Influence of Doping and Temperature on Solvatochromic Shifts in Optical Spectra of Carbon Dots

Solvatochromic shifts in nitrogen-doped and nitrogen–sulfur-co-doped carbon dots are studied by analyzing absorption, photoluminescence excitation and photoluminescence emission spectra, and their emission lifetimes in two different solvents, protic water (H2O) and aprotic dimethyl sulfoxide (DMSO). We identify three emission bands belonging to the sp2-hybridized core, the edge, and the functional surface groups of carbon dots, as well as surface-attached fluorophores that emit within the edge band energy range. Edge and surface bands show opposite solvatochromic shifts solely depending on the doping heteroatoms. We are able to reproduce emission shifts observed in DMSO by heating CDs in H2O from 7 to 87 °C, when the polarity and hydrogen-bonding strength of the solvent are reduced. Intrinsic edge band transitions are found to be strongly influenced by the solvent polarity, as charge transfer processes dominate. Surface band transitions are found to be influenced especially by hydrogen bonding between the carbon dots and the solvent. Together, these processes lead to characteristic, solvatochromic blue and red shifts of the emission bands. Furthermore, we observe strong emission quenching in the edge band but emission enhancement in the surface band of carbon dots in DMSO. This is attributed to quenched organic fluorophores that are formed during the carbon dot synthesis, leaving only intrinsic edge band emission while the radiative decay in the surface band is enhanced. As a result, the edge band of nitrogen–sulfur-co-doped carbon dots switches from an excitation-independent, fluorophore-like emission to an excitation-dependent emission associated with intrinsic edge states.