Core and Surface Electronic States and Phonon Modes in SiC Quantum Dots Studied by Optical Spectroscopy and Hybrid TDDFT

Abstract

The surface of the SiC quantum dot (QD) is a complex two-dimensional system that comprises fruitful surface reconstruction and passivation which affect its chemistry and photophysics. Here, we report the novel yellow fluorescence (2.19 eV) of the colloidal SiC QDs in the quantum confinement regime, which decays much slowly (lifetime > 70 ns) compared with the previously observed blue-green fluorescence (<15 ns). The semi-empirical and time-dependent density functional theory calculations reveal that it originates from the trapped surface states at the Si═O bond, whose energy levels lie in the forbidden gap of the SiC QD. There exists multiple splitting of the transverse and longitudinal optical phonon modes of the SiC QD, and there is an infrared active fingerprint surface mode belonging to the Si═O bond. The calculations agree well with the experiments. These findings highly improve our understanding of the surface properties of the SiC QDs.