Unraveling the Fluorescence Mechanism of Carbon Dots with Sub-Single-Particle Resolution.

Unlike quantum dots, photophysical properties of carbon dots (CDs) are not strongly correlated with particle size. The origin of CD photoluminescence has been related to sp2 domain size and the abundance of oxidized surface defects. However, direct imaging of surface-accessible spatially localized oxidized defects is still lacking. In this work, solvothermal-synthesized CDs are fractionated into different colors by polarity-based chromatography. We then study the mechanism of CD fluorescence by directly imaging individual CDs with sub-particle resolution by STM. Density of states imaging of CDs reveals that the graphitic core has a large bandgap that is inconsistent with observed fluorescence wavelength, whereas localized defects have smaller electronic gaps for both red-emitting dots (rCDs) and blue-emitting dots (bCDs). For individual bCDs within our laser tuning range we directly image optically active surface defects (ca. 1-3 nm in size) and their bandgaps, which agree with the emission wavelength of the ensemble from which the bCDs were taken. We find that the emissive defects are not necessarily the ones with the smallest gap, consistent with quantum yields less than unity (0.1-0.26). X-ray photoelectron spectroscopy and pH-dependent fluorescence titration show that oxygen-containing surface-accessible protonatable functional groups (e.g. phenolic -OH, -COOH) define the chemical identity of the defects. This observation explains why we detect neither long-lived optical excitation of the core nor a correlation between size and emission wavelength. Instead, control over the number of oxygen-containing defects defines the emission wavelength, with more oxidized defects at the surface producing redder emission wavelengths.