Charge Rearrangement Research Articles

Subnanometer Imaging of Adsorbate-Induced Electronic Structure Perturbation on Silicon Surfaces

Room-temperature scanning tunneling microscopy is utilized to explore the consequences of a single covalent bond formed between an organic molecule, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), and the Si(111)-7 × 7 surface at the atomic scale. Upon binding, both topographic imaging and spectroscopic techniques reveal significant charge rearrangement within the substrate that is delocalized from the organic adsorbate. With scanning tunneling spectroscopy, the spatial extent of this charge transfer is directly visualized and determined to extend up to 2 nm from the molecule predominately within half of the Si(111)-7 × 7 unit cell. Analysis of individual differential tunneling conductance spectra suggests that the charge transfer is mediated by the back-bonds of the silicon substrate.

Electronic structure of thiol-bonded self-assembled monolayers: Impact of coverage

In the present contribution, we investigate how the electronic structure of a Au(111) surface covered by a self-assembled monolayer (SAM) of conjugated molecules depends on the packing density of the SAM. To that aim, biphenylthiol-based molecules bearing electron-rich and electron-poor substituents are studied by means of slab-type band-structure calculations. We find that the screening effects arising from the presence of neighboring molecules decisively influence the impact of the substituent as well as the adsorption-induced charge rearrangements at the metal-molecule interface. This has profound consequences not only for the alignment of the molecular levels with the Fermi energy and the SAM-induced work-function modification but also contributes to the clarification of the nature of the thiol-gold bond. The presented findings have important implications for single-molecule transport, SAM-based chemical sensing, and electrode modification in organic electronics.

Understanding the properties of interfaces between organic self‐assembled monolayers and noble metals—a theoretical perspective

AbstractHighly ordered, self‐assembled monolayers (SAMs) on (noble) metal surfaces are of particular importance for both the emerging field of single‐molecule electronics as well as for improving the characteristics of more conventional organic (opto)electronic devices. In the first part of this article, we review the microscopic description of electrode work function modification induced by SAMs and the alignment of molecular states relative to the states in the metal. The focus will be on the Ag(111) surface and biphenyl SAMs involving different donor–acceptor head‐group substituents. The impact of different docking groups will also be discussed and the results will be compared to previous findings (primarily) for SAMs on Au(111). The second part of the article deals with an in‐depth description of charge rearrangements and the hybridization between molecular and metal states by projecting the electronic states of the combined system onto the orbitals of the isolated molecules. Furthermore, the sulfur–silver bond is analyzed by means of a crystal orbital occupation population analysis. Copyright © 2008 John Wiley & Sons, Ltd.

Imaging a coupled quantum dot-quantum point contact system

We have quantitatively studied the effect of charge traps on the electrical conductance of a quantum dot and a capacitively coupled quantum point contact. Using the sharp metallic tip of a low-temperature scanning force microscope as a scanned gate, we could localize the traps. The quantum point contact served as a charge detector and allowed us to distinguish single electron charging events in several traps from charging events on the dot. We used the quantum dot to analyze the tip-induced potential quantitatively and found its shape to be independent of the voltage applied to the tip within a certain range of parameters. We estimate that the trap density is below 0.1% of the doping density, that the charging energy of a trap is three times higher than that of the quantum dot, and that the interaction energy between the quantum dot and a trap is a significant portion of the dot’s charging energy. Possibly, such charge traps are the reason for frequently observed parametric charge rearrangements.

Dielectric relaxation mechanism of single crystal and polycrystal bismuth germanate

We report the results of investigation on the mechanism of dark conductivity and dielectric relaxation of Bi12GeO20 single crystals and dense ceramics. Alternating current electric characterization was performed in the temperature interval from 30 °C up to 730 °C and frequencies from 1 Hz to 13 MHz. The samples presented ohmic behavior even at high temperatures, for applied potentials up to 2.2 V. The electric conductivity of single crystals was thermally activated, with potential barriers changing from 0.75±0.05 eV to 1.4±0.2 eV at 500 °C. The same conduction mechanisms were observed for ceramic samples, with activation energies of 0.9±0.1 eV and 1.5±0.1 eV for charge transport through grains and grain boundaries, respectively. These energy values were related to defects arising from substitutional Bi3++h+ in Ge4+ sites. In the ceramic samples, the higher resistivity of the grain interfaces leads to a Maxwell–Wagner polarization of the grain boundaries due to local charge rearrangement. As a result, the overall permittivity of ceramics was higher than that observed for single crystals at frequencies corresponding to the grain boundary response.

Enhancement of thermoelectric performance in strongly correlated multilayered nanostructures

AbstractWe illustrate how the electronic charge reconstruction near the interfaces of metal–strongly‐correlated‐barrier–metal devices can lead to a significant enhancement of the thermoelectric response due to the breaking of particle‐hole symmetry. We show how one can achieve effective Seebeck coefficients larger than 150 µV/K and effective electronic figures‐of‐merit ZT higher than 1. Our work shows that electronic thermoelectric transport properties can be significantly enhanced from inhomogeneities created by the rearrangement of charge near the interfaces, and that bulk figures‐of‐merit cannot generically be employed to determine the figure‐of‐merit of a multilayered nanostructure. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Pb induced charge accumulation on InAs(1 1 1)B

The Pb/InAs(1 1 1)B interface has been studied by synchrotron radiation photoelectron spectroscopy (SR-PES) of valence band and In4d, As3d and Pb5d core levels. Room temperature deposition of ∼1 ML of Pb on InAs(1 1 1)B leads to an ordered overlayer that induces a metallic channel at the surface, as seen through a weak emission in the vicinity of the Fermi level. Its narrow localization in reciprocal space supports the formation of a two-dimensional free electron gas (2DEG) in the surface region. It is proposed that the adsorbed metal layer swaps the initial polarisation of the surface and thus pulls electrons back to the surface. This charge re-arrangement increases the charge density in the accumulation layer and reduces the screening length and thus the depth of the potential well at the surface.

Charge Rearrangement and Screening in a Quantum Point Contact

Compressibility measurements are performed on a quantum point contact (QPC). Screening due to mobile charges in the QPC is measured quantitatively, using a second point contact. These measurements are performed from pinch-off through the opening of the first few modes in the QPC. While the measured signal closely matches a Thomas-Fermi-Poisson prediction, deviations from the classical behavior are apparent near the openings of the different modes. Density functional calculations attribute the deviations to a combination of a diverging density of states at the opening of each one-dimensional mode and exchange interaction, which is strongest for the first mode.

Oxygen vacancies and induced changes in the electronic and magnetic structures ofLa0.66Sr0.33MnO3: A combinedab initioand photoemission study

The effect of oxygen vacancies on the electronic and magnetic properties of ${\mathrm{La}}_{0.66}{\mathrm{Sr}}_{0.33}\mathrm{Mn}{\mathrm{O}}_{3}$ (LSMO) has been investigated by means of ab initio calculations within the density-functional formalism combined with photoemission. The simulations show that the introduction of oxygen vacancies causes a shift of the valence-band features toward higher binding energies and an increase of the degree of covalency of Mn bondings. The Mn magnetic moments undergo some changes, keeping, however, the situation relatively close to the ideal nondefective system: in none of the different vacancy configurations, a drastic charge or spin rearrangement occurs. There is, though, an important vacancy-induced drawback: half-metallicity, typical of the perfectly stoichiometric system, is generally lost due to defective bands that cross the Fermi level. Photoemission experiments performed on epitaxial thin films of LSMO with different contents of oxygen vacancies grown by pulsed laser deposition essentially confirm theoretical predictions. Our findings clearly indicate that the control over oxygen deficiency should therefore be experimentally achieved to avoid unwanted consequences in terms of spin-injection efficiency.

The specific oxidation mechanism in the initial stages of O 2 adsorption on submonolayer Ba covered W(1 1 0) surface

The initial oxidation of a polarized 0.35-ML Ba-covered W(1 1 0) surface has been investigated in detail by high-resolution photoemission spectroscopy using synchrotron radiation. Upon small exposures (⩽0.15 L) to O 2, both interfacial W 4f (Ba-coordinated W) and Ba 4d core levels shift simultaneously toward smaller binding energies and then fix during 0.15–0.25 L, finally disappear at 0.6 L. The concurrent negative shifts of both interfacial W 4f and Ba 4d peaks can be attributed to be a consequence of the increased occupancy of 5d states. This explanation is supported by the variations of 5d stats in valence band spectra upon the initial oxidation. Our results clearly demonstrate that charge rearrangement is the dominant effect in the surface reactivity of our oxidized surface. We also found that this charge redistribution is associated with a structural change during oxidation, which is in contrary to previous studies of alkali-metal promotion. That is, oxygen is first chemisorbed atop the polarized Ba adlayer, then incorporates beneath Ba adatoms to form a covalent Ba–O–W complex upon further dosage, and reacts strongly with the Ba overlayer to become an ionic Ba +2O −2 surface dipole layer at 0.6 L.

Structure of Hydrophobic Hydration of Benzene and Hexafluorobenzene from First Principles

We report on the aqueous hydration of benzene and hexafluorobenzene, as obtained by carrying out extensive (>100 ps) first principles molecular dynamics simulations. Our results show that benzene and hexafluorobenzene do not behave as ordinary hydrophobic solutes, but rather present two distinct regions, one equatorial and the other axial, that exhibit different solvation properties. While in both cases the equatorial regions behave as typical hydrophobic solutes, the solvation properties of the axial regions depend strongly on the nature of the pi-water interaction. In particular, pi-hydrogen and pi-lone pair interactions are found to dominate in benzene and hexafluorobenzene, respectively, which leads to substantially different orientations of water near the two solutes. We present atomic and electronic structure results (in terms of Maximally Localized Wannier Functions) providing a microscopic description of benzene- and hexafluorobenzene-water interfaces, as well as a comparative study of the two solutes. Our results point at the importance of an accurate description of interfacial water to characterize hydration properties of apolar molecules, as these are strongly influenced by subtle charge rearrangements and dipole moment redistributions in interfacial regions.

Toward a physical understanding of electron‐sharing two‐center bonds. I. General aspects

In 1916, Lewis and Kossel laid the empirical ground for the electronic theory of valence, whose quantum theoretical foundation was uncovered only slowly. We can now base the classification of the various traditional chemical bond types in a threefold manner on the one- and two-electron terms of the quantum-physical Hamiltonian (kinetic, atomic core attraction, electron repulsion). Bond formation is explained by splitting up the real process into two physical steps: (i) interaction of undeformed atoms and (ii) relaxation of this nonstationary system. We aim at a flexible bond energy partitioning scheme that can avoid cancellation of large terms of opposite sign. The driving force of covalent bonding is a lowering of the quantum kinetic energy density by sharing. The driving force of heteropolar bonding is a lowering of potential energy density by charge rearrangement in the valence shell. Although both mechanisms are quantum mechanical in nature, we can easily visualize them, since they are of one-electron type. They are however tempered by two-electron correlations. The richness of chemistry, owing to the diversity of atomic cores and valence shells, becomes intuitively understandable with the help of effective core pseudopotentials for the valence shells. Common conceptual difficulties in understanding chemical bonds arise from quantum kinematic aspects as well as from paradoxical though classical relaxation phenomena. On this conceptual basis, a dozen different bond types in diatomic molecules will be analyzed in the following article. We can therefore examine common features as well as specific differences of various bonding mechanisms.

Structure, Bonding, and Lowest Energy Transitions in Unsymmetrical Squaraines: A Computational Study

Natural resonance theory (NRT) and natural bond orbital (NBO) analysis have been carried out on a simple symmetrical and an unsymmetrical substituted squaraine with a view of understanding the structure of the latter type of squaraines. It is found that there are some fundamental differences in the structure and bonding between these two types of squaraines particularly in the resonance weights and delocalization energies. These differences are expected to reflect in the low energy transitions and charge transfer in these squaraines. To investigate this, the nature of the lowest energy transitions occurring on excitation in unsymmetrical squaraines has been studied using high-level symmetry adapted cluster-configuration interaction method (SAC/SAC-CI) and compared with reported experimental observations. In general the agreement with the experimental data is very good. The transition dipole moment always lies on the pi-backbone and is quite large in magnitude. The ground state dipole moment in some cases does not change in the excited state upon excitation while in some other cases there is a large reduction/enhancement in the magnitude indicative of some charge rearrangement in this direction. Inclusion of the solvent using the IEFPCM model, a slightly better agreement with the experiment is found in some cases. Studies are carried out with a different basis set and it is found that the change in basis set has very little effect on the transition energies. In the case of weak side donor groups attached to the central ring the larger charge transfer to the central acceptor ring in general takes place from the O- atoms of the squarylium moiety while in the case of strong donors the charge transfer from the O- atoms to the central rings drop down. We have not observed any correlation between the charge transfer in the excited state to the central ring from the side donor groups and the lowest energy excitation in the molecules. Reduction of the HOMO-LUMO gap (an indication of increase of the diradicaloid character) always leads to a bathochromic shift.

Effects of n-type doping on active Fe sites in ion implanted Fe in InP

An n-type doping significantly increases the number of active Fe atoms substituting In (FeIn) in Fe-implanted InP. We address the origin of this doping effect by investigating the relative stability of neutral and charged FeIn states with first-principles theoretical methods. The present results show that donor-acceptor pairs have direct stabilizing effects on FeIn related to charge rearrangements strongly localized at the Fe site and involving d orbitals. The resulting microscopic description of the doping effects accounts for the experimental findings and provides a significant guideline for tuning electronic and optical properties of Fe implanted InP and InP-based compounds.

Cooperative effects and dipole formation at semiconductor and self-assembled-monolayer interfaces

Dipole formation processes at self-assembled monolayers of benzene derivatives chemisorbed on the Si(111) surface are investigated from first principles. The surface dipole is shown to be sensitive to the molecular coverage and dominated by intramolecular charge rearrangement due to long-range, cooperative behavior. This cooperative behavior suppresses substrate contributions to dipole formation.

Excitation of the L Intermediate of Bacteriorhodopsin: Electric Responses to Test X-Ray Structures

The L intermediate of bacteriorhodopsin was excited, and its electrical response was measured. Two positive components were found in it with respect to the direction of proton pumping: an unresolved fast component, and a slower one ( τ = 7 μs) of small amplitude. The fast component was assigned to a charge motion corresponding to reisomerization of the retinal moiety, whereas the slow one was attributed to charge rearrangements reestablishing the ground state. Because three x-ray crystallographic structures have recently been reported for the L intermediate, it seemed important to calculate the intramolecular dipole moment changes associated to bR→L for all three structures, so as to compare them with similar quantities determined from the electrical signals. The results are discussed in terms of amino acid side chains possibly contributing to the observed effect. We propose to use electrical signals as a verification tool for intermediate structures of the photocycle, and thus for molecular models of proton pumping.

On the Interplay between CH···O and OH···O Interactions in Determining Crystal Packing and Molecular Conformation: An Experimental and Theoretical Charge Density Study of the Fungal Secondary Metabolite Austdiol (C12H12O5)

The total experimental electron density rho(r), its Laplacian inverted delta(2)rho(r), the molecular dipole moment, the electrostatic potential phi(r), and the intermolecular interaction energies have been obtained from an extensive set of single-crystal X-ray diffracted intensities, collected at T = 70(1) K, for the fungal metabolite austdiol (1). The experimental results have been compared with theoretical densities from DFT calculations on the isolated molecule and with fully periodic calculations. The crystal structure of (1) consists of zigzag ribbons extended along one cell axis and formed by molecules connected by both OH...O and CH...O interactions, while in a perpendicular direction, adjacent molecules are linked by short CH...O intermolecular contacts. An extensive, quantitative study of all the intra- and intermolecular H...O interactions, based not only on geometrical criteria, but also on the topological analysis of rho(r), as well as on the evaluation of the pertinent energetics, allowed us (i) to assess the mutual role of OH...O and CH...O interactions in determining molecular conformation and crystal packing; (ii) to identify those CH...O contacts which are true hydrogen bonds (HBs); (iii) to determine the relative hydrogen bond strengths. An experimental, quantitative evidence is given that CH...O HBs are very similar to the conventional OH...O HBs, albeit generally weaker. The comparison between experimental and theoretical electric dipole moments indicates that a noticeable charge rearrangement occurs upon crystallization and shows the effects of the mutual cooperation of HBs in the crystal. The total intermolecular interaction energies and the electrostatic energy contribution obtained through different theoretical methods are reported and compared with the experimental results. It is found that the new approach proposed by Spackman, based on the use of the promolecular charge density to approximate the penetration contribution to intermolecular electrostatic energies, predicts the correct relative electrostatic interaction energies in most of the cases.

The Fate of Dicationic States in Molecular Clusters of Benzene and Related Compounds

Calculations employing density functional theory indicate that, rather than undergoing fragmentation, dicationic clusters of benzene, hexafluorobenzene, and naphthalene produced by sequential one-electron or sudden double-ionization experiments on the neutrals can relax via the formation of inter-ring covalent C-C bonds, along with a series of proton transfers that enable a substantial reduction of inter- and intramolecular Coulomb repulsions. The theoretically predicted chemically bound structures correspond to deep local energy minima on the potential energy surface pertaining to the lowest electronic state of the dications and can therefore be regarded as metastable (kinetically long-lived) species. This discovery invalidates on theoretical grounds the liquid-droplet model of multiply charged clusters and sheds very unexpected light on possible consequences in chemistry of the intermolecular Coulombic decay (ICD) mechanism [Cederbaum, L. S.; et al. Phys. Rev. Lett. 1997, 79, 4778; Jahnke, T.; et al. Phys. Rev. Lett. 2004, 93, 163401] for deep inner-valence ionized states. Propagation of charge rearrangement reactions and proton transfers to several monomers may eventually lead to the formation of rather extended dicationic assemblies.

Noninvasive detection of charge rearrangement in a quantum dot in high magnetic fields

We demonstrate electron redistribution caused by magnetic field on a single quantum dot measured by means of a quantum point contact as non-invasive detector. Our device which is fabricated by local anodic oxidation allows to control independently the quantum point contact and all tunnelling barriers of the quantum dot. Thus we are able to measure both the change of the quantum dot charge and also changes of the electron configuration at constant number of electrons on the quantum dot. We use these features to exploit the quantum dot in a high magnetic field where transport through the quantum dot displays the effects of Landau shells and spin blockade. We confirm the internal rearrangement of electrons as function of the magnetic field for a fixed number of electrons on the quantum dot.

Theoretical study of structures and stability of the hydrogen-bonded complexes between 2-hydroxybenzonitrile (o-cyanophenol) and CO

The structures and stability of the hydrogen-bonded complexes between o-cyanophenol and CO: ( syn) o-CNPhOH⋯CO ( syn) o-CNPhOH⋯OC ( anti) o-CNPhOH⋯CO and ( anti) o-CNPhOH⋯OC have been investigated employing ab initio and DFT calculations at different basis sets. Full geometry optimisation was made for the complexes studied. The nature of the hydrogen bonding in the complexes and the influence of CO on the properties of o-cyanophenol has been investigated. The corrected values of the dissociation energy for each hydrogen-bonded complex have been calculated in order to estimate the strength of the hydrogen bonds. Having in mind the corrected values of the dissociation energy, the studied hydrogen-bonded complexes can be ordered according their stability as follows: ( anti) o-CNPhOH⋯CO>( syn) o-CNPhOH⋯CO>( anti) o-CNPhOH⋯OC>( syn) o-CNPhOH⋯OC. The hydrogen bonding between o-cyanophenol and CO leads to changes in the geometrical parameters (bond lengths and angles) of the monomers. These changes depend on the strength of the hydrogen bonds. It was established that the hydrogen bonding leads to a charge rearrangement in the monomers. The most sensitive to the complexation are the atoms, taking part in the hydrogen bonding.