Growth, structural and optical properties of orderly SnO2 microhemispheres on patterned sapphire substrates

Abstract

One-dimensional nanoscaled materials, such as nanotubes, nanowires and nanobelts, have attracted a great deal of attention in recent years because of their unique electronic, optical, and mechanical properties. Their potential applications are found in next generation devices, functional materials, and sensors. A material of particular interest is stannic oxide (SnO2), which is a novel oxide semiconductor material for ultraviolet and blue luminescence devices due to its wide band gap of 3.6 eV at room temperature. SnO2 can also be widely used in many fields, such as gas sensors, optoelectronic devices, and transparent conductive glass, because of its high optical transparency in the visible range, low resistivity, and higher chemical and physical stability. In recent years, one-dimensional nanostructures of SnO2 materials, such as nanobelts, nanotubes, and nanowires, have been reported. However, the preparations of orderly SnO2 micro/nanostructures have been rarely reported. In this paper, orderly SnO2 microhemispheres with different sizes are grown on patterned sapphire substrates by a traditional chemical vapor deposition method without using any catalyst. The patterned sapphire substrates are cleaned by using a standard sapphire wafer cleaning procedure. High-purity metallic Sn powders (99.99%) and oxygen gas are used as Sn and oxygen sources, respectively. The flow rate of high-purity Ar carrier gas is controlled at 200 sccm, and the oxygen reactant gas with a flow rate of 100 sccm is introduced into the system. In the growth process, the whole system is kept at 1000℃ for 30 min. The surface morphologies, structural and optical properties of the SnO2 microhemispheres are investigated by the field emission scanning electron microscope (HITACHI S4800), the X-ray diffraction with a Cu Kup radiation (0.15418 nm), the optical absorption spectroscope (UV-3600 UV-VIS-NIR, Shimadzu), and the photoluminescence spectroscope with an excitation source of He-Cd laser (=325 nm) to identify the As related acceptor emission, respectively. These results show that the diameters of SnO2 microhemispheres become larger, and the crystal quality is degraded with the increase of Sn powder mass. The special selective growth of SnO2 microhemisphere on a patterned sapphire substrate is found. In addition, we also find that the optical band gaps of the samples A-D are all redshifted with the increase of Sn powder mass. The shrinkage of Eg in the absorption spectrum should be partly attributed to the degradation of crystal quality because of excess Sn sources. This growth method of SnO2 microhemisphere provides a feasible and effective way of preparing the high density, orderly arrangement of SnO2 micro/nanostructures.