Optical and magnetic properties of Cr-doped ZnS nanocrystallites

Cr-doped ZnS (molar Cr:(Cr + Zn) between 0.51% and 19.69%) nanocrystallites have been prepared through co-precipitation method. The x-ray diffraction, transmission electron microscopy, and selected area electron diffraction results show that all the samples are in sphalerite structure with average particle size about 3 nm. No impurity phase relating to Cr element is found in all the samples. X-ray photoelectron spectroscopy spectra reveal that Cr was incorporated into ZnS lattice as Cr3+. It can be seen in the UV-visible absorption spectra that, besides the intrinsic band-gap absorption of ZnS below 370 nm, there are another two absorption bands (at 425 nm and 595 nm, respectively) in the visible light range, which are the characteristic bands of Cr3+. Photoluminescence spectroscopy was also used to characterize corresponding luminescence properties of the nanocrystallites. The band-edge emission in photoluminescence spectroscopy exhibits blue-shift as the concentration of Cr increases, and several emission peaks concerned with surface states and zinc vacancies were found in the wavelength range of 400 nm ∼ 500 nm. It is confirmed that Cr doping will bring about a considerable amount of zinc vacancies. Magnetic measurements indicated that all the samples are paramagnetic and the calculated effective magnetic moments μeff of Cr3+ were close to the theoretical value of 3.87 μB. Accordingly, it seems that zinc vacancies give no contribution to the overall magnetic response of the samples.