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Carbon dot-based lasers
CHAPTER
Carbon dot-based lasers: an introductory survey
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Songnan Qu, Ding Zhou, Zhen Tian, Di Li Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun, China
1. Introduction As there are various kinds of synthetic routines and precursors for CDs, the accurate structures and photoluminescence (PL) mechanisms of CDs are complicated and controversial. It is generally accepted that the intrinsic (bandgap related) transitions of CDs (including GQDs) are driven by the graphitic core size composed of conjugated sp2-domains, which is the real domination of quantum confinement effect [1,2]. Theoretical calculations and relative reported experimental results have demonstrated that the bandgap of CDs can be tuned by modulating the size of conjugated sp2-domains (Fig. 1.1) [3]. At the same time, doping and the surface state can also dramatically affect their PL properties with the easy introduction of heteroatoms (N, S, P, et al.) and the sufficient surface groups on CDs, especially oxygen
FIG. 1.1 Calculated emission wavelength (nm) using TDDFT method in vacuum as a function of the diameter of GQDs. The solid line is the linear fitting of zigzag-edged GQDs (G1eG6). The indicated diameter is the average of the horizontal and vertical dimensions [3]. Nanoscale Semiconductor Lasers. https://doi.org/10.1016/B978-0-12-814162-5.00001-7 Copyright © 2019 Elsevier Inc. All rights reserved.
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related functional groups, such as carboxyl and hydroxyl. To date, PL of CDs has been demonstrated in the blue and green spectral regions with a maximum PL quantum yield (QY) above 80% [4,5]. Recently, the realization of strong emission in the red to near-infrared (NIR) region has been extensively investigated and preliminarily achieved [6e8]. Kang et al. developed alkali-assisted electrochemical fabrication of CDs and separated them by column chromatography. They found different-sized CDs yielded different emission colors of blue, green, yellow, and red PL respectively. Their theoretical calculations based on the size of sp2-domains agreed well with the optical results [9]. Lau et al. reported the broadband emission from the GQDs covering 300 to >1000 nm and demonstrated that the broadband emission was attributed to the layered structure of the GQDs that contained a large conjugated system and provides extensive delocalized p electrons. The deep UV emission comes from the localized p electron in double bonds (mainly C]C). The visible emission is caused by the partial conjugated p electrons in the GQDs. As for NIR emission, the conjugated p electrons in the layered structure of the GQDs facilitate the NIR absorption [10]. Lin et al. synthesized full-colour tunable (blue lem ¼ 435 nm, green lem ¼ 535 nm and red lem ¼ 604 nm) CDs using phenylenediamine isomers as precursors in a solvothermal method in ethanol. Also, in this work, an emission redshift with increasing size and nitrogen content was observed [11]. Qu et al. prepared full-color emissive (lpeak: 448e638 nm), and finally CDs (namely CDs) from the same precursors citric acid and urea by tuning the solvents of water, glycerol, and dimethylformamide (DMF) in solvothermal conditions. They demonstrate how different solvents affect the dehydration and carbonization processes occurring during the high-temperature solvothermal reaction, resulting in the different overall sizes of the resulting CDs and thus dimensions of conjugated sp2-domains, determining their different emission colors (Fig. 1.2) [12]. Fan et al.
FIG. 1.2 Left: a possible growth mechanism for CDot-water, CDot-glycerol, and CDot-DMF. Right: photographs of the CDs solutions synthesized a three-solvent mixture (samples A, B, C, D, E, F, G) under daylight (top) and UV light (bottom) [12].
2. Mechanism of CDs
reported bright multicolor bandgap fluorescent CDs from blue to red with a QY up to 75% for blue fluorescence. The as-prepared CDs are nitrogen doped, highly surfacepassivated, and have a high degree of crystallinity [13]. Sun et al. found that emission of the CDs can be tuned by controlling the reaction conditions such as increasing the ratios of CA/urea and reaction temperature. The emissions are shifted from blue to red in the case of increasing effective conjugation length and the amount of surface functional groups, such as eCOOH [14]. Recently, enlarging the sp2-domain together with relative doping or surface engineering promised CDs with efficient orange and red emissions with the highest PL QY up to 46% and 53% [7,15]. These tunable and efficient bandgap transitions endow CDs to be promising gain medium to achieve lasing.
2. Mechanism of CDs Spontaneous emission is a transition from excited state to ground state without external influence, which was widely used to understanding photoluminescence mechanism in CDs. Fluorescent CDs were demonstrating great potential applications in bio-imaging [16e20] and light emitting diodes [12,21]. Recently, CDs shows another application in lasing materials [22e25]. Stimulated emission is a transition from excited state to ground state with association of an external electromagnetic filed, which produces an additional photon with same direction, phase and polarization as the incident photon. Stimulated emission can be modeled mathematically by Einstein relationship. Ultrafast pump-probe measurement, such as femtosecond time-resolved transient absorption (TA) spectroscopy, is a fundamental research for stimulated emission dynamics in CDs. After pump light excitation, electrons in ground states are pumped into excited states resulting in the ground state bleaching (GSB) feature in TA spectrum, due to the Pauli exclusion principle. There are three possible depopulation channels for electrons in excited states are spontaneous emission, stimulated emission (SE) and excited state absorption (ESA), respectively. In general, spontaneous emission cannot be detected in TA spectrum because of independence of probe light. Stronger stimulated emission signal in TA spectrum (Fig. 1.3) was observed in microwave synthesized CDs by L. Wang et al., due to its high PL quantum yield [26,27]. TA kinetic traces of CDs were well fitted by multi-exponential functions. The short lifetime components of 1e2 ps and long lifetime components of 4e5 ns were attributed to intrinsic state and molecule like state. The origin of green luminescence in these CDs and graphene quantum dots (GQDs) by bottom-up and top-down methods was attributed to the special edge states, which were related to functional groups with C]O. Recently, the PL of single CDs was proved from electric dipole emission center via the scanning of an azimuthally polarized laser beam (APLB) at focal region, as shown in Fig. 1.4 [28]. Created new photon by stimulated emission has the same phase, frequency, polarization and direction with incident photon. Anisotropic
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FIG. 1.3 TA spectra for (A) microwave-synthesized CDs, (B) electrochemically synthesized CDs, and (C) solvothermal synthesized GQDs at 400 nm excitation. ESA, excited-state absorption; GSB, ground-state bleaching; SE, stimulated emission. (D) Femtosecond time-resolved PL dynamics of CDs (probed at 520 nm) and GQDs (probed at 530 nm) at 400 nm excitation [27].
stimulated emission in CDs was observed by using polarization dependent femtosecond TA spectroscopy, indicating transition dipole moment in CDs [29]. The dipole-dipole interaction between CDs and polar solvent molecules induced redshift of emission peak in time resolved spectroscopy, so-called solvation relaxation. The obtained solvation relaxation times were ranged from 0.9 ps to 83.6 ps, dependent on viscosity and water content of solvents. The orientational relaxation time constants of CDs were enhanced from 49.2 ps to 793.5 ps with increasing the viscosity of solvent, as shown in Fig. 1.5. Additionally, the orientational relaxation was also dependent on the proton the proton donation capability of solvent. P. Jing et al. demonstrated that the dipole emission center in CDs is relevant to electron transition between localized state of dopant atoms (N, O) and delocalized state of sp2 C domain [29]. When population inversion in CDs is realized and the rate of stimulated emission exceeds that of absorption, then the optical amplification in CDs can be achieved [23e25].
3. Applications of carbon dots in lasing
(A)
(B)
(C)
(D)
(E)
(F)
(G)
(H)
(I)
(J)
(K)
(L)
(M)
(N)
(O)
(P)
(Q)
FIG. 1.4 (A) Single CDs excited with an azimuthally polarized laser beam. Inset shows a theoretically calculated excitation pattern for identical experimental conditions assuming a linear horizontal dipole. The orientation of the dipole is indicated by the double arrow. (B)(G) Consecutive images of the same sample area recorded by using an azimuthally polarized laser beam (scanning direction updown). Each double-lobe pattern corresponds to the same CND. The images show PL intermittency (D) and single-step photobleaching (F) of the particle. Scanning direction is top-down. (H)(Q) Defocused images of single CDs: experimental data (H)(L) and fitted patterns (M)(Q), respectively. All the patterns correspond to the emission of a single fixed dipole [28].
3. Applications of carbon dots in lasing The photoluminescence (PL) properties of Carbon-Dots show great potential in the applications of lasing. According to W. F. Zhang and colleagues at Hong Kong Polytechnic University, CDs can be synthesized by laser irradiation methods. By uniformly dispersing the CDs into N-Methylpyrrolidone (NMP), they can find some interesting results. White-light amplified spontaneous emission is found from the mixture when applying laser excitation at 266 nm. The peak wavelength of the emission spectra is found to be around 450 nm with linewidth at about 120 nm. The NMP can advance the emission efficiency of CDs over the broad spectrum, for the excitation energy is captured by the NMP and then transferred to the CDs, not absorbed by the CDs. Comparing to not using the NMP, optical gain per peak power of the mixture at around 450 nm is 39% higher, which is around 64 cm1 MW1 [30]. Later, they also observed lasing from CDs, which were dispersed into a layer of poly(ethylene glycol) coated on the surface of optical fibers under 266 nm optical excitation. This is due to the enhancement of photoluminescence intensity via the esterification of carboxylic groups of the CDs, and the formation of high-Q cylindrical microcavities to support second-type whispering gallery modes, where Q-factor is found to be >1.3 103 (Fig. 1.6) [23].
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FIG. 1.5 Top viewed TA spectra of CDs in water, when polarizations of pump and probe were set to (A) parallel and (B) perpendicular. (C) The difference spectrum between parallel and perpendicular TA spectra. (D) TA kinetic traces of CDs probed at 550 nm. (Black, red and green lines represent the parallel, perpendicular and magic angle polarizations. Blue line is the calculated anisotropy as a function of delay time). (E) Time-dependent polarization anisotropy of CDs in different solvents. (F) Reorientation times for CDs versus Viscosity of solvents [29].
FIG. 1.6 (A) Plot of lightelight curve and FWHM of the emission spectra of the CDs deposited on a quartz substrate, and (B) The corresponding emission spectra. The inset shows the photo of CDs on quartz substrate under laser excitation [30].
3. Applications of carbon dots in lasing
Moreover, a different method for lasing emission based on carbon-fluorescent materials has been used by the same research group. As shown in the report, the luminescent capability of CDs gained form the same functionalization process was studied and compared with the luminescent capability of graphene quantum dots (GQDs). Thanks to the geometrical advantages of CDs, such as larger surface area to volume ratio and smaller volume, the optical gain of GQDs is higher when compared to CDs. Dispersing a mixture of GQDs and TiO2 nanoparticles into ethanol, lasing emission was observed under optical excitation at 266 nm [31]. Similarly, Fan et al. demonstrated blue, green, and red random lasers with low-thresholds based on the NBE-T-CDs with very small Stokes shifts (9e16 nm) and small FWHM (30 nm). The blue, green, and red random lasers show remarkably low pump thresholds of 0.087, 0.052, and 0.048 mJ cm2 with corresponding narrow FWHM of 0.9, 0.37, and 0.82 nm, respectively [32]. The tunable amplified spontaneous emission (ASE) in GQD-doped cholesteric liquid crystal (CLC) was also observed by Y. T. Zhang and colleagues in Shanghai Institute of Microsystem and Information Technology. The GQDs are uniformly dispersed in CLC with a weight ratio of 0.5 wt% in CLC. Typical ASE can be triggered in the system under optical excitation, when pump energies are greater than 1.25 mJ cm2. At the long wavelength edge of the photonic bandgap, the emission peak moves from 662 to 669 nm when the working temperature changes from 50 to 90 C. The method of producing combined GQDs and CLC is simple and low-cost, with the resultant photastable and non-toxic. It is possible to fabricate ASE source and laser devices by combining the GQD gain material with the self-assembled CLC resonator (Figs. 1.7 and 1.8) [33].
FIG. 1.7 (A) PL spectra of the cylindrical microcavity laser coated with modified CDs (for 2 h of thermal treatment) in PEG 200 under optical excitation at 266 nm. The insets show the light curve (top-right corner) and photo (top-left corner) of the cylindrical microcavity laser. (B) Near field profile of the cylindrical microcavity laser under optical excitation at 266 nm [23].
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FIG. 1.8 (A) Blue, (B) green, and (C) red random lasing emission spectra from the NBE-T-CDs/ AueAg bimetallic porous nanowire composites excited by the 355, 480, and 532 nm laser pulses with a corresponding low pump energy density of 0.125, 0.075, and 0.057 mJ cm2, respectively. The insets are photographs of the blue, green, and red random lasing emission, respectively. (DeF) Plots of the blue (D), green (E), and red (F) integrated rando lasing emission intensity and FWHM of the narrow spikes in the random lasing emission spectra as a function of the pump energy density [32].
CDs fabricated by small molecules are utilized in generating laser beams as by up-bottom method. S. N. Qu and colleagues, who work for Changchun Institute of Optics Fine Mechanics and Physics Chinese Academy of Sciences, prepared high photoluminescent CDs though citric acid and urea. It is verified that the optical properties of CDs can be modulated by the dopant-N atom and sp2 C-contents. Blue emission was found in CDs prepared with the low urea mass ratio of 0.2:1 (CDs1), and the maximum PL quantum yield is 15%. Increasing sp2 C- and dopant-N atom contents, as determined in CDs prepared with high urea mass ratio of 2:1 (CDs2), lead to green emission (maximum PL quantum yield up to 36% in ethanol aqueous solution). Amplified spontaneous emission (ASE) can be found only in CDs2 ethanol aqueous solution. Green lasing emission can be observed from CDs2 ethanol aqueous solution in a linear long Fabry-Perot cavity, showing the possibility for using CDs2 as a gain medium for lasing. Comparing to C545T dye, CDs2 shows greater photostability As shown in the reports, the green emission from CDs2 is speculated to arise from electron-hole reconstructed(intrinsic state emission). Two main reasons of lasing emission are the high PL quantum yield and small overlap between absorption and emissions of CDs2 ethanol aqueous solution [24]. Besides CDs solution, the composites based on CDs have been found to be a novel lasing materials. By utilizing the CDs prepared by citric acid and urea as
3. Applications of carbon dots in lasing
mentioned above, H. Z. Liu and colleagues combined the as-prepared CDs into NaCl matrix in a simple method. The embedded CDs supplies luminescence centers to NaCl, so the hybrid crystals present the fluorescence centered at 510 nm under the illumination of 365 nm light. Meanwhile, the phosphorescence can last for about 314 ms after the 365 nm light was turned off. Furthermore, optical gain and lasing phenomenon has been observed from hybrid crystals. A weak spontaneous emission can be observed from the hybrid crystal when a low pump power is supplied, whereas 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 gives the whispering gallery mode (WGM) resonant cavity for lasing emission. That provides a new approach for achieving lasing materials (Figs. 1.9 and 1.10) [25].
FIG. 1.9 (A) Schematic diagram of experimental setup for optical pumping investigations of the CNP-based laser device (the black double arrows illustrate polarizations of the pumping laser and output laser). Photos of the operating CNP-based laser device under 355 nm laser pumping at (B) 30 kW cm2 and (C) 190 kW cm2 [24].
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FIG. 1.10 (A) 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) PL spectrum of an individual hybrid crystal under different pump powers, the inset shows the photograph of an excited hybrid crystal. (D) Relationship between the integrated emission intensity and the pump power of the hybrid crystals [25].
With the development of CDs-based laser, the corresponding possible mechanism has been proposed with the advance of CDs-based laser. Instead of high FLQYs, excitation wavelength-independent (lex-independent) photoluminescence characteristics are utilized to realize CDs-based light amplification, as reported by Y. S. Hu and colleagues. CDs with excitation wavelength-dependent (lex-dependent) PL characteristics and FLQYs as high as 99% and 96% were found not to exhibit amplified spontaneous emission (ASE), while those with lex-independent PL characteristics and FLQYs of only 38% and 82% realized ASE with low thresholds. The difficulty of achieving ASE using CDs with lex-dependent PL characteristics may lead to their high contents of CeOeH or CeOeC groups. These groups can attribute numerous localized electronic states within the np* gap, then decentralize the excited electrons, thus increase the difficulty of population inversion. In addition, the radiative transition rates and stimulated emission cross sections of CDs with lex-independent PL characteristics were found to be magnificantly higher than those of CDs with lex-dependent PL characteristics. As a practical structure for solid-state lasing devices, ASE in a planar waveguide structure was also demonstrated for the first time using CDs with lex-independent PL characteristics. These results
3. Applications of carbon dots in lasing
FIG. 1.11 ASE characteristics of CD1/PI film. (A) Normalized emission spectra of CD1/PI film with different pumping fluences. (B) Dependence of the output peak intensity and FWHM of CD1/PI on the pumping fluence. The inset in (B) depicts the operating device pumped at 355 nm for the CD1/PI film [34].
give us simple and effective guidelines for synthesizing and selecting CDs for low threshold lasing devices (Fig. 1.11) [34]. W. F. Zhang and colleagues at Shenzhen University, China, found the efficiency of multiphoton absorption was facilitated when the CDs were covalently coupled to organosilane chains. As a result, a large absorption coefficient of 1.16 106 cm5 per GW3 is obtained and four-photon luminescence under 1900 nm excitation is observed from the CDs at room temperature. Furthermore, by sandwiching a CDs film between a quartz substrate and a dielectric mirror, random lasing under three-photon (i.e. 1400 nm) excitation can be realized in a CDs laser. The formation of strongly confined microcavities, which arise from the non-uniform distribution of
FIG. 1.12 (A) Variations of PLQY (open black squares), and (B) PL intensity (open red circles), of the CDs@silica composites versus the concentration of CDs ethanol solution. Solid lines are solely provided as a guide to the eye. (B) Photograph of the xerogel-PDMS composites excited by an ultraviolet lamp. (C) Emission spectrum of a down-conversion white LED prototype entirely based on a powdered CDs@silica composite xerogel with a photograph of the working device (inset, panel (D)) [37].
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CHAPTER 1 Carbon dot-based lasers: an introductory survey
refractive indices inside the CDs film, is attributed to the realization of lasing emission [35]. For a young and emerging field, CDs (and their close relatives graphene quantum dots) have already made a significant impact in diverse scientific disciplines. Carbon-Dots have opened new avenues in photonics, electro-optics, and lasing fields. In particular, the unique luminescence properties of CDs, primarily the excitation-dependent emission phenomena, have been a prominent thread in this
FIG. 1.13 (A) The device structure comprising ITO/PEDOT:PSS (anode), MCBF-CDs (active emission layer), TPBi (ETL), and Ca/Al (cathode). The normalized PL spectra and the corresponding output EL spectra of (B) B-BF- CDs, (C) G-BF- CDs, (D) Y-BF- CDs, (E) OBF- CDs, and (F) R-BF- CDs thin films. The photographs in the insets of (BeF) display the close-up view of the surface emission from blue, green, yellow, orange, and red emission of monochrome LEDs [13].
References
field, making possible diverse experiments and analytical applications developed in the past several years. The optical and spectroscopic properties of Carbon-Dots will likely continue to shape the future development of these fields. Especially, the attractive chemical and photophysical properties of CDs have also provided impetus to research efforts aimed at creating mesoscale assemblies comprising CDs as luminescent elements for much more abundant materials and applications. Such materials also contain other chemical constituents having complementary roles, together producing multifunctional composite materials (Figs. 1.12 and 1.13). This field of research has been aided by the chemical stability and resilience of CDs to varied physical and chemical treatments. For instance, S. N. Qu and colleagues at Changchun Institute of Optics Fine Mechanics and Physics Chinese Academy of Sciences fabricated CDs@BaSO4 hybrid phosphors in an easy and low-cost process by sequentially assembling Ba2þ and SO4 2 ions onto the surface of CDs through electrostatic attraction, which exhibit excellent thermal and photostability [36]. They also prepared CDs@silica composites with high luminescence [37]. These as-prepared composites show great potential in down-conversion LEDs. Besides these achievements, L. Z. Fan and colleagues at Beijing Normal University successfully applied CDs in electroluminescent LEDs [13]. As presented in numerous publications, CDs’ luminescence has attracted much interests in technological development, holding the promise of CDs-based lasing equipment.
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[9] Li H, He X, Kang Z, Huang H, Liu Y, Liu J, Lian S, Tsang CHA, Yang X, Lee S-T. Water-soluble fluorescent carbon quantum dots and photocatalyst design. Angewandte Chemie International Edition 2010;49(26):4430e4. [10] Tang L, Ji R, Li X, Bai G, Liu CP, Hao J, Lin J, Jiang H, Teng KS, Yang Z, Lau SP. Deep ultraviolet to near-infrared emission and photoresponse in layered N-doped graphene quantum dots. ACS Nano 2014;8(6):6312e20. [11] Jiang K, Sun S, Zhang L, Lu Y, Wu A, Cai C, Lin H. Red, green, and blue luminescence by carbon dots: full-color emission tuning and multicolor cellular imaging. Angewandte Chemie International Edition 2015;54(18):5360e3. [12] Tian, Z.; Zhang, X.; Li, D.; Zhou, D.; Jing, P.; Shen, D.; Qu, S.; Zboril, R.; Rogach, A. L., Full-color inorganic carbon dot phosphors for white-light-emitting diodes. Advanced Optical Materials, https://doi.org/10.1002/adom.201700416. [13] Yuan F, Wang Z, Li X, Li Y, Tan Z a, Fan L, Yang S. Bright multicolor bandgap fluorescent carbon quantum dots for electroluminescent light-emitting diodes. Advanced Materials 2017;29(3). https://doi.org/10.1002/adma.201604436. [14] Miao X, Qu D, Yang D, Nie B, Zhao Y, Fan H, Sun Z. Synthesis of carbon dots with multiple color emission by controlled graphitization and surface functionalization. Advanced Materials 2017. https://doi.org/10.1002/adma.201704740. [15] Wang, Z.; Yuan, F.; Li, X.; Li, Y.; Zhong, H.; Fan, L.; Yang, S., 53% efficient red emissive carbon quantum dots for high color rendering and stable warm white-light-emitting diodes. Advanced Materials, https://doi.org/10.1002/adma.201702910. [16] Cao L, Wang X, Meziani MJ, Lu F, Wang H, Luo PG, Lin Y, Harruff BA, Veca LM, Murray D, Xie SY, Sun YP. Journal of the American Chemical Society 2007;129:11318. [17] Zhu S, Zhang J, Tang S, Qiao C, Wang L, Wang H, Liu X, Li B, Li Y, Yu W, Wang X, Sun H, Yang B. Advances in Functional Materials 2012;22:4732. [18] Yang ST, Cao L, Luo PG, Lu F, Wang X, Wang H, Meziani MJ, Liu Y, Qi G, Sun YP. Journal of the American Chemical Society 2009;131:11308. [19] Leme´nager G, De Luca E, Sun Y, Pompa PP. Nanoscale 2014;6:8617. [20] Chizhik AM, Stein S, Dekaliuk MO, Battle C, Li W, Huss A, Platen M, Schaap IAT, Gregor I, Demchenko AP, Schmidt CF, Enderlein J, Chizhik AI. Nano Letters 2016; 16:237. [21] Zhang X, Zhang Y, Wang Y, Kalytchuk S, Kershaw SV, Wang Y, Wang P, Zhang T, Zhao Y, Zhang H, Cui T, Wang Y, Zhao J, Yu WW, Rogach AL. ACS Nano 2013;7: 11234. [22] Sindeeva OA, Prikhozhdenko ES, Bratashov DN, Vostrikova AM, Atkin VS, Ermakov AV, Khlebtsov BN, Sapelkin AV, Goryacheva IYu, Sukhorukov GB. Carbon dot aggregates as an alternative to gold nanoparticles for the laser-induced opening of microchamber arrays. Soft Matter 2018;14:9012e9. [23] Zhang WF, Zhu H, Yu SF, Yang HY. Observation of lasing emission from carbon nanodots in organic solvents. Advanced Materials 2012;24:2263e7. [24] Qu S, Liu X, Guo X, Chu M, Zhang L, Shen D. Amplified spontaneous green emission and lasing emission from carbon nanoparticles. Advances in Functional Materials 2014; 24:2689e95. [25] Liu H, Wang F, Wang Y, Mei J, Zhao D. Whispering gallery mode laser from carbon dot-Nacl hybrid crystals. ACS Applied Materials and Interfaces 2017;9:18248e53. [26] Wang L, Zhu SJ, Wang HY, Wang YF, Hao YW, Zhang JH, Chen QD, Zhang YL, Han W, Yang B, Sun HB. Advanced Optical Materials 2013;1:264.
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Carbon dot-based lasers: an introductory survey
1
Songnan Qu, Ding Zhou, Zhen Tian, Di Li Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun, China
1. Introduction As there are various kinds of synthetic routines and precursors for CDs, the accurate structures and photoluminescence (PL) mechanisms of CDs are complicated and controversial. It is generally accepted that the intrinsic (bandgap related) transitions of CDs (including GQDs) are driven by the graphitic core size composed of conjugated sp2-domains, which is the real domination of quantum confinement effect [1,2]. Theoretical calculations and relative reported experimental results have demonstrated that the bandgap of CDs can be tuned by modulating the size of conjugated sp2-domains (Fig. 1.1) [3]. At the same time, doping and the surface state can also dramatically affect their PL properties with the easy introduction of heteroatoms (N, S, P, et al.) and the sufficient surface groups on CDs, especially oxygen
FIG. 1.1 Calculated emission wavelength (nm) using TDDFT method in vacuum as a function of the diameter of GQDs. The solid line is the linear fitting of zigzag-edged GQDs (G1eG6). The indicated diameter is the average of the horizontal and vertical dimensions [3]. Nanoscale Semiconductor Lasers. https://doi.org/10.1016/B978-0-12-814162-5.00001-7 Copyright © 2019 Elsevier Inc. All rights reserved.
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related functional groups, such as carboxyl and hydroxyl. To date, PL of CDs has been demonstrated in the blue and green spectral regions with a maximum PL quantum yield (QY) above 80% [4,5]. Recently, the realization of strong emission in the red to near-infrared (NIR) region has been extensively investigated and preliminarily achieved [6e8]. Kang et al. developed alkali-assisted electrochemical fabrication of CDs and separated them by column chromatography. They found different-sized CDs yielded different emission colors of blue, green, yellow, and red PL respectively. Their theoretical calculations based on the size of sp2-domains agreed well with the optical results [9]. Lau et al. reported the broadband emission from the GQDs covering 300 to >1000 nm and demonstrated that the broadband emission was attributed to the layered structure of the GQDs that contained a large conjugated system and provides extensive delocalized p electrons. The deep UV emission comes from the localized p electron in double bonds (mainly C]C). The visible emission is caused by the partial conjugated p electrons in the GQDs. As for NIR emission, the conjugated p electrons in the layered structure of the GQDs facilitate the NIR absorption [10]. Lin et al. synthesized full-colour tunable (blue lem ¼ 435 nm, green lem ¼ 535 nm and red lem ¼ 604 nm) CDs using phenylenediamine isomers as precursors in a solvothermal method in ethanol. Also, in this work, an emission redshift with increasing size and nitrogen content was observed [11]. Qu et al. prepared full-color emissive (lpeak: 448e638 nm), and finally CDs (namely CDs) from the same precursors citric acid and urea by tuning the solvents of water, glycerol, and dimethylformamide (DMF) in solvothermal conditions. They demonstrate how different solvents affect the dehydration and carbonization processes occurring during the high-temperature solvothermal reaction, resulting in the different overall sizes of the resulting CDs and thus dimensions of conjugated sp2-domains, determining their different emission colors (Fig. 1.2) [12]. Fan et al.
FIG. 1.2 Left: a possible growth mechanism for CDot-water, CDot-glycerol, and CDot-DMF. Right: photographs of the CDs solutions synthesized a three-solvent mixture (samples A, B, C, D, E, F, G) under daylight (top) and UV light (bottom) [12].
2. Mechanism of CDs
reported bright multicolor bandgap fluorescent CDs from blue to red with a QY up to 75% for blue fluorescence. The as-prepared CDs are nitrogen doped, highly surfacepassivated, and have a high degree of crystallinity [13]. Sun et al. found that emission of the CDs can be tuned by controlling the reaction conditions such as increasing the ratios of CA/urea and reaction temperature. The emissions are shifted from blue to red in the case of increasing effective conjugation length and the amount of surface functional groups, such as eCOOH [14]. Recently, enlarging the sp2-domain together with relative doping or surface engineering promised CDs with efficient orange and red emissions with the highest PL QY up to 46% and 53% [7,15]. These tunable and efficient bandgap transitions endow CDs to be promising gain medium to achieve lasing.
2. Mechanism of CDs Spontaneous emission is a transition from excited state to ground state without external influence, which was widely used to understanding photoluminescence mechanism in CDs. Fluorescent CDs were demonstrating great potential applications in bio-imaging [16e20] and light emitting diodes [12,21]. Recently, CDs shows another application in lasing materials [22e25]. Stimulated emission is a transition from excited state to ground state with association of an external electromagnetic filed, which produces an additional photon with same direction, phase and polarization as the incident photon. Stimulated emission can be modeled mathematically by Einstein relationship. Ultrafast pump-probe measurement, such as femtosecond time-resolved transient absorption (TA) spectroscopy, is a fundamental research for stimulated emission dynamics in CDs. After pump light excitation, electrons in ground states are pumped into excited states resulting in the ground state bleaching (GSB) feature in TA spectrum, due to the Pauli exclusion principle. There are three possible depopulation channels for electrons in excited states are spontaneous emission, stimulated emission (SE) and excited state absorption (ESA), respectively. In general, spontaneous emission cannot be detected in TA spectrum because of independence of probe light. Stronger stimulated emission signal in TA spectrum (Fig. 1.3) was observed in microwave synthesized CDs by L. Wang et al., due to its high PL quantum yield [26,27]. TA kinetic traces of CDs were well fitted by multi-exponential functions. The short lifetime components of 1e2 ps and long lifetime components of 4e5 ns were attributed to intrinsic state and molecule like state. The origin of green luminescence in these CDs and graphene quantum dots (GQDs) by bottom-up and top-down methods was attributed to the special edge states, which were related to functional groups with C]O. Recently, the PL of single CDs was proved from electric dipole emission center via the scanning of an azimuthally polarized laser beam (APLB) at focal region, as shown in Fig. 1.4 [28]. Created new photon by stimulated emission has the same phase, frequency, polarization and direction with incident photon. Anisotropic
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FIG. 1.3 TA spectra for (A) microwave-synthesized CDs, (B) electrochemically synthesized CDs, and (C) solvothermal synthesized GQDs at 400 nm excitation. ESA, excited-state absorption; GSB, ground-state bleaching; SE, stimulated emission. (D) Femtosecond time-resolved PL dynamics of CDs (probed at 520 nm) and GQDs (probed at 530 nm) at 400 nm excitation [27].
stimulated emission in CDs was observed by using polarization dependent femtosecond TA spectroscopy, indicating transition dipole moment in CDs [29]. The dipole-dipole interaction between CDs and polar solvent molecules induced redshift of emission peak in time resolved spectroscopy, so-called solvation relaxation. The obtained solvation relaxation times were ranged from 0.9 ps to 83.6 ps, dependent on viscosity and water content of solvents. The orientational relaxation time constants of CDs were enhanced from 49.2 ps to 793.5 ps with increasing the viscosity of solvent, as shown in Fig. 1.5. Additionally, the orientational relaxation was also dependent on the proton the proton donation capability of solvent. P. Jing et al. demonstrated that the dipole emission center in CDs is relevant to electron transition between localized state of dopant atoms (N, O) and delocalized state of sp2 C domain [29]. When population inversion in CDs is realized and the rate of stimulated emission exceeds that of absorption, then the optical amplification in CDs can be achieved [23e25].
3. Applications of carbon dots in lasing
(A)
(B)
(C)
(D)
(E)
(F)
(G)
(H)
(I)
(J)
(K)
(L)
(M)
(N)
(O)
(P)
(Q)
FIG. 1.4 (A) Single CDs excited with an azimuthally polarized laser beam. Inset shows a theoretically calculated excitation pattern for identical experimental conditions assuming a linear horizontal dipole. The orientation of the dipole is indicated by the double arrow. (B)(G) Consecutive images of the same sample area recorded by using an azimuthally polarized laser beam (scanning direction updown). Each double-lobe pattern corresponds to the same CND. The images show PL intermittency (D) and single-step photobleaching (F) of the particle. Scanning direction is top-down. (H)(Q) Defocused images of single CDs: experimental data (H)(L) and fitted patterns (M)(Q), respectively. All the patterns correspond to the emission of a single fixed dipole [28].
3. Applications of carbon dots in lasing The photoluminescence (PL) properties of Carbon-Dots show great potential in the applications of lasing. According to W. F. Zhang and colleagues at Hong Kong Polytechnic University, CDs can be synthesized by laser irradiation methods. By uniformly dispersing the CDs into N-Methylpyrrolidone (NMP), they can find some interesting results. White-light amplified spontaneous emission is found from the mixture when applying laser excitation at 266 nm. The peak wavelength of the emission spectra is found to be around 450 nm with linewidth at about 120 nm. The NMP can advance the emission efficiency of CDs over the broad spectrum, for the excitation energy is captured by the NMP and then transferred to the CDs, not absorbed by the CDs. Comparing to not using the NMP, optical gain per peak power of the mixture at around 450 nm is 39% higher, which is around 64 cm1 MW1 [30]. Later, they also observed lasing from CDs, which were dispersed into a layer of poly(ethylene glycol) coated on the surface of optical fibers under 266 nm optical excitation. This is due to the enhancement of photoluminescence intensity via the esterification of carboxylic groups of the CDs, and the formation of high-Q cylindrical microcavities to support second-type whispering gallery modes, where Q-factor is found to be >1.3 103 (Fig. 1.6) [23].
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FIG. 1.5 Top viewed TA spectra of CDs in water, when polarizations of pump and probe were set to (A) parallel and (B) perpendicular. (C) The difference spectrum between parallel and perpendicular TA spectra. (D) TA kinetic traces of CDs probed at 550 nm. (Black, red and green lines represent the parallel, perpendicular and magic angle polarizations. Blue line is the calculated anisotropy as a function of delay time). (E) Time-dependent polarization anisotropy of CDs in different solvents. (F) Reorientation times for CDs versus Viscosity of solvents [29].
FIG. 1.6 (A) Plot of lightelight curve and FWHM of the emission spectra of the CDs deposited on a quartz substrate, and (B) The corresponding emission spectra. The inset shows the photo of CDs on quartz substrate under laser excitation [30].
3. Applications of carbon dots in lasing
Moreover, a different method for lasing emission based on carbon-fluorescent materials has been used by the same research group. As shown in the report, the luminescent capability of CDs gained form the same functionalization process was studied and compared with the luminescent capability of graphene quantum dots (GQDs). Thanks to the geometrical advantages of CDs, such as larger surface area to volume ratio and smaller volume, the optical gain of GQDs is higher when compared to CDs. Dispersing a mixture of GQDs and TiO2 nanoparticles into ethanol, lasing emission was observed under optical excitation at 266 nm [31]. Similarly, Fan et al. demonstrated blue, green, and red random lasers with low-thresholds based on the NBE-T-CDs with very small Stokes shifts (9e16 nm) and small FWHM (30 nm). The blue, green, and red random lasers show remarkably low pump thresholds of 0.087, 0.052, and 0.048 mJ cm2 with corresponding narrow FWHM of 0.9, 0.37, and 0.82 nm, respectively [32]. The tunable amplified spontaneous emission (ASE) in GQD-doped cholesteric liquid crystal (CLC) was also observed by Y. T. Zhang and colleagues in Shanghai Institute of Microsystem and Information Technology. The GQDs are uniformly dispersed in CLC with a weight ratio of 0.5 wt% in CLC. Typical ASE can be triggered in the system under optical excitation, when pump energies are greater than 1.25 mJ cm2. At the long wavelength edge of the photonic bandgap, the emission peak moves from 662 to 669 nm when the working temperature changes from 50 to 90 C. The method of producing combined GQDs and CLC is simple and low-cost, with the resultant photastable and non-toxic. It is possible to fabricate ASE source and laser devices by combining the GQD gain material with the self-assembled CLC resonator (Figs. 1.7 and 1.8) [33].
FIG. 1.7 (A) PL spectra of the cylindrical microcavity laser coated with modified CDs (for 2 h of thermal treatment) in PEG 200 under optical excitation at 266 nm. The insets show the light curve (top-right corner) and photo (top-left corner) of the cylindrical microcavity laser. (B) Near field profile of the cylindrical microcavity laser under optical excitation at 266 nm [23].
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FIG. 1.8 (A) Blue, (B) green, and (C) red random lasing emission spectra from the NBE-T-CDs/ AueAg bimetallic porous nanowire composites excited by the 355, 480, and 532 nm laser pulses with a corresponding low pump energy density of 0.125, 0.075, and 0.057 mJ cm2, respectively. The insets are photographs of the blue, green, and red random lasing emission, respectively. (DeF) Plots of the blue (D), green (E), and red (F) integrated rando lasing emission intensity and FWHM of the narrow spikes in the random lasing emission spectra as a function of the pump energy density [32].
CDs fabricated by small molecules are utilized in generating laser beams as by up-bottom method. S. N. Qu and colleagues, who work for Changchun Institute of Optics Fine Mechanics and Physics Chinese Academy of Sciences, prepared high photoluminescent CDs though citric acid and urea. It is verified that the optical properties of CDs can be modulated by the dopant-N atom and sp2 C-contents. Blue emission was found in CDs prepared with the low urea mass ratio of 0.2:1 (CDs1), and the maximum PL quantum yield is 15%. Increasing sp2 C- and dopant-N atom contents, as determined in CDs prepared with high urea mass ratio of 2:1 (CDs2), lead to green emission (maximum PL quantum yield up to 36% in ethanol aqueous solution). Amplified spontaneous emission (ASE) can be found only in CDs2 ethanol aqueous solution. Green lasing emission can be observed from CDs2 ethanol aqueous solution in a linear long Fabry-Perot cavity, showing the possibility for using CDs2 as a gain medium for lasing. Comparing to C545T dye, CDs2 shows greater photostability As shown in the reports, the green emission from CDs2 is speculated to arise from electron-hole reconstructed(intrinsic state emission). Two main reasons of lasing emission are the high PL quantum yield and small overlap between absorption and emissions of CDs2 ethanol aqueous solution [24]. Besides CDs solution, the composites based on CDs have been found to be a novel lasing materials. By utilizing the CDs prepared by citric acid and urea as
3. Applications of carbon dots in lasing
mentioned above, H. Z. Liu and colleagues combined the as-prepared CDs into NaCl matrix in a simple method. The embedded CDs supplies luminescence centers to NaCl, so the hybrid crystals present the fluorescence centered at 510 nm under the illumination of 365 nm light. Meanwhile, the phosphorescence can last for about 314 ms after the 365 nm light was turned off. Furthermore, optical gain and lasing phenomenon has been observed from hybrid crystals. A weak spontaneous emission can be observed from the hybrid crystal when a low pump power is supplied, whereas 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 gives the whispering gallery mode (WGM) resonant cavity for lasing emission. That provides a new approach for achieving lasing materials (Figs. 1.9 and 1.10) [25].
FIG. 1.9 (A) Schematic diagram of experimental setup for optical pumping investigations of the CNP-based laser device (the black double arrows illustrate polarizations of the pumping laser and output laser). Photos of the operating CNP-based laser device under 355 nm laser pumping at (B) 30 kW cm2 and (C) 190 kW cm2 [24].
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FIG. 1.10 (A) 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) PL spectrum of an individual hybrid crystal under different pump powers, the inset shows the photograph of an excited hybrid crystal. (D) Relationship between the integrated emission intensity and the pump power of the hybrid crystals [25].
With the development of CDs-based laser, the corresponding possible mechanism has been proposed with the advance of CDs-based laser. Instead of high FLQYs, excitation wavelength-independent (lex-independent) photoluminescence characteristics are utilized to realize CDs-based light amplification, as reported by Y. S. Hu and colleagues. CDs with excitation wavelength-dependent (lex-dependent) PL characteristics and FLQYs as high as 99% and 96% were found not to exhibit amplified spontaneous emission (ASE), while those with lex-independent PL characteristics and FLQYs of only 38% and 82% realized ASE with low thresholds. The difficulty of achieving ASE using CDs with lex-dependent PL characteristics may lead to their high contents of CeOeH or CeOeC groups. These groups can attribute numerous localized electronic states within the np* gap, then decentralize the excited electrons, thus increase the difficulty of population inversion. In addition, the radiative transition rates and stimulated emission cross sections of CDs with lex-independent PL characteristics were found to be magnificantly higher than those of CDs with lex-dependent PL characteristics. As a practical structure for solid-state lasing devices, ASE in a planar waveguide structure was also demonstrated for the first time using CDs with lex-independent PL characteristics. These results
3. Applications of carbon dots in lasing
FIG. 1.11 ASE characteristics of CD1/PI film. (A) Normalized emission spectra of CD1/PI film with different pumping fluences. (B) Dependence of the output peak intensity and FWHM of CD1/PI on the pumping fluence. The inset in (B) depicts the operating device pumped at 355 nm for the CD1/PI film [34].
give us simple and effective guidelines for synthesizing and selecting CDs for low threshold lasing devices (Fig. 1.11) [34]. W. F. Zhang and colleagues at Shenzhen University, China, found the efficiency of multiphoton absorption was facilitated when the CDs were covalently coupled to organosilane chains. As a result, a large absorption coefficient of 1.16 106 cm5 per GW3 is obtained and four-photon luminescence under 1900 nm excitation is observed from the CDs at room temperature. Furthermore, by sandwiching a CDs film between a quartz substrate and a dielectric mirror, random lasing under three-photon (i.e. 1400 nm) excitation can be realized in a CDs laser. The formation of strongly confined microcavities, which arise from the non-uniform distribution of
FIG. 1.12 (A) Variations of PLQY (open black squares), and (B) PL intensity (open red circles), of the CDs@silica composites versus the concentration of CDs ethanol solution. Solid lines are solely provided as a guide to the eye. (B) Photograph of the xerogel-PDMS composites excited by an ultraviolet lamp. (C) Emission spectrum of a down-conversion white LED prototype entirely based on a powdered CDs@silica composite xerogel with a photograph of the working device (inset, panel (D)) [37].
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refractive indices inside the CDs film, is attributed to the realization of lasing emission [35]. For a young and emerging field, CDs (and their close relatives graphene quantum dots) have already made a significant impact in diverse scientific disciplines. Carbon-Dots have opened new avenues in photonics, electro-optics, and lasing fields. In particular, the unique luminescence properties of CDs, primarily the excitation-dependent emission phenomena, have been a prominent thread in this
FIG. 1.13 (A) The device structure comprising ITO/PEDOT:PSS (anode), MCBF-CDs (active emission layer), TPBi (ETL), and Ca/Al (cathode). The normalized PL spectra and the corresponding output EL spectra of (B) B-BF- CDs, (C) G-BF- CDs, (D) Y-BF- CDs, (E) OBF- CDs, and (F) R-BF- CDs thin films. The photographs in the insets of (BeF) display the close-up view of the surface emission from blue, green, yellow, orange, and red emission of monochrome LEDs [13].
References
field, making possible diverse experiments and analytical applications developed in the past several years. The optical and spectroscopic properties of Carbon-Dots will likely continue to shape the future development of these fields. Especially, the attractive chemical and photophysical properties of CDs have also provided impetus to research efforts aimed at creating mesoscale assemblies comprising CDs as luminescent elements for much more abundant materials and applications. Such materials also contain other chemical constituents having complementary roles, together producing multifunctional composite materials (Figs. 1.12 and 1.13). This field of research has been aided by the chemical stability and resilience of CDs to varied physical and chemical treatments. For instance, S. N. Qu and colleagues at Changchun Institute of Optics Fine Mechanics and Physics Chinese Academy of Sciences fabricated CDs@BaSO4 hybrid phosphors in an easy and low-cost process by sequentially assembling Ba2þ and SO4 2 ions onto the surface of CDs through electrostatic attraction, which exhibit excellent thermal and photostability [36]. They also prepared CDs@silica composites with high luminescence [37]. These as-prepared composites show great potential in down-conversion LEDs. Besides these achievements, L. Z. Fan and colleagues at Beijing Normal University successfully applied CDs in electroluminescent LEDs [13]. As presented in numerous publications, CDs’ luminescence has attracted much interests in technological development, holding the promise of CDs-based lasing equipment.
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