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
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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
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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
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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
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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
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[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.
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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
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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
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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
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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
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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
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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.
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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.
Conclusion
The opportunity for LEDs to become the lighting technology of the future depends upon the
discovery of better materials for emission and the solution of significant technical barriers involving
the materials associated with these devices. Technical advances are required to continue to improve
both the internal and external quantum efficiencies of these devices and to produce materials that
will sustain performance over product life. Through a better understanding of the materials, the
challenges of processing, product life performance, and packaging can be met to make the these
devices realize the promise of a more efficient light source with end user benefits not available
today.
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