Optical spectra and exciton radiative lifetimes in bulk transition metal dichalcogenides.

The optical response of layered transition metal dichalcogenides (TMDCs) exhibits remarkable excitonic properties which are important from both fundamental and device application viewpoints. One of these phenomena is the observation of intralayer/interlayer excitons. While much effort has been done to characterize excitons in monolayer TMDCs and their heterostructures, a quite limited number of works have addressed the exciton spectra of their bulk counterparts. In this work, we employ ab initio many-body perturbation calculations to investigate the exciton dynamics and spectra of bulk 2H-MX2 (M = Mo, W, and X = S, Se). For molybdenum-based systems, we find the presence of interlayer excitons at energies higher than the first bright exciton (XA), with non-negligible strength intensity. Our results also show that interlayer excitons in tungsten-based systems are almost degenerate in energy with XA and possess very small oscillator strengths when compared with molybdenum-based systems. At room temperature, and considering the thermal exciton fine-structure population for the XA-exciton, we estimate effective radiative lifetimes in the range of ∼4-14 ns. For higher energy excitons we predict longer effective lifetimes of tens of nanoseconds.

Me-graphane: tailoring the structural and electronic properties of Me-graphene via hydrogenation.

Graphene-based materials (GBMs) are a large family of materials that have attracted great interest due to potential applications. In this work, we applied first-principles calculations based on density functional theory (DFT) and fully atomistic reactive molecular dynamics (MD) simulations to study the structural and electronic effects of hydrogenation in Me-graphene, a non-zero bandgap GBM composed of both sp2 and sp3-hybridized carbon. Our DFT results show the hydrogenation can tune the electronic properties of Me-graphene significantly. The bandgap varies from 0.64 eV to 2.81 eV in the GGA-PBE approach, passing through metallic ground-states and a narrower bandgap state depending on the hydrogen coverage. The analyses of structural properties and binding energies have shown that all carbon atoms are in sp3 hybridization in hydrogenated Me-graphene with strong and stable C-H bonds, resulting in a boat-like favorable conformation for fully-hydrogenated Me-graphene. Our MD simulations have indicated that the hydrogenation is temperature-dependent for Me-graphene, and the covalent adsorption tends to grow by islands. Those simulations also show that the most favorable site, predicted by our DFT calculations, acts as trigger adsorption for the extensive hydrogenation.

Molybdenum defect complexes in bismuth vanadate.

Monoclinic bismuth vanadate (BiVO4) is a promising n-type semiconductor for applications in sunlight-driven water splitting. Several studies have shown that its photocatalytic activity is greatly enhanced by high concentrations of Mo and W dopants. In the present work, we performed ab initio calculations to assess the most favorable relative position between pairs of Mo dopants in BiVO4. Surprisingly, we verify that the lowest energy configuration for MoV pairwise defects in BiVO4 occurs on nearest-neighbor sites, despite the higher electrostatic repulsion and larger strain on the crystal lattice. Similar results were observed for WV defect pairs in W-doped BiVO4. We show that the origin of this effect lies in a favorable hybridization between the atomic orbitals of the impurities that is only verified when they are closest to each other, resulting in an enthalpy gain that overcomes the repulsive components of the pair formation energy. As a consequence, Mo and/or W doped BiVO4 are likely to present donor-donor defect complexes, which is an outcome that can be applied in experimental approaches for improving the photocatalytic activity of these metal oxides.

Mechanical behavior of bovine pericardium treated with hyaluronic acid derivative for bioprosthetic aortic valves.

We studied the mechanical behavior of bovine pericardium (BP) after anticalcification treatment using hyaluronic acid (HA) derivative. To simulate the physiological environment and stimulate the calcification process, the BP samples were immersed into simulated body fluid solution. We conducted scanning electron microscopy with energy dispersive X-ray spectrometry, and uniaxial mechanical tests of HA-treated and non-treated samples. Although our microstructural analyses indicated that the HA treatment actually prevents the formation of calcium phosphate deposits, the mechanical tests show significant increase of stiffness of the HA-treated samples. Using data from our mechanical tests as input parameters, we performed finite element (FE) computer simulations to estimate how this increase in the BP stiffness affects the stress distribution in the bioprosthetic leaflet. Although the maximum stress observed during the closing phase of the membrane in vivo is below the experimental yield stress in all cases we analyzed, our FE results indicate that increase of BP stiffness due to HA anticalcification treatment results in higher risk of disruption and failure of the leaflets in bioprosthetic heart valves. Since our FE results indicate that the commissure and the fixed edge are the regions that withstand the highest mechanical stresses during the closing phase, new designs of the valve might be efficient to enhance the endurance of the prosthesis. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 2273-2280, 2019.