Light source with quantum dots

Abstract

The invention provides a luminescent nano particles based luminescent material comprising a matrix of interconnected coated luminescent nano particles, wherein for instance wherein the luminescent nano particles comprise CdSe, wherein the luminescent nano particles comprise a coating of CdS and wherein the matrix comprises a coating comprising ZnS. The luminescent material according may have a quantum efficiency of at least 80% at 25 C., and having a quench of quantum efficiency of at maximum 20% at 100 C. compared to the quantum efficiency at 25 C.

Claims

1. A method for producing a luminescent nano particle based luminescent material, the method comprising: mixing coated luminescent nano particles, a second coating precursor system, and optionally a surfactant in a liquid; and heating the thus obtained mixture, wherein the luminescent nano particles are selected from the group consisting of semiconductor nano particles that are able to emit in the visible part of the spectrum, wherein the coated luminescent nano particles comprise a first coating comprising a first coating material, being different from the semiconductor material of the nano particles, wherein the first coating material is selected from the group consisting of M1.sub.x-M2.sub.y-M3.sub.z-A.sub.(x+2y+3z)/2 compounds, wherein M1 is selected from the group consisting of Na, Li, Mg, Cu, Ag and Au, wherein M2 is selected from the group consisting of Zn and Cd, wherein M3 is selected from the group consisting of Ga, As, In and Tl, wherein A is selected from the group consisting of O, S, Se, As, P, and Te, wherein x is in the range of 0-1, wherein y is in the range of 0-1, wherein z is in the range of 0-1, wherein at least one of x, y and z is larger than 0, and wherein the second coating precursor system comprises one or more precursors for forming a second coating on the coated luminescent nano particles, the second coating comprising a second coating material, being different from the first coating material, wherein the second coating material is selected from the group consisting of M4A, wherein M4 is selected from the group consisting of Al, Ca, Mg, Zn and Cd, wherein A is selected from the group consisting of Cl, F, O, S, Se and Te.

2. The method according to claim 1, further comprising separating the thus obtained luminescent material from the liquid and drying the luminescent material.

3. The method according to claim 1, wherein the second coating precursor system comprises one or more of Bis[bis(2-hydroxyethyl)dithio carbamato]zinc(II), 2-Mercaptopyridine N-Oxide Zinc Salt, (Toluene-3,4-dithiolato)zinc(II), Dibenzyl dithio carbamic Acid Zinc(II) Salt, Zinc(II) Dibutyl dithio carbamate, Diethyl dithio carbamic Acid Zinc Salt, Zinc Dimethyl dithio carbamate, Bis(tetrabutylammonium) Bis(1,3-dithiole-2-thione-4,5-dithiolato)zinc Complex.

4. The method according to claim 1, wherein the luminescent nano particles are selected from the group consisting of InP, CuInS.sub.2, CuInSe.sub.2, CdTe, CdSe, CdSeTe, AgInS.sub.2, AgInSe.sub.2, and ZnSe:Mn.

5. The method according to claim 1, wherein the first coating material is selected from the group consisting of Cu.sub.xZn.sub.yIn.sub.zS.sub.(x+2y+3z)/2, Cu.sub.xZn.sub.yIn.sub.zSe.sub.(x+2y+3z)/2, and CdS.

6. The method according to claim 1, wherein the second coating material is selected from the group consisting of ZnS, SiO.sub.2, MgS, ZnSe, ZnSSe, ZnO, Zn.sub.1-xMg.sub.xS.sub.ySe.sub.1-y, ZnSO.sub.3 and ZnSO.sub.4.

7. The method according to claim 1, wherein the luminescent nano particles comprise CdSe, wherein the first coating material comprises CdS and wherein the second coating material comprises ZnS.

8. A lighting unit comprising a light source configured to provide light source light in the UV or blue part of the visible spectrum and a luminescent material configured to absorb at least part of the light source light, wherein the luminescent material comprises a luminescent nano particles based luminescent material comprising a matrix of interconnected coated luminescent nano particles, wherein the luminescent nano particles are selected from the group consisting of semiconductor nano particles that are able to emit in the visible part of the spectrum, wherein the luminescent nano particles comprise a first coating comprising a first coating material, being different from the semiconductor material of the nano particles, wherein the first coating material is selected from the group consisting of M1.sub.x-M2.sub.y-M3.sub.z-A.sub.(x+2y+3z)/2 compounds, wherein M1 is selected from the group consisting of Na, Li, Mg, Cu, Ag and Au, wherein M2 is selected from the group consisting of Zn and Cd, wherein M3 is selected from the group consisting of Ga, As, In and Tl, wherein A is selected from the group consisting of O, S, Se, As, P, and Te, wherein x is in the range of 0-1, wherein y is in the range of 0-1, wherein z is in the range of 0-1, wherein at least one of x, y and z is larger than 0, wherein the matrix comprises a second coating comprising a second coating material, being different from the first coating material, wherein the second coating material is selected from the group consisting of M4A, wherein M4 is selected from the group consisting of Al, Ca, Mg, Zn and Cd, wherein A is selected from the group consisting of Cl, F, O, S, Se and Te, wherein the matrix of interconnected luminescent nano particles comprises spherical-joint structures, wherein one or more spherical parts comprise one or more coated luminescent nano particles, wherein the spherical parts are interconnected with joints comprising a material selected from the group consisting of M1.sub.x-M2.sub.y-M3.sub.z-A.sub.(x+2y+3z)/2 and M4A compounds, with M1, M2, M3, M4, A, x, y, z as defined above.

9. The lighting unit according to claim 8, wherein adjacent luminescent nano particles have a shortest distance (d) of at least 5 nm, and wherein the second coating has coating thicknesses (d2) in the range of 1-50 nm.

10. The lighting unit according to claim 8, wherein the luminescent material has a quantum efficiency of at least 80% at 25 C., and has a quench of quantum efficiency of at maximum 20% at 100 C. compared to the quantum efficiency at 25 C.

11. The lighting unit according to claim 8, wherein the luminescent material is comprised in a coating, and wherein the coating is configured to transmit at least part of the light source light, and wherein the light source comprises a LED.

12. The lighting unit according to claim 8, wherein the luminescent nano particles are selected from the group consisting of InP, CuInS.sub.2, CuInSe.sub.2, CdTe, CdSe, CdSeTe, AgInS.sub.2, AgInSe.sub.2, and ZnSe:Mn.

13. The lighting unit according to claim 8, wherein the first coating comprises a material selected from the group consisting of Cu.sub.xZn.sub.yIn.sub.zS.sub.(x+2y+3z)/2, Cu.sub.xZn.sub.yIn.sub.zSe.sub.(x+2y+3z)/2, ZnTeSe, and CdS.

14. The lighting unit according to claim 8, wherein the matrix comprises CdSe/CdS dots-in-rots nano particles.

15. The lighting unit according to claim 8, wherein the matrix comprises CdSe/CdS core-shell nano particles.

16. The lighting unit according to claim 8, wherein the second coating is selected from the group consisting of ZnS, SiO.sub.2, MgS, ZnSe, ZnO, Zn.sub.1-xMg.sub.xS.sub.ySe.sub.1-y, ZnSO.sub.3 and ZnSO.sub.4.

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) FIGS. 1a-1d schematically depict some embodiment of the luminescent material; and

(3) FIGS. 2a-2b schematically depicts an embodiment of the lighting unit.

(4) The drawings are not necessarily on scale.

(5) FIG. 3a displays a HRTEM image of nano-composite particles and FIG. 3b displays the same HRTEM image of nano-composite particles with black circles and and lines indicating the matrix of (here) CdSe/CdS with white lines indicating the ZnS coating;

(6) FIG. 4 displays the results of electron microscopy combined with EDXS;

(7) FIG. 5 shows the quantum yields temperature quenching for both drop caste normal CdSe/CdS/ZnS spherical dots and as-prepared CdSe/CdS/ZnS core-in-matrix composite;

(8) FIG. 6 shows a plot of the temperature depended photo luminescence intensity in air for CdSe/CdS quantum dots(lowest curve), CdSe/CdS/ZnS core-shell QDs (middle curve), and the CdSe/CdS/ZnS core-in-matrix composite as described herein (upper curve).

(9) FIG. 7 shows a plot of the lifetime (temperature depended quantum efficiency) at 80 C. in air for both CdSe/CdS (lower curve) rods and the CdSe/CdS/ZnS core-in-matrix composite as described herein (upper curve).

DETAILED DESCRIPTION OF THE EMBODIMENTS

(10) FIG. 1a schematically depicts a luminescent nano particles based luminescent material 100 comprising a coated matrix 10 of interconnected coated luminescent nano particles 20. The luminescent nano particles 20, such as CdSe QDs, are selected from the group consisting of semiconductor nano particles that are able to emit in the visible part of the spectrum. The luminescent nano particles 20 comprise a first coating 25 comprising a first coating material 125, such as CdS, being different from the semiconductor material of the nano particles. The matrix 10 comprises a second coating 35 comprising a second coating material 135, being different from the first coating material 125. Therefore, the term coated matrix 10 is herein applied. The table below gives a non limiting number of examples of combination of materials that can be used to make the coated matrix 10:

(11) TABLE-US-00001 Specific combination Alternative combination Core CdSe InP, CuInS.sub.2, CuInSe.sub.2, CdTe, CdSeTe, AgInS.sub.2, AgInSe.sub.2, ZnSe:Mn Shell CdS Cu.sub.xZn.sub.yInzS.sub.(x+2y+3z)/2, CU.sub.xZn.sub.yInzSe.sub.(x+2y+3z)/2, ZnSeTe Coating ZnS SiO.sub.2, MgS, ZnSe, ZnO, TiO.sub.2, Zn.sub.1xMg.sub.xS.sub.ySe.sub.1y, ZnSOx

(12) FIG. 1a shows a particle 101 of such luminescent nano particles based luminescent material 100. The matrix 10 comprises matrix material 110, which comprises the coated nano particles 20, i.e. the nano particles 20 with first coating 25, and the second coating material 35. Note that some parts of the matrix 10 may entirely consist of second coating material.

(13) Here, the matrix 10 comprises thus interconnected luminescent nano particles 20. The matrix 10 comprises spherical joint structures 50, wherein one or more spherical parts 51 comprise one or more coated luminescent nano particles 20. The spherical parts are interconnected with joints 52 comprising a material selected form the group consisting of M1.sub.x-M2.sub.y-M3.sub.z-A.sub.(x+2y+3z)/2, M4A and SiO.sub.2 compounds, such as for instance ZnS or CdS or a combination thereof.

(14) The distance between adjacent nano particles 20 within the matrix 10 is indicated with reference d. In general, this distance will be at least 5 nm. The thickness of the first coating layer 25 is indicated with reference d1; the thickness of the second coating 35 is indicated with reference d2. Reference L indicates the length of the above indicated joint 52. This length L of the joint 52 may for instance be in the range of 1-20 nm.

(15) The term different in the context of the second coating material being different from the first, or the first coating material being different from the luminescent material, especially indicates that the chemical composition of such second coating material is different from the first coating material and the chemical composition of the first coating material is different from the composition of the luminescent material.

(16) FIG. 1b schematically depicts a non-limiting number of possible types of luminescent material particles 101 of the luminescent nano particles based luminescent material 100 as described herein. Amongst others, tripods are schematically depicted. However, also matrices 10 are depicted wherein at least 50% of the (spheres 51 comprising) nano particles 20 are interconnected with at least two neighboring spheres 51, via joints 52.

(17) FIG. 1c schematically depicts an embodiment wherein the luminescent material 100 comprises luminescent material particles 101 which comprise dots-in-rods particles 41. The distance between the coated luminescent particles 20 is indicated with length L, wherein L is the distance between the coatings 25 of the adjacent luminescent material particles. Note that the coatings 25 are in this embodiment rod-shaped.

(18) FIG. 1d schematically depicts an embodiment wherein the luminescent material 100 comprises luminescent material particles 101 which comprise core-shell particles 42.

(19) The matrix 10 may also comprise a combination of both core-shell particles 42 and dots-in-rods particles 41.

(20) FIGS. 2a-2b schematically depict a non-limiting number of embodiments of a lighting unit 1 comprising a light source 2 configured to provide light source light 12 in the UV or blue part of the visible spectrum and the luminescent material 100 as described herein, configured to absorb at least part of the light source light 12. The luminescent nano particles based luminescent material 100 converts at least part of the light source light 12 into luminescent material light 101, and provides, optionally together with remaining light source light 12 lighting unit light 7. In FIG. 2a, an embodiment is depicted, wherein the luminescent nano particles based luminescent material 100 is comprises by an exit window 5 of the lighting unit. Dependent upon the type of light source 2, the type of luminescent nano particles based luminescent material 100, the amount and layer thickness of the luminescent nano particles based luminescent material 100, light source light 12 may be found downstream of the window 5, which is indicated with the dashed arrow. In FIG. 2a, the luminescent nano particles based luminescent material 100 is arranged at a non-zero distance from the light source 2. The distance is indicated with reference L2. In FIG. 2b, however, the distance L2 between the light source 2 and the luminescent material 100 is substantially zero. For instance, the luminescent nano particles based luminescent material 100 may be embedded in a resin on a LED light source.

(21) The exit window 5 may for instance be an organic and/or inorganic matrix, wherein the luminescent nano particles based luminescent material 100 is embedded. Alternatively or additionally, the luminescent nano particles based luminescent material 100 of the invention may be coated to such window 5.

EXPERIMENTAL

(22) Below we show an example experiment to obtain CdSe/CdS/ZnS nano composites with the described structure as well as the structure and optical characterization for the as-prepared material.

Example Experiment: CdSe/CdS/ZnS Nanocomposite Synthesis

(23) CdSe/CdS dots-in-rods nanoparticles are prepared according to literature processes and dispersed in 1-octadecene (ODE) with the concentration of 5 microM. 2 ml of the above QRs solution, 0.1 mmol zinc diethyldithiolcarbamate and 0.05 mmol hexydecylamine are mixed into 10 ml ODE in a 100 ml flask under Nz. The mixture is heated slowly under stirring to 180 C. and kept for 10 min. Then the solution is further heated to 240 C. and kept for 20 min. After synthesis, the solution is cooled down to room temperature and washed with ethanol and toluene for 2 times each. The washed particles are dispersed in 3 ml toluene and stored in closed bottle. The drop caste films of the particle are prepared by direct casting one drop of the particle solution on a glass plate and drying the drop in air.

Characterizations

(24) The structure and optical properties of the products could be easily detected. The structure could be characterized through the characterization methods of TEM, EDXS, XRD, ICPMS and XPS for the shape, type of components, crystal structure of components and ratio of the components. Here we used HRTEM to detect the structure, shape of the core-shell matrices.

(25) The image shown in FIGS. 3a-3b exhibits the lattice of the materials in different regions that give the evidence of the crystal structure of the components. FIG. 3a displays a HRTEM image of nano-composite particles and FIG. 3b displays the same HRTEM image of nano-composite particles with black circles and and lines indicating the matrix of (here) CdSe/CdS with the area within white lines and black lines indicating the ZnS coating.

(26) FIG. 4 displays the results of electron microscopy combined with EDXS: the concentration of the components in different regions on surface and inside matrices can be seen. FIG. 4, the left images shows a detailed HAADF STEM image. The red arrow indicates the line that was scanned during EDX spectra acquisition: at equidistant points along this line EDX spectra were acquired. The arrow indicates the scan direction.

(27) The upper right pane shows the intensity on the HAADF detector as a function of the position on the line. The lower right panes: EDX compositional profile. Along the vertical axis concentrations are given in mass %. Therefore when count in atomic concentration % the Zn has to be doubled because the molecule weight of Cd is near double of Zn. It is clearly visible in line scans that a higher intensity on the HAADF detector corresponds with a higher Cd and a lower Zn concentration. The special area chosen to make this scan is a group of the head of the CdSe/CdS rods which has the minimum Zn containing and may have some expose of CdS.

(28) In the large scare TEM, you mainly see the CdSe/CdS particles because the ZnS has lower contrast than CdSe/CdS. However, in the high resolution TEM (HRTEM), you can clearly see the ZnS lattice among the CdSe/CdS that crosslink all the particles. The CdSe/CdS has the average distance of 4 nm and the ZnS thickness is larger than 1 nm.

(29) The below table shows the XPS elements analysis of an CdSe/CdS/ZnS matrix:

(30) TABLE-US-00002 Sample C 1s Cd 3d N 1s O 1s S2p Se 3d Zn 3p -position (eV) 284.8 404.5 399.5 168.2 161.1 88.5 org NH SO.sub.4 Sulphide 144_A 86 0.6 1.3 3.2 0.1 4.5 <0.02 4.0 144_B 88 0.4 1.3 2.9 0.1 3.9 <0.02 3.7

(31) The table shows the apparent atomic concentrations (at %) in the materials at the two duplicate positions. Results are shown in rows 3 and 4. In the second row peak positions in eV are given. The most likely chemical assignment, based on the peak position, is given in the third row. From this table we can draw the conclusion that the CdSe concentration is rather low due to the much lower volume of CdSe nuclei in the CdS/ZnS matrices. A present of NH ligand on the surface of the particle and partly surface oxidization of the surface S to SO.sub.4. Zn has a much higher concentration than Cd and Cd+Zn=S which indicates clearly amount of CdS and ZnS in the matrices. A minor part of the surface S stay as SO4; it is common for CdS and ZnS QDs and will also stabilize the surface.

(32) The material was drop casted and studied the optical properties including quantum yields, thermal quenching and air stability/lifetime at 80 C. under the irradiation of a blue light at 450 nm with the power of 5 W/cm.sup.2. The drop caste core-shelled CdSe/CdS quantum rods have the maximum quantum yields of 60% and show slightly red shift of 2-5 nm compare to the particle in solution. The shift is caused by concentration quenching. The reported CdSe/CdS/ZnS quantum rods and polymer composites have quantum yields of about 15-75% according to the literature (2-4). Our CdSe/CdS/ZnS composites with dots-in-matrix structure have much enhanced quantum yields with up to 90% with no/minor shift caused by concentration quenching. FIG. 5 shows the quantum yields temperature quenching for both drop caste normal CdSe/CdS/ZnS spherical dots and as-prepared CdSe/CdS/ZnS core-in-matrix composite. The results show the highly reduced thermal quenching of the composites. The diamonds indicate normal CdSe/CdS/ZnS spherical core multi-shelled particles and the square indicate as prepared CdSe/CdS/ZnS nano composites as described herein.

(33) FIG. 6 shows a plot of the temperature depended photo luminescence intensity in air for CdSe/CdS quantum dots(lowest curve), CdSe/CdS/ZnS core-shell QDs (middle curve), and the CdSe/CdS/ZnS core-in-matrix composite as described herein (upper curve).

(34) FIG. 7 shows a plot of the lifetime (temperature depended quantum efficiency) at 80 C. in air for both CdSe/CdS (lower curve) rods and the CdSe/CdS/ZnS core-in-matrix composite as described herein (upper curve). Results show the highly improved stability of the composites.

(35) These particles can be used as phosphors in LED lighting in various configurations such as remote, vicinity and proximity for converting blue light to other colours including white.

(36) The herein presented ZnS matrices provide a thick layer of ZnS around the CdSe/CdS particles that confine the excitons within CdSe/CdS and stabilize them to reduce the thermal quenching and a distance among the CdSe cores that reduce the self-absorption and Forster energy transfer. Thin layer indicates less than 1.5 monolayer of the coating, meaning less than 0.5 nm of the shell thickness. Thick shelling is preferred because it will provide more unique properties as described above. The advantage of the proposed and presented structure, is not only the thickness of the ZnS layerwhich is provided by the ZnS matrices, but also a homogenous ZnS shelling due to the unique spherical joint structure. Only rod shape is not able to give such homogenous shelling. The total values of the ZnS could be analyzed and calculated by the element measurements such as XPS and ICPMS. In the case of the rods, an acceptable ZnS shelling should have the Zn:Cd ratio of >0.6; in the case of the present CdSe/CdS/ZnS matrices, the Zn:Cd ratio is larger than 2.

(37) Herein, a QDs-in-matrix system is proposed, to obtain a stable core-multi shell structure, in which the first shell material (herein also indicated as first coating material (such as CdS) forms a matrix (with different surface facets in specified regions). During the shelling with the second shell material (herein also indicated as second coating material) (such as ZnS), facets of material (CdS) and (ZnS) which are matched grow in a semispherical fashion and become linked by more straight regions which have another crystallographic direction. The structure allows the stable and homogenous growth of different shells on the core materials and lead to highly enhanced performance of the nanoparticles.

(38) Here we suggest a new QDs-in-matrix structure to achieve the desired properties and stability of the core-multi shell nanoparticles. Firstly, the core quantum dots (component A, for example CdSe) may be lattice matched grown into a matrix of one shell material (component B, for example CdS). This material (component B) shows a particular enhancement/function to the properties of core quantum dots. The whole matrices (A in B) are then coated with a second shell material (component C, for example ZnS) for further enhancement. During the shelling with the second shell material (component C), component C (for example ZnS) are lattice matched grown on facets of component B (for example CdS) in a semispherical fashion and linked by more straight regions which have another crystallographic direction. This structure leads to a reduced lattice stress between the component C and component B at the straight regions.