Quantum size effects in the optical properties of ligand stabilized aluminum nanoclusters

Here we describe an approach to the synthesis of small ligand stabilized Al nanoclusters by catalytic decomposition of alane using Ti(OⁱPr) ₄ as catalyst. The selected area electron diffraction (SAED) and elemental analysis are consistent with the presence of Al in the clusters. The cluster sizes are measured by the small-angle X-ray scattering method in air-free conditions. The absorption maximum exhibits red shifts when cluster sizes decrease from 4 to 1.5 nm. A two-layer Mie theory model indicates that the electron conductivity in the Al core is reduced due to a combination of quantum size effects and chemical interaction with the ligand shell resulting in the observed red shift with decreasing size. The red shift is shown to scale with the inverse radius in good agreement with a spill-out model. Furthermore, the results are consistent with time-dependent density functional simulations for a small ligand stabilized Al cluster. Remarkably, we find that the absorption maximum is significantly red-shifted compared with that expected from simulations based on the bulk dielectric constant. This is true even for the larger nanoclusters with diameters of 4 nm. This indicates that small ligand protected Al clusters behave significantly different from similar Ag and Au clusters.