Influence of Reaction Temperature on Optical Property of Mn-Doped ZnS Nanoparticles

Abstract

The reaction temperature has essential effect on quality of the product synthesized by hydrothermal method. We report here the variation of the optical characteristics of Mn-doped ZnS nanocrystallites prepared by mean of the stated method from Zn(CH$_{3}$COO)$_{2}$.2H$_{2}$O, Mn(CH$_{3}$COO)$_{2}$.4H$_{2}$O and Na$_{2}$S$_{2}$O$_{3}$.5H$_{2}$O as the precursors. The reaction temperature was set to vary from 120\r{}C to 240\r{}C at a constant reaction time of 15 hours. The XRD patterns showed that, for the reaction temperature range from 120 to 160\r{}C, the obtained products possessed a cubic $T_d^2 - F\overline 4 3m$ and a wurtzite $C_{6v}^4 - P6_3 mc$structure, in which the cubic phase was dominant. At the temperature range from 180 to 240\r{}C, the structures exhibited a cubic phase with the lattice constant increased from 5.41 to 5.43 {\AA}. The photoluminescence spectra showed that with the increase of reaction temperature from 120 to 240\r{}C the intensity of a blue band around 425 - 500 nm (attributed to both Zn, S vacancies) gradually decreased while the intensity of a yellow-orange band at 585 nm (attributed to the $^{4}$T$_{1}(^{4}$G) - $^{6}$A$_{1}(^{6}$S) transition of Mn$^{2 + }$ ions) was enhanced and reached maximum at 220\r{}C. The excitation spectra of the 585 nm band recorded at 160\r{}C showed a band at 335 nm which should be assigned to the near band-edge absorption. With increasing temperature to 200-240\r{}C the new bands appeared at 390, 430, 467, 494 nm. The intensity of these bands increased with temperature and achieved the maxima at 220\r{}C. They should be attributed to the absorption transitions of electrons from ground state $^{6}$A$_{1}(^{6}$S) to excited states$^{ 4}$E($^{4}$D); $^{4}$T$_{2}(^{4}$D); $^{4}$A$_{1}(^{4}$G) - $^{4}$E($^{4}$G); $^{4}$T$_{2}(^{4}$G) of Mn$^{2 + }$(3d$^{5})$ ions, respectively. The bands at 467, 494 nm only exposed clearly in the absorption spectra at 220\r{}C and 240\r{}C.