T. Wu

Nanyang Technological University, Tumasik, Singapore

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Publications (76)233.35 Total impact

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    ABSTRACT: Nitrogen-doped zinc oxide (ZnO) film has been grown by molecular beam epitaxy. The as-grown sample showed p-type conduction with a hole concentration of 3.1 x 10(17) cm(-3). After an annealing process in O2 at 600 degrees C for 30 min, p-type conduction was still remained, and the hole concentration of the film decreased to 6.8 x 10(16) cm(-3). Secondary ion mass spectroscopy revealed that the concentration of both nitrogen and hydrogen decreased after the annealing process. It is demonstrated that the intrinsic compensation source has been decreased after the annealing process. Because the variation trend of the hole concentration in the ZnO:N film is opposite to that of hydrogen and intrinsic defects, but in good accordance with nitrogen, the extrinsically substituted nitrogen (N(o)) should be the dominant factor that determines the conduction-type of the ZnO:N film.
    Journal of Nanoscience and Nanotechnology 05/2014; 14(5):3813-6. · 1.15 Impact Factor
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    ABSTRACT: We report the growth and magnetic properties of all-manganite superlattices composed of ultrathin double-exchange ferromagnetic La0.7Sr0.3MnO3 and noncollinear multiferroic TbMnO3 layers. Spontaneous magnetization and hysteresis loops are observed in such superlattices with individual La0.7Sr0.3MnO3 layers as thin as two unit cells, which are accompanied by pronounced exchange bias and enhanced coercivity. Our results indicate substantial interfacial magnetic coupling between spin sublattices in such superlattices, providing a powerful approach towards tailoring the properties of artificial magnetic heterostructures.
    Applied Physics Letters 01/2014; 104(15):152404-152404-5. · 3.79 Impact Factor
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    ABSTRACT: Low dimensionality, broken symmetry and easily-modulated carrier concentrations provoke novel electronic phase emergence at oxide interfaces. However, the spatial extent of such reconstructions - i.e. the interfacial "depth" - remains unclear. Examining LaAlO3/SrTiO3 heterostructures at previously unexplored carrier densities n2D ≥ 6.9 × 10(14) cm(-2), we observe a Shubnikov-de Haas effect for small in-plane fields, characteristic of an anisotropic 3D Fermi surface with preferential dxz,yz orbital occupancy extending over at least 100 nm perpendicular to the interface. Quantum oscillations from the 3D Fermi surface of bulk doped SrTiO3 emerge simultaneously at higher n2D. We distinguish three areas in doped perovskite heterostructures: narrow (<20 nm) 2D interfaces housing superconductivity and/or other emergent phases, electronically isotropic regions far (>120 nm) from the interface and new intermediate zones where interfacial proximity renormalises the electronic structure relative to the bulk.
    Scientific reports. 01/2014; 4:5338.
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    ABSTRACT: We report the magnetic and magnetotransport properties of (In0.985Nd0.015)2O2.89 thin films grown by pulse laser deposition. The clear magnetization hysteresis loops with the complementary magnetic domain structure reveal the intrinsic room temperature ferromagnetism in the as-prepared films. The strong sp-f exchange interaction as a result of the rare earth doping is discussed as the origin of the magnetotransport behaviours. A positive magnetoresistance (∼29.2%) was observed at 5 K and ascribed to the strong ferromagnetic sp-f exchange interaction in (In0.985Nd0.015)2O2.89 thin films due to a large Zeeman splitting in an external magnetic field of 50 KOe.
    Applied Physics Letters 01/2014; 104(20):202411-202411-5. · 3.79 Impact Factor
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    ABSTRACT: We study the magnetoelectric coupling for the [001]-oriented (LaMnO3)2/(BaTiO3)5/(SrMnO3)2 superlattice, by means of the density functional theory. An interesting transition between ferromagnetic ordering and antiferromagnetic ordering is demonstrated by switching ferroelectric polarization in short-period superlattice structure. The predicted ferroelectrically induced magnetic reconstruction is less sensitive to the choice of Coulomb-correction U within GGA + U scheme. A possible explanation is given in terms of the favorable effect of n-type SrMnO3/LaMnO3 interface. Our results suggest that a sizable magnetoelectric effect may be achieved in the short-period LaMnO3/BaTiO3/SrMnO3 superlattice, hence promising application in electrically controlled magnetic data storage.
    Journal of Applied Physics 01/2014; 116(7):074102-074102-5. · 2.21 Impact Factor
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    ABSTRACT: By use of optical pump-probe measurement, we study the relaxation dynamics of a multiferroic-ferromagnetic TbMnO3/La0.7Sr0.3MnO3 bilayer. The relaxation dynamics of both layers are well separated in time allowing us to investigate the magnetic coupling across the bilayer. We observe that the relaxation dynamics of the individual layers in the bilayer sample are the result of the interplay between the intrinsic magnetic order and the induced interfacial effect. Our data suggest the existence of induced ferromagnetic order in the TbMnO3 layer and antiferromagnetic order in the La0.7Sr0.3MnO3 layer.
    Applied Physics Letters 01/2014; 104(14):141602-141602-5. · 3.79 Impact Factor
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    ABSTRACT: A hysteretic in-plane magnetoresistance develops below the superconducting transition of LaAlO$_3$/SrTiO$_3$ interfaces for $\left|H_{/\!/}\right|<$ 0.15 T, independently of the carrier density or oxygen annealing. We show that this hysteresis arises from vortex depinning within a thin superconducting layer, in which the vortices are created by discrete ferromagnetic dipoles located solely above the layer. We find no evidence for finite-momentum pairing or bulk magnetism and hence conclude that ferromagnetism is strictly confined to the interface, where it competes with superconductivity.
    11/2013;
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    ABSTRACT: Self-powered ultraviolet photovoltaic effects have been investigated in metal/SrTiO3 Schottky junctions. The absorption edge of the SrTiO3 single crystal is about 389 nm. The peak photovoltage of the Pt/SrTiO3 junction is about 0.50 V at when it is illuminated by a KrF excimer laser with a wavelength of 248 nm. The peak photovoltages decrease with increasing the temperature due to the strong thermal fluctuation. Moreover, the peak photovoltages almost linearly increase with an increase in the power density of the laser at . The experimental results reveal it may be useful for self-powered ultraviolet detecting sensitive devices.
    EPL (Europhysics Letters) 09/2013; 103(5):57007. · 2.26 Impact Factor
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    ABSTRACT: The photoinduced insulator-metal phase transition and relaxation characteristics have been investigated in bare SrTiO3 single crystals. The photoinduced relaxation time constant after the irradiation shows an increase with increasing temperatures. The SrTiO3 single crystal has a cutoff wavelength and an absorption edge of spectrum at about 385 nm, which agrees well with the band gap. The photocurrent responsivity is 1.36 × 10−5 A/W at 300 nm wavelength. The relative change in resistance is more than above six orders at room temperature, possessing potential applications in ultraviolet sensitive and detecting devices.
    Journal of Applied Physics 07/2013; 114(3). · 2.21 Impact Factor
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    ABSTRACT: We report that room-temperature ferromagnetism emerges at the interface formed between ZnO nanowire core and Al2O3 shell although both constituents show mainly diamagnetism. The interface-based ferromagnetism can be further enhanced by annealing the ZnO/Al2O3 core-shell nanowires and activating the formation of ZnAl2O4 phase as a result of interfacial solid-state reaction. High-temperature measurements indicate that the magnetic order is thermally stable up to 750 K. Transmission electron microscopy studies reveal the annealing-induced jagged interfaces, and the extensive structural defects appear to be relevant to the emergent magnetism. Our study suggests that tailoring the spinterfaces in nanostructure-harnessed wide-band-gap oxides is an effective route towards engineered nanoscale architecture with enhanced magnetic properties.
    Applied Physics Letters 07/2013; 103(2). · 3.79 Impact Factor
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    ABSTRACT: The magnetic properties of manganite bilayers composed of G-type antiferromagnetic (AFM) SrMnO 3 and double-exchange ferromagnetic (FM) La 0.7 Sr 0.3 MnO 3 are studied. A spin-glass state is observed as a result of competing magnetic orders and spin frustration at the La 0.7 Sr 0.3 MnO 3 /SrMnO 3 interface. The dependence of the irreversible temperature on the cooling magnetic field follows the Almeida-Thouless line. Although an ideal G-type AFM SrMnO 3 is featured with a compensated spin configuration, the bilayers exhibit exchange bias below the spin glass freezing temperature, which is much lower than the Néel temperature of SMO, indicating that the exchange bias is strongly correlated with the spin glass state. The results indicate that the spin frustration that originates from the competition between the AFM super-exchange and the FM double-exchange interactions can induce a strong magnetic anisotropy at the La 0.7 Sr 0.3 MnO 3 /SrMnO 3 interface.
    · 3.77 Impact Factor
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    ABSTRACT: We report on the magnetic properties of cuprate/manganite bilayers composed of antiferromagnetic (AFM) La2CuO4 and ferromagnetic La0.70Sr0.30MnO3. The temperature dependent magnetization data indicate an interfacial spin-glass state. Furthermore, the bilayer exhibits significant enhancement of coercivity compared to the La0.70Sr0.30MnO3 single layer and reaches 705 Oe at 5 K. The dependence of coercivity on the AFM layer thickness indicates that the enhancement cannot be explained by the interfacial charge diffusion. Our results suggest that the exchange coupling between Mn and Cu spins and the magnetic frustration at the La2CuO4/La0.70Sr0.30MnO3 interface must be considered to harness the properties of cuprate/manganite heterostructures.
    Applied Physics Letters 01/2013; 102(3). · 3.79 Impact Factor
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    ABSTRACT: The magnetic properties of manganite bilayers composed of G-type antiferromagnetic (AFM) SrMnO3 and double-exchange ferromagnetic (FM) La0.7Sr0.3MnO3 are studied. A spin-glass state is observed as a result of competing magnetic orders and spin frustration at the La0.7Sr0.3MnO3/SrMnO3 interface. The dependence of the irreversible temperature on the cooling magnetic field follows the Almeida-Thouless line. Although an ideal G-type AFM SrMnO3 is featured with a compensated spin configuration, the bilayers exhibit exchange bias below the spin glass freezing temperature, which is much lower than the N´eel temperature of SMO, indicating that the exchange bias is strongly correlated with the spin glass state. The results indicate that the spin frustration that originates from the competition between the AFM super-exchange and the FM double-exchange interactions can induce a strong magnetic anisotropy at the La0.7Sr0.3MnO3/SrMnO3 interface.
    Physical Review B 01/2013; 87:054428. · 3.66 Impact Factor
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    ABSTRACT: The origins of the commonly observed green emission (GE) from ZnO nanostructures remain highly controversial despite extensive studies over the past few decades. Herein, through a comprehensive ultrafast optical spectroscopy study, new insights into its origin and the charge trapping dynamics at the GE centers in ZnO nanowires prepared by the vapor transport method are gained. Transient absorption spectroscopy (TAS) revealed a sub-band-gap absorption bleaching band arising from the state filling of the electrons in the conduction band and holes trapped in the GE centers. The GE originates from the recombination between the electrons in the conduction band and/or shallow donor levels and the holes trapped at the GE centers (which are located at similar to 0.88 eV above the valence band). Importantly, an ultrafast excitonic Auger-type hole trapping process to the GE centers occurring in a subpicosecond time scale was also uncovered by TAS-shedding new light on the mechanism behind the fast and efficient charge trapping of photoexcited carriers. The knowledge gained is crucial for the development of ZnO-based optoelectronic devices. DOI: 10.1103/PhysRevB.87.115309
    Physical review. B, Condensed matter 01/2013; 87(11). · 3.77 Impact Factor
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    ABSTRACT: We report on the interfacial magnetic coupling in manganite bilayers of collinear ferromagnetic La(0.7)Sr(0.3)MnO(3) and noncollinear multiferroic TbMnO(3). Exchange bias is observed at the Néel temperature of TbMnO(3) (~41 K) due to the onset of long-range antiferromagnetic ordering in the Mn spin sublattice. Interestingly, an anomalous plateau of exchange bias emerges at the ordering temperature of Tb spins (~10 K), and we ascribe this unique feature to the strong coupling between Tb and Mn spin sublattices in TbMnO(3), which in turn influences the magnetic coupling across the interface. On the other hand, the enhancement of coercivity in La(0.7)Sr(0.3)MnO(3)/TbMnO(3) shows monotonous temperature dependence. Our results illustrate a strong interfacial magnetic interaction at the La(0.7)Sr(0.3)MnO(3)/TbMnO(3) interface, highlighting the roles of competing spin orders, magnetic frustration, and coupling between multiple spin sublattices in artificial collinear/noncollinear spin heterostructures.
    Scientific Reports 01/2013; 3:1094. · 5.08 Impact Factor
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    ABSTRACT: We report on bandgap engineering of an emerging photovoltaic material of Cu2CdxZn1-xSnS4 (CCZTS) alloy. CCZTS alloy thin films with different Cd contents and single kesterite phase were fabricated using the sol-gel method. The optical absorption measurements indicate that the bandgap of the kesterite CCZTS alloy can be continuously tuned in a range of 1.55-1.09 eV as Cd content varied from x = 0 to 1. Hall effect measurements suggest that the hole concentration of CCZTS films decreases with increasing Cd content. The CCZTS-based solar cell with x = 0.47 demonstrates a power conversion efficiency of 1.2%. Our first-principles calculations based on the hybrid functional method demonstrate that the bandgap of the kesterite CCZTS alloy decreases monotonically with increasing Cd content, supporting the experimental results. Furthermore, Cu2ZnSnS4/Cu2CdSnS4 interface has a type-I band-alignment with a small valence-band offset, explaining the narrowing of the bandgap of CCZTS as the Cd content increases. Our results suggest that CCZTS alloy is a potentially suitable material to fabricate high-efficiency multi-junction tandem solar cells with different bandgap-tailored absorption layers. (C) 2013 AIP Publishing LLC.
    Journal of Applied Physics 01/2013; 114(18). · 2.21 Impact Factor
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    ABSTRACT: The heterostructure composed of Pr0.65(Ca0.75Sr0.25)0.35MnO3 and 0.7 wt. % Nb-doped SrTiO3 was fabricated. The heterostructure exhibits an asymmetric current-voltage relation similar to that of p-n junctions and an obvious photovoltaic effect with its maximum value of about 25.1 mV at T = 140 K. It is interesting that the magnetic field has a strong suppression effect on photovoltaic effect, and the maximum relative change of photovoltage under the magnetic field of 1 T is about 63% with the illumination of 15 mW/mm2 light (532 nm) at T ∼ 140 K. The physical mechanism is explained by the electronic phase separation mechanism.
    Applied Physics Letters 01/2013; 103(21):212401-212401-4. · 3.79 Impact Factor
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    ABSTRACT: Deciphering the intrinsic magnetic domain-wall (DW) resistivity of manganite materials by typical low-field magnetoresistance measurement is flawed due to the addition of different galvanomagnetic effects such as, colossal magnetoresistance, Lorentz force magnetoresistance, and anisotropic magnetoresistance (AMR). In this paper, by taking the advantage of rotational anisotropy and the stable rotation of the DW planes in half-metallic manganite La0.7Sr0.3MnO3 film, we deploy a remanent state resistance measurement technique to exclude all the field-dependent spurious effects from the intrinsic DW resistivity. To further refine its magnitude, we calculate the remanent state DW AMR by exploiting the three-dimensional micromagnetic simulation, which reveals a comparable but opposite contribution to the positive DW resistivity. From these results, we estimate the intrinsic DW resistance-area product in La0.7Sr0.3MnO3 to be 1.9×10−15 Ω·m2.
    Physical review. B, Condensed matter 11/2012; 86(18). · 3.77 Impact Factor
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    ABSTRACT: We report on ultraviolet photovoltaic effects in a BiFeO3/Nb-doped SrTiO3 heterostructure prepared by a pulsed laser deposition method. The heterostructure exhibits rectifying behaviors in the temperature range from 80 K to 300 K. The photovoltage of heterostructure is about 0.33 V at T = 80 K when it is illuminated by a KrF excimer laser with a wavelength of 248 nm. The peak photovoltages decrease with increasing the temperature because of the accumulation of photogenerated carriers. Moreover, the peak photovoltages of heterostructure almost linearly increase with an increase of the power density of laser at T = 300 K. The results reveal some properties that may be useful for possible applications in multiferroic photoelectric devices.
    Journal of Applied Physics 10/2012; 112(8). · 2.21 Impact Factor
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    ABSTRACT: Magnetic relaxation is ubiquitous in magnetic materials, and elucidation of the underlying mechanisms is important for achieving reliable device operations. Here, we systematically investigate the magnetic relaxation in compressively strained La0.7Sr0.3MnO3 thin films. Upon the removal of external magnetic field, the slow time-dependent increase of in-plane magnetization is correlated with the break-up of magnetic domains and the emergence of additional domain walls, whereas a reduction of magnetization for the initial short period dominates the magnetic relaxation at lower temperatures in thinner films. These relaxation effects underline the importance of domain dynamics in the properties of magnetic thin films.
    Applied Physics Letters 07/2012; 101(1). · 3.79 Impact Factor

Publication Stats

412 Citations
233.35 Total Impact Points

Institutions

  • 2008–2014
    • Nanyang Technological University
      • • School of Materials Science and Engineering
      • • Division of Physics and Applied Physics
      • • School of Physical and Mathematical Sciences
      Tumasik, Singapore
  • 2013
    • King Abdullah University of Science and Technology
      • Division of Physical Sciences and Engineering (PSE)
      Djidda, Makkah, Saudi Arabia
  • 2011
    • Bilkent University
      Engüri, Ankara, Turkey
  • 2010
    • University of Cambridge
      • Department of Physics: Cavendish Laboratory
      Cambridge, ENG, United Kingdom
  • 2000–2001
    • China Pharmaceutical University
      • Department of Pharmaceutics
      Nanjing, Jiangxi Sheng, China