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:
2014 |
Li, Suchun; Son, Young-Woo; Quek, Su Ying Large magnetoresistance from long-range interface coupling in armchair graphene nanoribbon junctions Journal Article APPLIED PHYSICS LETTERS, 105 (24), 2014, ISSN: 0003-6951. @article{ISI:000346643600051, title = {Large magnetoresistance from long-range interface coupling in armchair graphene nanoribbon junctions}, author = {Suchun Li and Young-Woo Son and Su Ying Quek}, doi = {10.1063/1.4904830}, times_cited = {0}, issn = {0003-6951}, year = {2014}, date = {2014-12-15}, journal = {APPLIED PHYSICS LETTERS}, volume = {105}, number = {24}, publisher = {AMER INST PHYSICS}, address = {1305 WALT WHITMAN RD, STE 300, MELVILLE, NY 11747-4501 USA}, abstract = {In recent years, bottom-up synthesis procedures have achieved significant advancements in atomically controlled growth of several-nanometer-long graphene nanoribbons with armchair-shaped edges (AGNRs). This greatly encourages us to explore the potential of such well-defined AGNRs in electronics and spintronics. Here, we propose an AGNR based spin valve architecture that induces a large magnetoresistance up to 900%. We find that, when an AGNR is connected perpendicularly to zigzag-shaped edges, the AGNR allows for long-range extension of the otherwise localized edge state. The huge magnetoresistance is a direct consequence of the coupling of two such extended states from both ends of the AGNR, which forms a perfect transmission channel. By tuning the coupling between these two spin-polarized states with a magnetic field, the channel can be destroyed, leading to an abrupt drop in electron transmission. (C) 2014 AIP Publishing LLC.}, keywords = {}, pubstate = {published}, tppubtype = {article} } In recent years, bottom-up synthesis procedures have achieved significant advancements in atomically controlled growth of several-nanometer-long graphene nanoribbons with armchair-shaped edges (AGNRs). This greatly encourages us to explore the potential of such well-defined AGNRs in electronics and spintronics. Here, we propose an AGNR based spin valve architecture that induces a large magnetoresistance up to 900%. We find that, when an AGNR is connected perpendicularly to zigzag-shaped edges, the AGNR allows for long-range extension of the otherwise localized edge state. The huge magnetoresistance is a direct consequence of the coupling of two such extended states from both ends of the AGNR, which forms a perfect transmission channel. By tuning the coupling between these two spin-polarized states with a magnetic field, the channel can be destroyed, leading to an abrupt drop in electron transmission. (C) 2014 AIP Publishing LLC. |
Quek, Su Ying; Khoo, Khoong Hong ACCOUNTS OF CHEMICAL RESEARCH, 47 (11), pp. 3250-3257, 2014, ISSN: 0001-4842. @article{ISI:000345262200006, title = {Predictive DFT-Based Approaches to Charge and Spin Transport in Single-Molecule Junctions and Two-Dimensional Materials: Successes and Challenges}, author = {Su Ying Quek and Khoong Hong Khoo}, doi = {10.1021/ar4002526}, times_cited = {0}, issn = {0001-4842}, year = {2014}, date = {2014-11-01}, journal = {ACCOUNTS OF CHEMICAL RESEARCH}, volume = {47}, number = {11}, pages = {3250-3257}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {CONSPECTUS: The emerging field of flexible electronics based on organics and two-dimensional (2D) materials relies on a fundamental understanding of charge and spin transport at the molecular and nanoscale. It is desirable to make predictions and shine light on unexplained experimental phenomena independently of experimentally derived parameters. Indeed, density functional theory (DFT), the workhorse of first-principles approaches, has been used extensively to model charge/spin transport at the nanoscale. However, DFT is essentially a ground state theory that simply guarantees correct total energies given the correct charge density, while charge/spin transport is a nonequilibrium phenomenon involving the scattering of quasiparticles.}, keywords = {}, pubstate = {published}, tppubtype = {article} } CONSPECTUS: The emerging field of flexible electronics based on organics and two-dimensional (2D) materials relies on a fundamental understanding of charge and spin transport at the molecular and nanoscale. It is desirable to make predictions and shine light on unexplained experimental phenomena independently of experimentally derived parameters. Indeed, density functional theory (DFT), the workhorse of first-principles approaches, has been used extensively to model charge/spin transport at the nanoscale. However, DFT is essentially a ground state theory that simply guarantees correct total energies given the correct charge density, while charge/spin transport is a nonequilibrium phenomenon involving the scattering of quasiparticles. |
Kim, Taekyeong; Darancet, Pierre; Widawsky, Jonathan R; Kotiuga, Michele; Quek, Su Ying; Neaton, Jeffrey B; Venkataraman, Latha Determination of Energy Level Alignment and Coupling Strength in 4,4′-Bipyridine Single-Molecule Junctions Journal Article NANO LETTERS, 14 (2), pp. 794-798, 2014, ISSN: 1530-6984. @article{ISI:000331343900061, title = {Determination of Energy Level Alignment and Coupling Strength in 4,4′-Bipyridine Single-Molecule Junctions}, author = {Taekyeong Kim and Pierre Darancet and Jonathan R Widawsky and Michele Kotiuga and Su Ying Quek and Jeffrey B Neaton and Latha Venkataraman}, doi = {10.1021/nl404143v}, times_cited = {0}, issn = {1530-6984}, year = {2014}, date = {2014-02-01}, journal = {NANO LETTERS}, volume = {14}, number = {2}, pages = {794-798}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {We measure conductance and thermopower of single Au-4,4'-bipyridine-Au junctions in distinct low and high conductance binding geometries accessed by modulating the electrode separation. We use these data to determine the electronic energy level alignment and coupling strength for these junctions, which are known to conduct through the lowest unoccupied molecular orbital (LUMO). Contrary to intuition, we find that, in the high-conductance junction, the LUMO resonance energy is further away from the Au Fermi energy than in the low-conductance junction. However, the LUMO of the high-conducting junction is better coupled to the electrode. These results are in good quantitative agreement with self-energy corrected zero-bias density functional theory calculations. Our calculations show further that measurements of conductance and thermopower in amine-terminated oligophenyl-Au junctions, where conduction occurs through the highest occupied molecular orbitals, cannot be used to extract electronic parameters as their transmission functions do not follow a simple Lorentzian form.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We measure conductance and thermopower of single Au-4,4'-bipyridine-Au junctions in distinct low and high conductance binding geometries accessed by modulating the electrode separation. We use these data to determine the electronic energy level alignment and coupling strength for these junctions, which are known to conduct through the lowest unoccupied molecular orbital (LUMO). Contrary to intuition, we find that, in the high-conductance junction, the LUMO resonance energy is further away from the Au Fermi energy than in the low-conductance junction. However, the LUMO of the high-conducting junction is better coupled to the electrode. These results are in good quantitative agreement with self-energy corrected zero-bias density functional theory calculations. Our calculations show further that measurements of conductance and thermopower in amine-terminated oligophenyl-Au junctions, where conduction occurs through the highest occupied molecular orbitals, cannot be used to extract electronic parameters as their transmission functions do not follow a simple Lorentzian form. |
Huang, Wen; Luo, Xin; Gan, Chee Kwan; Quek, Su Ying; Liang, Gengchiau Theoretical study of thermoelectric properties of few-layer MoS2 and WSe2 Journal Article 12 PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 16 (22), pp. 10866-10874, 2014, ISSN: 1463-9076. @article{ISI:000336781500069, title = {Theoretical study of thermoelectric properties of few-layer MoS_{2} and WSe_{2}}, author = {Wen Huang and Xin Luo and Chee Kwan Gan and Su Ying Quek and Gengchiau Liang}, doi = {10.1039/c4cp00487f}, times_cited = {12}, issn = {1463-9076}, year = {2014}, date = {2014-01-01}, journal = {PHYSICAL CHEMISTRY CHEMICAL PHYSICS}, volume = {16}, number = {22}, pages = {10866-10874}, publisher = {ROYAL SOC CHEMISTRY}, address = {THOMAS GRAHAM HOUSE, SCIENCE PARK, MILTON RD, CAMBRIDGE CB4 0WF, CAMBS, ENGLAND}, abstract = {Molybdenum disulfide (MoS2) and tungsten diselenide (WSe2) are prototypical layered two-dimensional transition metal dichalcogenide materials, with each layer consisting of three atomic planes. We refer to each layer as a trilayer (TL). We study the thermoelectric properties of 1-4TL MoS2 and WSe2 using a ballistic transport approach based on the electronic band structures and phonon dispersions obtained from first-principles calculations. Our results show that the thickness dependence of the thermoelectric properties is different under n-type and p-type doping conditions. Defining ZT(1st peak) as the first peak in the thermoelectric figure of merit ZT as doping levels increase from zero at 300 K, we found that ZT(1st peak) decreases as the number of layers increases for MoS2, with the exception of 2TL in n-type doping, which has a slightly higher value than 1TL. However, for WSe2, 2TL has the largest ZT(1st peak) in both n-type and p-type doping, with a ZT(1st peak) value larger than 1 for n-type WSe2. At high temperatures (T 4 300 K), ZT(1st peak) dramatically increases when the temperature increases, especially for n-type doping. The ZT(1st peak) of n-type 1TL-MoS2 and 2TL-WSe2 can reach 1.6 and 2.1, respectively.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Molybdenum disulfide (MoS2) and tungsten diselenide (WSe2) are prototypical layered two-dimensional transition metal dichalcogenide materials, with each layer consisting of three atomic planes. We refer to each layer as a trilayer (TL). We study the thermoelectric properties of 1-4TL MoS2 and WSe2 using a ballistic transport approach based on the electronic band structures and phonon dispersions obtained from first-principles calculations. Our results show that the thickness dependence of the thermoelectric properties is different under n-type and p-type doping conditions. Defining ZT(1st peak) as the first peak in the thermoelectric figure of merit ZT as doping levels increase from zero at 300 K, we found that ZT(1st peak) decreases as the number of layers increases for MoS2, with the exception of 2TL in n-type doping, which has a slightly higher value than 1TL. However, for WSe2, 2TL has the largest ZT(1st peak) in both n-type and p-type doping, with a ZT(1st peak) value larger than 1 for n-type WSe2. At high temperatures (T 4 300 K), ZT(1st peak) dramatically increases when the temperature increases, especially for n-type doping. The ZT(1st peak) of n-type 1TL-MoS2 and 2TL-WSe2 can reach 1.6 and 2.1, respectively. |