Engineering Photo-Luminescent Centers of Carbon Dots to Achieve Higher Quantum Yields

Abstract

Quantum yield (QY) of carbon dots (CDs) can be enhanced by improving the quality of CDs fabricated. Nonradiative transitions, because of various factors such as nonemissive surface states, defects, and poor choice of ligands, are major deterrents in achieving higher QYs. Such nonradiative transitions can be curtailed by modifying photo-luminescent (PL) centers of CDs with a careful control of synthesis conditions. Synthesis parameters such as reaction time, temperature, and ligand concentration play a major role in the quality of fabricated CDs. In order to understand the effect of reaction temperature and time on PL centers of CDs, carbon dots have been fabricated at temperatures ranging from 150 to 300 °C with reaction duration ranging from 11 to 60 min. White light-emitting CDs require broad full width at half-maximum (FWHM) emission spectra, which is achieved at higher synthesis temperatures and durations, where larger size distribution could be achieved leading to wide FWHM emission spectra (300–700 nm). However, in order to reduce nonemissive surface states, effective passivation of the surface is done by using hexadecylamine (HDA), which is also an auxochrome. HDA not only acts as a surface functionalizing agent, it also acts as a surface passivating agent. Bare CDs without surface functionalization show white light emission. However, blue to green emission is achieved in surface functionalized CDs (SFCDs). Furthermore, by surface functionalization of the CD surfaces, QY of the fabricated SFCDs is enhanced from 4 to 31% with an increase in the HDA/citric acid molar concentration ratio from 0 to 3. Henceforth, some preliminary experiments done on the fabricated CDs indicate that the CDs can successfully be applied as fluorescent markers.