Photoluminescence and ratiometric fluorescence temperature sensing abilities of zincate phosphors

Highlights

• Temperature sensing strategy based on dual transition metal is proposed.

• Designed Ti⁴⁺/Mn⁴⁺ co-doped phosphors via site-favorable occupation principle.

• Single-composition optical temperature indicator involving dual transition metal.

Abstract

Nowadays, the fluorescent materials applying to the temperature sensing field are mostly based on the rare-earth ions, scarcely non-rare-earth co-doped optical thermometers have been invested, let alone the phosphors with dual transition metal ions. Herein, we advocate a novel rare-earth free activated fluorescent material co-doped with dual transition metal ions and adopt the fluorescence intensity ratio-based strategy to measure the temperature. Firstly, this kind of optical material possesses the feasibility to be mass-produced for its intrinsic merits of economic rationality and nontoxicity. Besides, benefitting from the different thermal properties of sensitizer and activator ions, this material is supposed to be an excellent material for temperature sensing. It turns out that this Ti⁴⁺,Mn⁴⁺ coactivated optical material possesses an excellent temperature measuring performance with the optimal absolute sensitivity as high as 3.79% K⁻¹. What is more, the prominent signal discriminability and accurate temperature detection ability could be enabled deriving from two well-separated emission bands and excellent temperature resolution. Furthermore, the photoluminescence properties, the crystalline structure, the crystal field strength, and the viability of the Chromaticity Coordinates-based method to measure temperature are manifested in this paper as well. Overall, it is anticipated that this novel coactivated optical material with binary luminescent centers could be a promising candidate for temperature sensing, and this preliminary study would invoke researchers’ passion for exploring more dual activators-based optical thermometric materials in the absence of rare-earth ions.

Introduction

As a rudimentary variable, temperature plays a substantial role in meteorology, agriculture, as well as military fields [[1], [2], [3], [4], [5]]. Different from conventional contact thermometer, optical non-invasive thermometry with excellent luminescence signal detection and celerity response is more adaptive to brutal surroundings like rapid moving objects, and hyperfine sized surfaces [6,7]. Compared with other temperature measurements based on the material's fluorescence property, such as emission bandwidths [8], emission lifetime [9], spectrum phase shift and so on [10], fluorescence intensity ratio (FIR) is prioritized to be utilized in the temperature sensing field for its intrinsic merit of being insusceptible to non-temperature factors.

To date, considerable attention has been focused on the rare-earth (RE) ion systems as their abundant and available 4f–4f transitions that are sensitive to temperature [[11], [12], [13]]. Due to the innate boundedness of low accuracy, indistinct sensitivity and ordinary signal discriminability aroused from the small energy gap (200 cm⁻¹≤ΔE≤2000 cm⁻¹) between the thermally coupled levels, luminescent ratiometric technology based on thermally coupled level pairs of lanthanide ions has been gradually faded [[14], [15], [16]]. Some previous works also reported the single transition metal (TM) Mn⁴⁺/Cr³⁺ doped phosphors by utilizing the FIR of forbidden and allowed transitions between ²E→⁴A₂ and ⁴T₂→⁴A₂ as the temperature measurement criterion. The thermal sensitivity of such a system depends on the parabola's distortion belonging to the ground and excited states, which could be influenced by the crystal field strength. The susceptibility to thermal quenching, however, is restricted by the population of electrons in these two levels identifying with Boltzmann's law. Therefore, the intrinsic limitation of the maximum sensitivity stimulates us to work on the optical materials with dual luminescent centers [2]. So far, the FIR strategy-based materials have been tremendously investigated, including sole RE or TM ion combined with self-reference host [17], dual-emitting nanoscale temperature sensors base on co-doped dual RE ions or RE/TM ions [18,19], RE/quantum-dots hybrid nanostructures [20], dual luminescent centers of single lanthanide ion located at different crystal sites, and optical material containing different valences of doping RE ion [21].

In this article, we approached the neo-coactivated optical material involving dual TM ions with disparate temperature variations Ca₁₄Al₁₀Zn₆O₃₅:Ti⁴⁺,Mn⁴⁺, which can be abbreviated to CAZO: Ti⁴⁺,Mn⁴⁺. By adopting the variation of relatively temperature-susceptible Ti⁴⁺ emission intensity as the temperature detecting signal while the temperature-unimpressionable Mn⁴⁺ as the reference one, and in the absence of RE ions, Ca₁₄Al₁₀Zn₆O₃₅:Ti⁴⁺,Mn⁴⁺ phosphor with the advantages of being environmentally friendly and economically rationally is supposed to obtain a high temperature sensitivity. Although the Ti⁴⁺-activated phosphors were scarcely reported due to the unstable luminescence performance resulted from its naked 3d electron orbit (3d⁰), free from the inherent disadvantage of photon re-absorption phenomenon in RE ions, studies have proved that Ti⁴⁺ ion provides a broad emission range from ultraviolet to blue and can act as an excellent sensitizer ion to optimize the activator's luminescence property [4,22,23]. Meanwhile, due to the particular 3d³ electronic configuration, Mn⁴⁺ exhibited both outstanding thermal resistances resulted from strong electron-phonon coupling together with the existence of anti-Stokes, and high susceptibility to the coordination surroundings, especially the crystal field strength [[24], [25], [26]]. In addition, the zincate compound Ca₁₄Al₁₀Zn₆O₃₅ has already been corroborated as a splendid octahedral substrate for single-phase phosphors [22,[27], [28], [29]].

Hereby, the thermal sensitivity of such dual TM doped system highly relies on the distortion of the parabolas of the ground and excited states, caused by the influence of the crystal field strength. Benefitting from activator's sensibility to strong field strength, anti-Stokes phenomenon, and energy transfer from sensitizer to the activator, the thermal properties of doped ions vary, thus a high absolute sensitivity has been obtained as 3.79% K⁻¹. What is more, Mn⁴⁺ ion displayed a well-separated photoluminescence band originating from spin-forbidden ²E→⁴A₂ transition from the emission peak of Ti⁴⁺, providing an excellent signal discriminability and accurate temperature detection. Together with the outstanding thermal fatigue resistance, it is concluded that this novel dual activators-based optical thermometric material in the absence of RE ions is a promising candidate in temperature sensing application.