No abstract

We study the band valley modification induced by isotropic strain in monolayer WSe2 using the non-local van der Waals density functionals theory including the spin–orbit coupling effect. The dominant contributions of orbitals to the band extrema, spin splitting, and exciton diversity in monolayer WSe2 are visually displayed. The vertical shift of the d and p partial orbitals of W and Se atoms, respectively, at band edges under strain results in a notable reduction of the bandgap. Under tensile strain, the deformations of the band valleys lead to an additional appearance of optical excitons and the disappearance of momentum excitons. Therefore, the experimental observations of the changes in the radiation spectra such as the redshift of A and B excitons, blueshift of C and D excitons, enhancement of intensity, localization, and symmetrization of the exciton resonances can be explained thoroughly. Under compression, the band valley modification may lead to an additional appearance of momentum excitons and the disappearance of optical excitons. The compression is predicted to cause the blueshift of A and B excitons while it brings the redshift to C and D excitons. An asymmetric broadening and intensity de-enhancement of the exciton resonances are also found when a compression strain is applied. The modification of the band valleys can be explained by the enhancement/reduction of hybridization between orbitals under strain. These results offer new perspectives to comprehend the appearance/disappearance of the excitons in monolayer transition metal dichalcogenide materials upon mechanical perturbation.

In this work, we performed a density functional theory calculation to systematically investigate the adsorption and evaluate the adsorption performance of aromatic volatile organic compounds, benzene and toluene, on WSe2 monolayer. The most favourable adsorption configurations of gas molecules with the parallel orientation of the benzene ring to the substrate surface are explored by computing the binding energies as a function of spatial coordinates and carefully optimizing geometrical structures. The calculations pointed out that gas molecules could diffuse across the substrate along the diffusion paths with quite low diffusion potential barriers, about 180 meV for benzene and 130 meV for toluene molecules. We found that both gases are physisorbed on WSe2 monolayer with moderate adsorption energies, short recovery times, and large response lengths. The gas adsorption causes the bandgap reduction of 26 meV and a slight work function increase of the substrate. There is a charge transfer from the substrate onto the gas molecules, this may cause a resistance decrease of the p-type semiconductor substrate. WSe2 monolayer is quite sensitive to benzene and toluene, and could be suggested as an aromatic gas sensing material.

No abstract

Monitoring the exchanges of CO2 and energy between terrestrial ecosystems and the atmosphere is one of the approaches with high accuracy to quantify the contribution of tropical rainforest to climate. The results of the study in Nam Cat Tien forest ecosystem from 2012 to 2014 illustrate that this ecosystem plays the role of carbon sinks during 11 months in a years (over 90%) and the average uptake reaches 455,8 gC/m2. In addition, about 74% of the net radiation (155,8 W/m2) that the ecosystem obtainsis transformed into latent heat and about 23% into sensible heat thanks to the ability to restrict surface flow, fix groundwater and remain moisture at the high level.