Scanning Tunneling Microscopy Using Chemically Modified Tips

1-octadecanoic acid were observed with scanning tunnel- ing microscopy ... adsorbed.14 However, chemical modification of STM tips for the recognition ...
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Anal. Chem. 1998, 70, 255-259

Scanning Tunneling Microscopy Using Chemically Modified Tips Takashi Ito,† Philippe Bu 1 hlmann, and Yoshio Umezawa*

Department of Chemistry, School of Science, The University of Tokyo, Bunkyo-Ku, Tokyo 113, Japan

Monolayers of 1-octadecanol, 1-chlorooctadecane, and 1-octadecanoic acid were observed with scanning tunneling microscopy (STM). Chemical modification of STM tips allowed recognition of functional groups in monolayers of 1-octadecanol and 1-octadecanoic acid. Enhanced contrasts for hydroxyl and carboxyl residues were observed with gold tips modified with several aromatic mercaptans that can form hydrogen bonds (4-mercaptopyridine, 4-aminothiophenol, 4-hydroxythiophenol). On the other hand, no such enhancements were observed in monolayers of 1-chlorooctadecane. The enhanced contrasts for the OH and COOH groups seem to be due to electron tunneling through hydrogen bonds formed between the functional groups on tip and sample. These results suggest that tailored modification of STM tips may become a general method for the recognition of specific chemical species and functional groups in STM images. Since the mid-1970s, “tailored” chemical modification of electrode surfaces has been used in electrochemistry to control chemical reactions at electrodes.1,2 Until then, it was often not possible to hinder unwanted precipitation or adsorption processes on electrodes and control the overpotential necessary for desired reactions. Fouling of electrodes and slow electrochemical reaction rates resulted, which seriously affected electrochemical applications. Chemical modification of electrode surfaces has often led to improved experimental reproducibilities and selective acceleration of redox reactions of species of interest. The latter frequently involves specific recognition of chemical species through chemical interactions, such as charge-charge and hydrogen bond interactions. Just as in electrochemistry, electron transfer plays also a central role in scanning tunneling microscopy, which relies on the tunneling effect as operating principle.3-5 If an STM tip and a conducting sample are separated by only a few angstroms, their electronic wave functions overlap. Upon application of a bias * Corresponding author: (phone) +81-3-5802-2989; (fax) +81-3-5802-2989; (e-mail) [email protected]. † Research Fellow of the Japan Society for the Promotion of Science (JSPS). (1) Murray, R. W. In Electranalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1984; Vol. 13, pp 191-368. (2) Murray, R. W.; Ewing, A. G.; Durst, R. A. Anal. Chem. 1987, 59, 379A390A. (3) Wiesendanger, R. Scanning Probe Microscopy and Spectroscopy: Methods and Applications; University Press: New York, 1994. (4) Coratger, R.; Sivel, V.; Ajustron, F.; Beauvillain, J. Micron 1994, 25, 371385. (5) Ikai, A. Surf. Sci. Rep. 1996, 26, 261-332. S0003-2700(97)00498-8 CCC: $15.00 Published on Web 01/15/1998

© 1998 American Chemical Society

voltage, a tunneling current flows between tip and sample in the direction given by the sign of the applied voltage. Just as in electrochemistry, overlap of electronic wave functions (in this case those of tip and sample) is a requirement for the electron transfer. This analogy suggested to us that “tailored” chemical modification of STM tips should allow to discriminate chemical species through rational use of chemical interactions. It has been shown theoretically and observed experimentally that adsorption of atoms or molecules on a STM tip changes the tip structure6-8 and/or electronic states of the tip,6,9-14 and, therefore, the STM images also. For example, fullerene C60 adsorbed onto STM tips was reported to enhance atomically resolved images of HOPG (highly ordered pyrolytic graphite).7 It was hypothesized that C60 might provide a mechanical buffer between the tip and the sample surface, aid in sweeping away contaminants, decrease the contact area between tip and sample, and/or stabilize the gap between the tip and the surface.7 The adsorption of fullerene C60 on the tip results in images that represent a convolution of sample and tip structure.8 Such effects seem to be mainly due to changes in the tip structure. On the other hand, changes in the contrasts of STM images of S/Cu(1111)10 and S/Pt(111) surfaces11 as obtained when tips with adsorbed S atoms are used seem to be mainly the effect of changes in the electronic states of the tip. Similar examples include selective imaging of metal atoms by using tips with adsorbed O or S atoms12,13 and changes in the molecular image of NO on Rh(111) with tips onto which CO molecules adsorbed.14 However, chemical modification of STM tips for the recognition of chemical species and/or functional groups of organic molecules by “chemical interactions” like hydrogen bonds has so far not been reported. Because of its relevance for biological charge-transfer processes, electron transfer by tunneling through hydrogen bonds has (6) Sautet, P.; Dunphy, J. C.; Ogletree, D. F.; Joachim, C.; Salmeron, M. Surf. Sci. 1994, 315, 127-142. (7) Resh, J.; Sarkar, D.; Kulik, J.; Brueck, J.; Ignatiev, A.; Halas, N. J. Surf. Sci. 1994, 316, L1061-L1067. (8) Kelly, K. F.; Sarkar, D.; Prato, S.; Resh, J. S.; Hale, G. D.; Halas, N. J. J. Vac. Sci. Technol. 1996, B14, 593-596. (9) Chen, C. J. J. Vac. Sci. Technol. 1994, B12, 2193-2199. (10) Rousset, S.; Gauthier, S.; Siboulet, O.; Sacks, W.; Belin, M.; Klein, J. Phys. Rev. Lett. 1989, 63, 1265-1268. (11) McIntyre, B. J.; Sautet, P.; Dunphy, J. C.; Salmeron, M.; Somorjai, G. A. J. Vac. Sci. Technol. 1994, B12, 1751-1753. (12) Schmid, M.; Stadler, H.; Varga, P. Phys. Rev. Lett. 1993, 70, 1441-1444. (13) Ruan, L.; Besenbacher, F.; Stensgaard, I.; Laegsgaard, E. Phys. Rev. Lett. 1993, 70, 4079-4082. (14) Xu, H.; Ng, K. Y. S. Surf. Sci. 1996, 355, L350-L354.

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Figure 1. Chemical structures of the compounds 1-6.

recently attracted considerable attention.15-18 The measurement of rate constants for photoinduced electron transfers has shown that electron coupling modulated by hydrogen bonds can be larger than through σ bonds.17 We present here the first chemically modified STM tips that allow recognition of functional groups by what seems to be electron transfer through hydrogen bonds. As samples for observation, we used monolayers formed on HOPG by three kinds of primarily substituted hydrocarbons (C18H37OH, C18H37Cl, C17H35COOH). Such monolayers have been imaged previously with unmodified tips by several investigators.19-29 We examined the effect of tip modification on the STM images of these monolayers by self-assembling thiophenol, 4-mercaptopyridine, 4-aminothiophenol, 4-hydroxythiophenol, 3-mercaptopropionic acid, and 2-aminoethanethiol hydrochloride on gold tips. The results of the present study show that this chemical modification of STM tips allows one, based on hydrogen bond interactions, to recognize functional groups in monolayers of 1-octadecanol and 1-octadecanoic acid. EXPERIMENTAL SECTION The chemical structures of the thiols used in the present study are shown in Figure 1. 1-Octadecanol (C18H37OH), 1-chlorooctadecane (C18H37Cl), and 1-phenyloctane were purchased from Tokyo Kasei Kogyo Co. (Tokyo, Japan) and used without further purification. 1-Octadecanoic acid (C17H35COOH) was purchased from Wako Pure Chemical Industries (Osaka, Japan) and purified by recrystallization from n-hexane. Thiophenol (4), 4-mercapto(15) Turro, C.; Chang, C. K.; Leroi, G. E.; Cukier, R. I.; Nocera, D. G. J. Am. Chem. Soc. 1992, 114, 4013-4015. (16) Roberts, J. A.; Kirby, J. P.; Nocera, D. G. J. Am. Chem. Soc. 1995, 117, 8051-8052. (17) de Rege, P. J. F.; Williams, S. A.; Therien, M. J. Science 1995, 269, 14091413. (18) Cukier, R. I. J. Phys. Chem. 1995, 99, 16101-16115. (19) Rabe, J. P.; Buchholz, S. Science 1991, 253, 424-427. (20) McGonigal, G. C.; Bernhardt, R. H.; Yeo, Y. H.; Thomson, D. J. J. Vac. Sci. Technol. 1991, B9, 1107-1110. (21) Buchholz, S.; Rabe, J. P. Angew. Chem., Int. Ed. Engl. 1992, 31, 189-191. (22) Yackoboski, K.; Yeo, Y. H.; McGonigal, G. C.; Thomson, D. J. Ultramicroscopy 1992, 42-44, 963-967. (23) Elbel, N.; Roth, W.; Gu ¨ nther, E.; von Seggern, H. Surf. Sci. 1994, 303, 424-432. (24) Elbel, N.; Gu ¨ nther, E.; von Seggern, H. Appl. Phys. Lett. 1994, 65, 642644. (25) Gunning, A. P.; Kirby, A. R.; Mallard, X.; Morris, V. J. J. Chem. Soc., Faraday Trans. 1994, 90, 2551-2554. (26) Venkataraman, B.; Breen, J. J.; Flynn, G. W. J. Phys. Chem. 1995, 99, 66086619. (27) Venkataraman, B.; Flynn, G. W.; Wilbur, J. L.; Folkers, J.; Whitesides, G. M. J. Phys. Chem. 1995, 99, 8684-8689. (28) Hibino, M.; Sumi, A.; Hatta, I. Jpn. J. Appl. Phys. 1995, 34, 3354-3359. (29) Cyr, D. M.; Venkataraman, B.; Flynn, G. W.; Black, A.; Whitesides, G. M. J. Phys. Chem. 1996, 100, 13747-13759.

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pyridine (1), 4-aminothiophenol (2), 4-hydroxythiophenol (3), and 3-mercaptopropionic acid (5) from Tokyo Kasei Kogyo and 2-aminoethanethiol hydrochloride (6) from Wako Chemical Industries were used without further purification. Monolayers of 1-octadecanol and 1-octadecanoic acid were prepared by depositing a drop (∼10 µL) of a solution of these compounds in 1-phenyloctane onto a freshly cleaved surface of highly ordered pyrolytic graphite (HOPG; Digital Instruments, Santa Barbara, CA).19-21,23-29 Similar results were obtained with HOPG substrates from five different lots. 1-Chlorooctadecane monolayers were prepared likewise but without dilution in phenyloctane.26,27 STM tips were prepared from gold wire (0.3 mm diameter; Nilaco Co., Tokyo, Japan) by electrochemical etching in 3 M NaCl at ac 10 V.30 The tips were then washed with acetone in an ultrasonic bath or cleaned in “piranha solution” (7:3 concentrated H2SO4/30% H2O2. Caution: piranha solution reacts violently with organic compounds and should not be stored in closed containers). For the formation of SAMs on the tips, the latter were cleaned in piranha solution and immersed for more than 12 h in 1-10 mM ethanolic solutions (HPLC-grade ethanol, Wako Pure Chemical Industries) of the thiol.31 The tips were then rinsed with HPLCgrade ethanol and dried in a stream of nitrogen. STM measurements were carried out on a STA 330 microscope (Seiko Instruments, Inc., Tokyo, Japan) equipped with a 0.8 µm scan head. All images were obtained under ambient conditions at temperatures between 18 and 25 °C in the constant-current mode. Once the sample was mounted on the sample stage, STM images were obtained by scanning with the tip being immersed in the monolayer-forming solution. With each tip, STM measurements were performed for ∼50 min. Typical bias voltages were in the range of + 0.4 to + 1.0 V or -0.4 to -1.0 V, and the tunneling current was 0.3-1.0 nA. RESULTS AND DISCUSSION Upon placing a drop of the 1-phenyloctane solution of certain primarily substituted hydrocarbons on a HOPG substrate, monolayer films of the solutes spontaneously form at the solutionHOPG interface.19-21,23-29 Positioning of the STM tip and the approach of the tip to the sample last sufficiently long so that no additional waiting period is required before an organized monolayer can be observed by STM. Figure 2a shows a typical STM image of a 1-octadecanol monolayer, as observed with an unmodified gold tip. Lamella structures consisting of bright parallel bands were observed, each lamella being separated from the adjacent lamellae by dark borderlines. The length of the bright bands was 2.4 ( 0.2 nm, which agrees well with the length of a C18 carbon chain in all-trans conformation. This indicates that these bands correspond to octadecyl chains physisorbed on the graphite surface. The 1-octadecanol molecules are packed parallel to each other with the molecular axes oriented 60° relative to the dark lamella borders, forming a “herringbone” structure, as reported in earlier observations.19,21,23,25 The width of a lamella is ∼2.2 nm. The OH end groups cannot be distinguished from the hydrocarbon backbone. When unmodified gold tips were washed with acetone prior to imaging, the ends of the alkyl chains sometimes (30) Nam, A. J.; Teren, A.; Lusby, T. A.; Melmed, A. J. J. Vac. Sci. Technol. 1995, B13, 1556-1559. (31) Bryant, M. A.; Crooks, R. M. Langmuir 1993, 9, 385-387.

Figure 2. (a) STM image of 1-octadecanol (C18H37OH) physisorbed from a 1-phenyloctane solution onto HOPG, as obtained with an unmodified gold tip: image size 20 nm × 20 nm, total gray scale range 0.2 nm, sample bias voltage -1.0 V (sample negative), and tunneling current 1.0 nA. (b) STM image of 1-octadecanol physisorbed from a 1-phenyloctane solution onto HOPG, as obtained with a 4-mercaptopyridine-modified gold tip: image size 20 nm × 20 nm, total gray scale range 0.2 nm, sample bias voltage -1.0 V (sample negative), and tunneling current 0.7 nA. The height of the bright lines as compared to the rest of the lamellae was ∼0.04 nm. (c) Schematic illustration of the molecular arrangement as observed for the monolayer of 1-octadecanol. Open circles and bars represent hydroxyl groups and alkyl chains, respectively.

appeared brighter. This was the case for 3 of 27 tips that gave molecular images (11%, Table 1). With gold tips that had been cleaned in piranha solution to remove contaminants, on the other hand, such images were observed with none of the 91 tips that gave molecular images (0%, Table 1). Surprisingly, contrasts within molecules22,25 were not altered by changes in the bias voltage. These results suggest that images with enhanced

contrasts were obtained due to adsorption of probably organic contaminants onto the tips. Such impurities may be either not removed or even introduced by washing of the tips with acetone (vide infra) and may occasionally lead to contrast enhancements in a way similar to the intentional tip modification described below. Figure 2b shows an STM image of a 1-octadecanol monolayer as obtained with a tip modified with 1. Parallel bright lines separated by 4.4 ( 0.2 nm were observed. The height of these lines relative to the rest of the lamellae varied between 0.02 and 0.10 nm in the images of several identically prepared tips and samples (Table 1). Contrast enhancements as shown in Figure 2b were usually larger with the tips modified with 4-mercaptopyridine than with acetone-washed tips (vide supra), indicating a specific effect of 4-mercaptopyridine. The separation of the bright lines is twice the width of a lamella as seen in Figure 2a. This suggests that within a lamella all hydroxyl groups point into the same direction. Furthermore, the methyl groups of one lamella face the methyl groups of an adjacent lamella, while the hydroxyl groups form hydrogen bonds to the hydroxyl groups of another lamella (cf. scheme in Figure 2c). Images as shown in Figure 2b were obtained with 7 of 36 tips modified with 4-mercaptopyridine (19%, Table 1) under a fairly wide range of tunneling conditions (+0.4 to +1.0 V and -0.4 to -1.0 V bias voltage; 0.3-1.0 nA tunneling current). With the remaining 29 tips modified with 4-mercaptopyridine (81%), only images as the one shown in Figure 2a were observed. The low yield of images with enhanced contrasts may be due to difficulties in the control of the tip shape and/or insufficient coverage of the STM tip apex with thiol molecules. Because lateral interactions in SAMs of small-size aromatic mercaptans are weak,32 the stability of such monolayers is not expected to be very high. Indeed, after several images with enhanced contrasts had been acquired with a particular tip (i.e., after scanning for ∼3 min), the character of the images typically changed and images as shown in Figure 2a were obtained. This loss of enhanced contrast may be due to (i) removal of molecules from the STM tip apex during scanning,33 as also suggested in previous reports on tips with adsorbates,7,8,10,12-14 (ii) contamination of the tips, and/or (iii) changes in the shape of the tips during scanning. It is conceivable that enhanced contrasts can be observed again if the SAMs on the tips are regenerated by immersion of the tips into an ethanolic solution of the thiol. We are presently examining this possibility with the aim to better understand the reasons for the loss of enhanced contrasts after repeated use of modified tips. We further also examined how tip modification with 2-4 affects images of monolayers of 1-octadecanol. Results as shown in Figure 2b were observed with 8 of 26 tips (31%, Table 1) modified with 4-aminothiophenol and with 3 of the 31 tips (10%, Table 1) modified with 4-hydroxythiophenol. Whereas the effect of tip modification with 4-hydroxythiophenol was small at most, clearly enhanced image contrasts were obtained for modification with 4-aminothiophenol (Table 1). Interestingly, there was no effect of tip modification with thiophenol: no contrast enhancements as shown in Figure 2b were observed with 27 tips modified with thiophenol (Table 1). These results seem to reflect the (32) Chailapakul, O.; Crooks, R. M. Langmuir 1993, 9, 884-888. (33) Stranick, S. J.; Parikh, A. N.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 11136-11142.

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hydrogen bond basicity of the functional groups in these thiol derivatives: the hydrogen bond basicity decreases in the order of pyridine > aniline > phenol > benzene.34 Whereas enhanced contrasts in 1-octadecanol monolayers were observed with tips modified with 4-mercaptopyridine or 4-aminothiophenol, which have functional groups with strong hydrogen bond basicity, the small effect of tip modification with 4-hydroxythiophenol and the absence of an effect of tip modification with thiophenol seem to be the result of the small hydrogen bond basicity of their functional groups. These results all suggest that the enhanced contrasts in these STM images arise because of the presence of hydroxyl residues and are due to hydrogen bond interactions between the functional groups on tip and sample. Electron tunneling through hydrogen bonds, as recently observed repeatedly for photoinduced electron transfer,15-17 occurs when the electronic wave functions of the hydrogen bond donor and acceptor groups overlap.18 Whether these hydrogen bonds are strong, as those formed between hydroxyl groups and heteroatoms with a nonbonding electron pair, or whether weaker bonding between the hydroxyl groups and the aromatic π-orbital systems may also play a role for the electron transfer cannot be decided at present with certainty. However, because of their larger strength35 and concomitantly larger extent of electronic coupling, hydrogen bonds of the former type probably play a more important role in contrast enhancements than interactions of the latter type. Interestingly, no contrast enhancements were observed in images of 1-octadecanol monolayers when tips modified with 5 or 6 were used (see Table 1). The reason why only tip modification with aromatic mercaptans enhanced contrasts is not clear. STM images of 1-chlorooctadecane monolayers were acquired with unmodified and 4-mercaptopyridine-modified tips. The images as obtained with both types of tips were similar to those obtained previously with unmodified tips (data not shown).26,27 The absence of contrast enhancements for 1-chlorooctadecane monolayers does not seem surprising because no attractive interaction is expected between the pyridine and chlorine. Similar effects of tip modification as seen for 1-octadecanol monolayers were obtained when monolayers of 1-octadecanoic acid (C17H35COOH) were examined. Figure 3a shows an STM image as acquired with an unmodified gold tip. Lamella structures composed of bright bands were observed, and again each lamella was separated from adjacent lamellae by dark borderlines. The length of the bright bands was 2.6 ( 0.2 nm, which agrees well with the length of 1-octadecanoic acid. This agreement indicates that these bands correspond to alkyl chains oriented parallel to the graphite surface and perpendicular to the lamella borders (cf. scheme in Figure 3c).19,28 Similar images were reported previously. The COOH end groups were believed to appear as dark spots.28 Also in this system, we did not observe a bias voltage dependence of the image contrast.28 Most images were similar to that shown in Figure 3a. Only in some cases, both ends of the lamellae appeared slightly brighter than the central portion of the alkyl chains. When gold tips washed with acetone were used, increased contrasts were obtained with one of the 25 tips that gave molecular images (4%, Table 1). Similar images were also (34) Abraham, M. H. Chem. Soc. Rev. 1993, 22, 73-83. (35) Schneider, H.-J. Angew. Chem., Int. Ed. Engl. 1991, 30, 1417-1436.

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Figure 3. (a) STM image of 1-octadecanoic acid (C17H35COOH) physisorbed from a 1-phenyloctane solution onto HOPG, as acquired with an unmodified gold tip: image size 20 nm × 20 nm, total gray scale range 0.08 nm, sample bias voltage -0.7 V (sample negative), and tunneling current 0.7 nA. (b) STM image of 1-octadecanoic acid physisorbed from a 1-phenyloctane solution onto HOPG, as obtained with a 4-mercaptopyridine-modified gold tip: image size 20 nm × 20 nm, total gray scale range 0.2 nm, sample bias voltage +0.4 V (sample positive), and tunneling current 0.7 nA. The height of the bright lines as compared to the rest of the lamellae was ∼0.06 nm. (c) Schematic illustration of the molecular arrangement as observed for the monolayer of 1-octadecanoic acid. Open circles and bars represent carboxyl groups and alkyl chains, respectively.

observed with 3 of 90 tips cleaned in piranha solution (3%; Table 1). Again, contamination of the tip surface may explain these rare contrast enhancements. Figure 3b shows an STM image of a 1-octadecanoic acid monolayer as obtained with tips modified with 4-mercaptopyridine. Each lamella has two borders that appear much brighter than the

Table 1. Fractions of Tips with Which Enhanced Contrasts in Monolayers of 1-Octadecanol or 1-Octadecanoic Acid Were Observed, and the Corresponding Contrastsa 1-octadecanol monolayer tip modification tips cleaned with “piranha solution” tips cleaned with acetone 4-mercaptopyridine (1) SAM 4-aminothiophenol (2) SAM 4-hydroxythiophenol (3) SAM thiophenol (4) SAM 3-mercaptopropionic acid (5) SAM 2-aminoethanethiol hydrochloride (6) SAM

fraction 0b/91c

(0)d

3/27 (11) 7/36 (19) 8/26 (31) 3/31 (10) 0/27 (0) 0/20 (0) 0/24 (0)

contrast ht

1-octadecanoic acid monolayer (nm)a

0.03 ( 0.01 0.08 ( 0.03 0.06 ( 0.03 0.03 ( 0.01

fraction 3/90 (3) 1/25 (4) 6/36 (17) 1/23 (4) 6/28 (21) 0/26 (0) 0/25 (0) 1/22 (5)

contrast ht (nm) 0.02 ( 0.01 0.03 0.08 ( 0.07 0.03 0.03 ( 0.02 0.05

a Difference in height between alkyl region and enhanced region, as calculated for the highest contrasts obtained with tips that gave contrast enhancements (mean value ( standard deviation). b Number (n) of tips with which enhanced contrasts as shown in Figures 2b and 3b were observed. c Number (n0) of tips with which molecular images of the respective monolayers were observed. d Percentage () 100n/n0) of tips with which enhanced contrasts were observed.

alkyl chains (Figure 3b). The enhanced contrasts in these images seem to correspond to the COOH groups.28 Enhanced contrasts as shown in Figure 3b were obtained with 6 of 36 tips modified with 4-mercaptopyridine (17%, Table 1) and, thus, were significantly more frequent than in images observed with unmodified tips (vide supra). Contrast enhancements as shown in Figure 3b were usually larger with the tips modified with 4-mercaptopyridine than with unmodified tips (Table 1). Images similar to that in Figure 3b were observed with 1 of 23 tips (4%, Table 1) modified with 4-aminothiophenol, with 6 of 28 tips (21%, Table 1) modified with 4-hydroxythiophenol, and with one of 22 tips (5%, Table 1) modified with 2-aminoethanethiol hydrochloride. Again, no enhanced contrasts were observed with tips modified with thiophenol or 4-mercaptopropionic acid (Table 1). It seems that hydrogen bonding to 4-mercaptopyridine- or 4-hydroxythiophenolmodified tips makes recognition of the carboxyl groups possible. The hydrogen bond acidity of carboxylic acids is much larger than that of alkanols.34 This probably explains the enhanced contrasts in images of 1-octadecanoic acid monolayers obtained with 4-hydroxythiophenol-modified tips, while use of the same tips only gave small contrast enhancements in images of 1-octadecanol monolayers. However, in contrast to the case of the 1-octadecanol monolayers, the hydrogen bond basicity of the functional groups alone cannot explain these results fully. The hydrogen bond basicity decreasing in the order of pyridine > aniline > phenol cannot explain why enhanced images were observed more frequently for 4-hydroxythiophenol than for 4-aminothiophenol. The reason for the poor contrast enhancements when tips modified with 4-aminothiophenol are used is at present unclear. (36) Frisbie, D. R.; Rozsnyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071-2074. (37) Ito, T.; Namba, M.; Bu ¨ hlmann, P.; Umezawa, Y. Langmuir 1997, 13, 43234332 and references cited therein.

In Figure 3b, the width of the enhanced region is ∼1 nm, which is wider than that of the dark region corresponding to the COOH groups in Figure 3a (∼0.7 nm) and also wider than the width of the enhanced areas in monolayers of 1-octadecanol (∼0.6 nm). The reason for this large width may be related to the large strength of hydrogen bonds to the carboxyl group, resulting in electron tunneling across longer distances. CONCLUSIONS It was possible to recognize OH and COOH residues in monolayers of primarily substituted hydrocarbons by using STM tips modified with 4-mercaptopyridine, 4-aminothiophenol, or 4-hydroxythiophenol. The enhanced contrasts for the OH and COOH groups indicate hydrogen bond interactions between functional groups on tip and sample. To our knowledge, this is the first report on the influence of hydrogen bond interactions between tip and sample on STM imaging. Also chemical force microscopy takes advantage of chemically modified tips.36,37 The detection principles of STM and AFM with modified tips are however based on very different principles. The results presented here and the success of chemically modified electrodes in electrochemistry suggest that modification of STM tips may become a general method for the discrimination of chemical species and functional groups in STM images. ACKNOWLEDGMENT This work was supported by Grants from the Ministry of Education, Science and Culture, Japan, to Y.U. and a JSPS Fellowship to T.I. Received for review May 13, 1997. Accepted October 28, 1997.X AC970498W X

Abstract published in Advance ACS Abstracts, December 15, 1997.

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