Solvent-Engineered Strategy Synthesis of Multicolor Fluorescent Carbon Dots for Advanced Solid-State Lighting Applications

Nov. 17 , 2024photonics1

Abstract

At present, achieving tunable fluorescent carbon dots (CDs) with narrow full width at half-maximum (FWHM) and high fluorescence quantum yields (QYs) remains a significant challenge. In our study, l-tryptophan and o-phenylenediamine were utilized as precursors, systematically controlling their band gaps and surface states by varying the solvent type. The blue (B-CDs), yellow-green (YG-CDs), and red (R-CDs) fluorescent CDs were successfully prepared, with optimal excitation wavelengths (λ_ex) of 444, 537, and 597 nm, respectively. Especially, these multicolor CDs (M-CDs) exhibited impressive QYs of 53.69, 54.88, and 58.79%, and narrow FWHM of 71, 64, and 34 nm, respectively. Their distinct optical properties were achieved by manipulating the carbonization and dehydration processes through a solvent selection. The variations in optical properties were primarily attributed to increased amino nitrogen content, quantum size, and coordinated effects of surface oxidation states. Furthermore, M-CDs were successfully incorporated into polyvinyl alcohol (PVA) to produce transparent and flexible fluorescent films, demonstrating their excellent and stable optical quality. Finally, the potential of M-CDs in optoelectronic applications was showcased by fabricating bright light-emitting diodes (LEDs).

1. Introduction

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Carbon dots (CDs) are a fluorescent nanomaterial with significant potential for progression, offering diverse applications across various fields, including fluorescence sensing, information anticounterfeiting, biological imaging, and optoelectronic devices. (1−4) Their unique nanostructures and exceptional physical and chemical properties make them highly versatile. Previous studies have highlighted the abundance of surface groups on CDs, facilitating easy functionalization and modification. Moreover, their band gap energy and fluorescence emission are highly tunable based on the precursor selection and reaction conditions, allowing for the production of CDs emitting light across different colors. (5,6) The tunability of CDs luminescence is a subject of extensive interest, and systematic research is crucial to promoting the application of CDs in multiple fields. (7) The probable mechanisms such as quantum size effects, (8,9) nitrogen doping, (10,11) and surface states (12,13) were continuously explored. However, achieving color-tunable CDs often involves the collaborative control of multiple mechanisms, (14,15) leading to challenges in understanding the photoluminescence mechanism, especially in the context of multicolor CDs (M-CDs). (16)

Despite efforts to synthesize tunable CDs, challenges persist in synthesis strategies and fluorescence performance enhancement. (17) Most M-CDs require a more complex synthetic environment and are further separated using silica gel column chromatography. (18,19) Unfortunately, these methods consume time and large amounts of organic solvents, pose environmental and health risks, and are difficult to industrialize. Additionally, synthesized M-CDs often exhibit low quantum yields (QYs) and broad full width at half-maximum (FWHM), which severely restricts the applications in the biological imaging and light-emitting devices. (20,21) Low QYs signify significant nonradiative energy loss, which basically restrict the manufacture of high-performance CDs-based electroluminescent light-emitting diodes (LEDs), and it is usually improved through surface modification or passivation of functional groups. (22) Furthermore, previous reports emerge that the FWHM of most CDs exceeds 80 nm, which is not suitable for generating a wide-color gamut display with good efficiency. (23) Currently, colloidal semiconductor quantum dots represented by Cd²⁺ and Pb²⁺ have the characteristics of high QYs and narrow FWHM, making them a good choice for developing display and lighting technologies. (24) However, these quantum dots have been severely restricted due to harmful consequences. (25) Because of narrow FWHM, high QYs, and low toxicity, M-CDs become a popular alternative to heavy metal quantum dots in the electroluminescent LED field. To the best of our knowledge, few studies have reported the one-step synthesis of M-CDs with favorable optical properties using the same precursor under similar reaction parameters, free of organic solvents.

In this study, our M-CDs were created via a one-step solvothermal method using l-tryptophan (l-trp) and o-phenylenediamine (o-PD) as precursors with varying reaction solvents (acetic acid, water, and sulfuric acid). By modulation of solvent conditions, the dehydration and carbonization of the reaction were controlled to produce M-CDs with distinct particle sizes and functional group contents. The synthesized M-CDs, including blue (B-CDs), yellow-green (YG-CDs), and red CDs (R-CDs), exhibited relatively high QYs (53.69%, 54.88%, and 58.79%) and progressively narrowed FWHM (71, 64, and 34 nm). Most importantly, to address self-quenching of CDs in an aggregated state, the M-CDs were successfully dispersed into a polyvinyl alcohol (PVA) matrix, achieving the preparation of multicolor fluorescent films. Finally, by leveraging their high stability and bright luminescent performance, the M-CDs were integrated into LEDs, showcasing their potential in optoelectronic displays.

2. Materials and Methods

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The relevant information about experimental materials, instruments, and measurement of fluorescence QYs can be found in the Supporting Information.

2.1. Preparation of M-CDs

B-, YG-, and R-emissive CDs were acquired through a straightforward solvothermal process, utilizing o-PD and l-trp in different solvents (acetic acid, water, and sulfuric acid) under identical temperature conditions. The synthesis procedure for each CDs was as follows:

The synthesis of B-CDs: 178.3 mg of l-trp and 127.8 mg of o-PD (with a mass ratio of 1.4:1) were dissolved into 6.5 mL of ultrapure water and 6.5 mL of acetic acid. The mixture was ultrasonicated until fully dissolved, transferred to a Teflon-lined autoclave, and reacted at 160 °C for 6 h. At room temperature, the supernatant was filtered through a 0.22 μm filter membrane and dialyzed by a dialysis bag (MWCO = 500 Da) for 48 h, with regular water change every 4 h. The resulting dialyzed solution was then freeze-dried, collected, and stored.

The synthesis of YG-CDs: precursor composition was altered by exchanging the mass ratio of l-trp and o-PD to 127.8 and 178.3 mg, respectively. The mixture was dissolved in 10 mL of ultrapure water, and the subsequent steps were consistent with the procedure for the synthesis of B-CDs synthesis.

The synthesis of R-CDs: adjustments were made to the solvent system by utilizing 10 mL of ultrapure water and 1 mL of concentrated sulfuric acid. The remaining steps mirrored those of YG-CDs synthesis. Following synthesis, the sample was centrifuged at 9000 rpm for 5 min to discard the supernatant and added with ultrapure water to repeat the operation 2–3 times to obtain pure red CDs with solid particles.

2.2. Preparation of Multicolor Films

2 g portion of PVA was solubilized in 30 mL of ultrapure water and stirred for 35 min at 90 °C until fully dissolved. Subsequently, specific quantities of M-CDs were added to the PVA solution and stirred for an additional 10–15 min to ensure thorough mixing. The resulting mixtures were then slowly poured into clean Petri dishes and dried overnight at room temperature. By carefully peeling them off, we obtained homogeneous, transparent CDs/PVA films.

2.3. Preparation of LEDs

2 mL portion of each CDs solution was mixed with an appropriate amount of PVA solution. These resulting mixtures were then dropped onto LED chips (emitting wavelength of 365 nm and operating voltage of 3 V). The coated chips were subsequently dried at 70 °C for 4 h.

3. Results and Discussion

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3.1. Optimization of M-CDs Preparation Conditions

Our M-CDs were obtained by a one-step solvothermal method using l-trp and o-PD in different ratios within acetic acid, water, and sulfuric acid solutions. Compared with the general method, our method was simple and efficient. As a green synthetic method, the base solvent was water, so the method was environmentally friendly compared to the method using organic solvents such as formamide and N,N-dimethylformamide. In addition, the process of preparation of our M-CDs, with no column chromatography, was used to promote the development of the M-CDs industry. Meanwhile, it also overcame the drawbacks of synthesizing M-CDs with various precursors, such as inhomogeneity and inconsistency, which make it easy to provide a comparable explanation of the luminescence mechanism.

These synthesized M-CDs exhibited tunable photoluminescence (PL) emission (Figure 1), demonstrating the significant effect of solvent modulation on the fluorescence properties of M-CDs. To achieve high QYs of M-CDs, various reaction parameters were optimized, including the amount of acid in the solvent, reaction temperature, mass ratio of reactants, and reaction time. Initially, with the reaction temperature at 160 °C, the reaction time at 6 h, and the reactant mass ratio of 1:1.4, the influence of acid volume in the solvent on fluorescence QYs was then examined. For B-CDs and R-CDs, the optimal acid volume was found to be 6.5 and 1 mL, respectively, yielding the highest fluorescence QYs. Next, the reaction temperature, reactant mass ratio, and reaction time were optimized. Optimum temperature and time were 160 °C and 6 h, respectively. The highest QYs were observed with a mass ratio of 1.4:1 for B-CDs and 1:1.4 for both YG-CDs and R-CDs. Consequently, the optimized synthesis conditions resulting in the maximum QYs were 53.69%, 54.88%, and 58.79% for B-, YG-, and R-CDs, respectively (Figure S1a–c). The M-CDs in this study have both high QYs and narrow FWHM, which are considerably better than those reported in most literature (Table S1).

Figure 1. Schematic of the M-CDs preparation process.

3.2. Morphology and Structure of M-CDs

The morphology and particle size of M-CDs were examined by using HRTEM. Three types of CDs exhibited dispersed spherical structures without significant aggregation (Figure 2a–c). Further analysis of particle size distribution revealed that average sizes were 2.03, 2.50, and 5.23 nm for B-, YG-, and R-CDs, respectively (Figure 2d–f), indicating a gradual increase in particle size from blue to red, attributed to the quantum size effect, (26) where smaller particles exhibit blue emission and larger particles emit red emission. Additionally, XRD spectra displayed broad diffraction peaks at 21.1° , 21.6° , and 23.2° for the three CDs, respectively (Figure S2), indicating an amorphous carbon structure and their successful carbonization and formation. (27)

Figure 2. HRTEM photographs of (a) B-, (b) YG-, and (c) R-CDs. Size distributions of (d) B-, (e) YG-, and (f) R-CDs.

Subsequently, surface functional groups were investigated by using FTIR. M-CDs exhibited similar characteristic peaks, suggesting a comparable chemical composition across the different CDs (Figure S3a). Absorption peaks in the range of 1785–958 cm^–1 gradually weaken from B-CDs to R-CDs. Specifically, notable peaks included the broad O–H/N–H stretching vibrations around 3404 cm^–1, the peaks at 2923 cm^–1 were assigned to C–H, and in the range of 1747–1567 cm^–1, peaks were imputed to C═C/C═O. Absorption peaks at 1550–1330 cm^–1 were attributed to –C–N═/C–N. In R-CDs, the weak peak around 1400 cm^–1 corresponded to S═O and sulfur oxides, while peaks at 1177–975 cm^–1 indicated C–O bonds. Additionally, the peak at 752 cm^–1 was caused by the N–H bending vibration of the o-PD.

To gain deeper insights into the surface chemical properties and functional groups of M-CDs, XPS was conducted. The full XPS spectra of M-CDs displayed typical peaks at approximately 284.8, 399.5, and 530.6 eV, related to C 1s, N 1s, and O 1s, respectively. An additional peak at 167.4 eV in the R-CDs spectrum, corresponding to S 2p, (28,29) confirmed the successful incorporation of sulfur from the sulfuric acid solvent (Figure S3b).

Detailed information about the chemical states of the elements in M-CDs was further displayed. For all CDs, a characteristic peak was observed at 283.3 eV (C–C/C═C) (Figure 3a–c). For B- and YG-CDs, there are two peaks at 284.8 (C–N/C–O) and 286.8 eV (C═O), respectively (Figure 3a,b). For R-CDs, the C 1s spectrum exhibited peaks at 284.1 (C–N/C–O), 284.9 (C═O), and 288.1 eV (C–S) (Figure 3c). The N 1s spectra of B- and YG-CDs were decomposed into two peaks at 398.5 (pyridine N) and 399.7 eV (amide N), respectively. (30) These functional groups were observed at slightly different binding energies (397.3 and 398.7 eV) in R-CDs (Figure 3d–f). The O 1s spectra revealed peaks around 530.0 eV, associated with C═O bonds, while C–O bonding peaks were found at 531.2 eV (31) in YG- and R-CDs (Figure 3h,i) and at 532.1 eV in B-CDs (Figure 3g). The S═O peak at 532.6 eV in R-CDs further indicated sulfur incorporation. The S 2p spectra of R-CDs were deconvoluted into two peaks at 166.6 (C–S) and 167.9 eV (S═O) (Figure 3j). Altogether, the results were consistent with the FTIR spectra, corroborating the successful synthesis and functionalization of M-CDs.

Figure 3. (a–i) C 1s, N 1s, and O 1s XPS of B-, YG-, and R-CDs. (j) S 2p XPS spectra of R-CDs.

3.3. Optical Properties of M-CDs

B-, YG-, and R-CDs solutions appeared transparent in sunlight, exhibiting pale yellow, yellow, and pink, respectively. Under a UV lamp, they exhibited blue, yellow–green, and red fluorescences, respectively (Figure 4a).

Figure 4. (a) Pictures of M-CDs solution under daylight and UV light. (b) Absorption spectra of M-CDs.

To characterize the optical performance of M-CDs, UV absorption and fluorescence spectra were conducted. As shown in Figure 4b, B-, YG-, and R-CDs showed characteristic π → π* transitions at 200–250 nm. (32) Distinct UV–vis properties were observed due to variations in the surface chemical composition of the CDs. B-CDs exhibited significant UV absorption in the R-band at 267 and 273 nm, indicating the presence of n → π* transitions. Additionally, a shoulder peak at 287 nm suggested the existence of auxochromes with n electrons, (33) likely attributable to –NH₂ groups on the B-CDs surface. For YG-CDs, an absorption peak at 260 nm associated with the closed benzene ring structure on its surface, and B-band formation attributed to a π → π* transition was observed. (34) A peak at 427 nm indicated an n → π* transition containing a C═O bond. R-CDs showed absorption peaks at 284, 537, and 574 nm, with longer wavelengths corresponding to lower energy absorption bands, which were usually associated with the narrowing behavior of the electronic band gap, a feature observed in R-CDs. These absorption bands were derived from the π → π* transitions of the aromatic sp² hybridized structure (C═C/C–C) and n → π* transitions of C═O/C═N. (27)

Then, the PL spectra of the M-CDs were further examined. The maximum excitation wavelengths (λ_ex) for B-, YG-, and R-CDs were determined to be 370, 440, and 580 nm, respectively, with corresponding maximum emission wavelengths (λ_em) of 444, 537, and 598 nm (Figure 5a–c). All CDs exhibited single PL peaks with relatively narrow FWHM. Remarkably, as the emission shifted from B- to YG- and then to R-CDs, the FWHM decreased from 71 and 64 to 34 nm, indicating a progression toward sharper emission peaks. This narrow FWHM characteristic of CDs is advantageous for enhancing background-detected fluorescence contrast in bioimaging and the color saturation of luminescent devices. (35) Further analysis involved correlating their emission spectra with CIE coordinates, yielding values of (0.1495, 0.1031) for B-CDs, (0.2946, 0.5844) for YG-CDs, and (0.5891, 0.3943) for R-CDs (Figure 5d–f).

Figure 5. (a–c) PL spectra of M-CDs. (d–f) CIE coordinates of M-CDs. (g–i) 3D fluorescence matrices of M-CDs.

Three-dimensional excitation–emission fluorescence matrices of M-CDs were also plotted to elucidate the relationship between excitation and emission. The emission centers were clearly exhibited at (370, 444, 770), (440, 537, 849), and (580, 598, 813) for B-, YG-, and R-CDs, respectively (Figure 5g–i). Notably, B-CDs showed excitation wavelength dependence, with λ_ex ranging from 340 to 400 nm and λ_em red shifting from 444 to 447 nm. In contrast, YG-CDs exhibited λ_ex shifting from 380 to 460 nm without notable changes in the λ_em of 537 nm, indicating excitation wavelength independence. Similarly, R-CDs maintained predominantly λ_em at 598 nm when the excitation wavelength was from 500 to 590 nm, demonstrating excitation wavelength independence. The normalized spectral date of M-CDs further highlighted these characteristic fluorescence properties (Figure S4a–c).

The fluorescence characteristics of CDs are generally believed to be closely related to their surface states, since the surface functional groups mainly trap the excitons under excitation. (36) The radiative recombination of surface trap excitons imparts CDs with unique luminescent properties. The excitation wavelength dependence observed in B-CDs suggested that heterogeneous surface states provided various energy levels for photoluminescence with different fluorescence centers potentially arising from the defect states of functional groups. Conversely, the nonexcitation-dependent photoluminescence observed in YG- and R-CDs likely corresponds to the homogeneous surface states and their highly ordered graphite structures. (31)

Subsequently, the stability of the M-CDs was evaluated. All intensities slightly decreased after 90 min of continuous exposure to UV, indicating their excellent photostability and resistance to photobleaching (Figure S5a–c). Additionally, these CDs maintained their fluorescence intensity after 30 days of storage, demonstrating excellent temporal stability and suitability for long-term use in experimental studies and practical applications (Figure S5d–f).

3.4. Possible Luminescent Mechanisms of M-CDs

The luminescence mechanism of CDs is predominantly influenced by two key factors: the quantum size effect (carbon core-dominated luminescence) and surface state-derived luminescence (surface defects). Previous studies have established that short-wavelength emissions (blue and green) of CDs are primarily associated with the carbon core, whereas long-wavelength emissions (yellow, orange, and red) of CDs are closely linked to the surface states, particularly involving nitrogen-containing species. (37) N atoms are widely used as dopant atoms to regulate the optical and electronic properties of CDs. (38) Choose precursors with high N elements, such as amino acids, phenylenediamine, and their derivatives, to successfully achieve N doping.

The l-trp and o-PD selected in this study belong to aromatic amino acids and isomers of phenylenediamine, respectively. Their benzene ring structure provides a conjugated skeleton that makes it possible to form large conjugated planes, thereby affecting the emission wavelength and QYs of CDs. (39) So, when l-trp and o-PD are used as carbon and nitrogen sources in the reaction process, they not only form carbon nuclei with hybrid atoms but also introduce rich polar groups such as carboxyl and amino groups on their surfaces. The fluorescence performance of CDs will be controlled by functional groups, as the functional groups cling to the edges of the carbon core or interact with solvent molecules to create distinct surface states. The amino and carboxyl groups of three CDs underwent decomposition under high temperature and high pressure and combined with the amine group with high catalytic activity to produce a rigid and large sp² conjugated structure, with its edges connected to C═O, –OH, and –NH₂. Compared with B-CDs and YG-CDs, the addition of concentrated sulfuric acid for R-CDs could accelerate the oxidation and dehydration of the precursor and lead to the expansion of the conjugated system.

To elucidate the luminescence mechanisms of M-CDs, the fluorescence decay curves were first plotted, using biexponential fitting (Figure 6a–c). The decay process comprised short-lived components (τ₁) and long-lived components (τ₂), corresponding to the recombination of the carbon core and the surface states, respectively. (40) The average fluorescence lifetimes (τ_avg) were determined to be 4.42, 3.48, and 2.25 ns for the B-, YG-, and R-CDs, respectively. These findings indicated that the fluorescence of B- and YG-CDs predominantly originated from the emission of the nuclear state, while the increased proportion of τ₂ in R-CDs suggested a significant role of the surface state in red emission (Table S2).

Figure 6. Fluorescence lifetime for (a) B-, (b) YG-, and (c) R-CDs.

Furthermore, we analyzed the content of elements and chemical bonds of M-CDs synthesized by using different solvents to understand their tunable fluorescence emission. XPS revealed that R-CDs had a lower carbon content and higher oxygen content than B- and YG-CDs (Figure S6). This led to less core carbonization and more surface oxidation, consistent with the observed decrease in the sp² carbon content. Additionally, the C═O content of R-CDs was the highest among M-CDs, further demonstrating that a decrease in the graphitization of the surface structure led to an increase in surface defect sites (15) (Table S3). It was not difficult to observe that the pyridine N content in YG- and R-CDs was remarkably lower compared with B-CDs. Conversely, the amino nitrogen content gradually increased (Table S4). An increase in pyridine N is known to contribute to exciton capture and surface state changes, resulting in the blue shift in fluorescence emission. (41) So, the decrease of pyridine nitrogen and the increase of amino nitrogen were the two primary factors leading to the red shift in fluorescence emission.

Various forms of doping, including graphite N, pyridine N, amine, C═O, C–O, and C–S–C, could generate abundant defect sites in CDs, preventing the electrons from delocalizing around the entire CDs and inducing multicolor emission. (42) When compared to the use of acetic acid and water as solvents, the incorporation of sulfur in sulfuric acid-synthesized R-CDs generated C–S bonds, creating new defect sites and resulting in new light emission, specifically exhibiting a fluorescence red shift (Table S3).

Previous results indicated that differences in the hybridization degree of sp² and sp³ carbon affected the photoluminescence properties of CDs, enabling effective adjustment of fluorescence emission color. (43,44) In our research, Raman spectroscopy was further used to analyze the structure and crystallinity of M-CDs (Figure S7). The characteristic peaks around 1380 and 1581 cm^–1, representing the disordered sp³ carbon (D-band) and graphitized sp² carbon (G-band), (45) respectively, were observed in all CDs. The intensity ratio of the two bands (I_D/I_G) serves as an important surface analysis technique, convenient for evaluating the surface disorder and defect structure of carbon materials. (46,47) The I_D/I_G ratios were determined to be 0.60, 0.80, and 0.77 for B-, YG-, and R-CDs, respectively, indicating that YG- and R-CDs had lower sp² conjugated ordered carbon content and higher density of surface oxidation defects during solvent thermal carbonization. An increase in the number of defects was caused by the increase in amino nitrogen embedded in the sp² scaffold structure. This characterization aligned with the XPS analysis results, confirming the impact of surface groups on fluorescence properties.

To examine whether CDs synthesized with different solvents affected the band gap widths, solid-state UV absorption of CDs was characterized. The band gap energy of CDs is calculated by Eg(opt) = 1240/λ_edge, where λ_edge is the wavelength of the maximum absorption edge. (48) The calculated values of B-, YG-, and R-CDs were 2.21, 2.00, and 1.41 eV, respectively (Figure S8a–c). These values demonstrated that variations in surface functional groups affected the band gap of CDs. The particle size of M-CDs increased sequentially with the emission wavelength red-shifted, and the band gap between highest occupied molecular orbital and lowest occupied molecular orbital gradually decreased (Figure 7). This size-dependent band gap energy suggested that the quantum size was also a contributor to the red shift in fluorescence emission from CDs.

Figure 7. Diagram of tunable emission and energy levels of M-CDs.

The luminescence mechanism of M-CDs was analyzed in terms of electron leap energy levels. The carbon core primarily consisted of carbon atoms with different hybrid structures, while the surface states of CDs were mainly derived from oxygen-containing functional groups. The characterization of the microstructure, composition, and fluorescence properties of M-CDs confirmed that the red shift in the fluorescence emission was consistent with the increase in particle size and oxygen content. Therefore, the multicolor emission of M-CDs was largely influenced by both surface states and the quantum size effect. By modifying the reaction solvent, the formation structure of CDs could be regulated, achieve multilevel band gap adjustment, and promote the emission of multicolor fluorescent CDs.

In conclusion, M-CDs were synthesized using different solvents that led to variations in surface functional groups, which in turn affected the band gap and fluorescence properties of CDs. The observed red shift of the emission peak position, along with the increase in particle size and oxygen content, underscored the importance of surface states and quantum size effects in determining the emission characteristics of M-CDs. By carefully selecting reaction conditions and solvents, their optical behavior can be fine-tuned, enabling application in diverse fields such as bioimaging and optoelectronics.

3.5. Application of M-CDs

Solid-state photoluminescent materials are highly desirable for various applications, yet achieving single-component solid-state luminescent CDs through a one-step method presents significant challenges. Typically, such CDs suffered from severe aggregation-induced quenching, which diminished their luminescent properties. (49) To mitigate this issue, researchers often combined CDs with matrix materials to maintain a certain distance that inhibits self-quenching. In this study, PVA was chosen as the substrate for its superior transparency, tensile strength, and easy processability.

The solutions of M-CDs were mixed with PVA in the appropriate proportions to prepare composite fluorescent films. To our delight, these films remained clear and transparent under sunlight and exhibited bright fluorescent colors under ultraviolet light (Figure 8). This indicated that the PVA chains effectively prevent the stacking of π–π structures of the carbon nuclei, thereby resisting self-quenching in the aggregated state. (50) The polymer chains also hindered the aggregation of carbon nanoparticles while firmly fixing CDs and maintaining their stable fluorescence characteristics within the polymer matrix.

Figure 8. Photos of M-CDs films under daylight (top) and ultraviolet light (bottom).

This advantageous property allowed us to successfully create monochromatic optical devices by coating the CDs mixture on a 365 nm ultraviolet chip. As shown in Figure 9a, the three LEDs exhibited blue, yellow-green, and red, and relevant CIE coordinates were (0.21, 0.23), (0.33, 0.39), and (0.52, 0.46), respectively (Figure 9b). With the increase of the working current (from 60 to 700 mA), the electroluminescence spectra of the three CDs-LEDs remain unchanged but only an increase in the intensity (Figure S9). The color rendering index of B-, YG-, and R-CDs-based LEDs was 86.4, 90.9, and 60, respectively. The resultant CDs-based LEDs demonstrated significant potentiality in multicolor luminescence field.

Figure 9. (a) LED optical pictures of M-CDs. (b) CIE color coordinates of the M-CDs-based LEDs.

4. Conclusions

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In conclusion, this study successfully developed tunable fluorescent emission CDs with bright and stable luminescent properties by manipulating the dehydration and carbonization process of precursors through solvent control (acetic acid, water, and sulfuric acid). A comprehensive series of explorations proved that the choice of solvent significantly influenced the particle size of M-CDs and the nitrogen and oxygen doping content, which further influenced the band gap and shift in the emission position. Using PVA as a solid-state luminescent substrate, it was mixed with M-CDs to create clear and transparent tricolor solid-state lighting films. Ultimately, bright monochromatic LEDs were obtained with CIE coordinates of (0.21, 0.23), (0.33, 0.39), and (0.52, 0.46), respectively. This work contributed to the advancement of carbon-based materials for optical devices by developing tunable emission CDs with a superior optical performance.