Intrinsic and Extrinsic Fluorescence in Carbon Nanodots: Ultrafast Time‐Resolved Fluorescence and Carrier Dynamics

Abstract

Using a femtosecond resolved up‐conversion technique, the ultrafast carrier dynamics in fluorescent carbon nanodots are investigated in order to shed light on the mysterious origins of their fluorescence. These experiments reveal that the fluorescence of carbon nanodots consists of two spectral overlapped bands that can be ascribed to the intrinsic and extrinsic fluorescence. The intrinsic band exhibits a small bandwidth of 175 meV at 459 nm, and it is attributed to the sp2 nano domains. The extrinsic band originates from the surface states with a much broader bandwidth of 450 meV. The relaxation with time constants of a few picoseconds, tens of picoseconds, and a few nanoseconds are attributed to optical phonon scattering, acoustic phonon scattering, and carrier (e–h pair) recombination, respectively. A fast trapping is observed from the nano domains into the surface states with a time constant of 400 fs. The excitation wavelength‐dependent fluorescence can arise from the abundant carboxyl functional groups on the surface.

Electron–hole recombination dynamics in carbon nanodots

“…To explore the carrier dynamics in C-dots, the evolution of the picosecond time-resolved PL spectra of the as-prepared C-dots was monitored over a wavelength range of 325 to 465 nm at excitations of 273, 300, 343, and 369 nm. Each decay curve is fitted with a triple-exponential function; the best-fit parameters and average lifetimes are listed in Table 1 The Contrary to previous studies [17,18], this work shows that the average lifetimes detected at the fluorescence peak of 325 nm are longer than those detected at the fluorescence peak of 405 nm, when excited at 273 and 300 nm. This strongly indicates that the dual emissions centered at 325 and 405 nm do not originated from the combination of the intrinsic recombination radiation of carbogenic core and the relaxation of carriers onto the surface state.…”

“…surface states) [16][17][18][19][20]. Due to the relaxation of photogenerated carriers from the carbogenic core onto the surface states, the recombination lifetime progressively lengthens with increasing emission wavelength [18,21,22]. However, the fluorescence of C-dots is more complex than this simplified mechanism and it cannot explain several spectroscopic properties of C-dots, e.g.…”

Tailoring color emissions from N-doped graphene quantum dots for bioimaging applications

“…Yang et al provided further evidence from transient spectra. A charge separation could exist between sp 2 cluster and defect states, leading to a defect state luminescence in a dominated role for green emission GQDs 20,21,25,26 . However, there is rare report on the synthesis of highly luminescent GQDs with tailoring color emissions like inorganic nanocrystals.…”

Ultrafast optical spectroscopy of surface-modified silicon quantum dots: unraveling the underlying mechanism of the ultrabright and color-tunable photoluminescence

“…Ultrafast spectroscopy is a powerful tool for investigating the electronic structure, excitonic properties and carrier dynamics for various new types of fluorescent nanomaterials, for example, graphene oxide, 35,36 graphene quantum dots (QDs), 37 carbon nanodots, 38,39 polymer nanoparticles, 40 direct bandgap semiconductor QDs 41243 and Si NCs. 44249 In our previous work, we produced novel Si NCs with unprecedented ultrabright fluorescence and single-exponential decay.…”