xiaodai Lim and Gavin Kok Wai Koon and Da Zhan and Zexiang Shen and Barbaros Oezyilmaz and Chornghaur Sow}, doi = {10.1007/s12274-012-0262-x}, times_cited = {7}, issn = {1998-0124}, year = {2012}, date = {2012-11-01}, journal = {NANO RESEARCH}, volume = {5}, number = {11}, pages = {783-795}, publisher = {TSINGHUA UNIV PRESS}, address = {B605D, XUE YAN BUILDING, BEIJING, 100084, PEOPLES R CHINA}, abstract = {With self-assembly being an efficient and often preferred process to build micro- and nano-materials into ordered macroscopic structures, we report a simple method to assemble monolayer graphene onto densified vertically aligned carbon nanotube (CNT) micropillars en route to unique functional three-dimensional microarchitecture. This hybrid structure provides new means of studying strain induced in suspended graphene. The strain induced could be controlled by the size and number of supporting microstructures, as well as laser-initiated localised relaxation of the graphene sheet. The assembled structure is also able to withstand high-energy electron irradiation with negligible effect on the electrical properties of the hybrid system. The hybrid system was further functionalised with quantum dots on the CNTs with the assembled top graphene layer as a transparent electrode. Significant improvements in photocurrent were achieved in this system.}, keywords = {}, pubstate = {published}, tppubtype = {article} } With self-assembly being an efficient and often preferred process to build micro- and nano-materials into ordered macroscopic structures, we report a simple method to assemble monolayer graphene onto densified vertically aligned carbon nanotube (CNT) micropillars en route to unique functional three-dimensional microarchitecture. This hybrid structure provides new means of studying strain induced in suspended graphene. The strain induced could be controlled by the size and number of supporting microstructures, as well as laser-initiated localised relaxation of the graphene sheet. The assembled structure is also able to withstand high-energy electron irradiation with negligible effect on the electrical properties of the hybrid system. The hybrid system was further functionalised with quantum dots on the CNTs with the assembled top graphene layer as a transparent electrode. Significant improvements in photocurrent were achieved in this system. |