English

• Carbon dots (CDs), as emerging fluorescent carbon nanomaterials, have garnered much attention since their initial synthesis [1–3]. The exceptional optical properties, such as low toxicity and high stability of CDs, make them indispensable in bioimaging [4], nanomedicine [5], energy [6], and optoelectronic devices [7]. Compared to traditional fluorescent materials, the synthesis process of CDs is straightforward, and their precursors are widely available [8]. By carefully selecting precursors, it is possible to synthesize CDs with specific wavelengths, sizes, and surface states, thereby achieving the preparation of functionalized CDs [9]. The unique carbon core and polymer shell structure of CDs combine the advantages of quantum dots and organic compounds. By modulating their carbon core and shell, tunable quantum and molecular state luminescence can be achieved, further expanding the practical applications [10].

  In recent years, among the wide applications of carbon dots, electroluminescent light-emitting diodes (ELEDs) have attracted widespread attention and significant progress has been achieved [11]. Notably, remarkable progress has been made on blue ELEDs due to their advantages in phototherapy, optical communication, and the development of white LED technology [12]. Enhancing the efficiency of blue ELEDs is crucial, with a particular emphasis on improving the radiative recombination process. Researchers have employed various strategies to enhance the external quantum efficiency (EQE). For instance, Sargent et al. utilized surface engineering strategies to increase the quantum yield of blue CDs, achieving an EQE of 4% [13]. Yuan et al. developed CDs with thermally activated delayed fluorescence (TADF) properties, which enhanced the device’s EQE [14]. Additionally, our group has developed a series of hot exciton CDs through rational precursor selection, leveraging efficient hot exciton reverse intersystem crossing (RISC) processes to improve EQE [15]. However, these optical property modifications have not fully exploited the quantum state characteristics of CDs. Moreover, the complexity of precursor selection and reaction process design has limited the reporting of TADF and hot exciton CDs. Therefore, there is an urgent need to develop electroluminescent applications by utilizing the widespread properties of CDs.

  In addition to the RISC process, luminescent materials with exciton emission significantly contribute to radiative recombination in ELEDs, directly affecting the device’s internal quantum efficiency (IQE) [16–18]. Efficient exciton emission leads to a higher EQE, thus a greater proportion of injected charge carriers recombine to emit photons rather than undergoing non-radiative recombination. Research studies have shown that CDs typically exhibit high exciton binding energies, which stabilize excitons and make radiative recombination more likely compared to non-radiative processes [19]. As more electron-hole pairs recombine to emit light, this can result in higher IQE. Additionally, excitons with higher binding energies are less likely to dissociate into free carriers at elevated temperatures. This thermal stability ensures that ELEDs maintain stable luminescence and efficiency over a wider temperature range, thereby enhancing the performance and reliability of ELEDs under various operating conditions. Consequently, developing carbon dots with exciton emission characteristics is another effective approach to improving ELED efficiency [20,21].

  Based on this, we have developed a novel type of carbon dot with exciton emission characteristics, featuring high aqueous-phase quantum yield and solvent dependency, which can be used to construct blue CDs-based ELEDs. Variable-temperature fluorescence spectroscopy confirmed the large exciton binding energy of CDs, and the linear dependence of integrated intensity on excitation density demonstrated the decisive role of excitons in optical transitions. The blue ELEDs constructed with these CDs achieved an EQE of over 4%. This study introduces a new method for synthesizing and regulating the luminescence mechanism of CDs. Additionally, it proposes solutions for developing high-performance blue ELEDs.

  In this study, perylene tetracarboxylic dianhydride (PTCDA) was used as the raw material, and potassium carbonate as the catalyst to synthesize CDs via hydrothermal method. PTCDA has a perylene backbone with a large conjugated π structure, and its lowest singlet excited state is of the π-π^* type, making delocalized electrons easily excitable and conferring strong fluorescence in solution. Additionally, in an alkaline environment, the anhydride bonds of PTCDA hydrolyze into carboxyl groups, enhancing its water solubility. Potassium carbonate not only provides an alkaline environment but allows carbonate ions to attach to the surface of the CDs. When dispersed in water, PTCDA forms a red fluorescent solution, while the resulting CDs emit green light, indicating a clear distinction from the precursor (Fig. 1a). Subsequently, we conducted phase and structural characterization of the as-prepared CDs. Transmission electron microscopy (TEM) images revealed that CDs have a spherical structure and are uniformly dispersed (Fig. 1b), with an average particle size of 2.64 nm (Fig. S1 in Supporting information). The X-ray diffraction (XRD) pattern showed a characteristic diffraction peak of CDs at 25.4°, corresponding to the (002) crystal plane (Fig. 1c). The XRD displayed sharp peaks rather than traditional broad peaks, confirming the good crystallinity of CDs. High-resolution TEM (HRTEM) images showed clear lattice fringes, further demonstrating good crystallinity with an interplanar spacing of 0.35 nm, consistent with the XRD results (Fig. 1c insert) [22]. Meanwhile, the Raman spectrum of CDs shows a small I_D/I_G value, which confirms the good crystallinity (Fig. S2 in Supporting information). In the proton nuclear magnetic resonance (¹H NMR) spectrum, strong peaks at 7.7 and 8.3 ppm correspond to the large conjugated structures formed after polymerization, while some smaller peaks at 7.6 and 8.1 ppm correspond to small segments accumulated in the carbon core (Fig. 1d) [23]. Mass spectrometry (MS) revealed a large number of repeating fragment units (28), corresponding to the C=O structure. This occurs because the alkaline hydrolysis of anhydride bonds forms carboxyl groups, and the OH in the carboxyl groups dissociates first under ESI ionization, exposing a large amount of C=O (Fig. 1e). The mass peak at about 1900 Da corresponds to the large polycyclic aromatic hydrocarbons structure formed by precursor polymerization (Fig. S3 in Supporting information). Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) were used to investigate the surface functional groups of the CDs. Typically, the FTIR spectrum of CDs shows multiple peaks around at 3500 cm⁻¹, corresponding to the absorption of amino and hydroxyl groups. However, for the non-N-doped CDs developed in this study, only one characteristic hydroxyl absorption peak is present in this region. The peak at 1600 cm⁻¹ corresponds to C=O, while peaks at 1450 and 1350 cm⁻¹ correspond to C=C and C-O, respectively (Fig. 1f). XPS analysis of the CDs showed only C and O elements at 284.6 and 533.1 eV, respectively (Fig. S4 in Supporting information) [24]. Compared to the raw material, the CDs showed an increased carbon content, likely due to carbonization at high temperatures and the removal of CO₂ and water during the reaction [25]. High-resolution XPS indicated a higher content of C-O bonds in the CDs compared to the raw material, confirming the opening of anhydride bonds and the presence of surface structures on the CDs. We then used thermogravimetric analysis (TGA) and derivative thermogravimetric analysis (DTG) to characterize the core-shell structure of CDs. At 800 ℃, 30% of the original mass remained, attributed to the graphitized structure of the carbon core. The DTG curve showed three-stage degradation of CDs, differing from classical polymers. The first degradation at about 186 ℃ is due to the decomposition of groups on the CDs’ shell. Since no additional aliphatic precursors were added during the reaction, the shell layer of CDs lacks abundant polymer chains [26]. The second thermal degradation mainly corresponds to the decomposition of accumulated and crosslinked structures in the carbon core, and the third stage corresponds to the collapse of the conjugated graphitized structure in the carbon core (Fig. 1g) [27].

  Figure 1

  

  Figure 1.  Structural characterization of CDs: (a) Schematic showing the fabrication of the CDs, (b) TEM, (c) XRD patterns and HRTEM (insert), (d) ¹H NMR spectrum, (e) MS, (f) FTIR spectrum and (g) TGA spectrum of CDs.

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  Furthermore, we characterized the optical properties of CDs. The CDs exhibited excellent solubility and bright green fluorescence. The fluorescence emission spectrum of CDs showed excitation-independent emission, with a strong green emission peak at 511 nm and a shoulder peak at 550 nm (Fig. S5 in Supporting information). The UV–vis absorption spectrum of CDs in aqueous solution displayed two absorption bands: one at 250 nm, corresponding to the π-π^* transition of the conjugated structure in CDs, and the other between 400 and 500 nm, likely corresponding to the absorption of different energy levels created by the aggregation of the conjugated structure in the CDs (Fig. 2a). We then measured the absolute quantum yield of the purified CDs, which was found to be 86.76% (Fig. S6 in Supporting information). The time-resolved photoluminescence decay curves of CDs were measured under 450 nm excitation. The fluorescence intensity decay curves revealed the kinetics of electron deactivation from the excited state to the ground state through radiative and non-radiative pathways. The observed curves could be fitted to a biexponential function. The decay included a fast component (τ₁), attributed to the radiative recombination of intrinsic states, and two slower components (τ₂), resulting in an average lifetime of 7.5 ns (Fig. 2b). Interestingly, we diluted the as-prepared CDs in an aqueous solution and observed shifts in the fluorescence spectra at different concentrations. We labeled the three concentrations of CDs solutions as CDs-1 (0.01 mg/mL), CDs-2 (0.1 mg/mL), and CDs-3 (1 mg/mL). Upon dilution, a new emission peak emerged around 490 nm, which became more prominent with further dilution. Notably, the emission peak at 550 nm remained almost unchanged in intensity across the three concentrations. This led us to hypothesize that the 490 nm emission peak corresponds to the interaction between the shell and the solvent, with more of CDs shell being exposed upon dilution. The 511 nm emission peak corresponds to the aggregated state emission of the CDs, as this band dominates in high-concentration solutions (Fig. 2c). The absorption spectra at different concentrations showed consistent peak shapes and positions, with increasing absorption intensity as the concentration increased. The absorption peaks were fitted, revealing that they originated from stacking with strong vibrionic coupling (Fig. S7 in Supporting information). We also analyzed the fluorescence lifetimes of the CDs at three concentrations. As the concentration increased, the fluorescence lifetime gradually decreased. This is due to quenching effects that begin to shorten the fluorescence lifetime at moderate concentrations. Moreover, at higher concentrations, new non-radiative decay pathways (such as aggregation) are introduced, competing with fluorescence emission and further shortening the fluorescence lifetime (Fig. S8 in Supporting information).

  Figure 2

  

  Figure 2.  Optical properties of CDs: (a) Absorption spectra, PL excitation and emission spectra; (b) Fluorescence decay curve of CDs; (c) PL emission spectra of three CDs with different concentrations; (d) Fluorescence and afterglow spectra of solid CDs at 77 K; (e) Double logarithm fitting curve of the power-dependent emission intensity at room temperature of three CDs with different concentrations.

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  Next, we investigated the fluorescence mechanism of CDs. In the solid state, the CDs exhibited yellow emission with a wavelength of around 600 nm and a quantum yield of 16.38% (Figs. S9a and b in Supporting information). This is due to excessive resonance energy transfer and π-π interactions, leading to significant self-quenching in the solid state. The fluorescence lifetime of CDs in the solid state was 6.5 ns, which is shorter than in the solution (Fig. S9c in Supporting information). This shortening is mainly because resonance energy transfer competes with radiative transitions in the solid state, thus reducing the fluorescence lifetime. Further, we conducted temperature-dependent fluorescence experiments on the CDs aqueous solution. A large amount of potassium carbonate was added to the solution to lower the freezing point. At low temperatures, the solution froze into ice, and the sample exhibited solid-state emission. As the temperature increased, green emission appeared in the solution, with the highest green emission intensity observed at room temperature (Figs. S10 and S11 in Supporting information). Fitting the temperature-dependent spectra of the carbon dots yielded an exciton binding energy of 204 meV (Fig. S12 in Supporting information). The large exciton binding energy increases the likelihood of radiative recombination, effectively converting electrical energy into light and resulting in high electroluminescence efficiency. At 77 K, we measured the fluorescence and delayed spectra of the CDs in the solid state. The phosphorescence spectrum, delayed by 10 ms, showed an emission peak at 618 nm, indicating that the CDs do not possess hot exciton or thermally activated delayed fluorescence properties (Fig. 2d). The time-resolved emission spectra (TRES) were measured to further study the excited-state dynamics of CDs [28]. The transient fluorescence spectra of CDs at different concentrations were similar to the fluorescence spectra, reflecting the contribution of aggregation forms to the emission (Fig. S13 in Supporting information). We also measured the fluorescence lifetimes of CDs at different excitation wavelengths. As shown in Fig. S14 (Supporting information), the decay curves were similar across different excitation wavelengths, indicating that the decay process of the emission excited states did not change significantly over a range of wavelengths. The average fluorescence lifetime was only weakly dependent on the excitation wavelength and was not significantly related to the fluorescence quantum yield. To investigate the origin of radiative recombination in the CDs, we studied the relationship between the integrated fluorescence intensity and laser power density under 355 nm continuous-wave excitation for three different concentrations of CDs. As shown in Fig. 2e, the slopes (on a log-log scale) of the CDs solutions at different concentrations were close to 1. This indicates that the density of absorbed photons was proportional to the density of emitted photons, a characteristic feature of exciton recombination emission. The dependence of fluorescence on excitation energy also reflects different radiative recombination channels. Fluorescence increased linearly with the pump energy density, indicating that the increase in photons was due to exciton emission in the single-molecule recombination process. These results suggest that the carrier recombination mechanism in CDs is via exciton channels, and the exciton states determine the absorption and recombination processes of the CDs [29].

  Subsequently, we explored the application of CDs in the field of ELEDs. Despite their excellent water solubility, the CDs are poorly soluble in most organic solvents. Fortunately, they are sparingly soluble in 1,4-dioxane, where they exhibit blue emission with three peaks at 445, 474, and 508 nm (Fig. S15 in Supporting information). This is mainly because CDs surface contains abundant hydroxyl and carboxyl groups that can form hydrogen bonds. Polar solvents capable of forming hydrogen bonds can lead to the redistribution of electron density in the excited state, stabilizing it and causing a red shift in the emission. We then used poly(9-vinylcarbazole) (PVK) to encapsulate CDs and prepare thin films. PVK can act as an effective host matrix for CDs, helping to disperse them uniformly within the emitting layer, preventing aggregation caused quenching (ACQ), and improving the photoluminescence quantum yield (PLQY) of CDs-based films.

  The films exhibited fluorescence similar to that in 1,4-dioxane solution (Fig. 3a), with a quantum yield of 37.91% (Fig. 3b) and a lifetime of 4.82 ns (Fig. 3c). TRES showed that the CDs had the same fluorescence decay at different emission wavelengths. This indicates that the absorption and subsequent emission processes follow the same pathway, regardless of the specific emission wavelength, leading to consistent decay behavior (Fig. 3d). Next, we conducted ultraviolet photoelectron spectroscopy (UPS) experiments on the CDs-based films [30]. Using this information along with the optical bandgap, we calculated the highest occupied molecular orbital (HOMO) energy level and the lowest unoccupied molecular orbital (LUMO) energy level of the CDs films to be −5.9 eV and −3.3 eV, respectively (Fig. 3e). Atomic force microscopy (AFM) was used to examine the morphology and roughness of the carbon dot-based films, revealing that the prepared films were uniform and smooth (Fig. 3f).

  Figure 3

  

  Figure 3.  Optical properties of CDs-based films: (a) PL excitation and emission spectra, (b) quantum yield and (c) fluorescence decay curve of CDs-based films; (d) Fluorescence decay curves under different emission wavelengths, (e) absorption edge (inset), UPS spectra and (f) AFM of CDs film.

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  The 1,4-dioxane solution containing CDs and PVK demonstrated good film-forming properties. The spin-coated films exhibited bright solid-state luminescence, inspiring their use as the single emission layer in ELED devices. The ELED utilized a conventional device structure with indium tin oxide (ITO) as the anode, poly(3,4-ethylenedioxythiophene)polystyrene sulfonate (PEDOT: PSS) with PSS: Na solution as the hole injection layer (HIL), a PVK composite film as the emission layer, 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi) as the electron transport layer, and a LiF/Al bilayer cathode (Fig. 4a). With the addition of PSS: Na, the optimal HOMO level of the hole injection layer was −5.7 eV, ensuring better energy level alignment and improved hole injection (Fig. 4b) [31]. The thicknesses of the HIL, CDs-based emission layer, TPBi, and LiF/Al were approximately 40 nm, 30 nm, 40 nm, 1 nm, and 80 nm, respectively. Fig. 4c presents the typical electroluminescent (EL) spectrum; a high-brightness photograph of the ELED is also included. The ELED exhibited triple-peak emission at 444 nm, 473 nm, and 509 nm. As the operating voltage increased from 6.0 V to 9 V, there was no peak shift in the EL spectrum, demonstrating the spectral stability of the device. The International Commission on Illumination (CIE) color coordinates were (0.20, 0.23) (Fig. S16 in Supporting information). Fig. 4d shows the typical device brightness versus current density curve, with a turn-on voltage of 4.8 V and a brightness of 1290 cd/m². The B-LED’s external quantum efficiency (EQE) was 4.19% (Fig. 4e). Further evaluation of the device’s lifespan determined that a half-life (T₅₀) of 318 min (Fig. 4f), indicating a relatively high performance level for CDs-based blue ELEDs.

  Figure 4

  

  Figure 4.  Electroluminescent LED performance: (a) Device structure, (b) energy level diagram, (c) EL spectra, (d) current density–voltage–luminance, (e) EQE–current density, and (f) device stability of CDs-based ELEDs.

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  In summary, through the rational selection of precursors, we have developed exciton-emitting CDs. Variable-temperature fluorescence spectroscopy confirmed the large exciton binding energy of the as-prepared CDs. Additionally, from the linear dependence of integrated intensity on excitation density, we found that the emission spectrum is entirely attributable to exciton recombination behavior. By doping to adjust the energy level structure of the hole injection layer, the overall energy levels of the ELEDs devices showed a better match. The constructed CDs-based blue ELEDs achieved an external quantum efficiency of 4.19%, positioning them at the forefront of fluorescent carbon dot-based ELEDs. This study provides new insights into the luminescence mechanism of carbon dots and presents potential applications for solution-processed ELEDs.

  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.

  CRediT authorship contribution statement

  Yuan Liu: Writing – original draft, Conceptualization. Boyang Wang: Formal analysis, Data curation. Yaxin Li: Writing – review & editing. Weidong Li: Supervision, Funding acquisition. Siyu Lu: Writing – review & editing, Supervision, Funding acquisition.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (Nos. 22205058, 22105064, 52122308), the Funding Plan of Key Scientific Research Projects in Colleges and Universities of Henan Province (No. 23A150001), Doctoral Scientific Research Start-up Foundation from Henan University of Technology (No. 2021BS024), the Project of Youth Backbone Teachers of Henan University of Technology (No. 21421250), the Innovative Funds Plan of Henan University of Technology (No. 2022ZKCJ01).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110426.

  
  
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  Figure 1  Structural characterization of CDs: (a) Schematic showing the fabrication of the CDs, (b) TEM, (c) XRD patterns and HRTEM (insert), (d) ¹H NMR spectrum, (e) MS, (f) FTIR spectrum and (g) TGA spectrum of CDs.

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  Figure 2  Optical properties of CDs: (a) Absorption spectra, PL excitation and emission spectra; (b) Fluorescence decay curve of CDs; (c) PL emission spectra of three CDs with different concentrations; (d) Fluorescence and afterglow spectra of solid CDs at 77 K; (e) Double logarithm fitting curve of the power-dependent emission intensity at room temperature of three CDs with different concentrations.

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  Figure 3  Optical properties of CDs-based films: (a) PL excitation and emission spectra, (b) quantum yield and (c) fluorescence decay curve of CDs-based films; (d) Fluorescence decay curves under different emission wavelengths, (e) absorption edge (inset), UPS spectra and (f) AFM of CDs film.

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  Figure 4  Electroluminescent LED performance: (a) Device structure, (b) energy level diagram, (c) EL spectra, (d) current density–voltage–luminance, (e) EQE–current density, and (f) device stability of CDs-based ELEDs.

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