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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

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.
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f5: 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.

Mentions: GQDs were synthesized using different kinds of amine such as tertiary, secondary, and primary amine (mono and diamine) in order to investigate the effect of the types of amines. The GQDs-HMTA, GQDs-DEA, GQDs-EA and GQDs-EDA are used to denote the GQDs that were prepared using hexamethylene tetraamine (HMTA), diethylene amine (DEA), ethanol amine (EA), and ethylene diamine (EDA), respectively. Figure 5A–C showed TEM images of GQDs-HMTA, GQDs-DEA, and GQDs-EA. TEM images showed these three GQDs are monodispersed with a uniform diameter of 3.39 ± 0.69 nm, 4.53 ± 0.27 nm, 5.13 ± 0.47 nm, respectively. HR-TEM images confirmed that GQDs-HMTA showed a lattice fringe with 0.24 nm, which corresponds to the (1120) crystal phase of graphite. The corresponding SPM height image revealed a typical topographic height of 1–2.0 nm (Figure 5D–E), indicating that most of the GQDs consist of ca. 1–5 graphene layers1231. Raman spectrum (Supplementary Figure S6) was employed to further characterize the microstructure of GQDs-HMTA. GQDs-HMTA shows a disordered (D) band at 1359 cm−1, related to the presence of sp3 defects, and the crystalline (G) band at 1578 cm−1, related to in-plane vibration of sp2 carbon. The ratio of the intensities (ID/IG) of these characteristic bands can be used to correlate the structural properties of the carbon. The value of ID/IG is 0.9 for GQDs-HMTA, meaning that the as-prepared GQDs-HMTA have highly crystalline nature, which is consistence with the TEM results. The optical properties of GQDs-HMTA, GQDs-DEA and GQDs-EA are shown in Supplementary Figure S7. All GQDs exhibit a shoulder band at 235 nm and a clear absorption band ~340 nm, which is close to GQDs-U. The PL spectra show all GQDs have an emission at 450 nm that is excitation-independent. This further suggests that both the size and the surface state of these GQDs are uniform. The measured lifetime of GQDs-HMTA, -DEA, and -EA are 10, 7 and 6 ns, respectively, showing single exponential decay (Supplementary Figure S8).


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)

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

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

f5: 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.
Mentions: GQDs were synthesized using different kinds of amine such as tertiary, secondary, and primary amine (mono and diamine) in order to investigate the effect of the types of amines. The GQDs-HMTA, GQDs-DEA, GQDs-EA and GQDs-EDA are used to denote the GQDs that were prepared using hexamethylene tetraamine (HMTA), diethylene amine (DEA), ethanol amine (EA), and ethylene diamine (EDA), respectively. Figure 5A–C showed TEM images of GQDs-HMTA, GQDs-DEA, and GQDs-EA. TEM images showed these three GQDs are monodispersed with a uniform diameter of 3.39 ± 0.69 nm, 4.53 ± 0.27 nm, 5.13 ± 0.47 nm, respectively. HR-TEM images confirmed that GQDs-HMTA showed a lattice fringe with 0.24 nm, which corresponds to the (1120) crystal phase of graphite. The corresponding SPM height image revealed a typical topographic height of 1–2.0 nm (Figure 5D–E), indicating that most of the GQDs consist of ca. 1–5 graphene layers1231. Raman spectrum (Supplementary Figure S6) was employed to further characterize the microstructure of GQDs-HMTA. GQDs-HMTA shows a disordered (D) band at 1359 cm−1, related to the presence of sp3 defects, and the crystalline (G) band at 1578 cm−1, related to in-plane vibration of sp2 carbon. The ratio of the intensities (ID/IG) of these characteristic bands can be used to correlate the structural properties of the carbon. The value of ID/IG is 0.9 for GQDs-HMTA, meaning that the as-prepared GQDs-HMTA have highly crystalline nature, which is consistence with the TEM results. The optical properties of GQDs-HMTA, GQDs-DEA and GQDs-EA are shown in Supplementary Figure S7. All GQDs exhibit a shoulder band at 235 nm and a clear absorption band ~340 nm, which is close to GQDs-U. The PL spectra show all GQDs have an emission at 450 nm that is excitation-independent. This further suggests that both the size and the surface state of these GQDs are uniform. The measured lifetime of GQDs-HMTA, -DEA, and -EA are 10, 7 and 6 ns, respectively, showing single exponential decay (Supplementary Figure S8).

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