sylu2013@zzu.edu.cn (S. Lu). 1 These authors contributed equally to this work.
Received Date:
26 February 2025 Accepted Date:
18 July 2025 Revised Date:
14 July 2025 Available Online:
15 November 2025
Abstract:
Since the discovery of carbonized polymer dots (CPDs) two decades ago, this emerging family of carbon-based nanomaterials has rapidly risen to prominence. CPDs have found widespread applications in sensing, catalysis, energy, and biomedicine due to their flexible precursors and synthesis methods, tunable photoluminescence (PL) properties, and excellent biocompatibility. This report presents the advancements made in the realm of CPD precursors, elucidates their luminescence properties and underlying mechanisms, and explores the diverse applications of CPD-based materials. It comprehensively addresses key issues by delving into several interconnected chapters: Initially exploring the intriguing fluorescence and afterglow properties exhibited by CPDs, subsequently unraveling the complex luminescence mechanisms that underlie these phenomena, emphasizing the crucial aspect of controllable synthesis of CPDs, and ultimately culminating in the precise construction of composite materials tailored for applications in laser and electroluminescent devices. Furthermore, this report aims to provide communication and assistance for the controlled synthesis and expanded applications of CPDs.
Solution-processable nanofluorescent materials hold significant application value in fields such as light-emitting displays, bioimaging, and information anti-counterfeiting [1-3]. However, traditional materials, such as organic molecules, semiconductor quantum dots, and perovskite quantum dots, are plagued by issues of high toxicity, high cost, and complex preparation methods. Therefore, the development of novel, non-toxic, surface-modifiable nanofluorescent materials has become a research hotspot [4,5]. Carbonized polymer dots (CPDs), as a new type of nanofluorescent material, have garnered attention due to their low toxicity, good biocompatibility, and structural design flexibility. CPDs are prepared through cross-linking, condensation, and aromatization processes and exhibit tunable photoluminescence (PL) from UVB to near-infrared (NIR) [6-9]. These characteristics endow them with great potential in bioimaging [10-12], energy catalysis [13-15], and optoelectronic devices [16-18]. Although the luminescence mechanism of CPDs [19-21] has not been fully elucidated, studies indicate that their luminescence results from the combined action of conjugated units and surface functional groups. Given the abundance of surface functional groups and the complexity of their chemical structure, researchers are working to better understand the luminescence mechanism by studying the reaction processes, with the aim of characterizing the precise chemical structure of CPDs to reveal their luminescence mechanism. A design concept of "synergistic action of hybrid carbon-based core and surface micro/nanostructures" has been proposed, with the correlation between carbonization degree and luminescence mechanism being gradually uncovered. This has led to the development of precise control strategies for fluorescence and laser emission, significantly enhancing the performance of CPDs and their composite materials. These research achievements provide new insights into the controllable synthesis and application expansion of CPDs. However, clarifying the structure-property relationship to achieve controllable synthesis of CPDs remains a key scientific challenge.
2.
Preparation of CPDs
2.1
Synthesis methods of CPDs
Generally, CPDs are synthesized via two primary approaches: bottom-up and top-down methods. Bottom-up strategies involve molecular assembly from small organic precursors through solvothermal, hydrothermal, or microwave-assisted reactions, where doping agents can be incorporated to tailor optical properties. Top-down methods rely on the fragmentation of bulk carbon materials via physical or chemical techniques, such as laser ablation, electrochemical exfoliation, or acid oxidation, to yield nanoscale CDs. Both approaches enable post-synthetic modifications, including photoinduced doping, surface functionalization, or defect engineering, to optimize fluorescence efficiency and biocompatibility. Given that the synthesis methodologies of carbon dots have been comprehensively reviewed in prior literature [22-25], these details will be refrained from reiterating.
2.2
Precise construction and performance improvement
Strategies for the precise construction and performance enhancement of CPDs encompass optimizing reaction conditions, elemental doping, surface modification, and composite formation, enabling the tailored design of CPDs with specific structural and functional properties.
Surface modification significantly enhances the functionality of CPDs. Lai et al. prepared mannose-modified CPDs, which specifically bound to Escherichia coli with a detection limit of 100 CFU/mL in real samples [26]. These modifications highlight how tailored surface engineering expands the applications of CPDs in imaging, targeting, and sensing. Meanwhile, elemental doping significantly enhances the optical and functional properties of CPDs. Huang et al. demonstrated that nitrogen-phosphorus (N, P) co-doping in carbonized polymer dots (Arg-CPDs) enhanced spin-orbit coupling, minimized singlet-triplet energy gaps, and enabled room-temperature phosphorescence (RTP) with a 149 ms lifetime and visible afterglow, advancing encryption technologies (Figs. S1a and b in Supporting information) [27].
2.3
Controllable synthesis
Controllable synthesis of CPDs involves optimizing parameters like solvents, temperature, time, pH, and precursor ratios. Wang et al. demonstrated that polar solvents like water produce blue-emitting CPDs [28], while nonpolar solvents such as DMF induce nitrogen doping and extended conjugation, leading to red fluorescence and surface-adsorbed fluorophores, enabling precise fluorescence tuning for bioimaging. Moreover, CPDs-140 (140 ℃) and CPDs-220 (220 ℃) exhibited particle sizes of 5.1 and 6.1 nm, respectively, with emission controlled by surface states (477 nm) and carbon core states (441 nm). Higher temperatures promote surface state formation, but excessively high temperatures can destroy these states, shifting emission to carbon core dominance [29]. In 2024, He's group utilized the reactivity between tea polyphenols and o-phthalaldehyde to prepare CDs, achieving a red shift in fluorescence by adjusting pH, which altered precursor reaction pathways and produced fluorescent groups with varying conjugated structures and colors [30]. The choice of precursors also plays a critical role; multicolor CDs (MCDs) synthesized from l-glutamic acid and o-phenylenediamine (OPD) achieved tunable fluorescence from blue to NIR by varying precursor ratios (Figs. S1c-f in Supporting information) [31].
3.
Fluorescent CPDs
The classification of precursors and exploration of fluorescence mechanisms are critical for tuning the optical properties of CPDs. By selecting diverse precursors like organic molecules, biomass, or waste materials, the luminescent characteristics of CPDs can be precisely tailored, as their composition and structure directly influence fluorescence efficiency and wavelength. Understanding the luminescence mechanisms not only aids in optimizing CPD properties but also expands their applications in sensing, imaging, and catalysis. This part highlights key advancements in precursor selection and fluorescence mechanism research.
3.1
Organic molecules as precursors for CPDs
Organic molecular precursors, such as citric acid (CA), are widely used for CPD preparation due to their chemical diversity and structural flexibility. Both conjugated and non-conjugated molecules enable the tuning of CPDs' fluorescent properties through various reactions. For instance, CA reacts with amines under alkaline conditions, forming cross-linked polymer nanoparticles that aromatize into citrazinic acid, emitting blue fluorescence. These nanoparticles further carbonize into nanometer-sized CPDs, driven by the self-assembly of conjugated pyridine rings. This process highlights the role of organic precursors in controlling CPD properties. In 2013, Yang et al. used CA and ethylenediamine to form polymer-like CPDs, which were then carbonized to form high quantum yield (QY) CDs (Fig. 1c) [32]. Subsequently, Yu developed a general strategy to rapidly synthesize highly fluorescent CDs in large quantities by thermal treatment of various carbon precursors such as CA in the presence of monoethanolamine (MEA) and proposed the formation mechanisms, including polymerization, aromatization, nucleation, and growth (Fig. 1a) [33].
Figure 1
Figure 1.
(a) A proposed formation mechanism of N—C-dots prepared from citric acid in the MEA system. Copied with permission [33]. Copyright 2015, Royal Society of Chemistry Publishing Group. (b) Schematic of the synthesis processes of Na-CDs, ir-Na-CDs, and ir-Na-CDs@BSA. Copied with permission [35]. Copyright 2024, Wiley Publishing Group. (c) A synthetic route using citric acid and ethylenediamine: from ionization to condensation, polymerization, and carbonization. Copied with permission [32]. Copyright 2013, Wiley Publishing Group. (d) A synthetic route using citric acid and amine. Copied with permission [34]. Copyright 2016, American Chemical Society Publishing Group.
Qu's research group has been dedicated to adjusting the photophysical properties of CDs synthesized from CA and urea. Recently, Na-CDs with Na+1 functionalization was synthesized by introducing sodium carbonate into a mixture of CA and urea followed by solvent thermal treatment in formic acid, achieving unprecedentedly high red fluorescence and high photothermal conversion efficiency (PTCE) (Fig. 1d) [34]. Rogach's group formed high QY CDs using CA and three different nitrogen sources to investigate the effect of molecular precursors on their optical properties (Fig. 1b) [35].
On the other hand, starting from conjugated molecules, hydrothermal reactions lead to the generation of CPDs. Aromatic molecules couple to form larger conjugated molecules, and the increase in conjugation length narrows the HOMO-LUMO gap, leading to a red shift in luminescence [36-39]. Wang et al. reported the successful preparation of bright orange fluorescent carbon quantum dots (O—CQDs) using 1-amino-2-naphthol hydrochloride and CA by controlling different reaction conditions (Fig. 2a) [40]. Wang and Song's research groups utilized 5-amino-1,10-phenanthroline and salicylic acid as precursors to synthesize a novel type of CDs through solvothermal treatment, enabling simultaneous detection of Fe2+ and Fe3+ (Fig. 2b) [41]. In 2023, Song's group chose nitrogen-containing aromatic precursors and suitable linking molecules to achieve specific binding with Cd2+, producing luminescent CDs for a turn-on fluorescence probe for Cd2+ sensing and imaging (Fig. 2c) [42]. Song and Wang's group synthesized three types of CDs through the solvothermal reaction of OPD with three isomers of diphenol (Fig. 2d) [43]. They proposed a steric hindrance strategy to effectively adjust the formation and properties of the intramolecular hydrogen bonds in the CDs by varying the positions of the substituents on the precursor benzene ring.
Figure 2
Figure 2.
(a) Schematic of the Preparation of O—CQDs. Copied with permission [40]. Copyright 2022, American Chemical Society Publishing Group. (b) Synthesis process of T-CDs and C—CDs. Copied with permission [41]. Copyright 2023, Springer Nature Publishing Group. (c) The formation of OR-CDs. Copied with permission [42]. Copyright 2022, Elsevier Publishing Group. (d) Schematic of the synthesis process. Copied with permission [43]. Copyright 2023, Elsevier Publishing Group.
Exploring the utilization use of organic molecules precursors to achieve full-color emission from CDs is also worth investigating. Research indicates that the multicolor behavior of CDs is influenced by multiple factors, including the size of the CDs conjugated domains, the type or quantity of surface functional groups, electron transfer, fluorescence resonance energy transfer (FRET), and excitation dependence. In 2015, Lin reported the generation of three types of CPDs that emitted bright and stable red, green, and blue (RGB) luminescence under a single ultraviolet excitation (Fig. S2a in Supporting information) [44]. Xiong's group introduced a simple solvothermal method for synthesizing multicolored CDs using p-phenylenediamine (PPD) and urea as precursors [45]. The prepared red-emitting CDs exhibited a QY of approximately 24%, and the fully purified CDs displayed non-excitation-dependent luminescence ranging from blue to red, characterized by a single excitation peak and a single exponential lifetime (Fig. S2b in Supporting information). Qu et al. synthesized full-color luminescent CDs using CA and urea as common precursors through solvothermal synthesis in three different solvents (water, glycerol, and dimethylformamide) and their combinations (Fig. S2c in Supporting information) [46]. In 2021, we used CA and OPD to prepare full-spectrum emitting CDs by adjusting pH and hydrothermal temperature, guided by theoretical calculations. The study revealed that CD fluorescence depends on size and sp3/sp2 hybridization ratio: larger sizes and higher sp2 content cause red shifts. The resulting CDs covered the visible spectrum (413–635 nm) and were used to fabricate a white LED with CIE coordinates (0.33, 0.36), a color temperature of 5452 K, and a CRI of 88. This work advances research on full-spectrum CDs and their fluorescence mechanisms [47].
The construction of solid-state fluorescent (SSF) CPDs has progressed from dispersing CPDs in matrices to direct synthesis with SSF properties, culminating in AIE CPDs that counteract the ACQ effect. In 2017, Yang et al. achieved highly efficient full-color SSFCDs using a one-pot microwave-assisted method [48]. The polymeric network formed by crosslinking prevents the emission centers of SSFCDs from clustering, while non-conjugated sub-fluorophores help avoid π-π interaction quenching, marking a significant advancement in the SSF field (Fig. S3a in Supporting information). Lin's group used CA as a carbon source and six alkyl amines as carbonization solvents, employing a one-step solvothermal method to prepare multicolor SSFCDs [49]. The type of alkyl chain in the amine solvents significantly affected the interactions between solid-state CDs, thereby influencing their fluorescence wavelength and quantum efficiency (Fig. S3b in Supporting information). Liu reacted ortho-, meta-, and para-phenylenediamines with dithiosalicylic acid using acetic acid as the solvent, obtaining AIE CPDs in red (620 nm), green (520 nm), and blue (478 nm) (Fig. S3c in Supporting information) [50]. The AIE effect has been successfully applied to semiconductor nanoparticles and metal clusters, opening up new possibilities for the precise preparation of CD-based materials with multicolor solid-state fluorescence. A novel strategy to functionalize CDs using salicylaldehyde (SA)-type surface ligands with precise structures, achieved precise control over continuous full-color solid-state emission from blue to deep red light (covering a wavelength range of nearly 300 nm) under UV excitation (Fig. S3d in Supporting information) [51]. Mechanism studies revealed the conjugated hybridization process between aromatic ligands and the carbon core, forming new luminescent energy levels and playing a crucial role in regulating the emission bandgap. Furthermore, the developed full-color solid-state luminescent structures are not only universally applicable but also expand the synthetic strategies for solid-state luminescent CDs.
Red and NIR-emitting CPDs are gaining attention for their biocompatibility, strong tissue penetration, and high spatial resolution, making them ideal for biomedical imaging and diagnostics. For example, R-CDs synthesized with CA precursors and NH3·H2O exhibited a red shift from 423 nm to 667 nm due to N-doping and intermolecular amide bonding [52]. Similarly, pH-tunable CDs from phloroglucinol showed reversible green-to-deep-red emission, while pyrogallol-derived CDs achieved deep-red laser emission (672 nm) through protonation and aggregation [53]. Additionally, OPD-derived CDs enabled reversible NIR dual-wavelength laser emission, and PEI-modified CDs demonstrated potential in NIR angiography and bioimaging [54,55]. These examples highlight how structural and pH modulation can tailor CPDs for advanced biomedical applications. Qu's research group developed a material exhibiting efficient red fluorescence and a remarkable PTCE of 43% in the NIR-Ⅱ window. Complexation with BSA further enhanced the PLQY to 31% while retaining high PTCE. With low cytotoxicity and strong photothermal performance, this material holds great promise for NIR-Ⅱ photothermal tumor therapy (Fig. S4 in Supporting information) [35].
Chiral self-assembled CPDs have emerged as a promising class of nanomaterials due to their unique optical and structural properties. By leveraging chiral precursors or surface modifications, CDs can exhibit circularly polarized luminescence (CPL) and enantioselective behavior, making them valuable for applications in chiral sensing, catalysis, and optoelectronics. The self-assembly process is driven by non-covalent interactions, such as hydrogen bonding, π-π stacking, and chiral induction, which enable the formation of ordered nanostructures with enhanced chiroptical activity. Recent studies have demonstrated that controlling the size, surface chemistry, and assembly conditions of CDs can fine-tune their chiral properties. These advances highlight the potential of chiral self-assembled CDs in developing next-generation functional materials with tailored optical and molecular recognition capabilities, offering exciting opportunities for interdisciplinary research and technological innovation.
3.2
Biomass as precursors for CPDs
Biomass-derived CPDs, with higher biocompatibility and lower toxicity compared to chemical CPDs, are ideal for PL and bioimaging due to their large surface area, enhanced cellular uptake, and drug conjugation potential [56-58]. Rich in polysaccharides, proteins, and phospholipids, biomass materials like roots, flowers, leaves, fruits, seeds, and peels can be used to create CPDs with diverse medicinal and luminescent properties, making them excellent for drug delivery and bioapplications.
In 2015, Gopinath reported a simple one-step hydrothermal method to prepare CDs from coriander leaves. The obtained CDs were used as fluorescent probes for the sensitive and selective detection of Fe3+ ions (Fig. 3a) [59]. In 2020, Fan and others utilized common, inexpensive biomass waste such as citrus peels, ginkgo leaves, paulownia leaves, and magnolia flowers as carbon sources, employing an easy hydrothermal method to produce biomass CDs (Fig. 3b) [60]. When Fe3+ ions were added to the CDs solution, a fluorescence quenching effect was observed, leading to satisfactory results in detecting Fe3+ ions. Additionally, Shuang and others prepared bright blue fluorescent nitrogen-doped CDs (N—CDs) using astragalus as a carbon source through hydrothermal methods, without further processing (Fig. 3c). These N—CDs also have potential applications in detecting actual samples and intracellular imaging [61].
Figure 3
Figure 3.
(a) Schematic illustration depicting one-step synthesis of CDs from coriander leaves. Copied with permission [59]. Copyright 2015, Royal Society of Chemistry Publishing Group. (b) Schematic illustration of the synthesis of biomass CQDs from biomass waste by hydrothermal treatment and the mechanism of Fe3+ detection with as-prepared CQDs. Copied with permission [60]. Copyright 2019, Elsevier Publishing Group. (c) Schematic illustrating the synthesis methods of N—CDs. Copied with permission [61]. Copyright 2020, Royal Society of Chemistry Publishing Group. (d) Synthesis of CDs with avocado. Copied with permission [62]. Copyright 2021, Elsevier Publishing Group. (e) Schematic of the fabrication of J-CDs. Copied with permission [63]. Copyright 2022, Wiley Publishing Group.
In 2021, we synthesized two types of CPDs from avocado peel (CPDs-P) and pulp (CPDs-S) via a simple hydrothermal method (Fig. 3d) [62]. CPDs-P, with more oxygen-containing groups, showed superior Fe3+ detection performance. In traditional Chinese medicine, jujube is considered a good blood-tonifying agent. Based on this, we successfully reported the extraction of unique bio-CDs (J-CDs) from jujube using a hydrothermal method [63]. TEM data indicated that J-CDs had a uniform size and good dispersibility with an average particle size of 2.5 nm and exhibited good crystallinity (Fig. 3e). This small size allowed J-CDs to easily penetrate certain biological barriers. The synthesized J-CDs were composed of sp2/sp3 carbon atoms and oxygen/nitrogen groups, demonstrating good biocompatibility and membrane permeability. Considering their specific effects on erythropoiesis, the synthesized J-CDs may be a promising therapeutic agent for treating anemia, including cancer-related anemia.
The choice of biomass precursor plays a critical role in determining the morphology, surface chemistry, and optical properties of CPDs. For instance, nitrogen-rich precursors can enhance quantum yield and emission tunability, making CPDs suitable for bioimaging, while oxygen-rich precursors improve hydrophilicity and biocompatibility for drug delivery applications. Sulfur-containing precursors, on the other hand, can introduce dopants that boost catalytic or electrochemical performance for energy-related uses. Systematic trends in biomass precursor selection, combined with optimized carbonization conditions, provide valuable guidance for tailoring CPDs to specific applications.
3.3
Other precursors for CPDs
The environmental impact of plastic waste has spurred research into alternative precursors for carbon dots (CDs). Using waste plastic and nitric acid, we synthesized solid-state fluorescent CDs with tunable photoluminescence (PL), where increased oxidation led to emission red shifts. Similarly, ethanol-derived CDs, carbonized by sulfuric acid, were separated into blue, cyan, and yellow-emitting components, enabling multicolor LED fabrication (Fig. S7 in Supporting information). These studies highlight the potential of diverse precursors for tailoring CD properties, though further research is needed to clarify PL mechanisms and structural formation.
3.4
Large-scale preparation of CPDs
Currently, although researchers have achieved scaled-up production of CPDs using methods such as hydrothermal, microwave, and magnetic heating, these methods still face limitations such as long reaction times and low yield, making it difficult to produce CPDs at the kilogram scale under ambient temperature and pressure. Recent advances include Hou's efficient aldol condensation method for large-scale CD synthesis, Peng's microwave-hydrothermal approach for NIR CDs, and Ding's microwave-assisted pyrolysis for AIE-CDs. Additionally, we achieved room-temperature, ambient-pressure synthesis of multicolor CDs within 30 min, with fluorescence attributed to core structures and nitrogen content (Fig. S8 in Supporting information). These methods enable kilogram-scale production, paving the way for industrial applications such as high-performance LEDs.
4.
Afterglow CPDs
While fluorescent CDs are primarily classified by their precursor materials, which dictate their optical properties, long-afterglow CDs are categorized based on their luminescence mechanisms, emphasizing the diverse afterglow phenomena arising from variations in excited-state processes such as triplet-state transitions and energy transfer dynamics. By embedding CPDs into specific matrices or optimizing the selection of carbonization precursors and reaction conditions, CPDs with special structures can be prepared to achieve afterglow emission. Afterglow luminescent materials are those that produce light through radiative transitions of triplet excitons.
4.1
RTP based CPDs
Room temperature phosphorescence (RTP) has garnered significant attention due to their unique advantages in anti-counterfeiting, illumination, biology, and other fields, with continuous breakthroughs in the preparation of related materials. However, current RTP materials still struggle to overcome limitations such as complex preparation processes, high reagent costs, and toxicity. As emerging luminescent nanomaterials, CPDs possess excellent optical properties, small size, and low toxicity, which can effectively compensate for the shortcomings of traditional RTP materials. Currently, most RTP-based CPDs are composites formed through encapsulation strategies. CPDs without RTP properties are difficult to significantly enhance their lifetime and QY after encapsulation. Therefore, it is necessary to identify suitable precursors that allow for the synthesis of CPDs with good RTP characteristics, which can then be further enhanced through post-modification strategies.
In 2019, Lin reported the first preparation of CDs with dual emissions and aggregation-induced RTP characteristics (Fig. S9a in Supporting information) [64]. Through the screening of carbon precursors and preparation conditions, they used trimesic acid (TA) as the precursor and prepared the CDs via a one-pot hydrothermal method. Under 365 nm UV excitation, the TA-CDs exhibited unique white prompt and yellow RTP emissions, which could leverage the advantages of CDs while avoiding the drawbacks of traditional AIE and metal-free RTP materials. Yu and coworkers developed a CDs-in-zeolite strategy for preparing novel CD-based composites (Fig. S9b in Supporting information) [65]. They embedded CDs prepared from pre-designed organic precursors into an open-framework material with hexacoordinated manganese, resulting in an efficient red RTP CDs@MnAPO—CJ50 composite that exhibited strong red phosphorescence. In 2022, Bai and colleagues achieved the preparation of highly efficient blue RTP CDs through a molecular engineering strategy, with a blue light efficiency of 50.17% and a lifetime of 2.03 s (Fig. S9c in Supporting information) [66]. Experimental data indicated that the modification of surface functional groups helped the CDs form new (n, π*) configurations, thereby enhancing the singlet oxygen content (SOC), reducing the energy gap, and promoting the generation of intersystem crossing (ISC) processes. This work provided insights for future research on high-efficiency RTP CDs. Lin reported a simple method for preparing multicolor RTP CDs [67]. The synthesized CDs exhibited a fluorescence color change from blue to green in the solid state, with a significant RTP color change from cyan to yellow upon radiation exposure (Fig. S9d in Supporting information). This research not only presents a straightforward strategy for preparing multicolor RTP materials but also reveals the great potential of CDs in developing novel optical materials with unique properties.
In 2022, our team, collaborating with Yang's group, analyzed the cross-linking polymerization and carbonization process in CD formation using ethylenediaminetetraacetic acid tetrasodium salt (EDTA·4Na) as a precursor (Fig. S9e in Supporting information) [68]. The entangled polymer chains inhibited non-radiative transitions, endowing the CDs with RTP properties. Secondary cross-linking and carbonization with various polymers revealed that cross-linking generated abundant energy levels, promoting charge transfer between the polymer shell and CDs. The degree and strength of cross-linking controlled RTP performance, with tighter cross-linking enhancing phosphorescence emission. In 2023, our group made a groundbreaking achievement by utilizing the in-situ hydrolysis of tetraethyl orthosilicate (TEOS) to form covalent bonds with multicolor CDs for the preparation of phosphorescent materials (Fig. S9f in Supporting information) [69]. This method allows for the preparation of long-lived, multicolor phosphorescent CD composites without the need for heavy metal doping. Ultimately, blue, green, yellow, and red phosphorescent composites with emission wavelengths of 465, 500, 580, and 680 nm (below 177 K), respectively, are obtained. By fitting the measurement data, a phosphorescence lifetime of ~2.11 s and a QY of 36.68% are achieved. The tight encapsulation of CDs even isolates water and oxygen from the air, enhancing the stability of the RTP system and ensuring aqueous-phase phosphorescence. Based on their excellent phosphorescent properties, we have successfully applied them in information encryption and storage. The findings of this study will provide valuable reference for the development of long-lived, high QY multicolor RTP materials and contribute to advancing the practical applications of CD phosphorescent materials.
4.2
TADF based CPDs
Although numerous studies have reported RTP CPDs, their afterglow performance is far from meeting practical application requirements, especially with a lack of examples of NIR emission. Therefore, further enhancing the afterglow performance of CPDs and developing NIR afterglow is of great significance. Materials with both thermally activated delayed fluorescence (TADF) and RTP dual-mode afterglow can change their emission color through simple temperature adjustment, achieving a higher level of anti-counterfeiting. However, most current reports on CPDs afterglow focus on either RTP or TADF alone. Since TADF and RTP are two competing processes, it is quite challenging for a material to simultaneously exhibit both TADF and RTP. Developing a single-system material with multicolor RTP and TADF, allowing for adjustable emission colors and channels, is a challenging task.
Hu and his colleagues explored the preparation of a series of CD@Al2O3 materials by one-step calcination with varying reaction temperatures, demonstrating an ultra-broad room-temperature afterglow emission range of 376–619 nm (Fig. 4a). High-temperature treatment led to a reduction in surface functional groups and conjugation size of the CDs, causing the emission to blue-shift from red to ultraviolet [70]. The study also found that as the temperature increased, the room-temperature afterglow (RTA) emission mode transitioned from RTP to TADF, attributed to the enhanced crystallinity of the Al2O3 matrix, which stabilized the excited triplet state. Additionally, the continuous emission around 471 nm was linked to oxygen defects in the matrix. Shen's group developed a one-step solvothermal method to create temperature-responsive afterglow composites ( CB-Ⅰ) for optical anti-counterfeiting and sensing (Fig. 4b) [71]. Synthesized from ciprofloxacin and boric acid, CB-Ⅰ exhibited dual-mode afterglow (DF and RTP) with lifetimes of 180.5 and 193.5 ms at 438 and 490 nm, respectively, under UV light, along with blue fluorescence and cyan afterglow. At 80 K, it achieved an ultra-long phosphorescence lifetime of 1.11 s. Based on temperature adjustment, the composite demonstrated the ability to switch between single-mode and dual-mode afterglow, showcasing tunable afterglow colors.
Figure 4
Figure 4.
(a) Schematic representation of procedures for the preparation of CDs@Al2O3; images of the CDs@Al2O3 powders before and after turning off the 365 nm UV lamp irradiation and normalized afterglow spectra of CDs@Al2O3 under ambient conditions. Copied with permission [70]. Copyright 2024, Elsevier Publishing Group. (b) Corresponding photographs of CB-Ⅰ, CB-Ⅱ, CB-Ⅲ, CB-Ⅳ, CB-Ⅴ, CB-Ⅵ, and CB-Ⅶ on a piece of glass, respectively; digital photographs of information anticounterfeiting and encryption application of CB-Ⅰ and CB-Ⅳ powder under 365 nm UV light and data encryption application of CB-Ⅰ, CB-Ⅴ, and CB-Ⅶ powder. Copied with permission [71]. Copyright 2023, Elsevier Publishing Group. (c) Application of CDs@urea composites. Copied with permission [72]. Copyright 2024, Wiley Publishing Group. (d) Afterglow photos of B@B2O3, G@B2O3, R@B2O3 and NIR@B2O3 excited by 254 and 365 nm. Copied with permission [73]. Copyright 2024, Wiley Publishing Group.
Recently, we synthesized four CDs through structural regulation and achieved full-spectrum RTP with high brightness by encapsulating them with urea (Fig. 4c) [72]. The materials showed a maximum lifetime of 2141 ms and QY of 56.55%. CDs-Ⅳ@urea exhibited dual afterglow with red RTP at 622 nm and green TADF. Precursor conjugation and functional groups influenced nitrogen encapsulation, stabilizing triplet states and affecting the S1-T1 energy gap (ΔEST), enabling multicolor RTP. Strong encapsulation reduced ΔEST, promoting RISC and generating phosphorescence and TADF. This interaction stabilized the triplet state, making the material resistant to water, acids, and oxidizers. These afterglow materials show promise in multidimensional encryption and in vivo bioimaging. Collaborated with Yin team, we prepared blue fluorescent CDs (B-CDs), green fluorescent CDs (G-CDs), and red fluorescent CDs (R-CDs) using rhodamine 6G (Rh6G) as the raw material through a hydrothermal method [73]. Subsequently, BA was added to dehydrate and encapsulate the trichromatic CDs at high temperature. During this process, the multicolor CDs reacted with BA to form composite materials with blue, yellow-green, and red afterglow. The NIR afterglow emission originated from energy transfer induced by the encapsulation of R-CDs aggregates by B2O3. The enhancement of afterglow emission was attributed to the restriction of C-B covalent bonds and the encapsulation of CDs by B2O3, resulting in a QY of up to 37.67% for yellow-green afterglow. Ultimately, G@B2O3 was successfully applied to alternating current light-emitting diodes (AC-LEDs). Benefiting from the high QY of the afterglow composite material, the flickering characteristic of the AC-LED was nearly imperceptible to the naked eye. This research expands the potential applications of afterglow materials in optoelectronic devices (Fig. 4d).
5.
Luminescence mechanism
Currently, various precursors have been developed to prepare high-performance CDs, such as biomass, polymers, and organic molecules. However, the structure and PL mechanism of CDs remain controversial due to the difficulty in determining their composition as a result of numerous byproducts. The emission properties of CPDs remain a topic of debate, with several proposed mechanisms, including the quantum confinement effect, surface state emission, molecular state emission, and crosslink-enhanced emission (CEE). These mechanisms are not mutually exclusive and can coexist, often in complex and interconnected ways. For example, the quantum confinement effect may dominate in small-sized CPDs, while surface states or molecular fluorophores could play a more significant role in larger particles or those with abundant functional groups. Additionally, crosslink-enhanced emission may amplify luminescence efficiency regardless of the primary emission source. To elucidate these mechanisms, a combination of experimental approaches (e.g., UV–vis absorption, photoluminescence, time-resolved PL spectroscopy, TEM, XRD, XPS) and theoretical calculations (e.g., DFT, TD-DFT) can be employed to provide comprehensive insights into the energy levels, structural properties, and excited-state dynamics of CPDs. These methods, along with controlled experiments, help identify the dominant factors influencing luminescence and tailor CPDs for specific applications.
5.1
Fluorescence mechanisms
Various precursors, including biomass, polymers, and organic molecules, are used to prepare high-performance CDs. However, their structure and PL mechanisms remain debated due to complex byproducts and multiple proposed theories: quantum confinement, surface state emission, molecular state emission, and CEE. These mechanisms often coexist, with quantum confinement dominating in small CDs, while surface states or molecular fluorophores play larger roles in functionalized or larger particles. CEE can enhance luminescence regardless of the primary source. To clarify these mechanisms, experimental methods (e.g., UV–vis, PL, TEM, XRD, XPS) and theoretical calculations (e.g., DFT, TD-DFT) are combined to analyze energy levels, structures, and excited-state dynamics, enabling tailored CPDs for specific applications.
Zhou's group synthesized highly efficient G-CDs and R-CDs with a QY of up to 80% using perylene as a precursor [74]. The results indicated that the narrow bandgap of the CDs is influenced by their surface electronic states, which facilitates efficient red light emission. Chen prepared three types of CDs with different PL using microwave heating, resulting in blue, yellow, and orange emissions [75]. The increase in non-amino nitrogen can enhance fluorescence, while the rise of C=O/−CONH− groups can cause redshift in emission. The fluorescence color and intensity of the CDs are controlled by their surface states, primarily involving nitrogen-containing groups and the degree of oxidation (Fig. S11 in Supporting information) [28,76]. Soni et al. synthesized CDs using a hydrothermal method with OPD/HCl as the starting materials (Fig. 5a) [77]. They attributed the blue and green emissions to the aggregation of CDs and QXPDA. Sun successfully prepared multicolor-emitting CDs from a single molecule (OPD) [78]. They adjusted the building blocks by tuning the ratio of N, N-dimethylformamide and ethanol in the solvent. The results confirmed that the fluorescence of OPD-based CDs originated from surface molecular fluorophores, and the PL mechanism of OPD-based CDs belonged to molecular-state PL (Fig. 5c). In previous work, R-CDs were prepared from CA and its analogs with ammonia, revealing structural evolution and optical mechanisms (Fig. 5b) [52]. We analyzed OPD-based red-emitting CDs at the single-particle level, exploring their crosslinking and carbonization processes. Addressing the molecular state luminescence debate, we pioneered a single-particle testing system for CPDs, enabling precise measurement of their molecular weight and optical properties (Fig. 5d) [79]. Combined with ultra-low temperature single-particle Raman spectroscopy, we directly demonstrated that CPDs possess both the characteristics of quantum dots and polymers. Furthermore, we have achieved reversible switchable dual-wavelength laser emission in the NIR region through CDs derived from OPD with fine-tuning capabilities modulated by concentration or pH [54]. Furthermore, a model for CDs was established by comparing the laser characteristics of their fluorophore, 5,14-dihydroquinoline[2,3-b]acridine (DHQP). DHQP confined on the surface of CDs provided aggregation sites, and the conjugated carbon core facilitated multiple scattering, enhancing the light amplification process. The CDs exhibited an overall cross-linked confinement-enhanced emission effect, with higher PL quantum yield (PLQY), more stable laser emission, and a lower laser threshold. Therefore, it was verified that the luminescence of CDs originated from the particles themselves rather than small oligomeric molecules generated during the preparation process (Fig. 5e). This research not only provides valuable insights into the luminescence mechanism of CDs but also offers a promising direction for the rational design of tunable multi-wavelength laser gain materials.
Figure 5
Figure 5.
(a) Schematic diagram of the real experimental absorption and emission of light by the red, green and blue emissive components. Copied with permission [77]. Copyright 2021, Royal Society of Chemistry Publishing Group. (b) Schematic diagram of the formation of CDs through pyrolysis. Copied with permission [52]. Copyright 2022, Elsevier Publishing Group. (c) A schematic of three types of CDs reaction process at different solvent ratios of DMF and ethanol with OPD as the precursor. The dashed line indicates the possible reaction route without spectral evidence. The solid lines indicate the reaction routes. Copied with permission [78]. Copyright 2024, Wiley Publishing Group. (d) Formation energies of longitudinal and transverse growth of CDs from OPD. Core-shell structures formed by four kinds of CDs. Copied with permission [79]. Copyright 2022, Springer Nature Publishing Group. (e) DFT calculations and calculated PL spectra of DHQP with H aggregation. Copied with permission [54]. Copyright 2024, Elsevier Publishing Group.
The mechanism of afterglow luminescence mainly involves the formation and transformation of excited states. Under stimulation, electrons in the material are excited to high energy states, followed by a series of energy transfer and relaxation recesses. Under appropriate conditions, some excited state electrons can transfer energy through the triplet excited state (T1), resulting in DF, which allows the luminescence to persist after the excitation source is removed. Additionally, the formation of hydrogen bond networks in aqueous dispersions enhances the efficiency of RTP emission. By adjusting the composition and environment of the materials, such as the stability of Si-C covalent bonds and the competitive relationship of hydrogen bonds, it is possible to achieve color-variable and reversible luminescent properties.
Triplet excitons in organic materials are often quenched by dissolved oxygen in physiological environments, limiting their photostability. Lou and colleagues developed water-soluble NIR afterglow CDs with ultra-long lifetimes via a photo-oxidation method [80]. These CDs emitted afterglow at 670 and 720 nm, with lifetimes extending up to 5.9 h, significantly outperforming existing materials. The photo-oxidation process prevents oxygen quenching and triggers surface oxidation reactions, breaking and reconstructing bonds such as C=C. These releases stored chemical energy, enabling the CDs to function as "energy-storage batteries" and sustain prolonged afterglow emission (Fig. S12d in Supporting information). Hu and colleagues developed the first visible-light-excited material exhibiting TADF in aqueous solutions by encapsulating fluorine and nitrogen co-doped CDs (F, N—CDs) within silica nanoparticles (SiO2) [81]. This structure shields triplet excitons from quenching by water and oxygen, enabling long-lived TADF. Fluorine doping is crucial, as it induces a red shift and reduces the singlet-triplet energy gap via strong electron-withdrawing effects and the heavy atom effect (Fig. S12c in Supporting information). Additionally, Si-C covalent bonds stabilize triplet excited states, further promoting delayed fluorescence emission. The hydrogen bond network in aqueous dispersions facilitates RTP emission. Li and colleagues developed a stimulus-responsive CD nanomaterial (TPA-CDs/Si) with color-variable and reversible switching characteristics [82]. Although water typically quenches afterglow, TPA-CDs/Si transitions from blue to green upon water addition, showcasing its unique water-stimulated response. The interaction of Si-C covalent bonds stabilizes triplet excited states, promoting DF emission. Meanwhile, non-crystalline bound water forms a strong hydrogen bond network with carboxyl or hydroxyl groups, sustaining RTP emission in aqueous dispersions (Fig. S12e in Supporting information). Due to spin-forbidden transitions, interconversion between singlet and triplet states is rare, making RTP (T1→S0) and TADF (T1→S1→S0) challenging to achieve. Yang increased the content of functional groups (such as C=O and C=N) that facilitate intersystem crossing (ISC) and reverse intersystem crossing (RISC) processes through co-doping with copper and nitrogen, thereby forming dual luminescent centers that allow for subsequent modulation of the luminescent modes in composite materials (Fig. S12a in Supporting information) [83]. After synthesizing Cu, N—CDs@BA via a matrix-assisted method, the electronic defects in the matrix matched the energy levels of different luminescent centers in Cu, N—CDs, enabling Cu, N—CDs@BA to exhibit excitation-dependent characteristics that allow for the conversion between RTP and TADF. This ultimately facilitates advanced information encryption and anti-counterfeiting functions.
Liu's group successfully achieved RTP and TADF from CDs-modified amorphous silica using a one-pot sol-gel method, without the need for heavy atom dopants or removal of dissolved oxygen [84]. The key to this phenomenon lies in the transfer effect of electronic traps, which effectively capture and transport electrons between S1 and T1, increasing the rates of ISC and RISC, thereby stabilizing the triplet excited states and producing long-lived emission (Fig. S12b in Supporting information). Additionally, the covalent bonding between CDs and inorganic SiO2, along with the multiple effects of hydrogen bonding networks and the influence of inorganic defects on energy levels, collectively enhance the RTP and TADF characteristics, providing a novel explanation that combines inorganic defects with the triplet state mechanism of CDs. Nitrogen (N), phosphorus (P), sulfur (S), and other heteroatoms in biomass raw materials can achieve self-doping during the formation of CDs, leading to an increase in n-π* transitions and the formation of tunable surface electronic structures. In alkaline environments, green fluorescent and long-wavelength chitosan-based CQDs can be synthesized without organic solvents or halogens. The luminescent properties, linked to structure, focus on DF/RTP in GQDs embedded in PVA and BA, and their electrochemical performance with graphene oxide [85]. CDs' photoluminescence stems from sp2 aromatic domains and O/N functional groups, with short-wavelength emission from the carbon core and long-wavelength from surface defects (Fig. S12f in Supporting information).
The emission properties of CPDs remain a topic of debate, with several proposed mechanisms, including the quantum confinement effect, surface state emission, molecular state emission, and CEE. These mechanisms are not mutually exclusive and can coexist, often in complex and interconnected ways. For example, the quantum confinement effect may dominate in small-sized CPDs, while surface states or molecular fluorophores could play a more significant role in larger particles or those with abundant functional groups.
6.
CPDs lasers
CPDs, a promising laser material, are non-toxic, low-cost, and highly stable, driving advancements in miniaturized lasers. Research spans random lasers, whispering-gallery-mode lasers, plasmon resonance lasers, Fabry-Pérot lasers, and DBR microcavity single-mode lasers. These miniaturized solid-state lasers are vital for communication, micro/nano-processing, medicine, and biosensing, but high cost, toxicity, and instability have limited their progress. Thus, affordable, non-toxic, and stable laser gain materials are urgently needed. CPDs, zero-dimensional carbon nanoparticles under 10 nm, exhibit excellent light scattering and gain properties, enabling random laser emission through multiple scattering in mirrorless systems. (Fig. 6b) [86]. In 2012, Yu synthesized CDs in polyethylene glycol 200 (PEG200) and achieved enhanced PL peak intensity and wavelength tuning through the esterification of carboxyl groups on the CD surface [87]. By coating the modified CDs onto the surface of an optical fiber, they realized visible light region laser emission from a high-Q cylindrical microcavity under 266 nm pulsed light excitation (Fig. 6a). By further adjusting the content of various functional groups in CDs, the density of excited state energy levels can be tuned, thereby controlling the particle number inversion and reducing the laser threshold. In 2022, we prepared high-fluorescence quantum efficiency red-emitting CDs using commonly used non-toxic aliphatic small molecules as the carbon source, through graphitic nitrogen doping and surface modification/coating (Fig. 6d) [88]. We achieved low-threshold amplified spontaneous emission and solid-state single longitudinal mode red laser emission from CDs. Increasing graphitic nitrogen doping shifted emission from blue to red, confirmed by DFT simulations. PEG400 coating enhanced quantum efficiency to 65.5%. Laser pumping revealed low-threshold (8 µJ/cm2) emission and long gain lifetime (700 ps), enabled by a four-level system. A planar waveguide microcavity achieved high-stability single mode red laser emission, advancing miniaturized laser applications. In addition, Zhang developed silane CD/crystal hybrid materials using a one-pot solvothermal method with vinyltriethoxysilane (KH151), 1,3,5-benzenetricarboxylic acid (H3BTC), and ethanol (EtOH) as precursors (Fig. 6c) [89]. They achieved, for the first time, solid-state random laser emission in the near-UV to visible light region (315–600 nm) in CD-based materials. Meanwhile, under UV light irradiation, both the silane CD/crystal hybrid materials and the silane CDs exhibited cool white light emission. This new method of in-situ hybridization to prepare CD-transparent crystal materials offers advantages such as ease of preparation, broadband emission, tunability, and practical optical functions, making it a promising universal technology for designing novel optical hybrid materials and devices. Using CA and l-cysteine (l-Cys) as precursors, we prepared blue-emitting CDs (B-CDs) via microwave synthesis [90]. B-CDs' structure features carbon nanoparticles surface-modified with TPCA. Calculations showed radiative KR and σem positively correlate with laser threshold, peaking at B-CD2 (Fig. 6e). Optical pumping revealed laser thresholds decreased from 341.6 mJ/cm2 (B-CD1) to 165.5 mJ/cm2 (B-CD2), attributed to sp3 excited-state concentration and increased KR, enabling population inversion and light amplification. This study marks the first reduction in laser thresholds within a CD system, advancing micro-nano laser applications [54]. Using optical pumping technology, we achieved dual-wavelength laser emissions at 655 and 710 nm, which can be finely tuned and switched by controlling concentration and pH. After continuous operation for 6 h at twice the corresponding threshold, the laser intensity remained stable without attenuation (Fig. 6g
). Currently, there are only a few reports on lasers based on CDs, all of which have been achieved in solid-state or organic solutions. There remain significant challenges in achieving CD laser emission in aqueous solutions. Recently, Tang and Qu employed phthalic acid as a precursor and, with the assistance of boric acid and urea, used a solvent-free pyrolysis method to prepare CDs with a near 100% PLQY in aqueous solution [91]. They successfully achieved photo-pumped green laser emission in CD aqueous solutions for the first time with the aid of an F-P cavity (Fig. 6f).
Figure 6
Figure 6.
(a) The formation of high-Q cylindrical microcavities to support second-type whispering gallery modes. Copied with permission [87]. Copyright 2012, Wiley Publishing Group. (b) Representation of random lasing. Copied with permission [86]. Copyright 2019, Springer Nature Publishing Group. (c) Broad band random lasing from a F-P cavity. Copied with permission [89]. Copyright 2022, Wiley Publishing Group. (d) Single-Mode solid-statelaser with 0.14 nm linewidth and 14.8 dB SNR enabled by planar microcavity matching CDs' ASE Peak. Copied with permission [88]. Copyright 2021, Wiley Publishing Group. (e) Laser performance characterizations. Normalized emission spectra. Copied with permission [90]. Copyright 2023, Wiley Publishing Group. (f) Scheme of a FP parallel micro-resonator for CDs aqueous solution. Copied with permission [91]. Copyright 2024, Wiley Publishing Group. (g) Unveiling the photoluminescence mechanisms of carbon dots through tunable near-infrared dual-wavelength lasing; the squares represent different wavelengths, and the balls in squares represent different signal assignments. Copied with permission [54]. Copyright 2024, Elsevier Publishing Group.
In recent years, solution-processable lasers, especially those using CPDs, have gained attention for their spectral tunability, low thresholds, and flexibility, driving advancements in miniaturization and integration. These compact lasers are vital for information storage, communication, and biosensing. While CPDs show potential in anticounterfeiting and holographic displays due to their unique optical properties, their real-world device performance remains limited. Their optical signatures enable authentication and data encoding, while low toxicity ensures safe use. NIR lasers are particularly promising for space communication, laser radar, night vision, and clinical imaging/therapy, highlighting the importance of developing high-performance NIR-CPD lasers. In 2023, we synthesized full-color CDs (FC—CDs) with bright blue, green, yellow, red, deep-red, and NIR fluorescence (labeled as B-CDs, G-CDs, Y-CDs, R-CDs, DR-CDs, and NIR-CDs, respectively) [92]. The low content of sp3-hybridized carbon, high PLQY, and short fluorescence lifetime of the CDs were identified as crucial factors affecting their laser performance. The span from blue to NIR covers 238 nm, encompassing 140% of the national television standards committee color gamut. The low content of sp3-hybridized carbon introduced concentrated excited-state energy levels in the n-π* gap, enabling narrow FWHMs and population inversion. High KR facilitated optical amplification.
Finally, we successfully demonstrated color speckle-free laser imaging and high-quality dynamic holographic displays using these FC—CD lasers as light sources (Fig. S13d in Supporting information). Additionally, we successfully constructed logic gates (AND, OR, AND NOT) based on these tunable lasers [54]. This provides an effective approach for achieving tunable dual-wavelength lasers (Fig. S13c in Supporting information). Recently, vortex light with different topological charges and emission wavelengths was realized in CD random lasers [93]. Based on the spatial light modulator method, they not only achieved Laguerre-Gaussian mode vortex light with flexibly tunable topological charges ranging from 1 to 50, but also Bessel-Gaussian type vortex light with tunable topological charges from 1 to 10, and observed the self-healing characteristic of Bessel beams (Fig. S13a in Supporting information). Furthermore, they successfully realized polarization vortex light with topological charges ranging from 1 to 5 using a cascaded spatial light modulator method. Finally, these results were well verified in CD random lasers with different emission wavelengths covering the red, green, and blue spectral ranges. The integration of laser CDs with triethyl 1,3,5-benzenetricarboxylate (Et3BTC) represents a breakthrough in anti-counterfeiting technology. A dual random lasing medium is presented [94], which enhances security through the combination of materials, yielding lasing peaks around 325 and 450 nm, with an adjustable intensity ratio influenced by temperature, excitation area, and substrate bending (Fig. S13b in Supporting information).
7.
LEDs based on CPDs
LEDs are increasingly used in displays and lighting due to their high efficiency and stability. However, most traditional inorganic quantum dot and perovskite-based LEDs contain heavy metal ions, posing environmental risks. Although OLEDs contain non-toxic components, they are still hindered by complex manufacturing processes. Therefore, there is an urgent need to develop new low-toxicity, low-cost, and high-stability luminescent materials for electroluminescent LED devices. CPDs, as a new type of luminescent material, are gradually emerging in the LED field. However, the underlying mechanisms involving CPDs have received little attention. In 2023, Jian prepared deep-blue CDs with a luminescence center at 415 nm and a PLQY exceeding 60% through nitrogen doping (Fig. 7a) [95]. The CD-based light-emitting diodes (CLEDs) exhibited an external quantum efficiency (EQE) of 1.74% and a maximum brightness of 1155.0 cd/m2, with color coordinates close to the high-definition television HDTV standard. Currently, LEDs utilizing CDs mainly rely on their fluorescence properties, resulting in an exciton utilization rate of only 25% and limiting the EQE to a maximum of 5%. In the future, through methods such as phosphorescence, triplet-triplet annihilation, and TADF, CD-based LEDs are expected to achieve more efficient light emission. Furthermore, Tan successfully prepared two types of CDs with solid-state fluorescence properties by introducing triphenylamine groups with non-planar structures and high carrier mobility during the CD synthesis process in 2023 (Fig. 7b) [96]. These CDs not only exhibited significant solid-state luminescence properties but also possessed excellent solution processability, good film-forming ability, and high electron/hole mobility. Based on these CDs, high-performance multicolor electroluminescent LEDs, including orange and green LEDs, were constructed. The fabricated LEDs achieved maximum brightness, current efficiency, and turn-on voltage of 9450/4236 cd/m2, 1.57/2.34 cd/A, and 3.1/3.6 V, respectively. We developed 12 types of high EQE electroluminescent LEDs using CDs, leveraging their concentration-dependent optical properties (Fig. 7e) [97]. The EQE values for blue, cyan, and green devices are 2.04%, 2.62%, and 2.03%, respectively. Theoretical and experimental studies revealed that the high LED performance of CDs with low PLQYs stems from a rapid high-level reverse intersystem crossing (hRISC) process, efficiently converting excitons from high-energy triplet to singlet states and achieving near-100% exciton utilization efficiency (EUE). This work offers new insights into CD luminescent mechanisms and guides the development of cost-effective, high-performance CD-based LEDs. Recently, we obtained matrix-free TADF CDs using benzophenone derivatives and pyridine carboxaldehyde in an ethanol-sulfuric acid system [98]. This is also the first time that matrix-free TADF CDs have been developed in the field of CDs. The constructed electroluminescent diodes (ELEDs) achieved an external quantum efficiency exceeding 5.6%, which is the highest value reported for CD-based ELEDs. Benzophenone is a substance with a low n-π* excited state and extremely high S1-T1 transition efficiency, making it a nearly ideal scaffold structure for promoting ISC (Fig. 7f). However, due to its electron-withdrawing substituents that weaken fluorescence emission, it is necessary to select molecules with a large conjugated π structure, such as pyrene, as another scaffold. The lowest singlet excited state of pyrene is of the π-π* type, and its delocalized electrons are easily excited, making it easier to generate fluorescence and thereby compensating for the weakened fluorescence. Previous reports [65] have indicated that ethanol and sulfuric acid can react to form a cross-linked structure. This cross-linked structure can be used to construct a rigid network during the formation of CDs, effectively suppressing non-radiative transitions, thereby significantly stabilizing the triplet state and promoting the RISC process. Using these CDs as the emissive layer to construct ELEDs, yellow and white light devices with external quantum efficiencies of 5.68% and 1.70% respectively were obtained, which are the highest efficiencies reported so far for CD-based ELEDs by applying thermally activated exciton material design principles, one-dimensional fused-ring molecules are chosen as CD precursors. This structure typically features a large T2-T1 energy gap and a small S1-T2 energy gap [99]. Furthermore, specific heteroatoms such as nitrogen, sulfur, and oxygen can be introduced. Their lone pair electrons can introduce certain n-π* transition components into the excited-state orbitals, which is beneficial for enhancing the spin-orbit coupling between the singlet and triplet states. Perylene and perylene-3,4, 9,10-tetracarboxylic dianhydride (PTCDA) are chosen as precursors for CD synthesis. Compared to the precursors, the prepared B-CDs exhibit completely different photophysical processes and significant QYs (Fig. 7h). The reaction is carried out using DMF as the solvent and potassium persulfate as the catalyst. DMF can be activated by persulfate to generate amino radicals. A large number of amino and sulfate groups can also produce radicals from the large conjugated perylene molecules, thereby promoting rapid reactions and assisting in the synthesis of CDs. A more rational device structure and the RISC process in the emissive layer significantly enhance the external quantum efficiency of CD-based ELEDs. This study proposes a new method for regulating the triplet state of CDs, advancing their potential application in solution-processed ELEDs.
Figure 7
Figure 7.
(a) Device structure of CLED. Copied with permission [95]. Copyright 2023, Royal Society of Chemistry Publishing Group. (b) Optimal device structure and energy level diagram of the LEDs. Copied with permission [96]. Copyright 2023, Wiley Publishing Group. (c) L-I-V curves and ηc-current density (ηc-I) curves of the champion RT-CDs-LEDs device. Copied with permission [100]. Copyright 2023, Wiley Publishing Group. (d) The device structure of PeLEDs; cross-section SEM image of PeLEDs with CA/l-Arg CDs. Copied with permission [102]. Copyright 2024, American Chemical Society Publishing Group. (e) Energy diagram of the LED device structure and EL spectra of LEDs under turn-on voltage bias. Insets show photographs of the operating devices. Copied with permission [97]. Copyright 2022, Springer Publishing Group. (f) Matrix-free TADF CDs for electroluminescent light-emitting diodes achieving over 5.6% external quantum efficiency. Copied with permission [98]. Copyright 2024, American Chemical Society Publishing Group. (g) The structure of CDs-based devices. Copied with permission [101]. Copyright 2023, Wiley Publishing Group. (h) Energy diagram of the structure of the LED device; EL spectra of deep blue and blue ELEDs under turn-on voltage bias. Copied with permission [99]. Copyright 2024, Wiley Publishing Group.
Zhou and co-workers designed and synthesized a blue CQD luminescent material that spontaneously forms polymer chains on its surface, effectively suppressing the aggregation-induced fluorescence quenching effect, allowing the CQDs to maintain strong fluorescent emission in film form [100]. Furthermore, they designed a dual-host device structure to enhance carrier injection and transport, balancing carrier distribution, resulting in a high-brightness blue electroluminescent device based on CQDs (Fig. 7c).
Tan and colleagues utilized KIO4 as a strong oxidizing agent to catalyze the rapid cyclization reaction between OPD and 4-dimethylaminophenol in an aqueous phase, yielding the organic molecule 2-(dimethylamino)phenazine [101]. After purification and crystallization, this molecule self-assembled to form supramolecular CDs. The resulting RT-CDs exhibited good organic solubility, excellent solution processability, and high carrier mobility, making them suitable as the active luminescent layer in electroluminescent devices. Building on this, the researchers optimized the structure to achieve high-performance CD-based LEDs without the involvement of organic host materials (Fig. 7g).
CDs have a surface rich in functional groups, and we have found that they exhibit strong binding affinity with perovskite, further modulating its structure and photoelectric properties. For these reasons, well-modulated CDs can serve as a dual-functional additive, inhibiting the low-n phase and rebalancing hole-electron injection, thereby achieving ideal performance in quasi-2D PeLEDs. Recently, we synthesized CA/L-Arg CDs from CA and l-arginine via a one-step hydrothermal method [102]. Incorporating these CDs into the hole transport layer (HTL) suppressed the n = 1 phase in quasi-2D perovskites, resulting in a more concentrated phase distribution and increasing the PLQY from 52% to 71%. The CDs, synthesized from small molecules, exhibit poor conductivity and crosslink with insulating poly-styrenesulfonate (PSS), reducing the conductivity of the CDs-treated HTL (CDs-HTL) compared to the pristine HTL (p-HTL). This increased resistance aligns hole mobility with electron mobility in TPBi, enhancing exciton recombination efficiency (Fig. 7d). The quasi-2D PeLED with CDs-HTL achieved efficient sky-blue emission at 494 nm with an EQEmax of 21.07%, alongside excellent color stability and extended operational lifespan. This work offers a cost-effective, multifunctional CD additive for PeLEDs.
8.
Challenges and outlook
Within the family of carbon nanomaterials, fluorescent CPDs have emerged as a significant member of novel nanomaterials, prompting numerous researchers to investigate their properties and potential applications. CPDs exhibit several outstanding characteristics, including remarkable photostability, negligible cytotoxicity, excellent biocompatibility, convenient surface modification, and superior chemical inertness. These properties have led to their extensive use in various fields such as cell imaging, in vivo imaging, drug delivery, fluorescent sensing, photocatalysis, manufacturing of multicolor LEDs, energy conversion, and storage. However, there are still many challenges that require further exploration.
(1) The emission mechanisms of CPDs, evolving from molecular-state to size-dominant effects through structural carbonization, remain debated due to their complex internal structure, hindering further progress.
(2) Full-color CPDs, crucial for displays and encryption, struggle with achieving 100% quantum efficiency due to low purity or weak non-primary emission in existing strategies, highlighting the need for high-efficiency RGB-primary-emitting CPDs.
(3) NIR-Ⅱ CPDs, promising for biomedical applications, require advancements in precise synthesis, emission mechanism understanding, and improved quantum efficiency in aqueous solutions for optimal in vivo performance.
(4) Room-temperature afterglow materials like RTP and TADF face limitations in CPD systems, with challenges in achieving multi-color stimuli-responsive emissions, long-wavelength/short-wavelength afterglow, and bridging performance gaps with inorganic materials for advanced functionalities.
(5) CPDs are promising laser gain media, but underexplored factors affecting laser output and electrical pumping lasers hinder their development, necessitating further research into ultraviolet upconversion, NIR, and chiral lasers.
(6) High-performance CPD-based LEDs require precise energy-level tuning and deeper mechanistic insights into triplet-state stabilization and exciton utilization to overcome challenges in achieving white-light and NIR electroluminescence for practical applications.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Nos. 52203244, 22101267), the Key Scientific and Technological Project of Henan Province (No. 222102310683), and the China Postdoctoral Science Foundation (Nos. 2021M692905, 2024T170832), Natural Science Foundation of Henan Province (Nos. 242300421068, 242300421123).
Supplementary materials
Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.111612.
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Figure 1
(a) A proposed formation mechanism of N—C-dots prepared from citric acid in the MEA system. Copied with permission [33]. Copyright 2015, Royal Society of Chemistry Publishing Group. (b) Schematic of the synthesis processes of Na-CDs, ir-Na-CDs, and ir-Na-CDs@BSA. Copied with permission [35]. Copyright 2024, Wiley Publishing Group. (c) A synthetic route using citric acid and ethylenediamine: from ionization to condensation, polymerization, and carbonization. Copied with permission [32]. Copyright 2013, Wiley Publishing Group. (d) A synthetic route using citric acid and amine. Copied with permission [34]. Copyright 2016, American Chemical Society Publishing Group.
Figure 2
(a) Schematic of the Preparation of O—CQDs. Copied with permission [40]. Copyright 2022, American Chemical Society Publishing Group. (b) Synthesis process of T-CDs and C—CDs. Copied with permission [41]. Copyright 2023, Springer Nature Publishing Group. (c) The formation of OR-CDs. Copied with permission [42]. Copyright 2022, Elsevier Publishing Group. (d) Schematic of the synthesis process. Copied with permission [43]. Copyright 2023, Elsevier Publishing Group.
Figure 3
(a) Schematic illustration depicting one-step synthesis of CDs from coriander leaves. Copied with permission [59]. Copyright 2015, Royal Society of Chemistry Publishing Group. (b) Schematic illustration of the synthesis of biomass CQDs from biomass waste by hydrothermal treatment and the mechanism of Fe3+ detection with as-prepared CQDs. Copied with permission [60]. Copyright 2019, Elsevier Publishing Group. (c) Schematic illustrating the synthesis methods of N—CDs. Copied with permission [61]. Copyright 2020, Royal Society of Chemistry Publishing Group. (d) Synthesis of CDs with avocado. Copied with permission [62]. Copyright 2021, Elsevier Publishing Group. (e) Schematic of the fabrication of J-CDs. Copied with permission [63]. Copyright 2022, Wiley Publishing Group.
Figure 4
(a) Schematic representation of procedures for the preparation of CDs@Al2O3; images of the CDs@Al2O3 powders before and after turning off the 365 nm UV lamp irradiation and normalized afterglow spectra of CDs@Al2O3 under ambient conditions. Copied with permission [70]. Copyright 2024, Elsevier Publishing Group. (b) Corresponding photographs of CB-Ⅰ, CB-Ⅱ, CB-Ⅲ, CB-Ⅳ, CB-Ⅴ, CB-Ⅵ, and CB-Ⅶ on a piece of glass, respectively; digital photographs of information anticounterfeiting and encryption application of CB-Ⅰ and CB-Ⅳ powder under 365 nm UV light and data encryption application of CB-Ⅰ, CB-Ⅴ, and CB-Ⅶ powder. Copied with permission [71]. Copyright 2023, Elsevier Publishing Group. (c) Application of CDs@urea composites. Copied with permission [72]. Copyright 2024, Wiley Publishing Group. (d) Afterglow photos of B@B2O3, G@B2O3, R@B2O3 and NIR@B2O3 excited by 254 and 365 nm. Copied with permission [73]. Copyright 2024, Wiley Publishing Group.
Figure 5
(a) Schematic diagram of the real experimental absorption and emission of light by the red, green and blue emissive components. Copied with permission [77]. Copyright 2021, Royal Society of Chemistry Publishing Group. (b) Schematic diagram of the formation of CDs through pyrolysis. Copied with permission [52]. Copyright 2022, Elsevier Publishing Group. (c) A schematic of three types of CDs reaction process at different solvent ratios of DMF and ethanol with OPD as the precursor. The dashed line indicates the possible reaction route without spectral evidence. The solid lines indicate the reaction routes. Copied with permission [78]. Copyright 2024, Wiley Publishing Group. (d) Formation energies of longitudinal and transverse growth of CDs from OPD. Core-shell structures formed by four kinds of CDs. Copied with permission [79]. Copyright 2022, Springer Nature Publishing Group. (e) DFT calculations and calculated PL spectra of DHQP with H aggregation. Copied with permission [54]. Copyright 2024, Elsevier Publishing Group.
Figure 6
(a) The formation of high-Q cylindrical microcavities to support second-type whispering gallery modes. Copied with permission [87]. Copyright 2012, Wiley Publishing Group. (b) Representation of random lasing. Copied with permission [86]. Copyright 2019, Springer Nature Publishing Group. (c) Broad band random lasing from a F-P cavity. Copied with permission [89]. Copyright 2022, Wiley Publishing Group. (d) Single-Mode solid-statelaser with 0.14 nm linewidth and 14.8 dB SNR enabled by planar microcavity matching CDs' ASE Peak. Copied with permission [88]. Copyright 2021, Wiley Publishing Group. (e) Laser performance characterizations. Normalized emission spectra. Copied with permission [90]. Copyright 2023, Wiley Publishing Group. (f) Scheme of a FP parallel micro-resonator for CDs aqueous solution. Copied with permission [91]. Copyright 2024, Wiley Publishing Group. (g) Unveiling the photoluminescence mechanisms of carbon dots through tunable near-infrared dual-wavelength lasing; the squares represent different wavelengths, and the balls in squares represent different signal assignments. Copied with permission [54]. Copyright 2024, Elsevier Publishing Group.
Figure 7
(a) Device structure of CLED. Copied with permission [95]. Copyright 2023, Royal Society of Chemistry Publishing Group. (b) Optimal device structure and energy level diagram of the LEDs. Copied with permission [96]. Copyright 2023, Wiley Publishing Group. (c) L-I-V curves and ηc-current density (ηc-I) curves of the champion RT-CDs-LEDs device. Copied with permission [100]. Copyright 2023, Wiley Publishing Group. (d) The device structure of PeLEDs; cross-section SEM image of PeLEDs with CA/l-Arg CDs. Copied with permission [102]. Copyright 2024, American Chemical Society Publishing Group. (e) Energy diagram of the LED device structure and EL spectra of LEDs under turn-on voltage bias. Insets show photographs of the operating devices. Copied with permission [97]. Copyright 2022, Springer Publishing Group. (f) Matrix-free TADF CDs for electroluminescent light-emitting diodes achieving over 5.6% external quantum efficiency. Copied with permission [98]. Copyright 2024, American Chemical Society Publishing Group. (g) The structure of CDs-based devices. Copied with permission [101]. Copyright 2023, Wiley Publishing Group. (h) Energy diagram of the structure of the LED device; EL spectra of deep blue and blue ELEDs under turn-on voltage bias. Copied with permission [99]. Copyright 2024, Wiley Publishing Group.
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