Zn2+–Eu3+ energy transfer and calculation of Eu3+ 5D0 quantum efficiency

Fa-Bin Cao; Xing-Rong Wu; Liao-Sha Li

doi:10.1557/jmr.2011.279

Zn2+–Eu3+ energy transfer and calculation of Eu3+ 5D0 quantum efficiency

Published online by Cambridge University Press: 20 September 2011

Fa-Bin Cao ,

Xing-Rong Wu and

Liao-Sha Li

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Fa-Bin Cao*: Affiliation:
Anhui Provincial Key Laboratory for Metallurgical Engineering and Resources Recycling, Anhui University of Technology, Maanshan, Anhui 243002, People’s Republic of China
Xing-Rong Wu*: Affiliation:
Anhui Provincial Key Laboratory for Metallurgical Engineering and Resources Recycling, Anhui University of Technology, Maanshan, Anhui 243002, People’s Republic of China
Liao-Sha Li*: Affiliation:
Anhui Provincial Key Laboratory for Metallurgical Engineering and Resources Recycling, Anhui University of Technology, Maanshan, Anhui 243002, People’s Republic of China
*: a)Address all correspondence to these authors. e-mail: yjsun7410@yahoo.com.cn
b)e-mail: music@ahut.edu.cn
c)e-mail: liliaosha@ahut.edu.cn

Article contents

Abstract
INTRODUCTION
EXPERIMENTAL SECTION
RESULTS AND DISCUSSION
CONCLUSIONS
References

Abstract

With 2 mol% Zn2+ codoping and 2 mol% K+ charge compensation, the red-emitting phosphor [K0.8Y0.63Eu3+0.08Zn0.02][Mo0.2W0.8O4] was synthesized by solid-state reaction. X-ray powder diffraction spectrum indicates that it owns single phase. Through its emission spectra, excitation spectra, and fluorescence decay curves measured, its emission mechanism was mentioned and it was calculated for its partial J-O parameters and quantum efficiency of Eu3+ 5D0 energy level under 395 nm excitation. The results indicate that Eu3+ 5D0 → 7F2 red luminescence in the host can be excited by 395 nm, but its quantum efficiency can be improved in space and it has potential applications for white light-emitting diode as the red luminescent materials.

Keywords

Eu Optical properties Luminescence

Information

Type: Articles
Information: Journal of Materials Research , Volume 26 , Issue 18 , 28 September 2011 , pp. 2407 - 2413

DOI: https://doi.org/10.1557/jmr.2011.279 [Opens in a new window]
Copyright: Copyright © Materials Research Society 2011

I. INTRODUCTION

Nowadays a practical efficiency target is 100 lm W⁻¹ of a phosphor-converted light-emitting diode (LED) illumination, which is comparable to high-end white fluorescent lamps. Improving the quantum efficiency, stability, linearity, and temperature performance of phosphors are the most important issues to meet that target. Especially the improvement of the internal quantum efficiency of the presently applied materials can itself hardly contribute to that goal, since LED phosphors, such as YAG:Ce, (Ca,Sr)S:Eu, and (Ca,Sr)₂Si₅N₈:Eu, are already operating at the physical limit as far as the internal quantum efficiency is concerned. Namely, a drawback is the weak absorption of Eu³⁺ in the blue and even in the near-ultraviolet (UV).

The main strategy to solve that problem has focused on materials that have broad and intense charge-transfer (CT) absorption bands in the near-UV and are therefore capable of efficiently absorbing the emission from InGaN chips(370–420 nm). Such materials include oxysulfides, vanadates, molybdates, and tungstates, where the oxygen to Eu³⁺ CT band is very intense and broad and the position is at rather low energy, as for e.g., in Y₂O₂S:Eu or Gd₂O₂S:Eu.Reference Shionoya and Yen¹

A recent idea is to convert near-UV via the 4f–4f transitions of Eu³⁺ located at 394 nm (⁷F₀–⁵L₆), which is rather intense in tungstates and molybdates because they own large band gap (so as not to absorb the emitted radiation) and are stiff (easily excited lattice vibrations can lead to nonradiative relaxation, which increases the efficiency). Following the excitation process, Eu³⁺ relaxes nonradiatively to the ⁵D₀ state, which is the emitting energy level. For instance, the composition Gd₂Mo₃O₉ shows twice higher light output than Y₂O₂S:Eu for 395 nm excitation.Reference Zhao, Wang, Chen, Meng, Di, Ren and Yang²

Those results feed the hope that a Eu³⁺ phosphor, which is sufficiently strongly luminescent under 394 nm and/or 465 nm excitation, can be found in the future.

In our previous research work,Reference Cao, Tian, Chen, Xiao and Wu³^–Reference Cao, Li, Tian, Wu, Chen and Xiao¹⁰ (Ca,Sr)(Mo)O₄:Eu³⁺ and (Ca,Sr)(Mo,W)O₄:Eu³⁺ were prepared and its luminescent properties were improved to some degree by charge compensation and codoping under 394 nm excitation. But its emission abilities cannot meet the requirement of LED illumination. In this article, [K_0.8Y_0.63Eu³⁺_0.08Zn _0.02][Mo_0.2W_0.8O₄] was prepared, and its quantum efficiency and partial J-O parameters were calculated under 395 nm excitation. It is found that its emission ability is improved through Zn–Eu energy transfer.

II. EXPERIMENTAL SECTION

A. Synthesis

A series of red-emitting phosphors were synthesized by solid-state reaction. The starting materials were K₂CO₃ (A.R. grade), Eu₂O₃ (99.99%purity), MoO₃ (A.R. grade), H₂WO₄ (A.R. grade), Y₂O₃ (A.R. grade), and ZnCO₃ (A.R. grade). According to stoichiometric ratio, the reactants were weighed and ground, and the precursor was obtained. The precursor was prefired at 600 °C for 3 h and then fired at 900 °C for 4 h in the air. The red-emitting phosphor (pink-white substance) was gained.

B. Characteristics

The structures of the products were recorded by X-ray powder diffraction (XRD) employing CuK_α radiation at 40 kV and 250 mA. A step size of 0.02° (2θ) was used with a scan speed of 4 °/min. Excitation and emission spectra were measured by using a Hitachi F-4600 spectrometer (Hitachi Co. Ltd., Tokyo, Japan) equipped with a 150 W-xenon lamp under a working voltage of 500 V. The excitation and emission slits were set at 2.5 nm and scanning speed was 1200nm/min. All the measurements were performed at room temperature.

III. RESULTS AND DISCUSSION

A. Structural characterization of [K_0.8Y_0.63Eu³⁺_0.08Zn _0.02][Mo_0.2W_0.8O₄]

To characterize the phase purity and crystallinity of the sample, the XRD pattern of [K_0.8Y_0.63Eu³⁺_0.08Zn _0.02][Mo_0.2W_0.8O₄] is shown in Fig. 1. With K⁺, Y³⁺, Eu³⁺, and Zn²⁺ ions introduced in the host, the crystal structure of [K_0.8Y_0.63Eu³⁺_0.08Zn _0.02][Mo_0.2W_0.8O₄] is consistent with JCPDS card No. 29-0351, corresponding to the intrinsic diffraction patterns of CaMoO₄. The sample owns single phase, which have a tetragonal crystal structure with space group of I4₁/a(88).

FIG. 1.

X-ray powder diffraction pattern of [K_0.8Y_0.63Eu³⁺_0.08Zn_0.02][Mo_0.2W_0.8O₄].

In general, it is thought that ions replacing each other should abide by the laws that its ion radium, coordination number, and electronegativity are near to each others.Reference Pires and Davolos¹¹ With Eu³⁺, Y³⁺, K⁺, Zn²⁺, and W⁶⁺ ions coming into the host, Eu³⁺, Y³⁺, K⁺, and Zn²⁺ ions substitute partial Ca²⁺ ion sites and W⁶⁺ ions replace some Mo⁶⁺ ion positions. But, it is not clear for the doped ions coordination circumstances in the host lattice. To explore the coordination circumstances of introduced ions in the host, the calculations of the radius percentage difference between the introduced ions (Y³⁺/1.22, Eu³⁺/1.2, K⁺/0.82, Zn²⁺/1.65, W⁶⁺/2.36) and the substituted ions (Ca²⁺/1.00, Mo⁶⁺/2.16) in the host are summarized in Table I, based on the equation, D _r = 100*[R _m(CN) − R _i(CN)]/R _m(CN), where D _r is the radius percentage difference, CN the coordination number, R _m(CN) the radius of the host cation and R _i(CN) the radius of introduced ion. The values of D _r between Eu³⁺/Y³⁺/K⁺/Zn²⁺ and Ca²⁺ on six- and eight-coordinated sites are 5.3 and 4.8%/−2.0% and 8.9%/−37% and −22.3%/10% and 19.6%, respectively, while D _r between Mo⁶⁺ and W⁶⁺ on four- and six-coordinated sites are −2.4% and −1.7%. So, it is thought that the CN of Eu³⁺/Y³⁺/K⁺/Zn²⁺/W⁶⁺ is 8/6/4/8/6.

TABLE I.

Ionic radii difference percentage (D _r) between matrix cations and doped ions.

CN, Coordination number, the data for the effective ionic radii are from Ref. Reference Shannon12.

B. Spectral analysis of [K_0.8Y_0.63Eu³⁺_0.08Zn _0.02][Mo_0.2W_0.8O₄] and energy transfer mechanism

There are excitation spectrum of [K_0.8Y_0.63Eu³⁺_0.08Zn_0.02][Mo_0.2W_0.8O₄] under 614 nm monitoring and its emission spectrum under 395 nm excitation in Fig. 2. In the left excitation spectrum, there are one broad band from 200 to 450 nm (304 nm from the CT transitions of MoO₆, WO₆, and EuO₈,Reference Sheu, Chang, Kuo, Su, Wu, Lai, Tsai, Chi and Wu¹³ 362 nm from ⁷F₀ → ⁵D₄ transition, 377 nm and 381 nm from ⁷F₀ → ⁵L₇ transition, 395 nm from ⁷F₀ → ⁵L₆ transition) and two sharp peaks at 466 nm (splitting, 472 nm) and 534 nm (splitting, 527 nm/539 nm), respectively, corresponding to ⁷F₀ → ⁵D₂ transition and ⁷F₀ → ⁵D₁ transition. According to Ref. Reference Huh, Shim, Kim and Rag Do14, based on the excitation spectra, the assignments are given that the CT band of the MoO₆ groups covers the arrange of 225–375 nm and that of the WO₆ groups locates around 250–330 nm. Referring to the data reported in other Eu³⁺-doped phosphors,Reference Wang, Liang, Gong and Su¹⁵ the CT band of EuO₈ should be located around 250–320 nm. Under 614 nm monitoring, it is the most interesting that a broad band comes in being from 200 to 450 nm by uniting five peaks into one whole part because of CT band from host and 4f–4f transitions band of Eu³⁺ overlapping in [K_0.8Y_0.63Eu³⁺_0.08Zn _0.02][Mo_0.2W_0.8O₄]. In the right emission spectrum of [K_0.8Y_0.63Eu³⁺_0.08Zn_0.02][Mo_0.2W_0.8O₄] under 395 nm excitation, there are eight peaks attributed to ⁵D₀ → ⁷F₀ (581. nm, 232 a.u.), ⁵D₀ → ⁷F₁ (two peaks are situated at 590 and 596 nm, respectively, and their intensity are 1049 and 1343 a.u., respectively), ⁵D₀ → ⁷F₂ (614 nm/6379, 631 nm/493 a.u.), ⁵D₀ → ⁷F₃ (657nm, 216 a.u.), and ⁵D₀ → ⁷F₄ (693 nm/129 a.u., 708 nm/527 a.u.) in Table II. In general, ⁵D₀ → ⁷F_2,4,6 transitions are electric dipole transitions. In theory, electric dipole transitions are generally forbidden since the parity does not allow the transition between opposite parities, for example, 5d and 4f. But in the experiments, the forbidden is partly relieved because Eu³⁺ occupies a non-central-symmetric site when Eu³⁺ was doped in the host. So some narrow emission peaks are presented corresponding to ⁵D₀ → ⁷F_2,4 transitions in the experiments.

FIG. 2.

Excitation spectrum of [K_0.8Y_0.63Eu³⁺_0.08Zn_0.02][Mo_0.2W_0.8O₄] under 614 nm monitoring and its emission spectrum under 395 nm excitation.

TABLE II.

Transition positions and intensity of photoluminescence of [K_0.8Y_0.63Eu³⁺_0.08Zn_0.02][Mo_0.2W_0.8O₄] at ambient temperature.Footnote ^a

^a Under 395 nm excitation or under 614 nm monitoring.

Figure 3 depicts emission spectra of [K_0.8Y_0.63Eu³⁺_0.08Zn_0.02][Mo_0.2W_0.8O₄] and [K_0.8Y_0.65Eu³⁺_0.08][Mo_0.2W_0.8O₄] under 395 nm excitation. In the general, transition metal ions or others rare ions act on sensitizing luminous centers (Eu³⁺ ions) by codoping with rare ions in the host.Reference Lee, Luo, Cheng, Diau and Chen¹⁶^–Reference Solarz and Ryba-Romanowski¹⁹ With Zn²⁺ doped in the K–Y–Mo–W host, the luminescent intensity of [K_0.8Y_0.63Eu³⁺_0.08Zn_0.02][Mo_0.2W_0.8O₄] increases 138% (Zn²⁺) compared to the one of [K_0.8Y_0.65Eu³⁺_0.08][Mo_0.2W_0.8O₄]. It is investigated that Zn²⁺ codoping with Eu³⁺ in the materials can improve the emission properties of luminescent system because Zn²⁺ ions play the role of sensitizers in the host lattice, which can improve its emission efficiency.

FIG. 3.

Emission spectra of [K_0.8Y_0.63Eu³⁺_0.08Zn_0.02][Mo_0.2W_0.8O₄] and [K_0.8Y_0.65Eu³⁺_0.08][Mo_0.2W_0.8O₄] under 395 nm excitation.

According to no emission peaks of Zn²⁺ ions observed in the emission spectra while Zn²⁺ doped in the host improving the emission property of [K_0.8Y_0.63Eu³⁺_0.08Zn_0.02][Mo_0.2W_0.8O₄], one energy transfer mechanism is mentioned in Fig. 4. For energy transfer, the optimal concentration of sensitizers (Zn²⁺) is much lower than one of activators (Eu³⁺)Reference Zhang, Lu and Bo²⁰ and the most likely energy transfer path of Zn²⁺ → Eu³⁺ is energy transport of current carrier. Two mol% Zn²⁺ ions doped luminescent materials may bring about energy level (or named level trap). The 4f–4f transition of Eu³⁺ generates a lot of cavities under the UV excitation (process 1). A part of cavities at once combine with the electrons-excited to emit light (process 2), and other part of cavities are released from valence band and captured by Zn²⁺ trap level (process 3 and 4) by means of valence band. The cavities are released under thermal perturbation and absorbed by Eu³⁺ through valence band (process 5 and 6). Thus, this path improves the luminescence efficiency of Eu³⁺, and Zn²⁺ ions act as the sensitizers.Reference Shinn and Sibley²¹^–Reference Sabbagh Alvani, Moztarzadeh and Sarabi²³

FIG. 4.

Energy transfer path diagram of Zn²⁺ → Eu³⁺ in [K_0.8Y_0.63Eu³⁺_0.08Zn_0.02][Mo_0.2W_0.8O₄].

C. Charge compensation

With Zn²⁺ ions introduced into the K–Y–Mo–W host lattice, Zn²⁺ ions substitute Y³⁺ ions situation. Zn²⁺ ions divalent cations and Y³⁺ ions trivalent cations Zn²⁺ ions substituting Y³⁺ ions produces charge defect so [K_0.8Y_0.63Eu³⁺_0.08Zn_0.02][Mo_0.2W_0.8O₄] needs charge compensation. One Zn²⁺ ion occupies one Y³⁺ ion site, and one anion vacancy is introduced. In this work, with 2 mol% K⁺ used as charge compensators, the emission intensity of [K_0.8Y_0.63Eu³⁺_0.08Zn_0.02K_0.02][Mo_0.2W_0.8O₄] can be improved obviously, up to 120% compared to the one of [K_0.8Y_0.63Eu³⁺_0.08Zn_0.02][Mo_0.2W_0.8O₄] in Fig. 5. It is interesting that K⁺ ions are used as host compositions and charge compensators and its charge compensation model is one vacancy + Y³⁺ = Zn²⁺ + K⁺. In a word, Zn²⁺ ions destroying charge balance are attributed to Zn²⁺ ion substituting Y³⁺ ion site in unequal valence.

FIG. 5.

Emission spectra of [K_0.8Y_0.63 Eu³⁺_0.08Zn_0.02N_0.02][Mo_0.2W_0.8O₄] (N = K⁺ or no) under 395 nm excitation.

D. Calculating Ω_λ and quantum efficiency of Eu³⁺ ⁵D₀ energy level by J-O theory

As far as the Eu³⁺ ⁵D₀ → ⁷F_J (J = 0–6) transition is concerned, ⁵D₀ → ⁷F₁ is magnetic dipole transition, ⁵D₀ → ⁷F_2,4,6 electric dipole transitions, and ⁵D₀ → ⁷F_3,5 contain both electric dipole and magnetic dipole transition components. The emission of ⁵D₀ → ⁷F_5,6 transition cannot be observed in the experiments. Table III shows the average energies (cm⁻¹) of Eu³⁺ ⁵D₀ → ⁷F_J (J = 0–4) transitions.

TABLE III.

The average energies (cm⁻¹) and emission peaks situation/area of Eu³⁺ ⁵D₀ → ⁷F_J (J = 0–4) of [K_0.8Y_0.63Eu³⁺_0.08Zn_0.02][Mo_0.2W_0.8O₄].

To characterize transition rate between different energy levels of Eu³⁺ ions, a group of transition intensity parameters Ω_λ (λ = 2, 4) called J-O parameters are introduced into Judd-Ofelt theory. Ω_λ can be influenced by environment around Eu³⁺ ions. Especially, Ω₂ has a close relationship to the environment around Eu³⁺ ions. For the K–Y–Mo–W host, Ω₂ reflects structure symmetry, order property, and covalent character of K–Y–Mo–W interaction. In a word, the host system that has larger Ω₂ is in a more polarizable chemical environment and has stronger covalent character of K–Y–Mo–W interaction. Thus, transition intensity parameters can help us to know the lattice-sites occupied by Eu³⁺ ions in K–Y–Mo–W host and the microenvironment around Eu³⁺ ions.

Quantum efficiency of Eu³⁺ ⁵D₀ energy level can also be calculated by experimental data. The fluorescence lifetime τ of ⁵D₀, radiative transition rate A _R, and nonradiative transition rate A _N can be expressed as follows.

(1)

$\tau = {1 \over {A_{\rm{R}} + A_{\rm{N}} }}$

where A _R is the sum of each ⁵D₀ → ⁷F_J (J = 0–4) transition rate, viz. $\mathop\sum\limits_{J = 0,1,2,3,4} {A_{0 - } {_J} } = A_{0 - 0} + A_{0 - 1} + A_{0 - 2} + A_{0 - 3} \,+ A_{0 - 4} = A_{\rm{R}}$ . The A _0-J values are obtained by using the relation: $A_{0 - } {_J} = A_{0 - 1} \left( {{{S_{0 - J} } \mathord{\left/ {\vphantom {{S_{0 - J} } {S_{0 - 1} }}} \right. \kern-\nulldelimiterspace} {S_{0 - 1} }}} \right)\left( {{{\upsilon _{0 - J} } \mathord{\left/ {\vphantom {{\upsilon _{0 - J} } {\upsilon _{0 - 1} }}} \right. \kern-\nulldelimiterspace} {\upsilon _{0 - 1} }}} \right)$ , where S _0–J is the area under the curve related to the ⁵D₀–⁷F_J transition obtained from the spectral data, υ_0–J is the energy barycenter of the 0–J transition. Here A _0–J is the experimental coefficients of spontaneous emissions, namely A _0–1 is the Einstein’s coefficient of spontaneous emission between the ⁵D₀ and ⁷F₁ energy levels, which can be determined as about 50 s⁻¹ and as a reference to calculate the value of other A _0–J.Reference Lin, Tang and Zhang²⁴^–Reference Werts, Jukes and Verhoeven²⁶ So the quantum efficiency of Eu³⁺ ⁵D₀ energy level can be expressed by

(2)

$\eta = {{A_R } \over {A_R + A_{NR} }} = \tau \mathop\sum\limits_{J = 1,2,3,4} {A_{0 - } {_J }$

Fluorescence decay curves (measuring luminescence of ⁵D₀ → ⁷F₂) of ⁵D₀ energy level are measured under 395 nm excitation. The fluorescence decay curves are fitted by exponential decay to obtain the values of τ. Then the value of η would be obtained by Eq. (2). Table III depicts the average energies (cm⁻¹) and emission peaks situation/area of Eu³⁺ ⁵D₀ → ⁷F_J (J = 0–4) of KYEZMW, and Table IV shows the values of τ, A _R, A _R+A _N, and η of the sample.

$A_{0 - 1} \, = \,50\,{\rm s}^{ - 1} ,$

$A_{0 - } {_0} = A_{0 - 1} \left( {{{S_{0 - 0} } \mathord{\left/ {\vphantom {{S_{0 - 0} } {S_{0 - 1} }}} \right. \kern-\nulldelimiterspace} {S_{0 - 1} }}} \right)\left( {{{\upsilon _{0 - 0} } \mathord{\left/ {\vphantom {{\upsilon _{0 - 0} } {\upsilon _{0 - 1} }}} \right. \kern-\nulldelimiterspace} {\upsilon _{0 - 1} }}} \right) = 50\left\,( {{{2342} \mathord{\left/ {\vphantom {{2342} {17461}}} \right. \kern-\nulldelimiterspace} {17461}}} \right)\left[ {17212/\left( {16949 + 16779} \right)/2} \right] = 6.8{\rm{s}}^{ - 1} ,$

$A_{0 - } {_2} = A_{0 - 1} ( {{{S_{0 - 2} } \mathord{/ {\vphantom {{S_{0 - 2} } {S_{0 - 1} }}} \kern-\nulldelimiterspace} {S_{0 - 1} }}} )( {{{\upsilon _{0 - 2} } \mathord{/ {\vphantom {{\upsilon _{0 - 2} } {\upsilon _{0 - 1} }}} \kern-\nulldelimiterspace} {\upsilon _{0 - 1} }}}) = 50\,( {{{66823} \mathord{/ {\vphantom {{66823} {17461}}} \kern-\nulldelimiterspace} {17461}}} ){{[ {( {16297 + 15929} )/2} ]} \mathord{/ {\vphantom {{[ {( {16297 + 15929})/2}]}{[ {( {16949 + 16779} )/2} ]}}} \kern-\nulldelimiterspace} {[ {( {16949 + 16779} )/2} }} = 182.8{\rm{s}}^{ - 1} ,$

$A_{0 - } {_3} = A_{0 - 1} \left( {{{S_{0 - 3} } \mathord{\left/ {\vphantom {{S_{0 - 3} } {S_{0 - 1} }}} \right. \kern-\nulldelimiterspace} {S_{0 - 1} }}} \right)\left( {{{\upsilon _{0 - 3} } \mathord{\left/ {\vphantom {{\upsilon _{0 - 3} } {\upsilon _{0 - 1} }}} \right. \kern-\nulldelimiterspace} {\upsilon _{0 - 1} }}} \right) = 50\,\left( {{{3578} \mathord{\left/ {\vphantom {{3578} {17461}}} \right. \kern-\nulldelimiterspace} {17461}}} \right)\left( {{{15207} \mathord{\left/ {\vphantom {{15207} {16864}}} \right. \kern-\nulldelimiterspace} {16864}}} \right) = 9.2{\rm{s}}^{ - 1} ,$

$A_{0 - } {_4} = A_{0 - 1} \left( {{{S_{0 - 4} } \mathord{\left/ {\vphantom {{S_{0 - 4} } {S_{0 - 1} }}} \right. \kern-\nulldelimiterspace} {S_{0 - 1} }}} \right)\left( {{{\upsilon _{0 - 4} } \mathord{\left/ {\vphantom {{\upsilon _{0 - 4} } {\upsilon _{0 - 1} }}} \right. \kern-\nulldelimiterspace} {\upsilon _{0 - 1} }}} \right) = 50\,\left( {{{7756} \mathord{\left/ {\vphantom {{7756} {17461}}} \right. \kern-\nulldelimiterspace} {17461}}} \right)\left[ {\left( {14426 + 14116} \right)/2/16864} \right] = 8.7{\rm{s}}^{ - 1} ,$

$A_{\rm R} \, = \,A_{0 - 0} \, + \,A_{0 - 1} \, + A_{0 - 2} + \,A_{0 - 3} \, + \,A_{0 - 4} \, = \,257.5\,{\rm s}^{ - 1}$

$A_{\rm R} \, + \,A_{\rm N} \, = \,1/\tau \, = \,1000/0.546\, = \,1831.5\,{\rm s}^{ - 1}$

$\eta \, = \,A_{\rm R} /(A_{\rm R} \, + \,A_{\rm N} )^*100\% \, = \,(257.5/1831.5)^*100\% \, = \,14.06\%$

TABLE IV.

Partial J-O parameters, fluorescence lifetime, transition rates and quantum efficiency of [K_0.8Y_0.63Eu³⁺_0.08Zn_0.02][Mo_0.2W_0.8O₄], CaMoO₄:Eu³⁺ (Ref. Reference Lei and Yan27) and commercial Y₂O₃:Eu³⁺ Ref. Reference Liao, Xu and Qiu28) under 395 nm excitation.

Thus, the ratio between electric dipole transition rate and magnetic dipole transition rate can be expressed as follows.

(3)

${{A_{0 - J} } \over {A_{md} }} = {{e^2 k_J^3 } \over {S_{md} k_{md}^3 }}{{(n^2 + 2)^2 } \over {9n^2 }}\Omega _J {\langle} {\psi J\left\| {U^\lambda } \right\|} \right.\left. {\psi ^' J^' } \right{\rangle} ^2$

Transition rate of each energy level is in direct proportion to integral intensity of emission spectrum, namely ${{A_{0 - } {_J } \over {A_{md} }} = {{\int {I_J (k)dk} } \over {\int {I_{md} (k)dk} }}$ . Here e, k_J, J′ = 0, Ω_λ $\left{\langle} {\psi J\left\| {U^\lambda } \right\|} \right.\psi ^' \left. {J^' } \right{\rangle} ^2$ , k_md, n, h, and S_md are elementary charge, wave number of electric dipole transition luminescence, for the ⁵D₀ → ⁷F_J, transition intensity parameter, squared reduced matrix element of transition from $| {\psi ^' } \right\left. {J^' } \right{\rangle}$ to $\left{\langle} {\psi J |} \right.$ , wave number of ⁵D₀ → ⁷F₁ luminescence, refractive index of host 1.6,Reference Binnemans, Van Herck and Gờrller-Walrand²⁹ Planck constant, and the line strength of ⁵D₀ → ⁷F₁ transition 7.83 × 10⁻⁴² esu(c) cm²,Reference Boyer, Vetrone, Capobianco, Speghini and Bettinelli³⁰ respectively. The squared reduced matrix elements of ⁵D₀ → ⁷F_J (J = 2, 4) transitions are 0.0032 for J = 2 and 0.0023 for J = 4.Reference Lei and Yan²⁷^, Reference Kodaira, Claudia, Brito and Felinto³¹^, Reference Reisfeld, Greenberg, Brown, Drexhage and Jørgensen³² All parameters in this section used Gauss system of units. Then Ω_J can be calculated utilizing emission spectrum according to Eq. (3). Table IV shows the values of Ω₂ and Ω₄ (Ω₂ = 7.61 × 10⁻²⁰ cm² and Ω₄ = 0.713 × 10⁻²⁰ cm²).

It is noteworthy that the Ω₂ and Ω₄ intensity parameters for the KYEZMW phosphor are higher than that for CaMoO₄:Eu³⁺ phosphorsReference Lei and Yan²⁷ in Table IV, indicating the higher hypersensitive behavior of the ⁵D₀ → ⁷F₂ transition of Eu³⁺ ions in K–Y–Mo–W host matrix. Consequently, by comparing the Ω₂ intensity parameters between Mo and Mo–W systems, it can be concluded that the Mo–W system is in a more polarizable chemical environment. Therefore, this might suggest that the site symmetry occupied by the Eu³⁺ ions in molybdates-tungstates does not have the character of a centrosymmetric chemical environment. Moreover, the higher emission quantum efficiency is obtained from the [K_0.8Y_0.63Eu³⁺_0.08Zn_0.02][Mo_0.2W_0.8O₄] phosphors (η = 14.06%) than in CaMoO₄:Eu³⁺ phosphors (η = 12.78%), which is due to appreciably decrease in nonradiative decay rates from the ⁵D₀ level. But its quantum efficiency is much lower than commercial Y₂O₃:Eu³⁺’s (η = 55.6%) so there is a long way to go for application in the future.

IV. CONCLUSIONS

With 2 mol% Zn²⁺ ions doping and 2 mol% K⁺ ions charge compensation, [K_0.82Y_0.63Eu³⁺_0.08Zn_0.02][Mo_0.2W_0.8O₄] photoluminescent materials with good luminescent properties were synthesized by solid-state method and its possible energy transfer mechanism were mentioned. From the perspective of its crystal structure and emission mechanism, quantum efficiency is improved due to WO₆ introduced into the host and is still low for relaxes between energy levels occur so plenty of energy is lost in the transfer process. According to the characteristics of reduced matrix for ⁵D₀ → ⁷F_J transitions, J-O transition parameters Ω₂ and Ω₄ can be calculated, Ω₂ = 7.61 × 10⁻²⁰ cm² and Ω₄ = 0.713 × 10⁻²⁰ cm², from Eu³⁺ ions emission spectrum under 395 nm excitation and energy transfer efficiency is improved, η = 14.06% (compared to CaMoO₄, η = 12.78%). So Eu³⁺ ions can be used as an efficient spectroprobe in studying the microstructure of materials.

ACKNOWLEDGMENTS

The authors are grateful for Anhui Provincial Natural Science Foundation of Education Agency of China (Grant.No.KJ2011Z032 and Grant.No.KJ2009A69), Anhui Provincial Natural Science Foundation (Grant.No.090414150) of China, and Anhui Provincial International Cooperation of Science and Technology Agency of China (Grant No.09080703019).

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FIG. 1. X-ray powder diffraction pattern of [K0.8Y0.63Eu3+0.08Zn0.02][Mo0.2W0.8O4].

TABLE I. Ionic radii difference percentage (Dr) between matrix cations and doped ions.

FIG. 2. Excitation spectrum of [K0.8Y0.63Eu3+0.08Zn0.02][Mo0.2W0.8O4] under 614 nm monitoring and its emission spectrum under 395 nm excitation.

TABLE II. Transition positions and intensity of photoluminescence of [K0.8Y0.63Eu3+0.08Zn0.02][Mo0.2W0.8O4] at ambient temperature.a

FIG. 3. Emission spectra of [K0.8Y0.63Eu3+0.08Zn0.02][Mo0.2W0.8O4] and [K0.8Y0.65Eu3+0.08][Mo0.2W0.8O4] under 395 nm excitation.

FIG. 4. Energy transfer path diagram of Zn2+ → Eu3+ in [K0.8Y0.63Eu3+0.08Zn0.02][Mo0.2W0.8O4].

FIG. 5. Emission spectra of [K0.8Y0.63 Eu3+0.08Zn0.02N0.02][Mo0.2W0.8O4] (N = K+ or no) under 395 nm excitation.

TABLE III. The average energies (cm−1) and emission peaks situation/area of Eu3+5D0 → 7FJ (J = 0–4) of [K0.8Y0.63Eu3+0.08Zn0.02][Mo0.2W0.8O4].

TABLE IV. Partial J-O parameters, fluorescence lifetime, transition rates and quantum efficiency of [K0.8Y0.63Eu3+0.08Zn0.02][Mo0.2W0.8O4], CaMoO4:Eu3+ (Ref. 27) and commercial Y2O3:Eu3+ Ref. 28) under 395 nm excitation.

Article contents

Zn2+–Eu3+ energy transfer and calculation of Eu3+ 5D0 quantum efficiency

Abstract

Keywords

Information

I. INTRODUCTION

II. EXPERIMENTAL SECTION

A. Synthesis

B. Characteristics

III. RESULTS AND DISCUSSION

A. Structural characterization of [K0.8Y0.63Eu3+0.08Zn 0.02][Mo0.2W0.8O4]

B. Spectral analysis of [K0.8Y0.63Eu3+0.08Zn 0.02][Mo0.2W0.8O4] and energy transfer mechanism

C. Charge compensation

D. Calculating Ωλ and quantum efficiency of Eu3+ 5D0 energy level by J-O theory

IV. CONCLUSIONS

ACKNOWLEDGMENTS

References

REFERENCES

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Conflicting interests

A. Structural characterization of [K_0.8Y_0.63Eu³⁺_0.08Zn _0.02][Mo_0.2W_0.8O₄]

B. Spectral analysis of [K_0.8Y_0.63Eu³⁺_0.08Zn _0.02][Mo_0.2W_0.8O₄] and energy transfer mechanism

D. Calculating Ω_λ and quantum efficiency of Eu³⁺ ⁵D₀ energy level by J-O theory