Excitation-Independent Dual-Color Carbon Dots: Surface-State

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Excitation-Independent Dual-Color Carbon Dots: Surface-State Controlling and Solid-State Lighting Daqin Chen, Haobo Gao, Xiao Chen, Gaoliang Fang, Shuo Yuan, and Yong-Jun Yuan ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00675 • Publication Date (Web): 10 Aug 2017 Downloaded from http://pubs.acs.org on August 10, 2017

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Excitation-Independent Dual-Color Carbon Dots: Surface-State Controlling and Solid-State Lighting Daqin Chen*, Haobo Gao, Xiao Chen, Gaoliang Fang, Shuo Yuan, Yongjun Yuan College of Materials & Environmental Engineering, Hangzhou Dianzi University, Hangzhou, 310018, P. R. China

Abstract Long-wavelength orange-red emissions of carbon dots have recently attracted great attention due to their wide applications. Although it is possible to achieve long-wavelength luminescence by varying the incident excitation wavelength, excitation-independency is highly desired in terms of both practical applications and understanding emission mechanisms. In the present work, carbon dots with excitation wavelength independent orange and blue dual-color emissions were synthesized by a facile solvothermal route using p-phenylenediamine as carbon source and formamide as solvent. Structural and spectroscopic characterizations indicated that N- and O-related surface-state controlling via modifying reacting temperature/time were responsible for the dual-color emissions of carbon dots. Moreover, carbon solid film, retaining original orange emissions, was fabricated to explore its possible application as color converter in solid-state lighting. Impressively, by combining orange carbon film and yellow phosphor-in-glass with InGaN blue chip, light-emitting diode devices with improved color rendering index and correlated color temperature were successfully constructed.

Keywords Luminescence, Quantum dots, Optical materials, Light-emitting diodes, Carbon film, Display.

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Recently, luminescent carbon dots (C-dos) have attracted great attention due to their distinctive merits including low cytotoxicity, low cost, chemical inertness, low photobleaching and excellent biocompatibility.1-9 These advantages inspired extensive investigations on C-dots, enabling their promising applications in several fields, such as solid-stating lighting, laser, bioimaging, fluorescent ink as well as sensing.10-17 In these applications, the fluorescent colors in the visible full-spectral region ranging from blue to red are all required, especially in the long wavelength region from orange to red.18-22 As a consequence, it is highly urgent and desirable to develop color-tunable C-dot materials. So far, the reported C-dots usually emit an intense blue-green light and show excitation wavelength dependent long-wavelength emissions.23-25 Therefore, one has to adopt a series of different excitation light sources to achieve diverse emission colors and the long-wavelength (orange to red) emissions are generally weak,26 significantly limiting their practical applications. Notably, such excitation wavelength dependent multicolor luminescence of C-dots is generally originated from their diverse emitting states. In fact, C-dots show the bandgap transitions ascribing to the conjugated sp2-domains, and the bandgap of C-dots can be tuned by controlling the size of conjugated sp2-domains, resulting in different emission colors.25 On the other hand, O-, N-, and S- related defect-states on the surface of C-dots were also reported to contribute to the color-tunable luminescence of C-dots.27,28 Herein, we provide a facile solvothermal route using p-phenylenediamine (PP) as the carbon source and formamide (FA) as the solvent to synthesize luminescent C-dots. Surface-state controlling is easily realized by simply modifying solvothermal reacting temperature/time, which results in two-color (blue and orange) emissions for the as-fabricated C-dots. More importantly, these C-dots show an excitation 2


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wavelength independent behavior and can be converted into C solids to find promising application in solid-state lighting.

Results and Discussion

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Figure 1 (a, b) PLE/PL spectra and (c) absorption spectra of orange and blue C-dots, insets are the corresponding C-dot solutions and luminescent photographs under UV lamp excitation. Two-dimensional excitation-emission mapping for (d) the orange C-dots and (e) the blue C-dots. The luminescent colors of the fabricated C-dots are highly dependent on the solvothermal reaction time. As revealed in Figure 1a, photoluminescence (PL) spectrum demonstrates that the C-dots obtained by solvothrmal reaction at 200 oC for 1 h show broadband orange emission centered at about 580 nm. Further elongation of reaction time to 4 h, only blue emission located at about 420 nm is detected (Figure 1b). The quantum yields for the orange and blue emission are about 10-15% and 20-30%, respectively. Photoluminescence excitation (PLE) spectra exhibit distinct absorption features for the orange C-dots and blue ones. Three strong absorption bands at around 300, 402, and 460 nm are recorded by monitoring 580 nm emission for the orange C-dots, and a broad absorption band nearby 350 nm is detected by monitoring 420 nm emission for the blue C-dots. Different to the case of blue C-dots, the absorption spectrum of orange ones show extra broad absorption band in the range 3


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of 350-500 nm (Figure 1c). All these results indicate that the blue and orange emissions are originated from different emitting states of the as-fabricated C-dots. The influence of reaction temperature and time on optical properties of C-dots was further studied. With increase of temperature from 160 oC to 240 oC, the emission color of C-dots changes from orange to blue (Figure S1); with increase of reaction time, similar result can be achieved (Figure S2). The higher the reaction temperature, the shorter the reaction time to convert orange emission into blue one. Moreover, the emission color is not relevant to the adding content of p-phenylenediamine precursor (Figure S3). Therefore, it can be concluded that the change of emission color should be related to the modification of surface states of C-dots during solvothermal reaction. Impressively, both orange and blue C-dots show excitation wavelength independent emissions, as evidenced in Figure 1d and 1e. Although the intensity was varied, the peak position did not significantly changed upon the variation of excitation light wavelength (Figure S4), confirming that the orange and blue emissions of C-dots are originated from diverse individual emitting state, respectively. (a)

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Figure 2 TEM images of (a) orange C-dots (200 C/1h) and (b) blue ones (200oC/4h). Insets are the corresponding histograms of the particle size distribution of C-dots. (c, d) XPS full survey, high-resolution XPS of (e) C1s, (f) O1s, (g) N1s spectra for the orange and blue C-dots. (h) FTIR spectra of the related C-dot samples. To understand the luminescent mechanisms for the as-prepared C-dots, structural characterizations were carried out. Transmission electron microscope (TEM) images, presented in Figure 2a and 2b, reveal that both orange and blue C-dots are monodispersed without significant difference in particle sizes (2-3 nm). Evidently, the similar average sizes for these two kinds of C-dots indicate that the quantum size effect is not the dominant mechanism responsible for the conversion of emission color from orange to blue with increase of reaction temperature and time. X-ray photoelectron spectroscopy (XPS) measurements were performed to determine the actual compositions of the synthesized C-dot samples. C1s, N1s and O1s signals are clearly detectable in the XPS full survey spectra (Figure 2c, 2d), suggesting the existence of N and O elements in the present C-dots. With increase of reaction time, the N content and N/C ratio significantly increase, while no obvious change is found for the O/C ratio. In the high-resolution spectra, the C1s band consists of two peaks (Figure 2e), the dominant graphitic sp2 carbons (C=C/C-C) and minor carbonyl 5


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carbons (C=O).29 The O1s band can be mainly assigned to C=O (Figure 2f).30 Importantly, remarkable difference in the N1s band is found (Figure 2g), i.e., pyrrolic N exists in the orange C-dots while it is transformed into amine N with elongation of reaction time (blue C-dots).31 Fourier transform-infrared spectra (FTIR) (Figure 2h) further confirm that these C-dots possessed abundant hydrophilic groups such as O-H, stretching vibrations of C=C, C=O, and C=N bonds,32 being in agreement with XPS results. In addition, N-H group is obviously detected on the surface of the blue C-dots. Raman spectrum in the 1000-2000 cm-1 region (Figure S5), recorded under 532 nm laser excitation, shows two vibration bands, where G band (~1550 cm-1) attributes to graphitic sp2 carbon and D band (~1350 cm-1) assigns to surface state of carbon,18 confirming the existence of the disorder structures or defects on the surface of the present C-dots. (a)

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Figure 3 (a) A schematic growth process and the related structural models for the orange C-dots and blue ones. The proposed absorption (Abs) and emission (Em) 6


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mechanisms for (b) the orange C-dots and (c) the blue ones. Contour plots of time-resolved data for (d) the orange C-dots and (e) the blue ones. Insets are the corresponding decay curves by recording 580 nm and 420 nm emissions, respectively. As schematically illustrated in Figure 3a-3c, possible structural models responsible for the dual-color emissions and the related energy-state structures were proposed with the assistance of PLE/PL, absorption, TEM, XPS, FTIR and Raman spectra data. During the solvothermal process, p-phenylenediamine acting as the C and N source was dissolved in formamide solution, and assembled into O- and N-rich C-dots through a carbonization reaction (Figure 3a). Initially, the growth of sp2 C core followed by the formation of pyrrolic N (N-related defect state) on the surface will results in orange luminescence. However, the pyrrolic N group tends to be destroyed/carbonized with the elevation of temperature and the elongation of time due to its relatively low stability.31 As an alternative, inert amine N group (NH2-), together with abundant O-related defect states such as C=O and COOH, will be eventually formed on the surface of C-dots. As schematically demonstrated in Figure 3b, the excitation bands in the region of 250-320 nm are proposed to be assigned to the π→π* (HOMO→LUMO) transition of C=C bonds in sp2 C domain, while the excitation bands in the region of 320-420 nm and 420-500 nm are attributed to the transitions from π states to O- and N-related defect states on the surfaces of C-dots.25 Subsequently, the electrons in the π* and O defect states can non-radiatively relax to the N-related defect state to produce long-wavelength orange emission. Increasing reaction temperature and time will convert the activated N state into inert one (amine N) and make O-related defect state act as emitting state, where electrons will be 7


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accumulated and de-excite to ground states to produce blue emission (Figure 3c). In a further experiment, time-resolved fluorescence was carried out to evaluate the decay lifetime of C-dots using a time correlated single photon counting (TCSPC) technique. As shown in the insets of Figure 3d and 3e, the decay lifetime of orange emission is indeed different to (shorter than) that of blue one, confirming that the orange emission and blue one are originated from different emitting states. Notably, with increase of detecting emission wavelength from 550 nm to 650 nm for the orange C-dots, no obvious change of decay contour was evidenced (Figure 3d and Figure S6). Similar result can be found for the blue C-dots (Figure 3e and Figure S7). All these results verify that the orange and blue emissions are attributed to diverse individual emitting state, i.e., N-related defect state and O-related one, respectively. To gain more insight into the dual-color emissions, the influence of pH on the luminescence of C-dot solutions was investigated, as shown in Figure S8. Importantly, excitation wavelength independent emissions were retained for both the blue and orange C-dots dispersed in aqueous solution with different pH values. For the blue C-dot solution, the variation of pH value imposes no significant effect on emission position (Figure S8a-8c) as well as absorption characteristics (Figure S9a). For the orange C-dot solution, similar results can be found at the neutral or alkaline environment (Figure S8d, 8e); however, the orange luminescence and the corresponding absorption shift towards long-wavelength in the acidic environment (Figure S8f, Figure S9b). Based on the proposed energy-state diagram of orange C-dots (Figure 3b), the orange emission is assigned to the transition of N-related defect state to HOMO(π, no, nN) 8


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states and the long-wavelength absorption is originated from the transition of HOMO(π) state to N-related defect state. Since the shift of HOMO(π)→LUMO(π*) short-wavelength absorption is not obvious (Figure S9b), the emission/absorption red-shift of orange C-dots is probably attributed to the lowering of N-related emitting state under the influence of acidic environment. (a)

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Figure 4 Photographs of (a) C-dot PVP film and (b) the orange and blue luminescent solids under the irradiation of UV (365 nm) lamp. Contour plots of (c) excitation-emission mappings and (d) time-resolved decays (monitoring wavelength: 550-650 nm) for orange C solids obtained by varying the concentrations of C-dots in PVP from 0.1, 0.2 to 0.4 mg/mL (from top to bottom). Reprinted with permission from Hangzhou Dianzi University for use of the logo in (a). Currently, the commercial phosphor-converted white light-emitting-diode (pc-WLED) device was generally constructed by coupling InGaN blue chip with Ce: YAG yellow phosphors. Due to the shortage of red emitting component in the Ce3+: YAG microcrystals, this traditional strategy usually suffers from high correlated color temperature and low color rendering index.33 Worthy of notice, the excitation wavelength independent orange broadband emission of the present C-dots excitable by blue light may enable its possible application as color converter to improve 9


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correlated color temperature (CCT) and color rendering index (CRI) of WLED device. Figure S10 shows PLE and PL spectra of Ce: YAG phosphors and orange C-dots. Importantly, PLE spectra of Ce: YAG and C-dots have a large degree of overlap in the blue region and the orange emission of C-dots can effectively make up the shortage of red component for Ce: YAG phosphor, ensuring the promising application of the present C-dots in pc-WLED. Firstly, we prepared C films by incorporating C-dots into soli-state matrix (PVP) to facilitate its usage. As demonstrated in Figure 4a and 4b, different sizes and shapes of C-dot polymer films can be achieved and the original orange/blue emissions are well retained. The photostability of C-dots was then investigated. As revealed in Figure S11, the emission profile and intensity of orange C-dot film upon the excitation of 465 nm blue light was not remarkably changed when the exposure time is extended to 60 h, suggesting its excellent photostability. Furthermore, the influence of C-dot content (in film) on optical properties was studied (Figure 4c, 4d). Evidently, the excitation wavelength independent orange emission was not altered; however, it is found that the center emission wavelength gradually shifts towards long-wavelength with increase of C-dot content in PVP (Figure 4c), probably due to the re-absorption effect of C-dots. As demonstrated in the contour plots of time-resolved decays (Figure 4d), high C concentration in PVP indeed slightly decreases the decay rate of electrons from N-related emitting state to ground state.

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(a)

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Figure 5 (a) Schematic diagrams illustrating the construction of two kinds of prototype LED devices by integrating a 465 nm InGaN blue chip with orange C-dot film (C-LED) and C-dot-film/PiG (C/PiG-LED) color converters. EL spectra of the fabricated WLEDs (applied current: 350 mA) where the thickness of C-dot film gradually increases from 0.5, 1.0, 1.5 to 2.0 mm (from left to right): (b) C-LED and (c) C/PiG-LED. Insets of (b, c) are the corresponding WLEDs in operation. Table 1 Photoelectric parameters of C-LED and C/PiG-LED with different thicknesses of C-dot film under operating current of 350 mA C-LED 0.5 mm 1.0 mm 1.5 mm 2.0 mm

CCT (K) 14570 6331 3867 2158

Chromaticity coordinates (0.283,0.246) (0.323,0.272) (0.367,0.314) (0.470,0.358)

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Figure 6 (a) The variation of CIE coordinates for the (1) C-LED and (2) C/PiG-LED with increase of C-dot film thickness from 0.5, 1.0, 1.5 to 2.0 mm (from left to right). 11


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Dependence of (b) CCT and (c) CRI values on C-dot film thickness for both C-LED and C/PiG-LED. As a proof-of-concept experiment, LED devices are constructed by coupling orange C-dot film with an InGaN blue chip (named as C-LED, Figure 5a). EL spectra of the designed WLEDs driven by 350 mA operation current show a emission band peaking at 465 nm assigned to the blue chip and an orange broadband luminescence in the range of 480~780 nm originated from C-dot film (Figure 5b). Table 1 tabulates the corresponding photoelectric parameters including CCT, CRI, and chromaticity coordinates. Evidently, as the thickness of C-dot film increases, orange emission component relative to blue one monotonously enhances due to the increase of orange C-dot content in PVP. As a consequence, the shifting of color coordinates towards red region (Figure 6a), the decreasing of CCT from 15470 to 2158 (Figure 6b), the increasing of CRI from 54.7 to 73.7 (Figure 6c) for the C-LEDs with increase of C-dot film thickness are clearly observed. Correspondingly, the emitting color of C-LED changes from cold white to warm white, as demonstrated in the insets of Figure 5b. Notably, owing to the shortage of yellow component for the C-dot film, the optimal CRI value (73.3) for C-LED is still not high enough for practical application. Herein, Ce: YAG phosphor-in-glass is adopted as yellow color converter and is combined with orange C-dot film to constructed WLED. The Ce: YAG PiG plate was prepared via co-sintering Ce:YAG yellow commercial phosphor and TeO2-based low-melting glass, as previously reported by our group. The LED configuration by placing PiG adjacent to the chip followed by C-dot film is schematically shown in Figure 5a, and the influence of C-dot film thickness on optical performance of such C/PiG-LEDs was investigated, as exhibited in Figure 5c. Indeed, the yellow emitting component originated from Ce: YAG phosphors can effectively fill the gap between blue chip emission and orange C-dot emission, improving CCT up to 85 (Table 1, 12


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Figure 6c) and yielding pure white-light for the C/PiG-LEDs (insets of Figure 5c). Impressively, the movement of color coordinates almost overlaps with planckian locus (Figure 6a), CRI decreases from 5977 to 3089 (Figure 6b), CCT increases from 80.5 to 85.5 (Figure 6c) and luminous efficiency (LE) remains in the range of 60~80 lm/W with increase of C-dot film thickness, certainly confirming the promising application of the investigated orange C-dot film to improve the quality of WLED. Finally, the light stability of C/PiG-LED driven by 350 mA applied current under different working time intervals was examined. As tabulated in Table S2 and exhibited in Figure S12, no remarkable change/degradation of EL spectra, color coordinates, CCT, and CRI was detected with prolonging working time to 72 h, verifying the excellent color stability for the as-designed C-dots-based WLED.

Conclusions In summary, we have successfully prepared excitation-independent orange/blue dual-color C-dots by a solvothermal reaction method. TEM, XPS, FTIR, Raman, PLE/PL, absorption, and time-resolved results suggested that the orange and blue emissions were originated from diverse individual emitting state, i.e., N- and O-related defect-states on the surfaces of C-dots, respectively. Furthermore, orange C-dot films were fabricated by incorporating C-dots into PVP matrix and were confirmed to have promising applications in blue-light-excitable warm WLEDs. Specifically, the optimal C-dot-film/PiG-LED device shows a bright white light with CCT of 4250 K, CRI of 85, LE of 67 lm/W, color coordinates of (0.371,0.376), and superior color stability.

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Experimental Section Materials and Chemicals All the raw materials, including p-phenylenediamine, formamide, polyvinyl pyrrolidone (PVP, Mw=58000), ethyl acetate, ethanol, sodium hydroxide (NaOH) and HCl, were provided by Sinopharm Chemical Reagent Company and were used directly without further purification. Deionized (DI) water was adopted throughout the experiment.

Synthesis of dual-color C-dots In a typical synthetic procedure, p-phenylenediamine (0.25 mmol, 0.027 g) was dissolved in formamide (20 mL) solution and stirred to form a transparent solution. Then the solution was transferred into a 25 mL Teflon-lined stainless autoclave. After heating at 200 ℃ in oven for 1 h or 4 h and cooling down room temperate naturally, orange and blue suspensions were achieved respectively. The crude products were then purified with a silica column chromatography using the mixture of ethyl acetate and ethanol (4: 1) as the eluent. After removing solvents through rotary evaporation at 50 ℃ and further drying under vacuum, the purified C-dots could be finally obtained. Different reaction temperatures and times can also be used to prepare orange/blue C-dots by a similar procedure (see Table S1).

Preparation of C-dot films A slightly modified procedure reported by Yu et al. was adopted to prepare C-dot/polymer film.28 Firstly, a selected polyvinyl pyrrolidone (PVP) of 2.0 g was mixed with 10 mL formamide solution (C-dot content: 0.1~0.4 mg/mL) and stirred for 14


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30 min. Then the resulted mixture was transferred into a clean glass substrate and dried overnight under vacuum environment to get a solid composite film.

Fabrication of Ce3+: Y3Al5O12 (Ce: YAG) phosphor-in-glass (PiG) plate A low-temperature co-sintering technique was used to fabricate PiG plate.34-36 Commercial Ce:YAG yellow phosphors (1 wt%, XinLi Illuminant Co. Ltd) and TeO2-B2O3-Sb2O3-ZnO-Na2O low-melting glass were completely mixed and sintered in a platinum crucible at 600 °C for 20 min in ambient atmosphere. The melt was quenched into a pre-heated copper mold and then cooled to room temperature. The obtained PiG was finally polished and cut into φ10 mm disk with the thickness of 0.5 mm.

Construction of C-dot-based white light-emitting diode (WLED) Two kinds of LED devices were constructed by directly coupling the as-fabricate C-dot film on the InGaN blue chip or by coupling bi-layer C-dot-film/PiG color converters on the chip. Opaque silica gels were filled around the edges of device in order to avoid the leakage of blue light.

Characterization The actual compositions of C-dot samples were determined by X-ray photoelectron spectroscopy using a VG Scientific ESCA Lab Mark II spectrometer equipped with two ultra-high vacuum 6 (UHV) chambers. All the binding energies were referenced to the C1s peak of the surface adventitious carbon at 284.6 eV. Fourier transform infrared spectra were recorded on a Nicolet 6700 FTIR spectrometer in the range of 4000~400 cm−1 using the KBr pellet technique. Raman spectra were recorded 15


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on a Microscopic confocal Raman spectrometer (Renishaw Trade Co., Ltd, England) under the excitation 532 nm laser. Microstructure observation was performed on a JEOL JEM-2010 transmission electron microscope at 200 kV accelerating voltage. TEM specimen was prepared by directly drying a drop of a dilute formamide dispersion solution of C-dots on the surface of a copper grid. Absorption and emission spectra were recorded on an Edinburgh Instruments (EI) FS5 spectrofluorometer equipped with a continuous (150 W) and pulsed xenon lamps. Excitation-emission mappings of C-dots were recorded by continuously changing excitation wavelength with a fixed step of 1 nm, and the offset between excitation wavelength and emission one was set to be 30 nm to reduce scattering light. Quantum yield, defined as the ratio of emitted photons to absorbed ones, was determined by a spectrofluorometer (FS5) equipped with an integrating sphere. Time-resolved spectra of C-dots were detected on a fluorescent lifetime spectrometer (LifeSpec-II, EI) based on a time correlated single photon counting technique under the excitation of 375 nm or 475 nm picosecond laser. The lifetime values of C-dots were determined via the equation of

τ = ∫ I (t ) dt / I 0 , where I(t) is the time-related luminescence intensity and I0 is the peak intensity. Electroluminescence (EL) spectra, Commission Internationale de L’Eclairage (CIE) chromaticity coordinates, color rendering index, correlated color temperature

and

luminous

efficiency

of

the

constructed

devices

were

recorded/determined in a HAAS-2000 spectroradiometer (Everfine, P. R. China) under the operation current of 350 mA. All the experiments were performed at room temperature. 16


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ASSOSCIATED CONTENT Supporting Information. Table S1-S2 and Figure S1-S12. Extra optical absorption/PLE/PL spectra, excitation-emission mappings, time-resolved PL decay curves, Raman spectrum, stability of C-dot film and WLEDs, some experimental conditions for dual-color C-dots as well as photoelectric parameters of C/PiG-LED. This information is available free of charge via the Internet at http://pubs.acs.org/.

Author Information Corresponding author *

E-Mail: [email protected] (D. Q. Chen)

Acknowledgements This research was supported by Zhejiang Provincial Natural Science Foundation of China (LR15E020001), National Natural Science Foundation of China (51572065, 61372025, 51372172), 151 Talent's Projects in the Second Level of Zhejiang Province and the College Student's Activities of Science and Technology Innovation in Zhejiang Province (2016R407063).

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For Table of Contents Use Only

Excitation-Independent Dual-Color Carbon Dots: Surface-State Controlling and Solid-State Lighting Daqin Chen*, Haobo Gao, Xiao Chen, Gaoliang Fang, Shuo Yuan, Yongjun Yuan

C-dots and C-dot films with excitation independent orange/blue dual-color emissions were synthesized via controlling surface states and were demonstrated to have a promising application in solid-state lighting.

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Excitation-Independent Dual-Color Carbon Dots: Surface-State

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