This combined experimental and theoretical study seeks to determine the role that inductive effects have on hydrogen bonds by an investigation into the change in intramolecular hydrogen bond strength in 2-amino-1-trifluoromethylethanol (2ATFME) relative to that in 2-aminoethanol (2AE). Toward this end, the rotational spectra of the normal, 13C, and 15N isotopologues have been measured using Fourier transform microwave spectroscopy and fit to the rotational, quadrupole coupling, and centrifugal distortion constants of the Watson A-reduction Hamiltonian. Structural parameters used to characterize the strength of the intramolecular hydrogen bond have been determined from the experimental structures of both 2ATFME and 2AE as well as from MP2/6-311++G(d,p) calculations. A comparison of these parameters in 2ATFME with those of 2AE indicates that the electron-withdrawing trifluoromethyl CF3 group strengthens the hydrogen bond. These include a 4% decrease in the distance between the donor and acceptor heavy atoms of the hydrogen bond, a 6% increase toward linearity of the OH···N angle, and a 23% decrease of the COH···N torsional angle toward planarity in 2ATFME relative to 2AE. This trend toward increased intramolecular hydrogen bond strength in 2ATFME is also observed within the ab initio structures.

Beams of single C(+) ions are used for the incorporation of Si in the synthesis of thin films of SiC, which have a wide range of technological applications. We present a theoretical investigation of the use of C60 cluster beams to produce thin films of SiC on a Si substrate, which demonstrates that there are potential advantages to using C60(+) cluster ion beams over C(+) single ion beams. Molecular dynamics simulations of the multi-impact bombardment of Si with 20 keV normal incident C60 projectiles are performed to study the buildup of carbon and the formation of a region of Si-C mixing up to a fluence of 1.6 impacts/nm(2) (900 impacts). The active region of the Si solid is defined as the portion of target that contains almost all of the C atoms and the height ranges from 3 nm to more than 7 nm below the average surface height. The C fraction in the active region is calculated as a function of fluence, and a simple model is developed to describe the dependence of the C fraction on fluence. An analytic function from this model is fit to the data from the molecular dynamics simulations and extrapolated to predict the fluence necessary to achieve equilibrium conditions in which the C fraction is constant with fluence. The fraction of C atoms at equilibrium is predicted to be 0.19, and the fluence necessary to achieve 90% of this asymptotic maximum value is equal to 4.0 impacts/nm(2).

The delta layer depth response predicted by a simple statistical sputtering model is compared with molecular sputter depth profile data obtained on Langmuir-Blodgett delta layer systems. All input parameters of the statistical sputtering model are determined from low-fluence molecular dynamics simulations performed for 20-keV C-60 cluster bombardment of silicon, making the model de facto parameter free. It is found that both calculated and measured depth response functions can be parametrised by the semiempirical Dowsett expression. The resulting parameters (leading and trailing edge slope, full-width at half-maximum) agree surprisingly well with those determined from the measured depth profiles. Copyright (C) 2012 John Wiley & Sons, Ltd.

Molecular dynamics simulations of multi-impact bombardment of Si with 20-keV C-60 projectiles at normal incidence are performed for a total of 400 impacts, which corresponds to a fluence of of 7 x 10(13) C-60/cm(2). The surface is roughened by successive bombardment and achieves a steady-state root mean square roughness of 2.0 nm after about 100 impacts. There is a direct correlation between the local topography of the region around the impact point and the sputtered yield. The greatest yields of sputtered atoms are produced when the projectile impacts a mound, which is characterized by the height of the surface relative to the average surface height. When the projectile hits a local region corresponding to a crater with a height much less than the average surface height, the sputtered yield is very small. However, it is these trajectories that deposit carbon atoms at depths beneath the region from which atoms are sputtered, and are responsible for the buildup of carbon at the bottom of craters. Copyright (C) 2012 John Wiley & Sons, Ltd.

We present a simple statistical model describing the removal and relocation of material during a sputter depth profiling experiment. All input parameters are determined from low‐fluence molecular dynamics simulations, making the model de facto parameter free. The model can be used to extrapolate data from the molecular dynamics simulations to projectile fluences relevant to sputter depth profiling experiments. As a result, the erosion of the surface is calculated in terms of fluence‐dependent filling factors of different sample layers. Using input data determined for the 20‐keV C60 cluster bombardment of silicon, it is found that a steady‐state erosion profile is reached after removal of approximately 20 monolayer equivalents of material. Plotting the contribution of particles from a specific layer to the instantaneous sputtered flux, one can directly determine the delta layer response function predicted from such a model. It is shown that this function can be parameterized by the semiempirical Dowsett response function, and the resulting fitting parameters are compared with published depth profile data. The model is then used to study the role of different processes influencing the observed depth resolution. We find that the statistical nature of the sputtering process suffices to explain many features of experimentally measured delta layer depth profiles. Copyright © 2012 John Wiley & Sons, Ltd.

The delta layer depth response predicted by a simple statistical sputtering model (SSM) is compared with molecular sputter depth profile data obtained on Langmuir-Blodgett delta layer systems. All input parameters of the SSM are determined from low-fluence molecular dynamics simulations performed for 20-keV C60 cluster bombardment of silicon, making the model de facto parameter-free. It is found that both the calculated and measured depth response functions can be parametrized by the semi-empirical Dowsett expression. The resulting parameters (leading and trailing edge slope, full width half maximum) agree surprisingly well with those determined from the measured depth profiles.

The incorporation of carbon atoms into a silicon surface under bombardment with 40-keV C60+ ions is investigated using time-of-flight mass spectrometry of sputtered neutral and ionized SinCm clusters. The neutral particles emitted from the surface are post-ionized by strong field infrared photoionization using a femtosecond laser system operated at a wavelength of 1400/1700nm. From the comparison of secondary ion and neutral spectra, it is found that the secondary ion signals do not reflect the true partial sputter yields of the emitted clusters. The measured yield distribution is interpreted in terms of the accumulating carbon surface concentration with increasing C60 fluence. The experimental results are compared with those from recent molecular dynamics simulations of C60 bombardment of silicon.

An important factor that determines the possible lateral resolution in sputter depth profiling experiments is ion induced lateral displacement of substrate atoms. Molecular dynamics (MD) simulations are performed to model the successive bombardment of Si with 20keV C60 at normal incidence. A statistical analysis of the lateral displacement of atoms that originate from the topmost layer is presented and discussed. From these results, it is determined that the motion is isotropic and can be described mathematically by a simple diffusion equation. A “diffusion coefficient” for lateral displacement is determined to be 3.5Å2/impact. This value can be used to calculate the average lateral distance moved as a function of the number of impacts. The maximum distance an atom may move is limited by the time that it remains on the surface before it is sputtered. After 800 impacts, 99% of atoms from the topmost layer have been removed, and the average distance moved by these atoms is predicted to be 100Å. Although the behavior can be described mathematically by the diffusion equation, the behavior of the atoms is different than what is thought of as normal diffusion. Atoms are displaced a large distance due to infrequent large hops.

We have performed molecular dynamics (MD) simulations to investigate the effect of SiC bond formation on fluence‐dependent results in 20 keV C60 bombardment of Si. Sputter depth profiling experiments of C60 on Si have produced atypical results, which are thought to be caused by the strong covalent bonds that are formed between the C atoms in the projectile and Si atoms in the substrate. A recently developed scheme developed by Russo, et al. 8 has been adapted to perform MD simulations of 150 successive impacts of 20 keV C60 on Si, which corresponds to a total fluence of 2.64 × 1013 impacts/cm2. In order to isolate the effects of SiC bond formation, the same set of trajectories is calculated with and without the attractive SiC potential energy terms. When SiC bonds are able to form, nearly all the C atoms from the projectile are incorporated into the substrate. When the possibility of SiC bond formation is removed, most of the C atoms are backscattered into the vacuum. The cumulative result is that the substrate with SiC bonds contains a factor of twenty times more C atoms, which are located below the surface. Copyright © 2010 John Wiley & Sons, Ltd.

In this theoretical investigation, we combine the results of molecular dynamics (MD) Simulations with a simple Statistical sputtering model (SSM) in order to understand the factors limiting the optimum depth resolution achievable in sputter depth profiling experiments. The advantage of the SSM model is that it can be used to extrapolate the MD simulations toward the regime of high projectile fluence. First, a recently developed scheme [Russo, M. F., Jr.; Postawa, Z.; Garrison, B. J., J. Phys. Chem. C 2009, 113, 3270] is adapted to calculate 200 cumulative impacts of 20-keV C-60 bombardment on a Si substrate, which corresponds to a projectile fluence of 3.5 x 10(13) cm(-2). The following results are studied as a function of fluence: the development of surface topography, the amount and depth of origin of sputtered material, and the relocation of substrate particles, which produces interlayer mixing. Data from the MD simulations are used as input parameters in the SSM, which is able to reproduce results consistent with the MD simulations. A major finding from these studies is that the statistical nature of the sputtering process has a significant effect on the achievable depth resolution. The optimum delta layer response width for Si is estimated to be about 3 nm, while corresponding values of the order of 10 nm are predicted for molecular systems.

This article presents a theoretical study aimed at understanding the reactive nature of carbon in secondary ion mass spectrometry experiments of Si with keV C-60(+). Molecular dynamics simulations are performed to model the bombardment of three different substrates, Si, SiC, and diamond, with normal incident C-60 at kinetic energies ranging from 5 to 20 keV. Projectile atoms form strong covalent bonds with both Si and C and, therefore, are incorporated into the target material. Although these substrates have the same diamond lattice structure, they differ greatly in their cohesive energy and number density. The yield is found to be a factor of two times greater on SiC than on Si or diamond. The mesoscale energy deposition footprint model is used to understand the reasons for the differing, behaviors of the substrates as a result of C-60 bombardment.

Molecular dynamics simulations of the 20-keV C60 bombardment at normal incidence of Si, SiC, diamond and graphite targets were performed. The unique feature of these targets is that strong covalent bonds can be formed between carbon atoms from the C60 projectile and atoms in the solid material. The mesoscale energy deposition footprint (MEDF) model is used to gain physical insight into how the sputtering yields depend on the substrate characteristics. A large proportion of the carbon atoms from the C60 projectile are implanted into the lattice structure of the target. The sputtering yield from SiC is ∼twice that from either diamond or Si and this can be explained by both the region of the energized cylindrical tract created by the impact and the number density. On graphite, the yield of sputtered atoms is negligible because the open lattice allows the cluster to deposit its energy deep within the solid. The simulations suggest that build up of carbon with a graphite-like structure would reduce any sputtering from a solid with C60+ bombardment.

Depth profiling experiments using secondary ion spectrometry (SIMS) have shown effects that are characteristic to the pairing of the C60+ projectile with a Si target. Previous molecular dynamics simulations demonstrate that this unusual behavior is due to the fact that strong covalent bonds are formed between the C atoms in the projectile and the Si atoms in the target, which result in the implantation of carbon into the solid. The focus of this paper is to understand how the formation of chemical bonds affects the net sputtered yield. The results of molecular dynamics simulations of the keV bombardment of Si with C60, Ne60 and 12Ne60 at normal incidence are compared over a range of incident kinetic energies from 5 to 20keV. The net yields with Ne60 and 12Ne60 are significantly greater than with C60 at all incident kinetic energies, with 12Ne60 having the largest values. Application of the mesoscale energy deposition footprint (MEDF) model shows that the initial deposition of energy into the substrate is similar with all three projectiles. Snapshots of the initial pathway of the projectile atoms through the substrate show a similar lateral and vertical distribution that is centered in the region of the energy footprint. Therefore, the reason for the reduced yield with C60 is that the C atoms form bonds with the Si atoms, which causes them to remain in the substrate instead of being sputtered.

Molecular dynamics simulations of the sputtering of Si by keV C60 bombardment have been performed as a function of incident kinetic energy at two incident angles, normal incidence and 45°. Nearly all of the C atoms remain embedded in the surface after bombardment because the C atoms from the projectile form strong covalent bonds with the Si atoms in the target. At low incident kinetic energies, the sputtering yield of Si atoms is small and there is a net deposition of solid material from the projectile atoms. As the incident kinetic energy is increased, the yield of sputtered Si atoms increases. A transition occurs in which the yield of sputtered Si atoms exceeds the number of C atoms deposited, and there is a net erosion of the solid material. A significantly higher sputter yield is observed at an incident angle of 45° than at normal incidence, and therefore, the energy value is lower for the transition from net deposition to net erosion. This phenomenon is discussed in terms of the depth distribution of deposited energy, which is found to be shallower at an incident angle of 45°.

Molecular dynamics simulations of the sputtering of Si by C60 keV bombardment are performed in order to understand the importance of chemical reactions between C atoms from the projectile and Si atoms in the target crystal. The simulations predict the formation of strong covalent bonds between the C and Si atoms, which result in nearly all of the C atoms remaining embedded in the surface after bombardment. At low incident kinetic energies, little sputtering of Si atoms is observed and there is a net deposition of solid material. As the incident kinetic energy is increased, the sputtering yield of Si atoms increases. At 15 keV, the yield of sputtered Si atoms is more than twice the number of C atoms deposited, and there is a net erosion of the solid material.

Molecular dynamics simulations of the bombardment of a organic film on a gold substrate with Au and Au2 projectiles have been performed in order to understand the role of mass matching in the mechanisms for energy transfer. There will be an effective energy transfer from the projectile to the substrate atoms if the atom impacts the substrate unimpeded by molecules in the organic film. When a projectile atom hits an organic molecule, the process of fragmentation absorbs its incident kinetic energy and it enters the substrate region with little energy left. With Au2, there is a higher probability that one or more of the projectile atoms will hit a bare portion of the surface and initiate collision cascades with sufficient energy to result in ejection. Atoms in the top layer of the substrate lift off stable, intact molecules as they move out of the crystal.

Using the results of molecular dynamics (MD) simulations, we discuss the question of whether the observed difference in mass limits in secondary ion mass spectrometry (SIMS) and matrix assisted laser desorption ionization (MALDI) are inherently related to the underlying physics of ejection or rather insufficient experimentation. The simulations show clearly that the physics of large molecule emission in SIMS and MALDI is very different. Consequently, we conclude that larger molecules can be ejected in MALDI than in SIMS.

Sputtering of organic overlayers has been modeled using molecular dynamics computer simulations. The investigated systems are composed of benzene molecules condensed into one, two and three layers on an Ag{1 1 1} surface. The formed organic overlayers were bombarded with 4 keV Ar projectiles at normal incidence. The development of the collision cascade in the organic overlayer was investigated. The sputtering yield, mass, internal and kinetic energy distributions of ejected particles have been analyzed as a function of the thickness of the organic layer. The results show that all emission characteristics are sensitive to the variation of layer thickness. Although most of the ejected intact benzene molecules originate from the topmost layer, the emission of particles located initially in second and third layers is significant. The analysis indicates that the metallic substrate plays a dominant role in the ejection of intact organic molecules.