The complete Raman spectrum of SnO2 nanoparticles in presented and analyzed. In addition to the “classical” modes observed in the rutile structure, two other regions shown Raman activity for nanoparticles. The Raman bands in the low-frequency region are attributed to acoustic modes associated with the vibration of the individual nanoparticle as a whole. The high-frequency region is activated by surface disorder. A detailed analysis of these regions and the changes in the normal modes of SnO2 are presented as a function nanoparticle size.

The structural and optical properties of three different kinds of GaAs nanowires with 100% zinc-blende structure and with an average of 30% and 70% wurtzite are presented. A variety of shorter and longer segments of zinc-blende or wurtzite crystal phases are observed by transmission electron microscopy in the nanowires. Sharp photoluminescence lines are observed with emission energies tuned from 1.515 eV down to 1.43 eV when the percentage of wurtzite is increased. The downward shift of the emission peaks can be understood by carrier confinement at the interfaces, in quantum wells and in random short period superlattices existent in these nanowires, assuming a staggered band offset between wurtzite and zinc-blende GaAs. The latter is confirmed also by time-resolved measurements. The extremely local nature of these optical transitions is evidenced also by cathodoluminescence measurements. Raman spectroscopy on single wires shows different strain conditions, depending on the wurtzite content which affects also the band alignments. Finally, the occurrence of the two crystallographic phases is discussed in thermodynamic terms.

We study the influence of Nb doping on the ${\mathrm{TiO}}_2$ anatase-to-rutile phase transition, using combined transmission electron microscopy, Raman spectroscopy, x-ray diffraction and selected area electron diffraction analysis. This approach enabled anatase-to-rutile phase transition hindering to be clearly observed for low Nb-doped ${\mathrm{TiO}}_2$ samples. Moreover, there was clear grain growth inhibition in the samples containing Nb. The use of high resolution transmission electron microscopy with our samples provides an innovative perspective compared with previous research on this issue. Our analysis shows that niobium is segregated from the anatase structure before and during the phase transformation, leading to the formation of NbO nanoclusters on the surface of the ${\mathrm{TiO}}_2$ rutile nanoparticles.

This work reports the in-depth resolved Raman scattering analysis with different excitation wavelengths of ${\text{Cu}}_2{\text{ZnSnS}}_4$ layers. Secondary phases constitute a central problem in this material, particularly since they cannot be distinguished by x-ray diffraction. Raman spectra measured with 325 nm excitation light after sputtering the layers to different depths show peaks that are not detectable by excitation in the visible. These are identified with ${\text{Cu}}_3{\text{SnS}}_4$ modes at the surface region while spectra measured close to the back region show peaks from ZnS and ${\text{MoS}}_2$ . Observation of ZnS is enhanced by resonant excitation conditions achieved when working with UV excitation.

Molecular beam epitaxy Ga-assisted synthesis of GaAs nanowires is demonstrated. The nucleation and growth are seen to be related to the presence of a $\mathrm{Si}{\mathrm{O}}_2$ layer previously deposited on the GaAs wafer. The interaction of the reactive gallium with the $\mathrm{Si}{\mathrm{O}}_2$ pinholes induces the formation of nanocraters, found to be the key for the nucleation of the nanowires. With $\mathrm{Si}{\mathrm{O}}_2$ thicknesses up to $30\mathrm{nm}$ , nanocraters reach the underlying substrate, resulting into a preferential growth orientation of the nanowires. Possibly related to the formation of nanocraters, we observe an incubation period of $258\mathrm{s}$ before the nanowires growth is initiated.