Van der Waals torque induced by external magnetic fields

We present a method for inducing and controlling van der Waals torques between two parallel slabs using a constant magnetic field. The torque is calculated using the Barash theory of dispersive torques. In III-IV semiconductors such as $InSb$, the effect of an external magnetic field is to induce an optical anisotropy, in an otherwise isotropic material, that will in turn induce a torque. The calculations of the torque are done in the Voigt configuration, with the magnetic field parallel to the surface of the slabs. As a case study we consider a slab made of calcite and a second slab made of $InSb$. In the absence of magnetic field there is no torque. As the magnetic field increases, the optical anisotropy of $InSb$ increases and the torque becomes different from zero, increasing with the magnetic field. The resulting torque is of the same order of magnitude as that calculated using permanent anisotropic materials when the magnetic fields is close to 1 T.Comment: to appear in Journal of Applied Physic

A theoretical analysis of the role of defects in the adsorption of hydrogen sulfide on graphene

Density functional theory studies are reported to analyze the interaction between hydrogen sulfide (H2S) and graphene. The electron-ion interactions have been modeled using ultrasoft pseudopotentials and the exchange-correlation energies have been approximated by the method of the generalized gradient approximation in the parameterization of Perdew-Burke-Ernzerhof. Three graphene structures, one intrinsic and two with defects (vacancy and sustitution), and four H2S concentrations have been studied. The optimal geometries, binding energies, density of states (DOS) and charge density were obtained. In order to study the adsorption process three high symmetry sites were considered, namely, top, bridge, and center. The preferential adsorption structure corresponds to the center site in a physical way. The DOS of graphene-H2S systems shows a metallic behavior which coincides with the behavior of the isolated graphene. The geometrical structure of the graphene and the hydrogen sulfide remains unchanged

LDA approximation based analysis of the adsorption of O

Based on the density functional theory whitin the local density aproximation, we investigated the adsorption of the ozone molecule by the boron nitride sheet. To model the sheet we used a planar CnHm cluster; four high symmetry sites in the mesh were considered. A total energy calculation indicates that the boron nitride sheet remains planar and the ozone is adsorbed with an energy of 0.41 eV; the ozone reacts with the sheet forming an epoxy group and an oxygen molecule in an unstable configuration

Chitosan Coatings Modified with Nanostructured ZnO for the Preservation of Strawberries

Strawberries are highly consumed around the world; however, the post-harvest shelf life is a market challenge to mitigate. It is necessary to guarantee the taste, color, and nutritional value of the fruit for a prolonged period of time. In this work, a nanocoating based on chitosan and ZnO nanoparticles for the preservation of strawberries was developed and examined. The chitosan was obtained from residual shrimp skeletons using the chemical method, and the ZnO nanoparticles were synthesized by the close-spaced sublimation method. X-ray diffraction, scanning electron microscopy, electron dispersion analysis, transmission electron microscopy, and infrared spectroscopy were used to characterize the hybrid coating. The spaghetti-like ZnO nanoparticles presented the typical wurtzite structure, which was uniformly distributed into the chitosan matrix, as observed by the elemental mapping. Measurements of color, texture, pH, titratable acidity, humidity content, and microbiological tests were performed for the strawberries coated with the Chitosan/ZnO hybrid coating, which was uniformly impregnated on the strawberries’ surface. After eight days of storage, the fruit maintained a fresh appearance. The microbial load was reduced because of the synergistic effect between chitosan and ZnO nanoparticles. Global results confirm that coated strawberries are suitable for human consumption