Electronic structure calculations with GPAW: a real-space implementation of the projector augmented-wave method

Open source molecular modeling

The success of molecular modeling and computational chemistry efforts are, by definition, dependent on quality software applications. Open source software development provides many advantages to users of modeling applications, not the least of which is that the software is free and completely extendable. In this review we categorize, enumerate, and describe available open source software packages for molecular modeling and computational chemistry. An updated online version of this catalog can be found at https://opensourcemolecularmodeling.github.io.

Synthesis and characterization of metastable transition metal oxides and oxide nitrides

New routes to vanadium sesquioxide and tantalum oxide nitride (γ- and δ-phase) are presented. Phase pure V2O3 with bixbyite-type structure, a metastable polymorph, was obtained from vanadium fluoride hydrates at ~750 K. It crystallizes in the cubic crystal system in space group I a 3 ¯ $Ia\bar 3$ with lattice parameter a=939.30(5) pm. The catalytical properties of the corresponding oxide nitride phases and their oxidation and reduction solid-state kinetics were investigated. The preparation of γ-TaON as a phase pure sample can be realized by ammonolysis of X-ray amorphous tantalum oxide precursors at 1073 K. This metastable tantalum oxide nitride crystallizes in the monoclinic VO2(B)-type structure in space group C2/m. The same precursors can be used to synthesize the δ-modification with an anatase-type structure at 1023 K. It crystallizes in the tetragonal crystal system in space group I41/amd. A maximum yield of 82 m % could be obtained. The fundamental band gaps of the synthesized and of other metastable TaON polymorphs were calculated from first principles using the GW method. The present results are compared to experimental data and to previous calculations at hybrid DFT level.

Self-Interaction Corrected Functional Calculations of a Dipole-Bound Molecular Anion

A self-interaction corrected density functional is used to describe the ground state of the CH3CN(-) ion that includes a dipole bound electron with large spatial extent and low binding energy. Without the correction, some commonly used density functionals based on the generalized gradient approximation as well as hybrid functionals fail to give a bound ground state of the anion. A negative HOMO orbital energy of magnitude 0.013 eV is obtained using the self-interaction corrected PBE functional in good correspondence with the experimentally estimated binding energy of 0.019 eV. The dipole bound electron polarizes the CH3CN molecule and increases its dipole moment by 7% to 4.2 D. Because the computational effort increases slowly with system size, as the number of electrons cubed, the results presented here point to a viable approach to theoretical studies of dipole bound electrons in large and complex systems such as molecular clusters, biological systems, and solvated electrons.

Ab initio electronic structure of quasi-two-dimensional materials: A "native" Gaussian-plane wave approach

Ab initio electronic structure calculations of two-dimensional layered structures are typically performed using codes that were developed for three-dimensional structures, which are periodic in all three directions. The introduction of a periodicity in the third direction (perpendicular to the layer) is completely artificial and may lead in some cases to spurious results and to difficulties in treating the action of external fields. In this paper we develop a new approach, which is "native" to quasi-2D materials, making use of basis function that are periodic in the plane, but atomic-like in the perpendicular direction. We show how some of the basic tools of ab initio electronic structure theory - density functional theory, GW approximation and Bethe-Salpeter equation - are implemented in the new basis. We argue that the new approach will be preferable to the conventional one in treating the peculiarities of layered materials, including the long range of the unscreened Coulomb interaction in insulators, and the effects of strain, corrugations, and external fields.

Prospect of quantum anomalous Hall and quantum spin Hall effect in doped kagome lattice Mott insulators

Electronic states with non-trivial topology host a number of novel phenomena with potential for revolutionizing information technology. The quantum anomalous Hall effect provides spin-polarized dissipation-free transport of electrons, while the quantum spin Hall effect in combination with superconductivity has been proposed as the basis for realizing decoherence-free quantum computing. We introduce a new strategy for realizing these effects, namely by hole and electron doping kagome lattice Mott insulators through, for instance, chemical substitution. As an example, we apply this new approach to the natural mineral herbertsmithite. We prove the feasibility of the proposed modifications by performing ab-initio density functional theory calculations and demonstrate the occurrence of the predicted effects using realistic models. Our results herald a new family of quantum anomalous Hall and quantum spin Hall insulators at affordable energy/temperature scales based on kagome lattices of transition metal ions.

Defect-Tolerant Monolayer Transition Metal Dichalcogenides

Localized electronic states formed inside the band gap of a semiconductor due to crystal defects can be detrimental to the material's optoelectronic properties. Semiconductors with a lower tendency to form defect induced deep gap states are termed defect-tolerant. Here we provide a systematic first-principles investigation of defect tolerance in 29 monolayer transition metal dichalcogenides (TMDs) of interest for nanoscale optoelectronics. We find that the TMDs based on group VI and X metals form deep gap states upon creation of a chalcogen (S, Se, Te) vacancy, while the TMDs based on group IV metals form only shallow defect levels and are thus predicted to be defect-tolerant. Interestingly, all the defect sensitive TMDs have valence and conduction bands with a very similar orbital composition. This indicates a bonding/antibonding nature of the gap, which in turn suggests that dangling bonds will fall inside the gap. These ideas are made quantitative by introducing a descriptor that measures the degree of similarity of the conduction and valence band manifolds. Finally, the study is generalized to nonpolar nanoribbons of the TMDs where we find that only the defect sensitive materials form edge states within the band gap.

Reproducibility in density functional theory calculations of solids

The widespread popularity of density functional theory has given rise to an extensive range of dedicated codes for predicting molecular and crystalline properties. However, each code implements the formalism in a different way, raising questions about the reproducibility of such predictions. We report the results of a community-wide effort that compared 15 solid-state codes, using 40 different potentials or basis set types, to assess the quality of the Perdew-Burke-Ernzerhof equations of state for 71 elemental crystals. We conclude that predictions from recent codes and pseudopotentials agree very well, with pairwise differences that are comparable to those between different high-precision experiments. Older methods, however, have less precise agreement. Our benchmark provides a framework for users and developers to document the precision of new applications and methodological improvements.

Charge localization in a diamine cation provides a test of energy functionals and self-interaction correction

Density functional theory (DFT) is widely applied in calculations of molecules and materials. Yet, it suffers from a well-known over-emphasis on charge delocalization arising from self-interaction error that destabilizes localized states. Here, using the symmetric diamine N,N'-dimethylpiperazine as a model, we have experimentally determined the relative energy of a state with positive charge localized on one of the two nitrogen atoms, and a state with positive charge delocalized over both nitrogen atoms. The charge-localized state was found to be 0.33 (0.04) eV higher in energy than the charge-delocalized state. This provides an important test of theoretical approaches to electronic structure calculations. Calculations with all DFT functionals commonly used today, including hybrid functionals with exact exchange, fail to predict a stable charge-localized state. However, the application of an explicit self-interaction correction to a semi-local functional identifies both states and gives relative energy in excellent agreement with both experiment and CCSD(T) calculations.

Structure and role of metal clusters in a metal-organic coordination network determined by density functional theory

We present a comprehensive theoretical investigation of the structures formed by self-assembly of tetrahydroxybenzene (THB)-derivatives on Cu(111). The THB molecule is known to dehydrogenate completely during annealing, forming a reactive radical which assembles into a close-packed structure or a porous metal-coordinated network depending on the coverage of the system. Here, we present details on how the structures are determined by density functional theory calculations, using scanning tunneling microscopy-derived information on the periodicity. The porous network is based on adatom trimers. By analysing the charge distribution of the structure, it is found that this unusual coordination motif is preferred because it simultaneously provides a good coordination of all oxygen atoms and allows for the formation of a two-dimensional network on the surface.

Unravelling Site-Specific Photo-Reactions of Ethanol on Rutile TiO2(110)

Finding the active sites of catalysts and photo-catalysts is crucial for an improved fundamental understanding and the development of efficient catalytic systems. Here we have studied the photo-activated dehydrogenation of ethanol on reduced and oxidized rutile TiO₂(110) in ultrahigh vacuum conditions. Utilizing scanning tunnelling microscopy, various spectroscopic techniques and theoretical calculations we found that the photo-reaction proceeds most efficiently when the reactants are adsorbed on regular Ti surface sites, whereas species that are strongly adsorbed at surface defects such as O vacancies and step edges show little reaction under reducing conditions. We propose that regular Ti surface sites are the most active sites in photo-reactions on TiO₂.

Comparative Ab-Initio Study of Substituted Norbornadiene-Quadricyclane Compounds for Solar Thermal Storage

Molecular photoswitches that are capable of storing solar energy, so-called molecular solar thermal storage systems, are interesting candidates for future renewable energy applications. In this context, substituted norbornadiene-quadricyclane systems have received renewed interest due to recent advances in their synthesis. The optical, thermodynamic, and kinetic properties of these systems can vary dramatically depending on the chosen substituents. The molecular design of optimal compounds therefore requires a detailed understanding of the effect of individual substituents as well as their interplay. Here, we model absorption spectra, potential energy storage, and thermal barriers for back-conversion of several substituted systems using both single-reference (density functional theory using PBE, B3LYP, CAM-B3LYP, M06, M06-2x, and M06-L functionals as well as MP2 calculations) and multireference methods (complete active space techniques). Already the diaryl substituted compound displays a strong red-shift compared to the unsubstituted system, which is shown to result from the extension of the conjugated π-system upon substitution. Using specific donor/acceptor groups gives rise to a further albeit relatively smaller red-shift. The calculated storage energy is found to be rather insensitive to the specific substituents, although solvent effects are likely to be important and require further study. The barrier for thermal back-conversion exhibits strong multireference character and as a result is noticeably correlated with the red-shift. Two possible reaction paths for the thermal back-conversion of diaryl substituted quadricyclane are identified and it is shown that among the compounds considered the path via the acceptor side is systematically favored. Finally, the present study establishes the basis for high-throughput screening of norbornadiene-quadricyclane compounds as it provides guidelines for the level of accuracy that can be expected for key properties from several different techniques.

B40 fullerene as a highly sensitive molecular device for NH3 detection at low bias: a first-principles study

The adsorption of small molecules (NH3, N2, H2 and CH4) on all-boron fullerene B40 is investigated by density functional theory (DFT) and the non-equilibrium Green's function (NEGF) for its potential application in the field of single-molecular gas sensors. The high adsorption energies (-1.09 to -0.75 eV) of NH3 on different adsorption sites of the B40 surface indicate that NH3 strongly chemisorbs to B40. The charge transfer induced by the NH3 adsorption results in a modification of the density of states (DOS) of B40 near the Fermi level, and therefore changes its electronic transport properties. For all possible adsorption sites, the adsorption of NH3 exclusively leads to a decrease of the conductance of B40. Taking into consideration that the non-polar gas molecules (e.g. N2, H2 and CH4) are only physisorbed and show negligible effect on the conductance properties of B40, we would expect that B40 can be used as a single-molecular gas sensor to distinguish NH3 from non-polar gas molecules at low bias.

A Unified AMBER-Compatible Molecular Mechanics Force Field for Thiolate-Protected Gold Nanoclusters

We present transferable AMBER-compatible force field parameters for thiolate-protected gold nanoclusters. Five different sized clusters containing both organo-soluble and water-soluble thiolate ligands served as test systems in MD simulations, and parameters were validated against DFT and experimental results. The cluster geometries remain intact during the MD simulations in various solvents, and structural fluctuations and energetics showed agreement with DFT calculations. Experimental diffusion coefficients and crystal structures were also reproduced with sufficient accuracy. The presented parameter set contains the minimum number of cluster-specific parameters enabling the use of these parameters for several different gold nanoclusters. The parameterization of ligands can also be extended to different types of ligands.

Simple Screened Hydrogen Model of Excitons in Two-Dimensional Materials

We present a generalized hydrogen model for the binding energies (E_{B}) and radii of excitons in two-dimensional (2D) materials that sheds light on the fundamental differences between excitons in two and three dimensions. In contrast to the well-known hydrogen model of three-dimensional (3D) excitons, the description of 2D excitons is complicated by the fact that the screening cannot be assumed to be local. We show that one can consistently define an effective 2D dielectric constant by averaging the screening over the extend of the exciton. For an ideal 2D semiconductor this leads to a simple expression for E_{B} that only depends on the excitonic mass and the 2D polarizability α. The model is shown to produce accurate results for 51 transition metal dichalcogenides. Remarkably, over a wide range of polarizabilities the binding energy becomes independent of the mass and we obtain E_{B}^{2D}≈3/(4πα), which explains the recently observed linear scaling of exciton binding energies with band gap. It is also shown that the model accurately reproduces the nonhydrogenic Rydberg series in WS_{2} and can account for screening from the environment.

Evidence and implications of direct charge excitation as the dominant mechanism in plasmon-mediated photocatalysis

Plasmonic metal nanoparticles enhance chemical reactions on their surface when illuminated with light of particular frequencies. It has been shown that these processes are driven by excitation of localized surface plasmon resonance (LSPR). The interaction of LSPR with adsorbate orbitals can lead to the injection of energized charge carriers into the adsorbate, which can result in chemical transformations. The mechanism of the charge injection process (and role of LSPR) is not well understood. Here we shed light on the specifics of this mechanism by coupling optical characterization methods, mainly wavelength-dependent Stokes and anti-Stokes SERS, with kinetic analysis of photocatalytic reactions in an Ag nanocube–methylene blue plasmonic system. We propose that localized LSPR-induced electric fields result in a direct charge transfer within the molecule–adsorbate system. These observations provide a foundation for the development of plasmonic catalysts that can selectively activate targeted chemical bonds, since the mechanism allows for tuning plasmonic nanomaterials in such a way that illumination can selectively enhance desired chemical pathways.

The excitation of metal nanoparticles with light can lead to localized surface plasmon resonances, capable of driving chemical reactions in bound species. Here, the authors elucidate this mechanism and suggest that future plasmonic catalysts may be able to selectively activate specific chemical bonds.

Conformation and dynamics of the ligand shell of a water-soluble Au102 nanoparticle

Inorganic nanoparticles, stabilized by a passivating layer of organic molecules, form a versatile class of nanostructured materials with potential applications in material chemistry, nanoscale physics, nanomedicine and structural biology. While the structure of the nanoparticle core is often known to atomic precision, gaining precise structural and dynamical information on the organic layer poses a major challenge. Here we report a full assignment of ¹H and ¹³C NMR shifts to all ligands of a water-soluble, atomically precise, 102-atom gold nanoparticle stabilized by 44 para-mercaptobenzoic acid ligands in solution, by using a combination of multidimensional NMR methods, density functional theory calculations and molecular dynamics simulations. Molecular dynamics simulations augment the data by giving information about the ligand disorder and visualization of possible distinct ligand conformations of the most dynamic ligands. The method demonstrated here opens a way to controllable strategies for functionalization of ligated nanoparticles for applications.

The core structure of inorganic nanoparticles, stabilized by a passivating layer of organic molecules, is often known but information about the organic layer is tougher to derive. Here, the authors use NMR and computational methods to probe the ligand disorder and visualize possible ligand conformations.

Charge transfer and ultrafast nuclear motions: the complex structural dynamics of an electronically excited triamine

Three ionization centers make 1,3,5-trimethyl-1,3,5-triazacyclohexane (TMTAC) an interesting model system to study intramolecular charge transfer (CT). Because the molecule assumes a Cs symmetric, axial-equatorial-equatorial (aee) conformation in the ground state, there are two distinct types of the nitrogen atoms. We discovered that either nitrogen atom can be ionized independently so that two molecular cations exist with different (localized) charge distributions in the Franck-Condon region. The initially localized charge can delocalize via CT, provided the molecule acquires a suitable structural geometry. These proper structures are all found to have a common structural motif that supports CT via through-space-interaction. The structural dynamics and the CT process in Rydberg-excited TMTAC, where the molecular ion core closely resembles the ion, were probed by time-resolved Rydberg fingerprint spectroscopy. When TMTAC is excited at 230 nm to the Franck-Condon region of the 3s Rydberg state, the two types of nitrogen atom Rydberg chromophores give rise to distinct binding energy peaks. The sequential molecular responses that follow the Rydberg excitation manifest themselves as time-dependent changes of the binding energy and are observed by ionization at 404 nm. A fast transition with 103 fs time constant was attributed to a motion that leads to a local minimum of the charge-localized state on the Rydberg potential energy surface. Because a large amount of energy is deposited into the vibrational manifolds, the molecule continues to sample the potential energy surface and eventually reaches a dynamic equilibrium between charge-localized and charge-delocalized states. The forward and backward time constants were determined to be 1.02 ps and 4.09 ps, respectively. The binding energy spectrum also reveals the existence of an equilibrium among several charge-delocalized states. Quantum chemical calculations were carried out to find the stable minima of the ground state and the ion state. The binding energies of the Franck-Condon structures and the relaxed ion structures were calculated using the Perdew-Zunger self-interaction corrected DFT (PZ-SIC) method to assign the spectra at time zero and at equilibrium, respectively.

Dynamic Diglyme-Mediated Self-Assembly of Gold Nanoclusters

We report the assembly of gold nanoclusters by the nonthiolate ligand diglyme into discrete and dynamic assemblies. To understand this surprising phenomenon, the assembly of Au20(SC2H4Ph)15-diglyme into Au20(SC2H4Ph)15-diglyme-Au20(SC2H4Ph)15 is explored in detail. The assembly is examined by high-angle annular dark field scanning transmission electron microscopy, size exclusion chromatography, mass spectrometry, IR spectroscopy, and calorimetry. We establish a dissociation constant for dimer to monomer conversion of 20.4 μM. Theoretical models validated by transient absorption spectroscopy predict a low-spin monomer and a high-spin dimer, with assembly enabled through weak diglyme oxygen-gold interactions. Close spatial coupling allows electron delocalization between the nanoparticle cores. The resulting assemblies thus possess optical and electronic properties that emerge as a result of assembly.

Exploiting the locality of periodic subsystem density-functional theory: efficient sampling of the Brillouin zone

In order to approximately satisfy the Bloch theorem, simulations of complex materials involving periodic systems are made n(k) times more complex by the need to sample the first Brillouin zone at n(k) points. By combining ideas from Kohn-Sham density-functional theory (DFT) and orbital-free DFT, for which no sampling is needed due to the absence of waves, subsystem DFT offers an interesting middle ground capable of sizable theoretical speedups against Kohn-Sham DFT. By splitting the supersystem into interacting subsystems, and mapping their quantum problem onto separate auxiliary Kohn-Sham systems, subsystem DFT allows an optimal topical sampling of the Brillouin zone. We elucidate this concept with two proof of principle simulations: a water bilayer on Pt[1 1 1]; and a complex system relevant to catalysis-a thiophene molecule physisorbed on a molybdenum sulfide monolayer deposited on top of an α-alumina support. For the latter system, a speedup of 300% is achieved against the subsystem DTF reference by using an optimized Brillouin zone sampling (600% against KS-DFT).

Electron transport in molecular junctions with graphene as protecting layer

We present ab initio transport calculations for molecular junctions that include graphene as a protecting layer between a single molecule and gold electrodes. This vertical setup has recently gained significant interest in experiment for the design of particularly stable and reproducible devices. We observe that the signals from the molecule in the electronic transmission are overlayed by the signatures of the graphene sheet, thus raising the need for a reinterpretation of the transmission. On the other hand, we see that our results are stable with respect to various defects in the graphene. For weakly physiosorbed molecules, no signs of interaction with the graphene are evident, so the transport properties are determined by offresonant tunnelling between the gold leads across an extended structure that includes the molecule itself and the additional graphene layer. Compared with pure gold electrodes, calculated conductances are about one order of magnitude lower due to the increased tunnelling distance. Relative differences upon changing the end group and the length of the molecule on the other hand, are similar.

Quantized Evolution of the Plasmonic Response in a Stretched Nanorod

Quantum aspects, such as electron tunneling between closely separated metallic nanoparticles, are crucial for understanding the plasmonic response of nanoscale systems. We explore quantum effects on the response of the conductively coupled metallic nanoparticle dimer. This is realized by stretching a nanorod, which leads to the formation of a narrowing atomic contact between the two nanorod ends. Based on first-principles time-dependent density-functional-theory calculations, we find a discontinuous evolution of the plasmonic response as the nanorod is stretched. This is especially pronounced for the intensity of the main charge-transfer plasmon mode. We show the correlation between the observed discontinuities and the discrete nature of the conduction channels supported by the formed atomic-sized junction.

The Role of Hydrogen Bonds in the Stabilization of Silver-Mediated Cytosine Tetramers

DNA oligomers can form silver-mediated duplexes, stable in gas phase and solution, with potential for novel biomedical and technological applications. The nucleobase-metal bond primarily drives duplex formation, but hydrogen (H-) bonds may also be important for structure selection and stability. To elucidate the role of H-bonding, we conducted theoretical and experimental studies of a duplex formed by silver-mediated cytosine homopobase DNA strands, two bases long. This silver-mediated cytosine tetramer is small enough to permit accurate, realistic modeling by DFT-based quantum mechanics/molecular mechanics methods. In gas phase, our calculations found two energetically favorable configurations distinguished by H-bonding, one with a novel interplane H-bond, and the other with planar H-bonding of silver-bridged bases. Adding solvent favored silver-mediated tetramers with interplane H-bonding. Overall agreement of electronic circular dichroism spectra for the final calculated structure and experiment validates these findings. Our results can guide use of these stabilization mechanisms for devising novel metal-mediated DNA structures.

Controlling Electrical Conductance through a π-Conjugated Cruciform Molecule by Selective Anchoring to Gold Electrodes

Tuning charge transport at the single-molecule level plays a crucial role in the construction of molecular electronic devices. Introduced herein is a promising and operationally simple approach to tune two distinct charge-transport pathways through a cruciform molecule. Upon in situ cleavage of triisopropylsilyl groups, complete conversion from one junction type to another is achieved with a conductance increase by more than one order of magnitude, and it is consistent with predictions from ab initio transport calculations. Although molecules are well known to conduct through different orbitals (either HOMO or LUMO), the present study represents the first experimental realization of switching between HOMO- and LUMO-dominated transport within the same molecule.

Surface Tension Effects on the Reactivity of Metal Nanoparticles

We present calculated adsorption energies of oxygen on gold and platinum clusters with up to 923 atoms (3 nm diameter) using density functional theory. We find that surface tension of the clusters induces a compression, which weakens the bonding of adsorbates compared with the bonding on extended surfaces. The effect is largest for close-packed surfaces and almost nonexistent on the more reactive steps and edges. The effect is largest at low coverage and decreases, even changing sign, at higher coverages where the strain changes from compressive to tensile. Quantum size effects also influence adsorption energies but only below a critical size of 1.5 nm for platinum and 2.5 nm for gold. We develop a model to describe the strain-induced size effects on adsorption energies, which is able to describe the influence of surface structure, adsorbate, metal, and coverage.

Edge state magnetism in zigzag-interfaced graphene via spin susceptibility measurements

Development of graphene spintronic devices relies on transforming it into a material with a spin order. Attempts to make graphene magnetic by introducing zigzag edge states have failed due to energetically unstable structure of torn zigzag edges. Here, we report on the formation of nanoridges, i.e., stable crystallographically oriented fluorine monoatomic chains, and provide experimental evidence for strongly coupled magnetic states at the graphene-fluorographene interfaces. From the first principle calculations, the spins at the localized edge states are ferromagnetically ordered within each of the zigzag interface whereas the spin interaction across a nanoridge is antiferromagnetic. Magnetic susceptibility data agree with this physical picture and exhibit behaviour typical of quantum spin-ladder system with ferromagnetic legs and antiferromagnetic rungs. The exchange coupling constant along the rungs is measured to be 450 K. The coupling is strong enough to consider graphene with fluorine nanoridges as a candidate for a room temperature spintronics material.

Indication of non-thermal contribution to visible femtosecond laser-induced CO oxidation on Ru(0001)

We studied CO oxidation on Ru(0001) induced by 400 nm and 800 nm femtosecond laser pulses where we find a branching ratio between CO oxidation and desorption of 1:9 and 1:31, respectively, showing higher selectivity towards CO oxidation for the shorter wavelength excitation. Activation energies computed with density functional theory show discrepancies with values extracted from the experiments, indicating both a mixture between different adsorbed phases and importance of non-adiabatic effects on the effective barrier for oxidation. We simulated the reactions using kinetic modeling based on the two-temperature model of laser-induced energy transfer in the substrate combined with a friction model for the coupling to adsorbate vibrations. This model gives an overall good agreement with experiment except for the substantial difference in yield ratio between CO oxidation and desorption at 400 nm and 800 nm. However, including also the initial, non-thermal effect of electrons transiently excited into antibonding states of the O-Ru bond yielded good agreement with all experimental results.

Electric-Field Control of Interfering Transport Pathways in a Single-Molecule Anthraquinone Transistor

It is understood that molecular conjugation plays an important role in charge transport through single-molecule junctions. Here, we investigate electron transport through an anthraquinone based single-molecule three-terminal device. With the use of an electric-field induced by a gate electrode, the molecule is reduced resulting into a 10-fold increase in the off-resonant differential conductance. Theoretical calculations link the change in differential conductance to a reduction-induced change in conjugation, thereby lifting destructive interference of transport pathways.

Dielectric Genome of van der Waals Heterostructures

Vertical stacking of two-dimensional (2D) crystals, such as graphene and hexagonal boron nitride, has recently lead to a new class of materials known as van der Waals heterostructures (vdWHs) with unique and highly tunable electronic properties. Ab initio calculations should in principle provide a powerful tool for modeling and guiding the design of vdWHs, but in their traditional form such calculations are only feasible for commensurable structures with a few layers. Here we show that the dielectric properties of realistic, incommensurable vdWHs comprising hundreds of layers can be efficiently calculated using a multiscale approach where the dielectric functions of the individual layers (the dielectric building blocks) are computed ab initio and coupled together via the long-range Coulomb interaction. We use the method to illustrate the 2D-3D transition of the dielectric function of multilayer MoS2 crystals, the hybridization of quantum plasmons in thick graphene/hBN heterostructures, and to demonstrate the intricate effect of substrate screening on the non-Rydberg exciton series in supported WS2. The dielectric building blocks for a variety of 2D crystals are available in an open database together with the software for solving the coupled electrodynamic equations.

Carbon Dioxide Reforming of Methane using an Isothermal Redox Membrane Reactor

The continuous production of carbon monoxide (CO) and hydrogen (H₂) by dry reforming of methane (CH₄) is demonstrated isothermally using a ceramic redox membrane in absence of additional catalysts. The reactor technology realizes the continuous splitting of CO₂ to CO on the inner side of a tubular membrane and the partial oxidation of CH₄ with the lattice oxygen to form syngas on the outer side. La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) membranes evaluated at 840-1030 °C yielded up to 1.27 μmol _CO   s^-1 from CO₂, 3.77 μmol_H₂ g^-1 s^-1 from CH₄ , and CO from CH₄ at approximately the same rate as CO from CO₂. We compute the free energy of the oxygen vacancy formation for La_0.5Sr_0.5B_0.5B'_0.5O_3-δ (B, B'=Mn, Fe, Co, Cu) using electronic structure theory to understand how CO₂ reduction limits dry reforming of methane using LSCF and to show how the CO₂ conversion can be increased by using advanced redox materials such as La_0.5Sr_0.5MnO_3-δ and La_0.5Sr_0.5Mn_0.5Co_0.5O_3-δ .

Design Principles of Perovskites for Thermochemical Oxygen Separation

Separation and concentration of O2 from gas mixtures is central to several sustainable energy technologies, such as solar-driven synthesis of liquid hydrocarbon fuels from CO2 , H2 O, and concentrated sunlight. We introduce a rationale for designing metal oxide redox materials for oxygen separation through "thermochemical pumping" of O2 against a pO2 gradient with low-grade process heat. Electronic structure calculations show that the activity of O vacancies in metal oxides pinpoints the ideal oxygen exchange capacity of perovskites. Thermogravimetric analysis and high-temperature X-ray diffraction for SrCoO3-δ , BaCoO3-δ and BaMnO3-δ perovskites and Ag2 O and Cu2 O references confirm the predicted performance of SrCoO3-δ , which surpasses the performance of state-of-the-art Cu2 O at these conditions with an oxygen exchange capacity of 44 mmol O 2 mol SrCoO 3-δ(-1) exchanged at 12.1 μmol O 2 min(-1)  g(-1) at 600-900 K. The redox trends are understood due to lattice expansion and electronic charge transfer.

Rational design of metal nitride redox materials for solar-driven ammonia synthesis

Fixed nitrogen is an essential chemical building block for plant and animal protein, which makes ammonia (NH3) a central component of synthetic fertilizer for the global production of food and biofuels. A global project on artificial photosynthesis may foster the development of production technologies for renewable NH3 fertilizer, hydrogen carrier and combustion fuel. This article presents an alternative path for the production of NH3 from nitrogen, water and solar energy. The process is based on a thermochemical redox cycle driven by concentrated solar process heat at 700-1200°C that yields NH3 via the oxidation of a metal nitride with water. The metal nitride is recycled via solar-driven reduction of the oxidized redox material with nitrogen at atmospheric pressure. We employ electronic structure theory for the rational high-throughput design of novel metal nitride redox materials and to show how transition-metal doping controls the formation and consumption of nitrogen vacancies in metal nitrides. We confirm experimentally that iron doping of manganese nitride increases the concentration of nitrogen vacancies compared with no doping. The experiments are rationalized through the average energy of the dopant d-states, a descriptor for the theory-based design of advanced metal nitride redox materials to produce sustainable solar thermochemical ammonia.

Silver (I) as DNA glue: Ag(+)-mediated guanine pairing revealed by removing Watson-Crick constraints

Metal ion interactions with DNA have far-reaching implications in biochemistry and DNA nanotechnology. Ag⁺ is uniquely interesting because it binds exclusively to the bases rather than the backbone of DNA, without the toxicity of Hg²⁺. In contrast to prior studies of Ag⁺ incorporation into double-stranded DNA, we remove the constraints of Watson-Crick pairing by focusing on homo-base DNA oligomers of the canonical bases. High resolution electro-spray ionization mass spectrometry reveals an unanticipated Ag⁺-mediated pairing of guanine homo-base strands, with higher stability than canonical guanine-cytosine pairing. By exploring unrestricted binding geometries, quantum chemical calculations find that Ag⁺ bridges between non-canonical sites on guanine bases. Circular dichroism spectroscopy shows that the Ag⁺-mediated structuring of guanine homobase strands persists to at least 90 °C under conditions for which canonical guanine-cytosine duplexes melt below 20 °C. These findings are promising for DNA nanotechnology and metal-ion based biomedical science.

Strong Influence of Coadsorbate Interaction on CO Desorption Dynamics on Ru(0001) Probed by Ultrafast X-Ray Spectroscopy and Ab Initio Simulations

We show that coadsorbed oxygen atoms have a dramatic influence on the CO desorption dynamics from Ru(0001). In contrast to the precursor-mediated desorption mechanism on Ru(0001), the presence of surface oxygen modifies the electronic structure of Ru atoms such that CO desorption occurs predominantly via the direct pathway. This phenomenon is directly observed in an ultrafast pump-probe experiment using a soft x-ray free-electron laser to monitor the dynamic evolution of the valence electronic structure of the surface species. This is supported with the potential of mean force along the CO desorption path obtained from density-functional theory calculations. Charge density distribution and frozen-orbital analysis suggest that the oxygen-induced reduction of the Pauli repulsion, and consequent increase of the dative interaction between the CO 5σ and the charged Ru atom, is the electronic origin of the distinct desorption dynamics. Ab initio molecular dynamics simulations of CO desorption from Ru(0001) and oxygen-coadsorbed Ru(0001) provide further insights into the surface bond-breaking process.

Design Principles for Metal Oxide Redox Materials for Solar-Driven Isothermal Fuel Production

The performance of metal oxides as redox materials is limited by their oxygen conductivity and thermochemical stability. Predicting these properties from the electronic structure can support the screening of advanced metal oxides and accelerate their development for clean energy applications. Specifically, reducible metal oxide catalysts and potential redox materials for the solar-thermochemical splitting of CO2 and H2O via an isothermal redox cycle are examined. A volcano-type correlation is developed from available experimental data and density functional theory. It is found that the energy of the oxygen-vacancy formation at the most stable surfaces of TiO2, Ti2O3, Cu2O, ZnO, ZrO2, MoO3, Ag2O, CeO2, yttria-stabilized zirconia, and three perovskites scales with the Gibbs free energy of formation of the bulk oxides. Analogously, the experimental oxygen self-diffusion constants correlate with the transition-state energy of oxygen conduction. A simple descriptor is derived for rapid screening of oxygen-diffusion trends across a large set of metal oxide compositions. These general trends are rationalized with the electronic charge localized at the lattice oxygen and can be utilized to predict the surface activity, the free energy of complex bulk metal oxides, and their oxygen conductivity.