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Register a new userThe role of van der Waals and exchange interactions in high-pressure solid hydrogen
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We investigate the van der Waals interactions in solid molecular hydrogen structures. We calculate enthalpy and the Gibbs free energy to obtain zero and finite temperature phase diagrams, respectively. We employ density functional theory (DFT) to calculate the electronic structure and Density functional perturbation theory (DFPT) with van der Waals (vdW) functionals to obtain phonon spectra. We focus on the solid molecular $C2/c$, $Cmca$-12, $P6_3/m$, $Cmca$, and $Pbcn$ structures within the pressure range of 200 $<$ P $<$ 450 GPa. We propose two structures of the $C2/c$ and $Pbcn$ for phase III which are stabilized within different pressure range above 200 GPa. We find that vdW functionals have a big effect on vibrations and finite-temperature phase stability, however, different vdW functionals have different effects. We conclude that, in addition to the vdW interaction, a correct treatment of the high charge gradient limit is essential. We show that the dependence of molecular bond-lengths on exchange-correlation also has a considerable influence on the calculated metallization pressure, introducing errors of up to 100GPa.
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Recent research showed that the rotational degree of freedom in stacking 2D materials yields great changes in the electronic properties. Here we focus on an often overlooked question: are twisted geometries stable and what defines their rotational energy landscape? Our simulations show how epitaxy theory breaks down in these systems and we explain the observed behaviour in terms of an interplay between flexural phonons and the interlayer coupling, governed by Moire superlattice. Our argument applied to the well-studied MoS$_2$/Graphene system rationalize experimental results and could serve as guidance to design twistronics devices.
The adsorption of aromatic molecules on metal surfaces plays a key role in condensed matter physics and functional materials. Depending on the strength of the interaction between the molecule and the surface, the binding is typically classified as either physisorption or chemisorption. Van der Waals (vdW) interactions contribute significantly to the binding in physisorbed systems, but the role of the vdW energy in chemisorbed systems remains unclear. Here we study the interaction of benzene with the (111) surface of transition metals, ranging from weak adsorption (Ag and Au) to strong adsorption (Pt, Pd, Ir, and Rh). When vdW interactions are accurately accounted for, the barrier to adsorption predicted by standard density functional theory (DFT) calculations essentially vanishes, producing a metastable precursor state on Pt and Ir surfaces. Notably, vdW forces contribute more to the binding of covalently bonded benzene than they do when benzene is physisorbed. Comparison to experimental data demonstrates that some of the recently developed methods for including vdW interactions in DFT allow quantitative treatment of both weakly and strongly adsorbed aromatic molecules on metal surfaces, extending the already excellent performance found for gas-phase molecules.
Understanding charge transfer (CT) between two chemical entities and subsequent change in their charge densities is essential not only for molecular species but also for various low-dimensional materials. Because of their extremely high fraction of surface atoms, two-dimensional (2-D) materials are most susceptible to charge exchange and exhibit drastically different physicochemical properties depending on their charge density. In this regard, spontaneous and uncontrollable ionization of graphene in the ambient air has caused much confusion and technical difficulty in achieving experimental reproducibility since its first report in 2004. Moreover, the same ambient hole doping was soon observed in 2-D semiconductors, which implied that a common mechanism should be operative and apply to other low-dimensional materials universally. In this Account, we review our breakthroughs in unraveling the chemical origin and mechanistic requirements of the hidden CT reactions using 2-D crystals. We developed in-situ optical methods to quantify charge density using Raman and photoluminescence (PL) spectroscopy and imaging. Using gas and temperature-controlled in-situ measurements, we revealed that the electrical holes are injected by the oxygen reduction reaction (ORR): $O_{2}$ + $4H^{+}$ + $4e^{-}$ $rightleftharpoons$ $2H_{2}O$, which was independently verified by pH dependence in HCl solutions. In addition to oxygen and water vapor, the overall CT reaction requires hydrophilic dielectric substrates, which assist hydration of the sample-substrate interface. The interface-localized reaction allowed us to visualized and control interfacial molecular diffusion and CT by varing the 2-D gap spacing and introducing defects. The complete mechanism of the fundamental charge exchange summarized in this Account will be essential in exploring material and device properties of other low dimensional materials.
We use the diffusion quantum Monte Carlo to revisit the enthalpy-pressure phase diagram of the various products from the different proposed decompositions of H$_2$S at pressures above 150~GPa. Our results entails a revision of the ground-state enthalpy-pressure phase diagram. Specifically, we find that the C2/c HS$_2$ structure is persistent up to 440~GPa before undergoing a phase transition into the C2/m phase. Contrary to density functional theory, our calculations suggest that the C2/m phase of HS is more stable than the I4$_1$/amd HS structure over the whole pressure range from 150 to 400 GPa. Moreover, we predict that the Im-3m phase is the most likely candidate for H$_3$S, which is consistent with recent experimental x-ray diffraction measurements.
Sparse matter is characterized by regions with low electron density and its understanding calls for methods to accurately calculate both the van der Waals (vdW) interactions and other bonding. Here we present a first-principles density functional theory (DFT) study of a layered oxide (V2O5) bulk structure which shows charge voids in between the layers and we highlight the role of the vdW forces in building up material cohesion. The result of previous first-principles studies involving semilocal approximations to the exchange-correlation functional in DFT gave results in good agreement with experiments for the two in-plane lattice parameters of the unit cell but overestimated the parameter for the stacking direction. To recover the third parameter we include the nonlocal (dispersive) vdW interactions through the vdW-DF method [Dion et al., Phys. Rev. Lett. 92, 246401 (2004)] testing also various choices of exchange flavors. We find that the transferable first-principle vdW-DF calculations stabilizes the bulk structure. The vdW-DF method gives results in fairly good agreement with experiments for all three lattice parameters.
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