S. J. Wang

Tsinghua University, Peping, Beijing, China

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Publications (139)319.98 Total impact

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    ABSTRACT: It has been recently emphasized that the surface of a photocatalyst plays crucial roles on its photocatalytic performance. By nitridizing the top layer of the rutile TiO2(110) surface using reactive atomic N flux, we report direct experimental evidence of significantly enhanced photocatalytic activity under both ultraviolet (UV) and visible light irradiations. The visible light activity of a nitridized surface is found to be comparable to the UV light activity of the pristine surface. On the basis of X-ray photoemission spectroscopy (XPS) measurements and density-functional theory (DFT) calculations, top surface N doping efficiently narrows the local band gap, 2.0 eV, which accounts for the visible light activity. Under visible light excitation, nearly all free charges contribute to the photocatalytic reactions. The improvement of photocatalytic activity is attributed to the N 2p add-on shoulder at the valence band maximum (VBM) as well as the strong exchange-splitting at the surface that settles the N 2pz states inside the conduction band of the TiO2 matrix, which avails efficient charge transfer.
    The Journal of Physical Chemistry C. 01/2014; 118(2):994–1000.
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    ABSTRACT: Heavily Nb-doped anatase TiO 2 (TNO) thin films were prepared by pulsed dc magnetron sputtering using an Nb-doped TiO 2 target. The as-grown films exhibit high resistivity whose resistance decreases by ∼2 × 10 4 -fold upon vacuum annealing. The ∼40% Nb-doped anatase TiO 2 film shows a low resistivity of 5.7 × 10 −4 Ω cm and a high electron concentration of 3.07 × 10 21 cm −3 . Combined in situ x-ray photoelectron spectroscopy (XPS) measurement and density-functional theory (DFT) calculations show that oxygen interstitial (O int ) and Nb interstitial (Nb int ) defect clusters introduce localized shallow p -type accepter states that trap the electrons and reduce the conductivity. These defect clusters can be eliminated by vacuum annealing which is companied by outward diffusion of Nb. As a result, the trapped electrons backfill the Ti sites which are delocalized to promote conductivity.
    Materials Research Express. 01/2014; 1(1):015911.
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    ABSTRACT: Energy-band alignments for molybdenum disulphide (MoS2) films on high-k dielectric oxides have been studied using photoemission spectroscopy. The valence band offset (VBO) at monolayer MoS2/Al2O3 (ZrO2) interface was measured to be 3.31 eV (2.76 eV), while the conduction-band offset (CBO) was 3.56 eV (1.22 eV). For bulk MoS2/Al2O3 interface, both VBO and CBO increase by ∼0.3 eV, due to the upwards shift of Mo 4dz2 band. The symmetric change of VBO and CBO implies Fermi level pinning by interfacial states. Our finding ensures the practical application of both p-type and n-type MoS2 based complementary metal-oxide semiconductor and other transistor devices using Al2O3 and ZrO2 as gate materials.
    Applied Physics Letters 01/2014; 104(23):232110-232110-4. · 3.79 Impact Factor
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    ABSTRACT: Molybdenum is a refractory metal that is stable in a body-centred cubic structure at all temperatures before melting. Plastic deformation via structural transitions has never been reported for pure molybdenum, while transformation coupled with plasticity is well known for many alloys and ceramics. Here we demonstrate a structural transformation accompanied by shear deformation from an original <001>-oriented body-centred cubic structure to a <110>-oriented face-centred cubic lattice, captured at crack tips during the straining of molybdenum inside a transmission electron microscope at room temperature. The face-centred cubic domains then revert into <111>-oriented body-centred cubic domains, equivalent to a lattice rotation of 54.7°, and ~15.4% tensile strain is reached. The face-centred cubic structure appears to be a well-defined metastable state, as evidenced by scanning transmission electron microscopy and nanodiffraction, the Nishiyama-Wassermann and Kurdjumov-Sachs relationships between the face-centred cubic and body-centred cubic structures and molecular dynamics simulations. Our findings reveal a deformation mechanism for elemental metals under high-stress deformation conditions.
    Nature Communications 01/2014; 5:3433. · 10.02 Impact Factor
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    ABSTRACT: We report magnetotransport properties of double heterointerfaces in LaAlO3/SrTiO3/LaAlO3/SrTiO3(001) (LAO/STO/LAO/STO). A strong nonlinearity in the Hall resistivity is found when the temperature is below 80 K. This effect is attributed to multichannel conduction of interfacial charges generated in double heterostructures of LAO/STO where two-dimensional electron gas (2DEG) is produced. The multichannel conduction is confirmed by back gating modulation of Hall effect. Our result suggests the possibility to achieve coupled bilayer 2DEG layers in LAO/STO superlattices.
    Applied Physics Letters 01/2014; 105(1):011603-011603-4. · 3.79 Impact Factor
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    ABSTRACT: The American Nuclear Society (ANS) has presented the prestigious Edward Teller award to Dr. Bruce A. Remington during the 2011 IFSA conference due to his "pioneering scientific work in the fields of inertial confinement fusion (ICF), and especially developing an international effort in high energy density laboratory astrophysics" [1,2]. This is a great acknowledgement to the subject of high energy density laboratory astrophysics. In this context, we report here one experiment conducted to model solar flares in the laboratory with intense lasers [3]. The mega-gauss -scale magnetic fields produced by laser produced plasmas can be used to make magnetic reconnection topology. We have produced one table-top solar flare in our laboratory experiment with the same geometric setup as associated with solar flares.
    11/2013;
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    ABSTRACT: Using optical pump-terahertz probe spectroscopy, we studied the dynamic behaviors of photoinduced insulator-metal phase transition in vanadium oxide thin films with different oxygen stoichiometry. We found that the insulator-metal phase transition of vanadium dioxide is very sensitive to oxygen stoichiometry: the increased oxygen content in vanadium oxide will reduce the magnitude of phase transition and change the dynamics of the phase transition. The transient complex photoconductivity of vanadium oxide thin films is investigated and analyzed with Drude-Smith model, supplying insight of the dynamic process of phase transition in vanadium oxide thin films.
    Applied Physics Letters 10/2013; 103:151908. · 3.79 Impact Factor
  • Thin Solid Films 10/2013; · 1.87 Impact Factor
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    ABSTRACT: Owing to its high stability in aqueous solution and high quantum efficiency, tantalum based oxyntride (TaON) has attracted increasing attentions for application as visible light photocatalyst. However, despite the recent progress in photocatalytic studies, its bulk charge transport mechanisms are yet to be discovered because of the lack of single crystal sample. In this paper, high quality single crystalline TaON(100) thin film was prepared on cubic YSZ(100) surface by reactive RF magnetron sputtering to avail the understandings of charge transport mechanism so as to improve the device efficiency. The stoichiometry, crystal phase and structure were examined in situ by x-ray photoelectron spectroscopy (XPS) and ex situ by x-ray diffraction (XRD) and transmission electron microscopy (TEM). The TaON film crystallizes in monoclinic β-phase with its [010]/[001] directions aligned to those of the substrate. The small band gap of 2.5 eV as well as the high structure perfection suggests better performance for visible light water splitting. The method can be used to prepare other surface orientations to elucidate fundamental surface structure dependent photoactivities.
    Journal of Solid State Chemistry 08/2013; 204:27–31. · 2.04 Impact Factor
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    ABSTRACT: Low-temperature depositions of Si films from hydrogenated amorphous silicon (a-Si : H) to highly crystallized hydrogenated microcrystalline silicon (µc-Si : H) were realized by the low-frequency inductively coupled plasma (LF-ICP) technique, with low hydrogen dilution (50%) and without any intentional substrate heating. µc-Si : H films with a thin incubation layer (<100 nm), a small grain size (<14 nm), a weak [1 1 1] or [2 2 0] preferred orientation, and a low hydrogen content (4–8%) were obtained (at deposition rates ~0.4–0.7 nm s−1). The increased crystallinity due to the increasing rf power is accompanied by the improved structural order, lowered impurity contamination and decreased hydrogen content. The compactness of the µc-Si : H film is positively related to crystallinity and independent of the high microstructure factor (>0.8). Low-temperature growth of µc-Si : H is attributed to high atomic H flux and suppression of high-energy ion bombardment due to the high density of low-temperature electrons in the plasma. A µc-Si : H solar cell with a less dense intrinsic layer (on a SnO2 : F glass substrate) exhibits a high Voc (584 mV), showing great potential for photovoltaic applications.
    Journal of Physics D Applied Physics 05/2013; 46(21):215501. · 2.53 Impact Factor
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    ABSTRACT: We studied the transport of an intense electron beam produced by high intensity laser pulses through metals and insulators. Targets were irradiated at two different intensities, 1017 W/cm2 and 1019 W/cm2, at the laser facility Xtreme Light XL-III in Beijing, a Ti:Sa laser source emitting 40 fs pulses at 800 nm. The main diagnostic was Cu-Kα fluorescence imaging. Images of Kα spots have been collected for those two laser intensities, for different target thickness, and for different materials. Experimental results are analyzed taking into account both collisional and collective effects as well as refluxing at the edge of the target. The target temperature is evaluated to be Tc ∼ 6 eV for intensity I = 1017 W/cm2 (for all the tested materials: plastic, aluminium, and copper), and Tc ∼ 60 eV in aluminium and 120 eV in titanium for intensity I = 1019 W/cm2.
    Physics of Plasmas 03/2013; 20(3). · 2.38 Impact Factor
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    ABSTRACT: The complex structural transformation in crystals under static pressure or shock loading has been a subject of long-standing interest to materials scientists and physicists. The polymorphic transformation is of particular importance for iron (Fe), due to its technological and sociological significance in the development of human civilization, as well as its prominent presence in the earth's core. The martensitic transformation α→ε (bcc→hcp) in iron under shock-loading, due to its reversible and transient nature, requires non-trivial detective work to uncover its occurrence. Here we reveal refined microstructural fingerprints, needle-like colonies and three sets of {112}<111> twins with a threefold symmetry, with tell-tale features that are indicative of two sequential martensitic transformations in the reversible α→ε phase transition, even though no ε is retained in the post-shock samples. The signature orientation relationships are consistent with previously-proposed transformation mechanisms, and the unique microstructural fingerprints enable a quantitative assessment of the volume fraction transformed.
    Scientific Reports 01/2013; 3:1086. · 5.08 Impact Factor
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    ABSTRACT: Si3N4 thin films were grown on epitaxial graphene layers on 6H-SiC (0001) substrate, and the related interfacial properties have been studied by using high-resolution transmission electron microscopy (TEM), X-ray photoemission spectroscopy (XPS), and first-principles calculations. It is found that Si3N4 forms uniform coverage on graphene/SiC. The interaction between graphene and Si3N4 is weak, and the measured barrier height between them is 2.7 ± 0.1 eV, which is high enough to minimize the tunneling carriers. The in situ thermal treatment shows that the Si3N4/graphene/6H-SiC heterojunctions are thermally stable up to 800 °C. These results suggest promising application of Si3N4 as a dielectric in graphene-based electronic devices.
    The Journal of Physical Chemistry C. 10/2012; 116(42):22315–22318.
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    ABSTRACT: ZnO(0 0 2) epitaxial films have been successfully grown on Si(1 1 1) with CeO2 as a buffer layer by pulsed laser deposition. In spite of large lattice mismatch between ZnO and CeO2, good film quality was achieved, as proven by Fourier filtered high-resolution transmission electron microscopy (HRTEM) image, due to reduction in interface strain by domain matching epitaxy. The epitaxial relationship of ZnO and CeO2 on the Si substrate was determined to be (0 0 2)[2 1 0]ZnO||(1 1 1)[1 1 2] (1 1 1)[1 1 2]Si. The HRTEM images show low defect concentrations in both the deposited ZnO film and CeO2 layer. Ordered crack lines are observed on the surface of the ZnO film which are due to A-type and B-type stackings of CeO2 on Si(1 1 1). Sharp near-band edge emission at 3.253 eV was detected for the ZnO film through photoluminiscence measurement at room temperature.
    Journal of Physics D Applied Physics 09/2012; 45(41):415306. · 2.53 Impact Factor
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    ABSTRACT: We studied the ultrafast dynamic behavior of the photoinduced insulator-metal phase transition in VO(2) thin film using optical pump-terahertz probe spectroscopy with different excitation fluences and at different temperatures. We observed two processes in the insulator-metal phase transition in VO(2): a fast process and a slow process. The fast process is a nonthermal process, which is ascribed to the nucleation of the metal phase, while the slow process is strongly affected by temperature and is ascribed to the thermally driven growth and coalescence of metal domains in VO(2). The transient complex conductivity spectra at different delay times are also investigated.
    Journal of Physics Condensed Matter 09/2012; 24(41):415604. · 2.22 Impact Factor
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    ABSTRACT: The magnetic reconnection (MR) configuration was constructed by using two approaching laser-produced plasma bubbles. The characteristics of the MR current sheet were investigated. The driving energy of the laser pulse affects the type of the current sheet. The experiments present “Y-type” and “X-type” current sheets for larger and smaller driving energy, respectively. The energetic electrons were found to be well-collimated. The formation and ejection of plasmoid from the “Y-type” current sheet was expected to enhance the number of accelerated electrons.
    Journal of Plasma Physics 08/2012; 78(04):497-500. · 0.76 Impact Factor
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    ABSTRACT: We present on a systematic study of the contribution of surface plasmon (SP) coupling and light extraction toward emission enhancement of Platinum (Pt) nano-patterns capped MgZnO films. Time resolved Photoluminescence (PL) results indicate that the Pt coating can greatly reduces the non-radiative recombination rate by passivation of surface states, making the decay slow down. Temperature dependence of the integrated photoluminescence intensity reveals that the Pt nano-patterns can offer a large amount of light transfer and scattering, which enormously increase the light extraction efficiency up to 3.8-fold. These results indicate that the increased light extraction efficiency caused by surface modification via Pt coating rather than SP coupling plays a dominant role in increasing bandgap emission of MgZnO film.
    Optics Express 06/2012; 20(13):14556-63. · 3.55 Impact Factor
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    ABSTRACT: The effects of laser incidence angle on lateral fast electron transport at front target surface, when a plasma is preformed, irradiated by intense (>1018 W/cm2) laser pulses, are studied by Kα imaging technique and electron spectrometer. A horizontally asymmetric Kα halo, resulting from directional lateral electron transport and energy deposition, is observed for a large incidence angle (70°). Moreover, a group of MeV high energy electrons is emitted along target surface. It is believed that the deformed preplasma and the asymmetrical distribution of self-generated magnetic field, at large incidence angle, play an important role in the directional lateral electron transport.
    Laser and Particle Beams 03/2012; 30(01):39-43. · 2.02 Impact Factor
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    ABSTRACT: The annealing effects on the composition, structure and magnetism for Co overlayer deposited onto rutile TiO2 (100) surfaces at room temperature has been investigated. Room temperature ferromagnetic behaviour is exhibited for all samples, but saturation magnetic moment per Co atom decreases with increasing annealing temperature until 530°C. In-situ photoemission studies show that the reduction of the saturation magnetic moment is due to Co oxidation at high annealing temperature. However, the saturation magnetic moment increases after annealing the sample up to 700°C. The formation of a Co-Ti-O phase rather than Co metallic clusters observed by high-resolution transmission electron microscopy is suggested to be responsible for the increase of the saturation magnetic moment.
    International Journal of Modern Physics B 01/2012; 22(01n02). · 0.46 Impact Factor
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    ABSTRACT: Surface plasmon (SP) mediated emission from highly ordered Au-nanopattern–capped ZnCdO film has been studied by temperature-dependent photoluminescence (TDPL) from 10 to 300 K. It is found that the bandgap emission from Au-capped ZnCdO is stronger than that of bare ZnCdO throughout the entire temperature range owing to SP coupling and surface modification. The TDPLs of ZnCdO films with and without Au coatings exhibit an inconspicuous S-shape in the plot of emission peak energy (Ep) vs. temperature, signifying a weak carrier localization effect: Ep redshifts in the temperature range of 10–80 K, blueshifts in the range 80–140 K, and redshifts again in the range 140–300 K. It is interesting to find that the temperature-dependent behavior of the enhancement ratio can be divided into three temperature ranges. In the low- and high-temperature ranges, the enhancement ratio is not significantly changed with temperature. However, in the medium-temperature range, the enhancement ratio increases monotonically with temperature due to the excitons delocalization from the potential minima. These results indicate that excitons delocalization is an important prerequisite of the efficient plasmonic enhancement for SP mediated emission from ZnCdO system.
    EPL (Europhysics Letters) 01/2012; 99(2). · 2.26 Impact Factor

Publication Stats

778 Citations
319.98 Total Impact Points

Institutions

  • 2014
    • Tsinghua University
      Peping, Beijing, China
  • 2008–2013
    • Northeast Institute of Geography and Agroecology
      • • Shenyang National Laboratory for Materials Science
      • • Institute of Physics
      Beijing, Beijing Shi, China
    • Imperial College London
      • Department of Physics
      London, ENG, United Kingdom
  • 2006–2013
    • Chinese Academy of Sciences
      • Institute of Physics
      Peping, Beijing, China
  • 2000–2012
    • National University of Singapore
      • • Department of Materials Science and Engineering
      • • Department of Electrical & Computer Engineering
      • • Department of Physics
      Singapore, Singapore
  • 2010
    • Shanghai Jiao Tong University
      • Department of Physics
      Shanghai, Shanghai Shi, China
  • 2005–2010
    • Nanyang Technological University
      • School of Physical and Mathematical Sciences
      Singapore, Singapore
  • 2003
    • Hebei Medical University
      Chentow, Hebei, China