SUPERTETRAHEDRON PHOSPHOR FOR SOLID-STATE LIGHTING

Abstract

The invention provides a lighting unit (100) comprising a light source (10), configured to generate light source light (11) and a luminescent material (20), configured to convert at least part of the light source light (11) into luminescent material light (51), wherein the luminescent material (20) comprises a phosphor (40), wherein this phosphor comprises an alkaline earth aluminum nitride based material having a cubic crystal structure with T5 supertetrahedra, wherein the T5 supertetrahedra comprise at least Al and N, and wherein the alkaline earth aluminum nitride based material further comprises a luminescent lanthanide incorporated therein.

Claims

1. A lighting unit comprising a light source, configured to generate light source light and a luminescent material, configured to convert at least part of the light source light into luminescent material light, wherein the luminescent material comprises a phosphor, wherein this phosphor comprises an alkaline earth aluminum nitride based material having a cubic crystal structure with T5 supertetrahedra, wherein the T5 supertetrahedra comprise at least Al and N, and wherein the alkaline earth aluminum nitride based material further comprises a luminescent lanthanide incorporated therein, wherein the alkaline earth aluminum nitride based material comprises
[M.sub.mxyA.sub.a]{G.sub.gQ.sub.qD.sub.dN.sub.55ncR.sub.nC.sub.c}:ES.sub.x,RE.sub.y wherein M comprises one or more selected from the group consisting of Ca, Sr, Mg, Ba; A comprises one or more selected from the group consisting of Li, Na; G comprises one or more selected from the group consisting of Al, Ga, B, wherein G at least comprises Al; Q comprises one or more selected from the group consisting of Mg, Mn, Zn; D comprises one or more selected from the group consisting of Si, Ge; R comprises one or more selected from the group consisting of O, S; ES comprises one or more selected from the group consisting of Eu, Yb, Sm; RE comprises one or more selected from the group consisting of Ce, Pr, Nd, Sm, Eu (III), Gd, Tb, Dy, Ho, Er, Tm; 0<m30; 0x2; 0y1; 0<x+y3; 0<g39; 0q5; 0d12; 0n5; 0c12 26m+a30; g+q+d=39; 2(m+q)+3(y+g)+a+4d=165n+c.

2. The lighting unit according to claim 1, wherein (a) the luminescent lanthanide is selected from the group consisting of Eu (II), Sm, Yb, Ce, Pr, Nd, Sm, Eu (III), Gd, Tb, Dy, Ho, Er, and Tm, and wherein (b) the alkaline earth aluminum nitride based material is of the space Fd-3m.

3. The lighting unit according to claim 1, wherein the T5 supertetrahedra comprise AlN.sub.4 tetrahedra.

4. The lighting unit according to claim 1, wherein G=Al.

5. The lighting unit according to claim 1, wherein M comprises one or more of Ca, Sr, and Mg, wherein A comprises Li, wherein G at least comprises Al, wherein Q comprises Mg, wherein D comprises Si, wherein R comprises O, wherein Es comprises Eu, and wherein RE comprises Ce, wherein further x/y<0.1 or y/x<0.1, and wherein d=n=c=0.

6. The lighting unit according to claim 1, wherein the light source comprises a light emitting diode (LED), and wherein the alkaline earth aluminum nitride based material comprises M.sub.(20)A.sub.(8+2)Al.sub.39N.sub.55:EU, with in the range of 0-2.

7. The lighting unit according to claim 1, wherein the luminescent material further comprise one or more other phosphors selected from the group consisting of a divalent europium containing nitride luminescent material, a divalent europium containing oxonitride luminescent material, a trivalent cerium containing garnet and a trivalent cerium containing oxonitride, and wherein the light source is configured to generate blue light.

8. A phosphor comprising an alkaline earth aluminum nitride based material having a cubic crystal structure with T5 supertetrahedra, wherein the T5 supertetrahedra comprise at least Al and N, and wherein the alkaline earth aluminum nitride based material further comprises a luminescent lanthanide incorporated therein, wherein the alkaline earth aluminum nitride based material comprises
[M.sub.mxyA.sub.a]{G.sub.gQ.sub.qD.sub.dN.sub.55ncR.sub.nC.sub.c}:ES.sub.xRE.sub.y wherein M comprises one or more selected from the group consisting of Ca, Sr, Mg, Ba; A comprises one or more selected from the group consisting of Li, Na; G comprises one or more selected from the group consisting of Al, Ga, B, wherein G at least comprises Al; Q comprises one or more selected from the group consisting of Mg, Mn, Zn; D comprises one or more selected from the group consisting of Si, Ge; R comprises one or more selected from the group consisting of O, S; ES comprises one or more selected from the group consisting of Eu, Yb, Sm; RE comprises one or more selected from the group consisting of Ce, Pr, Nd, Sm, Eu (III), Gd, Tb, Dy, Ho, Er, Tm; 0<m30; 0x2; 0y1; 0<x+y3; 0<g39; 0q5; 0d12; 0n5; 0c12 26m+a30; g+q+d=39; 2(m+q)+3(y+g)+a+4d=165n+c.

9. The phosphor according to claim 8, wherein (a) the luminescent lanthanide is selected from the group consisting of Eu (II), Sm, Yb, Ce, Pr, Nd, Sm, Eu (III), Gd, Tb, Dy, Ho, Er, and Tm, and wherein (b) the alkaline earth aluminum nitride based material is of the space Fd-3m.

10. The phosphor according to claim 8, wherein the T5 supertetrahedra comprise AlN.sub.4 tetrahedra.

11. The phosphor according to claim 8, wherein G=Al.

12. The phosphor according to claim 8, wherein M comprises one or more of Ca, Sr, and Mg, wherein A comprises Li, wherein G at least comprises Al, wherein Q comprises Mg, wherein D comprises Si, wherein R comprises O, wherein Es comprises Eu, and wherein RE comprises Ce, wherein further x/y<0.1 or y/x<0.1, and wherein d=n=c=0.

13. The phosphor according to claim 8, wherein the phosphor comprises phosphor particles having a coating, wherein the coating comprises one or more coating selected from the group consisting of an AlPO.sub.4 coating, an Al.sub.2O.sub.3 coating and a SiO.sub.2 coating.

14. The phosphor according claim 8, wherein the alkaline earth aluminum nitride based material comprises M.sub.(20)A.sub.(8+2)Al.sub.39N.sub.55:EU, with in the range of 0-2.

15. An LCD display device comprising the lighting unit according claim 1 configured as backlighting unit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0081] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

[0082] FIGS. 1a-1d schematically depict some embodiments of the lighting unit; the drawings are not necessarily on scale;

[0083] FIG. 2 shows the powder XRD pattern (M) and simulated (S) diagram. The minor phase AlN is tagged with asterisks. Note that the simulated (S) and measured (M) are nearly identical. Reference D indicates the difference between measured (M) and simulated (S). Reference Ca1 indicates the calculated XRD reflection positions;

[0084] FIG. 3 shows a sketched structure of the invented phosphor showing the characteristic structural feature of connected T5 supertetrahedra with the general chemical sum M.sub.35X.sub.56 consisting of 35 regular AlN4-tetrahedra. The small AlN4-tetrahedra (single tetrahedron in a layer in the middle between the upper layer with four T5 supertetrahedra pointing up and the lower layer with four T5 supertetrahedra pointing down) acting as bridge between the T5-supertetrahedron structures. Each supertetrahedron is built up by corner-sharing AlN4 subunits;

[0085] FIGS. 4a-4c show sketched structures of the invented phosphor showing the three different Calcium coordination sites Ca1 (Wyckoff position 96h), Ca2 (Wyckoff position 48f) and Ca3 (Wyckoff position 16d) filling interstitial space between the T5-supertetrahedra-lattice framework structure. Two of the three different Calcium sites Ca1 (Wyckoff position 96h) and Ca3 (Wyckoff position 16d) coordinate octahedrally (FIG. 4a,c), the third Calcium site Ca2 (Wyckoff position 480 coordinates trigonal prismatically (FIG. 4b) The Ca3-site (Wyckoff position 16d) can be occupied partially.

[0086] FIGS. 5a-5b shows excitation and emission spectra, respectively, of Ca.sub.18.75xLi.sub.10.5Al.sub.39N.sub.55:Eu.sub.x at different Eu concentrations (the values in the graph are the molar doping levels corresponding to the x values indicated in the description below); and

[0087] FIG. 6 shows an EELS spectrum of the invented phosphor material. The energy loss region of LiK edge, Al-L.sub.2,3 and Al-L.sub.1 is shown. The background left to the LiK edge has been subtracted. EL indicates energy loss (in eV); I indicates the intensity (in arbitrary units).

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0088] FIG. 1a schematically depicts an embodiment of the lighting unit, indicated with reference 100, of the invention. The lighting unit comprises a light source 10, which is in this schematic drawing a LED (light emitting diode). In this embodiment, on top of the light source 10, here on the (light exit) surface 15, thus downstream of the light source 10, a luminescent material 20 is provided. This luminescent material 20 comprises phosphor as described herein, indicated with reference 40. By way of example, the lighting unit 100 further comprises, for instance for light extraction properties, a (transmissive) dome 61. This is an embodiment of a transmissive optical element 60, which is in this embodiment arranged downstream of the light source 10 and also downstream of the light conversion layer 20. The light source 10 provides light source light 11 (not indicated in the drawing), which is at least partly converted by the light conversion layer 20, at least by phosphor 40, into luminescent material light 51. The light emanating from the lighting unit is indicated with reference 101, and contains at least this luminescent material light 51, but optionally, dependent upon the absorption of luminescent material 50 also light source light 11. FIG. 1b schematically depicts another embodiment, without dome, but with an optional coating 62. This coating 62 is a further example of a transmissive optical element 60. Note that the coating 62 may in an embodiment be one or more of a polymeric layer, a silicone layer, or an epoxy layer. Alternatively or additionally a coating of silicon dioxide and/or silicon nitride may be applied. In both schematically depicted embodiment of FIGS. 1a-1b, the luminescent material 20 is in physical contact with the light source 10, or at least its light exit surface (i.e. surface 15), such as the die of a LED. In FIG. 1c, however, the luminescent material 20 is arranged remote from the light source 10. In this embodiment, the luminescent material 20 is configured upstream of a transmissive (i.e. light transmissive) support 30, such as an exit window. The surface of the support 30, to which the light conversion layer 20 is applied, is indicated with reference 65. Note that the luminescent material 20 may also be arranged downstream of the support 30, or at both sides of the support luminescent material 20 may be applied. The distance between the luminescent material 20 and the light source (especially its light exit surface 15) is indicated with reference d1, and may be in the range of 0.1 mm-10 cm. Note that in the configuration of FIG. 1c, in principle also more than one light source 10 may be applied. FIG. 1d is schematically the same as FIG. 1c, but now with a plurality of light sources 10. Optionally, the luminescent material is shaped into a self-supporting layer, such as a ceramic material. In such instance, the transmissive optical element 60 may not be necessary, but may nevertheless be present.

[0089] The novel phosphor disclosed herein is obtained by a solid-state reaction. For the preparation of Eu.sup.2+-doped Ca.sub.18.75Li.sub.10.5Al.sub.39N.sub.55 stoichiometric amounts of the starting materials Calcium hydride, Lithium nitride, Aluminum and Europium fluoride are mixed. The concentration of Eu.sup.2+ in the mixture is 0.5 mole % based on the Calcium amount. Subsequently, the mixture is heated in nitrogen for 5 hours at 1250 C.

[0090] The novel phosphor was indexed as cubic lattice from single crystal X-ray pattern (using Mo-K radiation) with the resulting formula Ca.sub.18.75Li.sub.10.5Al.sub.39N.sub.55. The crystallographic data of Ca.sub.18.75Li.sub.10.5Al.sub.39N.sub.55 are visible in table 1, the atomic coordinates, isotropic displacement parameters and Wyckoff positions in table 2 (both obtained from single-crystals).

TABLE-US-00001 TABLE 1 Crystallographic data of the invented phosphor Ca.sub.18.75Li.sub.10.5Al.sub.39N.sub.55 obtained from a single-crystal: Formula Ca.sub.18.75Li.sub.10.5Al.sub.39N.sub.55 Crystal system cubic Space group Fd-3m (no. 227) Lattice parameters/ a = b = c = 22.415(3) Cell volume/.sup.3 11263(2) Formula units/cell 8

TABLE-US-00002 TABLE 2 Atomic coordinates, isotropic displacement parameters (in .sup.2) and Wyckoff positions of Ca.sub.18.75Li.sub.10.5Al.sub.39N.sub.55: Eu.sup.2+, standard deviations in parentheses (obtained from a single-crystal): Atom Wyckoff Site SOF x y z U.sub.eq Ca1 96h ..2 0 0.35133 0.64867 0.01111(14) Ca2 48f.sup. 2.mm 0.2948 0.01019(16) Ca3 16d .3m 0.375 0.0139(8) Al1 96g ..m 0.27368 0.27368 0.17825 0.00701(17) Al2 96g ..m 0.27747 0.27747 0.37409 0.00713(18) Al3 48f.sup. 2.mm 0.43026 0.0075(2) Al4 32e .3m 0.04659 0.04659 0.04659 0.0071(3) Al5 32e .3m 0.17601 0.17601 0.17601 0.0071(3) Al6 8b 43m 0.0070(5) N1 96g ..m 0.02341 0.02341 0.32571 0.0082(4) N2 96g ..m 0.02561 0.02561 0.12678 0.0078(4) N3 96g ..m 0.07474 0.07474 0.47744 0.0084(4) N4 96g ..m 0.32478 0.32478 0.13032 0.0085(4) N5 32e .3m 0.32638 0.32638 0.32638 0.0068(7) N6 16c .3m 0 0.sup. 0.sup. 0.0088(11) N7 8a 43m 0.0091(15) Li1 192i.sup. 1 0.219 0.01804 0.206 0.32236 0.027(6) Li2 96h ..2 0.438 0 0.43483 0.56517 0.027(6)

[0091] The Rietveld refinement of the crystallographic data from powder XRD measurements (Fd-3m, Z=8, a=22.3609(3) ) confirmed the data obtained from single crystals (Fd-3m, Z=8, a=22.415(3) ). The powder XRD of the invented cubic phase is visible in FIG. 2. As minor phase, AlN is detected (marked with asterisks).

[0092] In FIG. 3, the structure of the invented phosphor is sketched showing the characteristic structural feature of a T5 supertetrahedral cluster with the general chemical sum M.sub.35X.sub.56 consisting of 35 regular AlN.sub.4-tetrahedra. The intermediate AlN.sub.4-tetrahedra dimers acting as bridge between the T5-supertetrahedron structures. There may be three different Calcium coordination sites and two different Lithium sites. Both cations (Ca.sup.2+ and Li.sup.+) are filling the interstitial space between the T5 supertetrahedra lattice framework structure.

[0093] Two of the three different Calcium sites Ca1 (Wyckoff position 96h) and Ca3 (Wyckoff postion 16d) coordinate octahedrally (FIG. 4a,4c), the third Calcium site Ca2 (Wyckoff postion 48f) coordinates trigonal prismatically (FIG. 4b) The Ca3-site (Wyckoff position 16d) can be occupied partially. As a consequence, the Calcium and Lithium content can vary between two extremes as follows:

Ca.sub.20Li.sub.8Al.sub.39N.sub.55Ca.sub.18Li.sub.12Al.sub.39N.sub.55

[0094] The general chemical formula is Ca.sub.(20)Li.sub.(8+2)Al.sub.30N.sub.55 (with in the range of 0-2). Compared to the published Na.sub.26Mn.sub.39O.sub.55 structure (published by Mller et al., see above), the quadratic pyramidal coordinating Na3-sites (Wyckoff position 96g) are not occupied by the adequate Calcium cations (Ca2-sites). However, these Calcium cations coordinate additionally in a trigonal prismatical position (Wyckoff position 48f) which is not found for the adequate Na-site in the oxide structure.

[0095] The photoluminescence spectra (FIG. 5b) of such a phosphor reveal a narrow red emission with a peak located at around 647 nm and a FWHM of about 1140 cm.sup.1. A broad absorption in the blue spectra region is visible in the reflection and photoluminescence excitation spectra (monitored at 650 nm).

[0096] It is believed that the larger Eu(II) ion preferably occupies the larger trigonal prismatically coordinated cation site (Ca2, Wyckoff position 48f). It is further believed that other larger alkaline earth cations like Sr(II) are incorporated preferably on this position.

[0097] Part or all of the Li(I) and Al(III) can e.g. be substituted by Mg(II) to form e.g. a material of composition Ca.sub.20xMg.sub.6[Al.sub.35Mg.sub.4N.sub.55]:Eu.sub.x. Part of the Ca(II) can e.g. be substituted by Sr(II) to shift the absorption and emission band positions of Eu(II) towards higher energies. Examples are e.g. compositions of stoichiometry Ca.sub.14ST.sub.6xLi.sub.8[Al.sub.30N.sub.55]:Eu.sub.x.

[0098] Part or all of the Eu(II) dopant can be replaced by Ce(III) which shows absorption centered in the 430-480 nm spectral range and emission in the 510-570 nm range. Increasing the Ce concentration shifts the emission towards longer wavelengths. If both activators, Eu(II) and Ce(III) are present in the structure, emission in the green to yellow and in the red spectral range is obtained. Charge compensation for Ce(III) replacing e.g. Ca(II) in the structure can be realized by e.g. adjusting the Ca/Li ratio in the structure. An example for such an embodiment is e.g. Ca.sub.18.5Li.sub.9.5Al.sub.39N.sub.55:Ce.sub.0.5. Another example is e.g. Ca.sub.18.4Li.sub.0.5Al.sub.39N.sub.55:Ce.sub.0.5Eu.sub.0.1.

[0099] Low oxygen contents of the claimed phosphors are being preferred to maximize the desired emission properties, however, smaller amounts of oxygen incorporated e.g. via the starting materials can be tolerated in the structure by e.g. formal substitution of [AlN] pairs in the T5 supertetrahedra structure by [MgO] pairs. Accordingly, Mn(II) or Zn(II) can be incorporated in the tetrahedral network.

[0100] Incorporation of tetravalent ions like Si in the T5 supertetrahedra network to e.g. further increase the lattice stability at elevated temperatures or to modify the host lattice band gap structure can be realized by e.g. formal substitution of [AlN] pairs in the T5 supertetrahedra structure by [SiC] pairs or by e.g. replacing part of the Ca(II) by monovalent Na. Examples for such compositions are e.g. Ca.sub.17.75Li.sub.10.5Si.sub.18 Al.sub.21C.sub.18N.sub.37:Eu.sub.1.0 or Ca.sub.13.75Na.sub.2Li.sub.10.5Si.sub.2Al.sub.37N.sub.55:Eu.sub.3.0.

[0101] Variying the Eu doping level leads to a slight shift in emission color due to change in emitted light reabsorption. FIG. 5b shows photoluminescence measurement data for Ca.sub.18.75xLi.sub.10.5Al.sub.39N.sub.55:Eu.sub.x (x=0.188, 0.094, 0.038). Lowering the Eu doping level leads to a slight blue shift of the emission band and a decrease in absorption strength in the UV to green spectral range. The excitation maximum is located at 525 nm in the green spectral range (see FIG. 5a and table 3).

TABLE-US-00003 TABLE 3 Emission characteristics as function of the dopin level of divalent Eu Doping level/mol % Emission peak/nm FWHM/cm.sup.1 0.2 646 1120 0.5 647 1140 1.0 650 1135

[0102] To confirm the presence of Li in the structure, EELS measurements were done in a transmission electron microscope (TEM) with an accelerating voltage of 300 kV. The LiK edge in FIG. 6 occurs at around 56.5 eV and a main peak at 61.6 eV. The Al-L.sub.2,3 and Al-L.sub.1 edges can be seen in the spectrum, but they overlap with the higher energy loss region of the LiK edge. The Al-L.sub.2,3 edge shows a maximum peak at 82.6 eV. The values of LiK and Al-L.sub.2,3 edges are in good accordance with data known from the literature (Li.sub.2CaSi.sub.2N.sub.4 and Li.sub.2SrSi.sub.2N.sub.4 A Synthetic Approach to Three-Dimensional Lithium Nitridosilicates, M. Zeuner, S. Pagano, S. Hug, P. Pust, S. Schmiechen, C. Scheu, W. Schnick, Eur. J. Inorg. Chem. 2010, 4945-4951; Near-Edge Structure of Metal-Alumina Interfaces, Scheu, C., et al., Electron Energy-Loss Microsc. Microanal. Microstruct. 6, 19-31, (1995)). The EELS measurements clearly show the presence of Li and Al being integrated in the structure. The EELS data were detected with an energy resolution of about 0.9-1.2 eV as determined by the FWHM of the zero loss peak. Reference O indicates the onset. The data were obtained with a dispersion of 0.3 eV/channel. The acquisition time was 10 s for the LiK, Al-L.sub.2,3 and Al-L.sub.1 edges. All data were corrected for channel-to-channel gain variation and dark current.

[0103] The pre-edge background of the LiK was extrapolated by the use of a 1.sup.st order-log-polynomial function and subtracted from the original spectra.