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Recent experiments on ion irradiation of carbon nanotubes have revealed a wealth of intriguing phenomena. However, in spite of the experimental progress, the production of irradiation-induced defects in multiwalled nanotubes (MWNTs) and their properties are not yet well understood. By employing molecular dynamics with analytical potentials we simulate irradiation of MWNTs with various noble-gas ions and calculate the ion ranges as a function of ion energy. We also use the conventional binary collision stochastic approach to estimate the ranges and compare the results obtained through the two methods. We further characterize the irradiation-induced defects which appear in MWNTs under both single ion impacts and high-dose bombardment. We finally study if, similarly to carbon onions, irradiation can give rise to transformations of nanotubes to nanodiamond rods and demonstrate that such transformations do not occur in MWNTs due to their tubular structure.

Recent theoretical results indicate that ion-beam mixing can be used to synthesize nanocomposite structures from immiscible elements, relying on a self-organization phenomenon at steady state under irradiation. According to this modeling, self organization requires that the range of the forced atomic relocations in displacement cascades exceeds a critical value. Experimental evidence supporting the formation of nanocomposites by this mechanism has been found in the immiscible system Ag‐Cu irradiated with 1 MeV Kr ions. To address this experimentally relevant model system, and to test the theoretical predictions, we study, by molecular dynamics ~MD!, the characteristics of irradiation mixing in a Ag‐Cu alloy subjected to bombardment with 62 keV He, 270 keV Ne, 500 keV Ar, and 1 MeV Kr ions. The distribution of atomic relocations measured by MD is then used to perform lattice kinetic Monte Carlo ~KMC! simulations of phase evolution, during which both thermal decomposition and irradiation mixing operate simultaneously. The KMC results show that, in the framework of this self-organization mechanism, a nanocomposite structure can be stabilized at steady state by irradiation with heavy ions ~Ne, Ar, and Kr!, but not with He ions. As the characteristic relocation range for He ions is half of that measured for the heavy ions, these results support the theoretical prediction of the existence of a critical relocation range for compositional patterning to take place under irradiation. © 2003 American Institute of Physics. @DOI: 10.1063/1.1540743#

Employing both quantum mechanical and empirical force models, we use molecular dynamics simulations to obtain sticking cross sections for CH3 radical chemisorption on unsaturated sites of carbon surfaces. Effects of the local atomic neighborhood on the chemisorption are examined for the comparison of the results with experiments. Our results show that the chemisorption of a CH3 radical onto a dangling bond is highly affected by the neighborhood of the unsaturated carbon atom sites. Notably, sticking cross sections of totally bare dangling bond sites at the surface and sites partly shielded by neighboring methyl groups are observed to differ by two orders of magnitude, (15.3±1.7) A2 and (0.2±0.1) A2, respectively. We describe a steering effect which explains the recent experimental observation that the sticking cross section can be larger than the average area per surface site.

311 defects are extended, rodlike defects that play a central role in the processing of Si during integrated circuit manufacturing. Diffuse x-ray scattering techniques provide a nondestructive means to detect defects in solids. However, to date there has been no knowledge of what the x-ray scattering pattern from 311 defects looks like. Using a recently introduced fully atomistic modeling scheme, the diffuse x-ray scattering patterns were calculated from 311 defects. The results demonstrate how 311 defects can be detected, how the main varieties of 311 defect can be distinguished, and how both the defect width and length can be derived from the scattering.

We study order transitions and defect formation in a model high-entropy alloy (CuNiCoFe) under ion irradiation by means of molecular dynamics simulations. Using a hybrid Monte-Carlo/molecular dynamics scheme a model alloy is generated which is thermodynamically stabilized by configurational entropy at elevated temperatures, but partly decomposes at lower temperatures by copper precipation. Both the high-entropy and the multiphase sample are then subjected to simulated particle irradiation. The damage accumulation is analyzed and compared to an elemental Ni reference system. The results reveal that the high-entropy alloy---independent of the initial configuration---installs a certain fraction of short-range order even under particle irradiation. Moreover, the results provide evidence that defect accumulation is reduced in the high-entropy alloy. This is because the reduced mobility of point defects leads to a steady state of defect creation and annihilation. The lattice defects generated by irradiation are shown to act as sinks for Cu segregation., 29 pages, 20 figures

Diffuse x-ray scattering is a powerful means to study the structure of defects in crystalline solids. The traditional analysis of diffuse x-ray scattering experiments relies on analytical and numerical methods which are not well suited for studying complicated defect configurations. We present here an atomistic simulation method by which the diffuse x-ray scattering can be calculated for an arbitrary finite-sized defect in any material where reliable interatomic force models exist. We present results of the method for point defects, defect clusters and dislocations in semiconductors and metals, and show that surface effects on diffuse scattering, which might be important for the investigation of shallow implantation damage, will be negligible in most practical cases. We also compare the results with x-ray experiments on defects in semiconductors to demonstrate how the method can be used to understand complex damage configurations.

Ion beam mixing was investigated in crystalline and amorphous semiconductors and metals using molecular dynamics simulations. The magnitude of mixing in an amorphous element compared to its crystalline counterpart was found to be larger by a factor of 2 or more. Mixing in semiconductors was found to be significantly larger than in a face-centered-cubic (fcc) metal of corresponding mass and atomic density. The difference in mixing between amorphous and crystalline materials is attributed to local relaxation mechanisms occurring during the cooling down phase of the cascade. Comparison of mixing in semiconductors and metals shows that short range structural order also has a significant influence on mixing. The mixing results in fcc metals indicate that the role of the electron–phonon coupling in the evolution of collision cascades may be less significant than previously thought.

Doping is a widely used method to enhance the properties of materials. Despite the recently increased understanding of the mechanisms of chemical erosion by low-energy hydrogen ions, the effect of doping on these types of processes is still not well understood. We study the erosion of Si-doped (0–30 at. %) carbon under 20 eV deuterium irradiation using molecular dynamics simulations. We show that the chemical sputtering of carbon decreases with increasing Si concentration. The reasons for the reduced sputtering yield lie in the longer Si–C interaction lengths and efficient dynamic rebonding of hydrocarbon species.

A common misconception is that the irradiation of solids with energetic electrons and ions has exclusively detrimental effects on the properties of target materials. In addition to the well-known cases of doping of bulk semiconductors and ion beam nitriding of steels, recent experiments show that irradiation can also have beneficial effects on nanostructured systems. Electron or ion beams may serve as tools to synthesize nanoclusters and nanowires, change their morphology in a controllable manner, and tailor their mechanical, electronic, and even magnetic properties. Harnessing irradiation as a tool for modifying material properties at the nanoscale requires having the full microscopic picture of defect production and annealing in nanotargets. In this article, we review recent progress in the understanding of effects of irradiation on various zero-dimensional and one-dimensional nanoscale systems, such as semiconductor and metal nanoclusters and nanowires, nanotubes, and fullerenes. We also consider the t...

The structure and thermal response of amorphous ion tracks formed along the [112¯0], [101¯0], and [0001]-directions in crystalline quartz have been investigated using small angle x-ray scattering. The radii of the ion tracks vary by about 5% (0.3 nm) for tracks along different crystallographic directions. Molecular dynamics simulations reproduce this anisotropy along the [101¯0] and [0001] directions and suggest that differences in thermal conductivity along these directions are partly responsible for this observation. Using in situ annealing, tracks along the [101¯0] and [0001] directions were shown to recrystallize during thermal annealing around 960–1020 °C with activations energies around 6 eV, while those along the [112¯0]-direction already disappeared at 640 °C with a significantly lower activation energy around 3–4 eV.

Carbon nanotubes are of wide interest due to their excellent properties such as tensile strength and electrical and thermal conductivity, but are not, when placed alone on a substrate, well resistant to mechanical wear. Diamond-like carbon (DLC), on the other hand, is widely used in applications due to its very good wear resistance. Combining the two materials could provide a very durable pure carbon nanomaterial enabling to benefit from the best properties of both carbon allotropes. However, the synthesis of high-quality diamond-like carbon uses energetic plasmas, which can damage the nanotubes. From previous works it is neither clear whether the quality of the tubes remains good after DLC deposition, nor whether the DLC above the tubes retains the high sp3 bonding fraction. In this work, we use experiments and classical molecular dynamics simulations to study the mechanisms of DLC formation on various carbon nanotube compositions. The results show that high-sp3-content DLC can be formed provided the deposition conditions allow for sidewards pressure to form from a substrate close beneath the tubes. Under optimal DLC formation energies of around 40–70 eV, the top two nanotube atom layers are fully destroyed by the plasma deposition, but layers below this can retain their structural integrity.

Using molecular dynamics, we have studied the behaviour of cylindrical [001]-oriented Fe and FeCr nanowires under uniaxial tensile strain with both an embedded atom method (EAM) and a Tersoff-like bond order potential. The mechanical properties were analysed and the deformation mechanism was studied and compared between the potentials. The effects of chromium content and size of the wire were studied. Both potentials show elongation by deformation twinning in the 〈111〉/{211} system resulting in a significantly stiffer and stronger [110]-axial nanowire. The pure iron nanowires are elastically softer than bulk iron and an addition of chromium has both a softening and weakening effect. The bond order potential shows a strong dependence on chromium concentration, while the dependence is considerably weaker for the EAM potential.

Classical molecular dynamics simulation was used to irradiate a GaN nanowire with rear-earth erbium (Er). Ten cumulative irradiations were done using an ion energy of 37.5 keV on a 10 × 10 nm2 surface area which corresponds to a fluence of 1 × 1013 cm−2. We studied the location and types of defects produced in the irradiation. Er implantation leads to a net positive (expansion) strain in the nanowire and especially at the top region a clear expansion has been observed in the lateral and axial directions. The lattice expansion is due to the hydrostatic strain imposed by a large number of radiation induced defects at the top of the NW. Due to the large surface-to-volume ratio, most of the defects were concentrated at the surface region, which suggests that the experimentally observed yellow luminescence (YL) in ion implanted GaN NWs arises from surface defects. We observed big clusters of point defects and vacancy clusters which are correlated with stable lattice strain and the YL band, respectively.

We present a method for determining the melting point of copper nanowires based on classical molecular dynamics simulations and use it to investigate the dependence of the melting point on wire diameter. The melting point is determined as the temperature at which there is a significant change in the fraction of liquid atoms in the wire, according to atomic bond angle analysis. The results for the wires with diameters in the range 1.5 nm to 20 nm show that the melting point is inversely proportional to the diameter while the cross-sectional shape of the wire does not have a significant impact. Comparison of results obtained using different potentials show that while the absolute values of the melting points may differ substantially, the melting point depression is similar for all potentials. The obtained results are consistent with predictions based on the semi-empirical liquid drop model.

The effects of irradiation by F, P, and PF4 on optical properties of GaN were studied experimentally and by atomistic simulations. Additionally, the effect of Ag was studied by simulation. The irradiation energy was 0.6 keV/amu for all projectiles. The measured photoluminescence (PL) decay time was found to be decreasing faster when irradiation was done by molecular ion compared to light ion irradiation. The PL decay time change is connected with the types of defect produced by different projectiles. Simulation results show that the light ions mainly produce isolated point defects while molecular and heavy ions produce clusters of point defects. The total amount of defects produced by the PF4 projectile was found to be very close to the sum of all defects produced in five individual cascades started by one P and four F single ions. This and the similar depth profiles of damage produced by molecular and light ion irradiations suggest that the defect clusters are one of the important reasons for fast PL decay. Moreover, the simulations of irradiation by Ag ions, whose mass is close to the mass of the PF4 molecule, showed that the produced defects are clustering in even bigger conglomerates compared to PF4 case. The latter has a tendency to split in the pre-surface region, reducing on average the density of the collision cascade.

We have studied defect production during single atomic and molecular ion irradiation having an energy of 50 eV/amu in GaN by molecular dynamics simulations. Enhanced defect recombination is found in GaN, in accordance with experimental data. Instantaneous damage shows non-linearity with different molecular projectile and increasing molecular mass. Number of instantaneous defects produced by the PF4 molecule close to target surface is four times higher than that for PF2 molecule and three times higher than that calculated as a sum of the damage produced by one P and four F ion irradiation (P+4×F). We explain this non-linearity by energy spike due to molecular effects. On the contrary, final damage created by PF4 and PF2 shows a linear pattern when the sample cools down. Total numbers of defects produced by Ag and PF4 having similar atomic masses are comparable. However, defect-depth distributions produced by these species are quite different, also indicating molecular effect.

We report on the observation of nanoscale density fluctuations in 2 μm thick amorphous SiO2 layers irradiated with 185 MeV Au ions. At high fluences, in excess of approximately 5 × 1012 ions/cm2, where the surface is completely covered by ion tracks, synchrotron small angle x-ray scattering measurements reveal the existence of a steady state of density fluctuations. In agreement with molecular dynamics simulations, this steady state is consistent with an ion track “annihilation” process, where high-density regions generated in the periphery of new tracks fill in low-density regions located at the center of existing tracks.

We performed classical molecular dynamics simulations of defect production in small-diameter hexagonal Si nanowires under Ar ion irradiation. Using irradiation energies of 30 eV to 10 keV, we find that for low energies the defect production in the nanowires may be enhanced by as much as a factor of 3 in comparison to bulk Si due to the large surface-to-volume ratio of the systems. Conversely, at higher energies the increased transmission of ions causes a significant decrease in defect production.

Low energy deposition of metal alloy nanoclusters is studied by molecular dynamics simulations. In a previous study, two mechanisms were introduced for epitaxial alignment of elemental clusters: The heating induced by the surface energy released upon impact and the thermally activated dislocation motion. In this study, these mechanisms are shown to dominate for Cu3Ag, Cu3Au, and Cu3Ni clusters as well. The question whether the alloyed nature of the system or the initial chemical ordering of the particles influences epitaxial alignment with a substrate is discussed. Chemical ordering is shown to have a negligible effect due to disordering occurring at the initial stages of deposition.

In this work, we perform atomistic molecular dynamics simulations to assess the properties of small helium vacancy (He-V) and pure He clusters in body-centered cubic Fe and in Fe90–Cr10 (Fe–10Cr) random alloy. The following two goals are pursued: determining diffusion mechanisms of He-V clusters occurring in dynamic simulations and revealing a possible influence of Cr on the mobility/stability of He-V clusters in the Fe–10Cr alloy. We also present a newly developed set of interatomic potentials for the Fe–Cr–He system, fitted to a set of specially performed density functional theory calculations. The obtained results show that the dissociation energies of the studied He-V clusters, as well as the migration energy of He interstitial, are not significantly affected in the alloy compared to pure Fe. It was found that small pure He clusters with sizes up to four atoms, that were assumed to be immobile in many previous studies devoted to He-release/accumulation kinetics, in fact, exhibit fast three dimensional...

We study the possibility of modifying the structure of a multiply twinned nanoparticle by ion irradiation. Molecular dynamics simulations are carried out for the prototypic case of a metastable icosahedral Pt particle bombarded with He and Xe ions in the energy range of 0.1-10 keV. A single xenon impact can be used to melt the particle. It can also induce partial melting, which causes a collapse of the twin boundary structure in the solid part and transformation to single crystalline morphology. Under He irradiation, we observe a saturation of the vacancy concentration, but no untwinning.

Ion irradiation causes damage in semiconductor crystal structures and affects charge carrier dynamics. We have studied the damage production by high-energy (100keV–10MeV) H, He, Ne, and Ni ions in GaAs and GaAs90N10 using molecular dynamics computer simulations. We find that the heavier Ne and Ni ions produce a larger fraction of damage in large clusters than H and He. These large clusters are either in the form of amorphous zones or (after room-temperature aging or high-temperature annealing) in the form of vacancy and antisite clusters. The total damage production in GaAs and GaAs90N10 is found to be practically the same for all the ions. A clearly smaller fraction of the damage in GaAs90N10 compared to GaAs is in large clusters, however. Our results indicate that experimentally observed differences in charge carrier lifetimes between light and heavy ion irradiations, and before and after annealing, can be understood in terms of the large defect clusters. An increasing amount of damage in large clusters...

A reactive interatomic potential based on an analytical bond-order scheme is developed for the ternary system W–C–H. The model combines Brenner’s hydrocarbon potential with parameter sets for W–W, W–C, and W–H interactions and is adjusted to materials properties of reference structures with different local atomic coordinations including tungsten carbide, W–H molecules, as well as H dissolved in bulk W. The potential has been tested in various scenarios, such as surface, defect, and melting properties, none of which were considered in the fitting. The intended area of application is simulations of hydrogen and hydrocarbon interactions with tungsten, which have a crucial role in fusion reactor plasma-wall interactions. Furthermore, this study shows that the angular-dependent bond-order scheme can be extended to second nearest-neighbor interactions, which are relevant in body-centered-cubic metals. Moreover, it provides a possibly general route for modeling metal carbides. © 2005 American Institute of Physics. DOI: 10.1063/1.2149492

The 311 defects play a crucial role in the damage healing and dopant redistribution which occurs during the annealing of an ion-beam-doped Si. Using grazing-incidence x-ray scattering we measure the type, length, and width of the 311 defects created with different annealing times. In particular, we show that measurements around (1.3 1.3 0) in reciprocal space can be used to determine all these quantities without the need for pristine reference samples. The results agree well with computer simulation predictions and transmission-electron-microscopy measurements, demonstrating that x-ray methods can be used as a nondestructive, rapid method to characterize the 311 defects.