Huckel Theory Research Articles

Qualitative Insights into the Transport Properties of Hückel/Möbius (Anti)Aromatic Compounds: Application to Expanded Porphyrins

Expanded porphyrins have been recently identified as promising candidates for conductance switching based on aromaticity and molecular topology changes. However, the factors that control electron transport switching across the metal–molecule–metal junction still need to be elucidated. For this reason, the transport properties of Huckel/Mobius (anti)aromatic compounds are investigated thoroughly in this work to gain qualitative understanding into the conductivity of these unique macrocycles. Starting from a polyene model, a simple counting rule is developed to predict the occurrence of quantum interference around the Fermi level at the Huckel level of theory. Next, the different approximations of Huckel theory are lifted, enabling the exploration of the influence of each of these approximations on the transport properties of expanded porphyrins. Along the way, a detailed study on the relationship between the conductance and aromaticity/topology has been undertaken. Even though it has been proposed that the...

High-Sensitivity Sensor Using C60-Single Molecule Transistor

In this paper, we use the Extended Huckel Theory coupled with a non-equilibrium green function to calculate the transport proprieties of molecular single electron transistor (MSET). We present the effect of electrode materials on the transport properties in an MSET using a Fullerene (C60) molecule. The operation and performance of this MSET as a gas sensor are studied for the first time using a Fullerene as sensing material. The MSET sensors exhibited a change in the electric characteristic when the sensors were exposed to different gases. The favorable NH3 response characteristics of the MSET sensors were observed from the change in the drain-source current as a function of voltage, when the sensors were exposed to various NH3 concentrations ranging from 40 to 200 ppb. A satisfactory selectivity and remarkable high sensitivity have been found. The present results make our MSET as a very powerful candidate for use in gas sensing/measuring instrument, which allows the detection of toxic gas at low concentration.

Nano-colloid electrophoretic transport: Fully explicit modelling via dissipative particle dynamics

Abstract In present study, a novel fully explicit approach using dissipative particle dynamics (DPD) method is introduced for modelling electrophoretic transport of nano-colloids in an electrolyte solution. Slater type charge smearing function included in 3D Ewald summation method is employed to treat electrostatic interaction. Moreover, capability of different thermostats are challenged to control the system temperature and study the dynamic response of colloidal electrophoretic mobility under practical ranges of external electric field in nano scale application ( 0.072 E 0.361 v / n m ) covering non-linear response regime, and ionic salt concentration (0.049 S C 0.69 [M]) covering weak to strong Debye screening of the colloid. The effect of different colloidal repulsions are then studied on temperature, reduced mobility and zeta potential which is computed based on charge distribution within the spherical colloidal EDL. System temperature and electrophoretic mobility both show a direct and inverse relationship respectively with electric field and colloidal repulsion. Mobility declining with colloidal repulsion reaches a plateau which is a relatively constant value at each electrolyte salinity for A i i > 600 in DPD units regardless of electric field intensity. Nose–Hoover–Lowe–Andersen and Lowe-Andersen thermostats are found to function more effectively under high electric fields ( E > 0.145 [ v / n m ] ) while thermal equilibrium is maintained. Reasonable agreements are achieved by benchmarking the radial distribution function with available electrolyte structure modellings, as well as comparing reduced mobility against conventional Smoluchowski and Huckel theories, and numerical solution of Poisson-Boltzmann equation.

Synthesis and characterization of oligobenzimidazoles: Electrochemical, electrical, optical, thermal and rectification properties

A series of benzimidazole monomers, (2-(2, 4-dihydroxyphenyl)-1H-benzimidazol-5-yl)(phenyl) methanone (BIKH), 2-(3-ethoxy-2-hydroxyphenyl)-1H-benzo [d]imidazole-5-yl) (phenyl) methanone (BIKE) and 2-(5-bromo-2-hydroxyphenyl)-1H-benzo [d]imidazole-5-yl) (phenyl) methanone (BIKB) were prepared by condensing three substituted aromatic aldehydes with 3, 4-diaminobenzophenone. In aqueous alkaline medium the benzimidazoles were converted in to oligomers by oxidative polycondensation using NaOCl as oxidant. The formation of monomers and oligomers were confirmed with 1H, 13C NMR, FT-IR, and UV–visible spectroscopic techniques. The oligomers were investigated for their optical, electrical, electrochemical and thermal properties. The electrochemical and optical band gaps of monomers and oligomers were calculated using both UV–visible spectroscopy and cyclic voltametry respectively. The band gap values of monomers are compared with band gap values obtained from quantum theoretical calculations with DFT. The electrical conductivity studies of iodine doped and undoped oligomers were done using two point probe technique. It is found that these values are showing good correlation with the charge densities on imidazole nitrogen obtained from Huckel method. The conductivity of oligomers increases with increase in iodine vapour contact time. The dielectric properties of oligomers have been investigated at different temperature and frequency. The dielectric measurement data were used to calculate the AC conductivity and activation energy of oligomers. Oligomer OBIKH is having greater thermal stability due to its number of chain propagation sites than other oligomers and is shown by its high carbines residue of around 60% at 600 °C in thermogravimetric analysis. I–V characteristics of oligobenzimidazole p-n diodes have shown good rectifying nature in the range −4 to 4 V.

Adsorption Site of Gas Molecules on Defective Armchair Graphene Nanoribbon Formed Through Ion Bombardment

High sensitivity and selectivity is desired in sensing devices. The aim of this study is to investigate the use of the ion bombardment process in creating a defect on graphene nanoribbons (GNR), which significantly affects sensing properties, in particular adsorption energy, charge transfer and sensitivity. A process has been developed to form the defect on the GNR surface using molecular dynamic (MD) with a reactive force field with nitrogen ion. The sensing properties were calculated using the extended Huckel theory when oxygen (O2) and ammonia (NH3) molecules are exposed to different areas on the defective site. Through simulation, it was found that the ion bombardment process formed various types of defects on the GNR surface. Most notably, molecules adsorbed on the ripple area considerably improve the sensitivity by more than 50%. This indicates that the defect on the armchair graphene nanoribbon (AGNR) surface can be a method to enhance graphene-based sensing performance.

Optical, thermal and electrical properties of polybenzimidazoles derived from substituted benzimidazoles

Three benzimidazole monomers synthesized by condensing various substituted phenolic aldehydes with 4-methylphenylenediamine were converted in to polymers by oxidative polycondensation. The structure of the monomers and polymers were confirmed by various spectroscopic techniques. Electronic distribution of molecular frontier orbitals and optimized geometries of monomers were calculated by Gaussian 09 package. The spectral results showed that the repeating units are connected through both CC and COC linkages. Both polymers and monomers are showing good fluorescence emission in blue region. The electrical conductivity of I2 doped PBIs was measured using two point probe technique. The conductivities of PBIs were compared on the basis of the charge densities obtained from Huckel method on imidazole nitrogen which is involved in iodine coordination. The conductivity of polymers increases with increase in iodine vapour contact time. The dielectric properties of the synthesized polymers have been investigated at different temperature and frequency. Among the PBIs, PBIOP is having greater thermal stability and is shown by high carbines residues of around 50% at 500 °C in thermogravimetric analysis.

Electrical characterization of C28 fullerene junctions formed with group 1B metal electrodes

We present an atomistic theory of electronic transport through single molecular junctions based on smallest stable fullerene molecule, C28. The electronic properties of single molecular junctions critically depend on the nature of electrode material. The two probe device is modeled by constraining C28 between two semi-infinite metal electrodes, from group 1B of periodic table, copper, silver and gold. We have highlighted the correlated phenomena of resonant conduction and current driven dynamics in molecular junctions using extendend Huckel theory in combination with non equilibrium Green’s function framework. We conclude strong dependence of conductance on transmissions, which leads to oscillating conductance spectrum. An interesting interplay between conducting channels and different degrees of spatial localization and delocalization of molecular orbitals is evinced. The physical origin of current and conductance of so-formed C28 molecular junctions is discussed in detail by analysing their density of states, transmission spectra, molecular orbital analysis, rectification ratio and molecular projected self consistent Hamiltonian eigen states at different operating voltages ranging from -2V to +2V.

Oxidative polycondensation of benzimidazole using NaOCl: Synthesis, characterization, optical, thermal and electrical properties of polybenzimidazoles

A series of polybenzimidazole polymers, poly-2-(1H-benzo[d] imidazole-2-yl) phenol (PBIP2), poly-3-(1H-benzo[d] imidazole-2-yl) phenol (PBIP3) and poly-4-(1H-benzo[d] imidazole-2-yl) phenol (PBIP4) were synthesized by oxidative polycondensation of benzimidazole monomers 2-(1H-benzo [d] imidazole-2-yl) phenol (BIP2), 3-(1H-benzo [d] imidazole-2-yl) phenol (BIP3) and 4-(1H-benzo [d] imidazole-2-yl) phenol (BIP4). The structure of benzimidazoles monomers and polybenzimidazoles (PBI) were confirmed by various spectroscopic techniques. The quantum theoretical calculations of band gap energy values of monomers were done with DFT and are compared with its optical band gap energy values. Fluorescence spectra of these compounds showed maximum emission in blue region. The electrical conductivity of PBIs was measured by four-point probe technique and showed good electrical response on iodine doping and conductivity increases with increase iodine doping time. The differences in conductivities among the three PBIs are in accordance with the charge density on imidazole nitrogens calculated by Huckel method. The high carbines residue (∼40%) at 500 °C in thermo gravimetric analysis shows that the PBIs are having reasonably good thermal stability. Polymers have recorded high dielectric constant at low applied frequency of 50 Hz at 393 K. The I–V characteristics of polybenzimidazoles p-n diodes showed rectifying nature with a typical forward to reverse current in the range −4 to 4 V. The high n values are caused by non homogeneities and effect of series resistance.

Effect of doping on the transport properties of single-walled carbon nanotube two probe systems

This paper reports the doping induced negative differential resistance (NDR) in chromium substituted zig-zag (4, 0) single walled carbon nanotube (SWCNT) devices and the effect of the doping level on this property by using non-equilibrium Green's function theory formalism in combination with semi-empirical extended Huckel theory (EHT) calculations of the Atomistic Tool Kit software in device mode. The results show that the tunneling current, number of NDR peaks and the conductance increases with an increase in the level of doping. The high peak to valley current ratio (PVR) of the three proposed models are presented in the form of tables for comparative studies. The highest peak to valley current ratio of 5.70 is found in the two atom chromium doped SWCNT geometry. It is believed that the explored chromium doped CNT devices in this study will find number of applications in the future, particularly for fast switching and high speed signal processing.

Estimation of aspect ratio of cellulose nanocrystals by viscosity measurement: influence of surface charge density and NaCl concentration

Cellulose nanocrystals (CNCs) with similar size and various surface charge densities were prepared by sulfuric acid hydrolysis and NaOH desulfation. The influence of surface charge density and NaCl concentration on the intrinsic viscosity of CNC suspensions and predicted aspect ratio were investigated by Ubbelohde viscometer. With decreased CNC surface charge density, the intrinsic viscosity initially decreased due to the electric double layers on the CNC surface and subsequently increased due to CNC aggregation. To screen electroviscous effect, NaCl was added into CNC suspensions. With increased NaCl concentration, the intrinsic viscosity of CNC suspensions first decreased and then increased. The aspect ratios of CNCs predicted by Batchelor equation from the minimum intrinsic viscosity were consistent with that measured by transmission electron microscopy. Suspensions of CNCs with higher surface charge density needed less NaCl to obtain minimum intrinsic viscosity. The NaCl content that should be added to the suspension to predict the actual physical aspect ratio of CNC can be estimated by Debye–Huckel theory, assuming that the Debye length is equal to the CNC diameter.

A comparison study of electrode material effects on the molecular single electron transistor

In this paper, the studies of Molecular Single electron Transistor (MSET) is presented. We have used the Extended Huckel Theory (EHT) coupled with non-equilibrium green's function (NEGF) formalism which is implemented in ATOMISTIC TOOLKIT package. The transport properties with different electrode materials have been studied for the first time using pentacene molecule. The presence of Coulomb blockade on I–V characteristics confirmed that our device operates as Single Electron Transistor. The negative differential resistance (NDR) effect and the coulomb Staircase state has been outlined. The transmission Spectra and density of states (DOS) dependence of the electrode has also been explored to analyze the I-V curves. The NDR behavior and size device make our MSET as a good candidate for use in gate logic circuit with low consumption.

Dynamical Approach to Multi-Equilibria Problems Considering the Debye–Hückel Theory of Electrolyte Solutions: Concentration Quotients as a Function of Ionic Strength

We recently described a dynamical approach to the equilibrium problem that involves the formulation of the kinetic rate equations for each species. The equilibrium concentrations are determined by evolving the initial concentrations via this dynamical system to their steady state values. This dynamical approach is particularly attractive because it can be extended easily to very large multi-equilibria systems and the effects of ionic strength also are easily included. Here we describe mathematical methods for the determination of steady state concentrations of all species with the consideration of their activities using several approximations of Debye–Huckel theory of electrolyte solutions. We describe the equations for a system that consists of a triprotic acid H3A and its conjugate bases. With these equations, two types of multi-equilibria systems were studied and compared to experimental data. The first system is exemplified by case studies of solutions of acetate-buffered acetic acid and the second system is exemplified by the hydroxide titration of citric acid. The discussion focuses on the effect of ionic strength on pH and on the amplification of acidity by ionic strength. Ionic strength effects are shown to cause significant deviations from the widely used Henderson–Hasselbalch equation.

The effect of electrodes on 11 acene molecular spin valve: Semi-empirical study

A new revolution in electronics is molecular spintronics, with the contemporary evolution of the two novel disciplines of spintronics and molecular electronics. The key point is the creation of molecular spin valve which consists of a diamagnetic molecule in between two magnetic leads. In this paper, non-equilibrium Green’s function (NEGF) combined with Extended Huckel Theory (EHT); a semi-empirical approach is used to analyse the electron transport characteristics of 11 acene molecular spin valve. We examine the spin-dependence transport on 11 acene molecular junction with various semi-infinite electrodes as Iron, Cobalt and Nickel. To analyse the spin-dependence transport properties the left and right electrodes are joined to the central region in parallel and anti-parallel configurations. We computed spin polarised device density of states, projected device density of states of carbon and the electrode element, and transmission of these devices. The results demonstrate that the effect of electrodes modifying the spin-dependence behaviours of these systems in a controlled way. In Parallel and anti-parallel configuration the separation of spin up and spin down is lager in the case of iron electrode than nickel and cobalt electrodes. It shows that iron is the best electrode for 11 acene spin valve device. Our theoretical results are reasonably impressive and trigger our motivation for comprehending the transport properties of these molecular-sized contacts.

Energy gaps of graphene clusters: the first-principles calculations based on high-throughput screening

With the enumeration of the triangular lattice fragments, we have systematically investigated the graphene clusters (CnHm, n = 14 – 24) with various sizes and shapes, whose structural stabilities and electronic properties are studied by the Hückel molecular orbital (HMO) method and the first-principles calculation. According to the formation energies, we show the structural stabilities of the clusters are closely related to the shape and size, as well as the chemical potential of hydrogen. The energy gaps obtained from the HMO method are in the same trend with the ones calculated by the first-principles calculations, indicating the effective screening of the gap minimum and maximum in a fast speed. There is a general decreasing of the energy gaps with the size increment due to the quantum confinement, meanwhile, the gaps are also highly dependent on the shape of the clusters for those with the same number of carbon atom.

Hückel Theory + Reorganization Energy = Marcus–Hush Theory: Breakdown of the 1/n Trend in π-Conjugated Poly-p-phenylene Cation Radicals Is Explained

Among the π-conjugated poly-p-phenylene wires, fluorene-based poly-p-phenylene (FPPn) wires have been extensively explored for their potential as charge-transfer materials in functional photovoltaic devices. Herein, we undertake a systematic study of the redox and optical properties of a set of FPPn (n = 2–16) wires. We find that, while their absorption maxima (νabs) follow a linear trend against cos[π/(n + 1)] up to the polymeric limit, redox potentials (Eox) show an abrupt breakdown from linearity beginning at n ∼ 8. These observations prompted the development of a generalized model to describe the unusual evolution of redox and optical properties of poly-p-phenylene wires. We show that the cos[π/(n + 1)], commonly expressed as 1/n, dependence of the properties of various π-conjugated wires has its origin in Huckel molecular orbital (HMO) theory, which however, fails to predict the evolution of the redox potentials of these wires, as the oxidation-induced structural/solvent reorganization is unaccounted...

Anisotropic Electronic Structure and Transport Properties of the H-0 Hyperhoneycomb Lattice

Carbon, being one of the most versatile elements of the periodic table, forms solids and molecules with often unusual properties. Recently, a novel family of three-dimensional graphitic carbon structures, the so-called hyperhoneycomb lattices, has been proposed, with the possibility of being topological insulators. In this work, we present electronic structure calculations for one member (H-0) of this family, using Density Functional Theory and non-equilibrium Green’s functions transport calculations to show that the H-0 structure should have strongly anisotropic electronic properties, being an insulator or a conductor depending on the crystalline orientation chosen for transport. Calculations in the framework of Extended Huckel Theory indicate that these properties can only be understood if one considers at least 2nd nearest-neighbor interactions between carbon atoms, invalidating some of the conclusions of Ref. 1, at least for this particular material.

Design of fullerene-based biomarker for detection of lead impurities

Abstract The fullerene-based molecular junction in a device configured for sensor applications in medical diagnostics has been evaluated in this study. The presence of highly carcinogenic agents such as lead is detected using this molecular device, which is configured and investigated using extended Huckel theory (EHT) and the non-equilibrium Green’s function. The electrical parameters that we calculated can be employed to determine the concentration of lead impurities by evaluating its transmission coefficients and molecular orbitals. Our results suggest that the alignment of current increases with increasing impurity level (i.e., of 6, 12, and 18 lead atoms), thus indicating sensor characteristics.

Effect of asymmetric molecule–electrode coupling and molecular bias on rectification in molecular junctions

In this research work, we compare the rectification trends of two symmetrical and one asymmetrical molecular junction formed with gold and silver electrodes bridging benzenedithiol molecule. The origin of rectification is attributed to both molecular bias drop and asymmetric molecule–electrode coupling. The electronic transport properties are computed by using semi-empirical extended Huckel method combined with non-equilibrium Green’s function framework. The results are fully rationalized by analysing the distribution of molecular orbitals with changing bias voltage, available density of states and area of transmission spectra spanned within bias window, transmission eigenstates and transmission pathways. We deduce through this work that the molecular rectification is not only the property of asymmetric molecule–metal coupling, but molecular bias also plays vital role in stemming asymmetric I–V characteristics. Our results suggest how to realize molecular rectification by using different electrode materials which act as Schottky barriers in molecular junctions that emulate p–n junction diode in semiconductor electronics.

Non-invasive cancer detection using molecular device based on aromatic molecules

Abstract Major advances in the molecular diagnostics of screening tissue and blood samples have fueled the search for biomarkers and diagnostic nanosensors that can detect abnormalities early in their development cycles. In this paper, we propose an aromatic molecular junction-based biomarker that can trace the concentration of lead in any live sample. A nanomolecular device based on an anthracenedithiol molecule was modeled. Its electrical transport attributes were computed using a semi-empirical extended Huckel theory combined with non-equilibrium Green’s function formalism. We observed that the current and conductance assay had prominent changes when the molecular junction device was exposed to variant concentrations of lead. Based on our results, we propose aromatic molecular junction-based biomarkers that can detect even minor adulterations of lead when plied at operating voltage range.

Effect of anchoring atom and electrostatic gating on the electronic transport properties in single molecular electronic devices

The effect of anchoring atom and electrostatic gate on the electron transport through gated thiophene single molecular device is studied by utilizing non-equilibrium Green’s function coupled with self-consistent extended Huckel theory. Gated gold–molecule–gold junctions are built using thiophene (Tp) molecule as functional component and sulphur (S) and selenium (Se) as anchoring atoms in field effect transistor (FET) configuration. The electron transport analysis of the gated thiophene single molecular device is investigated through the current–voltage and the electron transmission spectra. The results show that the anchoring atoms modulate the transport nature of these devices in a controlled manner. We find that the S–Tp–S device produces larger current than Se–Tp–Se device. Also we studied the effect of electrostatic gating on S–Tp–S and Se–Tp–Se device. We find that, positive bias or negative bias for Vg, will correspondingly, raise or lower the transmission coefficients T(E) in relation to the Fermi level (EF) for both the devices. Our results show that magnitude of Isd current varies more than one order for same Vsd over different Vg bias for S–Tp–S device, whereas for Se–Tp–Se device Isd current varies more than five times for same Vsd over different Vg bias. Se–Tp–Se device shows gate controlled NDR behavior. Finally, we demonstrated the application of using thiophene based single molecular FET to realize five basic logic gates very low Vsd bias. The key feature of the suggested design is the opportunity of demonstrating various logic gates with just one molecular unit transistor and demonstrated at very low Vsd bias.