Novel hetero-layered materials with tunable direct band gaps by sandwiching different metal disulfides and diselenides

"Although bulk hexagonal phases of layered semiconducting transition metal dichalcogenides (STMD) such as MoS2, WS2, WSe2 and MoSe2 exhibit indirect band gaps, a mono-layer of STMD possesses a direct band gap which could be used in the construction of novel optoelectronic devices, catalysts, sensors and valleytronic components. Unfortunately, the direct band gap only occurs for mono-layered STMD. We have found, using first principles calculations, that by alternating individual layers of different STMD (MoS2, WS2, WSe2 and MoSe2) with particular stackings, it is possible to generate direct band gap bi-layers ranging from 0.79 eV to 1.157 eV. Interestingly, in this direct band gap, electrons and holes are physically separated and localized in different layers. We foresee that the alternation of different STMD would result in the fabrication of materials with unprecedented optical and physico-chemical properties that would need further experimental and theoretical investigations.

Theoretical characterization of several models of nanoporous carbon

"Elastic, electronic and vibrational properties of seven models of nanoporous carbon are reported. The studied structures are periodic graphitic arrangements with heptagonal and octagonal rings of carbon, known as Schwarzites. The calculations were performed within a non-orthogonal tight binding framework which has been shown to be reliable for diamond, graphene layers, fullerenes and carbon nanotubes. In contrast with previous studies, each structure was properly relaxed, so that differences between each model must be assigned to intrinsic properties rather than to differences in their construction. Thermodynamic properties were calculated from the vibrational density of states.

Design of graphene electronic devices using nanoribbons of different widths

"We present a simple design of a field effect transistor based on graphene nanoribbons, taking advantage of the metallic and semiconductor nature of nanoribbons with different widths. Such device could be constructed by using lithography techniques. The conductance of the proposed device is obtained by using the Kubo formula, assuming a strong damping due to the substrate and imperfections of the lattice. By removing the control electrodes, the design could also be used as an electrical resistance.

Mejora de un Sistema de Gestión Logística para la reducción de los costos en la Empresa EYSM Ingeniería SAC de Callao, 2017

El presente trabajo de investigación se titula la mejora sistema de Gestión logística para la reducción de los costos en la empresa EYSM INGENIERIA SAC de Callao, 2017. Dicha empresa se desenvuelve en el rubro construcción y mantenimiento, la cual se encarga de desarrollar proyectos de instalación de estructuras en base a acero inoxidable y mantenimiento de maquinaria electromecánica en todo el territorio nacional. El objetivo principal de la investigación es que a través de mejorar el sistema de gestión logística se reduzcan los costos logísticos relacionados a la empresa EYSM INGENIERIA SAC optimizando dicho sistema y buscar mantener constantemente el funcionamiento. En la actualidad, la empresa cuenta con un exceso de costos logísticos por una mala gestión de estos atribuyendo costos ocultos por falta herramientas de control y mejora. Se planteó desarrollar la metodología SRM (sistema de gestión de las relaciones con los proveedores) para aumentar la eficiencia en la gestión de las compras y transporte y consecuentemente reducir los costos logísticos. El estudio es explicativo-experimental, diseño cuasi-experimental, evaluaciones pre test, para diagnosticar el estado anterior de la empresa, y postest para la evaluación si hubo una reducción en los costos si se mejoró el sistema de gestión logística

Sulfur Vacancy Related Optical Transitions in Graded Alloys of MoxW1-xS2 Monolayers

Engineering the electronic bandgap is of utmost importance in diverse domains ranging from information processing and communication technology to sensing and renewable energy applications. Transition metal dichalcogenides (TMDCs) provide an ideal platform for achieving this goal through techniques including alloying, doping, and creating in-plane or out-of-plane heterostructures. Here, we report on the synthesis and characterization of atomically controlled two-dimensional graded alloy of MoxW1-xS2, wherein the center region is Mo rich and gradually transitions towards a higher concentration of W atoms at the edges. This unique alloy structure leads to a continuously tunable bandgap, ranging from 1.85 eV in the center to 1.95 eV at the edges consistent with the larger band gap of WS2 relative to MoS2. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy showed the presence of sulfur monovacancy, VS, whose concentration varied across the graded MoxW1-xS2 layer as a function of Mo content with the highest value in the Mo rich center region. Optical spectroscopy measurements supported by ab initio calculations reveal a doublet electronic state of VS, which was split due to the spin-orbit interaction, with energy levels close to the conduction band or deep in the band gap depending on whether the vacancy is surrounded by W atoms or Mo atoms. This unique electronic configuration of VS in the alloy gave rise to four spin-allowed optical transitions between the VS levels and the valence bands. Our work highlights the potential of simultaneous defect and optical engineering of novel devices based on these 2D monolayers.Comment: 65 pages, 7 figures in main text. 21 figures in supplemental dat

Observation of magnetic edge state in graphene nanoribbons

"The electronic structure and spin magnetism for few-layer-graphene nanoribbons synthesized by chemical vapor deposition have been investigated using near-edge x-ray absorption fine structure (NEXAFS) and electron-spin resonance (ESR). For the pristine sample, a prepeak was observed below the pi* peak close to the Fermi level in NEXAFS, indicating the presence of additional electronic states close to the Fermi level. The intensity of this prepeak decreased with increasing annealing temperature and disappeared after annealing above 1500 degrees C. The ESR spectra, which proved the presence of localized spins, tracked the annealing-temperature-dependent behavior of the prepeak with fidelity. The NEXAFS and ESR results jointly confirm the existence of a magnetic edge state that originates from open nanographene edges. The disappearance of the edge state after annealing at higher temperatures is explained by the decrease in the population of open edges owing to loop formation of adjacent graphene edges.