Novel blue emitting Tm³⁺ activated lanthanum molybdate: Investigations on luminescence behaviour and Judd-Ofelt theoretical analysis

Highlights

• Novel Tm³⁺ doped blue phosphor synthesized by a simple technique.

• Oriented attachment mediated growth mechanism.

• Intense sharp blue emission at 453 nm.

• Highly asymmetric site occupancy from JO analysis.

• CIE co-ordinates fall in blue region.

Abstract

Novel blue emitting thulium doped La₂(MoO₄)₃ nanophosphors were synthesized by facile co-precipitation method and the spectroscopic features were systematically studied on the basis of Judd-Ofelt analysis. The tetragonal phase formation of Tm³⁺ doped La₂(MoO₄)₃ with space group I4₁/a is confirmed from the XRD analysis and the growth mechanism by oriented attachment is revealed by TEM analysis. The ¹D₂→³F₄ transition at 453 nm is the dominant emission peak of Tm³⁺ in the photoluminescence emission spectra and offers major contribution towards the intense blue emission of the phosphor material. The Judd-Ofelt intensity parameters exhibit the trend ${\text{Ω}}_2>{\text{Ω}}_4>{\text{Ω}}_6$, which point out the asymmetric site occupancy of Tm³⁺ ions in the host matrix. The estimated higher branching ratio for ¹D₂→³F₄ blue emission of Tm³⁺ signifies it as a potential laser emission transition. The CIE chromaticity co-ordinates of optimized La_1.94Tm_0.06(MoO₄)₃ nanophosphors falls in the blue region of the chromaticity diagram with co-ordinates (0.214,0.097).

Introduction

Multifaceted optical applications associated with rare earth luminescence are primarily devoted to their intense sharp multicolour emissions originating from 4f-4f intraconfigurational transitions. The lifting up of degeneracy in intra 4f forbidden transitions of rare earth ions by the influence of a strong crystal field offered by the host matrix is the key point of rare earth luminescence [1,2]. Obviously, solid state lighting technology demands highly efficient NUV or blue LED excitable red, green and blue phosphors with high efficiency and colour purity. Nowadays tremendous works are reported on Tb³⁺ and Eu³⁺ based green and red phosphors with desirable properties [[3], [4], [5], [6], [7], [8]]. However, the spectroscopic investigation on blue emitting phosphors is relatively less and most of them are characterized with broad blue emission bands. Foka et al. reported the photoluminescence studies on Ce³⁺ activated SrAl₂O₄ and Tabaza et al. reported Ce³⁺ activated MgAl₂O₄, which are UV excitable blue phosphors [9,10]. A broad emission band in the range 340–440 nm and a broad blue-green emission band centered at 490 nm were observed for Ce³⁺ activated SrAl₂O₄ and MgAl₂O₄ phosphors respectively. Pawade et al. synthesized Eu²⁺ doped SrMg₂Al₁₆O₂₇ blue phosphors with a broad excitation band centered at 324 nm and a broad emission band with a maximum at 465 nm [11]. Eu²⁺ doped Ca₂Sr(PO₄)₂ blue phosphor with a broad excitation band in the range 200–410 nm and a broad emission band in the range 410–500 nm with a maximum at 430 nm were synthesized and characterized by X. Wang et al. [12]. Another blue phosphor which is used in plasma display panels is Tm³⁺ activated lanthanum phosphate [13]. The material is excited either at 147 or 173 nm and the emission peaks are observed at 348, 362 and 452 nm.

Most of the blue emitting phosphors discussed above possess a broad emission band in the blue region. The advantage of Tm³⁺ activated phosphors is that, instead of a broad band, intense sharp peaks are observed in the blue region corresponding to the emission from ¹D₂ and ¹G₄ emitting levels. The sharp emission due to f-f transitions in Tm³⁺ ions is beneficial for enhancing the spectral purity of blue emission, compared with other broad band blue emitters. In addition, as these emitting levels are well separated from the next lower lying levels of Tm³⁺ ions, the possibility of non-radiative processes from these states through multiphonon relaxation is less, even in high phonon energy host matrices. Also, the sharp excitation peak of Tm³⁺ observed at 360 nm facilitates efficient UV excitation of the material. On account of all these facts, luminescence properties of Tm³⁺ based blue phosphors deserve much attention. The origin of blue emission of Tm³⁺ is attributed to ¹D₂→³F₄ and ¹G₄→³H₆ transitions corresponding to 453 and 473 nm emissions respectively [14,15]. The Tm³⁺ emission is highly sensitive to the nature of the host lattice in which they are embedded. There are few reports on the Tm³⁺ based luminescence in tungstate, molybdate and vanadate host lattices. L. Guerbous et al. reported host sensitized Tm³⁺ luminescence in LiIn(WO₄)₂ phosphors and J. Liao et al. investigated the spectroscopic and the morphological aspects of AWO₄ (A = Ca,Sr,Ba) blue phosphors [15,16]. The synthesis and the photoluminescence properties of Tm³⁺ doped Y_xGd_1-xVO₄ nanophosphors were studied by B.Yan et al. [14]. Likewise, the spectroscopic properties on the basis of Judd-Ofelt theoretical analysis of Tm³⁺ doped NaBi(MoO₄)₂ single crystals were carried out by N.V. Gusakova et al. [17]. D. Zhu et al. synthesized Tm³⁺/Dy³⁺ co-doped Ba_mSr_1−mMoO₄ polycrystals by a low temperature combustion method and investigated the luminescence properties of the material for the realization of white light generation [18].

Molybdate based crystals are proved to be one of the excellent host matrices for the incorporation of rare earth ions. They are self activated luminescent materials with strong absorption in the UV region and efficient energy transfer to the dopant ions via Mo–O charge transfer band absorption [19]. In addition, they have high thermal and chemical stability [20]. Generally, nanophosphors with superior emission properties synthesized by simple cost-effective synthesis technique deserve much attention. In the present work, lanthanum molybdate which is an outstanding candidate for the incorporation of luminescent rare earth ions is chosen as the host lattice and investigated the spectroscopic properties of the material. The detailed investigations on the Tb³⁺ doped green phosphors and the tunable dual colour emitting Tb–Eu co-doped La₂(MoO₄)₃ nanophosphors via energy transfer mechanism were reported in our previous works [21,22].

The synthesis and characterization of Tm³⁺ activated lanthanum molybdate nanophosphors and the JO analysis is reported for the first time. The technique adopted for the synthesis is a simple and cost-effective method. Due to the asymmetric site occupancy of Tm³⁺ in tetragonal La₂(MoO₄)₃ host, electric dipole transition will dominate and thus maximum emission intensity is assured for purely electric dipole blue emission transition of Tm³⁺ ions. Apart from the photoluminescence investigations, the studies are extended towards colorimetric analysis in order to get a complete picture of the emission properties of the material. In addition, there are no previous reports on JO analysis and the prediction of radiative parameters of Tm³⁺ activated La₂(MoO₄)₃ host matrix. Almost all the previous studies on Tm³⁺ doped in different matrices were solely concentrated on the optical characterizations and lacks morphological investigations [16,23]. However, in the present study the morphological features of the material were also investigated.