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. 2026 Sep 28. Online ahead of print. doi: 10.1039/d6ra06953c

Single-component white-light-emitting carbon quantum dots from lignite coal with adaptive broad colour-temperature tunability

Tuhin Mandal a,d, Rabi Narayan Senapati a, Ashish Kumar Ghosh b,d, Reginald Ebhin Masto a,d, Vikram Singh a,c,d,✉
PMCID: PMC13617754  PMID: 42807386

Abstract

Sustainable single-component white light emitters exhibiting tunable correlated colour temperature (CCT) while maintaining a nearly constant colour rendering index (CRI) (>75) are very scarce in the literature. In this connection, single-component white-emitting carbon quantum dots (CQDs) were synthesized from Indian lignite coal via a simple one-step dimethyl sulfoxide (DMSO)-assisted ultrasonication process at room temperature. The resulting 4.0 nm average-sized CQDs exhibited a broadband emission profile from 400–750 nm across the entire visible spectrum under 390 nm excitation. Remarkably, the CCT can be tuned over a wide range from near warm-white (4535 K) to daylight-white (11 258 K), simply by varying the amount of lignite precursor from 0.03 to 0.08 g, while maintaining the CRI within 74–80. The optimized CQDs displayed chromaticity coordinates of (0.27, 0.34), a CCT of 8612 K, and a CRI of 76. Furthermore, flexible CQD-mixed with polyvinyl alcohol (PVA) composite films and CQD-coated light-emitting diode (LED) chips were fabricated, demonstrating their practical utility in solid-state white-light-emitting devices. To the best of our knowledge, this is the first report of lignite-derived single-component white-emitting CQDs exhibiting controllable broad CCT tunability through simple precursor-mass modulation while preserving acceptable colour-rendering performance.

Sustainable single-component white light emitters exhibiting tunable correlated colour temperature (CCT) while maintaining a nearly constant colour rendering index (CRI) (>75) are very scarce in the literature.graphic file with name d6ra06953c-ga.webp

1. Introduction

Lignite, also called brown coal, is a low-ranked coal widely used for power generation and industrial applications because it is abundant and inexpensive.1 Global lignite reserves are estimated at roughly 200–220 billion tonnes, while India holds approximately 47.37 billion tonnes, representing around 12% of the nation's total coal reserves.2 However, its high moisture content and low energy density limit its transportation and utilization efficiency.3 In addition, lignite combustion releases sulphur dioxide, nitrogen oxides, and particulate matter, along with large quantities of ash, posing serious environmental concerns.4 Therefore, these challenges necessitate exploring alternative, value-added pathways for the effective utilization of lignite.

Recent advances in the development of nanomaterials from naturally abundant resources for optoelectronic applications have gained considerable attention.5,6 In this context, lignite represents a promising carbon-rich precursor for the development of advanced nanomaterials. This approach offers a route to valorise lignite as an alternative, low-cost resource while reducing dependence on scarce and expensive resources such as rare earths. Lignite typically contains 60–65% carbon (on a dry, ash-free basis), making it suitable for the synthesis of carbon-based nanomaterials.7,8 Among them, CQDs emerged as zero-dimensional nanomaterials with unique optical and electronic properties, including strong luminescence, high photostability, good aqueous dispersibility and tunable emission characteristics.9–14 Their emission behaviour can be precisely engineered through size modulation, surface functionalization, and heteroatom doping,15,16 enabling broadband and multicolour emission suitable for white-light applications. The intrinsic aromatic and aliphatic structures of lignite, along with naturally occurring heteroatoms, can facilitate controlled carbonisation and the formation of CQDs with balanced sp2/sp3 domain structures, tailored with nitrogen, oxygen and sulphur-containing functionalities that are favourable for broadband emission.17,18

Broad spectral emission across the visible region is essential for generating white-light-emission (WLE) based materials in optoelectronic applications, as it enables balanced colour output.19,20 Depending on CCT and spectral power distribution, white-light sources are generally classified as warm white, neutral white, cool white, and daylight white. Warm white light, typically exhibiting a CCT of approximately 2700–3500 K, appears yellowish. It is widely preferred for residential, hospitality, and ambient lighting applications because of its comfortable and relaxing visual appearance. Neutral white light generally falls within the CCT range of 3500–5000 K and provides a balance between visual comfort and colour discrimination. Cool white light, with a CCT of about 5000–6500 K, possesses a relatively bluish appearance. The cool white emitting devices are commonly used in offices, healthcare facilities, industrial environments, and outdoor illumination, where enhanced visibility and alertness are desired. Daylight white emission corresponds to higher CCT values, typically above 6500 K, and exhibits a more pronounced blue component, which is frequently used for high visual acuity, colour-critical inspection, display illumination, and the simulation of natural daylight conditions. Since the spectral characteristics of natural daylight vary with environmental and atmospheric conditions, daylight white illumination generally encompasses a broad range of higher CCT values (exceeding 10 000 K) and is designed to emulate the visual characteristics of natural daylight.21–23 Traditionally, tunable-CCT-based white-light emission has been achieved by combining multiple colour-emitting CQDs or by integrating CQDs with rare-earth-based inorganic materials. However, such multicomponent systems often involve complex fabrication procedures and may suffer from drawbacks including reabsorption, energy-transfer-induced self-absorption, spectral instability, and batch-to-batch variations, ultimately affecting device efficiency, colour consistency, and reproducibility.24–26 To demonstrate these challenges, the development of single-component white-light-emitting CQDs with tunable CCT using earth-abundant precursors and simple synthetic approaches has emerged as a promising strategy.

In this work, we report the synthesis of single-component white-emitting CQDs derived from lignite coal via a DMSO-assisted ultrasonication process in 10 min at room temperature. Owing to its intrinsic chemical structure with heteroatoms, lignite serves as an abundant and low-cost precursor for the production of white-emissive CQDs. The combined effect of lignite-derived surface functionalities and DMSO-assisted surface modification gives CQDs a broadband photoluminescence spanning 400–750 nm under 390 nm excitation, covering the entire visible region. Importantly, broad CCT tunability from 4535 K to 11 258 K was achieved by simply varying the lignite precursor amount, without altering the chemical composition or introducing secondary emitters, while maintaining a stable CRI of 74–80. The optimized white-emitting CQDs exhibit CIE chromaticity coordinates of (0.27, 0.34), a CCT of 8612 K, and a CRI of 76. In addition, CQD-based PVA composite films and LED chip coatings demonstrate the potential of these materials for versatile solid-state white-emitting lighting applications. By transforming lignite coal, a low-grade fossil carbon resource, into advanced optoelectronic materials, this work expands its value-added utilization and offers a promising route toward sustainable and circular materials development.

2. Experimental section

2.1. Chemicals & materials

Indian lignite coal samples were collected from two regions: Neyveli, Tamil Nadu (Lignite-I), and Panandhro, Gujarat (Lignite-II). The raw coal samples were crushed, powdered, and sieved (200 mesh) to obtain a particle size of 74 microns before chemical treatment. All chemicals used in this study were of analytical grade and were used without further purification. The solvents used, including heptane, hexane, toluene, ethanol, water, hydrogen peroxide, lactic acid, diethylenetriamine, ethylenediamine, acetone, acetonitrile, tetrahydrofuran, dimethylformamide, and DMSO, were purchased from Sisco Research Laboratories Pvt. Ltd Quinine hemisulfate monohydrate was purchased from 3A Sunrise Co., Ltd (China). The strong acids, such as sulfuric acid, nitric acid Whatman filter paper, were brought from Merck Pvt. Ltd for the study. Double-distilled water made in the lab was used for all solutions and analyses. The PVA was obtained from Thermo Fisher Scientific Pvt. Ltd to prepare polymeric film.

2.2. Instrumentation

The optical properties of the DMSO-dispersible CQDs were studied using UV-visible absorption and fluorescence spectroscopy. Absorption spectra were recorded on a Shimadzu UV-1900i spectrophotometer, whereas photoluminescence measurements were performed using a Hitachi F-7000 spectrofluorometer. The CQD morphology, including histograms, elemental maps, and microstructural features, was characterized by high-resolution transmission electron microscopy (HRTEM) using a Thermo Fisher Scientific Talos F200X G2 microscope. The crystalline structure of the CQDs was further evaluated by X-ray diffraction (XRD) using a Rigaku SmartLab diffractometer. Surface chemical functionalities were identified by Fourier transform infrared (FTIR) spectroscopy using a Shimadzu IRAffinity-1S instrument. The elemental composition, surface chemical states, and electronic environments of the CQDs were further examined by X-ray photoelectron spectroscopy (XPS) on a PHI 5000 VersaProbe III system. Fluorescence lifetime measurements were acquired in the nanosecond time domain using a HORIBA DeltaFlex time-correlated single-photon counting (TCSPC) spectrometer. Fluorescence emission under ultraviolet irradiation was visually inspected using a Spectroline ENF-260C/FE 6 W long-wave UV lamp. The inorganic composition of the coal sample was analyzed by X-ray fluorescence (XRF) spectroscopy using a Rigaku ZSX Primus IV X-ray Fluorescence Spectrophotometer. The ELTRA's TGA Thermostep thermogravimetric analyzer was used for proximate analysis of coal and for ultimate analysis, the elemental analyzer vario MAX Cube was used. For the experimental investigation of device fabrication potential, commercially available 5 mm, 4.0 V UV-LED chips and 3.0 V micro lithium cells were procured from the local market and utilized in the study. The chromaticity coordinates, CRI, CCT, and CQS of the CQD-based WLE were evaluated using CIE 1931 chromaticity software.

2.3. Synthesis of white-emitting CQDs

The study initially set out to explore lignite as a way to develop white-emitting carbon nanomaterials that could be produced on a large scale. Finding a suitable solvent that is both low-cost and low-toxicity for the development of white-emitting carbon materials remains a significant challenge. In this context, a diverse set of nonpolar, polar protic, and polar aprotic solvents was systematically shortlisted, including heptane, hexane, toluene, ethanol, water, hydrogen peroxide, lactic acid, diethylenetriamine, ethylenediamine, acetone, acetonitrile, tetrahydrofuran, and dimethylformamide and DMSO. Among the screened solvents, DMSO proved to be the most effective, possibly due to its high polarity and hydrogen-bond accepting ability. At first, 0.05 g of a 200-mesh lignite sample was weighed and dispersed in 5.0 mL of DMSO in a 15.0 mL glass vial. The glass vial was subsequently transferred to a sonication chamber and sonicated at 40 kHz at 30 °C for 10 minutes. The formation of CQDs is attributed to the synergistic effect of DMSO and ultrasonic cavitation. Owing to the relatively low degree of carbonization and high oxygen content of lignite, ultrasonication preferentially cleaves weak oxygenated and aliphatic linkages connecting condensed sp2-aromatic domains. The liberated aromatic nanofragments are subsequently stabilized and solvated by highly polar DMSO through strong solvent–surface interactions, preventing their reassembly into larger carbonaceous structures. This top-down exfoliation and fragmentation process yields nanosized carbon quantum dots decorated with abundant oxygen-containing functional groups, which not only improve colloidal stability but also generate emissive surface states responsible for the observed fluorescence.27,28 The colloidal CQDs solution obtained was passed through a 2.5 µm membrane filter, and the resulting filtrate was centrifuged at 8000 rpm for 10 minutes to remove the large particles. The diluted CQD solution (425 µg mL−1) displayed white emission under 365 nm UV illumination from a UV torch and was stored at 4 °C for subsequent use. The product yield of CQDs obtained from the synthesis process was approximately 10.2%. The developed WLE-CQDs were independently synthesized three times from each lignite sample (lignite-I and lignite-II) under identical experimental conditions. The observed WLEs were reproducibly obtained from both Lignite-I- and Lignite-II-derived CQDs, which exhibited nearly identical CIE coordinates, CCT, and CRI values, along with comparable product and quantum yields (Fig. S1 and Table S1). For the CCT tunability experiments, the ultrasonication protocol described above was followed, with the lignite precursor mass systematically varied (0.03, 0.05, and 0.08 g) while maintaining a constant DMSO volume of 5.0 mL. We also attempted DMSO-assisted solvothermal treatment at 160 °C and 180 °C; however, the produced CQDs exhibited less desirable CIE coordinates for WLE. Therefore, the solvothermal-synthesized CQDs were excluded from further optimization and application studies. Fig. 1 presents a schematic representation of the synthesis method used for white-emitting CQDs. The long-term photostability of the CQDs was evaluated by storing the samples in capped 15 mL glass vials under ambient laboratory conditions for two months. No noticeable change in the WLE intensity was observed after storage, indicating good long-term photostability under the tested conditions. Detailed descriptions of analytical methodologies for HRTEM, TCSPC, XRD, FTIR and XPS characterization are provided in the SI (SI 1.1, 1.2, 1.3, 1.4, and 1.5). A detailed description of the methodology used in the CIE 1931 software for determining the chromaticity coordinates, CCT, CRI, and colour quality scale (CQS) is also provided in SI 2.

Fig. 1. The schematic of the single-step ultrasonication synthesis of white fluorescent CQDs using lignite and DMSO as the carbon source and solvent, respectively.

Fig. 1

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2.4. Fabrication of white-emitting CQDs-coated PVA film

PVA was chosen to explore the solid-state implementation of the developed white-emitting CQDs because of its excellent optical transparency, polar, hydroxyl-rich matrix, good film-forming ability, and compatibility with carbon nanomaterials. Typically, 1.0 g of PVA powder was dissolved in 10 mL of double-distilled water under continuous stirring at 90 °C for 30 minutes, yielding a clear, homogeneous 10% (w/v) PVA solution. Then, the resulting transparent PVA solution was left for an additional 30 minutes at 50 °C to remove bubbles. The resulting mixture was cast onto a clean glass Petri dish and dried under ambient conditions overnight to obtain a flexible, transparent PVA film. For CQD coating, the dried PVA film was immersed in a DMSO-dispersed CQD solution (425 µg mL−1) for 3.0 h to enable diffusion and adsorption of CQDs into the polymeric matrix. Upon immersion in the DMSO-dispersed CQD solution, the PVA matrix undergoes partial solvent-induced swelling, enabling diffusion of CQDs into the polymer network. The abundant hydroxyl groups of PVA interact with the oxygen- and nitrogen-containing surface functionalities of the CQDs through hydrogen bonding, facilitating their uniform incorporation and immobilization within the matrix. After soaking, the film was removed and dried under ambient conditions to effectively entrap the CQDs and yield a stable fluorescent composite film. The actual CQD loading in the PVA matrix depends on the diffusion, adsorption, and retention of CQDs during immersion; therefore, the PVA/CQD mass ratio cannot be directly determined from the initial CQD dispersion concentration alone. For exploring the device fabrication potential, a commercial 5 mm, 4.0 V UV-LED chip was coated with a 10% (w/v) PVA solution using a simple dip-coating method. The LED chip was dipped into the PVA solution and allowed to dry at ambient temperature for 30 min. The coating procedure was repeated to obtain a second layer, resulting in a uniform PVA coating around the UV-LED. After formation of the PVA coating, the LED chip was immersed in the DMSO dispersion of CQDs (425 µg mL−1) for 30 min to facilitate the diffusion and adsorption of CQDs into the PVA layer. The CQD-loaded LED was subsequently removed from the dispersion and dried at ambient temperature to obtain a uniform luminescent coating. The resulting CQD-coated PVA film and CQD-PVA-coated LED chip exhibited uniform white emission and were subsequently used in solid-state WLE studies. The long-term photostability of the CQD-coated PVA film and the CQD-PVA-coated LED chip was evaluated by storing the samples in plastic zip bags, wrapped in tissue paper, under ambient laboratory conditions for two months. No noticeable change in WLE intensity was observed after the storage period, indicating good long-term photostability under the tested conditions.

3. Results & discussion

3.1. Physicochemical attributes of lignite samples

The physicochemical characterization of the lignite coal was undertaken prior to material synthesis because the elemental composition and chemical makeup of the feedstock are key factors influencing the structural and optical properties of the resulting CQDs. Accordingly, proximate and ultimate analyses were performed to establish the compositional profiles of the lignite samples. Both analyses provided valuable insights into the suitability of the lignite samples as precursors for the synthesis, as shown in Table 1. The low ash contents of lignite-I (12.2%) and lignite-II (7.5%) indicate a lower presence of mineral impurities, which is advantageous for the preparation of CQDs. The presence of excessive inorganic constituents can interfere with carbonization and adversely affect the optical properties of the resulting CQDs. The high volatile matter contents of both lignite-I and lignite-II (44.3% and 40.1%, respectively) suggest the presence of higher reactive organic components that can facilitate the formation of carbonaceous nanostructures during ultrasonication treatment.

Table 1. Assessment of the physicochemical attributes of lignite samples.

Lignite sample Proximate analysis Ultimate analysis
Moisture (%) Ash (%) Volatile matter (%) Fixed carbon Gross calorific value (cal g−1) C (%) H (%) S (%) N (%) O (%)
Lignite-I 15.4 12.2 44.3 28.2 4626.5 51.6 5.2 0.4 2.9 27.8
Lignite-II 20.4 7.5 40.1 31.4 4597 63.9 4.4 0.6 2.8 20.8
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The ultimate analysis also revealed distinct differences between the two lignite samples. The lignite-II had a higher carbon content (63.9%) and lower oxygen content (20.8%) than lignite-I (51.6% carbon and 27.8% oxygen). This result indicates lignite-II has a greater degree of coalification than lignite-I. In contrast, the elevated oxygen content of lignite-I suggests a higher abundance of oxygen-containing functional groups. These groups may serve as reactive sites during the formation of CQDs, and contribute to surface passivation and emissive surface states. Oxygen-rich functionalities are known to play an important role in tuning the optical characteristics of CQDs. A notable feature of both lignite samples is their comparable sulfur and nitrogen contents. The lignite-I contained 2.9% sulfur and 0.4% nitrogen, whereas lignite-II contained 2.8% sulfur and 0.60% nitrogen. These heteroatoms are of particular interest because they can be intrinsically incorporated into the carbon framework or surface functionalities of CQDs during synthesis. They lead to the generation of defect sites and localized electronic states that significantly influence photoluminescence behavior. The presence of sulfur- and nitrogen-containing functionalities is often associated with enhanced surface-state emission and may contribute to the generation of the broadband emission required for white-emitting CQDs. Considering the combined effects of their high carbon content, low mineral impurity levels, and appreciable sulfur and nitrogen concentrations, both lignite-I and lignite-II samples are promising sources for the synthesis of self-passivated heteroatom-functionalized CQDs. The elemental composition of these lignite samples is expected to facilitate the formation of diverse emissive centers and surface states, which are critical for achieving efficient photoluminescence and white-light emission. Therefore, the inherent sulfur and nitrogen in lignite coal provide a strategic advantage by serving as in situ heteroatom sources during CQD synthesis, reducing the need for additional dopant precursors.

3.2. Morphological study

HR-TEM analysis showed that the synthesized CQDs have uniform nanoscale sizes and stay evenly dispersed in DMSO without clumping over time. They appeared as tiny dot-like particles at the nanoscale, with some variation in their sizes (Fig. 2a and b). The size distribution analysis indicated that the CQDs had an average diameter of about 4.3 nm (Fig. 2c). Elemental analysis using HAADF-STEM showed how carbon, oxygen, nitrogen, and sulfur (indicated by red, green, blue and yellow, respectively) are distributed throughout the CQDs, even across particles of different sizes (Fig. 2d and S2). SAED patterns displayed faint bright, diffuse rings, indicating that the synthesized CQDs do not have a long-range crystalline structure and are mainly amorphous (Fig. 2e). The XRD pattern showed a broad, weak hump in the 15–30° range rather than distinct Bragg peaks. This broad peak confirms that the CQDs are amorphous nature and supports the findings from the SAED analysis (Fig. 3f). This broad feature is associated with the (002) reflection of sp2-hybridized carbon, and its wide shape is due to the extremely small crystallite size.

Fig. 2. Morphological and elemental composition of the developed CQDs: (a) HRTEM image captured at a 20 nm scale; (b) a higher-magnified HRTEM image at a 10 nm scale; (c) particle size distribution presented as a histogram; (d) elemental distribution obtained through HAADF-STEM analysis; (e) SAED analysis showing faint concentric diffraction rings; and (f) XRD pattern re-confirming the amorphous nature of the CQDs.

Fig. 2

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Fig. 3. Surface chemistry and functional groups analysis of the synthesized CQDs: (a) spectrum of FTIR study; (b) full XPS survey spectrum; high-resolution deconvoluted spectra of (c) C 1s, (d) O 1s, (e) N 1s and (f) S 2p.

Fig. 3

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The Raman analysis of CQDs dispersed in DMSO revealed an ID/IG ratio exceeding 1.5, confirming the predominantly amorphous carbon nature of the as-prepared CQDs (Fig. S3), consistent with the broad XRD pattern observed. However, the presence of a weak but distinguishable G band at ∼1580 cm−1 confirms the presence of residual sp2 hybridised carbon domains within the amorphous matrix. This observation is reasonable because the G band arises from sp2 C Created by potrace 1.16, written by Peter Selinger 2001-2019 C bonds and never fully vanishes even in heavily disordered carbon systems. The sharp peaks observed at ∼1040 cm−1 and ∼1424 cm−1 are attributed to the symmetric S–C stretching vibration and combination band of DMSO, respectively. As DMSO is a strong Raman scatterer, these features dominate the spectral background and partially suppress the inherently weak CQDs' signal. The broad feature centred around ∼1800 cm−1 is assigned to overlapping contributions from the second-order D + D″ combination band, characteristic of defect-rich nanostructured carbon,29 and surface carbonyl stretching vibrations from the oxygen-rich surface functional groups, as independently confirmed by FTIR and XPS analyses.

3.3. Surface functionalities study

FTIR spectroscopy revealed strong, broad absorption bands at approximately 3375 and 3250 cm−1, corresponding to the stretching vibrations of the phenolic hydroxyl (–OH) and amine (–NH) groups, respectively. The peak around 1650 cm−1 is associated with C Created by potrace 1.16, written by Peter Selinger 2001-2019 O stretching vibrations, while the bands near 1438 cm−1 indicate C–H bending modes. The peaks observed at 1026 and 680 cm−1 correspond to the bending vibrations of C–O and C–S bonds, respectively. The reduced and poorly resolved FTIR intensity in the 915–480 cm−1 region is likely due to a combination of overlapping fingerprint vibrations (C–O, C–C skeletal, aromatic C–H out-of-plane, and possible C–S contributions from residual DMSO), along with increased scattering and lower detector sensitivity in the low-wavenumber region for carbon-based nanomaterials (Fig. 3a). The XPS analysis was also carried out to investigate the surface chemistry and to further validate the FTIR results by examining elemental composition and surface atomic coordination. The XPS survey spectrum showed clear signals corresponding to carbon (284.0 eV), oxygen (531.0 eV), nitrogen (400.0 eV), and sulfur (169.8 eV and 227 eV). The respective atomic percentages were 60.3% for carbon, 38.7% for oxygen, 1.0% for nitrogen, and less than 0.1% for sulfur (Fig. 3b). The decomposition of C 1s spectra (Fig. 3c) reveals multiple distinct components at 284.7, 284.1, 286.5, and 285.6 eV, which can be assigned to C–C/C–H, CE001C, C Created by potrace 1.16, written by Peter Selinger 2001-2019 O, and C–O/C–S/C–N functional groups, respectively. The deconvoluted O 1s XPS profile (Fig. 3d) shows four distinct bands at 531 eV, 532 eV, 532.5 eV, and 533.8 eV corresponding to C Created by potrace 1.16, written by Peter Selinger 2001-2019 O, phenolic –OH, C–OH, and O–C Created by potrace 1.16, written by Peter Selinger 2001-2019 O functionalities, respectively. A higher-resolution view of the N 1s XPS spectrum (Fig. 3e) reveals four well-defined peaks at 398.6, 399.2, 400.0, and 401.1 eV. These are attributed to C–N Created by potrace 1.16, written by Peter Selinger 2001-2019 C, HN Created by potrace 1.16, written by Peter Selinger 2001-2019 C/–CONH–, pyridinic nitrogen, and graphitic nitrogen species, respectively. The deconvoluted S 2p XPS spectrum (Fig. 3f) shows five distinct peaks located at 163.6, 164.8, 166.2, 167.6, and 168.4 eV, corresponding to various sulfur-based chemical environments present in the sample. The first two peaks correspond to the spin–orbit split S 2p doublet (2p3/2 and 2p1/2), confirming the presence of C–S covalent bonding. In contrast, the higher-binding-energy peaks are associated with oxygenated sulfur species, including sulfinic (–C–SO2–), sulfonic (–C–SO3–), and sulfate (–C–SO4–) functional groups. The S 2s signal observed at around 227 eV further supports the presence of sulfur species identified from the S 2p XPS analysis.

3.4. Photophysical study

The effects of different solvents on the emission colour and photoluminescence spectra for developing WLE CQDs were systematically investigated. The photoluminescence spectra revealed consistent blue emission across solvents of varying polarity, including heptane (Hp), hexane (Hx), toluene (Tol), ethanol (EtOH), water (H2O), hydrogen peroxide (H2O2), lactic acid (LA), acetone (Acet), acetonitrile (MeCN), tetrahydrofuran (THF), and dimethylformamide (DMF). The photoluminescence spectra of ethylenediamine (EDA), diethylenetriamine (DETA), and DMSO exhibit broad type emission, resulting in a whitish fluorescence (Fig. 4a). However, near-white light emission spanning almost the entire visible spectrum (400–750 nm) was observed only in DMSO (Fig. 4a). The white-light nature of the emission was verified by deriving the CIE 1931 chromaticity coordinates (x, y) and spectrum from steady-state photoluminescence spectra using the Colour Calculator (Fig. S4 and Table S2). The colour coordinate values fall in the blue region (Fig. 4b), indicating dominant blue emission, likely originating from sp2 core-based emissive states. The CIE 1931 chromaticity coordinates of the emission in DMSO were calculated to be (0.27, 0.34), which are close to the ideal white-light coordinates of (0.33, 0.33) (Fig. 4b). This solvent-dependent fluorescence behavior of CQDs is governed by variations in solvent–surface interactions, which modulate the relative contributions of core (π–π*) and surface-state (n–π*) emissions. In nonpolar and highly polar protic/aprotic solvents, either weak interaction or excessive solvation may lead to predominantly blue emission from isolated sp2 emissive states. In contrast, DMSO, EDA, and DETA provide an optimal interaction environment that balances surface-state stabilization and relaxation pathways, resulting in broadened emission and near white-light output. Although inorganic components are present in the lignite precursor (Table S3), their concentration is very low, as confirmed by the proximate analysis (Table 1). Therefore, their contribution to the observed WLE is negligible, as evidenced by the broad emission profile, nanosecond-scale lifetimes, and strong dependence on the aggregation behavior of developed CQDs. The presence of J-aggregation in DMSO around 570–575 nm is also clearly evident in Fig. 4c. This could be attributed to the extension of the orange-red tail up to 750 nm, resulting in emission across almost the entire visible region (400–750 nm). Further studies of DMSO-dispersed CQDs showed that with increasing concentration from 53.10 µg mL−1 to 850.0 µg mL−1, the emission exhibited a red shift, which confirms the presence of J-aggregation and the formation of aggregation-induced emissive states. This red shifting indicating enhanced interparticle interactions or aggregation effects at higher concentrations of CQDs.30 The colour coordinates of the developed CQDs also shift from pure blue coordinates (0.19, 0.19) to whitish coordinates (0.34, 0.41) as the concentration increases (Fig. 4d). The presence of J-type aggregates in developed CQDs enhances electronic coupling between neighboring emissive centers, promoting the formation of lower-energy excitonic states and facilitating faster radiative relaxation pathways.31,32

Fig. 4. Spectral emission of synthesized CQDs at 390 nm excitation: (a) in different polar and non-polar solvents; (b) CIE chromaticity plot showing different colour coordinates in different solvents; (c) at varying concentrations ranging from 53.1 µg mL−1 to 850 µg mL; (d) CIE chromaticity plot illustrating the variation in colour coordinates with increasing concentration.

Fig. 4

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The UV-vis absorption spectrum of CQDs dispersed (425 µg mL−1) in DMSO shows a broad absorption spectrum spanning 250–700 nm, with two distinct edges at 275 nm and 350 nm (Fig. 5a, black spectrum). Both the absorption edges are attributed to π–π* transitions of C Created by potrace 1.16, written by Peter Selinger 2001-2019 C (sp2 carbon domains) and n–π* transitions of surface functional groups such as C Created by potrace 1.16, written by Peter Selinger 2001-2019 O, C Created by potrace 1.16, written by Peter Selinger 2001-2019 N, and S Created by potrace 1.16, written by Peter Selinger 2001-2019 O. Since the exact concentration of CQDs cannot be precisely determined, the optical density was maintained below 1.0 at the 390 nm excitation wavelength to minimize the inner filter effect. The semitransparent CQD solution in DMSO (425 µg mL−1) exhibited a distinct, continuous spectral coverage in the emission spectrum from blue to near-red (400–750 nm) upon excitation at 390 nm. The broad emission spectrum is consistent with the coexistence of intrinsic π–π* transitions, surface-state-mediated n–π* transitions, and aggregation-induced low-energy emissive states present in the developed DMSO-dispersed CQDs33 (Fig. 5a, blue spectrum). The WLE of the developed CQDs was further confirmed by the CIE chromaticity coordinates, and the calculated value was 0.27, 0.34, nearly matching the ideal white light co-ordinates of 0.33, 0.33 (Fig. 5b). The CIE chromaticity analysis also yielded a CCT of 8612 K and a CRI of 76, confirming the daylight-white emission characteristics of the synthesized CQDs. The quantum yield of the CQDs was calculated to be 21%, using quinine hemisulfate monohydrate as the standard reference solution (in 0.5 M sulphuric acid) at a 360 nm excitation wavelength. TCSPC analysis showed two distinct emissive components, centred at 480 nm and 570 nm, respectively (Fig. 5c and d). This indicates that the observed emissions are fluorescence-type and may arise from two structurally different cores. The 480 nm emission exhibits a slightly longer lifetime (2.74 ns) than the 570 nm emission (2.59 ns), suggesting differences in the relaxation dynamics of the two emissive states. The shorter lifetime of the 570 nm emission is consistent with a relatively faster decay process, potentially associated with surface-state-related or aggregation-induced emissive centres. Whereas the longer-lived 480 nm emission may originate from a more localized sp2 core-related emissive state. This interpretation is further supported by the UV-vis absorption features at ∼275 and ∼350 nm, the concentration-dependent red-shifted emission indicative of J-type aggregation, and the presence of abundant oxygen, nitrogen, and sulfur-containing surface functional groups.

Fig. 5. Optical properties and excited-state dynamics of the synthesized CQDs: (a) UV-vis absorption and fluorescence spectra (at excitation of 390 nm); (b) CIE chromaticity colour coordinates corresponding to the WLE of the CQDs; (c) tri-exponential fitting of the TCSPC decay profile recorded at excitation 405 nm and emission 480 nm; (d) tri-exponential fitting of the TCSPC decay profile recorded at excitation 405 nm and emission 570 nm.

Fig. 5

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The Tauc-derived optical transition energy of white-emitting CQDs was calculated to be 3.56 eV (Fig. 6a) using the Tauc equation:

(αhν)2 = A(hν − Eopt) 1

Here, α is the absorption coefficient, hν is the photon energy, A is the independent constant, and Eopt denotes the estimated optical transition energy. Considering the heterogeneous structural nature of CQDs, including the presence of diverse carbon-core domains, surface functionalities, and multiple emissive states, the obtained value of 3.56 eV is considered an approximate optical transition energy rather than a definitive fundamental band-gap value. This relatively high optical transition energy reflects the higher-energy electronic states associated primarily with the conjugated carbon framework. Furthermore, a detailed photophysical analysis showed that the synthesized CQDs exhibited adaptive CCT tunability with an almost constant CRI via controlled modulation of lignite mass and post-synthetic dilution. By varying the amount of lignite precursor while maintaining a constant DMSO volume of 5.0 mL during synthesis, the WLE characteristics of the CQDs could be systematically tuned. For CQDs synthesized from 0.08 g of lignite, the first dilution showed a warm-white emission with a CCT of 4535 K and a CRI of 80, whereas further dilution increased the CCT to 7504 K with a corresponding CRI of 74. Similarly, CQDs synthesized from 0.05 g of lignite exhibited CCT values of 5055 and 8612 K with CRI values of 79 and 76 for the first and second dilutions, respectively. For CQDs synthesized from 0.03 g of lignite, the CCT increased from 6078 K (CRI = 75) to 11 258 K (CRI = 74) upon dilution. As summarized in Table 2, both precursor loading and dilution significantly influence the WLE characteristics (from near warm-white to daylight-white) of the CQDs (Fig. 6c). A systematic increase in CCT was observed as lignite concentration decreased and dilution increased (Fig. 6b), indicating a progressive shift toward daylight-white emission. This observation suggests an enhanced contribution of the higher-energy emissive component relative to the lower-energy surface- or aggregation-induced emission. The behaviour is consistent with the dual-emission character identified by TCSPC analysis and the presence of multiple emissive states observed in the photoluminescence spectra. The CQDs exhibit two emissive components centred at 480 and 570 nm, corresponding to higher and lower-energy emissive states, respectively. At higher concentrations, the longer-wavelength (lower energy) emissive component becomes more prominent, leading to warmer WLE with lower CCT values. In contrast, dilution reduces the relative contribution of the lower-energy emission and enhances the spectral weight of the lower-wavelength (higher-energy) emissive state, resulting in progressively cooler WLE and higher CCT values.

Fig. 6. Electronic and colour tunable properties of the synthesized CQDs: (a) bandgap calculation using Tauc equation; (b) CCT-based chromaticity corresponding to the WLE of the CQDs; (c) tunable WLE of CQDs with correlated colour temperatures ranging from near warm to natural white under 365 nm UV excitation torch.

Fig. 6

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Table 2. The CCT tunability values through precursor concentration control and post-synthetic dilution of lignite-derived CQDs.

Lignite amount (g) First diluted CQDs concentration (µg mL−1) CCT (K) CRI CIE coordinates Second diluted CQDs concentration (µg mL−1) CCT (K) CRI CIE coordinates
0.08 1360 4535 80 0.37, 0.42 680 7504 74 0.28, 0.36
0.05 850 5055 79 0.34, 0.41 425 8612 76 0.27, 0.34
0.03 510 6078 75 0.31, 0.39 255 11 258 74 0.25, 0.32
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This observed CCT tunability is likely associated with the heterogeneous electronic structure of the CQDs, which comprises intrinsic sp2 carbon-core emissive domains and surface-functional-group-induced energy states.34 Changes in lignite precursor concentration modify the relative population and distribution of these emissive centers during CQD formation, thereby influencing the spectral balance of the emitted light. Consequently, the relative contributions of high and low-energy emission components can be adjusted, resulting in systematic tuning of the WLE characteristics from warm-white to cool-white.35 This tunability from near warm-white to daylight-white while maintaining CRI values in the range of 74–80 highlights the capability of lignite-derived CQDs to serve as versatile single-component white-light emitters with controllable photometric properties.

The quality of WLE from the synthesized CQDs was further evaluated using the CRI and CQS, two widely accepted metrics for assessing the fidelity of colour reproduction relative to a reference illuminant, such as natural daylight. Fig. 7a presents the individual CRI values for the 14 standard test colour samples (R1–R14), along with the general CRI (Ra). Most test colour samples exhibited acceptable to good colour-rendering performance. However, the R9 sample (strong saturated red) displayed a markedly negative value, indicating a poor deficiency in reproducing saturated red hues. Such behavior is commonly associated with an insufficient red spectral component in the emission profile of the light source. The CRI colour vector diagram shown in Fig. 7b displays the chromaticity shifts of the 14 test colour samples in the CIELAB a–b colour space. This is under the CQD-based white-light source relative to the reference illuminant. In the diagram, the red squares represent the ideal colour coordinates under the reference source, whereas the black circles correspond to the coordinates obtained under the CQD emission. Perfect colour rendering would result in a complete overlap of the two datasets. However, noticeable deviations are observed for several colour samples, particularly R9, which exhibits a substantial displacement consistent with its strongly negative CRI value. Furthermore, the test polygon is compressed in the red–orange region, confirming the poor reproduction of red tones. In contrast, green- and blue-related samples, such as R3 and R12, exhibit relatively smaller chromaticity shifts, indicating improved rendering fidelity in these spectral regions. The CQS analysis, shown in Fig. 7c, provides a complementary assessment of colour quality using 15 highly saturated test colour samples (Q1–Q15) along with the average CQS (Qa). The CQD-based white-light source demonstrates superior performance in the green–cyan region (Q6–Q8), whereas significantly lower scores are observed for the red and purple regions (Q12–Q13), highlighting reduced rendering capability for these colours. The corresponding CQS colour vector diagram in Fig. 7d maps the chromaticity coordinates of all 15 test colour samples in the CIELAB a–b space. Similar to the CRI analysis, the test polygon is generally smaller than the reference polygon, indicating that the emitted white light renders most colours with reduced saturation. The largest deviations are observed in the red (Q1, Q12, and Q13) and blue–purple (Q14 and Q15) regions, whereas the green–yellow region (Q5–Q7) exhibits the closest agreement with the reference illuminant, consistent with the higher CQS values obtained for these samples.

Fig. 7. Evaluation of the colour-rendering characteristics of the developed white-light-emitting CQDs: (a) individual CRI values for the 14-standard test colour samples (TCS01–TCS14); (b) CRI colour vector diagram depicting the corresponding chromaticity shifts; (c) individual CQS values for the 15 saturated test colour samples; and (d) CQS colour vector diagram depicting the corresponding chromaticity shifts.

Fig. 7

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Overall, the CQD-based white-light source achieved a general CRI (Ra) of approximately 75 and a CQS (Qa) of about 40, indicating moderate colour-rendering performance. Both CRI and CQS analyses consistently reveal a significant limitation in reproducing saturated red and purple colours, as evidenced by the strongly negative R9 value and the large chromaticity deviations observed in the corresponding vector diagrams. Conversely, green and blue–green colour samples exhibit comparatively better rendering performance. The colour vector analyses further demonstrate a systematic inward compression of the colour gamut relative to the reference illuminant, resulting in an overall desaturation of rendered colours. These results suggest that, although the developed CQDs are capable of generating white-light emission with reasonable colour fidelity, further spectral optimization, particularly enhancement of the red-emission component, is required to achieve high-quality solid-state lighting performance.

3.5. Mechanism of WLE

Based on the collective microscopic, spectroscopic, and photophysical results, the WLE of the CQDs is attributed to the synergistic contributions of intrinsic carbon-core states, surface-related emissive states, and aggregation-induced low-energy states. The HRTEM, Raman, UV-vis absorbance, and the TCSPC studies collectively support the presence of sp2-conjugated carbon domains and their contribution to the higher-energy emission. Upon excitation, excitons generated within the sp2-conjugated carbon domains can undergo radiative recombination either directly via intrinsic core states or via surface-associated states with different energy distributions. This process produces emissions spanning the blue spectral region.

The FTIR, XPS and solvent-dependent emission behavior further suggest that the relative population and emission characteristics of the surface-related states are strongly influenced by the local chemical environment. In most solvents, the emissive process is mainly driven by intrinsic carbon-core and surface-related states, resulting in predominantly blue emission. However, in DMSO, its highly polar aprotic nature and strong hydrogen-bond-accepting ability enable selective interactions with surface functionalities such as hydroxyl, amino, and carboxyl groups. Such solvent–surface interactions can alter the electronic distribution and stabilization of surface-localized states associated with heteroatom-containing functional groups. Consequently, additional lower-energy emissive pathways become accessible, giving rise to enhanced emission in the green-yellow spectral region.

The solvent-mediated modification of the CQD surface environment may also influence interparticle interactions while maintaining accessible aromatic sp2-conjugated domains. This facilitates the self-assembly of neighboring CQDs through π–π interactions between aromatic domains, potentially assisted by hydrogen-bonding interactions involving surface functional groups. The resulting intermolecular electronic coupling can promote the formation of J-type aggregated domains and generate energetically stabilized low-energy emissive states. The concentration-dependent red shift, together with the wavelength-resolved TCSPC results and the emergence of a distinct longer-lived decay component toward the red spectral region, provides further evidence for the contribution of aggregation-induced emissive species.

Accordingly, the formation of J-aggregated domains introduces additional low-energy states that coexist with the intrinsic carbon-core and surface-related emissive centers. Photoexcited carriers can therefore undergo multiple relaxation and radiative recombination pathways spanning a broad range of energies. As schematically illustrated in Fig. 8, the higher-energy blue emission is attributed predominantly to intrinsic carbon-core states associated with the sp2-conjugated domains, the intermediate green-yellow emission is associated primarily with heterogeneous surface-related states influenced by oxygen and nitrogen-containing functionalities, and the lower-energy orange-red emission originates from aggregation-induced states generated through intermolecular electronic coupling and J-type aggregation. The simultaneous contribution of these emissive channels results in a continuous broadband emission across the visible region, ultimately giving rise to the observed WLE.36

Fig. 8. Schematic illustration of the proposed mechanism of WLE from lignite-derived CQDs dispersed in DMSO.

Fig. 8

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3.6. Solid-state implementations

To evaluate the potential of the synthesized white-emitting CQDs for solid-state lighting applications, their WLE behavior was investigated by incorporating them into a PVA matrix (Fig. 10a and b). The photoluminescence spectrum of the CQDs incorporated in the PVA composite film exhibited a broad emission spanning the visible region, closely resembling the WLE observed in the corresponding colloidal dispersion (Fig. 9a). Under 390 nm UV excitation wavelength, the CIE chromaticity coordinates, CCT, and CRI of the composite film were measured as (0.26, 0.32), 10 374 K, and 75, respectively. These values are comparable to those of the CQD solution, which exhibited chromaticity coordinates of (0.27, 0.34), a CCT of 8612 K, and a CRI of 76, indicating that the PVA matrix effectively retains the intrinsic optical properties of the CQDs in the solid state (Fig. 9b). The slight changes in the chromaticity coordinates, CCT, and CRI of the CQD-incorporated PVA film can be attributed to CQDs–PVA interactions, changes in the local dielectric environment, and minor aggregation effects within the polymer matrix, which cause subtle modifications in the emission characteristics of the CQDs.

Fig. 10. Solid-State incorporation of CQDs into PVA film and CQD–PVA-coated LEDs.

Fig. 10

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Fig. 9. Solid state implementation of WLE from CQDs: (a) the fluorescence emission of CQDs solution and CQDs-soaked PVA film under 390 nm UV illumination, along with the corresponding inset images captured under each illumination condition; (b) the CIE chromaticity coordinates of both CQDs solution and CQDs-soaked PVA film under 390 nm UV excitation.

Fig. 9

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Furthermore, the CQDs incorporated PVA composite was coated onto a commercial UV LED chip and evaluated under applied electrical current using 3.0 V micro lithium cells (Fig. 10c). The prepared LED was intended primarily as a proof-of-concept demonstration of the practical applicability of the developed WLE-CQDs. The fabricated CQD-soaked, PVA-coated LED chip emitted bright white light, enabling clear visual recognition of a thumb under dark conditions with natural white illumination (Fig. 10e). In contrast, illumination using the bare UV LED chip produces a bluish-violet colour, which provides insufficient visible-light output for effective visual recognition of the thumb (Fig. 10d). These results demonstrate the capability of the CQDs soaked PVA film as an efficient colour-conversion layer for white-light-emitting device applications. A comparative study of the lignite-derived white-emitting CQDs and other reported white-emitting materials was conducted. The analysis in Table S4 revealed that the lignite-derived CQDs offer a superior sustainable material platform owing to their eco-friendly synthesis, low-cost and abundant precursor source, and comparable WLE characteristics relative to previously reported materials.

4. Conclusion

In summary, single-component white-emitting CQDs were successfully synthesized from Indian lignite coal via a simple, one-step, DMSO-assisted ultrasonication process at room temperature. Importantly, the white-light emission exhibited broad CCT tunability from near warm-white (4535 K) to daylight-white (11 258 K) via simple precursor mass control, eliminating the need for compositional modification or additional emissive components while maintaining a stable CRI of 74–80. The optimized CQDs solution displayed broadband emission across the visible region (400–750 nm), with CIE chromaticity coordinates of (0.27, 0.34), a CCT of 8612 K, and a CRI of 76. Comprehensive spectroscopic and photophysical investigations revealed that the observed WLE originates from the synergistic contributions of intrinsic carbon-core states, surface-related emissive states, and aggregation-induced low-energy emissive centres. The obtained CRI and CQS values indicate that the developed CQDs can generate white light with reasonable colour fidelity; however, further enhancement of the red-emission component would be beneficial for achieving superior solid-state lighting performance. Furthermore, the successful incorporation of the CQDs into the PVA matrix and UV-LED chip coating demonstrated their feasibility for solid-state white-light generation, highlighting their potential for application-specific everyday illumination. Although the product yield in the present study is relatively low (10.2%), the resulting CQDs exhibit a favorable quantum yield (21%) and excellent white-light-emitting photophysical properties; further optimization of the synthesis process to improve product yield and reduce solvent consumption will be explored in future work. This work not only demonstrates a sustainable route for converting low-value lignite coal into value-added photonic nanomaterials but also opens new opportunities to utilize inexpensive carbonaceous feedstocks in developing tunable white-light-emitting materials for next-generation lighting applications.

Author contributions

Conceptualization, methodology, experiment performing and writing of original draft (Tuhin Mandal); funding support, review and editing of the manuscript (Rabi Narayan Senapati); fluorescence spectrophotometer acquisition and study (Ashish Ghosh); supervision, reviewing and editing of the manuscript (Reginald Ebhin Masto); conceptualization, methodology, supervision, funding, reviewing, and editing draft (Vikram Singh).

Conflicts of interest

The authors declare no conflicts of interest.

Supplementary Material

RA-OLF-D6RA06953C-s001

Acknowledgments

The authors thank CSIR-Central Institute of Mining and Fuel Research, Dhanbad, for funding (MLP-190/2025-26) and for providing the facilities. The authors acknowledge the Central Research Facility, Indian Institute of Technology (ISM) Dhanbad, for the HRTEM, Time-resolved fluorescent spectroscopy, and XPS study. The authors would also like to thank Prof. Soumit Chatterjee, Department of Chemistry, Indian Institute of Technology (ISM) Dhanbad, for the acquisition of the Steady-state spectrophotometric study. The CSIR-IITR manuscript communication number is IITR/SECC-PME/MSS/2026/066.

Data availability

The data sets used and/or analyzed during the current study are available from the corresponding authors upon reasonable request.

Supplementary information (SI): the data supporting this article are included in the SI. SI: SI 1. Analytical methods: SI 1.1. Morphology study using HRTEM analysis, SI 1.2. Lifetime study using TCSPC analysis, SI 1.3. Inorganic composition of Lignite using XPS study; SI 1.4. Surface functional group study using FTIR; SI 1.5. Surface elemental composition study using XPS; Fig. S1. The optical properties of two different CQDs derived from lignite-I and lignite-II, Fig. S2. HAADF-STEM characterization of the developed CQDs: (a) HAADF-STEM image; (b) combined elemental maps of C, O, N, and S; (c) elemental map of carbon; (d) elemental map of nitrogen; (e) elemental map of oxygen; (f) elemental map of sulphur; Fig. S3. RGB colour contributions measured from the photoluminescence emission spectra of the as-synthesized CQDs derived from lignite; Table S1. Solvent-dependent CIE 1931 chromaticity co-ordinates, CRI and CCT values of the various CQDs; Table S2. The inorganic composition present in the Lignite samples, Table S3. The inorganic composition present in the Lignite samples. See DOI: https://doi.org/10.1039/d6ra06953c.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

RA-OLF-D6RA06953C-s001
RA-OLF-D6RA06953C-s001.pdf (1.8MB, pdf)

Data Availability Statement

The data sets used and/or analyzed during the current study are available from the corresponding authors upon reasonable request.

Supplementary information (SI): the data supporting this article are included in the SI. SI: SI 1. Analytical methods: SI 1.1. Morphology study using HRTEM analysis, SI 1.2. Lifetime study using TCSPC analysis, SI 1.3. Inorganic composition of Lignite using XPS study; SI 1.4. Surface functional group study using FTIR; SI 1.5. Surface elemental composition study using XPS; Fig. S1. The optical properties of two different CQDs derived from lignite-I and lignite-II, Fig. S2. HAADF-STEM characterization of the developed CQDs: (a) HAADF-STEM image; (b) combined elemental maps of C, O, N, and S; (c) elemental map of carbon; (d) elemental map of nitrogen; (e) elemental map of oxygen; (f) elemental map of sulphur; Fig. S3. RGB colour contributions measured from the photoluminescence emission spectra of the as-synthesized CQDs derived from lignite; Table S1. Solvent-dependent CIE 1931 chromaticity co-ordinates, CRI and CCT values of the various CQDs; Table S2. The inorganic composition present in the Lignite samples, Table S3. The inorganic composition present in the Lignite samples. See DOI: https://doi.org/10.1039/d6ra06953c.

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