Nano Research
Nano Res DOI 10.1007/s12274-014-0644-3
The photoluminescence mechanism in carbon dots (graphene quantum dots, carbon nanodots and polymer dots): current state and future perspective Shoujun Zhu, Yubin Song, Xiaohuan Zhao, Jieren Shao, Junhu Zhang and Bai Yang()
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The photoluminescence mechanism in carbon dots (graphene quantum dots, carbon nanodots and polymer dots): current state and future perspective
Shoujun Zhu, Yubin Song, Xiaohuan Zhao, Jieren Shao, Junhu Zhang and Bai Yang*
State Key Laboratory of Supramolecular Structure and Materials,
College
of Chemistry,
Jilin
University,
Changchun, 130012, P. R. China. E-mail:
[email protected]
Four respectable PL mechanisms for carbon dots (graphene quantum dots, carbon nanodots and polymer dots) have been confirmed: the carbon core state, surface state, molecule state as well as the crosslink enhanced emission (CEE) effect.
Nano Research DOI (automatically inserted by the publisher) Review Article
The photoluminescence mechanism in carbon dots (graphene quantum dots, carbon nanodots and polymer dots): current state and future perspective Shoujun Zhu, Yubin Song, Xiaohuan Zhao, Jieren Shao, Junhu Zhang and Bai Yang( )
Received: day month year
ABSTRACT
Revised: day month year
At present, the clear PL mechanism of carbon dots (CDs) is still open debate for the related researchers. Because of the variety of CDs, it is highly important to summarize the PL mechanism for these kinds of carbon materials, which can guide the effective synthesis routes and novel applications. This review will focus on the PL mechanism of the CDs. Three kinds of fluorescent CDs were involved: graphene quantum dots (GQDs), carbon nanodots (CNDs) and polymer dots (PDs). Four respectable PL principles have been confirmed: the quantum confinement effect or conjugated π-domains determined by carbon core, surface state determined by hybridization of the carbon backbone and the connected chemical groups, the molecule state determined by the solely fluorescent molecules connected on the surface or inner of the CDs, as well as the crosslink enhanced emission (CEE) effect. To give a thorough summary, the category and synthesis routes as well as the chemical/physical properties for CDs were shortly introduced in advance.
Accepted: day month year (automatically inserted by the publisher) © Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2014
KEYWORDS Carbon dots, graphene quantum dots, carbon nanodots, polymer dots, photoluminescence mechanism
1. Introduction Carbon materials are already well known for many years, which include graphite, diamond, fullerenes, carbon nanotube (CNT) and graphene. To make these kinds of materials fluorescent, their size and surface chemical groups should be carefully modulated. The as-prepared fluorescent carbon materials are always consisted of sp2/sp3 carbon, oxygen/nitrogen based groups and post-modified chemical groups. Up to now, many kinds of fluorescent carbon-based nanomaterials have been synthesized, including
carbon nanodots (CNDs) [1-2], fluorescent CNT [3], graphene oxide (GO) [4-5], graphene quantum dots (GQDs) [6-12], polymer dots (PDs) [13-15], nanodiamond [16-17] and so on. In this review, carbon dots (CDs) prepared by the chemical synthetic strategies are chosen as the main object to discuss, containing GQDs, CNDs and PDs. These three CDs possess similar photoluminescence (PL), while they are distinguished by the intrinsic inner structure and surface chemical groups. In detail, the synthesis of CDs can be divided into top-down nano-cutting method and bottom-up organic
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approaches. Top-down nano-cutting method generally includes cutting different carbon resources like GO, carbon fiber, CNT, fullerene and graphite electrode. Bottom-up organic approaches always contain carbonization of carbohydrate, self-assembly of polycyclic aromatic hydrocarbon (PAH) as well as organic synthesis from small molecules. At present, the clear PL mechanism of CDs is still an open debate for the related researchers [18]. Because of the variety of CDs, it is highly important to summarize the PL mechanism for these kinds of carbon materials, which can guide the effective synthesis routes and novel applications. This review will focus on the PL mechanism of the CDs; three kinds of fluorescent CDs were involved: graphene quantum dots (GQDs), carbon nanodots (CNDs) and polymer dots (PDs) (Fig. 1) [19]. Four respectable PL principles have been confirmed: the quantum confinement effect or conjugated π-domains determined by carbon core, surface state determined by hybridization of the carbon backbone and connected chemical groups, the molecule state determined by the solely fluorescent molecules connected on the surface or inner of the CDs as well as the crosslink enhanced emission (CEE) effect. To give a thorough summary, the category, synthesis routes and the chemical/physical properties for CDs were shortly introduced in advance. Due to the abundant and increasing reports about CDs, we apologize to the researchers whose important publications may be left out.
2. Category and synthesis routes 2.1 A classification of reported CDs CDs are the comprehensive definition for various nanosized carbon materials. In a broad sense, all nanosized materials, which are mainly comprised of carbon can be called as CDs. CDs always possess at least one dimension less than 10 nm and fluorescence as their instinct properties. The structure of CDs consisted of sp2/sp3 carbon and oxygen/nitrogen based groups or polymeric aggregations. In detail, CDs mainly contained GQDs, CNDs and PDs (Fig. 1). The GQDs possess single or few layers graphene and connected chemical groups on the edge. They are anisotropic with lateral dimension larger than the height. CNDs are always spherical and they are divided into carbon naonparticles without crystal
lattice and carbon quantum dots (CQDs) with obvious crystal lattice. As a result, the PL center is very different for different kinds of CNDs. The PDs is aggregated or cross-linked polymer, which is prepared from linear polymer or monomers. In addition, carbon core and the connected polymer chains can also assemble to form PDs. Due to the diversity of the CDs, there were lots of approaches to these CDs, mainly including the “top-down” cutting from different carbon source, and “bottom-up” synthesis from organic molecules or polymers as well as surface functionality or passivation. We called the “top-down” and “bottom-up” routes as “nano-methods”, which are distinguished from the precisely organic routes [20-21]. 2.2 “Top-down” cutting from different carbon source Generally, the CDs were obtained from oxide-cutting different carbon resources, such as graphite power [22], carbon rods [23], carbon fiber [24], carbon nanotube [25-26], carbon black [27] and even candle soot [28] (Fig. 2). These carbon materials possess perfect sp2 carbon structure and lack efficient band gap to give the fluorescence. To make these kinds of carbon source photoluminescence, the sizes and surface chemistry have to be carefully modulated. As a result, the most common methods were cutting with concentrated acid oxidizing (HNO3 or H2SO4/HNO3 mixture) [29]. In these processes, the bulk carbon materials were cut into small pieces, while the surface was modified by oxygen based groups. The resulted small carbon product was so-called GQDs, CQDs or CNDs. It should be noted that the two-step cutting routes have always be used to prepare GQDs. The first step is to convert a graphite based material to GO sheets (usually the modified Hummers method), and the second step is cutting the GO into GQDs with various methods. [30-31]. Other “Top-down” cutting routes contained electrochemistry [32-33], hydrothermal/solvothermal/special oxidation [30, 34], metal-graphite intercalation [35] as well as strong physical routes, such as arc discharge [36], laser ablation [37] and nanolithography by reactive ion etching (RIE) [38-39]. In the electrochemistry method, the graphite rod electrode broke up to form CQDs or GQDs during the electrochemical cutting processes. The applied electrolyte contains ethanol [23], ionic
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liquid [32], NaH2PO4 [40], tetrabutylammonium per-chlorate (TBAP) [41] or even PBS/water [42-43], etc. Electric field peeled off nanosized carbon from electrode through graphite layer intercalation and/or radical reaction. In hydrothermal/solvothermal/special oxidation methods, some oxidized carbon resources, such as GO and oxidized CNT, which possessed defect-based chemical group (oxygen-based groups), can be cut into pieces by hydrothermal/solvothermal at high temperature and pressure [30, 34]. Some special oxidation methods, such as photo-fenton reaction of GO to GQDs [44] can also break up GO. Specially, the GQDs prepared by mask-assisted RIE was an efficient approach to precisely control the size and surface chemistry of the CDs [38], which is an ideal model system to clarify the PL mechanism. 2.3 “Bottom-up” synthesis from organic molecules or polymers The “bottom-up” methods were efficient routes to produce fluorescent CDs in large scale (Fig. 2). For example, small molecules and polymer may undergo dehydration and further carbonization to form the CNDs and PDs. The applied molecules always possessed -OH, -COOH, -C=O and -NH2 groups, which can dehydrate in elevated temperature. There were lots of approaches to perform the dehydration and carbonization processes, such as hydrothermal [45], microwave [46], combustion [47], pyrolysis in concentrated acid [48], carbonization in microreactor [49], enhanced hydrothermal (microwave-hydrothermal [50], plasma-hydrothermal [51]) and so on. These formation processes are usually uncontrollable, resulting in CDs with polydispersity, but using designed precursors may accurately obtain the GQDs with desired molecules weight and size, for example, intramolecular oxidative polycyclic aromatic hydrocarbons (PAHs). Although the organic-synthesized GQDs are the perfect model to understand the PL mechanism of fluorescent carbon materials, the complicated synthesis method and the difference compared with common fluorescent CDs reduce the possibility for this issue [52]. 2.4 Surface functionality or passivation The prepared CDs always possess lots of reactive groups, which afford the possibility to be modified by other chemical groups. The functionality and
passivation can enhance the quantum yields (QY) of the CDs, change the PL emission and meet the special applications. The QY of raw CDs used to be very low, which is hard for application and even detection. Sun’s groups pioneered to utilize NH2-PEG passivation to elevate the PL QY of the CNDs [53]. Zhu et al. also used the similar method to enhance the PL properties of the GQDs [54]. Yang’s groups used the cross-linked route to enhance the PL properties of the PDs; The bare PEI possess little fluorescence while the cross-linked PDs have elevated PL QY [55]. The surface or edge modification has been also used to tune the PL emission of the CDs. For example, the green emission can be changed to blue one by the surface reduction [56]. 3. Chemical and physical properties 3.1 Chemical structure As mentioned in the category and synthesis routes section, there were diversiform fluorescent CDs and various synthesis routes to obtain these materials. As a result, the chemical structure of the CDs possesses diversity according to the different synthesis approaches. In detail, the GQDs possess single or few graphene layers and connected chemical groups on the edge. They are anisotropic with lateral dimension larger than the height. Due to the existence of carbon core, the GQDs possess certain crystallinity with average lattices 0.24 nm (Fig. 3a), which corresponds to (100) spacing of single graphene dots lied on the lacey support films [35]. CNDs are always spherical and they are divided into carbon naonparticles without crystal lattice and CQDs with obvious crystal lattice [57]. The typical interlayer distance of CQDs is ca. 0.34 nm, which corresponding to (002) spacing of the crystalline graphite (Fig. 3b). The PDs is aggregation/assemble or cross-linked polymer from linear non-conjugated polymers. In addition, the carbon core and the grafted polymer chains can also form the PDs. All of the CDs possess the connected or modified chemical groups on the surface, such as oxygen-based, amino-based groups or polymer chains, etc. The direct characterization methods for carbon core include high-resolution TEM (HRTEM), Raman spectroscope and XRD. For the grafting chemical groups, the FTIR, XPS, NMR, MALDI-TOF (Fig. 3c) and element analysis were
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used to determine the general structure [58]. As a result, these kinds of fluorescent CDs were not “pure” carbon materials, the hybridization and coefficient between the carbon core and surrounding chemical groups play the leading role in the PL behavior of the CDs. 3.2 Optical properties Despite the diversity of the structures, the CDs possess some similar optical properties on the absorption and fluorescence. Herein we would just summarize the common optical properties rather than consider some specific examples. The absorption of the CDs typically shows strong optical absorption in the UV region (230-320 nm), with a tail extending into the visible range. For carbon core, a maximum peak at ca. 230 nm is ascribed to π-π* transition of aromatic C-C bonds, while a shoulder at 300 nm attributes to n-π* transition of C=O bonds or other connected groups [59]. Besides, the connected chemical groups may contribute the absorption at UV-visible regions. The observed deviations in absorption spectra data, at least to some extent, indicate the differences of compositions or structures in different hybridization derivatives. The PL properties were the most concerned issue for CDs in view of investigation of the PL mechanism and novel applications. Generally, the emission spectra of the CDs are roughly symmetrical on the wavelength scale. The emission peak of CDs is usually wide with large stocks shift when compared with that of organic dyes. The emission peak position is always related to the excitation wavelength, which is call wavelength-dependence behaviors. It may result from the wide distributions of differently sized dots and surface chemistry, different emissive traps (salvation effect), or a mechanism currently unresolved [60]. Fortunately, the excitation-dependent PL behaviors can be applied in multi-color imaging applications [61-62] The PL of CDs is kind of property like that of semiconductor quantum dots (QDs), but these fluorescent nanoparticles possess many differences. Firstly, it seemed not efficient to tune the PL color by control the size of CDs. In most situations, the PL color of CDs is relative to the surface group rather than the size. The most common CDs have the strong PL from blue to green color, and a few CDs can possess optimal emission in long wavelength section
[23, 28, 63]. Another main difference between CDs and QDs is that the PL bandwidth of CDs is much wider. The wide peak may result from the inhomogeneity chemical structure and diverse PL centers. QY is the number of emitted photons relative to the number of absorbed photons. CDs possess rather low QY (even lower than 1%), when CDs was just discovered. After surface modification or passivation, the QYs can be increased dramatically. The enhace PL properties were attributed to the strongly PL centers on the surface, synergy by both carbon core and chemical goups or solely by fluorophores [64]. From then on, the QY of the CDs improved year by year, Generally, QY depences on the synthesis routes and the surface chemistry. Most of CDs possess good photostability, which is benefited from the carbon core based PL center. Neither blinking nor meaningful reduction in PL intensity were observed in such CDs after continuous exposure to excitation (Fig. 4a) [37]. However, for the CDs with molecule state emission, the PL intensity decreased dramatically after high power UV exposure [65]. Some special luminescence behaviors of CDs can be observed in some situation, for example electrochemical luminescence (Fig. 4b) [33], the ECL mechanism of the CDs was suggested to involve the formation of excited-state CDs via electron-transfer annihilation of negatively charged and positively charged CDs. Although the up-conversion PL (two-photon absorption and anti-Stokes PL) was reported [23, 54], it is quite important to establish a proper characterization system to investigate this kind of properties, because some so-called “up-conversion PL” in CDs could be due to the excitation of second-order diffraction light (wavelength λ/2) from the monochromators in the fluorescence spectrophotometer [66-67]. The amplified spontaneous green emission and lasing emission was also observed from CNDs (Fig. 4c) [68], the high PL quantum yield and small overlap between absorption and emission of CDs ethanol solution are the key factors in achieving lasing emission. The special optical properties may lead to novel application of different CDs. Besides direct characterization, there are several indirect approach prove the PL origin. The pH-dependent and solvent-dependent PL is very
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important to investigate the emission behaviors of CDs (Fig. 4d-e) [30, 34]. The molecule state was affected under both the strongly acidic and base atmospheres, while PL intensity of the carbon core-edge state may increase due to protonation or deprotonation of the functional groups. The PL quench behaviors of CDs is another important tool to understand the PL mechanism (Fig. 4f) [69]. 3.3 Biological toxicity The toxicity of CDs is an extraordinary concern because of their potential for bio-based applications [12, 19, 70-71]. Bioimaging-based applications of diagnostics in vitro/vivo must be non-toxic and biocompatible [72]. In the last several years, metal-based QDs bioimaging methodologies appeared, together with toxicity concerns for their intrinsically toxic elements like cadmium. Compared with metal-based QDs, GQDs are constituted by intrinsically non-toxic element, carbon, which makes them a particularly useful and promising bio-analytical tool. Toxicity studies have been conducted by various research groups, and CDs appear to have low toxicity in vitro and in vivo. So far, the inherent toxicities of CDs have been evaluated by the cell-viability assay. The results indicated that GQDs, CNDs and PDs possess excellent biocompatibility and low cytotoxicity [15, 29-30, 53, 73-75]. The carboxylated GQDs do not cause apparent toxicities in rats at different dosage (5 and 10 mg/kg) for 22 days as evidenced by blood biochemistry and hematological analysis [76]. No severe symptoms of inflammation were observed in the liver, kidney, spleen, heart, or lung at 22 days after the administration of the carboxylated GQDs nanoparticles. All the evidences point to the great potential of CDs for in vitro and in vivo imaging studies. 4. PL mechanism of GQDs GQDs were the simplest CDs possessing the structure of single layer carbon core with connected chemical groups on the surface or edge. As a result, GQDs were the ideal model to investigate the PL mechanism of CDs. To explain the PL mechanism of GQDs, the PL behavior of chemically derived GO should be introduced firstly, because GO is an important raw material for GQDs preparation and they possess similar chemical structures. GO contains
oxygen-based functional groups either on the basal plane or at the edges. Therefore, the 2-3 nm aromatic sp2 domains are surrounded by linearly aligned epoxy and hydroxyl-boned sp3 C-O matrix [5, 77]. In such a structure of GO, the fluorescent property is determined by the π states of the sp2 sites. The π and π* electronic levels of the sp2 clusters, which is influenced by the bandgap of σ and σ* states of the sp3 matrix, are strongly confined. Radiative recombination of electron-hole (e-h) pairs in such sp2 clusters can arouse the fluorescence [78]. Because of the existence of wide size distribution of sp2 domains in GO, the bandgaps of different sizes of sp 2 cover a wide range, leading to the wide PL emission spectrum from visible to near infrared (Fig. 5). Many groups investigated the fluorescence of GO and reduced GO (r-GO). For example, Luo et al proposed that the bond distortions may contribute the fluorescence of GO and r-GO [79]. Gokus et al. have observed visible luminescence in the oxygen plasma-treated graphene and attribute the emission to CO-related localized electronic states at the oxidation sites [4]. Furthermore, Galande et al. have studied the pH-dependent fluorescence of GO and suggest the emission of quasi-molecular fluorophores in such kinds of materials [80]. They found that the excited state of the fluorophore species protonated in acidic media, which makes the PL spectra different in acidic and basic solutions. This kind of quasi-molecular fluorophore is caused by the carboxylic acid groups that are electronically coupled with the surrounding graphene core sheets [81]. Similar to GO, GQDs possess more defect, oxygen groups and functional groups on the surface. The current fluorescence results of GO could be referred to understand these emissions in GQDs. Excitons in graphene have an infinite Bohr diameter. Thus, graphene fragments of any size will show quantum confinement effects. As a result, GQDs have a non-zero bandgap and PL on excitation. This bandgap is tunable by modifying the size and surface chemistry [6]. Considerable development in the preparation of GQDs has been witnessed in the last five years, researchers have figured out reasonable PL mechanism, which could be referred to surface/edge state and conjugated π-domains. 4.1 Surface/edge state in GQDs The surface/edge state contained triple carbene at the
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zigzag edges, oxygen based groups on the graphene core, and resonance of amine moieties and graphene core. When the graphene sheets are cut along different crystallographic directions, diverse types of edges (armchair and zigzag edges) can be obtained. The edge type plays an important role in determining the electronic, magnetic, and optical properties. Ritter et al. have stated that predominantly zigzag-edge GQDs with 7-8 nm average dimensions are metallic owing to the presence of zigzag edge states, and GNRs with a higher fraction of zigzag edges exhibit a smaller energy gap than a predominantly armchair-edge ribbon of similar width [82]. Radovic and Bockrath reported that the zigzag sites are carbene-like, with a triplet ground state being most common, whereas the armchair sites are carbyne-like, with a singlet ground state most common [83]. Pan et al. suggested that blue PL of hydrothermally GQDs might be attributed to free zigzag sites with a carbine-like triplet ground state described as σ1π1 [34]. Under acidic conditions, the free zigzag sites of the GQDs are protonated, forming a reversible complex between the zigzag site and H+, and leading to the breaking of the emissive triple carbene state and the PL quenching (Fig. 4b). To the contrary, the PL recovers because the free zigzag sites are restored under alkaline condition. Lin et al. prepared GQDs with size about 20 nm from exfoliating and disintegrating CNTs or graphite flakes [35]. The obvious single layer and clear zigzag edge were proved in AFM and bright field high resolution TEM. The new opened band gap arises from the triple carbenes at the zigzag edges, corresponding to the transitions from the HOMO to σ and π orbital of LOMO in triple carbenes. These two kinds of GQDs with size of 9.6 nm and 20 nm respectively possess the similar absorption and emission behavior, which proved that the PL mechanism was determined by the triple carbene at the zigzag edges instead of the quantum confinement effects (Fig. 6a). Pan et al. also used single-particle spectroscopic measurements to investigate the PL behaviors of the GQDs [84]. As schematically shown in Fig. 6b, photo-excited electrons through the π-π* transitions were proposed to relax into either the sp2 energy levels or the defect states (actually we called
surface state), giving rise to the blue or long-wavelength PL, respectively. The former emission might bear the discrete feature due to quantum confinement effect (QCE) of electrons inside the sp2 carbon domains. The latter emission is related to the hybrid structure by both the oxygen functional groups (at the edges and/or on the basal planes) and graphene core. Despite noticeable differences in the size and the number of layers from particle to particle, all of the GQDs studied possess almost the same spectral line shapes and peak positions. As a result, it suggested the PL of these GQDs were caused by surface state. Besides the oxygen based groups, the amine-based groups were also important composition for surface state in GQDs. In the work of Tetsuka et al, the ammonia-assisted hydrothermal method was used to prepare GQDs. The product is edge-terminated by a primary amine, allowing the electronic structure to be modified with the effective orbital resonance of amine moieties and graphene core [58]. For the GQDs with the same sizes, the emission wavelength increased with the contents of amine-groups. Furthermore, the GQDs possess high QY because of the reduction of carboxylic and epoxide groups which act as the non-radiative electron-hole recombination center. Combined the experimental results and ab initio calculations, the primary amine at the edge of GQDs have higher HOMO orbital than that with hydrogen-terminated due to the strong orbital interaction with the -NH2 groups. The resonance feature between the delocalized π orbital and the molecule orbital in the -NH2 groups results in a narrowing of the optical band gap. Such amino-contained GQDs have also been reported by other groups. Jin et al. reported that the functionalized GQDs exhibit a redshift in the PL emission spectrum, compared to the pre-existing GQDs (the PL emissions of the amine-functionalized GQDs also shifted with changes of the pH due to the protonation and deprotonation of the functional groups) [85]. Calculations from DFT illustrated that PL shifts resulted from charge transfers between the functional groups and GQDs, which can tune the band gap of the GQDs. The band gap of the GQDs decreases to 2.254 eV, when a GQD is functionalized by one amino. And the band gap will gradually decrease with the increasing number of -NH2 groups
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(Fig. 6c). Kumar et al. also investigated the PL behaviors of amino-functionalized GQDs. First principles calculations suggested that primary amine edge termination (NH2) resulted in formation of an additional interband ca. 3.28 eV within the energy gap due to p orbital hybridization of C-N atoms at the edge sites [86]. Feng and co-workers proved that 1, 2-ethylenediamine functionalization on the surface of GQDs can form a specific cyclic structure which facilitates the proton transfer from the ammonium moiety to the conjugated structure, and thus lead to the largest enhancement of fluorescence [87]. The mechanism of GQDs emission was further investigated with femtosecond transient absorption spectroscopy and femtosecond time-resolved fluorescence dynamics measured by a fluorescence upconversion technique, as well as a nanosecond time-correlated single-photon counting technique in Wang’s and Yang’s groups [88-89]. They found that two independent molecule-like states and dark intrinsic state existed in solvothermally synthesized GQDs, as shown in Fig. 6d. The intrinsic state is attributed to graphene core, and its PL locates at around 470 nm with a dominant short lifetime and low PLQY. Another two irrelevant molecule-like state (around 320 nm and 400 nm) provides a blue fluorescence with peak at ca. 430 nm and green fluorescence with peak at ca. 530 nm. The three kinds of emission states constitute the fascinating PL of green-fluorescence GQDs. In their other related work, the PL of GQDs was changed from the green emission to blue emission by tuning the surface chemistry [90]. Through the modification of GQDs, the connected alkylamines transferred the -COOH and epoxy of GQDs into -CONHR and -CNHR, both of which can reduce the non-radiative recombination induced by the -COOH and epoxy groups [91], and transfer GQDs from defect state emission into intrinsic state emission. In reduction method, the carbonyl, epoxy and amino moieties were changed into -OH groups, which suppress non-radiative process and further enhance integrity of π conjugated system (also reduce the defects). As a result, intrinsic state emission (contained both the intrinsic and blue molecule-like state observed in TA) plays the leading role instead of defect state emission. The blue- and green-color emissions of GQDs and GOQDs were also prepared by Seo et al [92]. They
have revealed that the green luminescence of GOQDs originates from defect states with oxygenous functional groups, whereas the blue luminescence of GQDs is dominated by intrinsic states in the high-crystalline structure. The surface/edge state of PL in the GQDs was also investigated by other groups [93-98]. For example, Lingam et al. found the evidence for edge-state PL in solvothermally synthesized GQDs. If the edge of GQDs was destroyed or disappeared, the PL intensity was decreased to vanish [99]. All of the similar work proved that the surface state (or edge state) was the key PL mechanism in these kinds of GQDs. In addition, for some cases of the GQDs, the functional surface state could also be counted to be special polyaromatic fluorophores. 4.2 Quantum confinement effect of conjugated π-domains in GQDs In the last section, the surface state played the leading role in the PL behavior of the related GQDs instead of the intrinsic graphene core state. However, for the GQDs with perfect graphene core and less surface chemical groups, the bandgap of conjugated π-domains was thought to be the true intrinsic PL center. A major feature of quantum dots is the QCE, which occurs when quantum dots are smaller than their exciton Bohr radius [100]. DFT calculations have shown that the bandgap of GQDs increases to approximately 2 eV in a GQD consisting of 20 aromatic rings and 7 eV for benzene (Fig. 7a) [5]. In other words, the PL emission of the GQDs can be tuned by the size of the conjugated π-domains. Typically, as the particles are smaller, the luminescence energies are blue-shifted to higher energy. First of all, the GQDs prepared by organic solution method should be introduced because they possess special electronic and optical properties, which are suitable models to investigate the PL of QCE. Recently, Li et al. reported the synthesis and optical characterization of colloidal GQDs with a uniform and tunable size through organic chemistry routes [21]. The prepared GQDs consisted of graphene moieties containing 168, 132, and 170 conjugated carbon atoms, respectively (Fig. 7b) [101]. The synthesis was based on oxidative condensation reactions developed by Scholl, Müllen, etc [102]. The GQDs consist of light atoms and thus have a small
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dielectric constant and weak spin-orbit coupling. These lead to strong carrier-carrier interactions and electronic states with well-defined spin multiplicity. As a result, GQDs have much larger energy band than other inorganic semiconductor QDs with similar sizes. That was why most of the GQDs possess PL at range of blue to green section. Furthermore, the size-dependent, discrete excitonic levels could significantly slow down the relaxation of high excited states in GQDs due to a phonon bottleneck. In addition, strong carrier-carrier interactions could lead to generation of more than one exciton with one photon absorbed, a process particularly useful for improving the efficiency of photon-generated carrier [103]. The singlet-triplet splitting of GQDs was determined to be ca. 175 meV, and intersystem crossing was so efficient that it competed with internal conversion among the states with the same multiplicity. As a result, the GQDs emitted both fluorescence and phosphorescence [101]. Since triplet states have a significantly longer lifetime, they could profoundly affect the chemical reactivity and other processes such as charge transfer or exciton migration in the GQDs-based system. Yang et al. investigated the photophysics of the organic synthesized GQDs (C42H18, C96H30, C132H34 and C222H42), and found that the intrinsic state depended on size, while the energy level offset between intrinsic state and edge state decided their optical properties (Fig. 7c) [104]. As a result, the green fluorescence of the C42H18, C96H30 not only depends on the size, but also results from bright edge state. If the energy level offset between intrinsic state and edge state was large enough, the fluorescence is dominant. If the energy level offset was small enough (meeting thermal activation condition, ca. kBT), the long carrier lifetime in intrinsic state will give a possibility for intersystem crossing from singlet excited state to triple excited state of edge state, such as above mentioned C132. In the cases of C132H34 and C222H42, the intrinsic state possibly decreased and was lower than the edge state; as a result, they lost the expected fluorescence. Then, the size dependent PL of GQDs prepared by “nano-methods” was introduced (The PL of these GQDs was not controlled by QCE). Peng et al. prepared three kinds of GQDs with different sizes of 1-4 nm, 4-8 nm, 7-11 nm via varying the reaction
temperatures, which emitted different PL changing from blue and green to yellow as shown in Fig. 8a-b [24]. GQDs prepared via acidic oxidation from carbon black showed PL transition from green to yellow as their sizes increased from 15 to 18 nm. Kim et al. presented size-dependent shape/edge-state variations of GQDs and visible PL showing anomalous size dependences (Fig. 8c) [105]. With varying the average size of GQDs from 5 to 35 nm, the peak energy of the absorption spectra monotonically decreases (Fig. 8d). The peak energy and the shape of the PL spectra are strongly dependent on the size of GQDs. All PL spectra show similar size-dependent peak shifts, almost irrespective of excitation wavelength except 470 nm. The PL peak energy decreases as diameter increases up to ca. 17 nm, consistent with the QCE. However, if the diameter is larger than ca. 17 nm, the PL peak energy increases with increasing diameter; in other words, the QCE no longer holds. This contradictory to QCE was originated from the edge changing, for diameter < 17 nm the majority of GQDs is of circular/elliptical shape with mixed edges of zigzag and armchair, but for diameter > 17 nm they are of polygonal shape mostly with armchair edge. These edges changing disturbed the evolution between QCE and the size of the GQDs. As mentioned above, the more armchair edges compared with zigzag edges could result in larger energy gap for graphene materials [82]. Most of the reported GQDs possessed the diameter over 5 nm and strongly visible PL emission. However, based on the DFT results, the bandgap energies of GQDs in the size range of over 5 nm are not beyond 1.0 eV [106]. The visible PL found in GQDs could be explained by minimization of thermalization due to electron-phonon scattering or by formation of excited-state relaxation channel resulting in inelastic light scattering by electric doping [100, 107]. By size-dependent shape/edge-state variations of GQDs, the electronic transitions can be modified in nanometer-sized GQDs to produce strong visible PL emissions in a controlled fashion. The high-energy PL of GQDs is especially efficient due to the unique properties of graphene: fast carrier-carrier scattering dominating over electron-phonon scattering. This facilitates direct recombination of excited e-h pairs producing
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Nano Res.
such high-energy PL before thermalization of the carriers with the lattice [108]. The high Coulomb scattering rate of graphene, which is attributed to the strongly reduced dielectric screening in the two-dimensional structure, is also essential for producing high-density non equilibrium carriers responsible for the strong e-h recombination. Moreover, because the surface/edge state-derived PL emissions are relatively brighter, there may be a general risk for their contaminating the observed bandgap fluorescence in the GQDs [86]. As a result, lots of experimentally observed PL of the GQDs was surface/edge state. For clear investigation on the QCE of the GQDs, the organic synthesis and nanolithography based routes to perfect sample will be highly desired in the near future. Besides, the chromatographic separation was also a powerful tool to investigate the PL mechanism of GQDs. For example, Zhu et al. pioneered the column chromatography to separate the GQDs with tuned oxidation degree, as a result, the separated GQDs possessed PL from blue to green [31]. Matsuda and co-worker developed the size-exclusion high performance liquid chromatography (HPLC) to separate the prepared GQDs [109]. Drastic change in PL spectra of GQDs from UV to red light region is observed by difference in their overall sizes. Discrete changes in emission wavelength indicate that the PL change comes from the differences in the population of small sp2 fragments with various sizes or shapes embedded in the GQDs. It’s also highly important to clarify the PL mechanism of GQDs by theoretical calculation, for example, Alam Sk et al. systematically investigated the PL properties of GQDs using DFT and TDDFT calculations [106]. It is revealed that the emission of zigzag-edged GQDs can cover the entire visible light spectrum by varying the diameter from 0.89 to 1.80 nm. Armchair edged and pyrrolic N-doping induced blue-shift, while the chemical functionalities and defects can cause the red-shift PL. Furthermore, the isolated inhomogenous sp2 domains can widen the PL peaks of GQDs. 5. PL mechanism of CNDs 5.1 Quantum size effect in carbon quantum dots (CQDs) There are a few studies concerned on the quantum
size effect in CNDs. We would rather called these kinds of CNDs as CQDs. Kang et al. have developed a current density-controlled electrochemical method to prepare CQDs, after further separation, the CQDs with 1.2-3.8 nm were obtained [23]. The PL properties vary sensitively with CQDs size (Fig. 9a), with small CQDs (1.2 nm, center) giving UV light emission, medium sized CQDs (1.5-3 nm) visible light emission (400-700 nm), and large CQDs (3.8 nm, center) near-infrared emission (Fig. 9b-j). To further confirm that and explain why these strong emissions came from the quantum-sized graphite fragment of CQDs, Kang et al. performed theoretical calculations to investigate the relationship between PL and cluster sizes. Fig. 9k shows the dependence of HOMO-LUMO gap on the size of the graphene fragments. As the size of the fragment increases, the gap decreases gradually, and the gap energy in the visible spectral range is obtained from graphene fragments with a diameter of 1.4-2.2 nm, which agrees well with the visible emission of CDs with diameters of
