Structural, band gap and photoluminescence behaviour of Mn-doped ZnS quantum dots annealed under Ar atmosphere

Abstract

Undoped and Mn doped ZnS quantum dots with Mn = 2, 4 and 6 % have been prepared through a simple chemical method, namely the chemical precipitation method and annealed under Ar atmosphere. The prepared samples have been analyzed using X-ray diffraction (XRD), scanning electron microscope, energy dispersive X-ray (EDX) spectra, transmission electron microscope and X-ray photoelectron spectroscopy (XPS), UV–visible spectrometer, Fourier transform infra red (FTIR) spectra and photoluminescence (PL) measurements. XRD pattern confirmed that all the samples had cubic structure and the average crystallite size varied in the range of 1–3 nm. The substitution of Mn into Zn–S matrix was supported by the variation in lattice parameters. The elemental composition of the samples with their nominal stoichiometry was verified by EDX analysis. XPS revealed the presence and incorporation of Mn2+ into ZnS lattice sites and the formation of a Mn2+–Zn2+–S combined structure. The higher transmittance observed at Mn = 2 % doped ZnS is useful for the opto-electronic device applications. The continues red shift of energy gap by Mn-doping is due to the direct energy transfer between semiconductor-excited states and the 3d levels of Mn2+ ions. The detected absorption bands around 672 cm−1 and 480–496 from FTIR spectra proved the presence of Mn–S specific vibrations. PL spectra showed the two strong and broad bands, UV band around 390 nm and blue emission band between 455 and 483 nm and a weak green emission around 543 nm. The suppression of blue emission by Mn-doping and the reduction of green emission were discussed based on surface defect sites and vacancies.