Yosuke Kanai

Role of semiconducting and metallic tubes in P3HT/carbon-nanotube photovoltaic heterojunctions: Density functional theory calculations

A density functional theory approach is employed to investigate poly-3-hexylthiophene (P3HT) interfaced with both a semiconducting and metallic carbon nanotube (CNT). For the semiconducting CNT, a type-II heterojunction can form, making such an interface desirable as a photovoltaic heterojunction. In contrast, with the metallic CNT, substantial charge redistribution occurs and the interaction is strongly enhanced. The built-in-potential is, however, quite small, and P3HT becomes electrostatically more attractive for electrons. These observations together indicate that, in a photovoltaic heterojunction based on a mixed CNT distribution, the majority of interfaces are with metallic CNTs and inefficient.

Mechanism of Thermal Reversal of the (Fulvalene)tetracarbonyldiruthenium Photoisomerization: Toward Molecular Solar–Thermal Energy Storage

In the currently intensifying quest to harness solar energy for the powering of our planet, most efforts are centered around photoinduced generic charge separation, such as in photovoltaics, water splitting, other small molecule activation, and biologically inspired photosynthetic systems. In contrast, direct collection of heat from sunlight has received much less diversified attention, its bulk devoted to the development of concentrating solar thermal power plants, in which mirrors are used to focus the sun beam on an appropriate heat transfer material. An attractive alternative strategy would be to trap solar energy in the form of chemical bonds, ideally through the photoconversion of a suitable molecule to a higher energy isomer, which, in turn, would release the stored energy by thermal reversal. Such a system would encompass the essential elements of a rechargeable heat battery, with its inherent advantages of storage, transportability, and use on demand. The underlying concept has been explored extensively with organic molecules (such as the norbornadiene-quadricyclane cycle), often in the context of developing photoswitches. On the other hand, organometallic complexes have remained relatively obscure in this capacity, despite a number of advantages, including expanded structural tunability and generally favorable electronic absorption regimes. A highly promising organometallic system is the previously reported, robust photo-thermal fulvalene (Fv) diruthenium couple 1 {l_reversible} 2 (Scheme 1). However, although reversible and moderately efficient, lack of a full, detailed atom-scale understanding of its key conversion and storage mechanisms have limited our ability to improve on its performance or identify optimal variants, such as substituents on the Fv, ligands other than CO, and alternative metals. Here we present a theoretical investigation, in conjunction with corroborating experiments, of the mechanism for the heat releasing step of 2 {yields} 1 and its Fe (4) and Os (6) relatives. The results of the combined study has enabled a rigorous interpretation of earlier and new experimental measurements and paint a surprising picture. First-principles calculations were employed based on spin unrestricted density functional theory (DFT) with a non-empirical gradient corrected exchange-correlation functional. Ultrasoft pseudopotentials were used to describe the valence-core interactions of electrons, including scalar relativistic effects of the core. Wavefunctions and charge densities were expanded in plane waves with kinetic energies up to 25 and 200 Rydberg, respectively. Reaction pathways were delineated with the string method, as implemented within the Car-Parrinello approach. This method allows for the efficient determination of the minimum energy path (MEP) of atomistic transitions and thus also saddle points (transition states, TSs), which are the energy maxima along the MEP. All geometries were optimized until all forces on the atoms were less than 0.02 eV/{angstrom}. The calculated structures of 1 and 2 were in good agreement with their experimental counterparts.

X‐ray Transient Absorption and Picosecond IR Spectroscopy of Fulvalene(tetracarbonyl)diruthenium on Photoexcitation

Caught in the light: The fulvalene diruthenium complex shown on the left side of the picture captures sun light, causing initial Ru-Ru bond rupture to furnish a long-lived triplet biradical of syn configuration. This species requires thermal activation to reach a crossing point (middle) into the singlet manifold on route to its thermal storage isomer on the right through the anti biradical.

Role of Four-Fold Coordinated Titanium and Quantum Confinement in CO2 Reduction at Titania Surface

Photocatalytic reduction of carbon dioxide (CO(2)) into hydrocarbons is an attractive approach for mitigating CO(2) emission and generating useful fuels at the same time. Titania (TiO(2)) is one of the most promising photocatalysts for this purpose, and nanostructured TiO(2) materials often lead to an increased efficiency for the photocatalytic reactions. However, what aspects of and how such nanomaterials play the important role in the improved efficiency are yet to be understood. Using first-principles calculations, reaction mechanisms on the surface of bulk anatase TiO(2)(101) and of a small TiO(2) nanocluster were investigated to elucidate the role of four-fold coordinated titanium atoms and quantum confinement (QC) in the CO(2) reduction. Significant barrier reduction observed on the nanocluster surface is discussed in terms of how the under-coordinated titanium atoms and QC influence CO(2) reduction kinetics at surface. It is shown that the reduction to CO can be greatly facilitated by the under-coordinated titanium atoms, and they also make CO(2) anion formation favorable at surfaces.

Testing the TPSS meta-generalized-gradient-approximation exchange-correlation functional in calculations of transition states and reaction barriers

We have studied the performance of local and semilocal exchange-correlation functionals [meta-generalized-gradient-approximation (GGA)-TPSS, GGA-Perdew-Burke-Ernzerhof (PBE), and local density approximation (LDA)] in the calculation of transition states, reaction energies, and barriers for several molecular and one surface reaction, using the plane-wave pseudopotential approach. For molecular reactions, these results have been compared to all-electron Gaussian calculations using the B3LYP hybrid functional, as well as to experiment and high level quantum chemistry calculations, when available. We have found that the transition state structures are accurately identified irrespective of the level of the exchange-correlation functional, with the exception of a qualitatively incorrect LDA prediction for the H-transfer reaction in the hydrogen bonded complex between a water molecule and a OH radical. Both the meta-GGA-TPSS and the GGA-PBE functionals improve significantly the calculated LDA barrier heights. The meta-GGA-TPSS further improves systematically, albeit not always sufficiently, the GGA-PBE barriers. We have also found that, on the Si(001) surface, the meta-GGA-TPSS barriers for hydrogen adsorption agree significantly better than the corresponding GGA-PBE barriers with quantum Monte Carlo cluster results and experimental estimates.

Site-Selective Passivation of Defects in NiO Solar Photocathodes by Targeted Atomic Deposition

For nanomaterials, surface chemistry can dictate fundamental material properties, including charge-carrier lifetimes, doping levels, and electrical mobilities. In devices, surface defects are usually the key limiting factor for performance, particularly in solar-energy applications. Here, we develop a strategy to uniformly and selectively passivate defect sites in semiconductor nanomaterials using a vapor-phase process termed targeted atomic deposition (TAD). Because defects often consist of atomic vacancies and dangling bonds with heightened reactivity, we observe-for the widely used p-type cathode nickel oxide-that a volatile precursor such as trimethylaluminum can undergo a kinetically limited selective reaction with these sites. The TAD process eliminates all measurable defects in NiO, leading to a nearly 3-fold improvement in the performance of dye-sensitized solar cells. Our results suggest that TAD could be implemented with a range of vapor-phase precursors and be developed into a general strategy to passivate defects in zero-, one-, and two-dimensional nanomaterials.

Charge separation in nanoscale photovoltaic materials: recent insights from first-principles electronic structure theory

In this feature article we focus on the key problem of charge separation in nano-scale photovoltaic materials; in particular recent theoretical/computational work based on first principles electronic structure approaches is presented and discussed. We review applications of state-of-the-art electronic structure calculations to nano-scale materials that enable charge separation between an excited electron and hole in so-called excitonic photovoltaic cells. Emphasis is placed on theoretical results that provide insight into experimentally observed processes, which are yet to be understood and do not appear to obey a single unique model but rather depend on atomistic details. Examples are provided that illustrate how computational approaches can be employed to probe new directions in materials design for inducing efficient charge separation. We also discuss the computational challenges in electronic structure theory for reliably predicting and designing new materials suitable for charge separation in photovoltaic applications.

Role of Charge Transfer in Water Diffusivity in Aqueous Ionic Solutions

We performed molecular dynamics simulations on four types of systems containing ion and solvating water. Two systems contained a cation (Na(+) or K(+)), and two other systems an anion (Cl(-) or I(-)). Classical molecular dynamics simulations were performed using three different force fields: a fixed charge force field, a polarizable force field that includes explicit polarization, and also a recently developed force field that includes polarization and charge transfer. These simulations were then compared to first-principles molecular dynamics simulations. While the first-principles simulations showed that the anions accelerated water translational diffusion, the cations slowed it down. In simulations with the classical force fields, only the force field that incorporates explicit charge transfer reproduced this ion-specific behavior. Additional simulations performed to understand the effect of charge transfer demonstrated that two competitive factors determine the behavior of water translational diffusion: the ions diminished charge accelerates water, while the net charge acquired by water either accelerates or slows down its dynamics. Our results show that charge transfer plays a crucial role in governing the water dynamics in aqueous ionic solutions.

Plane-wave pseudopotential implementation of explicit integrators for time-dependent Kohn-Sham equations in large-scale simulations

Explicit integrators for real-time propagation of time-dependent Kohn-Sham equations are compared regarding their suitability for performing large-scale simulations. Four algorithms are implemented and assessed for both stability and accuracy within a plane-wave pseudopotential framework, employing the adiabatic approximation to the exchange-correlation functional. Simulation results for a single sodium atom and a sodium atom embedded in bulk magnesium oxide are discussed. While the first-order Euler scheme and the second-order finite-difference scheme are unstable, the fourth-order Runge-Kutta scheme is found to be conditionally stable and accurate within this framework. Excellent parallel scalability of the algorithm up to more than a thousand processors is demonstrated for a system containing hundreds of electrons, evidencing the suitability for large-scale simulations based on real-time propagation of time-dependent Kohn-Sham equations.

Role of exchange in density-functional theory for weakly interacting systems: Quantum Monte Carlo analysis of electron density and interaction energy

We analyze the density-functional theory DFT description of weak interactions by employing diffusion and reptation quantum Monte Carlo QMC calculations, for a set of benzene-molecule complexes. While the binding energies depend significantly on the exchange-correlation approximation employed for DFT calculations, QMC calculations show that the electron density is accurately described within DFT, including the quantitative features in the reduced density gradient. We elucidate how the enhancement of the exchangeenergy density at a large reduced density gradient plays a critical role in obtaining accurate DFT description of weakly interacting systems. Weak interactions play an important role in numerous chemical, physical, and biological phenomena in nature 1, and vast opportunities exist for using weak interactions for various technological applications such as hydrogen storage for renewable energy and highly selective coatings for biochemical detectors 2,3. Our ability to accurately describe such interactions in theoretical calculations is important for advancing these technologically important fields. Density-functional theory DFT4,5 is a promising method for describing the electronic structure of realistic systems because of its applicability to a large class of materials ranging from molecules to solids, in terms of both accuracy and computational affordability. Weakly interacting systems, however, remain a challenging class of materials to describe accurately within the DFT approaches in practice 6. The difficulty has been attributed primarily to the dominant role of nonlocal correlation in describing weak interactions such as the van der Waals interaction, which is absent or incorrectly accounted for within many exchange-correlation XC approximations. There have been a number of efforts to either empirically or formally include nonlocal correlation in the XC approximation 7. In addition to this, a quantitative description remains highly challenging due to the pairing exchange part, which requires further investigation 8 and is in general considerably larger than the correlation part. In the context of improving the accuracy of DFT, quantum Monte Carlo QMC calculations have played an important role in the development of the XC approximation, starting with the seminal work of Ceperley and Alder on the homogeneous electron gas 9. With computational and methodological advances, it is now becoming possible for QMC to compute accurate electron densities for realistic systems. In this article, we employ QMC calculations to analyze the electron density and binding energies calculated from DFT in order to elucidate the role of exchange in the XC approximation for describing weak interactions. In spite of the severe XC approximation dependence of the binding energy, our QMC results show that both the electron density and the reduced density gradient RDG are described quite accurately by DFT. Using these results, we show that an enhancement of the exchange energy density at large RDG values plays a critical role in obtaining accurate binding energies. We demonstrate that the diverging behavior of this enhancement factor at large RDG among different exchange approximations leads to significant differences in the binding energy. Taken together, these results show that the exchange description in XC approximations needs to be improved if DFT is to describe quantitatively and correctly the physics of weakly interacting systems, even with an accurate inclusion of nonlocal correlation. Tailoring the exchange enhancement factor at large RDG for weak interactions might improve significantly the description while essentially leaving unaffected other types of interactions and avoiding the computationally expensive optimized effective potential approach to obtain the exact exchange.