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

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Functional Inorganic Materials and Devices

Tricolor White-Light Emitting Carbon Dots with Multiple-Cores@Shell Structure for WLED Application Tianyi Zhang, Feifei Zhao, Li Li, Bin Qi, Dongxia Zhu, Jianhua Lü, and Changli Lü ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 24 May 2018 Downloaded from http://pubs.acs.org on May 24, 2018

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ACS Applied Materials & Interfaces

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

KEYWORDS: carbon dots, multiple-cores@shell structure, tricolor emission, high quantum yields, pure white-light emission

ACS Paragon Plus Environment

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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 and multicolor emission are still huge challenges to be addressed for their practical use in different fields. Here, novel nitrogen doped multiple-cores@shell structured AC-CDs with tricolor emissions of red, green and blue were constructed via one pot hydrothermal method from 5-amino-1,10-phenanthroline (Aphen) and citric acid (CA) 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; CIE coordinate) was carried out from single carbon dot with QY of 29 % through regulating the excitation and concentration of multiple-cores@shell structured AC-CDs. In addition, because of their excellent PL properties, the white-emitting AC-CDs as emitting phosphor can be easily used in the fabrication of white light emitting diode (WLED) with good anti-photobleaching and temperature stability.


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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 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 and multicolor emission colors of CDs.30 Now many strategies, including using surface state and size control, solvatochromic effect, heteroatom doping and energy transfer, have been explored to develop the CDs with longwavelength and multicolor emission.3,5,6,9-16,19,28,31-37 White light-emitting materials have gained heightened attention due to their extensive applications in the fields of lighting and displays. Recently, many studies also focused on the


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fabrication of CDs-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 dots (CQDs)-based monochrome LEDs with stable emission color using CQDs directly as an active emission layer.35 Recently red emissive CQDs with high quantum yield was also prepared via a sequencial dehydrative condensation and dehydro-genative planarization (DCDP) route, and an ultraviolet-pumped CQD-based warm WLED was also carried out 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 blue-light emitters 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 (SSF) 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.13,33,4446,48

The white-emitting CDs have been prepared from oligomer polyamide resin as carbon

source via one-step 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 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 appropriate ratio of organic chromophore precursors.13 Especially, the pure white


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emissive CDs with tricolor emitting bands can be obtained via this strategy, but the absolute quantum yield of as-prepared 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 multiplecores@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 below 10 nm. However, the as-prepared AC-CDs by us is a special kind of carbon dots with a size greater than 10 nm due to the formation of the multiplecores@shell structure. Different from other multicolor CDs, it is the first report of core-shell structured CDs with individually adjustable triple primary colours (red, green and blue emission). The origin of different emissions and the related fluorescent mechanism of multiple-cores@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 white light-emitting diode (WLED). To the best of our knowledge, this is the highest QY yet reported for pure white-light emission from single CD.


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

EXPERIMENTAL SECTION Materials. Citric acid (CA, 99.5%) was purchased from Aladdin and 5-amino-1,10phenanthroline (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. TEM and HRTEM characterizations were performed on a JEOL-2100F electron microscope. 1H-NMR spectra were examined with an 500 MHz AVANCE Bruker spectrometer using deuterated d6-DMSO as the solvent. FTIR spectras were obtained using KBr disks on a Magna 560 FT-IR spectrometer. PL spectra were recorded on a Cary Eclipse spectrophotometer and UV-vis absorption spectra were collected using a SHIMADZU UV-2550


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UV-visible spectrophotometer. The X-ray photoelectron spectroscopy (XPS) spectra were performed with a Quantum 2000 spectrometer using non-monochromatized Al Ka excitation radiation. Photoluminescence quantum yields were measured in a calibrated integrating sphere in 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 oC for 7 h. After the reactors were cooled to room temperature naturally, the solution was filtrated through a microporous membrane (0.22  m) 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 10000 r/min 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 oC, and the synthetic procedure was the same as above, except the reaction temperature was set at 170 oC and 230 oC, 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

o

filtrated through a microporous membrane (0.22 m) 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.


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Preparation of Aphen derived CDs-2. Aphen (0.15 g) and 120 L concentrated hydrochloric acid were put into deionized water (15 mL) and 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 post-functionalized CDs-3. CDs-1 (1.35 g) and Aphen (0.15 g) were dissolved in deionized water (15.0 mL) and other synthetic procedure was the same as that for the preparation of AC-CDs. Fabrication of WLED from AC-CDs. We directly used the AC-CDs/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 pre-polymerized at 60 oC 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 oC for 1 h each, and finally at 120 oC for 0.5 h. The spectra of LEDs were measured by combining a Spectra scan PR650 spectro-photometer with an integrating sphere and a computer-controlled direct-current power supply Keithley model 2400 voltage current source under ambient condition at room temperature. The color of the light was identified by the CIE (Commission Internationale de L’ Eclairage 1931) calorimeter system.


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RESULTS AND DISCUSSION We investigated the effects of different reaction temperatures (170, 200 and 230 oC) 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 well-structured AC-CDs at low temperature of 170 oC, while the higher temperature at 230 oC 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 oC provided the wellstructured 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 post-functionalized CDs-3 which 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 nm and 0.18 nm, respectively. Above results indicate that the CDs-1 or CDs-2 can not form the large size of AC-CDs. It is well known that the size of CDs is usually below 10 nm, while the as-synthesized AC-CDs from Aphen and CA exhibited a larger size about 25 nm (Figure S2a), which is similar to that of CDs3 (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-cores@shell structure. The possible formation mechanism of the multiplecores@shell structured AC-CDs is proposed as shown in Figure 1. We speculated that the citric acid molecules firstly 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


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accompanied by amidation process and a certain degree of carbonization. Eventually, the multiple-cores@shell structured AC-CDs with relative large size were generated.

Figure 2. TEM images of AC-CDs obtained during different reaction times: (a) 45 min, (b) 1 h, (d) 2 h, (e) 3 h, (f) 7 h, and (c) is a high resolution image of (b). The insets of pictures (a, c, d, e) are the corresponding HRTEM images.


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To further prove the formation of the multiple-cores@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, CA firstly is carbonized to produce small CA-derived CDs (CA-CDs) with the clear lattice fringes 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 large spherical 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 CA-CDs are mainly wrapped by the shell layers derived from 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 AC-CDs were explored via Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS). Compared with CDs-1 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 AC-CDs. 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 triple different peaks, namely, C 1s (286 eV), N 1s (400 eV), and O 1s (531 eV), and the atomic content of C/O/N is 68.36 %, 28.98 % and 2.66 % from the elemental analysis. The high-resolution C 1s spectrum (Figure 3b) can be deconvoluted into


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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 spectrum (Figure 3d) shows two typical peaks at 532.1 and 534.1 eV for C=O and C-O.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 contains more N elements, while 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 time from the outside to the inside of AC-CDs, with the Aphen parts were etched away, the contents of N elements in AC-CDs gradually reduced with etching. Moreover, 1H-NMR data (Figure S4) of AC-CDs also further demonstrate that the Aphen moieties are present in the ACCDs due to the appearance of aromatic proton peaks from 6 ppm 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 low field. This should be attributed to the result of amidation reaction of Aphen and CA in the formation process of AC-CDs.


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Figure 3. (a) XPS survey spectrum and high-resolution XPS (b) C 1s, (c) N 1s, and (d) O 1s spectra of AC-CDs. As shown in Figure 4, the as-prepared multiple-cores@shell structured 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 concentration of AC-CDs in ethanol, with the increasing excitation wavelength from 300 to 400 nm, the intensities of the three emission bands at 430 nm (blue), 500 nm (green) and 630 nm (red) gradually enhance on the whole, especially, the extent of the increase for green and red emission peaks is more obvious.


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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 the AC-CDs possible to emit white light. However, as the excitation wavelength increases from 400 nm to 600 nm, the blue emission sharply weakens and the obviously enhanced green and red light-emission peaks are observed correspondingly, in particular, for high concentration samples. Actually, it can be seen that the multiple-cores@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), while as the concentration of AC-CDs increases, there is an evidently enhanced absorption band at 350-650 nm corresponding to the n→π* transitions of the C=O and C=N bonds (Figure 4b-d). To further understand the PL mechanism of multiple-cores@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 nm to 520 nm). Aphen derived CDs-2 displays different emission bands centered at 400 nm, 480 nm 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-cores@shell structured AC-CDs obtained from Aphen and CA and this also demonstrates that the AC-CDs are pure product rather than a mixture. However, as compared the PL characteristics of AC-CDs


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and CDs-3 in ethanol solutions (Figure 4 and Figure S5c, d), we can find that the PL spectra of the two samples have the 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 of 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 that of AC-CDs, and especially, their particle sizes also are more than 20 nm 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 the evidence supports our conjecture about the formation mechanism of the multiple-cores@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.


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Figure 4. UV-vis absorption (black line) and fluorescence emission spectra (color line) for ACCDs in ethanol solution with different concentrations of (a) 0.01, (b) 0.1, (c) 0.25 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.


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As shown in Figure 4d, the obvious triple emission peaks of AC-CDs with multiplecores@shell structure should originate from different structures of AC-CDs corresponding to different energy levels from core band, edge band and surface band, respectively.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 Due to 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 of new radiation complex channels and cause a wide range of emission energies and long-wavelength PL emission (Figure 4). Due to 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, that is edge band. The mechanism of green emission in AC-CDs can also be demonstrated by the green emission fluorescence decay curves with a bi-exponential decay (Figure S7 and Table S1), and this result also indicates that the green emission comes from the two components which are 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 AC-CDs could play a key role in the creation of surface state and cause red emission because the phenanthroline derived


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shell structure of AC-CDs has a large conjugated rigid sp2 domains, leading to the emergence of lower energy states with a larger red-shift emission.6,47

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).


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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 L, 1.0 M) were added in a certain concentration of AC-CDs solution (10 mL, 1.0 mg mL), the optical properties of AC-CDs obviously were changed due to the coordination of Aphen

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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 obviously weakened, while 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 nm 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, while 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 changes from mono-exponential decay to biexponential decay and 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 electrons distribution and thus the original red emission channel, or causes the destruction of some large conjugated rigid sp2 domains which 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 bi-


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exponential decay, and the percentage of 

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and 

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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 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).

Figure 6. (a) Relationship between the red emission intensity and the concentration of AC-CDs, (b) Optical photograph of the high concentration 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.


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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-CDs 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, while the red emission peak (630 nm) increases continuously. As the concentration is greater than 1.0 mg mL-1, a decrease in red emission intensity was observed 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 redemission 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, and thus affecting the PL emission. With the distance decrease between the AC-CDs 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 red-shift of 113 nm from that of green emission (500 nm), even with a red-shift of 213 nm from blue


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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 red-shift is ascribed to FRET or re-absorption, which always occurs in small interparticle distance 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 nm 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 also have 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 optical photograph of the bright red-emitting 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 multicolour LED. But here we are looking forward to exploring the applications of AC-CDs in the white LED based on their triple excitation-wavelength-independent adjustable emission bands of blue, green and red fluorescence. In order 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


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(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.

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).


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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 selfquenching effect of photoluminescence due to their aggregation.57,58 In this work, the obtained multi-cores/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 multi-cores/shell structured 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 ACCDs 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-CDs/HEMA monomers is easily polymerized into a monolith showing a bright white color under 400 nm UV lamp (Figure 6c, d), and its PL spectrum is not changed for a long time UV irradiation for 10 h under the same conditions (Figure 7a, b), which means a good antiphotobleaching, and the PL spectrum is also very stable below 100 oC (Figure 7c, d). It is worth noting that when the device temperature exceeds 100 oC, 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 oC, 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 dots 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


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potential to become a new class of advanced fluorescent materials. For the above reasons we have designed WLEDs based on UV chips and AC-CDs/PHEMA hybrid as phosphors. The device was coated on its surface of the solid-state light-emitting chips by the AC-CDs/PHEMA monolith obtained via in-situ polymerization of AC-CDs/HEMA as the LED phosphors. The electroluminescence (EL) spectrum of the entire device consists with triple emission peaks (Figure 8a). We performed a 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 white light emitting LED (Figure 8c) with a high color rendering index (CRI) 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 color LED phosphors.

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 AC-CDs/PHEMA hybrid applied as phosphor.


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CONCLUSIONS In summary, the nitrogen doped multiple-cores@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-cores@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-shell structured AC-CDs with core band (blue), edge band (green) and surface band (red), and 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 WLED device with a variety of excellent properties such as good antiphotobleaching and temperature stability. Our work for the first time put forward the synthesis of core-shell structured CDs, and this provides a new concept for the future design of multicolor fluorescent CDs for different applications.

ASSOCIATED CONTENT Supporting

Information.

Additional

Optical

properties,

structural

characterization,

luminescence stability tests and related applications of CDs. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION ACS Paragon Plus Environment

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Corresponding Author *E-mail: [email protected]

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

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Graphical abstract


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

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