Quantum behaviour of hydrogen and muonium in vacancy-containing complexes in diamond

Most solid-state electronic structure calculations are based on quantum electrons and classical nuclei. These calculations either omit quantum zero-point motion and tunnelling, or estimate it in an extra step. Such quantum effects are especially significant for light nuclei, such as the proton or its analogue, μ+. We propose a simple approach to including such quantum behaviour, in a form readily integrated with standard electronic structure calculations. This approach is demonstrated for a number of vacancy-containing defect complexes in diamond. Our results suggest that for the NHV- complex, quantum motion of the proton between three equivalent potential energy minima is sufficiently rapid to time-average measurements at X-band frequencies

Lattice dilatation of boron in silicon

AbstractRecent data on laser-annealed, boron-implanted silicon are analysed, and the results compared with earlier experiments and simple theory. Each substitutional boron decreases the total volume by about 90% of the volume per silicon atom in the perfect crystal

Exploiting the excited state

AbstractThe major ideas of microelectronics are associated with the electronic ground state, or with states thermally accessible at modest temperatures. Photonics, and many realisations of quantum devices, require excited states. Excited states are the basis of new processing methods for organic and inorganic systems. The natures of excited states can vary enormously and, especially in the wide gap materials, the processes involving excited states are extraordinarily varied. Can these states be controlled or exploited, rather than merely accessed in spectroscopy? Certainly electronic excitation can be used in materials modification, when the ideas of charge localisation and energy localisation are central. The basic processes of energy transfer, energy conversion, energy control, and control of phase exploit wide ranges of excitation intensity, and of spatial and temporal scale. Length scales can span extreme miniaturisation in lithography or quantum dots, mesoscopic scales similar to optical wavelengths, and human scales. Timescales range even more widely, from femtosecond plasmon responses, through picoseconds for self-trapping or pre-plume ablation, to many years for the lifetimes of device components.Semiconductor systems underly two relatively new areas of enormous potential. The first concerns the dynamics of quantum dots, especially the II–VI dots of a few hundred atoms for which confinement is significant, rather than the self-organised III–V dots for which the Coulomb blockade is crucial. The second is quantum information processing based on silicon-compatible quantum gates. In both cases, there are key issues of coherence, whether electronic, vibrational or explicitly quantal. In both cases, conventional intuitive models are insufficient. Yet the emerging picture is optimistic: the combination of small physical size and the variety of available excited states open up major opportunities

Mesoscopic modelling of bipolar charge evolution in CN-PPV LEDs

Since various chances are possible in the molecular structure of the repeat unit, substituted poly(para-phenylenevinylene) (PPV) has ben used as active component in light-emitting diodes (LEDs) to obtain light emission in a wide range of colours.A major aspect determining device performance is the competition between current flow, trapping and recombination within the polymer layer. By suitable Monte Carlo calculations, we have performed computer experiments in which bipolar charge carriers are injected at constant rate in polymer networks made of cyano-substituted PPV chains with variable length and orientation. The intra-molecular electronic properties used in these simulations were calculated by a quantum molecular dynamics method. In order to assess the influence of cyano-substitution on the properties of single-layer PPV LEDs, we have focused our attention on bipolar charge evolution in time. Specifically addressed are the differences in electric field strength needed for intra-molecular charge mobility of electrons and holes and their consequences at mesoscopic scale. (C) 2004 Elsevier B.V. All rights reserved

A simulation of the NiO/Ag interface with point defects

The NiO/Ag interface has been modelled using established simulation techniques, which have been modified to include the image interactions between the oxide ions and the induced charge in the metal. The energies of point defects near the interface were calculated and it was found that the surface rumpling was such that defects with a negative net charge were favoured. This will result in a space charge layer with excess cation vacancies which will cancel the interfacial potential. A low energy interface was modelled in which the cation sub-lattice of the second oxide plane was saturated with vacancies and Ni3+. ions. Such a structure may be responsible for the observed excess of oxygen near the NiO/Ni interface, and also for the low wetting angles of metals on NiO, compared with MgO

Theory of hydrogen in liquid and solid metals

AbstractA method for calculating the interatomic forces between isolated hydrogens and their host metal atoms is outlined. The method uses a semiempirical, molecular-orbital approach for a suitable cluster of atoms, with the empirical parameters fitted to experimental potential energy curves for diatomic molecules. Parameters suitable for hydrogen in liquid or solid Li and Na are given.The method is applied to the calculation of solvation energies of hydrogen in liquid Li and Na, where satisfactory agreement with experiment is obtained. Detailed potential energy surfaces are also found for H in solid Na and estimates are made of local mode frequencies, the stability of the tetrahedral sites, lattice relaxation, and effective charges, and atomic radii. Neither the anionic nor the protonic limit is appropriate. It has not proved possible to describe the potential energy surfaces in terms of a sum of twobody and volume-dependent terms alone

Effect of molecular properties on the performance of polymer light-emitting diodes

The performance of a single layer polymer light-emitting diode depends on several interdependent factors, although recombination between electrons and holes within the polymer layer is believed to play an important role. Our aim is to carry out computer experiments in which bipolar charge carriers are injected in polymer networks made of poly(p-phenylene vinylene) chains randomly oriented. In these simulations, we follow the charge evolution in time from some initial state to the steady state. The intra-molecular properties of the polymer molecules obtained from self-consistent quantum molecular dynamics calculations are used in the mesoscopic model. The purpose of the present work is to clarify the effects of intra-molecular charge mobility and energy disorder on recombination efficiency. In particular, we find that charge mobility along the polymer chains has a serious influence on recombination within the polymer layer. Our results also show that energy disorder due to differences in ionization potential and electron affinity of neighbouring molecules affects mainly recombinations that occur near the electrodes at polymer chains parallel to them.Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência, Tecnologia, Inovação” - POCTI/CTM/41574/2001Comunidade Europeia (CE). Fundo Europeu de Desenvolvimento Regional (FEDER

Dynamics of braze spreading

AbstractFundamental studies of liquid flow provide a useful framework for analysing the spreading behaviour of brazes. Three stages can be distinguished by analysing kinetic data for the spreading of a reactive NiP braze over FeCr workpieces. The first and last stages when the interfacial microstructure is relatively stable can be modelled by mathematical relationships developed to describe the flow of idealised non-reactive systems, while the intermediate stage can be related to the flow kinetics of active metal brazes over ceramics. We discuss the mechanisms responsible and argue that classification of kinetic behaviour could be of value in modelling other systems

Role of image forces in non-contact scanning force microscope images of ionic surfaces

AbstractWe consider the effect of the image interaction on the force acting between tip and surface in non-contact scanning force microscope experiments. This interaction is relevant when a conducting tip interacts with either a polar bulk sample or with a thick film grown on a conducting substrate. We compare the atomistic contribution due to the interaction between the microscopic tip apex and the sample with the macroscopic van der Waals and image contributions to the force on the tip for several representative NaCl clusters adsorbed on a metal substrate. We show that the microscopic force dominates above the plain (001) terrace sites and is solely responsible for image contrast. However, the image force becomes comparable to the microscopic force above the surface di-vacancy and dominates the interaction above a charged step

Theoretical study of the influence of the morphology in polymer-based devices functioning

It is well known that the morphology of polymer-based optoelectronic devices can influence their efficiency, since the ways that polymer chains pack inside the active layer can influence not only the charge transport but also the optic properties of the device. By using a mesoscopic model we carried out computer experiments to study the influence of the polymer morphology on the processes of charge injection, transport, recombination and collection by the electrodes opposite to those where the injection of bipolar charge carriers take place. Our results show that for polymer layers where the conjugated segments have perpendicular and random orientation relative to the electrodes surface, the competition between charge collection and charge recombination is affected when the average conjugation length of the polymer strands increase. This effect is more pronounced with the increase of the potential barrier at polymer/electrode interfaces that limit charge injection and increase charge collection. For these molecular arrangements the intra-molecular charge transport plays a major role in device performance, being this effect negligible when the polymer molecules have their axis parallel to the electrodes. Although the polymer morphology modelled in this work is far from real, we believe that our model can give some insights on the role of the microstructure on the functioning of polymer-based devices.European Community Fund (FEDER)Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência , Tecnologia, Inovação” – POCTI/CTM/41574/2001, CONC-REEQ/443/EEI/2005 e SFRH/BD/22143/200