Growth of copper and palladium on α-Al2O3(0001)

Surface Science 460 (2000) L510–L514 www.elsevier.nl/locate/susc

Surface Science Letters

Growth of copper and palladium on a-Al O (0001) 2 3 C.L. Pang, H. Raza, S.A. Haycock, G. Thornton * Surface Science Research Centre and Chemistry Department, Manchester University, Manchester M13 9PL, UK Received 20 March 2000; accepted for publication 5 May 2000

Abstract Non-contact atomic force microscopy (NC-AFM ) has been used to image the room-temperature growth of copper and palladium on the (1×1) and (E31×E31)R±9° terminations of a-Al O (0001). Three-dimensional (3D) clusters 2 3 of palladium are observed on both the (1×1) and the (E31×E31)R±9° terminations, with 3D clusters of copper observed on the reconstructed surface. There is evidence of step-edge-dominated growth of palladium on the (E31×E31)R±9° termination. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Aluminium oxide; Atomic force microscopy; Copper; Growth; Low index single crystal surface; Palladium

The growth and properties of metals on oxides are of considerable importance in applications ranging from catalysis to the electronics industry. Alumina is a particularly important substrate in this context, which has given rise to a number of single-crystal surface-science studies of the clean surface structure and associated metal growth. Work has focused on the surfaces of a-Al O and 2 3 thin films grown on metallic substrates. In addition to the (1×1) termination, the (0001) basal face of a-Al O adopts a number of reconstructions, 2 3 including the air-stable (E31×E31)R±9° termination. For the (1× 1) surface, surface X-ray diffraction (SXRD) data suggest a single-aluminium-layer bulk termination [1], while ab initio calculations predict vertical relaxations extending to the sixth layer [2]. For the (E31×E31)R±9° phase, the model proposed from SXRD consists * Corresponding author. Fax: +44-161-275-4971. E-mail address: [email protected] (G. Thornton)

of a tiling of domains bearing a close resemblance to that of two metal Al(111) planes separated by a hexagonal network of domain walls [3]. It is formed from the (1×1) phase by evaporation of the two upper layers of oxygen. A number of studies of metal growth on insulators have employed ultrathin films of oxides grown on metal substrates [4–6 ]. This allows scanning tunnelling microscopy (STM ) to be used as well as other techniques that make use of charged particles. These are difficult or impossible to use with the native surfaces of a-alumina because of its wide band gap. However, non-contact atomic force microscopy (NC-AFM ) can be used to study metal growth on insulating oxide surfaces, as demonstrated recently for palladium on MgO(001) [7]. In the present work we use NC-AFM to study the growth of copper and palladium on a-Al O (0001). For palladium we can com2 3 pare our results with those for growth on Al O –NiAl(110) [5,6 ], a study of which evidenced 2 3

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Volmer–Weber ( VW ) [i.e., three-dimensional (3D)] cluster growth, with palladium clusters decorating line defects. For growth on the (0001) basal plane of the native oxide, previous results have variously indicated VW [8] and two-dimensional island (2D-I ) [9] growth, with a contracted Pd(111) plane parallel to the substrate [8]. The substrate termination in these earlier studies was not specified in the reports. As for copper growth, the results of a recent surface extended X-ray absorption fine structure (SEXAFS) study suggest that copper adopts the VW growth mode on both the (1×1) and the (E31×E31)R±9° terminations. Growth was found to be consistent with the universal laws of formation of breath figures [10,11]. Our NC-AFM results provide direct evidence for 3D clustering of copper and palladium on the a-alumina basal face above a coverage of 0.3 monolayers (ML). Here, 1 ML is defined as the two-dimensional (2D) packing density of copper (1.77×1015 atoms cm−2) and palladium (1.52×1015 atoms cm−2). Measurements employed an atomic force microscope/scanning tunnelling microscope (Omicron GmbH ) operating at a base pressure of ≤1× 10−10 mbar. The NC-AFM feedback source was the frequency shift of the cantilever resonance. A frequency-modulation detector measures the frequency difference between the cantilever and a reference oscillator. NC-AFM topographic images were recorded by measuring the z position of the tip with constant frequency-shift feedback. A conducting silicon cantilever (Nanosensor GmbH ) with 10 N m−1 force constant and a resonance frequency of 270–300 kHz was used. The frequency shift, Df, used ranged from −13 Hz to −309 Hz. The cantilevers were cleaned in situ by argon-ion bombardment. Tungsten filaments were wrapped around copper and palladium wire for metal deposition with dose rates estimated at ~0.015 ML min−1 for copper, ~0.05 ML min−1 for palladium on the (E31×E31)R±9° surface and 0.75 ML min−1 for palladium on the (1×1) surface. Coverages were estimated from NC-AFM images with an error of ±50%. An NC-AFM image of the clean (E31× E31)R±9° surface is shown in Fig. 1. No atomic

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˚ ×1150 A ˚ ) of clean Fig. 1. NC-AFM image (1420 A a-Al O (0001)-(E31×E31)R±9°. Df=−309 Hz. Step heights 2 3 ˚ , (b) 5.7 A ˚ , (c) 2.0 A ˚ and (d) 6.0 A ˚ . An are as follows: (a) 2.0 A hexagonal depression is indicated by an arrow.

features can be distinguished although step edges can be seen, which principally lie along the [101: 0] and [112: 0] directions. These step edges in some cases outline shapes that appear hexagonal, as expected for the basal face. As illustrated by the model of the unit cell in Fig. 2, a 1/6 unit cell ˚ ) corresponds to the minimum separation (2.17 A between equivalent layers [12]. Hence the step

Fig. 2. Side view of the unit cell of a-Al O (0001). The mini2 3 mum separation between equivalent layers (1/6 unit cell= ˚ 2.17 A) is indicated. Aluminium atoms are depicted as small spheres, while oxygen atoms are represented by the large spheres.

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˚ × 413 A ˚ ) of 0.3 ML of copper on a-Al O (0001)-(E31×E31)R±9°. Df= Fig. 3. (a) 3D representation of an NC-AFM image (492 A 2 3 −28 Hz. (b) Line profile ( y) across a substrate step edge for 0.3 ML of copper on a-Al O (0001)-(E31×E31)R±9°. (c) Line profile 2 3 (z) across two copper clusters.

˚ and 6±0.5 A ˚, heights observed in Fig. 1, 2±0.5 A are consistent with single- and triple-height steps. Images recorded after depositing 0.3 ML of copper on a-Al O (0001)-(E31×E31)R±9° contain 2 3 more clearly resolved steps. An example is shown in Fig. 3, which contains a step of height ˚ (see Fig. 3b), consistent with a double4.5±0.5 A height step. These results are in line with contact mode AFM results for a-Al O (0001)-(1×1), 2 3 which contains steps that have heights in integer ˚ [12]. multiples of 2.1 A If we consider the copper clusters in the image ˚ in in Fig. 3, islands between the sizes of 3–8 A ˚ height and 20–30 A in diameter are distributed randomly on the surface. A height profile of a pair of clusters is shown in Fig. 3c. The island sizes carry a large uncertainty because NC-AFM cannot accurately quantify heights [13,14] and the diameters of clusters are overestimated with all scanning

probe micoscopies [15]. In that step-edge-dominated growth is not observed, the results lend some support to the breath figure growth mode proposed by Gautier-Soyer et al. [10], which requires that growth is not defect-dominated. We cannot, however, rule out the possibility that point defect nucleation plays a role. We were unable to directly corroborate the growth model because we could not measure the coverages with sufficient accuracy. Our images indicate that 3D islands are formed at relatively low coverages. However, it was not possible to image copper at coverages lower than 0.3 ML. Palladium and gold have been shown to grow with the 2D-I mechanism on TiO (110)2 (1×1) with the 2D-to-3D phase transition occurring at coverages as low as 0.1 ML [16,17]. It is not possible for us to rule out the possibility that small 2D islands of copper are formed prior to the 3D clusters.

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˚× Fig. 5. 3D representation of an NC-AFM image (500 A ˚ ) of 0.2 ML of palladium on a-Al O (0001)-(1×1). 500 A 2 3 Df=−62.5 Hz.

˚ × 800 A ˚ ) of 1.1 ML of pallaFig. 4. NC-AFM image (800 A dium on a-Al O (0001)-(E31×E31)R±9°. Df=−13 Hz. 2 3 Lines are drawn to guide the eye.

It is worth noting that, on the basis of ionscattering measurements, it has been suggested that hydroxyl species contaminate the (1×1) surface unless it is heated above 1400 K [18]. The effect of hydroxylation is as yet unclear for the (E31×E31)R±9° surface, although experimental and theoretical data for the (1×1) surface indicate that it increases wetting, leading to layer-by-layer growth of copper [19,20]. On this basis, our results indicate that the level of hydroxylation on our (E31×E31)R±9° is not sufficient to dramatically modify the growth characteristics. We were unable to resolve copper islands on the (1×1) surface at the coverages investigated; i.e., 0.3 ML, 1 ML and 1.4 ML. This probably arose from a charging effect, although we cannot rule out the possibility that a continuous layer of copper was formed, as evidenced for the hydroxylated surface by ion scattering [19]. Fig. 4 shows an NC-AFM image of 1.1 ML of palladium on (E31×E31)R±9°. This image shows 3D clusters with diameter in the range 40– ˚ and height in the range 3–6 A ˚ covering most 60 A of the surface, smaller than the cluster diameter of ˚ found on Al O –NiAl(110) at ~2 ML ~100 A 2 3

[6 ]. This discrepancy could arise from a difference in dose rate. The results in Fig. 4 suggest VW growth, although we cannot rule out the 2D-I or the Stanski–Krastinov (SK ) growth mode because we could not image the surface at submonolayer coverages. In contrast to copper growth, there is some evidence of step-dominated growth. Linear and curved arrangements of clusters can be seen, suggesting that clusters decorate the step edges. This is a similar result to that obtained for palladium growth on Al O –NiAl(110), where palla2 3 dium was found to align in linear and curved rows corresponding to step edges and grain boundaries [5]. The similarity of palladium growth on the (E31×E31)R±9° reconstruction and the thin film is not unexpected, in that platinum is predicted to have similar adsorption behaviour on both surfaces [21]. For palladium it proved possible to image clusters on the (1×1) termination, the quality of the images improving with increasing coverage. That shown in Fig. 5 corresponds to a coverage of 1.2 ML. The image evidences 3D clusters with a ˚ and a height of ~2–3 A ˚, diameter of ~30–40 A suggesting either the VW, 2D-I or SK growth mode. We were unable to distinguish between these growth modes because again we could not image the surface at submonolayer coverage. To summarise, we have provided strong evidence that copper adopts either the VW or the 2D-I growth mode on the (E31×E31)R±9° ter-

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mination of a-Al O (0001), with no evidence of 2 3 step-dominated growth. This lack of step-dominated growth lends credence to the proposed breath figure growth mode, a model that could be further tested with NC-AFM in conjunction with a more accurate monitor of the coverage. For palladium, 3D islands are observed on both a-alumina surfaces. Furthermore, for the (E31×E31)R±9° surface, a step-dominated growth of palladium is observed which is similar to the behaviour of palladium on thin-film Al O –NiAl(110). 2 3 Acknowledgement This work was funded by the EPSRC ( UK ).

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