Temperature-dependent photoluminescence of carbon quantum dots

Temperature-dependent photoluminescence (PL) and time-resolved PL (TRPL) of carbon quantum dots (CQD) were investigated to explain the possible source of luminescence. XPS, PL, and UV-Vis absorption measurements suggest that the nitrogen-related functional groups, emission, and absorption depend on the amount of the CQD in the film. The energy loss between the absorption and emission spectra indicates the presence of vibrational relaxation in CQD films. The 0-0 transition peaks of the CQD films exhibit blueshift with faster decay time as temperature increases. This temperature-dependent blueshift and lifetimes possibly indicate the presence of thermally activated hopping sites which was also seen in another organic semiconductors. These hopping sites are related to nitrogen-based functional groups which are confirmed by the presence of additional deconvoluted peaks in the C1s high resolution XPS measurements. These measurements show that the presence of the conjugated functional groups is the primary reason for the exciton diffusion dynamics in the CQD. However, the excessive amount of CQD in the film results increases reabsorption and re-emission process. Thus, limiting the temperature-dependent PL and TRPL behaviors of films with larger amount of CQD.