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The photoluminescence of graphene quantum dots (GQDs) is rigorously investigated due to their potential applications. However, GQDs from graphene oxide are inherently embedded with non-negligible defects and oxygen grafted onto the edge and basal plane, which induce a change of innate electronic structure in the GQDs. Thus, graphene oxide based GQDs can misrepresent the characteristic properties of primitive GQDs. Here we report the size and pH dependent photophysical properties of GQDs that minimize the content of oxygen and defects. From auger electron spectroscopy, the oxygen content of the GQDs with two different lateral sizes (∼2 nm and ∼18 nm) was probed and found to be ∼5% and ∼8%, respectively. Two common photoluminescence (PL) peaks were observed at 436 nm (the intrinsic bandgap) and 487 nm (the extrinsic bandgap) for both GQDs. The characteristic PL properties in extrinsic and intrinsic bandgaps examined by optical spectroscopic methods show that the emission peak was red-shifted and that the peak width was widened as the size increased. Moreover, the PL lifetime and intensity were not only reversibly changed by pH but also depended on the excitation wavelength. This is in line with our previous report and is ascribed to the size variation of sp2 subdomains and edge functionalization.

Nano-fillers have provided a big advantage for enhancing the performance of rubber composites through leading the synergy effects in the physical and chemical properties. However, despite various approaches having been explored, the process to make a homogenous and stable dispersion of nano-filler in the rubber matrix remains a major challenge in this field. Herein, we propose a simple and effective route for synthesizing nanocomposites of rubber with homogenous and stable dispersed low defect graphene flakes (l-GFs), which are prepared using l-GFs/SBR composites via aqueous-phase mixing of exfoliated l-GFs with SBR latex. The l-GFs embedded SBR matrix shows a remarkable improvement in the modulus and tensile strength even at the low loading rate, which is ascribed to the efficient dispersion of the l-GFs enhancing interfacial interaction with the rubber matrix. The integration of l-GFs into the SBR matrix significantly improves the thermal and electrical conductivities, as well as the gas barrier property of the rubber composites. This method is water-mediated, green and scalable, showing great potential for the production of various l-GFs-based rubber composites at an industrial level.