N. M. Harrison

On the prediction of band gaps from hybrid functional theory

Details of the band gap and band widths within materials are of fundamental importance to a wide range of applications. A hybrid scheme is used to predict the band gaps of a variety of materials. The electronic structure of silicon is examined in some detail and comparisons with alternative theories are made. Agreement with experimentally derived band gaps is at least as good as that obtained with sophisticated correlated calculations or perturbation theories. The functional is straightforward to implement, computationally efficient and produces ground state energy surfaces which are significantly more accurate than those computed using the best gradient corrected density functionals currently in use.

Electronic structure and magnetic properties of graphitic ribbons

First-principles calculations are used to establish that the electronic structure of graphene ribbons with zigzag edges is unstable with respect to magnetic polarization of the edge states. The magnetic interaction between edge states is found to be remarkably long ranged and intimately connected to the electronic structure of the ribbon. Various treatments of electronic exchange and correlation are used to examine the sensitivity of this result to details of the electron-electron interactions, and the qualitative features are found to be independent of the details of the approximation. The possibility of other stablization mechanisms, such as charge ordering and a Peierls distortion, are explicitly considered and found to be unfavorable for ribbons of reasonable width. These results have direct implications for the control of the spin-dependent conductance in graphitic nanoribbons using suitably modulated magnetic fields.

Materials science - The hardest known oxide

A material as hard as diamond or cubic boron nitride has yet to be identified, but here we report the discovery of a cotunnite-structured titanium oxide which represents the hardest oxide known. This is a new polymorph of titanium dioxide, where titanium is nine-coordinated to oxygen in the cotunnite (PbCl2) structure. The phase is synthesized at pressures above 60 gigapascals (GPa) and temperatures above 1,000 K and is one of the least compressible and hardest polycrystalline materials to be described.

Analytical Hartree–Fock gradients for periodic systems

We present the theory of analytical Hartree–Fock gradients for periodic systems as implemented in the code CRYSTAL. We demonstrate how derivatives of the integrals can be computed with the McMurchie–Davidson algorithm. Highly accurate gradients with respect to nuclear coordinates are obtained for systems periodic in 0, 1, 2, or 3 dimensions.

Structural transformations in graphene studied with high spatial and temporal resolution.

Graphene has remarkable electronic properties, such as ballistic transport and quantum Hall effects, and has also been used as a support for samples in high-resolution transmission electron microscopy and as a transparent electrode in photovoltaic devices. There is now a demand for techniques that can manipulate the structural and physical properties of graphene, in conjunction with the facility to monitor the changes in situ with atomic precision. Here, we show that irradiation with an 80 kV electron beam can selectively remove monolayers in few-layer graphene sheets by means of electron-beam-induced sputtering. Aberration-corrected, low-voltage, high-resolution transmission electron microscopy with sub-ångström resolution is used to examine the structural reconstruction occurring at the single atomic level. We find preferential termination for graphene layers along the zigzag orientation for large hole sizes. The temporal resolution can also be reduced to 80 ms, enabling real-time observation of the reconstruction of carbon atoms during the sputtering process. We also report electron-beam-induced rapid displacement of monolayers, fast elastic distortions and flexible bending at the edges of graphene sheets. These results reveal how energy transfer from the electron beam to few-layer graphene sheets leads to unique structural transformations.

Water chemistry on surface defect sites: Chemidissociation versus physisorption on MgO(001)

The following paper presents the results of a theoretical study that probed the chemistry of water at structural defects on the MgO (001) surface. The computational technique used was periodic Hartree–Fock (PHF) theory with density functional based correlation corrections. The adsorption energies for water adsorbed on isolated corner, edge, and surface sites on the MgO surface were compared to the hydroxylation energies for the same sites. As stated in a previous paper, the binding of water to the perfect surface is exothermic by 4.1‐5.6 kcal/mol whereas hydroxylating the perfect surface was endothermic by 24.5 kcal/mol. At step‐edge sites, the process of water adsorption is exothermic and comparable in magnitude to the hydroxylation of these sites. The binding energies associated with water bound to the step‐edge are 6.5–10.5 kcal/mol, and hydroxylation of this site is exothermic by 7.3 kcal/mol. At corner sites we find a strong preference for hydroxylation. The binding of water to a corner is exothermic...

Dynamics of single Fe atoms in graphene vacancies.

Focused electron beam irradiation has been used to create mono and divacancies in graphene within a defined area, which then act as trap sites for mobile Fe atoms initially resident on the graphene surface. Aberration-corrected transmission electron microscopy at 80 kV has been used to study the real time dynamics of Fe atoms filling the vacancy sites in graphene with atomic resolution. We find that the incorporation of a dopant atom results in pronounced displacements of the surrounding carbon atoms of up to 0.5 Å, which is in good agreement with density functional theory calculations. Once incorporated into the graphene lattice, Fe atoms can transition to adjacent lattice positions and reversibly switch their bonding between four and three nearest neighbors. The C atoms adjacent to the Fe atoms are found to be more susceptible to Stone-Wales type bond rotations with these bond rotations associated with changes in the dopant bonding configuration. These results demonstrate the use of controlled electron beam irradiation to incorporate dopants into the graphene lattice with nanoscale spatial control.

First-principles molecular dynamics simulation of water dissociation on TiO2 (110)

Abstract We have performed first-principles molecular dynamics calculations of water adsorption on TiO2 (110). We find that dissociative adsorption occurs at the fivefold-coordinated Ti site resulting in the formation of two types of hydroxyl group. The vibrational spectra calculated from this hydroxylated surface show that a clear stretch frequency is present for only one of these groups, with vibrations from the other hydroxyl broadened due to hydrogen bonding between the two hydroxyl groups.

A defective graphene phase predicted to be a room temperature ferromagnetic semiconductor

Theoretical calculations, based on the hybrid exchange density functional theory, are used to show that in graphene, a periodic array of defects generates a ferromagnetic ground state at room temperature for unexpectedly large defect separations. This is demonstrated for defects that consist of a carbon vacancy in which two of the dangling bonds are saturated with H atoms. The magnetic coupling mechanism is analysed and found to be due to an instability in the ?-electron system with respect to a long-range spin polarization characterized by alternation in the spin direction between adjacent carbon atoms. The disruption of the ?-bonding opens a semiconducting gap at the Fermi edge. The size of the energy gap and the magnetic coupling strength are strong functions of the defect separation and can thus be controlled by varying the defect concentration. The position of the semiconducting energy gap and the electron effective mass are strongly spin-dependent and this is expected to result in a spin asymmetry in the transport properties of the system. A defective graphene sheet is, therefore, a very promising material with an in-built mechanism for tailoring the properties of future spintronic devices.

An ab initio study of ZnO(101̄0)

Abstract The structure of the ZnO(1010) surface has been determined by using ab initio, all-electron total energy calculations. By employing local basis sets based on Gaussians, and a hybrid density functional (B3LYP), results in excellent agreement with experiment have been obtained for the geometric and electronic structure. The calculations predict a strong covalent character at the surface of the material that may indicate the mechanism behind the stability of the polar (0001) and (0001) surfaces of zincite.