Electron transfer mediating properties of self-assembled monolayers: A nanografting and conductive probe AFM study.

摘要

Molecular electronics have been attracting tremendous interest in recent years. One of the basic questions remaining unanswered, however, in this field is how to determine the "conductivity" of molecules. Conventional methods for measuring electron transport properties through metal-molecule-metal junctions lack the precise descriptions of the contact properties in the junction. Consequently, the "conductivities" of molecules reported in the literature are not consistent. In this dissertation we report a new experimental method for measuring relatively small differences in electron tunneling through two distinct monolayers. We place them side by side using scanning probe nanolithography and compare the tunneling currents by conductive probe atomic force microscopy under identical force, voltage, and tip contamination conditions. We demonstrate the validity of our approach by applying it to two isomeric molecules with similar length and functional groups, with only the position of two functional groups, one aromatic and the other aliphatic, being inverted with respect to each other. The relative values of the two tunneling currents, calculated using density functional theory in the Tersoff-Hamann approach, compare very well with the experimental data, providing us with an example of theory vs experiment agreement that is rather uncommon in this field.; In this dissertation we also use atomic force microscopy (AFM) to nanostructure and image 1, 10-decanedithiol (DDT, or C10S2) and biphenyl 4,4'-dithiol (BPDT) layers on Au (111) surfaces comparing them to those prepared by self-assembly. First, layers of dithiols are self-assembled from solution onto gold surfaces and are imaged with an AFM as a function of time to examine their growth kinetics. Secondly, 100 nm x 100 nm monolayer patches made of dithiol molecules are nanografted into a self-assembled monolayer inert matrix made of 1-decanethiol. Although nanografting of thiols routinely generates very compact layers with good height uniformity, nanostructuring of dithiols using this method always yields multilayers that form through intermolecular S-S bonds. We demonstrate two possible ways of tailoring, layer by layer, the structure of dithiols. First we form multilayers by nanografting, using then the AFM tip to gradually shave away the top layers. In the second we add antioxidant to the solution while doing nanografting to suppress the oxidative coupling of the --SH groups. We found that nanografting in the presence of excess amount of antioxidant can produce monolayers of dithiols. The so produced DDT (C10S2) monolayer patches are lower than what can be calculated by the 30°-tilt model, while the height of nanografted patches of BPDT closely corresponds to a vertical (standing-up) configuration. Finally, we use conductive-probe AFM to investigate the electron tunneling properties through BPDT multilayers. The molecules in these layers turn out to behave as conductive molecular wires, making these nanostructures good candidates for constructing molecular electronic devices.