Citric-acid-derived carbon nanodots are increasingly being explored as novel fluorescent nanomaterials due to their strong photoluminescence (PL). However, an accurate picture of the formation of carbon nanodots and an exhaustive structure–property correlation are still lacking. Herein we present a systematic investigation of the formation mechanism of carbon nanodots by following the pyrolysis of a citric acid–diethylenetriamine precursor at different temperatures. The collective nanodots are investigated by dynamic rheological measurements, exhibiting a strong pyrolytic temperature dependence of the viscoelastic properties. Atomic force microscopy, transmission electron microscopy, and Raman spectroscopy reveal that the synthesized “dots” at different pyrolytic temperatures are different in essence, and the transition of their chemical structure from molecular clusters to carbogenic nanoparticles during pyrolysis is highly verified. We find that a molecular fluorophore with intense PL predominates at low temperature (<250 °C), but a newly created quasi-molecular fluorophore with blue-shifted and decreased PL quantum yield predominates at high temperatures (300 °C). Time-resolved photoluminescence spectroscopy suggests that the strong PL suppression at high temperature is due mostly to a dramatic increase in the nonradiative decay rate of the quasi-molecular electronic state.

Recently, graphene nanodots (GNDs) have been frequently considered as inherently heterogeneous systems, leading to multicolor emission under a changeable excitation wavelength. However, an accurate picture of the GNDs and an exhaustive structure–property correlation are still lacking. Using a two dimensional photoluminescence excitation (2D-PLE) map, molecular orbital calculation, reduction level dependent PL analysis, absorption spectroscopy and time-resolved PL spectroscopy, three cases of quasi-molecular PL are determined in ethylenediamine (EDA) reduced GNDs, including the CO related electronic state, graphenol related electronic state and large π-conjugated domains. The graphenol structure is expected to be created via nucleophilic addition–elimination reactions between epoxide groups and EDA, contributing most to the blue-shifted and enhanced PL of GNDs. The multiple quasi-molecular PL provides deeper insights into the commonly called “excitation wavelength dependent PL”. An effort is made to utilize the heterogeneous photoluminescence through phosphor-based light-emitting diodes employing reduced GNDs as a phosphor, which are capable of converting blue light into white light.

Herein, we report the characterization of highly fluorescent citric-acid derived carbon dots (CACDs) synthesized by hydrothermal treatment of citric acid and diethylenetriamine below 200 °C. After being purified using a gel permeation chromatography cleanup system, the complexity and chemical composition of the CACDs were evaluated by liquid chromatography coupled with high-resolution Fourier transform ion cyclotron resonance mass spectrometry. The fluorophores consisted of five-membered ring fused 2-pyridones identified as the photoluminescence origin. M06-2X density functional calculations, surface tension and morphological studies suggested DETA@5CA serves as the main building block to fabricate supramolecular aggregates. Then we proposed that the dimeric and trimeric fluorophores coupled with DETA@5CA led to “dot” topologies in the CACDs solution under the effect of hydrogen bonding. In aqueous solution, the CACDs exhibited narrowly dispersed optical properties and a high fluorescent quantum yield (∼98%). Moreover, the supramolecular interaction induced CACDs have high sensitivity under various ambient conditions, such as pH, organic solvents and metal ions.

In this work, few layer graphene quantum dots (GQDs) with a size of 3–5 nm are purposely treated with highly concentrated aqueous NaBH4 solutions to obtain the reduced graphene quantum dots (rGQDs). Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy demonstrate that the number of carbonyl groups decreases but –OH related defects increase during chemical reduction. Green and weak emissions of original GQDs originate from carrier recombination in the disorder-induced localized state (mainly including carbonyl and carboxyl and epoxy groups). As the reduction degree increases, the photoluminescence (PL) quantum efficiency of GQDs increases dramatically from 2.6% to 10.1%. In the meantime, the PL peak position blue shifts rapidly, and full width at half maximum (FWHM) becomes narrower. Thus we can infer that graphenol topological defects (hydroxyl functionalized graphene) are gradually formed during reduction. Besides, graphenol defect related PL features a longer fluorescence lifetime, excitation wavelength dependence but less pH sensitivity.

In this work, few layer graphene quantum dots (GQDs) with a size of 3–5 nm are purposely treated with highly concentrated aqueous NaBH4 solutions to obtain the reduced graphene quantum dots (rGQDs). Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy demonstrate that the number of carbonyl groups decreases but –OH related defects increase during chemical reduction. Green and weak emissions of original GQDs originate from carrier recombination in the disorder-induced localized state (mainly including carbonyl and carboxyl and epoxy groups). As the reduction degree increases, the photoluminescence (PL) quantum efficiency of GQDs increases dramatically from 2.6% to 10.1%. In the meantime, the PL peak position blue shifts rapidly, and full width at half maximum (FWHM) becomes narrower. Thus we can infer that graphenol topological defects (hydroxyl functionalized graphene) are gradually formed during reduction. Besides, graphenol defect related PL features a longer fluorescence lifetime, excitation wavelength dependence but less pH sensitivity.