Nanotwinning and structural phase transition in CdS quantum dots

Nanotwin structures are observed in high-resolution transmission electron microscopy studies of cubic phase CdS quantum dots in powder form by chemical co-precipitation method. The deposition of thin films of nanocrystalline CdS is carried out on silicon, glass, and TEM grids keeping the substrates at room temperature (RT) and 200 degrees C by pulsed laser ablation. These films are then subjected to thermal annealing at different temperatures. Glancing angle X-ray diffraction results confirm structural phase transitions after thermal annealing of films deposited at RT and 200 degrees C. The variation of average particle size and ratio of intensities in Raman peaks I-2LO/I-1LO with annealing temperature are studied. It is found that electron-phonon interaction is a function of temperature and particle size and is independent of the structure. Besides Raman modes LO, 2LO and 3LO of CdS at approximately 302, 603, and 903 cm(-1) respectively, two extra Raman modes at approximately 390 and 690 cm(-1) are studied for the first time. The green and orange emissions observed in photoluminescence are correlated with phase transition.