Photocurrent generation in organic photovoltaics (OPVs) relies on the dissociation of excitons into free electrons and holes at donor/acceptor heterointerfaces. The low dielectric constant of organic semiconductors leads to strong Coulomb interactions between electron-hole pairs that should in principle oppose the generation of free charges. The exact mechanism by which electrons and holes overcome this Coulomb trapping is still unsolved, but increasing evidence points to the critical role of hot charge-transfer (CT) excitons in assisting this process. Here we provide a real-time view of hot CT exciton formation and relaxation using femtosecond nonlinear optical spectroscopies and non-adiabatic mixed quantum mechanics/molecular mechanics simulations in the phthalocyanine-fullerene model OPV system. For initial excitation on phthalocyanine, hot CT excitons are formed in 10(-13) s, followed by relaxation to lower energies and shorter electron-hole distances on a 10(-12) s timescale. This hot CT exciton cooling process and collapse of charge separation sets the fundamental time limit for competitive charge separation channels that lead to efficient photocurrent generation.

The decomposition of ethylene carbonate (EC) during the initial growth of solid-electrolyte interphase (SEI) films at the solvent-graphitic anode interface is critical to lithium ion battery operations. Ab initio molecular dynamics simulations of explicit liquid EC/graphite interfaces are conducted to study these electrochemical reactions. We show that carbon edge terminations are crucial at this stage, and that achievable experimental conditions can lead to surprisingly fast EC breakdown mechanisms, yielding decomposition products seen in experiments but not previously predicted.Improving the fundamental scientific understanding of lithium ion batteries 1-3 is critical for electric vehicles and efficient use of solar and wind energy. A key limitation in current batteries is their reliance on passivating solid electrolyte interphase (SEI) films on graphitic anode surfaces. 1-5 Upon first charging of a pristine battery, the large negative potential applied to induce Li + intercalation into graphite decomposes ethylene carbonate (EC, Fig. 1) molecules in the solvent, yielding a self-limiting, 30-50 nm thick, passivating SEI layer containing Li 2 CO 3 , lithium ethylene dicarbonate ((CH 2 CO 3 Li) 2 ), 2,4-6 and salt decomposition products. C 2 H 4 and CO gases have also been detected 7,8 and shown to come from EC. 9 Similar reactions occur during power cycling when the SEI film cracks and graphite is again exposed to EC. 2 If instead the solvent is pure propylene carbonate (PC), a stable SEI film does not materialize 1,2 and the battery fails. Our work shows that novel mechanisms for the initial stages of SEI-growth at electrode-electrolyte interfaces can be simulated within time scales accessible to ab initio molecular dynamics (AIMD), 10 which have successfully modelled liquid-solid interfaces. 11 AIMD is likely also applicable to shed light on cosolvent/additives which must decompose more readily than EC to alter and improve SEI structure, Li + transport, and passivating properties. 1,2 EC-decomposition mechanisms under electron-rich conditions have been proposed (e.g., Refs. 4,5) and investigated using gas cluster Density Functional Theory calculations with and without dielectric continuum approximation of the liquid environment. 12-15 Thus "EC − ", coordinated to Li + or otherwise, has been predicted to undergo ethylene carbon (C E )-oxygen (O 1 ) bond cleavage to form a more stable radical anion (Figs. 1a-b). The potential energy barrier involved is at least 0.33 eV. 12,14 Carbonyl carbon (C C )-O 1 bond-"breaking" (or elongation) in the gas phase EC − -Li + complex yields a lower barrier, but metastable products. 14 Unlike these previous work, AIMD simulations can include explicit liquid state environments and EC/graphite interfaces. Unlike classical force field-based simulations, 16,17 AIMD accounts for covalent bondbreaking. We apply the VASP code, 18,19 the PerdewBurke-Ernzerhof (PBE) functional, 20 Γ-point Brillouin zone sampling, 400 eV planewave energy cutoff, tritium masses for all protons to allow B...

Understanding the acid-base behavior of silica surfaces is critical for many nanoscience and bionano interface applications. Silanol groups (SiOH) on silica surfaces exhibit two acidity constants-one as acidic as vinegar-but their structural basis remains controversial. The atomic details of the more acidic silanol site govern not just the overall surface charge density at near neutral solution pH but also how ions and biomolecules interact with and bind to silica immersed in water. Using ab initio molecular dynamics simulations and multiple representative crystalline silica surfaces, we determine the deprotonation free energies of silanol groups with different structural motifs. We show that previously proposed motifs related to chemical connectivity or intersilanol hydrogen bonds do not yield high acidity. Instead, a plausible candiate for pK(a) = 4.5 silanol groups may be found in locally strained or defected regions with sparse silanol coverage. In the process, irreversible ring-opening reactions of strained silica trimer rings in contact with liquid water are observed.

Passivating lithium ion (Li) battery electrode surfaces to prevent electrolyte decomposition is critical for battery operations. Recent work on conformal atomic layer deposition (ALD) coating of anodes and cathodes has shown significant technological promise. ALD further provides well-characterized model platforms for understanding electrolyte decomposition initiated by electron tunneling through a passivating layer. First-principles calculations reveal two regimes of electron transfer to adsorbed ethylene carbonate molecules (EC, a main component of commercial electrolyte), depending on whether the electrode is alumina coated. On bare Li metal electrode surfaces, EC accepts electrons and decomposes within picoseconds. In contrast, constrained density functional theory calculations in an ultrahigh vacuum setting show that, with the oxide coating, e(-) tunneling to the adsorbed EC falls within the nonadiabatic regime. Here the molecular reorganization energy, computed in the harmonic approximation, plays a key role in slowing down electron transfer. Ab initio molecular dynamics simulations conducted at liquid EC electrode interfaces are consistent with the view that reactions and electron transfer occur right at the interface. Microgravimetric measurements demonstrate that the ALD coating decreases electrolyte decomposition and corroborates the theoretical predictions.

We manifest a significant influence of field direction and polarity on surface wetting, when the latter is tuned by application of an external electric field. Thermodynamics of field-induced filling of hydrocarbon-like nanopores with water is studied by open ensemble molecular simulation. Increased field strength consistently results in water-filling and electrostriction in hydrophobic nanopores. A threshold field commensurate with surface charge density of about one elementary charge per 10 nm2 suffices to render prototypical paraffin surfaces hydrophilic. When a field is applied in the direction perpendicular to the confining walls, the competition between orientational polarization and angle preferences of interfacial water molecules relative to the walls results in an asymmetric wettability of opposing surfaces (Janus interface). Reduction of surface free energy observed upon alignment of confinement walls with field direction suggests a novel mechanism whereby the applied electric field can operate selectively on water-filled nanotubes while empty ones remain unaffected.

Passivating solid-electrolyte interphase (SEI) films arising from electrolyte decomposition on low-voltage lithium ion battery anode surfaces are critical for battery operations. We review the recent theoretical literature on electrolyte decomposition and emphasize the modeling work on two-electron reduction of ethylene carbonate (EC, a key battery organic solvent). One of the two-electron pathways, which releases CO gas, is re-examined using simple quantum chemistry calculations. Excess electrons are shown to preferentially attack EC in the order (broken EC − ) > (intact EC − ) > EC. This confirms the viability of two electron processes and emphasizes that they need to be considered when interpreting SEI experiments. An estimate of the crossover between one-and two-electron regimes under a homogeneous reaction zone approximation is proposed. 1 arXiv:1307.3165v1 [physics.chem-ph] 11 Jul 2013 EC charge neutral ethylene carbonate c-EC − intact ethylene carbonate radical anion o-EC − ring-opened ethylene carbonate radical anion EDC ethylene dicarbonate BDC butylene dicarbonate k 1 bimolecular EC − recombination rate to form BDC k 2 unimolecular EC 2− decay rate k 3 unimolecular EC − ring-opening rate (C E -O E bond) k e rate of electron tunneling to EC k e rate of electron tunneling to EC − I. INTRODUCTION Solid electrolyte interphase (SEI) films on low voltage anode surfaces (e.g., graphite, Li metal, Si) are critical for lithium ion battery operations. 1-5 They arise from electrochemical reduction and subsequent breakdown of the organic solvent-based electrolyte which is metastable under battery charging voltage. Once formed, the SEI hinders electron tunneling from the anode and prevents further electrolyte decomposition while still permitting Li + ions to diffuse between the electrolyte and the anode. The electrolyte and electrode have to be matched to produce stable SEI films. For example, ethylene carbonate (EC) is essential for widely used graphitic anodes. Substantial experimental work has been performed to study the SEI structure and chemical composition, which is extremely complex and heterogeneous. The gases released during the first charging cycle, when the SEI is largely created, have also been analyzed. 5-21 Despite this, SEI formation mechanisms at the atomic lengthscale are difficult to elucidate by purely experimental means, and significant uncertainties remain. With some exceptions, proposed mechanisms have been indirectly inferred from SEI chemical composition and gas product distribution. Such analysis can be hampered by further reactions of initial electrolyte breakdown products 18 and even sample preparation procedures during ex-situ measurements. 17 Battery material surfaces are not clean or homogeneous, and differences in synthetic/experimental conditions likely contribute to SEI variations reported in different laboratories. For example, there are significant differences in the amount of CO gas 2reported. 5,[9][10][11][12][13]20 As will be discussed, CO release is the signature of a key SEI f...

We use atomistic simulations to address the question when capillary evaporation of water confined in a hydrocarbonlike slit is kinetically viable. Activation barriers and absolute rates of evaporation are estimated using open ensemble Monte Carlo-umbrella sampling and molecular dynamics simulations. At ambient conditions, the evaporation rate in a water film four molecular diameters thick is found to be of the order 10(5)(nm(2) s)(-1), meaning that water readily evaporates. Films more than a few nanometers thick will persist in a metastable liquid state. Dissolved atmospheric gas molecules do not significantly decrease the activation barrier.

Density functional theory (DFT) methods for calculating the quantum mechanical ground states of condensed matter systems are now a common and significant component of materials research. The growing importance of DFT reflects the development of sufficiently accurate functionals, efficient algorithms and continuing improvements in computing capabilities. As the materials problems to which DFT is applied have become large and complex, so have the sets of calculations necessary for investigating a given problem. Highly versatile, powerful codes exist to serve the practitioner, but designing useful simulations is a complicated task, involving intricate manipulation of many variables, with many pitfalls for the unwary and the inexperienced. We discuss several of the most important issues that go into designing a meaningful DFT calculation. We emphasize the necessity of investigating these issues and reporting the critical details.