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FLOW SYNTHESIS OF QUANTUM DOT
NANOCRYSTALS
CROSS REFERENCE TO RELATED
APPLICATION 5
This application is a continuation-in-part of U.S. patent application Ser. No. 09/751,670, filed Dec. 28, 2000, now abandoned the contents of which are incorporated herein by reference in their entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention resides in the field of nanocrystalline materials and processes for their manufacture. 15
2. Description of the Prior Art
Quantum-sized particles, i.e., those having diameters within the range of about 0.1 nm to about 50 nm, also known as quantum dots or nanocrystals, are known for the unique 20 properties that they possess as a result of both their small size and their high surface area. Some of these particles have unique magnetic properties that make the particles useful in ferro fluids, in magnetic tagging elements, and in electronic data systems such as recording media. Luminescent nanoc- 25 rystals are particularly useful as detectable labels such as oligonucleotide tags, tissue imaging stains, protein expression probes, and the like, in applications such as the detection of biological compounds both in vitro and in vivo. Luminescent nanocrystals offer several advantages over 30 conventional fluorophores, particularly for multiplexed and/ or high sensitivity labeling. Nanocrystals typically have larger absorption cross sections than comparable organic dyes, higher quantum yields, better chemical and photochemical stability, narrower and more symmetric emission 35 spectra, and a larger Stokes shift. Furthermore, the absorption and emission properties vary with the particle size and can be systematically tailored.
A variety of methods have been reported for the preparation of nanocrystals. These methods include inverse 40 micelle preparations, arrested precipitation, aerosol processes, and sol-gel processes. A method commonly used for the preparation of binary nanocrystals is one in which an organometallic and elemental set of nanocrystal precursors is injected into a hot solvent as the solvent is being stirred. 45 Product nucleation can begin immediately, but the injection causes a drop in the solvent temperature, which tends to halt the nucleation process. Nucleation and particle growth can be continued by heating the reaction mixture with further stirring, and the temperature can be dropped to stop the 50 reaction when the desired particle size is obtained. As a result, the success of this batchwise "stirred-pot" method is strongly affected by system parameters such as the initial temperature of the solvent, the injection temperature and in particular the injection rate, the stirring efficiency, the con- 55 centrations of the reactant materials, the length of time that the mixture is held at the reaction temperature, and the efficiency of the cooling both after injection and after the desired endpoint is achieved. Some of these parameters are difficult to control with precision, and this can lead to poor go reproducibility of the product. The lack of precise control also leads to nanocrystals with surfaces that are nonuniform, products that are readily degradable, and/or nanocrystals with low emission quantum yields.
The initial reaction conditions, i.e., the manner and con- 65 ditions under which the reaction is initiated, are particularly important in controlling the quality and uniformity of the
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product, far more so than in other types of synthesis. Stirred-pot methods suffer in this regard since there are limits to how rapidly and uniformly the temperature of the reaction mixture can be changed or otherwise controlled. The temperature drop that occurs upon injection of the precursors will vary with the precursor temperature prior to injection, the volume of precursor injected and its rate of injection, the volume of the heated solvent, and the stirring efficiency. The difficulty in cooling rapidly when terminating the reaction often means that a lower reaction temperature must be used as a means of avoiding excess reaction. Further difficulties with stirred-pot methods are that they often involve the injection of large volumes of flammable or pyrophoric materials at very high temperatures, or the rapid evolution of gases, all of which present safety hazards.
Control of the properties of nanocrystals by the application of coatings or shells has been reported, notably in International Patent Publication No. WO 99/26299 (PCT/ US98/23984), "Highly Luminescent Color-Selective Materials," Massachusetts Institute of Technology, applicant, international publication date May 27, 1999, and references cited therein. The application of an inorganic shell, for example, can increase the quantum yield of the nanocrystal as well its chemical stability and photostability. The techniques for applying a shell are stirred-pot techniques that are usually similar to those used for the preparation of the core. Like the diameter of the core, the thickness of the shell affects the properties of the finished product, and the thickness will vary with the same system parameters that affect the core. The difficulties in controlling these parameters in a stirred-pot system lead to difficulties in controlling the nature and quality of the final product.
SUMMARY OF THE INVENTION
The limitations and difficulties described above and others encountered in the preparation of nanocrystals are addressed by the present invention, which resides in processes and apparatus for the production of monodisperse luminescent semiconductor nanocrystals, for the application of a coating to nanocrystal cores, and for both. The manufacture of nanocrystals in accordance with this invention is accomplished by first dissolving or dispersing precursor materials capable of reacting to form nanocrystals in a solvent, for example a coordinating solvent, and introducing the resulting reaction mixture into a reaction tube that is embedded or immersed in a heat transfer medium. Likewise, the application of a coating to nanocrystal cores in accordance with this invention is accomplished by dispersing the nanocrystal cores in a solvent, for example a coordinating solvent, in which are dissolved the precursor materials for the coating, and introducing this reaction mixture into the reaction tube. In either case, the heat transfer medium maintains the reaction mixture at the desired reaction temperature, and the reaction mixture is passed continuously through the tube. The internal diameter of the tube is preferably small enough to promote rapid transfer of heat from the tube walls to the center of the fluid stream flowing through the tube and hence rapid heating of the continuously flowing stream to the reaction temperature. In addition to the tube diameter, the flow rate is varied and adjusted, and the tube length selected, to permit control of the reaction. Flow rate, temperature and pressure are all controllable, and in preferred embodiments the reaction is quenched by cooling the product stream upon its emergence from the reaction tube by any of various conventional cooling techniques.
Characteristic properties of the product stream, such as optical properties, electrical properties, magnetic properties,
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electromagnetic properties, and the like are detected and a comparison is made between the detected values and a predetermined or preselected target range that is indicative of the product quality sought to be achieved. Any discrepancy or deviation between the detected values and target range can then be used to adjust the variable reaction conditions, such as the temperature of the heat transfer medium, the flow rate of the reaction mixture through the tube, or both, until the product changes sufficiently that the detected values fall within or otherwise conform to the target range.
It has further been discovered that the final nanoparticle size, size distribution and yield can be controlled by introducing a reaction promoter into the reaction system under selected conditions such as exposure time and temperature. An example of a reaction promoter is air or generally any oxygen-containing gas (i.e., oxygen gas itself or a gas mixture containing molecular oxygen). The particle size, size distribution and yield affect the properties of the product stream listed above, i.e., the optical, electrical, magnetic, and electromagnetic properties, and deviations between the detected values and the target range can be reduced or eliminated by adjustment of the exposure time to the oxygen-containing gas, the temperature maintained during the exposure, or other characteristics of the exposure that can be varied. Exposure of the reaction mixture to the gas can be done before the reaction mixture enters the continuous-flow system or while the reaction mixture is in the continuous-flow system.
Reaction apparatus in accordance with this invention includes a thermally conductive reaction tube of sufficiently small internal diameter to accomplish effective heat transfer in the flowing stream, a heat transfer medium in thermal contact with the exterior of the reaction tube, a pump or other fluid-driving component for continuously supplying pressure to a reactant or precursor mixture to the reaction tube, a monitoring unit to evaluate, measure, or otherwise detect the properties of the product stream, preferably but not necessarily as the product stream leaves the reaction tube, as an indication of the nature and quality of the nanocrystals formed in the reaction mixture during its passage through the reaction tube, and optionally a control loop to adjust the reaction conditions in the tube or upstream of the tube to correct for any discrepances between the detected values and the target range.
Further details of these features and the various preferred embodiments of the several aspects of this invention are described below.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a process flow diagram of one embodiment of the present invention.
FIG. 2 is a process flow diagram of a second embodiment of the present invention.
FIG. 3 is a superimposed plot of emission spectra of nanocrystals formed in accordance with the present invention under various reaction conditions.
FIG. 4 is a further superimposed plot of emission spectra of nanocrystals formed in accordance with this invention under different sets of reaction conditions.
FIG. 5 is a still further superimposed plot of emission spectra of nanocrystals formed in accordance with this invention under still different reaction conditions.
DETAILED DESCRIPTION OF THE
INVENTION AND SPECIFIC EMBODIMENTS
The terms "semiconductor nanocrystal," "quantum dot," "QdotTM nanocrystal," or simply "nanocrystal" are used
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interchangeably herein and refer to an inorganic crystallite between about 1 nm and about 1000 nm in diameter, more typically between about 2 nm and about 20 nm (such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or
5 20 nm). A semiconductor nanocrystal is capable of emitting electromagnetic radiation upon excitation (i.e., the semiconductor nanocrystal is luminescent) and includes a "core" of one or more first semiconductor materials, and may be surrounded by a "shell" of a second semiconductor material,
10 according to terminology used in the art. A semiconductor nanocrystal core surrounded by a semiconductor shell is referred to as a "core/shell" semiconductor nanocrystal. The surrounding "shell" material typically has a bandgap energy that is larger than the bandgap energy of the core material
15 and can be chosen to have an atomic spacing close to that of the "core" substrate. The core and/or shell can be a semiconductor material including, but not limited to, those of the Groups II-VI (ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, MgS, MgSe, MgTe, CaS, CaSe, CaTe, SrS,
20 SrSe, SrTe, BaS, BaSe, BaTe, and the like) and III-V (GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, and the like) and IV (Ge, Si, and the like), and alloys or mixtures thereof.
The term "luminescence" denotes the emission of electromagnetic radiation (light) from an object. Luminescence
25 results from a system that is "relaxing" from an excited state to a lower state with a corresponding release of energy in the form of a photon. These states can be electronic, vibronic, rotational, or any combination of these three. The transition responsible for luminescence can be stimulated through the
30 release of energy stored in the system chemically or added to the system from an external source. The external source of energy can be of a variety of types including chemical, thermal, electrical, magnetic, electromagnetic, physical, or any other type excited by absorbing a photon of light, by
35 being placed in an electric field, or through a chemical oxidation-reduction reaction. The energy of the photons emitted during luminescence can be in a range of low-energy microwave radiation to high-energy x-ray radiation. Luminescence typically entails the release of photons in the range
40 of from IR to UV radiation.
The term "monodisperse" when describing particles denotes a population of particles of which a major portion, typically at least about 60%, more preferably from 75% to 90%, fall within a specified particle size range. A population
45 of monodisperse particles deviates less than 10% rms (rootmean-square) in diameter and typically less than 5% rms. In addition, upon exposure to a primary light source, a monodisperse population of semiconductor nanocrystals is capable of emitting energy in narrow spectral linewidths, as
50 narrow as 12 nm to 60 nm full width of emissions at half maximum peak height (FWHM), and with a symmetric, nearly Gaussian line shape. As one of ordinary skill in the art will recognize, the linewidths are dependent on, among other things, the size heterogeneity, i.e., monodispersity, of
55 the semiconductor nanocrystals in each preparation. Certain single semiconductor nanocrystal complexes have been observed to have FWHM as narrow as 12 nm to 15 nm.
The formation of nanocrystalline particles (or nanocrystal cores for encapsulation) in accordance with this invention is
60 done in a continuous-flow manner, and precursors known in the art may be used. Useful precursors are many and varied, depending on the type of nanocrystals to be prepared and the intended use of the nanocrystals. Among the various classes of nanocrystals are those that emit light, and examples are
65 those bearing the empirical formulae CdX or ZnX in which X is a chalcogen. Preferred chalcogens are S, Se and Te, with Se particularly preferred. Preferred nanocrystals are ZnS,
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ZnSe, ZnTe, CdS, CdSe, and CdTe. Reactants capable of forming nanocrystals of these materials are cadmium and zinc salts, cadmium and zinc oxides, and organocadmium and organozinc compounds as the source of the Cd and Zn, respectively, and elemental chalcogen or chalcogen- 5 containing compounds as the source of the chalcogen. Examples of cadmium and zinc salts are those in which the anion is acetate or other carboxylates (such as formate, decanoate, and alkanoates of intermediate chain lengths, as well as oxalate, maleonate, and adipate), acetylacetonate, 10 nitrates, nitrites, sulfates, sulfites, perchlorates, chlorates, carbonate, carbamate, phosphates (including substituted phosphates and particularly hexafluorophosphates), fluoride, chloride, bromide, iodide, hydroxide, and borates (including substituted borates such as tetrafluoroborate). Preferred salts 15 are acetates, carboxylates, acetylacetonates, perchlorates, carbonates, chlorides, bromides, iodides, hydroxides, and unsubstituted borates. While dimethyl cadmium is an example of an organocadmium compound. Examples of organocadmium and organozinc compounds are those in 20 which the organic moiety is methyl (such as dimethylcadmium), ethyl (such as diethyl zinc), butyl, phenyl, and combinations of these moieties.
The coating of pre-formed nanocrystal cores in accordance with this invention is likewise performed in a 25 continuous-flow manner by placing the cores in a suspension in which the precursors that form the coating are dissolved. Coatings of various compositions known in the art can be applied in this manner. One class of coatings are those serving to passivate the core surfaces to improve optical 30 properties such as quantum yield, and the precursors for these coatings are surface passivating reactants. Among this class are nanocrystals bearing the empirical formula ZnY in which Y is S, Se, Te, or a mixture thereof. The passivating reactants for this class include a Zn-containing reagent and 35 a reactant containing either S, Se, Te or combinations thereof.
Whether the reaction is a nanocrystal core-forming reaction or a coating reaction, the process is often facilitated by performing the reaction in the presence of a coordinating 40 solvent or by addition of a coordinating additive. The terms "coordinating solvent" and "coordinating additive" as used herein denote a solvent or other chemical additive that enters into molecular coordination with the atoms in the reactants that combine to form the nanocrystalline materials or the 45 reactants that combine to form the coatings on the nanocrystal core surfaces or with the nanocrystals themselves. The coordinating solvent thus enhances the solubility of the reactants while also serving as a means of modulating the reactivity of the precursors or the growing nanoparticles. A 50 wide range of solvents that function in this manner can be used, and preferred among these are alkyl phosphines, alkyl phosphine oxides, pyridines, furans, ethers, amines and alcohols. Coordinating solvents that are particularly preferred for cadmium chalcogenide nanocrystals are tri-n- 55 octylphosphine and tri-n-octylphosphine oxide. A coordinating solvent that is particularly preferred for zinc chalcogenide nanocrystals is hexadecylamine. In certain embodiments of the invention, notably Cd-Se systems, a mixture of tri-n-octylphosphine and tri-n-octylphosphine 60 oxide offers particular benefits, the former potentially serving as a preferential coordinator for Se and the latter for Cd.
The reactions performed in accordance with this invention are performed on a continuous-flow basis in the thermally conductive reaction tube. The tube is thermally conductive 65 in order to permit efficient heat transfer between the heat transfer medium surrounding the tube and the reaction
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mixture flowing through the tube. As will be well appreciated by those skilled in the art, the efficiency of the heat transfer is also dependent on the internal diameter and wall thickness of the tube and on the composition of the heat transfer medium surrounding the tube. As an alternative, the reactants can be preheated outside the reaction tube and combined immediately prior to entry into the tube.
While this invention is not intended to be limited to specific values for the diameter and length of the tube, the optimal values of these dimensions will be determined by considerations of the viscosity of the reaction mixture and the pressure drop needed to drive the reaction mixture through the tube, both of which will depend on the concentration of the reactants in the reaction mixture, the temperature, and other parameters. The optimal dimensions can be determined by routine experimentation, or by the use of relationships that are well known among those skilled in fluid dynamics. In general, however, successful results will be obtained with a reaction tube having an internal diameter of about 1.0 mm or less, and preferably within the range of about 0.1 mm to about 1.0 mm, and most preferably within the range of from about 0.25 mm to about 0.8 mm. The reaction tube will have a wall thickness great enough to provide dimensional stability and sturdiness to the tube but the wall may otherwise be as thin as possible. If the tube material has a high heat conductivity, the tube will permit rapid heat transfer and the choice of wall thickness will be of little importance. In some cases, such as a tube embedded in a solid temperature control block, the reaction tube may be continuous with the block, whereupon wall thickness of the tube itself is not a consideration.
Any temperature changes that are imposed on the reaction mixture during its passage through the reaction tube will likewise depend on the tube diameter and on the flow rate of the reaction mixture. Flow rates may vary, and the invention is not intended to be limited to specific flow rates. Effective results will be achieved however at flow rates within the range of from about 10 fiL per minute to about 1000 fiL per minute, preferably from about 30 ,wLper minute to about 300 fiL per minute. In certain embodiments of the invention, faster flow rates can be used.
The degree or extent of reaction also depends on the concentrations of the reactants, the length of the reaction tube, and the temperature and pressure of the reaction mixture inside the tube. As noted above, the temperature may be imposed by a heat transfer medium surrounding the tube itself or by preheating the reactants prior to their entry into the tube. None of these operating parameters are limited to specific values in this invention, and each may vary considerably in accordance with the type of product being prepared and the characteristics and qualities that are sought in the product. The appropriate selection of these parameters is a matter of routine skill to those experienced or familiar with batchwise processes for these reactions. The choice of the tube dimensions, for example, will depend on the desired flow rate and temperature as well as other parameters of the system. In most applications, it is contemplated that the reaction tube will be from about 3 cm to about 300 cm in length, preferably from about 10 cm to about 100 cm in length. Likewise, the most typical temperatures will be at least about 100° C, and preferably within the range of from about 100° C. to about 400° C, more preferably within the range of about 250° C. to about 400° C. These temperature ranges are applicable to both the nanocrystal core-forming reaction and the coating reaction.
As noted above, additional control of the reaction can be achieved by the introduction of a reaction promoter to the
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