Covellite CuS – Single crystal growth by chemical vapour transport (CVT) technique and characterization

The covellite CuS single crystals were grown by the chemical vapour transport (CVT) technique using iodine as a transporting agent. The energy dispersive analysis of X-rays (EDS) of the as-grown CuS single crystal showed them to be in perfect stoichiometry. The powder X-ray diffraction (XRD) of as-grown CuS single crystals confirmed hexagonal structure with lattice parameters, a=b=3.795 Å and c=16.34 Å. The strain and dislocation density calculated using XRD data were found to be 0.0016 lin−2 m−4 and 3.67×1015 lin m−2, respectively. The selected area electron diffraction (SAED) showed spot pattern, stating the as-grown CuS to be single crystalline in nature. The thermogravimetric (TG), differential thermal (DTA) and differential thermogravimetric (DTG) analysis of as-grown CuS single crystal in inert N2 atmosphere and in air showed them to be stable in the temperature range of ambient to 630 K in case of inert N2 atmosphere and up to 600 K in case of air. The activation energy (Ea), activation enthalpy (ΔH⁎), activation entropy (ΔS⁎) and Gibbs free energy change (ΔG⁎) of CuS single crystals were determined from the thermal curves using Coats–Redfern (C–R) relations. The analysis of optical absorption spectrum of the as-grown CuS single crystal showed them to possess direct bandgap of 1 eV. The photoluminescence spectra for excitation wavelengths, 325 nm, 350 nm and 375 nm showed that the CuS single crystal had two blue emission peaks and two green emission peaks. The Raman spectra of CuS single crystal showed sharp peak at 474 cm−1 and a less prominent peak at 265 cm−1. The sharp peak of 474 cm−1 is identified as due to S–S stretching mode of S2 ions. The study of d.c. electrical resistivity variation with temperature showed that the resistivity increases with temperature indicating metallic nature of the CuS single crystal. All results are deliberated in details.