열증착을 이용한 산화아연 나노와이어의 합성과 광학적 특성

Abstract

Until recent years, there has been increasing interest in the problem of synthesis of one-dimensional ZnO nanostructures due to their potential applications in electronics, optics, photonics, and sensing devices. Especially for ZnO nanowires which exhibit novel physical properties owing to their unique geometry with high aspect ratio. In fact, one always desires to fabricate high purity single-crystalline ZnO nanowires. For this reason, synthesizing method and structure characterization of ZnO nanowires have been investigated. In addition, the optical properties of the ZnO nanowires also have been studied in this thesis. Vertically well-aligned single-crystalline ZnO nanowire arrays have been synthesized successfully on both the ZnO thin film and the Si substrate by thermal evaporation deposition of Zn powders at 620 oC. The synthesized ZnO nanowires had diameters in the range of 80-250 nm, lengths over several micrometers, which were single crystalline in a hexagonal structure, showed a uniform morphology and faceted planes at the tips of the nanowires and grew along the [0001] direction. The SEM images showed that ZnO thin films seem to be a key for the control of orientation of the ZnO nanowires on the substrate. The ZnO thin films orientated preferably along (002) plane controlled the growth direction. In addition, the XRD results of the both ZnO nanowires grown on the ZnO thin film and on the Si substrate showed that the ZnO thin film could play an important role in improving crystal quality. Generally vertically-aligned ZnO nanowires were synthesized using metal catalyst via VLS growth mechanism. In this work, the well-aligned ZnO nanowires were grown on the ZnO thin films and directly on the Si substrate without catalyst or additives at the temperatures of 620 oC by thermal evaporation deposition. It was seen that the alignment and diameter of the nanowires could be controlled by the control of the alignment, thickness, and grain size of the ZnO thin film on the Si substrate. Detailed structural characterization of both the ZnO nanowires grown on the Si substrate and on the ZnO thin film was also investigated by using TEM, the TEM images and selected area electron diffraction patterns revealed a single-crystalline structure of both the ZnO nanowires grown on the Si substrate and the ZnO thin film. The experimental conditions which affected properties and morphologies of the ZnO nanostructures, such as growth temperature, carrier gas flow rate, the substrate position, were detailly examined. The morphologies and alignment of the ZnO nanostructures can be definitely controlled by adjusting such experimental conditions. The PL of both the ZnO nanowires grown on the ZnO thin film and Si substrate were studied at room temperature using a He-Cd laser line of 325 nm as the excitation source. The PL results reveal that the ZnO nanowire arrays synthesized in this work have excellent optical properties, which were confirmed by the evident in strong and sharp UV emission at 380 nm with a broad green emission at 489 nm. However, the optical properties of the ZnO nanowires grown on the ZnO thin films improved comparing with those of the ZnO nanowires directly grown on the Si substrate, for the reason, which could be contributed to role of ZnO thin film in improving crystal quality. Besides, in this work, ZnO thin films with orientated preferably along (002) plane were fabricated on Si substrate by a simple sol-gel method. ZnO thin films dominantly orientated along (002) plane were obtained by preheating the dip-coating films at temperatures from 300-550 oC for 20 min after each coating and postheating at temperatures from 600-700 oC for 1h. The heat-treatment conditions, the withdrawal speed of the substrate, and thickness were found to greatly affect the crystallographic orientation and morphology of the as-grown ZnO thin films. The technique growth of ZnO nanowires on the ZnO thin films may open up opportunities for fabricating 1D nanowire with a high quality, a vertical alignment, and a large-scale production.