Molecular junctions were fabricated consisting of a 3.7 nm thick layer of nitroazobenzene (NAB) molecules between a pyrolyzed photoresist substrate (PPF) and a titanium top contact which was protected from oxidation by a layer of gold. Raman spectroscopy, XPS, and AFM revealed that the NAB layer was 2-3 molecules thick and was bonded to the two conducting contacts by C-C and N-Ti covalent bonds. The current/voltage behavior of the PPF/NAB(3.7)/Ti junctions showed strong and reproducible rectification, with the current at +2 V exceeding that at -2 V by a factor of 600. The observed current density at +3 V was 0.71 A/cm 2 , or about 10 5 e -/s/molecule. The i/V response was strongly dependent on temperature and scan rate, with the rectification ratio decreasing for lower temperature and faster scans. Junction conductivity increased with time over several seconds at room temperature in response to positive voltage pulses, with the rate of increase larger for more positive potentials. Voltage pulses to positive potentials and back to zero volts revealed that electrons are injected from the Ti to the NAB, to the extent of about 0.1-1 e -/molecule for a +3 V pulse. These electrons cause an activated transition of the NAB into a more conductive quinoid state, which in turn causes an increase in conductivity. The transition to the quinoid state involves nuclear rearrangement which occurs on a submillisecond to several second time scale, depending on the voltage applied. The quinoid state is stable as long as the applied electric field is present, but reverts back to NAB within several minutes after the field is relaxed. The results are interpreted in terms of a thermally activated, potential dependent electron transfer into the 3.7 nm NAB layer, which brings about a conductivity increase of several orders of magnitude.

Carbon/molecule/metal molecular junctions were fabricated by metal deposition of titanium or copper on monolayers of nitroazobenzene ͑NAB͒, biphenyl, and nitrobiphenyl ͑NBP͒, and multilayers of NAB and NBP covalently bonded to an sp 2 carbon substrate. The electronic behavior of Ti junctions was extremely dependent on residual gas pressure during E-beam deposition, due to the formation of a disordered Ti oxyhydroxide deposit. The junction resistance decreased with decreasing residual gas pressure, and the hysteresis and rectification observed previously for relatively high deposition pressure was absent for pressures below 5 ϫ 10 −7 Torr. Deletion of the molecular layer resulted in low-resistance junctions for both high and low deposition pressures. Replacement of the Ti with Al with otherwise identical deposition conditions resulted in insulating junctions with much higher resistance and no rectification. Ti junctions made at low residual gas pressure had resistances and current/voltage characteristics similar to those of junctions with Cu top contacts, with the latter exhibiting high yield and good reproducibility. The current/ voltage characteristics of both the Ti and Cu junctions fabricated with low residual gas pressure were nonlinear and showed a strong dependence on the molecular layer thickness. The hysteresis and rectification previously observed for junctions fabricated at relatively high residual gas pressure depend on the combination of the NAB layer and the semiconducting TiO x film, with the TiO x layer conductivity depending strongly on formation conditions. Rectification and hysteresis in NAB/TiO x junctions may result from either redox reactions of the NAB and TiO x layers, or from electron injection into the conduction band of Ti oxide.

"Accelerated" chromate conversion coating (CCC) formulations are distinguished among all CCC types by the fact that they utilize a special chemical additive to increase coating weight, 1 or increase the rate of the film forming Cr VI to Cr III reduction reaction. 2 Ostensibly, accelerated CCCs are used on corrosion-prone Al-Cu-Mg and AlZn-Mg-Cu alloys to ensure maximum protection. 3 Ferricyanide [Fe(CN) 6 3Ϫ ] has been used as an accelerant in commercial CCC formulations since the 1960s. 1 Despite the long history, its role in coating formation has not been precisely established. Both Fe(CN) 6 3Ϫ and Fe(CN) 6 4Ϫ are readily detected in CCCs by various surface-sensitive techniques, supporting the notion that it contributes to coating weight by becoming part of the CCC film. 1 For example, Treverton and Davies used X-ray photoelectron spectroscopy (XPS) and ion-beam etching to study accelerated CCCs formed on 99.8% pure Al substrates. 4 Results indicated the presence of Fe(CN) 6 4Ϫ/3Ϫ concentrated in the near-surface regions. It was suggested that the compound was probably present as a CrFe(CN) 6 salt. However, the findings were insufficient to establish a clear role for Fe(CN) 6 3Ϫ in coating formation. In subsequent studies aimed at clarifying their earlier work, these workers prepared accelerated CCCs on 99.8% Al from a bath formulation based on commercial chemistries. 5 XPS results indicated that Fe(CN) 6 3Ϫ/4Ϫ was present throughout the coating but was concentrated in the near-surface region. Mixed metal cyanides identified in the earlier study were now not present. Additionally, evidence of ferrocyanide [Fe(CN) 6 4Ϫ ] was found. The authors equivocated on the significance of ferrocyanide determination since ferri-to ferrocyanide reduction in X-ray beams was known to occur. Despite these complications, it was proposed that the primary film growth reaction, chromate reduction, was stifled by adsorption of ferricyanide on the nascent CCC, thereby increasing the availability of free chromate for reaction with uncoated metal.Hagans and Haas specifically considered film formation on Cu and Fe intermetallic compounds (IMCs) in studies of accelerated CCC formation on 2024-T3. 6 Auger electron spectroscopy (AES), XPS, and ion-beam depth profiling were used to show that for coating formation times up to 3 min, the film formation rate on various phases in 2024-T3 decreased in the order: matrix phase > Cu IMC > Fe IMC. After 5 min of immersion time, coating thickness appeared to be similar on the different phases. These workers proposed that ferrocyanide interacted with Cu-rich IMCs to form Cu 4 Fe(CN) 6 or Cu 2 Fe(CN) 6 and that these compounds promoted corrosion resistance by eliminating the galvanic couples that would otherwise form between the noble particles and the matrix phase.Lytle et al. used a variety of surface analytical techniques, including X-ray absorption spectroscopy (XAS), X-ray absorption near edge spectroscopy (XANES), extended X-ray absorption fine structure (EXAFS), and Fourier transform infrared s...

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Ceramic hot-section turbine components allow higher temperature for up-rated power and improved fuel economy. The methodology used in arriving at a successful 200-hr engine demonstration of an all ceramic nozzle is presented along with current progress in demonstrating the remaining ceramic hot-section static components.