Tun Seng Herng

National University of Singapore, Tumasik, Singapore

Are you Tun Seng Herng?

Claim your profile

Publications (55)196.34 Total impact

  • Qipeng Zhao · Tun Seng Herng · Chun Xian Guo · Dieling Zhao · Jun Ding · Xianmao Lu
    [Show abstract] [Hide abstract]
    ABSTRACT: Thermoresponsive magnetic ionic liquids (MILs) with lower critical solution temperature (LCST) below 60 oC are synthesized. Magnetic separation of MILs from aqueous solution at temperatures above LCST is demonstrated.
    No preview · Article · Feb 2016 · RSC Advances
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Open-shell singlet diradicaloids recently have received much attention due to their unique optical, electronic and magnetic properties and promising applications in materials science. Among various diradicaloids, quinoidal π-conjugated molecules have become the prevailing designs. However, there still lacks fundamental understanding on how the fusion mode and pro-aromaticity/anti-aromaticity affect their diradical character and physical properties. In this work, a series of pro-aromatic benzo-thia-fused [n]thienoacenequinodimethanes (Thn-TIPS (n=1-3) and BDTh-TIPS) were synthesized and compared with the previously reported anti-aromatic bisindeno-[n]thienoacenes (Sn-TIPS, n=1-4). The ground-state geometric and electronic structures of these new quinoidal molecules were systematically investigated by X-ray crystallographic analysis, variable temperature NMR, ESR, SQUID, Raman, and electronic absorption spectroscopy, assisted by DFT calculations. It was found that the diradical character index (y0) increased from nearly zero for Th1-TIPS to 2.4% for Th2-TIPS, 18.2% for Th3-TIPS, and 38.2% for BDTh-TIPS, due to the enhanced aromatic stabilization. Consequently, with the extension of molecular size, the one-photon absorption spectra are gradually red-shifted, the two-photon absorption (TPA) cross section values increase, and the singlet excited state lifetimes decrease. By comparison with the corresponding anti-aromatic analogues Sn-TIPS (n=1-3), the pro-aromatic Thn-TIPS (n=1-3) exhibit larger diradical character, longer singlet excited state lifetime and larger TPA cross section value. At the same time, they display distinctively different electronic absorption spectra and improved electrochemical amphotericity. Spectroelectrochemical studies revealed a good linear relationship between the optical energy gaps and the molecular length in the neutral, radical cationic and dicationic forms. Our research work disclosed the significant difference between the pro-aromatic and anti-aromatic quinoidal compounds and provided guidance for the design of new diradicaloids with desirable properties.
    Full-text · Article · Jan 2016 · Chemical Science
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Properties of particles whose dimensions are of the nanoscale can be quite different from those of bigger dimension particles. We investigate here the effects of doping Sb5+ ions into ZnO nanorods (NR's) on the formation of magnetic moments. The pure ZnO and Sb-doped ZnO NR's were grown by the hydrothermal process. The fabricated nanorods were then studied using X-ray diffraction (XRD), ultraviolet-visible light spectroscopy (UV-vis), Scanning Electron Microscopy (SEM), Photoluminescence (PL) Spectrometer and Vibrating Sample Magnetometer (VSM) spectrometry. Substitution of Sb into the ZnO leads to the formation of a defect (SbZn+2VZn) complex state which can capture the oxygen p-orbital electrons of the oxygen atoms and form a spin polarized state. The PL was used to monitor the defect formation in the ZnO NR's when the Sb is doped in. As more Sb was doped in, the visible blue portion of the PL spectrum decreased while the visible red portion increased. Magnetization measurements (VSM) showed that the pure ZnO NR's possessed a magnetic moment which initially increased as Sb is doped in but decreased as more Sb was doped in. We obtain a value of 1.015 memu/g for the saturation magnetization of the pure ZnO nanorod.
    Full-text · Article · Jan 2016 · Journal of Magnetism and Magnetic Materials
  • [Show abstract] [Hide abstract]
    ABSTRACT: Open-shell singlet diradicaloids display unique electronic, non-linear optical and magnetic activity and could become novel molecular materials for organic electronics, photonics and spintronics. However, design and synthesis of diradicaloids with a significant polyradical character is a challenging task for chemists. In this article, we report our efforts toward tetraradicaloid system. A series of potential tetraradicaloids by fusion of two p-quinodimethane (p-QDM) units with naphthalene or benzene rings in different modes were synthesized. Their model compounds containing one p-QDM moiety were also prepared and compared. Their ground-state structures, physical properties and chemical reactivity were systematically investigated by various exper-imental methods such as steady-state and transient absorption, two-photon absorption, X-ray crystallographic analysis, electron spin resonance, superconducting quantum interference device and electrochemistry, assisted by density functional theory calculations. It was found that their diradical and tetraradical characters show a clear dependence on the fusion mode. Upon the introducing of more five-membered rings, the diradical characters greatly decrease. This difference can be explained by the pro-aromaticity/anti-aromaticity of the molecules as well as the intramolecular charge transfer. Our comprehensive studies provide a guideline for the design and synthesis of stable open-shell singlet polycyclic hydrocarbons with significant polyradical characters.
    No preview · Article · Dec 2015 · Journal of the American Chemical Society
  • Source
    Viveka Kalidasan · Xiao Li Liu · Tun Seng Herng · Yong Yang · Jun Ding
    [Show abstract] [Hide abstract]
    ABSTRACT: Magnetic hyperthermia is a fast emerging, non-invasive cancer treatment method which is used synergistically with the existing cancer therapeutics. We have attempted to address the current challenges in clinical magnetic hyperthermia-improved biocompatibility and enhanced heating characteristics, through a single combinatorial approach. Both superparamagnetic iron oxide nanoparticles (SPIONs) of size 10 nm and ferrimagnetic iron oxide nanoparticles (FIONs) of size 30 nm were synthesized by thermal decomposition method for comparison studies. Two different surface modifying agents, viz, Cetyl Trimethyl Ammonium Bromide and 3-Aminopropyltrimethoxysilane, were used to conjugate Bovine Serum Albumin (BSA) over the iron oxide nanoparticles via two different methods—surface charge adsorption and covalent amide bonding, respectively. The preliminary haemolysis and cell viability experiments show that BSA conjugation mitigates the haemolytic effect of the iron oxide nanoparticles on erythrocytes and is non-cytotoxic to the healthy Baby Hamster Kidney cells. It was observed from the results that due to better colloidal stability, the SAR value of the BSA-iron oxide nanoparticles is higher than the iron oxide nanoparticles without BSA, irrespective of the size of the iron oxide nanoparticles and method of conjugation. The BSA-FIONs seem to show improved biocompatibility, as the haemolytic index is less than 2 % and cell viability is up to 120 %, when normalized with the control. The SAR value of BSA-FIONs is 2300 W g−1 when compared to 1700 W g−1 of FIONs without BSA conjugation. Thus, we report here that BSA conjugation over FIONs (with a high saturation magnetization of 87 emu g−1) provide a single combinatorial approach to improve the biocompatibility and enhance the SAR value for magnetic hyperthermia, thus addressing both the current challenges of the same. Graphical Abstract
    Full-text · Article · Oct 2015 · Nano-Micro Letters
  • [Show abstract] [Hide abstract]
    ABSTRACT: Magnetic single-domain islands based on in-plane anisotropy (usually, shape anisotropy) and their dipole—dipole interactions have been investigated extensively in recent years. This has been driven by potential applications in magnetic recording, spintronics, magneto-biology, etc. Here, we propose a concept of outof-plane magnetic clusters with configurable domain structures (multi-flux states) via dipole—dipole interactions. Their flux stages can be switched through an external magnetic field. The concept has been successfully demonstrated by patterned [Co/Pd] islands. A [Co/Pd] multilayer exhibits a large perpendicular anisotropy, a strong physical separation, and uniform intrinsic properties after being patterned into individual islands by electron beam lithography. A threeisland cluster with six stable flux states has been realized by optimizing island size, thickness, gap, anisotropy, saturation magnetization, etc. Using [Co/Pd] multilayers, we have optimized the island structure by tuning magnetic properties (saturation magnetization and perpendicular anisotropy) using Landau—Liftshitz—Gilbert (LLG) simulation/calculation. Potential applications have been proposed, including a flexi-programmable logic device with AND, OR, NAND, and NOR functionalities and a magnetic domino, which can propagate magnetic current as far as 1 µm down from the surface via vertical dipole—dipole interactions. [Figure not available: see fulltext.] © 2015, Tsinghua University Press and Springer-Verlag Berlin Heidelberg.
    No preview · Article · Oct 2015 · Nano Research
  • [Show abstract] [Hide abstract]
    ABSTRACT: Magnetite Fe3O4 (ferrite) has attracted considerable interest for its exceptional physical properties: It is predicted to be a semimetallic ferromagnetic with a high Curie temperature, it displays a metal-insulator transition, and has potential oxide-electronics applications. Here, we fabricate a high-magnetization (> 1 Tesla) high-resistance (~0.1 Ω·cm) sub-nanostructured (grain size < 3 nm) Fe3O4 film via grain-size control and nano-engineering. We report a new phenomenon of spin-flipping of the valence-spin tetrahedral Fe3+ in the sub-nanostructured Fe3O4 film, which produces the high magnetization. Using soft X-ray magnetic circular dichroism and soft X-ray absorption, both at the Fe L3,2- and O K-edges, and supported by first-principles and charge-transfer multiple calculations, we observe an anomalous enhancement of double exchange, accompanied by a suppression of the superexchange interactions because of the spin-flipping mechanism via oxygen at the grain boundaries. Our result may open avenues for developing spin-manipulated giant magnetic Fe3O4-based compounds via nano-grain size control. [Figure not available: see fulltext.] © 2015, Tsinghua University Press and Springer-Verlag Berlin Heidelberg.
    No preview · Article · Aug 2015 · Nano Research
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Two pillared-layer coordination polymeric compounds, namely, [Co2(bpeb)2(obc)2]·DMF·5H2O (1) and [Co2(bpeb)(obc)2]·2DMF·H2O (2), have been synthesized using a long dipyridyl spacer ligand (1,4-bis[2-(4-pyridyl)ethenyl]benzene, bpeb), 4,4′-oxybisbenzoate (obc), and Co(NO3)2·6H2O under solvothermal conditions, using appropriate ratios of Co(II) to bpeb. In compound 1, the double pillared-layer structure has a dimeric repeating unit and exhibits rob topology, while single pillared-layer compound 2 has an unusual building block which is a linkage isomer of the well-known paddle-wheel structure and has a pcu topology. Twofold and threefold interpenetrations are observed in 1 and 2, respectively. The variable-temperature magnetic properties of 1 and 2 were also investigated. The double pillared-layer structure of 1 exhibits antiferromagnetic behavior while a relatively rare ferromagnetism has been observed for the single pillared-layer structure of 2.
    Full-text · Article · Aug 2015 · Crystal Growth & Design
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We use ionic liquid-assisted electric field effect to tune the carrier density in an electron-doped cuprate ultrathin film and cause a two-dimensional superconductor-insulator transition (SIT). The low upper critical field in this system allows us to perform magnetic field (B)-induced SIT in the liquid-gated superconducting film. Finite-size scaling analysis indicates that SITs induced both by electric and magnetic field are quantum phase transitions and the transitions are governed by percolation effects - quantum mechanical in the former and classical in the latter case. Compared to the hole-doped cuprates, the SITs in electron-doped system occur at critical sheet resistances (Rc) much lower than the pair quantum resistance RQ=h/(2e)2=6.45 k{\Omega}, suggesting the possible existence of fermionic excitations at finite temperature at the insulating phase near SITs.
    Full-text · Article · Jul 2015 · Physical Review B
  • O. Chichvarina · T. S. Herng · W. Xiao · X. Hong · J. Ding
    [Show abstract] [Hide abstract]
    ABSTRACT: Fe3O4 has been widely studied because of its great potential in spintronics and other applications. As a magnetic electrode, it is highly desired if magnetic anisotropy can be controlled. Here, we report the results from our systematic study on the magnetic properties of magnetite (Fe3O4) thin films epitaxially grown on various MgO substrates. Strikingly, we observed a prominent perpendicular magnetic anisotropy in Fe3O4 film deposited on MgO (111) substrate. When measured in out-of-plane direction, the film (40 nm thick) exhibits a well-defined square hysteresis loop with coercivity (Hc) above 1 kOe, while much lower coercivity was obtained in the in-plane orientation. In sharp contrast, the films deposited onto MgO (100) and MgO (110) substrates show in-plane magnetic anisotropy. These films exhibit a typical soft magnet characteristic—Hc lies within the range of 200–400 Oe. All the films showed a clear Verwey transition near 120 K—a characteristic of Fe3O4 material. In addition, a series of magnetoresistance (MR) measurements is performed and the MR results are in good agreement with the magnetic observations. The role of the substrate orientation and film thickness dependency is also investigated.
    No preview · Article · May 2015 · Journal of Applied Physics
  • O. Chichvarina · T. S. Herng · K. C. Phuah · W. Xiao · N. Bao · Y. P. Feng · J. Ding
    [Show abstract] [Hide abstract]
    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.
    No preview · Article · Jan 2015 · Journal of Materials Science
  • [Show abstract] [Hide abstract]
    ABSTRACT: A two-step chemical approach to synthesize high quality Fe3O4 nanodisc is reported. The magnetic hyperthermia properties of the nanodisc and isotropic nanoparticles are investigated systematically. The results suggest that the nanodisc shows much higher specific absorption rate (SAR) than isotropic nanoparticles. This is attributed to the parallel alignment of nanodisc with respect to the alternating current magnetic field, which is confirmed by good agreement between experimental results and micromagnetic simulation. It is found that such parallel alignment could enhance the SAR value by a factor of ≈2 with respect to the randomly oriented case. The above results indicate that the nanodisc provides an excellent thermal seed for magnetic hyperthermia. This study sheds the light on the magnetic hyperthermia mechanism of magnetic nanodisc and it also opens the window to explore high efficiency thermal seeds by controlling the orientation of magnetic nanostructures.
    No preview · Article · Dec 2014 · Advanced Functional Materials
  • Source

    Full-text · Dataset · Jul 2014
  • [Show abstract] [Hide abstract]
    ABSTRACT: The emission behavior of C-doped ZnO films, which were prepared by implantation of carbon into ZnO films, is investigated. Orange/red emission is observed for the films with the thickness of 60–100 nm. However, the film with thickness of 200 nm shows strong green emission. Further investigations by annealing bulk ZnO single crystals under different environments, i.e. Ar, Zn or C vapor, indicated that the complex defects based on Zn interstitials are responsible for the strong green emission. The existence of complex defects was confirmed by electron spin resonance (ESR) and low temperature photoluminescence ( PL ) measurement.
    No preview · Article · Jun 2014 · AIP Advances
  • [Show abstract] [Hide abstract]
    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.
    No preview · Article · Jan 2014 · Applied Physics Letters
  • Tong Li · Wen Xiao · Tun Seng Herng · Nina Bao · Jun Ding
    [Show abstract] [Hide abstract]
    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.
    No preview · Article · Jun 2013 · Journal- Korean Physical Society
  • [Show abstract] [Hide abstract]
    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.
    No preview · Article · Jun 2013 · Thin Solid Films
  • Source
    Amit Kumar · Tun Seng Herng · Kaiyang Zeng · Jun Ding
    [Show abstract] [Hide abstract]
    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.
    Full-text · Article · Sep 2012 · ACS Applied Materials & Interfaces
  • Source
    T S Herng · A Kumar · C S Ong · Y P Feng · Y H Lu · K Y Zeng · J Ding
    [Show abstract] [Hide abstract]
    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.
    Full-text · Article · Aug 2012 · Scientific Reports
  • Source
    T. S. Herng · A. Kumar · C. S. Ong · Y. P. Feng · Y. H. Lu · K. Y. Zeng · J. Ding
    [Show abstract] [Hide abstract]
    ABSTRACT: Investigation of the non-volatile resistance change in noncentrosymmetric compounds _ Supplementary Information
    Preview · Dataset · Aug 2012

Publication Stats

696 Citations
196.34 Total Impact Points

Institutions

  • 2011-2016
    • National University of Singapore
      • Department of Materials Science and Engineering
      Tumasik, Singapore
  • 2006-2010
    • Nanyang Technological University
      • School of Electrical and Electronic Engineering
      Tumasik, Singapore