Modeling the electroluminescence of atomic wires from quantum dynamics simulations.

Static and time-dependent quantum-mechanical approaches have been employed in the literature to characterize the physics of light-emitting molecules and nanostructures. However, the electromagnetic emission induced by an input current has remained beyond the realm of molecular simulations. This is the challenge addressed here with the help of an equation of motion for the density matrix coupled to a photon bath based on a Redfield formulation. This equation is evolved within the framework of the driven-Liouville von Neumann approach, which incorporates open boundaries by introducing an applied bias and a circulating current. The dissipated electromagnetic power can be computed in this context from the time derivative of the energy. This scheme is applied in combination with a self-consistent tight-binding Hamiltonian to investigate the effects of bias and molecular size on the electroluminescence of metallic and semiconducting chains. For the latter, a complex interplay between bias and molecular length is observed: there is an optimal number of atoms that maximizes the emitted power at high voltages but not at low ones. This unanticipated behavior can be understood in terms of the band bending produced along the semiconducting chain, a phenomenon that is captured by the self-consistency of the method. A simple analytical model is proposed that explains the main features revealed by the simulations. The methodology, applied here at a self-consistent tight-binding level but extendable to more sophisticated Hamiltonians such as density functional tight binding and time dependent density functional theory, promises to be helpful for quantifying the power and quantum efficiency of nanoscale electroluminescent devices.

Nanobubble Stability and Formation on Solid-Liquid Interfaces in Open Environments.

Are surface nanobubbles transient or thermodynamically stable structures? This question remained controversial until recently, when the stability of gas nanobubbles at solid-liquid interfaces was demonstrated from thermodynamic arguments in closed systems, establishing that bubbles with radii of hundreds of nanometers can be stable at modest supersaturations if the gas amount is finite. Here we develop a grand-canonical description of bubble formation that predicts that nanobubbles can nucleate and remain thermodynamically stable in open boundaries at high supersaturations when pinned to hydrophobic supports as small as a few nanometers. While larger bubbles can also be stable at lower supersaturations, the corresponding barriers are orders of magnitude above kT, meaning that their formation cannot proceed via heterogeneous nucleation on a uniform solid interface but must follow some alternative path. Moreover, we conclude that a source of growth-limiting mechanism, such as pinning or gas availability, is necessary for the thermodynamic stabilization of surface bubbles.

Water Self-Dissociation is Insensitive to Nanoscale Environments.

Nanoconfinement effects on water dissociation and reactivity remain controversial, despite their importance to understand the aqueous chemistry at interfaces, pores, or aerosols. The pKw in confined environments has been assessed from experiments and simulations in a few specific cases, leading to dissimilar conclusions. Here, with the use of carefully designed ab initio simulations, we demonstrate that the energetics of bulk water dissociation is conserved intact to unexpectedly small length-scales, down to aggregates of only a dozen molecules or pores of widths below 2 nm. The reason is that most of the free-energy involved in water autoionization comes from breaking the O-H covalent bond, which has a comparable barrier in the bulk liquid, in a small droplet of nanometer size, or in a nanopore in the absence of strong interfacial interactions. Thus, dissociation free-energy profiles in nanoscopic aggregates or in 2D slabs of 1 nm width reproduce the behavior corresponding to the bulk liquid, regardless of whether the corresponding nanophase is delimited by a solid or a gas interface. The present work provides a definite and fundamental description of the mechanism and thermodynamics of water dissociation at different scales with broader implications on reactivity and self-ionization at the air-liquid interface.

Fluorescence in quantum dynamics: Accurate spectra require post-mean-field approaches.

Real time modeling of fluorescence with vibronic resolution entails the representation of the light-matter interaction coupled to a quantum-mechanical description of the phonons and is therefore a challenging problem. In this work, taking advantage of the difference in timescales characterizing internal conversion and radiative relaxation-which allows us to decouple these two phenomena by sequentially modeling one after the other-we simulate the electron dynamics of fluorescence through a master equation derived from the Redfield formalism. Moreover, we explore the use of a recent semiclassical dissipative equation of motion [C. M. Bustamante et al., Phys. Rev. Lett. 126, 087401 (2021)], termed coherent electron electric-field dynamics (CEED), to describe the radiative stage. By comparing the results with those from the full quantum-electrodynamics treatment, we find that the semiclassical model does not reproduce the right amplitudes in the emission spectra when the radiative process involves the de-excitation to a manifold of closely lying states. We argue that this flaw is inherent to any mean-field approach and is the case with CEED. This effect is critical for the study of light-matter interaction, and this work is, to our knowledge, the first one to report this problem. We note that CEED reproduces the correct frequencies in agreement with quantum electrodynamics. This is a major asset of the semiclassical model, since the emission peak positions will be predicted correctly without any prior assumption about the nature of the molecular Hamiltonian. This is not so for the quantum electrodynamics approach, where access to the spectral information relies on knowledge of the Hamiltonian eigenvalues.

Tailoring Cooperative Emission in Molecules: Superradiance and Subradiance from First-Principles Simulations.

Cooperative optical effects provide a pathway to both the amplification (superradiance) and the suppression (subradiance) of photon emission from electronically excited states. These captivating phenomena offer a rich variety of possibilities for photonic technologies aimed at electromagnetic energy manipulation, including lasers and high-speed emitting devices in the case of superradiance or optical energy storage in that of subradiance. The employment of molecules as the building pieces in these developments requires a precise understanding of the roles of separation, orientation, spatial distribution, and applied fields, which remains challenging for theory and experiments. These questions are addressed here through ab initio quantum dynamics simulations of collective emission on the basis of a novel semiclassical formalism and time-dependent density functional theory. By establishing the configurations leading to decoherence and how the fine-tuning of a pulse can accumulate or release optical energy in H2 arrays, this report provides fundamental insight toward the design of real superradiant and subradiant devices.

Partition Constant of Binary Mixtures for the Equilibrium between a Bulk and a Confined Phase.

It is well-known that the thermodynamic, kinetic and structural properties of fluids, and in particular of water and its solutions, can be drastically affected in nanospaces. A possible consequence of nanoscale confinement of a solution is the partial segregation of its components. Thereby, confinement in nanoporous materials (NPM) has been proposed as a means for the separation of mixtures. In fact, separation science can take great advantage of NPM due to the tunability of their properties as a function of nanostructure, morphology, pore size, and surface chemistry. Alcohol-water mixtures are in this context among the most relevant systems. However, a quantitative thermodynamic description allowing for the prediction of the segregation capabilities as a function of the material-solution characteristics is missing. In the present study we attempt to fill this vacancy, by contributing a thermodynamic treatment for the calculation of the partition coefficient in confinement. Combining the multilayer adsorption model for binary mixtures with the Young equation, we conclude that the liquid-vapor surface tension and the contact angle of the pure substances can be used to predict the separation ability of a particular material for a given mixture to a semiquantitative extent. Moreover, we develop a Kelvin-type equation that relates the partition coefficient to the radius of the pore, the contact angle, and the liquid-vapor surface tensions of the constituents. To assess the validity of our thermodynamic formulation, coarse grained molecular dynamics simulations were performed on models of alcohol-water mixtures confined in cylindrical pores. To this end, a coarse-grained amphiphilic molecule was parametrized to be used in conjunction with the mW potential for water. This amphiphilic model reproduces some of the properties of methanol such as enthalpy of vaporization and liquid-vapor surface tension, and the minimum of the excess enthalpy for the aqueous solution. The partition coefficient turns out to be highly dependent on the molar fraction, on the interaction between the components and the confining matrix, and on the radius of the pore. A remarkable agreement between the theory and the simulations is found for pores of radius larger than 15 Å.

Doping and coupling strength in molecular conductors: polyacetylene as a case study.

The doping mechanisms responsible for elevating the currents up to eleven orders of magnitude in semiconducting polymer films are today well characterized. Doping can also improve the performance of nanoscale devices or single molecule conductors, but the mechanism in this case appears to be different, with theoretical studies suggesting that the dopant affects the electronic properties of the junctions. In the present report, multiscale time-dependent DFT transport simulations help clarify the way in which n-type doping can raise the current flowing through a polymer chain connected to a pair of electrodes, with the focus on polyacetylene. In particular, our multiscale methodology offers control over the magnitude of the chemical coupling between the molecule and the electrodes, which allows us to analyze the effect of doping in low and strong coupling regimes. Interestingly, our results establish that the impact of dopants is the highest in weakly coupled devices, while their presence tends to be irrelevant in low-resistance junctions. Our calculations point out that both the equalization of the frontier orbitals with the Fermi level and a small gap between the HOMO and the LUMO must result from doping in order to observe any significant increase of the currents.

Dissipative Equation of Motion for Electromagnetic Radiation in Quantum Dynamics.

The dynamical description of the radiative decay of an electronically excited state in realistic many-particle systems is an unresolved challenge. In the present investigation electromagnetic radiation of the charge density is approximated as the power dissipated by a classical dipole, to cast the emission in closed form as a unitary single-electron theory. This results in a formalism of unprecedented efficiency, critical for ab initio modeling, which exhibits at the same time remarkable properties: it quantitatively predicts decay rates, natural broadening, and absorption intensities. Exquisitely accurate excitation lifetimes are obtained from time-dependent DFT simulations for C^{2+}, B^{+}, and Be, of 0.565, 0.831, and 1.97 ns, respectively, in accord with experimental values of 0.57±0.02, 0.86±0.07, and 1.77-2.5 ns. Hence, the present development expands the frontiers of quantum dynamics, bringing within reach first-principles simulations of a wealth of photophysical phenomena, from fluorescence to time-resolved spectroscopies.

A simple approximation to the electron-phonon interaction in population dynamics.

The modeling of coupled electron-ion dynamics including a quantum description of the nuclear degrees of freedom has remained a costly and technically difficult practice. The kinetic model for electron-phonon interaction provides an efficient approach to this problem, for systems evolving with low amplitude fluctuations, in a quasi-stationary state. In this work, we propose an extension of the kinetic model to include the effect of coherences, which are absent in the original approach. The new scheme, referred to as Liouville-von Neumann + Kinetic Equation (or LvN + KE), is implemented here in the context of a tight-binding Hamiltonian and employed to model the broadening, caused by the nuclear vibrations, of the electronic absorption bands of an atomic wire. The results, which show close agreement with the predictions given by Fermi's golden rule (FGR), serve as a validation of the methodology. Thereafter, the method is applied to the electron-phonon interaction in transport simulations, adopting to this end the driven Liouville-von Neumann equation to model open quantum boundaries. In this case, the LvN + KE model qualitatively captures the Joule heating effect and Ohm's law. It, however, exhibits numerical discrepancies with respect to the results based on FGR, attributable to the fact that the quasi-stationary state is defined taking into consideration the eigenstates of the closed system rather than those of the open boundary system. The simplicity and numerical efficiency of this approach and its ability to capture the essential physics of the electron-phonon coupling make it an attractive route to first-principles electron-ion dynamics.

Electrochemically Generated Nanobubbles: Invariance of the Current with Respect to Electrode Size and Potential.

Gas-producing electrochemical reactions are key to energy conversion and generation technologies. Bubble formation dramatically decreases gas-production rates on nanoelectrodes, by confining the reaction to the electrode boundary. This results in the collapse of the current to a stationary value independent of the potential. Startlingly, these residual currents also appear to be insensitive to the nanoelectrode diameter in the 5 to 500 nm range. These results are counterintuitive, as it may be expected that the current be proportional to the circumference of the electrode, i.e., the length of the three-phase line where the reaction occurs. Here, we use molecular simulations and a kinetic model to elucidate the origin of current insensitivity with respect to the potential and establish its relationship to the size of nanoelectrodes. We provide critical insights for the design and operation of nanoscale electrochemical devices and demonstrate that nanoelectrode arrays maximize conversion rates compared to macroscopic electrodes with same total area.

Transport and Spectroscopy in Conjugated Molecules: Two Properties and a Single Rationale.

In the context of electron dynamics simulations, when the charge density of a molecule is subject to a perturbation in the form of a short electric field pulse, density fluctuations develop in time. In the absence of dissipation, these oscillations continue indefinitely, reflecting the resonances of the electronic system; as a matter of fact, from the Fourier transform of the time dependent dipole arising from them, the absorption spectrum of the molecule can be calculated. Since these oscillations are the result of the electrons moving through the molecular structrure, it seems plausible that they carry information on the transport properties of the system. This is the idea explored in the present article for the case of conjugated polymers. Specifically, we depart from a nonequilibrium state with the charge concentrated on the ends of the molecule, and estimate the currents flowing back and forth during the evolution of electron dynamics simulations. These show that the charge oscillates between the sides of the polymer with the predominance of a frequency that is coincident with one of the main bands in the absorption spectrum, which can be ascribed to a charge transfer transition. Thus, from the charge transfer band frequency appearing in the absorption spectrum, the molecular conductance of a conjugated molecule can be calculated. Also interestingly, we find that, while a perturbation excites all resonances of an electronic system, the form in which this perturbation is applied can be manipulated to determine the relative manifestation of the response. The electric field pulse excites all resonances according to the transition dipole moment and is then appropriate to produce the absorption spectrum. A charge separated initial state, however, specifically stimulates the charge transfer mode and is then suitable to calculate transport properties. This allows us to propose a simple approach to obtain molecular conductances and tunneling decay constants in agreement with results from much more demanding electronic structure techniques.

Multiscale approach to electron transport dynamics.

Molecular simulations of transport dynamics in nanostructures usually require the implementation of open quantum boundary conditions. This can be instrumented in different frameworks including Green's functions, absorbing potentials, or the driven Liouville von Neumann equation, among others. In any case, the application of these approaches involves the use of large electrodes that introduce a high computational demand when dealing with first-principles calculations. Here, we propose a hybrid scheme where the electrodes are described at a semiempirical, tight binding level, coupled to a molecule or device represented with density functional theory (DFT). This strategy allows us to use massive electrodes at a negligible computational cost, preserving the accuracy of the DFT method in the modeling of the transport properties, provided that the electronic structure of every lead is properly defined to behave as a conducting fermionic reservoir. We study the nature of the multiscale coupling and validate the methodology through the computation of the tunneling decay constant in polyacetylene and of quantum interference effects in an aromatic ring. The present implementation is applied both in microcanonical and grand-canonical frameworks, in the last case using the Driven Liouville von Neumann equation, discussing the advantages of one or the other. Finally, this multiscale scheme is employed to investigate the role of an electric field applied normally to transport in the conductance of polyacetylene. It is shown that the magnitude and the incidence angle of the applied field have a considerable effect on the electron flow, hence constituting an interesting tool for current control in nanocircuits.

Halide-Mediated Modification of Magnetism and Electronic Structure of α-Co(II) Hydroxides: Synthesis, Characterization, and DFT+U Simulations.

The present study introduces a comprehensive exploration in terms of physicochemical characterization and calculations based on density functional theory with Hubbard's correction (DFT+U) of the whole family of α-Co(II) hydroxyhalide (F, Cl, Br, I). These samples were synthesized at room temperature by employing a one-pot approach based on the epoxide route. A thorough characterization (powder X-ray diffraction, X-ray photoelectron spectroscopy, thermogravimetric analysis/mass spectroscopy, and magnetic and conductivity measurements) corroborated by simulation is presented that analyzes the structural, magnetic, and electronic aspects. Beyond the inherent tendency of intercalated anions to modify the interlayer distance, the halide's nature has a marked effect on several aspects. Such as the modulation of the CoOh to CoTd ratio, as well as the inherent tendency towards dehydration and irreversible decomposition. Whereas the magnetic behavior is strongly correlated with the CoTd amount reflected in the presence of glassy behavior with high magnetic disorder, the electrical properties depend mainly on the nature of the halide. The computed electronic structures suggest that the CoTd molar fraction exerts a minor effect on the inherent conductivity of the phases. However, the band gap of the solid turns out to be significantly dependent on the nature of the incorporated halide, governed by ligand to metal charge transfer, which minimizes the gap as the anionic radius becomes larger. Conductivity measurements of pressed pellets confirm this trend. To the best of our knowledge, this is the first report on the magnetic and electrical properties of α-Co(II) hydroxyhalides validated with in silico descriptions, opening the gate for the rational design of layered hydroxylated phases with tunable electrical, optical, and magnetic properties.

Mechanisms of Nucleation and Stationary States of Electrochemically Generated Nanobubbles.

Gas evolving reactions are ubiquitous in the operation of electrochemical devices. Recent studies of individual gas bubbles on nanoelectrodes have resulted in unprecedented control and insights on their formation. The experiments, however, lack the spatial resolution to elucidate the molecular pathway of nucleation of nanobubbles and their stationary size and shape. Here we use molecular simulations with an algorithm that mimics the electrochemical formation of gas, to investigate the mechanisms of nucleation of gas bubbles on nanoelectrodes, and characterize their stationary states. The simulations reproduce the experimental currents in the induction and stationary stages, and indicate that surface nanobubbles nucleate through a classical mechanism. We identify three distinct regimes for bubble nucleation, depending on the binding free energy per area of bubble to the electrode, Δγbind. If Δγbind is negative, the nucleation is heterogeneous and the nanobubble remains bound to the electrode, resulting in a low-current stationary state. For very negative Δγ, the bubble fully wets the electrode, forming a one-layer-thick micropancake that nucleates without supersaturation. On the other hand, when Δγbind > 0 the nanobubble nucleates homogeneously close to the electrode, but never attaches to it. We conclude that all surface nanobubbles must nucleate heterogeneously. The simulations reveal that the size and contact angle of stationary nanobubbles increase with the reaction driving force, although their residual current is invariant. The myriad of driven nonequilibrium stationary states with the same rate of production of gas, but distinct bubble properties, suggests that these dissipative systems have attractors that control the stationary current.

One-Dimensional Confinement Inhibits Water Dissociation in Carbon Nanotubes.

The effect of nanoconfinement on the self-dissociation of water constitutes an open problem whose elucidation poses a serious challenge to experiments and simulations alike. In slit pores of width ≈1 nm, recent first-principles calculations have predicted that the dissociation constant of H2O increases by almost 2 orders of magnitude [ Muñoz-Santiburcio and Marx, Phys. Rev. Lett. 2017 , 119 , 056002 ]. In the present study, quantum mechanics-molecular mechanics simulations are employed to compute the dissociation free-energy profile of water in a (6,6) carbon nanotube. According to our results, the equilibrium constant Kw drops by 3 orders of magnitude with respect to the bulk phase value, at variance with the trend predicted for confinement in two dimensions. The higher barrier to dissociation can be ascribed to the undercoordination of the hydroxide and hydronium ions in the nanotube and underscores that chemical reactivity does not exhibit a monotonic behavior with respect to pore size but may vary substantially with the characteristic length scale and dimensionality of the confining media.

Interplay of Coordination Environment and Magnetic Behavior of Layered Co(II) Hydroxichlorides: A DFT+U Study.

In this work we present a systematic computational study of the structural and magnetic properties of a layered family of Co(II) hydroxichlorides, obeying to the general formula Co(OH)2- xCl x(H2O) y. This solid contains both octahedral and tetrahedral cobalt ions, displaying a complex magnetic order arising from the particular coupling between the two kinds of metallic centers. Here, supercells representing concentrations of 12, 20, and 40% of tetrahedral sites were modeled consistently with the compositions reported experimentally. Our simulations show that the two types of cobalt ions tend to couple antiferromagnetically, giving rise to a net magnetic moment slightly out of the plane of the layers. The band gap reaches its minimum value of 1.4 eV for the most diluted fraction of tetrahedral Co(II) sites, going up to 2.2 eV when the content is 40%. Moreover, our results suggest that the presence of interlayer water stabilizes the material and at the same time strongly modifies the electronic environment of tetrahedral Co(II), leading to a further drop of the band gap. To our knowledge, this is the first theoretical investigation of this material.

Grand Canonical Investigation of the Quasi Liquid Layer of Ice: Is It Liquid?

In this study, the solid-vapor equilibrium and the quasi liquid layer (QLL) of ice Ih exposing the basal and primary prismatic faces were explored by means of grand canonical molecular dynamics simulations with the monatomic mW potential. For this model, the solid-vapor equilibrium was found to follow the Clausius-Clapeyron relation in the range examined, from 250 to 270 K, with a Δ Hsub of 50 kJ/mol in excellent agreement with the experimental value. The phase diagram of the mW model was constructed for the low pressure region around the triple point. The analysis of the crystallization dynamics during condensation and evaporation revealed that, for the basal face, both processes are highly activated, and in particular cubic ice is formed during condensation, producing stacking-disordered ice. The basal and primary prismatic surfaces of ice Ih were investigated at different temperatures and at their corresponding equilibrium vapor pressures. Our results show that the region known as QLL can be interpreted as the outermost layers of the solid where a partial melting takes place. Solid islands in the nanometer length scale are surrounded by interconnected liquid areas, generating a bidimensional nanophase segregation that spans throughout the entire width of the outermost layer even at 250 K. Two approaches were adopted to quantify the QLL and discussed in light of their ability to reflect this nanophase segregation phenomena. Our results in the µVT ensemble were compared with NPT and NVT simulations for two system sizes. No significant differences were found between the results as a consequence of model system size or of the working ensemble. Nevertheless, certain advantages of performing µVT simulations in order to reproduce the experimental situation are highlighted. On the one hand, the QLL thickness measured out of equilibrium might be affected because of crystallization being slower than condensation. On the other, preliminary simulations of AFM indentation experiments show that the tip can induce capillary condensation over the ice surface, enlarging the apparent QLL.

Chemical Reactivity and Spectroscopy Explored From QM/MM Molecular Dynamics Simulations Using the LIO Code.

In this work we present the current advances in the development and the applications of LIO, a lab-made code designed for density functional theory calculations in graphical processing units (GPU), that can be coupled with different classical molecular dynamics engines. This code has been thoroughly optimized to perform efficient molecular dynamics simulations at the QM/MM DFT level, allowing for an exhaustive sampling of the configurational space. Selected examples are presented for the description of chemical reactivity in terms of free energy profiles, and also for the computation of optical properties, such as vibrational and electronic spectra in solvent and protein environments.

Spectroscopy in Complex Environments from QM-MM Simulations.

The applications of multiscale quantum-classical (QM-MM) approaches have shown an extraordinary expansion and diversification in the last couple of decades. A great proportion of these efforts have been devoted to interpreting and reproducing spectroscopic experiments in a variety of complex environments such as solutions, interfaces, and biological systems. Today, QM-MM-based computational spectroscopy methods constitute accomplished tools with refined predictive power. The present review summarizes the advances that have been made in QM-MM approaches to UV-visible, Raman, IR, NMR, electron paramagnetic resonance, and Mössbauer spectroscopies, providing in every case an introductory discussion of the corresponding methodological background. A representative number of applications are presented to illustrate the historical evolution and the state of the art of this field, highlighting the advantages and limitations of the available methodologies. Finally, we present our view of the perspectives and open challenges in the field.

Stability and Vapor Pressure of Aqueous Aggregates and Aerosols Containing a Monovalent Ion.

The incidence of charged particles on the nucleation and the stability of aqueous aggregates and aerosols was reported more than a century ago. Many studies have been conducted ever since to characterize the stability, structure, and nucleation barrier of ion-water droplets. Most of these studies have focused on the free-energy surface as a function of cluster size, with an emphasis on the role of ionic charge and radius. This knowledge is fundamental to go beyond the rudimentary ion-induced classical nucleation theory. In the present article, we address this problem from a different perspective, by computing the vapor pressures of (H2O)nLi+ and (H2O)nCl- aggregates using molecular simulations. Our calculations shed light on the structure, the critical size, the range of stability, and the role of ion-water interactions in aqueous clusters. Moreover, they allow one to assess the accuracy of the classical thermodynamic model, highlighting its strengths and weaknesses.