Electronic and vibrational spectroscopic studies of gold-nanoclusters

Abstract

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.