Anharmonic calculations of vibrational spectra for molecular adsorbates: A divide-and-conquer semiclassical molecular dynamics approach.

The vibrational spectroscopy of adsorbates is becoming an important investigation tool for catalysis and material science. This paper presents a semiclassical molecular dynamics method able to reproduce the vibrational energy levels of systems composed by molecules adsorbed on solid surfaces. Specifically, we extend our divide-and-conquer semiclassical method for power spectra calculations to gas-surface systems and interface it with plane-wave electronic structure codes. The Born-Oppenheimer classical dynamics underlying the semiclassical calculation is full dimensional, and our method includes not only the motion of the adsorbate but also those of the surface and the bulk. The vibrational spectroscopic peaks related to the adsorbate are accounted together with the most coupled phonon modes to obtain spectra amenable to physical interpretations. We apply the method to the adsorption of CO, NO, and H2O on the anatase-TiO2 (101) surface. We compare our semiclassical results with the single-point harmonic estimates and the classical power spectra obtained from the same trajectory employed in the semiclassical calculation. We find that CO and NO anharmonic effects of fundamental vibrations are similarly reproduced by the classical and semiclassical dynamics and that H2O adsorption is fully and properly described in its overtone and combination band relevant components only by the semiclassical approach.

Theoretical investigation of the platinum substrate influence on BaTiO3 thin film polarisation.

Density functional theory calculations are performed to study the out-of-plane polarisation in BaTiO3 (BTO) thin films epitaxially grown on platinum. Prior to any polarisation calculation, the stability of the Pt(001)/BaTiO3(001) structure is thoroughly discussed. In particular, the nature of the Pt/BTO and BTO/vacuum interfaces is characterised. The growth of BTO is shown to start with a TiO2 layer while the nature of the surface termination does not broadly modify the stability. Therefore both upper terminations are considered when describing the ferroelectric behaviour in Pt/BTO interfaces. The geometric and electronic effects of the substrate on the polarisation are investigated. To isolate the electronic influence of platinum, the out-of-plane polarisation in Pt/BTO systems is compared to the one in isolated BTO slabs constrained to the same lattice mismatch induced by the epitaxial growth on platinum. The ferroelectric phase is favoured as soon as the thickness is larger than 23 Å, both for isolated and deposited BTO, for the smallest width. The Pt substrate will modify the size of polarisation domains, while an upper BaO layer through the use of asymmetric [TiO2/BaO] systems will induce an increase of the polarisation. One could take advantage of this experimentally.

The nature of the Pt(111)/α-Fe2O3(0001) interfaces revealed by DFT calculations.

Density functional theory calculations are performed to give a thorough description of structural, energetic, and electronic properties of Pt(111)/α-Fe2O3(0001) systems by spin-polarized calculations, accounting for the on-site Coulomb interaction. Toward the better understanding of Pt(111)/α-Fe2O3(0001) interfaces, two terminations of α-Fe2O3(0001) surface, namely, the single Fe- and the O3-termination, are considered and coupled with the four possible (top, hcp, fcc, and bridge) sites on Pt(111). The effect of the strain on clean hematite surfaces due to the lattice mismatch between the substrate and the overlayer is included in the analysis. Among the possible adsorption configurations, bridge sites are unstable, while the most favorable configurations are the ones at hollow sites. The stability of the interfaces is not only influenced by the termination of the overlayer but also influenced by the degree of its structural relaxation and the relative position of the first layer of O atoms in hematite with respect to Pt. To elucidate the different nature of the two terminations of the overlayer on Pt, projected density of states and 3D charge density difference plots are also discussed.

One Step Toward a New Generation of C-MOS Compatible Oxide P-N Junctions: Structure of the LSMO/ZnO Interface Elucidated by an Experimental and Theoretical Synergic Work.

Heterostructures formed by La0.7Sr0.3MnO3/ZnO (LSMO/ZnO) interfaces exhibit extremely interesting electronic properties making them promising candidates for novel oxide p-n junctions, with multifunctional features. In this work, the structure of the interface is studied through a combined experimental/theoretical approach. Heterostructures were grown epitaxially and homogeneously on 4″ silicon wafers, characterized by advanced electron microscopy imaging and spectroscopy and simulated by ab initio density functional theory calculations. The simulation results suggest that the most stable interface configuration is composed of the (001) face of LSMO, with the LaO planes exposed, in contact with the (112̅0) face of ZnO. The ab initio predictions agree well with experimental high-angle annular dark field scanning transmission electron microscopy images and confirm the validity of the suggested structural model. Electron energy loss spectroscopy confirms the atomic sharpness of the interface. From statistical parameter estimation theory, it has been found that the distances between the interfacial planes are displaced from the respective ones of the bulk material. This can be ascribed to the strain induced by the mismatch between the lattices of the two materials employed.