Source of Bright Near-Infrared Luminescence in Gold Nanoclusters

Abstract

Gold nanoclusters with near-infrared (NIR) photoluminescence (PL) have great potential as sensing and imaging materials in biomedical and bioimaging applications. In this work, Au₂₁(S<i>-</i>Adm)₁₅ and Au₃₈S₂(S<i>-</i>Adm)₂₀ are used to unravel the underlying mechanisms for the improved quantum yields (QY), large Stokes shifts, and long PL lifetimes in gold nanoclusters. Both nanoclusters show decent PL QY. In particular, the Au₃₈S₂(S<i>-</i>Adm)₂₀ nanocluster shows a bright NIR PL at 900 nm with QY up to 15% in normal solvents (such as toluene) at ambient conditions. The relatively lower QY for Au₂₁(S<i>-</i>Adm)₁₅ (4%) compared to that of Au₃₈S₂(S<i>-</i>Adm)₂₀ is attributed to the lowest-lying excited state being symmetry-disallowed, as evidenced by the pressure-dependent antispectral shift of the absorption spectra compared to PL, yet Au₂₁(S<i>-</i>Adm)₁₅ maintains some emissive properties due to a nearby symmetry-allowed excited state. Furthermore, our results show that suppression of nonradiative decay due to the surface "lock rings", which encircle the Au kernel and the surface "lock atoms" which bridge the fundamental Au kernel units (e.g., tetrahedra, icosahedra, etc.), is the key to obtaining high QYs in gold nanoclusters. The complicated excited-state processes and the small absorption coefficient of the band-edge transition lead to the large Stokes shifts and the long PL lifetimes that are widely observed in gold nanoclusters.