Preparation And Optical Properties Of CdS And Zn₂SnO₄ Micro/Nanostructures

Cadmium sulfide?CdS?has a direct band gap of 2.42 eV at room temperature and a small exciton Bohr radius of 2.5 nm.It is one of the best candidates for window materials of heterojunction solar cells to avoid recombination of photogenerated carriers and improve the efficiency of solar cells.In addition,it has important applications in light-emitting diodes,photocatalysis,solar energy conversion and photodegradation of water pollutants.Zinc stannate?Zn₂SnO₄?is an important n-type semiconductor material with high electron mobility,high conductivity and low visible light absorption.It has great potential applications in many fields,such as transparent conductive oxides,thin film photovoltaic devices and flat panel displays,thin film solar cells and selective gas sensors.In this paper,CdS nanobelts,Cd_1-xZn_xS nanorods,Cd_1-xZn_xS feather-like nanostructures,CdS/CdMoO₄ microwires and Zn₂SnO₄ tetrapods were synthesized by thermal evaporation method,and the mechanism of synthetic growth,Raman vibration spectrum and the characteristics of photoluminescence?PL?have been studied in detail.The specific contents are as follows:?1?The CdS nanobelts with different sizes were synthesized at deposition sites in different temperature zones by thermal evaporation of CdS powder,and their crystal structure,vibration and optical properties were characterized.X-ray diffraction?XRD?analysis shows that CdS nanobelts of different sizes belong to hexagonal wurtzite phase.The results of Raman spectroscopy clearly show the first-order?1LO?and second-order?2LO?longitudinal optical phonon modes of CdS.The results of the PL spectra show that the smaller CdS nanobelts have two emission bands:a weak bandgap emission and a wide emission band between 600 nm and 830 nm,while the larger CdS nanobelts have only one wide emission band between 500 nm and 700 nm.?2?The high-purity zinc powder and CdS powder were used as raw materials to synthesize Cd_1-x-x Zn_xS nanostructures by one-step thermal evaporation.Scanning electron microscope?SEM?showed that Cd_1-x-x Zn_xS nanorods were synthesized on the ceramic boat wall;large-scale Cd_1-xZn_xS feather-like nanostructures were synthesized on a silicon substrate.The results of Raman spectroscopy show that the 1LO and 2LO modes of the Cd_1-x-x Zn_xS ternary compound are shifted to the short wavelength direction compared with the Raman spectrum of pure CdS,because the Zn ions are incorporated.This leads to an increase in CdS structure fluctuations,phonon confinement,and substitution disorder.PL test results show that Cd_1-x-x Zn_xS nanorods have a wide emission band at 440⁶40 nm,and Cd_1-x-x Zn_xS feather-like nanostructures have a wide emission band at 450⁶40 nm.Different substrate properties,such as crystallinity,thermal expansion and lattice mismatch between the substrate and the material,play an important role in the growth of nanocrystalline structure.?3?CdS/CdMoO₄ composite microwires were successfully synthesized by two-step thermal evaporation.The first step is to evaporate the CdS powder and grow the CdS micronwires on the silicon substrate.The second step is to evaporate molybdenum disulfide?MoS₂?in the air atmosphere,and a layer of CdMoO₄ particles is coated on the surface of the CdS microwires.Results of the SEM and Raman spectroscopy indicate that the synthesized composite consists of CdS and CdMoO₄.PL test results show that CdS/CdMoO₄ has a green emission band at 510 nm.?4?Zn₂SnO₄ and zinc oxide?ZnO?tetrapod structures were synthesized by one-step thermal evaporation.The results of XRD and room temperature Raman spectroscopy indicate that the spinel phase Zn₂SnO₄ tends to grow in the higher temperature zone,while the lower temperature zone is more suitable for the growth of hexagonal wurtzite phase ZnO.The results of PL spectra show that the Zn₂SnO₄ tetrapod has an asymmetric wide emission band in the wavelength range of 450⁸00 nm,while the ZnO tetrapod has only one green light emission band at 513 nm.