Tunneling Decay Constant Research Articles

Plasmonic Imaging of Tuning Electron Tunneling Mediated by a Molecular Monolayer.

Probing and tuning the electron tunneling in metal electrode–insulator–metal nanoparticle systems provide a unique vision for understanding the fundamental mechanism of electrochemistry and broadening the horizon in practical applications of molecular electronics in many electrochemical systems. Here we report a plasmonic imaging technique to monitor the local double-layer charging of individual Au nanoparticles deposited on gold electrode separated by monolayer of n-alkanethiol molecules. The thickness of molecular monolayer tunes the tunneling kinetics and conductivity, which predicts the heterogeneous behavior on the modified electrode surface for different electrochemical systems. We studied the distance dependence of the electron tunneling and double layer charging processes by a plasmonic-based electrical impedance microscopy. By performing fast Fourier transform analysis of the recorded plasmonic image sequences, we can quantify the interfacial impedance of single nanoparticles and the tunneling decay constant of molecular layer. We further observed the electron neutralization dynamics during single-nanoparticle collisions on different surfaces. This optical readout of electron tunneling demonstrates an imaging approach to determine the electrical properties of metal electrode–insulator–metal nanoparticle systems, which include the electron tunneling mechanism and local impedance.

Transport and Spectroscopy in Conjugated Molecules: Two Properties and a Single Rationale.

In the context of electron dynamics simulations, when the charge density of a molecule is subject to a perturbation in the form of a short electric field pulse, density fluctuations develop in time. In the absence of dissipation, these oscillations continue indefinitely, reflecting the resonances of the electronic system; as a matter of fact, from the Fourier transform of the time dependent dipole arising from them, the absorption spectrum of the molecule can be calculated. Since these oscillations are the result of the electrons moving through the molecular structrure, it seems plausible that they carry information on the transport properties of the system. This is the idea explored in the present article for the case of conjugated polymers. Specifically, we depart from a nonequilibrium state with the charge concentrated on the ends of the molecule, and estimate the currents flowing back and forth during the evolution of electron dynamics simulations. These show that the charge oscillates between the sides of the polymer with the predominance of a frequency that is coincident with one of the main bands in the absorption spectrum, which can be ascribed to a charge transfer transition. Thus, from the charge transfer band frequency appearing in the absorption spectrum, the molecular conductance of a conjugated molecule can be calculated. Also interestingly, we find that, while a perturbation excites all resonances of an electronic system, the form in which this perturbation is applied can be manipulated to determine the relative manifestation of the response. The electric field pulse excites all resonances according to the transition dipole moment and is then appropriate to produce the absorption spectrum. A charge separated initial state, however, specifically stimulates the charge transfer mode and is then suitable to calculate transport properties. This allows us to propose a simple approach to obtain molecular conductances and tunneling decay constants in agreement with results from much more demanding electronic structure techniques.

Multiscale approach to electron transport dynamics.

Molecular simulations of transport dynamics in nanostructures usually require the implementation of open quantum boundary conditions. This can be instrumented in different frameworks including Green's functions, absorbing potentials, or the driven Liouville von Neumann equation, among others. In any case, the application of these approaches involves the use of large electrodes that introduce a high computational demand when dealing with first-principles calculations. Here, we propose a hybrid scheme where the electrodes are described at a semiempirical, tight binding level, coupled to a molecule or device represented with density functional theory (DFT). This strategy allows us to use massive electrodes at a negligible computational cost, preserving the accuracy of the DFT method in the modeling of the transport properties, provided that the electronic structure of every lead is properly defined to behave as a conducting fermionic reservoir. We study the nature of the multiscale coupling and validate the methodology through the computation of the tunneling decay constant in polyacetylene and of quantum interference effects in an aromatic ring. The present implementation is applied both in microcanonical and grand-canonical frameworks, in the last case using the Driven Liouville von Neumann equation, discussing the advantages of one or the other. Finally, this multiscale scheme is employed to investigate the role of an electric field applied normally to transport in the conductance of polyacetylene. It is shown that the magnitude and the incidence angle of the applied field have a considerable effect on the electron flow, hence constituting an interesting tool for current control in nanocircuits.

Plasmonic Measurement of Electron Transfer between a Single Metal Nanoparticle and an Electrode through a Molecular Layer.

We study electron transfer associated with electrocatalytic reduction of hydrogen on single platinum nanoparticles separated from an electrode surface with an alkanethiol monolayer using a plasmonic imaging technique. By varying the monolayer thickness, we show that the reaction rate depends on electron tunneling from the electrode to the nanoparticle. The tunneling decay constant is ∼4.3 nm-1, which is small compared to those in literature for alkanethiols. We attribute it to a reduced tunneling barrier resulting from biasing the electrode potential negatively to the hydrogen reduction regime. In addition to allowing study of electron transfer of single nanoparticles, the work demonstrates an optical method to measure charge transport in molecules electrically wired to two electrodes.

Nano-Particle VO2 Insulator-Metal Transition Field-Effect Switch with 42 mV/decade Sub-Threshold Slope

The possibility of controlling the insulator-to-metal transition (IMT) in nano-particle VO2 (NP-VO2) using the electric field effect in a metal-oxide-VO2 field-effect transistor (MOVFET) at room temperature was investigated for the first time. The IMT induced by current in NP-VO2 is a function of nano-particle size and was studied first using the conducting atomic force microscope (cAFM) current-voltage (I-V) measurements. NP-VO2 switching threshold voltage (VT), leakage current (Ileakage), and the sub-threshold slope of their conductivity (Sc) were all determined. The cAFM data had a large scatter. However, VT increased as a function of particle height (h) approximately as VT(V) = 0.034 h, while Ileakage decreased as a function of h approximately as Ileakage (A) = 3.4 × 10−8e−h/9.1. Thus, an asymptotic leakage current of 34 nA at zero particle size and a tunneling (carrier) decay constant of ~9.1 nm were determined. Sc increased as a function of h approximately as Sc (mV/decade) = 2.1 × 10−3eh/6 and was around 0.6 mV/decade at h~34 nm. MOVFETs composed of Pt drain, source and gate electrodes, HfO2 gate oxide, and NP-VO2 channels were then fabricated and showed gate voltage dependent drain-source switching voltage and current (IDS). The subthreshold slope (St) of drain-source current (IDS) varied from 42 mV/decade at VG = −5 V to 54 mV/decade at VG = +5 V.

Moiré structure of MoS2 on Au(111): Local structural and electronic properties

Monolayer islands of molybdenum disulfide (MoS2) on Au(111) form a characteristic moiré structure, leading to locally different stacking sequences at the S-Mo-S-Au interface. Using low-temperature scanning tunneling microscopy (STM) and atomic force microscopy (AFM), we find that the moiré islands exhibit a unique orientation with respect to the Au crystal structure. This indicates a clear preference of MoS2 growth in a regular stacking fashion. We further probe the influence of the local atomic structure on the electronic properties. Differential conductance spectra show pronounced features of the valence band and conduction band, some of which undergo significant shifts depending on the local atomic structure. We also determine the tunneling decay constant as a function of the bias voltage by a height-modulated spectroscopy method. This allows for an increased sensitivity of states with non-negligible parallel momentum k∥ and the identification of the origin of the states from different areas in the Brillouin zone.

Oligofluorene Molecular Wires: Synthesis and Single-Molecule Conductance

We report the synthesis and molecular junction conductance for a series of oligofluorenes to establish clear structure–property relationships for this electronically important material. We use a scanning tunneling microscopy based break-junction method (STM-BJ) to measure single-molecule conductance in oligofluorenes that vary in (a) the number of fluorene repeat units (n = 1–3), (b) bridge carbon substitution (dihydrogen, dimethyl, dihexyl, and didodecyl), and (c) linker-group termination (methyl sulfide versus primary amine). Conductance in oligofluorene molecular junctions is found to occur via tunneling, with a tunneling decay constant, β, of 0.31 per A, or equivalently, 2.6 per fluorene unit, consistent with other π-conjugated molecular wires. Simple tunnel coupling calculations for model Au2-oligofluorene molecular clusters are reported to validate experimental conductance measurements. Finally, molecular conductance distributions for methyl sulfide terminated oligofluorenes are observed to be extre...

Ultraflat Au nanoplates as a new building block for molecular electronics

We demonstrate the charge transport properties of a self-assembled organic monolayer on Au nanoplates with conductive probe atomic force microscopy (CP-AFM). Atomically flat Au nanoplates, a few hundred micrometers on each side, that have only (111) surfaces, were synthesized using the chemical vapor transport method; these nanoplates were employed as the substrates for hexadecanethiol (HDT) self-assembled monolayers (SAMs). Atomic-scale high-resolution images show molecular periodicity, indicating a well-ordered structure of the HDT on the Au nanoplates. We observed reduced friction and adhesion forces on the HDT SAMs on Au nanoplates, compared with Si substrates, which is consistent with the lubricating nature of HDT SAMs. The electrical properties, such as I–V characteristics and current as a function of load, were measured using CP-AFM. We obtained a tunneling decay constant (β) of 0.57 Å−1, including through-bond ( = 0.99 Å−1) and through-space ( = 1.36 Å−1) decay constants for the two-pathway model. This indicates that the charge transport properties of HDT SAMs on Au nanoplates are consistent with those on a Au (111) film, suggesting that SAMs on nanoplates can provide a new building block for molecular electronics.

Probing the rupture of a Ag atomic junction in a Ag-Au mixed electrode

We probed that the atomic junction in Ag part ruptures during stretching of atomic sized contacts of Ag-Au mixed electrodes, resulting in Ag-Ag electrodes through a scanning tunneling microscope breaking junction (STM-BJ) technique. We observed that the conductance and tunneling decay constant for a series of amine-terminated oligophenyl molecular junctions are essentially the same for the Ag-Au mixed and the Ag-Ag electrodes. We also found the molecular plateau length and the evolution patterns with the Ag-Au mixed electrodes are similar to those with Ag-Ag electrodes rather than the Au-Au electrodes in the molecular junction elongation. This result is attributed to the smaller binding energy of Ag atoms compared to that of Au atoms, so the Ag junction part is more easily broken than that of Au part in stretching of Ag-Au mixed electrodes. Furthermore, we successfully observed that the rupture force of the atomic junction for the Ag-Au mixed electrodes was identical to that for the Ag-Ag electrodes and smaller than that for the Au-Au electrodes. This study may advance the understanding of the electrical and the mechanical properties in molecular devices with Ag and Au electrodes in future.

Single-molecule conductance with nitrile and amino contacts with Ag or Cu electrodes

The single-molecule conductance of 1,4-dicyanobenzene (DCB), 1,4-benzenediamine (BDA) and 4,4'-biphenyldicarbonitrile (BPDC) with Ag and/or Cu electrodes is measured by electrochemical jump-to-contact STM-break junction. All single-molecule junctions present three sets of conductance values revealing different contact geometries. We observe that the single-molecule conductance of Ag-BDA-Ag junction is larger that of Ag-DCB-Ag junction, and DCB with Ag contacts are more conductive than that with Cu ones. This is related to a different electronic coupling between the molecules and the electrodes. Tunneling decay constants of 1.70 and 1.68 per phenyl group were found for Ag and Cu electrodes, respectively. The present study therefore shows that nitrile and amino groups can also be used as effective anchors for other metals than gold.

Molecular series-tunneling junctions.

Charge transport through junctions consisting of insulating molecular units is a quantum phenomenon that cannot be described adequately by classical circuit laws. This paper explores tunneling current densities in self-assembled monolayer (SAM)-based junctions with the structure Ag(TS)/O2C-R1-R2-H//Ga2O3/EGaIn, where Ag(TS) is template-stripped silver and EGaIn is the eutectic alloy of gallium and indium; R1 and R2 refer to two classes of insulating molecular units-(CH2)n and (C6H4)m-that are connected in series and have different tunneling decay constants in the Simmons equation. These junctions can be analyzed as a form of series-tunneling junctions based on the observation that permuting the order of R1 and R2 in the junction does not alter the overall rate of charge transport. By using the Ag/O2C interface, this system decouples the highest occupied molecular orbital (HOMO, which is localized on the carboxylate group) from strong interactions with the R1 and R2 units. The differences in rates of tunneling are thus determined by the electronic structure of the groups R1 and R2; these differences are not influenced by the order of R1 and R2 in the SAM. In an electrical potential model that rationalizes this observation, R1 and R2 contribute independently to the height of the barrier. This model explicitly assumes that contributions to rates of tunneling from the Ag(TS)/O2C and H//Ga2O3 interfaces are constant across the series examined. The current density of these series-tunneling junctions can be described by J(V) = J0(V) exp(-β1d1 - β2d2), where J(V) is the current density (A/cm(2)) at applied voltage V and βi and di are the parameters describing the attenuation of the tunneling current through a rectangular tunneling barrier, with width d and a height related to the attenuation factor β.

Electric transport properties of surface-anchored metal-organic frameworks and the effect of ferrocene loading.

Understanding of the electric transport through surface-anchored metal-organic frameworks (SURMOFs) is important both from a fundamental perspective as well as with regards to possible future applications in electronic devices. To address this mostly unexplored subject, we integrated a series of representative SURMOF thin films, formed by copper nodes and trimesic acid and known as HKUST-1, in a mercury-drop-based tunneling junction. Although the transport properties of these SURMOFs are analogous to those of hybrid metal-organic molecular wires, manifested by a very low value of the tunneling decay constant (β ≈ 0.006 Å(-1)), they are at the same time found to be consistent with a linear increase of resistance with film thickness. Upon loading of SURMOF pores with ferrocene (Fc), a noticeable increase in transport current was observed. A transport model and ab initio electronic structure calculations were used to reveal a hopping transport mechanism and to relate the changes upon Fc loading to those of the electronic and vibrational structures of the SURMOF films.

Length dependence of electron transport through molecular wires--a first principles perspective.

One-dimensional wires constitute a fundamental building block in nanoscale electronics. However, truly one-dimensional metallic wires do not exist due to Peierls distortion. Molecular wires come close to being stable one-dimensional wires, but are typically semiconductors, with charge transport occurring via tunneling or thermally-activated hopping. In this review, we discuss electron transport through molecular wires, from a theoretical, quantum mechanical perspective based on first principles. We focus specifically on the off-resonant tunneling regime, applicable to shorter molecular wires (<∼4-5 nm) where quantum mechanics dictates electron transport. Here, conductance decays exponentially with the wire length, with an exponential decay constant, beta, that is independent of temperature. Different levels of first principles theory are discussed, starting with the computational workhorse - density functional theory (DFT), and moving on to many-electron GW methods as well as GW-inspired DFT + Sigma calculations. These different levels of theory are applied in two major computational frameworks - complex band structure (CBS) calculations to estimate the tunneling decay constant, beta, and Landauer-Buttiker transport calculations that consider explicitly the effects of contact geometry, and compute the transmission spectra directly. In general, for the same level of theory, the Landauer-Buttiker calculations give more quantitative values of beta than the CBS calculations. However, the CBS calculations have a long history and are particularly useful for quick estimates of beta. Comparing different levels of theory, it is clear that GW and DFT + Sigma calculations give significantly improved agreement with experiment compared to DFT, especially for the conductance values. Quantitative agreement can also be obtained for the Seebeck coefficient - another independent probe of electron transport. This excellent agreement provides confirmative evidence of off-resonant tunneling in the systems under investigation. Calculations show that the tunneling decay constant beta is a robust quantity that does not depend on details of the contact geometry, provided that the same contact geometry is used for all molecular lengths considered. However, because conductance is sensitive to contact geometry, values of beta obtained by considering conductance values where the contact geometry is changing with the molecular junction length can be quite different. Experimentally measured values of beta in general compare well with beta obtained using DFT + Sigma and GW transport calculations, while discrepancies can be attributed to changes in the experimental contact geometries with molecular length. This review also summarizes experimental and theoretical efforts towards finding perfect molecular wires with high conductance and small beta values.

Electrical conductance of single-molecular junctions formed with palladium electrodes

We measured the conductance of a series of amine-terminated oligophenyl molecular junctions formed with palladium (Pd) electrodes by using a scanning tunneling microscope-based break-junction technique. For comparison, we also used Au electrodes to form a metal-molecular junction. We found that the molecular junctions formed with Pd electrodes had higher conductances than those formed with gold (Au) electrodes through a statistical analysis and a conductance histogram. Furthermore, the measured conductance decayed exponentially with molecular backbone length with a tunneling decay constant that was essentially the same for Pd and Au electrodes, which is attributable to the molecules that conduct through the highest occupied molecular orbital and the higher work function of Pd compared with Au. Because our work allows measuring the conductance of a molecule formed with various metal electrodes to be measured, it should be relevant to molecular junctions with various materials for future organic electronics on a molecular scale.

Odd-even effects in charge transport across n-alkanethiolate-based SAMs.

This paper compares rates of charge transport across self-assembled monolayers (SAMs) of n-alkanethiolates having odd and even numbers of carbon atoms (nodd and neven) using junctions with the structure M(TS)/SAM//Ga2O3/EGaIn (M = Au or Ag). Measurements of current density, J(V), across SAMs of n-alkanethiolates on Au(TS) and Ag(TS) demonstrated a statistically significant odd-even effect on Au(TS), but not on Ag(TS), that could be detected using this technique. Statistical analysis showed the values of tunneling current density across SAMs of n-alkanethiolates on Au(TS) with nodd and neven belonging to two separate sets, and while there is a significant difference between the values of injection current density, J0, for these two series (log|J0Au,even| = 4.0 ± 0.3 and log|J0Au,odd| = 4.5 ± 0.3), the values of tunneling decay constant, β, for nodd and neven alkyl chains are indistinguishable (βAu,even = 0.73 ± 0.02 Å(-1), and βAu,odd= 0.74 ± 0.02 Å(-1)). A comparison of electrical characteristics across junctions of n-alkanethiolate SAMs on gold and silver electrodes yields indistinguishable values of β and J0 and indicates that a change that substantially alters the tilt angle of the alkyl chain (and, therefore, the thickness of the SAM) has no influence on the injection current density across SAMs of n-alkanethiolates.

Tunneling Decay Constant of Alkanedicarboxylic Acids: Different Dependence on the Metal Electrodes between Air and Electrochemistry

In this paper, the different tunneling decay constants βN of alkanedicarboxylic acids depending on the metal electrodes in various surroundings are discussed. The conductance of alkanedicarboxylic acids (HOOC–(CH2)n–COOH, n = 2–4) contacting to Pd and Ag were systematically measured by the scanning tunneling microscopy (STM) break junction (STM-BJ) in air. A similar tunneling decay constant βN of about 1 per −CH2 was found for both metals, which might arise from the Fermi energy of electrodes pinning with the energy positions of molecular states under ambient atmosphere. However, the pinning effect can be destroyed by potential control in electrochemistry, and the βN is determined by the alignment of the molecular energy levels relative to the Fermi energy level of the electrodes, which well explains the order of βN,Pd < βN,Ag < βN,Cu in electrochemistry. The current work shows the important role of the surroundings in electron transport through molecular junctions.

Fabrications of insulator-protected nanometer-sized electrode gaps

We developed SiO2-coated mechanically controllable break junctions for accurate tunneling current measurements in an ionic solution. By breaking the junction, we created dielectric-protected Au nanoprobes with nanometer separation. We demonstrated that the insulator protection was capable to suppress the ionic contribution to the charge transport through the electrode gap, thereby enabled reliable characterizations of liquid-mediated exponential decay of the tunneling conductance in an electrolyte solution. From this, we found distinct roles of charge points such as molecular dipoles and ion species on the tunneling decay constant, which was attributed to local structures of molecules and ions in the confined space between the sensing electrodes. The device described here would provide improved biomolecular sensing capability of tunneling current sensors.

Conductance of Tailored Molecular Segments: A Rudimentary Assessment by Landauer Formulation

One of the strengths of molecular electronics is the synthetic ability of tuning the electric properties by the derivatization and reshaping of the functional moieties. However, after the quantitative measurements of single-molecule resistance became available, it was soon apparent that the assumption of negligible influence of the headgroup-electrode contact on the molecular resistance was oversimplified. Due to the measurement scheme of the metal--molecule-metal configuration, the contact resistance is always involved in the reported values. Consequently the electrical behavior of the tailored molecular moiety can only be conceptually inferred by the tunneling decay constant (βn in Rmeasured = R(n=0)e(βnN), where N is the number of repeated units), available only for compounds with a homologous series. This limitation hampers the exploration of novel structures for molecular devices. Based on the Landauer formula, we propose that the single-molecule resistance of the molecular backbones can be extracted. This simplified evaluation scheme is cross-examined by electrode materials of Au, Pd, and Pt and by anchoring groups of thiol (-SH), nitrile (-CN), and isothiocyanate (-NCS). The resistance values of molecular backbones for polymethylenes (n = 4, 6, 8, and 10) and phenyl (-C6H4-) moieties are found independent of the anchoring groups and electrode materials. The finding justifies the proposed approach that the resistance of functional moieties can be quantitatively evaluated from the measured values even for compounds without repeated units.

Laser synthesized gold nanoparticles for high sensitive strain gauges

We demonstrate high strain sensitivity property of gold nanoparticle (Au-NP) thin films fabricated on flexible polydimethylsiloxane (PDMS) substrates. This behavior is attributed to quantum tunneling effect that is highly dependent on nanoparticle separation. Au-NPs were synthesized in water by nanosecond laser ablation method. The clean surface providing high tunneling decay constant, size of the Au-NPs and Au-NPs aggregate clusters offer advantages for high sensitivity strain sensor. We prepared Au-NPs films on flexible PDMS substrate by using hands-on drop-cast method. To obtain high gauge factor (g factor), we investigated the nanoparticles concentration on the substrate. Laser-generated Au-NPs films demonstrated g factor of ∼300 for higher than 0.22% strain and ∼80 for the strain lower than 0.22% strain, which is favorably comparable to reported sensitivities for strain sensors based on Au-NPs. Mechanical characterizations for the prolonged working durations suggest long term stability of the strain sensors. We discuss several models describing conductance of films in low and high strain regimes.

Influence of an Atom in EGaIn/Ga2O3 Tunneling Junctions Comprising Self-Assembled Monolayers

We compared the electrical properties of self-assembled monolayers (SAMs) formed on template-stripped Au from two homologous series of five different oligo(phenylene)s bearing alkane thiol tails. The terminal phenyl ring is substituted by a 4-pyridyl ring in one series, thus the two differ only by the substitution of C–H for N. We formed tunneling junctions using the liquid metal eutectic Ga–In (EGaIn) as a nondamaging, conformal top contact that is insensitive to functional groups and measured the current-density, J, tunneling decay constants, β, and transition voltages, Vtrans. Conductance measurements alone did not sufficiently differentiate the two series of molecules. The length dependences of the two series of SAMs produced values of β of 0.44 and 0.42 Å–1 for pyridyl- and phenyl-terminated SAMs, respectively, which lie between the expected values for alkanethiolates and oligo(phenylene)s. The values of Vtrans were ∼0.3 V larger for the phenyl-terminated SAMs than for the pyridyl-terminated SAMs. A comparison of the values of J to highest occupied molecular orbital (HOMO) levels determined by density functional theory (DFT) calculations revealed an odd–even effect for the phenyl-terminated SAMs but not the pyridyl-terminated SAMs. Plots of Vtrans versus the measured shift in work function, measured with a Kelvin probe, reveal a roughly linear trend. Plots of the difference between HOMO and Fermi energies reveal a strong linear trend with two distinct series that clearly differentiate the two series of SAMs, even between SAMs with nearly identical HOMO levels, but only when the dipole-induced shift in vacuum level is considered. The influence of the electronic properties of the SAMs is clearly evident in the conductance data and highlights the importance of molecular dipoles in tunneling junctions comprising SAMs. Taken together, the data show that tunneling junctions incorporating EGaIn as a top contact are sensitive enough to differentiate SAMs that differ by the substitution of a single atom.