Healing mechanism of multi-vacancy defective graphene under carbon irradiation

The irradiation healing process of defective graphene was studied by reactive molecular dynamics simulation of injecting C atoms on a multi-vacancy graphene sheet. We studied the effect of environment temperature and incident energy of injected atoms on healing process of defective graphene. Our simulations show that a relatively high temperature (about 1600?K) is prerequisite for perfect healing of defective graphene. Moreover, an appropriate incident energy for injected atoms (0.16?eV/atom for ～1800?K) is also necessary for perfect healing, even under a suitable temperature for perfect healing. Defect structures, such as carbon chains and blister-like structures, will occur and hinder the healing process, if the adsorption process (determined basically by incident energy) is faster than the reorganization process (dominated by temperature). In addition, the temperature dependence of reorganization capability of graphene was further studied by molecular dynamics simulation of relaxing an intact graphene sheet with adsorbed atoms. The analysis of the evolution of various micro-structures, emerged during the reorganization simulations, is helpful for deeply understanding the healing mechanism of defective graphene sheet under carbon irradiation.     

Thermal evolution spectrometry of low energy inert gas ions injected into polycrystalline UO2

Low energy ions were injected into sintered UO₂ and the probabilities of entrapment measured, as a function of ion energy from 50 eV to 3 keV, by heating the targets after irradiation and recording the ratio of quantity of gas released to incident ion fluence. Values of entrapment probability increase from zero at low ion energies to about unity at 1 keV. The rate of thermal gas evolution was also recorded as a function of temperature during the post-bombardment target tempering, enabling deduction of activation energies for evolution. These activation energies are correlated with published data on the annealing of point and extended defects in the solid and with self diffusion energies.     

Spatial and energy distributions of particles sputtered from NiPd single crystals

Molecular dynamic simulation has been used to study the sputtering of NiPd single-crystal disordered compounds. Spatial distributions, as well as the energy of Ni and Pd particles sputtered from the NiPd (001) face have been investigated, for Ar bombarding ions of energies E₀ ranging from 0.1 keV to 5 keV, incident at different angles. The results have shown a predominant exit both of Ni and Pd atoms near the same close-packed directions, in contrast to what is observed for ordered binary compounds. For unchanged composition of the surface, the emission of Ni in the <011> close-packed directions is significantly greater than the emission of Pd. When the composition of top layers was changed, however, the opposite is observed and the exit of Pd prevails. Energy dependence and energy spectra of sputtered components have been analysed as well as the origin and the number of the generation of ejected particles. The observed trends are explained by the particular behaviour of correlated collisions in single crystals.     

Implantation parameters affecting aluminum nano-particle formation in alumina

The formation of nano-dimensional metallic Al precipitates in alumina due to the reduction of the host matrix as a result of ambient temperature ion implantation of Y ions is examined. The formation and growth of Al precipitates are dependent on both the Y ion dose and the energy available to the matrix, as reported here. Reducing the ion dose from 5 × 10¹⁶ to 2.5 × 10¹⁶ ions/cm² results in smaller precipitates; 10.7 ± 1.8 nm to 9.0 nm ± 1.2 nm, respectively, for incident ion energies of 150 keV, based upon particle size measurements obtained using energy filtered transmission electron microscopy. Below a fluence of 2.5 × 10¹⁶, particle formation is not detected. The energy available to the matrix was varied; first, by controlling the incident ion energy (varied between 60 and 150 keV) while holding the substrate at ambient temperature, and second, by controlling the substrate temperature (varied between 44 and 873 K) while holding the incident ion energy constant at 150 keV. Experiments conducted with incident ion energies of 110 keV or higher produce crystalline Al precipitates; whereas implantations at 100 keV produce amorphous Al particles and implantations at 60 keV produce no detectable precipitates. The implantations carried out as a function of temperature produce successively smaller precipitates with decreasing temperature to 77 K (6.7 ± 1.0 nm), below which no precipitates are detected. An Arrhenius activation energy for the formation of the aluminum precipitates of 1.7 kJ/mole has been calculated using the volume of precipitates formed as a function of inverse temperature. This low activation energy suggests that radiation enhanced diffusion (RED) is responsible for particle growth during these implantations.     

Molecular dynamics simulation of ion ranges in the 1–100 keV energy range

Binary collision approximation methods have been conventionally used to describe the slowing down of recoiling ions. In order to better understand the slowing-down process, molecular dynamics methods are more and more used in the literature. However, the computer capacity limits the usefulness of the methods in most practical cases where ion implantation in the 1–100 keV energy range is used. We present an efficient molecular dynamics method for calculating ion ranges and deposited energies in the recoil energy region 100 eV to 100 keV. By taking into account only the interactions that are involved in the slowing-down process, range and deposited energy distributions at higher energies can be simulated. The method is demonstrated by range calculations of 40 keV H atoms in Si, 40 keV He atoms in Ta and 100 eV to 10 keV Si atoms in Si.     

Numerical estimates for energy of sputtered target atoms and reflected Ar neutrals in sputter processes

During the sputtering process in Ar gas, the sputtered target atoms and the reflected Ar neutrals from the target have much higher energy than the background gas. In this study, the Thompson distribution and an updated Meyer model based on the elastic energy transfer between two colliding particles were used to obtain energy distributions and average energies for the sputtered metal atoms and the reflected Ar neutrals. An energy dependent elastic collision cross section was incorporated into Meyer’s model and a thermalization criterion based on power balance was used. Under typical sputtering conditions (0.5 mTorr and 1000 K Ar, 400 eV incident Ar ion), the model predictions indicate that for Cu, Ti and Ta targets, the sputtered metal atoms have initial average energies from 15 to 22 eV and thermalize with the background Ar gas between 10 and 20 collisions. The reflected Ar neutrals thermalize after about 10 collisions. Depending on the number of collisions, the energy dependent mean free path values of the sputtered metal atoms range from 300 to 100 cm while the mean free path values for the reflected Ar neutrals range from 200 to 100 cm.     

Principles and applications of ion beam techniques for the analysis of solids and thin films

This paper reviews in principle and by examples how a collimated mono-energetic and mono-atomic beam incident on a target provides information on its structure and composition when the energy of the back-scattered beam atoms, or of the particles generated by nuclear reactions, is analyzed. Examples are selected with particular emphasis on thin films and Si technology. For convenience, we define three different energy ranges of the incident beam (low energies from 1 to 6 keV, medium energies from 100 to 500 keV and high energies from 1 to 2 MeV) and discuss each range separately, according to the following table of contents.     

Modified morphology of graphene sheets by Argon-atom bombardment: molecular dynamics simulations

By a molecular dynamics method, we simulated the process of Argon-atom bombardment on a graphene sheet with 2720 carbon atoms. The results show that, the damage of the bombardment on the graphene sheet depends not only on the incident energy but also on the particle flux density of Argon atoms. To compare and analyze the effect of the incident energy and the particle flux density in the Argon-atom bombardment, we defined the impact factor on graphene sheet by calculating the broken-hole area. The results indicate that, there is an exponential accumulated-damage for the impact of both the incident energy and the particle flux density and there is a critical incident energy ranging from 20-30 eV/atom in Argon-atom bombardment. Different configurations, such as sieve-like and circle-like graphene can be formed by controlling of different particle flux density as the incident energy is more than the critical value. Our results supply a feasible method on fabrication of porous graphene-based materials for gas-storages and molecular sieves, and it also helps to understand the damage mechanism of graphene-based electronic devices under high particle radiation.     

Transport of sputtered atoms investigated by Monte Carlo method

Increasing attention has been paid to the sputtering process as a tool to deposit films and to the study of the interaction between the film properties and the deposition parameters. It is obvious that the energy and direction of these particles arriving at the substrate is in close relation with the transport process from the target to the substrate. This work deals with the computer simulation of the sputtered Ag atoms trajectories through the background gas in a diode-sputtering configuration. For that, we have developed a numerical model to simulate the transport process. We followed the three-dimensional trajectory of each sputtered atom separately and calculated the scattering angle and the energy loss if a collision took place. A statistical method, Monte Carlo simulations is used. The model predicts the flux of Ag atoms arriving at the substrate, their energies and angular distribution. The dependence of the deposition rates of Ag atoms on the gas pressure and the distance between target to substrate were investigated.     

The Effect of Electron Beam Irradiation on Electron Diffraction Patterns of Bi-Sr-Ca-Cu-O High- Tc Superconductors

The effect of electron irradiation having the energy of 75, 100, and 200 keV on structural modifications of Bi-2212 superconducting samples has been studied. For the last energy, the irradiation time from zero to 150 min was used. At a constant energy of the electrons, the observed phenomena consist in the disappearance of the incommensurate unidimensional modulation, in the decreasing of spots' intensity and their elongation along the equivalent crystallographic axis a, and even spot splitting with the occurrence of double extra spots, with the increase of the irradiation time.After electron irradiation with energy of 75 and 100 keV, the structural modifications lead to some spot patterns consisting of some planar lattices (in some cases a pseudo-tetragonal one) that are twisted on each other at different angles (8°, 13.6°, 19°, etc.) around the axis of the incident electron beam. For the irradiation at increased doses of thin microcrystals having reduced lateral dimensions, the electron diffraction spots were arranged in discrete or partial continuous Debye rings or continuous concentric Debye rings characteristic for the polycrystalline state.After electron irradiation with energy of 200 keV, the effects of electron irradiation on Bi-2212 samples depend strongly on irradiation fluence rate and time and consisted in the following: disordering defects in the diffraction patterns (disappearance of some spots, spot intensity modification, streaks occurrence, spot elongation); alteration and disappearance of incommensurate structural modification; conversion of single crystal particle areas into polycrystalline material; and quasi-amorphization.A simple approach based on the evaluation of the displacement yield of in-plane oxygen atoms vs. irradiation time for the different incident energy and electron fluence rates could explain the general trend of irradiation damage in HTS materials.     

The influence of sputtering on FeSi

The effect of different (0.5, 2 and 4 keV) Ar⁺ energy ions on (a) the composition of an FeSi surface, (b) the oxidation of the FeSi surface after bombardment, and (c) the segregation of silicon after bombardment, has been monitored by Auger electron spectroscopy. Silicon was found to be preferentially sputtered by the Ar⁺ ions at all the different energies during bombardment. This effect was more pronounced at the 0.5 keV ion energy bombardment. There was a slight increase in the oxidation rate from the higher to the lower Ar⁺ ion energy at which the sample was sputtered before oxidation. The rate of silicon diffusion to the surface at 593 K after the sample had been sputtered, was lower when the sample had been sputtered by 0.5 keV ions than by 2 keV ions.     

Controlled growth of Co nanofilms on Si(100) by ion-beam sputtering

The effect of process conditions on the properties of cobalt films grown on silicon by ion-beam sputtering is analyzed from the nucleation stage to film thicknesses corresponding to the properties of bulk material. The argon ion energy is shown to play a central role in determining the sputtering process. Sputtering a cobalt target with argon ions less than 0.8 keV in energy produces granular layers. The cobalt layers grown at argon ion energies above 1.2 keV are continuous even in the nucleation stage. The layers 1.2 to 2 nm in thickness have high resistivity and are comparable in magnetic properties to bulk material. The high-energy component of the total flux of cobalt atoms ejected from the target plays an important role in the initial stages of deposition, especially at argon ion energies from 1.2 to 2.2 keV. In the nucleation stage, the energy deposited by cobalt atoms in the silicon substrate facilitates the formation of a continuous layer in the initial stage of the process.     

Studies on sputtering process of multicomponent Zr-Ti-Cu-Ni-Be alloy thin films

Sputter deposition process of a multicomponent Zr-Ti-Cu-Ni-Be metallic alloy has been studied experimentally and by numerical simulations. Monte-Carlo simulations were performed using a model based on thermalization and diffusion of sputtered atoms. Incident energy and angle of sputtered atoms on substrate were obtained from simulations. The incident angular distribution was observed to be a normal distribution at all sputtering pressures. Average incident kinetic energy of the condensing atoms on the substrate was observed to be 0.2-0.3 eV indicating most of them are thermalized. Simulations were extended to predict compositional variations in films prepared at various process conditions. These results were compared with composition of films determined experimentally using Rutherford Backscattering Spectrometry (RBS). Contents of Zr, Ti, Cu and Ni quantified using RBS were in moderate agreement with the simulated composition. Be could not be quantified accurately by RBS largely due to very low energy peak of Be in the spectrum. These studies are shown to be useful in understanding the complexities in multicomponent sputtering.     

Controlled growth of Co nanofilms on Si(100) by ion-beam deposition

This paper examines the effect of ion-beam sputtering conditions on the nucleation of Co nanofilms on Si(100). The argon ion energy is shown to play a key role in determining the sputtering process. Sputtering a cobalt target with argon ions less than 0.8 keV in energy produces granular layers. The cobalt layers grown at Ar⁺ ion energies above 1.2 keV are continuous even in the nucleation stage. The layers 1.2 to 2 nm in thickness have high resistivity and are comparable in magnetic properties to bulk material. The high-energy component of the total flux of cobalt atoms ejected from the target plays an important role in the initial stages of deposition, especially at argon ion energies from 1.2 to 2.2 keV. In the nucleation stage, the cobalt atoms have a finite penetration depth in the silicon substrate, where they give up energy which facilitates the formation of a continuous layer in the initial stage of the process.     

Glow curve analysis applied to the discrimination of X ray versus proton irradiation

A module for proton track structure simulation in liquid water was implemented in the biophysical model PARTRAC. Simulated tracks of energy deposition events from the radiation under investigation were superimposed on a higher-order DNA target model describing the whole genome inside a human cell. The parameters controlling DNA damage from direct and indirect effects were adapted to agree with yields and pathway contributions derived from gamma ray irradiation experiments. Single and double strand break (DSB) induction was simulated for irradiations by protons, photons and electrons over a wide range of initial energies. The relative biological effectiveness for DSB induction after proton irradiation was found to rise from 1.2 at 5 keV.micron-1 to about 2.5 at 70 keV.micron-1. About half of this growth resulted from an increased production of DSB clusters associated with small (< 10 kbp) fragments.     

Cluster size dependence and yield linearity in cluster bombardment simulations of benzene

Cluster bombardment of a molecular solid, benzene, is modeled using molecular dynamics simulations in order to investigate the effect of projectile cluster size and incident energy on the resulting yield. Using the mesoscale energy deposition footprint (MEDF) model, we are able to model large projectiles with incident energies from 5 to 140 keV and predict trends in ejection yield. The highest ejection yield at 5 keV was observed at C 20 and C 60, but shifts toward larger clusters for higher energies. These trends are explained in terms of the MEDF model. For these projectiles, all of the incident energy is deposited in the near-surface region, which is optimal for the projectile energy to contribute to the ejection yield. Because the energy is deposited in the optimal position for contributing to the ejection process, the yields increase linearly with incident energy with a slope that is nearly independent of the cluster size.     

Deposition of optical thin films by ion beam sputtering

The use of sputtering from diode or magnetron sources has been investigated thoroughly in the last few years in order to replace traditional evaporation methods for optical thin film deposition. The kinetic energy of sputtered materials, higher than that of evaporated atoms, is one of the most important causes of the superior adherence, hardness and mechanical stability of sputtered thin films. Present technology evolution is tending to develop new techniques, allowing higher and more controllable energies of materials impinging on the substrate. In the ion beam sputter deposition (IBSD) technique the working pressure in the deposition chamber may be lower than 10^-2 Pa, so thermalization of sputtered materials is avoided and the energies of depositing atoms are higher than in plasma sputtering, where thermalization takes place. This work describes the investigations carried out for realizing optical treatments by means of IBSD. The apparatus used for this study is described with details of the experiments carried out and the results obtained in the deposition of TiO₂, Y₂O₃, Al₂O₃, SiO₂ and ZnS. The films are characterized optically, mechanically and for the determination of the damage threshold from 1064 nm laser radiation.     

Magnetron sputter deposition as visualized by Monte Carlo modeling

The Monte Carlo code SIMTRA, simulating the transport of atoms from the source to the substrate during physical vapor deposition (PVD), is used in several case studies to highlight important issues related to thin film sputter deposition. Atom collisions during gas-phase transport affect the energy distribution and the deposition profile of sputtered atoms. The model is compared with published models for the thermalization of sputtered atoms, and some features of this process are discussed. The vacuum chamber design can be easily implemented in the Monte Carlo code, and this possibility is used to discuss the use of shutters and masks, and the influence of the deposition geometry. The code can also be used to predict the composition when combing different sources, segmented targets, and during combinatorial synthesis of thin films. As the details of the transport are described, the velocity and the density of the gas-phase atoms can be calculated which can assist in the interpretation of several spectroscopic techniques such as laser induced fluorescence. Not only the energy loss of the transported atoms, but also their remaining energy upon arrival at the substrate is important as the incident energy strongly influences thin film growth. To illustrate the latter, the model is also used to study the growth of biaxially aligned thin films. The key parameters influencing the level of alignment can easily be retrieved using SIMTRA.     

Simulation of ion sputtering of copper clusters from single crystal graphite surface

The sputtering of clusters consisting of 13 and 75 copper atoms from a (0001) graphite surface bombarded by normally incident 200-eV Ar⁺ ions was studied by molecular dynamics method. The angular distribution of sputtered copper atoms, their energies, and the sputtering yields are discussed.     

Energy deposition control during cluster bombardment: a molecular dynamics view

Molecular dynamics simulations are performed to model C60 and Au3 bombardment of a molecular solid, benzene, in order to understand the energy deposition placement as a function of incident kinetic energy and incident angle. Full simulations are performed for 5 keV projectiles, and the yields are calculated. For higher energies, 20 and 40 keV, the mesoscale energy deposition footprint model is employed to predict trends in yield. The damage accumulation is discussed in relationship to the region where energy is deposited to the sample. The simulations show that the most favorable conditions for increasing the ejection yield and decreasing the damage accumulation are when most of the projectile energy is deposited in the near-surface region. For molecular organic solids, grazing angles are the best choice for achieving these conditions.