Rachel K. Smith

Control and placement of molecules via self-assembly

We use self- and directed assembly to pattern organic monolayers on the nanometre scale. The ability of the scanning tunnelling microscope to obtain both nanometre-scale structural and electronic information is used to characterize patterning techniques, to elucidate the intermolecular interactions that drive them and to probe the structures formed. We illustrate three successful approaches: (1) phase separation of self-assembled monolayers by terminal and internal functionalization, (2) phase separation of self-assembled monolayers induced by post-adsorption processing and (3) control of molecular placement by insertion into a self-assembled monolayer. These methods demonstrate the possibilities of patterning films by exploiting the intrinsic properties of the molecules. We employ these methods to prepare matrix-isolated samples to probe molecular electronic properties of single and bundled molecules.

Exploiting intermolecular interactions and self-assembly for ultrahigh resolution nanolithography

The combination of self-, directed, and positional assembly techniques, i.e., “bottom up” fabrication, will be essential for patterning and connecting future nanodevices. Systematic exploration of local intermolecular interactions on surfaces will permit their exploitation for the rational design of molecular-scale surface structures. We use the scanning tunneling microscope to probe the local behavior of self-assembled films at the nanometer scale. The ability to control the molecular placement within and by self-assembled monolayers is a means of patterning surfaces. A monolayer with customized features can be produced by manipulating the dynamics of film formation, which are heavily affected by the selectable intermolecular interactions of adsorbates and the structural components naturally occurring within the films. Additionally, the controlled placement and thickness of self-assembled multilayers created from alternating strata of α,ω-mercaptoalkanoic acids and coordinated metal ions can be developed...

Measurements and Mechanisms of Single-Molecule Conductance Switching

We have engineered and analyzed oligo(phenylene-ethynylene) (OPE) derivatives to understand and to control the bistable conductance switching exhibited by these molecules when inserted into saturated alkanethiolate and amidecontaining alkanethiolate self-assembled monolayers (SAMs) on Au{111}. By engineering the structures of the OPE derivatives, we have shown conductance switching to depend on hybridization changes at the molecule–substrate interface. In addition, we have demonstrated bias-dependent switching controlled by interactions between the dipole of the OPEs and the electric field applied between the scanning tunneling microscope tip and the substrate. These interactions are stabilized via intermolecular hydrogen bonding between the OPEs and host amide-containing SAMs.

Nanometer-Scale Electronics and Storage

The ability to control the placement of molecules is essential for the patterning and fabrication of nanoscale electronic devices. We apply selective chemistry and self-assembly in combination with conventional nanolithographic techniques to reach higher resolution, greater precision, and chemical versatility in the nanostructures that we create. We illustrate three successful approaches: (1) phase separation of self-assembled monolayers (SAMs) by terminal and internal functionalization, (2) phase separation of SAMs induced by post-adsorption processing and (3) control of molecular placement by insertion into a self-assembled monolayer. These methods demonstrate the possibilities of patterning films by exploiting the intrinsic properties of the molecules. We then employ these self-assembled monolayers as a means to isolate molecules with electronic function to determine the mechanisms of function, and the relationships between molecular structure, environment, connection, coupling, and function. Using self-assembly techniques in combination with scanning tunneling microscopy (STM) we are able to study candidate molecular switches individually and in small bundles. Alkanethiolate SAMs on gold are used as a host two-dimensional matrix to isolate and to insulate electrically the molecular switches. We then individually address and electronically probeeach moleculeusing STM. The conjugated molecules exhibit reversible conductance switching, manifested as a change in the topographic height in the STM images. The origins of switching and the relevant aspects of the molecular structure and environment required will be discussed.