Fig. 1 Two main approaches were adopted to prepare fluorescent GQDs: the 'top-down' splitting route from different carbon sources and 'bottom-up' method from small molecules or polymers.
Fig. 1 Two main approaches were adopted to prepare fluorescent GQDs: the 'top-down' splitting route from different carbon sources and 'bottom-up' method from small molecules or polymers.
Fig. 2 Quantum confinement effect of conjugated p-domains in GQDs. (a) Energy gap of p–p* transitions calculated based on DFT as a function of the number of fused aromatic rings (N). Reproduced with permission of Wiley-VCH from Ref. [32]. (b) Solubilization strategy for colloidal GQDs. Reproduced with permission from the American Chemical Society from Ref. [26]. (c) The suggested energy levels of C42H18, C96H30, C132, C132H34, and C222H42. The energy level of C132 was determined from the reported paper [26]. Reproduced by permission of Elsevier from Ref. [27].
Fig. 2 Quantum confinement effect of conjugated p-domains in GQDs. (a) Energy gap of p–p* transitions calculated based on DFT as a function of the number of fused aromatic rings (N). Reproduced with permission of Wiley-VCH from Ref. [32]. (b) Solubilization strategy for colloidal GQDs. Reproduced with permission from the American Chemical Society from Ref. [26]. (c) The suggested energy levels of C42H18, C96H30, C132, C132H34, and C222H42. The energy level of C132 was determined from the reported paper [26]. Reproduced by permission of Elsevier from Ref. [27].
Fig. 3 Surface/edge states in GQDs. (a) The oxygen groups were formed at the edge of GQDs during the splitting process. (b) The PL picture in green-fluorescence GQDs. The oxygen and amino groups on the edge determine the band gap of these GQDs. Reproduced with permission from the Royal Society of Chemistry from Ref. [14]. (c) Energy level structures to explain the optical behaviors of photoexcited electrons in GQDs, including their radiative recombination from discrete sp2-related states and continuous defect states, thermally activated decay into non-radiative traps, and non-radiative relaxation from higher- to lower- defect states. Reproduced with permission from the American Chemical Society from Ref. [29]. (d) HOMO and LUMO energy levels of GQD–(NH2)n. Black and blue lines indicate HOMO and LUMO levels, respectively. Reproduced with permission from the American Chemical Society from Ref. [31].
Fig. 3 Surface/edge states in GQDs. (a) The oxygen groups were formed at the edge of GQDs during the splitting process. (b) The PL picture in green-fluorescence GQDs. The oxygen and amino groups on the edge determine the band gap of these GQDs. Reproduced with permission from the Royal Society of Chemistry from Ref. [14]. (c) Energy level structures to explain the optical behaviors of photoexcited electrons in GQDs, including their radiative recombination from discrete sp2-related states and continuous defect states, thermally activated decay into non-radiative traps, and non-radiative relaxation from higher- to lower- defect states. Reproduced with permission from the American Chemical Society from Ref. [29]. (d) HOMO and LUMO energy levels of GQD–(NH2)n. Black and blue lines indicate HOMO and LUMO levels, respectively. Reproduced with permission from the American Chemical Society from Ref. [31].

To make carbon materials fluorescent, the size and surface chemistry are very important (they always consist of sp2 and sp3 carbons and post-modified chemical groups) [1]. Many kinds of fluorescent carbon-based nanomaterials have been synthesized, including carbon dots [2–5], fluorescent CNTs [6], graphene oxide (GO) [7], graphene quantum dots (GQDs) [8–11] and so on [12,13]. GQDs prepared by chemical synthetic strategies are chosen as the main subject to discuss. In detail, the synthesis of GQDs can be divided into 'top-down' splitting methods and 'bottom-up' organic approaches. Due to the special chemical composition of GQDs, they possess abundant surface groups and strong photoluminescence (PL). However, the most controversial issue is the PL mechanism. The dominant PL center of GQDs contains the quantum confinement effect of conjugated-domains,the surface/edge state in GQDs, as well as the synergistic effect of these two factors [1] (Table S1).

Synthesis routes of GQDs

The 'top-down' splitting method is a direct and efficient route to preparing GQDs. The most popular 'top-down' splitting route is acid-assisted-cutting from different carbon sources by H2SO4/HNO3: the carbon source (graphite power, carbon fiber, carbon nanotube or even coal) is firstly exfoliated and oxidized, and then undergoes splitting at the oxygen based defect position by a zipper mechanism (Fig. 1)[1]. Generally,the quantum yields (QYs) of GQDs by the acid assisted cutting routes are lower than 1% [14]. In an improved method, hydrothermal [8], solvothermal [15] and photo-fenton reactions [16] are applied to GO as the initial material, resulting in GQDs with elevated QYs due to surface modification. There are other 'top-down' splitting routes, involving electrochemistry [9], metal-graphite intercalation [17], and nanolithography by reactive ion etching (RIE) [18]. The 'bottom-up' methods are efficient routes to produce fluorescent GQDs, including the carbohydrate dehydration and organic synthesis approaches (Fig. 1). In the carbohydrate carbonization method, the GQDs are obtained using suitable small molecules or polymers for dehydration and further carbonization. Commonly used materials are citric acid and glucose [19]. These formation processes are usually uncontrollable, resulting in GQDs with polydispersity; however, the use of designed precursors, such as intramolecular oxidative polycyclic aromatic hydrocarbons (PAHs), may accurately obtain GQDs with the desired molecular weight and size [20,21]. The organic synthesis approach is precise but complicated, needing many steps to obtain GQDs in the large dimension.

Optical properties of GQDs

The absorption of GQDs is focused on the UV region. The peak at 230–270 nm is attributed to the pi-pi* transition, while the peak at ca. 320 nm belongs to the n-pi* transition. The absorption tail is regarded as the surface state, which is related to the hybrid structure between the graphene core and connected chemical groups. The most reported GQDs possess emission peaks in the blue to green region of the spectrum after excitation by UV light. Furthermore, the PL of GQDs is excitation-, pH- and solvent-dependent. The PL exhibits weak photo-bleaching and is highly stable under UV light [22]. 

Photoluminescence mechanism of GQDs

pi-Domain controlled quantum size effect in GQDs

The GQDs possess a graphene core and attached “uncertain” chemical groups. The PL is controlled by both the graphene core and the surrounding chemical groups. Specifically, the graphene core determines the intrinsic emission, while the attached chemical groups control the surface state [22]. For the GQDs with a perfect graphene core and fewer surface chemical groups, the bandgap of the conjugated -domains is thought to be the true intrinsic PL center. A major feature of quantum dots is the quantum confinement effect (QCE), which occurs when quantum dots are smaller than their exciton Bohr radius [23,24]. First of all, the GQDs prepared by the organic synthesis method should be introduced because they possess special electronic and optical properties that make them suitable models to investigate the PL of QCE [25]. Recently, Li et al. reported the synthesis of colloidal GQDs with a uniform and tunable size through organic chemistry routes [26] (Fig. 2a). The GQDs consist of light atoms and thus have a small dielectric constant and weak spinorbit coupling. These lead to strong carrier–carrier interactions and electronic states with a well-defined spin multiplicity. As a result, GQDs have a much larger energy band than other inorganic semiconductor QDs with similar sizes. That is why most GQDs possess PL in the blue to green region of the spectrum. Yang et al. investigated the photophysics ofthe organic synthesized GQDs, and found that the intrinsic state depends on their size, while the energy level offset between the intrinsic state and edge state determines their optical properties (Fig. 2b) [27]. If the energy level offset between the intrinsic state and edge state is large enough, the fluorescence is dominant. If the energy level offset is small enough, the long carrier lifetime in the intrinsic state offers a possibility for intersystem crossing from the singlet excited state to the triplet excited state of edge state [26]. In cases of C132H34 and C222H42, it is possible that the intrinsic states decrease to a lower energy level than the edge state, and as a result, lose the expected fluorescence. 

Edge and chemical groups controlled surface state in GQDs

For most GQDs prepared from 'top-down' routes, the PL center of the GQDs was suggested to be the surface state, which is related to the hybridization structure of the edge groups and the connected graphene core (Fig. 3a). The efficient edge groups for green emission are mainly carboxyl and amide groups while hydroxyl groups contribute to blue emission [14,28]. Zhu et al. proved the existence of the surface state of GQDs using the following three factors [14]: (1) the PL of the GQDs is enhanced by UV exposure, during which partial OH groups are converted to carboxyl groups; (2) the PL properties of GQDs can be further improved by a one-step solvothermal treatment, in which partial carboxyl groups are converted to amide groups, forming modified-GQDs (m-GQDs) with even stronger surface states; (3) reduced m-GQDs possess more OH groups than reduced GQDs, resulting in a greater number of blue PL centers. No matter which energy level of the LUMO band an electron is excited to, it finally relaxes into the surface state energy levels, which determine the PL properties of the GQDs (Fig. 3b). The quantum yield of the GQDs can be improved by three factors: enhancement of PL centers by edge modification, enrichment of the structure electron density through reduction, and suppression of non-radiative recombination by removing the epoxy groups. Pan et al. suggested that blue PL of GQDs might be attributed to free zigzag sites with a carbene/carbyne-like triplet ground state described as 11. With single-particle spectroscopic technology, the defect state has also been proven to determine the PL of GQDs (Fig. 3c) [29]. Tetsuka et al. prepared GQDs with edge-terminated by a primary amine, modifying the electronic structure to yield effective orbital resonance between the amine moieties and the graphene core [30]. Jin et al. reported that the functionalized GQDs exhibit a redshift in the PL emission compared to the pre-existing GQDs, and the PL emissions of the amine-functionalized GQDs also shift with changes in pH due to the protonation and deprotonation of the functional groups (Fig. 3d) [31]. 

Outlook

Other standpoints, such as molecule state, surface passivation, heteroatom doping, are also notable for understanding the PL mechanism of GQDs. A reasonable explanation of GQDs’ PL mechanism will be monumental not only in guiding synthesis of graphene materials with multi-colors, improving output, and enhancing quantum yields, but also in expanding their potential in nano-electronics, photovoltaic devices, bio-detection, as well as disease diagnostics/therapeutics.

The size and connected groups on GQDs are polydisperse, which make PL mechanism complicated. Fortunately, these inherent functionalized groups provide opportunities for tailoring their chemical structures as well as their optical and optoelectronic properties. The widely observed PL emission in GQDs may be a result of the quantum size effect and surface state. The surface state also contains triplet carbenes at the zigzag edges, attached chemical groups, surface defects, heteroatom doping in carbon lattice, and giant red-edge effect [7]. The influence of single- or multi-layer(s) in GQDs is another important issue in understanding the PL mechanism. Many reported GQDs are in fact not single-layered, so a comparison between single-layered and multi-layered GQDs is desired. The multi-layered GQDs should actually be carbon quantum dots (CQDs), which means that researchers should examine the PL mechanism of carbon-based materials and investigate it synthetically.

Acknowledgements

This work was supported by the NSFC under Grant nos. 21504029, 51373065, the 973 Program under Grant no. 2012CB933800, and the International Postdoctoral Exchange Fellowship Program (20150031).

This article was originally published in Nano Today 13 (2017) 10-14, doi: 10.1016/j.nantod.2016.12.006

Supplementary data

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.nantod.2016.12.006.

 

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