Size-Scalable Near-Infrared Photoluminescence in Gold Monolayer Protected Clusters
Patrick J. Herbert, Christopher J. Ackerson, Kenneth L. Knappenberger
Abstract
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.