Controllable nanogap fabrication on microchip by chronopotentiometry

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.

Application of Micro/Nanofabrication Techniques to On-Chip Molecular Electronics

Molecular electronics is a promising subject to overcome the size limitation of silicon-based electronic devices. In the past decades, various micro/nanofabrication techniques have been developed for constructing molecular junctions, and a number of breakthroughs are made in the characterizations and applications of the single-molecule device. The history and progress are reviewed in this article, laying emphasis on the recent works on the combination of micro/nanofabrication techniques with other techniques such as electrochemical deposition and surface-enhanced Raman spectroscopy (SERS). Some prototypical single-molecule devices such as molecular transistors are presented. Finally, the challenges and prospects in the fabrication of single-molecule devices are discussed.

Single-Molecule Conductance of a π-Hybridized Tripodal Anchor while Maintaining Electronic Communication

Direct hybridization between the π-orbital of a conjugated molecule and metal electrodes is recognized as a new anchoring strategy to enhance the electrical conductance of single-molecule junctions. The anchor is expected to maintain direct hybridization between the conjugated molecule and the metal electrodes, and control the orientation of the molecule against the metal electrodes. However, fulfilling both requirements is difficult because multipodal anchors aiming at a robust contact with the electrodes often break the π-conjugation, thereby resulting in an inefficient carrier transport. Herein, a new tripodal anchor framework-a 7,7-diphenyl-7H-benzo[6,7]indeno[1,2-b]thiophene (PBIT) derivative-is developed. In this framework, π-conjugation is maintained in the molecular junction, and the tripodal structure makes the molecule stand upright on the metal electrode. Molecular conductance is measured by the break junction technique. A vector-based classification and first-principles transport calculations determine the single-molecule conductance of the tripodal-anchoring structure. The conductance of the PBIT-based molecule is higher than that of the tripodal anchor having sp³ carbon atoms in the carrier transport pathway. These results demonstrate that extending the π-conjugation to the tripodal leg is an effective strategy for enhancing the conductivities of single-molecule junctions.

Digital image analysis for measuring nanogap distance produced by adhesion lithography

A simple digital image analysis for measuring nanogap distance produced by adhesion lithography is proposed. Adhesion lithography produces metal electrodes with sub-15 nm undulated space and μm to mm scale width without using electron beam lithography. Although the process has been rapidly improved in recent years, there has been no generalized procedure to evaluate the nanogap distance. In this study, we propose a procedure to evaluate a nanogap electrode with large width/gap distance ratios (>1000). The procedure is to determine the average distance of nanogap space from the area and the perimeter of the space by the analysis of the grayscale image. This procedure excludes any arbitrariness of the estimation and gives quantitative comparison of nanogap electrodes produced by different processes.

Atomic switches of metallic point contacts by plasmonic heating

Electronic switches with nanoscale dimensions satisfy an urgent demand for further device miniaturization. A recent heavily investigated approach for nanoswitches is the use of molecular junctions that employ photochromic molecules that toggle between two distinct isoforms. In contrast to the reports on this approach, we demonstrate that the conductance switch behavior can be realized with only a bare metallic contact without any molecules under light illumination. We demonstrate that the conductance of bare metallic quantum contacts can be reversibly switched over eight orders of magnitude, which substantially exceeds the performance of molecular switches. After the switch process, the gap size between two electrodes can be precisely adjusted with subangstrom accuracy by controlling the light intensity or polarization. Supported by simulations, we reveal a more general and straightforward mechanism for nanoswitching behavior, i.e., atomic switches can be realized by the expansion of nanoelectrodes due to plasmonic heating.

Mechanically controllable break junctions for molecular electronics

A mechanically controllable break junction (MCBJ) represents a fundamental technique for the investigation of molecular electronic junctions, especially for the study of the electronic properties of single molecules. With unique advantages, the MCBJ technique has provided substantial insight into charge transport processes in molecules. In this review, the techniques for sample fabrication, operation and the various applications of MCBJs are introduced and the history, challenges and future of MCBJs are discussed.

Nanogap electrode fabrication for a nanoscale device by volume-expanding electrochemical synthesis

A novel nanogap fabrication method using an electrochemical nanopatterning technique is presented. Electrochemical deposition of platinum ions reduces the microgap size to the sub-50-nm range due to the self-limited volume expansion of the electrodes. Additionally, the low crystallinity of platinum reduces the line edge roughness in the electrodes, whereas the high crystallinity of gold increases it. Current compliance, a buffered resistor, and a symmetric deposition strategy are used to achieve high reliability and practicality of nanogap electrodes. As a possible application, an organic thin-film transistor using the nanogap electrodes is also demonstrated.

The fabrication and characterization of adjustable nanogaps between gold electrodes on chip for electrical measurement of single molecules

This work reports on a new method to fabricate mechanically controllable break junctions (MCBJ) with finely adjustable nanogaps between two gold electrodes on solid state chips for characterizing electron transport properties of single molecules. The simple, low cost, robust and reproducible fabrication method combines conventional photolithography, chemical etching and electrodeposition to produce suspended electrodes separated with nanogaps. The MCBJ devices fabricated by the method can undergo many cycles in which the nanogap width can be precisely and repeatedly varied from zero to several nanometers. The method improves the success rate of the MCBJ experiments. Using these devices the electron transport properties of a typical molecular system, commercially available benzene-1,4-dithiol (BDT), have been studied. The I-V and G-V characteristic curves of BDT and the conductance value for a single BDT molecule established the excellent device suitability for molecular electronics research.

Solid state nanogaps for differential measurements of molecular properties

This paper demonstrates the production and probing of solid state nanogaps. These nanogaps can be inexpensively and controllably produced using a combination of molecular and standard photolithography. These nanogaps are implemented for chemical monitoring by using surface enhanced Raman spectroscopy to collect molecular information at the nanogap and current-voltage traces to probe the charge transport of the nanogap. These data show that the oligonucleotides used as the molecular resist are degraded, that some of the degraded oligonucleotides are removed, and then new oligonucleotides are adsorbed.

Nanoscale control and manipulation of molecular transport in chemical analysis

The ability to understand and control molecular transport is critical to numerous chemical measurement strategies, especially as they apply to mass-limited samples in nanometer-scale structures. The characteristics of nanoscale structures and devices highlighted in the examples discussed in this article include enhanced mass transport, accessing novel physical behavior, large surface-to-volume ratio, diminished background signals, and the fact that molecular characteristics can dominate the behavior of the structure. The control of nanoscale transport is physically embodied in different structures and experiments. Those structures and experiments highlighted here are featured because of their centrality (nanochannels and nanopores), their connection to more familiar macroscale phenomena (nanoelectrodes), and/or their ability to introduce control (stimulus-responsive materials) or because they represent especially interesting possibilities (stochastic sensing structures).

Stable atom-scale junctions on silicon fabricated by kinetically controlled electrochemical deposition and dissolution

Metallic atom-scale junctions (ASJs) constitute the natural limit of nanowires, in which the limiting region of conduction is only a few atoms wide. They are of interest because they exhibit ballistic conduction and their conductance is extraordinarily sensitive to molecular adsorption. However, identifying robust and regenerable mechanisms for their production is a challenge. Gold ASJs have been fabricated electrochemically on silicon using an iodide-containing medium to control the kinetics. Extremely slow electrodeposition or electrodissolution rates were achieved and used to reliably produce ASJs with limiting conductance <5 G(0). Starting from a photolithographically fabricated, Si(3)N(4)-protected micrometer-scale Au bridge between two contact electrodes, a nanometer-scale gap was prepared by focused ion beam milling. The opposing Au faces of this construct were then used in an open-circuit working electrode configuration to produce Au ASJs, either directly or by first overgrowing a thicker Au nanowire and electrothinning it back to an ASJ. Gold ASJs produced by either approach exhibit good stabilityin some cases being stable over hours at 300 Kand quantized conductance properties. The influence of deposition/dissolution potential and supporting electrolyte on the stability of ASJs are considered.