2 research outputs found
ESI-MS Identification of Abundant Copper–Gold Clusters Exhibiting High Plasmonic Character
The
protected noble-metal structures comprising 145 metal-atom
sites and 60 ligands are among the frequently identified larger metal-cluster
systems exploited in many avenues of research. Herein we report a
comparative electrospray ionization-mass spectrometry (ESI-MS) investigation
of the 60-fold thiolated Au<sub>144</sub> and CuAu<sub>144</sub> clusters,
in various positive charge-states, in conjunction with a density-functional
theoretical (DFT) analysis based upon the icosahedral Pd<sub>145</sub>-structure-type applicable to these systems. Samples rich in the
hexanethiolate-protected CuAu<sub>144</sub> clusters are obtained
via a single-phase reduction process. The predicted electronic structure
of the vacancy-centered Au<sub>144</sub>(SR)<sub>60</sub> system provided
a simple rationale for the limiting [4+] charge-state observed of
Au<sub>144</sub>, whereas the maximal [3+] charge detected on the
CuAu<sub>144</sub>(SR)<sub>60</sub> cluster can be explained if the
145th atom occupies the central site. Occupancy of the center-site
stabilizes the superatomic 3S-orbital, and thereby shifts the shell-closing
count from 82 to 84 free electrons. The DFT-calculated energetics
also predicts a strong (0.65 eV) preference for placing the smaller
Cu ion in this central site. Remarkably, the optical absorption spectra
of dilute tetrahydrofuran (THF) solutions feature a broad band centered
near 2.3 eV, in contrast to the previously reported “nonplasmonic”
response of sub-2.0-nm all-gold or -copper clusters. Other methods
(matrix-assisted laser desorption ionization mass spectrometry and
high-resolution electron microscopy) were used to investigate whether
aggregation phenomena might account for this observed plasmon emergence.
This unusual result points to the need to obtain highly purified samples
of copper-doped gold clusters of ca. 145 atoms total