T. S. Herng

National University of Singapore, Singapore, Singapore

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Publications (41)115.19 Total impact

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    ABSTRACT: The development of technologically important material zinc-blende ZnO has been hindered due to the difficulties inherent in obtaining a stable zinc-blende phase. In this paper, we fabricate the stable zinc-blende ZnO on Pt/Ti/SiO2/Si substrate through phase transformation from the originally wurtzite to the zinc-blende phase. X-ray diffraction data in combination with high-resolution TEM measurements provide the direct evidence on the formation of the well-defined zinc-blende structure with predominated (202) orientation. According to the experimental results and first principles calculations, the incorporation of titanium dopants into ZnO system favors the formation of the zinc-blende structure. The platinum (Pt) surface stabilizes the ZnO zinc-blende structure at the interface (thin film) due to its low ZnO/Pt interface energy, preventing the decomposition in ZnO wurtzite and Zn2TiO4. Additionally, magnetic and optical properties of the ZnO zinc-blende thin films are investigated. Unexpectedly, the film is found to exhibit magnetization of ~75 emu/cm3, while its ZnO wurtzite counterpart is non-ferromagnetic.
    Journal of Materials Science 01/2015; 50(1). · 2.31 Impact Factor
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    ABSTRACT: We study room temperature optics and electronic structures of ZnO:Cu films as a function of Cu concentration using a combination of spectroscopic ellipsometry, photoluminescence, and ultraviolet-visible absorption spectroscopy. Mid-gap optical states, interband transitions, and excitons are observed and distinguishable. We argue that the mid-gap states are originated from interactions of Cu and oxygen vacancy (Vo). They are located below conduction band (Zn4s) and above valence band (O2p) promoting strong green emission and narrowing optical band gap. Excitonic states are screened and its intensities decrease upon Cu doping. Our results show the importance of Cu and Vo driving the electronic structures and optical transitions in ZnO:Cu films.
    Applied Physics Letters 01/2014; 104(8). · 3.52 Impact Factor
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    ABSTRACT: Thin layers of Pb(Zr0.95Ti0.05)O3/Co ferrite (CFO) were deposited on Si(100)/SiO2/Ti/Pt(111) and SrTiO3(001) substrates using pulsed laser deposition technique. The phase and structural evolutions of the composites were characterized using X-ray diffractometry and transmission electron microscope. The magnetic and electric properties of the samples were evaluated by the hysteresis loop measurement systems. The results showed that the double layer of Pb(Zr0.95Ti0.05)O3/CFO coating on SrTiO3 substrate, exhibited a clear magnetic hysteresis loop as well as a saturated polarization. Magnetoelectric coefficient of this sample (αE) reaches to 244 × 10− 3 V/A (194 mV/cm Oe). Thin layer of Pb(Zr0.95Ti0.05)O3/CFO on Si(100)/SiO2/Ti/Pt(111) substrate was polycrystalline without texture and its maximum magnetoelectric coefficient was 167 × 10− 3 V/A.
    Thin Solid Films 06/2013; 537:76–79. · 1.87 Impact Factor
  • Tong Li, Wen Xiao, Tun Seng Herng, Nina Bao, Jun Ding
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    ABSTRACT: Significantly enhanced ferromagnetism and strong green emission were observed for Cu-doped ZnO films undergoing a hydrogenation process. After H2 treatment at 500 °C, the saturation magnetization of 2 at% Cu-doped ZnO film (50 nm) is significantly increased from ∼1.65 emu cm−3 (0.3 µB /Cu) to ∼11.7 emu cm−3 (1.5 µB /Cu). The areal magnetization of the hydrogenated Cudoped ZnO is dependent on the film thickness, thus suggesting that it is Cu dopants that play dominant roles rather than surface magnetism arising from OH attachment. Detailed XPS and Raman analysis demonstrated that H2 treatment may introduce more Cu (Cu1+-like) impurities and oxygen vacancies that coupled with each other, resulting in ferromagnetic ordering. Furthermore, strong green emission can be obtained in hydrogenated Cu-doped ZnO films. The green emission is unlikely related with Cu dopants, but H2 treatment can generate a unique structure with porous morphology and coexistence of complex defects that favors the green emission. The “green defects” are not simple O vacancies or H-incorporation and are stable after annealing at 700 °C under O2 atmosphere. The high room temperature ferromagnetism and strong green light emission of hydrogenated Cu-doped ZnO film pave a way to its novel applications in future spintronics and optoelectronic devices.
    Journal- Korean Physical Society 06/2013; 62(12). · 0.51 Impact Factor
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    Amit Kumar, Tun Seng Herng, Kaiyang Zeng, Jun Ding
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    ABSTRACT: The bipolar charge phenomenon in Cu and Co co-doped zinc oxide (ZnO) film samples has been studied using scanning probe microscopy (SPM) techniques. Those ZnO samples are made using a pulsed laser deposition (PLD) technique. It is found that the addition of Cu and Co dopants suppresses the electron density in ZnO and causes a significant change in the work function (Fermi level) value of the ZnO film; this results in the ohmic nature of the contact between the electrode (probe tip) and codoped sample, whereas this contact exhibits a Schottky nature in the undoped and single-element-doped samples. These results are verified by Kelvin probe force microscopy (KPFM) and ultraviolet photoelectron spectroscopy (UPS) measurements. It is also found that the co-doping (Cu and Co) can stabilize the bipolar charge, whereas Cu doping only stabilizes the positive charge in ZnO thin films.
    ACS Applied Materials & Interfaces 09/2012; 4(10):5276-80. · 5.90 Impact Factor
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    ABSTRACT: Coexistence of polarization and resistance-switching characteristics in single compounds has been long inspired scientific and technological interests. Here, we report the non-volatile resistance change in noncentrosymmetric compounds investigated by using defect nanotechnology and contact engineering. Using a noncentrosymmetric material of ZnO as example, we first transformed ZnO into high resistance state. Then ZnO electrical polarization was probed and its domains polarized 180° along the [001]-axis with long-lasting memory effect (>25 hours). Based on our experimental observations, we have developed a vacancy-mediated pseudoferroelectricity model. Our first-principle calculations propose that vacancy defects initiate a spontaneous inverted domains nucleation at grain boundaries, and then they grow in the presence of an electrical field. The propagation of inverted domains follows the scanning tip motion under applied electrical field, leading to the growth of polarized domains over large areas.
    Scientific Reports 08/2012; 2:587. · 5.08 Impact Factor
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    Nanoscience and Nanotechnology Letters 06/2012; 4(6):609-610. · 1.44 Impact Factor
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    ABSTRACT: Using a two-step fabrication technique (pulsed laser deposition (PLD) and H2 surface treatment), we fabricated ZnO thin films that could emit ultra-strong green emission with coexistence of random lasing phenomenon. After PLD deposition, the as-prepared undoped ZnO thin films (200–500 nm) were annealed in Ar 95%–H25% ambient at 500 °C. The H2 treatment led to the formation of a porous structure that creates substantial optical cavities (diameter ∼1.3 µm). Surprisingly, these optical cavities tremendously amplified the green emission rather than ultraviolet (UV) emission. There was insignificant change in emission intensity after high-temperature annealing (700 °C) in O2 and acetone dipping, indicating the samples are thermally and chemically stable. The samples exhibited a high quantum yield of 32%. We studied the origin of this ultra-strong green emission using low-temperature photoluminescence, extensive structural study and cyclic annealing. The results suggested that neither hydrogen nor VO plays a role in green emission. The green emission was attributed mainly to the complex defects and the presence of structural defects in the porous structure. In addition, we demonstrated the feasibility of large-scale green emission ZnO fabrication via micro-size patterning, paving a way to practical optoelectronic applications.
    Journal of Physics D Applied Physics 05/2012; 45(18). · 2.53 Impact Factor
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    ABSTRACT: The ferromagnetism in many carbon nanostructures is attributed to carbon dangling bonds or vacancies. This provides opportunities to develop new functional materials, such as molecular and polymeric ferromagnets and organic spintronic materials, without magnetic elements (for example, 3d and 4f metals). Here we report the observation of room temperature ferromagnetism in Teflon tape (polytetrafluoroethylene) subjected to simple mechanical stretching, cutting or heating. First-principles calculations indicate that the room temperature ferromagnetism originates from carbon dangling bonds and strong ferromagnetic coupling between them. Room temperature ferromagnetism has also been successfully realized in another polymer, polyethylene, through cutting and stretching. Our findings suggest that ferromagnetism due to networks of carbon dangling bonds can arise in polymers and carbon-based molecular materials.
    Nature Communications 03/2012; 3:727. · 10.74 Impact Factor
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    ABSTRACT: We have investigated the photoluminescence and electrical properties of a coherently coupled interface consisting of a ZnO layer grown on top of an oriented VO2 layer on sapphire across the phase transition of VO2. The band edge and defect luminescence of the ZnO overlayer exhibit hysteresis in opposite directions induced by the phase transition of VO2. Concomitantly the phase transition of VO2 was seen to induce defects in the ZnO layer. Such coherently coupled interfaces could be of use in characterizing the stability of a variety of interfaces in situ and also for novel device application.
    Applied Physics Letters 01/2012; 100:241907. · 3.52 Impact Factor
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    ABSTRACT: A series of undoped ZnO films of different thicknesses was grown on different substrates over a range of different temperatures and oxygen partial pressures. Notably, ferromagnetism was detected in very thin ZnO films (20 nm), and its magnetic ordering was also found to be thermally stable up to 800 °C. To our surprise, magnetic ordering was destroyed as the ZnO overlayer grew thicker, just as its in-plane compressive strain was released and the ZnO/substrate interface damaged by misfit dislocations. The source of magnetism was found to be due to neither defects in the bulk of the ZnO overlayer nor the bulk of the substrate. Experimental results showed that strain at the ZnO/substrate interface led to a strain-induced magnetic effect. Using first-principles ab intio calculation, we confirmed that strain at the ZnO/substrate interface stabilizes zinc vacancy defects, which are magnetic. Ferromagnetic ordering is a result of the coupling of unpaired electron spins originating from the oxygen 2p orbitals surrounding the zinc vacancies.
    The Journal of Physical Chemistry C. 12/2011; 116(1):610–617.
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    ABSTRACT: Nonmagnetic ZnO films were found to exhibit room temperature ferromagnetism after hydrogen annealing at elevated temperatures (100–500 °C), accompanied by (OH) bonds detection. The areal saturation magnetization Ms ( ∼ 1.1×10−5 emu cm−2) was insensitive to film thickness, suggesting surface magnetism. The attribution to OH bonds on surface was further supported when the ferromagnetism disappeared after a short immersion for 1 s in acid solution while ferromagnetism was relatively stable in basic environment. The alternative H2- and Ar-annealing can switch ferromagnetic “on” and “off” state, as the annealing under Ar atmosphere can reduce OH bond quantity significantly. First-principles calculations have further confirmed that OH-terminated ZnO surface belonging to the p31m two-dimensional space group has the lowest formation energy of −2.97 eV and a magnetic moment of 0.30 μB per OH due to unpaired magnetic moment of electrons occupying O 2p orbital. Insufficient surface OH concentration may result in antiferromagnetism and/or paramagnetism.
    Applied Physics Letters 04/2011; 98(15):152505-152505-3. · 3.52 Impact Factor
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    ABSTRACT: A mutual ferromagnetic and ferroelectric coupling (multiferroic behavior) in Cu-doped ZnO is demonstrated via deterministic control of Cu doping and defect engineering. The coexistence of multivalence Cu ions and oxygen vacancies is important to multiferroic behaviors in ZnO:Cu. The samples show clear ferroelectric and ferromagnetic domain patterns. These domain structures may be written reversibly via electric and magnetic bias.
    Advanced Materials 04/2011; 23(14):1635-40. · 14.83 Impact Factor
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    ABSTRACT: Multiferroic epitaxial films, include SrRuO3/Pb(Zr0.95Ti0.05)O3/CoFe2O4 has been successfully deposited on SrTiO3 substrate by pulsed-laser deposition technique. The results show that the prepared films exhibit a single phase. The Pb(Zr0.95Ti0.05)O3 (PZT) film was highly textured with (1 0 0) orientation and gives good ferroelectric properties with saturated polarization of 15 μC/cm2. The magnetic coercivity of CoFe2O4 film on Pb(Zr0.95Ti0.05)O3 has been dampened to 0.9 kOe. The anisotropic magnetically behavior of CoFe2O4 film was changed to isotropic by using high Zr concentrated PZT as underneath layer. Heterostructure films show a good ferromagnetic and ferroelectric coupling that lead to the large magnetoelectricity of 287 mV/cm Oe.
    Materials & Design. 04/2011; 32(4):2370–2373.
  • Journal of Nanoscience and Nanotechnology 03/2011; 11(3):2549-50. · 1.15 Impact Factor
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    ABSTRACT: To better understand the observation of room-temperature ferromagnetic ordering in O-deficient Cu:ZnO films [1], we calculated the configuration-averaged spectral function of ZnO with 2% Cu impurities and 1% O vacancies within the "LDA+U" approximation, solving the Hamiltonian only within the low energy Hilbert space, defined via the first-principles Wannier functions [2]. Based on these first principles results we proposed a microscopic "indirect double-exchange" model for the FM in Cu:ZnO that explains our main experimental findings.[4pt] [1] T.S. Herng et al, Phys. Rev. Lett. 105, 207201 (2010)[0pt] [2] T. Berlijn et al, arXiv:1004.1156 (2010)
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    ABSTRACT: We report direct evidence of room-temperature ferromagnetic ordering in O-deficient Cu:ZnO films by using soft x-ray magnetic circular dichroism and x-ray absorption [1]. Our measurements have revealed unambiguously two distinct features of Cu atoms associated with (i) magnetically ordered Cu ions present only in the oxygen-deficient samples and (ii) magnetically disordered regular Cu^2+ ions present in all the samples. We find that a sufficient amount of both oxygen vacancies and Cu impurities is essential to the observed ferromagnetism, and a non-negligible portion of Cu impurities is uninvolved in the magnetic order.[4pt] [1] T.S. Herng et al, Phys. Rev. Lett. 105, 207201 (2010)
    03/2011;
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    ABSTRACT: In this work, Pt nanocrystals (NCs) were embedded into different oxide films by codeposition of Pt and oxide at 400 °C and 10-8 Torr using a pulse laser deposition system. Films of Pt (25 mol. %) NCs embedded in (Al2O3, ZnO, or SnO2) matrix showed room temperature ferromagnetism (RTFM), while Pt (25 mol. %) NCs/(MgO or SiO2) films were not magnetic. The detailed study of Pt NCs/Al2O3 films with different concentrations of Pt revealed that RTFM depended on Pt NC size and conductivity of the film. Resistance-temperature study of this film showed that the hopping conduction may be associated with RTFM.
    Journal of Applied Physics 01/2011; 109. · 2.21 Impact Factor
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    ABSTRACT: The surface potential of undoped and copper-doped zinc oxide (ZnO:Cu) films has been studied using the Kelvin probe force microscopy at ambient condition. In contrast to the undoped ZnO with unipolar behavior, the ZnO:Cu film exhibits a bipolar surface potential behavior under a dc bias. The localized hole trapping phenomenon is attributed to the presence of Cu ions in ZnO films. With an appropriate amount of the Cu ions ( ∼ 8 at. %), the charge trapping is reasonably stable over a period of 20 h, which can be associated with the presence of oxygen vacancies. This coexistence of Cu ions and oxygen vacancies in ZnO gives rise to stable bipolar behavior, paving way to potential charge storage application.
    Applied Physics Letters 12/2010; 97(23):232103-232103-3. · 3.52 Impact Factor

Publication Stats

293 Citations
115.19 Total Impact Points

Institutions

  • 2010–2012
    • National University of Singapore
      • Department of Materials Science and Engineering
      Singapore, Singapore
  • 2006–2010
    • Nanyang Technological University
      • School of Electrical and Electronic Engineering
      Tumasik, Singapore