Tricolor White-Light-Emitting Carbon Dots with Multiple-Cores@Shell

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 19796−19805

Tricolor White-Light-Emitting Carbon Dots with MultipleCores@Shell Structure for WLED Application Tianyi Zhang, Feifei Zhao, Li Li, Bin Qi, Dongxia Zhu, Jianhua Lü, and Changli Lü* College of Chemistry, Northeast Normal University, Changchun 130024, P. R. China

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ABSTRACT: The past few years have witnessed the rapid development of carbon dots (CDs) due to their outstanding optical properties and a wide range of applications. However, the design and control of CDs with long-wavelength multicolor emission are still huge challenges to be addressed for their practical use in different fields. Here, novel nitrogen-doped multiple-core@shell-structured AC-CDs with tricolor emissions of red, green, and blue were constructed via one-pot hydrothermal method from 5-amino-1,10phenanthroline and citric acid as reactants and the growth process of AC-CDs was monitored with the reaction time in the synthetic system. The origin of different fluorescence emissions was explored using the unique coordination ability of the surface groups of AC-CDs. An obvious concentration dependence of fluorescent properties was observed for the as-prepared AC-CDs, and a highly fluorescent quantum yield (QY) of 67% for red emission at 630 nm can be obtained by adjusting concentration of AC-CDs. The pure white-light emission (0.33, 0.33; Commission Internationale de l’Elcairage coordinate) was carried out from single carbon dot with QY of 29% through regulation of the excitation and concentration of multiple-core@shell-structured AC-CDs. In addition, because of their excellent photoluminescent properties, the white-emitting AC-CDs as emitting phosphor can be easily used in the fabrication of whitelight-emitting diode with good anti-photobleaching and temperature stability. KEYWORDS: carbon dots, multiple-core@shell structure, tricolor emission, high quantum yields, pure white-light emission

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INTRODUCTION Since the first report that carbon dots (CDs) have been extracted from single-walled carbon nanotubes in 2004,1 CDs have proved to be one of the most momentous photoluminescent (PL) materials owing to their excellent luminescence properties, responsive fluorescence quenching/enhancement properties, chemical modification and functional integration, good biocompatibility, low toxicity, and low cost.2−12 Thousands of raw materials and hundreds of approaches have been researched for the preparation of CDs.13−19 The present synthetic methods for CDs have been mainly grouped into two classifications:20,21 top-down synthetic route, such as laser ablation, electrochemical exfoliation, and arc discharge,1,22−25 and bottom-up synthetic method, just like ultrasonic-assisted route, microwave pyrolysis, and hydrothermal means.6,11−18,26 Heteroatom doping is an attractive method for availably changing the intrinsic properties and modulating electronic density of CDs. Especially, it could effectively produce strong photoluminescence through introducing nitrogen, phosphorus, sulfur, boron, and other elements.5,7,19,26−29 However, the emission wavelength for most currently available CDs is in the blue-light region and the single blue-light emission unquestionably restricts their wider applications, such as biological imaging, full-color displays, and light-emitting diodes (LEDs). Therefore, it is highly desirable to acquire long-wavelength multicolor emissions of CDs.30 Now, many strategies, including using surface state and size control, solvatochromic effect, © 2018 American Chemical Society

heteroatom doping, and energy transfer, have been explored to develop the CDs with long-wavelength multicolor emission.3,5,6,9−16,19,28,31−37 White-light-emitting materials have gained heightened attention due to their extensive application in the fields of lighting and displays. Recently, many studies also focused on the fabrication of CD-based white-light-emitting materials.8,13,15,28,33−48 A facile method for achieving this goal is to mix red/green/blue emitting CDs in a transparent polymer matrix.15 Fan et al. also reported the first carbon quantum dot (CQD)-based monochrome LEDs with stable emission color using CQDs directly as an active emission layer.35 Recently, red emissive CQDs with high quantum yield (QY) were also prepared via a sequential dehydrative condensation and dehydrogenative planarization route and an ultraviolet-pumped CQD-based warm white-light-emitting diode (WLED) was also fabricated by using the high-quality red/green/blue emitting CQDs as phosphors.47 In addition, the white-light-emitting nanocomposites were fabricated by using CDs as primary bluelight emitters and combining other non-CD-based green and red-emitting phosphors, such as lanthanide complexes and CdTe QD@NaCl powders.8,43 Liu et al. synthesized the nitrogen-doped CDs with tunable solid-state fluorescence Received: March 1, 2018 Accepted: May 24, 2018 Published: May 24, 2018 19796

DOI: 10.1021/acsami.8b03529 ACS Appl. Mater. Interfaces 2018, 10, 19796−19805

Research Article

ACS Applied Materials & Interfaces

emitting diode (WLED). To the best of our knowledge, this is the highest QY yet reported for pure white-light emission from single CD.

through surface modification or interparticle spacing control, and they further achieved white-light emission by constructing dual-fluorescence morphologies in epoxy resin.28 However, it should be a greater technological challenge to realize white-light emission from a single carbon dot as emitting phosphor.13,33,44−46,48 The white-emitting CDs have been prepared from oligomer polyamide resin as the carbon source via onestep ultrasound at room temperature.33 Yang et al. reported the supersmall white-light-emitting carbon nanodots (∼0.5 nm) obtained by using the hydrothermal process from two amino acids of L-serine and L-tryptophan by controlling the temperature and pH value in the reaction system,48 but the above reported white emissions of CDs come from a very broad fluorescence emission band with a poor controllability, which makes it difficult to obtain pure white emission with Commission Internationale de L’Eclairage (CIE) coordinates (0.33, 0.33). Recently, Prato et al. have designed and synthesized tunable fluorescent CDs across the entire visible optical spectrum from 400 to 700 by choosing an appropriate ratio of organic chromophore precursors.13 Especially, the pure white emissive CDs with tricolor emitting bands can be obtained via this strategy but the absolute quantum yield of asprepared CDs is less than 8%. Therefore, it is highly desirable to achieve high quantum yield (QY) of white-light emissive CDs through the design and control of CD’s structure. In this work, the unique multicolor emissive carbon dots (AC-CDs) with multiple-core@shell structure were prepared through one-pot hydrothermal route from the 5-amino-1,10phenanthroline (Aphen) and citric acid (CA) as carbon sources (Figure 1). It is well known that the size of CDs is usually

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EXPERIMENTAL SECTION

Materials. Citric acid (CA, 99.5%) was purchased from Aladdin, and 5-amino-1,10-phenanthroline (Aphen, 97%) was purchased from Sigma-Aldrich. 2-Hydroxyethyl methacrylate (HEMA, 96%), 2,2′azobis(2-methylpropionitrile) (AIBN, 99%), and Zn(NO3)2 were purchased from Macklin. Characterization. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) characterizations were performed on a JEOL-2100F electron microscope. 1H NMR spectra were examined with an 500 MHz AVANCE Bruker spectrometer using deuterated dimethyl sulfoxide-d6 as the solvent. Fourier transform infrared (FTIR) spectras were obtained using KBr disks on a Magna 560 FTIR spectrometer. PL spectra were recorded on a Cary Eclipse spectrophotometer, and UV−vis absorption spectra were recorded using a SHIMADZU UV-2550 UV−visible spectrophotometer. The Xray photoelectron spectroscopy (XPS) spectra were recorded with a Quantum 2000 spectrometer using non-monochromatized Al Kα excitation radiation. Photoluminescence quantum yields were measured in a calibrated integrating sphere in the Edinburgh FLSP920 spectrometer. Fluorescence lifetimes were obtained with Edinburgh FLSP920 time-corrected single photon counting system. Preparation of AC-CDs. Aphen (0.15 g) and citric acid (1.35 g) were dissolved in deionized water (15 mL). The mixture was then transferred into a Teflon-lined stainless-steel reactor (25 mL) and heated at 200 °C for 7 h. After the reactors were cooled to room temperature naturally, the solution was filtrated through a microporous membrane (0.22 μm) to remove the large particles and dialyzed with a dialysis bag (retained molecular weight: 3500 Da) for 72 h to remove the unreacted small molecules. Finally, the product was soaked by deionized water and centrifuged at 10 000 rpm for 10 min twice to wash off impurities; then, it was concentrated by freeze-drying under vacuum to obtain the black solid of AC-CDs (0.13 g). We also synthesized the AC-CDs at 170 and 230 °C, and the synthetic procedure was the same as above, except the reaction temperature was set at 170 and 230 °C, respectively. Preparation of CA-Derived CDs-1. CA (1.5 g) was put into deionized water (15 mL), and then the mixture was transferred into a Teflon-lined stainless-steel reactor (25 mL) and heated to 200 °C for 7 h. After the reactors were cooled to room temperature naturally, the solution was filtrated through a microporous membrane (0.22 μm) to remove the large particles and dialyzed with a dialysis bag (retained molecular weight: 3500 Da) for 72 h to remove the unreacted small molecules. Finally, the product as a pale yellow solid was obtained by freeze-drying under vacuum. Preparation of Aphen-Derived CDs-2. Aphen (0.15 g) and 120 μL of concentrated hydrochloric acid were put into deionized water (15 mL) and the other synthetic procedure was the same as that for the synthesis of CDs-1, except the reaction time was set at 7 h. Preparation of Postfunctionalized CDs-3. CDs-1 (1.35 g) and Aphen (0.15 g) were dissolved in deionized water (15.0 mL) and the other synthetic procedure was the same as that for the preparation of AC-CDs. Fabrication of WLED from AC-CDs. We directly used the ACCDs/PHEMA hybrid as the phosphors coated onto the LED chip. GaN LED chips without phosphor coating were purchased from New Star Photoelectric CO., Ltd. The emission of GaN LED chip centered at 400 nm, and the operating voltage is 3.5 V. The prepared AC-CDs (13.3 mg) were dissolved into HEMA (10 mL), and then 10.0 mg of azobisisobutyronitrile (AIBN) as an initiator was added to the solution. The mixture was prepolymerized at 60 °C for 12 h, and then the viscous prepolymer was poured on the chip. The LED was placed in an oven at 70, 80, 90, and 100 °C for 1 h each and finally at 120 °C for 0.5 h. The spectra of LEDs were measured by combining a Spectra scan PR-650 spectrophotometer with an integrating sphere and a computer-controlled direct-current power supply Keithley model 2400

Figure 1. Schematic illustration of the preparation and application of multiple-core@shell-structured AC-CDs.

below 10 nm. However, the as-prepared AC-CDs by us are a special kind of carbon dots with a size greater than 10 nm due to the formation of the multiple-core@shell structure. Different from other multicolor CDs, it is the first report of core−shellstructured CDs with individually adjustable triple-primary colors (red, green, and blue emission). The origin of different emissions and the related fluorescent mechanism of multiplecore@shell-structured AC-CDs was discussed in combination with the unique coordination ability of Aphen on the surface of AC-CDs. A highly PL QY of 67% for red emission at 630 nm can be achieved from the as-prepared AC-CDs using the concentration effect. The pure white emission with an absolute QY as high as 29% can also be easily obtained from a single carbon dot through adjusting the proportion of triple-primary colors using the concentration effect of AC-CDs, and the excellent properties can be used to make the white-light19797


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Figure 2. TEM images of AC-CDs obtained during different reaction times: (a) 45 min, (b) 1 h, (d) 2 h, (e) 3 h, and (f) 7 h, and (c) is a highresolution image of (b). The insets of pictures (a), (c), (d), (e) are the corresponding HRTEM images. voltage current source under ambient conditions at room temperature. The color of the light was identified by the CIE (Commission Internationale de L’Eclairage 1931) calorimeter system.

the large size of AC-CDs. It is well known that the size of CDs is usually below 10 nm, whereas the as-synthesized AC-CDs from Aphen and CA exhibited a larger size of about 25 nm (Figure S2a), which is similar to that of CDs-3 (Figure S2d). Therefore, it is considered that the AC-CDs obtained from Aphen and CA under this reaction condition are a multiple small core material coated by the shell material, which is defined as multiple-core@shell structure. The possible formation mechanism of the multiple-core@shell-structured AC-CDs is proposed as shown in Figure 1. We speculated that the citric acid molecules first formed small carbon dots with a diameter below 5 nm in the reaction system due to their high reaction rate and then these small CA-CDs as cores reacted with Aphen accompanied by amidation process and a certain degree of carbonization. Eventually, the multiple-core@shellstructured AC-CDs with relative large size were generated. To further prove the formation of the multiple-core@shell structure of AC-CDs, we also monitored the growth process of AC-CDs in the synthetic system with the reaction time. As shown in Figure 2, it can be clearly observed that with the increase of reaction time during the reaction process of CA and Aphen, first, CA is carbonized to produce small CA-derived CDs (CA-CDs) with the clear lattice fringes of about 0.16 nm (Figure 2a), which is the same as CDs-1’s lattice fringes, and then many CA-CDs are aggregated to form a large spherical

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RESULTS AND DISCUSSION We investigated the effects of different reaction temperatures (170, 200, and 230 °C) on the morphologies of AC-CDs prepared from Aphen and CA with a molar ratio of 1:9. It was found that the reaction system had not yet begun to form wellstructured AC-CDs at the low temperature of 170 °C, whereas the higher temperature at 230 °C would lead to the existence of excessive carbonization and mutual polymerization between the AC-CDs in the system to form a diseased irregular structure (Figure S1) and only the reaction system at 200 °C provided the well-structured AC-CDs with a uniform size and distribution. As shown in Figure 2f, the AC-CDs are uniformly dispersed, individually spherical, and amorphous nanodots with an average diameter of 25 nm. We also designed and synthesized a series of CDs, including the CA-derived CDs-1, Aphen-derived CDs-2, and postfunctionalized CDs-3 that was prepared by the reaction of preformed CDs-1 with Aphen in the hydrothermal condition. From the TEM images of different CDs (Figure S2), it is very clear that both CDs-1 and CDs-2 are small size of carbon dots with a diameter below 10 nm and possess distinct lattice fringes of 0.16 and 0.18 nm, respectively. Above results indicate that the CDs-1 or CDs-2 cannot form 19798


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Figure 3. (a) XPS survey spectrum and high-resolution XPS of (b) C 1s, (c) N 1s, and (d) O 1s spectra of AC-CDs.

spectrum (Figure 3d) shows two typical peaks at 532.1 and 534.1 eV for CO and C−O, respectively.6,13 The above XPS data proved that the Aphen molecules have reacted with CA to generate AC-CDs and XPS laser etching was also used to verify the multiple-core@shell structure. The nitrogen content decreased from 2.66% for CDs before etching to 2.44% with etching for 300 s and then descended to 1.95% after etching for 600 s. This result should be due to the fact that AC-CDs have a multiple-core@shell structure. The Aphen moieties on the outer surface of AC-CDs serve as the main shell structure and contain more N elements, whereas the interior of the AC-CDs composed of CA-derived CDs (CA-CDs) without N elements and Aphen moieties should have lower N content. Thus, after different etching times from the outside to the inside of ACCDs with the Aphen parts were etched away, the contents of N elements in AC-CDs gradually reduced with etching. Moreover, 1 H NMR data (Figure S4) of AC-CDs also further demonstrate that the Aphen moieties are present in the AC-CDs due to the appearance of aromatic proton peaks from 6 to 10 ppm. When compared with the 1H NMR spectrum of pure Aphen, the signal peaks of aromatic protons originated from Aphen moieties in AC-CDs appreciably shifted to a low field. This should be attributed to the result of amidation reaction of Aphen and CA in the formation process of AC-CDs. As shown in Figure 4, the as-prepared multiple-core@shellstructured AC-CDs in ethanol solution exhibited interesting PL properties with the change of excitation wavelength and solution concentration. As the excitation wavelength is below 400 nm, the AC-CDs with low concentration (0.01 mg mL−1) mainly display a strong excitation-wavelength-independent blue emission at 430 nm accompanied by a shoulder peak centered at 500 nm and a weak red emission peak at 630 nm. At the same time, no matter what the concentration of AC-CDs in ethanol, with the increasing excitation wavelength from 300 to

assembled structure via the interaction or reaction between Aphen and CA-CDs accompanied by the carbonization and size-shrinkage of aggregates (Figure 2b−d). Finally, with the increasing reaction and carbonization degree, the inner CACDs are mainly wrapped by the shell layers derived from the carbonized Aphen structure probably because of the full carbon core−shell structure and reaction condition. The spectral data in the following discussion also indirectly proved this structure. The functional groups and chemical compositions of ACCDs were explored via Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS). Compared with CDs1 and 2, several characteristic absorption bands, including the stretching vibrations of O−H (3430 cm−1), N−H (3241 cm−1), unsaturated C−H (3066 cm−1), and CC (1560 cm−1), can be observed in the FTIR spectra (Figure S3) of CDs-3 and ACCDs. The strong vibration peak at 1720 cm−1 is mainly ascribed to amide linkage (CONH) due to the amidation reaction between Aphen and CA. Moreover, the stretching vibrations of C−N, C−O, and benzene originated from Aphen are also observed at 1430, 1220−1180, and 856−804 cm−1, respectively.48 The full-range XPS spectrum (Figure 3a) of AC-CDs shows three different peaks, namely, C 1s (286 eV), N 1s (400 eV), and O 1s (531 eV), and the atomic contents of C/O/N are 68.36, 28.98, and 2.66%, respectively, from the elemental analysis. The high-resolution C 1s spectrum (Figure 3b) can be deconvoluted into three peaks for C−C/CC (284.6 eV), C− O/C−N (286.3 eV), and CO (288.0 eV). Four distinct peaks can be identified in the N 1s spectrum (Figure 3c) at 398.8, 399.2, 400.6, and 402.0 eV attributed to pyridinic N, amino N, pyrrolic N, and graphitic N, respectively, indicating that the Aphen molecules have undergone a certain degree of carbonization in the formation process of AC-CDs because of the presence of amino N. In addition, the existence of amide group has been proved again by above XPS data. The O 1s 19799


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the n → π* transitions of the CO and CN bonds (Figure 4b−d). To further understand the PL mechanism of multiple-core@ shell-structured AC-CDs, the PL spectra of CDs-1, 2, and 3 were measured as shown in Figure S5. The CA-derived CDs-1 has a typical excitation-wavelength-dependent PL emission, and the fluorescence intensity increases first and then decreases mainly in the blue-green emission region (375−520 nm). Aphen-derived CDs-2 displays different emission bands centered at 400, 480, and 550 nm with the increasing excitation wavelength. The above results clearly show that the fluorescent characteristics of CDs-1 and CDs-2 are significantly different from the multiple-core@shell-structured AC-CDs obtained from Aphen and CA and this also demonstrates that the ACCDs are a pure product rather than a mixture. However, on comparing the PL characteristics of AC-CDs and CDs-3 in ethanol solutions (Figures 4 and S5c,d), we can find that the PL spectra of the two samples have similar triple emission bands with similar emission wavelengths except the peak shape. In addition, as the excitation wavelength increases, the postfunctionalized CDs-3 also exhibits a concentration effect for the PL emission and its PL intensity increases first and then decreases. From above analyses, there is almost no difference between AC-CDs and CDs-3 in their FTIR spectra (Figure S3). The XPS spectra (Figure S6) also show that the elemental compositions of CDs-3 are basically similar to those of ACCDs, and especially their particle sizes also are more than 20 nm, as observed from TEM images (Figure S2). The above results indicate that the formation mechanism and structure of the two samples are basically the same although their preparation methods are different. So all of the evidence supports our conjecture about the formation mechanism of the multiple-core@shell-structured AC-CDs prepared from Aphen and CA via one-pot hydrothermal route and obtained core− shell-structured AC-CDs should have multiple energy gaps. As shown in Figure 4d, the obvious triple emission peaks of AC-CDs with multiple-core@shell structure should originate from different structures of AC-CDs corresponding to different energy levels from core band, edge band, and surface band, respectively.49,50 A possible model of the energy level structures of AC-CDs responsible for the three emissions and the corresponding emission origin was proposed, as schematically shown in Figure 5a. We believe that the blue emission (430 nm) of AC-CDs is from the citric acid-derived multiple carbon cores with graphitizing sp2-networked carbogenic domain as the core band. These sp2 domains can cause obvious local distortion and then produce diverse energy gaps, which could be situated in the band tail of the π → π* transition from CC bonds and produce the bandgap energy of blue emission.51 Because of the formation of Aphen moiety in AC-CDs, they create numerous new energy levels between n−π* gaps (Figure 4). Hence, there may appear various new radiation complex channels and cause a wide range of emission energies and longwavelength PL emission (Figure 4). Because of the introduction of Aphen in AC-CDs, the amide bonds formed by the reaction of the carboxyl groups of citric acid and the amino groups of Aphen and many kinds of amide bonds will produce below the π* state, which is the origin of green emission, which is the edge band. The mechanism of green emission in AC-CDs can also be demonstrated by the green emission fluorescence decay curves with a biexponential decay (Figure S7 and Table S1), and this result also indicates that the green emission comes from the two components that are

Figure 4. UV−vis absorption (black line) and fluorescence emission spectra (color line) for AC-CDs in ethanol solution with different concentrations of (a) 0.01 mg mL−1, (b) 0.1 mg mL−1, (c) 0.25 mg mL−1, and (d) 1.0 mg mL−1. (e) 1931 CIE chromaticity diagram showing coordinates of AC-CDs under different conditions in Table S2 and (f) PL spectra of pure white-light-emitting sample 8 (concentration: 1.04 mg mL−1) with CIE coordinate (0.33, 0.33), inset: the corresponding photograph of sample 8 under 400 nm excitation.

400 nm, the intensities of the three emission bands at 430 nm (blue), 500 nm (green), and 630 nm (red) are gradually enhanced on the whole; especially, the extent of the increase for green and red emission peaks is more obvious. For example, when the concentration of AC-CDs reaches 1.0 mg mL−1, the PL intensities of three emission peaks reached an approximate level (Figure 4d). What surprised us is that the tricolor emission bands from blue to red lights almost cover the entire visible spectrum and this makes it possible for the AC-CDs to emit white light. However, as the excitation wavelength increases from 400 to 600 nm, the blue emission sharply weakens and the obviously enhanced green- and red-lightemission peaks are observed correspondingly, in particular, for high-concentration samples. Actually, it can be seen that the multiple-core@shell-structured AC-CDs display the excellent excitation-wavelength-independent red, green, and blue emissions simultaneously due to the existence of multiple energy gaps by regulating the excitation wavelength and concentration of AC-CDs. UV−vis absorption spectra in Figure 4 also clearly show the as-synthesized AC-CDs have different absorption bands. The observed absorption peaks at 231 and 273 nm for the low-concentration AC-CDs are due to the π → π* transition of the CC bonds (Figure 4a), whereas, as the concentration of AC-CDs increases, there is an evidently enhanced absorption band at 350−650 nm, corresponding to 19800


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obviously weakened, whereas the blue emission is basically unchanged after the addition of zinc ions. This corresponding change is also observed in the UV−vis absorption spectra (Figure 5c). The absorption peaks at 231 and 271 nm do not change significantly after the addition of metal ions, but the absorption band at 350−650 nm decreased significantly. From above results, it can be seen that the red and green emissions are mainly affected by the Aphen groups in combination with the changes in Figure 5, whereas the blue emission is unaffected by the coordination ions. Comparing the fluorescence decay curves of different emission bands in AC-CDs before and after the addition of zinc ions (Figure S7 and Table S1), it is found that the PL lifetime of 630 nm emission changes from monoexponential decay to biexponential decay and is shorter than that of the pore AC-CDs solution, indicating that the source of the 630 nm emission has changed from one component to two components.28 Thus, this result should be due to the coordination of Aphen groups in AC-CDs with Zn2+, which leads to a possible electron transfer that affects the surface electron distribution and thus the original red emission channel or causes the destruction of some large conjugated rigid sp2 domains that come from the surface state (Figure 5d,e).6,31,47 These changes will shorten the lifetime and provide a new emission component. The above phenomenon also confirms that the red emission comes from the Aphen groups, i.e., the surface state. The PL lifetime at 500 nm is still a biexponential decay, and the percentage of τ1 and τ2 is similar to that of pure AC-CDs, demonstrating that the source of the 500 nm emission has not been significantly affected by metal ions although the green-light emission intensity has an evident decline. However, the bond length of amino groups in Aphen moiety may be changed due to the coordination with Zn2+, which affects the Aphen group-related electron transition in the amide bond. As a result, the green emission is quenched obviously (Figure 5b). These phenomena also supported our above assumption that the green emission is related to the amide bonds, which is the edge band in AC-CDs (Figure 5a). Finally, the almost unchanged blue emission and its lifetime also proves that blue emission originates from the AC-CD’s cores derived from CA molecules because the core bond does not undergo the coordination reaction with Zn2+ (Figure 5d,e). The concentration effect of AC-CDs was also studied to provide a further insight into the red emission of AC-CDs (Figure S8). When the concentration of AC-CD solution increases from 0.01 to 2.0 mg mL−1, the intensity of blue emission peak (430 nm) decreases gradually excited by 400− 600 nm, and the green emission peak (500 nm) increases first and then decreases, whereas the red emission peak (630 nm) increases continuously. As the concentration becomes greater than 1.0 mg mL−1, a decrease in red emission intensity was observed, as shown in Figure 6. This fluorescence quenching of AC-CDs may be ascribed to excessive resonance energy transfer (RET) or direct π → π interactions like typical organic molecular chromophores.53,54 The above phenomenon for the concentration-induced fluorescence’s enchantment of AC-CDs solution (below 1.0 mg mL−1) has also been reported and explained by other studies.31 It is generally considered that the red-emission bandgap energy originated from the surface state of AC-CDs is due to the difference in the distribution of electrons between the surface and the interior. As a result, the different distances in the intermolecular interactions can affect the surface electrons’ distribution, thus affecting the PL emission. With the distance decreasing between the AC-CDs

Figure 5. (a) Energy level structures to explain the PL behaviors of the three different emissions from AC-CDs. (b) PL spectra and (c) UV− vis absorption spectra of AC-CDs in the absence or presence of Zn2+. Surface state structures of AC-CDs in the absence (d) or presence (e) of Zn2+ (the insets are the corresponding photographs of AC-CDs before and after coordination with Zn2+ under 400 nm excitation).

surface fluorophores and graphitizing cores.28 The red emission should originate from the phenanthroline part on the surface of the AC-CDs, i.e., the surface state. These phenanthroline moieties could serve as surface passivation and promote more effective radiative recombination, which results in a series of emission traps. Hence, the phenanthroline part groups in ACCDs could play a key role in the creation of a surface state and cause red emission because the phenanthroline-derived shell structure of AC-CDs has large conjugated rigid sp2 domains, leading to the emergence of lower energy states with a larger red-shift emission.6,47 It is well-known that Aphen is a class of molecules that have the ability to coordinate with metal ions,52 so we further have insight into the PL mechanism of the as-prepared AC-CDs using the coordination ability of Aphen moieties in AC-CDs, and, as expected, when the zinc ions (100 μL, 1.0 M) were added in a certain concentration of AC-CD solution (10 mL, 1.0 mg mL−1), the optical properties of AC-CDs obviously were changed due to the coordination of Aphen ligands with metal ions. As shown in Figure 5b, we can see that the red emission and green emission in the PL spectra of AC-CDs are 19801


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AC-CDs in ethanol solution. The as-prepared AC-CDs are expected to have a wide range of applications, including medical and biolabeling imaging and solid multicolor LED, but here we are looking forward to exploring the applications of AC-CDs in the white LED based on their triple excitation-wavelengthindependent adjustable emission bands of blue, green, and red fluorescence. To obtain white-light emission, we studied the CIE coordinates of AC-CDs in ethanol with different concentrations and excitations and found that under 400 nm excitation, as the concentration of the solution increases, the CIE coordinates gradually move from blue (0.18, 0.14) to red (0.67, 0.33) region. We optimized the optimal condition (Figure 4e and Table S2) and finally obtained pure white emission (0.33, 0.33; CIE coordinate) for the AC-CDs with a solution concentration of 1.04 mg mL−1. From the optical picture (Figure 4f), we can observe that the solution emits bright white emission under 400 nm excitation and its absolute PL quantum yield (QY) is 29%, which is the highest value recorded for pure white-emitting single carbon dot. Direct white-light emitter has very important lighting application and attracts a wide range of attention and interest.8,28,33,35,38−40,43,48,55,56 However, most of the carbon dots have a self-quenching effect of photoluminescence due to their aggregation.57,58 In this work, the obtained multicores/ shell-structured AC-CDs have concentration-enhanced effect for PL emission, especially red light, which can overcome the PL self-quenching of AC-CDs. In addition, the multicore/shellstructured single AC-CD also shows tricolor emissions so the white-light-emitting AC-CDs as phosphor can be designed to fabricate white LED. We introduced the AC-CDs into the monomer of hydroxyethyl methacrylate (HEMA), followed by in situ bulk polymerization to prepare polymer hybrid, which can be processed to meet the requirements for different applications. The white-light emission nanocomposites can be achieved under the UV LED excitation by adjusting the concentration of AC-CDs in the final polymer matrix. The mixture of AC-CD/HEMA monomers is easily polymerized into a monolith showing a bright white color under a 400 nm UV lamp (Figure 6c,d), and its PL spectrum is not changed with a long time UV irradiation for 10 h under the same conditions (Figure 7a,b), which means a good anti-photobleaching is achieved, and the PL spectrum is also very stable below 100 °C (Figure 7c,d). It is worth noting that when the device temperature exceeds 100 °C, the fluorescence intensity of the device has a slight decline. This result may be because that the glass transition temperature (Tg) of PHEMA is about 95 °C and the physical properties of PHEMA change at above Tg, which affects the fluorescence intensity of the device. These properties are essential features for the LED applications. Through the study of the fluorescence monoliths with AC-CDs, we found that it had many advantages over other organic dyes and semiconductor quantum dot dopants into the film or in the polymer, including easy manufacturing, high luminous characteristics, decentralized uniformity, and low toxicity, and these advantages make the AC-CDs have great potential to become a new class of advanced fluorescent materials. For the above reasons, we have designed WLEDs based on UV chips and AC-CD/PHEMA hybrid as phosphors. The device was coated on the surface of the solid-state light-emitting chips by the AC-CD/PHEMA monolith obtained via in situ polymerization of AC-CD/HEMA as the LED phosphors. The electroluminescence (EL) spectrum of the entire device consists of triple emission peaks (Figure 8a). We performed a

Figure 6. (a) Relationship between the red emission intensity and the concentration of AC-CDs, (b) optical photograph of the highconcentration AC-CDs in ethanol (1.0 mg mL−1) under sunlight and 532 nm excitation. Photographs of AC-CDs/PHEMA bulk hybrid under sunlight (c) and 400 nm excitation light with (d) bright pure white light.

because of the intermolecular interaction, their surface potentials are reduced by charge redistribution, thereby limiting the excitation of electron transfer to the surface.31 Fluorescence resonance energy transfer (FRET) process also may contribute to the concentration-induced fluorescence enchantment of AC-CDs. We can find from Figure 4a−d that with increasing concentration of AC-CDs in ethanol, the UV− vis absorption intensities of different energy bands, especially at 400−550 nm, obviously increase, while at the same time, the emission bands of 430 and 500 nm decrease gradually and the red emission at 630 nm increases continuously. Actually, the emission maximum at 630 nm of AC-CDs has a remarkable redshift of 113 nm from that of green emission (500 nm), similarly with a redshift of 213 nm from that of blue emission (430 nm). We considered that there should be a FRET process in the AC-CDs due to the above facts and the large spectral overlap between UV−vis absorption region (380−550 nm) and blue-green emission bands, especially, for the high-concentration systems of AC-CDs (Figure 4c,d).28,35,36 The FRET is usually described in terms of the Förster distance (R0) for convenience, and it is believed that the extra-large redshift is ascribed to FRET or reabsorption, which always occurs in small interparticle distances less than R0.28,36 To verify the FRET occurrence in AC-CDs, the fluorescence decay curves (Figure S7) for AC-CDs with low concentration (0.1 mg mL−1) and high concentration (1.0 mg mL−1) in ethanol solution were also measured. It can be seen from Table S1 that the lifetimes at 430 and 500 nm decrease with increasing solution concentration but the lifetime at 630 nm increases with increasing solution concentration, demonstrating the occurrence of FRET process which competes with radiative transition and then shortens the lifetime of blue and green emission.36 So, the concentration effect again shows that the red emission is derived from the surface state of AC-CDs. Using the concentration effect of AC-CDs, we have also optimized the best concentration (1.0 mg mL−1) for red emission (Figure 6) to achieve the high absolute QY up to 67%, which is the highest value reported for red-emitting AC-CDs.47 Figure 6b is an optical photograph of the bright red-emitting 19802


Research Article


and surface band (red) and are accompanied by a certain degree of FRET. The triple emission peaks of the AC-CDs can generate a pure white emission with an extremely high quantum yield of 29%, which can be used to construct a WLED device with a variety of excellent properties, such as good anti-photobleaching and temperature stability. Our work for the first time put forward the synthesis of core−shellstructured CDs, and this provides a new concept for the future design of multicolor fluorescent CDs for different applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b03529. Additional optical properties, structural characterization, luminescence stability tests, and related applications of CDs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Figure 7. (a) PL spectra of AC-CDs under different UV irradiation times, (b) graph of fluorescence intensity under diverse UV irradiation times, (c) PL spectra of AC-CDs under diverse temperature surroundings, and (d) graph of fluorescence intensity under diverse temperature surroundings (ex = 400 nm).

ORCID

Tianyi Zhang: 0000-0003-3160-9495 Changli Lü: 0000-0003-1978-0628 Notes

CIE coordinate fitting from the PL spectrum and got the coordinates of (0.33, 0.33) (Figure 8b). The results show that the monolith hybrid as phosphors can achieve a pure whitelight-emitting LED (Figure 8c) with a high color rendering index of 92 and luminous efficiency of 30.5 lm W−1.40,42,43,47,48 In addition, it is possible to obtain different colors of CIE coordinates by adjusting the concentration of AC-CDs in the polymer matrix, which can be used to fabricate different colored LED phosphors.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21574017). The authors also thank Lu He for helpful discussion and help.

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REFERENCES

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CONCLUSIONS In summary, the nitrogen-doped multiple-core@shell-structured AC-CDs with controllable fluorescence have been prepared by a facile one-pot hydrothermal method and the origin of different emissions were explored. An obvious concentration-induced fluorescence enchantment effect is observed in the AC-CDs, and the maximum PL QY of red emission for AC-CDs can reach 67% using this effect. The special multiple-core@shell structure can endow the AC-CDs with the triple emission bands, including blue (430 nm), green (500 nm), and red (630 nm) fluorescence. The triple emissions from the AC-CDs root in different positions of the core−shellstructured AC-CDs with core band (blue), edge band (green),

Figure 8. (a) EL spectrum, (b) CIE color coordinate, and (c) photograph of WLED operated at 3.5 V and commercial UV chip (400 nm) with ACCD/PHEMA hybrid applied as phosphor. 19803


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Tricolor White-Light-Emitting Carbon Dots with Multiple-Cores@Shell

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