Gold-doped silver nanoclusters with enhanced photophysical properties

We detail the characterization of atomically precise, luminescent silver and gold bimetallic nanoclusters (Ag and AgAuNCs) grown in the presence of bidentate lipoic acid (LA, the oxidized form) and dihydrolipoic acid (DHLA, the reduced form) ligands. We found that while doping AuNCs with Ag or Cu precursors using up to a 50% molar fraction (during growth) did not lead to any photoluminescence enhancement, doping of AgNCs with Au resulted in a six-fold enhancement of the PL emission compared to undoped AgNCs. The effect of doping is also reflected in the optical absorption and PL excitation spectra of the gold-doped NCs (AgAuNCs), where a clear blue shift in the absorbance features with respect to the pure AgNCs has been measured. Mass spectrometry measurements using ESI-MS showed that the AgNCs and Au-doped AgNCs had the compositions Ag-29(DHLA)(12) and Ag28Au(DHLA)(12), respectively. The bimetallic nature of the AgAuNC cores was further supported by X-ray Photoelectron Spectroscopy (XPS) measurements. Data showed that the binding energies of the Ag and Au atoms measured from the nanoclusters were shifted with respect to those of the Ag and Au metals. Furthermore, the change in the Ag binding energy was affected by the presence of Au atoms. DOSY-NMR measurements performed on both sets of nanoclusters yielded no change in the hydrodynamic radius measured for either set of NCs when capped with the same ligands.