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Luminescence performance of Cr3+ doped and Cr3+, Mn4+ co-doped La2ZnTiO6 phosphors
Journal Pre-proof Luminescence performance of Cr3+ doped and Cr3+ , Mn4+ co-doped La2 ZnTiO6 phosphors Jinhong Ou (Conceptualization) (Validation)Formal analysis Investigation) (Writing - original draft), Xiaoliang Yang (Conceptualization) (Methodology), Siguo Xiao (Writing - review and editing)
PII:
S0025-5408(19)32298-6
DOI:
https://doi.org/10.1016/j.materresbull.2019.110764
Reference:
MRB 110764
To appear in:
Materials Research Bulletin
Received Date:
6 September 2019
Revised Date:
28 December 2019
Accepted Date:
31 December 2019
Please cite this article as: Ou J, Yang X, Xiao S, Luminescence performance of Cr3+ doped and Cr3+ , Mn4+ co-doped La2 ZnTiO6 phosphors, Materials Research Bulletin (2020), doi: https://doi.org/10.1016/j.materresbull.2019.110764
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Luminescence performance of Cr3+ doped and Cr3+, Mn4+ codoped La2ZnTiO6 phosphors
Jinhong Ou, Xiaoliang Yang, Siguo Xiao*
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School of Physics and Optoelectronics, Xiangtan University, Hunan 411105, China *Author to whom correspondence should be addressed: Electronic mail: [email protected] Fax number: +8673158292468 Phone number: +8615573224832 Graphical abstract
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Cr3+ doped and Cr3+, Mn4+ co-doped La2ZnTiO6 phosphors have been prepared by a high-temperature solid-state method and their luminescence properties are investigated in detail. A unique luminescence performance of the Cr3+, Mn4+ co-doped LZT is found.
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Highlights:
‧ Luminescence properties of Cr3+ in La2ZnTiO6 are reported for the first
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‧ Total luminescence intensity is significantly improved by codoping with Mn4+. ‧ Cr3+，Mn4+ co-doped phosphor refers new approach to design novel wavelength detector. 1
Abstract
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Cr3+ doped and Cr3+, Mn4+ co-doped La2ZnTiO6 phosphors were prepared using solid-state reaction method and their luminescence performance has been investigated. La2ZnTiO6: Cr3+ phosphor exhibits red luminescence band centred at 740 nm when excited with light in spectral range from ultraviolet to blue and green (300 nm-500 nm). Both emissions at 740 nm (Cr3+) and at 710 nm (Mn4+) can be observed in the Cr3+, Mn4+ co-doped phosphor and its total luminescence intensity excited at 337 nm is enhanced to 1.6 times with the introduction of Mn4+ ions. The ratio of emission intensity at 710 nm to that at 740 nm in the co-doped phosphor increases monotonically as excitation wavelength increases from 330 to 400 nm. The performance of the developed phosphors in present work indicates that they might be used in indoor cultivation of LEDs and wavelength detection. Keywords: B. Luminescence, A. optical materials
1. Introduction
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Inorganic luminescent materials or the so-called ‘‘phosphors’’ have been continuously developed in the past century and widely used in many fields such as displays, sensors, solar cells, plant cultivation and solid-state lighting, etc. [1-10] Commonly inorganic phosphor are achieved by doping one or more luminescent dopants to a host material (crystal or glass). Rare earth (RE) ions like Tb3+, Eu3+, Ce3+and Eu2+, have always been the most widely used dopants in phosphors [11-14]. However, rare earth doped phosphors have some obvious disadvantages. Firstly, trivalent rare earth ions usually exhibit extremely narrow and weak absorption bands because of their parity-forbidden transition within the 4f configuration, resulting in the very low light absorption efficiency. Secondly, rare earth ions might have strong photon re-absorption in visible region due to their abundant transition channels and thus reduce the final light output of light devices. Thirdly, most rare earth elements tend to be expensive and some rare earth doped phosphors require harsh synthesis conditions. Therefore, developing novel inorganic phosphors without rare earth elements is meaningful for the future market. Transition metal ions, like Mn4+ and Cr3+, may be alternative dopants for rare earth free phosphors. When located in an octahedral coordination site, Mn4+ or Cr3+ ion is possible to be excited with near-ultraviolet and blue light due to their 4A2→4T1 and 4 A2→4T2 transitions. The luminescence wavelength significantly relies on the crystal field strength of the substituted sites and it can shift from red to deep-red in different crystal field environment. Up to now, Mn4+ or Cr3+ doped aluminates, tungstates, gallates, stannates and germinates such as Sr2ZnWO6:Mn4+ [15], MgAl2O4: Cr3+ [16], SrLaGaO4:Mn4+ [17], Li2Mg3SnO6:Mn4+ [18] and Mg3Ga2GeO8:Mn4+ [19] have been widely investigated. However, it is worth noting that titanates are also good host materials for transition metal doped phosphors. Besides the similar ionic radius between Ti4+ and Mn4+/Cr3+ ions, titanates usually contain abundant octahedral sites (Ti-O6), 2
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which provides suitable environment for the luminescence of Mn4+ and Cr3+ ions. Additionally, titanate phosphors doped with transition metal ions can be synthesized without expensive raw materials and complicated processes. As a matter of fact, recently there are some investigations on Mn4+ doped titanate phosphors like Mg2TiO4: Mn4+ [20], Li2MgTiO6: Mn4+ [21] and Gd2ZnTiO4: Mn4+ [22], etc. However, few investigations on Cr3+ doped titanate phosphors have been reported. [23]. In present work, Cr3+ doped as well as Mn4+, Cr3+ co-doped La2ZnTiO6 phosphors are prepared using a solid-state reaction method at high temperature. The crystal structure is characterized with X-ray diffraction (XRD). The excitation spectrum, emission spectrum and concentration dependent luminescence intensity and decay lifetime for the Cr3+ doped La2ZnTiO6 phosphor are investigated systematically. Especially, some novel luminescence properties have been found in the co-doped phosphors in addition to enhancing the luminescence intensity.
2.Experimental
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The La2ZnTi(1-x-y)O6: xCr3+, yMn4+ (x=0.003 − 0.015, y=0 − 0.0011) samples were synthesized by solid-state reaction method at high temperature. According to the chemical proportion, starting materials like TiO2, ZnO, La2O3, Cr2O3 and MnCO3 with analytical reagent (AR) grade were weighed carefully. The mixtures were put in a corundum mortar and ground thoroughly, and then preheated at 900 °C for 8 h in a muffle furnace. After cooling down to room temperature naturally, the reactants were milled again and then heated up to 1275 °C and held for 6 h. The prepared samples were naturally cooled to room temperature and ground into powder for further characterization. X-ray diffraction (XRD) patterns were measured by a diffract (XRD) meter (PIGAKV Ultima IV) using Cu Kα radiation in the 2θ range from 20° to 90°. A monochromator (Zolix Instrument) was applied to obtain the emission and excitation spectra, which equipped with a photomultiplier (PMTH-S1-CR131) and a xenon lamp of 150 W. Both the luminescence decay curves and the temperature dependent photoluminescence spectra with 337 nm excitation were measured with a FLS980 (Edinburgh) spectrometer and a pulse xenon lamp was used as the excitation light.
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Fig. 1. (a) XRD patterns for La2ZnTi(1-x-y)O6: xCr3+, yMn4+ (x=0.005, 0.007, 0.009; y=0.0007, 0.0009) phosphors. (b) Structure of La2ZnTiO6 unit cell.
3.Results and discussion
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The XRD patterns of La2ZnTi(1-x-y)O6: xCr3+, yMn4+ (x=0.005, 0.007, 0.009; y=0, 0.0005, 0.0007) samples and the ICSD card of La2ZnTiO6 (no.172755) are shown in Fig. 1(a). It can be found that there is no significant difference between the diffraction peaks of these samples and that of the pure La2ZnTiO6. At the same time, no peaks of impurities are observed, meaning that the addition of Mn4+ or Cr3+ has little influence on the crystal structure of these samples. La2ZnTiO6 belongs to double-perovskite with a space group of 𝑃121/𝑛1(14) [24]. The crystallographic structure of La2ZnTiO6 is given in Fig. 1(b). From Fig. 1(b) one may see that that Zn2+ and Ti4+ occupy the center positions of two adjacent octahedrons which are formed by O2-, respectively, and La3+ are in cavity sites surrounded by octahedrons. The ionic radii of Cr3+ (CN=6, 0.615 Å) and Mn4+ (CN = 6,0.53 Å) is more similar to that of Ti4+ (CN = 6, 0.61 Å) as comparing with that of Zn2+ (CN = 6, 0.74 Å) and La3+ (CN = 6, 1.03 Å, CN = 8, 1.16 Å). Hence 4
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Cr3+ and Mn4+ ions might tend to occupy the octahedral center sites of Ti4+ [25].
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Fig. 2. (a) Excitation spectra of La2ZnTi0.993O6 :0.007Cr3+. (b) Emission spectra of La2ZnTi0.993O6 :0.007Cr3+. (c) Tanabe–Sugano energy level diagram.
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The excitation spectrum and emission spectrum of the La2ZnTi0.993O6: 0.7%Cr3+ sample are presented in Fig. 2(a) and Fig. 2(b), respectively. As shown in Fig. 2(a), the excitation spectrum monitored at 740 nm shows an excitation band from 300 nm to 500 nm and presents two peaks at around 337 nm and 479 nm. The emission spectrum under 337 nm excitation in Fig. 2(b) shows an emission in 700 ~ 780 nm region with central peak at 740 nm. Cr3+ ions have an electronic configuration as 3d3. The Tanabe-Sugano energy level diagram shown in Fig. 2(c) can be used to express the energy levels of Cr3+ ion in an octahedron environment. The two peaks shown in the excitation spectra at about 337 nm and 479 nm should correspond to 4A2→4T1 and 4A2→4T2 transition, respectively. The emission energy level of Cr3+ will be different with different crystal field. When the crystal-field is strong enough, the 4T2 level of Cr3+ ion is above the 2E level, and the electrons transition from the 2E level back to the ground state and exhibits a sharp emission peak. Conversely, when the crystal-field is weak, there will be a board band emission caused by the 4T2 to 4A2 transition. From the emission spectra, the emission peak at 740 nm should originate from the 2E→4A2 transition. The emission bands on both sides of the peak at 740 nm correspond to anti-Stokes side-bands and Stokes side-bands, which due to the vibration of the 3d3 electrons when Cr3+ locates in an octahedral structure. The local crystal field strength of Cr3+ can be specifically obtained after calculating the 𝐷𝑞/𝐵 value with following formula: [26,27,28]
𝐷𝑞
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𝐵
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= (𝑥 2 −10𝑥)
(1)
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where 𝐷𝑞 can be calculated with following formula: 𝐷𝑞 =
𝐸( 4𝑇2 − 4𝐴2 ) 10
(2)
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and 𝑥 can be expressed as 𝑥=
𝐸( 4𝐴2 − 4𝑇1 )−𝐸( 4𝐴2 − 4𝑇2 ) 𝐷𝑞
(3)
base on the excitation spectra of the LZT: Cr3+, the value of 𝐸( 4𝐴2 − 4𝑇1 ) and 𝐸( 4𝐴2 − 4𝑇2 ) can be estimated to be about 29,673.59 cm-1 and 20,876.83 cm-1, respectively. And then the parameters 𝐷𝑞 and 𝐵 are 2088 and 896 cm-1, respectively. Hence the 𝐷𝑞/𝐵 value is 2.33 and exceeds 2.2, suggesting that Cr3+ ions locate within strong crystal field indeed. 6
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The Cr3+ concentration dependent emission spectra of LZT: Cr3+ samples with 337 nm excitation are measured and depicted in Fig. 3(a). It is found that with the contents of Cr3+ ion increases from 0.3% to 1.5%, there is no obvious change on the emission spectra except for the integral emission intensity. The relationship between integral intensity and Cr3+ ion is further shown in Fig. 3(b). The integral luminescence intensity increases monotonically until the concentration of Cr3+ gets 0.7%, and then begins to weak with Cr3+ concentration further increasing. The decrease of luminescence intensity at Cr3+ concentration beyond 0.7% should be caused by the concentration quenching. With Cr3+ content increasing, the distance between the neighboring Cr3+ ions decreases. Therefore, much more energy transfers between neighboring Cr3+ ions occur, and thus energy is more likely to transfer to luminescent killer sites, leading to the reduction of luminescence intensity at high doping concentration.
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Fig. 3. (a) Emission spectra of La2ZnTi(1-x)O6: xCr3+ (x = 0.003, 0.005, 0.007, 0.009, 0.011, 0.013, 0.015) (λex = 337 nm). (b) Dependence of integral intensity on Cr3+ concentration. (c)
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𝑙𝑛(𝐼/𝑥) vs. 𝑙𝑛(𝑥) relationship based on Eq. (5). (d) The decay curve of La2ZnTi(1-x)O6: xCr3+ (x = 0.003, 0.005, 0.007, 0.009, 0.011, 0.013, 0.015) at 740 nm with excitation at 337 nm.
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In order to determine the mechanism of energy transfer among the Cr3+ ions, the critical distance (𝑅𝑐 ) is approximated according to following relationship firstly: [29, 30] 3𝑉
𝑅𝑐 = 2 (4𝜋𝑥 𝑁) 𝑐
1 3
(4)
in which 𝑥𝑐 is the critical concentration of the activator ion, 𝑉 is the volume of the unit cell, and 𝑁 represents the number of sites that the Cr3+ ion can substitute in a unit cell. For LZT: Cr3+ samples, 𝑥𝑐 = 0.007, 𝑉 = 246.58 Å3 and 𝑁 = 4. The calculated 𝑅𝑐 is 25.62 Å and more than 5 Å. So the energy migration between neighboring Cr3+ ions in La2ZnTiO6 hosts is impossible to be caused by exchange interaction. Consequently, the energy migration mechanism for Cr3+ in LZT host should 8
be multipole–multipole interaction, which can be determined using following formula base on Dexter’s theory: [29, 30,31, 32] 𝐼 𝑥
𝜃 3
= 𝐾 [1 + 𝛽(𝑥) ]
−1
(5)
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where 𝐼 and 𝑥 are the luminescence intensity and the activator concentration, respectively. 𝐾 and 𝛽 are constants in the same host and excitation condition. when 𝜃 equals 6, 8 and 10 means that the multipole–multipole interaction is dipole-dipole, dipole-quadrupole and quadrupole-quadrupole interactions, respectively. The linear fitting plot of 𝑙𝑛(𝐼/𝑥) vs. 𝑙𝑛(𝑥) as shown in Fig. 3(c) can be used to approximate the value of the 𝜃. The fitted value of 𝜃 is about 4.08, most close to 6. It means that the dominant energy transfer mechanism for Cr3+ in LZT host is most likely the electric dipole-dipole interaction. Room temperature luminescence decay curves at 740 nm of La2ZnTi0.993O6: xCr3+ phosphors under 337 nm excitation are also measured, as shown in Fig. 3(d). The decay curves can be fitted with second order exponential equation: [33,34,35] (6)
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𝐼(𝑡) = 𝐴𝑟 𝑒𝑥𝑝(−𝑡/𝜏𝑟 ) + 𝐴𝑠 𝑒𝑥𝑝(−𝑡/𝜏𝑠 )
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In which 𝐼(𝑡) represents the luminescence intensity, 𝑡 is the time. 𝐴𝑟 and 𝐴𝑠 are constants corresponding to 𝜏𝑟 and 𝜏𝑠 , respectively. 𝜏𝑟 and 𝜏𝑠 represent rapid and slow lifetime values for exponential components, respectively. 𝐴𝑟 , 𝐴𝑠 , 𝜏𝑟 and 𝜏𝑠 values can be determined from the fitting lines of the decay curves, and then the effective lifetime constant (𝜏 ∗ ) may be calculated by following formula: [33,34,35] 𝜏 ∗ = (𝐴𝑟 𝜏𝑟2 + 𝐴𝑠 𝜏𝑠2 )/(𝐴𝑟 𝜏𝑟 + 𝐴𝑠 𝜏𝑠 )
(7)
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The effective decay times 𝜏 ∗ are calculated to be 287.08 μs, 279.83 μs, 269.63 μs, 256.13 μs, 249.82 μs, 241.08 μs and 225.02 μs for La2ZnTi(1-x)O6: xCr3+ with x= 0.003, 0.005, 0.007, 0.009,0.011, 0.013 and 0.015, respectively. The measured lifetime for Cr3+ emission decreases with the Cr3+ content increasing. This can be explained by the follow formula: [36] 1 𝜏∗
1
= 𝜏 + 𝐴𝑛𝑟 + 𝑃𝑡 0
(8)
in which 𝜏0 is the radiative lifetime, 𝐴𝑛𝑟 represents the nonradiative rate, 𝑃𝑡 refers to the energy transfer rate among Cr3+ ions. As Cr3+ concentration increases, the distance between Cr3+ ions decreases and thus the energy transfer rate 𝑃𝑡 increases. Consequently, the lifetime shortens at high Cr3+ concentration.[37] To further improve the luminescence performance of the phosphor, Mn4+ ions were co-doped into the LZT:0.7%Cr3+. The spectral characteristics of Mn4+ single-doped LZT samples have already been reported. It has an excitation band ranging from 300 to 9
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600 nm with two peaks at about 370 nm and 507 nm and can give emission band peaking at about 710 nm [38]. The emission spectra of the LZT:0.007Cr3+, xMn4+ (x=0.03%-0.11%) that excited at 337 nm are shown in Fig. 4(a). The emission of Mn4+ (at 710 nm) and Cr3+ (at 740 nm) are both observed. Fig. 4(b) further shows the relationship between the integral intensity and concentration of Mn4+. It’s found that when the concentration of Mn4+ ions is less than 0.07%, the integral intensity of Mn4+ emission centred at 710 nm increases with the enhancement of Mn4+ concentration, while the Cr3+ luminescence intensity is hardly affected by the doped Mn4+ ions. After the Mn4+ concentration exceeds 0.07%, the luminescence intensity of Mn4+ and Cr3+ both decreases due to the concentration quenching. Comparing with the Cr3+ single doped samples, the addition of Mn4+ ions largely extends the emission band and increases the total emission intensity. It is found that the integral intensity of the LZT:0.007Cr3+, 0.0007Mn4+ phosphor has been enhanced to 1.6 times as compared with that of the LZT:0.007Cr3+ phosphor. Fig. 4(c) gives the normalized excitation spectra of Mn4+ monitored at 710 nm and Cr3+ monitored at 740 nm for the LZT:0.007Cr3+, 0.0007Mn4+ sample. It is clearly seen that the ultraviolet light in a wider region can be converted into deep red in Mn4+, Cr3+ co-doped phosphor than that in Cr3+ singly doped one.
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Fig. 4. (a) Emission spectra of La2ZnTiO6: 0.007Cr3+, xMn4+ (x = 0.0003, 0.0005, 0.0007, 0.0009, 0.0011) with excitation at 337 nm. (b) Luminescence intensity as function of Mn4+ content. (c) Normalized excitation spectra of Mn4+ and Cr3+ in LZT.
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The thermal stability for the Mn4+/Cr3+ co-doped LZT, luminescence spectra for La2ZnTi0.9923O6: 0.007Cr3+, 0.0007Mn4+ sample under different temperature (300 K to 450 K) with 337 nm excitation are also measured, and the relationship between relative intensity and temperature is given in Fig. 5(a). The emission intensities of Mn4+ and Cr3+ both weaken as the temperature increases from 300 K to 450 K and the luminescence intensity for Cr3+ decreases faster than that for Mn4+. The temperature dependent luminescence behaviors are attributed to thermal quenching, which might be explained with the configurationally coordinate given in Fig. 5(b). In normal condition, the electrons of Mn4+ and Cr3+ transition from the 2E level to their ground states and emit photons. With the temperature going up from 300 K to 450 K, electrons are affected by thermal excitation and thus have greater possibility to go back to the ground state via non-radiative decay process. Consequently, the luminescence intensity declines with the temperature rising. [21] 11
The activation energy (∆𝐸) is further estimated by following equation: [39,40] 𝐼(𝑇 )
0 𝐼(𝑇) = 1+𝐴𝑒𝑥𝑝(−∆𝐸/𝑘
(9)
𝐵 𝑇)
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in which 𝐼(𝑇0 ) represents the initial intensity and 𝐼(𝑇) represents the luminescence intensity at temperature 𝑇. 𝐴 is constant and 𝑘𝐵 represents the Boltzmann constant and has a value as 8.629×10-5 eVK. According to the equation (9), ∆𝐸 can be estimated by the fitting plot of 𝑙𝑛[𝐼(0)/𝐼(𝑇) − 1] vs. 1⁄𝑘𝑇 as shown in Fig. 5(c). It is 0.44 eV for Mn4+ and 0.36 eV for Cr3+ in the LZT host, respectively.
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Fig. 5. (a) Normalized temperature dependent emission intensity of La2ZnTi0.9923O6: 0.007Cr3+, 0.0007Mn4+in Mn4+ (circle) and Cr3+ (square). (b) Diagram of schematic configuration
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coordinate illustrating thermal quenching of Mn4+ and Cr3+. (c) Relationship between 𝑙𝑛[𝐼(0)/ 𝐼(𝑇) − 1] and 1⁄𝑘𝑇 according to Eq. (9).
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The 700~750 nm far-red light can control the process of plant growth by adjusting the ratio of phytochrome PFR and PR. [41] This means that the developed phosphors in this work have potential application in indoor cultivation of LEDs. In addition, from Fig. 4(c) we may find that the luminescence intensity ratio between Mn4+ and Cr3+ ions might vary monotonically with the excitation wavelength varying from 330 to 400 nm. Therefore, the emission spectra of La2ZnTi0.9923O6: 0.007Cr3+, 0.0007Mn4+ sample are further carefully measured excited at different wavelengths (330 to 400 nm). The normalized luminescence spectra are showed in Fig. 6(a). The relationship between the integral intensity ratio 𝑅 (Mn4+ vs. Cr3+) and the excitation wavelength (330 – 400 nm) is presented in Fig. 6(b). It is found in Fig. 6(b) that the integral intensity ratio 𝑅 increases monotonically with the excitation wavelength increasing. With this performance the co-doped phosphor may act as wavelength detector, since the excitation wavelength can be determined when the integral intensity ratio 𝑅 is obtained by measuring the emission intensity of Mn4+ and Cr3+.[42] The wavelength measurement sensitivity S can be defined as 𝑑𝑅
𝑆 = 𝑑𝜆
𝑒𝑥
(10)
here 𝑅 is the integral intensity ratio of Mn4+ to Cr3+ and 𝜆𝑒𝑥 is the corresponding excitation wavelength. The sensitivity S as function of excitation wavelength 𝜆𝑒𝑥 is also plotted in Fig. 6(b). It is noted that the sensitivity S in 330-400 nm for the Mn4+, Cr3+ co-doped La2ZnTiO6 is evidently higher than that of the Mn4+, Cr3+ co-doped Li2MgTiO6 reported previously. However, it is noted in Fig. 5(a) that the luminescence intensity of Cr3+ in La2ZnTiO6 decreases faster than that of Mn4+ with temperature 13
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increasing, meaning that the integral intensity ratio 𝑅 will vary with the temperature. Therefore, maintaining a stable temperature for the phosphor is necessary to avoid error in wavelength measurement when it acts as wavelength detector.
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Fig. 6. (a) Normalized emission spectra of La2ZnTi0.9923O6: 0.007Cr3+, 0.0007Mn4+ sample excited at different wavelength (330 nm to 400 nm). (b) Relationship between the integral intensity ratio 𝑅 of Mn4+ to Cr3+ and excitation wavelength (square) and relationship between sensitivity S (triangle) and excitation wavelength.
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4. Conclusion
In brief, Cr3+ doped and Cr3+, Mn4+ co-doped LZT phosphors were synthesized with solid-state reaction process. Cr3+ doped LZT has a broad excitation band in region of 300 nm to 500 nm and can give an emission in region of 700 nm ~ 780 nm with central peak at 740 nm. Mn4+/Cr3+ co-doped phosphor not only gives emission of Mn4+ ions but also that of Cr3+ ions. The total luminescence intensity of the co-doped sample is enhanced compared with the Cr3+ singly doped one. In addition, the integral intensity ratio 𝑅 between the Mn4+ and Cr3+ emissions increases monotonically as the excitation wavelength increases from 330 to 400 nm. The excitation wavelength dependent 14
luminescence performance of the co-doped phosphor means that it is capable of detecting wavelength, referring a new possible approach to design future compact spectrometer. Author Statement Xiaoliang Yang: Conceptualization, Methodology. Jinhong Ou: Conceptualization, Validation, Formal analysis, Investigation, Writing Original Draft.
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Siguo Xiao: Writing - Review & Editing
Conflicts of interest There are no conflicts of interest
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Acknowledgments
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This work was supported by the National Science Foundation of China (No.11674272).
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137 (2015) 8936-8939. [13] Y. Xuan, X. Wang, C.S. Liu, X. Yan, Structure and luminescence properties of single-phased BaCa2Y6O12: Eu3+, Dy3+, ECS J. Solid State Sci. Technol. 3 (2014) R216-R221. [14] P. Pust, V. Weiler, C. Hecht, A. Tücks, A.S. Wochnik, A.K. Henß, ... W. Schnick, Narrowband red-emitting Sr [LiAl3N4]: Eu2+ as a next-generation LED-phosphor material, Nat. Mater. 13 (2014): 891. [15] R. Cao, X. Ceng, J. Huang, X. Xia, S. Guo, J. Fu, Adouble-perovskite Sr2ZnWO6:Mn4+ deep red phosphor: synthesis and luminescence properties, Ceram. Int., (2016) S0272884216312561.
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[21] Y. Jin, Y. Hu, H. Wu, H. Duan, L. Chen, Y. Fu, ... M. He, A deep red phosphor Li2MgTiO4: Mn4+ exhibiting abnormal emission: potential application as color converter for warm wLEDs, Chem. Eng. J. 288 (2016) 596-607. [22] H. Chen, H. Lin, Q. Huang, F. Huang, Y. Wang, A novel double-perovskite Gd2ZnTiO6: Mn4+ red phosphor for uv-based w-leds: structure and luminescence properties, J. Mater. Chem. C, 4 (2016) 2374-2381. [23] H. Zhu, M. Fang, Z. Huang, K. Chen, M. Guan, C. Tang, ... M. Wang, Novel chromium doped perovskites A2ZnTiO6 (A= Pr, Gd) Synthesis, crystal structure and photocatalytic activity under simulated solar light irradiation, Appl. Surf. Sci. 393 (2017) 348-356. [24] A. Fu, A. Guan, F. Gao, X. Zhang, L. Zhou, Y. Meng, H. Pan, A novel double perovskite 17
La2ZnTiO6: Eu3+ red phosphor for solid-state lighting: synthesis and optimum luminescence, Opt. Laser Technol. 96 (2017) 43-49. [25] R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 32 (1976) 751-767. [26] J. Xiang, J. Chen, N. Zhang, H. Yao, C. Guo, Far red and near infrared double-wavelength emitting phosphor Gd2ZnTiO6: Mn4+, Yb3+ for plant cultivation LEDs, Dyes Pigm. 154 (2018) 257-262.
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Res. Bull. 88 (2017) 258-265. [36] J. Zhou, Z. Xia, Synthesis and near-infrared luminescence of La3GaGe5O16: Cr3+ phosphors, RSC Adv. 4 (2014) 46313-46318. [37] D.Y. Wang, C.H. Huang, Y.C. Wu, T.M. Chen, BaZrSi3O9: Eu2+: a cyan-emitting phosphor with high quantum efficiency for white light-emitting diodes, J. Mater. Chem. 21 (2011) 10818-10822. [38] Y. Takeda, H. Kato, M. Kobayashi, H. Kobayashi, M. Kakihana, Photoluminescence properties of Mn4+-activated perovskite-type titanates, La2MTiO6: Mn4+ (M= Mg and Zn), Chem. Lett. 44 (2015) 1541-1543.
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19
PII:
S0025-5408(19)32298-6
DOI:
https://doi.org/10.1016/j.materresbull.2019.110764
Reference:
MRB 110764
To appear in:
Materials Research Bulletin
Received Date:
6 September 2019
Revised Date:
28 December 2019
Accepted Date:
31 December 2019
Please cite this article as: Ou J, Yang X, Xiao S, Luminescence performance of Cr3+ doped and Cr3+ , Mn4+ co-doped La2 ZnTiO6 phosphors, Materials Research Bulletin (2020), doi: https://doi.org/10.1016/j.materresbull.2019.110764
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Luminescence performance of Cr3+ doped and Cr3+, Mn4+ codoped La2ZnTiO6 phosphors
Jinhong Ou, Xiaoliang Yang, Siguo Xiao*
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School of Physics and Optoelectronics, Xiangtan University, Hunan 411105, China *Author to whom correspondence should be addressed: Electronic mail: [email protected] Fax number: +8673158292468 Phone number: +8615573224832 Graphical abstract
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Cr3+ doped and Cr3+, Mn4+ co-doped La2ZnTiO6 phosphors have been prepared by a high-temperature solid-state method and their luminescence properties are investigated in detail. A unique luminescence performance of the Cr3+, Mn4+ co-doped LZT is found.
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Highlights:
‧ Luminescence properties of Cr3+ in La2ZnTiO6 are reported for the first
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‧ Total luminescence intensity is significantly improved by codoping with Mn4+. ‧ Cr3+，Mn4+ co-doped phosphor refers new approach to design novel wavelength detector. 1
Abstract
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Cr3+ doped and Cr3+, Mn4+ co-doped La2ZnTiO6 phosphors were prepared using solid-state reaction method and their luminescence performance has been investigated. La2ZnTiO6: Cr3+ phosphor exhibits red luminescence band centred at 740 nm when excited with light in spectral range from ultraviolet to blue and green (300 nm-500 nm). Both emissions at 740 nm (Cr3+) and at 710 nm (Mn4+) can be observed in the Cr3+, Mn4+ co-doped phosphor and its total luminescence intensity excited at 337 nm is enhanced to 1.6 times with the introduction of Mn4+ ions. The ratio of emission intensity at 710 nm to that at 740 nm in the co-doped phosphor increases monotonically as excitation wavelength increases from 330 to 400 nm. The performance of the developed phosphors in present work indicates that they might be used in indoor cultivation of LEDs and wavelength detection. Keywords: B. Luminescence, A. optical materials
1. Introduction
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Inorganic luminescent materials or the so-called ‘‘phosphors’’ have been continuously developed in the past century and widely used in many fields such as displays, sensors, solar cells, plant cultivation and solid-state lighting, etc. [1-10] Commonly inorganic phosphor are achieved by doping one or more luminescent dopants to a host material (crystal or glass). Rare earth (RE) ions like Tb3+, Eu3+, Ce3+and Eu2+, have always been the most widely used dopants in phosphors [11-14]. However, rare earth doped phosphors have some obvious disadvantages. Firstly, trivalent rare earth ions usually exhibit extremely narrow and weak absorption bands because of their parity-forbidden transition within the 4f configuration, resulting in the very low light absorption efficiency. Secondly, rare earth ions might have strong photon re-absorption in visible region due to their abundant transition channels and thus reduce the final light output of light devices. Thirdly, most rare earth elements tend to be expensive and some rare earth doped phosphors require harsh synthesis conditions. Therefore, developing novel inorganic phosphors without rare earth elements is meaningful for the future market. Transition metal ions, like Mn4+ and Cr3+, may be alternative dopants for rare earth free phosphors. When located in an octahedral coordination site, Mn4+ or Cr3+ ion is possible to be excited with near-ultraviolet and blue light due to their 4A2→4T1 and 4 A2→4T2 transitions. The luminescence wavelength significantly relies on the crystal field strength of the substituted sites and it can shift from red to deep-red in different crystal field environment. Up to now, Mn4+ or Cr3+ doped aluminates, tungstates, gallates, stannates and germinates such as Sr2ZnWO6:Mn4+ [15], MgAl2O4: Cr3+ [16], SrLaGaO4:Mn4+ [17], Li2Mg3SnO6:Mn4+ [18] and Mg3Ga2GeO8:Mn4+ [19] have been widely investigated. However, it is worth noting that titanates are also good host materials for transition metal doped phosphors. Besides the similar ionic radius between Ti4+ and Mn4+/Cr3+ ions, titanates usually contain abundant octahedral sites (Ti-O6), 2
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which provides suitable environment for the luminescence of Mn4+ and Cr3+ ions. Additionally, titanate phosphors doped with transition metal ions can be synthesized without expensive raw materials and complicated processes. As a matter of fact, recently there are some investigations on Mn4+ doped titanate phosphors like Mg2TiO4: Mn4+ [20], Li2MgTiO6: Mn4+ [21] and Gd2ZnTiO4: Mn4+ [22], etc. However, few investigations on Cr3+ doped titanate phosphors have been reported. [23]. In present work, Cr3+ doped as well as Mn4+, Cr3+ co-doped La2ZnTiO6 phosphors are prepared using a solid-state reaction method at high temperature. The crystal structure is characterized with X-ray diffraction (XRD). The excitation spectrum, emission spectrum and concentration dependent luminescence intensity and decay lifetime for the Cr3+ doped La2ZnTiO6 phosphor are investigated systematically. Especially, some novel luminescence properties have been found in the co-doped phosphors in addition to enhancing the luminescence intensity.
2.Experimental
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The La2ZnTi(1-x-y)O6: xCr3+, yMn4+ (x=0.003 − 0.015, y=0 − 0.0011) samples were synthesized by solid-state reaction method at high temperature. According to the chemical proportion, starting materials like TiO2, ZnO, La2O3, Cr2O3 and MnCO3 with analytical reagent (AR) grade were weighed carefully. The mixtures were put in a corundum mortar and ground thoroughly, and then preheated at 900 °C for 8 h in a muffle furnace. After cooling down to room temperature naturally, the reactants were milled again and then heated up to 1275 °C and held for 6 h. The prepared samples were naturally cooled to room temperature and ground into powder for further characterization. X-ray diffraction (XRD) patterns were measured by a diffract (XRD) meter (PIGAKV Ultima IV) using Cu Kα radiation in the 2θ range from 20° to 90°. A monochromator (Zolix Instrument) was applied to obtain the emission and excitation spectra, which equipped with a photomultiplier (PMTH-S1-CR131) and a xenon lamp of 150 W. Both the luminescence decay curves and the temperature dependent photoluminescence spectra with 337 nm excitation were measured with a FLS980 (Edinburgh) spectrometer and a pulse xenon lamp was used as the excitation light.
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Fig. 1. (a) XRD patterns for La2ZnTi(1-x-y)O6: xCr3+, yMn4+ (x=0.005, 0.007, 0.009; y=0.0007, 0.0009) phosphors. (b) Structure of La2ZnTiO6 unit cell.
3.Results and discussion
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The XRD patterns of La2ZnTi(1-x-y)O6: xCr3+, yMn4+ (x=0.005, 0.007, 0.009; y=0, 0.0005, 0.0007) samples and the ICSD card of La2ZnTiO6 (no.172755) are shown in Fig. 1(a). It can be found that there is no significant difference between the diffraction peaks of these samples and that of the pure La2ZnTiO6. At the same time, no peaks of impurities are observed, meaning that the addition of Mn4+ or Cr3+ has little influence on the crystal structure of these samples. La2ZnTiO6 belongs to double-perovskite with a space group of 𝑃121/𝑛1(14) [24]. The crystallographic structure of La2ZnTiO6 is given in Fig. 1(b). From Fig. 1(b) one may see that that Zn2+ and Ti4+ occupy the center positions of two adjacent octahedrons which are formed by O2-, respectively, and La3+ are in cavity sites surrounded by octahedrons. The ionic radii of Cr3+ (CN=6, 0.615 Å) and Mn4+ (CN = 6,0.53 Å) is more similar to that of Ti4+ (CN = 6, 0.61 Å) as comparing with that of Zn2+ (CN = 6, 0.74 Å) and La3+ (CN = 6, 1.03 Å, CN = 8, 1.16 Å). Hence 4
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Cr3+ and Mn4+ ions might tend to occupy the octahedral center sites of Ti4+ [25].
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Fig. 2. (a) Excitation spectra of La2ZnTi0.993O6 :0.007Cr3+. (b) Emission spectra of La2ZnTi0.993O6 :0.007Cr3+. (c) Tanabe–Sugano energy level diagram.
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The excitation spectrum and emission spectrum of the La2ZnTi0.993O6: 0.7%Cr3+ sample are presented in Fig. 2(a) and Fig. 2(b), respectively. As shown in Fig. 2(a), the excitation spectrum monitored at 740 nm shows an excitation band from 300 nm to 500 nm and presents two peaks at around 337 nm and 479 nm. The emission spectrum under 337 nm excitation in Fig. 2(b) shows an emission in 700 ~ 780 nm region with central peak at 740 nm. Cr3+ ions have an electronic configuration as 3d3. The Tanabe-Sugano energy level diagram shown in Fig. 2(c) can be used to express the energy levels of Cr3+ ion in an octahedron environment. The two peaks shown in the excitation spectra at about 337 nm and 479 nm should correspond to 4A2→4T1 and 4A2→4T2 transition, respectively. The emission energy level of Cr3+ will be different with different crystal field. When the crystal-field is strong enough, the 4T2 level of Cr3+ ion is above the 2E level, and the electrons transition from the 2E level back to the ground state and exhibits a sharp emission peak. Conversely, when the crystal-field is weak, there will be a board band emission caused by the 4T2 to 4A2 transition. From the emission spectra, the emission peak at 740 nm should originate from the 2E→4A2 transition. The emission bands on both sides of the peak at 740 nm correspond to anti-Stokes side-bands and Stokes side-bands, which due to the vibration of the 3d3 electrons when Cr3+ locates in an octahedral structure. The local crystal field strength of Cr3+ can be specifically obtained after calculating the 𝐷𝑞/𝐵 value with following formula: [26,27,28]
𝐷𝑞
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𝐵
15(𝑥−8)
= (𝑥 2 −10𝑥)
(1)
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where 𝐷𝑞 can be calculated with following formula: 𝐷𝑞 =
𝐸( 4𝑇2 − 4𝐴2 ) 10
(2)
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and 𝑥 can be expressed as 𝑥=
𝐸( 4𝐴2 − 4𝑇1 )−𝐸( 4𝐴2 − 4𝑇2 ) 𝐷𝑞
(3)
base on the excitation spectra of the LZT: Cr3+, the value of 𝐸( 4𝐴2 − 4𝑇1 ) and 𝐸( 4𝐴2 − 4𝑇2 ) can be estimated to be about 29,673.59 cm-1 and 20,876.83 cm-1, respectively. And then the parameters 𝐷𝑞 and 𝐵 are 2088 and 896 cm-1, respectively. Hence the 𝐷𝑞/𝐵 value is 2.33 and exceeds 2.2, suggesting that Cr3+ ions locate within strong crystal field indeed. 6
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The Cr3+ concentration dependent emission spectra of LZT: Cr3+ samples with 337 nm excitation are measured and depicted in Fig. 3(a). It is found that with the contents of Cr3+ ion increases from 0.3% to 1.5%, there is no obvious change on the emission spectra except for the integral emission intensity. The relationship between integral intensity and Cr3+ ion is further shown in Fig. 3(b). The integral luminescence intensity increases monotonically until the concentration of Cr3+ gets 0.7%, and then begins to weak with Cr3+ concentration further increasing. The decrease of luminescence intensity at Cr3+ concentration beyond 0.7% should be caused by the concentration quenching. With Cr3+ content increasing, the distance between the neighboring Cr3+ ions decreases. Therefore, much more energy transfers between neighboring Cr3+ ions occur, and thus energy is more likely to transfer to luminescent killer sites, leading to the reduction of luminescence intensity at high doping concentration.
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Fig. 3. (a) Emission spectra of La2ZnTi(1-x)O6: xCr3+ (x = 0.003, 0.005, 0.007, 0.009, 0.011, 0.013, 0.015) (λex = 337 nm). (b) Dependence of integral intensity on Cr3+ concentration. (c)
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𝑙𝑛(𝐼/𝑥) vs. 𝑙𝑛(𝑥) relationship based on Eq. (5). (d) The decay curve of La2ZnTi(1-x)O6: xCr3+ (x = 0.003, 0.005, 0.007, 0.009, 0.011, 0.013, 0.015) at 740 nm with excitation at 337 nm.
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In order to determine the mechanism of energy transfer among the Cr3+ ions, the critical distance (𝑅𝑐 ) is approximated according to following relationship firstly: [29, 30] 3𝑉
𝑅𝑐 = 2 (4𝜋𝑥 𝑁) 𝑐
1 3
(4)
in which 𝑥𝑐 is the critical concentration of the activator ion, 𝑉 is the volume of the unit cell, and 𝑁 represents the number of sites that the Cr3+ ion can substitute in a unit cell. For LZT: Cr3+ samples, 𝑥𝑐 = 0.007, 𝑉 = 246.58 Å3 and 𝑁 = 4. The calculated 𝑅𝑐 is 25.62 Å and more than 5 Å. So the energy migration between neighboring Cr3+ ions in La2ZnTiO6 hosts is impossible to be caused by exchange interaction. Consequently, the energy migration mechanism for Cr3+ in LZT host should 8
be multipole–multipole interaction, which can be determined using following formula base on Dexter’s theory: [29, 30,31, 32] 𝐼 𝑥
𝜃 3
= 𝐾 [1 + 𝛽(𝑥) ]
−1
(5)
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where 𝐼 and 𝑥 are the luminescence intensity and the activator concentration, respectively. 𝐾 and 𝛽 are constants in the same host and excitation condition. when 𝜃 equals 6, 8 and 10 means that the multipole–multipole interaction is dipole-dipole, dipole-quadrupole and quadrupole-quadrupole interactions, respectively. The linear fitting plot of 𝑙𝑛(𝐼/𝑥) vs. 𝑙𝑛(𝑥) as shown in Fig. 3(c) can be used to approximate the value of the 𝜃. The fitted value of 𝜃 is about 4.08, most close to 6. It means that the dominant energy transfer mechanism for Cr3+ in LZT host is most likely the electric dipole-dipole interaction. Room temperature luminescence decay curves at 740 nm of La2ZnTi0.993O6: xCr3+ phosphors under 337 nm excitation are also measured, as shown in Fig. 3(d). The decay curves can be fitted with second order exponential equation: [33,34,35] (6)
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𝐼(𝑡) = 𝐴𝑟 𝑒𝑥𝑝(−𝑡/𝜏𝑟 ) + 𝐴𝑠 𝑒𝑥𝑝(−𝑡/𝜏𝑠 )
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In which 𝐼(𝑡) represents the luminescence intensity, 𝑡 is the time. 𝐴𝑟 and 𝐴𝑠 are constants corresponding to 𝜏𝑟 and 𝜏𝑠 , respectively. 𝜏𝑟 and 𝜏𝑠 represent rapid and slow lifetime values for exponential components, respectively. 𝐴𝑟 , 𝐴𝑠 , 𝜏𝑟 and 𝜏𝑠 values can be determined from the fitting lines of the decay curves, and then the effective lifetime constant (𝜏 ∗ ) may be calculated by following formula: [33,34,35] 𝜏 ∗ = (𝐴𝑟 𝜏𝑟2 + 𝐴𝑠 𝜏𝑠2 )/(𝐴𝑟 𝜏𝑟 + 𝐴𝑠 𝜏𝑠 )
(7)
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The effective decay times 𝜏 ∗ are calculated to be 287.08 μs, 279.83 μs, 269.63 μs, 256.13 μs, 249.82 μs, 241.08 μs and 225.02 μs for La2ZnTi(1-x)O6: xCr3+ with x= 0.003, 0.005, 0.007, 0.009,0.011, 0.013 and 0.015, respectively. The measured lifetime for Cr3+ emission decreases with the Cr3+ content increasing. This can be explained by the follow formula: [36] 1 𝜏∗
1
= 𝜏 + 𝐴𝑛𝑟 + 𝑃𝑡 0
(8)
in which 𝜏0 is the radiative lifetime, 𝐴𝑛𝑟 represents the nonradiative rate, 𝑃𝑡 refers to the energy transfer rate among Cr3+ ions. As Cr3+ concentration increases, the distance between Cr3+ ions decreases and thus the energy transfer rate 𝑃𝑡 increases. Consequently, the lifetime shortens at high Cr3+ concentration.[37] To further improve the luminescence performance of the phosphor, Mn4+ ions were co-doped into the LZT:0.7%Cr3+. The spectral characteristics of Mn4+ single-doped LZT samples have already been reported. It has an excitation band ranging from 300 to 9
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600 nm with two peaks at about 370 nm and 507 nm and can give emission band peaking at about 710 nm [38]. The emission spectra of the LZT:0.007Cr3+, xMn4+ (x=0.03%-0.11%) that excited at 337 nm are shown in Fig. 4(a). The emission of Mn4+ (at 710 nm) and Cr3+ (at 740 nm) are both observed. Fig. 4(b) further shows the relationship between the integral intensity and concentration of Mn4+. It’s found that when the concentration of Mn4+ ions is less than 0.07%, the integral intensity of Mn4+ emission centred at 710 nm increases with the enhancement of Mn4+ concentration, while the Cr3+ luminescence intensity is hardly affected by the doped Mn4+ ions. After the Mn4+ concentration exceeds 0.07%, the luminescence intensity of Mn4+ and Cr3+ both decreases due to the concentration quenching. Comparing with the Cr3+ single doped samples, the addition of Mn4+ ions largely extends the emission band and increases the total emission intensity. It is found that the integral intensity of the LZT:0.007Cr3+, 0.0007Mn4+ phosphor has been enhanced to 1.6 times as compared with that of the LZT:0.007Cr3+ phosphor. Fig. 4(c) gives the normalized excitation spectra of Mn4+ monitored at 710 nm and Cr3+ monitored at 740 nm for the LZT:0.007Cr3+, 0.0007Mn4+ sample. It is clearly seen that the ultraviolet light in a wider region can be converted into deep red in Mn4+, Cr3+ co-doped phosphor than that in Cr3+ singly doped one.
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Fig. 4. (a) Emission spectra of La2ZnTiO6: 0.007Cr3+, xMn4+ (x = 0.0003, 0.0005, 0.0007, 0.0009, 0.0011) with excitation at 337 nm. (b) Luminescence intensity as function of Mn4+ content. (c) Normalized excitation spectra of Mn4+ and Cr3+ in LZT.
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The thermal stability for the Mn4+/Cr3+ co-doped LZT, luminescence spectra for La2ZnTi0.9923O6: 0.007Cr3+, 0.0007Mn4+ sample under different temperature (300 K to 450 K) with 337 nm excitation are also measured, and the relationship between relative intensity and temperature is given in Fig. 5(a). The emission intensities of Mn4+ and Cr3+ both weaken as the temperature increases from 300 K to 450 K and the luminescence intensity for Cr3+ decreases faster than that for Mn4+. The temperature dependent luminescence behaviors are attributed to thermal quenching, which might be explained with the configurationally coordinate given in Fig. 5(b). In normal condition, the electrons of Mn4+ and Cr3+ transition from the 2E level to their ground states and emit photons. With the temperature going up from 300 K to 450 K, electrons are affected by thermal excitation and thus have greater possibility to go back to the ground state via non-radiative decay process. Consequently, the luminescence intensity declines with the temperature rising. [21] 11
The activation energy (∆𝐸) is further estimated by following equation: [39,40] 𝐼(𝑇 )
0 𝐼(𝑇) = 1+𝐴𝑒𝑥𝑝(−∆𝐸/𝑘
(9)
𝐵 𝑇)
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in which 𝐼(𝑇0 ) represents the initial intensity and 𝐼(𝑇) represents the luminescence intensity at temperature 𝑇. 𝐴 is constant and 𝑘𝐵 represents the Boltzmann constant and has a value as 8.629×10-5 eVK. According to the equation (9), ∆𝐸 can be estimated by the fitting plot of 𝑙𝑛[𝐼(0)/𝐼(𝑇) − 1] vs. 1⁄𝑘𝑇 as shown in Fig. 5(c). It is 0.44 eV for Mn4+ and 0.36 eV for Cr3+ in the LZT host, respectively.
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Fig. 5. (a) Normalized temperature dependent emission intensity of La2ZnTi0.9923O6: 0.007Cr3+, 0.0007Mn4+in Mn4+ (circle) and Cr3+ (square). (b) Diagram of schematic configuration
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coordinate illustrating thermal quenching of Mn4+ and Cr3+. (c) Relationship between 𝑙𝑛[𝐼(0)/ 𝐼(𝑇) − 1] and 1⁄𝑘𝑇 according to Eq. (9).
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The 700~750 nm far-red light can control the process of plant growth by adjusting the ratio of phytochrome PFR and PR. [41] This means that the developed phosphors in this work have potential application in indoor cultivation of LEDs. In addition, from Fig. 4(c) we may find that the luminescence intensity ratio between Mn4+ and Cr3+ ions might vary monotonically with the excitation wavelength varying from 330 to 400 nm. Therefore, the emission spectra of La2ZnTi0.9923O6: 0.007Cr3+, 0.0007Mn4+ sample are further carefully measured excited at different wavelengths (330 to 400 nm). The normalized luminescence spectra are showed in Fig. 6(a). The relationship between the integral intensity ratio 𝑅 (Mn4+ vs. Cr3+) and the excitation wavelength (330 – 400 nm) is presented in Fig. 6(b). It is found in Fig. 6(b) that the integral intensity ratio 𝑅 increases monotonically with the excitation wavelength increasing. With this performance the co-doped phosphor may act as wavelength detector, since the excitation wavelength can be determined when the integral intensity ratio 𝑅 is obtained by measuring the emission intensity of Mn4+ and Cr3+.[42] The wavelength measurement sensitivity S can be defined as 𝑑𝑅
𝑆 = 𝑑𝜆
𝑒𝑥
(10)
here 𝑅 is the integral intensity ratio of Mn4+ to Cr3+ and 𝜆𝑒𝑥 is the corresponding excitation wavelength. The sensitivity S as function of excitation wavelength 𝜆𝑒𝑥 is also plotted in Fig. 6(b). It is noted that the sensitivity S in 330-400 nm for the Mn4+, Cr3+ co-doped La2ZnTiO6 is evidently higher than that of the Mn4+, Cr3+ co-doped Li2MgTiO6 reported previously. However, it is noted in Fig. 5(a) that the luminescence intensity of Cr3+ in La2ZnTiO6 decreases faster than that of Mn4+ with temperature 13
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increasing, meaning that the integral intensity ratio 𝑅 will vary with the temperature. Therefore, maintaining a stable temperature for the phosphor is necessary to avoid error in wavelength measurement when it acts as wavelength detector.
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Fig. 6. (a) Normalized emission spectra of La2ZnTi0.9923O6: 0.007Cr3+, 0.0007Mn4+ sample excited at different wavelength (330 nm to 400 nm). (b) Relationship between the integral intensity ratio 𝑅 of Mn4+ to Cr3+ and excitation wavelength (square) and relationship between sensitivity S (triangle) and excitation wavelength.
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4. Conclusion
In brief, Cr3+ doped and Cr3+, Mn4+ co-doped LZT phosphors were synthesized with solid-state reaction process. Cr3+ doped LZT has a broad excitation band in region of 300 nm to 500 nm and can give an emission in region of 700 nm ~ 780 nm with central peak at 740 nm. Mn4+/Cr3+ co-doped phosphor not only gives emission of Mn4+ ions but also that of Cr3+ ions. The total luminescence intensity of the co-doped sample is enhanced compared with the Cr3+ singly doped one. In addition, the integral intensity ratio 𝑅 between the Mn4+ and Cr3+ emissions increases monotonically as the excitation wavelength increases from 330 to 400 nm. The excitation wavelength dependent 14
luminescence performance of the co-doped phosphor means that it is capable of detecting wavelength, referring a new possible approach to design future compact spectrometer. Author Statement Xiaoliang Yang: Conceptualization, Methodology. Jinhong Ou: Conceptualization, Validation, Formal analysis, Investigation, Writing Original Draft.
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Siguo Xiao: Writing - Review & Editing
Conflicts of interest There are no conflicts of interest
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Acknowledgments
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This work was supported by the National Science Foundation of China (No.11674272).
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