Semiconductor nanocrystals and methods of preparation

A method for preparing semiconductor nanocrystals is disclosed. The method comprises adding a precursor mixture comprising one or more cation precursors, one or more anion precursors, and one or more amines to a ligand mixture including one or more acids, one or more phenol compounds, and a solvent to form a reaction mixture, wherein the molar ratio of (the one or more phenol compounds plus the one or more acids plus the one or more amine compounds) to the one or more cations initially included in the reaction mixture is greater than or equal to about 6, and heating the reaction mixture at a temperature and for a period of time sufficient to produce semiconductor nanocrystals having a predetermined composition. Methods for forming a buffer layer and/or an overcoating layer thereover are also disclosed. Semiconductor nanocrystals and compositions including semiconductor nanocrystals of the invention are also disclosed. In certain embodiments, a semiconductor nanocrystal includes one or more Group IIIA and one or more Group VA elements.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the technical field of nanotechnology, and more particularly to semiconductor nanocrystals and methods for preparing same.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, there is provided a method for preparing semiconductor nanocrystals, the method comprising: adding a precursor mixture comprising one or more cation precursors, one or more anion precursors, and one or more amines to a ligand mixture including one or more acids, one or more phenol compounds, and a solvent to form a reaction mixture, wherein the molar ratio of (the one or more phenol compounds plus the one or more acids plus the one or more amine compounds) to the one or more cations initially included in the reaction mixture is greater than or equal to about 6, and heating the reaction mixture at a temperature and for a period of time sufficient to produce semiconductor nanocrystals having a predetermined composition.

In certain embodiments, at least one cation precursor comprises a Group III cation precursor and at least one anion precursor comprises a Group V anion precursor.

In certain preferred embodiments, the molar ratio of cation precursor(s) to anion precursor(s) in the precursor mixture is greater than 1. In certain of such embodiments, the molar ratio is in a range from greater than 1 to about 3. In certain embodiments the ratio is 2:1. Other ratios may be determined to be useful or desirable.

In certain embodiments, at least one of the one or more amine compounds comprises a primary amine. In certain embodiments, at least one of the one or more amine compounds comprises a secondary amine. In certain embodiments, at least one of the one or more amine compounds comprises a tertiary amine. In certain embodiments including one or more amine compounds, a mixture of amines can be included.

In certain embodiments, one or more phenol compounds comprise a monocyclic phenol or a polycyclic phenol.

In certain embodiments, at least one or the one or more phenol compounds comprises a mixture of two or more different phenol compounds.

In certain embodiments, one or more of the acids included in the mixture comprise a carboxylic acid compound.

In certain embodiments, each of the one or more acids comprises a carboxylic acid compound.

In certain embodiments including one or more carboxylic acid compounds in the mixture, at least one of the one or more carboxylic acid compounds further comprises one or more alkyl groups.

In certain embodiments including a carboxylic acid compound in the mixture, at least one of the one or more carboxylic acid compounds comprises CH3(CH2)nC(O)OH wherein n=1-18.

In certain embodiments including one or more carboxylic acid compounds in the mixture, at least one of the one or more carboxylic acid compounds further comprises one or more aromatic groups.

In certain embodiments, the precursor mixture further includes a solvent. In certain of such embodiments, a solvent can comprise a mixture of solvents. In certain of such embodiments, a solvent comprises a weakly coordinating solvent. In certain of such embodiments, a solvent comprises a non-coordinating solvent.

In certain embodiments, the precursor mixture is formed without heating. For example, in certain embodiments, the precursors can be admixed at room temperature. Preferably the precursor mixture is not heated or subjected to heat prior to being added to the reaction mixture to avoid nucleation prior to addition.

In certain embodiments, the molar ratio of (the one or more phenol compounds plus the one or more acids plus the one or more amine compounds) to the one or more cations is greater than about 10.

In certain embodiments, the molar ratio of (the one or more phenol compounds plus the one or more acids plus the one or more amine compounds) to the one or more cations is in a range from about 6 to about 20.

In certain embodiments, the molar ratio of (the one or more phenol compounds plus the one or more acids plus the one or more amine compounds) to the one or more cations is in a range from about 10 to about 15.

In certain embodiments, the reaction mixture temperature is in a range from about 150° C. to about 300° C.

In certain embodiments including a cation precursor comprising indium and an anion precursor comprising arsenic, the ligand mixture is at a temperature greater than or equal to about 250° C. when the precursor mixture is added thereto.

In certain embodiments, the method can further comprise isolating the nanocrystals from the reaction mixture.

In certain embodiments, the semiconductor nanocrystals in accordance with the invention comprise semiconductor nanocrystals that are capable of emitting light with a peak emission wavelength in a range from about 800 nm to about 2 microns upon excitation.

In certain embodiments, the semiconductor nanocrystals in accordance with the invention comprise semiconductor nanocrystals that are capable of emitting light with a peak emission wavelength in a range from about 1 micron to about 2 microns upon excitation.

In accordance with another embodiment of the present invention, the method can further comprise introducing additional ligand mixture and additional precursor mixture to the reaction mixture to grow the particles to a predetermined particle size, wherein the molar ratio of (the one or more phenol compounds plus the one or more acids plus the one or more amine compounds) to the one or more cations introduced into the reaction mixture during such growth is greater than or equal to about 6.

In certain embodiments, additional ligand mixture and additional precursor mixture is added in an incremental manner.

In certain embodiments, additional ligand mixture and additional precursor mixture is added in a continuous manner.

In certain embodiments, additional ligand mixture and additional precursor mixture is added in a semi-continuous manner.

Additional ligand mixture and additional precursor mixture are preferably introduced separately. In certain of such embodiments, additional ligand mixture and additional precursor mixture are added concurrently. Such additions are preferably at a predetermined rate.

In certain embodiments, additional ligand mixture and additional precursor mixture can be introduced separately, with additional ligand mixture being added prior to each introduction of additional precursor mixture.

In certain preferred embodiments, the molar ratio of cation(s) precursor to anion precursor in additional precursor mixture is greater than 1. In certain of such embodiments, the molar ratio is in a range from greater than 1 to about 3. In certain embodiments the ratio is 2:1. Other ratios may be determined to be useful or desirable.

In certain embodiments, the method can further comprise forming a buffer layer comprising a semiconductor material over a portion of the outer surface of the semiconductor nanocrystal.

In certain embodiments, forming the buffer layer comprises adjusting the temperature of the reaction mixture to a second temperature, adding buffer anion precursors and buffer cation precursors, and reacting the buffer precursors to form the semiconductor material. In certain of such embodiments, the buffer anion precursors and buffer cation precursors are not mixed together prior to addition.

In certain embodiments, the buffer layer does not completely cover the outer surface of the semiconductor nanocrystal on which it is formed.

Examples of semiconductor materials for inclusion in a buffer layer include, but are not limited to, Group II-VI compounds, Group III-V compounds, etc. Other non-limiting examples of semiconductor materials for inclusion in a buffer layer are described below. Other semiconductor materials may be used.

In certain preferred embodiments, the semiconductor nanocrystals are not isolated from the reaction mixture prior to forming the buffer layer.

In certain embodiments, the method further comprises forming an overcoating layer over the outer surface of a semiconductor nanocrystal.

In certain embodiments, a buffer layer is included on a portion of the outer surface of a semiconductor nanocrystal prior to forming an overcoating layer thereover.

In certain embodiments, forming an overcoating layer comprises adding overcoating anion precursors and overcoating cation precursors to the reaction mixture, and reacting the overcoating precursors to form the semiconductor material.

In certain embodiments, the overcoating precursors are not mixed together prior to addition.

In certain embodiments, each of the overcoating precursors are added at a predetermined rate, which may be the same or different.

In certain preferred embodiments, the semiconductor nanocrystals are not isolated prior to forming an overcoating layer.

In certain of such preferred embodiments, no additional acid, phenols, or amines are added in the overcoating step.

In certain of such preferred embodiments, it may be determined to be useful or desirable to add additional acid, phenols, amines, and/or mixtures thereof.

In accordance with another aspect of the present invention, there are provided semiconductor nanocrystals prepared in accordance with methods of the invention described in this application.

In accordance with another aspect of the present invention, there are provided compositions comprising semiconductor nanocrystals prepared in accordance with methods of the invention described in this application.

As used herein, “phenol compound” refers to any compound that includes at least one six-membered aromatic ring that is bonded directly to a hydroxyl group (—OH).

As used herein, an “amine compound” refers to any organic compounds that contains at least one basic nitrogen atom with a lone electron pair or compound that includes an organic functional group that contains at least one basic nitrogen atom with a lone electron pair.

As used herein, a “phosphonic acid compound” refers to a compound including at least one phosphonic acid group.

As used herein, a “carboxylic acid compound” refers to a compound including at least one carboxylic acid group.

The foregoing, and other aspects described herein, all constitute embodiments of the present invention.

It should be appreciated by those persons having ordinary skill in the art(s) to which the present invention relates that any of the features described herein in respect of any particular aspect and/or embodiment of the present invention can be combined with one or more of any of the other features of any other aspects and/or embodiments of the present invention described herein, with modifications as appropriate to ensure compatibility of the combinations. Such combinations are considered to be part of the present invention contemplated by this disclosure.

Additional information concerning the foregoing, and other information useful with the present inventions is provided below.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed. Other embodiments will be apparent to those skilled in the art from consideration of the description and drawings, from the claims, and from practice of the invention disclosed herein.

DETAILED DESCRIPTION OF THE INVENTION

Various aspects and embodiments of the present inventions will be further described in the following detailed description.

The present invention relates to methods for preparing semiconductor nanocrystals. The present invention further relates to semiconductor nanocrystals. The present invention further also relates to compositions comprising semiconductor nanocrystals.

In accordance with one aspect of the present invention, there is provided a method for preparing semiconductor nanocrystals, the method comprising adding a precursor mixture comprising one or more cation precursors, one or more anion precursors, and one or more amines to a ligand mixture including one or more acids, one or more phenol compounds, and a solvent to form a reaction mixture, wherein the molar ratio of (the one or more phenol compounds plus the one or more acids plus the one or more amine compounds) to the one or more cations that are initially included in the reaction mixture is greater than or equal to about 6, and heating the reaction mixture at a temperature and for a period of time sufficient to produce semiconductor nanocrystals having a predetermined composition.

Examples of phenol compounds includes, for example, but are not limited to, resorcinol or hydroquinone, bisphenol F, bisphenol A, 2,2′-dihydroxybenzophenone, 2,4-di-tert-butylphenol, 2-phenylphenol, 2,6-diphenylphenol, 2,6-dimethyl phenol, and 2,3-dimethyl phenol. Other phenols may also be determined to be useful or desirable by one of ordinary skill in the art.

In certain embodiments, one or more phenol compounds comprise a monocyclic phenol or a polycyclic phenol.

In certain embodiments, at least one or the one or more phenol compounds comprises a mixture of two or more different phenol compounds.

In certain embodiments, one or more of the acids included in the mixture comprise a carboxylic acid compound.

In certain embodiments, each of the one or more acids comprises a carboxylic acid compound.

In certain embodiments including one or more carboxylic acid compounds in the mixture, at least one of the one or more carboxylic acid compounds further comprises one or more alkyl groups.

In certain embodiments including a carboxylic acid compound in the mixture, at least one of the one or more carboxylic acid compounds comprises CH3(CH2)nC(O)OH wherein n=1-18.

In certain embodiments including one or more carboxylic acid compounds in the mixture, at least one of the one or more carboxylic acid compounds further comprises one or more aromatic groups.

Other acids may also be determined to be useful or desirable by one of ordinary skill in the art.

In certain embodiments, the precursor mixture comprising one or more cation precursors, one or more anion precursors, and one or more amines further includes a solvent.

A solvent for inclusion in the precursor mixture and/or the ligand mixture can comprise a single solvent or a mixture of solvents.

In certain embodiments, a solvent comprises a non-coordinating solvent.

Examples of non-coordinating solvents for use in the methods taught herein include, but are not limited to, squalane, octadecane, or any other saturated hydrocarbon molecule. Mixtures of two or more solvents can also be used. Other non-coordinating solvents for use in the methods taught herein can be readily ascertained by one of ordinary skill in the art. In certain embodiments, non-coordinating solvents comprise liquids having a dipole moment in the range form 0 to about 0.2 μ/D.

In certain embodiments, a solvent comprises a weakly coordinating solvent. Examples of weakly coordinating solvents include, for example, but are not limited to aromatic ether or solvent mixtures including an aromatic ether, e.g., DowTherm A. In certain embodiments, weakly coordinating solvents comprise liquids having a dipole moment in the range from 0.2 to 0.7 μ/D. Other non-coordinating solvents and weakly coordinating solvents compatible with the reaction conditions and that will not react with the semiconductor nanocrystals or ligands can be identified by one of ordinary skill in the art.

In certain embodiments, the molar ratio of (the one or more phenol compounds plus the one or more acids plus the one or more amine compounds) to the one or more cations initially included in the reaction mixture is greater than about 10.

In certain embodiments, the molar ratio of (the one or more phenol compounds plus the one or more acids plus the one or more amine compounds) to the one or more cations initially included in the reaction mixture is in a range from about 6 to about 20.

In certain, the molar ratio of (the one or more phenol compounds plus the one or more acids plus the one or more amine compounds) to the one or more cations initially included in the reaction mixture is in a range from about 10 to about 15.

In certain embodiments including a cation precursor comprises indium and an anion precursor comprises arsenic, for example, the molar ratio of (the one or more phenol compounds plus the one or more acids plus the one or more amine compounds) to the one or more cations initially included in the reaction mixture is preferably greater than 10, more preferably greater than 12.

Other ratios may also be determined to be useful or desirable by one of ordinary skill in the art.

In certain embodiments, one or more additional ligands in addition to ligands included in the ligand mixture can be included in the reaction mixture. Other ligands are discussed elsewhere herein.

In certain embodiments, the reaction mixture temperature is in a range from about 150° C. to about 300° C.

In certain embodiments, the reaction mixture temperature is in a range from about 180° C. to about 300° C.

In certain embodiments, the wherein the reaction mixture temperature is in a range from about 270° C. to about 300° C.

Other temperatures may also be determined to be useful or desirable.

Preferably the temperature is maintained substantially constant during the preparation of the semiconductor nanocrystals.

Preferably the temperature of the precursor mixture prior to introduction into the reaction mixture is at room temperature or a temperature that is below room temperature, but above the freezing point of the mixture.

In certain embodiments, the reaction is carried out in an inert atmosphere, where exposure to air is substantially, and preferably completely, precluded.

In certain of such embodiments, the method can further include isolating the nanocrystals from the reaction mixture.

In certain embodiments, the method comprises a method for preparing semiconductor nanocrystals comprising one or more Group III elements and one or more Group V elements. In certain of such embodiments, the method comprises, the method comprising: combining one or more Group III cation precursors, one or more Group V anion precursors, one or more ligands, one or more amine compounds, and a first solvent without heating to form a precursor mixture; adding the precursor mixture to a ligand mixture to form a reaction mixture, the ligand mixture comprising one or more acids, one or more phenol compounds, and a second solvent, wherein the molar ratio of (the one or more phenol compounds plus the one or more acid compounds plus the one or more amine compounds) to the one or more cations initially included in the reaction mixture is greater than or equal to about 6, wherein at the time of precursor mixture addition, the ligand mixture is at a temperature sufficient to initiate reaction of one or more Group III cation precursors with one or more of the Group V anion precursors.

Semiconductor nanocrystals prepared by the methods described herein can comprise one or more semiconductor materials. Examples of semiconductor materials include, but are not limited to, Group II-VI compounds (e.g., binary, ternary, and quaternary compositions), Group II-V compounds (e.g., binary, ternary, and quaternary compositions), Group III-VI compounds (e.g., binary, ternary, and quaternary compositions), Group III-V compounds (e.g., binary, ternary, and quaternary compositions), Group IV-VI compounds (e.g., binary, ternary, and quaternary compositions), Group compounds (e.g., binary, ternary, and quaternary compositions), Group II-IV-VI compounds (e.g., binary, ternary, and quaternary compositions), Group II-IV-V compounds (e.g., binary, ternary, and quaternary compositions), Group IV elements, and alloys including any of the foregoing, and/or a mixture including any of the foregoing. Semiconductor nanocrystals can also comprise one or more semiconductor materials that comprise ternary and quaternary alloys that include one or more of the foregoing compounds.

As used herein, “cation” refers to a metal element (e.g., a Group I (e.g., Cu, Ag, Au), Group II (e.g., Mg, Zn, Cd, Hg), Group III (e.g., B, Al, Ga, In, Tl), or Group IV (e.g., Si, Ge, Sn, Pb) element) included in a semiconductor nanocrystal; wherein the valence of the element can include zero. As used herein, “anion” refers to a non-metal element (e.g., a Group V (e.g., N, P, As, Sb, Bi) or Group VI (e.g., O, S, Se, Te) element) included in a semiconductor nanocrystal); wherein the valence of the element can include zero.

The size and composition of a semiconductor nanocrystal affects its characteristic spectral emission wavelength. Thus, the size and composition of a semiconductor nanocrystal is readily selected by one of ordinary skill in the art such that the semiconductor nanocrystal emits photons at a predetermined wavelength in the desired portion of the spectrum.

In certain embodiments, a semiconductor nanocrystal can comprise a semiconductor material comprising one or more elements of Group III (e.g., B, Al, Ga, In, TI) and one or more elements of Group V (e.g., N, P, As, Sb, Bi) (e.g., binary, ternary, and quaternary III-V compositions).

Examples of metal cation precursors include elements, covalent compounds, ionic compounds, and/or coordination complexes, that serve as a source for the desired metal element(s) in the resulting nanocrystal. For example, a metal cation precursor can constitute a wide range of substances, including, but not limited to, a metal oxide, a metal carbonate, a metal bicarbonate, a metal sulfate, a metal sulfite, a metal phosphate, metal phosphite, a metal halide, a metal carboxylate, a metal alkoxide, a metal thiolate, a metal amide, a metal imide, a metal alkyl, a metal aryl, other organometallics, a metal coordination complex, a metal solvate, and the like. For example, non-limiting examples of indium precursors include In(III) acetate, In(III) trifluoroacetate, trialkyl indium (InR3)(wherein R=methyl, ethyl, propyl, butyl, pentyl, hexyl, isopropyl, isobutyl, test-butyl, etc.); non-limiting examples of gallium precursors include Ga(III) acetate, Ga(III) trifluoroacetate, trialkyl gallium (InR3)(wherein R=methyl, ethyl, propyl, butyl, pentyl, hexyl, isopropyl, isobutyl, tert-butyl, etc.). Other Group III cation precursors can be readily ascertained by one of ordinary skill in the art.

Examples of anion precursors include elements, covalent compounds, or ionic compounds that serve as a source for a non-metallic element(s) in the resulting nanocrystal. Non-metallic precursors are most often selected from the elements themselves (oxidation state 0), covalent compounds, or ionic compounds of the non-metallic elements. For example, non-limiting examples of phosphorus precursors include P(SiR3)3wherein R=methyl, ethyl, propyl, butyl, pentyl, hexyl, isopropyl, isobutyl, tert-butyl, etc.); non-limiting examples of arsenic precursors include As(SiR3)3wherein R=methyl, ethyl, propyl, butyl, pentyl, hexyl, isopropyl, isobutyl, tert-butyl, etc.). Other Group V precursors can be readily ascertained by one of ordinary skill in the art.

In certain embodiments, semiconductor nanocrystals comprising binary materials, ternary materials, quaternary materials, and even more complex species may be prepared using the methods described herein, in which case more than one cation precursor and/or more than one anion precursor may be included.

In certain embodiments, a semiconductor nanocrystal can include a semiconductor material comprising one or more elements of Group III and one or more elements of Group V (e.g., binary, ternary, and quaternary III-V compositions).

In certain embodiments, a semiconductor nanocrystal can include a semiconductor material comprising one or more elements of Group II and one or more elements of Group VI (e.g., binary, ternary, and quaternary II-VI compositions).

A semiconductor nanocrystal may also be referred to herein as a semiconductor nanoparticle, a nanocrystal, a quantum dot or a QD.

A semiconductor nanocrystal is capable of emitting light upon excitation. A semiconductor nanocrystal can be excited by irradiation with an excitation wavelength of light, by electrical excitation, or by other energy transfer.

The emission from a semiconductor nanocrystal can be a narrow Gaussian emission band that can be tuned through the complete wavelength range of the ultraviolet, visible, or infra-red regions of the spectrum by varying the size of the semiconductor nanocrystal, the composition of the semiconductor nanocrystal, or both.

In certain embodiments, the semiconductor nanocrystals of the invention comprise semiconductor nanocrystals that are capable of emitting light with a peak emission wavelength in a range from about 800 nm to about 2 microns upon excitation.

In certain embodiments, the semiconductor nanocrystals of the invention comprise semiconductor nanocrystals that are capable of emitting light with a peak emission wavelength in a range from about 1 micron to about 2 microns upon excitation.

Photoluminescence quantum efficiency (also referred to as quantum efficiency, quantum yield or solution quantum yield) represents the percent of absorbed photons that are reemitted as photons upon excitation by irradiation with an excitation wavelength of light.

A nanocrystal is a nanometer sized particle, e.g., in the size range of up to about 1000 nm. In certain embodiments, a nanocrystal can have a size in the range of up to about 100 nm. In certain embodiments, a nanocrystal can have a size in the range up to about 20 nm (such as about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 nm). In certain embodiments, a nanocrystal can have a size less than 100 Å.

In certain embodiments, the method further comprise introducing additional amounts of ligand mixture and additional amounts of precursor mixture to the reaction mixture to increase or grow the size of the of the seed particles to a predetermined particle size, wherein the molar ratio of (the one or more phenol compounds plus the one or more acids plus the one or more amine compounds) to the one or more cations introduced into the reaction mixture during such growth is greater than or equal to about 6.

In certain embodiments, additional ligand mixture and additional precursor mixture is added in an incremental manner. For example, in certain of such embodiments, the additional ligand mixture and additional precursor mixture can be added as a series of incremental additions.

In certain embodiments, additional ligand mixture and additional precursor mixture is added in a continuous manner.

In certain embodiments, additional ligand mixture and additional precursor mixture is added in a semi-continuous manner.

Additional ligand mixture and additional precursor mixture are preferably introduced separately. In certain of such embodiments, additional ligand mixture and additional precursor mixture are added concurrently. Such additions are preferably at a predetermined rate.

In certain embodiments, additional ligand mixture and additional precursor mixture can be introduced separately, with additional ligand mixture being added prior to each introduction of additional precursor mixture.

Preferably, the molar ratio of cation(s) precursor to anion precursor in additional precursor mixture is greater than 1. In certain of such embodiments, the molar ratio is in a range from greater than 1 to about 3. In certain embodiments the ratio is 2:1. Other ratios may be determined to be useful or desirable.

Preferably the temperature of the additional precursor mixture prior to introduction into the reaction mixture is at room temperature or a temperature that is below room temperature, but above the freezing point of the mixture.

Temperatures useful for the reaction mixture during preparing of nanocrystals described above can also be used during introduction of additional ligand mixture and additional precursors to grow the nanocrystals. Other temperatures may also be determined to be useful or desirable.

Preferably the temperature is maintained substantially constant during the preparation of the semiconductor nanocrystals.

In certain embodiments, a semiconductor nanocrystal prepared in accordance with the present invention (prior to further inclusion of a buffer layer and/or an overcoating layer), has a size in a range from about 2 to about 10 nanometers, from about 2 to about 8 nanometers, from about 2 to about 4 nanometers, etc. Other sizes can also be prepared.

The size of a nanocrystal can be determined, for example, by direct transmission electron microscope measurement. Other known techniques can also be used to determine nanocrystal size.

Nanocrystals can have various shapes. Examples of the shape of a nanocrystal include, but are not limited to, sphere, rod, disk, tetrapod, other shapes, and/or mixtures thereof.

As discussed above, the method can further include formation of a buffer layer and/or an overcoating layer.

In certain embodiments, the method can further comprise forming a buffer layer comprising a semiconductor material over a portion of the outer surface of the semiconductor nanocrystal.

Preferably the nanocrystals are not isolated from the reaction mixture and the buffer layer is formed on the nanocrystals “in situ”.

In certain embodiments, forming a buffer layer comprises adjusting the temperature of the reaction mixture to a second temperature, adding buffer anion precursors and buffer cation precursors, and reacting the buffer precursors to form the semiconductor material.

In certain embodiments, the temperature of the reaction mixture is adjusted to a second temperature prior to adding the precursor materials for forming the buffer layer.

In certain embodiments, the second temperature is lower than the temperature of the reaction mixture during preparation of the semiconductor nanocrystals.

In certain embodiments wherein the buffer layer comprises a II-VI compound, the second temperature is less than 250° C., and preferably less than 200° C. In certain of such embodiments, the temperature is about 180° C. Other temperatures may, however, be determined to be useful or desirable.

In certain of such embodiments, the buffer anion precursors and buffer cation precursors are not mixed together prior to addition.

Semiconductor materials described herein can be included in a buffer layer.

Precursors for forming semiconductor materials that are described elsewhere in this application can be used in forming the buffer layer.

Each of the precursors may be included in a solvent prior to being added to the dispersion of nanocrystals over which a buffer layer is to be formed.

Solvents can comprise solvents described elsewhere in the application. Other solvents may also be determined to be useful or desirable.

In certain preferred embodiments, the buffer layer does not completely cover the outer surface of the semiconductor nanocrystal on which it is formed.

Preferably the buffer layer is no thicker than a monolayer.

Examples of semiconductor materials for inclusion in a buffer layer include, but are not limited to, Group II-VI compounds, Group III-V compounds, etc. Other non-limiting examples of semiconductor materials for inclusion in a buffer layer are described below. Other semiconductor materials may be used.

Preferably the buffer layer has a composition that is different from the composition of the nanocrystal over which it is disposed.

In certain embodiments, the method can further comprise forming an overcoating layer over the outer surface of a semiconductor nanocrystal.

In certain embodiments, a buffer layer is optionally included on a portion of the outer surface of a semiconductor nanocrystal prior to forming an overcoating layer thereover.

In certain embodiments, forming an overcoating layer comprises adding overcoating anion precursors and overcoating cation precursors to a dispersion of the nanocrystals to be overcoated, and reacting the overcoating precursors to form an overcoating material.

In certain preferred embodiments, the overcoating comprises a semiconductor material. More preferably such semiconductor material is different from the semiconductor material of the core nanocrystal and the buffer layer.

Preferably the nanocrystals are not isolated from the reaction mixture and the buffer layer is formed on the nanocrystals “in situ”.

In certain of such preferred embodiments, no additional acid, phenols, or amines are added in the overcoating step.

In certain of such preferred embodiments, it may be determined to be useful or desirable to add additional acid, phenols, amines, and/or mixtures thereof.

In certain embodiments, the overcoating precursors are not mixed together prior to addition.

In certain embodiments, each of the overcoating precursors are added at a predetermined rate, which may be the same or different from each other.

In certain embodiments, the overcoating precursors are added to the dispersion including nanocrystals to be overcoated, which dispersion is at a first overcoating temperature, and the temperature is then adjusted to a second overcoating temperature to grow the overcoating material to a predetermined thickness.

In certain embodiments, the first overcoating temperature is in a range from about 150° C. to about 200° C. In certain embodiments, 180° C. can be preferred.

The second overcoating temperature is preferably higher than the first overcoating temperature.

In certain embodiments, the second overcoating temperature is greater than 200° C., and preferably in a range from about 220° C. to about 280° C. In certain embodiments, 260° C. can be preferred.

After the desired overcoating layer is formed, the temperature of the growth medium is cooled to room temperature.

In certain preferred embodiments, the reaction vessel is transferred to a dry box using standard air-free techniques.

In accordance with another aspect of the present invention, there are provided semiconductor nanocrystals prepared in accordance with methods of the invention described in this application.

As discussed above, the method can further include formation of a buffer layer and/or an overcoating layer.

In certain embodiments, the buffer layer and or overcoating layer (which may also be referred to herein as a “shell”) comprise one or more semiconductor materials.

In certain embodiments, the one or more semiconductor materials are different from the semiconductor material included in the nanocrystal core.

In embodiments further including a buffer layer and an overcoating layer, the one or more semiconductor materials included in each is different from the semiconductor material included in the other.

In certain embodiments, an overcoating layer can comprise two or more layers of the same or different semiconductor materials. In certain embodiments, a layer can include a single semiconductor material or a mixture of two or more semiconductor materials with different compositions, particle sizes, and/or emission characteristics. A semiconductor material may comprise a compound, a doped compound, and/or an alloy. A nanocrystal core surrounded by a shell is also referred to as having a “core/shell” structure.

Examples of semiconductor materials for inclusion in a buffer layer and/or overcoating layer include, but are not limited to, a semiconductor material comprising one or more elements of Group II and one or more elements of Group VI (e.g., binary, ternary, and quaternary II-VI compositions). Other non-limiting examples of semiconductor materials include a semiconductor material comprising one or more elements of Group III and one or more elements of Group V (e.g., binary, ternary, and quaternary III-V compositions), Group IV-VI compounds (e.g., binary, ternary, and quaternary compositions), Group compounds (e.g., binary, ternary, and quaternary compositions), Group II-IV-VI compounds (e.g., binary, ternary, and quaternary compositions), Group II-IV-V compounds (e.g., binary, ternary, and quaternary compositions), Group IV elements, an alloy including any of the foregoing, and/or a mixture including any of the foregoing.

Overcoating thickness can be varied by growing a desired thickness of overcoating material. For example, an overcoating layer can have a thickness of at least one monolayer or more than about one monolayer. Preferably, the thickness is less than that at which quantum confinement is not achieved. The thickness is selected to achieve the predetermined characteristics of the core/shell nanocrystal. In certain embodiments, the thickness is in the range from greater than about 1 to about 20 monolayers. In certain embodiments, the thickness is in the range from greater than about 1 to about 10 monolayers. In certain embodiments, the thickness is in the range from greater than about 1 to about 5 monolayers. In certain embodiments, the thickness is in the range from about 1 to about 5 monolayers. In certain embodiments, the thickness is in the range from about 3 to about 5 monolayers. In certain embodiments, more than 20 monolayers can be grown. In certain embodiments, the overcoating layer has a thickness from about 2 to about 2.5 monolayers.

In certain embodiments, a nanocrystal includes a shell or overcoating layer formed over at least a portion, and preferably all, of the outer surface thereof.

As mentioned above, in certain embodiments, the nanocrystal can be isolated from the reaction mixture prior to optionally forming a buffer layer and/or overcoating layer thereover. In certain of such embodiments, isolated nanocrystals can be purified before a buffer layer and/or overcoating layer is formed.

In certain preferred embodiments, the formation of the buffer layer and overcoating layer is carried out in the same reaction vessel in which the core nanocrystal is formed.

In certain embodiments, a nanocrystal further includes one or more ligands attached to the surface of the nanocrystal. Such ligands can be derived from the reaction mixture during growth of the nanocrystal.

In certain embodiments, a nanocrystal core, and/or a nanocrystal shell comprises a colloidal growth process. Colloidal growth occurs by injection a cation precursor and an anion precursor into a hot solvent. This permits discrete nucleation and results in the controlled growth of macroscopic quantities of nanocrystals. The injection produces a nucleus that can be grown in a controlled manner to form a nanocrystal. The reaction mixture can be gently heated to grow and anneal the nanocrystal. Both the average size and the size distribution of the nanocrystals in a sample are dependent on the growth temperature. The growth temperature necessary to maintain steady growth increases with increasing average crystal size. The nanocrystal is a member of a population of nanocrystals.

In certain embodiments, the population of nanocrystals that can be obtained has a narrow, monodisperse distribution of particle sizes. The distribution of particle sizes can also be referred to as a size. Preferably, a monodisperse population of particles includes a population of particles wherein at least about 60% of the particles in the population fall within a specified particle size range. A population of monodisperse particles preferably deviate less than 15% rms (root-mean-square) in size and more preferably less than 10% rms and most preferably less than 5%.

A narrow size distribution of the nanocrystals comprising semiconductor material allows the possibility of light emission in narrow spectral widths. Monodisperse semiconductor nanocrystals have been described in detail in Murray et al. (J. Am. Chem. Soc., 115:8706 (1993)); in the thesis of Christopher Murray, “Synthesis and Characterization of II-VI Quantum Dots and Their Assembly into 3-D Quantum Dot Superlattices”, Massachusetts Institute of Technology, September, 1995; and in U.S. patent application Ser. No. 08/969,302 for “Highly Luminescent Color-Selective Materials” (now U.S. Pat. No. 6,322,901). The foregoing documents are hereby incorporated herein by reference in their entireties.

As mentioned above, a cation precursor can be an inorganic compound, an organometallic compound, or elemental metal. For example, in preparing a semiconductor material represented by the general formula MX, wherein the semiconductor material comprises one or more Group III cation precursors (represented by M in the above general formula), M comprises one or more of boron, aluminum, gallium, indium, and thallium. Additional chemical elements can optionally be further included. In preparing other semiconductor materials, the cation precursor is selected based on the composition of the desired semiconductor material. The anion precursor is a compound capable of reacting with the cation precursor to form a material with the general formula MX. For example, in preparing a semiconductor material comprising a Group V, element, X comprises one or more of nitrogen, phosphorus, arsenic, antimony, or bismuth. In preparing other semiconductor materials, X is selected based on the composition of the desired semiconductor material. For example, X donor can be a chalcogenide (Group VI) donor or a pnictide (Group V) donor, such as a phosphine chalcogenide, a bis(silyl) chalcogenide, dioxygen, an ammonium salt, or a tris(silyl) pnictide. Examples of suitable X donors include, but are not limited to, dioxygen, bis(trimethylsilyl) selenide ((TMS)2Se), octadecene-Se, trialkyl phosphine selenides such as (tri-n-octylphosphine) selenide (TOPSe) or (tri-n-butylphosphine) selenide (TBPSe), octadecene-Te, trialkyl phosphine tellurides such as (tri-n-octylphosphine) telluride (TOPTe) or hexapropylphosphorustriamide telluride (HPPTTe), bis(trimethylsilyl)telluride ((TMS)2Te), octadecene-S, bis(trimethylsilyl)sulfide ((TMS)2S), a trialkyl phosphine sulfide such as (tri-n-octylphosphine) sulfide (TOPS), an ammonium salt such as an ammonium halide (e.g., NH4Cl), tris(trimethylsilyl) phosphide ((TMS)3P), tris(trimethylsilyl) arsenide ((TMS)3As), or tris(trimethylsilyl) antimonide ((TMS)3Sb). In certain embodiments, the M donor (e.g., cation precursor) and the X donor (e.g., anion precursor) can be moieties within the same molecule.

Size distribution during the growth stage of the reaction can be estimated by monitoring the absorption or emission line widths of the particles. Modification of the reaction temperature in response to changes in the absorption spectrum of the particles allows the maintenance of a sharp particle size distribution during growth. Reactants can be added to the nucleation solution during crystal growth to grow larger crystals as well as to sharpen size distribution. For example, by stopping growth at a particular semiconductor nanocrystal average diameter, a population having an average semiconductor nanocrystal diameter of less than 150 Å can be obtained. A population of nanocrystals can have an average diameter of 15 Å to 125 Å. In addition, by stopping growth at a particular semiconductor nanocrystal average diameter and choosing the proper composition of the semiconducting material, the emission spectra of the semiconductor nanocrystals can be tuned continuously over the wavelength range of 300 nm to 5 microns, or from 400 nm to 800 nm.

Optionally, the particle size distribution of the nanocrystals can be further refined by size selective precipitation with a poor solvent for the nanocrystals, such as, for example, methanol/butanol. For example, nanocrystals can be dispersed in a solution of 10% butanol in hexane. Methanol can be added dropwise to this stirring solution until opalescence persists. Separation of supernatant and flocculate by centrifugation produces a precipitate enriched with the largest crystallites in the sample. This procedure can be repeated until no further sharpening of the optical absorption spectrum is noted. Size-selective precipitation can be carried out in a variety of solvent/nonsolvent pairs, including pyridine/hexane and chloroform/methanol. The size-selected nanocrystal population preferably has no more than a 15% rms deviation from mean particle size, more preferably 10% rms deviation or less, and most preferably 5% rms deviation or less.

Transmission electron microscopy (TEM) can provide information about the size, shape, and distribution of the semiconductor nanocrystal population. Powder X-ray diffraction (XRD) patterns can provide the most complete information regarding the type and quality of the crystal structure of the semiconductor nanocrystals. Estimates of size are also possible since particle diameter is inversely related, via the X-ray coherence length, to the peak width. For example, the diameter of the semiconductor nanocrystal can be measured directly by transmission electron microscopy or estimated from X-ray diffraction data using, for example, the Scherrer equation. It also can be estimated from the UV/Vis absorption spectrum.

As discussed herein, nanocrystals can further include other ligands attached thereto in addition to those derived from the one or more acids, phenols, and amines included in the reaction mixture. Ligands can optionally be exchanged after growth, if desired.

Examples of ligands include, but are not limited to, fatty acids and other carboxylic acid compounds, phosphonic acid compounds, amine compounds, phosphines, phosphine oxides, etc.

Examples of solvents include, but are not limited to, octadecene, squalene, methyl myristate, octyl octanoate, hexyl octanoate, and CH3(CH2)nC(O)O(CH2)mCH3wherein n=4-18 and m=1-8, dioctyl ether, and diphenyl ether, and mixtures of one or more solvents. In certain embodiments, a mixture can comprise a mixture, (including but not limited to a eutectic mixture) of biphenyl and diphenyl oxide, including, e.g., DOWTHERM A, available from the Dow Chemical Company. Other high boiling point ethers (e.g., BP>˜200° C.) may also be used. Such ethers can be aromatic ethers, aliphatic ethers or aromatic aliphatic ethers. Examples of additional ethers include, but are not limited to, dihexyl ether, diethyleneglycol dimethyl ether, diethyleneglycol dibutyl ether, triethyleneglycol dimethyl ether, tetraethyleneglycol dimethyl ether, butyl phenyl ether, benzyl phenyl ether, dibenzyl ether, ditolyl ether and isomers thereof. Mixtures of two or more solvents can also be used. Other solvents can be readily ascertained by one of ordinary skill in the art.

In certain embodiments, trioctyl phosphine (TOP) can be used as a solvent.

Examples of non-coordinating solvents for use in the methods taught herein include, but are not limited to, squalane, octadecane, or any other saturated hydrocarbon molecule. Mixtures of two or more solvents can also be used. Other non-coordinating solvents for use in the methods taught herein can be readily ascertained by one of ordinary skill in the art. In certain embodiments, non-coordinating solvents comprise liquids having a dipole moment in the range form 0 to about 0.2 μ/D. In certain embodiments, weakly coordinating solvents can be used. In certain embodiments, weakly coordinating solvents comprise liquids having a dipole moment in the range from 0.2 to 0.7 μD.

Coordinating solvents (e.g., TOP, etc.) can also be included in formation of a buffer layer and/or overcoating layer.

In certain embodiments, the reaction is carried out in an inert atmosphere, where exposure to air is substantially, and preferably completely, precluded.

The present invention will be further clarified by the following non-limiting examples, which are intended to be exemplary of the present invention.

EXAMPLES

Synthesis of NIR Emitting Quantum Dots

The 500 mL reactor is equipped with a manual stirrer and thermocouple. The remaining 3 ports are sealed with rubber septa. The stirring is turned on (set to 260 rpm) and the reactor placed under vacuum for 1 hour. After degassing, the reactor is backfilled with N2and heated to 285° C. using a heating mantle. Once the reactor has stabilized at 285° C. the stirring is adjusted to 400 rpm and the InAs core precursors are added via syringe in one portion:

InAs precursors for core growth:

After injection the reaction is monitored via UV/Vis/NIR absorption spectroscopy to monitor the progress of the core growth. Once the 1stexcitonic peak for the InAs nanocrystals has reached 790-800 nm the reaction is cooled to 180° C. and a 100 fold dilution of the crude core mixture is analyzed via UV/Vis/NIR absorption spectroscopy.

The stir rate is reduced to 333 rpm for the shelling process.

CdSe Buffer Layer Growth:1) 1.38 mL of 2M Trioctyl phosphine ligated Selenium (F.W. 78.96, Strem, 99.99%) is dissolved in 20 mL of trioctylphosphine (TOP) (F.W. 370.60, Strem,). This is added in one portion to the InAs core reaction and maintained at 180° C. for 10 minutes.2) 2.75 mL of 0.5M Cd-Oleate (In house prepared)/Oleic Acid (F. W. 282.46, Aldrich, Tech. Grade) solution is added in one portion to the 500 mL reactor and the solution is maintained at 180° C. for 30 minutes.
CdZnS Outer Shell Growth:

1) 2.33 g of dimethyl cadmium (F.W. 142.88, Strem) and 2.02 g of diethyl zinc (F.W. 123.49, Strem) are dissolved in 80 mL of TOP.

The slow addition of CdZnS shell precursor solutions to the 500 mL reactor is started at 180° C. and allowed to add to the reactor for 10 minutes (40 mL/hour). The reactor is then heated to 260° C. for the duration of the overcoating procedure (approximately 2 hours). The reaction is then cooled to room temperature and transferred to the dry box using standard air-free techniques.

The reaction mixture (circa 450 mL) is diluted with 2-propanol (IPA, fresh from solvent purification system) to a total volume of 2 L to induce flocculation of the InAs based nanoparticles. The mixture is centrifuged (3000 rpm/45 minutes) and the supernatant is discarded. The oily residue is triturated with fresh IPA and the resulting black slurry isolated via centrifugation (4000 rpm, 2 minutes). The black solids are re-suspended in hexanes (circa 150 mL) and filtered through a 0.4 μM filter. The colloidal InAs/CdSe/CdZnS quantum dot solution is then stored under stored under inert atmosphere.

Semiconductor nanocrystals (also referred to herein as “QDs”) prepared generally in accordance with Example 1 can provide PL of approximately 1000 nm+50, with a FWHM of about 100-130, and a PLQY of about 40-70%, as shown inFIG. 1.

Synthesis of NIR Emitting Quantum

The 500 mL reactor is equipped with a manual stirrer and thermocouple. The remaining 3 ports are sealed with rubber septa. The stirring is turned on (set to 260 rpm) and the reactor placed under vacuum for 1 hour. After degassing, the reactor is backfilled with N2and heated to 285° C. using a heating mantle. Once the reactor has stabilized at 285° C. the stirring is adjusted to 400 rpm and the InAs core precursors are added via syringe in one portion:

InAs precursors for core growth:

After injection the reaction allowed to stir at 285° C. for three hours.

Growth of Large InAs cores:

Additional ligand mixture and additional precursor mixture are added to the InAs core after 3 hrs of initial core growth. The additional Indium and Arsenic precursor mixture is added slowly via syringe pump over 1.5 hours. Additional ligand mixture is added in a single portion at three times during the slow addition of additional precursor mixture: at 0 min, 30 minutes, and 60 minutes. At the end of the addition, the 1 excitonic absorption feature should reach 1010-1025 nm via UV/Vis/NIR absorption spectroscopy. This size regime is ideal for continuing on with the shell growth procedure.

Additional InAs Precursors and Ligands to Grow Large Core:

FIG. 2shows a comparison of InAs particles before and after additional growth.

The half-width at half maximum of the first excitonic absorption shown inFIG. 2is narrower for the final (larger) particles than for the seed particles from which the final particles are prepared. This is indicative of the final particles having a narrower size distribution than that of the starting seed particles.

InAs semiconductor nanocrystals can be overcoated to provide a variety of wavelengths, including wavelengths greater than 1 micron. Semiconductor nanocrystal prepared generally in accordance with the procedure described in Example 2 are overcoated as follows without prior isolation.

1) 11.262 g of dimethyl cadmium (F.W. 142.88, Strem) and 1.08 g of diethyl zinc (F.W. 123.49, Strem) are dissolved in 200 mL of TOP.

The slow addition of CdZnS shell precursor solutions to the 500 mL reactor is started at 180° C. and allowed to add to the reactor for 10 minutes (40 mL/hour). 2 mL of Oleic Acid (distilled in dry box) is added and the reactor is then heated to 260° C. for the duration of the overcoating procedure (approximately 2 hours), The reaction is then cooled to room temperature and transferred to the dry box using standard air-free techniques.

The QDs are comprised of an InAs core surrounded by a semiconductor shell(s) of ZnS, CdZnS, or CdSe/CdZnS. The shell in this case possesses a wider bandgap confining the wavefunction of the excited state electron hole pair in the photoexcited core. A third, organic layer surrounds the semiconductor shell which prevents QDs from interacting and subsequently deactivating.

In another aspect of the present invention, there is provided a composition including the semiconductor nanocrystals taught herein.

In certain embodiments, the composition can comprise a particle, composition, formulation, powder, coatings, etc. described in International Application No. PCT/US2010/48285, of QD Vision, Inc., for “Particles Including Nanoparticles, Uses Thereof, And Methods”, filed 9 Sep. 2010, which is hereby incorporated herein by reference in its entirety.

In certain embodiments, the composition can comprise a formulation, coating, powder, composition, formulations, raw batch formulation, etc. described in International Application No. PCT/US2010/48291, of QD Vision, Inc., for “Formulations Including Nanoparticles”, filed 9 Sep. 2010, which is hereby incorporated herein be reference in its entirety.

Additional information that may be useful with the present invention is included in International Application No. PCT/US2007/13152 of Coe-Sullivan, et al., for “Light-Emitting Devices and Displays With Improved Performance”, filed 4 Jun. 2007; International Application No. PCT/US2008/007901 of Linton, et al., for “Compositions And Methods Including Depositing Nanomaterial”, filed 25 Jun. 2008; International Application No. PCT/US2009/002123 filed 3 Apr. 2009 of QD Vision, Inc. for “Light-Emitting Device Including Quantum Dots”, International Application No. PCT/US2010/51867 of QD Vision, Inc., filed 7 Oct. 2010, and International Application No. PCT/US2010/56397 of QD Vision, Inc., filed 11 Nov. 2010, and International Application No. PCT/US2007/024320, of QD Vision, Inc., for “Nanocrystals Including A Group Ma Element And A Group Va Element, Method, Composition, Device And Other Products”, filed 21 Nov. 2007. The disclosures of each of the foregoing applications are hereby incorporated herein by reference in their entireties.

As used herein, the singular forms “a”, “an” and “the” include plural unless the context clearly dictates otherwise. Thus, for example, reference to an emissive material includes reference to one or more of such materials.