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We have obtained cathodoluminescence (CL) emission from porous silicon (PSi) films by excitation with electrons with kinetic energies below 2 keV. We observed two types of CL emissions, a stable one and a non-stable one. The stable emission is characterized by a CL peak that is red shifted with respect to its photoluminescence (PL) peak. The other CL emission is the most promising in terms of its application to field emission display technology. We were able to tune this CL emission by controlling the average size of the nanoparticles. This size dependence shows that the mechanism for this type of CL emission can be associated to the quantum confinement and surface chemistry effects that are known to exist in the porous silicon system. To further prove this point we were able to correlate this CL emission with its PL counterpart. In addition, we present our preliminary results for the beginning of a proof-of-concept for a field emission display using carbon nanofibers as the electron emitter component and a “PSi paste” as the display component. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The structural bonding in a series of nanocrystalline carbon thin films grown under different substrate biasing conditions was analyzed using Raman spectroscopy in both the visible and ultraviolet (uv) regimes of the spectrum. The nanocrystalline carbon thin films studied were deposited by the hot-filament chemical vapor deposition (HFCVD) technique using a 2% concentration of methane in hydrogen. The films were deposited on molybdenum substrates under various substrate biasing conditions. A positive bias (forward) produced a continuous flow of electrons from the filament on to the substrate, whereas a negative bias (negative) caused the substrate to be bombarded with positive ions. Films were also grown under no bias, for comparison. Differences in the Raman spectra obtained with visible (λL = 514.5 nm) and uv (λL = 244.0 nm) excitation sources were investigated. Apart from the basic features consisting of D and G bands at around 1360 and 1580 cm−1 in the case of visible Raman spectra, an extra feature at around 1060 cm−1, denoted a T band, appeared in the uv Raman spectra only. Hence, uv Raman scattering measurements clearly revealed the presence of sp3-bonded carbon atoms. The position and its intensity ratio with respect to the G peak [(I(T)/I(G)] were used to provide a reliable means to measure the sp3 C bonding fraction, which is both semi-quantitative and non-destructive. The sp3 content estimated within the microstructure of n-C thin films was found to be around 60–80%. Further, probing the samples with two different photons (visible and uv) allowed us to estimate qualitatively the amount and clustering of sp2 sites. This technique provided a fast and reliable microstructural characterization of disordered carbons. The dispersion in the Raman features is specific to each carbon system and, therefore, can be used as a fingerprint. These findings point at the similarities of bias-assisted HFCVD nanocrystalline carbon (n-C) materials and the tetrahedrally bonded amorphous carbon (ta-C) materials grown by ion-beam assisted deposition (IBAD). Copyright © 2003 John Wiley & Sons, Ltd.

Oxygenated CdTe films prepared by r.f. sputtering have been shown to have band gap energies between 1.48 and 3.35 eV, depending on the amount of oxygen incorporated in the CdTe matrix. Samples with oxygen concentrations above 7 at.% are amorphous, whereas those with oxygen concentrations below 7 at.% are polycrystalline, as determined by x-ray and Raman measurements. The Raman spectra of the polycrystalline samples show that there is ca. 4 at.% Te in the form of inclusions under high pressure, which is relatively easy to detect owing to the enhanced Raman cross-section of Te. The Raman spectra of the amorphous samples is dominated by a single broad band at 159 cm −1. The fact that tellurates in general show a similar band, in spite of their different vibrational density of states, indicates that it corresponds to TeTe bonds in the amorphous matrix.

Temperature-dependent first-order Raman scattering studies in the range 30–290 K were carried out in LiCsSO4 in order to investigate its phonon spectra in relation to the phase transitions in the crystal. Complete spectra were obtained for the z(xx)y and z(xz)y geometries. A careful analysis of the spectra shows that both the low- and high-energy Raman modes exhibit variations either in frequency or intensity or both with temperature of the crystal, indicating that the D2k normal phase undergoes a transition at the Ti (202 K) and one to the commensurate phase at ca 160 K (Tc). Below the Tc, in the C2ν phase, the lattice mode at 44 cm−1 was observed to be a soft phonon with its frequency decreasing to 32 cm−1 at 30 K, indicating the possibility of a new structural phase transition of unlocking type in the crystal at a temperature Tu below 20 K. Another observation is the oscillating behaviour of Rayleigh scattering near the phase transition at the Ti.

Polarized first-order Raman spectra of BeO crystal were recorded at various temperatures to study the variations in frequencies and intensities associated with some of the phonons. At 35 K five modes were assigned to be fundamentals at 338.1 682.6, 726.7 and 1085.8 cm−1. The different scattering geometries were utilized to study the directional dependence of the dipolar phonons. Two peaks at 703.6 and 1093.7 cm−1. were the quasi-TO and LO modes, respectively. Both the frequency and the intensity of the phonons increase with decreasing temperature. The mode at 682.6 cm−1 was examined in more detail at several temperatures.