Influence of Alkali Metal Ions on Luminescence Behaviour of Ca0.5R1-x(MoO4)2:xLn3+ (R = Y, La), (Ln = Eu, Tb, Dy) Pulsed Laser Deposited Thin Phosphor Films

Abstract

Thin phosphor films of Ca0.5R1-x(MoO4)2:xLn3+, M+ (R3+ = La, Y), (Ln3+ = Eu, Tb, Dy) (M+ = Li+, K+ and Na+) were deposited on quartz substrates by pulsed laser deposition (PLD) technique by ablation of a stoichiometric monocrystal target. The deposition was carried out using an Nd‐YAG laser (λ = 1064 nm) in an ultra‐high vacuum (UHV) with an oxygen back pressure of 300 mTorr at 600°C substrate temperatures. The laser‐ablated films are optically active, as verified by the photoluminescence (PL) spectra, and the films exhibit smooth Stark levels. The photoluminescence of the Ca0.5R1-x(MoO4)2:xLn3+, M+ phosphors properties reveals characteristic visible emissions. Further, the co‐doping of alkali metal chlorides MCl (M = Na, K, Li) into the Ca0.5R1-x(MoO4)2:xLn3+, M+ phosphor greatly improves the luminescence intensity, which can be explained by charge compensation effect. The fluorescence lifetime and photometric coordinates are discussed in detail.

1. Introduction

The growth of good quality larger area thin film with homogeneous size distribution and morphology is still a demanding issue, and it is of significant attention towards the research fraternity. Remarkably, uniform micro/nano‐structures have been paying global attention due to its potential application in high‐performance luminescence and opto‐electronic device based on community their novel optical and electronic properties. To synthesize novel thin film materials such as molybdates [1, 2], tungstates [3], vanadates [4], and fluorides [5], copious prominent techniques are extensively adopted, for example chemical bath deposition (CBD), successive immersion layer adsorption reaction (SILAR), polymerization, electrodeposition, sputtering, metal‐organic chemical vapour deposition (MO‐CVD), molecular beam epitaxy (MBE), atomic layer deposition (ALD), pulsed laser deposition (PLD). In the midst of all, PLD is a multitalented method to prepare multiconstituent thin film materials in which raster examining of high‐energy pulsed laser ablates the target material and produces the plasma plume [6, 7]. In recent times, the PLD technique has created a widespread usage with an exceptionally astonishing result in materials preparation and fabrication of a device in the opto‐electronics field. Albeit the fabrication of optical quality of the thin films and waveguides using PLD technique with various technical hitches and burning issues, till date, these issues have undeniably been lucratively conquered, and quite a few good‐quality thin films were grown by PLD.

1.1. Recent research scenario

In contrast to conventional incandescent and fluorescent lamps, white light emitting diode (w‐LEDs) is seemed to be an optimistic solid‐state light source with a good‐quality energy conversion luminescence device [8]. By coalesce into the GaN blue LED chip with yellow emitting phosphor YAG:Ce³⁺ which yield a white light emission using the conventional technique [9–11]. Nevertheless, the deficient of red emission cog ends with low colour rendering index (CRI) and luminous efficacy of radiation (LER) which restricts their pertinence towards a few ambits [9, 12]. To conquer this hindrance, red or orange‐red emitting ion such as Pr³⁺, Sm³⁺, Cr³⁺ and Mn²⁺ is co‐doped with YAG:Ce³⁺ [10]. The other one is combining YAG:Ce³⁺ with red or orange‐red phosphors such as nitrides (M₂SiN₈:Eu²⁺), sulphides (CaS:Eu²⁺), oxynitrides (MSi₂O₂N₂:Eu²⁺) (where M = Ca, Sr) [10]. Furthermore, the enhancement of intense emission in the host material can be engendered by co‐doping of alkali metal‐chloride results in strong emission, which may possibly be an opportune and a generally suitable approach to acquire the phosphors with sufficient intensity and excellent efficiency are a great essential deal for prospective solid‐state lighting devices [2]. Therefore, it is necessary to discover a suitable phosphor material with a sufficient chemical permanence with enhanced efficiency. Rare earth‐doped phosphor materials are paying attention towards the research problems based on its applications in all the prospects of science and technology. Molybdates and tungstates with metallic elements form an essential class of phosphor materials. They belong to the scheelite family having a space group I_41/a. In both molybdate and tungstate family, the alkaline earth‐based rare‐earth‐activated double molybdates are very much highly significant efficient materials on the basis of its unique structural, optical properties have come across profound applications in technological aspects. Alkaline rare‐earth‐activated tungstates having a general formula ARE (MoO₄)₂ (RE = Y, La; A = Ba, Ca, Sr) are considered as better luminescent hosts investigated significantly for various purposes such as photocatalysts [11], displays [8] and acquire substantial hydrolytic and thermal permanence. Furthermore, the electroluminescent devices in the form of thin films from these micro/nano‐architectures are to be built for the white light emitting diode applications. Among the aforesaid variety of fabrication of thin film techniques, pulsed laser deposition (PLD) is a viable method [2, 13, 14]. Nowadays, to fabricate homogeneous and large‐scale thin films, laser rastering system attached into PLD technique has been used [13]. To consider the aspects of application, aforesaid reasons could make the PLD technique most unique and almost suitable for the growth and fabrication of good‐quality micro/nano‐thin films. It is interesting that the structural, optical, and photophysical properties of the micro/nano‐architectures could be compared with the thin film phosphors and its bulk [2, 15] counter‐parts.

In this viewpoint, we have prepared the single crystalline nano‐thin phosphor films of Ca_0.5R_1-x(MoO₄)₂:xLn³⁺ (R³⁺ = Y, La), (Ln³⁺ = Eu, Tb, Dy) with co‐doping of alkali metal chlorides (0.02 M of LiCl, KCl, NaCl) have been ablated on quartz substrates using the pulsed laser deposition method (PLD). For the first time, the luminescence properties of these alkali chloride‐activated phosphors are studied. The as‐prepared molybdate and tungstate powders were further deposited as thin films using the laser‐ablation by forming a ceramic target under oxygen atmosphere. Followed by the as‐prepared samples was analyzed using X‐ray diffraction (XRD), atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM), photoluminescence (PL) spectroscopy, photometric characteristics using commission internationale de i'eclairage (CIE) diagrams. The colour chromaticity coordinates and luminescence decay times have also been determined and discussed in reference to the effect of alkali metal ions.

2. Experimental details

By employing the PLD technique, for the first time, the nano‐thin phosphor films of Ca_0.5R_1-x(MoO₄)₂:xLn³⁺ (x = 0.16 M) (R³⁺ = Y, La), (Ln³⁺ = Eu, Tb, Dy) with co‐doping of alkali metal chlorides (0.02 M of LiCl, KCl, NaCl) were effectively coated on the quartz substrates by maintaining the substrate temperature of 600°C under oxygen atmospheric pressure (∼300 mTorr). Figure 1a–i shows the images of different experimental procedures engaged for the growth of thin films.

2.1. Preparation of ceramic target, cleaning of substrates and growth of Ca_0.5R_1-x(MoO₄)₂:xLn³⁺ nano‐thin phosphor films

To prepare a strong and extremely impenetrable ceramic (molybdate and tungstate) target for laser ablation, the starting precursors such as Na₂CO₃, La₂O₃, Y₂O₃, MoO₃ and Ln₂O₃ (Ln = Eu, Tb, Dy) were taken in stoichiometric ratios along with 0.02 M of alkali chlorides (LiCl, KCl and NaCl), followed by using the agate mortar pestle the powders were grounded for 2 h. The doping concentrations of Ln³⁺ were optimized in our previous work [15] and kept at constant (0.16 M) for all the Ln³⁺ ions. Without using any binders, the homogeneously mixed powders were pressed and pelletized in the form of disk (pellet) at a pressure of 6 tons. By eliminating the unstable contaminants, shun pores, crack, and endorse densification, to promote the diffusion in atomic level all through the preparation of target [16]. Furthermore, the as‐prepared pellet was annealed at 900°C for 3 h to achieve a very strong, stable and thick pellet having a diameter of about 2.5 cm, and thickness of about 0.4 cm is attained. Then, the annealed target is used for laser ablation.

Figure 1.

Photograph of different experimental methods demonstrating (a) homogeneously pelletized ceramic target, (b) high‐temperature annealed target, (c) cleaning of substrates, (d) target loading, (e) loading of substrates, (f) impurities removal in UHV chamber (glow discharge), (g) laser ablation, (h) target after ablation, (i) thin films after deposition.

The procedure for predeposition cleaning of substrates and growth of nano‐thin phosphor films were already discussed in detail on our previous work [2].

2.2. Characterization

The morphology of the product was analysed by field emission scanning electron microscope (FESEM‐SUPRA 55). Using atomic force microscopy (NTEGRA PRIMA‐NTMDT, USA), the surface topography of the thin phosphor films was studied. The crystal structure and phase purity of as‐synthesized phosphor were recognized and confirmed by PANalytical's X'Pert PRO Materials Research X‐ray Diffractometer (Almelo, USA) equipped with a CuKα radiation (λ = 0.154060 Å) at a scanning rate of 0.02°s⁻¹ in a 2θrange of 15°–60°. Further, down‐conversion PL excitation and emission studies and fluorescence decay time measurements were performed at room temperature using a Cary bench‐top spectrophotometer (AGILENT Instruments, USA).

3. Results and discussion

3.1. Morphological and X‐ray diffraction analysis

Figure 2a and b shows the scanning electron microscopy (SEM) images of Ca_0.5R_1-x(MoO₄)₂:xLn³⁺ (R³⁺ = Y, La), (Ln³⁺ = Eu, Tb, Dy) thin phosphor film co‐doped with Li⁺, K⁺ and Na⁺ metal ions prepared at 600°C with 300 mTorr. The as‐prepared phosphor film comprises homogeneous nearly circular grains with a typical grain size around 250 nm. Based on the AFM studies, the surface topography, line profile and roughness of the prepared thin film was estimated. Figures 3a and 4a show the 3D AFM image of thin film of Ca_0.5R(MoO₄)₂:Eu³⁺ (R = Y, La), and the scan was performed on 3 × 3 and 5 × 5 µm² area, respectively. From the 3D topographic image, the as‐prepared thin phosphor film comprises of polished surface with uniform arrangement of the particles and with less agglomeration. The average roughness along with root‐mean‐square (rms) of the as‐grown thin film was determined as 24.72 and 26.84 nm for Ca_0.5R_1-x(MoO₄)₂:xEu³⁺ (R³⁺ = Y, La). The 3D surface topography, 2D surface scan image, line profile, and histogram analysis are shown in Figures 3a–d and 4a–d, respectively. Based on the scaling process, the particle size would be reduced to nano‐scale in thin film which could be efficiently used for display applications. The optimization of maintaining different substrate temperature on rare‐earth doped phosphors has been well examined and reported in our earlier work [2].

Figure 2.

The SEM image (a and b) of Ca_0.5R_0.5(MoO₄)₂:Eu³⁺ (R³⁺ = Y, La) thin film.

The crystallinity and phase purity of the prepared products were examined using indexed powder X‐ray diffraction patterns Figure 5(a and b) for as grown thin film samples. The compound Ca_0.5Y_(1-x)(MoO₄)₂:xRE³⁺ crystallizes in the scheelite tetragonal crystal structure with a space group of I_41/a. The unit cell of Ca_0.5Y_(1-x)(MoO₄)² consists of [MoO₄]^2- anions and Ca²⁺ and Y³⁺/La³⁺ cations. In this phase, Mo sites are occupied by the molybdenum atoms (Mo⁶⁺) and located at the centres of tetrahedron and surrounded by four equivalent oxygen (O^2-) atoms. The divalent Ca²⁺ and trivalent Y³⁺/La³⁺ occupies the dodecahedral sites associated with the tetrahedral symmetry. The degree of dodecahedral and tetrahedral distortions is discussed in the previous work [2, 15, 17].

Figure 3.

The AFM images of Ca_0.5Y(MoO₄)₂:Eu³⁺ thin film (a) 3D surface topography, (b) 2D surface scan image, (c) line profile for vertical cross‐section in 3 × 3 µm² scan area and (d) histogram analysis.

In the powder XRD pattern, all the peaks are indexed perfectly which indicates a pure tetragonal phase having scheelite crystal structure and the planes (1 0 1), (1 1 2), (0 0 4), (2 0 0), (2 0 4), (2 2 0), (1 1 6) and (1 3 2) are in well accordance with the JCPDS card no. 82‐2369 of NaY(MoO₄)₂. No deleterious phases are found. The peak shift is not noticed with respect to doping. An intense peak with plane (1 1 2) is found at 28.95° [2, 15, 17].

3.2. Photoluminescence properties of laser‐ablated thin phosphor films:
Ca_0.5R_1-x(MoO₄)₂:xLn³⁺,M⁺ (R = Y, La; Ln = Eu, Tb and Dy; M = Li, K and Na)

3.2.1. Enhancement of luminescence intensity by the persuade of alkali metal ions in
Ca_0.5R_1-x(MoO₄)₂:xLn³⁺,M⁺

In the phosphor material, by introducing the alkali metal chlorides, nitrates or fluorides, which substantially increase the luminescence intensity owing to the charge compensation effect between unequal ions [18]. On our previous work in Ca_0.5R_1-x(MoO₄)₂:xLn³⁺ (R = Y, La) phosphor, the doping of alkali ions appreciably improves the emission properties by charge compensation using solid‐state reaction method [15, 19].

Figure 4.

The AFM images of Ca_0.5La(MoO₄)²:Eu³⁺ thin film (a) 3D surface topography, (b) 2D surface scan image, (c) line profile for vertical cross‐section in 5 × 5 µm² scan area and (d) histogram analysis.

Figure 5.

XRD patterns of (a) Ca_0.5Y(MoO₄)₂:Ln³⁺,Na⁺, (b) Ca_0.5La(MoO₄)₂:Ln³⁺,Na⁺ (Ln = Eu, Tb and Dy).

In our system, Eu³⁺, Tb³⁺, Dy³⁺ and M⁺ co‐doped in Ca_0.5R(MoO₄)₂ (R = Y, La) matrix would induce a distortion in lattice, and consequently, the lattice symmetry is desperately lowered [20]. The co‐doped Eu³⁺, Tb³⁺, Dy³⁺ and M⁺ at the Ca²⁺ sites in prepared thin film samples would play a role of dominance with enhanced luminescence intensity [21]. This is due to altering the symmetry and their surroundings in the locality of rare earth ions by adding the charge compensators of alkali metal ions [22]. Figures 6 and 7 show the PL emission spectra of Ca_0.5Y_1-x(MoO₄)₂:xLn³⁺,M⁺ (Ln = Eu, Tb and Dy; M = Li, K and Na) and Ca_0.5La_1-x(MoO₄)₂:xLn³⁺, M⁺ (Ln = Eu, Tb and Dy; M = Li, K and Na) thin film phosphors. The luminescence emission intensity is deliberately increased for Na⁺ ion co‐doped Ca_0.5R_1-x(MoO₄)₂:xLn³⁺ (R = Y, La; Ln = Eu, Tb and Dy). This could be owing to the charge compensation effect, and the proposed mechanism is Ca²⁺ → 2M⁺ (M = Li⁺, K⁺, Na⁺) [21, 22]. Furthermore, the ionic radius of Na⁺ (0.97 Å) is closer to the Ca²⁺ (1.12 Å) which is somewhat better than that of K⁺ (1.33 Å) and Li⁺ (0.59 Å) [3, 15]. Hence, there is an efficient replacement of Ca²⁺ ions by alkali metal ions. This forms the basis for the increased luminescence intensity, and obviously, Na⁺ is having the best charge compensation effect. The electronic configurations and its transitions are explained in the subsequent sections.

Figure 6.

PL emission spectrum of the thin film phosphor Ca_0.5Y(MoO₄)₂:Eu³⁺ co‐doped with various alkali metal ions [Li⁺(Green), K⁺(Red) and Na⁺(Blue)].

Figure 7.

PL emission spectrum of the thin film phosphor Ca_0.5La(MoO₄)₂:Eu³⁺ co‐doped with various alkali metal ions [Li⁺(Green), K⁺(Red) and Na⁺(Blue)].

3.2.2. Photoluminescence excitation studies

3.2.2.1. Ca_0.5Y_1-x(MoO₄)₂:xLn³⁺,Na⁺ (Ln = Eu, Tb and Dy)

Figure 8a shows the room temperature PL excitation spectra of Ca_0.5Y_1-x(MoO₄)₂:xLn³⁺ (Ln = Eu, Tb and Dy) thin film phosphors. The PL excitation spectrum of Ca_0.5Y_1-x(MoO₄)₂:xEu³⁺,Na⁺ is having a wavelength range of 225–575 nm. It is showing two regions with a broad band and intense sharp peaks. The broad band is located from 225 to 350 nm with a centre at ∼306 nm attributed to the O^2- to Eu³⁺ charge transfer band (CTB) and also designated as ligand‐to‐Eu³⁺ metal charge transfer transitions (LMCT) [23, 24]. Above 350 nm, intense sharp peaks are found at 362 nm (⁷F₀ → ⁵D₄), 382 nm (⁷F₀ → ⁵L₇), 395 nm (⁷F₀ → ⁵L₆), 416 nm (⁷F₀ → ⁵D₃), 465 nm (⁷F₀ → ⁵D₂) and 536 nm (⁷F₀ → ⁵D₁). Among which, the strong and intense peak is found at 395 nm. The characteristic configurations were attributed to the transition from the ⁷F₀ ground state of Eu³⁺ to the upper excited states (⁵D_1,2,3,4 and L_6,7). This strongest peak in UV region is more suitable for exciting Eu³⁺ ions.

Figure 8b depicts the room temperature excitation spectrum of Ca_0.5Y_1-x(MoO₄)₂:xTb³⁺,Na⁺ with a wavelength range of 270–390 nm in the UV region. It consists of two regions. One is from 270 to 360 nm, a highly intense and wide band designated as charge transfer band (CTB) having centred at ∼295 nm is ascribed to the charge transfer of molybdate host lattice [17]. The other is due to f‐f transitions of Tb³⁺ and its peak is at 376 nm (⁷F₆ → ⁵G₆) which is less intense than CTB. The energy transfer is being occurred from 4f⁸ to 4f⁷5d configuration of Tb³⁺ ions [17].

The excitation spectrum for the Ca_0.5Y_1-x(MoO₄)₂:xDy³⁺,Na⁺ thin film phosphor is shown in Figure 8c. The wavelength of the excitation spectrum ranges from 240 to 480 nm. The strong broad band is ranging from 240 to 340 nm bears a centre at ∼270 nm. Above 340 nm, the numerous intense f‐f transitions of Dy³⁺ ions are found. The f‐f transitions are located at 353 nm (⁶H_15/2 → ⁶P_7/2), 367 nm (⁶H_15/2 → ⁶P_5/2), 388 nm (⁶H_15/2 → ⁴I_3/2), 425 nm (⁶H_15/2 → ⁴G_11/2), 453 nm (⁶H_15/2 → ⁴I_15/2) and 475 nm (⁶H_15/2 → ⁴F_9/2) [25]. The highly intense peak is found at 353 nm which is the best candidate for exciting Dy³⁺ ions.

Figure 8.

PL excitation spectrum (a, b, and c) for the thin film phosphors Ca_0.5Y(MoO₄)₂:Ln3+ (Ln = Eu, Tb and Dy).

3.2.2.2. Ca_0.5La_1-x(MoO₄)₂:xLn³⁺,Na⁺ (Ln = Eu, Tb and Dy)

The room temperature PL excitation spectra of Ca_0.5La_1-x(MoO₄)₂:xLn³⁺ (Ln = Eu, Tb and Dy) thin phosphor films are illustrated in Figure 9a. The PL excitation spectrum of Ca_0.5Y_1-x(MoO₄)₂:xEu³⁺,Na⁺ comprises of wavelength range of 200–550 nm. It consists of two regions with a wide band and highly intense sharp peaks. The wide band is found from 220 to 350 nm with a centre at ∼278 nm ascribed to the O^2- to Eu³⁺ ligand‐to‐Eu³⁺ metal charge transfer transitions (LMCT) [19]. Above 350 nm, intense sharp peaks are found at 360 nm (⁷F₀ → ⁵D₄), 382 nm (⁷F₀ → ⁵L₇), 394 nm (⁷F₀ → ⁵L₆), 415 nm (⁷F₀ → ⁵D₃), 464 nm (⁷F₀ → ⁵D₂) and 535 nm (⁷F₀ → ⁵D₁). In that, the strongest and highly intense peak is found at 394 nm. This strongest peak in UV region is good for exciting Eu³⁺ ions.

Figure 9.

PL excitation spectrum (a, b, and c) for the thin film phosphors Ca_0.5La(MoO₄)₂:Ln³⁺ (Ln = Eu, Tb and Dy).

Figure 9b shows that the photoluminescence excitation spectrum of Ca_0.5La_1-x(MoO₄)₂:xTb³⁺,Na⁺ possess a wavelength range of 220–420 nm in the UV region. Among the two regions, the broad region is from 220 to 340 nm, ascribed to charge transfer band (CTB) which is centred at ∼278 nm. The other sharp peaks are due to f‐f transitions of Tb³⁺ with peaks at 369 nm (⁷F₆ → ⁵G₅) and 378 nm (⁷F₆ → ⁵G₆) is having lesser intensity than charge transfer band (CTB) [19].

The room temperature excitation spectrum for the Ca_0.5La_1-x(MoO₄)₂:xDy³⁺,Na⁺ thin film phosphor is depicted in Figure 9c. The range of the excitation spectrum is from 240 to 440 nm. The broad band ranges from 240 to 340 nm bears a centre at ∼271 nm. After 340 nm, a number of highly intense f‐f transitions of Dy³⁺ ions are located. The f‐f transitions are found at 352 nm (⁶H_15/2 → ⁶P_7/2), 367 nm (⁶H_15/2 → ⁶P_5/2), 388 nm (⁶H_15/2 → ⁴I_3/2) and 428 nm (⁶H_15/2 → ⁴G_11/2) [19]. The most intense peak that is situated at ∼352 nm is fit for exciting Dy³⁺ ions.

3.2.3. Photoluminescence emission studies

3.2.3.1. Ca_0.5Y_1‐x(MoO₄)₂:xLn³⁺,Na⁺ (Ln = Eu, Tb and Dy)

The room temperature PL emission spectra for Ca_0.5Y_1-x(MoO₄)₂:xLn³⁺,Na⁺ (Ln = Eu, Tb and Dy) thin phosphor films are shown in Figure 10a. The emission spectra monitored at 395 nm UV excitation for Ca_0.5Y_1-x(MoO₄)₂:xEu³⁺,Na⁺ illustrates a number of intra‐configurational 4f‐4f transitions arising from Eu^{3+ 5}D₀ excited state to the ⁷F_J (J = 1, 2, 3 and 4) ground states [26–28]. Upon other excitations and also with LMCT, there is no significant change in emission spectra. The strong and most intense emission peak is found at 616 nm upon 395 nm UV excitation is ascribed to the ⁵D₀ → ⁷F₂ electric‐dipole transition depicts hypersensitive red emission which is parity forbidden (ΔJ = 2) [26]. Also, it shows two sub‐peaks arises due to Stark energy splitting, that is (2J + 1) Stark components of J‐degeneracy splitting [27]. The predominant electric‐dipole transition signifies that Eu³⁺ ions are found at sites without inversion symmetry. The other transitions found at 587 nm (⁵D₀ → ⁷F₁) show orange emission owing to magnetic‐dipole transition. The other relatively weaker transitions are located at 655 nm (⁵D₀ → ⁷F₃) and 702 nm (⁵D₀ → ⁷F₄) [28]. The red emission to orange emission (R/O) ratio for the thin film phosphor is 5.2536. From these findings, it is evident that Ca_0.5Y_1-x(MoO₄)₂:xEu³⁺,Na⁺ possess scheelite tetragonal structure having C_3v site symmetry could be used for display applications.

The PL emission spectrum for Ca_0.5Y_1-x(MoO₄)₂:xTb³⁺,Na⁺ upon ∼295 nm UV excitation is shown in Figure 10b, comprises four PL emission bands having peaks at 489 nm (⁵D₄ → ⁷F₆), 545 nm (⁵D₄ → ⁷F₅), 587 nm (⁵D₄ → ⁷F₄) and 621 nm (⁵D₄ → ⁷F₃). In these emission peaks, the highly remarkable green colour is located at 545 nm related to the predominant transition ⁵D₄ → ⁷F₅ [27]. It is due to the energy transfer from the host which is populating only ⁵D₄ level. The dominant emission peak which is having two sub peaks is due to the Stark energy splitting and forms the suitable candidate for display applications.

Figure 10.

PL emission spectra (a, b, and c) of the thin film phosphors Ca_0.5Y(MoO₄)₂:Ln³⁺,M⁺ (Ln = Eu, Tb and Dy; M = Na).

The room temperature PL emission spectrum (Figure 10c) for the thin film phosphor Ca_0.5Y_1-x(MoO₄)₂:xDy³⁺,Na⁺ excited upon 353 nm excitation wavelength. The emission spectrum consists of two major peaks with respective peaks at 485 nm ascribed to magnetic dipole transition of (⁴F_9/2 → ⁶H_15/2) and 576 nm related to forced electric dipole transition of ⁴F_9/2 → ⁶H_13/2. The magnetic dipole transition is lesser sensitive to the coordination environment [27, 28]. The forced electric dipole transition is appeared only in the case of Dy³⁺ ions which are found at the local sites without inversion centre symmetry [28]. The respective blue emission having a centre at 485 nm is relatively lower intense than predominant yellow emission. The yellow‐to‐blue line ratio is 6.8026 which signifies that forced electric dipole transition is in dominance thereby indicating that the Dy³⁺ ions would found at the local sites with non‐inversion centre symmetry in the Ca_0.5Y_1-x(MoO₄)₂ host.

3.2.3.2. Ca_0.5La_1-x(MoO₄)₂:xLn³⁺,Na⁺ (Ln = Eu, Tb and Dy)

The room temperature PL excitation spectrum of Ca_0.5La_1-x(MoO₄)₂:xEu³⁺ thin phosphor films monitored at 394 nm excitation wavelength is shown in Figure 11a, consists of numerous intra‐configurational 4f‐4f transitions. As the Eu³⁺ concentration in the Ca_0.5La(MoO₄)₂ host lattices increases, the photoluminescence emission of the host is suppressed due to the overcoming of Eu³⁺ ions. The intensities of the different ⁵D₀ → ⁷F_J transitions might depend on the local symmetry of crystal field of Eu³⁺ ions [19]. The highly intense and strong emission peak is located at 615 nm upon 394 nm UV excitation corresponds to the ⁵D₀ → ⁷F₂ electric‐dipole transition showing hypersensitive red emission with parity forbidden (ΔJ = 2). The split‐up in the peaks is due to the Stark energy splitting, which is having (2J + 1) Stark components of J‐degeneracy splitting [28]. The predominant electric‐dipole transition implies that Eu³⁺ ions would be situated at sites with non‐inversion symmetry [15]. The transitions located at 586 nm show emission in the orange region which is associated with magnetic‐dipole transition (⁵D₀ → ⁷F₁) and is not affected by the chemical surroundings of Eu³⁺. The remaining transitions at 654 nm (⁵D₀ → ⁷F₃) and 701 nm (⁵D₀ → ⁷F₄) are the weakest ones. The red‐to‐orange emission (R/O) ratio of the phosphor is 5.5311 which suggests the sites symmetry of the respective Eu³⁺ ions. Based on these observations, it is suggested that Ca_0.5La_1-x(MoO₄)₂:xEu³⁺,Na⁺ might be a suitable phosphor candidate for solid‐state lighting applications.

The PL emission spectrum for Ca_0.5La_1-x(MoO₄)₂:xTb³⁺,Na⁺ recorded at ∼278 nm UV excitation is shown in Figure 11b which consists of four PL emission peaks at 489 nm (⁵D₄ → ⁷F₆), 545 nm (⁵D₄ → ⁷F₅), 585 nm (⁵D₄ → ⁷F₄) and 621 nm (⁵D₄ → ⁷F₃). Among these emission peaks, the sensitive green colour is located at 545 nm associated to the predominant transition ⁵D₄ → ⁷F₅ [19]. It is based on the energy transfer from the host populates only ⁵D₄ level. The dominant emission peak possesses two sub peaks which correspond to the Stark energy splitting.

The room temperature PL emission spectrum (Figure 11c) for the thin phosphor film Ca_0.5La_1-x(MoO₄)₂:xDy³⁺,Na⁺ excited with 352 nm excitation wavelength. The emission spectrum comprises of two major peaks with peak positions at 477 nm attributed to magnetic dipole transition of (⁴F_9/2 → ⁶H_15/2) and 570 nm corresponds to forced electric dipole transition of ⁴F_9/2 → ⁶H_13/2 [25]. The magnetic dipole transition is least sensitive to the coordination environment. The forced electric dipole transition would be found only in the case of Dy³⁺ ions situated at the local sites without inversion symmetry [19]. The emission in the blue region having a centre at 477 nm is having relatively least intense than predominant yellow emission. The yellow‐to‐blue line ratio is 6.0076 which implies that the forced electric dipole transition is most dominant hence indicating that the Dy³⁺ ions are situated at the local sites with non‐inversion symmetry in the Ca_0.5La_1-x(MoO₄)₂ host.

Figure 11.

PL emission spectra (a, b, and c) of the thin film phosphors Ca_0.5La(MoO₄)₂:Ln³⁺,M⁺ (Ln = Eu, Tb and Dy; M = Na).

3.3. Photometric characterization and decay‐time analysis

Figures 12a, b and 13a, b show the decay time profile and Commission Internationale de I'Eclairage (CIE) colour chromaticity coordinates of the Ca_0.5Y_1-x(MoO₄)₂:xLn³⁺,Na⁺ (Ln = Eu, Tb and Dy) and Ca_0.5La_1-x(MoO₄)₂:xLn³⁺,Na⁺ (Ln = Eu, Tb and Dy) phosphors. The CIE colour chromaticity coordinate of these phosphors was estimated and is given in Table 1. Furthermore, to know about the characteristic emission of these phosphors, the value of colour purity was derived by the equation [24]

$\text{Colourpurity}=\frac{\sqrt{{\left(x-x_i\right)}^2+{\left(y-y_i\right)}^2}}{\sqrt{{\left(x_d-x_i\right)}^2+{\left(y_d-y_i\right)}^2}}100$E1

where (x, y) is denoted as CIE chromaticity coordinate of the synthesized sample, (x_i, y_i) is the CIE white illumination, and (x_d, y_d) is the CIE chromaticity coordinate of the dominant wavelength. Thus, the colour purities of the Ca_0.5Y_1-x(MoO₄)₂:xEu³⁺,Na⁺, Ca_0.5Y_1-x(MoO₄)₂:xTb³⁺,Na⁺ and Ca_0.5Y_1-x(MoO₄)₂:xDy³⁺,Na⁺ phosphors are 90.0, 87.5 and 81.3%, respectively. Similarly, the colour purities of the Ca_0.5La_1-x(MoO₄)₂:xEu³⁺,Na⁺ Ca_0.5La_1-x(MoO₄)₂:xTb³⁺,Na⁺ and Ca_0.5La_1-x(MoO₄)₂:xDy³⁺,Na⁺ phosphors are 95.0, 91.9 and 81.7%. From the results, it is suggested that these phosphors with remarkable CIE chromaticity coordinate with high colour purities might be suitable for applications in display devices as the best red, green and yellow emitting phosphors.

Figure 12.

Luminescence decay profiles (a and b) for the thin film phosphors Ca_0.5R_1-x(MoO₄)₂:xLn³⁺ (R³⁺ = Y, La),Na⁺ (Ln = Eu, Tb and Dy).

Figure 13.

CIE diagram (a, b) for the thin film phosphors Ca_0.5Y(MoO₄)₂:Ln³⁺,Na⁺ (Ln = (A) Eu, (B) Tb and (C) Dy).

The representative PL decay curves for luminescence emission for the phosphors Ca_0.5Y_1-x(MoO₄)₂:xLn³⁺,Na⁺ (Ln = Eu, Tb and Dy) and Ca_0.5La_1-x(MoO₄)₂:xLn³⁺,Na⁺ (Ln = Eu, Tb and Dy) are shown in Figure 12a, b. This can be fitted well into a single exponential function [15, 27] as

$I=I_0\text{exp}\left(\frac{-t}{\tau }\right)$E2

where I₀ is the luminescence intensity at times t = 0 and τis its associated luminescence lifetime. The photometric quantities and luminescence decay time values are given in Table 1.

Phosphor	CCT (K)	CRI	Colour coordinates		LER (lm W^-1)	Colour purity (%)	τ(ms)
			x	y
Ca0.5Y(MoO4)2:Eu^3+,Na^+	1149	33	0.635	0.365	162	90.0	0.462
Ca0.5Y(MoO4)2:Tb^3+,Na^+	N/A	26	0.296	0.564	491	87.5	0.455
Ca0.5Y(MoO4)2:Dy^3+,Na^+	3658	18	0.414	0.478	525	81.3	0.172
Ca0.5La(MoO4)2:Eu^3+,Na^+	1196	42	0.656	0.343	321	95.0	0.481
Ca0.5La(MoO4)2:Tb^3+,Na^+	6850	12	0.251	0.570	530	91.9	0.485
Ca0.5La(MoO4)2:Dy^3+,Na^+	4195	16	0.404	0.485	462	81.7	0.187

Table 1.

Photometric parameters, color purity and luminescence decay time for the phosphors Ca_0.5R_1-x(MoO₄)₂:xLn³⁺, Na⁺ (R = Y, La; Ln = Eu, Tb and Dy).

3.4. Photoluminescence emission studies from nano‐architectures

The thin phosphor films grown from nano‐powder are being synthesized by the hydrothermal method, and the synthesis procedure is described previously by our group [17]. The luminescence emission intensity is being enhanced by co‐doping of alkali metal ions. Furthermore, for the co‐doping of alkali precursors, instead of using alkali chloride, alkali carbonates were taken and converted them into alkali nitrates. These alkali nitrates were co‐doped with the existing precursors following the hydrothermal method nano‐powders were synthesized and thin films were deposited from these powders [17]. The room temperature PL emission spectrum for Ca_0.5R_1-x(MoO₄)₂:xEu³⁺,Na⁺ (R = Y, La) as the representative thin phosphor films are shown in Figure 14. The emission spectra monitored at 395 nm UV excitation for both the phosphors shows a number of intra‐configurational f‐f transitions. The strong and most intense emission peak is found at 616 nm for Ca_0.5Y_1-x(MoO₄)₂:xEu³⁺,Na⁺ and 615 nm for Ca_0.5La_1-x(MoO₄)₂:xEu³⁺,Na⁺ is attributed to the ⁵D₀ → ⁷F₂ electric‐dipole transition possess hypersensitive red emission [15, 28, 29]. The Stark energy splitting is mildly shown for both the phosphors. It is noticed that the splitting of the electric dipole transition is uniform and homogeneous between the two thin phosphor films. The intensity of the spectral peaks for the nano‐thin phosphor film is nearly close to that of those from the bulk thin phosphor film, as peak intensity is related to reduced particle size and improved homogeneity [17, 29]. The dominant electric‐dipole transition suggests that Eu³⁺ ions are found at sites with non‐inversion symmetry [30]. The other transitions found at 587 nm (⁵D₀ → ⁷F₁) for Ca_0.5Y_1-x(MoO₄)₂:xEu³⁺,Na⁺ and 586 nm (⁵D₀ → ⁷F₁) for Ca_0.5La_1-x(MoO₄)₂:xEu³⁺,Na⁺ show orange emission due to magnetic‐dipole transition. The other relatively weaker transitions are found at 654 nm (⁵D₀ → ⁷F₃) and 702 nm (⁵D₀ → ⁷F₄) for both the nano‐thin phosphor films. The photometric parameters for both the phosphors are under further investigation. From these results, it is indicated that Ca_0.5R_1-x(MoO₄)₂:xEu³⁺,Na⁺ (R = Y, La) phosphors are best candidates for display applications.

Figure 14.

PL emission spectrum of the thin film phosphors Ca_0.5Y(MoO₄)₂:Eu³⁺,Na⁺ and Ca_0.5La(MoO₄)₂:Eu³⁺,Na⁺ prepared from nano‐phosphors.

4. Conclusion

In conclusion, the nano‐sized single crystalline Ca_0.5La_1-x(MoO₄)₂:xEu³⁺,M⁺ ceramic thin phosphor films deposited on quartz substrates by pulsed laser deposition technique using Nd‐YAG laser source in an ultra‐high vacuum (UHV). The FESEM images exhibited the spherical‐shaped phosphor particles. XRD patterns revealed the scheelite‐type crystal structure without any impurity phases. By using AFM, the surface topographies and distributions of grains were investigated. Upon optical excitation, Eu‐, Tb, and Dy‐doped Ca_0.5La_1-x(MoO₄)₂:xEu³⁺,M⁺ thin phosphor films showed characteristic emissions in the bright‐red, green and yellow regions, respectively. The obtained results suggested that the deposited thin film phosphors could serve as efficient materials for electroluminescence and display applications.

Conflict of interest

The authors declare that there is no conflict of interests regarding the publication of this chapter.

Acknowledgments

Work incorporated in this chapter was supported by Science and Engineering Research Board (SERB), (SR/FTP/PS‐135/2011) Govt. of India. The authors apologize for inadvertent omission of any pertinent references.