Solid State Phenomena Vol. 155 (2009) pp 113-143 © (2009) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/SSP.155.113 Online: 2009-05-14 Inorganic Phosphor Materials for Solid State White Light Generation Dimple P. Dutta and A. K. Tyagi Chemistry Division, Bhabha Atomic Research Center, Mumbai-400085, INDIA dimpled@barc.gov.in, aktyagi@barc.gov.in Keywords: Solid-state lighting, Inorganic Phosphors, Sono-chemical Abstract. Solid-state lighting (SSL) is emerging as a highly competent field and a possible alternative to existing lighting technologies. Development of a suitable phosphor is an important aspect of SSL. The aim of this review is to summarize status of Inorganic Phosphors towards SSL applications. Various examples have been taken from oxide, fluoride, nitride, sulfide and phosphate based host lattices. The important concepts like CIE coordinates and Color Correlated Temperature (CCT) will also be discussed. The sections encompasses of red, blue and green light emitting phosphors. The white light emitting phosphors will also be discussed in details. Introduction Solid-state lighting (SSL) is emerging as a highly competent field and a possible alternative to existing lighting technologies. The emphasis on research on SSL has gained tremendous momentum in the past decade due to their inherent advantages of high light efficiency, low energy consumption and long service lifetime compared to conventional light sources [1-5]. There are various approaches to get efficient solid state sources for white light generation. We can directly mix light from three (or more) monochromatic sources, red, green and blue (RGB), to produce a white source matching with the RGB sensors in the human eye. Another method is to use a blue LED to pump one or more visible light-emitting phosphors that has been integrated into the phosphor-converted LED (pc-LED) package. The pc-LED is designed to leak some of the blue light beyond the phosphor to generate the blue portion of the spectrum, while phosphor converts the remainder of the blue light into the red and green portions of the spectrum. We can also use an ultraviolet LED to pump a combination of red, green and blue phosphors in such a way that none of the pump LED light is allowed to escape. Each of these approaches has potential advantages and clear technical challenges. Mixing the emission from red, blue and green colored LEDs is the most straightforward technique since there is no quantum deficit arising from Stokes shift and hence offers infinitely graduated color and white point control. However, this form requires independent output power control on each LEDs, and moreover there is a gap in the operating voltage between them making the operation quite cumbersome. Phosphor-converted LEDs are the most common LED based white light source where a blue LED is used with a yellow emitting phosphor. The yellow light stimulates the red and green receptors of the eye, the resulting mix of blue and yellow light gives the appearance of white, the resulting shade often called "lunar white". In 1996, the white LEDs fabricated from blue LED chips combined with yellow phosphors Ce3+(YAG) were commercialized [6]. This phosphor-conversion white LED represented an innovation in solid-state lighting, because they were small, lightweight, had a long lifetime and was easy to operate. However, they were inherently less efficient than an RGB source because of the unavoidable energy loss concomitant with the wavelength-conversion of a photon from wavelength λ1 to λ2 with λ2 > λ1. The energy loss is particularly large for wavelengthconversion processes from the UV (400 nm) to the red (625 nm) where the loss is 36%. The third approach is to have UV-LEDs. In this case the UV light is completely adsorbed by the phosphors and the mixed RGB output appears white. The quantum deficit between the UV pump and the phosphors, especially the low-energy red phosphor, dissipates significant energy and makes this All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.ttp.net. (ID: 130.203.136.75, Pennsylvania State University, University Park, USA-09/10/15,04:09:07) 114 Synthesis, Characterization and Properties of Nanostructures approach inherently less efficient than either the RGB or the pc-LED schemes for generating white light. However, the UV-LED approach has the advantage that color can be controlled by the phosphor mix at least at one point in time and at one temperature and hence the color rendering should be excellent. The white LEDs have been used primarily for back lighting sources of liquid crystal displays (LCDs). The luminous efficiency (ηL) of the first white LEDs was only 5 lm/W at a forward-bias current of 20 mA. However, now the efficiency of white light LEDs has already surpassed that of incandescent lamps and is competitive with fluorescent lamps. Efficacies for commercial pc-LEDs using blue InGaN LEDs and Y3Al5O12:Ce3+(YAG:Ce) garnet-based phosphors can be greater than 80 lm/W for 1 W devices [7-9]. However, other requirements, such as lamp color, need to be met before pc-LEDs can extensively replace fluorescent or incandescent light sources. For example, the highest efficacy pc-LED lamps have color temperatures (CCTs) of 4500-6500 K, making them less acceptable as replacements for incandescent and halogen lamps that have CCTs of 2500-3200 K. In addition, for CCTs less than 6000 K, the color rendering index (CRI) of typical high efficacy pcLED lamps is less than 80, in comparison to CRIs of 100 for incandescent and halogen lamps and 82-85 for CFLs. Therefore, the phosphor materials play an important role in white-light LEDs. Ideally the conversion phosphors for white-light LEDs must combine high quantum efficiency and absorption for UV-blue radiation with the ability to withstand the high temperature generated by the LED without degrading and quenching the luminescence, and moreover it should be chemically stable. Thus, novel phosphor materials with improved properties are greatly in demand. In this article, we will trace the common inorganic phosphor materials for indicator and high power LEDs, address challenges in moving towards true solid state lighting sources using phosphors, summarize recent advances in red, blue, green and white light emitting phosphors, and conclude with a look at what the future might hold for Illumination with Solid State Lighting Technology. Phosphor evolution The use of phosphors by man probably started more than 2000 years ago when they were used in fireworks to modify the colour output. However, real phosphor development is a 20th century phenomena starting in the 1940s, and its more recent development is due to its application in cathode ray tubes (TVs, PC monitors, test equipment) and fluorescent lighting. Moreover, during the last five years white LEDs have become very important lighting sources and the importance of LED phosphors for white and coloured light generation must be considered an important market driver in the future. Phosphors are usually made from a suitable host material, to which an activator is added. The host materials are typically oxides, nitrides and oxynitrides, sulfides, selenides, halides or silicates of zinc, cadmium, manganese, aluminum, silicon, or various rare earth metals. The activators prolong the emission time (afterglow). Many phosphor powders are produced in lowtemperature processes, such as sol-gel and usually require post annealing at temperatures of ~1000 0 C, which is undesirable for many applications. However, annealing can be avoided by proper optimization of the growth process. Phosphors are now critical to the long-term performance of LED lighting, and the ideal phosphor material should have a broad excitation spectrum in the desired spectral region, narrow emission bands centered on suitable wavelengths, high quantum efficiency (>90%), high levels of absorption at the excitation wavelength, high temperature stability of emission spectra and quantum efficiency along with a suitable morphology. For phosphor-based white LEDs, the phosphor absorbs the short-wavelength emission from the primary LED chip and down-converts it to a longer-wavelength emission. For example, the first phosphor-based white LED used a blue GaInN LED pumping a YAG:Ce3+ yellow phosphor [10]. The phosphor density and thickness is chosen to transmit only a fraction of the blue light. Mixing yellow phosphorescence with the blue electroluminescence results in white light. With a growing demand for blue based white LEDs and the huge additional potential from the general lighting Solid State Phenomena Vol. 155 115 market, it is not surprising that there has been intense interest in phosphor research for red-bluegreen and white phosphors based on UV, blue or green excitation wavelengths. Efficacies for commercial phosphor converted LEDs (pc LEDs) using blue InGaN LEDs and Y3Al5O12:Ce3+ (YAG:Ce) garnet-based phosphors can be greater than 80 lm/W for 1W devices [11-13]. These efficacies are higher than compact fluorescent lamps (CFLs) and comparable to linear fluorescent lamps, and implementation of high efficacy pc LED lamps into general lighting could significantly reduce lighting energy consumption. However, other requirements, such as lamp color, need to be met before pc LEDs can extensively replace fluorescent or incandescent light sources. Achieving lower CCTs and higher CRIs requires red phosphors to compensate for the spectral deficiencies of standard pc LEDs. The color rendering index of white LEDs made by blue LED with yellowemitting phosphor method is low due to lack of red light component [11]. There are two ways to generate warm white light: One is to combine an UV chip with tricolor (red, green and blue) phosphors and the other is to compensate the red light deficiency of YAG:Ce3+-based LED with a separate red-emitting phosphor [12]. Research efforts in red phosphors The first practical visible-spectrum (red) LED was developed in 1962 by Nick Holonyak Jr., while working at General Electric Company [13]. Holonyak is seen as the "father of the light-emitting diode" [14]. M. George Craford, a former graduate student of Holonyak, invented the first yellow LED and improved the brightness of red and red-orange LEDs by a factor of ten in 1972 [15]. In 1976, T.P. Pearsall created the first high-brightness, high efficiency LEDs for optical fiber telecommunications by inventing new semiconductor materials specifically adapted to optical fiber transmission wavelengths [16]. The three-band white LEDs are believed to offer high Ra and are also regarded as the greatest potential for high efficiency solid-state lighting [17–20]. For excellent color rendering index and high efficiency, it is important to seek an efficient red phosphor with strong absorption in the near-UV or UV region. The well-known red-emitting phosphors for white LEDs are usually sulfide semiconductors such as Zn1-xCdxS:Ag [21,22], SrY2S4:Eu [23,24], CaS:Eu [25,26], SrS:Eu [26,27], Ca1-xSrxS:Eu [25,27], Ba2ZnS3:Mn [28], etc. However, they suffer from poor chemical and photo stability, high cost or unsatisfactory efficiency [29,30]. Efficient redemitting phosphors are still commercially limited to Eu2+ doped CaS and SrS, but their hygroscopic nature needs to be overcome by a complicated treatment [31]. Y2O2S:Eu also shows efficient red emission, but it has a high manufacturing cost due to the necessity of using expensive rare earth oxides Eu2O3 and Y2O3 [28]. For these reasons, there is an urgent need to explore the possibility of designing a stable red phosphor with intense absorption in the near-UV or UV spectral region to increase the overall white light efficiency and lifetime. Recently, some authors found that some red phosphor doped by Sm3+ can be excited effectively by UV light [31-34]. Consequently, a novel red phosphor Y2O2S:Sm3+ was fabricated by combustion method [35]. This phosphor can emit red light when excited by UV light (412 nm) or visible light (468 nm). Eu3+ doped phosphors, particularly in which the Eu3+ occupies a non-centrosymmetric site in the host, have been widely used as red phosphors due to the exhibited characteristic red emission corresponding to 5D0→7F2 transition of Eu3+ [36]. Consequently, the effect of Eu3+ doping in various host matrices has been studied and their photoluminescence properties have been analyzed [37- 43]. Eu3+ doped molybdate or tungstate phosphors have great potential for near-UV or UV LED as red phosphor due to the intense charge-transfer band absorption in UV region, effective f–f transition of Eu3+ at 394 nm and 465 nm and excellent thermal and hydrolytic stability, [30,20,44]. Gd2(MoO4)3 was researched as one member of an isotypic series of ferroelectric rare-earth molybdates and was first prepared by Borchardt [45]. Nassau et al. [46] gave a survey of the structure of tungstates and molybdates with the formula RE2(AO4)3 (A = Mo, W) along the rareearth series. In different prepared temperature Gd2(MoO4)3 has several structures [46]. The 116 Synthesis, Characterization and Properties of Nanostructures ferroelectric β’ phase of Gd2(MoO4)3 shares an orthorhombic structure (space group Pba2) and shows excellent thermal and hydrolytic stability [46]. Furthermore, it is reported that the site symmetry of the rare earth ions in this host lattice is C1 without inversion symmetry [47], that is in favor of the red emission (5D0 → 7F2 transition) of Eu3+ at ∼615 nm. Hence, the phosphor Eu-doped Gd2(MoO4)3 is expected to be a red phosphor with appropriate CIE chromaticity coordinates. Thus the Eu3+-doped Gd2(MoO4)3 phosphor is expected to be a red phosphor for GaN-based LED. Recently, a series of the red phosphor Gd2−xEux(MoO4)3 for GaN-based white-LED by sol–gel method has been reported [48]. Results indicate that phosphors Gd2−xEux(MoO4)3 prepared by sol– gel method have a narrow size distribution, favorable size, homogeneous shape and high luminescent intensity, which is better than those of the phosphors prepared by solid-state method. The phosphor Gd1.2Eu0.8(MoO4)3 shows a higher luminescent intensity than that of phosphor Y2O2S:0.05Eu3+ under the excitation of near-UV and blue light. It has also been observed that the quenching concentration in Gd2(MoO4)3:Eu3+ is much higher than in CaMoO4:Eu3+ due to the large distance between rare earth in the latter case [49]. In order to obtain more efficient phosphors for near UV-LED, two approaches were adopted to strengthen and broaden the absorption in ∼400 nm (the emission wavelength of GaN based LED). The first one is by co-doping Sm3+ and Eu3+ ions in the phosphor. It is well known that Eu3+/Sm3+ ions exhibit strong absorption at about 395 nm/405 nm in many host lattices [50,51], as a consequence, the absorptions around 400 nm are expected to be strengthened and broadened by this co-doping system. The other approach is by substituting some Gd3+ ions in the host with Y3+ ions. From the viewpoint of host compound, the spectroscopic line is expected to be narrow when the rare-earth ions enter the lattice sites of a pure host compound in general. However, when the host compound can form solid solutions by adjusting the cations or anions, the sub-lattice structure around the luminescent center ions will be expected to be somewhat diverse and therefore the spectroscopic lines of rare-earth ions are expected to be broadened [52]. Consequently, the phosphor Gd0.2Y0.572Eu1.2Sm0.028(MoO4)3 synthesized using solid state route showed broadened absorption around 400 nm and enhanced red emissions due to Eu3+ f–f transitions under ∼400 nm [53]. On a similar note, Bi3+ and Sm3+ co-doped NaEu(MoO4)2 phosphors were prepared by solid-state reaction technique [54]. The obtained NaEu0.76Bi0.20Sm0.04(MoO4)2 phosphor shows broadened excitation band around 400 nm, and enhanced red emissions due to Eu3+ f–f transitions under 400 nm light excitation. The CIE chromaticity coordinates (x = 0.66, y = 0.34) of the phosphor are close to NTSC standard values. However, more efficient red phosphors are needed to achieve an acceptable efficiency of WLED. Recently, new powellite type phases, CaRENbMoO8 (RE = Y, La, Nd, Sm or Bi) has been synthesized [55] and it is found that when doped with Eu3+ in CaLaNbMoO8, they give strong red emission spectra under near-UV or blue excitation wavelength. When compared with emission intensity from CaMoO4:Eu3+, the emission from CaLaNbMoO8:Eu3+ showed greater intensity values under the near-UV excitation wavelength [56]. Alkaline earth borate is an important luminescent material because of its excellent chemistry and thermal stabilization, facile synthesis and cheap raw material (H3BO3), so it has been extensively applied to phosphor for lamps. Since it can be efficiently excited by LED chips, there have been a few reports recently about this material applied in phosphor for white LEDs [57,58]. Moreover, with the condition of co-doping, the emission capability of phosphor can be efficiently enhanced [59]. The excitation and emission spectra of LiBaBO3:Sm3+ phosphor indicate that it can be effectively excited by UV LED, and emit 597 nm red light. The emission intensity of LiBaBO3:Sm3+ phosphor increases with increasing Sm3+ concentration, then decreases, and reaches the maximal value at 3 mol%, the concentration self-quenching mechanisms are the d-d interaction by Dexter theory. Emission intensity of LiBaBO3:Sm3+ was enhanced by doping charge compensation Li+, Na+, K+ [60]. Silicate based phosphor materials have also been explored as silicate material has good thermal and chemical stability, and Eu3+ is an ideal red phosphor activator for its 4f6 electronic configuration. Solid State Phenomena Vol. 155 117 Ca2SiO4:Eu3+ red phosphor materials synthesized by the flux fusion reaction method, with the main excitation band located at 400 nm, and dominated emission peak at 612 nm, was very suitable for the low-cost trichromatic WLED based on UV InGaN chip. Concentration quenching did not occur in Ca2SiO4:Eu3+ with the increase of concentration in Eu3+.The luminescent intensity of Ca2SiO4:Eu3+ with Li+ as a charge compensator was much stronger than that of phosphors with Na+, K+, and Cl–, respectively, as charge compensators [61]. Doped rare-earth tellurates (Ln2TeO6) are also suitable for use as inorganic phosphors. The La2TeO6 phosphors doped with Eu3+ have been synthesized and its photoluminescent (PL) properties were investigated. The excitation spectrum is dominated by three bands corresponding to the excitations of electrons from Eu3+ 4f ground state to different excited 4f levels of Eu3+. The emission spectrum is characterized by an intense peak centered at 616 nm due to 5D0 → 7F2 transition of Eu3+ ions [62]. However, the phosphor La2-xEuxTeO6 emits under the excitation of 254nm UV light. Consequently, doped Eu0.1GdxLa1.9-xTeO6 (0.02≤x≤0.1) powder phosphors were synthesized. Under the excitation of 395nm UV light, the emission spectrum exhibits an intense peak centered at 616nm [63]. Apart from various oxides, even fluorides and oxyfluorides are currently being explored as host materials for phosphors. Complex fluorides and oxyfluorides have low phonon energy due to the stretching vibration of host molecules, phosphors with this structure as hosts lead to high efficiency of luminescence. The fascinating property makes complex fluorides and oxyfluorides attractive in recent years. NaYF4 and YOF have aroused great interest among these luminescence materials [6475]. For example, NaYF4:Yb, Er3+ and YOF:Yb3+, Er3+ can emit one photon of shorter wavelength via absorption of two or more exciting photons with longer wavelengths via an energy-transfer upconversion (UC) process [64-66]. They are used in solid lasers [67], telecommunications [68], illumination [69], flat-panel displays, and biological detection and labeling [70-75]. When doped with Eu3+ or Tb3+, these compounds show down-conversion (DC) luminescence [76,77] and can be used in electronics, dielectrics, optics, optoelectronics, and photonics [78-80]. Eu3+-doped NaYF4 and YOF nanocrystallites were seen to be good red emitters [81]. Recently, nitridosilicates, oxonitridosilicates, or oxonitridoaluminosilicates, which are related to oxosilicates by formal exchanges of oxygen by nitrogen or/and silicon by aluminum, have been extensively studied as host lattices for phosphors, which exhibit unusual, interesting luminescence properties when activated by rare earth ions, such as M2Si5N8:Eu2+/Ce3+ [82], MSi2O2-δN2+2/3δ: Eu2+/Ce3+ (M = Ca, Sr, Ba) [87-89], CaSiN2:Eu2+/Ce3+ [90,91], MgSiN2:Eu2+ [92,93], MYSi4N7:Eu2+/Ce3+ (M = Sr, Ba) [94,95], MSixAl2-xO4-xNx:Eu2+ (M = Ca, Sr, Ba) [96], SiAlON:RE (RE = Eu2+, Ce3+, Yb2+, Tb3+, Pr3+, Sm3+) [97-102], SiAlON:Eu2+ [103], and CaAlSiN3:Eu2+ [104]. Most importantly, these phosphors emit visible light efficiently under near ultraviolet or blue-light irradiation and have superior thermal and chemical stability to their oxide and sulfide counterparts, allowing them to be used as down-conversion luminescent materials for white light-emitting diodes (LEDs). The luminescence properties of Eu2+ and Ce3+ in MSiN2 (M = Sr, Ba) have been studied. Eu is present as the divalent ion in both Eu-doped BaSiN2 and SrSiN2 samples due to the absence of sharp f-f transition lines characteristic for Eu3+ in their emission spectra. As a result, all the broad emissions in the sample of M1-xEuxSiN2 (0 < x ≤ 0.1; M = Sr, Ba) are essentially assigned to the 4f65d1→4f7 transition of the Eu2+ ion on the single M site. Ba1-xEuxSiN2 (0 < x ≤ 0.1) shows a broad emission band in the wavelength range of 500–750 nm with maxima from about 600 to 630 nm with increasing Eu2+ concentration, while Sr1-xEuxSiN2 (0 < x ≤0.1) shows a broad emission band in the wavelength range of 550–850 nm with maxima from 670 to 685 nm with increasing Eu2+ concentration. The high absorption and strong excitation bands of M1-xEuxSiN2 (0 < x ≤ 0.1; M = Sr, Ba) in the wavelength range of 300–530 m are very favorable properties for application as lightemitting-diode conversion phosphors [105]. Recently, a new class of red-emitting nitride phosphors has been developed [106-108] for white LED use, which requires high luminescence efficiency under excitation at near-ultraviolet to blue light from InGaN diode. In particular, CaAlSiN3:Eu2+ is 118 Synthesis, Characterization and Properties of Nanostructures one of the most promising phosphors for practical application which requires saturated red emission, chemical stability, small thermal quenching, and high quantum efficiency [107,108]. Also single crystals of Sr0.99Eu0.01AlSiN3 were prepared by heating at 2173 K under a high pressure, 190 MPa, N2 gas [109]. The emission peak from Eu2+ in Sr0.99Eu0.01AlSiN3 was observed at 610 nm by excitation with 455nm light. Eu2+-doped nitridosilicates M2Si5N8 with M = Ca, Sr, or Ba [110] (the so called 2-5-8 materials) are now being industrially used in pc-LEDs as highly efficient red phosphor materials. They afford conversion of the initial blue radiation of GaN-based primary LEDs into red light, thus, in combination with a green to yellow emitting converter material, achieving warm white light pc-LEDs with improved color rendition in the red spectral region. The low energy excitation and emission bands of these compounds are mainly due to the strong nephelauxetic effect of the covalently bound nitrogen coordinating the alkaline earth ions and Eu2+. Recently, the highly efficient nitridosilicate phosphors M2Si5N8 (M = Sr, Ba, Eu) for phosphorconverted pc- LEDs were synthesized at low temperatures using a novel precursor route involving metal amides M(NH2)2 [111]. Research efforts in green phosphors To obtain a full-colour display, red, green and blue-emitting phosphors with suitable colour coordinates are necessary. White LEDs can be made by coating near ultraviolet (NUV) emitting LEDs with a mixture of high efficiency europium based red and blue emitting phosphors plus green emitting copper and aluminium doped zinc sulfide (ZnS:Cu,Al). The phosphor conversion of LEDs light depends strongly on the efficient absorption of the blue or UV LED emission light. The strong absorption can be expected from dipole-allowed electron transitions in activator ions. The Eu2+ ion is a well known activated ion, which can be crystal field shifted in the spectral location of their absorption and emission lines. The shift of the Eu2+ emission band depends on the host lattice, covalency, and the strength of the crystal field. Thus phosphors possessing special luminescence properties can be designed and prepared by means of selecting proper host and activated ions. Alkaline earth sulfides serve as good host crystals to be substituted by luminescent rare earth ions. However, they have been hampered in practical application by their sensitiveness to water and atmospheric components. When sulfides are exposed to moisture they decomposed to carbonates or sulfates that eliminate the original luminescence [112]. Sulfide phosphors may also degrade under high energy UV or electron beam strike [113]. However, sulfide phosphors fit well for LED applications with adhesive seal and blue excitation. Strong blue absorptions of CaS:Ce3+, SrS:Eu2+ make them good candidates for LED applications [114]. Codoping Ce3+ in SrS:Eu2+ and CaS:Eu2+ yielded 28% and 18% emission enhancements, respectively, due to the efficient energy transfer from Ce3+ to Eu2+. The ternary compounds II-III2-S4 doped with rare earth ions have been studied for several years and were found to be very attractive for lighting and display applications, such as field emission display (FED) [115], wavelength converters in phosphor-converted light-emitting diodes (LEDs) for solidstate lighting [116] and most importantly EL [117-119]. Le Thi et al. [120] investigated different MS-Al2S3 systems (M =Ca, Sr, Ba) and described some luminescence properties of Eu2+ as dopant in these thioaluminates. The II-III2-(S,Se)4 ternary compounds were introduced as phosphors for thin-film electroluminescence (TFEL) by Benalloul et al. [121]. In particular, thiogallates doped with rare-earth ions, such as Ce3+ and Eu2+, have been studied for full-colour TFEL displays [119,122–124]. The Eu2+ doped SrGa2S4 compound is well known as an efficient green phosphor, with excellent colour coordinates (x = 0:26; y = 0:69), high lumen equivalent (560 lm/W) [125] and fast luminescence decay (480 ns) [126]. SrGa2S4 is a semiconducting material with a large bandgap (4.4 eV [127]) exhibiting an orthorhombic crystalline structure. This phosphor is used for fullcolour Thin Film ElectroLuminescence (TFEL) displays [110], Field Emission Displays (FED) Solid State Phenomena Vol. 155 119 [128], and Solid State Lighting (SSL) in phosphor converted Light EmittingDiodes (pc-LED) [129,130]. The dependence of luminescent properties on Eu2+ content for (Sr1-xEux)Ga2S4 was also studied [131]. When Eu2+ content increases, the emission peak shifts from 531 nm (x = 0.01) to 543 nm (x = 1) gradually. The peak shift can be explained in connection with the crystal field strength. Because the radium of Eu2+ is smaller than that of Sr2+, the crystal field becomes stronger with Eu2+ content increasing, thus the lowest component of the 4f65d1 configuration of the Eu2+ ion may shift to low energy, which results in the red shift of emission peak. Although all of Sr2+ ions are substituted by Eu2+ ions, the concentration quenching does not occur; EuGa2S4 emits very strong yellowish-green light, which indicates that the interaction between Eu2+ ions is weak. For the green emission, CaAl2S4: Eu is also a promising material. CaAl2S4:Eu2+ emission provides good CIE colour coordinates (x = 0:141; y = 0:721) for the green component in display applications [132]. Powder with a dopant concentration of 8.5 mol% shows the highest luminescence efficiency. Sr4Al14O25:Eu2+ is another well-known host for blue–green emitting phosphor [133]. Among all the lanthanides, in Ce and Eu, the lifetime of 5d–4f transitions are about three orders of magnitude shorter than the 4f–4f transitions lifetime exhibited by other lanthanides, and thus are of immense importance. Along with europium, Ce3+ is another important element of lanthanide series that has 4f1 configuration and is, therefore, capable of producing luminescence from red region to UVregion due to strong crystal field dependence of its 5d–4f transition energy. The blue–green emitting Sr4Al14O25:Ce3+ phosphor was prepared by the conventional solution combustion method [134]. Photoluminescence studies revealed emissions at 472 and 511 nm that correspond to the transition between lowest T2g level of the 5d state to the 2F5/2 and 2F7/2 ground state levels of the Ce3+. The excitation at 275 nm corresponds to O2-→Ce4+ charge transfer processes to lowest 5d state of Ce ion (T2g). Eu2+ doped MAl2O4 (M = Sr, Ca) is another well-known persistent phosphor [135,136 ]. SrAl2O4:Eu2+ has tridymite-like structure and good phosphorescence under UV irradiation [137] but gives very low persistence. So the incorporation of Dy ion into the SrAl2O4:Eu2+ is necessary to effectively enhance the long lasting phosphorescence as a consequence of forming a highly dense trapping level located at a suitable depth in relation to the thermal release rate at room temperature [138]. The SrAl2O4:Eu2+,Dy3+,B3+ phosphor was reported by Murayama et al. to have enhanced phosphorescence. When a solid-state reaction is applied to prepare aluminates, boron oxide (B2O3) is used as a flux to accelerate grain growth [139]. According to previous studies [140,141] the added B2O3 is substituted in the AlO4 framework of SrAl2O4, resulting in the enhancement of luminescence intensity at low concentration but the persistent phosphorescence suppressed with increasing the boron concentration. In the LED phosphor, the long persistent property is not necessary, but the enhanced luminescence intensity is needed. Hence (Sr1-xZnx)1-y(Al1.98,B0.02):Eu2+y phosphor was synthesized by spray pyrolysis and the luminescent properties were investigated [142]. Pure monoclinic SrAl2O4 phase was formed until the post-treatment temperature was increased up to 1200 °C. The Sr4Al14O25 phase appeared as a minor phase when the temperature was over 1300 °C. As a result, the intensity of the green emission at 520 nm had the maximum value at 1200 °C. The concentration quenching of the emission intensity was observed when the Eu2+ content was 10 mol %. It was confirmed that the B3+ codoping is very helpful and necessary to enhance the luminescence intensity, but the large content accelerated the formation of Sr4Al14O25 phase and thereby reduced the emission intensity. It was found that the substitution of Zn atoms instead of the strontium produced a new blue (460 nm) emission site that was stable at ambient temperature and appeared to play the role of sensitizer for the energy transfer. Consequently, the Zn substitution of less than 50% greatly improved the luminescence efficiency, especially in the excitation wavelength range from 380 to 420 nm. Therefore, the optimized phosphor (Sr0.6Zn0.4)0.9(Al1.98B0.02):Eu2+0.1 has potential for use as a green phosphor for ultraviolet LEDs. 120 Synthesis, Characterization and Properties of Nanostructures It is well-known that phosphors based on borates have attracted much attention due to their high stability, low synthesis temperature, and high ultraviolet and optical damage threshold [143-147]. Rare earth orthoborates LnBO3 (Ln = lanthanides and yttrium) have been proved to be very useful host lattices for the luminescence of Eu3+ and Tb3+, which have found wide applications in mercuryfree fluorescent lamps and various kinds of display devices [148-150]. As the most frequently used activator ions in luminescent materials, the Eu3+ and Tb3+ mainly show emissions due to transitions of 5D0→7FJ (J = 1, 2, 3, 4) in the red region and 5D4 → 7FJ (J= 6, 5, 4, 3) in the green region, respectively. Some phosphors of borate and phosphate compounds have a high absorption coefficient in the VUV regions and are chemically stable under sustained plasma conditions [151]. Borates doped with rare earth ions have attracted much attention due to its high UV transparency and nonlinear properties. Additionally, it has an exceptional optical damage threshold and is able to withstand the harsh conditions of vacuum discharge lamps or screens. Among the borate phosphors, LnBO3 (Ln=La– Lu, Y) orthoborates and huntite-type yttrium aluminium borate (YAl3(BO3)4 or YAB), which belongs to the double borates, are well known as efficient phosphors with excellent absorption efficiency under VUV excitation [152,153]. YAB can be improved by activation with various rare earth ions and Al3+ substitution by Ga3+ and Cr3+ ions. Therefore, borate compounds are viable candidates for VUV phosphors. Consequently, the VUV luminescent property of a novel Tb3+activated green-emitting phosphor with the chemical formula of LnGa3(BO3)4 (Ln=Y, Gd) was investigated [154]. Irrespective of the Tb3+ concentrations, a single-phase huntite-type gallium borate compound was formed via a conventional solid state reaction method. This phosphor exhibited a strong green emission with inner-shell transition of Tb3+ with a dominant peak centered at 545 nm under excitation of 147 nm. The very interesting feature of this borate compound is the possible existence of a high concentration of Tb3+ ions without concentration quenching. In addition, the VUV excitation spectrum showed strong charge transfer absorption efficiency at the proper Y3+/Gd3+ atomic ratio. Consequently, the emission of Tb3+ was induced by the energy transfer of VUV excitation to activator Tb3+ ions via the co-existing Y3+/Gd3+ in LnGa3(BO3)4 (Ln=Y, Gd). Similarly, Tb(1-x)BO3:xEu3+ (x = 0-1) microsphere phosphor was hydrothermally synthesized directly without further sintering treatment and reductive ambience for protection [155]. An efficient energy transfer occurred from Tb3+ to Eu3+ in TbBO3 host, which is ascribed to the energy overlap between Tb3+ and Eu3+ (Tb3+:5D4 → 7F6, 5, 4, 3 ↔ Eu3+:7F0, 1 → 5D0, 1, 2 + ∆E) and hexagonal crystal structure of TbBO3 host. The PL color of TbBO3:xEu3+ phosphors can be easily tuned from green, yellow, orange, to red-orange by changing the doping concentration (x) of Eu3+, making the materials have potential applications in fluorescent lamps for advertizing signs and other color display fields. Since early in 1962, phosphates with the general formula ABPO4 (where A is a monovalent cation and B is a divalent cation) have been of interest for their optical [156] or ferroelectric properties [157]. It is well-known that these phases crystallize into three types of basic structures [158,159] depending on the sizes of the cations, for example, (1) the Na2SO4 family, where both A and B are large enough to occupy eight- or ninecoordinated sites; (2) the stuffed tridymite materials, where B is sufficiently small enough to occupy a tetrahedral site; and (3) the olivine-related compounds, where both A and B are located in octahedral sites. The variety in these structures of the ABPO4 family makes it possible to fine-tune a specific physical property or to design a new useful material. Hence there is an increased interest in the synthsis of new efficient luminescent materials having structures derived from the ABPO4 family. Consequently, a novel green-yellow emitting phosphor LiZn1-xPO4:Mnx was synthesized which can be tuned from green to yellow under ultraviolet radiation by varying Mn2+ dopant in the host matrix [160]. Also NaCaPO4:Eu2+ phosphor has been synthesized by the conventional high temperature solid state reaction [161]. The emission spectrum has a single intense broad band centered at 505 nm, which appears green to naked eyes. The excitation spectrum couples well with the emission of UV LED chips (350–410 nm). When the concentration of doped Eu2+ is 0.05, the NaCaPO4:Eu2+ has the strongest emission intensity. The Solid State Phenomena Vol. 155 121 results indicate that the NaCaPO4:Eu2+ is a potential green phosphor for UV LEDs. The known commercial standards for green-emitting phosphors are, GdCeMgB5O10:Tb3+ and LaCePO4:Tb3+ [50]. These phosphors require high processing temperatures, high reducing atmospheres and high Tb3+ concentrations, for efficient performance. The aluminate and phosphate phosphors require high reducing atmospheres when compared to the present borates to reduce Tb4+ to Tb3+ because of differences in their crystal structure and the ionic environment. In practice it is understood that the reduction is enhanced in borate anionic networks under mild-reducing atmospheres when compared to phosphate or aluminate networks. The borate phosphor GdMgB5O10:Tb3+ does not require very high temperatures like the aluminate or the phosphate phosphors, but, to reduce Tb3+ concentration the borate requires the ion Ce3+ as sensitizer. The presence of the ion Ce3+ in all these phosphors causes instability if heated in air at room and at high temperatures, and hence causes problem during the baking process of lamp fabrication. To overcome these difficulties, a phosphor with a stable Tb3+ state which is capable of undergoing excitation with radiation of wavelength 254 nm (so as to avoid sensitizers like Ce3+ ion) and which can be synthesized at lower temperatures in air/mild-reducing atmospheres was needed. Hence, new Tb3+-activated (at the M or M0-sites) borates of the type A6MM’(BO3)6 [A = Sr; M = Gd, Y; M’ = Al, Ga, In, Sc and Y] and LaSr5MM’(BO3)6 [A = Sr, La; M = Y; M’ = Mg] [162-165] which can be used as alternatives to the above commercial green phosphors after suitable modifications have been developed. Several new green phosphors have been identified in Tb3+ activated hexaborates of the type Sr6M1-xTbxM’(BO3)6 [M = M’ = Gd, Y], Sr6TbM’(BO3)6 [M’ = Al, Ga, In, Sc] and LaSr5Y1-xTbxMg(BO3)6 where 0.05 ≤ x ≤ 1.0 [166]. These compounds were synthesized by the solid state reaction under reducing atmosphere. Photoluminescence excitation and emission studies on these borates show that the excitation and emission features are similar to the existing aluminate, borate and phosphate green phosphors applied in tri-color lpmv lamps. The hexaborate system has several advantages over the standard green phosphors viz., direct excitation with 254 nm, no sensitizer like Ce3+ ion which is unstable, low temperature for synthesis, stability of Tb3+-ion at low and high temperatures in air and hence these borates are identified as efficient new alternatives to the existing commercial green phosphors for use in tri-color lpmv lamps. Among the known inorganic phosphors, manganese-doped zinc silicate (Zn2SiO4:Mn) is an efficient green emitting phosphor widely used in cathode ray tubes (CRTs), fluorescent lamps, and plasma display panels (PDPs) due to its high luminescent efficiency and chemical stability, and it also presents the advantage of highly saturated color [167, 168]. Conventionally, the Mn2+-doped Zn2SiO4 phosphors are fabricated mainly by solid-state reaction (SSR) of starting powders, where the high firing temperatures required and the subsequent grinding and ball milling processes make it rather difficult to control the particle shape and sizes, on which the luminescent properties of phosphors strongly depend [169]. Recently, a facile route for preparing green light emitting Zn2SiO4:Mn phosphor via a low-temperature solid-state reaction method by using mesoporous silica SBA-15 as an active template has been developed [170]. Room temperature photoluminescence and decay kinetics of the phosphors show their strongly enhanced green emission and non-single exponential decay behavior with a long decay time. The phosphors used for near UV LEDs must show a strong and broad absorption at around 380–405nm and emit intense visible light under 380–405nm excitation. Silicate-based phosphors activated with Eu2+ are particularly suitable for this purpose. Eu2+ is a well-known activated ion. The emission of Eu2+ is strongly dependent on the host lattice and can be shifted from the UV to the red region of the electromagnetic spectrum [171]. Some Eu2+ doped silicates also have been applied in solid-state lighting, thanks to their excellent efficiencies and appropriate absorption bands. A great deal of work has been done on Eu2+ activation of these hosts. Blasse et al. have systemically reported the fluorescent properties of Eu2+ activated binary and ternary silicates [172]. Park et al. prepared Sr2SiO4:Eu2+ and Sr3SiO5:Eu2+ phosphors, and found stronger yellow emission than the conventional light-conversion phosphor YAG:Ce [173,174]. The introduction of Mg2+ or Ba2+ into Sr2SiO4:Eu2+ was found to increase the efficiency of Sr2SiO4:Eu2+ and to extend its excitation band 122 Synthesis, Characterization and Properties of Nanostructures to longer wavelengths [175]. Therefore, Eu2+ activated silicates are also expected to be suitable luminescent materials for LEDs. Recently, reports on Eu2+ activated Li2MSiO4 (M = Ca, Sr) phosphors for LEDs have shown that, under near-UV excitation, Li2CaSiO4:Eu2+ phosphor emits an intense bluish-green light [177], while Li2SrSiO4:Eu2+ phosphor emits an orange-yellow emission [177]. Using urea as fuel and boric as flux, a novel bluish green emitting phosphor Li2(Ba0.99,Eu0.01)SiO4:B3+ was successfully synthesized using a combustion method [178]. Luminescence measurements showed that the phosphors can be efficiently excited by ultraviolet (UV) to visible region, emitting a bluish green light with peak wavelength of 490 nm. The results showed that the boric acid was effective in improving the luminescence intensity of Li2(Ba0.99,Eu0.01)SiO4. Green Ba2SiO4:Eu2+ phosphors were also synthesized by solid-state reaction [179]. They are excellent phosphors for n-UV LED due to broad excitation band near the UV range, intense emission, short decay time and good stability. Furthermore, green phosphor-converted LEDs were successfully fabricated by pre-coating Ba2SiO4:Eu2+ phosphors onto 395 nm emitting InGaN chips. The green LEDs’ color coordinates are stable and their emission intensities increase for forward-bias currents from 5 to 50 mA. These results indicate that as-fabricated green LEDs are suitable for traffic lights and automotive displays according to the regulations of ITE and SAE. Green light emitting (Ba,Sr)2SiO4:Eu phosphor was known to be suitable for UV LED phosphor because it has short decay time and high luminescence characteristics under long wavelength UV. Especially, the chromaticity of the (Ba,Sr)2SiO4:Eu phosphor particles could be controlled by changing the ratio of barium and strontium of the host material. (Ba,Sr)2SiO4:Eu phosphor particles were prepared and optimized by the conventional solid state reaction method [180-185]. In order to improve the photoluminescence characteristics of (Ba,Sr)2SiO4:Eu phosphor, several rare earth materials were employed as co-dopant in the solid state reaction method [186]. Aluminum, yttrium and gadolinium co-dopants improved the photoluminescence intensities of the (Ba,Sr)2SiO4:Eu phosphor particles in the solid state reaction method. The morphology and luminescence characteristics of the phosphor particles are affected by the phosphor preparation process. The photoluminescence intensities of co-doped (Ba,Sr)2SiO4:Eu phosphor particles prepared by spray pyrolysis technique were about 120 to 143% of (Ba,Sr)2SiO4:Eu phosphor particles without codopant [187]. The highest photoluminescence intensity was achieved when the doping concentration of yttrium was about 1.7 times of the doping concentration of europium. The photoluminescence intensity of the sieved phosphor particles was comparable to that of the original (Ba,Sr)2SiO4:Eu phosphor particles. Calcium silicate is also a good host material for phosphors. The photoluminescence spectra show that Ca3Si2O7:Eu2+ phosphor is efficiently excited by UV–visible light in the wavelength range from 250 to 450nm and emits intensely green light with a broad peak at around 521nm [188]. Halide and silicate are the excellent matrices for Eu2+ activated phosphors. The combination of both the matrices, halosilicate, has several advantages, like low synthetic temperature, high chemical and physical stability. Sr4Si3O8Cl4:Eu2+ and Sr3.5Mg0.5Si3O8Cl4:Eu2+ phosphors prepared by a conventional solid state reaction, show an efficient bluish-green wide-band emission centering at 484 nm, which originates from the 4f5d1→4f7 transition of Eu2+ ion when excited by 370 nm nearultraviolet light [189]. The excitation spectra of the phosphors are a broad band extending from 250 nm to 400 nm. Mg2+ codoping greatly enhances the bluish– green emission of the phosphors. An LED was fabricated by coating the Sr3.5Mg0.5Si3O8Cl4:0.08Eu2+ phosphor onto an ~370 nmemitting InGaN chip. The LED exhibits bright bluish–green emission under a forward bias of 20 mA. The results indicate that Sr3.5Mg0.5Si3O8Cl4:0.08Eu2+ is a candidate as a bluish–green component for fabrication of NUV-based white LEDs. Also Eu2+-doped Ca10(Si2O7)3Cl2 halosilicate phosphors were synthesized by high temperature solid state method [190]. Owing to excellence in excitation spectrum profile, appropriate chromatic coordinates of (0.265, 0.520) and good temperature properties, it is expected to be applied as a new green phosphor for near-UV light emitting diodes (LEDs). Solid State Phenomena Vol. 155 123 Another promising new class of LED phosphor materials is the oxonitridosilicates, known as SIONS. The N3- in this lattice is a soft Lewis base, which results in a high covalency. This shifts the energy of the 4f-5d absorption and emission for Eu2+ and Ce3+ ions in these host lattices to sufficiently low energies. In addition, sions are known to be highly stable toward oxidation and hydrolysis [191]. The ability to tune the emission color of a luminescent material is of great importance for practical applications. The emission of green color of the phosphor Sr1-x-y2 2+ zCaxBaySi2O2N2:Euz + (0 ≤ x, y ≤1; 0.005 ≤ z ≤ 0.16) can be tuned in two ways: by changing Eu concentration and by substitution of the host lattice cation Sr2+ by either Ca2+ or Ba2+ [192]. Upon raising the Eu2+ concentration above 2%, a red shift in the emission is observed from 535 nm (0.5% Eu2+) to 554 nm (16% Eu2+). This is ascribed to energy migration and energy transfer between the dopant ions. Increasing the concentration of Eu2+, however, also results in a decrease of the quantum efficiency and luminescence quenching temperature, which makes this concept not suitable for color tuning aimed at application in white light LEDs. The concept of color tuning by changing the host lattice was found to be very promising. Replacing part of the host lattice cation Sr2+ with Ca2+ shows that the crystal structure is preserved up to 50% of ion exchange. A red shift in emission is observed while retaining the high (90%) quantum efficiency. The luminescence quenching temperature is however lower for the Eu2+ emission in the mixed (Sr,Ca) compounds. Admixture of Ba2+ to the host lattice shows an unexpected red shift of the Eu2+ emission. For the mixed composition with up to 50% Ba2+, the high quantum efficiency and a high thermal quenching temperature are maintained for the Eu2+ luminescence. On this basis, Ba2+ substitution is a promising method for shifting the emission of SrSi2O2N2:Eu to longer wavelengths. α-SiAlON ( isostructual with α-Si3N4)), based oxynitride phosphors are good phosphor host materials absorbing in the UV region [193]. The luminescence of Yb2+ in α-SiAlON is characterized by the shift of the absorption from UV to the visible spectral region and an intense green emission band centered at 549 nm. The luminescence occurring at such low energies can principally be ascribed to the large crystal field splitting and nephelauxetic effect as a result of the nitrogen-rich coordination of Yb2+ in α-SiAlON. The luminescence properties of Yb2+ are greatly dependent on the activator concentration and the chemical composition of the host lattice. This novel Ca-αSiAlON:Yb green phosphor is expected to be used for phosphor converted white LEDs. Research efforts in blue phosphors The current blue phosphor material for solid-state lighting based on near-UV LEDs is mainly BaMgAl10O17:Eu2+ (BAM) due to its ideal chromaticity coordinates and high emission efficiency. BAM phosphor is considerably less stable than the red and green emitting components, both during panel fabrication (thermal degradation) and panel operation (VUV damage). Both damages in this material can result in color shift and the loss of brightness. It has been documented in the literature that the thermal degradation is assigned to the oxidation of Eu2+ to Eu3+ [194-196]. On the other hand, it was recently reported that the mechanism of VUV damage in BAM phosphor is due to the migration of Eu2+ ions to metastable sites and the formation of color centers in the spinel layer [197]. To reduce the thermal damage of BAM phosphor, solutions such as the baking process of panel fabrication in a reducing environment and the coating of BAM phosphor particles have been proposed [198]. However, these problems still remain unsolved in practical use. Moreover, no precise solution has been given to overcome the degradation mechanism by VUV irradiation. Therefore, (Ba,Sr)3MgSi2O8, which has a Merwinite structure, was adopted as the host material of new blue emitting phosphor [199]. The phosphors were synthesized by a conventional solid state reaction using a flux. The photoluminescence properties of the phosphors were investigated under 147 nm VUV ray excitation. With the increase of Ba content in the composition, the emission spectrum was blue-shifted, indicating a central wavelength-tunable blue phosphor. The Ba/Sr ratio with the central wavelength at 450 nm, which is same as BAM phosphor, was 0.2. The optimum 124 Synthesis, Characterization and Properties of Nanostructures Eu2+ concentration showing the maximum intensity was 1.5 mol% based on the emission intensity under VUV irradiation. On the other hand, the brightness of the (Ba, Sr)3MgSi2O8:Eu2+ phosphor after heat treatment at 500°C was less affected than that of BAM. Therefore, (Ba,Sr)3MgSi2O8:Eu2+ phosphor could be regarded as a potential blue phosphor for PDP application. The optical properties of rare earth doped haloaluminates Sr3Al2O5Cl2:Ce3+,Li+ have been investigated [200]. The PLE is a broad band extending from 300 nm to 400 nm. The PL shows broad band, and can be resolved two emission bands peaking at 440 and 480 nm corresponding to the transitions of 5d states to 4f 2F5/2 and 2F7/2 of Ce3+ ion. The fluorescence decay curve consists of two exponential components and the lifetime is in the order of nanosecond. The emission intensity at 150 °C remains at about 80% of that at room temperature. Sr3Al2O5Cl2:Ce3+,Li+ phosphor can be a promising blue phosphor for near UV-excited white LEDs. BAM shows a poor absorption band around 400 nm, not well suitable for InGaN chips [201,202]. The chalcogenide has the smaller electronegative value element sulfur compared with the oxide. When Eu2+ or Ce3+ was doped into such chalcogenide hosts, the crystal field splitting of doped ions will be stronger, the absorption of the 4f–5d transitions may extend to the visible (400– 500 nm) area. So the Eu2+ or Ce3+ doped chalcogenide are very appropriate phosphors excited by near-UV or blue emitting diodes for solid-state lighting, such as red-emitting Ca1-xSrx(SySe1-y):Eu2+ [203], yellow-emitting CaGa2S4:Eu2+ [204,205], and Sr1–xCaxGa2S4:Eu2+ [206] phosphors. Photoluminescence properties of CaLaGa3S6O:Ce3+ were investigated comparatively with the commercial blue-emitting phosphor BaMgAl10O17:Eu2+ [207]. It shows a more perfect and efficient broad absorption band around the 398 nm emission of the commercial near ultraviolet light-emitting diodes (LEDs), and presents a comparable blue-emitting performance. The blue light emitting LED with the CIE chromaticity coordinates of (0.147, 0.089) was successfully fabricated by precoating CaLaGa3S6O:Ce3+ phosphor onto a 398 nm-emitting InGaN chip. All these results indicate that CaLaGa3S6O:Ce3+ is a promising blue phosphor candidate for white LEDs. BaZnOS, a recently synthesized layered Zn-containing oxysulfide by Clarke and his co-workers [208], belongs to the orthorhombic system. Structurally similar to the good host material SrZnO2 [209], BaZnOS consists of edge-sharing [ZnO2S2] tetrahedral layers separated by Ba atoms. The local coordination environment of Zn is the [ZnO2S2] tetrahedron. All the structural characteristics suggest that the compound may act as a promising host material for transition metal activators. The good chemical and thermal stability of the material are also very attractive. The blue-emitting Cudoped BaZnOS phosphor was successfully obtained by a conventional solid-state reaction in sealed fused silica tubes [210]. Under the excitation of UV radiation, the phosphor can efficiently give a blue emission centered at 430 nm, corresponding to the transition from conduction band edge to the excited state of Cu2þ in the BaZnOS host. The maximum emission intensity occurs at 0.08 mol% of the Cu doping content for both photoluminescence (PL) and X-ray excited luminescence. The optimized blue-emitting BaZnOS:Cu phosphor has a larger PL intensity than the well-known green emitting ZnO:Cu and blue-emitting ZnS:Cu phosphors. The excellent luminescence properties are tightly related to the appropriate direct band gap and the unique crystal structure of BaZnOS host. Among the phosphates, KBaPO4 host lattice is a potential blue-emitting phosphor because of its excellent thermal resistance (in operation) and good color purity [211,212]. A novel blue-emitting KBP:Eu2+ phosphor has been synthesized which can be excited with wide range of excitation (UV to VUV) [213]. The relative intensity of KBP:Eu2+ phosphor when compared to BAM:Eu2+ commercial (Nichia) phosphor are 65%, 122%, and 108% under 147, 254, and 365nm excitation, respectively. Hence, KBP:Eu2+ phosphor is an appropriate candidate for CCFLs, PDPs, and pcwhite LEDs. In addition, strong moisture resistance was obtained by providing a nano-sized silica coating chemically bonded on the KBP:Eu2+ phosphor. It is expected that this coating procedure can also be applicable to other samples having weak moisture resistance so as to broaden device application possibilities. Recently, Guo et al. [214] have reported a SrMg2(PO4)2: Eu2+, Mn2+ Solid State Phenomena Vol. 155 125 phosphor and investigated the energy transfer from Eu2+ to Mn2+ in the SrMg2(PO4)2 host. The phosphor SrMg2(PO4)2: Eu2+ emits bright blue luminescence with a peak wavelength at 423 nm under ultraviolet excitation at 363 nm [215]. Based on the experimental results and the theoretical calculation, it is identified that the dipole–dipole interaction plays a major role in the mechanism of concentration quenching of Eu2+ in SrMg2(PO4)2: Eu2+ phosphor. In addition, the obtained phosphor SrMg2(PO4)2:Eu2+ exhibits higher thermal stability than commercially available YAG: Ce3+ phosphor, especially above 390 K. Also the LiSrPO4:Eu2+ phosphor can be efficiently excited by UV-visible light from 250 to 440 nm, and it exhibited bright blue emission [216]. A2P2O7 (A=Ca, Sr, Ba; Mg, Zn) is a large family of pyrophosphate compounds, which have been found to crystallize in two structural types that can be predicted from the ionic radius of A cation. When radius of A2+ is smaller than 0.97Å(A=Mg, Zn), A2P2O7 is of the thortveitite structure, in which [P2O7] groups are in stagger configuration; and when the radius of A2+ is larger than 0.97Å (A=Ca, Sr, Ba), the [P2O7] groups are in eclipsed configuration [217]. The luminescent properties, especially the Eu2+-doped diphosphates, such as Ca2P2O7:Eu2+, α-Sr2P2O7:Eu2+, MgSrP2O7:Eu2+ and MgBaP2O7:Eu2+ were found to be efficient phosphors in the violet–blue region [218]. By introducing optical inert ions into the lattice, such as Zn2+, it is possible to tailor the luminescent properties. A new efficient blue phosphor, Eu2+ activated SrZnP2O7, has been synthesized at 1000 °C under reduced atmosphere [219]. Under ultraviolet excitation (200–400 nm), efficient Eu2+ emission peaked at 420nm was observed, of which the luminescent efficiency at the optimal concentration of Eu2+ (4 mol%) was estimated to be 96% as that of BaMgAl10O17:Eu2+. Hence, the SrZnP2O7:Eu2+ exhibit great potential as a phosphor in different applications, such as ultraviolet light emitting diode and photo-therapy lamps. Quest for white light using phosphors In comparison with the commercial white LEDs fabricated with a blue chip and the yellow phosphor Y3Al5O12:Ce3+, the white LEDs fabricated with near ultraviolet (n-UV) chips and red/green/blue tricolor phosphors can offer a higher efficient solid-state light [220]. Achieving lower CCTs and higher CRIs requires red phosphors to compensate for the spectral deficiencies of standard commercial pcLEDs. In principle, using a single white phosphor instead of phosphor blends could help to reduce some of this variability. One approach leading to a single good CRI/low CCT phosphor is a modification of the composition of Ce3+-doped garnets by creating additional “sites” for Ce3+ with redder emission while retaining part of the typical yellow-green Ce3+ emission in aluminate garnets. The Ce3+ emission can be strongly red-shifted when there is a larger crystal field splitting of the two lowest-energy 5d levels, as in Lu2CaMg2Si3O12 [221]. There is a distribution of higher and lower energy Ce3+ sites in this silicate garnet, but this does not lead to a single low CCT/good CRI phosphor due to inhomogeneous broadening that smears any distinction between high and low energy sites. Because the energy position of the lowest Ce3+ 5d level can be modified by the covalency and polarizability of Ce3+-ligand bonds [222], incorporating ligands with a lower electronegativity compared to O2- would lower the energy of the 5d levels and lead to distinct Ce3+ sites with redder emission. One example where incorporating ligands with lower electronegativity leads to a red shift in 4fN-15d1 f 4fN emission is (Sr,Ba,Ca)Al2-xSixO4-xNx:Eu2+ [223], where O2- anions are replaced by N3- with Si4+ charge compensation. A similar incorporation of N3- in YAG has been reported in ceramic sintering experiments with Si2N2O as the Si4+-N3source [224]. Si4+-N3- incorporation on Ce3+ doped RE3Al5O12:Ce3+ (RE ) Lu3+, Y3+, or Tb3+) garnet phosphors leads to distinct low-energy Ce3+ absorption and emission bands that are assigned to Ce3+ ions that have N3- in their local coordination [225]. The combination of the typical Ce3+ emission in garnets with the low energy Ce3+ emission band results in a broad emission spectrum suited for white LED lamps with low color temperatures and good color rendering using only a single phosphor. However, the low-energy Ce3+ emission band has stronger quenching at high temperatures, a potential limitation. 126 Synthesis, Characterization and Properties of Nanostructures The white LED fabricated with NUV chip and corresponding phosphor has higher color stability because all the colors are determined by the phosphors. Up till now, a few white phosphors suitable for NUV excitation have been reported [226-229], the phosphors with better optical properties are still in development. Rare-earth-ions-doped silicate hosts have demonstrated good photoluminescence properties and chemical–physical stabilities. Eu2+ in such kinds of host may emit various colors demanded by white lighting. Emission spectra of Eu2+-doped phosphors with nominal composition of 2SrO.MgO.xSiO2 (0.8≤ x ≤1.2) consist of a blue band (460 nm) and a yellow band (550 nm), the relative intensities of which change with the SiO2 composition x [230]. It is found that a combination of the blue and yellow bands can generate white light at x around 1 under near- ultraviolet excitation. A white light emitting diode fabricated using a GaN chip (λem = 400 nm) exhibited color coordinates of x = 0.33, y = 0.34, color rendering index of 85 and luminous efficiency of 6 lm/W. Ce-doped Y2SiO5 is an excellent blue-emitting phosphor. On the other hand, Tb-doped Y2SiO5 emits a green color with photoluminescence properties similar to Zn2SiO4: Mn and BaAl12O19 [231]. There are two monoclinic phases for Y2SiO5, X1 for large RE3+ ions (La–Tb) and X2 for small RE3+ ions (Dy–Sc) [232]. The X1 phase transforms to X2 above 1190 °C which exhibits more luminous intensity [233-236]. Ce3+ is a blue broad band emitter with the ability to transfer energy from the lowest 5d levels to Tb3+ when accommodated in the same host matrix. Yttrium silicate doped with Ce3+ or Tb3+ in powder or thin film [237-241] and energy transfer between these two ions has been well studied in recent years. Results confirm that yttrium silicate is an excellent host lattice for RE3+ for low voltage cathodoluminescence for FED_s and for electroluminescent devices. Characteristic electronic transitions measured from photoluminescence spectra at λem = 544 nm (5D4 →7F5) in Tb3+ and λem = 418 nm (5d→ 2F7/2) in Ce3+ show that an inductive energy transfer occurs when Tb3+ ions absorbs the energy from Ce3+ upon excitation with long-UV photons (λexc = 358–380 nm) [242]. The optimal concentration for the best luminescence intensity has been obtained in Y2SiO5:Ce0.0075,Tb0.025, and the closest to daylight white emission was found with a composition of Y2SiO5:Ce0.0075,Tb0.040. These activated phosphors with two rare earth ions in the yttrium silicate host (Y2SiO5) represent an efficient way to produce white-light emission close to D65 with chromaticity coordinates of x = 0.225 and y = 0.320. Also, silicate-based phosphors activated with Eu2+ and Mn2+ are very suitable. These phosphors show the blue or the green emissions from the 4f–5d transition of Eu2+ ions and the red emission from the 4T–6A transition of Mn2+ ions. The absorption and emission bands of activators are controlled by changing the crystal field or the covalence depending on site size, site symmetry and coordination number of activator ions [243]. The phosphor’s shape in white LED affects the luminescent efficiency because it influences the scattering of incident or emitting light. Phosphors with spherical shape are more effective for forming a good phosphor layer rather than irregular shaped one [244]. X3MgSi2O8: Eu2+, Mn2+ (X=Ba, Sr, Ca) phosphors with the mean particle size of 200 nm and the spherical shape show three emission colors under near-ultraviolet light: the blue and green colors from Eu2+ ions and the red color from Mn2+ ions [245]. Three emission bands show the different emission colors with changing X2+ cations which arises from two competing factors of the crystal field strength and the covalency. These phosphors with maximum excitation of around 375 nm can be applied as color-tunable phosphors for white-light-emitting diode based on ultraviolet/phosphor technology. Luminescent properties of CaAl2Si2O8 phosphors coactivated with Eu2+ and Mn2+ under photoexcitation has also been studied [246]. The spectroscopic data indicate that the Eu2+→ Mn2+ energy transfer process takes place in the host matrix of CaAl2Si2O8. The energy transfer from Eu2+ to Mn2+ has found to occur via a dipole-quadrupole mechanism. This phosphor can be systematically tuned to generate white light under ultraviolet radiation and it has been shown to exhibit the potential to act as a white-emitting phosphor for ultraviolet LEDs. Phosphate and borophosphate hosts have also been explored for white light emission. Eu2+ and Mn2+ co-doped (Sr,Ba)6BP5O20 phosphors were prepared by high temperature solid state reaction and it was observed that with the increasing content of Ba2+, color-coordinate parameters x, y Solid State Phenomena Vol. 155 127 change regularly, and lighting color moves from greenish blue, purplish blue to white [247]. Whiteemitting phosphors were finally obtained with color-coordinate x, y of 0.223 and 0.237, respectively. Similarly, the phosphates α-Sr(PO3)2, SrZn(P2O7), and α-Sr2(P2O7) were doped and codoped with Eu2+ and Mn2+ and photoluminescence studies were done on them [248]. Codoped αSr(PO3)2:Eu,Mn emits white light during excitation with an UV wavelength of 323 nm, which is accessible by UV LEDs based on AlGaN [249]. The rare-earth sesquioxide viz. gadolinium oxide (Gd2O3) has been shown to be a good host for the luminescence of rare-earth ions [250,251]. Eu3+ and Tb3+ ions are generally used as efficient luminescent centres in display applications [252]. Gadolinium oxide host and europium/dysprosium/terbium doped gadolinium oxide nano particles were synthesized using the sonochemical technique [253]. Sonochemical synthesis is based on acoustic cavitation resulting from the continuous formation, growth and implosive collapse of the bubbles in a liquid. This technique has been used successfully for the synthesis of various nanoparticles [254-255]. The triple doped samples showed multi color emission on single wavelength excitation. The photoluminescence results were correlated with the lifetime data to get an insight into the luminescence and energy transfer processes taking place in the system. The novel nano-crystalline Gd2O3:RE (RE = Dy, Tb) phosphor, on excitation at 247 nm resulted in a very impressive CIE chromaticity coordinates of x = 0.315 and y = 0.316, and correlated colour temperature of 6508 K which is very close to standard daylight. II-VI semiconducting nanocrystals (NCs) have recently emerged as better phosphors compared to traditional phosphors [256-258]. These nanophosphors have broader and stronger absorption and higher resistance to photooxidation compared to the common emissive materials, such as organic dyes and inorganic phosphors; also for the NCs with sizes less than 10 nm, loss of energy due to scattering is strongly reduced [259]. Another important aspect of these NCs is solution processability [260]; the surface of NCs can be functionalized using various organic molecules, making them soluble in both polar and nonpolar solvents. In recent years, transition-metal-doped nanocrystals have come up as a new class [261-264] of light-emitting materials that retain all of the advantages of undoped NCs and also overcome some of the intrinsic disadvantages such as self absorption and sensitivity to thermal, chemical, and photochemical disturbances compared to their undoped counterpart [265,266]. In a very recent article [267], two routes to white-light generation has been demonstrated. In one case, the core-shell-shell type of multilayer structure has been used. In this case, the material and the size of core and those of the shell are so chosen that emissions of different wavelengths from these two combine to give white light. In essence, this approach is similar to the one based on blending with all of the advantages and disadvantages of that method. The other approach involves generation of white light combining surface-state and band-edge emission, similar to the report [268] of Bowers II et al. Here the idea is to generate white light by combining surface-state emissions of nanocrystalline host and inner-core transitions from dopant centers. For example in case of Mn2+-doped CdS NCs; the addition of Mn2+ as a dopant helps in extending the emission to longer wavelengths. This approach offers several advantages. Because the surface-state emission as well as the inner-core transition at the dopant sites are relatively less sensitive to a size variation compared to the band-gap emissions, the chromaticity of the light generated is not critically dependent on the particle size or its distribution, thereby making it possible to use a sample with larger size distributions. Although neither the surface state nor the dopant emission can be tuned, the chromaticity of the white light can be significantly tuned by altering the relative proportion of each of these two emissions. Additionally, these NCs can be excited over a wide range of excitation wavelengths without disturbing the chromaticity of emission. Finally, this approach does not suffer from self-absorption because of substantial stokes shifts of the component emissions compared to absorption, therefore producing white light both as a dilute solution and in the solid form and proving itself to be an ideal material for a white-lightemitting intrinsic layer in a WLED. The generation of white light from a simple transition-metal- 128 Synthesis, Characterization and Properties of Nanostructures doped semiconducting nanocrystal, namely, Mn2+-doped CdS, by suitably tuning the relative surface-state emissions of the nanocrystal host and the dopant emission has been reported [269]. White light emitted by these nanocrystals remains unchanged both in solution form as well as in the solid state and can be excited by a wide range of UV lights without disturbing the chromaticity. This desirable property arises from the intrinsic separation of the absorption energy and the emission energies due to a large stokes shift, thereby avoiding the problem of self-absorption altogether. Similarly, Mn-doped ZnS nanorods synthesized on zinc foils by a solvothermal approach exhibited white light emission upon excitation in the UV range (300-330 nm) [270]. X-ray diffraction studies coupled with energy-dispersive X-ray analysis and X-ray photoelectron spectroscopy indicated the presence of a thin oxide layer on the Mn-doped ZnS nanorods. The emitted white light was found to be the result of blue, green, and orange emission bands. The blue bands at 400 and 459 nm were attributed to sulfur vacancies and surface states, respectively. The green band at 511 nm was associated with the singly ionized oxygen vacancy of the ZnO shell layer. The orange emission originated from the 4T1-6A1 transition of the Mn2+ ions. Thus, the emissions from the Mn-doped ZnS core and the outer ZnO shell combine together to produce the white light. However, in both cases, the measured quantum efficiency of about 2% is very small for any practical application at present; hopefully, the quantum efficiency can also be increased within this approach in the near future. Among the rare earth fluorides, cerium fluoride (CeF3) has been attracting increasing attention as an important fluorescent host material owing to its low vibrational energies and the subsequent minimization of the quenching of the excited state of the rare-earth ions [271]. Fluorides can serve as host materials for up-conversion (UC) and down conversion (DC) processes and can be applied in the fields of lighting, display technology and biolabelling as a consequence of the development of infrared laser diode [272]. Very recently, we have synthesized well crystalline CeF3 and CeF3 doped with Dy3+, Tb3+ and Eu3+ nanoparticles by sonochemical route [273]. The double doped samples showed characteristic emission of respective dopants (Dy3+ and Tb3+) when excited at the 4f→5d transition of Ce3+ [Figure 1]. The chromaticity coordinates for these samples were calculated and it was observed that the CeF3 co doped with Dy3+ and Tb3+ gave an emission very close to white light [Figure 2]. 3+ Dy Intensity (a.u.) 3+ Dy 3+ Tb 400 450 500 550 600 wavelength (nm) Figure 1: Emission spectrum of sonochemically synthesized CeF3:Dy:Tb. 650 Solid State Phenomena Vol. 155 129 Figure 2: Chromaticity diagram for sonochemically synthesized CeF3:Dy:Tb. All the phosphor materials discussed above, generate white light by the down conversion process. Down-conversion is the conversion of UV into visible light and is widely exploited in phosphors [274]. The up-conversion process is based on sequential absorption and energy transfer steps. This event is different from multiphoton absorption processes, requiring high excitation densities. Lanthanide ions are suitable candidates for up-conversion processes because of their energy levels [275-277]. To achieve an efficient, cost-effective and durable white light source we need (i) stable photocycle, (ii) cheap excitation (e.g., 980 nm CW laser) and efficient absorption,(iii) control over the intensity of red, green, and blue emission, and (iv) easy and cost-effective device fabrication. Recently it has been reported that white light can easily be generated from SiO2,ZrO2 sol-gel thin film made with Ln3+-doped nanoparticles codoped with Yb3+ ions [278]. The codoping with Yb3+ ions makes it possible to excite with 980 nm light only. Red, green, and blue emission was generated from three different lanthanide ions, that is, Er3+ (red as well as green), Eu3+ (red), and Tm3+ (blue) ions. The vapor phase synthesis of upconverting Y2O3 nanocrystals doped with Yb3+, Er3+, Ho3+, and Tm3+ to generate red, green, blue, and white light has also been reported [279]. Incorporating Er3+ within the Yb3+ doped Y2O3 nanocrystals produced orange and yellow upconversion luminescence (under 980 nm laser excitation) tunable by varying the Yb3+ concentration. The Yb3+, Er3+, and Tm3+ codoped Y2O3 nanocrystals exhibited nearly equal intensities of the red, green, and blue emissions upon 980 nm laser excitation. White light can be produced by adjusting the concentrations of the Ln3+ ions within the Y2O3 nanocrystals. 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