Long-lasting green, yellow, and red phosphorescence of carbon dots embedded on ZnAl2O4 nanoparticles synthesized by a combustion method

This work presents the luminescent, structural and morphological properties of carbon dots (CDs) synthesized on ZnAl2O4 (ZAO) by a simple combustion method. The samples were annealed at different temperatures (600 °C, 700 °C, 800 °C, and 900 °C) in order to change the concentration and size of the CDs on ZAO. The XRD patterns indicated that the annealed samples present a cubic phase (spinel structure). According to the TEM analysis, the CDs on ZAO (CZAO) have a particle size in the range of 2.0–4.0 nm. The annealing treatment, allowed to tune their luminescence emission from green (517 nm) to red (650 nm) under excitation with blue light (455 nm). Yellow, orange, and red phosphorescence were observed for the first time from CDs. The green/yellow and orange/red phosphorescence lasted 5 min and 15 min, respectively, which are the longest phosphorescence times reported for CDs. It is proposed that CDs' luminescence originates from the deep levels formed after the annealing treatment. Due to the color tunability presented by the CDs in fluorescence and phosphorescence, the CZAO samples can be useful for applications in biolabeling, bioimaging, lighting, and signaling for buildings and roads.