Light source with quantum dots

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 x -M2 y -M3 z -A (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 2 , CuInSe 2 , CdTe, CdSe, CdSeTe, AgInS 2 , AgInSe 2 , and ZnSe:Mn.

5. The method according to claim 1 , wherein the first coating material is selected from the group consisting of Cu x Zn y In z S (x+2y+3z)/2 , Cu x Zn y In z Se (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 2 , MgS, ZnSe, ZnSSe, ZnO, Zn 1-x Mg x S y Se 1-y , ZnSO 3 and ZnSO 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 x -M2 y -M3 z -A (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 x -M2 y -M3 z -A (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 (d 2 ) 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 2 , CuInSe 2 , CdTe, CdSe, CdSeTe, AgInS 2 , AgInSe 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 x Zn y In z S (x+2y+3z)/2 , Cu x Zn y In z Se (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 2 , MgS, ZnSe, ZnO, Zn 1-x Mg x S y Se 1-y , ZnSO 3 and ZnSO 4 .