Quek Su Ying
Degree: PhD
Position: Associate Professor
Affiliation: NUS - Department of Physics
Research Type: Theory
Office: S16-06-16
Email: phyqsy@nus.edu.sg
Website: http://www.physics.nus.edu.sg/staff/queksy.html
Research Interests:
First principles calculations (mean field and many-electron perturbation theory)
Interface science
Emerging materials
Electronic energy level alignment and transport
CA2DM Publications:
2025 |
Liu, Matthew W -J; Ulman, Kanchan Ajit; Zheng, Boyang; Jain, Arpit; Heintzelman, Daniel J; Wang, Ke; He, Wen; Dong, Chengye; Lu, Li-Syuan; Crespi, Vincent H; Quek, Su Ying; Robinson, Joshua A; Jr, Kenneth Knappenberger L Structure-Dependent Electronic Relaxation Dynamics of Two-Dimensional Silver Monolayers Journal Article NANO LETTERS, 25 (49), pp. 17145-17151, 2025, ISSN: 1530-6984. @article{ISI:001629007700001, title = {Structure-Dependent Electronic Relaxation Dynamics of Two-Dimensional Silver Monolayers}, author = {Matthew W -J Liu and Kanchan Ajit Ulman and Boyang Zheng and Arpit Jain and Daniel J Heintzelman and Ke Wang and Wen He and Chengye Dong and Li-Syuan Lu and Vincent H Crespi and Su Ying Quek and Joshua A Robinson and Kenneth Knappenberger L Jr}, doi = {10.1021/acs.nanolett.5c04723}, times_cited = {0}, issn = {1530-6984}, year = {2025}, date = {2025-12-01}, journal = {NANO LETTERS}, volume = {25}, number = {49}, pages = {17145-17151}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {The electronic relaxation dynamics of two-dimensional silver polar metal heterostructures (2D-PMets), isolated with two different Ag lattice structures, were studied with femtosecond transient absorption (fs-TA) spectroscopy. The two 2D Ag phases, called Ag-(1) and Ag-(2), differ in atomic packing density, which leads to phase-specific ultralow frequency (ULF) phonon modes and visible electronic absorption transitions. Time-resolved kinetic traces for both phases were fit to a biexponential decay function, with the first decay component pertaining to ultrafast electronic relaxation and the second corresponding to carrier-phonon scattering. The first decay time constant tau(1) is <400 fs for both phases. In contrast, carrier-phonon scattering exhibited lattice-specific and excitation wavelength-independent relaxation time constants; tau(2) similar to 2 ps for Ag-(1) and similar to 1 ps for Ag-(2). The shorter tau(2) in Ag-(2) is attributed to increased carrier-phonon scattering probability in more close-packed lateral structures. The results indicate that atomic-level structure controls energy flow in spatially confined 2D materials.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The electronic relaxation dynamics of two-dimensional silver polar metal heterostructures (2D-PMets), isolated with two different Ag lattice structures, were studied with femtosecond transient absorption (fs-TA) spectroscopy. The two 2D Ag phases, called Ag-(1) and Ag-(2), differ in atomic packing density, which leads to phase-specific ultralow frequency (ULF) phonon modes and visible electronic absorption transitions. Time-resolved kinetic traces for both phases were fit to a biexponential decay function, with the first decay component pertaining to ultrafast electronic relaxation and the second corresponding to carrier-phonon scattering. The first decay time constant tau(1) is <400 fs for both phases. In contrast, carrier-phonon scattering exhibited lattice-specific and excitation wavelength-independent relaxation time constants; tau(2) similar to 2 ps for Ag-(1) and similar to 1 ps for Ag-(2). The shorter tau(2) in Ag-(2) is attributed to increased carrier-phonon scattering probability in more close-packed lateral structures. The results indicate that atomic-level structure controls energy flow in spatially confined 2D materials. |
Zhang, Jingda; Quek, Su Ying Quantum Defects in 2D Transition Metal Dichalcogenides for Terahertz Technologies Journal Article ACS NANO, 19 (41), pp. 36204-36214, 2025, ISSN: 1936-0851. @article{ISI:001588895100001, title = {Quantum Defects in 2D Transition Metal Dichalcogenides for Terahertz Technologies}, author = {Jingda Zhang and Su Ying Quek}, doi = {10.1021/acsnano.5c06007}, times_cited = {0}, issn = {1936-0851}, year = {2025}, date = {2025-10-07}, journal = {ACS NANO}, volume = {19}, number = {41}, pages = {36204-36214}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Substitutional transition metal (TM) point defects have recently been controllably introduced in two-dimensional (2D) transition metal dichalcogenides. We identify quantum defect candidates through a first-principles materials discovery approach with 25 TM elements substituting Mo and W in 2D MoS2 and WSe2, respectively. We elucidate trends in the charge transition levels for these 50 systems and report the existence of defects with spin-triplet ground states and a zero-field splitting (ZFS) in the terahertz (THz) regime, in contrast to typical gigahertz values. These defects can couple to resonant near-infrared radiation, providing a route to applications as high-fidelity qubits controlled by spin-dependent optical transitions. The THz ZFS implies that these high-fidelity operations can take place at higher temperatures compared to the case for GHz qubits. Our results also point toward the possibility of realizing a single-photon THz emitter. This work broadens the scope of quantum defects, highlighting the opportunities for next-generation THz quantum technologies-an area of growing interest given the rapid advancement in the development of THz sources and detectors.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Substitutional transition metal (TM) point defects have recently been controllably introduced in two-dimensional (2D) transition metal dichalcogenides. We identify quantum defect candidates through a first-principles materials discovery approach with 25 TM elements substituting Mo and W in 2D MoS2 and WSe2, respectively. We elucidate trends in the charge transition levels for these 50 systems and report the existence of defects with spin-triplet ground states and a zero-field splitting (ZFS) in the terahertz (THz) regime, in contrast to typical gigahertz values. These defects can couple to resonant near-infrared radiation, providing a route to applications as high-fidelity qubits controlled by spin-dependent optical transitions. The THz ZFS implies that these high-fidelity operations can take place at higher temperatures compared to the case for GHz qubits. Our results also point toward the possibility of realizing a single-photon THz emitter. This work broadens the scope of quantum defects, highlighting the opportunities for next-generation THz quantum technologies-an area of growing interest given the rapid advancement in the development of THz sources and detectors. |
Cheng, Nicholas Lin Quan; Ulman, Kanchan Ajit; Quek, Su Ying Defect Engineering in Hexagonal Boron Nitride: Optical Properties of Stable Defect Complexes Arising from Boron Interstitials Journal Article ACS APPLIED MATERIALS & INTERFACES, 17 (16), pp. 24058-24070, 2025, ISSN: 1944-8244. @article{ISI:001466592300001, title = {Defect Engineering in Hexagonal Boron Nitride: Optical Properties of Stable Defect Complexes Arising from Boron Interstitials}, author = {Nicholas Lin Quan Cheng and Kanchan Ajit Ulman and Su Ying Quek}, doi = {10.1021/acsami.4c19034}, times_cited = {0}, issn = {1944-8244}, year = {2025}, date = {2025-04-14}, journal = {ACS APPLIED MATERIALS & INTERFACES}, volume = {17}, number = {16}, pages = {24058-24070}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Hexagonal boron nitride (hBN) is a wide-band-gap semiconductor that is promising as a host material for solid-state quantum technologies through defect engineering. It has been shown that boron atoms can be removed from the lattice upon irradiation by electrons or light ions, creating boron vacancies and boron interstitials. While the optical properties of boron-vacancy-derived defects have been studied extensively, little is known about the optical properties of boron-interstitial-derived defects. In this work, we use state-of-the-art first-principles calculations to predict the electronic and optical properties of boron interstitials (Bint) and defect complexes comprising Bint and substitutional carbon impurities at boron and nitrogen sites (CB and CN). These carbon impurities can be present in as-grown hBN and can also be introduced intentionally. We demonstrate that these complexes are expected to be stable at room temperature. Our GW-Bethe-Salpeter equation (BSE) calculations show that Bint-CB and Bint-CN have low-energy optical transitions that are isolated in energy, making them suitable as single-photon emitters. Together with constrained density functional theory calculations to capture the red shift due to emission, we predict that Bint-CB and Bint-CN have zero phonon lines at similar to 2.0 eV and similar to 2.6 eV, respectively. Defects involving Bint are likely to be the source of blue emitters recently observed in regions several microns away from ion-irradiated parts of hBN. Our work sheds light on these recent experiments and introduces a fresh perspective to the field of quantum emitters in hBN-we show that defects related to Bint are potential single-photon emitters that can be intentionally created in hBN.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Hexagonal boron nitride (hBN) is a wide-band-gap semiconductor that is promising as a host material for solid-state quantum technologies through defect engineering. It has been shown that boron atoms can be removed from the lattice upon irradiation by electrons or light ions, creating boron vacancies and boron interstitials. While the optical properties of boron-vacancy-derived defects have been studied extensively, little is known about the optical properties of boron-interstitial-derived defects. In this work, we use state-of-the-art first-principles calculations to predict the electronic and optical properties of boron interstitials (Bint) and defect complexes comprising Bint and substitutional carbon impurities at boron and nitrogen sites (CB and CN). These carbon impurities can be present in as-grown hBN and can also be introduced intentionally. We demonstrate that these complexes are expected to be stable at room temperature. Our GW-Bethe-Salpeter equation (BSE) calculations show that Bint-CB and Bint-CN have low-energy optical transitions that are isolated in energy, making them suitable as single-photon emitters. Together with constrained density functional theory calculations to capture the red shift due to emission, we predict that Bint-CB and Bint-CN have zero phonon lines at similar to 2.0 eV and similar to 2.6 eV, respectively. Defects involving Bint are likely to be the source of blue emitters recently observed in regions several microns away from ion-irradiated parts of hBN. Our work sheds light on these recent experiments and introduces a fresh perspective to the field of quantum emitters in hBN-we show that defects related to Bint are potential single-photon emitters that can be intentionally created in hBN. |
Liang, Haidong; Chen, Yuan; Loh, Leyi; Cheng, Nicholas Lin Quan; Litvinov, Dmitrii; Yang, Chengyuan; Chen, Yifeng; Zhang, Zhepeng; Watanabe, Kenji; Taniguchi, Takashi; Koperski, Maciej; Quek, Su Ying; Bosman, Michel; Eda, Goki; Bettiol, Andrew Anthony Site-Selective Creation of Blue Emitters in Hexagonal Boron Nitride Journal Article ACS NANO, 19 (15), pp. 15130-15138, 2025, ISSN: 1936-0851. @article{ISI:001465855100001, title = {Site-Selective Creation of Blue Emitters in Hexagonal Boron Nitride}, author = {Haidong Liang and Yuan Chen and Leyi Loh and Nicholas Lin Quan Cheng and Dmitrii Litvinov and Chengyuan Yang and Yifeng Chen and Zhepeng Zhang and Kenji Watanabe and Takashi Taniguchi and Maciej Koperski and Su Ying Quek and Michel Bosman and Goki Eda and Andrew Anthony Bettiol}, doi = {10.1021/acsnano.5c03423}, times_cited = {5}, issn = {1936-0851}, year = {2025}, date = {2025-04-12}, journal = {ACS NANO}, volume = {19}, number = {15}, pages = {15130-15138}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Hexagonal boron nitride (hBN) has been of great interest due to its ability to host several bright quantum emitters at room temperature. However, the identification of the observed emitters remains challenging due to spectral variability, as well as the lack of atomic defect structure information. In this work, we demonstrate the site-selective creation of blue emitters in exfoliated hBN flakes with high-energy ion irradiation. With the correlation analysis of cryogenic and temperature-dependent photoluminescence (PL) spectroscopy, we observe two zero phonon lines (ZPLs) at similar to 432.8 and 454.3 nm. Photoluminescence excitation (PLE) measurements further confirm the emission origins of the two prominent lines. Scanning transmission electron microscopy (STEM) reveals that the dominant defect structures present in ion-irradiated samples are vacancy-type (V x ) and adatom(intercalant)-type (A x ). Together with first-principles GW-BSE (Bethe-Salpeter equation) calculations, we deduce that the observed blue emissions are likely related to boron intercalants (Bint). Our results not only discover a group of blue emissions in hBN but also provide insights into the physical origin of the emissions with local atomic structures in hBN.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Hexagonal boron nitride (hBN) has been of great interest due to its ability to host several bright quantum emitters at room temperature. However, the identification of the observed emitters remains challenging due to spectral variability, as well as the lack of atomic defect structure information. In this work, we demonstrate the site-selective creation of blue emitters in exfoliated hBN flakes with high-energy ion irradiation. With the correlation analysis of cryogenic and temperature-dependent photoluminescence (PL) spectroscopy, we observe two zero phonon lines (ZPLs) at similar to 432.8 and 454.3 nm. Photoluminescence excitation (PLE) measurements further confirm the emission origins of the two prominent lines. Scanning transmission electron microscopy (STEM) reveals that the dominant defect structures present in ion-irradiated samples are vacancy-type (V x ) and adatom(intercalant)-type (A x ). Together with first-principles GW-BSE (Bethe-Salpeter equation) calculations, we deduce that the observed blue emissions are likely related to boron intercalants (Bint). Our results not only discover a group of blue emissions in hBN but also provide insights into the physical origin of the emissions with local atomic structures in hBN. |
2024 |
Loh, Leyi; Ho, Yi Wei; Xuan, Fengyuan; del Aguila, Andres Granados; Chen, Yuan; Wong, See Yoong; Zhang, Jingda; Wang, Zhe; Watanabe, Kenji; Taniguchi, Takashi; Pigram, Paul J; Bosman, Michel; Quek, Su Ying; Koperski, Maciej; Eda, Goki Nb impurity-bound excitons as quantum emitters in monolayer WS_2 Journal Article NATURE COMMUNICATIONS, 15 (1), 2024. @article{ISI:001360396900001, title = {Nb impurity-bound excitons as quantum emitters in monolayer WS_2}, author = {Leyi Loh and Yi Wei Ho and Fengyuan Xuan and Andres Granados del Aguila and Yuan Chen and See Yoong Wong and Jingda Zhang and Zhe Wang and Kenji Watanabe and Takashi Taniguchi and Paul J Pigram and Michel Bosman and Su Ying Quek and Maciej Koperski and Goki Eda}, doi = {10.1038/s41467-024-54360-5}, times_cited = {10}, year = {2024}, date = {2024-11-20}, journal = {NATURE COMMUNICATIONS}, volume = {15}, number = {1}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {Point defects in crystalline solids behave as optically addressable individual quantum systems when present in sufficiently low concentrations. In two-dimensional (2D) semiconductors, such quantum defects hold potential as versatile single photon sources. Here, we report the synthesis and optical properties of Nb-doped monolayer WS2 in the dilute limit where the average spacing between individual dopants exceeds the optical diffraction limit, allowing the emission spectrum to be studied at the single-dopant level. We show that these individual dopants exhibit common features of quantum emitters, including narrow emission lines (with linewidths <1 meV), strong spatial confinement, and photon antibunching. These emitters consistently occur within a narrow spectral range across multiple samples, distinct from common quantum emitters in van der Waals (vdW) materials that show large ensemble broadening. Analysis of the Zeeman splitting reveals that they can be attributed to bound exciton complexes comprising dark excitons and negatively charged Nb.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Point defects in crystalline solids behave as optically addressable individual quantum systems when present in sufficiently low concentrations. In two-dimensional (2D) semiconductors, such quantum defects hold potential as versatile single photon sources. Here, we report the synthesis and optical properties of Nb-doped monolayer WS2 in the dilute limit where the average spacing between individual dopants exceeds the optical diffraction limit, allowing the emission spectrum to be studied at the single-dopant level. We show that these individual dopants exhibit common features of quantum emitters, including narrow emission lines (with linewidths <1 meV), strong spatial confinement, and photon antibunching. These emitters consistently occur within a narrow spectral range across multiple samples, distinct from common quantum emitters in van der Waals (vdW) materials that show large ensemble broadening. Analysis of the Zeeman splitting reveals that they can be attributed to bound exciton complexes comprising dark excitons and negatively charged Nb. |
Xuan, Fengyuan; Lai, Mingrui; Wu, Yaze; Quek, Su Ying Exciton-Enhanced Spontaneous Parametric Down-Conversion in Two-Dimensional Crystals Journal Article 14 PHYSICAL REVIEW LETTERS, 132 (24), 2024, ISSN: 0031-9007. @article{ISI:001251520000012, title = {Exciton-Enhanced Spontaneous Parametric Down-Conversion in Two-Dimensional Crystals}, author = {Fengyuan Xuan and Mingrui Lai and Yaze Wu and Su Ying Quek}, doi = {10.1103/PhysRevLett.132.246902}, times_cited = {14}, issn = {0031-9007}, year = {2024}, date = {2024-06-14}, journal = {PHYSICAL REVIEW LETTERS}, volume = {132}, number = {24}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {We show that excitonic resonances and interexciton transitions can enhance the probability of spontaneous parametric down -conversion, a second -order optical response that generates entangled photon pairs. We benchmark our ab initio many -body calculations using experimental polar plots of second harmonic generation in NbOI 2 , clearly demonstrating the relevance of excitons in the nonlinear response. A strong double-exciton resonance in 2D NbOCl 2 leads to giant enhancement in the second order susceptibility. Our work paves the way for the realization of efficient ultrathin quantum light sources.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We show that excitonic resonances and interexciton transitions can enhance the probability of spontaneous parametric down -conversion, a second -order optical response that generates entangled photon pairs. We benchmark our ab initio many -body calculations using experimental polar plots of second harmonic generation in NbOI 2 , clearly demonstrating the relevance of excitons in the nonlinear response. A strong double-exciton resonance in 2D NbOCl 2 leads to giant enhancement in the second order susceptibility. Our work paves the way for the realization of efficient ultrathin quantum light sources. |
Qiu, Zhizhan; Han, Yixuan; Noori, Keian; Chen, Zhaolong; Kashchenko, Mikhail; Lin, Li; Olsen, Thomas; Li, Jing; Fang, Hanyan; Lyu, Pin; Telychko, Mykola; Gu, Xingyu; Adam, Shaffique; Quek, Su Ying; Rodin, Aleksandr; Neto, Castro A H; Novoselov, Kostya S; Lu, Jiong Evidence for electron-hole crystals in a Mott insulator Journal Article 13 NATURE MATERIALS, 23 (8), 2024, ISSN: 1476-1122. @article{ISI:001237790900002, title = {Evidence for electron-hole crystals in a Mott insulator}, author = {Zhizhan Qiu and Yixuan Han and Keian Noori and Zhaolong Chen and Mikhail Kashchenko and Li Lin and Thomas Olsen and Jing Li and Hanyan Fang and Pin Lyu and Mykola Telychko and Xingyu Gu and Shaffique Adam and Su Ying Quek and Aleksandr Rodin and Castro A H Neto and Kostya S Novoselov and Jiong Lu}, doi = {10.1038/s41563-024-01910-3}, times_cited = {13}, issn = {1476-1122}, year = {2024}, date = {2024-06-03}, journal = {NATURE MATERIALS}, volume = {23}, number = {8}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {The coexistence of correlated electron and hole crystals enables the realization of quantum excitonic states, capable of hosting counterflow superfluidity and topological orders with long-range quantum entanglement. Here we report evidence for imbalanced electron-hole crystals in a doped Mott insulator, namely, alpha-RuCl3, through gate-tunable non-invasive van der Waals doping from graphene. Real-space imaging via scanning tunnelling microscopy reveals two distinct charge orderings at the lower and upper Hubbard band energies, whose origin is attributed to the correlation-driven honeycomb hole crystal composed of hole-rich Ru sites and rotational-symmetry-breaking paired electron crystal composed of electron-rich Ru-Ru bonds, respectively. Moreover, a gate-induced transition of electron-hole crystals is directly visualized, further corroborating their nature as correlation-driven charge crystals. The realization and atom-resolved visualization of imbalanced electron-hole crystals in a doped Mott insulator opens new doors in the search for correlated bosonic states within strongly correlated materials.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The coexistence of correlated electron and hole crystals enables the realization of quantum excitonic states, capable of hosting counterflow superfluidity and topological orders with long-range quantum entanglement. Here we report evidence for imbalanced electron-hole crystals in a doped Mott insulator, namely, alpha-RuCl3, through gate-tunable non-invasive van der Waals doping from graphene. Real-space imaging via scanning tunnelling microscopy reveals two distinct charge orderings at the lower and upper Hubbard band energies, whose origin is attributed to the correlation-driven honeycomb hole crystal composed of hole-rich Ru sites and rotational-symmetry-breaking paired electron crystal composed of electron-rich Ru-Ru bonds, respectively. Moreover, a gate-induced transition of electron-hole crystals is directly visualized, further corroborating their nature as correlation-driven charge crystals. The realization and atom-resolved visualization of imbalanced electron-hole crystals in a doped Mott insulator opens new doors in the search for correlated bosonic states within strongly correlated materials. |
2022 |
Noori, Keian; Xuan, Fengyuan; Quek, Su Ying Origin of contact polarity at metal-2D transition metal dichalcogenide interfaces Journal Article NPJ 2D MATERIALS AND APPLICATIONS, 6 (1), 2022. @article{ISI:000876057300001, title = {Origin of contact polarity at metal-2D transition metal dichalcogenide interfaces}, author = {Keian Noori and Fengyuan Xuan and Su Ying Quek}, doi = {10.1038/s41699-022-00349-x}, times_cited = {5}, year = {2022}, date = {2022-10-14}, journal = {NPJ 2D MATERIALS AND APPLICATIONS}, volume = {6}, number = {1}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {Using state-of-the-art ab initio GW many-body perturbation theory calculations, we show that monolayer MoS2 on Au is a p-type contact, in contrast to the vast majority of theoretical predictions using density functional theory. The predominantly n-type behaviour observed experimentally for MoS2/Au junctions can be attributed to the presence of sulfur vacancies, which pin the Fermi level. GW calculations on WSe2/Au junctions likewise predict p-type contacts for pristine WSe2 and n-type contacts for junctions with selenium vacancies. Experimentally, WSe2/metal junctions are predominantly p-type or ambipolar, with p-type junctions being observed for selenium-deficient WSe2, suggesting that selenium vacancies are not effective in pinning the Fermi level for WSe2/metal junctions. We rationalize these apparently contradictory results by noting that selenium vacancies in WSe2 are readily passivated by oxygen atoms. Taken together, our state-of-the-art calculations clearly elucidate the relation between contact polarity and atomic structure. We show that non-local exchange and correlation effects are critical for determining the energy level alignment and even the contact polarity (in the case of MoS2 on Au). We further reconcile a large body of experimental literature on TMDC/metal contact polarities by consideration of the defect chemistry.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Using state-of-the-art ab initio GW many-body perturbation theory calculations, we show that monolayer MoS2 on Au is a p-type contact, in contrast to the vast majority of theoretical predictions using density functional theory. The predominantly n-type behaviour observed experimentally for MoS2/Au junctions can be attributed to the presence of sulfur vacancies, which pin the Fermi level. GW calculations on WSe2/Au junctions likewise predict p-type contacts for pristine WSe2 and n-type contacts for junctions with selenium vacancies. Experimentally, WSe2/metal junctions are predominantly p-type or ambipolar, with p-type junctions being observed for selenium-deficient WSe2, suggesting that selenium vacancies are not effective in pinning the Fermi level for WSe2/metal junctions. We rationalize these apparently contradictory results by noting that selenium vacancies in WSe2 are readily passivated by oxygen atoms. Taken together, our state-of-the-art calculations clearly elucidate the relation between contact polarity and atomic structure. We show that non-local exchange and correlation effects are critical for determining the energy level alignment and even the contact polarity (in the case of MoS2 on Au). We further reconcile a large body of experimental literature on TMDC/metal contact polarities by consideration of the defect chemistry. |
Zhang, Zhepeng; Liang, Haidong; Loh, Leyi; Chen, Yifeng; Chen, Yuan; Watanabe, Kenji; Taniguchi, Takashi; Quek, Su Ying; Bosman, Michel; Bettiol, Andrew A; Eda, Goki Optically Active Chalcogen Vacancies in Monolayer Semiconductors Journal Article 11 ADVANCED OPTICAL MATERIALS, 10 (23), 2022, ISSN: 2195-1071. @article{ISI:000852786000001, title = {Optically Active Chalcogen Vacancies in Monolayer Semiconductors}, author = {Zhepeng Zhang and Haidong Liang and Leyi Loh and Yifeng Chen and Yuan Chen and Kenji Watanabe and Takashi Taniguchi and Su Ying Quek and Michel Bosman and Andrew A Bettiol and Goki Eda}, doi = {10.1002/adom.202201350}, times_cited = {11}, issn = {2195-1071}, year = {2022}, date = {2022-09-12}, journal = {ADVANCED OPTICAL MATERIALS}, volume = {10}, number = {23}, publisher = {WILEY-V C H VERLAG GMBH}, address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY}, abstract = {Defect engineering of atomically thin semiconducting crystals is an attractive route to developing single-photon sources and valleytronic devices. For these applications, defects with well-defined optical characteristics need to be generated in a precisely controlled manner. However, defect-induced optical features are often complicated by the presence of multiple defect species, hindering the identification of their structural origin. Here, we report systematic generation of optically active atomic defects in monolayer MoS2, WS2, MoSe2, and WSe2 via proton-beam irradiation. Defect-induced emissions are found to occur approximate to 100 to 200 meV below the neutral exciton peak, showing typical characteristics of localized excitons such as saturation at high-excitation rates and long lifetime. Using scanning transmission electron microscopy, it is shown that freshly created chalcogen vacancies are responsible for the localized exciton emission. Density functional theory and ab initio GW plus Bethe-Salpeter-equation calculations reveal that the observed emission can be attributed to transitions involving defect levels of chalcogen vacancy and the valence band edge state.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Defect engineering of atomically thin semiconducting crystals is an attractive route to developing single-photon sources and valleytronic devices. For these applications, defects with well-defined optical characteristics need to be generated in a precisely controlled manner. However, defect-induced optical features are often complicated by the presence of multiple defect species, hindering the identification of their structural origin. Here, we report systematic generation of optically active atomic defects in monolayer MoS2, WS2, MoSe2, and WSe2 via proton-beam irradiation. Defect-induced emissions are found to occur approximate to 100 to 200 meV below the neutral exciton peak, showing typical characteristics of localized excitons such as saturation at high-excitation rates and long lifetime. Using scanning transmission electron microscopy, it is shown that freshly created chalcogen vacancies are responsible for the localized exciton emission. Density functional theory and ab initio GW plus Bethe-Salpeter-equation calculations reveal that the observed emission can be attributed to transitions involving defect levels of chalcogen vacancy and the valence band edge state. |
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 textitg-factor in Self-Intercalated 2D TaS_2 Journal Article SMALL, 18 (38), 2022, ISSN: 1613-6810. @article{ISI:000842350300001, title = {Giant textitg-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 = {4}, 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. |
He, Wen; Wetherington, Maxwell T; Ulman, Kanchan Ajit; Robinson, Joshua A; Quek, Su Ying Interface-Mediated Resonant Raman Enhancement for Shear Modes in a 2D Polar Metal Journal Article JOURNAL OF PHYSICAL CHEMISTRY C, 126 (34), pp. 14581-14589, 2022, ISSN: 1932-7447. @article{ISI:000844406700001, title = {Interface-Mediated Resonant Raman Enhancement for Shear Modes in a 2D Polar Metal}, author = {Wen He and Maxwell T Wetherington and Kanchan Ajit Ulman and Joshua A Robinson and Su Ying Quek}, doi = {10.1021/acs.jpcc.2c04433}, times_cited = {3}, issn = {1932-7447}, year = {2022}, date = {2022-08-18}, journal = {JOURNAL OF PHYSICAL CHEMISTRY C}, volume = {126}, number = {34}, pages = {14581-14589}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {2D polar metals synthesized by confinement heteroepitaxy at the SiC/graphene interface are covalently bound to the SiC substrate. In this work, we elucidate the importance of the SiC substrate, and specifically the Ga/Si interface, on the low frequency resonant Raman spectra of 2D Ga on SiC. The low-frequency Raman modes are dominated by in-plane shear modes in 2D Ga. We show that the frequency of these shear modes is modified by the presence of the substrate for few-layer Ga and that these shear modes couple strongly to the electronic states corresponding to the interface Ga and Si atoms. Consequently, resonant Raman enhancement occurs at laser incident energies that are resonant with the interband optical transitions involving these interface Ga and Si states. This resonant Raman enhancement is observed in laser-energy dependent measurements, an experimental signature of the strong electron-phonon coupling present in these 2D polar metals.}, keywords = {}, pubstate = {published}, tppubtype = {article} } 2D polar metals synthesized by confinement heteroepitaxy at the SiC/graphene interface are covalently bound to the SiC substrate. In this work, we elucidate the importance of the SiC substrate, and specifically the Ga/Si interface, on the low frequency resonant Raman spectra of 2D Ga on SiC. The low-frequency Raman modes are dominated by in-plane shear modes in 2D Ga. We show that the frequency of these shear modes is modified by the presence of the substrate for few-layer Ga and that these shear modes couple strongly to the electronic states corresponding to the interface Ga and Si atoms. Consequently, resonant Raman enhancement occurs at laser incident energies that are resonant with the interband optical transitions involving these interface Ga and Si states. This resonant Raman enhancement is observed in laser-energy dependent measurements, an experimental signature of the strong electron-phonon coupling present in these 2D polar metals. |
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 NbOI_2 Journal Article 104 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 = {104}, 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. |
Reed-Lingenfelter, Serrae N; Chen, Yifeng; Yarali, Milad; Charboneau, David J; Curley, Julia B; Hynek, David J; Wang, Mengjing; Williams, Natalie L; Hazari, Nilay; Quek, Su Ying; Cha, Judy J Compact Super Electron-Donor to Monolayer MoS_2 Journal Article 14 NANO LETTERS, 22 (11), pp. 4501-4508, 2022, ISSN: 1530-6984. @article{ISI:000810028900001, title = {Compact Super Electron-Donor to Monolayer MoS_2}, author = {Serrae N Reed-Lingenfelter and Yifeng Chen and Milad Yarali and David J Charboneau and Julia B Curley and David J Hynek and Mengjing Wang and Natalie L Williams and Nilay Hazari and Su Ying Quek and Judy J Cha}, doi = {10.1021/acs.nanolett.2c01167}, times_cited = {14}, issn = {1530-6984}, year = {2022}, date = {2022-06-08}, journal = {NANO LETTERS}, volume = {22}, number = {11}, pages = {4501-4508}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {The surface functionalization of two-dimensional (2D) materials with organic electron donors (OEDs) is a powerful tool to modulate the electronic properties of the material. Here we report a novel molecular dopant, Me-OED, that demonstrates record-breaking molecular doping to MoS2, achieving a carrier density of 1.10 +/- 0.37 X 10(14) cm(-2) at optimal functionalization conditions; the achieved carrier density is much higher than those by other OEDs such as benzyl viologen and an OED based on 4,4'-bipyridine. This impressive doping power is attributed to the compact size of Me-OED, which leads to high surface coverage on MoS2. To confirm, we study Bu-t-OED, which has an identical reduction potential to Me-OED but is significantly larger. Using field-effect transistor measurements and spectroscopic characterization, we estimate the doping powers of Me- and Bu-t-OED are 0.22-0.44 and 0.11 electrons per molecule, respectively, in good agreement with calculations. Our results demonstrate that the small size of Me-OED is critical to maximizing the surface coverage and molecular interactions with MoS2, enabling us to achieve unprecedented doping of MoS2.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The surface functionalization of two-dimensional (2D) materials with organic electron donors (OEDs) is a powerful tool to modulate the electronic properties of the material. Here we report a novel molecular dopant, Me-OED, that demonstrates record-breaking molecular doping to MoS2, achieving a carrier density of 1.10 +/- 0.37 X 10(14) cm(-2) at optimal functionalization conditions; the achieved carrier density is much higher than those by other OEDs such as benzyl viologen and an OED based on 4,4'-bipyridine. This impressive doping power is attributed to the compact size of Me-OED, which leads to high surface coverage on MoS2. To confirm, we study Bu-t-OED, which has an identical reduction potential to Me-OED but is significantly larger. Using field-effect transistor measurements and spectroscopic characterization, we estimate the doping powers of Me- and Bu-t-OED are 0.22-0.44 and 0.11 electrons per molecule, respectively, in good agreement with calculations. Our results demonstrate that the small size of Me-OED is critical to maximizing the surface coverage and molecular interactions with MoS2, enabling us to achieve unprecedented doping of MoS2. |
He, Wen; Wetherington, Maxwell T; Ulman, Kanchan Ajit; Gray, Jennifer L; Robinson, Joshua A; Quek, Su Ying Shear Modes in a 2D Polar Metal Journal Article JOURNAL OF PHYSICAL CHEMISTRY LETTERS, 13 (18), pp. 4015-4020, 2022, ISSN: 1948-7185. @article{ISI:000803568300007, title = {Shear Modes in a 2D Polar Metal}, author = {Wen He and Maxwell T Wetherington and Kanchan Ajit Ulman and Jennifer L Gray and Joshua A Robinson and Su Ying Quek}, doi = {10.1021/acs.jpclett.2c00719}, times_cited = {4}, issn = {1948-7185}, year = {2022}, date = {2022-05-12}, journal = {JOURNAL OF PHYSICAL CHEMISTRY LETTERS}, volume = {13}, number = {18}, pages = {4015-4020}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Low-frequency shear and breathing modes are important Raman signatures of two-dimensional (2D) materials, providing information on the number of layers and insights into interlayer interactions. We elucidate the nature of low-frequency modes in a 2D polar metal-2D Ga covalently bonded to a SiC substrate, using a first-principles Green's function-based approach. The low-frequency Raman modes are dominated by surface resonance modes, consisting primarily of out-of-phase shear modes in Ga, coupled to SiC phonons. Breathing modes are strongly coupled to the substrate and do not give rise to peaks in the phonon spectra. The highest-frequency shear mode blue-shifts significantly with increasing thickness, reflecting both an increase in the number of Ga layers and an increase in the effective interlayer force constant. The surface resonance modes evolve into localized 2D Ga modes as the phonon momentum increases. The predicted low-frequency modes are consistent with Raman measurements on 2D polar Ga.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Low-frequency shear and breathing modes are important Raman signatures of two-dimensional (2D) materials, providing information on the number of layers and insights into interlayer interactions. We elucidate the nature of low-frequency modes in a 2D polar metal-2D Ga covalently bonded to a SiC substrate, using a first-principles Green's function-based approach. The low-frequency Raman modes are dominated by surface resonance modes, consisting primarily of out-of-phase shear modes in Ga, coupled to SiC phonons. Breathing modes are strongly coupled to the substrate and do not give rise to peaks in the phonon spectra. The highest-frequency shear mode blue-shifts significantly with increasing thickness, reflecting both an increase in the number of Ga layers and an increase in the effective interlayer force constant. The surface resonance modes evolve into localized 2D Ga modes as the phonon momentum increases. The predicted low-frequency modes are consistent with Raman measurements on 2D polar Ga. |
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 NbOI_2 Journal Article 42 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 = {42}, 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. |