Synthesis and modulation of the optical properties of carbon quantum dots using microwave radiation

Carbon quantum dots have exhibited highly fluorescent characteristics as nanomaterials. Soluble in water and easily synthesized by multiple simple techniques, there are immense fabrication possibilities by permuting their properties via changing precursors, synthesis route, reaction parameters, etc. As economic and environment-friendly seed material, they are being viewed as an alternative to conventional fluorescent materials in myriad of applications including displays, cancer detection, drug delivery carriers in biomedicine, absorbing material in photovoltaics, etc. In this work, the hydrophilic carbon quantum dots were synthesized from the aqueous solution of citric acid and urea through microwave radiation for varying heat durations. The method is facile, faster and friendly to the environment without any need for high temperature and complicated chemical techniques. It was observed that the bandgap of the fabricated carbon quantum dots and its optical properties namely absorbance, photoluminescence enhanced with an increase in exposure of samples to heat up to an optimum limit, owing to the increase in density of states. However, further exposure to heat for longer duration degraded the absorbance and bandgap while photoluminescence gets saturated. Stokes' shift revealed that all the synthesized carbon quantum dots possess stable emission. This was reconfirmed from consistent emission peak positions under varying excitation in the samples. The absorbance and PL spectrum exhibited by the synthesized dots makes it a suitable material for boosting the performance of organic solar cell.