Characterization of carbon/nitroazobenzene/titanium molecular electronic junctions with photoelectron and Raman spectroscopy

Molecular junctions consisting of nitroazobenzene (NAB) chemisorbed to a substrate of pyrolyzed photoresist film (PPF) and a top contact of vapor-deposited titanium were examined with Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The thickness of the NAB layer varied from submonolayer to 4.5 nm, and a thin (1-3 nm) overlayer of Ti was deposited by electron beam deposition. Without Ti, the NAB surface Raman spectrum was sufficiently strong to observe previously unreported modes in the 500-1000-cm(-1) region, and the 1000-1700-cm(-1) region was sufficiently strong to observe the effects of metal deposition. Upon Ti deposition, the intensities of NAB modes associated with the nitro group decreased significantly, and the XPS indicated formation of a Ti-N bond. For the thicker NAB layers (1.9 and 4.5 nm), the intensities of the NO2 Raman modes partially recovered over a several-day period, but they remain depressed or absent in the submonolayer sample. The results indicate a reaction between condensing Ti atoms and the terminal NO2 group, probably to form a Ti-nitroso linkage between NAB and Ti. The result is a molecular junction with covalent bonding at both ends in the form of a C-C bond between PPF and NAB and a Ti-N bond to the top contact. The structural implications of the current results are interpreted in the context of recently reported functioning PPF/NAB/Ti molecular electronic junctions. In particular, the reaction between Ti and the nitro group appears to prevent short circuits resulting from incursion of Ti into the NAB layer.