First-principles studies on layered materials and their interaction with liquids

Abstract

We have addressed two problems in this thesis. In the first one, we have performed Density Functional Theory (DFT) along with K-edge X-ray Absorption Near-Edge Structure (XANES) calculations in order to probe the local environment of the X-ray absorbing carbon atoms in carbon allotrope systems, as well as in diamond-like systems. We have found that, in addition to the accordance with experimental results regarding both the 1s – π∗ and the 1s – σ∗ transition of graphitic systems, as well as the 1s–σ∗ transition of diamond systems, our results show that the diamond-like ones can be characterized through this technique by observing the X-ray polarized spectra. Moreover, we observed that diamondol changes from a direct to an indirect bandgap material (0.75 eV at Γ-point to 0.68 eV) when the 1s-electron is removed from the carbon atom. Also, we have found that in such a case, the valence band maximum changes from spin-down to spin-up state. Similar results were seen for the fluorinated system. We also observed that the π transition for both the single-covered materials are spin-down polarized. Regarding the double-covered systems (bidiamondol and bi-F-diamane), this is not observed, though bidiamondol does not show π transition, whereas the another one does. Concerning the second problem, the interaction of liquid acetonitrile (ACN) with the surface of Mo-based layered materials, we have approached it through Molecular Dynamics (MD) simulations followed by DFT-based calculations to assess the charge transfer in the solid-liquid interface. We were able to notice that the liquid ACN molecules are more concentrated near the MoS2 surface, but only physisorbed, and also that their molecular density acquires an ordering due to the solid surface. By DFT, we calculated the charge transfer between the solid surface and the ACN molecules. Our results did not agree with the experimental evidence, as we have seen that the solids lost electrons to the region where there are ACN molecules. In spite of this, qualitatively our results showed that MoS2 induced higher charge transfer compared to the MoO3. This trend may change when environmental effects are included in this model.