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In this work we show the feasibility of producing silicon nanocrystals by means of a new method based on the oxi-reduction of silicon dioxide induced by the presence of an impurity and annealing. The choice of magnesium as the impurity relies on its chemical properties of oxireducing the SiO2 matrix while avoiding the formation of Si-based compounds. The samples were obtained by 3×1016 and 1×1017 at/cm2 ion implantation into fused silicon dioxide followed by annealing in vacuum at 900 °C for 2 or 10 h. Rutherford backscattering spectrometry (RBS) characterized the chemical content and the Mg depth distribution. In all cases, photoluminescence measurements that showed a broad band starting around 1.8 eV with increasing emission intensity for lower energies, indicating the presence of Si nanocrystals. The analysis of the photoluminescence data in the framework of the quantum confinement theory suggests the existence of relatively large Si nanocrystals. The presence of these nanocrystals was also confirmed by Raman spectroscopy.
A study of the synthesis of Co nanodots by ion implantation was carried out. Silica was implanted with 35 keV Co+ ion beams with doses of 8×1015, 3×1016 and 1×1017 at/cm2 and transmission electron microscopy revealed the presence of spherical nanodots in these samples. Annealing in vacuum at 900 oC was used to change the size distribution of the nanodots. The annealed samples presented an absorption band related to the plasmon collective excitation of the metallic nanodots that redshifted for higher Co contents. The magnetic character of the samples was revealed by magnetic force microscopy measurements that showed the presence of randomly distributed structures with defined magnetization in the case of annealed samples. This work shows the feasibility of synthesizing Co nanodots with controlled size distribution.
Amorphous carbon films were deposited onto (100) Si crystals and onto ultra-pure Al foils by dc-magnetron sputtering with different Ar plasma pressures, from 0.17 to 1.4 Pa. We investigate the voids structure and the voids density in these films by means of small angle x-ray scattering (SAXS) and mass spectrometry of effused gases. The analysis of the effusion spectra provided clear evidence that films deposited at lower pressures are compact, while the films deposited at higher pressure present a more open structural arrangement, confirming density results obtained by using ion beam techniques. SAXS results reveal that the fraction of open volumes increases with the plasma pressure: a direct correlation between film density and open volume fraction is found. These different film microstructures could be explained by the existence of different bombarding regimes during film growth
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