Synthesis of TOPO-capped Mn-doped ZnS and CdS quantum dots

Abstract

TOPO (tri-n-octylphosphine oxide) capped Mn-doped ZnS and Mn-doped CdS nanoparticles have been prepared from bis(diethyldithiocarbamato)zinc(II) or bis(methylhexyldithiocarbamato)cadmium(II) and manganese dichloride. The nanoparticles obtained all show quantum size effects in their optical spectra with respect to the parent CdS and ZnS. There are clear differences in the photoluminescence of the ZnS (448 nm) and ZnS∶Mn (510 nm) samples. The level of Mn-doping can be differentiated by the photoluminescence intensity, all of the doped samples show an emission maximum at 510 nm. EPR spectra and ICP results confirm the quantitative incorporation of manganese into the ZnS quantum dots. Selected area electron diffraction (SAED), X-ray diffraction (XRD) and transmission electron microscopy (TEM) show the material to be hexagonal. The crystallinity of the material was also evident from high resolution transmission electron microscopy (HRTEM) which gave well-defined images of particles with clear lattice fringes. The PL spectrum of the doped CdS nanoparticles has an emission maximum at 585 nm as is usually attributed to the 4T1–6A1 electronic transition of manganese in a tetrahedral site. However the PL spectrum changes over time (weeks) to give a spectrum typical of the deep trap emission of CdS. SAED and XRD show both CdS and the Mn-doped CdS particles to be in the hexagonal phase. TEM and high resolution TEM show images of nanosize particles with clear lattice fringes. EPR spectra and ICP results confirm the presence of manganese in the CdS nanoparticles. EPR spectra showed that CdS∶Mn was not stable over longer periods of time with segregation of manganese at the surface of particles in older samples of CdS. By contrast, the ZnS∶Mn samples were quite stable for several weeks.