Eu3+-activated Ca3Mo0.2W0.8O6 red-emitting phosphors: A near-ultraviolet and blue light excitable platform for solid-state lighting and thermometer
Introduction
Considering its admirable merits (e.g., small volume, high luminous efficiency, low pollution, high brightness, etc.), the phosphor converted white light emitting diode (white-LED) plays an important role in the field of solid-state lighting [[1], [2], [3], [4], [5]]. Commercial white-LEDs are currently composed of blue-emitting InGaN chip and yellow Y3Al5O12:Ce3+ phosphors. However, they expose defects of the low color rending index (CRI < 80) and high correlated color temperature (CCT > 7000 K) owing to lack of red components in their emission spectra [[6], [7], [8], [9], [10]]. Aside from this commercial route, the combination of near-ultraviolet (NUV) chips, blue, green, and red light-emitting phosphors is employed to produce excellent white light with tunable color coordinate, high CRI and low CCT [11]. Significantly, searching for a novel NUV/blue light excitable single emitting phosphor is very important to achieve the high quality white-LED lamp. Although some red-emitting phosphors, such as Sr[LiAl3N4]:Eu2+, (Ca,Sr)S:Eu2+, Y2O2S:Eu3+ and K2TiF6:Mn4+, have been currently reported, the efficiency of the red phosphor is still much lower than that of the green and blue phosphors [[11], [12], [13], [14], [15]]. Moreover, nitrides require rigorous reaction conditions, including high reaction temperature and reduced atmosphere, while oxysulfides and sulfides display poor thermal and chemical stability [16,17]. Furthermore, the Mn4+ ions doped fluoride compounds suffer from low thermal stability and low water-resistance as well as a complex synthesis process [18,19]. Consequently, new red-emitting phosphors with high stability and efficiency which can absorb the emission wavelength of the NUV (350–410 nm) or blue (450–470 nm) chips is urgently developed [[20], [21], [22]].
On the other hand, non-contact optical thermometers, especially those based on rare-earth ions excited luminescent compounds, have been extensively studied because of their high sensitivity, high resolution and rapid response [23]. Generally, the temperature is measured by monitoring temperature-dependent photoluminescence parameters, such as, fluorescence intensity ratio (FIR), lifetime, bandwidth, etc [[24], [25], [26]]. As for the FIR-based optical thermometer, it depends on the emission intensity ratios of two thermal coupled levels (TCL) from rare-earth (RE) ions (Er3+, Dy3+, Tm3+, Ho3+, etc.) under different temperatures [27,28]. Furthermore, the dual-emitting centers (Bi3+-Eu3+, Tb3+-Eu3+ and Pr3+-Tb3+, etc.) strategy, which is relied on the diverse responses of these two ions to the temperature, were received much attention [[29], [30], [31]]. Besides, the optical thermometry was also realized by some researchers through monitoring the relation between lifetime and temperature. Manohar et al., reported that LaAlO3:Sm3+ nanophosphors had promising applications in optical thermometry by analyzing the temperature-dependent lifetime [32]. Through monitoring the lifetime at diverse temperatures, Li et al., also demonstrated that Ca8ZnLa(PO4)7:Tb3+,Eu3+ and Lu2MoO6:Sm3+ phosphors were potential candidates for optical thermometer [29,33]. In spite of these previously reported results, the investigation on the thermometric properties of the RE ions activated phosphors based on their lifetimes is still not enough. As a result, more efforts should be made.
Among trivalent RE ions, Eu3+ ion has received extensive attention due to intense red emission from the 5D0 → 7F2 transition [10,34]. However, the Eu3+ ion itself has defects with narrow excitation lines and small absorption cross-sections [35]. Therefore, selecting appropriate hosts, which can effectively absorb the excitation energy and then transfer it to the Eu3+ ions, is required. Recently, the double perovskites have gained widespread attention owing to their excellent physical properties (e.g., electrical behaviors, thermal stability, etc.) [11]. Generally, for A2B′B″O6 -type double perovskite compounds, the A-site cation is coordinated with eight oxygen ions to form BO8 dodecahedron without inversion centrosymmetry, while the B′ and B″ cations are coordinated with six oxygen ions to generate the BO6 octahedron with inversion centrosymmetry site [35,36]. As is known, the charge transfer bands (CTBs) of WO4 groups and WO6 groups are usually located at about 270–325 nm and 250–350 nm, respectively [35,37]. In comparison, the CTBs of MoO4 groups and MoO6 groups are usually sited in the wavelength range of 300–350 nm and 300–425 nm, respectively, since the Mo is more electronegative than W, and then lead to the energy level of the 5d orbital of W is higher than that of the 4d orbital of Mo [17]. Wang et al., have studied the effect of local structural change on the thermal quenching performance of the Ca3MoxW1-xO6:Eu3+ phosphors and found that the Ca3Mo0.2W0.8O6:Eu3+ exhibited the excellent luminescence performance [35]. In this work, considering the extended absorption band in the NUV-blue region (360–410 nm), we chose Ca3Mo0.2W0.8O6 (abbreviated as: CMWO) as the host so as to prepare the Eu3+-activated CMWO phosphors. We have studied the properties (i.e., phase composition, electric structure, photoluminescence (PL), decay time, internal quantum efficiency, and thermal stability) of resultant phosphors in detail. The white-LED lamps were fabricated to explore the suitability of the designed phosphors for solid-state lighting. In addition, the thermometric properties of the CMWO:xEu3+ phosphors were also investigated by discussing the temperature-dependent lifetime of Eu3+ ions.
Section snippets
Sample fabrication
The Ca3-xMo0.2W0.8O6 (CMWO:xEu3+; x = 0.00, 0.03, 0.05, 0.07, 0.09, 0.11, 0.12, 0.13, 0.15 and 0.17) phosphors were synthesized by high temperature solid-state reaction. The starting materials, CaO (99.99%), WO3 (99.995%), MoO3 (99.95%) and Eu2O3 (99.99%) were bought from Sinopharm Chemical Reagent Co., Ltd. Based on the corresponding stoichiometric ratio, the raw materials were weighed and ground for 30 min. Afterward, these mixtures were sintered at 1300 °C for 6 h with the heating ratio of
Crystal structure and phase identification
As shown in Fig. 1a, the Gd3WO6 compound is a typical monoclinic double-perovskite structure with the space group of P21/n. Based on the above description, A and B′ sites are Ca2+ ions, whereas B″ site is W6+ ions [35]. The A and B′ sites are occupied by the same element Ca2+, inferring that Ca2+ has two different environments based on the structural feature. The Ca2+ cation at A site without inversion centrosymmetry (C1) is surrounded by eight oxygen ions, while the cations (Ca2+, W6+) at B
Conclusions
In this work, the CMWO:xEu3+ red-emitting phosphors were successfully synthesized by a traditional high-temperature method. The resultant phosphors were found to be excitable by both NUV and blue light. Both Judd-Ofelt theory and the PL emission spectrum demonstrated that the Eu3+ occupied the sites without inversion symmetry in CMWO host lattices. The optimal doping concentration of Eu3+ ions in the selected host lattices was 9% mol and the NR energy transfer between Eu3+ ions was occupied by
CRediT authorship contribution statement
Junpeng Xue: Investigation, Conceptualization, Visualization, Writing - original draft. Hyeon Mi Noh: Data curation, Visualization. Sung Heum Park: Resources. Byung Chun Choi: Resources. Jung Hwan Kim: Writing - review & editing. Jung Hyun Jeong: Conceptualization, Project administration, Supervision. Peng Du: Writing - review & editing, Supervision. Weiguang Ran: Formal analysis, Validation. Mingjun Song: Formal analysis, Validation.
Declaration of competing interest
The authors declare no competing financial interest.
Acknowledgments
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (No.2018R1A2B6005179). The Eu3+ doped Ca3Mo0.2W0.8O6 phosphors were supplied by the K. C. Wong Magna Fund in Ningbo University (xkzw1507) and Functional Phosphor Bank at Pukyong National University (2017M3A9B8069470).
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