Tailoring periodic nanostructures of vicinal copper surfaces: Formation and evolution of oxygen-induced faceting on Cu(3 3 2)

Surface Science 603 (2009) 3081–3087

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Tailoring periodic nanostructures of vicinal copper surfaces: Formation and evolution of oxygen-induced faceting on Cu(3 3 2) C.E. ViolBarbosa a,*, J. Fujii a, G. Panaccione a, G. Rossi a,b a b

Laboratorio TASC-INFM-CNR, Area Science Park, S.S. 14, Km 163.5, I-34149 Trieste, Italy Dipartimento di Fisica, Università di Modena e Reggio Emilia, Via A. Campi 213/A, I-41100 Modena, Italy

a r t i c l e

i n f o

Article history: Received 20 March 2009 Accepted for publication 17 August 2009 Available online 22 August 2009 Keywords: Auto-assembly STM Nanowires Oxidation Vicinal surface

a b s t r a c t We report and model calculations of nanostripes formation of the Cu(3 3 2) surface obtained by oxygeninduced reconstruction. Scanning tunnelling microscope (STM) results with atomic resolution reveal alternate facets of clean Cu(1 1 1) and Cu(1 1 0)–O(21) along the [1 1 0] direction, with the same average direction of Cu(3 3 2). At the edge between the two facets, oxygen is absorbed in a pseudo threefold site of the unreconstructed Cu(3 3 2). Tuneable periodicity, from 3 to 10 nm, is obtained by controlled change of the surface treatment. We discuss the formation of the periodic nanostructures and the mechanism driving the reconstruction via model calculations. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction Self-organization and self-assembly at the nanoscale on metallic surfaces has proven to be a valid bottom-up approach to nanostructuring, alternative to the advanced capabilities of nanolithography [1]. One of the bottleneck of such bottom-up approach is the statistical nature of the resulting nanostructures, i.e. a fine tuning of several parameters controlling the process is mandatory in order to reliably reproduce the metastable nanostructures. Among metallic crystalline substrates, vicinal surfaces provide nanostructured templates with local atomic precision and high reproducibility at medium and long range [1]. Vicinal surfaces of a single crystal are high Miller-index surfaces that can be viewed as the average resulting from periodic arrangements of terraces and steps, i.e. facets with low Miller indexes [2]. Such surfaces present periodic local fields (stresses, special atomic sites at step edges and kinks) [3–5], often inducing selective sticking or reactivity to atomic and/or molecular adsorbates [6–9]. The faceting reconstruction of a vicinal surface, although not commonly observed in clean surfaces [2,10], can be induced by a specific adsorbate [11–18], thus representing a viable pathway to the goal of periodic nanostructures with tailored properties. Looking at copper vicinal surfaces, faceting induced by Ag adsorption on the Cu(2 2 3) surface has been reported, with alter-

* Corresponding author. Address: Laboratorio TASC-INFM-CNR, APE Beamline at Elettra, Area Science Park, S.S. 14, Km 163.5, I-34149 Trieste, Italy. Tel.: +39 040 3758408; fax: +39 040 226767. E-mail address: [email protected] (C.E. ViolBarbosa). 0039-6028/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2009.08.018

nated distribution of Ag facets and clean Cu stripes [13]. Moreover, control of the Ag thickness modifies the stripe width from 3 to 30 nm. In this case, the tuneability of the long range nanostructures has been described through the change of the elastic properties, where the presence of the absorbate acts as a trigger to produce faceting. In a similar system, Brandstetter et al. [17] were able to vary the distribution of the terrace width in Cu(19 19 1) (vicinal of Cu(1 1 0)), through oxygen exposure. In this case, the formation of Cu–O stripes perpendicular to the steps edges on the Cu(1 1 0) terraces leads to the extension or certain terraces at the expense of neighbouring ones. The process is ruled by the mobility of the surface atoms and it is strongly dependent upon the exposure and annealing temperature, i.e. the surface kinetics [17]. In order to deepen the understanding of both the faceting process and the formation of nanostructures, and in particular to explore what mechanism is the driving one between minimization of stress (surface elastic properties) or surface atoms mobility (surface kinetic properties), we present a combined experimental and theoretical study of the faceting driven by oxygen adsorption on the Cu(3 3 2) surface, a system where exposure to vicinal Cu(n,n,n  1) surfaces induces a reconstruction consisting of alternating clean Cu(1 1 1) and Cu(1 1 0)O(21) flat facets [18]. We show that a stable configuration at room temperature (RT) is found by careful control of both the oxygen dose and the sample temperature, and a reproducible self-assembly with average periodicity of 4, 6 or 10 nm is obtained. In this system, the oxygen coverage is determined by the ratio between (1 1 0) and (1 1 1) facets [18], resulting to be the same for all periodicities. The analysis as a

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function of exposure pressure and temperature suggest that the average periodicity is directly linked to the kinetic properties of the Cu–O chemistry and surface kinetics properties, rather than to the elastic interaction between the facets. The characterization of three periodic striped surfaces, obtained with an optimized protocol, has been carried out by means of lowenergy electron diffraction (LEED) and scanning tunnelling microscopy (STM). At the edge between the two facets we observe a monoatomic step where the oxygen is absorbed in a pseudo threefold site of the unreconstructed Cu(3 3 2). Ab initio calculations were performed to aid the interpretation of the STM data. Finally we assign the adsorption of the oxygen atom at the step edge (pseudo threefold site) as the trigger of the surface reconstruction by pushing the stepped surface to the instability with respect to the faceting. 2. Methods 2.1. Experimental The Cu(3 3 2) surface is vicinal to the Cu(1 1 1) with 10.0° miscut in the [1 1 2] direction and exhibits monoatomic steps with (1 1 1) microfacets and (1 1 1) terraces. According to the hardball model the terraces consist of 5 1/3 atomic rows (l = 12 Å). The sample was nominally miscut by 10° with 0.1° of accuracy and characterized by X-ray diffraction. The surface preparation was performed in the sample preparation module of the APE beamline of TASC-INFM at the Elettra storage ring laboratory in Trieste [19]. The clean surface was obtained in UHV (p < 1  1010 mbar) by repeated cycles of Ar-ion sputtering at 650 eV and annealing at 500 °C followed by slow cooling to RT, i.e. the temperature was reduced from 500 to 150 °C in at least 40 min. Clean Cu(3 3 2) surfaces, displaying parallel step edges and average terraces of 12 Å width, were exposed to oxygen while maintaining the substrate at fixed temperatures. Post annealing of 10 min follows all preparations. The oxygen exposure was carried out through of the backfilling for the preparation chamber to pressures from 5  108 to 2  107 mbar. The sample temperature was set by e-bombardment on the back side of sample and monitored by a thermocouple in contact with the sample and by an optical pyrometer. The surface composition was checked by Auger Electron Spectroscopy (AES) and the long range order was probed by LEED, while the medium and local ranges were probed by STM topography (in the latter case with atomic resolution). The tunnelling microscopy and spectroscopy measurements were carried out in a dedicated UHV STM chamber, with a base pressure lower than 1  1010 mbar, that is directly connected to the sample preparation module of the APE beamline. The STM topography was acquired by operating the STM in constant-current mode with an electron-etched tungsten tip. The sample was kept at room temperature during the STM measurements. 2.2. Calculation details STM simulations have been obtained from ab initio calculation. The size of the primitive cell of the oxidized faceted Cu(3 3 2) would be too large for our available computational resource, and we have modelled our system by a Cu(3 3 2) slab with the oxygen atom in the step edge position, Cu(3 3 2)O(step edge), and by a Cu(1 1 0) slab with the Cu–O chain position in the reconstructed surface, Cu(1 1 0)O(21). The Quantum-Espresso package [20] for first principle DFT calculations based on the generalized gradient approximation PW91-GGA [21] was adopted with a plane-wave cut energy of 400 eV. The interaction between ionic cores and valence electrons is described by an ultrasoft pseudo-potential;

where the 3d, 4s and 4p electrons are treated as valence electrons for copper, and the 2s and 2p are the valence electrons for oxygen. A fractional occupancy was calculated using a Gaussian smearing with a width of 7 meV. We obtained a cohesive energy of 3.60 eV for copper with a lattice constant of 3.63 Å; the experimental values measured for copper present a 3% lower cohesive energy and 0.5% smaller lattice constant [22]. The binding energy for oxygen molecule was calculated in 6.38 eV for oxygen molecule with a bond length of 1.23 Å; the experimental values measured for oxygen molecule present a 20% lower binding energy and about 1% smaller bond length [23]. The GGA typically overestimate the cohesive/binding energies, especially in isolate molecules [24]. Nevertheless since we are dealing with the relative stability of different structures, this overestimation will not affect the relative energy differences. We initially performed the calculation for 3 base systems: Cu(1 1 1), Cu(1 1 0), Cu(3 3 2). All systems were simulated by a seven-layer slab where the first three layers are allowed to relax in order to minimize the total energy. The vacuum region separating the periodic slabs is 30 Å thick. The reciprocal space was sampled with a mesh of 16  16 for Cu(1 1 1) and Cu(1 1 0) and 8  4 for Cu(3 3 2) Brillouin zone supercell. Computational details and convergence checks are the same as discussed in Ref. [25]. The convergence of total energy relative to the number of slab layers (k-points) is smaller than 10 meV (5 meV). We follow with the calculation of the two systems: Cu(3 3 2)O(step edge) and Cu(1 1 0)O(21). The number of layers and the vacuum thickness are the same as those of the base systems. The reciprocal space was sampled with a mesh of 4  2 and 16  8 in the Brillouin zone of Cu(3 3 2)O(step edge) and Cu(1 1 0)O(21) supercell. STM images are simulated by the integration in energy E (from e.Vb to Ef) of the local density of states LDOS(r,E) at each position r in a plane parallel to the surface [26]. This procedure actually simulates the STM constant-height mode, where the distance between the plane and the surface models the tip-surface separation distance. We assume that the tunnelling current in the constantheight mode mimics the variation of height in the constant-current mode in the present systems. 3. Results The exposure of the Cu(3 3 2) to oxygen induces the development of (1 1 0) facets. By varying the kinetics of the surface reaction, one obtains a variable width of the (1 1 0) facets, all aligned along the [1 1 0] direction, obtaining therefore a striped surface with alternating (1 1 0) facets and (1 1 1) terraces with different space periodicity. The temperature parameters are critical in determining the (1 1 0) facet sizes and therefore the periodicity of the reconstructed surface. We observe that a high oxygen pressures favours the formation of larger facets even if the total exposure (pressure  time) is kept constant, producing a broadening of the facet width distribution at temperature lower than 150 °C. The set of parameters optimized for the three reconstructions presented in this paper are summarized in Table 1. The reconstructions are labelled according to the periodicity of stripes as 4 nm OCu(3 3 2), 6 nm OCu(3 3 2), 10 nm OCu(3 3 2), displaying respectively an average periodicity (standard deviation) of 38 (7); 60 (13) Table 1 Dependence of the reconstruction periodicity with the exposure parameters. Periodicity average (nm)

3–4

5–7

10

O2 Exposure

5E8 5 100 °C 100 °C

1E7 5 100 °C 150 °C

1E7 5 150 °C 200 °C

Annealing

Pressure (mbar) Time (min) Temperature

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and 102 (18) Å. The statistic of the stripes size was performed in a set of STM images that sample an area of 500  500 nm. The STM topography of the three periodicities is presented in Fig. 1. The insets correspond to scans taken along the horizontal lines in the STM images: one observes a clear (1 1 0) facet (positive

Fig. 2. LEED image for the 4 nm OCu(3 3 2) (a), 6 nm OCu(3 3 2) (b). In (a) the hexagonal dashed line depicts the pattern of the (1 1 1) In (b), the LEED was pterrace. ffiffiffi taken at [1 1 0] direction. The ratio Dk(001)/Dk(110)  2 of the rectangular pattern indicates a (21) superstructure on the Cu(1 1 0) facet. The electron energy is 70 eV for (a) and 92 eV for (b). In (a) the spots contrast of rectangular pattern was locally enhanced.

Fig. 1. (100  100 nm) STM topographic images of the (a) 4 nm OCu(3 3 2); (b) 6 nm OCu(3 3 2) and (c) 10 nm OCu(3 3 2). Main crystallographic directions are indicated. The horizontal lines in each panel correspond to the direction of the line scans in the insets. Sample bias voltage was Vb = 1.45 V and the tunnelling current was 0.3–0.8 nA in all images.

slope) and (1 1 1) terrace (negative slope). The angle measured between the two slopes ranges from 28° to 35° depending on the tip condition. For all sample periodicities of the oxidized Cu(3 3 2), the observed LEED patterns present a hexagonal structure together with a rectangular pattern, corresponding respectively to the (1 1 1) terraces the (1 1 0) facets. In the case of 4 nm OCu(3 3 2) (Fig. 2a), the splitting of spot in the hexagonal pattern is resolved. The distance of the two split spots is about one third of the clean Cu(3 3 2), i.e. a periodicity of 36 Å in a direction perpendicular to the stripes, a value in good agreement with STM results. A change in the incidence angle of LEED allows to locate the (0, 0) specular reflection of the rectangular pattern at 35° from the [1 1 1] direction (Fig. 2b), confirming the expected angle between (1 1 1) and (1 1 0) plane, i.e. 35.26°. The ratio between the sides of the rectangular pattern in the LEED of (1 1 0) facet (Fig. 2b) indicates a periodicity of 5.1 and 3.6 Å along [1 1 0] and [0 0 1] directions, which is compatible with a (21) reconstruction on the Cu(1 1 0) facets. Fig. 3 displays high resolution STM images of the 4 nm OCu(3 3 2), revealing the presence of the O(21) reconstruction on the (1 1 0) facet. The STM images are similar to the usual ‘added row’ oxygen-induced reconstruction in Cu(1 1 0) [19,20], driven by the developing of O–Cu chains along the [0 0 1] direction. The negative sample bias probes the states below the Fermi energy Ef; the chosen values enhances the signal originating from the most energetically occupied antibonding state of the O–Cu chain, that is about 1.16 eV bellow Ef [27]. Fig. 3a shows the STM image of (1 1 0) facet for three different bias voltage (Vb). The O–Cu chains are displayed as bright stripes, equally spaced by 5.3 Å along the [1 1 0] direction. Above the stripes we observe an extra row of spots (dashed rectangle), also spaced by 5.3 Å, but shifted of 1.27 Å from the O–Cu chains in the [1 1 0] direction. The same feature was also observed in the 6 nm OCu(3 3 2) and 10 nm OCu(3 3 2). The bias voltage dependence of the STM images reveals a different local density of states between the stripes and the extra spots as due by a different bonding environment. In fact, when we set the fast scan direction along the [0 0 1] direction, the line scan along the O–Cu chain (Fig. 3b) suggests the presence of a monoatomic step between the (1 1 0) and (1 1 1) facet. The line profile (horizontal line) has been integrated in the [1 1 0] direction and shows three peaks separated by 3.6 Å, i.e. oxygen atoms in the (21) reconstruction on Cu(1 1 0) facet. A further peak, found distanced by 7.2 Å, is associated to the presence of oxygen atom at a monoatomic step edge. Combining the information from the STM images of Fig. 3a and b, we are able to associate the extra atomic row observed in Fig. 3a to oxygen atoms on fourfold hollow sites (or close to this position) at a step edge between the (1 1 0) facet

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Fig. 3. High resolution STM images of the (1 1 0) facet of the 4 nm OCu(3 3 2). (a) STM image with different sample bias: 1.48, 0.80 and 0.47 V. The solid line rectangle indicates the O–Cu chains. The distance between the chains is 5.3 Å. The dashed line rectangle shows the position of the spots attributed to the fourfold coordinated oxygen; (b) STM image scanned along [0 0 1], the inclination of the O–Cu chains is an artifact caused by drift effects. The line scan in (b) averages all lines along [0 0 1] direction (parallel to the blue line), indicating a presence of a monoatomic step between the (1 1 0) and (1 1 1) facets; (c) The comparison between a real STM image and proposed hard-ball model. The oxygen atoms (small red spheres) on the monoatomic step edge are placed in the fourfold site. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

and (1 1 1) terrace. A similar adsorption site for oxygen has been reported for the oxidized Cu(211) [28]. This atomic position is

reproduced in Fig. 3c via hard-sphere model, which is compared to the STM data.

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4. Discussion Our experimental results indicate that the average periodicities is strongly dependent on external parameters (exposure and annealing temperatures), hence suggesting that the elastic interaction plays a minor role in defining the long range order of the reconstruction. The elastic energy is a functional of the stress on the border of two phases, that is, the surface stress difference between the Cu(1 1 0)O(21) facet and the Cu(1 1 1) facet plus the monoatomic step. A possible explanation is that the surface stress of both facets have a small values near to the edge and therefore does not produce a well defined elastic energy minimum for a specific periodicity. The average periodicity is then ruled by the kinetic properties of the Cu–O chains growth on the (1 1 0) facets. Since the reconstruction process involves the transport of large amounts of Cu atoms, the stabilization of the size of a (1 1 0) facet by the assembling of the O–Cu chains is strongly dependent on surface mobility and, therefore, on temperature. In order to validate our interpretation of the STM data, we have performed STM simulation using ab initio calculation. Fig. 4 pre-

Fig. 4. Relaxed supercell used for simulation of the Cu(3 3 2)O(step edge) (a) and Cu(1 1 0)O(21) (b). The big spheres represent the Cu atoms while the small sphere represents the O atom. The green spheres indicate the nearest Cu atoms for the O atom. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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sents results of the calculation for the relaxed Cu(3 3 2)O(step edge) and Cu(1 1 0)O(21) supercells. In the Cu(3 3 2)O(step edge), the O atom relaxes from the fourfold site (equal distance from atoms 1 and 2) to a position closer to the step edge (pseudo threefold coordinated). The distance between the O atom and the nearest Cu atom in this site is 1.86 Å. The structural results obtained for Cu(1 1 0)O(21) system are similar to those presented in Ref. [25]. Fig. 5 shows the simulated image for Cu(3 3 2)O(step edge) (panel a and c) and for Cu(1 1 0)O(21) (panel b and d), to be compared with experimental results in Fig. 3a. The distance for Cu(3 3 2)O(step edge) was set in such way that the maximum intensity of the simulated image has the same value of Cu(1 1 0)O(21) at bias of Vb = 0.84 V. Fixing the plane distance for both systems, we obtain the variation of the integrated LDOS with Vb = 0.47 V (Fig. 5a and b) and Vb = 1.48 V (Fig. 5c and d). The intensity of the row of spots in the Cu(3 3 2) is higher (smaller) than the intensity of the O–Cu chains when the sample bias is 0.47 V (1.48 V). This confirms that the bias dependence of the STM images allows identifying the specific O sites, according to the model. In order to get a better insight into oxidation process, we have calculated the adsorption energy of oxygen on different sites in the Cu(3 3 2). The adsorption on the (1 1 1) terraces was calculated using a coverage of 0.25 ML on the Cu(1 1 1) slab. Such a coverage is low enough to avoid the influence of the repulsive interaction energy between oxygen atoms [29]. The adsorption energies are given with respect to the free oxygen atom. We obtained 4.96 and 4.81 eV for the fcc-hollow site and hcp-hollow site on Cu(1 1 1). The adsorption values in the step edge were calculated using the Cu(3 3 2) supercell shown in Fig. 4a. We obtained 5.40 and 5.01 eV for the pseudo threefold site and the fcc-hollow site in the upper edge. The pseudo threefold site at step edge is clearly the preferential absorption site, with a total electron transfer of 0.41 electron from copper to the oxygen, which indicates an ionic character for the bonding [25]. The large adsorption energy and the electron transfer from surface to the oxygen atom suggest the formation of a strong bond. STM data on Fig. 3 indicates the presence of oxygen at step edge on the unreconstructed stepped surface. Following this assumption we now discuss the influence on the faceting process of oxygen atoms on pseudo threefold site. The energy balance which rules the stability of vicinal surface is very delicate. The difference between the surface energy of the

Fig. 5. Simulation of STM for the Cu(3 3 2)O(step edge) (a and c), and for Cu(1 1 0)O(21) (b and d). A distance of 3 monoatomic layers for the STM simulation of the Cu(1 1 0)O(21) was set. The set (a)–(b) and (c)–(d) represent the STM image of the (1 1 0) facet shown in Fig. 3a, with the bias voltage of Vb = 1.48 and 0.47 V. The grey scale was independently normalized by the maximum intensity of each set.

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stepped and faceted surface at 0 K is in many cases of the order of meV [10,30,31]. Many theoretical studies have been performed to explain the stability of the vicinal surface with respect to the faceting. A common approach is to assume that a clean vicinal surface is stable with respect to the faceting when the surface energy (per unit of area) of the stepped surface is smaller than the surface energy (per unit of area) of the linear combination of the facets that keep the original orientation [30]. In Cu(3 3 2), the combination of (1 1 1) and (1 1 0) facets maintains the primitive orientation as modelled in Fig. 6. The size ratio between the facets (1 1 1) and (1 1 0) is equal to 2.46, resulting in an increase of 4% in the total area. The condition for the transformation of an area S of the Cu(3 3 2) surface into a (1 1 1) and (1 1 0) facets with areas S111 and S110 is then:

Sc332 > S111 c111 þ S110 c110

ð1Þ

To estimate the energy balance to the Cu(3 3 2) reconstruction and faceting we have calculated surface energy (c) for the respective surfaces: results are listed in Table 2, compared with results of recent theoretical work [31] using DFT calculation with higher sampling of reciprocal space. The calculated values for the linear combination of the surface energies of the relaxed Cu(1 1 1) and Cu(1 1 0) surfaces (cf ) are reported in the last line of Table 2. The linear combination constants are derived from the ratio between the area of facets and the total area of the surface projection on the (3 3 2) plane. One notes that the cf value is very near from the surface energy of the relaxed Cu(3 3 2). Calculation suggests that the vicinal surface is on the border of instability with respect to the faceting, whereas the unrelaxed Cu(3 3 2) surface would be fairly unstable with respect to the faceting (surface energy 1–2% higher than the linear combination cf ). This effect, also observed also for Cu(n 1 1) surfaces [30,31], indicates that the multilayer relaxations plays an important role in the stabilization of the stepped surface [31]. Hecquel [3] derived the elastic interaction between steps consisting in two terms of opposite sign: a negative term that is originated by the reaction within the bulk with respect to the surface relaxation, and the positive term which is proportional to the in-plane deformation in direction perpendicular to the steps. Fig. 7a shows the displacements of the first layer in z direction ([1 1 1]) originated by the relaxation of Cu(3 3 2) surface and its comparison with the relaxation of Cu(3 3 2)O(step). The oxygen in the pseudo three fold

Fig. 6. Geometry of the faceted and stepped (3 3 2) surface.

Fig. 7. Surface modification due the relaxation: displacement of the first layer in z direction ([1 1 1]) (a) and strain of the first layer along the y direction ([1 1 2]) (b). The inset in (a) is the side view of the stepped surface indicating the label position y.

clearly inhibits the inward displacement of the first layer. Fig. 7b shows the deformation (strain) of the first layer in the direction perpendicular to the steps [1 1 2]. As mentioned by Hecquet [3,32], for Cu(0 n 1), the presence of steps induces a compression (negative strain) of the terrace surface close to the step, resulting in a local reduction of tensile stress. We notice that the presence of the oxygen atom at pseudo threefold site increases the compression. We argue that theses significant changes in the atomic position of the first layers produced by the adsorption of the oxygen at step edge of the Cu(3 3 2) can contribute to the instability of the stepped surface before reconstruction. It is therefore clear from our results that both absorption of oxygen in pseudo threefold site and formation of the Cu–O chains strongly contribute on the forming (1 1 0) facets, the first acting as the trigger mechanism for faceting and the second stabilizing the reconstruction and defining the size of the (1 1 0) facet and therefore the periodicity. The absence of oxygen in the (1 1 1) terraces is explained by the small

Table 2 Surface energy (c) calculated for unrelaxed and relaxed slabs. The sampling grid was calculated using the Monkhorst–Pack mesh. The bottom row is the surface energy of the facet Cu(3 3 2) surface approached as the linear combination of Cu(1 1 0) and Cu(1 1 1).

a

Surface energy c (J/m2)

(Present work) GGA

DFT-FLAPWa

Unrelaxed

Relaxed

Grid

Unrelaxed

Relaxed

Grid

Cu(1 1 1) Cu(1 1 0) Cu(3 3 2) 0.30cCu(110) + 0.74cCu(111)

1.339 1.579 1.475

1.334 1.531 1.450 1.447

16  16 16  16 84

1.320 1.591 1.455

1.317 1.550 1.434 1.440

20  20 20  20 20  4

Ref. [31]: DFT employing the all-electron full potential linearised augmented plane-wave.

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sticking coefficient of this plane, about on hundredth of the Cu(1 1 0) [33]. In summary, we have shown that, by controlling the oxygen exposure parameters and kinetic conditions, it is possible to obtain surface reconstructions of the primitive Cu(3 3 2) vicinal surface that consist of alternated facets of Cu(1 1 1) and Cu(1 1 0)– O(21) with average periodicity from 3 to 10 nm, always maintaining the same average orientation of Cu(3 3 2). The strong dependence of the periodicity on the temperature indicates that the kinetic properties of the Cu–O stripes growth play the main role in producing a periodic arrangement of nanostripes. Also we have obtained direct images of the (1 1 0) facets where we found that the surface reconstruction is indeed composed by Cu(1 1 1) clean terraces and Cu(1 1 0)–O(21), and at the edge between the two facets, by a monoatomic step in which the oxygen is absorbed close to a fourfold coordinated site (pseudo threefold site). The adsorption of oxygen in such site in the unreconstructed Cu(3 3 2) could be responsible for triggering the faceting reconstruction. Acknowledgements We thank M. Peressi for valuable discussions. Computational resources have been provided by CINECA facility. References [1] J.V. Barth, G. Costantini, K. Kern, Nature 437 (2005) 671. [2] S. Rousset, F. Pourmir, J.M. Berroir, J. Klein, J. Lecoeur, P. Hecquet, B. Salanon, Surf. Sci. 422 (1999) 33. and references therein. [3] Pascal Hecquet, Surf. Sci. 561 (2004) 127. [4] F. Raouafi, C. Barreteau, M.C. Desjonquères, D. Spanjaard, Surf. Sci. 505 (2002) 183. [5] Yu Shu, Jian-Min Zhang, Ke-Wei Xu, Surf. Interface Anal. 39 (2006) 349. [6] Min Qiu, Pei-Lin Cao, Jin-hao Ruan, J. Phys.: Condens. Matter 89 (1996) 4867. [7] J. Shen, J.P. Pierce, E.W. Plummer, J. Kirschner, J. Phys.: Condens. Matter 15 (2003) R1. and references therein.

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