Investigation of the temperature dependence on the CuSn nanofibers and MoSe thin films

Abstract

CuSn nanomaterials are attracting much attention because they are abundant in nature and have excellent thermal and optical properties. However, there is still a lack of research on how characteristics change when heat was applied. In Chapter 2 of this thesis, we fabricated CuSn nanofibers (NFs) using electrospinning and investigated the physicochemical properties of NFs according to the calcination temperature. Among various methods, electrospinning have advantages that NFs having a diameter of several nanometers can be easily produced. First, NFs were prepared from precursor solutions with viscosity controlled using polyacrylonitrile (PAN). The solvent and polymer in the NFs were removed at the calcination temperature determined by the thermogravimetric/differential thermal analysis (TG/DTA). As a result of scanning electron microscope (SEM), it was confirmed that pores were formed on the surface of NFs when NFs was calcined at 500 oC, and the pores were filled by aggregation of exposed particles at 800 oC. Through the energy dispersive spectroscopy (EDS) of the CuSn500 NF, it was confirmed that Sn and Cu are the main materials inside and outside of the NF, respectively. At 500 °C, which is similar to the melting point of Sn, Sn particles were aggregated together and exist inside the NF. According to the X-ray diffraction (XRD), the crystallinity was the best at 800 oC and the CuSn NFs confirmed the monoclinic structure of Cu6Sn5. X-ray photoelectron spectroscopy (XPS) observed the presence of carbon-binding peaks, which confirmed that the polymer was carbonized at 800 oC. The work function of CuSn NFs calcined at 150 oC was calculated from ultraviolet spectroscopy (UPS) and was 4.19 eV. Transition metal and chalcogen elements have been studied extensively in recent years. In particular, MoS2, which is a combination of Mo and S in a ratio of 1: 2, has been applied to industries such as batteries and fuels cells. In this thesis, we have fabricated thin films using Se belonging to the same group as S. Se is less toxic than S and has excellent electrical characteristics. In Chapter 3 of this thesis, TFs were prepared at various ratios of Mo and Se at room temperature and the characteristics were investigated. First, the ratio of Mo to Se was varied to fabricate 200 nm of the same thickness. The relative elemental ratios of Mo and Se in the TFs were confirmed by energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy. And contact angle measurement results showed hydrophilic property of less than 90o. It was confirmed by scanning electron microscopy that the density of the surface increased as the ratio of Se increased. UPS, Kelvin probe, and 4-point probe measurements showed that the conductivity decreased and the work function increased with increasing Se ratio. This is due to the influence of Se having an electric conductivity value about 12 times lower than that of Mo. In the last chapter 4, we have studied the physicochemical properties of MoSe TFs obtained by previous studies according to various annealing temperatures. When heat treatment was performed at 600 to 900 oC, the particles of Mo and Se were observed on the surface of the TFs and visually observed by an optical microscope (OM). Mo showed dark seed and Se had bright nano-rod. TFs annealed at room temperature and 500 oC were observed by SEM. Se particles formed a layer at a temperature above the melting point (217 oC) and evaporated, which caused the Mo particles to be exposed to the surface. Se was evaporated and disappeared in TFs with low Se content, so that the silicon peak used as substrate can be confirmed by XRD and XPS. Both Mo and Se were oxidized at temperatures above 300 ℃. UPS and Kelvin probe also showed the highest work function at 200 oC. At annealing temperatures above 300 oC where Se evaporates, the tendency to decrease again due to the influence of Mo appears. Conductivity results also show a similar tendency to decrease again at 200 oC.