Chih-Ming Lin

High-performance electrochemical pseudo-capacitor based on MnO2 nanowires/Ni foam as electrode with a novel Li-ion quasi-ionic liquid as electrolyte

To achieve high energy and power densities, we developed a high-voltage 2.5 V electrochemical pseudo-capacitor with a maximum energy density of 304 W h kg−1 based on MnO2 nanowires electrodeposited on nickel foam as electrode with a novel Li-ion quasi-ionic liquid as electrolyte.

Facile electrochemical synthesis of 3D nano-architectured CuO electrodes for high-performance supercapacitors

With a simple electrochemical process, we prepared nano-architectured CuO electrodes with a 3D hierarchically porous structure and an excellent supercapacitive performance. These nano-architectured CuO electrodes were processed through co-deposition of a Ni–Cu layer on Ni foam, selective etching of Cu from the Ni–Cu film (leaving tentacle-like nanoporous Ni), and anodic deposition of CuO nanoribbons (NRs) on the tentacle-like nanoporous Ni/Ni foam substrate. Because of its unique nano-architecture, the prepared CuO nanoribbon-on-Ni-nanoporous/Ni foam (CNRNP) electrode shows exceptional performance of energy storage relative to a conventional version of the electrode. The CNRNP electrode has also a superior kinetic performance relative to CuO nanoflake-on-Ni foam (CNFNF) and flake-like CuO (FLC) electrodes. Besides its excellent cyclic stability, an exceptionally large specific capacitance of 880 and 800 F g−1 (deducting the substrate capacitance from the total) for the CNRNP electrode is obtained at scan rates of 10 and 200 mV s−1, respectively. The excellent pseudocapacitive characteristics of CNRNP electrodes associated with the variation of the Cu oxidation state during charge and discharge cycles were elucidated with in situ X-ray absorption near-edge structure (XANES) spectra.

Field Emission Properties of Gold Nanoparticle-Decorated ZnO Nanopillars

The structural and optoelectronic properties of ZnO nanopillars (ZnO-NPs) grown on Si substrates by the vapor transport deposition method were investigated. In particular, by varying the deposition duration and hence the morphology of the vertically aligned ZnO-NPs, the resultant field emission characteristics were systematically compared. In addition to identifying the advantageous field emission properties exhibited in the pencil-like ZnO-NPs, we observed that by adhering Au nanoparticles on the surface of the ZnO-NPs the turn-on field and the maximum current density can be drastically improved from 3.15 V/μm and 0.44 mA/cm(2) at 5 V/μm for the best ZnO-NPs to 2.65 V/μm and 2.11 mA/cm(2) at 5 V/μm for Au/ZnO-NPs, respectively. The enhancement of field emission characteristics that resulted from Au-nanoparticle decoration is discussed on the basis of charge-transfer-induced band structure modifications.

Enhanced visible photoluminescence from ultrathin ZnO films grown on Si-nanowires by atomic layer deposition

Bright room temperature visible emission is obtained in heterostructures consisting of approximately 3.5 nm thick ZnO ultrathin films grown on Si-nanowires produced by means of self-masking dry etching in hydrogen-containing plasma. The ZnO films were deposited on Si-nanowires by using atomic layer deposition (ALD) under an ambient temperature of 25 degrees C. The orders of magnitude enhancement in the intensity of the room temperature photoluminescence peaked around 560 nm in the present ZnO/Si-nanowire heterostructures is presumably due to the high aspect (surface/volume) ratio inherent to the Si-nanowires, which has, in turn, allowed considerably more ZnO material to be grown on the template and led to markedly more efficient visible emission. Moreover, the ordered nanowire structure also features an extremely low reflectance (approximately 0.15%) at 325 nm, which may further enhance the efficiency of emission by effectively trapping the excitation light.

Metal-ferroelectric (BiFeO3)-insulator (Y2O3)-semiconductor capacitors and field effect transistors for nonvolatile memory applications

Metal-ferroelectric-insulator-semiconductor capacitors and field effect transistors with Al/BiFeO3/Y2O3/Si structure were fabricated and characterized for nonvolatile memory applications. The capacitance-voltage curves exhibit a maximum clockwise memory window of 0.92 V. The minimum leakage current density is 2×10−7u2002A/cm2 at an applied voltage of 5 V. The capacitance-voltage memory window as a function of the sweep voltage range was investigated. The IDS-VGS curves of metal-ferroelectric-insulator-semiconductor transistors show a maximum memory window of 0.84 V. The drain current on/off ratio maintained more than three orders of magnitude after an elapsed time of 104u2002s.

Field Emission in Vertically Aligned ZnO/Si-Nanopillars with Ultra Low Turn-On Field

An effective method of fabricating vertically aligned silicon nanopillars (Si-NPs) was realized by using the self-assembled silver (Ag) nanodots as natural metal-nanomask during dry etching process. The obtained Si-NPs were preferentially aligned along the c-axis direction. Ultrathin ZnO films (~9 nm) were subsequently deposited on the Si-NPs by atomic layer deposition (ALD) to enhance the field emission property. The average diameter of the ZnO/Si-NPs is in the order of tens of nanometers, which enables efficient field emission and gives rise to marked improvement in the field enhancement factor, β. The turn-on field defined by the 10 μA/cm(2) current density criterion is ~0.74 V/μm with an estimated β ≈ 1.33×10(4). The low turn-on field and marked enhancement in β were attributed to the small radius of curvature, high aspect ratio, and perhaps more importantly, proper density distribution of the ZnO/Si-NPs.

Enhanced Free Exciton and Direct Band-Edge Emissions at Room Temperature in Ultrathin ZnO Films Grown on Si Nanopillars by Atomic Layer Deposition

Room-temperature ultraviolet (UV) luminescence was investigated for the atomic layer deposited ZnO films grown on silicon nanopillars (Si-NPs) fabricated by self-masking dry etching in hydrogen-containing plasma. For films deposited at 200 °C, an intensive UV emission corresponding to free-exciton recombination (~3.31 eV) was observed with a nearly complete suppression of the defect-associated broad visible range emission peak. On the other hand, for ZnO films grown at 25 °C, albeit the appearance of the defect-associated visible emission, the UV emission peak was observed to shift by ~60 meV to near the direct band edge (3.37 eV) recombination emission. The high-resolution transmission electron microscopy (HRTEM) showed that the ZnO films obtained at 25 °C were consisting of ZnO nanocrystals with a mean radius of 2 nm embedded in a largely amorphous matrix. Because the Bohr radius of free-exictons in bulk ZnO is ~2.3 nm, the size confinement effect may have occurred and resulted in the observed direct band edge electron-hole recombination. Additionally, the results also demonstrate order of magnitude enhancement in emission efficiency for the ZnO/Si-NP structure, as compared to that of ZnO directly deposited on Si substrate under the same conditions.

Annealing effect on the optical response and interdiffusion of n-ZnO/p-Si "111… heterojunction grown by atomic layer deposition

Optical and structural properties of n-ZnO films grown on a p-Si (111) substrate by atomic layer deposition were observed using in situ synchrotron x-ray diffraction during annealing. The photoluminescence showed a complicated photon response with increasing annealing temperature. In situ x-ray diffraction indicated the growth of grains for an annealing temperature from 500 to 800u2009°C with the orientation altering from polycrystalline to preferential (200). Measurements with a time-of-flight secondary-ion mass spectrometer indicated that the outgassing of hydrogen atoms and ZnO/Si interdiffusion behavior were correlated with the intensity and position of emissions in photoluminescence spectra.

Energy-dispersive x-ray diffraction and Raman scattering of Zn1-xMnxSe bulk crystals at high pressure

Energy-dispersive x-ray diffraction experiments were carried out to investigate the structure of phase transitions under high pressure. It was found that the zinc blende (B3) to rock salt (B1) phase transition pressures of Zn0.93Mn0.07Se and Zn0.76Mn0.24Se bulk crystals are found 11.8±1.5 and 9.9±0.5 GPa, respectively. The respective bulk moduli are 61.8±0.8 and 60.5±0.8 GPa. The pressure-induced zinc blende (ZB) to rock salt (RS) structure phase transition is interpreted as a signature of the semiconductor to metal transition for Zn1−xMnxSe. The above interpretation is further corroborated by the observation of the disappearance of the longitudinal optical phonon at the pressure where the ZB to RS structure transition occurs.

X-ray scattering and absorption spectroscopy study of the order-disorder transition of (FePt)/sub 1-x/Cu/sub x/ nanoparticles

(FePt)/sub 1-x/Cu/sub x/ nanoparticles were prepared by the simultaneous polyol reduction of platinum acetylacetonate and copper acetylacetonate, and the thermal decomposition of Fe carbonyl. The addition of copper reduces the transition temperature of disordered fcc structure to ordered L1/sub 0/ by about 50/spl deg/C-100/spl deg/C. Together with the X-ray powder diffraction (XRD) and X-ray absorption spectroscopy (XAS) results, the structure of FePtCu ternary alloy was verified and the Fe site was replaced by Cu.