Chemistry Reference
In-Depth Information
2 nm
FIGURE 10.2
HREM. image. of. a. truncated. octahedral. Au. cluster. deposited. on. MgO..
(Reprinted.with.permission.from.Pauwels,.B.,.Van.Tendeloo,.G.,.Bouwen,.W.,.Theil.Kuhn,.L.,.
Lievens,.P.,.Lei,.H.,.Hou,.M..
Phys. Rev. B
,.62,.10383,.2000..Copyright.2000.by.the.American.
Physical.Society.)
spectrum.occur.at.“magic.numbers”.due.to.extra.thermodynamic.or.kinetic.stability.at.
these.cluster.sizes,.which.may.be.explained.in.terms.of.the.electronic.or.atomic.pack-
ing.effects.[23]..As.x-ray.diffraction.is.generally.used.to.study.the.size,.structure,.and.
chemical.composition.of.particles.supported.on.surfaces.[24],.electron.diffraction.is.
widely.used.on.free.clusters.to.get.information.on.the.geometry,.average.size,.and.
temperature.[25]..Electron.and.scanning.probe.microscopes.are.powerful.techniques.
for.investigating.nanoparticles..Transmission.electron.microscopy.(TEM).is.useful.
for.studying.metal.nanoclusters.dispersed.onto.organic.substrates..It.is.possible.to.get.
resolution.down.to.the.Angstrom.level.by.high-resolution.TEMs.(HRTEM,.HREM),.
as.shown.in.Figure.10.2.[26]..In.scanning.transmission.electron.microscopy.(STEM),.
an.electron.beam.is.scanned.across.the.sample.[27]..Scanning.electron.microscopy.
(SEM).is.similar.to.STEM,.but.the.image.of.the.sample.surface.is.produced.by.the.
secondary.electrons.emitted.by.the.surface.following.excitation.by.the.primary.elec-
tron.beam.[28]..In.atomic.force.microscopy.(AFM),.a.ine.tip.is.brought.into.close.
contact.with.the.sample.and.senses.the.small.repulsive.force.between.the.probe.tip.
and. the. surface. [29],. while. in. the. scanning. tunneling. microscopy. (STM). the. ine.
tip.is.again.brought.extremely.close.to.the.surface.and.a.voltage.is.applied.between.
the.tip.and.the.sample.so.that.a.tunneling.current.lows.[30]..Since.the.binding.ener-
gies.of.metal.atom.core.electrons.distinguishably.change.with.the.atomic.number,.
high-energy.x-ray.radiation.is.particularly.useful.for.studying.metallic.nanoclusters.
[31]..X-ray.absorption.spectroscopy.(XAS),.extended.x-ray.absorption.ine.structure.
(EXAFS),. near-edge. x-ray. absorption. ine. structure. (NEX-AFS),. x-ray. absorption.
near-edge.structure.(XANES),.x-ray.photoelectron.spectroscopy.(XPS),.and.Auger.
electron. spectroscopy. (AES). are. useful. x-ray. spectroscopic. techniques. for. study-
ing.nanoclusters..Other.spectroscopic.techniques.such.as.ultraviolet-visible.(UV-vis).
[32],. infrared. (IR),. photoelectron,. surface-enhanced. Raman,. Mossbauer,. nuclear.
magnetic.resonance.(NMR),.and.electrochemical.NMR.(EC-NMR).spectroscopies.
are.also.used.for.the.characterization.of.metallic.nanoparticles..In.addition,.magnetic.
properties. of. free. TMC. can. be. studied. by. Stern-Gerlach. delection. experiments.
[33],. whereas. those. of. larger,. surface-supported. clusters. can. be. measured. using.
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