Chem. Mater. 2005, 17, 4939-4948
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Carbon/Molecule/Metal and Carbon/Molecule/Metal Oxide Molecular Electronic Junctions Rajendra Prasad Kalakodimi, Aletha M. Nowak, and Richard L. McCreery* Department of Chemistry, The Ohio State UniVersity, 100 West 18th AVenue, Columbus, Ohio 43210 ReceiVed March 31, 2005. ReVised Manuscript ReceiVed July 14, 2005
Molecular junctions were fabricated on the basis of a 1.7-4.5 nm thick layer of fluorene (FL) or nitroazobenzene (NAB) covalently bonded to a graphitic pyrolyzed photoresist film (PPF) substrate. The junction was completed with a top contact consisting of metallic Cu, TiO2, or aluminum(III) oxide (AlOx) and a final layer of Au. The current/voltage behavior of the junctions depended strongly both on the nature of the metal or metal oxide top layers and on the structure of the molecular layer. PPF/NAB/Cu/ Au and PPF/FL/Cu/Au junctions were highly conducting, with resistances of 0.3-1.7 Ω cm2, depending on the identity and thickness of the molecular layer. Substitution of Cu with either AlOx or TiO2 caused a large increase in junction resistance by 2-4 orders of magnitude, but also yielded rectifying junctions in the case of PPF/NAB(4.5)/TiO2(3.1)/Au. For a positive bias (PPF relative to Au) above +2 V, the NAB(4.5)/TiO2(3.1) junction became highly conductive, apparently due to injection of electrons into the TiO2 conduction band. PPF/NAB(4.5)/AlOx(3.3)/Au junctions exhibited symmetric i/V responses with very low currents, and capacitances consistent with those expected for a parallel plate capacitor with two dielectric layers. However, Raman spectroscopy of the NAB/AlOx junctions showed structural changes under negative bias corresponding to reduction of NAB, despite the absence of significant current flow. The changes were reversible and repeatable provided the bias was between -1.5 and +1.0, but partially irreversible when the bias excursion was negative of -1.5 V. Combined with a previous spectroscopic study of PPF/NAB/TiO2/Au junctions, the results imply a rectification mechanism based on electron transport through the NAB LUMO and the TiO2 conduction band, and possibly a Coulombic barrier resulting from reduction of the NAB in the molecular junction.
Introduction Many metal/molecule/metal molecular electronic junctions have been reported,1 most commonly based on self-assembled monolayer (SAM)2-9 and Langmuir-Blodgett10-13 structures. Of particular interest are those exhibiting rectification and conductance switching,14-16 due in part to potential * To whom correspondence should be addressed. Phone: (614) 292-2021. E-mail:
[email protected].
(1) McCreery, R. Chem. Mater. 2004, 16, 4477. (2) Zhou, C.; Deshpande, M. R.; Reed, M. A.; Jones, L.; Tour, J. M. Appl. Phys. Lett. 1997, 71, 611. (3) Chang, S.-C.; Li, Z.; Lau, C. N.; Larade, B.; Williams, R. S. Appl. Phys. Lett. 2003, 83, 3198. (4) Slowinski, K.; Majda, M. J. Electroanal. Chem. 2000, 491, 139. (5) 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. (6) Rampi, M. A.; Whitesides, G. M. Chem. Phys. 2002, 281, 373. (7) Haag, R.; Rampi, M. A.; Holmlin, R. E.; Whitesides, G. M. J. Am. Chem. Soc. 1999, 121, 7895. (8) Slowinski, K.; Fong, H. K. Y.; Majda, M. J. Am. Chem. Soc. 1999, 121, 7257. (9) Tran, E.; Rampi, M. A.; Whitesides, G. M. Angew. Chem., Int. Ed. 2004, 43, 3835. (10) Stewart, D. R.; Ohlberg, D. A. A.; Beck, P. A.; Chen, Y.; Williams, R. S.; Jeppesen, J. O.; Nielsen, K. A.; Stoddart, J. F. Nano Lett. 2004, 4, 133. (11) Chen, Y.; Ohlberg, D. A. A.; Li, X.; Stewart, D. R.; Williams, R. S.; Jeppesen, J. O.; Nielsen, K. A.; Stoddart, J. F.; Olynick, D. L.; Anderson, E. Appl. Phys. Lett. 2003, 82, 1610. (12) Chen, Y.; Jung, G.-Y.; Ohlberg, D. A. A.; Li, X.; Stewart, D. R.; Jeppesen, J. O.; Nielsen, K. A.; Stoddart, J. F.; Williams, R. S. Nanotechnology 2003, 14, 462. (13) Jaiswal, A.; Amaresh, R. R.; Lakshmikantham, M. V.; Honciuc, A.; Cava, M. P.; Metzger, R. M. Langmuir 2003, 19, 9043.
applications in microelectronics. We have investigated molecular junctions based on covalent bonding to a graphitic carbon substrate, with a metal17-19 or metal oxide20-22 top contact. The substrate is made by pyrolysis of commercial photoresist (pyrolyzed photoresist film, or PPF), and resembles a very flat (rms < 0.5 nm) form of glassy carbon, with a resistivity of 0.005 Ω cm.23,24 The molecular layer is bonded to the PPF by a covalent C-C bond, made by electrochemical reduction of a diazonium reagent, and the resulting mono- or multilayer has been characterized by (14) Collier, C. P.; Jeppesen, J. O.; Luo, Y.; Perkins, J.; Wong, E. W.; Heath, J. R.; Stoddart, J. F. J. Am. Chem. Soc. 2001, 123, 12632. (15) Heath, J. R.; Collier, C. P.; Mattersteig, G.; Raymo, F. M.; Stoddart, J. F.; Wong, E. Electrically Addressable Volatile Non-Volatile Molecular-Based Switching Devices. U.S. Patent 6,198,655, 2001. (16) Pease, A. R.; Jeppesen, J. O.; Stoddart, J. F.; Luo, Y.; Collier, C. P.; Heath, J. R. Acc. Chem. Res. 2001, 34, 433. (17) Anariba, F.; Steach, J.; McCreery, R. J. Phys. Chem. B 2005, 109, 11163. (18) Anariba, F.; McCreery, R. L. J. Phys. Chem. B 2002, 106, 10355. (19) Solak, A. O.; Ranganathan, S.; Itoh, T.; McCreery, R. L. Electrochem. Solid State Lett. 2002, 5, E43. (20) McCreery, R.; Dieringer, J.; Solak, A. O.; Snyder, B.; Nowak, A. M.; McGovern, W. R.; DuVall, S. J. Am. Chem. Soc. 2004, 126, 6200. (21) McCreery, R. L.; Dieringer, J.; Solak, A. O.; Snyder, B.; Nowak, A.; McGovern, W. R.; DuVall, S. J. Am. Chem. Soc. 2003, 125, 10748. (22) McGovern, W. R.; Anariba, F.; McCreery, R. J. Electrochem. Soc. 2005, 152, E176. (23) Ranganathan, S. Preparation, Modification and Characterization of a Novel Carbon Electrode Material for Applications in Electrochemistry and Molecular Electronics. Ph.D. Thesis, The Ohio State University, Columbus, OH, 2001. (24) Ranganathan, S.; McCreery, R. L. Anal. Chem. 2001, 73, 893.
10.1021/cm050689c CCC: $30.25 © 2005 American Chemical Society Published on Web 08/27/2005
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Raman spectroscopy,25-28 FTIR,29,30 XPS,28,31,32 AFM,30,33-35 and SIMS.36 Carbon-based molecular junctions exhibit current/voltage behavior which depends strongly on the structure and thickness of the molecular layer,17,18,37 and in certain cases exhibits strong rectification.20-22 The i/V behavior is reproducible and robust, with device yields exceeding 80% and endurance of thousands of voltage scans. In the course of developing carbon/molecule/titanium junctions, we observed a strong dependence of the i/V behavior on the residual gas pressure during Ti deposition.20-22 The involvement of titanium oxide in junction behavior was investigated by XPS depth profiling, and by comparison of Ti and Cu as metal top contacts. In addition, in situ Raman spectroscopy in functioning carbon/nitroazobenzene(NAB)/ titanium oxide/Au junctions revealed that a redox process occurred under applied bias, reducing the molecule and modulating the junction conductivity.26 We proposed that conductance switching was due to injection of electrons into the titanium oxide conduction band under positive bias (PPF relative to Au), which accompanied redox activity in the NAB/titanium oxide layers. The in situ spectroscopy clearly demonstrated structural changes within the junction in the presence of an applied electric field, but the connection between such changes and i/V behavior remains to be understood completely. The current work was undertaken to reveal the connections between structure and i/V behavior in carbon/molecule/metal/ Au junctions, particularly related to rectification in carbon/ NAB/titanium oxide/Au junctions. Cu and aluminum oxide top contacts are compared to titanium oxide to compare conducting and insulating contacts to those of semiconducting titanium oxide. In addition, three molecular layers were compared: an NAB monolayer, an NAB multilayer, and a fluorene (FL) monolayer. Fluorene was chosen as a second molecular structure which differed from NAB in its lack of a nitro or azo group to determine if those entities were determinants of junction electronic behavior. To clearly distinguish various junction designs, the molecular layer thickness and metal thickness will be given where appropriate. For example, PPF/NAB(4.5)/AlOx(3.3)/Au denotes a PPF substrate, a 4.5 nm thick NAB layer, a 3.3 nm thick aluminum(III) oxide layer, and a Au top layer to yield a total metal/metal oxide thickness of 10.0 nm. (25) (26) (27) (28) (29) (30) (31) (32) (33) (34) (35) (36) (37)
Nowak, A. M.; McCreery, R. L. Anal. Chem. 2004, 76, 1089. Nowak, A.; McCreery, R. J. Am. Chem. Soc. 2004, 126, 16621. Liu, Y.-C.; McCreery, R. L. Anal. Chem. 1997, 69, 2091. Liu, Y.-C.; McCreery, R. L. J. Am. Chem. Soc. 1995, 117, 11254. Stewart, M. P.; Maya, F.; Kosynkin, D. V.; Dirk, S. M.; Stapleton, J. J.; McGuiness, C. L.; Allara, D. L.; Tour, J. M. J. Am. Chem. Soc. 2004, 126, 370. Kariuki, J. K.; McDermott, M. T. Langmuir 2001, 17, 5947. Kuo, T.-C.; McCreery, R. L. Anal. Chem. 1999, 71, 1553. Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Saveant, J. M. J. Am. Chem. Soc. 1997, 119, 201. Anariba, F.; DuVall, S. H.; McCreery, R. L. Anal. Chem. 2003, 75, 3837. Kariuki, J. K.; McDermott, M. T. Langmuir 1999, 15, 6534. Brooksby, P. A.; Downard, A. J. Langmuir 2004, 20, 5038. Combellas, C.; Kanoufi, F.; Pinson, J.; Podvorica, F. Langmuir 2005, 21, 280. Ranganathan, S.; Steidel, I.; Anariba, F.; McCreery, R. L. Nano Lett. 2001, 1, 491.
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Experimental Section Molecular junctions were fabricated in a cross-junction configuration described previously,17 with the junction formed at the intersection of a PPF line and a vapor-deposited metal or metal/ metal oxide stripe. Insulating, flat substrates of silicon wafers (boron doped, 12-16 Ω cm) with a coating of 1500 Å of silicon nitride (Virginia Semiconductor, Inc.) were cut into 1 cm × 2 cm samples. Cut pieces were sonicated in acetone (Mallinckrodt AR, 99.7%) for 5 min, followed by a 30 s rinse in isopropyl alcohol (SigmaAldrich, 99.5%) and Nanopure water (18 MΩ cm, Barnstead), and then the samples were dried under a stream of argon gas. PPF films, which form the bottom contact for the molecular junctions, were prepared as described previously.18,24,38 The clean silicon nitride pieces were spin coated with the positive photoresist AZP4330RS (AZ Electronic Materials, Somerville, NJ), soft-baked at 90 °C for 20 min, and then cooled to room temperature before photolithography. Individual samples were then placed under a lithographic contact mask (Photo Sciences, Inc., Torrance, CA) with a pattern of four stripes 1 mm in width. A mercury arc lamp (model 68810, Oriel Corp., Stratford, CT) operating at 350 W was used to expose the photoresist to soft UV radiation for 120 s. Immediately after UV exposure, the samples were submerged into a 1:4 (v/v) solution of photoresist developer (AZ 400K, AZ Electronic Materials) in Nanopure water for 20-30 s, then rinsed with Nanopure water, and dried with argon. Pyrolysis was carried out as described previously at 1000 °C for 1 h in the presence of forming gas (95% nitrogen and 5% hydrogen). Prior to surface modification, the PPF samples were rinsed with acetonitrile (Sigma-Aldrich, 99.5%) which was treated with activated carbon filtered with 0.2 µm nylon filters (Millipore) prior to surface modification. Electrochemical derivatization was performed with a BAS 100 W potentiostat (Bioanalytical Systems, West Lafayette, IN), as described previously.21,33 An electrolyte solution consisting of a 1 mM concentration of the corresponding diazonium salt dissolved in acetonitrile solution with 0.1 M tetrabutylammonium tetrafluoroborate ((TBA)BF4; Sigma-Aldrich, 99.5%) was used for the electrochemical derivatization. A Ag/Ag+ (0.01 M AgNO3 and 0.1 M TBABF4 in acetonitrile) reference electrode (Bioanalytical Systems), calibrated with ferrocene to be +0.22 V vs aqueous SCE, was used for derivatization. A 4.5 nm thick NAB multilayer was deposited by four scans between +400 and -600 mV vs Ag/Ag+ at 200 mV/s. A 1.9 nm thick NAB monolayer was deposited by cycling PPF once between +400 and -200 mV vs Ag+/Ag at a scan rate of 200 mV/s. Similarly, a 1.7 nm thick FL monolayer was deposited by cycling PPF once in the corresponding diazonium salt solution between +400 and -800 mV vs Ag+/Ag at a scan rate of 200 mV/s. As has been discussed by several authors,33,34,36,39 diazonium reduction can lead to multilayers via attack of the initial monolayer by subsequently electrogenerated radicals. To verify molecular layer thickness, AFM “scratching” was used as described in detail elsewhere.33 Following surface modification, all the samples were immediately transferred to a clean filtered acetonitrile solution for 1 min to remove residual diazonium salt, then rinsed in acetonitrile, and dried with an argon stream. Following surface derivation and cleaning, the modified samples were loaded into a vacuum chamber for metal deposition through a shadow mask consisting of four 0.5 mm wide parallel lines. The mask and the samples were positioned on a rotating holder ∼50 cm from the crucible of an electron-beam source (Telemark, (38) Ranganathan, S.; McCreery, R. L.; Majji, S. M.; Madou, M. J. Electrochem. Soc. 2000, 147, 277. (39) Kiema, G. K.; Fitzpatrick, G.; McDermott, M. T. Anal. Chem. 1999, 71, 4306.
Carbon/Molecule/Metal or Metal Oxide Junctions Freemont, CA). Metal deposition details are provided in the Supporting Information, Table 2S, with nominal film thicknesses determined by a Telemark 860 deposition controller with quartz crystal microbalance. The AlOx film thickness was verified with AFM “scratching” 33 on a sample lacking the Au and molecular layers to be 3.26 ( 0.036 nm. For PPF/molecule/TiOx/Au junctions, nominally 3.0 nm of titanium was deposited at 7 × 10-6 Torr, and the thickness of the resulting titanium oxide deposit was verified with AFM to be 3.13 ( 0.03 nm. A slightly different procedure was followed to produce PPF/molecule/TiO2/Au junctions. After cryopumping to
