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Cite this: Phys. Chem. Chem. Phys., 2014, 16, 5763

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Plasmon spectroscopy of small indium–silver clusters: monitoring the indium shell oxidation† a a Emmanuel Cottancin,a Cyril Langlois,b Jean Lerme ´, Michel Broyer, a a Marie-Ange Lebeault and Michel Pellarin*

Owing to the very different electrovalences of indium and silver, nanoparticles made of these elements are among the simplest examples of hybrid plasmonic systems retaining a full metallic character. The optical properties of small indium–silver clusters are investigated here for the first time in relation to their structural characterization. They are produced in the gas phase by a laser vaporization source and co-deposited in a silica matrix. The optical absorption of fresh samples is dominated by a strong surface plasmon resonance (SPR) in the near UV, in an intermediate position between those of pure elements. A combination of SPR analysis and electron microscopy imaging provides evidence for the favourable surface segregation of indium. After a prolonged exposure to ambient air and because of the silica matrix porosity, changes in the SPR reflect the spontaneous formation of a dielectric indium oxide shell around a metallic silver core. The metallic character of indium can nevertheless be recovered by Received 5th December 2013, Accepted 30th January 2014

annealing under a reducing atmosphere. The reversibility of these processes is directly mirrored in

DOI: 10.1039/c3cp55135k

calculations. By controlling their oxidation level, In–Ag clusters can be considered as new candidates to

optical measurements through SPR shifts and broadenings as supported by multi-shell Mie theory extend SPR spectroscopy in the UV range and model plasmonic systems consisting of a silver particle of

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potentially very small size, fully protected by a dielectric oxide shell.

1. Introduction The high surface-to-volume atomic ratio in nanoparticles induces spectacular changes in their geometric and electronic structures, and therefore in their basic properties, as compared to those of the bulk materials. One of the most fascinating phenomena related to size reduction is the emergence of the localized surface plasmon resonance (SPR) in their optical absorption spectra.1–3 For relatively large nanostructures (a few tenths to a few hundreds of nanometers), this phenomenon is extensively exploited in the application field of nanoplasmonics. On the other hand, smaller systems commonly called ‘‘clusters’’ are especially interesting from a fundamental point of view because of the emergence of quantum size effects.4–7 Only a small number of metallic elements are suited for building spherical nanoplasmonic structures disclosing a well-defined a

Institut Lumie`re Matie`re, UMR5306 Universite´ Lyon 1-CNRS, Universite´ de Lyon, 69622 Villeurbanne Cedex, France. E-mail: [email protected] b MATEIS, UMR 5510, INSA-Universite´ Lyon 1-CNRS, INSA de Lyon, 69621 Villeurbanne Cedex, France † Electronic supplementary information (ESI) available: (1) Theoretical SPR response of oxidized indium clusters, (2) effective dielectric function of disordered bimetallic InAg clusters, (3) classical versus semi-quantal calculations: broadening of the SPR bands, (4) SPR in core shell In@Ag clusters: effect of a partial alloying. See DOI: 10.1039/c3cp55135k

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plasmon resonance from the near UV to the near IR range. The best ‘‘plasmonic’’ candidates are those having conduction electrons sufficiently weakly coupled to deep electronic bands to display a ‘‘quasi-free’’ behavior when subjected to an electromagnetic excitation. In this respect, alkalis are the simplest monovalent metal candidates. Their dielectric permittivity is almost perfectly described by the Drude–Sommerfeld model and a sharp SPR dominates their absorption spectrum in the visible range.8 Unfortunately, these elements are highly sensitive to oxidation and can only be studied with confidence under vacuum conditions, usually in the gas phase. Column III.A metals essentially differ from alkalis in their larger conduction electron density, which results in a larger plasma frequency and a shift of the SPR in the UV range. Although these systems are still sensitive to oxidation, they are less fragile than alkalis, especially for large sizes.9 As an illustration, aluminum nanostructures have recently been engineered for their application as nanoantennas operating in the visible range.10 Of course, monovalent coinage metals remain the most commonly used.7 Although gold is more robust than silver regarding oxidation and photoaging,11 the latter is interesting because of its relatively sharper SPR band which lies noticeably below the interband transition threshold. Recently, there has been a strong interest in bimetallic nanoparticles for which, in addition to size and shape, chemical composition and order can be used to tailor and control unseen properties.12

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This is particularly true in the field of catalysis,13 magnetism14 and of course optics, with the promise of a fine-tuning of the plasmon resonance by playing with the metal composition, as mainly evidenced for coinage metal based alloys.15,16 Nanoalloys of optical interest can also combine a coinage metal with a metal known for its special magnetic or chemical properties, resulting in systems suitable for magneto-optical17 or photochemical studies.18 Although pure transition metals themselves do not sustain any clear SPR in their nanoparticulate form, the plasmonic signature of the coinage metal is preserved on the condition of a phase separation between both metals.19 If an alloy is formed at the atomic scale, the optical spectra depend on the optical properties of the alloy as a specific new material and the plasmonic signature may vanish.20 On the other hand, optical investigations on nanoalloys made of two ‘‘plasmonic’’ metals other than coinage ones are very scarce, certainly because of difficulties encountered in their synthesis. In this work we discuss the special example of silver–indium nanoalloys which gives the opportunity to combine in a single nanoparticle two metals of very different electron densities. Owing to the trivalent character of indium, the SPR of pure clusters of this element is expected to be repelled in the UV range and significantly blue shifted as compared to the SPR of silver clusters. Combining both metals should thus lead to SPR modifications depending on the actual chemical structure of the cluster. In addition, attention shall be paid to the role of oxidation that consumes conduction electrons and prevents them from participating in collective excitations, especially in the case of indium because of its particularly high reactivity. The paper is organized as follows. In Section 2, we first outline the theoretical models (classical and semi-quantal) developed to interpret optical measurements. The techniques of sample elaboration and optical characterization are outlined in Section 3. Experimental results on pure silver, pure indium and bimetallic In–Ag clusters are presented in Section 4 with a structural analysis by electron microscopy. These measurements and the optical signature of a reversible indium oxidation in the most likely ‘‘silver core–indium shell’’ structure are discussed in relation to numerical simulations of the optical response.

2. Optical absorption of pure and bimetallic In–Ag clusters: theoretical background 2.1. Optical absorption cross-section of pure clusters: dielectric function and surface plasmon resonance Large enough nanoparticles of a pure metal are assumed to have the same local properties as the corresponding bulk material and especially the same dielectric permittivity e(o) as far as optical properties are concerned.2 The localized surface plasmon resonance that originates from the confinement of conduction electrons in metallic nanoparticles subjected to an electromagnetic excitation is then completely controlled by the spectral dependence of e(o) as reflected by the expression of the

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absorption cross-section (quasistatic approximation) of a spherical particle with radius R (R { c/o) embedded in silica sabs ðoÞ ¼

  3=2 9oeSiO2 ðoÞ 4 3 e2 ðoÞ (1) pR  2 c 3 e1 ðoÞ þ 2eSiO ðoÞ þe2 ðoÞ2 2

where e1(o) and e2(o) are the real and imaginary parts of e(o) respectively. The dielectric function of a quasi free electron bulk metal (jellium) can be expressed in the form e(o) = 1 + wC(o) where the dielectric susceptibility wC(o) describes the polarization/absorption properties of the conduction electron gas and reflects the optical transitions internal to the conduction band. The Coulomb repulsion between conduction electrons may be also screened by the polarization of the ionic cores (containing bound valence electrons) and light absorption involving transitions from filled core-electron bands to empty states in the conduction band may compete with the surface plasmon excitation. An additional complex term wIB(o), the so-called interband transition contribution, thus contributes to the total dielectric susceptibility of the metal (e(o) = 1 + wC(o) + wIB(o)). The gap between the top of the valence band and the Fermi level defines the threshold oIB above which these transitions are allowed. For silver (oIB D 3.9 eV) this screening and the hybridization between the 4d valence and the 5s4p conduction bands are important. In the case of indium, oIB is repelled in the far UV21 region and indium behaves in the visible range as a quasi-free (unscreened) trivalent electron metal ( wIB In(o) D 0). 2.2. Optical absorption cross-section of small pure clusters: intrinsic size effects Despite a higher surface to volume atomic ratio and the absence of long range crystalline order, the local bulk structure is still preserved in much smaller entities or clusters (D o 10 nm) and their optical response (SPR) is surprisingly well reproduced by the bulk dielectric function. In the absence of a general theory for describing the modifications of the dielectric screening by bound core electrons, the rough approximation of retaining a bulk value for the interband susceptibility (wIB(o)) is commonly accepted and justified on the basis of the general agreement between experimental observations and numerical simulations in various systems.2 The permittivity of small clusters will essentially deviate from the bulk, due to intrinsic size effects induced by the confinement of the conduction electron gas. From a classical point of view, scattering of electrons by the cluster surface actually reduces their mean free path and consequently increases their overall collision rate.22,23 This additional vF where vF is the Fermi velocity and l a contribution scales as ‘ limited mean free path for the electron motion. For a spherical particle of radius R, the collision rate can be written as gðo; RÞ ¼ vF gðoÞ þ g where g is a coefficient close to unity.2,22 According to R a previously published procedure, we introduce a size dependent dielectric function e(o,R) on the basis of an approximate but easy to handle parametrization.24 The interband dielectric

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susceptibility wIB(o) is first evaluated from tabulated data of the dielectric function (eexp(o)) for bulk Ag25 and In.26 The difference e(o)  wIB(o) is then assumed to consist of a Drude–Sommerfeld op2 ffi contribution in the form: eC ðoÞ ¼ 1  oðo þ igðoÞÞ op2 1 as it would be in the case of a perfect free oðo þ ig1 Þ conduction electron metal. g(o) is the frequency dependent optical collision rate that can be taken to be almost constant in the sffiffiffiffiffiffiffiffiffiffi re2 studied range (g(o) D gN), op ¼ is the plasma frequency m e eo where r is the bulk electron density, me the electron effective mass and e the electron charge. As it does not affect significantly the results presented in the following, me is taken as the mass of the free electron. The collision rate gN is treated here as an adjustable parameter to fulfill the arbitrary condition that eexp(o) and eC(o) + wIB(o) give SPR profiles of similar width when applying the Mie theory in the dipolar approximation (1). An approximately size dependent dielectric function is finally established in the form: op2 op2  , which presents eðo; RÞ ¼ eexp ðoÞ þ oðo þ ig1 Þ oðo þ igðRÞÞ the advantage of converging exactly towards the bulk experimental dielectric function for large particle sizes.19,27 Drude– Sommerfeld collision rates for indium and silver have thus Ag been estimated to be gIn N = 0.48 eV and gN = 0.065 eV respectively. The electron densities and Fermi velocities for indium and silver are rIn = 11.49  1022 cm3, rAg = 5.85  1022 cm3, 6 1 6 1 28 vIn and vAg Optical F = 1.74  10 m s F = 1.39  10 m s . scattering being negligible in the size range considered here, extinction and absorption cross-sections are almost identical and formula (1) will hold in a classical approach.

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of effective medium theories. The most simple approach consists in averaging the dielectric functions of both elements according to their volume ratio.15,20 However, using dielectric functions of pure elements implies that they form large enough domains inside the cluster so that local bulk properties can be relevant. This constraint will hardly hold in the case of inhomogeneous or partially diluted nanoalloys, all the more that 4 nm diameter clusters only contain between 700 and 800 atoms of each metal. Considering that silver and indium atoms contribute one and three delocalized conduction electrons respectively, a more reasonable empirical approach would be to construct an effective dielectric function eeff,C alloy on the basis of a Drude– Sommerfeld contribution including a weighted average of electron densities and collision rates. In the visible range, bare Coulomb interactions are essentially screened by the polarization of the silver ionic cores in a way that will depend on their spatial distribution. The central issue will be to determine the effective eff,C susceptibility wIB eff (o) that must be added to ealloy to account for these screening effects. We have also considered the possible surface oxidation of pure or alloyed core–shell clusters straightforwardly described by the generalized Mie theory for one and two spherical shells. In this study, the outermost shell essentially consists of amorphous indium oxide In2O3. This semi-conductor is transparent in the visible range and begins to absorb light from 3.5 eV (below 350 nm). As compared to metals, the negligible density of free electrons will not favour the emergence of a plasmon resonance in In2O3 confined nanodomains (Fig. S1 in ESI†), as it can also be concluded by inspecting the dielectric function of In2O3 and especially its constantly positive real part over the visible range.30 2.4.

2.3. Optical absorption of bimetallic In–Ag clusters: effective dielectric function Bimetallic In–Ag clusters may adopt various chemical structures and different theoretical descriptions have to be considered, still assuming the spherical symmetry for simplicity. In the case of a full surface segregation of one element, the generalized Mie theory for a core–shell system provides an analytical expression of the absorption cross-section which depends on the core radius (Rc), the shell thickness (d), the outer medium permittivity em and the permittivities of both metals (core and shell) accounting for intrinsic size effects as defined above. A vF modified collision rate gc ðRc Þ ¼ g1 þ g is introduced for the Rc core metal. The electrons of the shell are confined between two spherical interfaces and their effective collision rate can also be expressed analytically through an appropriate formula in the frame vF of a ballistic description (gs ¼ gs ðRc ; R ¼ Rc þ d Þ ¼ g1 þ g s ‘path pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi with ‘spath ¼ 3 d ðR2  Rc2 Þ).20,29 If there is no complete phase segregation, ‘‘alloyed’’ structures consist of a random distribution of indium and silver atoms. Defining an effective dielectric permittivity for such systems is a difficult problem widely discussed in the literature on the basis

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Classical versus semi-quantal calculations

For a better overview of specific size reduction effects, classical calculations have been complemented by semi-quantal counterparts, according to the formalism described in previous papers.5,6,24 Conduction electrons are treated quantum mechanically within the time dependent local density approximation (TDLDA) framework. In the case of silver, the dynamic screening by bound electrons is taken into account in a classical way through the interband transition contribution to the dielectric function (wIB(o)). The polarizing ionic background is chosen here to extend up to the cluster surface. As noticed before, this screening can be ignored in indium that will be considered as a quasi-perfect jellium metal. In practical density functional theory (DFT) calculations, an intrinsic width d is attributed to each single particle excitation lines (Lorentzian-shaped peaks with d as full width at half maximum (FWHM)). For a more convenient comparison between classical and semi-quantal spectra, these widths are taken equal to the Drude–Sommerfeld collision rates (d = 0.065 eV for silver and d = 0.48 eV for indium or indiumcontaining clusters). Contrary to classical calculations, the size dependence of the optical response (size effects) is intrinsically related to the quantum treatment of the conduction electron gas and originates from its spill-out beyond the edge of the positive ionic background (geometrical cluster surface).

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3. Sample preparation and spectroscopy: experimental methods

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3.1.

Matrix embedded clusters

Thin dielectric films containing metal clusters are synthesized using the low energy cluster beam deposition technique (LECBD).31 Clusters are produced in a conventional laser vaporization source. The second harmonic of a nanosecond Nd3+:YAG pulsed laser is focused at the surface of a metallic target generating an atomic plasma in the presence of a continuous flow of helium gas. The atomic vapor is rapidly cooled down by collisions with the inert gas (static pressure of a few tenths of mbars) which induces the nucleation and growth of small metal clusters (2 nm–6 nm in diameter). The particle–gas mixture expands into vacuum through a conical nozzle in the form of a supersonic beam towards a deposition chamber. Bimetallic clusters are essentially produced from an In0.5Ag0.5 alloyed target (50%–50% atomic composition, GoodFellow Inc.). Complementary experiments have also been realized with an In0.75Ag0.25 target. The laser vaporization process favors the growth of ‘‘nanoalloys’’ with an average composition that is known to preserve the target one.32 Clusters are finally deposited on a Suprasil substrate (fused silica, 1 cm  1 cm, 1 mm thick) simultaneously with the transparent amorphous silica matrix evaporated using an electron gun. The co-deposition process results in the formation of a thin dielectric silica film containing dispersed metallic clusters (200 nm–300 nm thickness). The volume fraction of cluster matter (q = 0.5–3%) is low enough to prevent significant coalescence and optical coupling between the nanoparticles. A small reference area of the sample is covered only by the pure dielectric film thanks to a suited shading of the cluster beam. The average cluster size is varied through the He carrier gas pressure. Cluster size distributions, morphologies and crystallographic structures can be independently controlled by High Resolution Transmission Electron Microscopy (HRTEM) or High Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADF-STEM) imaging, after their deposition on grids coated with an ultrathin amorphous carbon film (Agar Inc.). 3.2. Optical absorption measurements and SPR inhomogeneous broadening Optical transmission of nanocomposite thin films containing pure Ag, pure In or mixed In–Ag clusters is recorded with a double beam Perkin-Elmer spectrophotometer between 1.55 eV and 6.5 eV (l = 800 nm to 190 nm). It is normalized by measurements made through the sample area left blank. The silicon dioxide matrix is transparent in the spectral range of interest. Assuming that the nanocomposite film can be described as a homogeneous medium characterized by an effective dielectric c function eeff (o) = (neff (o) + i keff (o))2 (o ¼ 2p where l is the l wavelength in vacuum) and according to the Beer–Lambert   2p  keff ðoÞ  d ¼ law, the transmittance is TðoÞ ¼ exp 2  l expðaðoÞ  dÞ (keff (o) is the imaginary part of the effective refractive index, d the film thickness and a(o) the attenuation

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coefficient). In the weak absorption regime, the absorbance A = 1  T D ad will be reported in experimental spectra instead of a since the thickness d was not determined with enough precision. For diluted samples (q { 1), A(o) can be simply expressed from individual cluster absorption crosssections siabs as those classically defined by relation (1) d P 1 P (AðoÞ ¼  siabs ðoÞ ¼  siabs ðoÞ, V and S are the probed V i S i sample volume and surface and i indexes the N probed particles). Since the spectral profile of siabs depends on the size, shape and local dielectric environment of particle i, A(o) is proportional to the average of individual cross-sections over these parameter   N homo distributions AðoÞ ¼ sin ðoÞ , resulting in an inhomoS abs geneous broadening of SPR bands. A quantitative analysis of homo experimental spectra requires evaluation of sin (o) from abs i individual sabs and therefore the precise knowledge of cluster size, shape and environment distributions. It is easier to consider an effective cross-section seff abs(o) simply calculated from the mean values of these parameters since they are more accessible quantities. On average, the clusters are actually spherical, with a mean radius hRi determined from electron microscopy analysis. As regarding their local dielectric environment, the residual porosity of the silicon dioxide matrix may be responsible for local variations of the dielectric permittivity experienced by the embedded particles and can be mimicked by an effective dielectric permittivity eeff SiO2 ðoÞ. In the absence of direct measurements, the dielectric function of the silica matrix will be taken from Palik’s tables as the one of silica glass   33 glass eglass eSiO2 ðoÞ certainly overestimates eeff SiO2 ðoÞ but the SiO2 ðoÞ . induced SPR red-shifts are expected to be masked by the relatively important SPR broadening caused by local fluctuations. Optical response calculations are not intended to account for SPR inhomogeneous broadening but rather to disclose spectral shifts as a function of the particle structure or model parameters assumed in simulations. In the following, it will be sufficient to focus on a qualitative comparison between the measured absorbance A(o) and the calculated effective absorption cross-section seff abs(o). The SPR bands associated with in homo seff (o) have almost the same location and will abs(o) and sabs essentially differ in their width.

4. Chemical structure and SPR spectroscopy: results 4.1. Electron microscopy of mixed In–Ag clusters: core–shell structure and shell oxidation TEM structural characterization has been conducted on a spherical aberration corrected ETEM (Titan ETEM 80-300, FEI Company) with a modified S-Twin objective lens. The correction of spherical aberrations allows a point-to-point resolution of 0.10 nm. Fig. 1a and b shows the high resolution TEM image (HRTEM) of bimetallic clusters deposited on amorphous carbon, under the same source operating conditions as for the synthesis

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Fig. 1 (a) and (b) HRTEM images of two In0.5Ag0.5 clusters of different sizes deposited on an amorphous carbon film. On the right side of the figure, inner circles are drawn so as to enclose the silver cores. The outer circle radii (full lines) are deduced from the latter knowing the In/Ag atomic ratio and slightly off centred to reproduce the real particle shape. The outer dashed circles correspond to the hypothesis of a complete indium shell oxidation. The corresponding diameter values are indicated. (c) HAADF-STEM image of size selected In0.75Ag0.25 clusters. The shell geometry predicted from the cluster mass and composition is drawn in the inset. (d) HRTEM images of In0.75Ag0.25 clusters at different irradiation times with the electron beam. The signature of the In2O3 crystalline phase is shown in the squared frame.

of thin films. For convenience, selected sizes are somewhat larger than those studied by optical means but the following observations remain very general. One clearly distinguishes a crystalline core surrounded by a less contrasted and mostly amorphous shell. Some crystalline parts can however be distinguished in the shell (Fig 1b) and their lattice fringe periodicity can be attributed to (222) cubic In2O3 plane which is the most intense diffraction direction for this oxide. The core is definitely made of pure silver since the measured fringe spacing corresponds to the (111) lattice plane parameter for this element. Similar observations can be made whatever the size of deposited

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clusters. On the right part of Fig. 1a, an inner circle of radius 4.6 nm is superimposed on the initial image so as to delimit a spherical boundary of the silver core region, assuming an approximate spherical symmetry for this object. Considering the respective densities of bulk silver and indium (dAg = 10.49 g cm3 and dIn = 7.31 g cm3) and the relative atomic composition of mixed cluster (50%–50%), the particle diameter should be 6.25 nm under the hypothesis of an outer shell made of indium. A corresponding circle of this size is reported in Fig. 1a, slightly off centred so as to reproduce the non-perfect concentricity of the core and the shell. The overall size of the cluster is therefore correctly reproduced which is consistent with the formation of an Ag core/In shell geometry. However, this agreement is better if indium is replaced by less dense indium oxide (6.6 nm diameter dashed circle assuming dIn2O3 = 7.18 g cm3), confirming the initial suspicion of a shell consisting of poorly crystallized In2O3. The same analysis can be made for the larger cluster in Fig. 1b (10.3 nm outer diameter with a 7.2 nm diameter silver core). These conclusions are not only independent of the particle size but also of its composition as illustrated by HAADF-STEM images of In0.75Ag0.25 clusters in Fig. 1c. In this case, clusters were mass-selected (15% accuracy) according to the deposition technique described in ref. 34. The drawing on the left bottom corner shows the colored area of an ideal spherical core/shell cluster calculated for this atomic composition considering the most probable value of the selected cluster mass. It perfectly matches the microscopy images. Although indium and silver atoms have close atomic numbers (Z = 49 for indium and Z = 47 for silver), the noticeably smaller atomic density of indium, in its pure metallic or oxidized form, explains why the core is brighter than the shell in HAADF-STEM images. We were not able to perform a local chemical analysis (EDX) of the shell, but we have observed that, under the influence of electron beam irradiation and heating, amorphous In2O3 crystallizes under its cubic form and can be detected in HRTEM images (Fig. 1d). Since TEM grids are exposed to the open air before being analyzed, it is not surprising that the indium shell can be completely oxidized owing to the high reactivity of this element. Obtaining information about the structure of freshly deposited clusters is therefore very difficult. However, thermodynamic considerations may help to infer the most likely chemical configuration of the In–Ag system. Indium and silver are hardly miscible elements but their bulk phase diagram indicates that they can nevertheless form several intermetallic compounds which would be compatible with a partial atomic alloying.30,35 At room temperature, In2Ag or Ag2In phases are observed in InxAg(1x) alloys having compositions close to x D 0.5. However, owing to the large difference in indium and silver surface tension, the Ag core–In shell (Ag@In) structure should be favored, even if some disordered atomic alloying may occur at the interface. In any case, a preferential oxygen binding with indium atoms is expected to drive them towards the surface in contact with the oxidizing atmosphere, taking advantage of the relatively low surface tension of In2O3.36 This effect could explain the subsequent formation of an In2O3 shell and the silver enrichment of the core if this one was not initially pure (Ag2In for instance).

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4.2.

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Fresh sample absorption spectra

Optical absorption measurements for samples containing silver, indium and mixed (In0.5Ag0.5) clusters are shown in Fig. 2. The average values of the size distributions are obtained from TEM image analysis of reference samples and are estimated to be 4.2 nm, 3.9 nm and 4.5 nm respectively. We report linear absorption coefficient curves as explained in Section 3.2. The units are arbitrary since sample thicknesses and therefore cluster densities (q o 5%) have not been measured precisely. It is sufficient for a qualitative discussion because the total amount of cluster matter in the samples is low enough (see Section 3.1) to merely modify the SPR magnitude without changing its location and overall profile. Relative absorption measurements in Fig. 2 first show that the plasmon resonance of pure Ag and In clusters embedded in porous silica film peaks at about 3 eV and 4.8 eV respectively. The position of the SPR being known to red-shift for increasing values of the outer medium refractive index, the maximum absorption energies in Fig. 2 are consistent with directly comparable data taken from the literature for clusters embedded in various matrices. They are less abundant for indium37 than for silver.38,39 The SPR of mixed clusters containing the same amount of silver and indium atoms is intermediate between those of pure elements, regarding both its spectral position and width.

For clarity, the trace of absorption curves for Ag and In0.5Ag0.5 clusters is arbitrarily stopped towards the high energy side but a clear signal increase is observed in this spectral region, reflecting the presence of silver interband transitions. 4.3.

Pure metal clusters: comparison with calculations

Calculations of pure silver and pure indium absorption crosssections are displayed in Fig. 3, according to the methods described in Section 2. They are all performed assuming spherical clusters of the same size (D = 4 nm) embedded in a silica matrix. The difference with exact experimental cluster sizes is not awkward here, owing to the negligible role of size as compared to composition effects on the SPR profile. We have actually checked that the calculated SPR position and width are weakly sensitive to the exact cluster size in the range under consideration in these experiments (3.5 nm–5 nm). Compared to classical Mie theory, surface plasmon resonances are systematically red-shifted both in semi-quantal DFT calculations and in experiment because of the electron spillout.5,40 Indeed, due to their quantum character, the conduction electrons are not strictly confined in a spherical box but they are allowed to spill out beyond the surface defined by the geometrical cluster volume. A smooth surface profile of the electron density is actually known to induce a plasmon shift towards the low energy side for

Fig. 2 Experimental absorption spectra (not to scale) for pure Ag (thin black line), pure In (thick gray line), and mixed In0.5Ag0.5 clusters (thick black line) embedded in porous silica. To avoid line crossings, the plots of Ag and In0.5Ag0.5 cluster spectra have been interrupted on the high energy side. Absorption maxima are marked with vertical arrows.

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VIn2 O3  is set to 30% VIn2 O3 þ VIn (about 25% of the indium atoms are bounded with oxygen) (see Fig. S1 in ESI† for a more detailed analysis). The resulting SPR is red shifted and broadened, in better agreement with experimental observations. However, the broadening of the experimental SPR is mainly due to fluctuations in cluster shapes and above all to the matrix porosity. Theoretical predictions of the plasmon bands are consistent with experimental measurements, considering their inhomogeneous broadening (Fig. 2 and arrows in Fig. 3). As mentioned in Section 3.2, taking into account the silica matrix porosity would have slightly blue-shifted the calculated SPR bands. A better agreement with experiment is found for semiquantal calculations even if a partial oxidation may play a role for indium clusters.

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surface (shell) and its volume ratio 

4.4. Mixed In–Ag clusters: the optical signature of a core–shell structure

Fig. 3 Absorption cross-sections of pure silver and pure indium clusters embedded in silica. Solid and dashed lines indicate classical Mie theory accounting for intrinsic size effects and semi-quantal calculations, respectively. Vertical dashed arrows give the position of experimental absorption maxima and the horizontal bars the spectral spread for absorption values larger than 90% of these maxima. The spectrum predicted for partially oxidized indium clusters In@In2O3 (30% surface oxidation) is given in the upper part of the figure (solid gray curve, the gray dotted arrow indicates the corresponding SPR red shift).

simple and trivalent metals. This phenomenon is responsible for size dependent modifications of the SPR in the small cluster size range. The situation is more complicated for noble metal clusters due to an incomplete dielectric screening by the ionic cores close to the cluster surface.3,39,41 As expected, Fig. 3 shows that the experimental results are in better agreement with semiquantal calculations even if Mie calculations remain valuable and of useful simplicity. Optical spectra were recorded as quickly as possible after removing samples from the deposition chamber (air exposure time less than 30 minutes). However, repeated measurements at various delays give evidence of SPR changes that will be discussed in Section 4.5. The progressive formation of an outer dielectric metallic oxide shell having a refractive index larger than the silica matrix one may be responsible for a SPR red shift.42 This is especially true for pure indium which is more sensitive to oxygen than silver. As an illustration, a simulation of the optical response of partially oxidized indium clusters is shown in Fig. 3. The indium oxide is located at the cluster

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The optical response of mixed In0.5Ag0.5 clusters depends on their chemical structure, i.e. the spatial organization of silver and indium atoms. Under the hypothesis of an alloy formation, simulating the optical response of mixed clusters on the basis of the formalism evoked in Section 2.3 is hazardous. The SPR is actually very sensitive to the effective interband dielectric susceptibility wIB eff(o). Owing to the depth of the indium 4d electron band, wIB (o) will merely reflect the polarization/absorption properties of eff the silver ionic cores and will be correlated with the interband dielectric susceptibility of silver itself. Theoretical and experimental studies on the bulk phase only deal with dilute alloys (silver rich),21,43 and their conclusions are hardly generalizable to the case of an equally composed alloy. Only empirical approaches IB can be used to estimate wIB eff(o) from the knowledge of wAg(o). However, calculated SPR spectra appear to be too much modeldependent for supporting an unambiguous analysis of the present results. Tentative simulations are reported as ESI† (Fig. S2 and S3). The agreement found between experiment and calculations for a suited choice of wIB eff(o) is not necessarily significant even if it does not completely rule out the possibility of an alloyed structure on the mere basis of an optical characterization. Nevertheless, this hypothesis remains unlikely as regards TEM imaging (Section 4.1) and thermodynamic considerations discussed below. As a consequence, Fig. 4 shows classical Mie calculations for clusters having the same size as pure species discussed above (D = 4 nm) and for a complete surface segregation of silver and indium in the form of a core–shell morphology supported by TEM observations. For an indium core–silver shell segregated structure (notation In@Ag in the following), a clear resonance is predicted to peak at about 3.4 eV separated from a broad band in the UV range (bottom spectrum in Fig. 4). These features are reminiscent of the plasmonic response of silver and indium monometallic domains. In cluster samples, inhomogeneous broadening effects and the possible formation of a silver oxide surface layer are expected to induce a damping and a red-shift of the first sharp resonance (3.4 eV). Since the experimental SPR in Fig. 2 peaks around 3.75 eV, one can conclude that the difference between the

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Fig. 4 Absorption cross-sections of In0.5Ag0.5 clusters (4 nm in diameter) embedded in amorphous silica (50%–50% atomic composition) for Ag@In (top) and In@Ag (bottom) core–shell structures. Calculations are performed using the Mie theory including intrinsic size effects in the electron collision rates (full lines) or within the semi-quantal formalism (dashed lines). The gray dashed area indicates the region where the experimental absorption signal is larger than 90% of its maximum (Fig. 3).

measured and calculated peak positions is larger than 0.35 eV and that the In@Ag structure is unlikely in the present case. In the reverse situation where the core is made of silver (Ag@In), the calculated spectrum is closer to the experimental one (top spectrum in Fig. 4). As for the In@Ag geometry, the plasmonic signatures of silver and indium domains can be distinguished even if the silver contribution is blue-shifted and more quenched by coupling with the indium outer shell. Such mechanisms have already been described in the literature in the framework of the plasmon hybridization concept for instance, more especially in the case of larger nanoparticles.44 The sharper resonance on the low energy side is marked enough to survive inhomogeneous broadening effects in cluster films and its position is consistent with the maximum of absorption detected in the experiment (3.75 eV). A comparison between experimental and simulated spectra is in favor of the formation of phase segregated mixed clusters, the most likely made of a silver core surrounded by an indium shell (Ag@In). Similar qualitative conclusions are drawn from semi-quantal calculations. It must be noticed that for Ag (Fig. 3) and mixed In–Ag clusters (Fig. 4), a larger damping of the SPR is observed for classical Mie theory calculations as compared to

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semi-quantal ones. This can be first explained by the choice made for the size dependent electron collision rate in the classical formalism (g factor in the Drude part of the dielectric permittivity) that may be overestimated. Sharper resonances closer to semi-quantal results can be obtained when this size effect is neglected or minored (see Fig. S4 in ESI†). This is also consistent with an overall redshift of the plasmon resonances in semi-quantal calculations which results in a weaker coupling of conduction electron excitations (plasmon) with silver interband transitions above 3.9 eV. However, in both methods, a progressive broadening of the plasmon band is observed from pure silver to pure indium clusters and reflects the higher electronic collision rates of the trivalent metal (Fig. 3 and 4). This evolution is not totally masked by inhomogeneous broadening effects and is also visible in the experimental spectra (Fig. 2). As remarked in Section 4.1, electron microscopy is able to conclude in favor of an Ag@In2O3 core–shell structure, but it was not possible to observe non-oxidized or weakly oxidized clusters as those studied in fresh samples (Section 4.2). The alloy phase diagram does not predict a full phase separation of indium and silver in the bulk. However, such a conclusion has to be taken with caution as far as nanoscale systems are considered.45 Owing to their large relative number, surface atoms are less coordinated than the bulk ones and may actually induce a strong change in the expected chemical element distribution, even in the absence of oxygen. The surface segregation of indium suspected from the analysis of the experimental SPR spectrum (Fig. 4) could be the result of a much lower surface tension for this element (ss(In) = 0.56 J m2 (ref. 36) compared to ss(Ag) = 1.07 J m2 (ref. 46) for liquid-like droplets and room temperature extrapolation) and of a large crystalline lattice mismatch (atomic Wigner–Seitz radii rs(Ag) = 3.02 bohr and rs(In) = 3.46 bohr). Similar situations were also encountered for Cu–Ag,47 Ni–Ag,19,20,48 or Au–Pt49 nanoalloys for instance. Unfortunately, indium and silver have close atomic numbers and a cluster surface analysis by Low Energy Ion Scattering (LEIS) as previously reported for NiAg clusters is not feasible here.19 In relation to a possible limited phase separation, we have finally considered the approaching case where the core and the shell are not made of pure metals but of alloys, in the form AgnIn and InnAg respectively (n integer) as it can be suspected from an inspection of the bulk phase diagram. The agreement with experiment is all the better as n is large (quasi-pure metals). The case n = 2 which is characteristic of the most likely intermetallic phase in the bulk materials35 is illustrated in the ESI† (Fig. S5). For this composition, the resonance peak is located close to the one obtained for the core–shell structure from pure metals (Ag@In) but less marked. It would certainly be smoothed off by inhomogeneous broadening in cluster ensemble measurements. Nevertheless, this structure cannot be totally ruled out since HRTEM images do not give a definitive proof of an initial pure Ag/pure In core–shell arrangement. In any case, SPR measurements are more in line with the presence of quasi-pure metallic domains (Ag rich–In rich core–shell).

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Mixed In–Ag clusters: a reversible indium shell oxidation

The indium shell oxidation observed in clusters dispersed on TEM grids is the result of an aging process in ambient atmosphere. Despite of a slower kinetics, it should also take place in optical samples since clusters are not perfectly protected by the porous silica matrix. Delayed optical measurements actually probe the cluster surface oxidation through time dependent SPR shifts. As an illustration, Fig. 5a compares, on the same vertical scale, the absorption spectrum of a fresh sample made of In0.5Ag0.5 clusters embedded in silica (the same as in Fig. 2) with the spectrum recorded two months later. In this case, the SPR is strongly shifted (about 1 eV, corresponding to a wavelength shift of 125 nm) towards the low energy side and more than twice less intense. Contrary to what is commonly observed on oxidized clusters of an initially pure metal, the SPR is surprisingly narrowed. Although the time evolution of the plasmon resonance was not studied systematically, we have checked that sample aging is fast and moreover the absorption spectrum does not evolve linearly with time. The transformed spectrum in Fig. 5a could be almost obtained only after a few days of continuous exposure to air. On the basis of TEM observations, changes in the optical response can be understood in terms of indium oxidation (Section 4.4). If the indium shell is fully oxidized and the

protected silver core remains metallic, the final clusters consist in pure silver clusters surrounded by a dielectric shell. This explains both the quasi-Lorentzian-profile which is a characteristic of pure silver nanoparticles and the pronounced plasmon redshift because the In2O3 shell has a larger real refractive index than porous silica (

Plasmon spectroscopy of small indium-silver clusters: monitoring the indium shell oxidation.

Owing to the very different electrovalences of indium and silver, nanoparticles made of these elements are among the simplest examples of hybrid plasm...
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