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Not AvailableCarbon dots have received considerable attention due to their tunable emission. Single-particle techniques revealed that individual top-down and bottom-up green carbon dots can support several chromophores. In particular, several studies demonstrated that bottom-up synthesized carbon dots are typically made of amorphous carbon and are multichromophoric but may also just be chemically impure, with free dye in solution or polymerized in a carbon matrix. Carbon dots made by top-down precursors, however, are highly graphitic and more often single-chromophoric, begging the question if carbon dots made from bottom-up precursors could have similar optical properties compared to their top-down counterparts, if properly purified. Here, we compare green-emitting carbon dots made by two methods: top-down by chemical oxidation and bottom-up from small-molecule precursors in a solvothermal synthesis followed by rigorous purification. Such dots have cores of different crystallinity, but both types have oxidized surfaces. Just as ensemble absorption and emission spectra show only subtle differences, we find based on single-particle emission imaging that both types of carbon dots contain similar weights of carbon dots with single and multiple chromophores. Surprisingly, the carbon dots are optically similar, despite coming from opposing synthetic approaches. Although the majority of all carbon dots are single-chromophoric, top-down carbon dots are found to more likely have only one emitting chromophore, whereas bottom-up carbon dots are comparatively more multichromophoric. In the multichromophoric case, bottom-up carbon dots have on average a greater number of chromophores than top-down carbon dots. Our results showing that very differently made carbon dots with different structural properties exhibit strikingly similar emission properties reveal the important insight that out of structural heterogeneity emerges spectroscopic homogeneity. 

Time-resolved spectroscopy of plasmonic nanoparticles is a vital technique for probing their ultrafast electron dynamics and subsequent acoustic and photothermal properties. Traditionally, these experiments are performed with spectrally broad probe beams on the ensemble level to achieve high signal amplitudes. However, the relaxation dynamics of plasmonic nanoparticles is highly dependent on their size, shape, and crystallinity. As such, the inherent heterogeneity of most nanoparticle samples can complicate efforts to build microscopic models for these dynamics solely on the basis of ensemble measurements. Although approaches for collecting time-resolved microscopy signals from individual nanoparticles at selected probe wavelengths have been demonstrated, acquiring time-resolved spectra from single objects remains challenging. Here, we demonstrate an alternate method that efficiently yields the time-resolved spectra of a single gold nanodisk in one measurement. By modulating the frequency-doubled output of a 96 MHz Ti:sapphire oscillator at 8 kHz, we are able to use a lock-in pixel-array camera to detect photoinduced changes in the transmission of a white light continuum probe derived from a photonic crystal fiber to produce broadband femtosecond transmission spectra of a single gold nanodisk. We also compare the performance of the lock-in camera for the same single nanoparticle to measurements with a single-element photodiode and find comparable sensitivities. The lock-in camera thus provides a major advantage due to its ability to multiplex spectral detection, which we utilize here to capture both the electronic dynamics and acoustic vibrations of a single gold nanodisk following ultrafast laser excitation.