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Thin films of SnO 2 prepared by pulsed-laser deposition on R-cut sapphire substrates exhibit ferromagnetic properties at room temperature when they are doped with Cr, Mn, Fe, Co, or Ni, but not with other 3d cations. Extrapolated Curie temperatures are generally in excess of 500 K. The moment of the films is roughly independent of doping level, from 0.1-15 at. %, with a value per unit substrate area of 200± 100 B nm −2 . When magnetization is expressed as a moment per 3d dopant ion, it varies from more than the spin-only value at low concentrations to less than 0.2 B /ion near the percolation threshold. Greatest values are found for iron. The magnetization of the films is highly anisotropic with values when the field is applied perpendicular to the substrate more than double the in-plane values. There is little hysteresis except at high doping levels. The oxides are degenerate n-type semiconductors with a Hall mobility of 100 cm 2 V −1 s −1 and 1.4ϫ 10 19 carriers cm −3 in a one-band model, but no anomalous Hall effect or magnetoresistance was observed at room temperature. The data are discussed in relation to ͑a͒ the donor impurity-band model of ferromagnetism in semiconductors and ͑b͒ the magnetic defect model.

In this paper the formation and annihilation of defects produced in Si nanocrystals ͑nc͒ by ion-beam irradiation are investigated in detail. The luminescence properties of Si nanocrystals embedded in a SiO 2 matrix were used as a probe of the damaging effects generated by high-energy ion-beam irradiation. Samples have been irradiated with 2 MeV He ϩ , Si ϩ , Ge ϩ , and Au ϩ ions at different doses, in the range between 1ϫ10 9 /cm 2 and 1ϫ10 16 /cm 2 . With increasing the ion dose, the nc-related photoluminescence ͑PL͒ strongly decreases after a critical dose value, which depends on the ion mass. We have observed that the luminescence drop is accompanied by a concomitant lifetime quenching that marks the rise of new nonradiative phenomena, related to the damage left over by the ion beam. It is shown that the lifetime quenching alone cannot quantitatively explain the much stronger PL drop, but the total number of emitting centers has to diminish too. By assuming that a Si nc is damaged when it contains at least one defect inside its volume, we developed a model that relates the fraction of quenched nc to the total defect concentration in the film and to the value of the nc volume itself. This model is shown to be in good agreement with the experimental value of the quenched fraction of Si nc extracted from the luminescence and lifetime measurements. Moreover, we studied the recovery of the damaged Si nc by performing both isochronal and isothermal annealings. It is demonstrated that in slightly damaged Si nc a large variety of defects characterized by activation energies between 1 and 3 eV exists. On the contrary, the recovery of the PL properties of completely amorphized Si nc is characterized by a single activation energy, whose value is 3.4 eV. Actually, this energy is associated with the transition between the amorphous and the crystalline phases of each Si grain. The recrystallization kinetics of Si nanostructures is demonstrated to be very different from that of a bulk system. These data are presented and explained on the basis of the large surface/volume ratio characterizing low-dimensional Si structures.

The characteristics of Si nanocrystals (nc) and Er-doped Si nc embedded within Si/SiO2 Fabry–Perot microcavities are investigated. It is shown that very narrow (Δλ∼1.5 nm) and intense luminescence peaks can be obtained within the 600–1000 nm wavelength range for Si nc and at around 1.54 μm for Er-doped Si nc by properly varying the cavity resonance. The luminescence intensity of the on-axis emission is over an order of magnitude above that of similar samples without a cavity and the overall luminescence is confined within a 30° cone from the sample normal. The properties of these cavities are presented and the implications of the results discussed.

ZnO nanostructures were synthesized by a proteic sol-gel method, using zinc nitrate hexahydrate and gelatin as precursors. Size and shape evolution of ZnO nanostructures were achieved by annealing temperature in the range 250-1000 ºC. The crystalline structure, morphology and optical properties of the ZnO nanoparticles were characterized by X-Ray Diffraction (XRD), Raman Spectroscopy (RS), Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and room temperature Photoluminescence (PL). The result of structural characterization shows the formation of platelets and nanorods in the micro-scale and ZnO nanostructures with high quality hexagonal wurtzite crystal. Sharp peaks in RS after annealing temperature, related to wurtzite structure, were observed corroborating with XRD and TEM measurements. Room temperature PL spectra showed two contribution bands which peaked at ~380 nm, originating from the recombination of free excitons, and ~520 nm corresponding to the impurities and structural defects, like oxygen vacancies and zinc interstitial. The effects of annealing temperature in the structural and optical properties are detailed and the results compared among the experimental techniques. The high quality of the samples obtained by an alternative organic precursor method opens a low-cost route to technological applications of zinc oxide.