Interrelation of micro-strain, energy band gap and PL intensity in Ce doped ZnS quantum structures

Three-dimensional (3-D) quantum structures (QSs) of ZnS and Cerium (Ce) doped ZnS were synthesized via chemically affordable sol-gel process. Influence of Ce doping and thus induced micro-strain on the structural, morphological, and optical characteristics was explored. XRD confirmed the formation of single-phase zinc blende ZnS. Estimated average crystal size corresponding to highest intensity XRD peak (111) varied within 1.65-4.65 nm which are comparable with Bohr radius of ZnS. Due to the size mismatch between Ce and Zn, micro-strain and vacancies were found to be developed in host matrix of ZnS. Thermodynamic calculations validated an expansion and contraction in lattice parameter due to Ce doping. FTIR spectra confirmed the presence of different functional groups related to Zn and S. Photoluminescence (PL) emissions observed at 420, 461, 509 and 560 nm are related to the defect states such as interstitial sulfur, zinc interstitial, sulfur vacancies and zinc vacancies respectively. Rise of another emission peak in doped ZnS at 600 nm was due to 5d -> 4f energy level transitions in Ce(3+ )ions. Evolved micro-strain profile, PL intensity and energy band gap variation were analogous to each other with respect to doping concentration. Microscopic images confirmed the structural transformation to cuboidal shaped ZnS QSs with increase in doping concentration. EDX and XPS supported the elemental analysis along with oxidation states of the available elements such as Zn, S and Ce.