Aluminum nanocrystals evolving from cluster to metallic state: Size tunability and spectral evidence

Sub-3 nm aluminum (Al) nanocrystal is an emerging class of nanomaterial with properties distinct to noble metal nanoclusters. The complete solution synthesis of aluminum nanoclusters was recently reported, and their photoluminescence (PL) was observed for the first time. At the moment, there exists no method to tune the size of ultrasmall aluminum nanocrystals in solution thus no knowledge on the boundary state between aluminum nanoclusters to plasmonic nanoparticles. In this work, it is demonstrated a study of size-controlled solution synthesis of ultrasmall aluminum nanocrystals with size controlled between ~ 2.2 and ~ 3.8 nm. Increasing the size results in three sets of spectral responses: (1) absorption due to nascent plasmons generated at ~ 340 nm for larger particles, confirmed by Mie theory calculations; (2) significant decreased quantum yield of PL from ~ 7.8% to ~ 2.4%, indicating reduced quantum confinement effects and increased metallicity; (3) drop of fluorescence lifetime was observed, especially when the diameter of aluminum nanoparticles was changed from ~ 3.0 to ~ 3.8 nm. This study provides experimental evidence and insights to the transitional state between aluminum nanoclusters and plasmonic nanoparticles, which seems to occur at size larger than gold nanoclusters.