Gao Xingyu
Email Address
phygaoxy@nus.edu.sg
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Publication Molecular orientation and ordering during initial growth of copper phthalocyanine on Si(111)(2007-03-01) Wang, L.; Qi, D.; Liu, L.; Chen, S.; Gao, X.; Wee, A.T.S.; PHYSICSThe molecular orientation and order in the initial growth of copper phthalocyanine on Si(111) was investigated in situ by angular-dependent X-ray absorption spectroscopy. A transition from lying-down to standing-up configuration occurs in less than one monolayer. After the Si surface is passivated by a monolayer of the molecules, only the standing-up structure persists in subsequent growth. The molecular film also exhibits an order - disorder - order transition in the distribution of molecular tilt angle during growth. These transitions are related to the changes of interactions in the system at different stages. © 2007 American Chemical Society.Publication Tuning of C60 energy levels using orientation-controlled phthalocyanine films(2010-09-01) Mao, H.Y.; Wang, R.; Huang, H.; Wang, Y.Z.; Gao, X.Y.; Bao, S.N.; Wee, A.T.S.; Chen, W.; PHYSICS; CHEMISTRYThe interface electronic structure of C60 /CuPc and C 60 heterojunctions on SiO2 and highly oriented pyrolytic graphite has been studied using ultraviolet photoelectron spectroscopy, x-ray photoelectron spectroscopy, and synchrotron based photoelectron spectroscopy. Fermi level pinned to the negative integer charge transfer state of C 60 molecules on the standing CuPc film has been observed, while nearly vacuum-level alignment is observed for C60 on the lying CuPc film. We also found small vacuum-level shifts for C60 on both standing and lying F16 CuPc films, which can be attributed to the rearrangement of underlying F16 CuPc molecules. With the use of orientation-controlled CuPc and F16 CuPc thin films, C60 highest occupied molecular orbital energy levels relative to the substrate Fermi level can be tuned from 1.9 eV for C60 on the standing CuPc film to 1.0 eV on the standing F16 CuPc film. © 2010 American Institute of Physics.Publication Synthesis and magnetic properties of MnSb nanoparticles on Si-based substrates(2007) Zhang, H.; Kushvaha, S.S.; Chen, S.; Gao, X.; Qi, D.; Wee, A.T.S.; Wang, X.-S.; ELECTRICAL & COMPUTER ENGINEERING; PHYSICS; MATERIALS SCIENCE AND ENGINEERINGMnSb nanoparticles with average lateral diameters 〈d〉 from 5 to 30 nm are synthesized on ultra thin silicon nitride covered Si(111) by codeposition of Mn and Sb. These MnSb particles possess sharp interfaces and a NiAs-type lattice. The Mn 2p x-ray absorption spectroscopy analysis shows broad line shapes due to the itinerant Mn 3d states. The particles with 〈d〉 =5 and 8.5 nm are superparamagnetic at room temperature, while those of 〈d〉 =15 and 30 nm exhibit ferromagnetic behavior. © 2007 American Institute of Physics.Publication Deterministic conversion between memory and threshold resistive switching via tuning the strong electron correlation(2012) Peng, H.Y.; Li, Y.F.; Lin, W.N.; Wang, Y.Z.; Gao, X.Y.; Wu, T.; PHYSICSIntensive investigations have been launched worldwide on the resistive switching (RS) phenomena in transition metal oxides due to both fascinating science and potential applications in next generation nonvolatile resistive random access memory (RRAM) devices. It is noteworthy that most of these oxides are strongly correlated electron systems, and their electronic properties are critically affected by the electron-electron interactions. Here, using NiO as an example, we show that rationally adjusting the stoichiometry and the associated defect characteristics enables controlled room temperature conversions between two distinct RS modes, i.e., nonvolatile memory switching and volatile threshold switching, within a single device. Moreover, from first-principles calculations and x-ray absorption spectroscopy studies, we found that the strong electron correlations and the exchange interactions between Ni and O orbitals play deterministic roles in the RS operations.Publication Slow charge relaxation in ionizable alkanethiols and its role in modulating electric characteristics of molecules and passivated gold nanoparticles(2009-03-05) Xie, X.N.; Sivaramakrishnan, S.; Song, Q.; Gao, X.; K.-H Ho, P.; Ong, C.K.; Wee, A.T.S.; PHYSICS; NUS NANOSCIENCE & NANOTECH INITIATIVEThis work addresses the electric characteristics associated with the slow ionization relaxation in ionizable alkylthiols which are widely used as passivation ligands for gold nanoparticles (NPs). It is observed that reversible ionic motion under a cycling voltage induces highly transient current with strong hysteresis loop. The ionization and relaxation of alkanethiols is responsible for the buildup and reversal of an internal bias, and thus leads to capacitive and charge memory effect in the electric conduction of the molecules. The role of ligand ionization in the electric behavior of passivated gold NPs is also demonstrated. The slow ionic process allows for both capacitive and resistive conduction and can be used to regulate the charge transport through the Au NP film. Our results represent a novel charge conduction mechanism governed by ligands in the NP/molecule binary system and would find new applications in molecular electronics and NP-based memory and sensor devices. © 2009 American Chemical Society.Publication Plasma cleaning of contaminated optical components of synchrotron radiation beam-line(2009-12) Wei, W.; Wang, Q.; Wang, Y.; Yu, X.; Gao, X.; PHYSICS; SINGAPORE SYNCHROTRON LIGHT SOURCEThe plasma cleaning technique has been successfully developed to regularly remove the carbon contaminated layer on surfaces of all the optical components, installed in the vacuum chamber of the synchrotron radiation beam line, so as to ensure a good reflectance of the beam line, especially after a serious reduction of the photon flux near the carbon absorption edge and other wavelength ranges. In the technique, radio frequency (RF) plasma in dry oxygen and argon mixture produces large quantity of ozone, which oxidizes carbon contaminates into CO and CO2. The experimental results show that the plasma cleaning technique is capable of completely removing carbon contamination and maintaining ultra high vacuum of the beam line.Publication Molecular orientation-dependent ionization potential of organic thin films(2008-11-25) Chen, W.; Huang, H.; Chen, S.; Huang, Y.L.; Gao, X.Y.; Wee, A.T.S.; PHYSICS; NUS NANOSCIENCE & NANOTECH INITIATIVEOrientation-dependent ionization potentials (IPs) of organic thin films have been investigated with the combination of synchrotron-based high-resolution photoemission spectroscopy and near-edge X-ray absorption fine structure measurements. Organic thin films of copper(II) phthalocyanine (CuPc) and its fully fluorinated counterpart of copper hexadecafluorophthalocyanine (Fi 16CuPc) with well-controlled orientation are used as model systems. Both molecules lie flat on the graphite surface and stand upright on Au(111) terminated by self-assembled monolayer of octane-1-thiol. The IP for the standing-up CuPc thin film (IP = 4.75 eV) is 0.40 eV lower than that of the lying-down film (IP = 5.15 eV). In contrast, the IP of the standing up F 16CuPc (EP = 6.50 eV) is 0.85 eV higher than that of the lying-down film (IP = 5.65 eV). This reversed orientation dependence in IP is explained by the opposite surface dipoles in the standing-up CuPc and F16CuPc molecular layers originating from the different intramolecular dipolar bonds exposed at the surfaces, i.e., a net upward pointing surface dipole in the standing-up CuPc thin film that lowers the IP, and a net downward pointing surface dipole in the standing-up F16CuPc thin film that increases the IP. © 2008 American Chemical Society.Publication Orientation-controlled charge transfer at CuPc/ F16 CuPc interfaces(2009) Chen, W.; Chen, S.; Chen, S.; Li Huang, Y.; Huang, H.; Qi, D.C.; Gao, X.Y.; Ma, J.; Wee, A.T.S.; PHYSICS; CHEMISTRY; NUS NANOSCIENCE & NANOTECH INITIATIVEMolecular orientation-controlled charge transfer has been observed at the organic-organic heterojunction interfaces of copper-hexadecafluoro- phthalocyanine (F16 CuPc) or copper(II) phthalocyanine (CuPc) on both standing-up and lying-down CuPc or F16 CuPc thin films. In situ synchrotron-based photoemission spectroscopy reveals that the charge transfer at the standing F16 CuPc/CuPc or CuPc/ F16 CuPc interface is much larger than that at the lying F16 CuPc/CuPc or CuPc/ F16 CuPc interface. This can be explained by the orientation-dependent ionization potentials of well-ordered organic thin films, which place the highest-occupied molecular orbital of the standing CuPc film much closer to the lowest-unoccupied molecular orbital of the standing F16 CuPc film, facilitating stronger charge transfer as compared to that at the lying OOH interfaces. Our results suggest the possibility of manipulating interfacial electronic structures of organic heterojunctions by controlling the molecular orientation, in particular for applications in ambipolar organic field transistors and organic photovoltaics. © 2009 American Institute of Physics.Publication Atomic structure of the 6H-SiC(0 0 0 1) nanomesh(2005-12-10) Chen, W.; Xu, H.; Liu, L.; Gao, X.; Qi, D.; Peng, G.; Tan, S.C.; Feng, Y.; Loh, K.P.; Wee, A.T.S.; INSTITUTE OF ENGINEERING SCIENCE; PHYSICS; CHEMISTRYThe atomic structure of the carbon nanomesh template (the so-called 63×63R30° reconstruction) on the 6H-SiC(0 0 0 1) surface was investigated in detail by scanning tunneling microscopy (STM), low energy electron diffraction (LEED), synchrotron photoemission spectroscopy (PES) and density-functional theory (DFT) calculations. We propose that the origin of the carbon nanomesh template arises from the self-organization of excess carbon atoms forming a novel honeycomb arrangement atop the 6H-SiC(0 0 0 1) surface after annealing at 1100 °C. Two carbon nanomesh-related C 1s components are observed at binding energies of 285.1 eV (S1) and 283.8 eV (S 2) by angle-resolved synchrotron PES experiments. We assign the S2 component to carbon atoms that bond to one underlying Si atom, and the S1 component to carbon atoms bonded to other carbon atoms without Si-C bond formation. Further annealing results in the formation of crystalline graphitic layers on top of the surface. © 2005 Elsevier B.V. All rights reserved.Publication Thickness-dependent energy level alignment of rubrene adsorbed on Au(111)(2007) Wang, L.; Chen, S.; Liu, L.; Qi, D.; Gao, X.; Wee, A.T.S.; PHYSICSEnergy level alignment of rubrene adsorbed on Au(111) was studied by photoemission spectroscopy. After rubrene adsorption, the work function is reduced from 5.24 eV for clean Au to 4.31 eV, suggesting the invalidity of vacuum level alignment and the presence of a strong interfacial dipole. The frontier molecular orbital energies of rubrene are modified by electrode surface polarization in the submonolayer regime. As a consequence, the hole injection barrier is thickness dependent and varies from about 0.4 eV for a monolayer of rubrene to 0.9 eV for a thick layer. © 2007 American Institute of Physics.