M. Alatalo

The contribution made by lattice vacancies to the Wigner effect in radiation-damaged graphite

Models for radiation damage in graphite are reviewed and compared, leading to a re-examination of the contribution made by vacancies to annealing processes. A method based on density functional theory, using large supercells with orthorhombic and hexagonal symmetry, is employed to calculate the properties and behaviour of lattice vacancies and displacement defects. It is concluded that annihilation of intimate Frenkel defects marks the onset of recovery in electrical resistivity, which occurs when the temperature exceeds about 160 K. The migration of isolated monovacancies is estimated to have an activation energy of E(a) ≈ 1.1 eV. Coalescence into divacancy defects occurs in several stages, with different barriers at each stage, depending on the path. The formation of pairs ultimately yields up to 8.9 eV energy, which is nearly 1.0 eV more than the formation energy for an isolated monovacancy. Processes resulting in vacancy coalescence and annihilation appear to be responsible for the main Wigner energy release peak in radiation-damaged graphite, occurring at about 475 K.

First principles study of vacancies in Si

Abstract Silicon monovacancies in different charge states have been studied using ab initio molecular dynamics. The electronic structure and ionic positions have been calculated simultaneously using the Car-Parrinello -method. The accuracy of the method is analysed by studying bulk silicon with different cut-off energies for the plane-wave expansion of the electron wave functions. The electronic structures are calculated using pseudopotentials with s-nonlocality only and with s- and p-nonlocality. The two pseudopotential approximations used give different relaxation patterns. The relaxations around the vacancies are found to depend strongly on the charge state when the pseudopotential with s- and p-nonlocality is used. With s-nonlocality only, the charge state dependence of the distortions is small. From the total energies we obtain the ionization levels and the formation energies associated with different charge states.

Characteristics of S adsorption on Pd vicinal surfaces

Results of first principles calculations for two types of vicinals of Pd(1 1 1) and Pd(1 1 0), with 3-atom wide terraces, are reported. We discuss the structure of the stepped surfaces, and the preferred adsorption sites and adsorption energies for S. We show that the adsorbate can have considerable effect on the relaxations and registry of substrate atoms and that the effect is most pronounced for Pd(3 2 0) which also displays the strongest adsorption energies for S. 2003 Elsevier Science B.V. All rights reserved.

Positron annihilation at paramagnetic defects in semiconductors

Positron annihilation at vacancies in semiconductors is studied. The authors are interested in the formation of singlet and triplet positron-electron states, and especially, in their influence on the positron annihilation characteristics. These states are formed by a positron trapped by the vacancy and an unpaired (paramagnetic) electron in a deep level. They provide examples by determining the two ensuing positron lifetime components for vacancies in Si and GaAs. For the practical calculations it is shown how the self-consistent electron structures obtained by the pseudopotential-plane wave methods can be used. Finally, the implications are discussed from the experimental point of view.

Metallic contact between MoS2 and Ni via Au Nanoglue

A critical factor for electronics based on inorganic layered crystals stems from the electrical contact mode between the semiconducting crystals and the metal counterparts in the electric circuit. Here, a materials tailoring strategy via nanocomposite decoration is carried out to reach metallic contact between MoS2 matrix and transition metal nanoparticles. Nickel nanoparticles (NiNPs) are successfully joined to the sides of a layered MoS2 crystal through gold nanobuffers, forming semiconducting and magnetic NiNPs@MoS2 complexes. The intrinsic semiconducting property of MoS2 remains unchanged, and it can be lowered to only few layers. Chemical bonding of the Ni to the MoS2 host is verified by synchrotron radiation based photoemission electron microscopy, and further proved by first-principles calculations. Following the systems band alignment, new electron migration channels between metal and the semiconducting side contribute to the metallic contact mechanism, while semiconductor-metal heterojunctions enhance the photocatalytic ability.

Electronic structure calculations for substitutional copper and monovacancies in silicon

Two different computer program packages based on the self-consistent local-spin-density approximation—aimpro and vasp—are employed in this study of substitutional copper CuSi and monovacancies VSi in silicon, including the effects of their charge state. The programs differ in the types of basis sets and pseudopotentials they use, each with their own relative merits, while being similar in overall quality. This approach aims to reduce uncertainty in the results, particularly for small or subtle effects, where the risk is greatest that the conclusions are affected by artifacts specific to a particular implementation. The electronic structures of the two defects are closely related, hence they are expected to behave in a similar manner. For both defects structural distortions resulting in lower point group symmetries than Td (the highest possible) are found. This is in good agreement with the results of previous studies of VSi. Much less is known about symmetry-lowering effects for CuSi; however, the electronic levels of CuSi have been measured accurately, while those for VSi are less accessible. Calculating them is a challenging task for theory. The strategy we adopt, based purely on comparing total energies of supercells in different charge states, with and without model defects, reproduces the three known levels for CuSi reasonably well. Satisfactory results are also obtained for VSi in so far as can be judged for this more complex case.

A DFT study of the effect of SO4 groups on the properties of TiO2 nanoparticles

We present a study of the optical, electronic, and structural properties of TiO2 anatase-structured nanoparticles upon adsorption of SO4 groups, which are always present on the surface of the particles during the sulfate manufacturing method. Structural and electronic properties were studied using the density functional theory method (DFT), and optical properties were obtained by time-dependent DFT. It was found that SO4 groups alter both the geometric and electronic structure of TiO2 nanoparticles and change the photoabsorption characteristics. In particular, we find that η2-O2 type O-O moieties are formed due to the adsorption of 3 and 4SO4 groups.

Lateral diatomic two-dimensional artificial molecules: Classical transitions and quantum-mechanical counterparts

Structural properties of a finite number (

Correlation effects for positron annihilation with core and semicore electrons

N = 2 - 20

Adatom island diffusion on metal fcc(100) surfaces

) of point charges (classical electrons) confined laterally in a two-dimensional two-minima potential are calculated as a function of the distance (