Diffusion of protons and hydrogen atoms in representative two-dimensional materials is investigated. Specifically, density functional calculations were performed on graphene, hexagonal boron nitride (h-BN), phosphorene, silicene, and molybdenum disulfide (MoS 2) monolayers to study the surface interaction and penetration barriers for protons and hydrogen atoms employing finite cluster models. The calculated barrier heights correlate approximately with the size of the opening formed by the threefold open sites in the monolayers considered. They range from 1.56 eV (proton) and 4.61 eV (H) for graphene to 0.12 eV (proton) and 0.20 eV (H) for silicene. The results indicate that only graphene and h-BN monolayers have the potential for membranes with high selective permeability. The MoS 2 monolayer behaves differently: protons and H atoms become trapped between the outer S layers in the Mo plane in a well with a depth of 1.56 eV (proton) and 1.5 eV (H atom), possibly explaining why no proton transport was detected, suggesting MoS 2 as a hydrogen storage material instead. For graphene and h-BN, off-center proton penetration reduces the barrier to 1.38 eV for graphene and 0.11 eV for h-BN. Furthermore, Pt acting as a substrate was found to have a negligible effect on the barrier height. In defective graphene, the smallest barrier for proton diffusion (1.05 eV) is found for an oxygenterminated defect. Therefore, it seems more likely that thermal protons can penetrate a monolayer of h-BN but not graphene and defects are necessary to facilitate the proton transport in graphene. RECEIVED
We implement a well-established concept to consider dispersion effects within a Poisson-Boltzmann approach of continuum solvation of proteins. The theoretical framework is particularly suited for boundary element methods. Free parameters are determined by comparison to experimental data as well as high-level quantum mechanical reference calculations. The method is general and can be easily extended in several directions. The model is tested on various chemical substances and found to yield good-quality estimates of the solvation free energy without obvious indication of any introduced bias. Once optimized, the model is applied to a series of proteins, and factors such as protein size or partial charge assignments are studied.
et al.: Photoemission from Ordered Physisoi :bed Molecular Phases NdGraphite, N2 and CO/Ag(lll)like, and an extra LEED structure characteristic of a Xe(l11) monolayer is only seen at or close to monolayer completion [ll, 18,271. Conversely, the adsorption isotherms of the second xenon layer (ontop of the completed first one) is steplike, indicating a first order phase transition [8,11], because both the adsorption energy and the dipole repulsion are strongly reduced ( Table 1). The same was found for the third Xe layer on Pd(100) [S], and in this case even the critical temperatures of the 2D phase transitions in the second and third layer could be measured [S].In summary, we conclude that our new UV-photoemission data of Xe adsorbed at 58 K on Cu(lll), Ag(l11) and Ru(001), which provide a quantitative distinction between the coexisting 2D gas and 2D solid phase of xenon on these surfaces, confirm the anticipated substrate dependence of the equilibrium between both phases. The partial coverage of the equilibrium 2D gas phase increases the higher the adsorption energy and the greater the dipole moment per xenon atom is. This finding is corroborated by earlier results for Xe mono-and bilayer adsorption on palladium. This work was supported by the Deutsche Forschungsgemeinschaft through Sonderforschungsbereich 128. One of us (A. J.) is thankful for a followship from the Alexander-von-Humboldt-Stiftung.
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