Molecular engineering and measurements to test hypothesized mechanisms in single molecule conductance switching.

Six customized phenylene-ethynylene-based oligomers have been studied for their electronic properties using scanning tunneling microscopy to test hypothesized mechanisms of stochastic conductance switching. Previously suggested mechanisms include functional group reduction, functional group rotation, backbone ring rotation, neighboring molecule interactions, bond fluctuations, and hybridization changes. Here, we test these hypotheses experimentally by varying the molecular designs of the switches; the ability of the molecules to switch via each hypothetical mechanism is selectively engineered into or out of each molecule.

Molecular engineering of the polarity and interactions of molecular electronic switches.

We have investigated and learned to control switching of oligo(phenylene ethynylene)s embedded in amide-containing alkanethiol self-assembled monolayers on Au{111}. We demonstrate bias-dependent switching of the oligo(phenylene ethynylene)s as a function of the interaction between the dipole moment of the oligo(phenylene ethynylene)s and the electric field applied between the scanning tunneling microscope tip and the substrate. We are able to invert the polarity of the switches by altering their design-inverting their dipole moments.

Cross-step place-exchange of oligo(phenylene-ethynylene) molecules.

We have observed nitro-functionalized oligo(phenylene-ethynylene) molecules exhibiting motion up and down Au{111} substrate monatomic step edges within host self-assembled monolayers of n-alkanethiols, independent of previously observed conductance switching. Single molecules have been imaged with scanning tunneling microscopy to place-exchange reversibly between the top and bottom of monatomic substrate step edges.

Orthogonally functionalized oligomers for controlled self-assembly.

The synthesis of molecules terminated with complementary thiol-protecting groups is described. The target compounds contain functionalities on one end known to form self-assembled monolayers on metal surfaces while at the other end an intact thioacetate is present whereby self-assembly may again occur after an in situ deprotection. Self-assembly data is reported for selected compounds to assess their efficacy in surface adhesion.

Direct covalent grafting of conjugated molecules onto Si, GaAs, and Pd surfaces from aryldiazonium salts.

Using aryldiazonium salts that are air-stable and easily synthesized, we describe here a one-step, room-temperature route to direct covalent bonds between pi-conjugated organic molecules on three material surfaces: Si, GaAs, and Pd. The Si can be in the form of single crystal Si including heavily doped p-type Si, intrinsic Si, heavily doped n-type Si, on Si(111) and Si(100), and on n-type polycrystalline Si. The formation of the aryl-metal or aryl-semiconductor bond attachments was confirmed by corroborating evidence from ellipsometry, reflectance FTIR, XPS, cyclic voltammetry, and AFM analyses of the surface-grafted monolayers.