Carbon dots are cost-effective, environmental friendly, and biocompatible nanoparticles with many potential applications in optoelectronics and biophotonics. Their dual fluorescence bands were observed and could be attributed to core and surface state emission. We also conduct temperature-dependent fluorescence measurements from cryogenic to room temperatures. The dual emission bands exhibit similar temperature dependence. The strong electron–electron interactions and weak electron–phonon interactions could account for the very broad photoluminescence (PL) band even at 77 K. Our experimental results also suggest that carbon dots exhibit similar temperature behavior as metallic quantum dots (nanoclusters) but are different from inorganic semiconductor quantum dots. Here, for the first time, we present the temperature-dependent spectroscopic results to shed some light on the presently unclear fluorescence mechanism.

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.

Fluorescent Au 25 nanoclusters recently have drawn considerable research interest due to their unique properties and potential applications. Despite significant advances in their synthesis methods and application development, the origin of the fluorescence and underlying mechanism still remain unclear. In this work we investigate the fluorescence dynamics in BSA-protected Au 25 nanoclusters by time-resolved photoluminescence and transient absorption techniques covering picosecond to microsecond time scales. We demonstrate here that the red fluorescence consists of both prompt fluorescence and thermally activated delayed fluorescence, and the latter is more dominant. A small energy gap of 30 meV between the triplet and the singlet states was determined from our temperature-dependent time-resolved fluorescence measurement. Moreover, we elucidate that the absorption band at 2.34 eV corresponds to the HOMO− LUMO transition in this system due to the interaction between Au 25 NCs and BSA. We also show that an effective relaxation pathway exists from the higher excited state to the LUMO.

In this work, we investigated the temperature-dependent luminescence of bovine serum albumin-protected Au 25 nanoclusters and the correlation with their structure. Our experiments reveal that the red luminescence consists of two bands, namely, band I at 710 nm and band II at 640 nm. The temperature dependence of band I exhibits similarity to semiconductors, such as a red shift of emission and bandwidth broadening upon increasing temperatures due to electron−phonon and electron−defect/surface scattering. In contrast, band II exhibits different temperature dependence. It is concluded that band I exclusively originates from the icosahedral core of 13 Au(0) atoms and band II dominantly arises from the [−S−Au(I)−S−Au(I)−S−] staples. Moreover, with increasing temperatures, the intensity of band I and band II decreases. A similar activation energy was extracted, which is attributed to thermally activated defect/surface trapping. In addition, the relaxation from band II to band I was found to be inactive from 300 K down to 77 K.

We report here the temperature dependence studies of the fluorescence of histidine-protected Au 10 nanoclusters. We conclude from the experiments that the process involves thermally activated nonradiative trapping with an activation energy of 103 meV. At increased temperatures, the peak energy of fluorescence was found to exhibit a small blue shift, largely due to lattice torsional fluctuation, whereas the electron−phonon scattering plays a minor role. Moreover, the bandwidth of fluorescence also increased slightly as a result of enhanced surface/defect/impurity scattering. We suggest that electron−electron interactions and surface/ defect/impurity scattering play dominant roles in such a Au 10 nanocluster.