Scanning Tunneling Microscopy of Self-Assembled Phenylene Ethynylene Oligomers on Au(111) Substrates Karsten Walzer,* ,², | Eike Marx, ‡ Neil C. Greenham, ‡ Robert J. Less, § Paul R. Raithby, § and Kurt Stokbro* ,², ⊥ Contribution from Mikroelektronik Centret (MIC), Technical UniVersity of Denmark, Bldg. 345 East, DK-2800 Lyngby, Denmark, CaVendish Laboratory, Madingley Road, Cambridge, CB3 0HE, United Kingdom, and Department of Chemistry, UniVersity of Bath, Bath BA2 7AY, United Kingdom Received June 19, 2003; E-mail: ks@atomistix.com; walzer@iapp.de Abstract: In this paper, we report the self-assembly, electrical characterization, and surface modification of dithiolated phenylene-ethynylene oligomer monolayers on a Au(111) surface. The self-assembly was accomplished by thiol bonding the molecules from solution to a Au(111) surface. We have confirmed the formation of self-assembled monolayers by scanning tunneling microscopy (STM) and optical ellipsometry, and have studied the kinetics of film growth. We suggest that self-assembled phenylene ethynylene oligomers on Au(111) surfaces grow as thiols rather than as thiolates. Using low-temperature STM, we collected local current-voltage spectra showing negative differential resistance at 6 K. Introduction Phenylene ethynylene oligomers have attracted the interest of both physicists and chemists for their potential as the active element in future molecular electronic devices. Several ap- proaches have been made to study their electronic properties, both by experimental and theoretical methods. Among these are conductance measurements in nanopores, 1,2 break-junctions, 3 and various scanning tunneling microscope (STM) 4,5 or conduc- tive atomic force microscope (c-AFM) 6,7 setups, as well as theoretical studies using various approaches. 8-11 One phenomenon of special interest in molecular electronics is the so-called negative differential resistance (NDR), which is characterized by a decreasing current at increasing voltage. NDR is the basic principle of several electronic components, as the resonant tunneling diode (RTD) and the Gunn diode. Especially, the RTD can be used as the basis of a simple memory device. Therefore, molecules possessing NDR proper- ties may be utilized in molecular memory devices. NDR effects 1 as well as conductance switching 12 have already been observed for various oligomers. Our aim is to contribute to the knowledge about the NDR effect in such molecular films. We have selected a dithiolated phenylene ethynylene trimer with a nitro side group on the central ring (see Scheme 1, molecule 1). Its monothiolated analogue has shown NDR in nanopore experiments 2 and in c-AFM. 7 We use dithiols instead of monothiols, because they allow a well-defined adsorption of molecules in two-terminal devices, such as gold nanogaps 13 or break junctions, provided such gaps can be produced with the correct width. In contrast to nanopore experiments, where typically thousands of molecules contribute to the measurement, the STM permits a very local conductivity measurement that only contacts very few molecules at a time. Experimental Section An Au(111) single crystal was used as substrate for STM measure- ments, prepared by sputter-anneal cycles in ultrahigh vacuum (UHV). For ellipsometry experiments, larger-area atomically flat films of Au- (111) on mica were used. From our previous work we know that the two substrates have similar surface properties. The thioacetate-protected molecule was synthesized as shown in Scheme 1 using a modification of the route reported by Tour et al. 2,14 ² Mikroelektronik Centret (MIC), Technical University of Denmark. ‡ Cavendish Laboratory, Madingley Road, Cambridge. § Department of Chemistry, University of Bath. | Present address: Institute of Applied Photophysics, Technical Univer- sity of Dresden, D-01062 Dresden, Germany. Email: walzer@iapp.de. ⊥ Present address: Nanoscience Center, Copenhagen Univesity, Univer- sitetsparken 5D, DK-2100 Copenhagen 0, Denmark. (1) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999, 286, 1550. (2) Chen, J.; Wang, W.; Reed, M. A.; Rawlett, A. M.; Price, D. W.; Tour, J. M. Appl. Phys. Lett. 2000, 77, 1224. (3) Reichert, J.; Ochs, R.; Beckmann, D.; Weber, H. B.; Mayor, M.; von Lo ¨hneysen, H. Phys. ReV. Lett. 2002, 88, 176 804. (4) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L., II.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705. (5) 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. (6) Fan, F. F.; Yang, J.; Dirk, S. M.; Price, D. W., Jr.; Kosynkin, D. V.; Tour, J. M.; Bard, A. J. J. Am. Chem. Soc. 2001, 123, 2454. (7) Fan, F. 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. (8) Seminario, J. M.; Zacarias, A. G.; Tour, J. M. J. Am. Chem. Soc. 2000, 122, 3015. (9) Karzazi, Y.; Cornil, J.; Bre ´das, J. L. J. Am. Chem. Soc. 2001, 123, 10 076. (10) Kushmerick, J. G.; Holt, D. B.; Yang, J. C.; Naciri, J.; Moore, M. H.; Shashidhar, R. Phys. ReV. Lett. 2002, 89, 086 802. (11) Taylor, J.; Brandbyge, M.; Stokbro, K. Phys. ReV. Lett. 2002, 89, 138 301. (12) Donhauser, Z. J.; Mantooth, B. A.; Kelly, K. F.; Bumm, L. A.; Monell, 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. (13) Sazio, P. J. A.; Berg, J.; See, P.; Ford, C. J. B.; Lundgren, P.; Greenham, N. C.; Ginger, D. S.; Bengtsson, S.; Chin, S. N. Mater. Res. Soc. Symp. Proc. 2001, B2.3.1., 679. Published on Web 01/10/2004 10.1021/ja036771v CCC: $27.50 © 2004 American Chemical Society J. AM. CHEM. SOC. 2004, 126, 1229-1234 9 1229