Encapsulation of Carbon Dots in Silica Matrices Offers Narrow Emission in the Solid-State of Printed Fluorescent Inks

Narrow emission of carbon dots (CDs) in their solid state is indispensable in most optoelectronic applications, but its realization is hindered by fluorescence quenching and spectral broadening in the solid state. We synthesized CDs with a small full width at half-maximum of ∼30 nm and a high quantum yield of ∼80%, but their fluorescence is completely quenched in the solid state. To develop a proper strategy to avoid solid-state quenching and preserve narrow emission, we first investigated the structure and fluorescence origin of CDs. We identified that a CD consists of π-conjugated domains with oxygen-containing functional groups. Among them, carbonyl groups attached to a carbon core are found to play a major role in narrow orange emission. Considering the fluorescence origin, CDs were encapsulated in silica matrices by reacting a silane precursor with surface hydroxy groups that do not contribute to the emission process. It enables CDs in the uniform dispersion in silica to preserve their narrow emission in the solid state by avoiding aggregation. The encapsulated CD dispersion was used as fluorescent ink for optoelectronic applications. By the silk screen method and inkjet printing, the selected patterns were printed, and the patterns well preserved the narrow emission after solvent drying, demonstrating a potential of CDs in fluorescent ink for various optoelectronic applications.