Open Access
Add to BibSonomy
Abstract
C-dot-based composites with phosphorescence have been widely reported due to their attractive potential in various applications. But easy quenching of phosphorescence induced by oxygen or instability of matrices remained a tricky problem. Herein, we reported a Si-doped-CD (Si-CD)-based RTP materials with long lifetime by embedding Si-CDs in sulfate crystalline matrices. The resultant Si-CD@sulfate composites exhibited a long lifetime up to 1.07 s, and outstanding stability under various ambient conditions. The intriguing RTP phenomenon was attributed to the C = O bond and the doping of Si element due to the fact that sulfates could effectively stabilize the triplet states of Si-CDs, thus enabling the intersystem crossing (ISC). Meanwhile, we confirmed that the ISC process and phosphorescence emission could be effectively regulated based on the heavy atom effect. This research introduced a new perspective to develop materials with regulated RTP performance and high stability.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Phosphorescent materials with long-lived excited states, emitting longer incident wavelength than luminescence after the removal of the excitation source, are essential for a variety of optical applications such as photoelectricity [1–4], anti-counterfeiting [5–7], and information encryption [8–11]. Initially, at first, the afterglow phosphors attracted extensive attention are typically limited to metal-containing organometallic complexes [12–15]. Generally, most of the heavy-metals are iridium, platinum, ruthenium, etc., which can emit room-temperature phosphorescence (RTP) by enhancing the transition of singlet-to-triplet states but toxicity and high cost impede the practical applications [16]. The metal-free RTP organic compounds dominated by special organic moieties have also been reported, such as aromatic aldehyde, heavy halogen, nitrogen heterocycles, and deuterated carbon [17–19], but complexity of their synthesis process and poor stability of the triplet excited states limited their applications [20]. There is an imperative need for RTP materials with environment-friendliness, low cost, and environmental stability for achieving the purpose of applications under ambient conditions.
As an emerging zero-dimensional luminescent nanomaterials, carbon dots (CDs) have received much attention due to their superior optical properties, low toxicity and outstanding stability [21–24]. Recently, its phosphorescence was also achieved by populating the excited state (S1) to triplet state (T1) through the intersystem crossing induced by deduction of energy difference between S1 and T1 that originates from the enhancement of spin-orbit coupling when CDs were incorporated into matrices [25,26]. The CD-based RTP material of poly(vinyl alcohol) (PVA) embedded with fluorescent CDs was firstly discovered in 2013, and its phosphorescent lifetime was up to 380 ms [27]. Subsequently, based on the structural confinements, the matrices such as KAl(SO4)2.xH2O and polyurethane (PU) were successfully applied to produce RTP materials combined with CDs [28,29]. However, the above CD-based RTP materials showed phosphorescence lifetime of persistent emission bellow 1 s [20]. That the origination of phosphorescence was carbonyl functional groups on the CDs surface decided that their transition from triplet to the ground state was easily quenched by the non-radiative process. There was a cognition that the single matrix hardly offered strong rigidity, dense hydrogen bonding sites, and especially good oxygen barrier performance [20,30,31]. Inspired by this analyzed results, two-component matrices such as 2-naphthalenesulfonate added the sugars and salts, cyclodextrin/salt, have been demonstrated improvement of phosphorescence stability and even enhancement of phosphorescence lifetime over 1s [32,33].
Even though significant progress in stabilizing the triplet excited states of CDs has been achieved, there still existed easy quenching of phosphorescence induced by decomposition of matrices or destruction of ligands on the CDs surface [34,35], which impeded the practical applications especially outdoors. Therefore, to solve the increasingly severe problem of counterfeiting that challenges communal facilities, company property, and personal property around the world, it is highly desirable to develop novel phosphorescent materials with long lifetime and chemical and phosphorescent stability in various application conditions.
2. Experimental section
2.1 Synthesis of Si-CDs
This is the synthesis of the Si doped CDs (Si-CDs) according to our previous report [36]. In detail, silane (1.5 mL), ethanediamine (1.6 mL) and CA-Na (1.2 g) were added into deionized water (28 mL). Then, after the mixed solution was stirred for 30 min and transferred into a polytetrafluoroethylene hydrothermal reactor (40 mL) and heated at 200 °C for 12 h, the reaction solution was cooled to room temperature naturally, and filtered (using water phase needle filter with hole diameter of 0.22 µm), dialyzed for 12 h (using dialysis bag with a molecular weight of 1000), and suspension steamed at 75 °C under vacuum to obtain the Si-CDs.
2.2 Preparation of Si-CD@Ca/Sr/BaSO4
Si-CD@Ca/Sr/BaSO4 with mass fraction of appropriately 1-5% Si-CDs at interval of 1% was prepared by co-precipitation method. Firstly, Si-CDs (5, 10, 15, 20, 25 ml, respectively) aqueous solution (total 30 mL) and 3.4 g of Anhydrous calcium chloride was stirred for 10 min at room temperature, and then the solution with 4 g of Ammonium sulfate was slowly dropped within 10 min and continued to be stirred for 2 h. Finally, the solid-state Si-CD@CaSO4 was separated by suction filtration and washed with deionized water and ethanol several times, and dried at 60 °C for 3 h. According to the same procedure as Si-CD@CaSO4, the other Si-CD@SrSO4 and Si-CD@BaSO4 powder with various concentration of Si-CDs was prepared, respectively.
2.3 Characterization methods
High-resolution transmission electron microscope (HRTEM) was measured in JEOL-2010electron microscope. Fourier transform-infrared spectroscopy (FT-IR) was recorded by Nicolet Avatar 360 FT-IR spectrophotometer. X-ray photoelectron spectroscopy (XPS) was obtained using AXIS ULTRA DLD, Kratos. UV-Vis absorption spectrums were collected in a JASCO V-570 spectrophotometer. Photoluminescence spectra afterglow emission spectra and afterglow decay curves was characterized by an F-7000 Hitachi fluorescence spectrofluorometer. phosphorescence quantum yield (PQY) was collected in a Edinburgh FLS920 spectrophotometer equipped with an integrating sphere and a flash lamp. Temperature-dependent phosphorescence emission spectra, afterglow emission spectra, and afterglow decay curves were recorded by combining a heating apparatus (Oxford Instruments) with the same Hitachi F-7000 fluorescence spectrophotometer. Powder X-ray diffraction (XRD) was conducted by a persee XD-2X/M4600. Scanning electron microscope (SEM) was measured in a XL-30-ESEM (FEI). Transmission electron microscopy (TEM) images were obtained using a TECNAI12, Holland.
3. Results and discussion
Herein, we observed RTP of the Si-CDs based phosphorescent materials that incorporate N-rich Si-CDs into inorganic matrices by a electrostatic assembly method. We studied the origins of induced phosphorescence and proved that C = O bond of Si-doped CDs and heavy atoms effect were responsible for the phosphorescence in view of both phosphorescence intensities and lifetime decay profiles of Si-CD@ (Ca/Sr/Ba)SO4. To analyze comparatively, the Si-CDs with C = O bond (T-Si-CDs) were prepared by hydrothermal treatment of 1-[3-(trimethoxysilyl) propyl]urea at 200 °C for 12 h, the other Si-CDs (D-Si-CDs) without C = O bond were also prepared according to the synthesis procedure of T-Si-CDs except for the replacement of N-[3-(trimethoxysilyl)propyl]ethylenediamine. As shown in Fig. 1, SrSO4 assembled in the T-Si-CDs that were prepared using the silane (1-[3-(trimethoxysilyl) propyl]urea) with C = O presented strong phosphorescent emission, and the electrostatic assembly method guaranteed Si-CDs could be homogeneously dispersed into SrSO4 matrices which was beneficial to prevent quenching of phosphorescence induced by ambient conditions.
Fig. 1. Schematic illustration for the synthesis of two typical Si-CDs, and formation procedure and emissions of their composite powders.
As shown in Figs. 2(a) and 2(b), the HRTEM images showed the average size of 4.9 nm for T-Si-CDs and 5.2 nm for D-Si-CDs that had the same spacing of 0.23 nm agreed with the basal spacing of graphite. The high-resolution spectra of C 1s in Fig. 2(c) showed three peaks at 284.6, 285.3 and 288.0 eV for C-C, C-N, and C = O, respectively [36]. Obviously, there presented a higher C = O/C = C peak intensity ratio of T-Si-CDs than D-Si-CDs, which indicated the existence of C = O bonds on the surface of T-Si-CDs while the weak peak of C = O from D-Si-CDs should be assigned to the sodium citrate source. Moreover, the FT-IR spectra further demonstrated the difference between T-Si-CDs and D-Si-CDs in the stretching vibration peak at 1640 cm−1 corresponding to C = O bond [Fig. 2(d)] [37]. The above synthesized Si-CDs showed strong blue light emission with centered wavelength at appropriately 445 nm, but none of them had the phosphorescence emission. Based on the reported RTP of CDs that were proved to be the roots of C = N/C = O bonds, to achieve the phosphorescence of T-Si-CDs, we have assembled them into SrSO4 matrices.
Fig. 2. (a), (b) HRTEM images (inset: Lattice fringe spacing) of T-Si-CDs and D-Si-CDs, respectively. (c), (d) XPS and FT-IR patterns of T-Si-CDs and D-Si-CDs, respectively.
Interestingly, in the SEM images [Figs. 3(a) and 3(b)], there presented the difference in size and morphology between bare SrSO4 and SrSO4@Si-CDs, which was attributed to the presence of Si-CDs that acted as nucleus and surfactant in the electrostatic assembly process, leading to a more smooth quasi sphere and smaller size of SrSO4@Si-CDs. As shown in Fig. 3(c), the Si and N elements belonging to Si-CDs were observed which indicated that Si-CDs were successfully assembled into the matrices of SrSO4. Moreover, there was a XRD characteristic diffraction peak of Si-CDs at appropriately 23.5° [Fig. 3(d)] and C = O bond from FT-IR pattern in SrSO4@Si-CDs [Fig. 3(e)], which further confirmed the existence of Si-CDs in crystalline SrSO4 powder. Taken together, Si-CDs with C = O bonds were assembled into SrSO4 matrices by electrostatic approach, which enhanced the probability of the intersystem crossing and radiative pathway between singlet and triplet states that induced the occurrence of RTP.
Fig. 3. (a), (b) SEM images of SrSO4 and T-Si-CD@SrSO4 powders, respectively. (c) EDS mapping images of T-Si-CD@SrSO4 powder. (d), (e) XRD and FT-IR patterns of T-Si-CDs, SrSO4 and T-Si-CD@SrSO4, respectively.
Under the excitation of 365 nm, both T-Si-CD@SrSO4 and D-Si-CD@SrSO4 powders presented bright blue fluorescence emission with peak at appropriately 450 nm and non-excitation dependence, while only T-Si-CD@SrSO4 showed the significant green afterglow luminescence lasting appropriately up to 10 s after UV light turned off. However, this phenomenon was not observed for D-Si-CD@SrSO4 [Fig. 4(a)]. Except for the fluorescence of T/D-Si-CDs and solid-state fluorescence, as shown in Fig. 4(b), T-Si-CD@SrSO4 exhibited RTP with emission wavelength at 518 nm with 8.34% phosphorescence quantum yield (PLQY) which was much more intense than D-Si-CD@SrSO4. In addition, as shown in Fig. 4(c), the phosphorescence emission lifetime of T-Si-CD@SrSO4 reached up to 1.04 s long, leading to more application fields than phosphorescent materials with lower phosphorescence lifetime.
Fig. 4. (a) Digital photos of T-Si-CD@SrSO4 and D-Si-CD@SrSO4 under turn-on and turn-off of 365 nm UV lamp. (b) Phosphorescence spectra of T-Si-CD@SrSO4 and D-Si-CD@SrSO4 at excitation of 350 nm. (c) Phosphorescence decay curve of T-Si-CD@SrSO4. (d) UV-Vis absorption and RTP excitation spectra of T-Si-CD@SrSO4. (e) Temperature-dependent phosphorescence spectrum of T-Si-CD@SrSO4.
To confirm whether the origins of phosphorescence emission was from C = O bond, we characterized the UV absorption spectrum and the phosphorescence excitation spectrum of T-Si-CD@SrSO4. As shown in Fig. 4(d), there presented the identical in maximum peak at appropriately 350 nm between absorption spectrum and phosphorescence excitation spectrum, which could be attributed to the n-π* transition of the C = O group on the surface of the Si-CDs indicating the origins of the phosphorescence emission [38–41]. Furthermore, we prepared various CDs using carbon sources such as benzylurea and urea with C = O bonds and N-[3- (trimethoxysilyl)propyl]ethylenediamine without C = O bonds, and found that no phosphorescence phenomenon appeared when these CDs were assembled into sulfates, which demonstrated that the phosphorescence of Si-CDs was attributed to C = O bonds of Si-CDs and the doping of Si element in CDs for the reason that sulfates could effectively stabilize the triplet states of Si-CDs. Moreover, as shown in Fig. 4(e) the temperature-dependent phosphorescence emission spectra which exhibited the afterglow luminescence intensity of the T-Si-CD@SrSO4 gradually decreased as undertreated temperature enhanced, which demonstrated that the afterglow luminescence phenomenon belonged to phosphorescence emission instead of delayed fluorescence.
We further investigated stabilities of T-Si-CD@SrSO4 in order to achieve its practical application. As shown in Fig. 5(a), when the calcination temperature increased from 25°C to 375°C, the phosphorescence intensity of T-Si-CD@SrSO4 powder kept basically unchanged, even though it decreased dramatically while the temperature was above 375°C due to the dehydration and carbonization of the surface groups of T-Si-CDs, indicating outstanding thermal stability of phosphorescence emission for T-Si-CD@SrSO4. Since oxygen is a good quencher for phosphorescence [27,42,43], to study the phosphorescence stability of T-Si-CD@SrSO4 under oxygen atmosphere, we characterized its phosphorescence intensity under nitrogen and oxygen atmosphere, respectively. As shown in Fig. 5(b), the phosphorescence intensity of T-Si-CD@SrSO4 declined by less than 6.5% while only oxygen flow was introduced into the composites, which proved that the matrix of SrSO4 could effectively protect RTP from quenching affected by oxygen. Compared with the conventional phosphorescent dyes that were easily quenched by oxygen, it showed better oxygen-barred property than many carbon dots based composites [27–29,44]. According to the reported researches on QDs, photo especially UV radiation with high light energy could destroy the ligands on QDs surface causing fluorescence and phosphorescence quenching. However, as shown in Fig. 5(c), after the continuous radiation of 365 nm UV for 200 h, T-Si-CD@SrSO4 showed almost no decline of phosphorescence emission, which proved its excellent photostability attributed to the scattering and absorbing of SrSO4 matrix weakening the effect of UV radiation on fluorophores. Additionally, to impede phosphorescence quenching by outdoor environment, phosphorescent materials which requires exceedingly excellent acid and alkali resistance should be applied. Based on the background, we have studied phosphorescence emission under varying pH values shown in Fig. 5(d). When the pH value was above 7 or below 6, the phosphorescence intensity showed a tendency of slight decrease due to partial solubility of SrSO4 surface and the permeation of moisture, but the property of phosphorescence intensity still maintained high. Thus, T-Si-CD@SrSO4 had outstanding stability under high temperature, oxygen atmosphere, strong acid and alkali, and UV radiation, which indicated that T-Si-CD@SrSO4 was suitable for most of practical applications especially outdoors.
Fig. 5. (a) Phosphorescence intensity of T-Si-CD@SrSO4 after calcination at different temperatures for 1 h. (b) Nitrogen-oxygen responsive curves of phosphorescence of T-Si-CD@SrSO4. (c), (d) Phosphorescence intensities of T-Si-CD@SrSO4 under 365 nm UV radiation and various pH values, respectively.
To understand the optical process of RTP, we proposed the mechanism diagram as shown in Figs. 6(a) and 6(b). The energy gap of T-Si-CDs was 0.49 eV between S1 and T1 that was conductive to spin-orbit coupling and efficiency of ISC, indicating the occurrence of RTP. However, the phosphorescence emission tended to be completely quenched by oxygen and particle collision in water [Fig. 6(a)]. While under the protection of (Ca/Sr/Ba)SO4 matrices, the energy loss caused by intramolecular vibration and the effects resulting in the quenching mentioned above significantly decreased, leading to RTP due to the transitions from T1 to S0 as shown in Fig. 6(b). Moreover, we found that the RTP was related to the heavy atom effect (HAE) that a moderate HAE caused maximum phosphorescence emission, while excessive HAE would shorten phosphorescence lifetime and reduce phosphorescence intensity [45,46]. In this case, compared with T-Si-CD@CaSO4 and T-Si-CD@BaSO4, there was easier transition of S1-T1 by ISC for T-Si-CD@SrSO4, thus accelerating the ISC and enhancing its phosphorescence emission. To confirm this analysis, we characterized the phosphorescence spectra of three sulfates embedded with T-Si-CDs. Apparently, as shown in Fig. 6(c), T-Si-CDs assembled into SrSO4 matrix presented higher phosphorescence intensity than smaller atomic number of CaSO4 and bigger atomic number of BaSO4 matrices. In addition, the HAE generally enhanced as the atomic weight increased, which resulted in decrease in phosphorescence lifetime [47]. As shown in Fig. 6(d), the phosphorescence decay curves showed the phosphorescence lifetime of 1.07 s, 1.04 s and 0.82 s corresponding to Ca, Sr and Ba elements, respectively, which further demonstrated the effect of HAE on phosphorescence lifetime. Taken together, T-Si-CD-based sulfate composites presented the RTP originated from the C = O bonds and its restriction of the vibrational motions due to the reinforcing rigidity of matrices, and also exhibited the phosphorescence lifetime and phosphorescence intensity that was highly matched with HAE.
Fig. 6. (a), (b) Jablonski diagram of RTP and fluorescence of T-Si-CDs and T-Si-CD@SrSO4, respectively. (c), (d) Phosphorescence spectra and corresponding decay curves of T-Si-CD@MSO4(M = Ca, Sr, Ba) powders.
The advantages of T-Si-CD@SrSO4 long phosphorescence materials presented various attractive applications. Here, we have demonstrated a promising application of T-Si-CD@SrSO4 as a dual-mode functional composite for use in the encryption of information and anti-counterfeiting. As shown in Fig. 7(a), upon irradiation with a 365 nm UV lamp, the capital letters of “S”, “R”, “T”, “U”, “A”, “N”, Y” coated T-Si-CD@SrSO4 and D-Si-CD@SrSO4 could emit bright blue light, whereas the letters of “S”, “T”, “A”, “Y” emitted bright green light that kept nearly 10 s after turn-off of UV lamp, which indicated the potential application in information encryption. Moreover, as shown in Fig. 7(b), we mixed the composite powder with glycerol and drew a flower on the filter paper, similarly, wherein the petals coated with T-Si-CD@SrSO4 emitted blue and green light under turn-on and turn-off of UV lamp, and the leaves and branch coated with D-Si-CD@SrSO4 only emitted blue-fluorescence, which indicated the perspective of anti-counterfeiting. Importantly, the series of sulfate-based phosphorescence materials possessed outstanding stabilities so that they can be applied as main constitute of coatings, indicating its practical applications under various outdoor conditions that most of the reported phosphorescence materials don’t have.
Fig. 7. (a) Digital photos of information encryption made from T-Si-CD@SrSO4 (covered the letters: “S” “T” “A” “Y”) and D-Si-CD@SrSO4 (covered the letters: “R” “U” “N”). (b) Digital photos of anti-counterfeiting made from T-Si-CD@SrSO4 (covered the petals) and D-Si-CD@SrSO4 (covered the leaves and branch).
4. Conclusion
In summary, we have reported a Si-CD-based RTP materials with long lifetimes by embedding Si-CDs in sulfate crystalline matrices. The as-prepared Si-CD@sulfate composites displayed long lifetime up to 1.07 s, and exhibited outstanding stability in the case of high temperature, oxygen atmosphere, strong acid and alkali, and UV radiation because sulfate host matrices possessed outstanding physic-chemical stability and could effectively stabilize the triplet state by suppressing the non-radiative processes and hindering the oxygen and moisture quenching, which affirmed their potential applications in security protection especially outdoors. In addition, we have demonstrated that the RTP of Si-CD-based materials was attributed to C = O bonds of Si-CDs and the doping of Si element in CDs, and confirmed that HAE could effectively regulate the ISC from T1 to S1 and the phosphorescence lifetime and intensity. The discovery of a new class of Si-CD-based RTP materials with long lifetimes and high stability will promote both fundamental understanding and practical applications of RTP materials.
Funding
National Natural Science Foundation of China (21571067, 21671070).
Disclosures
The authors declare no competing financial interests.
References
1. L. L. Da Luz, R. Milani, J. F. Felix, I. R. B. Ribeiro, M. Talhavini, B. A. D. Neto, J. Chojnacki, M. O. Rodrigues, and S. A. Júnior, “Inkjet printing of lanthanide–organic frameworks for anti-counterfeiting applications,” ACS Appl. Mater. Interfaces 7(49), 27115–27123 (2015). [CrossRef]
2. N. M. Sangeetha, P. Moutet, D. Lagarde, G. Sallen, B. Urbaszek, X. Marie, G. Viau, and L. Ressier, “3D assembly of upconverting NaYF4 nanocrystals by AFM nanoxerography: creation of anti-counterfeiting microtags,” Nanoscale 5(20), 9587–9592 (2013). [CrossRef]
3. C. Yan, R. S. Hegde, I. Y. Phang, H. K. Lee, and Y. L. Xing, “Encoding molecular information in plasmonic nanostructures for anti-counterfeiting applications,” Nanoscale 6(1), 282–288 (2014). [CrossRef]
4. B. Yoon, J. Lee, I. S. Park, S. Jeon, J. Lee, and J. Kim, “Recent functional material based approaches to prevent and detect counterfeiting,” J. Mater. Chem. C 1(13), 2388–2403 (2013). [CrossRef]
5. A. G. J. Williams, “Photochemistry and Photophysics of Coordination Compounds: Platinum,” Top. Curr. Chem. 281, 205–268 (2007). [CrossRef]
6. C. Adachi, M. A. Baldo, M. E. Thompson, and S. R. Forrest, “Nearly 100% internal phosphorescence efficiency in an organic light-emitting device,” J. Appl. Phys. 90(10), 5048–5051 (2001). [CrossRef]
7. Z. Song, T. Lin, L. Lin, S. Lin, F. Fu, X. Wang, and L. Guo, “Invisible security ink based on water-soluble graphitic carbon nitride quantum dots,” Angew. Chem., Int. Ed. 55(8), 2773–2777 (2016). [CrossRef]
8. Q. Li, M. Zhou, M. Yang, Q. Yang, Z. Zhang, and J. Shi, “Induction of long-lived room temperature phosphorescence of carbon dots by water in hydrogen-bonded matrices,” Nat. Commun. 9(1), 734–738 (2018). [CrossRef]
9. P. Burner, A. D. Sontakke, M. Salaun, M. Bardet, J. M. Mouesca, S. Gambarelli, A. L. Barra, A. Ferrier, B. Viana, A. Ibanez, V. Maurel, and I. Gautier-Luneau, “Evidence of organic luminescent centers in sol–gel-synthesized yttrium aluminum borate matrix leading to bright visible emission,” Angew. Chem., Int. Ed. 56(45), 13995–13998 (2017). [CrossRef]
10. J. Liu, N. Wang, Y. Yu, Y. Yan, H. Zhang, J. Li, and J. Yu, “Carbon dots in zeolites: A new class of thermally activated delayed fluorescence materials with ultralong lifetimes,” Sci. Adv. 3(5), e1603171 (2017). [CrossRef]
11. Z. Tian, D. Li, E. V. Ushakova, V. G. Maslov, D. Zhou, P. T. Jing, D. Z. Shen, S. N. Qu, and A. L. Roach, “Multilevel data encryption using thermal-treatment controlled room temperature phosphorescence of carbon dot/polyvinylalcohol composites,” Adv. Mater. 5(9), 1800795 (2018). [CrossRef]
12. C. A. Mitchell, R. W. Gurney, S. H. Jang, and B. Kahr, “On the mechanism of matrix-assisted room temperature phosphorescence,” J. Am. Chem. Soc. 120(37), 9726–9727 (1998). [CrossRef]
13. Z. Q. Chen, Z. Q. Bian, and C. H. Huang, “Functional IrIII complexes and their applications,” Adv. Mater. 22(13), 1534–1539 (2010). [CrossRef]
14. C. Ulbricht, B. Beyer, C. Friebe, and A. Winter, “Recent developments in the application of phosphorescent Iridium(III) complex systems,” Adv. Mater. 21(44), 4418–4441 (2009). [CrossRef]
15. J. Mei, N. L. C. Leung, R. T. K. Kwok, J. W. Y. Lam, and B. Z. Tang, “Aggregation-induced emission: together we shine, united we soar,” Chem. Rev. 115(21), 11718–11940 (2015). [CrossRef]
16. S. Y. Tao, S. Y. Lu, Y. J. Geng, S. J. Zhu, S. A. T. Redfern, Y. B. Song, T. L. Feng, W. Q. Xu, and B. Yang, “Design of metal-free polymer carbon dots: A new class of room-temperature phosphorescent materials,” Angew. Chem., Int. Ed. 57(9), 2393–2398 (2018). [CrossRef]
17. D. Chaudhuri, E. Sigmund, A. Meyer, L. Rock, P. Klemm, S. Lautenschlager, A. Schmid, S. R. Yost, T. Van Voorhis, and S. Bange, “Metal-free oLED triplet emitters by side-stepping kasha’s rule,” Angew. Chem., Int. Ed. 52(50), 13449–13452 (2013). [CrossRef]
18. Z. An, C. Zheng, Y. Tao, R. Chen, H. Shi, T. Chen, Z. Wang, H. Li, R. Deng, X. Liu, and W. Huang, “Stabilizing triplet excited states for ultralong organic phosphorescence,” Nat. Mater. 14(7), 685–690 (2015). [CrossRef]
19. P. Avouris, W. M. Gelbart, and M. A. El-Sayed, “Nonradiative electronic relaxation under collision-free conditions,” Chem. Rev. 77(6), 793–833 (1977). [CrossRef]
20. Q. J. Li, M. Zhou, Q. F. Yang, Q. Wu, J. Shi, A. H. Gong, and M. Y. Yang, “Efficient room-Temperature Phosphorescence from Nitrogen-Doped Carbon Dots in Composite Matrices,” Chem. Mater. 28(22), 8221–8227 (2016). [CrossRef]
21. X. Y. Zhang, Y. Zhang, Y. Wang, S. Kalytchuk, S. V. Kershaw, Y. H. Wang, T. Q. Zhang, Y. Zhao, H. Z. Zhang, and T. Cui, “Color-switchable electroluminescence of carbon dot light-emitting diodes,” ACS Nano 7(12), 11234–11241 (2013). [CrossRef]
22. M. Zheng, S. Liu, J. Li, D. Qu, H. F. Zhao, X. G. Guan, X. L. Hu, Z. G. Xie, X. B. Jing, and Z. C. Sun, “Integrating oxaliplatin with highly luminescent carbon dots: an unprecedented theranostic agent for personalized medicine,” Adv. Mater. 26(21), 3554–3560 (2014). [CrossRef]
23. K. A. Fernando, S. Sahu, Y. Liu, W. K. Lewis, E. A. Guliants, A. Jafariyan, P. Wang, C. E. Bunker, and Y. P. Sun, “Carbon quantum dots and applications in photocatalytic energy conversion,” ACS Appl. Mater. Interfaces 7(16), 8363–8376 (2015). [CrossRef]
24. W. Shi, X. H. Li, and H. M. Ma, “Carbon quantum dots and applications in photocatalytic energy conversion,” Angew. Chem., Int. Ed. 51(26), 6432–6435 (2012). [CrossRef]
25. M. L. Mueller, X. Yan, J. A. McGuire, and L. S. Li, “Triplet states and electronic relaxation in photoexcited graphene quantum dots,” Nano Lett. 10(7), 2679–2682 (2010). [CrossRef]
26. R. J. Gui, H. Jin, Z. H. Wang, F. F. Zhang, J. F. Xia, M. Yang, S. Bi, and Y. Z. Xia, “Room-temperature phosphorescence logic gates developed from nucleic acid functionalized carbon dots and graphene oxide,” Nanoscale 7(18), 8289–8293 (2015). [CrossRef]
27. Y. H. Deng, D. X. Zhao, X. Chen, F. Wang, H. Song, and D. Z. Shen, “Long lifetime pure organic phosphorescence based on water soluble carbon dots,” Chem. Commun. 49(51), 5751–5753 (2013). [CrossRef]
28. X. W. Dong, L. M. Wei, Y. J. Su, Z. L. Li, H. J. Geng, C. Yang, and Y. F. Zhang, “Efficient long lifetime room temperature phosphorescence of carbon dots in a potash alum matrix,” J. Mater. Chem. C 3(12), 2798–2801 (2015). [CrossRef]
29. J. Tan, R. Zou, J. Zhang, W. Li, L. Q. Zhang, and D. M. Yue, “Large-scale synthesis of N-doped carbon quantum dots and their phosphorescence properties in a polyurethane matrix,” Nanoscale 8(8), 4742–4747 (2016). [CrossRef]
30. S. Menning, M. Kramer, B. A. Coombs, F. Rominger, A. Beeby, A. Dreuw, and U. H. F. Bunz, “Twisted tethered tolanes: Unanticipated long-lived phosphorescence at 77 K,” J. Am. Chem. Soc. 135(6), 2160–2163 (2013). [CrossRef]
31. F. Meinardi, “Superradiance in molecular H aggregates,” Phys. Rev. Lett. 91(24), 247401 (2003). [CrossRef]
32. M. D. Richmond and R. J. Hurtubise, “Analytical characteristics of beta-cyclodextrin/salt mixtures in room-temperature solid-surface luminescence analysis,” Anal. Chem. 61(23), 2643–2647 (1989). [CrossRef]
33. G. J. Niday and P. G. Seybold, “Matrix effect on the lifetime of room-temperature phosphorescence,” Anal. Chem. 50(11), 1577–1578 (1978). [CrossRef]
34. K. Jiang, L. Zhang, J. F. Lu, C. X. Xu, C. Z. Cai, and H. W. Lin, “Triple-mode emission of carbon dots: applications for advanced anti-counterfeiting,” Angew. Chem., Int. Ed. 55(25), 7231–7235 (2016). [CrossRef]
35. L. Q. Bai, N. Xue, X. R. Wang, W. Y. Shi, and C. Lu, “Activating efficient room temperature phosphorescence of carbon dots by synergism of orderly non-noble metals and dual structural confinements,” Nanoscale 9(20), 6658–6664 (2017). [CrossRef]
36. G. Q. Hu, Y. Q. Sun, Y. X. Xie, S. S. Wu, X. J. Zhang, J. L. Zhuang, C. F. Hu, B. F. Lei, and Y. L. Liu, “Synthesis of silicon quantum dots with highly efficient full-band UV absorption and their applications in antiyellowing and resistance of photodegradation,” ACS Appl. Mater. Interfaces 11(6), 6634–6643 (2019). [CrossRef]
37. I. E. Anderson, R. A. Shircliff, C. Macauley, D. K. Smith, B. G. Lee, S. Agarwal, P. Stradins, and R. T. Collins, “Silanization of low-temperature-plasma synthesized silicon quantum dots for production of a tunable, stable, colloidal solution,” J. Phys. Chem. C 116(6), 3979–3987 (2012). [CrossRef]
38. G. P. Yong, Y. M. Zhang, W. L. She, and Y. Z. Li, “Stacking-induced white-light and blue-light phosphorescence from purely organic radical materials,” J. Mater. Chem. 21(46), 18520–18522 (2011). [CrossRef]
39. X. Zhang, X. Chen, S. Kai, H. Y. Wang, J. Yang, F. G. Wu, and Z. Chen, “Highly sensitive and selective detection of dopamine using one-pot synthesized highly photoluminescent silicon nanoparticles,” Anal. Chem. 87(6), 3360–3365 (2015). [CrossRef]
40. B. Ghosh, Y. Masuda, Y. Wakayama, Y. Imanaka, J. I. Inoue, K. Hashi, K. Deguchi, H. Yamada, Y. Sakka, and S. Ohki, “Hybrid White Light Emitting Diode Based on Silicon Nanocrystals,” Adv. Funct. Mater. 24(45), 7151–7160 (2015). [CrossRef]
41. J. Tan, J. Zhang, W. Li, L. Zhang, and D. Yue, “Synthesis of amphiphilic carbon quantum dots with phosphorescence properties and their multifunctional applications,” J. Mater. Chem. C 4(42), 10146–10153 (2016). [CrossRef]
42. Y. Feng, J. Cheng, L. Zhou, X. Zhou, and H. Xiang, “Ratiometric optical oxygen sensing: a review in respect of material design,” Analyst 137(21), 4885–4901 (2012). [CrossRef]
43. G. Zhang, G. M. Palmer, M. W. Dewhirst, and C. L. Fraser, “A dual-emissive-materials design concept enables tumour hypoxia imaging,” Nat. Mater. 8(9), 747–751 (2009). [CrossRef]
44. Y. H. Chen, J. L. He, C. F. Hu, H. R. Zhang, B. F. Lei, and Y. L. Liu, “Room temperature phosphorescence from moisture-resistant and oxygen-barred carbon dot aggregates,” J. Mater. Chem. C 5(25), 6243–6250 (2017). [CrossRef]
45. T. VoDinh, Room temperature phosphorimetry for chemical analysis, 1984.
46. R. J. Hurtubise, Phosphorimetry: Theory, Instrumentation, and Applications, 2012.
47. Z. Y. Wang, Y. S. Xiao, Z. Hui, J. W. Y. Lam, T. Li, L. Ping, C. Wang, L. Yang, Z. Wang, and Z. Qiang, “Crystallization-induced phosphorescence of pure organic luminogens at room temperature,” J. Phys. Chem. C 114(13), 6090–6099 (2010). [CrossRef]
