Solvatochromism Unravels the Emission Mechanism of Carbon Nanodots

Abstract

High quantum yield, photoluminescence tunability, and sensitivity to the environment are hallmarks that make carbon nanodots interesting for fundamental research and applications. Yet, the underlying electronic transitions behind their bright photoluminescence are strongly debated. Despite carbon-dot interactions with their environment should provide valuable insight into the emitting transitions, they have hardly been studied. Here, we investigate these interactions in a wide range of solvents to elucidate the nature of the electronic transitions. We find remarkable and systematic dependence of the emission energy and kinetics on the characteristics of the solvent, with strong response of the photoexcited dots to hydrogen bonding. These findings suggest that the fluorescence originates from the radiative recombination of a photoexcited electron migrated to surface groups with holes left in the valence band of the crystalline core. Furthermore, the results demonstrate the fluorescence tunability to inherently derive from dot-to-dot polydispersity, independent of solvent interactions.

Excitation‐Dependent Photoluminescence from Single‐Carbon Dots

“…The superimposed spectra show good agreement with the ensemble spectrum; the small deviation for green excitation in the red (>600 nm) might arise from the low emission intensity of red emitting particles being too weak to be efficiently detected by single-dot spectroscopy, or from the previously reported local environment dependence of the CD PL. [16,32] d) Distribution of the full width at half maximum (FWHM) (top), and its relation to the emission peak wavelength of all recorded single-CD PL spectra (bottom). Colors indicate the different excitation wavelengths (see the legend).…”

“…[10,13,14] The emission tunability has been most prominently ascribed to selective excitation of subsets of CDs within the CD ensemble. [15][16][17]32] Indeed, multiple studies have shown that it is possible to tune the emission by using different types of CDs that are themselves excitation independent. [18][19][20] On the other hand, Pan et al [11] and Fu et al [21] have suggested that different emission sites within individual CDs are responsible for excitation-dependent emission.…”

Design of Carbon Dots Photoluminescence through Organo-Functional Silane Grafting for Solid-State Emitting Devices

“…The weak solvatochromism of the main emission band is also supporting the hypothesis that most of N atoms are located on the C-dots surface. Formation of carbon nitride core structures is, in fact, generally associated to a strong solvatochromism 30 . The shape of the signal shown in Fig.…”

Effect of Halogen Ions on the Photocycle of Fluorescent Carbon Nanodots

“…As an alternative to an ISC process, quenching could theoretically be due to an electron transfer from the halogen anion to the C-dots. However, this mechanism seems very unlikely considering the nature of the optical transition of these C-dots: as previously reported [10], photoexcitation of these C-dots leads to a charge separation event forming an electron trapped on a surface site (exposed to the environment), while the positive hole stays well-buried inside the core, not exposed to the solvent. Therefore, it seems unlikely that the C-dots are readily available to accept an electron from the anion, supposedly recombining with the positive hole in the core [10].…”

“…In recent years, fluorescent carbon nanoparticles (carbon nanodots-C-dots) have become a very debated topic in nanomaterial science due to their interesting optical characteristics [1][2][3][4]. Beyond their non-toxicity, ease of synthesis, highly functionalized surface, very versatile structural properties [5][6][7][8] and significant two photon absorption properties [9], C-dots usually display an intense fluorescence due to the radiative recombination of an electron with hole after photo-excitation, usually occurring on surface trap sites [1,[10][11][12] although not always [13]. This fluorescence is very sensitive to the external environment and strongly responds to changes in environmental parameters, as for example solvent polarity [10,14], or the presence of molecules or ions in solution [12,[15][16][17], leading to the use of C-dots as sensors in solution phase.…”