Luo Xin
Email Address
c2dluox@nus.edu.sg
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Publication Large Frequency Change with Thickness in Interlayer Breathing Mode-Significant Interlayer Interactions in Few Layer Black Phosphorus(AMERICAN CHEMICAL SOCIETY, 2015-06-01) Luo, Xin; Lu, Xin; Koon, Gavin Kok Wai; Neto, Antonio H Castro; Oezyilmaz, Barbaros; Xiong, Qihua; Quek, Su Ying; DEPT OF PHYSICS; Dr Quek Su Ying; CENTRE FOR ADVANCED 2D MATERIALS© 2015 American Chemical Society. Bulk black phosphorus (BP) consists of puckered layers of phosphorus atoms. Few-layer BP, obtained from bulk BP by exfoliation, is an emerging candidate as a channel material in post-silicon electronics. A deep understanding of its physical properties and its full range of applications are still being uncovered. In this paper, we present a theoretical and experimental investigation of phonon properties in few-layer BP, focusing on the low-frequency regime corresponding to interlayer vibrational modes. We show that the interlayer breathing mode A3g shows a large redshift with increasing thickness; the experimental and theoretical results agree well. This thickness dependence is two times larger than that in the chalcogenide materials, such as few-layer MoS2 and WSe2, because of the significantly larger interlayer force constant and smaller atomic mass in BP. The derived interlayer out-of-plane force constant is about 50% larger than that of graphene and MoS2. We show that this large interlayer force constant arises from the sizable covalent interaction between phosphorus atoms in adjacent layers and that interlayer interactions are not merely of the weak van der Waals type. These significant interlayer interactions are consistent with the known surface reactivity of BP and have been shown to be important for electric-field induced formation of Dirac cones in thin film BP. (Graph Presented).Publication Tunable inverted gap in monolayer quasi-metallic MoS2 induced by strong charge-lattice coupling(Nature Publishing Group, 2017-09-07) Yin, Xinmao; Wang, Qixing; Cao, Liang; Tang, Chi Sin; Luo, Xin; Zheng, Yujie; Wong, Lai Mun; Wang, Shi Jie; Quek, Su Ying; Zhang, Wenjing; Rusydi, Andrivo; Wee, Andrew T. S.; DEPT OF PHYSICS; CENTRE FOR ADVANCED 2D MATERIALS; CHEMISTRYPolymorphism of two-dimensional transition metal dichalcogenides such as molybdenum disulfide (MoS2) exhibit fascinating optical and transport properties. Here, we observe a tunable inverted gap (~0.50 eV) and a fundamental gap (~0.10 eV) in quasimetallic monolayer MoS2. Using spectral-weight transfer analysis, we find that the inverted gap is attributed to the strong charge-lattice coupling in two-dimensional transition metal dichalcogenides (2D-TMDs). A comprehensive experimental study, supported by theoretical calculations, is conducted to understand the transition of monolayer MoS2 on gold film from trigonal semiconducting 1H phase to the distorted octahedral quasimetallic 1T' phase. We clarify that electron doping from gold, facilitated by interfacial tensile strain, is the key mechanism leading to its 1H-1T' phase transition, thus resulting in the formation of the inverted gap. Our result shows the importance of charge-lattice coupling to the intrinsic properties of the inverted gap and polymorphism of MoS2, thereby unlocking new possibilities for 2D-TMD-based device fabrication. © 2017 The Author(s).Publication Scalable processing for realizing 21.7%-efficient all-perovskite tandem solar modules.(American Association for the Advancement of Science (AAAS), 2022-05-13) Xiao, Ke; Lin, Yen-Hung; Zhang, Mei; Oliver, Robert DJ; Wang, Xi; Liu, Zhou; Luo, Xin; Li, Jia; Lai, Donny; Luo, Haowen; Lin, Renxing; Xu, Jun; Hou, Yi; Snaith, Henry J; Tan, Hairen; Mui Koon Tan; CENTRE FOR ADVANCED 2D MATERIALS; SOLAR ENERGY RESEARCH INST OF S'POREChallenges in fabricating all-perovskite tandem solar cells as modules rather than as single-junction configurations include growing high-quality wide-bandgap perovskites and mitigating irreversible degradation caused by halide and metal interdiffusion at the interconnecting contacts. We demonstrate efficient all-perovskite tandem solar modules using scalable fabrication techniques. By systematically tuning the cesium ratio of a methylammonium-free 1.8-electron volt mixed-halide perovskite, we improve the homogeneity of crystallization for blade-coated films over large areas. An electrically conductive conformal "diffusion barrier" is introduced between interconnecting subcells to improve the power conversion efficiency (PCE) and stability of all-perovskite tandem solar modules. Our tandem modules achieve a certified PCE of 21.7% with an aperture area of 20 square centimeters and retain 75% of their initial efficiency after 500 hours of continuous operation under simulated 1-sun illumination.Publication Rapid and Nondestructive Identification of Polytypism and Stacking Sequences in Few-Layer Molybdenum Diselenide by Raman Spectroscopy(WILEY-V C H VERLAG GMBH, 2015-08-12) Lu, Xin; Utama, M Iqbal Bakti; Lin, Junhao; Luo, Xin; Zhao, Yanyuan; Zhang, Jun; Pantelides, Sokrates T; Zhou, Wu; Quek, Su Ying; Xiong, Qihua; DEPT OF PHYSICS; Dr Quek Su Ying; CENTRE FOR ADVANCED 2D MATERIALS© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Various combinations of interlayer shear modes emerge in few-layer molybdenum diselenide grown by chemical vapor deposition depending on the stacking configuration of the sample. Raman measurements may also reveal polytypism and stacking faults, as supported by first principles calculations and high-resolution transmission electron microscopy. Thus, Raman spectroscopy is an important tool in probing stacking-dependent properties in few-layer 2D materials.Publication Low Resistance Metal Contacts to MoS2 Devices with Nickel-Etched-Graphene Electrodes(American Chemical Society, 2015-01-27) Leong, Wei Sun; Luo, Xin; Li, Yida; Khoo, Khoong Hong; Quek, Su Ying; Thong, John TL; Dr Quek Su Ying; CENTRE FOR ADVANCED 2D MATERIALS; ELECTRICAL AND COMPUTER ENGINEERING; PHYSICS© 2014 American Chemical Society. We report an approach to achieve low-resistance contacts to MoS2 transistors with the intrinsic performance of the MoS2 channel preserved. Through a dry transfer technique and a metal-catalyzed graphene treatment process, nickel-etched-graphene electrodes were fabricated on MoS2 that yield contact resistance as low as 200 ω·μm. The substantial contact enhancement (∼2 orders of magnitude), as compared to pure nickel electrodes, is attributed to the much smaller work function of nickel-graphene electrodes, together with the fact that presence of zigzag edges in the treated graphene surface enhances tunneling between nickel and graphene. To this end, the successful fabrication of a clean graphene-MoS2 interface and a low resistance nickel-graphene interface is critical for the experimentally measured low contact resistance. The potential of using graphene as an electrode interlayer demonstrated in this work paves the way toward achieving high performance next-generation transistors.Publication Temperature- and Phase-Dependent Phonon Renormalization in 1T'-MoS2(2018-04-30) Sherman Jun Rong Tan; Soumya Sarkar; Xiaoxu Zhao; Xin Luo; Yong Zheng Luo; Sock Mui Poh; Ibrahim Abdelwahab; Wu Zhou; Thirumalai Venkatesan; Wei Chen; Su Ying Quek; Kian Ping Loh; CENTRE FOR ADVANCED 2D MATERIALS; ELECTRICAL AND COMPUTER ENGINEERING; CHEMISTRY; MATERIALS SCIENCE AND ENGINEERING; NUS NANOSCIENCE & NANOTECH INITIATIVEPolymorph engineering of 2H-MoS2, which can be achieved by alkali metal intercalation to obtain either the mixed 2H/1T′ phases or a homogeneous 1T′ phase, has received wide interest recently, since this serves as an effective route to tune the electrical and catalytic properties of MoS2. As opposed to an idealized single crystal-to-single crystal phase conversion, the 2H to 1T′ phase conversion results in crystal domain size reduction as well as strained lattices, although how these develop with composition is not well understood. Herein, the evolution of the phonon modes in Li-intercalated 1T′-MoS2 (LixMoS2) are investigated as a function of different 1T′-2H compositions. We observed that the strain evolution in the mixed phases is revealed by the softening of four Raman modes, Bg (J1), Ag (J3), E12g, and A1g, with increasing 1T′ phase composition. Additionally, the first-order temperature coefficients of the 1T′ phonon mode vary linearly with increasing 1T′ composition, which is explained by increased electron–phonon and strain–phonon coupling.Publication First-principles investigations of the atomic, electronic, and thermoelectric properties of equilibrium and strained Bi2Se3 and Bi2Te3 including van der Waals interactions(American Physical Society, 2012-11-27) Luo, Xin; Sullivan, Michael B; Quek, Su Ying; Dr Quek Su Ying; CENTRE FOR ADVANCED 2D MATERIALS; PHYSICS; CHEMISTRYBi2Se3 and Bi2Te3 are layered compounds of technological importance, being excellent thermoelectric materials as well as topological insulators. We report density functional theory calculations of the atomic, electronic, and thermoelectric properties of strained bulk and thin-film Bi2Se3 and Bi 2Te3, focusing on an appropriate description of van der Waals (vdW) interactions. The calculations show that the van der Waals density functional (vdW-DF) with Cooper's exchange (vdW-DFC09x) can reproduce closely the experimental interlayer distances in unstrained Bi 2Se3 and Bi2Te3. Interestingly, we predict atomic structures that are in much better agreement with the experimentally determined structure from Nakajima than that obtained from Wyckoff, especially for Bi2Se3, where the difference in atomic structures qualitatively changes the electronic band structure. The band structure obtained using the Nakajima structure and the vdW-DFC09x optimized structure are in much better agreement with previous reports of photoemission measurements, than that obtained using the Wyckoff structure. Using vdW-DFC09x to fully optimize atomic structures of bulk and thin-film Bi2Se3 and Bi2Te3 under different in-plane and uniaxial strains, we predict that the electronic bandgap of both the bulk materials and thin films decreases with tensile in-plane strain and increases with compressive in-plane strain. We also predict, using the semiclassical Boltzmann approach, that the magnitude of the n-type Seebeck coefficient of Bi2Te3 can be increased by the compressive in-plane strain while that of Bi2Se3 can be increased with tensile in-plane strain. Further, the in-plane power factor of n-doped Bi 2Se3 can be increased with compressive uniaxial strain while that of n-doped Bi2Te3 can be increased by compressive in-plane strain. Strain engineering thus provides a direct method to control the electronic and thermoelectric properties in these thermoelectric topological insulator materials. © 2012 American Physical Society.Publication Anomalous frequency trends in MoS2 thin films attributed to surface effects(American Physical Society, 2013-08-28) Luo, Xin; Zhao, Yanyuan; Zhang, Jun; Xiong, Qihua; Quek, Su Ying; DEPT OF PHYSICS; Dr Quek Su Ying; CENTRE FOR ADVANCED 2D MATERIALSThe layered dichalcogenide MoS2 has many unique physical properties in low dimensions. Recent experimental Raman spectroscopies report an anomalous blue shift of the in-plane E2g1 mode with decreasing thickness, a trend that is not understood. Here, we combine experimental Raman scattering and theoretical studies to clarify and explain this trend. Special attention is given to understanding the surface effect on Raman frequencies by using a force constants model based on first-principles calculations. Surface effects refer to the larger Mo-S force constants at the surface of thin film MoS2, which results from a loss of neighbours in adjacent MoS2 layers. Without surface effects, the frequencies of both out-of-plane A1g and in-plane E2g1 modes decrease with decreasing thickness. However, the E2g1 mode blue shifts while the A1g mode red shifts once the surface effect is included, in agreement with the experiment. Our results show that competition between the thickness effect and the surface effect determines the mechanical properties of two-dimensional MoS2, which we believe applies to other layered materials. © 2013 American Physical Society.Publication Effects of lower symmetry and dimensionality on Raman spectra in two-dimensional WSe2(American Physical Society, 2013-11-27) Luo, Xin; Zhao, Yanyuan; Zhang, Jun; Toh, Minglin; Kloc, Christian; Xiong, Qihua; Quek, Su Ying; DEPT OF PHYSICS; Dr Quek Su Ying; CENTRE FOR ADVANCED 2D MATERIALSWe report the observation and interpretation of new Raman peaks in few-layer tungsten diselenide (WSe2), induced by the reduction of symmetry going from three-dimensional (3D) to two-dimensional (2D). In general, Raman frequencies in 2D materials follow quite closely the frequencies of corresponding eigenmodes in the bulk. However, while the modes that are Raman active in the bulk are also Raman active in the thin films, the reverse is not always true due to the reduced symmetry in thin films. Here, we predict from group theory and density functional calculations that two intralayer vibrational modes, which are Raman inactive in bulk WSe2 in our experimental configuration become Raman active in thin film WSe2, due to reduced symmetry in thin films. This phenomenon explains the Raman peaks we observe experimentally at ∼310 and 176 cm-1 in thin film WSe2. Interestingly, the bulk B2g1 mode at ∼310 cm-1 that is Raman inactive can, in fact, be detected in Raman measurements under specific wavelengths of irradiation, suggesting that in this case, crystal symmetry selection rules may be broken due to resonant scattering. Both theory and experiment indicate that the E2g1 and B2g1 modes blueshift with decreasing thickness, which we attribute to surface effects. Our results shed light on a general understanding of the Raman/infrared activities of the phonon modes in layered transition metal dichalcogenide materials and their evolution behavior from 3D to 2D. © 2013 American Physical Society.Publication 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.