Unraveling Electron Dynamics in p-type Indium Phosphide (100): A Time-Resolved Two-Photon Photoemission Study.

Renewable ("green") hydrogen production through direct photoelectrochemical (PEC) water splitting is a potential key contributor to the sustainable energy mix of the future. We investigate the potential of indium phosphide (InP) as a reference material among III-V semiconductors for PEC and photovoltaic (PV) applications. The p(2 × 2)/c(4 × 2)-reconstructed phosphorus-terminated p-doped InP(100) (P-rich p-InP) surface is the focus of our investigation. We employ time-resolved two-photon photoemission (tr-2PPE) spectroscopy to study electronic states near the band gap with an emphasis on normally unoccupied conduction band states that are inaccessible through conventional single-photon emission methods. The study shows the complexity of the p-InP electronic band structure and reveals the presence of at least nine distinct states between the valence band edge and vacuum energy, including a valence band state, a surface defect state pinning the Fermi level, six unoccupied surface resonances within the conduction band, as well as a cluster of states about 1.6 eV above the CBM, identified as a bulk-to-surface transition. Furthermore, we determined the decay constants of five of the conduction band states, enabling us to track electron relaxation through the bulk and surface conduction bands. This comprehensive understanding of the electron dynamics in p-InP(100) lays the foundation for further exploration and surface engineering to enhance the properties and applications of p-InP-based III-V-compounds for, e.g., efficient and cost-effective PEC hydrogen production and highly efficient PV cells.

DFT-Assisted Investigation of the Electric Field and Charge Density Distribution of Pristine and Defective 2D WSe2 by Differential Phase Contrast Imaging.

Most properties of solid materials are defined by their internal electric field and charge density distributions which so far are difficult to measure with high spatial resolution. Especially for 2D materials, the atomic electric fields influence the optoelectronic properties. In this study, the atomic-scale electric field and charge density distribution of WSe2 bi- and trilayers are revealed using an emerging microscopy technique, differential phase contrast (DPC) imaging in scanning transmission electron microscopy (STEM). For pristine material, a higher positive charge density located at the selenium atomic columns compared to the tungsten atomic columns is obtained and tentatively explained by a coherent scattering effect. Furthermore, the change in the electric field distribution induced by a missing selenium atomic column is investigated. A characteristic electric field distribution in the vicinity of the defect with locally reduced magnitudes compared to the pristine lattice is observed. This effect is accompanied by a considerable inward relaxation of the surrounding lattice, which according to first principles DFT calculation is fully compatible with a missing column of Se atoms. This shows that DPC imaging, as an electric field sensitive technique, provides additional and remarkable information to the otherwise only structural analysis obtained with conventional STEM imaging.

Defect-Assisted Exciton Transfer across the Tetracene-Si(111):H Interface.

Exciton transfers are ubiquitous and extremely important processes, but often poorly understood. A recent example is the triplet exciton transfer in tetracene sensitized silicon solar cells exploited for harvesting high-energy photons. The present ab initio molecular dynamics calculations for tetracene-Si(111):H interfaces show that Si dangling bonds, intuitively expected to hinder the exciton transfer, actually foster it. This suggests that defects and structural imperfections at interfaces may be exploited for excitation transfer.

Diquat Based Dyes: A New Class of Photoredox Catalysts and Their Use in Aerobic Thiocyanation.

A series of new organic donor-π-acceptor dyes incorporating a diquat moiety as a novel electron-acceptor unit have been synthesized and characterized. The analytical data were supported by DFT calculations. These dyes were explored in the aerobic thiocyanation of indoles and pyrroles. Here they showed a high photocatalytic activity under visible light, giving isolated yields of up to 97 %. In addition, the photocatalytic activity of standalone diquat and methyl viologen through formation of an electron donor acceptor complex is presented.

Electron-Nuclear Coherent Coupling and Nuclear Spin Readout through Optically Polarized VB- Spin States in hBN.

Coherent coupling of defect spins with surrounding nuclei along with the endowment to read out the latter are basic requirements for an application in quantum technologies. We show that negatively charged boron vacancies (VB-) in hexagonal boron nitride (hBN) meet these prerequisites. We demonstrate Hahn-echo coherence of the VB- spin with a characteristic decay time Tcoh = 15 µs, close to the theoretically predicted limit of 18 µs for defects in hBN. Elongation of the coherence time up to 36 µs is demonstrated by means of the Carr-Purcell-Meiboom-Gill decoupling technique. Modulation of the Hahn-echo decay is shown to be induced by coherent coupling of the VB- spin with the three nearest 14N nuclei via a nuclear quadrupole interaction of 2.11 MHz. DFT calculation confirms that the electron-nuclear coupling is confined to the defective layer and stays almost unchanged with a transition from the bulk to the single layer.

Adatom mediated adsorption of N-heterocyclic carbenes on Cu(111) and Au(111).

The adsorption of N-heterocyclic carbenes (NHCs) on Cu(111) and Au(111) surfaces is studied with density-functional theory. The role of the molecular side groups as well as the surface morphology in determining the adsorption geometry are explored in detail. Flat-laying NHCs, as observed experimentally for NHC with relatively small side groups, result from the adsorption at adatoms and give rise to the so-called ballbot configurations, which are more stable than adsorption on flat surfaces and provide an efficient precursor for the formation of bis(NHC) dimers. On Au(111), the resulting (NHC)2 Au complexes are purely physisorbed and thus mobile. On the more reactive Cu(111), in contrast, the central Cu atom in the (NHC)2 Cu dimer is still covalently bound to the surface, resulting in a mobility, which has to be thermally activated.

Spin Polarization, Electron-Phonon Coupling, and Zero-Phonon Line of the NV Center in 3C-SiC.

The nitrogen-vacancy (NV) center in 3C-SiC, the analog of the NV center in diamond, has recently emerged as a solid-state qubit with competitive properties and significant technological advantages. Combining first-principles calculations and magnetic resonance spectroscopy, we provide thorough insight into its magneto-optical properties. By applying resonantly excited electron paramagnetic resonance spectroscopy, we identified the zero-phonon absorption line of the 3A2 → 3E transition at 1289 nm (within the telecom O-band) and measured its phonon sideband, the analysis of which reveals a Huang-Rhys factor of S = 2.85 and a Debye-Waller factor of 5.8%. The low-temperature spin-lattice relaxation time was found to be exceptionally long (T1 = 17 s at 4 K). All these properties make NV in 3C-SiC a strong competitor for qubit applications. In addition, the strong variation of the zero-field splitting in the range 4-380 K allows its application for nanoscale thermal sensing.

Tetracene Ultrathin Film Growth on Hydrogen-Passivated Silicon.

Inorganic-organic interfaces are important for enhancing the power conversion efficiency of silicon-based solar cells through singlet exciton fission (SF). We elucidated the structure of the first monolayers of tetracene (Tc), an SF molecule, on hydrogen-passivated Si(111) [H-Si(111)] and hydrogenated amorphous Si (a-Si:H) by combining near-edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS) experiments with density functional theory (DFT) calculations. For samples grown at or below substrate temperatures of 265 K, the resulting ultrathin Tc films are dominated by almost upright-standing molecules. The molecular arrangement is very similar to the Tc bulk phase, with only a slightly higher average angle between the conjugated molecular plane normal and the surface normal (α) around 77°. Judging from carbon K-edge X-ray absorption spectra, the orientation of the Tc molecules are almost identical when grown on H-Si(111) and a-Si:H substrates as well as for (sub)mono- to several-monolayer coverages. Annealing to room temperature, however, changes the film structure toward a smaller α of about 63°. A detailed DFT-assisted analysis suggests that this structural transition is correlated with a lower packing density and requires a well-chosen amount of thermal energy. Therefore, we attribute the resulting structure to a distinct monolayer configuration that features less inclined, but still well-ordered molecules. The larger overlap with the substrate wave functions makes this arrangement attractive for an optimized interfacial electron transfer in SF-assisted silicon solar cells.

Electron paramagnetic resonance study of ferroelectric phase transition and dynamic effects in a Mn2+ doped [NH4][Zn(HCOO)3] hybrid formate framework.

We present an X- and Q-band continuous wave (CW) and pulse electron paramagnetic resonance (EPR) study of a manganese doped [NH4][Zn(HCOO)3] hybrid framework, which exhibits a ferroelectric structural phase transition at 190 K. The CW EPR spectra obtained at different temperatures exhibit clear changes at the phase transition temperature. This suggests a successful substitution of the Zn2+ ions by the paramagnetic Mn2+ centers, which is further confirmed by the pulse EPR and 1H ENDOR experiments. Spectral simulations of the CW EPR spectra are used to obtain the temperature dependence of the Mn2+ zero-field splitting, which indicates a gradual deformation of the MnO6 octahedra indicating a continuous character of the transition. The determined data allow us to extract the critical exponent of the order parameter (ß = 0.12), which suggests a quasi two-dimensional ordering in [NH4][Zn(HCOO)3]. The experimental EPR results are supported by the density functional theory calculations of the zero-field splitting parameters. Relaxation time measurements of the Mn2+ centers indicate that the longitudinal relaxation is mainly driven by the optical phonons, which correspond to the vibrations of the metal-oxygen octahedra. The temperature behavior of the transverse relaxation indicates a dynamic process in the ordered ferroelectric phase.

Impact of finite-temperature and condensed-phase effects on theoretical X-ray absorption spectra of transition metal complexes.

The impact of condensed-phase and finite-temperature effects on the theoretical X-ray absorption spectra of transition metal complexes is assessed. The former are included in terms of the all-electron Gaussian and augmented plane-wave approach, whereas the latter are taken into account by extensive ensemble averaging along second-generation Car-Parrinello ab initio molecular dynamics trajectories. We find that employing the periodic boundary conditions and including finite-temperature effects systematically improves the agreement between our simulated X-ray absorption spectra and experimental measurements. © 2018 Wiley Periodicals, Inc.

Current density analysis of electron transport through molecular wires in open quantum systems.

The current density in molecular wires connected to contacts is investigated within the nonequilibrium Green's function formalism combined with the Landauer approach. Energy-dependent and total current density through a series of molecular junctions are calculated in real space representation. A rich variety of current patterns including pronounced ring currents ("vortices") are found even in the defect-free minimal building blocks of molecular devices. The influences of contact positions, functional groups as well as atomic defects on the transport properties are examined systematically for prototypical ortho-, meta-, and para-substituted benzenes as well as heteroaromatic systems. It is found that substitutional functional groups mainly shift the molecular levels and retain characteristic transport channels, while a significant change of electronic pathways and conductance is induced by hetero-aromaticity. The current distribution is used to calculate the static magnetic field distribution in the carbon-based conductors. © 2017 Wiley Periodicals, Inc.

[Cu6 (NGuaS)6 ]2+ and its oxidized and reduced derivatives: Confining electrons on a torus.

The hexanuclear thioguanidine mixed-valent copper complex cation [Cu6 (NGuaS)6 ]+2 (NGuaS = o-SC6 H4 NC(NMe2 )2 ) and its oxidized/reduced states are theoretically analyzed by means of density functional theory (DFT) (TPSSh + D3BJ/def2-TZV (p)). A detailed bonding analysis using overlap populations is performed. We find that a delocalized Cu-based ring orbital serves as an acceptor for donated S p electrons. The formed fully delocalized orbitals give rise to a confined electron cloud within the Cu6 S6 cage which becomes larger on reduction. The resulting strong electrostatic repulsion might prevent the fully reduced state. Experimental UV/Vis spectra are explained using time-dependent density functional theory (TD-DFT) and analyzed with a natural transition orbital analysis. The spectra are dominated by MLCTs within the Cu6 S6 core over a wide range but LMCTs are also found. The experimental redshift of the reduced low energy absorption band can be explained by the clustering of the frontier orbitals. © 2017 Wiley Periodicals, Inc.

Experimental and Theoretical High-Energy-Resolution X-ray Absorption Spectroscopy: Implications for the Investigation of the Entatic State.

High-energy-resolution-fluorescence-detected X-ray absorption near-edge structure (HERFD-XANES) spectroscopy is shown to be a sensitive tool to investigate the electronic changes of copper complexes induced by geometric distortions caused by the ligand backbone as a model for the entatic state. To fully exploit the information contained in the spectra gained by the high-energy-resolution technique, (time-dependent) density functional theory calculations based on plane-wave and localized orbital basis sets are performed, which in combination allow the complete spectral range from the prepeak to the first resonances above the edge step to be covered. Thus, spectral changes upon oxidation and geometry distortion in the copper N-(1,3-dimethylimidazolidin-2-ylidene)quinolin-8-amine (DMEGqu) complexes [CuI(DMEGqu)2](PF6) and [CuII(DMEGqu)2](OTf)2·MeCN can be accessed.

Manipulation resolves non-trivial structure of corrole monolayer on Ag(111).

Non-trivial arrangement of molecules within a molecular network complicates structure determination due to interdigitation, partial overlap, or stacking. We demonstrate that combined imaging and lateral manipulation with a scanning tunneling microscope resolves the intricate structure of a molecular network in two-dimensions in a straightforward manner. The network, formed by a monolayer of 5,10,15-tris(pentafluorophenyl)-corrole molecules on Ag(111), is manipulated for the first time with single-molecule precision. Our results reveal a shingle-like packing of partially overlapping corrole molecules. Density functional theory calculations support our findings.

A Bifunctional Electrocatalyst for Oxygen Evolution and Oxygen Reduction Reactions in Water.

Oxygen reduction and water oxidation are two key processes in fuel cell applications. The oxidation of water to dioxygen is a 4 H(+)/4 e(-) process, while oxygen can be fully reduced to water by a 4 e(-)/4 H(+) process or partially reduced by fewer electrons to reactive oxygen species such as H2O2 and O2(-). We demonstrate that a novel manganese corrole complex behaves as a bifunctional catalyst for both the electrocatalytic generation of dioxygen as well as the reduction of dioxygen in aqueous media. Furthermore, our combined kinetic, spectroscopic, and electrochemical study of manganese corroles adsorbed on different electrode materials (down to a submolecular level) reveals mechanistic details of the oxygen evolution and reduction processes.

The Cu2O2 torture track for a real-life system: [Cu2(btmgp)2O2](2+) oxo and peroxo species in density functional calculations.

Density functional theory (DFT) calculations of the equilibrium geometry, vibrational modes, ionization energies, electron affinities, and optical response of [Cu2(btmgp)2(µ-O)2](2+) (oxo) and [Cu2(btmgp)2(µ-η(2):η(2)-O2)](2+) (peroxo) are presented. Comprehensive benchmarking shows that the description of the oxo-peroxo energetics is still a torture track for DFT, but finds the molecular geometry to be comparatively robust with respect to changes in the exchange-correlation functionals and basis sets. Pure functionals favor the oxo core found experimentally, whereas hybrid functionals shift the bias toward the peroxo core. Further stabilization of peroxo core results from relaxing the spin degrees of freedom using the broken-symmetry (BS) approach. Dispersion effects, conversely, tend to favor the oxo configuration. Triple-zeta basis sets are found to represent a sensible compromise between numerical accuracy and computational effort. Particular attention is paid to the modification of the electronic structure, optical transitions, and excited-state energies along the transition path between the oxo and peroxo species. The excited-state potential energy surface calculations indicate that two triplet states are involved in the transition that stabilize the BS solution. Charge decomposition and natural transition orbital analyses are used for obtaining microscopic insight into the molecular orbital interactions. Here, the crucial role of guanidine π-interactions is highlighted for the stabilization of the Cu2O2 core.

Geometrical and optical benchmarking of copper guanidine-quinoline complexes: insights from TD-DFT and many-body perturbation theory.

We report a comprehensive computational benchmarking of the structural and optical properties of a bis(chelate) copper(I) guanidine-quinoline complex. Using various (TD-)DFT flavors a strong influence of the basis set is found. Moreover, the amount of exact exchange shifts metal-to-ligand bands by 1 eV through the absorption spectrum. The BP86/6-311G(d) and B3LYP/def2-TZVP functional/basis set combinations were found to yield results in best agreement with the experimental data. In order to probe the general applicability of TD-DFT to excitations of copper bis(chelate) charge-transfer (CT) systems, we studied a small model system that on the one hand is accessible to methods of many-body perturbation theory (MBPT) but still contains simple guanidine and imine groups. These calculations show that large quasiparticle energies of the order of several electronvolts are largely offset by exciton binding energies for optical excitations and that TD-DFT excitation energies deviate from MBPT results by at most 0.5 eV, further corroborating the reliability of our TD-DFT results. The latter result in a multitude of MLCT bands ranging from the visible region at 3.4 eV into the UV at 5.5 eV for the bis(chelate) complex. Molecular orbital analysis provided insight into the CT within these systems but gave mixed transitions. A meaningful transition assignment is possible, however, by using natural transition orbitals. Additionally, we performed a thorough conformational analysis as the correct description of the copper coordination is crucial for the prediction of optical spectra. We found that DFT identifies the correct conformational minimum and that the MLCTs are strongly dependent on the torsion of the chelate angles at the copper center. From the results, it is concluded that extensive benchmarking allows for the quantitative analyses of the CT behavior of copper bis(chelate) complexes within TD-DFT.

Geometrical and optical benchmarking of copper(II) guanidine-quinoline complexes: insights from TD-DFT and many-body perturbation theory (part II).

Ground- and excited-state properties of copper(II) charge-transfer systems have been investigated starting from density-functional calculations with particular emphasis on the role of (i) the exchange and correlation functional, (ii) the basis set, (iii) solvent effects, and (iv) the treatment of dispersive interactions. Furthermore (v), the applicability of TD-DFT to excitations of copper(II) bis(chelate) charge-transfer systems is explored by performing many-body perturbation theory (GW + BSE), independent-particle approximation and ΔSCF calculations for a small model system that contains simple guanidine and imine groups. These results show that DFT and TD-DFT in particular in combination with hybrid functionals are well suited for the description of the structural and optical properties, respectively, of copper(II) bis(chelate) complexes. Furthermore, it is found an accurate theoretical geometrical description requires the use of dispersion correction with Becke-Johnson damping and triple-zeta basis sets while solvent effects are small. The hybrid functionals B3LYP and TPSSh yielded best performance. The optical description is best with B3LYP, whereby heavily mixed molecular transitions of MLCT and LLCT character are obtained which can be more easily understood using natural transition orbitals. An natural bond orbital analysis sheds light on the donor properties of the different donor functions and the intraguanidine stabilization during coordination to copper(I) and (II).

Modeling intrinsic defects in LiNbO3 within the Slater-Janak transition state model.

Intrinsic point defects in LiNbO3, i.e., isolated Nb antisites and Li as well Nb vacancies, are investigated from first-principles within the Slater-Janak transition state model. Thereby the electronic structure of the investigated defects is calculated with hybrid exchange-correlation functionals. This approach allows for the calculation of charge transition levels without comparing the total energies of differently charged supercells. The obtained results are in agreement with previous hybrid density-functional theory calculations based on total-energy differences. Li and Nb vacancies can be formed in the V(-)(Li) and V(5-)(Nb) charge states only, as long as the host is not strongly p-type or n-type, respectively. NbLi antisites may capture one or two electrons, forming the defect states often referred to as small bound polaron and bi-polaron.

Activation of surface hydroxyl groups by modification of H-terminated Si(111) surfaces.

Chemical functionalization of semiconductor surfaces, particularly silicon oxide, has enabled many technologically important applications (e.g., sensing, photovoltaics, and catalysis). For such processes, hydroxyl groups terminating the oxide surface constitute the primary reaction sites. However, their reactivity is often poor, hindering technologically important processes, such as surface phosphonation requiring a lengthy postprocessing annealing step at 140 °C with poor control of the bonding geometry. Using a novel oxide-free surface featuring a well-defined nanopatterned OH coverage, we demonstrate that hydroxyl groups on oxide-free silicon are more reactive than on silicon oxide. On this model surface, we show that a perfectly ordered layer of monodentate phosphonic acid molecules is chemically grafted at room temperature, and explain why it remains completely stable in aqueous environments, in contrast to phosphonates grafted on silicon oxides. This fundamental understanding of chemical activity and surface stability suggests new directions to functionalize silicon for sensors, photovoltaic devices, and nanoelectronics.