Controlled synthesis and characterization of one-dimensional II-VI nanomaterials

Abstract

II-VI materials are compound semiconductors which contain the elements of group II and group VI that bond together (e.g. ZnO, ZnS, ZnSe, CdO, CdS, CdSe, etc.). It is well-known that II-VI materials are important optoelectronic, luminescent, and lasing materials because of the large range of electronic energy band gaps which they exhibit. In the past few years, it was discovered that one-dimensional (1D) nanostructures, such as nanorod, nanowire and nanoribbon, have attracted extensive attention because of their unusual quantum properties and potential applications. They are expected to be both the connecters and functional components applied in the future microscopic electronic and optical devices. While structures, morphologies and sizes of nanostructures plays an important role on these applications, development of the controlled growth and understanding of the mechanism of various one-dimensional semiconductor nanostructures with homogeneous structures and compositions should then be given more effort. This project presents a review on synthesis, structure and growth mechanisms of one-dimensional (1D) II-VI nanomaterials and the main focus is devoted to ZnO and ZnS. Zinc oxide (ZnO) is believed to have the largest amount of different morphologies apart from carbon nanotube (CNT) and zinc sulfide (ZnS) consists of the largest band gap among II-VI materials. Thus they were chosen to be studied in this project. By adjusting the experimental parameters like temperature, pressure and gas flow, different morphologies of the nanomaterials of ZnO and ZnS were obtained, namely, ZnO nanohollowspheres, ZnO nanowires, ZnO nanocombs, ZnO nanoflowers, ZnS nanowire and ZnS nanoribbon. VII These 1D nanomaterials were fabricated by using simple thermal evaporation and condensation method because of its low cost, convenience and simplicity. High purity powder source of ZnO or ZnS was put in an alumina oxide crucible and evaporated above an elevated temperature in the furnace system. Then the vapor flowed along the temperature gradient and condensed on the silicon substrate at which is a lower temperature region. The morphology of the nanoscale product was then analyzed with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Energy dispersive spectrometer (EDX) and X– ray diffraction (XRD) were then utilized to determine the composition. ZnO nanohollowspheres and ZnO nanocombs were chosen to conduct the photoluminescence (PL) measurement which reveals nanohollowsphere has a higher green emission than that of nanocomb. Finally, some of their growth mechanisms were also study. The formation of ZnO nanohollowspheres is mainly due to the use of nitrogen gas which has a higher thermal conductivity and two-step controlling evaporation method which causes the revaporation in the second stage. The metal droplet on the tip of the ZnO nanowire indicated that the mechanism is vapor-liquid-solid (VLS). As for the oriented growth of ZnO nanowires, it may be caused by the parabolic gas flow in the quartz tube. It is also discovered that the thickness of the stem of the nanocombs can be controlled through the adjustment of the pressure. Moreover, by observing the structure of the stem, the growth of ZnO nanocomb with a thick stem may be consist of two steps, which is different from Ren’s reported. The formation of nanoflower may be the effect of the large size of Au droplet and the high adhesive force between the catalyst and the substrate, which do not allow the nanoneedle to raise the droplet. For the ZnS nanowires and nanoribbon, hydrogen-assisted growth together with VLS mechanism which is supported by the discovery of the metal particles on the nanostructure was used to explain their growth process.