Hybrid organic–inorganic perovskites, and particularly CH3NH3PbI3 (MAPbI3), have emerged as a new generation of photovoltaic devices due to low cost and superior performance. The performance is strongly influenced by current–voltage hysteresis that arises due to ion migration, and the challenge remains how to suppress the ion migration and hysteresis. Our first-principles calculations demonstrate that the energy barriers to diffusion of the I–, MA+, and Pb2+ ions are greatly affected by dipole moments of the MA species. The energy barriers of the most mobile I– ion range from 0.06 to 0.65 eV, depending on MA orientation. The positively charged MA+ and Pb2+ ions diffuse along the dipole direction, while the negatively charged I– ion strongly prefers to diffuse against the dipole direction. By influencing ion migration, the arrangement of MA molecules can effectively modulate the current–voltage hysteresis intensity. The current work contributes to the fundamental understanding of the microscopic mechanism of ion migration in MAPbI3 and suggests means to suppress the hysteresis effect and optimize perovskite performance. By demonstrating in detail how the arrangement of the organic molecules can efficiently influence ion migration and, hence, amplitude of the current–voltage hysteresis, our results suggest that the hysteresis effect can be suppressed and the long-term performance of perovskites can be improved, if the organic molecules are arranged and stabilized in an antiferroelectric order.

Quantum dot (QD) solids represent a new type of condensed matter drawing high fundamental and applied interest. Quantum confinement in individual QDs, combined with macroscopic scale whole materials, leads to novel exciton and charge transfer features that are particularly relevant to optoelectronic applications. This Perspective discusses the structure of semiconductor QD solids, optical and spectral properties, charge carrier transport, and photovoltaic applications. The distance between adjacent nanoparticles and surface ligands influences greatly electrostatic interactions between QDs and, hence, charge and energy transfer. It is almost inevitable that QD solids exhibit energetic disorder that bears many similarities to disordered organic semiconductors, with charge and exciton transport described by the multiple trapping model. QD solids are synthesized at low cost from colloidal solutions by casting, spraying, and printing. A judicious selection of a layer sequence involving QDs with different size, composition, and ligands can be used to harvest sunlight over a wide spectral range, leading to inexpensive and efficient photovoltaic devices.

Ultrafast charge recombination in hematite (α-Fe2O3) severely limits its applications in solar energy conversion and utilization, for instance, in photoelectrochemical water splitting. We report the first time-domain ab initio study of charge relaxation dynamics in α-Fe2O3 with and without the oxygen vacancy (Ov) defect, using non-adiabatic molecular dynamics implemented within time-dependent density functional theory. The simulations show that the hole trapping is the rate-limiting step in the electron–hole recombination process for both neutral and ionized Ov systems. The electron trapping is fast, and the trapped electron are relatively long-lived. A similar asymmetry is found for the relaxation of free charge carriers: relaxation of photoholes in the valence band is slower than relaxation of photoelectrons in the conduction band. The slower dynamics of holes offers an advantage to water oxidation at α-Fe2O3 photoanodes. Notably, the neutral Ov defect accelerates significantly the charge recombination rate, by about a factor of 30 compared to the ideal lattice, due to the stronger electron-vibrational coupling at the defect. However, the recombination rate in the ionized Ov defect is decreased by a factor of 10 with respect to the neutral defect, likely due to expansion of the local iron shell around the Ov site. The Ov defect ionization in α-Fe2O3 photoanodes is important for increasing both electrical conductivity and charge carrier lifetimes. The simulations reproduce well the time scales for the hot carrier cooling, trapping and recombination available from transient spectroscopy experiments, and suggest two alternative mechanisms for the Ov-assisted electron–hole recombination. The study provides a detailed atomistic understanding of carrier dynamics in hematite, and rationalizes the experimentally reported activation of α-Fe2O3 photoanodes by incorporation of Ov defects.

We present a time-domain ab initio study of electron–hole recombination in pristine MAPbI3, and compare it to the trap mediated recombination in MAPbI3 with the iodine interstitial defect. Nonadiabatic molecular dynamics combined with time-domain density functional theory show that the iodine interstitial defect creates a subgap state capable of trapping both electrons and holes. Hole trapping occurs much faster than electron trapping or electron–hole recombination. The trapped hole survives for hundreds of nanoseconds, because, rather surprisingly, recombination of electrons with the trapped hole takes several times longer than recombination of electrons with holes in the valence band. Because the hole trap is relatively shallow, the hole can escape into the valence band prior to recombining with the electron. The differences are rationalized by variation in nonadiabatic electron–phonon couplings, phonon-induced pure-dephasing times and electronic energy gaps. The time-domain atomistic simulations contribute to understanding of the experimentally known defect-tolerance of perovskite solar cells, which is of great importance to the solar cell performance.

Interfacial electron transfer (ET) constitutes the key step in conversion of solar energy into electricity and fuels. Required for fast and efficient charge separation, strong donor–acceptor interaction is typically achieved through covalent chemical bonding and leads to fast, adiabatic ET. Focusing on interfaces of pyrene, coronene, and a graphene quantum dot (GQD) with TiO2, we demonstrate the opposite situation: covalent bonding leads to weak coupling and nonadiabatic (NA) ET, while through-space π-electron interaction produces adiabatic ET. Using real-time time-dependent density functional theory combined with NA molecular dynamics, we simulate photoinduced ET into TiO2 from flat and vertically placed molecules and GQD containing commonly used carboxylic acid linkers. Both arrangements can be achieved experimentally with GQDs and other two-dimensional materials, such as MoS2. The weak through-bond donor–acceptor coupling is attributed to the π-electron withdrawing properties of the carboxylic acid group. The calculated ET time scales are in excellent agreement with pump–probe optical experiments. The simulations show that the ET proceeds faster than energy relaxation. The electron couples to a broad spectrum of vibrational modes, ranging from 100 cm–1 large-scale motions to 1600 cm–1 C–C stretches. Compared to graphene/TiO2 heterojunctions, the molecule/TiO2 and GQD/TiO2 systems exhibit energy gaps, allowing for longer-lived excited states and hot electron injection, facilitating charge separation and higher voltage. The reported state-of-the-art simulations generate a detailed time-domain, atomistic description of the interfacial charge and energy transfer and relaxation processes, and demonstrate that the fundamental principles leading to efficient charge separation in nanoscale materials depend strongly and often unexpectedly on the type of donor–acceptor interaction. Understanding these principles is critical to the development of highly efficient photovoltaic and photocatalytic cells.

Two-dimensional transition metal dichalcogenides (TMDs) have appeared on the horizon of materials science and solid-state physics due to their unique properties and diverse applications. TMD performance depends strongly on material quality and defect morphology. Calculations predict that sulfur adatom and vacancy are among the most energetically favorable defects in MoS2 with vacancies frequently observed during chemical vapor deposition. By performing ab initio quantum dynamics calculations we demonstrate that both adatom and vacancy accelerate nonradiative charge carrier recombination but this happens through different mechanisms. Surprisingly, holes never significantly populate the shallow trap state created by the sulfur adatom because the trap is strongly localized and decoupled from free charges. Charge recombination bypasses the hole trap. Instead, it occurs directly between free electron and hole. The recombination is faster than in pristine MoS2 because the adatom strongly perturbs the MoS2 layer, breaks its symmetry, and allows more phonon modes to couple to the electronic subsystem. In contrast, the sulfur vacancy accelerates charge recombination by the traditional mechanism involving charge trapping, followed by recombination. This is because the hole and electron traps created by the vacancy are much less localized than the hole trap created by the adatom. Because the sulfur adatom accelerates charge recombination by a factor of 7.9, compared to 1.7 due to vacancy, sulfur adatoms should be strongly avoided. The generated insights highlight the diverse behavior of different types of defects, reveal unexpected features, and provide the mechanistic understanding of charge dynamics needed for tailoring TMD properties and building high-performance devices.Keywords: electron−hole recombination; nonadiabatic molecular dynamics; sulfur vacancy and adatom defects; time-dependent density functional theory; Transition metal dichalcogenides;

The nonequilibrium dynamics of excited electrons in metals is probed by ultrafast laser measurements. Using a real-time Kohn–Sham time-dependent density functional theory and nonadiabatic molecular dynamics, we report direct modeling of such experiments, rationalizing the observed temperature dependence. Focusing on thin gold films, we analyze the effect of temperature on film structure, electronic state densities, nonadiabatic electron–phonon coupling, elastic electron–phonon scattering times, and electron–phonon relaxation rates. The effective electron–phonon coupling constants calculated at different temperatures are in good agreement with the values deduced from experiments and an alternative theory. A temperature increase accelerates both inelastic and elastic electron–phonon scattering and allows a larger number of higher-frequency phonon modes to couple to the electronic subsystem. The inelastic electron–phonon coupling is largest between nearest states, indicating that carrier relaxation involves transitions over small energy increments. In contrast, the elastic electron–phonon scattering is strongest for pairs of electronic states that are distant in energy. The electron–phonon interactions exhibit mild energy dependence, with both nonadiabatic electron–phonon coupling and elastic electron–phonon scattering times decreasing with increasing electron excitation energy. The detailed ab initio analysis of the electron–phonon interactions emphasizes the nonequilibrium nature of the relaxation processes and provides important insights into the electron–phonon energy exchange in metal films in general.

An analytic model for the field dependence of charge mobility is developed within the long-range-correlated disorder model of a dipole glass. Release of a charge carrier from a deep state is considered as hopping drift and diffusion in a quasi-Coulomb potential well. The analytic results, containing only one numerical parameter obtained from independent simulation, are in good agreement with the fit to the Monte-Carlo simulations. The developed approach justifies applicability of the concept of the effective transport level for the modeling of organic materials with large molecular dipoles.

Interfacial electron transfer (ET) plays a key role in the operation of solar cells based on TiO2 sensitized with organohalide perovskites, since it leads to separation of the photogenerated electrons and holes into different materials. The reported experimental ET times range by over 3 orders of magnitude, from sub-200 fs to over 300 ps. Using nonadiabatic molecular dynamics combined with ab initio time-domain density functional theory, we demonstrate that ET at a CH3NH3PbI3/TiO2 interface can be complete within 100 fs, indicating that the longer time scales reflect other processes, such as charge and exciton diffusion in perovskite bulk. The electron injection is fast because the interaction between the donor and acceptor species is strong at ambient conditions. Photoexcitation directly at the interface can create a charge-separated state. Electrons generated farther away inject by a combination of adiabatic and nonadiabatic mechanisms. Thermally activated low frequency vibrational motions at the interface modulate the CH3NH3PbI3/TiO2 separation, creating opportunities for chemical bonding and generating channels for adiabatic ET. Higher-frequency modes create large nonadiabatic coupling. The interaction between perovskite and TiO2 is purely van der Waals at 0 K, whereas at ambient temperatures I–Ti covalent bonds can form transiently at the interface. The covalent bonding is particularly important for photoexcitation of charge-separated states and adiabatic ET. The ET occurs prior to nonradiative electronic energy losses that can lead to charge trapping and recombination. The ultrafast interfacial charge separation contributes to the high efficiencies of perovskite-sensitized TiO2 solar cells. The reported simulations provide a detailed time-domain atomistic description of the interfacial ET and advance our understanding of carrier dynamics in perovskite solar cells.

Superconducting pairing due to electron–phonon coupling is investigated in recent pump–probe experiments. Combining time-dependent density functional theory and nonadiabatic molecular dynamics, we report the first direct modeling of such experiments and show how the electron–phonon relaxation depends on chemical bonding, electron–phonon coupling, and electronic state density. The relaxation rate is determined primarily by the nonadiabatic charge–phonon coupling strength, which in turn depends on the strength of chemical interactions between the key atoms, reflected in the wave function delocalization. The differences in the electronic density of states constitute the secondary factor. Having obtained good agreement with the experimental data on YBa2Cu3O6.5, we predict that the relaxation slows if Y is replaced with Sc or Ba with Sr, while the relaxation accelerates if O is replaced with S, indicating that YBa2Cu3S6.5 can exhibit improved superconducting performance.

Hybrid organic–inorganic perovskites show impressive potential for photovoltaic applications and currently give rise to one of the most vibrant research areas in the field. Until recently, the electrostatic interactions between their organic and inorganic components were considered mostly for stabilization of the fragile perovskite structure. We study the effect of local interactions of polar C–N bonds in the organic layer on the nonradiative electron–hole recombination in the recently reported room-temperature ferroelectric hybrid perovskite, (benzylammonium)2PbCl4. Using nonadiabatic molecular dynamics and real-time time-dependent density functional theory, we show that ferroelectric alignment of the polar groups weakens the electron–phonon nonadiabatic coupling and inhibits the nonradiative charge recombination. The effect is attributed to suppression of contributions of higher frequency phonons to the electron–phonon coupling. The coupling is dominated in the ferroelectric phase by slower collective motions. We also demonstrate the importance of van der Waals interactions for the charge-phonon relaxation in the hybrid perovskite systems. Combined with the long-range charge separation achievable in the ferroelectric phase, the weakened electron–phonon coupling indicates that ferroelectric order in hybrid perovskites can lead to increased excited-state lifetimes and improved solar energy conversion performance.

Obtaining graphene (GRA) in industrial quantities is among the most urgent goals in today's nanotechnology. Elegant methods involve the oxidation of graphite with its subsequent solvent-assisted exfoliation. The reduction of graphene oxide (GO) is challenging leading to a highly-disordered oxygen-rich material. A particularly successful microwave-induced reduction of GO was reported recently (Science, 2016, 353, 1413–1416). We mimic the experiment by reactive molecular dynamics and establish the molecular mechanisms of reduction and their time scales as functions of temperature. We show that the rapid removal of oxygen groups achieved by microwave heating leaves GRA sheets intact. The epoxy groups are most stable within GO. They can rearrange into the carbonyl groups upon quick heating. It is important to avoid creating holes upon graphite oxidation. They cannot be healed easily and undermine GRA thermal stability and electronic properties. The edge oxygen groups cannot be removed by irradiation, but their effect is marginal on the properties of μm GRA sheets. We demonstrate that different oxygen groups are removed from GO at drastically different temperatures. Therefore, it is possible to obtain separate fractions, e.g. carbonyl-, hydroxyl- and carboxyl-free partially reduced GO. Our results guide the improvement of the GO reduction methods and can be tested directly by experiment.

Two-dimensional transition metal dichalcogenides (MX2, M = Mo, W; X = S, Se) hold great potential in optoelectronics and photovoltaics. To achieve efficient light-to-electricity conversion, electron–hole pairs must dissociate into free charges. Coulomb interaction in MX2 often exceeds the charge transfer driving force, leading one to expect inefficient charge separation at a MX2 heterojunction. Experiments defy the expectation. Using time-domain density functional theory and nonadiabatic (NA) molecular dynamics, we show that quantum coherence and donor–acceptor delocalization facilitate rapid charge transfer at a MoS2/MoSe2 interface. The delocalization is larger for electron than hole, resulting in longer coherence and faster transfer. Stronger NA coupling and higher acceptor state density accelerate electron transfer further. Both electron and hole transfers are subpicosecond, which is in agreement with experiments. The transfers are promoted primarily by the out-of-plane Mo–X modes of the acceptors. Lighter S atoms, compared to Se, create larger NA coupling for electrons than holes. The relatively slow relaxation of the “hot” hole suggests long-distance bandlike transport, observed in organic photovoltaics. The electron–hole recombination is notably longer across the MoS2/MoSe2 interface than in isolated MoS2 and MoSe2, favoring long-lived charge separation. The atomistic, time-domain studies provide valuable insights into excitation dynamics in two-dimensional transition metal dichalcogenides.

Advancing organohalide perovskite solar cells requires understanding of carrier dynamics. Electron–hole recombination is a particularly important process because it constitutes a major pathway of energy and current losses. Grain boundaries (GBs) are common in methylammonium lead iodine CH3NH3PbI3 (MAPbI3) perovskite polycrystalline films. First-principles calculations have suggested that GBs have little effect on the recombination; however, experiments defy this prediction. Using nonadiabatic (NA) molecular dynamics combined with time-domain density functional theory, we show that GBs notably accelerate the electron–hole recombination in MAPbI3. First, GBs enhance the electron–phonon NA coupling by localizing and contributing to the electron and hole wave functions and by creating additional phonon modes that couple to the electronic degrees of freedom. Second, GBs decrease the MAPbI3 bandgap, reducing the number of vibrational quanta needed to accommodate the electronic energy loss. Third, the phonon-induced loss of electronic coherence remains largely unchanged and not accelerated, as one may expect from increased electron–phonon coupling. Further, replacing iodines by chlorines at GBs reduces the electron–hole recombination. By pushing the highest occupied molecular orbital (HOMO) density away from the boundary, chlorines restore the NA coupling close to the value observed in pristine MAPbI3. By introducing higher-frequency phonons and increasing fluctuation of the electronic gap, chlorines shorten electronic coherence. Both factors compete successfully with the reduced bandgap relative to pristine MAPbI3 and favor long excited-state lifetimes. The simulations show excellent agreement with experiment and characterize how GBs and chlorine dopants affect electron–hole recombination in perovskite solar cells. The simulations suggest a route to increased photon-to-electron conversion efficiencies through rational GB passivation.

N-Heterocyclic carbenes (NHCs) are becoming increasingly popular ligand frameworks for nanocrystal surfaces; however, as of yet the nature of the NHC–nanocrystal interface remains unexplored across different material types. Here we report a facile synthetic route to NHC-stabilized metal and metal chalcogenide nanocrystals. It was observed that NHC–Ag nanocrystals are colloidally stable, but much less so than the corresponding NHC–Ag2E analogues. Comprehensive NMR studies suggest a dynamic NHC–nanocrystal interface for both NHC–Ag and NHC–Ag2S; however, density functional theory calculations reveal a much stronger binding affinity of the NHC ligands to Ag2S compared with Ag nanocrystals, which explains the superior colloidal stability of the metal chalcogenides. This offers new insight into the surface chemistry of neutral L-type carbenes in colloidal nanocrystal chemistry.

Excited state dynamics at the nanoscale requires treatment of systems involving hundreds and thousands of atoms. In the majority of cases, depending on the process under investigation, the electronic structure component of the calculation constitutes the computation bottleneck. We developed an efficient approach for simulating nonadiabatic molecular dynamics (NA-MD) of large systems in the framework of the self-consistent charge density functional tight binding (SCC-DFTB) method. SCC-DFTB is combined with the fewest switches surface hopping (FSSH) and decoherence induced surface hopping (DISH) techniques for NA-MD. The approach is implemented within the Python extension for the ab initio dynamics (PYXAID) simulation package, which is an open source NA-MD program designed to handle nanoscale materials. The accuracy of the developed approach is tested with ab initio DFT and experimental data, by considering intraband electron and hole relaxation, and nonradiative electron–hole recombination in a CdSe quantum dot and the (10,5) semiconducting carbon nanotube. The technique is capable of treating accurately and efficiently excitation dynamics in large, realistic nanoscale materials, employing modest computational resources.

It has been a long time that divergent behaviors were observed in many photocatalytic hydrogen evolution reactions (HER) on CdS and ZnS although the two photocatalysts have similar compositions and structures. For example, CdS itself is inactive and loading of cocatalysts is indispensable to achieve high efficiency of hydrogen evolution, but the reverse is true for ZnS. The underlying reasons are still unclear to date. The Volmer reaction of HER on catalysts is H+ + e− + * → H*, and its free energy (ΔGH* = ΔEH* + ΔEZPE − TΔS + eU; the adsorption energy, zero-point energy, entropy and potential energy are on the right side) is a good theoretical descriptor of the electrocatalytic HER activity from the electrocatalytic HER theory. In this paper, we firstly determined the most stable CdS and ZnS(110) termination under the conditions of photocatalytic HER, i.e., pure (110), by calculating the free energies of three reactions related to H2O dissociation on (110). Then we rationalized these behaviors by calculating the free energy of H* adsorption on pure and Pt loaded CdS and ZnS(110) at different pH. The performance of photocatalytic HER on CdS and ZnS was found to be determined jointly by the free energy of H* adsorption and the conduction band minimum (CBM) of the photocatalysts. On pure (110) with large ΔGH*, the photocatalytic HER is favored on ZnS due to its higher CBM; on Pt loaded (110) with small ΔGH*, the photocatalytic HER is favored on CdS due to its lower CBM. These results well explained the divergent behaviors observed in the photocatalytic HER on CdS and ZnS.

Gold nanoparticles distinguish themselves from other nanoparticles due to their unique surface plasmon resonance properties that can be exploited for a multiplicity of applications. The promise of plasmonic heating in systems of Au nanoparticles on transition metal oxide supports, for example, Au/TiO2, rests with the ability of the surface plasmon in Au nanoparticles to effectively transfer energy into the transition metal oxide. Here, we report a critical observation regarding Au nanoparticle (Au55) surface plasmon excitations, that is, the relaxation of the surface plasmon excitation is very slow, on the order of several picoseconds. Starting from five plasmon states in Au55 nanoparticles using nonadiabatic molecular dynamics simulations, we find that the relaxation time constant resulting from these simulations is ∼6.8 ps, mainly resulting from a long-lived intermediate state found at around −0.8 eV. This long-lived intermediate state aligns with the conduction band edge of TiO2, thereby facilitating energy transfer injection from the Au55 nanoparticle into the TiO2. The current results rule out the previously reported molecular-like relaxation dynamics for Au55.

The critical point, CP (T, P), of the phase diagram quantifies the minimum amount of kinetic energy needed to prevent a substance from existing in a condensed phase. Therefore, the CP is closely related to the properties of the fluid far below the critical temperature. Approaches designed to predict thermophysical properties of a system necessarily aim to provide reliable estimates of the CP. Vice versa, CP estimation is impossible without knowledge of the vapor phase behavior. We report ab initio Born–Oppenheimer molecular dynamics (BOMD) simulations of sodium and potassium chlorides, NaCl and KCl, at and above their expected CPs. We advance the present knowledge regarding the existence of ionic species in the vapor phase by establishing significant percentages of atomic clusters: 29–30% in NaCl and 34–38% in KCl. A neutral pair of counterions is the most abundant cluster in the ionic vapors (ca. 35% of all vaporized ions exist in this form). Unexpectedly, an appreciable fraction of clusters is charged. The ionic vapor composition is determined by the vapor density, rather than the nature of the alkali ion. The previously suggested CPs of NaCl and KCl appear overestimated, based on the present simulations. The reported results offer essential insights into the ionic fluid properties and assist in development of thermodynamic theories. The ab initio BOMD method has been applied to investigate the vapor phase composition of an ionic fluid for the first time.

Strong electrostatic interactions in ionic compounds make vaporization a complex process. The gas phase can contain a broad range of ionic clusters, and the cluster composition can differ greatly from that in the liquid phase. Room-temperature ionic liquids (RTILs) constitute a complicated case due to their ionic nature, asymmetric structure, and a huge versatility of ions and ionic clusters. This work reports vapor–liquid equilibria and vapor compositions of butylpyridinium (BPY) RTILs formed with hexafluorophosphate (PF6), trifluoromethanesulfonate (TF), and bis(trifluoromethanesulfonyl)imide (TFSI) anions. Unlike inorganic crystals, the pyridinium-based RTILs contain significant percentages of charged clusters in the vapor phase. Ion triplets and ion quadruplets each constitute up to 10% of the vapor phase composition. Triples prevail over quadruples in [BPY][PF6] due to the size difference of the cation and the anion. The percentage of charged ionic clusters in the gas phase is in inverse proportion to the mass of the anion. The largest identified vaporized ionic cluster comprises eight ions, with a formation probability below 1%. Higher temperature fosters formation of larger clusters due to an increase of the saturated vapor density.

Quasi-two-dimensional colloidal nanoplatelets (NPLs) have recently emerged as a class of semiconductor nanomaterials whose atomically precise monodisperse thicknesses give rise to narrow absorption and emission spectra. However, the sub-picosecond carrier dynamics of NPLs at the band edge remain largely unknown, despite their importance in determining the optoelectronic properties of these materials. Here, we use a combination of femtosecond transient absorption spectroscopy and nonadiabatic molecular dynamics simulations to investigate the early time carrier dynamics of CdSe/CdS core/shell NPLs. Band-selective probing reveals sub-picosecond Auger-mediated trapping of holes with an effective second-order rate constant of 3.5 ± 1.0 cm2/s. Concomitant spectral blue shifts that are indicative of Auger hole heating are found to occur on the same time scale as the sub-picosecond trapping dynamics, whereas spectral red shifts that emerge at low excitation densities furnish an electron-cooling time scale of 0.84 ± 0.09 ps. Finally, nonadiabatic molecular dynamics simulations relate the observed sub-picosecond Auger-mediated hole-trapping dynamics to a shallow trap state that originates from the incomplete passivation of dangling bonds on the NPL surface.Keywords: Auger carrier heating; carrier cooling; femtosecond transient absorption spectroscopy; quantum wells; ultrafast carrier dynamics

Cubane features one of the highest densities of covalent bonds among all known compounds. Kinetically stable, it constitutes an excellent candidate for efficient storage of large amounts of energy. We employ ab initio and semiempirical quantum-chemical methods to investigate systematically the amounts of energy that can be stored in cubane and its two derivatives known synthetically, octanitrocubane (8-CUB) and heptanitrocubane (7-CUB). Using nonequilibrium molecular dynamics, we establish the energy liberation pathways and molecular decomposition mechanisms. The reaction starts with spontaneous isomerizations of nitrocubanes into nitroannulenes and nitrosooxycubanes, subsequently producing NO and octaone C8O8. Unstable C8O8 decomposes within a picosecond, producing 8 CO molecules. All reactions are exothermic, giving rise to a sharp temperature increase. Ultimately, CO and NO recombine into CO2 and N2. According to the thermochemical calculations, 8-CUB stores 4145 kJ mol–1 of free energy, while 7-CUB stores 4346 kJ mol–1. The reported results foster consideration of cubanes for energy storage.

The Haber–Bosch process is the main industrial method for producing ammonia from diatomic nitrogen and hydrogen. We use a combination of ab initio thermochemical analysis and reactive molecular dynamics to demonstrate that a significant increase in the ammonia production yield can be achieved using hydroxylated graphene and related species. Exploiting the polarity difference between N2/H2 and NH3, as well as the universal proton acceptor behavior of NH3, we demonstrate a strong shift of the equilibrium of the Haber–Bosch process toward ammonia (ca. 50 kJ mol–1 enthalpy gain and ca. 60–70 kJ mol–1 free energy gain). The modified process is of significant importance to the chemical industry.

Alkali metals are known to form dimers, trimers, and tetramers in their vapors. The mechanism and regularities of this phenomenon characterize the chemical behavior of the first group elements. We report ab initio molecular dynamics (AIMD) simulations of the alkali metal vapors and characterize their structural properties, including radial distribution functions and atomic cluster size distributions. AIMD confirms formation of Men, where n ranges from 2 to 4. High pressure sharply favors larger structures, whereas high temperature decreases their fraction. Heavier alkali metals maintain somewhat larger fractions of Me2, Me3, and Me4, relative to isolated atoms. A single atom is the most frequently observed structure in vapors, irrespective of the element and temperature. Due to technical difficulties of working with high temperatures and pressures in experiments, AIMD is the most affordable method of research. It provides valuable understanding of the chemical behavior of Li, Na, K, Rb, and Cs, which can lead to development of new chemical reactions involving these metals.

The influence of bare and solvated cations imbedded inside single-walled carbon nanotubes (SWCNTs) on the SWCNT electronic properties is studied by ab initio electronic structure calculations. The roles of ion charge and ion solvation are investigated by comparing Li+ vs Mg2+ and Li+ vs its solvatocomplex with two ethylene carbonate (EC) molecules, [Li(EC)2]+. Two achiral nanotubes with similar radii but different electronic structure are considered, namely, the metallic, (15,15) armchair, and semiconducting, (26,0) zigzag, SWCNTs. The intercalation process is energetically favorable for both CNT topologies, with all bare cations and the solvatocomplex under investigation, with the doubly charged Mg2+ ion exhibiting the highest energy gain. Insertion of the bare ions into the SWCNTs increases the electronic entropy. The electronic entropy changes because the ions introduce new energy levels near the Fermi level. Those initially empty levels of the cations accept electron density and generate electronic holes in the valence band of both SWCNT topologies. As a consequence, the semiconducting (26,0) zigzag SWCNT becomes metallic, exhibiting hole conductivity. Solvation of the bare Li+ ion by EC molecules completely screens the influence of the ion charge on the SWCNT electronic properties, independent of the topology. The last fact validates the common practice of employing standard, nonpolarizable force field models in classical molecular dynamics simulations of electrolyte solutions interacting with CNTs. The strong dependence of the nanotube electronic properties on the presence of bare ions can be used for development of novel cation sensors for mass spectroscopy applications.

Experiments show both positive and negative changes in performance of hybrid organic–inorganic perovskite solar cells upon exposure to moisture. Ab initio nonadiabatic molecular dynamics reveals the influence of humidity on nonradiative electron–hole recombination. In small amounts, water molecules perturb perovskite surface and localize photoexcited electron close to the surface. Importantly, deep electron traps are avoided. The electron–hole overlap decreases, and the excited state lifetime increases. In large amounts, water forms stable hydrogen-bonded networks, has a higher barrier to enter perovskite, and produces little impact on charge localization. At the same time, by contributing high frequency polar vibrations, water molecules increase nonadiabatic coupling and accelerate recombination. In general, short coherence between electron and hole benefits photovoltaic response of the perovskites. The calculated recombination time scales show excellent agreement with experiment. The time-domain atomistic simulations reveal the microscopic effects of humidity on perovskite excited-state lifetimes and rationalize the conflicting experimental observations.

Ion association in solutions of lithium salts in mixtures of alkyl carbonates carries significant impact on the performance of lithium ion batteries. Focusing on lithium bis(oxalato)borate, LiBOB, in binary solvents based on ethylene carbonate, EC, we show that neither continuum nor discrete solvation approaches are capable of predicting physically meaningful results. So-called mixed or the discrete–continuum solvation approach, based on explicit consideration of an ion solvatocomplex combined with estimation of the medium polarization effect, is required in order to characterize the ion association at the quantitative level. The calculated changes of the Gibbs free energy are overestimated by nearly an order of magnitude by the purely continuum and purely discrete approaches, with the values having the opposite signs. The physically balanced discrete–continuum description predicts weak ion association. The numerical data obtained with density functional theory are validated using coupled-cluster calculations and experimental X-ray data. The study contributes to resolution of the challenge in solvation modeling in general, and develops a reliable, practical method that can be used to screen ion association in a broad range of ion–molecular mixtures for lithium ion batteries, especially for the solutions of LiBOB in EC based mixtures.

A free electron in solution, known as a solvated electron, is the smallest possible anion. Alkali and alkaline earth atoms serve as electron donors in solvents that mediate outer-sphere electron transfer. We report herein ab initio molecular dynamics simulations of lithium, sodium, magnesium, and calcium in liquid ammonia at 250 K. By analyzing the electronic properties and the ionic and solvation structures and dynamics, we systematically characterize these metals as electron donors and ammonia molecules as electron acceptors. We show that the solvated metal strongly modifies the properties of its solvation shells and that the observed effect is metal-specific. Specifically, the radius and charge exhibit major impacts. The single solvated electron present in the alkali metal systems is distributed more uniformly among the solvent molecules of each metal’s two solvation shells. In contrast, alkaline earth metals favor a less uniform distribution of the electron density. Alkali and alkaline earth atoms are coordinated by four and six NH3 molecules, respectively. The smaller atoms, Li and Mg, are stronger electron donors than Na and Ca. This result is surprising, as smaller atoms in a column of the periodic table have higher ionization potentials. However, it can be explained by stronger electron donor–acceptor interactions between the smaller atoms and the solvent molecules. The structure of the first solvation shell is sharpest for Mg, which has a large charge and a small radius. Solvation is weakest for Na, which has a small charge and a large radius. Weak solvation leads to rapid dynamics, as reflected in the diffusion coefficients of NH3 molecules of the first two solvation shells and the Na atom. The properties of the solvated electrons established in the present study are important for radiation chemistry, synthetic chemistry, condensed-matter charge transfer, and energy sources.

Developed 25 years ago, Tully’s fewest switches surface hopping (FSSH) has proven to be the most popular approach for simulating quantum-classical dynamics in a broad variety of systems, ranging from the gas phase, to the liquid and solid phases, to biological and nanoscale materials. FSSH is widely adopted as the fundamental platform to introduce modifications as needed. Significant progress has been made recently to enhance the accuracy and efficiency of the surface hopping technique. Various limitations of the standard FSSH—associated with quantum nuclear effects, interference and decoherence, trivial or “unavoided” crossings, superexchange, and representation dependence—have been lifted. These advances are needed to allow one to treat many important phenomena in chemistry, physics, materials, and related disciplines. Examples include charge transport in extended systems such as organic solids, singlet fission in molecular aggregates, Auger-type exciton multiplication, recombination and relaxation in quantum dots and other nanoscale materials, Auger-assisted charge transfer, nonradiative luminescence quenching, and electron–hole recombination. This Perspective summarizes recent advances in the surface hopping formulation of nonadiabatic dynamics and provides an outlook on the future of surface hopping.

Recent observations of excitonic coherences within photosynthetic complexes suggest that quantum coherences could enhance biological light harvesting efficiencies. Here, we employ optical pump–probe spectroscopy with few-femtosecond pulses to observe an excitonic quantum coherence in CdSe nanocrystals, a prototypical artificial light harvesting system. This coherence, which encodes the high-speed migration of charge over nanometer length scales, is also found to markedly alter the displacement amplitudes of phonons, signaling dynamics in the non-Born–Oppenheimer regime.

Hybrid organic/inorganic polymer/quantum dot (QD) solar cells are an attractive alternative to the traditional cells. The original, simple models postulate that one-dimensional polymers have continuous energy levels, while zero-dimensional QDs exhibit atom-like electronic structure. A realistic, atomistic viewpoint provides an alternative description. Electronic states in polymers are molecule-like: finite in size and discrete in energy. QDs are composed of many atoms and have high, bulk-like densities of states. We employ ab initio time-domain simulation to model the experimentally observed ultrafast photoinduced dynamics in a QD/polymer hybrid and show that an atomistic description is essential for understanding the time-resolved experimental data. Both electron and hole transfers across the interface exhibit subpicosecond time scales. The interfacial processes are fast due to strong electronic donor–acceptor, as evidenced by the densities of the photoexcited states which are delocalized between the donor and the acceptor. The nonadiabatic charge–phonon coupling is also strong, especially in the polymer, resulting in rapid energy losses. The electron transfer from the polymer is notably faster than the hole transfer from the QD, due to a significantly higher density of acceptor states. The stronger molecule-like electronic and charge-phonon coupling in the polymer rationalizes why the electron–hole recombination inside the polymer is several orders of magnitude faster than in the QD. As a result, experiments exhibit multiple transfer times for the long-lived hole inside the QD, ranging from subpicoseconds to nanoseconds. In contrast, transfer of the short-lived electron inside the polymer does not occur beyond the first picosecond. The energy lost by the hole on its transit into the polymer is accommodated by polymer’s high-frequency vibrations. The energy lost by the electron injected into the QD is accommodated primarily by much lower-frequency collective and QD modes. The electron dynamics is exponential, whereas evolution of the injected hole through the low density manifold of states of the polymer is highly nonexponential. The time scale of the electron–hole recombination at the interface is intermediate between those in pristine polymer and QD and is closer to that in the polymer. The detailed atomistic insights into the photoinduced charge and energy dynamics at the polymer/QD interface provide valuable guidelines for optimization of solar light harvesting and photovoltaic efficiency in modern nanoscale materials.

Recent experimental studies demonstrated that photocatalytic CO2 reduction by Ru catalysts assembled on N-doped Ta2O5 surface is strongly dependent on the nature of the anchor group with which the Ru complexes are attached to the substrate. We report a comprehensive atomistic analysis of electron transfer dynamics in electroneutral Ru(di-X-bpy) (CO)2Cl2 complexes with X = COOH and PO3H2 attached to the N–Ta2O5 substrate. Nonadiabatic molecular dynamics simulations indicate that the electron transfer is faster in complexes with COOH anchors than in complexes with PO3H2 groups, due to larger nonadiabatic coupling. Quantum coherence counteracts this effect, however, to a small extent. The COOH anchor promotes the transfer with significantly higher frequency modes than PO3H2, due to both lighter atoms (C vs P) and stronger bonds (double vs single). The acceptor state delocalizes onto COOH, but not PO3H2, further favoring electron transfer in the COOH system. At the same time, the COOH anchor is prone to decomposition, in contrast to PO3H2, making the former show smaller turnover numbers in some cases. These theoretical predictions are consistent with recent experimental results, legitimating the proposed mechanism of the electron transfer. We emphasize the role of anchor stability, nonadiabatic coupling, and quantum coherence in determining the overall efficiency of artificial photocatalytic systems.

We present a computational study of the dynamical and electronic structure origins of the impact of anchoring groups, PO3H2, COOH, and OH, on the efficiency of photochemical CO2 reduction in Ru(di-X-bpy)(CO)2Cl2/Ta2O5 systems. Recent experimental studies indicate that the efficiency may not directly correlate with the driving force for electron transfer (ET) in these systems, prompting the need for further investigation of the role of anchor groups. Our analysis shows that there are at least two key roles of the anchor in determining the efficiency of CO2 reduction by the Ru complex. First, depending on local steric interactions, different tilting angles and their fluctuations may emerge for different anchors, affecting the magnitude of the donor–acceptor coupling. Second, depending on localization of acceptor states on the anchor, determined by the anchor’s tendency to form conjugate subsystems, the yields of ET to the catalytic center may vary, directly affecting the photocatalytic efficiency. Finally, our calculations indicate that surface modeling with N-doping and many-body effects are needed to describe the ET process in the systems properly. N-doping imparts the Ta2O5 surface with a dipole moment, while Coulomb and exchange contributions to the electron–hole interaction can produce excitons that should be taken into account.

The novel approach to nonadiabatic quantum dynamics greatly increases the accuracy of the most popular semiclassical technique while maintaining its simplicity and efficiency. Unlike the standard Tully surface hopping in Hilbert space, which deals with population flow, the new strategy in Liouville space puts population and coherence on equal footing. Dual avoided crossing and energy transfer models show that the accuracy is improved in both diabatic and adiabatic representations and that Liouville space simulation converges faster with the number of trajectories than Hilbert space simulation. The constructed master equation accurately captures superexchange, tunneling, and quantum interference. These effects are essential for charge, phonon and energy transport and scattering, exciton fission and fusion, quantum optics and computing, and many other areas of physics and chemistry.

Rapid development in lead halide perovskites has led to solution-processable thin film solar cells with power conversion efficiencies close to 20%. Nonradiative electron–hole recombination within perovskites has been identified as the main pathway of energy losses, competing with charge transport and limiting the efficiency. Using nonadiabatic (NA) molecular dynamics, combined with time-domain density functional theory, we show that nonradiative recombination happens faster than radiative recombination and long-range charge transfer to an acceptor material. Doping of lead iodide perovskites with chlorine atoms reduces charge recombination. On the one hand, chlorines decrease the NA coupling because they contribute little to the wave functions of the valence and conduction band edges. On the other hand, chlorines shorten coherence time because they are lighter than iodines and introduce high-frequency modes. Both factors favor longer excited-state lifetimes. The simulation shows good agreement with the available experimental data and contributes to the comprehensive understanding of electronic and vibrational dynamics in perovskites. The generated insights into design of higher-efficiency solar cells range from fundamental scientific principles, such as the role of electron–vibrational coupling and quantum coherence, to practical guidelines, such as specific suggestions for chemical doping.

Femtosecond optical pump–probe spectroscopy is employed to elucidate the band-selective ultrafast carrier dynamics of few-layer MoS2. Following narrowband resonant photoexcitation of the exciton A transition, the subpicosecond to picosecond relaxation dynamics of the electron and the hole at the K valley are separately interrogated by a broadband probe pulse. The temporal evolution of the spectral first moment reveals nonexponential intravalley relaxation dynamics in the conduction band. Fluence dependence measurements suggest that this relaxation process is predominantly mediated by acoustic phonon emission. Intervalley scattering of carriers from the K valley to the extrema of the conduction and valence bands is also observed via the decay of the spectral zeroth moment. In addition, second-order Raman scattering leads to the emergence of sidebands in the normalized differential transmission spectra. The observed two-phonon energies and the fluence-dependent time constants suggest that the E1g longitudinal optical (LO) phonon and the LA phonon participate in intervalley scattering in the conduction and valence bands, respectively. Ab initio nonadiabatic molecular dynamics simulations yield time constants of 0.80 and 0.72 ps for intra- and intervalley electronic relaxation, respectively; the latter agrees well with experiment. Finally, the normalized differential transmission spectra reveal a two-electron shake-up satellite that originates from band-edge radiative recombination and the simultaneous excitation of a hole from Kv1 to Kv2. From its spectral position, a Kv1–Kv2 spin–orbit splitting of 1166 ± 1 cm–1 is deduced. The observation of the two-electron transition points to the existence of strong electron correlation in photoexcited few-layer MoS2.

Density functional theory calculations were executed to clarify the mechanism of the experimentally observed upward shift in conduction band minimum (CBM) and valence band maximum (VBM) of N-doped Ta2O5, which is used as a photosensitizer in CO2 reduction. Calculations reproduce well the experimental energy levels (with respect to vacuum) of nondoped Ta2O5 and N-doped Ta2O5. Detailed analyses indicate that N-doping induces formations of defects of oxygenated species, such as oxygen atom and surface hydroxyl group, in the Ta2O5, and the defect formations induce charge redistributions to generate excess negative charges near the doped nitrogen atoms and excess positive charges near the defect sites. When the concentration of the doped nitrogen atoms at the surface is not high enough to compensate positive charges induced at the surface defects, the remaining positive charges are compensated by the nitrogen atoms in inner layers. Dipole moments normal to the surface generated in this situation raise the CBM and VBM of Ta2O5, allowing photogenerated electrons to transfer from N-doped Ta2O5 to the catalytic active sites for CO2 reduction as realized with Ru complex on the surface in experiment.

It is well-known experimentally and theoretically that surface ligands provide additional pathways for energy relaxation in colloidal semiconductor quantum dots (QDs). They increase the rate of inelastic charge-phonon scattering and provide trap sites for the charges. We show that, surprisingly, ligands have the opposite effect on elastic electron–phonon scattering. Our simulations demonstrate that elastic scattering slows down in CdSe QDs passivated with ligands compared to that in bare QDs. As a result, the pure-dephasing time is increased, and the homogeneous luminescence line width is decreased in the presence of ligands. The lifetime of quantum superpositions of single and multiple excitons increases as well, providing favorable conditions for multiple excitons generation (MEG). Ligands reduce the pure-dephasing rates by decreasing phonon-induced fluctuations of the electronic energy levels. Surface atoms are most mobile in QDs, and therefore, they contribute greatly to the electronic energy fluctuations. The mobility is reduced by interaction with ligands. A simple analytical model suggests that the differences between the bare and passivated QDs persist for up to 5 nm diameters. Both low-frequency acoustic and high-frequency optical phonons participate in the dephasing processes in bare QDs, while low-frequency acoustic modes dominate in passivated QDs. The theoretical predictions regarding the pure-dephasing time, luminescence line width, and MEG can be verified experimentally by studying QDs with different surface passivation.Keywords: colloidal quantum dots; electron−phonon scattering; luminescence; multiple exciton generation; pure dephasing;

TiO2 sensitized with organohalide perovskites gives rise to solar-to-electricity conversion efficiencies reaching close to 20%. Nonradiative electron–hole recombination across the perovskite/TiO2 interface constitutes a major pathway of energy losses, limiting quantum yield of the photoinduced charge. In order to establish the fundamental mechanisms of the energy losses and to propose practical means for controlling the interfacial electron–hole recombination, we applied ab initio nonadiabatic (NA) molecular dynamics to pristine and doped CH3NH3PbI3(100)/TiO2 anatase(001) interfaces. We show that doping by substitution of iodide with chlorine or bromine reduces charge recombination, while replacing lead with tin enhances the recombination. Generally, lighter and faster atoms increase the NA coupling. Since the dopants are lighter than the atoms they replace, one expects a priori that all three dopants should accelerate the recombination. We rationalize the unexpected behavior of chlorine and bromine by three effects. First, the Pb–Cl and Pb–Br bonds are shorter than the Pb–I bond. As a result, Cl and Br atoms are farther away from the TiO2 surface, decreasing the donor–acceptor coupling. In contrast, some iodines form chemical bonds with Ti atoms, increasing the coupling. Second, chlorine and bromine reduce the NA electron–vibrational coupling, because they contribute little to the electron and hole wave functions. Tin increases the coupling, since it is lighter than lead and contributes to the hole wave function. Third, higher frequency modes introduced by chlorine and bromine shorten quantum coherence, thereby decreasing the transition rate. The recombination occurs due to coupling of the electronic subsystem to low-frequency perovskite and TiO2 modes. The simulation shows excellent agreement with the available experimental data and advances our understanding of electronic and vibrational dynamics in perovskite solar cells. The study provides design principles for optimizing solar cell performance and increasing photon-to-electron conversion efficiency through creative choice of dopants.Keywords: dopants; electron−hole recombination; nonadiabatic molecular dynamics; organohalide perovskites; time-domain density functional theory; TiO2;

Si, Ge, or Sn doped hematite (α-Fe2O3) photoanodes show significantly enhanced efficiency for photo-oxidization of water. We employed DFT+U to study the doping of α-Fe2O3 with group IV elements, i.e., Si, Ge, and Sn. From the calculated formation energies and chemical potentials, three key points are concluded. (1) Low oxygen pressure is favored for doping both substitutional and interstitial dopants. (2) Substitutional doping of the Fe atom at the lattice site is more stable than interstitial doping in the octahedral vacancies. (3) Most interestingly, Ge doping is found to be easiest among the three dopants. This result contradicts intuition based on atomic size and indicates that Ge doping should be more efficient than Si and Sn doping in increasing the charge carrier concentration. Incorporation of the dopants at the Fe site generates an electron polaron and the dopant with the +4 valence state by spontaneous transfer of one electron from the dopant atom to a surrounding Fe atom, according to the analyses of charge transition energy levels and density of states. We identify the factors affecting the charge transfer process. The study elucidates the dopants role in increasing the electrical conductivity of α-Fe2O3 and provides guidelines for designing new efficient photoanodes.

Understanding charge transfer reactions between quantum dots (QD) and metal oxides is fundamental for improving photocatalytic, photovoltaic, and electronic devices. The complexity of these processes makes it difficult to find an optimum QD size with rapid charge injection and low recombination. We combine time-domain density functional theory with nonadiabatic molecular dynamics to investigate the size and temperature dependence of the experimentally studied electron transfer and charge recombination at CdSe QD–TiO2 nanobelt (NB) interfaces. The electron injection rate shows strong dependence on the QD size, increasing for small QDs. The rate exhibits Arrhenius temperature dependence, with the activation energy of the order of millielectronvolts. The charge recombination process occurs due to coupling of the electronic subsystem to vibrational modes of the TiO2 NB. Inelastic electron–phonon scattering happens on a picosecond time scale, with strong dependence on the QD size. Our simulations demonstrate that the electron–hole recombination rate decreases significantly as the QD size increases, in excellent agreement with experiments. The temperature dependence of the charge recombination rates can be successfully modeled within the framework of the Marcus theory through optimization of the electronic coupling and the reorganization energy. Our simulations indicate that by varying the QD size, one can modulate the photoinduced charge separation and charge recombination, fundamental aspects of the design principles for high-efficiency devices.

Quantum confinement in nanoscale materials allows Auger-type electron–hole energy exchange. We show by direct time-domain atomistic simulation and analytic theory that Auger processes give rise to a new mechanism of charge transfer (CT) on the nanoscale. Auger-assisted CT eliminates the renown Marcus inverted regime, rationalizing recent experiments on CT from quantum dots to molecular adsorbates. The ab initio simulation reveals a complex interplay of the electron–hole and charge–phonon channels of energy exchange, demonstrating a variety of CT scenarios. The developed Marcus rate theory for Auger-assisted CT describes, without adjustable parameters, the experimental plateau of the CT rate in the region of large donor–acceptor energy gap. The analytic theory and atomistic insights apply broadly to charge and energy transfer in nanoscale systems.

A nonadiabatic (NA) molecular dynamics (MD) simulation requires calculation of NA coupling matrix elements, the number of which scales as a square of the number of basis states. The basis size can be huge in studies of nanoscale materials, and calculation of the NA couplings can present a significant bottleneck. A quantum-classical approximation, NAMD overestimates coherence in the quantum, electronic subsystem, requiring decoherence correction. Generally, decoherence times decrease with increasing energy separation between pairs of states forming coherent superpositions. Since rapid decoherence stops quantum dynamics, one expects that decoherence-corrected NAMD can eliminate the need for calculation of NA couplings between energetically distant states, notably reducing the computational cost. Considering several types of dynamics in a semiconductor quantum dot, we demonstrate that indeed, decoherence allows one to reduce the number of needed NA coupling matrix elements. If the energy levels are spaced closer than 0.1 eV, one obtains good results while including only three nearest-neighbor couplings, and in some cases even with just the first nearest-neighbor coupling scheme. If the energy levels are spaced by about 0.4 eV, the nearest-neighbor model fails, while three or more nearest-neighbor schemes also provide good results. In comparison, the results of NAMD simulation without decoherence vary continuously with changes in the number of NA couplings. Thus, decoherence effects induced by coupling to a quantum-mechanical environment not only provide the physical mechanism for NAMD trajectory branding and improve the accuracy of NAMD simulations, but also afford significant computational savings.

TiO2 sensitized with quantum dots (QDs) gives efficient photovoltaic and photocatalytic systems due to high stability and large absorption cross sections of QDs and rapid photoinduced charge separation at the interface. The yields of the light-induced processes are limited by electron–hole recombination that also occurs at the interface. We combine ab initio nonadiabatic molecular dynamics with analytic theory to investigate the experimentally studied charge recombination at the PbSe QD–TiO2 interface. The time-domain atomistic simulation directly mimics the laser experiment and generates important details of the recombination mechanism. The process occurs due to coupling of the electronic subsystem to polar optical modes of the TiO2 surface. The inelastic electron–phonon scattering happens on a picosecond time scale, while the elastic scattering takes 40 fs. Counter to expectations, the donor–acceptor bonding strengthens at an elevated temperature. An analytic theory extends the simulation results to larger QDs and longer QD–TiO2 bridges. It shows that the electron–hole recombination rate decreases significantly for longer bridges and larger dots and that the main effect arises due to reduced donor–acceptor coupling rather than changes in the donor–acceptor energy gap. The study indicates that by varying QD size or ligands one can reduce charge losses while still maintaining efficient charge separation, providing design principles for optimizing solar cell design and increasing photon-to-electron conversion efficiencies.Keywords: electron transfer; nonadiabatic molecular dynamics; PbSe quantum dots; time-dependent density functional theory; TiO2;

We show that the electronic properties of single walled carbon nanotubes (SWCNTs) can be tuned continuously from semiconducting to metallic by varying the location of ions inside the tubes. Focusing on the Li+ cation inside the (26,0) zigzag semiconducting and (15,15) armchair metallic SWCNTs, we found that the Li+-SWCNT interaction is attractive. The interaction is stronger for the metallic SWCNT, indicating in particular that metallic tubes can enhance performance of lithium-ion batteries. The electronic properties of the metallic SWCNT are virtually independent of the presence of ions: Li+ creates an energy level in the valence band slightly below the Fermi energy. On the contrary, the semiconducting SWCNT can be made metallic by placing ions close to the tube axis: Li+ generates a new bottom of the conduction band. Letting the ions approach SWCNT walls recovers the semiconducting behavior.Keywords: Carbon nanotubes; electronic entropy; Fermi level; Free Energy Functional; hole conductivity; lithium ion; metallic and semiconducting properties; Γ-point;

Femtosecond optical pump–probe spectroscopy with 10 fs visible pulses is employed to elucidate the ultrafast carrier dynamics of few-layer MoS2. A nonthermal carrier distribution is observed immediately following the photoexcitation of the A and B excitonic transitions by the ultrashort, broadband laser pulse. Carrier thermalization occurs within 20 fs and proceeds via both carrier–carrier and carrier–phonon scattering, as evidenced by the observed dependence of the thermalization time on the carrier density and the sample temperature. The n–0.37±0.03 scaling of the thermalization time with carrier density suggests that equilibration of the nonthermal carrier distribution occurs via non-Markovian quantum kinetics. Subsequent cooling of the hot Fermi–Dirac carrier distribution occurs on the ∼0.6 ps time scale via carrier–phonon scattering. Temperature- and fluence-dependence studies reveal the involvement of hot phonons in the carrier cooling process. Nonadiabatic ab initio molecular dynamics simulations, which predict carrier–carrier and carrier–phonon scattering time scales of 40 fs and 0.5 ps, respectively, lend support to the assignment of the observed carrier dynamics.Keywords: carrier−carrier scattering; carrier−phonon scattering; MoS2; nonthermal; quantum kinetics; ultrafast dynamics;

It has been a long time that divergent behaviors were observed in many photocatalytic hydrogen evolution reactions (HER) on CdS and ZnS although the two photocatalysts have similar compositions and structures. For example, CdS itself is inactive and loading of cocatalysts is indispensable to achieve high efficiency of hydrogen evolution, but the reverse is true for ZnS. The underlying reasons are still unclear to date. The Volmer reaction of HER on catalysts is H+ + e− + * → H*, and its free energy (ΔGH* = ΔEH* + ΔEZPE − TΔS + eU; the adsorption energy, zero-point energy, entropy and potential energy are on the right side) is a good theoretical descriptor of the electrocatalytic HER activity from the electrocatalytic HER theory. In this paper, we firstly determined the most stable CdS and ZnS(110) termination under the conditions of photocatalytic HER, i.e., pure (110), by calculating the free energies of three reactions related to H2O dissociation on (110). Then we rationalized these behaviors by calculating the free energy of H* adsorption on pure and Pt loaded CdS and ZnS(110) at different pH. The performance of photocatalytic HER on CdS and ZnS was found to be determined jointly by the free energy of H* adsorption and the conduction band minimum (CBM) of the photocatalysts. On pure (110) with large ΔGH*, the photocatalytic HER is favored on ZnS due to its higher CBM; on Pt loaded (110) with small ΔGH*, the photocatalytic HER is favored on CdS due to its lower CBM. These results well explained the divergent behaviors observed in the photocatalytic HER on CdS and ZnS.

Energetic materials, such as explosives, propellants, and pyrotechnics, are widely used in civilian and military applications. Nanoscale explosives represent a special group because of the high density of energetic covalent bonds. The reactive molecular dynamics (ReaxFF) study of nitrofullerene decomposition reported here provides a detailed chemical mechanism of explosion of a nanoscale carbon material. Upon initial heating, C60(NO2)12 disintegrates, increasing temperature and pressure by thousands of Kelvins and bars within tens of picoseconds. The explosion starts with NO2 group isomerization into C–O–N–O, followed by emission of NO molecules and formation of CO groups on the buckyball surface. NO oxidizes into NO2, and C60 falls apart, liberating CO2. At the highest temperatures, CO2 gives rise to diatomic carbon. The study shows that the initiation temperature and released energy depend strongly on the chemical composition and density of the material.