Published on Web 08/07/2003
Molecular Rectification and Conductance Switching in Carbon-Based Molecular Junctions by Structural Rearrangement Accompanying Electron Injection Richard McCreery,* Jon Dieringer, Ali Osman Solak,† Brian Snyder, Aletha M. Nowak, William R. McGovern, and Stacy DuVall Contribution from the Department of Chemistry, The Ohio State UniVersity, Columbus, Ohio 43210 Received May 19, 2003; E-mail:
[email protected] Abstract: 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/cm2, or about 105 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.
Introduction
The electronic properties of several types of molecular junctions have attracted significant interest, because they are fundamental to the growing area of molecular electronics. A metal/molecule/metal junction combines properties of traditional metallic conductors with those of single molecules or groups of molecules, thus raising the prospect of introducing molecular properties into electronic circuits.1-14 The mechanism of electron † Permanent address: Ankara University, Faculty of Science, Ankara, Turkey.
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transport across molecular junctions has been a very active topic of research in part because it involves concepts from electrochemistry, solid-state physics, and long-range electron transfer phenomena in chemical and biochemical systems. Electron tunneling (coherent, incoherent, and resonant), thermally induced hopping, transmission through Schottky barriers, and other processes have been invoked to explain the dependence of current through a molecular junction on applied voltage and temperature.15-17 While there is a wide range of behaviors observed for metal/molecule/metal junctions, there is also general agreement that the current/voltage (i/V) characteristics are critically dependent on molecular structure and conformation. The term “molecular junction” is generally reserved for cases where the molecular layer is one or a few monolayers thick, usually in the range of 1-5 nm. Their monolayer or near(12) Holmlin, R. E.; Haag, R.; Chabinyc, M. L.; Ismagilov, R. F.; Cohen, A. E.; Terfort, A.; Rampi, M. A.; Whitesides, G. M. J. Am. Chem. Soc. 2001, 123, 5075. (13) Slowinski, K.; Majda, M. J. Electroanal. Chem. 2000, 491, 139. (14) Ranganathan, S.; Steidel, I.; Anariba, F.; McCreery, R. L. Nano Lett. 2001, 1, 491. (15) Rampi, M. A.; Whitesides, G. M. Chem. Phys. 2002, 281, 373. (16) Hong, S.; Reifenberger, R.; Tian, W.; Datta, S.; Henderson, J.; Kubiak, C. P. Superlattices Microstruct. 2000, 28, 289. (17) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999, 286. 10.1021/ja0362196 CCC: $25.00 © 2003 American Chemical Society
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monolayer thickness distinguishes molecular junctions from widely studied metal/molecule/metal structures with thick (>10 nm) molecular films, such as organic light-emitting diodes and classical dielectric capacitors. To date, the vast majority of molecular junctions are based on metal-thiol self-assembled monolayers (SAMs)18-22 and Langmuir-Blodgett (L-B) films.23-27 Examples include polyphenyl and poly(phenylvinyl) thiol molecules chemisorbed on gold or silver,16,28 biphenyl thiols suspended between Au and titanium,29 and L-B structures on Al or Al oxide substrates.24,25,27 Molecular junctions constructed with SAM or L-B chemistry have exhibited interesting electronic effects, such as rectification,30 conductance switching,20,21,24 and negative differential resistance (NDR).17,31,32 An additional concept relevant to the current report is the injection of charge into a molecular layer, to alter its properties. Examples include redox-mediated electron transfer in redox polymer films,33-38 electron injection into conducting polymer films,39,40 and electron injection into molecular tunneling junctions.41-44 Redox polymer films have exhibited rectification and chemiluminescence, and their behavior has been explained in terms of electron transport between redox centers with or without accompanying motion of ions.37,45 The evidence for electron injection into “thick” junctions with 10-100 nm molecular layers is convincing; however, the involvement of electron injection into monolayers in molecular junctions has not yet been established unequivocally. In the case of monolayer
molecular junctions, which exhibit at least some electronic conduction, it is not obvious how to distinguish electron conduction through the junction from a “redox” event which results in stored charge in the monolayer. The former may involve many more electrons than the latter, making a redox event difficult to observe. The current report investigates the possibility of charge injection in a new type of molecular junction consisting of a graphitic carbon substrate, a covalently bonded monolayer, and a titanium/gold top contact. Nitroazobenzene was bonded covalently to a very flat carbon film by electrochemical reduction of a diazonium reagent. As noted in previous publications, the covalent carbon-carbon bond between the graphitic substrate and organic monolayer is stronger than Au/thiol or L-B bonds; hence more stable junctions might be possible.14,46,47 These carbon/molecule/mercury junctions exhibited conductance switching14,47 and interesting dependence on temperature,46 but the reproducibility of the junction properties limited the conclusions available with a mercury top contact. The Ti contact reported here appears to bond covalently to the top of the monolayer, resulting in covalent bonds at both interfaces of the monolayer with the contacts. Carbon molecule/Ti molecular junctions were investigated primarily to determine the electron transport mechanism(s) through the junction, but, in the process, a potentially valuable phenomenon involving electron injection into the monolayer was revealed.
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Experimental Section Pyrolyzed photoresist films (PPF) were prepared at 1000 °C as described previously,48,49 on polished silicon substrates. The Si was boron doped with a resistivity of 1 Ω cm, but the properties of the silicon did not appear to be important. Because the resistivity of the PPF is ∼0.005 Ω cm,48,49 the PPF carries most of the current and the Si acts merely as a flat, relatively low conductivity substrate. Approximately 2 × 3 cm samples of Si (1 mm thick) were cut from 4 in. wafers, then spin coated with AZ-P4330-RS positive photoresist (Clariant Corp., Somerville, NJ) until the uncured photoresist thickness was 5-7 µm. After pyrolysis, the PPF film was approximately 1-2 µm thick. Pyrolyzed samples were inspected visually for any obvious defects. Monolayer deposition by reduction of diazonium reagents in acetonitrile is prone to multilayer formation, because electrons may be able to transfer through the initial monolayer to reduce additional diazonium ions.14,50,51 Deposition conditions were chosen carefully, and the layer thickness was confirmed with AFM by observing the depth (in tapping mode) of an intentional scratch made in the monolayer with contact mode AFM, as described elsewhere.50 Nitroazobenzene (NAB) was chosen for this initial investigation of Ti junctions due to its strong Raman scattering52,53 and previous experience with PPF/NAB/Hg junctions.14,47 See Supporting Information for additional details on NAB deposition. The NAB film thickness was determined with AFM “scratching”50 to be 3.68 ( 0.31 nm, based on 2 independent samples, 3 scratches, and 58 determinations of NAB layer thickness. The rms roughness of the NAB layers was 0.68 nm, slightly greater than the 0.5 nm roughness of the PPF substrate. The length of one NAB molecule is 1.45 nm, so a 3.7 nm layer is 2-3 NAB molecules thick. (46) Anariba, F.; McCreery, R. L. J. Phys. Chem. B 2002, 106, 10355. (47) Solak, A. O.; Ranganathan, S.; Itoh, T.; McCreery, R. L. Electrochem. Solid-State Lett. 2002, 5, E43. (48) Ranganathan, S.; McCreery, R. L.; Majji, S. M.; Madou, M. J. Electrochem. Soc. 2000, 147, 277. (49) Ranganathan, S.; McCreery, R. L. Anal. Chem. 2001, 73, 893. (50) Anariba, F.; DuVall, S.; McCreery, R. L. Anal. Chem. 2003, 75, in press. (51) Kariuki, J. K.; McDermott, M. T. Langmuir 2001, 17, 5947. (52) Liu, Y.-C.; McCreery, R. L. Anal. Chem. 1997, 69, 2091. (53) Itoh, T.; McCreery, R. L. J. Am. Chem. Soc. 2002, 124, 10894. J. AM. CHEM. SOC.
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Figure 1. Voltammograms for a PPF/NAB(3.7)/Ti junction (area ) 0.00785 cm2) for a (0.2 V range. Plot a shows i/V curves obtained at 1, 10, and 100 V/s; curve b is at 0.1 V/s. Positive current corresponds to electrons flowing from the Ti to the PPF, through the NAB layer. All voltages are stated as the PPF potential relative to the Ti contact.
By analogy to similar multilayers formed from diethylaminophenyl diazonium reagents,51,54 the NAB molecules are presumably linked to each other by bonds between phenyl rings. After derivatization, the entire Si/PPF/monolayer sample was rinsed with acetonitrile that had been purified with activated carbon and filtered through 0.2 µm Millipore filters. Immediately after rinsing, the PPF sample with NAB monolayer was placed behind a contact mask consisting of 0.5 or 1.0 mm diameter holes in a 1/32 in. thick polystyrene sheet. The mask and sample were positioned on a rotating holder in the top of an electron beam evaporator chamber, facing down, approximately 50 cm from a crucible containing metallic titanium. Titanium has been used previously as a metallic contact for both semiconductor and molecular junctions in part because of its high reactivity.29,55 An automated electron beam apparatus (Telemark, Fremont, CA) vaporized Ti from the crucible with the beam current operating near the threshold for Ti evaporation. The current was controlled to yield a Ti deposition rate at the sample of 0.03 nm/s until a thickness of 3 nm resulted, and then the deposition was paused for 5 min to permit cooling. An additional 10 nm of Ti was then deposited at 0.1 nm/s, and then an additional 40 nm at 1.0 nm/s. Metal thickness was monitored in situ with a quartz crystal microbalance positioned next to the sample (Telemark Model 860 deposition controller). The approximate velocity of the evaporated Ti atoms was measured separately by passing the Ti beam through a rotating 1 mm diameter aperture and noting the displacement of the Ti spot on a target as a function of rotation speed. The result was an average velocity of 71 ( 13 m/s. Although this velocity corresponds to a kinetic energy of
