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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.

Understanding the fluorescence of complex systems such as small nanocrystals with various surface terminations in solution is still a scientific challenge. Here we show that the combination of advanced time-resolved spectroscopy and ab initio simulations, aided by surface engineering, is able to identify the luminescence centers of such complex systems. Fluorescent water-soluble silicon carbide (SiC) nanocrystals have been previously identified as complex molecular systems of silicon, carbon, oxygen, and hydrogen held together by covalent bonds that made the identification of their luminescence centers unambiguous. The aqueous solutions of molecular-sized SiC nanocrystals are exceedingly promising candidates to realize bioinert nonperturbative fluorescent nanoparticles for in vivo bioimaging, and thus the identification of their luminescent centers is of immediate interest. Here we present identification of two emission centers of this complex system: surface groups involving carbon–oxygen bonds and a defect consisting of silicon–oxygen bonds that becomes the dominant pathway for radiative decay after total reduction of the surface. The identification of these luminescent centers reconciles previous experimental results on the surface and pH-dependent emission of SiC nanocrystals and helps design optimized fluorophores and nanosensors for in vivo bioimaging.