46 research outputs found
Solvent Induced Proton Hopping at a Water-Oxide Interface
Despite widespread interest, a detailed understanding of the dynamics of
proton transfer at interfaces is lacking. Here we use ab initio molecular
dynamics to unravel the connection between interfacial water structure and
proton transfer for the widely studied and experimentally well-characterized
water-ZnO interface. We find that upon going from a single layer
of adsorbed water to a liquid multilayer changes in the structure are
accompanied by a dramatic increase in the proton transfer rate at the surface.
We show how hydrogen bonding and rather specific hydrogen bond fluctuations at
the interface are responsible for the change in the structure and proton
transfer dynamics. The implications of this for the chemical reactivity and for
the modelling of complex wet oxide interfaces in general are also discussed.Comment: 6 pages, 5 figure
Second-Harmonic Scattering as a Probe of Structural Correlations in Liquids
Second-harmonic scattering experiments of water and other bulk molecular
liquids have long been assumed to be insensitive to interactions between the
molecules. The measured intensity is generally thought to arise from incoherent
scattering due to individual molecules. We introduce a method to compute the
second-harmonic scattering pattern of molecular liquids directly from atomistic
computer simulations, which takes into account the coherent terms. We apply
this approach to large-scale molecular dynamics simulations of liquid water,
where we show that nanosecond second-harmonic scattering experiments contain a
coherent contribution arising from radial and angular correlations on a length
scale of < 1 nm, much shorter than had been recently hypothesized (Shelton, D.
P. J. Chem. Phys. 2014, 141). By combining structural correlations from
simulations with experimental data (Shelton, D. P. J. Chem. Phys. 2014, 141),
we can also extract an effective molecular hyperpolarizability in the liquid
phase. This work demonstrates that second-harmonic scattering experiments and
atomistic simulations can be used in synergy to investigate the structure of
complex liquids, solutions, and biomembranes, including the intrinsic
intermolecular correlations
Solvent Fluctuations and Nuclear Quantum Effects Modulate the Molecular Hyperpolarizability of Water
Second-Harmonic Scatteringh (SHS) experiments provide a unique approach to
probe non-centrosymmetric environments in aqueous media, from bulk solutions to
interfaces, living cells and tissue. A central assumption made in analyzing SHS
experiments is that the each molecule scatters light according to a constant
molecular hyperpolarizability tensor . Here, we
investigate the dependence of the molecular hyperpolarizability of water on its
environment and internal geometric distortions, in order to test the hypothesis
of constant . We use quantum chemistry calculations
of the hyperpolarizability of a molecule embedded in point-charge environments
obtained from simulations of bulk water. We demonstrate that both the
heterogeneity of the solvent configurations and the quantum mechanical
fluctuations of the molecular geometry introduce large variations in the
non-linear optical response of water. This finding has the potential to change
the way SHS experiments are interpreted: in particular, isotopic differences
between HO and DO could explain recent second-harmonic scattering
observations. Finally, we show that a simple machine-learning framework can
predict accurately the fluctuations of the molecular hyperpolarizability. This
model accounts for the microscopic inhomogeneity of the solvent and represents
a first step towards quantitative modelling of SHS experiments
Connection between water's dynamical and structural properties: insights from ab initio simulations
Among all fluids, water has always been of special concern for scientists
from a broad variety of research fields due to its rich behavior. In
particular, some questions remain unanswered nowadays concerning the
temperature dependence of bulk and interfacial transport properties of
supercooled and liquid water, e.g. regarding the fundamentals of the violation
of the Stokes-Einstein relation in the supercooled regime or the subtle
relation between structure and dynamical properties. Here we investigated the
temperature dependence of the bulk transport properties from ab initio
molecular dynamics based on density functional theory, down to the supercooled
regime. We determined from a selection of functionals, that SCAN better
describes the experimental viscosity and self-diffusion coefficient, although
we found disagreements at the lowest temperatures. For a limited set of
temperatures, we also explored the role of nuclear quantum effects on water
dynamics using ab initio molecular dynamics that has been accelerated via a
recently introduced machine learning approach. We then investigated the
molecular mechanisms underlying the different functionals performance and
assessed the validity of the Stokes-Einstein relation. We also explored the
connection between structural properties and the transport coefficients,
verifying the validity of the excess entropy scaling relations for all the
functionals. These results pave the way to predict the transport coefficients
from the radial distribution function, helping to develop better functionals.
On this line, they indicate the importance of describing the long-range
features of the radial distribution function.Comment: 10 pages, 5 figure
Connection between waterâs dynamical and structural properties: Insights from ab initio simulations
Among all fluids, water has always been of special concern for scientists from a wide variety of research fields because of its rich behavior. In particular, some questions remain unanswered today regarding the temperature dependence of bulk and interfacial transport properties of supercooled and liquid water, for example, regarding the fundamentals of the violation of the StokesâEinstein relation in the supercooled regime, or the subtle relation between structure and dynamical properties. We have studied the temperature dependence of the bulk transport properties from ab initio molecular dynamics based on density functional theory, down to the supercooled regime. We determined, from a selection of functionals, that the SCAN (strongly constrained and appropriately normed) functional best describes the experimental viscosity and self-diffusion coefficient, although we found disagreements at lower temperatures. For a limited set of temperatures, we also explored the role of nuclear quantum effects on water dynamics using ab initio molecular dynamics that was accelerated by a recently introduced machine learning approach. We then investigated the molecular mechanisms underlying the different functionalsâ performance and assessed the validity of the StokesâEinstein relation. We also explored the connection between structural properties and transport coefficients, verifying the validity of the excess entropy scaling relations for all functionals. These results pave the way for the prediction of the transport coefficients from the radial distribution function, thus helping to develop better functionals. In this respect, these results indicate the importance of describing the long-range features of the radial distribution function
Impact of confinement and polarizability on dynamics of ionic liquids
Polarizability is a key factor when it comes to an accurate description of different ionic systems. The general importance of including polarizability into molecular dynamics simulations was shown in various recent studies for a wide range of materials, ranging from proteins to water to complex ionic liquids and for solidâliquid interfaces. While most previous studies focused on bulk properties or static structure factors, this study investigates in more detail the importance of polarizable surfaces on the dynamics of a confined ionic liquid in graphitic slit pores, as evident in modern electrochemical capacitors or in catalytic processes. A recently developed polarizable force field using Drude oscillators is modified in order to describe a particular room temperature ionic liquid accurately and in agreement with recently published experimental results. Using the modified parameters, various confinements are investigated and differences between non-polarizable and polarizable surfaces are discussed. Upon introduction of surface polarizability, changes in the dipole orientation and in the density distribution of the anions and cations at the interface are observed and are also accompanied with a dramatic increase in the molecular diffusivity in the contact layer. Our results thus clearly underline the importance of considering not only the polarizability of the ionic liquid but also that of the surface
ăăŒăăčă»ăăłăéžă°ăăäșșăăèȘă : ăšăŒăăăăźäșșéć
The interfaces of neat water and aqueous solutions play a prominent role in many technological processes and in the environment. Examples of aqueous interfaces are ultrathin water films that cover most hydrophilic surfaces under ambient relative humidities, the liquid/solid interface which drives many electrochemical reactions, and the liquid/vapor interface, which governs the uptake and release of trace gases by the oceans and cloud droplets. In this article we review some of the recent experimental and theoretical advances in our knowledge of the properties of aqueous interfaces and discuss open questions and gaps in our understanding
Performance estimation and Variability from Random Dopant Fluctuations in Multi-Gate Field Effect Transistors : a Simulation Study
As the formation of nearly abrupt p-n junctions in aggressively scaled transistors has become a complex task, a novel type of device in which there are no junctions has recently been suggested (J. P. Colinge et al., Nature 2010). The device of interest is referred to as the junctionless transistor, and it has demonstrated excellent functionality, with the advantage of a simpler fabrication process than conventional FETs. Despite the remarkable performances exhibited by the junctionless transistor, this device has to be tested against variability before it may be produced in large scale. Hence, the study of how the fluctuations in the number and in the position of the dopant atoms affects a large number of devices has been developed in this work. Such variability source is referred to as Random Dopant Fluctuations (RDF) and it is among the most critical ones for conventional MOSFETs. Our view is that RDF ought to largely affect the junctionless transistors. Hence, in this work we mainly aim at investigating the impact of RDF in these type of devices. Firstly, we provide a detailed analysis on the performance of an ideal junctionless transistor with a uniform non-random doping concentration, by mean of simulations developed using a TCAD software. Secondly, we investigate the effects of RDF in the junctionless transistor, as the principal aim of our study. Here, we determine how the I-V characteristics are affected by the random dopants and we illustrate fundamental the causes of the variations. A first estimation of the impact of RDF is provided by the illustration of the threshold voltage and beta [1] distributions, and by the computation of the fundamental statistical quantities relating to the two parameters. A further and last estimation is provided by the comparison obtained studying RDF on the inversion mode FET