Preparation And Characterization Of Zno Nanorods

As a direct band-gap semiconductor material, ZnO has a wide hand gap almost the same as GaN, but a much higher exciton binding energy (60meV) than that of GaN. Therefore, ZnO has a higher light emitting efficiency and is considered to be an ideal material for ultra-violet (UV) light emitting devices. Since the optically pumped UV stimulated emission of ZnO at room-temperature was reported, ZnO has shown the great potential in applications in UV light emitting diode (LED) and laser diode (LD) areas.Based on the current research status and application prospects, as well as the equipments of our research group, we have successfully prepared ZnO nanorods on silicon substrate by the method of pulsed laser deposition (PLD), and investigated the structural and optical properties by various characterization techniques. The results show that the temperature of the substrate as well as the background pressure plays a crucial role in the fabrication of ZnO nanorods. We also analyzed the growth mechanism of ZnO nanorods according to the appearances of the nanorods synthesized under different conditions. Well oriented ZnO nanorods arrays have been prepared under the pressure of 2kPa and 600℃substrate temperature.We also successfully synthesized ZnO nanorods with various length-diameter ratios on different substrate by means of hydrothermal synthesis from formamide solution. The surface morphology characterization of those samples indicate that the substrate in the experiment have certain impact on the growth of ZnO nanorods. According to the surface morphology of the sample after different reaction time, we analyzed the growth process of the sample as well.We have characterized the luminescent properties of ZnO nanorods prepared by both PLD methods and hydrothermal mothods. The photoluminescence (PL) spectra of the samples detected at room temperature have shown that all the prepared ZnO nanorods have strong UV emission bands while no obvious green or yellow emissions are observed. There are two reasons may be responsible for this PL characteristics. First of all, no metallic catalysts (e. g., Au, Ni) were used to guide the growth of the nanorods, therefore the nanorods are pure crystals without impurities material. Moreover, under such growth conditions, the synthesized ZnO nanorods have excellent crystallinity and very few defects.We also used the temperature-dependent PL spectra to investigate the ZnO nanorods deposited by PLD and found out that the PL intensity of ZnO nanorods at low temperatures is much higher than that at room-temperature. The details of the exciton emission which corresponding to free excitons and bound excitons, as well as their phonon replicas are resolved at low temperature. As the temperature of the sample increases, the PL intensity decreases, meanwhile the bound excitons turn to free excitons due to the absorption of thermal energy. Therefore, the light emission from recombination of the free excitons turned to predominate in the photoluminescence spectroscope instead of that of bound excitons.Furthermore, we have carried out the optical-pumping lasing emission experiment at room temperature. The results show that the ZnO nanorods prepared by both methods have distinct threshold effect. Once the pumping energy are higher than the threshold, the stimulated emission other than spontaneous one will emerge in the wavelength range of 370-390nm. The line width of the stimulated emission peaks is less than 0.4nm, which is 1/40 of that of the spontaneous emission. The stimulated excitation spectrum is the superposition of multi-mode laser generated from many nanoraods due to the relatively large spot area of the excitation light.