Dongyang Wan
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phywdy@nus.edu.sg
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SPECIALTY RESEARCH INST/CTRS
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SCIENCE
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PHYSICS
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Publication Electronic and plasmonic phenomena at nonstoichiometric grain boundaries in metallic SrNbO3(Royal Society of Chemistry, 2020-03-28) Song, Dongsheng; Wan, Dongyang; Wu, Hong-Hui; Xue, Deqing; Ning, Shoucong; Wu, Ming; Venkatesan, Thirumalai; Pennycook, Stephen J. J.; DEPT OF PHYSICS; ELECTRICAL AND COMPUTER ENGINEERING; MATERIALS SCIENCE AND ENGINEERING; NUS NANOSCIENCE & NANOTECH INITIATIVEGrain boundaries could exhibit exceptional electronic structure and exotic properties, which are determined by a local atomic configuration and stoichiometry that differs from the bulk. However, optical and plasmonic properties at the grain boundaries in metallic oxides have rarely been discussed before. Here, we show that non-stoichiometric grain boundaries in the newly discovered metallic SrNbO3 photocatalyst show exotic electronic, optical and plasmonic phenomena in comparison to bulk. Aberration-corrected scanning transmission electron microscopy and first-principles calculations reveal that a Nb-rich grain boundary exhibits an increased carrier concentration with quasi-1D metallic conductivity, and newly induced electronic states contributing to the broad energy range of optical absorption. More importantly, dielectric function calculations reveal extended and enhanced plasmonic excitations compared with bulk SrNbO3. Our results show that non-stoichiometric grain boundaries might be utilized to control the electronic and plasmonic properties in oxide photocatalysis. © 2020 The Royal Society of Chemistry.Publication New Family of Plasmonic Photocatalysts without Noble Metals(American Chemical Society, 2019-03-14) Wan, Dongyang; Yan, Bixing; Chen, Jianqiang; Wu, Shuyang; Hong, Jindui; Song, Donasheng; Zhao, Xiaoxu; Chi, Xiao; Zeng, Shengwei; Huang, Zhen; Li, Changjian; Han, Kun; Zhou, Wenxiong; Cao, Yu; Rusydi, Andrivo; Pennycook, Stephen J.; Yan, Ping; Ariando; Xu, Rong; Xu, Qing-Hua; Wang, X. Renshaw; Venkatesan, Thirumalai; ELECTRICAL AND COMPUTER ENGINEERING; PHYSICS; CHEMISTRY; MATERIALS SCIENCE AND ENGINEERING; NUS NANOSCIENCE & NANOTECH INITIATIVE; CHEMICAL & BIOMOLECULAR ENGINEERINGEfficient photocatalysis is important for sustainable energy. Recently, an unconventional photocatalyst based on intrinsic plasmon, called intrinsic plasmonic photocatalyst (IPP), seems promising for higher efficiency in hydrogen evolution. This catalyst seems to benefit from the advantages of visible light absorption, plasmon-assisted hot carrier generation, and good catalytic stability over conventional semiconductor photocatalysts. In this work, we report the relative hydrogen evolution efficiency under visible light irradiation of a family of IPP based on alkaline earth niobates (MNbO3, where M = Ca, Sr, or Ba), with efficiency of CaNbO3 > SrNbO3 > BaNbO3. The contributions of electron-phonon coupling time constant and solar energy absorption to the hydrogen evolution efficiency are identified as key based on our comprehensive study and characterization of carrier density (1022 cm-3), plasmon absorption, carrier dynamics, and surface area. This study demonstrates a generic approach to create a family of IPPs and further validates the role of solar energy absorption by intrinsic plasmon resonance in the enhancement of photocatalytic efficiency. © 2019 American Chemical Society.Publication Comparison of Typical Photocatalytic Systems with Intrinsic Plasmonic Photocatalysts Based on Strontium Niobate for Water Splitting(Wiley-VCH Verlag, 2017-11-06) Wan, Dongyang; Asmara, Teguh C.; Rusydi, Andrivo; Venkatesan, T.; DEPT OF PHYSICS; ELECTRICAL AND COMPUTER ENGINEERING; SINGAPORE SYNCHROTRON LIGHT SOURCE; NUS NANOSCIENCE & NANOTECH INITIATIVEWe report in this Review the study of conventional semiconductor-based photocatalysts and the photocatalytic properties of SrNbO3. The history of photocatalysis, its basic principles, and the problems associated with its commercial application will be discussed. These problems can be partially alleviated by using plasmonic metal nanoparticle/semiconductor systems. Then, we will introduce a new intrinsic plasmonic photocatalyst, SrNbO3. Epitaxial films of SrNbO3 are grown as a metallic oxide by means of pulsed laser deposition. This unusual material has a band gap of 4.1 eV with a degenerate conduction band with a charge-carrier density exceeding 1022 cm?3. This large charge-carrier density gives rise to a plasmon that in turn causes a strong absorption in the red range of light; this was mistaken earlier by the community to arise from a mid-gap state. When oxygen is introduced into this system, a special planar defect is formed that progressively reduces the percolation of the metallic region. Within the metallic region an unusual coherent plasmonic effect is seen, which arises from strong electronic correlation. Such a strong optical absorption enables the material to generate hydrogen from water under solar irradiation. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, WeinheimPublication Strontium Niobate for Near Infrared Plasmonics(WILEY, 2019-07-01) Dutta, A.; Wan, D. Y.; Yan, B. X.; Shalaev, V. M.; Venkatesan, T.; Boltasseva, A.; ELECTRICAL AND COMPUTER ENGINEERING; NUS NANOSCIENCE & NANOTECH INITIATIVEPlasmonics has developed greatly over the past two decades, and a plethora of plasmonic materials has been explored for practical plasmonic devices across various applications. While noble metals such as gold and silver are the most widely used plasmonic materials, other metals such as aluminum, copper, and magnesium have also been proposed as building blocks for plasmonics. Transparent conducting oxides (TCOs) such as aluminum? and gallium?doped zinc oxide and tin?doped indium oxide have been suggested for plasmonic applications in the near infrared (NIR) spectrum. These materials have lower carrier concentration than noble metals and therefore have lower material losses at the NIR. In this paper, strontium niobate (SNO) is showcased as an addition to the material database for plasmonics in the NIR. SNO plasmonic properties are studied through experimental demonstration of hybrid plasmon resonances in SNO films. Such resonances are characterized by electromagnetic field concentration in a low?index dielectric, sandwiched between a high?index dielectric resonator and a plasmonic metal. The results show that although SNO bulk film has an order of magnitude greater carrier concentration than TCOs, its losses are only a few times higher than in TCO materials thus holding a promise for plasmonic applications in the NIR.Publication Controlling Kondo-like Scattering at the SrTiO3-based Interfaces(Nature Publishing Group, 2016) Han, K; Palina, N; Zeng, S.W; Huang, Z; Li, C.J; Zhou, W.X; Wan, D.-Y; Zhang, L.C; Chi, X; Guo, R; Chen, J.S; Venkatesan, T; Rusydi, A; Ariando, NUSNNI-NanoCore, National University of Singapore, Singapore, 117411, Singapore, Department of Physics, National University of Singapore, Singapore, 117542, Singapore, National University of Singapore Graduate School for Integrative Sciences and Engineering (NGS), 28 Medical Drive, Singapore, 117456, Singapore; ELECTRICAL AND COMPUTER ENGINEERING; PHYSICS; CHEMISTRY; MATERIALS SCIENCE AND ENGINEERING; NUS NANOSCIENCE & NANOTECH INITIATIVEThe observation of magnetic interaction at the interface between nonmagnetic oxides has attracted much attention in recent years. In this report, we show that the Kondo-like scattering at the SrTiO3-based conducting interface is enhanced by increasing the lattice mismatch and growth oxygen pressure PO2. For the 26-unit-cell LaAlO3/SrTiO3 (LAO/STO) interface with lattice mismatch being 3.0%, the Kondo-like scattering is observed when PO2 is beyond 1 mTorr. By contrast, when the lattice mismatch is reduced to 1.0% at the (La0.3Sr0.7)(Al0.65Ta0.35)O3/SrTiO3 (LSAT/STO) interface, the metallic state is always preserved up to PO2 of 100 mTorr. The data from Hall measurement and X-ray absorption near edge structure (XANES) spectroscopy reveal that the larger amount of localized Ti3+ ions are formed at the LAO/STO interface compared to LSAT/STO. Those localized Ti3+ ions with unpaired electrons can be spin-polarized to scatter mobile electrons, responsible for the Kondo-like scattering observed at the LAO/STO interface. © 2016, Nature Publishing Group. All rights reserved.Publication Strontium Niobate for Near-Infrared Plasmonics(WILEY, 2019-07-01) Aveek Dutta; Dongyang Wan; Bixing Yan; Vladimir M. Shalaev; Thirumalai Venkatesan; Alexandra Boltasseva; ELECTRICAL AND COMPUTER ENGINEERING; PHYSICSPlasmonics has developed greatly over the past two decades, and a plethora of plasmonic materials has been explored for practical plasmonic devices across various applications. While noble metals such as gold and silver are the most widely used plasmonic materials, other metals such as aluminum, copper, and magnesium have also been proposed as building blocks for plasmonics. Transparent conducting oxides (TCOs) such as aluminum? and gallium?doped zinc oxide and tin?doped indium oxide have been suggested for plasmonic applications in the near infrared (NIR) spectrum. These materials have lower carrier concentration than noble metals and therefore have lower material losses at the NIR. In this paper, strontium niobate (SNO) is showcased as an addition to the material database for plasmonics in the NIR. SNO plasmonic properties are studied through experimental demonstration of hybrid plasmon resonances in SNO films. Such resonances are characterized by electromagnetic field concentration in a low?index dielectric, sandwiched between a high?index dielectric resonator and a plasmonic metal. The results show that although SNO bulk film has an order of magnitude greater carrier concentration than TCOs, its losses are only a few times higher than in TCO materials thus holding a promise for plasmonic applications in the NIR.Publication Electron transport and visible light absorption in a plasmonic photocatalyst based on strontium niobate(Nature Publishing Group, 2017-04-19) Wan, D. Y.; Zhao, Y. L.; Cai, Y.; Asmara, T. C.; Huang, Z.; Chen, J. Q.; Hong, J.; Yin, S. M.|}Nelson, C. T.; Motapothula, M. R.; Yan, B. X.; Xiang, D.; Chi, X.; Zheng, H.; Chen, W.; Xu, R.; Ariando A.; Rusydi, A.; Minor, A. M.; Breese, M. B. H.; Sherburne, M.; Asta, M.; Xu, Q. -H.; Venkatesan, T.; ELECTRICAL AND COMPUTER ENGINEERING; PHYSICS; CHEMISTRY; SINGAPORE SYNCHROTRON LIGHT SOURCE; NUS NANOSCIENCE & NANOTECH INITIATIVESemiconductor compounds are widely used for photocatalytic hydrogen production applications, where photogenerated electron-hole pairs are exploited to induce catalysis. Recently, powders of a metallic oxide (Sr1-xNbO3, 0.03Publication Engineering covalently bonded 2D layered materials by self-intercalation(NATURE PUBLISHING GROUP, 2020-05-01) Zhao, Xiaoxu; Song, Peng; Wang, Chengcai; Riis-Jensen, Anders C; Fu, Wei; Deng, Ya; Wan, Dongyang; Kang, Lixing; Ning, Shoucong; Dan, Jiadong; Venkatesan, T; Liu, Zheng; Zhou, Wu; Thygesen, Kristian S; Luo, Xin; Pennycook, Stephen J; Loh, Kian Ping; Dr Zhao Xiaoxu; CENTRE FOR ADVANCED 2D MATERIALS; CHEMISTRY; MATERIALS SCIENCE AND ENGINEERING; NUS NANOSCIENCE & NANOTECH INITIATIVE© 2020, The Author(s), under exclusive licence to Springer Nature Limited. Two-dimensional (2D) materials1–5 offer a unique platform from which to explore the physics of topology and many-body phenomena. New properties can be generated by filling the van der Waals gap of 2D materials with intercalants6,7; however, post-growth intercalation has usually been limited to alkali metals8–10. Here we show that the self-intercalation of native atoms11,12 into bilayer transition metal dichalcogenides during growth generates a class of ultrathin, covalently bonded materials, which we name ic-2D. The stoichiometry of these materials is defined by periodic occupancy patterns of the octahedral vacancy sites in the van der Waals gap, and their properties can be tuned by varying the coverage and the spatial arrangement of the filled sites7,13. By performing growth under high metal chemical potential14,15 we can access a range of tantalum-intercalated TaS(Se)y, including 25% Ta-intercalated Ta9S16, 33.3% Ta-intercalated Ta7S12, 50% Ta-intercalated Ta10S16, 66.7% Ta-intercalated Ta8Se12 (which forms a Kagome lattice) and 100% Ta-intercalated Ta9Se12. Ferromagnetic order was detected in some of these intercalated phases. We also demonstrate that self-intercalated V11S16, In11Se16 and FexTey can be grown under metal-rich conditions. Our work establishes self-intercalation as an approach through which to grow a new class of 2D materials with stoichiometry- or composition-dependent properties.Publication The Mechanism of Electrolyte Gating on High-Tc Cuprates The Role of Oxygen Migration and Electrostatics(2017-09-25) Lingchao Zhang; Shengwei Zeng; Xinmao Yin; Teguh Citra Asmara; Ping Yang; Kun Han; Yu Cao; Wenxiong Zhou; Dongyang Wan; Chi Sin Tang; Andrivo Rusydi; Ariando; Thirumalai Venkatesan; ELECTRICAL AND COMPUTER ENGINEERING; PHYSICS; SINGAPORE SYNCHROTRON LIGHT SOURCE; NUS NANOSCIENCE & NANOTECH INITIATIVEElectrolyte gating is widely used to induce large carrier density modulation on solid surfaces to explore various properties. Most of past works have attributed the charge modulation to electrostatic field effect. However, some recent reports have argued that the electrolyte gating effect in VO2, TiO2, and SrTiO3 originated from field-induced oxygen vacancy formation. This gives rise to a controversy about the gating mechanism, and it is therefore vital to reveal the relationship between the role of electrolyte gating and the intrinsic properties of materials. Here, we report entirely different mechanisms of electrolyte gating on two high-Tc cuprates, NdBa2Cu3O7−δ (NBCO) and Pr2–xCexCuO4 (PCCO), with different crystal structures. We show that field-induced oxygen vacancy formation in CuO chains of NBCO plays the dominant role, while it is mainly an electrostatic field effect in the case of PCCO. The possible reason is that NBCO has mobile oxygen in CuO chains, while PCCO does not. Our study helps clarify the controversy relating to the mechanism of electrolyte gating, leading to a better understanding of the role of oxygen electro migration which is very material specific.Publication Electron-soft phonon scattering in n-type SrTiO3(American Physical Society, 2015-02-11) Zhou, W. X.; Zhou, J.; Li, C. J.; Zeng, S. W.; Huang, Z.; Ma, H. J. Harsan; Han, K.; Lim, Z. S.; Wan, D. Y.; Zhang, L. C.; Venkatesan, T.; Feng, Y. P.; PHYSICS; MATERIALS SCIENCE AND ENGINEERING; NUS NANOSCIENCE & NANOTECH INITIATIVESrTiO3 undergoes a cubic to tetragonal phase transition at Tc=105-110K, which can be described by a Brillouin zone corner ?25 (111) soft phonon. Even though clear anomalies in specific heat, thermal expansion coefficient, and sound velocity have been observed, the correlation between phase transition and electronic transport properties in n-type doped SrTiO3 is still controversial. Here, we report phase transition induced electronic transport anomaly in temperature dependence of the temperature coefficient of resistance (TCR) consistently observed in a wide variety of SrTiO3-based systems by detailed transport measurements and first-principles calculations. The observed TCR anomaly, which can be well fitted with the electron-?25 soft phonon scattering around Tc, is found to be caused by anomalies in both mobility and carrier density, with the former taking the dominant role. Moreover, the magnitude of the anomaly is found to decrease with increasing carrier density. These findings demonstrate the role of the electron-?25 soft phonon scattering in the conduction mechanism in SrTiO3-based systems. � 2016 American Physical Society.