Characterizing the local oxidation nanolithography on highly oriented pyrolytic graphite.

Here, we characterize the patterns obtained through local oxidation nanolithography (LON) on highly oriented pyrolytic graphite as the write bias, speed, and force were varied. Different types of patterns-bumps, cracked bumps, and trenches-were obtained and characterized using four shape descriptors-pattern width, pattern height, cut width and cut depth. With an increase in write bias the obtained pattern type varied from bumps to cracked bumps to trenches. The use of a bias above 7.25 V resulted in trenches with increased variability in shape descriptor values. Similarly, an increase in write speed demonstrated a transition from trenches to cracked bumps to bumps. An increase in write force from 75 to 150 nN showed a shift in the threshold voltage from 4.25 V to just under 3.75 V and formed cracked bumps instead of bumps. These findings help solve the mystery of why bumps were not reported at threshold voltages before 2008. We believe these findings will be enable uniform reproduction and report of LON pattern.

A comparison of sodium and hydrogen halides at the air-water interface.

New molecular models, parameterized to ab initio calculations, were developed to describe HBr and HI at the air-water interface. These were used to compare how the air-water interface influenced dissociation of NaX and HX, with X being Cl, Br, or I, and also their propensity for the interface. The polarizable multistate empirical valence bond method, which explicitly describes proton sharing, was used to model HX. Results showed that the air-water interface suppressed HX dissociation from a contact ion pair to a solvent separated to a greater degree than NaX dissociation. Furthermore, HX had a greater propensity for the interface than NaX, which was a consequence of the hydronium ion having a greatest interfacial activity of all species studied. As a consequence of this, the average configuration of dissociated HX, while in both contact ion and solvent separated ion pairs near the air-water interface, is with the dissociated hydrogen oriented more towards the air than the X atom.

HCl accommodation, dissociation, and propensity for the surface of water.

A new reactive and polarizable molecular model was developed to describe HCl dissociation in liquid water and used to investigate HCl behavior at the air-water interface. It was found that the mechanism of HCl accommodation at the air-water interface began with its hydrogen pointing toward the water as it approached from the air. This was followed by dissociation into a contact ion pair once solvated at the air-water interface with the hydronium oriented more toward the air than the chloride on average. In comparison with NaCl, HCl showed some similar behavior in that its contact ion pair was stabilized at the air-water interface in comparison with the bulk. However, dissociated HCl had a greater propensity for the air-water interface than NaCl due to the fact that the hydronium ion was more surface active than sodium.

Interfacial behavior of simple inorganic salts at the air-water interface investigated with a polarizable model with electrostatic damping.

New molecular models that incorporated polarizable interactions with electrostatic damping were developed to better understand the interfacial properties of aqueous electrolyte systems. The models were parameterized to give free energies of aqueous solvation and the change in activity with respect to concentration in agreement with experiment. Specifically, we investigated NaCl, NaBr, and NaI systems, finding anion propensity for the air-water interface was reduced in comparison with previously developed polarizable models. This coincided with a more negative surface excess than that given by previously developed polarizable models. Furthermore, we investigated the interfacial properties of SrCl2 aqueous systems, finding that strontium had a moderate enhancement in interfacial density in comparison with bulk, while still having a fairly large negative surface excess, in agreement with experimental results.

How intermolecular charge transfer influences the air-water interface.

The interfacial properties of three water models that allow for intermolecular charge rearrangement were examined with molecular dynamics simulations. They included the TIP4P water model, the TIP4P-FQ water model, which recently were modified to include intermolecular charge transfer [A. J. Lee and S. W. Rick, J. Chem. Phys. 134, 184507 (2011)]. Furthermore, another model with intermolecular charge transfer was developed for this work that was both flexible and polarizable. The effect of including intermolecular charge transfer is modest on most interfacial properties, including surface tension, electrostatic potential, interfacial dipole, and structure. However, a negative charge was found to build up at the air-water interface, but much smaller than has been measured experimentally.

Computational study of ion distributions at the air/liquid methanol interface.

Molecular dynamic simulations with polarizable potentials were performed to systematically investigate the distribution of NaCl, NaBr, NaI, and SrCl(2) at the air/liquid methanol interface. The density profiles indicated that there is no substantial enhancement of anions at the interface for the NaX systems, in contrast to what was observed at the air/aqueous interface. The surfactant-like shape of the larger more polarizable halide anions, which is part of the reason they are driven to air/aqueous interfaces, was compensated by the surfactant nature of methanol itself. These halide anions had on average an induced dipole of moderate magnitude in bulk methanol. As a consequence, methanol hydroxy groups donated hydrogen bonds to anions where the negatively charged side of the anion induced dipole pointed, and methyl groups interacted with anions where the positively charged side of the anion-induced dipole pointed. Furthermore, salts were found to disrupt the surface structure of methanol. For the neat air/liquid methanol interface, there is relative enhancement of methyl groups at the outer edge of the air/liquid methanol interface in comparison with hydroxy groups, but with the addition of NaX this enhancement was reduced somewhat. Finally, with the additional of salts to methanol, the computed surface potentials decreased, which is in contrast to what is observed in corresponding aqueous systems, where the surface potential increases with the addition of salts. Both of these trends have been indirectly observed with experiments. The surface potential trends were found to be due to the greater propensity of anions for the air/water interface that is not present at the air/liquid methanol interface.

Computational investigation of the influence of organic-aqueous interfaces on NaCl dissociation dynamics.

NaCl pairing and dissociation was investigated at the CCl(4)-water and 1,2-dichloroethane (DCE)-water interfaces, and compared with dissociation results in the bulk and at the air-water interface utilizing polarizable potentials. The transition path sampling methodology was used to calculate the rate constant for dissociation, while umbrella sampling was used to map out a free energy profile for NaCl dissociation. The results found that ion pairing was weakest at the organic-water interfaces, even weaker than in the water bulk. This is in contrast to what has been observed previously for the air-water interface, in which NaCl ion paring is stronger than in the bulk [C. D. Wick, J. Phys. Chem. C 113, 6356 (2009)]. A consequence of the weaker binding at the organic-water interfaces was that ion dissociation was faster than in the other systems studied. Interactions of the organic phase with the ions influenced the magnitude of the Cl(-) induced dipole moment, and at the organic-water interfaces, the average Cl(-) induced dipole was found to be lower than at the air-water interface, weakening interactions with Na(+). These weaker interactions were found to be responsible for the weaker ion pairing found at the organic-water interfaces.

The behavior of NaOH at the air-water interface: a computational study.

Molecular dynamics simulations with a polarizable multistate empirical valence-bond model were carried out to investigate NaOH dissociation and pairing in water bulk and at the air-water interface. It was found that NaOH readily dissociates in the bulk and the effect of the air-water interface on NaOH dissociation is fairly minor. Also, NaOH complexes were found to be strongly repelled from the air-water interface, which is consistent with surface tension measurements. At the same time, a very strong preference for the hydroxide anion to be oriented toward the air was found that persisted a few angstroms toward the liquid from the Gibbs dividing surface of the air-water interface. This was due to a preference for the hydroxide anion to have its hydrogen pointing toward the air and the fact that the sodium ion was more likely to be found near the hydroxide oxygen than hydrogen. As a consequence, the simulation results show that surfaces of NaOH solutions should be negatively charged, in agreement with experimental observations, but also that the hydroxide has little surface affinity. This provides the possibility that the surface of water can be devoid of hydroxide anions, but still have a strong negative charge.

Computational investigation of the first solvation shell structure of interfacial and bulk aqueous chloride and iodide ions.

Molecular dynamics simulations with polarizable interaction potentials were carried out to understand the solvation structure of chloride and iodide anions in bulk and interfacial water, showing qualitative similarities between the first solvation shell structures at the interface and bulk. For the more polarizable iodide, its solvation structure was found to be more anisotropic than chloride, and this trend persisted at both the interface and in the bulk. The anisotropy of the solvation structure correlated with polarizability, but it was also found to inversely correlate with anion size. When polarizability was reduced to near zero, a very small anisotropy in the water solvation structure around the ion still persisted. Polarizable anions were found to have on average an induced dipole in the bulk that was significantly larger than zero. This induced dipole resulted in the water hydrogen atoms having stronger interactions with the anions on one side of them, in which the dipole was pointing. In contrast, the other side of the anions, in which the induced dipole was pointing away from, had fewer water molecules present and, for the case of iodide, was rather devoid of water molecules all together at both the interface and in the bulk. This region formed a small cavity in the bulk, whereas at the air-water interface it was simply part of the air interface. In the bulk, this small cavity may be viewed as somewhat hydrophobic, and the need for the extinction of this cavity may be one of the major driving forces for the more polarizable anions to reside at the air-water interface.

Investigating hydroxide anion interfacial activity by classical and multistate empirical valence bond molecular dynamics simulations.

Molecular dynamics simulations were carried out to understand the propensity of the hydroxide anion for the air-water interface. Two classes of molecular models were used, a classical polarizable model and a polarizable multistate empirical valence bond (MS-EVB) potential. The latter model was parametrized to reproduce the structures of small hydroxide-water clusters based on proton reaction coordinates. Furthermore, nuclear quantum effects were introduced into the MS-EVB model implicitly by refitting its potential energy function to account for them. The final MS-EVB model showed reasonable agreement with experiment and ab initio molecular dynamics simulations for dynamical and structural properties. The free-energy profiles for both the classical and MS-EVB models were mapped out across the air-water interface, and the classical model gave a higher free energy at the interface with respect to bulk. However, the MS-EVB model gave little free-energy difference between when the hydroxide anion was in the bulk and when it was present at the air-water interface with its oxygen fully solvated and its hydrogen pointing toward the vapor. When the hydroxide oxygen started to desolvate, the free energy increased dramatically, suggesting that the hydroxide anion can be found in the interfacial region.

Electrostatic dampening dampens the anion propensity for the air-water interface.

Molecular dynamics simulations with polarizable potentials and electrostatic dampening were carried out to understand the influence of electrostatic dampening on the propensity of anions for the air-water interface. New anion molecular models incorporating these features were developed for this work. The results showed that electrostatic dampening reduced the average anion induced dipole in bulk water, in agreement with previous investigations [M. Masia, J. Chem. Phys. 128, 18 (2008)]. As a consequence, electrostatic dampening was found to significantly reduce, but not eliminate, the influence of polarizability on the anion propensity for the air-water interface. The Br(-) and I(-) models showed reduced propensity for the air-water interface with respect to previous models parametrized in a similar manner, but with no electrostatic dampening.

Solid, liquid, and interfacial properties of TiAl alloys: parameterization of a new modified embedded atom method model.

New interatomic potentials for pure Ti and Al, and binary TiAl were developed utilizing the second nearest neighbour modified embedded-atom method (MEAM) formalism. The potentials were parameterized to reproduce multiple properties spanning bulk solids, solid surfaces, solid/liquid phase changes, and liquid interfacial properties. This was carried out using a newly developed optimization procedure that combined the simple minimization of a fitness function with a genetic algorithm to efficiently span the parameter space. The resulting MEAM potentials gave good agreement with experimental and DFT solid and liquid properties, and reproduced the melting points for Ti, Al, and TiAl. However, the surface tensions from the model consistently underestimated experimental values. Liquid TiAl's surface was found to be mostly covered with Al atoms, showing that Al has a significant propensity for the liquid/air interface.

Distribution, structure, and dynamics of cesium and iodide ions at the H2O-CCl4 and H2O-vapor interfaces.

Molecular dynamics simulations utilizing many-body potentials of H2O-CCl4 and H2O-vapor interfaces were carried out at different cesium and iodide ion concentrations to compare ion distribution, interfacial orientational and structural properties, and dynamics. It was found that cesium was repelled by both interfaces, and iodide was active at both interfaces, but to a much greater degree at the H2O-vapor interface. At the interface, the iodide dipole was strongly induced, orienting perpendicular to the interface for both systems, leading to stronger hydrogen bonds with water. For the H2O-CCl4 interface, though, there was a compensation between these strong hydrogen bonds and short to moderate ranged repulsion between iodide and CCl4. Hydrogen bond distance and angular distributions showed weaker water-water hydrogen bonds at both interfaces, but generally stronger water-iodide hydrogen bonds. Both translational and rotational dynamics of water were faster at the interface, while for CCl4, its translational dynamics was slower, but rotational dynamics faster at the interface. For many of the studied systems and species, translational diffusion was found to be anisotropic at both interfacial and bulk regions.

Computational observation of enhanced solvation of the hydroxyl radical with increased NaCl concentration.

Classical molecular dynamics simulations with many-body potentials were carried out to quantitatively determine the effect of NaCl salt concentration on the aqueous solvation and surface concentration of hydroxyl radicals. The potential of mean force technique was used to track the incremental free energy of the hydroxyl radical from the vapor, crossing the air-water interface into the aqueous bulk. Results showed increased NaCl salt concentration significantly enhanced hydroxyl radical solvation, which should significantly increase its accommodation on water droplets. This has been experimentally observed for ozone aqueous accommodation with increased NaI concentration, but, to our knowledge, no experimental study has probed this for hydroxyl radicals. The origin for this effect was found to be very favorable hydroxyl radical-chloride ion interactions, being stronger than those for water-chloride.

Diffusion at the liquid-vapor interface of an aqueous ionic solution utilizing a dual simulation technique.

The recently proposed dual simulation technique [J. Phys. Chem. B 2004, 108, 6595.], with slight modification, was used to determine the diffusion coefficients for a variety of regions of a 2.2 M sodium chloride aqueous solution with a vapor-liquid interface. The diffusion of all species was shown to be isotropic far away from the interface, but at different regions in the interface, the diffusion coefficients parallel and perpendicular to the interface did not agree for water and chloride. Specifically, interfacial water diffusion parallel to the interface was significantly higher than diffusion perpendicular to the interface. Chloride ions showed even larger anisotropicity in its diffusion coefficient at the interface, with its perpendicular diffusion being similar to its bulk value, but parallel diffusion being much higher, corresponding to the region of highest chloride ion concentration. The origin for this was found to be hydrogen bonds with waters which are highly oriented perpendicular to the interface, somewhat impeding chloride ion diffusion perpendicular to the interface. While sodium ion diffusion increased at the interface, its interfacial concentration is low in that region, and its diffusion was fairly isotropic throughout all regions.

Transferable potentials for phase equilibria. 7. Primary, secondary, and tertiary amines, nitroalkanes and nitrobenzene, nitriles, amides, pyridine, and pyrimidine.

The transferable potentials for phase equilibria (TraPPE) force fields are extended to amine, nitro, nitrile, and amide functionalities and to pyridine and pyrimidine. In many cases, the same parameters for a functional group are used for both united-atom and explicit-hydrogen representations of alkyl tails. Following the TraPPE philosophy, the nonbonded interaction parameters were fitted to the vapor-liquid coexistence curves for selected one-component systems. Coupled-decoupled configurational-bias Monte Carlo simulations in the Gibbs ensemble were applied to neat (methyl-, dimethyl-, trimethyl-, ethyl-, diethyl-, or triethyl-)amine, nitromethane, nitroethane, nitrobenzene, acetonitrile, propionitrile, acetamide, propanamide, butanamide, pyridine, and pyrimidine. Excellent agreement with experimental results was found, with the mean unsigned errors being less than 1% for both the critical temperature and the normal boiling temperature. Similarly, the liquid densities at low reduced temperatures are reproduced to within 1%, and the deviation for the critical densities is about 4%. Additional simulations were performed for the binary mixtures of methylamine + n-hexane, diethyl ether + acetonitrile, 1-propanol + acetonitrile, and nitroethane + ethanol. With the exception of the methylamine/n-hexane mixture for which the separation factor is substantially overestimated, agreement with experiment for the other three mixtures is very satisfactory.

Temperature dependence of hydrogen bonding: an investigation of the retention of primary and secondary alcohols in gas-liquid chromatography.

Configurational-bias Monte Carlo simulations in the Gibbs Ensemble were carried out to investigate the analyte partitioning of n-pentane, n-hexane, n-heptane, 1-propanol, and 2-propanol into a dioctyl ether retentive (stationary) phase used in gas-liquid chromatography. The united-atom version of the TraPPE (transferable potentials for phase equilibria) force field was used to model all analytes and the solvent. The analyte partition coefficients, Gibbs free energies of transfer, and Kovats retention indexes were calculated at four different temperatures ranging from 303.15 to 348.15 K. Although hydrogen bonding is a major contributor to the retention of the alcohol analytes over the entire temperature range, its importance for the separation factor between the primary and secondary alcohol decreases substantially with increasing temperature. The enthalpies and entropies for hydrogen bond formation were also estimated from the temperature dependence of the corresponding equilibrium constants. In agreement with experimental measurements, it is observed that the hydrogen bond involving 1-propanol is enthalpically favored, but entropically disfavored compared to 2-propanol.

Bringing Reactivity to the Aggregation-Volume-Bias Monte Carlo Based Simulation Framework: Water Nucleation Induced by a Reactive Proton.

The development of the aggregation-volume-bias Monte Carlo based simulation technique has led to recent success in studying rare nucleation events, but thus far, this simulation method has been limited to nonreactive systems. This work presents the first application of this technique to study a reactive system of relevance to atmospheric chemistry, i.e., formation of water droplets in the presence of a reactive proton, by combining this approach with a multistate empirical valence bond (MSEVB) description of the excess proton (or the hydronium). It was shown that the ability for the hydronium to share its charge with adjacent water molecules changes dramatically with the cluster size, especially when clusters are small and the distribution of the charge is affected by the presence of an interface, emphasizing the need to use this more sophisticated MSEVB model for such a reactive system. In addition, the simulation results obtained from this system are compared to those with nonreactive hard-sphere ions of different sizes. Overall, the presence of a hydronium or ions appeared to dramatically change the free energy landscape of nucleation compared to the pure water system, leading to the formation of a stable precritical cluster. Although the free energy change due to the addition of the first few water molecules was shown to be very sensitive to the ionic details, the later portion of the free energy profile was found to be nearly independent of the nature of the ion.

Temperature effects on the retention of n-alkanes and arenes in helium-squalane gas-liquid chromatography. Experiment and molecular simulation.

Experiments and molecular simulations were carried out to study temperature effects (in the range of 323 to 383 K) on the absolute and relative retention of n-hexane, n-heptane, n-octane, benzene, toluene and the three xylene isomers in gas-liquid chromatography. Helium and squalane were used as the carrier gas and retentive phase, respectively. Both the experiments and the simulations show a markedly different temperature dependence of the retention for the n-alkanes compared to the arenes. For example, over the 60 K temperature range studied, the Kovats retention index of benzene is found to increase by about 16 or 18+/-10 retention index units determined from the experiments or simulations, respectively. For toluene and the xylenes, the experimentally measured increases are similar in magnitude and range from 14 to 17 retention index units for m-xylene to o-xylene. The molecular simulation data provide an independent method of obtaining the transfer enthalpies and entropies. The change in retention indices is shown to be the result of the larger entropic penalty and the larger heat capacity for the transfer of the alkane molecules.

Computational observation of pockets of enhanced water concentration at the 1-octanol/water interface.

Molecular dynamics simulations with polarizable potentials were carried out to investigate the 1-octanol-water interface in which a significant amount of water migrated into the 1-octanol phase. A region of enhanced water concentration, around three times the average concentration in water saturated 1-octanol, was present 18 Å from the Gibbs dividing surface into the 1-octanol phase. This coincided with two layers of 1-octanol molecules, forming a somewhat ordered bilayer with the first layer having its hydroxyl group pointed toward the water phase. The second layer of 1-octanol had hydroxy groups pointed in the opposite direction on average. A consequence of this was a region of high alkyl concentration and reduced polarity, as has been previously observed. Water structure in the octanol phase contracted as it approached the 1-octanol phase, opposite what was observed at the n-octane-water interface with polarizable potentials. In contrast, 1-octanol hydroxy structure expanded as it came in contact with water.