Karen Johnston

First-principles study of symmetry lowering and polarization in BaTiO3/SrTiO3 superlattices with in-plane expansion

The crystal structure and local spontaneous polarization of (BaTiO3)m/(SrTiO3)n superlattices is calculated using a first-principles density functional theory method. The in-plane lattice constant is 1% larger than the SrTiO3 substrate to imitate the relaxed superlattice structure and the symmetry is lowered to monoclinic space group Cm which allows polarization to develop along the [110] and [001] directions. The polarization component in the [110] direction is found to develop only in the SrTiO3 layers and falls to zero in the BaTiO3 layers, whereas the polarization in the [001] direction is approximately uniform throughout the superlattice. These findings are consistent with recent experimental data and first-principles results for epitaxially strained BT and ST.

Influence of van der Waals forces on the adsorption structure of benzene on silicon studied using density functional theory

Two different adsorption configurations of benzene on the

Adsorption structures of phenol on the Si(001)-(2X1) surface calculated using density functional theory

\mathrm{Si}(001)\text{\ensuremath{-}}(2\ifmmode\times\else\texttimes\fi{}1)

Polarization enhancement in short period superlattices via interfacial intermixing

surface, the tight-bridge and butterfly structures, were studied using density functional theory. Several exchange and correlation functionals were used, including the recently developed van der Waals density functional (vdW-DF), which accounts for the effect of van der Waals forces. In contrast to the Perdew-Burke-Ernzerhof (PBE), revPBE, and other generalized-gradient approximation functionals, the vdW-DF finds that, for most coverages, the adsorption energy of the butterfly structure is greater than that of the tight-bridge structure.

Properties of short polystyrene chains confined between two gold surfaces through a combined density functional theory and classical molecular dynamics approach

Several dissociated and two nondissociated adsorption structures of the phenol molecule on the Si(001)-(2X1) surface are studied using density functional theory with various exchange and correlation functionals. The relaxed structures and adsorption energies are obtained and it is found that the dissociated structures are energetically more favorable than the nondissociated structures. However, the ground state energies alone do not determine which structure is obtained experimentally. To elucidate the situation core level shift spectra for Si 2p and C 1s states are simulated and compared with experimentally measured spectra. Several transition barriers were calculated in order to determine, which adsorption structures are kinetically accessible. Based on these results we conclude that the molecule undergoes the dissociation of two hydrogen atoms on adsorption.

Modelling molecule–surface interactions—an automated quantum-classical approach using a genetic algorithm

The effect of intermixing at the interface of short period PbTiO3 SrTiO3 superlattices is studied using first-principles density functional theory. The results indicate that interfacial intermixing significantly enhances the polarization within the superlattice. This enhancement is directly related to the off-centering of Pb and Sr cations and can be explained through a discussion of interacting dipoles. This picture should be general for a wide range of multicomponent superlattices and may have important consequences for the design of ferroelectric devices.

Polymer adhesion : first-principles calculations of the adsorption of organic molecules onto Si surfaces

The properties of atactic short-chain polystyrene films confined between two parallel gold surfaces at a temperature of 503 K are investigated using a combination of density functional theory calculations and classical atomistic simulations. A classical Morse-type potential, used to describe the interaction between the polymer and the gold surface, was parameterized based on the results of density functional calculations. Several polystyrene films were studied, with thicknesses ranging from around 1–10 nm. The structural, conformational and dynamical properties of the films were analysed and compared to the properties of the bulk polystyrene systems. The dynamics of the polystyrene close to the surface was found to be significantly slower than in the bulk.

A hierarchical dualscale study of bisphenol-A-polycarbonate on a silicon surface: structure, dynamics and impurity diffusion

We present an automated and efficient method to develop force fields for molecule-surface interactions. A genetic algorithm (GA) is used to parameterise a classical force field so that the classical adsorption energy landscape of a molecule on a surface matches the corresponding landscape from density functional theory (DFT) calculations. The procedure performs a sophisticated search in the parameter phase space and converges very quickly. The method is capable of fitting a significant number of structures and corresponding adsorption energies. Water on a ZnO(0001) surface was chosen as a benchmark system but the method is implemented in a flexible way and can be applied to any system of interest. In the present case, pairwise Lennard Jones (LJ) and Coulomb potentials are used to describe the molecule-surface interactions. In the course of the fitting procedure, the LJ parameters are refined in order to reproduce the adsorption energy landscape. The classical model is capable of describing a wide range of energies, which is essential for a realistic description of a fluid-solid interface.

Computational study of (111) epitaxially strained ferroelectric perovskites BaTiO3 and PbTiO3

The structures and energetics of organic molecules adsorbed onto clean and H-passivated Si (001) - (2×1) surfaces have been calculated using density functional theory. For benzene adsorbed on the clean Si surface the tight-bridge structure was found to be stable and the butterfly structure metastable. Both carbonic acid, H2 C O3, and propane, C3 H8, dissociate on contact with the surface. Passivation of the Si surface with H atoms has a dramatic effect on the surface properties. The passivated surface is very inert and the binding energy of all the molecules is very weak.

Phase behaviour of self-assembled monolayers controlled by tuning physisorbed and chemisorbed states: A lattice-model view

A previously developed and studied coarse-grained model is used to investigate the properties of bisphenol-A-polycarbonate (BPA-PC) in contact with the Si(001)-(2 × 1) surface. The surface interaction potentials are based on density functional calculations. Both a smooth wall potential and a site-dependent wall potential were used to represent the surface. For both types of surface potential it was found that only the chain ends adsorb and the density profiles and conformations in each case are similar. The site-dependent surface slows the dynamics of the polymer at the interface by an order of magnitude compared to the bulk dynamics for the chain lengths considered. The diffusion of non-adsorbing impurity particles for both surface potentials was investigated and the concentration and dynamics of the impurity particles were analysed.