Examination of Optical Spectroscopy of Cadmium Chalcogenide Nanocrystals

High optical quality Cd chalcogenide (Te, Se, and S) quantum dots with average size at the near Bohr radius were synthesized using a colloidal chemical reaction. The Cd chalcogenide (Te, Se, and S) nanocrystals exhibited strong blue shift and discrete energy states which were significantly modified from bulk crystals. The discrete structure of energy states leads to a discrete absorption spectrum of quantum dots, which is in contrast to the continuous absorption spectrum of a bulk semiconductor. Atomic-like discrete energy states of exciton indicate a quantum confinement effect. The energy spacing between the first and second peaks of exciton absorption is less than 50 meV for bigger than ~4 nm quantum dot size, and is more than several hundred meV for ~2 nm size of CdTe quantum dots. The emission of CdSe nanocrystals tuned from near-infrared to dark blue by a reduction in the dot radius from 8 to 0.8 nanometers. The quantum confinement modifies the optical absorption, the photoluminescence, the radiative decay time, and the nonlinear optical properties of nanocrystals. The optical properties of quantum dots are also sensitive to the dielectric effects of surrounding environment, which can be utilized for the detection of nanoscale crack on the spacecraft.