Role of non-stoichiometric defects in optical properties of metal-selenide nanostructures

Abstract

Metal-selenide nanostructures from different semiconductor families such as ZnSe, SnSe, and CuxSey were synthesized with different Se/cation ratios. The effects of non-stoichiometric defects such as vacancies and interstitials on their optical properties were investigated. X-ray diffraction patterns (XRD) indicated that the Se/Zn > 1 and Se/Cu > 1 conditions led to creating heterostructures of ZnSe/Se and CuxSey/Se, while, the sample with Se/Sn > 1 condition only showed a single phase of SnSe with border XRD peaks. Scanning electron microscopy (SEM) images of the samples showed different morphology for the ZnSe and CuxSey nanostructures with different conditions, while, SnSe samples had nanoparticles (NPs) morphology in all conditions. High-resolution transmission electron microscopy (HRTEM) images of SnSe samples showed that the SnSe NPs with Se/Sn = 1.2 condition were single-crystalline NPs, without any defects, while, the SnSe NPs with Se/Sn = 0.8 condition included vacancy and stacking fault defects. The optical band-gap values of the samples were not significantly changed by different Se/cation ratios. More details of the effects of non-stoichiometric defects on the optical properties of the samples were obtained by photoluminescence (PL) spectroscopy. The PL results of any type of samples showed that the non-stoichiometric defects could change the PL spectrum feature. Valence band spectroscopy results showed a shift in valence-band maximum (VBM) position in Se/cation>1 ratio in comparison to the Se/cation<1 ratio. However, this shift was bigger in the SnSe NPs and it was proposed that this bigger shift could be due to spin-orbit coupling (SOC) interaction, while, a valence-band offset was responsible for the VBM shift in the other nanostructures due to their heterostructure nature.