Analysis of solid-state luminescence of carbon dots based on photoluminescence property change depending on the interparticle separation

Abstract

The PL mechanism of CDs in concentrated- and solid-state system was studied. First, we investigated the self-quenching origin of CDs by comparing solid-state luminescent CDs and solid-state quenched CDs. From the structural analysis, we confirmed that the content of sp$^2$ domains in CDs is highly related to their self-quenching in solid-state. With the suggestion on their self-quenching mechanism, we provided the synthetic strategy of CDs to reduce or avoid their self-quenching problem. Second, to verify the emission property changes of CDs depending on the interparticle separation, we studied the change in the PL behaviors of CDs by changing their concentration in dispersion system. With dual-color-emissive CDs exhibiting both blue- and red emission under UV light, we observed the concentration-dependent relative intensity change of each emission band. By replacing solvent species and changing pH, we confirmed that the separation of the CD particles plays an important role in determining their dominant emissive state. Finally, the effect of π-conjugated domain on the PL property change during interparticle separation variation was studied using three types of CDs with different degree of carbonization. In dilute dispersion, all the CDs possessed two emissive centers arising from surface- and core-state, but showed surface-state dominant PL features. With the increase of CD concentration, the CDs with higher contents of π-conjugated domain are more likely to change the dominant PL center from surface- to core-state due to the great suppression of surface-derived emission. In the aggregated state, the emission from surface-state was vanished, and the π-conjugated domain contents influenced the emission phenomenon of core-state. Based on the PL decay results, the mechanism on PL property change of CDs for different interparticle separation was suggested. The results of this study would bring meaningful advances in understanding and engineering the PL properties of CDs for extended applications.