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.

Establishing processing–structure–property relationships for monolayer materials is crucial for a range of applications spanning optics, catalysis, electronics and energy. Presently, for molybdenum disulfide, a promising catalyst for artificial photosynthesis, considerable debate surrounds the structure/property relationships of its various allotropes. Here we unambiguously solve the structure of molybdenum disulfide monolayers using high-resolution transmission electron microscopy supported by density functional theory and show lithium intercalation to direct a preferential transformation of the basal plane from 2H (trigonal prismatic) to 1T′ (clustered Mo). These changes alter the energetics of molybdenum disulfide interactions with hydrogen (ΔGH), and, with respect to catalysis, the 1T′ transformation renders the normally inert basal plane amenable towards hydrogen adsorption and hydrogen evolution. Indeed, we show basal plane activation of 1T′ molybdenum disulfide and a lowering of ΔGH from +1.6 eV for 2H to +0.18 eV for 1T′, comparable to 2H molybdenum disulfide edges on Au(111), one of the most active hydrogen evolution catalysts known.

Fluoroethylene carbonate (FEC) shows promise as an electrolyte additive for improving passivating solid-electrolyte interphase (SEI) films on silicon anodes used in lithium ion batteries (LIB).We apply density functional theory (DFT), ab initio molecular dynamics (AIMD), and quantum chemistry techniques to examine excess-electron-induced FEC molecular decomposition mechanisms that lead to FEC-modified SEI. We consider one-and two-electron reactions using cluster models and explicit interfaces between liquid electrolyte and model Li x Si y surfaces, respectively. FEC is found to exhibit more varied reaction pathways than unsubstituted ethylene carbonate.The initial bond-breaking events and products of one-and two-electron reactions are qualitatively similar, with a fluoride ion detached in both cases. However, most one-electron products are chargeneutral, not anionic, and may not coalesce to form effective Li + -conducting SEI unless they are further reduced or take part in other reactions. The implications of these reactions to silicon-anode based LIB are discussed.

Sai et al. Reply: In Ref.[1], we demonstrated that there exists a nontrivial correction, arising from the viscosity of the electron liquid [2], to the conductance of nanoscale junctions calculated within the adiabatic local-density approximation to time-dependent density-functional theory (DFT). This dynamical correction cannot be captured by any static DFT functional, even the exact one. To provide an estimate of these effects, we derived Eq. (14) for the dynamical viscous resistance and evaluated that ex

We show, using a tight-binding model and time-dependent density-functional theory, that a quasi-steady-state current can be established dynamically in a finite nanoscale junction without any inelastic effects. This is simply due to the geometrical constriction experienced by the electron wave packets as they propagate through the junction. We also show that in this closed nonequilibrium system two local electron occupation functions can be defined on each side of the nanojunction which approach Fermi distributions with increasing number of atoms in the electrodes. The resultant conductance and current-voltage characteristics at quasi-steady state are in agreement with those calculated within the static scattering approach.

We carry out first-principles density-functional calculations of the antiferrodistortive ͑AFD͒ and ferroelectric ͑FE͒ soft-mode instabilities in tetragonal SrTiO 3 , with the structural degrees of freedom treated in a classical, zero-temperature framework. In particular, we use frozen-phonon calculations to make a careful study of the anisotropy of the AFD and FE mode frequencies in the tetragonal ground state, in which an R-point AFD soft phonon has condensed. Because of the anharmonic couplings, the presence of this AFD distortion substantially affects both the AFD and FE mode frequencies. The AFD mode is found to be softer for rotations around a perpendicular axis (E g mode͒ than for rotations about the tetragonal axis (A 1g mode͒, in agreement with experimental results. The FE mode, on the other hand, is found to be softer when polarized perpendicular to the tetragonal axis (E u mode͒ than parallel to it (A 2u mode͒. The sign of this frequency splitting is consistent with the experimentally reported anisotropy of the dielectric susceptibility and other evidence. Finally, we present a discussion of the influence of various types of structural distortions on the FE instability and its anisotropy.

We report studies of ferroelectricity in ultra-thin perovskite films with realistic electrodes. The results reveal stable ferroelectric states in thin films less than 10Å thick with polarization normal to the surface. Under short-circuit boundary conditions, the screening effect of realistic electrodes and the influence of real metal/oxide interfaces on thin film polarization are investigated. Our studies indicate that metallic screening from the electrodes is affected by the difference in work functions at oxide surfaces. We demonstrate this effect in ferroelectric PbTiO3 and BaTiO3 films.The effect of size on thin-film ferroelectricity has been known for a long time, but has not been completely understood. Initial experiments and mean field calculations based on the Landau theory suggested that below a critical correlation volume 1 of electrical dipoles between 10-100 nm 3 , ferroelectricity vanishes due to intrinsic size effects. 2,3 For thin films with the polar axis perpendicular to the surface, incomplete compensation of surface charges creates a depolarizing field that has been shown to further reduce the polarization stability. 4,5 Recently, however, monodomain ferroelectric phases have been observed in very thin films, below ten unitcells thick. 6,7,8 Furthermore, Fong et al. showed that ferroelectric phases can be stable down to ∼12Å (three unit cells) in PbTiO 3 films by forming 180 • stripe domains, suggesting that no fundamental thickness limit is imposed by the intrinsic size effect in thin films. 9 This idea has been corroborated by ab initio calculations carried out on perovskite films, which tell that no critical thickness exists for polarization parallel to the surface 10 and that polarization perpendicular to the surface can exist in films three unit-cells thick if the depolarization field is artificially removed. 11,12 On the other hand, it has been found that the depolarization field plays a dominant role in reducing polarization normal to the surface and depressing ferroelectric transition temperatures in thin films. In a continuum model by Batra et al. 4 which includes the depolarization effect, a critical thickness of 100Å for perovskite films was analytically derived, assuming a Thomas-Fermi screening length of 1Å for the metal electrodes. Also, a recent first-principles calculation revealed that BaTiO 3 films with SrRuO 3 electrodes lose ferroelectricity below ∼24Å (6 unit cells), 13 thus suggesting that a minimum thickness limit exists for useful ferroelectric films. While indeed a minimum film thickness must be influenced by the polarization of the ferroelectrics and the screening length of the electrodes, 14 it is not yet clear whether the depolarizing field can ever be completely removed by realistic electrodes on ultrathin films, nor how monodomain thin film ferroelectricity is affected by the choice of electrodes and by the interactions at the metal/oxide interface.In this paper, we provide answers to these questions.In particular, we investigate how the critical thickness varies ...

We have developed and implemented a formalism for computing the structural response of a periodic insulating system to a homogeneous static electric field within density-functional perturbation theory (DFPT). We consider the thermodynamic potentials E(R, η, E ) and F (R, η, P), whose minimization with respect to the internal structural parameters R and unit cell strain η yields the equilibrium structure at fixed electric field E and polarization P, respectively. First-order expansion of E(R, η, E ) in E leads to a useful approximation in which R(P) and η(P) can be obtained by simply minimizing the zero-field internal energy with respect to structural coordinates subject to the constraint of a fixed spontaneous polarization P. To facilitate this minimization, we formulate a modified DFPT scheme such that the computed derivatives of the polarization are consistent with the discretized form of the Berry-phase expression. We then describe the application of this approach to several problems associated with bulk and short-period superlattice structures of ferroelectric materials such as BaTiO3 and PbTiO3. These include the effects of compositionally broken inversion symmetry, the equilibrium structure for high values of polarization, field-induced structural phase transitions, and the lattice contributions to the linear and the non-linear dielectric constants.