Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 22332−22338
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Polymer-Assisted Self-Assembly of Multicolor Carbon Dots as SolidState Phosphors for Fabrication of Warm, High-Quality, and Temperature-Responsive White-Light-Emitting Devices Chan Wang,† Tantan Hu,† Yueyue Chen, Yalan Xu, and Qijun Song*
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Key Laboratory of Synthetic and Biological Colloids, Ministey of Education, International Joint Research Center for Photoresponsive Molecules and Materials, School of Chemical & Material Engineering, Jiangnan University, Wuxi 214122, P. R. China S Supporting Information *
ABSTRACT: White-light-emitting devices (WLEDs) are considered to be a promising illumination source; especially, the WLEDs based on carbon dots (CDs) with white fluorescence have attracted extensive research interest. Herein, we report the design and implementation of solid white-light-emitting phosphors (WCDs@PS), which combine blue and orange emissive CDs (BCDs and OCDs) assisted by polystyrene (PS) through a self-assembly technique. Based on these phosphors (OCDs/BCDs = 1.2:1), the obtained WLEDs display a warm white light with International Commission on Illumination (CIE) coordinates of (0.35, 0.36), a high color rendering index of 93.2, a low correlated color temperature of 4075 K, and a luminous efficiency of up to 14.8 lm·W−1. Interestingly, these WLEDs exhibit temperature-dependent emission performance, whose light-emission spectrum can be adjusted in situ from white (λ ∼ 400−730 nm) to blue (λ ∼ 440 nm) in the range of 20−80 °C. A change in CIE coordinates from (0.35, 0.36) to (0.32, 0.23) was also observed. The temperature-driven tunable LEDs as a thermochromism device could broaden the application of CDs-based lighting systems in special displays. KEYWORDS: polymer-assisted self-assembly, multicolor carbon dots phosphors, high color rendering index, white-light-emitting devices, temperature sensitivity
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INTRODUCTION Solid-state white-light-emitting devices (WLEDs) have attracted a great deal of research interest due to their low power consumptions, long lifetime, fast response, and high luminous efficacy.1−9 They are the most promising solid illumination sources or devices to replace traditional filament bulbs and fluorescent lamps. WLEDs are usually obtained by mixing blue chips with a single yellow phosphor or combining near-UV chips with tricolor phosphors (red, green, and blue colors).9−16 However, in the former method, the created white-light color involves the hue isolation problem as the blue and yellow colors are complementary, as well as the significant dependence on forward current. Moreover, due to the lack of red color elements in light, the LED devices provide a low color rendering index (CRI) and high correlated color temperature (CCT). Although the WLEDs obtained from the latter method have better color rendering performance and higher luminous efficacy, they suffer the difficulty in packaging processes and regulating the optimal ratio of every phosphor. Furthermore, each phosphor has its own luminance decay and optical stability, which may induce a color shift, self-quenching, and reabsorption problem.17−19 To overcome these problems, further investigation of new light-emitting materials and their composites for device fabrication is urgently required. © 2019 American Chemical Society
Carbon dots (CDs) have emerged to be an ideal alternative for traditional rare-earth nanocrystals and semiconductor quantum dots because of their excellent luminescence properties, chemical inertness, easy functionalization, good photostability, low toxicity, and low cost.20−23 Many recent publications have proved the wide applicability of CDs-based white-light-emitting materials in electroluminescent LEDs. In general, CDs must be homogeneously dispersed within solid matrixes [i.e., poly(vinyl pyrrolidone), polyacrylamide, polyacrylic acid, poly(methyl methacrylate), and silica] to prevent luminescence quenching resulted from aggregation-induced self-absorption and complicated film-forming processes.9,24−28 For example, Tian et al. successfully mixed red/green/blueemitting CDs in a polydimethylsiloxane matrix to fabricate white-light-emitting phosphors with International Commission on Illumination (CIE) chromaticity coordinates of (0.34, 0.31), which are very close to the coordinates of natural white light (0.33, 0.33) marked by the standards of the International Commission on Illumination (CIE), 1931.9 However, these CDs-based WLEDs exhibited only a CCT and a CRI value of Received: March 11, 2019 Accepted: June 5, 2019 Published: June 5, 2019 22332
DOI: 10.1021/acsami.9b04345 ACS Appl. Mater. Interfaces 2019, 11, 22332−22338
Research Article
ACS Applied Materials & Interfaces
three-necked round-bottomed flask, followed by addition of 50.0 mmol styrene and 150 mL of deionized water. After stirring at room temperature for 30 min, the mixture was heated at 70 °C for 10 min under a nitrogen atmosphere. Subsequently, 10 mL of KSP solution (∼0.1 g KSP) was quickly added to the mixture. The reaction was then allowed to proceed for 8 h. The purified PS nanoparticles were obtained by centrifugation and dispersed in deionized water for further use. Synthesis of Multicolor CDs. The blue emissive CDs (BCDs) and orange emissive CDs (OCDs) were synthesized according to a modified literature method.7 Briefly, an amount of 1.0 g of CA and 2.0 g of urea were dissolved in 10 mL of DMF. Then, the mixture was transferred into a Teflon-lined stainless steel autoclave and the temperature was increased to 180 °C and maintained for 6 h. After cooling to the room temperature, the BCDs and OCDs were separated through silica column chromatography. Finally, the obtained BCDs and OCDs were re-dispersed in water and freezedried to a powder form for subsequent use. Preparation of the CDs@PS Composites. An amount of 100 mg of the CDs (BCDs and OCDs) was thoroughly mixed with 4 mL of PS solution and then stirred for 4 h at room temperature. The mixture was centrifuged at 10 000 rpm to remove unadsorbed CDs, and the precipitates were collected and freeze-dried for later use. To obtain a solid WCDs@PS, the mass ratios of OCDs/BCDs (i.e., 2:1, 1.7:1, 1.5:1 1.2:1, and 0.8:1) were tested, and a series of CDs@PS composites were obtained and named PCDs@PS (pink), LPCDs@PS (light-pink), LYCDs@PS (light-yellow), WCDs@PS (white), VCDs@PS (violet), and BCDs@PS (blue) based on their lightemitting color. Fabrication of WLEDs Based on WCDs@PS Composites. For fabricating WLEDs, OE6550A/B silica gel and CDs@PS powders were mixed in a mass ratio of 1:1, and then the mixture was deposited on the GaN-LED chip (peak wavelength ∼380 nm). Next, the chip was attached to the bottom of the LED pedestal. The device was dried at 150 °C for 1 h in air. After that, the as-prepared devices were exposed to different temperatures (20−80 °C) for the temperatureresponsive study. Note that the heat sinks were equipped during the packaging process of WLEDs to achieve effective heat dissipation, and the real-time temperature of the GaN-LED chip was monitored by Fotric 225s, and its temperature was about 70 °C during the test. Characterization. The UV−vis absorption spectra were recorded on a UV−vis spectrophotometer (Hitachi U-3900, Japan). A Horiba Jobin Yvon Fluoromax 4C-L spectrophotometer (France) was used to record the photoluminescence (PL) spectra. The samples were excavated by a picosecond diode laser (the excitation wavelengths were set to be 370 and 458 nm), and the fluorescence lifetime data was collected by the time-correlated single-photon counting (TCSPC) system. QE-2100 (Japanese Amnesty) with an integrating sphere was used to measure the absolute quantum yield. The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) experiments were performed on a Tecnai GI F20 UTWIN with an accelerating voltage of 200 KV. The scanning electron microscopy (SEM) image was obtained using Hitachi S4800. The AXIS ULTRA DLD (Shimadzu, Japan) spectrometer with Mg Kα excitation (1253.6 eV) was used to record the X-ray photoelectron spectra. Binding energy calibration was based on C 1s at 284.6 eV. The Fourier transform infrared (FT-IR) spectra were recorded on the Nicolet 6700 FT-IR spectrophotometer in the range of 4000−400 cm−1.
5048 K and 82.4, respectively, which are not favorable for highquality lighting. On the other hand, the development of WLEDs with tunable color is of great importance for displays, lighting fixtures, and communication technologies. Traditional methods are available to obtain different predefined colors by adjusting their emission wavelengths through complex material design and band-gap engineering.14,15,29−31 Zhang and coworkers have prepared pure white-light-emission materials through regulating the excitation and concentration of tricolor emitting CDs (blue, green, and red).32 However, to control the LEDs color in situ within a single device encountered great difficulties. Wang et al. demonstrated that graphene possessed tailored electronic and optical properties, which potentially provide a way to achieve this goal;33 nevertheless, a strategy for realizing in situ control of the color of LEDs based on whitelight-emitting CDs is still lacking, although it is highly desired. Polystyrene (PS) nanospheres could be an ideal solid matrix because of their high transparency, strong rigidity, good chemical resistance, and stability. Although PS has been reported as the hard template or carrier to prepare metal nanocrystals,34−36 it has not been used for assembling multicolor CDs. In this work, we demonstrated the first solid white-light-emitting phosphor (WCDs@PS) by combining blue and orange emissive CDs (BCDs and OCDs) assisted by PS through a self-assembly technique. PS acts not only as the linker to replace previously reported solid matrixes but also as the blocker to prevent intermolecular fluorescence resonance energy transfer and aggregation-caused quenching of multicolor CDs. Consequently, strong solid-state luminescence was obtained by simply controlling the PS size and the feed ratio of PS/CDs to precisely regulate the distance of multicolor CDs. When the mass fraction of multicolor CDs in CDs@PS was controlled to be 0.5 (OCDs/BCDs = 1.2:1, mass ratio), the WCDs@PS-based LEDs emitted bright-white light with the CIE coordinates of (0.35, 0.36), a low CCT of 4075 K, and a high CRI of 93.2. In addition, the device showed a high luminous efficiency (LE) of 14.8 lm·W−1. Interestingly, the fluorescent signal of WCDs@PS was reversibly responsive to temperature in the range of 20−80 °C; the light-emission spectrum of WCDs@PS-based WLEDs devices can be adjusted in situ from white (λ ∼ 400−730 nm) to blue (λ ∼ 440 nm) by conditioning the environmental temperature. Thus, like a chameleon, the temperature-driven tunable LEDs can be used as a thermochromism device. Hence, this work presents a promising way to construct CDs-based solid-state lighting systems and also a method for in situ tuning of emission wavelengths.
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EXPERIMENTAL SECTION
Materials and Chemicals. The commercially available GaN-LED chips with the emission wavelength centered at 380 nm were obtained from Advanced Optoelectronic Technology Inc (China). Chemicals including urea, methanol, N,N-dimethylformamide (DMF), and dichloromethane were acquired from Sigma-Aldrich. Potassium persulfate and styrene were obtained from Shanghai Titan Scientific Co., Ltd. Citric acid (CA) monohydrate was purchased from Tianjin Bodi Chemical Co., Ltd (China). N-Isopropyl acrylamide (NIPAM) was supplied by the Sinopharm Chemical Reagent Co., Ltd. (China). All of the reagents and materials were used as received without any further purification. Deionized water was used in all experiments. Preparation of Polystyrene (PS) Nanospheres. The PS nanospheres were prepared according to the previously described procedures.34 Briefly, 4.4 mmol NIPAM was dissolved in ∼15 mL of deionized water, and then the mixture was transferred to a 250 mL
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RESULTS AND DISCUSSION As reported in the previous studies, a facile method for achieving white-light-emitting phosphors is to combine lightemitting substances with complementary colors.3,38 For example, orange- and blue-emitting copper nanoclusters were combined to prepare WLEDs.3 Since the luminescence of WCDs nanocomposites may be quenched in their solid states, the common method is to dope phosphors into a solid matrix to keep solid-state luminescence for device applications.27,42,43 22333
DOI: 10.1021/acsami.9b04345 ACS Appl. Mater. Interfaces 2019, 11, 22332−22338
Research Article
ACS Applied Materials & Interfaces
composites; thus, the solid white-light-emitting phosphors (WCDs@PS) were facilely prepared by controlling the mass ratios of OCDs/BCDs and PS/WCDs. It is worth noting that the CIE coordinates of obtained WCDs@PS were (0.33, 0.30), which are very close to the values of natural white light (0.33, 0.33). The PL spectra of WCDs@PS are broad and span the most width of the visible spectrum, suggesting a promising applicability of WCDs@PS in WLEDs. To acquire further insight into the performance of WCDs@ PS, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM) analyses were carried out. As can be seen in Figures 2 and S3A, the resultant PS were orderly
In the present work, WCDs@PS in the solid state was realized by linking OCDs with BCDs by PS, which showed an intrinsic advantage to eliminate the quenching effects by isolation of emitters (Figure 1A). A colloid thermodynamic effect was
Figure 1. (A) Schematic illustration of the assembly of WCDs@PS nanocomposites. (B) Photographs of a series of CDs@PS composites with various mass ratio of OCDs and BCDs. (C) Emission colors of CDs@PS composites.
Figure 2. (A, B) Low- and high-resolution TEM images of pure PS, respectively. (C, D) Low- and high-resolution TEM images of CDs@ PS nanocomposites, respectively. (E) Photoluminescence (PL) spectra of BCDs@PS, OCDs@PS, and WCDs@PS nanocomposites under optimal excitation wavelength. (F) Photographs of BCDs (left), OCDs (middle), and WCDs@PS (right) nanocomposites, illuminated by a 365 nm UV lamp.
believed to be the driving force for coating the PS microspheres with other nanoparticles.39 The mass ratio of PS/WCDs and the particle size of PS are two important parameters to control the distance of multicolor CDs. Spherical PS with a uniform particle size (about 230 nm) and good dispersion property was prepared by a previously reported method.37 This particle size had been proven to be suitable for the assembly of nanoparticles. The effect of the PS/WCDs mass ratio on the fluorescent properties of WCDs@ PS composites was initially investigated. As shown in Figure S1, when the feed ratio increases from 1:1 to 2:1, the fluorescent intensities of peaks at 430 and 590 nm are decreased, as the excess PS produced a cloudy mixture and caused the fluorescence quenching of WCDs. However, insufficient PS (PS/WCDs = 1:2) leads to a low yield of WCDs@PS composites, although the fluorescent intensity still remained unchanged. Therefore, we set the feed ratio of PS/ WCDs to 1:1. The ratio of CDs to the complementary color has a significant effect on the PL emission wavelength of CDs@PS composites. Figure 1B,C shows the color changes obtained from CDs@PS composites with different OCDs/ BCDs ratios. When the molar ratios were adjusted from 2:1 to 1.2:1, the emitting colors were varied from pink to white when excited at 380 nm and blue color was observed by further decreasing the ratio to 0.8:1. The PL spectra of resultant CDs@PS composites are shown in Figure S2, and we obtained the corresponding CIE chromaticity coordinates from those emission curves. The emission color of six CDs@PS composites varied from orange to white and finally to blue, and the corresponding CIE coordinates also changed from (0.38, 0.36) to (0.33, 0.30) and finally to (0.25, 0.24), consistent with the color changes. Based on the above experiments, the mass ratios of 1.2:1 for OCDs/BCDs and 1:1 for PS/WCDs were optimal to obtain WCDs@PS
arranged and possessed almost the same diameter of approximately 230 nm. Moreover, the surfaces of PS nanospheres were smooth without foreign substance adhesion (Figure 2B). After being linked with OCDs and BCDs, a lot of small black dots can be seen on the surface, indicating that the multicolor CDs were successfully attached as shown in Figure 2D,E. The HRTEM images of WCDs@PS composites and CDs are provided in Figure S3B,C; the lattice fringes of CDs in composites are clearly visible, suggesting that the PS encapsulation did not affect the property of CDs. The PL experiments further confirmed the formation of composites. The two peaks located at 440 and 590 nm in the PL spectrum of WCDs@PS nanocomposites, respectively, belong to the emission peaks of BCDs and OCDs (Figure 2E). Photographs of BCDs, OCDs, and WCDs@PS show a bright fluorescence under UV irradiation (Figure 2F). The absolute PL quantum yield (QY) of WCDs@PS was calculated to be 25.0% when excited at 380 nm. The chemical compositions of WCDs@PS were examined by Fourier transform infrared (FT-IR) spectra and X-ray photoelectron spectroscopy (XPS). As shown in Figures S4 and S5, the XPS spectra revealed that the WCDs@PS consists of three elements, i.e., carbon, nitrogen, and oxygen.7,52 The C 1s spectrum can be fitted into four peaks for sp2 C(CC/C− C) at 284.8 eV, sp3 C(C−O/C−N) at 285.5 eV, −CO at 287.4 eV, and −COOH at 291.5 eV. For O 1s, the XPS spectra can be converted into C−O (533.2 eV) and CO (531.4 eV). The high-resolution N 1s spectrum contains two kinds of N species associated with amino N at 399.7 eV and pyridinic N at 22334
DOI: 10.1021/acsami.9b04345 ACS Appl. Mater. Interfaces 2019, 11, 22332−22338
Research Article
ACS Applied Materials & Interfaces 400.2 eV.40,41 In the FT-IR spectra of pure PS and WCDs@PS, the characteristic absorption bands of O−H (3207 cm−1), N− H (3416 and 3368 cm−1), unsaturated C−H (3066 cm−1), and CO (∼1700 cm−1) were observed (Figure S5).7 In the FTIR spectrum of WCDs@PS, the strong vibration peaks at 2400 and 1318 cm−1 can be ascribed to the CN and CO bonds, respectively, and the peaks at 1097 and 1029 cm−1 are produced from COC bond. Besides, the characteristic absorption band at 777 cm−1 was also observed, which can be ascribed to the N−H deformation vibration.41,52 These results are consistent with those obtained for pure CDs. Based on the above FT-IR and XPS analyses, it can be deduced that CDs and PS are successfully assembled together by electrostatic interaction instead of encapsulation, which also explain why the CDs can maintain their native properties and structures. The photostability was evaluated by radiating WCDs@PS nanocomposites with a 6 W UV lamp, and no photobleaching was observed for the WCDs@PS after continuous exposure for 12 h (Figure S6). The result demonstrated the excellent photostability of as-prepared WCDs@PS, which strongly support their application in WLEDs as the color-conversion layer. A room-temperature warm WLED was therefore fabricated by coating WCDs@PS on a 380 nm GaN chip, where the composite material acts as a white-light converter. The electroluminescence (EL) spectrum recorded for this device is broad and contains two emission peaks: one from the chip centered at 380 nm and the other from the WCDs@PS corresponding to the visible component in the range of 450− 700 nm (Figure 3B). Compared with the PL spectrum of
WLED featured a high CRI of 93.2, significantly higher than the typical CRI values (65−90) of previously reported CDsbased WLEDs (seen in Figure 3D and Table S1).9,13,17,19,45−50 The performance of our WLEDs device was also compared with that of the LEDs based on other fluorescent phosphors, and the details are listed in Table S2. It can be seen that the WCDs@PS-based WLEDs exhibited high quantum efficiency and good resistance to photobleaching and water. Considering the high CRI and low CCT values, our WCDs@PS phosphorbased WLEDs can be used as excellent optical media for highperformance solid-state lighting. The stability of a WLED device was another key issue to be considered for its practical application. Figure S8 shows the EL spectra recorded under 380 nm excitation. With the enhancement of the bias current from 20 to 80 mA in a step of 10 mA, only a marginal change in the emission spectrum was observed. The graph of the luminous efficiency (LE) versus operating time is shown in Figure S8B. It is obvious that the EL intensity remained above 65% of the initial value after continuous operation for 15 d, indicating that our WLEDs possess a good life span. The corresponding CCT, CRI, external quantum efficiency (EQE), and luminous efficiency (LE) were analyzed as a function of bias current, and the shift values of CCT, CRI, EQE, and LE were 439 K, 1.6, 0.11%, and 2.5 lm·W−1, respectively, as shown in Tables S3 and S4. Meanwhile, a shift in CIE coordinates from (0.3516, 0.3623) to (0.3508, 0.3661) was recorded, and it is worth noting that the CIE coordinates of (0.3508, 03661) are still in the warm white-light area (Table S3). The above data indicated that the resulting warm WLED device has high color chromatic stability against the increase of the bias current3,4 Therefore, we used our warm WLED device for practical illumination (Figure 3A), and the colorful candies exhibited their original colors under illumination. Compared with the apparent color under sunlight, there is nearly no difference, further demonstrating the high-quality lighting performance of our devices. It is highly desirable and challenging to be able to control the color in situ in a WLEDs device. An interesting phenomenon was found in our experiments that the solid WCDs@PS exhibited ratiometric fluorescence with temperature-tunable properties. Namely, as the temperature increased from 20 to 80 °C, the intensity of blue emission from BCDs located at 430 nm remained constant but the orange emission at 590 nm from OCDs was gradually reduced (Figure 4A,B). The two emission peaks were well separated with a wavelength difference of 160 nm, indicating an improved resolution for ratiometric detection and fluorescence imaging. The corresponding photographs of a series of WCDs@PS are shown in Figure S9. With the increase of the temperature, the system color changes from pink to yellow, then to purple, and eventually to blue under 365 nm UV illumination. The CIE coordinates also show the variation from (0.45, 0.32) to (0.36, 0.31) and finally to (0.29, 0.27), which are consistent with the fluorescent images. The PL emission spectra of BCDs@PS and OCDs@PS nanocomposites were also recorded within the same investigated temperature window as shown in Figures S11 and S12. The results indicated that the temperatureresponsive properties of WCDs@PS were generated from OCDs, which further confirm that PS as a good linker did not change the properties of CDs. The intensity ratio of the two emission wavelengths (I590/I430) was analyzed as a function of temperature in both forward and backward temperature modes, and the signal ratio displayed good linearity with
Figure 3. (A) Photographs of candies with different colors illuminated with a WCDs@PS-composite-based WLED. Inset: the photograph of colorful candies under natural light. (B) Emission spectrum of WLEDs based on the WCDs@PS composite phosphor. (C) CIE chromaticity coordinates. (D) Contrast of average CRI for recently reported CDs-based WLEDs.
WCDs@PS (Figure S7), the emission peaks shifted to the long-wavelength region because of self-absorption in the different output emission paths, which is similar to that in previous reports.44 The CIE color coordinates of this warm WLED with a correlated color temperature (CCT) of 4075 K were (0.35, 0.36) at 20 mA drive current, which was marked in the CIE 1931 color space (Figure 3C). In addition, the warm 22335
DOI: 10.1021/acsami.9b04345 ACS Appl. Mater. Interfaces 2019, 11, 22332−22338
Research Article
ACS Applied Materials & Interfaces
temperature controller as the lamp, and the colorful candies were chosen as testing subjects (seen in Figure 5). In these
Figure 4. Temperature dependence of the fluorescence intensity from CDs@PS nanocomposites in the solid state. (A) Fluorescent emission spectra recorded under an excitation of 380 nm with the increasing temperature from 20 to 80 °C in a step of 10 °C. (B) Fluorescent emission spectra (excitation 380 nm) for the decrease of temperature from 80 to 20 °C. (C) Fitted curve of the intensity ratio of 430−590 nm vs temperature. (D) Eight cycles of intensity variations measured at 20−80 °C.
Figure 5. (a1, b1, c1) Photographs of candies with different colors illuminated with a WCDs@PS-based WLED under different operating temperatures. (a2, b2, c2) Corresponding emission spectra of WLEDs. (a3, b3, c3) Corresponding CIE chromaticity coordinates.
experiments, our WCDs@PS-based WLEDs device exhibited high-quality lighting performance, which is quite suitable for illumination (Figure 3). When the temperature was increased to 40 °C, our WLEDs device emitted a bright-white light with CIE coordinates of (0.35, 0.32), which were still close to those of the ideal white light. As the temperature rose to 60 °C and 80 °C, the light-emission spectrum was changed from bright orange (∼600 nm) to blue (∼450 nm). The corresponding CIE coordinates were changed from (0.35, 0.29) to (0.32, 0.23), which have the same varied tendency with CIE coordinates of WCDs@PS nanocomposites. A possible explanation was that the color variance was attributed to the decreased PL intensity of ingredient OCDs at higher temperatures. Typical emission spectra recorded at different temperatures are shown in Figure S12; we can see that the intensity of the emission peak at about 590 nm is gradually reduced as the temperature is increased. To further prove this speculation, we fabricated an OCDs@PS-based LED device with a 380 nm GaN chip, and the EL spectra were recorded and are shown in Figure S15. When the temperature was increased from 20 to 80 °C, the fluorescence intensity of LEDs devices was gradually decreased, and the color of the emission light was changed from red to purple, due to the fluorescence quenching of OCDs. In addition, the results of CIE coordinates were consistent with the changes in the EL spectra. Based on these properties, our WCDs@PS-based WLEDs device as a temperature sensor will open up a new application field for LEDs.
temperature (Figure 4C). To ensure the reproducibility, the luminescence switching operations were repeated for eight consecutive cycles by multiple heating and cooling cycles between 20 and 80 °C, and the result indicated a good reversibility of the two-way switching processes by WCDs@PS (Figure 4D). The time-correlated single-photon counting (TCSPC) system was used to perform fluorescence lifetime experiments, and the fluorescence decay profiles were collected. As shown in Figure S13, the fluorescence lifetimes of BCDs@PS and OCDs@PS at room temperature are 3.8 and 4.9 ns and their quantum yields are calculated to be 10.7 and 15.2%, respectively. Nevertheless, changes in temperature do not affect the lifetimes of BCDs@PS and OCDs@PS. Unlike typical fluorescent materials, the temperature-sensitive characteristics do not follow the Boltzmann distribution. That is, as the temperature increases, the molecule collision frequency and the nonradiative transition rate increase, whereas the radiative transition rate remains constant, which, in turn, affects the lifetime and fluorescence intensity.23,51 The temperature may exert a great influence on the surface state emission rather than the intrinsic electronic transition mode of WCDs@PS. The chemical structures of a series of WCDs@PS were tested by FT-IR spectroscopy under different temperatures, and their positions of absorption peaks are not affected. However, the transmittances of two peaks at 3455 and 1709 cm−1, attributed to N−H and CO, respectively, are responsive to the temperature (Figure S14). We can preliminarily conclude that the reversible temperature-dependent fluorescent properties of WCDs@PS are controlled by surface functional groups.52 In view of the above-mentioned thermosensitive WCDs@PS phosphors, we investigated the application of WCDs@PSbased WLEDs device as a chameleon-like temperature sensor to realize special illumination. For instance, they may be used as work indicators in various electronic products or for detecting the temperature variation in industrial processes. To observe the visual effect, we combined our WLEDs with a
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CONCLUSIONS In summary, we reported the first demonstration of the solid white-light-emitting phosphors (WCDs@PS) by combining blue and orange emissive CDs (denoted BCDs and OCDs) assisted by polystyrene (PS) nanospheres through a selfassembly technique. When the molar ratio of OCDs and BCDs was 1.2:1, the obtained WCDs@PS composites emitted a highquality white light with CIE coordinates of (0.33, 0.30). This method not only overcame the disadvantages of solid matrixes but also successfully prevented the fluorescence resonance energy transfer between multicolor CDs. Based on the excellent stability and high-quality lighting performance of 22336
DOI: 10.1021/acsami.9b04345 ACS Appl. Mater. Interfaces 2019, 11, 22332−22338
Research Article
ACS Applied Materials & Interfaces
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WCDs@PS, a warm WLED was fabricated, and it exhibited CIE coordinates of (0.35, 0.36), a CCT of 4075 K, a CRI of 93.2, and a luminous efficiency of 14.8 lm·W−1, excited by 380 nm GaN chips. Significantly, the light-emitting signal of our WLEDs is reversibly responsive to the external environmental temperature with a good reproducibility, which can be used as a chameleon-like temperature-sensing device. This device opens up the possible application of carbon-based photonic devices in special illumination.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b04345. PL spectra and CIE chromaticity coordinates of CDs@ PS nanocomposites with various mass ratios of BCDs and OCDs; PL spectra of CDs@PS nanocomposites with different mass ratios of PS and CDs; SEM photographs of pure PS nanospheres; HRTEM images of WCDs@PS and WCDs; XPS spectra of WCDs@PS nanocomposites; FT-IR spectra of WCDs@PS nanocomposites and pure PS nanospheres; relative PL intensities of WCDs@PS composites; comparison of luminous properties of recently reported WLEDs based on CDs; PL spectra of WCDs@PS composites; EL spectra of WCDs@PS nanocomposites; CIE coordinates, CCT, CRI, EQE, and luminous efficiencies of the WCDs@PS-based WLEDs at different working currents; collected data of current, voltage, luminous flux, and LE; color coordinates and corresponding photograph of CDs@PS composites with the increasing temperature; PL spectra of WCDs@PS composites; PL spectra of BCDs@PS composites; PL spectra of OCDs@PS composites; fluorescence decay profiles of OCDs@PS and BCDs@PS; FT-IR spectra of WCDs@PS composites with the increasing temperature; and EL spectra and CIE chromaticity coordinates of OCDs@PS-based LEDs at different temperatures from 20 to 80 °C (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions †
C.W. and T.H. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the Postdoctoral Research Foundation of Jiangsu Province, China (No. 2018K022B), the China Postdoctoral Science Foundation (No. 2019M651688), the National First-Class Discipline Program of Food Science and Technology (JUFSTR20180301), and the 111 Project (B13025).
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