RESUMEN
In this study, we use differential phase contrast images obtained by scanning transmission electron microscopy combined with computer simulations to map the atomic electrostatic fields of MoS2 monolayers and investigate the effect of sulfur monovacancies and divancancies on the atomic electric field and total charge distribution. A significant redistribution of the electric field in the regions containing defects is observed, with a progressive decrease in the strength of the projected electric field for each sulfur atom removed from its position. The electric field strength at the sulfur monovacancy sites is reduced by approximately 50% and nearly vanishes at the divacancy sites, where it drops to around 15% of the original value, demonstrating the tendency of these defects to attract positively charged ions or particles. In addition, the absence of the sulfur atoms leads to an inversion in the polarity of the total charge distribution in these regions.
RESUMEN
This paper reports a high-resolution amplified optical sampling time-resolved spectroscopy technique for characterization of the chirp of laser diodes with no limitations on measuring chirp ranges. The chirp induced by slowly (25 Hz) and rapidly (>100 MHz) varying current waveforms was measured. Although the slowly varying case used a triangular current waveform to modulate a distributed feedback laser diode, a non-linear chirp was measured, and a chirp excursion of 71 GHz was found. For the rapidly varying case, the chirp response of three different laser diodes emitting around 1.55 µm were characterized with direct modulation square current pulses. Chirp excursions of up to 157 GHz were measured. From the measurements results, three thermal time constants, from tens of nanoseconds up to hundreds of microseconds, were obtained for each laser diode.
RESUMEN
This Engineering and Laboratory Note describes a simple, fast, cost-effective, and practical splicing technique for poly-methyl-methacrylate (PMMA) plastic optical fibers. We believe this technique can be useful for home networks, laboratory research, and educational uses. It employs a widely available, low-cost transparent adhesive combined with a suitable alignment sleeve. The fiber tips did not need to be cleaned or polished, and the polyethylene jacket did not need to be removed. The alignment between the cleaved fibers was performed by means of the jacket. Low insertion losses were measured for the main PMMA-based fiber types.
RESUMEN
The band structure and the optical conductivity of an ABA (Bernal-type) stacked graphene trilayer are calculated. It is shown that, under appropriate doping, a strong resonant peak develops in the optical conductivity, located at the frequency corresponding to approximately 1.4 times the interlayer hopping energy and caused by the 'nesting' of two nearly parabolic bands in the electronic spectrum. The intensity of this resonant absorption can be controlled by adjusting the gate voltage. The effect is robust with respect to increasing temperature.
RESUMEN
Two-dimensional crystals of semiconducting transition metal dichalcogenides absorb a large fraction of incident photons in the visible frequencies despite being atomically thin. It has been suggested that the strong absorption is due to the parallel band or 'band nesting' effect and corresponding divergence in the joint density of states. Here, we use photoluminescence excitation spectroscopy to show that the band nesting in mono- and bilayer MX2 (M=Mo, W and X=S, Se) results in excitation-dependent characteristic relaxation pathways of the photoexcited carriers. Our experimental and simulation results reveal that photoexcited electron-hole pairs in the nesting region spontaneously separate in k-space, relaxing towards immediate band extrema with opposite momentum. These effects imply that the loss of photocarriers due to direct exciton recombination is temporarily suppressed for excitation in resonance with band nesting. Our findings highlight the potential for efficient hot carrier collection using these materials as the absorbers in optoelectronic devices.
RESUMEN
Thermochromic VO2 thin films have successfully been grown on SiO2-coated float glass by reactive DC and pulsed-DC magnetron sputtering. The influence of substitutional doping of V by higher valence cations, such as W, Mo, and Nb, and respective contents on the crystal structure of VO2 is evaluated. Moreover, the effectiveness of each dopant element on the reduction of the intrinsic transition temperature and infrared modulation efficiency of VO2 is discussed. In summary, all the dopant elements--regardless of the concentration, within the studied range-- formed a solid solution with VO2, which was the only compound observed by X-ray diffractometry. Nb showed a clear detrimental effect on the crystal structure of VO2. The undoped films presented a marked thermochromic behavior, specially the one prepared by pulsed-DC sputtering. The dopants effectively decreased the transition of VO2 to the proximity of room temperature. However, the IR modulation efficiency is markedly affected as a consequence of the increased metallic character of the semiconducting phase. Tungsten proved to be the most effective element on the reduction of the semiconducting-metal transition temperature, while Mo and Nb showed similar results with the latter being detrimental to the thermochromism.
RESUMEN
In this study Mo-doped VO2 thin films have successfully been grown on SiO2-coated float glass by reactive DC magnetron sputtering. Structural analyses, carried out at room temperature, revealed single-phase monoclinic VO2 with preferred crystal orientation in the (002) direction, regardless the Mo doping concentration. The surface morphology of the films revealed elongated grains dispersed randomly with decreasing sizes for higher Mo doping concentrations. The measured roughness parameters showed an opposite tendency, i.e., increased for higher Mo doping contents. The optical analyses showed films with maximum optical transparencies in the visible range from 35 to 45% and decreased IR modulation capacity from 36 to 25% with increasing Mo content from 3 to 11%. The Mo dopant concentration in the film was found to have a linear influence on the phase transition temperatures, showing a decrease of about 3 degrees C per at% of Mo. Phase transition temperatures as low as 32 degrees C were obtained.
RESUMEN
A incontinencia urinaria de esforco (IUE) constitui uma afeccao ginecologica que encerra complexidade, nao so quanto ao diagnostico como tambem em relacao ao tratamento. Os recursos propedeuticos baseiam-se principalmente em dados de anamnese, exame ginecologico, teste do cotonete, uretrocistografia miccional e teste urodinamico. Mais recentemente, a ultra-sonografia (USG) se tem mostrado como metodo de inegavel valor para o estudo da mobilidade do colo vesical. O objetivo deste trabalho foi avaliar a mobilidade do colo vesical atraves da USG por via vaginal. A casuistica foi constituida por dois grupos: 35 mulheres incontinentes e 22 continentes. O diagnostico da IUE baseou-se em: queixa clinica,exame ginecologico e teste urodinamico. Atraves da USG,por via vaginal foi avaliado o deslocamento do colo vesical nos dois grupos. Os resultados mostraram que o colo vesical na mulher incontinente se encontra no repouso mais proximo da sinfise publica e ao esforco mais distante, quando comparado com o grupo continente. Em relacao ao deslocamento, os autores observaram que nas mulheres incontinentes e superior a 10m, com uma sensibilidade aproximadamente de 70 por cento. Concluem que este metodo nao invasivo pode ser empregado na propedeutica da IUE.
