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to.their.size-dependent.property.changes.[10]..Clusters.with.thousands.of.atoms.in.
the.range.of.10.nm.diameter.and.more.are.said.to.be.in.the.“scalable.regime”.where.
various. properties. change. smoothly. with. the. cluster. radius,. and. tend. to. the. bulk.
limit.as.the.size.(diameter).increases..On.the.other.hand,.in.the.so-called.non-scal-
able.regime,.where.clusters.have.no.more.than.a.few.hundred.particles.and.1-3.nm.
diameter,.changes.in.chemical.and.physical.properties.are.not.predictable.and.are.
strongly.size.dependent..In.this.non-scalable.size.regime,.the.removal.or.addition.
of.a.single.atom.to.the.cluster.can.result.in.signiicant.changes.to.properties.such.as.
geometric. and. electronic. structure,. binding. energy. (BE),. melting. temperature,. or.
catalytic.activity.[11].
Size-dependent.properties.make.clusters.a.separate.class.of.material..Currently,.
understanding.the.mechanism.that.determines.properties.of.small.particles,.predict-
ing.the.changes.of.such.properties.with.respect.to.the.bulk.or.to.the.molecular.limit,.
and. controlling. and. manipulating. objects. of. a. few. nanometer. in. size. are. the. key.
issues.of.nanoscience.and.nanotechnology.
Metal. clusters. are. “inite. groups. of. metal. atoms. participating. in. direct. metal-
metal. bonds”. [12].. Small. transition. metal. clusters. (TMC). attract. a. lot. of. attention.
because. of. their. potential. use. as. catalysts,. components. of. nanodevices,. magnetic.
data.storage.materials,.and.in.novel.cluster-based.materials..For.example,.platinum.
is.one.of.the.most.important.elements.in.the.heterogeneous.catalysis.of.hydrogena-
tion.as.well.as.in.the.catalysis.of.the.CO,.NO,.and.hydrocarbons..It.is.currently.the.
preferred.oxidation.and.reduction.catalyst.for.low-temperature.polymer.electrolyte.
membrane.(PEM).fuel.cells.[13].
10.1.2 e
xPerimental
m
ethodS
for
tmc
Metallic. clusters. can. be. generated. in. a. variety. of. ways:. in. the. gas. phase,. in. solu-
tion,.in.a.matrix,.or.supported.on.a.substrate.[5,14]..In.a.recent.review.by.Ferrando.
et.al..[15],.methods.for.generating.mono-.and.bimetallic.nanoclusters.were.listed.as.
(a). molecular. beams. [16]. where. interaction-free. clusters. are. produced. by. the. pro-
cesses. of. the. vaporization. of. atoms. or. molecules. in. the. gas. phase,. nucleation. of.
atoms.or.molecules.to.form.a.cluster.nucleus,.and.addition.of.more.atoms.or.mol-
ecules.to.the.initially.formed.nucleus;.(b).chemical.reductions.[17].where.metal.salts.
are.dissolved.in.an.appropriate.solvent.in.the.presence.of.a.surfactant.or.polymeric.
ligands,. which. passivate. the. cluster. surface;. (c). ion. implantation. (metal. nanoclus-
ters. embedded. in. insulating. matrices). [18];. (d). electrochemical. synthesis. [19]. for.
generating.mono-.and.bimetallic.nanoparticles.in.solution;.(e).radiolysis.[20].where.
metal.ions.are.reduced.by.γ-ray.irradiation.of.water;.(f).sonochemical.synthesis.[21].
where.nanoparticles.are.induced.by.“sonication”.of.a.solution.of.metal.ions.by.high-
intensity.ultrasound;.and.(g).biosynthesis.[22].by.the.self-assembly.of.biomolecules.
The.most.commonly.applied.techniques.for.the.characterization.of.nanoclusters.
are.mass.spectrometry,.electron.diffraction.microscopy,.and.x-ray.spectroscopy.as.
well.as.other.forms.of.spectroscopy..Clusters.in.a.molecular.beam.can.be.ionized.by.
electron.impact.or.laser.and.then.be.delected.by.an.electric.ield..Since.the.delections.
occur.with.respect.to.the.mass.of.the.clusters,.it.is.possible.to.study.the.mass.abun-
dance.of.clusters.in.the.cluster.beam..The.peaks.that.occur.in.the.mass.abundance.
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