Understanding the Fluorescence Mechanism of Carbon Dots through Single-Molecule Excited-State Imaging*

We employed scanning tunneling microscopy (STM) and single molecule absorption STM (SMA-STM) to study the fluorescence mechanism in carbon dots (Cdots). The red- and blue-fluorescing Cdot samples were previously purified and characterized in bulk with standard spectroscopic techniques. Carbon dots were deposited on a thin hybrid PtAu film over a sapphire substrate via solvent-based aerosol deposition. We first performed current imaging tunneling spectroscopy (CITS) to resolve the spatial electronic structure of the adsorbed Cdots, estimating the quantity of defects, whether they were active or inactive as optical absorbers, and measuring energy levels of dopants. Then we applied SMA-STM to spatially resolve electronically excited defects, and used scanning tunneling spectroscopy (STS) in situ to look at the density of states at these locations during laser excitation and otherwise. We observed significant spatial variations in the band structure and detected increased defect quantity as fluorescence red-shifts. We further identified the energy levels involved in fluorescence and ultimately discuss the most plausible fluorescence mechanism, which does not depend on the size of the Cdot.