Synthesis,Characterization, and Possible Applications of ZnO Nanocrystals

ZnO nanocrystals are synthesized using the quenching method. The properties of the grown nanocrystals are studied using ultraviolet-vis spectroscopy, x-ray diffraction, photoluminescence, electron diffraction, energy dispersive x-ray spectroscopy, and high resolution transmission electron microscopy. Current-voltage characteristics of the prepared samples are studied for investigating the possible application of the samples as switch and memristor.     

Structure and optical properties of nanostructured zinc oxide films with different growth temperatures

ZnO films were prepared by pulsed laser deposition (PLD) on glass substrate with temperature ranging from room temperature (RT) to 500 °C. All the films formed the hexagonal wurtzite structure and showed the c-axis (0 0 2) preferred orientation. The films deposited at 200 °C showed the narrowest full width at half maximum of both X-ray diffraction (XRD) and rocking curve, largest height grain size, smallest macrostress and least point defects. Meanwhile, it was found that the films deposited at 350 °C displayed the most intense diffraction peak in XRD and a strong UV emission while it showed the most intense defect-related green emission, fastest growth rate and larger macrostress. In addition, the cross section images showed all films grew with a columnar form along (0 0 2) orientation.     

Thermally stimulated currents as a measure of degradation of zinc oxide varistors

ZnO varistor degradation phenomena was studied using thermally stimulated current (TSC) method. It is believed that asymmetrical degradation of V-I characteristics caused by DC biasing is due to charge accumulation on the reverse biased side of ZnO grain barrier. The TSCs' measuring system and cryostat apparatus used in experiment were described. The thermally stimulated currents observed in this study for varistor samples doped with different amounts of Co, due to their recovery from asymmetrical electric stress, could not be controlled by movement of Zn interstitials with activation energy 0.55 v eV, as it is widely believed, for the TSC activation energy determined in this experiment was 0.39 v eV.     

Formation of ZnO nanodot arrays along the step edges on R-face sapphire by metalorganic chemical vapor deposition

Dot array formation of zinc oxide (ZnO) along the linear single steps was demonstrated on sapphire substrates under near atmosphere pressure by metalorganic chemical vapor deposition (MOCVD). Sapphire substrates of , (0 0 0 1) and planes (A-plane, C-plane and R-plane, respectively) were employed as templates for manufacturing the nanostructures. For highly controlling dot array formation the substrates were prepared by annealing at 1000 °C for 3 h in air after chemical treatment. The step arrays were easily prepared on C-plane and R-plane sapphire. The linearly aligned ZnO nanodot arrays were formed on R-plane sapphire along the step edges over several ten micrometers. The result can be attributed to the smaller number of dangling bonds on R-plane than on A-plane and C-plane, enhancing the surface diffusion length. Sapphire can be a good template for manipulating II–VI semiconductor on it to form nanostructures even at near atmosphere pressure by a conventional MOCVD.     

Strong exciton emission from ZnO microcrystal formed by continuous 532 nm laser irradiation

The Zinc oxide (ZnO) microcrystal is formed out of irradiated powder sample by a continuous-wave 532-nm laser with a high power of about 200 mW, and the microcrystal formation process is monitored by in situ Raman spectroscopy simultaneously. Scanning electron microscope image shows that multi-shaped ZnO microcrystal, including nano-rods and nano-flakes, is obtained near the brim of laser irradiated spot. The photoluminescence spectra of ZnO microcrystal are studied at both room temperature and low temperature of 10 K. With the ZnO microcrystal, we obtain that the peak intensity of near band-edge emission is at least 400 times stronger than that of deep-level emission at room temperature, and that up to fifth-order phonon replicas of free exciton emission are easily distinguished in the 10 K photoluminescence spectra. Both of them indicate that the ZnO microcrystal formed by intense laser irradiation has a very good crystalline structure.     

Pressure Dependence of Wurtzite ZnO Structure

From simultaneous X-ray diffraction and EXAFS experiments, structural properties in the Wurtzite phase of ZnO have been measured up to the transition pressure (9 GPa). These results are compared with first-principles calculations and discussed in terms of wurtzite mechanical stability at high pressure.     

Photoluminescence from ZnO-SiO2 opals with different sphere diameters and thicknesses

We systematically investigated the photoluminescence (PL) and transmittance characteristics of ZnO-SiO₂ opals with varied positions of the stop-band and film thicknesses. An improved ultraviolet (UV) luminescence was observed from ZnO-SiO₂ composites over pure ZnO nanocrystals under 325 nm He-Cd laser excitation at room temperature. The UV PL of ZnO nanocrystals in SiO₂ opals with stop-bands center of 410 nm is sensitive to the thickness of opal films, and the UV PL intensity increases with the film thickness increasing. The PL spectra of ZnO nanocrystals in SiO₂ opals with stop-bands center of 570 nm show a suppression of the weak visible band. The experimental results are discussed based on the scattering and/or absorbance in opal crystals.     

Effect of the oxygen pressure on the microstructure and optical properties of ZnO films prepared by laser molecular beam epitaxy

A series of ZnO films were prepared on the Si (1 0 0) or glass substrate at 773 K under various oxygen pressures by using a laser molecular beam epitaxy system. The microstructure and optical properties were investigated through the X-ray diffraction, Raman spectrometer, scanning electron microscope, ultraviolet–visible spectrophotometer and spectrofluorophotometer. The results showed that ZnO thin film prepared at 1 Pa oxygen pressure displayed the best crystalinity and all ZnO films formed a columnar structure. Meanwhile, all ZnO films exhibited an abrupt absorption edge near the wavelength of 380 nm in transmission spectra. With increasing the oxygen pressure, the transmission intensity changed non-monotonically and reached a maximum of above 80% at 1 Pa oxygen pressure, based on which the band gaps of all ZnO films were calculated to be about 3.259–3.315 eV. Photoluminescence spectra indicated that there occurred no emission peak at a low oxygen pressure of 10⁻⁵ Pa. With the increment of the oxygen pressure, there occurred a UV emission peak of 378 nm, a weak violet emission peak of 405 nm and a wide green emission band centered at 520 nm. As the oxygen pressure increased further, the position of UV emission peak remained and its intensity changed non-monotonically and reached a maximum at 1 Pa. Meanwhile the intensity of green emission band increased monotonically with increasing the oxygen pressure. In addition, it was also found that the intensity of UV emission peak decreased as the measuring temperature shifted from 80 to 300 K. The analyses indicated that the UV emission peak originated from the combination of free excitons and the green emission band originated from the energy level jump from conduction band to O_Zn defect.     

Surface reconstruction of ZnO nanowire arrays via solvent-evaporation-induced self-assembly

Vertically aligned ZnO nanowires (NWs) can be reconstructed on large scales by a solvent-evaporation-induced method. The morphologies of the nanowires are regulated by changing the concentration of the solution. Possible mechanism, which the compressive residual stresses and wires/wires self-attraction can be responsible for the surface reconstruction, is addressed. Furthermore, we compare the structural properties of ZnO nanowires before and after reconstructed.     

Zn(0.6Mn0.2Ni0.2Fe2O4/聚邻甲基苯胺复合物的制备和电磁性能

用化学沉淀法和原位聚合法分别制备了Zn0.6Mn0.2Ni0.2Fe2O4铁氧体纳米粒子和Zn0.6Mn0.2Ni0.2Fe2O4铁氧体/聚邻甲基苯胺复合微粒(ZMNF/POT).通过现代测试技术表征了样品组成、结构、形貌和电-磁性能.结果表明,POT对ZMNF粒子具有较好的包覆作用;复合物电导率与POT的含量成正比,而磁性能与ZMNF粒子的含量相关;在1～15MHz频段内,POT和ZMNF/POT复合物的介电损耗与其电导率表现出一致性;复合物具有可观的磁损耗,比磁介质型ZMNF的大,且ZMNF含量为31.74 wt%的复合物的磁损耗最大,有望作为屏蔽和吸收电磁波材料获得应用.