title: The dual roles of functional groups in the photoluminescence of graphene quantum dots.
abstract: The photoluminescent properties of graphene nanoparticle (named graphene quantum dots) have attracted significant research attention in recent years owing to their profound application potential. However, the photoluminescence (PL) origin of this class of nanocarbons is still unclear. In this paper, combining direct experimental evidence enabled by a facile size-tunable oxygenated graphene quantum dots (GQDs) synthesis method and theoretical calculations, the roles of the aromatic core, functional groups and disordered structures (i.e. defects and sp(3) carbon) in the PL of oxygenated GQDs are elucidated in detail. In particular, we found that the functional groups on GQDs play dual roles in the overall emission: (1) they enable π* → n and σ* → n transitions, resulting in a molecular type of PL, spectrally invariable with change of particle size or excitation energy; (2) similar to defects and sp(3) carbon, functional groups also induce structural deformation to the aromatic core, leading to mid-gap states or, in other words, energy traps, causing π* → mid-gap states → π transitions. Therefore, functional groups contribute to both the blue edge and the red shoulder of GQDs' PL spectra. The new insights on the role of functional groups in PL of fluorescent nanocarbons will enable better designs of this new class of materials.
authors.affiliation: Queensland Micro- and Nanotechnology Centre, Griffith University, Nathan Campus, Brisbane, QLD 4111, Australia. qin.li@griffith.edu.au and School of Engineering (Environmental), Griffith University, Nathan Campus, Brisbane, QLD 4111, Australia.
authors.forename: Shujun
authors.initials: S
authors.lastname: Wang
authors.affiliation: CSIRO Materials Science and Engineering, Gate 5, Normanby Road, Clayton, VIC 3168, Australia.
authors.forename: Ivan S
authors.initials: IS
authors.lastname: Cole
authors.affiliation: Department of Chemistry & Laboratory of Advanced Materials, Fudan University, Shanghai, 200433, P.R. China.
authors.forename: Dongyuan
authors.initials: D
authors.lastname: Zhao
authors.affiliation: Queensland Micro- and Nanotechnology Centre, Griffith University, Nathan Campus, Brisbane, QLD 4111, Australia. qin.li@griffith.edu.au and School of Engineering (Environmental), Griffith University, Nathan Campus, Brisbane, QLD 4111, Australia.
authors.forename: Qin
authors.initials: Q
authors.lastname: Li