Zhigao Hu

Effect of oxygen defects on ferromagnetic of undoped ZnO

Mn doped ZnO nano-particles were synthesized by a solution route and annealed in N2, O2, and Ar, respectively. X-ray diffraction and X-ray photoelectron spectroscopy measurements show that the samples possess typical wurtzite structure and have no other impurity phases. Magnetization loops for ZnO samples were measured and clearly show typical ferromagnetic saturation behavior. With the combination of defect analysis based on photoluminescence spectroscopy and first-principle calculations of the possible magnetic defect centers in Mn doped ZnO, the effect of defects on the nature and origin of ferromagnetism was investigated. The results suggest oxygen vacancies, especially singly ionized oxygen vacancies, play a crucial role in mediating ferromagnetism in the Mn doped ZnO system.

Structural, electronic band transition and optoelectronic properties of delafossite CuGa1−xCrxO2 (0 ≤ x ≤ 1) solid solution films grown by the sol–gel method

Pure phase CuGa1−xCrxO2 (0 ≤ x ≤ 1) films were prepared on (001) sapphire substrates by the sol–gel method. The structure, vibration modes, and compositions of the films were analyzed by X-ray diffraction, scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. It was found that the Cr-substituting induced the increase of the film’s roughness, changed the film’s internal structure, and made more crystal defects and grain boundaries. Due to the interatomic potential becoming weaker between Cu and O atoms with increasing the Cu–O bond length, the peak positions of the A1g and Ag phonon modes shifted toward a lower frequency with increasing x. The optical transmittance of the films approached about 60–80% in the visible region and the values of the direct band gap linearly decrease from 3.56 to 3.09 eV with increasing x. The Cr-introduction effects on the electronic band transition have been investigated in detail. The new energy state located at 0.17 eV above the top of the valence band is observed in the CuGa0.8Cr0.2O2 film, which can be derived from the defect energy level. It can induce the increment of the hole in the valence band, contribute to the electrical conductivity, and lower the thermal activation energy. Moreover, the CuGa0.8Cr0.2O2 film is found to be of the larger electrical conductivity of 0.071 S cm−1 at room temperature, which shows the promising application values, as compared to other CuGa1−xCrxO2 films.

Temperature dependence of electronic transitions and optical properties in multiferroic BiFeO3 nanocrystalline film determined from transmittance spectra

The ultraviolet-infrared transmittance spectra of BiFeO 3 nanocrystalline film have been studied in the temperature range 5.3–300 K. A redshift trend of the absorption edge and optical constants with increasing the temperature can be observed. Four interband electronic transitions can be uniquely assigned and strongly depend on the temperature. Moreover, two magnetic transitions located at about 150 and 200 K have been observed and can be interpreted as spin-reorientation transitions. It was found that the optical band gap decreases from 2.69 ± 0.01 to 2.65 ± 0.01 eV with increasing the temperature due to the modification of the electron-phonon interactions.

Optical constants and thermo-optic coefficients of nanocrystalline diamond films at 30–500°C

The refractive index and absorption index of nanocrystalline diamond (NCD) films were investigated using spectroscopic ellipsometry between 30 and 500°C. Due to their high transparency the experimental spectra could be well fitted in the subgap region using a single-oscillator model with a four-phase layered structure. The single-oscillator model yields a small optical absorption in the band gap region. The temperature dependence of dispersion of the refractive index over the photon energy range of 1.15–4.75eV was determined. Based on the Bose-Einstein model, a thermo-optic coefficient of (1∕n)(∂n∕∂T)=6.5×10−6K−1 at 300K was obtained for the NCD film in the near-infrared region.

Photoluminescence and low-threshold lasing of ZnO nanorod arrays

We report on the photoluminescence (PL) and lasing characteristics of ZnO nanorod arrays (NRAs) fabricated by hydrothermal process on nanocrystalline ZnO seeded Si and post-growth annealing. The morphology of the ZnO NRAs was examined by field emission scanning electron microscopy and the structure was characterized by x-ray diffraction, Fourier-transform infrared and Raman scattering spectroscopy. The properties of light emission were studied by continuous wave (CW) and 30 ps pulsed ultraviolet excitation. The ZnO NRAs consist of aligned nanorods and are nanocrystalline with wurtzite structure and c-axis orientation. At room temperature, the ZnO NRAs are capable of emitting strong CW PL and pulsed stimulated emission, with the latter showing obvious lasing characteristics. The threshold for lasing was observed to be ~16 kW/cm(2).

Optical properties of pulsed laser deposited rutile titanium dioxide films on quartz substrates determined by Raman scattering and transmittance spectra

Optical response of rutile TiO2 films grown under different laser energy by pulsed laser deposition has been investigated by Raman scattering and spectral transmittance. Dielectric functions in the photon energy range of 1.24–6.5 eV have been extracted by fitting the experimental data with the Adachi’s model [S. Adachi, Phys. Rev. B 35, 7454 (1987)]. The refractive index dispersion in the transparent region is mainly ascribed to the higher A1-A2 electronic transitions for the rutile TiO2 films. Owing to slightly different crystalline structures and film densities, the optical band gap linearly increases with increasing packing density. The phenomena were confirmed by different theoretical evaluation methods.

Oxygen-vacancy-related dielectric relaxation in BiFeO3 films grown by pulsed laser deposition

Two kinds of BiFeO3 (BFO) thin films with different oxygen stoichiometry are fabricated on (La, Sr)CoO3 coated silicon substrates. A Debye-like dielectric relaxation was observed in the samples thermally treated at 3 Pa oxygen. The frequency dependence of permittivity of the samples treated at 3 Pa oxygen can be fitted by a model containing the Debye-like dielectric response and the universal dielectric response. According to the model, the dielectric relaxation can be ascribed to the oxygen vacancy, and the possible influences from the interfacial polarization between BFO and electrodes have been excluded by the measurement of the dielectric responses of BFO films at different dc biased voltages. The calculated value of dc electric conductivity in BFO films from this model has the same order of magnitude as the published results. These results indicate that the existence of oxygen vacancy not only influences the leakage performance of BFO films but also affects the dielectric properties of BFO. The electrical performance of BFO films and devices can be improved by decreasing the density of oxygen vacancy.

Intrinsic evolutions of optical functions, band gap, and higher-energy electronic transitions in VO(2) film near the metal-insulator transition region

Transmittance spectra of (011) vanadium dioxide (VO2) film have been studied in the temperature range of 45–80 °C. Owing to increasing carrier concentration, the near-infrared extinction coefficient and optical conductivity around metal-insulator transition (MIT) rapidly increase with the temperature. Moreover, three electronic transitions can be uniquely assigned and show the hysteresis behavior near the MIT region. It was found that the optical band gap decreases from 0.457 to 0.042 eV before the MIT, then reduces to zero for the metal state. This confirms the fact that the a 1 g and e g π bands are moved close and finally overlap with the temperature.

Temperature dependence of optical band gap in ferroelectric Bi3.25La0.75Ti3O12 films determined by ultraviolet transmittance measurements

Optical properties of ferroelectric Bi3.25La0.75Ti3O12 (BLT) films on quartz have been investigated using ultraviolet-infrared transmittance spectra in the temperature range of 77–500K. The spectra can be divided into three distinctive photon regions between 1.1 and 6.5eV. It is found that the band gap Eg decreases from 3.88to3.77eV with the temperature. The parameters aB and ΘB of the Bose-Einstein model are 30.3meV and 218.7K, respectively. The band narrowing coefficient dEg∕dT is −2.65×10−4eV∕K at room temperature. The present results can be crucial for future application of ferroelectric BLT-based electro-optic and high temperature optoelectronic devices.

Enhanced photoelectrochemical activity of vertically aligned ZnO-coated TiO2 nanotubes

Vertically aligned ZnO-TiO2 hetero-nanostructures constructed of anatase TiO2 nanotubes (NTs) and wurtzite ZnO coatings are fabricated by atomic layer deposition of ZnO coatings on electrochemical anodization formed TiO2 NTs, and their photoelectrochemical activities are studied through photoelectrochemical and electrochemical characterization. Compared with bare TiO2 NTs, the transient photocurrent increases to over 1.5-fold for the annealed ZnO-coated TiO2 NTs under visible illumination. The ZnO-coated TiO2 NTs also show a longer electron lifetime, a lower charge-transfer resistance and a more negative flat-band potential than the bare TiO2 NTs, confirming the improved photoelectrochemical activity due to the enhanced charge separation.