Simple Electrostatic Model Research Articles

Static electric field enhancement in nanoscale structures

We study the effect of local atomic- and nano-scale protrusions on field emission and, in particular, on the local field enhancement which plays a key role as known from the Fowler-Nordheim model of electronic emission. We study atomic size defects which consist of right angle steps forming an infinite length staircase on a tungsten surface. This structure is embedded in a 1 GV/m ambient electrostatic field. We perform calculations based upon density functional theory in order to characterize the total and induced electronic densities as well as the local electrostatic fields taking into account the detailed atomic structure of the metal. We show how the results must be processed to become comparable with those of a simple homogeneous tungsten sheet electrostatic model. We also describe an innovative procedure to extrapolate our results to nanoscale defects of larger sizes, which relies on the microscopic findings to guide, tune, and improve the homogeneous metal model, thus gaining predictive power. Furthermore, we evidence analytical power laws for the field enhancement characterization. The main physics-wise outcome of this analysis is that limited field enhancement is to be expected from atomic- and nano-scale defects.

Slow Relaxation of Magnetization in an Isostructural Series of Zinc-Lanthanide Complexes: An Integrated EPR and AC Susceptibility Study.

We report the synthesis, structure, and spectroscopic and dynamic magnetic properties of a series of heterodinuclear complexes, [ZnLn(LH4 )2 ](NO3 )3 ⋅6 H2 O (Ln=Nd, Tb, Dy, Ho, Er, and Yb), with the singly deprotonated form of a new compartmentalized Schiff-base ligand, LH5 . The Ln(III) ions in these systems show a distorted square-antiprism geometry with an LnO8 coordination sphere. EPR spectroscopy and DC magnetic studies have shown that the anisotropic nature of the complexes is far more complicated than predicted on the basis of a simple electrostatic model. Among the investigated systems, only the Dy(III) derivative showed single-ion magnet behavior, in zero and an applied magnetic field, both in pure polycrystalline samples and in a series of polycrystalline samples with different degrees of dilution at the single-crystal level in the isostructural Y(III) derivative. The rich dynamics observed as functions of frequency, field, and temperature reveals that multiple relaxation mechanisms are at play, resulting in a barrier of 189 cm(-1) , which is among the highest reported for a dinuclear Zn-Dy system. Analysis of the dynamic behavior as a function of dilution degree further evidenced the persistence of non-negligible intermolecular interactions, even at the lowest concentration of 1 %.

Electrical tuning of Rashba spin-orbit interaction in multigated InAs nanowires

Indium arsenide nanowires (NWs) are a promising platform to fabricate quantum electronic devices, among other advantages they have strong spin-orbit interaction (SOI). The controlled tuning of the SOI is desired in spin-based quantum devices. In this study we investigate the possibility of tuning the SOI by electrostatic fields generated by a back gate and two side gates placed on the opposite sides of the NW. The strength of the SOI is analyzed by weak anti-localization effect. We demonstrate that the strength of the SOI can be strongly tuned up to a factor of 2 with the electric field across the NW, while the average electron density is kept constant. Furthermore, a simple electrostatic model is introduced to calculate the expected change of the SOI. Good agreement is found between the experimental results and the estimated Rashba-type SOI generated by the gate-induced electric field.

Contact Potentials, Fermi Level Equilibration, and Surface Charging.

This article focuses on contact electrification from thermodynamic equilibration of the electrochemical potential of the electrons of two conductors upon contact. The contact potential difference generated in bimetallic macro- and nanosystems, the Fermi level after the contact, and the amount and location of the charge transferred from one metal to the other are discussed. The three geometries considered are spheres in contact, Janus particles, and core-shell particles. In addition, the force between the two spheres in contact with each other is calculated and is found to be attractive. A simple electrostatic model for calculating charge distribution and potential profiles in both vacuum and an aqueous electrolyte solution is described. Immersion of these bimetallic systems into an electrolyte solution leads to the formation of an electric double layer at the metal-electrolyte interface. This Fermi level equilibration and the associated charge transfer can at least partly explain experimentally observed different electrocatalytic, catalytic, and optical properties of multimetallic nanosystems in comparison to systems composed of pure metals. For example, the shifts in the surface plasmon resonance peaks in bimetallic core-shell particles seem to result at least partly from contact charging.

Period of photoconductivity oscillations and charge dynamics of quantum dots in p–i–n GaAs/InAs/AlAs heterojunctions

Quantum oscillations of photoconductivity in p–i–n GaAs/InAs/AlAs quantum-dot heterojunctions have been studied. The dominating effect of the dynamics of charge accumulation of optically excited holes at quantum dots on the oscillation period and on the general evolution of the holes with a change in the illumination power has been shown within a simple electrostatic model. Investigation of the temperature dependence of the oscillating structure of the current–voltage characteristics has confirmed our interpretation.

A Surprisingly Simple Electrostatic Model Explains Bent Versus Linear Structures in M(+)-RG2 Species (M = Group 1 Metal, Li-Fr; RG = Rare Gas, He-Rn).

It is found that a simple electrostatic model involving competition between the attractive dispersive interaction and induced-dipole repulsion between the two RG atoms performs extremely well in rationalizing the M(+)-RG2 geometries, where M = group 1 metal and RG = rare gas. The Li(+)-RG2 and Na(+)-RG2 complexes have previously been found to exhibit quasilinear or linear minimum-energy geometries, with the Na(+)-RG2 complexes having an additional bent local minimum [A. Andrejeva, A. M. Gardner, J. B. Graneek, R. J. Plowright, W. H. Breckenridge, T. G. Wright, J. Phys. Chem. A, 2013, 117, 13578]. In the present work, the geometries for M = K-Fr are found to be bent. A simple electrostatic model explains these conclusions and is able to account almost quantitatively for the binding energy of the second RG atom, as well as the form of the angular potential, for all 36 titular species. Additionally, results of population analyses are presented together with orbital contour plots; combined with the success of the electrostatic model, the expectation that these complexes are all physically bound is confirmed.

Optical Activity in the Near-IR Region: The λ=980 nm Multiplet of Chiral Yb3+ Complexes.

We used a very simplified electrostatic model based on charge and polarizability of atoms and groups on an organic ligand around a lanthanide ion to predict the near-infrared electronic circular dichroism (NIR ECD) spectra of Yb3+ (a monoelectronic ion). We tuned our method by using two widely different complexes. The first was the heterobimetallic species CsYb(hfbc)4 [hfbc=(-)-3-heptafluorobutyrylcamphorate], in which the ligand is a diketonate and, as such, is endowed with a chromophore with strong UV absorption (π-π* ). Its oxygen atoms define a square antiprism, which provides a symmetric coordination polyhedron. The second system was Yb DOTMA [DOTMA=(1R,4R,7R,10R)-α,α',α'',α'''-tetramethyl-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid], a chiral Yb analogue of Gd DOTA (DOTA=1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid), in which the ligand lacks relevant electronic transitions and provides a dissymmetric cage. The relative weights of dynamic (ligand polarization) and static contributions to Yb NIR ECD were evaluated, and the spectra appear to have been well predicted by theory through the introduction of a heuristic weight factor. To validate the approach and to confirm the value of the weight factor, we applied it to two other compounds, namely, Na3 Yb(BINOLate)3 and Yb(BINOLAM)3 [BINOLate=2,2'-dihydroxy-1,1'-binaphthyl; BINOLAM=3,3'-bis(diethylaminomethyl)-1-1'-bi-2-naphthol].

Freestanding membranes of cross-linked gold nanoparticles: novel functional materials for electrostatic actuators.

Their tunable electrical, optical, and mechanical properties make freestanding membranes of organically cross-linked gold nanoparticles (GNPs) interesting materials for applications in micro- and nanoelectromechanical systems. Here, we demonstrate the application of α,ω-alkanedithiol-cross-linked GNP membranes as electrostatically driven actuators. The devices were fabricated by depositing these membranes (thickness 29-45 nm) onto cylindrical cavities (diameter ∼200 μm; depth ∼8-15 μm), which were lithographically patterned in a SU-8 resist. Applying voltages of up to ±40 V across the membrane and the silicon substrate deflected the membranes by several hundreds of nanometers, as measured by atomic force microscopy, confocal microscopy, and interferometry. A simple electrostatic model, which takes into account the membranes' mechanical properties, was used to interpret the experimental data.

ChemInform Abstract: Charging and Discharging at the Nanoscale: Fermi Level Equilibration of Metallic Nanoparticles

AbstractReview: 116 refs.

Numerical analysis of the resistance behavior of an electrostatically-induced graphene double junction

We present a numerical approach that we have developed in order to reproduce and explain the resistance behavior recently observed, as a function of the backgate voltage and of the position of a biased scanning probe, in a graphene flake in which a double p---n junction has been electrostatically induced. A simplified electrostatic model has been adopted to simulate the effect of gate voltages on the potential landscape, assuming for it a slow variation in space and using a simple capacitive model for the coupling between the electrodes and the graphene sheet. The transport analysis has then been performed with a solution of the Dirac equation in the reciprocal space coupled with a recursive scattering matrix approach. The efficiency of the adopted numerical procedure has allowed us to explore a wide range of possible potential landscapes and bias points, with the result of achieving a good agreement with available experimental data.

From Breathing Pores to Three-Dimensional Ionic Self-Assembly Under Electrochemical Control

Molecular self-assembly at the solid–liquid interface is highly developed, yet still faces several challenges, in particular related to reaching into the third dimension. Here, we demonstrate the spontaneous and reversible transition between two- and three-dimensional self-assembly of a charged polyaromatic molecule at the solid–liquid interface under electrochemical conditions, using in situ scanning tunnelling microscopy. By tuning the interfacial potential, we can selectively organise our target molecules in an open porous pattern, stretch these pores to accommodate another molecule and form an auto-host–guest structure, or stack the building blocks in a stratified bilayer. Using a simple electrostatic model, we rationalise which charge density is required to enable bilayer formation, and conversely, which molecular size/charge ratio is necessary in the design of new building blocks. Our findings could form the basis of electrochemically controlled dynamic host–guest systems and 3D structures such as artificial receptors.

A simplified electrostatic model for hydrolase catalysis

Toward the development of an electrostatic model for enzyme catalysis, the active site of the enzyme is represented by a cavity whose surface (and beyond) is populated by electric charges as determined by pH and the enzyme's structure. The electric field in the cavity is obtained from electrostatics and a suitable computer program. The key chemical bond in the substrate, at its ends, has partial charges with opposite signs determined from published force−field parameters. The electric field attracts one end of the bond and repels the other, causing bond tension. If that tension exceeds the attractive force between the atoms, the bond breaks; the enzyme is then a successful catalyst. To illustrate this very simple model, based on numerous assumptions, some results are presented for three hydrolases: hen-egg white lysozyme, bovine trypsin and bovine ribonuclease. Attention is given to the effect of pH.

Diarylethene Molecules on a Ag(111) Surface: Stability and Electron-Induced Switching

Diarylethene derivatives are photochromic molecular switches, undergoing a ring-opening/-closing reaction by illumination with light. The symmetry of the closed form is determined by the Woodward–Hoffmann rules according to which the reaction proceeds by conrotatory rotation in that case. Here, we show by a combined approach of scanning tunneling microscopy (STM) and density functional theory (DFT) calculations that the open isomer of 4,4′-(4,4′-(perfluorocyclopent-1-ene-1,2-diyl)bis(5-methylthiophene-4,2-diyl)dipyridine) (PDTE) retains its open form upon adsorption on a Ag(111) surface. It can be switched into a closed form, which we identify as the disrotatory cyclization product, by controlled manipulation with the STM tip. Evidence of an electric-field dependent switching process is interpreted on the basis of a simple electrostatic model, which suggests that the reaction proceeds via an “upright” intermediate state. This pathway thus strongly differs from the switching reaction in solution.

Motion of Br2 molecules in clathrate cages. A computational study of the dynamic effects on its spectroscopic behavior.

This work looks into the spectroscopic behavior of bromine molecules trapped in clathrate cages combining different methodologies. We developed a semiempirical quantum mechanical model to incorporate through molecular dynamics trajectories, the effect movement of bromine molecules in clathrate cages has on its absorption spectra. A simple electrostatic model simulating the cage environment around bromine predicts a blue shift in the spectra, in good agreement with the experimental evidence.

Investigating atomic contrast in atomic force microscopy and Kelvin probe force microscopy on ionic systems using functionalized tips

Noncontact atomic force microscopy (NC-AFM) and Kelvin probe force microscopy (KPFM) have become important tools for nanotechnology; however, their contrast mechanisms on the atomic scale are not entirely understood. Here we used chlorine vacancies in NaCl bilayers on Cu(111) as a model system to investigate atomic contrast as a function of applied voltage, tip height, and tip functionalization. We demonstrate that the AFM contrast on the atomic scale decisively depends on both the tip termination and the sample voltage. On the contrary, the local contact potential difference acquired with KPFM showed the same qualitative contrast for all tip terminations investigated, which resembled the contrast of the electrostatic field of the sample. We find that the AFM contrast stems mainly from electrostatic interactions but its tip dependence cannot be explained by the tip dipole alone. With the aid of a simple electrostatic model and by density functional theory, we investigate the underlying contrast mechanisms.

Molecular anisotropy analysis of single-ion magnets using an effective electrostatic model.

Simple electrostatic models have been shown to successfully rationalize the magnetic properties of mononuclear single molecule magnets based on f-elements and even to predict the direction of the magnetic anisotropy axis in these nanomagnets. In this Article, we go a step forward by showing that these models, conveniently modified to account for the covalency effects, are able to predict not only the easy axis direction but also the three components of the magnetic anisotropy. Thus, by using a lone pair effective charge (LPEC) model we can fully reproduce the angular dependence of the magnetic susceptibility in single crystals of pentamethylcyclopentadienyl-Er-cyclooctatetraene single-ion magnet. Furthermore, the parametrization of the ligands obtained in this study has been extrapolated to successfully reproduce spectroscopic data of a set of mononuclear lanthanoid complexes based on the same kind of ligands, thus emphasizing the predictive character of this model.

Squeezing, then stacking: from breathing pores to three-dimensional ionic self-assembly under electrochemical control.

We demonstrate the spontaneous and reversible transition between the two- and three-dimensional self-assembly of a supramolecular system at the solid-liquid interface under electrochemical conditions, using in situ scanning tunneling microscopy. By tuning the interfacial potential, we can selectively organize our target molecules in an open porous pattern, fill these pores to form an auto-host-guest structure, or stack the building blocks in a stratified bilayer. Using a simple electrostatic model, we rationalize which charge density is required to enable bilayer formation, and conversely, which molecular size/charge ratio is necessary in the design of new building blocks. Our results may lead to a new class of electrochemically controlled dynamic host-guest systems, artificial receptors, and smart materials.

Polyelectrolyte multilayer electrostatic gating of graphene field-effect transistors.

We apply polyelectrolyte multilayer films by consecutive alternate adsorption of positively charged polyallylamine hydrochloride and negatively charged sodium polystyrene sulfonate to the surface of graphene field effect transistors. Oscillations in the Dirac voltage shift with alternating positive and negative layers clearly demonstrate the electrostatic gating effect in this simple model system. A simple electrostatic model accounts well for the sign and magnitude of the Dirac voltage shift. Using this system, we are able to create p-type or n-type graphene at will. This model serves as the basis for understanding the mechanism of charged polymer sensing using graphene devices, a potentially technologically important application of graphene in areas such as DNA sequencing, biomarker assays for cancer detection, and other protein sensing applications.

Electrostatic model of solid-state capacitor with ionizable charge traps

We present the analysis of a simple electrostatic model of a solid-state capacitor with ionizable charge traps. It shows a charge–voltage characteristic resembling a chemical battery. With a given withstanding voltage and the same geometry, the energy density of the capacitor with ionizable traps will be lower than that of an ordinary capacitor with metal electrodes.

The role of electrostatic effects in determining the structure of LiF-graphene interfaces

Lithium fluoride (LiF) is a main component of the solid electrolyte interphase (SEI) in Li-ion batteries. Using density functional theory (DFT) calculations and development of a simple electrostatic model we study the structure and mechanism of LiF growth on graphene. We find that, independent of being in contact or not with the graphene surface, crystalline-LiF nanoclusters (NCs) with (100) facets grow in a three-dimensional mode. These findings are in agreement with recent experimental studies on graphene anodes. We also find that the stabilities of LiF NCs can be predicted by a simple electrostatic model for ionic NCs. Since the main components of the SEI layer are ionic compounds, this study shows how electrostatic effects can control the atomic structure of the SEI layer in Li-ion batteries.