Polar-step Driven Metal Nucleation and Growth: The Ag/ZnO(1010) Case

The morphology, epitaxy, band-alignment and optical response of silver nanoparticles on ZnO (1010) has been analysed by microscopy, photoemission, electron di(cid:27)raction and di(cid:27)er-ential re(cid:29)ectivity. At 700 K, clusters tend to nucleate and grow exclusively along [010] -oriented polar steps and corners, and not on (cid:29)at ter-races or non-polar [001] steps. This preference leads to a speci(cid:28)c epitaxy Ag (111)[110] (cid:107) ZnO [010](1010) with a large lattice mismatch along the dense row of the metal. The experiments have been rationalized through ab initio simulations. A much lower adsorption energy is obtained for the O-terminated steps with a charge transfer from silver to oxygen, that corroborates the variation of ionization energy observed in photoemission and the presence of cationic silver. This anisotropy of growth rever-berates in the plasmonic response of the metallic particles. The growth and epitaxy of metals on the polar (0001) and non-polar (1010) orientations of ZnO is discussed in terms of surface/step polarity compensation in the light of the present (cid:28)ndings.


Introduction
Encapsulated in complex stack of lms, silver layers are widely used by the glass industry as the active element for infrared reection in low-emissive or anti-solar coatings. 1,2Their mechanical resistance and ability to withstand dewetting upon annealing during windows tempering or shaping rely on the metal adhesion at the interfaces.Among the potential oxide substrates, ZnO wurtzite turned out to be one of the best candidates in terms of silver wetting.Beyond the argument of a reduced bandgap compared to bulk-insulating oxides 3 and of a good cristallization during sputtering deposition, several fundamental reasons have been put forward to explain the enhanced adhesion on ZnO. 4 The rst argument relies on the polarity of the (0001) orientation in the wurtzite structure 5 since the polycristalline ZnO layers on which silver is deposited display a strong c-axis bre texture.Among dierent mechanisms such as creation of vacancies or hydroxylation, charge transfer to the deposited metal should allow to circumvent the polar catastrophe i.e. the electrostatic divergence of the surface potential due to the alternate stack-ing of Zn cation and O anion planes along the [001] direction. 6,7The adhesion energy between Ag and the ideal, bulk-like ZnO(0001)-Zn and (0001)-O terminations was computed to be rather high. 8However, several mechanisms are at work to make the existing surfaces already polar-compensated before metal growth, which actually deeply modies their atomic and electronic structures and lower the adhesion with the Ag overlayer.The actual mechanisms are still debated and they depend on the specic surface termination (either Zn or O) and chemical environment; for instance, surface hydrogen load and departure from the ideal stoichiometry are quite entangled. 919good metal adhesion on an oxide substrate is often due to epitaxy and lattice matching between the two systems.In this respect, the case of Ag/ZnO(0001) is quite puzzling.Previous grazing incidence X-ray diraction measurements 20,21 on vapour-deposited Ag on ZnO single crystals showed that the metal adopts on both faces, Zn and O-terminated, a so-called hexagon/hexagon Ag[110](111) ZnO[100](0001) orientation with a lattice mismatch of -11 % despite an expected lower value (-3.8 %) for the 30 • -rotated orientation.This nding was assigned to a perfect coincidence of (9×9) unit cells of Ag every (8×8) surface unit cells of ZnO.Nevertheless, growing nanoparticles (NPs) that require a size of at least 2.6 nm to feel this coincidence were found stress-free at their bulk lattice parameter for coverage above half a monolayer.In parallel, small-angle X-ray scattering evidenced a quasi-2D growth and a good apparent wetting; at-top and highaspect ratio (diameter/height) (111)-oriented NPs cover rapidly the whole surface.Similar ndings in terms of epitaxial orientation and bidimensional growth morphology were also obtained for other face centred cubic metals such as Cu, 2225 Pd 26 or Pt. 27,28ZnO precipitates in Ag matrix share also the hexagon/hexagon orientation. 29But according to simulations, the hexagon/hexagon orientation is slightly more stable than the rotated one. 8e morphology, epitaxy and stress state of Ag NPs on the polar faces of ZnO run counter classical expectations about metal/oxide growth.As previously shown for Ag/Al 2 O 3 (0001) 30 or Ag/MgO(001), 31 beyond an undoubtful non-wetting 3D growth of low aspect ratio particles, the NPs lattice parameter is driven by the interplay between surface and interface stresses. 32Below a critical size of about 3 nm, a contraction or expansion is observed depending on the sign of the metal/oxide mist, while above this size the strain state obeys the "Laplace pressure" law.Such an unusual behaviour of Ag/ZnO(0001) NPs could be due to the large density of defects, in particular step edges, that exist on the polar faces of ZnO.Indeed, for vacuum annealed ZnO(0001)-Zn, a dense array of triangular pits and holes is observed by scanning tunneling microscopy, 10,33 which, according to atomistic simulations, 10,11,19 are non-stoichiometric and provide the required charge to heal the surface polarity.Noteworthy, the ZnO nanostructures exhibit [100]-oriented edges that are terminated by rows of O or Zn ions and are themselves polar.The morphology of the ZnO(0001)-Otermination is less clear-cut and the actual mechanism of the surface polarity compensation is still debated, although the saturation of half the O surface anions with protons is likely in the experimental conditions. 17As far as a metal growth is concerned, a likely explanation is a step-induced nucleation and quick lateral growth of the NPs with the alignment of the dense rows along the step edge. 34The copper NPs growth clearly goes with a charge transfer as determined by the evolution of band-bending and work function. 22,23Silver is cationic (anionic) on ZnO(0001)-O (ZnO(0001)-Zn) at low coverage, before adsorbing as a neutral species when the particle size increases and eventually form a Schottky barrier. 35Whether the charge transfer originates from healing the step or the surface polarity, and how it reinforces the metal-oxide chemical bonding remains anyway unclear.
In order to understand the role of the step edges on metal adsorption, the non-polar ZnO(1010) orientation is a better playground than ZnO(0001)-Zn and ZnO(0001)-O owing to the complexity of polarity compensation on the latter terminations.The ZnO(1010) surface is characterized by the presence of rectangular holes and pits with perpendicular steps of different terminations 3638 (Figure 1).While [001] steps expose compensated facets made of equal numbers of cations and anions, [010] steps are polar, either O or Zn terminated. 6,7The specic role of steps was already underlined in the case of Cu/ZnO(1010) 19,39 or Cu/ZnO(0001)-Zn. 25,34,40,41ucleation, epitaxy and band-alignment have been probed during Ag growth on ZnO(1010) by a combination of Scanning Tunneling Microscopy (STM), Low-Energy Electron Diraction (LEED) and Ultra-violet Photoemission Spectroscopy (UPS).The impact of the surface anisotropy and of the charge transfer on the plasmon resonances in the NPs was also explored at macroscopic scale by UV-visible polarized Surface Dierential Reectivity Spectroscopy (SDRS).Beyond shape and density anisotropies, 42,43 charging upon contact with the support and size reduction 44 may inuence plasmon resonances and de-excitation mechanisms, 4547 with inuences on the charge dy-namics, 48 like in the case of the material catalytic response. 49Experiments haves been supplemented herein by ab initio simulations in the framework of the Density Functional Theory (DFT) to analyse the formation energies of the various step terminations as well as the preferred Ag adsorption site and its charge state.

Experiments
Experiments were conducted into two separated vacuum vessels (Modena/Paris) with base pressures in the low 10 −10 mbar and both equipped with X-ray photoemission spectroscopy (XPS) and LEED facilities.ZnO(1010) single crystal surface (provided by SurfaceNet) was prepared by cycles of sputtering and annealing at 1100 K until reaching a satisfactory LEED pattern (Figure 1-b) and the lack of contaminants at the sensitivity of XPS.Silver was deposited from a temperature regulated thermal evaporation cell at a base pressure in the high 10 −10 mbar and keeping the substrate either at 300 or 700 K.The evaporation rate around 1 Å/min was previously calibrated in situ with a quartz microbalance.
A hemispherical analyzer (EA125 Omicron) working at normal emission and at constant pass energy was used (i) for XPS measurements under Al-Kα non-monochromatic X-ray excitation and (ii) for UPS under He I excitation (at an overall resolution of 0.1 eV).Binding energy scale was set on the Fermi level of a clean Ag reference crystal.The work function of metal deposit on ZnO was measured from the secondary electron cuto by polarizing the sample at -20 V to reaccelerate the low energy electrons.Samples were conductive enough to prevent any noticeable charge eects with all the above cited techniques.STM images were collected on an Omicron RT UHV AFM/STM with a chemically etched W tip. Images were analyzed by means of WSxM 50 and SPIP softwares.
The growth process of silver was followed in real-time by SDRS at an incident angle of 55 • .The experimental setup (previously described in Ref. 51) allowed recording simultaneously the s-and p-polarization states of light.The incident plane was aligned parallel or perpendicular to the [001] surface direction of ZnO(1010) to within a few degrees through electron diraction.The back face of the substrate was tarnished to avoid back reection.Shortly, SDRS consists in recording the relative variation of the sample reectivity in the UV-visible range (1.5-4.5 eV) by using the bare substrate signal as a reference.The incoming light excites localized surface plasmon absorption modes 5153 in the growing NPs, which position and intensities are highly sensitive to the morphology and to the electromagnetic interaction with the substrate and among the collections of NPs. 51,52Associated to dielectric modellings, 5459 this optical technique already allowed to describe growth nucleation, growth and coalescence processes of supported NPs and their shape evolution. 3032,60In s-polarization, the electric eld being parallel to the surface can only excite modes parallel to the surface.Since the electric eld is in the plane of incidence in p-polarization, both parallel and perpendicular active modes can be excited.Silver being a test bed for plasmonics, submonolyer sensitivity could be easily achieved owing to the contrast of dielectric function with the substrate and of the excitation of very intense plasmon resonances.
Ab initio simulations DFT calculations have been carried out by means of the Quantum Espresso suite, 61 within the Generalized Gradient Approximation (GGA), 62 in conjunction with the use of ultra-soft pseudo-potentials.Systematic tests showed that the convergence of the total energy, atomic forces and crystal parameters can be attained at 50 Ryd cuto energy on the plane-wave basis set.The computed values of the lattice constants a = 3.283 Å (3.25 Å), c = 5.296 Å (5.207 Å) slightly overestimate the experimental ones (in brackets) ) as usual in GGA; the parameter dening the relative position of O with respect to Zn along the [001] axis is u = 0.379.
The ZnO(1010) surfaces were simulated through slabs of variable thicknesses, consisting of M (1010) double layers, with 4 ≤ M ≤ 12, and a void space that is as thick as 12 planes.The surface Brillouin zone was sampled by a (5 × 3) grid, centred at Γ point.The slab total energy E(M ) can be written as the sum of the surface contribution and the bulk one as: where A 0 and σ 0 are the ZnO(1010) surface area and energy, respectively, and E bulk the energy of the crystalline bulk. 63From the previous formula, by means of a linear regression, both bulk and surface energies are obtained.After extensive checks, the computational set-up ensures the convergence of the surface energy within 0.03 J/m 2 2 meV/Å 2 .Vicinal surfaces with (1010) terraces and steps oriented along either [010] (polar) or [001] (non-polar) were simulated through shifted periodic boundary conditions. 64The energy for the formation of step per unit of length λ θ can be written as the dierence between the surface energies of the vicinal and the at surfaces as: where θ is the angle between the vicinal surface n θ and at surface n 0 normal directions, A θ and σ θ the vicinal surface area and energy, respectively, and a the lattice parameter along the step direction.In all slab calculations, the atoms in the two innermost layers were kept xed in their ideal bulk positions, whereas all the others were free to relax, until the atomic forces reduced to less than 2 meV/Å.By simulating several terrace widths (i.e.various θ), the step energy was found via a linear regression.Dierent kinds of step were simulated: (i) the non-polar step, parallel to [001], and (ii) two polar steps, either type-A or type-B (Figure 5).The resulting step energies vary between 0.10 eV/Å (for non-polar [001] steps) to 1.06 eV/Å (for type-A polar [010] steps) and 0.42 eV/Å (for type-B polar [010] steps).This hierarchy of energies matches with the trends observed by STM of step elongation along the non-polar [001] direction upon annealing at high temperatures. 65As the actual termination of the steps on ZnO(1010) are unknown from the experiments, we systematically simulated distinct edges and studied the rst stages of Ag adsorption, on dierent sites, in order to determine the most relevant stable adsorption congurations in thermodynamic equilibrium conditions.shows its hexagonal shape.a terrace (inset Figure 4-a) conrms the alignment of step edges with atomic rows separated by 5 Å, as previously reported for this surface. 37The apparent step height is found close to a multiple of h 3.3 Å, which corresponds to the bulk periodicity of ZnO bilayers perpendicular to the (1010) plane, with a preference for a single bilayer.

Results and discussion
When Ag is deposited on this surface, it spontaneously nucleates in the form of NPs.Apparent lateral size, height and density of the NPs obtained from STM grain analysis are reported in Tab. 1.Like for Cu/ZnO(1010), 39 the growth is clearly 3D since, at the early beginning, NPs have an height of 3-4 atomic planes.The hexagonal prismatic shape shown in the inset of Figure 4-e, with its tips aligned along the [010] direction of ZnO agrees with the epitaxy relationship found in LEED.But no clear shape anisotropy induced by nucleation and growth along the step edges was observed.The morphology strongly depends on the deposition temperature as conrmed by SDRS lineshapes (see below).When NPs are formed at 300 K, their apparent lateral size is between 2.5 and 4 nm and reaches an apparent complete coverage of the surface well below 1 Å if tip convolution is not accounted for.At 300 K, preferential nucleation occurs at step edges but some NPs appear randomly distributed all over the terraces as well.But when the growth temperature is raised up to 700 K, the NPs increase in size and their density is reduced as expected from thermally enhanced diusion.Astonishingly, they nucleate exclusively on corners or on polar     If the deposition is done at thermodynamic equilibrium, the isolated Ag atoms are preferentially adsorbed at the O-terminated [010] polar step, on the hollow sites.This is no longer the case of the Ag 2 dimer, which still adsorbs parallel to the O-terminated [010] polar step, but on the two bridge sites.The adsorption of a full, epitaxial Ag row along the step edge was simulated by using a (3 × 1) surface cell, with a periodically repeated Ag atom at distinct sites close to the step edge.Noteworthy, the dimer adsorption conguration is very similar to the most stable one for an entire Ag row along the O-terminated [010] step, apart from a smaller Ag-Ag distance (2.92 Å in Ag 2 instead of 3.28 Å in the case of epitaxial Ag row).Numerical calculations show two trends when considering the the number of Ag adatoms on the stepped ZnO(1010) surface.First, two isolated Ag adatoms are more stable than the Ag 2 dimer at the [010] steps (see Figure 6).∆H ad increases with the number of Ag up to the completion of a Ag row along the [010] step (where |∆H ad | is lower by roughly 20-30% than for the isolated Ag adatom) for the most stable congurations.The relative weakening of the adsorption strength with step coverage can be ascribed to the charged state of the adatoms close to the step, which results into the mutual electrostatic repulsion between themselves.Secondly, according to our simulations, the adsorption of Ag on the O-terminated [010] step is preferred to that on the Zn-terminated edge, whatever the step coverage.This is agreement with the experimental STM images (Figure 4), which show that the nucleation at polar step edges is not uniform, although the step termination could not be determined experimentally.From the comparison of the computed ∆H ad for the isolated Ag, the dimer and the entire row, one can conclude that the nucleation on hollow and bridge sites on O-terminated [010] steps are preferred if Ag is deposited mainly from gas-phase atoms.However, another question could arise: given a Ag adsorbed at Oterminated step edges, where does a second Ag atom go?By comparing the adsorption energies of a Ag 2 dimer at the step and of two Ag adatoms, the rst at the O-terminated [010] step and the second in another site, it appears that at the very rst stage of Ag deposition, the nucleation on distinct sites is favoured over the growth of a Ag row at the step edges.This could explain the observed lack of anisotropy of particle shape, such as elongation along the step edge.As the density of Ag adatoms at O-terminated [010] steps increases, there could be a competition between the adsorption of a second Ag atom along the O-terminated [010] step forming a dimer and the adsorption at Znterminated [010] steps.Clearly, this picture depends on the deposition conditions and kinetics.However, the simulations indicate that the nucleation on multiple sites appears to be favoured onto the sudden growth of Ag rows on few nucleation sites.This picture is consistent with the STM observations (Figure 4).

Charge transfer
The preferential nucleation on step edges can induce noticeable modications in the electronic properties and the dielectric response of the NPs.They were scrutinized by UPS and SDRS respectively.Up to 1 Å, valence band (VB) spectra (Figure 7-a) are characterized by a Zn 3d derived state feature at around 10 eV and a O 2p/Zn 4s one between 2 and 6 eV.Above this lm thickness value, the modications observed above 4 eV can be assigned to the appearance of Ag 4d derived states.In parallel, the density of states rises up close the Fermi level due to the developement of the Ag 5s metallic character of the clusters.The clear Fermi energy step in the NPs at a thickness of 10 Å validates a posteriori the calibration of binding energy scale.The lack of the Ag 5s derived states below 1 Å is not related to a non-metallic/metallic transition of the NPs but more likely due to nal state eects in small clusters in which the poor screening of the hole shifts upward the binding energy. 35he interpretation is supported by the presence of plasmon resonances, i.e. collective oscillations of the electronic cloud coupled to the incident eld, in SDRS (Figure 10).since the early stages of the growth.Oscillations on the 10 Å curve of Figure 7 remind the photoemissions observation made in the case of Ag growth on polar faces of ZnO 35 or on ZnO(0110) 66 and are assigned similarly to conned Ag 3d states in the NPs.The position of the VB edge was obtained as the crossing point between the linear t of the ank of the valence band and the background level.From this value, the shift of the VB edge, namely the variation of band bending in the oxide ∆V bb was determined (Figure 7-b,c).The corresponding positive variation points at a downward band bending that impacts also the core level positions (not shown).The maximum ∆V bb occurs at about 0.5 Å and goes to zero at 2 Å for NPs grown at 300 K; at 700 K, those onsets are shifted to 1 Å and 4 Å respectively, pointing at a coverage eect.To understand the origin of this evolution, the work function variation ∆Φ was also determined from the low-energy cut-o of secondary electron distribution.As a result, the ionization energy variation was calculated as ∆I = ∆Φ + ∆V bb .All quantities are reported in Figure 7. ∆I is negative in the range 0-2 Å for 300 K deposition, 0-4 Å for 700 K, when the downward band bending is observed.This indicates the presence of a positive surface dipole at the surface, due to a charge transfer between the Ag NPs to the oxide, with a positive end on the metal side.Above in thickness, silver adsorbs as a neutral species.The dierent ranges at which charge occurs at the two temperatures are related to the dierent nucleation densities and coverages as observed in STM.Most likely, ∆I and therefore the charge transfer drop to zero when the NPs saturate the steps.Since the dominant charging is positive and the NPs tend to decorate preferentially one type of the two polar borders, a preference to O-terminated borders is more likely as suggested by atomistic simulations.
To pinpoint the amplitude and direction of the charge transfer, electronic density for Ag adatoms adsorbed at step edge has been scrutinized in the simualtions.Since [010] steps are composed by rows of either O or Zn ions, a polarization normal to the step direction should develop.The internal electric eld bends the electron bands, until the levels of the nonbonding states on the O and Zn edges overlap.The O states are partially emptied and the Zn ones partially lled, providing the necessary charge compensation. 6,7The surface is therefore open-shell.Upon Ag adsorption, one can note an electron transfer from the Ag adatom to the O at the step edge (see Figure 8, right panel).In particular, the extended s-like Ag orbital is depleted, mainly towards p-like orbitals on the neighbouring O ions.This allows for the O at the step edge to recover their typical ionic conguration, while Ag is partially ionized and polarity compensated.As a matter of fact, the adsorption of the Ag 2 dimer in the bridge conguration restores the insulating character of the overall system.When Ag is adsorbed at the Zn-terminated [010] step, there is an electron population increase in the step region between the adatom and the neighboring Zn, at the expense of a d-like Ag orbital (see Fig-ure 8, left panel).This situation corresponds to a metallic-like binding between Ag and Zn.Experimentally, the order of magnitude of the transferred charge can be deduced from a planecapacitor modelling of the change of ionization energy: ∆I = eN p ⊥ / 0 . 67N and p ⊥ are the surface density of dipoles and their normal component while e, 0 have their usual meaning.For a p ⊥ dipole length given by the step height (see Figure 8-right), a value of ∼ 0.05 e/Ag is found.signal at 700 K for the two beam directions and the two polarization states in the submonolayer regime (Figures 10-a,b) and above (Figures 10-c,d).The spectra are characterized by intense plasmon absorptions in the growing Ag NPs; these are excited by the component of the electric eld parallel to the surface (both s-and p-polarizations) for the low energy peak (below 3 eV) and by the normal component (p-polarization only) for the high energy feature (above 3.7 eV).Strikingly, a dierence in position of the low-energy resonance between the two polarization states occurs in the submonolayer regime (Figures 10-a,c) but vanishes above (Figures 10-b,d).The peak is systematically red-shifted in p-polarization compared to s-polarization for a beam oriented along the [010] polar step edges and vice-versa for the [001] direction.To highlight this eect, the positions of all peaks have been extracted from a parabolic t of the extremum (Figure 11).The cross-over in position observed between 2-3 Å for the low energy peak between the two polarizations is less clear at room temperature than at 700 K, a nding which is conrmed by a direct inspection of the raw data of Figure 12 As already shown for Ag NPs on oxides, 31,51,52,56,68,69 the low energy resonance position is an extremely sensitive reporter of morphology; the more red-shifted the atter or the denser the particles.A less important blue shift is usually found for the high energy resonance with nanoparticle aspect ratio (diameter/height) and particle density.This rule of thumb is fullled when comparing peak positions at dierent growth temperatures (Figure 11) as expected from temperature-induced dewetting.As seen by STM (Fig. 4), no clear anisotropy of shape can put forward to explain the optical anisotropy at the earliest stages of the growth.Moreover, the role of the anisotropy of ZnO bulk dielectric constants can be safely ruled out to explain the present ndings on the low-energy plasmon peak.The detailed ellipsometry measurements of Ref. 70  showed that, for a photon energy in the band gap of ZnO where lies the peak of interest, the ordinary and extraordinary directions behave in a similar way.Optical anisotropy of ZnO appears mainly around the exciton absorption close to the band gap edge.At the opposite, the found shift between polarizations for the low energy resonance is compatible with the STM results of denser particles along the polar [010] step edge.Indeed, the dipole-dipole coupling between NPs 51,52,54,71 creates a depolarization eld that acts again the parallel plasmon oscillation and therefore red shifts the resonant frequency proportionally to the ratio of the oscillator strength to the spacing between NPs.It is worth noticing that the optical anisotropy disappears at large thickness when, as seen by STM, the surface is covered more randomly by the NPs.
The experimental spectra also show a feature slightly above the 3.2 eV band gap of ZnO (Figures 10,12).Its line shape and position in p-polarization is at odd with an eect of substrate thermoreectance 72 that could be due to a slight increase of substrate temperature when opening evaporation cell.To see if it matches with the excitation of modes of absorption in Ag NPs, 5153 SDRS spectra have been inverted using an already discussed Kramers-  Kronig algorithm 51,73 to obtain the interface susceptibilities 54 (IS).These complex dielectric lengths characterize the dielectric behaviour of the lm/substrate interface in the direction parallel (γ-IS) and perpendicular (β-IS) to the surface.Shortly, through such a treatment, the dielectric responses of the lm alone, parallel and perpendicular to the substrate, are desentangled from the known optical response of the bulk substrate 70 and from the mixture of electric eld directions in p-polarization spectra.As seen in the example of Figure 13-a,b, the imaginary part of parallel γ-IS is dominated (i) by an increasing and shifting parallel plasmon resonance 51 below 3 eV and (ii) by the bulk Ag interband transitions above 3.8 eV.Both absorption occurs in the Ag NPs.In fact, as already shown, 51 the experimental plasmon peak encompasses the excitation of dipole-like polarization modes A and B in at particles (see simulation Figure 13-d).But an absorption feature rises between 3.1 and 3.5 eV in the submonolayer regime before saturating at higher thickness at the opposite to the plasmon resonance (see Figure 13-c).Absent from simulations 57,58 with truncated spheroidal particles with bulk dielectric constants, 70,74 it parallels the imaginary part of the bulk dielectric function of ZnO.It is assigned to the modi- cation of absorption across the band gap of the substrate due to the charge transfer to the bulk.A likely hypothesis is a modulation of the electric eld in the ZnO space charge layer that parallels the band bending observed by UPS at the beginning of the growth.The impacted depth as given by the continuous lm behaviour of IS 51,54 Im(γ)/Im( ZnO ) amounts to a few nanometers, an order of magnitude that matches with the Debye length previously determined by photoemission in ZnO crystals prepared by sputter-annealing. 35The dierence of visibility of such a feature in the raw data at 300 and 700 K might be related to a coverage eect of step edge.

Conclusion
The growth of silver on the non-polar ZnO(1010) surface have been scrutinized by a combination of surface science techniques (LEED, STM, UPS and SDRS) and ab initio calculations.If not so clear cut at roomtemperature, at 700 K, tridimensional metallic clusters nucleate and growth exclusively at corners and polar [010]-oriented step edges with a preference towards one type of termination.In parallel, the metal aligns its dense row along the same direction despite a large lattice mismatch.According to atomistic simulations, metal adsorption is strongly thermodynamically favoured at step edges compared to at terrace sites with a strong preference for Oterminated ones.As seen in UPS at the early stages of growth and conrmed by calculations, a charge transfer occurs between the metal and terminal oxygen atoms giving rise to slightly cationic silver and partial healing of step polarity.At the opposite, the binding to Zn-terminated step is more metallic.This anisotropy of nucleation and growth gives rise to an in-plane anisotropic plasmonic response of the clusters.Beyond these resonances, the metal adsorption give rise to a band gap related optical feature that parallels the observed band-bending.This work puts forward the question of polar step healing as a potential source of apparent good metal spreading and of the astonishing epitaxial relationship of metals at the surfaces of ZnO.Further work is required to tackle the question of the actual impact of the epitaxy on the stress state of the NPs.

Figure 1 :
Figure 1: (a) Atomic ball model of the ZnO(1010) surface with oxygen anions in red and zinc cations in grey.Steps running along the [010] direction are either O-or Znterminated while perpendicular ones along the [001] direction are composed equally of both ions.Main directions are indicated by arrows.(b) LEED pattern of a ZnO(1010) clean surface (E B = 120 eV) with its rectangular unit cell ((5.207 × 3.25) Å 2 ).

Figure 2 :
Figure 2: Ball model of a slab model of ZnO vicinal surface with O (red) and Zn (gray).The slab consists here of 6 double ZnO(1010) layers and the steps run along the [010] direction, with step length equal to a.The step height h = a corresponds to a ZnO double layer and the terrace width to L = 2.5c, where a and c are the computed ZnO lattice parameters.n θ and n 0 are the normal directions to the vicinal surface and the (1010) terraces, respectively, and θ the angle of the vicinal surface as adopted in Equation 2. Shifted periodic boundary conditions are made apparent by the representation of two supercells along the shifted direction.
Figure 3: (a) LEED pattern (E B = 140 eV) of a 20 Å deposit of Ag/ZnO(1010) after annealing at 900 K and (b) scheme of the reciprocal space of bulk Ag(111) and ZnO(1010).Red (respectively blue) disks correspond to ZnO (respectively Ag) bulk Bragg diraction while open circles stand for bulk extinction in Ag that may be active in LEED.Dotted line stand for the LEED eld of view.
[010] borders; excluding coner sites, a slight preference for one termination (0.65 : 0.35, Figure 4-d) is observed assuming an alternance of Zn and O step termination from one terrace to the other.But neither apparent step height nor width statistics allow a clear assignment of the favored termination.The experimental diculty in determining the specic step termination (either O or Zn) and the precise Ag adsorption sites motivated us to run several rst-principle simulations, with distinct substrate congurations: (i) the at surface, (ii) the non-polar [001] step, (iii) the polar [010] step, either O-or Zn-terminated.For each substrate conguration, several likely non-equivalent adsorption sites for Ag atoms have been considered, in the limit of almost isolated adatoms or full Ag rows.The Ag adsorption enthalpy was computed through the dierence of the computed total energies between the surface with adatom(s), on the one the clean surface plus the Ag dimer, on the other side: ∆H ad = E(Ag : ZnO) − E(ZnO) − E(Ag 2 )/2.The dimer has been chosen as the reference system for silver, mainly for two reasons: Ag 2 is an "intermediate" system between the isolated atom and the bulk, very likely present in the experimental deposition by evaporation; moreover, the adsorption enthalpy can be directly compared to the case of hydrogen, where the molecule H 2 is the reference system.Firstly, let's focus on the adsorption of an isolated Ag atom.To that aim, a Ag atom per (3 × 3) surface supercell is considered, with Ag-Ag shortest distance between periodic images equal to 3a 9.85 Å.The adsorption enthalpy ∆H ad for selected congurations did not vary signicantly when considering larger 2D supercells.On the at ZnO(1010) terraces, there are two preferential adsorption sites, the bridge (BR) and hollow (HL) ones (see Figure 5).The BR site is more stable than the HL, with ∆H ad = −0.28eV/atom.In the BR conguration, the Ag binds to O (d Ag−O = 2.20 Å) and Zn (d Ag−Zn = 2.74 Å).On non-polar [001] steps, the preferred adsorption site is the hollow site at the step edge (see Figure 5-b), where Ag binds to two O (d Ag−O (s) = 2.35 Å and d Ag−O (t) = 2.40 Å) and two Zn (d Ag−Zn (s) = 2.95 Å and d Ag−Zn (t) = 2.80 Å)1 .For comparison, the bond lengths that are obtained by summing the ionic radii R ion and the co-1 For the sake of clarity, X (t) and X (s) denote atoms of type X at terraces and step edge, respectively

Figure 5 :
Figure 5: Synoptic of the preferred adsorption sites for a Ag adatom on ZnO(1010) (at surface and with steps).In the sketch of the adsorption congurations, white, red and grey balls stand for Ag, O and Zn atoms, respectively; the darker, the deeper into the bulk from the topmost layer.For each conguration, the corresponding adsorption enthalpy ∆H ad per Ag adatom is reported.

Figure 6 :
Figure 6: Computed adsorption enthalpies for Ag atoms at polar [010] steps on the ZnO(1010) surface.For the Ag 2 dimer, three local minima are shown: (i) the most stable BR-BR conguration, parallel to [010]-O; (ii) the HL-HL conguration, normal to [010]-O; (iii) the HL-HL conguration, parallel to [010]-Zn.Red, green and light gray balls stand for O, Zn and Ag atoms.

Figure 7 :
Figure 7: (a) UPS valence band spectra; the inset shows a zoom at the Fermi level.(b,c) Variations of band bending, work function and ionization energy as a function of Ag lm thickness for depositions at (b) 300 K and (c) 700 K.

Figure 8 :Figure 9 :
Figure 8: Electron charge density dierence after adsorption of Ag rows on bridge sites at Zn-terminated (left) and O-terminated (right) [010] steps.Regions where positive density increases (decreases) after adsorption are plotted in red (blue).The crystal directions are labelled and the step prole sketched by a dashed line.

Figure 11 :
Figure 11: Evolution of (a) low energy and (b) high energy plasmon peak positions.In Figure-(a), s and p signals are compared for various beam orientations and temperatures while only p-polarization is able to probe normal excitation.

Table 1 :
Ag nanoparticle morphology as obtained from STM image analysis.Thickness is estimated from quartz microbalance assuming a sticking coecient of one.Quantities are not corrected from tip convolution.
valent radii R cov are: R ion (O 2− ) + R ion (Ag + ) = 2.55 Å and R cov (Ag) + R cov (Zn) = 2.84 Å.The corresponding adsorption enthalpy is ∆H ad = −0.48eV/atom, thus about double compared to the at terrace.The adsorption strength is enhanced on [010] polar steps.On the Znterminated step, the HL site is favoured, with ∆H ad = −0.95eV/atom; Ag binds to 4 Zn, two on the step edge and two on the terrace (d Ag−Zn (s) = 2.74 Å, d Ag−Zn (t) = 2.76 Å).Overall, the preferred adsorption site for an isolated Ag is the HL site at the O-terminated [010] polar step, with ∆H ad = −1.87eV, thus slightly more stable than the BR site (∆H ad = −1.69eV) at the same step.In the HL site, Ag binds to four O, two on the edge and two on the terrace, forming also two loose bonds with Zn.It is worth noting that the adsorption enthalpy of Ag at the O-terminated [010] polar step, for both HL and BR sites, is more negative that its counterpart for hydrogen (∆H ad = −1.4eV/H atom); in contrast with the polar ZnO(0001)-O surface, silver adsorption is favoured over protonation.This has important consequences, as the onset of Ag adsorption at [010]-O steps on the ZnO(1010) surface could be practically unaected by the presence of residual hydrogen in the deposition chamber.