OXO- AND HYDROXO-BASED COMPOSITE INORGANIC LIGANDS FOR QUANTUM DOTS

Abstract

The invention provides a luminescent material (10) comprising quantum dots (100), wherein the luminescent material (10) further comprises a capping agent (110) coordinating to the quantum dots (10), wherein the capping agent comprises M.sub.xO.sub.y(OH).sub.z.sup.n, wherein M is selected from the group consisting of B, Al, P, S, V, Zn, Ga, Ge, As, Se, Nb, Mo, Cd, In, Sn, Sb, Te, Ta and W, wherein x1, y+z1, and wherein n indicates a positive or negative charge of the capping agent.

Claims

1. A particulate luminescent material comprising quantum dots, wherein the luminescent material further comprises a capping agent coordinating to the quantum dots, wherein the capping agent comprises M.sub.xO.sub.y(OH).sub.z.sup.n, wherein M is selected from the group consisting of B, Al, P, S, V, Zn, Ga, Ge, As, Se, Nb, Mo, Cd, In, Sn, Sb, Te, Ta and W, wherein x1, y+z1, wherein n indicates a positive or negative charge of the capping agent, wherein the particulate luminescent material comprises particles having an inorganic matrix hosting the quantum dots with inorganic capping agents.

2. The luminescent material according to claim 1, wherein M is selected from the group consisting of Al, V, Zn, Mo, Sn, and W.

3. The luminescent material according to claim 1, wherein capping agent comprises one or more of the aluminate ion (Al(OH).sub.4.sup.), the stannate ion (SnO.sub.3.sup., SnO.sub.3.sup.2, and SnO.sub.4.sup.4), the vanadate ion (VO.sub.3.sup.,VO.sub.4.sup.3), the molybdate ion (MoO.sub.4.sup.2), the tungstate ion (WO.sub.4.sup.2), the phosphate ion (PO.sub.4.sup.3), and the zincate ion (Zn(OH).sub.4.sup.2).

4. The luminescent material according to claim 1, wherein when also organic capping agents are coordinating to the quantum dots, the amount of organic capping agents is less than 5 wt. % relative to the total weight of quantum dots.

5. The luminescent material according to claim 1, wherein capping agent comprises the zincate ion (Zn(OH).sub.4.sup.2).

6. The luminescent material according to claim 5, wherein the luminescent quantum dots have an outer layer comprising an inorganic compound, wherein the inorganic capping agents comprise one or more of the aluminate ion (Al(OH).sub.4.sup.), the stannate ion (SnO.sub.3.sup., SnO.sub.3.sup.2, and SnO.sub.4.sup.4), the vanadate ion (VO.sub.3.sup.,VO.sub.4.sup.3), the molybdate ion (MoO.sub.4.sup.2), the tungstate ion (WO.sub.4.sup.2) the phosphate ion (PO.sub.4.sup.3), and the zincate ion (Zn(OH).sub.4.sup.2), and wherein one or more of the following applies (i) the inorganic salt of the inorganic matrix and an outer layer of the quantum dots have an element in common, (ii) the inorganic capping agent and the inorganic matrix have an element in common, and (iii) an outer layer of the quantum dots and the inorganic capping agent have an element in common.

7. The luminescent material according to claim 5, wherein the luminescent quantum dots are dispersed within the particles, wherein the particles have a number averaged particle size in the range of 0.5-40 m, and wherein the luminescent material comprises in the range of 0.01-5 wt. % quantum dots relative to the total weight of the luminescent material.

8. The luminescent material according to claim 5, obtainable by a method comprising: (i) providing luminescent quantum dots with an organic capping agent and providing in an exchange process said luminescent quantum dots with the inorganic capping agent in a first liquid, wherein the capping agent comprises M.sub.xO.sub.y(OH).sub.z.sup.n, wherein M is selected from the group consisting of B, Al, P, S, V, Zn, Ga, Ge, As, Se, Nb, Mo, Cd, In, Sn, Sb, Te, Ta and W, wherein x1, y+z1, and wherein n indicates a positive or negative charge of the capping agent.

9. The luminescent material according to claim 1, comprising a first liquid comprising said quantum dots with capping agent coordinating to the quantum dots.

10. A wavelength converter element comprising a host material with the luminescent material, according to claim 1, embedded therein.

11. A lighting device comprising a light source and a luminescent material, as defined in claim 1, wherein the light source is configured to illuminate the luminescent material.

12. A method for the production of a luminescent material based on quantum dots, the method comprising: (i) providing luminescent quantum dots with an organic capping agent and providing in an exchange process said luminescent quantum dots with the inorganic capping agent in a first liquid, wherein the capping agent comprises M.sub.xO.sub.y(OH).sub.z.sup.n, wherein M is selected from the group consisting of B, Al, P, S, V, Zn, Ga, Ge, As, Se, Nb, Mo, Cd, In, Sn, Sb, Te, Ta and W, wherein x1, y+z1, and wherein n indicates a positive or negative charge of the capping agent, wherein the exchange process comprises a phase transfer process.

13. The method according to claim 12, further comprising: (ii) precipitating in a co-precipitation process an inorganic salt comprising precipitated material from the first liquid, the precipitated material comprising said quantum dots hosted by the co-precipitated inorganic salt; (iii) separating in a separation process the precipitated material from the first liquid (20).

14. The method according to claim 13, wherein the luminescent quantum dots have an outer layer, wherein in the co-precipitation process two or more salts (M.sub.1-A.sub.1; M.sub.2-A.sub.2) are applied, wherein at least one of the salts and the outer layer have an element in common, and wherein the inorganic capping agent and one or more of the salts have an element in common.

15. The method according to claim 12, wherein the inorganic capping agents comprise one or more of the aluminate ion (Al(OH).sub.4.sup.), the stannate ion (SnO.sub.3.sup., SnO.sub.3.sup.2, and SnO.sub.4.sup.4), the vanadate ion (VO.sub.3.sup.,VO.sub.4.sup.3), the molybdate ion (MoO.sub.4.sup.2), the tungstate ion (WO.sub.4.sup.2), the phosphate ion (PO.sub.4.sup.3), and the zincate ion (Zn(OH).sub.4.sup.2).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0089] 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:

[0090] FIGS. 1a-1d schematically depict some aspects of the invention;

[0091] FIGS. 2a-2b schematically depict some aspects of a method for the production of the luminescent material;

[0092] FIGS. 3a-3b schematically depict some further aspects of the invention; and

[0093] FIG. 4 depicts emission spectra of quantum dots as described herein in heptane (left curve) and the same quantum dots in a ZnS matrix.

[0094] The schematic drawings are not necessarily on scale.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0095] Ideally, the ligand used is highly compatible with the surface of the qdots (which is in most cases ZnS), so sulphide based ligands are preferred. Some ligands are for example S.sup.2, HS.sup., SnS.sub.4.sup.4, Sn.sub.2S.sub.6.sup.4 but others are also possible (e.g. Se.sup.2, Te.sup.2, etc.). Generally a decrease in QE is observed upon exchanging the original organic ligands for those sulphide based inorganic ones. Relative low quantum efficiencies were found for ligands such as Sn.sub.2S.sub.4.sup.4, S.sup.2, HS.sup. and OH.sup. in aqueous systems.

[0096] What is proposed here is the use of composite anions of a different class, with the common composition of M.sub.xO.sub.y(OH).sub.z.sup.n (with M being an element capable of forming oxo- or hydroxo species, see table III for examples). As a special representative of this class, the zincate ion is presented: Zn(OH).sub.4.sup.2. This in an interesting ion since normally the combination of Zn.sup.2+ and hydroxide ion would result in the insoluble Zn(OH).sub.2. However, at very high pH (=high hydroxide concentration), this zincate ion is formed which is soluble again in water, forming an anion with a central metal ion that is also a component of the shell material most commonly used in qdots. It was observed that the high QE of the qdots with the original (organic) ligands was preserved to a large extent by using this new class of inorganic ligands. This material was also used to in a co-precipitation experiment of ZnCl.sub.2 and Na.sub.2S to prepare qdots in a ZnS matrix. Below, the invention is further elucidated in view of some specific embodiments and also reference examples. For instance, the hydoxide ligand and the Sn.sub.2S.sub.6.sup.4 ligand are used as references.

[0097] The organic ligands on the quantum dots are replaced by inorganic ligands, such as sulfide based ligands (e.g. Sn.sub.2S.sub.6.sup.4 or S.sup.2) or zinc based ligands (e.g. Zn(OH).sub.4.sup.2, which make them dispersible in water or other polar solvents such as DSMO or formamide. The inorganic ligands are preferably highly compatible with the ZnS shell (or other shell, or non-shell outer layer material) that is found on the majority of all modern quantum dot types. After the exchange and purification, a thick ZnS layer is deposited on those qdots by a simple precipitation procedure. Aqueous solutions of two water soluble salts (ZnCl.sub.2 and Na.sub.2S) are mixed, that form the insoluble ZnS in situ. The ZnS ultimately forms a matrix around the qdots, thereby forming a qdot/ZnS composite that can be applied as a generic micron-sized phosphor powder that is more stable against prevailing LED conditions while there is less or no need for additional hermetic sealing.

[0098] In addition to inorganic ligand exchanged qdots as described above, any other qdot type that is water dispersable (for example mercaptopropionic acid capped qdots or silica coated qdots) can be used as starting point for this co-precipitation method to incorporate the QDs in a second inorganic matrix.

[0099] In addition to ZnS, any other inorganic material that can be formed via the solution precipitation method (i.e. 2 or more water soluble materials that combine into 1 or more water insoluble materials).

[0100] Typically, quantum dots are obtained as zinc sulphide coated nanocrystals, surrounded by organic ligands such as oleate and dispersed into an organic solvent like toluene. The first step into creating qdots with inorganic ligand in an inorganic matrix (ILIM-qdots), is to exchange those organic ligands for inorganic ones. Typically sulphide based ions are used (S.sup.2, HS.sup., SnS.sub.4.sup.4, Sn.sub.2S.sub.6.sup.4) but others are also possible (e.g. OH.sup.). This exchange is schematically shown in the figure below.

[0101] The ligand exchange is schematically depicted in FIG. 1a (derived from Maksym V. Kovalenko et al., JACS 2010, 132, 10085-10092), with ref. 100 indicating the quantum dot, ref 107 indicating the organic ligand, and ref 110 indicating the inorganic ligand. The ligands depicted are only shown by way of example. Other ligands, organic as well as inorganic may also be chosen. In FIG. 1a, the symbol C.sub.n-T may indicate the hydrocarbon tail. The reference NC refers to nano crystal.

[0102] Ideally, the ligand used is highly compatible with the surface of the qdots (which is in most cases ZnS), so sulphide based ligands are preferred. In addition to inorganic ligand exchanged qdots as described above (which are preferred due to their inorganic nature), any other type of water dispersable qdots can be used as starting point for the inorganic matrix incorporation as described below. For example, qdots can also be made water soluble by exchanging the aliphatic ligands by charged ligands such as mercaptopropionic acid, or aminoethanethiol.

[0103] In addition to the inorganic and organic ligand water soluble qdots, also silica coated qdots can be incorporated with the method described below. Silica coated QDs can be obtained via the so-called reverse micelle method or Stober method and has been extensively studied (Koole et al., Chem. Mater. 2008, 20, p. 2503). However, the silica layer around qdots is amorphous, and therefore a less sufficient barrier to water and air. Thus, also silica coated qdots can be incorporated in a second, micron-sized inorganic matrix by the co-precipitation method described below. The surface of the silica coated qdots may need to be pretreated in order to act as suitable nucleation seed for the second matrix attachment.

[0104] After the ligand exchange, an inorganic matrix can be applied. Ideally, the inorganic matrix applied is highly compatible with the qdot surface and the inorganic ligand(s) used, so zinc sulphide (ZnS) is preferred, but other materials are certainly possible.

[0105] The method we apply here is using a simple precipitation approach whereby an insoluble salt (ZnS) is formed by combining two highly water-soluble salts (Na.sub.2S and ZnCl.sub.2). Combining aqueous solutions of these salts will result in a swift formation of a ZnS precipitate. The combination of the other two ions should result in a soluble salt again (NaCl in this case). As the qdots are (preferably) sulphide terminated, they can act as seeds for the growth of the ZnS, thereby resulting in a relatively thick coating of the qdots with ZnS. After washing (to remove NaCl and excess reactants) and drying, a fully inorganic material containing qdots can be obtained, as is schematically shown in FIG. 1b. This figure schematically represents the formation of a thick ZnS shell around (inorganic ligand) qdots via a simple precipitation procedure. In FIG. 1b, reference 110a indicates a layer of the inorganic ligands. This layer may not be a pure layer of ligands, but there may be a gradient change of the quantum dot particle to the bulk of the matrix, with a high concentration inorganic ligands close to the QD and substantially no inorganic ligand further away from the QD. Reference 12 indicates the co-precipitated particles obtained in the process. In general, these particles may be included a plurality of quantum dots. Reference 14 indicates the matrix or matrix material (i.e. the co-precipitated salt (material) wherein the QDs are dispersed. Reference 1000 indicates a luminescent layer or body comprising (particulate) luminescent material. This is herein also indicated as wavelength converter element 1000.

[0106] FIG. 1c schematically depicts the same as the right part of FIG. 1a, i.e. QD 100, but now with, by way of example, a zincate as capping ligand 110 coordinating to the QD. Several options are shown how the capping ligand may coordinate to the QD (cations). A (solid) luminescent material 10 will in general include a multitude of such quantum dot particles with ligands 110. Further, in reality each QD 100 will be surrounded by a plurality of ligands 110. FIG. 1d schematically depicts an embodiment of the luminescent material 10, wherein the QDs are (still) in the first liquid 20. For instance, a closed cuvette may contain the QDs 100 dispersed in the first liquid 20, with the capping agents or ligands 110 surrounding the QDs and facilitation solubility and/or dispersability. Such luminescent material may e.g. also be used in a lighting device (see below).

[0107] FIG. 2a very schematically depicts the quantum dots 100 being dispersed via the ligands 107 in the liquid 20. After co-precipitation (CP), a layer with precipitated material is obtained. This precipitated material is indicated with reference 107. With further processing, the precipitated material may e.g. result in particulate luminescent material 10 (see FIG. 1b) or e.g. in a wavelength converter element 1000 enclosing the particular luminescent material 10 with quantum dots. The wavelength converter element may include a host material 1014, such as a silicone or PMMA, etc., which surrounds the luminescent material particles 12. Hence, the matrix material of wavelength converter element will in general be of a material that is different from the precipitated salt material.

[0108] FIG. 2b schematically presents of the inorganic ligand exchange procedure with quantum dots. Here, QD refers to quantum dots, OL refers to organic liquid, IL refers to inorganic ligands, L refers to liquid (for inorganic ligands), t indicates time, and QD-IL-L indicates the quantum dots with inorganic ligands in the liquid. OL refers in the most right drawing/stage again to organic liquid.

[0109] FIG. 3a schematically depicts two types of quantum dots, though more types are possible (see also above), such as e.g. dot-in-rod quantum dots, which are also a type of core-shell QDs. The left QD 100 is a bare QD without shell. Here, the chemical composition of the outer layer may be substantially identical to the chemical composition of the rest of the quantum dot. The quantum dot here has organic ligands 107. The right particle is a so-called core-shell particle. The core is indicated with reference QDC and the shell, which is here also the outer layer 105, is indicated with reference S. Of course, also core-shell-shell or other type of quantum dot based particles may be applied.

[0110] FIG. 3b schematically 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. Together, they may for instance provide white lighting unit light 151. Referring to FIGS. 2a and 3b, the invention thus also includes wavelength converter elements enclosing luminescent material particles. The luminescent particles on their turn may include a precipitated salt enclosing quantum dots. The quantum dots may include core-shell type quantum dots (or other type of quantum dots). Further, between the quantum dots and the precipitated salt material, there may be a layer this is substantially based on the inorganic ligands with which the quantum dots were stabilized in the first liquid (before co-precipitation) of the inorganic salt. The luminescent material 10 may be arranged at a non-zero distance from the light source 160, though in other embodiments the luminescent material may arranged on an emissive surface (such as a LED die) of the light source 160.

EXAMPLES

Example 1

[0111] Commercially available quantum dots from Crystalplex (CdSe/CdS/ZnS core/shell/shell) with oleate ligands (emitting at 600 nm) were subjected to an inorganic ligand exchange by adding 0.25 mL of the qdot solution (5 mg/mL in toluene) to 1.75 mL n-heptane. The polar phase was made by 0.125 mL 1M (NH.sub.4).sub.4Sn.sub.2S.sub.6 in water to 2 mL of formamide (FA). The two phases were combined and stirred vigorously for 45 minutes. The organic layer was removed, and the FA phase was washed 4 times with n-heptane (1-2 mL). Finally the clear FA layer was collected and to this was added 3 mL of acetonitrile together with a few drops (ca. 15 L) of the inorganic ligand solution to precipitate the qdots.

[0112] After centrifugation and discarding the supernatant, the dots were redispersed into 1.3 mL 20 mM Na.sub.2S.9H.sub.2O in water. To this dispersion was added dropwise 1.3 mL of 20 mM ZnCl.sub.2 in water. A precipitate was formed that took all the qdots with it, i.e. the supernatant was optically clear and virtually colorless.

[0113] The resulting material was washed 3 times with water (3 mL) to remove NaCl, 2 times with acetone to remove the water and dried in vacuo. A highly colored brittle material was obtained that showed weak luminescence under UV light. Qdot concentration was estimated at 30 wt. % which probably results in concentration quenching.

Example 2

[0114] Commercially available quantum dots (CdSe/CdS/ZnS core/shell/shell) with oleate ligands (emitting at 575 nm) were subjected to an inorganic ligand exchange by adding 0.25 mL of the qdot solution (1 mg/mL in toluene) to 2 mL n-heptane. The polar phase was 2 mL of a 5 mg/mL solution of Na.sub.2S.9H.sub.2O in formamide (FA). The two phases were combined and stirred vigorously for 30 minutes. The organic layer was removed, and the FA phase was washed 4 times with n-heptane (1-2 mL). Finally the clear FA layer was collected and to this was added 3 mL of acetonitrile to precipitate the qdots.

[0115] After centrifugation and discarding the supernatant, the dots were redispersed into 0.25 mL of the 5 mg/mL Na.sub.2S solution in FA. The dots were still slightly agglomerated at this stage. To this dispersion was added 3 mL of water and 4 mL of 0.1 M Na.sub.2S.9H.sub.2O in water. Subsequently, 4 mL of 0.1 M ZnCl.sub.2 in water was added in a dropwise fashion, and an additional 4 mL of water. A precipitate was formed, taking all the qdots with it from the dispersion.

[0116] The resulting material was washed 3 times with water (7 mL) to remove NaCl, 2 times with acetone (7 mL) to remove the water and dried in vacuo. A salmon-pink brittle solid was obtained, that showed clear emission under UV light. The Qdot concentration was calculated at 0.6 wt. %. Quantum efficiency was measured to be 25% (original qdots dispersed in toluene were 80%). The material was gently crushed and studied under a fluorescence microscope, where it showed clear emission.

[0117] The flakes of material are glassy in appearance. They were further studied with high resolution SEM. The material was found to be composed of agglomerated nanospheres, 30-60 nm in diameter. No individual qdots (size 6-8 nm) were observed, so it appears that all of them are coated with ZnS and actually inside the nanograins. Stability measurements have been performed (in ambient air) with good results. From the SEM pictures it seems that all quantum dots are embedded in beads (nanospheres), with often a single quantum dot in a single bead instead of a plurality of quantum dots in a single bead.

Example 3

[0118] An aqueous solution of potassium zincate (K.sub.2[Zn(OH).sub.4]) was made by adding 3.125 mL of a 1M ZnCl.sub.2 solution to 5 mL of a 10M KOH solution (both in water). The resulting solution was diluted with water to a final concentration of 0.125M in Zn and 2M in KOH.

[0119] Commercially available quantum dots (CdSe/CdS/ZnS core/shell/shell) with oleate ligands (emitting at 575 nm) were subjected to an inorganic ligand exchange by adding 1 mL of the qdot solution (5 mg/mL in toluene) to 7 mL n-heptane. The polar phase was made by adding 1.6 mL of the 0.125M K.sub.2[Zn(OH).sub.4] and 2M KOH to 4.8 mL 1M KOH and 1.6 mL of H.sub.2O. The resulting polar phase is 8 mL of 0.0125 M K.sub.2[Zn(OH).sub.4] and 1M KOH. The two phases were combined and stirred vigorously for 1 hour. The organic layer was removed, and the FA phase was washed 4 times with n-heptane (1-2 mL). 1 mL of the resulting qdot dispersion was added to 12.5 mL of an aqueous 0.1M Na.sub.2S solution. Subsequently, 12.5 mL of 0.1 M ZnCl.sub.2 in water was added in a dropwise fashion, and an additional 4 mL of water. A precipitate was formed, taking all the qdots with it from the dispersion.

[0120] The resulting material was washed 4 times with water (10 mL) to remove NaCl, 2 times with acetone (10 mL) to remove the water and dried in vacuo. A salmon-pink brittle solid was obtained, that showed clear emission under UV light. The Qdot concentration was calculated at 0.5 wt. %. Quantum efficiency was measured to be 56% (original qdots dispersed in toluene were 80%). FIG. 4 shows the emission spectra of quantum dots as described herein in heptane (left curve) and the same quantum dots in a ZnS matrix. In the ZnS matrix the emission is shifted to lower energy. This may be due to a ligand and/or matrix effect. The emission spectrum of the quantum dots with inorganic ligands in water was also measured. That emission spectrum was substantially the same as the emission spectrum of the quantum dots in heptane.

Example 4

Sn.SUB.2.S.SUB.6..SUP.4 .Ligand

Example 4A

Reference

[0121] Commercially available quantum dots from Crystalplex (CdSe/CdS/ZnS core/shell/shell) with oleate ligands (emitting at 575 nm; QE 80%) were subjected to an inorganic ligand exchange by adding 0.25 mL of the qdot solution (5 mg/mL in toluene) to 1.75 mL n-heptane. The polar phase was made by adding 0.125 mL of a 0.1M aqueous solution of (NH.sub.4).sub.4Sn.sub.2S.sub.6 to 2 mL of water. The Sn.sub.2S.sub.6.sup.4 is a ligand known in the art. The two phases were combined and stirred vigorously for 1.5 hour. The organic layer was removed, and the aqueous phase was washed 4 times with n-heptane (3-4 mL). Finally the water layer was collected. No emission was observed under UV light.

[0122] This example illustrates that a typical ligand known in the art results in a large drop in the performance of the qdots.

Example 4B

[0123] To the ligand exchanged qdot dispersion of example 4A was added 1 ml of a 10M KOH solution in water. The dots flocculated and were centrifuged off. The sediment was redispersed in 0.25 ml 1M KOH. This resulted over the course of several days in an increase in quantum yield up to 17%. In general, with the sulphide ligands known in the art, an increase in QE was observed upon adding KOH during or after the ligand exchange.

[0124] Addition of smaller amounts of KOH also results in (slow) return of the emission: addition of 100 l 0.1M KOH to 100 l of a qdot dispersion made similar to 4A, resulted in an increase in QE from 0.5 to 14% after 3 days.

Example 4C

[0125] Ligand exchanges were performed as in example 4A, but instead of adding 2 ml of water to the (NH.sub.4).sub.4Sn.sub.2S.sub.6 solution, 2 ml of KOH solutions of various concentrations were used. After ligand exchange and washing with heptane, the QE of the aqueous layers was measured as such, after heptane washing and after sedimentation of the qdots with acetonitrile, followed by centrifugation/redispersion in water (to remove excess KOH). Results are listed in Table I, clearly showing a higher KOH concentration results in a higher QE. It is also clear that upon removing the excess of KOH the QE drops. The higher QE values after KOH removal for the higher starting concentrations of KOH are most likely due to the residual amount of KOH left in the dispersion after only one sedimentation/centrifugation/redispersion step. The QE is reduced further by repetition of this procedure. At KOH concentrations above 1M, the ionic strength of the water layer was so high the dots flocculated at the water/heptane interface.

TABLE-US-00001 TABLE I QE of Sn.sub.2S.sub.6.sup.4 ligand exchanged Qdots as a function of hydroxide concentration and processing QE (%) QE(%) [KOH] QE (%) after heptane after removal (mol/l) after LE washing of excess KOH 0 0 0.01 0.1 0.1 14 18 1 0.5 31 34 7 1.0 31 35 16

Example 5

Hydroxide Ligand

[0126] The hydroxide ion itself was also found to be able to exchange the original organic ligands from the qdots (as is also known from prior art). A ligand exchange with as described in example 4A but with Crystalplex (CdSe/CdS/ZnS core/shell/shell) with oleate ligands (emitting at 610 nm; QE 80%) and a 1M KOH solution resulted in the qdots in the aqueous layer with a QE of 20%.

Example 6

Preparation of the Zincate Ion Solution

[0127] A stock solution of K.sub.2Zn(OH).sub.4 in water was made by adding 3.125 mL of 1M ZnCl.sub.2 to 5.0 mL of 10M KOH (both in water). The resulting mixture was diluted to 25 mL with water in a volumetric flask, affording a stock solution which is 0.125M in Zn.sup.2+ and 2.0M in KOH.

Example 6A

Ligand Exchange with the Zincate Ion

[0128] A ligand exchange using the zincate ligand solution and commercially available quantum dots from Crystalplex (CdSe/CdS/ZnS core/shell/shell) with oleate ligands (emitting at 610 nm) was performed by using an apolar phase of 7 mL of n-heptane mixed with 1 mL of the qdot dispersion (5 mg/mL) and a polar phase consisting of 0.8 mL of the zincate stock solution and 7.2 mL of 1M KOH. The final concentrations in the polar phase were therefore 0.0125M in Zn.sup.2+ and 1.1M KOH. Both phases were mixed and stirred vigorously for 1 hour resulting in a transfer of the qdots to the polar phase. After demixing, the organic layer was removed, and the aqueous phase was washed 4 times with n-heptane (6-8 mL). The resulting aqueous dispersion was used as is. FTIR analysis showed the virtually complete replacement of the original organic ligands (less than 0.2% of the oleate ligand was still present). The QE of this dispersion was found to be up to 60% (original QE of the starting dots was 80% in heptane or 70% in toluene).

[0129] This example illustrates that with this new ligand system it is feasible to retain the original QE of the qdots to a large extent upon ligand exchange and transfer to a polar solvent. This high was reproduced several times

Example 6B

Ligand Exchange with the Zincate Ion (II)

[0130] Commercially available quantum dots from Crystalplex (CdSe/CdS/ZnS core/shell/shell) with oleate ligands (emitting at 610 nm) were subjected to an inorganic ligand exchange by adding 1 mL of the qdot solution (5 mg/ml in toluene) to 7 ml n-heptane. The polar phase was made by adding 1.6 ml 0.125 mol/l K.sub.2Zn(OH).sub.4 in KOH (KOH total 2 mol/l) to 4.8 mL 1.0 mol/l KOH and 1.6 ml water. This results in 8 ml polar phase with 0.025 mol/l Zn.sup.2+ and 1 mol/l KOH. The two phases were combined and stirred vigorously for 1 hour. The organic layer was removed, and the water phase was washed 3 times with n-heptane (ca. 8 ml). The QE of this dispersion was found to be 52% (original QE of the starting dots was 80% in heptane).

[0131] An overview of ligand exchanges with various concentrations of the zincate ligand and KOH is listed in Table II. At too high concentration of zinc and/or KOH, the ligand exchanged qdots are not colloidally stable (agglomeration is observed). At too low a zinc concentration this is also observed. When the KOH concentration is too low or the zinc concentration too high, the zincate solution is not stable (precipitation of Zn(OH).sub.2 observed).

TABLE-US-00002 TABLE II results of ligand exchanged qdots using solutions of zincate ions, as a function of the concentration of zinc and KOH. When a value for the QE is listed, a stable dispersion of ligand exchanged qdots was obtained. QE values measured several weeks after preparation, which has resulted in a drop in QE of 10-15%. [Zn.sup.2+]/[KOH].fwdarw. 0.25M 0.5M 1.0M 2.0M 0.125M zincate solution and LE qdots not colloidally stable 0.04 QE = 37% 0.0125 QE = 37% QE = 36% QE = 42% LE zincate qdots not solution colloidally not stable stable 0.004 LE qdots not colloidally stable

Example 7

Stannate Ligand

[0132] Commercially available quantum dots from Crystalplex (CdSe/CdS/ZnS core/shell/shell) with oleate ligands (emitting at 610 nm) were subjected to an inorganic ligand exchange by adding 0.25 mL of the qdot solution (5 mg/ml in toluene) to 1.75 ml n-heptane. The polar phase was made by adding 50 l 1M Na.sub.2SnO.sub.3 in water to 1.95 ml 1M KOH. The two phases were combined and stirred vigorously for 1 hour, upon which the qdots transferred to the aqueous layer. The organic layer was removed, and the water phase was washed 3 times with n-heptane (ca. 8 ml). The quantum efficiency was over 60%.

Example 8

Stannate Ligand

[0133] In another stannate example the ligand exchange was performed with the same type of quantum dots, also with oleate ligands. The apolar phase was heptane and the polar phase was 0.1 mol/l Na.sub.2SnO.sub.3 in 0.1 mol/l KOH. The ligands were exchange after 30 min stirring/shaking. As emulsion formation was observed, the mixture was centrifuged (only 5 min at 2000 RPM). The water phase orange, but turbid. The quantum efficiency (QE) was 68.7% at an emission maximum of 612.4 nm (2 h after starting the preparation).

Example 9

Stannate Ligand

[0134] In another stannate example the ligand exchange was performed with the same type of commercial quantum dots (as in Example 8), again capped with oleate ligands. The apolar phase was made by mixing 0.5 mL of the 5 mg/mL quantum dot dispersion with 1.5 mL of n-heptane. The polar phase was made by mixing 0.2 mL of an aqueous 1 M Na.sub.2SnO.sub.3 solution with 1.8 mL of water, resulting in a 0.1 M Na.sub.2SnO.sub.3 solution. The two phases were combined and shaken for 1 hour at room temperature. Some emulsion formation was observed, and the so the mixture was centrifuged (2 min at 2000 rpm) to assist the demixing. The water phase was orange but turbid. The quantum efficiency was 59% (measured same day as preparation).

Further Experiments

[0135] The organic ligands on the quantum dots are replaced by inorganic ligands, especially phosphate (PO.sub.4.sup.3) based. The ligand exchange makes the qdots dispersible in water or other polar solvents such as DSMO or formamide. We found that that with these ligands, a large drop in QE of the qdots can be prevented to a large extent. The drop in QE can be further minimised or even completely eliminated by performing the ligand exchange in water-free conditions. Hence, the following is described below:

[0136] the use of PO.sub.4.sup.3 as an inorganic ligand to achieve high quantum yield in quantum dots where the original ligands are exchanged for these ligands; and [0137] performing the ligand exchange in water-free conditions to achieve quantum yields with several ligands (PO.sub.4.sup.3, S.sup.2) to achieve quantum yields that are essentially the same of the qdot prior to ligand exchange, and increased shelf-life of the dispersion.

Example(s) 10

Phosphor Based Ligands

Example 10.1

Na.SUB.3.PO.SUB.4 .in Formamide, Water-Free

[0138] All processing and sample handling was done in a water-free glovebox environment using dried solvents and chemicals.

[0139] A stock solution of the inorganic ligand was made by dissolving 265 mg Na.sub.3PO.sub.4 (1.62 mmol) in 113 g (100 mL) of water-free formamide (Hydranal Formamide, Fluka). After stirring for 6 hours in a glovebox the solution was filtered through a 1.2 m syringe filter to remove any non-dissolved material. The resulting solution was 16 mM Na.sub.3PO.sub.4 in formamide.

[0140] Commercially available quantum dots from Crystalplex (CdSe/CdS/ZnS core/shell/shell) with oleate ligands (emitting at 610 nm; QE 80%) were subjected to an inorganic ligand exchange by adding 60 L of the qdot solution (50 mg/mL in toluene) to 5 g dodecane. As polar phase 10 g of the 16 mM Na.sub.3PO.sub.4 in formamide solution was used. The mixture was stirred vigorously for 16 hours. Then the organic phase was removed and the polar phase was washed once with n-heptane. After flocculation the qdots by mixing the polar phase with 10 mL ethanol, the suspension was centrifuged to separate the solvents from the precipitate. The qdots were redispersed in 1 mL dry formamide with a small amount of Na.sub.3PO.sub.4 (to stabilise the solution), the QE of this dispersion was found to be 80-85%, which is the same as the original qdots prior to ligand exchange in an organic solvent. QE was found to drop over time, but could be maintained by having a small amount of the ligand salt (Na.sub.3PO.sub.4) in the solution.

Example 10.2

Na.SUB.2.HPO.SUB.4 .in Formamide, Waterfree

[0141] As example 10.1, now with Na.sub.2HPO.sub.4 in dry formamide solution as the polar phase. After workup the QE was found to be 79%.

Example 10.3

Na.SUB.3.PO.SUB.4., Aqueous Processing

[0142] As example 10.1, but now in ambient conditions using water as the solvent. While the qdots transferred to the water layer, indicating a successful ligand exchange, the use of water lead to swift and extensive flocculation of the ligand exchanged quantum dots, with precluded further study.

Example(s) 11

Effect of Water

Example 11.1

Na.SUB.2.S.9H.SUB.2.O in Formamide, Ambient

[0143] This experiment was performed in ambient conditions with solvents that were not specifically dried. No special precautions to exclude water were taken.

[0144] A stock solution was made by solving 348.2 mg Na.sub.2S.9H.sub.2O (1.45 mmol) in 14.5 mL (16.4 g) formamide (0.1 mmol/mL; 24.01 mg/mL).

[0145] Commercially available quantum dots from Crystalplex (CdSe/CdS/ZnS core/shell/shell) with oleate ligands (emitting at 610 nm; QE 80%) were subjected to an inorganic ligand exchange by adding 1 mL of the qdot solution (5 mg/mL in toluene) to 7 mL n-Heptane. As polar phase 8 mL of fresh prepared Na.sub.2S.9H.sub.2O/FA stock solution was used (192 mg Na.sub.2S.9H.sub.2O). After stirring for 2 hours the QE of polar phase was measured to be 69%. After 10 days the QE dropped to 40.3%.

Example 11.2

Na.SUB.2.S.9H.SUB.2.O in a 50/50 Mix of Water and Formamide

[0146] As example 11.1, now using a 50/50 l/l mixture of water and formamide as the solvent. Ligand exchange was successful but the resulting QE was low: 17%.

Example 11.3

Na.SUB.2.S H.SUB.2.O in Water

[0147] As example 11.1, now using pure water as the solvent. Ligand exchange failed. The qdots became brown and heavily flocculated.

Example 11.4

Na.SUB.2.S in Formamide, Water-Free

[0148] All processing and sample handling was done in a water-free glovebox environment using dried solvents and chemicals.

[0149] A stock solution was made by solving 90 mg Na.sub.2S (1.153 mmol) in 11 mL water-free formamide (0.105 mmol/mL).

[0150] Commercially available quantum dots from Crystalplex (CdSe/CdS/ZnS core/shell/shell) with oleate ligands (emitting at 610 nm; QE 80%) were subjected to an inorganic ligand exchange by adding 0.1 mL of the qdot solution (50 mg/mL in toluene) to 3.9 mL n-heptane. 4 mL of fresh prepared (not older than 2 h) Na.sub.2S/formamide stock solution was used as polar phase (32.7 mg Na.sub.2S). After 30 minutes of stirring, all of the qdots moved to the polar phase. The QE of the polar phase was found to be 79%. After one week the QE dropped slightly to 72%.