- Email: [email protected]
Molecule-dependent topography determined by noncontact atomic force microscopy: carboxylates on TiO2(1 1 0)
Applied Surface Science 188 (2002) 257±264
Molecule-dependent topography determined by noncontact atomic force microscopy: carboxylates on TiO2(1 1 0) Hiroshi Onishi*, Akira Sasahara, Hiroshi Uetsuka, Taka-aki Ishibashi Surface Chemistry Laboratory, Kanagawa Academy of Science and Technology, KSP E-404, Takatsu-ku, Kawasaki, Kanagawa 213-0012, Japan Received 2 September 2001; accepted 10 September 2001
Abstract The feasibility of noncontact atomic force microscopy (NC-AFM) in single-molecule analysis is experimentally demonstrated. Carboxylates (RCOO ) with different R's were identi®ed molecule-by-molecule on the TiO2(1 1 0) substrate. The constant frequency-shift topography exhibited a good correlation with the physical topography of the carboxylates with R=H, CH3, C(CH3)3 and C=CH. The molecule-dependent topography was interpreted with the van der Waals force pulling the tip into the adsorbates. The constant frequency-shift topography of a series of ¯uorine-containing acetates (R=CH3, CHF2, and CF3) revealed that the intramolecular electric polarization perturbs the tip±molecule force through electrostatic couplings. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Atomic force microscopy; Surface structure; Morphology; Chemisorption; Carboxylic acid; Titanium oxide; Low index single crystal surfaces; Semiconducting surfaces
1. Introduction Noncontact atomic force microscopy (NC-AFM) employs an attractive force to construct the topography of materials. It offers an opportunity for singlemolecule sensing of materials which are important in chemical and biochemical applications regardless of conductivity. It was, however, not a trivial challenge to visualize individual organic adsorbates when the atom resolution was ®rst achieved on Si [1,2] and InP [3] surfaces in 1995. The force pulling the tip into the surface is less sensitive to the gap width (r) than the tunneling current utilized in scanning tunneling microscopy (STM), especially when chemically stable *
Corresponding author. Tel.: 81-44-819-2094; fax: 81-44-819-2095. E-mail address: [email protected] (H. Onishi).
molecules cover the surface. The attractive potential between two stable molecules with closed molecular orbitals is shallow and exhibits r 6 decay [4]. The high-resolution topography of formate (HCOO ) [5] was ®rst reported as a molecular adsorbate in 1997. Various compounds including a perylene-derivative (PTCDA) [6], C60 [7,8], adenine [9,10], DNA [11], acetate (CH3COO ) [12], alkanethiols [10,13], and a metal porphyrin (Cu-TBPP) [14] have been successfully imaged to date. We have proposed that a series of carboxylates (RCOO ) adsorbed on the TiO2(1 1 0) surface provide molecule-level standards with known physical and chemical properties to calibrate the microscope topography [15±18]. Our systematic studies on the constant frequency-shift topography of carboxylates with R=H, CH3, C(CH3)3, C=CH, CF3, and CHF2 described in this paper have revealed the mechanism behind the
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 9 3 5 - 7
258
H. Onishi et al. / Applied Surface Science 188 (2002) 257±264
high-resolution imaging of the molecular adsorbates; the van der Waals force is responsible for the observed molecule-dependent topography despite its longrange nature. The intramolecular electric polarization (permanent dipole moment), if it exists, perturbs the topography through electrostatic couplings with the tip. 2. The surface and molecules Carboxylates adsorbed on the (1 1 0) truncation of rutile TiO2 have been extensively studied as a prototype of organic materials interfaced with an inorganic metal oxide [19]. A carboxylic acid molecule (RCOOH) dissociates on this surface to a carboxylate (RCOO ) and a proton (H) at room temperature
[20] as illustrated in Fig. 1. A bridge con®guration of the adsorbed formate (R=H), acetate (R=CH3), and propanate (R=CH2CH3) is experimentally and theoretically evidenced where the pair of negatively charged oxygen atoms in RCOO coordinate two positively charged Ti atoms at the surface. The O± Ti distance and O±C±O angle of the formate were determined in the quantitative analysis of photoelectron diffraction [21], and a theoretical calculation [22] yielded a compatible length and angle. Electron-stimulated desorption [23] and X-ray absorption [24] studies revealed the C±C bond of the acetate and propanate perpendicular to the surface. We thus expect that the O±Ti bonds between the ionic adsorbate (RCOO ) and ionic substrate (TiO2) are so stable that the bridge con®guration is maintained on carboxylates with other R's.
Fig. 1. The TiO2 substrate and carboxylates imaged in this study: (a) top- and side-viewed ball model. Constituent atoms are represented by spheres of van der Waals or ionic radii. Filled and shaded balls represent Ti and O atoms in the substrate; (b) atom geometry of formate, acetate, pivalate, and propiolate bridge adsorbed on the TiO2 (1 1 0) surface. The O±Ti distance and O±C±O angle follow Ref. [21]. Other lengths and angles are similar to those in the corresponding molecules in the gas phase [28,29].
H. Onishi et al. / Applied Surface Science 188 (2002) 257±264
The adsorbed carboxylates make a monolayer with a 2 1 long-range order when 80% or more of the surface Ti sites are occupied [20]. The in-plane spacings of the adsorbates are regulated at 0.65 and 0.59 nm along the [1 1 0] and [0 0 1] directions in the 2 1 monolayer. By scanning a monolayer composed of carboxylates with different R's, the microscope topography of the R's can be quantitatively compared while minimizing tip-dependent artifacts. 3. Experimental The constant frequency-shift topography of molecule-covered TiO2 surfaces was observed at room temperature with a commercial UHV-compatible atomic force microscope (JEOL, JSPM-4500A). A silicon cantilever of the resonant frequency 310± 370 kHz and the force constant 14 N m 1 on the speci®cation sheet (NSCS11, NT-MDT) was occasionally sputter-cleaned with an argon ion beam accelerated at 1 keV. A constant-amplitude AC voltage was applied to the piezoactuator at the ®xed end of the cantilever. The peak-to-peak amplitude at the tip-end (2A0) was estimated by measuring the response of the tip±sample distance when the AC voltage was arti®cially changed. Figures in this paper were constructed with A0 2:7 3:6 nm.1 The frequency shift (Df) was measured by a frequency-modulation method. The molecule-by-molecule resolution was sensitive to the frequency shift and also to the bias voltage (Vs) externally applied between the surface and the grounded cantilever. The voltage was optimized to enhance the resolution with minimum frequency shifts near 100 Hz. We suspect that the optimum Vs re¯ects the electrostatic potential of the moleculecovered surfaces. The atom-scale resolution is most improved when the contact potential difference is canceled between the tip and the surface [25]. The 3D scales of the microscope were calibrated on a Si(1 1 1)- 7 7 surface. Microscope images were presented in gray scale without ®ltering, while cross-sections were measured on the images ®ltered by a nine-point median operation. 1 The amplitudes mentioned in our previous papers, Refs. [15± 17] were overestimated.
259
The 1 1 truncation of TiO2(1 1 0) substrates (7 1 0:3 mm3 , Earth Chemicals or Shinko-sha) were prepared by cycles of argon-ion sputtering and vacuum annealing at 900 K in the microscope [26,27]. A Si(1 1 1) plate clamped with the sample was resistively heated and the wafer temperature was monitored with an infrared pyrometer. The annealed 1 1 surface was cooled at room temperature and exposed to RCOOH vapor for several Langmuirs (1L 1 10 6 Torr s). The RCOO -covered surface was successively exposed to R0 COOH vapor to produce a monolayer of mixed compositions. The 2 1 order of the adsorbates was maintained in the mixed monolayer being independent of the ratio of the two compounds. This evidenced RCOO and R0 COO were adsorbed uniformly in the bridge form. Hence, the molecule-dependent corrugation observed in the mixed monolayers was ascribed to the different R's uprightly terminating the COO group on the surface. 4. Alkyl-substituted carboxylates Fig. 2(a) presents the constant frequency-shift topography of a formate monolayer. Individual formates (R=H) ordered on a 2 1 mesh were resolved as protrusions of a uniform brightness. The position of the adsorbates is marked in the middle panel. Filled circles represent unoccupied surface sites. The crosssection in the lower panel shows that the corrugation amplitude ¯uctuation was not more than 0.01 nm. The formate-covered surface was exposed to acetic acid (CH3COOH) vapor. Some formates were exchanged by acetates (R=CH3) supplied from the gas phase. The number of acetates could be controlled by the exposure time, while the total formate acetate coverage was maintained at the saturation. Brighter particles appeared in the image when the formate monolayer was exposed to acetic acid for 0.6 L, as shown in Fig. 2(b). The number of brighter spots increased with the exposure time to acetic acid. Hence, we assigned the brighter particles to the acetates having exchanged formates. Twenty-nine acetates and 188 formates were identi®ed in the topography. An isolated acetate and its surrounding formates exhibited an image height contrast of 0.06 nm in cross-sections, one of which is shown in the lower
260 H. Onishi et al. / Applied Surface Science 188 (2002) 257±264 Fig. 2. The constant frequency-shift topography of carboxylate monolayers prepared on the TiO2(1 1 0) surface: (a) pure formate monolayer, 10 10 nm2 , Df 132 Hz, V s 0:4 V, A0 3:4 nm; (b) formate±acetate mixed layer, 10 10 nm2 , Df 106 Hz, V s 0:3 V, A0 3:6 nm; (c) formate±pivalate mixed layer, 10 10 nm2 , Df 83 Hz, V s 0:2 V, A0 3:6 nm; (d): formate±propiolate mixed layer, 10 10 nm2 , Df 99 Hz, V s 0:1 V, A0 2:7 nm. The position of the molecules is illustrated in the middle panels. Open circles represent the taller molecules in the mixed monolayers. Cross-sections determined on the lines inserted in the images are shown in the lower panels.
H. Onishi et al. / Applied Surface Science 188 (2002) 257±264
panel. We estimated the accuracy of the height measurement to be 0.01 nm. Pivalate is terminated by a bulky R=C(CH3)3. Pivalate-containing formate monolayers were observed in a similar manner. Fig. 2(c) presents a constant frequency-shift topography of a mixed monolayer. Nine bright spots were surrounded by formates of the ordinary brightness. The bright particles were assigned to the pivalates based on the exposure-dependent population. The image height contrast of an isolated pivalate over the formates was determined to be 0.11 nm. Propiolate with C=CH is a needle-like adsorbate of a single-atom diameter. Fig. 2(d) shows the topography of a formate monolayer exposed to propiolic acid (HC=CCOOH) gas for 3 L. Bright and nonbright particles were again assigned to the propiolate and formate, respectively. The constant frequency-shift topography of the propiolate was 0.20 nm higher than that of the formate. The image topography of the formate, acetate, pivalate, and propiolate followed the order of the physical topography of the alkyl groups terminating the adsorbates. The physical topography of the target molecules is shown in Fig. 1(b) where C±H, C±C and C=C lengths are reasonably assumed at the values of the corresponding RCOOH molecules in the gas phase [28,29]. The top hydrogen atom of the formate is thereby located at 0.38 nm above the surface plane containing the Ti atom pair, while three equivalent hydrogen atoms of the acetate are elevated at 0.46 nm. The uppermost H atoms in the pivalate is lifted by 0.58 nm relative to the Ti plane. The H atom terminating the triple-bonded carbon chain in the propiolate is at 0.64 nm. Fig. 3 summarizes the observed heights relative to the formate as a function of the physical topography, i.e., the altitude of the topmost H atoms given in the model. The straight line ®ts the four observations. The altitude of the topmost H atoms hence serves as a good parameter to predict the constant frequency-shift topography. When we scaled the horizontal axis with other properties (molecular weight, the number of atoms in a molecule, or the number of electrons in valence states), the correlation became worse. On the other hand, if the tip apex traced the contour of an adsorbate composed of hard-sphere atoms, the image topography would reproduce the physical
261
Fig. 3. The constant frequency-shift topography of the alkylsubstituted carboxylates as a function of their physical topography given in the model of Fig. 1.
topography in the one-to-one ratio as shown by the broken line in Fig. 3. This was not the case, however. The slope of the ®tted line was 0.7. A slope less than unity indicates the long-range nature of the force responsible for the molecule-dependent contrast. The tip±molecule force re¯ected in the frequency shift is the sum of the forces between the tip apex and individual adsorbates. When the tip passes above a tall molecule isolated in short molecules, the feedback regulation pulls up the tip to keep the frequency-shift constant. Forces between the lifted apex and the surrounding molecules are reduced due to the increased tip±surface distance. The probe is pushed down to restore the forces lost. This qualitative picture predicts that the tip±molecule force remains nonzero at distances over the lateral separation of the carboxylates (0.59±0.65 nm). Indeed, two-dimensionally clustered acetates exhibited enhanced contrast over an isolated acetate [15] which is consistent with the prediction. Chemical bonding interactions cannot be important across such a wide tip±molecule gap, whereas contrast-enhancements on Si(1 1 1)- 7 7 are interpreted with the direct overlap of dangling bond orbitals [30,31]. The attractive component of van der Waals force is probable for the molecule-dependent topography of the alkyl-substituted carboxylates. A theoretical simulation with Lennard±Jones type forces de®ned by empirical parameters reproduced the propiolate topography [32]. The calculation yielded the tip-apex atom 0.4± 0.5 nm away from the top H atom of the adsorbate. The calculated strength of individual forces evidenced
262
H. Onishi et al. / Applied Surface Science 188 (2002) 257±264
the considerable contribution of adsorbates located at the second nearest sites or farther, which was as much as 50% of the contribution by the molecule at the ®rst nearest site just below the tip. The attractive component of the van der Waals force contains electrostatic terms caused by permanentdipole/permanent-dipole coupling, permanent-dipole/
induced-dipole coupling, and induced-dipole/induceddipole coupling (dispersion force). We expect that the four carboxylates are equivalent in terms of their permanent electric dipole, because the alkyl groups are nonpolar. Hence, the molecule-dependent image contrast of one carboxylate relative to another is ascribed to the dispersion force and/or the force created by the
Fig. 4. The constant frequency-shift topography of ¯uorine-substituted acetates adsorbed on the TiO2 surface: (a) pure acetate monolayer, 9 9 nm2 , Df 88 Hz, V s 0:2 V, A0 3:4 nm; (b) acetate±tri¯uoroacetate mixed layer, 9 9 nm2 , Df 75 Hz, V s 0:1 V, A0 3:5 nm; (c) pure tri¯uoroacetate layer, 9 9 nm2 , Df 81 Hz, V s 0:1 V, A0 3:4 nm. The position of the molecules is illustrated in the middle panels. Open circles represent the taller molecules in the mixed monolayers. Cross-sections determined on the lines inserted in the images are shown in the lower panels.
H. Onishi et al. / Applied Surface Science 188 (2002) 257±264
coupling between the permanent dipole on the tip and the induced dipole on the molecule. If we further assume that the tip apex of Si exhibits the smallest permanent dipole, the dispersion force remains dominant. We propose the dispersion force responsible for the molecule-dependent topography of a nonpolar group of atoms based on these results and considerations. The other terms of the van der Waals force, which is related to the permanent-dipole moment of adsorbates, may also contribute to the constant frequency-shift topography. Its possible contribution is examined in the following section. 5. Fluorine-substituted acetates The constant frequency-shift topography of acetate (R=CH3) and tri¯uoroacetate (R=CF3) was compared in Fig. 4 to deduce how the pure electrostatic dipolar coupling contributes to the topography. The strongly polarized C±F bonds in the latter molecule perturb the electrostatic ®eld over the molecule. Fig. 4(a) and (c) show the topography of the acetate-covered and the tri¯uoroacetate-covered surfaces. Individual molecules were resolved as particles of uniform brightness. Molecular images of two different brightnesses appeared on a mixed monolayer containing the acetates and tri¯uoroacetates as shown in Fig. 4(b). Seventy-one bright spots and 118 nonbright spots were ordered in the 2 1 symmetry. We assigned the bright and nonbright particles to the acetate and tri¯uoroacetate, respectively. The number of bright molecules increased with the exposure to CH3COOH. Adsorbate images assignable to a third species did not appear. Cross-sections in the lower panel showed the acetate was 0.04 nm higher than the tri¯uoroacetate in the constant frequency-shift topography. The molecule-dependent topography of the CH3and CF3-terminated molecules is attributed to the F atoms replacing the H atoms in the acetate. The F atoms in the tri¯uoroacetate are higher by 0.46 nm from the surface plane containing the Ti atom pair as well as the H atoms in the acetate as illustrated in Fig. 5, where the O±Ti distance and O±C±O angle are similar to those in the formate. The van der Waals radius of a F and H atom is 0.15±0.13 and 0.12 nm, respectively [33,34] in accordance with the increased
263
Fig. 5. The atom geometry of the acetate and tri¯uoroacetate adsorbed on the TiO2(1 1 0) surface. The O±Ti distance and O±C± O angle follow Ref. [21]. Other lengths and angles reproduce those in the corresponding molecules in the gas phase [28].
number of valence electrons on the F atom. Hence, the CF3-terminated acetates is identical to or slightly (<0.05 nm) taller than the CH3-terminated acetate in physical topography. The observed contrast in the constant frequency-shift topography was inverted. We ascribed the CF3-induced depression of the topography to the electrostatic ®eld perturbed by the dipole ®eld originating from the polarized C±F bonds. It has been established in organic chemistry that ¯uorine, the most electronegative element, pulls valence when bound to another element such as carbon. The moment of a gas-phase HCF3 molecule, 1.65 D 1D Debye 3:34 10 30 Cm [28], provides an estimate of the moment on the CF3 terminal group. The CF3 unit with the C±C bond perpendicular to the surface presents a dipole moment of 1.65 D oriented towards the surface as a ®rst approximation. On the other hand, the COO unit in the carboxylate is polarized as Od ±Cd±Od and displays a moment towards the vacuum. Indeed, the work function of the TiO2(1 1 0) surface was decreased by 0.9 eV when a formate monolayer covered the surface [35]. The strength of the moment is estimated at 0.9 D based on the number density of the formates ordered in the 2 1 symmetry, 2:60 1014 cm 2 . These estimations predict that the moment on the CF3 group almost compensates for the moment of the opposite orientation on the COO unit. The diminished moment composite over the molecule reduces the electrostatic force through the weakened couplings of the molecular moment with the induced and permanent moment on the tip. A more quantitative interpretation requires a theoretical work that estimates the electrostatic ®eld over the different molecules including the intrinsic Madelung potential of the ionic substrate.
264
H. Onishi et al. / Applied Surface Science 188 (2002) 257±264
It was also possible to distinguish di¯uoroacetate (R=CHF2) from tri¯uoroacetate in the constant frequency-shift topography [36]. This demonstrates that the composition of a single molecule is analyzed with one F-atom sensitivity. 6. Conclusions The systematic studies on the constant frequencyshift topography of carboxylates with R=H, CH3, C(CH3)3, C=CH, CF3, and CHF2 revealed the mechanism behind the high-resolution imaging of the chemically stable molecular adsorbates. The van der Waals force is primarily responsible for the moleculedependent topography. The permanent-dipole moment of the imaged molecule, if exists, perturbs the topography through electrostatic couplings with the tip. These results are a ®rst step towards the single-molecule analysis of the physical and chemical properties using NC-AFM. Acknowledgements The authors thank S. Kitamura at JEOL for his advice in operating the microscope. This work was partly supported by a Grant-in-Aid for Scienti®c Research from the Ministry of Education, Sciences, Sports, and Culture of Japan (No. 12750031). References [1] F.J. Giessibl, Science 267 (1995) 68. [2] S. Kitamura, M. Iwatsuki, Jpn. J. Appl. Phys. 34 (1995) L145. [3] Y. Sugawara, M. Ohta, H. Ueyama, S. Morita, Science 270 (1995) 1646. [4] J. Israelachvili, Intermolecular and Surface Forces, 2nd Edition, Academic Press, London, 1992. [5] K. Fukui, H. Onishi, Y. Iwasawa, Chem. Phys. Lett. 280 (1997) 296. [6] B. Gotsmann, C. Schmidt, C. Seidel, H. Fuchs, Eur. Phys. J. B 4 (1998) 267. [7] K. Kobayashi, H. Yamada, T. Horiuchi, K. Matsushige, Appl. Surf. Sci. 140 (1999) 281. [8] K. Kobayashi, H. Yamada, T. Horiuchi, K. Matsushige, Appl. Surf. Sci. 157 (2000) 228.
[9] T. Uchihashi, T. Okada, Y. Sugawara, K. Yokoyama, S. Morita, Phys. Rev. B 60 (1999) 8309. [10] T. Fukuma, K. Kobayashi, T. Horiuchi, H. Yamada, K. Matsushige, Appl. Phys. A 72 (2001) S109. [11] Y. Maeda, T. Matsumoto, T. Kawai, Appl. Surf. Sci. 140 (1999) 400. [12] K. Fukui, Y. Iwasawa, Surf. Sci. 464 (2000) L719. [13] T. Uchihashi, T. Ishida, M. Komiyama, M. Ashino, Y. Sugawara, W. Mizutani, K. Yokoyama, S. Morita, H. Tokumoto, M. Ishikawa, Appl. Surf. Sci. 157 (2000) 244. [14] Ch. Loppacher, M. Bammerlin, M. Guggisberg, E. Meyer, H.J. GuÈntherrodt, R. LuÈthi, R. Schlittler, J.K. Gimzewski, Appl. Phys. A 72 (2001) S105. [15] A. Sasahara, H. Uetsuka, H. Onishi, J. Phys. Chem. 105 (2001) 1. [16] A. Sasahara, H. Uetsuka, H. Onishi, Appl. Phys. A 72 (2001) S101. [17] A. Sasahara, H. Uetsuka, H. Onishi, Surf. Sci. 481 (2001) L437. [18] A. Sasahara, H. Uetsuka, H. Onishi, Phys. Rev. B, 64 (2001) 121406(R). [19] V.E. Henrich, P.A. Cox, The Surface Science of Metal Oxides, Cambridge University Press, Cambridge, 1994. [20] H. Onishi, T. Aruga, Y. Iwasawa, J. Catal. 146 (1994) 557. [21] S. Thevuthasan, G.S. Herman, Y.J. Kim, S.A. Chambers, C.H.F. Peden, Z. Wang, R.X. Ynzunza, E.D. Tober, J. Morais, C.S. Fadley, Surf. Sci. 401 (1998) 261. [22] P. KaÈkell, K. Terakura, Surf. Sci. 461 (2000) 191. [23] Q. Guo, I. Cocks, E.M. Williams, J. Chem. Phys. 106 (1997) 2924. [24] A. GutieÂrrez-Sosa, P. MartõÂnez-Escolano, H. Raza, R. Lindsay, P.L. Wincott, G. Thornton, Surf. Sci. 471 (2001) 163. [25] E. Meyer, L. Howald, R. LuÈthi, J. Haefke, M. RuÈetcshi, T. Bonner, R. Overney, J. Frommer, R. Hofer, H.-J. GuÈntherrodt, J. Vac. Sci. Technol. B 12 (1994) 2060. [26] H. Onishi, K. Fukui, Y. Iwasawa, Bull. Chem. Soc. Jpn. 68 (1995) 2447. [27] H. Onishi, Y. Iwasawa, Surf. Sci. 313 (1994) L783. [28] D.R. Lide (Ed.), CRC Handbook of Chemistry and Physics, 81st Edition, CRC Press, Boca Raton, 2000. [29] D.G. Lister, J.K. Tyler, Spectrochim. Acta A 28 (1972) 1423. [30] T. Uchihashi, Y. Sugawara, T. Tsukamoto, M. Ohta, S. Morita, Phys. Rev. B 56 (1997) 9834. [31] F.J. Giessibl, S. Hembacher, H. Bielefeldt, J. Mannhart, Science 289 (2000) 422. [32] A. Sasahara, H. Uetsuka, T. Ishibashi, H. Onishi, Appl. Surf. Sci., submitted for publication. [33] A. Bondi, J. Phys. Chem. 68 (1964) 441. [34] L. Pauling, The Nature of the Chemical Bonds, Cornell University Press, Ithaca, 1960, p. 260. [35] H. Onishi, T. Aruga, C. Egawa, Y. Iwasawa, Surf. Sci. 193 (1988) 33. [36] A. Sasahara, H. Uetsuka, H. Onishi, in press.
Molecule-dependent topography determined by noncontact atomic force microscopy: carboxylates on TiO2(1 1 0) Hiroshi Onishi*, Akira Sasahara, Hiroshi Uetsuka, Taka-aki Ishibashi Surface Chemistry Laboratory, Kanagawa Academy of Science and Technology, KSP E-404, Takatsu-ku, Kawasaki, Kanagawa 213-0012, Japan Received 2 September 2001; accepted 10 September 2001
Abstract The feasibility of noncontact atomic force microscopy (NC-AFM) in single-molecule analysis is experimentally demonstrated. Carboxylates (RCOO ) with different R's were identi®ed molecule-by-molecule on the TiO2(1 1 0) substrate. The constant frequency-shift topography exhibited a good correlation with the physical topography of the carboxylates with R=H, CH3, C(CH3)3 and C=CH. The molecule-dependent topography was interpreted with the van der Waals force pulling the tip into the adsorbates. The constant frequency-shift topography of a series of ¯uorine-containing acetates (R=CH3, CHF2, and CF3) revealed that the intramolecular electric polarization perturbs the tip±molecule force through electrostatic couplings. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Atomic force microscopy; Surface structure; Morphology; Chemisorption; Carboxylic acid; Titanium oxide; Low index single crystal surfaces; Semiconducting surfaces
1. Introduction Noncontact atomic force microscopy (NC-AFM) employs an attractive force to construct the topography of materials. It offers an opportunity for singlemolecule sensing of materials which are important in chemical and biochemical applications regardless of conductivity. It was, however, not a trivial challenge to visualize individual organic adsorbates when the atom resolution was ®rst achieved on Si [1,2] and InP [3] surfaces in 1995. The force pulling the tip into the surface is less sensitive to the gap width (r) than the tunneling current utilized in scanning tunneling microscopy (STM), especially when chemically stable *
Corresponding author. Tel.: 81-44-819-2094; fax: 81-44-819-2095. E-mail address: [email protected] (H. Onishi).
molecules cover the surface. The attractive potential between two stable molecules with closed molecular orbitals is shallow and exhibits r 6 decay [4]. The high-resolution topography of formate (HCOO ) [5] was ®rst reported as a molecular adsorbate in 1997. Various compounds including a perylene-derivative (PTCDA) [6], C60 [7,8], adenine [9,10], DNA [11], acetate (CH3COO ) [12], alkanethiols [10,13], and a metal porphyrin (Cu-TBPP) [14] have been successfully imaged to date. We have proposed that a series of carboxylates (RCOO ) adsorbed on the TiO2(1 1 0) surface provide molecule-level standards with known physical and chemical properties to calibrate the microscope topography [15±18]. Our systematic studies on the constant frequency-shift topography of carboxylates with R=H, CH3, C(CH3)3, C=CH, CF3, and CHF2 described in this paper have revealed the mechanism behind the
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 9 3 5 - 7
258
H. Onishi et al. / Applied Surface Science 188 (2002) 257±264
high-resolution imaging of the molecular adsorbates; the van der Waals force is responsible for the observed molecule-dependent topography despite its longrange nature. The intramolecular electric polarization (permanent dipole moment), if it exists, perturbs the topography through electrostatic couplings with the tip. 2. The surface and molecules Carboxylates adsorbed on the (1 1 0) truncation of rutile TiO2 have been extensively studied as a prototype of organic materials interfaced with an inorganic metal oxide [19]. A carboxylic acid molecule (RCOOH) dissociates on this surface to a carboxylate (RCOO ) and a proton (H) at room temperature
[20] as illustrated in Fig. 1. A bridge con®guration of the adsorbed formate (R=H), acetate (R=CH3), and propanate (R=CH2CH3) is experimentally and theoretically evidenced where the pair of negatively charged oxygen atoms in RCOO coordinate two positively charged Ti atoms at the surface. The O± Ti distance and O±C±O angle of the formate were determined in the quantitative analysis of photoelectron diffraction [21], and a theoretical calculation [22] yielded a compatible length and angle. Electron-stimulated desorption [23] and X-ray absorption [24] studies revealed the C±C bond of the acetate and propanate perpendicular to the surface. We thus expect that the O±Ti bonds between the ionic adsorbate (RCOO ) and ionic substrate (TiO2) are so stable that the bridge con®guration is maintained on carboxylates with other R's.
Fig. 1. The TiO2 substrate and carboxylates imaged in this study: (a) top- and side-viewed ball model. Constituent atoms are represented by spheres of van der Waals or ionic radii. Filled and shaded balls represent Ti and O atoms in the substrate; (b) atom geometry of formate, acetate, pivalate, and propiolate bridge adsorbed on the TiO2 (1 1 0) surface. The O±Ti distance and O±C±O angle follow Ref. [21]. Other lengths and angles are similar to those in the corresponding molecules in the gas phase [28,29].
H. Onishi et al. / Applied Surface Science 188 (2002) 257±264
The adsorbed carboxylates make a monolayer with a 2 1 long-range order when 80% or more of the surface Ti sites are occupied [20]. The in-plane spacings of the adsorbates are regulated at 0.65 and 0.59 nm along the [1 1 0] and [0 0 1] directions in the 2 1 monolayer. By scanning a monolayer composed of carboxylates with different R's, the microscope topography of the R's can be quantitatively compared while minimizing tip-dependent artifacts. 3. Experimental The constant frequency-shift topography of molecule-covered TiO2 surfaces was observed at room temperature with a commercial UHV-compatible atomic force microscope (JEOL, JSPM-4500A). A silicon cantilever of the resonant frequency 310± 370 kHz and the force constant 14 N m 1 on the speci®cation sheet (NSCS11, NT-MDT) was occasionally sputter-cleaned with an argon ion beam accelerated at 1 keV. A constant-amplitude AC voltage was applied to the piezoactuator at the ®xed end of the cantilever. The peak-to-peak amplitude at the tip-end (2A0) was estimated by measuring the response of the tip±sample distance when the AC voltage was arti®cially changed. Figures in this paper were constructed with A0 2:7 3:6 nm.1 The frequency shift (Df) was measured by a frequency-modulation method. The molecule-by-molecule resolution was sensitive to the frequency shift and also to the bias voltage (Vs) externally applied between the surface and the grounded cantilever. The voltage was optimized to enhance the resolution with minimum frequency shifts near 100 Hz. We suspect that the optimum Vs re¯ects the electrostatic potential of the moleculecovered surfaces. The atom-scale resolution is most improved when the contact potential difference is canceled between the tip and the surface [25]. The 3D scales of the microscope were calibrated on a Si(1 1 1)- 7 7 surface. Microscope images were presented in gray scale without ®ltering, while cross-sections were measured on the images ®ltered by a nine-point median operation. 1 The amplitudes mentioned in our previous papers, Refs. [15± 17] were overestimated.
259
The 1 1 truncation of TiO2(1 1 0) substrates (7 1 0:3 mm3 , Earth Chemicals or Shinko-sha) were prepared by cycles of argon-ion sputtering and vacuum annealing at 900 K in the microscope [26,27]. A Si(1 1 1) plate clamped with the sample was resistively heated and the wafer temperature was monitored with an infrared pyrometer. The annealed 1 1 surface was cooled at room temperature and exposed to RCOOH vapor for several Langmuirs (1L 1 10 6 Torr s). The RCOO -covered surface was successively exposed to R0 COOH vapor to produce a monolayer of mixed compositions. The 2 1 order of the adsorbates was maintained in the mixed monolayer being independent of the ratio of the two compounds. This evidenced RCOO and R0 COO were adsorbed uniformly in the bridge form. Hence, the molecule-dependent corrugation observed in the mixed monolayers was ascribed to the different R's uprightly terminating the COO group on the surface. 4. Alkyl-substituted carboxylates Fig. 2(a) presents the constant frequency-shift topography of a formate monolayer. Individual formates (R=H) ordered on a 2 1 mesh were resolved as protrusions of a uniform brightness. The position of the adsorbates is marked in the middle panel. Filled circles represent unoccupied surface sites. The crosssection in the lower panel shows that the corrugation amplitude ¯uctuation was not more than 0.01 nm. The formate-covered surface was exposed to acetic acid (CH3COOH) vapor. Some formates were exchanged by acetates (R=CH3) supplied from the gas phase. The number of acetates could be controlled by the exposure time, while the total formate acetate coverage was maintained at the saturation. Brighter particles appeared in the image when the formate monolayer was exposed to acetic acid for 0.6 L, as shown in Fig. 2(b). The number of brighter spots increased with the exposure time to acetic acid. Hence, we assigned the brighter particles to the acetates having exchanged formates. Twenty-nine acetates and 188 formates were identi®ed in the topography. An isolated acetate and its surrounding formates exhibited an image height contrast of 0.06 nm in cross-sections, one of which is shown in the lower
260 H. Onishi et al. / Applied Surface Science 188 (2002) 257±264 Fig. 2. The constant frequency-shift topography of carboxylate monolayers prepared on the TiO2(1 1 0) surface: (a) pure formate monolayer, 10 10 nm2 , Df 132 Hz, V s 0:4 V, A0 3:4 nm; (b) formate±acetate mixed layer, 10 10 nm2 , Df 106 Hz, V s 0:3 V, A0 3:6 nm; (c) formate±pivalate mixed layer, 10 10 nm2 , Df 83 Hz, V s 0:2 V, A0 3:6 nm; (d): formate±propiolate mixed layer, 10 10 nm2 , Df 99 Hz, V s 0:1 V, A0 2:7 nm. The position of the molecules is illustrated in the middle panels. Open circles represent the taller molecules in the mixed monolayers. Cross-sections determined on the lines inserted in the images are shown in the lower panels.
H. Onishi et al. / Applied Surface Science 188 (2002) 257±264
panel. We estimated the accuracy of the height measurement to be 0.01 nm. Pivalate is terminated by a bulky R=C(CH3)3. Pivalate-containing formate monolayers were observed in a similar manner. Fig. 2(c) presents a constant frequency-shift topography of a mixed monolayer. Nine bright spots were surrounded by formates of the ordinary brightness. The bright particles were assigned to the pivalates based on the exposure-dependent population. The image height contrast of an isolated pivalate over the formates was determined to be 0.11 nm. Propiolate with C=CH is a needle-like adsorbate of a single-atom diameter. Fig. 2(d) shows the topography of a formate monolayer exposed to propiolic acid (HC=CCOOH) gas for 3 L. Bright and nonbright particles were again assigned to the propiolate and formate, respectively. The constant frequency-shift topography of the propiolate was 0.20 nm higher than that of the formate. The image topography of the formate, acetate, pivalate, and propiolate followed the order of the physical topography of the alkyl groups terminating the adsorbates. The physical topography of the target molecules is shown in Fig. 1(b) where C±H, C±C and C=C lengths are reasonably assumed at the values of the corresponding RCOOH molecules in the gas phase [28,29]. The top hydrogen atom of the formate is thereby located at 0.38 nm above the surface plane containing the Ti atom pair, while three equivalent hydrogen atoms of the acetate are elevated at 0.46 nm. The uppermost H atoms in the pivalate is lifted by 0.58 nm relative to the Ti plane. The H atom terminating the triple-bonded carbon chain in the propiolate is at 0.64 nm. Fig. 3 summarizes the observed heights relative to the formate as a function of the physical topography, i.e., the altitude of the topmost H atoms given in the model. The straight line ®ts the four observations. The altitude of the topmost H atoms hence serves as a good parameter to predict the constant frequency-shift topography. When we scaled the horizontal axis with other properties (molecular weight, the number of atoms in a molecule, or the number of electrons in valence states), the correlation became worse. On the other hand, if the tip apex traced the contour of an adsorbate composed of hard-sphere atoms, the image topography would reproduce the physical
261
Fig. 3. The constant frequency-shift topography of the alkylsubstituted carboxylates as a function of their physical topography given in the model of Fig. 1.
topography in the one-to-one ratio as shown by the broken line in Fig. 3. This was not the case, however. The slope of the ®tted line was 0.7. A slope less than unity indicates the long-range nature of the force responsible for the molecule-dependent contrast. The tip±molecule force re¯ected in the frequency shift is the sum of the forces between the tip apex and individual adsorbates. When the tip passes above a tall molecule isolated in short molecules, the feedback regulation pulls up the tip to keep the frequency-shift constant. Forces between the lifted apex and the surrounding molecules are reduced due to the increased tip±surface distance. The probe is pushed down to restore the forces lost. This qualitative picture predicts that the tip±molecule force remains nonzero at distances over the lateral separation of the carboxylates (0.59±0.65 nm). Indeed, two-dimensionally clustered acetates exhibited enhanced contrast over an isolated acetate [15] which is consistent with the prediction. Chemical bonding interactions cannot be important across such a wide tip±molecule gap, whereas contrast-enhancements on Si(1 1 1)- 7 7 are interpreted with the direct overlap of dangling bond orbitals [30,31]. The attractive component of van der Waals force is probable for the molecule-dependent topography of the alkyl-substituted carboxylates. A theoretical simulation with Lennard±Jones type forces de®ned by empirical parameters reproduced the propiolate topography [32]. The calculation yielded the tip-apex atom 0.4± 0.5 nm away from the top H atom of the adsorbate. The calculated strength of individual forces evidenced
262
H. Onishi et al. / Applied Surface Science 188 (2002) 257±264
the considerable contribution of adsorbates located at the second nearest sites or farther, which was as much as 50% of the contribution by the molecule at the ®rst nearest site just below the tip. The attractive component of the van der Waals force contains electrostatic terms caused by permanentdipole/permanent-dipole coupling, permanent-dipole/
induced-dipole coupling, and induced-dipole/induceddipole coupling (dispersion force). We expect that the four carboxylates are equivalent in terms of their permanent electric dipole, because the alkyl groups are nonpolar. Hence, the molecule-dependent image contrast of one carboxylate relative to another is ascribed to the dispersion force and/or the force created by the
Fig. 4. The constant frequency-shift topography of ¯uorine-substituted acetates adsorbed on the TiO2 surface: (a) pure acetate monolayer, 9 9 nm2 , Df 88 Hz, V s 0:2 V, A0 3:4 nm; (b) acetate±tri¯uoroacetate mixed layer, 9 9 nm2 , Df 75 Hz, V s 0:1 V, A0 3:5 nm; (c) pure tri¯uoroacetate layer, 9 9 nm2 , Df 81 Hz, V s 0:1 V, A0 3:4 nm. The position of the molecules is illustrated in the middle panels. Open circles represent the taller molecules in the mixed monolayers. Cross-sections determined on the lines inserted in the images are shown in the lower panels.
H. Onishi et al. / Applied Surface Science 188 (2002) 257±264
coupling between the permanent dipole on the tip and the induced dipole on the molecule. If we further assume that the tip apex of Si exhibits the smallest permanent dipole, the dispersion force remains dominant. We propose the dispersion force responsible for the molecule-dependent topography of a nonpolar group of atoms based on these results and considerations. The other terms of the van der Waals force, which is related to the permanent-dipole moment of adsorbates, may also contribute to the constant frequency-shift topography. Its possible contribution is examined in the following section. 5. Fluorine-substituted acetates The constant frequency-shift topography of acetate (R=CH3) and tri¯uoroacetate (R=CF3) was compared in Fig. 4 to deduce how the pure electrostatic dipolar coupling contributes to the topography. The strongly polarized C±F bonds in the latter molecule perturb the electrostatic ®eld over the molecule. Fig. 4(a) and (c) show the topography of the acetate-covered and the tri¯uoroacetate-covered surfaces. Individual molecules were resolved as particles of uniform brightness. Molecular images of two different brightnesses appeared on a mixed monolayer containing the acetates and tri¯uoroacetates as shown in Fig. 4(b). Seventy-one bright spots and 118 nonbright spots were ordered in the 2 1 symmetry. We assigned the bright and nonbright particles to the acetate and tri¯uoroacetate, respectively. The number of bright molecules increased with the exposure to CH3COOH. Adsorbate images assignable to a third species did not appear. Cross-sections in the lower panel showed the acetate was 0.04 nm higher than the tri¯uoroacetate in the constant frequency-shift topography. The molecule-dependent topography of the CH3and CF3-terminated molecules is attributed to the F atoms replacing the H atoms in the acetate. The F atoms in the tri¯uoroacetate are higher by 0.46 nm from the surface plane containing the Ti atom pair as well as the H atoms in the acetate as illustrated in Fig. 5, where the O±Ti distance and O±C±O angle are similar to those in the formate. The van der Waals radius of a F and H atom is 0.15±0.13 and 0.12 nm, respectively [33,34] in accordance with the increased
263
Fig. 5. The atom geometry of the acetate and tri¯uoroacetate adsorbed on the TiO2(1 1 0) surface. The O±Ti distance and O±C± O angle follow Ref. [21]. Other lengths and angles reproduce those in the corresponding molecules in the gas phase [28].
number of valence electrons on the F atom. Hence, the CF3-terminated acetates is identical to or slightly (<0.05 nm) taller than the CH3-terminated acetate in physical topography. The observed contrast in the constant frequency-shift topography was inverted. We ascribed the CF3-induced depression of the topography to the electrostatic ®eld perturbed by the dipole ®eld originating from the polarized C±F bonds. It has been established in organic chemistry that ¯uorine, the most electronegative element, pulls valence when bound to another element such as carbon. The moment of a gas-phase HCF3 molecule, 1.65 D 1D Debye 3:34 10 30 Cm [28], provides an estimate of the moment on the CF3 terminal group. The CF3 unit with the C±C bond perpendicular to the surface presents a dipole moment of 1.65 D oriented towards the surface as a ®rst approximation. On the other hand, the COO unit in the carboxylate is polarized as Od ±Cd±Od and displays a moment towards the vacuum. Indeed, the work function of the TiO2(1 1 0) surface was decreased by 0.9 eV when a formate monolayer covered the surface [35]. The strength of the moment is estimated at 0.9 D based on the number density of the formates ordered in the 2 1 symmetry, 2:60 1014 cm 2 . These estimations predict that the moment on the CF3 group almost compensates for the moment of the opposite orientation on the COO unit. The diminished moment composite over the molecule reduces the electrostatic force through the weakened couplings of the molecular moment with the induced and permanent moment on the tip. A more quantitative interpretation requires a theoretical work that estimates the electrostatic ®eld over the different molecules including the intrinsic Madelung potential of the ionic substrate.
264
H. Onishi et al. / Applied Surface Science 188 (2002) 257±264
It was also possible to distinguish di¯uoroacetate (R=CHF2) from tri¯uoroacetate in the constant frequency-shift topography [36]. This demonstrates that the composition of a single molecule is analyzed with one F-atom sensitivity. 6. Conclusions The systematic studies on the constant frequencyshift topography of carboxylates with R=H, CH3, C(CH3)3, C=CH, CF3, and CHF2 revealed the mechanism behind the high-resolution imaging of the chemically stable molecular adsorbates. The van der Waals force is primarily responsible for the moleculedependent topography. The permanent-dipole moment of the imaged molecule, if exists, perturbs the topography through electrostatic couplings with the tip. These results are a ®rst step towards the single-molecule analysis of the physical and chemical properties using NC-AFM. Acknowledgements The authors thank S. Kitamura at JEOL for his advice in operating the microscope. This work was partly supported by a Grant-in-Aid for Scienti®c Research from the Ministry of Education, Sciences, Sports, and Culture of Japan (No. 12750031). References [1] F.J. Giessibl, Science 267 (1995) 68. [2] S. Kitamura, M. Iwatsuki, Jpn. J. Appl. Phys. 34 (1995) L145. [3] Y. Sugawara, M. Ohta, H. Ueyama, S. Morita, Science 270 (1995) 1646. [4] J. Israelachvili, Intermolecular and Surface Forces, 2nd Edition, Academic Press, London, 1992. [5] K. Fukui, H. Onishi, Y. Iwasawa, Chem. Phys. Lett. 280 (1997) 296. [6] B. Gotsmann, C. Schmidt, C. Seidel, H. Fuchs, Eur. Phys. J. B 4 (1998) 267. [7] K. Kobayashi, H. Yamada, T. Horiuchi, K. Matsushige, Appl. Surf. Sci. 140 (1999) 281. [8] K. Kobayashi, H. Yamada, T. Horiuchi, K. Matsushige, Appl. Surf. Sci. 157 (2000) 228.
[9] T. Uchihashi, T. Okada, Y. Sugawara, K. Yokoyama, S. Morita, Phys. Rev. B 60 (1999) 8309. [10] T. Fukuma, K. Kobayashi, T. Horiuchi, H. Yamada, K. Matsushige, Appl. Phys. A 72 (2001) S109. [11] Y. Maeda, T. Matsumoto, T. Kawai, Appl. Surf. Sci. 140 (1999) 400. [12] K. Fukui, Y. Iwasawa, Surf. Sci. 464 (2000) L719. [13] T. Uchihashi, T. Ishida, M. Komiyama, M. Ashino, Y. Sugawara, W. Mizutani, K. Yokoyama, S. Morita, H. Tokumoto, M. Ishikawa, Appl. Surf. Sci. 157 (2000) 244. [14] Ch. Loppacher, M. Bammerlin, M. Guggisberg, E. Meyer, H.J. GuÈntherrodt, R. LuÈthi, R. Schlittler, J.K. Gimzewski, Appl. Phys. A 72 (2001) S105. [15] A. Sasahara, H. Uetsuka, H. Onishi, J. Phys. Chem. 105 (2001) 1. [16] A. Sasahara, H. Uetsuka, H. Onishi, Appl. Phys. A 72 (2001) S101. [17] A. Sasahara, H. Uetsuka, H. Onishi, Surf. Sci. 481 (2001) L437. [18] A. Sasahara, H. Uetsuka, H. Onishi, Phys. Rev. B, 64 (2001) 121406(R). [19] V.E. Henrich, P.A. Cox, The Surface Science of Metal Oxides, Cambridge University Press, Cambridge, 1994. [20] H. Onishi, T. Aruga, Y. Iwasawa, J. Catal. 146 (1994) 557. [21] S. Thevuthasan, G.S. Herman, Y.J. Kim, S.A. Chambers, C.H.F. Peden, Z. Wang, R.X. Ynzunza, E.D. Tober, J. Morais, C.S. Fadley, Surf. Sci. 401 (1998) 261. [22] P. KaÈkell, K. Terakura, Surf. Sci. 461 (2000) 191. [23] Q. Guo, I. Cocks, E.M. Williams, J. Chem. Phys. 106 (1997) 2924. [24] A. GutieÂrrez-Sosa, P. MartõÂnez-Escolano, H. Raza, R. Lindsay, P.L. Wincott, G. Thornton, Surf. Sci. 471 (2001) 163. [25] E. Meyer, L. Howald, R. LuÈthi, J. Haefke, M. RuÈetcshi, T. Bonner, R. Overney, J. Frommer, R. Hofer, H.-J. GuÈntherrodt, J. Vac. Sci. Technol. B 12 (1994) 2060. [26] H. Onishi, K. Fukui, Y. Iwasawa, Bull. Chem. Soc. Jpn. 68 (1995) 2447. [27] H. Onishi, Y. Iwasawa, Surf. Sci. 313 (1994) L783. [28] D.R. Lide (Ed.), CRC Handbook of Chemistry and Physics, 81st Edition, CRC Press, Boca Raton, 2000. [29] D.G. Lister, J.K. Tyler, Spectrochim. Acta A 28 (1972) 1423. [30] T. Uchihashi, Y. Sugawara, T. Tsukamoto, M. Ohta, S. Morita, Phys. Rev. B 56 (1997) 9834. [31] F.J. Giessibl, S. Hembacher, H. Bielefeldt, J. Mannhart, Science 289 (2000) 422. [32] A. Sasahara, H. Uetsuka, T. Ishibashi, H. Onishi, Appl. Surf. Sci., submitted for publication. [33] A. Bondi, J. Phys. Chem. 68 (1964) 441. [34] L. Pauling, The Nature of the Chemical Bonds, Cornell University Press, Ithaca, 1960, p. 260. [35] H. Onishi, T. Aruga, C. Egawa, Y. Iwasawa, Surf. Sci. 193 (1988) 33. [36] A. Sasahara, H. Uetsuka, H. Onishi, in press.