Synthesis and Characterization of Controlled Size CdSe Quantum Dots by Colloidal Method.

Monodispersed and highly luminescence cadmium selenide (CdSe) quantum dots (QDs) have been prepared in a single pot by colloidal reaction method. The QDs were characterized using X-ray diffraction (XRD), Raman Spectroscopy, transmission electron microscope (TEM), energy dispersive spectroscopy (EDS), UV-visible absorption spectroscopy and photoluminescence (PL) spectroscopy to study the structural, morphological, compositional and optical properties. The growth temperature played an important role to control the particle size. The optical wavelength was found to be shifted systematically from 460 nm to 575 nm upon increasing the reaction temperature from 110 °C to 260 °C. The size of CdSe QDs, ~2-4 nm was estimated from absorption data. The emission tail exhibited at higher wavelength in PL measurement for the QDs synthesized for lower reaction temperature revealed the presence of surface trap-states. A cubic crystal structure of CdSe QDs was revealed by XRD analysis. The spherical QDs of size 2 to 4.5 nm were observed from TEM analysis for the samples prepared at 140 °C, 200 °C and 260 °C. The sizes of QDs obtained by TEM are in good agreement with the results obtained from optical and XRD data. High resolution transmission electron microscopy (HRTEM) confirmed the cubic crystal structure of CdSe QDs. The Selected area diffraction (SAD) pattern exhibited diffused ring corresponds to (111), (220) and (311) reflections of cubic structure of CdSe. The compositional analysis studied by EDS revealed the growth of nearly stoichiometric CdSe QDs. The LO1 vibrational mode observed about 202-205 cm-1 decreases the broadening systematically upon increasing the reaction temperature.