Barend J. Thijsse

On the size-dependent elasticity of silicon nanocantilevers: impact of defects

Recent measurements have indicated that the elastic behaviour of silicon nanocantilevers and nanowires is size-dependent. Several theoretical models have been proposed to explain this phenomenon, mainly focused on surface stress effects. However, discrepancies are found between experiments and theories, indicating that there could be other influences in addition to surface effects. One of the important issues, which was experimentally confirmed and has not been considered, is accounting for the fact that experimentally tested nanocantilevers and nanowires are not defect free. In this paper molecular dynamics (MD) is utilized to study the effects of defects on the elasticity of silicon. The effective Young’s modulus ˜ E of [100] and [110] oriented silicon nanoplates is extracted in the presence of defects, showing that such defects significantly influence the size-dependent behaviour in ˜ E. The MD results are compared with the results of continuum theory, showing that continuum theory holds, even for very small defects. Taking into account the surface effects, native oxide layers together with fabrication-induced defects, the experimental measurements can be explained. The studied example involved nanocantilevers, but can be extended to nanowires. (Some figures in this article are in colour only in the electronic version)

Revisiting the Al/Al?O? interface: Coherent interfaces and misfit accommodation

We study the coherent and semi-coherent Al/α-Al2O3 interfaces using molecular dynamics simulations with a mixed, metallic-ionic atomistic model. For the coherent interfaces, both Al-terminated and O-terminated nonstoichiometric interfaces have been studied and their relative stability has been established. To understand the misfit accommodation at the semi-coherent interface, a 1-dimensional (1D) misfit dislocation model and a 2-dimensional (2D) dislocation network model have been studied. For the latter case, our analysis reveals an interface dislocation structure with a network of three sets of parallel dislocations, each with pure-edge character, giving rise to a pattern of coherent and stacking-fault-like regions at the interface. Structural relaxation at elevated temperatures leads to a further change of the dislocation pattern, which can be understood in terms of a competition between the stacking fault energy and the dislocation interaction energy at the interface. Our results are expected to serve as an input for the subsequent dislocation dynamics models to understand and predict the macroscopic mechanical behavior of Al/α-Al2O3 composite heterostructures.

Collective and specific types of short and medium range order in metallic glasses

Abstract By comparing partial radial distribution functions of metallic glasses with those of their crystalline counterparts, we attempt to give an answer to the question as to which similarities exist between the short (and medium) range atomic order found in amorphous structures and that found in crystals. For this purpose the concept of “para-paracrystallinity” is used. It is found that the metal-metal distribution function of the crystal structure D0 e , which is the structure of, for example, metastable Fe 3 B, serves as a universal function for all binary (AB) glassy metals, not only for the A-A distribution function, but also for the A-B and B-B functions. The chemical type of the glass (e.g. metal-metalloid or metal-metal) appears to be of importance only for the short range order in the amorphous structure, but not for the median range order. The atomic sizes play a part only in geometrical scaling factors. Based on the evidence presented, an interpretation of the glassy state for binary alloys is given in terms of an essentially substitutional alloy with a D0 e -like organization.

Current-induced transition in atomic-sized contacts of metallic alloys

The scale of electronic devices will soon be reduced to a level where material properties will be very different from the bulk, necessitating research into very small scale devices. A metallic point contact is perhaps the most simple example of an atomic-sized device. It consists of a connection of a few atoms, typically between 1 and 1000, between two macroscopic electrodes. The size of the contact is therefore of the same order of magnitude as the Fermi wavelength of the electrons. In this limit, the conductance is related to the transmission of an electron wave through the system. For a free electron gas, the transmission only depends on the size of the constriction. As a result, the conductance in a 2-dimensional electron gas has been seen to increase by steps as a function of the width of the constriction 1,2 . The situation is more complicated in a metallic contact, where the transmission depends on the geometry of the contact as well as on the atomic structure of the atoms forming it. In the simplest case the contact consists of a single atom, where the conductance has been shown to be determined by the

Thin Ta films: growth, stability, and diffusion studied by molecular-dynamics simulations

Abstract We report results of classical molecular-dynamics simulations of bcc and β-Ta thin films. Thermal PVD film growth, surface roughness, argon ion bombardment, phase stability and transformation, vacancy and adatom diffusion, and thermal relaxation kinetics are discussed. Distinct differences between the two structures are observed, including a complex vacancy diffusion mechanism in β-Ta. Embedded atom method potentials, which were fitted to bcc properties, have been used to model the Ta–Ta interactions. In order to verify the application of these potentials to the more complex β-Ta structure, we have also performed density functional theory calculations. Results and implications of these calculations are discussed.

A partical algorithm for least-squares spline approximation of data containing noise

A simple yet robust algorithm is presented to approximate data containing noise by smooth spline functions. The method is easy to implement and fast. Experience has shown that in the great majority of cases no user decisions are required to obtain adequate approximants, which is a useful property for routine processing of experimental data. The statistical basis of the algorithm is a generalized version of the Durbin–Watson statistic, which performs well even in the presence of correlated noise. Examples are given to illustrate the power of the algorithm.

Establishing Uniform Acceptance in Force Biased Monte Carlo Simulations.

Uniform acceptance force biased Monte Carlo (UFMC) simulations have previously been shown to be a powerful tool to simulate atomic scale processes, enabling one to follow the dynamical path during the simulation. In this contribution, we present a simple proof to demonstrate that this uniform acceptance still complies with the condition of detailed balance, on the condition that the characteristic parameter λ = 1/2 and that the maximum allowed step size is chosen to be sufficiently small. Furthermore, the relation to Metropolis Monte Carlo (MMC) is also established, and it is shown that UFMC reduces to MMC by choosing the characteristic parameter λ = 0 [Rao, M. et al. Mol. Phys.1979, 37, 1773]. Finally, a simple example compares the UFMC and MMC methods.

An investigation of universal medium range order in metallic glasses

Abstract For three different metal-metalloid glasses and one metal-metal glass, it is demonstrated, using the so-called shift-effect, that the total radial distribution functions, measured by either X-ray diffraction or neutron diffraction, are all accurately reproduced by the same set of partial RDFs. The shift-effect, which has been experimentally shown to be true for Ni81B19, assumes that all partial RDFs are essentially identical, apart from an effective atomic diameter as the only scaling factor. Calculated and experimental RDFs are compared for amorphous FeMnBSi, FeNiP, PdNiP and TiNi. The atomic diameters that yield the best fits fall well within the ranges found for the same atoms in crystalline and amorphous metals of other compositions. Only when the RDF peak positions and peak heights are subjected to a very sensitive test, small composition-dependent discrepancies are detected. These are similar to those found in other investigations between the experimental partial RDFs for Ni81B19 and Fe80B20.

A reverse Monte Carlo study of amorphous Ti67Ni33

Abstract By applying the reverse Monte Carlo (RMC) simulation technique to X-ray and neutron diffraction data of amorphous Ti 67 Ni 33 , a set of atomic coordinates is obtained, which forms a realistic representation of the metallic glass structure. The atomic positions are analyzed in terms of partial radial distribution functions, nearest-neighbour and bond-angle distributions, and are compared with previously obtained RMC results for Ni 81 B 19 . Similarities and differences are discussed in terms of composition and atomic size.

A reverse Monte Carlo study of amorphous Ni81B19

Abstract The Reverse Monte Carlo algorithm (RMC) was used to construct a configuration of 1920 atoms representing the structure of amorphous Ni 81 B 19 . The input for the RMC-procedure consisted of three partial radial distribution functions obtained from neutron diffraction experiments. A good fit to these data could be obtained only when a correction for normalisation errors was allowed. It was found that a well-fitting RMC-configuration may still contain traces of its early history, which becomes evident only in the atomic positions themselves. The analysis of the final configuration, obtained after a start with random atomic positions, involves Voronoi-volumes, “bond” angles and number of nearest neighbours. Comparison with the related crystalline structures Ni 3 B and Fe 3 B strongly supports the conjecture that the metallic glass structure is built up from (distorted) prismatic units. The way in which these units are coupled is similar to that in the Fe 3 B crystal.