Quantum Yield of the Emission of Carbon Quantum Dots: Tricks Due to Solvent Polarity and Excitation Dynamics

Carbon quantum dots (CQDs) are recognized as promising fluorescent nanomaterials possessing high thermal stability, tuned photoluminescence, along with the low cost of their production. Quantum yield (QY) of photoluminescence (PL) of CQDs is known to vary largely depending on the synthetic procedures/ chemical composition of CQDs, but there is still no unique approach for explanation of the nature and QY of PL emission of CQDs. Particularly, an accompanying effect of the solvent environment on QY of the PL emission is paid less attention. In this work, we compare photophysical properties of CQDs synthesized by different methods that resulted in the different composition of CQDs. First, fluoralkylated CQDs (Fluocar® Nano) doped with F, N and O heteroatoms, were synthesized via solvothermal method using urea, anhydrous citric acid and 3-(trifluoromethyl) aniline as precursors. Second, hydrophilic CQDs were obtained by liquid-phase synthesis procedure using sucrose (“standard” CQDs). Depending on the synthesis procedure the PL emission properties were tuned according to the polarity of solvent the CQDs were dispersed in. It was found that QY of PL emission of Fluocar® Nano has a much stronger magnitude and absolute dependence on the solvent polarity compared to “standard” CQDs. Fluocar® Nano demonstrated QY of PL emission as high as 93% in dioxane and as low as 30% in aqueous dispersion, whereas QY of the “standard” CQDs varied within 0.7-2.3% when relative solvent polarities ranged from 0.164 to 1.0; however, their relative changes depending on the solvent polarity were similar (Fig.1). Dynamics of the PL emission during first minute of excitation in the form of PL relaxation to some saturated value was also strong for Fluocar® Nano dispersed in less polar solvents, while the emission was stable in a strongly polar aqueous medium. Tentative conclusion was made that the surface of CQDs plays a crucial role in tuning the PL emission, providing different concentration of surface traps that become gradually fed during excitation and different interactions of CQD and polar solvents, respectively. Figure 1