Crystal structure, energy gap and photoluminescence investigation of Mn²⁺/Cr³⁺-doped ZnS nanostructures by precipitation method

Mn added ZnS (Zn_0.97Mn_0.03S) and Mn–Cr-doped ZnS (Zn_0.95Mn_0.03Cr_0.02S) nanostructures were synthesized by co-precipitation process. XRD pattern confirmed the cubic phase with highest intensity along (111) orientation. The shrinkage of crystallite size from 36 Å (Zn_0.97Mn_0.03S) to 26 Å (Zn_0.95Mn_0.03Cr_0.02S) and the influence of Cr/Mn on microstructural, optical and photoluminescence properties in ZnS were investigated. The substitution of Cr in Zn_0.97Mn_0.03S lattice not only diminished the crystallite size and also produced more defect-associated luminescent activation centres. The elevated micro-strain from 9.71 × 10^–3 (Zn_0.97Mn_0.03S) to 13.11 × 10^–3 (Zn_0.95Mn_0.03Cr_0.02S) by Cr substitution is due to the decrease of size and the higher micro-strain at Cr = 2% is owing to the drop off of activation energy which is originated from higher electro-negativity of Cr ions than Zn²⁺ ions. The enhanced lattice parameters by Cr doping may be due to the coexistence of both Cr³⁺ ions and Cr²⁺ ions where the existence of Cr²⁺ ions is higher than Cr³⁺ ions and substitute Zn²⁺ basic ions with the ionic radius of 0.74 Å in the Zn–Mn–S host lattice. The presence of Zn²⁺, Mn²⁺ and Cr³⁺ ions in Zn–Mn–Cr–S lattice was confirmed by XPS spectra. SEM/TEM micrographs explored the microstructure and confirmed the sized reduction by Cr doping. The elevation in band gap from 3.50 eV (Zn_0.97Mn_0.03S) to 3.63 eV (Zn_0.95Mn_0.03Cr_0.02S, ∆E_g ~ 0.13 eV) by Cr addition was explained by Burstein–Moss effect and reduced crystallite size. The tuning of band gap and crystallite size of basic ZnS nanostructure by Mn/Cr substitution encourages these materials for modern electronic applications. FTIR spectra established the occurrence of Mn/Cr in Zn–S lattice by their characteristic bondings. The elevated yellowish-orange emission at 594 nm in Mn/Cr substituted ZnS is due to the exchange communication among the s–p electron states of Cr³⁺, Mn²⁺ and Zn²⁺ ions in Zn–S lattice. The inclusion of Mn /Cr provides an efficient control over modification of various emissions which suggests their applications in organic LED materials.