Downconversion process in Yb3+doped GdAG nanocrystals

Downconversion process in Yb3+doped GdAG nanocrystals

Journal of Luminescence 193 (2018) 70–72 Contents lists available at ScienceDirect Journal of Luminescence journal homepage: www.elsevier.com/locate...

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Journal of Luminescence 193 (2018) 70–72

Contents lists available at ScienceDirect

Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

Downconversion process in Yb3+doped GdAG nanocrystals ⁎

MARK

Robert Tomala , Karina Grzeszkiewicz, Dariusz Hreniak, Wieslaw Strek Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Wroclaw, Poland

A B S T R A C T The efficient near infrared 2F5/2→2F7/2 downconversion emission of Yb3+ was observed upon excitation of Gd3Al5O12:Yb3+ nanocrystals in UV range. Its integrated intensity was measured as a function of excitation power in the range 1–30 mW and compared for a low and high concentration of Yb ions. The model of luminescence dependency on absorbed pump power was applied to investigate the emission mechanism. It was found that the N parameter of that model, interpreted as the order of the process, was dependent on concentration of Yb ions. Moreover, the presence of Yb2+ has been confirmed. The mechanism of energy transfer processes leading to observed Yb3+ emission was discussed in terms of a sequence of successive cross relaxations via a presence of divalent and trivalent Yb ions: Gd3+→Yb2+→ (Yb3+)*.

1. Introduction

2. Experimental

A concept to apply luminescent materials for enhancing photoresponse of photovoltaic cells was subject of elaborated works by Prof. Renata Reisfeld [1]. One of the possibilities is downshifting of solar UV radiation into near infrared range accessible for silicon cells. The process can be based on multi-photon cutting mechanism via multi-ion cooperative relaxations proposed by Dexter [2] that can convert high energy photon into lower energy photons. Mechanism of quantum cutting phenomena in rare earth doped materials was intensively investigated in last years. This interest is associated with increase of efficiency of photoresponse of photovoltaic cells in third generation solar cells. Among the different lanthanide systems the energy relaxation by downconversion or downshifting mechanisms was investigated most frequently between Gd3+-Eu3+ [3] Pr3+ -Yb3+[4] Tb3+-Yb3+ [5–7], Eu3+- Yb3+ [8], and Er3+- Yb3+ [9]. In this work we report the observation of the energy transfer process from excited Gd3+ ions to neighbouring Yb3+ acceptors in Gd3Al5O12:20%Yb3+ nanocrystalline powder. The intense Yb3+ luminescence has been recorded under UV laser excitation at 266 nm. The role of Yb2+ ions in energy transfer from Gd3+ to Yb3+ in (Gd0.8Yb0.2)3Al5O12 nanocrystals was investigated. The mechanism of energy downshifting via quantum cutting is discussed in terms of sequence of different cross relaxation transitions: Gd3+ →Yb2+ → (Yb3+)*.

2.1. Preparation and structure



Corresponding author. E-mail address: [email protected] (R. Tomala).

http://dx.doi.org/10.1016/j.jlumin.2017.06.038 Received 13 May 2017; Accepted 17 June 2017 Available online 20 June 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.

Nanocrystalline powders of ytterbium (1 and 20 mol percent) doped gadolinium aluminum garnet (GdAG:Yb) were prepared by Pechini method. Gadolinium and ytterbium nitrate were prepared at first time. Stoichiometric amount of gadolinium oxide (99.999%) and ytterbium oxide (99.999%) were dissolved in an aqueous nitric acid. Then, the stoichiometric amount of aluminum nitrate nonahydrate solution (99.99%), citric acid (in molar ratio of 1:5 in respect of cations) and ethylene glycol (in molar ratio of 1:1 in respect of citric acid) were added to mixture aqueous of nitrate solution of gadolinium and ytterbium. The mixture was stirred for one hour. Then the solution was heated at 90 °C for 1 week to obtaining a brown resin. At final stage, the dried resin was heated at 1100 °C for 16 h. The structure of powders was confirmed by XRD measurement (see Fig. 1). The X-ray patterns were recorded using PANalytical X′Pert Pro powder diffractometer (Cu Kα1: 1.54060 Å). The diffraction peaks correspond to the reference pattern of cubic Gd3Al5O12 structure (PDF #01-073-1371). The average crystallite sizes calculated using Rietveld refinement method are 56 and 57 nm for samples doped with 1% and 20% of ytterbium, respectively. 2.2. Optical measurements Absorption spectra were measured with a Cary Varian 14 spectrophotometer. The excitation and emission spectra as well as luminescence decay curves were recorded using the FLS980 Fluorescence

Journal of Luminescence 193 (2018) 70–72

R. Tomala et al.

Fig. 1. The XRD patterns of (Gd0.8Yb0.2)3 Al5O12 nanocrystals annealed at 1000 °C.

Fig. 3. The emission of Gd3Al5O12: Yb3+ nanocrystals excited by 266 nm excitation An inset presents the influence of excitation power on the Yb3+ emission intensity of 2I5/2 → 2 I7/2 transition of (Gd0.8Yb0.2)3Al5O12 and (Gd0.8Yb0.01Y0.19)3Al5O12 nanocrystals excited with laser beam at 266 nm.

Spectrometer from Edinburgh Instruments equipped with 450 W Xenon lamp and 100 W Xenon pulse lamp and with detectors: standard photomultiplier Hamamatsu R928P and NIR PMT cooled by liquid nitrogen for near infrared emission.

excitation power of 266 nm laser irradiation on integrated intensity of F5/2 → 2F7/2 transition. This dependence is classically described by Nphoton power law PN where N is the order parameter determined by the slope of the log-log plot. One can see that the downconversion intensity increases in a log-log scale linearly with excitation power with a slope N=1.19 for high concentration and N=1.35 for low concentration of Yb ions. Assuming three photon cutting for downconversion process Gd3+→ 3(Yb3+)* the slope should be close to 1/3 [5]. The difference is surprising and suggests the different mechanism to be responsible for observed downshifted emission. The order parameter N is significantly larger than 1, N=1.35 for low concentration and decreased to N=1.19 for higher concentration of Yb ions. It means the one-photon process cannot be responsible solely for downconversion as it was discussed by Terra et al. [8] in Tb3+ - Yb3+ co-doped Calibo glasses where N=1.02. Therefore, there is a question, what the mechanism could be responsible for the energy downconversion in our case. To answer, it should be noted that the Yb3+ transition is accompanied by a weak intensity broad band centred at around 13,000 nm that may be assigned to the 2F7/2eg → 3F3 transition of Yb2+ ion. In a range 15,000–27,000 cm−1 there are located other weak intensity emission bands that could be assigned to Yb2+ ion following the results of Solomonov [10]. This observation lead to the inclusion of Yb2+ excited states into present considerations. The possibility of participation of Yb2+ excited states in energy transfer process and its bridging character between Gd3+ and Yb3+ ions was investigated by luminescence decay time measurements. The emission decay curves recorded for the emission of Yb3+ at 1 029 nm upon 266 nm excitation are shown in Fig. 4. The decay curves of emission intensity I(t) are strongly nonexponential. However, for both low and high concentration of Yb, it can be well fitted by biexponential decay function, where two decay times τ1 and τ2 are obtained 2

3. Results and discussion The absorption spectrum of Gd3Al5O12:Yb3+ nanocrystals with low (1%) and high (20%) Yb ions concentration measured at room temperature are shown in Fig. 2. There are recorded several sharp absorption bands assigned to trivalent Yb3+ and Gd3+ ions in the investigated range. The 2F7/2→2F5/2 transition of Yb3+ ion was observed at around 10,000 cm−1 as well as two group of bands assigned to the 8 S7/2→6P7/2, 5/2, 3/2 and 8S7/2→6IJ transitions of Gd3+ ion at 32,000 cm−1 and 36,400 cm−1 respectively. In addition, the two high intensity and one weaker broad bands were observed. Following Solomonov et al. [10] these bands may be assigned to divalent ytterbium ions Yb2+. The bands of Gd3+ are overlapped with the 3F3,4→2F5/2 eg and 3F3,4→2F5/2 t2 g transitions of Yb2+. A lack of absorption of Yb3+ dimers is presumably associated with overlap by absorption bands of Yb2+ ion. The luminescence spectrum of GdAG:Yb nanocrystals with low (1%) and high (20%) Yb ions concentration measured upon 266 nm excitation is shown in Fig. 3. The excitation pump directly the 6IJ state of Gd3+ and 2F7/2 t2 g of Yb2+. The most intense emission band located at 9 718 cm−1 for both samples is assigned to the 2F5/2→2F7/2 transition of Yb3+. Its shape and relative Stark level intensities are not changing in function of Yb concentration. Inset of Fig. 3 shows the influence of

I (t ) = k1 e−t / τ1 + k2 e−t / τ1

(1)

where the pre-exponential factors k1 and k2 are proportional to amounts of Yb2+ and Yb3+ ions, respectively. Based on this model, the decay curves were split into two exponential decays with two different amplitudes. The slow component was determined by decay time τ2=1.66 ms and the fast one τ1=0.12 ms for sample doped with 20% Yb. The slow decay time may be ascribed to trivalent Yb3+ ions, due to its well-known decay times of the order of ms, whereas the fast decay time may be associated with emission 3F2 → 3F3,3F4 of divalent ytterbium Yb2+ state that is located at the same energy range as the 2F7/ 2 3+ . The value of fast component increased with 2→ F5/2 emission of Yb

Fig. 2. Absorption spectra of Gd3Al5O12: Yb3+ nanocrystals measured at room temperature.

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Fig. 4. The luminescence decay of Yb3+ in (a) (Gd0.8Yb0.01Y0.19)3Al5O12 and (b) (Gd0.8Yb0.2)3Al5O12 nanocrystals measured at 1029 nm upon 266 nm excitation.

increasing amount of divalent Yb ions an amount of Yb3+ decreases and in result an effective number of cross relaxations between Yb2+ and Yb3+ decreases. Therefore we observe a lower order parameter N for higher concentration. 4. Conclusions The process of downconversion of UV excitation of Gd3+ into NIR emission of Yb3+ in Gd3Al5O12:x%Yb3+ nanocrystals was investigated. The experiments were performed for low x=0.01 and high x=0.2 concentration of Yb ions. The excitation power dependence characterized by the low power order parameter N significantly larger than 1 indicates that observed emission is not associated with the photon cutting process. It was found that concentration of Yb ions affects significantly the downconversion. The results were discussed assuming a presence of trivalent and divalent states of Yb in Gd3Al5O12 host. A presence of Yb2+ was confirmed by absorption and emission spectra. Since the UV irradiation excites simultaneously Gd3+ and Yb2+ states the different radiative and nonradiative relaxations may occur. The mechanism of downconversion was discussed in terms of multistep transitions mediated by cross-relaxation processes associated with excited states of Yb2+ and Yb3+ ions.

Fig. 5. The energy level diagrams of Yb3+, Gd3+ and Yb2+ ions in (Gd0.8Yb0.2)3 Al5O12 nanocrystals and the scheme of downconversion process. CR indicates cross relaxation step.

increasing Yb in contrast to value of slow component. Moreover with increasing Yb concentration amount of Yb2+ ions increased in relative to Yb3+ ions [11]. The scheme of energy levels of Yb3+, Gd3+ and Yb2+ ions in cubic crystal field is presented in Fig. 5. Following this diagram the mechanism of downconversion may be discussed in terms of successive relaxation transitions: Gd3+ →Yb2+ → (Yb3+)*. In the first step (1) the 266 nm pumping gives access to simultaneous excitation Gd3+ and Yb2+ ion corresponding to the 8S7/2→6IJ absorption transition of Gd3+ and 3F3,4→2F5/2t2 g absorption transition of Yb2+. Then the process of energy transfer from Gd3+ to Yb2+ and radiative and nonradiative relaxations take place. The level structure of Yb2+ allows to occur three different cross relaxation processes between Yb2+ emission transitions and Yb3+ ground state absorption as: (3) [4f135d(2F7/2t2 g)→4f135d(2F5/2eg)] ⟺(6)[ 2F7/2→2F5/2]; (4) [4f135d (2F5/2eg) →4f135d(2F7/2eg)]⟺ (6)[ 2F7/2→2F5/2]; and (7) [4f136s (3F2,1F3) → 4f136s(3F3,3F4)] ⟺(6) [ 2F7/2→2F5/2]. It means that with

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