Formation mechanism and optimization of highly luminescent N-doped graphene quantum dots

Abstract

Photoluminescent graphene quantum dots (GQDs) have received enormous attention because of their unique chemical, electronic and optical properties. Here a series of GQDs were synthesized under hydrothermal processes in order to investigate the formation process and optical properties of N-doped GQDs. Citric acid (CA) was used as a carbon precursor and self-assembled into sheet structure in a basic condition and formed N-free GQD graphite framework through intermolecular dehydrolysis reaction. N-doped GQDs were prepared using a series of N-containing bases such as urea. Detailed structural and property studies demonstrated the formation mechanism of N-doped GQDs for tunable optical emissions. Hydrothermal conditions promote formation of amide between -NH₂ and -COOH with the presence of amine in the reaction. The intramoleculur dehydrolysis between neighbour amide and COOH groups led to formation of pyrrolic N in the graphene framework. Further, the pyrrolic N transformed to graphite N under hydrothermal conditions. N-doping results in a great improvement of PL quantum yield (QY) of GQDs. By optimized reaction conditions, the highest PL QY (94%) of N-doped GQDs was obtained using CA as a carbon source and ethylene diamine as a N source. The obtained N-doped GQDs exhibit an excitation-independent blue emission with single exponential lifetime decay.

Figure 1. Characterizations of GQDs-NaOH.

(A) Representative TEM images of GQDs- NaOH. Inset panels show the HR-TEM image (left) and size (diameter) distribution (right). (B) SPM height image of the graphene nanoparticles. Line through some of the GQDs-NaOH shows the height profile. (C) UV-Vis spectrum (dash curve) and photoluminescence spectra (solid curves) of the graphene nanoparticles. (D) X-ray photoelectron spectroscopy (XPS) full scan survey and the high-resolution C1s spectra are shown in inset.

Figure 2. Representative TEM images of GQDs-U samples prepared at (A) 4 hours, (B) 6 hours, 8 hours, and (C) 24 hours.

The corresponding HR-TEM images and particle size distribution are shown as left and right insets, respectively.

Figure 3. XPS spectra of GQDs-U-4, GQDs-U-6, GQDs-U-8 and GQDs-U-24.

Column A is the full scan survey, column B is the high-resolution C1s XPS spectra, and column C is the high-resolution N1s XPS spectra.

Figure 4. Possible formation process of N-doped GQDs.

Figure 5. Representative TEM images (A–C), SPM images (D–F) and XPS spectra (G–I) of GQDs-HMTA, GQDs-DEA and GQDs-EA.

Left and right insets are the corresponding HR-TEM images and particles size distribution in A–C. The height profiles in D–F are inserted along the line cut. High resolution C 1s and N1s spectra are inserted in the full scan survey in G–I.

Figure 6. Characterizations of GQDs-EDA.

(A) Representative TEM images of GQDs-EDA. Insets: HR-TEM image (left) and particles size (diameter) distribution (right). (B) SPM height image of GQDs-EDA. (C) UV-Vis spectrum (black dash curve) and photoluminescence spectra. (D) XPS full scan survey. Inset: high-resolution C 1s and N1s XPS spectra.

Figure 7. Characterizations of GQDs-G and GQDs-T.

(A) Representative TEM images of GQDs-G. Inset: HR-TEM and particle size distribution. (B) Representative TEM images of GQDs-T. Inset: HR-TEM and particle size distribution. (C) UV-Vis spectra (dash line) and PL spectra (solid lines) of GQDs-G and (D) GQDs-TRIS.