Scielo RSS <![CDATA[Revista mexicana de física]]> http://www.scielo.org.mx/rss.php?pid=0035-001X20150002&lang=es vol. 61 num. 2 lang. es <![CDATA[SciELO Logo]]> http://www.scielo.org.mx/img/en/fbpelogp.gif http://www.scielo.org.mx <![CDATA[<b>Shape transformation from silver triangular nanoprisms to nanodisks</b>: <b>Raman characterization and sculpturing mechanism</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2015000200001&lng=es&nrm=iso&tlng=es The sculpturing action of chloride ions on silver nanoplates is investigated. Recent reports show that the shape transformation from silver triangular nanoprisms to nanodisks by addition of chloride ion occurs after a threshold concentration. In this work, a chemical study of this threshold concentration is presented. There is theoretical and experimental evidence that the morphological change only depends on the chloride ion concentration and not on the Cl-/Ag molar ratio. Besides, the chloride ion etching ability is attributed to the (AgCl)° complex, which controls the morphology change through a stepwise process where a silver atom is removed from the (110) plane, and is subsequently deposited on the (111) plane (i.e. from the nanoplate vertex to the basal plane). The threshold chloride ion concentration in the shape transformation coincides with the point where Ag+ ion and (AgCl)° concentrations are the same. On the other hand, the quantitative formation of AgCl-2 avoids the stepwise process, and then the colloidal system is destabilized. Furthermore, the analysis of the Raman spectra supports the transformation mechanisms. <![CDATA[<b>Optical, structural and morphological properties of CdS-CdCO<sub>3</sub></b> <b>films</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2015000200002&lng=es&nrm=iso&tlng=es CdS-CdCO3 thin films were grown by chemical bath deposition. Different constant deposition temperatures were employed in the range of 20-80°C. From X-ray Diffraction (XRD) results can be observed that intensity of CdS peak is abruptly reduced when deposition temperature is decreased. By SEM images the formation and change in shape and size of crystallites can be observed as temperature is decreased. The forbidden energy band gap was 2.4-4.1 eV, determined from optical absorption. The formation of products was further confirmed with FTIR studies. <![CDATA[<b>Atomic displacements effects on the electronic properties of Bi<sub>2</sub>Sr<sub>2</sub>Ca<sub>2</sub>Cu<sub>3</sub>O<sub>10</sub></b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2015000200003&lng=es&nrm=iso&tlng=es The displacements effects of the oxygen atom associated to the Sr-plane (O3) in the electronic properties of Bi2Sr2Ca2Cu3O10 (Bi-2223), have been investigated using density functional theory. We determined intervals of the O3 atomic positions for which the band structure calculations show that the Bi-O bands, around the high symmetry point M‾ in the irreducible Brillouin zone, emerge towards higher energies avoiding its contribution at Fermi level, as experimentally has been reported. This procedure does not introduce foreign doping elements into the calculation. Our calculations present a good agreement with the angle-resolved photoemission spectroscopy (ARPES) and nuclear magnetic resonance (NMR) experiments. The two options found differ in character (metallic or nonmetallic) of the Bi-O plane. There are not any experiments, to the best of our knowledge, which determine this character for Bi-2223. <![CDATA[<b>Estudio computacional de las energías de interacción de dímeros</b> <b><i>cis-trans</i> y <i>trans-trans</i> del ácido fórmico</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2015000200004&lng=es&nrm=iso&tlng=es En este trabajo, se construyeron las superficies de energía potencial (PES, por sus siglas en inglés) de dímeros cis-trans y trans-trans del ácido fórmico a partir de una versión modificada del método annealing simulado, además, las geometrías de equilibrio, energías de interacción, y energías de deslocalización, fueron calculadas en el nivel de teoría MP2/6-311++G(3df,2p). Las energías de interacción calculadas tuvieron desviaciones de hasta 38.0% al no considerar el error por superposición de bases (BSSE, por sus siglas en inglés) y la energía del punto cero (ZPE, por sus siglas en inglés). También, se calcularon las energías de deslocalización correspondientes a la interacción entre el donador y el aceptador de electrones de los enlaces de hidrogeno, siendo -64.4 kcal mol-1 el valor de la mínima energía de deslocalización para el dímero más estable. En general, se observe) que las energías de deslocalización disminuyeron con el aumento de la longitud de los enlaces de hidrógeno, y en particular, se encontró una correlación razonable para ln[E(2)] como una función exponencial de la longitud de los enlaces de hidrogeno. Adicionalmente, se encontró una buena correlación entre las energías de interacción y las energías de deslocalización: altos valores de E(2)tot correspondieron con las energías de interacción más exergónicas. Lo anterior, permitió aproximar las energías de interacción como función de las energías de deslocalización de los enlaces de hidrógeno en dímeros de ácido fórmico.<hr/>In this work, potential energy surface (PES) of cis-trans and trans-trans formic acid dimers has been sampled using a modified version of annealing simulated method, and the geometries, interaction energies and delocalization energies were calculated at MP2/6-311++G(3df,2p) level of theory. In our findings, the interaction energy had deviations of up to 38.0% if basis set superposition error (BSSE) and zero-point energy (ZPE) are not taken in account. Also, for the hydrogen bond formation, we calculated delocalization energy of proton donor - proton acceptor interaction, being -64.4 kcal mol-1 the minimum value for delocalization energy of the most stable dimer. Generally, delocalization energies diminished with increasing of hydrogen bond length, and particularly, a reasonable correlation coefficient was found for ln[E(2)] as a exponential function of the hydrogen bond length. In addition, we have found a good correlation between interaction energies and delocalization energies: high values of E(2)tot correspond to the most exergonic interaction energies. This finding allowed approximate the interaction energy as function of total delocalization energy of hydrogen bonds in formic acid dimers. <![CDATA[<b>Microestructura y propiedades eléctricas de bismuto y óxido de bismuto depositados por magnetrón sputtering UBM</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2015000200005&lng=es&nrm=iso&tlng=es En este trabajo, se fabricaron películas delgadas de Bismuto (Bi) y (Oxido de Bismuto (Bi2O3) a temperatura ambiente mediante la técnica de Sputtering con Magnetron Desbalanceado (UBM - UnBalance Magnetron) sobre sustratos de vidrio. Las propiedades microestructurales y eléctricas de las muestras fueron estudiadas mediante difracción de rayos X (XRD) y el método para Medición de Propiedades Físicas - PPMS (Physical Property Measurement System), respectivamente. La resistividad a oscuras del material fue medida para un rango de temperaturas entre 100 y 400 K. A partir de las medidas de XRD se observe) el carácter policristalino del Bi asociado a la presencia de fases por encima del pico principal, 2θ=26.42° y un crecimiento gobernado por una estructura Romboédrica. Los parámetros cristalinos fueron obtenidos para ambos compuestos de Bi y Bi2O3. A partir de los análisis de los espectros de la conductividad en función de la temperatura se estableció que el mecanismo de transporte que gobierna la región de altas temperaturas (T>300 K) es el de portadores térmicamente activados. A partir de las medidas de conductividad se encontró que las energías de activación para el Bi2O3 y Bi fueron de 0.0094 y 0.015 eV, respectivamente.<hr/>In this work, bismuth (Bi) and bismuth oxide (Bi2O3) thin films were prepared, at room temperature, by Sputtering Unbalanced Magnetron (UBM - Unbalance Magnetron) technique under glass substrates. Microstructural and electrical properties of the samples were studied by X-ray diffraction (XRD) and System for Measuring Physical Properties - PPMS (Physical Property Measurement System). Dark resistivity of the material was measured for a temperature range between 100 and 400 K. From the XRD measurements it was observed a polycrystalline character of the Bi associated to the presence of phases above the main peak, 2θ = 26.42° and a growth governed by a rhombohedral structure. Crystal parameters were obtained for both compounds, Bi and Bi2O3. From the analysis of the spectra of the conductivity as a function of temperature, it was established that the transport mechanism that governs the region of high temperature (T>300 K ) is thermally activated carriers. From conductivity measurements the activation energies were obtained of 0.0094 eV and 0.015 eV for Bi2O3 and Bi, respectively. <![CDATA[<b>Rod-like fluorescent halloysite nanotubes-silica composites</b>: <b>a novel colloidal system</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2015000200006&lng=es&nrm=iso&tlng=es An inexpensive novel method to produce a rod-like fluorescent colloidal system is presented in this work. The system consists of core-shell particles synthesized by using halloysite nanotubes as cores, and silica layers as shells on their surfaces. Unlike traditional protocols for producing silica core-shell particles, the method used in this work does not require a coupling agent. In addition, a fluorescent dye was incorporated to produce fluorescent composites. Due to their resulting morphology and the possibility of incorporating a fluorescent dye, they constitute a promising new colloidal model system for studying the physics of anisotropic colloidal suspensions experimentally, as well as a potential source of new materials. <![CDATA[<b>Thin-film transistors based on zinc oxide films by ultrasonic spray pyrolysis</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2015000200007&lng=es&nrm=iso&tlng=es The application of Zinc Oxide (ZnO) films by ultrasonic spray pyrolysis at 250, 300 and 450°C as active layer in thin-film Transistors (TFTs) is presented. The performance of the devices shows an unexpected behavior in function of the deposition temperature. The ZnO films were deposited from 0.2 M precursor solution of Zinc acetate in methanol, using air as carrier gas. 70nm-thick ZnO was deposited over 100 nm-thick aluminum electrodes patterned on 50 nm-thick thermally grown SiO2 on highly doped Si wafers. The highly doped Si wafer was used as the gate electrode. The ZnO TFTs at 250°C showed field-effect mobilities around of 0.05 cm²/Vs and threshold voltages of 8 V. <![CDATA[<b>Properties of Er<sub>2</sub>O<sub>3</sub> nanoparticles synthesized by a modified</b> <b>co-precipitation method</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2015000200008&lng=es&nrm=iso&tlng=es Er2 O3 nanoparticles were synthesized by co-precipitation with the addition of ascorbate as stabilizing agent. The nanoparticles had spherical shapes with a mean diameter of 32 nm and were allocated in clusters, as determined by XRD, AFM, and optical microscopy. Characteristic green and red emissions from Er³+ were recorded by pumping the nanoparticles at 525 nm, 805 nm, and 975 nm. However, the luminescence spectra show an enhancement of red emission for NIR pump wavelengths. We proposed this behavior was due to phonon-assisted depopulation mechanisms and energy transfer processes related to the different excitation schemes. <![CDATA[<b>Transmission and escape in finite superlattices with Gaussian modulation</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2015000200009&lng=es&nrm=iso&tlng=es We study the transmission and escape energies dependent as a function of the electron energy in superlattices where the barriers height is modulated by a Gaussian function and they are compared with those produced by regular superlattices where all the barriers have the same height. We use for the calculations the effective mass approximation using the transfer matrix formalism. For Gaussian systems with 7 and 9 barriers, the transmission coefficient has passbands with almost perfect transmission. The escape energies E = Er+ iΓ are situated near these transparency bands but they do not coincide with them and they can be far from the passbands. Er is the electron energy and Γ describe the width of the states. For these systems the escape states are very wide. In the case of regular systems there are transmission bands which present only resonance peaks with unit value. The escape states are narrow and coincide with these resonances much better than in the case of Gaussian superlattices but the coincidence is not perfect. For 3 barriers where the height of the lateral barriers is reduced gradually, the resonances transform to transparency bands and the width of the escape energies increases. Although there is no coincidence, we associate the increase of width of the escape energies with the formation of transparency bands.<hr/>Estudiamos la transmisión y energías de escape como función de la energía en superredes donde las alturas de las barreras están moduladas por una función gaussiana y son comparados con los producidos por una superred regular donde todas las alturas de las barreras es la misma. Para los cálculos utilizamos la aproximación de masa efectiva usando el formalismo de matrices de transferencia. Para sistemas gaussianos con 7 y 9 barreras, el coeficiente de transmisión tiene bandas de paso con transmisión casi perfecta. La energías de escape E = Er+ iΓ están situadas cerca de las bandas de paso. Er es la energía del electrón y Γ describe el ancho de los estados. Para estos sistemas los estados de escape son amplios. En el caso de los sistemas regulares existen bandas de transmisión que solo presentan picos de resonancia con valor unitario. Los estados de escape son estrechos y coinciden con las resonancias mucho mejor que en el caso de superredes gaussianas pero la coincidencia no es perfecta. Para 3 barreras donde la altura de las barreras laterales se reduce gradualmente, las resonancias se convierten a las bandas de transparencia y la anchura de las energías de escape aumenta. Aunque no hay una coincidencia, asociamos el aumento del ancho de las energías de escape con la formación de bandas de transparencia. <![CDATA[<b>Study of dynamical properties in</b> <b><i>β</i></b><b>-TCP/CH layers</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2015000200010&lng=es&nrm=iso&tlng=es β-Tricalcium phosphate/Chitosan (β-TCP/Ch) coatings were deposited on 316L stainless steel (316L SS) substrates by a cathodic electro-deposition technique at different coating compositions. The crystal lattice arrangements were analyzed by X-Ray diffraction (XRD), and the results indicated that the crystallographic structure of β-TCP was affected by the inclusion of the chitosan content. The changes in the surface morphology as a function ofincreasing chitosan in the coatings via scanning electron microscopy (SEM) and atomic force microscopy (AFM) showed the root-mean squares hardness of the β-TCP/Ch coatings decreased by further increasing chitosan percentage. The elastic-plastic characteristics of the coatings were determined by conducting nanoindentation test, indicating that the increase if chitosan percentage is directly related to increasing the hardness and elastic modulus of the β-TCP/Ch coatings. Tribological characterization was performed by scratch test and pin-on-disk test to analyze the changes in the surface wear the β-TCP/Ch coatings. Finally, the results indicated an improvement in the mechanical and tribological properties of the β-TCP/Ch coatings as a function of increasing of the chitosan percentage. <![CDATA[<b>Dimensional regularization with non Beta-functions</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2015000200011&lng=es&nrm=iso&tlng=es The most general method to regularize Feynman's integrals in quantum field theory is Dimensional Regularization, in which the most common way to evaluate the associated integral involves Beta functions. We present a new method to evaluate the integral through the residue theorem. We apply our method to a toy model on universal extra dimensions and show that radiative corrections changes the shift-mass between zero and Kaluza-Klein excited modes.<hr/>Regularización Dimensional suele ser el método más utilizado para regularizar integrales de Feynman, comúnmente en dicho método, las integrales asociadas implican funciones Beta. Presentamos un método para evaluar tales integrales mediante el Teorema del Residuo. El método se aplica sobre un modelo de juguete con dimensiones extras universales, observando como las correcciones radiativas envuelven un corrimiento de masa entre el modo cero y los modos de Kaluza-Klein. <![CDATA[<b>Design and construction of a gallium fixed-point blackbody at CENAM</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2015000200012&lng=es&nrm=iso&tlng=es For temperatures below silver fixed-point defined by the International Temperature Scale of 1990, a transfer radiation thermometer can be calibrated using either of two calibration schemes: a variable temperature blackbody with a standard platinum resistance thermometer as a reference, or with a set of fixed-point blackbodies. CENAM is presently working with the first scheme, and it is developing fixed-point blackbodies to have the capability to work with the second scheme too. For this purpose a gallium fixed-point blackbody to calibrate CENAM's transfer radiation thermometer was designed and constructed. The blackbody cavity has a cylindro-cone shape with effective emissivity equal to 0.9992 ± 0.0004 in the 8 μm to 14 μm wavelength range. The radiance temperature of the gallium fixed-point blackbody was estimated to have an expanded uncertainty of 54 mK, with a coverage factor k = 2. <![CDATA[<b>Improved method for extracting the equivalent circuit elements of a CRLH-TL unit cell</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2015000200013&lng=es&nrm=iso&tlng=es This paper presents an improved method to extract the parameters of an equivalent circuit model of a CRLH-TL unit cell. This method is based on the analysis of the unit cell equivalent circuit admittance and impedance frequency dependence. This procedure is validated using both, electromagnetic simulation data, and measured scattering parameters. The proposed methodology is easy, and has a better performance than the existing method.