Afterglow Luminescence in Wet-Chemically Synthesized Inorganic

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Afterglow Luminescence in Wet-Chemically Synthesized Inorganic Materials: Ultra-Long Room Temperature Phosphorescence Instead of Persistent Luminescence Atul D. Sontakke, Alban Ferrier, Pauline Burner, Vinicius Ferraz Guimarães, Mathieu Salaun, Vincent Maurel, Isabelle Gautier-Luneau, Alain Ibanez, and Bruno Viana J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b01702 • Publication Date (Web): 13 Sep 2017 Downloaded from http://pubs.acs.org on September 14, 2017

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The Journal of Physical Chemistry Letters

Afterglow Luminescence in Wet-Chemically Synthesized Inorganic Materials: Ultra-Long Room Temperature Phosphorescence Instead of Persistent Luminescence

Atul D. Sontakke,*,a Alban Ferrier,a,b Pauline Burner,c,d Vinicius F. Guimarães,c,d Mathieu Salaün,c,d Vincent Maurel,e Isabelle Gautier-Luneau,c,d Alain Ibanezc,d and Bruno Viana*,a

a

PSL Research University, Chimie ParisTech - CNRS, Institut de Recherche de Chimie Paris, 75005 Paris,

France b

Sorbonne University, UPMC University, Paris 06, 75005 Paris, France

c

CNRS, Inst NEEL, F-38042 Grenoble, France

d

Univ. Grenoble Alpes, Inst NEEL, F-38042 Grenoble, France

e

Univ. Grenoble Alpes, CEA, CNRS, INAC, SyMMES, F-38000 Grenoble, France

Keywords: phosphorescence; triplet-state stabilization; defect-related luminescence; dopantfree phosphors.

Corresponding authors *Email: [email protected] *Email: [email protected]

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ABSTRACT Wet-chemically synthesized amorphous yttrium-aluminum-borates (a-YAB) exhibit intense visible photoluminescence (PL). Preliminary investigations revealed a correlation of PL with the presence of carbon-related impurities; however their exact nature is still under investigation. These powders also exhibit afterglow luminescence that lasts for several seconds at room-temperature (RT). A comparison with persistent phosphors and phosphorescent dye revealed that the afterglow in a-YAB is a phosphorescence phenomenon and not the persistence luminescence, which is more common in inorganic solids. The unusual RT phosphorescence in a-YAB could be achieved due to triplet-state stabilization of trapped luminescent organic moieties in glassy matrix. This is indeed an important step forward in understanding the complex luminescence mechanism in such promising wet-chemically synthesized functional materials. Moreover, phosphorescence is detectable for over 10 s at RT suggesting rigid glassy inorganic matrix is more efficient in preserving phosphorescence at elevated temperatures, opening the path for several attractive applications including time-resolved bio-imaging and thermometry.

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Amorphous powders of yttrium-aluminum-borates (a-YAB) synthesized by wet-chemical methods exhibit intense photoluminescence (PL) in the visible spectral region. Its high PL quantum yield, stability and broadband profile with impressive color rendering ability make aYAB potential candidate for dopant-free solid-state lighting phosphor.1,2 The exact nature of luminescent species is yet to be fully understood, however, PL seems to exhibit some correlation with carbon presence,2 suggesting carbogenic impurities trapped during wetchemical synthesis might be responsible for luminescence properties. The interests of carbon impurities based luminescent materials were initiated after Green et al. reported white emitting phosphors obtained from silicate-carboxylate sol-gel precursor with PL QY of about 35%.3 This was followed by a series of researches in different oxides and oxynitrides obtained by various wet-chemical methods.1,4-7 Hayakawa et al. obtained 66.5% QY in Al2O3-SiO2 porous glasses,4 whereas Lin et al. showed 31% QY for bluish-white emission in BPO4/Ba obtained using sol-gel process.5 Ogi et al. reported tunable luminescent BCNO phosphors (carbon-doped borooxynitrides) with QY up to 79%.6 Amongst, the a-YAB powders obtained by the polymeric precursor method revealed highest QY of about 90%.2 Interestingly in majority of such reports, the luminescence was attributed to carbon substitutional cationic defects in crystalline or amorphous networks. Nevertheless, in certain cases it is attributed to structural defects,8 oxygen vacancies as well as carbonyl radicals4 or carbon nanoclusters, so called carbon-dots.9 In a-YAB powders, the electron paramagnetic resonance (EPR) suggested the presence of carbon-related radicals.2,10 To further understand the luminescence mechanism, it is worthwhile to explore other methods and find more correlations with known luminescent species. The PL emission and excitation (PLE), decay kinetics and afterglow luminescence can 3 ACS Paragon Plus Environment


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provide vital information characteristic to the emitting centers. In BCNO phosphors, previous reports revealed the presence of afterglow luminescence attributed to the slow release of trapped electrons to the emitting C substitutional defects, i.e. persistence luminescence.11 In aYAB powders, we also observed visible afterglow luminescence that lasts for several seconds after ceasing the excitation. Understanding the mechanism of afterglow luminescence can give important insights on the luminescent species in such dopant-free phosphors as well as unearth its additional functionalities. In this letter, we report a detailed investigation on afterglow in aYAB using a thorough comparison with standard afterglow phosphors, i.e. persistent phosphors and a typical phosphorescent dye molecule dispersed in a polymer thin film. We considered two a-YAB samples prepared using sol-gel (SG) and polymeric precursor (PP) methods, namely the SG-450 and PP-740, respectively (Supporting Information, SI, Experimental section). The samples represent optimum synthesis conditions for maximum luminescence intensities among the respective series. Fig. 1a shows the steady-state PL and PLE spectra of SG450 and PP-740 samples. SG-450 gives blue emission, whereas broad white emission is achieved in PP-740. This is due to the fact that the calcination temperature in SG-450 is 450°C but it is 740°C in PP-740, and the luminescence shape exhibits broadening with the increase in calcination temperature (SI, Fig. S1). This is due to the formation of additional luminescent species emitting at longer wavelengths with increasing calcination temperature. The PLE spectra showed distinct excitation bands for 400 nm emission monitoring in both the samples, while for longer wavelengths, such as for 550 nm emission in PP-740, the PLE revealed structure-less broad PLE profile. Similar observation has been reported in graphene oxide (GO) - reduced GO (rGO) suspensions exhibiting multiple emission centers.12 4 ACS Paragon Plus Environment

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Figure 1. Steady-state PL-PLE spectra (a); normalized afterglow (PhL) 1 s after ceasing the excitation compared to PL (b); color chromaticity of PL and afterglow emissions with corresponding digital images under Hg lamps excitation (c); PL decay d); afterglow decay (e); and the digital images of afterglow emissions (under 365 nm LED).

Fig. 1b shows the afterglow luminescence spectra under 254 nm and 365 nm excitation, respectively. Interestingly, the afterglow luminescence under 254 nm in SG-450 do not show noticeable change in spectral profile, but a distinct redshift is revealed under 365 nm excitation. In PP-740, the afterglow is red-shifted under both the excitations. It is to be noted that the steady-state PL profile do exhibit changes as a function of excitation wavelengths, but under 254 nm and 365 nm excitations, the PL profiles remain similar (Fig. 1c image and SI, Fig. S2). Fig. 1c presents the color chromaticity diagram of the steady-state PL and the corresponding afterglow luminescence in both the samples along with their digital images. Both samples reveal color shift in afterglow luminescence relative to the steady-state PL, although the shift is more prominent under 365 nm excitation.



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The PL decay reveals a fast decay and a slow afterglow kinetics in both samples. The fast decay exhibits lifetime of about 8-10 ns (Fig. 1d), whereas the slow component lasts for several seconds (Fig. 1e). In SG-450, the afterglow luminescence is relatively stronger and can be detected by eyes for more than 10 s (Fig. 1f). The fast decay is in the range of a typical fluorescence transition (109 s-1). The slow afterglow luminescence can be induced by two possibilities: the thermally activated slow release of trapped charges (electrons and/or holes) to the emitting centers, i.e. persistent luminescence or the forbidden electronic transitions, i.e. triplet to singlet phosphorescence. The former is commonly observed in inorganic materials but the latter is limited to molecular compounds. We compared the afterglow decay with standard persistent luminescent inorganic phosphors (extrinsic trapped charge recombination) and an organic phosphorescent dye (intrinsic forbidden triplet-singlet transition). SrAl2O4:Eu,Dy (SrAlO:Eu-Dy), Y3Al2Ga3O12:Ce,Cr (YAGG:CeCr), Y3Al5O12:Ce (YAG:Ce) and ZnGaO4:Cr (ZGO:Cr) were used for persistent luminescence,13 whereas NPB dye (N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine) embedded in polystyrene film (PS:NPB) was used for phosphorescence luminescence.14 Fig. 2a presents the afterglow decay kinetics as a function of excitation durations. The measurements were carried out at RT (293 K) for all samples except the PS:NPB for which the sample was cooled down to 273 K to achieve better phosphorescence intensity. Two different behaviors are clearly observed. Indeed, the persistent standards show rise in afterglow intensity with the increase in excitation duration, but no such increase could be witnessed in a-YAB powders that behave more similar to the phosphorescent dye (PS:NPB). To get better visualization, the decay profiles are shown in log-log scale in Fig. 2b. The afterglow intensity at one second is plotted as a


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function of excitation duration in Fig. 2c, revealing a rise in intensity for the persistent standards, but no change or a very slight decrease for a-YAB samples (PP-740 and SG-450) and the PS:NPB dye. Moreover, this behavior was maintained even at lower excitation intensity in aYAB powders (SI, Fig. S3).

Figure 2. PL decay recorded under 254 nm excitations for 5 s to 600 s irradiation duration (a); in log-log scale (b); and the afterglow intensity at 1 s (normalized for highest intensity) plotted as a function of excitation durations (c).

The increase in the afterglow intensity with excitation duration in persistent phosphors is due to the charging of defect traps, which slowly gets saturated with the excitation duration depending on the trap density, charging efficiency and the excitation flux. In phosphorescent molecules too, the singlet to triplet intersystem crossing is time dependent, but it is typically of millisecond order.15,16 The temperature dependence of afterglow luminescence normalized with respect to 7 ACS Paragon Plus Environment


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steady-state PL intensity is presented in Fig. 3a-d. It can be seen that the afterglow intensity at 1 s increases in SrAlO:Eu-Dy with the increase in temperature, while in case of a-YAB and PS:NPB dye, the afterglow intensity decreases (Fig. 3e). The increased initial afterglow in SrAlO:Eu-Dy is expected due to more efficient thermal release of trapped charges and is in good agreement with the thermoluminescence (TL) glow curve in Fig. 3f.17 Nevertheless, the TL curve of PP-740 sample shows the presence of weak trap distribution, but the influence of these trapped charge release is not significant over the dominant phosphorescence mechanism leading to an overall decrease in afterglow with temperature. However, the decrease in PP-740 follows relatively different dynamics due to the presence of both phosphorescence and persistence mechanism. No TL glow signals could be detected for SG-450, thereby eliminating the possibility of persistent mechanism.

Fig. 3: Temperature dependence of decay curves, λex: 254 nm for 60 s (a-d); afterglow intensity one second after ceasing the excitation with respect to temperature (e); and the TL glow curves (f). (TL curves for SG-450 and PP-740 are enlarged by a multiplication factor of 25 with respect to TL of SrAlO:Eu,Dy.)


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In general, the redshift in afterglow emission is usually more pronounced in phosphorescence than in the persistence luminescence (SI, Fig. S4). In a-YAB, it is evident that the afterglow emission is significantly red shifted relative to PL emission, except for SG-450 under 254 nm excitation, where the shift is small. One possibility can be the delayed fluorescence,18 however it needs an independent investigation in view of the complexity of a-YAB materials and is out of the primary interest of present letter. Nevertheless, it is clear from the results that the strong afterglow luminescence in a-YAB is due to the phosphorescence mechanism involving triplet to singlet forbidden transitions. These results help us understand that a-YAB powders exhibit active luminescent organic moieties trapped in the glassy matrix that are responsible for the afterglow luminescence. The relative intensity of afterglow is higher in SG-450 sample over the PP-740. Indeed, the thermogravimetric analysis revealed the total carbon residue presence is about 13000 ppm (1.3%) in SG-450, but only 200 ppm in PP-740.10 As the calcination temperature increases, the organic content decreases, which systematically reduces the afterglow intensity (SI, Fig. S5, S6).19 Phosphorescence phenomenon is rarely seen at RT. The weak oscillator strength for the spin forbidden triplet-singlet transition requires spin-flip and therefore it possesses slower transition probability. This in general, allows excited triplet state to quench non-radiatively, which has higher probability than the radiative transitions. In a-YAB powders, the phosphorescence is detectable for several seconds at RT, which is one of the highest value to be achieved. This is due to the stabilization of triplet state in molecular species well confined in the amorphous yttrium aluminum borate inorganic matrix.23 It is similar to the triplet-state stabilization in



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polymer matrices, but is relatively more efficient owing to the rigid encapsulation and lower multi-phonon relaxation probability in inorganic oxide matrix.14-16,20-22 This opens new possibilities of conserving phosphorescence efficiency at elevated temperatures that can, in particular be interesting for several applications.24 In bio-imaging, traditional fluorophore induces major difficulty with the tissue auto-fluorescence that reduces the signalto-noise ratio.21 The use of phosphorescent bio-markers stabilized in nano-glassy spheres and having phosphorescence emission compatible with biological windows could effectively avoid the tissue auto-fluorescence by time-resolved spectroscopy and provide excellent signal-tonoise imaging. Moreover, it allows great freedom over the probe size as the phosphorescence efficiency does not get influenced by probe size dimensions unlike in the case of persistent nano-phosphors, which are significantly weak compared to their bulk counterparts.25,26 Further advantages of phosphorescent bio-markers are their ability to be completely bio-compatible and non-toxic, wavelength selectivity and excellent temperature sensitivity, where the later also provides additional functionality for temperature gradient imaging and nano-thermometry.27 In summary, we successfully demonstrated the origin of afterglow luminescence in wetchemically synthesized a-YAB powders as the triplet to singlet phosphorescence. This is an important advancement in understanding the complex luminescence properties in a-YAB phosphors and similar wet-chemically synthesized inorganic materials, which will be helpful for their further improvements.

Supporting Information Detailed description on material synthesis and experimental procedure, PL of additional powders synthesized at varied calcination temperatures and under varied excitations, PL-PLE 10 ACS Paragon Plus Environment

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maps, PL and afterglow spectra in SrAlO:Eu-Dy and PS:NPB dye, afterglow decay curves of a-YAB samples and the XRD patterns of the investigated samples. Acknowledgements This work was carried out under the ANR LuminoPhor-LED project (ANR-14-CE05-0033).

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10. Burner, P. Nouvelle Génération de Luminophores pour L’éclairage par LED. Ph.D. Thesis, University Grenoble Alps, 2016. 11. Liu, X.; Qiao, Y.; Dong, G.; Ye, S.; Zhu, B.; Zhuang, Y.; Qiu, J. BCNO-based Long-Persistent Phosphor. J. Electrochem. Soc. 2009, 156, P81. 12. Chien, C. T.; Li, S. S.; Lai, W. J.; Yeh, Y. C.; Chen, H. A.; Chen, I. S.; Chen, L. C.; Chen, K. H.; Nemoto, T.; Isoda, S.; Chen, M.; Fujita, T.; Eda, G.; Yamaguchi, H.; Chhowalla, M.; Chen, C. W. Tunable Photoluminescence from Graphene Oxide. Angew. Chem. Int. Ed. 2012, 51, 6662-6666. 13. Li, Y.; Gecevicius, M.; Qiu, Long Persistent Phosphors - From Fundamentals to Applications. J. Chem. Soc. Rev. 2016, 45, 2090-2136. 14. Reineke, S.; Baldo, M. A. Room Temperature Triplet State Spectroscopy of Organic Semiconductors. Sci. Rep. 2014, 4, 3797. 15. Reineke, S.; Seidler, N.; Yost, S. R.; Prins, F.; Tisdale, W. A.; Baldo, M. A. Highly Efficient, Dual State Emission from an Organic Semiconductor. Appl. Phys. Lett. 2013, 103, 093302. 16. An, Z.; Zheng, C.; Tao, Y.; Chen, R.; Shi, H.; Chen, T.; Wang, Z.; Li, H.; Deng, R.; Liu, X.; Huang, W. Stabilizing Triplet Excited States for Ultralong Organic Phosphorescence. Nature Mater. 2015, 14, 685-690. 17. Botterman, Y.; Joos, J.J.; Smet, P.F. Trapping and Detrapping in SrAl2O4:Eu,Dy Persistent Phosphors: Influence of Excitation Wavelength and Temperature. Phys. Rev. B 2014, 90, 085147. 18. Jiang, K.; Wang, Y.; Kai, C.; Lin, H. Activating Room Temperature Long Afterglow of Carbon Dots via Covalent Fixation. Chem. Mater. 2017, 29, 4866-4873. 19. Aida, T.; Hirama, N.; Tsutsumi, Y. Thermal Behavior of Carboxylic Acid Functionality in Coal. ACSFuel 1997, 42(1), 0218-0221. 20. Lettinga, M. P.; Zuilhof, H.; van Zandvoort, M. A. M. J. Phosphorescence and Fluorescence Characterization of Fluorescein Derivatives Immobilized in Various Polymer Matrices. Phys. Chem. Chem. Phys. 2000, 2, 3697-3707. 21. Kwon, M. S.; Yu, Y.; Coburn, C.; Phillips, A. W.; Chung, K.; Shanker, A.; Jung, J.; Kim, G.; Pipe, K.; Forrest, S. R.; Youk, J. H.; Gierschner, J.; Kim, J. Suppressing Molecular Motions for Enhanced Room-Temperature Phosphorescence of Metal-free Organic materials. Nat. Comm. 2015, 6, 8947. 22. Hirata, S.; Totani, K.; Zhang, J.; Yamashita, T.; Kaji, H.; Marder, S. R.; Watanabe, T.; Adachi, C. Efficient Persistent Room Temperature Phosphorescence in Organic Amorphous Materials under Ambient Conditions. Adv. Fun. Mater. 2013, 23, 3386-3397. 23. Burner, P.; Sontakke, A. D.; Salaün, M.; Bardet, M.; Mouesca, J.-M.; Gambarelli, S.; Barra, A.-L.; Ferrier, A.; Viana, B.; Ibanez, A.; Maurel, V.; Gautier-Luneau, I. Evidence of Organic 12 ACS Paragon Plus Environment

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Figure 1. Steady-state PL-PLE spectra (a); normalized afterglow (PhL) 1 s after ceasing the excitation compared to PL (b); color chromaticity of PL and afterglow emissions with corresponding digital images under Hg lamps excitation (c); PL decay d); afterglow decay (e); and the digital images of afterglow emissions (under 365 nm LED). 116x89mm (600 x 600 DPI)

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Figure 2. PL decay recorded under 254 nm excitations for 5 s to 600 s irradiation duration (a); in log-log scale (b); and the afterglow intensity at 1 s (normalized for highest intensity) plotted as a function of excitation durations (c). 106x74mm (600 x 600 DPI)



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Fig. 3: Temperature dependence of decay curves, λex: 254 nm for 60 s (a-d); afterglow intensity one second after ceasing the excitation with respect to temperature (e); and the TL glow curves (f). (TL curves for SG-450 and PP-740 are enlarged by a multiplication factor of 25 with respect to TL of SrAlO:Eu,Dy.) 106x74mm (600 x 600 DPI)


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Afterglow Luminescence in Wet-Chemically Synthesized Inorganic

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