Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 16005−16014
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Highly Efficient Carbon Dots with Reversibly Switchable Green−Red Emissions for Trichromatic White Light-Emitting Diodes Biao Yuan,†,§ Shanyue Guan,† Xingming Sun,† Xiaoming Li,‡ Haibo Zeng,‡ Zheng Xie,*,† Ping Chen,† and Shuyun Zhou*,†
ACS Appl. Mater. Interfaces 2018.10:16005-16014. Downloaded from pubs.acs.org by UNIV OF KENTUCKY on 08/20/18. For personal use only.
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Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, 29 Zhongguancun East Road, Haidian District, Beijing 100190, China ‡ MIIT Key Laboratory of Advanced Display Materials and Devices, Institute of Optoelectronics & Nanomaterials, College of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China § College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049, China S Supporting Information *
ABSTRACT: Carbon dots (CDs) have potentials to be utilized in optoelectronic devices, bioimaging, and photocatalysis. The majority of the current CDs with high quantum yield to date were limited in the blue light emission region. Herein, on the basis of surface electron-state engineering, we report a kind of CDs with reversible switching ability between green and red photoluminescence with a quantum yield (QY) of both up to 80%. Highly efficient green and red solid-state luminescence is realized by doping CDs into a highly transparent matrix of methyltriethoxysilane and 3-triethoxysilylpropylamine to form CDs/gel glasses composites with QYs of 80 and 78%. The CDs/gel glasses show better transmittance in visible light bands and excellent thermal stability. A blue-pumped CDs/gel glasses phosphor-based trichromatic white light-emitting diode (WLED) is realized, whose color rendering index is 92.9. The WLED gets the highest luminous efficiency of 71.75 lm W−1 in CDs-based trichromatic WLEDs. This work opens a door for developing highly efficient green- and red-emissive switching CDs which were used as phosphors for WLEDs and have the tendency for applications in other fields, such as sensing, bioimaging, and photocatalysis. KEYWORDS: carbon dots, high quantum yield, switchable, gel glasses, LEDs
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have been fabricated.11,43,44 However, high QY and longwavelength (red)-emissive CDs are rarely reported, thus attracting much efforts to develop proper synthesis methods and explore the indistinct luminescence mechanism. Red emission CDs with QYs of 5.4% (680 nm) and 2.3% (640 nm) were synthesized from polythiophene derivatives via the hydrothermal method in 2014.45,46 Jiang et al.47 reported red emission CDs with a QY of 26.1% (centered at 603 nm) synthesized by using p-phenylenediamine as the precursor. Afterward, Xiong and co-workers increased the wavelength to 625 nm (QY: 24%) via the hydrothermal treatment of urea and p-phenylenediamine, followed by purification by column chromatography.48 The authors proposed that the large amounts of nitrogen doping and big sizes of particles are responsible for the mechanism of CDs with efficient red emission, which still remains unclear. Then, Qu et al. prepared
INTRODUCTION Luminescent materials, such as organic dyes,1 rare-earth phosphors,2,3 semiconductor quantum dots (QDs),4,5 perovskite QDs,6,7 and carbon dots (CDs),8−14 have drawn tremendous attention because of their excellent luminescent characteristics and outstanding diverse applications. The advantages of CDs, such as large surface area, easy functionalization, good photostability, and excellent biocompatibility, make them superior to fluorescent organic dyes and QDs, which demonstrate that CDs have the potential to be successfully used in white light-emitting diodes (WLEDs),15−21 optoelectronic devices,22−25 bioimaging,26−30 fingerprint information storage or anticounterfeiting,31−34 and photocatalysis.35−38 The photoluminescence (PL) intensity of the initially reported CDs was rather weak with a quantum yield (QY) of 1.6%.39,40 The PL QY of CDs has been remarkably enhanced by several methods recently, for example, modification or passivation on their surface and heteroatom doping.41,42 Blue-emissive CDs with a QY of more than 90% and green-emissive CDs (G-CDs) with a QY higher than 70% © 2018 American Chemical Society
Received: February 9, 2018 Accepted: April 17, 2018 Published: April 17, 2018 16005
DOI: 10.1021/acsami.8b02379 ACS Appl. Mater. Interfaces 2018, 10, 16005−16014
Research Article
ACS Applied Materials & Interfaces
Scheme 1. Schematic Illustration of Highly Efficient Green- and Red-Emissive Switching CDs Doped into MTES and APTES To Form CDs/Gel Glasses for Trichromatic WLEDs
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RESULTS AND DISCUSSION Perylene was refluxed with HNO3; then, the precipitate (3,4,9,10-tetranitroperylene) was separated by centrifugation, which was confirmed by 1H nuclear magnetic resonance (1H NMR) and Fourier transform infrared (FT-IR) spectra (Figure S1). The G-CDs were synthesized by the solvothermal treatment of 3,4,9,10-tetranitroperylene in ethanol solution with sodium hydroxide and collected via silica gel column chromatography to separate impurities. The nucleophilic substitution reactions are supposed to take place between 3,4,9,10-tetranitroperylene and OH− because of the four positively charged sites of −NO 2 groups of 3,4,9,10tetranitroperylene. In the control experiment, g-CDs were prepared by the solvothermal treatment of 3,4,9,10-tetranitroperylene in ethanol solution without alkali, followed by purification via silica gel column chromatography. The R-CDs were achieved from G-CDs by adding alkaline species. The morphology of G-CDs was characterized by the transmission electron microscopy (TEM) image, which was revealed in Figure 1a. The G-CDs achieve good dispersion and are uniform in size distribution. The high-resolution TEM (HRTEM) images (inset) of the CDs present that the specific spacing is 0.21 nm, which can be interrelated to the graphene (100) planes.27 The G-CDs are mainly monodispersed and limited in the range of 5−8 nm, and the average size is 6.53 nm, as shown in Figure 1b. The X-ray diffraction patterns (Figure S2a) of G-CDs demonstrate the high crystallinity and purity of G-CDs. The broad peak at 21.2° illustrates that the lattice spacing is 0.419 nm, which can be interrelated to the graphene (002) planes. The lattice spacing of G-CDs is more than that of graphite (0.334 nm) because of the higher amounts of oxygen and nitrogen.27 Atomic force microscopy (AFM) was further used to study the size and morphology of G-CDs (Figure 1c). It is clearly shown that the height range of G-CDs is 0.6−1.2 nm. The AFM data indicate that these CDs are almost singleor double-layered, which further confirm that CDs are graphited.57 The graphene fragment structure of G-CDs was also proven via the Raman spectrum (Figure S2b). The G peak at 1610 cm−1 is related to the E2g mode of the sp2-bonded carbon atoms in graphite, and the D peak at 1366 cm−1 represents disorder on the surface because of scattering.58,59 The large amounts of sp2-bonded carbon atoms reveal much more disorder on the surface. Then, sodium hydroxide (0.1 M) was added into the G-CDs ethanol solution to transform to red emission CDs, named as R-CDs. The R-CDs solution was further dialyzed by a dialysis bag (100−500 Da) for 48 h to separate the alkali and the R-CDs returned to G-CDs. This
CDs with orange emission (580 nm, QY: 46%) through the solvothermal treatment of urea and citric acid in dimethylformamide.49 Pure red emission CDs (640 nm) synthesized from citric acid in formamide via microwave-assisted heating with a QY of 22.9% were also reported.50 Furthermore, Wang and coworkers reported 53% efficient red-emissive CDs (R-CDs) prepared by innovating sequential dehydrative condensation and dehydrogenative planarization for warm WLEDs in 2017.51 The preparation of CDs with both red emission and high QY is still a challenging goal, which seriously hinders their development and widespread applications. Polycyclic aromatic hydrocarbons (PAHs) with good thermal stability and strong π−π interactions between molecules and tunable band gaps were regarded as outstanding precursors for CDs,52 but the QYs of CDs based on PAHs were not high enough.53−55 Therefore, it is essential to develop highly efficient CDs from PAHs. Surface electron-state engineering has been applied as an effective strategy in CDs to regulate the electronic structure and optical properties. A variety of electron-donating groups including −OH, −NH2, and −SH have been incorporated to enhance the p−π conjugation of the system.19,48,56 Here, we report highly fluorescent CDs with green and red emission switching using perylene as the precursor, as shown in Scheme 1. Substituted derivatives (3,4,9,10-nitroperylene) were synthesized from perylene with active −NO2-functionalized group in hot HNO3. Highly efficient G-CDs were achieved from the solvothermal treatment of 3,4,9,10-nitroperylene in an alkaline solution. The R-CDs were developed from G-CDs by adjusting their surface electronic state with addition of an alkali. The QYs of G-CDs and R-CDs were 81 and 80%, respectively. As far as we concern, the QYs of both G-CDs and R-CDs were the highest in the green and red emission region. Furthermore, the excellent luminescence of G-CDs and R-CDs in the solid state is successfully realized by doping CDs into a highly transparent matrix of methyltriethoxysilane (MTES) and 3triethoxysilylpropylamine (APTES) to form CDs/gel glasses. The highly efficient CDs/gel glasses show good transmittance in visible light bands and excellent thermal stability. By using such CDs/gel glasses as emitting phosphors, a blue-pumped CDs phosphor-based trichromatic warm WLED is realized. The WLED gets the highest luminous efficiency (LE) of 71.75 lm W−1 in CDs-based trichromatic WLEDs. This warm WLED also demonstrated good color chromatics with a color rendering index (CRI) of 92.9. 16006
DOI: 10.1021/acsami.8b02379 ACS Appl. Mater. Interfaces 2018, 10, 16005−16014
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) TEM images (inset: HRTEM images), (b) distribution of size, and (c) AFM images (inset: height of G-CDs along the white line) of G-CDs. (d) FT-IR spectra of g-CDs, G-CDs, and R-CDs. XPS C 1s spectra of (e) G-CDs and (g) R-CDs. XPS N 1s spectra of (f) G-CDs and (h) RCDs.
means that G-CDs and R-CDs were reversibly switchable. The average size and height of R-CDs were 6.45 and 0.7−1.4 nm (Figure S3), respectively, which are almost the same with GCDs. Furthermore, the chemical structures of these CDs were investigated by X-ray photoelectron spectroscopy (XPS) and FT-IR spectra, as shown in Figures 1d−h and S4. The stretching vibration bands of −NH2 (3364 and 3199 cm−1) and −NO2 (1521 and 1350 cm−1) of g-CDs are detected in Figure 1d. Compared with g-CDs, −OH of G-CDs at 3433 cm−1
demonstrates that the nucleophilic substitution reactions successfully occurred between OH− and 3,4,9,10-tetranitroperylene. Two specific bands of G-CDs at 1647 and 1616 cm−1 are observed, which correspond to the pyridinic N and pyrrolic N structures, respectively. Compared with G-CDs, the intensity of the stretching vibration of −OH of R-CDs is decreased, whereas the intensity of the pyrrolic N structure is increased. Meanwhile, the appearance of a new band at 1655 cm−1 can be related to CO in the quinone structure. The results can be 16007
DOI: 10.1021/acsami.8b02379 ACS Appl. Mater. Interfaces 2018, 10, 16005−16014
Research Article
ACS Applied Materials & Interfaces
Figure 2. (a) UV−vis absorption spectra of G-CDs and R-CDs. (b) PL spectra of G-CDs excited at different wavelengths [inset: the photo of G-CDs under daylight (left) and 365 nm UV light (right)]. (c) PL spectra of R-CDs excited at different wavelengths [inset: the photo of R-CDs under daylight (left) and 365 nm UV light (right)]. (d) Time-resolved PL spectra of G-CDs and R-CDs.
further supported by the XPS spectra (Figures 1e−h and S4). The strongest peak at 284.7 eV can be related to the C−C/C C group of g-CDs. The C−N group at 285.5 eV confirms the existence of N in g-CDs (Figure S4a). The XPS N 1s spectra of g-CDs (Figure S4b) can validate the existence of −NH2 and −NO2 in g-CDs. The pyrrolic N (398.5 eV), pyridinic N (400.1 eV), and C−OH group (286.3 eV) of G-CDs can also be found in Figure 1e,f, which is well-consistent with the results of the FT-IR spectrum. Compared with G-CDs, a new peak at 288.1 eV (CO) appeared in the XPS spectra of R-CDs (Figure 1g). This can further prove the quinone structure in R-CDs. The XPS O 1s spectra of G-CDs and R-CDs also confirm the quinone structure (Figure S4c,d). Compared with G-CDs, the amounts of pyrrolic N of R-CDs were increased. The corresponding data are shown in Figure 1h. UV−vis and fluorescence measurements were utilized to characterize the absorption and PL properties of G-CDs and RCDs. The high-energy region absorption band (
