Operando STM study of the interaction of imidazolium-based ionic liquid with graphite

Understanding interactions at the interfaces of carbon with ionic liquids (ILs) is crucially beneficial for the diagnostics and performance improvement of electrochemical devices containing carbon as active materials or conductive additives in electrodes and ILs as solvents or additives in electrolytes. The interfacial interactions of three typical imidazolium-based ILs, 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (AMImTFSI) ILs having ethyl (C2), butyl (C4) and octyl (C8) chains in their cations, with highly oriented pyrolytic graphite (HOPG) were studied in-situ by electrochemical scanning tunneling microscopy (EC-STM). The etching of HOPG surface and the exfoliation of graphite/graphene flakes as well as cation intercalation were observed at the HOPG/C2MImTFSI interface. The etching also takes place in C4MImTFSI at −1.5 V vs Pt but only at step edges with a much slower rate, whereas C8MIm+ cations adsorbs strongly on the HOPG surface under similar conditions with no observable etching or intercalation. The EC-STM observations can be explained by the increase in van der Waals interaction between the cations and the graphite surface with increasing length of alkyl chains

Linear response theory and optimal control for a molecular system under nonequilibrium conditions

In this paper, we propose a straightforward generalization of linear response theory to systems in nonequilibrium that are subject to nonequilibrium driving. We briefly revisit the standard linear response result for equilibrium systems, where we consider Langevin dynamics as a special case, and then give an alternative derivation using a change-of-measure argument that does not rely on any stationarity or reversibility assumption. This procedure moreover easily enables us to calculate the second order correction to the linear response formula (which may or may not be useful in practice). Furthermore, we outline how the novel nonequilibirum linear response formula can be used to compute optimal controls of molecular systems for cases in which one wants to steer the system to maximize a certain target expectation value. We illustrate our approach with simple numerical examples

Error Estimate of Short-Range Force Calculation in the Inhomogeneous Molecular Systems

In the present paper, we develop an accurate error estimate of the non-bonded short-range interactions for the inhomogeneous molecular systems. The root mean square force error is proved to be decomposed into three additive parts: the homogeneity error, the inhomogeneity error and the correlation error. The magnitude of the inhomogeneity error, which is dominant in the interfacial regions, can be more than one order of magnitude larger than the homogeneity error. This is the reason why a standard simulation with xed cut-off radius is either less accurate if the cut-off is too small, or wastes considerable computational effort if the cut-off is too large. Therefore, based on the error estimate, the adaptive cut-off and long-range force correction methods are proposed to boost the efficiency and accuracy of the simulation, respectively. The effectiveness of the proposed methods are demonstrated by molecular dynamics simulations of the liquid-vapor equilibrium and the nanoscale particle collision. Different roles of the homogeneity error and inhomogeneity error are also discussed

On the Numerical Accuracy of Ewald, Smooth Particle Mesh Ewald, and Staggered Mesh Ewald Methods for Correlated Molecular Systems

In this work, we develop the accurate error estimates for three state-of-art algorithms of long-range electrostatic interaction in inhomogeneous and correlated molecular systems. They are the Ewald summation, the smooth particle mesh Ewald (SPME) and the staggered mesh Ewald methods. Two branches of force computation, namely the ik- and analytical differentiation, are considered. All the estimates are developed by proposing a more general framework: if the error force is of pairwise form, then the root-mean-square force error is composed of three additive parts, the homogeneity error, the inhomogeneity error and the correlation error. Computationally scalable estimates (estimating the errors at the cost O(N log N)) are developed for all the considered algorithms. The effectiveness of the proposed estimates and the important role of the correlation error are carefully checked and demonstrated by example systems

Theoretical prediction on the structural, electronic, and polarization properties of tetragonal Bi₂ZnTiO₆

Author name used in this publication: C. H. Woo2008-2009 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Adaptive Resolution Simulation (AdResS): A Smooth Thermodynamic and Structural Transition from Atomistic to Coarse Grained Resolution and Vice Versa in a Grand Canonical Fashion

The AdResS method in molecular dynamics (MD) allows, in a grand canonical (GC) fashion, to change on-the-fly the number of degrees of freedom of a system, allowing to pass from atomistic (AT) to coarse-grained (CG) resolution and vice versa as a function of the position of the molecule in the simulation box. The coupling of resolutions is made in a thermodynamically consistent way, though in the current formulation, in the region where the molecule changes resolution, neither thermodynamic nor structural properties can be preserved. Here we propose an extension of the method where basic thermodynamic and structural properties can be systematically controlled also in the transition region; this assures a very smooth change from one molecular representation to the other. Moreover, we provide a rigorous argument which shows that if in the region where the molecules change resolution the radial distribution function (RDF) is the same as in the AT and CG region, then the AT region is, from the statistical point of view, equivalent to a subsystem embedded in a larger full AT system, at least up to a second order approximation

Reply to "Comment on 'Fano resonance for Anderson Impurity Systems' "

In a recent Comment, Kolf et al. (cond-mat/0503669) state that our analysis of the Fano resonance for Anderson impurity systems [Luo et al., Phys. Rev. Lett 92, 256602 (2004)] is incorrect. Here we want to point out that their comments are not based on firm physical results and their criticisms are unjustified and invalid.Comment: 1 page, 1 figure, to appear in PR

Grand-canonical-like molecular-dynamics simulations by using an adaptive-resolution technique

n this work, we provide a detailed theoretical analysis, supported by numerical tests, of the reliability of the adaptive resolution simulation (AdResS) technique in sampling the Grand Canonical ensemble. We demonstrate that the correct density and radial distribution functions in the hybrid region, where molecules change resolution, are two necessary conditions for considering the atomistic and coarse-grained regions in AdResS equivalent to subsystems of a full atomistic system with an accuracy up to the second order with respect to the probability distribution of the system. Moreover, we show that the work done by the thermostat and a thermodynamic force in the transition region is formally equivalent to balance the chemical potential difference between the different resolutions. From these results follows the main conclusion that the atomistic region exchanges molecules with the coarse-grained region in a Grand Canonical fashion with an accuracy up to (at least) second order. Numerical tests, for the relevant case of liquid water at ambient conditions, are carried out to strengthen the conclusions of the theoretical analysis. Finally, in order to show the computational convenience of AdResS as a Grand Canonical set up, we compare our method to the Insertion Particle Method (IMP) in its most efficient computational implementation. This fruitful combination of theoretical principles and numerical evidence candidates the adaptive resolution technique as a natural, general and efficient protocol for Grand Canonical Molecular Dynamics for the case of large systems

A Microcantilever-based Gas Flow Sensor for Flow Rate and Direction Detection

The purpose of this paper is to apply characteristics of residual stress that causes cantilever beams to bend for manufacturing a micro-structured gas flow sensor. This study uses a silicon wafer deposited silicon nitride layers, reassembled the gas flow sensor with four cantilever beams that perpendicular to each other and manufactured piezoresistive structure on each micro-cantilever by MEMS technologies, respectively. When the cantilever beams are formed after etching the silicon wafer, it bends up a little due to the released residual stress induced in the previous fabrication process. As air flows through the sensor upstream and downstream beam deformation was made, thus the airflow direction can be determined through comparing the resistance variation between different cantilever beams. The flow rate can also be measured by calculating the total resistance variations on the four cantilevers.Comment: Submitted on behalf of EDA Publishing Association (http://irevues.inist.fr/handle/2042/16838

Symmetry, complexity and multicritical point of the two-dimensional spin glass

We analyze models of spin glasses on the two-dimensional square lattice by exploiting symmetry arguments. The replicated partition functions of the Ising and related spin glasses are shown to have many remarkable symmetry properties as functions of the edge Boltzmann factors. It is shown that the applications of homogeneous and Hadamard inverses to the edge Boltzmann matrix indicate reduced complexities when the elements of the matrix satisfy certain conditions, suggesting that the system has special simplicities under such conditions. Using these duality and symmetry arguments we present a conjecture on the exact location of the multicritical point in the phase diagram.Comment: 32 pages, 6 figures; a few typos corrected. To be published in J. Phys.