Nitrogen In The Amorphous-germanium Network: From High Dilution To The Alloy Phase

In this work experimental data referring to the structural and optoelectronic characteristics of amorphous-germanium-nitrogen thin films are presented and discussed. The nitrogen content of the a-Ge:N samples, deposited by the rf-sputtering technique in an Ar+N2 atmosphere, was allowed to vary from typical impurity levels (less than 0.5 at. %) up to around 35 at. %. The material properties change depending on the nitrogen concentration, determined from a deuteron-induced nuclear reaction. The likely mechanisms of nitrogen incorporation into the solid phase are discussed, as well as the influence of the nitrogen content on the transport and optical properties of the films. A proportionality constant relating the total nitrogen concentration in the solid phase and the integrated absorption of the in-plane stretching vibration mode of the Ge-N dipole has been determined. It has been found that a close analogy exists between the general properties of a-Ge:N alloys and those measured in amorphous-silicon-nitrogen alloys. © 1993 The American Physical Society.4874560457

Effect of O2+, H2+ O 2+, and N2+ O2 + ion-beam irradiation on the field emission properties of carbon nanotubes

The effect of O2+, H2+ O 2+, and N2+ O2 + ion-beam irradiation of carbon nanotubes (CNTs) films on the chemical and electronic properties of the material is reported. The CNTs were grown by the chemical vapor deposition technique (CVD) on silicon TiN coated substrates previously decorated with Ni particles. The Ni decoration and TiN coating were successively deposited by ion-beam assisted deposition (IBAD) and afterwards the nanotubes were grown. The whole deposition procedure was performed in situ as well as the study of the effect of ion-beam irradiation on the CNTs by x-ray photoelectron spectroscopy (XPS). Raman scattering, field-effect emission gun scanning electron microscopy (FEG-SEM), and field emission (FE) measurements were performed ex situ. The experimental data show that: (a) the presence of either H2+ or N2 + ions in the irradiation beam determines the oxygen concentration remaining in the samples as well as the studied structural characteristics; (b) due to the experimental conditions used in the study, no morphological changes have been observed after irradiation of the CNTs; (c) the FE experiments indicate that the electron emission from the CNTs follows the Fowler-Nordheim model, and it is dependent on the oxygen concentration remaining in the samples; and (d) in association with FE results, the XPS data suggest that the formation of terminal quinone groups decreases the CNTs work function of the material. © 2011 American Institute of Physics.Fil:Escobar, M. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Goyanes, S.N. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Candal, R.J. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Alvarez, F. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Temperature-dependent optical bandgap of TiO2 under the Anatase and Rutile phases

A crucial step in developing new and/or more efficient devices relies on the detailed knowledge of the properties of their main (elemental or combined) parts. The same applies to titanium dioxide TiO2 that forms the basis of today numerous applications whose performances are determined by the presence or relative amount of the Anatase (A-) and Rutile (R-) most common phases of TiO2. Whereas the basic structural characteristics of these two polymorphs are very well-established, some of their optical aspects still deserve attention. With these ideas in mind this work presents a comprehensive investigation of the optical bandgap Egap of TiO2 under the Anatase and Rutile phases. The study considered pure A-TiO2 and R-TiO2 samples in the form of powder, and optical reflectance and Raman spectroscopy in the 83–823 K temperature range. In addition to reliable Egap values the study explores their temperature-dependent Egap(T) behavior that is expected to assist future advancements in the field of TiO2-based materials and devices (thin films, interfaces, low-dimensional systems, and photocatalytic and solar cells, for example)

Raman spectroscopy of lithium niobate (LiNbO3) − Sample temperature and laser spot size effects

The Raman spectrum of lithium niobate (LiNbO3) was investigated in the 83–823 K temperature range and as a function of different laser spot sizes. The measurements considered a Z-cut (congruent, undoped) LiNbO3 crystal, HeNe laser excitation (632.8 nm photons), and the zx,xyz¯ geometry. The main LiNbO3-related phonon modes were identified and analyzed in terms of their peak position (ω) and linewidth (γ), according to which it was possible to identify the contributions originating from anharmonic phonon-coupling and thermal lattice-expansion processes. The analyses of the temperature-dependent ωT and γT data took into account the 3- and 4-phonon model and the experimental Grüneisen and linear thermal expansion coefficients − as available from literature. Among all phonon modes of LiNbO3, the A1(TO4) mode presented a peculiar behavior (at room temperature) that depends on the laser spot size during the Raman measurements. The development of this forbidden mode has been associated with the congruent-photorefractive nature of LiNbO3 as well as with small variations in the geometry of polarization (as a result of changes in the laser spot size)