Chemistry Reference
In-Depth Information
0.45
0.40
0.35
0.30
3
4
Diameter (nm)
5
FIGURE 11.7
Total.magnetic.moment.per.atom.as.a.function.of.nanoparticle.diameter.(nm).
for.Au
n
.
S-linked.ligands,.whereas.weakly.coupled.linkers.such.as.N.do.not.affect.the.dia-
magnetic.behavior.of.Au.surfaces..In.particular,.it.was.shown.that.for.some.link-
ers. such. as. sulfur,. a. spin. symmetry. breaking. occurs. that. lowers. the. energy. and.
leads. to. preferential. spin. density. localization. on. the. gold. atoms. neighboring. the.
chemisorption. site.. These. preliminary. results. seem. to. be. in. agreement. with. the.
conjecture. proposed. by. Crespo. et. al.. [62]. that. the. interaction. between. S. and. Au.
orbitals. is. responsible. for. the. onset. of. magnetism. in. thiol-capped. gold. nanoclus-
ters..However,.in.order.to.conirm.this.proposal,.similar.calculations.using.the.FPT.
model.should.be.performed.on.larger.clusters.with.core-shell.structures.typical.of.
Au.nanoparticles.
Our.group.has.also.performed.electronic.structure.calculations.on.larger.clusters.
(Au
23
.and.Au
55
).in.order.to.reproduce.and.study.the.magnetic.behavior.of.high.elec-
tric. dipole. moment. thiopolypeptide. R-helix. linkers. (consisting. of. eight. l-glycine.
units),.chemisorbed.on.the.(111).Au.surfaces.[81]..The.wave.function.broken.sym-
metry.method.(BS-UDFT).was.used.for.this.purpose.[82]..The.results.of.this.study.
indicate.a.strong.correlation.between.the.magnetic.behavior.of.the.adsorbate-cluster.
system. and. the. orientation. of. the. electric. dipole. of. the. R-helix. and. charge. trans-
fer.at.the.molecule-metal.interface..Upon.chemisorption,.dipole.moments.may.be.
quenched.or.enhanced.with.respect.to.the.gas.phase.value..The.results.of.this.study.
indicate.that.the.strongest.reduction.in.dipole.moment.accompanied.with.net.charge.
transfer.from.the.Au.surface.leads.to.a.very.stable.magnetic.state..Additionally,.it.
was.found.that.the.magnetic.properties.of.these.systems.are.strongly.dependent.on.
the.size.and.geometrical.structure.of.the.Au.cluster.under.consideration.
In.addition.to.the.cluster.calculations,.we.have.also.studied.the.effect.of.benze-
nethiol.chemisorption.on.the.magnetic.properties.of.gold.ilms.[83].via.DFT.calcu-
lations. on. extended. systems.. Gold. ilms. have. been. modeled. by. constructing. slabs.
from. the. Au. (111). lattice,. containing. from. one. to. four. layers,. with. 3.×.3. atoms. in.
each.layer..Spin-polarized.DFT.calculations.have.been.performed.using.the.SIESTA.
package.[84]..This.code.allows.electronic.structure.calculations.of.extended.systems.
using.pseudopotentials.with.periodic.boundary.conditions,.where.numerical.atomic.
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