Huckel Theory Research Articles

All-Graphene Planar Self-Switching MISFEDs, Metal-Insulator-Semiconductor Field-Effect Diodes

Graphene normally behaves as a semimetal because it lacks a bandgap, but when it is patterned into nanoribbons a bandgap can be introduced. By varying the width of these nanoribbons this band gap can be tuned from semiconducting to metallic. This property allows metallic and semiconducting regions within a single Graphene monolayer, which can be used in realising two-dimensional (2D) planar Metal-Insulator-Semiconductor field effect devices. Based on this concept, we present a new class of nano-scale planar devices named Graphene Self-Switching MISFEDs (Metal-Insulator-Semiconductor Field-Effect Diodes), in which Graphene is used as the metal and the semiconductor concurrently. The presented devices exhibit excellent current-voltage characteristics while occupying an ultra-small area with sub-10 nm dimensions and an ultimate thinness of a single atom. Quantum mechanical simulation results, based on the Extended Huckel method and Nonequilibrium Green's Function Formalism, show that a Graphene Self-Switching MISFED with a channel as short as 5 nm can achieve forward-to-reverse current rectification ratios exceeding 5000.

A Study on the Mechanism for H2 Dissociation on Au/TiO2 Catalysts

The mechanism of H2 dissociation on supported gold catalysts was investigated due to their wide applications in hydrogenation reactions. The analysis of Huckel theory suggests O2––H+–H––Au as a transition-state structure for H2 dissociation on a Au/TiO2 system because of strong O2––H+ interaction. Subsequent density-functional theory (DFT) calculations verified that the O2––H+–H––Au structure at the perimeter of Au/TiO2 displays the lowest reaction barrier for H2 dissociation among various surface sites of gold, TiO2, and Au/TiO2 systems. In addition, the dissociation barrier on this O2––H+–H––Au structure is dependent on the charge on oxygen; TiO2-surface oxygen, not directly connected to gold, with less electrons leads to a lower barrier, compared to perimeter oxygen connected to gold, and a reduced TiOx/Au system should be less active due to more electrons on oxygen, which can explain recent experimental results.

Asymmetrical Molecular Junctions with different Alligator Clips

This paper describes different molecular junctions with different alligator clips connected to gold electrodes at nanometre-scale. Our objective was to determine the charge transport characteristics of these junctions and determine the optimum alligator clip pair. We scrutinised anthracenedithiol (ADT), anthracenediamine (ADA) and anthracenethiolamine (ATA) and computed their conductance parameters through our simulations via Semi Emperical Extended Huckel Theory Approach. We aimed at finding that whether symmetrical (same alligator clips at each side) or asymmetrical (different alligator clips at each side) is the optimised approach to connect the anthracene molecule with gold electrodes and found that different alligator clips i.e. thiol and amine exhibited more conduction than that of same clip pair i.e. ADT and ADA. We further examined the effect of changing alligator clips positions from para to meta and found varied quantum interference effects. Peak to valley current ratios (PVR) for ADT was found to be maximum as 7.05 whereas meta ATA exhibited lowest peak to valley current ratio of 1.44. 

ChemInform Abstract: Solution Phase Thermodynamics of Strong Electrolytes Based on Ionic Concentrations, Hydration Numbers and Volumes of Dissolved Entities

AbstractReview: 54 refs.

Electron Transport Properties of Si-Based Nanowires with Substitutional Impurities

The electron transport properties of ultrathin Si nanowires with various substitutional atoms located at different positions are theoretically studied by using the nonequilibrium Green function combined with the extended Huckel theory. The results show that there exists a close correlation between the symmetry of the current–voltage curves and the position of substitutional atoms of these nanowires. Various patterns of negative differential resistance (NDR) appear, which are also related to the substitutional position in the nanowires. Moreover, the effect of gate electrodes is revealed. By analyzing the transmission spectra, we find that gate voltages change the position of energy levels of nanowires, and thereby modulate the current of nanowire devices. This work provides a deep insight into the effect of substitutional position on the electron transport properties of ultrathin Si nanowires.

Investigation of Terminal Group Effect on Electron Transport Through Open Molecular Structures

The effect of terminal groups on the electron transport through metal-molecule-metal system has been investigated using nonequilibrium Green's function (NEGF) formalism combined with extended Huckel theory (EHT). Au-molecule-Au junctions are constructed with borazine and BCN unit structure as core molecule and sulphur (S), oxygen (O), selenium (Se) and cyano-group (CN) as terminal groups. The electron transport characteristics of the borazine and BCN molecular systems are analyzed through the transmission spectra and the current-voltage curve. The results demonstrate that the terminal groups modifying the transport behaviors of these systems in a controlled way. Our result shows that, selenium is the best linker to couple borazine to Au electrode and oxygen is the best one to couple BCN to Au electrode. Furthermore, the results of borazine systems are compared with that of BCN molecular systems and are discussed. Simulation results show that the conductance through BCN molecular systems is four times larger than the borazine molecular systems. Negative differential resistance behavior is observed with borazine-CN system and the saturation feature appears in BCN systems.

Theoretical model for the polarization molecular and Hückel treatment of PhosphoCyclopentadiene in an external electric field: Hirschfeld study

In this study we relate the Huckel method and molecular polarization of PhosphoCyclopentadiene (P-cyclopentadiene) with respect to the Cyclopentadienyl, and its consequent separation of charges of particular set of conjugated diene systems. The Huckel method and Molecular Polarization of P-cyclopentadiene is expressed as a function of the induced polarizability of Cyclopentadienylin an external electric field, presenting a technique to express the molecular polarizabilities and Huckel method of diene systems as a function of another in an external electric field, using local quantum similarity index (LQSI) based on the Hirschfeld partitioning in the framework of conceptual density functional theory, this index was introduced in the molecular polarization of cyclopentadienyl in an external electric field and in the secular determinant of the Huckel method applied to the Cyclopentadienylin order to express the molecular polarization and Huckel method as a function of P-cyclopentadiene using six local similarity index: Overlap, Overlap-Interaction, Coulomb, Coulomb-Interaction, Overlap-Euclidian distance and Coulomb-Euclidian distance.The topo-geometrical superposition approach (TGSA) was used as method of alignment, which allows us to obtain high results in the proposed LQSIs, this method to be a straightforward procedure to cope with the problem of relative orientation of the molecules when evaluating, developing a new technique that will allow us to study structural systems that differ in one atom in its structure, and proposing methodologies for future studies on a much broader range of systems in which the Huckel method and molecular polarization of two species that differ only in one atom in its structure can be approximated in this way.

Temperature Effects on Electrophoresis

We present a model capturing the important contributors to the effects of temperature on the observable electrophoretic mobilities of small ions, and on solution conductivity and pH. Our temperature model includes relations for temperature-dependent viscosity, ionic strength corrections, degree of ionization (pK), and ion solvation effects on mobility. We incorporate thermophysical data for water viscosity, temperature-dependence of the Onsager-Fuoss model for finite ionic strength effects on mobility, temperature-dependence of the extended Debye-Huckel theory for correction of ionic activity, the Clarke-Glew approach and tabulated thermodynamic quantities of ionization reaction for acid dissociation constants as a function of temperature, and species-specific, empirically evaluated correction terms for temperature-dependence of Stokes' radii. We incorporated our model into a MATLAB-based simulation tool we named Simulation of Temperature Effects on ElectroPhoresis (STEEP). We validated our model using conductivity and pH measurements across a temperature variation of 25-70 °C for a set of electrolytes routinely used in electrophoresis. The model accurately captures electrolyte solution pH and conductivity, including important effects not captured by simple Walden-type relations.

Solution phase thermodynamics of strong electrolytes based on ionic concentrations, hydration numbers and volumes of dissolved entities

This article summarizes the successful restoration of Arrhenius’ theory of electrolytic dissociation founded 130 years ago. His idea of using the conductivity ratio for the degree of dissociation was a breakthrough in interpreting the properties of dilute solutions. For higher concentrations, as it was not satisfactory, attempts were being made to improve the methods of obtaining the degrees of dissociation. However, the latter was prematurely replaced by the empirical activity coefficient correction factors. Subsequently, the success of the Debye–Huckel (D–H) theory of inter-ionic interaction between free ions in explaining the properties of dilute solutions was taken to mean complete dissociation at all concentrations and interpretation of solution properties amounted to extending the D–H equations by the gradual addition of more parameters. At the end of six decades the equations became so complex that they failed to provide a unified interpretation of the causes of non-ideality at all concentrations and a molecular insight into the observed phenomena. Therefore, the present author abandoned them and started investigating the available experimental data as such. Using the degrees of dissociation and surface and bulk hydration numbers evaluated from solution vapour pressures, the author was able to interpret solution properties based on the idea due to Arrhenius a century earlier. Presented here is a comparison of the complicated equations based on complete dissociation with the simple equations obtained based on partial dissociation, hydration and volumes of entities in the solution. All the main equations are summarized in the table.

Using Extended Hückel Theory as a Platform To Introduce Jahn–Teller Distortion: The Spontaneous Distortion of 1,3,5,7-Cyclooctatetraene from a Perfect Octagon

Herein, a slightly enhanced version of extended Huckel molecular orbital theory is applied to demonstrate the spontaneous distortion of 1,3,5,7-cyclooctatetraene from a perfect octagon, a consequence of the Jahn-Teller effect. The exercise is accessible to students who have been introduced to basic quantum mechanics and extended Huckel molecular orbital theory and is easily implemented with widely available algebraic manipulation software such as Maple or Mathematica. This exercise is suitable for inclusion in the physical chemistry curriculum at the upper-level undergraduate or introductory graduate level.

Rationalisation and visualisation of non-bonded interactions

Models of non-bonded interactions are crucial in structure-based drug design. Standard hydrogen bonds are well modelled through traditional molecular mechanics forcefields with their treatments of electrostatics, and functional forms, often based on abundant crystal structure data, to describe their geometries. But interactions - for example, hydrogen bonds with carbon as the donor, or so-called halogen interactions - are not well handled, or not handled at all. Here, we describe the use of extended Huckel (E-Huckel) theory, a very fast, low-level MO theory, to evaluate the geometries and energies of such non-standard interactions. These include successful modelling of halogen interactions for Cl, Br and I, which each display the so-called sigma hole in QM calculations, and likewise interactions between sulphur and oxygen. The E-Huckel modelling has been implemented in the 2011.10 version of the MOE (Molecular Operating Environment) software system. A pharmacophore feature type has been created for halogen interactions; an example of its use in finding scaffold replacements for thyronines (thyroid hormones), which employ halogen interactions involving iodine (Figure ​(Figure1)1) [1], will be presented. Figure 1 Triiodothyronine (T3) in its receptor binding site. (PDB 3GWS).

Analysis of Graphene Nanoribbons Passivated with Gold, Copper and Indium

This study investigates the effect of edge- passivation on graphene nanoribbons. The geometry of graphene is simple and regular, and infinite, planar structure can easily be created either by hand or by taking a single layer from the crystal structure of graphene. To create a device-like structure, the infinite sheet must be cut into a suitable shape. Such a shape, at least for electronic applications, and it is called graphene nanoribbon (GNR). A pristine graphene monolayer can be cut into elongated strips to form 1D structure, referred to as graphene nanoribbons (GNRs) which can be either metallic or semiconducting depending on the type and width of edges. On the base of series of simulations it is found that elements from Ist, IIIrd and IV th group are used as passivated elements with Armchair and Zigzag nanoribbons instead of Hydrogen. Best characteristics for zigzag nanoribbons are presented by elements from Ist group. All experiments are made with Gold and Copper. For armchair nanoribbons, best characteristic are shown by elements from III rd group. The experiment is made with Indium. For nanoribbon with zigzag shaped edge is used DFT (Density Functional Theory) with LDA (Local Density Approximation). The chiral index of such nanoribbon is (3, 3). For the calculations of armchair nanoribbon is used Extended Huckel method. The chiral index of such nanoribbon is (3, 0). In both cases the k-point are set to 1 x 1 x 100 for na, nb and nc, respectively. For nanoribbons with zigzag shaped edges, DFT calculations show that edge-state bands at Fermi level (EF) rise to a very large Density of States (DOS) at EF, while Density of States of the armchair nanoribbons shows an energy gap around Fermi level. After Band Structure and Density of State, Bloch State is calculated and plot. Bloch States can be used to investigate the symmetry of certain bands and how this may be releated to the transport properties. Looking at the respective Bloch function, the wave function at G and Z are real and there is a distinct difference between valence and conduction band Bloch functions. These findings can be useful for the prospective GNR-based devices.

Oxidative ammonolysis of 3(4)-methyl- and 3,4-dimethylpyridines using vanadium oxide catalysts

Oxidative ammonolysis of 3(4)-methyl- and 3,4-dimethylpyridines using vanadium oxide catalyst doped with Cr2O3, SnO2, and ZrO2 was studied. The yields of nitriles and conversion of the starting compounds were found to depend on the CH-acidity of the latter in the gas phase. The possible mechanisms of the formation of pyridine-3,4-dicarboxylic acid imide at the oxidative ammonolysis of 3,4-dimethylpyridine was discussed. The relation between the activity of the modified catalysts and the proton affinity of the vanadyl oxygen calculated by the extended Huckel method was established.

Complexation of Tungsten(VI) with Methyliminodiacetic Acid at Different Ionic Strengths

The results of this research give ionic strength dependence patterns for the complexation of tungsten(VI) with methyliminodiacetic acid at T = 298 K. The formation data reported in this work were obtained at different ionic strengths (0.1 < I/mol·dm−3 < 1.0) of sodium perchlorate on the basis of Job’s continuous variation method, which show that a 1:1 complex forms at pH = 7.50. UV experimental data were collected for the calculation of the stability constants according to the extended Debye–Huckel theory, the specific ion interaction theory and the parabolic model. Finally, these three models have been compared.

Correlation between Fourier series expansion and Hückel orbital theory

The Fourier series can be used to describe periodic phenomena such as the one-dimensional crystal wave function. By the trigonometric treatements in Huckel theory it is shown that Huckel theory is a special case of Fourier series theory. Thus, the conjugated π system is in fact a periodic system. Therefore, it can be explained why such a simple theorem as Huckel theory can be so powerful in organic chemistry. Although it only considers the immediate neighboring interactions, it implicitly takes account of the periodicity in the complete picture where all the interactions are considered. Furthermore, the success of the trigonometric methods in Huckel theory is not accidental, as it based on the fact that Huckel theory is a specific example of the more general method of Fourier series expansion. It is also important for education purposes to expand a specific approach such as Huckel theory into a more general method such as Fourier series expansion.

Tight-binding model of Mn12single-molecule magnets: Electronic and magnetic structure and transport properties

We describe and analyze a tight-binding model of single molecule magnets (SMMs) that captures both the spin and spatial aspects of the SMM electronic structure. The model generalizes extended Huckel theory to include the effects of spin polarization and spin-orbit coupling. For neutral and negatively charged Mn12 SMMs with acetate or benzoate ligands the model yields the total SMM spin, the spins of the individual Mn ions, the magnetic easy axis orientation, the size of the magnetic anisotropy barrier and the size of the HOMO-LUMO gap consistent with experiment. For neutral molecules the predicted spins and spatial locations of the HOMO are consistent with the results of density functional calculations. For the total spin and location of the LUMO density functional theory-based calculations yield varied results while the present model yield results consistent with experiments on negatively charged molecules. For Mn12 SMMs with thiolate- and methylsulphide-terminated benzoate ligands (Mn12-Ph-Th) we find the HOMO to be located on the magnetic core of the molecule, but (unlike for the Mn12 SMMs that have previously been studied theoretically), we predict the LUMO and near LUMO orbitals of Mn12-Ph-Th to be located on ligands. Therefore we predict that for these Mn12 SMMs resonant and off-resonant coherent transport via near-LUMO orbitals, not subject to Coulomb blockade, should occur. We propose that this effect can be used to identify specific experimentally realized SMM transistors in which the easy axis and magnetic moment are approximately parallel to the direction of the current flow. We also predict effective spin filtering by these SMMs to occur at low bias whether the transport is mediated by the HOMO that is on the magnetic core of the SMM or by near LUMO orbitals located on the nominally non-magnetic ligands.

Comparison of the Debye–Hückel and the Mean Spherical Approximation Theories for Electrolyte Solutions

The thermodynamics of electrolyte solutions has been investigated by many scientists throughout the last century. While several theories have been presented, the most popular models for the electrostatic interactions are based on the Debye–Huckel and mean spherical approximation (MSA) theories. In this paper we investigate the differences between the Debye–Huckel and the MSA theories, and comparisons of the numerical results for the Helmholtz energy and its derivatives with respect to temperature, volume and composition are presented. The investigation shows that the nonrestricted primitive MSA theory performs similarly to Debye–Huckel, despite the differences in the derivation. We furthermore show that the static permittivity is a key parameter for both models and that in many cases it completely dominates the results obtained from the two models. Consequently, we conclude that the simpler Debye–Huckel theory may be used in connection with electrolyte equations of state without loss of accuracy.

Experimental evidence of the validity of Bahe–Varela theory to describe the volumetric properties of ionic liquids

Bahe–Varela (B–V) theory, based in the pseudo-lattice model, explains the thermodynamics of electrolyte solutions over the whole composition range. Thus, Bahe in 1972 extended the Debye–Huckel theory, developed for the most diluted electrolytes, to more concentrated solutions taking into account long range inter-ionic Coulombian forces. The introduction of long range interactions in the model generates naturally a pseudo-lattice arrangement of the ions in the concentrated liquid. As Bahe’s description fails at very concentrated solutions, in 1997 Varela and co-workers extended that pseudo-lattice theory by including the short range interactions with solvent molecules and other ions. In recent years, with the discovery of ionic liquids, B–V theory has attracted an increasing interest because these compounds can be seen as very concentrated electrolyte solutions (in fact they can be seen as “solutions” in the limit of no solvent!). Following this line, Turmine and co-workers in 2007 adapt the B–V theory to explain the volumetric properties of ionic liquid solutions up to saturation (i.e. the pure compound). They applied the equation extracted to fit the variation of the partial molar volume of three different aqueous solutions of IL with good results.In the present paper, we extend that analysis for systems formed by eight ILs and two solvents: water and ethanol. We have used density measurements for six aqueous mixtures recently published, two with 1-alkyl-3-methyl imidazolium tetrafluoroborate at T=298K and four with 1-ethyl-3-methyl imidazolium alkyl sulphate at (288, 298, 308 and 318)K. Also we analyze seven mixtures with ethanol of the two same IL families and for the same temperatures than the aqueous systems. From the original data, we calculated the partial molar volume of the IL for all concentrations, being its variation lower than 2% for the aqueous systems (except that with EMIM-BF4, where it is 5%) and lower than 4% for those with ethanol. In spite of the simplicity of the Bahe–Varela equation, with only three fitting parameters, and of the three different forms of the partial molar volume versus concentration behaviour observed, the fitting quality of the B–V equation is excellent for all analyzed systems, which tends to confirm its universal validity.

Formation energies of native point defects in II–VI crystals

The formation energies of point defects in II–VI crystals have been evaluated from Hall measurement results and calculated by a thermodynamic method and the extended Huckel method. The energies obtained by different techniques are in satisfactory agreement with each other and with data in the literature.

Electrical conduction along porphyrin wires using the self-consistent extended-Huckel and non-equilibrium Green’s function methods

ABSTRACTIn recent years, first-principle electronic structure calculations have been carried out to investigate such diverse phenomena as charge transport in molecular wires, optical properties of quantum structures and in photonics. However, at this time the prohibitive computational cost does not allow for such calculations to be easily carried out on nano-scale device structures comprising thousands of atoms. In addition, there are issues relating to the applicability of these approaches to describing the excitations that ought to be involved in charge transport.Self-consistent extended Huckel theory (SC-EHT) has proved very effective in describing the band alignment at semiconductor interfaces, and optical properties of partially covered surfaces, as well as being employed in studying the electronic states of large molecules. We have developed a non-equilibrium Greens function (NEGF) SC-EHT code that may be applied to study charge transport through molecular wires. We study the transmission of a porphyrin molecule attached via thiol linkers to gold electrodes, compare our results with those obtained from density functional theory (DFT). We have studied the influence the thiol position on the Au substrate has on the conduction and the dependence of the electron transmission on the molecular conformation. In addition, we also report on the results of some preliminary investigations studying the influence of water on the conduction pathways.