# Patent

## Title
Highly luminescent nanostructures and methods of producing same

## Abstract

Highly luminescent nanostructures, particularly highly luminescent quantum dots, are provided. The nanostructures have high photoluminescence quantum yields and in certain embodiments emit light at particular wavelengths and have a narrow size distribution. The nanostructures can comprise ligands, including C5-C8 carboxylic acid ligands employed during shell formation and/or dicarboxylic or polycarboxylic acid ligands provided after synthesis. Processes for producing such highly luminescent nanostructures are also provided, including methods for enriching nanostructure cores with indium and techniques for shell synthesis.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a divisional of U.S. patent application Ser. No. 13/917,570, filed Jun. 13, 2013, which claims the benefit of the following prior provisional patent application: U.S. Ser. No. 61/667,147, filed Jul. 2, 2012, entitled “HIGHLY LUMINESCENT NANOSTRUCTURES AND METHODS OF PRODUCING SAME” by Wenzhuo Guo et al., which is incorporated herein by reference in its entirety for all purposes.


FIELD OF THE INVENTION
The invention pertains to the field of nanotechnology. More particularly, the invention relates to highly luminescent nanostructures, particularly highly luminescent nanostructures comprising an InP core and one or more shell layers. The invention also relates to methods of producing such nanostructures.
BACKGROUND OF THE INVENTION
Semiconductor nanostructures can be incorporated into a variety of electronic and optical devices. The electrical and optical properties of such nanostructures vary, e.g., depending on their composition, shape, and size. For example, size-tunable properties of semiconductor nanoparticles are of great interest for applications such as light emitting diodes (LEDs), lasers, and biomedical labeling. Highly luminescent nanostructures are particularly desirable for such applications
Quantum dots with a CdSe core have been produced that exhibit a high quantum yield. However, the intrinsic toxicity of cadmium raises environmental concerns which limit the future application of such cadmium-based nanoparticles. InP-based nanostructures have a similar emission range and are therefore an ideal substitute for CdSebased materials. High quantum yield InP nanostructures, however, have been difficult to obtain.
Methods for simply and reproducibly producing highly luminescent nanostructures, particularly highly luminescent InP nanostructures, are thus desirable. Among other aspects, the present invention provides such methods. A complete understanding of the invention will be obtained upon review of the following.
SUMMARY OF THE INVENTION
Methods for producing highly luminescent nanostructures are described, including methods of enriching indium nanostructure cores, methods using C5-C8 carboxylic acid ligands during shell synthesis, and methods for two-step growth of a layered ZnSxSe1-x/ZnS shell. Compositions related to the methods of the invention are also featured, including highly luminescent nanostructures with high quantum yields.
One general class of embodiments provides methods for production of core/shell nanostructures (e.g., quantum dots). In the methods, a nanostructure core is provided, and a shell surrounding the core is produced by providing one or more precursors and reacting the precursors in the presence of a C5-C8 carboxylic acid ligand to produce the shell. The carboxylic acid can be branched or, more preferably, unbranched. Optionally, the carboxylic acid is an unbranched monocarboxylic acid. The carboxylic acid can be saturated or unsaturated.
Thus, in one class of embodiments, the ligand is an alkyl carboxylic acid, preferably an unbranched alkyl carboxylic acid such as pentanoic acid, hexanoic acid, heptanoic acid, or octanoic (caprylic) acid. In other embodiments, the ligand is an alkenyl carboxylic acid or an alkynyl carboxylic acid, for example, 4-pentenoic acid.
The shell can be produced in the presence of a mixture of ligands, e.g., a C5-C8 carboxylic acid ligand and an additional ligand. In one class of embodiments, the additional ligand is a long chain fatty acid ligand, which long chain fatty acid comprises at least 12 carbon atoms.
In one aspect, the nanostructure core is a Group III-V core, e.g., an InP core. The shell typically comprises a material different than the core. In one aspect, the shell comprises a Group II-VI semiconductor, for example, ZnSxSe1-x, where 0≦x≦1. The shell optionally comprises more than one layer, which are optionally synthesized in different steps. Accordingly, in one class of embodiments, the shell comprises at least two layers and forming the shell includes providing a first set of one or more precursors and reacting them in the presence of the C5-C8 carboxylic acid ligand to produce a first layer of the shell, and then providing a second set of one or more precursors and reacting them in the presence of the C5-C8 carboxylic acid ligand to produce a second layer of the shell. Different layers typically comprise different materials. For example, the first layer can comprise ZnSxSe1-x, where 0<x<1 (e.g., where 0.25≦x≦0.75 or where x is about 0.5), and the second layer can comprise ZnS. Suitable precursors for shell formation are well known in the art. For example, suitable precursors for ZnSxSe1-x shell formation, where 0<x<1, include diethyl zinc, bis(trimethylsilyl)selenide, and hexamethyldisilthiane, and suitable precursors for ZnS shell formation include diethyl zinc and hexamethyldisilthiane.
If desired, the ligand(s) in which the shell was synthesized can be replaced by a different ligand. For example, in one class of embodiments, after shell formation the ligand is exchanged with a dicarboxylic or polycarboxylic acid ligand (e.g., dodecenyl succinic acid), which can further increase quantum yield.
The methods optionally include incorporating the nanostructures into a matrix, a phosphor, and/or a device (e.g., an LED, backlighting unit, downlight, or other display or lighting unit or an optical filter) after their synthesis. Essentially all of the features noted for the other methods and compositions herein apply to these methods as well, as relevant.
Nanostructures and compositions resulting from the methods are also a feature of the invention. Accordingly, one general class of embodiments provides a composition that includes a nanostructure (e.g., a quantum dot) and a C5-C8 carboxylic acid ligand bound to a surface of the nanostructure. The composition optionally includes a population of such nanostructures with bound ligand. The carboxylic acid can be branched or, more preferably, unbranched. Optionally, the carboxylic acid is an unbranched monocarboxylic acid. The carboxylic acid can be saturated or unsaturated.
Thus, in one class of embodiments, the ligand is an alkyl carboxylic acid, preferably an unbranched alkyl carboxylic acid such as pentanoic acid, hexanoic acid, heptanoic acid, or octanoic (caprylic) acid. In other embodiments, the ligand is an alkenyl carboxylic acid or an alkynyl carboxylic acid, for example, 4-pentenoic acid.
The composition optionally includes one or more additional ligands. For example, in one class of embodiments the composition also includes a long chain fatty acid ligand that comprises at least 12 carbon atoms bound to the surface of the nanostructure. In a preferred class of embodiments, the composition includes a dicarboxylic or polycarboxylic acid ligand bound to the surface of the nanostructure, e.g., dodecenyl succinic acid.
The nanostructure can comprise essentially any desired material, for example, a Group II-VI semiconductor, a Group III-V semiconductor, or a combination thereof. Thus, the nanostructure optionally comprises InP and/or ZnSxSe1-x where 0≦x≦1 (e.g., where x=0, x=1, 0<x<1, 0.25≦x≦0.75, or x is about 0.5). The nanostructure optionally includes at least one shell.
In one class of embodiments, the nanostructure comprises an InP core and a ZnSxSe1-x shell, where 0≦x≦1, e.g., where 0<x<1, where 0.25≦x≦0.75, or where x is about 0.5. For example, the nanostructure can be an InP/ZnSxSe1-x/ZnS core/shell quantum dot, where 0<x<1, e.g., where 0.25≦x≦0.75 or where x is about 0.5.
The nanostructure or population of nanostructures is optionally embedded in a matrix (e.g., an organic polymer, silicon-containing polymer, inorganic, glassy, and/or other matrix). In one class of embodiments, the nanostructure or population of nanostructures is incorporated into a device, for example, an LED, backlighting unit, downlight, or other display or lighting unit or an optical filter. Essentially all of the features noted for the other compositions and methods herein apply to these compositions as well, as relevant.
As noted, core/shell nanostructures can include more than one layer in their shell. For example, an InP core is advantageously coated with an intermediate layer of ZnSSe followed by an outer layer of ZnS. Accordingly, one general class of embodiments provides a population of InP/ZnSxSe1-x/ZnS core/shell quantum dots, where 0.25≦x≦0.75 (e.g., x is about 0.5) and where the ZnSxSe1-x shell is between about 0.3 and about 1.0 monolayer thick (e.g., about 0.5 monolayer thick) and the ZnS shell is between about 1.0 and about 3.0 monolayers thick (e.g., about 2 monolayers thick). In one class of embodiments, the ZnSxSe1-x shell is about 0.5 monolayer thick and the ZnS shell is about 2 monolayers thick.
The InP cores of the quantum dots optionally have an average diameter between about 1 nm and about 5 nm, e.g., between about 15 {acute over (Å)} and about 20 {acute over (Å)} or between about 25 {acute over (Å)} and about 30 {acute over (Å)}. The population of nanostructures is optionally embedded in a matrix and/or incorporated into a device, for example, an LED, backlighting unit, downlight, or other display or lighting unit or an optical filter. Essentially all of the features noted for the other compositions and methods herein apply to these compositions as well, as relevant.
Another general class of embodiments provides methods for production of core/shell nanostructures (e.g., quantum dots) where the shell comprises at least two layers. In the methods, an InP nanostructure core and a first set of one or more precursors are provided. The precursors are reacted to produce a first layer of the shell, which first layer comprises ZnSxSe1-x, where 0.25≦x≦0.75 (e.g., where x is about 0.5). A second set of one or more precursors is then provided and reacted to produce a second layer of the shell, which second layer comprises ZnS.
Optionally, the core has an average diameter between about 1 nm and about 5 nm. In one exemplary class of embodiments, the core has an average diameter between about 15 {acute over (Å)} and about 20 {acute over (Å)}. In another exemplary class of embodiments, the core has an average diameter between about 25 {acute over (Å)} and about 30 {acute over (Å)}.
Suitable precursors for shell formation are well known in the art. For example, suitable precursors for ZnSxSe1-x shell formation include, but are not limited to, diethyl zinc, bis(trimethylsilyl)selenide, and hexamethyldisilthiane. Suitable precursors for ZnS shell formation include, but are not limited to, diethyl zinc and hexamethyldisilthiane.
Thickness of the shell layers can be conveniently controlled by controlling the amount of precursor provided. Thus, in one class of embodiments, providing a first set of one or more precursors and reacting the precursors to produce a first layer of the shell comprises providing the one or more precursors in an amount whereby, when the reaction is substantially complete, the first layer is between about 0.3 and about 1.0 monolayer of ZnSxSe1-x thick, e.g., about 0.5 monolayer of ZnSxSe1-x thick. Similarly, in one class of embodiments providing a second set of one or more precursors and reacting the precursors to produce a second layer of the shell comprises providing the one or more precursors in an amount whereby, when the reaction is substantially complete, the second layer is between about 1.0 and about 3.0 monolayers of ZnS thick, e.g., about 2 monolayers of ZnS thick.
Essentially all of the features noted for the other methods and compositions herein apply to these methods as well, as relevant, for example, with regard to enrichment of the cores, use of ligand, and incorporation of the resulting nanostructures into a matrix, phosphor, and/or device.
Enrichment of InP nanostructures with indium can significantly enhance their quantum yield. Thus, one general class of embodiments provides methods for production of nanostructures. In the methods, InP nanostructures are produced in solution. After the InP nanostructures are removed from the solution in which they were produced, they are suspended in at least one solvent. The suspended nanostructures are contacted with a first precursor, which first precursor comprises In (indium). Suitable first precursors include, e.g., indium carboxylates such as indium laurate.
The nanostructures can be of essentially any desired size. For example, they can have an average diameter between about 1 nm and about 5 nm. In one exemplary class of embodiments, the InP nanostructures have an average diameter between about 15 {acute over (Å)} and about 20 {acute over (Å)}. In another exemplary class of embodiments, the InP nanostructures have an average diameter between about 25 {acute over (Å)} and about 30 {acute over (Å)}.
Following enrichment, the In-enriched InP nanostructures are optionally used as cores to which one or more shells are added to produce core/shell nanostructures (e.g., core/shell quantum dots). Thus, in one class of embodiments, after the suspended nanostructures have been contacted with the first precursor, one or more second precursors are added and reacted to form a shell. Typically, the shell comprises a material other than InP, for example, another Group III-V material such as GaP or a Group II-VI material such as ZnS or ZnSe. The shell optionally comprises more than one layer. For example, the resulting nanostructures can be InP/ZnSxSe1-x/ZnS core/shell quantum dots, where 0<x<1. Accordingly, in one exemplary class of embodiments, forming the shell includes providing a first set of one or more second precursors and reacting them to produce a first layer of the shell, which first layer comprises ZnSxSe1-x, where 0.25≦x≦0.75 (e.g., x is about 0.5), and then providing a second set of one or more second precursors and reacting them to produce a second layer of the shell, which second layer comprises ZnS. Suitable precursors for shell formation are well known in the art. For example, suitable precursors for ZnSSe shell formation include diethyl zinc, bis(trimethylsilyl)selenide, and hexamethyldisilthiane, and suitable precursors for ZnS shell formation include diethyl zinc and hexamethyldisilthiane.
Typically, the second precursors are reacted to form the shell in the presence of a ligand. The ligand can be essentially any of those known in the art, or, preferably, can be a C5-C8 carboxylic acid ligand, e.g., any of those C5-C8 carboxylic acid ligands described herein. As an example, the ligand can be an unbranched C5-C8 alkyl carboxylic acid such as hexanoic acid. If desired, the ligand(s) in which the InP nanostructure, InP core, and/or shell was synthesized can be replaced by a different ligand. For example, in one class of embodiments, after shell formation the ligand is exchanged with a dicarboxylic or polycarboxylic acid ligand (e.g., dodecenyl succinic acid).
Essentially all of the features noted for the other methods and compositions herein apply to these methods as well, as relevant, for example, with regard to enrichment of the cores, use of ligand, and incorporation of the resulting nanostructures into a matrix, phosphor, and/or device.
Highly luminescent nanostructures are also a feature of the invention. For example, one general class of embodiments provides a composition comprising a population of nanostructures, which population displays a photoluminescence quantum yield of 70% or greater, wherein a photoluminescence spectrum of the population has a full width at half maximum (FWHM) of 50 nm or less, and wherein the member nanostructures comprise InP. Optionally, the nanostructures are substantially free of Ga. The population can display a photoluminescence quantum yield of 75% or greater, 80% or greater, 85% or greater, or even 90% or greater.
The photoluminescence spectrum of the nanostructures can cover essentially any desired portion of the spectrum. For example, the photoluminescence spectrum of the population can have an emission maximum between 450 nm and 750 nm, e.g., between 500 nm and 650 nm. In one class of embodiments, the photoluminescence spectrum of the population has an emission maximum between 500 nm and 560 nm, e.g., between 500 nm and 540 nm (e.g., in the green portion of the spectrum). In one class of embodiments, the photoluminescence spectrum of the population has an emission maximum between 600 nm and 650 nm (e.g., in the red portion of the spectrum).
As noted, the size distribution of the nanostructures (as indicated, e.g., by the full width at half maximum of the photoluminescence spectrum) is narrow. Optionally, the photoluminescence spectrum of the population has a full width at half maximum of 45 nm or less, or even 40 nm or less or 35 nm or less.
The nanostructures optionally include at least one shell. In one aspect, the nanostructures are quantum dots, e.g., quantum dots with an InP core and one or more shells. In one class of embodiments, the nanostructures are InP/ZnSxSe1-x/ZnS core/shell quantum dots, where 0<x<1, e.g., where 0.25≦x≦0.75 or where x is about 0.5. In one class of embodiments, the ZnSxSe1-x layer of the shell is about 0.5 monolayer thick and the ZnS layer of the shell is about 2.0 monolayers thick. Optionally, the InP cores have an average diameter between about 1 nm and about 5 nm, e.g., between about 15 {acute over (Å)} and about 20 {acute over (Å)}. The average diameter of the quantum dots can be between about 1.8 nm and about 7.5 nm, e.g., between about 30 {acute over (Å)} and about 36 {acute over (Å)}.
The composition optionally includes one or more nanostructure ligands, e.g., those described herein and/or those known in the art. Thus, in one class of embodiments, the composition includes a C5-C8 carboxylic acid ligand bound to the nanostructures, e.g., an unbranched alkyl carboxylic acid such as pentanoic acid or hexanoic acid. In one class of embodiments, the composition includes a dicarboxylic or polycarboxylic acid ligand bound to the nanostructures, e.g., dodecenyl succinic acid.
The population of nanostructures is optionally embedded in a matrix (e.g., an organic polymer, silicon-containing polymer, inorganic, glassy, and/or other matrix, incorporated into a nanostructure phosphor, and/or incorporated into a device, for example, an LED, backlighting unit, downlight, or other display or lighting unit or an optical filter. Essentially all of the features noted for the other compositions and methods herein apply to these compositions as well, as relevant.
Another general class of embodiments provides a composition comprising a population of nanostructures, which population displays a photoluminescence quantum yield of 75% or greater, wherein the member nanostructures comprise InP and are substantially free of Ga. The population can display a photoluminescence quantum yield of 80% or greater, 85% or greater, or even 90% or greater.
The photoluminescence spectrum of the nanostructures can cover essentially any desired portion of the spectrum. For example, the photoluminescence spectrum of the population can have an emission maximum between 450 nm and 750 nm, e.g., between 500 nm and 650 nm. In one class of embodiments, the photoluminescence spectrum of the population has an emission maximum between 500 nm and 560 nm, e.g., between 500 nm and 540 nm (e.g., in the green portion of the spectrum). In one class of embodiments, the photoluminescence spectrum of the population has an emission maximum between 600 nm and 650 nm (e.g., in the red portion of the spectrum). The size distribution of the nanostructures can be relatively narrow. Thus, the photoluminescence spectrum of the population can have a full width at half maximum of 60 nm or less, e.g., 50 nm or less, 45 nm or less, or even 40 nm or less or 35 nm or less.
The nanostructures optionally include at least one shell. In one aspect, the nanostructures are quantum dots, e.g., quantum dots with an InP core and one or more shells. In one class of embodiments, the nanostructures are InP/ZnSxSe1-x/ZnS core/shell quantum dots, where 0<x<1, e.g., where 0.25≦x≦0.75 or where x is about 0.5. In one class of embodiments, the ZnSxSe1-x layer of the shell is about 0.5 monolayer thick and the ZnS layer of the shell is about 2.0 monolayers thick. Optionally, the InP cores have an average diameter between about 1 nm and about 5 nm, e.g., between about 15 {acute over (Å)} and about 20 {acute over (Å)}. The average diameter of the quantum dots can be between about 1.8 nm and about 7.5 nm, e.g., between about 30 {acute over (Å)} and about 36 {acute over (Å)}.
The composition optionally includes one or more nanostructure ligands, e.g., those described herein and/or those known in the art. Thus, in one class of embodiments, the composition includes a C5-C8 carboxylic acid ligand bound to the nanostructures, e.g., an unbranched alkyl carboxylic acid such as pentanoic acid or hexanoic acid. In one class of embodiments, the composition includes a dicarboxylic or polycarboxylic acid ligand bound to the nanostructures, e.g., dodecenyl succinic acid.
The population of nanostructures is optionally embedded in a matrix (e.g., an organic polymer, silicon-containing polymer, inorganic, glassy, and/or other matrix, incorporated into a nanostructure phosphor, and/or incorporated into a device, for example, an LED, backlighting unit, downlight, or other display or lighting unit or an optical filter. Essentially all of the features noted for the other compositions and methods herein apply to these compositions as well, as relevant.
Another general class of embodiments provides a composition comprising a population of nanostructures, which population displays a photoluminescence quantum yield of 65% or greater, wherein a photoluminescence spectrum of the population has an emission maximum between 600 nm and 650 nm, and wherein the member nanostructures comprise InP.
The nanostructures optionally include at least one shell. In one aspect, the nanostructures are quantum dots, e.g., quantum dots with an InP core and one or more shells. In one class of embodiments, the nanostructures are InP/ZnSxSe1-x/ZnS core/shell quantum dots, where 0<x<1, e.g., where 0.25≦x≦0.75 or where x is about 0.5. In one class of embodiments, the ZnSxSe1-x layer of the shell is about 0.5 monolayer thick and the ZnS layer of the shell is about 2.0 monolayers thick. Optionally, the InP cores have an average diameter between about 25 {acute over (Å)} and about 30 {acute over (Å)}. The average diameter of the quantum dots can be between about 40 {acute over (Å)} and about 46 {acute over (Å)}. The size distribution of the nanostructures can be relatively narrow. For example, the photoluminescence spectrum of the population can have a full width at half maximum of 45 nm or less.
The composition optionally includes one or more nanostructure ligands, e.g., those described herein and/or those known in the art. Thus, in one class of embodiments, the composition includes a C5-C8 carboxylic acid ligand bound to the nanostructures, e.g., an unbranched alkyl carboxylic acid such as pentanoic acid or hexanoic acid. In one class of embodiments, the composition includes a dicarboxylic or polycarboxylic acid ligand bound to the nanostructures, e.g., dodecenyl succinic acid.
The population of nanostructures is optionally embedded in a matrix (e.g., an organic polymer, silicon-containing polymer, inorganic, glassy, and/or other matrix, incorporated into a nanostructure phosphor, and/or incorporated into a device, for example, an LED, backlighting unit, downlight, or other display or lighting unit or an optical filter. Essentially all of the features noted for the other compositions and methods herein apply to these compositions as well, as relevant.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A shows transmission electron micrographs of InP/ZnSSe/ZnS nanostructures at low resolution and high resolution FIG. 1B.
FIGS. 2A-2E illustrate optical characterization of green-emitting InP/ZnSSe/S quantum dots. FIG. 2A presents quantum yield measurement based on fluorescein dye (fluorescein upper line, dots lower line). FIG. 2B presents absorption spectra of fluorescein dye. FIG. 2C presents photoluminescence spectra of fluorescein dye. FIG. 2D presents absorption spectra of InP/ZnSSe/S dots. FIG. 2E presents photoluminescence spectra of InP/ZnSSe/S dots.
FIGS. 3A-3E illustrate optical characterization of red-emitting InP/ZnSSe/S quantum dots. FIG. 3A presents quantum yield measurement based on rhodamine 640 dye (Rh 640, upper line; dots lower line). FIG. 3B presents absorption spectra of rhodamine 640 dye. FIG. 3C presents photoluminescence spectra of rhodamine 640 dye. FIG. 3D presents absorption spectra of InP/ZnSSe/S dots. FIG. 3E presents photoluminescence spectra of InP/ZnSSe/S dots.



DEFINITIONS
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention pertains. The following definitions supplement those in the art and are directed to the current application and are not to be imputed to any related or unrelated case, e.g., to any commonly owned patent or application. Although any methods and materials similar or equivalent to those described herein can be used in the practice for testing of the present invention, the preferred materials and methods are described herein. Accordingly, the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting.
As used in this specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a nanostructure” includes a plurality of such nanostructures, and the like.
The term “about” as used herein indicates the value of a given quantity varies by +/−10% of the value, or optionally +/−5% of the value, or in some embodiments, by +/−1% of the value so described. For example, “about 100 nm” encompasses a range of sizes from 90 nm to 110 nm, inclusive.
A “nanostructure” is a structure having at least one region or characteristic dimension with a dimension of less than about 500 nm, e.g., less than about 200 nm, less than about 100 nm, less than about 50 nm, or even less than about 20 nm or less than about 10 nm. Typically, the region or characteristic dimension will be along the smallest axis of the structure. Examples of such structures include nanowires, nanorods, nanotubes, branched nanostructures, nanotetrapods, tripods, bipods, nanocrystals, nanodots, quantum dots, nanoparticles, and the like. Nanostructures can be, e.g., substantially crystalline, substantially monocrystalline, polycrystalline, amorphous, or a combination thereof. In one aspect, each of the three dimensions of the nanostructure has a dimension of less than about 500 nm, e.g., less than about 200 nm, less than about 100 nm, less than about 50 nm, or even less than about 20 nm or less than about 10 nm.
The term “heterostructure” when used with reference to nanostructures refers to nanostructures characterized by at least two different and/or distinguishable material types. Typically, one region of the nanostructure comprises a first material type, while a second region of the nanostructure comprises a second material type. In certain embodiments, the nanostructure comprises a core of a first material and at least one shell of a second (or third etc.) material, where the different material types are distributed radially about the long axis of a nanowire, a long axis of an arm of a branched nanowire, or the center of a nanocrystal, for example. (A shell can but need not completely cover the adjacent materials to be considered a shell or for the nanostructure to be considered a heterostructure; for example, a nanocrystal characterized by a core of one material covered with small islands of a second material is a heterostructure.) In other embodiments, the different material types are distributed at different locations within the nanostmcture; e.g., along the major (long) axis of a nanowire or along a long axis of arm of a branched nanowire. Different regions within a heterostructure can comprise entirely different materials, or the different regions can comprise a base material (e.g., silicon) having different dopants or different concentrations of the same dopant.
As used herein, the “diameter” of a nanostructure refers to the diameter of a cross-section normal to a first axis of the nanostructure, where the first axis has the greatest difference in length with respect to the second and third axes (the second and third axes are the two axes whose lengths most nearly equal each other). The first axis is not necessarily the longest axis of the nanostructure; e.g., for a disk-shaped nanostructure, the cross-section would be a substantiallycircular cross-section normal to the short longitudinal axis of the disk. Where the cross-section is not circular, the diameter is the average of the major and minor axes of that cross-section. For an elongated or high aspect ratio nanostructure, such as a nanowire, the diameter is measured across a cross-section perpendicular to the longest axis of the nanowire. For a spherical nanostructure, the diameter is measured from one side to the other through the center of the sphere.
The terms “crystalline” or “substantially crystalline,” when used with respect to nanostructures, refer to the fact that the nanostructures typically exhibit long-range ordering across one or more dimensions of the structure. It will be understood by one of skill in the art that the term “long range ordering” will depend on the absolute size of the specific nanostructures, as ordering for a single crystal cannot extend beyond the boundaries of the crystal. In this case, “long-range ordering” will mean substantial order across at least the majority of the dimension of the nanostructure. In some instances, a nanostructure can bear an oxide or other coating, or can be comprised of a core and at least one shell. In such instances it will be appreciated that the oxide, shell(s), or other coating can but need not exhibit such ordering (e.g. it can be amorphous, polycrystalline, or otherwise). In such instances, the phrase “crystalline,” “substantially crystalline,” “substantially monocrystalline,” or “monocrystalline” refers to the central core of the nanostructure (excluding the coating layers or shells). The terms “crystalline” or “substantially crystalline” as used herein are intended to also encompass structures comprising various defects, stacking faults, atomic substitutions, and the like, as long as the structure exhibits substantial long range ordering (e.g., order over at least about 80% of the length of at least one axis of the nanostructure or its core). In addition, it will be appreciated that the interface between a core and the outside of a nanostructure or between a core and an adjacent shell or between a shell and a second adjacent shell may contain non-crystalline regions and may even be amorphous. This does not prevent the nanostructure from being crystalline or substantially crystalline as defined herein.
The term “monocrystalline” when used with respect to a nanostructure indicates that the nanostructure is substantially crystalline and comprises substantially a single crystal. When used with respect to a nanostructure heterostructure comprising a core and one or more shells, “monocrystalline” indicates that the core is substantially crystalline and comprises substantially a single crystal.
A “nanocrystal” is a nanostructure that is substantially monocrystalline. A nanocrystal thus has at least one region or characteristic dimension with a dimension of less than about 500 nm, e.g., less than about 200 nm, less than about 100 nm, less than about 50 nm, or even less than about 20 nm or less than about 10 nm. The term “nanocrystal” is intended to encompass substantially monocrystalline nanostructures comprising various defects, stacking faults, atomic substitutions, and the like, as well as substantially monocrystalline nanostructures without such defects, faults, or substitutions. In the case of nanocrystal heterostructures comprising a core and one or more shells, the core of the nanocrystal is typically substantially monocrystalline, but the shell(s) need not be. In one aspect, each of the three dimensions of the nanocrystal has a dimension of less than about 500 nm, e.g., less than about 200 nm, less than about 100 nm, less than about 50 nm, or even less than about 20 nm or less than about 10 nm.
The term “quantum dot” (or “dot”) refers to a nanocrystal that exhibits quantum confinement or exciton confinement. Quantum dots can be substantially homogenous in material properties, or in certain embodiments, can be heterogeneous, e.g., including a core and at least one shell. The optical properties of quantum dots can be influenced by their particle size, chemical composition, and/or surface composition, and can be determined by suitable optical testing available in the art. The ability to tailor the nanocrystal size, e.g., in the range between about 1 nm and about 15 nm, enables photoemission coverage in the entire optical spectrum to offer great versatility in color rendering.
A “carboxylic acid” is an organic acid having at least one carboxyl group. A C5 carboxylic acid includes five carbon atoms, a C6 carboxylic acid includes six carbon atoms, a C7 carboxylic acid includes seven carbon atoms, and a C8 carboxylic acid includes eight carbon atoms.
An “alkyl carboxylic acid” has the formula R—COOH, where R is an alkyl group.
An “alkenyl carboxylic acid” has the formula R—COOH, where R is an alkenyl group.
An “alkynyl carboxylic acid” has the formula R—COOH, where R is an alkynyl group.
An “alkyl group” is a functional group that includes only single-bonded carbon and hydrogen atoms. An alkyl group can be unbranched (i.e., linear or straight-chain), branched, or cyclic.
An “alkenyl group” includes only carbon and hydrogen atoms but contains at least one carbon-to-carbon double bond. An alkenyl group can be unbranched (i.e., linear or straight-chain), branched, or cyclic.
An “alkynyl group” contains only carbon and hydrogen atoms and includes at least one triple bond between two carbon atoms. An alkynyl group can be unbranched (i.e., linear or straight-chain), branched, or cyclic.
A “dicarboxylic acid” is a compound having two carboxylic acid moieties (e.g., two monocarboxylic acid moieties or one dicarboxylic acid moiety). A “polycarboxylic acid” is a compound having three or more carboxylic acid moieties.
A “fatty acid” is a monocarboxylic acid with an aliphatic tail (saturated or unsaturated, but including only carbon and hydrogen atoms).
A “precursor” in a nanostructure synthesis reaction is a chemical substance (e.g., a compound or element) that reacts, e.g., with another precursor, and thereby contributes at least one atom to the nanostructure produced by the reaction.
A “ligand” is a molecule capable of interacting (whether weakly or strongly) with one or more faces of a nanostructure, e.g., through covalent, ionic, van der Waals, or other molecular interactions with the surface of the nanostructure.
“Photoluminescence quantum yield” is the ratio of photons emitted to photons absorbed, e.g., by a nanostructure or population of nanostructures. As known in the art, quantum yield is typically determined by a comparative method using well-characterized standard samples with known quantum yield values.
Unless clearly indicated otherwise, ranges listed herein are inclusive.
A variety of additional terms are defined or otherwise characterized herein.
DETAILED DESCRIPTION
Methods for colloidal synthesis of a variety of nanostructures are known in the art. Such methods include techniques for controlling nanostructure growth, e.g., to control the size and/or shape distribution of the resulting nanostructures.
In a typical colloidal synthesis, semiconductor nanostructures are produced•by rapidly injecting precursors that undergo pyrolysis into a hot solution (e.g., hot solvent and/or surfactant). The precursors can be injected simultaneously or sequentially. The precursors rapidly react to form nuclei. Nanostructure growth occurs through monomer addition to the nuclei, typically at a growth temperature that is lower than the injection/nucleation temperature.
Surfactant molecules interact with the surface of the nanostructure. At the growth temperature, the surfactant molecules rapidly adsorb and desorb from the nanostructure surface, permitting the addition and/or removal of atoms from the nanostructure while suppressing aggregation of the growing nanostructures. In general, a surfactant that coordinates weakly to the nanostructure surface permits rapid growth of the nanostructure, while a surfactant that binds more strongly to the nanostructure surface results in slower nanostructure growth. The surfactant can also interact with one (or more) of the precursors to slow nanostructure growth.
Nanostructure growth in the presence of a single surfactant typically results in spherical nanostructures. Using a mixture of two or more surfactants, however, permits growth to be controlled such that non-spherical nanostructures can be produced, if, for example, the two (or more) surfactants adsorb differently to different crystallographic faces of the growing nanostructure.
A number of parameters are thus known to affect nanostructure growth and can be manipulated, independently or in combination, to control the size and/or shape distribution of the resulting nanostructures. These include, e.g., temperature (nucleation and/or growth), precursor composition, time-dependent precursor concentration, ratio of the precursors to each other, surfactant composition, number of surfactants, and ratio of surfactant(s) to each other and/or to the precursors.
Synthesis of Group II-VI nanostructures has been described, e.g., in U.S. Pat. Nos. 6,225,198, 6,322,901, 6,207,229, 6,607,829, 7,060,243, 7,374,824, 6,861,155, 7,125,605, 7,566,476, 8,158,193, and 8,101,234 and US patent application publications 2011/0262752 and 2011/0263062.
Although Group II-VI nanostructures such as CdSe/CdS/ZnS core/shell quantum dots can exhibit desirable luminescence behavior, as noted above, issues such as the toxicity of cadmium limit the applications for which such nanostructures can be used. Less toxic alternatives with favorable luminescence properties are thus highly desirable. Group III-V nanostructures in general, and InP-based nanostructures in particular, offer an ideal substitute for cadmium-based materials due to their compatible emission range.
Synthesis of Group III-V nanostructures has been described, e.g., in U.S. Pat. No. 5,505,928 to Alivisatos et al. entitled “Preparation of III-V semiconductor nanocrystals,” U.S. Pat. No. 6,306,736 by Alivisatos et al. entitled “Process for forming shaped group III-V semiconductor nanocrystals, and product formed using process,” U.S. Pat. No. 6,576,291, U.S. Pat. No. 6,821,337, U.S. Pat. No. 7,138,098, US patent application publication 2003/0214699 by Banin et al. entitled “Method for producing inorganic semiconductor nanocrystalline rods and their use,” Wells et al. (1989) “The use of tris(trimethylsilyl)arsine to prepare gallium arsenide and indium arsenide” Chem. Mater. 1:4-6, and Guzelian et al. (1996) “Colloidal chemical synthesis and characterization of InAs nanocrystal quantum dots” 69: 1432-1434.
In particular, synthesis of InP-based nanostructures has been described, e.g., in Xie et al. (2007) “Colloidal InP nanocrystals as efficient emitters covering blue to near-infrared” J. Am. Chem. Soc. 129:15432-15433, Micic et al. (2000) “Core-shell quantum dots of lattice-matched ZnCdSe2 shells on InP cores: Experiment and theory” J. Phys. Chem. B 104:12149-12156, Liu et al. (2008) “Coreduction colloidal synthesis of III-V nanocrystals: The case of InP” Angew. Chem. Int. Ed. 47:3540-3542, Li et al. (2008) “Economic synthesis of high quality InP nanocrystals using calcium phosphide as the phosphorus precursor” Chem. Mater. 20:2621-2623, Battaglia and Peng (2002) “Formation of high quality InP and InAs nanocrystals in a noncoordinating solvent” Nano Lett. 2:1027-1030, Kim et al. (2012) “Highly luminescent InP/GaP/ZnS nanocrystals and their application to white light-emitting diodes” J. Am. Chem. Soc. 134:3804-3809, Nann et al. (2010) “Water splitting by visible light: A nanophotocathode for hydrogen production” Angew. Chem. Int. Ed. 49:1574-1577, Borchert et al. (2002) “Investigation of ZnS passivated InP nanocrystals by XPS” Nano Lett. 2:151-154, Li and Reiss (2008) “One-pot synthesis of highly luminescent InP/ZnS nanocrystals without precursor injection” J. Am. Chem. Soc. 130:11588-11589, Hussain et al. (2009) “One-pot fabrication of high-quality InP/ZnS (core/shell) quantum dots and their application to cellular imaging” Chemphyschem 10:1466-70, Xu et al. (2006) “Rapid synthesis of high-quality InP nanocrystals” J. Am. Chem. Soc. 128:1054-1055, Micic et al. (1997) “Size-dependent spectroscopy of InP quantum dots” J. Phys. Chem. B 101:4904-4912, Haubold et al. (2001) “Strongly luminescent InP/ZnS core-shell nanoparticles” Chemphyschem 5:331-334, CrosGagneux et al. (2010) “Surface chemistry of InP quantum dots: A comprehensive study” J. Am. Chem. Soc. 132:18147-18157, Micic et al. (1995) “Synthesis and characterization of InP, GaP, and GalnP2 quantum dots” J. Phys. Chem. 99:7754-7759, Guzelian et al. (1996) “Synthesis of size-selected, surface-passivated InP nanocrystals” J. Phys. Chem. 100:7212-7219, Lucey et al. (2005) “Monodispersed InP quantum dots prepared by colloidal chemistry in a non-coordinating solvent” Chem. Mater. 17:3754-3762, Lim et al. (2011) “InP@ZnSeS, core@composition gradient shell quantum dots with enhanced stability” Chem. Mater. 23:4459-4463, and Zan et al. (2012) “Experimental Studies on Blinking Behavior of Single InP/ZnS Quantum Dots: Effects of Synthetic Conditions and UV Irradiation” J. Phys. Chem. C 116:394-3950. However, such efforts have had only limited success in producing InP nanostructures with high quantum yields.
In one aspect, the present invention overcomes the above noted difficulties (e.g., low quantum yield) by providing techniques for enriching InP nanostructures or nanostructure cores with indium. Ligands whose use in shell growth enhances quantum yield of the resulting core/shell nanostructures are described. Methods for two-step growth of a layered ZnSxSe1-x/ZnS shell are also described. Compositions related to the methods of the invention are also featured, including highly luminescent nanostructures with high quantum yields and narrow size distributions.
Nanostructure Core Enrichment
Enrichment of InP nanostructures with indium can significantly enhance their quantum yield. Without limitation to any particular mechanism, reactivity of the indium precursor is typically lower than that of the phosphorus precursor, resulting in nanostructures having inherent defects in the crystal structure since the ratio of In:P in the nanostructures is less than 1:1. Again without limitation to any particular mechanism, enrichment of such nanostructures with indium after their initial synthesis can “fill the holes” in the crystal lattice, increasing the In:P ratio (e.g., to 1:1 or essentially 1:1) and improving quality of the crystal structure. Enrichment increases quantum yield and also typically results in a red shift in emission wavelength.
Thus, one general class of embodiments provides methods for production of nanostructures. In the methods, InP nanostructures are produced, typically in solution. After the InP nanostructures are removed from the solution in which they were produced, they are suspended in at least one solvent. The suspended nanostructures are contacted with a first precursor, which first precursor comprises In. The solution in which the suspended nanostructures are contacted with the In-containing precursor is substantially free of any P-containing precursor, since the nanostructures are removed from the solution in which they were synthesized (and optionally washed before resuspension) and since no P-containing precursor is added to the suspended nanostructures. (It will be evident that, although the nanostructures are preferably removed from the solution in which they were produced before enrichment is performed, enrichment can instead be performed with the nanostructures remaining in their growth solution, and such embodiments are also a feature of the invention.)
The InP nanostructures can be produced as described hereinbelow (see, e.g., the sections entitled “Synthesis of InP Core for Green-Emitting Core/Shell Dots” and “Synthesis of InP Core for Red-Emitting Core/Shell Dots” in the Examples) or as known in the art (see, e.g., U.S. Pat. No. 7,557,028, U.S. Pat. No. 8,062,967, U.S. Pat. No. 7,645,397, and US patent application publication 2010/0276638 (e.g., Example 7), as well as the references listed hereinabove). The nanostructures can be of essentially any desired size. For example, the InP nanostructures can have an average diameter between about 1 nm and about 5 nm. In one exemplary class of embodiments, the InP nanostructures have an average diameter between about 15 {acute over (Å)} and about 20 {acute over (Å)}. In another exemplary class of embodiments, the InP nanostructures have an average diameter between about 25 {acute over (Å)} and about 30 {acute over (Å)}. Size can be determined as is known in the art, for example, using transmission electron microscopy and/or physical modeling.
The first (In-containing) precursor can be the same as, or different than, the In-containing precursor(s) employed during InP nanostructure synthesis. Suitable first precursors include, but are not limited to, InCl3, chloroindium oxalate, indium oxide, indium phenoxy, and trialkyl, trialkenyl, and trialkynyl indium compounds, among other compounds used in the art as precursors for nanostructure synthesis. In a preferred class of embodiments, the first precursor is an indium carboxylate (i.e., an indium (III) carboxylate), for example, indium acetate, indium stearate, or indium myristate. In a particularly preferred embodiment, the first precursor is indium laurate.
Since a red shift of absorption and photoluminescence wavelength indicative of nanostructure growth is observed upon enrichment with indium, this red shift can be monitored to ensure that an adequate amount of first precursor is provided. For example, a quantity of indium laurate precursor such that the amount of indium in the precursor added to the solution is 7% (mole %) of the amount of indium in the InP nanostructures in the solution has been observed to be the minimum required to produce a red shirt; additional precursor does not increase the red shift.
Suitable solvents are known in the art and include those commonly used for nanostructure synthesis, particularly non-coordinating solvents. Exemplary solvents include 1-octadecene, 1-decene, 1-dodecene, and tetradecane.
The temperature at which the nanostructures and first precursor are contacted is typically lower than that at which the nanostructures were grown, for example, 20-80° C. lower (e.g., 200-230° C. compared to a growth temperature of 230-300° C.). The solution containing the nanostructures and precursor is maintained at such temperature until enrichment is complete, e.g., as determined by monitoring absorption and luminescence. Without limitation to any particular mechanism, the red shift in absorption and photoluminescence wavelength initially observed is thought to represent coating of In on the nanostructures, followed by an increase in emission intensity thought to represent diffusion of In into the core without an additional change in the size of the nanostructures. Incubation of the solution containing the nanostructures and precursor can be performed until no further increase in brightness is observed (e.g., for 1-2 hours). The enrichment process can result in a significant increase in quantum yield of the nanostructures, e.g., 10% or more.
Following enrichment, the In-enriched InP nanostructures are optionally used as cores to which one or more shells are added to produce core/shell nanostructures (e.g., core/shell quantum dots). Thus, in one class of embodiments, after the suspended nanostructures have been contacted with the first precursor, one or more second precursors are added and reacted to form a shell. Typically, the shell comprises a material other than InP, for example, another Group III-Y material such as GaP or a Group II-VI material such as ZnS or ZnSe.
The shell optionally comprises more than one layer (i.e., the core is surrounded by more than one shell). For example, the resulting nanostructures can be InP/GaP/ZnS, InP/ZnSe/ZnS, InP/ZnSeTe/ZnS, InP/MnSe/ZnS, InP/CdS/ZnS, InP/ZnCdSe/ZnS, InP/ZnCdMgSe/ZnS, or InP/ZnSxSe1-x/ZnS nanostructures, or, preferably, InP/ZnSxSe1-x/ZnS core/shell quantum dots, where 0<x<1. A multilayer shell is optionally synthesized in several discrete steps, as described in greater detail hereinbelow; see, e.g., the sections entitled “Multilayered Shells,” “Indium Enrichment and Shelling Process for Green-Emitting Dots,” and “Indium Enrichment and Shelling Process for Red-Emitting Dots.” Accordingly, in one exemplary class of embodiments, forming the shell includes providing a first set of one or more second precursors and reacting them to produce a first layer of the shell, which first layer comprises ZnSxSe1-x, where 0<x<1, and then providing a second set of one or more second precursors and reacting them to produce a second layer of the shell, which second layer comprises ZnS. Preferably, 0.25≦x≦0.75, e.g., x is about 0.5.
Suitable precursors for shell formation are well known in the art. For example, suitable precursors for ZnSxSe1-x shell formation, where 0<x<1, include diethyl zinc, zinc carboxylates (e.g., zinc stearate or zinc hexanoate), bis(trimethylsilyl)selenide, elemental selenium (e.g., dissolved in tributylphosphine), hexamethyldisilthiane, and organic thiols (e.g., 1-dodecanethiol, tert-dodecylmercaptan, or 1-octanethiol), and suitable precursors for ZnS shell formation include diethyl zinc, zinc carboxylates (e.g., zinc stearate or zinc hexanoate), hexamethyldisilthiane, and organic thiols (e.g., 1-dodecanethiol, tertdodecylmercaptan, or 1-octanethiol). Shell layer thickness is optionally controlled by controlling the amount of precursor(s) provided, as described hereinbelow.
Typically, the second precursors are reacted to form the shell in the presence of a ligand. The ligand can be essentially any of those known in the art, or, preferably, can be a C5-C8 carboxylic acid ligand, e.g., any of those C5-C8 carboxylic acid ligands described hereinbelow; see the section entitled “C5-C8 Carboxylic Acid Ligands.” As an example, the ligand can be an unbranched C5-C8 alkyl carboxylic acid such as hexanoic acid.
If desired, the ligand(s) in which the InP nanostructure, InP core, and/or shell was synthesized can be replaced by a different ligand. For example, in one class of embodiments, after shell formation the ligand is exchanged with a dicarboxylic or polycarboxylic acid ligand (e.g., dodecenyl succinic acid).
A variety of suitable ligands (e.g., for core synthesis, shell synthesis, and/or post-synthesis exchange) are known in the art. As a few examples, in addition to those described herein, such ligands include (but are not limited to) dicarboxylic acid, polycarboxylic acid, dicarbinol, alcohol, amine, polymeric silicone, and/or organic polymeric ligands as described in US patent application publications 2010/0276638 and 2007/0034833, amine functional polystyrene ligands and polyethyleneimine or modified polyethyleneimine ligands as described in US patent application publication 2012/0113672, silsesquioxane ligands (including polyhedral oligomeric silsesquioxane and/or carboxylic acid ligands) as described in U.S. Pat. Nos. 7,267,875 and 7,585,564 and US patent application publication 2008/0118755, carboxylic acid siloxane polymer ligands as described in U.S. patent application Ser. No. 13/803,596, and alkylamine siloxane polymer ligands as described in U.S. provisional patent application 61/663,234 and U.S. patent application Ser. No. 13/803,596.
The resulting nanostructures (e.g., quantum dots, e.g., InP/ZnSxSe1-x/ZnS quantum dots) optionally display a high photoluminescence quantum yield, e.g., 65% or greater, 70% or greater, 75% or greater, 80% or greater, 85% or greater, or even 90% or greater. The photoluminescence spectrum of the nanostructures can cover essentially any desired portion of the spectrum. For example, the photoluminescence spectrum can have an emission maximum between 450 nm and 750 nm, e.g., between 500 nm and 650 nm, between 500 nm and 560 nm, or between 600 nm and 650 nm. The photoluminescence spectrum can have a full width at half maximum of 60 nm or less, e.g., 50 nm or less, 45 nm or less, or even 40 nm or less or 35 nm or less, reflecting the narrow size distribution of the nano structures.
The resulting nanostructures are optionally embedded in a matrix (e.g., an organic polymer, silicon-containing polymer, inorganic, glassy, and/or other matrix), used in production of a nanostructure phosphor, and/or incorporated into a device, e.g., an LED, backlight, downlight, or other display or lighting unit or an optical filter. Exemplary phosphors and lighting units can, e.g., generate a specific color light by incorporating a population of nanostructures with an emission maximum at or near the desired wavelength or a wide color gamut by incorporating two or more different populations of nanostructures having different emission maxima. A variety of suitable matrices are known in the art. See, e.g., U.S. Pat. No. 7,068,898 and US patent application publications 2010/0276638, 2007/0034833, and 2012/0113672. Exemplary nanostructure phosphor films, LEDs, backlighting units, etc. are described, e.g., in US patent application publications 2010/0276638, 2012/0113672, 2008/0237540, 2010/0110728, and 2010/0155749 and U.S. Pat. Nos. 7,374,807, 7,645,397, 6,501,091, and 6,803,719.
As another example, the resulting nanostructures can be used for imaging or labeling, e.g., biological imaging or labeling. Thus, the resulting nanostructures are optionally covalently or noncovalently bound to biomolecule(s), including, but not limited to, a peptide or protein (e.g., an antibody or antibody domain, avidin, streptavidin, neutravidin, or other binding or recognition molecule), a ligand (e.g., biotin), a polynucleotide (e.g., a short oligonucleotide or longer nucleic acid), a carbohydrate, or a lipid (e.g., a phospholipid or other micelle). One or more nanostructures can be bound to each biomolecule, as desired for a given application. Such nanostructure-labeled biomolecules find use, for example, in vitro, in vivo, and in cellulo, e.g., in exploration of binding or chemical reactions as well as in subcellular, cellular, and organismal labeling.
Nanostructures resulting from the methods are also a feature of the invention. Thus, one class of embodiments provides a population of InP nanostructures or nanostructures comprising InP cores in which the nanostructures or cores have an In:P ratio of essentially 1:1 (e.g., greater than 0.99:1). The nanostructures are optionally quantum dots.
It will be evident that the methods can be applied to synthesis of nanostructures other than InP nanostructures where the ratio of the two or more elements constituting the nanostructures does not reach its ideal during synthesis of the nanostructures. Accordingly, one general class of embodiments provides methods for production of nanostructures in which nanostructures comprising a first element and a second element are produced in solution. The nanostructures are removed from the solution in which they were produced and then suspended in at least one solvent. The suspended nanostructures are contacted with a precursor comprising the first element (but typically not with a precursor comprising the second element), at a temperature and for a time sufficient for enrichment of the nanostructures with the first element to occur as described above for indium.
Shell Formation
In one aspect, the highly luminescent nanostructures of the present invention include a core and at least one shell. The shell can, e.g., increase the quantum yield and/or stability of the nanostructures. Typically, the core and the one or more shells comprise different materials. The core is generally synthesized first, optionally enriched as described above, and then additional precursors from which the shell (or a layer thereof) is produced are provided.
Typically, the core and shell(s) are synthesized in the presence of at least one nanostructure ligand. Ligands can, e.g., enhance the miscibility of nanostructures in solvents or polymers (allowing the nanostructures to be distributed throughout a composition such that the nanostructures do not aggregate together), increase quantum yield of nanostructures, and/or preserve nanostructure luminescence (e.g., when the nanostructures are incorporated into a matrix). The ligand(s) for core synthesis and for shell synthesis can but need not be the same. Similarly, following synthesis, any ligand on the surface of the nanostructures can be exchanged for a different ligand with other desirable properties.
Suitable ligands for synthesis of nanostructure cores, including InP cores, are known in the art. Exemplary ligands include fatty acids (e.g., lauric, caproic, myristic, palmitic, stearic, and oleic acid), organic phosphine oxides (e.g., trioctylphosphine oxide (TOPO), triphenylphosphine oxide, and tributylphosphine oxide), and organic amines (e.g., dodecylamine and oleylamine).
Although ligands for synthesis of nanostructure shells are also known in the art, recent work at Nanosys has demonstrated that carboxylic acid ligands shorter than the fatty acids typically used for core synthesis result in core/shell nanostructures exhibiting higher quantum yields.
C5-C8 Carboxylic Acid Ligands
Accordingly, one general class of embodiments provides methods for production of core/shell nanostructures. In the methods, a nanostructure core is provided, and a shell surrounding the core is produced by providing one or more precursors and reacting the precursors in the presence of a C5-C8 carboxylic acid ligand to produce the shell. The carboxylic acid can be branched (e.g., having a one carbon branch) or, more preferably, unbranched. Without limitation to any particular mechanism, ligands with little steric hindrance (such as, e.g., unbranched carboxylic acid ligands) offer maximum nanostructure surface coverage, resulting in higher quantum yields. Optionally, the carboxylic acid is an unbranched monocarboxylic acid. The carboxylic acid can be saturated or unsaturated.
Thus, in one class of embodiments, the ligand is an alkyl carboxylic acid, preferably an unbranched alkyl carboxylic acid such as pentanoic acid, hexanoic acid, heptanoic acid, or octanoic (caprylic) acid. In other embodiments, the ligand is an alkenyl carboxylic acid or an alkynyl carboxylic acid, for example, 4-pentenoic acid.
The shell can be produced in the presence of a mixture of ligands, e.g., a C5-C8 carboxylic acid ligand and an additional ligand (whether added during shell synthesis or remaining in the reaction mixture from core synthesis). For example, shell synthesis can be performed with a C5-C8 carboxylic acid ligand and an additional carboxylic acid ligand, e.g., a fatty acid ligand. In one class of embodiments, the additional ligand is a long chain fatty acid ligand, which long chain fatty acid comprises at least 12 carbon atoms. The fatty acid can be saturated or unsaturated, and can comprise less than 30 or less than 20 carbon atoms. Suitable fatty acids include, e.g., oleic acid, lauric acid, myristic acid, palmitic acid, or stearic acid. To maximize quantum yield of the resulting nanostructures, the long chain fatty acid preferably constitutes less than 50 molar percent of the total ligand in the mixture, more preferably, less than 25 molar %, less than 10 molar %, or less than 5 molar %.
In one aspect, the nanostructure core is a Group III-V core, e.g., an InAs, GaP, GaN, InN, or GaAs core. In one class of embodiments, the nanostructure core is an InP core. The core can be produced as described hereinbelow (see, e.g., the sections entitled “Synthesis of InP Core for Green-Emitting Core/Shell Dots” and “Synthesis of InP Core for Red-Emitting Core/Shell Dots” in the Examples) or as known in the art (see, e.g., U.S. Pat. No. 7,557,028, U.S. Pat. No. 8,062,967, U.S. Pat. No. 7,645,397, and US patent application publication 2010/0276638 (e.g., Example 7), as well as the references listed hereinabove). The core can be of essentially any desired size. For example, the core optionally has an average diameter between about 1 nm and about 5 nm. In one exemplary class of embodiments, the core has an average diameter between about 15 {acute over (Å)} and about 20 {acute over (Å)}. In another exemplary class of embodiments, the core has an average diameter between about 25 {acute over (Å)} and about 30 {acute over (Å)}. Optionally, the resulting nanostructures are quantum dots.
The shell typically comprises a material different than the core. In one aspect, the shell comprises a Group II-VI semiconductor, for example, ZnSxSe1-x, where 0≦x≦1 (e.g., where x=0, x=1, 0<x<1, 0.25≦x≦0.75, or x is about 0.5), ZnS, ZnSe, ZnSeTe, MnSe, MgSe, CdS, CdSe, ZnCdSe, or ZnCdMgSe.
The shell optionally comprises more than one layer. A multilayer shell is optionally synthesized in several discrete steps, as described in greater detail hereinbelow; see, e.g., the sections entitled “Multilayered Shells,” “Indium Enrichment and Shelling Process for Green-Emitting Dots,” and “Indium Enrichment and Shelling Process for RedEmitting Dots.” Accordingly, in one class of embodiments, the shell comprises at least two layers and forming the shell includes providing a first set of one or more precursors and reacting them in the presence of the C5-C8 carboxylic acid ligand to produce a first layer of the shell, and then providing a second set of one or more precursors and reacting them in the presence of the C5-C8 carboxylic acid ligand to produce a second layer of the shell. Different layers typically comprise different materials. For example, the first layer can comprise ZnSxSe1-x, where 0<x<1 (e.g., where 0.25≦x≦0.75 or where x is about 0.5), and the second layer can comprise ZnS.
Suitable precursors for shell formation are well known in the art. For example, suitable precursors for ZnSxSe1-x shell formation, where 0<x<1, include diethyl zinc, zinc carboxylates (e.g., zinc stearate or zinc hexanoate), bis(trimethylsilyl)selenide, elemental selenium (e.g., dissolved in tributylphosphine), hexamethyldisilthiane, and organic thiols (e.g., 1-dodecanethiol, tert-dodecylmercaptan, or 1-octanethiol), and suitable precursors for ZnS shell formation include diethyl zinc, zinc carboxylates (e.g., zinc stearate or zinc hexanoate), hexamethyldisilthiane, and organic thiols (e.g., 1-dodecanethiol, tertdodecylmercaptan, or 1-octanethiol). Shell layer thickness is optionally controlled by controlling the amount of precursor(s) provided, as described hereinbelow.
If desired, the ligand(s) in which the shell was synthesized can be replaced by a different ligand. A variety of suitable ligands are known in the art, for example, the dicarboxylic acid, polycarboxylic acid, dicarbinol, alcohol, amine, polymeric silicone, and/or organic polymetic ligands described in US patent application publications 2010/0276638 and 2007/0034833, the amine functional polystyrene ligands and polyethyleneimine or modified polyethyleneimine ligands described in US patent application publication 2012/0113672, the silsesquioxane ligands (including polyhedral oligomeric silsesquioxane and/or carboxylic acid ligands) described in U.S. Pat. Nos. 7,267,875 and 7,585,564 and US patent application publication 2008/0118755, the carboxylic acid siloxane polymer ligands as described in U.S. patent application Ser. No. 13/803,596, and the alkylamine siloxane polymer ligands described in U.S. provisional patent application 61/663,234 and U.S. patent application Ser. No. 13/803,596. For example, in one class of embodiments, after shell formation the ligand is exchanged with a dicarboxylic or polycarboxylic acid ligand (e.g., dodecenyl succinic acid), which can further increase quantum yield.
The resulting nanostructures (e.g., quantum dots, e.g., InP/ZnSxSe1-x/ZnS quantum dots) optionally display a high photoluminescence quantum yield, e.g., 65% or greater, 70% or greater, 75% or greater, 80% or greater, 85% or greater, or even 90% or greater. The photoluminescence spectrum of the nanostructures can cover essentially any desired portion of the spectrum. For example, the photoluminescence spectrum can have an emission maximum between 450 nm and 750 nm, e.g., between 500 nm and 650 nm, between 500 nm and 560 nm, or between 600 nm and 650 nm. The photoluminescence spectrum can have a full width at half maximum of 60 nm or less, e.g., 50 nm or less, 45 nm or less, or even 40 nm or less or 35 nm or less.
The resulting nanostructures are optionally embedded in a matrix (e.g., an organic polymer, silicon-containing polymer, inorganic, glassy, and/or other matrix), used in production of a nanostructure phosphor, and/or incorporated into a device, e.g., an LED, backlighting unit, downlight, or other display or lighting unit or an optical filter, or bound to a biomolecule, as for the embodiments described above.
Nanostructures and compositions resulting from the methods are also a feature of the invention. Accordingly, one general class of embodiments provides a composition that includes a nanostructure (e.g., a quantum dot) and a C5-C8 carboxylic acid ligand bound to a surface of the nanostructure. The composition optionally includes a population of such nanostructures with bound ligand. As will be appreciated, “bound” ligand is ligand that interacts (whether weakly or strongly) with one or more faces of a nanostructure, e.g., through covalent, ionic, van der Waals, or other molecular interactions with the surface of the nanostructure. Bound ligand typically remains associated with the nanostructures when they are pelleted by centrifugation, precipitated, or otherwise fractionated. The carboxylic acid can be branched (e.g., having a one carbon branch) or, more preferably, unbranched. Optionally, the carboxylic acid is an unbranched monocarboxylic acid. The carboxylic acid can be saturated or unsaturated.
Thus, in one class of embodiments, the ligand is an alkyl carboxylic acid, preferably an unbranched alkyl carboxylic acid such as pentanoic acid, hexanoic acid, heptanoic acid, or octanoic (caprylic) acid. In other embodiments, the ligand is an alkenyl carboxylic acid or an alkynyl carboxylic acid, for example, 4-pentenoic acid.
The composition optionally includes one or more additional ligands. For example, in one class of embodiments the composition also includes a fatty acid ligand (saturated or unsaturated) bound to the surface of the nanostructure. Optionally, the fatty acid is long chain fatty acid that comprises at least 12 carbon atoms, and can comprise less than 30 or less than 20 carbon atoms. Suitable fatty acids include, e.g., oleic acid, lauric acid, myristic acid, palmitic acid, and stearic acid. To maximize quantum yield of the nanostructure, the long chain fatty acid preferably constitutes less than 50 molar % of the total ligand in the composition, more preferably, less than 25 molar %, less than 10 molar %, or less than 5 molar %. In a preferred class of embodiments, the composition includes a dicarboxylic or polycarboxylic acid ligand bound to the surface of the nanostructure, e.g., dodecenyl succinic acid. A large number of other suitable ligands are known in the art; see, e.g., the dicarboxylic acid, polycarboxylic acid, dicarbinol, alcohol, amine, polymeric silicone, and/or organic polymeric ligands described in US patent application publications 2010/0276638 and 2007/0034833, the amine functional polystyrene ligands and polyethyleneimine or modified polyethyleneimine ligands described in US patent application publication 2012/0113672, the silsesquioxane ligands (including polyhedral oligomeric silsesquioxane and/or carboxylic acid ligands) described in U.S. Pat. Nos. 7,267,875 and 7,585,564 and US patent application publication 2008/0118755, the carboxylic acid siloxane polymer ligands as described in U.S. patent application Ser. No. 13/803,596, and the alkylamine siloxane polymer ligands described in U.S. provisional patent application 61/663,234 and U.S. patent application Ser. No. 13/803,596.
The nanostructure can comprise essentially any desired material, for example, a Group II-VI semiconductor, a Group III-V semiconductor, or a combination thereof. Thus, the nanostructure optionally comprises InP, ZnSxSe1-x where 0≦x≦1 (e.g., where x=0, x=1, 0<x<1, 0.25≦x≦0.75, or x is about 0.5), ZnS, ZnSe, ZnSeTe, MnSe, MgSe, CdS, CdSe, ZnCdSe, ZnCdMgSe, InAs, GaP, GaN, InN, and/or GaAs. The nanostructure optionally includes at least one shell.
In one class of embodiments, the nanostructure comprises an InP core and a ZnSxSe1-x shell, where 0≦x≦1, e.g., where 0<x<1, where 0.25≦x≦0.75, or where x is about 0.5. For example, the nanostructure can be an InP/ZnSxSe1-x/ZnS core/shell quantum dot, where 0<x<1, e.g., where 0.25≦x≦0.75 or where x is about 0.5. The nanostructure (e.g., quantum dot, e.g., InP/ZnSxSe1-x/ZnS quantum dot) optionally has a photoluminescence quantum yield of 65% or greater, e.g., 70% or greater, 75% or greater, 80% or greater, 85% or greater, or even 90% or greater. The photoluminescence spectrum of the nanostructure can cover essentially any desired portion of the spectrum. For example, the photoluminescence spectrum can have an emission maximum between 450 nm and 750 nm, e.g., between 500 nm and 650 nm, between 500 nm and 560 nm, or between 600 nm and 650 nm. The photoluminescence spectrum for a population of the nanostructures can have a full width at half maximum of 60 nm or less, e.g., 50 nm or less, 45 nm or less, or even 40 nm or less or 35 nm or less.
The nanostructure or population of nanostructures is optionally embedded in a matrix (e.g., an organic polymer, silicon-containing polymer, inorganic, glassy, and/or other matrix). In one class of embodiments, the nanostructure or population of nanostructures is incorporated into a device, for example, an LED, backlighting unit, downlight, or other display or lighting unit or an optical filter. As noted above, exemplary matrices and devices are known in the art. Also as noted above, the nanostructure can be covalently or noncovalently bound to a biomolecule.
Multilayered Shells
As noted, core/shell nanostructures can include more than one layer in their shell. For example, an InP core is advantageously coated with an intermediate layer of ZnSSe followed by an outer layer of ZnS. ZnSSe has a smaller lattice mismatch with InP than does ZnS. Providing a thin intermediate layer of ZnSSe over the InP core thus increases quantum yield. Application of a ZnS outer layer further increases quantum yield and also enhances stability of the nanostructures.
Although graded ZnSSe shells have been described (Lim et al. (2011) “InP@ZnSeS, core@composition gradient shell quantum dots with enhanced stability” Chem. Mater. 23:4459-4463), synthesis of a layered ZnSSe/ZnS shell in at least two discrete steps provides greater control over thickness of the resulting layers. Similarly, synthesis of the core and the shell in different steps also provides greater flexibility, for example, the ability to employ different solvent and ligand systems in the core and shell synthesis. Multi-step synthesis techniques can thus facilitate production of nanostructures with a narrow size distribution (i.e., a small FWHM) and high quantum yield. Accordingly, one aspect of the invention provides methods for forming a shell comprising at least two layers, in which one or more precursors are provided and reacted to form a first layer, and then (typically when formation of the first layer is substantially complete) one or more precursors for formation of a second layer are provided and reacted.
One general class of embodiments provides methods for production of core/shell nanostructures (e.g., quantum dots) where the shell comprises at least two layers. In the methods, an InP nanostructure core and a first set of one or more precursors are provided. The precursors are reacted to produce a first layer of the shell, which first layer comprises ZnSxSe1-x, where 0<x<1 (preferably where 0.25≦x≦0.75, or where x is about 0.5). A second set of one or more precursors is provided and reacted to produce a second layer of the shell, which second layer comprises ZnS. Typically, the second set of precursors is provided after reaction of the first set to form a first layer is substantially complete (e.g., when at least one of the first precursors is depleted or removed from the reaction or when no additional growth is detectable).
The InP core can be produced as described hereinbelow (see, e.g., the sections entitled “Synthesis of InP Core for Green-Emitting Core/Shell Dots” and “Synthesis of InP Core for Red-Emitting Core/Shell Dots” in the Examples) or as known in the art (see, e.g., U.S. Pat. No. 7,557,028, U.S. Pat. No. 8,062,967, U.S. Pat. No. 7,645,397, and US patent application publication 2010/0276638 (e.g., Example 7), as well as the references listed hereinabove). The core can be of essentially any desired size. Optionally, the core has an average diameter between about 1 nm and about 5 nm. In one exemplary class of embodiments, the core has an average diameter between about 15 {acute over (Å)} and about 20 {acute over (Å)}. In another exemplary class of embodiments, the core has an average diameter between about 25 {acute over (Å)} and about 30 {acute over (Å)}.
Suitable precursors for shell formation are well known in the art. For example, suitable precursors for ZnSxSe1-x shell formation, where 0<x<1, include, but are not limited to, diethyl zinc, zinc carboxylates (e.g., zinc stearate or zinc hexanoate), bis(trimethylsilyl)selenide, elemental selenium (e.g., dissolved in tributylphosphine), hexamethyldisilthiane, and organic thiols (e.g., 1-dodecanethiol, tert-dodecylmercaptan, or 1-octanethiol). Suitable precursors for ZnS shell formation include, but are not limited to, diethyl zinc, zinc carboxylates (e.g., zinc stearate or zinc hexanoate), hexamethyldisilthiane, and organic thiols (e.g., 1-dodecanethiol, tert-dodecylmercaptan, or 1-octanethiol).
Thickness of the shell layers can be conveniently controlled by controlling the amount of precursor provided. For a given layer, at least one of the precursors is optionally provided in an amount whereby, when the growth reaction is substantially complete, the layer is of predetermined thickness. If more than one different precursor is provided, either the amount of each precursor can be so limited or one of the precursors can be provided in limiting amount while the others are provided in excess. Suitable precursor amounts for various resulting desired shell thicknesses can be readily calculated. For example, the InP core can be dispersed in solution after its synthesis and purification, and its concentration can be calculated, e.g., by UV/Vis spectroscopy using the Beer-Lambert law. The extinction coefficient can be obtained from bulk InP. The size of the InP core can be determined, e.g., by excitonic peak of UV/Vis absorption spectrum and physical modeling based on quantum confinement. With the knowledge of particle size, molar quantity, and the desired resulting thickness of shelling material, the amount of precursor can be calculated using the bulk crystal parameters (i.e., the thickness of one monolayer of shelling material).
In one class of embodiments, providing a first set of one or more precursors and reacting the precursors to produce a first layer of the shell comprises providing the one or more precursors in an amount whereby, when the reaction is substantially complete, the first layer is between about 0.3 and about 1.0 monolayer of ZnSxSe1-x thick, e.g., about 0.5 monolayer of ZnSxSe1-x thick. Typically, this thickness is calculated assuming that precursor conversion is 100% efficient. (As noted above, a shell can but need not completely cover the underlying material. Without limitation to any particular mechanism and purely for the sake of example, where the first layer of the shell is about 0.5 monolayer of ZnSxSe1-x thick, the core can be covered with small islands of ZnSxSe1-x or about 50% of the cationic sites and 50% of the anionic sites can be occupied by the shell material.) Similarly, in one class of embodiments providing a second set of one or more precursors and reacting the precursors to produce a second layer of the shell comprises providing the one or more precursors in an amount whereby, when the reaction is substantially complete, the second layer is between about 1.0 and about 3.0 monolayers of ZnS thick, e.g., about 2 monolayers of ZnS thick or about 2.8-3 monolayers thick. Given, e.g., that one monolayer of ZnS0.5Se0.5 is 3.19 {acute over (Å)} thick and one monolayer of ZnS is 3.12 {acute over (Å)} thick, thickness of the resulting layers in {acute over (Å)} can be calculated if desired.
Typically, the precursors are reacted to form the shell layers in the presence of a ligand. The ligand can be essentially any of those known in the art, or, preferably, can be a C5-C8 carboxylic acid ligand, e.g., any of those C5-C8 carboxylic acid ligands described above; see the section entitled “C5-C8 Carboxylic Acid Ligands.” As an example, the ligand can be an unbranched C5-C8 alkyl carboxylic acid such as hexanoic acid. If desired, the ligand(s) in which the shells were synthesized can be replaced by a different ligand. A variety of suitable ligands (e.g., for core synthesis, shell synthesis, and/or subsequent exchange) are known in the art. As a few examples, in addition to those described herein, such ligands include (but are not limited to) dicarboxylic acid, polycarboxylic acid, dicarbinol, alcohol, amine, polymeric silicone, and/or organic polymeric ligands as described in US patent application publications 2010/0276638 and 2007/0034833, amine functional polystyrene ligands and polyethyleneimine or modified polyethyleneimine ligands as described in US patent application publication 2012/0113672, silsesquioxane ligands (including polyhedral oligomeric silsesquioxane and/or carboxylic acid ligands) as described in U.S. Pat. Nos. 7,267,875 and 7,585,564 and US patent application publication 2008/0118755, carboxylic acid siloxane polymer ligands as described in U.S. patent application Ser. No. 13/803,596, and alkylamine siloxane polymer ligands as described in U.S. provisional patent application 61/663,234 and U.S. patent application Ser. No. 13/803,596. For example, in one class of embodiments, after shell formation the ligand is exchanged with a dicarboxylic or polycarboxylic acid ligand (e.g., dodecenyl succinic acid).
The resulting nanostructures (e.g., quantum dots, e.g., InP/ZnSxSe1-x/ZnS quantum dots) optionally display a high photoluminescence quantum yield, e.g., 65% or greater, 70% or greater, 75% or greater, 80% or greater, 85% or greater, or even 90% or greater. The photoluminescence spectrum of the nanostructures can cover essentially any desired portion of the spectrum. For example, the photoluminescence spectrum can have an emission maximum between 450 nm and 750 nm, e.g., between 500 nm and 650 nm, between 500 nm and 560 nm, or between 600 nm and 650 nm. The photoluminescence spectrum can have a full width at half maximum of 60 nm or less, e.g., 50 nm or less, 45 nm or less, or even 40 nm or less or 35 nm or less.
The resulting nanostructures are optionally embedded in a matrix (e.g., an organic polymer, silicon-containing polymer, inorganic, glassy, and/or other matrix), used in production of a nanostructure phosphor, and/or incorporated into a device, e.g., an LED, backlighting unit, downlight, or other display or lighting unit or an optical filter, or bound to a biomolecule, as described above.
Nanostructures resulting from the methods are also a feature of the invention. Thus, one class of embodiments provides a population of InP/ZnSxSe1-x/ZnS core/shell nanostructures, where 0<x<1, wherein the ZnSxSe1-x shell is between about 0.3 and about 1.0 monolayer thick (e.g., about 0.5 monolayer thick) and the ZnS shell is between about 1.0 and about 3.0 monolayers thick (e.g., about 2 monolayers thick or about 2.8-3 monolayers thick). In a preferred class of embodiments, 0.25≦x≦0.75, e.g., x is about 0.5. In one class of embodiments, the ZnSxSe1-x shell is about 0.5 monolayer thick and the ZnS shell is about 2 monolayers thick. In one class of embodiments, the ZnSxSe1-x shell is about 0.5 monolayer thick and the ZnS shell is about 2.8-3.0 monolayers thick.
In one class of embodiments, the nanostructures are quantum dots. The InP cores of the quantum dots optionally have an average diameter between about 1 nm and about 5 nm, e.g., between about 15 {acute over (Å)} and about 20 {acute over (Å)} or between about 25 {acute over (Å)} and about 30 {acute over (Å)}. The nanostructures (e.g., quantum dots) optionally display a high photoluminescence quantum yield, e.g., 65% or greater, 70% or greater, 75% or greater, 80% or greater, 85% or greater, or even 90% or greater. The photoluminescence spectrum of the nanostructures can cover essentially any desired portion of the spectrum. For example, the photoluminescence spectrum can have an emission maximum between 450 nm and 750 nm, e.g., between 500 nm and 650 nm, between 500 nm and 560 nm, or between 600 nm and 650 nm. The photoluminescence spectrum can have a full width at half maximum of 60 nm or less, e.g., 50 nm or less, 45 nm or less, or even 40 nm or less or 35 nm or less.
The nanostructures optionally have a ligand bound to their surface, for example, any of those ligands described hereinabove (e.g., a C5-C8 carboxylic acid ligand, a fatty acid ligand, a dicarboxylic acid ligand, and/or a polycarboxylic acid ligand). The population of nanostructures is optionally embedded in a matrix (e.g., an organic polymer, silicon-containing polymer, inorganic, glassy, and/or other matrix) and/or incorporated into a device, for example, an LED, backlighting unit, downlight, or other display or lighting unit, as described above. Also as noted above, the nanostructures can be covalently or noncovalently bound to a biomolecule.
Highly Luminescent Nanostructures
Employing the various techniques detailed herein (e.g., indium core enrichment, synthesis of ZnSSe/ZnS shells, and/or use of C5-C8 carboxylic acid ligands during shell synthesis followed by ligand replacement with dicarboxylic acid or polycarboxylic acid ligands) permits synthesis of nanostructures exhibiting quantum yields higher than those previously achieved in the art. Highly luminescent nanostructures are thus a feature of the invention.
One general class of embodiments provides a composition comprising a population of nanostructures, which population displays a photoluminescence quantum yield of 65% or greater, wherein the member nanostructures comprise InP. Optionally, the member nanostructures are substantially free of Ga. The population can display a photoluminescence quantum yield of 70% or greater, 75% or greater, 80% or greater, 85% or greater, or even 90% or greater. For example, the quantum yield can be between 75% and 95%, e.g., between 80% and 95% or between 85% and 95%.
The photoluminescence spectrum of the nanostructures can cover essentially any desired portion of the spectrum, particularly the visible spectrum. For example, the nanostructures can emit in the red, orange, yellow, green, blue, indigo, and/or violet range. Optionally, the photoluminescence spectrum of the population has an emission maximum between 450 nm and 750 nm, for example, between 500 nm and 650 nm. In one class of embodiments, the photoluminescence spectrum of the population has an emission maximum between 500 nm and 560 nm, e.g., between 500 nm and 540 nm (e.g., in the green portion of the spectrum). In one class of embodiments, the photoluminescence spectrum of the population has an emission maximum between 600 nm and 650 nm (e.g., in the red portion of the spectrum).
The size distribution of the nanostructures can be relatively narrow. Thus, the photoluminescence spectrum of the population can have a full width at half maximum of 60 nm or less, e.g., 50 nm or less, 45 nm or less, or even 40 nm or less or 35 nm or less.
The nanostructures optionally include at least one shell. In one aspect, the nanostructures are quantum dots, e.g., quantum dots with an InP core and one or more shells. In one class of embodiments, the nanostructures are InP/ZnSxSe1-x/ZnS core/shell quantum dots, where 0<x<1, e.g., where 0.25≦x≦0.75 or where x is about 0.5. In one class of embodiments, the ZnSxSe1-x layer of the shell is between about 0.3 and about 1.0 monolayer of ZnSxSe1-x thick, e.g., about 0.5 monolayer of ZnSxSe1-x thick. In one class of embodiments, the ZnS layer of the shell is between about 1.0 and about 3.0 monolayers of ZnS thick, e.g., about 2.0 monolayers of ZnS thick or about 2.8-3 mono layers thick.
The nanostructures can be of essentially any desired size. For example, the nanostructures can be InP/ZnSxSe1-x/ZnS core/shell quantum dots where the InP cores have an average diameter between about 15 {acute over (Å)} and about 20 {acute over (Å)}, e.g., where green emission is desired. As another example, the nanostructures can be InP/ZnSxSe1-x/ZnS core/shell quantum dots where the InP cores have an average diameter between about 25 {acute over (Å)} and about 30 {acute over (Å)}, e.g., where red emission is desired. The average diameter of the quantum dots can be between about 30 {acute over (Å)} and about 36 {acute over (Å)} or between about 40 {acute over (Å)} and about 46 {acute over (Å)}, again for green and red dots, respectively. Optionally, the average diameter of the quantum dots is between about 1.5 nm and about 10 nm (e.g., between about 1.5 nm and about 8 nm or between about 1.8 nm and about 7.5 nm) and/or the cores have an average diameter between about 1 nm and about 5 nm.
The composition optionally includes one or more nanostructure ligands, e.g., those described herein and/or those known in the art. Thus, in one class of embodiments, the composition includes a C5-C8 carboxylic acid ligand bound to the nanostructures. The carboxylic acid can be branched (e.g., having a one carbon branch) or, more preferably, unbranched. Optionally, the carboxylic acid is an unbranched monocarboxylic acid. The carboxylic acid can be saturated or unsaturated.
Thus, in one class of embodiments, the ligand is an alkyl carboxylic acid, preferably an unbranched alkyl carboxylic acid such as pentanoic acid, hexanoic acid, heptanoic acid, or octanoic (caprylic) acid. In other embodiments, the ligand is an alkenyl carboxylic acid or an alkynyl carboxylic acid, for example, 4-pentenoic acid. In one class of embodiments, the composition includes a fatty acid ligand (e.g., lauric, caproic, myristic, palmitic, stearic, or oleic acid).
In one class of embodiments, the composition includes a dicarboxylic or polycarboxylic acid ligand bound to the nanostructures, e.g., dodecenyl succinic acid. Optionally, the composition includes a C5-C8 carboxylic acid ligand (e.g., one of those described above) and a dicarboxylic or polycarboxylic acid ligand; a fatty acid ligand is optionally also included. A large number of other suitable ligands are known in the art; see, e.g., the dicarboxylic acid, polycarboxylic acid, dicarbinol, alcohol, amine, polymeric silicone, and/or organic polymeric ligands described in US patent application publications 2010/0276638 and 2007/0034833, the amine functional polystyrene ligands and polyethyleneimine or modified polyethyleneimine ligands described in US patent application publication 2012/0113672, the silsesquioxane ligands (including polyhedral oligomeric silsesquioxane and/or carboxylic acid ligands) described in U.S. Pat. Nos. 7,267,875 and 7,585,564 and US patent application publication 2008/0118755, the carboxylic acid siloxane polymer ligands as described in U.S. patent application Ser. No. 13/803,596, and the alkylamine siloxane polymer ligands described in U.S. provisional patent application 61/663,234 and U.S. patent application Ser. No. 13/803,596.
The population of nanostructures is optionally embedded in a matrix (e.g., an organic polymer, silicon-containing polymer, inorganic, glassy, and/or other matrix). In one class of embodiments, the population of nanostructures is incorporated into a device, for example, an LED, backlighting unit, downlight, or other display or lighting unit or an optical filter. As noted above, exemplary matrices and devices are known in the art. Also as noted above, the nanostructures can be covalently or noncovalently bound to a biomolecule.
EXAMPLES
The following sets forth a series of experiments that demonstrate growth of highly luminescent nanostructures, including indium enrichment of InP cores, synthesis of a two layer ZnSSe/ZnS shell in two steps using a short chain carboxylic acid ligand, and post-synthesis ligand exchange.
It is understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims. Accordingly, the following examples are offered to illustrate, but not to limit, the claimed invention.
Example 1
Synthesis of Highly Luminescent Quantum Dots
Synthesis of InP Core for Green-Emitting Core/Shell Dots
To produce the InP cores, 5 g of trioctylphosphine oxide (TOPO), 2.33 g of indium acetate (8 mmol), and 4.797 g of lauric acid (24 mmol) are added to a reaction flask. The mixture is heated to 160° C. for 40 minutes and then 250° C. for 20 minutes under vacuum. The reaction temperature is then increased to 300° C. under N2 flow. At this temperature, 1 g tris(trimethylsilyl)phosphine (4 mmol) in 3 g trioctylphosphine (TOP) is quickly injected into the reaction flask, and reaction temperature is kept at 260° C. After one minute, the reaction is stopped by removing the heating element, and the reaction solution is allowed to cool to room temperature. 8 ml toluene is added to the reaction mixture in the glove box, and the InP dots are precipitated out by addition of 25 ml ethanol to the mixture followed by centrifugation and decantation of the supernatant. The InP dots obtained are then dissolved in hexane. The dot concentration is determined by UV-Vis absorption measurement based on the bulk InP extinction coefficient at 350 nm.
For additional details on core synthesis, see Example 7 of US patent application publication 2010/0276638.
Indium Laurate Precursor Preparation
To produce an indium laurate precursor for indium core enrichment, 10 g of 1-octadecene (ODE), 146 mg indium acetate (0.5 mmol), and 300 mg lauric acid (1.5 mmol) are added to a reaction flask. The mixture is heated to 140° C. for 2 hours under vacuum to obtain a clear solution. This precursor solution is kept in a glove box at room temperature until needed. The indium laurate precipitates due to low solubility at room temperature; a desired amount of the indium laurate in ODE mixture is heated to about 90° C. to form a clear solution and then the desired amount of the precursor solution is measured out when needed.
Indium Enrichment and Shelling Process for Green-Emitting Dots
To achieve indium core enrichment, 25 mg InP cores in solution in hexane, 6 ml ODE, and 0.5 g of indium laurate in ODE (from the previous step) are added to a reaction flask. The mixture is put under vacuum for 10 minutes at room temperature. The temperature is then increased to 230° C. and held at that temperature for 2 hours.
The reaction temperature is then lowered to 140° C., and 0.7 g hexanoic acid is added to the reaction flask, which is held at 140° C. for 10 minutes.
A 0.5 monolayer thick ZnSSe shell is formed by drop-wise addition of 21 mg diethyl zinc, 13 mg bis(trimethylsilyl)selenide, and 10 mg hexamethyldisilthiane in 1 ml ODE to the reaction mixture. After ZnSSe precursor addition, the reaction temperature is held at 140° C. for 30 minutes.
A 2 monolayer thick ZnS shell is then formed by drop-wise addition of 110 mg diethyl zinc and 106 mg hexamethyldisilthiane in 2 ml ODE to the reaction mixture. Ten minutes after the ZnS precursor addition, the reaction is stopped by removing the heating element.
Synthesis of InP Core for Red-Emitting Core/Shell Dots
To produce the InP cores, 5 g of trioctylphosphine oxide (TOPO), 1.46 g of indium acetate (5 mmol), and 3.16 g of lauric acid (15.8 mmol) are added to a reaction flask. The mixture is heated to 160° C. for 40 minutes under nitrogen, and then 250° C. for 20 minutes under vacuum. The reaction temperature is then increased to 300° C. under N2 flow. At this temperature, 0.25 g tris(trimethylsilyl)phosphine (1 mmol) in 3 g 1-octadecene (ODE) is quickly injected into the reaction flask, and reaction temperature is kept at 260° C. After 5 minutes, the reaction is stopped by removing the heating element, and the reaction solution is allowed to cool to room temperature. 8 ml toluene is added to the reaction mixture in the glove box, and the InP dots are precipitated out by addition of 20 ml ethanol to the mixture followed by centrifugation and decantation of the supernatant. The InP dots obtained are then dissolved in hexane. The dot concentration is determined by UV-Vis absorption measurement based on the bulk InP extinction coefficient at 350 nm.
Indium Enrichment and Shelling Process for Red-Emitting Dots
To achieve indium core enrichment, 25 mg InP cores in hexane solution and 5 g indium laurate in ODE (from the step entitled “Indium Laurate Precursor Preparation”) are added to a reaction flask. The mixture is put under vacuum for 10 minutes at room temperature. The temperature is then increased to 230° C. and held at that temperature for 2 hours.
The reaction temperature is then lowered to room temperature, 0.7 g hexanoic acid is added to the reaction flask, and the temperature is increased to 80° C.
A 0.5 monolayer thick ZnSSe shell is formed by drop-wise addition of 14 mg diethyl zinc, 8 mg bis(trimethylsilyl)selenide, and 7 mg hexamethyldisilthiane in 1 ml ODE to the reaction mixture. After ZnSSe precursor addition, the reaction temperature is held at 80° C. for 10 minutes and then the temperature is increased to 140° C. and held for 30 minutes.
A 2 monolayer thick ZnS shell is then formed by drop-wise addition of 69 mg diethyl zinc and 66 mg hexamethyldisilthiane in 2 mL ODE to the reaction mixture. 10 minutes after the ZnS precursor addition, the reaction is stopped by removing the heating element.
DDSA Ligand Exchange Procedure for InP/ZnSeS/ZnS Dots (Green-Emitting or Red-Emitting)
After the shelling reaction, the reaction mixture is allowed to cool down to room temperature. White precipitate is removed by centrifugation, resulting in a clear solution of dots in ODE. 30-50 mg DDSA (dodecenyl succinic acid) is added to lml dot solution and heated for 3 to 15 hours at 90° C. After ligand exchange, the dots are precipitated by adding 6 ml ethanol to 1 ml dot solution, followed by centrifugation, decantation of the supernatant, and drying under vacuum. The dried dots are dissolved in hexane for quantum yield measurement.
Quantum Yield Measurement
On the basis of following equation, relative quantum yield of quantum dots is calculated using fluorescein dye as a reference for green-emitting dots at the 440 nm excitation wavelength and rhodamine 640 as a reference for red-emitting dots at the 540 nm excitation wavelength:




  
    Φ
    x
  
  =
  
    
      
        Φ
        ST
      
      ⁡
      
        (
        
          
            Grad
            x
          
          
            Grad
            ST
          
        
        )
      
    
    ⁢
    
      (
      
        
          η
          x
          2
        
        
          η
          ST
          2
        
      
      )
    
  




The subscripts ST and X denote the standard (reference dye) and the quantum dot solution (test sample), respectively. Φx is the quantum yield of the dots, and ΦST is the quantum yield of the reference dye. Grad=(I/A), I is the area under the emission peak (wavelength scale); A is the absorbance at excitation wavelength. η is the refractive index of dye or nanostructure in the solvent. See, e.g., Williams et al. (1983) “Relative fluorescence quantum yields using a computer controlled luminescence spectrometer” Analyst 108:1067.

Characterization of InP/ZnSSe/ZnS Quantum Dots

FIGS. 1A and 1B show TEM images of InP/ZnSSe/ZnS quantum dots synthesized basically as described above.
Optical characterization of green-emitting InP/ZnSSe/S quantum dots produced basically as described above is illustrated in FIGS. 2A-2E. FIGS. 2B and 2C present absorption and photoluminescence spectra of fluorescein dye. FIGS. 2D and 2E present absorption and photoluminescence spectra of InP/ZnSSe/S quantum dots. FIG. 2A illustrates measurement of quantum yield of the dots based on fluorescein.
Optical characterization of red-emitting InP/ZnSSe/S quantum dots produced basically as described above is illustrated in FIGS. 3A-3E. FIGS. 3B and 3C present absorption and photoluminescence spectra of rhodamine 640 dye. FIGS. 3D and 3E present absorption and photoluminescence spectra of InP/ZnSSe/S quantum dots. FIG. 3A illustrates measurement of quantum yield of the dots based on rhodamine 640.
Representative optical data for red- and green-emitting quantum dots produced basically as described above are presented in Table 1.







TABLE 1







Representative optical data for green- and


red-emitting InP/ZnSeS/ZnS quantum dots.









Emission maximum

Full width at half


wavelength, nm
Quantum yield, %
maximum, nm





525
75
40


546
85
50


636
65
45









Example 2
Zinc Sulfide Shell Formation Using an Organic Thiol Sulfur Precursor
Green-emitting InP cores are synthesized as described above. To achieve indium core enrichment, 25 mg InP cores in solution in hexane, 6 ml ODE, and 0.5 g of indium laurate in ODE (prepared as detailed above) are added to a reaction flask. The mixture is put under vacuum for 10 minutes at room temperature. The temperature is then increased to 230° C. and held at that temperature for 2 hours.
The reaction temperature is then lowered to 140° C., and 0.7 g hexanoic acid is added to the reaction flask, which is held at 140° C. for 10 minutes. A 0.5 monolayer thick ZnSSe shell is formed by drop-wise addition of 21 mg diethyl zinc, 13 mg bis(trimethylsilyl)selenide, and 10 mg hexamethyldisilthiane in 1 ml ODE to the reaction mixture over 2 min. After ZnSSe precursor addition, the reaction temperature is held at 140° C. for 30 minutes. The temperature is then increased to 200° C. A two monolayer thick ZnS shell is then formed by drop-wise addition of 110 mg diethyl zinc and 120 mg 1-dodecanethiol in 4 ml ODE to the reaction mixture over 40 min by syringe pump. Ten minutes after the ZnS precursor addition, the reaction is stopped by removing the heating element. DDSA ligand exchange is performed as described above.
Representative optical data for green-emitting quantum dots produced basically as described above are presented in Table 2.







TABLE 2







Representative optical data for green-


emitting InP/ZnSeS/ZnS quantum dots.









Emission maximum

Full width at half


wavelength, nm
Quantum yield, %
maximum, nm





527
74
41









While the foregoing invention has been described in some detail for purposes of clarity and understanding, it will be clear to one skilled in the art from a reading of this disclosure that various changes in form and detail can be made without departing from the true scope of the invention. For example, all the techniques and apparatus described above can be used in various combinations. All publications, patents, patent applications, and/or other documents cited in this application are incorporated by reference in their entirety for all purposes to the same extent as if each individual publication, patent, patent application, and/or other document were individually indicated to be incorporated by reference for all purposes.

## Claims

1. A method for production of core/shell, nanostructures, the method comprising:
providing a nanostructure core; and
producing a shell surrounding the core by providing one or more precursors and reacting the precursors in the presence of a C5-C8 carboxylic acid ligand to produce the shell, wherein a C5-C8 carboxylic ligand is bound to a surface of the nanostructure core, and wherein the core/shell nanostructures are InP/ZnSxSe1-x/ZnS core/shell quantum dots, where 0<x<1.

2. The method of claim 1, wherein the ligand is an alkyl carboxylic acid.

3. The method of claim 1, wherein the ligand is an unbranched alkyl carboxylic acid.

4. The method of claim 1, wherein the ligand is hexanoic acid.

5. The method of claim 1, wherein the ligand is pentanoic acid.

6. The method of claim 1, wherein the ligand is an alkenyl carboxylic acid or an alkynyl carboxylic acid.

7. The method of claim 1, wherein the ligand is 4-pentenoic acid.

8. The method of claim 1, wherein the ligand is an unbranched monocarboxylic acid.

9. The method of claim 1, wherein reacting the precursors in the presence of a C5-C8 carboxylic acid ligand comprises reacting the precursors in the presence of a C5-C8 carboxylic acid ligand and a long chain fatty acid ligand, which long chain fatty acid comprises at least 12 carbon atoms.

10. The method of claim 1, wherein the shell comprises at least two layers; and wherein providing one or more precursors and reacting the precursors in the presence of a C5-C8 carboxylic acid ligand to produce the shell comprises providing a first set of one or more precursors and reacting the precursors in the presence of the C5-C8 carboxylic acid ligand to produce a first layer of the shell, and then providing a second set of one or more precursors and reacting the precursors in the presence of the C5-C8 carboxylic acid Ligand to produce a second layer of the shell.

11. The method of claim 10, wherein 0.25<x<0.75.

12. The method of claim 11, Wherein x is about 0.5.

13. The method of claim 10, wherein the first set of precursors comprises diethyl zinc, bis(trimethylsilyl)selenide, and hexamethyldisilthiane.

14. The method of claim 10, wherein the second set of precursors comprises diethyl zinc and hexamethyldisilthiane.

15. The method of claim 1, comprising, after producing the shell, exchanging the ligand with a dicarboxylic or polycarboxylic acid ligand.

16. The method of claim 15, wherein the dicarboxylic or polycarboxylic acid ligand is dodecenyl succinic acid.