Tunneling Spectroscopy for Electronic Bands in Multi-Walled Carbon Nanotubes with Van Der Waals Gap

Various intriguing quantum transport measurements for carbon nanotubes (CNTs) based on their unique electronic band structures have been performed adopting a field-effect transistor (FET), where the contact resistance represents the interaction between the one-dimensional and three-dimensional systems. Recently, van der Waals (vdW) gap tunneling spectroscopy for single-walled CNTs with indium–metal contacts was performed adopting an FET device, providing the direct assignment of the subband location in terms of the current–voltage characteristic. Here, we extend the vdW gap tunneling spectroscopy to multi-walled CNTs, which provides transport spectroscopy in a tunneling regime of ~1 eV, directly reflecting the electronic density of states. This new quantum transport regime may allow the development of novel quantum devices by selective electron (or hole) injection to specific subbands.     

Site-directed electronic tunneling through a vibrating molecular network

The effect of electronic-nuclear coupling on electronic transport through a complex molecular network is studied. Electronic tunneling dynamics in a network of N donor/acceptor sites, connected by molecular bridges, is shown to be controlled by electronic-nuclear coupling at the bridges. Particularly, electronic coupling to an accepting nuclear mode at the contact site between the donor and the rest of the network is shown to affect the tunneling path selection to specific acceptors. In the "deep" tunneling regime, the network is mapped onto an N-level system using a recursive perturbation expansion, enabling analytical treatment of the electronic dynamics. The analytic formulation is applied for two model systems, demonstrating site-directed tunneling by electronic-nuclear coupling. Numerical simulations suggest that this phenomenon is not limited to the deep tunneling regime.     

Enhanced hopping conductivity in low band gap donor-acceptor molecular wires Up to 20 nm in length

We have measured the current-voltage characteristics of conjugated oligo-tetrathiafulvalene-pyromelliticdiimide-imine (OTPI) wires ranging in length from 2.5 to 20.2 nm, contacted by Au electrodes. OTPI wires were built from Au substrates using alternating donor (tetrathiafulvalene, TTF) and acceptor (pyromelliticdiimide, PMDI) building blocks linked via aryl imine groups. Metal-molecule-metal junctions consisting of approximately 100 wires in parallel were prepared by contacting the wire films with an Au-coated atomic force microscope tip. The long OTPI wires exhibit a narrow band gap (<1.5 eV) and multiple redox states, which facilitate carrier injection from the Au contacts for hopping transport. We observe the theoretically predicted change in direct current (DC) transport from tunneling to hopping as a function of systematically controlled wire length, as well as strongly enhanced wire conductivity (0.02 S/cm) in the hopping regime. Hopping conduction is confirmed by length-, temperature-, and field-dependent transport measurements. These nanoscale transport measurements illuminate the role of molecular length and bond architecture on molecular conductivity and open opportunities for greater understanding of hopping transport in conjugated polymer films.     

Construction of halogen and hydrogen bond‐based multicomponent nanostructures at liquid‐solid interface

The cooperative effect of hydrogen and halogen bonds on the 2‐dimensional molecular arrangement of highly oriented pyrolytic graphite has been studied by scanning tunneling microscopy. The scanning tunneling microscopy observations demonstrate that the self‐assembled hydrogen‐bonded molecular chicken‐wire networks of trimesic acid have been significantly transformed after annealing and the introduction of tribromobenzene guest molecules. Bromine atoms and carboxyl groups were found to form 2 different multicomponent structures via hydrogen and halogen bonds. Owing to the effect of halogen and hydrogen bonds, tribromobenzene with trimesic acid formed the 3‐fold symmetry networks.     

Conduction properties of bipyridinium-functionalized molecular wires

We present a detailed analysis of the coherent electron transport through a redox-active, 4,4'-bipyridinium (viologen)-functionalized molecular wire, which was studied in several recent experiments. Our calculations for the bare viologen predict conductances differing by 2 orders of magnitude depending on the contact geometry. For the alkyl-wired viologen unit, we obtain an exponential decay of the conductance with the wire length. Because this exponent also governs the conductance in the incoherent transport regime, comparison with experiments is legitimate and we find a good agreement. Furthermore, our calculations indicate that the experimentally observed conductance switching behavior is not amenable to an explanation inside a coherent transport picture. A possible incoherent mechanism is being discussed.     

Dark channels in resonant tunneling transport through artificial atoms

We investigate sequential tunneling through a multilevel quantum dot confining multiple electrons in the regime where several channels are available for transport within the bias window. By analyzing solutions to the master equations of the reduced density matrix, we give general conditions on when the presence of a second transport channel in the bias window quenches transport through the quantum dot. These conditions are in terms of distinct tunneling anisotropies which may aid in explaining the occurrence of negative differential conductance in quantum dots in the nonlinear regime.     

Site-directed electronic tunneling in a dissipative molecular environment

The ability to control electronic tunneling in complex molecular networks of multiple donor/acceptor sites is studied theoretically. Our past analysis, demonstrating the phenomenon of site-directed transport, was limited to the coherent tunneling regime. In this work we consider electronic coupling to a dissipative molecular environment including the effect of decoherence. The nuclear modes are classified into two categories. The first kind corresponds to the internal molecular modes, which are coupled to the electronic propagation along the molecular bridges. The second kind corresponds to the external solvent modes, which are coupled to the electronic transport between different segments of the molecular network. The electronic dynamics is simulated within the effective single electron picture in the framework of the tight binding approximation. The nuclear degrees of freedom are represented as harmonic modes and the electronic-nuclear coupling is treated within the time-dependent Redfield approximation. Our results demonstrate that site-directed tunneling prevails in the presence of dissipation, provided that the decoherence time is longer than the time period for tunneling oscillations (e.g., at low temperatures). Moreover, it is demonstrated that the strength of electronic coupling to the external nuclear modes (the solvent reorganization energy) controls the coherent intramolecular tunneling dynamics at short times and may be utilized for the experimental control of site-directed tunneling in a complex network.     

Elimination of the interfering effect of hydrogen on the determination of palladium by stripping voltammetry

The effect of hydrogen on the electrooxidation current of a palladium–hydrogen precipitate is studied. It is shown that the UV irradiation of the solution changes the mechanism of the formation of molecular hydrogen in the electrochemical deposition of palladium. A procedure is developed for determining palladium in platinum metal preconcentrates by stripping voltammetry.     

Influence of atmospheric oxygen on hydrogen detection on Pd using Kelvin probe technique

Influence of oxygen concentration in the measurement atmosphere on detection of hydrogen using Kelvin probe was studied. The studied material was a 100-μm-thick palladium foil, which was mounted in a 3D printed electrochemical flow cell. The used setup enables hydrogen loading with in-situ contact potential measurement of the hydrogen exit side of the Pd electrode. The hydrogen loading and unloading procedure, including insertion of different amounts of hydrogen into the Pd membrane and recording resulting values of contact potential difference, was performed at distinct oxygen concentrations ranging between 1 and 80 vol%. An increasing amount of oxygen in the atmosphere surrounding the hydrogen-loaded Pd electrode resulted in an accelerated removal of hydrogen from the Pd. The kinetics of this reaction was studied based on Kelvin probe measurements, and a reaction mechanism is discussed.     

Self-Consistent-Field potential-energy surfaces for hydrogen atom pairs within small palladium clusters

An ab initio electronic structure study is presented of hydrogen–hydrogen interactions in an electronic environment perturbed by the presence of palladium atom clusters. In particular, we investigated changes in the interatomic potential when the atomic centers are trapped inside an fcc palladium octahedral hole and when they are separated from each other by a (111) plane of palladium atoms. The (111) plane was modeled with a cluster of three palladium atoms. Self-consistent field (SCF ) level calculations were performed, and palladium atom pseudopotentials were employed to make the systems studied computationally tractable. For pairs of atoms placed within the octahedral hole, various lines of approach were considered [along the (100), (110), and (111) directions]. Lattice deformations and electronic excitations were examined for their effect on the interatomic potential.     

n-Type field-effect transistors made of an individual nitrogen-doped multiwalled carbon nanotube

We report on the fabrication and characterization of field-effect transistor based on an individual multiwalled nitrogen-doped carbon nanotube. Our measurements show that the N-doped carbon nanotubes have n-type properties. The contact properties of the tube and Pt electrodes are also studied in detail. Temperature dependence of two-terminal transport experiments suggests that transport is dominated by thermionic emission and tunneling through a 0.2 eV Schottky contact barrier.     

Redox reactions in nanocomposites on a sulfated polytetraphenylcalix[4]resorcinarene matrix

Redox reactions involving hydrogen and oxygen in the presence of hybrid nanocomposites containing palladium and copper or palladium and silver in a cis-tetraphenylcalix[4]resorcinarene polymer matrix were studied. In the composites containing palladium and copper, the redox transformations involved copper. In the composites with palladium and silver, the redox reactions involved the polymer matrix. The reductions in the metal-polymer nanocomposites were catalyzed by palladium.     

Thermoelectric transport properties in atomic scale conductors

The thermoelectric transport properties in atomic scale conductors consisting of a Si atom connected by two electrodes are investigated. It is found that both the electrical current and the heat current have two contributions, one from the voltage and the other from the temperature gradient. The quantities such as the Seebeck thermopower and the thermal conductance that characterize the thermoelectric transport properties of the tunnel atomic junction are studied quantitatively with a first-principles technique within the framework of Landauer-Buttiker formalism in the linear response regime. A finite thermopower only exists in a very narrow range where the energy derivative of the transmission function is nonzero. The thermopower anomaly is observed in the tunneling regime in this device but this does not violate the thermodynamic law with respect to the heat current.     

Higher Acenes by On‐Surface Dehydrogenation: From Heptacene to Undecacene

A unified approach to the synthesis of the series of higher acenes up to previously unreported undecacene has been developed through the on‐surface dehydrogenation of partially saturated precursors. These molecules could be converted into the parent acenes by both atomic manipulation with the tip of a scanning tunneling and atomic force microscope (STM/AFM) as well as by on‐surface annealing. The structure of the generated acenes has been visualized by high‐resolution non‐contact AFM imaging and the evolution of the transport gap with the increase of the number of fused benzene rings has been determined on the basis of scanning tunneling spectroscopy (STS) measurements.     

Palladium nanoparticles in aqueous solution: Preparation,properties, and effect of their size on catalytic activity

A method has been developed for the preparation of palladium nanoparticles with different sizes of up to 7 nm via the reduction of Pd(II) ions with hydrogen in an aqueous solution on seed metal nanoparticles (2.5 nm). The effect of the size of nanoparticles on their catalytic activity in methyl viologen reduction with molecular hydrogen in an alkaline medium has been studied. It has been found that the specific catalytic activity of palladium nanoparticles is independent of their size.     

Correlation between HOMO alignment and contact resistance in molecular junctions: aromatic thiols versus aromatic isocyanides

Understanding electron transport in metal-molecule-metal (MMM) junctions is of great importance for the advancement of molecular electronics. Critical factors that determine conductivity in a MMM junction include the nature of metal-molecule contacts and the electronic structure of the molecular backbone. We have studied the electronic transport property and the valence electronic structure on rigid, conjugated oligoacenes of increasing length with either thiol (-S) or isocyanide (-CN) linkers using conducting probe atomic force microscopy (CP-AFM) and ultraviolet photoelectron spectroscopy (UPS). We find that for these conjugated systems the Au-CN contact is more resistive than Au-S. The difference in contact resistance correlates with UPS measurements that show the highest-occupied molecular orbital (HOMO) of the isocyanide series is lower in energy (relative to the Fermi level of Au) than the HOMO of the thiol series, indicating the presence of a higher tunneling barrier at the contact for the isocyanide-linked molecules. By contrast, the difference in the HOMO positions for the two series of molecules does not appear to affect the length dependence of the junction resistance (i.e., the beta value = 0.5 A-1).     

Gap size dependent transition from direct tunneling to field emission in single molecule junctions

I/V characteristics recorded in mechanically controllable break junctions revealed that field emission transport is enhanced in single molecule junctions as the gap size between two nanoelectrodes is reduced. This observation indicates that Fowler-Nordheim tunneling occurs not only for intermolecular but also for intramolecular electron transport driven by a reduced energy barrier at short tunneling distances.     

Analytical results on ballistic transport in a periodic molecular wire

In this paper, we study the coherent electronic transport of a periodic quantum wire (P-QW) such as poly-acetylene connected to uniform metallic leads, within the tight-binding (TB) approach and in the ballistic regime. We have calculated the Green’s function (GF), density of states (DOS) and the coherent transmission coefficient (TC) fully exactly for a quantum wire. The quasi gap and the energy and wire-length dependence of the GF and conductance for the system are also derived. Finally, we obtain a non-linear equation which gives the bound state energies. Our calculation can be generalized to arbitrary leads and can be applied to molecular wires, polymers, nanocrystals, where results may be useful in designing future molecular electronic devices.     

Hydrogen diffusion transfer through an asymmetric three-layer vanadium membrane

Hydrogen diffusion transfer through a three-layer membrane has been studied within the framework of the lattice model under the Bragg?Williams approximation. A set of equations describing hydrogen transfer through a vanadium membrane coated with thin palladium layers has been derived taking into account the interactions of hydrogen atoms in the membrane layers. The obtained equations have been solved using the Mathcad-14 software package. It has been shown that the interaction between hydrogen atoms has a significant influence on hydrogen permeability at near-atmospheric pressures. It has been found that the permeability of the vanadium membrane is markedly higher than that of a palladium one at the same thickness. The effect of asymmetric vanadium membrane embrittlement has been shown to depend on the location of palladium layers with different thicknesses. The embrittlement of the vanadium membrane begins at higher pressures, when a thicker palladium layer is located at the inlet. It has been revealed that, for asymmetric membranes, the value of the diffusion flux of hydrogen atoms may depend on the transfer direction. At the same membrane thickness, the permeability of the asymmetric membrane is actually equal to that of a symmetrical membrane, provided that a thicker palladium layer is located at the inlet. At the opposite orientation, of the permeability of the asymmetric membrane is lower than that of the symmetric one.     

Chemical Reactions and Kinetics of the Carbon Monoxide Coupling in the Presence of

The chemical reactions and kinetics of the catalytic coupling reaction of carbon monoxide to diethyl oxalate were studied in the presence of hydrogen over a supported palladium catalyst in the gaseous phase at the typical coupling reaction conditions. The experiments were performed in a continuous flow fixed-bed reactor. The results indicated that hydrogen only reacts with ethyl nitrite to form ethanol, and kinetic studies revealed that the rate-determining step is the surface reaction of adsorbed hydrogen and the ethoxy radical (EtO-). A kinetic model is proposed and a comparison of the observed and calculated conversions showed that the rate expressions are of rather high confidence.