Whispering Gallery Mode Laser from Carbon DotâNaCl Hybrid

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Whispering gallery mode laser from carbon dots-NaCl hybrid crystals Hongzhen Liu, Fei Wang, Yunpeng Wang, Jingjing Mei, and Dongxu Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 15 May 2017 Downloaded from http://pubs.acs.org on May 16, 2017

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ACS Applied Materials & Interfaces

Whispering gallery mode laser from carbon dotsNaCl hybrid crystals Hongzhen Liu,1,2 Fei Wang, *,1 Yunpeng Wang,1 Jingjing Mei,1,2 Dongxu Zhao*,1 1

State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine

Mechanics and Physics, Chinese Academy of Sciences, No.3888 Dongnanhu Road, Changchun, 130033, People’s Republic of China 2

University of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China.

KEYWORDS: carbon dot, hybrid crystal, fluorescence, phosphorescence, laser

ABSTRACT: Carbon dots (CDs)-NaCl hybrid crystals are obtained by incorporating the CDs into NaCl matrix through a simple process. The embedded CDs have added the luminescence centers into NaCl, as the result, the hybrid crystals present the fluorescence centered at 510 nm under the illumination of 365 nm light. Meanwhile, the phosphorescence with an average lifetime of 314 milliseconds (ms) is achieved after the 365nm light was turned off. Furthermore, optical gain and lasing phenomenon has been observed from hybrid crystals. When the pump power is low, a weak spontaneous emission can be observed from the hybrid crystal, while the lasing action was observed under high pump power. The lasing threshold is found to be 0.08 mW and corresponding Q factor is calculated to be 447. The tiny cubic crystal in hybrid crystals

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offers the whispering gallery mode (WGM) resonant cavity for lasing emission. That has provided a new approach for realizing lasing materials.

INTRODUCTION Carbon dots (CDs), as a rising star among nano-materials, have attracted much attention of researchers due to their properties of water solubility, good stability, low toxicity, environmental friendliness and simple synthesis routes.1-5 Extensive applications such as patterning, sensor, bioimaging,1 florescence ink

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and coding8 have been achieved on CDs, which are benefited from

its excellent fluorescence properties. However, most CDs were used in aqueous solution, which has seriously limited their further applications in solid states. Though great interests have been focused on using CDs as emitting materials in lighting and display applications, 9-14 because the aggregation-induced quenching effects usually occur in CDs when drying them into films or powders,1,15-16 the performances of CDs based solid state luminescent devices are still unsatisfactory. The absence of luminescence in solid state has greatly limited the development of CDs materials. Therefore, great efforts of researchers were made to achieve the luminescence of CDs in solid state.8,14-18 Our group have previously reported solid state fluorescent CDs by dispersing CDs into polyvinyl alcohol (PVA) matrix.8 Qu et al. integrated the CDs with starch particles, and CD-based phosphors with QY of 50% were obtained.16 The N-doped CDs dispersed in polyurethane matrixes emit both fluorescence and phosphorescence.18 To prevent quenching of luminescence in solid state, the CDs were generally dispersed in polymer matrixes to achieve luminescent materials. In most cases, the reported CDs based solid state luminescent materials were hydro-gel films or powders, which are in amorphous states, and therefore less thermal and long-term stable for more specialized or wider range of possible applications under various conditions including high temperatures as well as high intensity illumination. Hence, to


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extend its application range, exploring new types of CDs based luminescent materials in crystalline phase is necessary. Eychmüller et al. have previously reported a series of solid state luminescent crystals by embedding fluorescent quantum dots (QDs) into a protective NaCl matrix.19-22 The incorporation of QDs into NaCl crystals have provided the advantages of protecting the QDs from the environment, forming a robust and photostable matrix and meanwhile retaining the favorable optical properties of the solution of QDs.19-21, 23 Inspired by the works, combining the CDs and NaCl together to form hybrid crystals would presents the properties of CDs and NaCl at the same time. The embedded CDs could add luminescent centers into NaCl crystals, meanwhile, the NaCl will offer a number of useful properties including isolating and protecting the CDs from aggregation-induced self-quenching, easily crystallizing to form a solid state luminescent material in crystalline phase, and good thermal and long-term stabilities. Then, the CDs based crystals which luminesce in solid state will be realized. In this work, microwave synthesized CDs were embedded into the protective matrix of NaCl crystals through a modified liquid-liquid diffusion-assisted crystallization (LLDC) process.19 The prepared CDs-NaCl hybrid crystals present fluorescence of CDs. Meanwhile, phosphorescence which centered at 519 nm with a long lifetime of 314 ms could also be observed from the crystals at room temperature. Furthermore, the NaCl crystals have provided the whispering gallery mode (WGM) resonant cavity to CDs, and lasing phenomenon with the threshold of 0.08 mW has been observed from the hybrid crystals under the pump source of a Ti: sapphire femtosecond laser, the corresponding Q factor is calculated to be 447. This work not only provides a feasible method to obtain high quality fluorescent and phosphorescent crystals of CDs, but also shows a new approach for realizing lasing materials.


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RESULTS AND DISCUSSION The CDs were prepared through a simple one-step microwave synthesis route. 4 As shown in the transmission electronic microscopy (TEM) image in Figure S1, the diameter of CDs is approximately 5 nm. The high resolution TEM (HRTEM) characterization reveals that their lattice spacing is 0.318 nm, which is in accordance with the previously reported values of CDs.4,24 Surface functional groups of the CDs were detected by Fourier transform infrared spectroscopy (FTIR) and shown in Figure S2. The broad absorption bands at 3100-3500 cm-1 originate from the functional groups of ν(O-H) and ν(N-H). These functional groups have improved the hydrophilicity and stability of the CDs in aqueous system. The absorption bands at 1600-1770 cm-1 can be assigned to ν(C=O) of carbonyl groups.4,17,24 The δ(CH2) groups are responsible for the absorption bands from 1350 to 1460 cm-1.4,24 The CDs dispersed in water emit green light under the excitation of 365 nm UV light (inset of Figure S3). However, when the CDs are deposited on a glass substrate and dried, the green emission is quenched (Figure S4). Like other CDs reported in previous works,1,4,15,16 the CDs present the excitation-dependent behavior (Figure S5). UV-vis absorption spectrum of CDs aqueous solution (Figure S3, red line) shows two peaks centered at 337 nm and 408 nm, which can be attributed to the n-π* transition of C=O bond.17 Also, this transition of C=O bond has leaded to the peak located at 400 nm in the photoluminescence excitation (PLE) spectrum (Figure S3, black line) of the CDs solution. After the LLDC process of embedding the CDs into NaCl matrix, the CDs-NaCl hybrid crystals were obtained. As shown in Figure 1, the hybrid crystals present the shape of quadrate, and the average size of hybrid crystals are approximately 2 mm. Under the illumination of 365 nm light, the hybrid crystals emit green light, which is shown in the photograph of Figure 1. Figure 2c shows the corresponding photoluminescence (PL) spectra of the hybrid crystals


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fabricated from different volumes of as-prepared CDs solution under the excitation of 360 nm. The spectra show one peak mainly at 510 nm, and the fluorescence intensity of hybrid crystals is enhanced with the increase in volume of added CDs solution. Yet, the PL spectrum of CDs aqueous solution under 360 nm illumination show one peak mainly at 506 nm (Figure S2). The minimal red-shift of the fluorescence peaks can be attributed to the change of the dielectric constant of the surrounding media.19-21 Figure 2a shows the HRTEM image of the hybrid crystals. Because the NaCl matrix would melt under the electron beam exposure, the NaCl matrix is hard to distinguish. However, single crystalline CDs can be seen clearly from the image, which indicates the well-dispersion and non-aggregation of CDs in NaCl crystals. Also, the scan electron microscopy (SEM) image in Figure 2b presents the smooth surface of the hybrid crystals. The X-ray diffraction (XRD) was used to confirm the crystallinity of the hybrid crystals. As shown in Figure 2d, the hybrid crystals made from different volumes of CDs all have the similar solid-state packing of cubic NaCl, indicating that the incorporation of CDs into NaCl may not disturb or modify the crystalline structure of the host material. The above results confirm that the CDs have been embedded into NaCl crystals successfully. It is worth noting that the hybrid crystal made from 120 µL CDs solution presents the weaker diffraction intensity than that of 100 µL, which may be attributed to the incorporation of more CDs in NaCl crystals. The crystals made from 100 µL CDs solution has the optimum performances in illumination, absorption (Figure S6) and construction. Phosphorescent materials have significantly broader opportunities in cellular imaging, light emitting and chemo-sensing applications due to the high yields and comparatively slower decay rates that benefited from their triplet excited states. The room temperature phosphorescence (RTP) can also be observed from the CDs-NaCl hybrid crystals. When the UV light illuminating


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on the crystals is turned off, the hybrid crystal still emit green light (Figure S7a), which can be observed by the naked eye for seconds. Figure S7c shows the phosphorescence spectrum of the hybrid crystals prepared by 100 µL CDs solution. As can be seen from the figure, the phosphorescence peak of the crystal is located at 519 nm, which corresponds with the fluorescence peak of the hybrid crystals in Figure 2c. The redshift of phosphorescent peak in comparison to the fluorescent peak can be attributed to the lower energy of T1 states than the corresponding S1 states. To reveal the phosphorescence lifetime of the hybrid crystals, timeresolved phosphorescence spectra were characterized and shown in Figure S7d. The decay curve of the hybrid crystals can fitted into a multi-exponential function with three lifetimes of 6.176 ms (21.04%), 77.2 ms (41.35%) and 371.7 ms (37.61%). The multiple phosphorescence lifetimes imply the present of various electronic transition processes, which originate from the various chemical environments of the carbonyls on the surface of CDs.8,17 The average lifetime can be calculated using the following equation: < >= ∑ ⁄∑

(1)

the calculated average lifetime of the hybrid crystal is 314 ms, and the lifetimes of the hybrid crystals prepared by different volumes of CDs solution were also characterized and shown in Fig. S8 and the inset of Fig. 7d. As can be seen from the figures, although fabricated by the different volumes of CDs solution, the lifetimes of the hybrid crystals are all around 310 s, which is nearly unchanged with the changing amount of embedded CDs. In order to explore the origin of the phosphorescence, the phosphorescence excitation spectrum and the absorption spectrum of the hybrid crystals were investigated. In the absorption spectrum of hybrid crystals (Figure S7e), the peak centered at 400 nm can be attributed to n–π* transition of C=O groups.17 The phosphorescence excitation spectrum at 516 nm shows a broad peak between 350 and 480 nm,


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which overlaps the absorption band of C=O groups. The singlet and triplet states of carbonyl groups are close in energy, and the spin-orbit coupling (SOC) is efficient,8,17,18 so the carbonyl groups are considered as the origin of RTP. To clarify the role of NaCl matrix played in the phosphorescence of hybrid crystals, the lifetimes of CDs solution and CDs thin films were also investigated. As shown in Figure S7f, the aqueous solution of CDs hardly has no lifetime, and the phosphorescence lifetime of CDs thin film is 0.041 ms, which is far shorter than that of hybrid crystals. Li et al. have reported the saltembedded carbon nanodots (S-CDs), which show a lifetime of 6.5 ns.23 The difference of lifetimes between the S-CDs and our work may be attributed to the different levels of aggregation in solid matrix. The aggregation level of S-CDs is higher than us by comparing the TEM images of hybrid crystals. The close spacing of CDs in solid states may suffer from the self-quenching processes, then result in a suboptimal efficiency. The ordered structure of NaCl and the long spacing between CDs, which are provided by the host NaCl matrix, could prevent the close contact interactions that facilitate the non-radioactive decay of excited molecules.25 In addition, The host NaCl matrix could also effectively keep the CDs from exposure to atmospheric O2 to form triplet oxygen, which is the major cause of phosphorescence quenching under ambient conditions.26 Therefore, another possible role of NaCl crystal is hindering the direct collisions between carbonyls and oxygen molecules, then facilitating the phosphorescence. When we grinded the hybrid crystals into powders, both the fluorescence and phosphorescence could also be observed (Figure S7b). The result confirms that the CDs are completely and steadily embedded into NaCl matrix. It is also suggests that the hybrid crystals have superior mechanical stability and component constancy, which can greatly broaden the applications of the CDs into the fields of long-term, high temperature and high intensity illumination.


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The room temperature lasing characteristics of the hybrid crystals were performed by a Newport Ti: sapphire femtosecond laser with 35 fs pulse-width, 1k Hz repetition frequency and 800 nm center wavelength. An optical parametrical amplifier was used to tune the output wavelength to 360 nm. The pump bean was focused onto the hybrid crystal by quartz lenses. The photograph of the experimental setup is shown in Figure 3a. The excitation laser was focused onto the hybrid crystal at a grazing angle, and the light probe was also set with the detecting angle of 45° normal to the hybrid crystal. As shown in Figure 3c, when the pump power is 0.07 mW, a weak spontaneous emission with the full width at half maximum (FWHM) of 7 nm can be observed. As the pump power increases, several sharp peaks with the FWHM of less than 1.8 nm emerge from the emission spectrum. With further increase in the pump power to 0.3 mW, more sharp peaks appear. A plot of integrated PL intensity of the peaks with pump power is shown in the Figure 3d. The intensity of which increases rapidly with the further increase in pump power and the lasing threshold is found to be 0.08 mW. The appearance of sharp peaks with narrow width upon increasing the pump power and the non-linear increase of the emission intensity indicate that the lasing phenomenon in the CDs-NaCl hybrid crystals has been observed. Lasing phenomena could not appear without the feedback of appropriate cavity. Various feedback mechanisms may account for the lasing, such as F-P cavity, WGM or random resonant effect.27-29 In our experiment, the feedback can be attributed to the WGM cavity formed by the NaCl. Figure 3b shows the SEM images of the grinded hybrid crystals, and the picture indicates that the CDs-NaCl hybrid crystals are integrated by the tiny cubic crystals, which could act as the WGM cavity (more SEM images are shown in Figure S11). The illustration depicting the lasing of WGM is shown in the inset of Figure 3b, in which the optical modes undergo total internal reflection around the walls of the tiny cubic crystals, thus has provided resonant pathways for


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optical gain. The Q factor is an important parameter to describe a laser cavity. For a lasing mode, the Q factor can be calculated with the following formula: Q = λ ∕ Δλ

(2)

where λ and ∆λ are the peak wavelength and its FWHM, respectively. In this experiment, the FWHM of the laser mode is about 1.15 nm at the wavelength of 514 nm in Figure 3c. Therefore, the corresponding Q factor is calculated to be 447. To understand the lasing observed in the hybrid crystals clearly, the mode spacing calculation of the tiny cubic crystals was performed according to the equation: ∆ = ⁄ − ⁄ = !2√2 − ⁄

(3)

where L is the path length of a round trip, l is the side length of the tiny cubic crystal, n is the refractive index of NaCl and dn/dλ denotes the dispersion relation. In this work, the l of the cubic crystals is approximately 20 μm, the n and dn/dλ of NaCl at 515 nm are 1.55 and -0.098751 µm-1, respectively. The calculated mode spacing ∆λmode is 2.9 nm, which matches well with the experimental mode spacing of 2.8 nm shown in Figure 3c. The optical mode number can be deduced through # = /

(4)

where N is the mode number, λ is the corresponding wavelength and L is the optical path length. The calculated mode number N511.9 nm of the peak at 511.9 nm is 182, then N514.7 nm and N517.5 nm are 181 and 180, separately. Most reported lasing phenomena were observed in the fluorescent materials, and the laser from high-efficient phosphor materials have not yet been achieved and reported. As depicted in Figure 4, the singlet excitons (S1), which are formed by pumping from the ground state (S0), are found to relax rapidly back to S0 and emit a photon. However, a small fraction of the S1 states


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undergo intersystem crossing (ISC), producing the lowest lying triplet state (T1). Due to the long lifetime of T1, its population dominates over that of other excited states.30 This quasi-steady-state triplet states are easy to be pumped to a higher-lying triplet state (Tn) under the optical pumping of the femtosecond laser, which is the origin of triplet absorption. Then, the Tn states undergo partial and non-radiative relaxations to T1. The relaxation from T1 to S0 belongs to radiative recombination processes. However, for realizing the radiative recombination from T1 to S0, the emitting cross-section of T1→S0 must be larger than the excited-state absorption cross-section of T1→Tn, that is %&' →)* > %+,-&' →&.

(5)

To meet the requirement, the materials need strong SOC effect, which is hard to be accomplished in phosphor materials.31,32 For the CDs-NaCl hybrid crystals, the central wavelength of the laser in Figure 3c is located near 510 nm, which is consistent with the fluorescence peak of the crystals in Figure 2c. Therefore, the triplet state haven’t participated in the lasing phenomenon of hybrid crystals.

CONCLUSION In summary, the CDs-NaCl hybrid crystals were achieved by incorporating the CDs into NaCl matrix through a modified LLDC process. The embedded CDs have added the luminescence centers into NaCl matrix, as the result, the hybrid crystals presented the fluorescence centered at 510 nm under the illumination of 365 nm light. The phosphorescence with an average lifetime of 314 ms was also achieved. What’s more important is that the tiny cubic crystal in hybrid crystals offered the WGM resonant cavity for optical gain and lasing phenomenon has been observed from hybrid crystals. When the pump power is low, weak spontaneous emission can be observed.


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With further increase in the pump power, more sharp peaks appear. The intensity of which increases rapidly with the further increase in pump power and the lasing threshold is found to be 0.08 mW and the Q factor is calculated to be 447, which has provided a new approach for realizing lasing materials.

EXPERIMENTAL Preparation of CDs: Critric acid monohydrate (99.5%) and MeOH were purchased from Sinopharm Chemical Reagents. Urea was purchased from Beijing Chemical Works. Deionized water was obtained from a water purification system with a resistivity of 18.25 mΩ cm. All materials were used as received without further purification or treatment. Citric acid (1.5 g) and urea (1.5 g) were added into 10 mL deionized water, then a homogeneous and colorless transparent solution was formed. The solution was heated in a microwave oven for 4 minutes, after which the transparent solution changed into solid-stated black product. The black product was collected and dispersed in 60 ml deionized water under ultrasonication for 30 min. Then the solution was filtered by dialysis membranes (MD34-500D) to remove larger and insoluble particles. After the dialysis, supernatant liquid was obtained, which contains fluorescent CDs. Preparation of Mixed Crystal: The CDs-NaCl hybrid crystals were prepared through the modified liquid-liquid diffusionassisted crystallization (LLDC) process.19 Firstly, 3 mL MeOH was taken and placed into a beaker. Then, 0.5 g NaCl, 1.7 mL deionized water and a certain volume of as-prepared CDs solution (20 µL, 40 µL, 60 µL, 80 µL, 100 µL and 120 µL) were mixed together to form a saturated solution of NaCl. The mixed solution was slowly injected to the bottom of the beaker


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and an aqueous layer was formed below the MeOH layer, which leaded to a stable interface between two liquids. Because water and MeOH are soluble with each other, the diffused MeOH in NaCl solution could gradually decrease the solubility of the salt, then the NaCl crystals together with CDs separated out from the mixed solution within 24 hours. Characterization: The morphology of samples was investigated by the field-emission scanning electron microscopy (FESEM, Hitachi S-4800). The high resolution transmission electron microscope (HRTEM) was obtained on a JEOL JEM-2100F equipment. The photoluminescence (PL) measurement was carried out with a Hitachi F-7000 fluorescence spectrophotometer, and the absorption spectra were carried out using a Shimadzu UV-3101 PC spectrophotometer. The Fourier transformed infrared (FTIR) spectrum was recorded with a Bruker VERTEX spectrometer. X-ray diffraction (XRD) patterns were collected with a Bruker D8 system. The time-resolved phosphorescence spectra were investigated using an Edinburgh FLS920 fluorescence spectrometer. The room temperature lasing characteristics of the hybrid crystals were performed by a Newport Ti: sapphire femtosecond laser with 35 fs pulse-width, 1k Hz repetition frequency and 800 nm center wavelength. An optical parametrical amplifier was used to tune the output wavelength to 360 nm, which was focused onto the hybrid crystal by quartz lenses. The lasing spectra were recorded by an ACTON SpectraPro 2300i spectrograph.


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FIGURES

Figure 1. Digital photographs of hybrid crystals under daylight (a), (c) and under UV illumination (b), (d).


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Figure 2. (a) HRTEM image of the hybrid crystal. (b) SEM image which shows the surface of hybrid crystal. (c) Fluorescence spectra of CDs-NaCl hybrid crystals formed from different volumes of as-prepared CDs solution. (d) XRD patterns of the hybrid crystals.


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Figure 3. (a) The photograph of the experimental setup to perform the room temperature lasing characteristics of the hybrid crystals. (b) SEM image of the grinded hybrid crystals, the inset shows the resonant pathways in the tiny cubic crystal. (c) The PL spectrum of an individual hybrid crystal under different pump powers, the inset shows the photograph of an excited hybrid crystal. (d)The relationship between the integrated emission intensity and the pump power of the hybrid crystals.


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Figure 4. The energy band diagram of hybrid crystals.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: TEM images of the prepared CDs, FTIR spectra of the CDs, fluorescence excitation spectra and absorption spectra of CDs aqueous solution, UV-Vis absorption spectra of the CDs-NaCl hybrid crystals, time-resolved phosphorescence spectra of CDs-NaCl hybrid crystals, fluorescence spectra of the CDs-NaCl hybrid crystals, phosphorescence spectra of the CDs-NaCl hybrid crystals, SEM images of the grinded hybrid crystals, energy band diagram of hybrid crystals.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected] Author Contributions H. L., F. W., and D. Z. designed the experiments. H. L. and Y. W. performed the experiments. H. L and F. W. wrote the manuscript. All of the authors discussed the results and commented on the manuscript. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China under Grant No.11504367, the Youth Scientific Research Foundation of Jilin province under Grant No. 20160520121JH. REFERENCES (1) Zhu, S.; Meng, Q.; Wang, L.; Zhang, J.; Song, Y.; H. Jin,; Zhang, K.; Sun, H.; Wang, H.; Yang, B. Highly Photoluminescent Carbon Dots for Multicolor Patterning, Sensors, and Bioimaging. Angew. Chem., Int. Ed. 2013, 52, 3953-3957.


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(2) Li, H.; He, X.; Kang, Z.; Huang, H.; Liu, Y.; Liu, J.; Lian, S.; Tsang, C.; Yang, X.; Lee, S. Water-Soluble Fluorescent Carbon Quantum Dots and Photocatalyst Design. Angew. Chem., Int. Ed. 2010, 49, 4430-4434. (3) Sun, Y.; Zhou, B.; Lin, Y.; Wang, W.; Fernando, K.; Pathak, P.; Meziani, M.; Harruff, B. a.; Wang, X.; Wang, H.; Luo, P.; Yang, H.; Kose, M. E.; Chen, B.; Mv, L.; Xie, S. QuantumSized Carbon Dots for Bright and Colorful Photoluminescence. J. Am. Chem. Soc. 2006, 128, 7756-7757. (4) Qu, S.; Wang, X.; Lu, Q.; Liu, X.; Wang, L. A Biocompatible Fluorescent Ink Based on Water-Soluble Luminescent Carbon Nanodots. Angew. Chem., Int. Ed. 2012, 124, 12381-12384. (5) Baker, S. N.; Baker, G. a. Luminescent Carbon Nanodots: Emergent Nanolights. Angew. Chem., Int. Ed. 2010,49, 6726-6744. (6) Chen, X.; Jin, Q.; Wu, L.; Tung, C.; Tang, X. J. Synthesis and Unique Photoluminescence Properties of Nitrogen-Rich Quantum Dots and Their Applications. Angew. Chem., Int. Ed. 2014, 53, 12542-12547. (7) Wang, F.; Xie, Z.; Zhang, B.; Liu, Y.; Yang, W.; Liu, C. Down- and Up-Conversion Luminescent Carbon Dot Fluid: Inkjet Printing and Gel Glass Fabrication. Nanoscale. 2014, 6, 3818-3823. (8) Deng, Y.; Zhao, D.; Chen, X.; Wang, F.; Song, H.; Shen, D. Long Lifetime Pure Organic Phosphorescence Based on Water Soluble Carbon Dots. Chem. Commun. 2013, 49, 5751-5753.


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(9) Mao, L.; Tang, W.; Deng, Z.; Liu, S.; Wang, C.; Chen, S. Facile Access to White Fluorescent Carbon Dots toward Light-Emitting Device. Ind. Eng. Chem. Res. 2014, 53, 64176425. (10) Choi, H., Ko, S.; Choi, Y.; Joo, P.; Kim, T.; Lee, B. R.; Jung, J.; Choi, H. J.; Cha, M.; Jeong, J.; Hwang, I.; Song, M. H.; Kim, B.; Kim, J. Y. Versatile Surface Plasmon Resonance of Carbon-Dot-Supported Silver Nanoparticles in Polymer Optoelectronic Devices Nat. Photonics. 2013, 7, 732-738. (11) Guo, X.; Wang, C.; Yu, Z.; Chen, L.; Chen, S. Facile Access to Versatile Fluorescent Carbon Dots toward Light-Emitting Diodes. Chem. Commun. 2012, 48, 2692-2694. (12) Kwon, W.; Lee, G.; Do, S.; Joo, T.; Rhee, S. Size-Controlled Soft-Template Synthesis of Carbon Nanodots toward Versatile Photoactive Materials. Small. 2014, 10, 506-513. (13) Li, X.; Liu, Y.; Song, X.; Wang, H.; Gu, H.; Zeng, H. ntercrossed Carbon Nanorings with Pure Surface States as Low-Cost and Environment-Friendly Phosphors for White-Light-Emitting Diodes. Angew. Chem., Int. Ed. 2014, 53, 1-7. (14) Zhang, X.; Zhang, Y.; Wang, Y.; Kalytchuk, S.; Kershaw, S. V.; Wang, Y.; Wang, P.; Zhang, T.; Zhao, Y.; Zhang, H.; Cui, T.; Wang, Y.; Zhao, J.; Yu, W.; Rogach, A. L. ColorSwitchable Electroluminescence of Carbon Dot Light-Emitting Diodes. ACS Nano 2013, 7, 11234-11241. (15) Deng, Y.; Chen, X.; Wang, F.; Zhang, X.; Zhao D.; Shen, D. Environment-Dependent Photon Emission From Solid State Carbon Dots and Its Mechanism. Nanoscale. 2014, 6, 1038810393.


19


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

Page 20 of 23

(16) Sun, M.; Qu, S.; Hao, Z.; Ji, W.; Jing, P.; Zhang, H.; Zhang, L.; Zhao, J.; Shen, D. Towards Efficient Solid-State Photoluminescence Based on Carbon-Nanodots and Starch Composites. Nanoscale. 2014, 6, 13076-13081. (17) Dong, X.; Wei, L.; Su, Y.; Li, Z.; Geng, H.; Yang, C.; Zhang, Y. Efficient Long Lifetime Room Temperature Phosphorescence of Carbon Dots in a Potash Alum Matrix. J. Mater. Chem. C. 2015, 3, 2798-2801. (18) Tan, J.; Zou, R.; Zhang, J.; Li, W.; Zhang, L.; Yue, D. Large-Scale Synthesis of N-Doped Carbon Quantum Dots and Their Phosphorescence Properties in a Aolyurethane Matrix. Nanoscale. 2016, 8, 4742-4747. (19) Adam, M.; Wang, Z.; Dubavik, A.; Stachowski, G. M.; Meerbach, C.; Soran-Erdem, Z.; Rengers, C.; Demir, H. V.; Gaponik, N.; Eychmüller, A. Liquid–Liquid Diffusion-Assisted Crystallization: A Fast and Versatile Approach Toward High Quality Mixed Quantum Dot-Salt Crystals. Adv. Funct. Mater. 2015, 25, 2638-2645. (20) Otto, T.; N.;

Müller, M.;

Mundra, P.;

Lesnyak, V.; Demir, H. V.;

Gaponik,

Eychmüller, A. Colloidal Nanocrystals Embedded in Macrocrystals: Robustness,

Photostability, and Color Purity. Nano Lett. 2012, 12, 5348-5354. (21) Muller, M.;

Kaiser, M.;

Stachowski, G. M.;

Resch-Genger, U.; Gaponik, N.;

Eychmüller, A. Photoluminescence Quantum Yield and Matrix-Induced Luminescence Enhancement of Colloidal Quantum Dots Embedded in Ionic Crystals. Chem. Mater. 2014, 26, 3231-3237.


20

Page 21 of 23

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


(22) Adam, M.; Erdem, T.; Stachowski, G. M.; Soran-Erdem, Z.; Lox, J. F. L.; Bauer, C.; Poppe, J.; Demir, H. V.; Gaponik, N.; Eychmüller, A. Implementation of High-Quality WarmWhite Light-Emitting Diodes by a Model-Experimental Feedback Approach Using Quantum Dot−Salt Mixed Crystals. ACS Appl. Mater. Interfaces. 2015, 7, 23364-23371. (23) Kim, T.; Wang, F.; McCormick, P.; Wang, L.; Brown, C.; Li, Q. Salt-Embedded Carbon Nanodots as a UV and Thermal Stable Fluorophore for Light-Emitting Diodes. Journal of Luminescence. 2014, 154, 5224-5230. (24) Qu, S.; Liu, X.; Guo, X.; Chu, M.; Zhang, L.; Shen, D. Amplified Spontaneous Green Emission and Lasing Emission from Carbon Nanoparticles. Adv. Funct. Mater. 2014, 24, 26892695. (25) Bolton, O.; Lee, D.; Jung, J.; Kim, J. Tuning the Photophysical Properties of Metal-Free Room Temperature Organic Phosphors via Compositional Variations in Bromobenzaldehyde / Dibromobenzene Mixed Crystals. Chem. Mater. 2014, 26, 6644-6649. (26) Mukherjee, S.; Thilagar, P. Recent Advances in Purely Organic Phosphorescent Materials. Chem. Commun. 2015, 51, 10988-11003. (27) Chen, R.; Ling, B.; Sun, X. W.; Sun, H. D. Room Temperature Excitonic Whispering Gallery Mode Lasing from High-Quality Hexagonal ZnO Microdisk. Adv. Mater. 2011, 23, 2199-2204. (28) Gargas, Daniel J.; Moore, Michael C.; Ni, Adrian.; Chang, S.; Zhang, Z.; Chuang, S.; Yang, P. Whispering Gallery Mode Lasing from Zinc Oxide Hexagonal Nanodisks. ACS nano. 2010, 4, 3270-3276,


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Page 22 of 23

(29) Li, J.; Lin, Y.; Lu, J.; Xu, C.; Wang, Y.; Shi, Z.; Dai, J. Whispering-Gallery Submicron Cavity and Graphene Improved Lasing Performance. ACS nano. 2015, 9, 6794-6800, (30) Yang, X.; Lee, C.; Westenhoff, S.; Zhang, X.; C, Neil.; Saturation, G. Relaxation, and Dissociation of Excited Triplet Excitons in Conjugated Polymers. Adv. Mater. 2009, 21, 916-919 (31) Chaudhuri, S.; Li, D.; Sigmund, E.; Wettach, H.; Hoger, S.; Lupton, John M. Plasmonic Surface Enhancement of Dual Fluorescence and Phosphorescence Emission from Organic Semiconductors: Effect of Exchange Gap and Spin–Orbit Coupling. Chem. Commun. 2012, 48, 6675-6677 (32) Chen, X.; Yu, R.; Wu, X.; Liang, D.; Jia, J.; Lu, Z. A Strongly Greenish-Blue-Emitting Cu4Cl4 Cluster with an Efficient Spin–Orbit Coupling (SOC): Fast Phosphorescence Versus Thermally Activated Delayed Fluorescence. Chem. Commun. 2016, 52, 6288-6291


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Whispering Gallery Mode Laser from Carbon DotâNaCl Hybrid

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