Growth and Characterization of Crystalline Silica Films on Pd(100

Nov 18, 2013 - Gregory S. Hutchings , Jin-Hao Jhang , Chao Zhou , David Hynek , Udo D. Schwarz , and Eric I. Altman. ACS Applied Materials & Interface...
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Growth and Characterization of Crystalline Silica Films on Pd(100) Eric I. Altman,*,†,‡ Jan Götzen,†,§ Niveditha Samudrala,†,‡ and Udo D. Schwarz†,§ †

Center for Interface Structure and Phenomena, Yale University, New Haven, Connecticut 06520, United States Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520, United States § Department of Mechanical Engineering and Materials Science, Yale University, New Haven, Connecticut 06520, United States ‡

ABSTRACT: Silica films grown on Pd(100) were characterized by Auger electron spectroscopy, low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM). While no evidence of long-range order could be detected for films grown below 600 K, STM images of these films nevertheless revealed flat surfaces through which the step-terrace structure of the substrate could be seen. Annealing the films in 10−6 Torr of O2 above 975 K resulted in crystalline bilayers that produced hexagonal LEED patterns with a periodicity twice that of the substrate and with one of the overlayer close-packed directions paralleling Pd[011]. The extent of the crystalline domains was limited to typically five repeat units along two of the three close-packed directions of the film but was tens of repeat units long along the third. The lattice matching to the substrate expands the spacing in the bilayer on Pd(100) compared to bulk crystalline SiO2 and bilayers observed on other substrates; as a consequence, it is suggested that the regular domain boundaries that form help relieve stress. The dominant features in high-resolution STM images were dark pores surrounded by six other pores; consistent with prior studies, these features are assigned to six-membered rings of corner-sharing SiO4 tetrahedra. Elongation of the pores at the domain boundaries is attributed to insertion of edge-sharing tetrahedra into the rings. Ab initio calculations on freestanding bilayers were performed to understand the effect of the substantial strain on the growth and structure of the film. The results indicate that relaxation orthogonal to the commensurate direction can greatly reduce the strain energy; as a consequence, the square substrate promotes epitaxial growth of crystalline SiO2 by providing an incommensurate direction along which the film can relax.

1. INTRODUCTION Silica and related silicates find widespread applications in catalysis, both as the active phase and as supports for other catalytic materials.1,2 Despite the technological importance of silica and silicate surfaces, it has been difficult to exploit advances in scanning probe microscopy and surface science to characterize the intrinsic reactivity of individual surface sites on these materials. As a result, knowledge of the active sites on silicate surfaces has been subject to the limitations of averaging techniques. A key source of this difficulty has been that the materials of greatest interest are either amorphous or grow as crystals smaller than ∼10 μm. Further, even if larger crystals could be grown, the interesting surface chemistry often occurs on internal surfaces, not external crystal surfaces. It can be argued, however, that since all silicates of interest in catalysis share the same motif involving corner-sharing SiO4 tetrahedra,3,4 studying a planar silica surface can offer great insight into silicate surface chemistry. While single crystal quartz might appear to be a good starting point for a model silica surface, quartz undergoes a series of phase transitions with temperature that can make it difficult to prepare highly ordered clean surfaces;4 its wide bandgap also makes it difficult to apply many surface science techniques. As a result, several groups have worked on growing thin silica layers on single crystal metal surfaces including Mo(110), Mo(112), Ni(111), Pd(100), Pt(111), and Ru(0001).5−16 Of particular note are recent studies on Ru(0001), which showed that silica © 2013 American Chemical Society

bilayers could be produced in amorphous and crystalline forms and imaged with atomic resolution using scanning probe microscopy (SPM).11,12,16 This was followed by reports of bilayer crystalline and amorphous silica on graphene and bilayer amorphous silica on Pt(111),11,15,17 suggesting that the bilayer may be a stable form of SiO2 that can be grown on any flat substrate provided the material is stable to high temperatures and is less reactive toward oxygen than silicon. In this paper, it will be shown that crystalline bilayers can also be grown on Pd(100) and that the lattice mismatch between the crystalline bilayers and Pd(100) creates regular, characteristic defects in the films. Prior studies have shown that ordered silica films tend to be constructed from hexagonal networks of corner-sharing SiO4 tetrahedra.10,12 For Ni(111), an intricate growth procedure was developed to produce epitaxial multilayers of β-quartz (0001).13 Meanwhile, on Mo(112) an ordered hexagonal monolayer could be formed, but attempts to grow thicker layers yielded rough surfaces with no discernible long-range order.10,18 The ordered monolayer on Mo(112) is bound to the substrate through Si−O−Mo bonds.6,10 In contrast, the hexagonal crystalline bilayer grown on Ru(0001) is a closed layer involving only Si−O−Si bonds and no dangling Received: October 11, 2013 Revised: November 15, 2013 Published: November 18, 2013 26144

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bonds.11,12 There are thus two key advantages to the bilayers: (1) since there are no direct chemical bonds to the substrate, the silica surface is minimally affected by the interface, and (2) the saturated bonds at the bilayer surface mimic the internal surfaces of zeolites and related materials. A previous attempt to grow model silica films on Pd(100) met with some success.14 It was found that flat SiO2 films up to ≈3 nm thick could be grown and imaged with STM, which revealed terraces separated by steps with heights well in excess of those expected for Pd(100). In addition, atomic-scale features with a periodicity of 0.36 nm were reported, though curiously no ordered low-energy electron diffraction (LEED) patterns were provided. This 0.36 nm spacing is, however, much less than the 0.5 nm for β-quartz (0001) on Ni(111) and the 0.542 nm found for the crystalline bilayer on Ru(0001).12,13 The reported results nevertheless suggested a simple method for growing ordered SiO2 films involving only Si deposition at modest temperatures (500 K) in 7.5 × 10−6 Torr of O2;14 growth on Ni(111), in contrast, requires alternate annealing in a hydrogen plasma and oxygen,13 and on Ru(0001), annealing above 1150 K is necessary.12 In this paper it will be shown that flat multilayer SiO2 films can indeed be grown on Pd(100) at modest temperatures but that annealing to high temperatures (at least 975 K) is necessary to yield crystalline bilayers similar to those recently seen on Ru(0001)11 and graphene.17 Although the initial multilayer films appear flat in STM images, their low conductivity makes them subject to inadvertent tip modification. After annealing, both LEED and STM reveal the formation of an ordered hexagonal SiO2 layer despite the square substrate. It is found that the size of the crystalline domains is small along the Pd[011] direction in which the film is in registry with the substrate but large along the direction in which it is incommensurate. Inspired by ab initio calculations of freestanding bilayers suggesting that contraction along the incommensurate directions can relieve much of the strain energy, this finding is attributed to stress relief via contraction along the incommensurate direction and through domain boundary formation along the commensurate direction where the substrate dictates the spacing of the film.

amorphous SiO2 bilayer that partially covered the surface. Throughout the remainder of this paper, coverages are reported in ML, where 2 ML would correspond to a surface uniformly covered by a silica bilayer. Electrochemically etched W tips were used for all STM measurements. The tips were heated in situ by electron bombardment prior to use. Images were recorded at tunneling currents between 0.1 and 1.0 nA; unless otherwise noted, the magnitude of the current did not affect the images over this range. Sample biases are reported such that a positive bias refers to tunneling into unoccupied states of the sample. All STM images shown in this paper were recorded at room temperature. 2.2. Theoretical Approach. Freestanding crystalline silica bilayers were modeled using the Crystal09 software package.22,23 Several different basis sets and functionals were tested; all lead to similar outcomes. The computational results presented in this paper were obtained using a 66-21G(d) basis set for Si and a 6-31G(d) basis set for oxygen,24 with the hybrid HF-DFT functional PW1PW.25 The PW1PW functional was chosen based on reports suggesting that it gives accurate results for oxides.26 Since experimental results did not show any evidence of long-range reconstructions or large deviations from hexagonal symmetry, the crystalline bilayer was modeled assuming a centered rectangular crystallographic unit cell with Cmmm layer group symmetry that allows for deviations from perfect hexagonal geometry by changing the angle between the primitive lattice vectors from 120°. Each crystallographic cell contained 8 Si and 16 O atoms, though symmetry dictates only one unique Si site and three unique O sites. The Monkhorst− Pack method was used to generate the k-space sampling points with a 15 × 15 × 1 mesh.17 The calculations were benchmarked by comparison to the structural and elastic properties of α-quartz. Although the bilayer more closely resembles β-tridymite and β-cristabolite, αquartz was chosen as the reference material since both βtridymite and β-cristabolite have significant degrees of shortrange disorder that are difficult to model.4,27 Since the computational work was motivated by understanding the impact of strain on the films, the comparison of calculated elastic constants with measured values for a well-defined material was an important benchmark. As illustrated in Table 1,

2. METHODS 2.1. Experimental Section. Experiments were performed using an ultrahigh-vacuum system that has been detailed previously.19,20 Briefly, the system is equipped with a doublepass cylindrical mirror analyzer for Auger electron spectroscopy (AES) measurements, LEED optics, an ion gun for sputter cleaning, a differentially pumped microwave plasma source used to generate atomic oxygen, a quartz crystal microbalance for measuring deposition rates, and a high-speed variable-temperature scanning tunneling microscope. The Pd(100) single crystal was prepared by cycles of argon ion bombardment and O2 and UHV annealing as described elsewhere.21 Some limited experiments were performed using a Pt(111) substrate; in this case the crystal was cleaned by cycles of sputtering and annealing in UHV and in 10−6 Torr of O2 followed by a final UHV anneal at 1075 K. Silicon was deposited by placing the sample a few centimeters from a silicon wafer heated hot enough to sublime Si. Deposition was carried out in (1−4) × 10−6 Torr of O2 with the sample temperature held between 550 and 600 K. The amount of SiO2 deposited was measured using a quartz crystal microbalance, which was calibrated through STM measurements of the fractional coverage of Pt(111) by an

Table 1. Comparison of Experimental and Computed Structural and Elastic Properties of α-Quartz property

experiment

B3LYP

PW1PW

lattice parameters (nm) C11 (GPa) C33 (GPa) C44 (GPa) C66 (GPa) C12 (GPa) C13 (GPa) C14 (GPa) modulus (GPa)

0.4916, 0.5409 86.8 106.4 58.0 39.8 7.2 12.0 17.9 37.5

0.4966, 0.5474 86.2 109.1 58.8 37.3 11.6 17.6 9.1 40.1

0.4930, 0.5437 81.5 107.3 57.4 34.3 12.8 14.6 9.0 38.7

the calculations do an excellent job of reproducing the structural properties of α-quartz and, for the most part, the elastic constants. The only substantial deviations are for the offdiagonal terms C12 and C14 that are over- and underestimated, respectively, by a factor of 2. Table 1 also indicates little difference between PW1PW and B3LYP calculations. 26145

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Figure 1. (a) Auger electron spectra taken at different stages of silica film preparation: (i) bare Pd(100) substrate; (ii) after deposition of 2.4 ML of Si at 550−600 K in 2 × 10−6 Torr of O2; (iii) after annealing the film in (ii) for 5 min at 1075 K in 2 × 10−6 Torr of O2; (iv) after annealing the film in (iii) for an additional 10 min at 1075 K in 2 × 10−6 Torr of O2; (v) reference spectrum of an oxidized Si(100) wafer. The labels and arrows indicate the positions of the main Si, Pd, and O peaks. (b) Auger electron difference spectra where the contribution of the Pd substrate was subtracted from the spectra for the silica films: (i) after deposition of 2.4 ML of Si at 550−600 K in 2 × 10−6 Torr of O2; (ii) after annealing the film in (i) for 5 min at 1075 K in 2 × 10−6 Torr of O2; (iii) after annealing the film in (ii) for an additional 10 min at 1075 K in 2 × 10−6 Torr of O2.

Figure 2. Changes in LEED patterns as a 2.4 ML thick silica film on Pd(100) was heated to progressively higher temperatures: (a) after deposition at 550−600 K; (b) after annealing at 975 K for 5 min in (1−2) × 10−6 Torr of O2; (c) after annealing for 5 min at 1075 K for 5 min in (1−2) × 10−6 Torr of O2.

For the films, the O KLL to Si LVV AES peak ratio varied between 0.48 and 0.66 while for the reference sample the Si LVV and O KLL peaks were roughly equal in intensity. This difference can be associated with the shorter electron mean free path at the ≈80 eV energy of the Si LVV peak versus the over 500 eV for the O KLL peak. That is, the signal for oxygen can extend far deeper into the bulk, and so an atomically thin film will give a relatively weaker oxygen signal compared to a bulk sample. This change in the relative intensities of the Si and O AES peaks for atomically thin films compared to bulk samples was estimated by taking the mean free paths as 0.6 nm for the Si LVV peak and 1.0 nm for the O KLL peak32 and using the 0.18 nm α-quartz monatomic step height to estimate the thickness of the silica layers.33 This calculation suggests that a bilayer film would see a 50% enhancement of the Si peak relative to the O peak compared to a bulk sample. As a consequence, the AES data confirm that the films are indeed SiO2. It should be pointed out that under the experimental conditions the interaction of oxygen with Pd is restricted to the surface, and so the O peak in the AES spectrum cannot be due to oxidation of the Pd substrate.21 In comparing the AES spectra in Figure 1a, a noticeable decrease in the intensity of the Si peak can be seen after more prolonged annealing: the Si LVV to Pd MNN peak ratio decreased from 0.19 following growth to 0.13 after annealing in 10−6 Torr of O2 for a total of 15 min at the highest temperature

3. RESULTS 3.1. Film Stoichiometry. The stoichiometries of the silica films were characterized at each processing step using AES. The Si LVV AES peak position and shape are extremely sensitive to the Si oxidation state: the main peak for elemental Si is centered 92 eV while that for SiO2 is positioned at 76 eV.28−30 Reduction of SiO2 leads to a characteristic defect peak at 89 eV.29 Despite some overlap with Pd peaks, Figure 1a shows that the Si AES peak recorded immediately after depositing 2.4 ML of Si in background oxygen is similar to that seen on a Si wafer oxidized at 1275 K for 10 min, which yields an amorphous Si layer roughly 10 nm thick,31 far in excess of the AES sampling depth. Further, annealing at temperatures as high as 1075 K in 10−6 Torr of O2 had no obvious effect on the Si peak shape or position. A clearer comparison to the SiO2 reference spectrum was obtained by subtracting the Pd contribution to the spectra for the silica-covered Pd samples. Difference spectra were generated by normalizing and aligning the spectra so that the intensities and positions of the main Pd MNN peak were identical; the alignment required interpolation of the data. The resulting difference spectra in Figure 1b show good agreement between the peak position and shape of the Si peak for the films on Pd and the reference SiO2 sample, with no evidence of the reported defect peak. One significant difference between the spectra for the films and the reference sample is the relatively weaker O KLL peak. 26146

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of 1075 K. More substantial declines in the Si peak were seen when the films were either annealed longer under the same conditions or annealed in lower oxygen pressures (1.5 × 10−7 Torr) at similar temperatures, suggesting that high temperatures and insufficient oxygen can lead to silica decomposition and migration of Si into the bulk Pd crystal. 3.2. Structural Characterization by LEED. Deposition of Si onto Pd(100) at 550−600 K in background oxygen did not lead to any evidence of an ordered overlayer. As illustrated by the LEED pattern in Figure 2a obtained immediately after depositing 2.4 ML of SiO2, only faint spots due to the Pd substrate could be detected. To determine if order could be induced by annealing, the surface was heated for 5 min to progressively higher temperatures in (1−2) × 10−6 Torr of O2 and then slowly cooled in background O2. The first evidence of order was seen after heating to 975 K. As shown in the faint pattern in Figure 2b, an inner ring of eight spots could be detected, followed by a more prominent ring of 12 spots with four of the spots coincident with the (1,0) spots of the Pd substrate, and finally several weak spots slightly further from the center. Annealing to 1075 K substantially increased the intensity of the LEED pattern. As illustrated in Figure 2c, a prominent ring of 12 spots can now be seen followed by a similar ring of 12 spots just slightly further from the center; curiously, the inner ring of eight spots in Figure 2b, as well as a few spots further from the center, could no longer be seen. More prolonged annealing at 1075 K further improved the LEED patterns. The LEED patterns in Figure 3a,b illustrate all of the diffraction spots that could be seen by varying the electron energy; Figure 3a was recorded using the same film in Figure 2 annealed for a total of 10 min at 1075 K in (1−2) × 10−6 Torr of O2, while Figure 3b was recorded after annealing a thicker film, 5.1 ML, for over 1 h at 1075 K in an oxygen background which, as described above, reduced the surface coverage. The schematic in Figure 3c shows that the LEED patterns can be reproduced by considering single scattering from two domains of a hexagonal structure rotated by 30°, each with a (1,1) spot coincident with a (1,0) spot of the square substrate. The translation from the reciprocal lattice to the real space lattice is illustrated in Figure 3d. Since the reciprocal surface lattice vectors of the overlayer intersect at 60° while those of the substrate are orthogonal, the coincidence of the (1,1) spots of the overlayer with the (1,0) spots of the substrate indicate that in real space one of the close-packed directions of the overlayer aligns with Pd[011] and that the surface lattice constant of the overlayer is twice that of Pd(100) or 0.550 nm. The 0.550 nm is expanded compared to the 0.542 nm spacing observed for the crystalline SiO2 bilayer on Ru(0001).16 In the matrix notation 2 0 . Apart from the the structure can be denoted − 1 3 periodicity of the pattern, it is also obvious in Figures 3a,b that the LEED spots are elongated along the [1 1] direction of the overlayer in reciprocal space. This suggests that the coherence length is short along both of the lattice vectors in real space, including the direction parallel to Pd[011], but long parallel to the third close-packed direction as illustrated in Figure 3d; the appropriateness of this picture is independently corroborated by the STM images in section 3.4. It should be pointed out that this model does not account for all of the spots seen in Figure 2b, in particular the central ring of eight spots. Instead, these features can be associated with an oxygen-induced (√5 × √5) R27° structure on bare patches of the Pd(100) substrate.21,34,35

(

Figure 3. (a) LEED pattern obtained from 2.4 ML of SiO2 deposited on Pd(100) and annealed for 10 min at 1075 K in (1−2) × 10−6 Torr of O2. The red and green lines highlight orthogonal reciprocal unit cells. (b) LEED pattern for 5.1 ML of SiO2 deposited onto Pd(100) and annealed for over 1 h at 1075 K in (1−2) × 10−6 Torr of O2. (c) Model LEED pattern constructed from two hexagonal domains, each with a (1 1) spot coincident with a substrate {1 0} spot. The square substrate is denoted by gray spots, one hexagonal domain by green spots, and the second hexagonal domain by red spots; the coincident spots are shaded purple. The lines indicate the reciprocal unit cells of the two hexagonal domains. The black circle indicates the edge of the LEED field of view at 158 eV. (d) Real space periodicities and epitaxial alignment inferred from the LEED patterns with gray circles indicating the Pd atoms and small shaded circles the lattice points of the overlayer. Dashed lines highlight domain walls.

It is not clear why these features were not seen in subsequent LEED images; however, oxygen on Pd is highly reactive and can readily be removed by reaction with background gases in the chamber.21,36 Finally, a hexagonal LEED pattern attributed to oxidation of Si impurities in Pd(100) has been reported; however, in this case an overlayer lattice constant of 0.46 nm was suggested.35,37 3.3. Morphology and Modification of As-Grown SiO2 Films. Figure 4a displays a typical wide-range image of the Pd(100) surface after depositing SiO2, in this case 5.1 ML, without further processing. The image shows an array of steps, a series of dots, and several jumps typically associated with tunneling instabilities. We note in particular the terrace in the middle of the image, which has an unusual finger shape, thereby strongly resembling those previously reported for multilayer SiO2 films on Pd(100).14 The height histogram for this image (Figure 4b) reveals that with the exception of the small terrace at the bottom corners of the image, the terraces are separated by close to 0.2 nm. This step height closely matches the 0.195 nm height expected for Pd(100) but is much less than the 0.28 nm step height previously reported for SiO2 films grown on Pd(100) under the same conditions.14 The widths of the peaks in the histogram suggest that the terraces have an average roughness of 0.035 nm, substantially larger than would be expected for a bare Pd(100) surface. Smaller range images revealed no evidence of atomic-scale structure, also in contrast

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Figure 4. (a) Scanning tunneling microscopy image of a 5.1 ML thick SiO2 on Pd(100) recorded immediately after deposition at 550−600 K in 2 × 10−6 Torr of O2. The sample bias was −2.0 V. (b) Histogram showing the height distribution in the STM image in (a). The dashed vertical lines are 0.2 nm apart.

Figure 5. Sequence of STM images of 5.1 ML of SiO2 deposited on Pd(100) recorded after annealing to 1025 K for 5 min in 1 × 10−6 Torr of oxygen. The arrows indicate the same location in each image, thereby highlighting by comparison the formation of step edges along the horizontal fast scan direction. The sample bias was −2.0 V for all images.

Figure 6. Successive STM images of 1.02 ML of SiO2 on Pt(111) annealed to 1075 K in 2 × 10−6 Torr of O2 for 10 min. The white areas are the amorphous silica bilayer, and the lower level is the bare Pt(111) substrate. (a) First image of the edge of a large silica island. (b) In the second image, isolated, elongated islands have been ripped from the edge of the large island. (c) After zooming out, dots can be seen decorating the edges of the previous scan window, the small islands can be seen adjacent to the original island, and additional lengthening of the islands along the horizontal fast scan direction can be seen. The sample biases were (a) −1.50, (b) −1.50, and (c) −2.0 V.

second, parallel dark depression appears (Figure 5c), and finally the two depressions merge to form a finger-shaped lower terrace. In addition, a linear array of dots can be seen toward the left edge of Figure 5d. Thus, long and thin terraces can be associated with inadvertent modification of the surface with the tip. In fact, we find that any incomplete SiO2 layer is susceptible to tip-induced modification, with alterations as dramatic as those in Figure 6 common. Silica layers on Ru(0001) are similarly susceptible to tip modification.38 Figure 6a shows a large Pt(111) terrace partly covered by an amorphous SiO2 bilayer produced by high temperature annealing in background oxygen. Even though it was the first image recorded on this spot, it already reveals horizontal gashes in the silica layer. Zooming out after a second scan of the same size had been completed reveals the appearance of islands elongated in the

to the previous report that implied an ordered structure with a periodicity of 0.36 nm.14 This finding is consistent with the previous section’s LEED results that also failed to uncover any indication of crystallinity for films prepared in this manner. Similar surface morphologies were observed when films in excess of 3 ML thick were annealed to 1075 K in background oxygen but not long enough for significant decomposition of the SiO2 layer. The sequence of images presented in Figure 5, which was recorded on the surface of one such film, reveals that successive imaging leads to changes in the step structure and ultimately culminates in a finger-like morphology similar to the one of Figure 4a. Starting with Figure 5a, a narrow depression can be seen in the lower terrace; notably, this depression parallels the fast scan direction of the microscope. In subsequent images, the depression widens (Figure 5b), a 26148

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Figure 7. Scanning tunneling microscopy images of a crystalline silica bilayer on Pd(100). (a) Large-scale image revealing step faceting. (b) Higher resolution image of the same surface. At this magnification, the crystalline structure can be followed across a faceted substrate step; the white arrows point to APBs on the lower terrace. The Pd[011] direction is identified from the step direction and the close-packed directions of the pores. Rotational domain boundaries are visible on the lower terrace at the bottom left and on the upper terrace near the bottom center of the image. (c) Another high-resolution image showing a series of APBs pointed to by the arrows. Note that in both (b) and (c) the atomic contrast was enhanced by cycling through the grayscale twice. Sample biases were −0.75 V in (a) and −1.75 V in (b) and (c).

Figure 8. (a) Close-up STM image of an APB. The white lines highlight the offset between the rows across the APB while the black parallelogram connects the first repeat units on each side of the APB. (b) Structural model of the APB. Gray circles represent Pd, red circles O, and blue circles Si; the darker red circles highlight O atoms at a lower depth. The dotted parallelogram connects the centers of the six-membered rings on either side of the APB, which are assigned to the prominent dark spots in the STM images. The green line toward the lower left highlights the path along which the side view in (c) was extracted.

the terraces, the resulting 0.015 nm difference would be difficult to distinguish. Further, Figure 6 reveals essentially the same modification of the SiO2 layer in the absence of any substrate steps. Therefore, it is concluded that at modest growth temperatures silica forms flat but atomically disordered films that conform to the step-terrace structure of the Pd(100) substrate. 3.4. STM Imaging of Crystalline Silica Layers. Annealing films with less than 3 ML of SiO2 at 1075 K in background oxygen dramatically changed the surface, as illustrated by the STM images in Figure 7. These images are from the surface whose AES spectrum is provided in curve iv of Figure 1a and whose LEED pattern is given in Figure 3a. The wide-range image in Figure 7a reveals a completely different step-terrace morphology from that seen in Figures 4 and 5. The step edges tend to be very straight and to facet at predominantly 90° angles, although several steps intersecting at 45° can also be seen; the 45° step toward the upper right of the image emerges from a screw dislocation. The step directions are consistent with the square symmetry of the substrate rather than the hexagonal overlayer; thus, the step-terrace structure reflects that of the Pd(100) substrate. Further, the image shows no evidence of features elongated along the fast scan direction, and so it is

fast scan direction (Figure 6b). Finally, after a tip change, zooming out further exposes a vertical line of dots along the edge of the original scan window and a straightening of the island edges along the horizontal fast scan direction. The data in Figures 5 and 6 were collected with sample biases between −1.5 and −2 V; similar observations were made at positive sample biases and biases as low as 0.5 V. At least over the range between 0.1 and 0.55 nA, varying the tunneling current did not have a noticeable effect on the surface modification; however, it was found that the ability to stably image incomplete layers was sensitive to the condition of the tip. The morphology of the surface after SiO2 deposition at 550− 600 K can now be understood, with (i) the finger-shaped terrace and the dots on the surface being a result of prior scanning and (ii) the straighter steps above and below the finger-shaped terrace reflecting steps on the Pd(100) substrate, consistent with the measured step heights. This would then suggest that the finger-shaped terrace represents an incomplete SiO2 layer; however, Figure 4b indicates that the step height down to the neighboring layer is indistinguishable from that of Pd(100). On first sight, this seems to imply that scanning also disrupts the Pd substrate. Step heights on silica, however, can be close to those of Pd(100), with the monatomic step height expected for α-quartz (0001) 0.18 nm;33 given the roughness of 26149

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bottom half of the bilayer; thus, there are no dangling bonds in the structure. A similar rotated tetrahedron model has been proposed for silica layers on Mo(112).40,41 The rotation of the tetrahedra would create a rumpling of the bilayer, which may account for the white or raised appearance of the pore walls adjacent to the APB. As illustrated in Figure 8, the model reproduces the experimental observations well; however, some uncertainties remain. The model gives a substantially larger aspect ratio for the parallelogram connecting the pores on opposite sides of the APB, 2.55 versus 2.40. This error cannot be explained by experimental artifacts such as drift alone since the measured spacing in Figure 8a along the overlayer [1 1] direction, which runs almost vertically in the image, is only 1% larger than that along [1 0]. Further, since the overlayer is only lattice matched to the substrate in one direction and since bulk silica framework materials typically deviate from ideal hexagonal symmetry,4,27 small distortions from a perfect hexagonal structure that would be difficult to detect are likely. Another consequence of the geometric mismatch between the film and substrate is that the registry of the Si and O atoms with the Pd substrate varies substantially across a domain as depicted in Figure 8b. Therefore, the assumption that the registry is preserved across the APB may be poor. As a result, the width of the APB may be governed more by the structure of the silica layer than the Pd atom spacing. The model thus proposes a general form of the APB; more detailed characterization of the structure that is beyond the scope of the current paper would be required to pinpoint the exact atomic positions. It should be pointed out that the model pictured in Figure 8b does not include the possibility of oxygen atoms adsorbed at the Pd/SiO2 interface. For Ru(0001), crystalline bilayer structures were observed with and without such oxygen depending on the treatment conditions.16 Since the AES measurements employed in this study could neither confirm nor exclude oxygen adsorbed at the Pd/SiO2 interface, the role of interfacial oxygen in forming the observed structures needs to be considered. Under the Si deposition and subsequent oxygen treatment conditions, the most likely oxygen phases on Pd(100) are (2 × 2) and (√5 × √5)R27°.21,34,36 The latter can be excluded, since in this case, step facets tend to rotate 27°,21 which they clearly do not (cf. Figure 7). On the other hand, while (2 × 2) phases would be consistent with the observed faceting, they would not markedly change the picture in Figure 8. For the crystalline bilayer on Ru(0001), the adsorbed O forms a (2 × 2) structure with the O occupying hollow sites on the Ru surface at the center of the hexagonal silica rings. No similar structure can form on Pd(100), however, because the silica film is commensurate in only one direction on this surface. If oxygen atoms at the interface adopted a (2 × 2) configuration, the gray balls that denote the structure of the underlying surface in Figure 8b would be replaced by a mesh twice as large. A domain boundary in the adsorbed layer would then be required to maintain the registry across the APB; however, as noted above, the registry varies across a domain in any event. Alternatively, the adsorbed O atoms could fill the centers of the silica rings, which would add an O atom to the repeat unit in Figure 8b but would otherwise not alter the picture. Thus, adsorbed O is not considered to play a decisive role in the silica structures that form. 3.5. Computational Results. The objective of the computational effort was to understand how a material that is generally considered difficult to distort can be strained by close to 4%, particularly when all the bonds are saturated and the

concluded that the completed crystalline layer can be imaged without tip modification. Higher resolution images of the surface are provided in Figures 7b,c. In both of these images, the steps are substrate steps and the gray scale is cycled through on each terrace to allow the atomic-scale features to be seen. The step in Figure 7b is faceted at 90°, and a close-packed direction of the overlayer on the upper (bottom right) and lower terraces parallels the step edge that runs predominantly left to right. Since the low-energy step directions on fcc(100) surfaces are [011] and [010], and only the former parallels a close-packed direction of the overlayer (see Figure 3), the substrate [011] direction can be identified as highlighted in the image in Figure 7b. The expected 30° rotational domain boundary can be seen on the higher terrace (lower right). In addition, several slightly brighter lines run almost vertically on the lower terrace; these are antiphase domain boundaries (APBs) that can be more clearly seen in Figure 7c. The lower terrace at the bottom and upper right of Figure 7c shows four almost evenly spaced APBs, with the domains separating the boundaries four and five repeat units wide. Carrying over the assignment of the Pd[011] direction from Figure 7b, it becomes apparent that the extent of the hexagonal domains is limited along the overlayer [1 0 ] and [0 1] directions but runs for long distances in the overlayer [1 1] direction, consistent with the LEED data. The result is an array of line defects on the surface. The structure of an APB is shown in more detail in the STM image in Figure 8a. We see from this image how the rows of dark holes or pores that run along Pd [011] are shifted across the APB by a little more than half the spacing between the rows in the Pd [01̅1] direction. This is consistent with a displacement of the domains by one substrate lattice constant in the Pd [01̅1] direction, which would ideally offset the rows of pores by √3/3 or 42% of their spacing along that direction. The dotted parallelogram that connects the closest pores across the APB suggests that the width of the domain boundary is 2.4 times the 2aPd spacing of the pores within the domain, where aPd is the spacing of the Pd(100) surface. The closest match to these dimensions that keeps the pores in the same registry with the substrate across the APB is a displacement of 5aPd in the Pd [011] direction, which yields a domain boundary width of 2.55 times the spacing within the SiO2 domains. The domains are connected by elongated dark pores that can also be seen in Figure 7c. Based on the above observations and the literature on the structure of crystalline silica layers,12,15,16,39 the model of the APB structure pictured in Figure 8b was constructed. Since (i) the SiO2 coverage was initially 2.4 ML but then fell after annealing (see Figure 1) and (ii) prior work indicated that only silica bilayers form on less reactive metal surfaces (e.g., Pt15), the model assumes that the crystalline silica film on Pd(100) is a bilayer. As on Ru(0001), the perfect bilayer takes the form of two sheets of corner-sharing SiO4 tetrahedra arranged in sixmembered rings parallel to the surface and linked by Si−O−Si bonds perpendicular to it; ideally, the two honeycomb sheets lay directly on top of one another. In the model of the APB, the stretched pores are formed by creating an elongated eightmembered ring. The eight-membered ring is constructed by inserting rotated SiO4 tetrahedra into the six-membered ring that allow linear connections to the neighboring tetrahedra. This leaves two oxygens per tetrahedron, shown in dark red in Figure 8b and illustrated in the cross section in Figure 8c, that are shared with a mirrored edge-sharing tetrahedron in the 26150

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Figure 9. (a) Top-down view of the equilibrium structure of the unstrained freestanding crystalline silica bilayer. The dark solid rectangle indicates the crystallographic unit cell and the dashed line the primitive, in this case hexagonal, unit cell. The labels a and b denote the two lattice parameters. Oxygen atoms are red and Si blue. (b) Perspective view of the repeat unit of the crystalline bilayer (outlined by the dashed line in (a)). The labels O1, O2, and O3 denote the three distinct oxygen sites when the cell is stretched or compressed along b and a is allowed to relax (O1 and O3 are equivalent for hexagonal geometries); all the Si atoms are equivalent. (c) Plot of the energy of a silica bilayer as a function of b lattice parameter strain. Relaxed refers to the case where a is free to vary and hexagonal to where hexagonal geometry is fixed. The dashed lines highlight the strain states for Ru, Pd, and Pt assuming 2x structures with the computed unstrained lattice parameter for the bilayer and the experimental lattice parameters for Ru, Pd, and Pt. (d) Plot of the strain along a and out of plane, z, as the b lattice parameter is strained. (e, f) The impact of b lattice strain on the O−Si−O bond angles and Si−O bond lengths. For these plots a is allowed to relax, and the oxygen atoms are identified in (b).

actual material the sheets rumple to avoid such bond angles.4,27 Interestingly, despite the energetic penalty of 180° Si−O−Si bond angles in bulk silica, the computed equilibrium structure for the bilayer includes a 180° Si−O−Si bond that links the two halves of the structure. Meanwhile, the other two Si−O−Si bond angles are 140.3°, which is within the range favored in bulk silica,27,42,43 and the O−Si−O bond angles are within 0.5° of the ideal tetrahedral bond angle (109.5°). The Si−O bond lengths match both previous reports for the bilayer and values found in bulk silica.12,16,17,42,43 Figures 9c−f illustrate what happens to the structure and energetics of the bilayer when it is strained to match the lattice constant of a substrate. For the plot of energy versus strain in Figure 9c, two scenarios are considered: (1) a hexagonal lattice is maintained by straining the bilayer equally along both primitive lattice vectors; (2) the b lattice parameter pictured in Figure 9a is strained but the a lattice parameter is allowed to relax. The former reflects the situation for growth on (111) substrates where the lattice can be matched in both directions while the latter can apply to (100) substrates where the bilayer can only be commensurate with the substrate in one direction. In both cases, the internal coordinates are allowed to relax to reach a local energy minimum. The plot in Figure 9c shows that allowing relaxation in the orthogonal direction reduces the

layer is considered to interact with substrates solely through weak van der Waals interactions.12,17 The computed equilibrium structure of the unstrained, freestanding bilayer is pictured in Figures 9a,b; the bond lengths and angles are listed in Table 2. The computed lattice parameter for the hexagonal Table 2. Computed Equilibrium Properties of the Freestanding Crystalline Silica Bilayer property

value

symmetry lattice constant (nm) energy (relative to α-quartz) Si−O bond lengths (nm) Si−O−Si bond angles (deg) O−Si−O bond angles (deg)

hexagonal 0.5298 0.111 eV/Si 0.1623, 0.1621 109.1, 109.8 140.3, 180

structure of 0.5298 nm is essentially the same as previously reported,17 though slightly larger than the 0.526 nm expected based on the most closely related bulk phase of silica, βtridymite, which includes similar sheets of six-membered rings of corner-sharing SiO4 tetrahedra. Note that the value for βtridymite is for an idealized structure with 180° Si−O−Si bond angles linking the sheets of corner sharing tetrahedra; in the 26151

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Figure 10. Plots of the impact of b lattice strain on the O3−Si−O3 bond angle (a) and Si−O3 bond length (b) for hexagonally constrained bilayers and bilayers where the a lattice parameter is allowed to relax.

Analysis of the impact of deviations from 120° on the model two domain LEED pattern in Figure 3c, and the experimental LEED data suggest that deviations larger than 1°−1.5° from 120° would have been detected. Thus, the relaxed model appears to overestimate the degree of distortion from hexagonal. There may be several causes for this overestimation. First, as noted in the Methods section, a couple of the offdiagonal elastic constants calculated for α-quartz are significantly off, suggesting that the chosen basis set may simply overestimate how easy it is to distort the crystal. Alternatively, as described above, the relaxation along a involves a tilting of the tetrahedra such that O1 and O3 are no longer coplanar; for the strain to match the Pd lattice, the O3 atoms fall 0.03 Å below the O1 atoms on the bottom surface. For a freestanding layer, this would have no impact on the energy of the system; however, for a supported layer this would substantially weaken 1/3 of the oxygen−surface bonds. Thus, it is suggested that the substrate can constrain the interfacial oxygen to be coplanar, thus constricting how much the bilayer may relax along a. Therefore, the curves in Figure 9c should be taken as upper and lower bounds for the strain energy for silica bilayer growth on geometrically mismatched substrates.

strain energy by more than a factor of 4, to less than 0.04 eV/Si for the nearly 4% strain to match Pd. This very modest number suggests that the bilayer is surprisingly compliant. Plots of the corresponding strain in the a lattice constant and in the thickness of the film are provided in Figure 9d, which indicates that relaxation occurs predominantly within the plane of the bilayer rather than out of plane. The plots of the O−Si−O bond angles (Figure 9e) and Si−O bond lengths (Figure 9f) show that the primary relaxation mechanism is a closing and opening of the O3−Si−O3 bond angle (see Figure 9b) as the bilayer is compressed and stretched along b, respectively. Meanwhile, all the Si−O bond lengths change by no more than 1% as the film is stretched by up to 4.5%. In addition to the distortion of the bond angles, the results indicate that the tetrahedra tilt slightly, by no more than 5°, when the film is strained in one direction. As a result, the bond connecting the two halves of the bilayer deviates from 180°, causing a rumpling of the layer such that the O3 site above (below) O1 on the top (bottom) surface when the layer is in tension. To help understand why allowing the a lattice parameter to relax reduces the strain energy by such a large factor, the influence of strain on bond lengths and angles were compared for biaxially strained bilayers where the hexagonal geometry is maintained and uniaxially strained bilayers where relaxation is allowed orthogonal to the strain direction; the results are provided in Figure 10. Since Figure 9 indicates that strain has the greatest effect on bonds involving O3, the O3−Si−O3 bond angles and Si−O3 bond lengths are plotted in Figures 10a,b. The results indicate that constraining the system to the hexagonal geometry greatly limits how much the Si−O3−Si bond can bend. As a result, tensile strain stretches the Si−O3 bond 3 times as much as when a is allowed to relax. Since bonds are much more difficult to stretch than to bend, the result is a much higher strain energy. Lastly, the results indicate that the bond angle connecting the two halves of the bilayer is always 180° when the geometry is hexagonal. As noted above, the situation where a is allowed to relax should more closely mimic what happens on Pd(100); since the bilayer is incommensurate with the substrate along a, there is no driving force to constrain it to match the substrate in this direction. As highlighted in Figure 9c, this would suggest that a compresses 2% when b is stretched by 3.8% to match Pd; this translates to a 117° angle between the primitive lattice vectors.

4. DISCUSSION Silica growth on Pd(100) either at temperatures below 975 K or coverages in excess of 2 ML leads to a flat layer that conforms to the step-terrace structure of the substrate but is disordered in three dimensions. It has been suggested that atomic scale features could not be detected on such surfaces with scanning probe microscopies (SPM) because the disorder perpendicular to the surface masks any atomic scale corrugation.12 In addition, it should be pointed out that these conformal films lack the pores that are the most prominent features in SPM images of the crystalline bilayers. Despite the inability to map the atomic structure of the surface of these films, they still have properties that make them interesting as a substrate for model supported catalyst studies. These include the potential to grow the films on any metal substrate that interacts more weakly with oxygen than Si, including inert metals such as Au that would not influence catalytic reactions, but cannot be annealed to high enough temperatures to form the bilayer structures. Further, the lack of any pores down to the metal surface and the ability to grow the films more than 26152

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to a level comparable to that experienced by the hexagonal layer on Ru(0001). This suggests that it may be possible to form crystalline silica bilayers on Pt(100) or Pt(110) and that it could be challenging to form the crystalline structure on Pd(111).

two layers thick can allow an unambiguous decoupling of the surface from the underlying metal. Still, the metal allows surface science methods to be applied while the flatness can enable atomic-scale characterization of supported clusters using SPM; if the issues with tip modification in STM cannot be overcome, NC-AFM would still be feasible. At coverages up to roughly 2 ML and temperatures in excess of 975 K, crystalline silica bilayers could be created on the Pd(100) surface. Comparing the structure of the crystalline bilayer to bulk crystalline forms of SiO2 provides insights into the influence of the metal substrate on the structures that form. The hexagonal crystalline bilayer structure most closely resembles the high temperature, ambient pressure silica phases β-tridymite and β-cristabolite, both of which are constructed of honeycomb sheets of six-membered rings of corner-sharing SiO4 tetrahedra. In contrast to the bilayer, in the bulk phases the tetrahedra alternate between pointing up and pointing down, which allows three-dimensional frameworks to be constructed. The honeycomb sheets stack in an hcp manner in β-tridymite and in an fcc manner in β-cristabolite. The lattice spacing in the hexagonal sheets is 0.505 nm in β-tridymite and 0.507 nm in β-cristabolite. These numbers, however, represent values averaged over structural heterogeneity.27,43 Based on the bond lengths of these materials, ideal, flat hexagonal silica sheets would have a spacing of 0.526 nm, which compares well with the 0.53 nm reported for silica bilayers on graphene.17 Therefore, the observed 0.550 nm spacing on Pd(100) clearly puts the bilayer under considerable tensile stress, suggesting that the frequent domain boundaries seen along Pd[011] where the film is in registry with the substrate are not due to nucleation and growth kinetics but are intrinsic features of the surface that allow stress relief as has been seen for alumina films on NiAl.44,45 The domains are long along the bilayer [1 1] direction because the film is not in registry with the square substrate in this direction and thus may relax by contracting, thereby creating a distorted hexagonal structure. The calculations for the freestanding bilayers indicate that the energy gained by relaxation in the incommensurate direction can be considerable. Interestingly, the crystalline bilayer has now been seen on Ru(0001) with a lattice constant of 0.542 nm12 and on Pd(100) with a lattice constant of 0.550 nm, but on Pt(111), where it would have a lattice constant of 0.554 nm in a (2 × 2) structure, only amorphous bilayers could be produced.15 It was suggested that the comparatively weak Pt−O interaction precluded the formation of the crystalline layer on Pt(111);15 as illustrated in Figure 9c, the additional 0.2 eV strain energy required to form the hexagonal bilayer on Pt(111) undoubtedly also plays a role. The interaction of oxygen with Pd, however, is much more similar to that with Pt rather than that with Ru. In fact, adsorbed oxygen on both Pd and Pt(111) form (2 × 2) structures from which oxygen desorption peaks near 750 K;46−48 thus, oxygen adsorption at the silica/metal interface is unlikely to account for the ability to from ordered structures on Pd(100) but not on Pt(111). Therefore, we propose that the square substrate actually aids the formation of the crystalline bilayer by providing an incommensurate direction along which stress can be relieved by contraction rather than APB formation. In contrast, on Pt(111) strain would need to be relieved along three equivalent directions leading to so many APBs that a crystalline description is no longer valid. The results in Figure 9c suggest that allowing the crystalline bilayer to relax along one direction can reduce the strain energy on Pt

5. SUMMARY Silica films grown on Pd(100) at modest temperatures were shown to lead to flat but atomically disordered films that conform to the step-terrace structure of the substrate. Annealing such films in oxygen at temperatures above 975 K can produce crystalline silica bilayers with hexagonal or nearly hexagonal symmetry. The crystalline bilayer forms epitaxially with one of its close-packed directions parallel to Pd[011] and a spacing twice that of the substrate along this direction. The interaction with the substrate expands the spacing of the bilayer compared to hexagonal SiO2 sheets seen in bulk crystalline phases of silica and silica bilayers previously seen on graphene and Ru(0001). Images of the crystalline bilayer revealed regular domain boundaries that limit the extent of the domains along Pd[011] where the bilayer is in registry with the substrate; it is proposed that these domain boundaries help relieve the tensile stress in the bilayer. In contrast, the domains are large along the overlayer [1 1] direction where the film is incommensurate with the substrate and thus can slightly contract without any energetic penalty due to a loss of registry with the substrate. To understand how the epitaxial strain may be relieved, ab initio calculations were performed on strained, freestanding silica bilayers. The results indicate that allowing the structure to distort from hexagonal as it is strained allows the stress to be primarily relieved by bending Si−O−Si bond angles rather than stretching the Si−O bonds. This reduces the strain energy 4fold compared to the scenario where the bilayer is constrained to match a hexagonal substrate. Therefore, despite the geometric mismatch between the crystalline silica bilayer and the Pd(100) substrate, the square substrate actually plays an important role in directing the structure of the overlayer. This finding helps explain why it has been possible to form amorphous but not crystalline silica bilayers on Pt(111).

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], Ph 203-432-4375 (E.I.A.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the US Department of Energy through Basic Energy Sciences Grant DE-FG02-06ER15834 and the National Science Foundation through the Yale Materials Research Science and Engineering Center Grant DMR-1119826. J.G. gratefully acknowledges financial support by the German Science Foundation (DFG) through GO 2093/ 1-1. The authors also acknowledge the help of M. Li, X. Zhu, G.H. Simon, O. E. Dagdeviren, and M. Herdiech in carrying out this work.

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REFERENCES

(1) Auerbach, S. M.; Carrado, K. A.; Dutta, P. K. Handbook of Zeolite Science and Technology; M. Dekker: New York, 2003. (2) Weitkamp, J.; Puppe, L. Catalysis and Zeolites: Fundamentals and Applications; Springer: Berlin, 1999. 26153

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Article

(3) Liebau, F. Structural Chemistry of Silicates; Springer-Verlag: Berlin, 1985. (4) Griffen, D. T. Silicate Crystal Chemistry; Oxford University Press: New York, 1998. (5) Shaikhutdinov, S.; Freund, H.-J. Ultrathin Silica Films on Metals: The Long and Winding Road to Understanding the Atomic Structure. Adv. Mater. 2013, 25, 49−67. (6) Chen, M. S.; Goodman, D. W. Ultrathin, Ordered Oxide Films on Metal Surfaces. J. Phys.: Condens. Matter 2008, 20, 264013−1− 264013−11. (7) Xu, X.; Goodman, D. W. The Preparation and Characterization of Ultra-thin Silicon Dioxide Films on a Mo(110) Surface. Surf. Sci. 1993, 282, 323−32. (8) Wendt, S.; Ozensoy, E.; Wei, T.; Frerichs, M.; Cai, Y.; Chen, M. S.; Goodman, D. W. Electronic and Vibrational Properties of Ultrathin SiO2 Films Grown on Mo(112). Phys. Rev. B 2005, 72, 115409−1− 115409−9. (9) Schroeder, T.; Giorgi, J. B.; Baumer, M.; Freund, H. J. Morphological and Electronic Properties of Ultrathin Crystalline Silica Epilayers on a Mo(112) Substrate. Phys. Rev. B 2002, 66, 165422−1− 165422−11. (10) Weissenrieder, J.; Kaya, S.; Lu, J. L.; Gao, H. J.; Shaikhutdinov, S.; Freund, H. J.; Sierka, M.; Todorova, T. K.; Sauer, J. Atomic Structure of a Thin Silica Film on a Mo(112) Substrate: a TwoDimensional Network of SiO4 Tetrahedra. Phys. Rev. Lett. 2005, 95, 076103−1−076103−4. (11) Lichtenstein, L.; Buchner, C.; Bing, Y.; Shaikhutdinov, S.; Heyde, M.; Sierka, M.; Wlodarczyk, R.; Sauer, J.; Freund, H. J. The Atomic Structure of a Metal-Supported Vitreous Thin Silica Film. Angew. Chem., Int. Ed. 2012, 51, 404−407. (12) Bing, Y.; Kaden, W. E.; Xin, Y.; Boscoboinik, J. A.; Martynova, Y.; Lichtenstein, L.; Heyde, M.; Sterrer, M.; Wlodarczyk, R.; Sierka, M.; et al. Thin Silica Films on Ru(0001): Monolayer, Bilayer and Three-dimensional Networks of [SiO4] Tetrahedra. Phys. Chem. Chem. Phys. 2012, 14, 11344−11351. (13) Kundu, M.; Murata, Y. Growth of Single-Crystal SiO2 Film on Ni(111) Surface. Appl. Phys. Lett. 2002, 80, 1921−1923. (14) Zhang, Z.; Jiang, Z.; Yao, Y.; Tan, D.; Fu, Q.; Bao, X. Preparation and Characterization of Atomically Flat and Ordered Silica Films on a Pd(100) Surface. Thin Solid Films 2008, 516, 3741−3746. (15) Xin, Y.; Bing, Y.; Anibal Boscoboinik, J.; Shaikhutdinov, S.; Freund, H. J. Support Effects on the Atomic Structure of Ultrathin Silica Films on Metals. Appl. Phys. Lett. 2012, 100, 151608−1− 151608−5. (16) Loffler, D.; Uhlrich, J. J.; Baron, M.; Yang, B.; Yu, X.; Lichtenstein, L.; Heinke, L.; Buchner, C.; Heyde, M.; Shaikhutdinov, S.; Freund, H. J.; Wlodarczyk, R.; Sierka, M.; Sauer, J. Growth and Structure of Crystalline Silica Sheet on Ru(0001). Phys. Rev. Lett. 2010, 105, 146104−1−146104−4. (17) Huang, P. Y.; Kurasch, S.; Srivastava, A.; Skakalova, V.; Kotakoski, J.; Krasheninnikov, A. V.; Hovden, R.; Mao, Q.; Meyer, J. C.; Smet, J.; et al. Direct Imaging of a Two-Dimensional Silica Glass on Graphene. Nano Lett. 2012, 12, 1081−1086. (18) Stacchiola, D. J.; Baron, M.; Kaya, S.; Weissenrieder, J.; Shaikhutdinov, S.; Freund, H. J. Growth of Stoichiometric Subnanometer Silica Films. Appl. Phys. Lett. 2008, 92, 011911−1− 011911−3. (19) Nakakura, C. Y.; Phanse, V. M.; Zheng, G.; Bannon, G.; Altman, E. I.; Lee, K. P. A High-Speed Variable-Temperature Ultrahigh Vacuum Scanning Tunneling Microscope. Rev. Sci. Instrum. 1998, 69, 3251−3258. (20) Li, M.; Altman, E. I. Formation and Interaction of Epitaxial Ge Structures on Ge(001). Phys. Rev. B 2002, 66, 115313−1−115313−10. (21) Zheng, G.; Altman, E. I. The Oxidation Mechanism of Pd(100). Surf. Sci. 2002, 504, 253−270. (22) Dovesi, R.; Saunders, V. R.; Roetti, C.; Orlando, R.; ZicovichWilson, C. M.; Pascale, F.; Civalleri, B.; Doll, K.; Harrison, N. M.; Bush, I. J.; et al. Crystal09 2.0.1 User’s Manual; University of Turin: Torino, 2013.

(23) Perger, W. F.; Criswell, J.; Civalleri, B.; Dovesi, R. Ab-initio Calculation of Elastic Constants of Crystalline Systems with the CRYSTAL Code. Comput. Phys. Commun. 2009, 180, 1753−1759. (24) Tosoni, S.; Civalleri, B.; Ugliengo, P. Hydrophobic Behavior of Dehydroxylated Silica Surfaces: A B3LYP Periodic Study. J. Phys. Chem. C 2010, 114, 19984−19992. (25) Bredow, T.; Gerson, A. R. Effect of Exchange and Correlation on Bulk Properties of MgO, NiO, and CoO. Phys. Rev. B 2000, 61, 5194−5201. (26) Peintinger, M. F.; Oliveira, D. V.; Bredow, T. Consistent Gaussian Basis Sets of Triple-Zeta Valence with Polarization Quality for Solid-state Calculations. J. Comput. Chem. 2013, 34, 451−459. (27) Keen, D. A.; Dove, M. T. Local Structures of Amorphous and Crystalline Phases of Silica, SiO2, by Neutron Total Scattering. J. Phys.: Condens. Matter 1999, 11, 9263−9273. (28) Chao, S. S.; Tyler, J. E.; Tsu, D. V.; Lucovsky, G.; Mantini, M. J. Auger Electron Spectroscopy Studies of Silicon Nitride, Oxide, and Oxynitride Thin Films: Minimization of Surface Damage by Argon and Electron Beams. J. Vac. Sci. Technol., A 1987, 5, 1283−1287. (29) Chao, S. S.; Tyler, J. E.; Takagi, Y.; Pai, P. G.; Lucovsky, G.; Lin, S. Y.; Wong, C. K.; Mantini, M. J. A Study of Chemical Bonding in Suboxides of Silicon Using Auger Electron Spectroscopy. J. Vac. Sci. Technol., A 1986, 4, 1574−1579. (30) Davis, L. E.; MacDonald, N. C.; Palmberg, P. W.; Riach, G. E.; Weber, R. E. Handbook of Auger Electron Spectroscopy, 2nd ed.; Physical Electronics Industries: Eden Prairie, MN, 1976. (31) Deal, B. E.; Grove, A. S. General Relationship for the Thermal Oxidation of Silicon. J. Appl. Phys. 1965, 36, 3770−3778. (32) O’Connor, D. J.; Sexton, B. A.; Smart, R. S. C. Surface Analysis Methods in Materials Science, 2nd ed.; Springer-Verlag: Berlin, 2003. (33) Steurer, W.; Apfolter, A.; Koch, M.; Sarlat, T.; Sondegard, E.; Ernst, W. E.; Holst, B. The Structure of the β-quartz (0001) Surface Investigated Using Helium Atom Scattering and Atomic Force Microscopy. Surf. Sci. 2007, 601, 4407−4411. (34) Vu, D. T.; Mitchell, K. A. R.; Warren, O. L.; Thiel, P. A. Tensor LEED Analysis of the Pd(100)-(√5x√5) R27°-O Surface Structure. Surf. Sci. 1994, 318, 129−138. (35) Orent, T. W.; Bader, S. D. LEED and ELS Study of the Initial Oxidation of Pd(100). Surf. Sci. 1982, 115, 323−334. (36) Zheng, G.; Altman, E. I. The Reactivity of Surface Oxygen Phases on Pd(100) Towards Reduction by CO. J. Phys. Chem. B 2002, 106, 1048−1057. (37) Bader, S. D.; Richter, L.; Orent, T. W. Auger-UPS Study of Intrinsic and Si-Stabilized Oxides of Palladium. Surf. Sci. 1982, 115, 501−512. (38) Shaikhutdinov, S. Personal communication. (39) Todorova, T. K.; Sierka, M.; Sauer, J.; Kaya, S.; Weissenrieder, J.; Lu, J. L.; Gao, H. J.; Shaikhutdinov, S.; Freund, H. J. Atomic Structure of a Thin Silica Film on a Mo(112) Substrate: A Combined Experimental and Theoretical Study. Phys. Rev. B 2006, 73, 165414− 1−165414−9. (40) Ricci, D.; Pacchioni, G. Structure of Ultrathin Crystalline SiO2 Films on Mo(112). Phys. Rev. B 2004, 69, 161307−1−161307−4. (41) Giordano, L.; Ricci, D.; Pacchioni, G.; Ugliengo, P. Structure and Vibrational Spectra of Crystalline SiO2 Ultra-thin Films on Mo(112). Surf. Sci. 2005, 584, 225−236. (42) Tucker, M. G.; Squires, M. P.; Dove, M. T.; Keen, D. A. Dynamic Structural Disorder in Cristobalite: Neutron Total Scattering Measurement and Reverse Monte Carlo Modelling. J. Phys.: Condens. Matter 2001, 13, 403. (43) Cope, E. R.; Dove, M. T. Evaluation of Domain Models for βCristobalite from the Pair Distribution Function. J. Phys.: Condens. Matter 2010, 22, 125401−1 - 125401−7. (44) Libuda, J.; Winkelmann, F.; Baumer, M.; Freund, H. J.; Bertrams, T.; Neddermeyer, H.; Muller, K. Structure and Defects of an Ordered Alumina Film on NiAl(110). Surf. Sci. 1994, 318, 61−73. (45) Simon, G. H.; König, T.; Rust, H. P.; Heyde, M.; Freund, H. J. Atomic Structure of the Ultrathin Alumina on NiAl(110) and Its Antiphase Domain Boundaries as Seen by Frequency Modulation 26154

dx.doi.org/10.1021/jp4101152 | J. Phys. Chem. C 2013, 117, 26144−26155

The Journal of Physical Chemistry C

Article

Dynamic Force Microscopy. New J. Phys. 2009, 11, 093009−1− 093009−16. (46) Zheng, G.; Altman, E. I. The Oxidation of Pd(111). Surf. Sci. 2000, 462 (1−3), 151−168. (47) Weaver, J. F.; Chen, J.-J.; Gerrard, A. L. Oxidation of Pt(111) by Gas-Phase Oxygen Atoms. Surf. Sci. 2005, 592, 83−103. (48) Zhdanov, V. P.; Kasemo, B. Simulation of Oxygen Desorption from Pt(111). Surf. Sci. 1998, 415, 403−410.

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