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) 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 similar to 2.2 to similar to 3.8 nm. Increasing the size results in three sets of spectral responses: (1) absorption due to nascent plasmons generated at similar to 340 nm for larger particles, confirmed by Mie theory calculations; (2) significant decreased quantum yield of PL from similar to 7.8% to similar 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 similar to 3.0 to similar to 3.8 nm. This study provides experimental evidence and insights to the transitional state between aluminum nanoclusters to plasmonic nanoparticles, which seems to occur at size larger than gold nanoclusters.

Automated summary

This study demonstrates the size-controlled solution synthesis of ultrasmall aluminum nanocrystals with sizes ranging from about 2.2 to about 3.8 nm. Increasing the size results in three sets of spectral responses: absorption of nascent plasmons generated at around 340 nm, decreased quantum yield of PL, and a drop in fluorescence lifetime, especially when the diameter of aluminum nanoparticles increases from around 3.0 to around 3.8 nm. This provides experimental evidence and insights into the transitional state between aluminum nanoclusters and plasmonic nanoparticles.