Synthesis of Subnanometric Metal Nanoparticles JavierCalvoFuentes 1 ,Jose ´ Rivas 2,3 and M.ArturoLo ´pez-Quintela 2 1 NANOGAPSUB-NM-POWDERS.A., Milladoiro–Ames(ACorun ˜a),Spain 2 LaboratoryofMagnetismandNanotechnology, InstituteforTechnologicalResearch,Universityof SantiagodeCompostela,SantiagodeCompostela, Spain 3 INL–InternationalIberianNanotechnology Laboratory,Braga,Portugal Synonyms Atomic cluster; Cluster; Nanocluster; Quantum cluster; Quantumdot Definition Metalatomicclustersconsistofgroupsofatomswith well-defined compositions and one or very few stable geometricstructures.Theyrepresentthemostelemen- tal building blocks in nature – after atoms – and are characterized by their size, comparable to the Fermi wavelengthofanelectron,whichmakesthemabridge betweenatomsandnanoparticlesorbulkmetals,with propertiesverydifferentfrombothofthem. Introduction Typicalmetalnanoparticleswithdimensionsfromtwoto several tens of nanometers show smoothly size- dependent properties. However, when particle size becomes comparable to the Fermi wavelength of an electron (0.52 nm for gold and silver), properties of metal clusters are dramatically different from what should be expected if they were due only to their high surface-to-volumeratio.Inthesesubnanometricspecies, quantumeffectsareresponsiblefortotallynewchemical, optical, and electronic properties such as, for example, magnetism,photoluminescence,orcatalyticactivity. Theterm nanoparticle usuallyreferstoanyparticle of bulk metal with dimensions in the nanoscale. Nanoparticles usually present a core-shell structure with a core of bulk metal surrounded by a shell of disordered atoms, and their enhanced properties are due mainly to their high surface/volume ratio. The term cluster is used in reference to subnanometric species consisting of well-defined structures of metal atoms stabilized by different types of protecting ligands, with sizes below approximately 1–2 nm. In general,clusterscanbedividedinto(1)largeclusters, consisting of a core formed by a number of metal atoms in the range 10–20 to 100–200 and aprotectingshellofstrongligandssuchasphosphines or thiols; and (2) small clusters formed by a reduced numberofatoms(2to10–20),whichdonotneedany strong stabilizing ligand and have almost all their atoms on the surface. Due to quantum effects, both kinds of clusters – large and small – present discrete energylevelsandanincreasingbandgapwithdecreas- ingsize(Fig.1). Because of this splitting of energies at the Fermi level, clusters show different properties to those of nanoparticles.Clusterpropertiesarehighlydependent on their sizes. As an example, the typical surface plasmon band observed in Ag-metal nanoparticles (Fig. 2a) disappears for clusters below approximately 1–2 nm (Figs. 2b and c corresponding to large and small clusters, respectively), indicating that all the conducting electrons are now frozen and the metal silverlosesitstypical metallic character.Atthesame time, a clear difference between large and small clusters can be observed: large clusters show a continuous decrease of the absorption band with somesmallbumps,similartotheabsorptiondisplayed bysemiconductors(Fig.2b),andsmallclustersdisplay well-definedabsorptionbandsindicatinga molecular- like behavior(Fig.2c). The presence of such size-dependent properties can be well represented on a 3D periodic table of elements – schematically depicted in Fig. 3 – where, between the atomic and the bulk state of every element, a whole range of new materials with their differentsize-dependentpropertiesappear. Cluster Structure It is very difficult to do precise calculations of the electronic structure of metal clusters, particularly when they are larger than just a small number of Synthesis of Subnanometric Metal Nanoparticles 2639 S S