Single-phase Ce3+–Mn2+–Tb3+ tri–codoped barium–yttrium–silicate phosphors

Ce³⁺–Mn²⁺–Tb³⁺ cooperative barium–yttrium-orthosilicate phosphors composed of Ba_{9-3m/2-n-3p/2}Ce_mMn_nTb_pY₂Si₆O₂₄ (m = 0.005–0.4, n = 0–0.5, p = 0–0.5) were prepared using a solid-state reaction. The X-ray diffraction patterns of the resultant phosphors were examined to index the peak positions. The photoluminescence (PL) excitation and emission spectra of the Ce³⁺–Mn²⁺–Tb³⁺ activated phosphors were clearly monitored. The dependence of the luminescent intensity of the Mn²⁺–Tb³⁺ co-doped Ba_9-3m/2Ce_mY₂Si₆O₂₄ host lattices on Ce³⁺ content (m = 0.025, 0.1) was also investigated. Co-doping Mn²⁺ into the Ce³⁺–Tb³⁺ co-doped host structure enabled energy transfer from Ce³⁺ to Mn²⁺; this energy transfer mechanism is discussed. The phosphors of Ce³⁺–Mn²⁺–Tb³⁺ doped Ba₉Y₂Si₆O₂₄ host lattice were prepared for efficient white-light emission under NUV excitation. With these phosphors, the desired CIE values including white region of the emission spectra were achieved.     

Development of a new blue CaMgSi2O6:Eu2+ phosphor for PDPs

Abstract— A blue‐light‐emitting Eu²⁺‐doped CaMgSi₂O₆ phosphor having a long lifetime for a plasma‐display panel (PDP) was developed. The CaMgSi₂O₆:Eu²⁺(CMS:Eu) phosphors show no luminance degradation during the baking process, and an equivalent photoluminescence peak intensity compared to that of the conventional blue‐phosphor BaMgAl₁₀O₁₇:Eu²⁺ (BAM) after baking. CMS: Eu shows a poor luminescent characteristic for the Xe excimer band excitation due to the lack of absorption. To introduce the absorption center for the Xe excimer band, we performed Gd‐codoping of CMS: Eu as a sensitizer and found a new excitation band around 172 nm, which originated from Gd³⁺. The test PDPs panels using synthesized CMS: Eu phosphor and CMS: Eu, Gd phosphor were examined to investigate the luminescent and aging characteristics of a Xe‐discharge excitation source. The CMS: Eu panel shows an emission peak intensity comparable to that of the BAM panel (i.e., a comparable stimuli L/CIEy, 93% of BAM), while the CMS: Eu, Gd panel shows poorer blue emission intensity compared to the BAM panel (up to 53% of total stimuli of BAM). The CMS: Eu panel and the CMS: Eu, Gd panel show less luminance degradation than the BAM panel under the aging test, and the panel retains 90% of its luminance after 300 hours of driving. It was found that CMS: Eu appears to be a candidate for a new blue PDP phosphor because of its longevity in a Xe‐discharge plasma environment.     

Synthesis and photoluminescent properties of orange-emitting Sm3+-activated KPb4(PO4)3 phosphor for LEDs

A series of Sm³⁺-doped KPb₄(PO₄)₃ phosphors have been successfully synthesized by high-temperature solid-state reaction method, and the structure, morphology and luminescent properties were investigated. The SEM images suggest that the prepared phosphor has an irregular morphology with a diameter of about 10 ~ 20 μm. Under near-ultraviolet (NUV) light (404 nm) excitation, all prepared phosphors KPb_4-xSm_xP₃O₁₂ (x = 0, 0.02, 0.04, 0.06, 0.08, 0.1, 0.15, 0.2, 0.3, 0.4 and 0.5) show the characteristic f → f emission bands of the Sm³⁺ activator. And the emission intensities have an upward trend with increasing the Sm³⁺ concentration when x is lower than 0.1. By monitoring 598 nm emission, the excitation spectrum of KPb_3.9Sm_0.1(PO₄)₃ contains a series of sharp bands in the range of 250 ~ 500 nm, which matches well with the NUV LED chip. The CIE coordinates of KPb_3.9Sm_0.1(PO₄)₃ phosphor was evaluated to be (0.5714, 0.4253), corresponding to orange color with a high color purity of about 90%.     

The influence of processing parameters on luminescent oxides produced by combustion synthesis

Rare-earth activated oxide phosphors have application in high energy photoluminescent (plasma panels) and cathodoluminescent (field emission devices) flat panel displays. These phosphors are composed of a highly insulating host lattice with fluorescence arising from the 3d→3d, 5d→4f or 4f→4f transitions in transition metal or rare earth ions. Fabrication of complex host compositions Y₂SiO₅, Y₃Al₅O₁₂, Y₂O₃, and BaMgAl₁₀O₂₇ along with controlled amounts of the activators (Cr³⁺, Mn²⁺, Ce³⁺, Eu²⁺, Eu³⁺, Tb³⁺, Tm³⁺) represent a challenge to the materials synthesis community. High purity, compositionally uniform, single phase, small and uniform particle size powders are required for high resolution and high luminous efficiency in the new flat panel display developments. This paper will review the synthesis techniques and present physical and luminescent data on the resulting materials.

1. Introduction

The visible-light-generating components of emissive, full color, flat panel displays are called phosphors. Phosphors are composed of an inert host lattice and an optically excited activator, typically a 3d or 4f electron metal. For application in the emerging full color, flat panel display industry, thermally stable, high luminous efficiency, radiation resistant, fine particle size powders are required. The demands of these newer technologies have produced a search for new materials and synthesis techniques to improve the performance of phosphors.Oxide phosphors were found to be optimal for field emission display (FED) and plasma panel display (PDP) devices. Compared with a cathode ray tube, an FED operates with lower energy (3–10 keV) but higher current density (1 mA/cm²) beams impinging on the phosphors. This requires more luminous efficient and thermally stable materials. Luminous efficiency is defined as the ratio of the energy out (lumens) to the input energy. Outgassing from the highly efficient sulfide based phosphors has been shown to degrade the cathode tips of the field emitter array and cause irreversible damage [1]. For PDPs, high energy photons (147 nm, 8.5 eV) impinge on the phosphor powders and cause a reduction in luminous efficiency of the display over time because of radiation damage induced in the material [2].Another requirement is on the particle size distribution: there is a maximum and minimum particle size limitation to the powders. For FED applications, about five particle layers are required to achieve optimal light output [3]. Large particles (>8 μm) require thicker layers, increasing the phosphor cost and also producing more light scattering. Additionally the pixel pitch (250 μm) places a maximum on particle size [4]. Alternatively, it was found that small particles (<0.2 μm) do not have high luminous efficiency arising from grain boundary effects [5]. The activator ion in the crystal is most efficient when located in the bulk material in a regular crystal field. Activators located on the surface or on the grain boundaries are thought to be non-luminescent or even luminescence quenching regions.For full color displays, three phosphor compositions are necessary to emit in the red (611–650 nm), green (530–580 nm) and blue (420–450 nm) regions of the visible spectrum. Some oxide based phosphors used in FEDs are the red-emitting (Y_1−xEu_x)₂O₃, the green-emitting (Y_1−xTb_x)₃Al₅O₁₂ and the blue-emitting (Y_1−xCe_x)₂SiO₅ [6]. For some PDPs the red-emitting component is (Y_1−xEu_x)₂O₃, the green-emitting is Zn_1−xMn_xSi₂O₅ and the blue-emitting is (Ba_1−xEu_x)MgAl₁₀O₁₇ [7]. Sulfide phosphors are also used in FEDs but suffer from the aforementioned degradation problems.

2. Phosphor synthesis techniques

Synthesis of oxide phosphors has been achieved by a variety of routes: solid-state reactions [8 and 9], sol–gel techniques [10], hydroxide precipitation [11], hydrothermal synthesis [12 and 13] and combustion synthesis [14, 15, 16 and 17]. Solid-state reactions are performed at high temperatures, typically around 1600°C, because of the refractory nature of the oxide precursors. For multielement compositions, an incomplete reaction is often obtained with undesirable precursor products present in the final product. This technique requires several heating and grinding steps in order to achieve well-reacted, small particle size phosphors. For sol–gel and hydroxide precipitation methods, dilute solutions of metallorganics or metal salts are reacted and condensed into an amorphous or weakly crystalline mass. The advantage of these methods is that atomically mixed powders are obtained in the as-synthesized condition and problems associated with incomplete reactions are avoided. However, these as-synthesized materials must also be heat treated to high temperatures to crystallize the desired phase and to achieve particle sizes greater than 0.2 μm. Hydrothermal synthesis is a low temperature and high pressure decomposition technique that produces fine, well-crystallized powders [13]. These powders must also be heat treated to high temperature to extract the maximum luminous efficiency. Combustion synthesis is a novel technique that has been applied to phosphor synthesis in the past few years. This technique produces highly crystalline powders in the as-synthesized state and will be described in more detail in Section 3.

3. Combustion synthesis of oxide phosphors

Combustion synthesis involves the exothermic reaction between metal nitrates and a fuel. Combustion synthesis is an important powder processing technique generally used to produce complex oxide ceramics such as aluminates [18, 19, 20 and 21], ferrites [22, 23, 24 and 25], and chromites [26 and 27]. The process involves the exothermic reaction of an oxidizer such as metal nitrates, ammonium nitrate, and ammonium perchlorate [28], and an organic fuel, typically urea (CH₄N₂O), carbohydrazide (CH₆N₄O), or glycine (C₂H₅NO₂).The combustion reaction is initiated in a muffle furnace or on a hot plate at temperatures of 500°C or less; much lower than the phase transition of the target material. In a typical reaction, the precursor mixture of water, metal nitrates, and fuel decomposes, dehydrates, and ruptures into a flame after about 3–5 min. The resultant product is a voluminous, foamy powder which occupies the entire volume of the reaction vessel. The chemical energy released from the exothermic reaction between the metal nitrates and fuel can rapidly heat the system to high temperatures (>1600°C) without an external heat source. Combustion synthesized powders are generally more homogeneous, have fewer impurities, and have higher surface areas than powders prepared by conventional solid-state methods [28].The mechanism of the combustion reaction is quite complex. The parameters that influence the reaction include: type of fuel, fuel to oxidizer ratio, use of excess oxidizer, ignition temperature, and water content of the precursor mixture. In general, a good fuel should react non-violently, produce non-toxic gases, and act as a complexant for metal cations [28]. Complexes increase the solubility of metal cations, thereby preventing preferential crystallization as the water in the precursor solution evaporates [29]. The adiabatic flame temperature, T_f, of the reaction is influenced by the type of fuel, fuel to oxidizer ratio, and the amount of water remaining in the precursor solution at the ignition temperature [27]. The flame temperature can be increased with the addition of excess oxidizer such as ammonium nitrate [28], or by increasing the fuel/oxidizer molar ratio. The following equation can be used to approximate the adiabatic flame temperature for a combustion reaction:

(1)

where ΔH_r and ΔH_p are the enthalpies of formation of the reactants and products, respectively, c_p is the heat capacity of products at constant pressure, and T₀ is 298 K. Measured flame temperatures are typically lower than calculated values of flame temperature as a result of heat loss. Table 1 lists the various phosphor compositions that were synthesized by combustion synthesis.

Table 1. Phosphor compositions obtained by combustion synthesis

An example of a stochiometric combustion reaction of yttrium, aluminum and terbium nitrate with carbohydrazide to form (Y_1−xTb_x)₃Al₅O₁₂ is:

(2)

(1−x)Y(NO₃)₃+5Al(NO₃)₃+3xTb(NO₃)₃+15CH₆N₄O→ (Y_1−xTb_x)₃Al₅O₁₂+42N₂+15CO₂+45H₂O.

When complete combustion occurs, the only gaseous products obtained are N₂, CO₂, and H₂O, making this an environmentally clean processing technique. The generation of gaseous products increases the surface area of the powders by creating micro- and nanoporous regions. For the earlier reaction, for every mole of solid produced, 102 mol of gas are produced.The difference in particle size with the use of different fuels depends upon the number of moles of gaseous products released during combustion. As more gases are liberated, the agglomerates are disintegrated and more heat is carried from the system thereby hindering particle growth. A greater number of moles of gas are produced in combustion reactions with carbohydrazide. If complete combustion is assumed, the gaseous product amounts liberated in combustion reactions with glycine, urea and carbohydrazide, are shown in Table 2. The reactions shown are for 2 mol of nitrate producing 1 mol of metal sesquioxide. If Y₃Al₅O₁₂ (YAG) is produced, the reactions must be multiplied by four, as 8 mol of nitrate are used in the reaction.

Table 2. Number of moles of gas produced for different fuels per mole of metal sesquioxide formed

The BET surface area for the as-synthesized YAG phosphors made with glycine, urea and carbohydrazide was measured to be 19, 22 and 25 m²/g, respectively [16], which is consistent with the increase in number of moles of gas produced.Fig. 1 shows the efficiency in lumens per watt (lm/W) as a function of electron accelerating voltage for (Y_1−xTb_x)Al₅O₁₂ made by solid-state, hydrothermal synthesis and combustion synthesis. The combustion synthesized YAG produced in this work has low-voltage cathodoluminescence efficiencies that are comparable to powders produced by other techniques. The efficiencies for all three phosphors were essentially the same at voltages below 600 V. At these voltages, the penetration depth of the incident electron beam is low, (0.004 nm at 600 V) exciting the surface layer of the phosphor particles. In the higher voltage regime, (>600 V), the penetration depth of the electron beam is greater (0.03 nm at 1 kV). The efficiencies of solid-state and hydrothermal synthesized (Y_1−xTb_x)Al₅O₁₂ at these voltages were approximately 1.0 lm/W greater than combustion synthesized (Y_1−xTb_x)Al₅O₁₂. This is because of the smaller crystallite size of these powders (60 nm) compared with hydrothermal and solid-state synthesized powders (100 nm).

Fig. 1. Effect of synthetic route on the low-voltage cathodoluminescence efficiency of (Y_1−xTb_x)3Al₅O₁₂.     

Quantum chemistry and QSPR study on relationship between crystal structure and emission wavelength of Eu2+‐doped phosphors

Abstract— The relationship between crystal structures and emission properties has been computationally investigated for Eu²⁺‐doped phosphors. The electronic structure of the Eu²⁺‐doped BaMgAl¹⁰O¹⁷ phosphor was analyzed by using the quantum chemistry method. The different effects of O and Ba atoms on the Eu 5d states were determined. The presence of O and Ba atoms increases and decreases the energy level of the Eu 5d orbital by forming anti‐bonding and bonding interactions, respectively. According to the electronic‐structure analysis, the structure index that represents the local geometrical information of the Eu atom was defined. The relationship between the crystal structures and the emission wavelengths of the 1 6 Eu²⁺‐doped oxide phosphors were studied by using the quantitative structure‐property relationship (QSPR). The QSPR model suggested that the both O and alkaline‐earth atoms around the Eu atom are of importance in the determination of the emission wavelength. The interaction between the Eu and the nearest O atoms make the Eu²⁺ emission wavelength short. On the other hand, the interaction from the alkaline‐earth atoms around the Eu atom lengthens the Eu²⁺emission wavelength. This evaluation method is useful in selecting the host material that indicates a desirable emission wavelength of the Eu²⁺‐doped phosphors.     

Investigation on luminescence of Na3Ca6(PO4)5:Eu2+ phosphor for LEDs

In this paper, a series of Na₃Ca_6(1−x)(PO₄)₅:xEu²⁺ (NCP:xEu²⁺, 0 ≤ x ≤ 4%) phosphors were prepared by conventional solid-state reaction method, and their photoluminescence properties were studied. Upon 365 nm excitation, the typical NCP:2%Eu²⁺ phosphor shows an asymmetric bluish green emission band with the dominant peak at 498 nm which could be attributed to the 4f⁶5d¹-4f⁷ transition of Eu²⁺. By measuring the time-resolved photoluminescence spectra, it reveals more than one Eu²⁺ emission center in the Eu²⁺-activated NCP phosphors. By monitoring 498 nm, the excitation spectrum of NCP:2%Eu²⁺ demonstrates a broad excitation band ranging from 240 to 450 nm, which can match well with the emission wavelength of the NUV LED chip. The SEM image shows that the average particle size of NCP:2%Eu²⁺ is about 19.4 µm. The above results imply that the NCP:Eu²⁺ phosphor could have potential application in LEDs.     

Eu2+‐doped AlN—SiC solid‐solution phosphors: Synthesis and cathodoluminescence properties

Abstract— Eu and Si co‐doped AlN was reported to be an interesting blue phosphor for field‐emission displays (FEDs). In this paper, SiC instead of Si³N⁴ was used as the Si source. Eu²⁺‐doped AlN—SiC phosphors were prepared by firing the powder mixtures of AlN, SiC, and Eu²O³ at 2050°C for 2 hours under 1‐MPa N². Solid solutions between AlN and SiC were formed in a wide range, promoting the solution of Eu²⁺ in AlN. The phosphors showed intense blue emissions under electron‐beam excitation, indicative of potential phosphors for FEDs.     

Red phosphor Li2Mg2(WO4)3: Eu3+ with lyonsite structure for near ultraviolet light-emitting diodes

A series of Eu³⁺-activated Li₂Mg₂(WO₄)₃ (LMW) materials were synthesized by high temperature solid state reactions. The phosphor can be effectively excited by 394 nm near ultraviolet light and emit intense red light with high color purity. Prepared phosphors can be indexed to LMW with particular lyonsite structure. The occupation of Eu³⁺ in LMW is selective. Most of Eu³⁺ comes into ¹A sites without inversion symmetry. The present research suggests that LMW is a suitable host for luminescence applications and Eu³⁺-activated LMW is a promising phosphor for phosphor-converted white light-emitting diodes.     

A new long-lasting phosphor Zr and Eu co-doped SrMg2(PO4)2

Zr⁴⁺- and Eu³⁺-codoped SrMg₂(PO₄)₂ phosphors were prepared by conventional solid-state reaction. Under the excitation of ultraviolet light, the emission spectra of Sr_0.95Eu_0.05Mg_2−2xZr_2xP₂O₈ (x = 0.0005-0.07) are composed of a broad emission band peaking at 500 nm from Zr⁴⁺-emission and the characteristic emission lines from the ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3 and 4) transitions of Eu³⁺ ions. These phosphors show the long-lasting phosphorescence. The emission color varies from red to white with increasing Zr⁴⁺-content. The white-light emission is realized in single-phase phosphor of Sr_0.95Eu_0.05Mg_2−2xZr_2xP₂O₈ (x = 0.07) by combining the Zr⁴⁺- and Eu³⁺-emission. The duration of the persistent luminescence of Sr_0.95Eu_0.05Mg_2−2xZr_2xP₂O₈ (x = 0.07) reaches nearly 1.5 h. The time at which the long-lasting phosphorescence intensity is 50% of its original value (T_0.5) is 410 s. The afterglow decay curves and the thermoluminescence spectra were measured to discuss this long-lasting phosphorescence phenomenon. The co-doped Zr⁴⁺ ions act as both the luminescence centers and trap-creating ions.     

Synthesis of Sm3+ and Dy3+ doped LaBWO6 for evaluation as potential materials in luminescent display applications

A series of Sm³⁺ and Dy³⁺ doped LaBWO₆ phosphors were synthesized by high temperature solid state reaction. Recorded XRD patterns proved that the titled compound in a single phase has been obtained. Sm³⁺ and Dy³⁺ doped LaBWO₆ could emit orange and white light, respectively. The optimal doping concentration of Sm³⁺ or Dy³⁺ was experimentally ascertained to be 6 mol%. The critical distance of energy transfer for Sm³⁺ or Dy³⁺ doped sample is 1.540 nm. In addition, there is no cross energy transfer between the Sm³⁺ and Dy³⁺ ions in the co-doped samples. The results indicated that the electric dipole–dipole interaction is predominant energy transfer mechanism for concentration quenching of Sm³⁺ or Dy³⁺ doped LaBWO₆ phosphor. The charge transfer band was observed in the excitation spectra of Sm³⁺ or Dy³⁺ doped LaBWO₆ phosphors. Present investigation indicated that Sm³⁺ and Dy³⁺ doped LaBWO₆ can be applied in solid state lighting and LaBWO₆ is a promising host for display applications.     

Dysprosium doped di-strontium magnesium di-silicate white light emitting phosphor by solid state reaction method

Dysprosium doped di-strontium magnesium di-silicate namely Sr₂MgSi₂O₇:Dy³⁺ phosphor was prepared by the solid state reaction method. The phase structure, surface morphology, particle size, elemental analysis was analyzed by using XRD, TEM, EDX and FTIR techniques. The EDX and FTIR spectra confirm the present elements in Sr₂MgSi₂O₇:Dy³⁺ phosphor. The optical properties of Sr₂MgSi₂O₇:Dy³⁺ phosphor was investigated utilizing thermoluminescence (TL), photoluminescence (PL), long lasting phosphorescence and mechanoluminescence (ML). Under the ultraviolet excitation, the emission spectra of Sr₂MgSi₂O₇:Dy³⁺ phosphor are composed of a broad band and the characteristic emission of Dy³⁺ peaking at 470 nm (blue), 575 nm (yellow) and 678 nm (red), originating from the transitions of ⁴F_9/2 → ⁶H_15/2, ⁴F_9/2 → ⁶H_13/2 and ⁴F_9/2 → ⁶H_11/2. CIE color coordinates of Sr₂MgSi₂O₇:Dy³⁺ are suitable as white light emitting phosphor. Decay graph indicate that this phosphor also contains fast decay and slow decay process. The peak of ML intensity increases linearly with increasing impact velocity of the moving piston. The possible mechanism of this white light emitting long lasting phosphor is also investigated.     

Synthesis of Bi and Gd doped ZnB2O4 for evaluation as potential materials in luminescent display applications

A series of Bi³⁺ and Gd³⁺ doped ZnB₂O₄ phosphors were synthesized with solid state reaction technique. X-ray diffraction technique was employed to study the structure of prepared samples. Excitation and emission spectra were recorded to investigate the luminescence properties of phosphors. The doping of Bi³⁺ or Gd³⁺ with a small amount (no more than 3 mol%) does not change the structure of prepared samples remarkably. Bi³⁺ in ZnB₂O₄ can emit intense broad-band purplish blue light peaking at 428 nm under the excitation of a broad-band peaking at 329 nm. The optimal doping concentration of Bi³⁺ is experimentally ascertained to be 0.5 mol%. The decay time of Bi³⁺ in ZnB₂O₄ changes from 0.88 to 1.69 ms. Gd³⁺ in ZnB₂O₄ can be excited with 254 nm ultraviolet light and yield intense 312 nm emission. The optimal doping concentration of Gd³⁺ is experimentally ascertained to be 5 mol%. The decay time of Gd³⁺ in ZnB₂O₄ changes from 0.42 to 1.36 ms.     

CaMoO4:RE3+,Yb3+,M+ phosphor with controlled morphology and color tunable upconversion

CaMoO₄:RE³⁺,Yb³⁺ (RE = Er, Ho, Tm) phosphors were successfully synthesized by a facile hydrothermal method. XRD patterns confirmed tetragonal structure under different RE³⁺ and M⁺ ions doping conditions. Particles shapes and sizes were confirmed by SEM and TEM analyses. Particles shape and size were well tuned by control of solution pH; spherical balls consisting of nano-grains at low pH of ∼2, rice grain shapes at moderate pH of ∼6, and thin flakes at higher pH of ∼12, were observed. Fine tunability of upconversion (UC) emission color was achieved by doping multiple RE³⁺ ions within a single CaMoO₄ host. Blue, green and orange upconverted emission were observed by doping Tm³⁺, Er³⁺ and Ho³⁺ in the CaMoO₄, respectively. Further, the emission colors were well tuned by the combination of Tm, Er and Ho ions and their concentrations. CaMoO₄:Tm³⁺,Ho³⁺,Yb³⁺ exhibited perfect white emission with well tunability from cool white to warm white colors. Substitution of part of Ca ions by M⁺ (M = Li, Na, K, Rb) ions affected the crystal field symmetry around RE³⁺ ions and hence changed the transition probabilities between their f–f transition levels, consequently intensified the UC intensities. The blue (Tm³⁺), green (Er³⁺), and orange (Ho³⁺) upconversion intensities of CaMoO₄:RE³⁺,Yb³⁺,0.10 K⁺ phosphors increased by 60, 50 and 40 folds compared to the unsubstituted analogues, respectively. The K substituted CaMoO₄:RE³⁺,Yb³⁺,K⁺ phosphors exhibited intense UC emissions visible by naked eye even pumped by less than 1 mW laser power and can have potential application in displays and variety of other applications.     

Structural characterization and optical properties of dysprosium doped strontium calcium magnesium di-silicate phosphor by solid state reaction method

Dysprosium doped strontium calcium magnesium di-silicate phosphor namely: SrCaMgSi₂O₇:Dy³⁺ was prepared by the solid state reaction method. The crystal structure of the prepared phosphor was an akermanite type structure, which belongs to the tetragonal crystallography with space group P${\overline{42}}_1$m. From field emission scanning electron microscopy (FESEM), agglomerations of particles were observed due to the high temperature synthesis process. The chemical composition of the sintered SrCaMgSi₂O₇:Dy³⁺ phosphor was confirmed by energy dispersive X-ray spectroscopy (EDX). Under the ultra-violet (UV) excitation, the characteristic emission of Dy³⁺ ions peaking at 478, 580 and 674 nm, originating from the transitions of ⁴F_9/2 → ⁶H_15/2, ⁴F_9/2 → ⁶H_13/2 and ⁴F_9/2 → ⁶H_11/2 in the 4f⁹ configuration of Dy³⁺. Commission International de I’Eclairage (CIE) color coordinates of SrCaMgSi₂O₇:Dy³⁺ are suitable as white light emitting phosphor. Decay graph indicate that this phosphor contains fast decay and slow decay process. The mechanoluminescence (ML) intensity of SrCaMgSi₂O₇:Dy³⁺ phosphor increases linearly with increasing impact velocity of the moving piston, which suggests that this phosphor can be used as sensors to detect the stress of an object. Thus the present investigation indicates that the piezo-electrically induced de-trapping model is responsible to produce ML in SrCaMgSi₂O₇:Dy³⁺ phosphor. The possible mechanism of white light emitting long lasting phosphor is also investigated.     

Rare‐earth materials for use in the dark

Abstract— Recently, it was found that some materials doped with rare‐earth ions show bright and long‐lasting phosphorescence. They do not include radioactive elements and can be safely used as luminous paints for use in the dark. Some of them are better than the traditional zinc sulfide doped with copper (ZnS:Cu). The most important rare‐earth materials with long‐lasting phosphorescence are aluminates such as alkaline‐earth aluminates MAl₂O₄:Eu²⁺, Dy³⁺ (M = Sr, Ca) and garnets Y₃Ga₅O₁₂:Tb³⁺, Gd₃Ga₅O₁₂:Tb³⁺, Cd₃Al₂Ge₃O₁₂:Tb³⁺, Cd₃M₂Ge₃O₁₂:Pr³⁺ (M = Al, Ge), Y₃Al_5?xGa_xO₁₂:Ce³⁺ (x = 3, 3.5). Some oxides such as InBO₃:Tb³⁺, Ba₂SiO₄:Dy³⁺ also show long‐lasting phosphorescence properties. Other sulfide materials include ZnS:Eu, Ca_xSr_1?x S:Bi, Tm, Cu or Ca_xSr_1?xS:Eu. Alkaline‐earth aluminates MAl₂O₄:Eu²⁺ (M = Mg, Ca, Sr, Ba) codoped with RE³⁺ (RE = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) were synthesized by using homogeneous precipitation method.     

Design and characterization of new lanthanide fluorides and their optical properties

Three kinds of lanthanide phosphors (La_xLu_1−xF₃: Eu³⁺, LaF₃–CaF₂:Eu³⁺ and LaF₃: Eu³⁺) have been successfully synthesized based on three different ways such as molten salts, co-precipitation, supersonic and microwave irradiations. The as-prepared powder materials all exhibited red luminescence. Their crystal structures or morphologies were studied by means of X-ray powder diffraction and scanning electronic microscope. Eu³⁺-doped LaF₃–CaF₂ phosphor can be emissive under excitation at longer wavelengths (466 and 533 nm) excitations. Supersonic and microwave irradiations have shortened the reaction time of LaF₃: Eu³⁺ crystals in 40 min under very low temperature (50 °C).     

Spectroscopic and photoluminescence properties of MgO:Cr3+ nanosheets for WLEDs

This work explores the synthesis of nanocrystalline MgO:Cr³⁺ (1–9 mol%) nanophosphors via solution combustion route at 400 °C. The nanophosphors were well characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier transform infra-Red (FTIR) spectroscopy. PXRD results confirm cubic phase and SEM micrographs indicate that the particles are highly porous and agglomerated. The TEM images show that the powder consists of spherical particles of size ∼5–15 nm. Upon 356 nm excitation the emission profile of MgO:Cr³⁺ exhibits an emission peak at 677 nm due to ²E_g → ⁴A_2g transition. It was observed that PL intensity increases with increase in Cr³⁺ concentration and highest PL intensity was observed for 3 mol% doped sample and afterward it decreases, attributed to concentration quenching. The resultant CIE chromaticity co-ordinates in the white region make the present phosphor highly useful for display applications and also for white light-emitting diodes (WLEDs).     

Optical temperature sensing behavior of enhanced green upconversion emissions from Er-Mo:Yb2Ti2O7 nanophosphor

The Er-Mo:Yb₂Ti₂O₇ nanocrystalline phosphor has been prepared by sol-gel method and used as an optical thermometry. By Mo codoping, the green upconversion (UC) emission intensity increased about 250 times than that of Er:Yb₂Ti₂O₇ under a 976 nm laser diode excitation. It indicates that such green enhancement arises from the high excited state energy transfer (HESET) with the |²F_7/2, ³T₂> state of Yb³⁺-MoO₄²⁻ dimer to the ⁴F_7/2 level of Er³⁺. The fluorescence intensity ratio (FIR) of the two green UC emissions bands was studied as a function of temperature in a range of 290-610 K, and the maximum sensitivity and the temperature resolution were approximately 0.0074 K⁻¹ and 0.1 K, respectively. It suggests that the Er-Mo:Yb₂Ti₂O₇ nanophosphor with a higher green UC emissions efficiency is a promising prototype for applications in optical temperature sensing.     

Enhanced PL and VUV absorption of BaZr(BO3)2:Eu3+ by Al3+ incorporation

Abstract— The photoluminescence (PL) and vacuum‐ultraviolet excitation (VUV) properties of BaZr(BO₃)₂ doped with the Eu³⁺ activator ion were studied as a new red phosphor for PDP applications. The excitation spectrum shows strong absorption in the VUV region with an absorption band edge at 200 nm. The charge‐transfer excitation band of Eu³⁺ was enhanced by co‐doping with an Al³⁺ ion into the BaZr(BO₃)₂ lattices. The PL spectrum shows the strongest emission at 615 nm, corresponding to the electric dipole ⁵D₀ → ⁷F₂ transition of Eu³⁺ in BaZr(BO₃)₂, which results in good red‐color purity.     

Study of the lifetime of phosphor screens formed via the settling and slurry methods under 7‐kV electron bombardment

Abstract— The lifetime of phosphor (ZnS:Ag,Cl) screens, excited by 7‐kV electron beams, which were formed via the settling method and the slurry method, respectively, were compared. The cathode‐ray tube (CRT) formed by using the settling method was found to have a better lifetime than that formed by the slurry method. Transmission electron microscopy (TEM) revealed that the phosphors were coated by a continuous glass film. Gas analysis showed a reduced emission for SO₂ gas from phosphors due to heating during the settling method as compared to the slurry method, indicating greater decomposition of ZnS by the slurry method. For low‐vacuum CRTs, the luminance degradation by electron irradiation was low for the settling method as compared to the slurry method. This indicates that the glass coating of the settling method produces a restraining effect on the decomposition of ZnS and prevents gas from attacking the phosphor surface, resulting in a better lifetime for the settling method.