Published on Web 03/09/2004
Structure-Dependent Charge Transport and Storage in Self-Assembled Monolayers of Compounds of Interest in Molecular Electronics: Effects of Tip Material, Headgroup, and Surface Concentration Fu-Ren F. Fan,† Yuxing Yao,‡ Lintao Cai,‡ Long Cheng,‡ James M. Tour,*,‡ and Allen J. Bard*,† Contribution from the Department of Chemistry and Biochemistry and Center for Nano- and Molecular Science and Technology, The UniVersity of Texas at Austin, Austin, Texas 78712-0165, and Department of Chemistry and Center for Nanoscale Science and Technology, Rice UniVersity, Houston, Texas 77005 Received May 6, 2003; E-mail:
[email protected] Abstract: The electrical properties of self-assembled monolayers (SAMs) on a gold surface have been explored to address the relation between the conductance of a molecule and its electronic structure. We probe interfacial electron transfer processes, particularly those involving electroactive groups, of SAMs of thiolates on Au by using shear force-based scanning probe microscopy (SPM) combined with currentvoltage (i-V) and current-distance (i-d) measurements. Peak-shaped i-V curves were obtained for the nitro- and amino-based SAMs studied here. Peak-shaped cathodic i-V curves for nitro-based SAMs were observed at negative potentials in both forward and reverse scans and were used to define the threshold tip bias, VTH, for electric conduction. For a SAM of 2′,5′-dinitro-4,4′-bis(phenylethynyl)-1-benzenethiolate, VII, VTH was nearly independent of the tip material [Ir, Pt, Ir-Pt (20-80%), Pd, Ni, Au, Ag, In]. For all of the SAMs studied, the current decreased exponentially with increasing distance, d, between tip and substrate. The exponential attenuation factors (β values) were lower for the nitro-based SAMs studied here, as compared with alkylthiol-based SAMs. Both VTH and β of the nitro-based SAMs also depended strongly on the molecular headgroup on the end benzene ring addressed by the tip. Finally, we confirmed the “memory” effect observed for nitro-based SAMs. For mixed SAMs of VII and hexadecanethiol, I, the fraction of the charge collected in the negative tip bias region that can be read out at a positive tip bias on reverse scan (up to 38%) depended on the film composition and decreased with an increasing fraction of I, suggesting that lateral electron hopping among molecules of VII occurs in the vicinity of the tip.
Introduction
Understanding how electrons flow through organic molecules is important in several areas: rationalizing electron transfer in organic and biological molecules; fabricating molecular electronic devices such as organic light-emitting devices (LED), memory devices, or field-effect transistors (FET); and developing single-molecule and single-electron devices.1 One preferred approach for the assembly of molecules into devices and their connection to the macroscopic world relies on molecular selfassembly, such as self-assembled monolayers (SAMs).2 Covalent attachment to metal or semiconductor surfaces can be achieved † ‡
The University of Texas at Austin. Rice University.
(1) See, for example: Tour, J. M. Molecular Electronics: Commercial Insights, Chemistry, DeVices, Architecture and Programming: World Scientific: New Jersey, 2003. Tour, J. M. Acc. Chem. Res. 2000, 33, 791, and references therein. Reed, M. A.; Tour, J. M. Sci. Am. 2000, 282, 86, and references therein. Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999, 286, 1550. Xue, Y.; Datta, S.; Hong, S.; Reifenger, R.; Henderson, J. I.; Kubiak, C. P. Phys. ReV. B 1999, 59, R7852, and references therein. (2) See, for example: Ulman, A. Chem. ReV. 1996, 96, 1533. Dubois, L. H.; Nuzzo, R. G. Annu. ReV. Phys. Chem. 1992, 43, 437. Finklea, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19, pp 110-335. 10.1021/ja0359815 CCC: $27.50 © 2004 American Chemical Society
readily. Advances in modern synthetic supramolecular chemistry offer a huge range of molecular structures with different properties. Recent developments in device fabrication techniques and different experimental approaches allow single or a small group of molecules to be manipulated and investigated electronically.3 Among them, solid-state metal-insulator-metal (MIM)4 junction and scanning probe microscopy (SPM)5 related techniques have frequently been used to study ultrathin films of organic materials. Several groups have used SPM to probe tunneling across molecules in thin organic films.6 By using conducting-probe AFM (CP-AFM),7 Frisbie and co-workers measured i-V curves for SAMs of alkanethiols on Au, and Rawlett and co-workers studied the negative differential resistance (NDR) effect1 for (3) See, for example: Cygan, M. T.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. J. Am. Chem. Soc. 1998, 120, 2721. Semaltianos, N. G. Chem. Phys. Lett. 2000, 329, 76. (4) See, for example: Collier, C. P.; Mattersteig, G.; Young, E. W.; Luo, Y.; Beverly, K.; Sampaio, J.; Raymo, F. M., Stoddart, J. F.; Heath, J. R. Science 2000, 289, 1172. (5) See, for example: Scanning Tunneling Microscopy and Spectroscopy, 2nd ed.; Bonnell, D. A., Ed.; VCH: New York, 2000. Scanning Probe Microscopy: Analytical Methods; Wiesendanger, R. Ed.; Springer-Verlag: Berlin, 1998. J. AM. CHEM. SOC. 2004, 126, 4035-4042
9
4035
Yao et al.
ARTICLES
molecules containing an electroactive nitro moiety.8 Recently developed simplified models for electron tunneling across molecules in MIM junctions relate the current with distance, d, according to eq 1.9
i ) i0 exp(-βd)
(1)
in which β is a structure-dependent attenuation factor that was originally introduced to describe the extent of wave function mixing between the molecule and the two metal contacts. Recently we reported results on the molecular electrical properties of SAMs of several compounds of interest in molecular electronics by using a tuning fork-based SPM technique combined with i-V and i-d measurements, for characterization and screening of compounds in an inert atmosphere.10 We reported the effects of electroactive substituent groups on the central benzene ring of oligo(phenylene ethynylene)s (OPEs) and oligophenylenes (OPs) and the molecular wire backbone on their electric properties, and charge storage on nitro-based molecules.10 In this paper we extend this earlier work and report systematic studies of interfacial electron transfer processes across SAMs of 2′-nitro-4,4′-bis(phenylethynyl)-1benzenethiolate with different headgroups. We also studied the effect of different tip materials on electric conduction of SAMs of compound VII. In addition, we confirm the previously proposed model for charge storage in nitro-based SAMs by studies of mixed monolayers of I and VII that suggests that the “memory” effect we observed is mainly associated with lateral electron hopping rather than with the stochastic switching observed for metal/molecule junctions.11 Experimental Section Materials. The syntheses of the compounds (Chart 1) have been described elsewhere.1,12 The gold substrate was prepared by cutting a single-crystal silicon wafer into a 6 × 16 mm2 sheet, which was then cleaned for 30 min in a hot (40 °C), fresh acidic hydrogen peroxide (3:1 H2SO4/H2O2, v/v) solution. It was then rinsed with flowing distilled water, EtOH, and acetone and dried in flowing ultrahigh-purity N2 gas. The gold films were deposited by thermal evaporation of 200 nm thick (6) See, for example: Langlais, V. I.; Schittler, R. R.; Tang, H.; Gourdon, A.; Joachim, C.; Gimzeski, J. K. Phys. ReV. Lett. 2000, 83, 2809. Datta, S.; Tian, W.; Hong, S.; Reifenger, R.; Henderson, J. I.; Kubiak, C. P. Phys. ReV. Lett. 1997, 79, 2530. Zhou, S.; Liu, Y.; Xu, Y.; Hu, Z. D.; Qiu, X.; Wang, C.; Bai, C. Chem Phys. Lett. 1998, 297, 77. Donhauser, Z. J.; Mantooth, B. A.; Kelly, K. F.; Bumm, L. A.; Monnell, J. D.; Stapleton, J. J.; Price, D. W., Jr.; Rawlett, A. M.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 2001, 292, 2303. Dunbar, T. D.; Cygan, M. T.; Bumm, L. A.; McCarty, G. S.; Burgin, T. P.; Reinerth, W. A.; Jones, L., II; Jackiw, J. J.; Tour, J. M.; Weiss, P. S.; Allara, D. L. J. Phys. Chem. B 2000, 104, 4880. Bumm, L. A.; Arnold, J. J.; Dunbar, T. D.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. B 1999, 103, 8122. (7) See, for example: Dai, H.; Wong, E. W.; Lieber, C. M. Science 1996, 272, 523. Alpersion, B.; Cohen, S.; Rubinstein, I.; Hodes, G. Phys. ReV. B 1995, 52, R17017. Klein, D.; McEuen, P. Appl. Phys. Lett. 1995, 66, 2478. (8) Wold, D. J.; Frisbie, C. D. J. Am. Chem. Soc. 2000, 122, 2970. Kelley, T. W.; Granstrom, E. L.; Frisbie, C. D. AdV. Mater. 1999, 11, 261. Wold, D. J.; Frisbie, C. D. J. Am. Chem. Soc. 2001, 123, 5549. Rawlett, A. M.; Hopson, T. J.; Nagahara, L. A.; Tsui, R. K.; Ramachandran, G. K.; Lindsay, S. M. Appl. Phys. Lett. 2002, 81, 3043. (9) See, for example: Ratner, M. A.; Davis, B.; Kemp, M.; Mujica, V.; Roitberg, A.; Yaliraki, S. Ann. N. Y. Acad. Sci. 1998, 852, 22. Segal, D.; Nitzan, A.; Davis, W. B.; Wasielewski, M. R.; Ratner, M. A. J. Phys. Chem. B 2000, 104, 3817. (10) (a) Fan, F.-R. F.; Yang, J.; Dirk, S. M.; Price, D. W., Jr.; Kosynkin, D.; Tour, J. M.; Bard, A. J. J. Am. Chem. Soc. 2001, 123, 2454. (b) Fan, F.-R. F.; Yang, J.; Cai, L.; Price, D. W., Jr.; Dirk, S. M.; Kosynkin, D. V.; Yao, Y.; Rawlett, A. M.; Tour, J. M.; Bard, A. J. J. Am Chem. Soc. 2002, 124, 5550. (11) Ramachandran, G. K.; Hopson, T. J.; Rawlett, A. M.; Nagahara, L. A.; Primak, A.; Lindsay, S. M. Science 2003, 300, 1413. Rawlett, A. M.; Hopson, T. J.; Nagahara, L. A.; Tsui, R. K.; Ramachandran, G. K.; Lindsay, S. M. Appl. Phys. Lett. 2002, 81, 3043. 4036 J. AM. CHEM. SOC.
9
VOL. 126, NO. 12, 2004
Chart 1.
Au onto the Si sheets with a 25 nm Cr adhesion layer at a rate of 1 Å/s under a vacuum of 2 × 10-6 Torr. The gold samples were finally stored in a N2 atmosphere. Before use, the gold substrates were cleaned by a UV/O3 cleaner (Boekel Industries, Inc., model 135500) for 10 min to remove organic contamination. After ultrasonic cleaning in EtOH for 20 min to remove the resulting gold oxide layer, the Au substrates were then rinsed with EtOH and acetone and then dried in flowing N2. This procedure was confirmed to provide a clean, reproducible gold surface.13 Methylene chloride was distilled from calcium hydride. Tetrahydrofuran was distilled from sodium/benzophenone ketyl. All other chemicals were used as received without further purification. SAM Preparation. SAMs were prepared by two reliable and reproducible methods: base promoted and acid promoted assemblies.14 In the base-promoted technique, the compound (1 mg) was dissolved in a corresponding solvent, e.g., EtOH, THF, or a mixture of acetone/ MeOH (2:1, v/v), in a 4 mL vial to a concentration of about 0.5 mM. Concentrated NH4OH (10 µL) was then added, and the mixture was incubated for 10 min to deprotect the thiolacetyl group. Addition of an excess of NH4OH (e.g., 40 µL) will lead to precipitation. A 200 µL sample of acetone/MeOH solution of 0.3 mM Cs2CO3 was also used for the deprotection. A clean gold substrate was then immersed into the solution at room temperature for a period of 20-24 h. In the acid-promoted method, the compound (1 mg) was dissolved in a solvent mixture of CH2Cl2/MeOH (2:1 by volume) in a 4 mL vial. Concentrated H2SO4 (50-70 µL) was then added, and the solution was incubated for 1-4 h to deprotect the thiolacetyl moiety. A clean gold substrate was then immersed into the solution at room temperature for a period of 20-24 h. All the solutions were freshly prepared, previously purged with N2 for an oxygen-free environment, and kept in the dark (12) Allara, D. L.; Dunbar, T. D.; Weiss, P. S.; Bumm, L. A.; Cygan, M. T.; Tour, J. M.; Reinerth, W. A.; Yao, Y.; Kozaki, M.; Jones, L., II. Annal. N.Y. Acad. Sci. 1998, 852, 349. Chen, J.; Wang, W.; Reed, M. A.; Rawlett, A. M.; Price, D. W.; Tour, J. M. Appl. Phys. Lett. 2000, 77, 1224. Tour, J. M.; Kozaki, M.; Seminario, J. M. J. Am. Chem. Soc. 1998, 120, 8486. Sonogashira, K. In Metal-catalyzed Cross-Coupling Reactions; Diederich, F., Stang, P. J., Eds.; Wiley-VCH: New York, 1998; pp 203-230. (13) Ron, H.; Matlis, S.; Rubinstein, I. Langmuir 1998, 14, 1116. Ron, H.; Rubinstein, I. J. Am. Chem. Soc. 1998, 120, 13444. (14) Cai, L.; Yao, Y.; Price, D. W.; Tour, J. M. Chem. Mater. 2002, 14, 2905.
Charge Transport and Storage in SAMs
Figure 1. Schematic representation of the measurement and formation of a MIM junction with a tuning fork-based SPM tip containing a SAM on Au.
during immersion to avoid photo-oxidation. After the assembly, the samples were removed from the solutions, rinsed thoroughly with methanol, acetone, and CH2Cl2, and finally blown dry with N2. Usually the SAMs of compounds IV and VII were prepared under acidic conditions, while the others were prepared using the basic condition. Mixed SAMs consisting of compounds I and VII were prepared by immersing clean Au substrates in mixed solutions containing the two molecules at different ratios for a period of 18 h. Compound VII was deprotected by the acid-promoted method as described above before mixing with the molecule I. Apparatus and Measurements. Monolayer thickness was determined using either a Rudolph series 431A ellipsometer or a Gaertner LSE ellipsometer. The He-Ne laser (632.8 nm) light was incident at 70° on the sample. Measurements were carried out before and immediately after monolayer adsorption. The length of the molecular wire was calculated from the sulfur atom to the furthest proton for the minimum energy extended forms by molecular mechanics. As reported by Tour et al.,15 an nf value of 1.55 and kf ) 0 were assumed for all the film thickness calculations and the Au-S-C bond angle was assumed to be linear at 0.24 nm for the Au-S bond length. The OPE system has a tilt angle of the long molecular axis of
