G. X. Hu

The Sol-Gel-Derived Nickel-Cobalt Oxides with High Supercapacitor Performances

Nickel cobalt (Ni-Co) oxides with various Ni/Co ratios are synthesized by using a sol-gel process. Electrochemical performance, microstructure, morphology and BET surface area are found strongly related to Ni and Co concentration. Increasing Co concentration changes the oxide microstructure from NiO crystal dominate structure (Ni:Co= 1:1 and 1:2) to Co 3 O 4 dominate structure (Ni:Co= 1:4 and 0:1). A maximum specific capacitance of 1539 Fg ―1 was obtained for Ni-Co (Ni:Co= 1:2) oxide at a current density of lAg ―1 , and this capacitance is similar to that of RuO 2 . A systematic study shows that the Ni-Co oxide (1:2) has a mesoporous structure with a high BET surface area of ∼315 m 2 g ―1 and porous size of ∼4.7 nm, which is favorable for the charge/ discharge process of a supercapacitor.

Effects of substrate on the structure and orientation of ZnO thin film grown by rf-magnetron sputtering

X-ray diffractions, Nomarski microscopy, scanning electron microscopy, and photoluminescence have been used to study the effects of substrate on the structure and orientation of ZnO thin films grown by rf-magnetron sputtering. GaAs(001), GaAs(111), Al2O3(0002) (c-plane), and Al2O3(11¯02) (r-plane) wafers have been selected as substrates in this study. X-ray diffractions reveal that the ZnO film grown on GaAs(001) substrate is purely textured with a high c-axis orientation while that grown on GaAs(111) substrate is a single ZnO(0002) crystal; a polycrystalline structure with a large-single-crystal area of ZnO(0002) is obtained on a c-plane Al2O3 substrate while a ZnO(112¯0) single crystal is formed on an r-plane Al2O3 substrate. There is absence of significant difference between the photoluminescence spectra collected from ZnO∕GaAs(001), ZnO∕GaAs(111), and ZnO∕Al2O3(0002), while the photoluminescence from ZnO∕Al2O3(11¯02) shows a reduced intensity together with an increased linewidth, which is, likely, due...

Annealing effects on electrical and optical properties of ZnO thin-film samples deposited by radio frequency-magnetron sputtering on GaAs (001) substrates

The effects of thermal annealing on Hall-effect measurement and photoluminescence (PL) from undoped n-type ZnO/GaAs thin-film samples have been studied. The evolutions of carrier concentration, electrical resistivity, and PL spectrum at various annealing conditions reveal that the dominant mechanism that affects the electrical and PL properties is dependent on the amount of thermal energy and the ambient pressure applied during the annealing process. At low annealing temperatures, annihilation of native defects is dominant in reducing the carrier concentration and weakening the low-energy tail of the main PL peak, while the GaAs substrate plays only a minor role in carrier compensations. For the higher temperatures, diffusion of Ga atoms from the GaAs substrate into ZnO film leads to a more n-type conduction of the sample. As a result, the PL exhibits a high-energy tail due to the high-level doping.

Effects of oxygen on low-temperature growth and band alignment of ZnO∕GaN heterostructures

Continuous ZnO thin films have been grown at low temperature (400°C) on GaN∕c-sapphire substrates in a radio-frequency magnetron-sputtering chamber employing a substoichiometric ZnO target with and without extra oxygen feeding. The effects of oxygen on the growth and band alignment of the ZnO∕GaN heterostructures were investigated by using scanning-electron microscopy, x-ray diffraction, photoluminescence and transmittance/absorbance, ultraviolet-resonant Raman scattering, and x-ray photoelectron spectroscopy. Very remarkable changes of the structural and optical properties resulted from the introduction of oxygen: the surface hexahedral facets were diminished; the size of the surface islands and, hence, the compressive strains were reduced; ultraviolet transparency of the ZnO film was enhanced, together with an increased band gap due to the reduced intrinsic shallow-donor defects; and hence, the free-electron concentration. The offset in valence bands of the ZnO∕GaN heterostructure was increased by ∼90me...

Surface optical phonon and A1(LO) in ZnO submicron crystals probed by Raman scattering: Effects of morphology and dielectric coating

The authors studied the surface optical phonon and A1(LO) in ZnO submicron crystals as well as ZnO single crystals with submicron surface structures by employing ultraviolet Raman scattering. The small variation in Raman frequency of A1(LO), when the growth condition is changed, is mainly caused by the crystal imperfection via two mechanisms: one is the frequency lowering by native defects and the other is the frequency increasing due to the tilt of c-axis of ZnO crystals away from the surface normal direction that leads to the mixing of A1(LO) and E1(LO). The evolution of Raman features, before and after dielectric coatings, in conjunction with the photoluminescence, provides clear-cut evidence for the observation of a surface optical phonon. The frequency of the observed surface mode is lowered by the existence of surface states, and the surface states can be effectively reduced by the dielectric coatings.

Radio-frequency magnetron sputtering and wet thermal oxidation of ZnO thin film

The authors studied the growth and wet thermal oxidation (WTO) of ZnO thin films using a radio-frequency magnetron sputtering technique. X-ray diffraction reveals a preferred orientation of [101¯0]ZnO(0002)∕∕[112¯0]Al2O3(0002) coexisted with a small amount of ZnO (101¯1) and ZnO (101¯3) crystals on the Al2O3 (0001) substrate. The ZnO (101¯1) and ZnO (101¯3) crystals, as well as the in-plane preferred orientation, are absent from the growth of ZnO on the GaAs(001) substrate. WTO at 550 °C improves the crystalline and the photoluminescence more significantly than annealing in air, N2 and O2 ambient; it also tends to convert the crystal from ZnO (101¯1) and ZnO (101¯3) to ZnO (0002). The evolution of the photoluminescence upon WTO and annealing reveals that the green and orange emissions, centered at 520 and 650 nm, are likely originated from oxygen vacancies and oxygen interstitials, respectively; while the 420 nm emission, which is very sensitive to the postgrowth thermal processing regardless of the subst...

Properties of ZnO influenced by P concentration

The properties of ZnO were found to be influenced by P concentration. Upon increasing P concentration in ZnO, up to small values of less than 0.1 at. %, the as-deposited ZnO is found to undergo a change from n-type to p-type character. The change is accompanied by a redshift of the optical band gap energy and an increase in the lattice constants of ZnO, which can be attributed to substitution of O by P. A further increase in P concentration, up to 0.4 at. %, leads to a decrease in both lattice constants and energy band gaps of the samples, compared to those of undoped ZnO. Additionally, the electrical conductivity of samples stopped increasing for high P concentrations and became an insulator at P=0.4 at. %. Possible mechanisms for these effects were investigated and discussed.