MORPHOLOGY AND ELECTRONIC STRUCTURE OF OXIDIZED SILICON NANOCLUSTERS

摘要

The dependence of quantum size effects on bonding structure in oxidized silicon nanoclusters is established by correlating photoluminescence data with photon-yield electronic structure measurements at the Advanced Light Source. The nanoclusters were synthesized using a laser ablation technique that utilizes a convective He environment to control the size of the particles. After removal from the growth chamber, our ex situ photoluminescence (PL) results indicate that, as the nanoclusters oxidize, the main PL peak moves from 1.83 eV to 1.94 eV in energy. The central focus of the present work is to establish the origin of the main PL peak, and to determine why its energy shifts as the nanoclusters are allowed to oxidize slowly in air. The changes in the morphology and bonding structure of the clusters were established using soft-x-ray fluorescence spectroscopy (SXF) and photon-yield near-edge x-ray absorption fine structure (NEXAFS) spectroscopy, which probe the element-specific density of occupied (SXF) and unoccupied (NEXAFS) states, respectively. Our conclusion is that the as-synthesized nanoclusters consist of a pure, crystalline Si core within a nearly pure SiO_2 shell, with little or no sub-oxides present. As the nanoclusters oxidize, the radius of the crystalline core decreases in size, which gives rise to the change in the position of the PL signal.