NANO LETTERS
Stochastic Variation in Conductance on the Nanometer Scale: A General Phenomenon
2003 Vol. 3, No. 11 1617-1620
Ronald A. Wassel, Ryan R. Fuierer, Namjin Kim, and Christopher B. Gorman* Department of Chemistry, North Carolina State UniVersity, Box 8204, Raleigh, North Carolina 27695-8204 Received August 27, 2003; Revised Manuscript Received October 3, 2003
ABSTRACT The relative conductance of two different electroactive thiol molecules (containing ferrocene and viologen headgroups) inserted into an n-alkanethiolate background SAM was tracked over time using a scanning tunneling microscope. Both types of inserted molecules exhibited stochastic variation in their conductance. This phenomenon of “blinking” thus appears to be quite general, despite the fact that these two molecules are structurally different from one another and from molecules in which this phenomenon had been studied previously. This behavior is most simply rationalized as conformational and/or orientational changes of one or a small collection of molecules over time.
The conductance of a nanometer-scale metal-moleculemetal junction is dependent on several factors. Notable among these is the conformation and orientation of the molecules in the junction and any conformational changes that these molecules can undergo with respect to the metallic contacts. Stochastic switching between various conductances should occur in any junction composed of a small enough collection of molecules. This phenomenon has been observed using a scanning tunneling microscope (STM) tip in contact with a self-assembled monolayer (SAM) as one of the metallic contacts. Weiss et al. observed conductance changes over time (“blinking”) in phenylene ethynylene-based oligomers inserted into a host SAM.1-5 This behavior was attributed to conformational changes within the molecule. Lindsay et al. also observed conductance changes over time in alkanedithiol SAMs on gold that had been capped with gold nanoparticles.6 This behavior was attributed to changes in the bonding between the molecule and the gold nanoparticle. This phenomenon has also been observed in nanometer-scale conducting polymer7 and water-containing8 junctions and in thin oxide layers.9,10 Although the mechanism for the variation in the conductance in each of these systems likely differs, each has the common feature that a nanometer-scale collection of molecules conduct the current. Because the current used in STM feedback is based on tunneling, an increase in the dimensions of the tip substrate gap is expected to result in an exponential change in the current. This phenomenon translates into order-of-magnitude changes in the tunneling current for angstrom changes in * Corresponding author. E-mail:
[email protected]. 10.1021/nl034710p CCC: $25.00 Published on Web 10/18/2003
© 2003 American Chemical Society
the gap dimensions. Given that any metal-molecule-metal junction is likely to have enough variability on this length scale, conductance changes in these junctions (and thus stochastic switching) should be completely general in this type of system. In this paper, we show that stochastic switching can be observed in two types of electroactive thiol molecules inserted into an n-alkanethiolate SAM on gold. Previously we have observed negative differential resistance (NDR, decreasing current with increasing bias) in these types of molecules when in a SAM.11 At the applied bias in which these molecules show NDR, they show an enhanced conductance compared to an n-alkanethiolate SAM background. Here, we show that this enhanced conductance behavior blinks on and off, presumably because of conformation and/ or orientation changes of the inserted molecules with the SAM over time. A series of STM images were collected over time that monitored the conductance of two types of electroactive thiol molecules adventitiously inserted into an n-alkanethiolate SAM. Samples were prepared using a protocol originally described by Weiss et al.12-15 In the first system, ferrocenylundecanethiol (FcC11SH)16 was inserted into a dodecanethiolate SAM (C12S-SAM) host matrix; in the other, a viologen-terminated thiol (MVC11SH)17 was inserted into a decanethiolate SAM (C10S-SAM) host matrix. Table 1 shows the molecules used for this study as well as their calculated heights on the surface. In the bias region in which NDR was observed for these electroactive molecules (ca. 1 V), they appeared as bright spots in the darker n-alkane-
Table 1. Structures of the Molecules Employed in SAMs Studied
a Calculated using standard bond lengths and angles and assuming a 30° tilt of the molecules on the Au substrate.
Figure 2. Series of STM images collected over time on (A) MVC11S- inserted into a C10S-SAM (70 × 70 nm2, +1.2-V setpoint bias, 10-pA setpoint current, z range ) 2 nm) and (B) FcC11S- inserted into a C12S-SAM (47 × 47 nm2, +1.0-V setpoint bias, 10-pA setpoint current, z range ) 2 nm). The circled regions labeled a-d are assigned to molecules that were found to switch conductance states several times.
Figure 1. STM images illustrating the bias-dependent height contrast of MVC11S- (500 × 500 nm2 lateral dimensions, z scale ) 3 nm) inserted into a C10S-SAM (10-pA setpoint current). (A) Image obtained at a 0.2-V setpoint bias. (B) Image obtained at a 1.0-V setpoint bias exhibiting increased apparent height for the inserted molecules versus the C10S-SAM.
thiolate SAM background matrix. At lower bias, the relative conductance of these two species was similar, and little or no contrast between these molecules was observed. This phenomenon had previously been illustrated for FcC11S-SAM regions that had been nanopatterned into a C12S-SAM.11,18 In Figure 1, the same behavior is shown for MVC11SH inserted into C10S-SAM. Specifically, at 0.2 V applied bias between the tip and substrate, little contrast was observed in an STM image of this sample. Increasing the applied bias 1618
to 1.0 V revealed a number of bright features of approximately molecular dimensions. These are assigned to adventitiously inserted MVC11SH molecules. Both of these types of inserted molecules changed their conductance over time relative to the n-alkanethiolate SAM background. Figure 2A shows a series of STM images of the same region of a sample in which the relative height (e.g., relative conductance) of several inserted MVC11SH molecules varied over time. Figure 2B shows similar behavior for inserted FcC11SH molecules. The molecules blinked independently of one another, consistent with the contention that the blinking was not due to the direction in which the STM was scanning (up and down scans could presumably represent two different orientations of the tip and therefore two different pathways for the current to tunnel through) or a tip perturbation applied consistently to each inserted molecule. Figure 3A shows a time-lapse series of images of one or a small collection of inserted MVC11SH molecules from which the apparent height of this region versus time could be quantified (Figure 3B). A similar series of images (Figure 3C) and an apparent height versus time plot (Figure 3D) were obtained in a similar fashion for inserted FcC11SH. The apparent height contrast between the higher and lower conductance states of MVC11S- and FcC11S- fluctuates Nano Lett., Vol. 3, No. 11, 2003
consistent conductance, it does illustrate the importance of addressing this issue in molecular-scale electronics studies. Experimental Details. SAM Preparation. Substrates were flame-annealed Au(111) facets on a gold bead formed at the end of a gold wire (Alfa Aesar, 99.999%). The wire was alloyed to a platinum foil (Alfa Aesar) base.11 Prior to monolayer deposition, the substrates were cleaned in piranha solution (3:1 H2SO4:H2O2 (30%) by volume). Caution: Piranha solution is corrosiVe. In addition, care should be taken not to store piranha solution for extended periods of time because of the formation of explosiVe oxides. nAlkanethiolate SAMs were prepared on gold by refluxing the substrates in 1 mM n-alkanethiol ethanolic solutions followed by a brief exposure to 1 mM ethanolic solutions of the electroactive guest species (FcC11SH or MVC11SH). This procedure allowed small groups of the guest species to insert into the defects sites within the host matrix.2,3,6 After copious rinsing in ethanol, the sample holder was mounted in a custom Kel-F fluid cell in preparation for scanning. STM Imaging. All STM experiments employed a Digital Instruments (Santa Barbara, CA) Nanoscope IIIa equipped with an E scanner and a low-current converter to obtain STM images at room temperature in dry nitrogen. The sample was then imaged with STM at a setpoint of 10 pA and at biases of 1.0 V for the inserted FcC11SH molecules and 1.2 V for the inserted MVC11SH molecules. Images were continuously captured to monitor the variation in the apparent height of that species. Because of thermal drift and creep in the STM piezoelectric imaging system, the ability to keep the tip imaging in the same region required large scan areas (100 nm to 500 nm) to compensate for the drift. Figure 3. (A) Time-lapse series of STM images for a set of inserted MVC11SH molecules. Images are each 24.6 × 24.6 Å2 and were used to calculate (B) the variation in apparent height for the bright region versus time. (C) Time-lapse series of STM images for a set of inserted FcC11SH molecules. Images are each 15.6 × 15.6 Å2 and were used to calculate (D) the variation in apparent height for the bright region versus time.
through the series of images in a manner similar to the previously reported stochastic switching behaviors described above. This behavior is consistent with both types of molecules assuming different conformations and/or orientations in the SAM. Notably, the magnitudes of the apparent height fluctuations are different when inserted MVC11S- and FcC11S- are compared. These observations support the hypothesis that conformational/orientational variations of these two different molecules give rise to the fluctuation. Stochastic variation in the relative conductance of molecules is likely a completely general phenomenon. It is illustrated here for two electroactive thiol molecules that are structurally quite different from one another and from molecules in which this phenomenon has been studied previously. This behavior is most simply rationalized as being due to conformational and/or orientational changes of one or a small collection of molecules over time. On the length scale relevant to molecular electronics, these changes will not be averaged out over a large collection of molecules. Although the work presented here does not indicate how many molecules will be required to create a junction with a Nano Lett., Vol. 3, No. 11, 2003
Acknowledgment. This work was supported by the Office of Naval Research (grant no. N00014-00-1-0633). References (1) Chen, J.; Reed, M. A.; Asplund, C. L.; Cassell, A. M.; Myrick, M. L.; Rawlett, A. M.; Tour, J. M.; Patten, P. G. V. Appl. Phys. Lett. 1999, 75, 624-626. (2) Donhauser, Z. J.; Mantooth, B. A.; Pearl, T. P.; Kelly, K. F.; Nanayakkara, S. U.; Weiss, P. S. Jpn. J. Appl. Phys., Part 1 2002, 41, 4871-4877. (3) 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. (4) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999, 286, 1550-1552. (5) Chen, J.; Wang, W.; Reed, M. A.; Rawlett, A. M.; Price, D. W.; Tour, J. M. Appl. Phys. Lett. 2000, 77, 1224-1226. (6) Ramachandran, G. K.; Hopson, T. J.; Rawlett, A. M.; Nagahara, L. A.; Primak, A.; Lindsay, S. M. Science 2003, 300, 1413-1416. (7) He, H. X.; Li, X. L.; Tao, N. J.; Nagahara, L. A.; Amlani, I.; Tsui, R. Phys. ReV. B 2003, 68, 045302. (8) Boussaad, S.; Xu, B. Q.; Nagahara, L. A.; Amlani, I.; Schmickler, W.; Tsui, R.; Tao, N. J. J. Chem. Phys. 2003, 118, 8891-8897. (9) Rogers, C. T.; Buhrman, R. A. Phys. ReV. Lett. 1985, 55, 859-862. (10) Jiang, X.; Dubson, M. A.; Garland, J. C. Phys. ReV. B 1990, 42, 5427-5432. (11) Gorman, C. B.; Carroll, R. L.; Fuierer, R. R. Langmuir 2001, 17, 6923-6930. (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. Ann. N.Y. Acad. Sci. 1998, 852, 349-370. (13) 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, 2721-2732. 1619
(14) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705-1707. (15) Weiss, P. S.; Bumm, L. A.; Dunbar, T. D.; Burgin, T. P.; Tour, J. M.; Allara, D. L. Ann. N.Y. Acad. Sci. 1998, 852, 145-168. (16) Uosaki, K.; Sato, Y.; Kita, H. Langmuir 1991, 7, 1510-1514.
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NL034710P
Nano Lett., Vol. 3, No. 11, 2003
