Highly Efficient Carbon Dots with Reversibly Switchable Green-Red

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Functional Nanostructured Materials (including low-D carbon)

Highly Efficient Carbon Dots with Reversibly Switchable Green-Red Emissions for Trichromatic White LEDs Biao Yuan, Shanyue Guan, Xingming Sun, Xiaoming Li, Haibo Zeng, Zheng Xie, Ping Chen, and Shuyun Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02379 • Publication Date (Web): 17 Apr 2018 Downloaded from http://pubs.acs.org on April 17, 2018

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ACS Applied Materials & Interfaces

Highly Efficient Carbon Dots with Reversibly Switchable Green-Red Emissions for Trichromatic White LEDs Biao Yuan,†, § Shanyue Guan,† Xingming Sun,† Xiaoming Li,‡ Haibo Zeng,‡ Zheng Xie,*† Ping Chen† and Shuyun Zhou*†

†

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

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, based on the surface electron state engineering, we report a kind of CDs with reversibly switching ability between 1

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green and red photoluminescence with quantum yield both up to 80%. Highly efficient green and red solid state luminescence are realized by doping CDs into highly

transparent

matrix

methyltriethoxysilane

and

3-Triethoxysilylpropylamine to form CDs/gel-glasses composites with quantum yield 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 phosphors based trichromatic white 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 to be applied in other field, such as sensing, bioimaging and photocatalysis.

KEYWORDS: carbon dots, high quantum yield, switchable, gel-glasses, LEDs

INTRODUCTION Luminescent materials have drawn tremendous attention because of the excellent luminescent characteristics and outstanding diversity applications, such as organic dyes,1 rare-earth,2-3 semiconductor quantum dots (QDs),4-5 perovskite QDs,6-7 and carbon dots (CDs).8-14 The advantages of CDs such as large surface area, easy to functionalize, good photostability and excellent biocompatibility make them superior to fluorescent organic dyes and QDs, which precipitates that CDs have potential to be 2


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successfully used in white light emitting diodes (WLEDs),15-21 optoelectronic devices,22-25

bioimaging,26-30

fingerprints

information

storage

or

anti-counterfeiting31-34 and photocatalysis.35-38 The photoluminescent (PL) intensity of initially reported CDs was rather weak with 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 surface and heteroatom doping.41-42 Blue emissive CDs with QY more than 90% and green emissive CDs with QY higher than 70% have been fabricated.11, 43-44 However, high QY and long-wavelength (red) emissive CDs are rarely reported thus attracting much efforts to develop proper synthesized methods and explore indistinct luminescence mechanism. Red emission CDs with QY of 5.4% (680 nm) and 2.3% (640 nm) were synthesized from polythiophene derivatives via hydrothermal in 2014.45-46 Jiang et al.47 reported CDs of red emission with QY of 26.1% (centered at 603 nm) synthesized by using p-phenylenediamine as the precursor. Afterwards Xiong and co-workers increased the wavelength to 625 nm (QY: 24%) via hydrothermal of urea and p-phenylenediamine and purified by column chromatography.48 The authors held that the large amounts of nitrogen doping and big sizes of particle are responsible for the mechanism of CDs with efficient red emission, which still remains unclear. Then Qu et al. prepared CDs with orange emission (580 nm, QY: 46%) through solvothermal treatment of urea and citric acid in DMF.49 Pure red emission CDs (640 nm) synthesized by citric acid in formamide via microwave-assisted heating with QY of 22.9 % were also reported.50 Furthermore, Wang and co-workers reported 53% efficient red emissive CDs 3



prepared by innovating a 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 great goal which seriously hinder their development and widespread applications. The polycyclic aromatic hydrocarbons (PAHs) with good thermal stability and strong π-π interactions between molecules and tunable bandgaps were regarded as outstanding precursor for CDs,52 but the QY of CDs based on PAHs were not high enough.

53-55

So it is essential to develop highly efficient CDs from PAHs.

The surface electron state engineering has been applied in CDs to regulate the electronic structure and optical properties as an effective strategy. 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 the 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 green emissive CDs (G-CDs) were achieved from the solvothermal of 3, 4, 9, 10-nitroperylene in alkaline solution. The red emissive CDs (R-CDs) were developed from G-CDs by adjusting their surface electronic state with adding alkali. The QY of G-CDs and R-CDs were 81% and 80%, respectively. As far as we concern, the QY of G-CDs and R-CDs are both the highest in green and red emission region. Furthermore, the excellent luminescence of G-CDs and R-CDs in solid state were successfully realized by 4


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doping CDs into highly transparent matrix of MTES and APTES to form CDs/gel-glasses. The highly efficient CDs/gel-glasses show good transmittance in visible light bands and excellent thermal stability. Using such CDs/gel-glasses as emitting phosphor, we have realized a blue-pumped CDs phosphors-based trichromatic warm WLED. The WLED gets the highest luminous efficiency of 71.75 lm W–1 in CDs-based trichromatic WLEDs. This warm WLED also demonstrated good color chromatics with a CRI of 92.9.

Scheme 1. Schematic illustration of highly efficient green and red emissive switching CDs doping into MTES and APTES to form CDs/gel-glasses for trichromatic WLED.

RESULTS AND DISCUSSION Perylene

was

refluxed

with

HNO3,

then

precipitate

(3,

4,

9,

10-tetranitroperylene) was separated by centrifuge, which was confirmed by 1H nuclear magnetic resonance (1H NMR) and Fourier transform infrared (FT-IR) spectrum (Figure S1). The G-CDs were synthesized by solvothermal of 3, 4, 9, 10-tetranitroperylene in ethanol solution with sodium hydroxide and collected via silica gel column chromatography to separate impurity. The nucleophilic substitution reactions are supposed to take place between 3, 4, 9, 5



10-tetranitroperylene and OH– due to the four positively charged sites of –NO2 groups of 3, 4, 9, 10-tetranitroperylene. In control experiment, g-CDs were prepared by solvothermal of 3, 4, 9, 10-tetranitroperylene in ethanol solution without alkali and 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 transmission electron microscopy (TEM) image, which was revealed in Figure 1a. The G-CDs achieve good dispersion and uniform in size distribution. The HRTEM images (inset) of the CDs presents that the specific spacing is 0.21 nm which is revealed to the graphene (100) planes.27 The G-CDs are mainly monodispersed and limited in 5-8 nm and average size is 6.53 nm as shown in Figure 1b. X-ray diffraction patterns (XRD) (Figure S2a) of G-CDs demonstrates that 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 G-CDs’ lattice spacing is more than graphite (0.334 nm) because of the higher amounts of oxygen and nitrogen.27 The atomic force microscope (AFM) was further applied to survey 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 indicates these CDs are almost single or double layered, which further confirms that CDs are graphited.57 The graphene fragment structure of G-CDs was also proved via 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 6


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represents disorder on the surface because of the scattering.58-59 The large amounts of sp2-bonded carbon atoms reveals much more than disorder of surface. Then, the sodium hydroxide (0.1 M) was added into G-CDs ethanol solution in order to transform to red emission CDs, named R-CDs. The R-CDs solution was further dialyzed by dialysis bag (100-500 Da) for 48 h to separate the alkali and the R-CDs returned to G-CDs. This means G-CDs and R-CDs were reversibly switchable. The average size and height of R-CDs were 6.45 nm and 0.7-1.4 nm (Figure S3), which is almost the same with G-CDs. Furthermore, the chemical structures of these CDs were investigated by X-ray photoelectron spectroscopy (XPS) and FT-IR spectra as shown in Figure 1d-h and Figure S4. The stretching vibration bands of –NH2 (3364 and 3199 cm–1) and –NO2 (1521 and 1350 cm–1) of g-CDs were detected in Figure 1d. Compared with g-CDs, the –OH of G-CDs at 3433 cm–1 demonstrates the nucleophilic substitution reactions were 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 are corresponding to pyridinic N and pyrrolic N structure. Compared with G-CDs, the intensity of stretching vibration of –OH of R-CDs is decreased while the intensity of pyrrolic N structure is increased. Meanwhile, the appearance of new band at 1655 cm−1 can be revealed to C=O in the quinone structure. The results can be further supported by the XPS spectra (Figure 1e-h and Figure 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 7



the existence of N in g-CDs (Figure S4a). The XPS N 1s spectra of g-CDs (Figure S4b) can validate the existence of the –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 and 1f, which is in well consistent with the results of the FT-IR spectrum. Compared with G-CDs, a new peak at 288.1 eV (C=O) 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 and S4d). Compared with G-CDs, the amounts of pyrrolic N of R-CDs were increased. The corresponding data are shown in Figure 1h.

8


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Figure 1. (a) TEM images (inset: HRTEM images), (b) distribution of size, (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 C1s spectra of (e) G-CDs and (g) R-CDs. XPS N1s spectra of (f) G-CDs and (h) R-CDs. 9



UV-vis and fluorescence measurements were utilized to characterize the absorption and photoluminescence properties of G-CDs and R-CDs. The high-energy region absorption band (< 350 nm) of the spectra is induced by the π-π* transitions in the C–N and C–C bonds. The specific absorption peak at 457 nm of G-CDs reveals large amounts of sp2-bonded carbon in G-CDs (Figure 2a), which is compatible with the Raman data (Figure S2b). Compared with G-CDs, R-CDs can display obvious visible absorption band, with strong red-shift from 457 nm to 543 nm, leading to shorter bandgap between HOMO and LUMO. The G-CDs emit green photoluminescence under different wavelength excitation, which shows excitation-wavelength-independent property (Figure 2b). The strongest emission wavelength of G-CDs is 515 nm excitated at 460 nm, with an absolute QY of 81%. R-CDs display red-shifted strong red luminescence (Figure 2c). The emission wavelength of R-CDs is 610 nm with absolute QY of 80% excitated at 560 nm. In the control experiment, 3, 4, 9, 10-tetranitroperylene was used to operate the solvothermal process without alkaline species. The UV-vis absorption and photoluminescence spectra of g-CDs were measured (Figure S5). The prepared g-CDs were green emission at 510 nm and its QY was 29% excited at 480 nm. Compared with g-CDs, the high QY of G-CDs is determined from the hydroxyl groups functionalized to G-CDs. The –OH is strong electron-donating group which contains heteroatom with unbonded p electrons. The –OH group can lead to the increase of the mobility of the electron cloud in the conjugate system which can enhance p-π 10


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conjugation between hydroxyl group and G-CDs. Furthermore, we explored the fluorescence lifetimes of G-CDs and R-CDs, respectively (Figure 2d). The corresponding parameters are shown in Table S1. The average lifetimes were fitted by a decay time model of two components. The lifetime of R-CDs is 8.26 ns, which is longer than G-CDs (5.74 ns) due to the increased density of the electron cloud on the surface and the narrow bandgap between HOMO and LUMO which was further explained.41

Figure 2. (a) UV-vis absorption spectra of G-CDs and R-CDs, respectively. (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.

11



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To further evaluate the alkali dependent photoluminescence mechanism, the optical properties of CDs in various alkaline ethanol solution with different concentrations were carried out (Figure 3a-d), such as ammonia (NH3H2O) and ethylenediamine (NH2CH2CH2NH2). The corresponding UV-vis absorption spectra of CDs with different alkaline solution were presented in Figure 3a and 3c. As the dosage of alkali increased, the specific absorption peak at 457 nm of G-CDs was decreased with the enhancement of the absorption band at 543 nm of R-CDs, indicating the improvement of the quinone structure in R-CDs. The CDs solution’s colour (insets) alters gradually from yellow to brown to red gradually. As shown in Figure 3b and 3d, the CDs in different alkaline ethanol solution exhibits diverse photoluminescence excited at 460 nm. The PL intensity of CDs in alkaline solvents at 515 nm was decreased and a new red emission peak at 610 nm appeared, which was influenced by alkali. In the meantime,

the

alkalinity

enhanced

(NH3H2O

Highly Efficient Carbon Dots with Reversibly Switchable Green-Red

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