WO2009026396A1 - Methods for preparing semiconductor nanoparticles - Google Patents

Abstract

Methods for the synthesis of semiconductor nanocrystals coated with an inorganic shell are described. The semiconductor nanocrystals exhibit reduced photoluminescent intermittency known as blinking.

Description

METHODS FOR PREPARING SEMICONDUCTOR NANOPARTICLES

RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Application No. 60/956,893, filed August 20, 2007. This application is expressly incorporated by reference in its entirety.

TECHNICAL FIELD

[0002] The invention relates to methods for the synthesis of semiconductor nanocrystals and, more specifically, methods for the synthesis of improved inorganic shells on semiconductor nanocrystals are disclosed which result in the reduction or elimination of intermittent photoluminescence know as 'blinking'.

BACKGROUND OF THE INVENTION

[0003] Colloidal semiconductor nanocrystals, or quantum dots, can exhibit bright, long-lived fluorescence, with an emission wavelength that is simply tuned by changing the size of the nanocrystal. This property makes them useful as biological labels and as the active medium in optoelectronic devices such as light-emitting diodes and lasers. Recent developments have yielded a wide range of uses of colloidal quantum dots in such various areas of biotechnology as cell labeling, cell tracking, in vivo imaging and DNA detection to name a few.

[0004] Optical microscopy of single immobilized quantum dots has revealed a phenomenon known as 'blinking' where the quantum dots alternate between "on" (fluorescing) and "off (non- fluorescing) states. This behavior is largely independent of other photostability characteristics such as initial quantum yield, loss of quantum yield (bleaching), and 'dilution dimming' associated with loss of photoluminescence at very low concentrations. It has been a limiting factor to their use in some technologies where observation of single nanocrystal labels is critical.

[0005] In one of the earliest published descriptions of blinking, Nirmal et al. (Nature, 383, (1996), 802-804) describes light emission from single fluorescing nanocrystals of cadmium selenide under continuous excitation turning on and off intermittently. Later, Alivisatos et al. (Nature Biotech., 22(1), (2004), 47-52) suggests that the blinking of colloidal quantum dots arises from the difficulties of growing a thick surrounding shell of high band-gap material to fully 'confine' the photo-generated charges within the nanocrystals. However, there is not a generally recognized explanation for blinking behavior. In the field of quantum cryptography, the main problem with using CdSe quantum dot fluorescence is the random duration of bright and dark periods. Brockman et al. (New J. ofPhys., 6, (2004), 99) names these long dark periods as the main restriction for the use of colloidal CdSe quantum dots as single photon emitters.

[0006] Molecular Beam Epitaxy is one reliable and consistent method of making well-formed semiconductor nanocrystals. However, these nanocrystals are only made in very small numbers and require extremely expensive instrumentation. This method does not lend itself to become a larger scale, bench-top technique.

[0007] Alivisatos et al. describe an irreversible, post- synthetic photochemical annealing process using an Ar+ laser and a surfactant, in which they observe core/shell nanorods exhibiting increased quantum efficiencies and are stable in air under visible or UV excitation (JACS, 124, (2002), 7136-7145). The author does not mention how the experiments affected the nanocrystals' blinking, though, and there is not a strong correlation between blinking characteristics and other photostability measures.

[0008] There are also reports of using irradiation in preparation of nanoparticles. For example, Jin et al. report using irradiation of CdSe nanocrystals in 1,2-dichlorobenzene to produce new structures. Nano Letters, vol. 8(5), 1318-22 (2008). However, the structures were hollow nanotubes, and were composed of CdCl₂, demonstrating that Cl radicals generated by the UV irradiation of dichlorobenzene actually extracted Se from the CdSe nanocrystals and replaced it with Cl. In the process, by a mechanism attributed to a Kirkendall effect the nanocrystal particle was transformed into a hollow tube larger in diameter than the initial nanocrystal.

[0009] Despite the progress that has been made in the field of nanocrystals, a commercially viable method for reduction or elimination of "blinking' remains unsolved. Thus, there exists a need for a process for the reduction or elimination of blinking in semiconductor nanocrystals.

SUMMARY OF THE INVENTION

[0010] The present invention provides methods of making semiconductor nanocrystals with reduced photoluminescent intermittency which is commonly referred to as 'blinking', by exciting the semiconductor nanocrystal core during the early stages of shell formation. The present invention also provides semiconductor nanocrystal produced by the methods described herein.

[0011] This invention provides nanocrystals with attenuated photoluminescent intermittency (i.e., reduced blinking) that can be made using affordable, bench top methods in large quantities. The excitation methods can readily be adapted for use in conventional methods for putting a shell onto a nanocrystal core to make improved core/shell nanocrystals.

[0012] One embodiment of the invention provides a composition comprising a semiconductor nanocrystal having sustained photoluminescence. Another embodiment provides a composition comprising a semiconductor nanocrystal substantially free of photoluminescent intermittency. In another embodiment the invention provides a composition comprising a semiconductor nanocrystal that exhibits reduced photoluminescent intermittency as compared with a semiconductor nanocrystal produced without excitation of its nanocrystal core.

[0013] The compositions can be produced as described below and/or comprise the limitations of the embodiments that follow.

[0014] Another embodiment of the present invention provides a method for the preparation of semiconductor nanocrystals coated with an inorganic shell, the method comprising:

(a) providing semiconductor nanocrystal cores;

(b) contacting the semiconductor nanocrystal cores with at least one shell precursor and a solvent to form a mixture;

(c) continuously passing the mixture through a tube; and

(d) applying excitation to the mixture to prepare semiconductor nanocrystals coated with an inorganic shell.

[0015] In some embodiments, the semiconductor nanocrystal cores comprise MgS, MgSe, MgTe, CaS, CaSe, CaTe, SrS, SrSe, SrTe, BaS, BaSe, BaTe, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, Al₂S₃, Al₂Se₃, Al₂Te₃, Ga₂S₃, Ga₂Se₃, Ga₂Te₃, In₂S₃, In₂Se₃, In₂Te₃, SnS, SnSe, SnTe, PbS, PbSe, PbTe, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, BP, Si, or Ge, and mixtures thereof.

[0016] In some embodiments, the first and second shell precursors are those which form inorganic shells which comprise MgO, MgS, MgSe, MgTc, CaO, CaS, CaSe, CaTe, SrO, SrS, SrSe, SrTe, BaO, BaS, BaSe, BaTe, ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, HgO, HgS, Al₂O₃, Al₂S₃, Al₂Se₃, Al₂Te₃, Ga₂O₃, Ga₂S₃, Ga₂Se₃, Ga₂Te₃, In₂O₃, In₂S₃, In₂Se₃, In₂Te₃, SiO₂, GeO₂, SnO, SnO₂, SnS, SnSe, SnTe, PbO, PbO₂, PbS, PbSe, PbTe, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, BP, and mixtures thereof.

[0017] In another more particular embodiment the solvent is an organic solvent and mixtures thereof. [0018] In another more particular embodiment the semiconductor nanocrystal coated with an inorganic shell exhibits attenuated photoluminescent intermittency.

[0019] In another more particular embodiment the semiconductor nanocrystal coated with an inorganic shell does not exhibit photoluminescent intermittency.

[0020] In another more particular embodiment the excitation is provided by use of an electromagnetic field.

[0021] In another more particular embodiment the excitation is provided by use of a laser.

[0022] In another more particular embodiment the excitation is provided by use of a UV light.

[0023] In another more particular embodiment the excitation is provided by use of a mercury light.

[0024] Another more particular embodiment further comprises the step of monitoring the semiconductor nanocrystal core coated with an inorganic shell product for desired properties.

[0025] Another more particular embodiment further comprises monitoring the semiconductor nanocrystal core coated with an inorganic shell for the property of blinking. Blinking can be monitored using known methods for observing individual nanocrystals during irradiation, such as by use of a microscope. Automated methods can also be used to evaluate intermittency.

[0026] Another more particular embodiment further comprises adjusting the semiconductor nanocrystal core coated with an inorganic shell by changing the temperature of the reaction tube, the flow rate, the strength of excitation, the equivalents of the shell precursors, addition rates of the shell precursors, or any or all of these, if needed to obtain the desired product.

[0027] Another embodiment of the present invention provides a method for the preparation of semiconductor nanocrystals coated with an inorganic shell, the method comprising:

(a) providing semiconductor nanocrystal cores;

(b) contacting semiconductor nanocrystal cores with at least one shell precursor and a solvent to form a mixture; and

(c) applying excitation to the mixture to prepare semiconductor nanocrystals coated with an inorganic shell.

[0028] In another more particular embodiment the semiconductor nanocrystal cores comprise MgS, MgSe, MgTe, CaS, CaSe, CaTe, SrS, SrSe, SrTe, BaS, BaSe, BaTe, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, Al₂S₃, Al₂Se₃, Al₂Te₃, Ga₂S₃, Ga₂Se₃, Ga₂Te₃, In₂S₃, In₂Se₃, In₂Te₃, SnS, SnSe, SnTe, PbS, PbSe, PbTe, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, BP, Si, or Ge, and mixtures thereof. [0029] In another more particular embodiment the first and second shell precursors are those which form inorganic shells which comprise MgO, MgS, MgSe, MgTe, CaO, CaS, CaSe, CaTe, SrO, SrS, SrSe, SrTe, BaO, BaS, BaSe, BaTe, ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, HgO, HgS, Al₂O₃, Al₂S₃, Al₂Se₃, Al₂Te, Ga₂O₃, Ga₂S₃, Ga₂Se₃, Ga₂Te₃, In₂O₃, In₂S₃, In₂Se₃, In₂Te₃, SiO₂, GeO₂, SnO, SnO₂, SnS, SnSe, SnTe, PbO, PbO₂, PbS, PbSe, PbTe, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, or BP, and mixtures thereof.

[0030] In another more particular embodiment the solvent is an organic solvent such as those described herein or a mixture of two or more of such solvents.

[0031] In another more particular embodiment the excitation is provided by use of an electromagnetic field.

[0032] In another more particular embodiment the excitation is provided by use of a laser.

[0033] In another more particular embodiment the excitation is provided by use of a UV light.

[0034] In another more particular embodiment the excitation is provided by use of a mercury light.

[0035] Another more particular embodiment further comprises monitoring the semiconductor nanocrystal core coated with an inorganic shell product for desired properties.

[0036] Another more particular embodiment further comprises monitoring the semiconductor nanocrystal core coated with an inorganic shell product for the property of blinking.

[0037] Another more particular embodiment further comprises adjusting the product by changing the temperature, the strength of excitation, the equivalents of the shell precursors, addition rates of the shell precursors, or any or all of these, if needed to obtain the desired product.

[0038] Another embodiment of the present invention provides a method for attenuating photoluminescent intermittency in a shell-coated semiconductor nanocrystal comprising: exciting a semiconductor nanocrystal core in a solution comprising at least one shell precursor;

coating the nanocrystal core with a shell formed by at least one shell precursor, thereby forming the shell-coated semiconductor nanocrystal;

wherein the shell-coated semiconductor nanocrystal exhibits attenuated photoluminescent intermittency. Optionally, the semiconductor nanocrystal core solution is heated to a temperature sufficient to promote shell formation while excitation is applied, and excitation may be provided by an electromagnetic irradiation with energy in the IR, UV or visible range.

[0039] Further embodiments of the present invention are provided in the detailed description.

DETAILED DESCRIPTION OF SELECTED EMBODIMENTS

Definitions:

[0040] While compositions and methods are described in terms of "comprising" various components or steps (interpreted as meaning "including, but not limited to"), the compositions and methods can also "consist essentially of or "consist of the various components and steps, such terminology should be interpreted as defining essentially closed- member groups.

[0041] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention is related. The following terms are defined for purposes of the invention as described herein.

[0042] The term "epitaxial" as used herein refers to semiconductor layers having the same crystalline orientation as the substrates on which they are gown,

[0043] The tern "sustained photoluminescence" as used herein indicates the steady output of light following excitation, such as fluorescence from a semiconductor nanocrystal, with limited or no photoluminescent intermittency. The degree of intermittency exhibited by a nanocrystal having sustained photoluminescence will be less than that of a nanocrystal produced by conventional colloidal or molecular beam epitaxy (MBE) methods.

[0044] "Nanocrystal" as used herein refers to a semiconductor nanocrystal that is fluorescent, and is typically from about 1 to about 100 nm, or from 1-50 nm, or from 1-20 nm in its largest dimension. It can refer to a nanocrystal having a crystalline core, or to a core/shell nanocrystal, or to a core/shell nanocrystal with associated surface moieties, layers, or ligands. 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 of this invention are generally bright fluorescent nanocrystals, and the nanoparticles prepared from them are typically also bright, e.g., having a quantum yield of at least about 20%, sometimes at least 30%, sometimes at least 40%, and sometimes at least 50% or greater. Nanocrystals generally require a surface layer of ligands to protect the nanocrystal from degradation in use or during storage. Suitable ligands are known in the art, and can be selected to impart desired surface properties, such as water solubility, to the nanocrystal.

[0045] "Nanoparticle" as used herein refers to a nanocrystal, frequently a core/shell nanocrystal, plus any tightly-associated organic coating or other material that may be on the surface of the nanocrystal. A nanoparticle includes a bare core/shell nanocrystal, as well as a core nanocrystal or a core/shell nanocrystal having a layer of, e.g., TOPO or other material that is not removed from the surface by ordinary solvation. A nanoparticle may have a layer of ligands on its surface which may further be cross-linked; and a nanoparticle may have other or additional surface coatings that modify the properties of the particle, for example, solubility in water or other solvents. Such layers on the surface are included in the term 'nanoparticle.'

[0046] A nanoparticle may comprise a nanocrystal.

[0047] Certain nanocrystals and nanoparticles may sometimes be referred to as "quantum dots."

[0048] "Water-soluble" is used herein to mean the item is 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-dispersable. A water-soluble nanoparticle is also considered hydrophilic, since its surface is compatible with water and with water solubility.

[0049] "Hydrophobic nanoparticle" as used herein refers to a nanoparticle that is 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; rather, they clump or precipitate from aqueous solutions.

[0050] 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. [0051] In particular, exemplary materials for use as semiconductor nanocrystals in the biological and chemical assays of the present invention 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, 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. The nanocrystals are typically prepared in two steps that produce two distinct layers, a core and a shell.

[0052] In some embodiments, the nanoparticle of this invention is a member of a monodisperse population of nanoparticles of like composition. Monodisperse means that the particles are similar in size, and fall within about 10% of a particular mean dimension. The monodisperse particle population in some embodiments is characterized in that it exhibits less than about 10% rms deviation in the diameter, or largest dimension, of the core. In some embodiments, the monodisperse particle population exhibits less than about 5% rms deviation in the diameter, or largest dimension, of the core.

[0053] Nanoparticle sizes are from about 1 to about 100 nm, sometimes from about 1 to about 40 nm, or from about 1 to about 20 nm in their largest dimension or diameter. The core of a nanocrystal may be a semiconductor nanocrystal core that is spherical or nearly so, or is rod shaped; and may be from about 1 to about 50 nm in its largest dimension.

[0054] The nanoparticles of this invention are generally fluorescent, due to the presence of a fluorescent nanocrystal core. The nanoparticles are often characterized by a fluorescence maximum in the visible spectrum, and frequently the fluorescence of a monodisperse population of nanocrystals of the invention is characterized in that when irradiated the population emits light for which the peak emission is in the spectral range of from about 470 nm to about 620 nm.

[0055] The nanoparticles are generally bright and stable, providing a quantum yield of greater than about 20%, or greater than about 30%, or greater than about 50%, or greater than about 70%.

[0056] 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 is 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.

[0057] The nanoparticles of the invention 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 affects fluorescence wavelength slightly, and has substantial effects on the quantum yield, fluorescence stability, and other photostability characteristics. In some embodiments, the nanocrystal has a semiconductor shell up to about 5 monolayers in thickness, or up to about 3 nm in thickness. In some embodiments, shells ranging from 4-6 monolayers of CdS and 2.5-4.5 monolayers of ZnS may be used. In some embodiments, the shell is thinner, and can be up to about one monolayer in thickness.

[0058] In some embodiments, a core semiconductor nanocrystal is 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 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.

[0059] 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 is desirably higher and the valence band is desirably lower than those of the core semiconductor nanocrystal. For semiconductor nanocrystal cores that emit energy in the visible (e.g., CdS, CdSe, CdTe, ZnSe, ZnTe, GaP, GaAs) or near IR (e.g., InP, InAs, InSb, PbS, PbSe), a material that has a bandgap energy in the ultraviolet regions may be used. Exemplary materials include ZnS, ZnTe, GaN, and magnesium chalcogenides, e.g., MgS, MgSe, and MgTe. For a semiconductor nanocrystal core that emits in the near IR, 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) J. Phys. Chem. B 101:9463, Hines et al. (1996) J. Phys. Chem. 100: 468-471, Peng et al. (1997) J. Am. Chem. Soc. 119:7019-7029, and Kuno et al. (1997) J. Phys. Chem. 106:9869. It is also understood in the art that the actual fluorescence wavelength for a particular nanocrystal core depends 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.

[0060] In some embodiments, the metal atoms of a shell layer on a nanocrystal core are selected from Cd, Zn, Ga and Mg. The second element in these semiconductor shell layers can be selected from S, Se, Te, P, As, N and Sb. In some embodiments, the semiconductor nanocrystal is a core/shell nanocrystal, and the core comprises metal atoms selected from Zn, Cd, In, Ga, and Pb. Some preferred nanocrystal cores include CdS, CdSe, InP, CdTe, ZnSe and ZnTe; and some preferred shell materials include ZnS, ZnSe, CdS, and CdSe.

[0061] The nanocrystal 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 nanoparticles 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. Typically the nanocrystals of interest are from about 1 nm to about 100 nm in diameter, and sometimes from about 1 to about 25 nm. 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 20 nm in its largest dimension.

[0062] 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 is less than about 10 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.

[0063] Blinking, or intermittency, is seemingly random, and is typically not rhythmic enough to be characterized by a frequency. It can be characterized by the percentage of time a nanocrystal remains 'on' under constant illumination, referred to herein as %On time; or it can be characterized by the average time a nanocrystal remains 'on' between intermittent dark phases, referred to herein as an average blink- free period. Depending upon the application, an increase in either parameter may be advantageous. Thus in some embodiments, the invention provides a method of producing a core/shell nanocrystal having an improved %On time, or an improved average blink-free period, or both. Improvements in these properties are measured by comparison of the excitation-treated nanocrystal compositions to ones prepared under identical conditions but without excitation during the shell-formation step to improve intermittency.

[0064] Without being bound by theory, it is believed that excitation of the core during deposition of the first monolayers of the shell contributes to stabilizing the interfacial region, possibly by promoting exchange of atoms of the shell layer with ones from the core to a limited extent, possibly driven by imperfections of epitaxial matching of the shell to the core. This may result from a Kirkendall effect, much like the mechanism invoked by Jin et al. to explain their nanocrystal to nanotube conversion reaction. Jin, et al., Nano Letters, vol. 8(5), 1318-22 (2008). Very small amounts of element exchange between the core and the newly-applied shell atoms may help reduce the extent of such imperfections, and improve the matching of core to shell, or reduce imperfections in the interface between core and shell.

[0065] In one embodiment, the invention provides a method to prepare a core/shell nanocrystal, comprising:

(a) providing semiconductor nanocrystal cores;

(b) contacting the semiconductor nanocrystal cores with at least one shell precursor and a solvent to form a mixture; and

(c) applying excitation to the mixture to prepare semiconductor nanocrystals coated with an inorganic shell.

[0066] The mixture of nanocrystal cores and shell precursor may be heated before or during excitation to a temperature sufficient to promote nanocrystal formation. Suitable temperatures are known in the art, as they are temperatures suitable for shell formation in the absence of irradiative excitation.

[0067] In some embodiments, the excitation comprises electromagnetic radiation in the IR or visible wavelength range. In other embodiments, excitation comprises electromagnetic radiation in the UV wavelength range.

[0068] Excitation may be provided by available methods known in the art, including incandescent lamps, UV lamps, IR lamps, mercury vapor lamps, and lasers.

[0069] In some embodiments, the nanocrystal core is contacted with a single shell precursor, even though the shell to be formed requires two shell precursors to form. This may permit atom exchange between the core's surface and the shell precursors. Alternatively, both first and second shell precursors may be present when excitation is applied. . [0070] For applications of the invention, the nanocrystal core may comprise, or may consist essentially of, one of the following or a mixture of two of the following materials: CdS, CdSe, CdTe, ZnSe, ZnTe, GaP, GaAs, InP, InAs, InSb, PbS, and PbSe.

[0071] In some embodiments of the invention, the shell comprises ZnS, ZnSe, ZnTe, GaN, MgS, MgSe, or MgTe. Suitable shell precursors for adding each of these shells to a nanocrystal core are known to those of skill in the art.

[0072] The irradiation or excitation applied during shell formation improves nanocrystal products by reducing intermittency, i.e., decreasing the extent to which blinking interferes with the usefulness of the nanocrystals. In some embodiments, the %On time or the blink-free period for the semiconductor nanocrystals coated with an inorganic shell produced by the method is increased by at least about 20% compared to semiconductor nanocrystals made without excitation during shell formation. In some embodiments, it is increased by at least 40% or by at least 50%. In some embodiments, blinking is substantially eliminated by the methods of the invention.

[0073] The methods of the invention can be applied to either batch-processing nanocrystal production methods, or to flow methods for applying a shell to a nanocrystal. In one aspect, the invention provides a method for producing a core/shell nanocrystal using a flow process, where the method comprises:

(a) providing a population of semiconductor nanocrystal cores;

(b) contacting the semiconductor nanocrystal cores with at least one shell precursor and a solvent to form a mixture;

(c) continuously passing the mixture through a tube at a temperature sufficient to support shell formation on the nanocrystal; and

(d) applying excitation to the mixture to prepare semiconductor nanocrystals coated with an inorganic shell.

[0074] The mixture of nanocrystal cores and shell precursor may be heated before or during excitation to a temperature sufficient to promote nanocrystal formation. Suitable temperatures are known in the art, as they are temperatures suitable for shell formation in the absence of irradiative excitation.

[0075] In some embodiments, the excitation comprises electromagnetic radiation in the IR or visible wavelength range. In other embodiments, excitation comprises electromagnetic radiation in the UV wavelength range. [0076] Excitation may be provided by available methods known in the art, including incandescent lamps, UV lamps, IR lamps, mercury vapor lamps, and lasers.

[0077] In some embodiments, the nanocrystal core is contacted with a single shell precursor, even though the shell to be formed requires two shell precursors to form. This may permit atom exchange between the core's surface and the shell precursors. Alternatively, both first and second shell precursors may be present when excitation is applied. .

[0078] For applications of the invention, the nanocrystal core may comprise, or may consist essentially of, one of the following or a mixture of two of the following materials: CdS, CdSe, CdTe, ZnSe, ZnTe, GaP, GaAs, InP, InAs, InSb, PbS, and PbSe.

[0079] In some embodiments of the invention, the shell comprises ZnS, ZnSe, ZnTe, GaN, MgS, MgSe, or MgTe. Suitable shell precursors for adding each of these shells to a nanocrystal core are known to those of skill in the art.

[0080] The irradiation or excitation applied during shell formation improves nanocrystal products by reducing intermittency, i.e., decreasing the extent to which blinking interferes with the usefulness of the nanocrystals. In some embodiments, the %On time or the blink-free period for the semiconductor nanocrystals coated with an inorganic shell produced by the method is increased by at least about 10% compared to semiconductor nanocrystals made without excitation during shell formation. In some embodiments, it is increased by at least 20%, or by at least 30%, or by at least 40%, or by at least 50%, or by at least 60%. In some embodiments, blinking is substantially eliminated by the methods of the invention.

[0081] In another aspect, the invention provides a method to produce a core/shell nanocrystal with reduced blinking, comprising applying excitation to the core during the process of forming the shell on the core. The nanocrystal cores suitable for use in these methods, and the excitation methods useful therefore are similar to those described above.

[0082] In another aspect, the invention provides a core/shell nanocrystal made by any one of the methods described herein. In some embodiments, this core/shell nanocrystal has improvement relative to ones made without the excitation during shell formation that is described herein. The improvement can be increased %On time, or average blink-free period, or both. These parameters are increased by at least about 10% compared to semiconductor nanocrystals made without excitation during shell formation. In some embodiments, they are increased by at least 20%, or by at least 30%, or by at least 40%, or by at least 50%, or by at least 60%. [0083] The continuous flow method and the batch method of synthesizing semiconductor nanocrystals are commonly used bench-top methods for making nanocrystals. These methods are readily modified as described herein to produce coated nanocrystals with superior inorganic shell coatings with the goals of reducing photoluminescence intermittency, which may also produce a more compact, and therefore smaller, nanocrystal.

[0084] In an embodiment which relates to the well-established continuous flow method, nanocrystal cores are obtained and combined with one or more inorganic shell coating precursors and the appropriate solvents to form a dispersion mixture. The inorganic shell may comprise one, two or three precursors such as those described herein. In many embodiments, the inorganic shell formation uses a first precursor comprising Mg, Ca, Sr, Ba, Zn, Cd, Hg, Al, Ga In, Si, Ge, Sn, Ga, or Pb, and a second precursor comprising S, Se, O, N, Te, P, As, or Sb; suitable precursors for each of these elements are known in the art. For purposes of the invention, a first precursor is sometimes selected from those comprising Zn, Cd, Ga, Mg, and In. The second shell precursor is sometimes selected from those comprising S, Se and O.

[0085] Any suitable solvent can be used; in some embodiments, the solvent comprises one or more trialkyl phosphines, trialkyl phosphine oxides, alkyl amines, dialkyl amines, dialkylphosphinic acids, alkyl phosphonic acids, or hydrocarbons; where each alkyl group is a C4- C24 hydrocarbon group comprising straight chain, branched, or cyclic groups, or combinations of these, and where each hydrocarbon group can contain up to two unsaturated bonds. Trioctylphosphine (TOP), trioctyl phosphine oxide (TOPO), decylamine, dioctylamine, tetradecylphosphonic acid (TDPA), octadecene, and the like are sometimes used.

[0086] The dispersion of cores plus shell precursor(s) and solvent is passed through a tube which is maintained at a temperature sufficiently high to initiate a reaction among the reactants thereby inducing the formation of an inorganic shell surrounding the nanocrystal core.

[0087] During the early stages of shell formation, the dispersion mixture is exposed to a source of excitation which can include, but is not limited to, an electromagnetic field, a laser, UV light, IR light, or a Mercury lamp. The excitation is maintained at an intensity and duration such that the shell growth is epitaxial to the extent that blinking is reduced or eliminated in the resulting product. This stage of the synthesis is commonly referred to by those skilled in the art as the 'ripening' stage. While improvement in epitaxial growth may occur, the pertinent change caused by the treatment methods for the purposes of the invention are improved intermittency properties. [0088] In some embodiments, the excitation may be applied to the cores in the presence of the first shell precursor, prior to addition of the second shell precursor, for an initial period. In some embodiments, the excitation may be applied to the cores in the presence of the second shell precursor, prior to addition of the first shell precursor. In some embodiments, both shell precursors are added prior to excitation.

[0089] In another embodiment, which relates to the well-established batch method, nanocrystal cores are obtained and combined with inorganic shell coating precursors and the appropriate solvents to form a dispersion mixture. This dispersion is heated and maintained at a temperature sufficiently high to initiate a reaction among the reactants thereby inducing the formation of an inorganic shell surrounding the nanocrystal core. During the early stages of shell formation, the dispersion mixture is exposed to a source of excitation which can include, but is not limited to, an electromagnetic field, a laser, UV light, IR light, or a Mercury lamp. The excitation is maintained throughout the ripening stage at an intensity and duration such that the shell growth is epitaxial to the extent that blinking is reduced or eliminated in the resulting product.

[0090] Excitation is provided by electromagnetic radiation, and can be applied by any suitable means. Excitation is typically done with radiation in the IR, visible, and UV ranges, for example using radiation having a wavelength between about 10 and 1,000 nm; or between about 100 nm and 10 micrometers; or between about 1 and 1000 micrometers.

[0091] Suitable intensity is provided by standard UV, IR and visible light sources, such as a UV lamp or IR lamp, or a mercury vapor light. To increase the effect of the methods described herein, a high intensity excitation source such as a laser may be used. Preferably, the intensity of excitation energy is at least about 2000 W/m² in the reaction zone where nanocrystal shell formation is to occur.

[0092] In some embodiments, the excitation may be applied to the cores in the presence of the first shell precursor, prior to addition of the second shell precursor, for an initial period. In some embodiments, the excitation may be applied to the cores in the presence of the second shell precursor, prior to addition of the first shell precursor. In some embodiments, both shell precursors are added prior to excitation.

[0093] The semiconductor nanocrystal cores used may be any from the following list: MgS, MgSe, MgTe, CaS, CaSe, CaTe, SrS, SrSe, SrTe, BaS, BaSe, BaTe, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, Al₂S₃, Al₂Se₃, Al₂Te₃, Ga₂S₃, Ga₂Se₃, Ga₂Te₃, In₂S₃, In₂Se₃, In₂Te₃, SnS, SnSe, SnTe, PbS, PbSe, PbTe, AlP, AlAs, AlSb, GaN, GO, GaAs, GaSb, InN, InP, InAs, InSb, BP, Si, or Ge, and mixtures thereof.

[0094] The inorganic shell coating precursors are chosen so that the final obtained coating may be any from the following list: MgO, MgS, MgSe, MgTe, CaO, CaS, CaSe, CaTe, SrO, SrS, SrSe, SrTe, BaO, BaS, BaSe, BaTe, ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, HgO, HgS, Al₂O₃, Al₂S₃, Al₂Se₃, Al₂Te₃, Ga₂O₃, Ga₂S₃, Ga₂Se₃, Ga₂Te₃, In₂O₃, In₂S₃, In₂Se₃, In₂Te₃, SiO₂, GeO², SnO, SnO₂, SnS, SnSe, SnTe, PbO, PbO₂, PbS, PbSe, PbTe, AlN, ALP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, or BP and mixtures thereof.

[0095] A solvent is chosen in which both inorganic shell precursors will be soluble. Mixtures of solvents may be employed for this purpose. Selection of suitable solvents for nanocrystal formation is within the ordinary level of skill in the art.

[0096] In one embodiment, the semiconductor nanocrystal cores, the first shell precursor, second shell precursor and solvent are simultaneously injected into the heated tube resulting in a dispersion mixture. Typically, a constant flow rate through the tube is maintained. The temperature of the tube is sufficiently high to initiate shell formation on the nanocrystal core. An ideal temperature range may be determined for each nanocrystal core size used to ensure that the size distribution of the cores remains constant and that shells with a high degree of crystallinity are formed. The actual temperature range used to coat the nanocrystal cores is dependent upon the relative stability of the precursors, the core, and inorganic shell coating composition. These parameters are readily determined by routine experimentation in view of extensive guidance and knowledge in the art, since the excitation described herein for reducing intermittency does not significantly alter the desired conditions for shell formation.

[0097] In each embodiment, a form of excitation is applied to the dispersion mixture during the ripening stage, when at least one shell precursor is present with the core. The source of excitation is that of an electromagnetic field, a laser, UV light, IR light, or Hg light. When the modified batch method is used, the vessel needs to be permeable to the form of excitation used. The reaction tube used in the modified continuous flow method must contain either a light-transparent window or be made entirely out of a transparent material when a light excitation source is used. When the excitation is an electromagnetic field, the reaction tube can be made of standard materials, such as glass. The optimal amount, intensity, duration and source of excitation is found experimentally depending on factors such as the semiconductor nanocrystal core used, the size of the core used, the desired thickness of the inorganic shell coating, and the shell precursors used. [0098] The product mixture can be monitored for the desired amount of coating coverage during the ripening process. If the amount, level, or uniformity of coating is not within the targeted coating range, adjustments can be made to the temperature, flow rate, equivalents of first and second shell precursors, addition rates, excitation intensity, or any variable to correct the deviation. Once the targeted coating thickness and quality is obtained, the shell growth is quenched.

[0099] Quenching the reaction, and thus ceasing the growth of the inorganic shell surrounding the nanocrystal core, is done, in one embodiment, by cooling the reaction mixture using various methods. The cooling can be accomplished by, among other methods, contacting a heat transfer medium with the reaction tube, by adding solvent to the product mixture of a sufficiently low temperature to quench the reaction, or by cooling the product mixture once it emerges from the reaction tube.

[00100] The extent of improvement in intermittency will depend upon the specific core, shell precursors and excitation used. In some embodiments, the excitation increases either %On time or average blink-free period, or both, by at least 5%, relative to untreated nanocrystals, or by at least 10%, or by at least 15%, or by at least 20%, or by at least 25%. In some embodiments, the increase is at least 20%. In some embodiments, the increase is at least 40%. In some embodiments, the increase is at least 50%. In some embodiments, the increase is about 100%.

[00101] The following examples are included to demonstrate preferred embodiments of the invention. It should be appreciated by those of skill in the art that the techniques disclosed in the examples which follow represent techniques discovered by the inventor(s) to function well in the practice of the invention, and thus can be considered to constitute preferred modes for its practice. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the scope of the invention.

EXAMPLES

Example 1

[00102] The following example is a method of overcooling CdSe core nanocrystals, which emit at 630 nm, with ZnS to produce core/shell nanoparticles that emit light at 652-658 nm.

[00103] Obtain a bottle of 630 nm cores to be overcoated. Place the bottle of 630 nm CdSe cores in a 60-70 ⁰C water bath to melt the solution. [00104] Preparation of degassed TOPO. Cool a 500 mL 3-neck round-bottom reaction flask which has been cleaned and dried at HO⁰C. Add 37g TOPO (trioctylphosphine oxide). Apply vacuum to remove air from the solid TOPO. Hold vacuum for 5 minutes and then fill the vessel with nitrogen. Degas the TOPO by heating to 18O⁰C under full vacuum for 1 hr.

[001051 Preparation of a TOP/TDPA/Cd solution. Cool a 100 mL 3-neck round-bottom flask which has been cleaned and dried at 100⁰C. Add 8g TDPA (tetradecylphosphonic acid). Apply vacuum to remove air from the solid TOPO. Hold vacuum for 5 minutes and then fill the vessel with nitrogen.

[00106] Isolation of Cores. Add 240 mL of ethanol to a 500 mL Erlenmeyer flask. While swirling gently, slowly inject 100 mL molten cores into the ethanol. Transfer the solution to two 250 mL flat bottom centrifuge tubes. Centrifuge for 5 minutes at 2500 rpm. Decant the liquid from the centrifuge tubes to waste and drain them by standing them upside down on a paper towel for about 1 minute. Immediately after draining, use at most 30 mL of reagent grade hexane to completely disperse the cores. Once the degassed reactor vessel is at 100⁰C, lower the heating mantle and, using a disposable syringe, transfer the cores to the degassed TOPO in the reactor. Gradually apply vacuum to the reactor to boil off the hexane. When the hexane has essentially been removed, refill the reactor with nitrogen and inject 30 mL of Decylamine. This injection begins the amine treatment process. The duration of amine treatment is 30-45 minutes. This amine treatment ends when the TOP/TDPA/Cd suspension is added.

[00107] Preparation and addition of ZnS solution. Place 76 mL TOP (Trioctylphosphine) into a bottle. Add 2g diethylzinc. Add 2g hexamethyldisilthiane. Draw the entire contents of the TOP/Zns bottle into a syringe.

[00108] Transfer of TOP/TDPA/Cd solution. When the amine treatment for cores in the 500 mL reactor is finished, transfer the quantity of TOP/TDPA/Cd solution into the reactor at 100°C.

[00109] Start shell reaction. Set the temperature controller on the 500 ml, reactor to 23O⁰C. Begin addition of the TOP/Zn/S stock solution dropwise at a rate of 1 drop/second. Once approximately 20 percent of the TOP/Zn/S solution has been added, excite the vessel and the reaction mixture with an Argon laser. Continue this excitation for the duration of the TOP/Zn/S addition. The total addition should take 3 hours. When all of the liquid has been dispensed, turn off the heating mantle. When the temperature drops below 11O⁰C, add 160 mL toluene and stir overnight. Nanocrystals prepared by these methods will exhibit an increase in %On time of at least 5%, and an increase in average blink-free period of at least 5%.

[00110] All of the compositions and/or methods disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While the compositions and methods of this invention have been described in terms of preferred embodiments, it will be apparent to those of skill in the art that variations may be applied to the compositions and/or methods in the steps or in the sequence of steps of the methods described herein without departing from the concept and scope of the invention. More specifically, it will be apparent that certain agents which are chemically related may be substituted for the agents described herein while the same or similar results would be achieved. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the scope and concept of the invention.

Claims

ClaimsWhat is claimed is:

1. A method for preparing a core/shell nanocrystal, comprising:

(a) providing semiconductor nanocrystal cores;

(b) contacting the semiconductor nanocrystal cores with at least one shell precursor and a solvent to form a mixture; and

(c) applying excitation to the mixture to prepare semiconductor nanocrystals coated with an inorganic shell.

2. The method of claim 1, where the excitation comprises electromagnetic radiation in the IR or visible wavelength range.

3. The method of claim 1, where the excitation comprises electromagnetic radiation in the UV wavelength range.

4. The method of any one of the preceding claims, wherein the excitation is provided by a laser.

5. The method of any one of the preceding claims, wherein the at least one shell precursor comprises a first shell precursor and a second shell precursor.

6. The method of any one of the preceding claims, wherein the nanocrystal core consists essentially of CdS, CdSe, CdTe, ZnSe, ZnTe, GaP, GaAs, InP, InAs, InSb, PbS, or PbSe.

7. The method of any one of the preceding claims, wherein the shell comprises ZnS, ZnSe, ZnTe, GaN, MgS, MgSe, or MgTe.

8. The method of any one of the preceding claims, wherein the %On time or the blink-free period for the semiconductor nanocrystals coated with an inorganic shell produced by the method is increased by at least about 10% compared to semiconductor nanocrystals made without excitation during shell formation.

9. A method for producing a core/shell nanocrystal, comprising:

(a) providing a population of semiconductor nanocrystal cores;

(b) contacting the semiconductor nanocrystal cores with at least one shell precursor and a solvent to form a mixture;

(c) continuously passing the mixture through a tube at a temperature sufficient to support shell formation on the nanocrystal; and

(d) applying excitation to the mixture to prepare semiconductor nanocrystals coated with an inorganic shell.

10. The method of claim 10, wherein the at least one shell precursor comprises a first shell precursor and a second shell precursor.

11. The method of claim 9 or claim 10, wherein the core consists essentially of CdS, CdSe, CdTe, ZnSe, ZnTe, GaP, GaAs, InP, InAs, InSb, PbS, or PbSe.

12. The method of any one of claims 9-11, wherein the shell comprises ZnS, ZnSe, ZnTe, GaN, MgS, MgSe, or MgTe.

13. The method of any one of claims 9-12, where the excitation comprises electromagnetic radiation in the IR or visible wavelength range.

14. The method of any one of claims 9-12, where the excitation comprises electromagnetic radiation in the UV wavelength range.

15. The method of any one of claims 9-14, wherein the excitation is provided by a laser.

16. A method for producing a core/shell nanocrystal with reduced blinking, comprising applying excitation to the core during the process of forming the shell on the core.

17. A core/shell nanocrystal made by the method of any one of the preceding claims.

18. The core/shell nanocrystal of claim 17, wherein at least one parameter selected from %On time and average blink-free period is increased by at least about 10% relative to that for a nanocrystal produced without excitation of the nanocrystal core mixture during shell formation.