Measurements and Mechanisms of Single-Molecule Conductance Switching

Single-Molecule Conductance Behavior of Molecular Bundles

Controlling the orientation of complex molecules in molecular junctions is crucial to their development into functional devices. To date, this has been achieved through the use of multipodal compounds (i.e., containing more than two anchoring groups), resulting in the formation of tri/tetrapodal compounds. While such compounds have greatly improved orientation control, this comes at the cost of lower surface coverage. In this study, we examine an alternative approach for generating multimodal compounds by binding multiple independent molecular wires together through metal coordination to form a molecular bundle. This was achieved by coordinating iron(II) and cobalt(II) to 5,5'-bis(methylthio)-2,2'-bipyridine (L1) and (methylenebis(4,1-phenylene))bis(1-(5-(methylthio)pyridin-2-yl)methanimine) (L2) to give two monometallic complexes, Fe-1 and Co-1, and two bimetallic helicates, Fe-2 and Co-2. Using XPS, all of the complexes were shown to bind to a gold surface in a fac fashion through three thiomethyl groups. Using single-molecule conductance and DFT calculations, each of the ligands was shown to conduct as an independent wire with no impact from the rest of the complex. These results suggest that this is a useful approach for controlling the geometry of junction formation without altering the conductance behavior of the individual molecular wires.

Molecular electronic junction transport: some pathways and some ideas

When a single molecule, or a collection of molecules, is placed between two electrodes and voltage is applied, one has a molecular transport junction. We discuss such junctions, their properties, their description, and some of their applications. The discussion is qualitative rather than quantitative, and focuses on mechanism, structure/function relations, regimes and mechanisms of transport, some molecular regularities, and some substantial challenges facing the field. Because there are many regimes and mechanisms in transport junctions, we will discuss time scales, geometries, and inelastic scattering methods for trying to determine the properties of molecules within these junctions. Finally, we discuss some device applications, some outstanding problems, and some future directions.

Functional molecules and assemblies in controlled environments: formation and measurements

The local environment of a functional molecule or nanoscale assembly has tremendous impact on it and thus can be used for functional control. In addition, the local environment is critical in the interface to the physical, chemical, and biological worlds beyond the assemblies that are the most common applications targeted. Functional measurements without local structural information lack key insight into both the details and the roles of the environment. This Account focuses on progress toward and challenges in the controlled assembly and measurements of functional nanostructures in well-defined environments. The study of single precise supramolecular assemblies in well-defined environments offers unique insights into both interactions and function. By designing interactions between molecules and controlling assembly conditions, we can create and place atomically precise nanostructures. The tools to test the structures targeted and to measure the function of these assemblies are just now being developed and becoming available. Advances in this field have depended on gaining access to measurements at this scale. In particular, we recognize but do not yet understand the critical role of the chemical and physical environment of the assemblies. Likewise, we are just now realizing the important role that the substrates to which the assemblies are attached play in these processes. In order to develop a predictive understanding and the ability to design and to optimize functional assemblies, we must elucidate the physical, chemical, and electronic couplings among the molecules in the assemblies and with their substrates. With a suite of atomic- and molecular-resolution analytical tools, we are able both to ascertain whether the targeted structures have been formed and to measure their function. One of the keys to our ability to determine structure and measure function has been the development and application of methods for the automated acquisition, analysis, and associations of thousands or tens of thousands of single-molecule/particle/assembly structural, dynamic, spectroscopic, and functional data points.