Carbon nanobuds form a class of hybrid structures consisting of carbon nanotubes onto which fullerene types of units are covalently grown. Due to higher electrophilicity and curvature of the fullerene moiety a carbon nanobud exhibits higher reactivity compared to a plain nanotube. In this paper we study how the electronic structure and transport properties of carbon nanobuds are affected by chemical modification. The studied model systems comprise carbon nanobuds that are chemically modified by attaching Li and F atoms as well as tetrathiafulvalene molecules. We use the density functional theory combined with Landauer-Buttiker electron transport formalism. According to the simulations, the attached units change the relative positions of the Fermi levels, creating a distinctive effect on the electronic transport properties along associated carbon nanotubes. In semiconducting nanotubes the change in the conductance is systematic and should be detectable in experiments. Hence, the carbon nanobuds are potential candidates for sensor applications.

The low-energy cluster beam deposition of Ge clusters on a Si surface was simulated using classical molecular dynamics. In an effort to find a suitable energy range to construct porous Ge films, the porosity of the resulting layers was mapped as a function of deposition energy. It was discovered that the energies of interest to produce porosities in the range of 30% to 70% were between about 10 meV and 500 meV per atom. Also, it became clear that the number of deposited clusters must be above 40 for the calculated porosities to be accurate. In addition, transmission electron microscope image simulations were performed on the deposited samples, and images of porous and non-porous layers were found to be distinctly different.

We have studied the geometrical structures and atomic distributions in small and intermediate size (⩽512 atoms) SimGem clusters by different optimizing methods. We found out that there is a strong tendency for Ge to segregate onto the surface. This can be attributed to arise from the differences in atomic sizes, surface energies and elastic constants. From those the former two seem to have a bigger effect.

We have studied the effect on ion irradiation in short period SimGen superlattices by using classical molecular dynamics. We have analyzed the degree of amorphization, mixing of layers, displacement lengths for atoms, etc., in order to find out what kind of effects collision cascades cause in the system. Particular interest was put on the effect of pseudomorphic strain. We found that the strain does not have any influence on the outcome in the bulk whereas on the surface there is a clear effect on the adatom production. Furthermore, we also found that there is an asymmetry in the impurity distribution in the layers that we concluded to be an atomic size effect.

Molecular dynamics with analytical potentials is commonly used to obtain the distribution of defects produced by energetic particles in elemental and compound semiconductors. Collision cascades simulated by model-potential molecular dynamics are used to collect statistical data on the defect distribution but the local structure in such materials as GaAs is commonly recognized to be unreliable in comparison to tight-binding or ab initio total energy calculations. These two methods, however, are not practical in simulations of collision cascades because of their large computational workload. In this paper, we analyse the properties of the basic point defects in GaAs as obtained by using different model potentials and compare them to recent ab initio calculations based on the density-functional theory (DFT) in the local-density approximation (LDA). The aim of this work is to evaluate how close the model potential molecular dynamics predictions are to the benchmark DFT results and which model potential most accurately predicts realistic local structures of point defects.

It has been experimentally observed that Ge will segregate to the surface during Si deposition on Ge/Si(100). We have studied the segregation process by the analytical potential molecular dynamics method. We concentrated on recognizing the segregation mechanism and the time scales involved in the phenomenon. Firstly, we showed that the analytical potential molecular dynamics method can be successfully used to study the processes leading to the segregation. We found that the segregation process is dominated by thermally activated processes at high temperature (1000 K) and that the time scale for the process is fairly short (∼1 ns). We describe the observed atom-level segregation mechanisms in detail.

We have studied the Si20 cluster deposition on the Si(001) surface at two different substrate temperatures (300 and 1000 K) with various cluster energies. At 300 K we did not find any kind of epitaxial growth. At 1000 K there was clear epitaxial growth for all energies used in the study. In our case an energy of 5 eV per atom for the cluster gives the fastest growth rate. We observed that during the epitaxial growth there is an amorphous film on the substrate and there is an advancing crystalline–amorphous interface below it.

Recent experiments on ion irradiation of heavy metals such as gold and silver have shown that very unusual surface configurations can be produced by the irradiation. Typically, the surface damage has the shape of a crater, similar to those produced by meteorite impacts. The crater shapes are, however, often highly asymmetric and can show extended adatom ridges extending far from the crater well. Using molecular dynamics simulations we show how such exotic atom arrangements are produced. We describe atomic bridges over a crater and illustrate a slingshot-like effect which can propel atom clusters far from an impact position to produce isolated adatom islands.

It has recently become clear that electron irradiation can recrystallize amorphous zones in semiconductors even at very low temperatures and even when the electron beam energy is so low that it cannot induce atomic displacements by ballistic collisions. We study the mechanism of this effect using classical molecular dynamics augmented with models describing the breaking of covalent bonds induced by electronic excitations. We show that the bond breaking allows a geometric rearrangement at the crystal-amorphous interface which can induce recrystallization in silicon without any thermal activation.

By using classical molecular dynamics technique we have simulated the effects of 5 keV Xe atoms impinging on the strained Ge(100) 2×1 surface. We found that large adatom islands are formed on top of the amorphous zones created by the cascades. We also found that lattice atoms around the molten zone move radially inwards and thus cause strain relief in the sample.

Although ion beam mixing has been studied intensively over the last 20 years, many questions about the fundamental mechanisms involved during mixing remain unresolved. We review here recent simulation and experimental work which provides answers to some of the lingering questions about mixing in elemental materials. The results make clear the specific role which thermodynamic material properties, the nature of atomic bonding and electron–phonon coupling can have on ion beam mixing. Agreement obtained by direct comparison of simulated and experimental mixing coefficients gives confidence in our results, indicating that the experimental mixing values in heavy metals can be understood predominantly on the basis of atomic motion in liquid-like zones, and that the role of the electron–phonon coupling on ion beam mixing is much smaller than previously thought.

The erosion of carbon by intensive hydrogen bombardment has been recently shown to decrease sharply at very high fluxes (similar to 10(19) ions/cm(2) s). This effect cannot be explained by standard sputtering or erosion models, yet understanding it is central for selection of fusion reactor divertor materials, and formulation of sputtering models for high-flux conditions. Using molecular dynamics computer simulations we now show that the effect is due to the buildup of a high hydrogen content at the surface, leading to a shielding of carbon atoms by the hydrogen. [S0163-1829(99)50344-5].

We have studied atomic mixing in silicon by the classical molecular dynamics method, and directly compared the simulated data to experimental measurements. The relative importance of ballistic collisions and heat spike to the mixing is considered. We obtain a fairly good agreement between experiments and simulations. The heat spike contribution to the total mixing seems to be much lower than that of ballistic collisions.

We have studied the ion beam – surface interactions with the classical molecular dynamics simulation method. The properties of the GaAs (001) surface predicted by the potential model were investigated. The structure and amount of defects created on the GaAs (001) and Ge (001) surfaces under 50 eV Ga and Ge ions, respectively, were investigated and compared. The defect creation for the GaAs system was found to differ considerably from that of the Ge system. Since Ga, As, and Ge have similar masses this illustrates the importance of chemical effects on damage production in low-energy ion irradiation.

Variable-temperature scanning tunneling microscopy is used to characterize surface defects created by 4.5 keV He ion bombardment of Si(001) at 80-294 K; surface defects are created directly by ion bombardment and by diffusion of bulk defects to the surface. The heights and areal densities of adatoms, dimers. and adatom clusters at 80 and 130 K an approximately independent of temperature and in reasonable agreement with molecular dynamics calculations of adatom production. At 180 K, the areal density of these surface features is enhanced by a factor of similar to 3. This experimental result is explained by the migration and surface trapping of bulk interstitials formed within similar to 2 nm of the surface.

A comparative molecular dynamics simulation study of collision cascades in two elemental semiconductors and five fee metals is performed to elucidate how different material characteristics affect primary defect production during ion irradiation. By using simulations of full 400 eV-10 keV collision cascades and contrasting the results on different materials with each other, we probe the effect of the mass, melting temperature, material strength, and crystal structure on the modification of the material due to the cascade. The results show that the crystal structure has a strong effect on many aspects of damage production, while other material characteristics are of lesser overall importance. In all materials studied, isolated point defects produced by the cascade are predominantly interstitials. In semiconductors, amorphous clusters are produced in the cascade core, whereas in metals most of the crystal regenerates, leaving only small vacancy-rich clusters. Large interstitial clusters found in a few events in the heavy metals were observed to form by the isolation of a high-density liquid zone during the recrystallization phase of a cascade.