Ultrafast nanometric imaging of energy flow within and between single carbon dots

Carbon dots are promising fluorescent nanomaterials, but their fluorescence yield is fairly low when measured in the bulk. By observing single dots directly using femtosecond single-nanometer resolution imaging, we find that some carbon dots fluoresce exceedingly well and others not at all. We propose a simple mechanism, which suggests that near-perfect fluorescence yield is possible by improved separation or synthesis, as indeed quantum yields of carbon dots have been improving.Time- and space-resolved excited states at the individual nanoparticle level provide fundamental insights into heterogeneous energy, electron, and heat flow dynamics. Here, we optically excite carbon dots to image electrontextendashphonon dynamics within single dots and nanoscale thermal transport between two dots. We use a scanning tunneling microscope tip as a detector of the optically excited state, via optical blocking of electron tunneling, to record movies of carrier dynamics in the 0.1textendash500-ps time range. The excited-state electron density migrates from the bulk to molecular-scale (1 nm2) surface defects, followed by heterogeneous relaxation of individual dots to either long-lived fluorescent states or back to the ground state. We also image the coupling of optical phonons in individual carbon dots with conduction electrons in gold as an ultrafast energy transfer mechanism between two nearby dots. Although individual dots are highly heterogeneous, their averaged dynamics is consistent with previous bulk optical spectroscopy and nanoscale heat transfer studies, revealing the different mechanisms that contribute to the bulk average.All study data are included in the article and/or SI Appendix.