Scanning Tunneling Microscopy with Chemically Modified Tips

Feb 2, 2001 - STM gold tips chemically modified with 4-mercaptopyridine (4MP) were found capable of discriminating zinc(II) 5,15-bis(4-octadecyloxyphe...
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Anal. Chem. 2001, 73, 878-883

Scanning Tunneling Microscopy with Chemically Modified Tips: Discrimination of Porphyrin Centers Based on Metal Coordination and Hydrogen Bond Interactions Takahito Ohshiro,† Takashi Ito,‡ Philippe Bu 1 hlmann,† and Yoshio Umezawa*,†

Department of Chemistry, School of Science, The University of Tokyo, Bunkyo-Ku, Tokyo, 133-0033, Japan, and Department of Chemistry, Science University of Tokyo, Shinjuku-Ku, Tokyo, 162-8601, Japan

STM gold tips chemically modified with 4-mercaptopyridine (4MP) were found capable of discriminating zinc(II) 5,15-bis(4-octadecyloxyphenyl)porphyrin (PorZn) from its metal-free porphyrin (Por-2H) and nickel(II) complexes (Por-Ni) in the mixed monolayers of these compounds, spontaneously formed at the solution/graphite interface. The porphyrin centers in STM images observed with 4MP-modified tips exhibited bright spots, while those measured with unmodified tips exhibited the porphyrin centers as dark depressions. The centers of Por-Zn were brighter than those of Por-2H and Por-Ni, thereby allowing the discrimination of Por-Zn from Por-2H or Por-Ni in mixed monolayers. The changes in the contrasts of porphyrin centers of Por-2H and Por-Zn/ Por-Ni were explained by facilitated electron tunneling due to hydrogen bond and metal coordination interactions, respectively, between porphyrin centers and the pyridyl group of 4MP on the tip. Scanning tunneling microscopy (STM) is one of the most powerful methods for surface analysis in that it often exhibits atomically resolved images of sample surfaces. In addition to the topography, STM images also reflect the electronic structure of surfaces by influencing contrasts of STM images.1,2 For example, in some cases, certain functional groups of sample molecules can be observed as bright spots.3,4 Cyr et al. investigated monolayers of several primarily substituted alkanes CH3(CH2)nX (X ) CH3, OH, COOH, NH2, SH, Cl, Br, I; n ) 16-28) on graphite and showed that the NH2, SH, Br, and I end groups are observed as brighter spots than the CH3, OH, COOH, and Cl end groups.3,4 Interestingly, accidental adsorbates at the apex of a tip occasionally allow the selective imaging of surface species.5-9 The chemical * Corresponding author: (phone) +81-3-5841-4351; (fax) +81-3-5841-8349; (e-mail) [email protected]. † The University of Tokyo. ‡ Science University of Tokyo. (1) Binnig, G.; Rohrer, H.; Gerber, C.; Weibel, E. Phys. Rev. Lett. 1982, 49, 57. (2) Wiesendanger, R. Scanning Tunneling Microscopy and Spectroscopy: Methods and Applications; Cambridge University Press: New York, 1994. (3) Cyr, D. M.; Venkataraman, B.; Flynn, G. W. Chem. Mater. 1996, 8, 1600. (4) Giancarlo, L. C.; Flynn, G. W. Acc. Chem. Res. 2000, 33, 491. (5) Ruan, L.; Besenbacher, F.; Stensgaard, I.; Laegsgaard, E. Phys. Rev. Lett. 1993, 70, 4079.

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interactions between the tips and the samples may affect tunneling currents because of the perturbation of the density of states at the tip and sample, which allows the discrimination of different surface species. Recently, we showed that the chemical modification of STM tips can be used for the recognition of functional groups that cannot be recognized with unmodified metal tips. Hydroxyl and carboxyl groups were able to be observed as bright spots with gold tips modified with self-assembled monolayers (SAMs) of 4-mercaptopyridine (4MP), 4-aminothiophenol, and 4-hydroxythiophenol10 or with polypyrrole,11 which can all form hydrogen bonds with OH and COOH groups. Ether groups (ROR) were observed as bright spots with gold tips modified with SAMs of 4-mercaptobenzoic acid,12 which can form hydrogen bonds with ethers. In contrast, OH, COOH, and ROR groups appeared darker than the alkyl chains when STM images are measured with unmodified metal tips. These results were explained by increases in the probability of electron tunneling at functional groups due to hydrogen bond interactions between the samples and the tips.10-12 In the present study, we extend this idea toward metal coordination interactions. Monolayers of metal-free porphyrin and metalloporphyrins formed at solution/graphite interfaces were used as samples for STM measurements with unmodified and chemically modified tips. STM images of several kinds of metalloporphyrins were examined with unmodified metal tips,13-15 and the centers of cobalt(II) porphyrins and phthalocyanines appeared bright whereas those of copper(II), zinc(II), nickel(II), and free base porphyrins were all observed as dark depressions. It was, however, difficult to discriminate between the latter type of complexes with unmodified metal tips.15 This paper describes the (6) Schmid, M.; Stadler, H.; Varga, P. Phys. Rev. Lett. 1993, 70, 1441. (7) Biedermann, A.; Schmid, M.; Varga, P. Fresenius J. Anal. Chem. 1994, 349, 201. (8) Bartels, L.; Meyer, G.; Rieder, K.-H. Appl. Phys. Lett. 1997, 71, 213. (9) Bartels, L.; Meyer, G.; Rieder, K.-H. Surf. Sci. 1999, 432, L621. (10) Ito, T.; Bu ¨ hlmann, P.; Umezawa, Y. Anal. Chem. 1998, 70, 255. (11) Ito, T.; Bu ¨ hlmann, P.; Umezawa, Y. Anal. Chem. 1999, 71, 1699. (12) Nishino, T.; Bu ¨ hlmann, P.; Ito, T.; Umezawa, Y., submitted for publication. (13) Lu, X.; Hipps, K. W.; Wang, X. D.; Mazur, U. J. Am. Chem. Soc. 1996, 118, 7197. (14) Hipps, K. W.; Lu, X.; Wang, X. D.; Mazur, U. J. Phys. Chem. 1996, 100, 11207. (15) Ohshiro, T.; Ito, T.; Bu ¨ hlmann, P.; Umezawa, Y., in preparation. 10.1021/ac001056e CCC: $20.00

© 2001 American Chemical Society Published on Web 02/02/2001

Figure 1. Chemical structures of the Por-2H, Por-Zn, Por-Ni, 4-mercaptopyridine (4MP), and thiophenol (TP).

use of tips modified with SAMs of 4MP, which is able to form not only hydrogen bonds to metal-free porphyrin but also coordination bonds to the metal center of metalloporphyrins. Images of pure and mixed monolayers of the porphyrins measured with 4MPmodified tips were compared to those measured with unmodified tips and the effect of the tip modification on the STM images was discussed. Imaging with chemically modified tips allowed the discrimination of zinc(II) 5,15-bis(4-octadecyloxyphenyl)porphyrin (Por-Zn) from its metal-free porphyrin (Por-2H) or nickel(II) complexes (Por-Ni) in their mixed monolayers on the basis of contrast changes at porphyrin centers, which are most probably due to electron tunneling mediated by hydrogen bond or metal coordination interactions. EXPERIMENTAL SECTION As sample molecules, Por-2H and zinc(II) and nickel(II) 5,15bis(4-octadecyloxyphenyl)porphyrin (Por-Zn and Por-Ni, respectively) were synthesized (chemical structures: Figure 1).15 1,2Dichlorobenzene (Tokyo Kasei Kogyo, Tokyo, Japan), the solvent for the porphyrin solutions, was used without further purification. STM tips were prepared from gold wire (0.25-mm diameter; Nilaco, Tokyo, Japan) by electrochemical etching in 3 M NaCl at ac 10 V or from Pt-Ir wire (0.25 mm, Digital Instruments, Santa Barbara, CA) by mechanical cutting. The tips were then washed 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 tips, the tips were cleaned in piranha solution and immersed for more than 12 h in 1-10 mM ethanolic solution (HPLC-grade ethanol, Wako Pure Chemical, Osaka, Japan) of 4MP (Aldrich Chemical, Milwaukee, WI) or thiophenol (TP; Wako Pure Chemical) (chemical structure, Figure 1).10 The tips were then rinsed with ethanol and dried in a stream of argon or nitrogen. STM measurements were carried out on a Nanoscope E (Digital Instruments). Monolayers of the porphyrins were prepared by depositing a drop (10 µL) of a 1,2-dichlorobenzene solution containing one or two of the porphyrins (concentration, 0.5-1.0 mM) onto a freshly cleaved basal plane of highly ordered pyrolytic graphite (HOPG; Digital Instruments). Within minutes after the deposition of the porphyrin solution, STM measurements

Figure 2. (a) STM image of Por-2H monolayer obtained with an unmodified tip (constant-current mode). Scan area, 25 × 25 nm2; bias voltage, 1144 mV (sample negative); tunneling current, 927 pA. (b) Cross-sectional profile along the dashed line in (a).

were performed at the solution/graphite interface by immersion of the scanning tip into the porphyrin solution. All images were obtained under ambient conditions at temperatures between 18 and 25 °C in a constant-current mode. With each tip, STM measurements were performed for ∼50 min. For STM imagings with unmodified and chemically modified tips, the bias voltage was in the range of -0.3 to -2.0 V and the tunneling current was 0.3-1.1 nA. The resulting tunneling gap resistance was 0.3-5 MΩ, which is similar to previously reported observations of organic molecules at the interface of HOPG and 1-phenyloctane.3,4 A dependence of image contrasts on the bias voltage and tunneling current in this voltage range was not observed. When STM measurements were attempted with bias voltages beyond this range, the resulting electrical noise was large and clear STM images of the porphyrins could not be obtained. RESULTS AND DISCUSSION First, we observed the monolayers of Por-2H, Por-Zn, and PorNi with unmodified tips. Figures 2a and 3a show typical STM images of monolayers of Por-2H and Por-Zn at the 1,2-dichlorobenzene solutions/HOPG interface as observed with unmodified tips. The images are periodic and clearly show the porphyrin rings. The diameter of the rings is 1.1 ( 0.2 nm, which is consistent with the diameter of the porphyrin ring as estimated from a CPK model. This indicates that Por-2H and Por-Zn adsorb horizontally on HOPG. The centers of Por-2H and Por-Zn appear as dark depressions (Figures 2b and 3b). For Por-Ni, quite similar images were observed with unmodified tips (data not shown). To quantify the depth of the dark depressions, we defined the height of the Analytical Chemistry, Vol. 73, No. 5, March 1, 2001

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Figure 3. (a) STM image of Por-Zn monolayer obtained with an unmodified tip (constant-current mode). Scan area, 25 × 25 nm2; bias voltage, 1238 mV (sample negative); tunneling current of 292 pA. (b) Cross-sectional profile along the dashed line in (a).

Figure 4. Height of the porphyrin center relative to that of the porphyrin ring (hcenter/hring) with unmodified (a-c) and 4MP-modified tips (d-j) for pure Por-2H, Por-Zn, and Por-Ni monolayers (a-f) and mixed monolayers Por-Zn/Por-2H and Por-Zn/Por-Ni (g-j). Each value of hcenter/hring was the "mean value ( standard deviation" calculated from all the values obtained from cross sections of porphyrin rings in three to four images.

porphyrin center, hcenter, as the vertical distance between the alkyl chain and the center of the porphyrin and the height of the porphyrin ring, hring, as the vertical distance between the alkyl chain and the porphyrin ring (see scheme in Figure 4). The ratios of hcenter and hring (hcenter/hring; “mean value ( standard deviation” calculated from all the values obtained for each set of the images) were 0.4 ( 0.1, 0.5 ( 0.1, and 0.4 ( 0.1 for Por-2H, Por-Zn, and Por-Ni (Figure 4a-c), respectively, suggesting that the molecular images of these porphyrins are very similar. 880 Analytical Chemistry, Vol. 73, No. 5, March 1, 2001

Figure 5. (a) STM image of Por-2H monolayer obtained with a 4MPmodified tip (constant-current mode). Scan area, 25 × 25 nm2; bias voltage, 908 mV (sample negative); tunneling current, 928 pA. (b) Cross-sectional profile along the dashed line in (a).

In contrast, gold tips modified with 4MP gave very different images. Figure 5a shows a STM image of a Por-2H monolayer as observed with a 4MP-modified tip. The size and shape of the porphyrin rings are very similar to those observed with unmodified tips (Figure 2a), but the central parts of the porphyrins appeared as bright spots (Figure 5b). STM images like the one shown in Figure 5a were observed with 6 of 17 modified tips, while the images obtained with another 11 modified tips were similar to the image shown in Figure 2a. The bright spots for the Por-2H centers were most probably induced by hydrogen bond interactions between the pyridyl group of 4-mercaptopyridine on the STM tip and the two nitrogen-bound hydrogens of Por-2H. The chemical interaction between the modified tip and the sample facilitates the tunneling current with chemical selectivity, resulting in the contrast enhancement at the porphyrin center. This result is consistent with the previously reported effect of STM tip modification with hydrogen bond acceptors.10,11 On the other hand, the observation of dark spots for the porphyrin centers with the remaining tips indicates that, despite the attempt of chemical modification, a 4MP molecule does not always appear to be present at the tip apex. This explanation is supported by the following observations: First, during repeated scanning of the same sample area, the appearance of the porphyrin centers sometimes suddenly changed from bright to dark and a few scans later to bright again. The sudden disappearance and appearance of the contrast enhancement at the porphyrin centers probably corresponds to the disappearance of the 4MP molecule from the tip apex and the move of a new 4MP molecule to the tip apex, respectively. Second, when a large bias voltage (>4.0 V) was applied during measurements with 4MP-modified tips, bright spots

Figure 6. (a) STM image of Por-Zn monolayer obtained with a 4MPmodified tip (constant-current mode). Scan area, 25 × 25 nm2; bias voltage, 1182 mV (sample negative); tunneling current, 509.8 pA. (b) Cross-sectional profile along the dashed line in (a).

immediately changed into dark depressions, and subsequently, only images similar to those obtained with bare tips were observed. Such a large bias voltage probably removes the 4MP monolayer from the gold tip, resulting in the disappearance of the contrast enhancement. Analogous observations indicating the absence of modifying molecules from the apex of chemically modified tips were previously also made with other SAM-modified tips10,12 Figure 6a shows a typical STM image of a Por-Zn monolayer as observed with a 4MP-modified tip. The size and shape of the porphyrin rings are very similar to those in Figure 3a (unmodified tip), but the centers of the porphyrins often appeared as bright spots (Figure 6b). Similarly, the centers of Por-Ni often appeared as bright spots when 4MP-modified tips were used (Figure 7a), whereas the centers of Por-Ni appeared as dark depressions very similar in images to the centers of Por-2H (Figure 2) measured with unmodified tips. STM images as shown in Figures 6a and 7a were observed with 8 of the 16 modified tips and with 6 of 25 modified tips, respectively. Again, the absence of 4MP on the tip apex explains the observation of dark porphyrin centers (vide supra). Under continuous scanning, no change in the monolayer structure was observed with the unmodified and chemically modified tips, which suggests that the pickup of the porphyrin from the surface did not occur under the present experimental conditions. Because selective contrasts at porphyrin centers in images with 4MP-modified tips were observed at the metal centers of Por-Zn and Por-Ni, they were most probably induced by the metal coordination interactions between the pyridyl group of 4MP on

Figure 7. (a) STM image of Por-Ni monolayer obtained with a 4MPmodified tip (constant-current mode). Scan area, 25 × 25 nm2; bias voltage, 1562 mV (sample negative); tunneling current, 203 pA. (b) Cross-sectional profile along the dashed line in (a).

the STM tip and central metals of the porphyrins. The metal coordination interactions probably modify the overlap of the local density of states of the sample and tip, resulting in the increase in tunneling current. Indeed, the stabilities of axial complexes of zinc(II) porphyrins are fairly high (e.g., K ) 103-105 M-1 for pyridine or piperidine in benzene or toluene as solvents16), and nickel(II) porphyrins also bind one or even two axial ligands (e.g., K1 ) 0.4 M-1, K2 ) 2.5 M-1 for axial complexation between nickel(II) tetraphenyloxyporphyrin and pyrrolidine in dichloromethane17). The formation and dissociation of axial complexes of zinc(II) porphyrins (e.g., on-rates of more than 108 M s-1 and offrates of ∼105 M s-1 for pyridines17) are so fast processes that metal coordination interactions can occur and fade as a tip moves over a metal center of the sample (millisecond order). The involvement of metal coordination interactions in the contrast changes is probably supported by the correlation between the stability of the axial complexes for zinc(II) and nickel(II) porphyrins and the extent of the contrast change, as discussed in the following. To compare the contrast changes for two kinds of porphyrins under identical conditions, mixed monolayers containing two types of porphyrins (Por-Zn/Por-2H and Por-Zn /Por-Ni) were investigated. Figure 8a shows a typical STM image of a mixed monolayer of Por-Zn and Por-2H as observed with a 4MP-modified tip. The monolayer was obtained by spontaneous adsorption from a solution containing Por-Zn and Por-2H with a molar ratio of 1.00: (16) Buchler, J. W. In Porphyrins and Metalloporphyrins; Smith, K. M., Ed.; Elsevier: Amsterdam, 1975; p 234. (17) Kadish, K. M.; Smith, K. M.; Guilard, R. The Porphyrin Handbook: Inorganic, Organometallic and Coordination Chemistry; Academic Press: New York, 2000; Chapter 15.

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Figure 8. (a) STM image of mixed monolayer of Por-Zn and Por2H (molar ratio of Por-Zn and Por-2H in the sample solution; PorZn:Por-2H ) 1.00:0.50) obtained with a 4MP-modified tip (constantcurrent mode). Scan area, 25 × 25 nm2; bias voltage, 809 mV (sample negative); tunneling current, 334 pA. (b) Cross-sectional profile along the dashed line in (a). (c) STM image of mixed monolayer of Por-Zn and Por-2H (molar ratio of Por-Zn and Por-2H in the sample solution; Por-Zn:Por-2H ) 1.0:2.0) obtained with a 4MP-modified tip (constantcurrent mode). Scan area, 25 × 25 nm2; bias voltage, 1114 mV (sample negative); tunneling current, 524 pA; range of vertical height in the image, 0.5 nm.

0.50 onto a freshly cleaved basal surface of HOPG. The central parts of all porphyrins were observed as bright spots, just as in monolayers of Por-Zn or Por-2H alone. However, “very” bright spots and “moderately” bright spots were observed in the same STM images (Figure 8b). In the scan area of 25 × 25 nm2 in Figure 8a, 77 very bright spots and 36 moderately bright spots can be seen, giving a ratio of 1.00:0.47 for the number of very bright spots and moderately bright spots, which is very close to the molar ratio of the Por-Zn to Por-2H in the sample solution (Por-Zn:Por-2H ) 1.00:0.50). When the concentration of Por-Zn in the Por-Zn/Por2H solution was decreased, the number of very bright spots also decreased. For a monolayer formed from a solution containing Por-Zn and Por-2H with a molar ratio of 1.0:2.0, 46 very bright spots and 86 moderately bright spots appear in a scan area of 25 × 25 nm2 (Figure 8c), giving a ratio of 1.0:1.9 for very bright spots and moderately bright spots. This indicates that in the above mixed monolayer the very bright spots are the centers of Por-Zn 882 Analytical Chemistry, Vol. 73, No. 5, March 1, 2001

and the moderately” bright spots are centers of Por-2H. Images with such bright spots were observed with 20 of 30 modified tips. In contrast, when STM measurements were performed with unmodified tips and tips modified with TP, all the porphyrin centers in the same mixed monolayer appear as dark depressions (hcenter/hring ) 0.4 ( 0.1, 0.5 ( 0.1 with unmodified and TP-modified tips, respectively), and as a result, the porphyrins were unable to be discriminated each other. The absence of contrast changes in STM images of the porphyrin monolayers observed with TPmodified tips is explained by the lack of a functional group that can form hydrogen bond and metal coordination interactions with the sample porphyrins. The brightness of very bright spots and moderately bright spots for the mixed Por-Zn/Por-2H monolayers can be described more quantitatively by using the ratio of the center height and the ring height of a porphyrin (hcenter/hring; vide supra and see scheme in Figure 4). The values of 2.5 ( 0.6 for the very bright spots (Figure 4h) and 1.4 ( 0.2 for the moderately bright spots (Figure 4g) are within an error indistinguishable from the corresponding values of 2.0 ( 0.2 (Figure 4e) for pure Por-Zn monolayers and 1.5 ( 0.2 for pure Por-2H monolayers (Figure 4d). This suggests that the metal coordination interaction between the central zinc of Por-Zn and the pyridyl group of 4MP on the tip increases the tunneling current more than the hydrogen bonds between the N-H hydrogen of Por-2H and the pyridyl group of 4MP on the tip and that each porphyrin can possibly be identified on the basis of their hcenter/hring. Figure 9a shows a typical STM image of a mixed monolayer formed from a solution containing Por-Zn and Por-Ni with a molar ratio of 1.00:0.33, which was observed with a 4MP-modified tip. Similarly, as for the Por-Zn and Por-2H mixed monolayers (Figure 8a), the central parts of the porphyrins appear as very bright spots and moderately bright spots. Figure 9a exhibits 76 very bright spots and 31 moderately bright spots in a scan area of 25 × 25 nm2, giving a ratio of 1.00:0.41 for the number of very bright spots to that of moderately bright spots, which is close to the molar ratio of the Por-Zn to Por-Ni in the sample solution (Por-Zn:PorNi ) 1.00:0.33). With increasing concentration of Por-Zn in the Por-Zn/Por-Ni solution, the number of very bright spots increased: For a monolayer formed from a solution containing PorZn and Por-Ni with a molar ratio of 1.00:0.20, 114 very bright spots and 25 moderately bright spots appear in a scan area of 25 × 25 nm2 (Figure 9c), giving a ratio of 1.00:0.22 for very bright spots and moderately bright spots. Upon a decrease in the concentration of Por-Zn in the Por-Zn/Por-Ni solution, the number of very bright spots decreased: For a Por-Zn:Por-Ni molar ratio of 1.0:5.0 in the solution, 19 very bright spots and 81 moderately bright spots are observed in a scan area of 25 × 25 nm2 (Figure 9d), giving a ratio of 1.0:4.3 for very bright spots and moderately bright spots. This indicates that very bright spots are of the Por-Zn and moderately bright spots are the Por-Ni centers. Images with center spots of different brightness were observed with 14 of 19 modified tips. Again, when STM measurements were performed with unmodified tips and TP-modified tips, the all porphyrin centers in the same mixed monolayer appeared as dark depressions (hcenter/hring ) 0.6 ( 0.2 and 0.6 ( 0.2 with unmodified and TP-modified tips, respectively) and the two types of porphyrins were unable to be distinguished..

hcenter/hring for Por-Zn than for Por-Ni is explained by the stronger metal coordination interaction for the former with the pyridyl group of 4-mercaptopyridine on the tip (vide supra). A large overlap of the electronic wave functions of the molecules on tip and sample results in a strong metal coordination interaction on one hand and a large enhancement of the tunneling current on the other. It is well known that the appearance of STM images of molecular adsorbates such as metal phthalocyanines18 and benzene19,20 on metal surfaces mainly reflects their lowest unoccupied or highest occupied molecular orbitals. In addition, Sautet et al. theoretically explained the effect of atomic adsorbates at the tip apex on the STM images on the basis of an analytical model that describes the influence of an adsorbate by an effective coupling between tip and surface and by a perturbation of the surface electronic levels.21,22 However, these theoretical results are not sufficient to explain the results of the present study, because in this case the electronic states resulting from the chemical interaction between sample molecules and the pyridyl group of 4-MP on the tip have to be taken into account. Experimentally, STS measurements2,23,24 may provide information on the electronic states originating from these chemical interactions. To quantitatively understand the selective contrast change observed in this study, more theoretical and experimental studies will be necessary. CONCLUSION

Figure 9. (a) STM image of mixed monolayer of Por-Zn and PorNi (molar ratio of Por-Zn and Por-Ni in the sample solution; Por-Zn: Por-Ni ) 1.00:0.33) with a 4MP-modified tip (constant-current mode). Scan area, 25 × 25 nm2; bias voltage, 1149 mV (sample negative); tunneling current, 322 pA. (b) Cross-sectional profile along the dashed line in (a). (c) STM image of mixed monolayer of Por-Zn and Por-Ni (molar ratio of Por-Zn and Por-Ni in the sample solution; Por-Zn:PorNi ) 1.00:0.20). Scan area, 25 × 25 nm2; bias voltage, 1327 mV (sample negative); tunneling current, 440 pA; range of vertical height in the image, 0.5 nm. (d) STM image of mixed monolayer of Por-Zn and Por-Ni (molar ratio of Por-Zn and Por-Ni in the sample solution; Por-Zn:Por-Ni ) 1.0:5.0). Scan area, 25 × 25 nm2; bias voltage, 1148 mV (sample negative); tunneling current, 524 pA, range of vertical height in the image, 0.5 nm.

The difference between the very and moderately bright spots in the Por-Zn/Por-Ni mixed monolayers can again be quantified by hcenter/hring values. The values were 2.3 ( 0.4 for the very bright spots (Figure 4j) and 1.5 ( 0.2 for the moderately bright spots (Figure 4i). As expected, these values fall in a range similar to that for pure Por-Zn monolayers (2.0 ( 0.2) and pure Por-Ni monolayers (1.5 ( 0.2), respectively (Figure 4e,f). The larger (18) Sautet, P.; Joachim, C. Surf. Sci. 1992, 271, 387. (19) Sautet, P.; Joachim, C. Ultramicroscopy 1992, 42-44, 115. (20) Fisher, A. J.; Blochl, P. E. Phys. Rev. 1993, 70, 3263. (21) Sautet, P.; Dunphy, J. C.; Ogletree, D. F.; Joachim, C.; Salmeron, M. Surf. Sci. 1994, 315, 127. (22) Sautet, P. Chem. Rev. 1997, 97, 1097. (23) Stroscio, J. A.; Feenstra, R. M. In Scanning Tunneling Microscopy; Stroscio, J. A., Kaiser, W. J., Eds.; Academic Press: San Diego, 1993; p 95. (24) Yu, E. T. Chem. Rev. 1997, 97, 1017.

Whereas the center of Por-2H and the metal centers of PorZn and Por-Ni were observed as dark depressions with unmodified and TP-modified tips, the centers of all three porphyrins were observed as bright spots when 4MP-modified tips were used. Hydrogen bond interactions for Por-2H and metal coordination interactions for Por-Zn and Por-Ni between 4MP on the tips and sample molecules seem to enhance the probability of electron tunneling, resulting in these contrast changes. In addition, the contrast changes in STM images obtained with 4MP-modified tips allow the discrimination of Por-Zn from Por-2H and Por-Ni. This is the first demonstration of STM images with contrast enhancements based on metal coordination interactions between tip and sample molecules. These results indicate that the modification of STM tips with metal-coordinating ligands may be another approach to discriminate chemical species and functional groups in STM images and, together with the use of hydrogen bond interactions, will become a general method for “chemically sensitive” STM. ACKNOWLEDGMENT The authors thank Satoshi Ito, Department of Chemistry, Faculty of Science, Ehime University, for providing 5,15-bis(4octadecyloxyphenyl)porphyrin. This work was supported by grants from the Ministry of Education, Science and Culture, Japan.

Received for review December 18, 2000.

September

6,

2000.

Accepted

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