F. M. Matinaga

Raman-spectroscopic evaluation of the long-range order in Ba(B1/3′B2/3″)O3 ceramics

Ordered Ba(B1/3′B2/3″)O3 ceramics are currently used as dielectric resonators for microwave and millimeter wave technologies. The degree of ordering is generally determined by x-ray techniques. In this work, we demonstrate that Raman spectroscopy can be used to evaluate the degree of long-range order of these materials. We also show how varying the degree of order of samples allows partial assignment of the optical vibrational modes.

Raman-spectroscopic study of lanthanide trifluorides with the β-YF3 structure

We have performed polarized Raman scattering measurements on several Czochralski-grown lanthanide trifluoride (LnF3) crystals presenting the β-YF3 structure (space group: Pnma). The phonon mode assignments for the DyF3 and LuF3 spectra are given for the first time. Besides this, the Raman spectra of TbF3, ErF3 and YbF3 complete and extend previous studies on these materials. The vibrational modes of LnF3 compounds are then correlated with those of the archetype β-YF3 crystal. The totally symmetrical fundamentals present a strong dependence on the scattering geometry, similar to what is shown by molecular crystals with weakly interacting molecules. Most of the Raman features are well behaved with the lanthanide ion substitution. Although the observed modes always involve all ions, it was possible to discern modes that are dominated by the motion of either the lanthanide or the fluorine ions. The intensity of some low frequency modes has proved to be quite dependent on the orthorhombic distortion of the quasi-hexagonal structure.

Raman scattering and X-ray diffraction investigations on hydrothermal barium magnesium niobate ceramics

Abstract Ba(B′1/3B″2/3)O3 perovskites are important ceramic materials used as resonators in microwave technologies due to their adequate dielectric properties. These properties are mainly determined by the characteristics of the optical polar phonons of the material. A few spectroscopic investigations of lattice vibrations in Ba(B′1/3B″2/3)O3 ceramics were realized in highly ordered samples. In this work, we use Raman spectroscopy and X-ray diffraction to investigate hydrothermal Ba(Mg1/3Nb2/3)O3–BMN–ceramics with different order degree. Spectroscopic and structural data showed sensitivity to the sample microstructural evolution during sintering. Besides, they allowed independent determination of the long range order degree of the materials. The complete Raman mode assignment for the disordered and ordered BMN structure is also presented.

Hopping conduction and persistent photoconductivity in Cu2ZnSnS4 thin films

The temperature dependence of electrical conductivity and the photoconductivity of polycrystalline Cu2ZnSnS4 were investigated. It was found that at high temperatures the electrical conductivity was dominated by band conduction and nearest-neighbour hopping. However, at lower temperatures, both Mott variable-range hopping (VRH) and Efros?Shklovskii VRH were observed. The analysis of electrical transport showed high doping levels and a large compensation ratio, demonstrating large degree of disorder in Cu2ZnSnS4. Photoconductivity studies showed the presence of a persistent photoconductivity effect with decay time increasing with temperature, due to the presence of random local potential fluctuations in the Cu2ZnSnS4 thin film. These random local potential fluctuations cannot be attributed to grain boundaries but to the large disorder in Cu2ZnSnS4.

Raman studies on 0.4 nm diameter single wall carbon nanotubes

Abstract We performed polarized Raman scattering studies on 0.4 nm diameter single wall carbon nanotubes (SWNTs) grown inside the pores of zeolite crystals, using several different laser lines (1.92⩽ E laser ⩽2.71 eV). The strong diameter-selective resonant behavior of typical SWNTs (1 d t 2 carbons phonon density of states superimposed on the characteristic SWNT Raman features. We show that the behavior of typical SWNTs can be used qualitatively to analyze the radial breathing mode (RBM), G -band and D -band spectra of the 0.4 nm diameter SWNTs.

On three dimensional self-organization and optical properties of InAs quantum-dot multilayers

We report on experiments aimed at producing three-dimensional self-organization in InAs quantum-dot multilayers embedded in GaAs. These InAs/GaAs quantum-dot multilayers have been grown by molecular beam epitaxy. Employing atomic force microscopy, we have analyzed the island density in samples with different number of periods of InAs/GaAs bilayers The results reveals a decrease and a tendency to saturation of the island density with an increase in the number of periods, as a three-dimensional self-organization characteristic of these samples. Optical properties of the samples are examined via photoluminescence spectroscopy. The evolution of the quantum-dot photoluminescence peak position indicates an increment in the mean size of the buried islands and a relative homogenization in size of the quantum dots, as the number of periods increases. The results of the optical measurements agree with the morphological data, and characterize a spatial process of self-organization, related to the increment of the nu...

Optical phonon modes and crystal structure of NaLaF4 single crystals

Polarized Raman scattering and infrared reflectivity measurements have been used to investigate the crystal structure of Czochralski-grown NaLaF4 single crystals. The phonon symmetries, the simultaneous presence of polar modes in the infrared and Raman spectra, as well as the observation of piezoelectric resonance, helped us to identify the P6¯ group as the correct one for this crystal. This material belongs to a family of sodium lanthanide tetrafluorides (NaLnF4) crystals, whose photoluminescence efficiency is comparable to LiYF4. Therefore, NaLaF4 crystals may be important in the development of diode pumped up-conversion solid-state lasers. The number and behavior of the observed optical phonon modes were analyzed in terms of group theory predictions for the group symmetry found. A few anomalies in the phonon characteristics are discussed in terms of cationic disorder in the crystal lattice.

Resonant Raman scattering characterization of carbon nanotubes grown with different catalysts

Single-walled carbon nanotube samples produced in the presence of different combinations of metal catalysts have been studied by resonant Raman spectroscopy. The diameter distribution of different samples has been determined by analysis of the laser excitation energy dependence of the tangential modes associated with metallic nanotubes. These modes are resonantly enhanced over a narrow range of the exciting energies, which shifts for different samples. The Raman cross-section expression has been used to fit the experimental Raman excitation profiles. This procedure was used to determine the mean value and the width of the distribution of diameters within each sample.

Vibrational Spectroscopy and Electron-Phonon Interactions in Microwave-Hydrothermal Synthesized Ba(Mn1/3Nb2/3)O3 Complex Perovskites

Ba(Mn1/3Nb2/3)O3 ceramics were synthesized by the microwave-hydrothermal process. Crystalline, single-phase, needle-like materials were obtained with high anisotropy and aspect ratio. Large crystals could be prepared from a direct combination of nanosized crystals under microwaves through an oriented attachment mechanism. Infrared and Raman spectroscopies showed perfect agreement with group-theoretical approaches for a trigonal, P3m1 structure. Franck-Condon modes were identified in Raman scattering experiments conducted in different wavelengths, laser powers, and temperatures. The results showed that the Mn ions exhibit a particular role in the lattice dynamics in complex perovskites.

Optical phonon characteristics of incommensurate and commensurate modulated phases of Bi3NbO7 ceramics

Raman and infrared-reflectivity spectroscopies were used to investigate the optical phonon modes of the cubic-incommensurate and tetragonal-commensurate phases of Bi3NbO7 ceramic samples. For both structures, the observed features could be well described by applying the factor-group method to average oxygen-deficient fluorite subcells; i.e., it was possible to predict the correct number and the activities of the observed phonon modes. The spectra showed rather broad bands, which are mainly for the cubic-incommensurate phase, probably because of the disorder of the oxygen vacancies and the folding of the Brillouin zone due to the modulation of the atomic positions. This can partially explain the higher quality factor and the potential interest in the tetragonal phase of this material for applications in microwave devices.