Silica coated quantum dots with improved quantum efficiency

Abstract

The invention provides a method for the production of a luminescent material, (10) based on coated quantum dots (100), comprising: (i) providing luminescent quantum dots (100) in a liquid medium (20) wherein the luminescent quantum dots (100) have an outer layer (105) comprising first cations and first anions; and (ii) providing in a coating process a coating (120) on the outer layer (105) of the quantum dots (100) in the liquid medium (20), wherein the coating (120) comprises a silica coating; wherein during the coating process, or after the coating process, or during and after the coating process, the liquid medium (20) comprises one or more of a third element and a fourth element, wherein the first cation and the third element belong to the same group of the periodic system, and wherein the first anion and the fourth element belong to the same group of the periodic system.

Claims

1. A method for the production of a luminescent material based on coated quantum dots, the method comprising: providing luminescent quantum dots in a liquid medium wherein the luminescent quantum dots have an outer layer comprising first element comprising cations and second element comprising anions; and providing in a coating process a coating on the outer layer of the quantum dots in the liquid medium, wherein the coating comprises at least one of a silica coating, a titania coating, an alumina coating, and a zirconia coating; wherein during the coating process, or after the coating process, or during and after the coating process, the liquid medium comprises one or more of a third element comprising ion and a fourth element comprising ion, wherein the first element and the third element belong to the same group of the periodic system, and are selected from the group of metal elements, wherein the second element and the fourth element belong to the same group of the periodic system, and are selected from the group of non-metal elements, and wherein the first element and the third element are each independently selected from the group consisting of Zn and Cd, and wherein the second element and the fourth element are each independently selected from the group consisting of S, Se and Te.

2. The method according to claim 1, wherein the one or more of the third element and the fourth element are available in the liquid medium in a concentration of at least of 10 mM.

3. The method according to claim 1, wherein one or more of the following applies: the third element and the first element are a same element, and the fourth element and the second element are a same element.

4. The method according to claim 1, wherein the coating process is executed in micelles containing said quantum dots.

5. The method according to claim 1, wherein during the coating process the liquid medium comprises the third element comprising ion and does substantially not comprise the fourth element comprising ion, and wherein after the coating process the liquid medium comprises the fourth element comprising ion.

6. The method according to claim 1, wherein the coated quantum dots obtainable with the method according to claim 1 have a luminescence maximum wavelength that is shifted relative to a luminescence maximum wavelength of coated quantum dots obtainable with such method in the absence of the third element comprising ion and fourth element comprising ion.

7. The method according to claim 1, further comprising embedding the coated quantum dots in a host material to provide a wavelength converter element, wherein the host material comprises a silicone.

8. A luminescent material based on coated quantum dots, the luminescent material comprising luminescent quantum dots having an outer layer comprising first element comprising cations and second element comprising anions; and a coating arranged on said outer layer, wherein the coating comprises at least one of a silica coating, a titania coating, an alumina coating, and a zirconia coating, and wherein the coating further comprises one or more of a third element and a fourth element, wherein the first element and the third element belong to the same group of the periodic system, and wherein the second element and the fourth element belong to the same group of the periodic system, and wherein the first element and the third element are each independently selected from the group consisting of Zn and Cd, and wherein the second element and the fourth element are each independently selected from the group consisting of S, Se and Te.

9. The luminescent material according to claim 8, wherein one or more of the following applies: the third element and the first element are a same element, and the fourth element and the second element are a same element.

10. The luminescent material according to claim 8, wherein the one or more of a third element and a fourth element are available in the coating in an amount of at least 100 ppm, respectively.

11. The luminescent material according to claim 8, wherein the one or more of a third element and a fourth element are available in the coating in a weight ratio to silicon in the range of 1:100-25:100, respectively.

12. The luminescent material according to claim 8, wherein the quantum dots have a shape selected from the group consisting of a sphere, a cube, a rod, a wire, a disk, and a multi-pod, wherein the coating has a thickness in the range of 1-50 nm, wherein the quantum dots are of the core-shell type with a core material differing from the shell material, wherein the core material is selected from the group consisting of ZnS, ZnSe, CdS, CdSe and InP, wherein the shell material is selected from the group consisting of ZnS, ZnSe, CdS, and CdSe, and wherein the third element is selected from the group consisting of Zn and Cd, and wherein A4 is selected from the group consisting of S and Se.

13. A wavelength converter element comprising a host material with the luminescent material embedded therein, the luminescent material according to claim 8 or obtainable by the method according to claim 1.

14. The wavelength converter element according to claim 13, wherein the host material comprises a silicone.

15. A lighting device comprising a light source and a luminescent material, as defined in claim 8, or a luminescent material obtainable by the method according to claim 1, or a wavelength converter element comprising said luminescent material, wherein the light source is configured to illuminate the luminescent material.

16. A lamp comprising at least one lighting device according to claim 15.

17. A luminaire comprising at least one lighting device according to claim 15 or at least one lamp according to claim 16.

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. 1a schematically depicts an embodiment of the reverse micelle method (copied from H. Ding, Y. Zhang, S. Wang, J. Xu, S. Xu and G. Li, Fe.sub.3O.sub.4@SiO.sub.2 Core/Shell Nanoparticles: The Silica Coating Regulations with a Single Core for Different Core Sizes and Shell Thicknesses, Chem. Mater., vol. 24, p. 4572-4580, 2012);

(3) FIG. 1b schematically depicts a quantum dot based luminescent material;

(4) FIG. 1c schematically depicts an embodiment of the luminescent material;

(5) FIG. 2 evolution of QE (squares) and emission peak wavelength (crosses) over time (t in seconds) upon exposure of blue light of a standard silica coated QD sample, without any salt addition; PP indicates peak position (in wavelength (nm));

(6) FIG. 3 evolution of QE (upper curves) and emission peak wavelength (lower curves) over time (t in seconds) upon exposure of blue light of silica coated QDs where either ZnCl.sub.2 (diamond (1); upper squares (2)) or Na.sub.2S (triangles (3); lower squares (4)) was added after the silica growth but before the washing procedure;

(7) FIG. 4 shows (A-C) TEM images of QDs incorporated in silica without addition of salts (4A), and where ZnCl.sub.2 was added to ammonia upfront (4B) a HAADF-TEM image is shown which more clearly shows the QDs, but also other small particles inside the shell. In (4C) a zoomed image is shown with red arrows indicating the very small (2-3 nm) particles (likely ZnCl.sub.2 or ZnO.sub.2) in the silica shell. (4D) shows a TEM image of silica particles where Na.sub.2S was added to the ammonia upfront. The image shows that the QDs are substantially not incorporated in this case;

(8) FIG. 5 evolution of QE (diamonds (1)) and emission peak wavelength (squares (2)) over time upon exposure of blue light of silica coated QDs where ZnCl.sub.2 was added to the ammonia before silica growth;

(9) FIG. 6 evolution of QE (triangles (1) and emission peak wavelength (crosses (2) over time upon exposure of blue light of silica coated QDs where ZnCl.sub.2 was added to the ammonia before silica growth, and Na.sub.2S was added after silica growth but before the washing procedure;

(10) FIG. 7 evolution of QE (diamonds (1)) and emission peak wavelength (squares (2)) over prolonged time (t in seconds; 180000 seconds=50 h) upon exposure of blue light of silica coated QDs where ZnCl.sub.2 was added to the ammonia before silica growth, and Na.sub.2S was added after silica growth but before the washing procedure; and

(11) FIG. 8 schematically depicts an embodiment of a lighting device.

(12) The schematic drawings amongst the above figures are not necessarily on scale.

(13) FIG. 9a shows a lamp according to the invention and FIG. 9b shows a luminaire according to the invention.

(14) FIGS. 10a-10b show stability measurements of Examples 8-9, respectively, with on the x-axis the time in hours and on the y-axis the emission intensity in arbitrary units (a.u.).

DETAILED DESCRIPTION OF THE EMBODIMENTS AND EXAMPLES

(15) FIG. 1a schematically depicts an embodiment of the reverse micelle method. Reference 31 indicates a QD, reference 32 indicates a ligand, such as oleic acid or hexadecylamine; reference 33 indicates a surfactant, such as Igepal CO-520, reference 34 indicates a silica formation catalyst such as ammonia, reference 35 indicates a silica precursor, such as TEOS, reference 36 indicates a hydrolyzed silica precursor, such as hydrolyzed TEOS, reference 37 indicates silica and reference 38 indicates a liquid medium, especially an organic apolar solvent, such as cyclohexane.

(16) In FIG. 1a, a schematic overview of most of stages in the reverse micelle method is shown. First the apolar solvent, such as cyclo hexane, and the surfactant Igepal CO-520 are mixed. Then the quantum dots, typically with oleic acid as ligands are added, which results in a ligand exchange with Igepal CO-520, which can be shown by nuclear magnetic resonance. After the addition of the quantum dots, ammonia (25% in water) is added which is the catalyst for silica formation (other catalyst can be e.g. dimethylamine), in which the higher the concentration the faster the silica growth takes place. At the moment of the addition of ammonia, micelles will form, in which ammonia is the inner volume of the micelles and Igepal CO-520 is the surfactant. Water (such as from ammonia) may also necessary for the hydrolysis reaction of the silica (with ethanol as reaction product), although it gets released again at the condensation reaction. In the following step (ammonia addition and TEOS addition can also be reversed) TEOS is added.

(17) This silica precursor gets hydrolyzed (partly), due to the ammonia and a ligand exchange will take place with the Igepal and/or native ligands on the quantum dot surface. This will also make the quantum dots water soluble and enables the QDs to be present within the hydrophilic cores of the micelles. Micelles are very dynamic which interchange rapidly, however the amount of quantum dots added should roughly match the typical amount of silica spheres would be produced in the same synthesis procedure without QD addition. In this way it is possible to get exactly one quantum dot per micelle. The fact that the QDs are in the middle of the silica spheres indicates that the QDs act as seeds for silica growth. After the ligand exchange by TEOS, the silica will grow with a speed depending on the ammonia concentration. After several days all TEOS molecules condensate and the growth stop.

(18) In another example one can use QDs that have charged ligands (such as MPA, mercaptopropionic acid) which allows dispersion of these QDs in water. For these QDs with charged ligands dispersed in water it was stated that due to the charge of the quantum dot, it pushed other quantum dots out of the micelle. This electrostatic repulsion in combination with the correct match of the amount of quantum dots and silica particles would yield a good precision of one quantum dot per silica shell.

(19) In this way, the silica coated QDs can be obtained. After the last stage, the silica coated QDs can, when desired, be retrieved from the liquid medium. An option is to add a precipitator, i.e. a material that induces flocculation and subsequent precipitation of the QDs. For instance, ethanol can be used. Thereafter, the QDs can be washed, with a second liquid medium, such as ethanol or another (organic) (polar) solvent (such as one or more of acetonitrile, isopropanol, acetone, etc.). Within ethanol, the QDs can be dispersed and stored. Alternatively, the QDs can be retrieved from the second liquid medium (see also above) and can be embedded in a host material (see also below).

(20) Further, after the silica growth also one or more post treatments may be applied. Optionally, a further salt treatment stage may be included. Further, the quantum dots may be subject to a thermal treatment, which may lead to a more stable QE.

(21) FIG. 1b schematically depicts a quantum dot based luminescent material 10. By way of example different types of QDs, indicated with reference 100, are depicted. The QD at the top left is a bare QD, without shell. The QD is indicated with C (core). The QD 10 at the right top is a core-shell particle, with C again indicating the core, and S indicating the shell. At the bottom, another example of a core-shell QD is schematically depicted, but a quantum dot in rod is used as example. Reference 105 indicates the outer layer, which is in the first example the core material at the external surface, and which is in the latter two embodiments the shell material at the external surface of the QD 100.

(22) FIG. 1c schematically depicts an embodiment of the luminescent material 10, but now the QDs 100 including the coating 120, especially an oxide coating, such as a silica coating. The thickness of the coating is indicated with reference d1. The thickness may especially be in the range of 1-50 nm. Especially, the coating 120 is available over the entire outer layer 105. Note however that a silica coating may be somewhat permeable. Note also that the outer layer 105 of the uncoated nanoparticle (i.e. not yet coated with the coating of the invention), is (in general) not an outer layer anymore after the coating process, as then an outer layer will be the outer layer of the coating 120. However, herein the term outer layer, especially indicated with reference 105, refers to the outer layer of the uncoated (core-shell) nanoparticle.

(23) Below, some examples are described in more detail.

(24) A typical QD-silica shell growth is performed by mixing 10 ml cyclohexane and 1.276 ml igepal co-520 in a 20 mL vial under vigorous stirring. 80 l TEOS, 1 ml cyclohexane and 12 l QDs in heptane (50 mg/ml, CdS core ZnS shell Crystal plex dots commercially obtained via Crystalplex Inc.) are mixed for around 7 minutes and are afterwards added to the cyclohexane/igepal mixture. After 15 minutes stirring, 150 l ammonia (25%) is added which initiates the reaction. This mixture was stirred vigorously for one minute to distribute the ammonia evenly over the formed micelles. 1 minute after the ammonia addition, stirring was stopped and the cup was stored in the dark for typically 2 days. The quantum dots used are of the core shell type, with the core being Cd(Se,S) and the shell being ZnS. These quantum dots can be used for both red and green, with the latter having smaller dimensions. These quantum dots are provided with alkyl ligands (especially oleic acid) and in solvent, such as hexane.

(25) To stop the silica growth, 2.5 ml ethanol was added after which the QD with silica coating will precipitate and can be collected by centrifugation. The precipitation was dissolved in 9 ml ethanol and centrifuged again to wash the sample and remove unwanted reagents. This was repeated for two more times. Finally the sample was stored in ethanol and sonication was used disperse the QD with silica coating until a clear solution was obtained.

(26) Silica growth takes place in the (basic) aqueous phase within the reverse micelle, and QDs can act as seeds for shell growth. Therefore, at the appropriate ratio of QDs and micelles it is possible to incorporate single QDs in the middle of individual silica particles of roughly 20-25 nm in diameter, see FIG. 4a. It appears that the QDs with a hydrophobic oleic acid capping, which disperse well in apolar solvents like toluene or heptanes, act as seeds in the aqueous phase of the reverse micelle. It seems that hydrolyzed TEOS molecules (i.e. SiO.sup. groups) can replace the oleic acid molecule (the native ligand). The TEOS-capped QDs have a much better affinity for the polar aqueous phase, and can therefore act as seed of silica growth.

(27) In the examples below, it is shown that the QE of QDs typically drops from 80% to 20-30% upon silica shell growth using the standard recipe. However, the QE can be improved to 50% if salts such as ZnCl.sub.2 or Na.sub.2S are added during or after silica shell growth. The light exposure effect (sometimes also called photobrightening effect) and peak wavelengths are also affected depending on the exact addition procedure and material. The results clearly show that the added salts have an effect on the QD performance. Eventually, a QE of above 70% could be measured (in air) using the modified silica shell growth procedure and applying a light exposure step.

Example 1: QE of Baseline QD-silica Samples

(28) As a baseline, first a silica shell was grown around QDs using the standard recipe as described above. Aliquots were taken during silica shell growth, and the QE and peak wavelength were measured. Table 1 gives an overview of the QE as function of time. It shows that the QE drops from 80% of the native dots, towards 20-30% after silica shell growth. The drop predominantly takes place within the first 5 minutes of silica shell growth, which confirms that this is due to a QD surface/ligand effect. The resulting silica coated QDs using this recipe are shown in FIG. 1.

(29) TABLE-US-00001 TABLE 1 evolution of the quantum efficiency and peak position during silica shell growth. Peak position Time after quenching [min] Quantum efficiency [%] [nm] Original dots 2.5 mg/ml 80.2 609.73 5 39.7 608.73 150 26.2 607.06 600 29.8 605.10 1147 29.7 604.47 2880 31.9 604.18 10080 19.2 604.09

(30) FIG. 2 shows how the QE of liquid samples of a typical baseline silica coated QD sample changes upon exposure of blue light. Moderate light exposure is observed, with a final QE up to 35%. The light exposure is typically accompanied by a red-shift in emission wavelength.

(31) The standard light exposure herein is executed by exposure to (blue) light at medium flux (1 W/cm.sup.2). In this way, the light exposure effect or photobrightening effect may be evaluated as sometimes the QE increases (photobrightening), or sometimes also decreases, under light exposure.

Example 2: Addition of ZnCl2 or Na2S after Silica Growth but before the Washing Procedure

(32) To investigate the influence of salts on the QE of QDs, ZnCl.sub.2 or Na.sub.2S were added after the silica shell growth was completed (after 20 hours of shell growth in the dark). For these experiments a silica shell growth experiment was performed as described in Example 1, but after 20 hours of shell growth the mixture was divided over several vials for multiple addition experiments on the same baseline QD-silica mixture. For both salts, 100 l of a certain concentration salt in water was added to the reverse micelle mixture under mild stirring. This was done before the washing procedure was performed, in other words the micelles were still intact. After 1 hour of stirring with the added salt, the mixture was washed using the standard procedure, and the particles were redispersed in Ethanol after which the QE was determined. The QE's are listed in Table 2, together with the peak wavelength of the QD emission spectrum. The reference samples have a QE of 33% and 35%, which is slightly higher than the result described above but within the same range. When ZnCl.sub.2 is added, a gradual increase in QE is observed with increasing salt concentration (up to 47%), together with a red-shift in the emission peak wavelength (up to 2 nm). Similarly, an increase in QE to 49% is observed upon addition of a 100 mM Na.sub.2S solution, however 400 mM shows an increase to only 39%. Albeit a small change, the emission peak wavelength shifts to the blue in this case by approximately 0.5-1 nm.

(33) The results above show that addition of a salt has an impact on the final QE and peak wavelength of the QDs. It suggests that the ions have the chance to diffuse through the (porous) silica particle towards the QD surface.

(34) TABLE-US-00002 TABLE 2 QE data of silica coated QDs with and without addition of 100 l of ZnCl.sub.2 or Na.sub.2S salt at various concentrations. Na.sub.2S ZnCl.sub.2 Quantum Concentration Quantum PL peak efficiency PL peak [mM] efficiency [%] [nm] [%] [nm] 0 (reference 1) 35 604.8 35 604.8 0 (reference 2) 33 604.5 33 604.5 25 36 604.8 n.m. n.m. 100 39 605.3 49 604.5 400 47 606.6 38 604.0 n.m. = not measured

(35) The QE of the silica coated QDs where ZnCl.sub.2 or Na.sub.2S was added after silica growth but before the washing procedure was also followed over time upon illumination with blue light. In both cases 100 l of 400 mM salt concentration was added. The QE of both samples start at 47% and 40%, in line with the results listed in table 2. The sample treated with ZnCl.sub.2 shows light exposure up to 60% QE, accompanied by a red-shift of 1 nm. The sample treated with Na.sub.2S first shows a dramatic drop in QE to 15%, after which it increases to 40%, again accompanied by a red-shift of in this case 2 nm. From these results it appears that a ZnCl.sub.2 treatment after silica growth is a good route.

Example 3: Addition of ZnCl2 and Na2S after Silica Growth but before the Washing Procedure

(36) Silica coated QDs from reference 1 were also used to add both ZnCl.sub.2 and Na.sub.2S, in different sequence, see Table 3. The results of the reference and ZnCl.sub.2-only and Na.sub.2S-only experiments are also given for comparison. When first ZnCl.sub.2 is added and then Na.sub.2S, a QE of 52% is measured, with a peak wavelength of 604.5 nm. When the salts are added in the reverse sequence, a QE of 41% is measured, with a peak wavelength of 605.2 nm. These results confirm the effect of salt addition on QE and peak wavelength, and show that the sequence in which the salts are added do impact the final optical properties of the QDs. The results show that in case of adding both salts, it is most favorable to add Na.sub.2S as last.

(37) TABLE-US-00003 TABLE 3 overview of QE upon addition of ZnCl.sub.2 and Na.sub.2S after silica growth but before the washing procedure Addition sequence QE [%] Peak position [nm] Reference 1 35 604.8 ZnCl.sub.2 100 mM 39 605.3 Na.sub.2S 100 mM 49 604.6 ZnCl.sub.2 => Na.sub.2S 100 mM 52 604.5 Na.sub.2S => ZnCl.sub.2 100 mM 41 605.2

Example 4: Addition of ZnCl2 or Na2S during Silica Shell Growth

(38) In another embodiment the ZnCl.sub.2 or Na.sub.2S were added during the silica growth, where the 100 mM solutions of theses salts were mixed with the ammonia upfront. In this experiment 13 mg (0.93*10.sup.4 mol) of ZnCl.sub.2 per 150 L of 35% ammonia was used. Similarly, 50.76 mg Na.sub.2S was added to 150 L ammonia 35%. Na.sub.2S is a strong base, so also lower ammonia concentration could be useful. TEM images of the results are displayed in FIG. 4 below, in which (A-C) represents the ZnCl.sub.2 addition and (D) the Na.sub.2S addition.

(39) The TEM results in FIG. 4 show that ZnCl.sub.2 addition to the ammonia before addition to the silica synthesis results in the formation of small nanoparticles in the shell of the silica particles. These particles could be ZnCl.sub.2 and/or ZnO particles, but there is no conclusive evidence for this. The QDs are however still well incorporated in the silica particles. In case Na.sub.2S is added to the ammonia before addition to the silica synthesis, it appears that the QDs are not incorporated into the silica particles, but attached to the outside of the silica particles. It is suggested that this is due to a too high pH during silica growth, because Na.sub.2S is a strong base by itself.

(40) The QE of the sample where ZnCl.sub.2 was added to the ammonia had a QE of 51% (peak at 606.9 nm) and a remake had a QE of 54% (peak at 606.4 nm). FIG. 5 below shows the that the QE of this sample did not show light exposure (but rather a drop) upon exposure to blue light, accompanied by a red-shift of 1.5 nm. This is somewhat remarkable because the sample where the ZnCl.sub.2 was added to the reaction mixture after silica growth (example 2) did show light exposure effects.

(41) Hence, the invention also provides a luminescent material comprising particles and/or agglomerates of particles, wherein the particles especially have dimensions in the range of 20-500 nm, such as especially 20-350 nm, wherein substantially each particle comprises a (single) quantum dot surrounded by a silica coating. As indicated above, the quantum dot can be of the core-shell type. The silica comprising shell comprises also one or more elements that are shared with the outer layer of the quantum dot. Especially, about at least 70 wt. % of the luminescent material may comprise such particles and/or agglomerates thereof. Hence, the individual particles may not necessarily be interconnected. The individual particles may agglomerate, but may not form a combination of quantum dots sharing a single silica coating. Part of the luminescent material, such as 30 wt. % or less may optionally be based on particles sharing two or more quantum dots.

Example 5: Addition of ZnCl2 during Silica Shell Growth, and Na2S after Silica Growth but before the Washing Procedure

(42) In yet another embodiment, QDs were coated with a silica shell where ZnCl.sub.2 was added together with the ammonia, and Na.sub.2S was added after silica growth but before the washing procedure. In this experiment, 13 mg of ZnCl.sub.2 was dissolved into the ammonia (35%) solution, of which 150 ul was added at the start of the silica growth. After the silica synthesis was completed (22 hours in the dark without stirring), 100 ul of 400 mM Na.sub.2S in water was added to the reaction mixture and stirred gently for 1 hour. Next, the reaction mixture was precipitated with ethanol and washed 3 times in ethanol. The QE of the resulting sample was 54% at a peak wavelength of 608 nm. FIG. 6 below shows that this sample does show light exposure up to 58% within 1400 s, together with a red-shift in emission peak wavelength of 0.7 nm. Note that this peak wavelength is clearly different from the sample described in example 4, where ZnCl.sub.2 addition without Na.sub.2S post treatment resulted in an initial peak at 606.5 nm.

(43) When the QE of the same sample was measured again, a QE of 66% was measured with a peak wavelength of 608.5 nm. When this sample was exposed to blue light for prolonged times (up to 2 days) light exposure was observed up to a QE of 76%, while the peak wavelength hardly shifts anymore; see FIG. 7.

Example 6: Application in Different Films of Red QD

(44) A Comparison between a QD dispersion in toluene and in silicone films (wavelength converter element). Samples A-E were treated with 40 L ZnCl.sub.2 (0.4 M) and/or 100 L Na.sub.2S (0.4 M). A sample F was treated with 40 L saturated ZnCl.sub.2 and 100 L Na.sub.2S (0.4 M)

(45) Both samples treated with light exposure (LE) had an increase of the QE towards 65-70%. The results are indicated in table 4:

(46) TABLE-US-00004 TABLE 4 QD in toluene and in film for red QDs Dispersion Film Dispersion Film t = 0 t = 0 t = 1 t = 7 t = 0 t = 0 t = 1 t = 7 days days days days days days days days QE QE QE QE .sub.max .sub.max .sub.max .sub.max Sample Treatment (%) (%) (%) (%) (nm) (nm) (nm) (nm) A SiO.sub.2 28.3 17.6 18.5 18.0 603.02 605.56 605.41 604.91 B SiO.sub.2 + ZnCl.sub.2 44.8 24.4 26.8 22.5 605.34 607.00 606.92 606.67 C SiO.sub.2 + Na.sub.2S 36.4 30.1 31.4 32.1 602.28 603.81 603.54 602.92 D SiO.sub.2 + ZnCl.sub.2 + 48.4 28.6 30.6 30.8 605.02 605.77 605.55 605.26 Na.sub.2S E SiO.sub.2 + ZnCl.sub.2 + 42.3 43.1 45.8 47.2 604.55 605.25 605.18 604.87 Na.sub.2S + PB F SiO.sub.2 + ZnCl.sub.2 + 56.4 24.7 27.7 28.5 609.58 609.15 609.13 608.47 Na.sub.2S + PB

(47) It appears that the samples treated with Na.sub.2S show constant QE in films. Further, ZnCl.sub.2 induces a redshift and Na.sub.2S induces a blueshift. Further, light exposure has a positive effect on the QE in the films. Sample F has the optimal treatment w.r.t. QE for dispersions, and sample E has the optimal treatment w.r.t. QE for films.

Example 6: Application in Different Films of Green QD

(48) The same experiment was applied, but now with green QDs (NC536A), indicated with samples G-L, see table 5:

(49) TABLE-US-00005 TABLE 5 QD in toluene and in film for green QDs Dispersion Film Dispersion Film t = 0 t = 0 t = 1 t = 7 t = 0 t = 0 t = 1 t = 7 days days days days days days days days QE QE QE QE .sub.max .sub.max .sub.max .sub.max Sample Treatment (%) (%) (%) (%) (nm) (nm) (nm) (nm) G SiO.sub.2 17.8 10.3 10.6 10.0 538.18 541.00 540.89 540.32 H SiO.sub.2 + ZnCl.sub.2 30.5 11.8 13.3 14.0 539.50 543.12 542.94 542.73 I SiO.sub.2 + Na.sub.2S 21.2 20.8 22.9 24.5 537.72 539.91 539.75 539.49 J SiO.sub.2 + ZnCl.sub.2 + 45.5 16.4 17.1 18.3 537.91 538.87 538.80 538.61 Na.sub.2S K SiO.sub.2 + ZnCl.sub.2 + 42.2 26.2 26.7 28.2 538.17 539.18 539.06 538.81 Na.sub.2S + PB L SiO.sub.2 + ZnCl.sub.2 + 45.9 20.2 21.8 24.3 545.13 544.60 544.51 544.58 Na.sub.2S + PB

(50) Also these samples show constant QE in films. Further, again it was perceived that ZnCl.sub.2 induces a redshift and that Na.sub.2S induces a blueshift. Also here it was observed that light exposure has a positive effect on the QE in the films. Sample K has the optimal treatment w.r.t. QE for dispersions and sample L has the optimal treatment w.r.t. QE for films.

(51) The results above demonstrate that silica coating of QDs results in a dramatic drop of QE from 80% to 20-30%. Addition of ZnCl.sub.2 and/or Na.sub.2S salts during or after silica growth can improve the QE up to 60% or higher. Light exposure can further increase the QE to above 75%. Without salt addition, light exposure is limited up to 35%. It is suggested that during silica growth, Zn and/or S ions can be removed from the QD surface, resulting in surface defect states, reducing QE. By adding Zinc and/or sulphide salts, (and/)or other relevant salts, during or after silica shell growth, these traps states may be recovered. The changes in QE but also shifts in peak wavelength support the idea that the Zn and/or Sulphide ions, (and/)or other relevant ions, can actually migrate into the silica sphere towards the QDs, and thereby affect the optical properties. It is expected the quantum efficiencies can even be further improved by finetuning temperatures, concentrations and salt combinations and or salt addition schemes.

(52) Below in table 6, the chemical used are defined:

(53) TABLE-US-00006 TABLE 6 List of chemicals used Chemical Abbreviation Manufacturer Purity M (g/mol) Cyclohexane C.sub.6H.sub.12 Merck 99.5% 84.16 Tetraethyl Si(OC.sub.2H.sub.5).sub.4 Sigma Aldrich 99.0% 208.33 orthosilicate Quantum dots QDs Crystalplex NC- 50 mg/ml 610-A_5 Quantum dots QDs Crystalplex NC- 50 mg/ml 536A Zinc Chloride ZnCl.sub.2 Sigma Aldrich 97% 36,315 Igepal CO-520 (C.sub.2H.sub.4O).sub.nC.sub.15H.sub.24O, Sigma Aldrich 441 n~5 Ammonia 35% in NH.sub.4OH Fisher Electronic (MOS) 35.04 water grade Sodium sulfide Na.sub.2S.sub.n9H.sub.2O Sigma Aldrich 99.99% 240.18 nonahydrate Formamide CH.sub.3NO Sigma Aldrich 99.5% 45.04 Ethanol C.sub.2H.sub.6O Bisolv Chimie semiconductor 46.0 (dehydrated) grade MOS PURANAL

(54) FIG. 8 depicts a lighting device 150 with a light source 160, configured to generate light source light 161. This light source light 161 is at least partly received with the luminescent material 10, for instance in the form of a layer or body 1000, or comprised by such layer or body 1000. This layer or body may also be indicated as wavelength converter element (see also FIG. 2a). The luminescent material 10 is optically coupled with the light source 160. The luminescent material absorbs at least part of the light source light 161 and converts this light source light 161 into luminescent material light. The light provided by the lighting device 150 is indicated with reference 151. This lighting device light 151 may at least include the light generated by the luminescent material 10 upon said excitation with the light source light 161, but may optionally also include said light source light 161. Reference d2 indicates the distance of the downstream arranged luminescent material, here embedded in the wavelength converter comprising. The distance may be non-zero, indicating e.g. a remote configuration. The distance may optionally also be zero.

(55) FIG. 9a shows a lamp 900 that comprises a lighting device according to FIG. 8. In an alternative embodiment the lamp 900 may comprise multiple lighting devices according to the invention.

(56) FIG. 9b shows a luminaire 950 that comprises a lighting device according to FIG. 8. In an alternative embodiment the luminaire 950 may comprise multiple lighting devices according to the invention, or one or more lamps according to FIG. 9a.

Example 7: Addition of Cd(NO3)2 during Silica Growth

(57) In yet another example, QDs were coated with a silica shell where of Cd(NO.sub.3).sub.2 was added together with the ammonia. In this experiment, 40 l of Cd(NO.sub.3).sub.2 in water added with 120 l ammonia (35%), at the start of the silica growth. After the silica synthesis was completed (22 hours in the dark without stirring). Next, the reaction mixture was precipitated with ethanol and washed 3 times in ethanol. After washing the reaction mixture was dispersed in 400 l toluene. The QE of the resulting sample was 71% at a peak wavelength of 609 nm. Note that this peak wavelength is clearly different from the sample described in example 4, where ZnCl.sub.2 addition resulted in an initial peak at 606.5 nm.

Example 8: Addition of Cd(NO3)2 during Silica Growth and Na2S Addition after Silica Growth but before the Washing Procedure

(58) In yet another example, QDs were coated with a silica shell where Cd(NO.sub.3).sub.2 was added together with the ammonia, and Na.sub.2S was added after silica growth but before the washing procedure. In this experiment, 0.5 M of Cd(NO.sub.3).sub.2 was added with 120 l of ammonia (35%) solution. After the silica synthesis was completed (22 hours in the dark without stirring), 100 l of 400 mM Na.sub.2S in water was added to the reaction mixture and stirred gently for 30 minutes followed by the addition of 100 l of 0.8M NaOH for 30 minutes. Next, the reaction mixture was precipitated with ethanol and washed 3 times in ethanol and redispersed in toluene. The QE of the resulting sample was 82% at a peak wavelength of 606 nm. When the quantum dots were incorporated in a silicone film, the QE dropped to 61%. In an accelerated degradation test the samples was measured in an accelerated testing protocol on a LED with drive current of 200 mA and a board temperature of 90 C. After the thermal quench, related to the high operating temperature the emitted intensity dropped <15% over 60 hours as shown in FIG. 10a.

Example 9: Addition of ZnCl2 and Na2S Addition after Silica Growth but before the Washing Procedure

(59) In yet another example, QDs were coated with a silica shell where ZnCl.sub.2 was added together with the ammonia, and Na.sub.2S was added after silica growth but before the washing procedure followed by the addition of base. In this experiment, 50 l 0.1M ZnCl.sub.2 was added with 150 l of the ammonia (35%) solution at the start of the silica growth. After the silica synthesis was completed (22 hours in the dark without stirring), 100 l of 400 mM Na.sub.2S in water was added to the reaction mixture and stirred gently for 30 minutes followed by addition of 50 l 0.8 M NaOH and another 30 minutes stirring. Next, the reaction mixture was precipitated with ethanol and washed 3 times in ethanol and redispersed in toluene. The QE of the resulting sample was 58% at a peak wavelength of 605 nm. The emission as a function of time is followed at 200 mA and 90 C., as shown in FIG. 10b. After initial photo brightening and thermal quenching effects, the red emission signal is constant within 10% for at least 150 hours.

(60) Note that the embodiment of Example 9 has a higher stability than the embodiment of Example 8, and also a smaller intensity drop is perceived.

Example 10: Analysis of Amounts of Ions after ZnCl2, Na2S, NaOH Treatment of QDs from Example 1 and Example 9

(61) ICP-MS analysis of a sample prepared according to example 1 was carried out and compared to that of sample prepared according to example 9.

(62) In order to release the elements of interest out of the matrix, the sample is destructed using a microwave digestion procedure. After dissolution, the sample is diluted to a known volume and a semi-quantitative measurement was performed in order to determine the present elements by means of Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES). Subsequently the determined elements are determined more precisely by means of ICP-OES.

(63) During ICP-OES analysis, the sample solutions are fed through a nebulizer. This produces an aerosol which is led into an argon plasma. In this plasma the solution is evaporated, atomized and excited, which produces element specific emission. The intensity and wavelength of the emission is used to determine the amounts of Cd, K, Na, S, Si and Zn present. Calibration is performed by comparison with the intensities produced by a matrix matched blank and at least four matrix matched calibration standard solutions (obtained by dilution of certified reference solutions).

(64) Each blank, calibration standard and sample solution contains corresponding amounts of internal standards to correct for system variations during measurement. Each ICP-OES measurement consists of multiple replicates and each is measured using several wavelengths. For quality control blanks and spike recovery experiments are taken along.

(65) The results of the quantitative ICP-OES analyses are presented in table 7. The concentrations are expressed in ratios of weight percentages (wt %) relative to the amount of silicon. Each sample is analyzed in triplicate. The amounts of Na, S and Zn, are significantly higher for Example 9 compared to Example 1. The Cd and K content are the same within experimental error while the amounts of Na, S and Zn, reach values up to 10-20% of the amount if Si

(66) TABLE-US-00007 TABLE 7 weight of Cd, K, Na, S and Zn relative to the weight of silicon. Element Example 1 Example 9 Cd 4.3 4.5 Na 0.3 17.9 S 1.8 11.2 Si 100 100 Zn 2.6 15.7

(67) The invention enables efficient (QD-converted) LED light sources at high flux densities, for example LED lamps, spot lights, outdoor lighting, automotive lighting and/or laser applications.