Excitation‐Dependent Photoluminescence from Single‐Carbon Dots

Abstract

Carbon dots (CDs) are carbon-based fluorescent nanoparticles that can exhibit excitation-dependent photoluminescence (PL) "tunable" throughout the entire visible range, interesting for optoelectronic and imaging applications. The mechanism underlying this tunable emission remains largely debated, most prominently being ascribed to dot-to-dot variations that ultimately lead to excitation-dependent ensemble properties. Here, single-dot spectroscopy is used to elucidate the origin of the excitation-dependent PL of CDs. It is demonstrated that already single CDs exhibit excitation-dependent PL spectra, similar to those of the CD ensemble. The single dots, produced by a facile one-step synthesis from chloroform and diethylamine, exhibit emission spectra with several characteristic peaks differing in emission peak position and spectral width and shape, indicating the presence of distinct emission sites on the CDs. Based on previous work, these emission sites are related to the sp subregions in the carbon core, as well as the functional groups on the surface. These results confirm that it is possible to integrate and engineer different types of electronic transitions at the nanoscale on a single CD, making these CDs even more versatile than organic dyes or inorganic quantum dots and opening up new routes toward light-emission engineering.

Excitons in Carbonic Nanostructures

“…The features characterizing heterogeneity in ensembles should be the result of the summation of the individual contributions. In another study [101], it was shown that intra-particle heterogeneity is still possible. However, it may be the result of forming associations among nanoparticles since they are able to aggregate (see Section 3.1).…”

“…Orange-red emission was also reported with the shift of excitation to longer wavelengths together with the strong decrease in intensity. This fact can be attributed to the heterogeneity of emitters [100] that appear on inter-particle [31] or intra-particle [101] levels. However, quite recently, scientists achieved strong emission in this long-wavelength region by variation of precursors and experimental conditions.…”

“…The possibility of selectively bleaching one of these emissive forms due to the fact of its much lower photostability was shown [161], as well as its quenching of inter-particle interactions [162]. Single-particle studies [101] have demonstrated that different resolved spectral forms can be attributed to individual particles, suggesting the possibility of integrating different types of electronic transitions within the nanoscale composition. Transition from one emissive form to another may also proceed in the course of synthesis and as a function of reaction temperature and, thus, it can be thought that carbon particles initially form an association of molecular fluorophores and then they attain the carbonic core with a dramatic change of the emission [88,91,163].…”

Carbon Nanodots: A Review—From the Current Understanding of the Fundamental Photophysics to the Full Control of the Optical Response

“…At the spectroscopic level, the large inhomogeneity of most CD samples is pointed out by many experimental pieces of evidence, such as: broad unstructured absorption or emission bands, non-exponential kinetic traces in fluorescence decay [180], and differences between ensemble and single-molecule experimental results [191]. Its structural origin has been related to several factors as a chemical variability of CDs surface structure or charge from dot to dot [106,192], broad distribution of the size or shape irregularity [62,92,170], variability of the density of emitting states on different dots [89], or presence of different types of CD aggregates in solution [193].…”

Molecular imaging with nanoparticles: the dwarf actors revisited 10 years later

“…These heterogeneities hinder analysis of the photoluminescence mechanisms of the dots, by causing inconsistencies in the observational data. In 2017, it was reported that single-dot spectroscopy demonstrated the presence on a single dot of a range of different emissive sites allowing generation of a whole spectrum of photoluminescence wavelengths from that single dot, and matching in the single dot the excitation-dependent spectra resembling those of the population of dots in the batch (van Dam et al 2017). It is to be expected that inconsistent data might be generated from even a single dot, or from a two slightly different dots.…”