| We have utilized scanning tunneling microscopy (STM) to probe structures relevant to electronics at the nanometer scale. In a series of several experiments we examined the properties of molecules of that are candidate, for molecular electronics device components, oligo-phenylene ethynylenes (OPEs). We investigated the behavior of these molecules as individuals, by isolating and supporting them in matrices of alkanethiolates. It was discovered that these molecules can switch reversibly between multiple conductance states, and the switching events are manifested as a change in apparent height of the molecules in STM images. We determined that the local environment of the isolated molecules strongly determines their switching activity. Based on these experimental results, we conclude that the switching that is observed is a result of motion of the molecule itself, specifically tilting of the molecule at the substrate-molecule bond. This motion can cause a change in the conductance and physical height of the molecule, both of which lead to changes in apparent height in STM images. We demonstrated that the STM probe tip can be used to deliberately switch the molecules, either through direct tip-molecule perturbations or via the tip's electric field.; In related experiments, we compared monolayers of functionalized and non-functionalized OPEs to examine the role of functional groups on the structure and electronic properties of the monolayers. We found that self-assembly of ordered monolayers of these molecules can be inhibited by the presence of functional groups on these molecules. We compared our current-voltage spectra over these monolayers with previously published data, and did not observe the negative differential resistance (NDR) that had been shown for these molecules. We conclude that NDR is not a property intrinsic to these molecules; rather it is measurement dependent.; To control the alkanethiolate matrix in which OPEs are isolated more effectively, we devised a vapor annealing method that allowed us to introduce new alkanethiolate matrix molecules around isolated OPEs. We used two-component monolayers to examine the adsorption and exchange of new matrix molecules being introduced from the vapor phase. We confirmed that vapor annealing was an effective technique to modify the local composition of alkanethiolate matrices and to control defect structure.; In other experiments, we developed a new tool for two-dimensional profiling of dopants on semiconductor surfaces, the alternating current STM (ACSTM). Using a difference frequency detection scheme, we demonstrated the ability of the ACSTM to discern between dopant type and dopant density on bare and patterned silicon substrates. The instrument was able to image dopant patterns with nanometer resolution. |
