Mn²⁺ ions substitution inducing improvement of optical performances in ZnAl₂O₄: Cr³⁺ phosphors: Energy transfer and ratiometric optical thermometry

Highlights

• ZnAl₂O₄ without inversion defects are fabricated through doping with Mn²⁺.

• Red emission is enhanced about 15.8 times in ZnAl₂O₄ crystals doped with Mn²⁺ ions.

• The mechanism composited of energy transfer and reducing inversion defects is present.

• The higher sensitivity for optical thermometry is obtained.

Abstract

Zn_1-xMn_xAl₂O₄:0.1 mol% Cr³⁺ (0.0$4\le \mathrm{x}\le 0.16$) phosphors with single spinel phase were synthesized by using sol–gel method and the structure, optical and temperature sensing performances were reported herein. The results of X-ray photoelectron spectra indicate that the inversion defects related to octahedral Zn are reduced and the crystal field surrounding Al changes with Mn²⁺ doping in ZnAl₂O₄ lattices. Mn²⁺/Cr³⁺ co-doped ZnAl₂O₄ nanophosphors reveal a green emission band assigned to Mn²⁺ and a series of red emission peaks assigned to Cr³⁺, respectively. With the concentration of Mn²⁺ increasing, the intensity of zero phonon line (R line) assigned to Cr³⁺ increases, reaching the maximum at the optimal Mn²⁺ concentration of $x=0.14$. The energy transfer from Mn²⁺ to Cr³⁺ is confirmed with the energy transfer efficiency of 83%. The separation between ²E(e_g) and ²E(t_g) of Cr³⁺ is enlarged due to Mn²⁺ dopants giving rise to a change of crystal field. The luminous intensity ratio between two separated emission peaks at 685 nm (R₃) and 689 nm (R₂) reveals an obvious temperature dependence. The relative sensitivity changes from 3.7 %K⁻¹ to 0.25 %K⁻¹ with the temperature increasing from 80 K to 310 K, which is much larger than that of ZnAl₂O₄:Cr³⁺ nanophosphors without Mn²⁺, indicating its good application prospect in optical thermometry.

Introduction

Temperature measurement is a basic and vital subject in agricultural and industrial manufacture, biology and personal daily life. Most of temperature measurement technologies require physical contact commonly, while noncontact temperature measurement is desired in numerous fields such as biological imaging, industrial manufacture, environment monitoring and so on[1], [2], [3]. Optical thermometry has been attracted more attentions due to its high spatial resolution, high detection sensitivity and rapid response in noncontact measurement process[4], [5], [6]. In many types of optical thermometers, the temperature sensing based on fluorescence intensity ratio (FIR) has been widely investigated recently due to its significant accuracy and reliability. The performance of temperature sensing can be evaluated by the relative sensitivity (S_r) which can be improved by optimizing the matrix materials and dopants[7], [8], [9], [10]. Most investigations are focused on the rare earth (RE) doped variety of host lattices due to their excellent optical performances and stable energy level transition, e.g. Ho³⁺-doped Na_0.5Bi_0.5TiO₃ ceramics (S_r = 1.03 %@377 K, 6.14 %@167 K)[11], Er³⁺-doped SrLaAlO₄ phosphors (S_r = 1.42 %K⁻¹@298 K)[12], Tm³⁺ and Yb³⁺ co-doped LiNbO₃ single crystal[13], Eu³⁺, Tb³⁺ co-doped Gd₂(MoO₄)₃ phosphors[14], and so on, have been reported recently.

While the expensive price of RE limits their application. Transition metals have been considered as the alternatives of RE due to their excellent optical properties and popular price[1], [15]. Some transition elements, such as Mn, Cr and so on, exhibit obvious temperature dependence of photoluminescence. Zhu et al. [16] reported the high sensitivity optical thermometry of Mn²⁺/Mn⁴⁺ co-doped BaAl₁₂O₁₉/SrAl₁₂O₁₉ system with the relative sensitivity of 4.37 % %K⁻¹@293 K. The FIR property of Mg₂TiO₄:Mn⁴⁺ phosphor with relative sensitivity of $0.0254$ %K⁻¹[17] and Sn²⁺/Mn²⁺ co-doped Sr₂P₂O₇ phosphor with relative sensitivity of 0.027 %K⁻¹[18].

Comparing to the Mn ions, Cr³⁺ has the more chemical stability, thus attracts more attentions recently. In the host materials with strong crystal field, the emission assigned to the transition from the ²E state to ⁴A₂ (ground state) is dominated, and the ²E level splits into ²E($\overline{E}$) and ²E($2\overline{A}$) levels due to Cr³⁺ replacing the Al³⁺ in distorted octahedral sites, giving rise to two lines of R₁ (²E$(2\overline{A})\to$⁴A₂, 687 nm) and R₂ (²E$(\overline{E})\to$⁴A₂, 689 nm), as shown in Fig. 1 (a). The N-lines are assigned to Cr³⁺ in the inversion defects environment. The luminous characteristics of Cr³⁺ are strongly affected by the thermal quenching effects and crystal field[19], [20], thus the temperature dependence of fluorescence intensity ratio, lifetime, emission wavelength is suitable for optical thermometry, and the feasibility of Cr-doped luminescence sensors for luminescence thermometry has been investigated in various materials, such as ZnGa₂O₄[21], Al₂O₃[22], YAlO₃[23], MgAl₂O₄[24], BeAl₂O₄[25] and so on. The Ueda et al. investigated the temperature dependence of the photoluminescence (PL) spectrum of Cr³⁺ doped ZnGa₂O₄, and reported the temperature measurement performance based on the fluorescence intensity ratio between ⁴T₂→⁴A₂ to ²E→⁴A₂ luminescence bands with a high S_r of 2.8 %/K at 310 K[26]. Glais et al. reported an optical thermometer of Cr and Bi co-doped ZnGa₂O₄ phosphors with the S_r of 1.7 %/K at 190 ℃[21]. However, the emission band assigned to ⁴T₂→⁴A₂ transition is difficult to observe at low temperatures in ZAO crystals. Furthermore, the emissions related to N lines, Stokes and Anti-Stokes reveal wide and overlap spectra and reveal same change trend with temperature decreasing, so it is difficult to confirm the integral intensity accurately. When Mn²⁺ enters zinc aluminate lattices, the crystal field surrounding Cr³⁺ should be changed, and gives rise to the ²E$(2\overline{A})$ level shifting furtherly, which results in a new emission (R₃ line) assigned to Cr³⁺ as shown in Fig. 1(b). It is interesting to investigate whether R₃ line is considered as a new reference signal.

In this paper, Mn²⁺/Cr³⁺ co-doped ZnAl₂O₄ (ZAO) phosphors were fabricated by common sol–gel method. The energy transfer process from Mn²⁺ to Cr³⁺ has been investigated with various concentrations of Mn²⁺. With the concentration of Mn²⁺ increasing, the reversion defects are reduced and the separation between ²E($\overline{E}$) and ²E($2\overline{A}$) level splits furtherly. The relative intensity of N lines assigned to Cr³⁺ surrounded inversion defects in spinel lattices is reduced, as well as the Stocks and anti-Stocks lines become weaker. The temperature dependence of photoluminescence spectra of Mn²⁺ and Cr³⁺ co-doped ZnAl₂O₄ phosphors reveals better temperature sensitivity than that of Cr³⁺ single doped ZnAl₂O₄ phosphors, indicating the potential application in optical thermometry.