Local Electrostatic Potential Research Articles

Interplay between charge ordering and geometric ferroelectricity in LuFe2O4/LuFeO3 superlattices

Oxide superlattices have drawn great attention owing to the intriguing coupling among elastic, electrical, and magnetic orderings at the interfaces and the emergence of improper ferroelectricity. Here, superlattices composed of hexagonal $\mathrm{LuFe}{\mathrm{O}}_{3}$ $(\mathrm{h}\text{\ensuremath{-}}\mathrm{LuFe}{\mathrm{O}}_{3})$ and $\mathrm{Lu}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ are investigated via density functional theory calculations. h-$\mathrm{LuFe}{\mathrm{O}}_{3}$ is a well-known multiferroic material that is stable only in thin film or doped bulk state, while $\mathrm{Lu}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ is a charge ordered (CO) material where the existence of ferroelectricity is still a controversy. We have found that the CO-induced polarizations in $\mathrm{Lu}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ layers coexist with the geometric polarizations in $\mathrm{h}\text{\ensuremath{-}}\mathrm{LuFe}{\mathrm{O}}_{3}$ layers in the ${(\mathrm{Lu}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4})}_{m}/{(\mathrm{LuFe}{\mathrm{O}}_{3})}_{n}$ superlattices with different periodicities, and the ferroelectric states are generally preferred over the antiferroelectric states for $\mathrm{Lu}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ in superlattices. The out-of-plane polarizations in h-$\mathrm{LuFe}{\mathrm{O}}_{3}$ and $\mathrm{Lu}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ layers tend to be aligned in parallel, and the overall polarization increases with the ratio of $\mathrm{h}\text{\ensuremath{-}}\mathrm{LuFe}{\mathrm{O}}_{3}$. The influence of layered polarizations on the local electrostatic potential is not significant except the detected small trend caused by the CO-induced polarization within a FeO bilayer. Additionally, the local electronic structures show that the Fermi level position in a certain layer can be tuned by the valences of Fe in this layer and the polarization distributions in neighboring layers. $\mathrm{Lu}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ layers sandwiched between thick $\mathrm{h}\text{\ensuremath{-}}\mathrm{LuFe}{\mathrm{O}}_{3}$ layers are more susceptible. The calculated configurations of the superlattices are supported by atomic-resolution transmission electron microscopy experiments. Our results pave the way for tunable ferroelectricity in superlattice systems and create a playground for manipulating the coupling between various degrees of freedom.

Heralded Spectroscopy Reveals Exciton-Exciton Correlations in Single Colloidal Quantum Dots.

Multiply excited states in semiconductor quantum dots feature intriguing physics and play a crucial role in nanocrystal-based technologies. While photoluminescence provides a natural probe to investigate these states, room-temperature single-particle spectroscopy of their emission has proved elusive due to the temporal and spectral overlap with emission from the singly excited and charged states. Here, we introduce biexciton heralded spectroscopy enabled by a single-photon avalanche diode array based spectrometer. This allows us to directly observe biexciton–exciton emission cascades and measure the biexciton binding energy of single quantum dots at room temperature, even though it is well below the scale of thermal broadening and spectral diffusion. Furthermore, we uncover correlations hitherto masked in ensembles of the biexciton binding energy with both charge-carrier confinement and fluctuations of the local electrostatic potential. Heralded spectroscopy has the potential of greatly extending our understanding of charge-carrier dynamics in multielectron systems and of parallelization of quantum optics protocols.

Polypyrrole derivatives for detection of toxic gases: A theoretical study

AbstractPolypyrrole (PPy) based compounds have been considered interesting materials for several technological applications, mainly due to their unique optoelectronic properties and high versatility of synthesis. In particular, the sensitivity of their electrical properties to external stimuli defines these polymers as promising candidates for chemical sensor applications. However, the demanding processes involved in the synthesis of 3‐4 substituted PPy derivatives hinders the development of optimized systems. In this report, electronic structure calculations were conducted for a set of branched PPy derivatives aiming to identify promising compounds for chemical sensors. The calculations were carried out in the framework of the density functional theory (DFT), by considering oligomeric systems. The influence of the side groups on the local reactivities and adsorption centers of the compounds were evaluated via condensed‐to‐atoms Fukui indexes and molecular electrostatic potentials. DFT‐based adsorption studies and fully atomistic reactive molecular dynamics (FARMD) simulations were conducted for selected systems considering toxic gases as analytes. The results point out PPy, PPy‐CCH and PPy‐CN as promising candidates for chemical sensor applications, presenting high potential for the detection of Cl2 and SO2. FARMD results indicate that the analysis of local reactivities and electrostatic potentials can be used to identify relevant adsorption centers and possible degradation processes.

Crystal structure, Hirshfeld surface and reactivity of novel ligand-LAT1 derived from dehydroacetic acid: intermolecular interactions with SARS-Cov-2/main protease

The crystals of new ligand, namely (3E)-6-methyl-3-{1-[(pyridin-3-ylmethyl) amino] ethylene}-2H-pyran-2, 4(3H)-dione) (LAT1), were synthesized using the evaporation solution technique. Single-crystal X-ray diffraction and physico-chemical characterization (ATR, proton and carbon-13 NMR and UV-Visible) of LAT1 were reported. In addition, Hirshfeld surface analysis (HSA) of the solid compound, structure optimization, Mulliken and NBO charges, global indices of reactivity, local reactivity descriptors and molecular electrostatic potential (MEP) of the ligand were investigated theoretically. XRD analysis showed that LAT1 crystallizes in the triclinic space group P-1 and the structure was stabilized through hydrogen bonds. HAS revealed that H…H (46.5%) and O…H (25.7%) contacts are in control of crystal stacking. The energy gap (4.679 eV) and reactivity descriptors indicate the stability of LAT1. The Mulliken and NBO charges showed that the protons have a positive charge and the heteroatoms exhibit negative charges. The Fukui function and MEP study revealed that the heteroatoms are the most reactive sites for an electophilic attack on the ligand. Molecular docking simulation shows that the significant binding affinity of LAT1 with SARS-CoV-2/Mpro is due to the formation of high number of hydrogen bonds.

Ionospheric control of space weather

Abstract. As proposed by Saka (2019), plasma injections arising out of the auroral ionosphere (ionospheric injection) are a characteristic process of the polar ionosphere at substorm onset. The ionospheric injection is triggered by westward electric fields transmitted from the convection surge in the magnetosphere at field line dipolarization. Localized westward electric fields result in local accumulation of ionospheric electrons and ions, which produce local electrostatic potentials in the auroral ionosphere. Field-aligned electric fields are developed to extract excess charges from the ionosphere. This process is essential to the equipotential equilibrium of the auroral ionosphere. Cold electrons and ions that evaporate from the auroral ionosphere by ionospheric injection tend to generate electrostatic parallel potential below an altitude of 10 000 km. This is a result of charge separation along the mirror fields introduced by the evaporated electrons and ions moving earthward in phase space.

On the non-dominance of counterions in the 1:z planar electrical double layer of point-ions

According to the dominance prescription of point-ions in the non-linear Poisson-Boltzmann theory, proposed by Valleau and Torrie almost 40 years ago, the microscopic and thermodynamic properties of an asymmetric binary electrolyte converge asymptotically to those of a completely symmetric electrolyte, in the limit of an infinite surface charge density of a planar electrode, if the properties of the counterions are the same in both instances. By using the Grahame equation and the non-linear Poisson-Boltzmann theory, we show here that this prescription is certainly exact for the mean electrostatic potential at the electrode's surface and for the capacitive compactness. Contrastingly, analytical and numerical solutions of the non-linear Poisson-Boltzmann equation show that, in the limit of an infinite surface charge density of the planar electrode, it is possible to observe finite differences between the local mean electrostatic potentials and electric fields associated to a 1:1 and a 1:z electrolyte at places near the electrode's surface. Thus, we prove here that even in the absence of ion correlations and ionic excluded volume effects, the counterions do not fully dominate the structural properties in the entire electrical double layer in the non-linear Poisson-Boltzmann picture, which is confirmed through comparisons with new Monte Carlo simulations.

Regulating the magnetic properties of seven-coordinated Dy(III) single-ion magnets through the effect of positional isomers on axial crystal-field.

Six Dy(III) single-ion magnets (SIMs) [Dy(n-OMe-bbpen)X] were synthesized by a solvothermal reaction with three positional isomers (ortho, meta, and para) of ligands n-OMe-H2bbpen and dysprosium halides DyX3, (n-OMe-H2bbpen = N,N'-bis(2-hydroxy-n-methoxybenzyl)-N,N'-bis(2-methylpyridyl)ethylenediamine; n = 3, X = Cl, 1; n = 3, X = Br, 2; n = 4, X = Cl, 3; n = 4, X = Br, 4; n = 5, X = Cl, 5; n = 5, X = Br, 6). Dynamic magnetic measurements revealed that the six complexes possess notably different effective barriers of magnetic reversal: 872.0 K (1), 1210.1 K (2), 137.9 K (3), 602.6 K (4), 907.0 K (5) and 1216.7 K (6). 6 showed the best performance as SIMs among the six Dy(III) complexes. Moreover, the magnetic hysteresis loops of 6 remained open at 21 K. The crystal structures indicate the switching of local symmetry around Dy(III) ion, aroused by the variation in intermolecular interactions and steric effects. This switch is primarily correlated with the distinction of magnetic properties. In addition, ab initio calculations confirmed that the different electrostatic potential around Dy(III) ion stemming from the electronic effect of the OMe-substituted group is another factor leading to the distinction in magnetic properties. This work warns us that when designing ligands for Dy-SIMs, the effect of positional isomerism on magnetic performance must be considered, which is one of the factors that can easily be overlooked.

Structural, physico-chemical landscapes, ground state and excited state properties in different solvent atmosphere of Avapritinib and its ultrasensitive detection using SERS/GERS on self-assembly formation with graphene quantum dots

Avapritinib is a complex molecule with many heterocyclic rings developed for the treatment of advanced PDGFRA D842V-mutant gastrointestinal stromal tumours. The manuscripts presents the study of the structure, ground state and excited state reactivity and stability indices in different solvent atmosphere using density functional theory in B3LYP/aug-cc-pVDZ level. Different molecular properties like reactivity surface, average local ionization energy, nuclear and electronic electrostatic potential, reduced density gradient assay, and localized molecular orbital analysis are studied in detail. The various intramolecular electron delocalisations were studied using natural orbital formalism and intermolecular interactions analysed by evaluating the extended of non-covalent interactions. Another interesting study is the enhancement in Raman activity of the compound when it forms a self-assembly with graphene quantum dots leading to SERS/GERS, which is studied by incorporating Grimme's dispersion into the previous functional. The electronic spectrum was studied in detail using time dependent density functional theory using long range corrected CAM-B3LYP functionals. The compound showed good light harvesting efficiency also. The local information entropy study provides uncertainty of electrons in spatial distribution.

Quantum dots in AA-stacked bilayer graphene

While electrostatic confinement in single-layer graphene and AA-stacked bilayer graphene is precluded by Klein tunneling and the gapless energy spectrum, we theoretically show that a circular domain wall that separates domains of single-layer graphene and AA-stacked bilayer graphene can provide bound states. Solving the Dirac-Weyl equation in the presence of a global mass potential and a local electrostatic potential, we obtain the energy spectrum of these states and the corresponding radial probability densities. Depending on the mass potential profile, regular bound states can exist inside the quantum dot and topological bound states at the domain wall. Controlling the electrostatic potential inside the quantum dot enables the simultaneous presence of both types of states. We find that the number of nodes of the radial wave function of the regular bound states inside the quantum dot is equal to the radial quantum number. The energy spectra of the bound states display anticrossings, reflecting coupling of electron- and holelike states.

Analyzing contrast in cryo-transmission electron microscopy: Comparison of electrostatic Zach phase plates and hole-free phase plates

Phase plates (PPs) are beneficial devices to improve the phase contrast of life-science objects in cryo-transmission electron microscopy (TEM). The development of the hole-free (HF) PP, which consists of a thin carbon film, has led to impressive results due to its ease in fabrication, implementation and application. However, the phase shift of the HFPP can be controlled only indirectly. The electrostatic Zach PP uses a strongly localized and adjustable electrostatic potential to generate well-defined and variable phase shifts between scattered and unscattered electrons. However, artifacts in phase-contrast TEM images are induced by the presence of the PP rod in the diffraction plane. We present a detailed analysis and comparison of the contrast-enhancing capabilities of both PP types and their emerging artifacts. For this purpose, cryo-TEM images of a standard T4-bacteriophage test sample were acquired with both PP types. Simulated images reproduce the experimental images well and substantially contribute to the understanding of contrast formation. An electrostatic Zach PP was used in this work to acquire cryo-electron tomograms with enhanced contrast, which are of similar quality as tomograms obtained by HFPP TEM.

Measurement of electrostatic tip-sample interactions by time-domain Kelvin probe force microscopy.

Kelvin probe force microscopy is a scanning probe technique used to quantify the local electrostatic potential of a surface. In common implementations, the bias voltage between the tip and the sample is modulated. The resulting electrostatic force or force gradient is detected via lock-in techniques and canceled by adjusting the dc component of the tip–sample bias. This allows for an electrostatic characterization and simultaneously minimizes the electrostatic influence onto the topography measurement. However, a static contribution due to the bias modulation itself remains uncompensated, which can induce topographic height errors. Here, we demonstrate an alternative approach to find the surface potential without lock-in detection. Our method operates directly on the frequency-shift signal measured in frequency-modulated atomic force microscopy and continuously estimates the electrostatic influence due to the applied voltage modulation. This results in a continuous measurement of the local surface potential, the capacitance gradient, and the frequency shift induced by surface topography. In contrast to conventional techniques, the detection of the topography-induced frequency shift enables the compensation of all electrostatic influences, including the component arising from the bias modulation. This constitutes an important improvement over conventional techniques and paves the way for more reliable and accurate measurements of electrostatics and topography.

Electrostatic charge generation on material surfaces from the evaporation of liquids

Electrostatic charging is a ubiquitous but poorly understood phenomenon. One of the difficulties with studying electrostatics is separating the different types of charging (triboelectric, inductive, or gas phase ion adsorption). We constructed an experiment to test how liquid film evaporation affects the charge on an insulator surface. Our results showed that when a liquid evaporates in an electric field, it leaves a charge on the substrate surface. It was also determined that the magnitude and polarity of the charge depends on the local electrostatic potential of the substrate and sufficient charge buildup leads to a dielectric breakdown of air, a potential source of hazardous sparks. We reveal through careful experiments that the charge is transferred via induction from electrical ground through the liquid film and onto the substrate's surface. Ambient fields were shown to be able to cause this type of charging, suggesting that failing to account for this phenomenon may explain the lack of reproducibility in previous electrostatic charging studies.

Electrostatic properties of inner nanopore surfaces of anodic aluminum oxide membranes upon high temperature annealing revealed by EPR of pH-sensitive spin probes and labels

Anodic aluminum oxide (AAO) membranes are versatile nanomaterials that combine the chemically stable and mechanically robust properties of ceramics with homogeneous nanoscale organization that can be tuned to desirable pore diameters and lengths. The AAO substrates feature high surface area that is readily accessible to large and small molecules, making these nanostructures uniquely suited for many possible applications. Examples include templated self-assembly of macroscopically aligned biological membranes and substrates for heterogeneous catalysis. For further development of such applications, one would like to characterize and tune the electrostatic properties of the inner pore surface as well as the local acidity within the nanochannels. Here, we employed electron paramagnetic resonance (EPR) spectroscopy of a small molecule – ionizable nitroxide – as a reporter of the average local acidity in the nanochannels and the local electrostatic potential in the immediate vicinity of the pore surface. The former was achieved by measuring EPR spectra of this molecular probe diffusing in an aqueous phase confined in the AAO nanochannels while for the latter the nitroxide was covalently attached to the hydroxyl group of the alumina surface. We show that the local acidity within the nanochannels is increased by as much as ≈1.48 pH units vs. the pH of bulk solution by decreasing the pore diameter down to ca. 31 nm. Furthermore, the positive surface charge of the as-prepared AAO could be decreased and even switched to a negative surface charge upon annealing the membranes first to 700 °C and then to 1200 °C. For as-prepared AAO, the local electrostatic potential reaches ψ= (163 ± 5) mV for the nitroxide label covalently attached to AAO and located about 0.5 nm away from the surface. Overall, we demonstrate that the acid-based properties of the aqueous volume confined by the AAO nanopores pores can be tuned by either changing the pore diameter from ca. 71 to 31 nm or by thermal annealing to switch the sign of the surface charge. These observations provide a simple and robust means to tailor these versatile high-surface-area nanomaterials for specific applications that depend on acid-base equilibria.

A Single H2O Molecule inside Hydrophobic Carbon Nanocavities: Effect of Local Electrostatic Potential

We have studied the rotational dynamics and orientation of a single water molecule trapped inside carbon nanocavities based on fullerene C60 possessing a hydrophobic potential surface. By modifying...

Local Grand Canonical Monte Carlo Simulation Method for Confined Fluids.

We describe a new local grand canonical Monte Carlo method to treat fluids in pores in chemical equilibrium with a reference bulk. The method is applied to Lennard-Jones particles in pores of different geometry and is shown to be much more accurate and efficient than other techniques such as traditional grand canonical simulations or Widom's particle insertion method. It utilizes a penalty potential to create a gas phase, which is in equilibrium with a more dense liquid component in the pore. Grand canonical Monte Carlo moves are employed in the gas phase, and the system then maintains chemical equilibrium by "diffusion" of particles. This creates an interface, which means that the confined fluid needs to occupy a large enough volume so that this is not an issue. We also applied the method to confined charged fluids and show how it can be used to determine local electrostatic potentials in the confined fluid, which are properly referenced to the bulk. This precludes the need to determine the Donnan potential (which controls electrochemical equilibrium) explicitly. Prior approaches have used explicit bulk simulations to measure this potential difference, which are significantly costly from a computational point of view. One outcome of our analysis is that pores of finite cross-section create a potential difference with the bulk via a small but nonzero linear charge density, which diminishes as ∼1/ln(L), where L is the pore length.

In-plane band bending in hexagonal monolayer WS2 by edge polarization

A triangular domain structure within hexagonal monolayer ${\mathrm{WS}}_{2}$ is found to be induced by edge polarization. Based on density-functional theory calculations, the local electrostatic potential over ${\mathrm{WS}}_{2}$ is investigated, and with the S-adatom and ${\mathrm{S}}_{2}$-addimer reconstructions at the W edges, the domains facing the W edges are found to be lower by up to 1.8 eV in the electrostatic potential than those facing the S edges. The edge-induced band bending in the terminated two-dimensional system is a two-dimensional analogy of the surface band bending effect in the terminated three-dimensional system. The largest potential difference is found to be comparable to the electronic gap of ${\mathrm{WS}}_{2}$, and near the edges, the W and S domains can be $n$- and $p$-type doped, respectively, by which the hexagonal monolayer ${\mathrm{WS}}_{2}$ can be applied to a bipolar electronic junction device.

Micromesoporous Nitrogen-Doped Carbon Materials Derived from Direct Carbonization of Metal-Organic Complexes for Efficient CO2 Adsorption and Separation.

Metal-organic complexes (MOCs) are considered as excellent precursors to prepare carbon materials, due to the fact that heteroatoms and functional groups can be naturally reserved in the resulting carbon materials through the carbonization. Herein, micromesoporous nitrogen-doped carbons MPNC-1 and MPNC-2 are successfully obtained by direct carbonization (800 °C, KOH activation) of metal-organic complexes DQA-1 and DQA-2. MPNC-1 and MPNC-2 exhibit high BET surface area (2368.9 and 2327.6 m2 g-1), pore volume (1.95 and 1.89 cm3 g-1), and N contents (17.2% and 12.3%). At 25 °C and 1 bar, MPNC-1 and MPNC-2 show high CO2 adsorption of 7.53 and 6.58 mmol g-1, the estimated CO2/N2 selectivity are 20.5 and 22.6, indicating excellent promise for practical CO2 adsorption and separation applications. Theoretical calculation indicates carbon surfaces with pyridinic-N, pyrrolic-N, and graphitic-N coexistence could strongly change the local electronic distribution and electrostatic surface potential, enhancing the CO2 adsorption with adsorption energy of -58.96 kJ mol g-1. Theoretical calculation also highlights that CO2 adsorption mechanism is electrostatic interaction with a large green isosurface between CO2 molecules and the carbon surface.

Quantifying work function differences using low-energy electron microscopy: The case of mixed-terminated strontium titanate

For many applications, it is important to measure the local work function of a surface with high lateral resolution. Low-energy electron microscopy is regularly employed to this end since it is, in principle, very well suited as it combines high-resolution imaging with high sensitivity to local electrostatic potentials. For surfaces with areas of different work function, however, lateral electrostatic fields inevitably associated with work function discontinuities deflect the low-energy electrons and thereby cause artifacts near these discontinuities. We use ray-tracing simulations to show that these artifacts extend over hundreds of nanometers and cause an overestimation of the true work function difference near the discontinuity by a factor of 1.6 if the standard image analysis methods are used. We demonstrate on a mixed-terminated strontium titanate surface that comparing LEEM data with detailed ray-tracing simulations leads to much a more robust estimate of the work function difference.

The role of chlorine substituents in lichexanthones properties: the ionic and halogen bond interactions

The effects of chlorine substituents (Cl substituents) on the structural and electronic properties of lichexanthones (LXs) such as aromaticity of rings, the intramolecular interactions and the complexation of LXs with Mg2+ and NH3 (the ionic and halogen bond interactions) have been investigated by the quantum mechanical calculations at the M06-2X/6-311++G(d,p) level of theory. The effects of Cl substituents on the intramolecular H-bond interactions strongly depend on their positions on the ring. The Cl substituents increase the E2 values of nX → π*CC interactions, obtained from the natural bond orbital analysis, reduce the electron densities on the center of the substituted ring, ρRCP in the results of atoms in molecules analysis, and decrease the aromaticity of the rings with respect to the aromatic fluctuation index. The stabilities of compounds depend on the aromaticity of rings, the O···H-bond strength, and especially the Cl···H interaction. Also, the effects of substituents on the local minimum values of electrostatic potential around the O atoms (Vmin) and therefore the binding energies of [LX···Mg]2+ and [LX···Mg·4H2O]2+ complexes strongly depend on the positions of the substituents. The aromaticity of rings A and C decrease and that of B increases after complexation with the Mg2+ ion. The electronic effects (and in some cases the steric effects) play a special role in the formation and stability of LX···NH3 complexes. The stabilities of the LX···NH3 complexes increase by increasing the natural charges and the local maximum electrostatic potential (Vmax−Cl) calculated, respectively, on and around the Cl atoms, and the ρCl−N values calculated at the Cl···N BCPs.

Effective theory approach to the Schrödinger-Poisson problem in semiconductor Majorana devices

We propose a method for solving the Schrodinger-Poisson problem that can be efficiently implemented in realistic 3D tight-binding models of semiconductor-based Majorana devices. The method is based on two key ideas: i) For a given geometry, the Poisson problem is only solved once (for each local orbital) and the results are stored as an interaction tensor; using this Green's function scheme, the Poisson component of the iteration procedure is reduced to a few simple summations. ii) The 3D problem is mapped into an effective multi-orbital 1D problem with molecular orbitals calculated self-consistently as the transverse modes of an infinite wire with the same electrostatic potential as the local electrostatic potential of the finite 3D device. To demonstrate the capabilities of our approach, we calculate the response of the system to an external magnetic field, the dependence of the effective chemical potential on the work function difference, and the dependence of the effective semiconductor-superconductor coupling on the applied gate potential. We find that different low-energy bands are characterized by similar effective couplings, which results in induced gap features characterized by a single energy scale. We also find that electrostatic effects are responsible for a partial suppression of the Majorana energy splitting oscillations. Finally, we show that a position-dependent work function difference can produce a non-homogeneous effective potential that is not affected by the screening due to the superconductor and is only partially suppressed by the charge inside the wire. In turn, this potential can induce trivial low-energy states that mimic the phenomenology of Majorana zero modes. Thus, any position-dependent work function difference (even at the 1% level) along the nanowire must be avoided through carefully engineered semiconductor-superconductor interfaces.