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Interaction of oxygen vacancies with O2 on a reduced SrTiO3(100)√5 × √5-R26.6° surface observed by STM
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Surface Science 318 (1994) 29-38
Interaction of oxygen vacancies with 0, on a reduced SrTiO,( 1OO)E X /%R26.6” surface observed by STM Hiroyuki Tanaka *, Takuya Matsumoto, Tomoji Kawai, Shichio Kawai TheInstitute of Scientific and Industrial Research, Osaka University, Mihogaoka, Ibaraki, Osaka 567 Japan Received
21 January; accepted for publication
27 June 1994
Abstract We have performed real space and real time observations of oxygen adsorption on a reduced SrTiO,(lOO)fi fi-R26.6” surface by scanning tunneling microscopy @TM) in ultrahigh vacuum. The STM image of adsorbed oxygen is observed as a hole on the 6 X 6 surface. Two adsorption phases have been observed. The initial phase is for the coverage up to about 0.01 ML (oxygen pressure (PO,) = 1 X 10v9 Torr): STM images have revealed that adsorbed oxygen migrate and/or desorb without oxidized oxygen vacancy defects in this surface, suggesting that oxygen is adsorbed as an 0, molecule with adsorption energy E, = 0.9 eV. The STM image can be understood by electron transfer from an oxygen vacancy to adsorbed oxygen. The second phase (PO, > 1 X 10m9 Torr) is for the coverage greater than 0.01 ML, in which adsorbed oxygen form islands and irreversibly oxidize the oxygen vacancy defects. X
1. Introduction
The surface of strontium titanate (SrTiO,) has been thoroughly studied due to its variety of roles in heterogeneous catalysis, photocatalysis, and so on [l]. In this material, oxygen vacancy defects on the surface are especially important as centers for chemical reaction. For these reasons, several types of chemisorption of atoms and molecules have been studied in terms of surface electronic and geometric structures [2,3]. Oxygen adsorption on the surface defect sites is especially interesting, since the adsorbed oxygen is expected to interact with the surface oxygen vacancies (O-vacancies).
* Corresponding
author.
0039-6028//%07.00 0 19 Elsevier Science B.V. All rights reserved SSDI 0039-6028(94)00391-L
Ultraviolet photoemission spectroscopy (UPS) studies have found that most of the insulating oxide surfaces are inert to 0, molecules [3]. The reason for this inertness to 0, must partly be that a ligand field is required to stabilize the O*electronic configuration. Another reason for the inertness of insulating oxide surfaces to 0, must be the absence of any electrons on the surface ion that are available for the transfer to adsorbed oxygen. On the other hand, conducting oxides having partially filled cation bands behave quite differently from insulating oxides. 0, is found to interact strongly with the defect surface of all metal oxides that have been studied 131. The excess electronic charge on the cations adjacent to an O-vacancy is available for transfer to adsorbed oxygen. O-vacancy sites make possible the disso-
H. Tanaka et al. / Surfhce Science 318
30
ciation of 0, even at or below room temperature. The 0 atoms replace the missing lattice 02- ions and thus heal out the oxygen vacancy defects [4]. The study of oxide surfaces is in its infancy partially due to a scarcity of experimental results. Particularly the initial stage of adsorption on the surface remains unclear. Some of the reasons for this are the difficulty of obtaining single crystals and the problems inherent in preparing wellcharacterized surfaces. Moreover, the point defects considerably influence the electronic structure of oxide surfaces and adsorption reactions at the surfaces as well. Previous methods such as UPS and low energy electron diffraction (LEED) integrate the local information over a lateral sampling area up to several pm. To elucidate a microscopic mechanism of surface reaction including point defects, the method with atomically resolved and real-time characterization of the surfaces is required. For this purpose real time scanning tunneling microscopy (STM) during adsorption on clean surfaces provides the atomistic mechanism of adsorption and reaction of oxide surface. We have already reported atomic-scale resolution STM/ STS observations of a clean surface of SrTiO,(lOO) annealed in UHV. Fig. 1 shows a
l
0
T i
O
(.:,I:) 0
0
vacancy Ti - 0 vacancy complex
Fig. 1. A model of the reduced SrTiO (100) surface which has O-vacancy defects ordering in fi X $.5 m the TiO, layer. The STM image corresponds to the localized surface state arising from the O-vacancy defects inside the gap.
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lnm Fig. 2. An atomic-scale-resolution constant current image of a 5 nm by 5 nm region of the SrTiO,(lOO)fix fi-R26.6 surface taken with a sample bias of -3.0 V and a tunneling current of 0.3 nA. The maxima in the STM image correspond to the localized surface state arising from the ordering of the O-vacancy defects in the TiO, top layer.
model of the 6 x fi-R26.6” structure in the TiO, top layer which has ordered O-vacancies [5]. The STM image of the 6 X fi structure corresponds to the localized surface state inside the gap arising from the O-vacancy defects. In this paper, we report the first real space observation of interaction of oxygen vacancies with 0, on SrTiO,(lOO)& X 6-R26.6” by STM carried out in ultrahigh vacuum (UHV) that shows the existence of two adsorbed phases. The initial phase is obtained for coverages up to about 0.01 ML; STM images have revealed that adsorbed 0, migrate and/or desorb without oxidizing Ovacancy defects, suggesting that 0, is adsorbed as a molecule. For coverages greater than 0.01 ML, the second phase, adsorbed oxygen forms islands and oxidizes the O-vacancy defects.
2. Experimental The STM measurement was performed using a USM301 (UNISOKU, Japan). The experiments
H. Tanaka et al. /Surface
were conducted in a UHV chamber at a base pressure of 5 x 10-i’ Torr. Mechanically formed Pt-Ir tips were used. The m-scale was calibrated by imaging a well-known Si(lll)-7 X 7 surface [6]. The STM chamber was equipped with a preparation chamber for sample heating and various surface treatments. RHEED experiments were conducted in the preparation chamber using the 25 kV beam from a RHG-1000 (PHYSITEC, Japan) electron gun. Polished and (NO)-oriented, plate-shaped crystals of SrTiO, were purchased from Earth-Jewelry Co. (Japan). The SrTiO, crystal was clamped on
(4
Science 318 (1994) 29-38
31
a Si heater mounted on a holder made of Ta and MO. The sample was preheated for degassing in the preparation chamber and annealed in the STM chamber at 1200°C for 2 min. The sample temperature was measured with an optical pyrometer. The chamber pressure during annealing did not exceed 1 X lo-* Torr. After the annealing, the sample was transferred to the STM head. It was possible to obtain STM images with atomic resolution within 3 h after the annealing [5]. Subsequently, oxygen adsorption experiments have been performed during STM imaging at room temperature.
M
c Loi
I
I
I
7nm
1
Fig. 3. A sequence of constant current STM images of the same area (20 nm by 20 nm) observed (a) before the start of oxygen exposure (V, = - 1 V, Zt = 0.2 nA). (b) 5 min after (V, = -0.5 V, Z, = 0.2 1~41and Cc) 13 min after (V, = - 1 V, Z, = 0.2 nA) the introduction of oxygen at Paz > 1 X 10e9 Torr.
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et al. /Surface
3. Results and discussion 3.1. Stage I: low 0, pressure exposure An atomic image of the clean surface of SrTiO,(lOO)& X &-R26.6” is shown in Fig. 2. This surface structure in the STM image corresponds to the ordered O-vacancy defects which induce the surface defect state inside the band gap El. Exposure to oxygen on this surface, at an oxygen pressure (PO,) of 1 x lop9 Torr and at room temperature during STM imaging, leads to reproducible changes in the STM images. Oxygen adsorption sites are observed as holes in the STM images as indicated with C, D and E in Fig. 3. The adsorbed oxygen change their adsorption sites and/or desorb from the surface. Figs. 3a, 3b and 3c are the images recorded before 0, ad-
Science 318 (1994) 29-38
sorption, 5 and 13 min after oxygen introduction at PO, = 1 X lop9 Torr, respectively. Dark spots indicated with A, B on the image in Fig. 3a are contaminants which are utilized as landmarks because they do not change their location on the surface. After the introduction of oxygen, a hole indicated with C appears in Fig. 3b and disappears as shown in Fig. 3c. Moreover, holes indicated with D and E newly appear in Fig. 3c and disappeared in the next image, which is not shown. The time evolution in these images suggests that adsorbed oxygen migrates on the surface and/or desorbs from the surface. The coverage (0) of adsorbed oxygen has reached equilibrium around 0.01 ML, under the conditions of oxygen exposure at PO, = 1 X lo-” Torr. The values, 0 = 0.01 ML and PO, = 1 X lo-” Torr give the order of the mean surface stay-time (7,) of 10 s (we assume the sticking coefficient is unity). The adsorption energy (E,) is derived from the equation 7a = 7. exp( -E,/kJ)
Fig. 4. The potential energy diagram for interaction steps between the SrTi0,(100)6 X fi-R26.6” surface and oxygen. This diagram is based on a diagram in Ref. [4] and our experimental results. Vo,, denotes surface oxygen vacancy and O,,, lattice oxygen at the surface.
,
where 7. is the constant of the period of vibration nominal to the surface which is estimated to be lo-” s at room temperature from some other experiments [8]. Using this equation, E, is estimated to be about 0.9 eV, which is quite a bit smaller than the Ti-0 bonding energy of 6.9 eV. It is suggested that oxygen adsorbs as a molecule on the surface under the conditions of 0 = 0.01 ML
H. Tanaka et al. /Surface
square lattice and is larger than that of an adsorbed oxygen image of a metal surface [71. Cross sections of these holes, shown underneath Figs. 5a and 5b indicate that the corrugations of the clean surfaces are about 0.025 nm, while the depths of the holes are about 0.06 and 0.08 nm, respectively. Although these values depend a little on the STM bias voltage, the values of the holes are much larger than that of the corrugation of the clean surface. The STM images of the 6 x 6 pattern correspond to the state of the periodic surface defects inside the gap induced from the ordered O-vacancy [5]. Therefore, the appearance of the dark spot represents the electron charge transfer from the O-vacancy defect states into adsorbed 0, molecules, that drive electrons away from the Fermi level resulting in no state inside the band gap. This picture is consistent with the kinetical experimental result.
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3.2. Stage II: higher 0, exposure In order to increase the oxygen coverage, PO, is raised to 1 X lo-* Torr. At this oxygen pressure, oxygen coverage increases in proportion to exposure time because the oxygen adsorption and desorption do not attain equilibrium. Fig. 6a shows an image obtained at a coverage of about 0.1 ML. At this coverage it was difficult to resolve and count the individual adsorbed oxygens because STM imaging becomes unstable. This difficulty is caused by the increase of an insulative region of the surface with increasing oxygen coverage. After recording the image of Fig. 6a, oxygen introduction is ceased in order to examine the behavior of adsorbed oxygen depending on various island sizes. Sequential STM imaging in the same region of the surface shows that oxygen coverage converges in a few minutes after the
(b)
(a)
0.1 511
0.00;
I
4 Xhm
8 Xhm
Fig. 5. (a) Filled (V, = - 1 V, 2, = 0.3 nA) and (b) empty (V, = 1 V, I, = 0.3 nA) state STM images of (a) 9 nm by 9 nm and (b) 10 nm by 10 nm areas with cross sections of the images underneath, respectively. Oxygen adsorption sites are resolved as holes in both states. These holes are much deeper than the corrugations of the fi x 6 structure.
H. Tanaka et al. /Surface
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Science 318 (1994) 29-38
(a)
03
island
I
I
1Onm Fig. 6. A sequence of current STM images (V, = 1 V, I, = 0.2 nA) of the same area (44 nm by 44 nm) observed (a) during oxygen introduction Paz > 1 x lo-’ Torr (0 = 0.1 ML) and (b) 20 min after the cessation of oxygen introduction.
cessation of oxygen introduction and that adsorbed oxygen which has not desorbed does not migrate on the surface. Fig. 6b is an image obtained 20 min after the cessation of oxygen introduction. This image is of the same region as that
size
Fig. 7. (a) A current STM image of a 44 nm by 44 nm region (V, = - 1 V, It = 0.2 nA) about 50 nm apart from the scanned area of Figs. 5a and 5b recorded at about 1 h after the cessation of oxygen introduction. (b) Island size distribution on the image of (a), the mean island size is 3.8 unit of the Js X 6 square cell.
of Fig. 6a. There are oxygen and oxygen islands which do not migrate or desorbe in Fig. 6b. This result suggests that the adsorption energy of these remaining oxygen and oxygen islands should be larger than that of the adsorbed oxygen molecules observed in stage I. Therefore, we believe that the dark spots of these remaining oxygen and oxygen islands correspond to the oxygen vacancy sites that have been oxidized due to the lowering
Fig. 8. Current STM image (V, = 1 V, I, = 0.2 nA) of a 44 nm by 44 nm region observed (a) 2 min (0 = 0.08 ML); (b) 4 min (0 = 0.13 ML); (c) 6 min (0 = 0.2 ML); (d) 8 min (0 > 0.3 ML); (e) 10 min (0 > 0.4 ML) after the introduction of oxygen at Paz > 1 X 10-s Torr.
H. Tanaka et al. /Surface
ro211
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(d)
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H. Tanaka et al. /Surface
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(b) ’ 060,
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island size
12
3 4
Science 318 (1994) 29-38
result indicates that the operation of STM imaging causes no effect upon the mechanism of formation of the oxygen and oxygen islands. However, there is a difference between Fig. 6b and Fig. 7a which arises from whether STM imaging scans were performed or not during oxygen introduction. Many bright spots with a lateral dimension of the order of 0.5 to 2 nm appear in Fig. 6b during the STM imaging. On the other hand, the number of such bright spots in Fig. 7a is smaller than in Fig. 6a. We believe these bright spots are the material transferred from the STM tip because STM imaging is unstable under the oxygen exposure. Furthermore, we have obtained images with higher resolution to analyze the oxygen island formation. We have conducted oxygen adsorption at PO, = 1 x 10e8 Torr for over 10 min during STM imaging. Figs. Sa-8e are images recorded 2,
5 6 7 6 9x)1112l9l415161716192021~~
i stand size Fig. 9. Island size distributions for the image of Figs. 7a and 7b (white bars) are compared with that of the calculated distribution (dark bars) by the random adsorption model at the coverage corresponding to each of the images.
of a potential barrier (Fig. 41 between chemisorption and oxidation by forming oxygen islands. There is a possibility that the behavior of these adsorbed oxygens is affected by the STM imaging operation such as a strong electric field between the sample and the tip. To examine this effect, a STM image of the region 50 nm apart from the scanning area of Figs. 6a and 6b is also observed, as shown in .Fig. 7a. Fig. 7b shows an island distribution of Fig. 7a. The unit of the island size is the unit cell of the 6 x d? structure. The islands include next-nearest neighbor sites. In Figs. 7a and 7b, oxygen and oxygen islands that do not migrate or desorb are observed. This
(b)
0.61-1
Xhm Fig. 10. (a) Constant current STM image (V, = 1 V, I, = 0.2 nA) of a 100 nm by 100 nm area of the surface exposed to oxygen at 1 x lo-* Torr for over 10 min. (b) A cross section of the surface indicated by the line in (a), indicating the surface roughness of 0.1 to 0.2 nm and a single unit cell (0.4 nm) step.
H. Tanaka et al. /Surface
4, 6, 8 and 10 min after beginning the introduction of oxygen at PO, = 1 X lo-* Torr. The island size distributions of Figs. 8a, 8b and 8c are compared with the results of calculations assuming a random adsorption model, as shown in Figs. 9a, 9b and 9c, respectively. In this model, migration and desorption of oxygen are neglected, i.e., once absorbed oxygen will not migrate or desorb. Adsorption on the occupied site is also neglected, i.e., oxygen impinging on an adsorbed site will be returned into vacuum leaving no effect on the gas-solid interface. The unit of the island size is the unit cell of the 6 x 6 structure. The is-
37
Science 318 (1994) 29-38
Table 1 The mean island sizes C&,) of Fig. 8 (a) (0 = 0.08 ML), (b) (0 = 0.13 ML) and (cl (~9= 0.2 ML) are compared with calculated results <&,J by the random adsorption model at each of the coverages B/ML 0.08 0.13 0.2
1.5 2.3 3.7
1.4 1.7 2.8
lands include next-nearest neighbor sites. At any coverage, the experimental distribution shifts to a large island size in comparison with the calcu-
Fig. 11. RHEED patterns from the SrTiO,(lOO)~ X 6-R26.6” surface observed with the electron beam incident along the 10211 azimuth, (a) before the introduction of oxygen and (b) 10 min after the introduction of oxygen at 1 X lo-’ Torr.
38
H. Tanaka et al. /Surfnce
lated distribution, as shown in Fig. 9. In addition, the mean island size at each oxygen coverage is represented in Table 1. At low coverage (0 = 0.08 ML), the s,,, is slightly larger than so, with increasing 8. These deviations imply that once oxygen forms large islands, adsorbed oxygen scarcely desorbs from the surface regardless of the cessation of adsorption. With further increasing of the coverage, STM imaging became unstable and atomic resolution is no longer obtained, as shown in Figs. 8d and 8e. Figs. 10a and lob show a STM image and its cross section after oxygen adsorption of about 1 ML. Although a surface step of a single unit cell (0.4 nm) of SrTiO, is observed, atomic-scale resolution cannot be obtained. This image shows that the fi x 6 structure has completely disappeared due to an oxygen adsorption of only 1 ML. From the cross section, the roughnesses are the order of 0.1 to 0.2 nm, which is about one order larger than those of corrugation of the 6 x 6 structure (see Fig. 5). This result suggests that the surface defect level from the Ovacancy vanished and surface conductivity became inhomogeneous by the oxygen adsorption on the fi x 6 surface. For this reason, we consider that adsorbed 0, molecules dissociate and oxidize the O-vacancy defects after forming islands. To elucidate the structure of this surface, RHEED measurements are conducted under the same conditions of the STM experiment. Figs. lla and llb show RHEED patterns with the electron beam incident along the [021] azimuth before and after oxygen adsorption, respectively. The pattern observed before introduction of oxygen consists of streaks lying on a zero-order Laue circle and of spots on a first-order Laue circle corresponding to the periodicity d = 0.87 nm of the fi X 6 structure. Upon introduction of oxygen PO, = 1 x lo-’ Torr, l/5 order streaks and spots from the fi x fi structure have become weak in intensity, while streaks and spots corresponding to the 1 X 1 structure have not changed. This indicates that the oxidation of O-vacancy defects extinguish the fi x 6 reconstructed structure; however, the surface retains a fundamental 1 X 1 structure.
Science 318 (1994) 29-38
4. Conclusion We have performed real space and real time STM observations of oxygen adsorption on a SrTi0,(100)6 x fi-R26.6” surface in UHV. The STM image of adsorbed oxygen is observed as a hole on the 6 x 6 surface. We found the existence of two adsorption phases depending upon the pressure of oxygen exposure. The initial phase is obtained for coverages up to about 0.01 ML (PO, = 1 x low9 Torr); STM images reveal that oxygen adsorbates migrate and/or desorb without interacting strongly with O-vacancy defects, suggesting that 0, is adsorbed as a molecule. At this PO,, the oxygen coverages reach equilibrium around 0.01 ML. From this result, the oxygen adsorption energy E, is estimated to be about 0.9 eV. For coverages greater than 0.01 ML (PO, > 1 X 10v9 Torr), the second phase is formed; that is, oxygen adsorbates form islands which never desorb from the surface even stopping oxygen exposure. These oxygen islands oxidize the Ovacancy defects. However, this oxidation occurs retaining the unit cell step and the 1 X 1 structure without ruining out the fundamental unit cell structure at the surface. We believe that STM studies will reveal the electronic properties and geometric structure of oxide surfaces at atomic level and that this information will provide more insight into chemical reaction on the oxide surface.
References PI J.G. Mavroides, J.A. Kafalas and D.F. Kolisar, Appl. Phys. Lett. 28 (1976) 241.
f21M. Tsukada, H. Adachi and C. Satoko, Prog. Surf. Sci. 14 (1983) 113. [31 V.E. Henrich, Rep. Prog. Phys. 48 (1985) 1481. [41 W. Gijpel, G. Rocker and R. Feierabend, Phys. Rev. B 28 (1983) 3427. El T. Matsumoto, H. Tanaka, T. Kawai and S. Kawai, Surf. Sci. 278 (1992) L153; H. Tanaka, T. Matsumoto, T. Kawai and S. Kawai, Jpn. J. Appl. Phys. 32 (1992) 1405. 161 G. Binig, H. Rohrer, Ch. Gerber and E. Weibel, Phys. Rev. Lett. 50 (1983) 120. [71 E. Kopatzki and R.J. Behm, Surf. Sci. 245 (1991) 255. B1 J. Frenkel, Z. Phys. 26 (1924) 117.
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Surface Science 318 (1994) 29-38
Interaction of oxygen vacancies with 0, on a reduced SrTiO,( 1OO)E X /%R26.6” surface observed by STM Hiroyuki Tanaka *, Takuya Matsumoto, Tomoji Kawai, Shichio Kawai TheInstitute of Scientific and Industrial Research, Osaka University, Mihogaoka, Ibaraki, Osaka 567 Japan Received
21 January; accepted for publication
27 June 1994
Abstract We have performed real space and real time observations of oxygen adsorption on a reduced SrTiO,(lOO)fi fi-R26.6” surface by scanning tunneling microscopy @TM) in ultrahigh vacuum. The STM image of adsorbed oxygen is observed as a hole on the 6 X 6 surface. Two adsorption phases have been observed. The initial phase is for the coverage up to about 0.01 ML (oxygen pressure (PO,) = 1 X 10v9 Torr): STM images have revealed that adsorbed oxygen migrate and/or desorb without oxidized oxygen vacancy defects in this surface, suggesting that oxygen is adsorbed as an 0, molecule with adsorption energy E, = 0.9 eV. The STM image can be understood by electron transfer from an oxygen vacancy to adsorbed oxygen. The second phase (PO, > 1 X 10m9 Torr) is for the coverage greater than 0.01 ML, in which adsorbed oxygen form islands and irreversibly oxidize the oxygen vacancy defects. X
1. Introduction
The surface of strontium titanate (SrTiO,) has been thoroughly studied due to its variety of roles in heterogeneous catalysis, photocatalysis, and so on [l]. In this material, oxygen vacancy defects on the surface are especially important as centers for chemical reaction. For these reasons, several types of chemisorption of atoms and molecules have been studied in terms of surface electronic and geometric structures [2,3]. Oxygen adsorption on the surface defect sites is especially interesting, since the adsorbed oxygen is expected to interact with the surface oxygen vacancies (O-vacancies).
* Corresponding
author.
0039-6028//%07.00 0 19 Elsevier Science B.V. All rights reserved SSDI 0039-6028(94)00391-L
Ultraviolet photoemission spectroscopy (UPS) studies have found that most of the insulating oxide surfaces are inert to 0, molecules [3]. The reason for this inertness to 0, must partly be that a ligand field is required to stabilize the O*electronic configuration. Another reason for the inertness of insulating oxide surfaces to 0, must be the absence of any electrons on the surface ion that are available for the transfer to adsorbed oxygen. On the other hand, conducting oxides having partially filled cation bands behave quite differently from insulating oxides. 0, is found to interact strongly with the defect surface of all metal oxides that have been studied 131. The excess electronic charge on the cations adjacent to an O-vacancy is available for transfer to adsorbed oxygen. O-vacancy sites make possible the disso-
H. Tanaka et al. / Surfhce Science 318
30
ciation of 0, even at or below room temperature. The 0 atoms replace the missing lattice 02- ions and thus heal out the oxygen vacancy defects [4]. The study of oxide surfaces is in its infancy partially due to a scarcity of experimental results. Particularly the initial stage of adsorption on the surface remains unclear. Some of the reasons for this are the difficulty of obtaining single crystals and the problems inherent in preparing wellcharacterized surfaces. Moreover, the point defects considerably influence the electronic structure of oxide surfaces and adsorption reactions at the surfaces as well. Previous methods such as UPS and low energy electron diffraction (LEED) integrate the local information over a lateral sampling area up to several pm. To elucidate a microscopic mechanism of surface reaction including point defects, the method with atomically resolved and real-time characterization of the surfaces is required. For this purpose real time scanning tunneling microscopy (STM) during adsorption on clean surfaces provides the atomistic mechanism of adsorption and reaction of oxide surface. We have already reported atomic-scale resolution STM/ STS observations of a clean surface of SrTiO,(lOO) annealed in UHV. Fig. 1 shows a
l
0
T i
O
(.:,I:) 0
0
vacancy Ti - 0 vacancy complex
Fig. 1. A model of the reduced SrTiO (100) surface which has O-vacancy defects ordering in fi X $.5 m the TiO, layer. The STM image corresponds to the localized surface state arising from the O-vacancy defects inside the gap.
(1994)
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lnm Fig. 2. An atomic-scale-resolution constant current image of a 5 nm by 5 nm region of the SrTiO,(lOO)fix fi-R26.6 surface taken with a sample bias of -3.0 V and a tunneling current of 0.3 nA. The maxima in the STM image correspond to the localized surface state arising from the ordering of the O-vacancy defects in the TiO, top layer.
model of the 6 x fi-R26.6” structure in the TiO, top layer which has ordered O-vacancies [5]. The STM image of the 6 X fi structure corresponds to the localized surface state inside the gap arising from the O-vacancy defects. In this paper, we report the first real space observation of interaction of oxygen vacancies with 0, on SrTiO,(lOO)& X 6-R26.6” by STM carried out in ultrahigh vacuum (UHV) that shows the existence of two adsorbed phases. The initial phase is obtained for coverages up to about 0.01 ML; STM images have revealed that adsorbed 0, migrate and/or desorb without oxidizing Ovacancy defects, suggesting that 0, is adsorbed as a molecule. For coverages greater than 0.01 ML, the second phase, adsorbed oxygen forms islands and oxidizes the O-vacancy defects.
2. Experimental The STM measurement was performed using a USM301 (UNISOKU, Japan). The experiments
H. Tanaka et al. /Surface
were conducted in a UHV chamber at a base pressure of 5 x 10-i’ Torr. Mechanically formed Pt-Ir tips were used. The m-scale was calibrated by imaging a well-known Si(lll)-7 X 7 surface [6]. The STM chamber was equipped with a preparation chamber for sample heating and various surface treatments. RHEED experiments were conducted in the preparation chamber using the 25 kV beam from a RHG-1000 (PHYSITEC, Japan) electron gun. Polished and (NO)-oriented, plate-shaped crystals of SrTiO, were purchased from Earth-Jewelry Co. (Japan). The SrTiO, crystal was clamped on
(4
Science 318 (1994) 29-38
31
a Si heater mounted on a holder made of Ta and MO. The sample was preheated for degassing in the preparation chamber and annealed in the STM chamber at 1200°C for 2 min. The sample temperature was measured with an optical pyrometer. The chamber pressure during annealing did not exceed 1 X lo-* Torr. After the annealing, the sample was transferred to the STM head. It was possible to obtain STM images with atomic resolution within 3 h after the annealing [5]. Subsequently, oxygen adsorption experiments have been performed during STM imaging at room temperature.
M
c Loi
I
I
I
7nm
1
Fig. 3. A sequence of constant current STM images of the same area (20 nm by 20 nm) observed (a) before the start of oxygen exposure (V, = - 1 V, Zt = 0.2 nA). (b) 5 min after (V, = -0.5 V, Z, = 0.2 1~41and Cc) 13 min after (V, = - 1 V, Z, = 0.2 nA) the introduction of oxygen at Paz > 1 X 10e9 Torr.
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et al. /Surface
3. Results and discussion 3.1. Stage I: low 0, pressure exposure An atomic image of the clean surface of SrTiO,(lOO)& X &-R26.6” is shown in Fig. 2. This surface structure in the STM image corresponds to the ordered O-vacancy defects which induce the surface defect state inside the band gap El. Exposure to oxygen on this surface, at an oxygen pressure (PO,) of 1 x lop9 Torr and at room temperature during STM imaging, leads to reproducible changes in the STM images. Oxygen adsorption sites are observed as holes in the STM images as indicated with C, D and E in Fig. 3. The adsorbed oxygen change their adsorption sites and/or desorb from the surface. Figs. 3a, 3b and 3c are the images recorded before 0, ad-
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sorption, 5 and 13 min after oxygen introduction at PO, = 1 X lop9 Torr, respectively. Dark spots indicated with A, B on the image in Fig. 3a are contaminants which are utilized as landmarks because they do not change their location on the surface. After the introduction of oxygen, a hole indicated with C appears in Fig. 3b and disappears as shown in Fig. 3c. Moreover, holes indicated with D and E newly appear in Fig. 3c and disappeared in the next image, which is not shown. The time evolution in these images suggests that adsorbed oxygen migrates on the surface and/or desorbs from the surface. The coverage (0) of adsorbed oxygen has reached equilibrium around 0.01 ML, under the conditions of oxygen exposure at PO, = 1 X lo-” Torr. The values, 0 = 0.01 ML and PO, = 1 X lo-” Torr give the order of the mean surface stay-time (7,) of 10 s (we assume the sticking coefficient is unity). The adsorption energy (E,) is derived from the equation 7a = 7. exp( -E,/kJ)
Fig. 4. The potential energy diagram for interaction steps between the SrTi0,(100)6 X fi-R26.6” surface and oxygen. This diagram is based on a diagram in Ref. [4] and our experimental results. Vo,, denotes surface oxygen vacancy and O,,, lattice oxygen at the surface.
,
where 7. is the constant of the period of vibration nominal to the surface which is estimated to be lo-” s at room temperature from some other experiments [8]. Using this equation, E, is estimated to be about 0.9 eV, which is quite a bit smaller than the Ti-0 bonding energy of 6.9 eV. It is suggested that oxygen adsorbs as a molecule on the surface under the conditions of 0 = 0.01 ML
H. Tanaka et al. /Surface
square lattice and is larger than that of an adsorbed oxygen image of a metal surface [71. Cross sections of these holes, shown underneath Figs. 5a and 5b indicate that the corrugations of the clean surfaces are about 0.025 nm, while the depths of the holes are about 0.06 and 0.08 nm, respectively. Although these values depend a little on the STM bias voltage, the values of the holes are much larger than that of the corrugation of the clean surface. The STM images of the 6 x 6 pattern correspond to the state of the periodic surface defects inside the gap induced from the ordered O-vacancy [5]. Therefore, the appearance of the dark spot represents the electron charge transfer from the O-vacancy defect states into adsorbed 0, molecules, that drive electrons away from the Fermi level resulting in no state inside the band gap. This picture is consistent with the kinetical experimental result.
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33
3.2. Stage II: higher 0, exposure In order to increase the oxygen coverage, PO, is raised to 1 X lo-* Torr. At this oxygen pressure, oxygen coverage increases in proportion to exposure time because the oxygen adsorption and desorption do not attain equilibrium. Fig. 6a shows an image obtained at a coverage of about 0.1 ML. At this coverage it was difficult to resolve and count the individual adsorbed oxygens because STM imaging becomes unstable. This difficulty is caused by the increase of an insulative region of the surface with increasing oxygen coverage. After recording the image of Fig. 6a, oxygen introduction is ceased in order to examine the behavior of adsorbed oxygen depending on various island sizes. Sequential STM imaging in the same region of the surface shows that oxygen coverage converges in a few minutes after the
(b)
(a)
0.1 511
0.00;
I
4 Xhm
8 Xhm
Fig. 5. (a) Filled (V, = - 1 V, 2, = 0.3 nA) and (b) empty (V, = 1 V, I, = 0.3 nA) state STM images of (a) 9 nm by 9 nm and (b) 10 nm by 10 nm areas with cross sections of the images underneath, respectively. Oxygen adsorption sites are resolved as holes in both states. These holes are much deeper than the corrugations of the fi x 6 structure.
H. Tanaka et al. /Surface
34
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(a)
03
island
I
I
1Onm Fig. 6. A sequence of current STM images (V, = 1 V, I, = 0.2 nA) of the same area (44 nm by 44 nm) observed (a) during oxygen introduction Paz > 1 x lo-’ Torr (0 = 0.1 ML) and (b) 20 min after the cessation of oxygen introduction.
cessation of oxygen introduction and that adsorbed oxygen which has not desorbed does not migrate on the surface. Fig. 6b is an image obtained 20 min after the cessation of oxygen introduction. This image is of the same region as that
size
Fig. 7. (a) A current STM image of a 44 nm by 44 nm region (V, = - 1 V, It = 0.2 nA) about 50 nm apart from the scanned area of Figs. 5a and 5b recorded at about 1 h after the cessation of oxygen introduction. (b) Island size distribution on the image of (a), the mean island size is 3.8 unit of the Js X 6 square cell.
of Fig. 6a. There are oxygen and oxygen islands which do not migrate or desorbe in Fig. 6b. This result suggests that the adsorption energy of these remaining oxygen and oxygen islands should be larger than that of the adsorbed oxygen molecules observed in stage I. Therefore, we believe that the dark spots of these remaining oxygen and oxygen islands correspond to the oxygen vacancy sites that have been oxidized due to the lowering
Fig. 8. Current STM image (V, = 1 V, I, = 0.2 nA) of a 44 nm by 44 nm region observed (a) 2 min (0 = 0.08 ML); (b) 4 min (0 = 0.13 ML); (c) 6 min (0 = 0.2 ML); (d) 8 min (0 > 0.3 ML); (e) 10 min (0 > 0.4 ML) after the introduction of oxygen at Paz > 1 X 10-s Torr.
H. Tanaka et al. /Surface
ro211
I
I
7nm UN
(d)
c
Loi-
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(4
35
H. Tanaka et al. /Surface
36 o.eo-
(4
8E 060. 2 0.40. 8
& 0.20. 91
o.oo-
island size
(b) ’ 060,
E’. 2 0.40. 8
island size
12
3 4
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result indicates that the operation of STM imaging causes no effect upon the mechanism of formation of the oxygen and oxygen islands. However, there is a difference between Fig. 6b and Fig. 7a which arises from whether STM imaging scans were performed or not during oxygen introduction. Many bright spots with a lateral dimension of the order of 0.5 to 2 nm appear in Fig. 6b during the STM imaging. On the other hand, the number of such bright spots in Fig. 7a is smaller than in Fig. 6a. We believe these bright spots are the material transferred from the STM tip because STM imaging is unstable under the oxygen exposure. Furthermore, we have obtained images with higher resolution to analyze the oxygen island formation. We have conducted oxygen adsorption at PO, = 1 x 10e8 Torr for over 10 min during STM imaging. Figs. Sa-8e are images recorded 2,
5 6 7 6 9x)1112l9l415161716192021~~
i stand size Fig. 9. Island size distributions for the image of Figs. 7a and 7b (white bars) are compared with that of the calculated distribution (dark bars) by the random adsorption model at the coverage corresponding to each of the images.
of a potential barrier (Fig. 41 between chemisorption and oxidation by forming oxygen islands. There is a possibility that the behavior of these adsorbed oxygens is affected by the STM imaging operation such as a strong electric field between the sample and the tip. To examine this effect, a STM image of the region 50 nm apart from the scanning area of Figs. 6a and 6b is also observed, as shown in .Fig. 7a. Fig. 7b shows an island distribution of Fig. 7a. The unit of the island size is the unit cell of the 6 x d? structure. The islands include next-nearest neighbor sites. In Figs. 7a and 7b, oxygen and oxygen islands that do not migrate or desorb are observed. This
(b)
0.61-1
Xhm Fig. 10. (a) Constant current STM image (V, = 1 V, I, = 0.2 nA) of a 100 nm by 100 nm area of the surface exposed to oxygen at 1 x lo-* Torr for over 10 min. (b) A cross section of the surface indicated by the line in (a), indicating the surface roughness of 0.1 to 0.2 nm and a single unit cell (0.4 nm) step.
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4, 6, 8 and 10 min after beginning the introduction of oxygen at PO, = 1 X lo-* Torr. The island size distributions of Figs. 8a, 8b and 8c are compared with the results of calculations assuming a random adsorption model, as shown in Figs. 9a, 9b and 9c, respectively. In this model, migration and desorption of oxygen are neglected, i.e., once absorbed oxygen will not migrate or desorb. Adsorption on the occupied site is also neglected, i.e., oxygen impinging on an adsorbed site will be returned into vacuum leaving no effect on the gas-solid interface. The unit of the island size is the unit cell of the 6 x 6 structure. The is-
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Table 1 The mean island sizes C&,) of Fig. 8 (a) (0 = 0.08 ML), (b) (0 = 0.13 ML) and (cl (~9= 0.2 ML) are compared with calculated results <&,J by the random adsorption model at each of the coverages B/ML 0.08 0.13 0.2
1.5 2.3 3.7
1.4 1.7 2.8
lands include next-nearest neighbor sites. At any coverage, the experimental distribution shifts to a large island size in comparison with the calcu-
Fig. 11. RHEED patterns from the SrTiO,(lOO)~ X 6-R26.6” surface observed with the electron beam incident along the 10211 azimuth, (a) before the introduction of oxygen and (b) 10 min after the introduction of oxygen at 1 X lo-’ Torr.
38
H. Tanaka et al. /Surfnce
lated distribution, as shown in Fig. 9. In addition, the mean island size at each oxygen coverage is represented in Table 1. At low coverage (0 = 0.08 ML), the s,,, is slightly larger than so, with increasing 8. These deviations imply that once oxygen forms large islands, adsorbed oxygen scarcely desorbs from the surface regardless of the cessation of adsorption. With further increasing of the coverage, STM imaging became unstable and atomic resolution is no longer obtained, as shown in Figs. 8d and 8e. Figs. 10a and lob show a STM image and its cross section after oxygen adsorption of about 1 ML. Although a surface step of a single unit cell (0.4 nm) of SrTiO, is observed, atomic-scale resolution cannot be obtained. This image shows that the fi x 6 structure has completely disappeared due to an oxygen adsorption of only 1 ML. From the cross section, the roughnesses are the order of 0.1 to 0.2 nm, which is about one order larger than those of corrugation of the 6 x 6 structure (see Fig. 5). This result suggests that the surface defect level from the Ovacancy vanished and surface conductivity became inhomogeneous by the oxygen adsorption on the fi x 6 surface. For this reason, we consider that adsorbed 0, molecules dissociate and oxidize the O-vacancy defects after forming islands. To elucidate the structure of this surface, RHEED measurements are conducted under the same conditions of the STM experiment. Figs. lla and llb show RHEED patterns with the electron beam incident along the [021] azimuth before and after oxygen adsorption, respectively. The pattern observed before introduction of oxygen consists of streaks lying on a zero-order Laue circle and of spots on a first-order Laue circle corresponding to the periodicity d = 0.87 nm of the fi X 6 structure. Upon introduction of oxygen PO, = 1 x lo-’ Torr, l/5 order streaks and spots from the fi x fi structure have become weak in intensity, while streaks and spots corresponding to the 1 X 1 structure have not changed. This indicates that the oxidation of O-vacancy defects extinguish the fi x 6 reconstructed structure; however, the surface retains a fundamental 1 X 1 structure.
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4. Conclusion We have performed real space and real time STM observations of oxygen adsorption on a SrTi0,(100)6 x fi-R26.6” surface in UHV. The STM image of adsorbed oxygen is observed as a hole on the 6 x 6 surface. We found the existence of two adsorption phases depending upon the pressure of oxygen exposure. The initial phase is obtained for coverages up to about 0.01 ML (PO, = 1 x low9 Torr); STM images reveal that oxygen adsorbates migrate and/or desorb without interacting strongly with O-vacancy defects, suggesting that 0, is adsorbed as a molecule. At this PO,, the oxygen coverages reach equilibrium around 0.01 ML. From this result, the oxygen adsorption energy E, is estimated to be about 0.9 eV. For coverages greater than 0.01 ML (PO, > 1 X 10v9 Torr), the second phase is formed; that is, oxygen adsorbates form islands which never desorb from the surface even stopping oxygen exposure. These oxygen islands oxidize the Ovacancy defects. However, this oxidation occurs retaining the unit cell step and the 1 X 1 structure without ruining out the fundamental unit cell structure at the surface. We believe that STM studies will reveal the electronic properties and geometric structure of oxide surfaces at atomic level and that this information will provide more insight into chemical reaction on the oxide surface.
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