Effect of Bimodal Size Distribution on Optical Properties of CdSe Nanocrystals

Abstract

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. INTRODUCTIONIn recent years, considerable research efforts have been made towards the fabrication, characterization and exploitation of semiconductor nanocrystals (NCs) for possible application in electroluminescent devices [1,2], biosensors [3,4] and imaging based on their optical properties [5]. However, the NC optical properties are often influenced by their size, shape, strain and their bimodal/ multimodal size distribution. Semiconductor NCs [6] with size less than or comparable to Bohr exciton [7] radius exhibit size quantization effect (SQE) where one can observe blue shifts in their optical absorption because of the discritization of electronic energy levels within the band gap. Some of the important optical properties associated with semiconductor NCs are as under:With increasing NC size, the absorption threshold shifts to longer wavelength [8]. Photoluminescence (PL) properties are size, size distribution and shape dependent. Large size NCs show band edge [9] PL i.e the carrier recombination is between the conduction band electrons and valence band hole where as for very small NCs, the PL exibit excitonic effect with a stokes shift. The PL involves a nonradiative transition of photoexcited electrons from the conduction band maxima to sub-bands followed by radiative transition from sub-band to valence band maxima. Hence, excitonic transitions in semiconductor NCs are generally observed at lower energies compared to the absorption edge. Due to large surface to volume ratio, a large hydrostatic pressure is being exerted on the NCs by which the crystalline lattices are strained and the effects can be observed in their optical properties. PL is red shifted in case of strained large NC and blue shifted for strained small NCs [10] along with a large blue shift in absorption too. Furthermore, at low temperature, different thermal contraction [11] between a substrate and NC thin film on it, affects the PL properties due to differential strain at low temperature [12,13]. However, the NC growth conditions are such t...