Interaction of adsorbates on strained metallic layers

Current Opinion in Solid State and Materials Science 5 (2001) 67–73

Interaction of adsorbates on strained metallic layers J. Hrbek a , *, R.Q. Hwang b b

a Brookhaven National Laboratory, Chemistry Department 555, Upton, NY 11973 -5000, USA Sandia National Laboratories, Surface Chemistry Department, Livermore, CA 94551 -0969, USA

Abstract To relieve strain, metal surfaces under tensile or compressive stress often develop periodic two-dimensional patterns of dislocations. Recently, we have seen new developments in the characterization of strained layers, their theoretical simulations and their applications for growth of self-assembled cluster arrays. The atomic resolution imaging of clean and adsorbate-covered strained surfaces using scanning tunneling microscopy has allowed the identification of reactive sites and the structural characterization of growing clusters.  2001 Elsevier Science Ltd. All rights reserved. Keywords: Scanning tunneling microscopy; Adsorbates; Metallic layers

1. Introduction Strain in metal surfaces is caused by abrupt termination of the bulk or mismatch of lattice constants for heteroepitaxially grown layers. The consequences are surface reconstructions of single-crystal surfaces or formation of dislocation patterns in metal films. These patterns are often periodic arrays with lattice spacing in |5 nm range. The best-known examples are the ‘herringbone’ reconstruction of a close-packed surface of gold single crystal [1] and the striped phase of a copper bilayer grown on the Ru(0001) surface [2]. Though the gold and copper reconstruction patterns appear distinct, they are made up of the same ‘building blocks’, that is misfit dislocations. Due to the lattice mismatch between the surface and subsurface layers, stacking faults form, which is an energetically efficient way to accommodate strain. Top layer atoms alternate periodically between fcc and hcp stacking that are separated by misfit dislocations. At these dislocations, atoms are displaced vertically in the transition areas. These misfit dislocations meet at the domain boundaries to form edge dislocations (Fig. 1). Atoms residing at dislocations have different coordination than the rest and their adsorption sites are locally distorted and should have higher reactivity toward adsorbates. Recent experiments with strained surface of a single *Corresponding author. Tel.: 11-631-344-4344; fax: 11-631-3445815. E-mail address: [email protected] (J. Hrbek).

crystal of Ru supplied direct evidence for modified adsorption activity [*3]. Local strain was created by subsurface Ar bubbles trapped under the surface of Ru(0001)

Fig. 1. Network of misfit dislocations in the 2nd and 3rd Cu layer on Ru(0001). 2.8 ML (monolayers) of copper were deposited at room temperature and flash-annealed to 700 K. STM image size 6003600 nm. Two straight steps separating three Ru terraces are running in the left–right direction. The 2nd layer has parallel double strips arranged in large domains, the 3rd layer is a mix of strip phase domains and triangular dislocation features.

1359-0286 / 01 / $ – see front matter  2001 Elsevier Science Ltd. All rights reserved. PII: S1359-0286( 00 )00026-7

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after ion sputtering and mild anneal. Round domes of |10 nm in diameter were clearly visible in STM images of a clean Ru surface. While the tops of the domes are laterally stretched, their heels are compressed. Oxygen adsorption is stronger on stretched tops and weakened around the dome perimeter. The authors have estimated that the oxygen binding energy is lowered by about 40 meV at the perimeter as compared to a flat terrace. Density functional calculations on a Ru metal slab [*4] confirmed the experimental results by showing that chemisorption energies vary substantially on strained lattices with the bond getting stronger as the lattice constant increases. This effect was explained by the stress-induced shifts in the metal d bands. The authors have also shown that the correlation is applicable to several important catalytic systems. In this review, we shall first discuss the use of gold herringbone surface as a template for adsorption and directed growth of clusters. Next, we will examine the interaction of a striped copper layer with oxygen and sulfur. This will be followed by a discussion of interactions on silver layers and by a few comments about strain and catalytic activity.

2. Reconstructed Au(111) surface The annealed Au(111) surface exhibits a long-range 223œ3 reconstruction with atoms in the top layer contracted laterally relative to the bulk. Adatoms alternate in registry with the subsurface layer creating parallel lines of misfit dislocations with periodicity 6.3 nm. To relieve the stress isotropically a superstructure is formed with stress domains alternating by 61208 in zigzag pattern usually referred to as a herringbone structure. Because the reduction of surface stress is the driving force for this reconstruction, the herringbone structure is expected to be sensitive to surface perturbations affecting the delicate balance of the stress distribution [1,5–7]. Vapor deposition of nickel on reconstructed Au(111) results in ordered arrays of two-dimensional Ni islands [5,*6]. The nickel islands are one layer high, positioned in the elbows of herringbone structure and laid some 7.3 nm apart in rows separated by 14 nm. The authors concluded that the nucleation mechanism is related to enhanced electronic interaction of diffusing Ni adatoms at the dislocation edges where they slow down and can be trapped with higher probability. The growth of highly ordered island arrays suggested unprecedented opportunity for the fabrication of self-assembled structures in parallel fashion with a well-defined spacing on nanometer scale. Working with very low Ni coverages Meyer et al. [*8] have shown that preferential islands formation at elbows is a two-step process. The interaction starts first with the place exchange of nickel adatoms with gold substrate atoms and is followed by the second step in which the

substitutional Ni atoms act as nucleation sites for diffusing Ni adatoms. The authors argued that a lower kinetic barrier for the place exchange at the edge dislocations rather than lower mobility causes the preferential nucleation. Very recent work [9] addressed the question of the islands composition, morphology and their thermal stability. The islands at submonolayer coverages are composed of intermixed Ni and Au and they have triangular shapes with opposite orientations on alternating elbows reflecting the energetics of island step-edges on the reconstructed Au substrate. Annealing to temperatures as low as 350 K causes a measurable decrease of the Ni surface concentration. The place exchange mechanism was also observed in Ni electrodeposition on reconstructed Au [10]. However, Ru electrodeposition shows a different type of site-selective nucleation. The Ru nucleation [11] proceeds exclusively in the fcc troughs of the reconstructed surface and welldefined adatom structures replicate the substrate pattern. Characteristic differences and similarities between the atomic scale growth mechanism during vapor deposition in ultrahigh vacuum and during electrodeposition are discussed in a recent review article [12]. A striking example of two-dimensional molecular selfassembly on reconstructed Au was described recently for adsorption of 1-nitronaphtalene [**13]. These polar molecules are p bonded and pseudochiral upon adsorption, and at low temperatures form supramolecular clusters of finite size, mostly decamers, at the elbows. With increasing coverage identical clusters preferentially fill fcc regions to form zigzag rows of decamers. For room temperature adsorption and coverages above 0.3 ML well-separated double chains in preferred orientation with respect to substrate are observed. The interaction among molecules is controlled by the molecular charge distribution and hydrogen bonding is the cause of self-assembly. The growth of cobalt clusters on a gold template has obvious advantages for magnetic applications due to the dots extremely small volume. Vertical self-assembled structures were prepared by sequential deposition steps of Co on Au to form the dot array first [*14]. Next the gold was deposited to fill in the space among Co dots. Additional Co deposited has grown selectively on the top of the buried Co dots. Vertical pillars with 2:1 aspect ratio were fabricated by repeating this cycle 16 times without disturbing the periodic pattern. Another very interesting use of reconstructed gold was published a few months ago [**15]. When the Mo is evaporated in a background pressure of H 2 S and subsequently annealed at 673 K, the Mo islands are chemically transformed into crystalline MoS 2 nanoclusters. The clusters have the triangular shape with a side length of |3 nm and their orientation reflects the substrate symmetry. One layer thick nanocrystals are lying flat on the surface with the basal (0001) plane of the MoS 2 crystal oriented parallel to the Au(111) substrate. By combining the atomic

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resolution images of individual clusters with density functional calculations and cluster reactivity towards atomic hydrogen, the authors were able to identify the S-edge termination of the cluster. Atomic hydrogen stripped off some S atoms at the edges creating S vacancies, the catalytically active sites for the hydrodesulfurization reaction.

3. Strained copper layer In case of Cu on Ru(0001) the first layer grows pseudomorphically and is strained due to a 5.5% expansion of the Cu near-neighbor spacing relative to its bulk value. A sequence of strain-relieved structures develops for thicker Cu films with distinct network of misfit dislocations in the second and third layers [2,*16]. The growth of Cu layers on Ru and the film relaxation continues to be a topic of great interest [17,18]. The corrugation pattern of the second layer resembles the reconstruction of the Au(111) surface (Fig. 2A). Three uniaxially relaxed domains have double stripes with periodicity |4.3 nm. Similar to the lines in the Au herringbone pattern, the bright stripes are misfit dislocations separating regions of fcc and hcp stacking. The strain is almost fully relaxed across the stripes while the Cu atoms are still under stress along the stripes. The meeting points of misfit dislocations form the domain boundaries made of elbows and U-turns. The extra row of Cu atoms terminates there and the ideal three-fold hollow sites are distorted into pseudo four-fold hollow. Exposing the striped copper surface to oxygen leads to substantial structural changes to the layer [*19]. At very small O 2 exposures oxygen atoms start to adsorb on the edge dislocations at the domain boundaries. At exposure of |0.05 L O 2 all pre-existing elbows and U-turns are decorated. One order of magnitude higher O 2 exposures are required to induce the nucleation of new edge dislocations that provide additional adsorption sites. Oxygen adatoms are cutting the copper stripes along the compact direction in a correlated manner. The density of cuts increases to a point where the stripes bend to form trigons and copper in the hcp area of the striped phase is etched away. Sulfur interaction with the copper bilayer in early stages is similar to oxygen copper system with one important exception. It was possible to resolve individual sulfur adatoms trapped at the dislocation edges [*20,*21] even at room temperature (Fig. 2B). Sulfur is arranged in straight rows of four to eight adatoms, depending on the type of the dislocation edge. The measured S–S distance of |0.55 nm is much longer than the length of the S–S chemical bond and implies the occupation of the next nearest-neighbor sites on the Cu surface. The high reactivity of the edge dislocations is related to the presence pseudo four-fold hollow adsorption site as discussed above.

Fig. 2. (A) The strip phase of the 2nd copper layer. 1.6 ML of Cu was deposited at room temperature, annealed to 700 K and imaged at room temperature. Image size 1003100 nm. (B) Different area of the same sample after adsorption of |0.005 ML of sulfur at room temperature. Image size 100386 nm.

With increasing sulfur coverage new edge dislocations are created by two different mechanisms: dissociation of pre-existing edge dislocations and formation of new dislocations. Neither of these processes has an analog in the bulk. The stripes are cut in regular intervals with sulfur arranged in rows. Short segments of the stripes between the cuts bend and form trigons. The fcc regions of trigons are bound by a dislocation loop that is made up of three

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Fig. 3. This 25325 nm STM image shows several self-organized structures of adsorbed sulfur on the striped Cu phase (0.02 ML S coverage). Up to eight S atoms in a non-bonding distance are arranged in straight lines. The S rows move around on a timescale of seconds at room temperature and create larger hexagons (several fragments are visible). With more sulfur available, equilateral triangles of 18 S atoms will form and self-organize into a close-packed array. The Cu film inside of triangles has the hcp stacking while the fcc area is inside of the bend copper stripes.

Fig. 4. A well-ordered extensive template of distorted hexagons formed by annealing of a S / Ru surface (0.02 ML of sulfur) to 700 K. Despite the rather high temperature treatment no sulfide formation is seen. Image size 1003100 nm.

edge dislocations and three curved stripes. Sulfur, decorating the dislocation edges, self-organize into a network of hexagons and eventually close-packed network of equilateral S-triangles made of 18 S adatoms that isolate the hcp stacking area (Fig. 3). Sulfur thus removes the uniaxial strain in copper domain and isolates the areas of hcp and fcc stacking. All these massive structural changes are happening on the strain copper layer at S coverages up to 0.05 ML with no evidence for Cu–S chemical bond formation. Annealing of this surface does not lead to formation of copper sulfide either; instead an almost perfect array of hexagons with sulfur rows and copper trigons covering whole Ru terraces forms (Fig. 4). The potential of using periodic patterns of dislocations in heteroepitaxial systems was demonstrated recently. The surface dislocations are often limiting the diffusion of adatoms and the effect can be used for the ordered heterogeneous nucleation within the features defined by dislocation patterns. The array of Fe quantum dots of narrow size distribution and very high number densities was prepared on the second layer of Cu supported on a Pt(111) substrate [10000122].

4. Strained Ag layers The lattice constant of Ag is 4.3% larger than that of Pt and the first layer of Ag deposited on Pt(111) exhibits parallel misfit dislocations at submonolayer coverages before transition to a pseudomorphic phase at the first layer saturation [*22,23,24]. The second layer forms a wellordered trigonal network of dislocations that relieves the compressive strain, with dislocations marking the regions of fcc and hcp stacking. At low temperatures the third layer of Ag nucleates preferentially within the fcc region away from dislocation network. The dislocations function in this case as effective repulsive barriers for diffusion. As the impinging Ag atoms are trapped and confined into the areas defined by dislocation walls, the size distribution of growing clusters is much narrower than that of randomly nucleating clusters. On Ru the lattice mismatch of Ag is 6.9% and the first layer exhibits a range of structures controlled by the local density in the layer [25]. Below the first layer saturation the Ag film relaxes to form an ordered rectangular array of threading dislocations (unit cell |436 nm). Sulfur exposure of strained layer triggers complex behavior in the Ag film as sulfur displaces Ag atoms and forms two-dimensional vacancy islands [*26,27]. Isolated holes with |3.5 nm in diameter are formed at very low T S (,0.05 ML). The vacancies, filled with S adsorbed directly on the Ru support, are quite mobile, hopping between the dislocations of the original film. As S coverage increases more holes are created until they form nearly perfect triangular lattice with a vacancy island separation of |5.3 nm that extends over the whole Ru terrace (Fig. 5). The islands

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Fig. 5. STM image of sulfur adsorbed on the 1st silver layer on Ru(0001). Three Ru terraces are separated by two steps. Sulfur displaces silver and forms a close-packed array of holes filled with S. A (232) structure of sulfur on a bare Ru surface and inside of the holes is clearly visible. Image size is 50346 nm, Ag coverage is 0.6 ML and average S coverage is 0.1 ML.

fluctuate around the equilibrium positions and their correlated motion reflects the strength of interactions between them. From time-resolved STM images that were used to monitor the vibrations of islands, the elastic constants of the lattice were obtained and the weak force responsible for the lattice stability quantified.

5. Relevance for catalysis The motivation for creating well-defined periodic structures on the nanometer scale through self-assembly comes from several different areas of science and technology. Here we would like to mention possible applications in the area of fundamental studies in catalysis. Most commercial catalysts are small metal clusters supported on oxides. The rates of many catalytic reactions are structure sensitive, i.e. the rate depends on the size and structure of the metal cluster [28]. Significant changes in reaction rate occur for cluster sizes ranging from 10 21 to 10 2 nm (10 1 to 10 3 atoms). Synthetic techniques for catalyst preparation though optimized to provide catalytic particles of targeted size with relatively narrow size distribution [28,29] are not able to furnish monodisperse clusters of tunable size in the size range relevant for optimal activity and selectivity. Catalysis and surface science communities are trying to fabricate relevant experimental models of supported catalysts that will remedy this situation. Different approaches such as photolithography for the controlled

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fabrication of discrete metal deposits on silicon surface or deposition of colloids on surfaces have been tried, however, the cluster size is in the micrometer range. Electron beam lithography, a serial and therefore more time consuming fabrication technique, allows the preparation of, e.g. Pt or Ag two-dimensional ordered arrays of particles [30] in the nanometer size range with the particle density of 10 9 cm 22 . Discoveries of the strain-relieved patterns that are spontaneously created on metal surfaces provide truly nanosized templates for parallel fabrication of ordered two-dimensional arrays [6,22]. The arrays of clusters are formed through nucleation of deposited adatoms on substrates with periodic patterns of dislocations. The densities of clusters can be as high as 10 12 cm 22 and their size distribution several fold narrower than that of random nucleation. The first example comes from the work of Lambert et al. [*31,32], who characterized Pd growth on reconstructed Au(111) surface and used the model Pd /Au catalyst in acetylene cyclotrimerization. The catalytic activity scales linearly with Pd coverage reaching maximum at 0.6 ML. Beyond that point the activity for benzene formation decreases presumably due to the appearance of a new binding site for benzene decomposition and the suppression of cyclization activity. A different approach described recently uses alloying of metals from the same column of the periodic table to induce surface stress and limit the differences in electronic effects of the individual components. Bertolini [33] prepared several PdPt and PdNi alloys of different compositions and tested their reactivity for the 1,3 butadiene hydrogenation reaction at higher pressures. In the case of highly strained Pd in the PdNi surface the large enhancement of catalytic reactivity is observed. In contrast Pd in the PdPt surface is almost without strain and its reactivity is comparable to that of pure Pd.

6. Conclusions There have been exciting developments in our abilities to characterize strain metal surfaces with unprecedented detail. Atomic resolution images of dislocation networks and their response to adsorption of different materials clearly showed the wealth of phenomena controlling the interactions. Early demonstrations of concepts of selfassembled nanostructures of metal and metal compound clusters will certainly initiate a lot of additional work. Their obvious application is the preparation of structures in size ranges where electron confinement results in dramatic changes of the electronic, magnetic and optical properties. Fundamental studies in catalysis will also benefit from our ability to prepare clusters of defined size, distribution composition and density. We will certainly see more work on self-assembly of supramolecular clusters with potential

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application in sensor development and chiral catalysis on surfaces. The next big challenge will be the characterization of metal clusters on oxides. Structural characterization of metal nanoclusters supported on oxide surfaces [*34] together with studies of their reactivity [*35,36] will certainly advance the catalysts design and will be beneficial in the development of many nanodevices.

Acknowledgements We would like to thank many colleagues at our home institutions for many stimulating discussions and to DOE for funding the program through Contracts Nos DE-AC298CH10886 and DE-AC04-94AL8500.

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[9] Cullen WG, First PN. Island shapes and intermixing for submonolayer nickel on Au(111). Surf Sci 1999;420:53–64. [10] Moller FA, Magnussen OM, Behm RJ. Overpotential-controlled nucleation of Ni island arrays on reconstructed Au(111) electrode surface. Phys Rev Lett 1996;77:5249–52. [11] Strbac S, Magnussen OM, Behm RJ. Nanoscale pattern formation during electrodeposition: Ru on reconstructed Au(111). Phys Rev Lett 1999;83:3246–9. [12] Magnussen OM, Behm RJ. Structure and growth in metal epitaxy on low-index Au surfaces – a comparison between solid / electrolyte and solid / vacuum interfaces. J Electroanal Chem 1999;467:258–69. [**13] Bohringer M, Morgernster K, Schneider W-D, Berndt R, Mauri F, De Vita A, Car R. Two-dimensional self-assembly of supramolecular clusters and chains. Phys Rev Lett 1999;83:324–7. At low coverages and temperatures the fcc elbows of herringbone reconstruction served as nucleation sites for 1-nitronaphtalene tetramers and decamers. Rows of decamers formed in the fcc regions of herringbone at higher coverage and well-ordered chains followed. Interplay between the substrate surface strain and hydrogen bonding together with electrostatic repulsion of molecules lead to the complex ordering phenomena. We expect that this work will inspire a lot of adsorption studies of organic molecules on templates like reconstructed gold. [*14] Fruchart O, Kalua M, Barthel J, Kirchner J. Self-organized growth of nanosized vertical magnetic Co pillars on Au(111). Phys Rev Lett 1999;83:2769–72. The first successful experiment using the herringbone template for fabrication of three dimensional Co pillars by self-assembly. [**15] Helveg S, Lauritsen JV, Laesgaard E, Stensgaard I, Norskov JK, Clausen BS, Topsoe H, Besenbacher F. Atomic-scale structure of single-layer MoS 2 nanoclusters. Phys Rev Lett 2000;84:951–4. The herringbone template was used to prepare the array of molybdenum disulfide clusters. The clusters were imaged with atomic resolution and the surface sites active in catalytic reaction were characterized by STM. This approach will certainly be used for synthesis of other metal compounds. [*16] Gunther C, Vrijmoeth J, Hwang RQ, Behm RJ. Strain relaxation in hexagonally close-packed metal–metal interfaces. Phys Rev Lett 1995;74:754–7. Four different stages of strain-relief layers of Cu on Ru(0001) were characterized by STM. As the Cu film thickness increases the tensile strain of the first pseudomorphic structure is relaxed first by unilateral contraction in the 2nd layer and almost isotropically in the 3rd one. [17] Ruebush SD, Couch RE, Thevuthasan S, Fadley CS. X-ray photoelectron diffraction study of thin films grown on clean Ru(0001) and O-precovered Ru(0001). Surf Sci 1999;421:205–36. [18] Zajonz H, Gibbs D, Baddorf AP, Jahns V, Zehner DM. Structure and growth of strained Cu films on Ru(0001). Surf Sci 2000;447:L141– 6. [*19] de la Figuera J, Pohl K, Schmid AK, Bartelt NC, Hwang RQ. Linking dislocation dynamics and chemical reactivity on strained metal films. Surf Sci 1998;415:L993–9. Oxygen reacts initially at the elbows with the Cu strip phase. Additional oxygen converts Cu stripes to an array of triangular fcc areas with bend dislocations and etches away the hcp regions of the Cu film. [*20] de la Figuera J, Pohl K, Schmid AK, Bartelt NC, Hrbek J, Hwang RQ. Multiplication of threading dislocations in the strained metal films under sulfur exposure. Surf Sci 1999;433–435:93–8. Sulfur atoms also decorate the elbows but do not etch the Cu film. As concentration of sulfur increases more elbows are formed by multiplication or by cutting the stripes. [*21] Hrbek J, de la Figuera J, Pohl K, Jirsak T, Rodriguez JA, Schmid AK, Bartelt NC, Hwang RQ. A prelude to surface chemical reaction: imaging the induction period of sulfur interaction with a strained Cu layer. J Phys Chem B 1999;103:10557–61. The first real space images of the induction period of a surface chemical reaction. A complex interplay between strain relief and reaction mechanism was

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