X.-S. Wang

National University of Singapore, Singapore, Singapore

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Publications (19)52.52 Total impact

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    ABSTRACT: The surface morphology of Mn, Sb and MnSb nanostructures grown on highly-oriented pyrolytic graphite (HOPG) has been studied in detail with scanning tunneling microscopy in ultrahigh vacuum. At the initial stage of growth, three dimensional (3D) clusters and islands of Mn, Sb and MnSb are formed at step edges and other defect sites. In middle stage, crystalline 2D islands and 1D nanorods of Sb can be observed together with crystalline 3D islands, whereas single- and double-layer cluster chains of Mn are obtained at room temperature (RT). MnSb crystallites form after deposition of both Mn and Sb on HOPG surface and annealing at T ~ 375 K. The ex-situ X-ray photoemission spectroscopy measurement revealed the formation of MnSb compound on graphite. Ferromagnetism was observed at RT for the 50-nm MnSb thin film using vibrating sample magnetometer.
    Thin Solid Films 09/2012; 520(23):6909–6915. · 1.87 Impact Factor
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    ABSTRACT: The atomic structures that develop as a function of coverage during deposition of Bi on Ag(111) have been studied using low-temperature scanning tunneling microscopy, low-energy electron diffraction, and ab initio calculations. The growth process involves two sequential stages. At low coverage, Bi atoms are incorporated into the topmost layer of Ag(111), resulting in the formation of an Ag2Bi alloy confined to the surface and ordered (√3×√3)R30° Ag2Bi islands supported on Ag(111). This mode of accommodation of Bi was found to be energetically favorable based on ab initio total-energy calculations. At coverage above a critical value of 0.55 monolayers, the Ag2Bi alloy phase gradually converts into an ordered Bi (p×√3) overlayer structure supported on Ag(111). We postulate that the dealloying transition is likely driven by compressive strain induced by incorporation of large-size Bi atoms into Ag at a high coverage and the subsequent lack of miscibility of Ag and Bi bulk phases. After completion of the dealloying process, Bi(110) thin films can be grown epitaxially on top of Ag(111) with a chemically abrupt interface.
    Physical review. B, Condensed matter 01/2011; 83(23). · 3.77 Impact Factor
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    ABSTRACT: The growth and surface morphology of indium and aluminum nanostructures on highly oriented pyrolytic graphite (HOPG) and molybdenum disulphide (MoS2) surfaces were investigated using scanning tunneling microscopy in ultra-high vacuum. Mostly triangular and hexagonal In islands with (111) orientation were obtained along the steps of HOPG at room temperature (RT). Triangular, round-shape and large irregular In islands were found on MoS2 surfaces at different growth conditions. Al island chains as well as isolated triangular islands were found on HOPG whereas nanoparticles and ramified Al islands were obtained on MoS2 at RT. The shapes of these self-assembled metal nanostructures were controlled by varying growth conditions. The different shapes of nanostructures on these inert substrates can be explained in terms of differences in energetic and kinetic properties of atoms and clusters of each elements as well as the nature of substrates.
    Thin Solid Films 01/2009; 517(16):4540-4547. · 1.87 Impact Factor
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    ABSTRACT: The growth of indium and aluminum nanostructures on molybdenum disulphide (MoS2)(0001) substrate has been studied using scanning tunneling microscopy in ultra-high vacuum. At low coverage and room temperature (RT), mostly ultra-thin (approximately 1.2-2 nm) triangular In islands were observed on MoS2. With increasing coverage or high flux, large coalesced irregular islands along with triangular and round-shaped ones of increased average height were found. Triangular and round-shaped islands were obtained after annealing the RT-deposited In on MoS2 sample at 450 K. At approximately 375 K, exclusively triangular In islands were observed. Al nanoparticles with diameter in 4-16 nm range were obtained after a low-flux deposited whereas ramified islands were observed in a high flux at RT. Ultra-thin (approximately 1.20-2 nm) Al islands and films were obtained on MoS2 after deposition at 500 K. These results demonstrate that the shape of In and Al nanostructures grown on MoS2 can be controlled in self-assembly by adjusting substrate temperature, deposition flux and amount.
    Journal of Nanoscience and Nanotechnology 06/2008; 8(5):2707-12. · 1.15 Impact Factor
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    ABSTRACT: The phase transition process from the Si(1 1 1)-(7 × 7) surface to the Cu/Si(1 1 1)-(5 × 5) surface structure has been studied by scanning tunnelling microscopy and synchrotron radiation photoemission spectroscopy. The nucleation and growth of Cu/Si(1 1 1)-(5 × 5) on the Si(1 1 1)-(7 × 7) surface progress gradually with the increase in Cu coverage. Cu deposition on the Si(1 1 1)-(7 × 7) surface at room temperature may only involve the saturation of the surface dangling bonds, whereas a new surface phase of Cu/Si(1 1 1)-(5 × 5) is formed upon annealing, which saturates at a Cu coverage of 0.9 ML. Our experiments clearly show the surface phase transition process of the (5 × 5) structure as a function of the Cu coverage and provide useful insight into the Cu/Si(1 1 1)-(5 × 5) structure.
    Journal of Physics D Applied Physics 04/2008; 41(9):095306. · 2.53 Impact Factor
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    ABSTRACT: Thiophene molecule selectively binds to the adjacent adatom-rest atom pair on the Si(111)-(7x7) surface through its alpha-carbon atoms, leading to the covalent attachment of a C-S-C linkage and remaining C=C (beta-carbon) bond onto the surface. Photoemission studies show that Cu atom readily adsorbs onto the S atom of the functional group to form the Cu-S molecular anchor in two forms: one points away from the thiophene C=C group; the other points toward the C=C group.
    The Journal of Chemical Physics 02/2008; 128(4):044706. · 3.12 Impact Factor
  • Z. Yan, S.S. Kushvaha, W. Xiao, X.-S. Wang
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    ABSTRACT: When antimony (mostly Sb4) is deposited on highly oriented pyrolytic graphite (HOPG), in situ scanning tunneling microscopy images reveal that three-dimensional (3D) spherical islands, quasi-2D films and 1D nanowires (NWs) are formed. The spherical islands develop into faceted crystallites in the later growth stage. The lattice parameters of the 2D and 3D structures are close to those of α-Sb bulk, whereas the NWs appear in a compressed state. The Laplace pressure, which can reach the GPa range in a nanostructure, is considered the driving force for the compressive lattice structures of Sb NWs. We found conditions of controlling the dimensionality of Sb nanostructures in their self-assembly on HOPG to a certain extent. At room temperature and with a low Sb flux, 3D islands grow exclusively. At a substrate temperature of 100°C, 2D and 1D structures are dominant when a high deposition flux is used, whereas only NWs are formed initially when a low flux is used. These results are explained in terms of different activation energies for Sb4 diffusion and conversion to a chemisorption or dissociation state on HOPG. As the temperature increases, the rate of conversion to the chemisorption or dissociation state increases more rapidly than that of diffusion since the chemisorption activation energy is much higher than the diffusion barrier of physisorbed Sb4, resulting in enhanced 2D and 1D structural nucleation and growth, which are further favored with the increase in deposition flux. The bonding nature of various Sb structures with a graphite surface and the conditions for growing aligned Sb NWs exclusively are discussed.
    Applied Physics A 07/2007; 88(2):299-307. · 1.69 Impact Factor
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    ABSTRACT: We demonstrate the growth of Fe-induced magic clusters on Si(1 1 1)-(7 × 7) template by in situ scanning tunneling microscopy (STM). These clusters form near a dimer row at one side of the half-unit cell (HUC); and with three different equivalent orientations. A cluster model comprising three top layer Si atoms bonded to six Fe atoms at the next layer in the 7 × 7 faulted-half template is proposed. The optimized cluster structure determined by first-principles total-energy calculation shows an inward-shifting of the three center Fe atoms. The clusters and the nearby center-adatoms of the next HUCs appear with a significantly reduced height below bias voltages 0.4 V in high resolution empty-state STM images, suggesting an energy gap opening near the Fermi level at these localized cluster and adatom sites. We explain the stabilization of the clusters on the 7 × 7 template using the gain in electronic energy as the driving force for cluster formation.
    Surface Science 06/2007; 601(12):2486–2490. · 1.84 Impact Factor
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    ABSTRACT: MnSb 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.
    Applied Physics Letters 05/2007; 90(20):202503-202503-3. · 3.79 Impact Factor
  • G. Q. Xu, S. Chen, X. S. Wang, A. T. S. Wee
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    ABSTRACT: The Si mass transport taking place during the formation of the Cu∕Si(111)-(5×5) surface phase has been studied using scanning tunneling microscopy. From the measurement of the areas occupied by various structural domains and the quantitative consideration of the Si mass balance, the top Si atom density in the Cu∕Si(111)-(5×5) phase is found to be 0.96 monolayer. The Cu∕Si(111)-(5×5) structure is suggested to consist of a planar CuSi overlayer with an atomic ratio of Cu:Si close to 1:1. Further Cu deposition beyond the formation of Cu∕Si(111)-(5×5) phase results in the formation of Cu nanocrystals on the intermediate (5×5) layer.
    Physical Review B. 02/2007; 75(7).
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    ABSTRACT: We present a study on the adsorption and thermal decomposition of C60 on Co covered Si(111)-7 × 7 using scanning tunneling microscopy and X-ray photoelectron spectroscopy. Co-induced magic clusters grown on Si(111)-7 × 7 are identified as a possible adsorption site where 51 ± 3% of C60 molecules adsorb at room temperature. On Co/Si(111)-7 × 7, C60 molecules start to decompose at 450 °C, and are completely dissociated to form SiC by 720 °C. This temperature is significantly lower than 910 °C at which C60 completely dissociates on clean Si(111)-7 × 7. This is a possible low temperature method for growing crystalline SiC films using C60 as a precursor molecule.
    Applied Surface Science 01/2007; · 2.54 Impact Factor
  • X.-S. Wang, S. S. Kushvaha, Z. Yan, W. Xiao
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    ABSTRACT: One-dimensional (1D) semimetals (e.g., antimony and bismuth) are excellent thermoelectric materials. We demonstrate the self-assembly of 1D Sb nanowires on highly oriented pyrolytic graphite, along with structures of other dimensionality. Comparing with an Sb crystal in ambient condition, our scanning tunneling microscopy analysis indicates that these Sb nanowires have a compressed lattice structure, which is likely formed under the Laplace pressure that can be quite large in a nanostructure. The conditions for growing aligned semimetal nanowires exclusively are discussed.
    Applied Physics Letters 06/2006; 88(23):233105-233105-3. · 3.79 Impact Factor
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    ABSTRACT: The electronic structure of cobalt-induced magic clusters grown on Si(111)-(7×7) is investigated by scanning tunneling microscopy, scanning tunneling spectroscopy, and real-space multiple-scattering calculations. Topographical images of a half unit cell of Si(111)-(7×7) with the cluster acquired at low bias voltages of ±0.4V show greatly reduced cluster heights; however, the heights of the corner adatoms are unchanged, indicative of the highly localized nature of the charge distribution. Spectroscopic studies of the clusters indicate a band gap of ˜0.8eV , suggesting localized nonmetallic behavior. The opening of such a band gap is suggested to be a stabilizing factor for the observed magic clusters. A 65-atom Co-Si cluster is constructed to calculate the momentum- and element-projected density of states. The calculated result identifies that the intense state below the Fermi level at -1.75V in the experimental spectroscopic curve is primarily due to localized 3d orbitals of Co atoms in the magic clusters.
    Physical review. B, Condensed matter 05/2006; · 3.77 Impact Factor
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    ABSTRACT: Thiophene selectively binds to the pair of adjacent Si adatom and Si rest atom on the Si(111)-(7×7) surface, leading to the covalent attachment of C–S–C linkages onto the surface. Cu atoms are found to preferentially adsorb onto the S atoms of the functional C–S–C groups in the formation of copper nanoclusters on the thiophene-mediated Si(111)-(7×7) surface.
    Applied Physics Letters 03/2006; 88(12):123106-123106-3. · 3.79 Impact Factor
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    ABSTRACT: We have studied the size, shape transition, and nucleation sites of self-assembled cobalt silicide clusters on Si(111)−7×7 using scanning tunneling microscopy. Step decoration of solid-phase-epitaxy grown clusters (350–1000 °C) is observed on surfaces with narrow terrace widths (25–40 nm). Experiments repeated on surfaces with wide (>70 nm) terraces reveal significant differences with respect to shape, size, and growth morphology; a bimodal size distribution is observed due to cluster shape transition from irregular clusters to flat-top islands. The control of silicide island nucleation could have potential applications in electrode formation for molecular electronics and nanodevice fabrication.
    Applied Physics Letters 01/2006; 88(2):023121-023121-3. · 3.79 Impact Factor
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    ABSTRACT: Cobalt-induced magic clusters have been grown on Si (111) - (7×7) . In situ scanning tunneling microscopy studies show that the clusters are off-center in the 7×7 half-unit-cell breaking the local threefold symmetry, and thereby distinguishing this system from other group-III magic clusters. Another unique feature of these clusters is that they are accompanied by voids created by two missing adatoms. Supported by first-principles total energy calculations, we propose a model to explain the atomic structure and formation mechanism of these clusters. The reactive nature of Co leads to the liberation of substrate Si atoms which eventually bond with Co to form the clusters.
    Physical review. B, Condensed matter 11/2005; · 3.77 Impact Factor
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    ABSTRACT: The well-defined and patterned copper clusters formed on the Si(111)-(7×7) surface have been employed as a template for selective binding of molecules, forming ordered molecular nanostructures. Scanning tunneling microscopic studies show that thiophene molecules preferentially bind to the copper clusters through the S–Cu interaction involving S lone-pair electrons. Large-scale two-dimensional thiophene molecular nanostructures can be obtained using this patterned assembly technique. Our experiments demonstrate the feasibility for controllable growth of ordered molecular nanostructures on Si(111) surface.
    Applied Physics Letters 10/2004; 85(14):2926-2928. · 3.79 Impact Factor
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    ABSTRACT: The formation of self-assembled one-dimensional molecular nanostructures on Si(100) is studied by scanning tunneling microscopy. Using the Ge-covered Si(100)-2×n surface as a tempting template, we have obtained large-scale one-dimensional styrene molecular nanostructures. The styrene molecules selectively bond to the Ge/Si(100)-2×n surface through the interactions between one C=C bond of the vinyl group and a Ge=Ge dimer to form C–Ge linkages via a [2+2] cycloaddition. This result may provide a pathway for forming controlled organic nanostructures on the Si(100) surfaces. © 2004 American Institute of Physics.
    Applied Physics Letters 01/2004; 84(3):401-403. · 3.79 Impact Factor
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    ABSTRACT: The binding and interaction of Cu on Si(111)-(7×7) at room temperature have been studied using scanning tunneling microscopy. Self-organized growth of ordered Cu nanostructures was observed. At low coverage, the two-dimensional Cu nanostructures are predominantly assembled at the faulted halves of the (7×7) unit cells. High coverage leads to Cu-binding on the unfauled halves. Our results also show that the two-dimensional nanostructure consists of six Cu-atoms with three on the center adatoms and the others on the rest atoms within one half of a unit cell. The Cu nanostructure formed may have potential applications in the fabrication of nanoscale devices and chemical modification of semiconductor surfaces.
    Surface Science 01/2003; 531(3). · 1.84 Impact Factor