Queensland Micro- and Nanotechnology Centre, Griffith University, Nathan Campus, Brisbane, QLD 4111, Australia.
CSIRO Materials Science and Engineering, Gate 5, Normanby Road, Clayton, VIC 3168, Australia.
Nanoscale. 2016 Apr 14;8(14):7449-58. doi: 10.1039/c5nr07042b.
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.
近年来，由于石墨烯量子点（即石墨烯纳米粒子）具有深远的应用潜力，其发光性质引起了人们的极大关注。然而，这类纳米碳的发光起源仍不清楚。本文通过简便的可调控尺寸的含氧石墨烯量子点（GQDs）合成方法获得的直接实验证据，并结合理论计算，详细阐明了芳香核、官能团和无序结构（即缺陷和 sp(3) 碳）在含氧 GQDs 发光中的作用。特别是，我们发现 GQDs 上的官能团在整体发射中起双重作用：（1）它们使 π* → n 和 σ* → n 跃迁成为可能，从而产生分子类型的发光，其光谱不随颗粒尺寸或激发能的变化而变化；（2）与缺陷和 sp(3) 碳类似，官能团也会引起芳香核的结构变形，导致隙态或能量陷阱，从而引发 π* → 隙态 → π 跃迁。因此，官能团对 GQDs 发光光谱的蓝边和红肩都有贡献。这些关于官能团在荧光纳米碳发光中作用的新见解将能够更好地设计这一新类材料。
