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Ni electrochemical epitaxy on unreconstructed Au(111): An in-situ STM study
Surface Science 631 (2015) 135–140
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Surface Science journal homepage: www.elsevier.com/locate/susc
Ni electrochemical epitaxy on unreconstructed Au(111): An in-situ STM study F. Lecadre a, A. Damian a,1, I. Braems b, F. Maroun a,⁎, P. Allongue a a b
Physique de la Matière Condensée, Ecole Polytechnique, CNRS, 91128 Palaiseau, France ICMMO, Université Paris-Sud, CNRS, 91405 Orsay, France
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Available online 1 July 2014 Keywords: Au(111) reconstruction Ni moiré model Epitaxial growth Scanning tunnelling microscopy Electrodeposition
a b s t r a c t Gold (111) is a widely used substrate in vacuum and in electrolytes for studying the initial stages of metal growth. Its surface reconstruction is unavoidable using common preparation procedures in vacuum and this affects the growth mechanism of metal overlayers. In the electrochemical environment, the same surface reconstruction is obtained at potentials negative of the Au(111) potential of zero charge, whereas above this potential the surface structure is (1 × 1). In this paper we present a preparation procedure which allows blocking the Au step edges by Pd islands, inhibiting the formation of the Au(111) reconstruction and yielding the unreconstructed Au(111) surface (1 × 1) at any electrochemical potential. Using in-situ scanning tunnelling microscopy, we compare the Ni/Au(111) electrochemical epitaxy on reconstructed and on unreconstructed Au(111). Observations show that performing Ni growth on the (1 × 1) surface drastically improves the long-range order of the (8 × 8) moiré pattern created by the lattice mismatch between the Au substrate and Ni adlayer. Detailed characterizations of the moiré structure and of its electrochemical dependence provide new insights into the relative energy landscape of the multiple adsorption sites of the Ni atoms within the (8 × 8) unit cell. The possibility of stabilizing a Au(111)-1 × 1 surface opens up new perspectives for studying metal growth and molecular organisation. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The Au(111) surface is a widely used substrate in ultra high vacuum (UHV) and electrolytes for surface science studies as nucleation and growth processes, and molecular organisation on surfaces. This versatility stems at least from the two facts that the Au(111) surface (i) is inert and stable at ambient and in liquids, allowing the use of an easy surface preparation method, and (ii) has interesting electronic properties. The surface preparation at ambient consists in flame annealing the Au sample in air during few minutes [1] yielding atomically flat terraces which are typically few tens of nanometers wide, ideally suited for scanning tunnelling microscopy (STM) studies [2–5]. In addition,pthe ffiffiffi annealing process induces the formation of the well-known 22 × 3 surface reconstruction with its regular well-defined zigzag pattern accompanied by the uniaxial compression by ~ 4% along b110N. This structure has been characterized in details in the UHV [6] and in liquids [7,8]. pffiffiffi The topmost Au atoms occupy a variety of sites within the 22 × 3 unit
⁎ Corresponding author. E-mail address: [email protected] (F. Maroun). 1 Present address: Eramet Research, 1 Avenue Albert Einstein, BP 120, 78193 Trappes, France.
http://dx.doi.org/10.1016/j.susc.2014.06.014 0039-6028/© 2014 Elsevier B.V. All rights reserved.
cell, from three-fold hcp and fcc hollow sites to bridge ones. In addition, surface dislocations are present pffiffiffi at the “elbows” of the zigzag reconstruction pattern. The 22 × 3 Au(111) reconstruction allows relaxing the surface intrinsic tensile stress [9]. In the UHV, the aforementioned surface dislocations have a strong impact on the nucleation and the growth of epitaxial layers because they act as preferential nucleation sites for the growth of various metals such as Ni, Co, Fe and Pd [10–15]. Atomically resolved UHV-STM images gave strong evidence that place exchange occurs at these surface dislocations and that the embedded foreign atoms act as preferential nucleation centres for subsequent overlayer growth. The regular arrangement of the elbows of the Au reconstruction has been exploited to create ordered arrays of metallic nano-islands [10,12,15]. Similar preferential nucleation at the reconstruction elbows was reportedpin ffiffiffi the case of Ni and Co electrochemical deposition on the 22 × 3 Au(111) surface [16,17]. The electrochemical environment allows also the stabilization of the Au(111)-(1 × 1) surface at potentials more positive than the potential of zero charge (PZC) of Au(111) due to anion specific adsorption (~ −0.2 V vs mercury pffiffiffisulphate electrode) [8]. This potential induced phase transition 22 × 3 ↔ (1 × 1) around the PZC is reversible and it has been observed in different electrolytes [8,18,19]. As a consequence, the electrochemical growth of noble metals (Pt, Pd or Ag) may be performed on the (1 × 1) surface because their standard
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redox potentials are more positive than the Au(111) PZC [20,21]. In the case of less noble metals such as iron group metals (Fe, Co, Ni), which standard redox potentials are more negative than the Au(111) PZC, pffiffiffi the electrochemical growth could only be investigated on 22 × 3 Au(111) surface [17,22,23]. Metal deposition on the reconstructed Au(111) surface raises also the question of the reconstruction survival under the deposited layer with the possible Au alloying with the deposited metal. In this work we present a method allowing to keep the Au(111) surface unreconstructed in a large electrochemical potential window, including potentials more negative than the Au(111) PZC. Using this method, we revisit the Ni electrochemical growth on Au(111) and show that the atomic structure of the gold topmost layer has a dramatic impact on the Ni microstructure: on Au(111)-(1 × 1), a particularly well defined (8 × 8) moiré structure develops on the long range and no intermixing is present in contrast to observations on the reconstructed surface. Further STM data are presented to discuss the position of the Ni steps with respect to the moiré pattern. 2. Experimental details A top hat cut Au(111) single crystal (5 mm diameter, Mateck) was used as substrate for in situ STM imaging. Before each experiment, the crystal was electrochemically oxidized in 1 M HClO4 (1 min) and subsequently dipped in 10% HCl (10 s) to remove the oxide. The single crystal was flame-annealed and cooled down in air just before starting the experiments. The reference electrode was a saturated mercury/mercurous sulphate electrode (MSE). All potentials are hereafter quoted against MSE. All solutions were prepared with reagent grade chemicals using bidistilled ultrapure water (18.2 MΩ cm). Pd deposition took place in the STM cell as follows: i) Immersion of the Au crystal at +0.3 VMSE in a solution composed of 0.1 M K2SO4 + 1 mM H2SO4 + 1 mM KCl. At this potential, the Au(111) surface reconstruction is lifted and the excess 4% Au atoms are expelled from the topmost Au plane. The presence of Cl− ions increases the mobility of the excess Au adatoms, allowing Au adatom diffusion to the neighbouring steps, preventing the formation of Au islands. ii) The solution is exchanged under potential control (+0.3 VMSE) with a K2PdCl4 solution of composition 0.1 M H2SO4p+ ffiffiffi 10pμM ffiffiffi K2PdCl4. At this potential the PdCl2− anions form a ( 7 × 7) 4 R19.1 ordered adlayer covering the entire Au surface. The effective Pd coverage of this adlayer is ~0.14 [20]. iii) Pd deposition is induced by a potential step from +0.3 VMSE to 0 VMSE. The adsorbed PdCl24 − anions are first reduced to Pd adatoms and a slow reduction of PdCl24 − from the solution is then occurring. iv) Deposition is stopped by exchanging the solution by 0.1 M K2SO4 + 1 mM H2SO4 + 1 mM KCl, keeping the sample potential at 0 VMSE. Ni was deposited by stepping the potential from 0 V MSE to potentials ≤− 1.18 VMSE in a solution composed of 0.125 mM to 1 mM NiSO4 in 0.1 M K2SO4 + 1 mM H2SO4 + 1 mM KCl (supporting electrolyte). Ni deposition rate was varied between 0.24 and 20 ML/min. Ni deposition was stopped by quickly exchanging the plating solution by the supporting electrolyte under potential control. The same Ni deposition procedure has been applied for bare and for Pd covered Au(111) surface. Consequently, in the case of bare Au, the surface structure is first (1 × 1) at 0 VMSE and starts reconstructing when the potential is stepped negatively to deposit Ni, i.e., Au reconstruction takes place in parallel with Ni deposition although at a higher speed (see below). In situ electrochemical STM experiments were performed using a home-built scanning tunnelling microscope. Tungsten tunnelling tips were prepared by electrochemical etching in 2 M NaOH and coated
with Apiezon wax. The tip potential during STM imaging was typically at −0.95 VMSE. STM images (512 × 512 pixels, 3 min per image) were recorded in constant current mode with a tunnelling current of 0.5– 5 nA. The typical Ni/Au(111) potential during STM imaging − 1.15 VMSE. Images are presented as top views after plane fit correction using SPIP software (Image Metrology). 3. Results and discussion The flame annealed Au(111) single crystal surface pffiffiffi imaged in the supporting electrolyte at −0.6 VMSE presents a 22 × 3 reconstruction as shown in the STM image of Fig. 1a. Two Au terraces separated by an atomic step (highlighted by a white line) may be observed in this image. The height contrast of each terrace has been enhanced independently to better visualize the reconstruction. The height modulation is ~0.2 Å and the reconstruction pattern has a lateral period of 63 Å in agreement with previous observations [7]. On the long-range the surface reconstruction makes a zigzag pattern also called herringbone pattern which is identical to that obtained in ultra high vacuum [6]. In this solution, the Au(111) surface becomes (1 × 1) at potentials more positive than −0.2 VMSE. According to surface in situ X-ray scattering pffiffiffiexperiments, the kinetics of the reconstruction process (1 × 1) → 22 × 3 critically depends on the solution composition and on the applied potential [19]. In 0.01 M NaCl, an electrolyte close to that used in this work, the reconstruction process reaches a plateau in less than 10 s at potentials ~−1.2 VMSE. During the formation of the surface reconstruction, the 4% extra Au adatoms most probably originate from the Au steps through a detachment process. This is consistent with previous STM studies where it has been concluded that the reconstruction process starts close to the Au terrace edges [8]. Consequently, by preventing the Au atoms from detaching from the steps to go into the topmost Au plane, one should be able to maintain the gold surface unreconstructed at potential b PZC. This is the basic idea proposed in this work for stabilizing the Au(111)-(1 × 1) surface in an extended electrochemical potential range. To hinder Au adatom detachment from the steps, we decorated the Au steps by depositing a Pd submonolayer on the Au(111) surface at 0 VMSE, i.e., in a potential range where the Au(111) surface is (1 × 1). Fig. 1b shows a typical STM image acquired at − 0.8 VMSE (b PZC) of such a Pd/Au(111) substrate (Pd coverage ~ 0.4). The Pd islands are atomically flat and monatomic high. They preferentially nucleate at the Au steps and completely wet them in agreement with the literature [20]. Some Pd islands are also found in the middle of the Au terraces. The contrast of the Au(111) terraces in Fig. 1b has been enhanced to demonstrate pffiffiffi that they remain unreconstructed at −0.8 VMSE although the 22 × 3 structure is the thermodynamically stable configuration at this potential. Only a few nanometric depressions (two of them are indicated by an arrow) are observed. They are assigned to limited place exchange of Pd adatoms with Au atoms of the topmost layer as has been observed in UHV [24]. We found that a Pd coverage down to ~0.2 ML is enough to hinder the formation of the Au(111) surface reconstruction (see for example Fig. 2c). For Pd coverage b 0.2, the wetting of the Au steps by Pd may become locally incomplete and our observations show the formation of an incomplete surface reconstruction limited to a few pairs of reconstruction lines starting from the step edges (images not shown). In the following, the Au(111) surface with its steps decorated by Pd islands will be referred to as AuPd(111)-(1 × 1), whereas the bare surface will be referred to the reconstructing Au(111) surface. Fig. 2 compares the morphology of Ni submonolayer deposits on reconstructing Au(111) (Fig. 2a–b) and on AuPd(111)-(1 × 1) surfaces (Fig. 2c–d). The Ni growth rate was in the range 0.2–20 ML/min (see conditions in the figure caption). On both surfaces Ni deposits are composed of monatomic high islands. On the reconstructing Au(111) surface (Fig. 2a–b) the Ni islands present locally a quasi periodic height modulation while other regions present a very disordered height
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Fig. 1. (a) In situ STM image of Au(111) surface recorded at −0.6 VMSE in the supporting electrolyte. Image size is 200 × 200 nm2. (b) In situ STM image of the AuPd(111)-(1 × 1) surface (Pd 2 coverage 0.4) recorded at −0.8 VMSE in the pffiffiffisupporting electrolyte. Image size is 191 × 184 nm . In both images, the height contrast range has been decreased to ~1 Å independently for each Au terrace. The gold surface is 22 × 3 reconstructed in (a) and is (1 × 1) in (b). The arrows in (b) indicate two depressions in the Au terrace.
corrugation (see Ni areas surrounded by a black line). The quasi hexagonal symmetry of the periodic height modulation results from a moiré structure, which is induced by the −14% lattice mismatch between Ni and Au and which shows that the Ni layer is in epitaxy with Au(111).
The average distance between two moiré maxima equals 22 Å,pin ffiffiffi agreement with former studies of Ni electrodeposition on the 22 × 3 reconstructed Au(111) surface [22,25]. The lateral extension of this height modulation is however limited to a few periods only (see arrows
Fig. 2. (a) In situ STM image of Au(111) reconstructed surface covered by 0.5 ML of Ni deposited at −1.24 VMSE at an average rate of 1.4 ML/min (the Ni2+ concentration in the supporting electrolyte is 0.125 mM). Arrows indicate ordered moiré pattern regions. Rectangles indicate depressions in the Au terrace. The two islands with step edges enhanced by a black line correspond to disordered Ni regions. (b) Image of ~1 ML of Ni deposited at a rate of 20 ML/min on Au(111) (the Ni2+ concentration in the supporting electrolyte is 1 mM). Arrows indicate ordered moiré pattern regions. The two areas surrounded by dashed white lines correspond to disordered Ni regions. The inset corresponds to the auto-correlation image showing the absence of long range order. (c) In situ STM image of Au(111) covered by 0.2 ML of Pd surface and by 0.75 ML of Ni deposited at −1.24 VMSE at an average rate of 0.4 ML/min (the Ni2+ concentration in the supporting electrolyte is 0.125 mM). The three black lines separated by 60° indicate the dense directions of the Ni moiré pattern. (d) Image of ~0.9 ML of Ni deposited at − 1.18 VMSE at an average rate of 0.24 ML/min on AuPd(111) (the Ni2+ concentration in the supporting electrolyte is 0.2 mM). The inset corresponds to the autocorrelation image showing the presence of long range order. In all images the Au terrace and Ni layer height contrasts have been enhanced to better observe their surface structure. All images have been acquired at a sample potential of −1.15 VMSE and their size is 100 nm × 100 nm.
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indicating such regions). As a result the auto-correlation image is essentially featureless (Fig. 2b inset). By comparing Fig. 2a and Fig. 2b, one may conclude that the proportion of the Ni deposit presenting a disordered height modulation does not depend on the deposition rate in the range 1.5 ML/min (Fig. 2a) to 20 ML/min (Fig. 2b). Also noticeable, Fig. 2a clearly shows that the uncovered Au(111) is entirely reconstructed. The reconstruction pattern is however strongly perturbed as compared to that in Fig. 1a and numerous nanometre depressions (some of them are indicated by white rectangles) are observed. They are probably due to a Ni–Au place exchange process. Ni–Au place exchange was only reported previously at the elbows of the Au(111) reconstruction [16]. When Ni is deposited on the AuPd(111)-(1 × 1) surface (Fig. 2c–d), a quasi perfect hexagonal moiré pattern is observed over all the Ni monolayers. This remarkable long range ordering of the moiré pattern is confirmed by the autocorrelation image (Fig. 2d inset) where several oscillation orders are clearly observed. We performed a detailed analysis of the orientation of the Ni moiré directions on images similar to Fig. 2c and found that it varies by ±10° with respect to the average orientation given in the lower left corner of the image 2c. The same ordering of the moiré pattern is also observed on the Pd deposit (see area with enhanced contrast in Fig. 2d). As discussed elsewhere, the smaller Ni coverage on Pd arises from a lower binding energy of Ni on Pd [26]. Finally, it is worth highlighting that the uncovered Au terraces in Fig. 2c remain unreconstructed and only present some rare depressions similar to those observed on the initial AuPd(111) surface (Fig. 1b). We inspected the morphology of the Au terrace after Ni dissolution because this may give insights into the possible Ni/Au intermixing [23]. The above Ni deposits were dissolved at a very slow rate by progressively increasing the electrochemical potential up to −0.95 VMSE. Fig. 3a shows the partial dissolution of the Ni deposit presented in Fig. 2a. The black lines in this image recall the initial contours of some of the Ni islands before dissolution. Similar to the Au(111) surface which remained bare upon Ni deposition, Fig. 3a evidences that the Au(111) terrace which was formerly covered by Ni presents numerous nanometre depressions and a perturbed surface reconstruction. This indicates that the formation of the Au reconstruction takes place before significant Ni deposition. Indeed, we have observed by STM (data not shown) that a significant density of Au reconstruction pairs of lines is appearing a few seconds after a potential step from 0 VMSE to −1.2 VMSE. In addition, this observation is consistent with X-ray diffraction measurements where less than 10s were necessary to form the reconstruction at a potential of −1.2 VMSE [19].
Fig. 3a also shows that significant intermixing occurs between Ni and Au. In the case of Ni on AuPd(111)-(1 × 1) surface, partial (Fig. 3b) and complete (Fig. 3c) Ni dissolution leaves the Au(111) surface as atomically smooth as before Ni deposition (the dark grey islands in Fig. 3c are the pre-deposited Pd islands), except for a single Au reconstruction line pair in the middle of the image (see arrow in Fig. 3c). Fig. 3b and 3c originates from the same experiment but not from the same sample region. Consequently, the remaining Pd islands in Fig. 3c cannot be found in Fig. 3b. However, the features in-plane orientation may be directly compared. The Au(111) reconstruction line in Fig. 3c is found to be perpendicular to one of the Ni moiré direction in Fig. 3b. This is in agreement with the orientation of the Ni lattice with respect to the Au one (see below). Fig. 3c shows also that no significant intermixing between Ni and Au is present. Since in addition the Au surface is (1 × 1), Ni moiré may be ordered on the long range. It is interesting to examine the Ni step morphology. In the case of Ni islands on reconstructing Au(111) surface, the Ni steps are irregular (Fig. 2a). In the case of Ni/AuPd(111)-(1 × 1), Ni islands present steps that are rather straight and oriented parallel to the six average orientations of the moiré pattern (indicated by the black lines separated by 60° in Fig. 2c). The Ni step morphology during Ni dissolution is also interesting: the Ni steps remain irregular in the case of Ni on reconstructing Au(111) surface (Fig. 3a). In the case of Ni/AuPd(111)-(1 × 1), the Ni steps remain rather straight and are mainly oriented parallel to the Ni moiré main directions (Fig. 3b, dashed lines). The straightness of the Ni steps in the case of Ni/AuPd(111)-(1 × 1) indicates that they correspond to energetically favourable step directions, i.e., most probably dense atomic rows. Consequently, the parallelism between the Ni steps and the moiré average orientation suggests that the moiré average orientation corresponds to a Ni lattice not rotated with respect to the Au one. The orientation of the Ni moiré (Fig. 3b) with respect to the Au(111) reconstruction line (Fig. 3c) is a further support to this conclusion. The ±10° variation of the moiré orientation with respect to the average one is assigned a slight disorientation of the Ni atomic lattice with respect to the Au one. Using the general formula given in Ref. [27], we indeed calculate that a 1° atomic lattice disorientation of the Ni lattice with respect to the Au one yields ~8° moiré rotation associated with a slight decrease of the moiré period (less than 1 Å). Fig. 4a–b shows a model of the Ni atomic positions within the moiré superstructure on Au(111)-(1 × 1) (Fig. 4a top view, Fig. 4b side view). This model results from Monte Carlo simulations followed by quenched Molecular Dynamic simulations explained in details in Ref. [25]. Briefly, the calculations are based on N-body interatomic potential that is
Fig. 3. (a) In situ STM image of the Ni deposit of Fig. 2a during Ni dissolution at −1 VMSE. The black lines indicate the edges of the Ni islands before dissolution. Image size is 80 × 80 nm2. (b) In situ STM image of the Ni deposit of Fig. 2c during Ni dissolution at −0.95 VMSE. The dashed black lines highlight the Ni step directions. Note the triangular shape of the Ni islands. Image size is 100 × 100 nm2. (c) In situ STM image of the Ni deposit of Fig. 2c after complete Ni dissolution at −0.95 VMSE. The remaining Pd islands were formed during the preparation of the AuPd(111)-(1 × 1) surface. In both images, the height contrast of the Au terrace has been increased to ease the surface structure observation. Image size is 75 × 75 nm2.
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Fig. 4. (a) Model (top view) of the Ni moiré structure determined by Monte Carlo simulations. The dashed white lines correspond to a Ni dense atomic row running close to Ni atoms in hollow sites. (b) Cross section of the moiré structure along line AA′ in (a) showing the different Ni adsorption sites. (c) A zoomed STM image showing the most commonly encountered position of the Ni step edges in the valleys of the moiré pattern. (d) A zoomed STM image showing the less commonly encountered cases where the Ni step crosses the moiré maxima (see arrows). (e) Scheme showing (1) an island (A) on a flat substrate and (2) the same island divided into two islands (B) and (C) along a line creating two additional steps.
derived from the second moment approximation of the tight-binding scheme. The Ni moiré pattern was computed starting from a perfectly non-corrugated Ni(111) monolayer with equal in-plane interatomic distance. We considered a Ni monolayer atomic density given by the (8 × 8) moiré unit cell. In addition, we did not take into account the presence of an adsorbate overlayer. Although such description of the Ni moiré is not as precise as the one that may be obtained by ab-initio calculations, in the absence of the latter in the literature, our calculations are a reasonable basis for understanding the energetic trends of the Ni moiré. To fit with our experimental observations, no rotation of the Ni lattice with respect to Au topmost plane was allowed which yield moiré dense directions parallel to the dense atomic rows of Ni and Au. The unit cell is a 8 × 8 overlayer matching the period of 22 Å measured in STM images. The Ni atoms adopt a variety of adsorption sites within the 8 × 8 unit cell, from three-fold hollow to on top sites. An important result of this model is that the Ni–Ni spacing distance is significantly varying within the unit cell whereas an average Ni–Ni distance of 2.56 Å is generally inferred from standard ball models: The Ni–Ni distance equals ~ 2.8 Å for Ni atoms sitting in three-fold hollow sites and ~2.4 Å for Ni atoms sitting close to on top sites (the bulk Ni interatomic distance is 2.49 Å). We also examined in detail the position of the Ni steps within the moiré pattern unit cell as observed in our STM images. On as deposited Ni islands, we found that a large majority of the Ni steps (~90% of total length) present the configuration shown in Fig. 4c, i.e., the step is running along the valley of the moiré pattern. In the remaining 10% of the cases, other configurations may be found such as the one in Fig. 4d, where the step runs also through the moiré maxima. According to the model in Fig. 4a, the above observations show that energetically favoured Ni island edges run through the valleys of the moiré pattern,
i.e., are mainly composed of Ni atoms sitting close to the dashed lines shown in Fig. 4a. In other words, the Ni atoms sitting on or close to hollow and bridge sites are obviously energetically more stable to form a step than those for example on-top sites. In order to determine the origin of the specific step position within the Ni moiré pattern unit cell, we discuss the energetic of a step in a very simplified scheme. At the atomic scale, the step energy per unit length E may be determined by estimating the energy difference between state (1) a single monoatomic island (A) on a flat substrate, and state (2) the same island divided into two smaller islands (B) and (C) creating two additional steps (see Fig. 4e). Note that the total number of atoms is identical in both states and we consider that the islands (A), (B) and (C) have the same registry with the substrate. E is given by Ebond ∗ N, where Ebond is the energy of the in-plane broken bonds per step atom and N the linear atom density along the step. In the case of a pseudomorph growth, the nearest neighbour in-plane interatomic distance is identical for all the surface atoms and for all in-plane directions. Consequently, the in-plane interatomic bond energy Ebond does not depend on the atomic position. Therefore, the energetically favoured step direction is determined by the atomic direction which minimizes N, which corresponds to steps parallel to the dense atomic rows. In the case of Ni/AuPd(111)-(1 × 1), Ni is not pseudomorph with Au(111) but due to its epitaxial growth on Au, the observed step directions run also along the Ni dense atomic rows. We also observed that the position of the Ni step within a moiré pattern unit cell is in 90% of the cases along the moiré valleys and not along the moiré maxima. Since N is identical for a step running along the valleys and that along the maxima, one may conclude that the average Ebond is different for Ni atoms in the moiré valleys (Ni in hollow and bridge sites) and at the moiré maxima (Ni in on-top and bridge sites). Our results thus show that the Ni–Ni in-plane interatomic bond energy is smaller for Ni
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atoms in hollow sites and larger for Ni in on-top sites. This difference most probably originates from the main conclusion of our moiré model, i.e., the Ni–Ni in-plane interatomic distance is larger for Ni on hollow sites and smaller on on-top sites. Such model-experiment comparison was made possible thanks to the quasi defect free and the long range order of the Ni moiré on the AuPd(111)-(1 × 1) surface. 4. Conclusion In this work we have demonstrated that one can maintain the Au(111) surface unreconstructed at potential negative of the PZC by decorating the Au atomic steps with ~0.2 ML of Pd. We also compared the Ni epitaxy on reconstructing and unreconstructed Au(111) surfaces. Our in-situ STM observations show that the gold substrate atomic structure has a strong impact on the microstructure of the Ni monolayer. On the (1 × 1) surface, the 8 × 8 moiré pattern formed by the Ni lattice is remarkably well defined on the long-range and covers the entire Ni layer. In addition, we found that no Ni–Au intermixing occurs. These observations contrast with those of Ni on reconstructed Au(111). A careful examination of the Ni step edge configuration on unreconstructed Au(111) further demonstrates that the energetically more favourable step positions are those running along Ni atoms sitting close to hollow sites. These findings are in agreement with an atomic model of the moiré cell obtained by Monte Carlo simulations. The possibility of stabilizing a Au(111)-(1 × 1) surface opens up new perspectives for studying metal growth and molecular organization on this surface.
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Surface Science journal homepage: www.elsevier.com/locate/susc
Ni electrochemical epitaxy on unreconstructed Au(111): An in-situ STM study F. Lecadre a, A. Damian a,1, I. Braems b, F. Maroun a,⁎, P. Allongue a a b
Physique de la Matière Condensée, Ecole Polytechnique, CNRS, 91128 Palaiseau, France ICMMO, Université Paris-Sud, CNRS, 91405 Orsay, France
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Available online 1 July 2014 Keywords: Au(111) reconstruction Ni moiré model Epitaxial growth Scanning tunnelling microscopy Electrodeposition
a b s t r a c t Gold (111) is a widely used substrate in vacuum and in electrolytes for studying the initial stages of metal growth. Its surface reconstruction is unavoidable using common preparation procedures in vacuum and this affects the growth mechanism of metal overlayers. In the electrochemical environment, the same surface reconstruction is obtained at potentials negative of the Au(111) potential of zero charge, whereas above this potential the surface structure is (1 × 1). In this paper we present a preparation procedure which allows blocking the Au step edges by Pd islands, inhibiting the formation of the Au(111) reconstruction and yielding the unreconstructed Au(111) surface (1 × 1) at any electrochemical potential. Using in-situ scanning tunnelling microscopy, we compare the Ni/Au(111) electrochemical epitaxy on reconstructed and on unreconstructed Au(111). Observations show that performing Ni growth on the (1 × 1) surface drastically improves the long-range order of the (8 × 8) moiré pattern created by the lattice mismatch between the Au substrate and Ni adlayer. Detailed characterizations of the moiré structure and of its electrochemical dependence provide new insights into the relative energy landscape of the multiple adsorption sites of the Ni atoms within the (8 × 8) unit cell. The possibility of stabilizing a Au(111)-1 × 1 surface opens up new perspectives for studying metal growth and molecular organisation. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The Au(111) surface is a widely used substrate in ultra high vacuum (UHV) and electrolytes for surface science studies as nucleation and growth processes, and molecular organisation on surfaces. This versatility stems at least from the two facts that the Au(111) surface (i) is inert and stable at ambient and in liquids, allowing the use of an easy surface preparation method, and (ii) has interesting electronic properties. The surface preparation at ambient consists in flame annealing the Au sample in air during few minutes [1] yielding atomically flat terraces which are typically few tens of nanometers wide, ideally suited for scanning tunnelling microscopy (STM) studies [2–5]. In addition,pthe ffiffiffi annealing process induces the formation of the well-known 22 × 3 surface reconstruction with its regular well-defined zigzag pattern accompanied by the uniaxial compression by ~ 4% along b110N. This structure has been characterized in details in the UHV [6] and in liquids [7,8]. pffiffiffi The topmost Au atoms occupy a variety of sites within the 22 × 3 unit
⁎ Corresponding author. E-mail address: [email protected] (F. Maroun). 1 Present address: Eramet Research, 1 Avenue Albert Einstein, BP 120, 78193 Trappes, France.
http://dx.doi.org/10.1016/j.susc.2014.06.014 0039-6028/© 2014 Elsevier B.V. All rights reserved.
cell, from three-fold hcp and fcc hollow sites to bridge ones. In addition, surface dislocations are present pffiffiffi at the “elbows” of the zigzag reconstruction pattern. The 22 × 3 Au(111) reconstruction allows relaxing the surface intrinsic tensile stress [9]. In the UHV, the aforementioned surface dislocations have a strong impact on the nucleation and the growth of epitaxial layers because they act as preferential nucleation sites for the growth of various metals such as Ni, Co, Fe and Pd [10–15]. Atomically resolved UHV-STM images gave strong evidence that place exchange occurs at these surface dislocations and that the embedded foreign atoms act as preferential nucleation centres for subsequent overlayer growth. The regular arrangement of the elbows of the Au reconstruction has been exploited to create ordered arrays of metallic nano-islands [10,12,15]. Similar preferential nucleation at the reconstruction elbows was reportedpin ffiffiffi the case of Ni and Co electrochemical deposition on the 22 × 3 Au(111) surface [16,17]. The electrochemical environment allows also the stabilization of the Au(111)-(1 × 1) surface at potentials more positive than the potential of zero charge (PZC) of Au(111) due to anion specific adsorption (~ −0.2 V vs mercury pffiffiffisulphate electrode) [8]. This potential induced phase transition 22 × 3 ↔ (1 × 1) around the PZC is reversible and it has been observed in different electrolytes [8,18,19]. As a consequence, the electrochemical growth of noble metals (Pt, Pd or Ag) may be performed on the (1 × 1) surface because their standard
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redox potentials are more positive than the Au(111) PZC [20,21]. In the case of less noble metals such as iron group metals (Fe, Co, Ni), which standard redox potentials are more negative than the Au(111) PZC, pffiffiffi the electrochemical growth could only be investigated on 22 × 3 Au(111) surface [17,22,23]. Metal deposition on the reconstructed Au(111) surface raises also the question of the reconstruction survival under the deposited layer with the possible Au alloying with the deposited metal. In this work we present a method allowing to keep the Au(111) surface unreconstructed in a large electrochemical potential window, including potentials more negative than the Au(111) PZC. Using this method, we revisit the Ni electrochemical growth on Au(111) and show that the atomic structure of the gold topmost layer has a dramatic impact on the Ni microstructure: on Au(111)-(1 × 1), a particularly well defined (8 × 8) moiré structure develops on the long range and no intermixing is present in contrast to observations on the reconstructed surface. Further STM data are presented to discuss the position of the Ni steps with respect to the moiré pattern. 2. Experimental details A top hat cut Au(111) single crystal (5 mm diameter, Mateck) was used as substrate for in situ STM imaging. Before each experiment, the crystal was electrochemically oxidized in 1 M HClO4 (1 min) and subsequently dipped in 10% HCl (10 s) to remove the oxide. The single crystal was flame-annealed and cooled down in air just before starting the experiments. The reference electrode was a saturated mercury/mercurous sulphate electrode (MSE). All potentials are hereafter quoted against MSE. All solutions were prepared with reagent grade chemicals using bidistilled ultrapure water (18.2 MΩ cm). Pd deposition took place in the STM cell as follows: i) Immersion of the Au crystal at +0.3 VMSE in a solution composed of 0.1 M K2SO4 + 1 mM H2SO4 + 1 mM KCl. At this potential, the Au(111) surface reconstruction is lifted and the excess 4% Au atoms are expelled from the topmost Au plane. The presence of Cl− ions increases the mobility of the excess Au adatoms, allowing Au adatom diffusion to the neighbouring steps, preventing the formation of Au islands. ii) The solution is exchanged under potential control (+0.3 VMSE) with a K2PdCl4 solution of composition 0.1 M H2SO4p+ ffiffiffi 10pμM ffiffiffi K2PdCl4. At this potential the PdCl2− anions form a ( 7 × 7) 4 R19.1 ordered adlayer covering the entire Au surface. The effective Pd coverage of this adlayer is ~0.14 [20]. iii) Pd deposition is induced by a potential step from +0.3 VMSE to 0 VMSE. The adsorbed PdCl24 − anions are first reduced to Pd adatoms and a slow reduction of PdCl24 − from the solution is then occurring. iv) Deposition is stopped by exchanging the solution by 0.1 M K2SO4 + 1 mM H2SO4 + 1 mM KCl, keeping the sample potential at 0 VMSE. Ni was deposited by stepping the potential from 0 V MSE to potentials ≤− 1.18 VMSE in a solution composed of 0.125 mM to 1 mM NiSO4 in 0.1 M K2SO4 + 1 mM H2SO4 + 1 mM KCl (supporting electrolyte). Ni deposition rate was varied between 0.24 and 20 ML/min. Ni deposition was stopped by quickly exchanging the plating solution by the supporting electrolyte under potential control. The same Ni deposition procedure has been applied for bare and for Pd covered Au(111) surface. Consequently, in the case of bare Au, the surface structure is first (1 × 1) at 0 VMSE and starts reconstructing when the potential is stepped negatively to deposit Ni, i.e., Au reconstruction takes place in parallel with Ni deposition although at a higher speed (see below). In situ electrochemical STM experiments were performed using a home-built scanning tunnelling microscope. Tungsten tunnelling tips were prepared by electrochemical etching in 2 M NaOH and coated
with Apiezon wax. The tip potential during STM imaging was typically at −0.95 VMSE. STM images (512 × 512 pixels, 3 min per image) were recorded in constant current mode with a tunnelling current of 0.5– 5 nA. The typical Ni/Au(111) potential during STM imaging − 1.15 VMSE. Images are presented as top views after plane fit correction using SPIP software (Image Metrology). 3. Results and discussion The flame annealed Au(111) single crystal surface pffiffiffi imaged in the supporting electrolyte at −0.6 VMSE presents a 22 × 3 reconstruction as shown in the STM image of Fig. 1a. Two Au terraces separated by an atomic step (highlighted by a white line) may be observed in this image. The height contrast of each terrace has been enhanced independently to better visualize the reconstruction. The height modulation is ~0.2 Å and the reconstruction pattern has a lateral period of 63 Å in agreement with previous observations [7]. On the long-range the surface reconstruction makes a zigzag pattern also called herringbone pattern which is identical to that obtained in ultra high vacuum [6]. In this solution, the Au(111) surface becomes (1 × 1) at potentials more positive than −0.2 VMSE. According to surface in situ X-ray scattering pffiffiffiexperiments, the kinetics of the reconstruction process (1 × 1) → 22 × 3 critically depends on the solution composition and on the applied potential [19]. In 0.01 M NaCl, an electrolyte close to that used in this work, the reconstruction process reaches a plateau in less than 10 s at potentials ~−1.2 VMSE. During the formation of the surface reconstruction, the 4% extra Au adatoms most probably originate from the Au steps through a detachment process. This is consistent with previous STM studies where it has been concluded that the reconstruction process starts close to the Au terrace edges [8]. Consequently, by preventing the Au atoms from detaching from the steps to go into the topmost Au plane, one should be able to maintain the gold surface unreconstructed at potential b PZC. This is the basic idea proposed in this work for stabilizing the Au(111)-(1 × 1) surface in an extended electrochemical potential range. To hinder Au adatom detachment from the steps, we decorated the Au steps by depositing a Pd submonolayer on the Au(111) surface at 0 VMSE, i.e., in a potential range where the Au(111) surface is (1 × 1). Fig. 1b shows a typical STM image acquired at − 0.8 VMSE (b PZC) of such a Pd/Au(111) substrate (Pd coverage ~ 0.4). The Pd islands are atomically flat and monatomic high. They preferentially nucleate at the Au steps and completely wet them in agreement with the literature [20]. Some Pd islands are also found in the middle of the Au terraces. The contrast of the Au(111) terraces in Fig. 1b has been enhanced to demonstrate pffiffiffi that they remain unreconstructed at −0.8 VMSE although the 22 × 3 structure is the thermodynamically stable configuration at this potential. Only a few nanometric depressions (two of them are indicated by an arrow) are observed. They are assigned to limited place exchange of Pd adatoms with Au atoms of the topmost layer as has been observed in UHV [24]. We found that a Pd coverage down to ~0.2 ML is enough to hinder the formation of the Au(111) surface reconstruction (see for example Fig. 2c). For Pd coverage b 0.2, the wetting of the Au steps by Pd may become locally incomplete and our observations show the formation of an incomplete surface reconstruction limited to a few pairs of reconstruction lines starting from the step edges (images not shown). In the following, the Au(111) surface with its steps decorated by Pd islands will be referred to as AuPd(111)-(1 × 1), whereas the bare surface will be referred to the reconstructing Au(111) surface. Fig. 2 compares the morphology of Ni submonolayer deposits on reconstructing Au(111) (Fig. 2a–b) and on AuPd(111)-(1 × 1) surfaces (Fig. 2c–d). The Ni growth rate was in the range 0.2–20 ML/min (see conditions in the figure caption). On both surfaces Ni deposits are composed of monatomic high islands. On the reconstructing Au(111) surface (Fig. 2a–b) the Ni islands present locally a quasi periodic height modulation while other regions present a very disordered height
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Fig. 1. (a) In situ STM image of Au(111) surface recorded at −0.6 VMSE in the supporting electrolyte. Image size is 200 × 200 nm2. (b) In situ STM image of the AuPd(111)-(1 × 1) surface (Pd 2 coverage 0.4) recorded at −0.8 VMSE in the pffiffiffisupporting electrolyte. Image size is 191 × 184 nm . In both images, the height contrast range has been decreased to ~1 Å independently for each Au terrace. The gold surface is 22 × 3 reconstructed in (a) and is (1 × 1) in (b). The arrows in (b) indicate two depressions in the Au terrace.
corrugation (see Ni areas surrounded by a black line). The quasi hexagonal symmetry of the periodic height modulation results from a moiré structure, which is induced by the −14% lattice mismatch between Ni and Au and which shows that the Ni layer is in epitaxy with Au(111).
The average distance between two moiré maxima equals 22 Å,pin ffiffiffi agreement with former studies of Ni electrodeposition on the 22 × 3 reconstructed Au(111) surface [22,25]. The lateral extension of this height modulation is however limited to a few periods only (see arrows
Fig. 2. (a) In situ STM image of Au(111) reconstructed surface covered by 0.5 ML of Ni deposited at −1.24 VMSE at an average rate of 1.4 ML/min (the Ni2+ concentration in the supporting electrolyte is 0.125 mM). Arrows indicate ordered moiré pattern regions. Rectangles indicate depressions in the Au terrace. The two islands with step edges enhanced by a black line correspond to disordered Ni regions. (b) Image of ~1 ML of Ni deposited at a rate of 20 ML/min on Au(111) (the Ni2+ concentration in the supporting electrolyte is 1 mM). Arrows indicate ordered moiré pattern regions. The two areas surrounded by dashed white lines correspond to disordered Ni regions. The inset corresponds to the auto-correlation image showing the absence of long range order. (c) In situ STM image of Au(111) covered by 0.2 ML of Pd surface and by 0.75 ML of Ni deposited at −1.24 VMSE at an average rate of 0.4 ML/min (the Ni2+ concentration in the supporting electrolyte is 0.125 mM). The three black lines separated by 60° indicate the dense directions of the Ni moiré pattern. (d) Image of ~0.9 ML of Ni deposited at − 1.18 VMSE at an average rate of 0.24 ML/min on AuPd(111) (the Ni2+ concentration in the supporting electrolyte is 0.2 mM). The inset corresponds to the autocorrelation image showing the presence of long range order. In all images the Au terrace and Ni layer height contrasts have been enhanced to better observe their surface structure. All images have been acquired at a sample potential of −1.15 VMSE and their size is 100 nm × 100 nm.
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indicating such regions). As a result the auto-correlation image is essentially featureless (Fig. 2b inset). By comparing Fig. 2a and Fig. 2b, one may conclude that the proportion of the Ni deposit presenting a disordered height modulation does not depend on the deposition rate in the range 1.5 ML/min (Fig. 2a) to 20 ML/min (Fig. 2b). Also noticeable, Fig. 2a clearly shows that the uncovered Au(111) is entirely reconstructed. The reconstruction pattern is however strongly perturbed as compared to that in Fig. 1a and numerous nanometre depressions (some of them are indicated by white rectangles) are observed. They are probably due to a Ni–Au place exchange process. Ni–Au place exchange was only reported previously at the elbows of the Au(111) reconstruction [16]. When Ni is deposited on the AuPd(111)-(1 × 1) surface (Fig. 2c–d), a quasi perfect hexagonal moiré pattern is observed over all the Ni monolayers. This remarkable long range ordering of the moiré pattern is confirmed by the autocorrelation image (Fig. 2d inset) where several oscillation orders are clearly observed. We performed a detailed analysis of the orientation of the Ni moiré directions on images similar to Fig. 2c and found that it varies by ±10° with respect to the average orientation given in the lower left corner of the image 2c. The same ordering of the moiré pattern is also observed on the Pd deposit (see area with enhanced contrast in Fig. 2d). As discussed elsewhere, the smaller Ni coverage on Pd arises from a lower binding energy of Ni on Pd [26]. Finally, it is worth highlighting that the uncovered Au terraces in Fig. 2c remain unreconstructed and only present some rare depressions similar to those observed on the initial AuPd(111) surface (Fig. 1b). We inspected the morphology of the Au terrace after Ni dissolution because this may give insights into the possible Ni/Au intermixing [23]. The above Ni deposits were dissolved at a very slow rate by progressively increasing the electrochemical potential up to −0.95 VMSE. Fig. 3a shows the partial dissolution of the Ni deposit presented in Fig. 2a. The black lines in this image recall the initial contours of some of the Ni islands before dissolution. Similar to the Au(111) surface which remained bare upon Ni deposition, Fig. 3a evidences that the Au(111) terrace which was formerly covered by Ni presents numerous nanometre depressions and a perturbed surface reconstruction. This indicates that the formation of the Au reconstruction takes place before significant Ni deposition. Indeed, we have observed by STM (data not shown) that a significant density of Au reconstruction pairs of lines is appearing a few seconds after a potential step from 0 VMSE to −1.2 VMSE. In addition, this observation is consistent with X-ray diffraction measurements where less than 10s were necessary to form the reconstruction at a potential of −1.2 VMSE [19].
Fig. 3a also shows that significant intermixing occurs between Ni and Au. In the case of Ni on AuPd(111)-(1 × 1) surface, partial (Fig. 3b) and complete (Fig. 3c) Ni dissolution leaves the Au(111) surface as atomically smooth as before Ni deposition (the dark grey islands in Fig. 3c are the pre-deposited Pd islands), except for a single Au reconstruction line pair in the middle of the image (see arrow in Fig. 3c). Fig. 3b and 3c originates from the same experiment but not from the same sample region. Consequently, the remaining Pd islands in Fig. 3c cannot be found in Fig. 3b. However, the features in-plane orientation may be directly compared. The Au(111) reconstruction line in Fig. 3c is found to be perpendicular to one of the Ni moiré direction in Fig. 3b. This is in agreement with the orientation of the Ni lattice with respect to the Au one (see below). Fig. 3c shows also that no significant intermixing between Ni and Au is present. Since in addition the Au surface is (1 × 1), Ni moiré may be ordered on the long range. It is interesting to examine the Ni step morphology. In the case of Ni islands on reconstructing Au(111) surface, the Ni steps are irregular (Fig. 2a). In the case of Ni/AuPd(111)-(1 × 1), Ni islands present steps that are rather straight and oriented parallel to the six average orientations of the moiré pattern (indicated by the black lines separated by 60° in Fig. 2c). The Ni step morphology during Ni dissolution is also interesting: the Ni steps remain irregular in the case of Ni on reconstructing Au(111) surface (Fig. 3a). In the case of Ni/AuPd(111)-(1 × 1), the Ni steps remain rather straight and are mainly oriented parallel to the Ni moiré main directions (Fig. 3b, dashed lines). The straightness of the Ni steps in the case of Ni/AuPd(111)-(1 × 1) indicates that they correspond to energetically favourable step directions, i.e., most probably dense atomic rows. Consequently, the parallelism between the Ni steps and the moiré average orientation suggests that the moiré average orientation corresponds to a Ni lattice not rotated with respect to the Au one. The orientation of the Ni moiré (Fig. 3b) with respect to the Au(111) reconstruction line (Fig. 3c) is a further support to this conclusion. The ±10° variation of the moiré orientation with respect to the average one is assigned a slight disorientation of the Ni atomic lattice with respect to the Au one. Using the general formula given in Ref. [27], we indeed calculate that a 1° atomic lattice disorientation of the Ni lattice with respect to the Au one yields ~8° moiré rotation associated with a slight decrease of the moiré period (less than 1 Å). Fig. 4a–b shows a model of the Ni atomic positions within the moiré superstructure on Au(111)-(1 × 1) (Fig. 4a top view, Fig. 4b side view). This model results from Monte Carlo simulations followed by quenched Molecular Dynamic simulations explained in details in Ref. [25]. Briefly, the calculations are based on N-body interatomic potential that is
Fig. 3. (a) In situ STM image of the Ni deposit of Fig. 2a during Ni dissolution at −1 VMSE. The black lines indicate the edges of the Ni islands before dissolution. Image size is 80 × 80 nm2. (b) In situ STM image of the Ni deposit of Fig. 2c during Ni dissolution at −0.95 VMSE. The dashed black lines highlight the Ni step directions. Note the triangular shape of the Ni islands. Image size is 100 × 100 nm2. (c) In situ STM image of the Ni deposit of Fig. 2c after complete Ni dissolution at −0.95 VMSE. The remaining Pd islands were formed during the preparation of the AuPd(111)-(1 × 1) surface. In both images, the height contrast of the Au terrace has been increased to ease the surface structure observation. Image size is 75 × 75 nm2.
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Fig. 4. (a) Model (top view) of the Ni moiré structure determined by Monte Carlo simulations. The dashed white lines correspond to a Ni dense atomic row running close to Ni atoms in hollow sites. (b) Cross section of the moiré structure along line AA′ in (a) showing the different Ni adsorption sites. (c) A zoomed STM image showing the most commonly encountered position of the Ni step edges in the valleys of the moiré pattern. (d) A zoomed STM image showing the less commonly encountered cases where the Ni step crosses the moiré maxima (see arrows). (e) Scheme showing (1) an island (A) on a flat substrate and (2) the same island divided into two islands (B) and (C) along a line creating two additional steps.
derived from the second moment approximation of the tight-binding scheme. The Ni moiré pattern was computed starting from a perfectly non-corrugated Ni(111) monolayer with equal in-plane interatomic distance. We considered a Ni monolayer atomic density given by the (8 × 8) moiré unit cell. In addition, we did not take into account the presence of an adsorbate overlayer. Although such description of the Ni moiré is not as precise as the one that may be obtained by ab-initio calculations, in the absence of the latter in the literature, our calculations are a reasonable basis for understanding the energetic trends of the Ni moiré. To fit with our experimental observations, no rotation of the Ni lattice with respect to Au topmost plane was allowed which yield moiré dense directions parallel to the dense atomic rows of Ni and Au. The unit cell is a 8 × 8 overlayer matching the period of 22 Å measured in STM images. The Ni atoms adopt a variety of adsorption sites within the 8 × 8 unit cell, from three-fold hollow to on top sites. An important result of this model is that the Ni–Ni spacing distance is significantly varying within the unit cell whereas an average Ni–Ni distance of 2.56 Å is generally inferred from standard ball models: The Ni–Ni distance equals ~ 2.8 Å for Ni atoms sitting in three-fold hollow sites and ~2.4 Å for Ni atoms sitting close to on top sites (the bulk Ni interatomic distance is 2.49 Å). We also examined in detail the position of the Ni steps within the moiré pattern unit cell as observed in our STM images. On as deposited Ni islands, we found that a large majority of the Ni steps (~90% of total length) present the configuration shown in Fig. 4c, i.e., the step is running along the valley of the moiré pattern. In the remaining 10% of the cases, other configurations may be found such as the one in Fig. 4d, where the step runs also through the moiré maxima. According to the model in Fig. 4a, the above observations show that energetically favoured Ni island edges run through the valleys of the moiré pattern,
i.e., are mainly composed of Ni atoms sitting close to the dashed lines shown in Fig. 4a. In other words, the Ni atoms sitting on or close to hollow and bridge sites are obviously energetically more stable to form a step than those for example on-top sites. In order to determine the origin of the specific step position within the Ni moiré pattern unit cell, we discuss the energetic of a step in a very simplified scheme. At the atomic scale, the step energy per unit length E may be determined by estimating the energy difference between state (1) a single monoatomic island (A) on a flat substrate, and state (2) the same island divided into two smaller islands (B) and (C) creating two additional steps (see Fig. 4e). Note that the total number of atoms is identical in both states and we consider that the islands (A), (B) and (C) have the same registry with the substrate. E is given by Ebond ∗ N, where Ebond is the energy of the in-plane broken bonds per step atom and N the linear atom density along the step. In the case of a pseudomorph growth, the nearest neighbour in-plane interatomic distance is identical for all the surface atoms and for all in-plane directions. Consequently, the in-plane interatomic bond energy Ebond does not depend on the atomic position. Therefore, the energetically favoured step direction is determined by the atomic direction which minimizes N, which corresponds to steps parallel to the dense atomic rows. In the case of Ni/AuPd(111)-(1 × 1), Ni is not pseudomorph with Au(111) but due to its epitaxial growth on Au, the observed step directions run also along the Ni dense atomic rows. We also observed that the position of the Ni step within a moiré pattern unit cell is in 90% of the cases along the moiré valleys and not along the moiré maxima. Since N is identical for a step running along the valleys and that along the maxima, one may conclude that the average Ebond is different for Ni atoms in the moiré valleys (Ni in hollow and bridge sites) and at the moiré maxima (Ni in on-top and bridge sites). Our results thus show that the Ni–Ni in-plane interatomic bond energy is smaller for Ni
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atoms in hollow sites and larger for Ni in on-top sites. This difference most probably originates from the main conclusion of our moiré model, i.e., the Ni–Ni in-plane interatomic distance is larger for Ni on hollow sites and smaller on on-top sites. Such model-experiment comparison was made possible thanks to the quasi defect free and the long range order of the Ni moiré on the AuPd(111)-(1 × 1) surface. 4. Conclusion In this work we have demonstrated that one can maintain the Au(111) surface unreconstructed at potential negative of the PZC by decorating the Au atomic steps with ~0.2 ML of Pd. We also compared the Ni epitaxy on reconstructing and unreconstructed Au(111) surfaces. Our in-situ STM observations show that the gold substrate atomic structure has a strong impact on the microstructure of the Ni monolayer. On the (1 × 1) surface, the 8 × 8 moiré pattern formed by the Ni lattice is remarkably well defined on the long-range and covers the entire Ni layer. In addition, we found that no Ni–Au intermixing occurs. These observations contrast with those of Ni on reconstructed Au(111). A careful examination of the Ni step edge configuration on unreconstructed Au(111) further demonstrates that the energetically more favourable step positions are those running along Ni atoms sitting close to hollow sites. These findings are in agreement with an atomic model of the moiré cell obtained by Monte Carlo simulations. The possibility of stabilizing a Au(111)-(1 × 1) surface opens up new perspectives for studying metal growth and molecular organization on this surface.
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