We present a first principles density functional theory study of microscopic properties of hydrogen defect centres in diamond. Several configurations, involving interstitial hydrogen impurities, have been considered either forming with other defects, such as hydrogen defects and vacancies. The atomic structures, and hyperfine parameters of hydrogen result compared with the experimental data on electrically active centres in synthetic diamond. Based on Local density functional theory our calculations are in excellent agreement with one interpretation of electron paramagnetic resonance of hydrogen in diamond.

The data obtained recently from combined deep-level-transient spectroscopy (DLTS), local vibrational mode (LVM) spectroscopy and ab-initio modeling studies on structure, electronic properties, local vibrational modes, reconfiguration and diffusion paths and barriers for trivacancy (V3) and trivacancy-oxygen (V3O) defects in silicon are summarized. New experimental results on the introduction rates of the divacancy (V2) and trivacancy upon 4 MeV electron irradiation and on the transformation of V3 from the fourfold coordinated configuration to the (110) planar one upon minority carrier injection are reported. Possible mechanisms of the transformation are considered and discussed.

The preferred location of boron in oxidized free-standing Si nanoparticles was investigated using a first-principles density functional approach. The nanoparticles were modeled by a silicon core about 1.5 nm in diameter surrounded by an outer shell of SiO2 with a thickness of about 0.5 nm, and considered negatively charged. The calculated formation energies indicate that B is equally stable in the Si core and in the SiO2 shell, showing preference for interface sites. This indicates that, in contrast with phosphorus, the ratio of the boron concentration in the silicon core to that of the silicon shell will not be improved over one upon thermal annealing.

Effects of the ethylene carbonate (EC) solvent on Li insertion and diffusion in Si anodes are studied using density functional theory. On both (100) and (111) reconstructed surfaces of Si, a semi-dissociated (SD) configuration of EC is stable and most favorable for Li insertion, lowering its barrier by up to 0.2 eV vs a clean surface. The less stable molecular adsorption has little effect on Li insertion and diffusion, while the surface ketone formed by dissociating the SD configuration at a cost of 0.6 eV has a strong detrimental effect on Li insertion, increasing its barrier by up to 0.4 eV.

First-principles calculations are used to investigate the structure, electronic and optical properties of silicon nanocystals with chlorine-passivated surface. The nanocrystals considered were approximately spherical, with diameters between 1.5 and 3.0 nm. We show that the nanocrystals with chlorinated surface have a smaller bandgap, lower optical absorption threshold, and greater ionization energy and electron affinity than hydrogenated silicon nanocrystals of the same size

The mechanical and electrical properties of graphite and related materials such as multilayer graphene depend strongly on the presence of defects in the lattice structure, particularly those which create links between adjacent planes. We present findings which suggest the existence of a new type of defect in the graphite or graphene structure which connects adjacent planes through continuous hexagonal sp² bonding alone and can form through the aggregation of individual vacancy defects. The energetics and kinetics of the formation of this type of defect are investigated with atomistic density functional theory calculations. The resultant structures are then employed to simulate high resolution transmission electron microscopy images, which are compared to recent experimental images of electron irradiation damaged graphite.

Calculations have been performed to investigate the possibility for hydrogen adsorption in the manganese containing metal-organic framework MOF-73. A supercell of 348 atoms in total were employed and the computer code Aimpro with the LDA functional PW92 was used to relax the different structures in the study. The results clearly show that MOF-73 is a suitable candidate for hydrogen storage since the coordination/binding energy for a hydrogen molecule to the MOF structure falls in the range of a few tens of kJmol-1.

Diamond has many extreme physical properties and it can be used in a wide range of applications. In particular it is a highly effective particle detection material, where radiation damage is an important consideration. The WAR9 and WAR10 are electron paramagnetic resonance centres seen in irradiated, nitrogen-containing diamond. These S = 1/2 defects have C2v and C1h symmetry, respectively, and the experimental spectra have been interpreted as arising from nitrogen split-interstitial centres. Based upon the experimental and theoretical understanding of interstitial nitrogen defect structures, the AIMPRO density functional code has been used to assess the assignments for the structures of WAR9 and WAR10. Although the calculated hyperfine interaction tensors are consistent with the measured values for WAR9, the thermal stability renders the assignment problematic. The model for the WAR10 centre yields principal directions of the hyperfine tensor at variance with observation. Alternative models for both centres are discussed in this paper, but no convincing structures have been found.

Strontium titanate is a promising dielectric material for device applications including capacitors and gate dielectrics. However, oxygen vacancies, which are inevitable donor defects mobile under bias at room temperature, lead to undesirable leakage current in SrTiO3 thin films. Epitaxially grown SrTiO3 on lattice mismatched substrates leads to strained SrTiO3, inducing structural phase transitions from a cubosymmetric non-ferroelectric geometry to tetragonal and orthorhombic structures, depending upon the sign of the strain. In this study, density functional calculations have been performed to determine the impact of isotropic biaxial tensile strain in a (001) plane upon the phase of SrTiO3 and the activation energy for the migration of oxygen vacancies in such strained SrTiO3. The phase transition of the host material yields anisotropy in oxygen vacancy diffusion for diffusion within and between planes parallel to the strain. We found a general reduction in the barrier for diffusion within and normal to the plane of tensile strain. The inter-plane diffusion barrier reduces up to 25% at high values of strain. The variation in the barrier corresponding to in-plane diffusion is smaller in comparison to inter-plane diffusion. Finally, we reflect upon how the interplay between lattice strain with native defects plays a crucial role in the conduction mechanism of thin film, strained SrTiO3

The modification of the electronic structure of silicon nanocrystals using an organic dopant, 2,3,5,6- tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4-TCNQ), is investigated using first-principles calculations. It is shown that physisorbed F4-TCNQ molecules have the effect of oxidizing the nanocrystal, attracting the charge density towards the F 4-TCNQ-nanocrystal interface, and decreasing the excitation energy of the system. In periodic F4-TCNQ/nanocrystal superlattices, F 4-TCNQ is suggested to enhance exciton separation, and in the presence of free holes, to serve as a bridge for electron/hole transfer between adjacent nanocrystals.