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Highly luminescent carbon nanoparticles as yellow emission conversion phosphors
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Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Highly luminescent carbon nanoparticles as yellow emission conversion phosphors Xuegang Zheng a,b,c, Hailong Wang a,n, Qian Gong b, Lichun Zhang d, Guangliang Cui c, Qingshan Li d, Li Chen c, Fuquan Wu a, Shumin Wang b a
Provincial Key Laboratory of Laser Polarization and Information Technology, Department of Physics, Qufu Normal University, Qufu 273165, China State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Shanghai 200050, China c Institute of Condensed Matter Physics, Linyi University, Linyi 276005, China d School of Physics and Optoelectronic Engineering, Ludong University, Yantai 264025, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 20 September 2014 Accepted 29 December 2014
We report the novel synthesis of luminescent carbon nanoparticles (CNPs) and their applications as yellow emission conversion phosphors. CNPs were obtained in a hydrothermal process using hexamethylenetetramine (HMT)/glucose mixed solution as precursor. The mixed glucose is found to facilitate the decomposition of HMT, which leads to rapid and low-temperature growth of CNPs. CNPs (quantum yield exceeding 58%) solutions exhibit strong blue-green emission with ultraviolet light illumination and possess typical excitation-dependent photoluminescence (PL) behavior. CNPs were also coated onto ultraviolet and blue LEDs. Broad yellow emission was both achieved with ultraviolet or blue light excitation, indicating such CNPs phosphors can be used in fluorescent lamps or white LEDs. The nontoxic nature and broad yellow emission indicates potential applications of CNPs for phosphor-based white light-emitting devices. & 2015 Published by Elsevier B.V.
Keywords: Carbon materials Nanoparticles Yellow emission Phosphors White light emitting devices
1. Introduction The development of white light emitting devices is very important in liquid-crystal displays, full-color displays, and normal lighting sources in our life [1]. Generally, three kinds of white light emitting devices have been proposed including incandescent light bulbs, fluorescent lamps and white LEDs, among which fluorescent lamps are widely used as normal lighting source due to high efficiency and low cost, but waste fluorescent lamps often give rise to serious environmental problems. Phosphors containing rareearth metals (such as YAG:Ce or Sr3SiO5 phosphors) are toxic. It can be anticipated that if a new nontoxic alternative can completely or partially replace conventional rare-earth phosphors, environmental issues aroused by rare-earth metal are expected to be greatly minimized. In recent years, people paid much attention on carbon nanomaterials such as carbon nanotubes, graphene and CNPs. Carbon materials is environmentally and biologically compatible, low-cost and chemically stable [2]. There have been some successful demonstrations on CNPs as color-converted phosphors [3–5]. Once CNPs are successively used as the alternates to the
traditional heavy-metal-based quantum dots in white-light conversion phosphors, a revolution in lighting will be coming. To be power-efficient phosphors, CNPs with three key features must be apparent, involving: (1) easy handling, rapid and lowtemperature growth; (2) high quantum yield; (3) broad yellow emission spectra. To this date, CNPs have been prepared by various methods, including laser ablation [6,7], electrochemical exfoliation [8], combustion thermal oxidation [9], supported synthetic [10], microwave assisted hydrothermal dehydration of carbohydrates [11,12]. In contrast, decomposition of organic compounds using hydrothermal method is likely to be extended significantly in the mass production of CNPs. Various carbohydrates are used for CNPs production. Tang et al. [5] reported glucose-derived graphene quantum dots, but the quantum yields were as low as to be 7–11%. Earlier times, we used hexamethylenetetramine (HMT) as single precursor to grow fluorescent carbon nanospheres with quantum yield of 35% [3]. In this article, we investigated the growth of fluorescent CNPs using HMT/glucose mixed solution as precursor for the first time. Rapid and lowtemperature fabrication of CNPs was demonstrated with the help of adding glucose. The quantum yield is as high as 58.3%. We also
n
Corresponding author. Tel./fax: þ 86 5375051321. E-mail address: [email protected] (H. Wang).
obtained broad yellow emission using CNPs as yellow conversion phosphors excited by ultraviolet or blue light.
http://dx.doi.org/10.1016/j.matlet.2014.12.138 0167-577X/& 2015 Published by Elsevier B.V.
Please cite this article as: Zheng X, et al. Highly luminescent carbon nanoparticles as yellow emission conversion phosphors. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2014.12.138i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96
X. Zheng et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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2. Experimental
3. Results and discussion
2.1. Sample preparation
Fig. 1a shows the obtained CNPs solutions from HMT or HMT/ glucose mixed solution. Judged by the changes of solutions color, the mixed glucose was found to facilitate the decomposition of HMT and CNPs growth. The formation of CNPs is schematically shown in Fig. 1b, involving rings opening, carbonization, nucleation, growth of carbon dots and aggregation. HMT is rather stable with an admantane-like structure [14], and Glucose in water exhibits closed (ring) forms and is also stable. While HMT and glucose were mixed and heated, the amine derived from HMT will react with the aldehyde group of glucose and forms azomethine [15]. With chain-like structure, azomethines are not stable during heating, which leads to the rapid and low-temperature synthesis of CNPs. The quantum yield of 58.3% was obtained for those CNPs synthesized at 160 1C, which is higher than those synthesized at 99 1C or produced by other precursors [4,5,16]. We also performed TEM measurements with or without glucose as precursor. Fig. 1c shows TEM image of CNPs synthesized from HMT at 160 1C for 3 h. The spherical CNPs have a size distribution of 260–300 nm in diameter. We have explored other different precursors to grow CNPs by the same hydrothermal method, but such large submicron-scale carbon spheres have not been obtained. We do not fully understand the detailed formation mechanism, but we think it is related with the unique characteristics of HMT [3]. Fig. 1d shows TEM images of CNPs synthesized from HMT/glucose at 160 1C, revealing overlapping spherical shapes with a size of 30 nm in diameter. The highresolution TEM image (Fig. 1d, inset) shows the carbon spheres composed of 2nm carbon dots. The rapid decomposition of HMT
All chemicals were purchased from Alfa Aesar. HMT and HMT/ glucose mixed solution (100:1molar ratio of HMT: glucose) were respectively prepared and transferred into a 50 mL hydrothermal reactor. Distilled water was used as solvent. The reactors were heated in a thermostatic oven and kept at 160 1C or 99 1C for 3 h. Subsequently, the reactor was cooled to room temperature and the solution was removed out from the vessel for characterizations. To demonstrate the yellow light conversion, a few drops of concentrated CNPs solution were coated onto commercially LEDs. The applied voltage for the LEDs was 3.2 V.
2.2. Characterizations We performed Transmission electron microscopy (TEM) measurements on JEOL, JEM-2100 F with an accelerating voltage of 200 kV. Fluorescence emission and excitation spectra of CNPs aqueous solution were recorded using a Cary eclipse fluorescence spectrophotometer with Xe lamp as an excitation source. The UVvis spectra were obtained on a Shimadzu UV-3600 UV-vis spectrophotometer. The quantum yield was measured according to the established procedure [13]. Absolute values were calculated using the standard reference sample of Rhodamine B solution known as 0.9 while diluted with distilled water. Fourier transform infrared (FTIR) spectra were obtained by Nicolet 560 spectrometer.
Fig. 1. (a) Photographs of CNPs solutions with white light illumination. A and B correspond to HMT solution and HMT/glucose mixed solution heated at 160 1C or 99 1C for three hours. (b) Scheme for the formation of CNPs by hydrothermal method. (c) TEM image of CNPs synthesized from HMT. (d) TEM images of CNPs synthesized from HMT/ glucose. The inset: high-resolution TEM image. (e) FTIR spectra of different samples, including HMT, glucose and CNPs synthesized from HMT or HMT/glucose.
Please cite this article as: Zheng X, et al. Highly luminescent carbon nanoparticles as yellow emission conversion phosphors. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2014.12.138i
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X. Zheng et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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by adding glucose as well as simultaneous, homogeneous heating provide uniform nucleation and growth of carbon dots, which contributes to the formation of monodispersed carbon dots [5]. FTIR spectroscopy was used to investigate the bonding composition of CNPs and its functional groups, as shown in Fig. 1e. Whether with glucose as precursor or not, an obvious C¼C stretching at 1637 cm-1
3
is observed, which is a primary band due to sp2 carbon in CNPs [5], and it is absent from the source. The broad peak centered at 3407 cm-1 reveals the presence of O-H group [7]. The absorptions at 1389 and 2929 cm-1 also reveal the existence of C-H [5,7]. Fig. 2a depicts the UV-Vis, photoluminescence (PL) excitation and PL spectrum of the dilute CNPs solution. CNPs solution exhibits
Fig. 2. (a) UV-Vis absorption, PL excitation and PL spectra of CNPs solution. (b) PL spectra of CNPs solution excited by various wavelengths. (c) Emission spectra of ultraviolet LEDs without (black line) and with (red line) CNPs coating.
Fig. 3. (a) Emission spectra of blue LED before and after CNPs coating. Insets: photographs of the electrified blue LED with thin (left) or thick (right) CNPs coating. (b) The CIE chromaticity coordinates for the illuminating LEDs of (a).
Please cite this article as: Zheng X, et al. Highly luminescent carbon nanoparticles as yellow emission conversion phosphors. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2014.12.138i
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X. Zheng et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
strong blue-green emission under ultraviolet light with naked eyes (the inset of Fig. 1a). Two absorption bands at 220 and 270nm are attributable to π-πn transition of C═C and n-πn transition of C═O, respectively [5]. A bright blue-green PL emission with a peak at around 470 nm was observed upon excitation of CNPs aqueous solution at 400 nm. Fig. 2b shows the PL spectra of CNPs solution excited by various wavelengths. CNPs solution exhibits typical excitation wavelength-dependent properties as previous reported [11,16]. To demonstrate the yellow light conversion, a few drops of concentrated CNPs solution were coated onto a commercial ultraviolet LED, and the emission spectra were examined as show in Fig. 2c. The uncoated LED emission shows a strong peak centered at 395nm. After coating, the intensity of ultraviolet light weakens, accompanied with the presence of a broad yellow band centered at 550nm. The full width at half maximum (FWHM) is as broad as 100nm, indicating that the obtained CNPs can be used as yellow emission conversion phosphors. The coating absorbs a portion of the ultraviolet radiation and converts it to broad yellow emission. Usually, commercially white LEDs originate from blue LEDs and yellow light-emitting phosphors [8]. White emission is the combination of blue and phosphor-converted yellow light. We coated a blue LED with the obtained CNPs and the emission spectra are shown in Fig. 3a. The coated blue LED emitted warm white light. With CNPs layer getting thick, the color output shifts red (inset of Fig. 3b). As shown in Fig. 3b, CIE chromaticity coordinates shift from A(0.176, 0.034) to B(0.331, 0.263) and C(0.452,0.456), demonstrating that CNPs phosphors are capable of converting blue light into white light. The output color can be tuned by the thickness of CNPs coating. Normal sunlight color temperature shifts from 5200 to 5500 K. Here the color temperature of A, B and C corresponds to 1855 K, 5469 K and 3087 K, respectively. Among them, the emission of B is closer to the sunlight. CNPs exhibit excellent yellowconverted properties as conventional phosphors. The nontoxic nature and yellow emission from CNPs will make this material promising for fluorescent lamps and white LEDs. In conclusion, highly fluorescent CNPs were synthesized and used as yellow emission phosphors: (1) we first explored rapid
and low-temperature CNPs fabrication using HMT/glucose mixing solution as precursor; (2) the quantum yield of the obtained CNPs is as high as 58.3%; (3) more importantly, broad yellow emission was both achieved with ultraviolet light or blue light illumination, indicating such CNPs phosphors can be used in fluorescent lamps or white LEDs. This study provides a new potentiality and insight into environmentally friendly white light-emitting devices.
Acknowledgments We thank financial support from the National Natural Science Foundation of China (Grant Nos. 61321492,11274151 and 51431004), the Key Research Program of the CAS (Grant no. KGZD-EW-804), Open Research Fund Program of the State Key Laboratory of LowDimensional Quantum Physics (Grant No. 20120924) and Key Disciplines of Condensed Matter Physics of Linyi University References [1] Yin SN, Wang CF, Yu ZY, Wang J, Liu SS, Chen S. Adv Mater 2011;23:2915–9. [2] Baker SN, Baker GA. Angew Chem Int Ed 2010;49:6726–44. [3] Zheng XG, Wang HL, Gong Q, Chen L, Wang K, Wang SM. Mater Lett 2014;126:71–4. [4] Guo X, Wang C, Yu ZY, Chen L, Chen S. Chem Commun 2012;48:2692–4. [5] Tang L, Ji R, Cao X, Lin J, Jiang H, Li X, et al. ACS Nano 2012;6:5102–10. [6] Sun YP, Zhou B, Lin Y, Wang W, Fernando KAS, Pathak P, et al. J Am Chem Soc 2006;128:7756–7. [7] Hu SL, Niu KY, Sun J, Yang J, Zhao NQ, Du XW. J Mater Chem 2009;19:484–8. [8] Zheng LY, Chi YW, Dong YQ, Lin JP, Wang BB. J Am Chem Soc 2009;131:4564–5. [9] Bourlinos AB, Stassinopoulos A, Anglos D, Zboril R, Karakassides M, Giannelis EP. Small 2008;4:455–8. [10] Liu RL, Wu DQ, Liu SH, Koynov K, Knoll W, Li Q. Angew Chem Int Ed 2009;48:4598–601. [11] Zhu H, Wang X, Li Y, Wang Z, Yang F, Yang X. Chem Commun 2009;34:5118–20. [12] Wang X, Qu K, Xu B, Ren J, Qu X. J Mater Chem 2011;21:2445–50. [13] Jaiswal A, Ghosh SS, Chattopadhyay A. Chem Commun 2012;48:407–9. [14] Blazevic N, Kolbah D. Synthesis 1979;3:161–76. [15] Adabiardakani A, Hakimi M, Kargar H. World Appl Program 2012;2:472–6. [16] Zhai X, Zhang P, Liu C, Bai T, Li W, Dai L, et al. Chem Commun 2012;48:7955–7.
Please cite this article as: Zheng X, et al. Highly luminescent carbon nanoparticles as yellow emission conversion phosphors. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2014.12.138i
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Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Highly luminescent carbon nanoparticles as yellow emission conversion phosphors Xuegang Zheng a,b,c, Hailong Wang a,n, Qian Gong b, Lichun Zhang d, Guangliang Cui c, Qingshan Li d, Li Chen c, Fuquan Wu a, Shumin Wang b a
Provincial Key Laboratory of Laser Polarization and Information Technology, Department of Physics, Qufu Normal University, Qufu 273165, China State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Shanghai 200050, China c Institute of Condensed Matter Physics, Linyi University, Linyi 276005, China d School of Physics and Optoelectronic Engineering, Ludong University, Yantai 264025, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 20 September 2014 Accepted 29 December 2014
We report the novel synthesis of luminescent carbon nanoparticles (CNPs) and their applications as yellow emission conversion phosphors. CNPs were obtained in a hydrothermal process using hexamethylenetetramine (HMT)/glucose mixed solution as precursor. The mixed glucose is found to facilitate the decomposition of HMT, which leads to rapid and low-temperature growth of CNPs. CNPs (quantum yield exceeding 58%) solutions exhibit strong blue-green emission with ultraviolet light illumination and possess typical excitation-dependent photoluminescence (PL) behavior. CNPs were also coated onto ultraviolet and blue LEDs. Broad yellow emission was both achieved with ultraviolet or blue light excitation, indicating such CNPs phosphors can be used in fluorescent lamps or white LEDs. The nontoxic nature and broad yellow emission indicates potential applications of CNPs for phosphor-based white light-emitting devices. & 2015 Published by Elsevier B.V.
Keywords: Carbon materials Nanoparticles Yellow emission Phosphors White light emitting devices
1. Introduction The development of white light emitting devices is very important in liquid-crystal displays, full-color displays, and normal lighting sources in our life [1]. Generally, three kinds of white light emitting devices have been proposed including incandescent light bulbs, fluorescent lamps and white LEDs, among which fluorescent lamps are widely used as normal lighting source due to high efficiency and low cost, but waste fluorescent lamps often give rise to serious environmental problems. Phosphors containing rareearth metals (such as YAG:Ce or Sr3SiO5 phosphors) are toxic. It can be anticipated that if a new nontoxic alternative can completely or partially replace conventional rare-earth phosphors, environmental issues aroused by rare-earth metal are expected to be greatly minimized. In recent years, people paid much attention on carbon nanomaterials such as carbon nanotubes, graphene and CNPs. Carbon materials is environmentally and biologically compatible, low-cost and chemically stable [2]. There have been some successful demonstrations on CNPs as color-converted phosphors [3–5]. Once CNPs are successively used as the alternates to the
traditional heavy-metal-based quantum dots in white-light conversion phosphors, a revolution in lighting will be coming. To be power-efficient phosphors, CNPs with three key features must be apparent, involving: (1) easy handling, rapid and lowtemperature growth; (2) high quantum yield; (3) broad yellow emission spectra. To this date, CNPs have been prepared by various methods, including laser ablation [6,7], electrochemical exfoliation [8], combustion thermal oxidation [9], supported synthetic [10], microwave assisted hydrothermal dehydration of carbohydrates [11,12]. In contrast, decomposition of organic compounds using hydrothermal method is likely to be extended significantly in the mass production of CNPs. Various carbohydrates are used for CNPs production. Tang et al. [5] reported glucose-derived graphene quantum dots, but the quantum yields were as low as to be 7–11%. Earlier times, we used hexamethylenetetramine (HMT) as single precursor to grow fluorescent carbon nanospheres with quantum yield of 35% [3]. In this article, we investigated the growth of fluorescent CNPs using HMT/glucose mixed solution as precursor for the first time. Rapid and lowtemperature fabrication of CNPs was demonstrated with the help of adding glucose. The quantum yield is as high as 58.3%. We also
n
Corresponding author. Tel./fax: þ 86 5375051321. E-mail address: [email protected] (H. Wang).
obtained broad yellow emission using CNPs as yellow conversion phosphors excited by ultraviolet or blue light.
http://dx.doi.org/10.1016/j.matlet.2014.12.138 0167-577X/& 2015 Published by Elsevier B.V.
Please cite this article as: Zheng X, et al. Highly luminescent carbon nanoparticles as yellow emission conversion phosphors. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2014.12.138i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96
X. Zheng et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
2. Experimental
3. Results and discussion
2.1. Sample preparation
Fig. 1a shows the obtained CNPs solutions from HMT or HMT/ glucose mixed solution. Judged by the changes of solutions color, the mixed glucose was found to facilitate the decomposition of HMT and CNPs growth. The formation of CNPs is schematically shown in Fig. 1b, involving rings opening, carbonization, nucleation, growth of carbon dots and aggregation. HMT is rather stable with an admantane-like structure [14], and Glucose in water exhibits closed (ring) forms and is also stable. While HMT and glucose were mixed and heated, the amine derived from HMT will react with the aldehyde group of glucose and forms azomethine [15]. With chain-like structure, azomethines are not stable during heating, which leads to the rapid and low-temperature synthesis of CNPs. The quantum yield of 58.3% was obtained for those CNPs synthesized at 160 1C, which is higher than those synthesized at 99 1C or produced by other precursors [4,5,16]. We also performed TEM measurements with or without glucose as precursor. Fig. 1c shows TEM image of CNPs synthesized from HMT at 160 1C for 3 h. The spherical CNPs have a size distribution of 260–300 nm in diameter. We have explored other different precursors to grow CNPs by the same hydrothermal method, but such large submicron-scale carbon spheres have not been obtained. We do not fully understand the detailed formation mechanism, but we think it is related with the unique characteristics of HMT [3]. Fig. 1d shows TEM images of CNPs synthesized from HMT/glucose at 160 1C, revealing overlapping spherical shapes with a size of 30 nm in diameter. The highresolution TEM image (Fig. 1d, inset) shows the carbon spheres composed of 2nm carbon dots. The rapid decomposition of HMT
All chemicals were purchased from Alfa Aesar. HMT and HMT/ glucose mixed solution (100:1molar ratio of HMT: glucose) were respectively prepared and transferred into a 50 mL hydrothermal reactor. Distilled water was used as solvent. The reactors were heated in a thermostatic oven and kept at 160 1C or 99 1C for 3 h. Subsequently, the reactor was cooled to room temperature and the solution was removed out from the vessel for characterizations. To demonstrate the yellow light conversion, a few drops of concentrated CNPs solution were coated onto commercially LEDs. The applied voltage for the LEDs was 3.2 V.
2.2. Characterizations We performed Transmission electron microscopy (TEM) measurements on JEOL, JEM-2100 F with an accelerating voltage of 200 kV. Fluorescence emission and excitation spectra of CNPs aqueous solution were recorded using a Cary eclipse fluorescence spectrophotometer with Xe lamp as an excitation source. The UVvis spectra were obtained on a Shimadzu UV-3600 UV-vis spectrophotometer. The quantum yield was measured according to the established procedure [13]. Absolute values were calculated using the standard reference sample of Rhodamine B solution known as 0.9 while diluted with distilled water. Fourier transform infrared (FTIR) spectra were obtained by Nicolet 560 spectrometer.
Fig. 1. (a) Photographs of CNPs solutions with white light illumination. A and B correspond to HMT solution and HMT/glucose mixed solution heated at 160 1C or 99 1C for three hours. (b) Scheme for the formation of CNPs by hydrothermal method. (c) TEM image of CNPs synthesized from HMT. (d) TEM images of CNPs synthesized from HMT/ glucose. The inset: high-resolution TEM image. (e) FTIR spectra of different samples, including HMT, glucose and CNPs synthesized from HMT or HMT/glucose.
Please cite this article as: Zheng X, et al. Highly luminescent carbon nanoparticles as yellow emission conversion phosphors. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2014.12.138i
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by adding glucose as well as simultaneous, homogeneous heating provide uniform nucleation and growth of carbon dots, which contributes to the formation of monodispersed carbon dots [5]. FTIR spectroscopy was used to investigate the bonding composition of CNPs and its functional groups, as shown in Fig. 1e. Whether with glucose as precursor or not, an obvious C¼C stretching at 1637 cm-1
3
is observed, which is a primary band due to sp2 carbon in CNPs [5], and it is absent from the source. The broad peak centered at 3407 cm-1 reveals the presence of O-H group [7]. The absorptions at 1389 and 2929 cm-1 also reveal the existence of C-H [5,7]. Fig. 2a depicts the UV-Vis, photoluminescence (PL) excitation and PL spectrum of the dilute CNPs solution. CNPs solution exhibits
Fig. 2. (a) UV-Vis absorption, PL excitation and PL spectra of CNPs solution. (b) PL spectra of CNPs solution excited by various wavelengths. (c) Emission spectra of ultraviolet LEDs without (black line) and with (red line) CNPs coating.
Fig. 3. (a) Emission spectra of blue LED before and after CNPs coating. Insets: photographs of the electrified blue LED with thin (left) or thick (right) CNPs coating. (b) The CIE chromaticity coordinates for the illuminating LEDs of (a).
Please cite this article as: Zheng X, et al. Highly luminescent carbon nanoparticles as yellow emission conversion phosphors. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2014.12.138i
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X. Zheng et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
strong blue-green emission under ultraviolet light with naked eyes (the inset of Fig. 1a). Two absorption bands at 220 and 270nm are attributable to π-πn transition of C═C and n-πn transition of C═O, respectively [5]. A bright blue-green PL emission with a peak at around 470 nm was observed upon excitation of CNPs aqueous solution at 400 nm. Fig. 2b shows the PL spectra of CNPs solution excited by various wavelengths. CNPs solution exhibits typical excitation wavelength-dependent properties as previous reported [11,16]. To demonstrate the yellow light conversion, a few drops of concentrated CNPs solution were coated onto a commercial ultraviolet LED, and the emission spectra were examined as show in Fig. 2c. The uncoated LED emission shows a strong peak centered at 395nm. After coating, the intensity of ultraviolet light weakens, accompanied with the presence of a broad yellow band centered at 550nm. The full width at half maximum (FWHM) is as broad as 100nm, indicating that the obtained CNPs can be used as yellow emission conversion phosphors. The coating absorbs a portion of the ultraviolet radiation and converts it to broad yellow emission. Usually, commercially white LEDs originate from blue LEDs and yellow light-emitting phosphors [8]. White emission is the combination of blue and phosphor-converted yellow light. We coated a blue LED with the obtained CNPs and the emission spectra are shown in Fig. 3a. The coated blue LED emitted warm white light. With CNPs layer getting thick, the color output shifts red (inset of Fig. 3b). As shown in Fig. 3b, CIE chromaticity coordinates shift from A(0.176, 0.034) to B(0.331, 0.263) and C(0.452,0.456), demonstrating that CNPs phosphors are capable of converting blue light into white light. The output color can be tuned by the thickness of CNPs coating. Normal sunlight color temperature shifts from 5200 to 5500 K. Here the color temperature of A, B and C corresponds to 1855 K, 5469 K and 3087 K, respectively. Among them, the emission of B is closer to the sunlight. CNPs exhibit excellent yellowconverted properties as conventional phosphors. The nontoxic nature and yellow emission from CNPs will make this material promising for fluorescent lamps and white LEDs. In conclusion, highly fluorescent CNPs were synthesized and used as yellow emission phosphors: (1) we first explored rapid
and low-temperature CNPs fabrication using HMT/glucose mixing solution as precursor; (2) the quantum yield of the obtained CNPs is as high as 58.3%; (3) more importantly, broad yellow emission was both achieved with ultraviolet light or blue light illumination, indicating such CNPs phosphors can be used in fluorescent lamps or white LEDs. This study provides a new potentiality and insight into environmentally friendly white light-emitting devices.
Acknowledgments We thank financial support from the National Natural Science Foundation of China (Grant Nos. 61321492,11274151 and 51431004), the Key Research Program of the CAS (Grant no. KGZD-EW-804), Open Research Fund Program of the State Key Laboratory of LowDimensional Quantum Physics (Grant No. 20120924) and Key Disciplines of Condensed Matter Physics of Linyi University References [1] Yin SN, Wang CF, Yu ZY, Wang J, Liu SS, Chen S. Adv Mater 2011;23:2915–9. [2] Baker SN, Baker GA. Angew Chem Int Ed 2010;49:6726–44. [3] Zheng XG, Wang HL, Gong Q, Chen L, Wang K, Wang SM. Mater Lett 2014;126:71–4. [4] Guo X, Wang C, Yu ZY, Chen L, Chen S. Chem Commun 2012;48:2692–4. [5] Tang L, Ji R, Cao X, Lin J, Jiang H, Li X, et al. ACS Nano 2012;6:5102–10. [6] Sun YP, Zhou B, Lin Y, Wang W, Fernando KAS, Pathak P, et al. J Am Chem Soc 2006;128:7756–7. [7] Hu SL, Niu KY, Sun J, Yang J, Zhao NQ, Du XW. J Mater Chem 2009;19:484–8. [8] Zheng LY, Chi YW, Dong YQ, Lin JP, Wang BB. J Am Chem Soc 2009;131:4564–5. [9] Bourlinos AB, Stassinopoulos A, Anglos D, Zboril R, Karakassides M, Giannelis EP. Small 2008;4:455–8. [10] Liu RL, Wu DQ, Liu SH, Koynov K, Knoll W, Li Q. Angew Chem Int Ed 2009;48:4598–601. [11] Zhu H, Wang X, Li Y, Wang Z, Yang F, Yang X. Chem Commun 2009;34:5118–20. [12] Wang X, Qu K, Xu B, Ren J, Qu X. J Mater Chem 2011;21:2445–50. [13] Jaiswal A, Ghosh SS, Chattopadhyay A. Chem Commun 2012;48:407–9. [14] Blazevic N, Kolbah D. Synthesis 1979;3:161–76. [15] Adabiardakani A, Hakimi M, Kargar H. World Appl Program 2012;2:472–6. [16] Zhai X, Zhang P, Liu C, Bai T, Li W, Dai L, et al. Chem Commun 2012;48:7955–7.
Please cite this article as: Zheng X, et al. Highly luminescent carbon nanoparticles as yellow emission conversion phosphors. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2014.12.138i
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