Tianying Yan

Experimental and simulation study of neon collision dynamics with a 1-decanethiol monolayer

A study of the energy accommodation of neon colliding with a crystalline self-assembled 1-decanethiol monolayer adsorbed on Au(111) is presented. The intensity and velocity dependencies of the scattered neon as a function of incident angle and energy were experimentally measured. Scattering calculations show good agreement with these results, which allows us to examine the detailed dynamics of the energy and momentum exchange at the surface. Simulation results show that interaction times are, at most, a few picoseconds. Even for these short times, energy exchange with the surface, both normal and in-plane, is very rapid. An important factor in determining the efficiency of energy exchange is the location at which the neon collides with the highly corrugated and structurally dynamic unit cell. Moreover, our combined experimental and theoretical results confirm that these are truly surface collisions in that neon penetration into the organic boundary layer does not occur, even for the highest incident energies explored, 560 meV.

Molecular Dynamics Simulation of the Energetic Room-Temperature Ionic Liquid, 1-Hydroxyethyl-4-amino-1,2,4-triazolium Nitrate (HEATN)

Molecular dynamics (MD) simulations have been performed to investigate the structure and dynamics of an energetic ionic liquid, 1-hydroxyethyl-4-amino-1,2,4-triazolium nitrate (HEATN). The generalized amber force field (GAFF) was used, and an electronically polarizable model was further developed in the spirit of our previous work (Yan, T.; Burnham, C. J.; Del Popolo, M. G.; Voth, G. A. J. Phys. Chem. B 2004, 108, 11877). In the process of simulated annealing from a liquid state at 475 K down to a glassy state at 175 K, the MD simulations identify a glass-transition temperature region at around 250-275 K, in agreement with experiment. The self-intermediate scattering functions show vanishing boson peaks in the supercooled region, indicating that HEATN may be a fragile glass former. The coupling/decoupling of translational and reorientational ion motion is also discussed, and various other physical properties of the liquid state are intensively studied at 400 K. A complex hydrogen bond network was revealed with the calculation of partial radial distribution functions. When compared to the similarly sized 1-ethyl-4-methyl-1,4-imidazolium nitrate ionic liquid, EMIM+/NO3-, a hydrogen bond network directly resulting in the poorer packing efficiency of ions is observed, which is responsible for the lower melting/glass-transition point. The structural properties of the liquid/vacuum interface shows that there is vanishing layering at the interface, in accordance with the poor ion packing. The effects of electronic polarization on the self-diffusion, viscosity, and surface tension of HEATN are found to be significant, in agreement with an earlier study on EMIM+/NO3- (Yan, T.; Burnham, C. J.; Del Popolo, M. G.; Voth, G. A. J. Phys. Chem. B 2004, 108, 11877).

A computer simulation model for proton transport in liquid imidazole.

A multistate empirical valence bond (MS-EVB) model is developed to simulate proton transport in liquid imidazole. This approach allows proton transfer to simultaneously occur on both reaction sites (donor and acceptor) of the imidazole molecule. The underlying imidazole and imidazolium models are described by the generalized Amber force field (GAFF). The imidazole MS-EVB model was parametrized to reproduce the ab initio proton shuttling potential energy surface (PES) of a protonated imidazole dimer in the gas phase. In bulk phase at 393 K, the MS-EVB simulation yields a proton diffusion coefficient of 0.20 A(2)/ps and a Grotthuss hopping rate of 1/36 ps(-1). Both results are in good agreement with experimental results. Despite the prevalence of a classical-like imidazolium structure with highly localized protonic charge, charge delocalization is not a negligible process in the simulations. Rather, it is shown to enhance the rate of proton diffusion by approximately 40% through Grotthuss shuttling. Analysis of the EVB states reveals that the imidazolium ions first solvation shell by imidazole molecules is highly ordered through the formation of hydrogen bonds, while the second solvation shell is highly disordered. Together with the importance of charge delocalization, this result suggests that reorientation of imidazole rings in the second solvation shell is the rate-limiting step for proton transfer.

A washboard with moment of inertia model of gas-surface scattering

A washboard with moment of inertia (WBMI) model for gas atom scattering from a flexible surface is proposed and applied. This model is a direct extension of the washboard model [J. Chem. Phys. 92, 680 (1990)] proposed for gas atom scattering from relatively rigid, corrugated surfaces. In addition, a moment of inertia is incorporated in the original washboard model to describe the flexibility of softer, more highly corrugated surfaces such as polymer or liquid surfaces. The moment of inertia of the effective surface object introduces a dependence of the efficiency of energy transfer on the position and direction of impact, a feature that has been shown to be critical by molecular dynamics simulations. The WBMI model is solved numerically by Monte Carlo integration, which makes the implementation of multiple impacts between a colliding atom and the surface very efficient. The model is applied to Ne and Ar atoms scattering from an alkylthiolate self-assembled monolayer surface and reproduces the major results obtained by classical trajectory simulation of the same system, i.e., a bimodal translation energy distribution P(E(f)) with the low-energy component well-fit with a Boltzmann distribution, but with a temperature that may (Ar) or may not (Ne) be the same as the surface temperature. This indicates that the WBMI model, with well-motivated physical assumptions and simplified interaction, reveals many of the major aspects of the gas-surface collision dynamics, though it does not take into account the real-time dynamics explicitly.

Origin of the Boltzmann translational energy distribution in the scattering of hyperthermal Ne atoms off a self-assembled monolayer

A classical trajectory simulation is performed to study the dynamics of Ne-atom scattering off an n-hexyl thiolate self-assembled monolayer (SAM). The energy distribution of the scattered Ne-atoms may be deconvoluted into a Boltzmann component based on the surface temperature and a remaining non-Boltzmann high energy component. The former component becomes as large as 100% at low Ne-atom incident energies and appears to assume a high incident energy asymptotic value of ∽0.2–0.3 for incident angles less than 50°. This Boltzmann component does not arise from a Ne–SAM intermediate, since the vast majority of the collisions are direct with only one inner turning point in the Ne+SAM motion. Instead, it has varying contributions from trajectories which directly scatter from and those that penetrate the SAM. The translation (T)→vibration (V) energy transfer mechanism for the former class of trajectories may have similarities to exit-channel T→V coupling in models of unimolecular dissociation. SAM penetration becomes more important as the initial translational energy Ei is increased, resulting in both efficient and inefficient energy transfer. The latter results from repulsive forces as the Ne-atom is ‘‘ expelled’’ from the SAM. For incident angles less than 50° the size of the Boltzmann component scales with the total incident energy Ei reflecting equivalent energy transfer probabilities from the normal and parallel components of Ei. Such a result is consistent with the corrugated SAM surface. Penetration of the SAM scales with the normal component of Ei for small incident angles θi and Ei . Both the Boltzmann component to the transitional energy distribution of the scattered Ne atom, P(Ei) and SAM penetration become negligible for an increase in θi from 45° to 60°, suggesting an abrupt transition in the collision dynamics. The angular distributions for the scattered Ne-atoms are random for low initial energies Ei and θi, reflecting the surface corrugation and not the presence of an actual Ne–SAM intermediate. At high Ei and θi the scattering is non-random, preferring to remain in the incident plane with the parallel component of Ei conserved. This study shows there are ambiguities in associating a trapping desorption intermediate with statistical gas–surface scattering attributes, such as a Boltzmann component, in the translational energy distribution and a random angular distribution.

Structure and transport properties of the LiPF6 doped 1-ethyl-2,3-dimethyl-imidazolium hexafluorophosphate ionic liquids: a molecular dynamics study.

Molecular dynamics simulations have been performed on 1-ethyl-2,3-dimethyl-imidazolium hexafluorophosphate (EMMIPF(6)) ionic liquids (ILs) doped with different molar ratios of LiPF(6) at 523.15 K and 1 bar. Ionic conductivity, self-diffusion coefficients, density, and viscosity predicted by MD simulations were found to be in good agreement with previous studies. Structural analysis shows that the Li(+) cation is strongly coordinated by the F atom of the PF(6)(-) anion, and the number of F atoms coordinated with a Li(+) cation in the first solvation shell is about six for all molar ratios of LiPF(6)/EMMIPF(6) 0.05, 0.15, 0.30, and 0.50. The coordination number of the PF(6)(-) anion within the first solvation shell of Li(+) cation is about four, which tends to increase slightly when the salt concentration is increased. The two-dimensional radial-angular distribution study shows that the Li(+)-PF(6)(-) complex tends to form the C(2v) conformation at low salt concentration, whereas C(4v) conformation becomes important at higher salt concentration. It is found that the aggregation of Li(+)-PF(6)(-) complexes occurs in all four molar ratios, whereas ionic conductivity decreases and viscosity increases at higher salt concentration. The residence time correlation of PF(6)(-) within the first solvation shell of Li(+) shows a strong memory effect. The Li(+)-hopping function further shows that the hopping of Li(+) is strongly affected by its environment with different exchange rates of the PF(6)(-) anions for the structure diffusion, and the system of 0.5 LiPF(6)/EMMIPF(6) molar ratio has the slowest hopping rate.

Energetics, transition states, and intrinsic reaction coordinates for reactions associated with O(3P) processing of hydrocarbon materials

Electronic structure calculations based on multiconfiguration wave functions are used to investigate a set of archetypal reactions relevant to O(3P) processing of hydrocarbon molecules and surfaces. These include O(3P) reactions with methane and ethane to give OH plus methyl or ethyl radicals, O(3P) + ethane to give CH3O + CH3, and secondary reactions of the OH product radical with ethane and the ethyl radical. Geometry optimization is carried out with CASSCF/cc-pVTZ for all reactions, and with CASPT2/cc-pVTZ for O(3P) + methane/ethane. Single-point energy corrections are applied with CASPT2, CASPT3, and MRCI + Q with the cc-pVTZ and cc-pVQZ basis sets, and the energies extrapolated to the complete basis set limit (CBL). Where comparison of computed barriers and energies of reaction with experiment is possible, the agreement is good to excellent. The best agreement (within experimental error) is found for MRCI + Q/CBL applied to O(3P) + methane. For the other reactions, CASPT2/CBL and MRCI + Q/CBL predictions differ from experiment by 1-5 kcal/mol for 0 K enthalpies of reaction, and are within 1 kcal/mol of the best-estimate experimental range of 0 K barriers for O(3P) + ethane and OH + ethane. The accuracy of MRCI + Q/CBL is limited mainly by the quality of the active space. CASPT2/CBL barriers are consistently lower than MRCI + Q/CBL barriers with identical reference spaces.

Molecular dynamics simulation of LiTFSI-acetamide electrolytes: structural properties.

The liquid structures of nonaqueous electrolytes composed of lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and acetamide, with LiTFSI/acetamide molar ratios of 1:2, 1:4, and 1:6, were studied by molecular dynamics simulations. The simulations indicate that the Li+ cations prefer to be six-coordinate by the sulfonyl oxygen atoms of the TFSI- anions and the carbonyl oxygen atoms of the acetamide molecules, rather than by the most electronegative nitrogen atom of the TFSI- anion. Therefore, close Li+-TFSI- contact pairs exist in the system. The TFSI- anion prefers to provide only one of four possible oxygen atoms to coordinate to the same Li+ cation. Three conformations (cis, trans, and gauche) of the TFSI- anions were found to coexist in the liquid electrolyte. At high salt concentrations, the TFSI- anions mainly adopt the gauche conformation in order to provide more oxygen atoms to coordinate to different Li+ cations, while simultaneously reducing the repulsion among the Li+ cations. On the other hand, the fraction of TFSI- anions adopting the cis conformation is largest for the system with the molar ratio of 1:6, in which many clusters, mainly composed of the Li+ cations and the TFSI- anions, are immersed in the acetamide molecules. The size and charge distribution of clusters were also investigated. In the system with the molar ratio of 1:2, nearly all of the ions in the PBC (periodic boundary conditions) box aggregate into a bulky cluster that gradually disassembles into small clusters with decreasing salt concentration. The addition of acetamide molecules was found to effectively relax the liquid electrolyte structure, and the system with the molar ratio of 1:4 was found to exhibit a more homogeneous liquid structure than the other two electrolyte systems with molar ratios of 1:2 and 1:6.

Towards predicting the power conversion efficiencies of organic solar cells from donor and acceptor molecule structures

In this study, we developed a multiscale simulation framework to estimate the power conversion efficiencies of bulk heterojunction organic solar cells based on the molecular structures of the donor and acceptor. Firstly, we proposed a way to estimate the density of states (DOS) of the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) in organic thin films based on quantum calculations, and verified the Gaussian-like DOS in the organic semiconductors. Secondly, the electronic couplings in these thin films were calculated. By adding PC71BM molecules, although the donor–donor couplings are not altered significantly, the charge mobility is enhanced via additional donor–acceptor and acceptor–acceptor couplings. Thirdly, random walk simulations were performed to estimate the charge carrier mobilities. Finally, by incorporating the calculated energy levels, mobilities and DOS of these bulk heterojunctions into the numerical model developed, we obtained the working curves and power conversion efficiencies, which are in general consistence with experiment results. This study builds the foundation for the computation of power conversion efficiencies of organic solar cells using fully atomistic simulations.

Development of mean-field electrical double layer theory*

In order to understand the electric interfacial behavior, mean field based electric double layer (EDL) theory has been continuously developed over the past 150 years. In this article, we briefly review the development of the EDL model, from the dimensionless Gouy–Chapman model to the symmetric Bikerman–Freise model, and finally toward size-asymmetric mean field theory models. We provide the general derivations within the framework of Helmholtz free energy of the lattice–gas model, and it can be seen that the above-mentioned models are consistent in the sense that the interconversion among them can be achieved by reducing the basic assumptions.