Charge Density Model Research Articles

Experimental charge density of ferrocenyl derivative of β-lactam

A single crystal of a ferrocenyl derivative – cis-4-(2-ferrocenylphenyl)-3-hydroxyazetidin-2-one (FER), combining two chemically and biologically important systems (ferrocene and β-lactam), has been analysed from the structural, energetic and experimental charge density distribution perspectives. Investigated compound crystallizes in the chiral orthorhombic P212121 space group. Molecules of FER are arranged in a “zig-zag” manner and participate in a network of N–H⋯O and O–H⋯O hydrogen bonds as well as C–H···π contacts. The R22(9) ring motif of H-bonds, atypical for the mono−β−lactams, was observed. An experimental charge density model was obtained using the Hansen-Coppens multipole formalism. Energy density descriptors at bond critical points allowed classification of Fe–C interactions in FER as intermediate type with some features of covalency. The charge distribution for the ferrocenyl moiety is analogous to those previously reported, however in the case of the β-lactam substantial differences are present. Conjugation of ferrocenyl with β-lactam does not significantly affect the charge density distribution in either of these fragments. The unique R22(9) system of H-bonds was found to elude a typical topological H-bond classification.

Large-area nanostructured surfaces with tunable zeta potentials

Through a refined fabrication protocol based on block-copolymer self-assembly we synthesize nanostructured surfaces with conical nanopillars of different height (60, 120, and 200 nm) in hexagonal arrays with uniform spacing (∼50 nm) over large areas (> cm2). While the nanostructured surfaces fabricated on silicon substrates display superhydrophilic behavior, superhydrophobic properties are attained by coating with octadecyltrichlorosilane (OTS). Negative zeta potentials for all the studied surfaces are reported by electrokinetic flow measurements with aqueous KCl solutions at different concentrations (1 and 10 mM) and pH values between 4 and 8. While the surface nanostructure reduces the zeta potential magnitude, the hydrophobic OTS coating enhances it. Experimental results can be accounted for by a site-dissociation model for the surface charge density. The reported wetting properties and zeta potential tunability makes the studied surfaces particularly relevant for applications such as energy conversion and storage, membrane-based water treatment and molecular separation.

Coordination bonding in dicopper and dichromium tetrakis(μ-acetato)-diaqua complexes: Nature, strength, length, and topology.

Geometry optimization, energetics, electronic structure, and topology of electron density of dicopper (I) and dichromium (II) tetrakis(μ-acetato)-diaqua complexes are studied focusing on the metal-metal interactions. The performance of broken symmetry (BS) single-determinant ab initio (Hartree-Fock, Møller-Plesset perturbation theory to the second and third orders, coupled clusters singles and doubles) and density functional theory (BLYP, B3LYP, B3LYP-D3, B2PLYP, MPW2PLYP) methods is compared to multideterminant ab initio (CASSCF, NEVPT2) methods as well as to the multipole model of charge density from a single-crystal X-ray diffraction experiment (Herich et al., Acta Cryst. 2018, B74, 681-692). In vacuo DFT geometry optimizations (improper axial water ligand orientation) are compared against the periodic ones. The singlet state is found to be energetically preferred. J coupling of (I) becomes underestimated for all ab initio methods used, when compared to experiment. It is concluded that the strength of the direct M─M interactions correlates closely with the J coupling magnitude at a given level of theory. The double potential well character of (II) and of the dehydrated form of (II) are considered with respect to the Cr─Cr distance. The physical effective bond order of the metal-metal interaction is small (below 0.1 e) in (I) and moderate (0.4 e) in (II). The CASSCF results overestimate the electron density of the metal-metal bond critical point by 20% and 50% in (I) and (II), respectively, when compared to the multipole model. © 2019 Wiley Periodicals, Inc.

Experimental Insights into the Electronic Nature, Spectral Features, and Role of Entropy in Short CH3···CH3 Hydrophobic Interactions.

Hydrophobic interactions are often explored in solution-state aggregation of molecules. However, an experimental electron density description about these interactions is still lacking. Here, we report a systematic study on the electronic nature of methyl···methyl hydrophobic interactions in a series of multicomponent crystals of biologically active molecules. Charge density models based on high-resolution X-ray diffraction allow the visualization of subtle details of electron density features in the interaction region. Our study classifies these interactions as atypical group···group interactions in contrast to σ-hole interactions, which are stabilized by the minimized electrostatic repulsion and maximized dispersion forces. For the first time, we quantified the solid-state entropic contribution from the torsional mode of the methyl groups in stabilizing these interactions by thermal motion analysis based on neutron diffraction as well as variable-temperature crystallography. The carbon atoms in methyl···methyl interactions show a unique upfield chemical shift in the 13C solid-state NMR signal.

Spin resolved electron density study of YTiO3 in its ferromagnetic phase: signature of orbital ordering.

The present work reports on the charge and spin density modelling of YTiO3 in its ferromagnetic state (T C = 27 K). Accurate polarized neutron diffraction and high-resolution X-ray diffraction (XRD) experiments were carried out on a single crystal at the ORPHÉE reactor (LLB) and SPRING8 synchrotron source. The experimental data are modelled by the spin resolved pseudo-atomic multipolar model (Deutsch et al., 2012 ▸). The refinement strategy is discussed and the result of this electron density modelling is compared with that from XRD measured at 100 K and with density functional theory calculations. The results show that the spin and charge densities around the Ti atom have lobes directed away from the O atoms, confirming the filling of the t 2g orbitals of the Ti atom. The d xy orbital is less populated than d xz and d yz , which is a sign of a partial lift of degeneracy of the t 2g orbitals. This study confirms the orbital ordering at low temperature (20 K), which is already present in the paramagnetic state above the ferromagnetic transition (100 K).

Classical and quantum charge density model of nano scale doped DG MOSFETs below onset of strong inversion

Classical and quantum inversion charge density models are derived below threshold for a doped double-gate (DG) MOSFET. Since our charge density model includes a 2D analytical solution of channel potential considering the contribution of both mobile charge and doping concentration terms, the proposed models are also investigated slightly above threshold of the device having different channel lengths and validated by 2D TCAD (Technology Computer Aided Design) simulation. Quantum Confinement Effects are incorporated in quantum model by considering a 1D infinite rectangular well with a perturbation applied. Finally charge models are implemented in suitable drain current equation to study the electrical behavior of the device.

Mobility extraction for short channel UTBB-FDSOI MOSFETs under back bias using an accurate inversion charge density model

In this work we measure the inversion charge density for short UTBB-FDSOI MOSFETs (down to 45 nm) using an accurate method for the extraction of the parasitic components which take into account the effect of the back bias. Based on these results, we propose a modification of the inversion charge density model that uses the simplified Lambert function in order to include the back bias effect. This model is then validated with the experimental data for different gate lengths (from 185 nm to 45 nm). It is shown that this inversion charge model is valid for the mobility extraction and gives very good assessment of the mobility if we know only the IV-characteristic, the threshold voltage (under back bias effect) and the inversion charge capacitance. Finally, this method is used to extract the mobility for short channel devices (down to 40 nm) and it is found that the short channel mobility suffers from degradation even for large back biases. For the shortest device, it is demonstrated that the main source of mobility degradation is caused by oxide charges and interface states located near the S/D extensions and then it is deduced that the mobility reduction for large back biases is related to neutral defects from Si-crystal close to the channel edge.

Temperature influence on mobility and charge density model of photovoltaic cells

abstract Photovoltaic (PV) devices that capture the energy provided by the sun have great potential as renewable energy sources. However, the input parameters such as the luminous intensity and temperature of the solar cells tend to influence the operating characteristics in the solar panels. The inherent physical processes that cannot be altered limit the efficiency of photovoltaic cells, as well as temperatures above the ideal value of operation. We propose a diode model to represent a PV cell, presenting good approximation for a commercial silicon cell. In this work we report the model with the aid of Coughey Thoma's equation to obtain the results, which indicate decrease in the mobility of loads and subsequently evaluation in terms of the load carrier diffusion lengths in regions with ideal temperatures between 293.15 K to 301.15 K (20 °C and 28 °C). Results were compared with local temperature between 298.15 K at 308.15 K (25 °C and 35 °C), in a region with a PV plant in operation in the West of Bahia.

Finite ion size effect on the force and energy of the double-layer interaction between two parallel similar plates at arbitrary separations in an electrolyte solution

Expressions are derived for the force and energy of the double-layer interaction between two parallel similar plates with arbitrary surface charge densities or surface potentials at arbitrary separations in an electrolyte solution, by taking into account the finite ion size effect on the basis of the full expression of the activity coefficients of electrolyte ions derived by Carnahan and Starling. The results for two models of the double-layer interaction, that is, the constant surface potential and constant surface charge density models, are given. It is shown that in the constant surface potential model, at large plate separations, the magnitudes of the double-layer interaction force and energy increase as the total ion volume fraction increases but this tendency reverses at small plate separations, while in the constant surface charge density model, they always increase as the total ion volume fraction increases.

Crystal structure, interaction energies and experimental electron density of the popular drug ketoprophen.

The crystal and molecular structure of the pure (S)-enantiomer of the popular analgesic and anti-inflammatory drug ketoprophen (α-ket) is reported. A detailed aspherical charge-density model based on high-resolution X-ray diffraction data has been refined, yielding a high-precision geometric description and classification of the O-H⋯O interactions as medium strength hydrogen bonds. The crystal structure of the racemic form of ketoprophen (β-ket) was also redetermined at 100 K, at 0.5 Å resolution. A previously unreported disorder (10% occupancy) was discovered. In contrast to the racemic β-ket case, the (S)-enantiomer crystallizes with two independent molecules in the asymmetric unit with two distinct conformations. The major difference between the β-ket and α-ket crystal forms lies in the formation of distinct hydrogen-bonded motifs: a closed ring motif in β-ket versus infinite chains of hydrogen bonds in the chiral α-ket structure. However, the overall crystal packing of both forms is surprisingly similar, with close-packed layers of antiparallel-oriented benzo-phenone moieties bound by C-H⋯π interactions. Notably, the most important stabilizing term in the total lattice energies in both instances proved to be the dispersion related to these interactions. Both forms of the title compound (α- and β-ket) were additionally characterized by differential scanning calorimetry and thermogravimetric analysis.

An analytical effective excess charge density model to predict the streaming potential generated by unsaturated flow

The self-potential (SP) method is a passive geophysical method that relies on the measurement of naturally occurring electrical field. One of the contributions to the SP signal is the streaming potential, which is of particular interest in hydrogeophysics as it is directly related to both the water flow and porous medium properties. The streaming current is generated by the relative displacement of an excess of electrical charges located in the electrical double layer surrounding the minerals of the porous media. In this study, we develop a physically based analytical model to estimate the effective excess charge density dragged by the water flow under partially saturated conditions. The proposed model is based on the assumption that the porous media can be represented by a bundle of tortuous capillary tubes with a fractal pore size distribution. The excess charge that is effectively dragged by the water flow is estimated using a flux averaging approach. Under these hypotheses, this new model describes the effective excess charge density as a function of saturation and relative permeability while also depending on the chemical and interface properties, and on petrophysical parameters of the media. The expression of the model has an analytical single closed-form which is consistent with a previous model developed from a different approach. The performance of the proposed model is then tested against previous models and different sets of laboratory and field data from the literature. The predictions of the proposed model fits fairly well the experimental data and shows improvements to estimate the magnitude of the effective excess charge density over the previous models. This new model proposes a simple and efficient way to model the streaming current generation for partially saturated porous media.

Modeling and simulation of DC and microwave characteristics of AlInN(AlGaN)/AlN/GaN MOSHEMTs with different gate lengths

AbstractIn this paper, direct current (DC) and microwave characteristics are modeled and analyzed for AlInN(AlGaN)/AlN/GaN metal‐oxide‐semiconductor high electron mobility transistors with different gate lengths. The 2‐dimensional electron gas sheet charge density (ns) model is developed by considering polarization effect and flat‐band voltage with lowest occupied energy level (E0) in the quantum well. The drain current, transconductance, and gate charge model is developed by incorporating approximation of ns as proposed in this paper. Then, the cut‐off frequency is calculated from the transconductance and gate capacitance model. The analytical model results are in good agreement with Technology Computer Aided Design simulation and experimental results from literatures for DC and microwave characteristics for the proposed gate lengths (0.1, 0.2, and 0.3 μm). From these characteristics, it is also observed that AlInN/AlN/GaN MOSHEMT shows superior DC and radio frequency performance as compared with AlGaN/AlN/GaN MOSHEMT. AlInN/AlN/GaN MOSHEMT produces high drain current (2.34 A/mm) and high cut‐off frequency (124 GHz) at smaller gate length.

A method to estimate statistical errors of properties derived from charge-density modelling.

Estimating uncertainties of property values derived from a charge-density model is not straightforward. A methodology, based on calculation of sample standard deviations (SSD) of properties using randomly deviating charge-density models, is proposed with the MoPro software. The parameter shifts applied in the deviating models are generated in order to respect the variance-covariance matrix issued from the least-squares refinement. This `SSD methodology' procedure can be applied to estimate uncertainties of any property related to a charge-density model obtained by least-squares fitting. This includes topological properties such as critical point coordinates, electron density, Laplacian and ellipticity at critical points and charges integrated over atomic basins. Errors on electrostatic potentials and interaction energies are also available now through this procedure. The method is exemplified with the charge density of compound (E)-5-phenylpent-1-enylboronic acid, refined at 0.45 Å resolution. The procedure is implemented in the freely available MoPro program dedicated to charge-density refinement and modelling.

Corotation Plasma Environment Model: An Empirical Probability Model of the Jovian Magnetosphere

We developed a new empirical model for corotating plasma in the Jovian magnetosphere. The model, named the corotation plasma environment model version 2 (CPEMv2), considers the charge density, velocity vector, and ion temperature based on Galileo/plasma system (PLS) ion data. In addition, we develop hot electron temperature and density models based on Galileo/PLS electron data. All of the models provide respective quantities in the magnetic equator plane of 9–30 $\mathrm {R}_{J}$ , while the charge density model can be extended to 3-D space. A characteristic feature of the CPEM is its support of the percentile as a user input. This feature enables us to model extreme conditions in addition to normal states. In this paper, we review the foundations of the new empirical model, present a general derivation algorithm, and offer a detailed formulation of each parameter of the CPEMv2. As all CPEM parameters are of the analytical form, their implementation is straightforward, and execution involves the use of a small number of computational resources. The CPEM is flexible; for example, it can be extended, as new data (from observations or simulation results) become available. The CPEM can be used for the mission operation of the European Space Agency’s mission to Jupiter, JUpiter ICy moons Explorer (JUICE), and for future data analyses.

Self-assembly mechanism based on charge density topological interaction energies

The packing interactions have been evaluated in the context of the self-assembly mechanism of crystal growth and also for its impacts on the aromaticity of the trimesate anion. The structure of ethylammonium trimesate hydrate (1) measured at 100 K and a charge density model, derived in part from theoretical structures, is reported. Theoretical structure factors were obtained from the geometry-optimized periodic wave function. The trimesic acid portion of 1 is fully deprotonated and participates in a variety hydrogen bonding motifs. Topological analysis of the charge density model reveals the most significant packing interactions and is then compared to a complementary analysis performed by the Hirshfeld surface method. The results presented herein demonstrate that in organic salt crystals the small structural motifs are most stable and once formed as stand-alone structures, may direct the self-assembly process. Moreover, when intermolecular interactions supported by the electrostatic forces are analyzed, the care must be taken with interpretation of the results of Hirshfeld surface analysis for organic salts crystals.

Analytical Model Development for Unified 2D Electron Gas Sheet Charge Density of AlInN/GaN MOSHEMT

Abstract We have developed a unified analytical model for computation of 2D electron gas sheet charge density in AlInN/GaN metal-oxide-semiconductor high electron mobility transistor device structure. This model has been developed by incorporating the variation in lowest three energy sub-bands and Fermi level energy in the quantum-well with respect to gate voltage. We noticed that the dependency of lowest sub-band energy with Fermi energy having two fields, which are the lowest sub-band energy is greater and lesser than the Fermi level energy. According to these two fields, we have developed the fermi energy and sheet charge density expressions in each field. By combining each field of the models, developed a unified 2D electron gas sheet charge density model. The Fermi level and sheet charge density are interdependent in the model development. The developed model results are compared with TCAD simulation results and obtain a good consistency between them. This model is fitted to other metal-oxide-semiconductor high electron mobility transistor devices also with modifications in related physical values.

Multi-temperature study of potassium uridine-5'-monophosphate: electron density distribution and anharmonic motion modelling.

Uridine, a nucleoside formed of a uracil fragment attached to a ribose ring via a β-N1-glycosidic bond, is one of the four basic components of ribonucleic acid. Here a new anhydrous structure and experimental charge density distribution analysis of a uridine-5'-monophosphate potassium salt, K(UMPH), is reported. The studied case constitutes the very first structure of a 5'-nucleotide potassium salt according to the Cambridge Structural Database. The excellent crystal quality allowed the collection of charge density data at various temperatures, i.e. 10, 100, 200 and 300 K on one single crystal. Crystal structure and charge density data were analysed thoroughly in the context of related literature-reported examples. Detailed analysis of the charge density distribution revealed elevated anharmonic motion of part of the uracil ring moiety relatively weakly interacting with the neighbouring species. The effect was manifested by alternate positive and negative residual density patterns observed for these atoms, which `disappear' at low temperature. It also occurred that the potassium cation, quite uniformly coordinated by seven O atoms from all molecular fragments of the UMPH- anion, including the O atom from the ribofuranose ring, can be treated as spherical in the charge density model which was supported by theoretical calculations. Apart from the predominant electrostatic interactions, four relatively strong hydrogen bond types further support the stability of the crystal structure. This results in a compact and quite uniform structure (in all directions) of the studied crystal, as opposed to similar cases with layered architecture reported in the literature.

Compact Modeling of Gate Capacitance in III–V Channel Quadruple-Gate FETs

In this paper, we present a compact model for charge density and gate capacitance for low effective mass channel material based quadruple-gate FETs (QGFETs). The proposed model accounts for the effect of quantum capacitance and conduction band non-parabolicity, which are important in FETs comprised of the low effective mass channel material. In modeling of QGFET, we propose and use a new form of Fermi–Dirac integral of order $-$ 1/2, which matches closely with the numerical data. Our model for the charge density and gate capacitance is compared with three-dimensional technology computer aided design (TCAD) simulations data, and shows excellent match. The proposed explicit compact model can be easily employed in efficient exploration of circuits based on the low effective mass QGFET nanowires.

Asymmetry of current-voltage characteristics of ion-exchange membranes: Model of charge density of fixed groups linear by membrane thickness

The model of a finely porous membrane with a linear distribution of charges of its fixed groups along its depth is used in the work to explain the phenomenon of asymmetry of current-voltage characteristics of perfluorinated MF-4SK membranes surface–modified by polyaniline or halloysite nanotubes. A new exact analytical method of solution of a system of transport equations is suggested that is based on the assumption of the monotonous behavior of the electric potential inside the membrane and allows finding current- voltage characteristics (CVCs) of the membrane using a system of two implicit algebraic equations. Algebraic equations for limiting currents are obtained for the first time for different orientations of the anisotropic membrane in an electrodyalisis cell and existence of two inflection points in CVCs is explained when its less charge side is oriented towards the anode.

The effect of dielectric constant and work function on triboelectric nanogenerators: Analytical and numerical study

In this study, an analytical model of triboelectric charge density and transferred charge density for simple contact-mode triboelectric nanogenerators (TENG) is constructed. The model comprises the characteristics of triboelectric materials, such as dielectric constants and work function, and structural parameters such as the thickness of the dielectric material and the separation distance between the metal and dielectric. The model presented in this paper is the analytical interpretation of the metal to dielectric contact-mode TENG and is intended to serve as a guideline for the design of TENGs in various applications.