Long-range electron transfer through monolayers and bilayers of alkanethiols in electrochemically controlled Hg[bond]neling junctions

Interface of gallium-based liquid metals: oxide skin, wetting, and applications

Gallium-based liquid metals (GaLMs) are promising for a variety of applications-especially as a component material for soft devices-due to their fluidic nature, low toxicity and reactivity, and high electrical and thermal conductivity comparable to solid counterparts. Understanding the interfacial properties and behaviors of GaLMs in different environments is crucial for most applications. When exposed to air or water, GaLMs form a gallium oxide layer with nanoscale thickness. This "oxide nano-skin" passivates the metal surface and allows for the formation of stable microstructures and films despite the high-surface tension of liquid metal. The oxide skin easily adheres to most smooth surfaces. While it enables effective printing and patterning of the GaLMs, it can also make the metals challenging to handle because it adheres to most surfaces. The oxide also affects the interfacial electrical resistance of the metals. Its formation, thickness, and composition can be chemically or electrochemically controlled, altering the physical, chemical, and electrical properties of the metal interface. Without the oxide, GaLMs wet metallic surfaces but do not wet non-metallic substrates such as polymers. The topography of the underlying surface further influences the wetting characteristics of the metals. This review outlines the interfacial attributes of GaLMs in air, water, and other environments and discusses relevant applications based on interfacial engineering. The effect of surface topography on the wetting behaviors of the GaLMs is also discussed. Finally, we suggest important research topics for a better understanding of the GaLMs interface.

Molecular Electronics: Creating and Bridging Molecular Junctions and Promoting Its Commercialization

Molecular electronics is driven by the dream of expanding Moore's law to the molecular level for next-generation electronics through incorporating individual or ensemble molecules into electronic circuits. For nearly 50 years, numerous efforts have been made to explore the intrinsic properties of molecules and develop diverse fascinating molecular electronic devices with the desired functionalities. The flourishing of molecular electronics is inseparable from the development of various elegant methodologies for creating nanogap electrodes and bridging the nanogap with molecules. This review first focuses on the techniques for making lateral and vertical nanogap electrodes by breaking, narrowing, and fixed modes, and highlights their capabilities, applications, merits, and shortcomings. After summarizing the approaches of growing single molecules or molecular layers on the electrodes, the methods of constructing a complete molecular circuit are comprehensively grouped into three categories: 1) directly bridging one-molecule-electrode component with another electrode, 2) physically bridging two-molecule-electrode components, and 3) chemically bridging two-molecule-electrode components. Finally, the current state of molecular circuit integration and commercialization is discussed and perspectives are provided, hoping to encourage the community to accelerate the realization of fully scalable molecular electronics for a new era of integrated microsystems and applications.

Ionogel-Electrode for the Study of Protein Tunnel Junctions under Physiologically Relevant Conditions

The study of charge transport through proteins is essential for understanding complicated electrochemical processes in biological activities while the reasons for the coexistence of tunneling and hopping phenomena in protein junctions still remain unclear. In this work, a flexible and conductive ionogel electrode is synthesized and is used as a top contact to form highly reproducible protein junctions. The junctions of proteins, including human serum albumin, cytochrome C and hemoglobin, show temperature-independent electron tunneling characteristics when the junctions are in solid states while with a different mechanism of temperature-dependent electron hopping when junctions are hydrated under physiologically relevant conditions. It is demonstrated that the solvent reorganization energy plays an important role in the electron-hopping process and experimentally shown that it requires ≈100 meV for electron hopping through one heme group inside a hydrated protein molecule connected between two electrodes.

σ-σ Stacked supramolecular junctions

Intermolecular charge transport plays an essential role in organic electronic materials and biological systems. To date, experimental investigations of intermolecular charge transport in molecular materials and electronic devices have been restricted to conjugated systems in which π-π stacking interactions are involved. Herein we demonstrate that the σ-σ stacking interactions between neighbouring non-conjugated molecules offer an efficient pathway for charge transport through supramolecular junctions. The conductance of σ-σ stacked molecular junctions formed between two non-conjugated cyclohexanethiol or single-anchored adamantane molecules is comparable to that of π-π stacked molecular junctions formed between π-conjugated benzene rings. The current-voltage characteristics and flicker noise analysis demonstrate the existence of stacked molecular junctions formed between the electrode pairs and exhibit the characteristics of through-space charge transport. Density functional theory calculations combined with the non-equilibrium Green's function method reveal that efficient charge transport occurs between two molecules configured with σ-σ stacking interactions.

Semiconducting Langmuir-Blodgett Films of Porphyrin Paddle-Wheel Frameworks for Photoelectric Conversion

Understanding the photocurrent transportation within porphyrin-containing metal-organic frameworks (PMOFs) will be a critical step in applying these materials in light-harvesting molecular devices in the future. Two copper porphyrin paddle-wheel frameworks (Cu-PPFs) were employed to study the influence of metal ions coordinated into the porphyrin ligands on conductivity and photoelectron transfer capability. To compare the electronic and optical properties of both materials, we prepared an ultrathin film of each PPF via a Langmuir-Blodgett method. The resulting films exhibited uniform morphology and single-crystalline domains, in addition to photoelectric conversion capabilities. We confirmed that both Cu-PPFs have semiconducting properties with an optical band gap of around 2.7 eV. The current density generated by both Cu-PPFs was studied through a mercury drop junction approach. We observed a slightly higher conductivity from the Cu-PPF film consisting of metalloporphyrins than the one without copper doping in the porphyrin centers. In addition, the copper-ion-coordinated porphyrins were found to be more favorable for facilitating photoinduced electron transfer from the Cu-PPF film to a conductive glass substrate. This work presents a new approach of combining thin film fabrication and electro-heterojunction measurement to study electron transfer within an ultrathin film.

Understanding interface (odd–even) effects in charge tunneling using a polished EGaIn electrode

Charge transport across large area molecular tunneling junctions is widely studied due to its potential in the development of quantum electronic devices.

Photoswitchable Sn-Cyt c Solid-State Devices

Electron transfer across proteins plays an important role in many biological processes, including those relevant for the conversion of solar photons to chemical energy. Previous studies demonstrated the generation of photocurrents upon light irradiation in a number of photoactive proteins, such as photosystem I or bacteriorhodopsin. Here, it is shown that Sn-cytochrome c layers act as reversible and efficient photoelectrochemical switches upon integration into large-area solid-state junctions. Photocurrents are observed both in the Soret band (λ = 405 nm) and in the Q band (λ = 535 nm), with current on/off ratios reaching values of up to 25. The underlying modulation in charge-transfer rate is attributed to a hole-transport channel created by the photoexcitation of the Sn-porphyrin.

One-Nanometer Thin Monolayers Remove the Deleterious Effect of Substrate Defects in Molecular Tunnel Junctions

Defects in self-assembled monolayer (SAMs) based junctions cause the largest deviation between predicted and measured values of the tunnelling current. We report the remarkable, seemingly counterintuitive finding that shorter, less-ordered SAMs provide, unlike taller crystalline-like SAMs, higher quality tunnelling barriers on defective substrates, which points to self-repair of liquid-like SAMs on defects. The molecular dynamics show that short-chain molecules can more easily rotate into low-density boundary regions and smoothen out defects than thick solid-like SAMs. Our findings point to an attractive means of removing their deleterious effects simply by using flexible molecules.

Bias induced transition from an ohmic to a non-ohmic interface in supramolecular tunneling junctions with Ga2O3/EGaIn top electrodes

This study describes that the current rectification ratio, R ≡ |J|(-2.0 V)/|J|(+2.0 V) for supramolecular tunneling junctions with a top-electrode of eutectic gallium indium (EGaIn) that contains a conductive thin (0.7 nm) supporting outer oxide layer (Ga2O3), increases by up to four orders of magnitude under an applied bias of >+1.0 V up to +2.5 V; these junctions did not change their electrical characteristics when biased in the voltage range of ±1.0 V. The increase in R is caused by the presence of water and ions in the supramolecular assemblies which react with the Ga2O3/EGaIn layer and increase the thickness of the Ga2O3 layer. This increase in the oxide thickness from 0.7 nm to ∼2.0 nm changed the nature of the monolayer-top-electrode contact from an ohmic to a non-ohmic contact. These results unambiguously expose the experimental conditions that allow for a safe bias window of ±1.0 V (the range of biases studies of charge transport using this technique are normally conducted) to investigate molecular effects in molecular electronic junctions with Ga2O3/EGaIn top-electrodes where electrochemical reactions are not significant. Our findings also show that the interpretation of data in studies involving applied biases of >1.0 V may be complicated by electrochemical side reactions which can be recognized by changes of the electrical characteristics as a function voltage cycling or in current retention experiments.

Evolution of conformational order during self-assembly of n-alkanethiols on Hg droplets: an infrared spectromicroscopy study

This Letter describes Fourier-transform infrared spectroscopy evidence for the evolution of conformational order and coverage during the formation of n-alkanethiol monolayers on microdroplets of mercury from the solution phase. At the highest coverages obtained by self-assembly, the monolayer is characterized by predominantly all-trans conformational order. For partial monolayers obtained at arbitrarily quenched incubation periods, we find a continuous evolution of the chain conformational order with monolayer coverage. Analyzing these results in light of previously reported models from X-ray scattering reveals a complex self-assembly process in which the density-dependent evolution of the chain conformational order is coupled with that of molecular orientation and density.

Mechanism of rectification in tunneling junctions based on molecules with asymmetric potential drops

This paper proposes a mechanism for the rectification of current by self-assembled monolayers (SAMs) of alkanethiolates with Fc head groups (SC(11)Fc) in SAM-based tunneling junctions with ultra-flat Ag bottom electrodes and liquid metal (Ga(2)O(3)/EGaIn) top electrodes. A systematic physical-organic study based on statistically large numbers of data (N = 300-1000) reached the conclusion that only one energetically accessible molecular orbital (the HOMO of the Fc) is necessary to obtain large rectification ratios R ≈ 1.0 × 10(2) (R = |J(-V)|/|J(V)| at ±1 V). Values of R are log-normally distributed, with a log-standard deviation of 3.0. The HOMO level has to be positioned spatially asymmetrically inside the junctions (in these experiments, in contact with the Ga(2)O(3)/EGaIn top electrode, and separated from the Ag electrode by the SC(11) moiety) and energetically below the Fermi levels of both electrodes to achieve rectification. The HOMO follows the potential of the Fermi level of the Ga(2)O(3)/EGaIn electrode; it overlaps energetically with both Fermi levels of the electrodes only in one direction of bias.

Effect of headgroup on electrical conductivity of self-assembled monolayers on mercury: n-alkanethiols versus n-alkaneselenols

The relative efficiencies of electron tunneling across self-assembled monolayers (SAMs) of n-alkanethiols and n-alkaneselenols, CH(3)-(CH(2))(n)-XH, where n = 8, 9, 11, and X = S or Se, deposited on mercury electrodes were measured via electroreduction of Ru(NH(3))(6)(3+) in aqueous solutions. Electron tunneling rates across the monolayer films decay exponentially with the monolayer thickness with a tunneling coefficient, beta = 1.1 +/- 0.1 per CH(2) irrespective of the identity of the -XH headgroup. Electron tunneling rates across n-alkanethiol monolayers are ca. 4-fold larger than the rates measured across n-alkaneselenol monolayers containing the same number of carbon atoms, signifying the importance of headgroup/metal contact resistance in electron transfer through SAMs on mercury.

Influence of Defects on the Electrical Characteristics of Mercury-Drop Junctions: Self-Assembled Monolayers of n-Alkanethiolates on Rough and Smooth Silver

This paper compares the structural and electrical characteristics of self-assembled monolayers (SAMs) of n-alkanethiolates, SCn (n = 10, 12, 14), on two types of silver substrates: one used as-deposited (AS-DEP) by an electron-beam evaporator, and one prepared using the method of template-stripping. Atomic force microscopy showed that the template-stripped (TS) silver surfaces were smoother and had larger grains than the AS-DEP surfaces, and reflectance-absorbance infrared spectroscopy showed that SAMs formed on TS substrates were more crystalline than SAMs formed on AS-DEP substrates. The range of current densities, J (A/cm2), measured through mercury-drop junctions incorporating a given SAM on AS-DEP silver was, on average, several orders of magnitude larger than the range of J measured through the same SAM on TS silver, and the AS-DEP junctions failed, on average, 3.5 times more often within five current density-voltage (J-V) scans than did TS junctions (depending on the length of the alkyl chains of the molecules in the SAM). The apparent log-normal distribution of J through the TS junctions suggests that, in these cases, it is the variability in the effective thickness of the insulating layer (the distance the electron travels between electrodes) that results in the uncertainty in J. The parameter describing the decay of current density with the thickness of the insulating layer, beta, was either 0.57 A-1 at V = +0.5 V (calculated using the log-mean of the distribution of values of J) or 0.64 A-1 (calculated using the peak of the distribution of values of J) for the TS junctions; the latter is probably the more accurate. The mechanisms of failure of the junctions, and the degree and sources of uncertainty in current density, are discussed with respect to a variety of defects that occur within Hg-drop junctions incorporating SAMs on silver.

Conductance of α-Helical Peptides Trapped within Molecular Junctions

Self-assembled monolayers of alpha-helical peptides on a gold surface were employed as model systems for the investigation of mediated electron transfer. The peptides contained 14, 15, 16, and 17 amino acid residues. The measurements of electron transmission through single molecules of helical peptides were performed using scanning tunneling spectroscopy (STS). The molecules were trapped between the gold tip and the substrate. Electrical contact between the molecule and the gold probe was achieved by the use of peptides containing thiol groups present at each end of the helix. The conductance behavior of the peptides was examined as a function of tip-substrate distance at fixed bias voltage. Measurements performed with peptides containing different numbers of amino acid residues indicate that the distance dependence of electron transmission through an alpha-helix is weaker than that through simple n-alkyl bridges.

Measurement of current-induced local heating in a single molecule junction

We have studied the current-induced local heating effects in single molecules covalently bound to two electrodes by measuring the force required to break the molecule-electrode bonds under various conditions. The breakdown process is thermally activated, which is used to extract the effective temperature of the molecular junction as a function of applied bias voltage. We have also performed first-principles calculations of both local heating and current-induced force effects, and the results are in good agreement with the experimental findings.

Interface geometry and molecular junction conductance: geometric fluctuation and stochastic switching

Metal/molecule/metal transport junctions can transport charge in the elastic scattering (Landauer) regime if the injection gap is large and the molecule is relatively short. Stochastic switching and broad conduction peak distributions have been observed in such junctions. We examine the effect of altering interface geometry on transport, using density functional calculations. For most structures, variations in conductance of order 0-300% are found, but when an atomic wire of Au binds to the molecule, symmetry changes can modify currents by a factor of 10(3).

Experimental Study of the Interplay between Long-Range Electron Transfer and Redox Probe Permeation at Self-Assembled Monolayers: Evidence for Potential-Induced Ion Gating

Evidence for the competition between long-range electron transfer across self-assembled monolayers (SAMs) and incorporation of the redox probe into the film is reported for the electroreduction of Ru(NH(3)) at hydroxyl- and carboxylic-acid-terminated SAMs on a mercury electrode, by using electrochemical techniques that operate at distinct time scales. Two limiting voltammetric behaviors are observed, consistent with a diffusion control of the redox process at mercaptophenol-coated electrodes and a kinetically controlled electron transfer reaction in the presence of neutral HS-(CH(2))(10)-COOH and HS-(CH(2))(n)()-CH(2)OH (n = 3, 5, and 10) SAMs. The monolayer thickness dependence of the standard heterogeneous electron transfer rate constant shows that the electron transfer plane for the reduction of Ru(NH(3)) at hydroxyl-terminated SAMs is located outside the film | solution interface at short times. However, long time scale experiments provide evidence for the occurrence of potential-induced gating of the adsorbed structure in some of the monolayers studied, which takes the form of a chronoamperometric spike. Redox probe permeation is shown to be a kinetically slow process, whose activation strongly depends on redox probe concentration, applied potential, and chemical composition of the intervening medium. The obtained results reveal that self-assembled monolayers made of mercaptobutanol and mercaptophenol preserve their electronic barrier properties up to the reductive desorption potential of a fully grown SAM, whereas those of mercaptohexanol, mercaptoundecanol, and mercaptoundecanoic acid undergo an order/disorder transition below a critical potential, which facilitates the approach of the redox probe toward the electrode surface.

Dielectric properties and frequency response of self-assembled monolayers of alkanethiols

This paper presents dielectric properties of alkanethiol self-assembled monolayers (SAMs) under an ac electric field. Using a Hg-SAM/SAM-Hg junction, we measured the ac impedance of alkanethiol SAMs using a sinusoidal perturbation of 30 mV (peak-to-peak) with frequency ranging from 1 Hz to 1 MHz at zero bias. Semicircles at higher frequencies and at middle frequencies along with Warburg lines at lower frequencies were observed in complex plane impedance plots, that is, Nyquist plots. The frequency response of SAMs was analyzed by modeling the junction using an equivalent circuit and fitting the Nyquist plots. The semicircles at higher frequencies are attributed to the effect of the SAM/SAM interfaces, and the ones at middle frequencies are attributed to the effect of alkanethiol SAMs. The comparison in the plots of the imaginary part of the impedance Z against frequency for the bare Hg electrodes (in pure ethanal) and the SAM-covered Hg electrodes (in alkanethiol solution) supports the analysis. The Warburg lines are attributed to a certain ionic impurity. The dielectric loss spectra are further analyzed. Chain-length-dependent peaks, which correspond to different relaxation mechanisms, at higher frequencies and middle frequencies were observed in the spectra of the dissipation factor (tan delta vs frequency). The peaks move to small frequency with the increase of chain length of alkanethiols. Using a correlation of peak position with the chain length, we then derived active energies of 39-99 meV for alkanethiol SAMs of C7-C18 under an ac electric field.

Electrochemical wiring of α,ω-alkanedithiol molecules into an electrical circuit

The intrinsic electrical conductivity of alpha, omega-alkanedithiol increases if both ends of the molecule are covalently bonded to metallic contacts.

Measurement of single-molecule resistance by repeated formation of molecular junctions

The conductance of a single molecule connected to two gold electrodes was determined by repeatedly forming thousands of gold-molecule-gold junctions. Conductance histograms revealed well-defined peaks at integer multiples of a fundamental conductance value, which was used to identify the conductance of a single molecule. The resistances near zero bias were 10.5 +/- 0.5, 51 +/- 5, 630 +/- 50, and 1.3 +/- 0.1 megohms for hexanedithiol, octanedithiol, decanedithiol, and 4,4' bipyridine, respectively. The tunneling decay constant (betaN) for N-alkanedithiols was 1.0 +/- 0.1 per carbon atom and was weakly dependent on the applied bias. The resistance and betaN values are consistent with first-principles calculations.