Experimental and computational insights into luminescence in atomically precise bimetallic Au6−nCun(MPA)5 (n = 0–2) clusters

Abstract

Bimetallic nanoclusters (NCs) have emerged as a new class of luminescent materials for potential applications in sensing, bio-imaging, and light-emitting diodes (LEDs). Here, we have synthesized gold–copper bimetallic nanoclusters (AuCu NCs) using a one-step co-reduction method and tuned the emission wavelength from 520 nm to 620 nm by changing the [Cu²⁺]/[Au³⁺] molar ratio. The quantum yield (QY) increases from 6% to 13% upon incorporation of the Cu atom in the Au NCs. MALDI-TOF mass spectrometric analysis reveals that the composition of the Au NCs is Au₆(MPA)₅, and the bimetallic nanocluster is Au₄Cu₂(MPA)₅, where 3-mercaptopropionic acid (MPA) is used as the capping ligand. Furthermore, we investigated the optimized structures of the as-synthesized NCs using density functional theory (DFT) along with analysis of the preferable adsorption sites using Fukui functions. We report the HOMO–LUMO gap, which is consistent with the experimentally observed red shift in the UV-Vis absorption features of the Au NCs upon copper doping. XPS studies suggest the formation of intermixing of states between the 5d orbitals of Au and the 3d orbitals of Cu in the AuCu NCs after incorporating Cu atoms into the Au NCs, which is corroborated by the DFT calculations on electronic charge transfer from the Cu to the Au atom in the NCs. The coupling between Au(I) and Cu(I) facilitates the formation of a low-lying mixed Au(I)–Cu(I) energy state. This study elaborates on the impact of Cu doping on the excited-state relaxation dynamics of AuCu NCs.