Electron tunneling in molecular junctions

This thesis describes approximately four years of fundamental research in the field of molecular electronics. Data are obtained on nanoscopic metal---molecule---metal junctions using conducting-probe atomic force microscopy (CP-AFM). The CP-AFM method involves a conductive AFM probe that is brought into contact with a molecular monolayer that has been self-assembled on a conductive substrate. Tunneling currents are measured to describe charge transport through a small number of molecules. While there are many architectures for creating such molecular tunnel junctions, CP-AFM offers a quick and simple route to formation of nanoscale junctions void of pinholes, with the ability to change contact metals, and measure and control junction compression.; Four experiments are highlighted in this manuscript as contributions to the field. The first deals with effects of the type of connection between electrode and molecule (i.e., either a chemical bond or a physical contact), electrode work function, and applied bias. The breadth of this experiment focuses on a series of alkanethiol and alkanedithiol monolayers of varying length in order to characterize both the length dependence and the extrapolated contact resistance. These parameters are then used in the context of conventional theory to extract transmission coefficients relating to tunneling transport through different contacts and as the result of different metals.; The second experiment focuses on issues of reproducibility in CP-AFM measurements by performing measurements on Au/decanethiol/Au junctions under a variety of experimental conditions. The third experiment focuses on current rectification (asymmetry with applied voltage) of decanethiol junctions in comparison to a perfluorinated analogue (SHC2H4C 8F17). Alkanethiol monolayers are known to create surface dipoles that decrease the work function of the substrate that they are deposited upon. The opposite shift and rectification direction occurs for the perfluorinated analogue, indicating a correlation of the surface dipole and observed asymmetry.; Finally, applied load is varied in the fourth experiment similar to nanoindentation. Measurements are performed on alkanethiols other than decanethiol, with various loading rates and maximum applied loads. The data are modeled using conventional contact mechanics to estimate film modulus and plastic deformation.