Graphene quantum dots from chemistry to applications Materials Today Chemistry

Graphene quantum dots (GQDs) have been widely studied in recent years due to its unique structure- related properties, such as optical, electrical and optoelectrical properties. GQDs are considered new kind of quantum dots (QDs), as they are chemically and physically stable because of its intrinsic inert carbon property. Furthermore, GQDs are environmentally friendly due to its non-toxic and biologically inert properties, which have attracted worldwide interests from academic and industry. In this review, a number of GQDs preparation methods, such as hydrothermal method, microwave-assisted hydrothermal method, soft-template method, liquid exfoliation method, metal-catalyzed method and electron beam lithography method etc . , are summarized. Their structural, morphological, chemical composition, optical, electrical and optoelectrical properties have been characterized and studied. A variety of elemental dopant, such as nitrogen, sulphur, chlorine, ﬂ uorine and potassium etc., have been doped into GQDs to diversify the functions of the material. The control of its size and shape has been realized by means of preparation parameters, such as synthesis temperature, growth time, source concentration and catalyst etc. As far as energy level engineering is concerned, the elemental doping has shown an introduction of energy level in GQDs which may tune the optical, electrical and optoelectrical properties of the GQDs. The applications of GQDs in biological imaging, optoelectrical detectors, solar cells, light emitting diodes, ﬂ uorescent agent, photocatalysis, and lithium ion battery are described. GQD composites, having optimized contents and properties, are also discussed to extend the applications of GQDs. Basic physical and chemical parameters of GQDs are summarized by tables in this review, which will provide readers useful information. © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).


History of graphene quantum dots development
Carbon, which is one of the most abundant elements, has been widely researched since 19th century and has experienced a continuous surge in its studies. The entire development of carbon material to date is shown in Fig. 1. In the year 1859, Brodie [1] synthesized the graphite oxide to ascertain the molecular weight of graphite via some hazardous experiments, and coined its name 'Graphon'. At that time, the properties of graphite oxide were not described until 1918 by Kohlschütter and Haenni [2]. Further study on the structural property of graphite oxide was carried out by Bernal using single-crystal diffraction measurements [3] in 1924. As the characterization technology enhances, the electronic properties of graphite were first investigated by Wallace [4] and he introduced the idea of graphene in 1947. One year later, Ruess and Vogt [5] successfully detected a few layers graphite via transmission electron microscopy (TEM). In 1957, Hummers and Offeman [6] developed a new method, which is safer and more efficient than Brodie's method, in the preparation of graphite oxide. The idea of a single layer graphite sheet, which was initially described by Boehm [7] in 1962 and by some theoretical physicists [8], having a massless Dirac equation in 1984 was considered implausible in that era. During the development of carbon materials, a football-shaped fullerene was discovered by Smally [9] in 1985, which represented eureka moment in the scientific world, leading to the award of the Nobel Prize in 1996. This is the first carbon allotropy which inspires scientists around the world in exploiting carbon materials. In 1987, Mouras [10] introduced the word 'graphene' to represent single sheets of graphite although stable single-layer graphite sheet still remained non-existent by theoretical calculations. Another carbon allotropy so-called carbon nanotubes (CNTs) was later discovered by Iijima [11] in 1991. More than a decade later, an astonishing experiment on graphene was carried out by Geim and Novoslov [12] in Manchester University, where a monolayer graphene was developed using a Scotch tape technique in 2004. Their work has led to many exciting scientific researches into the two dimensional material, which could offer many novel applications. In 2010, a Nobel Prize was awarded to Geim and Novoslov for their experimental work on graphene. It was again awarded to researchers working on carbon materials. To date, the different graphitic forms, such as 0D fullerene, 1D carbon nanotubes, 2D graphene and 3D graphite, had successfully been discovered.
Since the discovery of graphene, the properties and applications of graphene have been extensively studied and exploited. However, researchers have realized many limitations of graphene, such as zero bandgap and low absorptivity etc. Structural modification of graphene was subsequently investigated to overcome the shortcomings. Graphene quantum dots (GQDs) were fabricated by Ponomarenko and Geim [13] in 2008 based on the previous work on carbon dots (CDs) by Xu [14] et al., in 2004. The GQDs are different from CDs because they possess graphene lattice inside the dots, which are smaller than 100 nm in size and less than 10 layers thick [15]. CDs are usually quasi-spherical carbon nanoparticles having a size of less than 10 nm [15]. The GQDs have many novel properties, such as its unique fluorescent properties discovered by Pan [16] et al., in 2010, due to its quantum confinement effect. The ability to modulate the properties of GQDs is important if the material is to be used in diverse applications. Modification of GQDs properties by means of doping was first investigated by Zhao [17] and collaborators in 2012 involving the use of nitrogen as dopant in GQDs. Besides, GQDs exhibit better solubility than CNTs. This is due to the large edge effect of GQDs, which can be modified by functional groups, unlike the CNTs that are limited by its one dimensional characteristics.
Graphene exhibits an infinite exciton Bohr radius [18]. However, GQD is a zero-dimensional material by converting two-dimensional graphene. This resulted in quantum confinement and edge effects, which the crystal boundary significantly modifies electron distribution due to the reduced dimension of the crystal to nanometer scale. Additionally, GQDs have non-zero bandgap, which is different from graphene. Previous theoretical calculations as well as optical and electrical experiments have demonstrated the existence of bandgap in GQDs. The semi-metal graphene can turn into either semiconductor or insulator GQDs. The broadening of the optical absorption of graphene has led to higher energy spectral due to the opening of the bandgap in GQDs. GQDs also exhibit different chemical and physical properties when compared with other carbon-based materials, such as CD, CNTs, fullerene and graphene etc., due to its special edge and quantum confinement effects. In the field of nanomaterials, many of the physical and chemical properties of nanomaterials are usually determined by the fabrication methods. Based on the GQDs preparation methods developed in recent years, these methods can be classified into two categories, namely top-down and bottom-up, according to the precursor material. As shown in the central part of Fig. 2, top-down method refers to direct cutting of graphene-related materials (such as graphene [16], graphene oxide [19], carbon nanotubes [20], carbon fibers [21], carbon black [22], graphite powder [23] and coal [24] etc.) into quantum size via various processes. In bottom-up approach, graphene-like smaller polycyclic aromatic hydrocarbons (PAHs) molecular precursors (such as benzene [25], hexaperi-hexabenzocoronene [26], glucose [27] and fullerene [28] etc.) are converted to GQDs by stepwise chemical reaction. In addition to quantum confinement and edge effects, GQDs exhibited many important characteristics, such as stable PL, non-toxicity, good solubility, surface grafting, biocompatibility and inertness [29], as shown in middle rings in Fig. 2. Some of these characteristics are overwhelmingly superior to traditional semiconductor quantum dots, hence prompted many potential applications of GQDs in solar cells [30e34], photodetectors [35e40], bioimagings [41e44], fluorescent agents [45e48], light-emitting diodes [49] (LEDs), batteries [50], sensors [51,52], drug carriers [53e58] and photo-/electro-catalytic [59] and so forth, as shown in marginal ring of Fig. 2.
The two effects, namely edge and quantum confinement effects, belong to the basic characteristics of GQDs. The unique chemical and physical properties of GQDs, such as non-toxicity, solubility, surface grafting, biocompatibility, inertness and stable PL, are results of its basic characteristics. Based on these properties, a series of applications are developed.

The outlooks of graphene quantum dots
The research on GQDs is still at an early stage, and many challenges of GQDs have yet to be addressed, as shown in Fig. 3. Although there are many important advantages and potential applications of GQDs, further research to enhance the properties of the material is required in order to meet the application requirements. Therefore, the studies of GQDs have been on-going to address the five urgent issues as stated in Fig. 3.
To meet the industry requirements, it is necessary to mass produce GQDs at relatively low cost. However, the product yield of GQDs using current preparation methods [60] is considerably low (<10% mostly), therefore new methods, such as photo-Fenton reaction method (highest yield of 45% [60]) and facile fabrication technique using coals [24], that can improve the product yield need to be exploited. Besides, the reported quantum yields (QYs) of GQDs are mostly at a range between 2% and 22.9% [60], which is much lower than the traditional semiconductor QDs. Thus the low QYs presented a challenge. Recent researches showed that the QYs of GQDs can be enhanced by surface modifications, such as the  formation of carbon-metal nano-composites [61]. In addition, the electronic and optical properties of GQDs, such as band gap [62] and fluorescence [63,64] etc., are dependent on their sizes and shapes. Therefore, the modulation of the intrinsic properties of GQDs is dependent on the accuracy in controlling its sizes and shapes etc. Conditions of fabrication methods are keys to solving this issue. GQDs have displayed various unique phenomena, such as up-conversion PL [65e67]. However, the current mechanisms were mainly acquired according to the optical behaviors leading to confusing results due to the different preparation conditions, hence there is a lack of understanding on the exact PL mechanism. Consequently, some forms of standard measurements coupled with theoretical studies should be carried out to better understand the PL phenomenon. To date, most of the reported PL colors of GQDs were ranged from blue to yellow [60]. The narrow spectral coverage of GQDs is limiting its applications in optoelectronic devices. The expanding of the spectral coverage of GQDs to all visible wavelengths and even near-infrared (NIR) is an important area of research in the future. Some groups [27,68,69] have obtained the NIR PL spectrum by doping nitrogen into GQDs.
The future of GQDs is more promising once the above challenges have been solved. In this review, we discuss the current preparation methods of GQDs in Section 2, and the functionalization of GQDs, such as heteroatoms doping, sizes and shapes controlling and forming its composite with other functional materials, in Section 3. In Section 4, some applications related to GQDs will be reviewed. Finally, a conclusion will be presented.

Preparation methods for graphene quantum dots
According to the current fabrication methods of GQDs as reported in literatures, the syntheses of GQDs can be classified into two main categories, namely top-down and bottom-up preparation approaches. As shown in Fig. 4, top-down approach involves direct cleaving of bulk carbon materials into nanoscale GQDs via liquid exfoliation and electron beam lithography techniques etc. Such approach has the advantages of abundant raw materials and would usually produce oxygen-containing functional groups at the edge, thus facilitating their solubility and functionalization. However, this approach has also suffered from some disadvantages, such as low yield, large density of defects, and non-controllable of size and shape and so on. While as the bottom-up approach is based on the growth of appropriate molecular precursors, such as small molecules and polymers, into nano-sized GQDs by hydrothermal, microwave-assisted hydrothermal, soft-template and metalcatalyzed methods etc. Such bottom-up approach possesses advantages, such as fewer defects and controllable of size and morphology; contrary to the top-down approach. However, the bottom-up approach has suffered from poor solubility, small dot size and aggregration issue etc. In the next context, detailed preparation methods are presented. Bottom-up methods will be presented first and then followed by top-down.

Bottom-up methods
As previously mentioned, bottom-up methods are based on growth of the sources including graphene-like smaller polycyclic aromatic hydrocarbons (PAHs) and appropriate molecules into GQDs. However, this method can be subdivided into four main routes, which cover hydrothermal method, microwave-assisted hydrothermal method, soft-template method and metal-catalyzed method, according to the way external energy is provided and the characteristics of fabrication.

Hydrothermal method
Hydrothermal method involves various techniques of crystallizing substances from high-temperature aqueous solutions to high vapor pressures. The fabrication of single-crystalline GQDs through the hydrothermal method has been demonstrated by many research groups. In early 2012, Dong et al. [69] successfully fabricated GQDs, having a size of~15 nm, using citric acid (CA) as source in the hydrothermal method that produced a photoluminescence (PL) quantum yield (PLQY) of 9.0%. The mechanism of synthesizing GQDs using CA via hydrothermal method is depicted in Fig. 5a. The atomic force microscopy (AFM) image of GQDs prepared by Dong and co-workers is displayed in Fig. 5b, which indicated that the GQDs were mostly nano-sheets of~15 nm in size with height in the range of 0.5e2.0 nm. Using CA and ethylenediamine (EDA) as carbon source materials, Yang's group [70] successfully produced nitrogen doped GQDs (N-GQDs) with a size of 5e10 nm, which exhibited relatively high PLQY of 75.2%. They suggested that the high PLQY was possibly due to two factors: the higher production yield (60%e70%), and the large number of surface defects, groups and edges from N-doping using EDA. Below is an equation related to the PLQY, which is expressed as [75,76]: where F is QY, K is the slope determined by the curves, h is the refractive index of the solvent, st refers to a standard with known quantum yield and c indicates unknown samples. For the aqueous solution, h c /h st was 1. Besides, Dong [71] et al. also prepared the N-GQDs using CA as precursor, and the resultant size was between 2 and 8 nm as indicated by the TEM image of the N-GQDs shown in Fig. 5d. The fringes of the carbon lattice can be clearly seen in the inset of Fig. 5d. Additionally, Chen and co-workers [77] fabricated amine-functionalized N-GQDs using CA and tris(hydroxymethyl) aminomethane (Tris-HMA), and its PLQY was 59.2%. CA as precursor has been widely adopted in the growth of GQDs using hydrothermal method [78,79]. Apart from CA, some graphene-like smaller polycyclic aromatic hydrocarbons (PAHs) can also be used as precursors in the preparation of GQDs. The GQDs emitting bright green fluorescence with high yield of 63% were synthesized with pyrene via hydrothermal method by Wang et al. [72]. Fig. 5e depicts the entire procedures. The dispersion showed highly uniform lateral size as indicated in TEM of GQDs in Fig. 5f. Fig. 5g showed the defect-free GQDs with spacing of 0.21 nm corresponding to that of graphene (100) planes. Furthermore, distinguishable benzene rings, which were well arranged in a hexagonal honeycomb carbon network, can be seen in the inset of Fig. 5g. Guo's group [73] prepared GQDs with tunable PL emission by hydrothermal approach using 1,5-dinitronaphthalene as raw material as shown in Fig. 5h. Interestingly, the amine-functionalized GQDs exhibited a distinct tunable PL emission from bluish green to yellow by changing the pH level from 5 to 10, as shown in Fig. 5i. This phenomenon can be attributed to the protonation and deprotonation of the amine-groups in acid or alkaline solutions. Broadening of the PL emission has been a challenge for previous materials. However, Zhao et al. [74] addressed the challenge by the formation of GQDs through pyrolysis of L-glutamic acid and the fabrication process is shown in Fig. 5j. The PL emission of GQDs fabricated by the raw material was tunable to NIR in the range from 800 to 850 nm. The morphology of the GQDs is shown in Fig. 5k, and high crystallinity of the GQDs is shown in inset of

Microwave-assisted hydrothermal method
Hydrothermal growth of GQDs usually takes long time. Thus, a fast and effective method was exploited by assisting with microwave, so-called microwave-assisted hydrothermal (MAH) method. The previously mentioned hydrothermal usually takes several hours, thus the method is not ideal for industrial mass production of GQDs. It is possible to shorten the growth duration of GQDs to a few minutes or even seconds by assisting with microwave.
Yang [80] co-author reported the fabrication of fluorine-doped GQDs by using MAH method and the growth of F-GQDs is depicted in Fig. 6a. The high crystallinity of the F-GQDs with an in-plane lattice spacing of 0.214 nm is shown in the HRTEM image (Fig. 6c). According to the size distribution, the size of F-GQDs was 2.38 ± 0.04 nm by Gaussian fitted calculation. The PL of F-GQDs was shifted to green (seen in Fig. 6b) as compared to simple GQDs, though the size of the prepared F-GQDs was much smaller than that of most GQDs. According to their analysis, the red-shifted PL might be ascribed to the  [70]; (d) TEM image of N-GQDs prepared by using CA, inset is high-resolution transmission electron microscopy (HRTEM) [71]; (e) Synthetic procedure of OH-functionalized GQDs (OH-GQDs) based on the source of pyrene via hydrothermal treatment [72]; (f) and (g) show the TEM and HRTEM image of GQDs prepared by using pyrene, respectively. The insets of (f) and (g) are lateral size distribution and fast Fourier transform (FFT) pattern, respectively [72]; (h) Schematic illustration of the hydrothermal synthesis of GQDs with bifunctional groups based on the source of 1,5-dinitronaphthalene [73]; (i) Digital photography of the GQDs solutions synthetized by using 1,5-dinitronaphthalene at different pH conditions under 365 nm UV irradiation [73]; (j) and (k) present the schematic diagram and the TEM image of GQDs which was synthetized through the pyrolysis of L-glutamic acid, respectively. The inset of (k) is the typical single GQDs with a lattice parameter of 0.246 nm [74].
doping of fluorine. The relationship between size of GQDs prepared by MAH method and the heating time was studied by Tang [63,81] et al. As shown in Fig. 6d, the glucose molecules dehydrated to form nucleation crystal of GQDs in the first step, and then the glucose and chemically active functional groups attached at the edges or the surface of growing GQDs. Finally, as-grown GQDs with certain size were prepared at different duration of microwave heating. Fig. 6e shows a series of GQD solutions, which were prepared by various heating times, irradiated by the ambient (top) and UV lamp (bottom). The emission color of the solutions are obviously changed. To understand this phenomenon, the diameter of the GQDs as a function of heating time was characterized using TEM and the curve plotted in Fig. 6f. The diameter of the GQDs increased monotonically from 1.5 to 3.9 nm when the heating time increased from 1 to 9 min, therefore increasing the heating times would lead to further growth of the GQDs. Moreover, the MAH method provided GQDs with uniform size distribution as shown in Fig. 6g and h. Feng et al. [82] also synthesized the GQDs and separated the A-(aqueous phase) and O-(organic phase) GQDs) by inducing the solvent. The whole separation of GQD solution and related structure of A-and O-GQDs is represented in Fig. 6i. Zhao and co-workers [83] obtained the oxygen-rich GQDs by MAH method and the level of doping with nitrogen was at 5.7%.
Hence, the MAH method combined the advantages of both hydrothermal and microwave techniques. The microwave heating provides simultaneous, homogeneous and fast heating, which has led to uniform size distribution of quantum dots and non-surface passivation.

Soft-template method
A novel method, known as soft-template, was developed for facile, low-cost and environmental friendly fabrication of GQDs. It provides nanoscale reaction cavity without complicated separation and purifying processes. Therefore, this route is in favor for mass production.
Yang and co-author [84] prepared GQDs by the soft-template approach. The TATB which is a planar and highly symmetric molecule with six strong intramolecular hydrogen bonds between eNH 2 and eNO 2 functional groups, and possesses a graphitic-like layered structure was used as the carbon source and template. Firstly, the TATB was annealed in the thermal process, which resulted in breaking down of various chemical bonds, along with the generation of expanding gases (e.g., NO, NO 2 , and H 2 O). Subsequently, the graphitic-like TATB multilayer developed into single layers as a result of the expanding gas. Finally, oxidative exfoliation  [63]; (g) and (h) present the TEM and the diameter distribution of GQDs, respectively [81]; (i) Schematic and the corresponding photographs of the interface separation process and detailed structures of the obtained A-and O-GQDs [82]. was employed to obtain dispersed N-GQDs. The preparation process of GQDs using TATB is displayed in Fig. 7a. Morphology and size of GQDs via the soft-template route using TATB is as shown in Fig. 7b. The AFM image shows well-dispersed and few-layer GQDs. One of the main advantages of soft-template method is controllable uniform size of GQDs. Müllen's [26] research group achieved monodispersed disk-like GQDs of~60 nm using HBC as precursor via the soft-template method. Detailed steps on the preparation are shown in Fig. 7c. The HBC molecules were condensedly stacked followed by the formation of a graphitic framework with fewer defects during the pyrolysis step. Subsequently, the artificial graphite was oxidized and exfoliated with a modified Hummers method, and then reduced with hydrazine. Finally, the GQDs prepared by using HBC as template was obtained. Homogeneous nanodisks of~60 nm in diameter and 2e3 nm in thickness were observed from the AFM image (seen in Fig. 7d and e). These GQDs demonstrated strong blue PL emission under excitation at 365 nm as shown in Fig. 7f. Apart from using the polymer as template, some small molecules were also used. For instance, Gao and co-workers [85] utilized small molecular carbon disulphide as template to fabricate GQDs doped with sulphur. As shown in Fig. 7g, the mixture liquid was burnt to form the carbon powder, which was then exfoliated to GQDs. The uniformity and dispersity of the GQDs were characterized by TEM (seen in Fig. 7h) showing an average diameter of 2.46 nm according to statistic calculation from Fig. 7i. Besides, Do et al. [86] used CA as the template to synthesize GQDs, which demonstrated high-quantum yield of 83%. It is worth mentioning that the PL intensity in a long-wavelength range (>500 nm) was observed to increase with increasing nitrogen content.
In brief, the soft-template can be a candidate for solving at least one of the challenges of GQDs, such as low-product yield.

Metal-catalyzed method
Metal-catalyzed method is a scarce route for preparation of GQDs. Loh et al. [28] utilized ruthenium as metal catalyst and used C 60 as a precursor to synthesize GQDs (seen in Fig. 8a). Interestingly, this method can modify the shape of GQDs by the annealing temperature, as shown in Fig. 8bec. The scanning tunnelling microscopy (STM) measurement was carried out on the GQDs grown on the surface of Ru (0001) at different annealing temperature for certain duration. The triangular and hexagonal shaped GQDs were obtained by annealing at 725 K and 825 K for 2 min respectively, as shown in Fig. 8deg. However, special structure of raw material and metal-catalyst were necessary for the fabrication of GQDs using this method, thus this route is uncommon. Fig. 7. Schematic diagrams and related characterized results of GQDs by using soft-template approach. (a) Illustration of the formation process of GQDs from single-layer 1,3,5-Triamino-2,4,6-trinitrobenzene (TATB) intermolecular condensation by soft-template route [84]; (b) AFM image of GQDs which was synthesized by using TATB deposited on a freshly cleaved mica surface. Inset: height profile along the dark line in the AFM image [84]. (c) Processing diagram for fabrication of GQDs via soft-template method based on the carbon source of Hexa-peri-hexabenzocoronene (HBC) [26]; (dee) and (f) AFM topography images of GQD prepared by using HBC and height profiles along the line in the images, and optical photograph obtained under excitation at 365 nm UV lamp, respectively [87]; (g) Schematic representation of the GQDs prepared by soft-template route [85]; (h) and (i) show the TEM and diameter distribution of GQDs fabricated by former method, respectively [85].

Top-down methods
An opposite route of the bottom-up method is top-down method, according to the relationship between precursors and products. Top-down approach usually fabricates the GQDs by breaking down the bulk material by physical or chemical means. The first GQDs was developed using this approach, thus top-down routes is very effective in the discovery of new materials and research of its structure and properties. Among top-down routes, liquid exfoliation method, by means of hydrothermal, electrochemical, oxidation and ultrasonic etc., and electron beam lithography method are commonly used routes. There are other less common routes, such as magnetron sputtering technique, which will also be described in this section.

Liquid exfoliation method
Nowadays liquid-phase exfoliation is often used for the preparation of graphene, where intercalation of graphite was performed using strong oxidizing agents, followed by expansion of graphite layers via thermal or chemical techniques. Such synthesis approach has increasingly been used in the preparation of GQDs by many groups [87e91]. The preparation of GQDs usually required the use of acid in cutting graphene, as shown in Fig. 9a. Wu and co-workers [16] produced GQDs that demonstrated blue luminescent via the liquid exfoliation route. The GQDs exhibited uniform size distribution with new PL results. Bulk carbon materials have been used in the synthesis of GQDs. For instance, carbon fibers (CF) was used as the precursor in the preparation of GQDs by Peng's [21] liquid exfoliation approach, as shown in Fig. 9b. AFM measurement indicated that the GQDs were consisting of 1e3 graphene layers (seen in Fig. 9c). Their results also showed that the PL emission color was influenced by the synthesis temperature. Besides, Tour et al. [24] managed to increase the product yield of GQDs through the use of coal via the same method as shown in Fig. 9d.
Similarly, Li [92] used coal for the preparation of GQDs. Fig. 9e shows the morphology and single crystalline of the GQDs. However, there was nonuniformity in the size distribution of the GQDs as observed from the TEM image, which could be caused by the fabrication conditions. On the other hand, graphite [97,98] has been considered the best source in the preparation of GQDs. Wang and co-authors [93] fabricated GQDs by sonication of graphite in liquid. Fig. 9f shows the complete preparation process of the GQDs based on the use of graphite as source. The TEM image of the GQDs showed an average lateral size of 2.5 nm (Fig. 9g), and the HRTEM indicated good crystallinity of the GQDs with lattice spacing of 0.335 nm corresponding to the c-axis orientation, as shown in Fig. 9h. Similar experiment was also carried out by Pei et al. [96] A more facile route was recently developed by Zuo and co-workers [94]. As shown in Fig. 9i, cotton was first annealed and then followed by liquid exfoliation. GQDs doped with fluorine were subsequently obtained. Similar shape of PGNFs to the CF can also be used as the source for GQDs in liquid exfoliation method, as demonstrated by Li's [95] group. The SEM image in Fig. 9j shows high-purity platelet graphite nanofibers (PGNFs) with diameters of 10e40 nm and zigzag-like surfaces. The TEM images of the prepared GQDs are shown in Fig. 9k and l. The CeC bond length in the GQDs was 0.146 nm which is close to the ideal CeC bond length for graphite (0.142 nm). The GQDs also exhibited good crystallinity as indicated in the inset of Fig. 9l. Similar to Zuo' work, Zhao and coauthors [99] also used the cotton as the carbon source in the preparation of GQDs. The difference was the use of chlorine as dressing agent, thus the GQDs were doped with chlorine (Cl-GQDs) showing many distinctive properties from pure GQDs, such as the intensity and peak of UVevis absorption. The overall fabrication process of GQDs is shown in Fig. 10a. Apart from 3D graphite, 2D graphene and 0D fullerene, the 1D carbon nanotube can also be cut into GQDs. In the reported work of Dong et al. [20], SWCTs with an average diameter less than 2 nm and a length of 5e15 mm were cut by means of chemical oxidation using a three-step sediment to form the GQDs1 and GQDs2 respectively, as shown in Fig. 10b. Interestingly, the aqueous solutions of GQDs1 and GQDs2 were yellow under white light, but their UVevis absorption spectra are dissimilar (seen in Fig. 10h). Among the optical properties of GQDs1 and GQDs2, their PL characteristics were obviously different.
However, the former liquid exfoliation assisted both the hydrothermal and chemical oxidation. A new technique, known as electrochemical exfoliation, was recently introduced to the liquid exfoliation method. Huang and co-workers [100] used the electrochemical workstation to prepare GQDs with higher product yield by using graphene paper as anode and platinum (Pt) sheet as counter electron. During the electrochemical oxidation and cutting process, the colorless electrolyte solution quickly turned black with product yield of 28% (after 120 min) in an ammonia solution because of the rapid generation of GQDs, as shown in Fig. 10c. A possible mechanism was proposed suggesting the graphene sheets or graphite flakes rapidly broke apart due to the harsh electrochemical exfoliation conditions in the strong electrolyte system, hence leading to the low yield of GQDs. However, in a weak electrolyte environment, the relatively prolonged oxidation interaction and suppressed intercalation induced exfoliation resulted in the continuous and efficient formation of GQDs, as displayed in Fig. 10d. Fig. 10e shows the TEM image of GQDs using electrochemical route, which produced homogeneous and good crystallinity GQDs. Kun and collaborators [101] also synthesized GQDs doped with boron (B-GQDs) by using CF as anode and Pt as cathode in the electrochemical workstation, as shown in the left of Fig. 10f. Their preparation process of GQDs and a detailed diagram showing the formation mechanism is represented on the right of Fig. 10f. The UVevisible absorption spectra of the above GQDs synthesized with different ratios of H 2 O/ IL (e.g. 0-a, 15%-b and 30%-c) is shown in Fig. 10g. A clear red shift from 265 to 348 nm was observed when the ratio of H 2 O/IL increased from 0% to 30%, this was attributed to a new hydrogenbonded network formed that resulted in a change to the internal organization and the liquid structure of the ILs. The inset of Fig. 10g shows the fluorescence of GQDs solutions at different concentrations under the 365 nm lamp illumination. It showed that the PL properties were influenced by the ratio of H 2 O/IL. Graphene oxide  [21]; (c) AFM image of GQDs by cutting CF [21]; (d) Schematic illustration of the synthesis of GQDs by using coal [24]; (e) TEM image of the GQDs by using coal via liquid exfoliation method, the inset is the HRTEM image of the GQDs [92]; (f) Schematic presentation of the preparation of GQDs by sonication-assisted liquid exfoliating graphitic carbon precursor [93]; (g) and (h) show TEM and HRTEM images of GQDs prepared by sonication-assisted exfoliation route, respectively (the inset in panel (g) is the magnified TEM image of GQDs) [93]; (i) Illumination of entire fabricated process of GQDs through the cotton [94]; (j) SEM image of platelet graphite nanofibers (PGNFs) [95]; (k) and (l) display the TEM of GQDs by using PGNFs, the inset of (l) represents a FFT pattern [95].
has also been used as carbon source in the preparation of GQDs as shown by the work of Teng et al. [102]. The TEM image, as shown in Fig. 11aec, indicates homogenous GQDs with good crystallinity were prepared by the liquid exfoliation method.
Generally, the liquid exfoliation route occurs in the liquid and the properties of products are often determined by the process environment and conditions. Therefore, the chemical and physical properties of the GQDs can be precisely controlled by modifying the fabrication parameters.
A novel technique, different from liquid exfoliation, was exploited in the fabrication of GQDs by Liu's [103] research group via the use of magnetron sputtering. As shown in Fig. 11d, a composite film comprising of GQDs and ZnO is formed at the substrate after magnetron sputtering on the target of graphite and ZnO composite. Subsequently, the composite film is treated with acid and then dialyzed to separate the GQDs from ZnO. Fig. 11e shows the TEM image of the highly crystalline and uniform GQDs.

Electron beam lithography method
Electron beam lithography method was first used in the preparation of GQDs [13]. However, it was not widely used due to the requirement of expensive equipment. Besides, the size of GQDs was limited by the processable scale of lithography technology and the diameter of GQDs was arbitrary modified. Nevertheless, this method opened up an important field of 0D carbon materials, which have so far generated great excitement within the scientific world. Table 1 provides a summary on the related properties of GQDs produced by current state-of-the-art bottom-up and top-down methods. It is worth noting that the product yield of GQDs via bottom-up approach is generally higher than the top-down approach. Furthermore, the physical and chemical properties of GQDs can be easily modified when prepared using bottom-up approach as compared to the top-down. Also, the choice of source for bottom-up route is wider than the opposite route. In any case, the selected method would very much depend on your applications.

Functionalization of graphene quantum dots
Pure GQDs have many limitations which restrict its application. In order to expand its application in a variety of fields, GQDs can be functionalized by various methods to customize its properties for specific applications. Functionalization of GQDs could modify its optical, chemical and electronic properties, which allow the materials to be used in wide-ranging applications. As shown in Fig. 12, doping with heteroatoms, forming composites with inorganic materials or polymers, and the controlling of size and shape of GQDs can be considered as functionalization. The basic properties of GQDs are changed via those means, which could potentially address some of the challenges faced by GQDs. Research into the functionalization of GQDs remains a hot-topic in the study of carbon nanomaterials. In this section, we will introduce the various functionalization methods in the following sequence: doping with a variety of heteroatoms in Section 3.1, the controlling of size and shape to tune the properties of GQDs in Section 3.2, and lastly the improved performances of GQDs via forming composites will be described in Section 3.3.

Doping of graphene quantum dots via a variety of heteroatoms
Doping of semiconducting materials is an essential process in the semiconductor industry as it can modulate the basic physical, chemical and electronic properties of the materials, hence making them useful in device applications. Similarly, functionalization of GQDs by means of doping can modify its optical, electronic and chemical properties for novel applications. In general, we can categorize the doping method into three groups depending on the number of doping atoms, as shown in Fig. 13a. An effective doping can tune the optical, chemical, electrochemical and electronic etc. properties of GQDs. It is possible to characterize the effect of doping, for example, through determining the change in the color of PL, the conductivity and water solubility of the doped GQDs. The various heteroatoms doping and doping methods will be introduced in this section. The effect of doping GQDs resulting in the change of its basic properties will be described as well. To date, the heteroatoms used in the doping of GQDs include potassium (K), sodium (Na), boron (B), nitrogen (N), phosphorus (P), oxygen (N), sulphur (S), selenium (Se), fluorine (F) and chlorine (Cl), as shown in top-left diagram of Fig. 12. Besides, these atoms can be co-doped into the GQDs, which can enhance certain properties of GQDs. The change in properties could cause by the electronegativity and other basic properties of the dopant atoms. The doping of GQDs with single, double and multiple heteroatoms will be presented as follows.

GQDs doped with single heteroatom
Single heteroatom doping of GQDs has shown to alter the properties of the GQDs. This section will describe the changes in the properties of GQDs by doping with single atom and less emphasis is given to the doping methods.
Boron atom is near by the carbon in the periodic table, hence it can be easily introduced into the GQDs. Wang and co-workers [104] demonstrated the doping of B into GQDs via MAH method, as shown in Fig. 13b. TEM images reveal non-uniform dispersion of B-GQDs and high-quality crystalline structure as shown in Fig. 13c and d respectively. Interestingly, obvious absorption peaks ocurred at 230 nm, 260 nm and 320 nm, which was different from undoped GQDs, are shown in Fig. 13e. The prominent peaks at 230 nm was ascribed to the transition of the p-p* aromatic sp 2 domains, while the other two weaker peaks were attributed to C] O n-p* transition. The B-doped GQDs have shown to enhance its PL intensity as demonstrated by Yang et al. [105]. They obtained high quantum yield of 95% from their B-GQDs, which were fabricated by evaporating B-GQDs borax aqueous solution. Highly intensive fluorescence of their B-GQDs under 405 nm excitation is shown in Fig. 13feh. The B-GQDs were also synthesized by means of bottomup approach as demonstrated by Tam and co-authors [106], as shown in Fig. 13i. The electrocatalytic activity has been enhanced by B doping into GQDs. Apart from doping GQDs with B element, Qian et al. [107] doped GQDs with K atom and the resultant absorption spectrum is shown in Fig. 13j. Two peaks were apparent at 222 nm Similar results were also obtained by doping GQDs with Na atom as studied by Yazdi and co-workers [108]. They reported blue-shift of the absorption peaks from their Na-doped GQDs. Hence, the doping of B, K and Na into GQDs can tune the optical absorption to short wavelength and enhance its PL intensity.
Apart from the above-mentioned elements, other elements were also widely researched. Among all the elements, N atom doping of GQDs is considered the earliest [17] and widest [70,77,85,109e134] studies performed on doped GQDs. The N-GQDs were fabricated by solid-to-solid process through using melamine as carbon and nitrogen sources which were carried out by Zhu et al. [135]. The preparation process is shown in Fig. 14a. Interestingly, this method of preparation has produced high quality N-GQDs, as shown in Fig. 14bec. Fig. 14c shows a lattice spacing of 0.24 nm from the TEM image. In addition, UVevis absorption spectra of N-GQDs were studied, as represented in Fig. 14d. The observed three absorption peaks were due to the aromatic sp 2 domains around 210 nm, n-p* transition absorption peak around 240 nm and band-gap transition absorption peak around 270 nm, while the long tail extending to the visible range spectrum. It is important to note that the band gap can be tuned through controlling the preparation conditions, such as doping concentration, preparation temperature and pH level. The band gap structure of N-GQDs prepared by the solid-to-solid method is displayed in Fig. 14e. The double plotted graphs showing the dependent of band gap on doping concentration and synthesis temperature is shown in Fig. 14f. From the graphs, we can deduce that the doping concentration decreased with the synthesis temperature, while the band gap was significantly influenced by the doping concentration due to the high electronegativity. Furthermore, when the N-GQDs were in an acidic environment, the graphitic N was gradually protonated and then showed a weaker negative induction effect than that of un-protonated graphitic N. This resulted in a high p electron cloud density, which led to a narrowed band gap. This phenomenon is represented in Fig. 14g, which shows the decreasing band gap of N-GQDs by reducing the pH level in the N-GQDs at a doping concentration of 5.5 at%. In addition, Sun and coworkers [136] fabricated high-crystalline N-GQDs via cutting graphene oxide (GO) precursor (refer to Fig. 14h), and the quantum yield was significantly improved to 74% through controlling the size and nitrogen doping. Microwave-assisted hydrothermal technique was used by Tang and co-workers [27] in the preparation of N-GQDs (refer to Fig. 14i), the resultant absorption peak was modified and exhibited at near-infrared region instead of situating at yellow  [105]; (i) Diagram of preparation of B-GQDs through hydrothermal route [106]; (j) and (k) show UVevis absorption and TEM image of K doped into GQDs (K-GQDs), respectively. The inset of (j) is optical image of aqueous K-GQDs solution under UV 365 nm excitation [107]. [142,143] or blue [144,145] regions. A novel metal catalytic strategy was also used to prepare N-rich GQDs by Lv's research group [137]. Fig. 14j displays the optical properties of their N-rich GQDs, which revealed excellent luminescence behavior. Huang [138] demonstrated the formation of N-GQDs via hydrothermal method using CA and dicyandiamide as carbon and nitrogen sources. Their technique demonstrated a high quantum yield of 36.5%. The strategy is illustrated in Fig. 14k. Up-conversion of PL was observed in N-GQDs prepared using hydrothermal method. This means that the emission wavelength of the N-GQDs was shorter than the excitation wavelength as studied by Li et al. [139]. The resultant PL curves for the N-GQDs are shown in Fig. 14l. An excellent quantum yield of 94% for N-GQDs was achieved by Sun and co-workers [146].
Apart from the N doping, P-doped GQDs was also studied [93]. Liu [140] fabricated P-doped GQDs (P-GQDs) having good stability and dispersion, and exhibited wide visible-light absorption region. Fig. 14m shows the synthesis process of P-GQDs. It is possible to tailor the electronic properties of P-GQDs via the process. P-GQDs can also be prepared using an electrochemical route as performed by Ma and co-workers [141] to investigate the free radical scavenging ability of P-GQDs. UVevis absorption of the DPPH in water with P-GQDs before and after the scavenging period was measured and is shown in Fig. 14n. The inset represents the change in color of the DPPH solution from purple to yellowish which indicated the excellent scavenging ability of P-GQDs.
Group Ⅵ and Ⅶ elements were also studied for doping of the GQDs. Li et al. [147] doped the GQDs with S atom using hydrothermal method and the preparation process is shown in Fig. 15a. TEM image, shown in Fig. 15b, indicates S-GQDs with uniform size were prepared. The electron diffraction pattern of the S-GQD indicates both in-plane (d 2 ¼ 0.213 nm) and basal plane (d 1 ¼ 0.344 nm) diffractions, as shown in Fig. 15c. Jin et al. [148] analyzed the optical variation of S-GQDs. Fig. 15d shows the fluorescence of S-GQDs at different preparation temperatures. Strong luminescence was observed for S-GQDs prepared at a temperature of 170 C. Their finding suggested that the excessively high temperature introduced a large number of S atoms, which subsequently covered the surface of S-GQDs resulting in the decrease of the fluorescence intensity. Moreover, the UVeVis absorption spectra of S-GQDs were compared with undoped GQDs, as shown in Fig. 15e. For S-GQDs, there existed a new absorption peak at 350 nm, which was absent from the undoped GQDs. In addition, the intensity of the overall absorption wavelength of S-GQDs was larger than the undoped GQDs. Interestingly, Wang and co-workers [149] prepared S-GQDs using durian (i) Diagram illustration of N-GQDs by using microwave-assisted hydrothermal routine [27]; (j) Optical characters of N-rich GQDs, UVevis absorption (black line), PLE spectrum (green line) and PL spectrum (red line), inset shows photograph of N-rich GQDs under UV 365 nm lighting [137]; (k) The illustration on the formation of N-GQDs by utilizing hydrothermal treatment [138]; (l) Up-conversion PL spectrum of N-GQDs at different excitation wavelength [139]; (m) Synthetic process and structural illustration for P doped GQDs (P-GQDs) [140]; (n) UVevis absorption of the 2,2-diphenyl-1-picrylhydrazyl (DPPH) with water P-GQDs which is fabricated by electrochemical routine before (1d) and after (5d) the scavenging period, inset is the color of the DPPH before and after the scavenging period [141].
flesh as precursor in a bottom-up approach. TEM image of the S-GQDs and its preparation process are shown in top part of Fig. 15f. Importantly, the PL characteristics of the S-GQDs were strongly determined by the doping concentration, as shown in bottom part of Fig. 15f. The color of PL from the S-GQDs can be tuned from blue to yellow, which is distinctively different from other elemental doping. Besides, the quantum yield was high and can reach up to 79% for S-GQDs exhibiting yellow PL. Group Ⅵ elemental doping of GQDs, such as O and Se doped GQDs, were also prepared to form GOQDs and Se-GQDs. Yang and co-workers [150] prepared Se-GQDs by using GOQDs as precursors. Fig. 15g shows the uniform size and high-quality crystalline of their Se-GQDs, which displayed yellow luminescence with a quantum yield of 29% (shown in Fig. 15h). Group Ⅶ elements, such as Cl and F, were also investigated for doping GQDs. Li and co-workers [151] prepared Cl doped GQDs (Cl-GQDs) using the process shown in Fig. 15k. TEM image of the Cl-GQDs is shown in Fig. 15i. Multicolor light emission was demonstrated from the Cl-GQDs as shown in Fig. 15l. Besides, Zhao et al. [99] reported the optical absorption peaks of Cl-GQDs occurred in deep-UV unlike the undoped GQDs. Zuo and co-workers [94] prepared F-doped GQDs, however its properties were rarely studied.
In brief, single elemental doping route has been studied extensively and in great depth, but most studies have been focussed on the N-GQDs. It would be of interest to explore other doping elements.

GQDs doped with double heteroatom
The doping of double atoms was initiated to utilize the advantages of various doped elements and has been performed by researchers around world [160e172]. UVevis absorption spectra of S and N co-doped GQDs (S, N-GQDs), which were prepared by Qu et al. [152], is shown in Fig. 16a. Three absorption bands were found at 338 nm, 467 nm and 557 nm, which were different from previous report. The extending absorption bands were attributed by the introduction of S and N atoms. The PL quantum yields for S, N-GQDs were 61%, 45% and 8% at wavelength of 340 nm, 440 nm and 540 nm, which represented blue, green and red emission, respectively. They explained the PL process of S, N-GQDs using the proposed mechanism shown in Fig. 16b. There were O, N, S states, which could relate to the p* orbital of the C]O (excitation energy was above 3.10 eV), C]N (excitation energy was above 2.75 eV) and C]S (excitation energy was above 1.90 eV) bonds in the S, N-GQDs. The observed cyan light under excitation wavelength of 440 nm could due to the overlapping of the N and O states in some degree. Moreover, the S p* orbital and S state, which were formed by introducing S doping, contributed to a weaker electronegativity. As a result, S, N-GQDs can absorb lower energy and emit light with longer wavelength. A high quantum yield of 87.8% for S, N-GQDs had been realized by using one-step bottom-up molecular fusion in a hydrothermal process [153] as shown in the diagram of Fig. 16c. The S, N-GQDs prepared by this method displayed bright blue fluorescence with quantum yield of 23.2%. They have successfully demonstrated the co-doping of S and N atoms into the lattice of GQDs. Kim's group [154] fabricated various doped GQDs by dissolving organic materials in different solvents. As shown in Fig. 16d, catecholamine was first dissolved in water and N, N-dimethyl sulfoxide (DMSO). N-GQDs and S, N-GQDs were then synthesized using microwave-assisted hydrothermal method. Besides, Li et al. [155] fabricated N, S-GQQDs via a facile, cheaper, and environmental friendly hydrothermal method using ammonia, S powder as dopant and GQDs mixed precursor. A relatively high QY of 41% was obtained from the S, N-GQDs and the TEM image on the materials showed uniform size as well as highcrystalline structure (refer to Fig. 16eef). In addition, the PL properties of N-GQDs and S, N-GQDs were investigated. PL emission of the NS-GQDs exhibited a clear blue shift of 54 nm as compared to a red shift observed from the N-GQDs (refer to Fig. 16g). However, such findings have been debated. Interestingly, Xu and co-workers [156] used a porous polythiophene-derived carbon as the sulphur source, while HNO 3 vapor was presented as the scissor and the nitrogen source, to prepare the S, N-GQDs using the novel method, as shown in Fig. 16h. The color of PL can be modified from blue to yellowish-green by varying the synthesis temperature between 150 C and 180 C as shown in Fig. 16i. Qu et al. [158] demonstrated that the introduction of S atom could alter the surface states of GQDs resulting in the extension of the absorption band to visible region (refer to Fig. 16k). S, N co-doped GQDs showed different emission colors under excitation of 420e520 nm as shown in the insets of Fig. 16k. To date, the main researches have been focussed on the S, N co-doped GQDs, but P, N co-doped has been less studied. Chen's [157] research group synthesized N and P co-doped GQDs from a single biomolecule precursor via a simple strategy, as illustrated in Fig. 16j. Such P, N-GQDs exhibited high quality structure and excellent optical characteristics (refer to Fig. 16l and m respectively). As shown in Fig. 16m, the absorption band was extended and the prominent PL peak occurred at 460 nm while a shoulder peak was found at 360 nm. Such optical characteristic was attributed to the introduction of S atoms. Limited work was reported on N, B codoped into GQDs. However, Favaro [159] and co-authors prepared N, B co-doped GQDs to enhance the electrochemical activity and they noted a clear decrease in the overpotential as a function of dopant concentration according to the sequence: N> B > B,N.
In summary, the co-doped technique was indeed able to control the properties of GQDs. However, the choice of the doping elements usually depends on the application of GQDs.

GQDs doped with multiple atoms
In addition to the double heteroatom doped GQDs, multiple atom doping also paly important role in some field such as batteries, solar cells and so forth. Pillai and co-workers [173] fabricated F, S, N-doped GQDs, the QY could reach up to 70%. The fabrication procedure and TEM images are presented in Fig. 17 (aec). Another group [174] also prepared F, S, N-doped GQDs (see Fig. 17 (def)) and utilized them to optimize the performance of dye-sensitized solar cells with power conversion efficiency (PCE) of 11.7% ± 0.22 and a fill factor (FF) of 71 %, and a QY of 70%. The tested properties of the solar cells can be seen in Fig. 17 (g). In a word, the multiple atom doped GQDs is an interesting approach, it should be extensively studied in the future. Fig. 18 is a plot to present the absorption peaks of various doped GQDs in the literature and the electronegativity of the dopant. It is noted that the absorption peaks are mostly concentrate on the blue region. The absorption peak can be extended from deep UV to red t by using S, N or N doping.

Tunable properties of graphene quantum dots by controlling size and shape
Another strategy to tune the optical, physical and chemical properties of GQDs is via controlling the size and shape of the GQDs. Several methods have been applied to modify the size and morphology of GQDs. Fig. 19a shows a decrease of the energy band gap with increasing size of the GQDs, hence demonstrating a relationship between energy band gap and size of the materials. Besides, Chen and coworkers [175] studied the relationship between the band gap and size using density-functional theory (DFT) and time-dependent DFT calculations as shown in Fig. 19b. It was suggested that the PL of a large GQD, consisting of heterogeneously hybridized carbon network, was essentially determined by the embedded small sp 2 clusters isolated by sp 3 carbons. There was a slight difference between theoretical prediction and experimental work in the degree of tuning the band gap by changing the size of GQDs, however the trend was in good agreement as studied by Ye and co-workers [176]. The band gap of GQDs, which influences its photoluminescent, can be tuned by tailored GQDs size via chemical methods.

Modification of GQDs bandgap via controlling size and shape
Based on the above-mentioned theoretical studies, numerous experimental studies have been performed to investigate the modification of GQDs bandgap. As shown in Fig. 19c, GQDs were capable of emitting light ranging from blue-green (2.9 eV) to orange-red (2.05 eV), depending on its size, functionalities and defects. Kim's [19] research group studied the relationship between PL and size of GQDs. Anomalous size dependences on the visible PL was also found in their experiment, which could due to shape and edge-state variations in the GQDs, as shown in Fig. 19dee. Both the size and morphology of GQDs were investigated to study their effect on tunable bandgaps. For example, studies were performed on GQDs with different shapes (e.g. circular to polygonal) and the corresponding edge-state variations as the size of GQD increases suggested that the PL behaviors were attributed to the novel feature of GQDs. Furthermore, morphology of GQDs can solely influence its band gap. Sarkar and co-workers [177] performed theoretical studies on the relationship between the shape of GQD and its band gap, as shown in Fig. 19f. Besides, they also explored the possibility of  The properties of GQDs modified by its size and shape have been investigated using different methodologies, such as Raman scattering [178], density of states [179], magnetic properties [180,181], faraday optical rotation (FOR) [182] and optical characterizers [130,183,184]. This topic on band gap engineering of GQDs via tailoring its size and shape has attracted great interests from both theoretical [185,186] and experimental [18,187] researchers. This is a promising technique in extending the application of GQDs in the near future.

Graphene quantum dot composites to optimize performances
There are several drawbacks of GQDs, such as difficult to assemble into film and poor conductivity etc. The formation of GQD composites could overcome some of the challenges and enhance the performances of the material, thus leading to many novel applications. As shown in Fig. 20a, polymer materials [191,192] and nano-materials [193e198] can be used to form composite with GQDs. Follow the sequence of organic and inorganic materials   [175]; (c) The curves of band gap with the increase with the size of GQDs, inset shows the color of PL tuneable with the size of GQDs [176]; (d) and (e) represent the UVevis spectra and PL of GQDs with various sizes, inset is digital picture of different luminescence of GQDs with different sizes [19]; (f) The energy gap as a function of total number of conjugated carbon atoms (N) and various morphologies for type-1 GQD (dash line with black circle), type-2 GQD (dash line with red square), type-3 GQD (dash line with green triangle) and type-4 (dash line with blue star), insets represent GQDs with different morphologies [177]. composites, the forming of composites with GQDs and other materials will be described.
Organic materials have a lot of merits, such as easy to form film, high carrier mobility and so on. These virtues could complement the performances of GQDs. Wen and co-workers [188] fabricated fluorescent organosilane-functionalized GQDs (Si-GQDs) and subsequently embedding them into mesoporous hollow silica spheres as bio-label for the first time. The entire preparation process is shown in Fig. 20b. TEM image, shown in Fig. 20c, displays the high crystalline quality of Si-GQDs and the characteristic of GQDs. Chen and co-workers [189] developed composite of GQDs and polypyrrole (PPy) as counter electrode of dye-sensitized solar cells (DSSCs) to optimize the performances of the solar cells. The fabrication procedure is presented in Fig. 20d. The composite formed a compact and uniform electrode as shown in Fig. 20e. As the composite film of GQD and PPy consisted of porous structures, there were more active sites resulting in higher charge transfer rate. The performance of DSSCs was greatly enhanced by the composite film and achieved the highest PCE of 5.27%, which was~20% higher than that of plain PPy (4.46%) based DSSCs. There is no doubt that the composites of GQDs and organic materials have enormous potential in future. Interestingly, Zor et al. [190] prepared a novel multifunctional composite consisting of magnetic silica beads, GQD and molecularly imprinted polypyrrole. The formation of the fabricated composite material is shown in Fig. 20f. The composite exhibited enhanced optical and magnetic properties. For example, the absorption spectrum of the composite was extended to greater wavelength, as displayed in Fig. 20g. Whereas, the inset of Fig. 20g indicates the obvious difference in the color of aqueous solution of GQDs and mSGP. The enhanced properties of GQD composite would benefit many applications. There has been a growing interest in developing new GQD composites in recent years. Malik's [199] group prepared nanofiber composite, which consisted of GQDs and polyaniline, as supercapacitor electrode materials. The composite material led to an enhanced current density of 1 Ag -1 as well as cyclic stability. The preparation process of the composite is shown in Fig. 21a. Qu et al. [200] fabricated composite of 0D GQDs and 3D graphene (3DG) by assembling them via a novel benign electrochemical method, as shown in Fig. 21b. The GQDs-3DG composite material was then used to develop supercapacitor with enhanced performances, which relied on the high surface area and increased active sites of the GQDs.
Apart from the composites of GQDs and organic materials, many inorganic materials have been used to form GQD composites to improve the performances of devices. Dhar and co-workers [201] demonstrated the composite of ZnO and GQDs can enhance the performance of UV photodetector with superior external quantum efficiency (EQE) of~57681% (at -1 V bias) at wavelength of~340 nm. The device structure is shown in Fig. 21c. The composite of ZnO nanorods/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and GQDs exhibited far better performances than GQD modified ZnO NRs and pure ZnO NRs. Other shape of ZnO, such as leaf-shaped ZnO (L-ZnO) [202], was also used in forming composite with GQDs. TEM image of the L-ZnO and GQDs composite is as  [189]; (f) and (g) display the schematic representation of the magnetic silica beads/graphene quantum dots/molecularly imprinted polypyrrole (mSGP) composites and spectroscopic characterization of the mSGP composites, inset of (g) is the optical picture of water solutions of GQDs (left) and mSGP (right) [190].
shown in Fig. 21d. UVevis absorption of the composite showed an increase in the absorbance intensity as well as extended spectrum (refer to Fig. 21e). In the same manner, TiO 2 and GQDs nanocomposites were fabricated by Bu [203]. TEM image of the nanocomposites is shown in Fig. 21f. The nanocomposites were used as photocatalysts, which exhibited extremely high photocatalytic activity due to the larger energy barrier between NGQDs and BeTiO 2 , resulting in enhanced light absorption and higher electron density. Ji et al. [204] assembled lithium ion battery using composite of MnV 12 and GQDs. The fabrication process is shown in Fig. 21g. The bar shaped composites can maximize the interfacial contact area and improve charging/discharging behavior, thus optimizing the performances of the lithium ion battery. Another interesting experiment [205] was performed by assembling Mn 3 O 4 and GQDs into nanocomposites, which can change the energy level of the materials instead of doping with heteroatoms or controlling the size and shape of GQDs. As shown in Fig. 21h, the multi-walled carbon nanotubes (MWCNTs), acted as the GQDs' precursor, were mixed with the Mn 3 O 4 to form the composites which exhibited intensive Raman scattering. This could be used in bioimaging to discriminate cancer cells and normal cells. Due to the large surface area to volume ratio of the GQDs, the composite materials are frequently used as catalysts. Wu et al. [206] fabricated the composites of GQDs and Au nanoparticles as catalysts. TEM image and structure of the composites are displayed in Fig. 21i and j, respectively.
In brief, GQD composites have shown to improve and optimize the performances of devices. Novel functionalities of GQDs can be developed using techniques, such as doping, controlling size/shape and composites, which would open up new applications for GQDs.

The applications of graphene quantum dots
The extensive properties of GQDs (and functionalized GQDs) would benefit many applications in a variety of fields. As shown in Fig. 22, GQDs have found many potiential applications in fields, such as medical [214e219], optical [85,220e224] and energy [139,225e229], which will impact our quality of life and attract significant commercial interests. Much research has been carried out to understand the unique properties of GQDs and tailoring its  [199]; (b) Cartoon representation of fabricating a symmetrical supercapacitor based on the GQDse3DG composite material [200]; (c) Schematic diagram on the structure of UV photodetector using composites of ZnO and GQDs as active layer [201]; (d) and (e) show SEM image and UVevis absorption of composites of leaf-templated ZnO and GQDs, respectively [202]; (f) TEM image of black TiO2 [203]; (g) Schematic illustration of the fabrication of composite of a manganese vanadium oxide and (GQDs) using a molecular manganese vanadate precursor [204]; (h) The synthesis route for the GQDeMn3O4 nanocomposite using MWCNTs as the precursor of GQDs [205]; (i) and (j) represent TEM image and the structure of composites of Au nanoparticles and GQDs [206].
properties using different techniques, such as doping, controlling of size/shape and developing composite material and so forth.
In the next decade, GQDs may become a popular material for advanced technology. They have already been exploited in a number of applications, such as biological imaging [230,231], drug delivery [232,233], photodetector [234,235], LEDs [236e238] and battery [239] etc. In this section, we will present the current applications of GQDs in medical, optical and energy-related fields. Each of these fields will be subdivided into detailed applications.

Medical applications
Due to the biocompatibility and nontoxicity of the GQDs, the nanomaterials have been exploited for medical applications and demonstrated some excellent performances, which could potentially replace some traditional materials in this field. There have been continuous efforts in exploiting GQDs for other novel medical applications due to the recent promising outcomes in the field.

Biological imaging
GQDs have been used in biological imaging for medical diagnostic as they can assist in locating cancerous cells and determining if drugs have been delivered to targeted cells as well as locating the drugs within cells [242]. The strong and tunable PL of GQDs allow the materials to be used in biological imaging. Pu and co-workers [240] used the modified GQDs as efficient fluorescent probes for highly selective detection. The entire fabrication process is shown in Fig. 23a. Firstly, the GQDs were stripped from graphene oxide sheets and amine-modified. Anti-human IgG antibodies were conjugated with Fe 3 O 4 nanoparticles to detect casts in urine. The modified GQDs were then linked on to the surface of anti-IgG functionalized Fe 3 O 4 nanoparticles. Finally, the Fe 3 O 4 /GQDs fluorescent probes were added into the sample to detect casts through fluorescent imaging. For comparing the efficiency of fluorescent probe using GQDs and Fe 3 O 4 /GQDs, a series of pre-quantitative casts (2, 4, 8, and 10 casts/ml) were analyzed by microscopy and fluorescent imaging assay, and the results are shown in Fig. 23bed. At the same concentration of casts, the counts of fluorescent imaging assay from Fe 3 O 4 /GQDs (Fig. 23d) were approximately 10 times higher than that of microscopy (Fig. 23b) and GQDs only (Fig. 23c). Their results indicated the detection efficiency of this fluorescent imaging assay was higher than microscopy mainly due to the magnetic enrichment of Fe 3 O 4 /GQDs nanocomposites. Such magnetic enrichment was efficient and simple.
Yang and co-workers [241] demonstrated that GQDs can be widely utilized for biological imaging in different kinds of cells. They prepared GQDs with uniform size and strong yellow emission by using top-down method. Three different kinds of stem cells, namely neurospheres cells (NSCs), pancreas progenitor cells (PPCs) and cardiac progenitor cells (CPCs), were used in the studied and cultured in different mediums. On the day of treatment, the cells were incubated with GQDs at a final concentration of 25 mg mL À1 for 24 h at 37 C. Subsequently, the cells were washed three times with PBS and then fixed with 4% paraformaldehyde for 20 min at room temperature. Fig. 23f, h and j show the confocal fluorescent images of NSCs, PPCs and CPCs with an excitation wavelength of 405 nm, respectively. As shown in Fig. 23e, g and i, the morphology of the cells can be discerned from the internalized GQDs. These results demonstrated that the GQDs can be used in different cells for biological imaging. However, there was slight cytotoxicity of GQDs in these cells as observed in Fig. 23k. The average cell viability of NSCs and CPCs was above 80% after 3 days culturing with GQDs at a concentration of 100 mg mL À1 , and was about 65% for PPCs.
In summary, the biocompatibility, strong fluorescence and nontoxicity of the GQDs have been investigated and they are an excellent choice for biological imaging in medical applications.

Drug delivery
Compact drug delivery nanosystems with multifunctional features are currently of considerable interest for cancer therapy owing to their improved dose tolerance and therapeutic efficacy [243]. GQDs have many attractive properties, such as sp 2 carbons, rich in p electrons and having a variety of functional groups (e.g.

carboxyl, hydroxyl, carbonyl and epoxy).
This is in addition to the reduced size of GQDs and different chemical reactivity as compared to other graphene based materials, such as GO or CNTs [245]. These characteristics would make GQDs an ideal candidate for drug delivery platform through p-p interactions and easy functionalization via the oxygen functional groups [243].
Hu and co-workers [243] fabricated SCNAs by employing GQDs as the substrate of the vehicles, as shown in Fig. 24a. Both GQDs and SCNAs were incubated with RG2 cells (a rat glioma cell line) at various concentrations for 24 h in order to evaluate the effects of the particles on cell viability. As shown in Fig. 24bee, CLSM images of RG2 cells incubated for 0.5 h showed the presence of SCNAs in the cytoplasm, and with increasing incubation time to 4 h, most of the particles were surrounding the cell nuclei. In addition to the fluorescence signal from DOX, the fluorescence of the GQDs could also be observed in RG2. When comparing the fluorescence signals between DOX and GQDs, most of them actually overlapped, which indicated good affinity in the cell. A detailed schematic illustration of the drug delivery is represented in Fig. 24f. The effectiveness of GQDs as drug delivery was studied by Some [244] et al. They used graphene-derivatives (such as DGO and GQD) as nanovectors for the delivery of the hydrophobic anticancer drug Cur and performed clinical studies using the technique. As shown in Fig. 24g, while tumor growth was initially inhibited to some extent (blue line) in the control group treated with free Cur, the tumor size did eventually increase. In stark contrast, the DGO-Cur and GQD-Cur groups exhibited a remarkable inhibition of tumor growth. Both GQD-Cur and DGO-Cur treated mice survived more than 14 days with almost no observable increase in tumor size (yellow and green lines). The photographic images showing the size of the tumor in response to the treatments can be seen in the inset. Besides, GQDs can also be used in gene delivery due to its favorable properties. For instance, Zhang's [248] and Guo's [249] team demonstrated such delivery by using GQDs.
In brief, the preliminary work on using GQDs as drug delivery system demonstrated the superior stability, good biocompatibility and excellent therapeutic performance of the material, which is a promising candidate for in-vivo therapeutic agent.

Photodynamic therapy
Photodynamic therapy (PDT), also known as photochemotherapy, is a form of phototherapy involving the use of light and a photosensitizing chemical substance that applied in conjunction to elicit cell death by molecular oxygen (phototoxicity). An interesting phenomenon that GQDs can generate the reactive oxygen species (ROS) in tumor cells was observed [250]. In addition, the upconversion characteristic of GQDs can convert low energy NIR light to UVevisible light, which has been widely examined for biological applications. Therefore, GQDs can be considered excellent materials for photodynamic therapy. Xing and co-workers [246] explained the mechanism of PDT (refer to Fig. 25a). The effectiveness of PDT using GQDs was investigated through clinical evaluation as shown in Fig. 25g. Furthermore, photoinduced cytotoxicity and oxidative stress of GQDs were also studied by Yin et al. [247] as shown in Fig. 25bee. Their findings suggested GQDs as anti-oxidants and pro-oxidants upon irradiation because they have the potential to become potent anti-oxidants for controlling ROSinduced cell damage.

Photothermal therapy
Apart from the above-mentioned PDT, another therapeutic technique has been performed using GQDs. Photothermal therapy (PTT) refers to the use of electromagnetic radiation (e.g. often in infrared wavelengths) for the treatment of various medical conditions, such as cancer. This approach is an extension of photodynamic therapy, in which a photosensitizer is excited with a specific band of light. This activation brings the sensitizer to an excited state where it then releases vibrational energy in the form of heat that kills the targeted cells.
Unlike photodynamic therapy, PTT does not require oxygen to interact with the target cells or tissues. Current studies on PTT are investigating the use of longer wavelength light, which is less energetic, and therefore less harmful to surrounding cells and tissues. For instance, Zhu and co-workers [58] fabricated DOX-MMSN/GQDs which were used as multifunctional platform for controlled drug delivery, magnetic hyperthermia and photothermal therapy, as shown in Fig. 25f. Many more studies have recently been carried out, such as from Lee's [253] and Yang's [254] research group. In short, GQDs has enormous prospect in medicine therapy and researchers are working tirelessly to develop advanced and novel therapeutic technologies based on the materials.

Antimicrobial materials
Infectious diseases caused by bacteria continue to be one of the greatest health problems worldwide, afflicting millions of people annually. Antibacterial materials are widely used in daily life and can effectively protect the public health [255]. GQDs have some remarkable behavior, which has been exploited as antimicrobial materials. For example, GQDs can produce toxicity under special wavelength irradiation.
This was demonstrated by Wang et al. [251] as they fabricated antimicrobial materials using GQDs. The clinical results of N-GQDs (5.1%)-AbLPS-treated-bacteria in E. coli under 670 nm laser photoexcitation are shown in Fig. 26aec, which indicated the GQDs played an important role in killing the bacteria. Qu and co-workers [252] designed an antibacterial system based on GQDs and low dose of H 2 O 2 . The system, represented in Fig. 26d, benefitted from the peroxidase-like activity and excellent biocompatibility of GQDs. Clinical studies were performed to assess the efficacy of the designed antibacterial system (shown in Fig. 26eei) in wound disinfection. GQDs band-aids were prepared and showed excellent antibacterial property in-vivo with the assistance of low concentration of H 2 O 2 , which indicated that GQDs band-aids have potential use for wound disinfection.
As shown, GQDs have been studied for use in the medical applications due to the many unique properties of the materials.
There remain much potential of GQDs in this field that has yet to be fully explored, which could lead to many more exciting applications. Many of the above-mentioned applications are still at its early stage of development however they have given us some ideas on the future of nanomedicine based on GQDs.

Optical applications
Due to the unique optical properties of the GQDs, different optodevices have been developed, such as photodetector [265,266], light emitting diodes [267e270] and photocatalysis [271e274] etc. This is based on some special characteristics of GQDs, for example, upconversion, strong photoluminescence and tuning of energy gap by controlling size/shape, which are different from traditional quantum dots, such as ZnO QDs and TiO 2 QDs etc. Different functionalization techniques of the GQDs were studied and achieved excellent performances in related applications. There are many potiential applications of GQDs in the optical field, which will be the topic for the following sections. Following the main lines of optoelectrical detector, LEDs and photocatalysis, many interesting researches related to these applications will be described.

Optoelectrical detector
Photodetectors or photosensors are sensors of light or electromagnetic energy [275], and they play an important role in national defense, real-time monitoring and space exploration and so on. Due to the ease of functionalization and uncommon optical properties of GQDs, many research groups have worked on the nanomaterials  [247]. (f) Schematic illustration of the preparation process of the doxorubicin loading GQDs-capped magnetic mesoporous silica (DOX-MMSN/ GQDs) nanoparticles and synergistic therapy combined with controlled drug release, magnetic hyperthermia, and photothermal therapy [58]; (g) Growth curves of mouse mammary tumor cell (4T1 tumors) in each group, inset is typical photographs of tumor tissue after each treatment [246]. and developed various methods to enhance the performance of photodetectors by utilizing GQDs. Among these methods, GQDs composited with traditional materials, such as silicon, ZnO or P3HT, with different dimensions, for example, Si nanowire [256] (Fig. 27a), Si nanoparticles [257] (Fig. 27b), Si pyramid [258] (Fig. 27dee), ZnO nanorods [259,260] (Fig. 27c and f) and P3HT [261] (Fig. 27g), have been investigated for use in photodetectors. In addition, GQDs have also been used to form hybrid composites with other 2D materials, such as graphene [35,38,263,264] (Fig. 27i, j, m and n), MoS 2 [262] (Fig. 27h) and WSe 2 [133] (Fig. 27l) etc. It is rare to use pure GQDs as an active layer, even though they have been attempted by some groups, due to the absorption peak position of GQDs is mainly located at the ultraviolet range, hence the photodetector would generally respond to short wavelength range, as shown in Fig. 27k, which led to deep ultraviolet photodetector [39] using pure GQDs. Interestingly, Tang and co-workers [27] developed a photodetector that exhibited photoresponse over UV to NIR range by doping pure GQDs with nitrogen. Such novel technique is generally known as Tang-Lau method. Table 2 provides a detailed list of merits of the different GQDs based photodetectors. According to the listed merits, it is apparent that there is a great prospect in using GQDs based photodetector in the near future. It has indeed inspired researchers to perform further research and development work in using GQDs as an active layer in the photodetector.

Light emitting diodes
Luminescence is a phenomenon where light is emitted from a material excited by an external energy. Such phenomenon has major scientific and technological roles in a variety of applications [275e277]. With a significant portion of global energy consumption going towards lighting, it is important to develop efficient illumination-grade lighting technologies [278]. The excellent optical properties of GQDs have been exploited in LEDs. There are two main approaches in the application of GQDs in LEDs. The first approach is to coat LEDs with GQDs to modify its lighting intensity and emitting wavelength (Fig. 28a) [279e282]. Second approach is to develop emitting layer using GQDs within the structure of LEDs (Fig. 28b) [283,284].
GQDs have been frequently used as phosphors coated on to the surface of LEDs, which led to the modification of color [285] (refer to Fig. 28c) as well as enhancement in intensity of emitting lights [286,287] (refer to Fig. 28feh). Besides, the GQDs can also be used as the active layer by forming composite with other emitting materials to improve the performance of LEDs. The intensity of LEDs irradiation can be controlled by changing the content of GQDs in the composites [49] (refer to Fig. 28d and e). Furthermore, the color of GQDs based LEDs can be tuned by functionalizing the GQDs with different functional group agents [288e290] (refer to Fig. 28iem).
The recent research has demonstrated that GQDs can be applied in LEDs using different approaches, namely as phosphors or active  (dee) Schematic and SEM image of the photodetector by hybridization with GQDs and Si [259]; (f) Representation of photodetector consisting of ZnO nanorods and GQDs [260]; (g) Diagram of photovoltaic detector based P3HT/GQDs [261]; (h) Schematic of a MoS 2 -GQDs heterostructure phototransistor [262]; (i) Schematic representation of a graphene/NGQDs hybrid phototransistor [263]; (j) Diagram of photodetector hybridization with GQDs and graphene [38]; (k) Schematic illustration of the pure GQDs photodetector with asymmetric AgeAu electrodes [39]; (l) Schematic illustration of an ML WSe 2 /N-GQDs photodetector [133]; (m) Schematics of a typical GQDs photodetector device [264]; (n) Photoresponsivity and specific detectivity versus laser wavelengths of 254e940 nm for the GQDs/Graphene hybrid photodetector [35]; (o) Schematic diagram of the N-GQDs photodetector [27]. layers. Another important aspect of GQDs based LEDs is that the color and intensity of the emitting light can be tuned by modifying the GQDs and its composite materials. In brief, the application of the GQDs on LEDs is fast becoming a reality in the near future.

Photocatalysis
Photocatalysis, which is the acceleration of a chemical reaction by light in the presence of a catalyst, has attracted considerable attention due to its application in renewable energy source. For example, clean energy can be generated by water-splitting into H 2 and O 2 using sunlight [291e295]. A visible light sensitive material capable of splitting water into H 2 and O 2 is critical in photocatalytic water-splitting [296], as shown in Fig. 29a. Another important criterion for such application is the use of low-cost, scalable and sustainable materials as photocatalysts. Carbon materials, which are low-cost, abundant and environmentally friendly, have been investigated as photocatalysts for cost-effective hydrogen production. Among the carbon materials, GQDs stand out as the best candidate for photocatalysis applications due to its many outstanding properties. For example, GQDs are 0D semiconductors with a bandgap that can be tailored by varying its size/shape and edge chemistry. Moreover, they exhibit large surface area, high electron mobility and can provide a direct path for the transport of photo-generated charge carriers, which will increase the lifetime of electronehole pairs and upconversion behavior. Different methods to enhance photocatalysis performances have been investigated recently [77,297e302]. Photocatalysts comprising of GQDs composites with other inorganic materials in different shapes, such as nanoparticle [308e310], nanobelt [307] and nanowire [310,311] have been synthesized and studied. As shown in Fig. 29b, the composites of GQDs and CdS were fabricated, where the GQDs were attached on to the surface of CdS nanoparticles. This resulted in enhancing the intensity of absorption for visible light and the mechanism was explained in detail by Lei and co-workers [303]. Titanium dioxide (TiO 2 ), which has been frequently used as catalyst, was hybridized with GQDs and the hybridization mechanism was proposed by Yang and co-workers [304], as shown in Fig. 29c. Li [172] et al. explained the possible mechanism for the photocatalytic H 2 evolution of S,N-GQD/TiO 2 composites under UVevis light irradiation using the band gap diagram shown in Fig. 29d. Composites of GQDs and ZnO NWs were also prepared for used as photocatalysts under solar irradiation [311], as illustrated in  [49]; (e) Luminous efficiencies and emission spectra of the devices, inset shows the emission spectra of LEDs composed of pure PVK (red dot line) and with 3 wt % (dark dot line) [49]; (f) and (g) show schematic cross section of the InGaN/InAlGaN UV LEDs coated with the GQDs and optical micrographs of the InGaN/InAlGaN LEDs without (top) and with (bottom) deposition of GQDs under an injection current of 100 mA, respectively [286]; (h) Photographs of the fabricated white LED lamp with no coating (left) and coated (middle) with GQDs as well as LEDs coated with GQDs ON states (right) [287]; (i) Structure of LEDs with GQDs as active layer [288]; (jem) Electroluminescence spectra of the LEDs with different functionalized GQDs, each inset show the photos of LEDs [288]. Fig. 29e. Moreover, the upconversion behavior of GQDs can also be observed in the composites of GQDs and g-C 3 N 4 [306]. It can enhance the intensity of visible light by converting the NIR spectra to visible range, as shown in Fig. 29f. Nanobelts possess greater surface areas, as compared to nanowire, hence increasing the loading of GQDs to enhance photocatalytic performances. Jang et al. [307] prepared ZnS nanobelts decorated with GQDs for photodegradation reaction of rhodamine B, as shown in Fig. 29g.
The unique properties of GQDs have been exploited in the above-mentioned optical applications. With the continuous efforts in understanding the properties of GQDs and developing new functionalities of the nanomaterials, it is anticipated that new and novel optical applications will continue to be developed in the near future.

Energy-related applications
As the growth of population and the reduction of resources continue, it becomes an urgent quest to explore new and sustainable energy for the future. As carbon is one of the most abundant materials in the world, it can be considered as an excellent candidate for energy-related applications. Solar energy is clean and renewable energy that can be converted in to electrical energy to support our daily life. However, solar energy is only available in the day but not at night. Therefore, energy storage is an important topic in energy applications and it remains one of the major challenges in renewable energy. In the following sections, the use of GQDs in solar cells and energy storage battery are presented.

Solar cells
Solar cells, which can covert sunlight to electricity, provide clean and renewable energy source. It is also an essential energy source for society which has scarce resources to generate electricity using conventional methods. The characteristics of GQDs, such as the downconversion, strong fluorescence, intensive absorption at UV range and easily-functionalization etc., have led to development of different solar cells using GQDs to enhance the performances of the cells. Some of the solar cells were based on traditional silicon solar cells [322], polymer solar cells [31,323e332] and other novel solar cells [183,333e345] etc.
Hybrid of Si and GQDs remain the leader in all solar cells as its power conversion efficiency was significantly enhanced to 16.55% via utilizing the properties of GQDs, such as strong absorption at UV range [32,312] (Fig. 30a and e), down-shift energy [33,313] (Fig. 30b [303]; (c) Simulation model of the cross-interaction in TiO 2 between carbon atoms and lattice oxygen (TieO) [304]; (d) Proposed mechanism for the photocatalytic H 2 evolution of S,N-GQD/TiO 2 composites under UVevis light irradiation [172]; (e) Schematic illustration of splitting mechanism over the N-GQDs/ZnO NW heterostructure under simulated solar light irradiation [305]; (f) Photocatalytic mechanism for hydrogen evolution over NGQDs/g-C 3 N 4 under visible light irradiation [306]; (g) Schematic for a plausible decomposition mechanism of GQDs/ZnS nanocomposite under light irradiation [307]. and f) and so on. Besides, the GQDs composites with other inorganic materials, such as ZnO [137,171,314,346e348], TiO 2 [315,316,349] and some classical solar materials [350e356], have also been reported, the power conversion efficiency of optimized devices were enhanced to 1.26%, 7.19% and 35% respectively. The structures of these solar cells are shown in Fig. 30c, d and g. Apart from using GQDs as active materials, they have been exploited for other functional layers, such as fluorescent layer of all-weather solar cells [30,357,358] (Fig. 30h), transporting layer [203] (Fig. 30j) and buffer layer [320] (Fig. 30m) etc. Organic materials were also used to form composites with GQDs [318] [359,360] (Fig. 30k) due to the ease of functionalization of the GQDs. Different functionalized types of the GQDs were investigated by Kim and co-workers [317] (refer to Fig. 30i). Their findings showed that enhanced optical absorptivity by rich functional groups in GQDs increased short-circuit current, while the improved conductivity of reduced GQDs led to an increase of fill factors and power conversion efficiency of up to 7.6%. The merit of organic solar cells is its flexibility, thus related research were studied by Shin and co-workers [319]. They have found that the GQDs enhanced the bending flexibility of the organic solar cells, as shown in Fig. 30l.
Generally, the GQDs and organic materials were utilized to form the bulk heterojunction (BHJ) in organic solar cells [319,321], as shown in Fig. 30k and n. In summary, GQDs have been applied as hybrid materials in both inorganic and organic solar cells. Besides, the performance of solar cells was enhanced by the special properties of the GQDs. Thus, the exploration of novel characteristics of GQDs will continue to make contribution towards the solar cells technology.

Energy storage battery
Most renewable energy sources, especially those harvested from nature, require storage via battery, thus it is important to develop supercapacitor battery technology that exhibit high efficiency, long-term cycling life, low weight, high energy density, and longterm stability to deal with the energy demand. The excellent properties of GQDs, such as large surface area and ease of functionalization, provide many advantages for energy storage applications. As shown in Fig. 31a, the environmentally friendly and high performance battery is composed of GQDs and the GQDs play vital role in the enhancement of battery performances.  [33], (e) [313] and (f) [32] show the GQDs/Si solar cells by taking advantage of downconversion of GQDs [32,33]; (c) Diagram of a typical quantum dot sensitized solar cells (QDSSC) based ZnO/GQDs device [314]; (d) Schematic structure of TiO 2 /CdS/GQDsÀnS 2À /Sn 2e Sb 2 Se 3 /NiO/C-fabric cell [315]; (g) The schematic device structure of NCQDs-sensitized TiO 2 solar cell [316]; (h) Diagram of all-weather GQDs solar cell [30]; (i) Schematic of a BHJ solar cell based on GQDs with different edge functional groups [317]; (j) Representation of the formation process of the GQDs@PEDOT coreÀshell nanostructure (CSNS) [203]; (k) Schematic structure of typical polymer solar cells based on GQDs [318]; (l) Normalized PCEs of the Ag NWs mixed with GQDs/graphene transparent conductive electrodes/flexible organic solar cells (TCE/FOSCs) as functions of bending radius. The inset shows real images of the FOSCs at different radium [319]; (m) Schematic structure of an inverted polymer solar cell with GQDsÀCs2CO3 buffer layer, inset shows the molecular structure of Cs 2 CO 3 [320]; (n) Schematic of organic photovoltaic (OPV) device with a GQDs-incorporated PEDOT:PSS layer [321].
Li ion and Na ion batteries have been widely investigated due to their high capacity, long cycle life and low cost. However, GQDs can be used as transporting materials to enhance the electrical properties of the battery [361] by introducing them into the electrolytes, as shown in Fig. 31b. In Li ion battery, TiO 2 has frequently been used as anodes. Wei and co-workers [362] fabricated hierarchical TiO 2 imbedded with GQDs (Fig. 31c), which were then applied to Li ion battery, and they reported a high specific capacity and excellent rate capability (160.1 mAhg À1 at 10 C after 500 cycles). In addition, Li [363] at el synthesized GQDs composites with Si nanoparticles, as shown in Fig. 31d. They reported that the GQDs coated surface resulted in an improved with bicontinuous electron and Li/Na ion transfer channels [361]; (c) Schematic illustration of the electrochemical reaction process of TiO2/GQDs [362]; (d) The process for synthesis of GQDs/Si nanoparticles [363]; (e) and (f) show the structure and magnified structure of GQDs-sodium thiosulfate/ carbon black (GQDs-S/CB), respectively [50]; (g) and (h) Conceptual illustrations of liquid electrolyte and Gel polymer electrolytes-a polymer framework comprising PAV and incorporated PMMA: Quantum Dots (GPE-PAVM:QD), respectively [364]; (i) Schematic configuration of S/CB (left) and GQDs-S/CB (right) employed as a cathode in a LieS battery [50]; (j) Schematics of the fabrication process of CuO-based nanowire electrodes [365]; (k) Rate performance with coulombic efficiency of CuO, Cu, and GQDs (CCG) and CuO, Cu nanowires (CC) anodes [365]. electrical conductivity as well as effectively prevented (the capacity remained at 3068 mAhg À1 after 100 cycles at 100 mAg À1 ) the direct contact of silicon surface with the electrolyte molecules. Interestingly, Park and co-workers [50] coated both carbon black and GQDs on sodium thiosulfate ( Fig. 31e and f) and investigated their characteristics, as well as demonstrated an excellent performance on discharge capacity of~1000 mAhg À1 even after 100 cycles. They discussed the structure of the cathode has a significant impact on the irreversible loss of high-order polysul-fides (HOPSs) during repeated battery operations, as shown in Fig. 31i. Furthermore, Teng et al. [364] demonstrated that GQDs can enhance the Li ion mobility in a gel polymer electrolyte (GPE) for Li ion batteries (LIBs) (delivering discharge capacity of 155 mAhg À1 ) and the proposed mechanism of the enhancement is represented in Fig. 31g and h. Another experiment that can clearly account for the important role of GQDs in enhancing the performance of Li ion battery (having specific capacity of 780 mAhg À1 ) was performed by Fan and co-workers [365]. The preparation process of their electrodes is shown in Fig. 31j. The plot, shown in Fig. 31k, reveals the different battery performances due to the different electrode structures.
As reported, the use of GQDs in batteries has led to improved performances in the battery. Importantly, such improvement in performances using GQDs is very cost-effective. Therefore the application of GQDs in battery to provide better energy storage is practical and commercially viable.

Fuel cell
Fuel cell can generate electricity through an electrochemical reaction of hydrogen fuel with oxygen. Unlike battery, fuel cell can provide a continuous supply of electricity as long as there is a constant source of fuel and oxygen (usually from air) to sustain the chemical reaction. Such energy technology is sustainable and environmentally friendly. There are different types of fuel cell [366e368] but they all consist of an anode and a cathode, which are essential components of the fuel cell. In the past, the electrode materials were mainly made of precious metals, such as Pd, Pt, Ag and Au etc, hence leading to expensive fuel cell. Recent studies have demonstrated the use of functionalized GQDs as fuel cell electrodes [369,370]. Inner charge transfer of the functionalized GQDs has resulted in the enhancement of the catalytic activity and stability. Since then, there has been an increase in research activities to exploit functionalized GQDs as cathode materials for fuel cell.

Conclusions
In this review, the history of the GQDs is introduced. Most of the work performed so far indicates that the applications of GQDs are still at the early stage of development and have been progressing rapidly. Numerous methods in fabricating the GQDs have been reviewed. The different unique properties of the GQDs have been discussed. This included various functionalization routes of the GQDs to optimize or enhance its properties, such as water solubility, optical absorption and conductivity etc. Since the discovery of graphene, limited progress on the application of graphene has been achieved. It was understood that this is due to some practical problems of graphene, such as zero band gap and low absorptivity and so forth. The development of GQDs has therefore become an important catalyst for the application of graphene. Undoubtedly, this has led to significant research efforts in GQDs. As demonstrated by previous work, GQDs are becoming imperative functional materials that can find applications in medical, optical and energyrelated fields. However, understanding of the mechanism in some of the GQDs applications remains unclear and requires further investigations. Nevertheless, this area of research has received much attention by researchers due to the enormous benefits of the GQDs can bring in the different applications.
In conclusion, this review has provided the developments, fabrication methods, different functionalization routes and applications of the GQDs with detailed discussions in those contexts. It also suggested the future direction for the development of GQDs.