Flow synthesis of quantum dot nanocrystals
View Patent ↗Nanocrystals are synthesized with a high degree of control over reaction conditions and hence product quality in a flow-through reactor in which the reaction conditions are maintained by on-line detection of characteristic properties of the product and by adjusting the reaction conditions accordingly. The coating of nanocrystals is achieved in an analogous manner.
1. A process for the preparation of monodisperse luminescent semiconductor nanocrystals having detectable properties within a target range, said process comprising:
(a) combining nanocrystal-forming reactants comprising (i) a member selected from the group consisting of cadmium salts, zinc salts, cadmium oxide, zinc oxide, organocadmium compounds, and organozinc compounds and (ii) a member selected from the group consisting of an elemental chalcogen and a chalcogen-containing compound, with a solvent and a reaction promoter to form a solution;
(b) continuously passing said solution at a selected flow rate through a thermally conductive reaction tube that is maintained at a temperature sufficiently high to initiate a reaction among said reactants, thereby producing a product mixture containing nanocrystals;
(c) monitoring said product mixture to detect properties of said nanocrystals that are indicative of the degree to which said nanocrystals possess desired characteristics; and
(d) comparing the value of said properties thus detected with said target range and adjusting either the temperature of said reaction tube, the flow rate of said solution, or both, if needed to correct any deviation between said value of said detected properties and said target range.
2. A process in accordance with claim 1 in which said properties thus detected are optical properties.
3. A process in accordance with claim 1 further comprising cooling said product mixture between steps (b) and (c) to a temperature sufficiently low to quench said reaction.
4. A process in accordance with claim 3 in which said cooling is achieved by a heat transfer medium in contact with said reaction tube.
5. A process in accordance with claim 1 in which step (c) is performed upon emergence of said product mixture from said reaction tube.
6. A process in accordance with claim 3 in which said cooling is performed by combining additional solvent with said product mixture, said additional solvent being at a temperature and a proportion relative to said product mixture sufficient to achieve a final temperature sufficiently low to quench said reaction.
7. A process in accordance with claim 2 in which said optical features are photoluminescent emission spectra, and step (c) comprises irradiating said product mixture with light and detecting wavelength spectra of photoluminescent energy emitted from said nanocrystals.
8. A process in accordance with claim 2 in which said optical features are absorbance, and step (c) comprises irradiating said product mixture with light and detecting absorbance spectra of said nanocrystals.
9. A process in accordance with claim 2 in which said optical features are light scattering, and step (c) comprises irradiating said product mixture with light and detecting the presence of light scattering by said nanocrystals.
10. A process in accordance with claim 1 in which said thermally conductive reaction tube is a coiled tube cast in a solid block of heat conductive metal.
11. A process in accordance with claim 1 in which temperature maintenance of said reaction tube is achieved by a heat transfer medium maintained at a temperature of at least about 100° C.
12. A process in accordance with claim 11 in which said heat transfer medium is maintained at a temperature of from about 100° C. to about 400° C.
13. A process in accordance with claim 1 in which said reactant (i) is a member selected from the group consisting of dimethyl cadmium and cadmium acetate.
14. A process in accordance with claim 1 in which said chalcogen is a member selected from the group consisting of sulfur, selenium, and tellurium.
15. A process in accordance with claim 1 in which said chalcogen is selenium.
16. A process in accordance with claim 1 in which said nanocrystal comprises a member selected from the group consisting of ZnS, ZnSe, ZnTe, CdS, CdSe, and CdTe.
17. A process in accordance with claim 1 in which said solvent is a member selected from the group consisting of alkyl phosphines, alkyl phosphine oxides, pyridines, furans, ethers, amines, and alcohols.
18. A process in accordance with claim 1 in which said solvent is a member selected from the group consisting of tri n octylphosphine and tri n octylphosphine oxide.
19. A process in accordance with claim 1 in which said solvent is a mixture of tri n octylphosphine and tri n octylphosphine oxide.
20. A process in accordance with claim 11 in which step (d) comprises adjusting the temperature of said heat transfer medium.
21. A process in accordance with claim 1 in which step (d) comprises adjusting the flow rate of said solution.
22. A process for the coating of nanocrystals with a passivating coating to achieve coated nanocrystals having detectable properties within a target range, said process comprising:
(a) combining nanocrystal cores with surface passivating reactants capable of forming a shell wherein the shell comprises a member selected from the group consisting of CdS, CdSe, ZnS, ZnSe, ZnTe, and alloys and mixtures thereof, and a solvent to form a dispersion;
(b) continuously passing said dispersion through a thermally conductive reaction tube maintained at a temperature sufficiently high to initiate a reaction among said passivating reactants, thereby producing a product mixture containing nanocrystals coated with a passivating coating;
(c) monitoring said product mixture to detect properties of said nanocrystals that are indicative of the degree to which said nanocrystals possess desired characteristics; and
(d) comparing values of said properties thus detected with said target range and adjusting the temperature of said reaction tube, the flow rate of said solution, or both, if needed to correct any deviation between said values of said detected properties and said target range.
23. A process in accordance with claim 1 further comprising heating said nanocrystal-forming reactants prior to step (a).
24. A process in accordance with claim 1 wherein said solvent is a coordinating solvent.
25. A process in accordance with claim 1 wherein step (a) comprises combining said nanocrystal-forming reactants with a coordinating additive in addition to said solvent and said reaction promoter.
26. A process in accordance with claim 1 wherein said reaction promoter is a member selected from the group consisting of oxygen and a reducing agent.