Graphene quantum dots (GQDs) represent single-layer to tens of layers of graphene of a size less than 30 nm, consisting of nanometer-scaled graphene particles with sp2-sp2 carbon bonds. GQDs are fragments limited in size, or domains, of a single-layer two-dimensional graphene crystal. Spectral studies have found that in almost all cases, GQDs are not single-layer graphene domains, but multi-layer formations. GQDs have advantages of nontoxicity, good solubility, stable photoluminescence, and better surface grafting, making them promising candidates for replacing inorganic QDs.
Fig.1. Multicolor fluorescent graphene quantum dots
Nowadays, several techniques have been developed to prepare GQDs including electron beam lithography, chemical synthesis, electrochemical preparation, graphene oxide reduction, C60 catalytic transformation, microwave assisted hydrothermal method (MAH), Soft-Template method, hydrothermal method and ultrasonic exfoliation method. For example, the synthetic scheme for GQDs is shown in Fig.2. Monolayer graphene oxide quantum dots (GOQDs), as the precursor with high yield and purity, are produced by acid-assisted cleavage of carbon black. And then, the GQDs sample can be obtained by annealing GOQDs in argon atmosphere at 1010 °C for 1 h.
Fig.2. Synthetic scheme for GQDs.
The black and red balls denote carbon and oxygen atoms, respectively.
(npj Quantum Materials 2017, 5)
GQDs are becoming advanced multifunctional materials for their unique optical, electronic, spin, and photoelectric properties induced by the quantum confinement effect and edge effect. GQDs have various important applications in bioimaging, cancer therapeutics, temperature sensing, drug delivery, surfactants, LEDs lighter converters, photodetectors, solar cells, photoluminescent material and biosensors fabrication. Due to their exceptional properties such as low toxicity, stable photoluminescence, chemical stability and pronounced quantum confinement effect, GQDs are considered as a novel material for biological, opto-electronics, energy and environmental applications. In recent studies, additional excellent properties of GQDs such as high transparency and high surface area have been proposed for energy and display applications. Because of the large surface area, electrodes using GQDs are applied for capacitors and batteries, and the conductivity of GQDs is higher than that of graphene oxide.