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Scanning tunneling microscopy study of terminal oxygen structures on WO3(1 0 0) thin films
Surface Science 579 (2005) 175–187 www.elsevier.com/locate/susc
Scanning tunneling microscopy study of terminal oxygen structures on WO3(1 0 0) thin films M. Li a
a,*
, A. Posadas b, C.H. Ahn b, E.I. Altman
a
Department of Chemical Engineering, Yale University, 9 Hillhouse Ave., Mason 102, New Haven, CT 06520, United States b Department of Applied Physics, Yale University, New Haven, CT 06520, United States Received 12 November 2004; accepted for publication 7 February 2005
Abstract Scanning tunneling microscopy (STM) was used to characterize the surface reconstructions on epitaxial WO3(1 0 0) thin films on LaAlO3(1 0 0) in a reducing environment. As the films were annealed between 600 and 770 K, a myriad of surface structures related to terminal oxygen were observed. Upon initial reduction the surface was covered with small c(2 · 2), p(2 · 2), c(4 · 2), and poorly ordered terminal oxygen terraces all coexisting with (1 · 1) islands. Further reduction caused large flat terraces of poorly ordered terminal oxygen to coexist with strand terminated p(n · 2) terraces with n = 3–5. Continued reduction led to a zigzag arrangement on top of the p(n · 2) surface, half-height p(2 · 2) and c(4 · 2) islands, and a local (15 · 2) structure. The latter three structures could only be explained by crystallographic shearing of the surface plane. In contrast to higher annealing temperatures, the exclusively p(n · 2) terminated surface characterized by alternating strands and troughs was not observed, suggesting that at lower temperatures crystallographic shear competes with the bulk migration responsible for trough formation as the dominant surface reduction mechanism. 2005 Elsevier B.V. All rights reserved. Keywords: Tungsten oxide; Epitaxy; Surface relaxation and reconstruction; Scanning tunneling microscopy
1. Introduction It has long been known that the reactivity and selectivity of transition metal oxide partial oxida*
Corresponding author. Tel.: +1 203 4324332; fax: +1 203 4324387. E-mail address: [email protected] (M. Li).
tion catalysts are often structure sensitive with generic defects frequently implicated as the reactive sites [1–8]. There has been only a limited amount of work in which the range of structures produced on an oxide surface by the oxidation and reduction that is an inherent part of the catalytic cycle on these materials has been characterized on an atomic level [9–11] and definitively
0039-6028/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2005.02.007
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linked with specific reaction pathways [9,12,13]. Therefore, we have been using scanning tunneling microscopy (STM) in conjunction with temperature programmed desorption (TPD) to draw connections between oxide surface structure and reactivity [12,14–16]. One of the materials we have focused on is WO3. Tungsten trioxide is an important material not only in catalysis [17,18], but also in gas sensing [19,20], and electrochromic devices [21,22]. Prior work on WO3(1 0 0) and (0 0 1) surfaces revealed two reduction mechanisms: (1) removal of terminal oxygen; and (2) migration of reduced W cations into the bulk. On WO3(1 0 0) thin films, we showed that the vacancies left by bulk migration can organize into troughs creating a new series of reconstructions [23,24]. These reconstructions did not alter the favored reaction pathway for adsorbed alcohols, dehydration to alkenes, but did promote this reaction at much lower temperatures than stoichiometric WO3 (0 0 1) surfaces [12]. In this paper, the transition from reduction by removal of terminal oxygen to bulk migration of reduced cations will be shown, and a new reduction mechanism, shearing of the surface plane will be revealed. Tungsten trioxide crystallizes in a network of corner sharing WO6 octahedra. Distortions and tilting of the octahedra result in deviations from the ideal cubic ReO3 structure [25–29]. A number of phases are observed that are distinguished by differences in the distortions. Between 290 and 630 K monoclinic c-WO3 with lattice constants a = 0.7297 nm, b = 0.7539 nm, c = 0.7688 nm and b = 90.91 is favored; STM experiments have been performed within this temperature range. In c-WO3 there are eight WO6 octahedra per monoclinic unit cell and so the distance between neighboring W atoms averages 0.375 nm. Along the h1 0 0i directions the structure can be pictured as alternating WO2 and O planes. Studies of WO3(0 0 1) single crystals revealed several surface structures depending on the surface treatment. A surface terminated by a long-range superstructure attributed to arrays of W6O18 clusters was observed in our previous study [30]. Paradoxically, reduction was required before the stoichiometric c(2 · 2) structure could be observed [30–32]. The c(2 · 2) structure with half of the ter-
minal oxygens removed is considered ‘‘fully oxidized’’ since it is autocompensated with all W cations in the 6+ oxidation state. Subsequent reduction led to p(2 · 2) [31,32], (6 · 2) and c(4 · 2) [30] structures with 1/4 ML terminal oxygen and half of the exposed W cations reduced to 5+. Further reduction produced bare WO2 terraces with (1 · 1) periodicities, no terminal oxygens, and all surface W reduced to 5+ [33,34]. These surface structures were defined with respect to an idealized ReO3 cubic unit cell with a lattice constant l = 0.375 nm; the same terminology will be used in this paper. Recently we showed that a series of p(n · 2) (n = 3, 4, 5) reconstructions could be induced by reducing p(2 · 2) surfaces of WO3(1 0 0) thin films [11,23]. The p(n · 2) surfaces were characterized by strands nl apart running along the [0 1 0] and [0 0 1] directions. The strands were explained by an added row model constructed from two octahedron-wide terraces with all terminal oxygens removed. Prolonged reduction converted the p(n · 2) surfaces to a (1 · 1) surface with fractional height steps attributed to bulk shear planes intersecting the surface [11]. Thus the results revealed close relationships between surface stoichiometry, structure, and morphology and bulk oxygen deficiency in the thin films. This stimulated the present detailed study of the structural and morphological changes that occur in the regime where terminal oxygen exists on the surface. In this paper it will be shown that initial reduction of [1 0 0]-oriented WO3 thin films leads to rough surfaces with small c(2 · 2), p(2 · 2), c(4 · 2), and poorly ordered terminal oxygen terraces coexisting with reduced (1 · 1) terraces. Continued reduction led to smoother surfaces with large poorly ordered terminal oxygen terraces coexisting with the stranded p(n · 2) reconstructions. Prior to producing a completely strand-covered surface, further reduction produced terminal oxygen in a zigzag pattern on top of the strands and half-height p(2 · 2) and c(4 · 2) islands attributed to crystallographic shear of the surface plane. The results demonstrate that reduction affects the structure and morphology of WO3 surfaces through three distinct mechanisms: (1) removal of terminal oxygen; (2) migration of reduced species into the bulk; and (3)
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crystallographic shearing of the surface plane. At lower temperatures these three mechanisms compete, leading to a myriad of local atomic arrangements.
2. Experimental The [1 0 0]-oriented WO3 films were grown on LaAlO3(1 0 0) substrates by radio frequency-magnetron sputtering as described previously [23]. The LaAlO3 substrates were polished on one side for film growth and cut into 5 · 10 · 0.5 mm3 pieces. Film thicknesses ranged from 60 to 80 nm. The quality of the films degraded after repeated thermal cycling and so the results reported in this paper are for several different films. All of the structures reported were seen on more than one film and the general sequence of structures seen on annealing was reproducible, although the specific conditions (e.g. annealing temperature and time) required to produce a given structure varied from film to film, presumably due to variations in film thickness and initial stoichiometry. Surface characterization by STM, low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) was performed in a separate ultra-high-vacuum system (UHV) [35]. The sample temperature was measured by clamping a K-type thermocouple housed in a thin Ta tube against the surface [35]. The films were cleaned either in a side chamber by heating to 820 K in oxygen pressures up to 104 Torr or annealing in the main chamber with NO2 (1 · 106 Torr) between 770 and 820 K [23]. Samples were considered clean when atmospheric contaminants such as C and S were below the AES detection limit. Traces of potassium impurities were always detected with AES after the films were cleaned. As WO3 films tend to be more easily reduced than single crystals at similar treatments inside the UHV chamber, reduced WO3 films were ÔreoxidizedÕ by annealing in a furnace at 670 K for hours with flowing O2. A similar treatment was previously applied to WO3 single crystals and led to rough surfaces that transformed into flat c(2 · 2) surfaces after reduction in vacuum [30]. After reoxidation and clean-
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ing in the vacuum chamber, the p(2 · 2) structure dominated the thin film surfaces. Electrochemically etched W STM tips were cleaned by electron beam bombardment prior to use. A tunneling current of 0.5 nA was used. Only empty state STM images corresponding to positive sample biases are presented in this paper as filled state imaging was not stable.
3. Results Fig. 1 shows STM images of a ÔreoxidizedÕ WO3(1 0 0) thin film recorded at room temperature after annealing at 770 K in O2 (3 · 105 Torr) for 13 h. The surface appears rough with small irregularly shaped terraces as shown in Fig. 1(a). Zooming in on the area in the dashed box in Fig. 1(a) reveals small p(2 · 2) domains on the terraces as shown in Fig. 1(b), suggesting the presence of terminal oxygen atoms. A p(2 · 2) unit cell is highlighted in Fig. 1(b) and labeled on the same area in Fig. 1(a). Meanwhile, a c(4 · 2) domain was detected on the same surface to the left of the p(2 · 2) structure as highlighted by a c(4 · 2) unit cell in Fig. 1(c). It was also noticed that the lower terrace near the c(4 · 2) domain was covered by a lower density of bright spots arranged in no clearly discernible pattern as marked by arrows. The height difference between the dots and the maxima in the c(4 · 2) domain in the upper terrace corresponded to the WO3 monatomic step height. In some areas of the same surface shown in Fig. 1(a), larger terraces were detected. Atomically resolved images of these terraces revealed a two domain c(4 · 2)/c(2 · 4) structure as highlighted in the small scale image in Fig. 1(d). Although the surface was predominantly covered by structures associated with terminal oxygen species, (1 · 1) islands elongated along the [0 0 1] and [0 1 0] directions were also observed, indicating that the surface oxygen concentration was not equilibrated over a long range. The surface shown in Fig. 1 gave a diffuse p(2 · 2) LEED pattern with a high background that can be attributed to the small size of the domains and the mix of p(2 · 2), c(4 · 2), (1 · 1) structures as well as the observation of poorly ordered domains.
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Fig. 1. The STM images of a ÔreoxidizedÕ WO3(1 0 0) thin film annealed in O2 (3 · 105 Torr) at 770 K for 13 h. (a) A large scale image with c(4 · 2), p(2 · 2) and terraces terminated by poorly ordered oxygens coexisting with (1 · 1) islands; (b,c) high resolution images of oxygen structures in the dashed and solid boxes in (a) respectively; (d) an atomically resolved image showing the c(4 · 2) and c(2 · 4) domains on the same surface in (a). The imaging biases were 2.5 V for (a), (b) and (c), and 1.75 V for (d).
Because the density of states at the bottom of the conduction band of metal oxides tends to be concentrated at the metal atoms, unoccupied state STM images of metal oxides often emphasize the position of the metal atoms. For WO3{1 0 0} surfaces, however, an analysis of the band structure of WO3 led Jones et al. [32] to conclude that the greater state density above the W atoms was insufficient to offset the height of the terminal oxygen atoms above the WO2 plane, and so they attributed bright spots in STM images of the p(2 · 2) and c(2 · 2) structures to the terminal oxygen atoms. Subsequently, Tanner et al. [16] reported images showing diffusion of the bright spots and denuded patches adjacent to c(2 · 2) domains that
could only be explained by attributing the bright spots to terminal oxygen. In contrast, when the O and W atoms are coplanar as in a (1 · 1) terrace, the maxima in STM images have been assigned to the W atoms [33]. The similar appearance of the poorly ordered dots in Fig. 1 to the terminal oxygen atoms in the p(2 · 2) and c(4 · 2) domains, and the monatomic step between these dots and the c(4 · 2) domain indicates that these features are also due to terminal oxygen atoms. Previously, we showed that reduction of p(2 · 2) WO3(1 0 0) thin film surfaces led to the p(n · 2) strand-terminated surfaces prior to the (1 · 1) surface [11,23]. Fig. 2 shows STM images obtained after a film was annealed between 730
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Fig. 2. (a) An STM image obtained after an as-grown film was annealed in NO2 (1 · 106 Torr) between 730 and 770 K for 8 h; (b) image obtained by zooming in on the bottom of (a); (c) a different area on the same surface. The imaging biases were 2.5 V for (a), 2 V for (b) and 1.75 V for (c).
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and 770 K in NO2 (1 · 106 Torr) for 8 h following growth. The surface was covered predominantly by p(n · 2) strands as shown in Fig. 2(a). The scattered white clusters on top of the strands are attributed to surface contamination. At the bottom of Fig. 2(a) a less corrugated area characterized by irregular trenches can also be seen. Zooming in on these areas (Fig. 2(b)) surprisingly revealed the c(2 · 2) structure associated with a stoichiometric WO3 surface and the (1 · 1) structure assigned to a W2O5 surface stoichiometry. Thus the surface stoichiometry varied dramatically over a span of only 5 nm, again indicating that the oxygen concentration on the surface was not equilibrated over a long range. Two distinct types of trenches were observed on the terraces in Fig. 2(b). The first, marked by solid arrows, were 0.17 ± 0.02 nm deep, or slightly less than half the monatomic step height on WO3(1 0 0). Although the density of states above the Fermi level in WO3 is concentrated at the W atoms [36], prior studies have shown that the difference in state density between the W and O sites is insufficient to offset the height of terminal O atoms above the adjacent W atoms [30,32]. As a result, terminal O atoms appear raised above bare (1 · 1) terraces by a little less than half the monatomic step height. Thus, this first class of trenches is attributed to missing terminal oxygen atoms. The second type of trenches, marked by the dashed arrow in Fig. 2(b), were deeper: 0.31 ± 0.01 nm, close to the monatomic step height. Therefore, these trenches can be considered as steps down to a lower oxygen-terminated terrace. Alternatively, the deeper trenches can be viewed as missing W–O octahedra versus missing terminal O atoms for the shallower trenches. The right half of Fig. 2(c) shows a wider range view of a larger oxygen terminated terrace. On this larger terrace the trenches show a strong tendency to run along the [0 0 1] direction; these are the shallower trenches attributed to missing terminal oxygens. When the film in Fig. 2 was annealed for another 75 min in NO2 (1 · 106 Torr) at 720 K terraces with terminal oxygen were still observed, though the terminal oxygen concentration was much lower and only poor ordering was seen. The image in Fig. 3(a) shows a low density of
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Fig. 3. (a) The surface after the film in Fig. 2 was annealed at 720 K for another 75 min in NO2 (1 · 106 Torr); (b) the surface of another as-grown film prepared by annealing at 700 K in O2 (1 · 106 Torr) for 6 h followed by NO2 (1 · 106 Torr) annealing at 710 K for 75 min. The imaging biases were 2.5 V for (a) and (b).
poorly ordered white balls on the surface. Their height above the neighboring gray areas was 0.16 nm, consistent with terminal oxygen. On the terrace in Fig. 3(a) the terminal oxygen concentration was 0.4 nm2 compared to 3.57 nm2 for c(2 · 2) and 1.79 nm2 for p(2 · 2) and c(4 · 2) indicating that the terrace is reduced compared to these structures. The image shows that the oxygen atoms tended to form chains less than 10 atoms long with the oxygen atoms spaced 2·
apart; oxygen atoms less than 2· apart were never observed. In this case the two straight trenches are 0.32 nm deep, much closer to the monatomic step height than the terminal oxygen height suggesting that the trenches correspond to missing octahedra. As shown in Fig. 3(b), similar structures were seen on another as-grown film prepared by 6 h of annealing at 700 K in O2 (1 · 106 Torr) followed by 75 min annealing at 710 K in NO2 (1 · 106 Torr). In this case the terminal oxygen concentration is higher, 0.84 nm2, and the oxygen atoms tend to arrange in small p(2 · 2) patches. In both Fig. 3(a) and (b) holes 0.32 nm deep were observed, a couple of these are marked in each image by arrows. Again this is much closer to the monatomic step height suggesting that these holes correspond to missing octahedra. The WO3(1 0 0) surface was terminated almost exclusively by p(n · 2) strands after prolonged reduction by annealing in UHV at 770 K for 13 h as shown in Fig. 4(a). In this case, the same film pictured in Fig. 1 was ÔreoxidizedÕ prior to the UHV anneal. Interestingly, several narrow bands were detected with bright dots arranged in a zigzag pattern as highlighted by the arrows in Fig. 4(a). The height of these dots was 0.16 nm above the neighboring strands, the expected height for terminal oxygens above a (1 · 1) surface, suggesting that the bright dots are terminal oxygen atoms. The pattern is typically staggered in adjacent rows leading to a repeat distance of 7· perpendicular to the rows. A structural model of the area highlighted by the dashed box in Fig. 4(a) is provided in Fig. 4(b) and (c). In the model, the raised zigzag bands are associated with terminal oxygen as indicated by the small white circles at the apices of the shaded octahedra, while the neighboring perpendicular strands take on the previously proposed added row structure. The model shows the gaps between the zigzag bands as missing terminal oxygen atoms; however, it is difficult to determine from the image if they correspond to missing oxygen or missing octahedra. If the gaps are missing octahedra then this structure is simply due to oxygen adsorption on top of p(n · 2) strands. Structures associated with terminal oxygen atoms were still seen after reducing the film in
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Fig. 5. An STM image obtained after the surface in Fig. 4 was annealed in UHV at 770 K for an additional 18 h. The imaging bias was 2.5 V.
Fig. 4. (a) An STM image of the surface after the same film in Fig. 1 was ÔreoxidizedÕ and then annealed in UHV at 770 K for 13 h. The imaging bias was 2.6 V. (b,c) Structural model of the zigzag pattern enclosed in the dashed box in (a). The white dots at the apices of the shaded octahedra indicate the presence of terminal oxygen.
Fig. 4(a) by annealing in UHV at 770 K for an additional 18 h. As highlighted in Fig. 5, elevated c(4 · 2) islands with a typical size of 12 · 6 nm2 were resolved 0.16 nm above neighboring strands. The island heights were consistent with terminal oxygen on top of a WO2 plane at the same height as the adjacent strands. Meanwhile pairs of rows of bright dots spaced 2· apart were detected between strands; at the right of Fig. 5 the dots on neighboring rows form a local p(2 · 2) pattern while at the left the dots are staggered in a zigzag pattern. Although the dots appear similar to termi-
nal oxygen atoms, in this case they are actually 0.04 nm lower than the adjacent strands. The same sorts of structures pictured in Fig. 5 were seen on other films as shown in Fig. 6 for a ÔreoxidizedÕ film annealed in O2 (1 · 106 Torr) between 630 and 720 K for 18 h. In this case the c(4 · 2) island at the top of Fig. 6(a) is the expected 0.16 nm above the nearby strands. Interestingly, the left edge of the c(4 · 2) islands appeared different from the oxygen atoms in the island as highlighted by the arrow in Fig. 6(a). Similar, featureless white bands were also seen bordering the one terminal oxygen atom-wide island also pointed to by an arrow in Fig. 6(a). In addition, Fig. 6(a) shows more extended domains of dots in a p(2 · 2) pattern between the strands. The arrangement of the dots suggest that they correspond to terminal oxygen atoms; however, they are again only 0.04 nm lower than the neighboring strands making them much too high to be on a lower terrace but much too low to be on the same terrace as the strands. The expected 0.37 nm monatomic step height was measured between the p(2 · 2) domains at the right and center of Fig. 6(a). Fig. 6(b) shows another area of the same surface. Here the lower gray terrace is predominantly p(2 · 2) with a few of the strands that make up p(n · 2) structures appearing slightly higher. A
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Fig. 6. Scanning tunneling microscopy images of a surface prepared by annealing a ÔreoxidizedÕ film in O2 (1 · 106 Torr) between 630 and 720 K for 16 h. (a,b) were taken at different areas of the same surface. The imaging biases were 2.5 V for (a) and 2.25 V for (b).
couple of small c(4 · 2) islands, and single and double chains of dots can also be seen as marked by arrows. These appear 0.16 nm above the strands suggesting that these are due to terminal oxygen. The images in Fig. 7 provide further insight into the structures seen in Figs. 5 and 6. In this case a ÔreoxidizedÕ film was annealed at 770 K in O2 (1 · 105 Torr) for 10 h and then at 620 K in NO2 (1 · 106 Torr) for 5 h. The surface was predominantly covered by the strands that ultimately
make up the p(n · 2) structures while p(2 · 2) and c(4 · 2) domains could also be seen. In Fig. 7(a), the white p(2 · 2) islands are 0.36 nm higher than the gray strands and slightly lower than the pair of white strands in the center of the image. While the height of the white strands is consistent with a monatomic step, the p(2 · 2) islands are more than a factor of two too high to be terminal oxygen on a WO2 plane at the same level as the gray strands and 0.17 nm too low to be terminal oxygen on the next highest WO2 plane. Fig. 7(b) shows a more detailed view of the island pointed to by the arrow in Fig. 7(a) and the area surrounding it. The vacancy at the left edge of the p(2 · 2) island was 0.15 nm deep, the expected apparent height difference between a terminal oxygen and a WO2 plane. In addition, the arrow points to a less corrugated band at a domain boundary in the p(2 · 2) island. Meanwhile in the lower level of Fig. 7(b) a similar band can be seen separating two single rows of spots separated by 2·. These bands are roughly half as wide as the strands that make up the p(n · 2) reconstructions indicating that they are distinct features. Atomically resolved STM images of the bands and the rows of 2· spots are provided in Fig. 7(c) and (d); these images were recorded near the area marked by a square in Fig. 7(a). The spots, highlighted by a dashed circle in Fig. 7(c), are slightly lower (0.04 nm) than the adjacent strands and appear similar to spots in p(2 · 2) and c(4 · 2) domains attributed to terminal oxygen. The corrugation along the rows is 0.072 nm which is typical for oxygen-terminated domains. The depth of the vacancy marked by an arrow is 0.16 nm, slightly less than the depth of the trenches (0.18 nm) between the strands, but typical of terminal oxygen vacancies. The bands between the rows of spots appear electronically flatter and slightly (0.02 nm) lower. Still, a weak 2· periodicity can be discerned along the bands. The lower corrugation (0.038 nm) makes the features appear more disk-like, as highlighted by the dashed square in Fig. 7(c), rather than the balls seen on the neighboring rows. In Fig. 7(c) and (d) the rows of disks and balls alternate with a 5· spacing between rows of balls. The registry of the rows is staggered to create a local (15 · 2) periodicity as
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Fig. 7. Images obtained after a ÔreoxidizedÕ film was annealed in O2 (1 · 105 Torr) at 770 K for 10 h followed by NO2 (1 · 106 Torr) annealing at 620 K for 5 h. (a) Wide range image; (b) higher resolution image taken at the center of (a); (c,d) atomically resolved images recorded near the square box in (a). The imaging biases were 0.8 V for (a), 1 V for (b), 0.9 V for (c) and 0.8 V for (d).
outlined by the dashed rectangle in Fig. 7(c). Fig. 7(d) is imaged near the area in Fig. 7(c) and the same strand is marked by a dashed line in both images. The same (15 · 2) unit cell is also outlined in Fig. 7(d). A proposed structural model for the local (15 · 2) structure in Fig. 7(c) and (d) is given in Fig. 8. The model shows the area between the two strands marked in Fig. 7(d). As described previously, these two strands are constructed from an added double row of octahedra, shown shaded in dark gray in Fig. 8(a), with all terminal oxygen atoms removed [23]. Oxygen atoms are removed from the edges of every other octahedron to create the 2· periodicity revealed by LEED [23]. As shown in the side view in Fig. 8(b), the strands
are half the monatomic step height above an oxygen-terminated terrace. The rows of balls arranged in a 2· periodicity appear the same as terminal oxygen atoms in their corrugation, height above vacancies, and spacing and so these are assigned to terminal oxygen atoms. To account for their height in the STM images, the oxygen atoms, drawn as white circles, are placed atop a WO2 plane sheared along [0 1 0] from the bulk corner sharing position to an edge sharing position as shown from the top view in Fig. 8(a). Thus the terminal oxygen atoms are placed nominally at the same height as the strands as shown in the side view in Fig. 8(b). Relaxations and the greater state density around W atoms than O atoms can explain the slightly lower height seen in the STM images
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Fig. 8. Structural model of the local (15 · 2) structure in Fig. 7(d). The (15 · 2) unit cell, terminal oxygen and disk-like features in Fig. 7(a) are also outlined.
[36]. Two types of vacancies are proposed in Fig. 8b with the one on the left caused by a missing octahedron and the one in the middle by a missing terminal oxygen atom. The model shows that the height of terminal oxygens above both types of vacancies is half the height of the WO6 octahedron as seen from Fig. 8(b), in agreement with the STM measurement from Fig. 7(c). The stoichiometry of the top two layers for the three octahedron-wide sheared plane with terminal oxygen atoms shown in Fig. 8(a) is W2O5, the same as the strands and the (1 · 1) structure, and consistent with seeing these structures simultaneously. In the model, the disks are pictured as terminal oxygen atoms on top of a single row of octahedra sheared along [0 0 1] rather than [0 1 0]. This puts the disks at essentially the same height as the strands and the oxygen atoms on top of the three atom wide sheared plane. Because the supporting octahedra share only one edge with the sheared row, this structure is slightly more oxygen rich with a stoichiometry of W6O17 for the sheared row and the
two rows of supporting octahedra. The lower corrugation in the STM images may be due to electronic differences between a single sheared row and a wider sheared plane. Essentially the same model can be used to explain the reduced height c(4 · 2) and p(2 · 2) islands in Figs. 5, 6 and 7(b) as well as the domain boundary in Fig. 7(b). In these cases the sheared planes are large enough to support p(2 · 2) and c(4 · 2) arrangements of terminal oxygens. For an infinite sheared plane with a quarter monolayer of terminal oxygen the stoichiometry of the top two layers would be W8O19 or slightly reduced compared to a (1 · 1) surface.
4. Discussion The results indicate that the WO3(1 0 0) surface can respond to reduction in three distinct ways: (1) removal of terminal oxygen; (2) shearing of the surface plane; and (3) migration of reduced W5+ into the bulk. The first mechanism accounts for
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the p(2 · 2), c(4 · 2), and (6 · 2) structures with 1/ 4 ML of terminal oxygen. The shearing of the surface plane can account for similar oxygen terminated structures with half-height steps. Finally, bulk migration explains the appearance of monatomic pits and trenches when oxygen terminated structures were reduced. The organization of these pits and trenches into ordered troughs led to the p(n · 2) stranded reconstructions. Evidence of bulk migration was previously seen on WO3 (0 0 1) single crystals [30]. In this case, rough surfaces were observed after cleaving that gradually gave way to ordered arrays of clusters and then to flat terraces that spread across the surface as the crystals were reduced; reoxidizing the crystals restored them to their original rough state. The smoothing and roughening were attributed to bulk migration and segregation and reoxidation of reduced W5+. The three reduction routes lead to the observed myriad of structures. Further, the results indicate that under the temperature range studied here, 600–770 K, the mechanisms do not occur sequentially as the films are oxidized and reduced but rather in competition. This creates complex surface morphologies such as the local (15 · 2) periodicity seen in Fig. 7(c) and (d) where structures with similar stoichiometries exist side-by-side. The surface morphology is further complicated by the observation that the surface oxygen concentration may be equilibrated over only a short range leading to the stoichiometric c(2 · 2) structure existing near reduced (1 · 1) terraces. All of these factors lead to surfaces that are far from equilibrium when the environment is changed from oxidizing to reducing. Since the reactivity of WO3 surfaces can depend on the surface structure this suggests that at moderate temperatures transients in reactivity may occur when the conditions are changed in a catalytic reactor and similarly that transients may occur in sensor response when conditions are changed from oxidizing to reducing [37]. We previously proposed that the formation of the p(n · 2) stranded reconstructions can be explained by the bulk migration of reduced W5+ [23]. As the c(4 · 2) and p(2 · 2) structures are reduced, the W5+ concentration builds up on the surface; when the surface chemical potential of
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W5+ exceeds that in the bulk, W cations start to migrate into the bulk leading to pits that ultimately organize to form the ordered troughs of the stranded p(n · 2) phases. The observation of monatomic pits and troughs on the oxygen terminated terraces supports this view. Further the pits on the terraces with poorly ordered terminal oxygen suggests that this surface state could be a metastable precursor to the p(n · 2) phases. In the previously proposed model of the p(n · 2) structures, terminal oxygen was placed in the troughs between the added rows to account for the observed trough depth of 0.18 nm, the narrowing of the troughs with continued reduction, and the observation of the structures before the (1 · 1) surface when a p(2 · 2) surface was reduced. Thus with increasing terminal oxygen removal, reorganization of the remaining terminal oxygen into the pits and ordering of the pits into ordered arrays of troughs can transform these terraces into the p(n · 2) structures. In our prior study focused on the p(n · 2) structures, LEED and STM revealed the following sequence of structures when a WO3(1 0 0) thin film was reduced by annealing between 800 and 1000 K: p(2 · 2); p(5 · 2); p(4 · 2); p(3 · 2); and (1 · 1) with bulk shear planes intersecting the surface. In contrast, the fractional height islands attributed to shearing of the surface plane were seen after annealing at lower temperatures, only 620 K for Fig. 7. Since the stoichiometries of these structures are similar, this suggests that the stranded reconstructions are lower in energy than sheared surface planes and, therefore, that the sheared surface planes are also a metastable state. In this case, the temperature may be too low for W5+ to rapidly migrate deep into the bulk and so the film responds to reduction by crystallographic shear of the surface plane rather than migration of reduced species into the bulk. Prior studies of WO3{1 0 0} thin films and single crystals indicate that the favored initial reduction pathway for stoichiometric surfaces is removal of terminal oxygen. The results presented here indicate that after the terminal oxygen coverage is decreased below 1/4 ML that the reduction pathway depends on temperature. At high temperatures, above 800 K, bulk migration of
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reduced species and surface diffusion of the resulting vacancies is sufficiently fast to allow the formation of ordered arrays of strands and troughs into p(n · 2) structures with n decreasing from 5 to 3 with continued reduction. In contrast, at lower temperatures terminal oxygen removal can continue allowing poorly ordered terminal oxygen domains to form with coverages less than 0.25 ML, and the surface plane can shear to form reduced terraces with terminal oxygen.
5. Summary The sequential reduction of WO3(1 0 0) thin films between 600 and 770 K leads to various surface reconstructions involving terminal oxygen. Initial reduction causes the removal of terminal oxygen to produce small terraces of c(2 · 2), p(2 · 2), p(4 · 2) and poorly ordered terminal oxygen coexisting with (1 · 1) islands. The surface thus exhibits a dramatic spatial variation in stoichiometry over a short range. Such a rough surface can be reduced via the bulk migration of W5+ to create large terraces of poorly ordered terminal oxygen with pits and trenches which could be the metastable precursor of p(n · 2) phases. Further reduction via the crystallographic shearing of the surface causes terminal oxygen to arrange into half-height p(2 · 2), c(4 · 2) and local (15 · 2) islands. The results suggest that reduction can affect the surface structure and morphology via three pathways: (1) removal of terminal oxygen; (2) migration of reduced species into the bulk; and (3) crystallographic shear. At lower temperatures (<800 K) and terminal coverages below 1/4 ML crystallographic shearing of the surface plane competes with the other two pathways to produce metastable oxygen terminated structures with half-height steps.
Acknowledgments The authors acknowledge the help of Weiwei Gao and Jun Wang in carrying out this work. A. Posadas and C.H. Ahn acknowledge support from NSF (DMR-0134721). This project is sup-
ported by the Department of Energy through Basic Energy Sciences grant number DE-FG0298ER 14882.
References [1] B.C. Gates, Catalytic Chemistry, Wiley, New York, 1992. [2] H.H. Kung, M.C. Kung, in: D.D. Eley, H. Pines, P.B. Weisz (Eds.), Advances in Catalysis, vol. 33, Academic Press, New York, 1985, p. 159. [3] C.J. Machiels, W.H. Cheng, U. Chowdry, W.E. Farneth, F. Hong, E.M. McCarron, A.W. Sleight, Appl. Catal. 25 (1986) 249. [4] J.M. Tatiboue¨t, J.E. Germain, J. Catal. 72 (1981) 375. [5] J.M. Tatiboue¨t, J.E. Germain, J.C. Volta, J. Catal. 82 (1983) 240. [6] U. Chowdry, A. Ferretti, L.E. Firment, C.J. Machiels, F. Ohuchi, A.W. Sleight, R.H. Staley, Appl. Surf. Sci. 19 (1984) 360. [7] R.S. Weber, J. Phys. Chem. 98 (1994) 2999. [8] A. Michalak, K. Hermann, M. Witko, Surf. Sci. 366 (1996) 323. [9] U. Diebold, Surf. Sci. Rep. 48 (2003) 53. [10] M. Li, W. Hebenstreit, U. Diebold, M.A. Henderson, D.R. Jennison, Faraday Discuss. 114 (1999) 245. [11] M. Li, E.I. Altman, A. Posadas, C.H. Ahn, Thin Solid Films 446 (2003) 238. [12] M. Li, W. Gao, A. Posadas, C.H. Ahn, E.I. Altman, J. Phys. Chem. B 108 (2004) 15259. [13] H. Over, Y.D. Kim, A.P. Seitsonen, S. Wendt, E. Lundgren, M. Schmid, P. Varga, A. Morgante, G. Ertl, Science 287 (2000) 1474. [14] R.E. Tanner, E.I. Altman, A. Sasahara, H. Onishi, Y. Liang, J. Phys. Chem. B 106 (2002) 8211. [15] R.E. Tanner, Y. Liang, E.I. Altman, Surf. Sci. 506 (2002) 251. [16] R.E. Tanner, P. Meethunkij, E.I. Altman, J. Phys. Chem. B 104 (2000) 12315. [17] A. Gutie´rrez-Alejandre, J. Ramı´rez, G. Busca, Catal. Lett. 56 (1998) 29. [18] Y. Wang, Q. Chen, W. Yang, Z. Xie, W. Xu, D. Huang, Appl. Catal. A 250 (2003) 25. [19] D.-S. Lee, K.-H. Nam, D.-D. Lee, Thin Solid Films 375 (2000) 142. [20] H. Kawasaki, J. Namba, K. Iwatsuji, Y. Suda, K. Wada, K. Ebihara, T. Ohshima, Appl. Surf. Sci. 197–198 (2002) 547. [21] A. Bessiere, C. Marcel, M. Morcrette, J.M. Tarascon, V. Lucas, B. Viana, N. Baffier, J. Appl. Phys. 91 (2002) 1589. [22] G. Fang, D. Li, B. Yao., J. Phys. D: Appl. Phys. 34 (2001) 2260. [23] M. Li, E.I. Altman, A. Posadas, C.H. Ahn, Surf. Sci. 542 (2003) 22. [24] M. Li, E.I. Altman, A. Posadas, C.H. Ahn, J. Vac. Sci. Technol. A 22 (2003) 1682.
M. Li et al. / Surface Science 579 (2005) 175–187 [25] E. Salje, Acta Crystallogr. B 33 (1977) 574. [26] E. Salje, K. Viswanathan, Acta Crystallogr. A 31 (1975) 356. [27] B.O. Loopstra, P. Boldrini, Acta Crystallogr. 21 (1966) 158. [28] E. Salje, S. Rehmann, F. Pobell, D. Morris, K.S. Knight, Herrmannsdo¨rfer, T. Dove, J. Phys. C: Solid State Phys. 9 (1997) 6563. [29] R. Diehl, G. Brandt, Acta Crystallogr. B 34 (1978) 1105. [30] R.E. Tanner, E.I. Altman, J. Vac. Sci. Technol. A 19 (2001) 1502. [31] F.H. Jones, K. Rawlings, J.S. Foord, P.A. Cox, R.G. Egdell, J.B. Pethica, B.M.R. Wanklyn, Phys. Rev. B 52 (1995) 14392.
187
[32] F.H. Jones, K. Rawlings, J.S. Foord, R.G. Egdell, J.B. Pethica, B.M.R. Wanklyn, S.C. Parker, P.M. Oliver, Surf. Sci. 359 (1996) 107. [33] F.H. Jones, R.A. Dixon, A. Brown, Surf. Sci. 369 (1996) 343. [34] R.A. Dixon, J.J. Williams, D. Morris, J. Rebane, F.H. Jones, R.G. Egdell, S.W. Downes, Surf. Sci. 399 (1998) 199. [35] C.Y. Nakakura, V.M. Phanse, G. Zheng, G. Bannon, E.I. Altman, K.P. Lee, Rev. Sci. Instrum. 69 (1998) 3251. [36] D.W. Bullett, J. Phys. C: Solid State Phys. 16 (1983) 2197. [37] M. Li, A. Posadas, C.H. Ahn, E.I. Altman, J. Phys. Chem. B 108 (2004) 15259.
Scanning tunneling microscopy study of terminal oxygen structures on WO3(1 0 0) thin films M. Li a
a,*
, A. Posadas b, C.H. Ahn b, E.I. Altman
a
Department of Chemical Engineering, Yale University, 9 Hillhouse Ave., Mason 102, New Haven, CT 06520, United States b Department of Applied Physics, Yale University, New Haven, CT 06520, United States Received 12 November 2004; accepted for publication 7 February 2005
Abstract Scanning tunneling microscopy (STM) was used to characterize the surface reconstructions on epitaxial WO3(1 0 0) thin films on LaAlO3(1 0 0) in a reducing environment. As the films were annealed between 600 and 770 K, a myriad of surface structures related to terminal oxygen were observed. Upon initial reduction the surface was covered with small c(2 · 2), p(2 · 2), c(4 · 2), and poorly ordered terminal oxygen terraces all coexisting with (1 · 1) islands. Further reduction caused large flat terraces of poorly ordered terminal oxygen to coexist with strand terminated p(n · 2) terraces with n = 3–5. Continued reduction led to a zigzag arrangement on top of the p(n · 2) surface, half-height p(2 · 2) and c(4 · 2) islands, and a local (15 · 2) structure. The latter three structures could only be explained by crystallographic shearing of the surface plane. In contrast to higher annealing temperatures, the exclusively p(n · 2) terminated surface characterized by alternating strands and troughs was not observed, suggesting that at lower temperatures crystallographic shear competes with the bulk migration responsible for trough formation as the dominant surface reduction mechanism. 2005 Elsevier B.V. All rights reserved. Keywords: Tungsten oxide; Epitaxy; Surface relaxation and reconstruction; Scanning tunneling microscopy
1. Introduction It has long been known that the reactivity and selectivity of transition metal oxide partial oxida*
Corresponding author. Tel.: +1 203 4324332; fax: +1 203 4324387. E-mail address: [email protected] (M. Li).
tion catalysts are often structure sensitive with generic defects frequently implicated as the reactive sites [1–8]. There has been only a limited amount of work in which the range of structures produced on an oxide surface by the oxidation and reduction that is an inherent part of the catalytic cycle on these materials has been characterized on an atomic level [9–11] and definitively
0039-6028/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2005.02.007
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linked with specific reaction pathways [9,12,13]. Therefore, we have been using scanning tunneling microscopy (STM) in conjunction with temperature programmed desorption (TPD) to draw connections between oxide surface structure and reactivity [12,14–16]. One of the materials we have focused on is WO3. Tungsten trioxide is an important material not only in catalysis [17,18], but also in gas sensing [19,20], and electrochromic devices [21,22]. Prior work on WO3(1 0 0) and (0 0 1) surfaces revealed two reduction mechanisms: (1) removal of terminal oxygen; and (2) migration of reduced W cations into the bulk. On WO3(1 0 0) thin films, we showed that the vacancies left by bulk migration can organize into troughs creating a new series of reconstructions [23,24]. These reconstructions did not alter the favored reaction pathway for adsorbed alcohols, dehydration to alkenes, but did promote this reaction at much lower temperatures than stoichiometric WO3 (0 0 1) surfaces [12]. In this paper, the transition from reduction by removal of terminal oxygen to bulk migration of reduced cations will be shown, and a new reduction mechanism, shearing of the surface plane will be revealed. Tungsten trioxide crystallizes in a network of corner sharing WO6 octahedra. Distortions and tilting of the octahedra result in deviations from the ideal cubic ReO3 structure [25–29]. A number of phases are observed that are distinguished by differences in the distortions. Between 290 and 630 K monoclinic c-WO3 with lattice constants a = 0.7297 nm, b = 0.7539 nm, c = 0.7688 nm and b = 90.91 is favored; STM experiments have been performed within this temperature range. In c-WO3 there are eight WO6 octahedra per monoclinic unit cell and so the distance between neighboring W atoms averages 0.375 nm. Along the h1 0 0i directions the structure can be pictured as alternating WO2 and O planes. Studies of WO3(0 0 1) single crystals revealed several surface structures depending on the surface treatment. A surface terminated by a long-range superstructure attributed to arrays of W6O18 clusters was observed in our previous study [30]. Paradoxically, reduction was required before the stoichiometric c(2 · 2) structure could be observed [30–32]. The c(2 · 2) structure with half of the ter-
minal oxygens removed is considered ‘‘fully oxidized’’ since it is autocompensated with all W cations in the 6+ oxidation state. Subsequent reduction led to p(2 · 2) [31,32], (6 · 2) and c(4 · 2) [30] structures with 1/4 ML terminal oxygen and half of the exposed W cations reduced to 5+. Further reduction produced bare WO2 terraces with (1 · 1) periodicities, no terminal oxygens, and all surface W reduced to 5+ [33,34]. These surface structures were defined with respect to an idealized ReO3 cubic unit cell with a lattice constant l = 0.375 nm; the same terminology will be used in this paper. Recently we showed that a series of p(n · 2) (n = 3, 4, 5) reconstructions could be induced by reducing p(2 · 2) surfaces of WO3(1 0 0) thin films [11,23]. The p(n · 2) surfaces were characterized by strands nl apart running along the [0 1 0] and [0 0 1] directions. The strands were explained by an added row model constructed from two octahedron-wide terraces with all terminal oxygens removed. Prolonged reduction converted the p(n · 2) surfaces to a (1 · 1) surface with fractional height steps attributed to bulk shear planes intersecting the surface [11]. Thus the results revealed close relationships between surface stoichiometry, structure, and morphology and bulk oxygen deficiency in the thin films. This stimulated the present detailed study of the structural and morphological changes that occur in the regime where terminal oxygen exists on the surface. In this paper it will be shown that initial reduction of [1 0 0]-oriented WO3 thin films leads to rough surfaces with small c(2 · 2), p(2 · 2), c(4 · 2), and poorly ordered terminal oxygen terraces coexisting with reduced (1 · 1) terraces. Continued reduction led to smoother surfaces with large poorly ordered terminal oxygen terraces coexisting with the stranded p(n · 2) reconstructions. Prior to producing a completely strand-covered surface, further reduction produced terminal oxygen in a zigzag pattern on top of the strands and half-height p(2 · 2) and c(4 · 2) islands attributed to crystallographic shear of the surface plane. The results demonstrate that reduction affects the structure and morphology of WO3 surfaces through three distinct mechanisms: (1) removal of terminal oxygen; (2) migration of reduced species into the bulk; and (3)
M. Li et al. / Surface Science 579 (2005) 175–187
crystallographic shearing of the surface plane. At lower temperatures these three mechanisms compete, leading to a myriad of local atomic arrangements.
2. Experimental The [1 0 0]-oriented WO3 films were grown on LaAlO3(1 0 0) substrates by radio frequency-magnetron sputtering as described previously [23]. The LaAlO3 substrates were polished on one side for film growth and cut into 5 · 10 · 0.5 mm3 pieces. Film thicknesses ranged from 60 to 80 nm. The quality of the films degraded after repeated thermal cycling and so the results reported in this paper are for several different films. All of the structures reported were seen on more than one film and the general sequence of structures seen on annealing was reproducible, although the specific conditions (e.g. annealing temperature and time) required to produce a given structure varied from film to film, presumably due to variations in film thickness and initial stoichiometry. Surface characterization by STM, low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) was performed in a separate ultra-high-vacuum system (UHV) [35]. The sample temperature was measured by clamping a K-type thermocouple housed in a thin Ta tube against the surface [35]. The films were cleaned either in a side chamber by heating to 820 K in oxygen pressures up to 104 Torr or annealing in the main chamber with NO2 (1 · 106 Torr) between 770 and 820 K [23]. Samples were considered clean when atmospheric contaminants such as C and S were below the AES detection limit. Traces of potassium impurities were always detected with AES after the films were cleaned. As WO3 films tend to be more easily reduced than single crystals at similar treatments inside the UHV chamber, reduced WO3 films were ÔreoxidizedÕ by annealing in a furnace at 670 K for hours with flowing O2. A similar treatment was previously applied to WO3 single crystals and led to rough surfaces that transformed into flat c(2 · 2) surfaces after reduction in vacuum [30]. After reoxidation and clean-
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ing in the vacuum chamber, the p(2 · 2) structure dominated the thin film surfaces. Electrochemically etched W STM tips were cleaned by electron beam bombardment prior to use. A tunneling current of 0.5 nA was used. Only empty state STM images corresponding to positive sample biases are presented in this paper as filled state imaging was not stable.
3. Results Fig. 1 shows STM images of a ÔreoxidizedÕ WO3(1 0 0) thin film recorded at room temperature after annealing at 770 K in O2 (3 · 105 Torr) for 13 h. The surface appears rough with small irregularly shaped terraces as shown in Fig. 1(a). Zooming in on the area in the dashed box in Fig. 1(a) reveals small p(2 · 2) domains on the terraces as shown in Fig. 1(b), suggesting the presence of terminal oxygen atoms. A p(2 · 2) unit cell is highlighted in Fig. 1(b) and labeled on the same area in Fig. 1(a). Meanwhile, a c(4 · 2) domain was detected on the same surface to the left of the p(2 · 2) structure as highlighted by a c(4 · 2) unit cell in Fig. 1(c). It was also noticed that the lower terrace near the c(4 · 2) domain was covered by a lower density of bright spots arranged in no clearly discernible pattern as marked by arrows. The height difference between the dots and the maxima in the c(4 · 2) domain in the upper terrace corresponded to the WO3 monatomic step height. In some areas of the same surface shown in Fig. 1(a), larger terraces were detected. Atomically resolved images of these terraces revealed a two domain c(4 · 2)/c(2 · 4) structure as highlighted in the small scale image in Fig. 1(d). Although the surface was predominantly covered by structures associated with terminal oxygen species, (1 · 1) islands elongated along the [0 0 1] and [0 1 0] directions were also observed, indicating that the surface oxygen concentration was not equilibrated over a long range. The surface shown in Fig. 1 gave a diffuse p(2 · 2) LEED pattern with a high background that can be attributed to the small size of the domains and the mix of p(2 · 2), c(4 · 2), (1 · 1) structures as well as the observation of poorly ordered domains.
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Fig. 1. The STM images of a ÔreoxidizedÕ WO3(1 0 0) thin film annealed in O2 (3 · 105 Torr) at 770 K for 13 h. (a) A large scale image with c(4 · 2), p(2 · 2) and terraces terminated by poorly ordered oxygens coexisting with (1 · 1) islands; (b,c) high resolution images of oxygen structures in the dashed and solid boxes in (a) respectively; (d) an atomically resolved image showing the c(4 · 2) and c(2 · 4) domains on the same surface in (a). The imaging biases were 2.5 V for (a), (b) and (c), and 1.75 V for (d).
Because the density of states at the bottom of the conduction band of metal oxides tends to be concentrated at the metal atoms, unoccupied state STM images of metal oxides often emphasize the position of the metal atoms. For WO3{1 0 0} surfaces, however, an analysis of the band structure of WO3 led Jones et al. [32] to conclude that the greater state density above the W atoms was insufficient to offset the height of the terminal oxygen atoms above the WO2 plane, and so they attributed bright spots in STM images of the p(2 · 2) and c(2 · 2) structures to the terminal oxygen atoms. Subsequently, Tanner et al. [16] reported images showing diffusion of the bright spots and denuded patches adjacent to c(2 · 2) domains that
could only be explained by attributing the bright spots to terminal oxygen. In contrast, when the O and W atoms are coplanar as in a (1 · 1) terrace, the maxima in STM images have been assigned to the W atoms [33]. The similar appearance of the poorly ordered dots in Fig. 1 to the terminal oxygen atoms in the p(2 · 2) and c(4 · 2) domains, and the monatomic step between these dots and the c(4 · 2) domain indicates that these features are also due to terminal oxygen atoms. Previously, we showed that reduction of p(2 · 2) WO3(1 0 0) thin film surfaces led to the p(n · 2) strand-terminated surfaces prior to the (1 · 1) surface [11,23]. Fig. 2 shows STM images obtained after a film was annealed between 730
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Fig. 2. (a) An STM image obtained after an as-grown film was annealed in NO2 (1 · 106 Torr) between 730 and 770 K for 8 h; (b) image obtained by zooming in on the bottom of (a); (c) a different area on the same surface. The imaging biases were 2.5 V for (a), 2 V for (b) and 1.75 V for (c).
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and 770 K in NO2 (1 · 106 Torr) for 8 h following growth. The surface was covered predominantly by p(n · 2) strands as shown in Fig. 2(a). The scattered white clusters on top of the strands are attributed to surface contamination. At the bottom of Fig. 2(a) a less corrugated area characterized by irregular trenches can also be seen. Zooming in on these areas (Fig. 2(b)) surprisingly revealed the c(2 · 2) structure associated with a stoichiometric WO3 surface and the (1 · 1) structure assigned to a W2O5 surface stoichiometry. Thus the surface stoichiometry varied dramatically over a span of only 5 nm, again indicating that the oxygen concentration on the surface was not equilibrated over a long range. Two distinct types of trenches were observed on the terraces in Fig. 2(b). The first, marked by solid arrows, were 0.17 ± 0.02 nm deep, or slightly less than half the monatomic step height on WO3(1 0 0). Although the density of states above the Fermi level in WO3 is concentrated at the W atoms [36], prior studies have shown that the difference in state density between the W and O sites is insufficient to offset the height of terminal O atoms above the adjacent W atoms [30,32]. As a result, terminal O atoms appear raised above bare (1 · 1) terraces by a little less than half the monatomic step height. Thus, this first class of trenches is attributed to missing terminal oxygen atoms. The second type of trenches, marked by the dashed arrow in Fig. 2(b), were deeper: 0.31 ± 0.01 nm, close to the monatomic step height. Therefore, these trenches can be considered as steps down to a lower oxygen-terminated terrace. Alternatively, the deeper trenches can be viewed as missing W–O octahedra versus missing terminal O atoms for the shallower trenches. The right half of Fig. 2(c) shows a wider range view of a larger oxygen terminated terrace. On this larger terrace the trenches show a strong tendency to run along the [0 0 1] direction; these are the shallower trenches attributed to missing terminal oxygens. When the film in Fig. 2 was annealed for another 75 min in NO2 (1 · 106 Torr) at 720 K terraces with terminal oxygen were still observed, though the terminal oxygen concentration was much lower and only poor ordering was seen. The image in Fig. 3(a) shows a low density of
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Fig. 3. (a) The surface after the film in Fig. 2 was annealed at 720 K for another 75 min in NO2 (1 · 106 Torr); (b) the surface of another as-grown film prepared by annealing at 700 K in O2 (1 · 106 Torr) for 6 h followed by NO2 (1 · 106 Torr) annealing at 710 K for 75 min. The imaging biases were 2.5 V for (a) and (b).
poorly ordered white balls on the surface. Their height above the neighboring gray areas was 0.16 nm, consistent with terminal oxygen. On the terrace in Fig. 3(a) the terminal oxygen concentration was 0.4 nm2 compared to 3.57 nm2 for c(2 · 2) and 1.79 nm2 for p(2 · 2) and c(4 · 2) indicating that the terrace is reduced compared to these structures. The image shows that the oxygen atoms tended to form chains less than 10 atoms long with the oxygen atoms spaced 2·
apart; oxygen atoms less than 2· apart were never observed. In this case the two straight trenches are 0.32 nm deep, much closer to the monatomic step height than the terminal oxygen height suggesting that the trenches correspond to missing octahedra. As shown in Fig. 3(b), similar structures were seen on another as-grown film prepared by 6 h of annealing at 700 K in O2 (1 · 106 Torr) followed by 75 min annealing at 710 K in NO2 (1 · 106 Torr). In this case the terminal oxygen concentration is higher, 0.84 nm2, and the oxygen atoms tend to arrange in small p(2 · 2) patches. In both Fig. 3(a) and (b) holes 0.32 nm deep were observed, a couple of these are marked in each image by arrows. Again this is much closer to the monatomic step height suggesting that these holes correspond to missing octahedra. The WO3(1 0 0) surface was terminated almost exclusively by p(n · 2) strands after prolonged reduction by annealing in UHV at 770 K for 13 h as shown in Fig. 4(a). In this case, the same film pictured in Fig. 1 was ÔreoxidizedÕ prior to the UHV anneal. Interestingly, several narrow bands were detected with bright dots arranged in a zigzag pattern as highlighted by the arrows in Fig. 4(a). The height of these dots was 0.16 nm above the neighboring strands, the expected height for terminal oxygens above a (1 · 1) surface, suggesting that the bright dots are terminal oxygen atoms. The pattern is typically staggered in adjacent rows leading to a repeat distance of 7· perpendicular to the rows. A structural model of the area highlighted by the dashed box in Fig. 4(a) is provided in Fig. 4(b) and (c). In the model, the raised zigzag bands are associated with terminal oxygen as indicated by the small white circles at the apices of the shaded octahedra, while the neighboring perpendicular strands take on the previously proposed added row structure. The model shows the gaps between the zigzag bands as missing terminal oxygen atoms; however, it is difficult to determine from the image if they correspond to missing oxygen or missing octahedra. If the gaps are missing octahedra then this structure is simply due to oxygen adsorption on top of p(n · 2) strands. Structures associated with terminal oxygen atoms were still seen after reducing the film in
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Fig. 5. An STM image obtained after the surface in Fig. 4 was annealed in UHV at 770 K for an additional 18 h. The imaging bias was 2.5 V.
Fig. 4. (a) An STM image of the surface after the same film in Fig. 1 was ÔreoxidizedÕ and then annealed in UHV at 770 K for 13 h. The imaging bias was 2.6 V. (b,c) Structural model of the zigzag pattern enclosed in the dashed box in (a). The white dots at the apices of the shaded octahedra indicate the presence of terminal oxygen.
Fig. 4(a) by annealing in UHV at 770 K for an additional 18 h. As highlighted in Fig. 5, elevated c(4 · 2) islands with a typical size of 12 · 6 nm2 were resolved 0.16 nm above neighboring strands. The island heights were consistent with terminal oxygen on top of a WO2 plane at the same height as the adjacent strands. Meanwhile pairs of rows of bright dots spaced 2· apart were detected between strands; at the right of Fig. 5 the dots on neighboring rows form a local p(2 · 2) pattern while at the left the dots are staggered in a zigzag pattern. Although the dots appear similar to termi-
nal oxygen atoms, in this case they are actually 0.04 nm lower than the adjacent strands. The same sorts of structures pictured in Fig. 5 were seen on other films as shown in Fig. 6 for a ÔreoxidizedÕ film annealed in O2 (1 · 106 Torr) between 630 and 720 K for 18 h. In this case the c(4 · 2) island at the top of Fig. 6(a) is the expected 0.16 nm above the nearby strands. Interestingly, the left edge of the c(4 · 2) islands appeared different from the oxygen atoms in the island as highlighted by the arrow in Fig. 6(a). Similar, featureless white bands were also seen bordering the one terminal oxygen atom-wide island also pointed to by an arrow in Fig. 6(a). In addition, Fig. 6(a) shows more extended domains of dots in a p(2 · 2) pattern between the strands. The arrangement of the dots suggest that they correspond to terminal oxygen atoms; however, they are again only 0.04 nm lower than the neighboring strands making them much too high to be on a lower terrace but much too low to be on the same terrace as the strands. The expected 0.37 nm monatomic step height was measured between the p(2 · 2) domains at the right and center of Fig. 6(a). Fig. 6(b) shows another area of the same surface. Here the lower gray terrace is predominantly p(2 · 2) with a few of the strands that make up p(n · 2) structures appearing slightly higher. A
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Fig. 6. Scanning tunneling microscopy images of a surface prepared by annealing a ÔreoxidizedÕ film in O2 (1 · 106 Torr) between 630 and 720 K for 16 h. (a,b) were taken at different areas of the same surface. The imaging biases were 2.5 V for (a) and 2.25 V for (b).
couple of small c(4 · 2) islands, and single and double chains of dots can also be seen as marked by arrows. These appear 0.16 nm above the strands suggesting that these are due to terminal oxygen. The images in Fig. 7 provide further insight into the structures seen in Figs. 5 and 6. In this case a ÔreoxidizedÕ film was annealed at 770 K in O2 (1 · 105 Torr) for 10 h and then at 620 K in NO2 (1 · 106 Torr) for 5 h. The surface was predominantly covered by the strands that ultimately
make up the p(n · 2) structures while p(2 · 2) and c(4 · 2) domains could also be seen. In Fig. 7(a), the white p(2 · 2) islands are 0.36 nm higher than the gray strands and slightly lower than the pair of white strands in the center of the image. While the height of the white strands is consistent with a monatomic step, the p(2 · 2) islands are more than a factor of two too high to be terminal oxygen on a WO2 plane at the same level as the gray strands and 0.17 nm too low to be terminal oxygen on the next highest WO2 plane. Fig. 7(b) shows a more detailed view of the island pointed to by the arrow in Fig. 7(a) and the area surrounding it. The vacancy at the left edge of the p(2 · 2) island was 0.15 nm deep, the expected apparent height difference between a terminal oxygen and a WO2 plane. In addition, the arrow points to a less corrugated band at a domain boundary in the p(2 · 2) island. Meanwhile in the lower level of Fig. 7(b) a similar band can be seen separating two single rows of spots separated by 2·. These bands are roughly half as wide as the strands that make up the p(n · 2) reconstructions indicating that they are distinct features. Atomically resolved STM images of the bands and the rows of 2· spots are provided in Fig. 7(c) and (d); these images were recorded near the area marked by a square in Fig. 7(a). The spots, highlighted by a dashed circle in Fig. 7(c), are slightly lower (0.04 nm) than the adjacent strands and appear similar to spots in p(2 · 2) and c(4 · 2) domains attributed to terminal oxygen. The corrugation along the rows is 0.072 nm which is typical for oxygen-terminated domains. The depth of the vacancy marked by an arrow is 0.16 nm, slightly less than the depth of the trenches (0.18 nm) between the strands, but typical of terminal oxygen vacancies. The bands between the rows of spots appear electronically flatter and slightly (0.02 nm) lower. Still, a weak 2· periodicity can be discerned along the bands. The lower corrugation (0.038 nm) makes the features appear more disk-like, as highlighted by the dashed square in Fig. 7(c), rather than the balls seen on the neighboring rows. In Fig. 7(c) and (d) the rows of disks and balls alternate with a 5· spacing between rows of balls. The registry of the rows is staggered to create a local (15 · 2) periodicity as
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Fig. 7. Images obtained after a ÔreoxidizedÕ film was annealed in O2 (1 · 105 Torr) at 770 K for 10 h followed by NO2 (1 · 106 Torr) annealing at 620 K for 5 h. (a) Wide range image; (b) higher resolution image taken at the center of (a); (c,d) atomically resolved images recorded near the square box in (a). The imaging biases were 0.8 V for (a), 1 V for (b), 0.9 V for (c) and 0.8 V for (d).
outlined by the dashed rectangle in Fig. 7(c). Fig. 7(d) is imaged near the area in Fig. 7(c) and the same strand is marked by a dashed line in both images. The same (15 · 2) unit cell is also outlined in Fig. 7(d). A proposed structural model for the local (15 · 2) structure in Fig. 7(c) and (d) is given in Fig. 8. The model shows the area between the two strands marked in Fig. 7(d). As described previously, these two strands are constructed from an added double row of octahedra, shown shaded in dark gray in Fig. 8(a), with all terminal oxygen atoms removed [23]. Oxygen atoms are removed from the edges of every other octahedron to create the 2· periodicity revealed by LEED [23]. As shown in the side view in Fig. 8(b), the strands
are half the monatomic step height above an oxygen-terminated terrace. The rows of balls arranged in a 2· periodicity appear the same as terminal oxygen atoms in their corrugation, height above vacancies, and spacing and so these are assigned to terminal oxygen atoms. To account for their height in the STM images, the oxygen atoms, drawn as white circles, are placed atop a WO2 plane sheared along [0 1 0] from the bulk corner sharing position to an edge sharing position as shown from the top view in Fig. 8(a). Thus the terminal oxygen atoms are placed nominally at the same height as the strands as shown in the side view in Fig. 8(b). Relaxations and the greater state density around W atoms than O atoms can explain the slightly lower height seen in the STM images
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Fig. 8. Structural model of the local (15 · 2) structure in Fig. 7(d). The (15 · 2) unit cell, terminal oxygen and disk-like features in Fig. 7(a) are also outlined.
[36]. Two types of vacancies are proposed in Fig. 8b with the one on the left caused by a missing octahedron and the one in the middle by a missing terminal oxygen atom. The model shows that the height of terminal oxygens above both types of vacancies is half the height of the WO6 octahedron as seen from Fig. 8(b), in agreement with the STM measurement from Fig. 7(c). The stoichiometry of the top two layers for the three octahedron-wide sheared plane with terminal oxygen atoms shown in Fig. 8(a) is W2O5, the same as the strands and the (1 · 1) structure, and consistent with seeing these structures simultaneously. In the model, the disks are pictured as terminal oxygen atoms on top of a single row of octahedra sheared along [0 0 1] rather than [0 1 0]. This puts the disks at essentially the same height as the strands and the oxygen atoms on top of the three atom wide sheared plane. Because the supporting octahedra share only one edge with the sheared row, this structure is slightly more oxygen rich with a stoichiometry of W6O17 for the sheared row and the
two rows of supporting octahedra. The lower corrugation in the STM images may be due to electronic differences between a single sheared row and a wider sheared plane. Essentially the same model can be used to explain the reduced height c(4 · 2) and p(2 · 2) islands in Figs. 5, 6 and 7(b) as well as the domain boundary in Fig. 7(b). In these cases the sheared planes are large enough to support p(2 · 2) and c(4 · 2) arrangements of terminal oxygens. For an infinite sheared plane with a quarter monolayer of terminal oxygen the stoichiometry of the top two layers would be W8O19 or slightly reduced compared to a (1 · 1) surface.
4. Discussion The results indicate that the WO3(1 0 0) surface can respond to reduction in three distinct ways: (1) removal of terminal oxygen; (2) shearing of the surface plane; and (3) migration of reduced W5+ into the bulk. The first mechanism accounts for
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the p(2 · 2), c(4 · 2), and (6 · 2) structures with 1/ 4 ML of terminal oxygen. The shearing of the surface plane can account for similar oxygen terminated structures with half-height steps. Finally, bulk migration explains the appearance of monatomic pits and trenches when oxygen terminated structures were reduced. The organization of these pits and trenches into ordered troughs led to the p(n · 2) stranded reconstructions. Evidence of bulk migration was previously seen on WO3 (0 0 1) single crystals [30]. In this case, rough surfaces were observed after cleaving that gradually gave way to ordered arrays of clusters and then to flat terraces that spread across the surface as the crystals were reduced; reoxidizing the crystals restored them to their original rough state. The smoothing and roughening were attributed to bulk migration and segregation and reoxidation of reduced W5+. The three reduction routes lead to the observed myriad of structures. Further, the results indicate that under the temperature range studied here, 600–770 K, the mechanisms do not occur sequentially as the films are oxidized and reduced but rather in competition. This creates complex surface morphologies such as the local (15 · 2) periodicity seen in Fig. 7(c) and (d) where structures with similar stoichiometries exist side-by-side. The surface morphology is further complicated by the observation that the surface oxygen concentration may be equilibrated over only a short range leading to the stoichiometric c(2 · 2) structure existing near reduced (1 · 1) terraces. All of these factors lead to surfaces that are far from equilibrium when the environment is changed from oxidizing to reducing. Since the reactivity of WO3 surfaces can depend on the surface structure this suggests that at moderate temperatures transients in reactivity may occur when the conditions are changed in a catalytic reactor and similarly that transients may occur in sensor response when conditions are changed from oxidizing to reducing [37]. We previously proposed that the formation of the p(n · 2) stranded reconstructions can be explained by the bulk migration of reduced W5+ [23]. As the c(4 · 2) and p(2 · 2) structures are reduced, the W5+ concentration builds up on the surface; when the surface chemical potential of
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W5+ exceeds that in the bulk, W cations start to migrate into the bulk leading to pits that ultimately organize to form the ordered troughs of the stranded p(n · 2) phases. The observation of monatomic pits and troughs on the oxygen terminated terraces supports this view. Further the pits on the terraces with poorly ordered terminal oxygen suggests that this surface state could be a metastable precursor to the p(n · 2) phases. In the previously proposed model of the p(n · 2) structures, terminal oxygen was placed in the troughs between the added rows to account for the observed trough depth of 0.18 nm, the narrowing of the troughs with continued reduction, and the observation of the structures before the (1 · 1) surface when a p(2 · 2) surface was reduced. Thus with increasing terminal oxygen removal, reorganization of the remaining terminal oxygen into the pits and ordering of the pits into ordered arrays of troughs can transform these terraces into the p(n · 2) structures. In our prior study focused on the p(n · 2) structures, LEED and STM revealed the following sequence of structures when a WO3(1 0 0) thin film was reduced by annealing between 800 and 1000 K: p(2 · 2); p(5 · 2); p(4 · 2); p(3 · 2); and (1 · 1) with bulk shear planes intersecting the surface. In contrast, the fractional height islands attributed to shearing of the surface plane were seen after annealing at lower temperatures, only 620 K for Fig. 7. Since the stoichiometries of these structures are similar, this suggests that the stranded reconstructions are lower in energy than sheared surface planes and, therefore, that the sheared surface planes are also a metastable state. In this case, the temperature may be too low for W5+ to rapidly migrate deep into the bulk and so the film responds to reduction by crystallographic shear of the surface plane rather than migration of reduced species into the bulk. Prior studies of WO3{1 0 0} thin films and single crystals indicate that the favored initial reduction pathway for stoichiometric surfaces is removal of terminal oxygen. The results presented here indicate that after the terminal oxygen coverage is decreased below 1/4 ML that the reduction pathway depends on temperature. At high temperatures, above 800 K, bulk migration of
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reduced species and surface diffusion of the resulting vacancies is sufficiently fast to allow the formation of ordered arrays of strands and troughs into p(n · 2) structures with n decreasing from 5 to 3 with continued reduction. In contrast, at lower temperatures terminal oxygen removal can continue allowing poorly ordered terminal oxygen domains to form with coverages less than 0.25 ML, and the surface plane can shear to form reduced terraces with terminal oxygen.
5. Summary The sequential reduction of WO3(1 0 0) thin films between 600 and 770 K leads to various surface reconstructions involving terminal oxygen. Initial reduction causes the removal of terminal oxygen to produce small terraces of c(2 · 2), p(2 · 2), p(4 · 2) and poorly ordered terminal oxygen coexisting with (1 · 1) islands. The surface thus exhibits a dramatic spatial variation in stoichiometry over a short range. Such a rough surface can be reduced via the bulk migration of W5+ to create large terraces of poorly ordered terminal oxygen with pits and trenches which could be the metastable precursor of p(n · 2) phases. Further reduction via the crystallographic shearing of the surface causes terminal oxygen to arrange into half-height p(2 · 2), c(4 · 2) and local (15 · 2) islands. The results suggest that reduction can affect the surface structure and morphology via three pathways: (1) removal of terminal oxygen; (2) migration of reduced species into the bulk; and (3) crystallographic shear. At lower temperatures (<800 K) and terminal coverages below 1/4 ML crystallographic shearing of the surface plane competes with the other two pathways to produce metastable oxygen terminated structures with half-height steps.
Acknowledgments The authors acknowledge the help of Weiwei Gao and Jun Wang in carrying out this work. A. Posadas and C.H. Ahn acknowledge support from NSF (DMR-0134721). This project is sup-
ported by the Department of Energy through Basic Energy Sciences grant number DE-FG0298ER 14882.
References [1] B.C. Gates, Catalytic Chemistry, Wiley, New York, 1992. [2] H.H. Kung, M.C. Kung, in: D.D. Eley, H. Pines, P.B. Weisz (Eds.), Advances in Catalysis, vol. 33, Academic Press, New York, 1985, p. 159. [3] C.J. Machiels, W.H. Cheng, U. Chowdry, W.E. Farneth, F. Hong, E.M. McCarron, A.W. Sleight, Appl. Catal. 25 (1986) 249. [4] J.M. Tatiboue¨t, J.E. Germain, J. Catal. 72 (1981) 375. [5] J.M. Tatiboue¨t, J.E. Germain, J.C. Volta, J. Catal. 82 (1983) 240. [6] U. Chowdry, A. Ferretti, L.E. Firment, C.J. Machiels, F. Ohuchi, A.W. Sleight, R.H. Staley, Appl. Surf. Sci. 19 (1984) 360. [7] R.S. Weber, J. Phys. Chem. 98 (1994) 2999. [8] A. Michalak, K. Hermann, M. Witko, Surf. Sci. 366 (1996) 323. [9] U. Diebold, Surf. Sci. Rep. 48 (2003) 53. [10] M. Li, W. Hebenstreit, U. Diebold, M.A. Henderson, D.R. Jennison, Faraday Discuss. 114 (1999) 245. [11] M. Li, E.I. Altman, A. Posadas, C.H. Ahn, Thin Solid Films 446 (2003) 238. [12] M. Li, W. Gao, A. Posadas, C.H. Ahn, E.I. Altman, J. Phys. Chem. B 108 (2004) 15259. [13] H. Over, Y.D. Kim, A.P. Seitsonen, S. Wendt, E. Lundgren, M. Schmid, P. Varga, A. Morgante, G. Ertl, Science 287 (2000) 1474. [14] R.E. Tanner, E.I. Altman, A. Sasahara, H. Onishi, Y. Liang, J. Phys. Chem. B 106 (2002) 8211. [15] R.E. Tanner, Y. Liang, E.I. Altman, Surf. Sci. 506 (2002) 251. [16] R.E. Tanner, P. Meethunkij, E.I. Altman, J. Phys. Chem. B 104 (2000) 12315. [17] A. Gutie´rrez-Alejandre, J. Ramı´rez, G. Busca, Catal. Lett. 56 (1998) 29. [18] Y. Wang, Q. Chen, W. Yang, Z. Xie, W. Xu, D. Huang, Appl. Catal. A 250 (2003) 25. [19] D.-S. Lee, K.-H. Nam, D.-D. Lee, Thin Solid Films 375 (2000) 142. [20] H. Kawasaki, J. Namba, K. Iwatsuji, Y. Suda, K. Wada, K. Ebihara, T. Ohshima, Appl. Surf. Sci. 197–198 (2002) 547. [21] A. Bessiere, C. Marcel, M. Morcrette, J.M. Tarascon, V. Lucas, B. Viana, N. Baffier, J. Appl. Phys. 91 (2002) 1589. [22] G. Fang, D. Li, B. Yao., J. Phys. D: Appl. Phys. 34 (2001) 2260. [23] M. Li, E.I. Altman, A. Posadas, C.H. Ahn, Surf. Sci. 542 (2003) 22. [24] M. Li, E.I. Altman, A. Posadas, C.H. Ahn, J. Vac. Sci. Technol. A 22 (2003) 1682.
M. Li et al. / Surface Science 579 (2005) 175–187 [25] E. Salje, Acta Crystallogr. B 33 (1977) 574. [26] E. Salje, K. Viswanathan, Acta Crystallogr. A 31 (1975) 356. [27] B.O. Loopstra, P. Boldrini, Acta Crystallogr. 21 (1966) 158. [28] E. Salje, S. Rehmann, F. Pobell, D. Morris, K.S. Knight, Herrmannsdo¨rfer, T. Dove, J. Phys. C: Solid State Phys. 9 (1997) 6563. [29] R. Diehl, G. Brandt, Acta Crystallogr. B 34 (1978) 1105. [30] R.E. Tanner, E.I. Altman, J. Vac. Sci. Technol. A 19 (2001) 1502. [31] F.H. Jones, K. Rawlings, J.S. Foord, P.A. Cox, R.G. Egdell, J.B. Pethica, B.M.R. Wanklyn, Phys. Rev. B 52 (1995) 14392.
187
[32] F.H. Jones, K. Rawlings, J.S. Foord, R.G. Egdell, J.B. Pethica, B.M.R. Wanklyn, S.C. Parker, P.M. Oliver, Surf. Sci. 359 (1996) 107. [33] F.H. Jones, R.A. Dixon, A. Brown, Surf. Sci. 369 (1996) 343. [34] R.A. Dixon, J.J. Williams, D. Morris, J. Rebane, F.H. Jones, R.G. Egdell, S.W. Downes, Surf. Sci. 399 (1998) 199. [35] C.Y. Nakakura, V.M. Phanse, G. Zheng, G. Bannon, E.I. Altman, K.P. Lee, Rev. Sci. Instrum. 69 (1998) 3251. [36] D.W. Bullett, J. Phys. C: Solid State Phys. 16 (1983) 2197. [37] M. Li, A. Posadas, C.H. Ahn, E.I. Altman, J. Phys. Chem. B 108 (2004) 15259.