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Physical Chemistry Chemical Physics

Optical Properties of NanoAlloys

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DOI: 10.1039/C5CP00498E

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Giovanni Barcaro1, Luca Sementa1, Alessandro Fortunelli1,* and Mauro Stener2,3,* 1

CNR-ICCOM & IPCF, Consiglio Nazionale delle Ricerche, via G. Moruzzi 1, 56124, Pisa, Italy

2

Dipartimento di Scienze Chimiche e Farmaceutiche, Università di Trieste, via L. Giorgieri 1, I-

34127, Trieste, Italy 3

Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali, INSTM, Unità

di Trieste

Abstract The optical properties of multi-component metal nanostructures (or nanoalloys) are the subject of an intense and rapidly growing experimental and theoretical activity. In this perspective article, we first provide a survey of the most recent developments in the field, concerning both theoretical methods, especially at the first-principles level, and novel results, distinguishing for convenience of presentation the sub-field of monolayer-protected multi-component metal clusters from the other alloy nanosystems. We then discuss a few general concepts which can be drawn from this survey, and offer a few suggestions on the most promising directions for future research. We hope that making the point in this fast developing field will provide a framework and a perspective useful to trigger future studies and advancements.

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Introduction

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Mixing two or more different metals to obtain novel geometries and tune fundamental properties of metal nanostructures, thus producing alloy nanostructures (or nanoalloys) 1- 3, represents an appealing opportunity for nanoscience and nanotechnology. Among the many intriguing effects

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associated with alloying on the structural, optical, catalytic, magnetic, etc. properties of nanoscale metals 4, optical phenomena represent an important subject, due to their scientific interest and technological applications 5. In this perspective article we focus and try to make the point on the optical properties of nanoalloys. Since an extensive overview of the synthesis and optical properties of metal nanostructures, including hybrid and mixed systems, has been given not long ago in several review articles 6- 13, in the following we assume these as a basis, and give an update of the developments and achievements of the last 4-5 years.

In detail, one of the topics that has recently greatly expanded and is currently under rapid development is the use of first-principles Time-Dependent Density-Functional Theory (TDDFT) techniques to predict optical properties of metal nanostructures. Section 2 is then dedicated to a brief survey of this topic. Moreover, a sub-field evolving in a particularly tumultuous fashion among alloy nanosystems is that of monolayer-protected, size-selected clusters 14-

30

. For clarity of

presentation, we then single out these latter systems apart, so that in Section 3 we discuss other nanosystems, i.e., nanoclusters and nanorods or nanowires both free and in a less interacting environment, while Section 4 will be devoted to monolayer-protected alloy clusters, also providing a literature review of the explosive growth in this sub-field. Some general principles will be extracted from an analysis of this literature survey in Section 5, while Section 6 will present conclusions and perspective outlook, pointing out which in our opinion are the most challenging and most promising directions for future research.

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2 Theoretical methods

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A comprehensive review about the theoretical approaches to describe plasmons by electronic structure methods has been published recently13. It is worth noting that up to 2011 most of the contributions came from simplified (semi-classical) models, whereas only very few contributions

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from conventional quantum chemistry using Time-Dependent Density-Functional Theory (TDDFT) method were reported. In recent years, instead, these latter approaches have been improved and represent very promising computational tools for future studies.

More specifically, the theoretical description of the optical properties of nanoalloys is not a trivial issue, for a series of reasons. First, even the basic physics of the phenomenon is sensitive to the size of the system. When the size is beyond the electron mean path (>10 nm) a classical or semiclassical electrodynamics model is usually adequate to treat the phenomenon since the cluster can be properly treated as a conducting medium. In contrast, for smaller sizes quantum confinement effects are important and therefore the treatment must be at the level of quantum mechanics. The classical models are based on the extension of Mie theory, based on numerical approaches like Discrete Dipole Approximation (DDA) 31. A significant difficulty is represented, at the classical level, by the choice of the dielectric function of the model. In fact, while for pure systems the bulk dielectric function is a natural choice, for alloys the situation is more complicated. In presence of segregation (core-shell, multilayer, Janus particles) the dielectric function of the pure components can be employed in the respective regions (with the problem of how to describe the interfacial region), while in presence of thorough intermixing of different elements the choice is less obvious: it is not trivial how to ‘mix’ the dielectric functions of different materials. The third difficulty is related to the accuracy of the quantum mechanical treatment. The most prominent optical phenomenon which theory is expected to reproduce is the Surface Plasmon Resonance (SPR), which is a collective phenomenon and therefore cannot be described by a single particle model, but rather entails coupling many excited configurations. At the moment the only practical method which is able to describe collective excitation modes and simultaneously be computationally competitive to be applied to clusters containing at least one hundred metal atoms is TDDFT, which is now well established in the quantum chemistry community. Current TDDFT implementations range from conventional chemical formalism based on molecular orbitals like in the Casida procedure 32, to plane waves or Cartesian grid with time evolution methods to solve TDDFT equations. TDDFT methods are able, in general, to tackle systems consisting of up to a few hundred metal atoms, 33 although in a very recent study a cluster containing even 1414 Au atoms has been investigated 34. 3

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limited number of eigenvalues which can be extracted with Davidson diagonalization procedure (and thus the limited excitation energy interval that can be explored) but allows a detailed analysis of the nature of the resonance. On the contrary, time evolution methods can easily reach high excitation energies but are not able to analyze the nature of the transitions. It is worth mentioning

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that in addition to TDDFT and classical methods, also hybrid approaches have been developed, especially to study the coupling between a single molecule treated quantum mechanically and a metal particle treated classically35.

In this context it is important to underline the importance of the choice of the exchange-correlation (xc-) functional, which is always a critical issue for (TD)DFT. In particular it is well known since the first TDDFT investigations 36, that the asymptotic behaviour of the exchange-correlation potential is crucial to obtain accurate excitation energies. This is not a trivial problem, since the correct asymptotic behaviour is governed by exchange interaction, which gives rise to a Coulomb tail (-1/r), while common Local Density Approximation (LDA) and Generalized Gradient Approximation (GGA) xc-functionals decrease exponentially at large distances. Two examples of model xc-potentials with correct asymptotic behavior which perform well for TDDFT are: the Leeuwen-Baerends1994 (LB94) one 37 and the statistical average of orbital potentials (SAOP) 38. As a general rule, validation with respect experimental data should be always done whenever possible as a preliminary calculation 39. To this effect it is important to have at one’s disposal accurate experimental data on systems with well-established structure, a condition which unfortunately very seldom occurs. Moreover, it is also important to keep in mind which is the ultimate goal of theory: we believe in fact that theory should not be limited to a mere simulation/reproduction of experiment, but should also provide in addition a rationalization of the spectrum in terms of electronic structure. To this purpose TDDFT offers two main tools: the conventional analysis and decomposition of the excited-state wave function in terms of one-electron excited configurations (density matrix formulation of DFT) and the induced density (density formulation of DFT). 40 Plots of the induced density are particularly useful to provide a visual analysis of excitations with a real plasmonic (collective) character, i.e., when they are made up of a combination of very many singleelectron modes. An interesting example of the usefulness of the induced density is an investigation on the interaction among a 2D array of gold nanowires 41. In this study the photoabsorption of the transversal plasmon was shown to be very sensitive to the inter-wire distance, and the appearance of ‘hot spots’ between them has been demonstrated by checking the shape of induced density. Another very powerful analysis tool is the Transition Component Mapping (TCM) developed by Hakkinen 4

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to a bidimensional representation of the induced density matrix, equivalent to the one-electron excited configurations coefficients. In this way it is possible to identify which configurations are involved at a given excitation energy, and therefore assign them to specific initial and final states. Unfortunately, in this perspective article we do not have space to discuss in detail the results of

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these analysis, and we defer this scrutiny to a future report.

Another topic, intimately related with nanosized systems which has been so far largely unexplored via the use of TDDFT methods in nanoalloys, is the coupling among plasmonic particles. Such phenomenon has been exhaustively considered in a recent review12 and still poses challenging fundamental questions to be investigated at the quantum chemical level, due to the increase in this size of the system (at least doubling), and the need of describing accurately both individual components and their interaction.

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3 Alloy nanostructures in a non-strongly interacting environment

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The theoretical studies on the optical properties on nanoalloys are rather wide, and many combinations of alloy pairs have been considered. However the most studied system is by far the Ag-Au pair. This is not surprising: coinage metals are the most suitable to produce intense SPR in

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practically useful systems and in addition gold and silver are totally miscible with each other. In the pioneering work of Broyer and coworkers 42,43 the photoabsorption of a series of (AuxAg1-x) clusters in the size range 1.5 – 5 nm in diameter dispersed in an alumina matrix was measured and compared with model jellium TDLDA calculations, where LDA is the Local Density Approximation xc-functional. The jellium TDLDA method employed in that work used a crude approximation of TDDFT, where the point charge nuclei were treated as a uniform background charge (jellium) and the dielectric function was taken as the experimental one of the bulk pure metal, while for intermediate compositions, a model dielectric function built as the arithmetic (molar-weighted) average of the pure metals was employed. In the experiment on particles with cluster size around 2.3 nm reported in Fig. 1(b), a clear but rather weak plasmon resonance appears at ~2.45 eV for pure gold, while as the silver concentration increases the plasmon gains in intensity and is shifted to higher energy, up to ~2.95 eV for pure silver. In practice, the plasmon evolves smoothly with respect to the composition. It is worth noting that the TDLDA jellium model was able to simulate properly the spectrum of both the pure Ag and Au clusters (with the reduced intensity of Au ascribed to the screening effect of the d–band), and the model was also able to describe the intermediate compositions. More recently two different TDDFT studies 44,45 have been reported in the literature, in both cases the calculated spectra were in qualitative agreement with experiment. In particular in Ref.44 not only the composition but also the chemical ordering effect on the spectrum of 147-atom bare clusters with cubo-octahedral geometry was studied, employing core-shell, multi-shell and maximum mixing candidate structures differing in their chemical ordering. It was then found that chemical ordering has a marginal effect on the plasmonic features, whose energy position and intensity is essentially governed by the composition, see Figure 1. In more detail, for intermediate compositions the clusters [Au55Ag92]5+ and [Au92Ag55]5+ were considered, and by varying the chemical ordering at fixed composition it was found that the plasmon displays a double peak structure, which is very pronounced for core-shell and multi-shell structures, much less for the mixed structures. It is worth mentioning that optical absorption of AgAu nanoclusters has been previously studied with various models (Drude theory, electron gas and DFT) 46- 49 considering size and composition effects.

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View Article Online The Ag-Cu nanolloy is another important system which has been studied by several authors, in DOI: 10.1039/C5CP00498E

particular Ag32Cu6 at the TDDFT level 50 or smaller clusters (7 atoms) by ab-initio correlated methods (SAC-CI) or TDDFT (13 atoms). 51- 53 The effect of the presence of copper in silver clusters causes a strong reduction of the plasmon intensity50. This is not surprising since the relative position of d and s bands in Cu is more similar to gold than to silver. Exotic helical Ag-Cu 13-atom clusters

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have also been studied at the TDDFT level 54 finding that both the structure and the position of the Cu single atom dopant appreciably influenced the optical spectrum. The AgNi system has been studied both experimentally and theoretically. 55 From the chemical ordering point of view it is well established that Ag-Ni nanoparticles at small sizes assume a coreshell structure with silver on the surface, 56 which is in agreement with the fact that Ni is not miscible with Ag in the bulk. Pure Ni is not plasmonic, but intermediate compositions of Ag-Ni nanoparticles give a plasmon resonance which is broader and blue-shifted with respect to silver. For fixed composition, a blue shift with respect to increasing size is found both experimentally and theoretically. More recently, the photoabsorption of Ag-Ni clusters has been studied by TDDFT calculations on clusters consisting at most 13 atoms, which are too small to give rise to plasmonic behaviour, so in this respect further studies on larger systems would be interesting 57-59. Notwithstanding, Rabilloud and coworkers were able to identify precise trends due to the presence of Ni in Ag cluster, which can be summarized as follows: the presence of Ni reduces the intensity and increases the density of transitions in the energy range up to 4.5 eV, moreover the position of Ni atoms is crucial to determine the features of photoabsorption spectra.

The Ag-Co system has been studied experimentally as well as theoretically at the level of Mie theory.55 Its behavior was found to be very similar to Ag-Ni, with the only exception of a lesser importance of size effects. In fact at fixed composition the plasmon in Ag-Co was found to be rather size-insensitive, at variance with the Ag-Ni case where a blue shift with increasing size was found. Ag-Pt has been also considered both experimentally and theoretically. 60 Interestingly, no plasmon is found in the alloy nanoparticles in the experiment, unless very large cluster size are reached, in contrast with semiclassical calculations which give rise to plasmons even at small sizes. This puzzling behaviour has recently been addressed by TDDFT, 61 showing that in ~150-atom clusters already few Pt dopant atoms are sufficient to strongly quench the silver plasmon, with a concomitant blue shift of the absorption peak position. Also interestingly, the location of the Pt atoms in the structural framework has been shown to be important in tuning the effect of quenching 7

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the dipolar external field they are more effective to reduce the plasmon intensity than when they are inside the cluster or in the equatorial region. This is consistent with the surface localization of the SPR.

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Recently, the Ag-In nanoalloy has been studied experimentally and with a semi-quantal model.62 This system is very interesting because the Ag-In alloy exhibits a strong plasmon peak whose energy is in between the plasmon resonances of pure Ag and In clusters. Since the In plasmon lies about 2 eV above that of Ag, this system is very promising to tune plasmons in the UV.

Finally it is worth mentioning that the photoabsorption of the ‘magic’ cluster WAu12 has been studied by TDDFT including Spin-Orbit (SO) coupling 63. This work has shown that spin-orbit (SO) effects are very important to assess the spectrum when the energy range is dominated by individual transitions, since SO splits and shifts these. However when many transitions are close to each other in energy, SO effects are washed out in the convoluted spectrum.

Before leaving this section, it is worth recalling that nanowires and nanorods represent very interesting systems. The optical response of these strongly anisotropic systems is a superposition of bulk-like plasmons, albeit confined in one dimension (1D), in the direction parallel (longitudinal) to the wire axis, and finite-size SPR in the two direction orthogonal (transversal) to the wire axis. 64 Indeed, transversal excitation of Au nanowires strongly resemble those of Au nanoparticle of similar cross-section41. For the longitudinal mode, it should be considered that the frequency of the bulk plasmon is expected to be reduced by a factor 1/3 when confined to 1D. This reduces the damping due to s/d band mixing and enhance absorption thus transforming a weakly plasmonic metal such as Au into a more effective absorbed, closer to Ag. 65,66,67

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4 Monolayer-protected clusters

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A rapidly growing sub-field in the context of metal nanostructures and nanoalloy systems is represented by clusters for which a precise chemical composition can be established and possess sufficient stability to be used in practical applications. These compounds in fact combine the unique

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electronic, catalytic, etc. properties of metal nanoclusters with a well-determined stoichiometry which narrows down response dispersion and allows chemical specificity. The interesting sizes range from several to some hundred atoms, with the upper limit currently set at the recently synthesized Faradaurate-940 or Au~940±20(SR)~160±4. 68 While coated or ligand-protected clusters, e.g., using phosphine ligands 69, have been known since a long time, the synthesis of monolayerprotected nanoclusters (or nanocrystal molecules or nanomolecules as they are variedly called) has strongly developed since the mid '90, when thiolate-passivated systems were introduced14-16, followed by systems passivated by selenolate18 and other ligands. A number of excellent review articles and book chapter have summarized the explosive interest in this field17-30, some of them dealing with alloyed clusters, see e.g. Refs.21,23,28. Indeed, during the revision stage of the present perspective article, a further review has appeared on-line 70.

In the context of the present perspective article, these systems are of interest because they possess distinct UV-vis absorption24 and luminescence 71,72 spectra. This gives rise to a huge wealth of applications, only very partially already realized, in disparate fields such as: catalysis 73 and photocatalysis 74,25, nano-electronics 75, biomarkers, electro-chemistry 76, chemical sensors 77, polymer composites with fluorescence resonance energy transfer 78, etc. The applications in nano-medicine are very intriguing 79- 83, and in many of these applications light/matter interaction plays a crucial role 84, such as in biosensors 85,86, bioimaging 87, cancer treatment 88,89, etc. As will be shown in the following, alloy nanoclusters can be very advantageous in this context – just to make an example, in terms of controlled release in anti-cancer or anti-microbial applications 90 .

From the experimental point of view, pioneering work on the synthesis of monolayer-protected binary nanoclusters was conducted by Murray, Dass, Negishi, Jin, Pradeep, Tsukuda, Khanna, Sen, and coworkers, producing as first examples: Au24Pd(SC2H4Ph)18 91, Pd1Au24(SC12H25)18 92, Au25nAgn(SC12H25)18

mercaptosuccinic

(n = 0–11) 93, (Au-Ag)144(SR)60 94, Ag7Au6@H2MSA (where H2MSA is acid, 95

(Ag-Au)38(SR)24 96,

Ag4Ni2(DMSA)4

(DMSA

=

meso-2,3-

97

dimercaptosuccinic acid) . These earlier reports have been followed by an explosive activity, as reviewed below. 9

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Although not the topic of this perspective, it can be mentioned that advances in synthetic protocols are achieved steadily in the field, see e.g. Refs. 98- 100,22,101- 105, trans-metalation106, liquid chromatography 107, structural characterization 108, antigalvanic reduction 109 , to mention a few, also to underline the fact that purity is especially important in view of the comparison with theoretical

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simulations. Additionally – as is always true in the field of alloy nanosystems – achieving a precise location of the different elements within the structural framework, i.e., the issue of chemical ordering or compositional structure, represents a great challenge at the experimental level. Despite the fact that, as we argue below, compositional structure may be a bit less important than in other fields such as catalysis as far as optical properties are concerned, the presence of a multitude of ‘homotops’ 110, i.e., compositional structure isomers corresponding to different arrangements of the metal elements in a given structural framework, can blur specificity and increase response dispersion. Discussing the methods and developments required to fully master this delicate synthetic issue would deserve a treatment on its own and lies outside the scope of this perspective and our own expertise. Here, it is simply mentioned as an important challenge in part still to be dealt with.

From the theoretical point of view, the first applications of rigorous TDDFT methods to predict the response of these species were reported in Refs. 111,112, although Ref.111 utilized a Au38(SR)24 geometry which was different from the one later determined experimentally 113. In these first articles, electronic or super-atom shell closure was used as a main theoretical concept also in connection with the prediction of optical response 114- 117,118. Energetically, electronic shell closure effects can play an important role in stabilizing specific sizes of ultra-small clusters. However, in analogy with bare metal nanoclusters 119, these effects – amounting to stabilization energies of the order of 1 eV at most – will become progressively less important in larger clusters. Moreover, detailed analysis of TDDFT simulations19 shows that the electronic structure and optical response of ultra-stable ligand-protected metal clusters is actually the result of an interplay among several contributions: (i) electronic shell closure (which translates in a substantial HOMO-LUMO gap) deriving from the filling of "super-atom-like" states given by the superposition of the metal valence electron orbitals; (ii) the influence of the ligands field on such metallic valence electrons, determining a partial oxidation of the outer metal atoms – with these two contributions included into the super-atom analysis, but also: (iii) the contribution of molecular orbitals other than those of the metallic valence electrons and in particular orbitals localized on the Au-S external-metal/ligand shell. Although the exact extent of ionic and covalent contribution to the bonding between the outer 10

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their wave functions are strongly involved in the electronic states around the Fermi level and therefore in low-energy electronic excitations, as indeed pictorially demonstrated by plots of the induced density33.

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In the context of optical response, an old question is at which size metallic features fully develop, curiously asked for an alloy Ag20Au18(PPh3)12Cl14 compound in one of the earliest studies 121. More recently 122, the synthesis of water-soluble glutathione-stabilized gold nanoclusters containing a wide range of Au atoms: from ~10 to ~1000, and their separation into 26 groups with distinct optical spectra was reported, allowing the direct observation of size-dependent transition from molecule-like to plasmonic optical behavior. In another recent study 123, based on excited-state dynamics measurements, Au144(SR)60

was advocated as the smallest metal-like monolayer-

protected Au cluster.

In the following, we briefly review the work done in this field, cataloguing clusters by size (all metal atoms are considered as a single element for simplicity).

4.1 Au13 and around

Ag7Au6@H2MSA clusters (where H2MSA is mercaptosuccinic acid) were synthesized according to a three-step procedure95. This produced monodispersed particles which were stable for months in aqueous phase at low temperatures (below 283 K) and in solid state at room temperature. The materials, in both solution and solid states, showed bright emission. A variety of techniques were used to determine the stoichiometry of the complex, which resulted to be Ag7Au6(MSA)10. The geometric and electronic structure of this system were studied theoretically and a distorted icosahedral geometry was proposed. TDDFT simulated absorption spectra based on this geometry agreed well with the experimental results, showing strong resonant excited states at excitation wave lengths coinciding with the experimentally measured values of 350 and 692 nm. Position and intensity of the luminescence peaks were studied as a function of the ligand nature; the behaviour of this system is in tune with that of Au25-based nanoclusters 124 . In another report 125 a different synthetic method was proposed to prepare Ag7Au6@H2MSA, and its high red fluorescence shown to be very sensitive (enhanced) – and in a highly selective way –, to the presence of Al3+ ions, so as to be promising for Al3+ sensing in aqueous solution, with a 11

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shown to promote aggregation and luminescent emission of Au(I)–cysteine complexes thus achieving great sensing selectivity126, and that the fluorescent response of Au nanocluster coated by glutathione with diameter less than 2 nm was found to be sensitive to the presence of low concentration of Fe3+, Cu2+, and Hg2+ ions (quenching), but the introduction of ethylene diamine

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tetra-acetic acid (EDTA) as a chelator set fluorescence "on" again for Fe3+, Cu2+, only, not for Hg2+, thus achieving multiplex sensing 127 . (Ag-Au)13(SR)10 clusters with R = CH3 have been studied at the theoretical level 128 . The aim of this study was to interpret experimental results on Au6Ag7@H2MSA recalled above95. Several binary isomers were investigated, whose structural framework was based on a [Au13(SR)10]+ motif proposed in a previous theoretical study on thiolate-protected pure Au clusters with composition AuN(SR)M, N = 12-20, M = 9-16 129, in which also optical and chiroptical properties had been calculated. Analogously, both the optical and chiroptical spectra were simulated for (AgAu)13(SR)10 clusters. The most stable homotop was found to exhibit a Ag6 octahedral core surrounded by dimer [Au2(SR)3] and trimer [Au3(SR)4)] units. The comparison of the optical spectrum with the experimental one of Au6Ag7@H2MSA was fair. Au13CuN(SR/R’)12 clusters with N= 2, 4, 8 were synthesized (with R/R’ mixed thiolate and phosphine ligands bearing pyridyl groups), their structure determined by X-ray measurements and characterized via DFT and TDDFT calculations 130. These compounds exhibit an icosahedral Au13 core face-capped by Cu atoms, so that they are not classical alloy systems. In a successive report 131 [Au12+nCu32(SR)30+n]4− anions with n = 0, 2, 4, 6 and R = PhCF3 were synthesized and characterized by X-ray single-crystal analysis and DFT calculations. The Keplerate Au12@Cu20 metal core in these systems more closely corresponds to a traditional alloy system. The agreement between measured and simulated optical spectra was fair.

Finally, (Ag-Au)N@GSH (with GSH glutathione and N = characterized via optical and CD spectroscopy

132

15, 18, 24) were synthesized and

. The proposed average stoichiometry was:

Au12.2Ag2.8(SG)13, Au14.4Ag3.6(SG)14, and Au17.6Ag7.4(SG)18. The average stoichiometry suggested that Ag alloying is easier for the larger clusters, and that Ag prefers occupying inner sites. An analysis of the optical spectra suggested that the presence of Ag changes the electronic structure and response of the clusters and might weaken circular dichroism, but the likely presence of a mixture of different homotops made an in-depth analysis difficult. 12

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4.2 Au25

A substantial amount of work has been devoted to the nanoalloy family derived from Au25(SR)18,

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whose structure has been determined second in the field of monolayer-protected clusters 133,118.

MAu24(SR)18 clusters with M a dopant metal atom belong to this family. The anionic form of this species is a magic cluster with an electronic shell closure at 8 free electrons. Doping with a single M = Pd atom was achieved as the first example of alloyed nanocrystal molecule91, and its structure determination has correspondingly been the first example of X-ray resolved structure of a thiolateprotected nanoalloy92. It should be underlined that the Pd-doped cluster is crystallized in a neutral charge state, not as an anion, at variance with the pure Au species and with predictions based on the superatom model 134. The Pd atom replaces an Au atom at the center of the cluster, producing what can be seen as a core-shell chemical ordering, as determined experimentally92,135 in perfect agreement with theoretical predictions134,135. Being (the first and probably) the simplest example of a thiolate-protected nanoalloy, the agreement between measured UV-vis and TDDFT simulated spectra92 is good, as illustrated in Fig. 3 taken from Figs. 4a and 5b of Ref.92. Pd-doping is beneficial to increases the chemical stability of this compound 136, but also to accelerate ligand exchange processes 137. In passing, a similar increase in chemical stability was also found in a PdAu10(PPh3)8Cl2

doped

analogue

of

a

well-studied

phosphine-coordinated

pure-gold

Au11(PPh3)8Cl2 cluster 138, and for a PdAu12(PR3)8Cl4 doped analogue of a Au13(PR3)8Cl4 cluster136. It can also be added that the catalytic activity of these system for the aerobic oxidation of benzyl alcohol is improved with respect to the pure Au25 cluster, albeit tested after calcination73, that however does not destroys the metal core motif and preserves the metal cluster identity basically simply getting rid of the ligand shell 139, thus representing an interesting path to the synthesis of size-selected supported alloy clusters. Au25-nAgn(SR)18 clusters were studied at both the experimental and theoretical levels93,140- 143,144. Mixed Au25-nAgn(SR)18 clusters with R = C12H25 or R = CH2CH2Ph and up to n = 11 were produced and their optical absorption and photoemission spectra recorded93. Ag atoms were argued to occupy inner sites. The absorption spectra evolved in a continuous manner as a function of Ag content, while the luminescence main peak moved from ~1060 nm to lower wave lengths. Total energy and TDDFT calculations were performed on Au25-nAgn(SH)18 clusters140, confirming the energetic preference of Ag for inner sites and predicting a continuous increase of an absorption band at 2.5 13

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higher-energy peak that corresponds to interband transitions from 3 eV to above 3.5 eV. These results were qualitatively confirmed by a later theoretical study on Au25-nAgn(SCH3)18 clusters141, although the use of a different xc-functional did not allow a quantitative comparison. A successive combined theoretical and experimental study on Au25-nAgn(SCH2CH2Ph)18 clusters142 gave results

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also in qualitative agreement with previous work, although it is interesting to observe that in this case an enhancement of optical absorption cross-section was not apparent. In a latest study, the atomic positions of Ag dopants in Au25−nAgn(SR)18 was finally determined by X-ray crystallography143. It was thus unambiguously proved that Ag is selectively incorporated into the 12 vertexes of the icosahedral core, with an average composition of Au18.3Ag6.7. An increase of absorption intensity at around ~470 nm was also demonstrated. Theoretical calculations suggested that absorption spectra can be quite sensitive to the precise location of Ag species within the framework, i.e., that different homotops can exhibit appreciably different spectra, but a deeper analysis of the origins and full rationalization of this behavior was not presented. These results are in agreement with the conclusions of a combined experimental (via extended X-ray absorption fine structure, EXAFS) and theoretical study144, in which the initial stage of doping Ag atoms into Au25(SCH2CH2Ph)18 clusters was investigated and a location of Ag on the vertexes of the icosahedral core was determined. Finally, it is worthwhile adding that, while studying the reactivity of Au25(SR)18 with Ag+, Cu2+, and Pb2+ ions by monitoring absorbance and fluorescence spectra in addition to voltammetric measurements, Murray and coworkers 145 proposed that the monolayerprotected Au25(SR)18- anion in solution can be oxidized by the metal ions, hence forming an adduct that is then seen in the mass spectra as, e.g., Au25-nAgn(SR)18, n =1-3, species. No accompanying calculations yet confirm these hypotheses despite their possible importance to shed light on the growth and the mechanisms of ligand exchange in monolayer-protected species. Au25-nCun(SR)18 systems protected by both thiolate ligands 146,144 and selenolate ligands 147 were investigated, with n up to 5 in the thiolated case and n up to 9 in the selenolated case. In the thiolate case146 experiment showed that single Cu doping slightly shifts the absorption peaks to lower energy. Theoretical calculations presented in the same article predicted that the most stable position for Cu is in the 12-atom icosahedral shell with an accompanying significant structural distortion of the cluster but in this case a blue shift of optical bands is predicted, contrary to experimental observations. However, if Cu is in the center of the cluster, a location higher in energy by 0.4 eV with respect to the location in the icosahedral shell, a red-shift is predicted in agreement with experiment. The authors attribute this mismatch to kinetic trapping, a hypothesis which awaits to be 14

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the cluster due to Cu doping. In a later work144, a combined experimental (via EXAFS) and theoretical study of the initial stage of Cu doping into Au25(SCH2CH2Ph)18 clusters was presented. The analysis of experimental data showed that the most probable location of Cu is in the staples, despite concurrent DFT calculations indicate a preference for the icosahedral vertex for a single Cu

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dopant. Aerobic oxidation under the reaction conditions was invoked to explain the discrepancy between theory and experiment. In the case of Au25-nCun(SeC8H17)18 selenolate clusters, increasing the number of Cu dopant atoms slightly shifts absorption and simultaneously photoluminescence emission to longer wavelengths and lower energies147.

PtAu24(SR)18 species were also obtained and characterized via UV-vis spectroscopy and DFT calculations 148,149. A Pt-centered icosahedral core structure was predicted from calculations. The main interest of this nanoalloy combination is in catalysis. Nevertheless, interestingly, a blue-shift of the lowest-energy absorption peak is observed in the experimental optical spectrum, albeit apparently without a decrease in intensity, in fair agreement with TDDFT predictions. An appreciable effect of alloying seems to be present on the catalytic activity of this compound. Alternative syntheses have been developed for Au25-nAgn@MHA clusters 150, where MHA is 6mercaptohexanoic acid. A continuous doping of Ag was obtained up to n = 11, with the Ag atoms argued to be located in the cluster core. Correspondingly, the cluster UV-vis absorption spectra showed an evolving a distinct band at ~470 nm. The luminescence properties of these clusters were strongly enhanced 151 via aggregation-induced emission 152. In a different approach [Au25-nAgn(PPh3)10(SC2H4Ph)5Cl2]2+ both pure and binary clusters with a mixed ligand shell were produced and their photoluminescence properties determined 153. The major result was that photoluminescence was very weak (quantum yield less than 1%) up to x = 12, whereas the Au12Ag13 composition strongly increased the quantum yield to a high value of 40%. The structure of these species was determined to coincide with that of the parent x=0 cluster 154, i.e., a bi-icosahedral rod-like shape. Modifications of the ligand shell to make the cluster water soluble while keeping its high photoluminescence are under study by the authors of Ref. 153.

Finally, using an antigalvanic reduction approach, it was possible to add 2 Ag atoms on the surface of Au25(SC2H4Ph)18 thus producing a Au25Ag2(SC2H4Ph)18 cluster 155. The agreement between the simulated and experimental UV-vis spectrum was good, as expected, and interestingly this cluster 15

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activity in the hydrolysis of carboxylic esters) with respect to the pure parent compound.

4.3 Au38

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The structure of a pure Au38(SCH2CH2Ph)24 complex is known113,156, which belongs to the class of Au38(SR)24 compounds first synthesized in Ref. 157. Au38-nAgn(SCH2CH2Ph)24 alloy nanomolecules were synthesized and purified according to a two-step procedure96. MALDI-TOF was used to discriminate between different stoichiometries (ranging from Au29Ag9 to pure Au38) in the nanoalloyed particles, and it was observed that Ag doping does not change the overall stoichiometry of the cluster. UV-VIS spectroscopy was used to investigate the effect of silver inclusion (up to 6 Ag atoms) on the optical response of the clusters. Incorporation of silver resulted in a broadening and blue-shift of the main peak with respect to the pure gold system. While the blue shift is consistent with theoretical expectations, the broadening might be due to the presence of several isomers, and is different with respect to the (Au-Ag)144(SR)60 case (see below), in which Ag doping seems to produce an appearance of plasmonic features. Pd2Au36(SC2H4Ph)24 was also synthesized and purified 158. Assuming that the structure of the cluster is the same of the parent Au38(SC2H4Ph)24113,156 compound, the two Pd atoms were hypothesized to be accommodated at the center of the two merged icosahedra, in tune with the substitution of the central atom that had been demonstrated in the case of PdAu24(SR)18 cluster due to strain release92. For similar homology reasons, the Pd2Au36(SC2H4Ph)24 complex resulted to be more stable than its pure counterpart and also more stable than a singly substituted PdAu37(SC2H4Ph)24 species which was also observed in the mass spectrum as a minority species. In another report 159 (Ag-Au)@BSA clusters (with BSA bovine serum albumin) were synthesized, and their steady state and time resolved excited state luminescence profiles measured. Due to the synthetic procedure, the exact mass composition was not determined, but MALDI mass spectroscopy indicated that alloy clusters were likely formed, of size similar to the parent pure clusters (~31- and ~38-atom Ag and Au clusters with molar ratio 1:1). No distinct peaks were observed in the optical absorption spectrum. However, the alloy clusters exhibited rather different luminescence profiles compared to the parent clusters.

4.4 Au130 & Au144 16

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Surprisingly, to the best of our knowledge doping has not yet been considered for the Au102(SR)44 compound which was actually the first nanocrystal molecule for which X-ray crystal structure was determined 160. However, in several articles Dass and coworkers pioneered the investigation of doped Au144(SR)60 systems. Considering Ag as a dopant species, (AuAg)144(SR)60 clusters were

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synthesized94, finding that up to 53 Ag atoms can replace Au atoms, keeping the same composition as demonstrated by electrospray-ionization mass spectrometry (ESI-MS). A preliminary theoretical investigation

was

conducted

on

these

systems 161,162,

followed

by extensive

TDDFT

simulations 163,164. These calculations assumed that Ag-doping did not affect the structural framework of the cluster, which may be suggested by the constancy of the overall composition, and used the geometry proposed in Ref. 165, which however has not been validated so far by X-ray crystal measurements. Later reports in the Dass group extended this line of investigation to substitution by copper: (AuCu)144(SR)60 166, and palladium: (AuPd)144(SR)60 167.

One question which is debated in this context is the effect of alloying on the optical response of Au144(SR)60 systems. Experimentally, the Ag-alloyed clusters exhibit a significant enhancement of absorption intensity in the 400-550 nm region of the spectrum94. However, according to the first TDDFT calculations on these systems163 no significant enhancement was predicted up to 54-60 Ag substitutions, except for a modest increase in the region below 400 nm, as illustrated in Fig. 4 taken from Fig.1 of Ref.163. Successively, TDDFT calculations using a different xc-functional164 found a modest increase especially around 500 nm when 60 Ag atoms in two different homotops (a symmetric one also considered in Ref.163 and a “random” one not previously investigated) were introduced in the structural framework. This mismatch between theory and experiment has been discussed in Ref.163 as an example of the need of a more thorough experimental and theoretical characterization to resolve this paradox and achieve a good agreement and therefore in-depth understanding of optical properties upon alloying. Possible sources of mismatch are: (1) geometry: the geometry proposed for Au144(SR)60 should be refined or it changes with Ag doping or the distribution of Ag atoms within the framework is different from the one so far hypothesized, (2) the xc-functional used in TDDFT calculations should be better validated against experimental data, (3) the experimentally observed peaks (especially the low-energy ones) arise not from individual clusters but from plasmonic interactions among them12,41: both aggregation and plasmonic interactions can in fact change with the cluster composition.

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found when Cu progressively substituted Au up to a 1:1 elemental composition ratio166 in (AuCu)144(SR)60 clusters. It was also suggested that Cu doping can change the cluster composition, eventually favoring (Au-Cu)145(SR)60 species, which was advocated as the first reported example of this phenomenon. These results are currently under investigation by several theoretical groups and it

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is likely that further information will soon be available. Indeed during the revision stage of present perspective article, two articles appeared on (Au-Cu)144(SR)60 clusters. In one article 168, (AuCu)144(SR)60 clusters were synthesized and subjected to UV-vis-NIR, MALDI-TOF and ESI mass spectroscopy and STEM characterization, and theoretical analysis. The strong enhancement of optical absorption and the birth of a plasmon-like feature at ~520 nm was confirmed. Theoretical calculations were performed on a CuAu144(SR)60 cluster with a geometry taken from the current structural model of Au144(SR)60, and it was found that the preferred Cu location was in the center of the cluster. However, STEM images showed a broad distribution of very different species, which was tentatively attributed to electron-beam-induced damage effects. In another article 169, DFT and TDDFT calculations were performed on a much wider set of CuAu144(SR)60 and CuAu143(SR)60 homotops. With some similarities to the Au25-nCun(SR)18 system recalled above144,146, theory predicted that the most stable position for Cu is in the ligand shell in which however no plasmonic peak is observed. In contrast, when Cu replaced Au in the core of the cluster, the simulated optical absorption spectra showed a significant increase of intensity in the region below 600 nm, in tune with experimental observations. Aerobic oxidation of clusters with Cu in the ligand shell144 was proposed to resolve the issue of thermodynamic stability, although in Ref.144 aerobic oxidation in Au25-nCun(SR)18 clusters was hypothesized to rationalize exactly the opposite discrepancy, i.e., location of Cu in the ligand shell despite theory suggested that location in the metal core was thermodynamically more favorable. In contrast to the Ag and Cu cases, incorporation of 1 to 3 Pd atoms replacing Au to give (Au-Pd)144(SR)60 systems produces a decrease in the optical features167. (AuAg)130(SR)50 alloyed particles have also been obtained 170 via an etching process of larger aggregates (conducted at high temperature in excess of thiol) followed by size-exclusion chromatography. During the etching process, also Au144 and Au137 clusters were formed, confirming the stability of these sizes, too. The process of core-size conversion towards stable sizes was repeated by using different ligands, as a further confirmation of the stability of the observed sizes. Following the same synthetic protocol used in the case of 144-atom and 38-atom particles, alloyed Au-Ag clusters (up to an Au:Ag ratio of 1:0.15) were obtained. Both pure and alloyed nanoparticles were successively investigated by UV-vis spectroscopy. Pure gold nanoparticles showed three 18

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broadening due to homotop blurring can be hypothesized here). The structure of this cluster is still unknown and no theoretical calculations support these findings. Larger clusters have finally been investigated. In Ref. 171, Ag-Au clusters coated by 11-

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mercaptoundecanoic acid have been prepared. Apart from a non-monotonous shift of the absorption bands toward higher energy, clusters with a molar ratio estimated around Ag:Au = 1:2 exhibited a three-fold increase in fluorescence reaching a quantum yield of 6.81%. Analogously, using cystidine as a template, Ag-Au nanoclusters with a diameter of 1.50 ± 0.31 nm were synthesized 172, and proved to be highly fluorescent, with a quantum yield of ∼9%, and an average lifetime ∼6.07 μs.

4.5 Ag-based systems

Doping of Ag-based monolayer-protected clusters has also been achieved, despite the greater difficulty in obtaining chemically stable systems when Ag is the base metal element.

Khanna, Sen and coworkers synthesized, determined the crystal structure, and measured the UV-vis spectrum of Ag4Ni2(DMSA)4 (DMSA = meso-2,3-dimercaptosuccinic acid)97, to the best of our knowledge this representing the first structural characterization of a ligand-protected binary clusters. Later, they also synthesized and measured the UV-vis spectrum of Ag4Pt2(DMSA)4 173 and simulated via TDDFT/PBE calculations the spectra of the Ni and Pt nanoalloys and additionally that of a hypothetical Pd analogue. The agreement between theory and experiment was fair for the Ni compound, much less for the Pt analogue. The TDDFT prediction was that the replacement of Ni by Pd and then Pt led to a progressive blue shift of optical absorption, in tune with findings on bare Ag-Pt clusters61 . Doping with magnetic elements had been previously investigated at the theoretical level 174- 176, and applications as high-performance spin filters had been suggested176. The related field of magnetoplasmonics is indeed very appealing 177 and TDDFT calculations could be performed to predict such effects by extending e.g. those of Ref. 61 to the spin-unrestricted case.

Recently, Pradeep and coworkers reported the synthesis of a selenolate­protected Ag-Pd alloy cluster anion 178, and measured its UV­vis spectrum. The authors suggest a one-pot synthesis 19

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different optical response (different colours). The clusters resulted stable for more than one month at ambient conditions. Electrospray ionization mass spectrometry (ESI-MS) was used to characterize the stoichiometry of the three clusters. Among these three, one was precisely identified, having an isotope distribution of the molecular ion peak showing an exact match with the

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corresponding calculated one: using a variety of experimental techniques, this cluster was identified as Ag5Pd4(SePh)12-. A model for the structure of this particle is still missing and no theoretical calculations have so far accompanied these findings.

Pure and alloyed M12Ag32(SR)30 (M=Ag or Au) nanoparticles have been obtained via wet-chemistry routes 179. Both pure and alloyed particles, which were stable in ambient conditions in their solid form for several months, were crystallized and their structure was identified: it consisted of a central dodecahedron of 32 atoms capped by six pyramidal-like Au2S5 units, whose structure is new with respect to that of the classical RS(M-SR-)x oligomeric unit characterizing many other metallorganic complexes of similar dimension. This structure, carrying a charge of -4 in both the pure and the alloyed case, was simulated in the same work using aromatic fluorinated ligands at the DFT/PBE level, finding a good agreement with the experimental structure. Simulation of the UV-VIS spectrum for the pure case demonstrates also a fair agreement with the experimental spectra. Electronic analysis confirms that the stabilization of the cluster is favored by a shell closure according to a super-atom model for both pure and alloyed particles.

In the case of

[Au12Ag32(SR)30]4-, Au and Ag were revealed to be distributed in a core-shell fashion rather than a random fashion, with all Ag atoms located on the surface of the clusters. A detailed theoretical analysis of these systems has been conducted 180 by varying the nature of the ligand, the overall charge of the system and by considering various homotops: simulation of the UV-VIS spectra produced a good agreement with the experiment also for the alloyed case. Finally, it was also theoretically predicted that doping with a central manganese magnetic atom results in a protected maximally magnetic nanocluster.

4.6 Chirality More recently, the issue of chirality has been tackled, see Ref. 181 for the discovery of this effect in monolayer-protected clusters and Ref. 182 for a latest review. Theoretically, it is possible to predict the circular dicroism (CD) spectra 183 of monolayer-protected metal clusters 184, and calculations of this property are becoming more common, although statistics is still limited and many more 20

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activity, whether an inherently chiral metallic core, the dissymmetric field effect, or the chiral footprint model, has been long debated 185. Computational results on Au36(SC2H4Ph)24 showed that – at least in this case of an achiral metal core and thiolate ligands – the geometrical arrangement of the Au–S protecting shell plays a decisive role 186 . This is in tune with experiments on Au25(SR)18

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clusters 187,188 .

Let us briefly review the literature in this field.

As mentioned above, (Ag-Au)N@GSH clusters with N = 15, 18, 24 have been characterized via CD spectroscopy132.

We also recall that chiroptical spectra have been predicted for thiolate-protected pure Au clusters with composition AuN(SR)M, N = 12-20, M = 9-16129 and for (Ag-Au)13(SR)10 clusters with R = CH3128 . Additionally, for the 38-atom doped clusters discussed above, Pd2Au36(SC2H4Ph)24 pure enantiomers have been prepared and isolated 189 finding that Pd doping affected the CD spectra. Particularly interesting was the finding that the kinetics of racemization accelerated with doping: the activation enthalpy for this process decreased from 29 to 20 Kcal/mol while the activation entropy surprisingly changed sign from 10 to -12 cal/mol in passing from Au38(SC2H4Ph)24 to Pd2Au36(SC2H4Ph)24. Since racemization is connected with ligand mobility and possibly exchange, this technique represent an effective way to obtain information on such ligand dynamic phenomena.

Finally, the CD spectra of both the Au144(SR)60 and Au84Ag60(SR)60 species were predicted in Ref.164, finding a remarkable overall similarity between the two cases, but also a range of transitions where the CD signals of the pure and alloyed species were of opposite sign.

In summary, it is not easy to derive principles in this field, due the limited statistics and comparison theory/experiment.

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5 Discussion

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The previous survey and its wide breadth despite it is limited to the latest years demonstrate the great interest associated with the optical properties of alloy nanostructures. Indeed, both scientific advancements and technological applications benefit from these developments, in many different

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fields4-30,73-89 such as: catalysis and photocatalysis, nano-electronics, biomarkers, electro-chemistry, chemical sensors, polymer composites with fluorescence resonance energy transfer, biosensors, bioimaging, cancer treatment, anti-microbial applications, etc.

From an analysis of the results of Secs. 3 and 4, in perfect agreement with previous work and analysis, it is possible to identify some trends which are typical of pure clusters but remain valid for nanoalloys as well. For pure systems the three factors determining the feature of optical absorption spectrum are: 1) element, 2) size, 3) structure.

The nature of the element is crucial to the optical properties. Indeed, despite the fact that from the point of view of Mie theory all metals should be similar, as all have mobile and polarizable conduction-band electrons, in transition metals the plasmonic response of conduction (s-type) electrons is strongly damped by the coupling with d-band electrons5. This can be clearly seen by comparying Ag with Au. In Ag the d-band is well below the Fermi energy and therefore the plasmon is very intense and emerges at smaller size with respect to Au, which is still a noble metal but in which the fact that the d-band (although fully occupied) is closer to the s-band due to relativistic effects makes that SPR are often significantly less intense especially in small nanoparticles. In short, electrons in silver clusters behave more like a free electron gas, giving rise to well-defined, intense and sharp plasmonic features, while in gold clusters interband transitions somewhat decrease effects such as intensity and sensitivity to shape.4 Chemical inertness is also an important factor in the choice of metals for plasmonic applications: the great reactivity of e.g. alkali and other simple metals makes that in practice nearly exclusively noble and quasi-noble metals are considered in applications.

The position of SPR peaks usually decreases in energy with increasing size. For example, looking at the evolution from molecular-like to plasmon-like behavior as a function of the size in Ag tetrahedral clusters, a linear dependence of the main peak positions with respect to the inverse size of the cluster was observed, in agreement with classical electrodynamics models 190. It should be recalled in this connection that photoabsorption is an extensive phenomenon, i.e., its integrated 22

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the Thomas−Reiche−Kuhn (TRK) sum rule which states that the integral of oscillator strengths over the whole electronic spectrum must equal the number of electrons. The size of the nanoparticle will thus determine the overall intensity: as a volume effect for bulk plasmons, as a surface area effect

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for SPR.

The structure is a third important parameter in determining the optical absorption features. However, it is usually found that fine structural details are somewhat less important with respect to the overall shape of the nanostructure. For example, a comparison of nanoparticles with nearly spherical shape39 showed that they exhibit similar absorption spectra, although details such as the presence of fcc(111) vs. (100) surfaces can finely tune and qualify this general expectation. Clearly, if the shape is very different from spherical, such as in tetrahedra190, tip effects can significantly shift the position of absorption peaks. Again, such a shift is much larger in Ag than in Au tetrahedral structures, because the more polarizable and free-electron like excitation of the former element will produce the most intense effect39 .

Once structure and size are fixed, in Ag-based nanoalloys two further general findings can be pointed out from an analysis of the results in Sec. 3,4:

1) the photoabsorption of alloys is essentially governed by chemical composition, and at zeroth order it can be predicted as a linear interpolation of the spectra of the two limiting (pure) cases as a function of composition: this is true in bare nanostructures42-44,45 but it is often confirmed also in monolayer-protected clusters (see above);

2) chemical ordering introduces variations in this general behavior: in some cases these can be appreciable50 and interesting, and attempts at rationalizing have been successful.61

Clearly these expectations will be especially valid if elemental intermixing is homogeneous, whereas segregated systems (such as the various kinds of core-shell arrangements) obviously introduce interfaces and can probably be better seen as the interaction of two (or more) separated components.

Moreover variations due to alloying will be most effective if the dopant species are placed in the positions where the SPR is mostly concentrated. For example, if plasmon is concentrated at the 23

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subsurface, as demonstrated by the disruptive effect of Pt atoms in Ag-Pt nanoclusters61. Another example is that alloying will be most effective in tetrahedral clusters if done at the tip of the cluster: in other words, if chemical substitution occurs in proximity of the ‘hot spots’ of the cluster it is expected to have a larger effect on plasmonic features (this however still awaits confirmation by

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explicitly calculations).

Cu doping of monolayer-protected Au clusters seems to represent an exception to these simple expectations. Indeed, the strong enhancement of optical absorption and the birth of a plasmon-like feature at ~520 nm found in (Au-Cu)144(SR)60 clusters166,168,169 interestingly suggests that in some systems alloying can produce synergic effects on the optical response. The interplay of chemical composition with geometric and compositional structure is certainly a topic which deserves more thorough and systematic studies in the future.

To summarize, what can certainly be concluded is that optical properties of nanoalloys are sensitive to the geometrical structure. Therefore to have a meaningful and sound comparison of these properties between theory and experiment the true geometric and compositional structure of the system should be available, a condition which unfortunately seldom occurs, although many efforts are being done in this direction and the situation will hopefully improve in a next future.

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6 Outlook

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As apparent from the previous survey and discussion, the field of optical properties of alloy nanosystems is very active, and a rich wealth of novel results is constantly been produced so that the present picture will probably be rapidly surpassed by new developments. It is interesting to note

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for example that the chemistry of these fascinating alloyed systems is far from being fully explored, and novel structures in addition to novel phenomena and processes are continuously discovered, which will likely pose new challenges to both theoretical and experimental elucidation. Nevertheless, we think it worthwhile to tentatively draw a few points and possible directions for future research from the current status and present knowledge.

In our opinion a most important point is that, while founding principles in the optical field have been established for pure species6-13, phenomena associated with alloying are not yet fully clarified. Some concepts and guiding principles have been singled out in Section 5. However, to put fundamentals of structure/property relationships on solid grounds, a punctual comparison between experiment and theory is strongly needed but unfortunately partly lacking. The reasons of this unsatisfying situation are varied.

On the experimental side, these are connected especially with the difficulty of producing precise geometrical information for alloy nanostructures, both in terms of framework and compositional structure or chemical ordering, despite recent achievements in a few promising examples97,143. Often, the presence of a multitude of homotops blurs chemical specificity, possibly produces structural changes, and makes comparison with theory and in-depth understanding uncertain. This issue is felt acutely in the field of monolayer-protected clusters, in which a precise, one-to-one comparison between theory and experiment on well-defined and well-characterized molecule-like species can in principle be made. In our opinion, issues such as, e.g., the faithfulness reproduction of the reaction mixture by mass spectrometric techniques via comparison and validation with parallel microscopy characterization have not fully investigated yet.

From the computational point of view this makes the important process of method validation difficult or impossible tout court. For example, although a reasonable agreement between experimental and simulated optical absorption spectra can be found, a systematic validation to single out which is the most accurate xc-functional has not been conducted yet. It is worth recalling here that alloy systems are intrinsically more complicated for theory, due to the need of precisely 25

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quantitative, predictive accuracy is sought for. It can also be recalled that fine interactions such as spin-orbit coupling may play a role to describe fine details63,191. For clusters on which structural and optical information is available with a good degree of certainty the results are comforting91,92, but this at present represents more an exception than a rule (with kinetic trapping or other phenomena

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sometimes invocated to justify discrepancies), and – regardless – a richer statistics is definitively needed.

Also, investigations and comparison between theory and experiment should be extended to more exotic phenomena than linear optical absorption. For example, already the field of chiroptical properties and the simulation of circular dicroism spectra185 is far from being extensively studied in the field of nanoalloys, although recent studies have proved how very useful information can be obtained when enantiomer separation is achieved189 .

Excited state dynamics is another topic which offers great promises. While experiment has provided exciting results, see e.g. Refs.71,72, theoretical work in this field is very limited due to the complexity of predicting the evolution of electronic excited states not simply for molecular species 192 but in composite metal-cluster/ligand systems. Experimentally, very interesting is e.g. the observation of a sudden jump in photoluminescence from ≤1% to 40% in changing the composition of [Au25-nAgn(PPh3)10(SC2H4Ph)5Cl2]2+ clusters from n = 12 to n = 13153, which points to a synergic effects of Ag dopants, or the enhancement of luminescence via aggregation-induced emission151 .

Another field in which experiments still need to provide a clear picture and theory is expected to give an important contributions is magnetoplasmonics177. This field is challenging due to the necessity to couple the effect of a magnetic field on the optical response. At the moment simplified models employing Drude theory193 have been able to describe quantitatively the experiment for gold clusters larger than 10 nm, while the régime of smaller sizes still represents a challenge for theory, and might be treated by extending present approaches to the spin-unrestricted case.61

Finally, given the intimate relationships between structural properties (including chemical ordering) and optical response, theory is called to answer fundamental questions not only on static properties but also on structural dynamics, such as elemental interdiffusion, as recently achieved for bare clusters 194,195,196, and ligand exchange and mobility. These topics have only been explored in a very limited degree, but will probably represent one of the main avenues of future developments. 26

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Given the great interest in this field, it is easy to predict that these challenges will be taken up in the next future, and – judging from the explosive literature growth reviewed above – it is likely that important advancements in both knowledge of these fascinating systems and control of their

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properties in technological applications will develop at a very fast pace.

Acknowledgements Networking from the MP0903 COST Action is gratefully acknowledged.

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Figure captions

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Figure 1. Photoabsorption spectra of cube-octahedral bimetallic clusters with increasing silver concentration. (a) Calculated TDDFT LB94 spectra, (b): experimental spectra. Reprinted with

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permission from Ref. 44. Copyright 2011 American Chemical Society. Figure 2. (a) Calculated TDDFT photoabsorption spectra of [Pt20Ag72]2-, the nature of the most intense transitions in terms of excited configurations is reported in the insets boxes. (b) Calculated TDDFT photoabsorption spectra of [Pt20Ag126]4+ (green line), [Pt12Ag134]4+ (blue line) and [Ag134]4+ (red line). Discrete lines have been convoluted with Gaussian functions with FWHM = 0.12 eV. Reprinted with permission from Ref. 61.Copyright 2014 American Chemical Society.

Figure 3. (a) Schematic picture of the optimized structure of neutral Pd1@Au24(SC12H25)18, (b) corresponding experimental (3) and simulated (3') spectra. Reproduced with permission from Ref.92 . Copyright © 2010, Royal Society of Chemistry.

Figure 4. Top image: experimental UV-vis spectra of (Au–Ag)144(SR)60 nanocrystal molecules in CH2Cl2 for Au:Ag precursor ratios of 1:0 (red), 1:0.25 (green), 1:0.50 (blue), 1:0.66 (magenta) and 1:0.75 (light blue) in the starting material (Exp). Images starting from the second top to bottom: calculated TDDFT photoabsorption spectra of (Au–Ag)144(SH)60 alloy nanomolecules – the Ag-Au composition and a picture of the structure of the metal framework are reported in the insets (SH groups not shown for clarity). Ag atoms colored in gray and Au atoms in yellow. Reproduced with permission from Ref.163 . Copyright © 2015, Royal Society of Chemistry.

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Figure 1

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a)

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Figure 3

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Figure 4

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Optical properties of nanoalloys.

The optical properties of multi-component metal nanostructures (or nanoalloys) are the subject of an intense and rapidly growing experimental and theo...
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