Stable Carbon Dots from Microwave-Heated Carbon Nanoparticles Generating Organic Radicals for In Situ Additions

Abstract

Carbon dots (CDots) are small carbon nanoparticles with effective surface passivation by organic functionalization. In the reported work, the surface functionalization of preexisting small carbon nanoparticles with N-ethylcarbazole (NEC) was achieved by the NEC radical addition. Due to the major difference in microwave absorption between the carbon nanoparticles and organic species such as NEC, the nanoparticles could be selectively heated via microwave irradiation to enable the hydrogen abstraction in NEC to generate NEC radicals, followed by in situ additions of the radicals to the nanoparticles. The resulting NEC-CDots were characterized by microscopy and spectroscopy techniques including quantitative proton and 13C NMR methods. The optical spectroscopic properties of the dot sample were found to be largely the same as those of CDots from other organic functionalization schemes. The high structural stability of NEC-CDots benefiting from the radical addition functionalization is highlighted and discussed.

On Carbon “Replacing” the Core in Classical Semiconductor Core/ZnS Quantum Dots

“…[6,9,17] The effects are both characterized by the same dramatic enhancement in fluorescence emission brightness or quantum yields and the substantial narrowing of the emission spectra, yet without any major changes in optical absorptions (Figures 4 & 5). Also, shared between the core/shell nanostructure and the classical CDots are the rather characteristic excitation wavelength dependencies of fluorescence emissions (Figure 5, which in classical CDots are established as being due to a distribution of emissive excited states in individual dots [14,17] ), again indicative of their very similar photoexcited state properties and processes and by extension largely the same surface passivation effects on CNPs due to either the organic surface functionalization or the surface capping with a nanoscale semiconductor like ZnS.…”

“…Experimentally, CNPs were harvested from commercially acquired carbon nanopowder sample by using the established processing protocol. [14] For the ZnS capping, CNPs were dispersed in a mixture of TOPO and hexadecylamine, followed by the mixing with zinc nitrate. The mixture was heated to 180 °C in an oil bath with vigorously stirring under nitrogen protection, and to the mixture was added drop-wise dodecanethiol to form ZnS for the capping of CNPs.…”

“…[8,18,19] In the mechanistic framework for the photoexcitation and the excited state properties and processes of CDots, [8,17,19] in which the core CNPs are solely responsible for the observed optical absorptions due to the common use of only colorless organic molecules for the CNP surface Figure 5 The optical absorption spectra (ABS) and the characteristic excitation wavelength dependence of fluorescence spectra (FLSC, from left to right corresponding to excitation wavelengths of 400 nm to 540 nm in 20 nm increment) for CNP/ZnS (upper) and the EDA-CDots [lower, EDA = 2,2'-(ethylenedioxy)bis(ethylamine)]. [14] functionalization, the initial processes following photoexcitation are likely rapid charge transfers and separation to result in the separated electrons and holes. Such transient processes and species are conceptually and phenomenologically analogous to those found in photoexcited CdSe and other semiconductor nanoparticles.…”