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NIR photoluminescence of bismuth-doped CsCdBr3 – The first ternary bromide phase with a univalent bismuth impurity center
Author’s Accepted Manuscript NIR photoluminescence of bismuth-doped CsCdBr3 – the first ternary bromide phase with a univalent bismuth impurity center Alexey N. Romanov, Alexander A. Veber, Daria N. Vtyurina, Mikhail S. Kouznetsov, Ksenia S. Zaramenskikh, Igor S. Lisitsky, Zukhra T. Fattakhova, Elena V. Haula, Pavel A. Loiko, Konstantin V. Yumashev, Vladimir N. Korchak
PII: DOI: Reference:
www.elsevier.com/locate/jlumin
S0022-2313(15)00402-0 http://dx.doi.org/10.1016/j.jlumin.2015.07.020 LUMIN13470
To appear in: Journal of Luminescence Received date: 12 March 2015 Revised date: 1 July 2015 Accepted date: 15 July 2015 Cite this article as: Alexey N. Romanov, Alexander A. Veber, Daria N. Vtyurina, Mikhail S. Kouznetsov, Ksenia S. Zaramenskikh, Igor S. Lisitsky, Zukhra T. Fattakhova, Elena V. Haula, Pavel A. Loiko, Konstantin V. Yumashev and Vladimir N. Korchak, NIR photoluminescence of bismuth-doped CsCdBr3 – the first ternary bromide phase with a univalent bismuth impurity center, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2015.07.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
NIR photoluminescence of bismuth-doped CsCdBr3 – the first ternary bromide phase with a univalent bismuth impurity center
Alexey N. Romanova,*, Alexander A. Veberb, Daria N. Vtyurinaa*, Mikhail S. Kouznetsovc, Ksenia S. Zaramenskikhc, Igor S. Lisitskyc, Zukhra T. Fattakhovaa, Elena V. Haulaa, Pavel A. Loikod, Konstantin V. Yumashevd, Vladimir N. Korchaka
a
N.N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, 4 Kosygina Street,
119991; b
Universität Erlangen-Nürnberg, Lehrstuhl für Glas und Keramik, Martensstr.5, 91058
Erlangen, Germany; c
State Scientific-Research and Design Institute of Rare-Metal Industry “Giredmet” JSC, 5-1
B.Tolmachevsky lane, 119017 Moscow, Russia; d
Center for Optical Materials and Technologies, Belarusian National Technical University, 65/17
Nezavisimosti Ave., 220013 Minsk, Belarus; *Corresponding author, e-mail: [email protected]; Phone: +7(905)5970866
Abstract Single crystals of ternary bromide phase CsCdBr3 doped with univalent bismuth cations are prepared for the first time by the Bridgman method. Bi+ impurity center emits a broadband long-lived near-infrared photoluminescence with a maximum at ~1053 nm. The characteristics of this photoluminescence and its relations with the energy spectrum of Bi+ impurity center are discussed. A comparison of Bi+ photoluminescence in CsCdBr3 and ternary chlorides (studied previously) is performed.
1
Keywords: univalent bismuth, photoluminescence, ternary bromide
2
Introduction During the recent years, a broadband long-lived near-infrared (NIR) photoluminescence (PL) that was observed in different bismuth-doped materials [1-3] attracted a lot of attention due to an unusual chemistry of this element, in particular, its ability to form the subvalent (low valent) cations and polycations. It was shown that several of such subvalent bismuth forms (Bi+ [4-11], Bi53+ [12-16], Bi28+ [16]) demonstrate the broadband NIR PL. The optical properties of these cations along with the discussion on the other possible luminescent centers in bismuth-doped materials can be found in a recent review [17]. Univalent bismuth impurity, observed in several chloride [6-10], iodide [18, 19] and nonhalide crystal phases [11, 20-25], represents the simplest form of NIR photoluminescent subvalent bismuth center. It possesses the 6p2 open shell electronic configuration with the 3P0 ground state and 3P1, 3P2 low lying excited states. The energy gap between the ground state and the excited 3P1, 3P2 ones results from a significant spin orbit effect that is relevant to all heavy metal ions. An additional splitting of degenerate 3P1 and 3P2 states by the crystal field produces the energetic spectrum of Bi+ impurity in crystals which determines its optical properties [9, 26]. Due to a large estimated Bi+ ionic radius it is believed that it can isomorphically substitute the heavy alkali cations (K+, Rb+, Cs+) [27], and, in fact, several alkali-bearing halide phases (RbPb2Cl5, KAlCl4, KMgCl3, CsCdCl3, CsI) containing Bi+ impurity center were prepared and display the long-lived broadband NIR PL [6-10,18,19]. While the optical characteristics of Bi+ in all the studied ternary chloride phases were similar with the excitation spectra having two maxima in the orange-red region of spectrum and broad photoluminescence band with a maximum at 900–1030 nm (at 300 K), the corresponding properties of the bismuth-doped CsI were rather different. This can be due to an influence of the different halide ion in Bi+ surrounding or different crystal symmetry (CsI is cubic while all ternary chlorides are of lower symmetry crystal systems). To elucidate the influence of different halide ions on the parameters of NIR luminescence of Bi+ impurity it would be advisable to prepare the Bi+-doped ternary halide with anions, different from chloride. In the present study, the ternary bromide CsCdBr3 phase was chosen for Bi+-doping for several reasons. First of all, Bi+-doped ternary bromides were never prepared before and in case of Bi+:CsCdBr3 it would be also possible to compare its optical properties with a closely related
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doped chloride phase Bi+:CsCdCl3 which was studied previously [9]. In fact, CsCdCl3 and CsCdBr3 are both crystallized in the P63/mmc space group (hexagonal system) and point symmetry groups of Bi+ (in the sites of Cs+) environment are: D3h and C3v (two nonequivalent positions) in the case of CsCdCl3 and D3h in the case of CsCdBr3 [28-30]. Secondly, before the incorporation into the crystal phase, the univalent bismuth should be prepared in the melt by reversible reaction of Bi(3+) compounds with a metal bismuth. It was found that in case of bromides, the synproportionation reaction: BiX3 + 2Bi ↔ 3BiX (X = Cl, Br)
(1)
is shifted to the formation of univalent halide to a greater extent than in the case of chlorides (at least in a gas phase) [31,32]. So one should expect that higher concentrations of Bi+ active centers can be attained in the bromide crystal phase in comparison with the chloride one. This circumstance can be important for the design of future optical materials, based on the Bi+ impurity center. Finally, CsCdBr3 is an easy to handle non-hygroscopic material with a large transparency window in the IR, which makes it an ideal host for NIR PL optical centers incorporation. Experimental Single crystalline samples of Bi+-doped CsCdBr3 (Fig. 1) were grown by Bridgman method in the sealed fused silica container (inner diameter: 26 mm). The initial batch was composed from CdBr2 (99.99%; 54.5 M parts), CsBr (99.99%; 44.5 M parts), BiBr3 (99.999%; 1 M part) and an excess of metallic bismuth. The CdBr2 was taken in excess, relative to the stoichiometry of CsCdBr3 formation to increase the Lewis acidity of the melt and to facilitate the Bi+ formation via the synproportionation reaction, Eq. (1) [9]. For the sake of comparison, the single crystals of undoped CsCdBr3 were also grown by the similar procedure from stoichiometric (1:1) mixture of CdBr2 and CsBr. In one preparation the liquid bromine was also added to the container to ensure the slightly oxidative atmosphere above the melt (20μL of Br2 to the 83 cm3 of filling inert gas over the melt). All operations of batch mixing and container filling were carried out in the argon-filled dry box (< 2 ppm H2O, <10 ppm O2).
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Optical absorption spectrum was measured with Varian CARY-5000 spectrophotometer; a thin (thickness: 200 μm) polished sample of Bi:CsCdBr3 crystal was used. NIR PL spectra were obtained with SDH-IV spectrometer (Solar LS), equipped with G9212-512 InGaAs linear image sensor (Hamamatsu). A set of compact laser diodes and diode-pumped solid state lasers with the emission wavelengths of 405, 532, 635, 660 and 690 nm was used for the excitation of NIR PL. Osram HBO 150W Xe-lamp combined with LOMO MDR-12 monochromator was used as variable wavelength light source to obtain the luminescence excitation spectra. The measurements were performed at the temperatures of 300 and 77 K and all emission and excitation spectra were corrected for the spectral response of the experimental setup. For a time-resolved studies of NIR PL of Bi:CsCdBr3 crystal, ns optical parametric oscillator tuned to ~628 nm was used as an excitation source. PL was collected by a wide-aperture lens and reimaged to the input slit of MDR-12 monochromator and then detected with a fast Hamamatsu G5851 photodetector and 500 MHz digital oscilloscope. The decay measurements were performed in the spectral range of 970–1250 nm. Results and Discussion The single crystalline specimens suitable for optical measurements were cut from the bottom side of the as-grown boule. The samples were yellow-green colored (see Fig. 1). The coloration of the material results from the existence of splitted absorption band in the visible (with the peaks centered at ~610 nm and 675 nm) and a strong absorption band in the UV spanning from 350 to 420 nm (with a peak at ~378 nm), see Fig. 2. An intense broadband NIR PL (Fig. 3,4) was observed under the excitation in the green and red spectral ranges (λex = 532, 635, 660 or 690 nm). The shape of the PL spectrum and the position of its maximum (1053 nm at 300 K) are independent on the excitation wavelength (see Fig. 3). A typical temporal decay plot for NIR PL of Bi:CsCdBr3 crystal is shown in Fig. 5 in a semi-log scale (inset shows the dependence of the characteristic decay time τ on the PL wavelength). Within the whole emission band, the decay is single-exponential. The decay time τ is nearunchanged when the PL wavelength is tuned from 970 to 1250 nm, and equal to 250±6 μs.
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NIR PL cannot be observed under the excitation at 405 nm thus indicating that strong UV-absorption band is inactive in the luminescence photoexcitation. Indeed, the PL excitation spectrum (Fig. 6) displays only two bands with the maxima at ~632 and 687 nm (at 300 K). The position of these peaks corresponds to ones in the visible part of the absorption spectrum. These experimental facts allow us to make a conclusion that a single active center is responsible for the NIR PL centered at ~1053 nm, as well as optical absorption in the red spectral range. Previously, the luminescence of bismuth-doped CsCdBr3 was studied by Wolfert and Blasse [33]. Contrary to our preparation that utilizes a reductive environment with an excess of Bi metal, they had prepared their samples in an oxidative atmosphere (with an excess of bromine) to ensure that all bismuth ions enter the crystal phase as Bi3+. For such a Bi3+-doped CsCdBr3 specimens they observed only one emission center with the excitation maximum at 4.33 eV (286 nm) and emission maximum at 2.12 eV (585 nm) which is completely quenched at T > 50 K. This luminescent center was assigned to Bi3+ in Cd2+ position and it is clear that it is not involved in the NIR PL and UV absorption at 378 nm discussed here. The luminescent centers observed previously in the undoped CsCdBr3 also cannot be invoked to explain the observed spectral features in our case [33-35]. Thus, the single NIR photoluminescent center that we observed in our experiments is related to the subvalent bismuth cations formed in the reductive environment. In fact, the characteristics of the NIR photoluminescent center in bismuth-doped CsCdBr3 closely resemble the optical properties of univalent bismuth cation Bi+ in the related ternary chloride CsCdCl3 (see Table 1 for a comparison) [9]. The positions of the maximum in the NIR-photoluminescence spectrum and two maxima in the excitation spectrum for Bi+-doped CsCdBr3 are all red-shifted in comparison with Bi+:CsCdCl3. A similar red-shift was previously observed in Bi+ absorption spectrum in bromide eutectic melt (AlBr3–NaBr) relative to the chloride eutectic composition (AlCl3–NaCl) [36]. These facts indicate that the substitution of chloride ions by bromide ones in the environment of Bi+ cation reduces the energies of sublevels of the 3P1 and 3P2 manifolds. We adopt here, that two bands in the visible range of the absorption spectrum and in the luminescence excitation spectrum of Bi+-doped ternary halides result from the excitation to the sublevels of crystal-field splitted 3P2 state, while the PL is the result of radiative transitions from the sublevels of the 3P1 state to the 3P0 ground state [9, 11, 26].
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Assuming that Bi+ enters the crystal lattice of CsCdBr3 in the sites of Cs+, the single optical center with a local symmetry of D3h and coordination number of 12 will be formed in this way. Among all previously studied Bi+-doped ternary halides the Bi+:CsCdBr3 one represent the simplest case of a single optical center in a high-symmetry crystal surrounding. The general diagram of low-lying electronic energy states of Bi+ ion in the D3h crystal environment is shown in Fig. 7. The sequence of the splitted sublevels within the 3P1 and 3P2 manifolds is somewhat arbitrary to the moment. Upon cooling to 77 K two bands in the PL excitation spectrum become sharper, but no additional peaks are resolved (Fig. 6). The Gaussian fit of the excitation spectrum reveals that up to 3 Gaussians are needed to fit the intense high-energy band, while the less intense low energy band can be approximated with only one Gaussian. It is reasonable to suggest that the high-intensity band is originated from the solely electric dipole (ED) allowed transition A'1→E' and its complex structure is the manifestation of Jahn-Teller effect of E-e type in the excited degenerate E' state [37]. The ED forbidden transition A'1→E'' can also contribute to this band. The weak low-energy band with a near-Gaussian shape is probably the result of the A'1→A'1 ED forbidden transition to the solely nondegenerate state of the 3P2 manifold. At 77 K, the spectrum of NIR PL also becomes sharper (relative to 300 K, see Fig. 4) and its maximum shifts to the longer wavelength (1078 nm at 77 K vs. 1053 nm at 300 K). Two Gaussians are needed to approximate both the 77 K and 300 K emission spectra. At 77 K the low-energy Gaussian becomes slightly blue-shifted (9277 cm-1 vs. 9197 cm-1 at 300 K) and its relative intensity increases. The high-energy Gaussian experiences a significant red-shift (9376 cm-1 vs. 9738 cm-1 at 300 K) and it is weakened at 77 K. This large temperature variation implies the complex nature of potential energy surface for the emissive state that can also be the result of Jahn-Teller effect in the E'' state of 3P1 manifold. An additional measurement in extended temperature range is needed to elucidate the details of Bi+ photoluminescence in CsCdBr3. The characteristic decay time of Bi+ NIR PL in CsCdBr3 is shorter than in CsCdCl3, see Table 1. This can be the result of larger oscillator strengths for optical transitions from the ground to the excited states in the bromide systems. This increased oscillator strengths are also
7
manifested as enlarged molar absorption of Bi+ in AlBr3–NaBr eutectic relative to the chloride one (AlCl3–NaCl) [36]. Turning our attention to the intense absorption band in UV (at 378 nm), we can suggest that it has no relation to the Bi+ impurity studied here or Bi3+ and intrinsic centers in CsCdBr3 reported previously [33-35]. To elucidate the origin of this band an additional measurements on single crystalline undoped CsCdBr3 were accomplished. Two specimens were produced by Bridgman method in neutral (inert atmosphere) and oxidizing (atmosphere with bromine vapors) conditions. It was found that the absorption band at 378 nm is still intense in specimen, prepared in the neutral conditions (6.9 cm-1 absorption coefficient at maximum), while oxidative atmosphere significantly diminishes its intensity (maximal absorption - 1.1 cm-1). Also, this band is more pronounced in the specimens, which cut from the upper part of the boules. Neither Bi+ absorption bands at 632 and 687 nm nor NIR PL were noticed in undoped CsCdBr3. Therefore, it can be concluded that the band at 378 nm is not associated with bismuth doping and originates from the crystal defects, resulted from the partial reduction of Cd2+. This phenomenon is well known and mixed valence cadmium compounds were already invoked to interpret the UV and visible absorption in the partially reduced cadmium halide melts [38]. Conclusions Univalent bismuth cation isomorphically substitutes Cs+ in CsCdBr3 ternary bromide, producing the unique NIR PL center with the D3h local symmetry. The excitation and emission spectra of Bi+ in CsCdBr3 resembles closely the same characteristics of ternary chlorides, but the maxima of the emission and excitation spectra are shifted to the longer wavelengths in ternary bromide. A complex structure of emission and excitation spectra and its marked dependence on the temperature may be the manifestation of Jahn-Teller effect in the degenerate E' and E'' low lying excited states of Bi+. The prepared Bi+:CsCdBr3 single crystalline specimens appear to be a non-hygroscopic material with a large transparency window in the IR and broadband PL in the NIR. This ternary bromide and other similar halide phases doped with univalent bismuth can be promising materials for broadband amplification of NIR emission, as well as wavelength-tunable lasers.
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Acknowledgements The research was supported by the Ministry of Education and Science of the Russian Federation (research project number 14.604.21.0131, unique identifier RFMEFI60414X0131). The corresponding author A.N. Romanov is the researcher at the N.N. Semenov Institute of Chemical Physics.
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References [1] Y. Fujimoto and M. Nakatsuka, Jpn. J. App. Phys., 40 (2001) L279. [2] Y. Fujimoto and M. Nakatsuka, Appl. Phys. Lett., 82 (2003) 3325. [3] E.M. Dianov, Quantum Electron. 42 (2012) 754. [4] X.G. Meng, J.R. Qiu, M.Y. Peng, D.P. Chen, Q.Z. Zhao, X.W. Jiang, C.S. Zhu, Opt. Express, 13 (2005) 1628. [5] X.G. Meng, J.R. Qiu, M.Y. Peng, D.P. Chen, Q.Z. Zhao, X.W. Jiang, C.S. Zhu, Opt. Express, 13 (2005) 1635. [6] A.G. Okhrimchuk, L.N. Butvina, E.M. Dianov, N.V. Lichkova, V.N. Zagorodnev, K.N. Boldyrev, Opt. Lett. 33 (2008) 2182. [7] A.A. Veber, A.N. Romanov, O.V. Usovich, Z.T. Fattakhova, E.V. Haula, V.N. Korchak, L.A. Trusov, P.E. Kazin, V.B. Sulimov, V.B. Tsvetkov, Appl. Phys. B 108 (2012) 733. [8] A.N. Romanov, A.A. Veber, Z.T. Fattakhova, O.V. Usovich, E.V. Haula, L.A. Trusov, P.E. Kazin, V.N. Korchak, V.B. Tsvetkov, V.B. Sulimov, J. Lumin. 134 (2013) 180. [9] A.N. Romanov, A.A. Veber, Z.T. Fattakhova, , M.S. Kouznetsov, K.S. Zaramenskikh, I.S. Lisitsky, V.N. Korchak, V.B. Tsvetkov, V.B. Sulimov, J. Lumin. 149 (2014) 292. [10] A.A. Veber, A.N. Romanov, O.V. Usovich, Z.T. Fattakhova, E.V. Haula, V.N. Korchak, L.A. Trusov, P.E. Kazin, V.B. Sulimov, V.B. Tsvetkov, J. Lumin. 151 (2014) 247. [11] A.N. Romanov, A.A. Veber, D.N. Vtyurina, Z.T. Fattakhova, E.V. Haula, D.P. Shashkin, V.B. Sulimov, V.B. Tsvetkov, V.N. Korchak, J. Mater. Chem. C 3 (2015) 3592. [12] H.-T. Sun, Y. Sakka, H. Gao, Y. Miwa, M. Fujii, N. Shirahata, Z. Bai and J.-G. Li, J. Mater. Chem. (21) 2011 4060. [13] R. Cao, M. Peng, L. Wondraczek and J. Qiu, Opt. Express 20 (2012) 2562. [14] R. Cao, M. Peng, J. Zheng, J. Qiu, and Q. Zhang, Opt. Express 20 (2012) 18505. [15] H.-T. Sun, B. Xu, T. Yonezawa, Y. Sakka, N. Shirahata, M. Fujii, J. Qiu and H. Gao, Dalton Trans. 41 (2012) 11055. [16] H.-T. Sun, Y. Sakka, N. Shirahata, H. Gao and Tetsu Yonezawa, J. Mater. Chem. 22 (2012) 12837. [17] H.-T. Sun, J. Zhou and J. Qiu, Prog. Mater. Sci. 64 (2014) 1.
10
[18] L. Su, H. Zhao, H. Li, L. Zheng, G.Ren, J. Xu, W. Ryba-Romanowski, R. Lisiecki, P.Solarz, Opt. Lett. 36 (2011) 4551. [19] L. Su, H. Zhao, H. Li, L. Zheng, X. Fan, X. Jiang, H. Tang, G. Ren, J. Xu, W. RybaRomanowski, R. Lisiecki, P. Solarz, Opt. Mater. Exp. 2 (2012) 757. [20] L. Su, J. Yu, P. Zhou, H. Li, L. Zheng, Y. Yang, F. Wu, H. Xia, J. Xu, Opt. Lett. 34 (2009) 2504. [21] J. Ruan, L. Su, J. Qiu, D. Chen, J. Xu, Opt. Express 17 (2009) 5163. [22] C. Li, Z. Song, J. Qiu, Z. Yang, X. Yu, D. Zhou, Z. Yin, R. Wang, Y. Xu, Y. Cao, J. Lumin. 132 (2012) 1807. [23] H.-T. Sun, Y. Matsushita, Y. Sakka, N. Shirahata, M. Tanaka, Y. Katsuya, H. Gao, K. Kobayashi, J. Am. Chem. Soc. 134 (2012) 2918. [24] H.-T. Sun, Y.Sakka, N. Shirahata, Y. Matsushita, K. Deguchi, T. Shimizu, J. Phys. Chem. C 117 (2013) 6399. [25] H.-T.Sun, A. Hosokawa, Y. Miwa, F. Shimaoka, M. Fujii, M. Mizuhata, S. Hayashi, S. Deki, Adv. Mater. 21 (2009) 3694. [26] H.L. Davis, N.J. Bjerrum, G.P. Smith, Inorg. Chem. 6 (1967) 1172. [27] A.N. Romanov, F.V. Grigoriev, V.B. Sulimov, Comput. Theor. Chem. 1017 (2013) 159. [28] J.R. Chang, G.L. McPherson, J.L. Atwood, Inorg. Chem. 14 (1975) 3079. [29] G.L. McPherson, A.M. McPherson, J.L. Atwood, J. Phys. Chem. Solids 41 (1980) 495. [30] J.M. Leger, A.M. Redon, C. Andraud, F. Pelle, Phys. Rev. B 41 (1990) 9276. [31] D. Cubicciotti, J. Phys. Chem. 64 (1960) 791. [32] D. Cubicciotti, J. Phys. Chem. 64 (1960) 1506. [33] A. Wolfert, G. Blasse, J. Solid State Chem. 55 (1984) 344. [34] R. Demirbilek Spektroskopische Untersuchungen am CsCdBr3. TU Darmstadt. Ph.D. Thesis (2002). [35] C. Andraud, F. Pelle, J.P. Denis, O. Pilla, B. Blanzat, J. Phys.: Condens. Matter 1 (1989) 1511. [36] N.J. Bjerrum, H.L. Davis, G.P. Smith, Inorg. Chem. 6 (1967) 1603. [37] I.B. Bersuker, The Jahn-Teller Effect, Cambridge University Press (2006). [38] R.A. Potts, R.D. Barnes, J.D. Corbett, Inorg. Chem. 7 (1968) 2558.
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Figure captions Figure 1 a) Boule of Bi+-doped CsCdBr3, produced by the Bridgman method. b) single crystalline specimen of Bi+-doped CsCdBr3 , prepared for optical measurements. Figure 2 Optical absorption spectrum of Bi:CsCdBr3 single crystalline specimen. Figure 3 Photoluminescence emission spectra of Bi+-doped CsCdBr3, excited at different wavelengths (300K). Figure 4 Photoluminescence emission spectra of Bi+-doped CsCdBr3 at 77 and 300 K; Gaussian decompositions (2 Gaussians) of both spectra are shown.
Figure 5 Photoluminescence decay plot for Bi+-doped CsCdBr3 (excitation wavelength is 628 nm, PL wavelength is 1060 nm); inset shows the dependence of characteristic decay time on the emission wavelength within the photoluminescence band.
Figure 6 Photoluminescence excitation spectra of Bi+-doped CsCdBr3 at 77 and 300 K; Gaussian decomposition (4 Gaussians) of the low temperature spectrum is shown.
Figure 7 Diagram of the lowest excited states of Bi+ impurity center (6p2 configuration) in CsCdBr3, produced by simultaneous action of spin-orbit interaction and crystal field of D3h symmetry; inset shows the first coordination sphere of Bi+ impurity ion (in the place of Cs+, coordination number CN = 12).
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Table 1. Characteristics of NIR photoluminescence of Bi+ impurity center in CsCdBr3 and CsCdCl3 ternary halides. Crystal phase
CsCdBr3
CsCdCl3
77 K
300 K
77 K
300 K
Maximum in PL emission spectrum, nm
1078
1053
1032
976
Maximum in PL
major
630
632
617
615
excitation spectrum, nm
minor
683
689
640
653
250
400
360
PL decay time, μs
Highlights:
Single crystals of Bi+-doped ternary bromide CsCdBr3 were prepared. Broadband NIR photoluminescence was observed from Bi+-doped CsCdBr3. Single optical center is responsible for NIR emission in Bi+-doped CsCdBr3.
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PII: DOI: Reference:
www.elsevier.com/locate/jlumin
S0022-2313(15)00402-0 http://dx.doi.org/10.1016/j.jlumin.2015.07.020 LUMIN13470
To appear in: Journal of Luminescence Received date: 12 March 2015 Revised date: 1 July 2015 Accepted date: 15 July 2015 Cite this article as: Alexey N. Romanov, Alexander A. Veber, Daria N. Vtyurina, Mikhail S. Kouznetsov, Ksenia S. Zaramenskikh, Igor S. Lisitsky, Zukhra T. Fattakhova, Elena V. Haula, Pavel A. Loiko, Konstantin V. Yumashev and Vladimir N. Korchak, NIR photoluminescence of bismuth-doped CsCdBr3 – the first ternary bromide phase with a univalent bismuth impurity center, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2015.07.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
NIR photoluminescence of bismuth-doped CsCdBr3 – the first ternary bromide phase with a univalent bismuth impurity center
Alexey N. Romanova,*, Alexander A. Veberb, Daria N. Vtyurinaa*, Mikhail S. Kouznetsovc, Ksenia S. Zaramenskikhc, Igor S. Lisitskyc, Zukhra T. Fattakhovaa, Elena V. Haulaa, Pavel A. Loikod, Konstantin V. Yumashevd, Vladimir N. Korchaka
a
N.N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, 4 Kosygina Street,
119991; b
Universität Erlangen-Nürnberg, Lehrstuhl für Glas und Keramik, Martensstr.5, 91058
Erlangen, Germany; c
State Scientific-Research and Design Institute of Rare-Metal Industry “Giredmet” JSC, 5-1
B.Tolmachevsky lane, 119017 Moscow, Russia; d
Center for Optical Materials and Technologies, Belarusian National Technical University, 65/17
Nezavisimosti Ave., 220013 Minsk, Belarus; *Corresponding author, e-mail: [email protected]; Phone: +7(905)5970866
Abstract Single crystals of ternary bromide phase CsCdBr3 doped with univalent bismuth cations are prepared for the first time by the Bridgman method. Bi+ impurity center emits a broadband long-lived near-infrared photoluminescence with a maximum at ~1053 nm. The characteristics of this photoluminescence and its relations with the energy spectrum of Bi+ impurity center are discussed. A comparison of Bi+ photoluminescence in CsCdBr3 and ternary chlorides (studied previously) is performed.
1
Keywords: univalent bismuth, photoluminescence, ternary bromide
2
Introduction During the recent years, a broadband long-lived near-infrared (NIR) photoluminescence (PL) that was observed in different bismuth-doped materials [1-3] attracted a lot of attention due to an unusual chemistry of this element, in particular, its ability to form the subvalent (low valent) cations and polycations. It was shown that several of such subvalent bismuth forms (Bi+ [4-11], Bi53+ [12-16], Bi28+ [16]) demonstrate the broadband NIR PL. The optical properties of these cations along with the discussion on the other possible luminescent centers in bismuth-doped materials can be found in a recent review [17]. Univalent bismuth impurity, observed in several chloride [6-10], iodide [18, 19] and nonhalide crystal phases [11, 20-25], represents the simplest form of NIR photoluminescent subvalent bismuth center. It possesses the 6p2 open shell electronic configuration with the 3P0 ground state and 3P1, 3P2 low lying excited states. The energy gap between the ground state and the excited 3P1, 3P2 ones results from a significant spin orbit effect that is relevant to all heavy metal ions. An additional splitting of degenerate 3P1 and 3P2 states by the crystal field produces the energetic spectrum of Bi+ impurity in crystals which determines its optical properties [9, 26]. Due to a large estimated Bi+ ionic radius it is believed that it can isomorphically substitute the heavy alkali cations (K+, Rb+, Cs+) [27], and, in fact, several alkali-bearing halide phases (RbPb2Cl5, KAlCl4, KMgCl3, CsCdCl3, CsI) containing Bi+ impurity center were prepared and display the long-lived broadband NIR PL [6-10,18,19]. While the optical characteristics of Bi+ in all the studied ternary chloride phases were similar with the excitation spectra having two maxima in the orange-red region of spectrum and broad photoluminescence band with a maximum at 900–1030 nm (at 300 K), the corresponding properties of the bismuth-doped CsI were rather different. This can be due to an influence of the different halide ion in Bi+ surrounding or different crystal symmetry (CsI is cubic while all ternary chlorides are of lower symmetry crystal systems). To elucidate the influence of different halide ions on the parameters of NIR luminescence of Bi+ impurity it would be advisable to prepare the Bi+-doped ternary halide with anions, different from chloride. In the present study, the ternary bromide CsCdBr3 phase was chosen for Bi+-doping for several reasons. First of all, Bi+-doped ternary bromides were never prepared before and in case of Bi+:CsCdBr3 it would be also possible to compare its optical properties with a closely related
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doped chloride phase Bi+:CsCdCl3 which was studied previously [9]. In fact, CsCdCl3 and CsCdBr3 are both crystallized in the P63/mmc space group (hexagonal system) and point symmetry groups of Bi+ (in the sites of Cs+) environment are: D3h and C3v (two nonequivalent positions) in the case of CsCdCl3 and D3h in the case of CsCdBr3 [28-30]. Secondly, before the incorporation into the crystal phase, the univalent bismuth should be prepared in the melt by reversible reaction of Bi(3+) compounds with a metal bismuth. It was found that in case of bromides, the synproportionation reaction: BiX3 + 2Bi ↔ 3BiX (X = Cl, Br)
(1)
is shifted to the formation of univalent halide to a greater extent than in the case of chlorides (at least in a gas phase) [31,32]. So one should expect that higher concentrations of Bi+ active centers can be attained in the bromide crystal phase in comparison with the chloride one. This circumstance can be important for the design of future optical materials, based on the Bi+ impurity center. Finally, CsCdBr3 is an easy to handle non-hygroscopic material with a large transparency window in the IR, which makes it an ideal host for NIR PL optical centers incorporation. Experimental Single crystalline samples of Bi+-doped CsCdBr3 (Fig. 1) were grown by Bridgman method in the sealed fused silica container (inner diameter: 26 mm). The initial batch was composed from CdBr2 (99.99%; 54.5 M parts), CsBr (99.99%; 44.5 M parts), BiBr3 (99.999%; 1 M part) and an excess of metallic bismuth. The CdBr2 was taken in excess, relative to the stoichiometry of CsCdBr3 formation to increase the Lewis acidity of the melt and to facilitate the Bi+ formation via the synproportionation reaction, Eq. (1) [9]. For the sake of comparison, the single crystals of undoped CsCdBr3 were also grown by the similar procedure from stoichiometric (1:1) mixture of CdBr2 and CsBr. In one preparation the liquid bromine was also added to the container to ensure the slightly oxidative atmosphere above the melt (20μL of Br2 to the 83 cm3 of filling inert gas over the melt). All operations of batch mixing and container filling were carried out in the argon-filled dry box (< 2 ppm H2O, <10 ppm O2).
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Optical absorption spectrum was measured with Varian CARY-5000 spectrophotometer; a thin (thickness: 200 μm) polished sample of Bi:CsCdBr3 crystal was used. NIR PL spectra were obtained with SDH-IV spectrometer (Solar LS), equipped with G9212-512 InGaAs linear image sensor (Hamamatsu). A set of compact laser diodes and diode-pumped solid state lasers with the emission wavelengths of 405, 532, 635, 660 and 690 nm was used for the excitation of NIR PL. Osram HBO 150W Xe-lamp combined with LOMO MDR-12 monochromator was used as variable wavelength light source to obtain the luminescence excitation spectra. The measurements were performed at the temperatures of 300 and 77 K and all emission and excitation spectra were corrected for the spectral response of the experimental setup. For a time-resolved studies of NIR PL of Bi:CsCdBr3 crystal, ns optical parametric oscillator tuned to ~628 nm was used as an excitation source. PL was collected by a wide-aperture lens and reimaged to the input slit of MDR-12 monochromator and then detected with a fast Hamamatsu G5851 photodetector and 500 MHz digital oscilloscope. The decay measurements were performed in the spectral range of 970–1250 nm. Results and Discussion The single crystalline specimens suitable for optical measurements were cut from the bottom side of the as-grown boule. The samples were yellow-green colored (see Fig. 1). The coloration of the material results from the existence of splitted absorption band in the visible (with the peaks centered at ~610 nm and 675 nm) and a strong absorption band in the UV spanning from 350 to 420 nm (with a peak at ~378 nm), see Fig. 2. An intense broadband NIR PL (Fig. 3,4) was observed under the excitation in the green and red spectral ranges (λex = 532, 635, 660 or 690 nm). The shape of the PL spectrum and the position of its maximum (1053 nm at 300 K) are independent on the excitation wavelength (see Fig. 3). A typical temporal decay plot for NIR PL of Bi:CsCdBr3 crystal is shown in Fig. 5 in a semi-log scale (inset shows the dependence of the characteristic decay time τ on the PL wavelength). Within the whole emission band, the decay is single-exponential. The decay time τ is nearunchanged when the PL wavelength is tuned from 970 to 1250 nm, and equal to 250±6 μs.
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NIR PL cannot be observed under the excitation at 405 nm thus indicating that strong UV-absorption band is inactive in the luminescence photoexcitation. Indeed, the PL excitation spectrum (Fig. 6) displays only two bands with the maxima at ~632 and 687 nm (at 300 K). The position of these peaks corresponds to ones in the visible part of the absorption spectrum. These experimental facts allow us to make a conclusion that a single active center is responsible for the NIR PL centered at ~1053 nm, as well as optical absorption in the red spectral range. Previously, the luminescence of bismuth-doped CsCdBr3 was studied by Wolfert and Blasse [33]. Contrary to our preparation that utilizes a reductive environment with an excess of Bi metal, they had prepared their samples in an oxidative atmosphere (with an excess of bromine) to ensure that all bismuth ions enter the crystal phase as Bi3+. For such a Bi3+-doped CsCdBr3 specimens they observed only one emission center with the excitation maximum at 4.33 eV (286 nm) and emission maximum at 2.12 eV (585 nm) which is completely quenched at T > 50 K. This luminescent center was assigned to Bi3+ in Cd2+ position and it is clear that it is not involved in the NIR PL and UV absorption at 378 nm discussed here. The luminescent centers observed previously in the undoped CsCdBr3 also cannot be invoked to explain the observed spectral features in our case [33-35]. Thus, the single NIR photoluminescent center that we observed in our experiments is related to the subvalent bismuth cations formed in the reductive environment. In fact, the characteristics of the NIR photoluminescent center in bismuth-doped CsCdBr3 closely resemble the optical properties of univalent bismuth cation Bi+ in the related ternary chloride CsCdCl3 (see Table 1 for a comparison) [9]. The positions of the maximum in the NIR-photoluminescence spectrum and two maxima in the excitation spectrum for Bi+-doped CsCdBr3 are all red-shifted in comparison with Bi+:CsCdCl3. A similar red-shift was previously observed in Bi+ absorption spectrum in bromide eutectic melt (AlBr3–NaBr) relative to the chloride eutectic composition (AlCl3–NaCl) [36]. These facts indicate that the substitution of chloride ions by bromide ones in the environment of Bi+ cation reduces the energies of sublevels of the 3P1 and 3P2 manifolds. We adopt here, that two bands in the visible range of the absorption spectrum and in the luminescence excitation spectrum of Bi+-doped ternary halides result from the excitation to the sublevels of crystal-field splitted 3P2 state, while the PL is the result of radiative transitions from the sublevels of the 3P1 state to the 3P0 ground state [9, 11, 26].
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Assuming that Bi+ enters the crystal lattice of CsCdBr3 in the sites of Cs+, the single optical center with a local symmetry of D3h and coordination number of 12 will be formed in this way. Among all previously studied Bi+-doped ternary halides the Bi+:CsCdBr3 one represent the simplest case of a single optical center in a high-symmetry crystal surrounding. The general diagram of low-lying electronic energy states of Bi+ ion in the D3h crystal environment is shown in Fig. 7. The sequence of the splitted sublevels within the 3P1 and 3P2 manifolds is somewhat arbitrary to the moment. Upon cooling to 77 K two bands in the PL excitation spectrum become sharper, but no additional peaks are resolved (Fig. 6). The Gaussian fit of the excitation spectrum reveals that up to 3 Gaussians are needed to fit the intense high-energy band, while the less intense low energy band can be approximated with only one Gaussian. It is reasonable to suggest that the high-intensity band is originated from the solely electric dipole (ED) allowed transition A'1→E' and its complex structure is the manifestation of Jahn-Teller effect of E-e type in the excited degenerate E' state [37]. The ED forbidden transition A'1→E'' can also contribute to this band. The weak low-energy band with a near-Gaussian shape is probably the result of the A'1→A'1 ED forbidden transition to the solely nondegenerate state of the 3P2 manifold. At 77 K, the spectrum of NIR PL also becomes sharper (relative to 300 K, see Fig. 4) and its maximum shifts to the longer wavelength (1078 nm at 77 K vs. 1053 nm at 300 K). Two Gaussians are needed to approximate both the 77 K and 300 K emission spectra. At 77 K the low-energy Gaussian becomes slightly blue-shifted (9277 cm-1 vs. 9197 cm-1 at 300 K) and its relative intensity increases. The high-energy Gaussian experiences a significant red-shift (9376 cm-1 vs. 9738 cm-1 at 300 K) and it is weakened at 77 K. This large temperature variation implies the complex nature of potential energy surface for the emissive state that can also be the result of Jahn-Teller effect in the E'' state of 3P1 manifold. An additional measurement in extended temperature range is needed to elucidate the details of Bi+ photoluminescence in CsCdBr3. The characteristic decay time of Bi+ NIR PL in CsCdBr3 is shorter than in CsCdCl3, see Table 1. This can be the result of larger oscillator strengths for optical transitions from the ground to the excited states in the bromide systems. This increased oscillator strengths are also
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manifested as enlarged molar absorption of Bi+ in AlBr3–NaBr eutectic relative to the chloride one (AlCl3–NaCl) [36]. Turning our attention to the intense absorption band in UV (at 378 nm), we can suggest that it has no relation to the Bi+ impurity studied here or Bi3+ and intrinsic centers in CsCdBr3 reported previously [33-35]. To elucidate the origin of this band an additional measurements on single crystalline undoped CsCdBr3 were accomplished. Two specimens were produced by Bridgman method in neutral (inert atmosphere) and oxidizing (atmosphere with bromine vapors) conditions. It was found that the absorption band at 378 nm is still intense in specimen, prepared in the neutral conditions (6.9 cm-1 absorption coefficient at maximum), while oxidative atmosphere significantly diminishes its intensity (maximal absorption - 1.1 cm-1). Also, this band is more pronounced in the specimens, which cut from the upper part of the boules. Neither Bi+ absorption bands at 632 and 687 nm nor NIR PL were noticed in undoped CsCdBr3. Therefore, it can be concluded that the band at 378 nm is not associated with bismuth doping and originates from the crystal defects, resulted from the partial reduction of Cd2+. This phenomenon is well known and mixed valence cadmium compounds were already invoked to interpret the UV and visible absorption in the partially reduced cadmium halide melts [38]. Conclusions Univalent bismuth cation isomorphically substitutes Cs+ in CsCdBr3 ternary bromide, producing the unique NIR PL center with the D3h local symmetry. The excitation and emission spectra of Bi+ in CsCdBr3 resembles closely the same characteristics of ternary chlorides, but the maxima of the emission and excitation spectra are shifted to the longer wavelengths in ternary bromide. A complex structure of emission and excitation spectra and its marked dependence on the temperature may be the manifestation of Jahn-Teller effect in the degenerate E' and E'' low lying excited states of Bi+. The prepared Bi+:CsCdBr3 single crystalline specimens appear to be a non-hygroscopic material with a large transparency window in the IR and broadband PL in the NIR. This ternary bromide and other similar halide phases doped with univalent bismuth can be promising materials for broadband amplification of NIR emission, as well as wavelength-tunable lasers.
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Acknowledgements The research was supported by the Ministry of Education and Science of the Russian Federation (research project number 14.604.21.0131, unique identifier RFMEFI60414X0131). The corresponding author A.N. Romanov is the researcher at the N.N. Semenov Institute of Chemical Physics.
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References [1] Y. Fujimoto and M. Nakatsuka, Jpn. J. App. Phys., 40 (2001) L279. [2] Y. Fujimoto and M. Nakatsuka, Appl. Phys. Lett., 82 (2003) 3325. [3] E.M. Dianov, Quantum Electron. 42 (2012) 754. [4] X.G. Meng, J.R. Qiu, M.Y. Peng, D.P. Chen, Q.Z. Zhao, X.W. Jiang, C.S. Zhu, Opt. Express, 13 (2005) 1628. [5] X.G. Meng, J.R. Qiu, M.Y. Peng, D.P. Chen, Q.Z. Zhao, X.W. Jiang, C.S. Zhu, Opt. Express, 13 (2005) 1635. [6] A.G. Okhrimchuk, L.N. Butvina, E.M. Dianov, N.V. Lichkova, V.N. Zagorodnev, K.N. Boldyrev, Opt. Lett. 33 (2008) 2182. [7] A.A. Veber, A.N. Romanov, O.V. Usovich, Z.T. Fattakhova, E.V. Haula, V.N. Korchak, L.A. Trusov, P.E. Kazin, V.B. Sulimov, V.B. Tsvetkov, Appl. Phys. B 108 (2012) 733. [8] A.N. Romanov, A.A. Veber, Z.T. Fattakhova, O.V. Usovich, E.V. Haula, L.A. Trusov, P.E. Kazin, V.N. Korchak, V.B. Tsvetkov, V.B. Sulimov, J. Lumin. 134 (2013) 180. [9] A.N. Romanov, A.A. Veber, Z.T. Fattakhova, , M.S. Kouznetsov, K.S. Zaramenskikh, I.S. Lisitsky, V.N. Korchak, V.B. Tsvetkov, V.B. Sulimov, J. Lumin. 149 (2014) 292. [10] A.A. Veber, A.N. Romanov, O.V. Usovich, Z.T. Fattakhova, E.V. Haula, V.N. Korchak, L.A. Trusov, P.E. Kazin, V.B. Sulimov, V.B. Tsvetkov, J. Lumin. 151 (2014) 247. [11] A.N. Romanov, A.A. Veber, D.N. Vtyurina, Z.T. Fattakhova, E.V. Haula, D.P. Shashkin, V.B. Sulimov, V.B. Tsvetkov, V.N. Korchak, J. Mater. Chem. C 3 (2015) 3592. [12] H.-T. Sun, Y. Sakka, H. Gao, Y. Miwa, M. Fujii, N. Shirahata, Z. Bai and J.-G. Li, J. Mater. Chem. (21) 2011 4060. [13] R. Cao, M. Peng, L. Wondraczek and J. Qiu, Opt. Express 20 (2012) 2562. [14] R. Cao, M. Peng, J. Zheng, J. Qiu, and Q. Zhang, Opt. Express 20 (2012) 18505. [15] H.-T. Sun, B. Xu, T. Yonezawa, Y. Sakka, N. Shirahata, M. Fujii, J. Qiu and H. Gao, Dalton Trans. 41 (2012) 11055. [16] H.-T. Sun, Y. Sakka, N. Shirahata, H. Gao and Tetsu Yonezawa, J. Mater. Chem. 22 (2012) 12837. [17] H.-T. Sun, J. Zhou and J. Qiu, Prog. Mater. Sci. 64 (2014) 1.
10
[18] L. Su, H. Zhao, H. Li, L. Zheng, G.Ren, J. Xu, W. Ryba-Romanowski, R. Lisiecki, P.Solarz, Opt. Lett. 36 (2011) 4551. [19] L. Su, H. Zhao, H. Li, L. Zheng, X. Fan, X. Jiang, H. Tang, G. Ren, J. Xu, W. RybaRomanowski, R. Lisiecki, P. Solarz, Opt. Mater. Exp. 2 (2012) 757. [20] L. Su, J. Yu, P. Zhou, H. Li, L. Zheng, Y. Yang, F. Wu, H. Xia, J. Xu, Opt. Lett. 34 (2009) 2504. [21] J. Ruan, L. Su, J. Qiu, D. Chen, J. Xu, Opt. Express 17 (2009) 5163. [22] C. Li, Z. Song, J. Qiu, Z. Yang, X. Yu, D. Zhou, Z. Yin, R. Wang, Y. Xu, Y. Cao, J. Lumin. 132 (2012) 1807. [23] H.-T. Sun, Y. Matsushita, Y. Sakka, N. Shirahata, M. Tanaka, Y. Katsuya, H. Gao, K. Kobayashi, J. Am. Chem. Soc. 134 (2012) 2918. [24] H.-T. Sun, Y.Sakka, N. Shirahata, Y. Matsushita, K. Deguchi, T. Shimizu, J. Phys. Chem. C 117 (2013) 6399. [25] H.-T.Sun, A. Hosokawa, Y. Miwa, F. Shimaoka, M. Fujii, M. Mizuhata, S. Hayashi, S. Deki, Adv. Mater. 21 (2009) 3694. [26] H.L. Davis, N.J. Bjerrum, G.P. Smith, Inorg. Chem. 6 (1967) 1172. [27] A.N. Romanov, F.V. Grigoriev, V.B. Sulimov, Comput. Theor. Chem. 1017 (2013) 159. [28] J.R. Chang, G.L. McPherson, J.L. Atwood, Inorg. Chem. 14 (1975) 3079. [29] G.L. McPherson, A.M. McPherson, J.L. Atwood, J. Phys. Chem. Solids 41 (1980) 495. [30] J.M. Leger, A.M. Redon, C. Andraud, F. Pelle, Phys. Rev. B 41 (1990) 9276. [31] D. Cubicciotti, J. Phys. Chem. 64 (1960) 791. [32] D. Cubicciotti, J. Phys. Chem. 64 (1960) 1506. [33] A. Wolfert, G. Blasse, J. Solid State Chem. 55 (1984) 344. [34] R. Demirbilek Spektroskopische Untersuchungen am CsCdBr3. TU Darmstadt. Ph.D. Thesis (2002). [35] C. Andraud, F. Pelle, J.P. Denis, O. Pilla, B. Blanzat, J. Phys.: Condens. Matter 1 (1989) 1511. [36] N.J. Bjerrum, H.L. Davis, G.P. Smith, Inorg. Chem. 6 (1967) 1603. [37] I.B. Bersuker, The Jahn-Teller Effect, Cambridge University Press (2006). [38] R.A. Potts, R.D. Barnes, J.D. Corbett, Inorg. Chem. 7 (1968) 2558.
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Figure captions Figure 1 a) Boule of Bi+-doped CsCdBr3, produced by the Bridgman method. b) single crystalline specimen of Bi+-doped CsCdBr3 , prepared for optical measurements. Figure 2 Optical absorption spectrum of Bi:CsCdBr3 single crystalline specimen. Figure 3 Photoluminescence emission spectra of Bi+-doped CsCdBr3, excited at different wavelengths (300K). Figure 4 Photoluminescence emission spectra of Bi+-doped CsCdBr3 at 77 and 300 K; Gaussian decompositions (2 Gaussians) of both spectra are shown.
Figure 5 Photoluminescence decay plot for Bi+-doped CsCdBr3 (excitation wavelength is 628 nm, PL wavelength is 1060 nm); inset shows the dependence of characteristic decay time on the emission wavelength within the photoluminescence band.
Figure 6 Photoluminescence excitation spectra of Bi+-doped CsCdBr3 at 77 and 300 K; Gaussian decomposition (4 Gaussians) of the low temperature spectrum is shown.
Figure 7 Diagram of the lowest excited states of Bi+ impurity center (6p2 configuration) in CsCdBr3, produced by simultaneous action of spin-orbit interaction and crystal field of D3h symmetry; inset shows the first coordination sphere of Bi+ impurity ion (in the place of Cs+, coordination number CN = 12).
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Table 1. Characteristics of NIR photoluminescence of Bi+ impurity center in CsCdBr3 and CsCdCl3 ternary halides. Crystal phase
CsCdBr3
CsCdCl3
77 K
300 K
77 K
300 K
Maximum in PL emission spectrum, nm
1078
1053
1032
976
Maximum in PL
major
630
632
617
615
excitation spectrum, nm
minor
683
689
640
653
250
400
360
PL decay time, μs
Highlights:
Single crystals of Bi+-doped ternary bromide CsCdBr3 were prepared. Broadband NIR photoluminescence was observed from Bi+-doped CsCdBr3. Single optical center is responsible for NIR emission in Bi+-doped CsCdBr3.
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