Structural and optical properties of ligand-free noble metal nanoclusters synthesized by ns-pulsed laser ablation at the argon-water interface.

Fully inorganic ligand-free noble metal nanoclusters (NCs) are a challenging system to obtain due to the fact that conventional chemical synthesis introduces surface-bound contaminants. Here, we present a one-step method for synthesizing highly pure, photoluminescent gold (Au) and silver (Ag) NCs by using Pulsed Laser Ablation at the argon-water interface to ensure an entirely CO2-free ablation environment. The colloids generated contain a mixture of larger plasmonic nanoparticles and small photoluminescent NCs that have been separated by centrifugation to yield ultrasmall NCs with a mean radius of about 1.8 nm (Au) and 1.4 nm (Ag). Comprehensive analysis by means of TEM, HRTEM, XRD, XPS, surface-enhanced Raman spectroscopy (SERS), and steady-state and time-resolved photoluminescence was performed to assess the absence of carbonaceous contaminants and to give a clear structural, optical, and chemical characterization of the two systems. XPS measurements demonstrated that the surface of the nanostructures exhibited predominantly metallic contributions. However, the silver nanostructures exhibited approximately 20% oxidation, while the gold nanostructures exhibited approximately 4%. Both Au and Ag NCs exhibit similar photoluminescence in both the UV and visible (blue) regions, with Quantum Yields (QY) around 1.2% (Au) and 2.0% (Ag) in the blue region. Time-resolved measurements reveal nanosecond-scale decays with two components: a fast decay of 2-3 ns and a slower decay of 6-9 ns. Notably, the fast component contributes ≈80% of the decay for Au NCs, but only ≈50% for Ag NCs, indicating differences in excited-state relaxation pathways specific to the metal, probably associated with the different degree of oxidation. Surprisingly, UV-Vis absorption spectroscopy reveals a strong absence of the conventional Localized Surface Plasmon Resonance (LSPR) band, showing instead a monotonic, molecular-like interband absorption profile. We hypothesize that this anomalous suppression of the LSPR peaks may be linked to unique features of the PLAL process, such as surface oxidation and a polycrystalline structure, which may artificially trigger quantum-size effects in 3.5 nm particles by confining the electronic response within subcrystalline grains. These fully inorganic NCs remain stable in water, providing a clean platform for understanding the fundamental properties of light behavior in noble metal nanostructures.