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Recent Progress in Rare-earth Ion-activated Molybdate-based Phosphors for Versatile Applications
Sk. Khaja Hussain, Yongbin Hua
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DOI: 10.22661/AAPPSBL.2020.30.2.26

Recent Progress in Rare-earth Ion-activated Molybdate-based Phosphors for Versatile Applications


*E-mail: jsyu@khu.ac.kr


Nowadays, rare-earth ion-activated molybdate-based inorganic phosphor materials have drawn considerable attention in the research field of white light-emitting diodes, optical thermometers (up- and down-conversion) and field-emission displays, because of their good luminescent efficiency and low auto-fluorescence characteristics. The molybdate-based phosphors maintain novel structural relation properties, consisting of a charge transfer band from ligand to its transition metal ions (Mo6+ group). Thus, this review focuses on the recent development of a powder form of inorganic phosphor compositions composed of molybdenum group elements and a study of their luminescence behavior, cathodoluminescence properties and application as optical thermometers. This could provide a new research prospect and future directions to investigate the novel molybdenum-based phosphor composites for versatile applications.


Tremendous research has been focused on phosphor-converted white light-emitting diodes (WLEDs) for next-generation solid-state lighting under ultraviolet and near-ultraviolet excitations. Phosphor-converted WLEDs have several advantages in terms of high luminescent efficiency, long lifetime, high color rendering index (CRI > 80) and low correlated color temperature (CCT < 4000 K), which is more suitable in our daily life than conventional lamps [1, 2]. On the other hand, in recent times, many new technologies such as field-emission displays (FEDs), plasma display panels, liquid crystal displays, electroluminescence devices, etc. have been employed to improve the energy efficiency and image quality of flat panel displays [3, 4]. Among them, FED is one of the important and most promising next-generation flat panel display technologies owing to its good contrast ratio, display performance, low power consumption, long lifetime, etc. Rare-earth ion-doped inorganic phosphor materials have the above promising features and can fulfill the requirements of FED with high-luminesce efficiency under low voltages of 5 to 7 kV (to excite electrons) and high current which is around 10-100 mA [1, 3-6]. In general, the inorganic host materials which include sulfur in their composition have several limitations in their usage for FEDs. In fact, phosphors containing sulfur (Gd2O2S:Tb3+, (Zn/Cd)S:Cu, SrGa2S4:Eu3+/Ce3+, Y2O2S:Eu3+ and ZnS:Ag) are frequently decomposed and release sulfide gas under high energy electron bombardment due to dissociation of the cation-sulfur bond. Thus, these features limit their performance from the application point of view. In order to achieve good display performance without sulfur in the phosphor composition, rare-earth ion-induced inorganic materials have been investigated for FED applications in recent years. Host materials that contain WO42-, MoO42-, GeO44-, SnO62-, TiO62- and NbO42- groups are efficient materials for FED applications. Of these materials, the molybdate-based phosphor materials have attracted much interest due to low photon energy and strong ultraviolet absorption with good thermal stability [3-6].

In addition, since conventional contact thermometers suffer from tardy response time in a changeable environment, it is difficult to satisfy people's everyday needs. Therefore, a new strategy, based on rare-earth ion-activated phosphors and using the fluorescence intensity ratio (FIR) technique, has been proposed for non-contact up- or down-conversion optical thermometers. It involves comparing the emission intensity between a pair of thermally-coupled levels, such as single rare-earth ions doped with Er3+ (4S3/2, 2H11/2), Tm3+ (3F2/3, 3H4), Ho3+ (5F2/3, 3K8) or rare-earth ions co-doped with different response times as a function of temperature like Sm3+/Tb3+, Eu3+/Tb3+, Bi3+/Eu3+, Pr3+/Tb3+ and so on [7-12]. Eventually, the FIR value, absolute sensing sensitivity (Sa) and relative sensing sensitivity (Sr) can be estimated by the following expressions [13, 14]:




This review provides a brief summary of the recent progress with molybdenum-based phosphor materials for versatile applications such as optical temperature sensors, WLEDs and FEDs, respectively.


The phosphor composite materials are prepared by solid-state, solvothermal, hydrothermal, co-precipitation, combustion, sol-gel and pechini-type sol-gel methods. In the various preparation processes, mostly solid-state, hydrothermal or solvothermal and sol-gel methods offer versatile morphological properties for the phosphors. For example, in the solid-state reaction and sol-gel methods, the particles could exist mostly in microstructures. Similarly, hydrothermal or solvothermal methods could provide smart morphologies with uniform sizes and shapes. These special morphological features are further beneficial for higher luminescence and optical properties. Fig. 1 shows a simple schematic representation for the preparation methods for molybdate-based phosphors that have been used in recent years. Initially, the raw materials are mixed manually (soli-state) or uniformly dissolved in aqueous solutions (sol-gel or hydrothermal) and the reaction is progressed at elevated temperatures; later it is collected and calcined at a higher temperature to obtain the pure phase form. The resulting molybdate-based phosphor powder was applied to versatile applications such as LED, optical thermometer and FED applications.


Fig. 1: Schematic representation of the preparation process for molybdate phosphors and their versatile applications.


Up-Conversion Optical Thermometers

Optical temperature measurement for molybdate-based phosphors using the principle of FIR has received extensive attention due to its fast response, high sensitivity and high spatial resolution. S. Sinha et al. described the up-conversion (UC) properties of Gd2Mo3O9:Er3+/Yb3+ phosphors with high sensing properties for optical thermometers [15]. The effect of Er3+ ion concentration on the sensing efficiency was investigated and the temperature-dependent UC measurements were carried out. As shown in Fig. 2(a-d), the UC spectra were recorded for the different concentrations of Er3+ ions (0.3, 1, 2 and 3 mol%) at three temperatures of 300, 390 and 460 K under constant excitation power (15 W cm-2). The variation in the green emission intensity was further studied by FIR principle as a function of temperature. Furthermore, the sensitivity of the material was evaluated for all the concentrations of the samples. The maximum sensitivity (10.57 횞 10-3 K-1 at 450 K) was noticed for the lower concentration of Er3+ ions (0.3 mol%). When the concentration was raised to 3 mol%, the sensitivity was reduced (10.57 횞 10-3 K-1 at 450 K) due to the increase of energy transfer between the adjacent Er3+ ions via cross-relaxation process (4F7/2 + 4I11/24F9/2 + 4F9/2) [15]. The different sensitivity values for different concentrations of Er3+ ions are further shown in Fig. 2(e-h).


Fig. 2: (a-d) Temperature-dependent UC emission spectra for green emission bands of Gd2Mo3O9:Er3+/Yb3+ (Er3+ = 0.3 to 3 mol%) phosphors excited by 980 nm. (e-h) Variation of absolute temperature sensitivity (S) as a function of temperature within 300-480 K. (Reprinted with permission from Ref. 15, copyright 2016, RSC Advances).

Down-Conversion Optical Thermometers

Recently, S.S. Perera et al. prepared molybdate type NaLa1-xDyx(MoO4)2 phosphors for down-conversion optical thermometers and compared the sensing properties with wolframate type NaLa1-yDyy(WO4)2 phosphors [16]. With increasing temperature, the FIR values increased due to the different response time of Dy3+ transitions (4I15/2 6H15/2, 4F9/26H15/2) for various temperatures as shown in Fig. 3(a). The Sa and Sr values for those samples were investigated as shown in Fig. 3(b) and (c). It is obvious that Sa increases with increase in temperature. The maximum value was found to be 0.00098 K-1 at 700 K, which is higher than that for wolframate type NaLa1-yDyy(WO4)2 phosphors. However, the Sr value for NaLa1-xDyx(MoO4)2 phosphors would be lower than that for NaLa1-y Dyy(WO4)2 phosphors with the same doping concentration as shown in Fig. 3(c). In addition, the sensing properties of those samples were affected by the doping concentration as displayed in Fig. 3(d). The cycling performance of the samples was studied as illustrated in Fig. 3(e), implying that the sensing performance was very stable for optical thermometer applications. The recent studies of up- and down-conversion optical thermometers and the corresponding absolute and relative sensing properties of the molybdate-based phosphors are summarized in Table 1.


Fig. 3: (a) FIR, (b) Sa and (c) Sr for the NaLa0.95Dy0.05(MoO4)2, NaLa0.975Dy0.025(WO4)2, Na5La0.9Dy0.1(MoO4)4 and Na5La0.9Dy0.1(WO4)4 phosphors. (d) Sr values at 350 and 700 K as a function of host composition and Dy3+ concentration. (e) FIR values for NaDy(MoO4)2 and Na5La0.5Dy0.5(WO4)4 as a function of heating-cooling cycle number. (Reprinted with permission from Ref. 16, copyright 2019, Journal of Materials Chemistry C).

Table 1. Optical thermometer performance in molybdate-based phosphors.


























































A series of Sm3+-activated CaGd2(MoO4)4 phosphors were synthesized by L. Zhou et al.[23]. The optimum doping concentration of Sm3+ was found to be 3 mol%, resulting in Commission Internationale de I'Eclairage (CIE) chromatic coordinates of (0.627, 0.372), close to the values for commercial red-emitting Y2O2S:Eu3+ phosphors of (0.622, 0.351) as shown in Fig. 4(a). Furthermore, the packaged LED was fabricated using CaGd2(MoO4)4:Sm3+, and the commercial blue-emitting BAM:Eu2+ and green-emitting (Ba, Sr)2SiO4:Eu2+ phosphors covering a near-ultraviolet (NUV) chip with an emitting wavelength of 385 nm. The packaged device emitted a dazzling white light with a high CRI value of 82.6 while the CIE chromaticity coordinates were found to be (0.305, 0.318) as shown in Fig. 4(b). CRI and CCT values for recently reported molybdate-based phosphors are shown in Table 2.


Fig. 4: CIE chromaticity coordinates of (a) a powder and (b) a packaged LED device. (Reprinted with permission of Ref. 23, copyright 2019, Journal of American Ceramic Society).


Table 2. Molybdate-based phosphors for WLEDs.
































To check the potential of molybdenum-based phosphors for FED applications, their cathodoluminescence (CL) properties have been investigated under low electron beam excitation and accelerating voltages. In general, in phosphor materials, at different accelerating voltages (fixed filament current) and filament currents (constant accelerating voltage), the CL emission intensity is enhanced. The increment or enhancement of CL emission intensity is due to the deeper penetration depth and interaction volume of the incident electrons in the corresponding host material. When the electron energy is increased, the CL emission of the phosphor can be increased. The electron penetration depth in CL can be further estimated by the following formula [28]:


In the above expression, ρ is the charge density (gm/cm3), A represents the molar mass, z indicates the number of electrons in the molecule and E is the accelerating voltage. Hou et al. and Sinha et al. studied the CL emission properties of CaMoO4:Ln3+(Ln = Eu, Tb, Dy) and CaMoO4:Er3+/Yb3+ molybdenum-based phosphors under different accelerating voltages and various filament currents [29, 30]. Sinha et al. discussed the CL emission of CaMoO4:Er3+/Yb3+ phosphors and further it was compared with the effect of doping with Na+/K+ ions. The doping of Na+/K+ ions in CaMoO4:Er3+/Yb3+ phosphors further increased the CL emission at operating filament currents and voltages as shown in Fig. 5. As disused briefly, due to the superior features of molybdate-based phosphors and doping with Na+/K+ ions, the CL emission intensity of CaMoO4:Ln3+ phosphors was increased.


Fig. 5: (a) Comparative CL spectra of CM and CMKNa; variation of CL intensity with (b) accelerating voltage and (c) filament current in CMKNa phosphor. (Reprinted with permission from Ref. 30, copyright 2017, New Journal of Chemistry)

Similarly, in recent years, our group also studied the enhancement of CL emission properties of molybdate-based Na0.5Gd0.5MoO4 red-emitting phosphors at various filament currents (34-55 關A) and accelerating voltages (5-9 keV) for FED applications [31]. For the Na0.5Gd0.5MoO4:0.25Eu3+ phosphor, the values of A, ρ and Z were 248.72, 3.79 and 188, respectively. Moreover, molybdenum-containing monolayer transitional metal dichalcogenide (MoS2) semiconductors showed enhanced CL and PL emissions in recent times. The CL emission intensity was stronger (500-fold) when the MoS2 was sandwiched by two layers of hexagonal boron nitride (van der Walls heterostructure configuration). The CL and PL emission peaks of MoS2 heterostructure were located at 1.831 eV as shown in Fig. 6. The CL mapping intensity for the MoS2 heterostructure can be seen in the inset of Fig. 6.


Fig. 6: CL and PL spectra of the monolayer MoS2 in the top-hBN/MoS2/bottom-hBN. Inset is the corresponding CL intensity mapping. (Reprinted with permission from Ref. 32, copyright 2017, Nano Letters)


Phosphors are irreplaceable inorganic materials in the field of WLED and FED devices. Temperature is a significant physical quantity in numerous fields, and it affects the spectroscopic and luminescent behavior of phosphor materials. Molybdates are suitable as host materials for phosphors with superior luminescence, temperature sensing and CL properties due to their high stability and relatively low energy consumption. In this review, we have summarized briefly the importance of molybdate-based phosphors for high-quality WLED applications and the study of their CL properties as a function of accelerating voltage and filament current for high-efficiency FED and optical thermometer applications.


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Sk. Khaja Hussain is currently working as a Postdoctoral Researcher at the Department of Chemical Engineering, Kyung Hee University, Republic of Korea. He received his bachelor's (B.Sc.) and Masters (M.Sc.) degrees in Chemistry from Acharya Nagarjuna University, Andhra Pradesh, India. Afterwards, he joined as a junior scientist (2011) Sequent Scientific Ltd., New Mangalore, India. He also received his Doctor of Philosophy (Ph. D.,) degree in Aug 2019 in the field of hybrid supercapacitors for energy storage under the supervision of Prof. Jae Su Yu at Kyung Hee University, Republic of Korea. His current research focuses on the design and chemical synthesis of inorganic nanostructures and functional hierarchical materials for energy storage devices and phosphor-converted white light-emitting diodes.

Yongbin Hua obtained his bachelor's degree (B.Sc.) at Shaoguan University, Guangdong province, China. He is a Master & Ph.D combined student under the supervision of Prof. Jae Su Yu at Kyung Hee University, Republic of Korea. His current research major is luminescent materials and their applications, including solid-state lighting, white light-emitting diodes, optical temperature sensors, flexible display films, anti-counterfeiting marks and FED devices.

Jae Su Yu is Professor at Department of Electronic Engineering and Director of the Institute for Wearable Convergence Electronics, Kyung Hee University, Republic of Korea. He received his Ph.D. degree in Optoelectronic Engineering from the Department of Information and Communications at Gwangju Institute of Science and Technology, Republic of Korea, in 2002. Afterwards, he joined the Center for Quantum Devices, Northwestern University, USA, as a Postdoctoral Fellow (2002-2006). He was a visiting professor in John A. Roger's group, University of Illinois Urbana-Champaign, USA. Until now, he has authored/co-authored more than 450 papers in SCI(E) indexed journals. His research group mainly works on the development of advanced materials for phosphors, energy storage/conversion, optoelectronic and biological application.

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