Synthesis of Red, Green, and Blue Carbon Quantum Dots and Construction of Multicolor Cellulose‐Based Light‐Emitting Diodes

Light‐emitting diodes (LEDs) are widely used in lighting and display applications. Carbon quantum dots (CQDs), which have high biocompatibility, high resistance to photobleaching, and full‐spectrum luminescence, have inherent advantages as fluorescent materials for LED devices. Herein, multicolor CQDs are prepared by a new reagent engineering strategy due to the difference of effective conjugate length and the surface electron‐withdrawing groups of CQDs. White CQDs are realized by mixing blue, green, and red CQDs proportionally. Then, the aggregation‐caused quenching phenomenon of CQDs is suppressed through the hydrogen‐bonding network of cellulose nanofibrils (CNFs). Multicolor fluorescent films are prepared from CQDs and CNFs by simple mixing and casting methods. Finally, thin‐film encapsulation based on the photosensitive resin ABPE‐10 coating can be realized and rapidly assembles into fluorescent films with different light‐emitting colors into LED devices, leading to have superior thermal performance compared with conventional LEDs. White LEDs have excellent white‐light illumination performance, with Commission Internationale de L’Eclairage color coordinates of (0.33, 0.37), a correlated color temperature of 5688 K, and a color rendering index of 86. This strategy provides a convenient and scalable pathway for low‐cost, environmentally friendly, and high‐performance CQDs‐based LEDs.


Introduction
Eco-friendly, power-saving, and extremely stable solid-state lighting and displays are consistent with green and sustainability objectives.White light-emitting diodes (W-LEDs) are characterized by high energy efficiency, long service life, fast response time, high reliability, small size, and simple structure, which make it promising for the next generation of ideal illumination and display equipment. [1]An ideal W-LED usually consists of a ultraviolet light-emitting diode (UV-LED) chip, a three-color system, and encapsulation materials, and this combination improves the color rendering index (CRI) and chromaticity stability of the W-LED. [2]The technology of the UV-LED chip is relatively mature and static.Tri-color systems, such as red, blue, and green, are usually constructed by mixing appropriate proportions of tri-color fluorescent materials, which are an important part of the color conversion of Light-emitting diodes (LEDs) are widely used in lighting and display applications.Carbon quantum dots (CQDs), which have high biocompatibility, high resistance to photobleaching, and full-spectrum luminescence, have inherent advantages as fluorescent materials for LED devices.Herein, multicolor CQDs are prepared by a new reagent engineering strategy due to the difference of effective conjugate length and the surface electron-withdrawing groups of CQDs.White CQDs are realized by mixing blue, green, and red CQDs proportionally.Then, the aggregation-caused quenching phenomenon of CQDs is suppressed through the hydrogen-bonding network of cellulose nanofibrils (CNFs).Multicolor fluorescent films are prepared from CQDs and CNFs by simple mixing and casting methods.Finally, thin-film encapsulation based on the photosensitive resin ABPE-10 coating can be realized and rapidly assembles into fluorescent films with different light-emitting colors into LED devices, leading to have superior thermal performance compared with conventional LEDs.White LEDs have excellent white-light illumination performance, with Commission Internationale de L'Eclairage color coordinates of (0.33, 0.37), a correlated color temperature of 5688 K, and a color rendering index of 86.This strategy provides a convenient and scalable pathway for low-cost, environmentally friendly, and high-performance CQDs-based LEDs.many LEDs.Semiconductor quantum dots and rare earth materials are commonly used as fluorescent materials, but they suffer from difficulties in synthesis, low stability, and high toxicity. [3]herefore, it is important to find alternative materials with nontoxic, environmentally friendly, low-cost, and suitable optical properties.Meanwhile, the traditional dispensing encapsulation method in the LED preparation process has some problems, such as easy precipitation of fluorescent materials, low phosphor conversion efficiency, and poor heat dissipation. [4]s a new type of carbon material, carbon quantum dots (CQDs) show unique photoluminescence (PL) properties and are characterized by low toxicity, high stability, tunable emission, easy functionalization due to OH and COOH groups on the surface of CQDs, and finally their low cost, [5] which gives CQDs great advantages and future prospects for application in the field of W-LEDs.However, a major difficulty is the full-spectrum tunability of the emission and the singularity of the excitation wavelength.Currently, most CQDs can only produce blue and green emission, and it is difficult to realize red or even longer wavelengths and multicolor emission with single wavelength excitation.If CQDs can be synthesized in a scalable manner and fluorescence can cover the entire visible spectrum, they can be excellent color conversion materials for all emission types of LEDs.The nature of the solvent during CQD synthesis has been reported to severely affect the degree of dehydration and carbonization of the precursor, with water as the most prevalent medium, and most CQDs prepared in aqueous solutions show only blue emission due to their high polarity. [6]6a,7] In addition, some hydrothermal treatments under acidic conditions can also lead to a redshift of the emission by increasing the degree of carbonation and expanding the conjugated sp 2 structure of CQDs. [8]Therefore, it is crucial to choose the right precursors, solvents, and synthesis techniques to design simple and controllable synthesis strategies for CQDs with tunable PL (especially emitting long wavelength fluorescence), as well as to clarify the relationship between their structure and PL properties.However, due to the inherent self-aggregation of nanoscale CQDs (size < 10 nm), CQDs have a pronounced aggregation-caused quenching (ACQ) phenomenon; [9] at high concentrations and in the solid state, their fluorescence emission is easily quenched, which greatly limits their application in solidstate emission.
Meanwhile, the encapsulation materials and methods are crucial to the performance of LEDs.Epoxy resins, silicone, and polyvinyl alcohol are usually used as encapsulation materials due to their high transparency and appropriate mechanical strength.However, these materials are difficult to degrade, have high coefficients of thermal expansion, and have the tendency to turn yellow during thermal processing.These properties generate large amounts of e-waste due to the rapid turnover and wear of electronic products, leading to the need for new green alternative encapsulation materials for LEDs.In addition, the traditional dispensing method of encapsulation has some problems, such as ease of precipitation of fluorescent materials, low phosphor conversion efficiency, and poor heat dissipation. [4]herefore, the thin-film encapsulation method of remote fluorescence has been proposed by researchers. [2,10]10a] Cellulose, a linear polymer made of D-glucose linked by β-1-4 glycosidic bonds, is the most abundant naturally occurring polymer on earth and is renewable, fully biodegradable, and has excellent biocompatibility and anisotropic thermal conductivity. [11]The cellulose backbone has many regularly distributed hydroxyl groups, which can easily construct hydrogen-bonding networks.Cellulose nanofibrils (CNFs) developed from cellulose are widely used in the preparation of transparent film materials due to their unique aspect ratio, nanoscale size, and abundant surface hydroxyl groups.Therefore, a CNF film is used to replace the commonly used nondegradable encapsulation materials, but the poor water barrier property of CNF film limits its application.However, improving the water resistance of CNFs by simple process control and at the same time realizing the preparation of solid-state fluorescent CNF films is of vital practical significance for the development of large-scale LED devices.
The acrylic resin ABPE-10 is a transparent, colorless, and amphiphilic light-curing resin with a phenyl and carboxylic ester structure.The simple coating on the surface of a CNF film enables the oxygen atom in the carboxylic acid ester structure of ABPE-10 to form a hydrogen bond network with the hydroxyl groups of CNF, and the phenyl and carboxylate esters of ABPE-10 are arranged on the surface of the material to improve the hydrophobicity and water barrier properties of the composite material after light curing. [12]Moreover, due to the fast UV light-curing properties of ABPE-10 (≈3-5 min), fast thin-film encapsulation at room temperature can be achieved by coating ABPE-10 on the surface of CNF film when preparing LED devices, which is ideal for mass production.
In this work, a new reagent engineering strategy was reported to obtain CQDs with significantly stable and tunable fluorescence emission, emitting from blue to red, using aminophenol as a precursor.Our study demonstrates that the particle size and surface functional groups of CQDs can be easily adjusted by controlling the reaction reagents, thus regulating the fluorescence wavelength.To fundamentally understand the mechanism of the influence of these reagents, we analyzed the trends in the particle size of CQDs as well as the surface electron-donating and electron-absorbing groups.These studies demonstrated that the emission wavelength of the CQDs was positively correlated to the size of the particles and the number of electron-withdrawing groups.The effectiveness of effective conjugation length on the positive correlation of emission wavelength was also elucidated.Then, multicolor and superior fluorescent composite films were prepared by simple mixing and casting methods based on the hydrogen bonding between CQDs and CNFs.Multicolor LED devices were assembled into a UV-LED chip, three-color fluorescent composite CNF films, and encapsulation of these materials by coating ABPE-10 on the surface of fluorescent CNF films, which realized thin-film encapsulation, provided excellent processability, and mechanical strength, anisotropic thermal conductivity, water resistance, and degradability of LEDs.This work provides a novel, versatile, and convenient technique for the synthesis of high-performance multicolor luminescent CQDs and offers great potential for the development of high heat dissipation and environmentally friendly LED devices.

Morphological and Structural Characterizations of CQDs
In this work, we demonstrated a reagent engineering approach to synthesize three typical CQDs with red, green, and blue fluorescence, denoted as R-CQDs, G-CQDs, and B-CQDs according to their luminescence colors, by solvent heat treatment of aminophenol in ethanol solvent and different reagents (Scheme 1, see Experimental section for more details).The yields of B-CQDs, G-CQDs, and R-CQDs are 60.47%, 21.30%, and 3.55%, respectively (Table S1, Supporting Information).The microscopic morphologies of the three CQDs were observed by transmission electron microscopy (TEM) and atomic force microscopy (AFM).The TEM images show three CQDs all exhibit monodisperse and uniform spherical-like nanoparticles (Figure 1a-c).For B-CQDs, G-CQDs, and R-CQDs, their average particle sizes are 2.29, 3.56, and 4.31 nm, respectively.The highresolution TEM images show three CQDs have a highly crystallized graphitic carbon-like structure and similar high-resolution lattice fringes with a lattice spacing of 0.21 nm corresponding to the (100) graphite plane (Figure 1d-f ). [13]AFM images also illustrate that these CQDs are monodisperse and uniform spherical particles (Figure 1g-i).For B-CQDs, G-CQDs, and R-CQDs, their heights are ≈2-3, 2-4, and 4-6 nm, respectively.These results suggest that the emission wavelengths are redshifted due to the quantum confinement effect as the particle size of the CQDs increases.
The composition and structure of CQDs were determined by X-Ray diffraction (XRD), Raman spectra, X-Ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR), and nuclear magnetic resonance (NMR).The XRD spectra of the three CQDs exhibit a distinct broad peak focused at 24°and a weak peak centered at 41°, attributed to the (002) and (100) graphitized structures with interlayer spacings of 0.34 and 0.21 nm (Figure 3a). [14]The peak at 24°is the (002) characteristic diffraction peak of graphite, suggesting that the interlayer stacking of CQDs (0.34 nm) is very close to that of graphite (≈ 0.35 nm); [15] this confirms that the highly conjugated sp 2 domains are tightly packed (0.35 nm) in the core part of CQDs and dominate.The peak at 41°corresponds to the (100) characteristic diffraction peak of graphite, which is consistent with the TEM results.Their graphitic structures are reflected in the Raman spectra.The intensity ratios of the crystalline G-band at 1572 cm À1 and the disordered D-band at 1343 cm À1 (I G /I D ) are 1.06, 1.14, and 1.16, respectively (Figure 3b).The increase in the ratio implies enhanced graphitization degree from B-CQDs to R-CQDs, indicating a gradual increase in the size of the sp 2 domains, which is consistent with the above TEM results.These results further indicate that CQDs have a highly crystalline graphene-like core structure.In addition, the emission wavelength is red-shifted as the sp 2 domain of CQDs increases.
XPS was utilized to analyze the surface structure of CQDs.The full XPS spectra of all three CQDs show three characteristic peaks, including C 1 s (285 eV), N 1 s (400 eV), and O 1 s (532 eV), suggesting that they are comprised of C, N, and O elements (Figure 2a,e,i).For the B-CQDs and G-CQDs to R-CQDs, the ratio of N/C is decreased from 0.23 and 0.03 to 0.02, and the ratio of O/C is increased from 0.19 and 0.28 to 0.26, respectively (Table S2, Supporting Information).The high-resolution C 1 s XPS spectra of three CQDs can be fitted to the three C species, sp 2 C (C═C/C─C) at 284.63 eV, sp 3 C (C─O/C─N) at 285.74 eV, and C═O/C═N at 288.61 eV (Figure 22b,f,j). [16]The percentages of C─C/C═C:C─O/C─N: C═O/C═N content of B-CQDs, G-CQDs to R-CQDs are 45.58:49.17:5.25,84.40:10.46:5.15,and 84.67:10.24:5.10,respectively, which indicate that the effective degree of conjugation of G-CQDs and R-CQDs is larger than that of B-CQDs (Table S3, Supporting Information).Meanwhile, the effective conjugation length of R-CQDs is larger than that of G-CQDs because the particle size of R-CQDs is larger than that of G-CQDs.This clearly shows that an increase in the effective conjugation length induces a redshift in the emission wavelength of CQDs.11b,17] From B-CQDs and G-CQDs to R-CQDs, the C─O content is decreased from 36.04% to 22.68%, and the number of C═O functional groups is increased significantly from 63.96% to 77.32% (Table S4, Supporting Information).The high-resolution N 1 s XPS spectra of three CQDs show two peaks corresponding to C─N═C (399.19 eV) and N─H (400.22 eV) (Figure 2d,h,l). [18]The C═N content increased from 41.62% (B-CQDs) to 72.66% (R-CQDs) and the N─H content decreased from 58.38% (B-CQDs) to 27.34% (R-CQDs) (Table S5, Supporting Information).These results indicate that O and N groups are important functional groups that induce luminescence.From B-CQDs and G-CQDs to R-CQDs, the content of electron-donating groups ─OH and ─NH 2 decreases, while the number of electron-withdrawing C═O and C═N significantly increases, reflecting that more electron-withdrawing groups originating from doping reagents grafted on the surfaces of these CQDs can increase the redshift of the emission wavelengths of CQDs.
The FT-IR spectra of the three CQDs show similar characteristic peaks at 3000-3500, 1606, 1412, 1203, and 1010 cm À1 corresponding to the stretching vibrations of the O─H/N─H, C═O/C═N, C═C, C─N, and C─O functional groups, respectively. [19]The results indicate the presence of conjugated aromatic domain structures as well as O─ and N-containing functional groups (Figure 3c).These results demonstrate that the three CQDs are co-functionalized by electron-donating groups (e.g., ─OH and ─NH 2 ) and electron-withdrawing groups (C═O and C═N) from aminophenol and doping reagents.NMR spectra ( 1 H and 13 C) were used to investigate the functional groups of the CQDs (Figure 3d,e).In the 1 H NMR spectra, the observed signals at 0.8-1.8ppm belong to the alkane H (─CH 2 ─ and ─CH 3 ), and the peaks at 1.90-2.45ppm correspond to H on the C adjacent to C═O. [18] The signal at 3.7-4.5 ppm corresponds to amino H, the peak centered at 5.03 ppm is attributed to hydroxyl H, and the signal at 9-12 ppm is attributed to carboxy/carbonyl H. [20] These nonaromatic hydrogens are located on the surface and edges of CQDs.Aromatic H signals were detected at 6.0-8.3 ppm, which can be attributed to the plasmonic resonance of the graphitized core. [18,21]In the 13 [2,18,21] The results show that CQDs have a two-part composition of graphitized carbon cores and surface states.

Optical Properties of CQDs
Figure 4 shows the results of the optical characterization of the three CQDs.For B-CQDs, the emission wavelength shifts from 380 to 550 nm when the excitation wavelength is varied from 300 to 470 nm (Figure 4a), indicating the excitation wavelength dependence of B-CQDs.The excitation wavelengths of G-CQDs are mainly in the range of 350 to 490 nm, and the emission wavelengths are ≈510 nm (Figure 4b), indicating that G-CQDs exhibit excitation-independent emission.For R-CQDs, there is a broad emission range in the excitation wavelength range of 420 to 550 nm, where the emission is centered at ≈605 nm (Figure 4c).These results suggest that G-CQDs and R-CQDs have excitation-independent emission.Figure 4d-f shows the results of UV-vis absorption spectra, PL emission, and photoluminescence excitation (PLE) spectra of the three CQDs tested along the dashed lines in the corresponding excitation-emission matrices of Figure 4a-c.1a] The broad absorption peaks of CQDs at greater than 300 nm are due to the n-π* transition in the oxygen/nitrogen functional groups and sp 2 structural domains of CQDs.B-CQDs show an exciton absorption band at 345 nm, while the optimal emission wavelength of B-CQDs is near 460 nm, which corresponds to an excitation wavelength of 390 nm.The PLE of the B-CQDs is near to the absorption band at 345 nm, which indicates that UV light can excite the blue emission of B-CQDs.G-CQDs have two absorption bands at 394 and 442 nm, and the PLE spectrum corresponding to the 510 nm emission wavelength covers the absorption bands at 394 and 442 nm, which implies that UV light can excite the green emission of G-CQDs.R-CQD exhibits two absorption peaks at 381 and 533 nm, and the PLE spectrum corresponding to the 605 nm emission covers two absorption bands at 381 and 533 nm, indicating that UV light can excite the red emission of R-CQDs.Thus, B-CQDs, G-CQDs, and R-CQDs can be excited by UV light and produce blue, green, and red fluorescence, respectively.As shown in Figure S1 and Table S1, Supporting Information, the absolute photoluminescence quantum yield (PLQY) values of B-CQDs, G-CQDs, and R-CQDs are 62.79%, 44.65%, and 20.88%, respectively, which are comparable to the PLQY values previously reported for blue, green, and red fluorescent CQDs. [22]The peak wavelength of the PL excitation spectra of the three CQDs coincides with the peak wavelengths of the corresponding exciton absorptions, suggesting that their emission is mainly assigned to the n-π* transition of the aromatic sp 2 structural domains containing C═O and C═N bonds [23] and that the emission of the three CQDs originates from band-edge exciton state decay rather than defect state decay. [24]It is worth noting that the self-absorption of all three CQDs is relatively small because the overlap between the exciton absorption and emission spectra is small, which is favorable for efficient fluorescence emission. [2]The gradual increase in the size of the CQDs from 2.29 to 4.31 nm is consistent with the increase in the corresponding PL wavelengths and first exciton absorption bands (Figure 5a), elucidating that quantum confinement effects lead to the redshift of the absorption and emission spectra.The number of electron-withdrawing groups of CQDs is positively correlated to the PL wavelength and the absorption band of the first exciton (Figure 5b), suggesting that electronwithdrawing groups also lead to the redshift of the absorption and emission spectra.
The fluorescence mechanism of CQDs was further explored, and the fluorescence lifetime of CQDs was tested by timeresolved fluorescence spectrometry.20a] The average lifetimes of B-CQDs, G-CQDs, and R-CQDs are 4.16, 5.28, and 4.83 ns, respectively (Figure S2a-c and Table S6, Supporting Information).The weight of τ 1 is greater than that of τ 2 for B-CQDs and R-CQDs, indicating that their fluorescence is dominated by the carbon-core state, and the opposite is true for G-CQDs, whose fluorescence is dominated by the surface state.Meanwhile, the changes in τ 1 and τ 2 weights indicate that the carbon core and surface state play a joint role in the emission redshift.The temperature (298-498 K)-dependent fluorescence spectra of CQDs were characterized use variable-temperature PL spectroscopy (Figure 4g-i).For all CQDs, the PL intensity decreases with increasing temperature due to the increasing probability of nonradiative recombination.The emission peaks of each CQDs decay at a specific rate, indicating that they have different PL centers (Figure 4j-l).The PL spectra show a slight redshift with increasing temperature, which is ascribed to the decrease in the bandgap of CQDs with increasing temperature. [25]In addition, these CQDs maintain long-term photostability (>10 h) under UV irradiation (Figure S2d-f, Supporting Information), which is beneficial to CQD-based long-life technical devices.These CQDs will be universally applicable to advanced LEDs due to their uniquely tunable fluorescence characteristics, as well as excellent light and thermal stability.
The bandgap energies of CQDs were calculated by the equation E g opt = 1240/λ edge , where λ edge is the wavelength of the maximum absorption edge.The bandgap energies of the B-CQDs, G-CQDs, and R-CQDs are 2.90, 2.41, and 1.97 eV, respectively (Table S7, Supporting Information).With the gradual decrease in the bandgap of CQDs from 2.90 to 1.97 eV, the exciton emission peak of CQDs is redshifted from 460 to 605 nm, and the size of CQDs increases from 2.29 to 4.31 nm, further demonstrating the quantum confinement effect of CQDs (Figure S3, Supporting Information).To further determine the energy band structure of the three CQDs, the highest occupied molecular orbital (HOMO) was determined by UV photoelectron spectroscopy (UPS), and the lowest unoccupied molecular orbital (LUMO) was obtained by calculations based on the bandgap energies and the HOMO (Figure S4a-c and Table S7, Supporting Information).From B-CQDs and G-CQDs to R-CQDs, there is a similar upward trend in the energy levels of CQDs, from 5.76 to 6.21 eV for HOMO and from 2.86 to 4.24 eV for LUMO, which directly elucidates the interband transitions of the CQDs (Figure 5c, S4d and Table S7, Supporting Information). [26]

Characteristics of Fluorescent CNPs
In order to manufacture white LED (W-LED), three types of CQDs were mixed in different ratios to obtain white CQD (W-CQDs).Among all mass ratios, the W-CQDs have the widest emission spectral range and the greatest fluorescence intensity when the mass ratio of B-CQDs, G-CQDs and R-CQDs is 5:1:5 (Figure S5, Supporting Information).Generally, there is no fluorescence emission from CQDs in the solid state, which is due to the carbon core centers of CQDs being mainly π-π conjugated structures, leading to agglomeration and π-π stacking in the solid state and the creation of the ACQ phenomenon.To inhibit the ACQ of CQDs, a method was constructed to disperse and immobilize CQDs through hydrogen-bonding interactions between CQDs and CNFs in this study.CQDs and CNFs were mixed uniformly in a certain ratio, and then four-color fluorescent films, B-CNP, G-CNP, R-CNP, and W-CNP, were prepared by the casting method (Scheme 1).The UV-Vis absorption spectra show that the absorption peaks of B-CNP, G-CNP, and R-CNP are consistent with those of the corresponding CQDs solutions, indicating that the CQDs are immobilized in the membranes by the hydrogen bonding in CNF (Figure 6a).Under 365 nm UV light, B-CNP, G-CNP, R-CNP, and W-CNP showed uniform emission in blue, green, red, and white (Figure 6b).The absolute PLQYs of B-CNP, G-CNP, and R-CNP are 10.30%, 5.05%, and 2.21%, respectively, which show good emission properties (Table S8, Supporting Information).The PL emission spectra of B-CNP, G-CNP, and R-CNP are shown in Figure 6c, and the emission peak positions of B-CNP, G-CNP, and R-CNP are consistent with those of the corresponding CQDs solutions, which demonstrate that the CQDs are successfully dispersed by the CNF network and inhibited the ACQ.These results indicate that the chemical structure of CQDs in CNP is unchanged compared to the corresponding CQDs solution.Compared with the pure CNF film, the XRD patterns of the CNPs show graphite (002) plane diffraction peaks, which proves that the CQDs are successfully immobilized by the CNF hydrogen bonding network (Figure 6d).In addition, mapping confirms that the elements of C and O are uniformly distributed in CNPs, indicating that CQDs are well dispersed in CNF without aggregation (Figure S6, Supporting Information).The transmittance and haze of the four CNPs were measured in the range of 400-800 nm.As shown in Figure 6e,f, all four CNPs exhibit high optical properties, with transparency of 84.04%, 68.59%, 66.27%, and 57.15% and haze of 59.46%, 68.73%, 65.80%, and 62.99% for CNP, B-CNP, G-CNP, and R-CNP, respectively, at 600 nm.These results indicate that the introduction of CQDs decreases the transparency of CNP and increases the haze of CNP, especially the CNP combined with CQDs shows a large haze covering the entire visible spectrum.Due to their high transmittance and haze, CNPs can be used promisingly for certain light management applications.Moreover, the PL spectrum of W-CNP extends into most of the visible light region (Figure 6c).The mechanical properties and water resistance of fluorescent CNPs are important indices of LED packaging materials.The stress-strain curves, tensile strength, Young's modulus, and elongation at break of CNP are shown in Figure 6g and Table S9, Supporting Information.The tensile strength, Young's modulus, and elongation at break of CNP are 62.40 AE 1.32 MPa, 4.92 AE 1.02 GPa, and 5.97 AE 0.64%, respectively.Compared with CNP, the tensile strength and elongation at break of B-CNP, G-CNP, R-CNP, and W-CNP decrease slightly, while the Young's modulus increases slightly, which is attributed to the fact that the sp 2 structure of the CQDs disrupts the hydrogen bonding between the CNFs, but the hardness of the composites is increased.The tensile strength, Young's modulus, and elongation at break of CNP-Co are 57.00AE 2.81 MPa, 5.84 AE 0.58 GPa, and 8.60 AE 0.81%, respectively (Figure 6g and Table S9, Supporting Information).Compared with CNP, the tensile strength of CNP-Co slightly decreases, while the elongation at break increases, indicating that the coating of ABPE-10 has little effect for the mechanical properties of CNPs.Moreover, the water contact angle of CNP-Co increases from 68.0°to 105.6°after coating with ABPE-10 resin, and the water contact angle of the fluorescent CNPs is similar to that of CNP-Co (Figure 6h).In addition, since ABPE-10 is a photosensitive resin, room temperature curing is possible.Therefore, surface coating with ABPE-10 resin is an effective and simple technique to improve CNPs performance, realize thin film encapsulation, and assist in the rapid assembly of LEDs.

Performance of LEDs
A 365 nm UV-LED chip was chosen as the excitation light source of the LED device.The four-color LED devices (B-LED, G-LED, R-LED, and W-LED) were obtained by light-curing fast thin-film encapsulation technology, and their structures are shown in Figure S7, Supporting Information.The B-LED, G-LED, and R-LED show uniform blue, green, and red emission in the on-state, respectively, and the electroluminescence (EL) emission peaks are essentially the same as those of the corresponding CQDs and CNPs, with CIE coordinates of (0.17, 0.18), (0.28, 0.45), and (0.51, 0.43), respectively, (Figure 7a-c).The W-LED exhibits excellent white light illumination performance with Commission Internationale de L'Eclairage (CIE) color coordinates of (0.33, 0.37), correlated color temperature (CCT) of 5688 K, and CRI of 86 (Figure 7d), which are comparable to the W-LED reported in recent years, [6a,8b] especially the CRI of 86, which is higher than the standard for indoor lighting (>80). [27]The EL emission spectra of W-LED cover the entire visible region from 400 to 750 nm, which is comparable to sunlight (Figure 7d).When W-LED illuminates the color card stock under dark conditions, the color card stock color is clearly discernible and shows the true color of the object well, reflecting the high color rendering of W-LED (Figure 7e).In addition, the emission spectra of W-LEDs do not change significantly after 48 h of continuous operation, proving that W-LEDs have good optical stability (Figure S8, Supporting Information).The luminous efficiency of B-LED, G-LED, R-LED, and W-LED was further measured.The luminous efficiency is 1.98, 2.35, 1.66, and 1.86 lm W À1 , respectively, (Table S10, Supporting Information).
LEDs in working applications are often accompanied by the generation of heat energy.An increase in the LED working temperature will lead to a reduction in light-emitting materials and light-emitting performance; therefore, a good heat dissipation effect of LEDs has broad application prospects.Figure 8 shows the temperature variation of LEDs prepared by thin-film encapsulation and the dispensing method of encapsulation with silica gel when operated at 4.5 V.When the temperature of the LEDs was increased to 50, 70, 80, 100, and 110 °C, the time required for the LEDs encapsulated by the film was 18, 30, 40, 71, and 96 s, respectively (Figure 8a), and the time required for the LEDs encapsulated by the dispensing method was 13, 25, 32, 52, and 67 s, respectively (Figure 8b).The results show that the time required for LEDs encapsulated in thin-film packaging is longer than that of LEDs encapsulated by the dispensing method when the LEDs reach the same temperature.Therefore, the heat dissipation effect of LEDs encapsulated in thin-film packaging is better than that of the dispensing method, which is attributed to the excellent thermal conductivity of cellulose in all phases and the heat dissipation space between the chip and the encapsulation film of LEDs prepared in thin-film packaging.The temperature change after LEDs is disconnected from the voltage also has a significant impact on the life of the LEDs. Figure S9 and S10, Supporting Information, show the temperature change graphs of LEDs encapsulated in thin-film packages and LEDs encapsulated by the dispensing method when the temperature is reduced from 110 to 30 °C, respectively.The results show that the thin-film encapsulated LEDs require less time to dissipate heat, which further proves that the thin-film encapsulated LEDs have superior heat dissipation performance compared to LEDs encapsulated by the dispensing method.The excellent heat dissipation performance proves that CNP-Co-encapsulated LEDs have great application potential.

Conclusions
We developed a new and convenient reagent engineering strategy to modulate the fluorescence emission of CQDs, emitting from blue to red light, by regulating the particle size of CQDs and the content of electron-donor/electron-withdrawing groups on the surface of CQDs.It was demonstrated that the effective conjugate length and the number of electron-withdrawing groups (C═O, C═N, etc.) on the surface of CQDs are positively correlated with the PL wavelength and synergistically contribute to the redshift of the emission wavelength of CQDs.Moreover, it was proven that the hydrogen-bonding network of CNF can effectively disperse CQDs and inhibit the aggregation-induced quenching of CQDs.Multicolor fluorescent films can be easily prepared by tape casting due to the hydrogen bonding between CQDs and CNFs, as well as the excellent film-forming and processing properties of CNFs.The study also shows that thin-film encapsulation based on the photosensitive resin ABPE-10 coating can be realized and rapidly assembles fluorescent films with different light-emitting colors into LED devices.W-LEDs with a CIE of (0.33, 0.37), a CCT of 5688 K, and a CRI of 86 were achieved.Compared with conventional LEDs, the assembly structure and method of the present work W-LEDs have superior heat dissipation performance and water resistance.In conclusion, this work provides a novel, versatile, and convenient technique for the preparation of high-performance multicolor CQDs and a convenient and scalable way to explore low-cost, environmentally friendly, and high-performance CQDs-based LEDs.

Experimental Section
Materials: m-Aminophenol, p-aminophenol, 2-hydroxy-2-methylpropiophenone (light curing agent, analytically pure), and hydroxyethyl methacrylate (active agent, analytically pure) were obtained from Aladdin Reagent Co. Ltd. (Shanghai, China).Formamide, terephthalic acid, acetic acid, dichloromethane, and methanol were obtained from Sigma-Aldrich (St. Louis, MO, USA).Quartz sand (300 mesh) for column chromatography was purchased from Aladdin Reagent Co. Ltd. (Shanghai, China).Preparation of CQDs: First, 0.2 g m-aminophenol, 10 mL formamide, and 10 mL anhydrous ethanol were mixed homogeneously and transferred to a Teflon-lined reactor (50 mL), which was heated at 180 °C for 12 h.At the end of the reaction, the reaction was cooled to room temperature.The resulting liquid product was filtered through a 0.22 μm nylon microporous membrane.After removing a large amount of solvent in rotary evaporation, the separation and purification of products were further carried out by silica gel column chromatography, and the eluent was a mixture of dichloromethane/methanol with a volume ratio of V (dichloromethane): V (methanol) = 100:1.Finally, the collected blue fluorescent solution was freeze-dried to obtain the solid powder of blue emissive CQDs (B-CQDs).According to the above preparation process, green emissive CQDs (G-CQDS) were obtained, except that the reaction reagents were composed of 0.2 g m-aminophenol, 0.1 g terephthalic acid, and 20 mL ethanol, and the eluent in the separation and purification process was a mixture of dichloromethane/methanol with a volume ratio of V (dichloromethane): V (methanol) = 10:1.The other conditions are the same as that for the synthesis of B-CQDs.
For the preparation of red emissive CQDs R-CQDs, the reaction reagents were composed of 0.2 g p-aminophenol, 10 mL acetic acid, and 10 mL ethanol, and the temperature of the reaction was 200 °C.The eluent in the separation and purification process was a mixture of dichloromethane/methanol with a volume ratio of V (dichloromethane): V (methanol) = 50:1.Other reaction conditions and processes were the same as those for B-CQDs.In addition, white emissive CQDs (W-CQDs) were prepared by mixing B-CQDs, G-CQDs, and R-CQDs powders at a ratio of 5:1:5.
Preparation of CNFs/CQDs Composite Films: A 12 g CNFs (2 wt%) suspension was mixed with 5 mg of B-CQDs, G-CQDs, R-CQDs, and W-CQDs powders and stirred at room temperature for 12 h.The mixtures were then transferred to a mold and vacuum dried at 60 °C for 12 h to prepare fluorescent films with different emissions.The resulting fluorescent films were labeled blue CNF film (B-CNP), green CNF film (G-CNP), red CNF film (R-CNP), and white CNF film (W-CNP) according to their emission.
Fabrication of LED Devices: The emission peak of the UV-LED chip is 365 nm.The B-CNP was cut into a size similar to that of the UV-LED chip, and then the B-CNP was taken to cover the surface of the chip.Meanwhile, to realize LED film encapsulation and improve the water resistance of LED, a layer of UV-curable ABPE-10 resin was coated on the surface of B-CNP and irradiated with a 365 nm UV lamp for 3 min for light-curing assembly, and the resulting LED device was named B-LED.Based on the above process, green-, red-, and white-emitting LED devices were obtained which were labelled G-LED, R-LED, and W-LED.A schematic of the synthesis of red, green, and blue CQDs and the corresponding fluorescent films and LEDs is shown in Scheme 1.
Material Characterization: The microscopic morphology of CQDs was characterized using TEM (FEI TECNAI G2 F30, USA) and AFM (Hitachi 5100 N, Japan).The crystalline structures of CQDs, CNPs and CNFs were investigated using XRD (Rigaku Japan SMARTLAB 3KW model).The chemical structure and functional group compositions of CQDs were analyzed using 13 C and 1 H NMR spectrometry (Bruker Avance III HD500, Germany), FT-IR (Bruker TENSOR II, Germany), and K-alpha þ X-Ray photoelectron spectrometry (XPS; Thermo Fisher Scientific, USA), respectively.The fluorescence properties of CQDs and CNPs were analyzed by a fluorescence spectrometer (Edinburgh FS5, UK).The absolute PLQY of CQDs and CNPs were measured using an integrating sphere on a fluorescence spectrometer (FS5, Edinburgh, UK).The fluorescence lifetimes and variable temperature fluorescence profiles of CQDs were measured using a steady-state/transient fluorescence spectrometer (Edinburgh FLS980, UK).The UV-Vis absorption spectra, transmittance and haze of CQDs and CNP were determined using a UV spectrophotometer (Hitachi u-4100 UV-Vis, Japan).UPS measurements were carried out using an hv = 21.2 eV He I source (Thermo Fisher Nexsa, USA).
C NMR spectra, signals in the range of 30-45 ppm are associated with aliphatic sp 3 C atoms, while signals in the range of 100-170 ppm are attributed to sp 2 C atoms, where signals observed at 163 ppm indicate that the sp 2 C atoms are bonded to ─OH, and signals at 167 ppm correspond to C═O/ C═N.

Figure 5 .
Figure 5. a) Optical properties of CQDs.PL wavelength and first exciton absorption band versus CQDs particle size.b) PL wavelength and first exciton absorption band versus electron-withdrawing groups (C═O and C═N) of CQDs.c) HOMO and LUMO energy levels versus CQDs particle size.

Figure 7 .
Figure 7. a) Applications of CQDs.EL spectra of B-LED, b) G-LED, c) R-LED (insets are emission photos of the corresponding LEDs in the on-state).d) CIE color coordinates of W-LED (inset shows EL spectra of W-LEDs and emission photos of on-state), and e) color of cardstock under W-LED illumination.

Figure 8 .
Figure 8.Heat Dissipation Performance of LEDs.a) Infrared thermal images of LEDs with the thin-film package when lit. b) Infrared thermal image of LEDs with the dispensing package when lit.