The Influence of Pd-Atom Substitution on Au<sub>25</sub>(SC<sub>8</sub>H<sub>9</sub>)<sub>18</sub> Cluster Photoluminescence
Patrick J. Herbert, Marcus A. Tofanelli, Christopher J. Ackerson, Kenneth L. Knappenberger
Abstract
The photoluminescence (PL) of Au25(SC8H9)18 and Au24Pd(SC8H9)18 was investigated using variable-field magnetic circular photoluminescence (VH-MCPL) spectroscopy. The comparison of low-temperature (4.5 K) PL spectra measured for both nanoclusters revealed an ≈40 meV blueshift of intraband emission upon Pd substitution. Compared to that for the the Au25(SC8H9)18 cluster, the degree of circular-polarization for the Au24Pd(SC8H9)18 cluster increased for the low energy (<1.65 eV) portion of the PL spectrum, but decreased for the higher energy (>1.65 eV) spectral region. MCPL spectra included three distinct components, underlying the global PL spectrum, for both clusters. Variable-field analysis of each MCPL component revealed an increase in spin–orbit coupling (Landé g-factor) magnitudes for radiative transitions of the Pd-substituted cluster with respect to Au25(SC8H9)18. Variable-temperature (VT) PL spectroscopy yielded reduced integrated global PL intensity and increased electron-vibrational coupling for Au24Pd(SC8H9)18, as compared to Au25(SC8H9)18. The results indicate that Pd-substitution for Au in Au25(SC8H9)18 results in increased angular momenta for metal–metal intraband transitions; the Landé g-factor for these transitions increased by ≈55% upon substitution, as compared to an approximate 19% increase for ligand-based transitions. The increased angular momentum translates to a 30% increase in electron–phonon coupling constants for intraband transitions, but only a 3% increase for ligand-based transitions. As a result, Pd substitution leads to less efficient metal–metal (intraband) radiative emission for Au24Pd (SC8H9)18 than for Au25(SC8H9)18.