Color-Tunable Carbon Dots Possessing Solid-State Emission for Full

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Color-Tunable Carbon Dots Possessing Solid-State Emission for Full-Color Light-Emitting Diodes Applications Tanglue Feng, Qingsen Zeng, Siyu Lu, Xianju Yan, Junjun Liu, Songyuan Tao, Mingxi Yang, and Bai Yang ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01010 • Publication Date (Web): 03 Nov 2017 Downloaded from http://pubs.acs.org on November 5, 2017

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Color-Tunable Carbon Dots Possessing Solid-State Emission for Full-Color Light-Emitting Diodes Applications Tanglue Feng†, Qingsen Zeng†, Siyu Lu‡, Xianju Yan†, Junjun Liu†, Songyuan Tao†, Mingxi Yang† and Bai Yang*† †

State Key Laboratory of Supramolecular Structure and Materials, college of

chemistry, Jilin University, Changchun, 130012, P. R. China. ‡

College of Chemistry and Molecular Engineering, Zhengzhou University, 100 Kexue

Road, Zhengzhou 450001 China Corresponding author: [email protected] (B. Yang)

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ABSTRACT In this paper, a unique strategy is proposed to modulate fluorescence color of carbon dots (CDs) under both aqueous solution and solid state for bright multi-color light-emitting

diodes (LEDs)

applications.

We

report

facile

synthesis of

dual-peak-emissive CDs with self-quenching-resistant character under solid state through hydrothermal method, and investigate the origins of dual-peak emission for the first time. In addition, based on the unique dual-peak-emissive phenomenon, acid-mediated PL behaviors were realized by changing the pH value of hydrothermal precursors. We realized the color-tunable fluorescence under aqueous solution from blue (B-CDs) to yellow-green (YG-CDs) color, and solid state from yellow to orange-red color. The changed PL behaviors are attributed to the more conjugated structure inside CDs due to elevated carbonization degree. Furthermore, the red-shifted fluorescence from aqueous solution to solid state is ascribed to supramolecular crosslinking between adjacent particles. Thus, by manipulating the supramolecular crosslinking degree of YG-CDs in PVA, series of luminescent blue-shift CDs/PVA composites phosphor were obtained. Finally, we fabricate the almost full-color and white color LEDs with decent performances, which indicate their potential for solid-state lighting applications.

KEYWORDS: carbon dots, photoluminescence, color-tunable, dual-peak-emission, full-color, light-emitting diodes.

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INTROUCTION For the past few years, phosphor-based light emitting diodes (LEDs) have been the subject of extensive research to achieve potential applications in multi-color display, low-cost back-lighting in liquid-crystal displays and next generation lighting sources for our daily life.1-2 In recently years, great progress has been made to design and fabricate high-performance rare earth ion-based phosphor LEDs. However, these phosphors were generally synthesized from expensive raw materials at high reaction temperature, which are detrimental to cost saving.3-4 More recently, the colloidal semiconductor quantum dots (QDs) have been considered as promising candidates for phosphor-based LEDs due to high photoluminescence quantum yield (PLQY), narrow full-width at haft-maximum (FWHM) and easily tunable band gap.5-6 However, severe toxicity of Cd and Pb-containing QDs has greatly hindered their practical application in LEDs.1,

7-8

Thus, it is highly desired to develop excellent

light-emitting materials with great optical stability for light and heat, cheap raw materials and low toxicity to replace the rare earth ion and QDs in phosphor-based LEDs. The emerging and light-emitting carbon dots (CDs) seem to be promising alternatives to rare earth ion and semiconductor QDs. CDs are considered to be quasi-spherical carbon nanomaterials with size less than 10 nm.9-10 It have inspired extensive research interests since discovered serendipitously in 2004 by arc-discharge methods11 owing to outstanding properties, such as bright fluorescence, good water-solubility, high biocompatibility, easy functionalization, low toxicity, etc.9, 12-15 Those excellent properties enable CDs with enormous potential in diverse applications including sensors16-18, bioimaging19-21, theranostic22-23, light-emitting diodes1-2, 8, 24-26 and efficient photocatalysts27-28 and so on. However, the majority of reported CDs generally present blue or green color fluorescence in aqueous solution, with the CDs under solid state exhibiting weak even no photoluminescence due to aggregation-induced

luminescence

quenching,

which

greatly

obstacle

the

development of bright CDs-based multi-color solid state phosphor LEDs.29 Currently, 3


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there were some reported methods to modulate the PL color of CDs, especially the achievement of long-wavelength emission, such as, the introduction of conjugated precursors23,

30-31

, surface modification32-34, solvent engineering35-37, additive

strategy38-40 and the using of special preparation methods41, etc. However, the self-quenching-resistant solid-state emission of CDs were rarely involved except for a few reported blue or green-emitting solid state CDs42-46. Thus, it is highly urgent to develop new methods to achieve color-tunable fluorescence under both aqueous solution and solid state in one CDs systems, especially the achievement of self-quenching-resistant the long-wavelength solid-state emission for bright multi-color phosphor LEDs. Herein, a unique strategy is proposed to modulate fluorescence color of CDs under both aqueous solution and solid state for bright multi-color LEDs applications. We synthesized bright dual-peak-emissive B-CDs with self-quenching-resistant character under solid state through hydrothermal method from p-aminosalicylic acid and citric acid via hydrothermal method, and explored the mechanism of dual-peak-emission for the first time. Then, based on the unique dual-peak emissive property, acid-mediated PL behaviors were realized by changing the pH value of hydrothermal precursors. We obtained the color-tunable fluorescence under aqueous solution from blue (B-CDs) to yellow-green (YG-CDs) color. Surprisingly, the solid-state B-CDs and YG-CDs exhibit red-shifted fluorescence of yellow and orange-red color respectively. The different PL behaviors between B-CDs and YG-CDs were investigated in depth. What’s more, the red-shifted solid state fluorescence versus their aqueous solution state are ascribed to supramolecular crosslinking

between

adjacent

particles.

Therefore,

by

manipulating

the

supramolecular crosslinking degree of YG-CDs in PVA, series of luminescent blue-shift CDs/PVA composites phosphor were obtained. Finally, almost full-color and white CDs/PVA phosphor-based LEDs with decent device performances were achieved. The acid-mediated PL behaviors based on dual-peak-emissive behavior represent a new approach to control the optical properties of CDs, and provide with more opportunities in photoelectric applications. 4


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RESULTS AND DISCUSSION The origin of dual-peak emission of B-CDs The synthetic process of B-CDs is shown in experiment section, and its optical properties were explored by UV-vis spectra and PL spectra. The UV-vis spectra (Figure 1a) of B-CDs present four absorption band located at 274 nm, 300-400 nm and 497 nm assigned to π→π* transition corresponding to the aromatic C=C bond, n→π* transition related to the C=O/C=N bond, and other aromatic structure respectively.47-49 The PL properties of B-CDs aqueous solution is shown in Figure 1b. It exhibits bright dual-peak-emission band, with presenting bright blue color fluorescence under UV lamp (inset of Figure 1b). The strongest blue and yellow-green-band emission locate at 452 nm and 525 nm respectively, with showing PLQY of 42.1% and 49.8% respectively. From the strongest excitation spectra in Figure 1c, the maximum excitation wavelength (498 nm) of yellow-green-emitting band of B-CDs agree well with the corresponding excitonic absorption peak (497 nm) while such phenomenon is not observed in the blue-emitting band, suggesting that the yellow-green emitting band was characterized by band-edge excition-state decay while the blue emitting band was the result of surface or defect-state decay.8,

50

Furthermore, fluorescence lifetime measurement (Figure S1 and Table 1) indicate obviously different PL lifetime between blue (9.78 ns) and yellow-green (6.19 ns) emission band of B-CDs. These results manifest that the dual-peak-emissive band of B-CDs originate from absolutely different PL centers. The origin of dual-peak-emission of the B-CDs were investigated in detail afterwards. It’s confirmed that the blue-emissive band of B-CDs originate from individual p-aminosalicylic acid, because the hydrothermally as-prepared CDs from the individual p-aminosalicylic acid can also generate similar dual-peak fluorescence emission (Figure S2), but a lower PLQY of 17.2% is obtained for blue emitting band. So we conclude that the existence of CA in the synthesis process of B-CDs can tremendously enhance the fluorescence and simultaneously not change the PL behaviors. Additionally, we find that the incorporation of CA into hydrothermal 5


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system could boost the stability of CDs solution as shown in Figure S3. The introduction of CA containing multi-oxygen-containing groups can increase the dispersibility of B-CDs in aqueous solution, while individual p-aminosalicylic acid including benzene ring is detrimental to the stabilization of as-obtained CDs in water after hydrothermal treatment.

Figure 1. The absorption spectra, emission spectra, and the strongest excitation -emission spectra of B-CDs (a-c) and YG-CDs (d-f). The inset in (a) and (d) is the enlarged absorption spectra. The inset in (b) and (e) show the aqueous solution photoluminescence photographs of B-CDs and YG-CDs under UV lamp, respectively.

Table 1. The fluorescence lifetime of two emission bands of B-CDs and YG-CDs.

In order to further explore the origin of dual-peak-emission phenomenon, we investigate the effect of solvent polarity on the PL properties of B-CDs (Figure S4a, 4b). The blue-emissive band of B-CDs shows obvious solvent-dependent PL behaviors. However, the yellow-green-emissive band exhibits solvent-independent PL 6


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behaviors. In other words, the PL center of blue-emitting component is susceptible to solution polarity compared to that of yellow-green-emitting component,51 which are caused by the different PL centers of two fluorescence component. It is indicated that blue and yellow-green-emissive band of the B-CDs originate from molecule state and carbon-core state, respectively. That is, because the PL properties of molecule state are easily influenced by solvent environment due to the bareness of the fluorescence center, while the carbon-core state coated by abundant molecule and surface groups possess a relatively stable chemical environment. The influences of pH values on the PL behaviors of B-CDs (Figure S4c, 4d) also support the synergistic PL mechanism of molecule state and carbon-core state.8 What’s more, the different response of the two emissive bands of B-CDs for Fe (III) ion as shown in Figure S5 also verifies this synergistic PL mechanism, based on similar reason as the solvent-dependent PL behaviors. To verify the synergistic PL mechanism of molecule state and carbon-core state, the hydrothermal temperature for the synthesis of B-CDs were changed from 160oC to 240oC (with interval temperature of 20oC). According to the UV-vis spectra (Figure S6), the absorption bands at 200-300 nm and 300-400 nm assigned to the π→π* transition and n→π* transition, respectively, both enhance with the increase of temperature, which result from the elevated carbonization degree. What’s more, the two emission bands of B-CDs get closer with the increase of preparation temperature as shown in Figure S7 and Table S1. In addition, the fluorescence intensity ratio of blue emission band to yellow-green emission shows the tendency of firstly increasing then decreasing (Figure S7, 8). The increase of hydrothermal temperature at 160-200oC contribute to the emission of molecule state due to the more sufficient dehydration.52 When the hydrothermal temperature further elevate (200-240oC), the molecule state fluorescence tends to decline due to deep carbonization, transferring into carbon-core state. Moreover, it should be noted that B-CDs synthesized at the 160oC of hydrothermal temperature present relatively higher absolute PLQY (Table S2) compared to other higher temperatures-derived CDs owning to lower absorption intensity (Figure S6). Thus, we conclude that the elevation of hydrothermal 7


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temperature will lead to the reduction of molecule state emission, causing the enhancement of carbon-core state emission due to the higher carbonization degree, which can also be verified by XPS results (Figure S9 and Table S3). From the XPS analysis results, with the increase of hydrothermal temperature, the contents of C-C/C=C and C=O tend to increase due to the higher carbonization degree.

Acid-mediated PL behaviors under both aqueous solution and solid state There were some previous reports8, 39-40, 53-54 about acid-mediated carbonization process at the synthesis process of CDs. According to the synergistic PL mechanism (molecule state and carbon-core state) of B-CDs, It was expected that the addition of acid to hydrothermal system can also influence the formation of molecule and carbon-core state, then PL properties due to the acid-mediated carbonization degree of precursors. We synthesized another CDs by the addition of acid while keep other conditions constant. The original (2.1) and more acidic pH value (1.0) of precursor solution derived CDs (named as B-CDs and YG-CDs, respectively) were investigated in detail as below. The optical properties of B-CDs and YG-CDs aqueous solution were explored by UV-vis spectra and PL spectra, etc. As same as B-CDs, the UV-vis spectra of YG-CDs also give multiple absorption bands at 270 nm, 382 nm and 507 nm. However, the absorption intensity of YG-CDs is higher than B-CDs as shown in Figure 1a,1d, which possibly arise from the higher carbonization degree under more acidic hydrothermal environment.53 The PL properties of B-CDs and YG-CDs are shown in Figure 1b,1e. The two CDs samples exhibit entirely different aqueous solution PL properties, with the aqueous solution of B-CDs and YG-CDs presenting bright blue and yellow-green color fluorescence under UV lamp, respectively (inset of Figure 1b, 1e). Typically, obvious dual-peak-emission can be observed in the fluorescence spectra of B-CDs. However, in the PL spectra of YG-CDs, there exists almost single yellow-green emitting band with strong emission peak locating at 528 nm, and the 8


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strongest excitation spectra of YG-CDs (Figure 1f) shows that the maximum excitation wavelength (506 nm) of yellow-green-emitting band of YG-CDs also agrees well with the corresponding excitonic absorption peak (507 nm) as B-CDs do. The approximate emission peak and same match of strong excitation peak and excitonic absorption peak between yellow-green-emitting band of B-CDs and YG-CDs, demonstrate that the yellow-green-emitting band of B-CDs and YG-CDs both originate from conspecific carbon-core state, and the carbon-core state dominate completely the PL behaviors of YG-CDs. The strongest emission wavelength and absolute PLQY of B-CDs and YG-CDs were summarized in Table S4. It should be noted that the PLQY of YG-CDs is lower than that of B-CDs owning to higher carbonization degree-generated strong absorption. Time-resolved PL spectra (Figure S1) were used to further elucidate the PL’s distinction between B-CDs and YG-CDs. As shown in Table 1, the blue emission band exhibits entirely different PL lifetime between the aqueous solution B-CDs (9.78 ns) and YG-CDs (6.14 ns), and similar phenomenon are also observed in yellow-green emission band.

Figure 2. The solid state photographs of B-CDs and YG-CDs under daylight (a) and UV lamp (b). The PL spectra of solid-state B-CDs (c) and YG-CDs (d).

To our delight, the B-CDs and YG-CDs exhibit self-quenching-resistant yellow color and orange-red color fluorescence under solid state as shown in Figure 2a-d. 9


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Typically, the B-CDs and YG-CDs synthesized at 200oC of hydrothermal temperature present brighter solid fluorescence emission compared with B-CDs and YG-CDs prepared at other higher temperatures, which are possibly caused by the insufficient (160oC) or excessive (240oC) carbonization degree. The absolute PLQY of solid-state B-CDs and YG-CDs were measured to be 1.04% and 0.42%, respectively. The difference of fluorescent color of the CDs between aqueous solution state and solid state possibly is caused by following factors. Under solid state, adjacent CDs particles get closer even crosslinking by supramolecular interaction55-56, such as H-bond (formed between hydroxyl and amino group), electrostatic interaction and proper π-π interaction. The supramolecular crosslinking under solid state may induce the emission of narrow bandgap which can’t show photoluminescence (PL) under aqueous state due to the continual energy loss by the non-radiative pathway resulting from the vibration and rotation of the narrow-bandgap-fluorophore. Nevertheless, the PL of these narrow bandgap can be enhanced by the supramolecular crosslinking under solid state, which is named as the supramolecular crosslink-enhanced emission (SCEE) effect.57 What’s more, supramolecular crosslinking possibly induce the fabrication of new luminescent bandgap, then red-shifted emission peak. The supramolecular-induced crosslinking effects in CDs will be further discussed in fluorescent solid-state CDs/PVA composites, later. The reasons of entirely different optical properties between B-CDs and YG-CDs caused by the addition of acid in the hydrothermal process are thought-provoking and need further to be illuminated. The different PL behaviors of B-CDs and YG-CDs may originate from different conjugation sp2 structure in carbon core, because the different pH value of mixed precursors aqueous solution could influence the existence form of chemical group (such as, amino and carboxyl group) in the process of carbonization, which probably result in different carbonization degree then affect the electron structure of conjugated system in carbon-core.53 The morphology of B-CDs and YG-CDs were characterized by transmission electron microscopy (TEM) and atomic force microscope (AFM). Figure 3a and 3b show TEM image of the as-prepared B-CDs and YG-CDs, both of which exhibit 10


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quasi-spherical shape and had a uniform disperse. The size distribution histogram (Figure S10) indicate that the diameters of the as-prepared B-CDs and YG-CDs are 2.4 nm and 5.8 nm, respectively. That is, the size of YG-CDs is larger than that of B-CDs, which indicate that the more acidic hydrothermal environment could facilitate the nucleation and growth of CDs. The high-resolution TEM (HRTEM) images (the inset of Figure 3a, 3b) reveal that the B-CDs contain not lattice fringes while the YG-CDs contain lattice fringes with the lattice spacing of ca. 0.3 nm. There is more ordered carbon-core structure inside YG-CDs after undergoing successive polymerization, dehydration and carbonization19, 53-54, 58, which are also verified by XRD patterning (Figure S11). These results agree well with our supposition that the more acidic hydrothermal environment probably contribute to sufficient carbonization of CDs then more ordered carbon-core structure.53 What’s more, the Raman spectra (Figure S12) of B-CDs and YG-CDs show that the YG-CDs have higher G/D ratio in contrast to B-CDs. The higher graphitization degree in YG-CDs indicates the higher carbonization degree. The AFM images of B-CDs and YG-CDs were shown in (Figure 3c-f), the heights of both two CDs are about 2.0-4.0 nm.

Figure 3. TEM images of B-CDs (a) and YG-CDs (b), the insets in the TEM images are the corresponding high-resolution TEM images. AFM images (10×10μm) of B-CDs (c) and YG-CDs (d), the insets (e) and (f) below the AFM images show the corresponding height profile along the lines.

11


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Fourier transfer infrared (FT-IR) and X-photoelectron spectroscopy (XPS) characterization were further used to investigate the surface compositions of the two CDs. From the FT-IR results (Figure 4a), the stretching vibrations of O-H and NH2 at 3300-3500 cm-1, the stretching vibration of C=O at 1720 cm-1 and the stretching vibrations of C=C, C=N at 1500-1680 cm-1, are observed in both B-CDs and YG-CDs.59-60 Moreover, we observe that the content ratio of C=C to C=O of YG-CDs is higher than that of B-CDs, which further demonstrate that the two kinds of CDs have different conjugation sp2 structure in carbon core. It is consistent well with the results that the YG-CDs possess higher carbonization degree in comparison with the B-CDs. The FT-IR analysis results could be further proved by X-ray photoelectron spectroscopy (XPS) analysis (Figure 4b-i, Table 2). Above results illuminate that different structure and compositions between B-CDs and YG-CDs produce dominated effects on the changed PL behaviors. Typically, the more acidic hydrothermal pH value can cause the protonation of chemical groups, then increase carbonization degree, therefore changing PL properties.

Figure 4. The FT-IR spectrum (a) of B-CDs and YG-CDs. The XPS full survey of B-CDs (b) and YG-CDs (c). The XPS analysis results (C1s, N1s, O1s) of B-CDs (d-f) and YG-CDs (g-i). 12


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Table 2. The content of various chemical bonds in B-CDs and YG-CDs.

Multi-color solid-state emission for LEDs applications In virtue of the color-tunable PL properties between B-CDs and YG-CDs, especially the different PL characters under the solid state, we fabricated the almost full-color luminescent CDs/polyvinyl alcohol (PVA) composites film by adjusting the concentration of CDs in PVA as shown in experiment section. From the optical photograph (Figure 5) and fluorescence spectra (Figure S13), the blue, green, and yellow-green color emitting CDs/PVA composite films can be obtained by adjusting the concentration of B-CDs in PVA. Likewise, the green, white, yellow, orange and orange-red color emitting CDs/PVA composite films are achieved by dispersing YG-CDs in PVA. The optimal absolute PLQY of blue, yellow-green, yellow, white and orange-red CDs/PVA composites films were measured to be 4.60%, 8.65%, 6.53%, 3.04% and 0.52%, respectively. The emission color of the composites films is related to the concentration of CDs in PVA. The lower concentration the CDs possessed in PVA matrix, the bigger tendency the CDs/PVA composite show for fluorescence color of CDs aqueous solution. On the contrary, the CDs/PVA composites tend to present fluorescence color of solid-state CDs. Those results elucidate that the CDs present dual-fluorescence morphologies42,

61

of aqueous

solution and solid states. The red-shifted solid state emission of CDs originate from supramolecular crosslinking, and any factors which can alter the crosslinking degree may change PL behaviors of CDs as shown in Figure S13. Significantly, employing the CDs with short-wavelength aqueous solution emission and simultaneous long-wavelength solid state emission, we propose a universal strategy to obtain 13


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full-color luminescent solid state composite by manipulating the crosslinking degree of CDs in matrix.

Figure 5. The photograph of B-CDs/PVA and YG-CDs/PVA composites films with different CDs concentrations under daylight (a) and UV lamp (b).

Figure 6. The photoluminescence photograph, emission spectrum and the color coordinates (CIE) of the blue (a), yellow-green (b), yellow (c), orange-red (d) and white (e) color phosphor-based LEDs. 14


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Then, the almost full-color and white color phosphor-based LEDs were also achieved by coating CDs/PVA composite as presented in experiment section. From the Figure 6, the almost full-color (blue, yellow-green, yellow, orange-red) and white color phosphor LEDs with decent performance are obtained as presented in Table 3. Emission spectra of these phosphor LEDs show CIE color coordinates of (0.21, 0.11), (0.31, 0.41), (0.41, 0.48), (0.48, 0.35) and (0.29, 0.30), respectively, and the white color LEDs show fairish color rendering index (CRI) of 83. What’s more, it is worth mentioning that the luminance of yellow-green, orange-red and white color-based phosphor LEDs reached to almost 5000, 10000 and 5000 cd·m-2 respectively. Compared to previous reported CDs-phosphor LEDs by other groups30, 35, 62-63, the luminous efficiency of LEDs in this work is relatively low, but this work propose a universal method to obtain simultaneously almost full-color and white color LEDs from CDs with dual-peak emission and solid-state long-wavelength emission character. Table 2. The device performances of almost full-color and white color CDs/PVA-based phosphor LEDs.

CONCLUSSIONS In summary, bright and dual-peak-emissive B-CDs with self-quenching-resistant character under solid state is synthesized from p-aminosalicylic acid and CA via hydrothermal method. The dual-peak-emissive behaviors are investigated for the first time

by

detailed

spectroscopy

characterization.

We

elucidate

that

the

dual-peak-emission band of blue and yellow-green color originate from the molecule 15


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state and carbon-core state respectively, which are verified by different response of blue and yellow-green-emitting band for solvent, pH, hydrothermal temperature and Fe3+. Then, based on the unique dual-peak-emissive behaviors, the color-tunable aqueous solution fluorescence from blue (B-CDs) to yellow-green (YG-CDs) color are realized by the addition of acid into the hydrothermal solution, with their solid state both showing red-shifted emission of yellow and orange-red color, respectively due to supramolecular crosslinking between adjacent CDs particles. The changed PL behaviors mainly stem from more conjugated sp2 structure of carbon core as a result of the elevated carbonation degree during hydrothermal process. Finally, by adjusting the supramolecular crosslinking degree of B-CDs and YG-CDs in PVA, we obtain the almost full-color emissive CDs/PVA composite films with different CDs concentration, then the almost full-color and white phosphor LEDs with decent device performances. The acid-mediated PL properties under aqueous solution and solid state based on the dual-peak-emission of CDs, represent a new approach to controlling the optical properties of CDs, and propose a universal strategy to obtain full-color luminescent solid state composite by manipulating the crosslinking degree of CDs in matrix, which indicate their potential for solid-state multi-color LEDs applications.

EXPERIMENTAL SECTION Chemicals. P-aminosalicylic acid (99%) were purchased from J&K Scientific Ltd.

Anhydrous

citric

acid

(98%)

were

obtained

from

Aladdin.

Concentrated hydrochloric acid (HCl, AR) were obtained from Beijing Chemical Works. Polyving akohol (PVA) (1788 87.0-89.0 wt.%) were purchased from Aladdin. Characterization. Transmission electron microscopy (TEM) measurement were performed on JEM-2100F microscope operated at 200 kV using the super thin carbon films. The heights of samples were measured by Atomic Force Microscope (AFM) using a SA400HV with a Seiko SPI3800N controller. Fourier Transfer infrared (FT-IR) spectra was acquired from Nicolet AVATAR 360 FT-IR spectrophotometer. PL spectra were obtained by Shimadzu RF-5301 PC spectrometer, and UV-vis absorption spectra 16


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were gained using Shimadzu UV-3101PC spectrophotometer. Elemental analysis were accomplished by the Elementar Vario micro cube. X-Ray photoelectron spectroscopy (XPS) was investigated by ESCA LAB 250, and the deconvolution processing of XPS spectra were performed on XPSPEARK Version 4.1 software. Time-resolved fluorescence spectra were carried out using FLS920 time-corrected single photon counting system. The XRD pattern were obtained by X-ray diffractometer (Empyrean). Raman spectrum were measured by a LabRAM ARAMIS Smart Raman Spectrometer with a HeNe laser as the excitation line of 785 nm. Quantum Yield Measurements. Absolute PLQY measurement were performed in FLS920 spectrometer equipped with a calibrated integrating sphere. We conducted the test light from FLS920 spectrometer to the sphere. The PL QY was determined by the ratio between photons emitted and absorbed by CDs. The aqueous solution of CDs was placed in a cuvette to measure its QY, while the solvent water was used as a blank sample for the reference measurement. Preparation of B-CDs and YG-CDs. For the preparation of B-CDs, p-aminosalicylic acid (2 mmol) and CA (2 mmol) were dissolved in 20 ml water, Then the mixture was transferred to a poly (tetrafluoroethylene) (Teflon)-lined autoclave followed by being heated at 160oC for 8 h. Then the as-prepared production was naturally cooled down to room temperature and filtered through a 0.22 µm filter membrane to remove the large particles. As a result, the brown solution was obtained. To remove the excess precursor and by-products, the solutions were further dialyzed against DI water through a dialysis bag with a molecular weight cut off of 500-1000 for 24 h. After removing solvent and freeze-drying further, a red brown powder was obtained and stored at 4oC. The as-obtained CDs samples were redispersed in DI water for further characterization. The preparation process of YG-CDs was same as that of B-CDs except for adjusting pH of mixture precursor aqueous solution to 1.0 by adding concentrated HCl. After heated at 160oC for 8 h followed by post-treatment, a dark solution was obtained. Fabrication of Freestanding Luminescence Films. 40 mL pristine as-prepared B-CDs aqueous solution was condensed to 5 mL, then the concentrated B-CDs 17


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aqueous solution with different volumes (10μL, 25μL, 50μL, 100μL, 250μL, 500μL, 1mL) were mixed respectively with polyvinyl alcohol (PVA ) aqueous solution (5% by weight) with total volumes of CDs/PVA aqueous solution being 3.0 mL. The mixed solution was vortexed for 2 min in Vortex generator, followed by being drop cast on plastic culture disk with the diameter of 3.5 cm. The culture disks containing CDs/PVA mixed solution were placed on flat table and left overnight under ambient conditions for evaporating the solvent. The resulting freestanding films were then peeled off and stored in desiccator for further using. Similar procedures were followed to fabricate luminescent YG-CDs/PVA composites films. Fabrication of Multi-color LEDs from B-CDs and YG-CDs. GaN LEDs chips without phosphor coating were purchased from Advanced Optoeletronic Technology CO., Ltd. The emission peak of GaN LEDs chips locate at 365 nm with the operating voltage of 4.0 V. Detailly, 40 mL pristine as-prepared YG-CDs aqueous solution was condensed to 5 mL, then the concentrated YG-CDs aqueous solution with different volumes (10μL, 25μL, 50μL, 100μL, 250μL, 500μL, 1mL) were mixed respectively with PVA aqueous solution (5% by weight) with total volumes of CDs/PVA aqueous solution being 3.0 mL, and the mixed solution was vortexed for 2 min in Vortex generator, The as-obtained mixture solution was coated dropwise onto UV-LEDs chips with about 2-4 drops. Then, those LEDs chips with CDs/PVA nanocomposite phosphors were naturally dried at room temperature overnight. As a results, yellow-green, yellow, orange-red and white color-based phosphor LEDs were achieved. Similar procedures were followed to fabricate blue color-based phosphor LEDs with 100μL pristine as-prepared B-CDs aqueous solution mixed in PVA with total volumes of 3.0 mL. Device Characterization. The emission 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 voltage current source under ambient condition at room temperature. The color of the light was identified by the CIE (Commission Internationale de L’Eclairage1931) calorimeter system. All measurements were performed under dark condition. 18


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ASSOCIATED CONTENT Supporting Information Time-resolved PL spectra, extra PL spectra and absorption, spectra peak position and intensity results, absolute PLQY results, XPS data and analysis results, size distribution results of TEM images, XRD data, Raman spectra.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (B. Yang) Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT This work was financially supported by the National key research and development program of china (2016YFB0401701), the National Science Foundation of china (NSFC)

under

grant

Nos.

51433003,

51373065,

21504029

and

JLU Science and Technology Innovative Research Team 2017TD-06.

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Color-Tunable Carbon Dots Possessing Solid-State Emission for Full

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