Showing papers on "Conductance published in 2011"

Modeling the conductance and DNA blockade of solid-state nanopores.

Abstract: We present measurements and theoretical modeling of the ionic conductance G of solid-state nanopores with 5-100 nm diameters, with and without DNA inserted into the pore. First, we show that it is essential to include access resistance to describe the conductance, in particular for larger pore diameters. We then present an exact solution for G of an hourglass-shaped pore, which agrees very well with our measurements without any adjustable parameters, and which is an improvement over the cylindrical approximation. Subsequently we discuss the conductance blockade ΔG due to the insertion of a DNA molecule into the pore, which we study experimentally as a function of pore diameter. We find that ΔG decreases with pore diameter, contrary to the predictions of earlier models that forecasted a constant ΔG. We compare three models for ΔG, all of which provide good agreement with our experimental data.

Revealing the molecular structure of single-molecule junctions in different conductance states by fishing-mode tip-enhanced Raman spectroscopy

Abstract: The conductance of single-molecule junctions may be governed by the structure of the molecule in the gap or by the way it bonds with the leads, and the information contained in a Raman spectrum is ideal for examining both. Here we demonstrate that molecule-to-surface bonding may be characterized during electron transport by 'fishing-mode' tip-enhanced Raman spectroscopy (FM-TERS). This technique allows mutually verifiable single-molecule conductance and Raman signals with single-molecule contributions to be acquired simultaneously at room temperature. Density functional theory calculations reveal that the most significant spectral change seen for a gold-4,4′-bipyridine-gold junction results from the deformation of the pyridine ring in contact with the drain electrode at high voltage, and these calculations suggest that a stronger bonding interaction between the molecule and the drain may account for the nonlinear dependence of conductance on bias voltage. FM-TERS will lead to a better understanding of electron-transport processes in molecular junctions. The conductance of single-molecule junctions is affected by the structure of the molecule and how it is bound to the electrodes, which may be examined using Raman spectroscopy. Liuet al. have developed 'fishing-mode' tip-enhanced Raman spectroscopy, which allows the simultaneous determination of conductance and Raman spectra.

Strain dependent resistance in chemical vapor deposition grown graphene

Abstract: The strain dependence of conductance of monolayer graphene has been studied experimentally here. The results illustrate the notable transitions: the slight increase, the dramatic decrease, and the sudden dropping of the conductance by gradually increasing the uniaxial strain. The graphene conductance behaves reversibly by tuning of the elastic tensile strain up to 4.5%, while it fails to recover after the plastic deformation at 5%. The change in conductance due to strain is surprisingly high, which indicates the potential applications in electromechanical devices.

Single-Molecule Junctions Based on Nitrile-Terminated Biphenyls: A Promising New Anchoring Group

Abstract: We present a combined experimental and theoretical study of the electronic transport through single-molecule junctions based on nitrile-terminated biphenyl derivatives. Using a scanning tunneling microscope-based break-junction technique, we show that the nitrile-terminated compounds give rise to well-defined peaks in the conductance histograms resulting from the high selectivity of the N-Au binding. Ab initio calculations have revealed that the transport takes place through the tail of the LUMO. Furthermore, we have found both theoretically and experimentally that the conductance of the molecular junctions is roughly proportional to the square of the cosine of the torsion angle between the two benzene rings of the biphenyl core, which demonstrates the robustness of this structure-conductance relationship.

Self-consistent GW calculations of electronic transport in thiol- and amine-linked molecular junctions

Abstract: The electronic conductance of a benzene molecule connected to gold electrodes via thiol, thiolate, or amino anchoring groups is calculated using nonequilibrium Green functions in combination with the fully self-consistent GW approximation for exchange and correlation. The calculated conductance of benzenedithiol and benzenediamine is one-fifth that predicted by standard density functional theory (DFT), in very good agreement with experiments. In contrast, the widely studied benzenedithiolate structure is found to have a significantly higher conductance due to the unsaturated sulfur bonds. These findings suggest that more complex gold-thiolate structures where the thiolate anchors are chemically passivated by Au adatoms are responsible for the measured conductance. Analysis of the energy level alignment obtained with DFT, Hartree-Fock, and GW reveals the importance of self-interaction corrections (exchange) on the molecule and dynamical screening at the metal-molecule interface. The main effect of the GW self-energy is to renormalize the level positions; however, its influence on the shape of molecular resonances also affects the conductance. Non-self-consistent ${\text{G}}_{0}{\text{W}}_{0}$ calculations, starting from either DFT or Hartree-Fock, yield conductance values within 50% of the self-consistent GW results.

Benzenedithiol: A Broad-Range Single-Channel Molecular Conductor

Abstract: More than a decade after the first report of single-molecule conductance, it remains a challenging goal to prove the exact nature of the transport through single molecules, including the number of transport channels and the origin of these channels from a molecular orbital point of view. We demonstrate for the archetypical organic molecule, benzenedithiol (BDT), incorporated into a mechanically controllable break junction at low temperature, how this information can be deduced from studies of the elastic and inelastic current contributions. We are able to tune the molecular conformation and thus the transport properties by displacing the nanogap electrodes. We observe stable contacts with low conductance in the order of 10–3 conductance quanta as well as with high conductance values above ∼0.5 quanta. Our observations show unambiguously that the conductance of BDT is carried by a single transport channel provided by the same molecular level, which is coupled to the metallic electrodes, through the whole c...

Highly conducting π-conjugated molecular junctions covalently bonded to gold electrodes.

Abstract: We measure electronic conductance through single conjugated molecules bonded to Au metal electrodes with direct Au-C covalent bonds using the scanning tunneling microscope based break-junction technique. We start with molecules terminated with trimethyltin end groups that cleave off in situ, resulting in formation of a direct covalent σ bond between the carbon backbone and the gold metal electrodes. The molecular carbon backbone used in this study consist of a conjugated π system that has one terminal methylene group on each end, which bonds to the electrodes, achieving large electronic coupling of the electrodes to the π system. The junctions formed with the prototypical example of 1,4-dimethylenebenzene show a conductance approaching one conductance quantum (G(0) = 2e(2)/h). Junctions formed with methylene-terminated oligophenyls with two to four phenyl units show a 100-fold increase in conductance compared with junctions formed with amine-linked oligophenyls. The conduction mechanism for these longer oligophenyls is tunneling, as they exhibit an exponential dependence of conductance on oligomer length. In addition, density functional theory based calculations for the Au-xylylene-Au junction show near-resonant transmission, with a crossover to tunneling for the longer oligomers.

An MCBJ case study: The influence of π-conjugation on the single-molecule conductance at a solid/liquid interface.

Abstract: π-Conjugation plays an important role in charge transport through single molecular junctions We describe in this paper the construction of a mechanically controlled break-junction setup (MCBJ) equipped with a highly sensitive log I–V converter in order to measure ultralow conductances of molecular rods trapped between two gold leads The current resolution of the setup reaches down to 10 fA We report single-molecule conductance measurements of an anthracene-based linearly conjugated molecule (AC), of an anthraquinone-based cross-conjugated molecule (AQ), and of a dihydroanthracene-based molecule (AH) with a broken conjugation The quantitative analysis of complementary current–distance and current–voltage measurements revealed details of the influence of π-conjugation on the single-molecule conductance

Enhanced thermoelectric properties in graphene nanoribbons by resonant tunneling of electrons

Abstract: Strongly enhanced thermoelectric properties are predicted for graphene nanoribbons (GNRs) with optimized pattern By means of nonequilibrium Green's function atomistic simulation of electron and phonon transport, we analyze the thermal and electrical properties of perfect GNRs as a function of their width and their edge orientation to identify a strategy likely to degrade the thermal conductance while retaining high electronic conductance and thermopower An effect of resonant tunneling of electrons is detected in mixed GNRs consisting of alternate zigzag and armchair sections To fully benefit from this effect and from strongly reduced phonon thermal conductance, a structure with armchair and zigzag sections of different widths is proposed It is shown to provide a high thermoelectric factor of merit $\mathit{ZT}$ exceeding unity at room temperature

Controlling single-molecule conductance through lateral coupling of pi orbitals

Abstract: In recent years, various single-molecule electronic components have been demonstrated1. However, it remains difficult to predict accurately the conductance of a single molecule and to control the lateral coupling between the π orbitals of the molecule and the orbitals of the electrodes attached to it. This lateral coupling is well known to cause broadening and shifting of the energy levels of the molecule; this, in turn, is expected to greatly modify the conductance of an electrode–molecule–electrode junction2,3,4,5,6. Here, we demonstrate a new method, based on lateral coupling, to mechanically and reversibly control the conductance of a single-molecule junction by mechanically modulating the angle between a single pentaphenylene molecule bridged between two metal electrodes. Changing the angle of the molecule from a highly tilted state to an orientation nearly perpendicular to the electrodes changes the conductance by an order of magnitude, which is in qualitative agreement with theoretical models of molecular π-orbital coupling to a metal electrode. The lateral coupling is also directly measured by applying a fast mechanical perturbation in the horizontal plane, thus ruling out changes in the contact geometry or molecular conformation as the source for the conductance change. The conductance of a single molecule can be reversibly tuned by mechanically changing the tilt angle between the molecule and contact electrodes.

Highly Conducting pi-Conjugated Molecular Junctions Covalently Bonded to Gold Electrodes

Abstract: We measure electronic conductance through single conjugated molecules bonded to Au metal electrodes with direct Au-C covalent bonds using the scanning tunneling microscope based break-junction technique. We start with molecules terminated with trimethyltin end groups that cleave off in situ resulting in formation of a direct covalent sigma bond between the carbon backbone and the gold metal electrodes. The molecular carbon backbone used in this study consist of a conjugated pi-system that has one terminal methylene group on each end, which bonds to the electrodes, achieving large electronic coupling of the electrodes to the pi-system. The junctions formed with the prototypical example of 1,4-dimethylenebenzene show a conductance approaching one conductance quantum (G0 = 2e2/h). Junctions formed with methylene terminated oligophenyls with two to four phenyl units show a hundred-fold increase in conductance compared with junctions formed with amine-linked oligophenyls. The conduction mechanism for these longer oligophenyls is tunneling as they exhibit an exponential dependence of conductance with oligomer length. In addition, density functional theory based calculations for the Au-xylylene-Au junction show near-resonant transmission with a cross-over to tunneling for the longer oligomers.

Atomic-scale engineering of electrodes for single-molecule contacts

Abstract: The conductance of a single C60 molecule depends on the number of atoms in the electrodes that are in direct contact with the molecule.

Conductance of a Helical Edge Liquid Coupled to a Magnetic Impurity

Abstract: Transport in an ideal two-dimensional quantum spin Hall device is dominated by the counterpropagating edge states of electrons with opposite spins, giving the universal value of the conductance, $2{e}^{2}/h$. We study the effect on the conductance of a magnetic impurity, which can backscatter an electron from one edge state to the other. In the case of isotropic Kondo exchange we find that the correction to the electrical conductance caused by such an impurity vanishes in the dc limit, while the thermal conductance does acquire a finite correction due to the spin-flip backscattering.

Anomalous Aharonov-Bohm Conductance Oscillations from Topological Insulator Surface States

Abstract: We study Aharonov-Bohm (AB) conductance oscillations arising from the surface states of a topological insulator nanowire, when a magnetic field is applied along its length. With strong surface disorder, these oscillations are predicted to have a component with anomalous period Φ(0)=hc/e, twice the conventional period. The conductance maxima are achieved at odd multiples of 1/2Φ(0), implying that a π AB phase for electrons strengthens the metallic nature of surface states. This effect is special to topological insulators, and serves as a defining transport property. A key ingredient, the surface curvature induced Berry phase, is emphasized here. We discuss similarities and differences from recent experiments on Bi2Se3 nanoribbons, and optimal conditions for observing this effect.

Environmental Control of Single-Molecule Junction Transport

Abstract: The conductance of individual 1,4-benzenediamine (BDA)-Au molecular junctions is measured in different solvent environments using a scanning tunneling microscope based point-contact technique. Solvents are found to increase the conductance of these molecular junctions by as much as 50%. Using first principles calculations, we explain this increase by showing that a shift in the Au contact work function is induced by solvents binding to undercoordinated Au sites around the junction. Increasing the Au contact work function reduces the separation between the Au Fermi energy and the highest occupied molecular orbital of BDA in the junction, increasing the measured conductance. We demonstrate that the solvent-induced shift in conductance depends on the affinity of the solvent to Au binding sites and also on the induced dipole (relative to BDA) upon adsorption. Via this mechanism, molecular junction level alignment and transport properties can be statistically altered by solvent molecule binding to the contact surface.

Do Molecular Conductances Correlate with Electrochemical Rate Constants? Experimental Insights

Abstract: We measured single-molecule conductances for three different redox systems self-assembled onto gold by the STMBJ method and compared them with electrochemical heterogeneous rate constants determined by ultrafast voltammetry It was observed that fast systems indeed give higher conductance Monotonous dependency of conductance on potential reveals that large molecular fluctuations prevent the molecular redox levels to lie in between the Fermi levels of the electrodes in the nanogap configuration Electronic coupling factors for both experimental approaches were therefore evaluated based on the superexchange mechanism theory The results suggest that coupling is surprisingly on the same order of magnitude or even larger in conductance measurements whereas electron transfer occurs on larger distances than in transient electrochemistry

Interfacial thermal transport in atomic junctions

Abstract: We study ballistic interfacial thermal transport across atomic junctions. Exact expressions for phonon transmission coefficients are derived for thermal transport in one-junction and two-junction chains, and verified by numerical calculation based on a nonequilibrium Green's function method. For a single-junction case, we find that the phonon transmission coefficient typically decreases monotonically with increasing freqency. However, in the range between the point of equal frequency spectrum and that of equal acoustic impedance, it first increases then decreases, which explains why the Kapitza resistance calculated from the acoustic mismatch model is far larger than the experimental values at low temperatures. The junction thermal conductance reaches a maximum when the interfacial coupling equals the harmonic average of the spring constants of the two semi-infinite chains. For three-dimensional junctions, in the weak coupling limit, we find that the conductance is proportional to the square of the interfacial coupling, while for a intermediate coupling strength the conductance is approximately proportional to the interfacial coupling strength. For two-junction chains, the transmission coefficient oscillates with the frequency due to interference effects. The oscillations between the two envelope lines can be understood analytically, thus providing guidelines for designing phonon frequency filters.

Single-Molecule Conductance of Pyridine-Terminated Dithienylethene Switch Molecules

Abstract: We have investigated the conductance of individual optically switchable dithienylethene molecules in both their conducting closed configuration and nonconducting open configuration, using the technique of repeatedly formed break-junctions. We employed pyridine groups to link the molecules to gold electrodes in order to achieve relatively well-defined molecular contacts and stable conductance. For the closed form of each molecule, we observed a peak in the conductance histogram constructed without any data selection, allowing us to determine the conductance of the fully stretched molecules. For two different dithienylethene derivatives, these closed-configuration conductances were (3.3 ± 0.5) × 10–5G0 and (1.5 ± 0.5) × 10–6G0, where G0 is the conductance quantum. For the open configuration of the molecules, the existence of electrical conduction via the molecule was evident in traces of conductance versus junction displacement, but the conductance of the fully stretched molecules was less than the noise fl...

Dependence of single-molecule conductance on molecule junction symmetry.

Abstract: The symmetry of a molecule junction has been shown to play a significant role in determining the conductance of the molecule, but the details of how conductance changes with symmetry have heretofore been unknown. Herein, we investigate a naphthalenedithiol single-molecule system in which sulfur atoms from the molecule are anchored to two facing gold electrodes. In the studied system, the highest single-molecule conductance, for a molecule junction of 1,4-symmetry, is 110 times larger than the lowest single-molecule conductance, for a molecule junction of 2,7-symmetry. We demonstrate clearly that the measured dependence of molecule junction symmetry for single-molecule junctions agrees with theoretical predictions.

Charge regulation in nanopore ionic field-effect transistors.

Abstract: We studied the ionic conductance through Al${}_{2}$O${}_{3}$ nanopore transistors to probe the surface charge density and its dependence on the applied gate field. The observed conductance modulations are entirely attributable to the electrostatic field effect, and their dependence on pH, ionic strength, and gate voltage is described by a quantitative model. Importantly, these experiments revealed how reactive surface groups dominate the response of nanofluidic field-effect devices via a chemical effect called charge regulation. A quantitative understanding of this effect enables the development of new nanofluidic technologies.

Optical modulation of molecular conductance.

Abstract: A novel scanning probe microscope stage permits break junction measurements of single molecule conductance while the molecules are illuminated with visible light. We studied a porphyrin-fullerene dyad molecule designed to form a charge separated state on illumination. A significant fraction of illuminated molecules become more conductive, returning to a lower conductance in the dark, suggesting the formation of a long-lived charge separated state on the indium–tin oxide surface. Transient absorption spectra of these molecular layers are consistent with formation of a long-lived charge separated state, a finding with implications for the design of molecular photovoltaic devices.

Phosphorus and phosphorus-nitrogen doped carbon nanotubes for ultrasensitive and selective molecular detection

Abstract: A first-principles approach is used to establish that substitutional phosphorus atoms within carbon nanotubes strongly modify the chemical properties of the surface, thus creating highly localized sites with specific affinity towards acceptor molecules. Phosphorus–nitrogen co-dopants within the tubes have a similar effect for acceptor molecules, but the P–N bond can also accept charge, resulting in affinity towards donor molecules. This molecular selectivity is illustrated in CO and NH3 adsorbed on PN-doped nanotubes, O2 on P-doped nanotubes, and NO2 and SO2 on both P- and PN-doped nanotubes. The adsorption of different chemical species onto the doped nanotubes modifies the dopant-induced localized states, which subsequently alter the electronic conductance. Although SO2 and CO adsorptions cause minor shifts in electronic conductance, NH3, NO2, and O2adsorptions induce the suppression of a conductance dip. Conversely, the adsorption of NO2 on PN-doped nanotubes is accompanied with the appearance of an additional dip in conductance, correlated with a shift of the existing ones. Overall these changes in electric conductance provide an efficient way to detect selectively the presence of specific molecules. Additionally, the high oxidation potential of the P-doped nanotubes makes them good candidates for electrode materials in hydrogen fuel cells.

Role of wetting and nanoscale roughness on thermal conductance at liquid-solid interface

Abstract: Using non-equilibrium molecular dynamics simulations in which a temperature gradient is imposed, we determine the thermal resistance of a model liquid-solid interface with and without nanoscale roughness. Our simulations reveal that in both cases the key factor controlling interfacial thermal conductance is the strength of the bonding between liquid and solid atoms. In addition, we show that in the case of rough surfaces, the nanoscopic wetting-dewetting transition has a strong signature in the interfacial thermal conductance. Interestingly, the smooth and rough interfaces, despite distinct geometry, show quantitatively the same dependence of the interfacial conductance on the value of the interfacial adhesion.

Bonding and pressure-tunable interfacial thermal conductance

Abstract: Stiffness of interfacial bonding between two materials plays a major role in controlling the thermal conductance of the interface. We use nonequilibrium molecular dynamics simulations to study interfacial thermal conductance at an epitaxial interface between two fcc crystals with interatomic interactions described by Lennard Jones (LJ) potentials. The interface stiffness was varied by two different methods: (i) application of pressure and (ii) direct change of the interfacial bonding strength by varying the depth of potential well parameter of the LJ potential. Our results show that when the interfacial bonding strength is low, interfacial stiffness increases linearly with pressure due to the anharmonicity of atomic interactions. Consequently, the interfacial conductance increases, first proportionally to interfacial stiffness, and then it saturates at a high value. Quantitatively similar behavior is observed when the stiffness of the interfacial bonding is increased by directly varying the depth of the potential well parameter of the LJ potential. By contrast, when the interfacial bonding strength is high, thermal conductance is almost pressure independent and in fact slightly decreases with increasing pressure. This decrease can be explained by the change of overlap between the vibrational density of states (DOS) in the two crystalline materials.

Field-effect modulation of conductance in VO2 nanobeam transistors with HfO2 as the gate dielectric

Abstract: We study field-effect transistors realized from VO2 nanobeams with HfO2 as the gate dielectric. When heated up from low to high temperatures, VO2 undergoes an insulator-to-metal transition. We observe a change in conductance (∼6%) of our devices induced by gate voltage when the system is in the insulating phase. The response is reversible and hysteretic, and the area of hysteresis loop becomes larger as the rate of gate sweep is slowed down. A phase lag exists between the response of the conductance and the gate voltage. This indicates the existence of a memory of the system and we discuss its possible origins.

Implications of cross-species interactions on the temperature dependence of Kapitza conductance

Abstract: We investigate the behavior of Kapitza conductance at interfaces between two Lennard-Jones fcc solids as a function of the range and strength of cross-species interactions via molecular dynamics simulations. It is found that decreasing either of these quantities leads to a reduction in the slope of linear temperature dependence of Kapitza conductance, suggesting a corresponding decrease in the probability of inelastic phonon-phonon interactions. To further explore the mechanisms responsible for such behavior, we calculate the phonon density of states and spectral temperature of each of the monolayers adjacent to the interface. It is found that the reduction of the range and strength of cross-species interactions leads to a softening of the density of states near the interface, while the spectral temperature calculations provide further evidence that such reductions decrease the probability of inelastic phonon scattering. These findings help explain varying accounts of the temperature dependence of Kapitza conductance observed in previous works.

Interfacial thermal conductance of transfer-printed metal films.

Abstract: IO N Deterministic assembly of microdevices by transfer-printing is an advanced manufacturing technology that enables the heterogeneous integration of disparate materials on largearea substrates.[1–5] All active electronic devices generate heat as a byproduct of their operation and thermal management of transfer-printed assemblies must be a consideration whenever the heat flux is large. While the thermal conductivities of most materials used in microelectronics are well known, the thermal conductance of interfaces formed by transfer-printing is unknown. We report studies of the thermal conductance of interfaces formed by transfer-printing of Au and Au(Pd) alloy thin films, 100 μm × 100 μm in area and 100 nm thick, on amorphous SiO2 , hydrogen-terminated Si(001), and singlecrystal Al2O3 substrates. We find that the thermal conductance Gt of transfer-printed interfaces spans a relatively small range, 10 10 kW cm−2. The thermal conductance G of interfaces formed by physical vapor deposition of metal films on dielectric substrates has been studied extensively. G is the transport coefficient that relates the heat flux JQ to the temperature drop ΔT at an interface, JQ = GT . The observed values span a large range, from a low conductance of G ≈ 10 MWm−2K−1 for Bi deposited on hydrogen-terminated diamond[6] to a high conductance[7] of G ≈ 700 MWm−2K−1 for epitaxial TiN/MgO. Often,

Changes in induced polarization associated with the sorption of sodium, lead, and zinc on silica sands.

Abstract: Low-frequency dielectric spectroscopy can be measured in terms of a conductance and a phase lag between the electrical current and the electrical field. This conductance and phase lag can be written as into a complex conductivity with both an in-phase and quadrature components that are frequency dependent. In sands, the low-frequency (10 mHz-40 kHz) spectra of the complex conductivity are dominated by the polarization of the electrical double layer (especially the internal part of the electrical double layer called the Stern layer) and the Maxwell-Wagner polarization (typically above 100 Hz). We present a polarization that is able to explain the complex conductivity spectra including the grain size distribution, the porosity, and the complexation of the mineral surface with the ions of the pore water. To test this model, we investigate the sorption of various cations (Na, Pb, Zn) characterized by different affinities with the surface of silica. Sand column experiments were carried out to see the change in the complex conductivity during the advective/dispersive transport of a lead nitrate solution and a zinc sulfate solution, replacing a sodium chloride solution in the pore space of the sand. The complex conductivity model is able to explain the change of the phase over time.

Influence of anisotropy on thermal boundary conductance at solid interfaces

Abstract: We investigate the role of anisotropy on interfacial transport across solid interfaces by measuring the thermal boundary conductance from 100 to 500 K across Al/Si and Al/sapphire interfaces with different substrate orientations. The measured thermal boundary conductances show a dependency on substrate crystallographic orientation in the sapphire samples (trigonal conventional cell) but not in the silicon samples (diamond cubic conventional cell). The change in interface conductance in the sapphire samples is ascribed to anisotropy in the Brillouin zone along the principal directions defining the conventional cell. This leads to resultant phonon velocities in the direction of thermal transport that vary nearly 40% based on crystallographic direction.

Large Conductance Changes in Peptide Single Molecule Junctions Controlled by pH

Abstract: Because electron transfer is often highly sensitive to bridge length and molecular conformation, changes in these can have a large impact on the conductance of single molecule electrical junctions. In this study, pH is used to control the conformation and effective bridge length of single molecule junctions containing the peptide sequence H(EL)5C (where H stands for histidine, E for glutamic acid, L for leucine, and C for cysteine). The ionizable glutamic acid residues in this peptide result in an oligo-peptide structure highly sensitive to pH. At low pH, the H(EL)5C bridge exists in its more compact α-helical state, while at high pH, deprotonation leads to electrostatic repulsion between the charged carboxylate groups of the glutamic acid residues, promoting more extended conformations. An scanning tunneling microscopy-based method is used to measure the single molecule conductance of Au|H(EL)5C|Au two-terminal junctions in buffered electrolyte solution at low pH (2) and higher pH (6.9). In its more comp...