Ultrasmooth Ru(0001) Films as Templates for Ceria Nanoarchitectures

Article pubs.acs.org/crystal

Ultrasmooth Ru(0001) Films as Templates for Ceria Nanoarchitectures Marc Sauerbrey,† Jan Höcker,† Meikel Wellbrock,† Marco Schowalter,† Jon-Olaf Krisponeit,†,‡ Knut Müller-Caspary,† Andreas Rosenauer,†,‡ Gang Wei,§ Lucio Colombi Ciacchi,‡,§ Jens Falta,†,‡ and Jan Ingo Flege*,†,‡ †

Institute of Solid State Physics, University of Bremen, Otto-Hahn-Allee, 28359 Bremen, Germany MAPEX Institute for Materials and Processes, University of Bremen, 28359 Bremen, Germany § Bremen Center for Computational Materials Science, Am Fallturm 1, 28359 Bremen, Germany ‡

W Web-Enhanced Feature * S Supporting Information *

ABSTRACT: Single crystalline magnetron sputter-deposited Ru(0001) epitaxial thin films on c-plane sapphire were prepared and used as a template for reactive CeO2 growth. Low-energy electron microscopy and diffraction, as well as transmission electron microscopy and atomic force microscopy, experiments were performed to investigate the crystallinity and morphology of the prepared films. Multiple cycles of Ar+ sputtering and high-temperature annealing produces films of exceptional surface quality. High-temperature reactive ceria growth leads to perfectly aligned triangular single-crystalline CeO2(111) islands of extraordinary morphological and structural homogeneity. At the chosen growth conditions, ceria nucleation takes place only at V-shaped surface defects on the otherwise atomically flat Ru terraces, opening up the possibility to influence the nucleation by introducing artificial surface defects using standard etching techniques. Due to their high crystallinity and extraordinary surface quality, these substrates present a low-cost alternative to Ru single crystals for model studies in heterogeneous catalysis and also allow for the use of destructive investigation techniques and irreversible surface modifications.

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INTRODUCTION Rare-earth oxides are well-known for their interesting materials properties and have been under intensive investigation over the last decades.1,2 Rare-earth oxides, and especially cerium oxide, deposited on transition metal substrates have become a large playground for a wide variety of surface science studies and in heterogeneous catalysis.3 However, the exact microscopic growth mechanisms and interfacial interactions are still not fully understood. In order to gain deeper insights into these processes, which largely influence the resulting materials properties, the availability of essentially perfect substrates is considered crucial. Specifically, it has been shown that the strength of interaction between the CeOx film and the substrate4 varies quite significantly between the different transition metals,5 which in turn leads to differences in the respective oxidation states, crystal structures, and morphologies of the ceria overlayers.6−14 Based on a large number of surface chemistry studies, it has also been shown that not only the oxidation state but especially the exposed crystal facet, and thus the crystallographic orientation of the oxide surface, is an important tuning knob in the process toward the fabrication of customized catalysts.15,16 In terms of model catalyst architectures, cerium © 2016 American Chemical Society

oxide on Ru is especially interesting because of the high degree of ordering of the ceria islands as well as its low miscibility in Ru;13,17,18 therefore, on this substrate the intrinsic structural properties of ceria become particularly prominent. Ceria islands grow in triangular shape since CeO2 crystallizes in the fluorite structure (space group Fm3̅m), which exhibits a 3-fold symmetry along the (111) axis. To manipulate the properties of the ceria islands, it is crucial to gain fundamental insights into the nucleation and growth dynamics. For such fundamental studies, usually commercially available single crystals are used. However, these substrates are expensive and generally not suitable for destructive investigation methods and irreversible surface manipulation. Inspired by approaches of using magnetron-sputtered epitaxial Ru films as a template for graphene growth by Sutter et al.,19 we investigated in situ the preparation necessary to obtain Ru thin films with comparable or even better crystallinity and morphology than commercially available Ru single crystals. We then used this application-oriented substrate to study the Received: February 4, 2016 Revised: May 28, 2016 Published: June 24, 2016 4216

DOI: 10.1021/acs.cgd.6b00192 Cryst. Growth Des. 2016, 16, 4216−4224

Crystal Growth & Design

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high-temperature deposition of ceria, which allowed an unrivaled revelation of the nucleation and growth dynamics on a microscopic scale. Furthermore, we provide first results on the nanoarchitecture of ceria islands using focused ion beam patterning of our substrates.

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EXPERIMENTAL SECTION

Al2O3(0001) substrates (Crystec) with a misorientation less than 0.1° were used as growth templates for the Ru thin films. The sputter deposition was performed in a vacuum chamber with a base pressure better than 10−8 Torr, using a commercial MeiVac MAK magnetron sputtering source with a Ru sputter target (purity 99.99%). Before Ru deposition, the Al2O3(0001) substrates were cleaned by heating in vacuum to 650 °C for 20 min. The Ru(0001) films were subsequently deposited on the Al2O3(0001) substrates by magnetron sputtering at a sample temperature of 630 °C in an argon pressure of about 4 × 10−3 Torr. The Ru samples were then transferred through air to a commercial Elmitec LEEM III low-energy electron microscopy system with a base pressure better than 2 × 10−10 Torr. This system is equipped with various apertures for microillumination low-energy electron diffraction (μLEED), allowing the acquisition of μLEED patterns from regions as small as 250 nm in diameter. To obtain high surface quality, the Ru films were post-treated by several cycles of Ar+ sputtering and high-temperature annealing in ultrahigh vacuum (UHV) and subsequently in an oxygen background pressure. The final annealing step was carried out in situ to observe the smoothening of the Ru surface in real-time. Ceria growth experiments were conducted in situ, evaporating cerium metal from a homemade electron beam evaporator out of a Mo crucible in an oxygen partial pressure of 5 × 10−7 Torr. Transmission electron microscopy (TEM) specimens were prepared with a FEI Nova 200 focused ion beam (FIB)/scanning electron microscope (SEM) system. In order to protect the ceria islands, a thin carbon layer was sputtered onto the sample before FIB preparation. After the FIB lift-out, the lamella was further etched in a Technoorg Linda, model IV ion mill system using Ar+ ions with an energy of 400 eV. An FEI Titan 80/300 electron microscope equipped with a corrector system for spherical aberration of the objective lens as well as a Fischione model 300 high-angle annular dark field (HAADF) detector was operated at an acceleration voltage of 300 kV for the investigations.

Figure 1. Analysis of the as-prepared and the post-treated Ru surface. (a) LEEM image of the untreated Ru surface after sputter deposition on Al2O3. The inset shows the respective LEED pattern recorded at 40 eV. (b) LEEM image and LEED pattern (inset) of the post-treated Ru surface. (c) AFM image of the post-treated surface. (d) AFM line profiles from the as prepared and the post-treated Ru surface (orange arrow in panel c).

ordered, essentially single-grain Ru film without any rotational domains. The roughness of the untreated Ru film was determined ex situ by atomic force microscopy (AFM). A representative height profile is shown in Figure 1d. It confirms that the surface is flat on a large scale, but exhibits a considerable roughness on the angstrom scale. For removing any contamination resulting from the transfer process as well as to reduce the surface roughness and improve their crystalline quality, the Ru films were treated with several cycles of Ar+ sputtering and postannealing up to 1050 °C both in UHV and subsequently in an oxygen background pressure of 2 × 10−6 Torr. The corresponding LEEM, LEED, and AFM images are shown in Figure 1b,c. After the surface treatment, micrometer-sized terraces can be observed in LEEM and AFM. Also the surface roughness was decreased significantly as indicated by the significantly decreased inelastic background in the LEED pattern. This finding is corroborated by the AFM height profile after annealing (Figure 1d), revealing atomically flat terraces that are only separated by single atomic steps. However, also randomly distributed surface defects of different sizes can be found on the sample surface, appearing as black dots in the LEEM image in Figure 1b. During the in situ observation of the Ru step-flow in the annealing process, some of these defects were found to act as pinning centers for the Ru surface steps. AFM measurements reveal a hexagonal outline and V-shaped height profile of the Ru defects (see Figure 2). Due to the 6-fold symmetry of the Ru, we assume an alignment of the defect edges with the main azimuthal crystallographic axes of the film. Similar defect shapes can also be observed in, for example, GaN films deposited on sapphire substrates, which are created by the threading dislocations of the sapphire substrates.20,21 In reference to these defects, we call the observed holes in the Ru film V-pits. Comparing the surface quality of the sputter-deposited films with the surfaces of commercially available low-miscut Ru single crystals,13,22 it follows that the sputter-deposited films are

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RESULTS AND DISCUSSION In this section, we will first present our findings regarding the preparation, post-treatment, and characterization of the Ru films followed by a detailed ceria growth study on the optimized Ru templates. Based on our findings regarding the nucleation of ceria islands, a phenomenological growth model is proposed. We then present our first results on ceria growth on prepatterned Ru substrates. Ru Thin Films on Al2O3(0001): Morphology and Structure. First, we investigated the untreated sputterdeposited Ru film with LEEM and μLEED. Figure 1a shows a LEEM image of the as-prepared sample surface, as well as the corresponding μLEED image recorded from a 5 μm area. The LEEM image suggests an already flat surface on the micrometer scale but with a significant small-scale roughness judging from the grainy appearance of the image. Notably, no terraces or atomic steps are visible. The surface roughness leads to significant inelastic electron scattering, resulting in a diffuse but intense background in the upper left quarter of the μLEED image (inset in Figure 1a). However, a weak but sharp characteristic 6-fold Ru diffraction pattern in only one azimuthal orientation can be observed in the LEED image. Together, these findings indicate the presence of an already 4217



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place at the V-pits. The Ru surface exhibits about 2.1 defects/ μm2 before ceria deposition and about 1.7 ceria islands/μm2 after deposition, which means that approximately 80% of the Ru surface defects act as nucleation seeds for the ceria islands. Interestingly, no nucleation can be observed at Ru steps as previously reported by Kaemena et al. for CeOx growth on a Ru(0001) single crystal at comparable preparation parameters.13 The V-pits remain observable at the later stages of the growth, even when each V-pit has already been completely enclosed and overgrown by the respective island. Closer inspection of many ceria islands reveals that the V-pit is usually not located in the center of the island but closer to one of its edges. In the following, this edge will be denoted as the island base (see Figure 3c). As shown in Figure 3b, the base of each triangle can be oriented in one of six different directions (numbers 1−6), rotated by multiples of 60° with respect to each other as a consequence of the hexagonal shape of the Vpits. However, due to their 3-fold symmetry, only two different triangle orientations can be distinguished when only the outer shape of the islands is taken into account. To explain this behavior, we propose the following nucleation and growth scenario as illustrated in Figure 3c. Upon initial Ce deposition, highly mobile CeOx species are present on the surface, exhibiting a diffusion length in the range of the mean V-pit distance. These mobile species agglomerate in the V-pits and sequentially fill them. Due to the disturbed ordering within the defects, the ceria filling the V-pits is most likely amorphous or polycrystalline (cf. Figure 3c, step 2). After

Figure 2. Characterization of surface defects by AFM. (a) AFM image (vertical deflection channel) of defects in the untreated sputterdeposited Ru thin films. Laterally hexagonal-shaped defects can be observed on the surface (marked by orange circles). (b) V-shaped height profiles taken from hexagonal defects shown in panel a (marked with arrows) using the height channel of the image.

at least similar with respect to morphology, crystallinity, and roughness, rendering the films well suitable as a replacement for expensive single crystal substrates. Reactive CeO2 Growth. Using the high quality sputterdeposited Ru(0001) films as substrates, we have followed the growth of ceria at high temperature in situ (see LEEM movie S1). Ceria Nucleation. Figure 3a shows the same sample area as Figure 1b but after deposition of about 0.17 MLE (monolayer equivalent; 1 MLE corresponds to a Ce areal density of 7.89 × 1014 cm−2) of ceria. It is obvious that ceria nucleation only takes

Figure 3. Evaluation of the ceria island orientation. (a) LEEM composite image (one taken directly before ceria island nucleation and another after deposition of about 0.17 MLE of CeO2 at the same sample position as shown in Figure 1b) demonstrating the spatial correlation of the island nuclei and the location of the substrate defects. Step edges are marked in orange. Two triangle orientations (marked with red and yellow triangles) can be observed on a single Ru terrace. (b) LEEM image at the same coverage highlighting the ceria morphology on a larger scale. The V-pits remain visible on the growing ceria islands as a dark spot. The inset shows a ceria island growing at a V-pit and a schematic to highlight the position of the V-pit with respect to the island base. The six different directions (rotated by 60° against each other) from the island base (indicated by the V-pit) to the opposite corner are highlighted by the numbers 1−6 and the arrows in the LEEM image. Based on our AFM analysis (Figure 7), the islands are about 5 nm high. (c) Schematic drawing of the filling of a V-pit with ceria and the subsequent nucleation and growth of a well-ordered ceria triangle at the V-pit boundary. 4218



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Figure 4. Analysis of ceria growth dynamics (shutter open at 0 s). (a) LEEM time-lapse series of two merging islands with similar edge length. As soon as the islands get in contact with each other they start the formation of a larger (combined) triangle. First the base of the new triangle is formed (denoted as line filling). Then the area between the two islands is rapidly filled. The defect marked by the red circle hinders the growth of the island, which is why the large triangle cannot be completed. (b, c) Lateral ceria island growth velocity. (b) The growth velocity of the small island marked with the yellow circle in the inset has been analyzed. The mean growth velocity is 10.1 nm2/s. (c) The integral growth velocity of the two islands (marked by the yellow ellipse) strongly increases after the islands touch. The jumps in the plots are due to focus changes in LEEM, which lead to an apparent difference in the island area.

the filling, also some species decorate the hexagonally shaped boundary of the V-pit. Due to the alignment of these boundaries parallel to the crystal axes of the Ru substrate, highly favored adsorption sites are generated, determining the orientation of the islands. As the island’s edge becomes longer than the edge of the defect, corner adsorption sites are formed between the triangular island and the polycrystalline cerium oxide, leading to a quick incorporation of the V-pit into the island. After this incorporation, the island forms an equilateral triangle, and the three side facets keep growing outward. Interestingly, the nucleation center remains visible as a dark spot on the island. This is most likely caused by the disturbed crystal structure at the nucleation center, locally reducing the electron reflectivity. Ceria Growth Dynamics. Enabled by the in situ capabilities of the LEEM, we investigated the growth dynamics of the ceria

islands in more detail. A representative time-lapse sequence is presented in Figure 4a (also see movie S2 for further details). Figure 4b shows a plot of the increase of surface area over time of a free-standing ceria island (marked by the yellow circle in the inset). Besides the jumps in the curve, which are caused by focus adjustments during the experiment, the island exhibits a linear increase of the area, yielding a growth velocity of about 10 nm2/s. This behavior can be explained by an effective 2D, row-by-row growth model of the ceria islands’ side facets as illustrated in Figure 5a. When a CeOx precursor hits the edge of an island, it diffuses along the edge to find an energetically favorable position. After adsorption, it provides at least one corner adsorption site on the island’s edge. It is to be expected that it is energetically favorable for further precursors to adsorb at these corner sites along the edge until the row is completed. This requires a diffusion length of the Ce metal or CeOx 4219



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increases significantly (see Figure 4c). While the growth rate before contact is approximately 22.3 nm2/s, it increases by a factor of about 1.6 after both islands get in direct contact. Remarkably, the growth rate is again constant. The increase in growth velocity can again be explained in the row-by-row growth model, assuming that the above-described growth is limited by the adsorption of precursors and especially by the formation of edge sites. The schematic drawing in Figure 5b depicts the configuration at the contact point of the two triangles, revealing the formation of a corner adsorption site with four nearest neighbors (4N). Such 4N corner sites are available until the area between the triangles is completely filled. Therefore, no new adsorption sites need to be created before a new row can be filled. It is intuitive that adsorption at such 4N corner sites is energetically even more favorable than at the previously discussed 3N corner sites. The increase in growth velocity is now understandable since as long as 4N sites are available, the formation of new adsorption sites is not a growth-limiting factor. The growth series in Figure 4 highlights that the ceria growth in the area between the two islands is hindered by a surface defect (cf. Figure 4a, 2781 s), which leads to the formation of two separate apexes instead of the islands merging completely into a single large triangle. Other than that, the only evidence for the large island being composed of two separate islands are the two dark spots that remain visible at the nucleation center of each individual island. Despite the significant lattice mismatch of ceria and Ru (∼40%) and the resulting inequality of the nucleation sites, no grain boundary or step edge can be observed in the contact region. Apparently, the possible mismatch of the island lattices in the contact region does not affect the merging of the islands.

Figure 5. Sketch of ceria island growth model. (a) Row-by-row growth. CeOx precursors diffuse along the edge of the island until they find an under-coordinated site. New nucleation of “corner seed sites” only takes place after a row has been completed. (b) 2D growth model of islands after getting in contact. 3N/4N: three/four nearest neighbor sites. The 4N corner site remains present until the area between the islands is filled.

precursors to be in the range of the mean distance of the V-pits as well as a limited supply of material on the surface. Hence, this model suggests that it is energetically favorable to fill up one row before a new nucleation site is formed, leading to a linear increase of the area of an island. Figure 4a shows a time-lapse sequence of two islands of similar edge length getting in contact during growth (LEEM movie S2). As soon as contact between the two islands is established, the integral growth velocity of the two islands

Figure 6. Analysis of ceria islands. (a) Large-scale LEEM image of the sample surface after CeO2 deposition at 900 °C. Only two different triangle orientations can be distinguished. The triangles are either pointing up (red) or down (yellow). This shows the strong correlation between the two lattices as well as the large scale homogeneity of the Ru substrate. (b) μLEED image of the sample (illuminated area 5 μm in diameter). The sharp and intense CeO2(111) diffraction pattern indicates high crystallinity of the islands; the absence of off-axis rotational domains demonstrates a perfect azimuthal alignment of the ceria lattice with the Ru substrate. (c) High-resolution LEEM image of merged ceria island shown in Figure 4. Step edges are visible on the top facet. (d) I(V)-LEEM analysis of the large CeO2 island in panel c and identification by comparison to available fingerprints24,26 from the literature. The I(V) curves have been recorded from areas marked by red and blue filled circles in panel c. 4220



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Figure 7. AFM evaluation of island heights and sizes. (a) Height distribution taken from a representative sample area (cf. inset, AFM vertical deflection channel). The mean height of the islands is about 5.3 nm. (b) Aspect ratio plot of ceria islands. The majority of the islands are between 12 and 18 trilayers (TL) thick while 18 TL seems to be the asymptotic limit of the height distribution. The island heights were determined from the AFM height channel.

already stated above, the nucleation of the triangles at the Ru Vpits can consistently explain this finding. On the large ceria island in Figure 6c, thin dark lines are observable. Using representative AFM measurements from comparatively large islands, these lines can readily be identified as single or multiple trilayer steps. Moreover, the ceria islands in our AFM images were identified automatically by numeric means, and the results regarding the height and respective area of each individual island were used to compile a histogram as depicted in Figure 7. The ceria islands show a mean height of about 5.3 nm. The evaluation of the aspect ratio of the islands proves a preferential lateral growth after reaching a certain height rather than a continued, truly 3D growth mechanism. Consequently, our statistical analysis also demonstrates that large islands are on average only slightly higher than smaller islands following a strongly nonlinear relation of height versus size. At the growth conditions employed, the island height distribution exhibits an asymptotic trend toward the limit of 18 ceria trilayers. Whereas it is not possible to directly determine the absolute height of the islands during the growth process in LEEM, it is possible to detect changes in the height of objects due to concomitant changes in focus conditions. On that basis, we determined that the vertical growth of the island is predominantly limited to the first few minutes after nucleation. After that initial period, almost only lateral growth is observed, consistent with the outcome of the statistical analysis of the AFM images. Further structural characterization was carried out using TEM. Figure 8 shows a high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image of the structure. In this mode, only electrons scattered to high angles (30−300 mrad) are detected. The scattered intensity strongly depends on the atomic number of the element and hence leads to a strong Z-contrast in the resulting image, because Z differs at least by 14 among the elements present here. The Al2O3 substrate, the Ru layer, a CeO2 island, and the C and Pt protection layers are clearly visible due the different atomic numbers of the elements in the layers. The Ru layer has a homogeneous thickness of about 76 ± 2 nm, and the intensity distribution within the Ru layer is homogeneous indicating a single-crystalline nature. Further supporting evidence is provided by selected area electron diffraction (SAED) taken at different positions across the specimen. It is found that the layer is oriented close to a [1̅21̅0] zone axis of Ru, and no

In the case of two islands with significantly different edge lengths coalescing, the result can be slightly different from what has been described above. While the lateral growth behavior is essentially the same, the resulting island morphology can be altered significantly. The contact region as well as the original island boundary remains visible as step edges or grain boundaries (Figure S3). Since no significant strain relaxation in the vertical direction of the islands can be deduced from TEM measurements (see next section), it is highly unlikely that this effect is caused by a strain difference between the islands. Hence, we attribute this visible effect to a significantly different height of the islands. As will be shown in the following paragraph, AFM measurements reveal that on average the smaller islands are not as high as the larger ones. The height difference ranges up to 2 nm, which is equivalent to approximately six trilayers (O−Ce−O, dTL = 3.124 Å) of CeO2(111). In the merging process, the original outlines remain visible as step edges since the 4N adsorption sites are apparently more favorable than the adsorption sites at newly formed step edges on top of the islands. Morphology and Structural Characterization of the Ceria/Ruthenium/Alumina Heterostructure. As shown in a previous study13 and nicely observable in Figure 3a,b, at the chosen deposition parameters cerium oxide grows in a Volmer−Weber-like growth mode. The I(V)-LEEM fingerprint23 shown in Figure 6d enables us to identify the stoichiometry and crystal phase as CeO2(111), in very good agreement with previous studies employing resonant X-ray photoemission and ab initio scattering theory,24 as well as X-ray absorption spectroscopy.25 μLEED patterns confirm the I(V) analysis and reveal the well-known p(1.4 × 1.4) reconstruction of ceria with respect to the Ru(0001) lattice (cf. Figure 6b).6,8 In contrast to previous studies of ceria growth on Ru(0001) single crystals,13,17 the LEED image does not show any off-axis rotational domains of the ceria islands, thus indicating a perfect azimuthal alignment with the Ru lattice. The representative large-scale LEEM image in Figure 6a also serves to uncover the preferential orientation of triangles on a large scale. They are either pointing up (red) or down (yellow). In agreement with the LEED measurements, no other rotational domains are observed. Since both orientations can be found on a single terrace and also at islands in direct contact with each other (marked in orange) the A-B stacking of the Ru substrate cannot be responsible for this behavior. However, as 4221



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parameters, a theoretical coincidence lattice of 5/7 (about 1% compressive strain of ceria) is predicted. However, a coincidence lattice of 7/10 (about 1% tensile strain of ceria) would also result in a similar absolute mismatch and in fact fits better to the experimental findings (see Figure 9). A relaxation of that

Figure 9. Analysis of the coincidence of the ceria and Ru lattices. In the Fourier-filtered STEM image arrows mark the lattice planes in the ceria and the Ru film for the case of a 5/7 (black) and 7/10 (red) coincidence, respectively. The 7/10 coincidence agrees better with the image.

Figure 8. (a) Large-scale and (b) high-resolution STEM image of sample prepared at 900 °C. A homogeneous Ru film with a thickness of about 70 nm with an abrupt interface to the underlying sapphire substrate is observed. The high-resolution image (b) shows the high crystallinity of the ceria island and the Ru substrate. A rather abrupt interface between ceria and Ru can be observed. The slightly blurry appearance of the interface can be explained by the projection of multiple sharp interfaces with an offset of one or two atomic Ru layers between each through the TEM specimen. We explain this roughening on the otherwise flat Ru substrate by a ceria-induced roughening transition of the Ru substrate underneath the ceria island.

strain from the bottom to the top of the island cannot be observed since the resulting difference in the ceria lattice constant is within the error margin of the measurement. Prepatterning of Ru Films: Introduction of Artificial Nucleation Sites. Being able to control the location of nucleation as well as the nucleating phase would offer interesting new possibilities to tailor nanostructured devices and their properties. As described above, nucleation of ceria islands could only be observed at the surface defects of the Ru film at the chosen growth parameters. Therefore, we explored this route by deliberately introducing artificial surface defects, as, for example, holes (Figure 10a) and lines (Figure 10c) of different size using Ga ion etching in a FIB system.

additional reflection has been observed. The respective orientation of the Al2O3 substrate is found to be close to the [011̅0] zone axis. Intriguingly, the epitaxial relationship between the Ru layer and the Al2O3 substrate is different from the one reported by Sutter and co-workers.19 This might be attributed to different preparation conditions of the Ru film. However, assuming a lattice coincidence of 2:1, a mismatch between Al2O3 and Ru of only 2% is determined for our samples, whereas a respective mismatch of about 13% is found in the case of Sutter et al. for their crystallographic relation. Hence, a possibly more stable configuration has been prepared in our samples. In the high-resolution STEM (HRSTEM) image shown in Figure 8, the first 2−3 atomic layers of the CeO2 look like a superposition of the Ru and the CeO2 lattice. This can be explained by the projection of a seemingly rough CeO2/Ru interface through the 40 nm thick TEM specimen. The specimen thickness was determined by a comparison of the measured normalized HAADF-STEM intensity in the Ru layer with a series of frozen lattice simulations, similar to the approach suggested by Rosenauer et al.27 Since the bare Ru substrate exhibits very few steps on the micrometer scale, we attribute this apparently rough interface to a mixture of locally well-ordered, abrupt Ru/ceria interfaces underneath the ceria island, which are separated by single atomic Ru surface steps. This points to a ceria-induced roughening transition of the Ru surface. A similar effect has been reported by Aulická et al. for the ceria-covered Cu(110) surface at 500 °C.28 From the lattice

Figure 10. (a) Prepatterned, cleaned Ru substrate. (b) Sample after cerium oxide deposition. In this image, nucleation of a “dark ceria” phase is observed at the artificial holes in the Ru film. However, in this case the nucleating ceria is not the CeO2(111), but most likely the CeO2(001) phase. (c) The artificial line defects are mostly decorated by CeO2(001) (cf. the square patch in the lower right corner), but adjacent nucleation of CeO2(111) is also observed. 4222



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The artificial holes produced with the FIB are about 250− 300 nm in diameter, whereas the intrinsic holes exhibit a diameter in the order of 50 nm. These large artificial holes were found to remain stable and relatively easy to locate in the LEEM system after the substrates were again Ar+ sputtered and annealed to remove any residual contamination from the ex situ patterning process. In the growth experiments, ceria islands are seen to nucleate at both intrinsic and artificial defects. However, as shown in Figure 10b, the ceria islands that grow from the artificial defects exhibit a different contrast with respect to the substrate, indicating that this is actually a different ceria phase. Close inspection of isolated dark ceria islands at trench-like defects (Figure 10c) reveals that these islands exhibit a square-like shape, corroborating the notion of a different cerium oxide orientation, that is, the CeO2(001) face. Indeed, this CeO2(001) phase has very recently been shown to occur also in highly stepped regions of Ru(0001) single crystals following ceria deposition at high temperature29 or high-temperature annealing.30 The artificial line defects in Figure 10c correspond to massive step bunches, which are highly active in oxygen dissociation and incorporation31 and, representing a local oxygen reservoir, may therefore act as efficient nucleation centers for CeO2(001).29,32 As described above, the size and especially the shape of the defects determine the orientation of the ceria islands. Apparently, the artificial defects do not offer the same properties as the intrinsic defects, rendering them inactive for seeding of controlled nucleation of well-ordered CeO2(111) islands. Hence, to control the nucleation density of CeO2(111), it would be necessary to introduce more intrinsic defects, which could be achievable by introducing defects in the Al2O3 substrate before Ru film growth, for example, by Ar+ sputtering. Yet maybe even more interesting is the nucleation of CeO2(001) at the artificial line defects in Figure 10c. Using a very gentle ion beam producing very dense line defects just a few nanometers wide should enhance the quality and density of the (001) oriented ceria islands, enabling basic structural as well as model catalytic studies for unraveling the structure−activity relation of the polar ceria(001) surface.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b00192. Descriptions of the two time lapse videos and LEEM image sequence showing the merging of two ceria islands of significantly different size (PDF) W Web-Enhanced Features *

LEEM movies showing the reactive high-temperature growth of triangular ceria islands on sputter-deposited Ru(0001) (movie S1) and the merging of two similar-sized ceria islands (movie S2).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: fl[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Knut Müller-Caspary acknowledges funding from the Deutsche Forschungsgemeinschaft (DFG) under Contract No. MU 3660/1-1. Furthermore, Jan Ingo Flege thanks the European COST Action CM1104 for support. This research has been supported by the Institutional Strategy of the University of Bremen, funded by the German Excellence Initiative.

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REFERENCES

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CONCLUSION High-quality Ru(0001) films prepared by magnetron sputter deposition are suitable as growth templates for the synthesis of highly ordered, single-crystalline, nanometer-thin ceria(111) microparticles. Our surface science studies allowed novel insights into the growth dynamics of these CeO2(111) islands. At the chosen preparation parameters, the islands exhibit a predominantly 2D growth with a linear increase in surface area, which we explain within a row-by-row growth model. TEM adds further evidence of the high crystalline quality of both CeO2 and Ru and also reveals sharp interfaces between sapphire and Ru as well as between Ru and cerium oxide. Vshaped surface defects on the Ru substrate are identified as the only nucleation centers for the ceria islands. In a combined topdown−bottom-up approach, we have used these new insights to manipulate the nucleation of ceria islands by introducing artificial surface defects using focused ion beam substrate patterning. Owing to the scalability of sputter deposition, our chosen process to prepare templated ceria nanostructures is a promising approach toward the controlled fabrication of ceria nanostructures. 4223



Article

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Ultrasmooth Ru(0001) Films as Templates for Ceria Nanoarchitectures

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