Spectroscopic characterization of carbon based molecular electronic junctions

Carbon based molecular electronic junctions were prepared by covalently bonding a molecular layer to a conducting graphitic carbon substrate through the electrochemical reduction of aryl diazonium tetrafluoroborate salts. The "top-contact" is made by electron beam evaporation of metals, such that the molecular layer is sandwiched between the two electrodes. The carbon substrate is fabricated by pyrolyzing commercially available photoresist in a reducing atmosphere. It is possible to pattern the photoresist before pyrolysis by simple photolithographic techniques, thus introducing the potential to prepare patterned carbon films to support various analytical applications.; An investigation was conducted using Raman spectroscopy and X-ray Photoelectron Spectroscopy (XPS) to evaluate the interaction between vapor deposited metals and the molecular layer. Evidence for the formation of a covalent bond between the molecular layer and the metal top-contact was found in molecular junctions incorporating a layer of 4-nitroazobenzene (NAB) and vapor deposited titanium. A strong interaction between the NO2 group of the NAB and the deposited titanium was observed using Raman Spectroscopy, in that the peaks associated with the NO2 group decrease in intensity immediately following deposition. The nature of this interaction was realized from the XPS spectra where a peak in the N1s region characteristic of a Ti-N bond was observed.; Bias-induced structural changes of the 4.5 nm thick NAB layer within a PPF/NAB/Ti/Au molecular electronic junction were observed using Raman spectroscopy through the semi-transparent Ti/Au top-contact. To our knowledge, this experiment is unprecedented in the literature. The observed spectral changes are consistent with a reversible redox process over a +3 to -1 volt range (PPF relative to Ti/Au). At potentials negative of -1V the NO2 group is permanently reduced to a para-substituted amine. This structure undergoes a reversible redox process over a +3 to -3 volt range. The structure of the reduced NAB film was deduced by comparing the spectrum to one that was collected from a NAB-modified PPF surface that was reduced electrochemically and a PPF surface derivatized with dimethylamino-azobenzene.; The structure of the NAB layer was also monitored as a function of applied voltage within a molecular electronic junction using a semitransparent Al/Au top-contact with Raman spectroscopy. The NAB-layer undergoes similar structural changes in the PPF/NAB/Al/Au junction, however the spectral changes are detected at different potentials. To try to account for this difference, the identity of the metal species was examined with XPS depth profile analysis. The aluminum layer consists of a single metal oxide species, where as the titanium layer is comprised of multiple metal oxide species.