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Chemical Identification of Carboxylate Surfactants with One-Fluorine-Atom Sensitivity Achieved by Noncontact Atomic Force Microscopy Akira Sasahara,* Hiroshi Uetsuka, and Hiroshi Onishi Surface Chemistry Laboratory, Kanagawa Academy of Science and Technology, KSP East 404, 3-2-1 Sakado, Takatsu-ku, Kawasaki, Kanagawa 213-0012, Japan Received May 10, 2003. In Final Form: June 16, 2003 Monolayers composed of difluoroacetate (CF2HCOO-) and trifluoroacetate (CF3COO-) were prepared on an atomically flat surface of titanium oxide. The chemical identity of individual adsorbed molecules was determined in the mixed monolayers by noncontact atomic force microscopy. The permanent dipole moment of the fluorine-substituted terminal groups perturbed the microscope topography via the electrostatic coupling with the probe tip.
Introduction Noncontact atomic force microscopy (NC-AFM)1 is a new and promising method to analyze individual molecules adsorbed on a solid surface. It employs the force pulling a probe tip into the surface to regulate tip-surface distance. To detect the tip-surface force less than 1 nN, a cantilever with a tip at one end is oscillated with its resonant frequency, f0. The force pulling the tip perturbs the oscillation, and the frequency shifts to the lowfrequency side. The topography of the surface is determined by keeping the frequency shift (∆f) constant. By doing this, atom- or molecule-scale resolution has been achieved on Si,2,3 CaF2,4,5 Al2O3,6,7 TiO2,8,9 and organic molecules.10-13 We systematically examined the constant frequencyshift topography of alkyl-terminated carboxylates (HCOO-, CH3COO-, (CH3)3CCOO-, and HCtCCOO-) chemisorbed on an atomically flat surface of TiO2.14,15 The observed topography exhibited a good correlation with the physical topography of the terminal groups. The van der Waals force between the molecule and the tip was claimed to have caused the molecule-dependent contrast. In addition, the permanent dipole moment of an adsorbed molecule perturbed the microscope topography via electrostatic * To whom correspondence should be addressed. Phone: +8144-819-2048. Fax: +81-44-819-2095. E-mail:
[email protected]. (1) Morita, S., Wiesendanger, R., Meyer E., Eds. Noncontact Atomic Force Microscopy; Springer-Verlag: Berlin, 2002. (2) Giessibl, F. J. Science 1995, 267, 68. (3) Kitamura, S.; Iwatsuki, M. Jpn. J. Appl. Phys. 1995, 34, L145. (4) Reichling, M.; Barth, C. Phys. Rev. Lett. 1999, 83, 768. (5) Foster, A. S.; Barth, C.; Shluger, A. L.; Reichling, M. Phys. Rev. Lett. 2001, 86, 2373. (6) Sasahara, A.; Uetsuka, H.; Onishi, H. Jpn. J. Appl. Phys. 2000, 39, 3773. (7) Barth, C.; Reichling, M. Nature 2001, 414, 54. (8) Fukui, K.; Onishi, H.; Iwasawa, Y. Phys. Rev. Lett. 1997, 79, 4202. (9) Tanner, R. E.; Sasahara, A.; Liang, Y.; Altman, E. I.; Onishi, H. J. Phys. Chem. B 2002, 106, 8211. (10) Fukui, K.; Onishi, H.; Iwasawa, Y. Chem. Phys. Lett. 1997, 280, 296. (11) Gotsmann, B.; Schmidt, C.; Seidel, C.; Fuchs, H. Eur. Phys. J. B 1998, 4, 267. (12) Kobayashi, K.; Yamada, H.; Horiuchi, T.; Matsushige, K. Appl. Surf. Sci. 1999, 140, 281. (13) Uchihashi, T.; Okada, T.; Sugawara, Y.; Yokoyama, K.; Morita, S. Phys. Rev. B 1999, 60, 8309. (14) Sasahara, A.; Uetsuka, H.; Onishi, H. Surf. Sci. 2001, 481, L437. (15) Sasahara, A.; Uetsuka, H.; Onishi, H. Appl. Surf. Sci. 2002, 188, 265.
coupling with the induced moment on the tip, as was demonstrated on trifluoroacetate (CF3COO-) adsorbed on the TiO2 surface.16 In the present study, the constant frequency-shift topographies of difluoroacetate (CF2HCOO-) and trifluoroacetate were compared to demonstrate the sensitivity of NC-AFM in detecting the permanent dipole moment caused by one C-F bond in an adsorbed molecule. The (110) surface of rutile TiO217,18 and carboxylates chemisorbed on this surface19 has been extensively studied as an atomically flat surface of metal oxide covered by organic adsorbates. Figure 1a shows the structure of the TiO2(110) surface. The row of Ti4+ cations and the row of O2- anions alternately lie parallel to the [001] direction. A carboxylic acid (RCOOH) dissociates on the surface to yield a carboxylate anion (RCOO-) and proton. The carboxylate is chemisorbed on a pair of Ti ions. A longrange ordered monolayer of the carboxylates is formed at room temperature when the Ti ions are fully occupied. An electron-stimulated desorption study found the C-C bond of adsorbed acetate perpendicular to the surface.20 We assume the same geometry of the difluoroacetate and trifluoroacetate as illustrated in Figure 1a. The atom coordinate in Figure 1b follows the Ti-O distance and O-C-O angle of formate (HCOO-).21 Experimental Procedures All experiments were carried out in a commercial ultrahigh vacuum microscope (JSPM4500A, JEOL). The base pressure of the system was kept below 3 × 10-8 Pa. The sample preparation chamber was equipped with an Ar+ sputtering gun (IG35, OCI), low-energy electron diffraction optics (BDL600, OCI), and X-ray photoelectron spectrometer (TM50045, JEOL). Constant frequency-shift topography was obtained at room temperature with a conductive silicon cantilever (NSCS11, NT-MDT). The canti(16) Sasahara, A.; Uetsuka, H.; Onishi, H. Phys. Rev. B 2001, 64, 121406(R). (17) Henrich, V. E.; Cox, P. A. The Surface Science of Metal Oxides; Cambridge University Press: New York, 1994. (18) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (19) Onishi, H. In Chemistry of Nanomolecular Systems; Nakamaura, T., Matsumoto, T., Tada, H., Sugiura, K., Eds.; Springer-Verlag: Berlin, 2003; p 75. (20) Guo, Q.; Cocks, I.; Williams, E. M. J. Chem. Phys. 1997, 106, 2924. (21) Thevuthasan, S.; Herman, G. S.; Kim, Y. J.; Chambers, S. A.; Peden, C. H. F.; Wang, Z.; Ynzunza, R. X.; Tober, E. D.; Morais, J.; Fadley, C. S. Surf. Sci. 1998, 401, 261.
10.1021/la0348012 CCC: $25.00 © 2003 American Chemical Society Published on Web 07/15/2003
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Figure 1. Models of a substrate and the molecules employed. (a) Five trifluoroacetates and a difluoroacetate adsrobed on the TiO2(110)-(1 × 1) surface. The filled small and large spheres represent Ti and O atoms, respectively. Oxygen atoms are shaded according to their depth. Rectangles with a dotted line show unit cells. (b) The atom geometry of the molecules. From left to right: difluoroacetate, trifluoroacetate, and formate. lever was oscillated by a piezo actuator, which was supplied with a square-form voltage with constant amplitude.3 The resonant frequency and force constant of the cantilever were ∼300 kHz and ∼14 N/m. The TiO2(110)-(1 × 1) surface was prepared by repeated Ar+ sputtering and vacuum annealing at 900 K. A TiO2(110) wafer (Shinko-sha, 7 × 1 × 0.3 mm3) was supported on the sample holder with a Si wafer (7 × 1 × 0.3 mm3) as a resistive heater. The temperature of the TiO2 wafer was monitored by an infrared pyrometer (TR630, Minolta). Contamination of the TiO2(110) surface was under the detection limit of X-ray photoelectron spectroscopy. Carboxylic acid gas was dosed to the TiO2 surface in the preparation chamber after purification by several freezepump-thaw cycles. The exposed sample was moved to the microscope chamber after the carboxylic acid gas was evacuated. Microscope images of 256 × 256 pixels are presented without filtering and smoothing. Cross sections were determined on the images filtered by a nine-point median operation.
Results and Discussion Figure 2a is the NC-AFM image of the TiO2(110)-(1 × 1) surface exposed to 3 L (1 L ) 10-6 Torr‚s) of trifluoroacetic acid gas. Trifluoroacetates were observed as spots with a regular corrugation arranged in a (2 × 1) periodicity. The dark area is a vacant site where the molecule was not adsorbed. The sample bias voltage (Vs) externally applied between the surface and the grounded cantilever was optimized to obtain high-contrast images with minimum frequency shifts. The resolution in the NC-AFM image is most improved when the contact potential difference is canceled between the tip and the sample.22,23 We suspect that the optimum Vs reflects the surface potential of the molecule-covered surfaces. The cross section shows that the fluctuation in the height of the molecules was no more than (0.01 nm. A mixed monolayer of the difluoroacetate and trifluoroacetate was prepared by exposing the TiO2(110)-(1 × 1) surface partially covered with trifluoroacetate to difluoroacetic acid gas. Figure 2b is the NC-AFM image of the TiO2 surface exposed to 1.8 L of trifluoroacetic acid gas after several repetitions of the experiments. Some bright spots were observed in addition to the nonbright spots. The numbers of bright spots, nonbright spots, and vacant sites were 15, 117, and 57, respectively. When this surface was exposed to 1.2 L of difluoroacetic acid gas, the number of spots increased, as is shown in Figure 2c. The spots were ordered in a (2 × 1) periodicity. The numbers of (22) Meyer, E.; Howald, L.; Lu¨thi, R.; Haefke, J.; Ru¨etschi, M.; Bonner, T.; Overney, R.; Frommer, J.; Hofer, R.; Gu¨ntherodt, H.-J. J. Vac. Sci. Technol., B 1994, 12, 2060. (23) Howald, L.; Lu¨thi, R.; Meyer, E.; Gu¨ntherodt, H.-J. Phys. Rev. B 1995, 51, 5484.
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bright spots, nonbright spots, and vacant sites were 42, 138, and 15, respectively. The ratio of the bright spots to all the adsorption sites increased from 0.08 to 0.22, while that of the vacant sites decreased from 0.30 to 0.08. The ratio of the nonbright spots slightly increased from 0.62 to 0.70. This indicates that the bright spots filled the vacant sites, and the preadsorbed nonbright spots remained on the surface during the additional exposure to the difluoroacetic acid gas. From these results, we assigned the bright spots and nonbright spots to the difluoroacetates and trifluoroacetates, respectively. The difluoroacetates in Figure 2b can be contamination originating from difluoroacetic acid remaining on the chamber wall. The apparent height difference between the difluoroacetate and trifluoroacetate was determined to be ∼0.03 nm from the cross sections. The distances in both molecules between the Ti atom and the F atom at the top end are equal when the bridge configuration is assumed. It has been revealed on Cu surfaces that the O-C-O angle and the Cu-O distance in the acetate are insensitive to the fluorine substitution.24,25 The CF2H and CF3 groups are thought to rotate around the C-C axis sufficiently faster than our acquisition time (51.2 ms/line). Hence, the molecules should show a uniform height in the image if the tip traced the simple contour determined by the van der Waals radii of the component atoms. However, this was not the case. Therefore, the observed height difference between the difluoroacetate and trifluoroacetate is due to the strength of the interaction force between the molecules and the tip. The tip detected stronger interaction above the difluoroacetate and was driven away from the molecule by the feedback loop. A covalent bond between the molecules and the Si atom at the end of the tip is not probable because the molecules are terminated with H or F atoms bonded with C atoms. We attribute the heterogeneous corrugation of the CF2H- and CF3-terminated molecules to the difference in the tip-molecule interaction induced by an electrostatic field. Here, we consider the component of the permanent dipole moment of the molecules normal to the surface. The OCO part is polarized as Oδ--Cδ+-Oδ- and gives a dipole moment directed toward the vacuum. TiO2 is an ionic crystal. When we allocate a +4e and -2e charge to the Ti and O atoms, a permanent dipole moment was expected from the substrate toward the vacuum on the adsorption site where a Ti4+ cation is exposed to the surface with an O2- anion underneath. Consequently, the dipole moment that originated from the OCO part and the substrate are both oriented toward the vacuum. Because fluorine is the most electronegative element and attracts electron density when bound to another element, the C-F bond is polarized as Cδ+-Fδ-. The dipole moment of the CF3 part can be analogized to be 1.65 D (1 D ) 3.34 × 10-30 C‚m) from the dipole moment of the CF3H molecule.26 When we assume that the dipole moment of the CF2H group is given by the vectorial summation of the dipole moments of the two C-F and C-H bonds, the perpendicular component of the CF2H dipole moment was estimated to be two-thirds that of the CF3 group, 1.1 D. Here, the dipole moment of the C-H group was neglected. The moment of the CF3 and CF2H groups is oriented in (24) Weiss, K.-U.; Dippel, R.; Schindler, K.-M.; Gardner, P.; Fritzsche, V.; Bradshaw, A. M.; Kilcoyne, A. L. D.; Woodruff, D. P. Phys. Rev. Lett. 1992, 69, 3196. (25) Johnston, S. M.; Rousseau, G.; Dhanak, V.; Kadodwala, M. Surf. Sci. 2001, 477, 163. (26) Lide, D. R., Ed. CRC Handbook of Chemistry and Physics, 81st ed.; CRC Press: Boca Raton, FL, 2000.
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Figure 2. NC-AFM images of the carboxylate-covered TiO2 surfaces (10 × 10 nm2). Open and filled circles in the middle panels represent difluoroacetates and trifluoroacetates, respectively. Cross sections determined along the lines in the middle panels are shown below. (a) Neat monolayer of trifluoroacetate. ∆f ) -143 Hz, Vs ) +0.4 V, peak-to-peak amplitude of oscillation of the cantilever (Ap-p) ) 6.5 nm. (b) TiO2 surface exposed to 1.8 L of trifluoroacetic acid gas. ∆f ) -172 Hz, Vs ) 0 V, Ap-p ) 6.5 nm. (c) The surface shown in part b after exposure to 1.2 L of difluoroacetic acid gas. ∆f ) -165 Hz, Vs ) +0.1 V, Ap-p ) 6.6 nm.
Figure 3. NC-AFM images of carboxylate-covered TiO2 surfaces (10 × 10 nm2). Cross sections determined along the lines in the images are shown below. (a) Neat monolayer of formate.14 ∆f ) -132 Hz, Vs ) +0.4 V, Ap-p ) 6.7 nm. (b) Formate monolayer exposed to difluoroacetic acid gas. ∆f ) -125 Hz, Vs ) +0.5 V, Ap-p ) 6.7 nm. (c) Formate monolayer exposed to trifluoroacetic acid gas. ∆f ) -40 Hz, Vs ) +0.4 V, Ap-p ) 6.8 nm.
the opposite direction of that of the OCO group and substrate. Thus, the difluoroacetate and trifluoroacetate give different perturbation to the tip that originated from the dipole moment of one C-F bond. The permanent dipole-induced dipole and permanent dipole-permanent dipole interactions between the molecule and the tip can be responsible for the apparent height difference of the molecules. On the basis of this assumption, the acetate (CH3COO-), which has no C-F bond, is expected to be observed as taller than the difluoroacetate and trifluoroacetate. This was indeed the case. The apparent height difference between the trifluoroacetate and acetate was ∼0.05 nm,16 which was larger than the difference between the difluoroacetate and trifluoroacetate. We further demonstrated that identification of the difluoroacetate and trifluoroacetate is possible in topographical images of mixed monolayers separately pre-
pared. Because adsorbed carboxylates migrate across the TiO2(110) surface at room temperature,27 the formate was used to fix the difluoroacetate and trifluoroacetate. Figure 3a is the NC-AFM image of the neat formate monolayer prepared by exposing the TiO2(110)-(1 × 1) surface to formic acid gas.14 The formates were observed as protrusions with equal height arranged in a (2 × 1) symmetry. When the formate-covered surface was exposed to 0.3 L of difluoroacetic acid gas, brighter spots appeared in the NC-AFM image, as is shown in Figure 3b. Because the number of bright spots increased with the exposure time, we assigned them to the difluoroacetates that replaced the formates. The cross section showed that the difluoroacetate was taller than the formate by ∼0.09 nm. Figure 3c is the NC-AFM image of the mixed monolayer of the formate and trifluoroacetate prepared in a similar manner. (27) Onishi, H.; Iwasawa, Y. Langmuir 1994, 10, 4414.
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The bright spots were assigned to trifluoroacetate, and the image-height difference between the formate and the trifluoroacetate was determined to be ∼0.06 nm. The difference in the image heights of difluoroacetate and trifluoroacetate separately determined on Figure 3b,c was ∼0.03 nm, which was a reproduction of that obtained in the mixed monolayer of the two. The present study shows that the NC-AFM topographical image sensitively reflects the difference in dipolar coupling between an adsorbed molecule and the tip with the sensitivity of one C-F bond. The identification of the molecules was attainable even in the images obtained by the separate experiments. The results should be extended
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to the more-detailed analysis of a single molecule. Distinction of functional groups and their atom geometries in a large admolecule may be achieved, when the sensitivity in the detection of the tip-molecule force is improved. Acknowledgment. This work was supported by Core Research for Evolutional Science and Technology (CREST) from the Japan Science and Technology Corporation (JST). LA0348012
