We have measured nonresonant and resonant Raman-scattering spectra from ZnO nanocrystals with an average diameter of 20 nm. Based on our experimental data and comparison with the recently developed theory, we show that the observed shifts of the polar optical-phonon peaks in the resonant Raman spectra are not related to the spatial phonon confinement. The very weak dispersion of the polar optical phonons in ZnO nanocrystals does not lead to any noticeable redshift of the phonon peaks for 20-nm nanocrystals. The observed phonon shifts have been attributed to the local heating effects. We have demonstrated that even the low-power ultraviolet laser excitation, required for the resonant Raman spectroscopy, can lead to the strong local heating of ZnO nanocrystals. The latter causes significant ͑up to 14 cm −1 ͒ redshift of the optical-phonon peaks compared to their position in bulk crystals. Nonresonant Raman excitation does not produce noticeable local heating. The obtained results can be used for identification of the phonons in the Raman spectra of ZnO nanostructures.

The carrier recombination processes in ZnO quantum dots ͑ϳ4 nm in diameter͒, ZnO nanocrystals ͑ϳ20 nm in diameter͒ and bulk ZnO crystal have been studied using photoluminescence ͑PL͒ spectroscopy in the temperature range from 8.5 to 300 K. The obtained experimental data suggest that the ultraviolet PL in ZnO quantum dots originates from recombination of the acceptor-bound excitons for all temperatures. In the larger size ZnO nanocrystals, the recombination of the acceptor-bound excitons is the dominant contribution to PL only at low temperature ͑T Ͻ 150 K͒. For higher temperatures ͑T Ͼ 150 K͒, PL is mostly due to recombination of the donor-bound excitons. Recombination processes in ZnO quantum dots and nanocrystals differ from those in bulk ZnO mainly because of the large surface-to-volume ratio in both types of nanoparticles and, consequently, a large number of acceptor defects near the surface. No strong inhomogeneous broadening has been observed in ultraviolet PL from ZnO quantum dots. Our results shed light on the carrier-recombination processes in ZnO quantum dots and nanocrystals, and can be used for the ZnO nanostructure optimization for the proposed optoelectronic and spintronic applications.

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Fabrication and magnetic characteristics of hybrid double walled nanotube of ferromagnetic nickel encapsulated conducting polypyrrole Appl. Phys. Lett. 90, 093122 (2007); 10.1063/1.2710748 Preparation of silicon carbide nanotubes by hydrothermal method

Recently proposed thermoelectric applications of quantum dot superlattices made of different material systems depend crucially on the values of the electrical and thermal conductivities in these nanostructures. We report results of the measurements of Hall mobility and thermal conductivity in a set of Ge 0.5 Si 0.5 /Si quantum dot superlattices. The average measured in-plane Hall mobility for the undoped Ge/Si quantum dot superlattices on a p-type substrate is 233.5 cm 2 V −1 s −1 at room temperature and 6.80 ϫ 10 3 cm 2 V −1 s −1 at 77 K. The average value of the thermal conductivity measured by 3 method is about 10 W/mK at room temperature and 3.5 W/mK at 77 K. In the low-temperature region, the thermal conductivity is proportional to T 0.7 − T 0.9 . Relatively large values of the carrier mobility and its temperature dependence suggest that the carrier transport in the investigated structures is likely of the band conduction type rather than hopping type. The thermal conductivity of the Ge 0.5 Si 0.5 /Si quantum dot superlattices is strongly reduced and has its peak value shifted toward the high temperatures as compared to the constituent bulk materials. Obtained results can be used for Ge x Si 1−x /Si quantum dot superlattice structure optimization for the high-temperature thermoelectric applications.