Enhancing white light-emitting diode performance with an ultra-wide spectrum ZnS:Mn-CDs@SiO₂ dual core@shell composite

Abstract

Recent progress in the field of carbon dots (CDs) has highlighted their significant potential, attributed to their exceptional optical properties and diverse applications, most notably in the realm of white light-emitting diodes (WLEDs). CDs are recognized for their broad spectrum of emission, adjustable fluorescence, and excellent thermal stability, making them ideal candidates for use in WLEDs. However, the challenge lies in synthesizing CDs that can emit long-wavelength, multicolor light from a single host material. This research presents a highly efficient dual-core–shell structured white-emitting phosphor, denoted as ZnS:Mn-CDs@SiO₂, which has achieved an impressive photoluminescence quantum yield (PLQY) of 25.8% and an expansive full width at half maximum (FWHM) of 131 nm. The successful implementation of this novel material has led to the creation of WLEDs with Commission Internationale de l'Eclairage (CIE) coordinates at (0.32, 0.38) and a correlated color temperature (CCT) of 5854.3 K, marking a significant stride in the development of high-performance WLEDs.

---

1. Introduction

White light-emitting diodes (WLEDs) have emerged as leading contenders to replace conventional incandescent and fluorescent lamps, owing to their superior attributes such as high power conversion efficiency, extended lifespan, and eco-friendliness.^1–4 However, the common commercial combination of yellow-emitting Y₃Al₅O₁₂:Ce³⁺ and blue-emitting InGaN chips in white WLEDs has limitations, as it lacks effective green and red emission sources. This results in cooler color temperatures, lower saturation, and poor color fidelity, making it unsuitable for high-accuracy color applications^5–7 Additionally, the application of Y₃Al₅O₁₂:Ce³⁺ is significantly restricted by its temperature sensitivity, which causes performance fluctuations under varying conditions.^8–10 In summary, its narrow emission spectrum, limited excitation wavelength range, and high production costs hinder its widespread adoption in broader markets.^11–13 To overcome the shortcomings of Y₃Al₅O₁₂:Ce³⁺ materials, researchers have developed alternative fluorescent materials suitable for lighting and display applications, such as metal oxide materials,^14–17 quantum dots (QDs),^18–20 perovskites,^21–23 metal–organic frameworks,²⁴etc. These materials can be utilized to fabricate WLEDs through several different methods: blending with RGB three-color raw materials, stacking with a monochrome conversion film, or employing them directly as a wide half-width single-matrix phosphor. Approaches that involve mixed phosphors and single-color film stacking may encounter challenges such as alterations in environmental stability, reabsorption issues between different components, and difficulties in precisely controlling the incorporation ratio.^25–27 As a result, researchers are increasingly interested in single-matrix white fluorescent materials with wide half-width emission characteristics.^28–30 This type of material offers a range of benefits, including a broad visible light spectrum, the absence of phase separation and color aging, and the elimination of the need for additional color-matching materials,^3,28,31 thereby reducing the complexity of ratio and color reabsorption concerns. WLEDs that utilize a single white phosphor with a broad half-peak width emission characteristic have shown improved color uniformity, higher color rendering index (CRI), and enhanced luminous efficiency.^32–34 This approach holds promise for addressing the limitations of traditional phosphor systems. Conventional single white luminescent substrates are often based on inorganic materials containing rare earth or other toxic, rare or precious metals, such as Ca₃Eu₂B₄O₁₂,¹⁶ YTaO₄:Bi³⁺,¹⁷ BAPPIn_1.996Sb_0.004Cl₁₀,³⁵ and CsPbBr₃.³⁶ These materials face challenges related to limited availability, environmental concerns, high costs, and complex synthesis processes.^37–41 There is an urgent need to develop single-matrix white phosphors that are abundant, eco-friendly, and cost-effective and possess excellent optical properties to meet the requirements of WLEDs.

In recent years, quantum dot materials, including III–V (e.g., InP^42,43), II–VI (e.g., ZnS^18,20), I–III–VI (e.g., CuInS^44,45), and CDs,⁴⁶ have gained widespread attention in lighting and display applications due to their high photoluminescence efficiency, low heat generation, excellent color tunability, and long service life. Among these, CDs stand out not only for these advantages but also for their superior cost-effectiveness, abundant raw materials, and the potential for large-scale synthesis at the kilogram level, making them particularly promising for WLED applications.^47,48 Early attempts by Tang et al. to create CD-based WLEDs involved using yellow wide-bandgap emitting CDs on blue GaN chips.⁴⁹ However, these initial CD-based WLEDs had issues with low CRI and excessive blue light emission, which could be harmful to the human retina. Yuan et al. addressed these issues by employing red/green/blue emitting mixed CDs as the white light-emitting layer in WLEDs.⁵⁰ Despite the promise of mixed CDs for white light emission, challenges such as complex preparation, inconsistent stability, and self-absorption and self-quenching persist. There is growing interest in single-matrix CDs with broad half-peak width emission, especially those with dual fluorescence/phosphorescence properties.^51–55 For example, Yang's group achieved a white light emission efficiency of 25% with blue-yellow fluorescent or phosphorescent dual-emitting CDs.⁵⁶ Li et al. reported a boric acid-wrapped CD composite with a half-peak width of up to 180 nm, demonstrating excellent white light emission.²⁹ However, there are limited reports on single-component white light CD materials, and they still face issues such as high preparation costs, photobleaching, and a lack of potential for large-scale production. The development of single-matrix white emitting CD materials is therefore a pressing need.

Building on our previous research, the development of CDs@Silica materials with fluorescence/phosphorescence properties emitting blue/cyan light has been reported.⁵⁷ Encapsulating CD materials within a robust SiO₂ matrix greatly enhances their resistance to photobleaching when exposed to UV light irradiation and improves their environmental stability against water and oxygen. It is possible to extend the emission wavelength range by integration of II–VI semiconductor nanocrystals into the CDs@SiO₂ materials. In this study, a dual-core structure phosphor, SiO₂-wrapped ZnS:Mn and CDs (ZnS:Mn-CDs@SiO₂), was designed and synthesized to achieve white light emission. The fluorescence characteristics of ZnS:Mn and CDs in the SiO₂ matrix complement each other, resulting in a pure white fluorescence emission. The dual-core structure material exhibits an impressive half-peak width of up to 131 nm and CIE chromaticity coordinates of (0.32, 0.32). As a white light phosphor, this material has been successfully used in the fabrication of WLEDs with a high color rendering index of 74.5, a color temperature of 5854.3 K, and a luminous efficiency of 30.52 lm W⁻¹. These results highlight the strong potential of this material for a variety of applications in white light illumination.

2. Experimental section

2.1. Materials and chemicals

Zinc acetate (Zn(OAc)₂, Analytical Reagent (AR) grade) was purchased from Shanghai Macklin Biochemical Co., Ltd. Sodium hydroxide (NaOH, AR grade) was purchased from Xilong Scientific. Tetraethyl orthosilicate (TEOS, AR grade) was purchased from Energy Chemical. Manganese chloride tetrahydrate (MnCl₂·4H₂O), chitosan, hydrochloric acid (HCl), sodium sulfide nonahydrate (Na₂S·9H₂O), ethanol, hydrofluoric acid (40% HF, AR grade) and N,N-dimethylformamide (DMF) were purchased from Sinopharm Chemical Reagent Co., Ltd, China. All the chemical reagents were used as received without further purification.

2.2. Materials synthesis

2.2.1 Synthesis of CDs@SiO₂. In a typical procedure, 0.25 mmol chitosan was introduced into a solution consisting of 2.14 mL of ethanol, 2.7 mL of DMF, 3 mL of 0.15 M NaOH, and 4 mL of TEOS. The mixture was then subjected to continuous stirring at a temperature of 80 °C until majority of the solvent had evaporated. Subsequently, the resultant mixture was annealed at a temperature of 540 °C for a duration of 3 hours. The final product obtained from this process was designated as CDs@SiO₂.

2.2.2 Synthesis of ZnS:Mn. First of all, 0.9 mmol of Zn(OAc)₂ and 0.1 mmol of MnCl₂·4H₂O were dissolved in 10 mL of deionized water and stirred for 20 minutes within an ice bath to yield Solution A. In parallel, Solution B was prepared by dissolving 0.25 mmol of chitosan in 2 mL of diluted hydrochloric acid (5.6 mol L⁻¹). Subsequently, the entirety of Solution B was slowly added into Solution A, and the combined mixture was vigorously agitated at ambient temperature for a duration of 1 hour. Following this, 12 mL of Na₂S·9H₂O (0.1 M) with a concentration of 0.1 M was added to the amalgamated solution, allowing the reaction to proceed for an additional 2 hours. Finally, the precipitate was subjected to centrifugation and rinsed three times with deionized water to lower the concentration of chitosan, which served dual roles as a surfactant and a carbon source in the subsequent reaction step.

2.2.3 Synthesis of ZnS:Mn-CDs@SiO₂. The product obtained in section 2.2.2 was mixed with 2.14 mL of ethanol, 2.7 mL of DMF, 3 mL of NaOH (0.15 M), and 4 mL of TEOS. The resulting mixture was stirred continuously at 80 °C until the liquid components were completely evaporated. Next, the dried product placed into an alumina crucible and then processed in a muffle furnace under an air atmosphere. The heating was done at a gradual rate of 5 °C per minute, starting from room temperature and reaching up to 540 °C. The product was kept at this high temperature for a duration of 3 hours before being allowed to cool down naturally to room temperature. Other calcination temperatures (i.e., 440 °C, and 640 °C) were also adopted for the synthesis of the composite materials to study the effect of the calcination temperature on the fluorescent properties. The sample obtained under 540 °C was named ZnS:Mn-CDs@SiO₂ unless otherwise stated.

2.3. Fabrication of WLEDs

The ZnS:Mn-CDs@SiO₂ phosphor was blended with a resin precursor, which consisted of silicone resin A and silicone resin B mixed in a mass ratio of 1 : 4. The mixture was then stirred thoroughly to ensure that all air bubbles were removed. Next, this well-mixed combination was drop-coated onto a 365 nm UV-LED chip. To finalize the preparation of the white light LED device, the coated chip was subjected to vacuum drying at a temperature of 60 °C.

2.4. Characterization of materials

Transmission electron microscopy (TEM), high-resolution HRTEM, bright-field TEM and the corresponding energy dispersive X-Ray Spectroscopy (EDX) elemental mappings images were obtained on a FEI Talos F200x instrument with a field emission gun operating at 200 kV. Powder X-Ray diffraction (XRD) data were measured on a RIGAKU Ultima IV powder X-ray diffractometer at a scanning rate of 3° min⁻¹ in the 2θ range from 10° to 70° using Cu Kα radiation (λ = 1.5406 Å) operating at an operation voltage and current maintained at 40 kV and 40 mA. The surface elemental states of the ZnS:Mn@CDs@SiO₂ composite material were analyzed by X-ray photoelectron spectroscopy (XPS, VG ESCALAB 250) with a monochromatized Al kα X-ray source (15 kV, 150 W, 500 nm, pass energy 20 eV). The fluorescence emission spectra were conducted utilizing the Edinburgh FLS980 spectrophotometer equipped with a xenon arc lamp (Xe 900) as the excitation source. Fourier transform infrared (FTIR) spectra were recorded in the range 400–4000 cm⁻¹ on a Nicolet iS50 FT/IR spectrophotometer. Luminescence characteristics of wLEDs were characterized with an LED tester (HP 9000, from Hangzhou Hopoo Light and Color Technology Co., Ltd) having an integrating sphere. PLQY was gained directly by the absolute PL quantum yield measurement system (XPQY-EQE, Guangzhou Xipu Photoelectric Technology Co., Ltd).

3. Results and discussion

3.1. Morphology and structural analysis of ZnS:Mn-CDs@SiO₂

To synthesize the ZnS:Mn-CDs@SiO₂ material, it is essential to choose an appropriate calcination temperature. If the temperature is too low, chitosan will not carbonize; however, if it is too high, ZnS will oxidize into ZnO that can further react with SiO₂, eventually forming weak fluorescence Zn₂SiO₄ (Fig. S1 and S2†). Optical images of the samples prepared at different temperatures reveal that the color of the samples under daylight gradually shifts from light yellow to white as the calcination temperature increases. Among these samples, only the one fabricated at 540 °C show strong white emission under the excitation of 365 nm UV light (Fig. S1†). Typical TEM images of the sample fabricated under 540 °C illustrate sub-micron particles composed of plenty of smaller nanoparticles (Fig. 1a). The brighter-contrast SiO₂ edges and the darker-contrast ZnS:Mn nanocrystals were clearly visible in further high-resolution TEM observations (Fig. 1b). The high-resolution TEM examination revealed a spacing of 0.31 nm in the bulk region of the nanocomposite, which aligns well with the d-spacing corresponding to the (111) crystallographic plane of ZnS.^58,59 The XRD pattern of the nanocomposite shows three broad diffraction peaks at 28.8°, 48.5° and 57.1° (Fig. S2a†), matching well with the characteristic diffraction peaks of the zinc blende ZnS (JCPDS no.77-2100). CDs are not observed in the TEM images in Fig. 1a and b is due to the weak contrast between CDs and ZnS:Mn. To identify the presence of CDs in the composite material, we conducted FTIR spectroscopy characterization on the ZnS:Mn-CDs@SiO₂ (Fig. S2b†). The fingerprint FTIR peaks of chitosan-derived CDs at 2909–3706 cm⁻¹ and at 1636 cm⁻¹, ascribed to the stretching vibration peaks of O–H/N–H and amino groups, respectively, were observed, consistent with the results published in the literature.⁶⁰ Additionally, signals for Si–O–C and Si–C covalent bonds, indicating connections between CDs and silica, were detected at 1062 cm⁻¹ and 795 cm⁻¹.^57,61 To further confirm the presence of CDs within the nanocomposite, we performed HF etching to remove the outer layers of SiO₂ and ZnS:Mn nanocrystals. The resulting CDs were isolated and enriched via centrifugation, followed by characterization. TEM images showed several uniformly dispersed nanodots with an average size of approximately 3.7 nm, providing further evidence of CDs within the material (Fig. S3†). Moreover, the high-angle annular dark-field (HAADF) and energy-dispersive spectroscopy (EDS) images of the nanocomposite present that the elemental constituents—Zn, S, Mn, C, Si, and O—are uniformly dispersed throughout the composite (Fig. 1c–i), further attesting to the successful encapsulation of ZnS:Mn and CDs within the SiO₂ matrix.

Fig. 1 (a) TEM image and size distribution histograms of ZnS:Mn-CDs@SiO₂ (scale bar: 200 nm), (b) lattice fringes of ZnS:Mn-CDs@SiO₂ (scale bar: 2 nm), (c–i) HAADF image and corresponding EDX mapping of the elemental distribution (scale bar: 200 nm).

To understand the chemical composition and electronic state of the ZnS:Mn-CDs@SiO₂ composite, it was subjected to XPS characterization. The survey scan validated the presence of Zn, S, Mn, C, Si, and O elements in the sample (Fig. S4†). Characteristic binding energy peaks for Zn²⁺ were identified at 1045.2 for Zn 2p1/2 and 1022.1 eV for Zn 2p3/2, respectively, while those for S²⁻ were found at 163.2 for S 2p1/2 and 162 eV for S 2p3/2, respectively (Fig. 2a and b). The high-resolution XPS spectrum for Mn²⁺ (Fig. 2c) exhibited a peak at 642.2 eV corresponding to Mn 2p3/2, and another peak near 654 eV for Mn 2p1/2 of Mn²⁺, both associated with Mn²⁺. The high-resolution C 1s spectrum (Fig. 2d) revealed three distinct carbon species at 288.6 eV for O–C=O, at 285.6 eV for C–O, and at 284.8 eV for C=C/C–C. The deconvoluted spectrum of Si 2p, presented in Fig. 2e, showed a clear peak at 103.6 eV, which is indicative of Si–O bonding within the SiO₂. Furthermore, the O 1s spectrum (Fig. 2f) featured two prominent peaks at 532.9 and 532 eV, which are ascribed to Si–O/C–O and C=O bonds, respectively. The XPS data collectively confirm the successful incorporation of all intended elements within the ZnS:Mn-CDs@SiO₂ composite, a result that aligns with the findings from TEM and EDX analyses as shown in Fig. 1.

Fig. 2 High-resolution XPS spectra of (a) Zn 2p, (b) S 2p, (c) Mn 2p, (d) C 1s, (e) Si 2p and (f) O 1s.

3.2. Optical properties and emitting mechanisms of ZnS:Mn-CDs@SiO₂

To investigate the optical properties and luminescence mechanisms of the synthesized ZnS:Mn-CDs@SiO₂, we conducted fluorescence spectroscopy, UV diffuse reflectance (DRS) and fluorescence lifetime analyses on the CDs@SiO₂, ZnS:Mn, and ZnS:Mn-CDs@SiO₂ materials (Fig. 3a–f). Under 325 nm UV irradiation, CDs@SiO₂ exhibited a broad emission peak centered at 450 nm, with an FWHM of 115 nm (Fig. 3a, blue line). The ZnS:Mn nanocrystals showed a double-peak emission, with peaks at 435 nm and 602 nm, corresponding to the S vacancy defects and ⁴T₁(G) to ⁶A₁(S) transition of Mn²⁺, respectively (Fig. 3a, red line).^62,63 The ZnS:Mn-CDs@SiO₂ displayed an even broader fluorescence emission peak located at 486 nm with an FWHM of approximately 131 nm (Fig. 3a, black line). In the CIE 1931 color coordinate diagram, the emissions of CDs@SiO₂, ZnS:Mn, and ZnS:Mn-CDs@SiO₂ correspond to the coordinates (0.22, 0.24), (0.55, 0.39), and (0.32, 0.32), respectively, displaying cyan, orange, and precise white light emission (Fig. 3b). We hypothesize that the broad peak emission observed in the ZnS:Mn-CDs@SiO₂ material arises from the combined contributions of its luminescent units, CDs@SiO₂ and ZnS:Mn. The broad FWHM value, reaching up to 131 nm, is a critical factor in achieving the white light emission characteristics of the ZnS:Mn-CDs@SiO₂ phosphor.

Fig. 3 Optical properties and emission mechanisms of CDs@SiO₂, ZnS:Mn, and ZnS:Mn-CDs@SiO₂: (a) fluorescence emission spectra with excitation wavelength of 325 nm, (b) the corresponding International CIE 1931 color coordinate diagram, (c) diffuse reflectance spectra, (d–f) fluorescence lifetime, (g) the potential luminescence mechanism diagram.

DRS was adopted to investigate the absorption characteristics and band gap relationship of ZnS:Mn-CDs@SiO₂ and its components (Fig. 3c and Fig. S5†). The DRS spectrum of the CDs@SiO₂ exhibits two distinct absorption peaks at 255 nm and 366 nm, which are attributed to the π → π* transition of the C=C bond and the n → π* transition of the C=O bond, respectively (Fig. 3c, blue line).^57,64,65 The corresponding energy level diagram shows that the band gaps of CDs@SiO₂ are 2.65 eV and 3.68 eV, respectively (Fig. S5a†). Due to the low concentration of Mn²⁺ doping (0.5%, Table S1†), the DRS spectrum of ZnS:Mn primarily reflects the absorption characteristics of the ZnS host, with an absorption edge at 310 nm and a weak d–d transition absorption peak from Mn²⁺ at 690 nm (Fig. 3c, red line), corresponding to a band gap of 3.54 eV (Fig. S5b†).^66,67 In the DRS spectrum of ZnS:Mn-CDs@SiO₂, a broad absorption band in the range of 255–370 nm formed by the overlap of ZnS:Mn and CDs (Fig. 3c, black line). These results suggest that the two components in ZnS:Mn-CDs@SiO₂ maintain their individual absorption properties. To understand the interaction between the two fluorescent components – CDs and ZnS:Mn nanocrystals, fluorescence lifetime measurements were performed (Fig. 3d–f and Table S2†). The results reveal that the average lifetimes at the respective emission peaks are 6.76 ns for CDs@SiO₂ and 7.92 ns for ZnS:Mn (Fig. 3d and e). Notably, under the same testing conditions, the fluorescence lifetime of the CDs in ZnS:Mn-CDs@SiO₂ is significantly enhanced to 7.8 ns, while the lifetime of ZnS:Mn is shortened to 6.07 ns (Fig. 3f). This indicates that there is a strong interaction between the two fluorescent components in the ZnS:Mn-CDs@SiO₂ composite, more specifically, the photoexcited electron transfer occurs from the ZnS:Mn component to the CDs. Through electron transfer between ZnS:Mn and CDs, the orange and blue emissions reach equilibrium and combine to produce white fluorescence (Fig. 3g, left). The luminescence mechanism is illustrated on the right side of Fig. 3g. For the ZnS:Mn component, the photoexcited electrons undergo three primary transitions: (1) a small fraction of electrons in the conduction band (CB) are transferred to the S defect states of ZnS:Mn via internal conversion (IC), emitting weak blue fluorescence when returning to the valence band (VB); (2) the majority of the excited electrons are transferred to the ⁴T₁(G) energy level of Mn²⁺ through intermediates ⁴E(D) and ⁴T₂(G) via IC and energy transfer (ET), eventually relaxing to the ground state ⁶A₁(S) of Mn²⁺ and emitting orange fluorescence;^63,68,69 (3) the remaining electrons relax to the singlet state (S₁) of the CDs via intersystem crossing (ISC). In the CD component, the excited electrons return to the ground state (S₀) together with the electrons transferred from ZnS:Mn via ISC and emit blue fluorescence with enhanced intensity. The combination of the blue light and orange light are responsible for the white emission with CIE coordinates of (0.32, 0.32) for the ZnS:Mn-CDs@SiO₂ composite.

Furthermore, the luminescence efficiency and photobleaching resistance of the ZnS:Mn-CDs@SiO₂ composite are critical for its potential use in WLED applications. Results indicated that the PLQY of ZnS:Mn-CDs@SiO₂ was 25.8% (Fig. S6a†). To assess its photostability, ZnS:Mn-CDs@SiO₂ was exposed to a 365 nm UV lamp with power of 6 W for 10 hours. The changes in fluorescence and phosphorescence spectra were monitored at various time intervals (Fig. S6b†). Upon long-time UV irradiation, the ZnS:Mn-CDs@SiO₂ composite exhibits excellent photobleaching resistance, with minimal fluctuation in its fluorescence spectrum throughout the irradiation period. After 10 hours of exposure, the fluorescence intensity decreased by only approximately 6%. These superior white emission properties position ZnS:Mn-CDs@SiO₂ as a promising candidate for application in the field of WLEDs.

3.3. Application in white light-emitting diodes

Due to the exceptional optical properties of the ZnS:Mn-CDs@SiO₂ composite, it is considered a promising candidate for the use as a color conversion layer in the development of WLED prototypes. To fabricate a WLED, a specific quantity of the ZnS:Mn-CDs@SiO₂ phosphor powder is homogeneously dispersed within a silica gel matrix and then uniformly applied onto the surface of an InGaN UV-LED chip, thereby creating the WLED device. The electroluminescence (EL) analysis of the tested WLED device was conducted under the conditions of voltage of 3.39 V and current of 0.12 A (Fig. 4). The EL image and spectral data show that the device emits strong white light during power-on, and its emission spectrum covers a wide range from 400 nm to 700 nm (Fig. 4a and b). Through the analysis of CIE chromaticity coordinates, the CIE coordinates of the device were measured to be (0.32, 0.38), indicating that its white light spectrum is located in the ideal position of the color space and conforms to the standard white light distribution (Fig. 4c).⁷⁰ The photoelectric performance of the fabricated WLED device was further investigated under varying voltage and current conditions (Table S3†). The results demonstrated a color rendering index (R_a) of 78.1, indicating excellent color rendering performance and the ability to accurately reproduce object colors. The CRI was measured at 74.5, further confirming its effective color rendering capability in practical applications. The average correlated color temperature (CCT) was 5854.3 K, which closely approximates the color temperature of natural white light, thereby meeting the requirements for general lighting applications. The luminous efficacy was recorded at 31.84 lm W⁻¹, while the power efficiency reached 40.6%, highlighting the device's high energy conversion efficiency from electrical to optical energy. In addition, compared with other reported single-matrix carbon dot WLED devices (Table S4†), this device shows a strong balance in color reproduction, color temperature and luminous efficiency, and is more suitable for application scenarios such as general lighting that require efficient natural white light.

Fig. 4 (a) Photograph of the white light-emitting diode (WLED) lamp, (b) electroluminescent spectrum of the WLEDs with InGaN UV (365 nm) and (c) commission Internationale de l’éclairage (CIE) 1931 color coordinate triangle.

4. Conclusions

In summary, we have successfully developed a straightforward and scalable synthetic approach to produce highly efficient white-emitting ZnS:Mn-CDs@SiO₂ phosphors, characterized by an impressive overall PLQY of 25.8% and an FWHM of up to 131 nm. From the analysis of the physical structure and spectral data it has been elucidated that the white emission is a result of the synergistic effect between the SiO₂-encapuslated CDs and the ZnS:Mn component. WLEDs were subsequently fabricated by integrating the ZnS:Mn-CDs@SiO₂ phosphor into UV-LED chips with an excitation wavelength of 365 nm. The resulting WLEDs exhibited excellent white light characteristics, with CIE coordinates, CCT luminous efficacy of (0.32, 0.38), 5854.3 K and 31.84 lm W⁻¹, respectively. This study not only contributes to the advancement of highly efficient white light materials for WLED applications, but also paves the way for a viable, gram-scale synthesis method for white-emitting phosphors. The findings have significant implications for the future development of energy-efficient lighting technologies and the broader field of optoelectronics.

Author contributions

M. L., Q. H. J., and Y. H. Z. conceived the idea. Q. H. J. and M. L. designed experiments and synthesized materials. S. Y. and H. X. C. conducted transmission and fluorescence spectroscopy. M. L., Y. Q. Z., and H. X. C. assisted in data analysis. Q. H. J., M. L., and Y. H. Z. wrote the manuscript and all authors reviewed it.

Data availability

All data needed for evaluating the conclusions in the paper are present in the paper and its ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the Recruitment Program of Global Experts, National Natural Science Foundation of China (Grant No. 62175033 and 61775040), the Fujian Science & Technology Innovation Laboratory for Optoelectronic Information (Grant No. 2021ZZ126), and Fuzhou University for the financial support.

References

1. Y. Shirasaki, G. J. Supran, M. G. Bawendi and V. Bulović, Emergence of colloidal quantum-dot light-emitting technologies, Nat. Photonics, 2013, 7, 13–23.
2. L. Wang, R.-J. Xie, T. Suehiro, T. Takeda and N. Hirosaki, Down-conversion nitride materials for solid state lighting: recent advances and perspectives, Chem. Rev., 2018, 118, 1951–2009.
3. Y. Chen, M. Zheng, Y. Xiao, H. Dong, H. Zhang, J. Zhuang, H. Hu, B. Lei and Y. Liu, A self–quenching–resistant carbon–dot powder with tunable solid–state fluorescence and construction of dual–fluorescence morphologies for white light–emission, Adv. Mater., 2016, 312–318.
4. H. Li, L. Li, L. Mei, W. Zhao, X. Zhou, H. Gao, X. Luo and Y. Hua, Double-perovskite LaBaZnTa_1(-xMnxO₆ phosphors for wide-range optical thermometer and high-quality WLED devices, J. Lumin., 2024, 270, 120566.
5. L. Xia, Q. Xiao, X. Ye, W. You and T. Liang, Erosion behavior and luminescence properties of Y₃Al₅O₁₂:Ce³⁺–embedded calcium bismuth borate glass–ceramics for WLEDs, J. Am. Ceram. Soc., 2019, 102, 2053–2065.
6. X. Shiqing, S. Liuzheng, Y. Zhang, J. Haidong, Z. Shilong, D. Degang, W. Huanping and W. Baoling, Effect of fluxes on structure and luminescence properties of Y₃Al₅O₁₂:Ce³⁺ phosphors, J. Rare Earths, 2009, 27, 327–329.
7. L. Feng, Y. Li, H. Xiong, S. Wang, J. Wang, W. Ding, Y. Zhang and F. Yun, Freestanding GaN-based light-emitting diode membranes on Y₃Al₅O₁₂:Ce³⁺ crystal phosphor plate for efficient white light emission, Appl. Phys. Express, 2016, 9, 081003.
8. Q. Shao, H. Li, Y. Dong, J. Jiang, C. Liang and J. He, Temperature-dependent photoluminescence studies on Y_2.93−xLn_xAl₅O₁₂:Ce_0.07 (Ln = Gd, La) phosphors for white LEDs application, J. Alloys Compd., 2010, 498, 199–202.
9. L. Wang, T. Seto and Y. Wang, A new efficient deep-red-emission phosphor Al₂O₃:Cr³⁺/Y₃Al₅O₁₂:Ce³⁺ for plant growth, Dalton Trans., 2021, 50, 3542–3549.
10. H. Nakamura, K. Shinozaki, T. Okumura, K. Nomura and T. Akai, Massive red shift of Ce³⁺ in Y₃Al₅O₁₂ incorporating super-high content of Ce, RSC Adv., 2020, 10, 12535–12546.
11. Z. Zhou, S. Liu, F. Wang and Y. Liu, Synthesis of Y₃Al₅O₁₂:Ce³⁺ phosphors by a modified impinging stream method: a crystal growth and luminescent properties study, J. Phys. D: Appl. Phys., 2012, 45, 195105.
12. L. Wang, X. Zhang, Z. Hao, Y. Luo, X.-j. Wang and J. Zhang, Enriching red emission of Y₃Al₅O₁₂:Ce³⁺ by codoping Pr³⁺ and Cr³⁺ for improving color rendering of white LEDs, Opt. Express, 2010, 18, 25177–25182.
13. H. S. Jang, J. H. Kang, Y.-H. Won, S. Lee and D. Y. Jeon, Mechanism for strong yellow emission of Y₃Al₅O₁₂:Ce³⁺ phosphor under electron irradiation for the application to field emission backlight units, Appl. Phys. Lett., 2007, 90, 071908.
14. Y. Zhang, J. Mao, P. Zhu and G. Wang, Tunable multicolor luminescence in vanadates from yttrium to indium with enhanced luminous efficiency and stability for its application in WLEDs and indoor photovoltaics, Nano Res., 2023, 16, 11486–11494.
15. P. Zhu, W. Wang, H. Zhu, P. Vargas and A. Bont, Optical properties of Eu³⁺-doped Y₂O₃ nanotubes and nanosheets synthesized by hydrothermal method, IEEE Photonics J., 2018, 10, 1–10.
16. G.-H. Li, N. Yang, J. Zhang, J.-Y. Si, Z.-L. Wang, G.-M. Cai and X.-J. Wang, The non-concentration-quenching phosphor Ca₃Eu₂B₄O₁₂ for WLED application, Inorg. Chem., 2020, 59, 3894–3904.
17. Y. Fu, X. Wang and M. Peng, Tunable photoluminescence from YTaO₄:Bi³⁺ for ultraviolet converted pc-WLED with high chromatic stability, J. Mater. Chem. C, 2020, 8, 6079–6085.
18. W.-J. Zhang, C.-Y. Pan, F. Cao, H. Wang and X. Yang, Highly bright and stable white-light-emitting cadmium-free Ag, Mn co-doped Zn–In–S/ZnS quantum dots and their electroluminescence, J. Mater. Chem. C, 2018, 6, 10233–10240.
19. W.-J. Zhang, C.-Y. Pan, F. Cao, H. Wang and X. Yang, Bright violet-to-aqua-emitting cadmium-free Ag-doped Zn–Ga–S quantum dots with high stability, Chem. Commun., 2018, 54, 4176–4179.
20. W.-J. Zhang, C.-Y. Pan, F. Cao, H. Wang, Q. Wu and X. Yang, Synthesis and electroluminescence of novel white fluorescence quantum dots based on a Zn–Ga–S host, Chem. Commun., 2019, 55, 14206–14209.
21. Y. Zhang, H. Cheng, X. Chen and Y. Zheng, o-Phenylenediamine Doped Tin-Based Two-Dimensional Perovskite for Light-Emitting Device Applications, J. Phys. Chem. C, 2023, 127, 23827–23834.
22. P. Zhu, S. Thapa, H. Zhu, S. Wheat, Y. Yue and D. Venugopal, Composition engineering of lead-free double perovskites towards efficient warm white light emission for health and well-being, J. Alloys Compd., 2023, 960, 170836.
23. P. Zhu, S. Thapa, H. Zhu, D. Venugopal, A. Sambou, Y. Yue, S. S. Dantuluri and S. Gangopadhyay, Solid-state white light-emitting diodes based on 3D-printed CsPbX₃-resin color conversion layers, ACS Appl. Electron. Mater., 2023, 5, 5316–5324.
24. L. Xu, Y. Li, Q. Pan, D. Wang, S. Li, G. Wang, Y. Chen, P. Zhu and W. Qin, Dual-mode light-emitting lanthanide metal-organic frameworks with high water and thermal stability and their application in white LEDs, ACS Appl. Mater. Interfaces, 2020, 12, 18934–18943.
25. M. Nyman, L. E. Shea-Rohwer, J. E. Martin and P. Provencio, Nano-YAG:Ce mechanisms of growth and epoxy-encapsulation, Chem. Mater., 2009, 21, 1536–1542.
26. X. Mi, H. Shi, Z. Wang, L. Xie, H. Zhou, J. Su and J. Lin, Luminescence properties of M₃(VO₄)₂:Eu³⁺(M = Ca, Sr, Ba) phosphors, J. Mater. Sci., 2016, 51, 3545–3554.
27. P. Cai, X. Wang, H. J. Seo and X. Yan, Bluish-white-light-emitting diodes based on two-dimensional lead halide perovskite (C₆H₅C₂H₄NH₃)₂PbCl₂Br₂, Appl. Phys. Lett., 2018, 112, 153901.
28. P. He, Y. Shi, T. Meng, T. Yuan, Y. Li, X. Li, Y. Zhang, L. Fan and S. Yang, Recent advances in white light-emitting diodes of carbon quantum dots, Nanoscale, 2020, 12, 4826–4832.
29. Q. Li, Y. Li, S. Meng, J. Yang, Y. Qin, J. Tan and S. Qu, Achieving 46% efficient white-light emissive carbon dot-based materials by enhancing phosphorescence for single-component white-light-emitting diodes, J. Mater. Chem. C, 2021, 9, 6796–6801.
30. H.-W. Lin, A. Ablez, Z.-H. Deng, Z.-H. Chen, Y.-C. Peng, Z.-P. Wang, K.-Z. Du and X.-Y. Huang, A ligand-incorporating strategy towards single-component white light in ionic zero-dimensional indium chlorides, J. Mater. Chem. C, 2024, 12(14), 5184–5190.
31. T. Meng, T. Yuan, X. Li, Y. Li, L. Fan and S. Yang, Ultrabroad-band, red sufficient, solid white emission from carbon quantum dot aggregation for single component warm white light emitting diodes with a 91 high color rendering index, Chem. Commun., 2019, 55, 6531–6534.
32. Q. Han, W. Xu, C. Ji, G. Xiong, C. Shi, D. Zhang, W. Shi, Y. Jiang and Z. Peng, Multicolor and Single-Component White Light-Emitting Carbon Dots from a Single Precursor for Light-Emitting Diodes, ACS Appl. Nano Mater., 2022, 5, 15914–15924.
33. W. Zhao, H. Wen, X. Yang and X. Qiao, Preparation and luminescent properties of KBaYSi₂O₇:RE³⁺ (RE = Dy, Eu) single-host phosphors for white LED, Mater. Lett., 2020, 261, 127104.
34. Z.-W. Zhang, A.-J. Song, S.-T. Song, J.-P. Zhang, W.-G. Zhang and D.-J. Wang, Synthesis and luminescence properties of novel KSrPO₄: Dy³⁺ phosphor, J. Alloys Compd., 2015, 629, 32–35.
35. J.-H. Wei, J.-F. Liao, L. Zhou, J.-B. Luo, X.-D. Wang and D.-B. Kuang, Indium-antimony-halide single crystals for high-efficiency white-light emission and anti-counterfeiting, Sci. Adv., 2021, 7, eabg3989.
36. S. Liu, G. Shao, L. Ding, J. Liu, W. Xiang and X. Liang, Sn-doped CsPbBr₃ QDs glasses with excellent stability and optical properties for WLED, Chem. Eng. J., 2019, 361, 937–944.
37. H. Zhang, H. Zhang, A. Pan, B. Yang, L. He and Y. Wu, Rare earth-free luminescent materials for WLEDs: recent progress and perspectives, Adv. Mater. Technol., 2021, 6, 2000648.
38. C. C. Lin, A. Meijerink and R.-S. Liu, Critical red components for next-generation white LEDs, J. Phys. Chem. Lett., 2016, 7, 495–503.
39. B. Chen, N. Pradhan and H. Zhong, From Large-Scale Synthesis to Lighting Device Applications of Ternary I–III–VI Semiconductor Nanocrystals: Inspiring Greener Material Emitters, J. Phys. Chem. Lett., 2018, 9, 435–445.
40. H. Zhang, G.-Y. Ding, A. Yousaf, L. Chen, X.-L. Wang, G.-G. Shan, C.-Y. Sun and Z.-M. Su, A typical 2D covalent organic polymer as multifunctional sensor and assemble a WLED, J. Solid State Chem., 2021, 297, 122101.
41. H. Yuce, T. Guner, S. Dartar, B. U. Kaya, M. Emrullahoglu and M. M. Demir, BODIPY-based organic color conversion layers for WLEDs, Dyes Pigm., 2020, 173, 107932.
42. S. Kim, T. Kim, M. Kang, S. K. Kwak, T. W. Yoo, L. S. Park, I. Yang, S. Hwang, J. E. Lee and S. K. Kim, Highly luminescent InP/GaP/ZnS nanocrystals and their application to white light-emitting diodes, J. Am. Chem. Soc., 2012, 134, 3804–3809.
43. P. Yu, S. Cao, Y. Shan, Y. Bi, Y. Hu, R. Zeng, B. Zou, Y. Wang and J. Zhao, Highly efficient green InP-based quantum dot light-emitting diodes regulated by inner alloyed shell component, Light: Sci. Appl., 2022, 11, 162.
44. B. Chen, H. Zhong, M. Wang, R. Liu and B. Zou, Integration of CuInS₂-based nanocrystals for high efficiency and high colour rendering white light-emitting diodes, Nanoscale, 2013, 5, 3514–3519.
45. C. C. Hsiao, Y. S. Su and S. R. Chung, Fabrication of CuInS₂/ZnS quantum dots-based white light-emitting diodes with high color rendering index. In Sixteenth International Conference on Solid State Lighting and LED-based Illumination Systems, SPIE, (2017, September), Vol. 10378, pp. 62–67.
46. B. Zhao and Z. a. Tan, Fluorescent carbon dots: fantastic electroluminescent materials for light–emitting diodes, Adv. Sci., 2021, 8, 2001977.
47. S. Zhu, Y. Song, X. Zhao, J. Shao, J. Zhang and B. Yang, The photoluminescence mechanism in carbon dots (graphene quantum dots, carbon nanodots, and polymer dots): current state and future perspective, Nano Res., 2015, 8, 355–381.
48. F. Yuan, S. Li, Z. Fan, X. Meng, L. Fan and S. Yang, Shining carbon dots: Synthesis and biomedical and optoelectronic applications, Nano Today, 2016, 11, 565–586.
49. L. Tang, R. Ji, X. Cao, J. Lin, H. Jiang, X. Li, K. S. Teng, C. M. Luk, S. Zeng and J. Hao, Deep ultraviolet photoluminescence of water-soluble self-passivated graphene quantum dots, ACS Nano, 2012, 6, 5102–5110.
50. F. Yuan, Z. Wang, X. Li, Y. Li, Z. a. Tan, L. Fan and S. Yang, Bright multicolor bandgap fluorescent carbon quantum dots for electroluminescent light-Emitting diodes, Adv. Mater., 2017, 29, 1604436.
51. Q. Li, M. Zhou, Q. Yang, Q. Wu, J. Shi, A. Gong and M. Yang, Efficient room-temperature phosphorescence from nitrogen-doped carbon dots in composite matrices, Chem. Mater., 2016, 28, 8221–8227.
52. J. Tan, Y. Ye, X. Ren, W. Zhao and D. Yue, High pH-induced efficient room-temperature phosphorescence from carbon dots in hydrogen-bonded matrices, J. Mater. Chem. C, 2018, 6, 7890–7895.
53. K. Jiang, X. Gao, X. Feng, Y. Wang, Z. Li and H. Lin, Carbon dots with dual-emissive, robust, and aggregation-induced room-temperature phosphorescence characteristics, Angew. Chem., Int. Ed., 2020, 59, 1263–1269.
54. J. Joseph and A. A. Anappara, Cool white, persistent room-temperature phosphorescence in carbon dots embedded in a silica gel matrix, Phys. Chem. Chem. Phys., 2017, 19, 15137–15144.
55. K. Jiang, Y. Wang, Z. Li and H. Lin, Afterglow of carbon dots: mechanism, strategy and applications, Mater. Chem. Front., 2020, 4, 386–399.
56. T. Yuan, F. Yuan, X. Li, Y. Li, L. Fan and S. Yang, Fluorescence–phosphorescence dual emissive carbon nitride quantum dots show 25% white emission efficiency enabling single-component WLEDs, Chem. Sci., 2019, 10, 9801–9806.
57. H. Cheng, S. Chen, M. Li, Y. Lu, H. Chen, X. Fang, H. Qiu, W. Wang, C. Jiang and Y. Zheng, Ultra-stable dual-color phosphorescence Carbon-Dot@ Silica material for advanced anti-counterfeiting, Dyes Pigm., 2023, 208, 110827.
58. J. Luciano-Velázquez, Y. Xin, Y.-f. Su, C. I. Quiles-Vélez, S. A. Cruz-Romero, G. E. Torres-Mejías, J. Rivera-De Jesús and S. J. Bailón-Ruiz, Synthesis, characterization, and photocatalytic activity of ZnS and Mn-doped ZnS nanostructures, MRS Adv., 2021, 6, 252–258.
59. M. A. Avilés, J. M. Córdoba, M. J. Sayagués and F. J. Gotor, Synthesis of Mn²⁺-doped ZnS by a mechanically induced self-sustaining reaction, J. Mater. Sci., 2020, 55, 1603–1613.
60. Y. Yang, J. Cui, M. Zheng, C. Hu, S. Tan, Y. Xiao, Q. Yang and Y. Liu, One-step synthesis of amino-functionalized fluorescent carbon nanoparticles by hydrothermal carbonization of chitosan, Chem. Commun., 2012, 48, 380–382.
61. C. Sun, Y. Zhang, Y. Wang, W. Liu, S. Kalytchuk, S. V. Kershaw, T. Zhang, X. Zhang, J. Zhao and W. W. Yu, High color rendering index white light emitting diodes fabricated from a combination of carbon dots and zinc copper indium sulfide quantum dots, Appl. Phys. Lett., 2014, 104, 261106.
62. K. Sooklal, B. S. Cullum, S. M. Angel and C. J. Murphy, Photophysical properties of ZnS nanoclusters with spatially localized Mn²⁺, J. Phys. Chem., 1996, 100, 4551–4555.
63. Q. Pan, D. Yang, Y. Zhao, Z. Ma, G. Dong and J. Qiu, Facile hydrothermal synthesis of Mn doped ZnS nanocrystals and luminescence properties investigations, J. Alloys Compd., 2013, 579, 300–304.
64. Y. Gao, H. Zhang, S. Shuang and C. Dong, Visible-light-excited ultralong-lifetime room temperature phosphorescence based on nitrogen-doped carbon dots for double anticounterfeiting, Adv. Opt. Mater., 2020, 8, 1901557.
65. J. Di, J. Xiong, H. Li and Z. Liu, Ultrathin 2D photocatalysts: electronic-structure tailoring, hybridization, and applications, Adv. Mater., 2018, 30, 1704548.
66. T. Trindade, P. O'Brien and N. L. Pickett, Nanocrystalline semiconductors: synthesis, properties, and perspectives, Chem. Mater., 2001, 13, 3843–3858.
67. M. A. Avilés, J. Córdoba, M. J. Sayagués and F. Gotor, Synthesis of Mn²⁺-doped ZnS by a mechanically induced self-sustaining reaction, J. Mater. Sci., 2020, 55, 1603–1613.
68. H. Labiadh, B. Sellami, A. Khazri, W. Saidani and S. Khemais, Optical properties and toxicity of undoped and Mn-doped ZnS semiconductor nanoparticles synthesized through the aqueous route, Opt. Mater., 2017, 64, 179–186.
69. D. Jiang, L. Cao, G. Su, H. Qu and D. Sun, Luminescence enhancement of Mn doped ZnS nanocrystals passivated with zinc hydroxide, Appl. Surf. Sci., 2007, 253, 9330–9335.
70. Z. Zhou, P. Tian, X. Liu, S. Mei, D. Zhou, D. Li, P. Jing, W. Zhang, R. Guo and S. Qu, Hydrogen peroxide-treated carbon dot phosphor with a bathochromic-shifted, aggregation-enhanced emission for light-emitting devices and visible light communication, Adv. Sci., 2018, 5, 1800369.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi01099j

---