Down Conversion Materials for SolidState Lighting

J. Am. Ceram. Soc., 97 [5] 1327–1352 (2014) DOI: 10.1111/jace.12943 © 2014 The American Ceramic Society

Journal

Review: Down Conversion Materials for Solid-State Lighting Joanna McKittrick‡† and Lauren E. Shea-Rohwer§

‡

Dept. Mechanical and Aerospace Engineering and Materials Science and Engineering Program, UC San Diego, 9500 Gilman Dr., La Jolla, California 93093-0411 §

Sandia National Laboratories, P.O. Box 5800, MS-1425, Albuquerque, New Mexico 87185-1425

The wavelength down conversion approach to solid-state lighting (SSL) uses down conversion materials to produce visible light when excited by near-UV or blue emission from InGaN LEDs. This review discusses two classes of down conversion materials: phosphors and semiconductor quantum dots (QDs). Strong absorption of the excitation wavelength; high luminous efficacy of radiation, which enables white light with a high color rendering index and a low correlated color temperature; high quantum efficiency; and thermal and chemical stability are some of the criteria for down converters used in SSL. This review addresses the challenges in the development of down converters that satisfy all these criteria. We will discuss the advantages and disadvantages of several phosphor compositions for blue and near-UV LEDs. The use of core/shell architectures to improve the photoluminescence and moisture resistance of phosphors is presented. QDs are another class of down conversion materials for near-UV and blue LEDs. Strategies to improve the photostability and reduce the thermal quenching of QDs include strain-graded core/shell interfaces and alloying. We discuss Cd-containing II–VI QDs, and Cd-free III–V and I–III–VI QDs and their potential for SSL applications. Finally, a description of different methods to integrate the phosphors and QDs with the LED is given.

I.

Introduction

L

IGHTING accounts for 22% of the electricity use and 8% of all energy use in the United States, and if predictions on solid-state lighting (SSL) are correct, this technology could reduce the electricity used for lighting by 33% in the next decade.1 The three major traditional lighting sources are incandescent, fluorescent, and high-intensity discharge lamps. Figure 1 shows the distribution of these light sources in various lighting applications for the United States.2 Incandescent

D. J. Green—contributing editor

Manuscript No. xxxxx. Received September 20, 2013; approved March 8, 2014. † Author to whom correspondence should be addressed. e-mail: [email protected]

lighting is preferred for residential applications, whereas fluorescent lighting is primarily used in commercial buildings. High-intensity discharge lamps are typically used for street and stadium lighting. Lighting technology is the last frontier that has not been affected by solid-state advances. Incandescent light bulbs have been virtually unchanged since Thomas Edison invented them over 120 yrs ago. An incandescent bulb is one of the most energy inefficient components in daily use because less than 5% of the power consumed by the bulb is converted into light (the rest is lost as heat). The luminous efficacy of an incandescent light source, defined as lumens emitted divided by the input power (Watts), is 15 lm/W for a 60 W bulb. These bulbs also have a limited lifetime. Fluorescent lighting is more efficient (50– 100 lm/W), but the tubes are fragile and bulky. Fluorescent lighting is slowly penetrating the incandescent market with the introduction of compact fluorescent bulbs, which can be inserted into incandescent bulb sockets. The largest problem with fluorescent lighting is the lower color rendering index (CRI). The CRI is a measure of the ability of a light source to reproduce the colors of various objects in comparison to sunlight. The sun is defined as having a CRI of 100, the same as an incandescent bulb. Fluorescent lighting has typical CRI values in the range 80–90. Figure 2 shows the spectral energy distribution for the sun, a fluorescent lamp and an incandescent lamp.3 In addition, in fluorescent tubes, there is approximately 3–5 mg of mercury, rendering a disposal problem. Solid-state lighting is based on inorganic or organic lightemitting diodes (LED). In inorganic SSL, which is the subject of this review, the LED is basically a p–n junction, composed of an InGaN alloy that can emit wavelengths from 360 to 470 nm (near-UV to blue), depending on the In concentration (see Fig. 3).4 Although LED technology is not new—the first commercial LEDs were introduced in the 1960s—development of a blue-emitting LED was elusive. It was not until the 1990s that a blue-emitting LED based on InGaN was introduced to the market.5,6 This invention gave rise to the possibility of producing white light using only solid-state components. Green–yellow LEDs are obtained by increasing the In concentration in the InGaN active region, but have much lower external quantum efficiencies (EQEs) than the near-UV/blue LEDs. Red LEDs used in SSL are based on AlInGaP alloys.

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Fig. 3. Band gap energy and lattice constant for w€ urtzite-structured GaN, AlN, and InN. Adapted from Ref. [4]. Fig. 1. Distribution for incandescent, fluorescent, and high-intensity discharge lamp market share. Data taken from Ref. [2].

Figure 4(a) shows the multiple LED approach, which is based on combining the emissions from three or more monochromatic LEDs to produce white light. Such devices could generate white light with a high lm/W, but packaging the LEDs and color mixing optics could be a cumbersome and expensive process. In addition, the luminous efficacy of green InGaN and red AlInGaP LEDs have been shown to decrease with increasing temperature,7 which leads to a reduction in device efficiency and unstable color properties. This review focuses on the wavelength down conversion approach to SSL, which uses near-UV or blue LEDs, combined with down conversion materials to produce various colors in the visible spectrum. Down converters absorb light of a shorter wavelength and emit light at a longer wavelength. Up converters do the opposite, and have application in lasers and displays. The wavelength down conversion approach is represented in Figs. 4(b)–(d). In Fig. 4(b) a “light sandwich” is shown where the blue light from the LED is

Fig. 2.

used to excite red- and green-emitting phosphors. The challenge here is to develop red- and green-emitting phosphors that can be excited by a blue LED light at 450–470 nm. Figure 4(c) shows the commercially available LED design— the blue LED light excites a yellow-emitting phosphor [(Y1
xCex)3Al5O12, YAG:Ce] and the combination results in a white light; Fig. 4(d) shows a design using a near-UV-emitting LED with a triblend of red-, green-, and blue-emitting phosphors in a remote phosphor configuration. The advantages of the blue LED plus yellow-emitting phosphors approach over the near-UV LED and red, green, and blue (RGB) phosphors approach are a lower cost and easier fabrication; the disadvantages are lower CRI, lower luminous efficacy, and lower chromatic stability under different driving currents.8 Figure 5 shows the Commission Internationale de l’Eclairage chromaticity diagram that maps the color space the human eye can detect.9 White light can be created from adding appropriate amounts of RGB light or from adding two colors. The “TV phosphors” are shown as the apices of

Spectral energy distribution for different light-emitting sources. Taken from Ref. [3].

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(b)

(c)

(d)

Fig. 4. Possible design configurations for a white-emitting LED. (a) Three red-, green-, and blue-emitting diodes (simplest but most expensive), (b) the “light sandwich” where the blue light from the LED excites green- and red-emitting phosphors, (c) the commercial design—a blueemitting diode with phosphor particles suspended in a polymer above the diode, (d) near UV LED using a triblend of phosphors in a remote phosphor configuration [J.K. Han, J. McKittrick, and J. Talbot, unpublished (2013)].

the triangle shown in the diagram and the dotted line shows how a mixture of two colors, blue and yellow, can give a white color. Another important characteristic of white light is the correlated color temperature (CCT). The CCT ranges from ~3000 K (warm white) to ~6000 K (cool white), as shown in Fig. 5. In the United States, most people prefer warm lighting for the home and cool lighting in the office environment. The advantages of using white LEDs for lighting are numerous: they do not require fragile glass tubes (vacuum or gas) such as other lighting types; the lifetime is much longer (up to two orders of magnitude over incandescent); there is less toxic waste in the processing or use; and finally, they are compact and present endless design possibilities. However, there are several drawbacks. First, they are costly; second, the CRI is not optimal; and finally, their luminous efficacy is not yet high enough to compete with existing lighting technologies. Table I lists lighting sources, their luminous efficacy, CRI, cost, and lifetime.1,10,11 As shown, SSL is expected to outperform incandescent and fluorescent lamps during the next decade in terms of efficacy, CRI, and lifetime. The lifetime is projected to be 100 000 h. Currently, the cost and the availability of high-power (>1 W) lamps are the drawbacks to this technology. The efficacy of the lamps must also be improved to over 150 lm/W to compete with fluorescent or high-intensity discharge lighting.12–14

II.

Down Conversion Materials for SSL

In this review, we distinguish between two classes of down conversion materials: phosphors and semiconductor nanocrystals, or quantum dots (QDs). Phosphors have a localized

luminescent center, whereas in QDs, the luminescence arises from quantum confinement of excitons. Phosphors are currently used in commercial SSL devices that are based on wavelength down conversion. QDs are an attractive alternative to phosphors, but are not currently used as down converters in SSL applications. Before discussing the requirements of down conversion materials for SSL, it is worthwhile to differentiate between phosphors and QDs.

(1) Phosphors Background Most phosphors are composed of an inert host lattice, usually a wide band gap material (e.g., oxides, nitrides, and sulfides) and a small amount of a dopant ion (also referred to as an activator), or a functional group as the luminescent center. The dopants are usually rare earth or transition metal elements. Phosphor compositions are typically written as host:activator. The luminescent centers are classified according to the optical absorption electronic transitions15: 1. 2. 3. 4.

4fn?4fn
15d (most rare earth ions); 4fn?4fn, 5fn?5fn (rare earth and actinide ions); 3dn?3dn; 4dn?4dn (transition metal ions) Charge transfer between an anion p-electron and an empty cation orbital (e.g., VO43
, WO42
, and MoO42
).

The configurational coordinate model is useful to explain the optical transitions in a localized luminescence center. The optical properties can be explained on the basis of potential energy curves, each of which represents the total energy of the molecule in its ground or excited state as a function of the configurational coordinate (Fig. 6). The total energy

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Fig. 5. CIE chromaticity diagram (color space) which maps out the color that is perceived by the human eye.9 Every color has an independent x, y coordinate. The points along the curved boundary of the diagram represent saturated colors with their wavelengths (in nanometers) shown in blue font. The blackbody curve is shown in the center of the diagram with the color temperature, Tc (in K) of white light. The triangle defines the color gamut of a television screen that utilizes red, green, and blue phosphors having chromaticity coordinates that are located at the corners of the triangle. The dotted line shows how mixing blue and yellow can produce white light.

is the sum of the electron energy and ion energy. The luminescent ion and its nearest neighbor are selected, ignoring the effects of other ions. The curves represent the potential energy of the ground and excited state of the luminescent center. The x axis is the interatomic distance between the luminescent ion and the surrounding anions. Excitation of the luminescent center occurs through energy absorption from A?B. Undesirable nonradiative relaxation (phonons) takes place from B?C with emission occurring from C?D followed by nonradiative relaxation from D?0. Thermal activation can take place from C?E. In this case, nonradiative relaxation takes place from E?0. Phosphors are typically micrometer-sized powders that can be produced using a wide assortment of methods, such as solid-state reaction, chemical precipitation, sol–gel, combustion, hydrothermal/solvothermal syntheses—the same methods used for producing other ceramic powders. Nanosized phosphors (nanophosphors, 606 nm) of a commercial broad red-emitting Eu2+-doped phosphor [Fig. 10(a)] is ~119 lm/Wem which is about half of the LER that would be obtained from the narrowband Eu3+-doped phosphor shown in Fig. 10(b) whose LER is ~235 lm/Wem.30 The product of the photopic curve and the emission spectrum is plotted in Figs. 10(a) and (b), and is used to calculate the LER [see Eq. (1)]. The narrow red emission from Eu3+ is due to parity-forbidden 4f?4f transitions that have low oscillator strength (~10
6), and consequently, low blue absorbance. Increasing the blue absorbance remains a challenge for Eu3+doped phosphors being considered for SSL. This section describes several phosphor families that employ Eu2+ and Ce3 dopants and their development for

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blue LED-based SSL devices. Progress toward the development of Eu3+- and Mn4+-doped phosphors with narrowband red emission is also discussed. (A) Yellow-Emitting YAG:Ce3+: YAG:Ce was the first phosphor used for white SSL.28 This phosphor absorbs blue light, in the range ~440–460 nm, and emits yellow light, in the range of ~520–640 nm. YAG crystallizes in a cubic garnet structure that can be described by the formula C3A2D3O12. The Y ions occupy dodecahedral C sites and the Al ions occupy both tetrahedral D and octahedral A sites.31 Emission can be red-shifted by partially substituting Gd3+ for Y3+, although a high concentration of Gd3+ also causes strong thermal quenching of quantum efficiency.32 Emission can also be blue-shifted by substituting Ga3+ for Al3+,32,33 but this also increases thermal quenching. White LEDs using YAG:Ce3+ alone cannot achieve a CCT below 5000 K. In fact, the CRI drops from about 80 at a CCT of 8000 K to roughly 70 for lower CCTs. An approach that has been investigated to improve the color properties of YAG:Ce-based LEDs involves codoping YAG:Ce. When YAG:Ce3+ is codoped with red-emitting Pr3+,34 Ce3+ transfers energy to Pr3+ which emits narrow red emission at ~610 nm. White LEDs that contain Ce3+, Pr3+-codoped YAG have a higher CRI than those containing YAG:Ce3+, but the emission from both Ce3+ and Pr3+ decreases as the Pr3+ concentration increases. As a result, the luminous efficacy (lumens per Watt of electrical power) of the white LED is reduced. When Y in YAG:Ce3+, Eu3+ is replaced with Tb, the emission spectrum extends into the red region, but the QY is significantly lower than that of YAG:Ce.35 (B) Broadband Red-Emitting Eu2+-Doped Phosphors: Eu2+ exhibits red emission in sulfide hosts. Ca1
xSrxS:Eu2+can be excited by 460 nm blue light, and has a strong emission peak that can be tuned to 615 nm,33 a wavelength region that is ideal for increasing the CRI of LEDs based on YAG:Ce. SrY2S4:Eu2+can be excited by 465 nm light, but emits in the deeper red, with a strong emission peak at 640 nm,36 to which the eye is almost three times less sensitive. These sulfide phosphors are all moisture sensitive and bleach by oxidation, so create stability issues for SSL devices. Another class of red phosphors is based on Eu2+-doped nonsulfide hosts, particularly the nitridosilicates.37–41 In these M2Si5N8 (M = Eu, Ba, Sr, Ca) host lattices, Eu2+ emission is sensitive to the size of the M cation and can be tuned from yellow to deep red by varying the occupancy of the M site.39,40 These orthorhombic lattices are comprised of threedimensional networks of corner-sharing SiN4 tetrahedra. Half of the nitrogen atoms are connected to two silicon atoms, and the other half is connected to three silicon atoms.41 Eu can occupy either of two 10-coordinated sites, which differ in their net positive charge, resulting in a broad emission comprised of two emission bands.37,40 The emission peaks can range from 570 to 680 nm, 609 to 680 nm, and 515–605 nm in Eu2+-doped Ba2Si5N8, Sr2Si5N8, and Ca2Si5N8, respectively.39 Sr2Si5N8:Eu2+ emits in the red and is reported to have a quantum yield exceeding 90%,68 but this high QY is largely offset by the deep red emission of this phosphor, which leads to low LER. Sr2Si5N8:Eu2+ is susceptible to thermal degradation, which has been attributed to the oxidation of Eu2+. The addition of Ba to the host lattice was found to increase oxidation resistance.42 White-emitting LEDs with the Sr,Ba-containing phosphor showed only 5.6% degradation versus 24% when Ba was absent.43 The aluminonitridosilicate phosphor Eu2+-doped CaAlSiN3 has shown potential for SSL applications.42 CaAlSiN3 crystallizes in the orthorhombic structure and is made up of overlaid planes of six-membered rings of corner-sharing SiN4 and AlN4 tetrahedra. Each plane is oriented 180° from the planes above and below it forming a three-dimensional framework.44 Eu2+ substitutes for Ca2+, which is coordi-

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Fig. 9. Energy level diagrams of Ce3+ and Eu2+ dopants, which are broad emitters and Eu3+, a narrow band emitter [L.E. Shea-Rohwer, unpublished (2013)]. See also Ref. [15]. Circles denote the energy levels from which luminescence is observed. For electromagnetic radiation in a vacuum, the wavenumber (cm
1) is proportional to the frequency and photon energy, so wavenumber is typically used as energy units in spectroscopy.

nated by 5 N ions. At an Eu concentration of 1.6 mol%, CaAlSiN3 has broad, deep-red emission that peaks at 660 nm.44 Reducing the Eu concentration to 0.05 mol% blue-shifts the emission peak to 634 nm, but the QY decreases significantly. The excitation spectrum of CaAlSiN3: Eu2+ is sufficiently broad to encompass all blue LED wavelengths currently being considered, but if this phosphor is used in conjunction with a green-emitting phosphor, unwanted absorption of the green light will occur. A more promising nitridoaluminosilicate is (Sr,Ca)AlSiN3. It was found that the emission peak could be blue-shifted from 650 to 620 nm by increasing the strontium concentration to 0.9 mol%.45 The PL intensity of (Sr,Ca)AlSiN3 is reported to be unaffected by temperatures up to 180°C.45 (C) Broadband Red-Emitting Ce3+-Doped Phosphors: Another class of red phosphors is based on Ce3+doped hosts such as CaSiN246 and Lu2CaMg2(Si,Ge)3O12.47 In CaSiN2 hosts, the Ce3+ emission peaks at 625 nm, but its broad excitation spectrum, extending from 425 to 575 nm, would strongly absorb the YAG:Ce emission. Its quantum yield is relatively low, ~40% under 515 nm excitation,46 and its emission extends into the deep red. In Lu2CaMg2(Si, Ge)3O12 hosts, the Ce3+ emission peaks at 605 nm,47 and also extends into the deep red. Therefore, these phosphors, though of interest for improving the CRI of the current generation of white LEDs, do not have the narrow linewidth needed for highly efficient SSL devices.11,48 (D) Narrowband Red-Emitting Eu3+-Doped Phosphors: Phosphors doped with Eu3+ emit narrowband red emission at ~610 nm due to the 5D0?7F2 electric dipole transition when the Eu3+ occupies noncentrosymmetric sites in the host lattice. While the Eu3+ emission color and linewidth (FWHM ~4 nm) are ideal for lighting, the parity-forbidden 4f–4f transitions have low oscillator strength (~10
6), leading to low blue light absorption. The low absorption of Eu3+ is a nonissue in fluorescent lighting because the 254 nm Hg line excites the Eu3+ ions through the Eu–O charge transfer (CT) band. With the goal of adapting Eu3+-doped phosphors to

SSL, researchers have investigated the following approaches: shifting the CT band edge from UV to blue and identifying host lattices in which Eu3+ can be efficiently excited with blue light. Charge-transfer band shift approach: Vanadates, molybdates, and tungstates have broad UV absorption due to M–O charge transfer (M = V, Mo, W). It has been reported that these CT bands can be red-shifted to enable excitation of Eu3+ with InGaN LED wavelengths.49–51 In YVO4:Eu3+, UV excitation is absorbed by the vanadate groups, which transfer the excitation energy to the Eu3+ ions resulting in red emission. When YVO4:Eu3+ is codoped with Bi3+, the V–O CT band edge red-shifts by ~80 nm from 340 to 420 nm49 due to Bi–O absorption. However, the QY of the Bi3+-containing phosphors is low. In studies of AMO4 and A3MO6 (A = Ca, Sr, Ba and M = Mo, W) and Ln2MoO6 (Ln = Y, Gd) host lattices, it was found that the CT band edge in the molybdates is at longer wavelengths than in the tungstates, and that when the Mo and W are coordinated by six O atoms instead of four, the CT band edge shifts to longer wavelengths.51 The orthorhombic molybdates (Eu3+-doped Ca3MoO6 and Y2MoO6) were found to be the most intense red-emitting phosphors, with narrowband emission at 615 nm, but the QY of these phosphors has not been reported.51 Identifying host lattices that enable efficient blue excitation of Eu3+: Another approach to red emitters that can be excited with blue LEDs involves identifying host lattices in which the Eu3+ is more efficiently excited by direct 4f–4f transitions than through CT band excitation. These host lattices must also have noncentrosymmetric sites for Eu3+ to occupy so that the electric-dipole 5D0?7F2 transition which emits red light at ~610 nm is favored over the magnetic dipole 5D0?7F1 transition, which emits orange light. Narrowband red emission has been reported from EuKNaTaO5 excited at 535 nm52; and La3NbO7 under 395 and 465 nm excitation.53 The QYs of these materials were not reported, but the PL was more intense under excitation with visible

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(a)

(b)

Fig. 10. Luminous efficacy of radiation (LER) in lumens per Watt of light emitted for (a) Eu2+-doped commercial phosphor.30 Reproduced by permission of The Electrochemical Society. (b) Eu3+-doped LaTaO4 phosphor made at Sandia National Laboratories.

light than UV light. Eu3+-doped pyrochlore tantalate nanophosphors were found to have narrow red emission with a QY as high as 78% under blue excitation.54 Orthotantalates (LaTaO4) were found to have QYs above 80% under blue excitation.55,56 Research on Eu3+-doped tantalate pyrochlore nanophosphors K,RETa2O7 (RE = Lu, Y) and K1
2x(Gd, Eu)Ta2O7
x (x = 1/3) showed that the QY and the blue absorbance linewidth are affected by the type of rare-earth ion and lattice distortion. The Lu and Y analogs crystallize in the AB2O7 structure where the potassium and rare-earth ions occupy the eight-coordinated A site and tantalum occupies the six-coordinated B-site. Rietveld refinement of the Lu and Y analogs revealed K, Lu/Y, and europium are disordered on the eightcoordinate A site, whereas Ta resides on the six-coordinate B-site. At a concentration of 10 at.% Eu3+, the QY of K, LuTa2O7 is 67%; and 63% for K,YTa2O7. When RE = Gd, the eight-coordinate A site is half-occupied with Gd plus the

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Eu and the potassium resides in a partially occupied site  0.079 nm away from the Gd site and  0.15 nm from the partially occupied oxygen site. As a result, K and Gd are disordered on the A site, but the potassium is seven-coordinate and off-center toward the vacant oxygen site. It is presumably this structural difference that makes the blue excitation linewidth of the Gd analog broader (~9 nm) than those of the Y and Lu analogs (Fig. 11). A broader blue absorption linewidth enables excitation with a wider range of LED wavelengths. At a Eu3+ concentration of 17.5%, the QY of the Gd pyrochlore is 78% under blue excitation and shows only 9% thermal quenching at 150°C. Comparable thermal quenching was reported for Y2Ti2O7:Eu pyrochlores at 150°C.57 The preliminary results discussed above show the potential of Eu3+ phosphors for SSL. Their main limitation for SSL use is their low blue absorbance. In powder form this is a real limitation, but can be overcome by fabricating the phosphors as transparent wafers ~1 mm thick to increase the optical path length of blue light to an optimal value. (E) Mn4+-Doped Red-Emitting Phosphors: Mn4+doped, narrowband red-emitting phosphors are currently being investigated for SSL. The spin-allowed, parity-forbidden nature of the Mn4+ transitions leads to absorption bands that are broader than those of Eu3+ and narrower than those of Eu2+. Mn4+-doped phosphors typically exhibit two absorption bands that peak in the near-UV and blue-green spectral regions due to the 4A2?4T1 and 4A2?4T2 transitions, respectively. The emission is due to the 2E?4A2 transition and is weakly dependent on the crystal field. There has been recent work on Mn4+-doped K2SiF6 and K2TiF6 phosphors.58 These phosphors have QYs ~80% and low thermal quenching.58 The emission from Mn4+ when incorporated into K2SiF6 has narrowband emission peaks at 612, 630, and 646 nm. Another Mn4+-doped phosphor that has been investigated for SSL is 3.5MgO–0.5MgF2–GeO2.59 In this host lattice, Mn4+ emits at 659 nm, and therefore has a lower LER than is desired for SSL. (F) Broadband Eu2+-Doped Green-Emitting Phosphors: We now turn to phosphors that have potential in the configuration shown in Fig. 4(b). The blue LED light needs to excite green-emitting phosphors (along with the above-mentioned red-emitting ones), which are discussed below. Broadband green-emitting phosphors that can be excited with blue LED wavelengths (440–470 nm) include Eu2+doped thiogallates, orthosilicates, oxonitridosilicates, and oxonitridoaluminosilicates. The emission from Eu2+ ions is due to the 4f65d?4f7 transitions, which are parity-allowed and thus have a high oscillator strength (10
2). The 5d orbitals are spatially diffuse and their energy levels depend on the crystal field of the surrounding ions. This dependence on the crystal field enables tuning of the emission color. (a) Thiogallates: In the general family of ternary sulfides MIIAM2IIIS4, the spectral position of Eu2+ emission is a strong function of the crystal environment,60,61 shifting from blue–green (493 nm) to green (535 nm) to yellow (565 nm) with decreasing MII cation size (Ba, Sr, and Ca), respectively, and with increasing MIII cation size (Al, Ga, and In). The green emission from the thiogallate SrGa2S4:Eu2+ has a FWHM of 50 nm and a quantum yield of ~80%. This phosphor has been used in single-phosphor LEDs to create green light from blue LED excitation40; and in two-phosphor (green and red) white LEDs.32 The main disadvantages of the thiogallates are their chemical and thermal instability. SrGa2S4:Eu2+ suffers from severe moisture sensitivity and significant thermal quenching at device operating temperatures. Some thiogallate phosphors are also susceptible to emission spectral shifts and spectral broadening at elevated temperatures. For example, measurements on BaGa2S4:Eu2+ show an emission peak shift from 503 nm (FWHM = 42 nm) to 488 nm (FWHM = 70 nm)61 when the temperature is increased from 77 to 450 K.

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Fig. 11. Blue absorption linewidths of Eu3+-doped rare-earth tantalate pyrochlores as a function of the rare-earth ion (Lu, Gd, Y). Reprinted with permission from Ref. [54]. Copyright 2009 American Chemical Society.

(b) Orthosilicates: The family of silicates that crystallize in the orthorhombic structure, (M2+)2SiO4 where M = Ba,Sr have received interest for potential use in SSL due to the ability to tune their emission colors by controlling the Ba/Sr ratio.62 For instance, in (Sr1.9
xBax)SiO4:Eu0.1, as the Ba concentration increases, the emission maximum shifts from 585 nm (for x = 0) to 519 nm (for x = 1.9).63 Under blue excitation, these phosphors have the highest PL intensity when x = 1.6, giving broad green emission due to the Eu2+ ions that occupy 10-coordinated sites in the lattice.64 Eu2+ also occupies nine-coordinated sites in the orthosilicates, which are only excitable with UV light.64 The Ba/Sr ratio also affects the thermal quenching behavior, with an increase in Ba concentration (x > 1.2) leading to a decrease in the thermal stability.63 The thermal quenching of (Ba/ Sr)2SiO4 is reported to be 30% at 150°C.65 Figure 12 shows the thermal quenching behavior of some silicate phosphors compared to garnet types and nitrides.66 The most thermally stable is Lu3Al5O12, with the least stable the orthosilicate types. There has been some success in reducing the thermal quenching with TiO2 coatings as demonstrated for Ba, Mgcodoped Sr2SiO4.67 (c) Nitride-Based Phosphors: Oxonitridosilicates, MIISi2O2N2 (M = Ca, Sr), have been identified more recently as a promising new class of phosphors with strong blue absorption, high quantum efficiency, and excellent thermal stability.40,68,69 These materials are comprised of (Si2O2N2)2
that is made up of layers of SiON3 tetrahedra.70 The orientation of silicate layers was found to affect the particle morphology and the peak emission wavelength of SrSi2O2N2: Eu2+ phosphors.71,72 Green emission from SrSi2O2N2:Eu2+ (FWHM = 78 nm, kmax = 538 nm) has been reported to exhibit quantum efficiencies >90%, even at temperatures above 200°C.68 The thermal quenching temperature of this phosphor is reported to be >230°C.70

The green-emitting nitridoaluminosilicate b-SIALON has an emission peak at 538 nm whose FWHM (55 nm) is narrower than that of other Eu2+-doped green emitters.73 The chemical formula of this lattice is Si6
zAlzOzN8
z, where z is the number of Al–O pairs that substitute for Si–N pairs.38 The value of z affects the phase purity and particle size and is typically ≤4.2. Like the oxonitridosilicates, this phosphor has low thermal quenching up to 150°C.38 The oxonitridoand aluminonitrido-silicate families are considered the most promising green-emitting phosphors for SSL and continue to be optimized. To summarize, the most promising green- and red-emitting compositions for blue-emitting LEDs are listed in Table III.

(5) Phosphors for Near-UV LED Approach Near-UV-LEDs (370–410 nm) display less current droop compared with the 450–470 nm blue-emitting LEDs, enabling better light extraction efficiency. Another advantage is that near-UV LEDs have less binning compared to blueemitting ones. Binning results from the variation in the emission wavelength across the wafer. The near-UV approach uses the “remote phosphor” configuration, where the phosphor is uniformly deposited onto a transparent substrate that is mounted above the LED, instead of being mixed in an epoxy dome, thus improving the light extraction efficiency by reducing the losses due to backscattering of the incident and emitted light into the chip (Fig. 4d). Another improvement to the extraction efficiency can be realized by using nanoparticles that will reduce light scattering. In addition, the ability to significantly improve the CRI and color temperature exists with the use of phosphor blends. The same requirements for phosphors for blue-emitting LEDs apply for the near-UV LEDs; the main challenge is to find multiple phosphor compositions that can be blended to achieve optimal CRI and CCT.

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Fig. 12. Thermal quenching of luminous output for several phosphor compositions. Data taken from Ref. [66].

Table III. Composition

Most Promising Phosphor Compositions for Blue and UV LED-Based Solid-State Lighting Excitation wavelength (nm)

Emission peak (nm)

QY (%)

Reference

Blue-emitting LEDs Green- to yellow-emitting SrSi2O2N2:Eu2+ b-SIALON Y3Al5O12:Ce3+ Orange- to red-emitting Sr2Si5N8:Eu2+ Blue-emitting LiCaPO4:Eu2+ Sr5(PO4)3Cl:Eu2+ (Sr,Ba)3Al4F:Eu2+ Green- to yellow-emitting (Ba,Sr)2SiO4:Eu2+ SrSi2O2N:Eu2+ Ba2MgSi2O7:Eu2+ Orange- to red-emitting Sr3SiO5:Eu2+

Sr2Si5N8:Eu2+ CaAlSiN3

450 450 450 450

539 535 546 625 Near UV-emitting LEDs

90 73 89 >90

400 345 400

475 445 472

330–380 390 380

510–570 570 505

400

580

96

400 450

610–680 650

95 75

(A) Identifying Phosphors for Near-UV-Emitting LEDs: There have been few investigations of phosphors that can be efficiently excited by near-UV wavelengths. One way to identify new phosphors is an empirical approach that

[70] [42] [136] [40,68]

88 90 82

[91,92] [85,137] [124,138]

88–95 91 92

[64,139] [123,124] [114] [127], [M.E. Hannah, A. Piquette, M. Anc, J. McKittrick, J. Talbot, J.K. Han, and K.C. Mishra, unpublished work (2013)] [39,140,141] [142,143]

has been developed by Dorenbos74–78 for rare-earth activators in wide band gap hosts. It was observed that the emission energy, Ee(Re3+), from a rare-earth ion is given by the following equation:

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Downconverters for SSL

Ee ðRe3þ Þ ¼ EF ðRe3þ Þ
DðAÞ
SðAÞ

(2)

Here, EF(Re3+) is the free ion energy, D(A) is the crystalfield depression in host A, and S(A) describes the Stokes shift in host A. Similar relations have been found to be valid for divalent ions. The validity of these expressions has been established both empirically and by a priori calculations for rare-earth ions in solids.74–79 Values of EF(Re3+) are available for most ions.79 An extensive list of D(A) has been tabulated by Dorenbos for trivalent rare-earth ions and a somewhat smaller list for divalent ions.80 As the above equation is based on observed transitions, the host–activator ion combinations chosen by this approach should lead to the discovery of new phosphors in a reliable manner. However, this approach does not indicate the required host in which the quantum efficiency will be high enough to meet the goal of >150 lm/W, nor does it address thermal or chemical stability. However, it is a starting point to explore new host:activator compositions. The phosphors developed to date have been based on Eu2+ or Ce3+ activation, due to the broad-band excitation

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range. In particular, emission from Eu2+ can range from UV to deep red, depending on the host, as shown in Fig. 13. The other 3+ rare earths yield narrow-band excitation ranges, which then are not efficiently excited due to the low oscillator strengths and the small emission wavelength change of the LED during operation (binning). (B) Blue-Emitting Phosphors: Blue-emitting phosphor compositions are critical as most blends contain a large proportion of the blue component. Most compositions have Eu2+ activators with emission peaks ranging from 420 to 490 nm. The best hosts are in the phosphate, halophosphate, halo-silicate, and silicate groups. Blue emission has been reported for nitrides and oxynitrides activated with Eu2+ or Ce3+, but the quantum efficiencies are low.81–83 The compositions with the highest QY are the halophosphates (apatites) having a generic formula A5(PO4)3Cl:Eu2+ (A = Sr, SCAP; Ba, BCAP; Ca, CCAP; or a mixture of the alkaline earth elements), which have QYs > 90%.84,85 The influence of Cl content on the PL emission intensity of Ca5(PO4)3Cl:Eu2+ [Fig. 14(a)] shows concentration quenching-type behavior indicating an optimal Cl concentration. Figure 14(b) shows the PL excitation and emission spectra of SCAP, CCAP, and BCAP, demonstrating strong absorption

Fig. 13. Emission wavelength of Eu2+-activated compounds. The horizontal dashed line at 360 nm separates the compounds in which Eu2+ shows f–f narrow line emission from the compounds showing d–f broadband emission. Line emission is limited to a small selection of fluoride and oxide compounds. Taken from Ref. [80].

(a)

(b)

Fig. 14. (a) Photoluminescence emission spectra of Ca5.17(PO4)3Clx:0.01Eu2+ with different Cl fractions (kex = 395 nm). (b) Photoluminescence excitation spectra (left, kem = 457, 447, and 436 nm) and emission spectra (right, kex = 395 nm) of Ca5.15(PO4)3Cl5:0.03Eu2+, Sr5.15(PO4)3Cl5:0.03Eu2+, and Ba5.15(PO4)3Cl5:0.03Eu2+. Taken from Ref. [86].

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Journal of the American Ceramic Society—McKittrick and Shea-Rohwer

in the 400 nm region and strong blue emission between 436– 457 nm, with CCAP as strongest emitter.86 CCAP displays little thermal quenching, retaining ~90% of the room temperature emission intensity at 200°C.87 Another phosphate family, ABPO4, A = alkali halide (Li, Na, or K) and B = alkaline earth elements (Ca, Sr, or Ba), have structures that depend on the sizes of the A and B ions (hexagonal, orthorhombic, or monoclinic).88–90 The peak emission wavelengths for the various compositions are shown in Fig. 15. Although the emission is strong under the excitation wavelength of 380 nm, the QY values [Figs. 15(a)–(c)] and peak emission wavelength [Fig. 15(d)] vary significantly. LiCaPO4 has the highest QY, whereas LiSrPO4 has the lowest. The emission wavelength ranges from 420 to 514 nm. The QY for LiCaPO4 ranges from 73% to 88%, depending on the processing condition.91,92 The micrometer-sized phosphors produced by solid-state reaction have higher QY values compared to the submicrometer-sized powders produced by sol–gel synthesis. The thermal stability of this family is excellent, retaining 80% of the room temperature luminescence intensity at 200°C.91 A high QY silicate is M3MgSi2O8:Eu2+ (M = Ca, Sr, Ba), where the SiO4 tetrahedra are connected in layers of MgO4 tetrahedra. The excitation spectrum consists of a broad band from 200 to 400 nm [J.K. Han, J. McKittrick, and J. Talbot, unpublished (2013)]. Blasse et al.93,94 studied the PL properties from Eu2+-activated M3MgSi2O8 ternary system (M: Ba, Sr, Ca), which showed a systematic decrease in peak emission wavelength depending on the M cation size. Barry95 studied mixtures of Eu2+-activated Ba3MgSi2O8, Sr3MgSi2O8, and Ca3MgSi2O8, and found that Ba3MgSi2O8 had the highest PL emission intensity and the shortest emission wavelength (437 nm, QY = 80%). Ca3MgSi2O8 had the lowest intensity and the longest peak emission wavelength (475 nm, QY = 40%) with Sr3MgSi2O8 falling between the two (458 nm). Sr3MgSi2O8 formed ideal solid solutions with

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both Ba3MgSi2O8 and Ca3MgSi2O8, but Ba3MgSi2O8 and Ca3MgSi2O8 did not, leading to an intermediate compound, BaCa2Si2O8. A solubility limit of x = 0.33 was found for (Sr1
xBax)3MgSiO8 and (Ca1
xBax)3MgSiO8,96 similar to the limit in (Ca1
xBax)3MgSiO8 found by Barry.95 The Ba-rich compositions were determined to have the trigonal glaseritetype structure, whereas the Ba-poor compositions have the monoclinic merwinite-type structure. Another 2D-layered SiO4–MgO4 structure is Sr2MgSi2O7:Eu2+, which shows emission at 476 nm97,98; however, the QY is low (15%).99 Other silicates are Li2MSiO4:Eu2+ (M = Ca, Ba). For Li2(Ca0.99Eu0.01)SiO4, there is a broad excitation band centered around 400 nm with emission at 460 nm.100 For Li2(Ba0.99Eu0.01)SiO4:B0.06, peak emission occurred at 490 nm.101 The addition of boric acid to the synthesis method increased the luminous intensity. Chalcogenides have smaller electronegativity values compared to oxides resulting in stronger crystal field splitting of Eu2+ or Ce3+ and the absorption band should extend into the visible. CaLaGa3S6O:Ce3+ phosphors were prepared by first converting precursor oxides to sulfides, then using a solid-state reaction to form the compound. There is a broad absorption band centered around 400 nm and two emission bands at 442 and 478 nm with an intensity of ~70% of that of the commercial blue-emitter (BaMgAl10O17:Eu2+).102 (C) Green/Yellow-Emitting Phosphors: There are numerous compositions with Eu2+-activated halo-silicates and silicates, and Ce3+ activated oxynitrides and halo-aluminates having the highest QY. (Sr1
xMx)3AlO4F:Ce3+ (M = Ca, Sr, Ba) has a tetragonal structure isostructural with Ce3CoCl5 with two different Sr2+ sites, having alternating layers of Sr(I)F3+ and Sr (II)2AlO43
.103,104 For x = 0, there is no absorption in the UV, but addition of Na resulted in the appearance of a strong absorption centered around 400 and increased the peak emission wavelength from 495 nm (10%) to 550 nm (20%) without

(a)

(b)

(c)

(d)

Fig. 15. Photoluminescence emission spectra of ABPO4:Eu2+ (B = Ca, Sr, or Ba) with quantum yields (%). (a) A = Li, (b) A = Na, and (c) A = K. (d) Peak emission wavelength as a function of composition. kex = 380 nm. Adapted from Ref. [91].

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Downconverters for SSL

significantly changing the absorption band.105 For x = 0.33 and y = 0.008, (Sr0.992
x
yBaxCey)3AlO4F has a broad excitation band centered around 400 nm and emission peak at 502 nm with a QY = 95% [Fig. 16(a)].103 The luminous intensity degraded by 50% at 200°C, comparable to YAG:Ce3+. A higher emission intensity was found for M = Ca then for M = Ba, as shown in Fig. 16(b) for a Ce concentration of y = 0.005.106 Unfortunately, these powders have poor chemical stability and degrade on contact with moisture.108 Solid solutions of Ce3+-activated (Sr2M)(AlO4F)1
x– (Sr2M)(SiO5)x (M = Sr, Ba) emit in the range 474–552 nm. The excitation and emission spectra for Sr2Ba(AlO4F)1
x (SiO5)x:Ce3+ are shown in Figs. 16(c) and (d). As x increases, the maximum peak excitation (398–409 nm) and emission (523–552 nm) wavelengths increase. The QY decreases and then increases with increasing x, which has a maximum of 70% and a minimum of 54%.107 Orthosilicates of M2SiO4:Eu2+ (M = Sr, Ba) have been found to have high QY, are thermally stable, and are relatively easy to synthesize.64,109–112 They have excellent absorption around 400 nm and emit from 510 to 570 nm, depending on the Sr/Ba ratio. The QY is reported to range from 88% to 95%. However, the luminescence intensity for M = Sr and Ca degrades to ~30% of the room temperature intensity at 140°C.113 Ba2MgSi2O7:Eu2+ demonstrates both high QY (92%) and good thermal stability. Figure 17(a) shows the excitation and emission spectra, illustrating the two absorption bands at 300

(a)

(c)

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and 395 nm and a broad emission band at 505 nm. Figure 17(b) shows that the compound is thermally stable, retaining 80% of the original emission intensity at 130°C. Substitution of Ca for Ba results in an increase in the peak emission wavelength (from 505 to 541 nm) with a significant reduction in QY (15%).115 However, substitution of Zn for Mg did not change the absorption or emission spectra.116 Orthorhombic Na3(Y1
xScx)Si3O9:Eu2+ phosphors show broad excitation bands between 250 and 400 nm and emission from 499 to 511 nm, depending on x, with a maximum absorption and emission for x = 0.4, as shown in Fig. 18.117 However, the emission intensity at 200°C was 20% of that at room temperature. Some potential aluminates include MAl2O4:Eu2+ (M = Sr, Ba), which absorb adequately around 400 nm and emit ~500 nm. Emission at 498 nm was found for BaAl2O4:Eu2+ that was formed in a thermal carbon-reducing atmosphere.118 The luminescence intensity of the 520 nm peak of SrAlO4: Eu2+ increased by 60% by the substitution of ~1% B on the Al sites.119 Addition of Zn to the B-doped powders (substituting for Sr) further increased the emission intensity by 80%,119 with a strong absorption band remaining from 350 to 400 nm. SrMgAl10O17:Eu2+, Mn2+ compositions were found to be color tunable from blue to green, depending on the Eu/Mn ratio. With no Mn, the peak emission wavelength was 470 nm with a broad absorption band from 250 to 400 nm. Addition of 4 to 36 at.% Mn led to the appearance of a green emission (b)

(d)

Fig. 16. (a) Excitation and emission spectra of (i) (Sr0.992
xBaxCe0.008)3AlO4F:Ce3+ (x = 0.33) at room temperature compared with a (ii) commercial YAG:Ce3+ phosphor. The PL emission spectrum of (Sr0.992
xBaxCe0.008)3AlO4F:Ce3+ (x = 0.33) at 77 K is also displayed. Taken from Ref. [103]. (b) Relative emission intensity of Sr3
x
3y/2MxCeyAlO4F as a function of M content (fixed Ce concentration, y = 0.005). Taken from Ref. [106]. Sr1.975Ce0.025Ba(AlO4F)1
x(SiO5)x: (c) excitation spectra and (d) emission spectra. Excitation/emission spectra were recorded using the maximum emission/excitation wavelength. Taken from Ref. [107].

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Vol. 97, No. 5

(b)

Fig. 17. Properties of Ba2MgSi2O7:Eu2+: (a) excitation (dash-dot line, kem = 505 nm) and emission spectra (solid line, kex = 395 nm). The emission spectrum of a near-UV chip is also shown (violet) and (b) temperature-dependent relative emission intensity. Taken from Ref. [114].

(a)

(b)

Fig. 18. Excitation and emission spectra Na3(Y1
xScx)Si3O9:Eu2+ phosphors: (a) as a function of Sc concentration (x = 0–1) and (b) as a function of Eu2+ concentration. Taken from Ref. [117].

band centered at 515 nm. As the concentration of Mn increased, the blue emission intensity decreased and was eventually eliminated at the highest concentration.120 Several excellent nitride or oxynitride compositions have been identified. b-SIALON (Si6
zAlzOzN8
z) activated with Eu2+ shows strong green emission under UV excitation, with a maximum peak wavelength of 535 nm with a QY = 54%. It is highly stable, retaining 90% of the room temperature emission intensity at 150°C.121 SrSi2O2N2:Eu2+ has a triclinic structure with SiON3
tetrahedra in a layered arrangement.122 There are four Sr sites coordinated with six oxygen ions. It has a broad excitation band from 350 to 450 nm and emission peak of 540 nm with a quantum efficiency of 90%.68,123 For MSi2O2
dN2+2/3d:Eu2+ (M = Ca, Sr, Ba) the d values ranged from 0 (M = Ca and Ba) to 1 (M = Sr). All three compounds were found to be monoclinic. The absorption spectra is maximum at ~400 nm and the peak emission wavelength is 580 nm (Ca), 570 nm (Sr), and 500 nm (Ba) for 10 mol% Eu.123 Other work has demonstrated that the emission color in (Sr1
xMx)Si2O2N2:Eu2+ (M = Ca, Ba) can be altered by changing the Eu concentration or by substituting Ca or Ba in the host lattice.124 CaSi2O2N2 has a monoclinic structure and BaSi2O2N2 has an orthorhombic structure. For (Sr1
yEuy)Si2O2N2, as y increased the peak emission wavelength increased from 535 to 554 nm and the QY varied between 58% (y = 0.16) and 91% (y = 0.02). The emission intensity is 90% of the room temperature value at 230°C. At 2% Eu, the peak emission wavelength was 560 nm for CaSi2O2N2 and 494 nm for BaSi2O2N2. For (Sr1
xCax)Si2O2N2:2%Eu2+, as x increased the peak emission wavelength increased from 538 to 555 nm, whereas the QY varied between 81% (x = 0.75) and 93% (y = 0.10–0.25). For (Sr1
xBax)Si2O2N2:2%Eu2+, as x

increased the peak emission wavelength increased from 538 to 564 nm, whereas the QY varied between 66% (x = 0.75) and 91% (y = 0.05, 0.5). (D) Red/Orange-Emitting Phosphors: Good red-emitting phosphors have been very difficult to develop. There are two main reasons for this: there are very few hosts in which Eu2+ or Ce3+ emit in the red, and historically most redemitting phosphors have been based on Eu3+, which is not an optimal activator for white-emitting LEDs. The forbidden 4f?4f transitions in Eu3+ have low oscillator strength (100 W/cm2, 150°C). In this section, two techniques to improve the photo- and thermal stability of core/shell QDs for SSL are discussed: strain-graded core/ shell interfaces and alloyed QDs. (A) II–VI Cd-Based Core/Shell QDs: The II–VI Cdcontaining QDs have been the most widely studied for luminescence-based applications, and are covered in recent review articles.18,21,23–25 Despite the high QYs that have been achieved from some QD cores,163–165 QD cores do not have long-term stability in solution nor are they protected from photo-oxidation. Passivating the QD cores with shells improves the photostability,160,161 thermal stability,166,167 quantum yield,168,169 and processability. However, the shells that passivate the QD cores create significant lattice mismatch strains, so their use can create significant core and shell stresses that can lead to the progressive nucleation of defects that act as nonradiative recombination centers and thus reduce the quantum yield. Strain-graded and alloyed

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Journal of the American Ceramic Society—McKittrick and Shea-Rohwer

(homogeneous and composition gradient) core/shell interfaces are able to reduce or eliminate such effects. The synthesis of core/shell QDs has advanced due to the development of Successive Ion Layer Absorption and Reaction (SILAR),170,171 a heterogeneous nucleation technique that uses more environmentally benign, air-stable precursors such as CdO and S, Se, and Te powders, and enables precise control over the composition of each atomic shell. In a typical SILAR synthesis of core/shell CdSe/CdS QDs, Cd, and S precursors are alternately injected into a solution of CdSe QDs at ~240°C.170 This reaction temperature favors the heterogeneous nucleation of CdS on the CdSe cores, rather than the homogeneous nucleation of CdS particles, which occurs at lower reaction temperatures. SILAR also enables the synthesis of QDs with multiple shells [Fig. 25(a)] and QD alloys [Fig. 25(b)]. (B) Strain-Graded II–VI Core/Shell Interfaces: ZnS is an ideal outer shell material for semiconductor QDs. Under continuous UV irradiation, a ZnS shell was found to impart greater photo-oxidation resistance to CdSe QDs than a CdS shell,171 which was attributed to the larger band gap difference between CdSe and ZnS, which lowers the density of photogenerated charge carriers on the QD surface. The fabrication of CdSe and CdTe QDs with an outer shell of ZnS requires a strain-graded approach because of the large lattice mismatch between ZnS and Cd(Se,Te). To minimize the lattice mismatch strains, spherical CdSe cores have been coated with CdS or ZnSe, followed by ZnS.172 Compared to core/ shell CdSe/ZnS, the core/shell/shell QDs were found to have

(a)

Vol. 97, No. 5

a narrower size distribution; improved crystallinity of the ZnS outer shell; control over the particle shape; and QYs as high as 80% (for CdSe/ZnSe/ZnS). Coating Type-II CdTe/ CdSe (
6.6% lattice mismatch) QDs with a type-I CdS shell (
4.0% lattice mismatch), followed by a ZnS shell (
7.0% lattice mismatch) yielded red-emitting QDs having reversible thermal quenching at 100°C with no thermal degradation.30 Figure 26 shows the PL emission of CdTe, CdTe/CdSe, and CdTe/CdSe/CdS QDs. The emission peak progressively redshifts as the number of shells increases due to the reduction in quantum confinement energy of the electron in the shell. Photostable CdSe nanorods have been demonstrated by coating them with CdS followed by ZnS shells.173 Spherical CdTe cores have been coated with CdS, followed by ZnS to produce core/shell/shell QDs that can be dispersed in water and have enhanced photostability.174 In another approach, the growth of multiple shell layers on 2–3 nm CdSe resulted in QDs up to 20 nm in size, having shells of ZnS or CdS or CdxZnyS alloys and have shown dramatic improvements in chemical and photostability and suppressed blinking.175 (C) Alloyed II–VI QDs: Ternary and quaternary alloyed QDs having homogeneous or graded compositions are interesting for lighting applications due to their composition-tunable band gap, enabling emission colors that are not obtainable from the parent binary semiconductors.176 CdSeTe QDs have comparable QYs and FWHM as core/ shell CdTe/CdSe QDs, but with red-shifted emission peaks.176 ZnxCd1
xSe QDs are blue- and green-emitting with QYs ranging from 70% to 85%.177 Quaternary alloys (ZnxCd1
xSySe1
y) have been reported with QYs of 40%– 60% and emission ranging from 440 to 650 nm.178 Shell interdiffusion in amine-capped CdSe/CdS/Cd0.5ZnS0.5/ZnS QDs led to a QY of 70%–85%171 due to the relaxation of the lattice strain. Water-soluble CdSe/ZnxCd1
xS QDs have been reported, with QYs of 35%.179 Most alloyed QDs are synthesized using core/shell synthesis methods.176,180–187 Alloyed QDs can be synthesized to be homogeneous or to have a composition gradient. Homogeneous CdSeTe QDs are synthesized under Cd-deficient conditions, with the final composition determined by the Se/Te molar ratio and the reactivities of Se and Te with Cd.176 Gradient alloys are formed under Cd-rich conditions.176 The faster reaction rate of Te with Cd results in a Te-rich core and a Se-rich surface. Homogeneous alloys can also be

(b)

Fig. 25. TEM images of (a) QD consisting of a CdTe core on which 10 atomic layers of CdSe have been deposited; and (b) composition-graded CdSe–ZnSe QDs [L.E. Shea-Rohwer, unpublished (2013)].

Fig. 26. Photoluminescence of CdTe cores; core/shell CdTe/CdSe; and core/shell/shell CdTe/CdSe/CdS quantum dots under 450 nm excitation. The PL red-shifts with each shell layer. Taken from Ref. [30]. Reproduced by permission of The Electrochemical Society.

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Downconverters for SSL

produced by heating core/shell QDs so that the semiconductors interdiffuse. Blue-emitting ZnxCd1
xS alloys with a Cdrich interior and Zn-rich outer layer have been reported.188 These alloys were synthesized using a noninjection approach. The ZnxCd1
xS alloys had a QY of 23%. Core/ shell Cd1
xZnxS/ZnS QDs were reported to have QYs as high as 80%.189,190 In this study, the shells were grown onto the alloyed QDs using similar procedures for shell growth on core QDs, but at higher temperatures during shell growth (310°C instead of 250°C). The temperature increase caused interdiffusion of the alloy and the shell, resulting in an increase in the QY. (D) Cd-Free QDs: Most work has been done on the Cd-based II–VI QDs, but the use of cadmium and other heavy metals in lighting applications is currently regulated under the EU Restriction of Hazardous Substances directive. The exemption on cadmium levels >100 ppm for SSL and displays is due to expire on July 1, 2014, so there is a growing need for suitable Cd-free alternatives.191 Two Cd-free material systems have shown significant promise for SSL, with efficient, narrow linewidth emission tunable over a wide range of the visible spectrum. These are as follows: (1) III–Vbased materials, of which InP has the greatest promise and (2) I–III–VI materials, such as CuInS2. InP/ZnS core/shell QDs (a Type I system) show emission maxima from 500 to 630 nm with QYs as high as 42%192,193 and as high as 70%,194 depending on the details of the synthesis. InP/GaP/ZnS QDs with extremely thin GaP layers195 have emission from 500 to 600 nm, show much improved thermal stability at 150°C (in comparison to InP/ZnS), and exhibit QYs as high as 85%. Seeded growth of InP on ZnSe nanocrystals results in ZnSe/InP/ZnS core/shell/shell heterostructures,196 which also give tunable emission from the green to the red with QYs as high as 60%. InP QDs are synthesized using solution-based methods similar to those used for the II–VI semiconductor QDs.197 Current syntheses of InP and InP/ZnS utilize P(SiMe3)3 as the phosphorus source, which is injected into an indium precursor solution containing indium chloride, stearic acid, zinc undecylenate, hexadecylamine, and 1-octadecene198,199; or combined with indium myristate, zinc stearate, dodecanethiol, and 1-octadecene, and subsequently heated to 300°C.194 InP and InP/ZnS QDs were synthesized at lower temperatures by injecting a solution of P(SiMe3)3, octylamine, and 1octadecene into a solution of indium acetate, myristic acid, and 1-octadecene at 180°C.200 InP shells were grown on ZnSe by slow addition of a mixture of In-tributyl phosphine and P (SiMe3)3 in octadecene to a degassed solution of ZnSe QDs, octadecene, and palmitic acid.201 For GaP shells, GaCl3 and oleic acid in octadecene has been used as a precursor.202 Currently, efforts are being made to scale up the reaction volumes192,193 and to better understand the experimental parameters that most affect the final product.201 ZnS–CuInS2 alloyed nanocrystals with Type I ZnS shells are another promising Cd-free system. CuInS2 QDs have a size-tunable emission range from 660 to 780 nm,202 which is far too red for SSL applications. However, alloying CuInS2 with ZnS to form cores and then applying a ZnS shell compositionally broadens the emission range from 518 to 810 nm, with published QYs as high as 56%–80%.203–205 CuInSe2 QDs typically emit in the near-IR,206 but can be made red-emitting through stoichiometry and size variations.207 The I–III–VI chalcopyrite semiconductor QDs, for example, CuInS2, are synthesized using solution-based methods and are typically coated or alloyed with II–VI semiconductors such as ZnS. Core/shell CuInS2/ZnS QDs were prepared in a two-step process where the core QDs were prepared by reacting indium acetate, copper iodide, myristic acid, and dodecanethiol in octadecene at 230°C, followed by the dropwise injection of a solution of zinc stearate and zinc ethylxanthate.202,204 To produce ZnS–CuInS2 alloyed QDs, copper, indium, and zinc acetates in stearic acid and octadecene were

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heated at 230°C in the presence of dodecanethiol, followed by injection of a sulfur-octadecene solution.203

III.

Phosphor/Quantum Dot Integration with the LED

The down converters must be coupled with the LED using methods that maximize the light extraction from the device. The luminescent material can be encapsulated in a polymer matrix; fabricated into plates or deposited onto transparent substrates, then bonded to the LED chip; or deposited onto the LED chip.

(1) Phosphor Integration (A) Polymer Encapsulation: In early commercial white LEDs, the LED chip and phosphor were encapsulated in a polymer, as shown in Fig. 27. Several significant problems were found with this LED design. First, the light output is not uniform as blue light “escapes” and is observed at the edges of the diode, thereby reducing the CRI. Secondly, the manufacturing steps are complicated. For example, the YAG:Ce phosphor is mixed with epoxy to form the “epoxy dome” surrounding the LED chip. The micrometer-sized phosphors lead to strong backscattering of the emission into the LED chip; poor beam collimation; and absorption losses in the phosphor itself. Uniformity of the mixture on the LED chip is difficult to achieve, as shown in Fig. 27, and this leads to significant variability in the color properties of such white LEDs. The use of nanoparticle phosphors would significantly reduce or eliminate multiple scattering. Transparent dispersions of YAG:Ce nanoparticles have been demonstrated.209,210 The transparency indicates that the particles are unagglomerated. Dispersions of nano-YAG:Ce show remarkably low light scattering. Recently, transparent films fabricated from nano-YAG:Ce3+ particles were found to have 82% transmission of 525 nm light compared to 3.8% transmission at 525 nm for films containing micrometer-sized YAG:Ce3+ particles.210 In another study, encapsulation of nano-YAG:Ce in epoxy did not cause quenching of the PL or spectral shifts.209 (B) Glass-Ceramic and Sintered Ceramic Phosphors: White LEDs that are based on polymer-encapsulated phosphors have short lifetimes because the polymer degrades when subjected to heat and moisture. The yellowing of the polymer resin that results leads to significant deterioration of the LED’s color quality. The glass-ceramic phosphor approach enables plate-like phosphors that can be integrated with LED chips without the use of polymer encapsulants. YAG:Ce glass-ceramic phosphors were prepared by melting

Fig. 27. Cross-sectional view of a blue-emitting LED with the phosphor particles suspended above, embedded in an epoxy dome. Adapted from Ref. [208].

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Journal of the American Ceramic Society—McKittrick and Shea-Rohwer

a mixture of SiO2, Y2O3, Al2O3, and Ce2O3 in a Pt crucible at 1500°C–1600°C for 5 h; casting the molten glass into a mold, followed by annealing at 1200°C–1500°C. The resulting YAG:Ce glass-ceramic was found to have ten times the thermal conductivity of epoxy resin, and maintained its color properties under temperature–humidity conditions, outperforming YAG:Ce-epoxy.211,212 Commercial white LEDs are now utilizing YAG:Ce in the form of plates instead of powders, which enables greater control over the color properties.213 Dense, translucent, luminescent ceramic (LumiramicTM, Philips, Eindhoven, the Netherlands) plates (~150 lm thick) are fabricated by sintering fine-grained powders. The plates are optically characterized and then matched with blue LED chips (whose peak emission wavelength varies) to achieve the desired color properties. Further improvements in the color quality are achieved by coating red-emitting phosphors onto the YAG:Ce plates.214 Transparent YAG:Ce plates (~100–800 lm thick) have been fabricated by vacuum sintering submicrometer YAG:Ce particles synthesized using coprecipitation.215 The sintered ceramic fabrication technique developed for YAG:Ce has been extended to other SSL phosphors. For instance, Eu2+-doped phosphors—green-emitting SrSi2O2N2 and amber-emitting (Ba, Sr)2Si5N8—have been fabricated as plates7 and used in warm white LEDs. (C) Electrophoretic Deposition: In addition to mixing the phosphor in a polymer to form a dome surrounding the LED [Fig. 28(a)], or forming glass-ceramic phosphor plates, there are two other methods to couple the phosphor with the diode: electrophoretic deposition (EPD) of the powders onto the diode directly [conformal distribution, shown in Fig. 28(b)] or on transparent substrates placed above the diode [remote phosphor configuration, Fig. 28(c)]. Electrophoretic deposition is a technique in which charged particles dispersed in a liquid are deposited onto a substrate under the force of an applied electric field [Fig. 29(a)]. EPD has many advantages besides its benign processing conditions, such as a high uniformity, the possibility of producing deposits fast and continuously, and a low level of contamination. Applications for this technology include components for batteries and solid oxide fuel cells, and wear and biocompatible coatings.217,218 Recent reviews of EPD, particularly considering nanoparticles, have been published.219–221

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Typically, the deposit thickness must be optimized by altering the bath chemistry, particle loading, applied voltage, and deposition time, so that it is thick enough to ensure full conversion (no unwanted near-UV light escapes) but not so thick as to reduce light output. As opposed to films for display technologies, films for SSL require 10–40 lm thickness for full conversion, which can create processing problems. Thick films typically require longer deposition times. At long times, settling of the powders has to be taken into account as the concentration of particles in the bath decreases with time. In addition, as the thickness increases, the effective electric field decreases due to the resistance of the insulating film. Other requirements are that the screen must be uniform to give consistent optical performance, the packing density should be optimized for the best light output, and the amount of nonluminescent materials (binders) should be minimized. Finally, the deposit must have sufficient adhesion strength to withstand handling during manufacturing, as well as during use. For producing white SSL devices, EPD has been used to deposit micrometer-sized phosphors onto substrates placed above an LED. Y3Al5O12:Ce3+ phosphor was deposited onto a substrate on top of blue LEDs.222 In other work, both layered and blended phosphor films using Eu-activated Ca-aSiAlON, b-SiAlON, and CaAlSiN3 (yellow-, green-, and redemitting, respectively) were deposited on a substrate that was placed on top of a blue LED.223 Recently, EPD was used to deposit both layered and blended phosphor films that generated white light using Eu2+-activated Sr2
xCaxSi5N8, Ba2SiO4, LiCaPO4, (Sr0.75Ba0.25)2SiO4, and (Sr0.5Ba0.5)3SiO5 (red-, green-, blue-, yellow-, and orange-emitting, respectively) with near-UV LEDs.106,216

(a)

(a) (b)

(b)

(c) (c)

Fig. 28. Possible distribution of phosphor powders. (a) Uniform distribution (current commercial design), (b) conformal distribution where the phosphor is deposited directly on the LED, and (c) remote phosphor distribution in which the phosphor particles are deposited on a transparent substrate located above the LED. Taken from Ref. [216].

Fig. 29. (a) Electrophoretic deposition setup. The phosphor powders are suspended in an electrolyte with an anode and cathode suspended in the solution. After a voltage is applied, the particles migrate either to an electrode and deposit particle by particle. (b) Top view SEM images of a conformal layer of phosphor that was deposited directly onto the LED chip. (c) Cross section of LED and phosphor layer, showing that three to four particle layers thick were deposited. Adapted from Ref. [16].

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Electrophoretic deposition can be readily adapted for coating a single composition or a triblend phosphor mixture. Figure 29(b) shows a SEM micrograph of a conformal layer of phosphors deposited directly on an LED chip by EPD. In Fig. 29(c), the cross section of the LED with the phosphor layer illustrates that three to four particle layers were deposited. These results indicate that EPD is an ideal technique to deposit either microcrystalline or nanocrystalline phosphors, either as a conformal layer or in a remote phosphor configuration.

(2) Quantum Dot Integration Quantum dots are not currently used in commercial white LEDs for SSL, but several studies have found that they can be polymer encapsulated and integrated with InGaN chips. UV aging studies of encapsulated QDs indicate that shells are essential for the QD stability in LEDs.160 The PL of epoxy-encapsulated CdSe/CdS/ZnS multishell QDs remained unchanged after 330 h of UV exposure (3 mW/cm2), whereas the encapsulated CdSe cores showed a 32 nm blue-shift and spectral broadening after 144 h. Red LEDs made from these CdSe/CdS/ZnS-epoxy composites dispensed onto a 390 nm InGaN LED had a power conversion efficiency, PCE (defined as the ratio of the emission power of the QDs to the absorbed UV power), of 48% which is better than red LEDs made with phosphors (PCE of 80%); high luminous efficacy of radiation, which enables a high color rendering index and a low correlated color temperature; thermal stability up to 150°C; chemical/moisture stability; photostability; low thermal quenching; long lifetime; and a small and uniform particle size to prevent multiple scattering.

Wide band gap oxides, nitrides, and oxynitrides have been identified as suitable host lattices for SSL phosphors. Currently, SSL phosphors employ Ce3+ and Eu2+ activators, having broad absorption bands that span the LED wavelengths of interest for SSL. However, the broad absorption is accompanied by broadband emission, which can extend into the deep red spectral region where the eye is insensitive. Depending on the host, Ce3+ and Eu2+ can yield emission colors across the visible spectrum. The challenge is to find high quantum efficiency compositions with the optimal color coordinates. The phosphors doped with these ions that have shown the most potential, thus far, for SSL are listed in Table III. There is a need for narrow red-emitting phosphors that satisfy the criteria for SSL. Eu3+ emits at ~610–615 nm which is optimal for warm white lighting, but has weak nearUV and blue light absorbance. Mn4+ dopants have broader absorption bands and narrow red emission, but host lattices in which Mn4+ exhibits optimal red emission have not been identified. Finding new narrow band red-emitting phosphors with strong near-UV and blue absorbance would significantly increase the luminous efficacy of SSL devices. Mostly micrometer-sized phosphors were discussed in this review, but there is interest in nanosized phosphors, which do not multiply scattering events. Reducing the particle size of phosphors to the nanoscale typically leads to a reduction in quantum yield (QY). A core/shell approach has been found to improve the photoluminescence emission intensity and QY of nanophosphors. Coating the nanophosphors with SiO2 shells not only passivates the surface defects that lead to nonradiative recombination but also reduces agglomeration and improves the moisture stability of the nanophosphors. Quantum dots have potential use in SSL, due to their high quantum yield, narrowband and tunable emission. However, improving the photostability and reducing the thermal quenching must be demonstrated if QDs are to find use in white SSL LEDs having high flux and temperatures (>100 W/cm2, 150°C). With advancements in QD synthesis such as the SILAR method, the traditional core/shell QDs are being replaced by new QD structures such as core/multishell QDs with strain-graded interfaces and alloyed QDs. Preliminary studies of such QD structures have shown promising improvements in photostability and reduced thermal quenching. Most QDs contain Cd, which poses toxicity concerns. Cdfree alternatives such as InP- and CuInS2-based QDs have shown promise. The narrowband red emission from InPbased QDs has been combined with green and yellow phosphor emissions to produce warm white light. Polymer encapsulation and electrophoretic deposition are methods of integrating the down conversion materials with LEDs. Early white LEDs were encapsulated by phosphors in epoxy domes. Current white LEDs utilize glass-ceramic and sintered ceramic phosphor plates, eliminating the problems with polymers such as photodegradation. QDs would also benefit from inorganic encapsulation processes.

Acknowledgments We thank Prof. Jan Talbot and Dr. Jae Ik Choi (UC San Diego), Dr. Jinkyu Han (Brookhaven National Lab), Drs. Kailash Mishra, Mark Hannah, Alan Piquette and Maria Anc (OSRAM-Sylvania Central Research) for valuable discussions. This work was supported by the U.S. Department of Energy grant DE-EE0002003. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. Work at Sandia was funded by Sandia’s Solid-State Lighting Science Energy Frontier Research Center and the U.S. Department of Energy, Office of Basic Energy Sciences.

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x-3y/2MxCeyAlO4F (M = Ca, Ba, 0 ≤ x ≤ 0.9, 0.001 ≤ y ≤ 0.05) Phosphors,” J. Lumin., 132, 2213–6 (2012). 107 K. A. Denault, N. C. George, S. R. Paden, S. Brinkley, A. A. Mikhailovsky, J. Neuefeind, S. P. DenBaars, and R. Seshadri, “A Green-Yellow Emitting Oxyfluoride Solid Solution Phosphor Sr2Ba(AlO4F)1
x(SiO5)x:Ce3+ for Thermally Stable, High Color Rendition Solid State White Lighting,” J. Mater. Chem., 22, 18204–13 (2012). 108 N. C. George, K. A. Denault, and R. Seshadri, “Phosphors for SolidState White Lighting,” Ann. Rev. Mater. Res., 43, 2.1–2.21 (2013). 109 J. K. Han, M. E. Hannah, A. Piquette, J. Micone. G. A. Hirata, J. B. Talbot, K. C. Mishra, and J. McKittrick, “Europium-Activated Barium/Strontium Silicates for Near-UV Light Emitting Diode Applications,” J. Lumin., 133, 184–7 (2013). 110 J. H. Lee and Y. J. Kim, “A Correlation Between a Phase Transition and Luminescent Properties of Sr2SiO4: Eu2+ Prepared by a Flux Method,” J. Ceram. Proc. Res., 10, 81–4 (2009). 111 J. K. Han, M. Hannah, G. A. Hirata, J. B. Talbot, K. C. Mishra, and J. McKittrick, “Structure Dependent Luminescence Characterization of Green– Yellow Emitting Sr2SiO4:Eu2+ Phosphors for Near UV LEDs,” J. Lumin., 132, 106–10 (2012). 112 J. K. Park, M. A. Lim, C. H. Kim, H. D. Park, J. T. Park, and S. Y. Choi, “White Light-Emitting Diodes of GaN-Based Sr2SiO4:Eu and the Luminescent Properties,” Appl. Phys. Lett., 82, 683–5 (2003). 113 J. S. Kim, Y. H. Park, S. M. Kim, J. C. Choi, and H. L. Park, “Temperature-Dependent Emission Spectra of M2SiO4:Eu2+ (M = Ca, Sr, Ba) Phosphors for Green and Greenish White LEDs,” Solid State Comm., 133, 445–8 (2005). 114 X. Zhang, J. Zhang, R. Wang, and M. Gong, “Photo-Physical Behaviors of Efficient Green Phosphor Ba2MgSi2O7:Eu2+ and Its Application in LightEmitting Diodes,” J. Am. Ceram. Soc., 93, 1368–71 (2010).

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115 L. Jiang, C. Chang, D. Mao, and C. Feng, “Concentration Quenching of Eu2+ in Ca2MgSi2O7:Eu2+ Phosphor,” Mater. Sci. Eng., B, 103, 271–5 (2003). 116 S. S. Yao, L. H. Xue, Y. Y. Li, Y. You, and Y. W. Yan, “Concentration Quenching of Eu2 + in a Novel Blue–Green Emitting Phosphor: Ba2ZnSi2O7: Eu2+,” Appl. Phys. B, 96, 39–42 (2009). 117 Z. Xia, J. Zhou, and Z. Mao, “Near UV-Pumped Green-Emitting Na3(Y, Sc)Si3O9:Eu2+ Phosphor for White-Emitting Diodes,” J. Mater. Chem. C, 1, 5917–24 (2013). 118 M. Peng and G. Hong, “Reduction from Eu3+ to Eu2+ in BaAl2O4:Eu Phosphor Prepared in an Oxidizing Atmosphere and Luminescent Properties of BaAl2O4:Eu,” J. Lumin., 127, 735–40 (2007). 119 K. Y. Jung, H. W. Lee, and H.-K. Jung, “Luminescent Properties of (Sr, Zn)Al2O4:Eu2+,B3+ Particles as a Potential Green Phosphor for UV LEDs,” Chem. Mater., 18, 2249–55 (2006). 120 G. Ju, Y. Hu, L. Chen, and X. Wang, “Photoluminescence Properties of Color-Tunable SrMgAl10O17:Eu2+, Mn2+ Phosphors for UV LEDs,” J. Lumin., 132, 1792–7 (2012). 121 N. Hirosaki, R. J. Xie, K. Kimoto, T. Sekiguchi, Y. Yamamoto, T. Suehiro, and M. Mitomo, “Characterization and Properties of Green-Emitting b-SiAlON:Eu2+ Powder Phosphors for White Light-Emitting Diodes,” Appl. Phys. Lett., 86, 211905, 3pp (2005). 122 H. H€ oppe, A. F. Stadler, O. Oeckler, W. Schnick, “Ca[Si2O2N2]—A Novel Layer Silicate,” Angew. Chem., 43, 5540–2 (2004). 123 Y. Q. Li, A. C. A. Delsing, G. de With, and H. T. Hintzen, “Luminescence Properties of Eu2 + -Activated Alkaline-Earth Silicon-Oxynitride MSi2O2
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144 K. Y. Jung, C. H. Lee, and Y. C. Kang, “Effect of Surface Area and Crystallite Size on Luminescent Intensity of Y2O3:Eu Phosphor Prepared by Spray Pyrolysis,” Mater. Lett., 59, 2451–6 (2005). 145 E. Zych, C. Brecher, and J. Glodo, “Kinetics of Cerium Emission in a YAG:Ce Single Crystal: The Role of Traps,” J. Phys. Condens. Matter, 12, 1947–58 (2000). 146 W. Zhao, S. Anghel, C. Mancini, D. Amans, G. Boulon, T. Epicier, Y. Shi, X. Q. Feng, Y. B. Pan, V. Chani, and A. Yoshikawa, “Ce3+ Dopant Segregation in Y3Al5O12 Optical Ceramics,” Opt. Mater., 33, 684–7 (2011). 147 L. A. Diaz-Torres, E. De la Rosa, P. Salas, and H. Desirena, “Enhanced Cooperative Absorption and Upconversion in Yb3+ Doped YAG Nanophosphors,” Opt. Mater., 27, 1305–10 (2005). 148 Y. Liu, W. Luo, R. Li, and X. Chen, “Spectroscopic Evidence of the Multiplesite Structure of Eu3+ Ions Incorporated in ZnO Nanocrystals,” Opt. Lett., 32, 566–8 (2007). 149 K. Kompe, H. Borchert, J. Storz, A. Lobo, S. Adam, T. Moller, and M. 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Downconverters for SSL

171 R. Xie, U. Kolb, J. Li, T. Basche, and A. Mews, “Synthesis and Characterization of Highly Luminescent CdSe-Core CdS/Zn0.5Cd0.5S/ZnS Multishell Nanocrystals,” J. Am. Chem. Soc., 127, 7480–8 (2005). 172 D. V. Talapin, I. Mekis, S. Gotzinger, A. Kornowski, O. Benson, and H. Weller, “CdSe/CdS/ZnS and CdSe/ZnSe/ZnS Core-Shell-Shell Nanocrystals,” J. Phys. Chem. B, 108, 18826–31 (2004). 173 L. Manna, E. C. Scher, L.-S. Li, and A. P. Alivisatos, “Epitaxial Growth and Photochemical Annealing of Graded CdS/ZnS Shells on Colloidal CdSe Nanorods,” J. Am. Chem. Soc., 124, 7136–45 (2002). 174 Y. He, H.-T. Lu, L.-M. Sai, Y.-Y. Su, M. Hu, C.-H. Fan, W. Huang, and L.-H. Wang, “Microwave Synthesis of Water-Dispersed CdTe/CdS/ZnS Core-Shell-Shell Quantum Dots with Excellent Photostability and Biocompatibility,” Adv. Mater., 20, 3416–21 (2008). 175 Y. Chen, J. Vela, H. Hton, J. L. Casson, D. J. Werder, D. A. Bussian, V. I. Klimov, and J. A. Hollingsworth, “‘Giant’ Multishell CdSe Nanocrystal Quantum Dots with Suppressed Blinking,” J. Am. Chem. Soc., 130, 5026–7 (2008). 176 R. E. Bailey and S. Nie, “Alloyed Semiconductor Quantum Dots: Tuning the Optical Properties Without Changing the Particle Size,” J. Am. Chem. Soc., 125, 7100–6 (2003). 177 M. D. Regulacio and M.-Y. Han, “Composition-Tunable Alloyed Semiconductor Nanocrystals,” Acc. Chem. Res., 43 [5] 621–30 (2010). 178 Z. T. Deng, H. Yan, and Y. Liu, “Band gap Engineering of QuarternaryAlloyed ZnCdSSe Quantum Dots via a Facile Phosphine-Free Colloidal Method,” J. Am. Chem. Soc., 131 [49] 17744–5 (2009). 179 Z. Fang, L. Liu, J. Wang, and X. Zhong, “Depositing a ZnxCd1
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xS Nanocrystals with Highly Narrow Luminescence Spectral Width,” J. Am. Chem. Soc., 125, 13559–63 (2003). 185 Y. C. Li, M. F. Ye, C. H. Yang, X. H. Li, and Y. F. Li, “Compositionand Shape-Controlled Synthesis and Optical Properties of ZnxCd1
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Journal of the American Ceramic Society—McKittrick and Shea-Rohwer

Joanna McKittrick has a B.S. in Mechanical Engineering from the University of Colorado, a M.S. in Materials Science and Engineering from Northwestern University and a Ph.D. in Materials Science and Engineering from MIT. She has worked in the area of luminescent materials for 20 years, first on cathode- and photoluminescent phosphors for heads up, field emission and plasma displays. More recently, her work has focused on luminescent materials for solid-state lighting, including the development of a phosphorless hetero junction and synthesis and characterization of new phosphor compositions that are efficiently excited by near UV LED light. She also has research interests in structural biological materials.

Vol. 97, No. 5

Lauren E Shea-Rohwer is currently a Principal Member of Technical Staff at Sandia National Laboratories in Albuquerque, NM. She received her Ph.D. in Materials Science from the University of California, San Diego in 1997. Her research interests include synthesis and characterization of nanoscale luminescent materials; the effects of the surface and interface characteristics on the quantum yield, photoluminescence decay, and thermal quenching of core/shell semiconductor quantum dots; phosphors having complex decay dynamics; narrowband redemitting phosphors; and rare-earth-free phosphors. She coorganized the 2nd International Symposium on Inorganic & Organic Luminescent Materials for Light-Emitting Diodes (Electrochemical Society). She is co-editor of the Handbook of Luminescence, Display Materials, and Devices, American Scientific Publishers (2003). She served as Chair of the Luminescence and Display Materials division of the Electrochemical Society.