Density Functional Theory Calculations on Cobalt and Platinum Transition Metal Clusters - Metallic Systems: A Quantum Chemist's Perspective - page 342

10.1 INTRODUCTION

10.1.1 t ranSition m etal c luSterS and t heir i mPortance
Nanoparticles,. which. are. aggregates. of. identical. or. different. atoms. or. molecules.
ranging.from.a.few.to.many.millions,.are.called.atomic.or.molecular.clusters.[1-6]..
They.constitute.a.separate.class.of.materials.in.between.the.bulk.on.one.hand.and.the.
molecular.state.on.the.other..Their.chemical,.optical,.electrical,.physical,.and.mag-
netic.properties.can.be.signiicantly.different.from.the.respective.bulk.materials..For.
instance,.while.bulk.gold.appears.yellow.in.color,.nanosized.gold.particles.appear.
in.red,.or.while.bulk.gold.does.not.react.with.oxygen,.gold.clusters.do..Thus,.mate-
rials. that. are. commonly. chemically. inert. in. bulk. can. become. catalytically. active.
[7],. nonmagnetic. bulk. materials. can. show. magnetic. behavior. when. scaled. down.
to. nanostructures. [8],. and. metallic. materials. can. become. semiconductors. [9].. The.
main. reasons. for. such. property. changes. are. as. follows:. in. the. regime. of. clusters,.
gravitational. forces. become. negligible. and. electromagnetic. forces. begin. to. domi-
nate,. quantum. mechanical. effects. dominate. the. motion. and. energy. instead. of. the.
classical.mechanics,.the.surface-to-volume.ratio.becomes.much.greater,.and.random.
molecular.motion.becomes.more.important..In.contrast.to.molecules,.nanoclusters.
do.not.have.a.ixed.size.or.composition..For.example,.the.vitamin.C.molecule.(see.
Figure.10.1a).contains.six.oxygen,.six.carbon,.and.eight.hydrogen.atoms,.which.are.
placed.at.certain.positions..On.the.other.hand,.platinum.clusters.(see.Figure.10.1b.
and.c).may.contain.any.number.of.constituent.particles.and.present.a.variety.of.mor-
phologies.for.a.ixed.size.
Clusters.can.be.classiied.in.many.different.ways:.they.are.either.homogeneous.
(composed.of.only.one.type.of.atom.or.molecule).or.heterogeneous.(composed.of.
different.types.of.atoms.or.molecules)..They.may.be.neutral.or.charged..They.can.
be.held.together.by.ionic.bonds.as.in.NaCl.clusters,.covalent.bonds.as.in.B.clusters,.
van.der.Waals.forces.as.in.He.or.C 60 .clusters,.or.metallic.bonds.as.in.Au.and.Pt.clus-
ters..Since.the.electrons.in.small.metal.clusters.are.not.as.free.as.electrons.of.bulk.
metals,.the.metallic.bonds.holding.small.metal.clusters.together.are.more.likely.to.
be.covalent.bonds..Another.useful.categorization.of.clusters.is.deined.with.respect.

(a) (b) (c)

FIGURE 10.1 Geometric.structures.of.vitamin-C.molecule.and.two.isomers.of.Pt 13 .clusters..
(a).Vitamin-C,.(b).Pt 13 .(1 st .isomer),.and.(c).Pt 13 .(second.isomer).