Local Charge Density Research Articles

Charge fluctuations on the sunlit surface of airless bodies and their role in dust levitation

The most likely cause of levitation of dust particles above the illuminated surface of airless bodies is the electrostatic mechanism. However, there are two essential problems to explain this effect. The first one is how a large electric charge (several units or even dozens of elementary charges e) which is required for levitation in the field of the electrical double layer, can be accumulated on a dust particle lying on the surface. The second problem is to find out the nature of the force that detaches dust particle from the surface overcoming the adhesive van der Waals force. The paper shows that the density of charge, which arises on a lit surface due to the photoeffect, is very small only when averaged over regions of a macroscopic size. On a submicron scale, the surface appears to be a collection of chaotic “spots” having positive or negative charges. These spots arise due to fluctuations in the fluxes of the photoelectrons taking off and falling back onto the surface, and the modulus of charge density within these spots is millions of times larger than the value of the charge density averaged over macroscopic regions. The number of positively charged spots is only slightly larger than the number of the negatively charged spots, which explains the smallness of the average charge density. The local charge density inside the spots is sufficient to allow the particles about 0.1 μm or less to get a charge sufficient to levitate them within the electric double layer at an altitude about 1 μm or more, where the field strength of the double layer is on the order of 10 Vm−1. The electrostatic field directly above the center of a charge spot is proportional to the local rather than average charge density and appears to be several orders of magnitude greater. Therefore, the force acting on a dust particle lying interior to a charge spot directly on the surface is proportional to the square of the local charge density and its magnitude can reach a tenth of pico-newton, which is probably sufficient for a particle to overcome the adhesive van der Waals force and take off.

Electric double layers with surface charge modulations: Exact Poisson-Boltzmann solutions.

Poisson-Boltzmann theory is the cornerstone for soft matter electrostatics. We provide exact analytical solutions to this nonlinear mean-field approach for the diffuse layer of ions in the vicinity of a planar or a cylindrical macroion. While previously known solutions are for homogeneously charged objects, the cases worked out exhibit a modulated surface charge-or equivalently, surface potential-on the macroion (wall) surface. In addition to asymptotic features at large distances from the wall, attention is paid to the fate of the contact theorem, relating the contact density of ions to the local wall charge density. For salt-free systems (counterions only), we make use of results pertaining to the two-dimensional Liouville equation, supplemented by an inverse approach. When salt is present, we invoke the exact two-soliton solution to the 2D sinh-Gordon equation. This leads to inhomogeneous charge patterns, that are either localized or periodic in space. Without salt, the electrostatic signature of a charge pattern on the macroion fades exponentially with distance for a planar macroion, while it decays as an inverse power law for a cylindrical macroion. With salt, our study is limited to the planar geometry and reveals that pattern screening is exponential.

Real-space charge-density imaging with sub-ångström resolution by four-dimensional electron microscopy.

The distribution of charge density in materials dictates their chemical bonding, electronic transport, and optical and mechanical properties. Indirectly measuring the charge density of bulk materials is possible through X-ray or electron diffraction techniques by fitting their structure factors1-3, but only if the sample is perfectly homogeneous within the area illuminated by the beam. Meanwhile, scanning tunnelling microscopy and atomic force microscopy enable us to see chemical bonds, but only on the surface4-6. It remains a challenge to resolve charge density in nanostructures and functional materials with imperfect crystalline structures-such as those with defects, interfaces or boundaries at which new physics emerges. Here we describe the development of a real-space imaging technique that can directly map the local charge density of crystalline materials with sub-ångström resolution, using scanning transmission electron microscopy alongside an angle-resolved pixellated fast-electron detector. Using this technique, we image the interfacial charge distribution and ferroelectric polarization in a SrTiO3/BiFeO3 heterojunction in four dimensions, and discover charge accumulation at the interface that is induced by the penetration of the polarization field of BiFeO3. We validate this finding through side-by-side comparison with density functional theory calculations. Our charge-density imaging method advances electron microscopy from detecting atoms to imaging electron distributions, providing a new way of studying local bonding in crystalline solids.

Pore connectivity effects on the internal surface electric charge of mesoporous silica

Nano-scale confinements within mesoporous systems develop overlapping electric double layers (EDL) such that the existing theoretical models cannot predict the electric potential distributions and resulting surface charges. In addition, ionic conditions undergo local variation through connections between the pore voids and pore throats. For the first time in literature, we studied the charging behavior of mesoporous silica in terms of the pore to throat size ratio (Rpt) to characterize the pore connectivity effects, in addition to porosity (є) and pore size (H). Both local and average surface charge densities inside mesoporous silica were examined by varying these parameters systematically. Results showed that the magnitude of surface charge density decreased with increasing EDL overlap and decreasing connectivity effects. We formulized this behavior and developed an extended model to predict mesoporous silica’s internal charge as a function of porosity, pore size, and pore to throat size ratio.

Measuring Local Electric Fields and Local Charge Densities at Electrode Surfaces Using Graphene-Enhanced Raman Spectroscopy (GERS)-Based Stark-Shifts.

We report spectroscopic measurements of the local electric fields and local charge densities at electrode surfaces using graphene-enhanced Raman spectroscopy (GERS) based on the Stark-shifts of surface-bound molecules and the G band frequency shift in graphene. Here, monolayer graphene is used as the working electrode in a three-terminal potentiostat while Raman spectra are collected in situ under applied electrochemical potentials using a water immersion lens. First, a thin layer (1 Å) of copper(II) phthalocyanine (CuPc) molecules are deposited on monolayer graphene by thermal evaporation. GERS spectra are then taken in an aqueous solution as a function of the applied electrochemical potential. The shifts in vibrational frequencies of the graphene G band and CuPc are obtained simultaneously and correlated. The upshifts in the G band Raman mode are used to determine the free carrier density in the graphene sheet under these applied potentials. Of the three dominant peaks in the Raman spectra of CuPc (i.e., 1531, 1450, and 1340 cm-1), only the 1531 cm-1 peak exhibits Stark-shifts and can, thus, be used to report the local electric field strength at the electrode surface under electrochemical working conditions. Between applied electrochemical potentials from -0.8 V to 0.8 V vs NHE, the free carrier density in the graphene electrode spans a range from -4 × 1012 cm-2 to 2 × 1012 cm-2. Corresponding Stark-shifts in the CuPc peak around 1531 cm-1 are observed up to 1.0 cm-1 over a range of electric field strengths between -3.78 × 106 and 1.85 × 106 V/cm. Slightly larger Stark-shifts are observed in a 1 M KCl solution, compared to those observed in DI water, as expected based on the higher ion concentration of the electrolyte. Based on our data, we determine the Stark shift tuning rate to be 0.178 cm-1/ (106 V/cm), which is relatively small due to the planar nature of the CuPc molecule, which largely lies perpendicular to the electric field at this electrode surface. Computational simulations using density functional theory (DFT) predict similar Stark shifts and provide a detailed atomistic picture of the electric field-induced perturbations to the surface-bound CuPc molecules.

Ab initio investigation of the elastic properties of bismuth-based alloys

Density functional theory has been used to investigate the elastic properties of various Bi-based alloy materials as potential replacements of lead-based solders. We compare calculated quantities which can be used to determine the effectiveness of our proposed replacements, such as the bulk $(K)$, shear $(G)$, and Young's $(E)$ moduli. We also computed the Pugh ratio $(\ensuremath{\gamma}=K/G)$, a quantity that is used to estimate the ductility of a solid, for each of the alloys. The effect of spin-orbit coupling on these materials has also been investigated. By analyzing the changes in charge density as Bi is substituted, we link the predicted changes in ductility in these materials to atomistic descriptors; specifically, delocalization of the charge density is known to be a signature of increased ductility, whereas more localized charge density is typically associated with brittleness. Through Bader charge analysis we provide quantitative insight into the uniformity of charge distribution and how this alters the ductility of these alloys. By using density functional theory to calculate the elastic properties of different ${\mathrm{Bi}}_{1\ensuremath{-}x}{Z}_{x}$ alloys $(Z$ = Sb, Te, In, Sn; $0\ensuremath{\le}\phantom{\rule{4pt}{0ex}}x\phantom{\rule{4pt}{0ex}}\ensuremath{\le}1)$, we correlate changes in ductility and corresponding changes in the electronic structure of a material. We have found that low concentrations of Sn and Te, substituted into the hexagonal crystal structure of Bi, induce significant increases in ductility.

Generality and specificity of the binding behaviour of lysozyme with pectin varying in local charge density and overall charge

We examined the similarities and differences of the binding behavior of lysozyme (lys) with pectin varying in local charge density (blockwise distribution and statistical distribution of methoxyl groups, BP and SP) and similar degree of methoxylation (DM). The interaction at ionic strength I = 0.01 and pH 5.1 was found to be mainly electrostatic, associated with an exothermic enthalpy change. BP which is more inclined to self-association binds lys forming strongly associated complex particles with average sizes much larger (10–20 μm) than that of SP. The latter formed with lys small (0.6–3 μm) complex particles or liquid droplets depending on DM. The critical Iset and pHset values for BP/lys systems, above which complexes do not form, are not sensitive to DM (approximately 0.1 and 11.5 respectively), whereas for SP/lys systems the critical Iset and pHset values decrease significantly with increasing DM. The effect of I on complexation (CP) of lys with BP and SP has a nonmonotonic character. The distribution of BP and SP within the complex particles is irregular and is different for pectin with low and high DM values. The binding constant and stoichiometry ratio (lys/pectin) of the systems with SP are slightly higher than those of BP at the same DM, and these values decrease sharply (50 times) with increasing DM. The large positive changes in entropy in SP/lys systems suggest that the interactions in these systems tend to be more hydrophobic in character at high DM. On the other hand, with BP, a negative enthalpy change, especially for pectin with low charge, suggests the formation of a large quantity of hydrogen bonds or van der Waals interactions during CP.

Activating Graphyne Nanosheet via sp-Hybridized Boron Modulation for Electrochemical Nitrogen Fixation.

Exploring new metal-free catalysts with high activity for nitrogen reduction reaction (NRR) is highly desirable but remains a big challenge. Graphyne (GY) is a typical two-dimensional carbon material with many excellent properties. However, the NRR has rarely been envisaged on a GY-based metal-free catalyst up to now. Density functional theory calculations reveal that although pristine GY is inactive for N2 reduction, boron modulation can endow it with efficient activity toward NRR. Natural bond orbitals analysis, spin/charge density distributions, and free energy change diagrams are performed and discussed. Three boron doping formats including sp2-substituted, sp-substituted, and adsorbed configuration are considered. The obtained data show sp-substitution will induce local moderate spin and charge densities at the boron site on the GY surface, which is convenient for N2 adsorption and activation, and conductive to N-related intermediates formation and transformation. Moreover, the incorporated sp-hybridized boron can provide one empty p orbital and one occupied p orbital around itself, which plays a key role as an electron reservoir to accept electrons from and donate electrons to the adsorbed N-related species, and thus facilitate N2 reduction and ammonia synthesis. Henceforth, it provides more opportunities for preparing GY and other carbon materials as efficient catalysts toward renewable energy conversion and storage.

Short self‐assembling cationic antimicrobial peptide mimetics based on a 3,5‐diaminobenzoic acid scaffold

AbstractInfections caused by Staphylococcal and Micrococcal species represent a major public health burden. Although treatments do exist, these tend to be associated with cytotoxic effects; furthermore, the emergence of antimicrobial resistance presents an immediate challenge. New classes of active compounds are required to address these threats to human health. Here we present a de novo peptidomimetic strategy that produces self‐assembling cationic antimicrobials. To identify a candidate compound with bactericidal activity, a small library of 8 peptidomimetics comprising ultrashort peptide sequences attached to a 3,5‐diaminobenzoic acid scaffold was generated and tested against Micrococcus luteus and Staphylococcus aureus. Self‐assembly appears to be the driving force for increased potency, likely by contributing to increased local surface charge density and peptide mass and producing a multivalent effect that enhances electrostatic interactions with negatively charged bacterial membranes, causing membrane disruption. The most active library member C7 forms patched micellar nanoparticles and has an activity higher than that of known natural antimicrobial peptides against M luteus. C7 also shows activity comparable to that of gramicidin S and the standard antibiotic vancomycin used in antibacterial therapy, but with a greater selectivity index. Importantly, C7 is also nontoxic and nonhemolytic, unlike the currently administered vancomycin, which can cause acute renal failure, and gramicidin S, which is highly hemolytic in nature. The short sequence length, ease of design, convenient synthesis strategy, and presence of a substitutable hydrophobic residue that enables self‐assembly into different nanostructures make this model compound highly attractive for generating cost‐effective, rapid‐acting peptide‐based antimicrobials.

Cover Picture: Covalency‐Driven Preservation of Local Charge Densities in a Metal‐to‐Ligand Charge‐Transfer Excited Iron Photosensitizer (Angew. Chem. Int. Ed. 31/2019)

Cover Picture: Covalency‐Driven Preservation of Local Charge Densities in a Metal‐to‐Ligand Charge‐Transfer Excited Iron Photosensitizer (Angew. Chem. Int. Ed. 31/2019)

First-principles study of mechanical and electronic properties of bent monolayer transition metal dichalcogenides

The mechanical and electronic properties of transition metal dichalcogenide (TMD) monolayers corresponding to transition groups IV, VI, and X are explored under mechanical bending from first principles calculations using the strongly constrained and appropriately normed (SCAN) meta-GGA (MGGA). SCAN provides an accurate description of the phase stability of the TMD monolayers. Our calculated lattice parameters and other structural parameters agree well with experiment. We find that bending stiffness (or flexural rigidity) increases as the transition metal group goes from IV to X to VI, with the exception of PdTe$_2$. Variation in mechanical properties (local strain, physical thickness) and electronic properties (local charge density, band structure) with bending curvature is discussed. The local strain profile of these TMD monolayers under mechanical bending is highly non-uniform. The mechanical bending tunes not only the thickness of the TMD monolayers but also the local charge distribution as well as the band structures, adding more functionalization options to these materials.

Covalency-Driven Preservation of Local Charge Densities in a Metal-to-Ligand Charge-Transfer Excited Iron Photosensitizer.

Covalency is found to even out charge separation after photo‐oxidation of the metal center in the metal‐to‐ligand charge‐transfer state of an iron photosensitizer. The σ‐donation ability of the ligands compensates for the loss of iron 3d electronic charge, thereby upholding the initial metal charge density and preserving the local noble‐gas configuration. These findings are enabled through element‐specific and orbital‐selective time‐resolved X‐ray absorption spectroscopy at the iron L‐edge. Thus, valence orbital populations around the central metal are directly accessible. In conjunction with density functional theory we conclude that the picture of a localized charge‐separation is inadequate. However, the unpaired spin density provides a suitable representation of the electron–hole pair associated with the electron‐transfer process.

Electrostatically localized proton bioenergetics: better understanding membrane potential

In Mitchell's chemiosmotic theory, membrane potential Δψ was given as the electric potential difference across the membrane. However, its physical origin for membrane potential Δψ was not well explained. Using the Lee proton electrostatic localization model with a newly formulated equation for protonic motive force (pmf) that takes electrostatically localized protons into account, membrane potential has now been better understood as the voltage difference contributed by the localized surface charge density([HL+]+∑i=1n[MLi+]) at the liquid-membrane interface as in an electrostatically localized protons/cations-membrane-anions capacitor. That is, the origin of membrane potential Δψ is now better understood as the electrostatic formation of the localized surface charge density that is the sum of the electrostatically localized proton concentration [HL+] and the localized non-proton cations density ∑i=1n[MLi+] at the liquid membrane interface. The total localized surface charge density equals to the ideal localized proton population density [HL+]0 before the cation-proton exchange process; since the cation-proton exchange process does not change the total localized charges density, neither does it change to the membrane potential Δψ. The localized proton concentration [HL+] represents the dominant component, which accounts about 78% of the total localized surface charge density at the cation-proton exchange equilibrium state in animal mitochondria. Liquid water as a protonic conductor may play a significant role in the biological activities of membrane potential formation and utilization.

Modulation of Molecular Spatial Distribution and Chemisorption with Perforated Nanosheets for Ethanol Electro-oxidation.

Integrating thermodynamically favorable ethanol reforming reactions with hybrid water electrolysis will allow room-temperature production of high-value organic products and decoupled hydrogen evolution. However, electrochemical reforming of ethanol has not received adequate attention due to its low catalytic efficiency and poor selectivity, which are caused by the multiple groups and chemical bonds of ethanol. In addition to the thermodynamic properties affected by the electronic structure of the catalyst, the dynamics of molecule/ion dynamics in electrolytes also play a significant role in the efficiency of a catalyst. The relatively large size and viscosity of the ethanol molecule necessitates large channels for molecule/ion transport through catalysts. Perforated CoNi hydroxide nanosheets are proposed as a model catalyst to synergistically regulate the dynamics of molecules and electronic structures. Molecular dynamics simulations directly reveal that these nanosheets can act as a "dam" to enrich ethanol molecules and facilitate permeation through the nanopores. Additionally, the charge transfer behavior of heteroatoms modifies the local charge density to promote molecular chemisorption. As expected, the perforated nanosheets exhibit a small potential (1.39 V) and high Faradaic efficiency for the conversion of ethanol into acetic acid. Moreover, the concept in this work provides new perspectives for exploring other molecular catalysts.

Laser-stimulated birefringence of Ga2O3-La2S2.95Se0.05 glasses for optical recording of information

Abstract We show an opportunity to change the birefringence of Ga2O3 – La2S2.95Se0.05 glasses doped by Er3+ and La3+ ions at λ = 1150 nm wavelength of cw He-Ne laser under illumination of the femtosecond laser. The variation of the refractive indices was found during illumination by 180 fs laser with frequency about 65 MHz (at wavelength λ = 1045 nm). The laser-stimulated birefringence has been explored versus temperature. The observed laser induced and temperature dependences of the birefringence confirm a huge contribution of anharmonic phonon subsystems which is typical for chalcogenide glasses. Due to an occurrence of local space charge density asymmetry, the efficiency of birefringence is enhanced. The anisotropy effect exists only during the illumination and is diminished during 10…12 s after switching off of the photo-inducing laser treatment. There is observed also a correlation between the occurred grating patterns and the values of the birefringence.

Atomic electrostatic maps of 1D channels in 2D semiconductors using 4D scanning transmission electron microscopy

Defects in materials give rise to fluctuations in electrostatic fields that reflect the local charge density, but imaging this with single atom sensitivity is challenging. However, if possible, this provides information about the energetics of adatom binding, localized conduction channels, molecular functionality and their relationship to individual bonds. Here, ultrastable electron-optics are combined with a high-speed 2D electron detector to map electrostatic fields around individual atoms in 2D monolayers using 4D scanning transmission electron microscopy. Simultaneous imaging of the electric field, phase, annular dark field and the total charge in 2D MoS2 and WS2 is demonstrated for pristine areas and regions with 1D wires. The in-gap states in sulphur line vacancies cause 1D electron-rich channels that are mapped experimentally and confirmed using density functional theory calculations. We show how electrostatic fields are sensitive in defective areas to changes of atomic bonding and structural determination beyond conventional imaging.

ZnO/GaN heterojunction based self-powered photodetectors: Influence of interfacial states on UV sensing

Gallium Nitride (GaN) and Zinc Oxide (ZnO) are well established semiconductors with their heterostructures opening avenues for the future of next generation sensing and opto-electronics technologies due to their low lattice mismatch and high exciton energy. ZnO/GaN heterostructure based ultraviolet photodetectors with complimentary material properties are expected to yield optimum efficiency, though their performance has remained low due to challenges related to interfacial properties. Inadequate analysis of ZnO/GaN interfacial properties/states viz. electronic structure, band offsets, localized charge density and defect states associated with overlayer (i.e. ZnO) thickness, and their influence on device performance has remained as an underestimated issue. Interestingly, literature reports a huge anomaly in the valence band offset (VBO) at ZnO/GaN interfaces, which being an effective measure of charge transport assist in the optimization of photodetector efficiency. Therefore, in the present report, we have fabricated ZnO/GaN heterostructure (with variable ZnO thickness) based Schottky barrier photodectors and investigated the dependence of photosensitivity & other device parameters on interfacial states/properties. We have witnessed a peak responsivity & detectivity of 225 mA/W−1 & 4.83 (×1013) Jones and a high speed photoswitching associated with the band offset, barrier height & defect states at the ZnO/GaN heterojunction. The underlying scientific phenomenon (e.g. interfacial dipole strength, charge accumulation etc.) leading to perturbations & discontinuities in interfacial/electronic states were also correlated and discussed in detail.

Electrostatic Adsorption of Uniformly Charged Sphere Ions Confined between Two Charged Plates

The theoretical model, which is based both on the modified fundamental-measure theory for the hard-sphere contribution and on the mean-field approximation for the cross correlation between the hard sphere contribution and Coulomb interaction, has been developed for studying the electrostatic adsorption of uniformly charged sphere ions. The Poisson equation by ions carrying charges that are spatially separated has been solved. The present theory shows an excellent agreement with the simulation data even for the large hard-sphere ions and is better than the modified Poisson- Boltzmann model. The calculated result shows that (i) the local particle and charge densities are strongly related to the charge distribution of ions, and that (ii) at large and strongly surfaceassociated ion the cross correlation between the hard sphere contribution and Coulomb interaction as well as the hard-sphere contribution becomes important, whereas the hard sphere contribution of charged ions relatively dominants at the weakly surface-associated ion.

Synthesis of Novel α-amidino Carboxylic Acids and their Use as H-Bond Catalysts in Strecker Reaction.

A wide variety of synthesized amidine derivatives are bioactive compounds. They show a vast range of medical properties. Therefore, a simple route for synthesis of novel class of amidine derivatives called amidino carboxylic acids and their use as catalysts in Strecker reaction has been reported in the current work. The stability, local charge density and hydrogen bond parameters were calculated for eight derivatives with different substituents. In order to synthesize these amidino carboxylic acids, we initially prepared Knovenogel condensation products via the reaction of isatin derivatives with malonitrile. When the reaction was performed in water, the resulting nitrile groups of malonitrile derivatives was hydrolyzed with HOAC/ H2SO4 to generate the desired amide groups. The amide groups in resulting compound converted to amine groups with two Hoffman rearrangements in the presence of NaOH/Br2. Further neutralization led to the final zwitterionic α-amidino carboxylic acids. In the next step, the catalytic activity of these compounds as H-bond donor catalyst was investigated in Strecker reaction. The overall yields of the derivatives with substituent on the aromatic ring of starting isatins are higher than that for the overall yields of nitrogen-substituted isatins. The reaction of 5-nitro isatin with the next reagent gives lower yield in aryl-substituted products. An increase of catalytic activity is observed by rising the electron-withdrawing power of the aromatic ring substituents., The presence of nitro group in the structure of catalyst caused a large increase of catalytic activity in Strecker reaction. DFT calculations at B3LYP/6-31++g(d,p) and Lanl2dz level of theory showed that these compounds act as single H-bond catalysts and higher yields were obtained for complexes with stronger hydrogen bond. A simple and efficient method for synthesis of ɑ-amidino carboxylic acids was developed in this research. These compounds have been used as a single H-bond donor catalyst in the Strecker reaction. DFT calculations were carried out to confirm the experimental results. The obtained data from computations are in good agreement with experimental results.

Direct spectroscopic evidence for mixed-valence Tl in the low carrier-density superconductor Pb1−xTlxTe

Upon doping with Tl the narrow band-gap semiconductor PbTe exhibits anomalously high-temperature superconductivity despite a very low carrier-density as well as signatures of the Kondo effect despite an absence of magnetic moments. These phenomena have been explained by invoking $2e$ fluctuations of the valence of the Tl dopants, but a direct measurement of the mixed valency implied by such a mechanism has not been reported to date. In this work we present the unambiguous observation of multiple valences of Tl in Tl-doped PbTe via photoemission spectroscopy measurements. It is shown via a quantitative analysis that the suppression of the carrier density in compositions exhibiting superconductivity and Kondo-like behavior can be accounted for by mixed valency, thus arguing against a self-compensation scenario proposed elsewhere for this material and strengthening the case for valence fluctuation models. In addition to the identification of ${\mathrm{Tl}}^{+}$ and ${\mathrm{Tl}}^{3+}$ a possible third intermediate local charge-density is suggested by full fits to the data, the origins of which are also discussed but remain unclear.