Fluorine-Passivated Silicon Nanocrystals: Surface Chemistry versus Quantum Confinement

The technological importance of silicon nanocrystals (Si NCs) largely originates from their remarkable optical properties. However, the detailed mechanisms for the optical behavior of Si NCs remain controversial. The controversy mainly centers on the interplay between quantum confinement and surface chemistry. By using a model system of fluorine (F)-passivated Si NCs in the framework of density functional theory, we demonstrate how both quantum confinement and surface chemistry impact the optical properties of Si NCs with sizes (diameters) < 2 nm. It is found that the effect of surface chemistry on both the excitation energy and emission energy of a Si NC becomes more significant as F coverage at the NC surface increases. For a Si NC whose size changes from 1.7 to 1.4 nm at both ground state and excited state, high F coverage (>50%) increasingly enables the effect of surface chemistry to prevail over that of quantum confinement. For a very small Si NC (<1.4 nm) at ground state, however, the quantum confinement effect is dominant no matter how large F coverage is. At the excited state, F coverage actually helps to confine excitons inside the very small Si NC. We show that medium F coverage leads to enhanced radiative recombination for Si NCs in most cases. But, 100% F coverage weakens the radiative recombination of Si NCs. The current work urges that surface chemistry must be taken into account together with quantum confinement in the analysis of the optical behavior of Si NCs.