posted on 2021-08-04, 16:13authored byPatrick
J. Herbert, Christopher J. Ackerson, Kenneth L. Knappenberger
Near-infrared photoluminescence of
a series of three gold monolayer
protected clusters (MPCs) with volumes spanning 50–200 Å3 was studied by using variable-temperature photoluminescence
(VT-PL) spectroscopy. The three MPCs, which included Au20(SC8H9)15-diglyme, Au25(SC8H9)18, and Au38(SC12H25)24, all exhibited temperature-dependent
intensities that reflected a few-millielectronvolt energy gap that
separated bright emissive and dark nonradiative electronic states.
All clusters showed increased PL intensities upon raising the sample
temperature from 4.5 K to a cluster-specific value, upon which increased
sample temperature resulted in emission quenching. The increased PL
in the low-temperature range is attributed to thermally activated
carrier transfer from dark to bright states. The quenching at elevated
temperatures is attributed to nonradiative vibrational relaxation
through Au–Au stretching of the MPCs metal core. Importantly,
the results show evidence of a common and size scalable metal-centered
intraband PL mechanism that is general for ultrasmall metal nanoclusters,
which are expected to show nonscalable optical properties.