Host sensitized near infrared to visible photon upconversion in 1% Re4+ doped Cs2NaYbBr6

Journal of Alloys and Compounds 374 (2004) 60–62

Host sensitized near infrared to visible photon upconversion in 1% Re4+ doped Cs2 NaYbBr6 Annina Aebischer, Hans U. Güdel∗ Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, 3000 Bern 9, Switzerland

Abstract Cs2 NaYbBr6 doped with 1% Re4+ was studied by optical absorption and luminescence spectroscopy. In the cubic host lattice the Re4+ ions substitute for the octahedrally coordinated Yb3+ . Visible by eye red Re4+ [2 T2g ] upconversion (UC) luminescence centered around 14 000 cm−1 is observed upon Yb3+ [2 F5/2 ] near infrared (NIR) excitation around 10 805 cm−1 between 10 and 300 K. An energy transfer (ET) step from Yb3+ [2 F5/2 ] to Re4+ [2 Eg /2 T1g ] followed by an energy transfer upconversion (ETU) process on the Re4+ is shown to be the mechanism responsible for this phenomenon. For Yb3+ excitation at 10 805 cm−1 with a power density of 1.3 kW/cm2 the VIS/NIR emitted photon ratio is 5% at 10 K. © 2003 Elsevier B.V. All rights reserved. Keywords: Phosphors; Crystal growth; Nonlinear optics; Light absorption; Luminescence

1. Introduction The combination of lanthanide (Ln) and transition metal (TM) ions in the same host crystal has long been used for sensitization purposes. As an example, the crystals for flashlight pumping in Nd3+ lasers typically contain Cr3+ as a sensitizer. The broadband spin-allowed d–d absorptions of Cr3+ are able to capture a much bigger portion of the flashlight than the sharpline f–f absorptions of Nd3+ with their small absorption cross-sections. Energy transfer (ET) from Cr3+ to Nd3+ can then occur both radiatively and nonradiatively. It would therefore appear that in mixed Ln/TM systems. TM is always the sensitizer and Ln the activator. In the present paper we report on a system in which the parts are reversed. In Re4+ doped Cs2 NaYbBr6 excitation of Yb3+ can lead to Re4+ luminescence. This comes about because both Yb3+ and Re4+ are somewhat atypical Ln and TM representatives, respectively. In Fig. 1 we show their energy levels up to 15 000 cm−1 . With a (4f)13 electron configuration Yb3+ has only one f–f excited state, i. e. 2 F5/2 at an energy around 10 000 cm−1 . All the f–f oscillator strength is thus concentrated in the 2 F7/2 ↔ ∗ Corresponding author. Tel.: +41-31-631-42-49; fax: +41-31-631-43-99. E-mail address: [email protected] (H.U. Güdel).

0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.11.065

2F

5/2 transition. Since there are no further absorptions up to the UV, Yb3+ is widely used as an upconversion (UC) sensitizer in mixed Ln/Ln containing materials with Tm3+ [1], Er3+ [2], Dy3+ , Ho3+ [3], Nd3+ , Pr3+ [4] as the active UC ions. Both upconversion phosphor and laser materials are based on this principle. In our title material Yb3+ is part of the host, and the host itself acts as a sensitizer. Re4+ is a (5d)3 TM ion. The crystal field is so strong, even in a bromide environment there are no spin-allowed d–d bands below 15 000 cm−1 . As a result of the high covalency of the Re–Br bond, the electron repulsion parameters are so strongly reduced that all the doublet states deriving from the (T2g )3 electron configuration lie in the near infrared (NIR) and visible (VIS) spectral range, see Fig. 1. We have previously shown that Re4+ in an octahedral F− , Cl− , and Br− environment is capable of NIR to VIS photon upconversion [5–7]. This is due to the fact that the Re4+ excitations shown in Fig. 1, being simple spin flips, couple very weakly to the lattice, and bear a lot of similarity to the f–f excitations in lanthanides. In Re4+ doped Cs2 ZrCl6 [6] and Cs2 ZrBr6 [7] excitation into the Re4+ [2 Eg /2 T1g ] multiplet between 8000 and 10 000 cm−1 leads to red Re4+ [7 (2 T2g )] emission at all temperatures between 10 and 300 K. This is a rather inconvenient excitation region, both in the laboratory and for possible applications. For Yb3+ excitation at around 10 300 cm−1 , on the other hand, there exist highly efficient laser diodes, and

A. Aebischer, H.U. Güdel / Journal of Alloys and Compounds 374 (2004) 60–62 -1

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Fig. 1. Energy level diagram of Yb3+ (left) and Re4+ (right) in 1% Re4+ : Cs2 NaYbBr6 . The emitting metastable states and the ground states are highlighted by stronger lines. All the relevant radiative and nonradiative transitions involved in the sensitization and upconversion processes are represented by full and other arrows, respectively. ET, ETU and CR stand for energy transfer, energy transfer upconversion and cross-relaxation, respectively.

in the laboratory a Ti3+ : sapphire laser can be used. The energetic disposition of levels, see Fig. 1, is such that a good spectral overlap of the Yb3+ [2 F5/2 ] emission profile with Re4+ [2 Eg /2 T1g ] absorptions should lead to efficient Yb3+ → Re4+ energy transfer. The objective of the present study is to explore, whether Yb3+ sensitized photon upconversion on Re4+ does occur and what its mechanism and efficiency is. The crystal used in this study was grown at 850 ◦ C by the Bridgman technique using CsBr, NaBr, YbBr3 , and Cs2 ReBr6 as starting materials. Details about the spectroscopy can be found in [7].

2. Results and discussion 2.1. Sensitization Fig. 2c shows the 15 K absorption spectrum of Re4+ : Cs2 NaYbBr6 with an estimated Re4+ concentration of 1%. The Re4+ concentration was estimated on the basis of 300 K absorption spectra and published ε values at 300 K [8]. Absorptions for both Yb3+ and Re4+ are readily identified and labeled accordingly in Fig. 2c. The Re4+ absorption lines are inhomogeneously broadened compared to those in Re4+ : Cs2 ZrBr6 [7] due to the charge compensations and lattice distortions necessary to incorporate Re4+ (63 pm radius) into the host lattice at the Yb3+ (87 pm radius) sites. The Yb3+ [2 F7/2 → 2 F5/2 ] absorption band between 10 000 and 11 000 cm−1 is highly structured. In the octahedral coordination of the elpasolite structure Yb3+ [2 F5/2 ] splits into a 8 and a 7 spinor. A great deal of the observed structure is thus due to vibronic interactions. Unusually, intense vibronic sidebands have been observed for these 2 F7/2 ↔ 2F 3+ in many other lattices [9,10]. 5/2 transitions of Yb Excitation at 10 805 cm−1 , the position of an Yb3+ absorption maximum at 15 K (see dashed arrow in Fig. 2c), leads to the luminescence spectrum shown in Fig. 2d. It is dominated by Re4+ emission with only 1% of the totally

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Energy (cm ) Fig. 2. (a) 15 K upconversion excitation spectrum, detection is at 13 051 cm−1 (arrow in Fig. 2b); (b) 15 K upconversion luminescence spectrum, excitation is at 10 146 cm−1 (full arrow in Fig. 2c); (c) 15 K absorption spectrum of 1% Re4+ : Cs2 NaYbBr6 in the NIR/VIS range; and (d) NIR and VIS luminescence spectrum of 1% Re4+ : Cs2 NaYbBr6 at 10 K. The excitation is into Yb3+ [7 (2 F5/2 )] at 10 805 cm−1 (dashed arrow in Fig. 2c). Inset: temperature-dependence of the VIS/NIR emitted photon ratio for 10 805 cm−1 laser excitation with 1.3 kW/cm2 power density.

emitted photons emitted by Yb3+ . Confirmation that Yb3+ excitation leads to Re4+ emission is provided by the 15 K upconversion excitation spectrum shown in Fig. 2a. The excitation lines exactly coincide with the Yb3+ [2 F5/2 ] absorption lines in Fig. 2c. From these observations we conclude that the Yb3+ [2 F5/2 ] to Re4+ [2 Eg /2 T1g ] sensitization step is highly efficient in this crystal. This is a remarkable result, considering that we have a Re4+ doping level of 1%. The Yb3+ [2 F5/2 ] excitation is mobile and can be considered as excitonic in Cs2 NaYbBr6 . In addition, the Yb3+ → Re4+ excitation trapping step must be very efficient. We have observed a similarly high ET rate in the analogous Re4+ doped CsNaYbCl6 [11]. 2.2. Upconversion The NIR excited Re4+ [7 (2 T2g )] emission around 13 000 cm−1 (see Fig. 2d), which is visible by eye up to

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room temperature, is clear evidence that photon upconversion does take place. In Fig. 2b this upconversion emission spectrum is enlarged, and we see, besides the dominant Re4+ [Γ7 (2 T2g )] band, a much weaker Re4+ [8 (2 T2g )] band around 14 500 cm−1 . This behavior is very similar to that observed in Re4+ : Cs2 ZrBr6 [7]. In analogy to that system and for energetic reasons we conclude that the mechanism depicted in Fig. 1 is at work in our case. Two Yb3+ excitations are consecutively injected into Re4+ by nonradiative Yb3+ [2 F5/2 ] to Re4+ [2 Eg /2 T1g ] energy transfer. Population density accumulates in the Re4+ [8 (2 T1g )] level, and by an energy transfer mechanism upconversion (ETU) to Re4+ [8 (2 T2g )] takes place. The main relaxation mechanism is by multiphonon relaxation to Re4+ [7 (2 T2g )], see curly arrow in Fig. 1. In contrast to fluorides and chlorides, in which this is quantitative, in a bromide there is a competing cross-relaxation (CR) (see Fig. 1) mechanism bypassing Re4+ [7 (2 T2g )], and there is about 1% radiative Re4+ [8 (2 T2g )] emission to the ground state (see Fig. 2b). The main VIS emitting state is Re4+ [7 (2 T2g )], and the branching ratio of VIS Re4+ [7 (2 T2g )] → Re4+ [8 (4 A2g )] to IR Re4+ [7 (2 T2g )] → Re4+ [2 T1g /2 Eg ] is about 1:2 [7]. Fig. 2d shows that at 10 K for 10 805 cm−1 Yb3+ [2 F5/2 ] excitation with a laser power density of 1.3 kW/cm2 the ratio of VIS Re4+ [7 (2 T2g )] to NIR Re4+ [8 (2 T1g )] emitted photons is 5%. This is about an order of magnitude higher than for Re4+ [6 (2 T1g )] excitation at 9563 cm−1 and a power density of 415 W/cm2 [7]. This big difference cannot fully be explained by the higher laser power, and we conclude that we can create a higher Re4+ [8 (2 T1g )] population density in our Yb3+ host sensitized 1% Re4+ : Cs2 NaYbBr6 than in 2.5% Re4+ :Cs2 ZrBr6 [7]. As shown in the inset of Fig. 2d the VIS/NIR emitted photon ratio for 1.3 kW/cm2 laser excitation at 11 805 cm−1 into Yb3+ [2 F5/2 ] remains roughly constant between 10 and 100 K, and then it drops to 0.2% at 300 K. This drop is similar to the one observed in Re4+ : Cs2 ZrBr6 [7], but there it is less pronounced. We ascribe this to a higher efficiency of the nonradiative loss processes in the title compound, mainly resulting from the distorted Re4+ sites. The measured VIS/NIR photon ratio is a measure of the overall efficiency of the upconversion process. From studies on 2.5%

Re4+ : Cs2 ZrBr6 [7] we know that at 10 K only about 18% of all the ions excited into Re4+ [8 (2 T2g )] emit a VIS photon. This is a consequence of the strong CR process and the strong interexcited state emission Re4+ [7 (2 T2g )] → Re4+ [2 T1g /2 Eg ], see Fig. 1. Therefore, we have to multiply our observed VIS/NIR ratio by roughly 6 to get a measure of the combined efficiencies of the sensitization and UC processes. From Section 2.1 and [6,7] we know that both of these processes are extremely efficient. The main losses occur after the upconversion step. Unfortunately the VIS/NIR branching ratio at Re4+ [7 (2 T2g )] of 1:2 is rather intrinsic and not easy to engineer by chemical variation. The thermal losses could possibly be reduced by choosing a host lattice which is more adapted to the tetravalent Re4+ guest ion.

Acknowledgements This work was financially supported by the Swiss National Science Foundation. We also thank the Portland-CementStiftung.

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