Direct Observation of Transition from Solid-State to Molecular-Like Optical Properties in Ultrasmall Silicon Carbide Nanoparticles

We employ time-dependent photoluminescence (PL) and steady-state PL excitation (PLE) measurements to study the size-dependent optical properties of ultrasmall silicon carbide (SiC) nanoparticles (NPs). We find that the nature of the optical transition transforms from solid-state indirect gap to molecular-like as the diameter of spherical SiC NPs is reduced from 4-6 to 1-3 nm with a smooth transition in between. We deduce the radiative lifetimes of SiC NPs that are well supported by ab initio time-dependent density functional theory calculations on realistically large SiC NPs with realistic surface terminations, including the solvation effects.