For years bismuth (Bi) has appealed to a broad community of scientists due to its peculiar electronic, optical, and more recently plasmonic and photocatalytic properties, which enable both the understanding of basic science phenomena and the development of a wide range of applications. In spite of this interest, a comprehensive spectral analysis of the dielectric function (ε = ε 1 + jε 2 ) of bulk Bi from the far infrared (IR) to the ultraviolet (UV) region is not available. So far, the data have been reported in limited spectral ranges and show a wide dispersion that is especially notorious for the IR region. In this work we report ε for Bi in a wide spectral range from 0.05 to 4.7 eV (24.8 to 0.3 μm, far IR to UV). ε is extracted from spectroscopic ellipsometry measurements of excellent quality (dense and smooth) Bi films by using the transfer matrix formalism and Kramers−Kronig consistent analysis. The higher quality and accuracy of the obtained ε compared with the literature data is demonstrated. The analysis and use of this reference bulk dielectric function provides crucial information for the exploration and understanding of the optical, plasmonic, and photocatalytic properties of Bi nanostructures. From its analysis, it is evidenced that the optical properties of Bi in the mid wave IR-to-UV are driven only by interband transitions, which are responsible for the dominant absorption band peaking at about 0.8 eV. Therefore, the plasmonic behavior and the photocatalytic performance of Bi nanostructures in the visible and UV are likely driven by these interband transitions that make ε 1 turn negative in this region without the need of exciting free carriers. Furthermore, classical electrodynamic simulations using the obtained ε show a strong size dependence for the optical extinction of Bi nanospheres in the far IR-to-near IR with Mie-like resonances broadly tunable across this region.
The formation and subsequent growth of crystalline silicon nanoclusters ͑Si-ncs͒ in annealed silicon-rich silicon oxides ͑SRSOs͒ were studied by glancing angle x-ray diffraction. SRSO samples with Si concentrations ͑y͒ of 0.40, 0.42, and 0.45 were grown by inductively coupled plasma-enhanced chemical-vapor deposition ͑PECVD͒. Samples with y = 0.42 grown by electron-cyclotron-resonance PECVD were also studied. Annealing treatments were performed at temperatures ͑T͒ of 900, 1000, and 1100°C for times ͑t͒ between 0.5 and 3 h in flowing Ar. As-grown SRSO films did not present signs of Si clusters ͑amorphous or crystalline͒; however, ͑111͒, ͑220͒, and ͑311͒ Bragg peaks corresponding to c-Si were clearly seen after annealing at 900°C for the y = 0.45 sample, but only barely seen for the y = 0.42 and undetected for the y = 0.40 samples. For T = 1000°C, all studied SRSO samples clearly showed the c-Si diffraction peaks, which became narrower with increasing t and T. From the width of the Si ͑111͒ peaks, the mean size of Si-ncs and their dependence on T and t was determined. Activation energies were deduced from the T dependence by fitting the results to two growth models of Si precipitates in an a-SiO 2 matrix reported in the literature. The activation energies qualitatively agree with values deduced from transmission electron microscopy studies of annealed SRSO reported in the literature. However, they are significantly lower than Si diffusion activation energies available in the literature for SiO 2 with low excess Si. A broad feature is also observed in the x-ray diffractograms for as-grown samples with low y, which shifts to the peak position corresponding to a-SiO 2 with increasing T. This behavior is explained by the formation of a well-defined a-SiO 2 phase with increasing T, where mixed Si-O 4−n Si n ͑n = 1,2,3͒ tetrahedra in the as-grown alloy are gradually converted into Si-O 4 and Si-Si 4 as phase separation of Si and SiO 2 proceeds. From the measured Si ͑111͒ peak positions, small Si-ncs are found to be tensilely strained by as much as ϳ0.8%. This effect becomes insignificant as Si-ncs become larger with increasing y or T.
Due to the presence of strong magnetic resonances, high refractive index dielectric nanoantennnas have shown the ability to expand the methods available for controlling electromagnetic waves in the subwavelength region. In this work, we experimentally demonstrate that an asymmetric dielectric dimer made of silicon can lead to highly directional scattering depending on the excitation wavelength, due to the interference of the excited magnetic resonances. A back focal plane imaging system combined with a prism coupling technique enables us to explore the scattering profile parallel to the substrate. The directivity of scattering along the substrate is high enough to produce selective guiding of light along the substrate. These results showing tunable control of directional scattering will encourage the realization of novel optical applications, such as optical nanocircuitry.
Cerium-doped silicon oxides with cerium concentrations of up to 0.9 at. % were deposited by electron cyclotron resonance plasma enhanced chemical vapor deposition. Bright cerium related photoluminescence, easily seen even under room lighting conditions, was observed from the films and found to be sensitive to film composition and annealing temperature. The film containing 0.9 at. % Ce subjected to anneal in N2 at 1200 °C for 3 h showed the most intense cerium-related emission, easily visible under bright room lighting conditions. This is attributed to the formation of cerium silicate [Ce2Si2O7 or Ce4.667 (SiO4)3O], the presence of which was confirmed by high resolution transmission electron microscopy.
Rare earth (Tb or Ce)-doped silicon oxides were deposited by electron cyclotron resonance plasma-enhanced chemical vapour deposition (ECR-PECVD). Silicon nanocrystals (Si-ncs) were formed in the silicon-rich films during certain annealing processes. Photoluminescence (PL) properties of the films were found to be highly dependent on the deposition parameters and annealing conditions. We propose that the presence of a novel sensitizer in the Tb-doped oxygen-rich films is responsible for the indirect excitation of the Tb emission, while in the Tb-doped silicon-rich films the Tb emission is excited by the Si-ncs through an exciton-mediated energy transfer. In the Ce-doped oxygen-rich films, an abrupt increase of the Ce emission intensity was observed after annealing at 1200∘C. This effect is tentatively attributed to the formation of Ce silicate. In the Ce-doped silicon-rich films, the Ce emission was absent at annealing temperatures lower than 1100∘C due to the strong absorption of Si-ncs. Optimal film compositions and annealing conditions for maximizing the PL intensities of the rare earths in the films have been determined. The light emissions from these films were very bright and can be easily observed even under room lighting conditions.
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