Kian Ping Loh
Degree: PhD
Position: Professor
Affiliation: NUS – Department of Chemistry
Research Type: Experiment
Office: MD1-14-03E
Email: chmlohkp@nus.edu.sg
Contact: (65) 6516 4402
Website: http://carbonlab.science.nus.edu.sg/
Research Interests:
2D materials growth and processing
Energy storage and conversion
Synthesis of 2D organic materials
Graphene
Optoelectronics
CA2DM Publications:
2023 |
Wang, Zishen; You, Jing-Yang; Chen, Chuan; Mo, Jinchao; He, Jingyu; Zhang, Lishu; Zhou, Jun; Loh, Kian Ping; Feng, Yuan Ping Interplay of the charge density wave transition with topological and superconducting properties Journal Article NANOSCALE HORIZONS, 8 (10), pp. 1395-1402, 2023, ISSN: 2055-6756. @article{ISI:001033202800001, title = {Interplay of the charge density wave transition with topological and superconducting properties}, author = {Zishen Wang and Jing-Yang You and Chuan Chen and Jinchao Mo and Jingyu He and Lishu Zhang and Jun Zhou and Kian Ping Loh and Yuan Ping Feng}, doi = {10.1039/d3nh00207a}, times_cited = {0}, issn = {2055-6756}, year = {2023}, date = {2023-07-18}, journal = {NANOSCALE HORIZONS}, volume = {8}, number = {10}, pages = {1395-1402}, publisher = {ROYAL SOC CHEMISTRY}, address = {THOMAS GRAHAM HOUSE, SCIENCE PARK, MILTON RD, CAMBRIDGE CB4 0WF, CAMBS, ENGLAND}, abstract = {Exotic phenomena due to the interplay of different quantum orders have been observed and the study of these phenomena has emerged as a new frontier in condensed matter research, especially in the two-dimensional limit. Here, we report the coexistence of charge density waves (CDWs), superconductivity, and nontrivial topology in monolayer 1H-MSe2 (M = Nb, Ta) triggered by momentum-dependent electron-phonon coupling through electron doping. At a critical electron doping concentration, new 2 x 2 CDW phases emerge with nontrivial topology, Dirac cones, and van Hove singularities. Interestingly, these 2 x 2 CDW phases are also superconducting. Our findings not only reveal a route towards realizing nontrivial electronic characters by CDW engineering, but also provide an exciting platform to modulate different quantum states at the confluence of CDWs, superconductivity, nontrivial topology, and electron-phonon coupling.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Exotic phenomena due to the interplay of different quantum orders have been observed and the study of these phenomena has emerged as a new frontier in condensed matter research, especially in the two-dimensional limit. Here, we report the coexistence of charge density waves (CDWs), superconductivity, and nontrivial topology in monolayer 1H-MSe2 (M = Nb, Ta) triggered by momentum-dependent electron-phonon coupling through electron doping. At a critical electron doping concentration, new 2 x 2 CDW phases emerge with nontrivial topology, Dirac cones, and van Hove singularities. Interestingly, these 2 x 2 CDW phases are also superconducting. Our findings not only reveal a route towards realizing nontrivial electronic characters by CDW engineering, but also provide an exciting platform to modulate different quantum states at the confluence of CDWs, superconductivity, nontrivial topology, and electron-phonon coupling. |
Liu, Chaofei; Zhang, Xiuying; Wang, Xinyun; Wang, Ziying; Abdelwahab, Ibrahim; Verzhbitskiy, Ivan; Shao, Yan; Eda, Goki; Sun, Wanxin; Shen, Lei; Loh, Kian Ping Ferroelectricity in Niobium Oxide Dihalides NbOX2 (X = Cl, I): A Macroscopic- to Microscopic-Scale Study Journal Article ACS NANO, 17 (8), pp. 7170-7179, 2023, ISSN: 1936-0851. @article{ISI:000969846300001, title = {Ferroelectricity in Niobium Oxide Dihalides NbOX2 (X = Cl, I): A Macroscopic- to Microscopic-Scale Study}, author = {Chaofei Liu and Xiuying Zhang and Xinyun Wang and Ziying Wang and Ibrahim Abdelwahab and Ivan Verzhbitskiy and Yan Shao and Goki Eda and Wanxin Sun and Lei Shen and Kian Ping Loh}, doi = {10.1021/acsnano.2c09267}, times_cited = {0}, issn = {1936-0851}, year = {2023}, date = {2023-04-10}, journal = {ACS NANO}, volume = {17}, number = {8}, pages = {7170-7179}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {2D materials with ferroelectric and piezoelectric properties are of interest for energy harvesting, memory storage and electromechanical systems. Here, we present a systematic study of the ferroelectric properties in NbOX2 (X = Cl, I) across different spatial scales. The in-plane ferroelectricity in NbOX2 was investigated using transport and piezoresponse force microscopy (PFM) measurements, where it was observed that NbOCl2 has a stronger ferroelectric order than NbOI2. A high local field, exerted by both PFM and scanning tunneling microscopy (STM) tips, was found to induce 1D collinear ferroelectric strips in NbOCl2. STM imaging reveals the unreconstructed atomic structures of NbOX2 surfaces, and scanning tunneling spectroscopy was used to probe the electronic states induced at defect (vacancy) sites.}, keywords = {}, pubstate = {published}, tppubtype = {article} } 2D materials with ferroelectric and piezoelectric properties are of interest for energy harvesting, memory storage and electromechanical systems. Here, we present a systematic study of the ferroelectric properties in NbOX2 (X = Cl, I) across different spatial scales. The in-plane ferroelectricity in NbOX2 was investigated using transport and piezoresponse force microscopy (PFM) measurements, where it was observed that NbOCl2 has a stronger ferroelectric order than NbOI2. A high local field, exerted by both PFM and scanning tunneling microscopy (STM) tips, was found to induce 1D collinear ferroelectric strips in NbOCl2. STM imaging reveals the unreconstructed atomic structures of NbOX2 surfaces, and scanning tunneling spectroscopy was used to probe the electronic states induced at defect (vacancy) sites. |
Chen, Zehong; Peng, Xinwen; Chen, Zhongxin; Li, Tingzhen; Zou, Ren; Shi, Ge; Huang, Yongfa; Cui, Peng; Yu, Jian; Chen, Yuling; Chi, Xiao; Loh, Kian Ping; Liu, Zhaoqing; Li, Xuehui; Zhong, Linxin; Lu, Jun Mass Production of Sulfur-Tuned Single-Atom Catalysts for Zn-Air Batteries Journal Article ADVANCED MATERIALS, 35 (15), 2023, ISSN: 0935-9648. @article{ISI:000941941800001, title = {Mass Production of Sulfur-Tuned Single-Atom Catalysts for Zn-Air Batteries}, author = {Zehong Chen and Xinwen Peng and Zhongxin Chen and Tingzhen Li and Ren Zou and Ge Shi and Yongfa Huang and Peng Cui and Jian Yu and Yuling Chen and Xiao Chi and Kian Ping Loh and Zhaoqing Liu and Xuehui Li and Linxin Zhong and Jun Lu}, doi = {10.1002/adma.202209948}, times_cited = {0}, issn = {0935-9648}, year = {2023}, date = {2023-03-02}, journal = {ADVANCED MATERIALS}, volume = {35}, number = {15}, publisher = {WILEY-V C H VERLAG GMBH}, address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY}, abstract = {Single-atom catalysts (SACs) show great potential for rechargeable Zn-air batteries (ZABs); however, scalable production of SACs from sustainable resources is difficult owing to poor control of the local coordination environment. Herein, lignosulfonate, a by-product of the papermaking industry, is utilized as a multifunctional bioligand for the mass production of SACs with highly active M-N-4-S sites (M represents Fe, Cu, and Co) via strong metal-nitrogen/sulfur coordination. This effectively adjusts the charge distribution and promotes the catalytic performance, leading to highly durable and excellent performance in oxygen reduction and evolution reactions for ZABs. This study paves the way for the industrial production of cost-effective SACs in a sustainable manner.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Single-atom catalysts (SACs) show great potential for rechargeable Zn-air batteries (ZABs); however, scalable production of SACs from sustainable resources is difficult owing to poor control of the local coordination environment. Herein, lignosulfonate, a by-product of the papermaking industry, is utilized as a multifunctional bioligand for the mass production of SACs with highly active M-N-4-S sites (M represents Fe, Cu, and Co) via strong metal-nitrogen/sulfur coordination. This effectively adjusts the charge distribution and promotes the catalytic performance, leading to highly durable and excellent performance in oxygen reduction and evolution reactions for ZABs. This study paves the way for the industrial production of cost-effective SACs in a sustainable manner. |
Abdelwahab, Ibrahim; Tilmann, Benjamin; Zhao, Xiaoxu; Verzhbitskiy, Ivan; Berte, Rodrigo; Eda, Goki; Wilson, William L; Grinblat, Gustavo; de Menezes, Leonardo S; Loh, Kian Ping; Maier, Stefan A Highly Efficient Sum-Frequency Generation in Niobium Oxydichloride NbOCl2 Nanosheets Journal Article ADVANCED OPTICAL MATERIALS, 11 (7), 2023, ISSN: 2195-1071. @article{ISI:000928804500001, title = {Highly Efficient Sum-Frequency Generation in Niobium Oxydichloride NbOCl_{2} Nanosheets}, author = {Ibrahim Abdelwahab and Benjamin Tilmann and Xiaoxu Zhao and Ivan Verzhbitskiy and Rodrigo Berte and Goki Eda and William L Wilson and Gustavo Grinblat and Leonardo S de Menezes and Kian Ping Loh and Stefan A Maier}, doi = {10.1002/adom.202202833}, times_cited = {0}, issn = {2195-1071}, year = {2023}, date = {2023-02-05}, journal = {ADVANCED OPTICAL MATERIALS}, volume = {11}, number = {7}, publisher = {WILEY-V C H VERLAG GMBH}, address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY}, abstract = {Parametric infrared (IR) upconversion is a process in which low-frequency IR photons are upconverted into high-frequency ultraviolet/visible photons through a nonlinear optical process. It is of paramount importance for a wide range of security, material science, and healthcare applications. However, in general, the efficiencies of upconversion processes are typically extremely low for nanometer-scale materials due to the short penetration depth of the excitation fields. Here, parametric IR upconversion processes, including frequency doubling and sum-frequency generation, are studied in layered van der Waals NbOCl2. An upconversion efficiency of up to 0.004% is attained for the NbOCl2 nanosheets, orders of magnitude higher than previously reported values for nonlinear layered materials. The upconverted signal is sensitive to layer numbers, crystal orientation, excitation wavelength, and temperature, and it can be utilized as an optical cross-correlator for ultrashort pulse characterization.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Parametric infrared (IR) upconversion is a process in which low-frequency IR photons are upconverted into high-frequency ultraviolet/visible photons through a nonlinear optical process. It is of paramount importance for a wide range of security, material science, and healthcare applications. However, in general, the efficiencies of upconversion processes are typically extremely low for nanometer-scale materials due to the short penetration depth of the excitation fields. Here, parametric IR upconversion processes, including frequency doubling and sum-frequency generation, are studied in layered van der Waals NbOCl2. An upconversion efficiency of up to 0.004% is attained for the NbOCl2 nanosheets, orders of magnitude higher than previously reported values for nonlinear layered materials. The upconverted signal is sensitive to layer numbers, crystal orientation, excitation wavelength, and temperature, and it can be utilized as an optical cross-correlator for ultrashort pulse characterization. |
2022 |
Chi, Xiao; Mandal, Lily; Liu, Cuibo; Fauzi, Angga Dito; Chaudhuri, Anindita; Whitcher, Thomas J; Jani, Hariom Kirit; Chen, Zhongxin; Xi, Shibo; Diao, Caozheng; Naradipa, Muhammad Avicenna; Yu, Xiaojiang; Yang, Ping; Castro-Neto, Antonio Helio; Breese, Mark B H; Loh, Kian Ping; Venkatesan, Thirumalai Venky; Rusydi, Andrivo Unravelling a new many-body large-hole polaron in a transition metal oxide that promotes high photocatalytic activity Journal Article NPG ASIA MATERIALS, 14 (1), 2022, ISSN: 1884-4049. @article{ISI:000764082700003, title = {Unravelling a new many-body large-hole polaron in a transition metal oxide that promotes high photocatalytic activity}, author = {Xiao Chi and Lily Mandal and Cuibo Liu and Angga Dito Fauzi and Anindita Chaudhuri and Thomas J Whitcher and Hariom Kirit Jani and Zhongxin Chen and Shibo Xi and Caozheng Diao and Muhammad Avicenna Naradipa and Xiaojiang Yu and Ping Yang and Antonio Helio Castro-Neto and Mark B H Breese and Kian Ping Loh and Thirumalai Venky Venkatesan and Andrivo Rusydi}, doi = {10.1038/s41427-022-00364-w}, times_cited = {0}, issn = {1884-4049}, year = {2022}, date = {2022-12-01}, journal = {NPG ASIA MATERIALS}, volume = {14}, number = {1}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {A many-body large polaron, which is important for both fundamental physics and technological applications, has been predicted to occur in bismuth vanadate (BiVO4). Herein, using a combination of high-resolution spectroscopic ellipsometry, X-ray absorption spectroscopy at the V L-3,L-2- and O K-edges, and high-resolution X-ray diffraction supported by theoretical calculations, we reveal a new many-body large-hole polaron in W-doped BiVO4 films and the interplay of the large-hole polaron and indirect bandgap when determining the photocatalytic activity. With various W doping concentrations and temperatures, anomalous spectral weight transfers in the complex dielectric function are observed, revealing electronic correlations, particularly the on-site Coulomb interactions of O p (U-pp) and V d (U-dd), and screening in BiVO4. Due to the distortion of BiO8 dodecahedra and U-dd, Bi 6 s is lifted to the top of the valance band, which results in the formation of an indirect bandgap and a large-hole polaron. The large-hole polaron is found to form as a localized midgap state, consisting of O p hybridized with the V d and Bi sp orbitals, and this is important when determining the high photocatalytic activity of BiVO4 Our results show the importance of the interplay among the charge, orbital, and lattice degrees of freedom in forming the many-body large-hole polaron, which improves the conductivity and results in a transition metal oxide with high photocatalytic activity.}, keywords = {}, pubstate = {published}, tppubtype = {article} } A many-body large polaron, which is important for both fundamental physics and technological applications, has been predicted to occur in bismuth vanadate (BiVO4). Herein, using a combination of high-resolution spectroscopic ellipsometry, X-ray absorption spectroscopy at the V L-3,L-2- and O K-edges, and high-resolution X-ray diffraction supported by theoretical calculations, we reveal a new many-body large-hole polaron in W-doped BiVO4 films and the interplay of the large-hole polaron and indirect bandgap when determining the photocatalytic activity. With various W doping concentrations and temperatures, anomalous spectral weight transfers in the complex dielectric function are observed, revealing electronic correlations, particularly the on-site Coulomb interactions of O p (U-pp) and V d (U-dd), and screening in BiVO4. Due to the distortion of BiO8 dodecahedra and U-dd, Bi 6 s is lifted to the top of the valance band, which results in the formation of an indirect bandgap and a large-hole polaron. The large-hole polaron is found to form as a localized midgap state, consisting of O p hybridized with the V d and Bi sp orbitals, and this is important when determining the high photocatalytic activity of BiVO4 Our results show the importance of the interplay among the charge, orbital, and lattice degrees of freedom in forming the many-body large-hole polaron, which improves the conductivity and results in a transition metal oxide with high photocatalytic activity. |
Telychko, Mykola; Noori, Keian; Biswas, Hillol; Dulal, Dikshant; Chen, Zhaolong; Lyu, Pin; Li, Jing; Tsai, Hsin-Zon; Fang, Hanyan; Qiu, Zhizhan; Yap, Zhun Wai; Watanabe, Kenji; Taniguchi, Takashi; Wu, Jing; Loh, Kian Ping; Crommie, Michael F; Rodin, Aleksandr; Lu, Jiong Gate-Tunable Resonance State and Screening Effects for Proton-Like Atomic Charge in Graphene Journal Article NANO LETTERS, 2022, ISSN: 1530-6984. @article{ISI:000871063800001, title = {Gate-Tunable Resonance State and Screening Effects for Proton-Like Atomic Charge in Graphene}, author = {Mykola Telychko and Keian Noori and Hillol Biswas and Dikshant Dulal and Zhaolong Chen and Pin Lyu and Jing Li and Hsin-Zon Tsai and Hanyan Fang and Zhizhan Qiu and Zhun Wai Yap and Kenji Watanabe and Takashi Taniguchi and Jing Wu and Kian Ping Loh and Michael F Crommie and Aleksandr Rodin and Jiong Lu}, doi = {10.1021/acs.nanolett.2c02235}, times_cited = {0}, issn = {1530-6984}, year = {2022}, date = {2022-10-10}, journal = {NANO LETTERS}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {The ability to create a robust and well-defined artificial atomic charge in graphene and understand its carrier dependent electronic properties represents an important goal toward the development of graphene-based quantum devices. Herein, we devise a new pathway toward the atomically precise embodiment of point charges into a graphene lattice by posterior (N) ion implantation into a back-gated graphene device. The N dopant behaves as an in-plane proton-like charge manifested by formation of the characteristic resonance state in the conduction band. Scanning tunneling spectroscopy measurements at varied charge carrier densities reveal a giant energetic renormalization of the resonance state up to 220 meV with respect to the Dirac point, accompanied by the observation of gate-tunable long-range screening effects close to individual N dopants. Joint density functional theory and tight-binding calculations with modified perturbation potential corroborate experimental findings and highlight the short-range character of N-induced perturbation.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The ability to create a robust and well-defined artificial atomic charge in graphene and understand its carrier dependent electronic properties represents an important goal toward the development of graphene-based quantum devices. Herein, we devise a new pathway toward the atomically precise embodiment of point charges into a graphene lattice by posterior (N) ion implantation into a back-gated graphene device. The N dopant behaves as an in-plane proton-like charge manifested by formation of the characteristic resonance state in the conduction band. Scanning tunneling spectroscopy measurements at varied charge carrier densities reveal a giant energetic renormalization of the resonance state up to 220 meV with respect to the Dirac point, accompanied by the observation of gate-tunable long-range screening effects close to individual N dopants. Joint density functional theory and tight-binding calculations with modified perturbation potential corroborate experimental findings and highlight the short-range character of N-induced perturbation. |
Wang, Wenhui; Ghosh, Tanmay; Yan, Hongwei; Erofeev, Ivan; Zhang, Kun; Loh, Kian Ping; Mirsaidov, Utkur The Growth Dynamics of Organic-Inorganic Metal Halide Perovskite Films Journal Article JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 144 (39), pp. 17848-17856, 2022, ISSN: 0002-7863. @article{ISI:000861815500001, title = {The Growth Dynamics of Organic-Inorganic Metal Halide Perovskite Films}, author = {Wenhui Wang and Tanmay Ghosh and Hongwei Yan and Ivan Erofeev and Kun Zhang and Kian Ping Loh and Utkur Mirsaidov}, doi = {10.1021/jacs.2c06022}, times_cited = {0}, issn = {0002-7863}, year = {2022}, date = {2022-09-21}, journal = {JOURNAL OF THE AMERICAN CHEMICAL SOCIETY}, volume = {144}, number = {39}, pages = {17848-17856}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Organic-inorganic metal halide perovskite films have emerged as potential candidate materials for photoelectric devices because of their superior optoelectronic properties. The performance of these devices depends on the quality of perovskite films defined by their grain size, crystallinity, and absence of pinholes. While solution-based processing is the most cost-effective and scalable approach to producing these films, the impact of the process parameters on the film quality and nanoscale details of these processes are unknown. Specifically, it is unclear how perovskites grow from a liquid precursor to form solid-phase nanocrystals and how these nanocrystals arrange to form a uniform film. Here, using liquid-phase transmission electron microscopy (TEM), we show how perovskite nanocrystals nucleate from a precursor solution and then grow and coalesce to form a polycrystalline film. Furthermore, we show how additives, such as urea, can improve the film crystallinity by increasing perovskite solubility, which induces the dissolution and subsequent redeposition of smaller crystals onto larger grains. Our approach to studying the growth of perovskite films provides an important insight into improving the synthesis of perovskites and other technologically relevant crystalline films.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Organic-inorganic metal halide perovskite films have emerged as potential candidate materials for photoelectric devices because of their superior optoelectronic properties. The performance of these devices depends on the quality of perovskite films defined by their grain size, crystallinity, and absence of pinholes. While solution-based processing is the most cost-effective and scalable approach to producing these films, the impact of the process parameters on the film quality and nanoscale details of these processes are unknown. Specifically, it is unclear how perovskites grow from a liquid precursor to form solid-phase nanocrystals and how these nanocrystals arrange to form a uniform film. Here, using liquid-phase transmission electron microscopy (TEM), we show how perovskite nanocrystals nucleate from a precursor solution and then grow and coalesce to form a polycrystalline film. Furthermore, we show how additives, such as urea, can improve the film crystallinity by increasing perovskite solubility, which induces the dissolution and subsequent redeposition of smaller crystals onto larger grains. Our approach to studying the growth of perovskite films provides an important insight into improving the synthesis of perovskites and other technologically relevant crystalline films. |
Ma, Teng; Chen, Hao; Yananose, Kunihiro; Zhou, Xin; Wang, Lin; Li, Runlai; Zhu, Ziyu; Wu, Zhenyue; Xu, Qing-Hua; Yu, Jaejun; Qiu, Cheng Wei; Stroppa, Alessandro; Loh, Kian Ping Growth of bilayer MoTe2 single crystals with strong non-linear Hall effect Journal Article NATURE COMMUNICATIONS, 13 (1), 2022. @article{ISI:000854873600017, title = {Growth of bilayer MoTe_{2} single crystals with strong non-linear Hall effect}, author = {Teng Ma and Hao Chen and Kunihiro Yananose and Xin Zhou and Lin Wang and Runlai Li and Ziyu Zhu and Zhenyue Wu and Qing-Hua Xu and Jaejun Yu and Cheng Wei Qiu and Alessandro Stroppa and Kian Ping Loh}, doi = {10.1038/s41467-022-33201-3}, times_cited = {0}, year = {2022}, date = {2022-09-17}, journal = {NATURE COMMUNICATIONS}, volume = {13}, number = {1}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {The reduced symmetry in strong spin-orbit coupling materials such as transition metal ditellurides (TMDTs) gives rise to non-trivial topology, unique spin texture, and large charge-to-spin conversion efficiencies. Bilayer TMDTs are non-centrosymmetric and have unique topological properties compared to monolayer or trilayer, but a controllable way to prepare bilayer MoTe2 crystal has not been achieved to date. Herein, we achieve the layer-by-layer growth of large-area bilayer and trilayer 1T ' MoTe2 single crystals and centimetre-scale films by a two-stage chemical vapor deposition process. The as-grown bilayer MoTe2 shows out-of-plane ferroelectric polarization, whereas the monolayer and trilayer crystals are non-polar. In addition, we observed large in-plane nonlinear Hall (NLH) effect for the bilayer and trilayer T-d phase MoTe2 under time reversal-symmetric conditions, while these vanish for thicker layers. For a fixed input current, bilayer T-d MoTe2 produces the largest second harmonic output voltage among the thicker crystals tested. Our work therefore highlights the importance of thickness-dependent Berry curvature effects in TMDTs that are underscored by the ability to grow thickness-precise layers.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The reduced symmetry in strong spin-orbit coupling materials such as transition metal ditellurides (TMDTs) gives rise to non-trivial topology, unique spin texture, and large charge-to-spin conversion efficiencies. Bilayer TMDTs are non-centrosymmetric and have unique topological properties compared to monolayer or trilayer, but a controllable way to prepare bilayer MoTe2 crystal has not been achieved to date. Herein, we achieve the layer-by-layer growth of large-area bilayer and trilayer 1T ' MoTe2 single crystals and centimetre-scale films by a two-stage chemical vapor deposition process. The as-grown bilayer MoTe2 shows out-of-plane ferroelectric polarization, whereas the monolayer and trilayer crystals are non-polar. In addition, we observed large in-plane nonlinear Hall (NLH) effect for the bilayer and trilayer T-d phase MoTe2 under time reversal-symmetric conditions, while these vanish for thicker layers. For a fixed input current, bilayer T-d MoTe2 produces the largest second harmonic output voltage among the thicker crystals tested. Our work therefore highlights the importance of thickness-dependent Berry curvature effects in TMDTs that are underscored by the ability to grow thickness-precise layers. |
Wang, Ziying; Wang, Zishen; Feng, Yuan Ping; Loh, Kian Ping Probing the Origin of Chiral Charge Density Waves in the Two-Dimensional Limits Journal Article NANO LETTERS, 2022, ISSN: 1530-6984. @article{ISI:000856557800001, title = {Probing the Origin of Chiral Charge Density Waves in the Two-Dimensional Limits}, author = {Ziying Wang and Zishen Wang and Yuan Ping Feng and Kian Ping Loh}, doi = {10.1021/acs.nanolett.2c02723}, times_cited = {0}, issn = {1530-6984}, year = {2022}, date = {2022-09-13}, journal = {NANO LETTERS}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Chirality generates spontaneous symmetry breaking and profoundly influences the topology, charge, and spin orders of materials. The chiral charge density wave (CDW) exhibits macroscopic chirality in the achiral crystal during the spontaneous electronic phase transitions. However, the mechanism of chiral CDW formation is shrouded in controversy. In this work, we report that two-dimensional H-phase TaS2 synthesized by molecular-beam epitaxy (MBE) shows a predominantly chiral CDW phase. Scanning tunneling microscopy (STM) imaging of the CDW reconstruction spots reveals a clockwise or anticlockwise intensity variation along the STM-imaged spots. First-principles calculations further show that the rotational symmetry of the momentum-dependent electronphonon coupling is broken, giving rise to chirality. Our work provides new insights into the physical origin of the chiral charge-ordered states, shedding light on a general ordering rule in chiral CDWs.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Chirality generates spontaneous symmetry breaking and profoundly influences the topology, charge, and spin orders of materials. The chiral charge density wave (CDW) exhibits macroscopic chirality in the achiral crystal during the spontaneous electronic phase transitions. However, the mechanism of chiral CDW formation is shrouded in controversy. In this work, we report that two-dimensional H-phase TaS2 synthesized by molecular-beam epitaxy (MBE) shows a predominantly chiral CDW phase. Scanning tunneling microscopy (STM) imaging of the CDW reconstruction spots reveals a clockwise or anticlockwise intensity variation along the STM-imaged spots. First-principles calculations further show that the rotational symmetry of the momentum-dependent electronphonon coupling is broken, giving rise to chirality. Our work provides new insights into the physical origin of the chiral charge-ordered states, shedding light on a general ordering rule in chiral CDWs. |
Wang, Ziying; Wang, Zishen; Zhou, Xin; Fu, Wei; Li, Haohan; Liu, Chaofei; Qiao, Jingsi; Quek, Su Ying; Su, Chenliang; Feng, Yuanping; Loh, Kian Ping Giant g-factor in Self-Intercalated 2D TaS2 Journal Article SMALL, 18 (38), 2022, ISSN: 1613-6810. @article{ISI:000842350300001, title = {Giant \textit{g}-factor in Self-Intercalated 2D TaS_{2}}, author = {Ziying Wang and Zishen Wang and Xin Zhou and Wei Fu and Haohan Li and Chaofei Liu and Jingsi Qiao and Su Ying Quek and Chenliang Su and Yuanping Feng and Kian Ping Loh}, doi = {10.1002/smll.202201975}, times_cited = {0}, issn = {1613-6810}, year = {2022}, date = {2022-08-21}, journal = {SMALL}, volume = {18}, number = {38}, publisher = {WILEY-V C H VERLAG GMBH}, address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY}, abstract = {Central to the application of spintronic devices is the ability to manipulate spins by electric and magnetic fields, which relies on a large Lande g-factor. The self-intercalation of layered transitional metal dichalcogenides with native metal atoms can serve as a new strategy to enhance the g-factor by inducing ferromagnetic instability in the system via interlayer charge transfer. Here, scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) are performed to extract the g-factor and characterize the electronic structure of the self-intercalated phase of 2H-TaS2. In Ta7S12, a sharp density of states (DOS) peak due to the Ta intercalant appears at the Fermi level, which satisfies the Stoner criteria for spontaneous ferromagnetism, leading to spin split states. The DOS peak shows sensitivity to magnetic field up to 1.85 mV T-1, equivalent to an effective g-factor of approximate to 77. This work establishes self-intercalation as an approach for tuning the g-factor.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Central to the application of spintronic devices is the ability to manipulate spins by electric and magnetic fields, which relies on a large Lande g-factor. The self-intercalation of layered transitional metal dichalcogenides with native metal atoms can serve as a new strategy to enhance the g-factor by inducing ferromagnetic instability in the system via interlayer charge transfer. Here, scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) are performed to extract the g-factor and characterize the electronic structure of the self-intercalated phase of 2H-TaS2. In Ta7S12, a sharp density of states (DOS) peak due to the Ta intercalant appears at the Fermi level, which satisfies the Stoner criteria for spontaneous ferromagnetism, leading to spin split states. The DOS peak shows sensitivity to magnetic field up to 1.85 mV T-1, equivalent to an effective g-factor of approximate to 77. This work establishes self-intercalation as an approach for tuning the g-factor. |
Jiang, Yucheng; He, Anpeng; Luo, Kai; Zhang, Jinlei; Liu, Guozhen; Zhao, Run; Zhang, Qing; Wang, Zhuo; Zhao, Chen; Wang, Lin; Qi, Yaping; Gao, Ju; Loh, Kian Ping; Wee, Andrew T S; Qiu, Cheng-Wei Giant bipolar unidirectional photomagnetoresistance Journal Article PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 119 (27), 2022, ISSN: 0027-8424. @article{ISI:000838706400010, title = {Giant bipolar unidirectional photomagnetoresistance}, author = {Yucheng Jiang and Anpeng He and Kai Luo and Jinlei Zhang and Guozhen Liu and Run Zhao and Qing Zhang and Zhuo Wang and Chen Zhao and Lin Wang and Yaping Qi and Ju Gao and Kian Ping Loh and Andrew T S Wee and Cheng-Wei Qiu}, doi = {10.1073/pnas.2115939119}, times_cited = {0}, issn = {0027-8424}, year = {2022}, date = {2022-07-05}, journal = {PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA}, volume = {119}, number = {27}, publisher = {NATL ACAD SCIENCES}, address = {2101 CONSTITUTION AVE NW, WASHINGTON, DC 20418 USA}, abstract = {Positive magnetoresistance (PMR) and negative magnetoresistance (NMR) describe two opposite responses of resistance induced by a magnetic field. Materials with giant PMR are usually distinct from those with giant NMR due to different physical natures. Here, we report the unusual photomagnetoresistance in the van der Waals heterojunctions of WSe2/quasi-two-dimensional electron gas, showing the coexistence of giant PMR and giant NMR. The PMR and NMR reach 1,007.5% at -9 T and -93.5% at 2.2 T in a single device, respectively. The magnetoresistance spans over two orders of magnitude on inversion of field direction, implying a giant unidirectional magnetoresistance (UMR). By adjusting the thickness of the WSe2 layer, we achieve the maxima of PMR and NMR, which are 4,900,000% and -99.8%, respectively. The unique magnetooptical transport shows the unity of giant UMR, PMR, and NMR, referred to as giant bipolar unidirectional photomagnetoresistance. These features originate from strong out-of-plane spin splitting, magnetic field-enhanced recombination of photocarriers, and the Zeeman effect through our experimental and theoretical investigations. This work offers directions for high-performance light-tunable spintronic devices.NMR)}, keywords = {}, pubstate = {published}, tppubtype = {article} } Positive magnetoresistance (PMR) and negative magnetoresistance (NMR) describe two opposite responses of resistance induced by a magnetic field. Materials with giant PMR are usually distinct from those with giant NMR due to different physical natures. Here, we report the unusual photomagnetoresistance in the van der Waals heterojunctions of WSe2/quasi-two-dimensional electron gas, showing the coexistence of giant PMR and giant NMR. The PMR and NMR reach 1,007.5% at -9 T and -93.5% at 2.2 T in a single device, respectively. The magnetoresistance spans over two orders of magnitude on inversion of field direction, implying a giant unidirectional magnetoresistance (UMR). By adjusting the thickness of the WSe2 layer, we achieve the maxima of PMR and NMR, which are 4,900,000% and -99.8%, respectively. The unique magnetooptical transport shows the unity of giant UMR, PMR, and NMR, referred to as giant bipolar unidirectional photomagnetoresistance. These features originate from strong out-of-plane spin splitting, magnetic field-enhanced recombination of photocarriers, and the Zeeman effect through our experimental and theoretical investigations. This work offers directions for high-performance light-tunable spintronic devices.NMR) |
Abdelwahab, Ibrahim; Tilmann, Benjamin; Wu, Yaze; Giovanni, David; Verzhbitskiy, Ivan; Zhu, Menglong; Berte, Rodrigo; Xuan, Fengyuan; de Menezes, Leonardo S; Eda, Goki; Sum, Tze Chien; Quek, Su Ying; Maier, Stefan A; Loh, Kian Ping Giant second-harmonic generation in ferroelectric NbOI2 Journal Article NATURE PHOTONICS, 16 (9), pp. 644-+, 2022, ISSN: 1749-4885. @article{ISI:000819314600001, title = {Giant second-harmonic generation in ferroelectric NbOI_{2}}, author = {Ibrahim Abdelwahab and Benjamin Tilmann and Yaze Wu and David Giovanni and Ivan Verzhbitskiy and Menglong Zhu and Rodrigo Berte and Fengyuan Xuan and Leonardo S de Menezes and Goki Eda and Tze Chien Sum and Su Ying Quek and Stefan A Maier and Kian Ping Loh}, doi = {10.1038/s41566-022-01021-y}, times_cited = {0}, issn = {1749-4885}, year = {2022}, date = {2022-06-30}, journal = {NATURE PHOTONICS}, volume = {16}, number = {9}, pages = {644-+}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {Implementing nonlinear optical components in nanoscale photonic devices is challenged by phase-matching conditions requiring thicknesses in the order of hundreds of wavelengths, and is disadvantaged by the short optical interaction depth of nanometre-scale materials and weak photon-photon interactions. Here we report that ferroelectric NbOI2 nanosheets exhibit giant second-harmonic generation with conversion efficiencies that are orders of magnitude higher than commonly reported nonlinear crystals. The nonlinear response scales with layer thickness and is strain- and electrical-tunable; a record >0.2% absolute SHG conversion efficiency and an effective nonlinear susceptibility chi((2))(eff) in the order of 10(-9) m V-1 are demonstrated at an average pump intensity of 8 kW cm(-2). Due to the interplay between anisotropic polarization and excitonic resonance in NbOI2, the spatial profile of the polarized SHG response can be tuned by the excitation wavelength. Our results represent a new paradigm for ultrathin, efficient nonlinear optical components.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Implementing nonlinear optical components in nanoscale photonic devices is challenged by phase-matching conditions requiring thicknesses in the order of hundreds of wavelengths, and is disadvantaged by the short optical interaction depth of nanometre-scale materials and weak photon-photon interactions. Here we report that ferroelectric NbOI2 nanosheets exhibit giant second-harmonic generation with conversion efficiencies that are orders of magnitude higher than commonly reported nonlinear crystals. The nonlinear response scales with layer thickness and is strain- and electrical-tunable; a record >0.2% absolute SHG conversion efficiency and an effective nonlinear susceptibility chi((2))(eff) in the order of 10(-9) m V-1 are demonstrated at an average pump intensity of 8 kW cm(-2). Due to the interplay between anisotropic polarization and excitonic resonance in NbOI2, the spatial profile of the polarized SHG response can be tuned by the excitation wavelength. Our results represent a new paradigm for ultrathin, efficient nonlinear optical components. |
Wang, Xinyun; Zhao, Yuzhou; Kong, Xiao; Zhang, Qi; Ng, Hong Kuan; Lim, Sharon Xiaodai; Zheng, Yue; Wu, Xiao; Watanabe, Kenji; Xu, Qing-Hua; Taniguchi, Takashi; Eda, Goki; Goh, Kuan Eng Johnson; Jin, Song; Loh, Kian Ping; Ding, Feng; Sun, Wanxin; Sow, Chorng Haur Dynamic Tuning of Moire Superlattice Morphology by Laser Modification Journal Article ACS NANO , 16 (5), pp. 8172-8180, 2022, ISSN: 1936-0851. @article{ISI:000812148900091, title = {Dynamic Tuning of Moire Superlattice Morphology by Laser Modification }, author = {Xinyun Wang and Yuzhou Zhao and Xiao Kong and Qi Zhang and Hong Kuan Ng and Sharon Xiaodai Lim and Yue Zheng and Xiao Wu and Kenji Watanabe and Qing-Hua Xu and Takashi Taniguchi and Goki Eda and Kuan Eng Johnson Goh and Song Jin and Kian Ping Loh and Feng Ding and Wanxin Sun and Chorng Haur Sow}, doi = {10.1021/acsnano.2c01625}, times_cited = {0}, issn = {1936-0851}, year = {2022}, date = {2022-05-24}, journal = {ACS NANO }, volume = {16}, number = {5}, pages = {8172-8180}, publisher = {AMER CHEMICAL SOC }, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA }, abstract = {In artificial van der Waals (vdW) layered devices, twisting the stacking angle has emerged as an effective strategy to regulate the electronic phases and optical properties of these systems. Along with the twist registry, the lattice reconstruction arising from vdW interlayer interaction has also inspired significant research interests. The control of twist angles is significantly important because the moire periodicity determines the electron propagation length on the lattice and the interlayer electron-electron interactions. However, the moire periodicity is hard to be modified after the device has been fabricated. In this work, we have demonstrated that the moire periodicity can be precisely modulated with a localized laser annealing technique. This is achieved with regulating the interlayer lattice mismatch by the mismatched lattice constant, which originates from the variable density of sulfur vacancy generated during laser modification. The existence of sulfur vacancy is further verified by excitonic emission energy and lifetime in photoluminescence measurements. Furthermore, we also discover that the mismatched lattice constant has the equivalent contribution as the twist angle for determining the lattice mismatch. Theoretical modeling elaborates the moire-wavelength-dependent energy variations at the interface and mimics the evolution of moire morphology. }, keywords = {}, pubstate = {published}, tppubtype = {article} } In artificial van der Waals (vdW) layered devices, twisting the stacking angle has emerged as an effective strategy to regulate the electronic phases and optical properties of these systems. Along with the twist registry, the lattice reconstruction arising from vdW interlayer interaction has also inspired significant research interests. The control of twist angles is significantly important because the moire periodicity determines the electron propagation length on the lattice and the interlayer electron-electron interactions. However, the moire periodicity is hard to be modified after the device has been fabricated. In this work, we have demonstrated that the moire periodicity can be precisely modulated with a localized laser annealing technique. This is achieved with regulating the interlayer lattice mismatch by the mismatched lattice constant, which originates from the variable density of sulfur vacancy generated during laser modification. The existence of sulfur vacancy is further verified by excitonic emission energy and lifetime in photoluminescence measurements. Furthermore, we also discover that the mismatched lattice constant has the equivalent contribution as the twist angle for determining the lattice mismatch. Theoretical modeling elaborates the moire-wavelength-dependent energy variations at the interface and mimics the evolution of moire morphology. |
Wang, Xinyun; Zhao, Yuzhou; Kong, Xiao; Zhang, Qi; Ng, Hong Kuan; Lim, Sharon Xiaodai; Zheng, Yue; Wu, Xiao; Watanabe, Kenji; Xu, Qing-Hua; Taniguchi, Takashi; Eda, Goki; Goh, Kuan Eng Johnson; Jin, Song; Loh, Kian Ping; Ding, Feng; Sun, Wanxin; Sow, Chorng Haur Dynamic Tuning of Moire Superlattice Morphology by Laser Modification Journal Article ACS NANO, 16 (5), pp. 8172-8180, 2022, ISSN: 1936-0851. @article{ISI:000820334500001, title = {Dynamic Tuning of Moire Superlattice Morphology by Laser Modification}, author = {Xinyun Wang and Yuzhou Zhao and Xiao Kong and Qi Zhang and Hong Kuan Ng and Sharon Xiaodai Lim and Yue Zheng and Xiao Wu and Kenji Watanabe and Qing-Hua Xu and Takashi Taniguchi and Goki Eda and Kuan Eng Johnson Goh and Song Jin and Kian Ping Loh and Feng Ding and Wanxin Sun and Chorng Haur Sow}, doi = {10.1021/acsnano.2c01625}, times_cited = {0}, issn = {1936-0851}, year = {2022}, date = {2022-05-16}, journal = {ACS NANO}, volume = {16}, number = {5}, pages = {8172-8180}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {In artificial van der Waals (vdW) layered devices, twisting the stacking angle has emerged as an effective strategy to regulate the electronic phases and optical properties of these systems. Along with the twist registry, the lattice reconstruction arising from vdW interlayer interaction has also inspired significant research interests. The control of twist angles is significantly important because the moire periodicity determines the electron propagation length on the lattice and the interlayer electron-electron interactions. However, the moire periodicity is hard to be modified after the device has been fabricated. In this work, we have demonstrated that the moire periodicity can be precisely modulated with a localized laser annealing technique. This is achieved with regulating the interlayer lattice mismatch by the mismatched lattice constant, which originates from the variable density of sulfur vacancy generated during laser modification. The existence of sulfur vacancy is further verified by excitonic emission energy and lifetime in photoluminescence measurements. Furthermore, we also discover that the mismatched lattice constant has the equivalent contribution as the twist angle for determining the lattice mismatch. Theoretical modeling elaborates the moire-wavelength-dependent energy variations at the interface and mimics the evolution of moire morphology.}, keywords = {}, pubstate = {published}, tppubtype = {article} } In artificial van der Waals (vdW) layered devices, twisting the stacking angle has emerged as an effective strategy to regulate the electronic phases and optical properties of these systems. Along with the twist registry, the lattice reconstruction arising from vdW interlayer interaction has also inspired significant research interests. The control of twist angles is significantly important because the moire periodicity determines the electron propagation length on the lattice and the interlayer electron-electron interactions. However, the moire periodicity is hard to be modified after the device has been fabricated. In this work, we have demonstrated that the moire periodicity can be precisely modulated with a localized laser annealing technique. This is achieved with regulating the interlayer lattice mismatch by the mismatched lattice constant, which originates from the variable density of sulfur vacancy generated during laser modification. The existence of sulfur vacancy is further verified by excitonic emission energy and lifetime in photoluminescence measurements. Furthermore, we also discover that the mismatched lattice constant has the equivalent contribution as the twist angle for determining the lattice mismatch. Theoretical modeling elaborates the moire-wavelength-dependent energy variations at the interface and mimics the evolution of moire morphology. |
Wu, Yaze; Abdelwahab, Ibrahim; Kwon, Ki Chang; Verzhbitskiy, Ivan; Wang, Lin; Liew, Weng Heng; Yao, Kui; Eda, Goki; Loh, Kian Ping; Shen, Lei; Quek, Su Ying Data-driven discovery of high performance layered van der Waals piezoelectric NbOI2 Journal Article NATURE COMMUNICATIONS, 13 (1), 2022. @article{ISI:000779784400014, title = {Data-driven discovery of high performance layered van der Waals piezoelectric NbOI_{2}}, author = {Yaze Wu and Ibrahim Abdelwahab and Ki Chang Kwon and Ivan Verzhbitskiy and Lin Wang and Weng Heng Liew and Kui Yao and Goki Eda and Kian Ping Loh and Lei Shen and Su Ying Quek}, doi = {10.1038/s41467-022-29495-y}, times_cited = {0}, year = {2022}, date = {2022-04-07}, journal = {NATURE COMMUNICATIONS}, volume = {13}, number = {1}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {The recent thrust toward flexible nanoscale devices creates a need for two-dimensional piezoelectric materials. Here, the authors find large piezoelectric response in NbOI2 flakes ranging from 4 nm to the bulk.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The recent thrust toward flexible nanoscale devices creates a need for two-dimensional piezoelectric materials. Here, the authors find large piezoelectric response in NbOI2 flakes ranging from 4 nm to the bulk. |