WO2010040100A2 - Methods for preparing nanocrystals using electron transfer agents - Google Patents

Abstract

Compositions and methods for the preparation of core nanocrystals are provided, using mismatched precursors and two or more electron transfer agents to independently control the nucleation and growth phases of particle formation.

Description

METHODS FOR PREPARING NANOCRYSTALS USING

ELECTRON TRANSFER AGENTS

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit of priority to U.S. Provisional Application serial no. 61/102,589 filed October 3, 2008, which is hereby incorporated by reference in its entirety.

Statement of Rights to Inventions Made Under Federally Sponsored Research

[0001] The embodiments disclosed herein were made, in part, with government support under cooperative agreement No. 70NANB4H3053 with the National Institute of Standards and Technology and the U.S. Department of Commerce. The government may have certain rights in the disclosed embodiments.

Technical Field

[0002] This disclosure provides methods for synthesizing nanoparticles using two electron transfer agents. More particularly, the disclosure provides methods for preparing nanoparticles which are useful in a variety of fields including biology, analytical and combinatorial chemistry, medical diagnostics, genetic analysis, solar energy conversion, and displays.

Background

[0003] Semiconductor nanocrystals have unique optical properties in between that of single molecules and bulk matter and are increasingly important for detecting, tracking, and observing single molecules and microscopic biological and biochemical structures in a variety of experimental protocols in biology, chemistry, medicine, and genetics. They provide fluorescent signals that are easily observed and are bright enough for practical observations to be made with single nanocrystals.

[0004] A number of methods for the formation of core nanocrystals from metal-anion binary salts are known in the art. These methods can generally be divided into classes based on the type of reactants employed and the presumed mechanism that arises based on how the oxidation states of the reactants compare.

[0005] In the first approach, the metal and nonmetal components that are reacting with each other are both provided in their neutral atomic form. For example, Murray, et al., J. Am. Chem. Soc, 1993, 115: 8706, described the reaction of dimethylcadmium (Me₂Cd) and trioctylphosphine selenide (TOPSe), which release neutral cadmium (Cd⁰) and selenium (Se⁰) atoms in solution respectively, so that no electron transfer is required to make their oxidation states match. Because the reactants are in suitable form to react with each other, this situation is considered a 'match' of oxidation states: neither needs to be oxidized or reduced for a reaction to occur, and no net electron imbalance results. Such reactions generally proceed very rapidly, because the cadmium and selenium atoms react instantly upon collision to form cadmium selenide (CdSe).

[0006] In a second category, the metal and nonmetal components are both provided in their ionic forms. For example, Peng, et al., J Am. Chem. Soc, 2001, 123: 183, described the preparation of CdSe and cadmium telluride (CdTe) using cadmium oxide (CdO) as the cadmium ion source, in the presence of TOPO and a phosphonic acid ligand, such as hexylphoshonic acid (HPA) tetradecylphosponic acid (TDPA), or octylphosphonic acid (OPA). Cadmium salts release Cd²⁺ ions in solution, while bis(trimethylsilyl)sulfide (TOPSe) releases Se^2", in solution. These reactions also proceed very rapidly, since the cadmium and sulfur ions also can react instantly to form cadmium sulfide (CdSe). This reaction type is considered a 'match', because again no oxidation or reduction of either species is required, and they can react in an appropriate stoichiometry to produce a neutral product.

[0007] In each of these categories, the extreme reactivity of the intermediates toward each other makes it difficult to control the particle size, particle yield and particle size dispersity. The reacting species, once released in solution, will react at a diffusion-controlled rate, or very nearly that fast. In some instances, the use of ligands or solvents may slow the reaction somewhat, but these approaches have not provided a general approach to controlling particle formation.

[0008] In particular, for example, it is often impossible to prevent such reactions from forming new nanocrystals, referred to as nucleation, which can make it difficult to control a reaction that is intended to add a shell to an existing nanocrystal. It is typically necessary to form a shell on a nanocrystal for use in certain applications, since the shell greatly enhances the chemical and photo-stability of the nanocrystal core. The shell is usually made of a different and complementary semiconductor material from the underlying core nanocrystal; thus if the shell- forming reaction results in nucleation, it forms new nanocrystals with a different composition from what is desired mixed in with the desired ones, and it is extremely difficult to separate the nanocrystals once they are formed as a mixture.

[0009] In a third approach, mismatched precursors may be chosen such that one precursor provides a neutral atom in solution under the reaction conditions, while the other precursor provides an ion. For example, a mixture of cadmium alkylphosphonate, which is a source of Cd²⁺ ions, and trioctylphosphine selenide (TOPSe), which is a source of Se⁰, might be employed as mismatched precursors. Such precursors cannot react to form a neutral species unless an electron transfer agent is present to adjust the oxidation state of one of the reactive species to provide 'matched' species capable of undergoing reaction. For example, a reductant could be used to add electrons to Cd²⁺ to provide two non- ionic species (i.e., Cd⁰ and Se⁰), or it could add electrons to Se⁰ to provide two ionic species (i.e., Cd²⁺ and Se^2"). Either way, once the atomic species are 'matched', their reaction can proceed, but the reaction cannot proceed without such an electron transfer agent. Alternatively, two ionic species having the same charge (i.e., two cations or two anions) would also be 'mismatched.' For example, mismatched precursors that provide two cationic species could be used, where one species is reduced to provide an anionic species capable of undergoing a 'matched' reaction. For example, Se²⁺ or Se⁴⁺ could be reduced to provide selenide anion Se^2", which could undergo reaction with a metal cation species, such as Cd²⁺. In another example, two cationic species could both be reduced to neutral species. Figure 1 depicts the stepwise process of nucleation and growth to illustrate how a reductant participates.

[0010] In another example, an oxidant could be used as the electron transfer agent, in a reaction between a neutral species and an anionic species. For example, Cd⁰ and Se^"2 could be used as mismatched precursors, wherein an oxidant is used to oxidize Se^"2 to Se⁰, giving two neutral species capable of undergoing a 'matched' reaction. The need for this electron transfer process and agent has been largely overlooked: because of the small scale and the complexity of the reactions involved, the role of the electron transfer agent is often performed by impurities either present in starting materials or accidentally generated in situ.

[0011] Some reactions having added electron transfer agents have been reported: for example, Zehnder and Treadway, US Patent No. 7,147,712, described the use of a promoter, which could be oxygen or a reducing agent, to promote and control nucleation and accelerate crystal growth. A single reductant was added to initiate nucleation, or initial formation of nanocrystals, and facilitate growth of the nanocrystals once nucleation had occurred. This approach provided control over the particle yield and over the ultimate particle size. However, because the same reductant was used for both the nucleation and growth phases, separation of the two phases could be achieved only by indirect means.

[0012] There remains a need for methods for manufacturing nanocrystal products in a high product yield and with a high level of control over particle size and particle dispersity, and also a need for separately controlling the nucleation and growth phases of nanocrystals. SUMMARY

[0013] Provided herein are methods to control and promote nanocrystal growth under conditions that avoid or minimize nucleation using two electron transfer agents to temporally separate the nucleation and growth phases. These methods provide for the preparation of nanocrystals which exhibit product reproducibility and controlled properties as well as novel compositions of such nanocrystals. The methods of this disclosure can provide independent control over the nucleation and growth phases of nanocrystal preparation.

[0014] The embodiments provided herein advantageously provide nanocrystals with reproducible product characteristics through the use of electron transfer agents which independently control nanocrystal formation and growth. The methods are robust because they may reproducibly provide nanocrystals even in the presence of some variation in the rates of addition of reagents to nanocrystal preparation reactions. In some embodiments, two reductants are used to control the process of nanocrystal formation.

[0015] In one aspect, a method for producing a population of nanocrystals, comprising: providing a mixture comprising: a first precursor; a second precursor, wherein the first precursor and the second precursor have mismatched oxidation states; a strong electron transfer agent in an amount sufficient to produce a desired amount of nucleation; and a weak electron transfer agent which is different than the strong electron transfer agent; and heating the mixture to a temperature for a period of time sufficient to induce formation of the population of nanocrystals.

[0016] In another aspect, a method for producing nanocrystals comprising: providing a mixture comprising a first precursor and a second precursor, wherein the first precursor and the second precursor have mismatched oxidation states; adding a sub- stoichiometric amount of a strong electron transfer agent to the mixture in an amount sufficient to produce a desired amount of nucleation; optionally heating together the mixture to produce the desired amount of nucleation; adding a weak electron transfer agent to the mixture in an amount sufficient to produce a desired amount of nanocrystal growth; and optionally heating the mixture for a period of time sufficient to produce the desired amount of nanocrystal growth.

[0017] In still another aspect, a method of producing nanocrystals comprising: providing a mixture comprising a first precursor, a second precursor, and a third precursor, wherein the first and second precursors have mismatched oxidation states, and wherein the third precursor has a matched oxidation state to the first precursor or the second precursor; and optionally heating the mixture to a temperature for a period of time sufficient to induce formation of nanocrystals.

[0018] Other aspects and advantages of the present methods and compositions will be apparent from the more detailed description below, by reference to certain embodiments thereof, and in further view of the examples included herein. It is to be understood that the terminology used herein is for the purpose of describing specific embodiments only and is not intended to be limiting.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 illustrates that two separate reductants can be used to promote nucleation followed by growth in the formation of a ZnTe nanocrystal.

[0020] Figure 2 illustrates the use of a weak reductant for the growth phase of nanocrystals, which in this embodiment arises from the precursor compound itself. Zinc undecylenate is an example of a precursor compound that also provides a weak reductant as an unsaturated carboxylate group. As shown by the higher absorbance levels on the right in Figure 2, the particle yield was higher for zinc undecylenate than for zinc stearate. Zinc undecylenate promotes crystal growth by providing a weak reductant group.

DETAILED DESCRIPTION

[0021] The embodiments described herein may be understood more readily by reference to the following detailed description and the Examples included herein. It is to be understood that the terminology used herein is for the purpose of describing specific embodiments only and is not intended to be limiting.

[0022] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which the embodiments disclosed herein belongs.

[0023] As used herein, "a" or "an" means "at least one" or "one or more."

[0024] As used herein, "about" means that the numerical value is approximate and small variations would not significantly affect the practice of the disclosed embodiments. Where a numerical limitation is used, unless indicated otherwise by the context, "about" means the numerical value can vary by ±10% and remain within the scope of the disclosed embodiments.

[0025] "Nanoparticle" as used herein refers to any particle with at least one major dimension in the nanosize range. Typically, a nanoparticle has at least one major dimension ranging from about 1 to 1000 nm.

[0026] Examples of nanoparticles include a nanocrystal, such as a core/shell nanocrystal, plus any tightly-associated organic coating or other material that can be on the surface of the nanocrystal. A nanoparticle can also include a bare core/shell nanocrystal, as well as a core nanocrystal or a core/shell nanocrystal having a layer of, e.g., TDPA, OPA, TOP, TOPO or other material that is not removed from the surface by ordinary solvation. A nanoparticle can have a layer of ligands on its surface which can further be cross-linked; and a nanoparticle can have other or additional surface coatings that can modify the properties of the particle, for example, increasing or decreasing solubility in water or other solvents. Such layers on the surface are included in the term 'nanoparticle.'

[0027] "Nanocrystal" as used herein can refer to a nanoparticle made out of an inorganic substance that typically has an ordered crystalline structure. It can refer to a nanocrystal having a crystalline core (core nanocrystal), or to a core/shell nanocrystal. Typically, a nanocrystal has a core diameter ranging from 1-100 nm in its largest dimension, preferably between about 1 to 50 nm in its largest dimension.

[0028] A core nanocrystal is a nanocrystal to which no shell has been applied; typically it is a semiconductor nanocrystal, and typically it is made of a single semiconductor material. It can have a homogeneous composition, or its composition can vary with depth inside the nanocrystal. Many types of nanocrystals are known, and methods for making a nanocrystal core and applying a shell to it are known in the art. The nanocrystals disclosed herein are frequently bright fluorescent nanocrystals, and the nanoparticles prepared from them are typically also bright, e.g., having a quantum yield of at least about 10%, sometimes at least about 20%, sometimes at least about 30%, sometimes at least about 40%, and sometimes at least about 50% or greater. It can be advantageous for nanocrystals to have a surface layer of ligands to protect the nanocrystal from degradation in use or during storage.

[0029] "Quantum dot" as used herein refers to a nanocrystalline particle made from a material that in the bulk is a semiconductor or insulating material, which has a tunable photophysical property in the near ultraviolet (UV) to far infrared (IR) range.

[0030] "Water-soluble" is used herein to mean the item can be soluble or suspendable in an aqueous-based solution, such as in water or water-based solutions or buffer solutions, including those used in biological or molecular detection systems as known by those skilled in the art. While water-soluble nanoparticles are not truly 'dissolved' in the sense that term is used to describe individually solvated small molecules, they are solvated and suspended in solvents that are compatible with their outer surface layer, thus a nanoparticle that is readily dispersed in water is considered water-soluble or water-dispersible. A water-soluble nanoparticle can also be considered hydrophilic, since its surface is compatible with water and with water solubility. [0031] "Hydrophobic nanoparticle" as used herein refers to a nanoparticle that can be readily dispersed in or dissolved in a water-immiscible solvent like hexanes, toluene, and the like. Such nanoparticles are generally not readily dispersed in water.

[0032] "Hydrophilic" as used herein refers to a surface property of a solid, or a bulk property of a liquid, where the solid or liquid exhibits greater miscibility or solubility in a high-dielectric medium than it does in a lower dielectric medium. By way of example, a material that is more soluble in methanol than in a hydrocarbon solvent such as decane would be considered hydrophilic.

[0033] Nanoparticles can be synthesized in shapes of different complexity such as spheres, rods, discs, triangles, nanorings, nanoshells, tetrapods, nanowires and so on. Each of these geometries have distinctive properties: spatial distribution of the surface charge, orientation dependence of polarization of the incident light wave, and spatial extent of the electric field. In some embodiments, the nanocrystals disclosed herein are roughly spherical.

[0034] In some embodiments, a nanoparticle as provided herein may be a core/shell nanocrystal having a nanocrystal core covered by a semiconductor shell. The thickness of the shell can be adapted to provide desired particle properties. The thickness of the shell may affect fluorescence wavelength, quantum yield, fluorescence stability, and other photostability characteristics.

[0035] The nanocrystal core and shell can be made of any suitable metal and non-metal atoms that are known to form semiconductor nanocrystals. Suitable semiconductor materials for the core and/or shell include, but are not limited to, ones including Group 2-16, 12-16, 13-15 and 14 element-based semiconductors such as ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, MgS, MgSe, MgTe, CaS, CaSe, CaTe, SrS, SrSe, SrTe, BaS, BaSe, BaTe, GaN, GaP, GaAs, GaSb, InP, InAs, InSb, AlS, AlP, AlSb, PbS, PbSe, Ge and Si and ternary and quaternary mixtures thereof. Typically, the core and the shell of a core/shell nanocrystal are composed of different semiconductor materials, meaning that at least one atom type of a binary semiconductor material of the core of a core/shell is different from the atom types in the shell of the core/shell nanocrystal. The nanocrystal core and shell can be made of any suitable metal and non-metal atoms that are known to form semiconductor nanocrystals. Semiconductor nanocrystals may be made using techniques known in the art. See, e.g., U.S. Pat. Nos. 6,048,616, 5,990,479, 5,690,807, 5,505,928 and 5,262,357, as well as International Patent Publication No. WO 99/26299, published May 27, 1999. These methods typically produce nanocrystals having a coating of hydrophobic ligands on their surfaces which protect them from rapid degradation. [0036] Nanocrystals can be characterized by their percent quantum yield of emitted light. For example, the quantum yield for the nanocrystals disclosed herein can be greater than about 10%, greater than about 20%, greater than about 30%, greater than about 40%, greater than about 50%, greater than about 60%, greater than about 70%, greater than about 80%, greater than about 90%, and ranges between any two of these values. The quantum yield is typically greater than about 30%, and preferably greater than 50%, greater than 70% and sometimes greater than 80%.

[0037] In some embodiments, the metal atoms of a shell layer on a nanocrystal core are selected from Cd, Hg, Zn, Be, Al, Ga, Mn, Cu and Mg. The second element in these semiconductor shell layers can be selected from S, Se, Te, O, P, As, N and Sb

[0038] The nanoparticle can be of any suitable size; typically, it is sized to provide fluorescence in the UV- Visible portion of the electromagnetic spectrum, since this range is convenient for use in monitoring biological and biochemical events in relevant media. The relationship between size and fluorescence wavelength is well known, thus making nanocrystals smaller may require selecting a particular material that gives a suitable wavelength at a small size, such as InP as the core of a core/shell nanocrystal designed to be especially small.

[0039] In frequent embodiments, the nanocrystals described herein are from about 1 nm to about 100 nm in diameter, sometimes from about 1 to about 50 nm in diameter, and sometimes from about 1 to about 25 nm in diameter. More specific ranges of sizes for nanocrytals can include, but are not limited to: about 0.5 nm to about 5 nm, about 1 nm to about 50 nm, about 2nm to about 50 nm, about 1 nm to about 20 nm, about 2 nm to about 20 nm, or from about 2 to about 10 nm. More specific size examples for nanocrystals can include, but are not limited to: about 0.1 nm, about 0.5 nm, about 1 nm, about 2 nm, about 3 nm, about 4 nm, about 5 nm, about 6 nm, about 7 nm, about 8 nm, about 9 nm, about 10 nm, about 11 nm, about 12 nm, about 13 nm, about 14 nm, about 15 nm, about 16 nm, about 17 nm, about 18 nm, about 19 nm, about 20 nm, about 25 nm, about 30 nm, about 35 nm, about 40 nm, about 45 nm, about 50 nm, and ranges between any two of these values. For a nanocrystal that is not substantially spherical, e.g. rod-shaped, it may be from about 1 to about 100 nm, or from about 1 nm to about 50 nm, from about 1 nm to about 25 nm, from about 1 nm to 10 nm, or sometimes from about 1 nm to 5 nm in its smallest dimension.

[0040] In some embodiments, provided herein is a nanocrystal core that can be less than about 10 nm in diameter, or less than about 7 nm in diameter, or less than about 5 nm in diameter. The small size of these nanocrystals can be advantageous in many applications, particularly because the nanocrystals disclosed herein are unexpectedly bright for their size. [0041] A typical single-color preparation of nanoparticles has crystals that are preferably of substantially identical size and shape. Nanocrystals are typically thought of as being spherical or nearly spherical in shape, but can actually be any shape. Alternatively, the nanocrystals can be non-spherical in shape. For example, the nanocrystal's shape can change towards oblate spheroids for redder colors. It is preferred that at least about 60%, at least about 70%, at least about 80%, at least about 90%, at least about 95%, and ideally about 100% of the particles are of the same size. Size deviation can be measured as root mean square ("rms") of the diameter, with less than about 30% rms, preferably less than about 20% rms, more preferably less than about 10% rms. Size deviation can be less than about 10% rms, less than about 9% rms, less than about 8% rms, less than about 7% rms, less than about 6% rms, less than about 5% rms, or ranges between any two of these values. Such a collection of particles can sometimes be referred to as being "monodisperse". One of ordinary skill in the art will realize that particular sizes of nanocrystals, such as of semiconductor nanocrystals, are generally obtained as particle size distributions.

[0042] It is well known that the color (emitted light) of the semiconductor nanocrystal can be "tuned" by varying the size and composition of the nanocrystal. Nanocrystals can absorb a wide spectrum of wavelengths, and emit a narrow wavelength of light. The excitation and emission wavelengths are typically different, and non-overlapping. The nanoparticles of a monodisperse population may be characterized in that they produce a fluorescence emission having a relatively narrow wavelength band. Examples of emission widths (FWHM) include less than about 200 nm, less than about 175 nm, less than about 150 nm, less than about 125 nm, less than about 100 nm, less than about 75 nm, less than about 60 nm, less than about 50 nm, less than about 40 nm, less than about 30 nm, less than about 20 nm, and less than about 10 nm. The width of emission is preferably less than about 50 nm, and more preferably less than about 35 nm at full width at half maximum of the emission band (FWHM). The emitted light preferably has a symmetrical emission of wavelengths. The emission maxima can generally be at any wavelength from about 200 nm to about 2,000 nm. Examples of emission maxima can include, but are not limited to: about 200 nm, about 400 nm, about 600 nm, about 800 nm, about 1,000 nm, about 1,200 nm, about 1,400 nm, about 1,600 nm, about 1,800 nm, about 2,000 nm, and ranges between any two of these values. In certain embodiments, a green color is desirable, so a wavelength in the green region is selected.

[0043] The nanoparticles can have surface coatings adding various functionalities. For example, the nanocrystals can be coated with lipids, phospholipids, fatty acids, polynucleic acids, polyethylene glycol, primary antibodies, secondary antibodies, antibody fragments, protein or nucleic acid based aptamers, biotin, streptavidin, proteins, peptides, small organic molecules, organic or inorganic dyes, precious or noble metal clusters.

[0044] Spectral characteristics of nanoparticles can generally be monitored using any suitable light-measuring or light-accumulating instrumentation. Examples of such instrumentation are CCD (charge-coupled device) cameras, video devices, CIT imaging, digital cameras mounted on a fluorescent microscope, photomultipliers, fluorometers and luminometers, microscopes of various configurations, and even the human eye. The emission can be monitored continuously or at one or more discrete time points. The photostability and sensitivity of nanoparticles allow recording of changes in electrical potential over extended periods of time.

[0045] In some embodiments, the nanoparticle provided herein can be a member of a monodisperse population of nanoparticles of like composition. The monodisperse particle population in some embodiments can be characterized in that it exhibits less than about 30% rms, preferably less than about 20% rms, more preferably less than about 10% rms deviation in the diameter, or smallest dimension, of the core. In some embodiments, the monodisperse particle population exhibits less than about 5% or less than about 3% rms deviation in the diameter, or smallest dimension, of the core. One of ordinary skill in the art will realize that particular sizes of nanocrystals, such as of semiconductor nanocrystals, are actually obtained as particle size distributions.

[0046] The nanoparticles of a monodisperse population may be characterized in that they produce a fluorescence emission having a relatively narrow wavelength band. In some embodiments, the monodisperse particle population can be characterized in that when irradiated the population emits light in a bandwidth of less than about 60 nm full width at half maximum (FWHM), or less than about 50 nm FWHM, and sometimes less than about 40 nm FWHM.

[0047] In some embodiments, a core semiconductor nanocrystal can be modified to enhance the efficiency and stability of its fluorescence emissions, prior to ligand modifications described herein, by adding an overcoating layer or shell to the semiconductor nanocrystal core. Having a shell may be preferred, because surface defects at the surface of the semiconductor nanocrystal core can result in traps for electrons, or holes that degrade the electrical and optical properties of the semiconductor nanocrystal core, or other non-radiative energy loss mechanisms that either dissipate the energy of an absorbed photon or at least affect the wavelength of the fluorescence emission slightly, resulting in broadening of the emission band. An insulating layer at the surface of the semiconductor nanocrystal core can provide an atomically abrupt jump in the chemical potential at the interface that eliminates energy states that can serve as traps for the electrons and holes. This results in higher efficiency in the luminescent processes. [0048] Suitable materials for the shell include semiconductor materials having a higher bandgap energy than the semiconductor nanocrystal core. In addition to having a bandgap energy greater than the semiconductor nanocrystal core, suitable materials for the shell should have good conduction and valence band offset with respect to the core semiconductor nanocrystal. Thus, the conduction band can be desirably higher and the valence band can be desirably lower than those of the core semiconductor nanocrystal. For semiconductor nanocrystal cores that emit energy in the visible (e.g., CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, GaP, GaAs, GaN) or near IR (e.g., InP, InAs, InSb, PbS, PbSe), a shell material that has a bandgap energy in the ultraviolet regions may be used. Exemplary shell materials can include, but are not limited to: CdS, CdSe, InP, InAs, ZnS, ZnSe, ZnTe, GaP, GaN, and magnesium chalcogenides, e.g., MgS, MgSe, and MgTe. For a semiconductor nanocrystal core that emits in the near IR, shell materials having a bandgap energy in the visible, such as CdS or CdSe, may also be used. The preparation of a coated semiconductor nanocrystal may be found in, e.g., Dabbousi et al. (1997) /. Phys. Chem. B 101:9463, Hines et al. (1996) /. Phys. Chem. 100: 468-471, Peng et al. (1997) /. Am. Chem. Soc. 119:7019-7029, and Kuno et al. (1997) /. Phys. Chem. 106:9869. It is also understood in the art that the actual fluorescence wavelength for a particular nanocrystal core can depend upon the size of the core as well as its composition, so the categorizations above are approximations, and nanocrystal cores described as emitting in the visible or the near IR can actually emit at longer or shorter wavelengths depending upon the size of the core.

[0049] Where a core/shell fluorescent semiconductor nanocrystal is used, it is sometimes advantageous to make the nanoparticle as small as practical; thus in some embodiments, the nanocrystal can be less than about 20 nm in diameter, and often less than about 8 nm, and sometimes less than about 6 nm in diameter, and in some embodiments, the nanocrystal is less than about 5 nm in diameter or size, or less than 4 nm in diameter or size.

[0050] Nanocrystal precursors can sometimes be referred to as a first precursor and a second precursor. The first precursor can be a metal-containing salt, such as a halide, carboxylate, phosphonate, carbonate, hydroxide, or diketonate, or a mixed salt thereof (e.g., a halo carboxylate salt, such as Cd(halo)(oleate)), of a metal, in which the metal can be, e.g., Cd, Zn, Mg, Be, Mn, Cu, Co, Pb Hg, Al, Ga, In, or Tl. The second precursor can be, e.g., O, S, Se, Te, N, P, As, or Sb. The second precursor mixture can include an amine, such as a primary amine (e.g., a Cs-C_2O alkyl amine). The second precursor can include, for example, a phosphine chalcogenide, a bis(silyl) chalcogenide, a dioxygen species, an ammonium salt, or a tris(silyl) phosphine, or the like. [0051] In one embodiment, the first precursor and the second precursor can be combined by contacting a metal or a metal-containing salt, and a reducing agent to form a metal-containing precursor. The reducing agent can include an alkyl phosphine, a 1,2-diol or an aldehyde, such as a C₆-C₂O alkyl diol or a C₆-C₂O aldehyde.

[0052] Examples of suitable metal-containing salts can include, but are not limited to cadmium acetylacetonate, cadmium iodide, cadmium bromide, cadmium chloride, cadmium hydroxide, cadmium carbonate, cadmium acetate, cadmium oxide, zinc acetylacetonate, zinc iodide, zinc bromide, zinc chloride, zinc hydroxide, zinc carbonate, zinc acetate, zinc oxide, magnesium acetylacetonate, magnesium iodide, magnesium bromide, magnesium chloride, magnesium hydroxide, magnesium carbonate, magnesium acetate, magnesium oxide, mercury acetylacetonate, mercury iodide, mercury bromide, mercury chloride, mercury hydroxide, mercury carbonate, mercury acetate, aluminum acetylacetonate, aluminum iodide, aluminum bromide, aluminum chloride, aluminum hydroxide, aluminum carbonate, aluminum acetate, gallium acetylacetonate, gallium iodide, gallium bromide, gallium chloride, gallium hydroxide, gallium carbonate, gallium acetate, indium acetylacetonate, indium iodide, indium bromide, indium chloride, indium hydroxide, indium carbonate, indium acetate, thallium acetylacetonate, thallium iodide, thallium bromide, thallium chloride, thallium hydroxide, thallium carbonate, or thallium acetate. Suitable metal-containing salts also include, for example, carboxylate salts, such as oleate, stearate, myristate, and palmitate salts, mixed halo carboxylate salts, such as M(halo)(oleate) salts, as well as phosphonate salts.

[0053] Alkyl can be a branched or unbranched saturated hydrocarbon group of 1 to 100 carbon atoms, preferably 1 to 30 carbon atoms, such as methyl, ethyl, n-propyl, isopropyl, n- butyl, isobutyl, t-butyl, octyl, decyl, tetradecyl, hexadecyl, eicosyl, tetracosyl and the like, as well as cycloalkyl groups such as cyclopentyl, cyclohexyl and the like. Optionally, an alkyl can contain 1 to 6 linkages selected from the group consisting of -O-, -S-, -M- and -NR- where R is hydrogen, or C1-C8 alkyl or lower alkenyl.

[0054] Prior to combining the metal-containing salt with the second precursor, the metal- containing salt can be contacted with a coordinating solvent to form a metal-containing precursor. Typical coordinating solvents include alkyl phosphines, alkyl phosphine oxides, alkyl phosphonic acids, alkyl phosphinic acids, or carboxylic acid containing solvents; however, other coordinating solvents, such as pyridines, furans, and amines may also be suitable for the nanocrystal production. Examples of suitable coordinating solvents include pyridine, tri-n-octyl phosphine (TOP) and tri-n-octyl phosphine oxide (TOPO). Also included are acid containing solvents, such as oleic acid, stearic acid, myristic acid, palmitic acid, TDPA, OPA, and the like. The coordinating solvent can include a 1,2-diol or an aldehyde. The 1,2-diol or aldehyde can facilitate reaction between the metal-containing salt and the second precursor and improve the growth process and the quality of the nanocrystal obtained in the process.

[0055] The second precursor is generally a chalcogenide donor or a Group V element, such as a phosphine chalcogenide, a bis(silyl) chalcogenide, dioxygen, an ammonium salt, or a tris(silyl) phosphine. Suitable second precursors include dioxygen, elemental sulfur, bis(trimethylsilyl) selenide ((TMS)₂Se), trialkyl phosphine selenides such as (tri-n- octylphosphine) selenide (TOPSe) or (tri-n-butylphosphine) selenide (TBPSe), trialkyl phosphine tellurides such as (tri-n-octylphosphine) telluride (TOPTe) or hexapropylphosphorustriamide telluride (HPPTTe), bis(trimethylsilyl)telluride ((TMS)₂Te), sulfur, bis(trimethylsilyl)sulfide ((TMS)₂S), a trialkyl phosphine sulfide such as (tri-n- octylphosphine) sulfide (TOPS), tris(dimethylamino) arsine, an ammonium salt such as an ammonium halide (e.g., NH₄Cl), tris(trimethylsilyl) phosphide ((TMS)₃P), tris(trimethylsilyl) arsenide ((TMS)₃As), or tris(trimethylsilyl) antimonide ((TMS)₃Sb). In certain embodiments, the first precursor and the second precursor can be moieties within the same molecule.

[0056] "Coordinating solvents" as used herein refers to a solvent such as TOP, TOPO, TDPA, OPA, carboxylic acids, and amines, which are effective to coordinate to the surface of a nanocrystal. 'Coordinating solvents' also include phosphines, phosphine oxides, phosphonic acids, phosphinic acids, amines, and carboxylic acids, which are often used in growth media for nanocrystals, and which form a coating or layer on the nanocrystal surface. They exclude hydrocarbon solvents such as hexanes, toluene, hexadecane, octadecene, and the like, which do not have heteroatoms that provide bonding pairs of electrons to coordinate with the nanocrystal surface. Hydrocarbon solvents that do not contain heteroatoms such as O, S, N or P to coordinate to a nanocrystal surface are referred to herein as non-coordinating solvents. Note that the term 'solvent' is used in its ordinary way in these terms: it refers to a medium that supports, dissolves, or disperses materials and reactions between them, but which does not ordinarily participate in or become modified by the reactions of the reactant materials.

Precursors

[0057] The formation of nanoparticles generally involves two distinct phases: the first phase, nucleation, requires a substantial number of precursors to coalesce into a nuclei (i.e., nucleation) while the second phase, growth, involves the addition of precursors to the existing nuclei. When precursor atoms are matched in type (i.e., both are nonionic (neutral), or one is cationic and the other is anionic), they usually react very rapidly. Such rapid reaction often produces extensive nucleation, even where nucleation is undesirable, and may result in the formation of particle populations lacking uniform particle size because growth and nucleation can occur simultaneously.

[0058] Independent control over these two formation stages is valuable, because the nucleation phase determines the nanoparticle yield, while the growth phase determines the ultimate size of the nanoparticle.

[0059] It is important to separate the nucleation phase from the growth phase in nanocrystal formation so that all the nanocrystals form at roughly the same time and then all grow together for the same amount of time to result in a uniform distribution of nanoparticle sizes, providing a monodisperse population of core nanocrystals. Uniform size is difficult to achieve if new tiny nuclei form after other particles have formed and grown for a period of time.

[0060] Provided herein are methods for separating the nucleation and growth stages, with enhanced control over each individual phase.

[0061] The present methods control of the two phases of particle formation is achieved by the use of 'mismatched' precursors. 'Mismatched' precursors cannot react without either addition or loss of electrons such that both precursors in solution are present in either a complementary ionic state or a neutral state; otherwise, they could not combine to form a neutral product. The lack of reactivity of 'mismatched' precursors in the absence of an electron transfer agent, such as a reducing agent or an oxidizing agent, can be exploited to temporally control the nucleation and growth phases of particle formation by controlling the amount and nature of the electron transfer agent present in the reaction mixture, along with the mismatched precursors.

[0062] The precursors provided herein will be considered 'mismatched' if one precursor provides a neutral species for nanocrystal formation in solution while the other precursor provides an ionic species in solution under the reaction conditions used, or if the precursors each provide an ionic species having the same charge (i.e., two cations or two anions). Examples of mismatched precursors include a precursor that provides a cation species paired with a precursor that provides a non-ionic (i.e., neutral) species or another cation, or a precursor that provides an anionic species paired with a precursor that provides a non- ionic (i.e., neutral) species or another anionic species. The relevant species is the reacting species present in the shell-forming reaction conditions.

[0063] In some embodiments, a dual oxidant system may be used. In other embodiments, a dual reductant system may be used. In still other embodiments, mixed electron transfer agent system (i.e., one oxidant and one reductant) may be used. In some such embodiments, a set of precursors may be used where one provides a neutral species and is the other provides a cationic species. In this system, a reductant may be added to reduce the neutral species to an anion, or to reduce the cationic species to a neutral oxidation state.

[0064] In some embodiments, the first precursor may comprise a metal atom M and the second precursor may comprise a nonmetal atom X, wherein the precursors are selected so that their oxidation states are mismatched.

[0065] The nanocrystal core and shell can be made of any suitable metal and non-metal atoms that are known to form semiconductor nanocrystals. Suitable semiconductor materials for the core and/or shell include, but are not limited to, ones including Group 2-16, 12-16, 13-15 and 14 element-based semiconductors, such as, e.g., ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, MgS, MgSe, MgTe, CaS, CaSe, CaTe, SrS, SrSe, SrTe, BaS, BaSe, BaTe, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, AlAs, AlP, AlSb, PbS, PbSe, Ge and Si and binary, ternary and quaternary mixtures thereof.

[0066] The selection of the composition of the semiconductor nanoparticle affects the characteristic spectral emission wavelength of the particle. Thus, a particular composition of a nanoparticle as provided herein may be selected based upon the spectral region being monitored. For example, semiconductor nanocrystals that emit energy in the visible range include, but are not limited to, CdS, CdSe, CdTe, ZnSe, ZnTe, GaP, and GaAs. Semiconductor nanocrystals that emit energy in the near IR range include, but are not limited to, InP, InAs, InSb, PbS, and PbSe. Finally, examples of semiconductor nanocrystals that emit energy in the blue to near-ultraviolet include, but are not limited to, ZnS and GaN. In each case, additional tuning of the fluorescence wavelength can be achieved to a degree by a shell added over the nanocrystal core.

[0067] Precursors useful as the "first" precursor in the methods disclosed herein include compounds containing elements from Groups 2 and 12 of the Periodic Table of the Elements (e.g., Zn, Cd, Hg, Mg, Ca, Sr, Ba, and the like), compounds containing elements from Group 13 of the Periodic Table of the Elements (Al, Ga, In, and the like), and compounds containing elements from Group 14 of the Periodic Table of the Elements (Si, Ge, Pb, and the like). Many forms of the precursors can be used in the methods of disclosed herein.

[0068] Examples of compounds useful as the first precursor can be organometallic compounds such as alkyl metal species, or salts such as metal halides, metal acetates, metal carboxylates, metal phosphonates, metal phosphinates, metal oxides, or other salts. In some embodiments, the first precursor provides a neutral species in solution. For example, alkyl metal species such as diethylzinc (Et₂Zn) or dimethyl cadmium are typically considered to be a source of neutral zinc atoms (Zn⁰) in solution. In other embodiments, the first precursor provides an ionic species (i.e., a metal cation) in solution. For example, zinc chloride (ZnCl₂) and other zinc halides, zinc acetate (Zn(OAc)₂) and zinc carboxylates are typically considered to be sources of Zn ⁺ cations in solution.

[0069] By way of example only, suitable first precursors providing neutral metal species include dialkyl metal sources, such as dimethyl cadmium (Me₂Cd), diethyl zinc (Et₂Zn), and the like. Suitable first precursors providing metal cations in solution include, e.g., cadmium salts, such as cadmium acetate (Cd(OAc)₂), cadmium nitrate (Cd(NO3)₂), cadmium oxide (CdO), and other cadmium salts; and zinc salts such as zinc chloride (ZnCl₂), zinc acetate (Zn(OAc)₂), zinc oleate (Zn(oleate)₂), zinc chloro(oleate), zinc undecylenate, zinc salicylate, and other zinc salts. In some embodiments, the first precursor is salt of Cd or Zn. In some embodiments, it is a halide, acetate, carboxylate, or oxide salt of Cd or Zn. In other embodiments, the first precursor is a salt of the form M(O₂CR)X, wherein M is Cd or Zn; X is a halide or O₂CR; and R is a C₄-C₂₄ alkyl group that is optionally unsaturated. Other suitable forms of Groups 2, 12, 13 and 14 elements useful as first precursors are known in the art.

[0070] Precursors useful as the "second" precursor in the methods disclosed herein include compounds containing elements from Group 16 of the Periodic Table of the Elements (e.g., S, Se, Te, and the like), compounds containing elements from Group 15 of the Periodic Table of the Elements (N, P, As, Sb, and the like), compounds containing elements from Group 14 of the Periodic Table of the Elements (Ge, Si, and the like) and compounds containing elements from Group 17 of the Periodic Table of the Elements (Halides). Many forms of the precursors can be used in the methods disclosed herein. It will be understood that in some embodiments, the second precursor will provide a neutral species in solution, while in other embodiments the second precursor will provide an ionic species in solution.

[0071] It should be appreciated that in certain embodiments, a nanoparticle core and/or shell can be comprised of more than two precursors. For example, the same methods described herein can be applied to form ternary nanoparticles using three precursors, quartenary nanoparticles using four precursors, etc.

[0072] When the first precursor comprises a metal cation, the second precursor will preferably provide an uncharged (i.e., neutral) non-metal atom in solution. In frequent embodiments, when the first precursor comprises a metal cation, the second precursor contributes a neutral chalcogen atom, most commonly S°, Se⁰ or Te⁰.

[0073] Suitable second precursors for providing a neutral chalcogen atom include, for example, elemental sulfur (often as a solution in an amine, e.g., decylamine, oleylamine, or dioctylamine, or an alkene, such as octadecene), and tri-alkylphosphine adducts of S, Se and Te. Such trialkylphosphine adducts are sometimes described herein as R₃P=X, wherein X is S, Se or Te, and each R is independently H, or a C1-C24 hydrocarbon group that can be straight-chain, branched, cyclic, or a combination of these, and which can be unsaturated. Exemplary second precursors of this type include tri-n (butylphosphine)selenide (TBP=Se), tri-n- (octylphosphine)selenide (TOP=Se), and the corresponding sulfur and tellurium reagents, TBP=S, TOP=S, TBP=Te and TOP=Te. These reagents are frequently formed by combining a desired element, such as Se, S, or Te with an appropriate coordinating solvent, e.g., TOP or TBP. Precursors that provide anionic species under the reaction conditions are typically used with a first precursor that provides a neutral metal atom, such as alkylmetal compounds and others described above or known in the art.

[0074] In some embodiments, the second precursor provides a negatively charged non-metal ion in solution (e.g., S^"2, Se^"2 or Te^"2). Examples of suitable second precursors providing an ionic species include silyl compounds such as bis(trimethylsilyl)selenide ((TMS)₂Se), bis(trimethylsilyl)sulfide ((TMS)₂S) and bis(trimethylsilyl)telluride ((TMS)₂Te). Also included are hydrogenated compounds such as H₂Se, H₂S, H₂Te; and metal salts such as NaHSe, NaSH or NaHTe. In this situation, an oxidant can be used to oxidize a neutral metal species to a cationic species that can react with the anionic precursor in a 'matched' reaction, or an oxidant can be used increase the oxidation state of the anionic precursor to provide a neutral species that can undergo a 'matched' reaction with a neutral metal species.

[0075] Other exemplary organic precursors are described in U.S. Pat. Nos. 6,207,299 and 6,322,901 to Bawendi et al., and synthesis methods using weak acids as precursor materials are disclosed by Qu et al., (2001), Nano Lett., l(6):333-337, the disclosures of each of which are incorporated herein by reference in their entirety.

[0076] Both the first and the second precursors can be combined with an appropriate solvent to form a solution for use in the methods of disclosed herein. The solvent or solvent mixture used to form a first precursor solution may be the same or different from that used to form a second precursor solution. The precursors can be dissolved separately, or they can be combined together into a single solution.

[0077] Heating of the mixture can be performed before mixing or after mixing of the precursors together, and either before or after combining the solvent and/or precursors with the weak reducing agent. In some embodiments, a first precursor, a second precursor and a weak reductant are all combined, optionally in a suitable solvent or mixture of solvents, to form a reaction mixture, and the reaction mixture is subsequently heated to a suitable temperature prior to the addition of the strong reductant. [0078] The order and rate of addition of the precursors is generally not critical to the methods of disclosed herein.

[0079] In some embodiments, the precursors are thus added at a rate that is limited only by practical factors associated with maintaining the desired reaction temperature, and addition of the precursors can be done as quickly as temperature control permits. Similarly, the precursors can all be present in the reaction mixture when it is heated to a desired reaction temperature, and one or both of the electron transfer agents can be added after the operating temperature has been reached. In some embodiments, at least the strong electron transfer agent is not added to the reaction mixture before it reaches the desired reaction temperature.

Coordinating Solvents

[0080] Suitable coordinating solvents include, by way of illustration and not limitation, hydrocarbons, amines, alkyl phosphines, alkyl phosphine oxides, carboxylic acids, ethers, furans, phosphoacids, pyridines and mixtures thereof. The solvent may actually comprise a mixture of solvents, often referred to in the art as a "solvent system". In preferred embodiments, the solvent comprises at least one coordinating solvent. In some embodiments, the solvent system comprises a secondary amine and a trialkyl phosphine (e.g., TBP or TOP), phosphonic acid (e.g., TDPA OPA, ) or a trialkylphosphine oxide (e.g., TOPO)

[0081] A coordinating solvent might be a mixture of an essentially non-coordinating solvent such as an alkane and a ligand as defined below.

[0082] Suitable hydrocarbons include alkanes, alkenes and aromatic hydrocarbons from 10 to about 30 carbon atoms; examples include octadecene and squalane. The hydrocarbon may comprise a mixture of alkane, alkene and aromatic moieties, such as alkylbenzenes (e.g., mesitylene).

[0083] Suitable amines include, but are not limited to, monoalkylamines, dialkylamines, and trialkylamines, for example trioctylamine, dioctylamine, octylamine, oleylamine, decylamine, dodecylamine, hexyldecylamine, and so forth. Alkyl groups for these amines typically contain about 6-24 carbon atoms per alkyl, and can include an unsaturated carbon-carbon bond, and each amine typically has a total number of carbon atoms in all of its alkyl groups combined of about 10-30 carbon atoms.

[0084] Exemplary alkyl phosphines include, but are not limited to, the trialkyl phosphines, tri-n-butylphosphine (TBP), tri-n-octylphosphine (TOP), and so forth. Alkyl groups for these phosphines contain about 6-24 carbon atoms per alkyl, and can contain an unsaturated carbon- carbon bond, and each phosphine has a total number of carbon atoms in all of its alkyl groups combined of about 10-30 carbon atoms.

[0085] Suitable alkyl phosphine oxides include, but are not limited to, the trialkyl phosphine oxide, tri-n-octylphosphine oxide (TOPO), and so forth. Alkyl groups for these phosphine oxides contain about 6-24 carbon atoms per alkyl, and can contain an unsaturated carbon-carbon bond, and each phosphine oxide has a total number of carbon atoms in all of its alkyl groups combined of about 10-30 carbon atoms.

[0086] Exemplary fatty acids include, but are not limited to, stearic, oleic, palmitic, myristic and lauric acids, as well as other carboxylic acids of the formula R-COOH, wherein R is a C₆-C₂₄ hydrocarbon group and can contain an unsaturated carbon-carbon bond..

[0087] Exemplary ethers and furans include, but are not limited to, tetrahydrofuran and its methylated forms, glymes, and so forth.

[0088] Suitable phosphonic and phosphinic acids include, but are not limited to hexylphosphonic acid (HPA), tetradecylphosphonic acid (TDPA), and octylphosphonic acid (OPA), and are frequently used in combination with an alkyl phosphine oxide such as TOPO. Suitable phosphonic and phosphinic acids are of the formula RPO₃H₂ or R₂PO₂H, wherein each R is independently a C6-C24 hydrocarbon group and can contain an unsaturated carbon-carbon bond.

[0089] Exemplary pyridines include, but are not limited to, pyridine, alkylated pyridines, nicotinic acid, and so forth.

[0090] Suitable alkenes include, e.g., octadecene, squalene and other C4-C30 hydrocarbons that are unsaturated.

[0091] Solvents can be used alone or in combination. TOP-TOPO solvent systems are commonly utilized in the art, as are other related (e.g., butyl) systems. For example, TOP and TOPO can be used in combination to form a cadmium solution, while TOP, alone, can be used to form a selenium solution (e.g., TOP + cadmium acetate, or TOP + cadmium nitrate).

[0092] Technical grade solvents can be used, and benefits can be obtained from the existence of beneficial impurities in such solvents, e.g. TOP, TOPO or both. In certain embodiments, the solvent comprises at least one coordinating solvent. In one preferred embodiment, the solvent is pure. Typically, this means that the solvent contains less than 10 vol %, and more preferably less than 5 vol % of impurities that can function as electron transfer agents. Therefore, solvents such as TOPO at 90% or 97% purity and TOP at 90% purity are particularly well suited for use in the methods disclosed herein, and solvents that are greater than 99% pure are preferred. [0093] The presence of minor amounts of impurities can provide unexpected sources of electron transfer agents, and these can defeat the objectives of the embodiments disclosed herein if they promote nucleation of the mismatched shell precursors. Moreover, a particular reagent may be a weak reducing/oxidizing agent for one system and a strong reducing/oxidizing agent, or an ineffective reducing/oxidizing agent, in a different system: weak and strong are necessarily dependent upon the specific shell precursors being used for the nanocrystal core-forming reaction, as well as the solvent and temperature being used.

[0094] For example, in some systems, an unsaturated bond provided by the solvent or one of the precursors can be a weak reducing/oxidizing agent, as discussed herein; in other systems, it might be ineffective as a weak reducing/oxidizing agent, and a weak reducing/oxidizing agent would be added to promote growth of the nanocrystal, even if that unsaturated bond were present, such as in a metal salt containing an unsaturated group like oleate.

[0095] In order to accurately control the two phases of nanocrystal formation, it is thus sometimes desirable to identify and account for the presence of any strong or weak reducing/oxidizing agents in the solvents and reagents used for these methods. Therefore, in some embodiments, a reagent to be used in the methods described herein is evaluated for its effect in the particular system. The suitability of a reagent, solvent, reducing agent, or precursor for the present methods can be determined by testing it to see whether that substance functions as, or contains impurities that function as, a strong reducing/oxidizing agent in the system of interest. Where a reagent functions as or contains impurities that function as a strong reducing/oxidizing agent, the reagent would be removed, replaced, or further purified for the methods disclosed herein.

Ligands

[096] In one preferred embodiment, ligands are included in the reaction. Ligands are compounds that complex with a precursor and/or a nanoparticle. Suitable ligands include, by way of illustration and not limitation, phospho-acids such as hexylphosphonic acid and tetradecylphosphonic acid (TDPA), octylphosphonic acid (OPA), carboxylic acids such as isomers of octadecanoic acid, amines, amides, alcohols, and ethers. In some cases, the ligand and the solvent can be the same.

Electron Transfer Agent: Reductants

[097] In the embodiments disclosed herein, control of the nucleation and growth phases of particle formation may be achieved by the use of mismatched precursors which cannot react without addition or loss of electrons by an electron transfer agent. The use of two separate electron transfer agents, in particular one or more reductants, makes it possible to independently promote nucleation or growth to a desired extent, and to improve temporal separation of these two formation phases. This approach allows independent control over particle yield and particle size, and also potentially produces particles with a more narrow size distribution.

[098] As used herein, a "strong" or "stronger" reductant (reducing agent) refers to a reductant that is capable of promoting nucleation, or initiation of particle formation, under the specific conditions of the reaction in which it is employed. A "weak" or "weaker" reductant refers to a reducing agent that is not capable of promoting substantial nucleation or initiation of particle formation under the specific conditions employed, but may be capable of promoting particle growth under those conditions.

[099] It will be understood by one of skill in the art that whether a particular reductant is a strong reductant or weak reductant is context specific, and will depend upon the particular reaction conditions in which it is employed. Electron transfer can proceed more readily at the surface of a growing particle (i.e. during the growth phase) than on a free ion in solution, as required for nucleation. Trial reductants can be categorized as strong or weak for a given system (nanocrystal- forming reaction) by determining whether the trial reductant behaves as a strong or weak reducing agent in those conditions. One can determine if a particular trial reducing agent is a strong reducing agent by contacting the particular precursors of interest with the trial reducing agent under the appropriate reaction conditions in the absence of any primary nanocrystal (added nanocrystal in the initial reaction mixture), so nucleation can be observed if it occurs: generally, if nucleation occurs at a significant rate, e.g., at a rate that is at least about 50% higher than the rate of nucleation in the absence of the trial reducing agent, the trial reducing agent is promoting nucleation and can be considered a strong reducing agent in that system. If the rate of nucleation is not significantly increased by the presence of the trial reducing agent, it is not a strong reducing agent in that system.

[0100] One can determine if a particular trial reducing agent is a weak reducing agent by contacting the particular precursors of interest with the trial reducing agent under shell-forming reaction conditions in the presence of a primary (added) nanocrystal of the same type as that formed by the precursors: generally, if nanocrystal growth occurs at an increased rate, e.g., at a rate that is at least twice as high as the rate of growth in the absence of the trial reducing agent, the trial reducing agent can be considered a reducing agent suitable for promoting nanocrystal growth. It thus may be a suitable weak reducing agent, provided it does not function as a strong reducing agent in the system of interest. [0101] A trial reducing agent can be considered a weak reducing agent if it is suitable for promoting nanocrystal growth by the test above, but is not a strong reducing agent in the particular system of interest. Because the relative strength of a reducing agent is dependent upon these factors, this functional categorization of reducing agents is a useful method to categorize a weak or strong reducing agent that can be applied to any particular trial reductant by routine testing.

[0102] Because electron transfer can proceed more readily at the surface of a growing particle (i.e. during the growth phase) than on a free ion in solution, as required for nucleation, a stronger reductant can be required for nucleation than for growth. This difference in reactivity allows the two phases to be separated by providing a small amount of a strong electron transfer agent, which can be rapidly consumed during nucleation, and a larger amount of a weaker electron transfer agent, which can enable continuing growth. The core nanocrystal size can be determined readily during the growth phase by monitoring the fluorescence wavelength.

[0103] In the embodiments provided herein, the extent of nucleation can be regulated by controlling the amount of the strong reductant used in the reaction. In some embodiments, a strong reductant can be added in an amount sufficient to promote a desired amount of nucleation. In other embodiments, the strong reductant can be added in a sub-stoichiometric amount relative to the precursor(s) to be reduced. In some such embodiments, the amount of strong reductant added can be less than about one-tenth, about one-tenth, less than about two-tenths, less than about three- tenths, or less than about four- tenths of what is needed for a stoichiometric reaction of the nanocrystal precursors. In some specific embodiments, the amount of strong reductant added can be about one-tenth of what is needed for a stoichiometric reaction.

[0104] Once the strong reductant has been consumed, which can occur rapidly at an appropriate reaction temperature, additional formation of small nuclei essentially ceases. Thus, limiting the amount of the strong reducing agent allows separation of the nucleation phase from the growth phase. This approach can allow independent control over particle yield and particle size, and also can potentially produce particles with a more narrow size distribution. Because the precursors remain mismatched, continued growth of the nanocrystal nuclei requires addition of a second electron transfer agent. Addition of a weak reductant allows particle growth, but does not promote further nucleation, thus providing particles of uniform size.

[0105] Because control of the reaction can achieved by use of electron transfer agents, it is not essential to carefully control rates of addition of precursors into the reaction, as has been done in some nanocrystal preparation methods. The nanocrystal precursors can be added in one portion, as quickly as desired without causing too much cooling of the reaction mixture; slow addition of a precursor to prevent undesired formation of new nuclei is not necessary in many embodiments of these methods. Indeed, the precursors and optionally the weak reducing agent can be combined in a suitable solvent and heated to the desired reaction condition without producing nanocrystal formation, which is initiated by addition of the strong reducing agent.

[0106] In preferred embodiments, the strong reductant can be added at an operating temperature sufficient for nucleation to occur. At such a temperature, it is believed that a rapid burst of nucleation occurs upon addition of the strong reductant, thereby rapidly consuming the reductant. Under these conditions, all core nanocrystals form at roughly the same time and then all grow together for the same amount of time to result in a uniform distribution of particle sizes, providing a monodisperse particle population.

[0107] It will be understood by one of skill in the art that whether a particular reductant is a strong reductant or weak reductant depends on the particular reaction conditions in which the reductant is employed.

[0108] Suitable reducing agents can include, by way of illustration and not limitation, chemical compounds such as tertiary phosphines, secondary phosphines, primary phosphines (e.g., diphenylphosphine, dicyclohexylphosphine, and dioctylphosphine); amines (e.g., decyl- and hexadecylamine); hydrazines; hydroxyphenyl compounds (e.g., hydroquinone and phenol); hydrogen; hydrides (e.g., sodium borohydride, lithium triethyl borohydride, sodium hydride and lithium aluminum hydride, and the like); metals (e.g., mercury and potassium); boranes (e.g., THF:BH₃ and B₂H₆); aldehydes (e.g., benzaldehyde and butyraldehyde); alcohols and thiols (e.g., ethanol and thioethanol); reducing halides(e.g., I^" and I₃ ^"); alkenes (e.g., oleic acid); alkynes; and polyfunctional reductants, i.e., a single chemical species that contains more than one reductant moiety, each reductant moiety having the same or different reducing capacity, such as tris- (hydroxypropyl)phosphine and ethanolamine); and so forth.

[0109] Typically, hydrides (metal hydrides like aluminum hydrides or metal borohydrides) and boranes function as strong reductants. Other reducing agents can function as either a strong reductant or a weak reductant, depending on the specific reaction conditions. For example, an alkylphosphine can function as a strong reductant in a synthesis of CdSe, but would be a weak reductant in a synthesis of ZnTe. Still other reductants, such as alkenes, alkynes, amines, and the like are typically weak reducing agents.

[0110] In some embodiments, the weak reductant can be provided by a component of one of the precursors. For example, an unsaturated carboxylate group, such as an oleate, can serve as a weak reductant for purposes of the embodiments disclosed herein. Figure 2 depicts a reaction wherein a Zn²⁺ species is reacting with a mismatched tellurium precursor, TOPTe. In the first graph on the left, the Zn²⁺ salt is a saturated salt, so no weak reductant was present. In the second graph, on the right, the salt includes an unsaturated carboxylic acid group, which provides a reductant. The particle yield was low for both reactions, indicating the need for a strong reductant to promote efficient nucleation; however, the reaction with the unsaturated carboxylate salt serving as a weak reducing agent clearly provides faster nanocrystal formation.

[0111] In certain embodiments, to provide a weak reductant to promote nanocrystal growth, the metal-containing precursor can sometimes be provided as a metal carboxylate salt of the form M(O₂C-R' )_n, where M is the metal, n is an integer from 1-3 that is determined by the oxidation state of the metal atom, and R' is a C₄-C₁00 unsaturated hydrocarbon group. In other embodiments, the salt can comprise one such unsaturated carboxylate counterion and one or more other counterions, for example halide ions. These provide convenient ways to provide a weak reductant without adding additional materials to the reaction mixture, and ensure that the reaction stoichiometry provides at least one weak reductant molecule per precursor atom. However, in some systems it may be necessary to determine that these function as weak and not as strong reductants.

[0112] In other embodiments, a solvent, such as an alkene, alkyne, or amine solvent, can function as the weak reducing agent. This approach may be particularly useful where the use of a large excess of the weak reducing agent is desirable. However, in some systems it may be necessary to determine that the solvent functions as a weak and not as a strong reductant.

[0113] It is expected that there may be particular advantages associated with the use of an electrochemical system (cathode-anode system) as the reducing agent, i.e., the cathode would serve as a source of electrons. By utilizing an electrode as a source of reducing equivalents, coulombic equivalents can be readily counted and their rate of delivery directly controlled. Use of electrodes also allows for controlling both the physical localization of reduction events, as well as the potential for direct formation of particle arrays at the electrode surface. Since the cathode will be positioned within the reaction chamber, the material selection is preferably one that will not react with the precursors, ligands or coordinating solvents. The anode will typically be positioned outside of the reaction vessel so material selection is not limited and any well known anode material can be used. Exemplary cathode materials include platinum, silver, or carbon. An exemplary method for delivering reducing equivalents to the cathode includes the use of a constant current or potentiostat in a two-electrode (working and counter) or three-electrode (working, counter, and reference) configuration.

[0114] The selection of suitable reductants for a particular combination of precursors is within the level of skill in the art. Electron Transfer Agent: Oxidants

[0115] In the embodiments disclosed herein, control of the nucleation and growth phases of particle formation may be achieved by the use of mismatched precursors which cannot react without addition or loss of electrons by an electron transfer agent. The use of two separate electron transfer agents, in particular one or more oxidants, makes it possible to independently promote nucleation or growth to a desired extent, and to improve temporal separation of these two formation phases. This approach allows independent control over particle yield and particle size, and also potentially produces particles with a more narrow size distribution.

[0116] As used herein, a "strong" or "stronger" oxidant (oxidizing agent) refers to a oxidant that is capable of promoting nucleation, or initiation of particle formation, under the specific conditions of the reaction in which it is employed. A "weak" or "weaker" oxidant refers to a oxidizing agent that is not capable of promoting nucleation or initiation of particle formation under the specific conditions employed, but may be capable of promoting particle growth under those conditions.

[0117] It will be understood by one of skill in the art that whether a particular oxidant is a strong oxidant or weak oxidant is context specific, and will depend upon the particular reaction conditions in which it is employed. Electron transfer often proceeds more readily at the surface of a growing particle (i.e. during the growth phase) than on a free ion in solution, as required for nucleation. Trial oxidants can be categorized as strong or weak for a given system (nanocrystal- forming reaction) by determining whether the trial oxidant behaves as a strong or weak oxidizing agent in those conditions. One can determine if a particular trial oxidizing agent is a strong oxidizing agent by contacting the particular precursors of interest with the trial oxidizing agent under the appropriate reaction conditions in the absence of any primary nanocrystal (added nanocrystal in the initial reaction mixture), so nucleation can be observed if it occurs: generally, if nucleation occurs at a significant rate, e.g., at a rate that is at least about 50% higher than the rate of nucleation in the absence of the trial oxidizing agent, the trial oxidizing agent is promoting nucleation and can be considered a strong oxidizing agent in that system. If the rate of nucleation is not significantly increased by the presence of the trial oxidizing agent, it is not a strong oxidizing agent in that system.

[0118] One can determine if a particular trial oxidizing agent is a weak oxidizing agent by contacting the particular precursors of interest with the trial oxidizing agent under shell-forming reaction conditions in the presence of a primary (added) nanocrystal of the same type as that formed by the precursors: generally, if nanocrystal growth occurs at an increased rate, e.g., at a rate that is at least twice as high as the rate of growth in the absence of the trial oxidizing agent, the trial oxidizing agent can be a oxidizing agent suitable for promoting nanocrystal growth. It thus can be a suitable weak oxidizing agent, provided it does not function as a strong oxidizing agent in the system of interest.

[0119] A trial oxidizing agent can be a weak oxidizing agent if it is suitable for promoting nanocrystal growth by the test above, but is not a strong oxidizing agent in the particular system of interest. Because the relative strength of a oxidizing agent is dependent upon these factors, this functional categorization of oxidizing agents is a useful method to categorize a weak or strong oxidizing agent that can be applied to any particular trial oxidant by routine testing.

[0120] Because electron transfer often proceeds more readily at the surface of a growing particle (i.e. during the growth phase) than on a free ion in solution, as required for nucleation, a stronger oxidant is required for nucleation than for growth. This difference in reactivity allows the two phases to be separated by providing a small amount of a strong electron transfer agent, which is rapidly consumed during nucleation, and a larger amount of a weaker electron transfer agent, which enables continuing growth. The core nanocrystal size can be determined readily during the growth phase by monitoring the fluorescence wavelength.

[0121] In the methods provided herein, the extent of nucleation can be regulated by controlling the amount of the strong oxidant used in the reaction. In some embodiments, a strong oxidant is added in an amount sufficient to promote a desired amount of nucleation. In preferred embodiments, the strong oxidant is added in a sub- stoichiometric amount relative to the precursor(s) to be reduced. In some such embodiments, the amount of strong oxidant added can be less than about one-tenth, about one-tenth, less than about two-tenths, less than about three-tenths, or less than about four- tenths of what is needed for a stoichiometric reaction of the nanocrystal precursors. In specific embodiments, the amount of strong oxidant added can be about one-tenth of what is needed for a stoichiometric reaction.

[0122] Once the strong oxidant has been consumed, which occurs rapidly at an appropriate reaction temperature, additional formation of small nuclei essentially ceases. Thus, limiting the amount of the strong oxidizing agent allows separation of the nucleation phase from the growth phase. This approach allows independent control over particle yield and particle size, and also potentially produces particles with a more narrow size distribution. Because the precursors remain mismatched, continued growth of the nanocrystal nuclei requires addition of a second electron transfer agent. Addition of a weak oxidant allows particle growth, but does not promote further nucleation, thus providing particles of uniform size.

[0123] Because control of the reaction can be achieved by use of electron transfer agents, typically oxidizing agents, it is not essential to carefully control rates of addition of precursors into the reaction, as has been done in some nanocrystal preparation methods. The nanocrystal precursors can be added in one portion, as quickly as desired without causing too much cooling of the reaction mixture; slow addition of a precursor to prevent undesired formation of new nuclei is not necessary in many embodiments of these methods. Indeed, the precursors and optionally the weak oxidizing agent can be combined in a suitable solvent and heated to the desired reaction condition without producing nanocrystal formation, which is initiated by addition of the strong oxidizing agent.

[0124] In preferred embodiments, the strong oxidant can be added at an operating temperature sufficient for nucleation to occur. At such a temperature, it is believed that a rapid burst of nucleation occurs upon addition of the strong oxidant, thereby rapidly consuming the oxidant. Under these conditions, all core nanocrystals can form at roughly the same time and then all grow together for about the same amount of time to result in a uniform distribution of particle sizes, providing a monodisperse particle population.

[0125] It will be understood by one of skill in the art that whether a particular oxidant is a strong oxidant or weak oxidant depends on the particular reaction conditions in which the oxidant is employed. Suitable oxidizing agents can include, by way of illustration and not limitation, chemical compounds such as: potassium nitrate; salts of hypochlorite, chlorite, chlorate, perchlorate and other analogous halogen compounds; tert-butyl hypochlorite; halogens such as fluorine, chlorine, bromine, and iodine; permanganate salts and compounds; cerium ammonium nitrate; hexavalent chromium compounds such as chromic and dichromic acids and chromium trioxide, pyridinium chlorochromate (PCC), and chromate/dichromate compounds; peroxide compounds; Tollens' reagent; sulfoxides; persulfuric acid; oxygen; ozone; osmium tetroxide; nitric acid; nitrous oxide; silver (I) compounds; copper (II) compounds; molybdenum (IV) compounds; iron (III) compounds; manganese (IV) compounds; N-Methylmorpholine-N-Oxide and other N-oxides; trimethylamine N-oxide; 3- chloroperoxybenzoic acid, and other peroxy acids; or peracetic acid.

[0126] In some embodiments, the weak oxidant is provided by a component of one of the precursors.

[0127] It is expected that there may be particular advantages associated with the use of an electrochemical system (cathode-anode system) as the oxidizing agent, i.e., the cathode would serve as a source of electrons. By utilizing an electrode as a source of oxidizing equivalents, coulombic equivalents can be readily counted and their rate of delivery directly controlled. Use of electrodes also allows for controlling both the physical localization of oxidation events, as well as the potential for direct formation of particle arrays at the electrode surface. Since the cathode will be positioned within the reaction chamber, the material selection is preferably one that will not react with the precursors, ligands or coordinating solvents. The anode will typically be positioned outside of the reaction vessel so material selection is not limited and any well known anode material can be used. Exemplary cathode materials include platinum, silver, or carbon. An exemplary method for delivering oxidizing equivalents to the cathode includes the use of a constant current or potentiostat in a two-electrode (working and counter) or three-electrode (working, counter, and reference) configuration.

Methods of Producing Nanocrystals Using Strong and/or Weak Electron Transfer Agents

[0128] Provided herein are methods of producing nanocrystals, using mismatched precursors in the presence of added electron transfer agents. In some embodiments, two different electron transfer agents are used to separately control the nucleation and growth phases of particle formation.

[0129] In one aspect, provided herein is a method of producing a nanocrystal or population thereof, the method comprising: (a) providing a mixture comprising a first precursor, a second precursor, a first (i.e., strong) electron transfer agent (for example, in an amount sufficient to form the desired level of nucleation), a second (i.e., weak) electron transfer agent (for example, in an amount sufficient to form the desired level of nanocrystal growth), and optionally a solvent (such as a coordinating solvent); and (b) heating the mixture to a temperature for a period of time sufficient to induce formation of nanocrystals.

[0130] In some embodiments, the nanocrystal formation reaction occurs in a continuous flow reactor system. In other embodiments, the nanocrystal formation reaction occurs in a batch reactor system.

[0131] In certain embodiments, the first and the second electron transfer agents are oxidants. In other embodiments, the first and the second electron transfer agents are reductants. In some embodiments the first electron transfer agent is an oxidant and the second electron transfer agent is a reductant, or vice versa.

[0132] In some embodiments, the oxidation states of the first precursor or the second precursor is changed to a neutral state by the strong and weak electron transfer agents. In other embodiments, the oxidation states of the first precursor and the second precursor are matched by the strong and weak electron transfer agents.

[0133] In certain embodiments, the method further comprises step (c), cooling the mixture to stop further growth of the nanocrystals or diluting the mixture to stop further growth of the nanocrystals. In some embodiments, the method further comprises a step of isolating the nanocrystals produced by the method. In other embodiments, the method further comprises a step of adding a shell to the nanocrystals, either with or without isolation.

[0134] The components of the reaction mixture (i.e., a first precursor, a second precursor, a first reductant, and a second reductant) can be added in any order, optionally in a solvent or mixture of solvents, and the reaction can be heated prior to and/or during addition of one or more of the components of the mixture. The precursors are frequently combined with an appropriate solvent or mixture of solvents to form a solution for use in the methods disclosed herein. The solvents for the first precursor and second precursor may be the same or different.

[0135] In some embodiments, a mixture comprising a first precursor, a second precursor, a first electron transfer agent, a second electron transfer agent, and optionally a solvent, is formed and the mixture is then heated to a temperature for a period of time sufficient to induce formation of nanocrystals.

[0136] In frequent embodiments, the first electron transfer agent is a strong electron transfer agent and the second electron transfer agent is a weak electron transfer agent under the reaction conditions employed, as further described herein.

[0137] In other embodiments, a mixture comprising a first precursor, a second precursor, a weak electron transfer agent, and optionally a solvent, is heated; a strong electron transfer agent is added in an amount sufficient to promote nucleation to a desired extent, and the reaction mixture is heated at a temperature and for a time sufficient to induce formation of nanocrystals.

[0138] In further embodiments, a mixture comprising a first precursor, a weak electron transfer agent, and optionally a first solvent, is heated; a second precursor, optionally in a second solvent (which may be the same or different from the first solvent) is added to the heated mixture; then a strong electron transfer agent is added in an amount sufficient to promote nucleation, and heating is continued at a temperature and for a time sufficient to induce formation of nanocrystals.

[0139] In other embodiments, a mixture comprising a first precursor, a second precursor, a strong electron transfer agent, and optionally a solvent, is heated at a temperature sufficient to promote formation of nucleation crystals; then a weak electron transfer agent is added to the mixture to promote particle growth, and the reaction mixture is heated further at a temperature and for a time sufficient to induce formation of nanocrystals.

[0140] In still other embodiments, a mixture comprising a first precursor, a second precursor, and optionally a solvent, is heated to a temperature sufficient for nucleation to occur in the presence of a strong electron transfer agent, then a strong electron transfer agent and a weak electron transfer agent are added simultaneously to the heated mixture, followed by continued heating at a temperature for a time sufficient to induce formation of nanocrystals. [0141] In preferred embodiments, the first electron transfer agent and the second electron transfer agent are different. In particularly preferred embodiments, the first electron transfer agent is a strong oxidant/reductant and the second electron transfer agent is a weak oxidant/reductant, as further described herein. The first electron transfer agent and the second electron transfer agent may independently be a chemical oxidant/reductant or a cathode.

[0142] In another aspect, a method of producing a nanocrystal or population thereof, is provided; the method comprising: providing a mixture comprising a first precursor and a second precursor, wherein the first precursor and the second precursor have mismatched oxidation states; adding a sub- stoichiometric amount of a strong electron transfer agent to the mixture in an amount sufficient to produce a desired amount of nucleation; optionally heating together the mixture to produce the desired amount of nucleation; adding a weak electron transfer agent to the mixture in an amount sufficient to produce a desired amount of nanocrystal growth; and optionally heating the mixture for a period of time sufficient to produce the desired amount of nanocrystal growth.

[0143] In certain embodiments, the strong and the weak electron transfer agents are oxidants. In other embodiments, the strong and the weak electron transfer agents are reductants. In some embodiments the strong electron transfer agent is an oxidant and the strong electron transfer agent is a reductant, or vice versa.

[0144] In certain embodiments, the strong electron transfer agent is provided in an amount sufficient to form the desired level of nucleation. In certain embodiments, the weak electron transfer agent is provided in an amount sufficient to form the desired level of nanocrystal growth.

[0145] In some embodiments, the oxidation states of the first precursor or the second precursor is changed to a neutral state by the strong and weak electron transfer agents. In other embodiments, the oxidation states of the first precursor and the second precursor are matched by the strong and weak electron transfer agents.

[0146] In another aspect, a method of producing a core nanocrystal or population thereof, is provided; the method comprising:

[0147] (a) providing a first mixture comprising a first precursor, a second precursor, and optionally a solvent; (b) heating the first mixture to a temperature that is sufficiently high to promote nucleation in the presence of a strong electron transfer agent; (c) adding a strong electron transfer agent to provide a second mixture, wherein the strong electron transfer agent is added in an amount sufficient to promote nucleation; and (d) heating the second mixture to a temperature for a period of time sufficient to induce formation of nanocrystals.

[0148] In some embodiments, the method further comprises step (e), cooling the second mixture to stop further growth of the nanocrystals, or diluting the reaction mixture to stop further growth. Optionally, the method may further comprise a step of isolating the nanocrystals from the reaction mixture. The method may also optionally comprise a step of adding a shell to the core nanocrystals from the reaction mixture or to isolated core nanocrystals.

[0149] In preferred embodiments, the first mixture is maintained at a temperature sufficiently high to promote nucleation during the addition of the strong electron transfer agent.

[0150] In preferred embodiments, the method further comprises the addition of a weak electron transfer agent, wherein the weak electron transfer agent is added before, simultaneously with, or after addition of the strong electron transfer agent.

[0151] In some embodiments, the first mixture further comprises a weak electron transfer agent. In some such embodiments, the weak electron transfer agent is provided by the solvent or by an unsaturated group present on one of the precursors.

[0152] In other embodiments, step (c) further comprises addition of a weak electron transfer agent before or simultaneously with addition of the strong electron transfer agent. In some such embodiments, the weak electron transfer agent is added simultaneously and separately from the strong electron transfer agent.

[0153] In further embodiments, step (c) further comprises addition of a weak electron transfer agent after addition of the strong electron transfer agent. In some such embodiments, the weak electron transfer agent is added following a period of time sufficient to allow formation of nucleation crystals. In other embodiments, the weak electron transfer agent is added after the strong electron transfer agent but prior to the completion of the nucleation phase.

[0154] In certain embodiments, the strong and the weak electron transfer agents are oxidants. In other embodiments, the strong and the weak electron transfer agents are reductants. In some embodiments the strong electron transfer agent is an oxidant and the strong electron transfer agent is a reductant, or vice versa.

[0155] In some embodiments, the oxidation states of the first precursor or the second precursor is changed to a neutral state by the strong and weak electron transfer agents. In other embodiments, the oxidation states of the first precursor and the second precursor are matched by the strong and weak electron transfer agents.

[0156] In yet another aspect, provided herein is a method for producing a nanocrystal or population thereof, comprising: (a) providing a mixture comprising: (i) a first precursor; (ii) a second precursor, wherein the first precursor and the second precursor have mismatched oxidation states; (iii) a strong electron transfer agent; (iv) a weak electron transfer agent which is different than the strong electron transfer agent; and (v) optionally one or more solvents; and (b) heating the mixture to a temperature for a period of time sufficient to induce formation of nanocrystals. [0157] In a further aspect, a small amount of 'matched' precursors can be used along with a mismatched pair in a larger amount, suitable for supporting growth after nucleation has occurred. The 'matched' precursors are capable of reacting immediately, thus inducing nucleation in the absence of a strong electron transfer agent. Once the 'matched' precursor is consumed, the presence of a weak electron transfer agent in the reaction mixture can be used to support growth, without additional nucleation. For example, a small amount of a more reactive zinc precursor like diethyl zinc can be used with R₃P=Se, in combination with a larger amount of a Zn²⁺ precursor. The Zn⁰ precursor, e.g. diethyl zinc, can be used in sufficient quantity to induce a desired amount of nucleation. Once it has been consumed, which occurs quickly at an appropriate reaction temperature, the Zn²⁺ precursor is present in sufficient quantity for a desired extent of growth, to produce a desired nanocrystal size in the presence of a weak electron transfer agent. The nanocrystal size can be determined readily during the growth phase by monitoring the fluorescence wavelength.

[0158] In one aspect, provided herein is a method of producing a nanocrystal or population thereof, the method comprising: (a) providing a mixture comprising a first precursor, a second precursor, a third precursor, and optionally a solvent, wherein the first and second precursors have mismatched oxidation states, and wherein the third precursor has a matched oxidation state to the first precursor or the second precursor; and (b) heating the mixture to a temperature for a period of time sufficient to induce formation of nanocrystals.

[0159] In some embodiments, the mixture further comprises a weak electron transfer agent. In some such embodiments, the weak reducing agent is added before, simultaneously with, or after the addition of the third precursor.

[0160] In certain embodiments, the weak electron transfer agent is an oxidant. In other embodiments, the weak electron transfer agent is a reductant.

[0161] In some embodiments, the nanocrystal formation reaction occurs in a continuous flow reactor system. In other embodiments, the nanocrystal formation reaction occurs in a batch reactor system.

[0162] In another aspect, provided herein is a method of producing a core nanocrystal or population thereof, the method comprising: (a) providing a first mixture comprising a first precursor, a second precursor, and optionally a solvent, wherein the first and second precursors have mismatched oxidation states; (b) heating the first mixture to a temperature that is sufficiently high to promote nucleation in the presence of a third precursor, wherein the third precursor has a matched oxidation state with the first precursor or the second precursor; (c) adding a third precursor in an amount sufficient to promote nucleation; and (d) heating the second mixture to a temperature for a period of time sufficient to induce formation of nanocrystals.

[0163] In some embodiments of the methods described herein, the strong electron transfer agent is a chemical reductant selected from the group consisting of tertiary phosphines; secondary phosphines; primary phosphines; amines; hydrazines; hydroxyphenyl compounds; hydrogen; hydrides; metals; boranes; aldehydes; alcohols; thiols; reducing halides; and polyfunctional reductants.

[0164] In some embodiments of the methods described herein, the strong electron transfer agent is a chemical oxidant such as: potassium nitrate; salts of hypochlorite, chlorite, chlorate, perchlorate and other analogous halogen compounds; tert-butyl hypochlorite; halogens such as fluorine, chlorine, bromine, and iodine; permanganate salts and compounds; cerium ammonium nitrate; hexavalent chromium compounds such as chromic and dichromic acids and chromium trioxide, pyridinium chlorochromate (PCC), and chromate/dichromate compounds; peroxide compounds; Tollens' reagent; sulfoxides; persulfuric acid; oxygen; ozone; osmium tetroxide; nitric acid; nitrous oxide; silver (I) compounds; copper (II) compounds; molybdenum (IV) compounds; iron (III) compounds; manganese (IV) compounds; N-Methylmorpholine-N-Oxide and other N-oxides; trimethylamine N- oxide; 3-chloroperoxybenzoic acid, and other peroxy acids; or peracetic acid.

[0165] In other embodiments of the methods described herein, the strong oxidant/reductant is a cathode. In some such embodiments, the cathode is made of a material selected from the group consisting of platinum, silver, and carbon.

[0166] In frequent embodiments of the present methods, the strong oxidant/reductant is added in a sub- stoichiometric amount. In some such embodiments, the amount of strong oxidant/reductant added is less than about one-tenth, about one-tenth, less than about two-tenths, less than about three-tenths, or less than about four-tenths what is needed for a stoichiometric reaction. In preferred embodiments, the amount of strong oxidant/reductant added is less about one-tenth of what is needed for a stoichiometric reaction. In preferred embodiments, the strong oxidant/reductant is added at an operating temperature sufficient for nucleation to occur.

[0167] In certain embodiments of the present methods, the reaction mixture is heated to a temperature sufficient to promote nucleation, and is held at constant temperature while the strong oxidizing/reducing agent is added.

[0168] In some embodiments of the methods described herein, the weak electron transfer agent is a chemical reductant selected from the group consisting of tertiary phosphines; secondary phosphines; primary phosphines; amines; hydrazines; hydroxyphenyl compounds; hydrogen; hydrides; metals; boranes; aldehydes; alcohols; thiols; reducing halides; and polyfunctional reductants.

[0169] In some embodiments of the methods described herein, the weak electron transfer agent is a chemical oxidant such as: potassium nitrate; salts of hypochlorite, chlorite, chlorate, perchlorate and other analogous halogen compounds; tert-butyl hypochlorite; halogens such as fluorine, chlorine, bromine, and iodine; permanganate salts and compounds; cerium ammonium nitrate; hexavalent chromium compounds such as chromic and dichromic acids and chromium trioxide, pyridinium chlorochromate (PCC), and chromate/dichromate compounds; peroxide compounds; Tollens' reagent; sulfoxides; persulfuric acid; oxygen; ozone; osmium tetroxide; nitric acid; nitrous oxide; silver (I) compounds; copper (II) compounds; molybdenum (IV) compounds; iron (III) compounds; manganese (IV) compounds; N-Methylmorpholine-N-Oxide and other N-oxides; trimethylamine N- oxide; 3-chloroperoxybenzoic acid, and other peroxy acids; or peracetic acid.

[0170] In other embodiments of the methods described herein, the weak oxidant/reductant is a cathode. In some such embodiments, the cathode is made of a material selected from the group consisting of platinum, silver, and carbon.

[0171] In some embodiments of the methods provided herein, the solvent is selected from the group consisting of hydrocarbons, amines, alkyl phosphines, alkyl phosphine oxides, carboxylic acids, ethers, furans, phospho- acids, pyridines, and mixtures thereof. In some such embodiments, the solvent comprises a mixture of solvents. In frequent embodiments, the reaction mixture comprises at least one solvent, preferably at least one coordinating solvent.

[0172] In specific embodiments, solvent mixtures comprising an alkyl phosphine and an alkyl phosphine oxide, such as TOP/TOPO are used. In other embodiments, solvent mixtures comprising an amine, in particular a secondary amine, and an alkyl phosphine or alkyl phosphine oxide are used. For example, dioctylamine may be used in combination with TBP, TOP or TOPO. Examples of specific solvents include, for example, TOPO, TOP, tributylphosphine, decylamine, dioctylamine, oleylamine, octadecane, squalane, oleic acid, stearic acid, tetradecylphosphonic acid, and mixtures thereof.

[0173] In some embodiments, the first precursor comprises a metal atom, and the second precursor does not contain a metal atom. In specific embodiments, the first precursor can contribute a metal cation to core formation when in the heated reaction mixture. In some such embodiments, the first precursor can be a salt of Cd or Zn. In specific embodiments, the first precursor can be a halide, acetate, carboxylate, phosphonate, or oxide salt of Cd, Zn, In or Ga.

[0174] In some embodiments, the second precursor can contribute an uncharged non-metal atom for core formation when it is in the heated reaction mixture. In specific embodiments, the second precursor can be a group of the form R₃P=X, wherein X is S, Se or Te, and each R is independently H, or a C₁-C₁Oo hydrocarbon group. In specific embodiments, the second precursor is tri-n (butylphosphine)selenide (TBP=Se), tri-n-(octylphosphine)selenide (TOP=Se), (butylphosphine)sulfide (TBP=S), tri-n-(octylphosphine)sulfide (TOP=S), (butylphosphine)telluride (TBP=Te), or tri-n-(octylphosphine)telluride (TOP=Te).

[0175] In frequent embodiments, no other precursors are present besides the first precursor and the second precursor.

[0176] In certain embodiments, a third precursor is present besides the first precursor and the second precursor. In some such embodiments, the third precursor provides a reactive species which has a matched oxidation state with the first precursor or the second precursor. In some such embodiments, the third precursor provides a neutral metal species. For example, the third precursor may be a dialkylmetal precursor (e.g., Et₂Zn or Me₂Cd) that provides a neutral metal species, e.g., Zn⁰ or Cd⁰. In other embodiments, the third precursor may contribute a charged non- metal atom to core formation. For example, the third precursor may provide S^2" or Se^2".

[0177] The nanocrystal core can be made of any suitable metal and non-metal atoms that are known to form semiconductor nanocrystals, as described herein. In particular embodiments, the core can comprise CdSe, CdS, CdTe, InP, InAs, ZnS, ZnSe, ZnTe, GaP or a mixture thereof.

[0178] In a further aspect, provided herein is a core nanocrystal produced by one of the methods described herein.

[0179] In the methods described herein, the heating step(s) is typically conducted at a temperature that is sufficient to induce temporally discrete homogeneous nucleation, which results in the formation of a monodisperse population of individual nanocrystals. Typically, the heating step is conducted at a temperature within the range of about 150-350⁰C, more preferably within the range of about 220-350⁰C. In addition, the mixing and heating steps can be conducted in a vessel that is evacuated and filled and/or flushed with an inert gas such as nitrogen. The filling can be periodic or the filling can occur, followed by continuous flushing for a set period of time. In some embodiments, the mixing step can involve a cooling step prior to exposure to the first or second reductant, for example, cooling to a temperature within the range of about 50 to 150⁰C.

[0180] It is understood that the above ranges are merely exemplary and are not intended to be limiting in any manner as the actual temperature ranges may vary, dependent upon the relative stability of the reductants, precursors, ligands, and solvents. Higher or lower temperatures may be appropriate for a particular reaction. The determination of suitable time and temperature conditions for providing nanoparticles is within the level of skill in the art using routine experimentation. [0181] It can be advantageous to conduct the nanocrystal-forming reactions described herein with exclusion of oxygen and moisture. In some embodiments the reactions are conducted in an inert atmosphere, such as in a dry box. The solvents and reagents are also typically rigorously purified to remove moisture and oxygen and other impurities, and are generally handled and transferred using methods and apparatus designed to minimize exposure to moisture and/or oxygen. In addition, the mixing and heating steps can be conducted in a vessel that is evacuated and filled and/or flushed with an inert gas such as nitrogen. The filling can be periodic or the filling can occur, followed by continuous flushing for a set period of time. Purity of solvents and reagents is sufficient if they achieve the desired shell formation reaction and do not introduce strong reductants into the reaction.

[0182] The solvent for these reactions often comprises an amine, with hexadecylamine as one typical example, and dioctylamine as another suitable example. Amine solvents can be difficult to purify sufficiently for use with especially sensitive systems such as for ZnTe nanocrystal core reactions, because of the especially high sensitivity the reaction components and products exhibit toward moisture and air. While nanocrystal preparations are typically done with purified solvents and reagents, and under inert atmosphere, further special precautions and steps are taken for purifying amine solvents used to prepare ZnTe nanocrystals. The amine to be used as solvent for these reactions is placed in a flask which is repeatedly evacuated then filled with anhydrous inert atmosphere. Anhydrous NaOH or KOH, having been dried under vacuum at over 100⁰C, is then added to the amine solvent, and the suspension is stirred for at least 8 hours. The amine is filtered under inert atmosphere to remove the solids, and the amine is then distilled under inert atmosphere, and stored under an inert atmosphere.

[0183] The synthesis of ZnTe can be a particularly good example for implementing the methods disclosed herein, because both Zn²⁺ and Te⁰ can be quite difficult to reduce free in solution, rendering nucleation quite difficult without the addition of a strong reductant. In the absence of a strong electron transfer agent, no nucleation is observed (i.e., no particles form) in the mixture of ZnCl₂ and Bu₃P=Te. This may be due to mismatch between the oxidation states of the tellurium and zinc precursors being used: the zinc chloride is in a +2 oxidation state under the reaction conditions, but the tellurium precursor Bu₃P=Te provides Te⁰ under the reaction conditions. It is believed that the uncharged tellurium species cannot readily react with Zn⁺ , and that reduction of either Te⁰ to Te^2", or of Zn⁺² to Zn⁰ is required for reaction to occur. Addition of a strong electron transfer agent (i.e., a reductant) permits reaction to occur, probably by reducing Te⁰ to Te^2". The procedure provided in Example 1 uses lithium triethylborohydride (LiEt₃BH) as the strong nucleation-promoting reductant. Oleic acid serves as the weak reductant in this process, and permits continuing growth after the LiEt₃BH has been consumed, even when LiEt₃BH is added at considerably less than stoichiometric levels.

[0184] In addition to the enhanced control over particle size and yield, a further benefit of the present methods derives from the scale and manufacturability improvements inherent in the embodiments disclosed herein. Most current practices for zinc chalcogenide particle synthesis employ diethylzinc, a Zn⁰ source which evolves considerable amounts of gas, adding variability and limiting manufacturability. When zinc salts are employed, their resistance to reduction typically requires use of a gas-evolving strong reductant. The present methods can permit sub- stoichiometric use of such a reductant. In the example below, the amount of strong reductant added is one-tenth what is needed for a stoichiometric reaction. Moreover, as discussed above, the methods render the nanocrystal synthesis more reliable because it is less dependent upon things like rates of addition of nanocrystal precursors, since the reductants provide controlled reaction rates.

Methods of Using the Nanocrystals Provided Herein

[0185] The core nanocrystals made by the methods provided herein may be used to form core/shell nanocrystals, using conditions known to those of skill in the art.

[0186] In addition, nanocrystals made by these methods can be further modified by modifications of the ligands present on the nanocrystal surface as is known in the art. For example, the ligands on the surface of the nanocrystal can be exchanged for other ligands to introduce new properties such as water solubility to the nanocrystals. Methods for making nanocrystals with water- solubilizing ligand coatings are known in the art. For example, Adams, et al. provides methods to make water-soluble nanocrystals by applying a coating of amphipathic polymeric material to the surface of a hydrophobic nanocrystal. U.S. Patent No. 6,649,138. The methods start with a hydrophobic nanocrystal, such as one described herein having a coating of hydrophobic ligands, such as trialkyl phosphines, trialkyl phosphine oxides, alkylamines, or alkylphosphonic acids. To this is added an outer layer comprised of a multiply amphipathic dispersant molecule comprising at least two hydrophobic domains and at least two hydrophilic domains. In some embodiments, the amphiphilic polymer comprises an acrylic acid or methacrylic acid polymer having some acrylic acid groups converted into amides with hydrophobic amine groups, such as monoalkyl amines or dialkylamines having at least 4-12 carbons per alkyl group; and having some free carboxylic acid groups to promote water solubility. These and other suitable amphiphilic dispersants suitable for such use are described at columns 14-18 of Adams, which is incorporated herein by reference. [0187] Thus in one aspect, disclosed embodiments provide a nanocrystal as described herein with a coating of amphiphilic dispersant as described in Adams, et al. The nanocrystals are thereby rendered water-soluble, making them suitable for use in a variety of methods in which nanocrystals such as quantum dots are known to be used. The solubilized nanocrystals and methods of making them are disclosed herein.

[0188] Other methods for rendering nanocrystals are described by Naasani, et al., in U.S. Patent 6,955,855 and U.S. Patent No. 7,198,847. These methods involve coating the nanocrystal with small water- solubilizing ligands, such as imidazole-containing compounds like dipeptides. Suitable imidazole-containing compounds are described at column 7 of the '855 Naasani patent.

[0189] By the term "imidazole-containing compound" is meant, for purposes of the specification and claims to refer to a molecule that has at least one imidazole group (e.g., imidazole ring) available for binding a metal such as zinc or other metal cation, or substrate containing such cation. In that part respect, preferably at least one imidazole moiety is in a terminal position with respect to the structure of the molecule. Generally, imidazole ring nitrogens frequently serve as coordinating ligand to operably bind a metal ion such as zinc or cadmium. In one embodiment, the imidazole-containing compound comprises an amino acid, or two or more amino acids joined together (e.g., known in the art as "peptidyl" or "oligopeptide"), which may include, but is not limited to, histidine, carnosine, anserine, baleine, homocarnosine, 1-methylhistidine, 3- methythistidine, imidazolysine, imidazole-containing ornithine (e.g., 5-methylimidazolone), imidazole-containing alanine (e.g., (beta)-(2-imidazolyl)-L(alpba) alanine), carcinine, histamine, and the like. Imidazole-containing amino acids may be synthesized using methods known in the art (see, e.g., Stankova et al., 1999, J. Peptide Sci. 5:392-398, the disclosure of which is herein incorporated by reference).

[0190] By the term "amino acid" is meant, as known in the art and for purposes of the specification and claims, to refer to a compound containing at least one amino group and at least one carboxyl group. As known in the art, an amino group may occur at the position adjacent to a carboxyl group, or may occur at any location along the amino acid molecule. In addition to at least one imidazole moiety, the amino acid may further comprise one or more additional reactive functionalities (e.g., amino, thiol, carboxyl, carboxamide, etc.). The amino acid may be a naturally occurring amino acid, a synthetic amino acid, a modified amino acid, an amino acid derivative, an amino acid precursor, in D (dextro) form, or in L (levo) form. Examples of derivatives may include, but is not limited to, an N-methylated derivative, amide, or ester, as known in the art, and where consistent with the functions of the amino acid as a coating as described herein (e.g., imparts water-solubility, buffers sufficiently in a pH range between about pH 6 and about pH 10, functions as a coat which can increase fluorescence intensity, and has one or more reactive functionalities that may be used to operably bind molecular probe). An amino acid of the aforementioned amino acids may be used in a preferred embodiment, and a preferred amino acid may be used separately in the composition of the disclosed embodiments to the exclusion of amino acids other than the preferred amino acid. Histidine is a particularly preferred imidazole-containing compound for coating the functionalized, fluorescent nanocrystals.

[0191] Ligands on the nanocrystals disclosed herein can also be cross-linked to increase the stability of the nanocrystal composition and improve its characteristics. The surface coating of ligands on the nanocrystals disclosed herein can be cross-linked using methods described by Naasani, using various cross-linking agents. Preferred cross-linking agents for use in the disclosed embodiments include those described by Naasani, et al., including tris(hydroxymethyl)phosphine (THP) and tris(hydroxymethyl)phosphino-propionate (THPP). Nanocrystals having water- solubilizing ligand coatings that are cross-linked are thus another embodiment disclosed herein.

[0192] Nanocrystals made by these methods can be used in methods for tracking molecules that are known in the art. For example, they can be linked to various target molecules by known methods. Commonly, they are linked to an affinity molecule or used in further transformations. Such further transformations can be used to introduce onto the surface of a nanocrystal a selected target (or cargo) molecule of interest, such as an antibody or other specific affinity molecule. Methods for attaching such affinity molecules to a fluorescent carrier are known in the art and can readily be adapted for use in the present methods: see, e.g., U.S. Patent No. 6,423,551, which also describes some bi-functional agents that can be used to link the surface of a nanocrystal to a target molecule and to a nanocrystal surface. These methods can also be used to introduce a number of, or a layer of, functionalized molecules on the surface of a nanocrystal, where the functionalized molecules can provide new surface properties to the nanoparticle, such as water-dispersability. In some embodiments, nanocrystals modified for attachment of an affinity molecule that can be used to detect a desired target compound, cell or cellular organelle, are provided.

[0193] The modified nanocrystals can be linked to an affinity molecule for use in methods to track, identify, or localize molecules of interest that the affinity molecule can bind to, demonstrating that the molecule of interest is present and where it is distributed or localized. The nanocrystals can also be used in binding experiments to visualize distribution of molecules that the affinity molecule recognizes. Selection of a suitable affinity molecule is within the ordinary level of skill in the art once a target compound is identified; for example, conventional methods can be used to produce or identify an antibody suitable to specifically bind to a target molecule of interest. The antibody can thus be linked to the nanocrystals disclosed herein, which can then be used to identify the presence, location, or movements of the target compound, using the nanocrystal as a fluorescent label. In some embodiments, a method is provided to identify or track a target molecule, by linking a suitable affinity molecule that selectively binds to the target molecule to a nanocrystal, and permitting the nanocrystal linked to the affinity molecule to contact the target molecule. Tracking or detection can be achieved by using conventional methods for tracking a fluorescent labeled moiety, such as by use of a fluorescence imaging system, microscope or camera.

[0194] In some embodiments, a functionalized nanocrystal as described herein, is provided. The nanocrystal can be linked to an affinity molecule selected to bind specifically to a target molecule of interest. Optionally, the nanocrystal linked to the affinity molecule can be bound to the target molecule of interest to form a fluorescently labeled complex. Target molecules of interest include proteins, enzymes, receptors, nucleic acids, hormones, and cell surface antigens characteristic of specific types of cells.

[0195] Many of the techniques and procedures described or referenced herein are well understood and commonly employed using conventional methodology by those skilled in the art. As appropriate, procedures involving the use of commercially available kits and reagents are generally carried out in accordance with manufacturer defined protocols and/or parameters unless otherwise noted.

[0196] The discussion of the general methods given herein is intended for illustrative purposes only. Other alternative methods and embodiments will be apparent to those of skill in the art upon review of this disclosure.

[0197] A group of items linked with the conjunction "or" should not be read as requiring mutual exclusivity among that group, but rather should also be read as "and/or" unless expressly stated otherwise. Although items, elements, or components of the embodiments disclosed herein may be described or claimed in the singular, the plural is contemplated to be within the scope thereof unless limitation to the singular is explicitly stated.

[0198] All patents, patent applications, patent publications, journal articles and other references cited herein are each hereby incorporated by reference in their entireties.

[0199] As used in the claims and specification, the words "comprising" (and any form of comprising, such as "comprise" and "comprises and "comprised"), "having" (and any form of having, such as "have" and "has"), "including" (and any form of including, such as "includes" and "include"), or "containing" (and any form of containing, such as "contains" and "contain"), are inclusive or open-ended and do not exclude additional, unrecited elements or method steps. [0200] The following examples are provided as further guidance regarding the making and using of the methods disclosed herein, and are not to be construed as limiting the various embodiments of those methods.

Example 1 Formation of ZnTe Core Nanocrystals

[0201] All reagents employed were anhydrous and all manipulations were performed under an inert atmosphere, except as noted. Zinc chloride (685 mg, 5 mmol) was weighed into a 250 mL round bottom flask with two 14/20 joints and a ground glass stopcock. Dioctylamine (35 mL) and oleic acid (1.6 mL, 5 mmol) were added and the flask was equipped with a magnetic stir bar. The glass stopcock was greased and closed, one glass joint was plugged with a rubber septum, and one joint was equipped with an adaptor and a stainless steel thermocouple, which was connected to a temperature controller modulating a 180 W heating mantle. The flask was placed in the heating mantle and the stopcock was connected to a source of flowing nitrogen and opened. The temperature controller was set to heat the flask to 115⁰C and held at that temperature with gentle stirring until the zinc salt dissolved. The flask was then evacuated and refilled with nitrogen three times.

[0202] The temperature was subsequently increased to 230⁰C, whereupon 15 mL of a 1 M solution of tellurium in tributylphosphine was added by syringe through the septum, cooling the flask contents slightly. When the temperature returned to 220⁰C, 1 mL of a 1 M solution of lithium triethylborohydride in tetrahydrofuran was added rapidly via syringe. The temperature was increased to 240⁰C and held for between 15 sec and 30 min, until the desired particle size was reached, then rapidly cooled. What is claimed is:

Claims

1. A method for producing a population of nanocrystals, comprising:

providing a mixture comprising:

a first precursor;

a second precursor, wherein the first precursor and the second precursor have mismatched oxidation states;

a strong electron transfer agent in an amount sufficient to produce a desired amount of nucleation; and

a weak electron transfer agent which is different than the strong electron transfer agent; and

heating the mixture to a temperature for a period of time sufficient to induce formation of the population of nanocrystals.

2. The method for producing a population of nanocrystals, as recited in claim 1, wherein the nanocrystal formation reaction occurs in a batch reactor system.

3. The method for producing a population of nanocrystals, as recited in claim 1, wherein the nanocrystal formation reaction occurs in a continuous flow reactor system.

4. The method for producing a population of nanocrystals, as recited in claim 1, wherein the oxidation states of the first precursor or the second precursor is changed to a neutral state by the strong and weak electron transfer agents.

5. The method for producing a population of nanocrystals, as recited in claim 1, wherein the oxidation states of the first precursor and the second precursor are matched by the strong and weak electron transfer agents.

6. The method for producing a population of nanocrystals, as recited in claim 1, wherein the strong and weak electron transfer agents are reductants.

7. The method for producing a population of nanocrystals, as recited in claim 1, wherein the strong and weak electron transfer agents are oxidants.

8. The method for producing a population of nanocrystals, as recited in claim 1, wherein one or more of the precursors and electron transfer agents are heated before mixing.

9. The method for producing a population of nanocrystals, a recited in claim 6, wherein the strong and weak reductants are selected from the group consisting of tertiary phosphines, secondary phosphines, primary phosphines, amines, hydrazines, hydroxyphenyl compounds, hydrogen, hydrides, metals, boranes, aldehydes, alcohols, thiols, reducing halides, polyfunctional reductants, and mixtures thereof.

10. The method for producing a population of nanocrystals, a recited in claim 7, wherein the strong and weak oxidants are selected from the group consisting of potassium nitrate; salts of hypochlorite, chlorite, chlorate, perchlorate and other analogous halogen compounds; tert-butyl hypochlorite; halogens; permanganate salts and compounds; cerium ammonium nitrate; hexavalent chromium compounds; peroxide compounds; Tollens' reagent; sulfoxides; persulfuric acid; oxygen; ozone; osmium tetroxide; nitric acid; nitrous oxide; silver (I) compounds; copper (II) compounds; molybdenum (IV) compounds; iron (III) compounds; manganese (IV) compounds; N-Methylmorpholine-N- Oxide; trimethylamine N-oxide; 3-chloroperoxybenzoic acid, peroxy acids; peracetic acid, and mixtures thereof.

11. The method for producing a population of nanocrystals, as recited in claim 1 , wherein the strong reductant is a cathode.

12. The method for producing a population of nanocrystals, as recited in claim 1, wherein the weak reductant is a cathode.

13. The method for producing a population of nanocrystals, as recited in claim 1, further including a solvent in the mixture

14. The method for producing a population of nanocrystals, as recited in claim 13, wherein the solvent is selected from the group consisting of hydrocarbons, amines, alkyl phosphines, alkyl phosphine oxides, carboxylic acids, ethers, furans, phospho-acids, pyridines, and mixtures thereof.

15. The method for producing a population of nanocrystals, as recited in claim 13, wherein the solvent is a coordinating solvent.

16. The method for producing a population of nanocrystals, as recited in claim 1, further comprising cooling the mixture.

17. The method for producing a population of nanocrystals, as recited in claim 1, wherein the first precursor is selected from a group of salts consisting of Cd, Zn, Ga, In, Al, Pb, Ge, Si, Hg, Mg, Ca, Sr, Ba, and mixtures thereof.

18. The method for producing a population of nanocrystals, as recited in claim 1, wherein the second precursor is R₃P=X, wherein X is S, Se or Te, and each R is independently H, or a C₁-C₂₄ hydrocarbon group.

19. The method for producing a population of nanocrystals, as recited in claim 1, wherein the second precursor is S, Se, or Te dissolved in an alkylphosphine, an alkene, or an amine.

20. The method for producing a population of nanocrystals, as recited in claim 1, wherein the second precursor is dissolved in tributylphosphine or trioctylphosphine.

21. The method for producing a population of nanocrystals, as recited in claim 1, further comprising: optionally isolating the population of nanocrystals; and applying a shell to each of the population of nanocrystals.

22. The method for producing a population of nanocrystals, as recited in claim 21, wherein the shell material is selected from ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, MgS, MgSe, MgTe, CaS, CaSe, CaTe, SrS, SrSe, SrTe, BaS, BaSe, BaTe, and mixtures thereof.

23. The method for producing a population of nanocrystals, as recited in claim 21, wherein the shell material is selected from the group consisting of GaN, GaP, GaAs, GaSb, InP, InAs, InSb, AlS, AlP, AlSb, PbS, PbSe, Ge, Si, and mixtures thereof.

24. The method for producing a population of nanocrystals, as recited in claim 1, wherein the first or the second precursor can server as the weak electron transfer agent.

25. The method for producing a population of nanocrystals, as recited in claim 1, wherein the weak electron transfer agent is provided in an amount sufficient for the desired nanocrystal growth.

26. A method for producing nanocrystals comprising: providing a mixture comprising a first precursor and a second precursor, wherein the first precursor and the second precursor have mismatched oxidation states; adding a sub- stoichiometric amount of a strong electron transfer agent to the mixture in an amount sufficient to produce a desired amount of nucleation; optionally heating together the mixture to produce the desired amount of nucleation; adding a weak electron transfer agent to the mixture in an amount sufficient to produce a desired amount of nanocrystal growth; and optionally heating the mixture for a period of time sufficient to produce the desired amount of nanocrystal growth.

27. The method for producing nanocrystals, as recited in claim 26, wherein the nanocrystal formation reaction occurs in a batch reactor system.

28. The method for producing nanocrystals, as recited in claim 26, wherein the nanocrystal formation reaction occurs in a continuous flow reactor system.

29. The method for producing nanocrystals, as recited in claim 26, wherein the oxidation states of the first precursor or the second precursor is changed to a neutral state by the strong and weak electron transfer agents.

30. The method for producing nanocrystals, as recited in claim 26, wherein the oxidation states of the first precursor and the second precursor are matched by the strong and weak electron transfer agents.

31. The method for producing nanocrystals, as recited in claim 26, wherein the strong and weak electron transfer agents are reductants.

32. The method for producing nanocrystals, as recited in claim 26, wherein the strong and weak electron transfer agents are oxidants.

33. A method of producing nanocrystals comprising:

providing a mixture comprising a first precursor, a second precursor, and a third precursor, wherein the first and second precursors have mismatched oxidation states, and wherein the third precursor has a matched oxidation state to the first precursor or the second precursor; and

optionally heating the mixture to a temperature for a period of time sufficient to induce formation of nanocrystals.

34. The method of producing nanocyrstals, as recited in claim 33, wherein the mixture further comprises a weak electron transfer agent.

35. The method of producing nanocrystals, as recited in claim 34, wherein the weak electron transfer agent is a reducing agent.

36. The method of producing nanocrystals, as recited in claim 34, wherein the weak electron transfer agent is an oxidizing agent.

37. The method of producing nanocrystals, as recited in claim 33, wherein the third precursor is added in an amount sufficient to promote the desired amount of nucleation.

38. The method of producing nanocrystals, as recited in claim 34, wherein the weak electron transfer agent is added in an amount sufficient to promote the desired amount of nanocrystal growth.

39. The method of producing nanocrystals, as recited in claim 33, further including:

forming a pre-mixture containing the first precursor and the second precursor prior to the addition of the third precursor; and

optionally heating the pre-mixture prior to the addition of the third precursor.