vdW Surface Research Articles

Exploration of adsorption mechanism of 2-phosphonobutane-1,2,4-tricarboxylic acid onto kaolinite and montmorillonite via batch experiment and theoretical studies

Two clay minerals, kaolinite (Kaol) and montmorillonite (Mt) with different crystal structures were chosen to investigate the comparative adsorption of 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC) through batch control experiments and theoretical studies. The systematical isotherm and kinetic studies agreed with Langmuir model and pseudo-second-order model, confirming a monolayer and chemisorption interaction process, respectively. The maximum removal capacities of Kaol and Mt for PBTC were 72.297 mg/g and 121.163 mg/g at pH=3.0 and T=298 K, respectively. Furthermore, the adsorption mechanisms were investigated by molecular dynamic (MD) simulations and density functional theory (DFT). The Interface force field (IFF) was firstly introduced into Materials Studio package to explore the microscopic mechanism of clay mineral interface. The dynamics behaviors verified that the oxygen (O) atom of carboxyl group has stronger affinity at the external surface of Mt, which consistent with the experimental data well. For DFT calculations, quantitative analysis around molecular van der Waals (vdW) surface was adopted to predict reactive sites for the electrophilic reaction. Independent Gradient Model (IGM) and Hirshfeld surface analyses in Multiwfn indicated that the high adsorption effect mainly attributes to hydrogen bond action. These findings improve our ability to explore the related properties occurring at the interface of different clay minerals.

Pressure-viscosity relation of 2,2,4-trimethylhexane predicted using all-atom TEAM force field

The pressure-dependent viscosity of 2,2,4-trimethylhexane is predicted using all-atom TEAM force field and the non-equilibrium periodic perturbation (PP) method. The viscosities predicted using the PP method do not show Newtonian plateau in the precision range of prediction for high-viscosity fluids. Using four alkane molecules as test cases, we found that our previously published force field, developed for ambient pressure conditions, is inaccurate for predicting viscosities under elevated pressures. Two approaches were attempted to enhance the pressure transferability of the force field: one involves moving the non-bond interaction site of hydrogen closer to that of attached carbon, which smooths the van der Waals (vdW) surface of molecule, and the other involves replacing the LJ-12-6 function by LJ-9-6, making the repulsion softer. Both approaches enhanced the pressure transferability; however, the former works well below 500 MPa, while the latter is valid under pressures up to 1 GPa.

Organic Compounds of Actinyls: Systematic Computational Assessment of Structural and Topological Properties in [AnO2(C2O4) n](2 n-2)- (An = U, Np, Pu, Am; n = 1-3) Complexes.

Exploring the bonding features between organics and actinide elements is a fundamental topic in nuclear waste separation. In this work, [AnO2(C2O4) n](2 n-2)- (An = U, Np, Pu, and Am; n = 1-3) complexes have been characterized by density functional theory. The actinyl oxalate complexes are found to exhibit the typical An-Oyl, An-Oeq bonds and Oyl-An-Oyl angles. Interatomic interaction analyzed by electron density difference, charge decomposition analysis, charges population, bond order, electron localization function, and quantum theory of atom in molecules indicates that An-Oeq bonds are ionic (closed-shell) bonding interaction with a small degree of covalent character. The similarities and differences between isomers have been discussed in the actinide coordination chemistry, and the orbital interactions also have been investigated through total, partial, and overlap population density of state diagrams. Besides, the electrostatic potential was used to predict the adsorption sites on the molecular vdW surface.

Highly crystalline synthesis of tellurene sheets on two-dimensional surfaces: Control over helical chain direction of tellurene

Recent results have identified two-dimensional (2D) tellurene as a potential van der Waals (vdW) material for thermoelectric and optoelectronics applications owing to their pseudo-one-dimensional (anisotropic) behavior and structure. While hydrothermal synthesis is suitable for yielding potentially crystalline materials, it is well known for its incompatibility with manufacturing and scaling. There is an urgent need for synthesis of tellurene sheets in vdW layered form with the desired crystallinity as well as controlled crystalline anisotropy using more conventional and scalable processes. Here, we report on the synthesis of tellurene sheets onto a variety of surfaces of vdW-bonded 2D layered crystals in their trigonal layered form with controlled tellurene chain (anisotropy) direction using horizontal physical vapor deposition method. Our systematic tellurene growth studies on six different vdW surfaces show that GaS and GaSe surfaces enable highly crystalline and self-oriented tellurene sheets. Detailed cross-sectional transmission electron microscopy (TEM), angle-resolved Raman spectroscopy, scanning electron microscopy, and energy-dispersive spectroscopy measurements provide fundamental insights into the growth dynamics and characteristics of tellurene. First-principles calculations and cross-sectional TEM measurements suggest that tellurene chains are well-oriented along the GaSe armchair lattice direction owing to the much-reduced total energy of the system and a stronger degree of coupling across adjacent layers. Synthesized tellurene sheets exhibit remarkable structural anisotropy, as evidenced by angle-resolved Raman measurements, and the overall results open a clear pathway for layer-by-layer growth of tellurene sheets with controlled crystalline anisotropy using conventional synthesis techniques.

Synthesis, spectral characterization and X-ray crystal structure studies of 3-(benzo[d][1,3]dioxol-5-yl)-5-(3-methylthiophen-2-yl)-4,5-dihydro-1H-pyrazole-1-carboxamide: Hirshfeld surface, DFT and thermal analysis

A novel pyrazole derivative, 3-(benzo[d][1,3]dioxol-5-yl)-5-(3-methylthiophen-2-yl)-4,5-dihydro-1H-pyrazole-1-carboxamide was synthesized and characterized by elemental analysis, FT-IR, NMR (1H and 13C), MS, UV–visible spectra and finally the structure was confirmed by the single crystal X-ray diffraction studies. The title compound (C16H15N3O3S) crystallized in the triclinic crystal system, with the space group Pī. A dihedral angle of 65.84(1)° between the pyrazole and the thiophene rings confirms the twisted conformation between them. The X-ray structure revealed that the pyrazole ring adopts an E-form and an envelope conformation on C7 atom. The crystal and molecular structure of the title compound is stabilized by inter molecular hydrogen bonds. The compound possesses three dimensional supramolecular self-assembly, in which CH⋯O and NH⋯O chains build up two dimensional arrays, which are extended to 3D network through CH···Cg and CO···Cg interactions. The structure also exhibits intramolecular hydrogen bonds of the type NH⋯N and π···π stacking interactions, which contributes to the crystal packing. Further, Hirshfeld surface analysis was carried out for the graphical visualization of several short intermolecular interactions on the molecular surface while the 2D finger-print plot provides percentage contribution of each individual atom-to-atom interactions. The thermal decomposition of the compound has been studied by thermogravimetric analysis. The molecular geometries and electronic structures of the compounds were fully optimized, calculated with ab-initio methods by HF, DFT/B3LYP functional in combination of different basis set with different solvent environment and the structural parameters were compared with the experimental data. The Mulliken atomic charges and molecular electrostatic potential on molecular van der Waals (vdW) surface were calculated to know the electrophilic and nucleophilic regions of the molecular surface. Nonlinear optical properties of the title compound were also discussed based on the polarizability and hyperpolarizability values.

Substructure-activity relationship studies on antibody recognition for phenylurea compounds using competitive immunoassay and computational chemistry

Based on the structural features of fluometuron, an immunizing hapten was synthesized and conjugated to bovine serum albumin as an immunogen to prepare a polyclonal antibody. However, the resultant antibody indicated cross-reactivity with 6 structurally similar phenylurea herbicides, with binding activities (expressed by IC50 values) ranging from 1.67 µg/L to 42.71 µg/L. All 6 phenylurea herbicides contain a common moiety and three different substitutes. To understand how these three different chemical groups affect the antibody-phenylurea recognition activity, quantum chemistry, using density function theory (DFT) at the B3LYP/6-311++ G(d,p) level of theory, was employed to optimize all phenylurea structures, followed by determination of the 3D conformations of these molecules, pharmacophore analysis, and molecular electrostatic potential (ESP) analysis. The molecular modeling results confirmed that the geometry configuration, pharmacophore features and electron distribution in the substituents were related to the antibody binding activity. Spearman correlation analysis further elucidated that the geometrical and electrostatic properties on the van der Waals (vdW) surface of the substituents played a critical role in the antibody-phenylurea recognition process.

Understanding of the conformational flexibility and electrostatic properties of coumarin derivatives in the active site of S. cerevisiae α-glucosidase

This study has been carried out to understand the nature of conformational flexibility and electrostatic properties of polyhydroxyl coumarins derivatives. When these compounds present in the active site of S. cerevisiae α-glucosidase, the lactone rings of the molecules are flanking out, while all benzene rings are embedded deep inside the binding cavity. These hydroxyl groups can interact with the surrounding amino acids by hydrogen bonds easily. When the hydroxyl groups at the C9 of benzene ring are replaced by methoxy groups, there is no evident influence on the hydrogen bonding interactions with the surrounding amino acid Asp68 and Lys155. However, their Laplacian values of electron densities of hydroxyl O–H bonds are obviously decreased in the active site, which suggests concentrated electron densities. In general, most of the electron densities of chemical bonds become more depleted after docking with the S. cerevisiae α-glucosidase, implying strong interactions with the surrounding amino acids. For polyhydroxyl coumarin derivatives, the global maximum values of the molecular electrostatic potential on molecular vdW surfaces stem from hydrogen atoms of the hydroxyl groups. However, the values are decreased evidently and stem from the different atoms in both phases while methoxy group is introduced. These fine details at electronic level allow to better understand the exact interactions between natural coumarins derivatives and target protein.

Ph(R)IF⋯HF (R = Me, Et, iPr, tBu) interaction: A strong hydrogen bond between hypervalent iodine compounds and HF

For the first time, the intermolecular hydrogen bonding interactions between polyvalent iodine compounds and hydrogen fluoride (HF) have been studied using M06-2X/SDD-6-311++G(d, p) method. The computational results show that, compared with iodine(I) compound IF, both the iodine(III) compound Ph(Et)IF and the iodine(V) compound Ph(Et)3IF have strong hydrogen bonding interactions with HF and the binding energies are about -21.00kcal/mol. Geometry analysis, natural bond orbital (NBO) analysis, energy decomposition analysis, electrostatic potential (ESP) analysis on molecular vdW surface and topological analysis for the bond critical points (BCPs) suggest that these strong interactions are not only electrostatic but also highly covalent in nature. In addition, our computational studies indicate that owing to the lower electronegativity of Cl and Br atoms relative to that of F atom, the compounds Ph(Et)ICl and Ph(Et)IBr have much weaker hydrogen bonding interactions with HF than Ph(Et)IF. Moreover, the electronic effects of substituents at the iodine center of the hypervalent iodine (III) compounds on the strength of the hydrogen bonds have also been studied and the results show that the alkyl groups such as methyl, ethyl, isopropyl and tert-butyl groups can enhance the hydrogen bonding interaction obviously relative to other types of substituents. And the hydrogen bond in complex Ph(iPr)IF⋯HF is very strong with binding energy of −22.44kcal/mol.

A comprehensive density functional theory study on molecular structures of (5, 5) carbon nanotube doped with B, N, Al, Si, P, Co, and Ni

Armchair single-walled carbon nanotubes (SWNTs), which were doped with B, N, Al, Si, P, Co, and Ni, have been studied using computational simulations based on density functional theory (DFT). The topological analysis and the electron localization function show that the nature of the interaction between carbon atoms and the dopant X atoms is not purely covalent or an ionic. In order to gain a deeper understanding of the interaction between X atoms and C atoms, calculations of natural bond orbital (NBO) analysis, bond order analysis, atomic charge analysis, and electrostatic potential (ESP) on the van der Waals (vdW) surfaces of molecules are required. The Natural population analysis (NPA), Hirshfeld and atomic dipole moment corrected Hirshfeld (ADCH) atomic charges and molecular electrostatic potential maps on vdW surfaces of C79H20Xs show that ESP values are in good agreement with ADCH values. The calculations of The Laplacian bond order (LBO), Mayer bond order (MBO) and Fuzzy bond order (FBO) illustrate that LBO has a strong correlation with the bond length. In addition, the calculations show that Fermi energies of the pristine C (5, 5) carbon nanotubes (CNTs) and all doped CNTs are equal to the energies of their highest occupied molecular orbital (HOMO). This work presents a comparison about the bonding characteristic between the doped atoms (X) and carbon atoms.

Relevance of the thermodynamic ERAS model for liquid mixtures with components exhibiting a small degree of self-association

The thermodynamic extended real associated solution (ERAS) model is reviewed and modified to accommodate weakly associating components. ERAS is a theoretical scheme applied to binary liquid mixtures whereby certain physically meaningful parameters such as the association equilibrium constants, species reduced volume and energy terms may be determined by optimization against the experimental excess molar volumes and enthalpies. The implicit assumption used for any non-associating molecule B is to routinely set the hydrogen-bonding energy ΔhB*, reaction volume ΔvB* and association constants KB to zero. It is proven here that such assignments lead to singularities in the ERAS parameters that are featured in the excess state functions used. A series of optimizations using the results of conductor-like screening model for real solvents (COSMO-RS) on the excess enthalpy HE of dimethoxymethane in binary mixtures with a series of alcohols are determined, and the parameters ΔhB*, ΔhAB* (mixed enthalpy of reaction) and XAB (interaction parameter) are determined. It is shown that the stipulated range of XAB values from ERAS theory requires minor modification if the above assumptions are not made. The magnitude of the computed variables are shown to be physically reasonable. The Bondi group averaged estimates for si, the ratio of the van der Waals (vdW) surface area to the vdW volume for molecule i, are contrasted against the more specific and reliable derivations from quantum calculations when calculating the excess functions. The degree of correlation is improved using the latter si values. We conclude that the results of the optimizations indicate that the ERAS model can be extended even to weakly self-associating components previously considered impossible in especially engineering thermodynamical applications, with the above modifications and refinements.

A New DelPhi Feature for Modeling Electrostatic Potential around Proteins: Role of Bound Ions and Implications for Zeta-Potential.

A new feature of the popular software DelPhi is developed and reported, allowing for computing the surface averaged electrostatic potential (SAEP) of macromolecules. The user is given the option to specify the distance from the van der Waals surface where the electrostatic potential will be outputted. In conjunction with DelPhiPKa and the BION server, the user can adjust the charges of titratable groups according to specific pH values, and add explicit ions bound to the macromolecular surface. This approach is applied to a set of four proteins with "experimentally" delivered zeta (ζ)-potentials at different pH values and salt concentrations. It has been demonstrated that the protocol is capable of predicting ζ-potentials in the case of proteins with relatively large net charges. This protocol has been less successful for proteins with low net charges. The work demonstrates that in the case of proteins with large net charges, the electrostatic potential should be collected at distances about 4 Å away from the vdW surface and explicit ions should be added at a binding energy cutoff larger than 1-2kT, in order to accurately predict ζ-potentials. The low salt conditions substantiate this effect of ions on SAEP.

Synthesis, Crystal Structure, and DFT Study of Ethyl 1-(2-(Hydroxyimino)-2-phenylethyl)-3-phenyl-1H-pyrazole-5-carboxylate

The crystal structure of ethyl 1-(2-(hydroxyimino)-2-phenylethyl)-3-phenyl-1H-pyrazole-5-carboxylate has been determined by X-ray single crystal diffraction. The crystal of the title compound is the monoclinic space group P2/c with unit cell parameters of a=8.634(2) Å, b=9.616(2) Å, c=22.190(3) Å, β=99.2652°, V=1818.3(4) Å3, and Z=4. The dihedral angles formed by the planes of the central pyrazole ring and the adjacent benzene rings are 73.60(7)° and 3.55(7)°, respectively. The combination of the weak intermolecular C-H⋯O and N-H⋯O hydrogen-bonding interactions stabilizes the crystal packing. The geometries of its Z and E isomers and the corresponding transition state (TS), as well as the dimer of its Z isomer, are optimized using the B3LYP hybrid functional coupled with def-TZVP triple-zeta polarized basis set. The bond angles and bond lengths of the optimized structure of Z dimer are very consistent with those of its single crystal parameters. Double-hybrid functional PWPB95-D3 in combination with very highly accurate basis set def2-QZVP is employed to evaluate accurate energy of each isomer and TS. The calculated equilibrium constant between Z and E isomers corresponds to the [Z]/[E] ratio of 4.29. Mulliken atomic charges and electrostatic potential (ESP) on molecular van der Waals (vdW) surface are calculated in order to study and predict the intermolecular interactions. The molecular total energies and frontier orbital analysis are also discussed.

Comparison of the directionality of the halogen, hydrogen, and lithium bonds between HOOOH and XF (X = Cl, Br, H, Li)

Detailed electrostatic potential (ESP) analyses were performed to compare the directionality of halogen bonds with those of hydrogen bonds and lithium bonds. To do this, the interactions of HOOOH with the molecules XF (X = Cl, Br, H, Li) were investigated. For each molecule, the percentage of the van der Waals (vdW) molecular surface that intersected with the ESP surface was used to roughly quantify the directionality of the halogen/hydrogen/lithium bond associated with the molecule. The size of the region of intersection was found to increase in the following order: ClF < BrF < HF < LiF. The maximum ESP in the region of intersection, V S, max, was observed to become more positive according to the sequence ClF < BrF < HF < LiF. For ClF and BrF, the positive electrostatic potential was concentrated in a very small region of the vdW molecular surface. On the other hand, for HF and LiF, the positive electrostatic potential was more diffusely scattered across the vdW surface than for ClF and BrF. Also, the optimized geometries of the dipolymers HOOOH··· XF (X = Cl, Br, H, Li) indicated that halogen bonds are more directional than hydrogen bonds and lithium bonds, consistent with the results of ESP analyses.

Parameterization for molecular Gaussian surface and a comparison study of surface mesh generation.

The molecular Gaussian surface has been frequently used in the field of molecular modeling and simulation. Typically, the Gaussian surface is defined using two controlling parameters; the decay rate and isovalue. Currently, there is a lack of studies in which a systematic approach in the determination of optimal parameterization according to the geometric features has been done. In this paper, surface area, volume enclosed by the surface and Hausdorff distance are used as three criteria for the parameterization to make the Gaussian surface approximate the solvent excluded surface (SES) well. For each of these three criteria, a search of the parameter space is carried out in order to determine the optimal parameter values. The resulted parameters are close to each other and result in similar calculated molecular properties. Approximation of the VDW surface is also done by analyzing the explicit expressions of the Gaussian surface and VDW surface, which analysis and parameters can be similarly applied to the solvent accessible surface (SAS) due to its geometric similarity to the VDW surface. Once the optimal parameters are obtained, we compare the performance of our Gaussian surface generation software TMSmesh with other commonly used software programs, focusing primarily on mesh quality and fidelity. Additionally, the Poisson-Boltzmann solvation energies based on the surface meshes generated by TMSmesh and those generated by other software programs are calculated and compared for a set of molecules with different sizes. The results of these comparisons validate both the accuracy and the applicability of the parameterized Gaussian surface.

Quantitative relationships for the prediction of the vapor pressure of some hydrocarbons from the van der Waals molecular surface

A quantitative structure - property relationship (QSPR) modeling of vapor pressure at 298.15 K, expressed as log (VP / Pa) was performed for a series of 84 hydrocarbons (63 alkanes and 21 cycloalkanes) using the van der Waals (vdW) surface area, SW/Å2, calculated by the Monte Carlo method, as the molecular descriptor. The QSPR model developed from the subset of 63 alkanes (C1-C16), deemed as the training set, was successfully used for the prediction of the log (VP / Pa) values of the 21 cycloalkanes, which was the external prediction (test) subset. A QSPR model was also developed for a series composed of all 84 hydrocarbons. Both QSPR models were statistically tested for their ability to fit the data and for prediction. The results showed that the vdW molecular surface used as molecular descriptor (MD) explains the variance of the majority of the log (VP / Pa) values in this series of 84 hydrocarbons. This MD describes very well the intermolecular forces that hold neutral molecules together. The clear physical meaning of the molecular surface values, SW/Å2, could explain the success of the QSPR models obtained with a single structural molecular descriptor.

Study on the droplet impact on hydrophobic surface in terms of van der Waals surface tension model

Research on the droplet impact on a hydrophobic surface is of important theoretical significance and engineering value, both in mesoscopic fluid mechanics and interactions between microfluid and special materials. The van der Waals (vdW) equation of state relates the pressure to the temperature and the density of the fluid, and gives the long-range attractive force and short-range repulsive force between particles. The van der Waals equation of state can be used to describe the surface tension between liquid and vapor. As a pure meshless particle method, the smoothed particle hydrodynamic (SPH) method can use the vdW equation of state written in SPH form of N-S equations to describe the surface tension. The vdW surface tension mode is validated by simulating the coalescence of two equally sized static droplets in vacuum. Repellant of the hydrophobic surface is derived from a core potential. By combining the vdW surface tension and the repulsive force of the surface, the phenomenon of a liquid droplet impact with a certain initial velocity on the hydrophobic surface is studied. The SPH model is not only capable to describe the spreading of the droplet after it contacts the surface, but also clearly reproduces the springback, bouncing and secondary impact of the droplet. During the deformation of the droplet, the inertia force impels the spreading process of the droplet whilst the springback and bouncing behavior is dominated by the surface tension. The simulated results are in good agreement with the published experimental observations and VOF simulated results, indicating that the way we treat the surface tension and the repulsive force of the hydrophobic surface is effective and applicable in droplet impact surface problems. The impact velocity and liquid viscosity are considered to be two important factors that affect the deformation of the droplet after it contacts the surface. By varying the impact velocity within a certain range it is concluded that the maximum liquid-solid contact area increases as the impact velocity grows, and the bounced droplet will leave the surface when the velocity is big enough. Another comparison between different liquid viscosities shows that the maximum contact area decreases as the liquid viscosity increases because of the viscous dissipation, and the droplet barely rebound when the liquid viscosity is big enough.

Architectured van der Waals epitaxy of ZnO nanostructures on hexagonal BN

Heteroepitaxy of semiconductors on two-dimensional (2-d) atomic layered materials enables the use of flexible and transferable inorganic electronic and optoelectronic devices in various applications. Herein, we report the shape- and morphology-controlled van der Waals (vdW) epitaxy of ZnO nanostructures on hexagonal boron nitride (hBN) insulating layers for an architectured semiconductor integration on the 2-d layered materials. The vdW surface feature of the 2-d nanomaterials, because of the surface free of dangling bonds, typically results in low-density random nucleation–growth in the vdW epitaxy. The difficulty in controlling the nucleation sites was resolved by artificially formed atomic ledges prepared on hBN substrates, which promoted the preferential vdW nucleation–growth of ZnO specifically along the designed ledges. Electron microscopy revealed crystallographically domain-aligned incommensurate vdW heteroepitaxial relationships, even though ZnO/hBN is highly lattice-mismatched. First-principles theoretical calculations confirmed the weakly bound, noncovalent binding feature of the ZnO/hBN heterostructure. Electrical characterizations of the ZnO nanowall networks grown on hBN revealed the excellent electrical insulation properties of hBN substrates. An ultraviolet photoconductor device using the vdW epitaxial ZnO nanowall networks/hBN heterostructure was further demonstrated as an example of hBN substrate-based device applications. The architectured heteroepitaxy of semiconductors on hBN is thus expected to create many other device arrays that can be integrated on a piece of substrate with good electrical insulation for use in individual device operation. Scientists have demonstrated a new approach for fabricating nanoarchitectured semiconductors on dielectric hexagonal boron nitride (BN). The Korean team's approach involves forming atomic ledges on hexagonal BN substrate to realize shape- and morphology-controlled van der Waals (vdW) epitaxy of ZnO nanostructures on hexagonal BN insulating layers. The combination of hexagonal BN and vdW epitaxy seems like a perfect match – hexagonal BN has highly suitable properties for use as insulating substrates in electronic and optoelectronic devices, while vdW epitaxy is well suited for fabricating high-quality epitaxial semiconductor/hexagonal BN heterostructures. However, it has been difficult to control nucleation sites in vdW epitaxy. The researchers' method overcomes this problem through promoted vdW nucleation growth of ZnO along the artificially formed ledges. The team anticipates that the approach could be extended to many other combinations of semiconductors and layered insulating (or dielectric) nanomaterials. Architectured ZnO nanostructures were grown by van der Waals (vdW) heteroepitaxy on hexagonal BN (hBN) layers with artificially patterned atomic ledges. Electron microscopic and theoretical computational analyses presented non-covalent epitaxial features of domain-aligned incommensurate ZnO/hBN heterostructure. The vdW epitaxial ZnO/hBN heterostructures exhibited excellent electrical insulation of hBN, and were applied to fabricate the ultraviolet photodetector devices, as an example of functional optoelectronic device applications.

Nanocomposite structures grown by inserting ionic salt RbNO3 into van der Waals gaps of III–VI compound layered semiconductors

The p-GaSe<RbNO3> and n-InSe<RbNO3> nanocomposite materials were prepared by inserting ionic salt RbNO3 from the liquid phase between layers of the layered crystals. Using this method a new technique will be available to fabricate nanocomposite structures with different morphologies. We have obtained self-organized structures that consist of a layered matrix and arrays of nanorings and nanowires formed from solid ionic salt RbNO3 nanocrystals on the atomically smooth van der Waals (0001) surfaces of layered semiconductor crystals. Atomic force microscopy, infrared spectroscopy, X-ray diffraction and impedance spectroscopy are used to characterize morphology, chemical composition, and structural and electrical properties of the grown materials. In this work, we study the role of structural properties in the electron/ionic carrier transport in the anisotropic nanocomposite structures. The enhancement in DC conductivity with respect to the pure high-resistance GaSe crystals observed for the nanocomposites is explained on the basis of the creation of nanoscale defects on the VDW surface. The maximum enhancement in DC conductivity was observed for GaSe<RbNO3> nanocomposites containing Ga2O3–RbNO3 branched nanowire networks.

Accuracy of continuum electrostatic calculations based on three common dielectric boundary definitions.

We investigate the influence of three common definitions of the solute/solvent dielectric boundary (DB) on the accuracy of the electrostatic solvation energy ΔGel computed within the Poisson Boltzmann and the generalized Born models of implicit solvation. The test structures include small molecules, peptides and small proteins; explicit solvent ΔGel are used as accuracy reference. For common atomic radii sets BONDI, PARSE (and ZAP9 for small molecules) the use of van der Waals (vdW) DB results, on average, in considerably larger errors in ΔGel than the molecular surface (MS) DB. The optimal probe radius ρw for which the MS DB yields the most accurate ΔGel varies considerably between structure types. The solvent accessible surface (SAS) DB becomes optimal at ρw ~ 0.2 Å (exact value is sensitive to the structure and atomic radii), at which point the average accuracy of ΔGel is comparable to that of the MS-based boundary. The geometric equivalence of SAS to vdW surface based on the same atomic radii uniformly increased by ρw gives the corresponding optimal vdW DB. For small molecules, the optimal vdW DB based on BONDI + 0.2 Å radii can yield ΔGel estimates at least as accurate as those based on the optimal MS DB. Also, in small molecules, pairwise charge-charge interactions computed with the optimal vdW DB are virtually equal to those computed with the MS DB, suggesting that in this case the two boundaries are practically equivalent by the electrostatic energy criteria. In structures other than small molecules, the optimal vdW and MS dielectric boundaries are not equivalent: the respective pairwise electrostatic interactions in the presence of solvent can differ by up to 5 kcal/mol for individual atomic pairs in small proteins, even when the total ΔGel are equal. For small proteins, the average decrease in pairwise electrostatic interactions resulting from the switch from optimal MS to optimal vdW DB definition can be mimicked within the MS DB definition by doubling of the solute dielectric constant. However, the use of the higher interior dielectric does not eliminate the large individual deviations between pairwise interactions computed within the two DB definitions. It is argued that while the MS based definition of the dielectric boundary is more physically correct in some types of practical calculations, the choice is not so clear in some other common scenarios.

MODELING THE INFLUENCE OF SURFACE EFFECT ON INSTABILITY OF NANO-CANTILEVER IN PRESENCE OF VAN DER WAALS FORCE

Surface effect often plays a significant role in the pull-in performance of nano-electromechanical systems (NEMS) but limited works have been conducted for taking this effect into account. Herein, the influence of surface effect has been investigated on instability behavior of cantilever nano-actuator in the presence of van der Waals force (vdW). Three different methods, i.e. an analytical modified Adomian decomposition (MAD), Lumped parameter model (LPM) and numerical solution have been applied to solve the governing equation of the system. The results demonstrate that surface effect reduces the pull-in voltage of the system. Moreover, surface energy causes the cantilever nano-actuator with the assigned parameter to deflect as a softer structure. It is found that while surface effect becomes important for low values of the cantilever nano-actuator thickness, vdW attraction is significant for low initial gap values. Surprisingly, the increase in the initial gap, enhances the contribution of surface effect in pull-in instability of the system while reduces the contribution of vdW attraction. Furthermore, the minimum initial gap and the detachment length of the cantilever nano-actuator that does not stick to the substrate due to vdW force and surface effect has been approximated. A good agreement has been observed between the values of instability parameters predicted via these three methods. Whilst compared to the instability voltage predicted by numerical solution, the pull-in voltage obtained by MAD series and LPM method is overestimated and underestimated, respectively.