Molecular origin of photoluminescence of carbon dots: aggregation-induced orange-red emission

Abstract

The molecular origin of the photoluminescence of carbon dots (CDs) is not known. This restricts the design of CDs with desired optical properties. We have synthesized CDs starting from carbohydrates by employing a simple synthesis method. We were able to demonstrate that the CDs are composed of aggregated hydroxymethylfurfural (HMF) derivatives. The optical properties of these CDs are quite unique. These CDs exhibit an excitation-independent PL emission maximum in the orange-red region (λ ∼ 590 nm). These CDs also exhibit excitation as well as monitoring wavelength-independent single exponential PL decay. These observations indicate that only one type of chromophore (HMF derivative) is present within the CDs. Several HMF derivatives are aggregated within the CDs; therefore, the aggregated structure cause a large Stokes shift (∼150 nm). By several control experiments, we showed that the same aggregated chromophore unit (HMF derivative), and not the individual fluorophores, is the fluorescing unit. The emission maximum and the single exponential PL lifetime are independent of the polarity of the medium. The existence of a low-lying trap state could be reduced quite significantly. A model has been proposed to explain the interesting steady state and dynamical photoluminescence behaviour of the CDs. As the molecular origin of their photoluminescence is known, CDs with desired optical properties can be designed.

Towards the photophysical studies of humin by-products

“…Moreover, the excitation spectra at λ em = 410 and 560 nm ( Figure 3B) are almost identical to those observed in the ACN solution for the same λ em which indicates that the same two fluorophores 2 and 3 are present in both solutions. The reported excitation and emission spectra of a highlyconcentrated ACN solution of furfural exhibit similar peaks to those presented in this work, greatly supporting the presence of furfural-based derivatives in our humins samples [27]. Finally, based on the previous results we attribute the emission peak (λem = 375 nm) in the pentane fraction to the same species responsible for the emission peak at λ em = 330 nm in the ACN solution (fluorophore 1).…”

Fluorescent carbon dots from mono- and polysaccharides: synthesis, properties and applications

“…The work by Mandal et al has also recently sought to provide some insights into nanoparticle formation and PL mechanism for sugar-derived CDs [47]. The team studied the reaction between sucrose and H 3 PO 4 to afford excitation-independent orange-red emissive CDs (Ex = 365 nm), which were readily soluble in organic solvents such as DCM and MeCN (Scheme 18).…”

Excitons in Carbonic Nanostructures

“…In a number of brilliant studies, it was shown that, in C-dots formed by thermal treatment of citric acid + amine, the aromatic fluorophores and their associates are observed well before the carbonization occurs [18,[88][89][90][91]. Moreover, the fluorescent C-dots may totally consist of organic dyes [92][93][94]. As we seen below, these facts may suggest the principle of formation of ordered H-aggregated structures.…”