Anal. Chem. 2003, 75, 1089-1093
Scanning Tunneling Microscopy with Chemically Modified Gold Tips: In Situ Reestablishment of Chemical Contrast Joel A. Olson and Philippe Bu 1 hlmann*
Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455
A method was developed for the reestablishment of chemical contrast in STM images obtained with chemically modified gold tips. Such tips display selective chemical contrast, which allows the selective imaging of specific species on the sample surface. Chemically modified STM tips can be fabricated by forming a self-assembled monolayer (SAM) on an electrochemically etched gold tip. One difficulty with this method thus far has been the relatively short lifetime of SAM-treated tips. The method described here utilizes the brief application of a high bias voltage between the sample and the tip to cause SAM molecules to reoccupy the tip apex, thereby allowing the tips to display selective chemical contrast in imaging. These treatments consist of applying a +1.9-V sample bias for 0.5-10 min under tunneling conditions. The usable lifetime of SAM-modified tips could be increased by more than 2 orders of magnitude, from hours to at least a month, dramatically increasing the efficiency of using SAM-modified gold tips. SAM molecules can also be removed from the tip apex by application of a negative sample bias (-2.0 V for 0.5-10 min) making it possible to alternate between conventional STM images and STM images with chemically enhanced contrasts. Scanning tunneling microscopy (STM) is a powerful method of surface analysis that generally provides the spatial disposition of atoms and molecules with high resolution. 1 However, it often does not provide direct discrimination of chemical species. Therefore, recent work has been directed toward chemically selective imaging by various means. Interestingly, adsorbates at the apex of a STM tip can allow the selective imaging of surface species. 2-6 One recently developed method for the chemical * Corresponding author. E-mail:
[email protected]. (1) Binnig, G.; Rohrer, H.; Gerber, C., Weibel, E. Phys. Rev. Lett. 1982, 49, 57. Scanning Tunneling Microscopy I, 2nd ed.; Gu ¨ ntherodt, H. J., Wiesendanger, R., Eds.; Springer-Verlag: New York, 1994. (2) Rousset, S.; Gauthier, S.; Siboulet, O.; Sacks, W.; Belin, M.; Klein, J. Phys. Chem. B 1989, 63, 1265. Ruan, L.; Besenbacher, F.; Stensgaard, I. Laegsgaard, E. Phys. Rev. Lett. 1993, 70, 4079. Schmid, M.; Stadler, H.; Varga, P. Phys. Rev. Lett. 1993, 70, 1441. McIntyre, B. J.; Sautet, P.; Dunphy, J. C.; Salmeron, G. A.; Somorjai, G. A. J. Vac. Sci. Technol. B 1994, 12, 1751. Xu, H.; Ng, K. Y. S. Surf. Sci. 1996, 355, L350. Kelly, K. F.; Sarkar, D.; Prato, S.; Resh, J. S.; Hale, G. D.; Halas, N. J. J. Vac. Sci. Technol. B 1996, 14, 593. Bartels, L.; Meyer, G.; Rieder, K. H. Appl. Phys. Lett. 1997, 71, 213. Xu, Q.-M.; Wan, L.-J.; Yin, S.-X.; Wang, C.; Bai, C.-L. J. Phys. Chem. B. 2001, 105, 10465. Hahn, J. R.; Ho, W. Phys. Rev. Lett. 2001, 87, 6102. (3) Ito, T.; Bu ¨ hlmann, P.; Umezawa, Y. Anal. Chem. 1998, 70, 255. 10.1021/ac025955+ CCC: $25.00 Published on Web 01/24/2003
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discrimination of specific atoms and functional groups in STM images is based on the control of interactions between samples and chemically modified tips, in particular, gold tips modified with self-assembled monolayers (SAMs) of thiols 3-5 or with polypyrrole. 6 The molecules that form the SAMs are selected in such a way that their terminal functionality interacts with a chemical functionality of interest on the surface, for example, by hydrogen bond formation3,4,6 or by metal-ligand coordination.5 As previously reported, these chemical tip-sample interactions cause the interacting functional groups or atoms to appear as bright (or “contrast-enhanced”) areas, clearly distinguishable in STM images (e.g., Figure 1a or Figure 2a, upper half).3-6 While quantitative models to explain chemical contrast enhancement of this type have not been developed yet, experimental evidence3-6 suggests that it results from chemically selective tip-sample interactions that perturb the density of states at the tip and sample. A drawback of SAM-modified gold tips has been the relatively low rate of successful tip fabrication. So far, only a limited fraction of all prepared tips has shown contrast enhancement. And for those that did, the usable lifetime was no more than a few hours before contrast enhancement was lost (see Figure 2 and the discussion in the text below). Presumably, the disappearance of contrast enhancement is due to the loss of SAM species from the STM tip apex. This short lifetime makes experiments at atmospheric pressure tedious (since tips must be frequently replaced) and UHV STM virtually impossible. Here we report a novel method for restoring tips that have lost their contrast enhancement capability, thus extending the tip lifetime from a few hours to at least a month and possibly much longer. EXPERIMENTAL SECTION Monolayers of 1-octadecanol (Tokyo Kasei Kogyo Co., Tokyo, Japan) or 1-octadecanoic acid (Wako Pure Chemical Industries, Osaka, Japan) were formed on highly oriented pyrolytic graphite (HOPG, grade ZYH, Advanced Ceramics Corp., Lakewood, OH) by spontaneous adsorption from solutions in 1-phenyloctane (Aldrich, Milwaukee, WI) solutions (∼5 mg/mL). STM tips were prepared as reported previously3,4 by electrochemical etching of gold wire (0.25 mm, Alfa Aesar, Ward Hill, MA) in 3 M NaCl at (4) Nishino, T.; Bu ¨ hlmann, P.; Ito, T.; Umezawa, Y. Surf. Sci. Lett. 2001, 490, L579. Nishino, T.; Bu ¨ hlmann, P.; Ito, T.; Umezawa, Y. Phys. Chem. Chem. Phys. 2001, 3, 1867. (5) Ohshiro, T.; Ito, T.; Bu ¨ hlmann, P.; Umezawa, Y. Anal. Chem. 2001, 73, 878. (6) Ito, T.; Bu ¨ hlmann, P.; Umezawa, Y. Anal. Chem. 1999, 71, 1699.
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Figure 1. (a) STM image of a monolayer of 1-octadecanoic acid on a HOPG substrate, collected with a chemically modified gold tip (4mercaptopyridine). The terminal lone pair of the SAM pyridine interacts with the hydrogen of the surface carboxylic acid species, resulting in an apparently raised region over the carboxyl groups. (b) STM image of a monolayer of 1-octadecanoic acid on HOPG, collected with a bare gold tip. The molecules form dimers and lie flat on the surface, as depicted schematically. The carboxyl groups appear as characteristic dark rows. (c) Schematic representation of 1-octadecanoic acid dimer. (d) Structure of 4-mercaptopyridine.
ac 10 V, rinsing with pure water (18.2 MΩ), and washing in an ultrasonic bath in pure water for ∼15 min. SAMs on the tips were prepared by placing the etched tips into a ∼2 mM ethanolic solution (USP, AAPER Alcohol and Chemical Co., Shelbyville, KY) of 4-mercaptopyridine (Aldrich) for >12 h, rinsing with ethanol, and drying in a stream of nitrogen. The tips were stored in air until use. STM measurements were conducted under ambient conditions at room temperature using a Molecular Imaging PicoSPM scanning head (Phoenix, AZ) and a RHK SPM 1000 control electronics unit (Troy, MI) with the STM tip directly immersed in one of the 1-phenyloctane solutions of the monolayerforming molecules. Images were collected in constant-current mode (0.7 nA). RESULTS/DISCUSSION Figure 1b shows an image of a monolayer of 1-octadecanoic acid as collected with a bare, chemically unmodified gold tip. The 1-octadecanoic acid forms a dimer species, which is schematically represented as shown in Figure 1c. Due to van der Waals interactions between the alkane chains and the HOPG, the alkane chains lie flat on the HOPG. 8 The individual methylene groups
are clearly visible. Characteristic rows of dark features are observed where the 1-octadecanoic acid molecules interact with their neighbors through their carboxyl groups. These apparent depressions are presumably due to a decrease in the available density of states in the region of the carboxyl moieties. Figure 1a shows an image of a monolayer of 1-octadecanoic acid collected with a gold tip that was chemically modified with a SAM of 4-mercaptopyridine (shown in Figure 1d). The 4-mercaptopyridine forms a SAM on the surface of the gold tip such that the pyridine electron lone pair is oriented away from the tip and toward the sample surface. Note that the image now shows rows of bright features that appear raised, rather than depressed as in the case of an unmodified tip. The raised features occur at the carboxyl functionalities of the 1-octadecanoic acid. As previously stated, such an image is termed “contrast enhanced” and is the result of the interaction of the terminal lone pair of the 4-mercaptopyridine and the carboxyl groups on the surface. Thus, the carboxyl groups are recognized selectively due to their chemical interaction (H-bonding) with the chemically modified STM tip. These results are consistent with those reported previously.3
(7) Sautet, P. Chem. Rev. 1997, 97, 1097. (8) Rabe, J. P.; Buchholz, S. Science 1991, 253, 424. McGonical, G. C.; Bernhardt, R. H.; Yeo, Y. H.; Thomson, D. J. J. Vac. Sci. Technol. B 1991, 9, 1107. Liang, W.; Whangbo, M.-H.; Wawkuschewski, A.; Cantow, H.-J.; Maganov, S. N. Adv. Mater. 1993, 5, 817. Bucher, J.-P.; Roeder, H.; Kern,
K. Surf. Sci. 1993, 289, 370. Venkataraman, B.; Flynn, G. W.; Wilbur. J.; Folkers, J. P.; Whitesides, G. M. J. Phys. Chem. 1995, 99, 8684. Cyr, D. M.; Venkataraman, B.; Flynn, G. W. Chem. Mater. 1996, 8, 1600. Hibino, M.; Sumi, A.; Tsuchiya, H.; Hatta, I. J. Phys. Chem. B 1998, 102, 4544. Giancarlo, L. C.; Flynn, G. W. Acc. Chem. Res. 2000, 33, 491.
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Figure 3. STM image of a 1-octadecanoic acid monolayer on HOPG, showing the establishment of contrast enhancement. Initially, the image is not contrast-enhanced (top; COOH groups appear dark), indicating an STM tip with a bare apex. This is followed by tip conditioning (horizontal arrows; +1.9 V, 15 s), accompanied by tip instability. Subsequently, the image is contrast-enhanced (bottom; COOH groups appear bright), indicating the presence of a SAM molecule at the tip apex.
Figure 2. (a) STM image of a monolayer of 1-octadecanoic acid on HOPG substrate (sample bias +1.6 V; tunneling current 0.91 nA). Initially (region I), contrast enhancement is observed. Roughly halfway through image collection, the tip becomes unstable (poor resolution, region II). Then the image appears as that of a normal, non-contrastenhanced monolayer of 1-octadecanoic acid (region III). The loss of contrast enhancement is presumably the result of the egress of the SAM molecule from the tip apex. (b) Hypothetical representation of the tip apex: In region I, a SAM molecule occupies the apex of the tip; in region III the tip apex is bare.
A drawback of this approach is the spontaneous loss of contrast enhancement that is typically observed after a period of continuous scanning. Figure 2a shows an image of a monolayer of 1-octadecanoic acid on a HOPG substrate, with the carboxyl rows indicated by arrows. The image was collected with a gold tip that had been treated with 4-mercaptopyridine. Initially (region I), the image shows the characteristic raised rows that result from selective chemical contrast enhancement; this indicates the presence of a SAM molecule at the tip apex as shown schematically in Figure 2b (I). Roughly halfway through the image collection (region II) the image blurs, which is indicative of tip instability. Finally, in region III, the image shows rows of dark (recessed) features that are characteristic of an unmodified tip, as shown schematically in Figure 2b (III). Another drawback of SAM-modified tips has been that, upon initial use, many modified tips do not provide images with contrast enhancement, i.e., images in which OH and COOH groups appeared as bright regions (Figure 1a). Instead, such images resemble those measured with unmodified tips (Figure 1b), with the OH and COOH groups appearing as dark depressions.
Previously, tips that did not display contrast enhancement upon initial use were discarded. However, we show here that chemically modified tips that have never had or have lost the ability to provide contrast enhancement can be conditioned to subsequently provide contrast enhancement. Our method takes advantage of the fact that a chemically treated tip that does not display contrast enhancement still possesses SAM molecules on areas of its surface other than at the tip apex. If those SAM molecules can be directed toward the tip apex, contrast enhancement can be established. To achieve reoccupation of the tip apex, we utilized tip conditioning by adjusting the bias applied between the tip and the sample. After imaging for at least 30 min, the tips that showed molecular resolution (but no contrast enhancement) were subjected to a positive sample bias of +1.9 V for a duration of 0.5-10 min. This in situ tip conditioning was performed while tunneling and, in most cases, while holding the tip stationary. After each voltage treatment, an image was collected to assess the conditioning results. If contrast enhancement was not observed in the subsequent image, the tip was subjected to another voltage treatment. However, one voltage treatment often sufficed. Tips that lost the ability to show contrast enhancement after continuous scanning over several hours were subjected to the same bias treatment. To illustrate the process of tip restoration, an image of a monolayer of 1-octadecanoic acid was collected at a particularly slow scan speed (6 min/image; Figure 3). The upper half of the image, obtained at an imaging bias of +0.67 V, does not display contrast enhancement (carboxyl rows are indicated by arrows at the top and bottom of the image). The tip was then treated by increasing the sample bias to +1.9 V for 15 s (denoted by the large arrows in Figure 3), causing the horizontal stripe in the image. Finally, the sample bias was decreased to its original value of +0.67 V. The lower part of the image, collected after the Analytical Chemistry, Vol. 75, No. 5, March 1, 2003
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Figure 4. STM image of a 1-octadecanol monolayer on HOPG with a chemically modified gold tip. The bright lines (indicated by arrows) are the result of the H-bond interactions between the SAM terminus on the tip and the hydroxyl group of the alcohol and manifest chemical contrast enhancement (sample bias +0.74 V).
treatment, shows the carboxyl groups of octadecanoic acid as bright regions, manifesting selective contrast enhancement. Note that this image is somewhat noisy due to the unusually slow scan speed that was necessary to allow sufficient time for the period of posttreatment tip instability to subside within a small portion of the image. Images collected at typical scan speeds (
