Nanocomposite Bi/TiO2 multilayer thin flms deposited by a crossed beam laser ablation confguration

Articulo relacionado con la caracterizacion de materialesA crossed beam pulsed laser deposition confguration was used to prepare nanocomposites Bi/TiO2 thin flms on two different substrates. The multilayered system was formed by depositing TiO2 and Bi layers alternately. In order to embed the Bi nanostructures in TiO2, the subsequent TiO2 layers were synthesized using a constant number of laser pulses (3000) corresponding to a thickness of approximately 21 nm. The Bi nanostructures were deposited on the TiO2 layers alternately by irradiating the Bi target with 30, 100, 200, and 300 laser pulses. In this way, the Bi nanostructures were embedded inside the TiO2 matrix. A total of 8 samples with bismuth and one reference, with TiO2 only, were produced. Transmission Electron Microscopy (TEM) showed that nearly spherical nanoparticles (NPs) were obtained at lower number of pulses, whereas at 300 pulses a quasi-percolated nanostructured Bi flm was obtained. X-Ray Photoelectron Spectroscopy (XPS) revealed that the TiO2 layers were not afected due to the bismuth presence. Raman Spectroscopy showed vibrational features characteristic of the rutile phase for the titania layer. The Raman spectrum of the multilayer prepared using 300 laser pulses on the bismuth, suggests that the Bi layer is formed by a mixture of metallic Bi, and α-Bi2O3. The Ultraviolet–Visible Spectroscopy reveals that no substantial changes are presented in the transmittance spectra indicating similar optical properties of the diferent deposits. Finally, the photoluminescence emission spectra indicate that the substrate position in the deposition chamber afects the electronic structure of the material.A. Martínez-Chávez thanks CONACyT for the scholarship granted (No. 815785). K. Esquivel and L. Escobar thank the Engineering Faculty-UAQ for the fnancial support granted through the Attention to national problems fund and the FONDEC-UAQ-2021. We greatly appreciate the collaboration of R. Basurto in performing the XPS measurements

Photocatalytic Activity under Simulated Sunlight of Bi-Modified TiO 2

The synthesis of Bi-modified TiO2 thin films, with different Bi contents, is reported. The obtained materials were characterized by energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy (RS), X-ray diffraction (XRD), photoluminescence (PL), and diffuse reflectance spectroscopy (DRS), in order to obtain information on their chemical composition, vibrational features, and optical properties, respectively. Compositional characterization reveals that the bismuth content can be varied in an easy way from 0.5 to 25.4 at. %. Raman results show that the starting material corresponds to the anatase phase of crystalline TiO2, and Bi addition promotes the formation of bismuth titanates, Bi2Ti2O7 at Bi contents of 10.4 at. % and the Bi4Ti3O12 at Bi contents of 21.5 and 25.4 at. %. Optical measurements reveal that the band gap narrows from 3.3 eV to values as low as 2.7 eV. The photocatalytic activity was tested in the degradation reaction of the Malachite Green carbinol base dye (MG) as a model molecule under simulated sunlight, where the most relevant result is that photocatalytic formulations containing bismuth showed higher catalytic activity than pure TiO2. The higher photocatalytic activity of MG degradation of 67% reached by the photocatalytic formulation of 21.5 at. % of bismuth is attributed to the presence of the crystalline phase perovskite-type bismuth titanate, Bi4Ti3O12

Evidence for self-sputtering during pulsed laser deposition of Zn

4 pags. ; 4 figs.Thin films of Zn have been prepared by pulsed laser deposition with a KrF excimer laser (248 nm). The laser energy density (E.D.) on the target has been varied in the 1 to 5 J/cm2 range. The results show that as the E.D. increases the material distribution changes. For low E.D.(≤ 1.6 J/cm2)the maximum of the distribution is at the substrate center, for intermediate E.D. it is displaced to the side, and a clear minimum appears at the center of the substrate for the higher E.D.(≥4.5 J/cm2). The growth velocity at the center of the substrate reaches a maximum value for E.D. of 2.8 J/cm2, and decreases for higher E.D. as a result of the competition between deposition and self-sputtering. Virtually a zero growth velocity is obtained for E.D. above 4.5 J/cm2. The self-sputtering process is most likely responsible for the increase of the film surface roughness as a function of the laser E.D. The low cohesive energy formetal Zn, compared to other metals (Fe, Ag, Cu) is correlated with the high efficiency of the self-sputtering for this material.This work was partially supported by MCYT (Spain) under TIC2002-03235 project and by Consejo Nacional de Ciencia y Tecnologia de M´exico (CONACYT). J. G. H. acknowledges a scholarship from the I3P program from the CSIC.Peer reviewe

Effect of gas pressure on the growth of selenium thin films by pulsed laser deposition

The effect of an inert gas pressure on the structure of selenium thin films has been systematically investigated in the pulsed laser deposition process. The ablated material is deposited on Au (111) gold thin films for its characterization by atomic force microscopy (AFM). Analysis of the surface morphology shows that instead of the formation of a uniform Se thin film on top of Au (111) terraces, as it occurs in high vacuum, the film grows as two dimensional ellipsoid shaped aggregates. The size of these Se aggregates increases significantly with the gas pressure and reaches a maximum at pressures of similar to1.5 Torr, and subsequently decreases with further increase of the gas pressure. This effect is probably due to the fact that the kinetic energy of the impinging species decreases as pressure increases, thus impeding diffusion on the substrate surface. However, further increase in the pressure prevents the Se species from being deposited on the substrate resulting in a decrease in size of the aggregates

Potential of bismuth nanoparticles embedded in a glass matrix for spectral-selective thermo-optical devices

© 2014 AIP Publishing LLC. The optical transmission at a fixed visible wavelength of Bi nanoparticles embedded in a dielectric is known to show a sharp hysteretic evolution as a function of the temperature due to the reversible melting-solidification of the nanoparticles. In this work, we explore the temperature-dependent optical response of Bi nanoparticles embedded in a doped germanate glass (GeO2-Al2O3-Na2O) in a broad range from the visible to the near infrared. The transmission contrast induced by melting of the nanoparticles is shown to be strongly wavelength-dependent and evolves from positive to negative as the wavelength increases. This behaviour is well modelled using effective medium calculations, assuming that the nanoparticles size, shape, and distribution are unmodified upon melting, while their dielectric function turns from that of solid Bi to that of liquid Bi thus modifying markedly their optical response. These results open a route to the spectral tailoring of the thermo-optical response of Bi nanoparticles-based materials, which can be profitable for the engineering of wavelength-selective thermo-optical modulators and filters with optimized amplitude of modulation and wavelength dependence.This work has been partially supported by FP7-NMP-2010-Eu-Mexico Grant Agreement No. 263878: Functionalities of Bismuth based Nanostructures (BisNano) and by from the Spanish Ministry of Economy and Competitiveness under Project No. TEC2012-38901-C02-01. J.T. acknowledges a Juan de la Cierva Grant No. JCI-2009–05098.Peer Reviewe

Anharmonic effects in Mg2X (X = Si, Ge, Sn) compounds studied by Raman spectroscopy

Measurements of the first-order Raman scattering of Mg2X (X = Si, Ge, Sn) as a function of temperature and pressure are presented. The volume contribution (implicit effect) to the frequency shift with temperature is computed and compared to the lattice-anharmonicity contribution (explicit effect). The latter contribution turns out to be larger than the former. The values of the implicit fraction (ratio of the volume to the total effect) for all three materials are found to be smaller than 0.5 and to depend on temperature. These findings are indicative of a predominantly covalent bonding in these materials.Le spectre Raman du premier ordre des composés Mg2X (X = Si, Ge, Sn) a été mesuré en fonction de la température et de la pression. L'effet de la dilatation (effet implicite) sur le déplacement en fréquence induit par la température est calculé et comparé à l'effet de l'anharmonicité du réseau (effet explicite). Il est observé que l'effet explicite est plus important que l'implicite. Les valeurs de la fraction implicite (rapport de l'effet de dilatation à l'effet total) sont inférieures à 0,5 pour les trois composés et dépendent de la température. Ces valeurs sont consistantes avec le caractère covalent prédominant dans ces matériaux

Size effects investigated by Raman spectroscopy in Bi nanocrystals

Bismuth nanocrystals, embedded in amorphous germanium thin films and synthesized by pulsed-laser deposition, are investigated by Raman spectroscopy. Raman spectra are strongly dependent upon the size of the deposited nanocrystals. For the larger nanocrystals, two peaks corresponding to the optical phonons A1g and Eg can be observed. As the size of the particles decreases, the Eg mode shifts to higher frequencies while the A1g mode shifts to lower frequencies. Such behavior is a result of the curvature of the respective phonon-dispersion curves in Bi. A third size-dependent low-frequency mode has also been observed and attributed to acoustic phonons confined in Bi nanocrystals. ©1999 The American Physical Society.The present work has been partially supported by the Consejo Nacional de Ciencia y Tecnologı´a of Me´xico and the Consejo Superior de Investigaciones Cientı´ficas of Spain through the agreement CONACYT-CSIC for travel support.Peer Reviewe