Molecular dynamics computer simulations are used to model kiloelectronvolt cluster bombardment of pure hydrocarbon [polyethylene (PE) and polystyrene (PS)] and oxygen-containing [paraformaldehyde (PFA) and polylactic acid (PLA)] polymers by 20 keV C60 projectiles at a 45° impact angle to investigate the chemical effect of oxygen in the substrate material on the sputtering process. The simulations demonstrate that the presence of oxygen enhances the formation of small molecules such as carbon monoxide, carbon dioxide, water, and various molecules containing C═O double bonds. The explanation for the enhanced small molecule formation is the stability of carbon and oxygen multiple bonds relative to multiple bonds with only carbon atoms. This chemistry is reflected in the fraction of the ejected material that has a mass not higher than 104 amu. For PFA and PLA, the fraction is approximately 90% of the total mass, whereas for PE and PS, it is less than half.

A model for predicting depth profiles due to energetic particle bombardment based on the RMS roughness of the system and the sputtering yield is proposed. The model is an extension of the macroscopic transport model proposed previously [Tuccitto, N., Zappala, G., Vitale, S., Torrisi, A., and Licciardello, A. J. Phys. Chem. C 2016, 120, 9263−9269]. The model is used to reconstruct the experimental depth profiles of a NiCr heterostructure due to bombardment by C60, SF5, O2, and Ga.

Molecular dynamics simulations, in which atomic and molecular solids are bombarded by Arn (n = 60–2953) clusters, are used to explain the physics that underlie the “universal relation” of the sputtering yield Y per cluster atom versus incident energy E per cluster atom (Y/n vs E/n). We show that a better representation to unify the results is Y/(E/U0) versus (E/U0)/n, where U0 is the sample cohesive energy per atom or molecular equivalent, and the yield Y is given in the units of atoms or molecular equivalents for atomistic and molecular solids, respectively. In addition, we identified a synergistic cluster effect. Specifically, for a given (E/U0)/n value, larger clusters produce larger yields than the yields that are only proportional to the cluster size n or equivalently to the scaled energy E/U0. This synergistic effect can be described in the high (E/U0)/n regime as scaling of Y with (E/U0)α, where α > 1.Keywords: cluster sputtering; molecular dynamics; secondary ion mass spectrometry;

Molecular dynamics (MD) simulations have been performed for 10 keV C60 bombardment of an octane molecular solid at normal incidence. The results are analyzed using the steady-state statistical sputtering model (SS-SSM) to understand the nature of molecular motions and to predict a depth profile of a δ-layer. The octane system has sputtering yield of ∼150 nm3 of which 85% is in intact molecules and 15% is fragmented species. The main displacement mechanism is along the crater edge. Displacements between layers beneath the impact point are difficult because the nonspherically shaped octane molecule needs a relatively large volume to move into and the molecule needs to be aligned properly for the displacement. Since interlayer mixing is difficult, the predicted depth profile is dominated by the rms roughness and the large information depth because of the large sputtering yield.

Molecular dynamics simulations of repetitive bombardment of solids by keV cluster beams have generated so much data that easy interpretations are not possible. Moreover, although the MD simulations remove 3–4 nm of material, that is not sufficient material to determine a depth profile. The recently developed steady-state statistical sputtering model (SS-SSM) uses information from the MD simulations and incorporates it into a set of differential equations to predict a depth profile. In this study the distributions that provide the input to the SS-SSM are compared for simulations of 15 keV bombardment of Ag(1 1 1) by C60, Au3 and Ar872 cluster beams.

The analytical steady-state statistical sputtering model (SS-SSM) is utilized to interpret molecular dynamics (MD) simulations of depth profiling of Ag solids with keV cluster beams of C60 and Au3 under different incident energy and angle conditions. Specifically, the results of the MD simulations provide the input to the SS-SSM and the result is a depth profile of a delta layer. It has been found that the rms roughness of each system correlates with the total displacement yield, a new quantity introduced in this study that follows naturally from the SS-SSM. The results indicate that the best depth profiles occur when the displacement yield is low and the sputtering yield is high. Moreover, it is determined that the expected value of the delta layer position as calculated from a depth profile rather than the peak position in the depth profile is the best indicator of the actual delta layer position.

Recently a “divide and conquer” approach was developed to model by molecular dynamics (MD) simulations dynamic secondary ion mass spectrometry (SIMS) experiments in order to understand the important factors for depth profiling. Although root-mean-square (rms) roughness can be directly calculated from the simulations, calculating depth profiles is beyond the current capability of the MD simulations. The statistical sputtering model (SSM) of Krantzman and Wucher establishes the foundation for connecting information from the MD simulations to depth profiles. In this study, we revise the SSM to incorporate more extensive information from the MD simulations in the steady-state region, thus presenting the steady-state statistical sputtering model (SS-SSM). The revised model is utilized to interpret MD simulations of 20 keV C60 bombardment of Ag at normal incidence as well as the effect of sample rotation on depth profiling.

The use of cluster beams in secondary ion mass spectrometry enables molecular depth profiling, a technique that is essential to many fields. The success of the technique often hinges upon the chemical nature of the substrate, the kinetic energy and incident angle of the primary cluster ion beam, and the sample temperature. It has been shown experimentally that the quality of depth profiles can be improved with cobombardment by a C60 cluster beam and a low-energy argon (Ar) beam. We present molecular dynamics simulations to elucidate the mechanistic reasons for the improved molecular depth profiles with an aim of understanding whether this cobombardment approach is generally applicable. We conclude that the low-energy Ar beam breaks up the surface topology created by the C60 beam, increasing the sputtering yield and reducing the buildup of chemical damage. The simulations also suggest that an equivalent result could be achieved without the Ar cobombardment by optimizing the conditions of the C60 beam.Keywords: C60; molecular depth profile; molecular dynamics; secondary ion mass spectrometry;

A recently formulated reactive Monte Carlo (RxMC) algorithm has been applied to determine the differential dissolution probability in topographically distinct crystallographic faces of β-cristobalite. Although the contribution of edge, kink, and step sites to the overall dissolution from a crystal is well-established, the effect of hydrogen bonding on flat terrace sites is less understood. In an earlier study (J. Phys. Chem. 111, 5169, 2007), 40 β-cristobalite surfaces were shown to have distinct intrasurface hydrogen bonding patterns between the hydroxylated surface groups. This study is an extension of the previous work with the aim to identify the role intrasurface hydrogen bonding has toward overall dissolution from these surfaces. Five surfaces have been chosen with varying ratios of Q2 (two surface OH groups per Si site) and Q3 (one surface OH group per Si site) groups. The results successfully show that pure Q2 {100} terrace sites with intrasurface hydrogen are less reactive than non-hydrogen bonded pure Q3 {111} terrace sites. The newly formulated RxMC algorithm for silica−water dissolution highlights the unique features of the terrace sites and distinguishes one surface from the other.

A newly developed Monte Carlo (MC) algorithm designed to study the complex interplay of dissolution and precipitation reactions on mineral surface is presented. This algorithm utilizes existing advanced reactive and configurational-biased MC techniques with new protocols specific for mineral−water interfaces. This time-independent methodology is especially advantageous for studying the kinetically slow quartz−water dissolution process. The aim is to use this method to understand the role of the local arrangement of reactive sites and surface topography in the surface evolution during dissolution. The simulations were performed in neutral pH medium, and two possible dissolution mechanisms were tested. The results indicate that out of the direct and stepwise mechanisms, the direct mechanism leads to complete dissolution that is not experimentally observed in the natural environment. On the other hand, the stepwise dissolution is more realistic, as it resembles the experimentally observed steady-state dissolution of the quartz−water system. These simulations identify the least coordinated surface sites (Q1) as the primary reactive site for hydrolysis and precipitation. Other surface sites (Q2 and Q3) also undergo hydrolysis, but they are sterically hindered and are turned passive by precipitating Q1 groups. The conclusions from the simulations are dominated by the surface topology of quartz; thus, we believe that the results are applicable for other polymorphs of silica and other protonation conditions.

The early stages of C60 bombardment of octane and octatetraene crystals are modeled using molecular dynamics simulations with incident energies of 5−20 keV. Using the AIREBO potential, which allows for chemical reactions in hydrocarbon molecules, we are able to investigate how the projectile energy is partitioned into changes in potential and kinetic energy as well as how much energy flows into reacted molecules and internal energy. Several animations have been included to illustrate the bombardment process. The results show that the material near the edge of the crater can be ejected with low internal energies and that ejected molecules maintain their internal energies in the plume, in contrast to a collisional cooling mechanism previously proposed. In addition, a single C60 bombardment was able to create many free and reacted H atoms which may aid in the ionization of molecules upon subsequent bombardment events.

Using 20-keV C60 on a Ag sample, multiple cluster bombardment events have been performed with molecular dynamics simulations. The purpose of this investigation is to develop a protocol for making depth profiling simulations tractable, as well as to examine the topographical effects which arise due to multiple impacts on smooth surfaces. The results show that when the total yield is equivalent to the removal of one atomic layer (0.24 nm), the distribution of the ejected particles is spread throughout the top 5.5 nm of the sample. Examples of individual bombardment events on the damaged surface exhibit a diversity of dynamics that is not observed on flat surfaces. Using the computational methods outlined, we have been able to run depth profiling simulations on a large-scale system.

Experiments and modeling studies have identified thermal, chemical, and mechanical processes as likely sources of laser ablation in polymeric materials. In our earlier study, molecular dynamics simulations with an embedded Monte Carlo based reaction scheme have been used to investigate the role of each of these processes separately, in a poly(methyl methacrylate) substrate irradiated by ultraviolet lasers. In the present study, using the same substrate, we allow for interactions among these processes, to better model the real ablation process, and investigate how it affects the system evolution during ablation. In the purely thermal case, chemical reactions are allowed to occur via the thermo-mechanical bond break and radical formation leading to subsequent gas formation. Similarly, in purely photochemical cases, additional bond breaks and reactions occur due to high temperatures and stresses. In all cases, it is observed that the ablation process is extended over longer time scales (≫ laser pulse width) resulting in higher yields. The thermo-mechanical bond breaks and ensuing reactions ensure that the substrate remains hotter for a long time, causing more fragmentation and ejection of the substrate. The mechanism of ejection for all thermal and chemical pathways is found to be both the thermo-mechanical in nature, driven by critical fraction of broken bonds, as well as chemical in nature, governed by near complete disintegration of the polymer matrix into monomers, small polymer fragments, and gas molecules. The formation of volatile gases, just underneath the surface assists in the ejection of the fragmented substrate. In all cases, secondary damage due to thermo-mechanical bond break process stimulated by high temperature and stresses, and by chemistry of the polymeric substrate, is found to contribute significantly more than the direct laser photochemical or photothermal damage, to the substrate and plume evolution as well as total ablation yield.

Laser ablation harnesses photon energy to remove material from a surface. Although applications such as laser-assisted in situ keratomileusis (LASIK) surgery, lithography, and nanoscale device fabrication take advantage of this process, a better understanding the underlying mechanism of ablation in polymeric materials remains much sought after. Molecular simulation is a particularly attractive technique to study the basic aspects of ablation because it allows control over specific process parameters and enables observation of microscopic mechanistic details. This Account describes a hybrid molecular dynamics−Monte Carlo technique to simulate laser ablation in poly(methyl methacrylate) (PMMA). It also discusses the impact of thermal and chemical excitation on the ensuing ejection processes. We used molecular dynamics simulation to study the molecular interactions in a coarse-grained PMMA substrate following photon absorption. To ascertain the role of chemistry in initiating ablation, we embedded a Monte Carlo protocol within the simulation framework. These calculations permit chemical reactions to occur probabilistically during the molecular dynamics calculation using predetermined reaction pathways and Arrhenius rates. With this hybrid scheme, we can examine thermal and chemical pathways of decomposition separately. In the simulations, we observed distinct mechanisms of ablation for each type of photoexcitation pathway. Ablation via thermal processes is governed by a critical number of bond breaks following the deposition of energy. For the case in which an absorbed photon directly causes a bond scission, ablation occurs following the rapid chemical decomposition of material. A detailed analysis of the processes shows that a critical energy for ablation can describe this complex series of events. The simulations show a decrease in the critical energy with a greater amount of photochemistry. Additionally, the simulations demonstrate the effects of the energy deposition rate on the ejection mechanism. When the energy is deposited rapidly, not allowing for mechanical relaxation of the sample, the formation of a pressure wave and subsequent tensile wave dominates the ejection process. This study provides insight into the influence of thermal, chemical, and mechanical processes in PMMA and facilitates greater understanding of the complex nature of polymer ablation. These simulations complement experiments that have used chemical design to harness the photochemical properties of materials to enhance laser ablation. We successfully fit the results of the simulations to established analytical models of both photothermal and photochemical ablation and demonstrate their relevance. Although the simulations are for PMMA, the mechanistic concepts are applicable to a large range of systems and provide a conceptual foundation for interpretation of experimental data.

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 C20 and C60, 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.

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.

Molecular dynamics simulations are used to elucidate mechanisms of ablation in dopant-polymer systems. In one set of simulations, a uniform distribution of thermal absorbers are added to a polymethyl methacrylate substrate and are excited. Chemical decomposition occurs in the regions near the absorbers. Ejection of large pieces of substrate then follows the thermo-chemical breakdown of material. In another set of simulations, an absorbing cluster is embedded in the polymethyl methacrylate substrate at a depth of 50 or 250 Å. Only the particles comprising the cluster are excited during the laser pulse. Ejection of material is initiated upon the fracture of the cluster and the cleavage of the surrounding polymer bonds with little chemical damage during the process. These two mechanisms of ejection suggest different pathways of ablation in doped polymer materials.

Using a molecular dynamics technique we demonstrate that the formation of holes in the d-shell can be a mechanism initiating sputtering of metastable metal atoms with a closed outer s-shell. The formation of d-holes is a result of electron promotion in the energetic collisions in the cascade developing in solid under fast ion bombardment.

Molecular dynamics simulations are used to elucidate mechanisms of ablation in dopant-polymer systems. In one set of simulations, a uniform distribution of thermal absorbers are added to a polymethyl methacrylate substrate and are excited. Chemical decomposition occurs in the regions near the absorbers. Ejection of large pieces of substrate then follows the thermo-chemical breakdown of material. In another set of simulations, an absorbing cluster is embedded in the polymethyl methacrylate substrate at a depth of 50 or 250 Å. Only the particles comprising the cluster are excited during the laser pulse. Ejection of material is initiated upon the fracture of the cluster and the cleavage of the surrounding polymer bonds with little chemical damage during the process. These two mechanisms of ejection suggest different pathways of ablation in doped polymer materials.