WO2005022120A2

WO2005022120A2 - Process for producing nanocrystals and nanocrystals produced thereby

Info

Publication number: WO2005022120A2
Application number: PCT/US2004/007138
Authority: WO
Inventors: Erik C. Scher; Mihai Buretea
Original assignee: Nanosys, Inc.
Priority date: 2003-03-11
Filing date: 2004-03-10
Publication date: 2005-03-10
Also published as: WO2005022120A3; EP1601612A2; AU2004269297A1; JP2006521278A; CA2518352A1

Abstract

A process for producing nanocrystals and nanocrystals produced thereby . The process comprises contacting a metal precursor with a mixture, which includes a coordinating solvent and optionally a surfactant and metal catalyst, to form a first precursor mixture. The process also comprises heating the first precursor mixture to a first temperature and contacting the first precursor mixture with a second precursor mixture, which may be a Group V or Group VI precursor, to form a reaction mixture at a second temperature. The process further comprises heating the reaction mixture at a third temperature to grow nanocrystals, whereby the second temperature is no more than about 15°C lower than the first temperature. The figure is a flowchart depicting the nanocrystal production process.

Description

PROCESS FOR PRODUCING NANOCRYSTALS AND NANOCRYSTALS PRODUCED THEREBY

BACKGROUND OF THE INVENTION Field of the Invention

[0001] The present invention relates to the field of semiconductor nanocrystals and to a process for preparing same.

Description of Background Art

[0002] Nanocrystals have gained a great deal of attention for their interesting and novel properties in electrical, chemical, optical and other applications. Such nanomaterials have a wide variety of expected and actual applications, including use as semiconductors for nanoscale electronics, optoelectronic applications in emissive devices, e.g., nanolasers, LEDs, etc., photovoltaics, and sensor applications, e.g., as nanoChemFETS.

[0003] While commercial applications of the molecular, physical, chemical and optical properties of nanocrystals are beginning to be realized; commercially viable processes for the production of a wide variety of nanocrystals have been limited. Both the starting materials used and the conditions under which the nanocrystals are grown are commercially prohibitive. The chemical reaction used to produce nanocrystals involves nanocrystal nucleation and growth. Lack of control over the nucleation event and growth phase in synthetic process has prevented the production of a wide variety of nanocrystal types.

[0004] Nanocrystals of semiconductors are traditionally formed by the fast injection of pyrophoric precursors into hot coordinating solvents. U.S. Patent No. 6,225,198 Bl to Alivisatos et al., the foil disclosure of which is hereby incorporated by reference in its entirety for all purposes, discloses a process for the formation of rod-shaped II- VI semiconductor nanocrystals. In the disclosed method, a cold solution (-10 °C ) of a Group II metal and Group VI element is injected into a binary surfactant mixture heated to temperatures around 360 °C to initiate nanocrystal nucleation, which reduces the reaction temperature to around 300 °C. The nanocrystals are grown at temperatures about 50-70 °C lower than the nucleation temperature. A variation in temperature drop of as little as 5 °C leads to different growth rates and different size, shape and structure nanocrystals can result.

[0005] Published U.S. Patent Application No. 20020066401 to Peng et al., the full disclosure of which is hereby incorporated by reference in its entirety for all purposes, discloses a method of synthesizing colloidal nanocrystals, in which a Group LT metal compound is combined with a coordinating solvent and heated to temperatures around 360 °C. A cold solution of a Group VI element is injected to initiate nucleation, which reduces the reaction temperature to around 300 C. The nanocrystals are grown at temperatures about 50-70 C lower than the nucleation temperature. A variation in temperature drop of as little as 5 °C leads to different growth rats and different size, shape and structure nanocrystals can result.

[0006] Accordingly, it would be desirable to have a process of producing nanocrystals that is commercially viable, offering greater control, predictability and reproducibility, as well as a process that is amenable to the production of a wide variety of semiconductor nanocrystal shapes and types.

SUMMARY OF THE INVENTION

[0007] The present invention relates to processes for producing nanocrystals. An embodiment comprises: contacting a metal precursor with a mixture comprising a coordinating solvent to form a first precursor mixture; heating the first precursor mixture to a first temperature; contacting the first precursor mixture with a second precursor mixture comprising one of a Group V and Group VI compound to form a reaction mixture at a second temperature; and heating the reaction mixture at a third temperature to grow nanocrystals; whereby the second temperature is no more than about 15 °C lower than the first temperature. Alternatively, the second temperature is no more than about 10 °C, 7 °C, 5 °C, 3 °C or 1 °C lower than the first temperature.

[0008] A further embodiment of the present invention comprises: contacting a metal precursor with a mixture comprising a coordinating solvent and a metal catalyst to form a first precursor mixture; heating the first precursor mixture to a first temperature; contacting the first precursor mixture with a second precursor mixture comprising one of a Group V and Group VI compound to form a reaction mixture at a second temperature; and heating the reaction mixture at a third temperature to grow nanocrystals; whereby the second temperature is no more than about 15 °C lower than the first temperature. Alternatively, the second temperature is no more than about 10 °C, 7 °C, 5 °C, 3 °C or 1 °C lower than the first temperature.

[0009] Another embodiment of the present invention relates to a composition of rod-shaped DI-V nanocrystals having at least about 50% hexagonal crystal structure and an aspect ratio of at least about 4:1. Alternatively, the composition of rod-shaped HI-V nanocrystals has at least about 70%, 80%, 90% or 95% hexagonal crystal structure and an aspect ratio of at least about 4:1.

[0010] Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The advantages of the invention will be realized and attained by the structure and particularly pointed out in the written description and claims hereof as well as the appended drawings.

[0011] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. BRIEF DESCRIPTION OF THE FIGURES

[0012] The accompanying drawings, which are included to illustrate exemplary embodiments of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention, h the drawings: [0013] FIG. 1 is a flow chart depicting the traditional process of producing nanocrystals. [0014] FIG. 2(a)-(b) are flowcharts depicting the preparation of the first precursor mixture in accordance with the present invention. [0015] FIG. 3(a)-(b) are flowcharts depicting the preparation of the second precursor mixture in accordance with the present invention. [0016] FIG. 4 is a flow chart depicting the process of producing nanocrystals in accordance with the invention. [0017] FIG. 5 depicts a pyramid-shaped II- VI or ITI-V type nanocrystal 500 having cubic crystal structure with four faces 502-508. [0018] FIG. 6 depicts a tetrapod-shaped II- VI or ILT.-V type nanocrystal 600 having cubic crystal structure in the center pyramid region 500 and hexagonal crystal structure in the four arms 602-608. [0019] FIG. 7 depicts a rod-shaped H-VI or JJLI-V type nanocrystal 700.

[0020] FIG. 8a is a Transmission Electron Microscope (TEM) micrograph of rod-shaped CdSe nanocrystals, produced in accordance with the present invention. [0021] FIG. 8b shows an X-ray diffraction (XRD) pattern taken of the rod- shaped CdSe nanocrystals. The x-axis is in degrees 2Θ and x-ray source is Cu- Kα radiation. [0022] FIG. 9 is a series of three TEM micrographs showing the production of tetrapod-shaped CdSe nanocrystals in accordance with the present invention. [0023] FIG. 10 shows the XRD patterns for samples of tetrapods with different arm lengths taken from four different nanocrystal syntheses.

[0024] The present invention will now be described with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. Additionally, the left-most digit(s) of a reference number identifies the drawing in which the reference number first appears.

DETAILED DESCRIPTION OF THE INVENTION

[0025] Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings.

[0026] Many variables contribute to the shape of the produced nanocrystal. For example, the variables include the reaction mixture temperature, the concentration of precursor compounds, the molar ratio of the precursor compounds and the concentration and type of surfactant and coordinating solvent. The inventors have discovered that a minimum, reproducible and predictable temperature change between the first and second temperatures affords maximum control and reproducibility in nanocrystal synthesis. This control has allowed for the production of a wide range of nanocrystal types and shapes, including shaped nanocrystal types that were not possible using previous processes known in the art.

[0027] FIG. 1 illustrates the traditional process of producing CdSe nanocrystals. The process comprises mixing, 106, a surfactant, 102, and a phosphine oxide, 104, and heating, 108, the mixture to produce a precursor mixture, 110. The process further comprises contacting, 118, simultaneously, a cadmium salt, 116, cooled below room temperature, and a selenium-phosphine complex, 114, also cooled below room temperature, to form a reaction mixture, 120, at a second temperature. The second temperature is at least about 30 °C to about 70 °C lower than the first temperature. The term "about" includes the specified number ±5%. For example, "about 400 °C" includes 380-420 °C. The reaction mixture is further processed by heating, 122, the reaction mixture to form nanocrystals and isolating, 124, the nanocrystals, to produce a nanocrystal composition, 126.

[0028] The present invention comprises a process for producing nanocrystals of II- VI or TJJ-V semiconductors, which offers control over the nanocrystal nucleation event and growth phase, and in turn the shape and size of the nanocrystal, by minimizing the temperature change between the first and second temperatures. Examples of II- VI or jJJ-V semiconductor nanocrystals made according to the present invention include: any combination of an element from Group II, such as Zn, Cd and Hg, with any element from Group VI, such as S, Se, Te, Po, of the Periodic Table; and any combination of an element from Group DX such as B, Al, Ga, hi, and Tl, with any element from Group V, such as N, P, As, Sb and Bi, of the Periodic Table. Specific examples include, but are not limited to ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, GaN, GaP, GaAs, InN, InP and As nanocrystals. The present invention also allows for control over the resulting shape and size of the nanocrystals. Examples of shapes that are made according to the invention include, but are not limited to, spheres, rods, arrowheads, teardrops and tetrapods.

[0029] Nanocrystals made in accordance with the present invention will optionally be subjected to further processing. For example, surface chemistry modifications are optionally made to the nanocrystals of the present invention. Examples of surface modification include, but are not limited to, the addition of coating layers and the addition of shells over the nanocrystals of the present invention. The coating layers and/or shells can be any material, for example, semiconductor materials, or the like. Such processing steps are known to one of skill in the art, see, for example, U.S. Patent Nos. 6,201,229 Bl and 6,322,901 Bl, the foil disclosures of which are hereby incorporated by reference in their entirety for all purposes. [0030] FIG. 2a illustrates one embodiment of the present invention, comprising first contacting a metal precursor, 200, with a mixture comprising a coordinating solvent, 202, to form a first precursor mixture, 204. The metal precursor can be any metal compound that comprises an element from Group II or Group HJ of the periodic table, such as a metal oxide, metal salt or organometallic complex. Metal oxides for use in the present invention include oxides of the elements Zn, Cd, Hg, B, Al, Ga, In and Tl. Examples of metal oxides include but are not limited to CdO, ZnO, Al O₃ and frι O . Metal salts for use in the present invention include salts of the elements Zn, Cd, Hg, B, Al, Ga, In and Tl. Examples of metal salts include, but are not limited to, metal halides, metal carboxylates, metal carbonates, metal sulfates and metal phosphates, such ZnF₂, ZnCl₂, ZnBr₂, Znl₂, Zn(acetate)₂, ZnSO , CdF₂, CdCl₂, CdBr₂, Cdl₂, Cd(acetate)₂, Cd(OH)₂, Cd(NO₃)₂, Cd(BF₄)₂, CdSO₄, CdCO₃, A1F₃, A1C1₃, AlBr₃, A1I₃, Al(OH)₂(CO₂CH₃), AlNH₄(SO₄)₂, Al(OH)₃, Al(NO₃)₃, Al(ClO₄)₃, AlPO₄, Al₂(SO₄)₃, GaF₃, GaCl₃, GaBr₃, Gal₃, Ga(NO₃)₃, Ga(ClO₄)₃, Ga₂(SO₄)₃, hιF₃, fr Cl₃, h Br₃, Iπl₃, h (NO₃)₃, hι(C10₄)₃ and In(acetate)₃. Organometallic complexes for use in the present invention include any organometallic complex of the elements Zn, Cd, Hg, B, Al, Ga, In and Tl. Examples of organometallic complexes include, but are not limited to, complexes between Group II or Group HI elements and alkyl, haloalkyl, alkenyl, alkynyl, aryl, alkoxyl, alkenoxyl and aryloxyl groups. Specific examples of organometallic complexes include, but are not limited to, dialkylzinc, dialkylcadmium, dialkylmercury, trialkylaluminum, trialkylgallium and trialkylindium, including Zn(CH₃)₂, Zn(CH₂CH₃)₂, Cd(CH₃)₂, Cd(CH₂CH₃)₂, Hg(CH₃)₂, Hg(CH₂CH₃)₂, A1(CH₃)₃, A1(CH₂CH₃)₃, Ga(CH₃)₃ Ga(CH₂CH₃)₃, h (CH₃)₃ and h (CH₂CH₃)₃.

[0031] Coordinating solvents for use in the present invention include solvents that can coordinate to metals and have boiling points greater than 150 °C. Preferably, the solvent has a decomposition temperature above 300 C. Examples of coordinating solvents for use in the present invention include those with the formula X=Y(R) wherein X is selected from the group consisting of O and S, or alternatively, X does not exist; Y is selected from the group consisting of N and P; and each R is selected from the group consisting of alkyl and haloalkyl. If Y is N, then X does not exist. It is understood by one of skill in the art that the nitrogen atom, N, is not pentavalent under conditions of present invention, and therefore, X cannot exist if N is Y and N is bonded to three R groups. Alkyl is used herein to refer to any branched or unbranched saturated hydrocarbon chain with 4 to 40 carbon atoms. Haloalkyl is used herein to refer to alkyl chains substituted by any number of halogen atoms such as CI, F, Br and I. Examples include perfluorooctyl (-C₈F₁ ) and pentadecafluorooctyl (-CH₂CF₁₅). Examples of coordinating solvents include but are not limited to trioctylamine, trihexylphosphine, trihexylphosphine oxide, trioctylphosphine, trioctylphosphine oxide, tridecylphosphine, tridecylphosphine oxide, tridodecylphosphine, tridodecylphosphine oxide, tritetradecylphosphine, tritetradecylphosphine oxide, trihexadecylphosphine, trihexadecylphosphine oxide, and trioctadecylphosphine, trioctadecylphosphine oxide. Referring back to FIG. 2a, the mixture comprising a coordinating solvent, 202, optionally further comprises a surfactant. Surfactant is used herein to refer to any molecule that interacts dynamically with the surface of a II- VI or IU-V semiconductor nanocrystal. A surfactant is understood to act dynamically with a nanocrystal surface if the surfactant is capable of removing and/or adding molecules to the nanocrystal, or alternatively, if the surfactant is capable of adhering, adsorbing or binding to the nanocrystal surface. The surfactants include alkylcarboxcylic acids, alkylamines, alkylamine oxides, sulphonates, sulphonic acids, phosphonates, phosphonic acids, phosphinic acids, phosphine oxides and polymers thereof. Examples include hexylphosphonic acid, octylphosphonic acid, decylphosphonic acid, dodecylphosphonic acid and phosphonate esters and polymers of the phosphonic acids, including dimers, trimers, tetramers, pentamers, hexamers, heptamers, etc. of the phosphonic acid.

[0033] In another embodiment of the present invention, as shown in FIG. 2a, the first precursor mixture, 204, further comprises a metal catalyst, 250. The metal catalyst facilitates nanocrystal nucleation and/or growth. Metal catalysts for use in the present invention include, but are not limited to, colloidal metal nanoparticles. Metal nanoparticles for use in the invention include any metal nanoparticles that facilitate the anisotropic growth of II- VI or HI-V semiconductor nanocrystals, for example, gold. Other metals for use in the present invention include any of the transition metals from the Periodic Table, including, but not limited to, copper, silver, nickel, palladium, platinum, cobalt, rhodium, iridium, iron, ruthenium, osmium, manganese, chromium, molybdenum, tungsten, vanadium, niobium, tantalum, titanium, zirconium and hafiiium. The metal nanoparticles can be any shape, preferably spheres, and have sizes in the range of about 1 to about 50 nanometers.

[0034] Referring to FIG. 2a, the first precursor mixture, 204, is heated, 206, to a first temperature, which is sufficient enough to initiate nanocrystal synthesis. For example, the first temperature is about 200 to 500 °C. Alternatively, the first temperature is about 250 to 450 °C. Alternatively, the first temperature is about 290 to 400 °C. The first precursor mixture heated to a first temperature, 208, is used in the nanocrystal synthesis immediately, or alternatively, there is a time delay, 210, wherein the first precursor mixture is held for a period of time at the first temperature before using it further. The period of time is in the range of about 5 minutes to about 12 hours. Alternatively, the first precursor mixture heated to a first temperature, 208, is cooled, 212, to a temperature of about 0 °C to about 100 °C and then optionally heated, 214, to form the first precursor mixture at the first temperature, 208.

[0035] FIG. 2b illustrates another embodiment of the present invention. Contacting the metal precursor, 200, with a surfactant, 216, and optionally a coordinating solvent, 217, forms the first precursor mixture, 204, further comprising a metal precursor complex, 226. For example, a surfactant such as hexylphosphonic acid and a metal precursor such as CdO react to form a Cd- hexylphosphonic acid precursor complex.

[0036] The first precursor mixture, 204, is optionally heated, 218. The metal precursor complex, 226, is further optionally isolated, 220, by cooling the metal precursor mixture and/or adding a solvent, for example methanol, capable of precipitating the metal precursor complex. Optionally, the process comprises the steps of purifying, 222, and drying, 224, the metal precursor complex, 226. Contacting the metal precursor complex, 226, with a coordinating solvent, 217, and optionally a surfactant, 216, and optionally a metal catalyst, 250, forms the first precursor mixture, 204. Heating, 228, the first precursor mixture, 204, to a first temperature forms the first precursor mixture at a first temperature, 208.

[0037] FIG. 3a illustrates another embodiment of the present invention. A second precursor mixture, 302, comprises one of a Group V and Group VI compound, 304. Group V compound is used herein to refer to any compound that comprises a Group V element of the Periodic Table. Elements from Group V of the Periodic Table include N, P, As, Sb and Bi. Examples of Group V compounds include, but are not limited to, N(TMS)₃, P(TMS)₃, As(TMS)₃, Sb(TMS)₃ and Bi(TMS)₃, wherein TMS refers to the trimethylsilyl group - Si(CH₃)₃; N(CH₃)₃, N(CH₂CH₃)₃, P(CH₃)₃, P(CH₂CH₃)₃, As(CH₃)₃, As(CH₂CH₃)₃, Sb(CH₃)₃, Sb(CH₂CH₃)₃, Bi(CH₃)₃ and Bi(CH₂CH₃)₃. Group VI compound is used herein to refer to any compound that comprises a Group VI element of the Periodic Table. Elements from Group VI of the Periodic Table include O, S, Se, Te and Po. Examples of Group VI compounds include, but are not limited to, elemental chalcogens such as S, Se, Te and Po.

[0038] Referring back to FIG. 3a, the second precursor mixture, 302, optionally further comprises a coordinating solvent, 217, for example, a trialkylphosphine oxide such as trioctylphosphine oxide or tritetradecylphosphine oxide. Optionally, the second precursor mixture further comprises a surfactant, 216, for example, hexylphosphonic acid. Alternatively, the second precursor mixture, 302, comprises no coordinating solvent, 217, or surfactant, 216. Preferably, the second precursor mixture, 302, comprises about 70-100%» Group V or VI precursor compound. Preferably, the amount of coordinating solvent, 217, and optional surfactant, 216, used is such that when the second precursor mixture, 302, contacts the first precursor mixture heated to a first temperature, 208, and forms a reaction mixture at a second temperature, as described below, the second temperature is not more than about 15 °C lower than the first temperature.

[0039] Referring back to FIG. 3a, the second precursor mixture, 302, is optionally heated, 310, to form a second precursor mixture at a temperature about 25 to 400 °C, 312. Preferably, the second precursor mixture, 302, is heated to a temperature such that when it contacts the first precursor mixture heated to a first temperature, 208, and forms a reaction mixture at a second temperature, as described below, the second temperature is not more than about 15 °C lower than the first temperature.

[0040] FIG. 3b illustrates a further embodiment of the present invention. A fractional amount of second precursor mixture, 302, is optionally diluted with surfactant, 216, and coordinating solvent, 217, to form a diluted second precursor mixture, 318. The diluted second precursor mixture, 318, comprises a different concentration of Group V or Group VI compound, 304, than the second precursor mixture, 302. The diluted second precursor mixture, 318, however, has the same volume as the second precursor mixture, 302. Thus, the present invention allows for changing the molar ratio between the two elements in II- VI and HI-V type semiconductor nanocrystals, without losing control, predictability and reproducibility over the nanocrystal synthesis. This process of dilution can be repeated any number of times. Varying the fractional amounts of second precursor mixture, 302, produces any number of diluted second precursor mixtures, all having varying concentrations of Group V or Group VI compound, 304, but all having constant volume. Optionally, heating, 316, the diluted second precursor mixture, 318, forms a diluted second precursor mixture at a temperature about 25 to 400 °C, 320.

[0041] Alternatively, the molar ratio between the two elements in II- VI and HI-V type semiconductor nanocrystals is varied by changing the amount and/or concentration of metal precursor, used in the first precursor mixture. Or alternatively, the amount and/or concentration of metal precursor used and the concentration of the Group V or Group VI compound used in the second precursor mixture are both varied.

[0042] FIG. 4 illustrates a further embodiment of the present invention. Contacting, 402, the first precursor mixture, 208, with a second precursor mixture, 312, or optionally, a diluted second precursor mixture, 320, forms a reaction mixture at a second temperature, 404. The contacting, 402, can be performed by any means known to one of ordinary skill in the art. For example, the second precursor mixture is rapidly injected into the first precursor mixture. The second temperature is no more than about 15 °C lower than the first temperature, alternatively, the second temperature is no more than about 10 °C, 7 °C, 5 °C, 3 °C, or 1 °C lower than the first temperature. Alternatively, there is no temperature change, meaning the first and second temperatures are equal. After the contacting, there is an optional time delay, 406, wherein the reaction mixture is held at the second temperature for a period of time. This period of time is about 10 seconds to about 10 minutes. The process further comprises heating, 408, the reaction mixture, 404, at a third temperature to form a reaction mixture at a third temperature, 410, and to grow nanocrystals. The third temperature can be any temperature that allows for the controlled growth of nanocrystals in a predetermined and defined crystal structure. For example, the nanocrystals are grown at a temperature of about 100 to about 450 °C. The reaction mixture is heated at the third temperature for a period of time to grow the nanocrystals. The length of time for the heating, 408, is in the range of about one minute to about one hour. [0043] In another embodiment of the present invention, the process shown in FIG. 4, further comprises contacting, 412, the reaction mixture heated to a third temperature, 410, with additional second precursor mixture, 312, or optionally, diluted second precursor mixture, 320. The additional second precursor mixture can be added all at once. Alternatively, the additional second precursor mixture or diluted second precursor mixture can be added in a series of additions. Alternatively, the additional second precursor mixture or diluted second precursor mixture can be added slowly and constantly over the course of the heating, 408.

[0044] The process shown in FIG. 4, further comprises isolating, 414, the nanocrystal composition, 416, from the reaction mixture. The isolating can be performed by any method known to one of ordinary skill in the art. One example of isolating comprises cooling the reaction mixture to room temperature, adding a sufficient amount of polar solvent, such as methanol, isopropanol or acetone and collecting the nanocrystals by any method such as filtration or centrifύgation. The nanocrystals can be separated by size and shape. Preferably, the nanocrystals have a narrow size and shape distribution and require no size or shape separation. The nanocrystals in composition 416 vary not more than about 20% in size. Alternatively, the nanocrystals in composition 416 vary not more than about 15%, 10% or 5% in size. Not more than about 20% of the nanocrystals in composition 416 have varying shape. Alternatively, not more than about 15 >, 10% or 5% of the nanocrystals in composition 416 have varying shape.

[0045] Nanocrystals can be separated according to size and shape by any method known to one of skill in the art. For example, the nanocrystals can be separated according to size by passing a composition of nanocrystals through filters having progressively smaller pores. Filters can have pore sizes in the range of about 100 nm to about 10 μm. Alternatively, the nanocrystals can be separated using shape selective precipitation. The addition of a different polarity solvent to a solution of nanocrystals precipitates less soluble nanocrystals, while the shapes that are more soluble remain in solution.

[0046] Controlling the temperature change between the first and second temperatures allows for precise control over the temperature of the reaction mixture, and thus precise control over the crystal structure in which the nanocrystals will nucleate and grow. The controlled temperature change also allows for a wider range of suitable first temperatures because the second temperature remains sufficiently high to grow the nanocrystals in the desired crystal structure after the reaction mixture is formed. In addition, the wider range of first temperatures allows for the use of a wider variety of reagents, coordinating solvents and surfactants, thus reducing manufacturing costs.

[0047] Producing II- VI type nanocrystals in an anisotropic shape depends on nucleating and/or growing the material in a particular crystal structure. Anisotropic is used herein to mean nanocrystals having properties that differ according to the direction of measurement. For example, anisotropic rod- shaped nanocrystals have anisotropic aspect ratios of about 2:1 to about 10:1 or greater. An aspect ratio of 2: 1 for a rod-shaped nanocrystal means the length of the rod is 2 times the width of the rod. An example is shown in FIG. 5, wherein rod-shaped nanocrystal, 500, has length, 504, and width, 502. Aspect ratio can be measured by any method known to one of skill in the art, for example, High Resolution or Low Resolution Transmission Electron Microscopy (HRTEM or TEM, respectively), scanning electron microscopy (SEM) or atomic force microscopy (AFM).

[0048] Crystal structure is used herein to mean the geometric arrangement of the points in space at which the atoms of the nanocrystal occur. As a specific example of crystal structure, CdSe, like other LT-VI nanocrystals, forms hexagonal and cubic crystal structures. FIG. 6 depicts a nanocrystal, 600, in a pyramid-shape, resulting from nucleation and/or growth in the cubic crystal structure, with four faces, 602, 604, 606, and 608. A higher temperature is required to nucleate CdSe and other IJ-VI nanocrystals in the hexagonal crystal structure than the cubic crystal structure. Likewise, it also requires a higher temperature to grow CdSe and other nanocrystals in the hexagonal crystal structure than in the cubic crystal structure. Nucleation and growth of CdSe and other II- VI and JJJ-V nanocrystals in the hexagonal crystal structure leads to anisotropic rod-shaped nanocrystals. The crystal structure of the nanocrystal can be determined by any process known to one of skill in the art and includes, but is not limited to, X-ray crystallography, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and solid state nuclear magnetic resonance (SSNMR). Preferably, X-ray crystallography or TEM is used. A further embodiment of the present invention comprises a process for producing anisotropic rod-shaped II- VI and JH-V nanocrystals. The process comprises contacting a first precursor mixture heated to a first temperature, with a second precursor mixture to form a reaction mixture at a second temperature. The first and second temperatures are sufficiently high to nucleate and/or grow II- VI or UI-V nanocrystals in the hexagonal crystal structure. The reaction mixture is then heated to a third temperature, which is also sufficiently high to grow the II- VI or HI-V nanocrystals in the hexagonal crystal structure. The second temperature is no more than about 15 °C lower than the first temperature, alternatively, the second temperature is no more than about 10 °C, 7 °C, 5 °C, 3 °C, or 1 °C lower than the first temperature. Alternatively, there is no temperature change, meaning the first and second temperatures are equal. For no extended period of time, therefore, does the temperature for nucleation or growth drop below that which is required to nucleate and/or grow the II- VI or HI-V nanocrystals in the hexagonal crystal structure. The phrase "extended period of time" is used herein to mean a period of time on the same order as the time required for the growth of nanocrystals. For example, if nanocrystals are grown for 10 minutes, then an extended period of time would be in the range of about one minute to about 10 minutes. [0050] In yet a further embodiment, the present invention comprises a process for producing tetrapod-shaped II- VI and HI-V nanocrystals. The process comprises contacting a first precursor mixture, which is heated to a first temperature, with a second precursor mixture to form a reaction mixture at a second temperature. The first and second temperatures are sufficiently high to nucleate II- VI and HI-V nanocrystals in the cubic crystal structure. The reaction mixture is then heated to a third temperature, which is sufficiently high to grow the H-VI and HI-V nanocrystals in the hexagonal crystal structure. The second temperature is no more than about 15 °C lower than the first temperature, alternatively, the second temperature is no more than about 10 °C, 7 °C, 5 °C, 3 °C, or 1 °C lower than the first temperature. Alternatively, there is no temperature change, meaning the first and second temperatures are equal. For no extended period of time, therefore, does the temperature for nucleation drop below that which is required to nucleate the H- VI and HI-V nanocrystals in the cubic crystal structure. And at no time, therefore, does the temperature for growth drop below that which is required to grow the H-VI and HI-V nanocrystals in the hexagonal crystal structure. As shown in FIG. 7, this process results in a tetrapod-shaped nanocrystal, 700, having 4 rod-shaped arms, 702, 704, 706, and 708 (each having hexagonal crystal structure) extending from each corresponding face, 602, 604, 606, and 608 of the center of the nanocrystal, 600 (each having cubic crystal structure).

[0051] FIG. 8a is a Transmission Electron Microscope (TEM) micrograph of rod-shaped CdSe nanocrystals, produced in accordance with the present invention. The micrograph shows the nanocrystals have rod shape with uniform length and aspect ratio. FIG. 8b shows an X-ray diffraction (XRD) pattern taken of the rod-shaped CdSe nanocrystals. The XRD pattern shows the nanocrystals are formed in the hexagonal crystal structure.

[0052] FIG. 9 is a series of three TEM micrographs showing the production of tetrapod-shaped nanocrystals over time in accordance with the present invention. The series of micrographs shows the precise control the process of the present invention offers over the length of the arms on each tetrapod. The reaction progresses as shown from top to bottom and shows increasing length of the tetrapod arms. FIG. 10 shows the XRD patterns for samples of tetrapods taken over time during the nanocrystal synthesis. The bottom XRD patterns show that early on in the process, the nanocrystals have a larger portion of the cubic crystal structure present. As the process progresses with time, each XRD pattern (moving up on the graph) shows an increasing amount of hexagonal crystal structure in the nanocrystals, corresponding to the growth of arms on the tetrapods. This data confirms the nanocrystals nucleate in the cubic crystal structure, forming the pyramid shaped center, but the nanocrystals grow in the hexagonal crystal structure to produce the rod-shaped arms of the tetrapods. Another embodiment of the present invention comprises a process for producing rod-shaped HI-V nanocrystals with at least about 50% hexagonal crystal structure and aspect ratio of at least about 4:1. Alternatively, the rod- shaped HI-V nanocrystals have at least about 60%, 70%, 80%, 90% or 95% hexagonal crystal structure and aspect ratio of at least about 4:1. The process comprises contacting a metal precursor comprising a Group HI element of the Periodic Table, with a mixture comprising a coordinating solvent, and a metal catalyst to form a first precursor mixture. The process further comprises heating the first precursor mixture to a first temperature. The first temperature is sufficiently high to nucleate and/or grow HI-V nanocrystals in the hexagonal crystal structure. The process further comprises contacting the first precursor mixture with a second precursor mixture comprising a Group V compound to form a reaction mixture at a second temperature, and heating the reaction mixture at a third temperature to grow nanocrystals. The second temperature is no more than about 15 °C lower than the first temperature, and at no time does the temperature drop below that which is required to grow the HI-V nanocrystals in the hexagonal crystal structure. This process of employing a metal catalyst and a minimum temperature change is especially useful for the isotropic growth of rod-shaped HT-V nanocrystals.

[0054] Another embodiment of the present invention, therefore, relates to a composition of rod-shaped HI-V nanocrystals having at least about 50%) hexagonal crystal structure and an aspect ratio of at least about 4:1. Alternatively, the composition of rod-shaped HI-V nanocrystals have at least about 60%, 70%, 80%, 90% or 95%> hexagonal crystal structure and aspect ratio of at least about 4:1. It is preferable to produce rod-shaped nanocrystals having no cubic crystal structure, because the areas having cubic crystal structure act as stacking faults such that the shape of the nanocrystal is not a straight rod but a zigzag-shaped rod. This zigzag shape can adversely affect the optical and electronic properties of the nanocrystal. The percentage of crystal structure for a particular nanocrystal can be determined by any method known to those of ordinary skill in the art. For example, measuring the amount of the nanocrystal in one crystal structure to the total amount of the nanocrystal, or by measuring the ratio of crystal structure in the produced nanocrystal to that of a nanocrystal pure in one crystal structure determines the percentage of crystal structure. X-ray diffraction patterns of nanocrystals pure in crystal structure are known to those of ordinary skill in the art and can be made, for example, theoretically, in silico or experimentally.

[0055] The nanocrystals of the present invention have usefol optical and electronic properties that can be applied in a variety of devices. Examples of devices include, but are not limited to electrooptic devices, such as white light sources, light emitting diodes (LED), photorefractive devices, RF filters, such as those for optical data storage, communication and photovoltaic devices, such as those for solar energy conversion.

[0056] In a device, the nanocrystals are deposited on a substrate, for example, an electrode, or sandwiched between two or more substrates. Substrates for use in the present invention include, but are not limited to silicon and other inorganic semiconductors, for example, ZnO, TiO and In₂O₃-SnO₂ (ITO); polymers such as semiconductive polymers, for example, polyphenylenevmylene; and glass, such as ITO-coated glass. Methods for applying the nanocrystals to a substrate surface are well known to those of ordinary skill in the art. For example, the nanocrystals are applied from solution via spin coating.

[0057] The nanocrystals can be deposited neat or as a mixture comprising the nanocrystals. The mixture further comprises materials that include, but are not limited to electrooptical and semiconductive organic and inorganic molecules and polymers. Specific examples of molecules and polymers include, but are not limited to amines, such as triarylamines and polymers or dendrimers thereof; inorganic semiconductors, such as GaAs, InP and TiO ; polyarylenes, such as polythiophene, polypyrrole, polyphenylene, and polyfluorene, and polyarylvinylenes, such as polyphenylenevmylene and polythienylvinylene.

[0058] Nanocrystals are deposited as a single layer or as multilayers. A layer comprises only one type of nanocrystal, for example, H-VI rods. Alternatively, a layer comprises two or more different types of nanocrystals. For example, a layer comprises two, three, four, five, six, seven, eight, nine, ten, etc. different types of nanocrystals. As a non-limiting example of a layer comprising three different types of nanocrystals, a layer comprises H-VI rods, H-VI tetrapods and HI-V rods. When nanocrystals are deposited in multilayers, each layer comprises the same type of nanocrystal. Alternatively, when nanocrystals are deposited in multilayers, each layer comprises a different type of nanocrystal. Layer thickness is about 10 nm to about 1000 μm. Preferably, the layer thickness is about 50 μm to about 100 μm. Layer thickness can be measured by any method known to one of ordinary skill in the art, for example, atomic force microscopy (AFM) or scanning electron microscopy (SEM).

[0059] The nanocrystals are oriented on the electrode surface in one direction. Alternatively, the nanocrystals are randomly oriented. The nanocrystals are oriented by any method known to those of skill in the art. For example, the nanocrystals are oriented under an applied electrical, optical or magnetic field, or the nanocrystals are oriented mechanically by fluid flow orientation. [0060] The following examples are illustrative, but not limiting, of the method and compositions of the present invention. Other suitable modifications and adaptations of the variety of conditions and parameters normally encountered in nanocrystal synthesis and which are obvious to those skilled in the art are within the spirit and scope of the invention.

EXAMPLE 1

[0061] High quality CdSe rods were prepared by admixing about 0.74g octadecylphosphonic acid (ODPA), about 3.23g of trioctylphosphine oxide (TOPO) and about 0.095g of CdO into a 3-neck flask. The flask was degassed and about 1.5 lg of trioctylphosphine (TOP) was added to form a first precursor mixture, hi a separate flask, a selenium precursor mixture (Se:TOP) was prepared with about 10%) selenium by weight. About 0.1 lg Se:TOP mixture was added to about 0.41g of TOP for a total weight of about 0.52g. The first precursor mixture was heated to about 320 °C. The new selenium precursor mixture with additional TOP was injected into the heated first precursor mixture to nucleate CdSe nanocrystals and form the reaction mixture. The temperature of the reaction mixture dropped to about 315 °C upon injection. The reaction mixture was heated at about 315 °C for about 15 minutes to produce high quality wurzite CdSe rods.

EXAMPLE 2

[0062] High quality CdTe tetrapods were prepared by admixing about 0.40g octadecylphosphonic acid (ODPA), about 3.63g of trioctylphosphine oxide (TOPO) and about 0.050g of CdO into a 3-neck flask. The flask was degassed by heating under vacuum and about 1.50g of trioctylphosphine (TOP) was added to form a first precursor mixture. In a separate flask, a tellurium precursor mixture (Te:TOP) was prepared with about 10% tellurium by weight. About 0.16g Te:TOP mixture was added to about 0.39g of TOP for a total weight of about 0.55g. The first precursor mixture was heated to about 320 °C. The new tellurium precursor mixture with additional TOP was injected into the heated first precursor mixture to nucleate CdTe nanocrystals and form the reaction mixture. The temperature of the reaction mixture dropped to about 315 C upon injection. The reaction mixture was heated at about 315 °C for about 15 minutes to produce high quality CdTe tetrapods. It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined in the appended claims. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

Claims

Whatls Claimed Is:

1. A process for producing nanocrystals, comprising: (a) contacting a metal precursor with a mixture comprising a coordinating solvent to form a first precursor mixture; (b) heating the first precursor mixture to a first temperature; (c) contacting the first precursor mixture with a second precursor mixture comprising one of a Group V and Group VI compound to form a reaction mixture at a second temperature; and (d) heating the reaction mixture at a third temperature to grow and thereby produce said nanocrystals; wherein the second temperature is no more than about 15 °C lower than the first temperature.

2. The process of claim 1, wherein said precursor is a metal oxide, metal salt or organometallic complex comprising a Group H or Group HI element.

3. The process of claim 2, wherein said Group H element is selected from the group consisting of Zn, Cd and Hg.

4. The process of claim 2, wherein said Group HI element is selected from the group consisting of B, Al, Ga, hi and Tl.

5. The process of claim 2, wherein said metal oxide is selected from the group consisting of CdO, ZnO, Al O₃, Ga O and In₂O₃.

6. The process of claim 2, wherein said metal salt is selected from the group consisting of ZnF , ZnCl₂, ZnBr₂, Znl , Zn(acetate)₂, ZnSO₄, CdF₂, CdCl₂, CdBr₂, Cdl₂, Cd(acetate)₂, Cd(OH)₂, Cd(NO₃)₂, Cd(BF₄)₂, CdSO₄, CdCO₃, A1F₃, AICI3, AlBr₃, A1I₃, Al(OH)₂(CO₂CH₃), AlNH₄(SO₄)₂, Al(OH)₃, Al(NO₃)₃, Al(ClO₄)₃, AlPO₄, Al₂(SO₄)₃, GaF₃, GaCl₃, GaBr₃, Gal₃, Ga(NO₃)₃, Ga(ClO₄)₃, Ga₂(SO₄)₃, InF₃, InCl₃, InBr₃, Inl₃, In(NO₃)₃, In(ClO₄)₃ and In(acetate)₃.

7. The process of claim 2, wherein said organometallic complex is selected from the group consisting of dialkylzinc, dialkylcadmium, dialkylmercury, trialkylaluminum, trialkylgallium and trialkylindium.

8. The process of claim 7, wherein said organometallic complex is selected from the group consisting of Zn(CH₃)₂, Zn(CH₂CH₃)₂, Cd(CH₃)₂, Cd(CH₂CH₃)₂, Hg(CH₃)₂, Hg(CH₂CH₃)₂, A1(CH₃)₃, A1(CH₂CH₃)₃, Ga(CH₃)₃ Ga(CH₂CH₃)₃, In(CH₃)₃ and In(CH₂CH₃)₃.

9. The process of claim 1, wherein said coordinating solvent has a boiling point of about 50 °C to about 500 °C.

10. The process of claim 9, wherein said coordinating solvent has the formula X=Y(R)₃ wherein: X is selected from the group consisting of O and S or X does not exist; Y is selected from the group consisting of N and P; each R is independently selected from the group consisting of alkyl having 6 to 20 carbon atoms and haloalkyl; wherein if Y is N then X does not exist.

11. The process of claim 10, wherein said coordinating solvent is selected from the group consisting of trioctylamine, trihexylphosphine, trihexylphosphine oxide, trioctylphosphine, trioctylphosphine oxide, tridecylphosphine, tridecylphosphine oxide, tridodecylphosphine, tridodecylphosphine oxide, tritetradecylphosphine, tritetradecylphosphine oxide, trihexadecylphosphine, trihexadecylphosphine oxide, and trioctadecylphosphine, trioctadecylphosphine oxide.

12. The process of claim 1, wherein said first precursor mixture further comprises a surfactant.

13. The process of claim 12, wherein said surfactant is selected from the group consisting of alkylcarboxcylic acids, alkylamines, alkylamine oxides, sulphonates, sulphonic acids, phosphonates and their polymers, phosphonic acids and their polymers, phosphinic acids and their polymers and phosphine oxides and their polymers.

14. The process of claim 13, wherein said surfactant is selected from the group consisting of dimers, trimers, tetramers, pentamers, hexamers, heptamers, octamers, nonamers and decamers of phosphonic acids.

15. The process of claim 13, wherein said surfactant is selected from the group consisting of hexylphosphonic acid, octylphosphonic acid, decylphosphonic acid, dodecylphosphonic acid, tefradecylphosphomc acid and polymers thereof.

16. The process of claim 1, wherein the first temperature is about 250 to about 450 °C.

17. The process of claim 1, wherein the first temperature is about 290 to about 400 °C.

18. The process of claim 1, further comprising after (b) and before (c): holding the first precursor mixture at a first temperature for a time of about 5 minutes to about 12 hours.

19. The process of claim 1, further comprising after (b) and before (c): (1) cooling the first precursor mixture to a temperature of about 0 °C to about 100 °C; and (2) heating the first precursor mixture to said first temperature.

20. The process of claim 1, wherein said first precursor mixture further comprises metal nanoparticles with diameters of about 1 nm to about 50 nm.

21. The process of claim 20, wherein said metal nanoparticles are gold nanoparticles.

22. The process of claim 1, wherein said first precursor mixture further comprises a metal precursor complex.

23. The process of claim 22, wherein said metal precursor complex comprises: (1) a Group H or Group HI metal; and (2) a surfactant selected from the group consisting of a phosphonic acid, a dimer of a phosphonic acid, a trimer of a phosphonic acid, and a polymer of a phosphonic acid.

24. The process of claim 22, further comprising after (a): (1) isolating the metal precursor complex from the first precursor mixture; and (2) contacting the metal precursor complex with a mixture comprising a coordinating solvent to form the first precursor mixture.

25. The process of claim 24, further comprising after (1) and before

(2): purifying the metal precursor complex.

26. The process of claim 24, wherein said mixture comprising a coordinating solvent further comprises a metal catalyst.

27. The process of claim 26, wherein said metal catalyst is gold nanoparticles.

28. The process of claim 27, wherein said gold nanoparticles have diameter of about 1 nm to about 50 nm.

29. The process of claim 1, wherein said Group V compound is selected from the group consisting of N[Si(CH₃)₃]₃, P[Si(CH₃)₃]₃, As[Si(CH₃)₃]₃, Sb[Si(CH₃)₃]₃, Bi[Si(CH₃)₃]₃, N(CH₃)₃, N(CH₂CH₃)₃, P(CH₃)₃, P(CH₂CH₃)₃, As(CH₃)₃, As(CH₂CH₃)₃, Sb(CH₃)₃, Sb(CH₂CH₃)₃, Bi(CH₃)₃ and Bi(CH₂CH₃)₃.

30. The process of claim 1, wherein said Group VI compound comprises an elemental chalcogen.

31. The process of claim 30, wherein said elemental chalcogen is selected from the group consisting of S, Se and Te.

32. The process of claim 1, wherein (c) further comprises contacting the first precursor mixture with the second precursor mixture heated to a temperature of about 25 °C to about 250 °C to form the reaction mixture at the second temperature.

33. The process of claim 1, wherein the second temperature is about 235 °C to about 500 °C.

34. The process of claim 1, wherein the second temperature is about 275 °C to about 350 °C.

35. The process of claim 1, wherein the second temperature is no more than about 10 °C lower than the first temperature.

36. The process of claim 1, wherein the second temperature is no more than about 5 C lower than the first temperature.

37. The process of claim 1, wherein the second and third temperatures are about the same.

38. The process of claim 1, wherein the third temperature is about 200-500 °C.

39. The process of claim 1, further comprising after (d): contacting the reaction mixture with a second precursor mixture comprising one of a Group V and Group VI compound.

40. The process of claim 1, further comprising after (d): forming on a substrate a thin film comprising nanocrystals.

41. The process of claim 40, wherein said thin film further comprises a polymer.

42. The process of claim 40, wherein said thin film is 100 nm to 100 μm.

43. The process of claim 40, wherein said substrate is a semiconductor.

44. The process of claim 43, wherein said semiconductor is selected from the group consisting of silicon, a polymer, ITO coated glass and TiO₂.

45. A composition comprising nanocrystals produced by the process of claim 1.

46. The composition of claim 45, wherein said nanocrystals are selected from the group consisting of ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, GaN, GaP, GaAs, InN, InP and InAs nanocrystals.

47. The composition of claim 45, wherein said nanocrystals have a rod, tetrapod, arrowhead, teardrop or rice shape.

48. A composition of rod-shaped HI-V nanocrystals having at least about 50%) hexagonal crystal structure and an aspect ratio of at least about 4:1.

49. The composition of claim 48, wherein said nanocrystals are selected from the group consisting of GaN, GaP, GaAs, InN, InP and InAs nanocrystals.

50. The composition of claim 48, wherein said nanocrystals have at least about 70% hexagonal crystal structure and an aspect ratio of at least about 4:1.

51. The composition of claim 48, wherein said nanocrystals have at least about 90% hexagonal crystal structure and an aspect ratio of at least about 4:1.