J. Phys. Chem. B 2004, 108, 15735-15737
15735
Oxygen-Atom Vacancies Imaged by a Noncontact Atomic Force Microscope Operated in an Atmospheric Pressure of N2 Gas Akira Sasahara,*,†,‡,§ Shin-ichi Kitamura,| Hiroshi Uetsuka,†,⊥ and Hiroshi Onishi†,§ Surface Chemistry Laboratory, Kanagawa Academy of Science and Technology, KSP, 3-2-1 Sakado, Takatsu-ku, Kawasaki, Kanagawa 213-0012, Japan, and JEOL Ltd., 3-1-2 Musashino, Akishima, Tokyo 196-0021, Japan ReceiVed: April 6, 2004; In Final Form: July 23, 2004
The topography of the rutile TiO2(110) surface was observed with a noncontact atomic force microscope (NC-AFM) in an atmospheric pressure of N2. Oxygen-atom rows arrayed with an interval of 0.65 nm and individual oxygen-atom vacancies in the rows were resolved in constant frequency-shift topography, though the cantilever oscillation was damped by the viscous resistance of the dense ambient N2 gas.
Introduction
Experimental Section
Noncontact atomic force microscope (NC-AFM) is a promising tool for the surface characterization and atom fabrication of nonconductive materials. This scanning-probe method, which does not rely on electron tunneling, detects weak forces that pull a tip held away from the surface by 1 nm or less.1 A cantilever with the tip on its free end is oscillated at its resonant frequency. The tip-surface force causes a shift of the frequency ∆f, which is sensitively detected by a frequency modulation method.2 The surface topography is reproduced in an imaging scan where the tip-surface distance is regulated to keep ∆f constant. Atomically resolved images were attained on inorganic semiconductors,3-7 insulators,8-10 and organic adsorbates.11-14 Chemical analysis of molecular adsorbates was further demonstrated by detecting the electrostatic force generated by a polarized group of atoms (CF3).15 Atom manipulation has also been achieved as an extension of imaging.16 To date, true atom-scale imaging of atom vacancies has been accomplished only in an ultrahigh vacuum. The quality factor (Q) of the cantilever oscillation in resonance exceeds 10 000 in a vacuum, which is suitable for highly sensitive detection of the force. On the other hand, operation in a dense gaseous or liquid environment is necessary for chemical and biological applications. Spatial resolution in such environments, however, has been limited to the order of nanometers.17-19 Monatomic steps and kinks were resolved on a calcite surface in water.20 In the present work, individual atom vacancies on TiO2 are resolved in an atmospheric pressure of N2, though the viscous resistance of the vapor reduces Q to 500. Oxygen-atom vacancies are thought to play a crucial role in redox reactions on metal oxides.21 The vacancies resolved under atmospheric pressure show the potential for in-situ imaging of the redox reactions by this force-based microscope.
A TiO2(110) surface was prepared in an ultrahigh vacuum microscope (JSPM-4500A, JEOL) equipped with an argon ion gun (IG35, OCI), low-energy electron diffraction optics (BDL600, OCI), and an X-ray photoelectron spectrometer (TM50045, JEOL). A TiO2(110) wafer (7 × 1 × 0.3 mm3, Shinko-sha) was clamped with a Si plate as a heater. The (1 × 1) phase was prepared by argon ion sputtering at room temperature and vacuum annealing at 900 K. The temperature of the wafer was monitored with an infrared pyrometer (TR630, Minolta). Any contamination left on the sputter-annealed surface was below the detection limit of X-ray photoelectron spectrometer. Constant frequency-shift topography was obtained with a conductive silicon cantilever (NSC12, MikroMasch) at room temperature. The resonant frequency and force constant of the cantilever were specified as ∼300 kHz and ∼14 N/m. The sample bias voltage Vs was optimized to cancel the contact potential difference between the tip and the surface, where topography of high contrast was yielded with a minimum frequency shift.22
* To whom correspondence should be addressed. Phone: +81-78-8035674. FAX: +81-78-803-5674. E-mail:
[email protected]. † Kanagawa Academy of Science and Technology. ‡ Present address: Core Research for Evolutional Science and Technology, Japan Science and Technology Agency, Kawaguchi Center Building, 4-1-8, Honcho, Kawaguchi 332-0012, Japan. § Present address: Department of Chemistry, Faculty of Science, Kobe University, 1-1 Rokkodai, Nada-ku, Kobe, Hyogo 657-8501, Japan. | JEOL, Ltd. ⊥ Present address: Technology Research and Development Department, General Technology Division, Central Japan Railway Co., 1545-33 Ohyama, Komaki, Aichi 485-0801, Japan.
Results and Discussions Figure 1 illustrates the accepted structure of the (1 × 1) phase of the rutile TiO2(110) surface.23,24 The topmost layer consists of oxygen-atom rows and titanium-atom rows parallel to the [001] direction. The oxygen-atom rows are separated from each other by 0.65 nm. The height of a single step is 0.33 nm. Figure 2 shows the constant frequency-shift topography of the sputterannealed TiO2(110)-(1×1) surface observed in a vacuum of 3 × 10-8 Pa. Topographic images are presented without filtering, while cross-sections were measured on each image smoothed by a nine-point median filter. Crystalline terraces and islands separated by single-height steps were seen in the wide-scan image in Figure 2a. When the scan size was reduced to 30 × 30 nm2 in Figure 2b, oxygen-atom rows were resolved parallel to the [001] direction. Depressions occasionally interrupting the rows, some of which are marked with arrows, were assigned to oxygen-atom vacancies. In the image in Figure 2b are shown 30 such vacancies. Bright bumps marked with arrowheads are TiOx clusters frequently formed on a sputter-annealed (1 × 1) surface.25 Individual oxygen atoms were resolved in a 10 × 10-nm2 image shown in Figure 2c. The interval between atoms
10.1021/jp0484940 CCC: $27.50 © 2004 American Chemical Society Published on Web 09/14/2004
15736 J. Phys. Chem. B, Vol. 108, No. 40, 2004
Sasahara et al.
Figure 1. (1 × 1) phase of the rutile (110) surface. Small and large spheres represent Ti and O atoms, respectively. The oxygen atoms are shaded according to their depth. The dotted line shows a unit cell. Figure 3. Constant frequency-shift topography of the TiO2(110)(1 × 1) surface obtained in N2 of 105 Pa. Arrows in b and c indicate oxygen-atom vacancies. Clusters of TiOx are marked with arrowheads in b. (a) 100 × 100 nm2, ∆f ) -136 Hz, Vs ) +1.2 V, Ap-p ) 6.8 nm. (b) 30 × 30 nm2, ∆f ) -139 Hz, Vs ) +1.2 V, Ap-p ) 6.8 nm. (c) 10 × 10 nm2, ∆f ) -240 Hz, Vs ) +1.2 V, Ap-p ) 6.8 nm.
Figure 4. Oscillation amplitude of the cantilever as a function of oscillation frequency. The cantilever used in the imaging scans of Figures 2 and 3 was freely oscillated (a) in the vacuum and (b) in the ambient N2.
Figure 2. Constant frequency-shift topography of the TiO2(110)(1 × 1) surface obtained in a vacuum. Arrows in b and c indicate oxygen-atom vacancies. Clusters of TiOx are marked with arrowheads in b. (a) 100 × 100 nm2, ∆f ) -96 Hz, Vs ) +0.9 V, the peak-topeak amplitude of cantilever oscillation (Ap-p) ) 6.0 nm. (b) 30 × 30 nm2, ∆f ) -102 Hz, Vs ) +0.9 V, Ap-p ) 6.0 nm. (c) 10 × 10 nm2, ∆f ) -136 Hz, Vs ) +0.9 V, Ap-p ) 6.0 nm.
is 0.30 nm along the row axis consistent with the structure Figure 1. These features reproduced those reported in a previous study.6 The microscope chamber was filled with five-nine N2 gas of 105 Pa after the images in Figure 2 had been observed.
Constant frequency-shift topography images obtained in the ambient N2 are shown in Figure 3. The feedback regulation of the tip-sample distance was less stable than in a vacuum. The tip occasionally touched the TiO2 surface as a result. The widescan image in Figure 3a revealed steps and terraces similar to those shown in Figure 2a. In the narrow-scan images of Figure 3b,c, atom vacancies were resolved as depressions on oxygenatom rows. The image in Figure 3b has the same dimension as that in Figure 2b, and the number of O-vacancies identified (33) is almost identical. Individual atoms were not resolved along the oxygen-atom rows even in the 10 × 10-nm2 image shown in Figure 3c. Figure 4 shows the oscillation amplitude spectrum of the cantilever when it is retracted far from the surface. The amplitude peaked at resonant frequency (f0) of 308.1 and 307.4 kHz in the vacuum and in the N2 atmosphere. The full width of the peak (∆υ), where the amplitude of the oscillation reduces to 1/x2 of the maximum was 0.022 and 0.57 kHz in the
Oxygen-Atom Vacancies Imaged by Noncontact AFM
J. Phys. Chem. B, Vol. 108, No. 40, 2004 15737
Figure 5. Frequency shift as a function of bias voltage observed (a) before and (b) after the imaging of Figure 3.
respective environments. The quality factor of the mechanical oscillation (Q) is related to f0 and ∆υ as follows,
Q)
f0 ∆υ
(1)
The calculated Q ) 14 000 in the vacuum reduced to 500 in N2 due to the viscous damping of the oscillation. The atom vacancies resolved with such a dramatically reduced Q suggests that the actual resolution of the microscope is determined by noises in the frequency detection and feedback regulation circuits,26 rather than by the accuracy of the cantilever oscillation directly related with Q. The thermally induced, stochastic vibration of the cantilever perturbs the externally excited, resonant oscillation. Assuming that the end of the cantilever thermally vibrates at temperature T, the root-mean-square amplitude of the thermal vibration (∆A) is expressed as27
∆A )
x
2kBTB πkf0Q
(2)
where kB, B, and k are Boltzmann constant, the measurement bandwidth, and the spring constant of the cantilever, respectively. In our imaging scans with k ) 14 N/m, B ) 650 Hz,27,28 and T ) 300 K, ∆A was estimated to be 5 and 28 fm in the vacuum and in the ambient N2, respectively. These lengths are much less than the vertical and lateral resolutions observed in Figures 2 and 3. Finally undesired adsorption of chemicals is considered on the tip and on the surface. Possible adsorbates are H2O, O2, CO2, and light hydrocarbons from the N2 ambient. Previous work indicates these compounds are only weakly adsorbed on the TiO2 surface at room temperature.29,30 Indeed, features assignable to such adsorbed species were absent in the images of Figure 3. On the other hand, they are readily adsorbed on the tip made of silicon. To check the possible contamination on the tip, the contact potential difference between the tip and the surface was measured. The frequency shift was recorded as a function of the bias voltage with a constant tip-surface distance (Figure 5). Parts a and b of Figure 5 show the curve taken after imaging for Figure 2 and Figure 3, respectively. The parabolic ∆f - Vs curves resulted from the electrostatic force Fel. Assuming the tip and surface to be two electrodes in a Kelvin probe separated by z,
Fel ) -
1 ∂C (V - Vs)2 2 ∂z
(3)
where C and V are the capacitance and contact potential difference between the tip and surface, respectively. The observed curves presented a minimum at +0.976 V in the
vacuum and at +0.979 V in the N2 ambient. If the tip was fully covered with polarized chemicals, the contact potential difference is expected to be affected. This was not the case. The observed shift as small as 0.003 eV can be ascribed to the heterogeneous potential over the TiO2 surface31 or simply to experimental scatter. The other feature of the ∆f - Vs plot is curvature. The different curvatures in a and b reflect the tip shape perturbed during the imaging scans of Figures 2 and 3. Equation 3 relates the curvature with ∂C/∂z. Curve b was more sensitive to Vs than curve a, indicating a larger ∂C/∂z and thus a less sharp shape. The tip accidentally touched the surface to be collapsed during continued scans. Acknowledgment. The authors acknowledge helpful discussions with Dr. Chi L. Pang. This work was supported by Core Research for Evolutional Science and Technology (CREST) of Japan Science and Technology Agency (JST). References and Notes (1) Noncontact Atomic Force Microscopy; Morita, S., Wiesendanger, R., Meyer, E., Eds.; Springer: Berlin, Germany, 2002. (2) Albrecht, T. R.; Gru¨tter, P.; Horne, D.; Rugar, D. J. Appl. Phys. 1991, 69, 668. (3) Giessibl, F. J. Science 1995, 267, 68. (4) Kitamura, S.; Iwatsuki, M. Jpn. J. Appl. Phys. 1995, 34, L145. (5) Sugawara, Y.; Ohta, M.; Ueyama, H.; Morita, S. Science 1995, 270, 1646. (6) Fukui, K.; Onishi, H.; Iwasawa, Y. Phys. ReV. Lett. 1997, 79, 4202. (7) Namai, Y.; Fukui, K.; Iwasawa, Y. J. Phys. Chem. B 2003, 107, 11666. (8) Reichling, M.; Barth, C. Phys. ReV. Lett. 1999, 83, 768. (9) Foster, A. S.; Barth, C.; Shluger, A. L.; Reichling, M. Phys. ReV. Lett. 2001, 86, 2373. (10) Barth, C.; Reichling, M. Nature 2001, 414, 54. (11) Fukui, K.; Onishi, H.; Iwasawa, Y. Chem Phys. Lett. 1997, 280, 296. (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) Onishi, H.; Sasahara A.; Uetsuka, H.; Ishibashi, T. Appl. Surf. Sci. 2002, 188, 257. (15) Sasahara, A.; Uetsuka, H.; Onishi, H. Phys. ReV. B 2001, 64, 121406(R). (16) Oyabu, N.; Custance, O Ä .; Yi, I.; Sugawara, Y.; Morita, S. Phys. ReV. Lett. 2003, 90, 176102. (17) Kobayashi, K.; Yamada, H.; Matsushige, K. Appl. Surf. Sci. 2002, 188, 430. (18) Nishi, R.; Houda, I.; Kitano, K.; Sugawara, Y.; Morita, S. Appl. Phys. A 2001, 72 [Suppl.], S93. (19) Jarvis, S. P.; Uchihashi, T.; Ishida, T.; Tokumoto, H.; Nakayama, Y. J. Phys. Chem. B 2000, 104, 6091. (20) Ohnesorge, F.; Binnig G. Science 1993, 260, 1451. (21) Ertl, G. In Impact of Surface Science on Catalysis; Gates, B. C., Kno¨zinger, H., Eds.; Academic Press: San Diego, CA, 2000; p 49. (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) Henrich, V. E.; Cox, P. A. The Surface Science of Metal Oxides; Cambridge University Press: New York, 1994; p 44. (24) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (25) Bennett, R. A.; Stone, P.; Price, N. J.; Bowker, M. Phys. ReV. Lett. 1999, 19, 3831. (26) Morita, S.; Sugawara, Y. Appl. Surf. Sci. 1999, 140, 406. (27) Giessibl, F. J. In Noncontact Atomic Force Microscopy; Morita, S., Wiesendanger, R., Meyer, E., Eds.; Springer: Berlin, Germany, 2002; p 11. (28) The O atoms on the TiO2(110)-(1 × 1) surface arranged at intervals of 0.30 nm were resolved in an image obtained at a scan speed of 195 nm/s. Hence the measurement bandwidth is more than 195/0.3 ) 650 Hz. (29) Onishi, H.; Aruga, T.; Egawa, C.; Iwasawa, Y. J. Chem. Soc., Faraday Trans. 1 1989, 85, 2597. (30) Suzuki, S.; Yamaguchi, Y.; Onishi, H.; Sasaki, T.; Fukui, K.; Iwasawa, Y. J. Chem. Soc., Faraday Trans. 1998, 94, 161. (31) Sasahara, A.; Uetsuka, H.; Onishi, H. Surf. Sci. 2003, 529, L245.
