SHELLING OF HALIDE PEROVSKITE NANOPARTICLES FOR THE PREVENTION OF ANION EXCHANGE

Abstract

A core/shell semiconductor nanoparticle structure comprises a core comprising a halide perovskite semiconductor and a shell comprising a semiconductor material that is not a halide perovskite (and that is substantially free of halide perovskites). The halide perovskite semiconductor core may be of the form AMX.sub.3, wherein: A is an organic ammonium such as CH.sub.3NH.sub.3.sup.+, (C.sub.8H.sub.17).sub.2(CH.sub.3NH.sub.3).sup.+, PhC.sub.2H.sub.4NH.sub.3.sup.+, C.sub.6H.sub.11CH.sub.2NH.sub.3.sup.+ or 1-adamantyl methyl ammonium, an amidinium such as CH(NH.sub.2).sub.2.sup.+, or an alkali metal cation such as Li.sup.+, Na.sup.+, K.sup.+, Rb.sup.+ or Cs.sup.+; M is a divalent metal cation such as Mg.sup.2+, Mn.sup.2+, Ni.sup.2+, Co.sup.2+, Pb.sup.2+, Sn.sup.2+, Zn.sup.2+, Ge.sup.2+, Eu.sup.2+, Cu.sup.2+ or Cd.sup.2+; and X is a halide anion (F.sup., Cl.sup., Br.sup., I.sup.) or a combination of halide anions.

Claims

1. A core/shell semiconductor nanoparticle comprising: a core comprising a halide perovskite semiconductor; and a shell substantially surrounding the core and comprising a semiconductor material that is not a halide perovskite, wherein the shell is substantially free of halide perovskites.

2. The core/shell semiconductor nanoparticle of claim 1, wherein the shell comprises BaTiO.sub.3, SrTiO.sub.3, BiFeO.sub.3, LaNiO.sub.3, CaTiO.sub.3, PbTiO.sub.3 or LaYbO.sub.3.

3. The core/shell semiconductor nanoparticle of claim 1, wherein the shell comprises a group IIB-VIB semiconductor material or a group IV-VI semiconductor material.

4. The core/shell semiconductor nanoparticle of claim 1, wherein the shell comprises ZnS or PbS.

5. The core/shell semiconductor nanoparticle of claim 1, wherein the core comprises a halide perovskite semiconductor of the form AMX.sub.3, wherein A is an organic ammonium, an anidinium or alkali metal cation, M is a divalent metal cation, and X is a halide anion.

6. The core/shell semiconductor nanoparticle of claim 5, wherein A is CH.sub.3NH.sub.3.sup.+, (C.sub.8H.sub.17).sub.2(CH.sub.3NH.sub.3).sup.+, PhC.sub.2H.sub.4NH.sub.3.sup.+, C.sub.6H.sub.11CH.sub.2NH.sub.3.sup.+, 1-adamantyl methyl ammonium, CH(NH.sub.2).sub.2.sup.+, Li.sup.+, Na.sup.+, K.sup.+, Rb.sup.+ or Cs.sup.+.

7. The core/shell nanoparticle of claim 5, wherein M is Mg.sup.2+, Mn.sup.2+, Ni.sup.2+, Co.sup.2+, Pb.sup.2+, Sn.sup.2+, Zn.sup.2+, Ge.sup.2+, Eu.sup.2+, Cu.sup.2+ or Cd.sup.2+.

8. The core/shell nanoparticle of claim 5, wherein X is F.sup., Cl.sup., Br.sup.or I.sup., or a combination thereof.

9. A composition comprising: a first population of core/shell semiconductor nanoparticles; a second population of core/shell semiconductor nanoparticles different from the first population of core/shell semiconductor nanoparticles, and a matrix, wherein the first population of core/shell semiconductor nanoparticles comprises core/shell semiconductor nanoparticles comprising a first core comprising a first halide perovskite semiconductor, and a first shell substantially surrounding the core and comprising a semiconductor material that is not a halide perovskite, wherein the first shell is substantially free of halide perovskites, and the second population of core/shell semiconductor nanoparticles comprises core/shell semiconductor nanoparticles comprising a second core comprising a second halide perovskite semiconductor different from the first halide perovskite semiconductor, and a second shell substantially surrounding the core and comprising a semiconductor material that is not a halide perovskite, wherein the second shell is substantially free of halide perovskites.

10. The composition of claim 9, wherein the matrix comprises a resin.

11. The composition of claim 9, wherein the first population of core/shell semiconductor nanoparticles emits light of a first wavelength and the second population of core/shell semiconductor nanoparticles emits light at a second wavelength when excited by a light source emitting at a third wavelength.

12. The composition of claim 11, wherein the first wavelength is within the green region of the electromagnetic spectrum and the second wavelength is within the red region of the electromagnetic spectrum.

13. The composition of claim 11, wherein the light source is a light-emitting diode.

14. The composition of claim 11, wherein the third wavelength is within the ultraviolet or blue region of the electromagnetic spectrum.

15. A solution comprising: a first population of core/shell semiconductor nanoparticles; a second population of core/shell semiconductor nanoparticles; and a solvent, wherein the first population of core/shell semiconductor nanoparticles comprises core/shell semiconductor nanoparticles comprising a first core comprising a first halide perovskite semiconductor, and a first shell substantially surrounding the core and comprising a semiconductor material that is not a halide perovskite, wherein the first shell is substantially free of halide perovskites, and wherein the second population of core/shell semiconductor nanoparticles comprises core/shell semiconductor nanoparticles comprising a second core comprising a second halide perovskite semiconductor that is different from the first halide perovskite semiconductor, and a second shell substantially surrounding the core and comprising a semiconductor material that is not a halide perovskite, wherein the second shell is substantially free of halide perovskites.

16. The solution of claim 15, wherein the first population of core/shell semiconductor nanoparticles emits light of a first wavelength and the second population of core/shell semiconductor nanoparticles emits light at a second wavelength when excited by a light source emitting at a third wavelength.

17. The solution of claim 15, wherein the first wavelength is within the green region of the electromagnetic spectrum and the second wavelength is within the red region of the electromagnetic spectrum.

18. The solution of claim 15, wherein the light source is a light-emitting diode.

19. The solution of claim 15, wherein the third wavelength is within the ultraviolet or blue region of the electromagnetic spectrum.

20. A light-emitting device comprising the core/shell nanoparticle of claim 1.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

[0012] FIG. 1 is a cross-sectional view showing the structure of certain core/shell nanocrystals according to the invention.

[0013] FIG. 2 shows UV-vis and PL spectra of (A) cores and (B) core/shell nanoparticles. The UV-vis exciton peak and the emission wavelength of the core/shell material are red-shifted compared to the un-passivated cores.

DETAILED DESCRIPTION OF THE INVENTION

[0014] Herein, core/shell nanoparticles comprising a halide perovskite semiconductor core and a semiconductor shell of a material other than a halide perovskite are described, wherein the shell may act as a barrier to prevent migration of the halide anion(s) from the core, thus preventing anion exchange when more than one type of halide perovskite nanoparticle are combined in a solution or matrix. A cross-sectional view of the structure of such core/shell nanoparticles is shown in FIG. 1. The invention is particularly suitable for combining different colors of nanoparticles for use in display devices.

[0015] As used herein, the term halide perovskite means a material of the form AMX.sub.3, where A is an organic ammonium such as, but not restricted to, CH.sub.3NH.sub.3.sup.+, (C.sub.8H.sub.17).sub.2(CH.sub.3NH.sub.3).sup.+, PhC.sub.2H.sub.4NH.sub.3.sup.+, C.sub.6H.sub.11CH.sub.2NH.sub.3.sup.+ or 1-adamantyl methyl ammonium, an amidinium such as, but not restricted to, CH(NH.sub.2).sub.2.sup.+, or an alkali metal cation such as, but not restricted to, Li.sup.+, Na.sup.+, K.sup.+, Rb.sup.+ or Cs.sup.+; M is a divalent metal cation such as, but not restricted to, Mg.sup.2+, Mn.sup.2+, Ni.sup.2+, Co.sup.2+, Pb.sup.2+, Sn.sup.2+, Zn.sup.2+, Ge.sup.2+, Eu.sup.2+, Cu.sup.2+ or Cd.sup.2+; and X is a halide anion (F.sup., Cl.sup., Br.sup., I.sup.) or a combination of halide anions.

[0016] The halide perovskite nanocrystal cores may be synthesised by any method. The synthesis of halide perovskite nanocrystals is well known in the prior art. For instance, colloidal syntheses of halide perovskite nanocrystals have been reviewed by Veldhuis et al. [S. A. Veldhuis, P. P. Boix, N. Yantara, M. Li, T. C. Sum, N. Mathews and S. G. Mhaisalkar, Adv. Mater., 2016, 28, 6804] For organo-halide perovskite nanocrystals (for example where A is an ammonium ion), synthesis is typically conducted below about 80 C., via a ligand-assisted re-precipitation method. Here, polar solvents that can dissolve inorganic lead and ammonium halide salts are injected into a polar solvent in the presence of coordinating ligands that stabilize the nanoparticles. CsPbX.sub.3 nanocrystals are typically synthesized by hot-injection at temperatures above about 150 C.

[0017] In some embodiments, the core/shell nanocrystal comprises a shell of a wider band gap semiconductor material. Table 1 shows the band gap of various halide perovskite semiconductors, as determined by ab initio calculations. [L. Lang, J.-H. Yang, H.-R. Liu, H.J. Xiang and X.G. Gong, Phys. Lett. A, 2014, 378, 290]

TABLE-US-00001 TABLE 1 E.sub.g, eV X = Cl X = Br X = I CH.sub.3NH.sub.3SnX.sub.3 1.94 1.31 0.75 CH.sub.3NH.sub.3PbX.sub.3 1.98 1.48 0.95 CsSnX.sub.3 1.19 0.80 0.49 CsPbX.sub.3 1.83 1.32 0.86

[0018] In some embodiments, the shell material comprises a perovskite crystal structure but does not comprise halide ions, such that the core and shell share a compatible crystallographic phase or similar lattice type to facilitate epitaxial growth of the shell layer and to minimize lattice strain at the core/shell interface. Suitable materials include, but are not restricted to: BaTiO.sub.3, SrTiO.sub.3, BiFeO.sub.3, LaNiO.sub.3, CaTiO.sub.3, PbTiO.sub.3 and LaYbO.sub.3. The shelling of nanoparticles with materials such as BaTiO.sub.3 has been described in the prior art. [P. N. Oliveira, D. Alanis, R. D. Bini, D. M. Silva, G. S. Dias, I. A. Santos, L. F. Ctica, R. Guo and A. S. Bhalla, Integrated Ferroelectrics, 2016, 174, 88]

[0019] In other embodiments, the shell material comprises a semiconductor that does not comprise the perovskite crystal structure. Suitable materials include, but are not restricted to: [0020] IIA-VIB (2-16) material, incorporating a first element from group 2 of the periodic table and a second element from group 16 of the periodic table and also including ternary, quaternary and higher order materials and doped materials, such as, but not restricted to: MgS, MgSe, MgTe, CaS, CaSe, CaTe, SrS, SrSe, SrTe; [0021] IIB-VIB (12-16) material incorporating a first element from group 12 of the periodic table and a second element from group 16 of the periodic table and also including ternary, quaternary and higher order materials and doped materials, such as, but not restricted to: ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe; [0022] II-V material incorporating a first element from group 12 of the periodic table and a second element from group 15 of the periodic table and also including ternary, quaternary and higher order materials and doped materials, such as, but not restricted to: Zn.sub.3P.sub.2, Zn.sub.3As.sub.2, Cd.sub.3P.sub.2, Cd.sub.3As.sub.2, Cd.sub.3N.sub.2, Zn.sub.3N.sub.2; [0023] III-V material incorporating a first element from group 13 of the periodic table and a second element from group 15 of the periodic table and also including ternary, quaternary and higher order materials and doped materials, such as, but not restricted to: BP, AlP, AlAs, AlSb; GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, AlN, BN;

[0024] III-IV material incorporating a first element from group 13 of the periodic table and a second element from group 14 of the periodic table and also including ternary, quaternary and higher order materials and doped materials, such as, but not restricted to: B.sub.4C, Al.sub.4C.sub.3, Ga.sub.4C;

[0025] III-VI material incorporating a first element from group 13 of the periodic table and a second element from group 16 of the periodic table and also including ternary, quaternary and higher order materials and doped materials, such as, but not restricted to: Al.sub.2S.sub.3, Al.sub.2Se.sub.3, Al.sub.2Te.sub.3, Ga.sub.2S.sub.3, Ga.sub.2Se.sub.3, In.sub.2S.sub.3, In.sub.2Se.sub.3, Ga.sub.2Te.sub.3, In.sub.2Te.sub.3; [0026] IV-VI material incorporating a first element from group 14 of the periodic table and a second element from group 16 of the periodic table and also including ternary, quaternary and higher order materials and doped materials, such as, but not restricted to: PbS, PbSe, PbTe, Sb.sub.2Te.sub.3, SnS, SnSe, SnTe;

[0027] material incorporating a first element from any group in the d-block of the periodic table, and a second element from group 16 of the periodic table, and optionally including any element from group 13 of the periodic table to form ternary, quaternary and higher order materials and doped materials, such as, but not restricted to: NiS, CrS, CuInS.sub.2, CuInSe.sub.2, CuGaS.sub.2, CuGaSe.sub.2, Cu.sub.2ZnSnS.sub.4.

[0028] The method of shell growth is unrestricted. Methods for shelling nanocrystal cores are well-known in the art. For example, the growth of a ZnS shell on InP core nanocrystals is described in U.S. Pat. No. 7,588,828 the contents of which are hereby incorporated by reference in their entirety.

[0029] There is growing concern regarding the use of heavy metals in consumer goods such as electronics products. The EU's Restriction of the Use of Certain Hazardous Substances (RoHS) Directive 2002/95/EC limits the amounts of heavy metals that can be used in electrical and electronic appliances. Similar legislation is being adopted across the globe. In some embodiments, the shell material is free of heavy metals. When the core of the nanoparticle contains one or more heavy metals, a heavy metal-free shell layer may prevent leaching of heavy metal ions from the core, which could be encountered, for example, in landfill environment or in biological systems.

[0030] When two or more types of the halide perovskite nanocrystals described herein are combined in a solution or matrix (for example a resin), the shell layer may act as a physical barrier to prevent anion exchange between the halide ions. This may enable two or more types (colors) of halide perovskite nanocrystals to be used to form, for example, a light-emitting device while retaining their distinct emissive properties. For example, green-emitting core/shell halide perovskite nanoparticles and red-emitting core/shell halide perovskite nanoparticles may be combined in a solution or matrix and incorporated into a device that emits white light when irradiated by a blue-emitting light source (such as a blue-emitting LED). Blue-, green- and red-emitting core/shell halide perovskite nanoparticles may be combined in a solution or matrix and incorporated into a device that emits white light when irradiated by an ultraviolet-emitting light source (such as an ultraviolet-emitting LED). Further, the shell layer may help to eliminate defects, dangling bonds and trap states from the halide perovskite core surface to improve the photoluminescence QY and stability of the nanocrystals.

EXAMPLES

Example 1

Synthesis of Core/Shell CsPbBr.SUB.3./ZnS Quantum Dots

[0031] First, a solution of Cs-oleate was prepared by mixing Cs.sub.2CO.sub.3 (0.407 g) with octadecene (20 mL) and oleic acid (1.55 mL), in a 50-mL 3-neck flask. The resulting mixture was dried for 1 hour at 120 C., and then heated under N.sub.2 to 150 C. until all the Cs.sub.2CO.sub.3 was dissolved. The solution was kept at 140 C. to prevent solidifying.

[0032] CsPbBr.sub.3 cores were synthesized by mixing PbBr.sub.2 (69 mg) and octadecene (5 mL) in a 25-mL 3-neck flask. The cloudy suspension was heated to 120 C. under nitrogen. Subsequently, oleic acid (0.5 mL) and oleylamine (0.5 mL) were injected and the solution was stirred until the PbBr.sub.2 dissolved completely. The reaction mixture was heated to 180 C. and Cs-oleate was injected. After 5 seconds, the cloudy, yellow mixture was cooled with compressed air. An aliquot of the crude solution had the following optical properties: photoluminescence maximum (PL.sub.max)=507 nm, full-width at half-maximum (FWHM)=27.5 nm, photoluminescence quantum yield (PLQY)=36%. The reaction solution was centrifuged and the particles were precipitated by adding acetone to the supernatant. The resulting pellet was re-dispersed in hexane.

[0033] For the shelling, 1 mL of core solution was mixed with octadecene (5 mL), zinc acetate (36.7 mg) and dodecanethiol (0.12 mL) and the resulting mixture was degassed at room temperature for 5 minutes. The flask was refilled with nitrogen and heated to 180 C. Once the temperature reached 180 C., the reaction was cooled again. An aliquot of the crude solution had the following optical properties: PL.sub.max=519 nm, FWHM=17 nm, PLQY=32%. The emission wavelength was significantly red-shifted compared to the un-passivated cores.

Example 2

Synthesis of Core/Shell CsPbBr.SUB.3./PbS QDs

[0034] First, a solution of Cs-oleate was prepared by mixing Cs.sub.2CO.sub.3 (0.407 g) with octadecene (20 mL) and oleic acid (1.55 mL), in a 50-mL 3-neck flask. The resulting mixture was dried for 1 hour at 120 C., and then heated under N.sub.2 to 150 C. until all the Cs.sub.2CO.sub.3 was dissolved. The solution was kept at 140 C. to prevent solidifying.

[0035] CsPbBr.sub.3 cores were synthesized by mixing PbBr.sub.2 (69 mg) and octadecene (5 mL) in a 25-mL 3-neck flask. The cloudy suspension was heated to 120 C. under nitrogen. Subsequently, oleic acid (0.5 mL) and oleylamine (0.5 mL) were injected and the solution was stirred until the PbBr.sub.2 dissolved completely. The reaction mixture was heated to 180 C. and Cs-oleate was injected. After 5 seconds, the cloudy, yellow mixture was cooled with compressed air. An aliquot of the crude solution had the following optical properties: PL.sub.max=507 nm, FWHM=27.5 nm, PLQY=36%. The reaction solution was centrifuged and the particles were precipitated by adding acetone to the supernatant. The resulting pellet was re-dispersed in hexane.

[0036] For the shelling, 0.4 mL of core solution was mixed with octadecene (5 mL) and dodecanethiol (0.2 mL) and the resulting mixture was degassed at room temperature for 5 minutes. The flask was refilled with nitrogen and heated to 120 C. Once the temperature reached 120 C., the reaction was held for 10 minutes before cooling to room temperature. An aliquot of the crude solution had the following optical properties: PL.sub.max=520 nm, FWHM=18 nm, PLQY=21%. The emission wavelength was significantly red-shifted compared to the un-passivated cores.

Example 3

Synthesis of Core/Shell CsPbBr.SUB.3./PbS Quantum Dots

[0037] First, a solution of Cs-oleate was prepared by mixing Cs.sub.2CO.sub.3 (0.407 g) with octadecene (20 mL) oleic acid (1.55 mL), into a 50-mL 3-neck flask. The resulting mixture was dried for 1 hour at 120 C., and then heated under N.sub.2 to 150 C. until all the Cs.sub.2CO.sub.3 dissolved. The solution was kept at 140 C. to prevent solidifying.

[0038] CsPbBr.sub.3 cores were synthesized by mixing PbBr.sub.2 (69 mg) and octadecene (5 mL) in a 25-mL 3-neck flask. The cloudy suspension was heated to 120 C. under nitrogen. Subsequently, oleic acid (0.5 mL) and oleylamine (0.5 mL) were injected and the solution was stirred until the PbBr.sub.2 dissolved completely. The reaction mixture was heated to 180 C. and Cs-oleate was injected. After 5 seconds, the cloudy, yellow mixture was cooled with compressed air. An aliquot of the crude solution had the following optical properties: PL.sub.max=512 nm, FWHM=20 nm, PLQY=31%. The reaction solution was centrifuged and the particles were precipitated by adding acetone to the supernatant. The resulting pellet was re-dispersed in hexane.

[0039] For the shelling, 1 mL of core solution was mixed with octadecene (3 mL), PbBr.sub.2 (73 mg) and dodecanethiol (0.2 mL) and the resulting mixture was degassed at room temperature for 5 minutes. The flask was refilled with nitrogen and heated to 180 C. Once the temperature reached 120 C., the reaction was held at this temperature for 30 minutes and cooled again. An aliquot of the crude solution had the following optical properties: PL.sub.max=514 nm, FWHM=19 nm, PLQY=83%. The emission wavelength was slightly red-shifted and the PLQY increased significantly compared to the un-passivated cores. The UV-vis and PL spectra of core and core/shell species are shown in FIG. 2.

[0040] By enabling two or more types of halide perovskite nanocrystals to be processed together, the present invention allows such nanocrystals to be processed more easily, and at lower cost, than prior art methods wherein each type of halide perovskite nanocrystal must be separately encapsulated prior to mixing.

[0041] All-inorganic perovskite nanoparticles are unstable in polar solvents, while organometallic halide perovskite nanoparticles are unstable in the presence of humidity and high temperatures [H. C. Yoon, H. Kang, S. Lee, J. H. Oh, H. Yang and Y. R. Do, ACS Appl. Mater. Interfaces, 2016, 8, 18189]. The addition of a non-halide perovskite shell layer to the halide perovskite core nanoparticles may help to overcome some of these stability issues.

[0042] The foregoing presents particular embodiments embodying the principles of the invention. Those skilled in the art will be able to devise alternatives and variations which, even if not explicitly disclosed herein, embody those principles and are thus within the scope of the invention. Although particular embodiments of the present invention have been shown and described, they are not intended to limit what this patent covers. One skilled in the art will understand that various changes and modifications may be made without departing from the scope of the present invention as literally and equivalently covered by the following claims.