sp²–sp³-Hybridized Atomic Domains Determine Optical Features of Carbon Dots

Abstract

Carbon dots (CDots) are a promising biocompatible nanoscale source of light, yet the origin of their emission remains under debate. Here, we show that all the distinctive optical properties of CDots, including the giant Stokes shift of photoluminescence and the strong dependence of emission color on excitation wavelength, can be explained by the linear optical response of the partially sp2-hybridized carbon domains located on the surface of the CDots’ sp3-hybridized amorphous cores. Using a simple quantum chemical approach, we show that the domain hybridization factor determines the localization of electrons and the electronic bandgap inside the domains and analyze how the distribution of this factor affects the emission properties of CDots. Our calculation data fully agree with the experimental optical properties of CDots, confirming the overall theoretical picture underlying the model. It is also demonstrated that fabrication of CDots with large hybridization factors of carbon domains shifts their emission to the red side of the visible spectrum, without a need to modify the size or shape of the CDots. Our theoretical model provides a useful tool for experimentalists and may lead to extending the applications of CDots in biophysics, optoelectronics, and photovoltaics.

Excitons in Carbonic Nanostructures

“…Commonly, fluorescent carbon nanoparticles are characterized by a high amount of carbon-carbon bonding, predominantly in sp 2 hybridization of the graphene type [55], but also by the features of disordered or diamond-type sp 3 hybridized structures [1,56]. This immediately suggests that the π-electronic conjugation must exist along their graphene sheets, but it may be irregular and disrupted into small fragments [57].…”

“…Graphene [116,117] or graphene oxide [118,119] were found to be very efficient in quenching all the emission of the dyes attached to them. Meanwhile, it is known that the fluorescent moiety of C-dots demonstrates strong electron-donor ability when interacting with external acceptors [55,120]. Moreover, their emission demonstrates the electron transfer quenching on interaction with other carbon materials such as graphene oxide and carbon nanotubes [121].…”

“…But their synthesis based on well-defined molecular precursors is preferable, since it is the way to achieve functional carbon materials with controlled surface chemistry, mesoscopic morphology, and microstructure [70]. Commonly studied C-dots are formed by carbon-carbon bonding predominantly in the graphene-type sp 2 hybridization [55], but they reveal a relatively high amount of the diamond-type sp 3 hybridization (or disorder) of carbon atoms [1,56]. This suggests that the π-electronic conjugation must exist along the graphene sheets, but this conjugation may be irregular, especially in view of the presence of disrupting hydroxyl and carbonyl groups [57].…”

“…Regarding the sizes of these particles, they vary from several to tens of nanometers and may exhibit broad variation in shapes. The structures of spherical (or almost spherical) shapes are usually considered to have zero dimensionality (0-D) which gave them the name "dots" [53] even if they are the fragments of two-dimensional structures of graphene (G-dots) [33,54] or graphene oxide (GO-dots) [12].Commonly, fluorescent carbon nanoparticles are characterized by a high amount of carbon-carbon bonding, predominantly in sp 2 hybridization of the graphene type [55], but also by the features of disordered or diamond-type sp 3 hybridized structures [1,56]. This immediately suggests that the π-electronic conjugation must exist along their graphene sheets, but it may be irregular and disrupted into small fragments [57].…”

Self-assembly of carbon based nanoparticles films by Langmuir-Blodgett method

“…[127][128][129][130][131] There are characterized by the following properties: quasi-spherical shape with lateral dimension smaller than 10 nm, amorphous structure-dominantly composed of sp 3 hybridized carbon, good chemical stability, tunable photoluminescence and optical band gap, resistance to photo-bleaching and low cytotoxicity under ambient light conditions. [132][133][134][135][136][137][138][139][140][141][142][143][144] Graphene quantum dots (GQDs) are photoactive nanomaterials produced by top down method from graphite or carbon nanotubes. [145][146][147][148][149] They have disk like shape with average diameter in the range of 2-20 nm and are composed mainly of sp 2 hybridized carbon and because of that are more crystalline.…”