Sulfur-incorporated carbon quantum dots with a strong long-wavelength absorption band

In this work, we synthesize sulfur-incorporated carbon quantum dots (S-CQDs) and report the effect of sulfur on their electronic structure. Sulfur provides the density of states or emissive trap states for photoexcited electrons, and hence improves absorbance and photoluminescence intensity in the long-wavelength (similar to 500 nm) regime. The formation of the emissive trap states in the band-gap is confirmed by time-resolved emission decay spectroscopy. It is revealed that the emissive trap states prolong the fluorescence lifetime of low energy (similar to 2.5 eV) photoexcited electrons. To explore further the change in the band-gap energy, we demonstrate charge transport in S-CQD films that serve as the channels of field-effect transistors (FETs). The turn-on voltage of the S-CQD-based FETs decreases with the increase of the sulfur concentration, which is consistent with the optical changes. Our results establish a technical basis to incorporate heterogeneous atoms into CQDs and examine the related changes made to their optoelectronic properties. This method would open up new prospects to control the band-gap energy of CQDs in mild conditions, and hence promote their applications in imaging agents and optoelectronic devices.