The “ON-OFF” microwave reaction time technique: A novel strategy to impact the fluorescence of multi-color emissive carbon dots

Multi-colored fluorescent carbon dots (CDs) were synthesized via the microwave-assisted technique using citric acid (CA). To the best of our knowledge, the “ON-OFF” microwave-assisted technique was investigated for synthesis CDs for the first time. The impact of the “ON-OFF” strategy was monitored on the morphological, structural, and optical properties of the CDs. By increasing the reaction time from 10 to20 min, a reduction in the particle size was detected from 4.1 ± 0.6 to 1.6 ± 0.2 nm, respectively using a high-resolution transmission electron microscope (HRTEM). A blue shift was detected in the photoluminescence (PL) as a function of the reaction time with optimal fluorescence intensity at 15 min. The quantum yields (QY) were 15.2 %, 22.2 %, and 11.9 % for the reaction time 10, 15, and 20 min, respectively. Three electronic transitions were observed from the UV-Vis spectra, and their corresponding energy band gap (Eg) were calculated using Tauc plot. The first transition was a reaction time-dependent in the range of 5.1–5.8 eV. While the other transitions were reaction time-independent and kept constant at 4.9 and 4.6 eV. A model was hypothesized to correlate the particle size reduction with the reaction time during the formation of the CDs.