Conduction modulation of π-stacked ethylbenzene wires on Si(100) with substituent groups

For the realization of molecular electronics, one essential goal is the ability to systematically fabricate molecular functional components in a well-controlled manner. Experimental techniques have been developed such that π-stacked ethylbenzene molecules can now be routinely induced to self-assemble on an H-terminated Si(100) surface at precise locations and along precise directions. Electron transport calculations predict that such molecular wires could indeed carry an electrical current, but the Si substrate may play a considerable role as a competing pathway for conducting electrons. In this work, we investigate the effect of placing substituent groups of varying electron donating or withdrawing strengths on the ethylbenzene molecules to determine how they would affect the transport properties of such molecular wires. The systems consist of a line of π-stacked ethylbenzene molecules covalently bonded to a Si substrate. The ethylbenzene line is bridging two Al electrodes to model current through the molecular stack. For our transport calculations, we employ a first-principles technique where density functional theory (DFT) is used within the non-equilibrium Green’s function formalism (NEGF). The calculated density of states suggest that substituent groups are an effective way to shift molecular states relative to the electronic states associated with the Si substrate. The electron transmission spectra obtained from the NEGF–DFT calculations reveal that the transport properties could also be extensively modulated by changing substituent groups. For certain molecules, it is possible to have a transmission peak at the Fermi level of the electrodes, corresponding to high conduction through the molecular wire with essentially no leakage into the Si substrate.     

Effect of the linkage position in ruthenium dyes on the photoelectrochemical perfomance

A series of newly designed Ru-dyes constructed with [Ru(bpy)(bpp)Cl]⁺ (bpp?=?2,6-bis(N-pyrazolyl)pyridine, bpy?=?bipyridine) as template were evaluated by studying the frontier molecular orbital energy, charge distribution, absorption spectrum, light harvesting efficiency, as well as photogenerated current density by using (time-dependent) density functional theory. The calculation results reveal that the light harvesting efficiency and current density are enhanced with only one linkage group attached onto the 4 site of pyridine ligand. Mounting the linkage groups to 3 or 3′ position on pyridine ligand introduces super strong UV absorption.     

Limiting current density and water dissociation in bipolar membranes

The behaviour of bipolar membranes in NaCl and Na₂SO₄ solutions is discussed. The membranes are characterized in terms of their limiting current densities. Below the limiting current density the electric current is carried by salt ions migrating from the transition region between the anion and the cation exchange layer of the bipolar membrane. In steady state these ions are replaced by salt ions transported from the bulk solutions into the transition region by diffusion and migration due to the fact that the ion-exchange layers are not strictly permselective. When the limiting current density is exceeded, the salt transport from the transition region can no longer be compensated by the transport into the region and a drastic increase in the membrane resistance and enhanced water dissociation is observed. This water dissociation is described as being a combination of the second Wien effect and the protonation and deprotonation of functional groups in the membrane. The limiting current density is calculated from a mass balance that includes all components involved in the transport. The parameters used in the mathematical treatment are the diffusion coefficients of salt ions and water, the ion mobilities in the membrane, the fixed charge densitiy of the membrane, the pK_b values of the functional groups and the solution bulk concentrations.     

Spin-related electronic pathway through single molecule on Au(111)

Spin properties of organic molecules have attracted great interest for their potential applications in spintronic devices and quantum computing. Fe-tetraphenyl porphyrin (FeTPP) is of particular interest for its robust magnetic properties on metallic substrates. FeTPP is prepared in vacuum via on-surface synthesis. Molecular structure and spin-related transport properties are characterized by low-temperature scanning tunneling microscope and spectroscopy at 0.5 K. Density functional theory calculations are performed to understand molecular adsorption and spin distribution on Au(111). The molecular structure of FeTPP is distorted upon adsorption on the substrate. Spin excitations of FeTPP are observed on the Fe atom and high pyrrole groups in differential conductance spectra. The calculated spin density distribution indicates that the electron spin of FeTPP is mainly distributed on the Fe atom. The atomic transmission calculation indicates that electrons transport to substrate is mediated through Fe atom, when the tip is above the high pyrrole group.     

Transport properties of graphene nanoribbon-based molecular devices

The electronic and transport properties of an edge-modified prototype graphene nanoribbon (GNR) slice are investigated using density functional theory and Green's function theory. Two decorating functional group pairs are considered, such as hydrogen-hydrogen and NH(2)-NO(2) with NO(2) and NH(2) serving as a donor and an acceptor, respectively. The molecular junctions consist of carbon-based GNR slices sandwiched between Au electrodes. Nonlinear I-V curves and quantum conductance have been found in all the junctions. With increasing the source-drain bias, the enhancement of conductance is quantized. Several key factors determining the transport properties such as the electron transmission probabilities, the density of states, and the component of Frontier molecular orbitals have been discussed in detail. It has been shown that the transport properties are sensitive to the edge type of carbon atoms. We have also found that the accepter-donor functional pairs can cause orders of magnitude changes of the conductance in the junctions.     

Influence of functional groups on charge transport in molecular junctions

Using density functional theory (DFT), we analyze the influence of five classes of functional groups, as exemplified by NO(2), OCH(3), CH(3), CCl(3), and I, on the transport properties of a 1,4-benzenedithiolate (BDT) and 1,4-benzenediamine (BDA) molecular junction with gold electrodes. Our analysis demonstrates how ideas from functional group chemistry may be used to engineer a molecule's transport properties, as was shown experimentally and using a semiempirical model for BDA [Nano Lett. 7, 502 (2007)]. In particular, we show that the qualitative change in conductance due to a given functional group can be predicted from its known electronic effect (whether it is sigma/pi donating/withdrawing). However, the influence of functional groups on a molecule's conductance is very weak, as was also found in the BDA experiments. The calculated DFT conductances for the BDA species are five times larger than the experimental values, but good agreement is obtained after correcting for self-interaction and image charge effects.     

Dependence of the conduction of a single biphenyl dithiol molecule on the dihedral angle between the phenyl rings and its application to a nanorectifier

The transport properties of a biphenyl dithiol (BPD) molecule sandwiched between two gold electrodes are studied using the nonequilibrium Green's function method based on the density functional theory. In particular, their dependence on the dihedral angle (phi=90 degrees -180 degrees ) between two phenyl rings is investigated. While the dihedral-angle dependence of the density of states projected on the BPD molecular orbitals is small, the transport properties change dramatically with phi. The transmission at the Fermi energy exhibits a minimum at phi=90.0 degrees and greatly increases with phi. The ratio of the maximum obtained at phi=180 degrees to the minimum exceeds 100. As an application of this characteristic transport behavior, a BPD molecule functionalized with NH(2) and NO(2) groups is considered. It is found that this molecule works as a nanorectifier.     

Lattice density functional theory of molecular diffusion

A density functional theory of diffusion is developed for lattice fluids with molecular flux as a functional of the density distribution. The formalism coincides exactly with the generalized Ono-Kondo density functional theory when there is no gradient of chemical potential, i.e., at equilibrium. Away from equilibrium, it gives Fick's first law in the absence of a potential energy gradient, and it departs from Fickian behavior consistently with the Maxwell-Stefan formulation. The theory is applied to model a nanopore, predicting nonequilibrium phase transitions and the role of surface diffusion in the transport of capillary condensate.     

Protonation effects on electron transport through diblock molecular junctions:A theoretical study

Diblock oligomers are widely used in molecular electronics. Based on fully self-consistent nonequilib-rium Green's function method and density functional theory, we study the electron transport properties of the molecular junction with a dipyrimidinyl-diphenyl (PMPH) diblock molecule sandwiched between two gold electrodes. Effects of different kinds of molecule-electrode anchoring geometry and protona-tion of the PMPH molecule are studied. Protonation leads to both conductance and rectification en-hancements. However, the experimentally observed rectifying direction inversion is not found in our calculation. The preferential current direction is always from the pyrimidinyl to the phenyl side. Our calculations indicate that the protonation of the molecular wire is not the only reason of the rectification inversion.     

Interacting nanoparticles with functional surface groups

Functionalized nanoparticles with ionizable groups have generated a large variety of structures with important potential applications in technology. The nature of their interactions is crucial to determining their solubility and to exploring assemblies with diverse symmetries. Here, we use a molecular theory to describe the interactions between two nanoparticles coated with short polymer chains that contain ionizable (functional) end‐groups immersed in aqueous salt solution. It is shown here that the fraction of ionized functional groups in the system depends on factors such as the ionic strength and pH of solution, grafting density of polymer chains, the chain length, as well as the separation distance between the nanoparticles. The interactions between two neighboring nanoparticles influence the charge regulation of the end‐groups, which consequently induces an asymmetric distribution of these charged end‐groups on the nanoparticles, and thus confers a preferred directionality in nanoparticle–nanoparticle interactions. We show that the charge regulating system is less repulsive than an equivalent system with a fixed charge distribution. This is due to a decrease in the charge density of the weak acid end‐groups, to avoid a local increase in counterion confinement (condensation) in the region between neighboring nanoparticles, when their separation decreases. The anisotropic degree of ionization found in our results can be used to design aggregates of nanoparticles with reduced symmetries. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Photoinduced electron transport process in electrochemical cell I. Phenomenological description

The process of electron transport plays an essential role in the fundamental phenomena of life like photosynthesis, respiration and vision as well as in photoelectronic devices. However, the molecular mechanisms of the electron way and factors governing the transport rate in such systems are still unclear. Several groups have reported theoretical approaches for searching the mechanisms by using statistical mechanics, coherent dynamics and quantum mechanics. The current density vector inside the semiconducting layer is determined. In this paper we consider the problem of transport of electron promoted in the electrochemical cell constructed of two electrodes with the dye molecules immersed in. We describe the process of electron promotion by refractive light wave on the vacuum–semiconductor boundary as well as on the semiconducting electrode and the dye molecule layer in terms of extended phenomenological electrodynamics formalism. The results of our theoretical model show that such a theoretical approach will give more information on the mechanism of electron transport and will give insight in the determination of some electric features of materials.     

Transfer of electrolyte ions and water dissociation in anion-exchange membranes under intense current conditions

Polarization characteristics of electromembrane systems (EMS) based on the Russian commercial heterogeneous membranes MA-40 and MA-41, the anion-exchange heterogeneous membrane AMH (Mega, Czech Republic), and the modified membrane MA-40M are studied by the method of rotating membrane disk in dilute sodium chloride solutions. The effective transport numbers of ions are found; the partial voltammetric characteristics (VAC) with respect to chloride and hydroxyl ions are measured; the limiting current densities are calculated as a function of the membrane disk rotation rate. In terms of the theory of the overlimiting state of EMS, based on experimental VAC and the dependences of the effective transport numbers on the current density, the following internal parameters of systems under study are calculated: the space charge and electric field strength distribution over the diffusion layer and the membrane. It is shown that water dissociation can be virtually completely eliminated by substituting chemically stable quaternary ammonium groups inert with respect to water dissociation in the surface layer of a heterogeneous anion-exchange membrane MA-40 for the active ternary and secondary functional amino groups. The maximum electric field strength values at the membrane/solution interface, which were found in the framework of the theory of over-limiting state, turned out to be close for all anion-exchange membranes studied, namely, (7?C9) × 10⁶ V/cm. This suggests that it is the nature of ionogenic groups in the surface layer rather than the field effect that plays the decisive role in the membrane ability to accelerate the water dissociation reaction. It is proved experimentally that in highly intense current modes of the electrodialysis process, the thermal hydrolysis of quaternary ammonium bases occurs in strongly basic MA-41 and AMH membranes by the Hofmann reaction to form ternary amino groups catalytically active in water dissociation reaction. Based on the concept on the catalytic mechanism of water dissociation, the fraction of ternary amino groups formed by thermal hydrolysis in the surface layer (the space charge region) of monopolar anion-exchange membranes MA-41 and AMH is assessed quantitatively as 0.7 and 6.5%, respectively.     

The impact of diperfluorophenyl and thienyl substituents on the electronic structures and charge transport properties of the fused thiophene semiconductors

The charge transport properties of 3 fused thiophene semiconductors, end-capped with diperfluorophenylthien-2-yl (DFPT) groups (DFPT-thieno[2′, 3′:4, 5]thieno[3, 2-b]thieno[2, 3-d]thiophene (TTA), DFPT-dithieno[2, 3-b:3′, 2′-d]thiophenes (DTT), and DFPT-thieno[3, 2-b]thiophene (TT)), are explored via density functional theory (DFT). To gain a better understanding of the impact of diperfluorophenyl and thienyl substituents on the electronic structures and charge transport properties of these molecules, the geometric structures, reorganization energy, frontier molecular orbitals, molecular ionization potentials and electron affinities, absorption spectra of the corresponding molecules including diperfluorophenyl (DFP)-TTA, DFP-DTT, DFP-TT, dithienyl (DT)-TTA, DT-DTT, DT-TT as well as their parent molecules (TTA, DTT and TT) are investigated for comparison. The calculated results show that introducing perfluorophenyl groups to the fused thiophenes could be a good strategy to promote electron transport, while the insertion of additional thiophene rings to DFP-end-capped derivatives could further extend the π-conjugation and enhance the charge transport properties of DFPT-end-capped analogs.     

A Two‐Dimensional Mesoporous Polypyrrole–Graphene Oxide Heterostructure as a Dual‐Functional Ion Redistributor for Dendrite‐Free Lithium Metal Anodes

Guiding the lithium ion (Li‐ion) transport for homogeneous, dispersive distribution is crucial for dendrite‐free Li anodes with high current density and long‐term cyclability, but remains challenging for the unavailable well‐designed nanostructures. Herein, we propose a two‐dimensional (2D) heterostructure composed of defective graphene oxide (GO) clipped on mesoporous polypyrrole (mPPy) as a dual‐functional Li‐ion redistributor to regulate the stepwise Li‐ion distribution and Li deposition for extremely stable, dendrite‐free Li anodes. Owing to the synergy between the Li‐ion transport nanochannels of mPPy and the Li‐ion nanosieves of defective GO, the 2D mPPy‐GO heterostructure achieves ultralong cycling stability (1000 cycles), even tests at 0 and 50 °C, and an ultralow overpotential of 70 mV at a high current density of 10.0 mA cm^?2, outperforming most reported Li anodes. Furthermore, mPPy‐GO‐Li/LiCoO₂ full batteries demonstrate remarkably enhanced performance with a capacity retention of >90 % after 450 cycles. Therefore, this work opens many opportunities for creating 2D heterostructures for high‐energy‐density Li metal batteries.     

Recognition of tandem sialic acid residues by CD38: a theoretical study

The electronic structures of gangliosides are described using semiempirical and ab inito molecular orbital theories as well as the density functional theory to clarify the causative factors of the differences in inhibitory effects and to elucidate the recognition mechanisms of the enzyme. Our results suggest that CD38 is likely to recognize the two phosphate groups in NAD and the two carboxyl groups in tandem sialic acid residues of gangliosides. The recognition mechanisms of the substrate are proposed based on the good correlation found between the orbital energy of the highest occupied molecular orbital of the gangliosides and the degree of the inhibitory effect.     

Electrokinetic transport of nanoparticles in functional group modified nanopores

Nanopore detection is a hot issue in current research. One of the challenges is how to slow down the transport velocity of nanoparticles in nanopores. In this paper, we propose a functional group modified nanopore. That means a polyelectrolyte brush layer is grafted on the surface of the nanopore to change the surface charge properties. The existing studies generally set the charge density of the brush layer to a fixed value. On the contrary, in this paper, we consider an essential property of the brush layer: the volume charge density is adjustable with pH. Thus, the charge property of the brush layer will change with the local H⁺ concentration. Based on this, we established a mathematical model to study the transport of nanoparticles in polyelectrolyte brush layer modified nanopores. We found that pH can effectively adjust the charge density and even the polarity of the brush layer. A larger pH can reduce the transport velocity of nanoparticles and improve the blockade degree of ion current. The grafting density does not change the polarity of the brush charge. The larger the grafting density, the greater the charge density of the brush layer, and the blockade degree of ion current is also more obvious. The polyelectrolyte brush layer modified nanopores in this paper can effectively reduce the nanoparticle transport velocity and retain the essential ion current characteristics, such as ion current blockade and enhancement.     

Orbital views of molecular conductance perturbed by anchor units

Site-specific electron transport phenomena through benzene and benzenedithiol derivatives are discussed on the basis of a qualitative Hu?ckel molecular orbital analysis for better understanding of the effect of anchoring sulfur atoms. A recent work for the orbital control of electron transport through aromatic hydrocarbons provided an important concept for the design of high-conductance connections of a molecule with anchoring atoms. In this work the origin of the frontier orbitals of benzenedithiol derivatives, the effect of the sulfur atoms on the orbitals and on the electron transport properties, and the applicability of the theoretical concept on aromatic hydrocarbons with the anchoring units are studied. The results demonstrate that the orbital view predictions are applicable to molecules perturbed by the anchoring units. The electron transport properties of benzene are found to be qualitatively consistent with those of benzenedithiol with respect to the site dependence. To verify the result of the Hu?ckel molecular orbital calculations, fragment molecular orbital analyses with the extended Hu?ckel molecular orbital theory and electron transport calculations with density functional theory are performed. Calculated results are in good agreement with the orbital interaction analysis. The phase, amplitude, and spatial distribution of the frontier orbitals play an essential role in the design of the electron transport properties through aromatic hydrocarbons.     

First-principles investigation of the asymmetric contact effect on current-voltage characteristics of a molecular device

The properties of electronic transport in an electronic device composed of a spatially symmetric phenyldithiolate molecule sandwiched between two gold electrodes with asymmetric contact are investigated by the first-principles study. It is found that the I-V and G-V characteristics of a device show significant asymmetry and the magnitudes of current and conductance depend remarkably on the variation of molecule-metal distance at one of the two contacts. Namely, an asymmetric contact would lead to the weak rectifying effects on the current-voltage characteristics of a molecular device. We also calculate self-consistently other microscopic quantities such as the local density of states, the total density of states, and the distribution of charges in the asymmetric molecular models under the applied bias. The results show that the highest-occupied molecular orbital (HOMO) is responsible for the resonant tunneling and the shifting of the HOMO due to the charging of the device under the bias voltage is the intrinsic origin of asymmetric I(G)-V characteristics.     

Quantum mechanical study of the coupling of plasmon excitations to atomic-scale electron transport

The coupling of optical excitation and electron transport through a sodium atom in a plasmonic dimer junction is investigated using time-dependent density functional theory. The optical absorption and dynamic conductance is determined as a function of gap size. Surface plasmons are found to couple to atomic-scale transport through several different channels including dipolar, multipolar, and charge transfer plasmon modes. These findings provide insight into subnanoscale couplings of plasmons and atoms, a subject of general interest in plasmonics and molecular electronics.