Nonlinear Optical Investigations in Nine-Atom Silver Quantum Clusters and Graphitic Carbon Nitride Nanosheets

Absorption saturation due to surface plasmon resonance affects the optical limiting efficiency of metal nanoparticles (NPs) by raising the limiting threshold to higher laser fluences. It has been shown that in gold, compared to the larger NPs, smaller quantum clusters (QCs) exhibit better optical limiting with lower limiting thresholds due to the absence of absorption saturation. Here we report optical limiting properties of two novel materials, namely, nine-atom silver (Ag₉) QCs and graphitic carbon nitride (GCN) nanosheets. The relatively large nonlinear absorption of Ag₉ QCs compared to Ag NPs is revealed from open-aperture Z-scan measurements carried out using 532 nm, 5 ns laser pulses. Optical nonlinearity in the QCs arises mostly from free carrier absorption and a relatively weak saturable absorption. The superior limiting efficiency of Ag₉ QCs is complemented by excellent chemical stability, which makes silver quantum clusters ideal candidates for optical limiting applications. The two-dimensional sheet-like structure of GCN is ideal for grafting metals and semiconductors, and we show that even though the nonlinearity of pristine GCN is low it can be improved substantially by grafting lightly with Ag₉ QCs.