We report here a synthesis method for single-crystal monodisperse GeO2 nanocubes in a reverse micelle system. Hydrolysis of germanium tetrachloride (GeCl4) in a micelle system produces GeO2 nanocubes in the presence of oleylamine and cetyltrimethylammonium (CTAB). The average edge length of the cubes varies from 50 to 520 nm depending on the concentration of CTAB in the solution. The perfection and monodispersity of the nanocubes are greatly improved by decreasing the pH value of the water solution. Different morphologies such as capsules can be prepared by using octane as the oil phase. The BFDH and Hartman-Perdok methods were used to predict the morphology evolution during the growth of GeO2 nanocubes.

High-pressure behaviors of wurtzite-type hexagonal CoO nanocrystals were investigated by in situ highpressure synchrotron radiation X-ray diffraction measurements up to 57.4 GPa at ambient temperature. It is found that bulk modulus of the hexagonal CoO phase is about 115 GPa at zero pressure. During compression, the hexagonal CoO phase transfers into rocksalt-type cubic phase in the pressure range of 0.8-6.9 GPa. The volume collapse accompanied by the transition was estimated to be about 20%. This is irreversibly phase transformation; that is, the cubic CoO phase remains after pressure release. Based on the data of peak width vs pressure, a cubic-to-rhombohedral phase transition was detected for the nanocrystalline cubic CoO phase with the transition pressure of about 36 GPa, lower than 43 GPa for bulk cubic CoO phase. The bulk modulus of the nanocrystalline cubic CoO phase of about 258 GPa is larger than 180 GPa for the corresponding bulk cubic CoO phase.