S,N-Codoped oil-soluble fluorescent carbon dots for a high color-rendering WLED

Quan Wang a, Yixun Gao a, Boyang Wang b, Yuanyuan Guo a, Umar Ahmad c, Yanqing Wang d, Yao Wang *a, Siyu Lu *b, Hao Li *a and Guofu Zhou a
aGuangdong Provincial Key Laboratory of Optical Information Materials and Technology, Institute of Electronic Paper Displays, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, P. R. China. E-mail: wangyao@m.scnu.edu.cn; haoli@scnu.edu.cn
bHenan Institute of Advanced Technology, College of Chemistry, Zhengzhou University, Zhengzhou 450001, P. R. China. E-mail: sylu2013@zzu.edu.cn
cDepartment of Chemistry, Faculty of Science and Arts and Promising Centre for Sensors and Electronic Devices, Najran University, Najran 11001, Kingdom of Saudi Arabia
dVaritronix (He Yuan) Display Technology Limited, He Yuan 517000, P. R. China

Received 3rd January 2020 , Accepted 26th February 2020

First published on 26th February 2020


Carbon dots (CDs) have attracted widespread attention in light-emitting-diode (LED) applications owing to their environmental friendliness, easy processing and unique optical properties. For the practical use of CDs in LEDs, the controllable synthesis of high performance oil-soluble carbon dots is still in urgent demand. In this study, S,N-codoped oil-soluble carbon dots (S,N-OCDs) with an average size of 3.38 nm were prepared by solvothermal reaction of acetone, dimethyl trithiocarbonate (DMTTC) and nitric acid. Characterization results reveal that both the conjugated aromatic π systems from the aldol condensation and highly active free radicals from DMTTC essentially dominate the development of these yellow-green emitting S,N-OCDs, which possess high carbonization, quantum yield (21.08%) and oil-solubility. Furthermore, a white light-emitting-diode (WLED) was fabricated through mixing the obtained S,N-OCDs and epoxy and drop-casting the mixture on the surface of gallium nitride (GaN)-based blue chips. The as-prepared WLED showed excellent color rendering properties (CCT of 5389 K, CIE coordinates of (0.33, 0.30), and CRI of 88.38).


Introduction

As a new generation of solid-state lighting, white light-emitting-diodes (WLEDs) have blossomed into the most common lighting source because of their superiorities of long operational life, high color rendering index and energy conversion compared to traditional incandescent and fluorescent lamps.1,2 Among many proposed design schemes for WLEDs, the combination of yellow-emitting phosphors with gallium nitride (GaN)-based blue chips is nowadays widely adopted in commercial applications.3,4 Compared with the multiple phosphor-strategy, the employment of single-phase phosphors such as Y3Al5O12:Ce3+ is easier for manufacture and packaging, without concerning the ratio of multiple phosphors. However, the high reaction temperature (over 1200 °C) and non-regeneration limit the widespread applications of single-phase phosphors.5,6 Meanwhile, the low color rendering index (CRI) of the WLED based on these single-phase phosphors also hinders their applications in high-quality lighting and displays.3 Hence, the quantum dot-based WLED has been developed to solve the above-mentioned problems, although toxic substances such as CdS and CdSe will inevitably be introduced.7,8 Overall, it is still required to find environment-friendly color converting materials for fabricating the next generation of WLEDs.

Very recently, carbon dots (CDs), as novel luminescent carbon nanoparticles with the size of less than 10 nm, have started to attract more and more attention from related researchers. CDs offer outstanding advantages of adjustable illumination range, high photostability and low toxicity.9,10 Therefore, carbon dots are expected to be potential replacements for phosphors in WLEDs. For example, CD/polymer composites have been utilized to obtain high CRIs for WLEDs.11–13 However, there are still at least two challenges that CDs face as color converting materials in WLEDs. That is, (i) to reach a high color rendering index (CRI > 85), and (ii) most of the reported CDs are hydrophilic, which is difficult for the fabrication of LEDs or optoelectronic devices under a strictly oil-soluble condition.14–16 Apparently, oil-soluble CDs with a high CRI have been pursued for applications in WLEDs.

Up to now, surface passivation and chemical oxidation have been common approaches to improve the properties of CDs, while they are not suitable for large-scale modification due to the complex processes.17,18 Heteroatom-doping as a simple and efficient method to improve luminescence has attracted intensive attention. Heteroatom-doping will not only adjust the band gap but also modify the electron density of CDs.19,20 Particularly, the S atom will further narrow the band gap, since it has lower electronegativity,21 and induce unique synergistic effects together with the N atom, which enables higher luminescence properties and longer wavelength emission. On the other hand, long chain alkanes are usually selected as surface deactivators or capping molecules to prepare oil-soluble CDs. However, this will inevitably cause the lack of active groups on the surface and shorter conjugated lengths in the carbon core of the as-prepared CDs, weakening the fluorescence properties of the CDs.22,23 We speculated that small molecules with multiple methyl groups would meet the requirements of fluorescence properties and oil-solubility of CDs. Therefore, precursors simultaneously containing S and N atoms and multiple methyl groups were considered as the best choice for the synthesis of high performance oil-soluble CDs.

In this study, acetone was employed as the carbon source and solvent to form CDs through aldol condensation.24 Dimethyl trithiocarbonate (DMTTC) was chosen as the sulfur (S) source for its high sulfur content and nitric acid was selected as the nitrogen (N) source owing to its good reactivity and acidity for the aldol condensation reaction. Different characterization techniques showed that the S,N-codoped oil-soluble carbon dots (S,N-OCDs) with multiple methyl groups were synthesized by a one-step solvothermal reaction. The as-prepared S,N-codoped oil-soluble carbon dots exhibited bright yellow-green emission (quantum yield: 21.08%), featuring a large emission wavelength range from 455 to 585 nm. Furthermore, the single-component S,N-OCDs were used as a color converting material to fabricate a WLED, which showed excellent color rendering properties (CCT of 5389 K, CIE coordinates of (0.33, 0.30), and CRI of 88.38).

Experimental section

Materials

Dimethyl trithiocarbonate (C3H6S3; DMTTC, 98%) was purchased from Sigma-Aldrich (USA). Acetone (C3H6O; ≥99.5%) was obtained from Guangzhou Chemical Reagent Co., Ltd (Guangzhou, P. R. China). Nitric acid (HNO3; 68%) was obtained from Tianjin Zhiyuan Chemical Reagent Co., Ltd (Tianjin, P. R. China). Rhodamine 6G (99%, laser grade) was purchased from Acros Organics (New Jersey, USA). GaN LED chips were provided by Shenzhen Looking Long Technology Co., Ltd (Shenzhen, P. R. China). A stainless steel autoclave was purchased from Hua Sin Science Co., Ltd (Guangzhou, P. R. China). Epoxy was purchased from Hunan Baxiongdi New Material Co., Ltd (Changsha, P. R. China). All other chemical reagents were analytically pure and directly used without further purification. In addition, milipore filter (0.22 μm, nylon) was obtained from Shanghai Titan Scientific Co., Ltd (Shanghai, P. R. China). Dialysis bags (RC, molecular weight cut-off: 1000 Da) were obtained from Shanghai green Bird Science & Technology Development Co., Ltd (Shanghai, P. R. China).

Synthesis of S,N-OCDs

S,N-OCDs were synthesized by a one-step solvothermal method. Typically, 111 μL of DMTTC and 50 μL of HNO3 were mixed together in 10 mL of acetone, and then poured into a stainless steel autoclave with poly(tetrafluoroethylene) lining (total capacity: 25 mL). After heating at 220 °C under a sealed condition for 10 hours, the autoclave was naturally cooled to room temperature. The resulting black-brown solution was first filtered through a 0.22 μm milipore filter to remove large particles and aggregates, and further purified by water dialysis for 3 days. Finally, the pure S,N-OCDs were obtained by lyophilization.

Characterization of S,N-OCDs

The morphology of the S,N-OCDs was observed using transmission electron microscopy (TEM) operated at 200 kV (JEM-2010, JEOL, Japan). Here, the particle sizes of the S,N-OCDs were measured using nanomeasure software, and their distribution was calculated using the Gaussian model. Crystal structure was determined by X-ray powder diffraction (XRD; D8 Advance, Bruker, Germany), using Cu-Kα as the incident radiation. Surface chemical groups were characterized by Fourier transform infrared (FT-IR) spectrophotometry (Vertex 70, Bruker, Germany) in the spectral range of 4000–400 cm−1 with the resolution of 2 cm−1 using the potassium bromide (KBr) plate method. Element composition was analyzed by using X-ray photoelectron spectroscopy (XPS; Escalab 250Xi, Thermo Fisher, UK) with a mono X-ray source with Al Kα as the incident radiation. Binding energy calibration was based on C 1s at 284.8 eV. Elemental analysis was carried out using an organic elemental analyzer (EA-3000, Eurovector, Italy). Ultraviolet-visible (UV-vis) absorption spectra were obtained using a UV-vis spectrophotometer (UV-1750, Shimadzu, Japan) via placing the sample in a 10 mm optical path length quartz fluorescence cuvette. Photoluminescence (PL) spectra were obtained using a luminescence spectrometer with a slit width of 5 mm (RF-6000, Shimadzu, Japan).

Calculation of photoluminescence (PL) quantum yield and lifetime

The QY of the S,N-CDs was calculated according to the following equation:
φx = φst(Ix/Ist)(ηx2/ηst2)(Ast/Ax)
for which the deionized water solution of Rhodamine 6G [QY = 0.95; excitation wavelength (Ex) = 488 nm] was chosen as the standard. Here, φ is the QY, I is the measured integrated emission intensity, η is the refractive index, and A is the optical density. The subscript “st” refers to the Rhodamine 6G standard, and “x” refers to the deionized water solution of the S,N-OCDs as the sample. In order to minimize reabsorption effects, the absorption in the 10 mm quartz cuvette was kept below 0.10 at the excitation wavelength (460–500 nm). The PL lifetime decays were recorded and calculated using a Fluorolog-3 fluorescence spectrophotometer (Horiba, USA).

Fabrication and characterization of a WLED

In order to fabricate a WLED based on S,N-OCDs, commercial GaN LED chips with the peak wavelength centered at 460 nm were employed. The operating voltage and working current of the GaN LED chips were 4.0 V and 20 mA. Typically, the S,N-OCD powders were homogeneously dispersed into epoxy and then drop-casted on the surface of the LED chip. Finally, the product was obtained by drying at room temperature for 6 hours. Its photoelectric properties were measured by using an integrating sphere spectroradiometer system (LTS-1500, MulanSphere, P. R. China).

Results and discussion

Optimization of preparation conditions

In this work, S,N-OCDs were produced by a simple solvothermal method using acetone, dimethyl trithiocarbonate and nitric acid as precursors. As two key preparation factors, both reaction time and temperature have priority in being optimized for high fluorescence performance S,N-OCDs. First of all, the formation temperature was tested at 160, 180, 200, 220 and 240 °C, respectively, for the same reaction time of 10 hours. Fig. 1A displays a marked effect of temperature on the QY of the resulting S,N-OCDs. In the temperature range from 160 to 220 °C, the QY reached its peak at 220 °C, followed by a large reduction with rising temperature. Fig. 1B further testifies the direct influence of reaction time on the QY of the resulting S,N-OCDs at the optimum temperature of 220 °C. Obviously, the QY gradually increased up to a maximum with the reaction time of 10 hours, and a following decrease was observed for longer times. Fig. S1 (ESI) displays the PL emission spectra of the optimized S,N-OCDs. Thus, the heating temperature of 220 °C and the reaction time of 10 hours were determined as the optimized preparation conditions for S,N-OCD formation.
image file: d0tc00016g-f1.tif
Fig. 1 Correlation curves between QY of S,N-OCDs and two key preparation factors: heating temperature for the same reaction time of 10 hours (A), and reaction time at the same heating temperature of 220 °C (B).

Morphologies of S,N-OCDs

As shown in Fig. 2A and B, the as-synthesized granular S,N-OCDs were well dispersed with high density, and they exhibited an average particle size of approximately 3.38 nm with a very narrow distribution. The high-resolution TEM image showed that most particles are amorphous carbon particles without any lattices; a few particles possess clear lattice fringes with the average lattice spacing of 0.21 nm (Fig. 2C). Distinctly, this was consistent with the in-plane lattice spacing of graphene (100), indicating the graphite-like structure of the S,N-OCDs.25 In Fig. 2D, a similar structure of the S,N-OCDs was found: the weak peak of graphite phase 100 centered at 41° represents the mentioned interlayer spacing of 0.21 nm.26 This also implies that there are only a few lattice fringes in the S,N-OCDs. Another broad diffraction peak located at 20° indicated a highly amorphous carbon phase.24 There was also no obvious D or G band in the Raman spectra of the S,N-OCDs because of a low carbon-lattice-structure content (Fig. S2, ESI).
image file: d0tc00016g-f2.tif
Fig. 2 Morphology characterization of the S,N-OCDs: TEM image (A), particle size distribution (B), high-resolution TEM image (C), and XRD pattern (D).

Surface chemical characterization of the S,N-OCDs

The chemical composition and structure of the resulting S,N-OCDs were analyzed in different ways, especially the surface chemical elements and groups for directly determining the luminescence properties. Fig. 3A displays the FTIR analysis of the S,N-OCDs. There was a broad absorption band around 3255 cm−1 that was attributed to the stretching vibration of a nitrogen–hydrogen single bond (N–H).27 Two strong peaks (2957–2925 and 1450–1372 cm−1) were, respectively, assigned to the stretching and bending vibrations of the carbon–hydrogen single bond (C–H) of methyl groups, which would result in good oil-solubility. A few absorption bonds around 1661, 1601 and 1120 cm−1 corresponded to the stretching vibrations of the carbon–nitrogen (C[double bond, length as m-dash]N) or carbon–oxygen double bond (C[double bond, length as m-dash]O), carbon–carbon double bond (C[double bond, length as m-dash]C), and carbon–nitrogen single bond (C–N), respectively.28 Particularly, two characteristic peaks at 1187 and 1038 cm−1 were ascribed to the carbon–hydrogen (C–O) or carbon–sulfur single bond (C–S), and sulfonic groups (–SO3), respectively.29 The manifestations of S,N-codoping were amino and –SO3 groups on the surface of the carbon core.
image file: d0tc00016g-f3.tif
Fig. 3 Surface chemical characterization of the S,N-OCDs: FT-IR spectrum (A); high resolution XPS spectra of C 1s (B), N 1s (C) and S 2p (D) of the prepared S,N-OCDs.

Besides, EA and XPS provided more chemical information on the S,N-OCDs. Fig. S3 (ESI) shows that the abundances of component carbon, oxygen, nitrogen and sulfur on the surface of the S,N-OCDs reached 80.15%, 13.71%, 3.87% and 2.27%, respectively. Herein, the high surface C content was very noticeable and exactly in accord with the EA results (Table S1, ESI), but the reverse of the expected high doping content of S and N. The high-resolution C 1s spectrum was further differentiated into three peaks at 284.8, 286.24 and 288.9 eV corresponding to C 1s states in the carbon–carbon bond (C–C)/C[double bond, length as m-dash]C, C–O/C–N/C–S and C[double bond, length as m-dash]O/C[double bond, length as m-dash]N bonds, respectively (Fig. 3B).26,30 The high-resolution O 1s spectrum contained two peaks at 532.1 and 533.3 eV corresponding to C[double bond, length as m-dash]O and C–O bonds, respectively (Fig. S3, ESI).31 Similarly, pyridinic N, graphitic N, and amino N, were represented at 398.5, 399.3 and 400.1 eV in the N 1s spectrum, respectively (Fig. 3C).11 In the S 2p spectrum, shown in Fig. 3D, two peaks at 163.9 and 165.1 eV corresponded to the C–S–C covalent bond of S 2p3/2 and S 2p1/2 spectra, and another peak at 167.9 eV was attributed to sulfone bridges (–C–SOx–C–).32,33 These elemental signals were in complete accord with the FT-IR spectrum, supporting the partial S,N-codoping.

Optical properties of S,N-OCDs

As shown in Fig. 4A, the absorption peak in the range from 230 to 250 nm was attributed to the π–π* transition of conjugated C[double bond, length as m-dash]C in the core of the S,N-OCDs,34,35 while the peak ranging from 290 to 320 nm was attributed to the n–π* transition of the carbon–heteroatom double bond (C[double bond, length as m-dash]X, X[double bond, length as m-dash]O or N).34–36Fig. 4B displays the optical photographs of the S,N-OCD solution (in acetone) under daylight and 460 nm excitation light. According to the fluorescence emission spectra at different excitation wavelength (λex), these S,N-OCDs exhibited an obvious excitation-dependent effect with a maximum photoluminescence (PL) emission at 545 nm (λex = 480 nm), and we further highlighted this effect by normalization (upper left inset in Fig. 4C). The strong yellow-green luminescence with a high QY of 21.08% at 480 nm originated from the inherent quantum size effect and special surface excited states induced by surface functional groups (i.e. amino and –SO3). Moreover, the PL decay curve of the S,N-OCDs was obtained by fitting with a single-exponential function (Fig. 4D). As calculated, the half-life time of the S,N-OCDs was up to 43.99 ns, which depends on the luminescence center with the electronic conjugated structure rather than the surface modification, as supported by the UV absorption of the sp2 aromatic domains around 245 nm.30
image file: d0tc00016g-f4.tif
Fig. 4 Photoluminescence characterization of the S,N-OCDs: UV-vis absorption spectrum (black; CDs concentration: 0.1 mg mL−1), PL excitation spectrum (red), and PL emission spectrum (blue) (A), photographs of the S,N-OCD solution under daylight (left) and 460 nm excitation light (right) (B), PL emission spectra with different excitation wavelengths (inset: normalized emission intensity curves) (C), and time-resolved fluorescent decay curve (D).

Formation mechanism of S,N-OCDs

As mentioned above, the resulting highly carbonized and conjugated structure has been confirmed by the ultrahigh carbon content, obvious UV absorption of conjugated C[double bond, length as m-dash]C, and amorphous carbon phase of the core. Meanwhile, the low S,N-codoping content and some proven surface functional groups were also taken into consideration to construct a realistic model of the S,N-OCDs: a highly conjugated carbon core with a few functional groups containing S or N atoms (Fig. 5). Both the effective conjugation length and the sp2 domain size had a great impact on the energy gap of the π–π* orbitals; in other words, the luminescence performance.37–39 As the major component, acetone would first react with a single conjugated unit by aldol condensation, which then coupled with others to form the crude polymer-like CDs.24 During this process, a small amount of nitric acid not only provided heteroatoms for doping, but also produced highly active free radicals to accelerate the “polymerization”.40 At the same time, DMTTC also acted as an effective cross-linker to fuse conjugated units by dynamic covalent transfer.41 Both of them had a strong enhancement effect on the “polymerization” of the acetone-aldolized precursors, almost the same as reversible additive-fragment transfer polymerization (RAFT), contributing to the relatively low contents of S,N-codoping.
image file: d0tc00016g-f5.tif
Fig. 5 Diagram of formation mechanism of the S,N-OCDs at high temperature and pressure.

Application of the S,N-OCDs in a WLED

Different from most water-soluble CDs, the resulting S,N-codoped carbon dots were oil-soluble, which facilitated the simple fabrication of a WLED. Especially, once integrated into a WLED device, these single-component S,N-OCDs displayed a matched emission spectrum within the whole visible region to emit bright white light directly (Fig. 6C and D). In this emission range from 440 to 780 nm, the peak before 480 nm belonged to the blue light source, and the one located at 480–780 nm belonged to the S,N-OCDs. Typically, as shown in Fig. 6B, the Commission Internationale de L’Eclairage (CIE) color coordinates of the WLED device were (0.33, 0.30), which are very close to those of pure white light (0.33, 0.33). Correspondingly, its color rendering index (CRI) and correlated color temperature (CCT) were 88.38 and 5389 K, respectively.
image file: d0tc00016g-f6.tif
Fig. 6 Construction and characterization of the S,N-OCD-based WLED device: structural diagram (A), CIE color coordinates (B), lighting photo (C), and emission spectrum with upper right inset showing a real photo of the WLED lamp (D).

The overall comparison between S,N-OCDs and other similar works is presented in Table 1 and Table S2 (ESI). These results are surely solid proof of the excellent luminescence properties of the S,N-OCDs in WLED devices, which were promising to perfect the color design of WLEDs effective for green lighting applications.

Table 1 Comparison of luminescence performance characteristics of CDs and their modified WLEDs
Sample CDs WLED
Ex. WL (nm) Em. WL (nm) QY (%) Color converter CCT (K) CRI/Ra CIE Color coordinates Working current/operating voltage Ref.
Notes: “CDs” and “WLED” are the abbreviations of carbon dots and white light-emitting-diode, respectively; “Ex. WL” and “Em. WL” are the abbreviations of excitation and emission wavelengths, respectively, and all the listed values are the optimal ones that were reported in the corresponding references; “QY” is the abbreviation of quantum yield; “CCT”, “CRI” and “CIE” are the abbreviations of correlated color temperature, color rendering index and Commission Internationale de L’Eclairage, respectively; “CQDs” and “GQDs” are the abbreviations of carbon quantum dots and graphene quantum dots, respectively; “PVA”, “PMMA” and “PVP” represent polyvinyl alcohol, polymethyl methacrylate, and polyvinyl pyrrolidone, respectively.
S,N-CDs 415 475 51.4 S,N-CDs/silicone 5227 (0.34, 0.38) 3.6 V 42
S,N-CDs 400 500 16.5 S,N-CDs/CaAlSiN3:Eu2+ 3863 86.9 (0.38, 0.39) 20 mA/3 V 43
S,N-CQDs 400 514 28 O-CQDs & G-CQDs/PMMA 44
450 611 16.7
S,N-GQDs 360 440 51.2 S,N-GQDs/YAG:Ce3+ composites with PVA 7342 74.6 (0.296, 0.311) 350 mA 45
N-CDs 420 520 5.3 o-CDs/r-CDs/PVP 3319 80.0 (0.394, 0.323) 3 V 46
540 610 11.8
N-CDs 375 444 67.06 N-CDs/optical encapsulant mixture 4290 (0.38, 0.42) 3.2 V 47
N-CDs 560 685 33.96 N-CDs/CDs/PMMA 4957 81.9 (0.34, 0.37) 20 mA/4 V 11
N-CDs 365 441 9.65 CDs/starch composites 5462 (0.33, 0.37) 3 V 12
N-GQDs 375 460 42 N-GQDs/PVA composites 4690 71 (0.32, 0.35) 48
GQDs 375 472 6.40 GQD/agar composites 5532 72.0 (0.33, 0.38) 20 mA 49
S,N-OCDs 480 545 21.08 S,N-CDs/epoxy composites 5389 88.4 (0.33, 0.30) 20 mA/4 V This work


Conclusions

In summary, we developed S,N-OCDs by a one-step solvothermal reaction of acetone, DMTTC and nitric acid. Through the optimization study, the best preparation time and temperature were determined as 10 hours and 220 °C. The obtained S,N-OCDs were spherical with an average size of about 3.38 nm, in a highly amorphous carbon phase. The corresponding surface chemical analyses further testified that the as-prepared S,N-OCDs were highly carbonized and conjugated, with a few functional groups on their surface. Based on this construction, the S,N-OCDs exhibited obvious excitation wavelength-dependence in the visible region with the emission maximum around 545 nm and a high QY of 21.08%. The formation mechanism of the S,N-OCDs was considered to be an aldol condensation, which was promoted by nitric acid and DMTTC. Furthermore, the S,N-OCDs were integrated into a WLED to emit almost pure white light with a CCT of 5389 K and CRI of 88.38. It is expected that the S,N-OCD/epoxy composites in this study will become promising color converting materials used in environment-friendly WLEDs.

Conflicts of interest

The authors report no conflicts of interest in this work.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51973070, 51673007, and 51773069), Guangdong Provincial Key Laboratory of Optical Information Materials and Technology (No. 2017B030301007), Major Project of Education Bureau of Guangdong Province, Innovative Team Project of Education Bureau of Guangdong Province, Science and Technology Program of Guangzhou (No. 2019050001), Startup Foundation from SCNU, MOE International Laboratory for Optical Information Technologies and the 111 Project. The authors would also like to acknowledge the support of the Ministry of Education, Kingdom of Saudi Arabia (Collaboration Grant PCSED-004-18).

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Footnotes

Electronic supplementary information (ESI) available. See DOI: 10.1039/d0tc00016g
These authors contributed equally to this work.

This journal is © The Royal Society of Chemistry 2020