NANO LETTERS
Probing the Effects of Conjugation Path on the Electronic Transmission through Single Molecules Using Scanning Tunneling Microscopy
2005 Vol. 5, No. 4 783-785
Kasper Moth-Poulsen,† Lionel Patrone,‡,§ Nicolai Stuhr-Hansen,† Jørn B. Christensen,† Jean-Philippe Bourgoin,‡ and Thomas Bjørnholm*,† Nano-Science Center, UniVersity of Copenhagen, UniVersitetsparken 5, 2100 Copenhagen Ø, Denmark, and CEA/DSM/DRECAM/SerVice de Chimie Mole´ culaire, Baˆ t. 125, CEA/Saclay, F-91191 Gif-sur-YVette Cedex, France Received January 6, 2005; Revised Manuscript Received February 10, 2005
ABSTRACT A systematic study of the relationship between the molecular structure of a series of thiol end-capped oligo-phenylenevinylenes (OPVs) and the coherent electronic transmission at the single molecule level was measured by scanning tunneling microscopy (STM). This reveals a significant change in the electronic transparency of various OPV derivatives due to the insertion of a methylene spacer group or due to nitro group substitution. Apparently, changes in the conjugation path through the central benzene ring from para to meta substitution does not have a profound effect on the electronic transparency of the molecules.
Fundamental investigations of the relationship between molecular and electronic structure at the single molecule level are crucial for the future development of molecular electronics. A number of previous investigations have addressed this issue in the coherent tunneling regime using two terminal devices and oligo-phenyleneethynylenes1-5 or oligo-thiophenes6-8 as the molecular systems. Very recently three terminal devices have also been applied to probe single molecules in the sequential tunneling regime (Coulomb blockade).9-11 In the present paper we report the first systematic studies of the electronic transmission properties of single oligo-phenylenevinylene molecules which represent one of the key structural motifs encountered in molecular electronics. The relationship between the molecular structure of a series of thiol end-capped oligo-phenylenevinylenes depicted in Table 1 and the coherent electronic transmission at the single-molecule level was measured by scanning tunneling microscopy (STM). To allow direct comparisons with previous studies1,2,6 we have adopted the method initially reported by Weiss and co-workers1 where single π-conjugated molecules are embedded in a dodecanethiol matrix. First, a self-assembled monolayer of dodecanethiol was prepared on freshly deposited gold on mica by immersion * Corresponding author. Email:
[email protected]. † University of Copenhagen, Denmark. ‡ CEA/Saclay, France. § Present address: CNRS-L2MP UMR 6137, ISEN-Toulon, Maison des Technologies, Place Georges Pompidou, 83000 Toulon, France. 10.1021/nl050032q CCC: $30.25 Published on Web 03/03/2005
© 2005 American Chemical Society
into a 10-3 mol L-1 solution of dodecanethiol in ethanol for several hours, followed by carefully rinsing with ethanol. The sample was then annealed in the same solution of dodecanethiol at 60 °C for 2 h, followed by annealing in air at 100 °C for 2 h. Second, the sample was immersed into 5 × 10-4 mol L-1 solutions of the molecules 1-10, respectively. The STM experiments were performed at ambient conditions using a custom-made microscope.6 Pt/Ir cut tips were used, and the bias voltage was applied to the sample with respect to the grounded tip. The STM images were recorded in the constant current mode. The imaging conditions were chosen so that the dodecanethiol lattice was resolved, i.e., with a typical tunneling resistance Rth g 100 GΩ. Under these conditions, the ability to work at low tunneling current in the picoampere range allowed us to apply a relatively small bias (+0.8 V) in air. Within these conditions, the tip remains outside the film. The experiments were performed at the same conditions (bias, temperature, humidity) for each molecule in order to be able to compare their behavior. To analyze the STM topographic data and translate them in terms of a phenomenological comparison of transport properties, taking into account the differences in the lengths of the compared molecules, we have used a simple model12 of the tunneling process through a single molecule, which describes the
Table 1. Comparison of Molecular Length, Apparent Height and β Values13,a
Figure 2. STM micrographs. (top) Self-assembled monolayer of dodecanethiol on gold. (bottom) Molecule 3 (white spots) inserted in dodecanethiol monolayer.
a
The full width at half maximum of the measured apparent height distribution was considered for each molecule 1-7. Concerning molecules 8 and 9, apparent height data were taken from ref 2. This allowed us to calculate the corresponding error bars on the βi, which are reported for molecules 1-9.
Figure 1. Model of the tunneling through the organic molecules.13
transconductance of a molecule as G ) G0 exp(-βi.hi), where i represents the molecule under investigation (1-10) (Figure 1). In this expression, valid in the low bias regime, G0 denotes the contact conductance defined by the molecule-metal coupling, βi the electronic decay constant within the molecule, and hi the geometric height of the molecule. The corresponding values of the electronic decay parameter, βi, 784
extracted from the average of measurements on more than 100 single molecules (for each compound 1-6), is listed in Table 1. Figure 2 illustrates two typical STM micrographs obtained before and after insertion of the phenylenevinylene molecules in the dodecanethiol monolayer. It is clearly seen that the π-conjugated molecules protrude in the domain boundaries of the dodecanethiol matrix (Figure 2 bottom). Furthermore, the apparent heights of the phenylenevinylene molecules are in all cases higher than expected from simple geometrical considerations as seen from Table 1. This clearly shows that the tunneling barriers caused by the conjugated molecules, as expected, are much lower than the barriers caused by the surrounding dodecanethiol molecules. Comparisons of the β values for molecules 1 and 2 shows that insertion of a methylene spacer between the sulfur atom and the π-conjugated moiety substantially modifies the transport. In that particular comparison, we do expect the β values for the π-conjugated part of the molecules to be identical. Making this assumption, we can express the difference as follows: the insertion of a methylene spacer lowers the tunneling probability through the entire molecule by a factor of 6, emphasizing the profound effect of the chemical nature of the contact to the metal electrode.6,7 Further comparisons of molecules 3 and 4 shows that nitro substitution increases the mean β-value from 0.63 to 0.78 following the same trend found for oligo-phenyleneethynylenes.2 The comparisons of 4 and 5 show that a para or meta coupling through the central benzene ring has very Nano Lett., Vol. 5, No. 4, 2005
little effect on the β values. Unexpectedly, the insertion of a second nitro substituent (5 vs 6) decreases the mean β value from 0.80 to 0.72. Finally, the gross comparisons to data reported from other experiments reveal a clear indication that transmission through the oligo-phenylenevinylenes is significantly enhanced compared to oligo-thiophenes and oligophenyleneethynylenes of similar length, in agreement with previous work of Kushmerick et al.15 We note that the above comparisons are based on the assumption that the preexponential factor in the transconductance formula, G0, and the geometrical orientation and the local environment do not change significantly from molecule to molecule. This crude approximation seems reasonable when working within a family of molecules that are all bound to the gold surface through the same type of chemical bond. The limit of the validity of the above assumptions is possibly indicated by the fact that we do not extract the same β-value for compounds 1 and 3. We have revealed a significant change in the electron transparency of various OPV derivatives due to the insertion of a methylene spacer group in the contact region (1 vs 2) and by nitro substitution (3 vs 4). Surprisingly, change in the conjugation path through the central benzene ring from para to meta substitution (4 vs 5) does not have a profound effect on the electronic transparency of the molecules. Acknowledgment. Financial support from the EU-IST program NANOMOL and the Danish research councils is acknowledged. Supporting Information Available: Synthesis and characterization of the compounds 1-6. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) 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, 1705-170. Cygan, M. T.; Dunbar, T. D.; Arnold, J. J.; Bumm, L. A.; Shedlock, N. F.; Burgin, T. P.; Jones, L.; Allara, D. L.; Tour, J. M.; Weiss, P. S. J. Am. Chem. Soc. 1998, 120, 27212732.
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(2) Donhauser, Z. J.; Mantooth, B. A.; Kelly, K. F.; Bumm, L. A.; Monnell, J. D.; Stapleton, J. J.; Price, D. W.; Rawlett, A. M.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 2001, 292, 2303-2307. (3) Fan, F. R. F.; Yang, J. P.; Cai, L. T.; Price, D. W.; Dirk, S. M.; Kosynkin, D. V.; Yao, Y. X.; Rawlett, A. M.; Tour, J. M.; Bard, A. J. J. Am. Chem. Soc. 2002, 124, 5550-5560. (4) Mayor, M.; Weber, H. B.; Reichert, J.; Elbing, M.; von Hanisch, C.; Beckmann, D.; Fischer, M. Ang. Chem. Int. Ed. 2003, 42, 58345838. (5) Walzer, K.; Marx, E.; Greenham, N. C.; Less, R. J.; Raithby, P. R.; Stokbro, K. J. Am. Chem. Soc. 2004, 126, 1229-1234. (6) Patrone, L.; Palacin, S.; Bourgoin, J. P.; Lagoute, J.; Zambelli, T.; Gauthier, S. Chem. Phys. 2002, 281, 325-332. (7) Patrone, L.; Palacin, S.; Charlier, J.; Armand, F.; Bourgoin, J. P.; Tang, H.; Gauthier, S. Phys. ReV. Lett. 2003, 91, 96802. (8) Kergueris, C.; Bourgoin, J. P.; Palacin, S.; Esteve, D.; Urbina, C.; Magoga, M.; Joachim, C. Phys. ReV. B 1999, 59, 12505-12513. (9) Park, J.; Pasupathy, A. N.; Goldsmith, J. I.; Chang, C.; Yaish, Y.; Petta, J. R.; Rinkoski, M.; Sethna, J. P.; Abruna, H. D.; Mceuen, P. L.; Ralph, D. C. Nature 2002, 417, 722-725. (10) Kubatkin, S.; Danilov, A.; Hjort, M.; Cornil, J.; Bredas, J. L.; StuhrHansen, N.; Hedegard, P.; Bjørnholm, T. Nature 2003, 425, 698701. (11) Park, H.; Park, J.; Lim, A. K. L.; Anderson, E. H.; Alivisatos, A. P.; McEuen, P. L. Nature 2000, 407, 57-60. (12) Bumm, L. A.; Arnold, J. J.; Dunbar, T. D.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. B 1999, 103, 8122-8127. (13) Since the STM operates in constant current mode, Ggap1Gmol1 ) Ggap2Gmol2. This two-layer tunnel junction model may be applied since the tip remains outside the molecular film. The molecular β values for the conjugated molecules can be estimated from the apparent height of the molecule, assuming that G1 ≈ G2 (ref 12) by using literature values for R, β1, and h1 and the calculated length h2 of the conjugated molecule (see Table 1). We note that assuming G1 ≈ G2 is a crude approximation: it does not however change the qualitative conclusion reached from the analysis. (14) The length of the molecules was calculated using PM3-type calculations. The conjugated molecules were assumed to be oriented perpendicular to the gold surface. The thickness of the dodecanethiol monolayer is assumed to be 14 Å, corresponding to a tilt angle of 30°.6 The apparent heights were measured using a STM operating in constant current mode (1, 9 pA) and +0, 78 V, except for molecules 8 and 9, which were measured at 1.0 pA and -1.0 V; values taken from ref 2. The β values were calculated using the simple box model as described in Figure 1, using decay constants for air, R ) 2.3 Å-1 and dodecanethiol, β ) 1.2 Å-1 (ref 12). (15) Kushmerick, J. G.; Holt, D. B.; Pollack, S. K.; Ratner, M. A.; Yang, J. C.; Schull, T. L.; Naciri, J.; Moore, M. H.; Shashidhar, R. J. Am. Chem. Soc. 2002, 124, 10654-10655.
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