Abstract
The invention provides a method for providing luminescent particles (100) with a hybrid coating, the method comprising (i) providing a first coating layer (110) onto the luminescent particles (100) by application of a sol-gel coating process, thereby providing coated luminescent particles; and (ii) providing a second coating layer (120) onto the coated luminescent particles by application of an atomic layer deposition process. The invention also provides luminescent particles (100) comprise a luminescent core (102), a first coating layer (110) having a first coating layer thickness (d1) in the range of 50-300 nm, and a second coating layer (120) having a second coating layer thickness (d2) in the range of 5-250 nm.
Claims
1. A method for providing luminescent particles with a hybrid coating, the luminescent particles comprising a core of a nitride luminescent material and oxide layer at the surface of the core, the method comprising: depositing a first coating layer onto the oxide layer of the luminescent particles by application of a sol-gel coating process, thereby providing coated luminescent particles; and depositing a second coating layer onto the coated luminescent particles by application of an atomic layer deposition process using a metal oxide precursor selected from a group of metal oxide precursors of metals selected from the group consisting of Al, Hf, Ta, Zr, Ti and Si, the atomic layer deposition process forming a multilayer comprising alternating layers comprising Ta.sub.2O.sub.5 and Al.sub.2O.sub.3.
2. The method according to claim 1, wherein the luminescent material selected from the SrLiAl.sub.3N.sub.4:Eu.sup.2+ class.
3. The method according to claim 1, wherein the sol-gel coating process comprises: providing a mixture of an alcohol, ammonia, water, the luminescent particles and a metal alkoxide precursor while agitating the mixture, and allowing the first coating to be formed on the luminescent particles, wherein the metal alkoxide precursor is selected from the group consisting of a titanium alkoxide, a silicon alkoxide, and an aluminum alkoxide; and retrieving the luminescent particles from the mixture and subjecting the luminescent particles to a heat treatment to provide the coated luminescent particles.
4. The method according to claim 1, wherein in the sol-gel coating process a silicon alkoxide precursor is used, wherein the silicon alkoxide precursor is selected from the group of compounds consisting of: ##STR00004## wherein R1, R2, R3 are selected from the group consisting of hydrolysable alkoxy moieties and R4 is selected from the group consisting of C1-C6 linear alkyl moieties, hydrolysable alkoxy moieties, and a phenyl moiety.
5. The method according to claim 4, wherein the silicon alkoxide precursor is selected from the group consisting of ##STR00005## and wherein in the atomic layer deposition process a metal oxide precursor selected from the group consisting of Al(CH.sub.3).sub.3, HAl(CH.sub.3).sub.2, Hf(N(CH.sub.3).sub.2).sub.4, Hf(N(CH.sub.2CH.sub.3).sub.2).sub.4, Hf[N(CH.sub.3)(CH.sub.2CH.sub.3)].sub.4, TaCl.sub.5, Ta(N(CH.sub.3).sub.2).sub.5, Ta{[N(CH.sub.3)(CH.sub.2CH.sub.3)].sub.3N(C(CH.sub.3).sub.3)}, ZrCl.sub.4, Zr(N(CH.sub.3).sub.2).sub.4, TiCl.sub.4, Ti(OCH.sub.3).sub.4, Ti(OCH.sub.2CH.sub.3).sub.4, SiCl.sub.4, H.sub.2N(CH.sub.2).sub.3Si(OCH.sub.2CH.sub.3).sub.3, and Si(OCH.sub.2CH.sub.3).sub.4, and an oxygen source selected from the group consisting of H.sub.2O and O.sub.3 are applied.
6. The method of claim 1 wherein depositing the first coating layer comprises depositing the first coating layer having a first coating layer thickness (d1) in a range of about 20 nm to about 500 nm; and wherein depositing the second coating layer comprises depositing the second coating layer having a second coating layer thickness (d2) in a range of about 5 nm to about 250 nm.
7. The method of claim 1, wherein the luminescent particles are smaller than 1 micron.
8. A method for providing luminescent particles with a hybrid coating, the luminescent particles comprising a core of a nitride luminescent material and oxide layer at the surface of the core, the method comprising: depositing a first coating layer onto the oxide layer of the luminescent particles by application of a sol-gel coating process, thereby providing coated luminescent particles; and depositing a second coating layer onto the coated luminescent particles by application of an atomic layer deposition (ALD) process, the atomic layer deposition process forming a multilayer comprising alternating layers, the alternating layers comprising a first ALD layer comprising Al.sub.2O.sub.3 and a second ALD layer comprising a metal oxide selected from a group consisting of Ta.sub.2O.sub.5, ZrO.sub.2, TiO.sub.2, Y.sub.2O.sub.3, Nb.sub.2O.sub.5, and HfO.sub.2.
9. The method of claim 8 wherein the second ALD layer comprises a metal oxide selected from a group consisting of Ta.sub.2O.sub.5, HfO.sub.2 and ZrO.sub.2.
10. The method of claim 9 wherein a thickness of each of the first and second ALD layers is less than 10 nm.
11. The method of claim 10 wherein the luminescent material is selected from the SrLiAl.sub.3N.sub.4:Eu.sup.2+ class.
12. The method of claim 10 wherein depositing the first coating layer comprises depositing the first coating layer having a first coating layer thickness (d1) at least 50 nm; and wherein depositing the second coating layer comprises depositing the second coating layer having a second coating layer thickness (d2) in a range of about 5 nm to about 250 nm.
13. The method of claim 8 wherein the second ALD layer comprises Ta.sub.2O.sub.5.
14. The method of claim 8 wherein the second ALD layer comprises HfO.sub.2.
15. The method of claim 8 wherein a thickness of each of the first and second ALD layers is less than 10 nm.
16. The method of claim 15 wherein the luminescent material is selected from the SrLiAl.sub.3N.sub.4:Eu.sup.2+ class.
17. The method of claim 15 wherein depositing the first coating layer comprises depositing the first coating layer having a first coating layer thickness (d1) at least 50 nm; and wherein depositing the second coating layer comprises depositing the second coating layer having a second coating layer thickness (d2) in a range of about 5 nm to about 250 nm.
18. The method of claim 8 wherein the luminescent material is selected from the SrLiAl.sub.3N.sub.4:Eu.sup.2+ class.
19. The method of claim 8 wherein depositing the first coating layer comprises depositing the first coating layer having a first coating layer thickness (d1) at least 50 nm; and wherein depositing the second coating layer comprises depositing the second coating layer having a second coating layer thickness (d2) in a range of about 5 nm to about 250 nm.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) 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:
(2) FIG. 1 schematically depicts a lighting device;
(3) FIG. 2a schematically depicts luminescent powder particles having a sol-gel first coating; FIGS. 2b-2d schematically depict some further aspects of the particulate luminescent material;
(4) FIGS. 3a-3b show SEM images of SiO.sub.2 coated powder after drying showing the surface structure of a single grain; FIG. 3c show TEM pictures of a hybrid coated particle;
(5) FIG. 4a shows the relative light output (LO) as a function of degradation time (in hours) for phosphor powder before (SiO.sub.2 only) and after ALD coating (Al.sub.2O.sub.3 on SiO.sub.2); degradation conditions: 60° C./100% relative humidity: ALD-1: 20 nm Al.sub.2O.sub.3 on phosphor; ALD-2: 40 nm Al.sub.2O.sub.3 on phosphor; ALD-3: 20 nm Al.sub.2O.sub.3 deposited on SiO.sub.2 coating; SiO.sub.2-1: sol-gel SiO.sub.2 coating on phosphor (basis of ALD-3);
(6) FIG. 4b shows the relative light output (LO) as a function of degradation time given in hours (85° C./100% RH); ALD-3: 20 nm Al.sub.2O.sub.3 on SiO.sub.2 coating; ALD-4: 20 nm Al.sub.2O.sub.3/Ta.sub.2O.sub.5 nanolaminate; deposited on thin SiO.sub.2 layer (<10 nm); ALD-5: 20 nm Al.sub.2O.sub.3/Ta.sub.2O.sub.5 nanolaminate; deposited on SiO.sub.2 coating; ALD-6: 20 nm Al.sub.2O.sub.3/HfO.sub.2 nanolaminate; deposited on SiO.sub.2 coating;
(7) FIG. 4c shows the relative light output (LO) as a function of degradation time given in hours (85° C./100% RH); ALD-3 and ALD-6 samples as described above; ALD-7: 20 nm Al.sub.2O.sub.3/HfO.sub.2 nanolaminate on thin SiO.sub.2 layer (<10 nm), nanolaminate design: 4×[1.5 nm Al.sub.2O.sub.3/3.5 nm HfO.sub.2]; ALD-8: 10 nm Al.sub.2O.sub.3/HfO.sub.2 nanolaminate on thin SiO.sub.2 layer (<10 nm), nanolaminate design: 2×[1.5 nm Al.sub.2O.sub.3/3.5 nm HfO.sub.2]. The sol-gel SiO.sub.2 coatings in general have a layer thickness in the range of 150-200 nm, unless indicated otherwise. The thin SiO.sub.2 layers, indicated with thicknesses <10 nm in general will have a mean layer thickness in the range of about 1-10 nm. The schematic drawings are not necessarily on scale.
DETAILED DESCRIPTION OF THE EMBODIMENTS
(8) FIG. 1 schematically depicts a lighting device 20 comprising a light source 10 configured to generate light source radiation 11, especially one or more of blue and UV, as well as a wavelength converter 30 comprising the luminescent material 1 with particles as defined herein. The wavelength converter 30 may e.g. comprise a matrix, such as a silicone or organic polymer matrix, with the coated particles embedded therein. The wavelength converter 30 is configured to (wavelength) convert at least part of the light source radiation 11 into wavelength converter light 21, which at least comprises wavelength converter light 31 and optionally also light source radiation 11. The wavelength converter light 31 at least includes luminescence from the herein described coated particles. However, the wavelength converter 30 may optionally include also one or more other luminescent materials. The wavelength converter 30, or more especially the luminescent material 1, may be arranged at a non-zero distance d3, such as at a distance of 0.1-100 mm. However, optionally the distance may be zero, such as e.g. when the luminescent material is embedded in a dome on a LED die. The distance d3 is the shortest distance between a light emitting surface of the light source, such as a LED die, and the wavelength converter 30, more especially the luminescent material 1.
(9) FIG. 2a schematically depicts luminescent powder particles having a sol-gel first coating forming a static powder bed during ALD of a second coating. The particles are indicated with references 100 and the sol-gel coating or first coating layer is indicated with reference 110. The luminescent cores are indicated with reference 102, and may include e.g. micrometer dimensional particles of a luminescent nitride or sulfide phosphor, but may also include other (smaller) material such as luminescent nanoparticles (see further FIG. 2c). As schematically shown in FIG. 2a, the outer shape of the first coating layer 110 may have a somewhat pocked shape, as was found in SEM (see further FIG. 3b). By way of example, the smaller particles in FIG. 2a indicate e.g. ALD precursor (see further below). Reference 100a is used to indicate the luminescent particles 100 only having the sol-gel first coating layer 110.
(10) FIGS. 2b-2d schematically depict some further aspects of the particulate luminescent material; FIG. 2b shows a luminescent material 1, here by way of example two particles with luminescent cores 102, and a first coating layer 110 (formed by sol-gel coating), having a thickness d1, and a second coating layer 120 (formed by ALD), having a thickness d2. The thicknesses are not necessarily on scale. The possible identations in the first coating layer 110 are not depicted. The thickness d1 may especially be a mean thickness, averaged over the first coating layer 110; likewise this may apply to the second thickness d2, etc. (see also below).
(11) FIG. 2c schematically depicts an embodiment wherein the luminescent core 102 includes a luminescent nanoparticle, here by way of example a quantum dot 130. The quantum dot in this example comprises a quantum rod with a (semiconductor) core material 106, such as ZnSe, and a shell 107, such as ZnS. Of course, other luminescent nanoparticles may also be used. Such luminescent quantum dot 130 can also be provided with the hybrid coating.
(12) As indicated above, the coating layer may include multi-layers; especially the second coating layer 120 may include a multi-layer coating. This is schematically shown in FIG. 2d, wherein the second coating layer 120 comprises an ALD multi-layer 1120, with layers 1121. References 1121a, 1121b and 1121c schematically indicate the individual layers, which may e.g. alternating Al.sub.2O.sub.3 layers (by way of example 1121b) and Ta.sub.2O.sub.5 layers (by way of example 1121a,1121c), respectively. Reference d2 indicates the thickness of the entire second coating layer 120. The individual ALD layers may e.g. have thicknesses in the range of 0.5-20 nm.
(13) FIG. 2d indicates with references 17, 27, 37, 47 and 57 the surfaces of the respective layers. As indicated above, the layer thicknesses described herein are especially average layer thicknesses. Especially at least 50%, even more especially at least 80%, of the area of the respective layers have such indicated layer thickness. Hence, referring to the thickness d2 between surface 17 and surface 47, below at least 50% of surface 17, a layer thickness in the range of e.g. 5-250 nm may be found, with the other less than at least 50% of the surface area 17 e.g. smaller or larger thicknesses may be found, but in average d2 of the second coating (multi-)layer 120 is in the indicated range of 5-250. Likewise, this may apply to the other herein indicated thicknesses. For instance, referring to the thickness d1 between surface 47 and surface 57, this thickness may over at least 50% of the area of 47 be in the range of 20-500 nm, with the other less than at least 50% of the surface area 47 e.g. smaller or larger thicknesses may be found, but in average d1 of the first layer 110 is in the indicated range of 5-500 nm, such as especially 20-500.
(14) FIGS. 2a-2d schematically depict luminescent particles 100 having a single nucleus. However, optionally also aggregates encapsulated with the first and the second coating layer may be formed. This may especially apply for quantum dots as luminescent cores.
(15) FIGS. 3a-3b show SEM images of SiO.sub.2 coated powder after drying showing the surface structure of a single grain, with especially FIG. 3b, measured with a SEM with a higher resolution, showing the pocked surface, which is herein also indicated as “moth-eye”. FIG. 3c show TEM pictures of a hybrid coated particle. Reference M in the left figure indicates a TEM matrix wherein the particles are embedded, such as a resin, for the purpose of TEM measurements. Reference 120 indicates a multi-layer second coating layer, here a 3-layer multi-layer on the first coating layer 110. In the right TEM figure it can be seen that there is an transition layer or intermediate layer 105 between the luminescent core 102 and the first coating layer 110. Here, a non-oxidic luminescent material was applied. The intermediate layer is an oxide layer (i.e. oxide formation at the surface of the non-oxidic luminescent material, such as a nitride luminescent material).
(16) FIG. 4a shows the relative light output as a function of degradation time (in hours) for phosphor powder before (SiO.sub.2 only) and after ALD coating (Al.sub.2O.sub.3 on SiO.sub.2); degradation conditions: 60° C./100% relative humidity: ALD-1: 20 nm Al.sub.2O.sub.3 on phosphor; ALD-2: 40 nm Al.sub.2O.sub.3 on phosphor; ALD-3 20 nm Al.sub.2O.sub.3 deposited on SiO.sub.2 coating; SiO.sub.2-1: sol-gel SiO.sub.2 coating on phosphor (basis of ALD-3). It is clear that only sol-gel coated material or only ALD coated material is inferior to the hybrid coating.
(17) FIG. 4b shows the relative light output as a function of degradation time given in hours (85° C./100% RH); ALD-3: 20 nm Al.sub.2O.sub.3 on SiO.sub.2 coating; ALD-4: 20 nm Al.sub.2O.sub.3/Ta.sub.2O.sub.5 nanolaminate; deposited on thin SiO.sub.2 layer (<10 nm); ALD-5: 20 nm Al.sub.2O.sub.3/Ta.sub.2O.sub.5 nano laminate; deposited on SiO.sub.2 coating; ALD-6: 20 nm Al.sub.2O.sub.3/HfO.sub.2 nano laminate; deposited on SiO.sub.2 coating. Amongst others, from these drawings can be concluded that ALD multi-layers of Al.sub.2O.sub.3 and a second oxide provide superior behavior over a “simple” Al.sub.2O.sub.3 ALD coating. The ALD-3 sample in FIG. 4a is the same as in FIG. 4b; the measurement conditions (temperature) were however different.
(18) FIG. 4c shows the relative light output (LO) as a function of degradation time given in hours (85° C./100% RH); ALD-3 and ALD-6 samples as described above; ALD-7 with 20 nm Al.sub.2O.sub.3/HfO.sub.2 nanolaminate on thin SiO.sub.2 layer (<10 nm) (nanolaminate design: 4×[1.5 nm Al.sub.2O.sub.3/3.5 nm HfO.sub.2]) and ALD-8 with 10 nm Al.sub.2O.sub.3/HfO.sub.2 nanolaminate on thin SiO.sub.2 layer (<10 nm), nanolaminate design: 2×[1.5 nm Al.sub.2O.sub.3/3.5 nm HfO.sub.2]. It is clear that thicker sol-gel layers and/or more stacked nano laminates provide better results than those with a thin sol-gel layer or a multi-layer stack with only a few layers. ALD-5 and ALD-6 have sol-gel coatings in the range of about 100-200 nm.
EXPERIMENTAL
(19) SiO.sub.2 Coating of Luminescent Powder
(20) For this experiment, 10 g SrLiAl.sub.3N.sub.4:Eu phosphor powder was stirred in 100 ml EtOH in a 3-necked glass flask under constant flow of nitrogen gas. After addition of 100 μl tetramethoxysilane (TMOS) and 15 ml 25% NH.sub.3 solution 8.5 g tetraethoxysilane (TEOS) dissolved in 50.4 g EtOH were added and the suspension was stirred for at least 3 hours.
(21) The coated powder was filtered and washed with EtOH. Finally the coated powder was dried for 24 h at 200° C. in air. FIG. 3a shows the SEM image of the SiO.sub.2 coated powder after drying showing the surface structure of a single grain.
(22) ALD Coating of SiO.sub.2 Coated Luminescent Powder
(23) ALD coatings were made on 5 g scale in a Picosun Oy R150 ALD reactor with a POCA™ sample holder. The following precursor materials were used to deposit either single material ALD coatings (Al.sub.2O.sub.3) or multi-layered nanolaminates (Al.sub.2O.sub.3/Ta.sub.2O.sub.5, Al.sub.2O.sub.3/HfO.sub.2): trimethylaluminum for Al, tris(ethylmethylamido)(tert.-butylimido)tantalum(V) for Ta, tetrakis(ethylmethylamido)hafnium for Hf, and H.sub.2O for O. Pure Al.sub.2O.sub.3 coatings of 20 nm and 40 nm thickness were deposited at 300° C. The deposition temperature was lowered to 250° C. to either make 20 nm or 100 nm thick Al.sub.2O.sub.3/Ta.sub.2O.sub.5 nanolaminates (n×[5 nm Al.sub.2O.sub.3+5 nm Ta.sub.2O.sub.5]; n=2 or 10) or 20 nm thick Al.sub.2O.sub.3/HfO.sub.2 nanolaminates (4×[1.5 nm Al.sub.2O.sub.3+3.5 nm HfO.sub.2]). To check for agglomerates, all powders were dry-sieved after ALD coating with a 63 nm POM sieve.
(24) Moisture Stability Test
(25) To test and compare the moisture stability of phosphor powders (with and without ALD coating), phosphor/silicone slurries with 2 vol.-% phosphor were prepared. The homogenous slurries were deposited in a sample holder made of Teflon®. After hardening the silicone at 150° C. in air, the quantum efficiency and absorption at 440 nm were measured and used to calculate the light output LO=A*QE. After the initial measurement, the sample holders were placed in gas-tight glass bottles that also contained some ml deionized water (no direct contact between water and sample, excess water present to prevent complete evaporation). The sealed container was placed in a drying chamber at a temperature between 50-100° C. After typical intervals of ˜25 h, degradation was stopped and the samples were removed to measure QE and absorption. The light output relative to t=0 was plotted (FIG. 4a) to describe the hydrolysis related degradation of the phosphor powders.
(26) Further moisture stability tests were performed with different examples of coated material as shown in FIG. 4b. Amongst others, the following particles were prepared, with as luminescent core a nitride luminescent material:
(27) TABLE-US-00002 1st coating 2nd coating 1 SiO.sub.2 2x [5 nm Al.sub.2O.sub.3/5 nm Ta.sub.2O.sub.5], 20 nm total thickness 2 (<10 nm) 10x [5 nm Al.sub.2O.sub.3/5 nm Ta.sub.2O.sub.5], 100 nm total thickness 3 SiO.sub.2 2x [5 nm Al.sub.2O3/5 nm Ta.sub.2O.sub.5], 20 nm total thickness 4 (~100- 10x [5 nm Al.sub.2O.sub.3/5 nm Ta.sub.2O.sub.5], 100 nm total thickness 200 nm)
(28) From amongst others the data on these samples it could be concluded that especially good results may be obtained with a first coating layer of at least 50 nm.
(29) FIG. 3c shows a TEM image of the 3rd sample. On the left, the AlO.sub.x/TaO.sub.x-multilayer is shown and on the right the SiO.sub.x/grain interface. This magnification was the highest possible to use to image the grain's edge: delamination of the interfacial layer like visible in the right image (see accolade) occurred quite quickly. The interfacial layer appears to be oxidized grain material (see EDS results). The bright line in between the grain and the oxidized part of the grain represents less dense (or absence of) material. In the left image, the darker layers correspond with TaO.sub.x, whereas the brighter ones correspond with AlO.sub.x. The AlO.sub.x-layer adjacent to the SiO.sub.x-layer can be discerned. A further series of samples was made and tested, similar as described above:
(30) TABLE-US-00003 ALD-7 20 nm Al2O3/HfO2 nanolaminate on thin SiO2 layer (<10 nm), nanolaminate design: 4 x [1.5 nm Al2O3/3.5 nm HfO2] ALD-8 10 nm Al2O3/HfO2 nanolaminate on thin SiO2 layer (<10 nm), nanolaminate design: 2 x [1.5 nm Al2O3/3.5 nm HfO2] ALD-9 10 nm Al2O3/Ta2O5 nanolaminate on thin SiO2 layer (<10 nm), nanolaminate design: 2 x [5 nm Al2O3/5 nm Ta2O5]
(31) Experimental results concerning the light output dependence on the time at elevated temperatures and 100% RH are shown in FIG. 4c (see above). With respect to ALD-9, after an initial decrease (see also ALD-5), the time stability is very good. Further, ALD-5 was tested at 100% RH at different temperatures. At about 800 hours, the relative intensities (relative to 0 hours) were about 98% (at 85° C.), about 97% (at 100° C.), and about 92% (at 130° C.). This shows that very stable phosphors have been obtained which can withstand severe conditions, without substantial decrease in intensity.
