NANO LETTERS
Nanometer-Scale Patterning of Oxygen Molecules Adsorbed on TiO2 Surface by an Atomic Force Microscope with a Conductive Cantilever
2002 Vol. 2, No. 9 925-927
Kenkichiro Kobayashi,* Yasumasa Tomita, and Shinya Yoshida Department of Materials Science, Faculty of Engineering, Shizuoka UniVersity, 3-5-1, Johoku, Hamamatsu, 432-8561 Japan Received June 3, 2002; Revised Manuscript Received July 8, 2002
ABSTRACT Oxygen molecules are selectively adsorbed on an area of TiO2 surface on which the cantilever has been scanned at −5.0 V in O2 atmosphere. The spatial resolution of 22 nm is estimated from the width of a pattern constructed from adsorbed oxygen molecules. The letters “YT” consisting of adsorbed oxygen molecules are written on the TiO2 surface in an area of 1000 × 1000 nm. The letter “Y” is erased after scanning at 10 V under ultraviolet irradiation.
The manipulation of atoms on the solid surfaces by an atomic force microscope with a conductive cantilever (AFMC) has attracted considerable attention from viewpoints of the fabrication of nanometer-scale devices.1-3 The manipulation of atoms by AFMC was mainly based on field assisted techniques.4 For instance, (1) Au atoms were evaporated from the Au-coated cantilever under a strong electric field,2 and (2) semiconductor surface was oxidized by applying a positive electric field.5,6 Nevertheless, the mechanisms of field assisted reactions by AFMC have not been revealed in detail. Thus, we have studied the field-assisted adsorption of oxygen molecules on the TiO2 surface, using AFMC. In previous papers,2,3 we found that oxygen molecules were adsorbed on the TiO2 surface by scanning a conductive cantilever at a negative voltage in O2 atmosphere. In this letter, some patterns constructed from adsorbed oxygen molecules are drawn on the TiO2 surface. The spatial resolution of the adsorbed oxygen molecules is estimated from the width of the patterns. For an application to nanolithography, we demonstrate writing and erasing of letters “YT” in an area of 1000 × 1000 nm, with adsorbed oxygen molecules. For the preparation of an n-type TiO2, a single crystalline TiO2 (110) was reduced at 800 °C for 4 h in 2% H2 atmosphere. An In metal was deposited on a rear surface of the TiO2 crystal for making an ohmic contact. Topographic and conductive images were measured with an atomic force microscope (SPI 3800N, Seiko). The TiO2 crystal was placed in an ambient controlled chamber into which ultraviolet irradiation was introduced through an optical fiber. The light 10.1021/nl025629+ CCC: $22.00 Published on Web 07/17/2002
© 2002 American Chemical Society
source was a high pressure-mercury lamp of 100 W. Ultraviolet irradiation containing mainly the line at 365 nm was supplied through a filter (U-360), which was transparent at wavelengths of 300 to 400 nm. A cutoff filter, which is transparent at wavelengths > 500 nm, was set in the entrance of a photodetector. The conductive cantilever was Au-coated Si3N4 (force constant 0.18 N/m and resonance frequency of 25 kHz). For measurements of current vs voltage characteristics, a bias voltage was swept from -10.0 to 10.0 V at a sweep rate of 0.6 V/s, where a positive voltage was defined when the TiO2 was positively polarized with respect to the cantilever. Topographic and conductive images were simultaneously taken at a scanning frequency of 1 Hz by a contact mode in which a repulsive force of 0.18 × 10-9 N was maintained during scanning. Current vs voltage characteristics at the TiO2 and Aucoated cantilever were typical of the Schottky junction; no current flowed at a positive bias voltage. Distinct current was not observed at -2.0 V or less, so that conductive images were taken at -2.3 V. Topographic and conductive images of the untreated TiO2 surface in Ar atmosphere are shown in the left of Figure 1. The surface roughness of the untreated TiO2 is of order of 1 nm. As seen in the conductive image, current of about 100 nA flows over the TiO2 surface. No changes in topographic and conductive images occur after several scans at -2.3 V in Ar atmosphere. To enhance the adsorption of oxygen molecules on the TiO2 surface, an area of 200 × 200 nm around center was scanned at -5.0 V in O2 atmosphere. Subsequently the ambient gas was changed from O2 to Ar, and at the same time bias voltage was varied
Figure 1. Topographic (top) and conductive (bottom) images of the TiO2 surface before oxygen adsorption (left), after oxygen adsorption (middle), and after photodesorption of oxygen (right). Oxygen adsorption was carried out at -5.0 V in O2 atmosphere. Topographic and conductive images were taken at -2.3 V in Ar atmosphere.
from -5.0 to -2.3 V. Topographic and conductive images in an area of 1000 × 1000 nm are shown in the middle of Figure 1. A noticed feature of the topographic image is that an upheaval of 1 nm appears in the area of 200 × 200 nm on which the cantilever has been scanned at -5.0 V. In the conductive image, the zero current appears in the area of 200 × 200 nm. In the topographic image, the upheaval of 1 nm is extremely large compared with the size of an adsorbed oxygen molecule itself, so that the upheaval is ascribed to another attractive force. The plausible attractive force is due to an electrostatic force induced from negative charges of adsorbed species, e.g., O2-, O-, and O2-. As will be discussed later, adsorbed species are easily removed even at room temperature under ultraviolet irradiation, indicating no break of O-O bonds. Accordingly, O2- seems to be a reasonably adsorbed species.7 In the conductive image, zero current is caused by both an increase of the barrier height at the interface between TiO2 and the cantilever8 and an increase of the distance between TiO2 and the cantilever. The dependence of a force between TiO2 and the cantilever upon the distance suggests that the cantilever is in contact with the TiO2 surface, even when no current flows. Thus, the zero current is ascribed to the increase of the barrier height induced by adsorbed oxygen molecules. The area of 200 × 200 nm was scanned at 10 V under ultraviolet irradiation. The resultant images are shown in the right of Figure 1. No upheaval is seen at the center of the topographic image. Zero-current area disappears in the conductive image. This indicates that adsorbed oxygen 926
Figure 2. Topographic image of the TiO2 surface in a narrow area of 100 × 100 nm after the oxygen adsorption has been performed at four positions. At each position, an area of 5 × 5 nm was scanned at -5.0 V in O2 atmosphere. The insertion is the cross-section profile of the TiO2 surface on which oxygen molecules are adsorbed. Nano Lett., Vol. 2, No. 9, 2002
Figure 3. Topographic image after the vector scan in which letters “YT” was written at -5 V in O2 atmosphere (left). Topographic image after the vector scan in which letter “Y” was overwritten at +10 V in Ar atmosphere under ultraviolet irradiation (right).
molecules are removed by applying a positive voltage under ultraviolet irradiation. The desorption of oxygen molecules under ultraviolet irradiation, i.e., photodesorption, can be explained by the following mechanism: holes generated in the valence band of the TiO2 under ultraviolet irradiation are injected into the chemisorption states of O2-, and then the holes are recombined with electrons taking part in the adsorption.9 To determine the spatial resolution of adsorbed oxygen molecules, field-assisted adsorption of oxygen was carried out in a narrow area: Four areas of 5 × 5 nm have been scanned at -5.0 V in O2 atmosphere, and subsequently a topographic image in an area of 100 × 100 nm was taken at zero bias voltage in Ar atmosphere. The obtained topographic image is shown in Figure 2. A cross-section profile of the TiO2 surface, on which oxygen molecules are adsorbed, is also included in Figure 2. The cross-section profile of adsorbed oxygen molecules is bell-shape, which is deviated from a rectangle. The height is about 1 nm, and the full width at a half-maximum is 22 nm, which is about 4 times the scanning width. The broadening of the profile is ascribed to a lateral electrostatic interaction between the cantilever and the negative charge of O2-, located on the area of 5 × 5 nm. Using a vector scan in which arbitrary patterns were drawn with the cantilever, we wrote the letters “YT” on the TiO2 surface. During the vector scan, a voltage of -5.0 V was applied to the TiO2 surface in O2 atmosphere. The topographic image after the vector scan is shown in the left of Figure 3. It should be noted that the letters “YT” appear in the topographic image. The letters “YT” have the height of about 6 nm and the line width of 20 nm, which is almost the same as the spatial resolution obtained in Figure 2. To examine the selective removal of adsorbed oxygen molecules, we carried out the vector scan in which the letter “Y” was overwritten at +10 V in the dark. The vector scan leads to no changes of topographic images. In contrast, overwriting Nano Lett., Vol. 2, No. 9, 2002
“Y” at +10 V under ultraviolet irradiation induces a drastic change of a topographic image (right in Figure 3). It is worthwhile to note that the letter “Y” is erased from the topographic image, although some blurs are seen in the image. This suggests that the desorption of oxygen molecules is controlled on a nanometer scale by means of an electric field and ultraviolet irradiation. The topographic images of Figures 1, 2, and 3 are not degraded for 15 h or more in either O2 or Ar atmosphere; the nanometer-scale patterns constructed from adsorbed oxygen molecules are stable at room temperature. This means that the further adsorption of oxygen molecules is negligible at room temperature in the dark. The reason is due to low concentration of electrons at the TiO2 surface, because the Fermi level at the surface is pinned at the deep levels of the chemisorption states of O2-. Such stable patterns may be applicable to memory storage. As seen in Figure 2, four patterns can be memorized in the area of 100 × 100 nm; the memory density corresponds to 250 Gbits/inch.2 The memory density may increase by improving the spatial resolution of adsorbed oxygen molecules. References (1) Kobayashi, K. J. Mater. Synth. Process. 1998, 6, 249. (2) Kobayashi, K.; Tomita, Y.; Yoshida, S. Trans. Mater. Res. Soc. Jpn. 2000, 25, 253. (3) Kobayashi, K.; Tomita, Y.; Morimoto, J. Mater. Sci. Lett. 2000, 19, 173. (4) Po¨tzschke, R. T.; Staikov, G.; Lorenz, W.J.; Wiesbeck, W. J. Electrochem. Soc. 1999, 146, 141. (5) Yasutake, M.; Ejiri, Y.; Hattori, T. Jpn. J. Appl. Phys. 1993, 32, L1021. (6) Matsuzaki, Y.; Yamada, A.; Konagai, M. J. Cryst. Growth, 2000, 209, 509. (7) Lu, G.; Linsebigler; A.; Yates, J. T., Jr. J. Chem. Phys. 1995, 102, 4657. (8) Kobayashi, K.; Nakata, H.; Matsushima, S.; Okada, G. J. Phys. Chem. 1995, 99, 999. (9) Perkins, C. L.; Henderson, M. A. J. Phys. Chem. B 2001, 105, 3856.
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