Electronic and vibrational spectroscopic studies of gold-nanoclusters
Gold nanoclusters are a peculiar new material with properties between that of bulk
metal and single atoms. They show size-dependent evolution of optical and electronic
properties sensitive to the change of single atom or single electron in the system.
Understanding the relationship between structure and properties is important to
both fundamental scientific question of the role of the quantum size effects in small
metal-clusters, and to application of nanoclusters in e.g. photonics, nanoelectronics,
and heterogeneous catalysis.
In this thesis four spectroscopic studies of monolayer-protected gold-nanoclusters
are presented. The optical gap of the Au144(PET)60 and (Au/Ag)144(PET)60 (PET:
phenylethylthiolate) nanoclusters were measured by absorption spectroscopy in the
infra-red (IR) region in order to validate the proposed Au144-cluster model, and to
measure the effect of the silver content to the optical gap of the cluster. For pure gold
Au144 optical gap of 2200 cm−1(0.27 eV) was measured in good agreement with the gap
calculated from the model with density functional theory (DFT). For bimetallic Au/Ag-
clusters the gap was found in the range 1150–2100 cm−1(0.16–0.20 eV), indicating the
gap is only moderately affected by doping Au with Ag.
The structure of the Au144(PET)60 single crystals was studied with IR-microscopy. The
IR-spectrum of the single crystals were found to be significantly broadened compared
to the reference PET and Au144(PET)60 in solution. The inhomogeneous broadening
was explained via randomly packed structure of the ligands on the cluster surface in
the crystals.
The model-structure proposed for the Au68(mMBA)32 -cluster (mMBA: meta-mercaptobenzoic acid) obtained from combination of transmission electron microscopy and
DFT was verified with IR-spectroscopy. The measured optical gap (2500 cm−1, 0.31 eV)
of the cluster was smaller than excpected for the cluster size, which is explained by
the disordered structure of the cluster.
In order to quantify the dependence of the solubility of Au102(pMBA)44-nanocluster
(pMBA: para-mercaptobenzoic acid) on the protonation state of the pMBA ligands,
we measured the number of protonated ligands on the cluster at different pH and the
acid constant pKa using acid-base and IR-titration. The pKa was measured to be 6.18,
two units higher than for free pMBA. The cluster was found to be insoluble in water
when less than fifth, and truly water-soluble when more than half of the ligands are in
deprotonated state.
...
Publisher
University of JyväskyläISBN
978-951-39-6516-7ISSN Search the Publication Forum
0357-346XKeywords
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