Quantum dot compositions

Abstract

It has been discovered that certain silicon-containing, surface-modifying ligands can be used to make semiconductor nanoparticles (quantum dots) more compatible with polysiloxanes. Quantum dots dispersed in a polysiloxane matrix may be used, for example, in light-emitting devices to alter the emission spectrum of such devices.

Claims

1. A composition comprising: at least one quantum dot comprising a semiconductor material; surface ligands disposed on the surface of the quantum dot, the surface ligand comprising a polysiloxane; and a capping agent disposed on the surface of the quantum dot, the capping agent selected from the group consisting of phosphines, phosphine oxides, alkyl phosphonic acids, alkyl-amines, aryl-amines, pyridines, long chain fatty acids and thiophenes.

2. The composition recited in claim 1 wherein the polysiloxane has a formula selected from the group consisting of:
RSi(CH.sub.3).sub.2O(Si(CH.sub.3).sub.2O).sub.nSi(CH.sub.3).sub.3,
RSi(CH.sub.3).sub.2O(Si(CH.sub.3).sub.2O).sub.nSi(CH.sub.3).sub.2 and
R(Si(R.sub.2R.sub.3)O).sub.n(Si(R.sub.3R.sub.4)O).sub.m where n and m are integers and R, R.sub.1, R.sub.2, R.sub.3, and R.sub.4 are aliphatic groups.

3. The composition recited in claim 2 wherein at least one of R, R.sub.1, R.sub.2, R.sub.3, and R.sub.4 are substituted with a functional group that chemically binds to the surface of the quantum dot.

4. The composition recited in claim 3 wherein the functional group is selected from the group consisting of thiols, carboxylates, and amines.

5. The composition recited in claim 2 wherein at least one of R, R.sub.1, R.sub.2, R.sub.3, and R.sub.4 is a substituted (CH.sub.2).sub.xCH.sub.3 chain, wherein x is 2 to 30.

6. The composition recited in claim 2 wherein at least one of R, R.sub.1, R.sub.2, R.sub.3, and R.sub.4 is an unsubstituted (CH.sub.2).sub.xCH.sub.3 chain, wherein x is 2 to 30.

7. The composition recited in claim 2 wherein the number of repeating units, n, is 2 to 30.

8. The composition recited in claim 1 wherein the quantum dot is a cadmium-free quantum dot.

9. The composition recited in claim 1 wherein the surface ligands comprise compounds having the chemical formula: HOOC(CH.sub.2).sub.10Si(CH.sub.3).sub.2O(Si(CH.sub.3).sub.2O).sub.ySi(CH.sub.3).sub.2C.sub.4H.sub.9, wherein y is an integer.

10. The composition recited in claim 1 wherein the surface ligands comprise compounds having the chemical formula: HOOCC.sub.18H.sub.36-PDMS where PDMS is polydimethylsiloxane.

11. A lighting device comprising a light-emitting diode (LED) encapsulated in a matrix, the matrix comprising: a polysiloxane; and a composition dispersed in the polysiloxane, the composition comprising: at least one quantum dot comprising a semiconductor material; surface ligands disposed on the surface of the quantum dot, the surface ligand comprising a polysiloxane; and a capping agent disposed on the surface of the quantum dot, the capping agent selected from the group consisting of phosphines, phosphine oxides, alkyl phosphonic acids, alkyl-amines, aryl-amines, pyridines, long chain fatty acids and thiophene.

12. A polymer film comprising a polymer polysiloxane matrix having dispersed therein a composition, the composition comprising: at least one quantum dot comprising a semiconductor material; surface ligands disposed on the surface of the quantum dot, the surface ligand comprising a polysiloxane; and a capping agent disposed on the surface of the quantum dot, the capping agent selected from the group consisting of: phosphines, phosphine oxides, alkyl phosphonic acids, alkyl-amines, aryl-amines, pyridines, long chain fatty acids and thiophene.

13. A polymer bead comprising a polysiloxane matrix having dispersed therein a composition comprising: at least one quantum dot comprising a semiconductor material; surface ligands disposed on the surface of the quantum dot, the surface ligand comprising a polysiloxane; and a capping agent disposed on the surface of the quantum dot, the capping agent selected from the group consisting of: phosphines, phosphine oxides, alkyl phosphonic acids, alkyl-amines, aryl-amines, pyridines, long chain fatty acids and thiophene.

Description

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

(1) FIG. 1 is a schematic illustration of a process for making polyalkylsiloxane-modified QDs, according to an embodiment of the invention.

(2) FIG. 2 contains photographs of quantum dots in viscous polydimethylsiloxane resin without additional surface ligand (top left vial) and with surface ligand (top right vial) and their corresponding films after curing at 50 C. for 24 hours under nitrogen.

(3) FIG. 3 shows dispersions of QDs in low viscosity polydimethylsiloxane with and without using additional surface ligands according to the invention

(4) FIG. 4 shows spectra of pristine (dashed line) and C.sub.22H.sub.45-polydimethyl-siloxane-treated (solid line) QD/crosslinked polydimethylsiloxane films.

DETAILED DESCRIPTION OF THE INVENTION

(5) Inorganic, silicon-based polymers, referred to as polysiloxanes, are often used for high performance coatings due to their excellent resistance to heat, UV radiation and oxidation. An example of a polysiloxane is polydimethylsiloxane (PDMS available as SYLGARD 184, DOW CORNING CORPORATION 2200 WEST SALZBURG ROAD MIDLAND MICHIGAN 48686). Several functional groups such as acrylate and epoxy can be incorporated in the polysiloxanes, providing flexibility to make cross-linked films and having other desired properties (e.g., high solids/low volatile organic compounds, good weatherability, excellent resistance to corrosion). Polysiloxanes may provide a cost-effective alternative to traditional organic coatings. Unfortunately, QDs generally disperse poorly in most polysiloxanes because the prior art surface ligands often used on the QDs are generally not compatible with the polysiloxane matrix. As a result, films and other structures made of QDs dispersed in polysiloxanes exhibit less than ideal QYs.

(6) It has been discovered that silicon-containing surface-modifying ligands can be used to render QDs more compatible with polysiloxanes. The silicon-containing surface-modifying ligands can be represented by structures 1-3:
RSi(CH.sub.3).sub.2O(Si(CH.sub.3).sub.2O).sub.nSi(CH.sub.3).sub.3(1)
RSi(CH.sub.3).sub.2O(Si(CH.sub.3).sub.2O).sub.nSi(CH.sub.3).sub.2R.sub.1(2)
R(Si(R.sub.2R.sub.3)O).sub.n(Si(R.sub.3R.sub.4)O).sub.m(3)

(7) Where n is an integer ranging from 2 to 30 and m is an integer, and R, R.sub.1, R.sub.2, R.sub.3, and R.sub.4 are aliphatic groups, with or without end-functionality. For example, one or more of R, R.sub.1, R.sub.2, R.sub.3, and R.sub.4 can be substituted with a functional group that chemically binds to the surface of QDs. Examples of such functional groups include thiols, carboxylates, and amines. According to certain embodiments, any of RR.sub.4 can be a substituted or unsubstituted (CH.sub.2).sub.CH.sub.3 chain. The number of repeating units, x, can be from 2 to 30, 2 to 20, 2 to 10, or 2 to 5, for example.

(8) Examples of suitable silicon-containing surface-modifying ligands are HOOC(CH.sub.2).sub.10Si(CH.sub.3).sub.2O(Si(CH.sub.3).sub.2O).sub.ySi(CH.sub.3).sub.2C.sub.4H.sub.9 and HOOCC.sub.18H.sub.36-PDMS (both available from Gelest, Inc., Morrisville, Pa.). Other suitable silicon-containing surface-modifying ligands according to structures 1-3 may be synthesized by techniques familiar to those skilled in the art. For example, esterification of a carboxylated polysiloxane with an aliphatic alcohol may be used to obtain aliphatic-substituted polysiloxanes. Alcohols, such as CH.sub.3(CH.sub.2).sub.nCH.sub.2OH, tert-butanol, and isoborneol may be used to make different aliphatic-terminated polysiloxanes useful as surface-modifier ligands. Example 1 below describes the preparation of such a ligand by the esterification reaction between a carboxy-terminated PDMS and lauryl alcohol.

(9) FIG. 1 illustrates a procedure for preparing polysiloxane-based films of QDs, as described herein. According to the procedure illustrated in FIG. 1, polysiloxane ligand prepared as described above is added to a container 101. The polysiloxane ligand will typically be a liquid, and can be added to the container neat or in a suitable solvent. The container can be a vial or flask, for example. It may be beneficial to purge the container with inert gas, such as N.sub.2. QDs can then be added to the container 102. The present disclosure is not limited to any specific types of QDs. However, due to their sensitivity, cadmium-free QDs benefit particularly from the methods disclosed herein. Examples of cadmium free QDs include QDs based on alloys of In, P, Zn, and S available from Nanoco Technologies Ltd. (Manchester U.K.) under the CFQD trademark.

(10) A person of skill in the art will appreciate that, in many cases, the coordination about the final inorganic surface atoms in any core, core/shell or core/multishell, doped or graded QD is typically incomplete, with highly reactive, non-fully coordinated atoms acting as dangling bonds on the surface of the particle, which can lead to particle agglomeration. This problem is typically overcome by passivating (capping) the bare surface atoms with protecting organic groups (capping agents). The capping agent may be the solvent in which the nanoparticles have been prepared or may be added to the reaction mixture in which the QDs are prepared. Ligands of this kind include, but are not limited to, mono- or multi-dentate ligands such as phosphines (trioctylphosphine, triphenylphosphine, t-butylphosphine, etc.), phosphine oxides (trioctylphosphine oxide, triphenylphosphine oxide, etc.), alkyl phosphonic acids, alkyl-amines (hexadecylamine, octylamine, etc.), aryl-amines, pyridines, long chain fatty acids and thiophenes.

(11) When QDs are mixed with the silicon-containing surface-modifying ligands disclosed herein, those ligands adhere to the surface of the QDs in one of two general modes. The first mode in which the silicon-containing surface-modifying ligands may adhere to the QD surface is by ligand exchange, whereby the silicon-containing surface-modifying ligands replace some of the capping agent molecules already present on the QD surface. This mode of adhesion is particularly favored for silicon-containing surface-modifying ligands that contain a surface-binding species, such as a thiol. The second mode is by intercalation of the silicon-containing, surface-modifying ligands within the sheath formed by the capping agent molecules already present on the surface of the QD.

(12) Once the surfaces of the QDs have been modified using the silicon-containing surface-modifying ligands, the QDs may be mixed with a silicon-containing polymer 103. Examples of suitable silicon-containing polymers include PDMS, such as SYLGARD 184, available from Dow Corning. A crosslinker and/or hardener may be added also and the mixture mixed thoroughly. The polymer/QD mixture may then be used to make films, beads, or any other polymer-based geometry 104. Method of forming films or beads will be apparent to those of skill in the art. For example, spin coating, drop coating, evaporative coating, printing, doctor blading, etc. may be used. Films may be cured at elevated temperatures, for example 50 C., or as otherwise known in the art.

(13) As shown in the examples below, when films are QDs are modified with silicon-containing ligands, the QDs disperse much more readily in silicones and yield better, and more consistent, films.

Example 1: Synthesis of Aliphatic-Terminated Polydimethylsiloxane Surface Ligands

(14) Monocarboxy-terminated PDMS (10 g, 6.67 mmol) was added to a three-neck, round-bottom flask equipped with a cooling tower and stirrer. Lauryl alcohol (1.49 g, 8 mmol) was then added to the flask under nitrogen and the mixture was dissolved in dichloromethane (DCM, 180 mL). Dicyclohexylcarbodiimide (DCCI) (1.376 g, 6.67 mmol) and pyrrolidino pyridine (99.8 mg, 0.667 mmol) were then added and the mixture was stirred under nitrogen overnight. The mixture was then refluxed using a 50 C. water bath for 6 hours before being washed with deionized water (3250 mL). The organic phase was then dried with Na.sub.2SO.sub.4 and DCM was removed under low pressure. The liquid so obtained was washed with methanol and then re-dissolved in hexane before the insoluble part in hexane solution was removed by filtration. Finally, the hexane removed under vacuum affording 6.89 g clear liquid.

Example 2:Films of QDs in PDMS Resin

(15) Red-emitting core/shell QDs having a core containing In, P, Zn, and S and having a shell based on ZnS were prepared as disclosed in U.S. Pat. No. 7,867,556, the entire contents of which are incorporated herein by reference. A visual comparison of the QDs in viscous PDMS resin (SYLGARD184, Dow Corning) without additional surface ligand and with surface ligand and their corresponding films after curing at 50 C. for 24 hours under nitrogen showed that the surface ligand (C.sub.22H.sub.45-PDMS) obtained by above-described esterification reaction clearly improved the dispersion of the red quantum dots in the PDMS resin and film.

(16) FIG. 3 illustrates the poor non-homogeneous dispersions of red QDs in low viscosity, unfunctionalized PDMS without additional surface ligands (A-C), a homogeneous dispersion of red QDs after treating with a liquid functionalized polysiloxane (D) and a homogeneous dispersion of (D) in low viscosity unfunctionalized PDMS showing the use of a functionalized polysiloxane to improve dispersion in unfunctionalized polysiloxanes; (A) QDs that were dried and mixed with PDMS; (B) QDs that were dried before addition of PDMS solution in toluene and mixing; (C) a QD dispersion in toluene that was mixed with low viscosity PDMS in toluene (heating at 50 C. did not improve this dispersion); (D) a good dispersion of red QDs in mercapto-functional silicone after mixing the QDs toluene dispersion with 4-6 mole % mercaptopropyl methylsiloxane-dimethylsiloxane copolymer with heating at 50 C. under nitrogen for 48-60 hours and toluene removed using high vacuum; (E) a clear solution of QD obtained when (D) was diluted and mixed with low viscosity, unfunctionalized PDMS.

(17) FIG. 4 shows spectra obtained using a LABSPHERE integrating sphere (LABSPHERE INC. 231 Shaker Street NORTH SUTTON NEW HAMPSHIRE 03260) of pristine (dashed line) and C.sub.22H.sub.45-PDMS-treated (solid line) cadmium-free QD/crosslinked PDMS (SYLGARD 184) films. It is clear that the ratio of the emission peak area to the excitation peak area of the C.sub.22H.sub.45-PDMS-treated cadmium-free QD film is higher than that of the pristine cadmium-free QD film which is in agreement with the fact that the treated cadmium-free quantum dots dispersed better in the PDMS. The QE of pristine (dashed line) and C.sub.22H.sub.45-PDMS-treated (solid line) cadmium-free QD/crosslinked PDMS films were 38 and 52% respectively.

(18) TABLE-US-00001 TABLE 1 Quantum yield of the pristine and surface ligand-treated red QD in silicone oils or films (QDs in PDMS (ca. 12 mg/1 mL resin)). QY Sample-QD in: Solution Film Appearance Toluene 70 Transparent homogeneous distribution. HOOCC.sub.10H.sub.21-PDMS-C.sub.4H.sub.9 38 QD partly aggregated. SYLGARD 184 62 54 QD aggregation. HOOCC.sub.18H.sub.37-PDMS-treated QDs 62 55 Opaque/ in SYLGARD 184 homogeneous distribution. C.sub.22H.sub.45-PDMS-treated QDs in 63 59 Opaque/ SYLGARD 184 homogeneous distribution. MPMS-PDMS treated QDs (not 55 Transparent/ diluted in low viscosity PDMS) homogeneous distribution. Notes: HOOCC.sub.10H.sub.21-PDMS-C.sub.4H.sub.9 is an abbreviation for monocarboxy-terminated polydimethylsiloxane (HOOC(CH.sub.2).sub.10Si(CH.sub.3).sub.2O((Si).sub.2O).sub.nSi(CH.sub.3).sub.2C.sub.4H.sub.9, viscosity = 20 cps) available from ABCR GmbH (76187 Karlsruhe, Germany); C.sub.18H.sub.37-PDMS is an abbreviation for octadecyl-terminated polydimethylsiloxane (ABCR); C.sub.22H.sub.45-DMS is an abbreviation for the ligand obtained from esterification reaction of HOOCC.sub.10H.sub.21-PDMS-C.sub.4H.sub.9 with lauryl alcohol and MPMS-PDMS is an abbreviation for 4-6 mole % mercaptopropyl methylsiloxane-dimethylsiloxane copolymer (from ABCR GmbH). The QD/PDMS films were typically cured at 50 C. for 24 hours in a nitrogen atmosphere.

(19) The embodiments disclosed herein provide the following advantages: they provide an alternative way to prepare QD/polymer films or QD dispersions in silicone oils for display and lighting applications with comparable performance and new properties; QD polysiloxane films are as flexible as films prepared using lauryl methacrylate (LMA) as a monomer and trimethyloyl propane trimethacrylate (TMPTM) as a cross-linker and the polysiloxane film's matrix is more stable against heat, UV, corrosion and oxidation; and there are fewer environmental and health-related issues since polysiloxanes have high solid content, low VOC and low toxicity compared to traditional polymeric materials obtained from (meth)acrylates and polyurethanes.

(20) Although particular embodiments have been shown and described, it should be understood that the above discussion is not intended to limit the present invention to these embodiments. Persons 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.