Microwave growth and tunable photoluminescence of nitrogen-doped graphene and carbon nitride quantum dots

Siyong Gu; Chien-Te Hsieh; Yasser Ashraf Gandomi; Jeng-Kuei Chang; Ju Li; Jianlin Li; Houan Zhang; Qing Guo; Kah Chun Lau; Ravindra Pandey

doi:10.1039/C9TC00233B

Microwave growth and tunable photoluminescence of nitrogen-doped graphene and carbon nitride quantum dots†

Siyong Gu,

^a Chien-Te Hsieh,

*^bc Yasser Ashraf Gandomi,

^ci Jeng-Kuei Chang,

*^de Ju Li,

^d Jianlin Li,

^f Houan Zhang,^a Qing Guo,

^gh Kah Chun Lau^h and Ravindra Pandey

^g

Author affiliations

* Corresponding authors

^a Fujian Provincial Key Laboratory of Functional Materials and Applications, School of Materials Science and Engineering, Xiamen University of Technology, Xiamen 361024, P. R. China

^b Department of Chemical Engineering and Materials Science, Yuan Ze University, Taoyuan 32003, Taiwan
E-mail: cthsieh@saturn.yzu.edu.tw

^c Department of Mechanical, Aerospace, and Biomedical Engineering, University of Tennessee, Knoxville, TN 37996, USA

^d Department of Nuclear Science and Engineering and Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

^e Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan
E-mail: jkchang@nctu.edu.tw

^f Energy and Transportation Science Division, Oak Ridge National Laboratory, TN 37831, USA

^g Department of Physics, Michigan Technological University, Houghton, MI 49931, USA

^h Department of Physics and Astronomy, California State University, Northridge, CA 91330, USA

ⁱ Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA

Abstract

Tunable photoluminescent nitrogen-doped graphene and graphitic carbon nitride (g-C₃N₄) quantum dots are synthesized via a facile solid-phase microwave-assisted (SPMA) technique utilizing the pyrolysis of citric acid and urea precursors. The atomic ratio, surface functionalization, and atomic structure of as-prepared quantum dots strongly depend on the ratio of citric acid to urea. The quantum dots have a homogeneous particle size and tend to form a circle and/or ellipse shape to minimize the edge free energy. The atomic ratio of surface nitrogen to carbon (N/C) in the quantum dots can reach as high as 1.74, among the highest values reported in the literature. The SPMA technique is capable of producing high-quality quantum dots with photoluminescence (PL) emission at various wavelengths on a pilot scale. The atomic structures of the N-doped graphene and g-C₃N₄ quantum dots are explored using molecular dynamics simulations. Increasing the urea concentration increases the tendency of in-plane N (i.e., quaternary N) substitution over that of other amino functionalizations, such as pyrrolic and pyridinic N. The PL emission can be precisely tuned via a one-step SPMA method by adjusting the precursor composition. A high quantum yield of 38.7% is achieved with N-doped graphene quantum dots, indicating the substantial influence of the N- and O-rich edge groups on the enhancement of PL efficiency. A bandgap structure is proposed to describe the interstate (π*–π) transition of quantum dots. This work introduces a novel approach for engineering the chemical composition and atomic structure of graphene and g-C₃N₄ quantum dots, facilitating their research and applications in optical, electronic, and biomedical devices.

Journal of Materials Chemistry C

Microwave growth and tunable photoluminescence of nitrogen-doped graphene and carbon nitride quantum dots†

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Microwave growth and tunable photoluminescence of nitrogen-doped graphene and carbon nitride quantum dots

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