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

Qu D, Zheng M, Zhang L, Zhao H, Xie Z, Jing X, Haddad RE, Fan H, Sun Z - Sci Rep (2014)

Bottom Line: The intramoleculur dehydrolysis between neighbour amide and COOH groups led to formation of pyrrolic N in the graphene framework.N-doping results in a great improvement of PL quantum yield (QY) of GQDs.The obtained N-doped GQDs exhibit an excitation-independent blue emission with single exponential lifetime decay.

View Article: PubMed Central - PubMed

Affiliation: 1] State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Changchun 130033, Jilin, P. R. China [2] University of Chinese Academy of Science, Beijing 100000, P. R. China.

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.

No MeSH data available.


Related in: MedlinePlus

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.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC4061557&req=5

f6: 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.

Mentions: Although the PL QY of GQDs-EA show the highest PL QY among the primary, secondary and tertiary amine, the GQDs-U series samples show much higher PL QY comparing with the GQDs-EA. We increased the amount of EA, no significant improvement of PL QY was obtained. (Supplementary Table 1) Since urea could be treated as a diamine that gives us a hint that diamine may be a better dopant than monoamine. Then we chose ethylene diamine (EDA) as a dopant for preparing N doped GQDs. TEM images of GQDs-EDA, as shown in Figure 6A, reveal that the GQDs-EDA are uniform and have a diameter of 2.30 ± 0.31 nm. The HR-TEM images exhibit clear lattice fringe of 0.24 nm, which discloses that the GQDs-EDA also have a graphite nature. AFM image of GQDs-EDA (Figure 6B) discloses the sheet structure, which is about ~1 nm in thickness. UV-Vis spectrum of GQDs-EDA exhibits 2 clear absorption bands at 235 and 340 nm, which are the same as GQDs-U. A strong and bright blue emission at 450 nm that is excitation-independent emission, was observed in the PL spectrum. GQDs-EDA exhibits a much longer lifetime of 14 ns that is almost double longer than GQDs-U. The full scan XPS spectrum (Figure 6D) shows three signals at 284, 399 and 521 eV that are attributed to C 1s, N1s, and O1s respectively. The RN/C is 0.59, which is higher than any other N doped GQDs in this report. High-resolution C 1s spectrum revealed that there exist three types of C including sp2 C (C = C), sp3 C (C-C, C-N, and C-O), and oxidized C (C = O). Two types of N, pyrrolic N and graphite N are shown in the high-resolution N 1s spectrum. These results indicate that the extent of doping of GQDs-EDA increases from monoamine to diamine. Due to the higher doping degree, the PL QY of GQDs-EDA also exhibits very high PL QY of 94%, which is the highest PL QY of GQDs to our knowledge. Comparing with GQDs, N doping introduces a new surface state labeled as the N-state2534. Electrons trapped by the N-state are able to facilitate a high yield of radiative recombination and depress non-radiative recombination. The density of N-state strongly affects the PL QY of GQDs. The higher N doping degree is in GQDs, the higher density of N-state is. From above results, the N doped GQDs are hardly obtained from tertiary amine as dopant because it has no active N-H. As a dopant, primary amine is better than secondary amine; diamine is a better dopant than monoamine.


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

Qu D, Zheng M, Zhang L, Zhao H, Xie Z, Jing X, Haddad RE, Fan H, Sun Z - Sci Rep (2014)

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.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4061557&req=5

f6: 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.
Mentions: Although the PL QY of GQDs-EA show the highest PL QY among the primary, secondary and tertiary amine, the GQDs-U series samples show much higher PL QY comparing with the GQDs-EA. We increased the amount of EA, no significant improvement of PL QY was obtained. (Supplementary Table 1) Since urea could be treated as a diamine that gives us a hint that diamine may be a better dopant than monoamine. Then we chose ethylene diamine (EDA) as a dopant for preparing N doped GQDs. TEM images of GQDs-EDA, as shown in Figure 6A, reveal that the GQDs-EDA are uniform and have a diameter of 2.30 ± 0.31 nm. The HR-TEM images exhibit clear lattice fringe of 0.24 nm, which discloses that the GQDs-EDA also have a graphite nature. AFM image of GQDs-EDA (Figure 6B) discloses the sheet structure, which is about ~1 nm in thickness. UV-Vis spectrum of GQDs-EDA exhibits 2 clear absorption bands at 235 and 340 nm, which are the same as GQDs-U. A strong and bright blue emission at 450 nm that is excitation-independent emission, was observed in the PL spectrum. GQDs-EDA exhibits a much longer lifetime of 14 ns that is almost double longer than GQDs-U. The full scan XPS spectrum (Figure 6D) shows three signals at 284, 399 and 521 eV that are attributed to C 1s, N1s, and O1s respectively. The RN/C is 0.59, which is higher than any other N doped GQDs in this report. High-resolution C 1s spectrum revealed that there exist three types of C including sp2 C (C = C), sp3 C (C-C, C-N, and C-O), and oxidized C (C = O). Two types of N, pyrrolic N and graphite N are shown in the high-resolution N 1s spectrum. These results indicate that the extent of doping of GQDs-EDA increases from monoamine to diamine. Due to the higher doping degree, the PL QY of GQDs-EDA also exhibits very high PL QY of 94%, which is the highest PL QY of GQDs to our knowledge. Comparing with GQDs, N doping introduces a new surface state labeled as the N-state2534. Electrons trapped by the N-state are able to facilitate a high yield of radiative recombination and depress non-radiative recombination. The density of N-state strongly affects the PL QY of GQDs. The higher N doping degree is in GQDs, the higher density of N-state is. From above results, the N doped GQDs are hardly obtained from tertiary amine as dopant because it has no active N-H. As a dopant, primary amine is better than secondary amine; diamine is a better dopant than monoamine.

Bottom Line: The intramoleculur dehydrolysis between neighbour amide and COOH groups led to formation of pyrrolic N in the graphene framework.N-doping results in a great improvement of PL quantum yield (QY) of GQDs.The obtained N-doped GQDs exhibit an excitation-independent blue emission with single exponential lifetime decay.

View Article: PubMed Central - PubMed

Affiliation: 1] State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Changchun 130033, Jilin, P. R. China [2] University of Chinese Academy of Science, Beijing 100000, P. R. China.

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.

No MeSH data available.


Related in: MedlinePlus