Stable indium-containing semiconductor nanocrystals
Nanocrystals having an indium-based core and methods for making them and using them to construct core-shell nanocrystals are described. These core-shell nanocrystals are highly stable and provide higher quantum yields than known nanocrystals of similar composition, and they provide special advantages for certain applications because of their small size.
1. A method for making a nanocrystal having a core consisting essentially of InP and a shell consisting essentially of ZnS, the method comprising:
contacting a phosphorus source with an indium source in a non-coordinating solvent to form an InP core; and
contacting the InP core with a zinc source, wherein the zinc source is a C 5 -C 20 alkylcarboxylate salt of zinc, and a sulfur source, wherein the sulfur source is in a medium that comprises a coordinating solvent, to form a ZnS shell directly on the InP core.
2. The method of claim 1 wherein the sulfur source is added after the zinc source.
3. The method of claim 1 wherein the medium does not contain a phosphine or phosphine oxide.
4. The method of claim 1 , further comprising cleaning the InP core prior to application of the ZnS shell.
5. The method of claim 1 , further comprising contacting the core of InP with the zinc source and the sulfur source in a medium that comprises an alkylamine.
6. The method of claim 1 wherein oxygen is excluded from the nanocrystal preparation until after the shell has been formed.
7. The method of claim 1 wherein the indium source is a C 5 -C 20 alkylcarboxylate salt of indium.
8. The method of claim 1 wherein the phosphorus source is a trialkylsilyl phosphine.
9. The method of claim 1 wherein the zinc source is a lipophilic zinc salt.
10. The method of claim 1 wherein the sulfur source is elemental sulfur.
11. The method of claim 1 wherein the non-coordinating solvent is a hydrocarbon.
12. The method of claim 11 wherein the hydrocarbon is 1-octadecene, squalane, octadecane, or tetradecane.
13. The method of claim 1 wherein the coordinating solvent is a C 5 -C 20 alkylamine.
14. The method of claim 1 wherein the sulfur source is added to a reaction mixture containing InP core nanocrystals and the zinc source, while the mixture is at a temperature above 200° C.
15. The method of claim 1 wherein the thickness of the ZnS shell is at least 0.5 nm.