The merger of electrochemistry and molecular electronics

Learning Conductance: Gaussian Process Regression for Molecular Electronics

Experimental studies of charge transport through single molecules often rely on break junction setups, where molecular junctions are repeatedly formed and broken while measuring the conductance, leading to a statistical distribution of conductance values. Modeling this experimental situation and the resulting conductance histograms is challenging for theoretical methods, as computations need to capture structural changes in experiments, including the statistics of junction formation and rupture. This type of extensive structural sampling implies that even when evaluating conductance from computationally efficient electronic structure methods, which typically are of reduced accuracy, the evaluation of conductance histograms is too expensive to be a routine task. Highly accurate quantum transport computations are only computationally feasible for a few selected conformations and thus necessarily ignore the rich conformational space probed in experiments. To overcome these limitations, we investigate the potential of machine learning for modeling conductance histograms, in particular by Gaussian process regression. We show that by selecting specific structural parameters as features, Gaussian process regression can be used to efficiently predict the zero-bias conductance from molecular structures, reducing the computational cost of simulating conductance histograms by an order of magnitude. This enables the efficient calculation of conductance histograms even on the basis of computationally expensive first-principles approaches by effectively reducing the number of necessary charge transport calculations, paving the way toward their routine evaluation.

Nanofabrication Techniques in Large-Area Molecular Electronic Devices

The societal impact of the electronics industry is enormous—not to mention how this industry impinges on the global economy. The foreseen limits of the current technology—technical, economic, and sustainability issues—open the door to the search for successor technologies. In this context, molecular electronics has emerged as a promising candidate that, at least in the short-term, will not likely replace our silicon-based electronics, but improve its performance through a nascent hybrid technology. Such technology will take advantage of both the small dimensions of the molecules and new functionalities resulting from the quantum effects that govern the properties at the molecular scale. An optimization of interface engineering and integration of molecules to form densely integrated individually addressable arrays of molecules are two crucial aspects in the molecular electronics field. These challenges should be met to establish the bridge between organic functional materials and hard electronics required for the incorporation of such hybrid technology in the market. In this review, the most advanced methods for fabricating large-area molecular electronic devices are presented, highlighting their advantages and limitations. Special emphasis is focused on bottom-up methodologies for the fabrication of well-ordered and tightly-packed monolayers onto the bottom electrode, followed by a description of the top-contact deposition methods so far used.

Tracking Dynamic Doping in a Solid-State Electrochromic Device: Raman Microscopy Validates the Switching Mechanism

Solid-state electrochromic devices often need appropriate characterization to establish the real working mechanism for optimization and diagnosis. Raman mapping has been used here to track "dynamic doping", an important concept in organic electronics and in polythiophene-based solid-state electrochromic devices to understand and validate the mechanism of bias-induced redox-driven color switching. The proposed method demonstrates the live formation and movement of polarons which is best suited for in situ solid-state Raman spectroelectrochemistry. A 2-fold approach has been adopted here for this (1) by fabricating a working device in cross bar geometry followed by in situ spectroscopy to demonstrate the device functioning and (2) by carrying out Raman mapping from a device in custom-designed thin-film-transistor-like geometry to track and actually "see" the mechanism spectroscopically.

Versatile electrochemical approaches towards the fabrication of molecular electronic devices

We highlight state-of-the-art electrochemical approaches for diazonium electroreduction on various electrodes that may be suitable for flexible molecular electronic junctions.

Charge transport and energy storage at the molecular scale: from nanoelectronics to electrochemical sensing

This tutorial review considers how the fundamental quantized properties associated with charge transport and storage, particularly in molecular films, are linked in a manner that spans nanoscale electronics, electrochemistry, redox switching, and derived nanoscale sensing.

Introducing mesoscopic charge transfer rates into molecular electronics

It has been demonstrated that the concept of mesoscopic rate is able to establish a bridge between electrochemical and molecular electronic concepts.

Adsorption of Expanded Pyridinium Molecules at the Electrified Interface and Its Effect on the Electron-Transfer Process

Adsorption properties of a series of redox-active expanded pyridinium molecules were studied at an electrified interface by cyclic and alternating current voltammetry methods. It was shown that the adsorbed state can sufficiently block N-pyramidalization of the pyridinium redox center of 2',6'-diphenyl-[4,1':4',4''-terpyridin]-1'-ium tetrafluoroborate (2), leading to a change of the mechanism from a single two-electron-transfer process to stepwise transfer of two electrons. Chemically locked molecules 1, 9-(pyridin-4-yl)benzo[ c]benzo[1,2]quinolizino[3,4,5,6- ija][1,6]naphthyridin-15-ium tetrafluoroborate (ring fusion), and 3, 3,5-dimethyl-2',6'-diphenyl-[4,1':4',4''-terpyridin]-1'-ium tetrafluoroborate (steric hindrance) do not enable N-pyramidalization of the redox center upon electron transfer (ET) and serve as references. It was shown that 1 follows Langmuir-type adsorption around a potential of zero charge and that 1-3 form a close-packed film with some repulsive interactions between individual molecules at potentials where ET takes place. It has been suggested that all three molecules lie flat on the electrode surface, with the lowest free energy of adsorption found for 2. Maximum surface concentration Γ* equal to (1.4 ± 0.1) × 10^-10 mol·cm^-2 was found for 1, (1.5 ± 0.1) × 10^-10 mol·cm^-2 for 2, and (1.6 ± 0.1) × 10^-10 mol·cm^-2 for 3. These findings will help to clarify the role of molecular contacts with conducting substrate in the single-molecule electron-transport measurements of 1-3 during the metal-molecule-metal junction formation process.

Structure Controlled Long-Range Sequential Tunneling in Carbon-Based Molecular Junctions

Carbon-based molecular junctions consisting of aromatic oligomers between conducting sp² hybridized carbon electrodes exhibit structure-dependent current densities (J) when the molecular layer thickness (d) exceeds ∼5 nm. All four of the molecular structures examined exhibit an unusual, nonlinear ln J vs bias voltage (V) dependence which is not expected for conventional coherent tunneling or activated hopping mechanisms. All molecules exhibit a weak temperature dependence, with J increasing typically by a factor of 2 over the range of 200-440 K. Fluorene and anthraquinone show linear plots of ln J vs d with nearly identical J values for the range d = 3-10 nm, despite significant differences in their free-molecule orbital energy levels. The observed current densities for anthraquinone, fluorene, nitroazobenzene, and bis-thienyl benzene for d = 7-10 nm show no correlation with occupied (HOMO) or unoccupied (LUMO) molecular orbital energies, contrary to expectations for transport mechanisms based on the offset between orbital energies and the electrode Fermi level. UV-vis absorption spectroscopy of molecular layers bonded to carbon electrodes revealed internal energy levels of the chemisorbed films and also indicated limited delocalization in the film interior. The observed current densities correlate well with the observed UV-vis absorption maxima for the molecular layers, implying a transport mechanism determined by the HOMO-LUMO energy gap. We conclude that transport in carbon-based aromatic molecular junctions is consistent with multistep tunneling through a barrier defined by the HOMO-LUMO gap, and not by charge transport at the electrode interfaces. In effect, interfacial "injection" at the molecule/electrode interfaces is not rate limiting due to relatively strong electronic coupling, and transport is controlled by the "bulk" properties of the molecular layer interior.

Electrical characterization of single molecule and Langmuir-Blodgett monomolecular films of a pyridine-terminated oligo(phenylene-ethynylene) derivative

Monolayer Langmuir-Blodgett (LB) films of 1,4-bis(pyridin-4-ylethynyl)benzene (1) together with the "STM touch-to-contact" method have been used to study the nature of metal-monolayer-metal junctions in which the pyridyl group provides the contact at both molecule-surface interfaces. Surface pressure vs area per molecule isotherms and Brewster angle microscopy images indicate that 1 forms true monolayers at the air-water interface. LB films of 1 were fabricated by deposition of the Langmuir films onto solid supports resulting in monolayers with surface coverage of 0.98 × 10(-9) mol·cm(-2). The morphology of the LB films that incorporate compound 1 was studied using atomic force microscopy (AFM). AFM images indicate the formation of homogeneous, monomolecular films at a surface pressure of transference of 16 mN·m(-1). The UV-vis spectra of the Langmuir and LB films reveal that 1 forms two dimensional J-aggregates. Scanning tunneling microscopy (STM), in particular the "STM touch-to-contact" method, was used to determine the electrical properties of LB films of 1. From these STM studies symmetrical I-V curves were obtained. A junction conductance of 5.17 × 10(-5) G 0 results from the analysis of the pseudolinear (ohmic) region of the I-V curves. This value is higher than that of the conductance values of LB films of phenylene-ethynylene derivatives contacted by amines, thiols, carboxylate, trimethylsilylethynyl or acetylide groups. In addition, the single molecule I-V curve of 1 determined using the I(s) method is in good agreement with the I-V curve obtained for the LB film, and both curves fit well with the Simmons model. Together, these results not only indicate that the mechanism of transport through these metal-molecule-metal junctions is non-resonant tunneling, but that lateral interactions between molecules within the LB film do not strongly influence the molecule conductance. The results presented here complement earlier studies of single molecule conductance of 1 using STM-BJ methods, and support the growing evidence that the pyridyl group is an efficient and effective anchoring group in sandwiched metal-monolayer-metal junctions prepared under a number of different conditions.

Electrochemical formation of an ultrathin electroactive film from 1,10-phenanthroline on a glassy carbon electrode in acidic electrolyte

The electrochemical reduction of 1,10-phenanthroline in aqueous acidic electrolyte at a glassy carbon electrode led to the covalent modification of the electrode. Thereafter, the deposited film can be switched to an electroactive form by electrochemical oxidation. An electroactive film can be also generated by alternate reductive and oxidative voltammetric cycling in a 1,10-phenanthroline/aqueous sulfuric acid solution. First, the electrochemical procedure for the formation of a film is presented. Second, the morphology and chemical structure of 1,10-phenanthroline coatings were investigated by atomic force microscopy, time-of-flight secondary ion mass spectrometry, X-ray photoelectron spectroscopy, and electrochemical techniques. The ultrathin (<15 nm) electrodeposited films consist of oligomeric species. The coatings deposited in alternate and/or continuous reductive and oxidative steps contain oxygen atoms incorporated into the oligomer backbone. The preliminary results point out the formation of a dione derivative that is responsible for the electroactivity of the grafted layer.

Localized in situ generation of diazonium cations by electrocatalytic formation of a diazotization reagent

A new one-step electrochemical approach for the localized generation of diazonium cations in the diffusion layer of an electrode by taking advantage of the electrocatalytic properties of the electrode for the formation of the diazotization agent (nitrite) is proposed. Once nitrite anions are formed by electrocatalytic reduction of nitrate, they immediately react with an arylamine to produce the corresponding diazonium cations, which can be electrochemically readily reduced at the electrode surface. By this method, spontaneous modification of the electrode surface can be avoided. Furthermore, because the potential of the electrochemical nitrate reduction depends strongly on the nature of the electrode material, we also demonstrate that selective grafting can be achieved on a surface, which consists of two different materials: copper-gold or copper-glassy carbon substrates.