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碳点的高性能化和功能化改性:方法、特性与展望

李程浩 ,  刘亚敏 ,  卢彬 ,  萨拉乌拉 ,  任先艳 ,  孙亚平

化学进展 ›› 2022, Vol. 34 ›› Issue (3) :499-518

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化学进展 ›› 2022, Vol. 34 ›› Issue (3) :499-518 DOI: 10.7536/PC210303
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碳点的高性能化和功能化改性:方法、特性与展望

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Toward High-Performance and Functionalized Carbon Dots: Strategies, Features, and Prospects

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摘要

碳点作为一种新型碳纳米材料,由于其出色的光学性能、低毒性、良好的生物相容性和易修饰性而被广泛应用于各个领域。为了满足不同领域的需求,几种用以调控碳点光学性能的方法已被提出,例如杂原子掺杂、半导体量子点掺杂、聚合物钝化和改性以及主-客体构建。其中,杂原子掺杂是通过单原子或多原子引入电子给体或受体改变其相邻碳原子的电子密度来增加荧光强度;半导体量子点也可与碳点进行复合提升电子分离效率而起到荧光增强的效果;就聚合物改性而言,聚合物不仅可以对碳点表面实施钝化和功能化,而且其固态(或固化)薄膜可以提供紧密的空间促进碳点表面的辐射跃迁起到荧光增强的效果。此外,由碳点-染料和多孔材料-碳点构成的两种主要的主-客体结构中,前者不仅对碳点的荧光发射强度有着促进的作用,更使得碳点具备了显著的红/近红外荧光发射性能,后者对固态发光碳点不仅提供了可能性和设计的灵活性,且为打开碳点新的应用领域提供了机会。本文将围绕四种碳点功能化的方法逐步展开讨论,并介绍相应碳点的光学性能、发光机理和潜在应用;论述功能化碳点的研究现状,并展望功能化碳点的研究方向。

Abstract

Carbon dots (CDots) emerging as a nova in carbon nanomaterials are applied in various fields due to their unique optical property, low toxicity, high biocompatibility, and designable flexibility. Several strategies for controlling the optical properties of CDots have been proposed to meet demands, including heteroatom doping, semiconductor quantum dot doping, polymer passivation and modification, and host-guest constructing. Electron donors or electron acceptors are introduced by doping a single heteroatom or multiple heteroatoms to affect the electron density of neighboring carbon atoms, hence enhancing the fluorescence emission intensity. Semiconductor quantum dots can also form composites with CDots to improve the electron hole separation efficiency, thus enhancing fluorescence. In terms of polymer modification, the polymer can passivate and functionalize the CDots' surface, and (cured) polymer film can provide tight spaces to promote the radiation transition on the surface of CDots to enhance the fluorescence. Furthermore, dye-CDots and porous material-CDots represent the main host-guest structures. The former shows positive effects on the fluorescence emission of CDots, especially performing better in red/near-infrared emission. The latter is highly designable and contributes to solid-state fluorescence, paving the way for advanced applications. This review elucidates four types of functionalized CDots and summarizes their optical properties, photoluminescence mechanisms, and potential applications. Finally, the development and current situation of functionalized CDots are discussed, and the future research direction of functionalized CDots has also been prospected.

Contents

1 Introduction

2 Heteroatom-doped carbon dots

2.1 N-doped carbon dots

2.2 Other donor atom-doped carbon dots

2.3 B-doped carbon dots

2.4 Lanthanide atom-doped carbon dots

2.5 Multiple atom-codoped carbon dots

3 Semiconductor quantum dot-doped carbon dots and their composites

4 Polymer functionalized carbon dots and their composites

4.1 Polymer as a surface layer of carbon dots for fluorescence enhancement

4.2 Polymer as a matrix in carbon dot-containing composites

4.3 Polymer as a precursor of carbon dots

5 Host-guest carbon dots

5.1 Host-guest structure composed of carbon dots and dyes

5.2 Host-guest structure composed of carbon dots and porous materials

6 Summary and outlook

Graphical abstract

关键词

碳点 / 杂原子 / 半导体量子点 / 聚合物 / 主-客体结构 / 红光/近红外光发射

Key words

carbon dots / heteroatom / semiconductor quantum dots / polymer / host-guest structure / red/near-infrared emission

引用本文

引用格式 ▾
李程浩, 刘亚敏, 卢彬, 萨拉乌拉, 任先艳, 孙亚平. 碳点的高性能化和功能化改性:方法、特性与展望[J]. 化学进展, 2022, 34(3): 499-518 DOI:10.7536/PC210303
Chenghao Li, Yamin Liu, Bin Lu, Ulla Sana, Xianyan Ren, Yaping Sun. Toward High-Performance and Functionalized Carbon Dots: Strategies, Features, and Prospects[J]. Progress in Chemistry, 2022, 34(3): 499-518 DOI:10.7536/PC210303

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1 Introduction

Carbon dots (CDots) have recently attracted everincreasing attention as the rising stars of the carbon nanomaterial family. Their excellent performances, such as fascinating optical properties[1⇓⇓~4], simple preparation[5⇓~7], good biocompatibility[8], and low toxicity[1], have been driving their explored wide applications in bioimaging and medical diagnosis[9], energy-related devices[10], photocatalysts[11], and environmental sensors[12]. Thus, it is important to perfect the fluorescence performances of CDots, attaining a wide spectrum of emissions from ultraviolet to infrared, outstanding electron and hole transfer properties, and high fluorescence quantum yield (QY). Great efforts have been initiated in this regard, including heteroatom doping, semiconductor quantum dot doping, polymer functionalization, and host-guest synergy.

Generally, any passivated defect plays an intrinsic role in improving the performance of CDots, acting as an excitation trap. Heteroatoms such as nitrogen (N), phosphorus (P)[13], boron (B)[14], fluorine (F)[15], and sulfur (S)[16] have been extensively used to enhance the emission properties of CDots. Specifically, they have such remarkable effects on the QY that the QY has increased by more than 10% and even up to 80% (Table 1). Moreover, the donor (e.g., N, P, S, and F) or acceptor (e.g., B) atoms can theoretically enhance the electron or hole density of their neighboring carbon atoms and change the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO-LOMO) gap, causing the emission to redshift. Unlike the above-mentioned heteroatoms, more recently, lanthanide atoms are used not only to functionalize CDots to improve the optical properties but also to endow lanthanide-doped CDots with multiple functions like photothermal therapy and magnetic resonance/fluorescence multimodal imaging (MRI/FI).

Besides heteroatom doping, some inorganic salts, such as semiconductor quantum dots, could serve as the secondary doping effect by acting as a hard yet looser shell. Sun's group found that the semiconductor quantum dots can improve the emission intensity of CDots to the extent that it is competitive with commercially available semiconductor quantum dots[17]. Moreover, a reversed core/shell structure, in which the CDots are used as a shell and play a vital role in improving semiconductor performance, such as photocatalytic efficiency.

Polymers have been found to be suitable as passivation layers, matrices, and precursors. The fluorescence QY of CDots has considerably improved polymer passivation. Sun's group first reported polymer's role in CDots that emitted fluorescence depending on simple surface passivation from PEG1500N, PPEI-EI[18], and crosslinked polyethylene glycol (PEG)[19]. Later, they further demonstrated that polymer matrices could provide a more compressive surface passivation layer that is beneficial for radiative recombination of the surface-trapped electrons and holes, resulting in fantastic fluorescence properties and applications. When embedded in polymer matrices, CDots are uniformly dispersed, which prevents the aggregated quenching phenomenon of solid CDots nanoparticles. Furthermore, the polymer can partially serve as a precursor for the carbonization of CDots, contributing to the growth of the carbon core.

More recently, a new configuration, “host-guest,” conceptually similar to endofullerenes, has been proposed by Sun's group[20]. Beyond high photoluminescence (PL) intensity, host-guest CDots can emit fluorescence in red/NIR designed and prepared with red/NIR dyes encapsulated as guests in the carbon nanoparticle core. This improvement may lead CDots to new fields such as photoelectric conversion devices. The photoelectric conversion capability of CDots depends heavily on the wavelength width of optical absorption and emission. Moreover, porous materials (PMs) such as zeolites, metal-organic frameworks (MOFs), and porous carbons, have been reported as hosts to enable CDots to emit in a solid state[21].

As mentioned above, among the several studies on CDots already in the literature, strategies by doping or incorporating organic molecules, inorganic ions, or polymers have been accepted smartly to increase the PL intensity and even broaden the scope of optical absorption and emission. This paper summarizes, compares, and discusses the dominant methods for improving the properties, broadening the application field, and elucidating the photophysical process of CDots, including heteroatom-doping, semiconductor quantum dot doping, polymer coating, and introducing a guest (or host) structure. This review provides a way to prepare CDots. Performances other than PL of those modified CDots related to some growing applications of their composites were also discussed.

2 Heteroatom-doped carbon dots

2.1 N-doped carbon dots

Heteroatom-doped CDots showing excellent PL properties have attracted more attention and demonstrated immense potential for diverse applications. Among several heteroatoms, the N atom is the most prevailing one contributing to the emission redshift and enhancement. Generally, N-doped CDots (N-CDots) are always synthesized in the “bottom-up” method. There are more than 10 kinds of nitrogen-containing chemicals reported as N sources, such as folic acid[22], ammonium citrate[23], and ethylenediamine[24]. Some of them act as only nitrogen sources that along with carbon sources as precursors, whereas others are used as precursors directly.

2.1.1 N-doped carbon dots refer to fluorescence enhancement

It has been demonstrated that the N atom can increase the electron density of its neighboring carbon atoms, providing a new radiation recombination pathway and thus tailoring the band structure. For instance, Yi et al. synthesized N-CDots with QY approximately 81% by hydrothermal treatment on trans-aconitic acid and diethylenetriamine, which were used as fluorescent probes for Fe3+ detection[32]; Xu et al. facilely synthesized N-CDots with PL and QY up to 50.2% via the carbonization of sodium alginate and melatonin[33]. Nevertheless, the carbon core structure or the passivation surface among the resulting CDots must be significantly different because of the distinct thermal stability and decomposition behavior of different nitrogen-containing precursors. Table 1 shows the reported N-CDots showing a significant difference in PL intensity and emission color; N-CDots extracted from a different carbonized precursor containing nitrogen sources possess the QY in a wide range of 10%~80%.

Nevertheless, the exact mechanism of the N atom to CDots fluorescence efficiency CDots remains unknown. For example, amino N in CDots has a superior PL efficiency[32]; Pyrrole N has been reported to participate in the protonation process to enhance the fluorescence emission[33]; Pyridine N enhances the conjugated system in CDots and thus improves the PL intensity of CDots due to the formation of cyclic imines[34]. Graphitic N increases the PL efficiency by effectively tailoring the electronic energy level, which is corresponded to optical transitions[35].

After Ren's group had paid great efforts, the pyridine N proved to be beneficial to increasing the QY of N-CDots, probably caused by the enhancement of electron exchange of complex luminescent centers and the formation and introduction of the formation and introduction of electron traps[36]. The result was disclosed on the basis of two kinds of well-designed nitrogen-doped CDots (N-CDots-1 and N-CDots-2) that contain the same N/C ratio and are common in the content of sp2 hybridized carbon atoms and particle size (Fig.1).

Generally, the fluorescence intensity is positively correlated with N atoms. Taking the N-CDots listed in Table 1 as examples. One of them possessing N atoms accounting for 40.3% of C atoms, shows the highest QY of 46.2%. Ren's group confirmed the contribution of the number of N atoms and prepared the N-CDots with a QY of 34.2% when the N/C ratio is 39.9%[36]. Overall, a great number of pyridine N can enhance the fluorescence emission of CDots.

2.1.2 N-doped carbon dots refer to emission redshift

Despite reports of red-emissive N-CDots and desirable N-doped graphene dots (N-GDots) emitting in a wide range from deep ultraviolet to NIR, it has been proven that N atoms in CDots have more effects on enhancing the PL emission intensity than tailoring the PL emission into NIR[25]. As far as the N-GDots were concerned, as shown in Fig.2, the relatively larger size is favorable to NIR absorption and emission. The NIR emitting ability is attributed to the larger conjugated system containing extensive delocalized π electrons in the layered N-GDots.

As previously stated, N-CDots are often synthesized by nitrogen-containing organics. The reported nitrogen sources for preparing red emission CDots can be classified into formamide, urea, and phenylenediamine, and the former two were extensively reported earlier than the last one. These nitrogen sources also show the ability to produce red emission N-CDots with a high fluorescence QY. Table 1 shows some examples of N-CDots with red emission greater than 600 nm.

2.1.2.1 Formamide

Formamide is a nitrogen-containing solvent that is widely used as a nitrogen source and as a solvent to provide good solubility to most carbon sources. Carbon sources are usually dissolved in formamide, and then, the solution is subjected to solvothermal treatment. N-CDots derived in this way usually possess a high fluorescence QY in red emission. In 2016, Pan et al. treated glutathione in formamide at 160 ℃ for 1 h to synthesize N-CDots[28], in which formamide played an important role in emission redshift with a QY of 16.8% at 683 nm when excited at 420 nm. Subsequently, the N-CDots were used in fluorescent bioimaging, showing the great potential of red emission N-CDots applied in vivo. In another work, Sun et al. reported a facile method for preparing N-CDots with a high QY of 22.9% at 640 nm. They treated citric acid in formamide via microwave-assisted heating at 160 ℃ for 1 h and 120 ℃ for another 1 h[29]. They prove the importance of formamide in synthesizing N-CDots that exhibited high photothermal conversion efficiency. Formamide-derived red N-CDots exhibited excellent biocompatibility in vivo and photothermal therapy in vitro.

2.1.2.2 Urea

Urea is a famous nitrogen source that has frequently been reported recently and can produce N-CDots successfully together with a carbon source. The optical properties of the produced N-CDots could be tailored by changing the molar ratio of the carbon source to urea and the reaction temperature. Miao et al. synthesized a series of N-CDots using citric acid and urea as precursors[37]. N-CDots with blue, green, and red emissions were prepared by tailoring the molar ratio and reaction temperature. More interestingly, as the ratio and temperature increased, the emission redshifted gradually. This work provides a feasible approach in developing colorful light-emitting devices using N-CDots prepared from citric acid and urea. In 2018, Bao et al. engaged in theragnostic with red N-CDots[9].

Similarly, citric acid and urea were used as precursors to synthesize red N-CDots, treated in dimethyl sulfoxide at 160 ℃ for 4 h. The accumulation of N-CDots in tumor tissue showed strong red fluorescence and high photothermal conversion efficiency of 59.2%. The tumors were eliminated after intravenous injecting red N-CDots followed by irradiation under a 655 nm laser. Consequently, urea is also a good choice for red emission N-CDots with strong thermostability and photostability.

2.1.2.3 Phenylenediamine

Phenylenediamine is an emerging nitrogen source compared with formamide and urea, which have many isomers and derivatives. Jiang et al. reported a facile method to synthesize blue, green, and red N-CDots using o-phenylenediamine, m-phenylenediamine, and p-phenylenediamine, respectively, since 2015[31]. Three kinds of N-CDots possessed emissions of three primary colors under single-wavelength ultraviolet irradiation (λex = 365 nm), both in solution and polymer matrix. N-CDots prepared from phenylenediamine were a big breakthrough in several applications related to full-color display because of their unique optical features. Wang et al. reported the success of preparing red emission N-CDots via a solvothermal method using o-phenylenediamine and dopamine as raw materials in 2019[38]. The as-prepared N-CDots had a high QY of 33.96% even in an aqueous solution compared with other reported red emission N-CDots synthesized in an organic solution. Also in 2019, Ye et al. synthesized red emission N-CDots using p-phenylenediamine,o-phenylenediamine,and dopamine at 200 ℃ via a one-pot hydrothermal method[39]. The red N-CDots-based fluorescent probes exhibited a reversible response to pH and high photostability, demonstrating the possibility that red N-CDots could be applied in vivo because of high sensitivity and low cytotoxicity. Although the toxicity of phenylenediamine is an issue before synthesizing N-CDots, converting phenylenediamine to N-CDots can yet be regarded as appropriate. As a result of the harsh treatment, these N-CDots are non-toxic and keep good cell permeability.

2.1.3 Photoluminescence mechanism of N-doped carbon dots

Fang et al. investigated four types of N-CDots prepared from citric acid and four amino group-containing molecules precursors via a hydrothermal method at 220 ℃ for 3 h[40]. They proposed that the prepared N-CDots with pyridine-derivatives and defects that contributed to luminescence, implying that the emission spectra of the N-CDots are determined not only by the isolated conjugating units but also by specific structures formed during the heat treatment process of precursors. Nitrogen sources such as formamide, urea, and phenylenediamine have great potential to convert to specific structures or be embedded in CDots during the reaction.

Sun and his group proposed that the intrinsic fluorescence origin of commercial red-emissive N-CDots is a dye-like substance mixed with a very low content of carbon[41]. Moreover, they compared N-CDots derived from citric acid and formamide treated at 160 ℃ for 1 h and at 300 ℃ for 2 h, respectively. The absorption curve suggests a very low carbon content until the precursors were carbonized at 300 ℃, suggesting that a low temperature in conjunction with a short carbonization time was incapable of producing the so-called “red emission N-CDots” because of the poor carbon content in the carbon core.

Some researchers have studied the structural transformation of carbon and nitrogen sources under different temperatures. Xiong et al. comprehensively summarized the molecular fluorophores' influence on N-CDots by taking citric acid-based CDots as examples[42]. Several specific organic fluorescent intermediates were collected when treating the combined precursors at temperatures from 130 ℃ to 180 ℃ no more than 2 h. These intermediates mainly contribute to the fluorescence predominated. Another result proved reasonable by hydrothermal synthesized N-CDots using citric acid and ethylenediamine as precursors, which produced fluorescent citrazinic derivatives that gave a very high QY to the N-CDots[43].

The organic fluorophores derived during the carbonization of the precursors possess specific absorption properties that contribute to the PL emission. In some reported work, it is the dye-like organic fluorophore attributed to the emission of the N-CDots rather than the carbon core, as their spectroscopic results suggest. Their UV-vis absorption spectra showed strong characteristic absorption peaks belonging to dye-like fluorophores conversely, but feeble broad absorption band approximately 480 nm belonging to π-plasmons in the carbon nanoparticles. Their PL spectra, however, show narrow emission spectra independent of excitation wavelengths.

Given the general understanding of the synthesis process of N-CDots that the formation of carbon nanoparticles at high temperatures must come after the pyrolysis of the precursor and polymerization of the resulting intermediate radical, molecular fluorophores would be expected to form at early stages. In other words, enough high carbonization temperature is necessary to form the carbon core, which is the main part of CDots; otherwise, great deals of molecular fluorophores would come into being during the synthesis instead of true CDots.

2.2 Other donor atom-doped carbon dots

Other heteroatoms (e.g., S, P, and F) are effective in enhancing the PL emission intensity by adjusting the electron density of their neighboring carbon atoms, although they have been relatively reported less than N atoms. The bottom-up method mainly prepared the single S, P, or F-doped CDots. The reported sulfur sources varied from sulfur-containing compounds, such as thiomalic acid[44] and sulfuric acid[45], to natural organisms like alfalfa[46] and L-cysteine[47]. S-doped CDots (S-CDots) reported in 2013 emitting blue color with a QY of 11.8% were prepared using sulfuric acid as a sulfur source and thiomalic acid as a carbon source[44]; other S-CDots showing an extremely high QY of approximately 67% were reported through sodium thiosulfate that was selected as a sulfur source[48]. The reported S-CDots uniformly emit blue fluorescence only, although they show a big difference in emission intensity.

What is worth mentioning is that phosphorus doping can improve the emission efficiency and tune the emission wavelength of CDots. In concentrated phosphoric acid at 80 ℃~90 ℃, for example, carbonized carbohydrates yielded yellow and red emission CDots[49]. Other work has similar results through a comparative study[45]; sulfuric acid and phosphoric acid were used to carbonize the same starting precursor, sucrose, leading to the isolation of CDots with blue (440 nm) and green (515 nm) emissions, respectively. In terms of fluorescence intensity, phosphorus plays an important role. Liu et al. prepared some P-doped CDots (P-CDots) with QY of 10.5%~21.7% using phytic acid as the precursor[50]. Phosphorous bromide combined with hydroquinone, as a phosphorus and carbon source, respectively, resulted in P-CDots exhibiting a QY of up to 25%. Phosphorus doping demonstrated an easy way to tune the emission wavelength of CDots. Table 2 shows some cases of the reported S or P-doped CDots.

By far, nitrogen was widely accepted for the emission of CDots, but the effects of other heteroatoms like sulfur or phosphorus must be further investigated. Ren's group conducted a comparative study on the effect of S and P by tuning the chain structure of a polymer precursor combined with a theoretical compute applying two one-layer models on the basis of coronene units containing phosphate and sulfonate group, respectively (Fig.3a)[51]. The results revealed that the phosphate group (as P source) has a better effect on improving the luminescence efficiency of CDots compared with the sulfonate group (as S source), shown by a narrower electronic gap (defined as the energy difference between, the HOMO-LUMO). The HOMO-LUMO gap of the model containing phosphate group is 3.67 eV, which is less than that containing sulfonate group 3.89 eV (Fig.3b).

2.3 B-doped carbon dots

There have been several successful instances of B-doped carbon materials, such as graphene, carbon nanotubes, fullerene, and diamond, in which B atoms can tune the electronic and optical properties of the carbon matrix. Similarly, boron can be well incorporated into CDots, thereby acting as an effective dopant. Thus far, the incorporation of boron into CDots or GDots has been achieved by hydrothermal/solvothermal method[52,53], microwave[54], arc-discharge in the gas phase (GDots)[55], or electrochemically (GDots)[56]. Boric acid was the most common boron source; nevertheless, other boron-containing compounds such as boron tribromide[57] and 4-vinylphenylboronic acid[58] were also used on occasion. Table 2 shows some of the reported B-doped CDots (B-CDots).

Regarding the PL, B increases the fluorescence emission intensity. When exposed to ultraviolet excitation, the B-CDots are strongly fluorescent in the blue region. Moreover, when compared with either undoped CDots or the N-CDots, B-CDots result in significant enhancement of nonlinear optical response CDots[54]; In other word, B-CDots prepared by microwave treatment of an aqueous solution such as citric acid, boric acid, and urea show a 25-fold enhancement of nonlinear optical response (in terms of χ(3)/C*). Table 3 lists the detailed values of the nonlinear optical parameters.

Besides PL, an excellent magnetic property of B-doped graphene dots (B-GDots) was reported in 2017, following the early theoretical and experimental studies of metal-free graphene. Theoretical studies have shown the ferromagnetic property of metal-free graphene due to various defects in the material, such as vacancies, topological defects, modification of the crystal lattice, and hydrogen chemisorption. Several subsequent studies have confirmed ferromagnetism and paramagnetism in graphene, reduced graphene oxide, and graphite[59,60]. When it comes to GDots, their paramagnetic properties were established after defects from B atoms were introduced[58]. The B-GDots exhibit positive contrast (brightening signaling) and possess a high r1 value of approximately 18.3 mM-1·S-1 that is higher than the clinical MRI contrast agent (r1 = 3.4~4.1 mM-1·S-1) (Fig.4). Because of the intrinsic excitation wavelength-tunable PL property lower toxicity than either semiconductor quantum dots or metal (e.g., Mn, Gd)-containing magnetic materials, B-GDots may serve as an MRI/FI probe.

2.4 Lanthanide-doped carbon dots

Lanthanide doping has is effective in functionalizing or enhancing the optical properties of CDots. When Gd was doped to CDots, for example, an MRI-guided photothermal chemotherapy agent was constructed using Gd-CDots nanoplatform that delivers chemotherapy drugs and a photothermal agent[65].

Furthermore, the Gd-CDots show a maximum emission peak at 437 nm with a fluorescence QY of 26.84%, showing high photothermal conversion efficiency at 808 nm in NIR emission.

Ren et al. prepared Gd-CDots after the pyrolysis treatment of Gd chelates at different temperatures[66]. Under a proper high temperature, an MRI/FI agent was obtained and showed excellent wavelength-tunable fluorescence properties with the QY of 8.9% and a superparamagnetic nature with the r1 of 6.4 mM·L-1·S-1. Yang et al. synthesized La-CDots with a higher PL QY of 12.42% and a longer lifetime of 9.80 ns than conventional CDots for the sensitive and selective detection of Fe3+ and cell imaging, showing that La doping may result in new electron transfer processes and improve the PL property of CDots[67]. Zhou et al. prepared Eu-CDots for fluorescent visual detection[68]. The fluorescence of Eu-CDots would gradually redshift by coordinating with dipicolinic acid due to the 5D0 to 7FJ (J = 0,1,2) transition of Eu. Zhang et al. synthesized Yb-CDots with a QY of 51.48% when excited at 450 nm[69]. CDots act as a visible emitter and an “antenna” for sensitizing the NIR emission of Yb.

To summarize, lanthanide-doped CDots have attracted more and more interest recently, especially because of their unique optical and spectral properties, such as long fluorescence lifetimes, narrow line-like emission bands, and large stokes shift. The lanthanide mainly affects both functionalizing the CDots and enhancing the fluorescence emission of CDots.

2.5 Multiple atom-codoped carbon dots

Multiple heteroatom doping has been proven effective in improving the PL properties of CDots due to the synergistic effect of heteroatoms. Several couples of heteroatoms have been reported, such as N, S; N, P; N, B; and others (such as S, P and N, F, S). Examples of codoped CDots have been presented in a review[70]. Hence, here we just discuss the synergistic effect between the doped heteroatoms by simple examples and focus on codoped CDots that possess fluorescence QY of more than 50%.

2.5.1 N- and S-codoped carbon dots

N and S codoping is the first effort to further enhance the PL intensity of CDots. Taking the first synthesized N, S-CDots as an example, when the L-cysteine was used as N and S source and treated using hydrothermal method accompanying citric acid, the prepared N, S-CDots exhibited a very high fluorescence QY of 73% and excitation independent emission.[47] The paper confirmed and demonstrated the prominent status of the surface state rather than electronic conjugate structures.

It seemed likely that the introduction of S atoms would enhance the effect of N atoms on the properties of the CDots through a synergy effect. The synergy effect has been thoroughly explained in the literature, as shown in Fig.5, where a comparative study between O-CDots (precursor: citric acid monohydrate), N-CDots (precursor: citric acid monohydrate and sulfur-free amino acid, glycine), and N, S-CDots (citric acid monohydrate and L-cysteine) was conducted[47]. The O-CDots have different kinds of surface states (labeled as O-states), the nitrogen doping introduces a new kind of surface state (labeled as the N-state). Electrons trapped by the newly formed surface N-states can facilitate a high yield of radiative recombination. The introduced S atoms eliminated the O-states and enhanced the N-states, resulting in the original O-states being neglected in the N, S-CDots. The fluorescence QY of O-CDots increases from 5.3% to 73.0% after doping with N and S atoms. Several other N, S-CDots possess fluorescence QY higher than 50% (Table 4).

2.5.2 N- and P-codoped carbon dots

P atom is the next codoping heteroatom after S atom to study the synergistic effect among the doped heteroatoms. Barman et al. proposed the first strategy to understand the effect of P doping on the photophysical behavior of nitrogen-containing CDots using steady-state and time-resolved spectroscopy[71]. They discovered that adding an extra electron to CDots for n-type doping (phosphorus dopant) favors the radiative relaxation pathways, which improves photophysical properties. In other words, the formation of p-n-type photochemical diodes via N and P codoping into CDots increased QY greatly[72]. For example, the fluorescence QY values, lifetime, and radiative rate constants increase from 64%, 12.77 ns, and 5.05 × 107 s-1 to 70%, 13.0 ns, and 5.40 × 107 s-1, respectively. Simultaneously, the nonradiative decay rate constant slightly decreases from 2.78 × 107 s-1 to 2.30 × 107 s-1. Some similar results are reported by other groups and are listed in Table 4.

Interestingly, N and P codoping into the CDots causes bright dual emissions that center at blue and green color regions, respectively, as shown in Table 4. The origin may be from (1) defects induced by P doping or N and P codoping which can lead to the presence of a highly localized state close to the Fermi level[73,74] and (2), possibly, codoping of N and P affects the degree of polymerization and graphitization. A higher graphitization degree induces a redshift of fluorescence resulting from the enhanced degree of π conjugation and the decreased band gaps. The green emission of N, P-CDots further demonstrates that phosphorus doping can tune the emission wavelength of CDots.

2.5.3 N- and B-codoped carbon dots

It has been well accepted that introductions of heteroatoms into carbon nanomaterials can tune the conduction or valence band position of the doped carbon material, resulting in altered functions, where N and P were widely used as electron donors while B was used as electron acceptors. Thus, it is fascinating to explore what will happen to CDots when both their conduction and valence band positions are tuned when simultaneously doped by N and B atoms.

For this purpose, Barman et al. designed a comparative study between N, P-CDots and N, B-CDots. They found that photophysical properties (radiative decay rate and average decay time) are increased for the former because of extra electron incorporation for n-type doping (P dopant) to CDots, which favors the radiative relaxation pathways. By contrast, B (p-type dopant) codoping to nitrogen-containing CDots favors the nonradiative electron hole recombination pathways due to the incorporation of excess holes. Therefore, QY, radiative rate, and average decay time decrease[71].

Perhaps, this is why reported N, B-CDots are hardly the same as N, S/P-codoped ones, with QY rarely exceeding 50%. Regardless of the type of precursor used, when oxidized by boric acid, the prepared CDots show the highest QY approximately 36.5%, as shown in Table 4.

Nevertheless, the N and B-CDots may be the best candidates for use in solar cells and optoelectronic devices because of their catalysis to charge transfer process from both sides of electron transfer and hole transfer.

2.5.4 Other heteroatom-codoped carbon dots

Obviously, there is a synergistic effect between the heteroatoms of electron donors that can improve the fluorescence performance of CDots. Except for the above-mentioned N, S; N, P; and N, B couples, there are other samples. For example, Loi et al. reported that the fluorescence for CDots produced with a mixture of acids (H2SO4/H3PO4) was 15% and 20% brighter than that produced with pure H2SO4 or H3PO4, respectively[45]. The N, F, S-CDots showing an excellent PL QY of 70% were collected by Kundu et al. after microwave treatment of multiwalled carbon nanotubes in a customized ionic liquid[75], which indicated that the reason for such high PL QY would attribute to the interaction of defect clusters and dopants.

Kang's group also performed another interesting work. They reported N, O-CDots with an absolute QY of 88.9% in ethanol[76]. Dehydrative condensation and dehydrogenative planarization tend to obtain N, O-CDots with a large π-conjugated structure, attributed to interactions among carboxyl, amino, and hydroxyl groups. From what was discussed above, N atoms as electron donors positively affect the fluorescence, and other heteroatoms codoping may help increase the fluorescence by influencing electron transferring.

3 Semiconductor quantum dot-doped carbon dots and their composites

Although heteroatoms have been proven successful in enhancing the fluorescence properties of CDots, Sun's group discovered a novel passivation agent, that is, semiconductor quantum dots, which significantly enhanced the emission intensity of CDots to a level comparable with commercially available semiconductor quantum dots. In 2008, ZnO-CDots or ZnS-CDots were prepared in an aqueous suspension of CDots with Zn(CH3COO)2 via hydrolysis with NaOH or precipitation with Na2S, respectively. After the organic molecules functionalized the ZnO-CDots or ZnS-CDots (e.g., PEG1500N), both dots exhibited strong PL under one- or two-photon excitations, competitive to commercially available ZnS dots[17].

In 2011, the effect of semiconductor quantum dots was further studied by the same group on the basis of ZnS-CDots and TiO2-CDots, primarily attributed to the enhancement of the formation of emissive states[88]. They reported that by combining nanoscale semiconductors with an organic functionalization passivation layer on the surface, the CDots exhibited fluorescence emission with a QY of up to 78% (Fig.6). These hybrid dots were much more fluorescent than their counterparts, and the observed high PL QY was elucidated because of the combined surface passivation effect on the core carbon nanoparticles by both the inorganic compound and the organic functionalization.

Although the semiconductor coating/doping is proven to be effective for improving the overall fluorescence properties, there is another kind of semiconductor quantum dot/CDots composites attracting more and more attention in several fields, where the CDots are attached on the surface of a semiconductor; furthermore, the composite shows more attractive performances than either CDots or semiconductor dots in terms of photocatalysis[89] and light harvesting[90]. For example, the CDots modified ZnO nanocrystal exhibits a photocatalytic degradation efficiency of rhodamine B under visible light increased by 433.3% compared with the pure ZnO because of its higher absorbance in the visible region[91].

Yu et al. also prepared CuS-CDots with a broad absorption band from 500 to 900 nm[92]. Recently, Wu et al. reported TiO2-CDots film with the ability of photocatalytic oxidation[93], in which CDots generated excited electrons and transferred the electrons to TiO2. Other TiO2-CDots for degrading methyl orange with a degradation efficiency of up to 91.04% in 24 min was designed by Zhang's group[94].

It has been well acknowledged that CDots can either act as a photoelectron reservoir to store and shuttle the photogenerated electrons from the semiconductor or serve as an organic dye-like “photosensitizer”[95,96]. The high absorption capacity of carbon materials is also expected to improve the photocatalytic process. Moreover, the CDots with high visible light absorption capacity collaborate well with the semiconductor dots that strongly absorb UV radiation and are promising in several new fields, such as bacterial inhibition[97], energy transfer[98], and electronic devices[99].

In conclusion, carbon-semiconductor hybrid dots in different structural configurations behave quite differently in terms of the optical properties and photoinduced redox properties. In the configuration of each small carbon nanoparticle coated/doped with a small amount of semiconductor, the semiconductor is more like a “co-surface passivation agent” and seems very effective. By contrast, the composites of semiconductor-coated CDots present different properties, with the carbon domains serving the functions for the harvesting of photons.

4 Polymer functionalized carbon dots and their composites

4.1 Polymer as a surface layer of carbon dots for fluorescence enhancement

Polymers have been used as crucial components to synthesize or modify CDots and thus increase the PL efficiency of CDots or provide new functionalities. In terms of increasing the PL efficiency, common polymer passivation agents include diamine-terminated oligomeric poly-(ethylene glycol) (PEG-NH2), polyethyleneimine (PEI)[100], and poly(propionyl-ethyleneimine-co-ethyleneimine) (PPEI-EI)[18].

Moreover, the crosslinking of the surface passivation layer could be applied to ensure structural stability and thus further increase the PL efficiency of CDots. Mechanistically, the fluorescence emissions in CDots may be attributed to the radiative recombination of the electrons and holes[18]. Here, the surface passivation layer plays a critical role in bright fluorescence emissions. Consequently, the structural stability of the surface passivation layer is expected to affect the fluorescence performance of CDots. As reported by Sun's group, after the PEG1500N functionalized CDots were crosslinked by dimethyl pimelimidate, their fluorescence brightness is increased by times[19].

A compressive surface passivation layer in CDots as implied by the same mechanistic picture could enhance the passivation effect for brighter fluorescence emissions, as indicated by the results obtained from CDots in a more restrictive environment, such as films. Sun's group first proved such a significant effect by dispersing the CDots into a poly(vinyl alcohol) (PVA) film matrix[101]. The compression effect on the surface passivation layer results in more favorable radiative recombinations of electrons and holes trapped on the surface of carbon particles.

4.2 Polymer as a matrix in carbon dot-containing composites

Polymers, in a sense, are desired candidates for the functionalization of the CDots because of their diversity and flexibility in structures and properties. Nevertheless, CDots can emit in solid state when incorporated into a polymer matrix, which is required for CDots to be applicable to various devices[102]. Solid-state CDots can easily aggregate and cause fluorescence quenching because of the absence of a polymer matrix. Consequently, polymer renders CDots to remain close but avoid aggregation, whereas the polymer matrix enables electrons to flow freely among CDots.

CDots have been used as a multistimuli-responsive probe, multifunctional hydrogel, molecular imprinting material, white light emitters, and conducting composites[103]. Notably, Sachdev et al. developed a chitosan-based hydrogel composed of highly fluorescent CDots and loaded with an anticancer drug called 5-fluorouracil. In this way, it was possible to combine the merits of both CDots and 5-fluorouracil on a common platform for monitoring the cellular uptake and therapeutic effects[104].

When applied as light emitters, CDots are always incorporated into a polymer matrix (e.g., PVA, PMMA (poly (methyl methacrylate)) to form transparent films[105⇓~107]. Chen et al. have reported white-light-emitting polymer composite films prepared by conjugating CDots and lanthanide complexes into a PMMA matrix[108]; Bhunia et al. reported a transparent film that exhibits tunable light emission through in situ synthesis of polymer matrices with CDots present[109].

Lee et al. reported cured CDots/epoxy composites[110], and they found that CDots solid-state composites show more stable PL emission than the CDots solution (the QY kept more than 80% after 8 weeks). The reason is that the epoxy resin matrix helps CDots be uniformly dispersed rather than get aggregated, which prevents the CDots from quenching.

Beyond their high PL efficiency, substantial photoinduced redox properties of CDots are opening up new opportunities for these quantum dot-like nanomaterials in light harvesting and related applications, leading to the beginning of studies on CDots/polymer composites as devices. CDots are both excellent electron donors and excellent electron acceptors; in appearance, the PL in CDots could be quenched efficiently by electron acceptor (e.g., 4-nitrotoluene, 2,4-dinitrotoluene) or donor molecules (e.g., N,N-diethylaniline) in solution.

Poly(N-vinylcarbazole) (PVK), for example, was selected as a representative of electronic polymer, which was functionalized on the surface of CDots by Sun's group to study the potentials in optoelectronic devices compared with C60[111]. The results showed that in comparing PVK-CDots and PVK-C60, the former was more absorptive and therefore more effective in photon harvesting across the visible spectral region. Moreover, the PVK-CDots exhibited significant photoinduced electron-accepting characteristics and, simultaneously, also extraordinary electron-donating abilities that were not available to PVK-C60 (Fig.7). Since C60-based composites with electronic polymers, including PVK, have found significant applications in optoelectronic devices and systems, similar or more valuable uses of the CDots functionalized by electronic polymers like PVK may be envisaged.

Chen et al. reported that using CDots as the electron transport layers improved the performances and prolonged the life of organic solar cells[112]. The CDots were prepared using citric acid and ethylenediamine as precursors, showing classical excitation-dependent properties. The wide applicability of CDots in organic photovoltaic cells was confirmed when using the CDots as the electron transport layers in polymer solar cells and the poly(3-hexylthiophene): phenyl-C61-butyric acid methyl ester as the donor. Notably, the devices with CDots (0.5 mg·mL-1) give an average power conversion efficiency (PCE) of 3.42% (best PCE 3.52%) with a Voc (open circuit voltage) of 0.609 V, an FF (fill factor) of 54.8%, and a Jsc (short circuit current) of 9.98 mA·cm-2.

Liu et al. introduced CDots into an active layer to increase the PCE of polymer solar cells[113]. The performances of doped devices were improved because of the accelerated photoinduced charge separation and transport from the CDots, and the highest PCE was increased by 28.2%.

Jian et al. developed CDots reinforced polypyrrole nanowires via an electrostatic self-assembly strategy for high-performance supercapacitors[114]. The CDots composite electrode displayed a specific capacitance of 306 F·g-1 at a current density of 0.5 A·g-1 and retained 66.8% capacitance when the current density increased by 80 times.

Su et al. studied the effect of CDots on free radical polymerization using N-vinyl-2-pyrrolidone and 2-hydroxyethyl acrylate as monomers[115]. They noticed that CDots could serve as reductants in the system to form redox initiator couples with $\mathrm{S}_{2} \mathrm{O}_{8}^{2-}$. The decomposition activation energy of $\mathrm{S}_{2} \mathrm{O}_{8}^{2-}$ efficiently was lowered by CDots because of electron transfer between the CDots and $\mathrm{S}_{2} \mathrm{O}_{8}^{2-}$. Simultaneously, the CDots transfer an electron to the monomers, allowing free radical polymerization. The composites showed an absolute PL of QY up to 60%, indicating that the redox properties enable CDots to initiate polymerization under mild conditions.

4.3 Polymer as a precursor of carbon dots

Polymers are excellent candidates for synthesizing CDots as precursors because of their flexibility in molecular structure, molecular weight, and topological structure. Natural polymers, such as lignin and chitosan, and synthetic polymers, such as PEG, PEI, and polythiophene, are widely used. Some examples and the corresponding details have been summarized in a review[103].

Another wonderful opportunity provided by the polymer precursors brought to investigate the fluorescence mechanism of CDots, benefiting from their tunable chain structure and molecular weight. For instance, Ren's group tried discovering the crucial factor controlling the emission intensity of CDots using two series of well-designed P- and S-codoped carbon dots (P/S-CDots) with variable particle sizes and individual chemical structures in the carbonaceous core and surface[51]. The chain structure and molecular weight of their precursor were prepared via free radical polymerization using acrylic phosphate, sodium methylallyl sulfonate, and acrylic acid as monomers. They found that the concentration of sp2 hybrid carbon atoms in the carbonaceous core played the most nontrivial role in affecting the PL intensity, followed by heteroatom's doping amount.

5 Hostguest carbon dots

5.1 Hostguest structure composed of carbon dots and dyes

Recently, dyes have been applied to modify CDots for various purposes. For example, Guldi's group used 11,11,12,12-tetracyano-9,10-anthra-p-quinodimethane (TCAQ) to modify CDots (TCAQ-CDots)[116], in which the TCAQ promotes strong electronic interactions in both the ground and electronically excited states. The TCAQ enables the charge separation in CDots, although the electron-accepting TCAQ causes the QY quenching from 18% to 1%. What's more, dyes are also effective in improving the performances of CDots involving fluorescence enhancement, especially in red/NIR emission.

As discussed in former sections, CDots, after deliberate surface functionalization, can attain a high fluorescence efficiency of more than 50% and are mostly fluorescent over the entire visible spectrum, depending on excitation wavelengths. Nevertheless, the observed emissions in blue/green are generally much stronger than those in red/NIR. CDots were designed and prepared through red/NIR dyes incorporated on the surface or encapsulated as guests in the core of carbon nanoparticles to solve this problem. The latter host-guest configuration, conceptually similar to endofullerenes, was first proposed by Sun's group[20]. More specifically, the cores in CDots were considered a “solid-state pool” (vs. the cavity in a fullerene) to trap or encapsulate chromophoric species of strong red/near-IR absorption and emissions (Fig.8). Three kinds of host-guest like CDots-dye (G@CDots) nano-particles were synthesized via microwave irradiation of a mixture of PEG and dyes, where PEG was converted to CDots as the host while cresyl violet, nile blue (NB), and zinc phthalocyanine as the guest, respectively. Three G@CDots emitted fluorescence at 750, 660, and 690 nm when excited at 570 nm, respectively. They all exhibited the excitation wavelength dependence because of their solid-like environment around that coming from hosting carbon nanoparticles.

Lately, in 2017, Sun's group further revealed the advanced nature of G@CDots by taking NB@CDots as an example and demonstrating their potential in bioimaging sensing[117]. The fluorescence QY of the guest NB in the NB@CDots is sufficiently bright, which is 18% ± 2% at 530 nm excitation, approximately 20 times that of free NB (approximately 1%) in aqueous solution. Also, the fluorescence lifetime is 2.2 ns, which is much longer than that of free NB molecules in water. Additionally, the NB@CDots show little toxicity and can be taken up by cells showing bright red fluorescence in the cell cytoplasm.

Because of the excellent red/NIR emission performances of the dyes, the idea of broadening the emission of CDots from blue to red and even NIR by dyes will be prevalent. This effort will broaden the reach of the already advancing research and rapidly promote the development of CDots.

5.2 Host-guest structure composed of carbon dots and porous materials

CDots usually serve as hosts, which seems like a “solid-state pool” for accommodating dyes that serve as guests; by contrast, PMs such as zeolites, MOFs, and porous carbons are usually applied as frameworks to modify CDots. Reasonably, the application of PMs can prevent CDots quenching in solid state because the CDots nanoparticles can be dispersed uniformly in the matrix of PMs instead of aggregating. The PMs have plenty of tight spaces, making it possible for efficient energy transfer and forming pathways from energy donors to acceptors since CDots can get close enough in the tight spaces.

Yu's group reported a “dot-in-zeolite” composite material[118] with a high QY up to 52.14% and an ultralong lifetime of 350 ms. The zeolitic matrices can effectively stabilize the triplet state by suppressing the nonradiative processes and hindering the oxygen quenching. Additionally, they reported and studied CDots@Zeolites composites with naked-eye visible multiplexing in another work[119]. Specifically, introducing C=O/C=N/N—H bonds or confined-domain environments to CDots is beneficial to strengthening the interactions between CDots and the zeolite framework, leading to a stable QY and a long lifetime.

MOFs are also extensively used as frameworks for CDots. Gao et al. prepared N, S-CDots and Eu-MOFs composites[120], which could detect water content in an organic solution by calculating the relative intensity of blue and red emissions. Wang et al. successfully synthesized CDots@ZIF-8 composites via facile calcination[121]. In stable ZIF-8 matrices, CDots could realize tunable fluorescence by adjusting calcination time. MOFs provide stable frameworks and porous structures for CDots and pave the way for advanced fluorescence-based applications.

Nonmetal oxide is also considered an ideal PM for the solid-state fluorescence of CDots. He et al. synthesized a core-satellite structure with CDots surrounding the quasispherical amorphous SiO2[122]. SiO2-CDots with this structure exhibit a QY of 35% and successfully avoid aggregation and gelation. Notably, amino groups on CDots as bridges have a high affinity with SiO2, enhancing both fluorescence and phosphorescence.

In conclusion, host-guest structures composed of CDots and PMs can achieve fluorescence in solid state. PMs are stable enough and provide matrices or frameworks to prevent CDots from aggregation quenching in solid state. Moreover, the host-guest structures could be designed flexibly, promising more advanced fluorescence-based applications.

6 Summary and outlook

In this review, we described ways for improving the fluorescence performances of CDots on the basis of different molecules such as heteroatom doping, semiconductor quantum dots doping, polymer coating, and dye incorporation. Donor heteroatoms (e.g., nitrogen, phosphorus, and sulfur) are widely used because of their roles in increasing the electron density of their neighboring carbon atoms, providing a new radiation recombination pathway, and improving emission intensity. As an acceptor, B atom does increase the fluorescence emission intensity, but the B-doped CDots usually show strong fluorescence in the blue region upon UV excitation. Moreover, multiple heteroatoms with significant synergistic effects enable CDots to possess fluorescence QY of more than 50%. Inorganic salts and polymers also contribute to the passivation effects. Moreover, the passivation effects can be enhanced in a more restrictive environment, such as a crosslinked polymer or film, enhancing fluorescence efficiency. Additionally, dye shows positive effects on the emission intensity and works well in terms of red/NIR emission. Red/NIR-CDots meet the challenge that the specific fluorescence mechanism of high-efficiency red/NIR-CDots remains unknown. Most synthesized CDots display strong blue or green emission, covering the weak red emission. In other words, the QY of most red/NIR-CDots is still low in an aqueous solution, and therefore, the large-scale preparation of such red/NIR-CDots remains restricted. Although several papers have reported red/NIR-CDots and their applications, the success of obtaining highly efficient red/NIR-CDots is sometimes by chance. In the future, researchers should pay more attention to material selectivity, the preparation process, and especially the fluorescence mechanism of red/NIR-CDots.

Later, we would like to share our perspectives on some critical issues regarding the PL mechanism and future hot topics of CDots. CDots have been attracting extensive attention since 2004, which are explored in many fields because of their promising advantages, such as facile preparation, high fluorescence QY, excellent biocompatibility, and wide-range emission. Fluorescence ranging over the entire visible spectra is crucial for successfully implementing CDots in practical applications, such as fluorescent probes, bioimaging, devices, sensors, and catalysts. There is no doubt that efficient red-emitting capability is the key. Although a few attempts have been reported for achieving red-emissive CDots using the bottom-up method and using nitrogen-containing composites such as those composed of citric acid and formamide, citric acid and urea, and phenylenediamine as precursors. It is still difficult to universally achieve high-efficient red-emissive CDots through structural design until the fluorescence mechanism of CDots is elucidated.

Although heteroatom doping has become a primary method to improve the fluorescence properties of CDots, the effects are dependent. The focus is still on the precursors containing different heteroatoms. The relationship between the “doping structure-electronic structure-fluorescence performance” of CDots is poorly understood because of insufficient results. Hence, it is a major challenge, and thus, it is very urgent to explore and understand the structure-property relationships in CDots, especially including the effect of structural and surface doping on optical and other properties of the CDots.

Exploring the intrinsic relationship between the doping and the optical properties of the CDots and the relationship between the electronic structures and the fluorescence properties of the CDots is critical. Among various methods, theoretical simulations and calculations are promising. The first principle has been successfully utilized to investigate the electronic structure of carbon nanomaterials and has shown its tendency to depict the structure of CDots, such as computing the absorption and fluorescence spectra of GDots using density theory methods and predicting the electronic structure and optical properties of amorphous carbon nanodots using density functional theory methods.

The intrinsic mechanism of the fluorescence of the CDots remains a topic of debate, which need more attention. Although numerous studies considering quantum confinement, surface trap emission, variable oxidation states, aggregation-induced emission, amorphous carbon clusters and even recently proposed molecular luminescence theory have been done, the fluorescence origins of these CDots remain elusive. Also, attention should be paid to the implicit reason for the bright fluorescence of CDots. Further research is needed to push the development of such CDots that can meet a specific demand through the specific heteroatom-doping or modification.

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基金资助

西南科技大学博士启动基金(16ZX7139)

龙山人才项目(18LZX675)

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