Chemistry Reference
In-Depth Information
orbitals.are.used.as.basis.sets.to.solve.the.single-particle.Kohn-Sham.equations..Our.
calculations. were. performed. within. the. PBE-generalized. gradient. approximation.
of.DFT,.using.norm-conserving.Troullier-Martins.pseudopotentials.[85].as.imple-
mented. in. the. SIESTA. software.. We. use. a. double-zeta. basis. set. with. polarization.
orbitals.on.all.the.atoms.[86]..The.real-space.mesh.used.for.the.calculation.of.the.
charge.density.and.evaluation.of.real-space.integrals.is.deined.by.a.“mesh.cut-off”.
parameter,.deining.the.maximum.energy.of.plane.waves.that.can.be.represented.on.
the.mesh.without.aliasing..This.parameter.was.set.to.350.Ry.for.all.calculations..The.
supercell.dimensions.were.kept.ixed.for.all.calculations.(
a = b
.=.8.65.Å,.
c
.=.30.Å).
Figure.11.8.depicts.a.plot.of.the.variation.of.the.magnetic.moment.with.the.num-
ber.of.layers.in.the.gold.ilms..As.observed.in.this.plot,.the.total.magnetic.moment.
decays.exponentially.as.the.number.of.layers.increases,.approaching.a.value.of.zero.
for.four.layers..This.magnetic.behavior.has.recently.been.observed.experimentally.in.
gold.ilms.[4]..In.the.case.of.the.four-layer.supercell.capped.with.benzenethiol,.our.
calculations.show.an.onset.of.a.inite.magnetic.moment.of.approximately.0.025.μ
B
.
per.absorbed.molecule..The.fact.that.this.value.is.smaller.than.the.experimental.one.
(on.the.order.of.10.μ
B
;.see.Ref..[64].for.details).is.not.surprising.given.the.limitation.
of.the.size.of.the.unit.cell.used.in.our.calculations.and.also.due.to.the.fact.that.our.
calculations.do.not.explicitly.include.important.spin-orbit.coupling.effects..Finally,.
population.analysis.of.this.system.suggests.that.the.majority.of.“spin.up”.electrons.
are. predominantly. localized. on. the. surface. and. shared. equally. among. the. atoms..
Small.“diamagnetic”.contributions.on.the.bulk.atoms.are.also.found.
Overall,.our.theoretical.calculations.show.that.chemisorbed.thiolates.on.gold.sur-
faces.induce.magnetism,.basically.due.to.a.local.Pauli.repulsion.between.the.sulfur.
and. the. gold. atoms. in. the. neighborhood. of. the. chemisorption. site.. However,. the.
results.also.show.that.bare.Au.clusters.have.an.intrinsic.tendency.to.exhibit.magnetic.
behavior,.suggesting.that.it.is.quite.possible.that.the.origin.of.chemisorption-induced.
magnetism.on.Au.surfaces.might.indeed.be.different.from.a.simple.spin.symmetry.
breaking.on.the.surface.atoms.in.Au.clusters.with.core-shell.structures..It.should.be.
noted.that.despite.the.interesting.results.of.this.study,.other.aspects.of.this.problem.
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1
2 3
Number of layers
4
FIGURE 11.8
Net.magnetic.moment.versus.the.number.of.layers.in.the.supercell.
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