Optical studies of colloidal II-VI semiconductor nanocrystals: Pressure-induced phase transitions

Nanocrystalline semiconductors and bulk semiconductors show several differences in their physical properties. Among these is an enhanced phase stability for the nanocrystals. This study was undertaken to investigate the structural phase stability of colloidal CdS nanocrystals under high pressure.; Bulk CdS has a wurtzite structure at ambient pressure and changes phase to the rocksalt structure at higher pressure. This is a first order phase transition that occurs at 27 kbar. However, colloidal CdS nanocrystals do not show such an abrupt phase transition change at the bulk pressure point.; The intensity of the Raman peaks is used as a tool to monitor the major differences in the spectra due to phase transition. The phase transition pressure point was determined by observing the Raman intensity to go to zero as the wurtzite phase change to rock salt. The phase transition pressure at room temperature is {dollar}sim{dollar}27 kbar for bulk CdS, {dollar}sim{dollar}40 kbar for fresh made CdS colloids, {dollar}sim{dollar}70 kbar for aged CdS colloids, and {dollar}sim{dollar}40 kbar for CdS powders. The decrease of the intensity of the Raman peak for the CdS colloids can be due to both off resonance and the occurrence of phase transition. By incorporating Raman and photoluminescence measurements we ascertain that for aged CdS colloids the slow intensity decreases after 40 kbar are mainly due to the phase transition and not due to off resonance conditions.; This elevated phase transition pressure for the colloids suggests an enhancement of the structural phase stability. To account for this elevated phase transition pressure in nanocrystals, model-I simulating a high concentration of defects (volume effect) and model-II simulating surface effects were proposed. Using kinetic theory and thermodynamics of phase transitions, it is found that model-I is more plausible.; Other major results found are as follows: (i) We achieved resonance effects in the nanocrystals by pressure tuning the electronic band gap energies at a fixed excitation energy. (ii) From the pressure coefficients in photoluminescence spectra the defect is identified to be a deep level defect. (iii) No hysteresis effect after releasing the pressure in the nanocrystal is observed.