Spectral-optic response of ZnS and mixed CdxZnyS nanoclusters on synthetic mordenites

Abstract

The spectral and optical response of homogeneously dispersed semiconductor nanoclusters of ZnS and of members of the ternary CdxZnyS system grown on synthetic Na-mordenite hosts is investigated. Samples were prepared by ion exchange using aqueous ZnSO4 and mixed CdCl2 and ZnSO4 solutions followed by treatment with flowing hydrogen sulfide. Unlike the hydrothermal method, nanoclusters formed in both the primary and the secondary porosity without large external clusters. Measurements of the absorption spectra by diffuse reflectance spectroscopy in the UV-visible range (ABS) and of the infrared (IR), Raman, and photoluminescence (PL) spectra were made as functions of relative Zn and Cd concentrations. Noticeable blue shifts in the absorption edges in the ABS spectra of the semiconductor-zeolite samples with respect to those of pure ZnS and CdS and a nanoparticle average size value smaller than the Bohr radius of the first exciton are consistent with the strong quantum confinement effects observed in the PL spectra. For an excitation wavelength of 250 nm, three well-defined emission bands at the photon energies 3.14, 4.34, and 5.4 eV were observed

they are attributed to trapped luminescence and to the high-energy 1S-1P and 1S-1D transitions, respectively. Luminescence near the band edge, attributable to 1S-1S excitonic emission was not observed, indicating very poor passivation of the surface states. All samples in the ternary CdxZnyS system exhibited a similar band centered at 395 nm corresponding to an energy value higher than the band-edge energies E-g

no red shifts were observed when the Cd content was increased, and the intensity of all PL bands were sensitive to the composition, being higher when the Zn content decreased. This behavior is attributed to complex nanocluster structures where smaller nanoparticles of ZnS are embedded in a host of Cd ions. Possible mechanisms for the observed luminescence are discussed using schematic energy level diagrams.