Synthesis and Photoluminescence Studies on Nanocrystalline Content Zns Film

Semiconductor nanoparticles with particle sizes less than few nm to a few hundred nm size exhibit size dependent energy gap between the top of occupied states and the bottom of unoccupied states. The size quantization effect is studied by the optical absorption measurements. We have studied the optical properties of ZnS nanocrystalline semiconductors prepared by a chemical route technique by mixing aqueous solutions of ZnCl2 and Na2S in the presence of mercaptoethenol. The photoluminescence is a process in which the material is excited by short wavelength electromagnetic wave and de-excitation causes emission of light. In many cases, photo-luminescence is obtained by band-to-band transition and photoluminescence peaks at the same photon energy as the absorption edge. In nanocrystals, as the size of the crystal is reduced, photoluminescence peaks has been found to shift towards shorter wavelength. The size of nanoparticles can be estimated using theoretical models relating the increase in band gap with the radius of particle. ZnS nanoparticles of different sizes have been prepared and their size has been estimated by shift in photoluminescence peak using effective mass approximation model and hyper-bolic band model.

The XRD studies indicate that the nanocrystalline powder and film specimens of ZnS are cubic in nature having zinc-blende structure. The sizes have been obtained in the range of 2 to 10 nm. Absorption spectra of powder and film specimens have shown blue-shift in absorption edge as compared to their bulk counterpart indicating increased band gap energy due to quantum confinement effect.

It can be seen from the spectra that there is practically uniform absorption in the Visible range (800 nm - 390 nm). Absorption increases suddenly in the UV region. For ZnS–I, sample sudden increase in the absorption occurred at about 260 nm similarly onset of absorption is obtained at 250 nm, 240 nm, 230 nm and 220 nm for ZnS–II, ZnS–III, ZnS–IV and ZnS–V samples, respectively. The values of Eg are obtained as 4.7 eV, 4.9eV, 5.2eV, 5.4eV & 5.6eV, respectively for the samples ZnS–I to ZnS-V corresponding to the absorption edge with increasing the capping agent concentration.