title: 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.
authors.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.
authors.forename: Dan
authors.initials: D
authors.lastname: Qu
authors.affiliation: State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Changchun 130033, Jilin, P. R. China.
authors.forename: Min
authors.initials: M
authors.lastname: Zheng
authors.affiliation: State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Changchun 130033, Jilin, P. R. China.
authors.forename: Ligong
authors.initials: L
authors.lastname: Zhang
authors.affiliation: State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Changchun 130033, Jilin, P. R. China.
authors.forename: Haifeng
authors.initials: H
authors.lastname: Zhao
authors.affiliation: State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Changchun 130022, Jilin, P. R. China.
authors.forename: Zhigang
authors.initials: Z
authors.lastname: Xie
authors.affiliation: State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Changchun 130022, Jilin, P. R. China.
authors.forename: Xiabin
authors.initials: X
authors.lastname: Jing
authors.affiliation: Center for Micro- Engineering and Materials, Department of Chemical and Nuclear Engineering, University of New Mexico, Albuquerque, NM 87106, United State.
authors.forename: Raid E
authors.initials: RE
authors.lastname: Haddad
authors.affiliation: 1] Center for Micro- Engineering and Materials, Department of Chemical and Nuclear Engineering, University of New Mexico, Albuquerque, NM 87106, United State [2] Sandia National Laboratories, Advanced Materials Laboratory, Albuquerque, NM 87106, United State.
authors.forename: Hongyou
authors.initials: H
authors.lastname: Fan
authors.affiliation: State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Changchun 130033, Jilin, P. R. China.
authors.forename: Zaicheng
authors.initials: Z
authors.lastname: Sun