| Pressure-induced 4- to 6-coordinate phase transitions were investigated in both II-VI and III-V semiconductors as a function of crystallite size. The transition from the low pressure wurtzite phase to the high pressure rock salt phase in CdSe nanocrystals occurred with a distinct change in optical features, and rock salt CdSe was found to be an indirect gap semiconductor even in finite size. The change in optical features, combined with high pressure X-ray diffraction, was used to follow the phase transition kinetics of CdSe crystallites. CdSe nanocrystals transform from one structure to another by single nucleation events, and the transformations obey simple unimolecular kinetics. Barrier heights were observed to increase with increasing nanocrystal size. InP and InAs nanocrystals were also found to transform via single nucleation and have a larger range of metastability than the corresponding extended solids. The effects of consecutive pressure cycles on CdSe nanocrystals were investigated, and the transformation pressures and hysteresis were consistent on each iteration. The compressibility of InP nanocrystals and CdSe nanocrystals were compared to the respective bulk semiconductors, and the observed deviations from the bulk were examined in relation to the quality of the surrounding pressure medium. |
