pubs.acs.org/Langmuir © 2010 American Chemical Society
Terminal Alkynes as an Ink or Background SAM in Replacement Lithography: Adventitious versus Directed Replacement Eric Z. Tucker and Christopher B. Gorman* Department of Chemistry, North Carolina State University, Box 8204, Raleigh, North Carolina 27695-8204 Received April 26, 2010. Revised Manuscript Received July 8, 2010 Self-assembled monolayers (SAMs) comprised from n-alkanethiols and terminal alkynes were subjected to solutions containing ferrocene-terminated thiol, thioacetate, and terminal alkyne. The rate and extent of chemical exchange were monitored by scanning tunneling microscopy (STM). In several cases, a rate constant for exchange could be obtained by fitting to a model for exchange. In each case where this could be accomplished, a different rate model gave the best fit to the data, suggesting that the mechanism of exchange depended on either or both the original SAM and the incoming molecule. In scenarios where the rate of exchange was slow, directed exchange was accomplished via STM tip-induced lithographic patterning (replacement lithography). The extent of exchange was independent of the incoming molecule, suggesting that tip-induced desorption was the limiting factor in this process.
Introduction Self-assembled monolayers of alkanethiols on gold provide a convenient way to add functionality across surfaces or in localized areas.1 Other types of SAMs on gold and other substrates have been explored. However, these systems do not rival alkanethiols on gold for most studies or applications because alkanethiol SAMs are simple to form and generally are stable.2 However, SAMs of alkanethiols on gold do have some limitations. Thiols desorb from gold at elevated temperatures (approximately 80-100 C).3 Also, for electronic applications, a linker with π-type character may facilitate greater orbital overlap and thus greater electronic coupling between the molecule and the metal substrate.4 Thus, it has become of interest to explore other binding group SAMs on gold where there could be improved stability and better electronic coupling. One system that has begun to show some promise in forming useful SAMs are terminal alkynes on gold. A theoretical study has shown that binding of ethynylbenzene molecules to gold is energetically favorable.5 In addition, another study investigated diethynylbenzene on gold nanoparticle by surface-enhanced Raman scattering (SERS), and the results supported a nearvertical configuration of the molecule.6 Another report gave evidence of the presence of alkynes on relatively large, flat gold surfaces where the presence of a densely packed film was indicated by large water drop contact angles, ellipsometry, electrochemical blocking, and infrared spectroscopy.7 When exploring new types of SAMs, one question of interest is how they can be patterned using various lithography methods. *To whom correspondence should be addressed. E-mail: chris_gorman@ ncsu.edu. (1) Schreiber, F. Prog. Surf. Sci. 2000, 65(5-8), 151–256. (2) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105(4), 1103–1169. (3) Delamarche, E.; Michel, B.; Kang, H.; Gerber, C. Langmuir 1994, 10(11), 4103–4108. (4) Liu, J. Q.; Gooding, J. J.; Paddon-Row, M. N. Chem. Commun. 2005, No.5, 631–633. (5) Ford, M. J.; Hoft, R. C.; McDonagh, A. J. Phys. Chem. B 2005, 109(43), 20387–20392. (6) Lim, J. K.; Joo, S. W.; Shin, K. S. Vib. Spectrosc. 2007, 43(2), 330–334. (7) Zhang, S.; Chandra, K. L.; Gorman, C. B. J. Am. Chem. Soc. 2007, 129(16), 4876–4877.
Langmuir 2010, 26(18), 15027–15034
Figure 1. Schematic of the replacement process. The white bars represent the background SAM, and the red bars represent the ink.
Here, it is of specific interest to explore how alkyne SAMs can be patterned using replacement lithography. The basic methodology of replacement lithography is illustrated in Figure 1.8 A scanning tunneling microscope (STM) is used to desorb alkanethiol from a SAM by applying an elevated bias between the tip and sample. This desorption occurs in the presence of another molecule, which can bind to the exposed gold surface. This additional thiol acts as an ink that fills these vacated areas. One key advantage of this technique is it can be applied to many other binding group SAMs or substrates. Recently, this technique has been extended to other substrate metals such as platinum and palladium.9 In addition, it has been shown that by comparing the amount of replacement induced at a series of different biases between different SAMs on metals one can probe the ease at which one SAM is replaced relative to another slightly different SAM, which gives an idea of the relative stability of the two systems toward replacement lithography.9,10 When a SAM is exposed to a solution containing a different, SAM-forming molecule, spontaneous replacement can occur. It has been shown that thiolate SAMs exposed to another thiol undergo adventitious replacement. Richardson et al. reported the adventitious replacement of a n-octanethiol SAM on gold by a (8) Gorman, C. B.; Carroll, R. L.; He, Y. F.; Tian, F.; Fuierer, R. Langmuir 2000, 16(15), 6312–6316. (9) Williams, J. A.; Gorman, C. B. J. Phys. Chem. C 2007, 111(34), 12804–12810. (10) Lewis, M. S.; Gorman, C. B. J. Phys. Chem. B 2004, 108(25), 8581–8583.
Published on Web 08/26/2010
DOI: 10.1021/la101676h
15027
Article
Tucker and Gorman
semifluorinated n-octanethiol exposed to the surface in solution.11 In addition, the reverse was also shown when a semifluorinated n-octanethiol SAM was exposed to a solution of n-octanethiol. The exchange process was monitored by photoelastic modulation infrared reflection adsorption spectroscopy (PM-IRRAS) and STM. Moreover, the adventitious replacement of adamantanethiol SAMs on gold by dodecanethiol has been illustrated. The bulky molecules in this SAM are easily displaced, and the mechanism and the kinetics of this exchange process were elucidated.12 Furthermore, during the process of replacement lithography, when 11-ferrocenylundecanethiol or 11-ferrocenylundecanethioacetate was exposed to the background alkanethiolate SAM, adventitious replacement has been observed.13 Comparing the amounts of adventitious replacement of a SAM exposed to a competing molecule gives additional information about the relative stability of the SAM. In this report we describe the results of an experiment in which a terminal alkyne is used as either the ink or the background SAM in replacement lithography (directed approach) and adventitious replacement (undirected approach). In the development of replacement lithography, it is of interest to incorporate molecules containing other binding groups (here, the terminal acetylene group) using this technique as components of either the initial or replacement SAMs. In addition, the propensity of adventitious replacement of both dodecyne (C10) and dodecanethiolate (C12S-) SAMs is explored.
Results and Discussion A. Characterizing Adventitious Replacement. The relative reactivity of a terminal alkyne SAM and an alkanethiol SAM toward adventitious (undirected) replacement was probed with STM. In previous work, we have shown that, at slightly elevated biases (ca. 1 V), alkanethiolate SAMs containing a ferrocenyl headgroup display enhanced tunneling compared to methylterminated alkanethiols when bound to gold.14 This enhancement may be due to resonant tunneling between the tip and ferrocenyl headgroup,15 although other explanations have been offered.16 In any event, the higher rate of tunneling manifests itself as an increase in the apparent height of the ferrocenyl-terminated SAM compared to the methyl-terminated SAM regions. This height contrast allows one to distinguish the ferrocenyl-containing molecules from methyl-terminated regions of the SAM. In addition, we previously showed that alkanethiol and alkanethioacetate molecules in a solution covering an alkanethiol SAM can adventitiously replace into defect sites in the SAM.13,14 By using an ink containing a binding group with an affinity for gold and ferrocene tail groups that cause increased height contrast, (11) Patole, S. N.; Baddeley, C. J.; O’Hagan, D.; Richardson, N. V. J. Phys. Chem. C 2008, 112(36), 13997–14000. (12) Saavedra, H. M.; Barbu, C. M.; Dameron, A. A.; Mullen, T. J.; Crespi, V. H.; Weiss, P. S. J. Am. Chem. Soc. 2007, 129(35), 10741–10746. (13) Fuierer, R. R.; Carroll, R. L.; Feldheim, D. L.; Gorman, C. B. Adv. Mater. 2002, 14(2), 154–157. (14) Gorman, C. B.; Carroll, R. L.; Fuierer, R. R. Langmuir 2001, 17(22), 6923– 6930. (15) (a) Wassel, R. A.; Credo, G. M.; Fuierer, R. R.; Feldheim, D. L.; Gorman, C. B. J. Am. Chem. Soc. 2004, 126, 295–300. (b) Wassel, R. A.; Fuierer, R. R.; Kim, N.; Gorman, C. B. Nano Lett. 2003, 3(11), 1617–1620. (16) (a) He, J.; Lindsay, S. M. J. Am. Chem. Soc. 2005, 127(34), 11932–11933. (b) Tivanski, A. V.; Walker, G. C. J. Am. Chem. Soc. 2005, 127(20), 7647–7653.
15028 DOI: 10.1021/la101676h
adventitious replacement could be monitored by STM. The relative rates and extent of replacement give an idea of the relative stability of a C12S- and C10 SAM. It was expected that adventitious replacement would occur to a greater extent in the SAM with more defects and/or a lower thermodynamic stability. In sections B and C adventitious replacement experiments were performed as follows. SAMs of either C12S- (section B) or C10 (section C) on gold were imaged in air. Then, in separate experiments, solutions of 11-ferrocenylundecanethioacetate (molecule 1),17 11-ferroceynylundecanethiol (molecule 2), or FcC11 (molecule 3) were placed onto the C10 and C12SSAMs, covering the surface and the tip. After adjusting the relative humidity to ca. 47%, imaging was continued in dodecane to examine the amount of adventitious replacement of the molecule in solution into the initial SAM. When lower concentrations of the replacing molecule were used, a solution of the replacing molecule was slowly pumped over the sample to ensure that its concentration remained constant during the experiment (e.g., the replacement did not deplete the molecule in solution). Figure 2 shows a series of STM images that follow the replacement process. Panels A and B are STM images of SAMs of C12Sand C10 before the solution of the replacing molecule was added. Pits (vacancy islands) were present in both C12S- and C10 SAMs. The remaining panels in Figure 2 are images that were taken after 50 μM solution of molecule 1 (thioacetate) in dodecane was exposed to these samples for 20 min (Figure 2C,D) and 60 min (Figure 2E,F). The rate and extent of replacement were obtained quantitatively by taking STM images collected at different replacement times and calculating the percent of the area occupied by the replacing molecule. Histograms of the height values were constructed and fit to a sum of two Gaussian functions (Figure 3B). The Gaussian curve with the lower apparent height was assigned to the background, and the curve with the higher apparent height was assigned to the replaced areas. In this process, regions containing pits and step edges were avoided. However, to the extent that they existed, they were counted as unreplaced regions as shown by the annotations in Figure 3B. Thus, the extent of replacement is systematically underestimated by this error which is estimated at
