Optical properties of ZnS nanocrystals/KH2PO4 dielectric crystal

Abstract

The properties of highly luminescent II-VI semiconductor nanoparticles have been extensively investigated to the application field. Generally the optical properties of crystals become extremely different with miniaturization of size [1]. Nanocrystals (NCs) are having an increasing importance due to their influence in different properties due to the quantum confinement stimulated by size decreasing [2]. Electron and phonon confinement is possible by II-VI NCs semiconductor when the size of particles tend to Bohr radius of the bulk crystal exciton showing new physical properties. These intriguing phenomena have been found new applications in telecommunication and Photonics [3]. Potassium dihydrogenophosphate KH2PO4 (KDP) has been recently used as a host matrix of ZnS semiconductor NCs as it is a transparent crystal in the UV-Visible region. KDP is a well known dielectric material for its nonlinear optical and electro-optical properties [4, 5] and well suited for frequency conversion [6]. In the present work, KH2PO4 (KDP) crystals doped with ZnS nanocrystals have been grown in aqueous solution supersaturated at 50°C using the lowering temperature method. X-ray diffraction spectra have shown that ZnS inclusions have been obtained with an average size of about 6.41 nm. The optical gap (Eg=4.01 eV) was deduced from the UV-Vis absorption spectra. The gap shift compared with ZnS bulk crystals (Eg=3.91 eV) could be due to the nanometric size of ZnS particles, revealing the intrinsic quantum confinement effect of these nanocrystals. Nevertheless, following experimental results from structural and optical characterisation the estimation of the ZnS particles size data shows a large dispersion, which must be taking into account. Furthermore, the possibility of embedding II-VI ZnS nanoparticles in KDP crystalline matrix has been proved and these materials present nanoparticles emitting light.