${\mathrm{SiO}}_2$ films containing Si nanocrystals (nc-Si) and Er were prepared and their photoluminescence (PL) properties were studied. The samples exhibited luminescence peaks at 0.81 and 1.54 μm, which could be assigned to the electron-hole recombination in nc-Si and the intra-4$f$ transition in ${\mathrm{Er}}^{\text{3+}},$ respectively. Correlation between the intensities of the two luminescence peaks was studied as functions of Er concentration and excitation power. The present results clearly demonstrate that excitation of ${\mathrm{Er}}^{\text{3+}}$ occurs through the recombination of photogenerated carriers spatially confined in nc-Si and the subsequent energy transfer to ${\mathrm{Er}}^{\text{3+}}$ .

Photoluminescence (PL) from surface-oxidized Si nanocrystals (nc-Si) was studied as a function of the size. The size of nc-Si was comparable with or larger than the Bohr radius of free excitons in bulk Si crystal (5 nm). In contrast to smaller surface-oxidized nc-Si (typically as small as a few nanometers in diameter), these relatively large nc-Si exhibited PL properties with strong size dependence. A high-energy shift of the PL peak from the vicinity of the bulk band gap to the visible region was observed. This PL shift was accompanied by a shortening of the PL lifetime and an increase in the exchange splitting energy of excitons. These size dependences indicate that the PL originates from the recombination of excitons confined in nc-Si. The differences in the PL properties between H-terminated and surface-oxidized nc-Si are also discussed.

We report on the oxygen vacancy induced ferromagnetism (FM) at and above room temperature in undoped TiO₂ nanoporous nanoribbons synthesized by a solvothermal route. The origin of FM in as-synthesized and vacuum annealed undoped nanoribbons grown for different reaction durations followed by calcinations was investigated by several experimental tools. X-Ray diffraction pattern and micro-Raman studies reveal the TiO₂(B), TiO₂(B)-anatase, and anatase–rutile mixed phases of TiO₂ structure. Field emission scanning electron microscopy and transmission electron microscopy observations reveal nanoribbons with uniform pore distribution and nanopits/nanobricks formed on the surface. These samples exhibit strong visible photoluminescence associated with oxygen vacancies and a clear ferromagnetic hysteresis loop, both of which dramatically enhanced after vacuum annealing. Direct evidence of oxygen vacancies and related Ti³⁺ in the as-prepared and vacuum annealed TiO₂ samples are provided through X-ray photoelectron spectroscopy analysis. Micro-Raman, infrared absorption and optical absorption spectroscopic analyses further support our conclusion. The observed room temperature FM in undoped TiO₂ nanoribbons is quantitatively analyzed and explained through a model involving bound magnetic polarons (BMP), which include an electron locally trapped by an oxygen vacancy with the trapped electron occupying an orbital overlapping with the unpaired electron (3d¹) of Ti³⁺ ion. Our analysis interestingly shows that the calculated BMP concentration scales linearly with concentration of oxygen vacancies and provides a stronger footing for exploiting defect engineered ferromagnetism in undoped TiO₂ nanostructures. The development of such highly porous TiO₂ nanoribbons constitutes an important step towards realizing improved visible light photocatalytic and photovoltaic applications of this novel material.

We have succeeded in observing the size dependent photoluminescence (PL) from Ge nanocrystals (nc-Ge) with 0.9–5.3 nm in average diameter ${(d}_{\mathrm{ave}})$ in the near-infrared region. The nc-Ge were fabricated by rf cosputtering of Ge and ${\mathrm{SiO}}_2$ and post annealing at $800°\mathrm{C}.$ It was found that the sample with $d_{\mathrm{ave}}=5.3\mathrm{}\mathrm{nm}$ shows a PL peak at about 0.88 eV. With decreasing the size, the PL peak shifted to higher energies and reached 1.54 eV for the sample with $d_{\mathrm{ave}}=0.9\mathrm{}\mathrm{nm}.$ It was also found that the PL intensity increases drastically with decreasing the size. The observed strong size dependence of the PL spectra indicates that the observed PL originates from the recombination of electron-hole pairs confined in nc-Ge.