Determination of the molecular electrical properties of self-assembled monolayers of compounds of interest in molecular electronics.

In this communication we report preliminary results with a tuning fork-based scanning probe technique combined with current-voltage (i-V) measurements, for the rapid characterization and screening of monolayer films in an inert atmosphere. We studied self-assembled monolayers (SAMs) of a variety of molecules with different structures that are being considered for possible application in molecular electronic devices and devised a high throughput analysis method for their characterization. There is currently a high level of interest in the electrical properties of isolated molecules especially in the use of unconventional substances and single molecules or SAMs to construct electronic devices.1-3 Advances in synthetic supramolecular chemistry, coupled with recent developments in device fabrication techniques and scanning probe techniques,4 allow single molecules to be manipulated and investigated electronically. Synthetic chemistry is mature enough to offer a huge range of molecular structures with different properties. A challenge has been to develop reliable and fast screening methods to characterize electronic properties of molecules and to be able to correlate the electrical behavior of the molecules with their structure. A few initial efforts have used long molecular wires across lithographically patterned proximal gold-coated probes separated by approximately 10 nm, but these studies were unreliable and not suitable for molecules shorter than the array gap.1,3 The Langmuir-Blodgett technique was also used to prepare a single molecular layer which was sandwiched between Al and Ti/Al contacts to form a device.5 Other efforts employed a nanopore arrangement or mechanically controllable break junctions6 where electronic measurements were performed between adjustable proximal point contacts. Still others have been performed in nanopores on a structure7 that has a metal top contact formed by vacuum evaporation, an active SAM, and a metal bottom contact. These techniques have provided interesting results, but the preparation of such nanostructures is time-consuming and fabrication-intensive. An attractive approach is to utilize a conducting atomic force microscope (AFM) tip8 as one of the contacts to form a metal-molecule-metal junction. We show here that a tuning fork-based scanning probe microscope (SPM)9 for rapid probe positioning combined with i-V measurements can be used to characterize and screen a large variety of molecules with different electrical properties. In the work reported here the molecules of interest were assembled on a flat gold substrate and then studied by this technique in a controlled environment. The basic principle of the device is illustrated in Figure 1. The tip, sharpened by electrochemical etching, as used for STM tips, is attached to a small tuning fork. The tuning fork is excited by an attached piezoelectric element, generally oscillating in the region of 33 to 100 kHz, and is used for rapid approach of the tip to the SAM. When the tip just contacts the SAM surface, the amplitude and frequency of the oscillation decrease, and this can be used to sense the presence of a surface. This same technique is used with many near-field scanning optical microscopy (NSOM) instruments to maintain tip position. Thus, the tip can be moved to the substrate and positioned fairly rapidly. The tip is then retracted slightly (about 10 nm) and moved to a different location on the SAM. The potential of the tip is swept with respect to the substrate over the desired potential range, and the current is recorded, as the tip is again approached toward the SAM, this time in small steps (e.g., 2 A). Before the tip contacts the molecules in the SAM, essentially no current flows. Upon contact, when the potential across the SAM containing electron-donating or electronwithdrawing groups attains a characteristic bias voltage, a current flows through the film. The magnitude of the current that flows in the i-V curve is a function of the conductance of the molecules. The synthesis of the compounds reported here and the preparation and characterization of the SAM have been described elsewhere10,11 (see Supporting Information). The i-V measurements were made in an argon atmosphere on seven compounds (Table 1).12 Figure 2 shows typical i-V characteristics of compounds I, II, and IV when the tip first contacts the surface of a SAM. For the alkylthiol (I) only the expected tunneling behavior at biases beyond about (4.8 V is seen (Figure 2A). Figure 2B shows the i-V curve of a SAM of 2′-ethyl-4,4′bis(phenylethynyl)-1-benzenethiolate (II) on gold. When the negative scan reaches about 2.8 V, a peaked current response of a few pA is observed. This type of response has been observed previously in similar studies with nanopore junctions and has been † The University of Texas at Austin. ‡ Rice University. (1) Tour, J. M. Acc. Chem. Res. 2000, in press. (2) Jortner, J.; Ratner, M. Molecular Electronics; Blackwell: London, 1997. (3) Metzger, R. M. Acc. Chem. Res. 1999, 32, 950. (4) See, e.g.: (a) Cygan, M. T.; Dunbar, T. D.; Arnold, J. J.; Bumm, L. A.; Shedlock, N. F.; Burgin, T. P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. J. Am. Chem. Soc. 1998, 120, 2721. (b) Semaltianos, N. G. Chem. Phys. Lett. 2000, 329, 76. (5) (a) Collier, C. P.; Wong, E. W.; Beloradsky, M.; Raymo, F. M.; Stoddart, J. F.; Kuekes, P. J.; Williams, R. S.; Heath, J. R. Science 1999, 285, 391. (b) Wong, E. W.; Collier, C. P.; Beloradsky, M.; Raymo, F. M.; Stoddart, J. F.; Heath, J. R. J. Am. Chem. Soc. 2000, 122, 5831. (6) Reed, M. A.; Zhou, C.; Muller, C. J.; Burgin, T. P.; Tour, J. M. Science 1997, 278, 252. (7) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999, 286, 1550. (8) (a) Wold, D. J.; Frisbie, C. D. J. Am. Chem. Soc. 2000, 122, 2970. (b) Kelley, T. W.; Granstrom, E. L.; Frisbie, C. D. AdV. Mater. 1999, 11, 261. (c) Dai, H.; Wong, E. W.; Lieber, C. M. Science 1996, 272, 523. (d) Alpersion, B.; Cohen, S.; Rubinstein, I.; Hodes, G. Phys. ReV. B 1995, 52, R17017. (e) Klein, D.; McEuen, P. Appl. Phys. Lett. 1995, 66, 2478. (9) Karrai, K.; Grober, R. D. Appl. Phys. Lett. 1995, 66, 1842. (10) Chen, J.; Wang, W.; Reed, M. A.; Rawlett, A. M.; Price, D. W.; Tour, J. M. Appl. Phys. Lett. 2000, 77, 1224. (11) Kosynkin, D.; Tour, J. M. Org. Lett., submitted for publication. (12) The number of molecules contacting the tip estimated from the tip radius (∼5 nm), the van der Waals radius (∼ 0.35 nm) of the molecules for a deformation depth of 0.2 nm of the SAM was about 35. Figure 1. Schematic representation of the measurement and formation of the metal-molecule-metal junction with a tuning fork-based SPM tip contacting the SAM on Au (not to scale). Ar atmosphere. 2454 J. Am. Chem. Soc. 2001, 123, 2454-2455