How to Understand and Interpret Current Flow in Nanopore/Electrode Devices

Nanopore-based single-molecule sensors have become an important class of analytical devices that have in some cases already reached the market place. Traditionally operated in a two-electrode configuration, devices with three or more electrodes have emerged recently, for example with a view on switching the transport properties of the nanopore or even tunneling-based detection of analytes with the ultimate goal of inexpensive and ultrafast DNA sequencing. How do these additional electrodes affect the current distribution in the cell and hence the sensor performance? This is significantly less clear and thus in focus here. We use impedance modeling of a prototypical three-electrode nanopore sensor and show that, depending on the conditions, standard experimental device characterization is severely affected by the presence of the third electrode. On the other hand, the simulations also provide guidelines on how to avoid such complications, identify “safe” operating conditions, and design criteria for optimized nanopore sensors

Multivariate Approach to Single-Molecule Thermopower and Electrical Conductance Measurements

We report a method using scanning tunneling microscope single molecular break junction to simultaneously measure and correlate the single-molecule thermopower and electrical conductance. In contrast to previously reported approaches, it does not require custom-built electronics and takes advantage of a trace-by-trace calibration of the thermal offset at the Au/Au contact, thus greatly facilitating thermoelectric measurements at the single-molecule level. We report measurements of three molecules, 1,4-di­(4-(ethynyl­(phenylthioacetate))) benzene, 1,8-octanedithiol, and 4,4′-bipyridine, and determine single-molecule Seebeck coefficients of 12(3), 5(2), and −5(2) μV K–1, respectively. Furthermore, the method statistically correlates the Seebeck voltage offset, electrical conductance, and stretching displacement of the single-molecule junction and allows for direct comparison with current-distance spectroscopy results obtained at constant bias

Mapping the Ion Current Distribution in Nanopore/Electrode Devices

Solid-state nanopores with integrated electrodes have interesting prospects in next-generation single-molecule biosensing and sequencing. These include “gated” nanopores with a single electrode integrated into the membrane, as well as two-electrode designs, such as a transversal tunneling junction. Here we report the first comprehensive analysis of current flow in a three-electrode device as a model for this class of sensors. As a new feature, we observe apparent rectification in the pore current that is rooted in the current distribution of the cell, rather than the geometry or electrostatics of the pore. We benchmark our results against a recently developed theoretical model and define operational parameters for nanopore/electrode structures. Our findings thus facilitate the rational design of such sensor devices

Intrinsic Multistate Switching of Gold Clusters through Electrochemical Gating

The electrochemical behavior of small metal nanoparticles is governed by Coulomb-like charging and equally spaced charge-transfer transitions. Using electrochemical gating at constant bias voltage, we show, for the first time, that individual nanoparticles can be operated as multistate switches in condensed media at room temperature, displaying distinct peak features in the tunneling current. The tunneling conductance increases with particle charge, suggesting that solvent reorganization and dielectric saturation become increasingly important

Branched Redox-Active Complexes for the Study of Novel Charge Transport Processes

The syntheses and electrochemical/optical properties of some branched and linear 1,1′-substituted ferrocene complexes for molecular electronics are described. Metal centers were extended (and where relevant, connected) by arylethynyl spacers functionalized with <i>m-</i>pyridyl, <i>tert-</i>butylthiol (S^<i>t</i>Bu), and trimethylsilyl (TMS) moieties. Such systems provide two well-defined molecular pathways for electron transfer and hold interesting prospects for the study of new charge transport processes, such as quantum interference, local gating, and correlated hopping events

Single Molecule Ionic Current Sensing in Segmented Flow Microfluidics

Herein, we describe the integration of two glass nanopores into a segmented flow microfluidic device with a view on enhancing the functionality of label free, single molecule nanopore sensors. Within a robust and mechanically stable platform, individual droplet compositions are distinguished before single molecule translocations from the droplet are detected electrochemically via the Coulter principle. This result is highly significant, combining the sensitivity of single molecule methods and their ability to overcome the clouding of the ensemble average with the “isolated microreactor” benefits of droplet microfluidics. Furthermore, devices as presented here provide the platform for the development of systems where the injection and extraction of single molecules allow droplet composition to be controlled at the molecular level

Combined Impact of Denticity and Orientation on Molecular-Scale Charge Transport

Reducing the dimensions of electronic devices to the nanoscale is an important objective with significant scientific and technical challenges. In molecule-based approaches, the orientation of the molecule and coordination to electrodes (denticity) can dramatically affect the electrical properties of the junction. Typically, higher conductance is associated with shorter transport distances and stronger molecule–electrode coupling; however, this is not always the case, as highlighted in this study. We focused on 7,7,8,8-tetracyanoquinodimethane (TCNQ) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) molecules and have used the scanning tunneling microscopy break junction (STM-BJ) method to measure the electrical conductance of single molecules bridged between gold electrodes with different molecular orientations and with varying denticities. In conjunction with the experiments, density functional theory (DFT) and nonequilibrium Green’s function (NEGF) calculations were performed to determine the conductance of four distinct molecular configurations. The calculated conductances show how different configurations and denticities influence the molecular orbital offsets with respect to the Fermi level and provide assignments for the experimental results. Surprisingly, lower denticity results in higher conductance, with the highest predicted molecular conductance being 0.6 G0, which is explained by the influence of molecule–electrode coupling on the energy of molecular orbitals relative to the Fermi level. These results highlight the importance of molecular geometry and binding configuration of the molecule to the electrode. Consequently, our findings have profound ramifications for applications in which orbital alignment is critical to the efficiency of charge transport, such as in dye sensitized solar cells, molecular switches, and sensors

Mechanism of Electrochemical Charge Transport in Individual Transition Metal Complexes

We used electrochemical scanning tunneling microscopy (STM) and spectroscopy (STS) to elucidate the mechanism of electron transport through individual pyridyl-based Os complexes. Our tunneling data obtained by two-dimensional electrochemical STS and STM imaging lead us to the conclusion that electron transport occurs by thermally activated hopping. The conductance enhancement around the redox potential of the complex, which is reminiscent of switching and transistor characterics in electronics, is reflected both in the STM imaging contrast and directly in the tunneling current. The latter shows a biphasic distance dependence, in line with a two-step electron hopping process. Under conditions where the substrate/molecule electron transfer (ET) step is dominant in determining the overall tunneling current, we determined the conductance of an individual Os complex to be 9 nS (Vbias = 0.1 V). We use theoretical approaches to connect the single-molecule conductance with electrochemical kinetics data obtained from monolayer experiments. While the latter leave some controversy regarding the degree of electronic coupling, our results suggest that electron transport occurs in the adiabatic limit of strong electronic coupling. Remarkably, and in contrast to established ET theory, the redox-mediated tunneling current remains strongly distance dependent due to the electronic coupling, even in the adiabatic limit. We exploit this feature and apply it to electrochemical single-molecule conductance data. In this way, we attempt to paint a unified picture of electrochemical charge transport at the single-molecule and monolayer levels

High Precision Fabrication and Positioning of Nanoelectrodes in a Nanopore

A simple and versatile method for the direct fabrication of tunneling electrodes with controllable gap distance by using electron-beam-induced deposition (EBID) is presented. We show that tunneling nanogaps smaller than the minimum feature size realizable by conventional EBID can be achieved with a standard scanning electron microscope. These gaps can easily be embedded in nanopores with high accuracy. The controllability of this fabrication method and the nanogap geometry was verified by SEM and TEM imaging. Furthermore, tunneling spectroscopy in a group of solvents with different barrier heights was used to determine the nanogap functionality. Ultimately, the presented fabrication method can be further applied for the fabrication of arrays of nanogap/nanopores or nanogap electrodes with tunable electrode materials. Additionally, this method can also offer direct fabrication of nanoscale electrode systems with tunable spacing for redox cycling and plasmonic applications, which represents an important step in the development of tunneling nanopore structures and in enhancing the capabilities of nanopore sensors