Helder S. Domingos

Carbon allotropes and strong nanotube bundles

We have used density functional pseudopotential calculations and molecular dynamics to predict new carbon structures of high stability. The new phases are strongly bound and involve the smallest radius nanotubes. It was found that it is possible to covalently link smallest/larger, smallest/smallest radius nanotubes together as well as larger nanotube/C20 1D-chains, resulting in extremely large interlinkage and consequent increase in the resistance to slippage. This procedure may enable the construction of extremely stiff nanotube bundles capable of making full use of the tensile properties of individual nanotubes, while enhancing the crystallinity of the material. Some of the carbon allotropes studied are the lowest energy non-diamond sp3 hybridized structures ever found.

Electronic properties of a grain boundary in Sb-doped ZnO

The electronic properties of a Σ = 13 32.2° [0001] tilt grain boundary in ZnO have been investigated using first-principles calculations. Two atomic models for the boundary have been considered, one of which contains structural units that are consistent with those observed for this orientation using electron microscopy. Doping both the grain boundary models with antimony reveals a strong driving force for segregation. Analysis of the electronic densities of states, bond populations and Mulliken charges shows that antimony creates a localized impurity state in the grain boundary and acts as a donor dopant. The resulting charge accumulation at the grain boundary together with the presence of local bonds that are metallic in character, will influence the mechanism for charge transport across the interface and this is discussed in relation to varistor applications.

Electronic structure of twist grain-boundaries in ZnO and the effect of Sb doping

Abstract We have investigated the properties of two Σ=7 [0 0 0 1] twist grain boundaries in ZnO using ab initio plane wave pseudopotential density functional theory. The calculations confirm the stability of the atomic models and enable a comparison between two energetically similar boundaries which exhibit bond stretching and bending. It is shown that shallow states exist as a consequence of the distortions in the grain boundaries, but no deep states. One of the boundaries was doped with a substitutional Sb impurity and it was seen that electron localisation takes place in the form of a weak Sb-metal host bond across the interface raising the possibility of further stabilisation of the boundary.

The Formation of Defect Complexes in a ZnO Grain Boundary

The microscopic properties a ZnO grain boundary containing extrinsic point defects are studied using a density functional computational approach. The results show that the grain boundary acts as a sink for native defects, such as the zinc vacancy and the oxygen interstitial, and also for bismuth substitutional impurities. The defects tend to accumulate at under-coordinated sites in the boundary core and prefer to form small clusters. In particular the segregation of Bi promotes the formation of the other native defects by lowering their formation energies in the boundary. Individually, the native defects and the Bi impurity do not produce deep interface states in the band gap which are electrically active. However, when the defects cluster to form a BiZn-VZn-Oi complex, new gap states are created of acceptor type. It is suggested that these new states are caused by defect interactions which compensate one another resulting in the depletion of an occupied impurity state and new bond formation. The results are discussed in terms of the Schottky barrier model commonly used to describe the electrical characteristics of ZnO varistors.

The effects of doping a grain boundary in ZnO with various concentrations of Bi

We have made a systematic study of the Bi-decoration process in a Σ = 13 [0001] tilt grain boundary in ZnO by first-principles calculations. This grain boundary is taken as a model system for studying the microscopic properties of commercial Bi-doped ZnO-varistors. The calculations show that the decoration process is strongly site dependent and that there is a considerable segregation energy for the Bi-atoms at low concentration. Increasing the concentration lowers the segregation energy which sets an upper limit of approximately 32% for the Bi-concentration in this grain boundary. This implies that the Bi-atoms stay in the grain boundary region rather than diffusing into the ZnO grains during the manufacturing process, but the maximum Bi-concentration is limited which is consistent with the experimental observations. Bi-impurities in ZnO act as donors at low impurity concentration, but a localized Bi-Bi-bond is formed at higher Bi-concentration in the grain boundary. This Bi-state is located in the band gap of ZnO and it may be responsible for the varistor effect observed in Bi-decorated grain boundaries.

Charge accumulation and barrier formation at grain boundaries in ZnO decorated with bismuth

Density functional plane-wave pseudopotential calculations have been performed on two high-angle grain boundaries in ZnO which have been decorated with various quantities of Bi. The results show that both grain boundaries, which have significantly different structures, can accommodate up to about 30% of substitutional Bi in qualitative agreement with experimental observations. The segregation of Bi to the boundaries results in local charge accumulation which is localized within Bi–Bi bonds or on Bi atoms. The charge accumulation in both boundaries results in fluctuations in potential across the interface and the formation of a barrier to electron transport. However, there is no evidence for a deep acceptor level usually associated with the Schottky barrier model. The present results suggest an alternative mechanism in which electrons are trapped in Bi–Bi bonds and depleted in an external field. However, defect states have not been ruled out and it is suggested that if they exist they are caused by more complex defects than those considered here.

Electronic Structure of a Bi-Doped Σ = 13 Tilt Grain Boundary in ZnO

We have investigated Bi doping in the bulk and in a Σ = 13 tilt grain boundary in ZnO using ab-initio DFT-calculations. We obtain a negative segregation energy suggesting that bismuth accumulates in the grain boundary. The Bi-atom causes considerable atomic displacements in the grain boundary increasing the local Bi–O bond length and attracting an O-atom on the opposite side of the structural unit in the grain boundary. The results suggest the formation of a Bi–rich phase in the grain boundary. The Bi-atoms act as donors and the conduction electrons are quasi-localised in the grain boundary region.

Theoretical Investigations of Interfaces in Electroceramic Materials

Some recent computer modeling studies of grain boundaries in electroceramics are reviewed. The report focuses on rutile and zinc oxide, which both have electronic device applications and describes computational methodologies employed in these investigations, both classical and quantum mechanical. Although the number of calculations is limited, there are indications that undoped symmetric grain boundaries in rutile and zinc oxide are not electrically active whereas doped boundaries do exhibit interface states in the bandgap, which will influence the electrical properties of the material.

Segregation Effects at a High-Angle Twist Boundary in ZnO

Ab initio density functional plane-wave pseudopotential calculations were performed for a Σ = 7 (θ = 21.79°) [0001] twist boundary in ZnO with and without the presence of Sb impurities. The segregation energies revealed a significant driving force for segregation and it was shown that the formation of an Sb monolayer was favoured. Decreased coordination in the boundary core suggested a trend towards the formation of an intergranular phase. The impurity states caused by the monolayer were located within the band gap and higher in energy relative to the state produced by a single impurity. Charge transfer to the Sb monolayer was observed indicating a possible enhancement of the grain boundary potential barrier.

On the stability of Met-Car analogue clusters in the solid state

Ab initio density functional plane-wave calculations are performed for a number Met-Car analogue clusters of special stability which are assembled to form hypothetical three-dimensional structures and linear chains. The stabilization energies of various competing structures are determined and their atomic and electronic properties are predicted. Some of the solid phases are found to be metallic (B8C12 and Si8C12) while others are semiconducting (N8C12) which suggests a range of new potential applications. The B8C12 phase is also considered to be a possible high-Tc superconductor. The solid phases of P8C12 are not predicted to be stable. The results demonstrate the viability of assembling new solid phases from clusters of various Met-Car analogues.