Effect of Bimodal Size Distribution on Optical Properties of CdSe Nanocrystals

Effect of bimodal size distribution (BSD) on optical properties of electrodeposited CdSe nanocrystalline thin films is reported. Grazing angle X-ray diffraction identifies two different nanocrystalline size regimes with both compressive and tensile strains coexisting in the same sample. The two size regimes change their size with the change of electrodeposition current density or temperature. Optical absorption studies reveal two absorption edges corresponding to the two different sizes. Presence of two well separated photoluminescence (PL) bands due to excitonic recombination supports the presence of BSD in the sample which is also confirmed from temperature dependent PL measurements. The origin of the two PL bands are attributed to the transfer of thermally activated carriers between two size regimes accompanied by excitonic recombination. Micro Raman scattering of nanocrystalline CdSe at 300K showed softening of Raman phonon modes due to small size nanocrystallites but do not reflect the BSD effect.