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:
2021 |
Rajabpour, Siavash; Vera, Alexander; He, Wen; Katz, Benjamin N; Koch, Roland J; Lassauniere, Margaux; Chen, Xuegang; Li, Cequn; Nisi, Katharina; El-Sherif, Hesham; Wetherington, Maxwell T; Dong, Chengye; Bostwick, Aaron; Jozwiak, Chris; van Duin, Adri C T; Bassim, Nabil; Zhu, Jun; Wang, Gwo-Ching; Wurstbauer, Ursula; Rotenberg, Eli; Crespi, Vincent; Quek, Su Ying; Robinson, Joshua A Tunable 2D Group-III Metal Alloys Journal Article ADVANCED MATERIALS, 33 (44), 2021, ISSN: 0935-9648. @article{ISI:000693328500001, title = {Tunable 2D Group-III Metal Alloys}, author = {Siavash Rajabpour and Alexander Vera and Wen He and Benjamin N Katz and Roland J Koch and Margaux Lassauniere and Xuegang Chen and Cequn Li and Katharina Nisi and Hesham El-Sherif and Maxwell T Wetherington and Chengye Dong and Aaron Bostwick and Chris Jozwiak and Adri C T van Duin and Nabil Bassim and Jun Zhu and Gwo-Ching Wang and Ursula Wurstbauer and Eli Rotenberg and Vincent Crespi and Su Ying Quek and Joshua A Robinson}, doi = {10.1002/adma.202104265}, times_cited = {0}, issn = {0935-9648}, year = {2021}, date = {2021-09-04}, journal = {ADVANCED MATERIALS}, volume = {33}, number = {44}, publisher = {WILEY-V C H VERLAG GMBH}, address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY}, abstract = {Chemically stable quantum-confined 2D metals are of interest in next-generation nanoscale quantum devices. Bottom-up design and synthesis of such metals could enable the creation of materials with tailored, on-demand, electronic and optical properties for applications that utilize tunable plasmonic coupling, optical nonlinearity, epsilon-near-zero behavior, or wavelength-specific light trapping. In this work, it is demonstrated that the electronic, superconducting, and optical properties of air-stable 2D metals can be controllably tuned by the formation of alloys. Environmentally robust large-area 2D-InxGa1-x alloys are synthesized byConfinement Heteroepitaxy (CHet). Near-complete solid solubility is achieved with no evidence of phase segregation, and the composition is tunable over the full range of x by changing the relative elemental composition of the precursor. The optical and electronic properties directly correlate with alloy composition, wherein the dielectric function, band structure, superconductivity, and charge transfer from the metal to graphene are all controlled by the indium/gallium ratio in the 2D metal layer.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Chemically stable quantum-confined 2D metals are of interest in next-generation nanoscale quantum devices. Bottom-up design and synthesis of such metals could enable the creation of materials with tailored, on-demand, electronic and optical properties for applications that utilize tunable plasmonic coupling, optical nonlinearity, epsilon-near-zero behavior, or wavelength-specific light trapping. In this work, it is demonstrated that the electronic, superconducting, and optical properties of air-stable 2D metals can be controllably tuned by the formation of alloys. Environmentally robust large-area 2D-InxGa1-x alloys are synthesized byConfinement Heteroepitaxy (CHet). Near-complete solid solubility is achieved with no evidence of phase segregation, and the composition is tunable over the full range of x by changing the relative elemental composition of the precursor. The optical and electronic properties directly correlate with alloy composition, wherein the dielectric function, band structure, superconductivity, and charge transfer from the metal to graphene are all controlled by the indium/gallium ratio in the 2D metal layer. |
Yin, Tingting; Ulman, Kanchan Ajit; Liu, Sheng; del Aguila, Andres Granados; Huang, Yuqing; Zhang, Lifa; Serra, Marco; Sedmidubsky, David; Sofer, Zdenek; Quek, Su Ying; Xiong, Qihua Chiral Phonons and Giant Magneto-Optical Effect in CrBr3 2D Magnet Journal Article ADVANCED MATERIALS, 33 (36), 2021, ISSN: 0935-9648. @article{ISI:000677213000001, title = {Chiral Phonons and Giant Magneto-Optical Effect in CrBr_{3} 2D Magnet}, author = {Tingting Yin and Kanchan Ajit Ulman and Sheng Liu and Andres Granados del Aguila and Yuqing Huang and Lifa Zhang and Marco Serra and David Sedmidubsky and Zdenek Sofer and Su Ying Quek and Qihua Xiong}, doi = {10.1002/adma.202101618}, times_cited = {0}, issn = {0935-9648}, year = {2021}, date = {2021-07-24}, journal = {ADVANCED MATERIALS}, volume = {33}, number = {36}, publisher = {WILEY-V C H VERLAG GMBH}, address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY}, abstract = {Phonons with chirality determine the optical helicity of inelastic light scattering processes due to their nonzero angular momentum. Here it is shown that 2D magnetic CrBr3 hosts chiral phonons at the Brillouin-zone center. These chiral phonons are linear combinations of the doubly-degenerate E-g phonons, and the phonon eigenmodes exhibit clockwise and counterclockwise rotational vibrations corresponding to angular momenta of l = +/- 1. Such E-g chiral phonons completely switch the polarization of incident circularly polarized light. On the other hand, the non-degenerate non-chiral A(g) phonons display a giant magneto-optical effect under an external out-of-plane magnetic field, rotating the plane of polarization of the scattered linearly polarized light. The corresponding degree of polarization of the scattered light changes from 91% to -68% as the magnetic field strength increases from 0 to 5 T. In contrast, the chiral E-g modes display no field dependence. The results lay a foundation for the study of phonon chirality and magneto-optical phenomena in 2D magnetic materials, as well as their related applications, such as the phonon Hall effect, topological photonics, and Raman lasing.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Phonons with chirality determine the optical helicity of inelastic light scattering processes due to their nonzero angular momentum. Here it is shown that 2D magnetic CrBr3 hosts chiral phonons at the Brillouin-zone center. These chiral phonons are linear combinations of the doubly-degenerate E-g phonons, and the phonon eigenmodes exhibit clockwise and counterclockwise rotational vibrations corresponding to angular momenta of l = +/- 1. Such E-g chiral phonons completely switch the polarization of incident circularly polarized light. On the other hand, the non-degenerate non-chiral A(g) phonons display a giant magneto-optical effect under an external out-of-plane magnetic field, rotating the plane of polarization of the scattered linearly polarized light. The corresponding degree of polarization of the scattered light changes from 91% to -68% as the magnetic field strength increases from 0 to 5 T. In contrast, the chiral E-g modes display no field dependence. The results lay a foundation for the study of phonon chirality and magneto-optical phenomena in 2D magnetic materials, as well as their related applications, such as the phonon Hall effect, topological photonics, and Raman lasing. |
Loh, Leyi; Chen, Yifeng; Wang, Junyong; Yin, Xinmao; Tang, Chi Sin; Zhang, Qi; Watanabe, Kenji; Taniguchi, Takashi; Wee, Andrew T S; Bosman, Michel; Quek, Su Ying; Eda, Goki Impurity-Induced Emission in Re-Doped WS2 Monolayers Journal Article NANO LETTERS, 21 (12), pp. 5293-5300, 2021, ISSN: 1530-6984. @article{ISI:000668003400052, title = {Impurity-Induced Emission in Re-Doped WS_{2} Monolayers}, author = {Leyi Loh and Yifeng Chen and Junyong Wang and Xinmao Yin and Chi Sin Tang and Qi Zhang and Kenji Watanabe and Takashi Taniguchi and Andrew T S Wee and Michel Bosman and Su Ying Quek and Goki Eda}, doi = {10.1021/acs.nanolett.1c01439}, times_cited = {0}, issn = {1530-6984}, year = {2021}, date = {2021-06-11}, journal = {NANO LETTERS}, volume = {21}, number = {12}, pages = {5293-5300}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Impurity doping is a viable route toward achieving desired subgap optical response in semiconductors. In strongly excitonic two-dimensional (2D) semiconductors such as transition metal dichalcogenides (TMDs), impurities are expected to result in bound-exciton emission. However, doped TMDs often exhibit a broad Stokes-shifted emission without characteristic features, hampering strategic materials engineering. Here we report observation of a well-defined impurity-induced emission in monolayer WS2 substitutionally doped with rhenium (Re), which is an electron donor. The emission exhibits characteristics of localized states and dominates the spectrum up to 200 K. Gate dependence reveals that neutral impurity centers are responsible for the observed emission. Using GW-Bethe-Salpeter equation (GW-BSE) calculations, we attribute the emission to transitions between spin-split upper Re band and valence band edge.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Impurity doping is a viable route toward achieving desired subgap optical response in semiconductors. In strongly excitonic two-dimensional (2D) semiconductors such as transition metal dichalcogenides (TMDs), impurities are expected to result in bound-exciton emission. However, doped TMDs often exhibit a broad Stokes-shifted emission without characteristic features, hampering strategic materials engineering. Here we report observation of a well-defined impurity-induced emission in monolayer WS2 substitutionally doped with rhenium (Re), which is an electron donor. The emission exhibits characteristics of localized states and dominates the spectrum up to 200 K. Gate dependence reveals that neutral impurity centers are responsible for the observed emission. Using GW-Bethe-Salpeter equation (GW-BSE) calculations, we attribute the emission to transitions between spin-split upper Re band and valence band edge. |
Liu, Sheng; Wu, Yaze; Liu, Xue; del Aguila, Andres Granados; Xuan, Fengyuan; Chaturvedi, Apoorva; Zhang, Hua; Quek, Su Ying; Xiong, Qihua Light-matter interactions in high quality manganese-doped two-dimensional molybdenum diselenide Journal Article SCIENCE CHINA-MATERIALS, 64 (10), pp. 2507-2518, 2021, ISSN: 2095-8226. @article{ISI:000649231600001, title = {Light-matter interactions in high quality manganese-doped two-dimensional molybdenum diselenide}, author = {Sheng Liu and Yaze Wu and Xue Liu and Andres Granados del Aguila and Fengyuan Xuan and Apoorva Chaturvedi and Hua Zhang and Su Ying Quek and Qihua Xiong}, doi = {10.1007/s40843-020-1641-9}, times_cited = {0}, issn = {2095-8226}, year = {2021}, date = {2021-05-08}, journal = {SCIENCE CHINA-MATERIALS}, volume = {64}, number = {10}, pages = {2507-2518}, publisher = {SCIENCE PRESS}, address = {16 DONGHUANGCHENGGEN NORTH ST, BEIJING 100717, PEOPLES R CHINA}, abstract = {Introducing magnetic dopants into two-dimensional transition metal dichalcogenides has recently attracted considerable attention due to its promising applications in spintronics and valleytronics. Herein we realized manganese-doped molybdenum diselenide (MoSe2) single crystal via chemical vapor transport (CVT) reaction, containing up to 2.9% (atomic concentration) Mn dopants, and investigated the light-matter interaction in these samples. We observed a suppressed trion intensity, a longer photoluminescence lifetime, and prominent blue- and red-shift of E2g2 (in-plane) and A(1g) (out-of-plane) Raman modes, respectively. Moreover, the Mn dopants increase the valley Zeeman splitting of the MoSe2 monolayer by similar to 50%, while preserving the linear dependence on magnetic field. First-principles calculations indicate that the spin-polarized deep level defect states are formed due to the Mn substitutional dopants in the MoSe2 lattice. The resulting defect potential favors the funnelling of excitons towards the defects. The Mn dopants reduce the magnitude of the interatomic force constants, explaining the red-shift of the A(1g) mode. The Mn atoms and their immediate Mo and Se neighbors carry significant magnetic moments, which enhance the observed exciton g-factors due to the exchange interactions affecting defect-bound excitons.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Introducing magnetic dopants into two-dimensional transition metal dichalcogenides has recently attracted considerable attention due to its promising applications in spintronics and valleytronics. Herein we realized manganese-doped molybdenum diselenide (MoSe2) single crystal via chemical vapor transport (CVT) reaction, containing up to 2.9% (atomic concentration) Mn dopants, and investigated the light-matter interaction in these samples. We observed a suppressed trion intensity, a longer photoluminescence lifetime, and prominent blue- and red-shift of E2g2 (in-plane) and A(1g) (out-of-plane) Raman modes, respectively. Moreover, the Mn dopants increase the valley Zeeman splitting of the MoSe2 monolayer by similar to 50%, while preserving the linear dependence on magnetic field. First-principles calculations indicate that the spin-polarized deep level defect states are formed due to the Mn substitutional dopants in the MoSe2 lattice. The resulting defect potential favors the funnelling of excitons towards the defects. The Mn dopants reduce the magnitude of the interatomic force constants, explaining the red-shift of the A(1g) mode. The Mn atoms and their immediate Mo and Se neighbors carry significant magnetic moments, which enhance the observed exciton g-factors due to the exchange interactions affecting defect-bound excitons. |
2020 |
Yarali, Milad; Zhong, Yiren; Reed, Serrae N; Wang, Juefan; Ulman, Kanchan A; Charboneau, David J; Curley, Julia B; Hynek, David J; Pondick, Joshua; Yazdani, Sajad; Hazari, Nilay; Quek, Su Ying; Wang, Hailiang; Cha, Judy J Near-Unity Molecular Doping Efficiency in Monolayer MoS2 Journal Article ADVANCED ELECTRONIC MATERIALS, 7 (2), 2020, ISSN: 2199-160X. @article{ISI:000602553300001, title = {Near-Unity Molecular Doping Efficiency in Monolayer MoS_{2}}, author = {Milad Yarali and Yiren Zhong and Serrae N Reed and Juefan Wang and Kanchan A Ulman and David J Charboneau and Julia B Curley and David J Hynek and Joshua Pondick and Sajad Yazdani and Nilay Hazari and Su Ying Quek and Hailiang Wang and Judy J Cha}, doi = {10.1002/aelm.202000873}, times_cited = {0}, issn = {2199-160X}, year = {2020}, date = {2020-12-28}, journal = {ADVANCED ELECTRONIC MATERIALS}, volume = {7}, number = {2}, publisher = {WILEY}, address = {111 RIVER ST, HOBOKEN 07030-5774, NJ USA}, abstract = {Surface functionalization with organic electron donors (OEDs) is an effective doping strategy for 2D materials, which can achieve doping levels beyond those possible with conventional electric field gating. While the effectiveness of surface functionalization has been demonstrated in many 2D systems, the doping efficiencies of OEDs have largely been unmeasured, which is in stark contrast to their precision syntheses and tailored redox potentials. Here, using monolayer MoS2 as a model system and an organic reductant based on 4,4'-bipyridine (DMAP-OED) as a strong organic dopant, it is established that the doping efficiency of DMAP-OED to MoS2 is in the range of 0.63 to 1.26 electrons per molecule. The highest doping levels to date are also achieved in monolayer MoS2 by surface functionalization and demonstrate that DMAP-OED is a stronger dopant than benzyl viologen, which is the previous best OED dopant. The measured range of the doping efficiency is in good agreement with the values predicted from first-principles calculations. This work provides a basis for the rational design of OEDs for high-level doping of 2D materials.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Surface functionalization with organic electron donors (OEDs) is an effective doping strategy for 2D materials, which can achieve doping levels beyond those possible with conventional electric field gating. While the effectiveness of surface functionalization has been demonstrated in many 2D systems, the doping efficiencies of OEDs have largely been unmeasured, which is in stark contrast to their precision syntheses and tailored redox potentials. Here, using monolayer MoS2 as a model system and an organic reductant based on 4,4'-bipyridine (DMAP-OED) as a strong organic dopant, it is established that the doping efficiency of DMAP-OED to MoS2 is in the range of 0.63 to 1.26 electrons per molecule. The highest doping levels to date are also achieved in monolayer MoS2 by surface functionalization and demonstrate that DMAP-OED is a stronger dopant than benzyl viologen, which is the previous best OED dopant. The measured range of the doping efficiency is in good agreement with the values predicted from first-principles calculations. This work provides a basis for the rational design of OEDs for high-level doping of 2D materials. |
Noori, Keian; Quek, Su Ying; Rodin, Aleksandr Hydrogen adatoms on graphene: The role of hybridization and lattice distortion Journal Article PHYSICAL REVIEW B, 102 (19), 2020, ISSN: 2469-9950. @article{ISI:000588226400003, title = {Hydrogen adatoms on graphene: The role of hybridization and lattice distortion}, author = {Keian Noori and Su Ying Quek and Aleksandr Rodin}, doi = {10.1103/PhysRevB.102.195416}, times_cited = {0}, issn = {2469-9950}, year = {2020}, date = {2020-11-11}, journal = {PHYSICAL REVIEW B}, volume = {102}, number = {19}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {By performing a detailed study of hydrogen adsorbates on graphene using density functional theory (DFT), we propose a general tight-binding (TB) formalism for a simultaneous treatment of multiple impurities of arbitrary species. To elucidate the details of the hydrogen-graphene bonding, we systematically examine the effects of hybridization and deformation on the band structure and the spectral function. An enhanced understanding of the binding mechanisms leads to a TB model whose predicted spectral function compares favorably with the DFT calculations on the scale of the supercell, as well as the individual adsorbates and carbon atoms. The computational load of our model scales with the number of impurities, not their separation, making it especially useful for experimentally relevant clustered impurity configurations that are too computationally expensive for DFT. The formalism described here allows for the treatment of Anderson impurities and impurities that bind to multiple carbon atoms.}, keywords = {}, pubstate = {published}, tppubtype = {article} } By performing a detailed study of hydrogen adsorbates on graphene using density functional theory (DFT), we propose a general tight-binding (TB) formalism for a simultaneous treatment of multiple impurities of arbitrary species. To elucidate the details of the hydrogen-graphene bonding, we systematically examine the effects of hybridization and deformation on the band structure and the spectral function. An enhanced understanding of the binding mechanisms leads to a TB model whose predicted spectral function compares favorably with the DFT calculations on the scale of the supercell, as well as the individual adsorbates and carbon atoms. The computational load of our model scales with the number of impurities, not their separation, making it especially useful for experimentally relevant clustered impurity configurations that are too computationally expensive for DFT. The formalism described here allows for the treatment of Anderson impurities and impurities that bind to multiple carbon atoms. |
Xu, Runrun; Xuan, Fengyuan; Quek, Su Ying Spin-Dependent Tunneling Barriers in CoPc/VSe2 from Many-Body Interactions Journal Article 12 JOURNAL OF PHYSICAL CHEMISTRY LETTERS, 11 (21), pp. 9358-9363, 2020, ISSN: 1948-7185. @article{ISI:000589920000062, title = {Spin-Dependent Tunneling Barriers in CoPc/VSe_{2} from Many-Body Interactions}, author = {Runrun Xu and Fengyuan Xuan and Su Ying Quek}, doi = {10.1021/acs.jpclett.0c02944}, times_cited = {12}, issn = {1948-7185}, year = {2020}, date = {2020-11-05}, journal = {JOURNAL OF PHYSICAL CHEMISTRY LETTERS}, volume = {11}, number = {21}, pages = {9358-9363}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Mixed-dimensional magnetic heterostructures are intriguing, newly available platforms to explore quantum physics and its applications. Using state-of-the-art many-body perturbation theory, we predict the energy level alignment for a self-assembled monolayer of cobalt phthalocyanine (CoPc) molecules on magnetic VSe2 monolayers. The predicted projected density of states on CoPc agrees with experimental scanning tunneling spectra. Consistent with experiment, we predict a shoulder in the unoccupied region of the spectra that is absent from mean-field calculations. Unlike the nearly spin-degenerate gas-phase frontier molecular orbitals, the tunneling barriers at the interface are spin-dependent, a finding of interest for quantum information and spintronics applications. Both the experimentally observed shoulder and the predicted spin-dependent tunneling barriers originate from many-body interactions in the interface-hybridized states. Our results showcase the intricate many-body physics that governs the properties of these mixed-dimensional magnetic heterostructures and suggests the possibility of manipulating the spin-dependent tunneling barriers through modifications of interface coupling.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Mixed-dimensional magnetic heterostructures are intriguing, newly available platforms to explore quantum physics and its applications. Using state-of-the-art many-body perturbation theory, we predict the energy level alignment for a self-assembled monolayer of cobalt phthalocyanine (CoPc) molecules on magnetic VSe2 monolayers. The predicted projected density of states on CoPc agrees with experimental scanning tunneling spectra. Consistent with experiment, we predict a shoulder in the unoccupied region of the spectra that is absent from mean-field calculations. Unlike the nearly spin-degenerate gas-phase frontier molecular orbitals, the tunneling barriers at the interface are spin-dependent, a finding of interest for quantum information and spintronics applications. Both the experimentally observed shoulder and the predicted spin-dependent tunneling barriers originate from many-body interactions in the interface-hybridized states. Our results showcase the intricate many-body physics that governs the properties of these mixed-dimensional magnetic heterostructures and suggests the possibility of manipulating the spin-dependent tunneling barriers through modifications of interface coupling. |
Li, Xing; Xu, Hai-Sen; Leng, Kai; Chee, See Wee; Zhao, Xiaoxu; Jain, Noopur; Xu, Hai; Qiao, Jingsi; Gao, Qiang; Park, In-Hyeok; Quek, Su Ying; Mirsaidov, Utkur; Loh, Kian Ping Partitioning the interlayer space of covalent organic frameworks by embedding pseudorotaxanes in their backbones Journal Article NATURE CHEMISTRY, 12 (12), pp. 1115-+, 2020, ISSN: 1755-4330. @article{ISI:000584339900001, title = {Partitioning the interlayer space of covalent organic frameworks by embedding pseudorotaxanes in their backbones}, author = {Xing Li and Hai-Sen Xu and Kai Leng and See Wee Chee and Xiaoxu Zhao and Noopur Jain and Hai Xu and Jingsi Qiao and Qiang Gao and In-Hyeok Park and Su Ying Quek and Utkur Mirsaidov and Kian Ping Loh}, doi = {10.1038/s41557-020-00562-5}, times_cited = {0}, issn = {1755-4330}, year = {2020}, date = {2020-11-02}, journal = {NATURE CHEMISTRY}, volume = {12}, number = {12}, pages = {1115-+}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {Mono- or few-layer sheets of covalent organic frameworks (COFs) represent an attractive platform of two-dimensional materials that hold promise for tailor-made functionality and pores, through judicious design of the COF building blocks. But although a wide variety of layered COFs have been synthesized, cleaving their interlayer stacking to obtain COF sheets of uniform thickness has remained challenging. Here, we have partitioned the interlayer space in COFs by incorporating pseudorotaxane units into their backbones. Macrocyclic hosts based on crown ethers were embedded into either a ditopic or a tetratopic acylhydrazide building block. Reaction with a tritopic aldehyde linker led to the formation of acylhydrazone-based layered COFs in which one basal plane is composed of either one layer, in the case of the ditopic macrocyclic component, or two adjacent layers covalently held together by its tetratopic counterpart. When a viologen threading unit is introduced, the formation of a host-guest complex facilitates the self-exfoliation of the COFs into crystalline monolayers or bilayers, respectively.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Mono- or few-layer sheets of covalent organic frameworks (COFs) represent an attractive platform of two-dimensional materials that hold promise for tailor-made functionality and pores, through judicious design of the COF building blocks. But although a wide variety of layered COFs have been synthesized, cleaving their interlayer stacking to obtain COF sheets of uniform thickness has remained challenging. Here, we have partitioned the interlayer space in COFs by incorporating pseudorotaxane units into their backbones. Macrocyclic hosts based on crown ethers were embedded into either a ditopic or a tetratopic acylhydrazide building block. Reaction with a tritopic aldehyde linker led to the formation of acylhydrazone-based layered COFs in which one basal plane is composed of either one layer, in the case of the ditopic macrocyclic component, or two adjacent layers covalently held together by its tetratopic counterpart. When a viologen threading unit is introduced, the formation of a host-guest complex facilitates the self-exfoliation of the COFs into crystalline monolayers or bilayers, respectively. |
Qian, Cheng; Zhou, Weiqiang; Qiao, Jingsi; Wang, Dongdong; Li, Xing; Teo, Wei Liang; Shi, Xiangyan; Wu, Hongwei; Di, Jun; Wang, Hou; Liu, Guofeng; Gu, Long; Liu, Jiawei; Feng, Lili; Liu, Yuchuan; Quek, Su Ying; Loh, Kian Ping; Zhao, Yanli JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 142 (42), pp. 18138-18149, 2020, ISSN: 0002-7863. @article{ISI:000580559000038, title = {Linkage Engineering by Harnessing Supramolecular Interactions to Fabricate 2D Hydrazone-Linked Covalent Organic Framework Platforms toward Advanced Catalysis}, author = {Cheng Qian and Weiqiang Zhou and Jingsi Qiao and Dongdong Wang and Xing Li and Wei Liang Teo and Xiangyan Shi and Hongwei Wu and Jun Di and Hou Wang and Guofeng Liu and Long Gu and Jiawei Liu and Lili Feng and Yuchuan Liu and Su Ying Quek and Kian Ping Loh and Yanli Zhao}, doi = {10.1021/jacs.0c08436}, times_cited = {0}, issn = {0002-7863}, year = {2020}, date = {2020-10-21}, journal = {JOURNAL OF THE AMERICAN CHEMICAL SOCIETY}, volume = {142}, number = {42}, pages = {18138-18149}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Covalent organic frameworks (COFs) are an emerging class of crystalline porous polymers with tailor-made structures and functionalities. To facilitate their utilization for advanced applications, it is crucial to develop a systematic approach to control the properties of COFs, including the crystallinity, stability, and functionalities. However, such an integrated design is challenging to achieve. Herein, we report supramolecular strategy-based linkage engineering to fabricate a versatile 2D hydrazone-linked COF platform for the coordination of different transition metal ions. Intra- and intermolecular hydrogen bonding as well as electrostatic interactions in the antiparallel stacking mode were first utilized to obtain two isoreticular COFs, namely COF-DB and COF-DT. On account of suitable nitrogen sites in COF-DB, the further metalation of COF-DB was accomplished upon the complexation with seven divalent transition metal ions M(II) (M = Mn, Co, Ni, Cu, Zn, Pd, and Cd) under mild conditions. The resultant M/COF-DB exhibited extended p-conjugation, improved crystallinity, enhanced stability, and additional functionalities as compared to the parent COF-DB. Furthermore, the dynamic nature of the coordination bonding in M/COF-DB allows for the easy replacement of metal ions through a postsynthetic exchange. In particular, the coordination mode in Pd/COF-DB endows it with excellent catalytic activity and cyclic stability as a heterogeneous catalyst for the Suzuki-Miyaura cross-coupling reaction, outperforming its amorphous counterparts and Pd/COF-DT. This strategy provides an opportunity for the construction of 2D COFs with designable functions and opens an avenue to create COFs as multifunctional systems.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Covalent organic frameworks (COFs) are an emerging class of crystalline porous polymers with tailor-made structures and functionalities. To facilitate their utilization for advanced applications, it is crucial to develop a systematic approach to control the properties of COFs, including the crystallinity, stability, and functionalities. However, such an integrated design is challenging to achieve. Herein, we report supramolecular strategy-based linkage engineering to fabricate a versatile 2D hydrazone-linked COF platform for the coordination of different transition metal ions. Intra- and intermolecular hydrogen bonding as well as electrostatic interactions in the antiparallel stacking mode were first utilized to obtain two isoreticular COFs, namely COF-DB and COF-DT. On account of suitable nitrogen sites in COF-DB, the further metalation of COF-DB was accomplished upon the complexation with seven divalent transition metal ions M(II) (M = Mn, Co, Ni, Cu, Zn, Pd, and Cd) under mild conditions. The resultant M/COF-DB exhibited extended p-conjugation, improved crystallinity, enhanced stability, and additional functionalities as compared to the parent COF-DB. Furthermore, the dynamic nature of the coordination bonding in M/COF-DB allows for the easy replacement of metal ions through a postsynthetic exchange. In particular, the coordination mode in Pd/COF-DB endows it with excellent catalytic activity and cyclic stability as a heterogeneous catalyst for the Suzuki-Miyaura cross-coupling reaction, outperforming its amorphous counterparts and Pd/COF-DT. This strategy provides an opportunity for the construction of 2D COFs with designable functions and opens an avenue to create COFs as multifunctional systems. |
Wang, Juefan; Quek, Su Ying Isolated flat bands and physics of mixed dimensions in a 2D covalent organic framework Journal Article NANOSCALE, 12 (39), pp. 20279-20286, 2020, ISSN: 2040-3364. @article{ISI:000579877200016, title = {Isolated flat bands and physics of mixed dimensions in a 2D covalent organic framework}, author = {Juefan Wang and Su Ying Quek}, doi = {10.1039/d0nr04428h}, times_cited = {0}, issn = {2040-3364}, year = {2020}, date = {2020-10-21}, journal = {NANOSCALE}, volume = {12}, number = {39}, pages = {20279-20286}, publisher = {ROYAL SOC CHEMISTRY}, address = {THOMAS GRAHAM HOUSE, SCIENCE PARK, MILTON RD, CAMBRIDGE CB4 0WF, CAMBS, ENGLAND}, abstract = {We demonstrate that it is possible to rationally incorporate both an isolated flat band and the physics of zero dimensions (0D), one dimension (1D), and two dimensions (2D) in a single 2D material. Such unique electronic properties are present in a recently synthesized 2D covalent organic framework (COF), where "I"-shaped building blocks and "T"-shaped connectors result in quasi-1D chains that are linked by quasi-0D bridge units arranged in a stable 2D lattice. The lowest unoccupied conduction band is an isolated flat band, and electron-doping gives rise to novel quantum phenomena, such as magnetism and Mott insulating phases. The highest occupied valence band arises from wave functions in the quasi-1D chains. Examples of mixed dimensional physics are illustrated in this system. The strong electron-hole asymmetry in this material results in a large Seebeck coefficient, while the quasi-1D nature of the chains leads to linear dichroism, in conjunction with strongly bound 2D excitons. We elucidate strategies to design and optimize 2D COFs to host both isolated flat bands and quantum-confined 1D subsystems. The properties of the 2D COF discussed here provide a taste of the intriguing possibilities in this open research field.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We demonstrate that it is possible to rationally incorporate both an isolated flat band and the physics of zero dimensions (0D), one dimension (1D), and two dimensions (2D) in a single 2D material. Such unique electronic properties are present in a recently synthesized 2D covalent organic framework (COF), where "I"-shaped building blocks and "T"-shaped connectors result in quasi-1D chains that are linked by quasi-0D bridge units arranged in a stable 2D lattice. The lowest unoccupied conduction band is an isolated flat band, and electron-doping gives rise to novel quantum phenomena, such as magnetism and Mott insulating phases. The highest occupied valence band arises from wave functions in the quasi-1D chains. Examples of mixed dimensional physics are illustrated in this system. The strong electron-hole asymmetry in this material results in a large Seebeck coefficient, while the quasi-1D nature of the chains leads to linear dichroism, in conjunction with strongly bound 2D excitons. We elucidate strategies to design and optimize 2D COFs to host both isolated flat bands and quantum-confined 1D subsystems. The properties of the 2D COF discussed here provide a taste of the intriguing possibilities in this open research field. |
Nisi, Katharina; Subramanian, Shruti; He, Wen; Ulman, Kanchan Ajit; El-Sherif, Hesham; Sigger, Florian; Lassauniere, Margaux; Wetherington, Maxwell T; Briggs, Natalie; Gray, Jennifer; Holleitner, Alexander W; Bassim, Nabil; Quek, Su Ying; Robinson, Joshua A; Wurstbauer, Ursula Light-Matter Interaction in Quantum Confined 2D Polar Metals Journal Article ADVANCED FUNCTIONAL MATERIALS, 31 (4), 2020, ISSN: 1616-301X. @article{ISI:000578331800001, title = {Light-Matter Interaction in Quantum Confined 2D Polar Metals}, author = {Katharina Nisi and Shruti Subramanian and Wen He and Kanchan Ajit Ulman and Hesham El-Sherif and Florian Sigger and Margaux Lassauniere and Maxwell T Wetherington and Natalie Briggs and Jennifer Gray and Alexander W Holleitner and Nabil Bassim and Su Ying Quek and Joshua A Robinson and Ursula Wurstbauer}, doi = {10.1002/adfm.202005977}, times_cited = {0}, issn = {1616-301X}, year = {2020}, date = {2020-10-15}, journal = {ADVANCED FUNCTIONAL MATERIALS}, volume = {31}, number = {4}, publisher = {WILEY-V C H VERLAG GMBH}, address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY}, abstract = {This work is a systematic experimental and theoretical study of the in-plane dielectric functions of 2D gallium and indium films consisting of two or three atomic metal layers confined between silicon carbide and graphene with a corresponding bonding gradient from covalent to metallic to van der Waals type.k-space resolved free electron and bound electron contributions to the optical response are identified, with the latter pointing towards the existence of thickness dependent quantum confinement phenomena. The resonance energies in the dielectric functions and the observed epsilon near-zero behavior in the near infrared to visible spectral range, are dependent on the number of atomic metal layers and properties of the metal involved. A model-based spectroscopic ellipsometry approach is used to estimate the number of atomic metal layers, providing a convenient route over expensive invasive characterization techniques. A strong thickness and metal choice dependence of the light-matter interaction makes these half van der Waals 2D polar metals attractive for quantum engineered metal films, tunable (quantum-)plasmonics and nano-photonics.}, keywords = {}, pubstate = {published}, tppubtype = {article} } This work is a systematic experimental and theoretical study of the in-plane dielectric functions of 2D gallium and indium films consisting of two or three atomic metal layers confined between silicon carbide and graphene with a corresponding bonding gradient from covalent to metallic to van der Waals type.k-space resolved free electron and bound electron contributions to the optical response are identified, with the latter pointing towards the existence of thickness dependent quantum confinement phenomena. The resonance energies in the dielectric functions and the observed epsilon near-zero behavior in the near infrared to visible spectral range, are dependent on the number of atomic metal layers and properties of the metal involved. A model-based spectroscopic ellipsometry approach is used to estimate the number of atomic metal layers, providing a convenient route over expensive invasive characterization techniques. A strong thickness and metal choice dependence of the light-matter interaction makes these half van der Waals 2D polar metals attractive for quantum engineered metal films, tunable (quantum-)plasmonics and nano-photonics. |
Xuan, Fengyuan; Quek, Su Ying Valley Zeeman effect and Landau levels in two-dimensional transition metal dichalcogenides Journal Article 22 PHYSICAL REVIEW RESEARCH , 2 (3), 2020. @article{ISI:000604153900004, title = {Valley Zeeman effect and Landau levels in two-dimensional transition metal dichalcogenides }, author = {Fengyuan Xuan and Su Ying Quek}, doi = {10.1103/PhysRevResearch.2.033256}, times_cited = {22}, year = {2020}, date = {2020-08-17}, journal = {PHYSICAL REVIEW RESEARCH }, volume = {2}, number = {3}, publisher = {AMER PHYSICAL SOC }, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA }, abstract = {This paper presents a theoretical description of both the valley Zeeman effect (g-factors) and Landau levels in two-dimensional H-phase transition metal dichalcogenides (TMDs) using the Luttinger-Kohn approximation with spin-orbit coupling. At the valley extrema in TMDs, energy bands split into Landau levels with a Zeeman shift in the presence of a uniform out-of-plane external magnetic field. The Landau level indices are symmetric in the K and K' valleys. We develop a numerical approach to compute the single-band g-factors from first principles without the need for a sum over unoccupied bands. Many-body effects are included perturbatively within the GW approximation. Nonlocal exchange and correlation self-energy effects in the GW calculations increase the magnitude of single-band g-factors compared to those obtained from density functional theory. Our first-principles results give spin- and valley-split Landau levels, in agreement with recent optical experiments. The exciton g-factors deduced in this work are also in good agreement with experiment for the bright and dark excitons in monolayer WSe2, as well as the lowest-energy bright excitons in MoSe2-WSe2 heterobilayers with different twist angles. }, keywords = {}, pubstate = {published}, tppubtype = {article} } This paper presents a theoretical description of both the valley Zeeman effect (g-factors) and Landau levels in two-dimensional H-phase transition metal dichalcogenides (TMDs) using the Luttinger-Kohn approximation with spin-orbit coupling. At the valley extrema in TMDs, energy bands split into Landau levels with a Zeeman shift in the presence of a uniform out-of-plane external magnetic field. The Landau level indices are symmetric in the K and K' valleys. We develop a numerical approach to compute the single-band g-factors from first principles without the need for a sum over unoccupied bands. Many-body effects are included perturbatively within the GW approximation. Nonlocal exchange and correlation self-energy effects in the GW calculations increase the magnitude of single-band g-factors compared to those obtained from density functional theory. Our first-principles results give spin- and valley-split Landau levels, in agreement with recent optical experiments. The exciton g-factors deduced in this work are also in good agreement with experiment for the bright and dark excitons in monolayer WSe2, as well as the lowest-energy bright excitons in MoSe2-WSe2 heterobilayers with different twist angles. |
Li, Xing; Qiao, Jingsi; Quek, Su Ying; Loh, Kian Ping The Origin of Dual Emission in Antiparallel-Stacked Two-Dimensional Covalent Organic Frameworks Journal Article ACS MATERIALS LETTERS, 2 (6), pp. 654-657, 2020. @article{ISI:000538342300013, title = {The Origin of Dual Emission in Antiparallel-Stacked Two-Dimensional Covalent Organic Frameworks}, author = {Xing Li and Jingsi Qiao and Su Ying Quek and Kian Ping Loh}, doi = {10.1021/acsmaterialslett.0c00119}, times_cited = {0}, year = {2020}, date = {2020-06-01}, journal = {ACS MATERIALS LETTERS}, volume = {2}, number = {6}, pages = {654-657}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Most previous reports on covalent organic frameworks (COFs) are centered on eclipsed (or presumably eclipsed) structures, while the properties peculiar to the antiparallel stacked structure are little known. Recently, we discover that two-dimensional (2D) acylhydrazone COFs with alkyoxy side chains adopt an antiparallel stacked structure. This provides us with the opportunity to re-examine the origin of dual emission in such COFs in the context of an antiparallel stacked structure, in lieu of an earlier proposal based on incorrectly assigned eclipsed stacked structure. We discover that the intralayer and interlayer hydrogen bonding in the antiparallel stacked structure work synergistically to restrict intermolecular bond rotation, thus turning on fluorescence. The interlayer hydrogen bonding between the acyl oxygen and the side-chain hydrogen is stronger in the dual emissive COF, compared to those in the single emissive ones, underscoring the roles of competing interhydrogen and intrahydrogen bonds in the excited-state conformers.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Most previous reports on covalent organic frameworks (COFs) are centered on eclipsed (or presumably eclipsed) structures, while the properties peculiar to the antiparallel stacked structure are little known. Recently, we discover that two-dimensional (2D) acylhydrazone COFs with alkyoxy side chains adopt an antiparallel stacked structure. This provides us with the opportunity to re-examine the origin of dual emission in such COFs in the context of an antiparallel stacked structure, in lieu of an earlier proposal based on incorrectly assigned eclipsed stacked structure. We discover that the intralayer and interlayer hydrogen bonding in the antiparallel stacked structure work synergistically to restrict intermolecular bond rotation, thus turning on fluorescence. The interlayer hydrogen bonding between the acyl oxygen and the side-chain hydrogen is stronger in the dual emissive COF, compared to those in the single emissive ones, underscoring the roles of competing interhydrogen and intrahydrogen bonds in the excited-state conformers. |
Fu, Wei; Qiao, Jingsi; Zhao, Xiaoxu; Chen, Yu; Fu, Deyi; Yu, Wei; Leng, Kai; Song, Peng; Chen, Zhi; Yu, Ting; Pennycook, Stephen J; Quek, Su Ying; Loh, Kian Ping Room Temperature Commensurate Charge Density Wave on Epitaxially Grown Bilayer 2H-Tantalum Sulfide on Hexagonal Boron Nitride Journal Article 27 ACS NANO, 14 (4), pp. 3917-3926, 2020, ISSN: 1936-0851. @article{ISI:000529895500017, title = {Room Temperature Commensurate Charge Density Wave on Epitaxially Grown Bilayer 2H-Tantalum Sulfide on Hexagonal Boron Nitride}, author = {Wei Fu and Jingsi Qiao and Xiaoxu Zhao and Yu Chen and Deyi Fu and Wei Yu and Kai Leng and Peng Song and Zhi Chen and Ting Yu and Stephen J Pennycook and Su Ying Quek and Kian Ping Loh}, doi = {10.1021/acsnano.0c00303}, times_cited = {27}, issn = {1936-0851}, year = {2020}, date = {2020-04-28}, journal = {ACS NANO}, volume = {14}, number = {4}, pages = {3917-3926}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {The breaking of multiple symmetries by periodic lattice distortion at a commensurate charge density wave (CDW) state is expected to give rise to intriguing interesting properties. However, accessing the commensurate CDW state on bulk TaS, crystals typically requires cryogenic temperatures (77 K), which precludes practical applications. Here, we found that heteroepitaxial growth of a 2H-tantalum disulfide bilayer on a hexagonal-boron nitride (h-BN) substrate produces a robust commensurate CDW order at room temperature, characterized by a Moire superlattice of 3 X 3 TaS2 on a 4 X 4 h-BN unit cell. The CDW order is confirmed by scanning transmission electron microscopy and Raman measurements. Theoretical calculations reveal that the stabilizing energy for the CDW phase of the monolayer and bilayer 2H-TaS2-on-h-BN substrates arises primarily from interfacial electrostatic interactions and, to a lesser extent, interfacial strain. Our work shows that engineering interfacial electrostatic interactions in an ultrathin van der Waals heterostructure constitutes an effective way to enhance CDW order in two-dimensional materials.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The breaking of multiple symmetries by periodic lattice distortion at a commensurate charge density wave (CDW) state is expected to give rise to intriguing interesting properties. However, accessing the commensurate CDW state on bulk TaS, crystals typically requires cryogenic temperatures (77 K), which precludes practical applications. Here, we found that heteroepitaxial growth of a 2H-tantalum disulfide bilayer on a hexagonal-boron nitride (h-BN) substrate produces a robust commensurate CDW order at room temperature, characterized by a Moire superlattice of 3 X 3 TaS2 on a 4 X 4 h-BN unit cell. The CDW order is confirmed by scanning transmission electron microscopy and Raman measurements. Theoretical calculations reveal that the stabilizing energy for the CDW phase of the monolayer and bilayer 2H-TaS2-on-h-BN substrates arises primarily from interfacial electrostatic interactions and, to a lesser extent, interfacial strain. Our work shows that engineering interfacial electrostatic interactions in an ultrathin van der Waals heterostructure constitutes an effective way to enhance CDW order in two-dimensional materials. |
Noori, Keian; Biswas, Hillol; Quek, Su Ying; Rodin, Aleksandr Graphene-mediated interaction between hydrogen adsorbates Journal Article PHYSICAL REVIEW B, 101 (11), 2020, ISSN: 2469-9950. @article{ISI:000519990500007, title = {Graphene-mediated interaction between hydrogen adsorbates}, author = {Keian Noori and Hillol Biswas and Su Ying Quek and Aleksandr Rodin}, doi = {10.1103/PhysRevB.101.115421}, times_cited = {0}, issn = {2469-9950}, year = {2020}, date = {2020-03-18}, journal = {PHYSICAL REVIEW B}, volume = {101}, number = {11}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {Interaction between adsorbed hydrogen atoms in graphene is studied using a combination of DFT and the path integral formalism. Our results reveal a complex nonmonotonic interaction profile. We show that the strength and sign of the interaction are dictated by the adsorbate arrangement, as well as the system doping. The path integral approach given here allows one to compute energies and densities in an efficient manner without relying on exact diagonalization.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Interaction between adsorbed hydrogen atoms in graphene is studied using a combination of DFT and the path integral formalism. Our results reveal a complex nonmonotonic interaction profile. We show that the strength and sign of the interaction are dictated by the adsorbate arrangement, as well as the system doping. The path integral approach given here allows one to compute energies and densities in an efficient manner without relying on exact diagonalization. |