| Considerable attention has been devoted to developing molecular-scale devices for applications in future electronic and sensing circuits. Several recent theoretical and experimental results have shown that small variations in the structure of the molecular junction (e.g., molecular conformation, contact geometry, bonding, single molecule vs. monolayer, etc.) can cause significant changes in the electrical properties of the device. Thus, the relationship between the electronic properties and the detailed structure of the junction must be understood before it is possible to use molecules in these applications.;In order to develop an improved understanding of the structure-property relationship, this thesis developed a new nanoscale crossed-wire molecular device that is comprised of a well-ordered monolayer of several hundred molecules sandwiched between a lithographically-defined bottom Ti/Au metal bottom contact and a single Au metal nanowire top contact. This molecular device structure has several attractive features. First, the single metal nanowire top contact reduces deleterious junction artifacts that can arise during the fabrication process (e.g., metal nanofilaments). Second, many devices can be integrated in parallel on the same substrate (potentially up to 100), which allows meaningful comparisons between theory and experiment. Third, the crossed-wire device facilitates variable and low-temperature current-voltage (I-V) measurements to study the dominant conduction mechanisms of different molecular junction. Finally, the device structure is amenable to Inelastic Electron Tunneling Spectroscopy (IETS) that allows a direct and in-situ measurement of the vibronic properties that correspond to different electronic states of the molecular junctions.;Electrical and spectroscopic measurements were conducted using three different molecules. First, saturated alkanethiol molecules were investigated to evaluate the new device structure because these σ-bonded molecules with simple structure have been studied extensively in previous experiments. Second, conjugated oligo(phenylen-eethynylene) molecular wire molecules were used to study the effect of π-conjugation on the electronic properties of molecular junction devices. Last, the bistable switching properties of thiol-substituted methylated oligoaniline SAM (self-assembled monolayer) junction devices were investigated by comparing I-V characteristics and IET spectra collected in high- and low-conductance states of the junction. |
