Graphene quantum dots (GQDs), mostly composed of carbon atoms, have been attracting attention ad a next generation light-emitting material since they are eco-friendly, earth-abundant, and not containing hazardous elements such as cadmium. However, contrary to semiconductor quantum dots including heavy metals, GQDs are difficult to realize luminescence other than blue light from the intrinsic band gap with relatively low efficiency. Green emission can be achieved from the defect level, however the efficiency is even lower than blue light. Active researches have been conducted over the past several years to control the emission wavelength of GQDs, but all of the principles are explained by luminescence from the defect level. When a large number of defect levels are formed, emission may occur after a transition from the level corresponding to the intrinsic band gap to the defect level, but the probability of non-radiative emission is increased and the efficiency is lowered. Moreover, the emission wavelength region is not constant depending on the excitation energy since the emission is not from the intrinsic band gap. In the case of excitation-dependent emission with low efficiency, the application to the electronic device as a light-emitting material is limited, therefore more studies should be implemented to overcome the limitation. Therefore, in order to overcome the limitations of the previously reported researches, this paper is focused on clarifying the principle of efficiently blue-emissive GQDs prepared by graphite intercalation compounds (GICs) developed in our previous research (named as GQD in this work) through accurate analysis. Based on the principle, we further visualize the advantages of intrinsic band gap of the GQD in various applications, and control optical characteristics by controlling the intrinsic band gap of GQDs.
The photoluminescence mechanism of GQDs on a small region of the isolated $sp^2$ carbon hexagons in graphene matrix known as subdomain is still controversial due to lack of convincing evidence. Therefore, the emission characteristics of three types of GQDs prepared through various synthesis methods were thoroughly analyzed and compared using various analytical methods including Auger electron spectroscopy (AES) and reflection electron energy loss spectroscopy (REELS). From the analysis, the reason why GQD prepared from GICs having low oxidation level has excellent blue luminescence due to intrinsic band gap has been described. Density functional theory (DFT) calculations show that it is stable to preferentially form small subdomains with 4-7 carbon hexagons with lower formation energy than larger subdomains. The fact that GQD which is weakly oxidized in the vicinity of the potassium ion through GICs has an experimental band gap of 3.1 eV obtained by REELS in a good agreement with the calculated band gap (2.89 ~ 3.13 eV) is observed. On the other hand, highly oxidized graphene quantum dots by other methods are very strong and randomly oxidized, making it difficult to form energetically favorable subdomains and have weak red-shifted luminescence characteristics. In addition, we found that nearly identical photoluminescence properties to GQD are observed in the intercalation treated centimeter-sized graphene. This clearly demonstrates that the origin of the blue emission in GQDs is from the subdomain rather than the physical size of the GQDs.
GQDs having a wide intrinsic band gap and low defect level are excellent in both light absorption and light emission, so that they take possibility to be employed in various categories of applications. Therefore, the blue-emitting GQD having a wide and discrete band gap obtained from GICs were decorated onto titanium dioxide ($TiO_2$) which can be efficiently applied in various directions with its great light absorption property. By employing GQD to $TiO_2$ nanoparticles (NPs) and three-dimensionally nanostructured monolithic $TiO_2$, $TiO_2$/GQD heterostructures with enhanced light absorption in both UV and visible ranges compared to pristine $TiO_2$ are obtained, and the principle of the enhanced absorption is explained by charge transfer and charge injection, respectively. The prepared $TiO_2$/GQD heterostructures show improved UV blocking effect when applied to sunscreen and solar cell requiring great light absorption. In addition, the heterostructures enable efficient electron extraction and charge separation from a well-aligned band structure, demonstrating its applicability as an electron transport layer and photocatalytic material for solar cells.
GQDs having such a wide band gap can improve the efficiency in solar cell, photocatalyst, etc. However, application of the light emitting devices has limitation since their emission wavelength is limited to blue. Therefore, we intended to efficiently control the intrinsic band gap of GQDs. By confirming that the reaction occurs pre-dominantly at the site of potassium in the production of GQD from GICs, we verified the location of potassium ions within GICs affect the size of subdomain. From this observation, the position of potassium within the graphite layers have been manipulated by controlling organic acids which is intercalated simultaneously with potassium in order to control the subdomain size. GICs in which organic acid molecules having various lengths were intercalated between potassium and graphite was prepared, and among them, GQDs exhibiting cyan emission were obtained from GICs using succinic acid. The cyan luminescent GQDs prepared by simultaneous insertion of succinic acid and potassium ion exhibit emission peak near 450 nm and have almost no excitation wavelength dependence which is the main characteristic of emission from intrinsic band gap. The result is considered to be a clear evidence regarding the possibility of engineering the intrinsic band gap of GQDs through GIC method.