Identification of trapping and recombination levels, structure, morphology, photoluminescence and optical absorption behavior of alloyed ZnxCd1−xS quantum dots

Abstract

Cubic ZnxCd1−xS nanoparticles (NPs) synthesized by chemical precipitation method with average crystallite size of 2.9 ± 0.2 nm were characterized using X-ray diffraction (XRD), X-ray fluorescence (XRF), high-resolution transmission electron microscope (HRTEM), UV–vis absorption, Fourier transform infrared red (FTIR), photoluminescence (PL) emission and Raman spectroscopy. XRD analysis demonstrated systematic shift of diffraction peaks to higher diffraction angles accompanied by a decrease in the lattice parameters with increasing Zn content (x); this confirmed the formation and homogeneity of ZnxCd1−xS nanoalloys. In addition, the dependence of lattice parameters and Raman shift on x showed linear behavior and good agreement with Vegard's law. TEM images of ZnxCd1−xS NPs revealed nearly spherical shape NPs, relatively narrow particle size distribution and standard deviation in the range 1.8–3.4%.; as well as HRTEM images showed well resolved diffraction crystalline planes of zinc-blende cubic type structure. Analysis of optical absorption spectra showed blue shift of both the direct optical band gap from 3.25 to 4.05 eV and excitonic absorption shoulder from 2.56 to 3.88 eV with increasing x, confirming the homogeneity of ZnxCd1−xS alloyed semiconductor NPs. At excitation wavelength 325 nm, the deconvoluted structural defects related PL emission bands are broadened and revealed stronger PL intensity than that at 370 nm. Furthermore, increasing x resulted in PL enhancement accompanied by blue shift of green emission band centered at 515 nm–445 nm. To explain composition dependent PL emission process; trapping and recombination localized levels in ZnxCd1−xS alloyed NPs were identified quantitatively and an energy band diagram was suggested.