Molecules as Components of Electronic Devices - American Chemical

Chapter 2

Downloaded by UNIV OF MISSOURI COLUMBIA on May 23, 2013 | http://pubs.acs.org Publication Date: February 20, 2003 | doi: 10.1021/bk-2003-0844.ch002

Using Probe Lithography and Self-Assembled Monolayers To Investigate Potential Molecular Electronics Systems R. Lloyd Carroll, Ryan Fuierer, and ChristopherB.Gorman* Department of Chemistry, North Carolina State University, Raleigh,NC27695

The pursuit of Molecular Electronics - that is, molecules that can be used as transistors, switches, rectifiers, and even logic gates - is a large and growing field of study. A fundamental phenomenon displayed by many essential molecular electronics components is that of Negative Differential Resistance (NDR). It is expected that molecules with accessible redox states (i.e., electroactive species) should display NDR. Scanning Probe Lithography was used to fabricate isolated nanostructures of redox-active molecules, and these were observed to display strongNDRunder ambient conditions . 1

The implementation of Molecular Electronics has two general requirements: the identification and preparation of molecular species with well-defined electronic properties and the ability to position, find, and interact with the molecules. Scanning Probe Lithography offers a method of prototyping molecular candidates, using the tip as both pencil and probe. Scanning Probe Lithography is useful to position small numbers of molecules with precision, find them in a background of inert molecules, and test their electronic properties while in a well-defined, surface bound nanostructure. Probe Lithography

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© 2003 American Chemical Society In Molecules as Components of Electronic Devices; Lieberman, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

11 techniques utilizing both Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM) have been demonstrated. " Our work has focused on the implementation of in-situ replacement lithography utilizing the STM , with imaging and lithography under an inert organic fluid such as dodecane. There are several benefits to such an in-situ process. Foremost, the organic fluid provides a method of mass transport, in that interesting molecules can be dissolved in the fluid and replaced into an existent surface. Moreover, inert organic fluids of very low polarity minimize leakage currents between the tip and sample, allowing imaging at very low tunneling currents. The process of in situ replacement lithography is straightforward. A selfassembled monolayer (or SAM) of an alkylthiolate (typically formed from decanethiol or dodecanethiol) is prepared on afreshlyannealed Au(l 11) surface. This SAM provides a well-ordered, largely crystalline surface into which other functionalized molecules can be replaced. An approximately 1 mM solution of a second functional thiol in dodecane is added before imaging. Imaging the SAM (typically at 1 V and 5-10 pA tunneling current) under the solution does not effect replacement of the second thiol. Raising the bias between tip and sample above some threshold voltage (typically ~3 V) promotes replacement Setting this bias and moving the tip in some computer-controlled pattern causes desorption of the thiolate SAM beneath the tip, and allows adsorption of the solvated thiol into the vacancies created (Figure 1). Upon completion of the pattern, the bias is again lowered to allow imaging of the surface without causing replacement. Line resolution using this technique is 10-15 nm . 2 8


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Figure 1. Schematic of replacement step in Scanning Probe Lithography

Figure 2 illustrates the characteristics of a ferrocene-terminated undecanethiolate SAM (henceforth "Fc") formed by replacement into an inert background of dodecanethiolate SAM. At low bias, the replaced Fc S A M is largely indistinguishable from the background. However, upon increasing the bias, the Fc S A M appears to increase in height, compared to the

In Molecules as Components of Electronic Devices; Lieberman, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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dodecanethiolate. This apparent height contrast indicates an enhanced flow of current through the Fc molecules, compared to the dodecanethiolate.

Figure 2. Bias Dependent Contrast exhibited by Fc replaced into dodecanethiolate SAM. STM images: 350nm scan size, indicated imaging parameters, 2nm z-scale, under dodecane

To investigate the origin of the bias dependent contrast, 1024 I-V curves were collected over a patterned surface composed of a square of Fc replaced into a background of decanethiolate (Figure 3, inset), under dodecane. The I-V curves were collected by turning off the feedback between tip and surface, then sweeping from positive bias to negative bias, in a specified range. The current response was collected and stored. The graph in Figure 3 shows two I-V curves.. The solid line is a typical curve selected from within the replaced Fc region. The non-linearity in the I-V curve is a phenomenon called Negative Differential Resistance, or NDR. The actual mechanism giving rise to the NDR in this system has not been established with certainty, but presumably, it arises due to resonant tunneling through accessible redox states in the Fc molecules. This is a similar phenomenon to that found in resonant tunneling diodes (RTDs). The dashed line is a typical curve selected from the decanethiolate region, outside the replacement square. Since the decanethiolate does not contain low-lying, accessible redox states, resonant tunneling in this bias range was not expected. Indeed, no NDR was observed. No apparent NDR behavior was observed when other, non-electroactive SAMs (e.g., those composed of carboxylic acid 13


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terminated alkylthiols) were probed. Similar NDR behavior was observed in both air and under dodecane solution.

Figure 3. NDR in Fc SAM. The inset is an STM image (700nm scan size, 400nm replaced region, collected at 1V, 5pA, 1Hz scan rate, under dodec a replaced region of Fc SAM in a background of decanethiolate SAM. The t I-V curves are representative curvesfromthe two regions of the surface: solid line is collected over the Fc SAM, and the dashed line is collected o decanethiolate SAM. (Adaptedfrom reference 1. Copyright 2001 Americ Chemical Society.)

From the I-V curves of Fc, it is apparent that the bias dependent contrast exhibited by Fc during imaging at higher biases arises as a consequence of the difference in current flow through the two types of molecules. Over Fc, as the tip-sample bias approaches the position of NDR, enhanced current flow through the Fc molecule leads to retraction of the tip, which is displayed as a larger apparent height contrast between Fc and decanethiolate. Upon examination of the I-V curves collected in many experiments, it was observed that the position at which the NDR occurs shifts slightly from curve to curve. This shift in NDR has been observed by others " . Given that the I-V 10

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14 curves are collected in a two electrode configuration (that is, tip and substrate), there is no chemical reference, and it is expected that the NDR position should shift somewhat during an experiment. To determine the variability of the response, a homogeneous Fc SAM was prepared and I-V curves collected over the SAM, under dodecane. A histogram of the observed peak position in a data set of 1015 curves is shown in Figure 4. A Gaussian fit of the histogram gives a mean peak position of 1.5 V with a full-width at half maximum of 0.1 V .


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Figure 4. Histogram ofposition of NDR in an Fc SAM, under dodecane. (Adapted from reference 1. Copyright 2001 American Chemical Society.)

It was hypothesized that other redox-active molecules would also display molecular NDR. Galvinol-terminated hexanethioacetate ("Gal", Figure 5) was provided by Professor David Shultz, and used in a "two-ink" experiment in which first Fc-SAM and then subsequently Gal-SAM regions were defined with replacement lithography into a dodecanethiolate S A M . As shown in the two panels of Figure 5 collected at low and high biases under dodecane, both molecules exhibit bias dependent contrast, within similar ranges. Further work is underway to synthesize other redox-active species, with particular effort towards those with accessible redox states that are different from Fc and Gal.



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Figure 5. Bias dependent contrast in Fc and Gal in a "two-ink" experiment. STM images: 600x600nm, indicated bias, 10pA 1Hz scan rate. (Adaptedfrom reference 1. Copyright 2001 American Chemical Society) t

Acknowledgments This work was supported in part by the Air Force Office of Scientific Research MURI Program in Nanoscale Chemistry, and by the National Science Foundation (CAREER Award, DMR-9600138 and CHE-9900072). References (1) Gorman, C. B.; Carroll, R. L.; Fuierer, R. R. Langmuir 2001, 17(22), 69236930. (2) Schoer, J. K.; Zamborini, F. P.; Crooks, R. M. J. Phys. Chem. 1996, 100, 11086-11091. (3) Schoer, J. K.; Crooks, R. M. Langmuir 1997, 13, 2323-2332. (4) Xu,S.;Liu,G.Y.Langmuir 1997, 13, 127-129. (5) Mizutani, W.; Ishida, T.; Tokumoto, H. Langmuir 1998, 14, 7197-7202. (6) Chen, J.; Reed, Μ. Α.; Asplund, C. L.; Cassell, A. M.; Myrick, M. L.; Rawlett, A. M.; Tour, J. M.; Van Patten,P.G.Appl.Phys. Lett. 1999, 75, 624-626. (7) Piner, R. D.; Zhu, J.; Xu,F.;Hong, S. H.; Mirkin, C. A. Science 1999, 283, 661-663. (8) X u , S.; Miller, S.; Laibinis, P. E.; Liu, G. Y. Langmuir 1999, 15, 72447251. (9) Gorman, C. B.; Carroll, R. L.; He, Y. F.; Tian, F.; Fuierer, R. Langmuir 2000,16,6312-6316. (10) Chen, J.; Reed, Μ. Α.; Rawlett, A. M.; Tour, J. M. Science 1999, 286, 1550-1552. (11) Kinne, M.; Barteau, M. A. Surf. Sci. 2000, 447, 105-111. (12) Han, W. H.; Durantini, Ε. N.; Moore, Τ. Α.; Moore, A. L.; Gust, D.; Rez, P.; Leatherman, G.; Seely, G. R.; Tao, N. J.; Lindsay, S. M. J. Phys. Chem. Β 1997, 101,10719-10725. (13) Sze, S. M. Physics of SemiconductorDevices;2nd ed.; John Wiley & Sons: New York, 1981.


Molecules as Components of Electronic Devices - American Chemical

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