Unveiling Carrier Relaxation Mechanism in Protonated/Deprotonated Carbon Dots and Their Solvent Effects via Ultrafast Spectroscopy

The intricate nature of the surface structure of carbon dots (CDs) hinders a comprehensive understanding of their emission behavior. In this study, we employ two types of CDs created through acid-alkali treatments, one with surface protonation and the other with surface deprotonation, with the objective of investigating the impact of these surface modifications on carrier behavior using ultrafast spectroscopy techniques. TEM, XRD, FTIR and Raman spectra demonstrate the CDs' structure, featuring graphitic core and abundant surface functional groups. XPS confirms the successful surface modifications of CDs via protonation and deprotonation. Ultrafast transient absorption (TA) spectroscopy reveals that deprotonation modification may decelerate the relaxation process, thereby increasing the visible PL quantum yields (PLQY). Conversely, protonation may accelerate the relaxation process due to the induced low-energy absorption band, resulting in self-absorption and reduced PLQY. Furthermore, TA analysis of CDs in mixed solvents with different proportions of ethanol shows the beneficial effect of ethanol in decelerating the relaxation process, leading to an increased PLQY of 33.7 % for deprotonated CDs and 22.1 % for protonated CDs. This study illuminates the intricate relationship between surface deprotonation/protonation modifications and carrier behavior in CDs, offering a potential avenue for the design of high-brightness CDs for diverse applications.