Photometric parameters, color purity and luminescence decay time for the phosphors Ca0.5R1-
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.
Keywords
- rare‐earth and alkaline activated
- thin phosphor films
- pulsed laser deposition
- surface morphology
- luminescence
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 I41/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 Ca0.5R1-
2. Experimental details
By employing the PLD technique, for the first time, the nano‐thin phosphor films of Ca0.5R1-
2.1. Preparation of ceramic target, cleaning of substrates and growth of Ca0.5R1-x (MoO4)2:x Ln3+ 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.
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 2
3. Results and discussion
3.1. Morphological and X‐ray diffraction analysis
Figure 2a and b shows the scanning electron microscopy (SEM) images of Ca0.5R1-
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 Ca0.5Y(1-
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:
Ca0.5R1-x (MoO4)2:x Ln3+,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
Ca0.5R1-x (MoO4)2:x Ln3+,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 Ca0.5R1
In our system, Eu3+, Tb3+, Dy3+ and M+ co‐doped in Ca0.5R(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 Ca0.5Y1-
3.2.2. Photoluminescence excitation studies
3.2.2.1. Ca0.5Y1-x (MoO4)2:x Ln3+,Na+ (Ln = Eu, Tb and Dy)
Figure 8a shows the room temperature PL excitation spectra of Ca0.5Y1-
Figure 8b depicts the room temperature excitation spectrum of Ca0.5Y1-
The excitation spectrum for the Ca0.5Y1-
3.2.2.2. Ca0.5La1-x (MoO4)2:x Ln3+,Na+ (Ln = Eu, Tb and Dy)
The room temperature PL excitation spectra of Ca0.5La1-
Figure 9b shows that the photoluminescence excitation spectrum of Ca0.5La1-
The room temperature excitation spectrum for the Ca0.5La1-
3.2.3. Photoluminescence emission studies
3.2.3.1. Ca0.5Y1‐x(MoO4)2: xLn3+,Na+ (Ln = Eu, Tb and Dy)
The room temperature PL emission spectra for Ca0.5Y1-
The PL emission spectrum for Ca0.5Y1-
The room temperature PL emission spectrum (Figure 10c) for the thin film phosphor Ca0.5Y1-
3.2.3.2. Ca0.5La1-x (MoO4)2:x Ln3+,Na+ (Ln = Eu, Tb and Dy)
The room temperature PL excitation spectrum of Ca0.5La1-
The PL emission spectrum for Ca0.5La1-
The room temperature PL emission spectrum (Figure 11c) for the thin phosphor film Ca0.5La1-
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 Ca0.5Y1-
where (
The representative PL decay curves for luminescence emission for the phosphors Ca0.5Y1-
where
Phosphor | CCT (K) | CRI | Colour coordinates | LER (lm W-1) | Colour purity (%) | ||
---|---|---|---|---|---|---|---|
Ca0.5Y(MoO4)2:Eu3+,Na+ | 1149 | 33 | 0.635 | 0.365 | 162 | 90.0 | 0.462 |
Ca0.5Y(MoO4)2:Tb3+,Na+ | N/A | 26 | 0.296 | 0.564 | 491 | 87.5 | 0.455 |
Ca0.5Y(MoO4)2:Dy3+,Na+ | 3658 | 18 | 0.414 | 0.478 | 525 | 81.3 | 0.172 |
Ca0.5La(MoO4)2:Eu3+,Na+ | 1196 | 42 | 0.656 | 0.343 | 321 | 95.0 | 0.481 |
Ca0.5La(MoO4)2:Tb3+,Na+ | 6850 | 12 | 0.251 | 0.570 | 530 | 91.9 | 0.485 |
Ca0.5La(MoO4)2:Dy3+,Na+ | 4195 | 16 | 0.404 | 0.485 | 462 | 81.7 | 0.187 |
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 Ca0.5R1-
4. Conclusion
In conclusion, the nano‐sized single crystalline Ca0.5La1-
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.
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