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.

Gold Nanoparticle Layers on Polystyrene Microspheres of Controlled Structure and Electrokinetic Properties

Formation of positively charged gold nanoparticle layers on polystyrene microparticles (PSMs600) was studied using the electrokinetic and the concentration depletion methods based on atomic force microscopy (AFM) and scanning electron microscopy (SEM) imaging. Primarily, the dependence of electrophoretic mobility of microparticles on the gold nanoparticle concentration in the suspension was measured. These results were quantitatively interpreted in terms of the three-dimensional electrokinetic model. This allowed to derive a formula for calculating the coverage of nanoparticles under in situ conditions whose validity was confirmed by direct SEM imaging of deposited gold nanoparticles (AuNPs). Additionally, the maximum coverage of gold nanoparticles for various ionic strengths was determined using a concentration depletion method based on AFM imaging of residual particles deposited on the silica substrate. The maximum coverage increased with ionic strength attaining a value of 0.35 for the ionic strength of 3 × 10^-3 M. This effect was attributed to the decreasing range of lateral electrostatic interactions among deposited particles. The electrokinetic properties of the gold nanoparticle layers were also evaluated in pH cycling experiments that confirmed their stability. Beyond significance to basic science, the new data acquired in this work confirm the feasibility of preparing gold nanoparticle layers on polymer microparticles characterized by a controlled structure, coverage, and electrokinetic properties.

Molecular rectifiers based on five-coordinate iron(iii)-containing surfactants

The state-of-the-art of metallorganic-based molecular rectification is reviewed with an emphasis on asymmetric five-coordinate Fe^III-containing surfactants in electrode|LB film|electrode assemblies.

Ultrafast Charge Transfer Pathways Through A Prototype Amino-Carboxylic Molecular Junction

Charge transport properties of a vertically stacked organic heterojunction based on the amino-carboxylic (A-C) hydrogen bond coupling scheme are investigated by means of X-ray resonant photoemission and the core-hole clock method. We demonstrate that hydrogen bonding in molecular bilayers of benzoic acid/cysteamine (BA/CA) with an A-C coupling scheme opens a site selective pathway for ultrafast charge transport through the junction. Whereas charge transport from single BA layer directly coupled to the Au(111) is very fast and it is mediated by the phenyl group, the interposition of an anchoring layer of CA selectively hinders the delocalization of electrons from the BA phenyl group but opens a fast charge delocalization route through the BA orbitals close to the A-C bond. This evidences that hydrogen bonding established upon A-C recognition can be exploited to spatially/orbitally manipulate the charge transport properties of heteromolecular junctions.

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.