CHARACTERIZATION OF CdSe_xS_(1-x) QUANTUM DOTS USING XRD, UV-VIS ABSORPTION AND RAMAN SPECTROSCOPY MEASUREMENTS

摘要

CdSe_xS_(1-x) quantum dots (QDs) have been grown in borosilicate glass matrix using two step annealing technique and are characterized by AFM, XRD, UV-Vis absorption, HRTEM and Raman spectroscopy measurements. AFM studies show that nearly spherical clusters of about 5-10 nanocrystals were grown. From XRD measurements, it is found that the nanocrystals are grown in hexagonal phase. From HRTEM measurements, it is observed that the size of QDs increases from 5.1 to 9.5 nm with increasing annealing duration from 3 to 10 hrs. Blue shift in absorption spectrum indicates that quantum confinement increases with decreasing annealing durations. Raman spectrum of CdSe_xS_(1-x) QDs shows two mode type of behavior i.e., CdSe like longitudinal optical phonon mode (LO_1) and CdS like longitudinal optical phonon mode (LO_2). It has been observed that with increasing the size of QDs from 5.1 to 9.5 nm, LO_1 shifts from 205 to 210 cm~(-1) while LO_2 shifts from 278 to 291 cm~(-1). The shift in the LO_1 and LO_2 Raman peaks towards high frequencies has been interpreted due to increase in the lattice compressive strain with the increase in QDs size. The effect of phonon negative dispersion on the shift of LO_1 and LO_2 is found to be comparatively insignificant. It is also found that the ratio of intensity of overtones to the fundamental mode increases from 0.34 to 0.48 with decreasing QD size. This indicates that the electron_phonon coupling increases with the decrease in size of QD due to increasing overlap of electron and hole wave function in small size QDs. It is further observed that the frequency of the surface optical phonon mode remains unchanged as the size of QD changes but the intensity decreases with the increase in the size of QDs finally merging in LO mode Raman spectrum.