Preparation And Optical Properties Of CdS One - Dimensional Nanomaterials

One-dimensional (1D) semiconductor nanostructures have attracted significant attention because their anisotropic geometry results in unique physical properties, which offer great potential application in nanolasers, thermal conductors, and photonic integration, various optoelectronic devices. As an important II-VI semiconductor material, CdS has a direct band gap of2.45eV at room temperature and is regarded as a possible candidate for. optoelectronic applications in the visible spectrum range. Therefore, many works have been devoted to fabricating various CdS1D nanostructures with controlled morphology, size, orientation, and crystallinity.In this paper, we explore the preparation conditions of CdS1D nanostructures, and then successfully prepared CdS nanowires, nanobelts and CdS/ZnS:Mn core-shell heterostructure nanowires.The detailed discussions are presented as follows.Firstly, different sizes of CdS nanobelts were synthesized at800,850, and900°C by the thermal evaporation of CdS powders on Au-coated silicon substrates and were used to study the size effects of Raman scattering and photoluminescent spectra. The Raman spectra of CdS nanobelts clearly exhibit first-and second-order longitudinal optical (LO) Raman peaks, surface phonon peaks, and multiphonon processes when excited using a wavelength of532nm. The mechanism of exciton-phonon coupling was observed to be mainly associated with the Frohlich interaction, and the coupling strength of the exciton-phonon increases with increasing lateral size. Compared with a larger CdS nanobelt, a narrower nanobelt exhibits a larger tensile strain. Recombination of free excitons (FX), excitons bound to neutral impurities (A0X), and donor-acceptor pairs (DAP) were identified from a low-temperature PL spectrum. At temperatures below～123K, a red shift of the FX energy occurs with decreasing lateral size due to a larger uniaxial tensile strain; at temperatures above～123K, a red shift of the FX energy occurs with increasing lateral size because of the reabsorption of the emitted light inside the thicker belt, indicating that the FX energy is a ffected by both the tensile strain and the surface-depletion-induced quantum confinement (the reabsorption of the emitted light) in the nanobelt. Secondly, both normal and ripple-like CdS microbelts (MBs) were prepared and their optical properties were investigated by photoluminescent and Raman measurements. One emission at-510nm for normal CdS MBs and two emissions around513and725nm for ripple-like one were observed. The longitudinal optical phonon mode (1LO) at～304cm-1and its overtone mode (2LO) at～605cm-1, multiphonon processes and surface phonon peaked at281cm-1were observed, the larger integrated intensities ratio of2LO to1LO indicates a strong exciton-phonon coupling interaction. Furthermore, more than4times larger enhanced Raman scattering for the ripple-like MBs than the normal one were observed, and the enhanced Raman scattering is mainly associated with the surface defects, which lead to a strong Frohlich interaction through the charge transfer, not by the deformation potential through surface plasmon enhancement.Thirdly, core-shell heterostructure nanowires of CdS and ZnS:Mn that The Mn doping concentration in the prepared ZnS:Mn target was2%were synthesized in two steps which were thermal evaporation of CdS powders and pulsed laser deposition (PLD). CdS nanowires were grown at800℃by the thermal evaporation of CdS powders on Au-coated silicon substrates, then the CdS nanowires were immediately overcoated with ZnS:Mn via PLD of different coating time. The XRD spectra show that wurtzite-type ZnS is more obvious with increasing the coating time, and typical branched and crossed nanowires appear prominently with the coating time, the diameters of the nanowires are in the range of70-600nm. Two dominant emission bands located at～508and～590nm were observed from the RT photoluminescence (PL) of CdS/ZnS:Mn core-shell heterostructure.