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  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B5/00—Single-crystal growth from gels
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  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
  • C30B29/10—Inorganic compounds or compositions
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• • C—CHEMISTRY; METALLURGY
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  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
  • C30B29/10—Inorganic compounds or compositions
  • C30B29/46—Sulfur-, selenium- or tellurium-containing compounds
  • C30B29/48—AIIBVI compounds wherein A is Zn, Cd or Hg, and B is S, Se or Te
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  • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
  • C30B29/60—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape characterised by shape
  • C30B29/605—Products containing multiple oriented crystallites, e.g. columnar crystallites
• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B7/00—Single-crystal growth from solutions using solvents which are liquid at normal temperature, e.g. aqueous solutions
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N15/00—Investigating characteristics of particles; Investigating permeability, pore-volume, or surface-area of porous materials
  • G01N15/02—Investigating particle size or size distribution
  • G01N15/0205—Investigating particle size or size distribution by optical means, e.g. by light scattering, diffraction, holography or imaging
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N15/00—Investigating characteristics of particles; Investigating permeability, pore-volume, or surface-area of porous materials
  • G01N2015/0038—Investigating nanoparticles

Definitions

• the present invention provides new compositions containing colloidal nanocrystals in which the as-prepared nanocrystals are substantially monodiserse, and hiving high photoluminescence quantum yields.
• This invention also encompasses new synthetic method for the synthesis of substantially monodisperse colloidal nanocrystals using new preparative methods that afford tunable crystal size, shape, and size/shape distribution.
• non- coordinating solvents in the synthetic process, these procedures constitute easier, less expensive, safer, and more environmentally “green” methods than those currently in use.
• This invention is generally applicable to any II- VI or III-V semiconductor material, and should be useful in generating metal,and nonmetal compounds as well.
• the advantages of the use of non-coordinating solvents are demonstrated herein using semiconductor nanocrystals as the examples.
• High quality colloidal nanocrystals are nanometer sized fragments formed in solution with well-controlled size, shape, surface structures, and excellent chemical processability.
• chemical processability means that nanocrystals can be treated as solution species.
• Colloidal nanocrystals are of
• ATLLIB0284834.4 great interest for industrial applications and academic studies because of their unique size dependent properties and flexible processing chemistry.
• colloidal nanocrystals particularly of semiconductor materials, continue to exhibit tremendous promise for developing advanced materials, and have attracted great interest for their utility in fundamental research.
• These nanocrystal-based emitters can be used for many purposes, such as light-emitting diodes, lasers, biomedical tags, photoelectric devices, solar cells, catalysts, and the like.
• the lack of adequate synthetic methods for preparing high quality nanocrystals has hampered progress in this area, and delayed the timely development of advanced applications for these unique materials.
• Present synthetic schemes for semiconductor nanocrystals including various organometallic approaches and their inorganic alternatives, are sometimes irreproducible and often provide crystals that are low in quality, possess high polydispersities, and may be plagued by impurities.
• CdSe nanocrystals using dimethyl cadmium (Cd (CH 3 ) 2 ) as the cadmium precursor is now well developed (Murray et al., J. Am. Chem. Soc. 1993, 115, 8706-8715; Barbera-Guillem, et al., U.S. Patent No. 6,179,912; Peng et al., Nature 2000, 404, 69-61; Peng et al.., J. Am. Chem. Soc. 1998, 120, 5343-5344).
• Cd (CH 3 ) 2 dimethyl cadmium
• Cd(CH 3 ) 2 is explosive by releasing large amounts of gas.
• the Cd(CH ) 2 related synthesis methods require very restrictive equipment and conditions and, thus, are not ideal for large-scale synthesis.
• CdSe is the only compound for which nanocrystals having a relatively monodisperse size distribution can be directly synthesized (Peng, et al., J. Am. Chem. Soc. 1998, 120, 10, 5343-5344). Peng, et al. reported that the size distribution of CdSe
• ATLLIB0284834.4 nanocrystals can approach monodispersity (polydispersity index, PDI * 1), by controlling the monomer concentration in the initial reaction solution, and that CdSe nanocrystal size could be controlled by adjusting the time for crystal growth.
• PDI * 1 monodispersity index
• CdSe nanocrystal size could be controlled by adjusting the time for crystal growth.
• the improved method would be amenable to syntheses in the air, rather than requiring an inert atmosphere, and it would use solvents that are liquid at room temperature to provide lower costs relative to current methods. If possible, the new method would also impart the ability to control the size of the nanocrystals produced, without sacrificing the desired narrow size distribution.
• the present invention demonstrates that, despite the general belief that coordinating solvents are necessary for preparing semiconductor nanocrystals, these materials may in fact be prepared in non-coordinating solvents. Therefore, this invention exhibits the desired features described above by providing synthetic methods that produce high quality, small, and highly monodisperse semiconductor nanocrystals.
• the present invention addresses the current limitations in the availability of high-quality colloidal nanocrystals by providing colloidal nanocrystals that, in
• the high monodispersity that can be achieved in this invention is seen in the photoluminescence emission line of the nanocrystals, which can have a full width at half maximum (FWHM) as narrow as 23-24 run, with typical FWHM values of around 18-25 nm.
• the as-prepared nanocrystals described herein are further characterized by the photoluminescence quantum yield (PL QY) of up to about 60%.
• As-prepared nanocrystals ranging in size from about 1-6 nm in average diameter are typical, with the size range of these nanocrystals being very monodisperse, with 4-5 nm sizes being commonly prepared.
• this invention encompasses new products and devices incorporating these nanocrystals, such as light-emitting diodes, biological labeling agent such as biomedical tags, photoelectric devices including solar cells, catalysts, lasers, and the like.
• prior nanocrystal syntheses typically require additional processing or size sorting steps after crystallization to achieve the desired size, size distribution, and other properties of the sample.
• the present invention affords improved sizes, size distributions, photoluminescence quantum yields, and related physical and chemical properties for colloidal nanocrystals in their "as-prepared” state, without the need for size sorting or further processing steps.
• the present invention further addresses the current limitations in synthesizing monodisperse semiconductor nanocrystals by establishing that coordinating solvents are not intrinsically required for the synthesis of high quality semiconductor nanocrystals.
• This concept is used to develop new synthetic methods that afford, very selectively, tunable crystal sizes/shapes and size/shape distributions.
• non-coordinating solvents allows more environmentally innocuous precursors and ligands to be employed.
• this new and reproducible synthetic method is significantly “greener” and less expensive than the existing schemes since the typical organophosphine/organophosphine oxide coordinating solvents are supplanted by non-coordinating solvents such as octadecene (ODE).
• ODE octadecene
• one embodiment of this invention involves a method of synthesizing semiconductor nanocrystals by combining a cation precursor, a ligand, and a non-coordinating solvent to form a cation-ligand complex and then admixing an anion precursor dissolved in a non-coordinating solvent with the cation-ligand complex at a temperature sufficient to form nanocrystals.
• the cation precursors can be elements, covalent compounds, or ionic compounds, including coordination complexes or a metal salt, that serve as a source for the electropositive element or elements in the resulting nanocrystal. When feasible, inexpensive and safe compounds such as metal oxides are preferred.
• Anion precursors can also be elements, covalent compounds, or ionic compounds that serve as a source for the electronegative element or elements in the resulting nanocrystal.
• These definitions anticipate that ternary compounds, quaternary compounds, and even more complex species may be prepared using the methods disclosed herein, in which case more than one cation precursor and/or more than one anion precursor are typically required.
• the methods disclosed herein are applicable to nanocrystals prepared using cation precursor compounds of the group II metals (for example, Zn, Cd or Hg), the group III metals (for example, Al, Ga, or In), the group IV metals (for example, Ge, Sn or Pb), or the transition metals (for example, Ti, Zr, Hf, N, ⁇ b, Ta, Cr, Mo, W, Mn, Tc, Re, Fe, Co, ⁇ i, Pd, Pt, Rh, and the like).
• the group II metals for example, Zn, Cd or Hg
• the group III metals for example, Al, Ga, or In
• the group IV metals for example, Ge, Sn or Pb
• transition metals for example, Ti, Zr, Hf, N, ⁇ b, Ta, Cr, Mo, W, Mn, Tc, Re, Fe, Co, ⁇ i, Pd, Pt, Rh, and the like.
• the cation precursor can constitute a wide range of substances, such as a metal oxide, a metal carbonate, a metal bicarbonate, a metal sulfate, a metal sulfite, a metal phosphate, metal phosphite, a metal halide, a metal carboxylate, a metal alkoxide, a metal thiolate, a metal amide, a metal imide, a metal alkyl, a metal aryl, a metal coordination complex, a metal solvate, a metal salt, and the like.
• the ligand is selected from fatty acids, amines, phosphines, phosphine oxides, or phosphonic acids.
• Anion precursors are most often selected from the element itself
• ATLLIB02 848344 (oxidation state 0), covalent compounds, or ionic compounds of the group V elements (N, P, As, or Sb), the group VI elements (O, S, Se or Te), the group VII elements (F, CI, Br, or I).
• highly monodisperse CdS nanocrystals may be prepared by dissolving the cation precursor CdO in the non- coordinating solvent octadecene (ODE) through its reaction with oleic acid (OA; C ⁇ 8 H 34 O 2 ) at elevated temperatures.
• ODE non- coordinating solvent octadecene
• OA oleic acid
• the CdO, OA, ODE mixture was maintained at around 300°C, while a solution of the anion precursor, elemental sulfur in ODE, was swiftly injected into the hot solution. This hot mixture was then allowed to cool to about 250°C to allow the growth of the CdS nanocrystals.
• This readily reproducible preparation can be carried out either under argon or open to air.
• nearly monodisperse CdTe nanocrystals may be prepared by dissolving CdO in ODE through its reaction with oleic acid (OA) at elevated temperatures.
• the CdO, OA, ODE mixture was maintained at around 270-300°C, while a solution of the anion precursor, elemental tellurium reacted with trihexylphosphine (THP) with a Te : THP molar ratio as about 1 : 1.1 in ODE, was swiftly injected into the hot solution. This hot mixture was then allowed to cool to about 250°C to allow the growth of the CdTe
• THP trihexylphosphine
• ATLLIB02 84834.4 nanocrystals This readily reproducible preparation can be carried out under argon.
• the shape of the as-prepared CdTe nanocrystals can be tuned between mondisperse dots, rods, and branched shapes by varying the ligands.
• nearly monodisperse CdSe nanocrystals may be prepared by dissolving CdO in ODE through its reaction with stearic acid (SA) at elevated temperatures.
• the CdO, SA, ODE mixture was cooled down to room temperature and hexadecylamine (HDA) was added into the mixture as a co-ligand.
• the reaction mixture was consequently heated up to and maintained at around 270-300°C, while a solution of the anion precursor, elemental selenium reacted with tributylphosphine (TBP) with a Se : TBP molar ratio as about 1 : 1.1 in ODE, was swiftly injected into the hot solution.
• TBP tributylphosphine
• non-coordinating solvent systems presents significant design advantages in the preparation of nanocrystals, because these solvents allow the reactivity of precursor monomers to be tuned by simply varying the ligand concentration in solution. This tunable reactivity provides the necessary balance between crystal nucleation and crystal growth, which is the key for controlling the size and size distribution of the resulting nanocrystals. In practice, such tunability has the great potential to promote the synthesis of various semiconductor nanocrystals to the level of that of the well-developed CdSe nanocrystals in coordinating solvents.
• ATLLIB02 848344 size-selected samples prepared through existing coordinating-solvent methods is significantly worse than that of the as-prepared nanocrystals through the new methods described herein.
• the quality of nanocrystals synthesized in non-coordinating solvents is at least comparable to those prepared by traditional organometallic synthesis in coordinating solvents, yet they are produced using far less dangerous and less toxic materials.
• Non-coordinating solvents afford further practical advantages to the synthetic methods disclosed here.
• ODE is liquid, rather than a solid as many coordinating solvents are, thereby contributing to the excellent processability of this synthetic system.
• the non-coordinating solvent based synthesis of this invention generally takes about 3-4 hours per sample preparation, which is significantly faster than the existing schemes using coordinating solvents (3-7 days per sample preparation for InP nanocrystals).
• Yet another aspect of the present invention is the development of a method of synthesizing monodisperse semiconductor nanocrystals using non- coordinating solvents.
• Still another aspect of this invention is discovery of a method to control the polydispersity index (PDI) of semiconductor nanocrystals during their synthesis.
• PDI polydispersity index
• Still another aspect of this invention is the development of a method for the synthesis of shape-controlled nanocrystals, such as rods and branched nanocrystals, which are monodisperse on all three dimensions.
• a further aspect of this invention is the development of a procedure for controlling the average size of the semiconductor nanocrystals prepared using non-coordinating solvents.
• Another feature of this invention involves methods for improving the quality of nanocrystals synthesized in non-coordinating solvents, such that their quality is comparable or better than those prepared by traditional organometallic synthesis in coordinating solvents.
• Another aspect of the present invention is the development of methods for synthesizing monodisperse CdS, CdSe, CdTe, ZnSe, InP, and InAs, without the need for size sorting.
• Yet a further aspect of this invention is to develop synthetic methods for nanocrystals that may be carried out in reaction vessels open to the air, without the need for inert atmosphere.
• An additional aspect of this invention is developing a synthetic scheme for monodisperse nanocrystals that using non-coordinating solvents that are liquid at room temperature, and processing and recycling procedures that contribute to the excellent processibility of the system, and are amendable to recycling procedures.
• Still another aspect of this invention is the development of synthetic procedures for preparing nanocrystals that allow more environmentally innocuous precursors, ligands, and solvents to be employed, and that are convenient, less expensive, safer, faster, and more environmentally "green” than methods currently used.
• FIGURES Figure 1 exemplifies both the UV-Vis absorption spectra and photoluminescence (PL) spectra of the "as-prepared" CdS nanocrystals with different sizes, without any size separation or processing.
• Figure 2 is a representation of the X-ray diffraction pattern (top), TEM image (bottom, left), and the related size distribution diagram of the CdS nanocrystals (bottom, right).
• Figure 3 demonstrates the temporal evolution of the absorption spectrum of the CdS nanocrystals grown in ODE with different oleic acid concentrations. The absorption peaks dedicated to a magic sized nanocluster are marked with a star.
• Figure 4 illustrates the following. Left: Spectroscopic demonstration of the separation of CdS nanocrystals from oleic acid and unreacted cadmium oleate. Right: Temporal evolution of the monomer concentrations in ODE with different oleic acid concentrations.
• Figure 5 illustrates the TEM images of the as-prepared CdTe nanocrystals. Top: Dot-shaped CdTe nanocrystals. Bottom: Tetrapole-shaped CdTe nanocrystals.
• Figure 7 demonstrates the temporal evolution of UV-Vis spectra of InP nanocrystals grown at 270°C with different In:MA ratios in ODE, where MA is myristic acid.
• Figure 8 is a representation of the UV-Vis spectra of InP nanocrystals grown by multiple injections. A secondary injection was performed after the nanocrystals grew for 5-10 minutes without changing the absorption spectrum.
• FIG. 9 represents the following. Left: PL, photoluminescence excitation (PLE), and UN-Nis spectra of InP nanocrystals. Right-Top: Transmission Electron Microscope (TEM) image of InP nanocrystals. Right-Bottom: XRD pattern of InP nanocrystals. Figure 10 demonstrates the temporal evolution of UV-Vis spectra of the as-prepared InAs nanocrystals grown in the non-coordinating solvent.
• TEM Transmission Electron Microscope
• the present invention provides substantially monodisperse semiconductor nanocrystals, as well as new synthetic methods for selectively producing monodisperse nanocrystals. These methods provide tunable crystal sizes/shapes and size/shape distributions, are versatile and highly selective, and often provide higher quality crystals than present methods. It is established herein that coordinating solvents are not intrinsically required for the synthesis of high quality semiconductor nanocrystals, notwithstanding the conventional notion that coordinating solvents are necessary for preparing semiconductor nanocrystals.
• oleic acid (OA) was used as a ligand and octadecene (ODE) acted as the solvent for both cadmium precursor (cadmium oleate, formed in situ by the reaction of CdO with oleic acid) and sulfur precursor
• ATLLIB02 84834.4 (element sulfur). Apart from these four very commonly used compounds, no other chemical is required.
• the non-coordinating solvent used to prepare CdS nanocrystals was octadecene (ODE; C ⁇ 8 H 36 ), which is a liquid at room temperature and boils at about 320 °C.
• Oleic acid (OA; C] 8 H 34 O 2 ) a natural surfactant, was chosen as the ligand for stabilizing the resulting nanocrystals and the cation precursors.
• the usual precursors were CdO and elemental sulfur, both of which are natural minerals.
• An ODE solution of elemental sulfur can be used as the sulfur precursor solution for the formation of CdS nanocrystals.
• CdO was dissolved in ODE through its reaction with oleic acid (OA) at elevated temperatures.
• OA oleic acid
• the CdO, OA, ODE mixture was maintained at around 300°C, while a sulfur solution (elemental sulfur in ODE) was swiftly injected into the hot solution.
• This hot mixture was then allowed to cool to about 250°C to allow the growth of the CdS nanocrystals.
• This preparation can be performed under an argon flow without degassing the reaction
• ATIXIB0284834.4 system, and can even be performed in air without sacrificing the quality of the nanocrystals. Reaction progress was monitored by UN- Vis absorption spectroscopy and photoluminescence (PL) spectra by sampling aliquots from the reaction flask.
• Useful non-coordinating solvents for preparing high quality semiconductor nanocrystals are generally selected using the following guidelines. First, the solvent should possess a relatively high boiling point (around 300°C or higher) for growing highly crystalline nanocrystals and a relatively low melting point (approximately 20°C or lower) for a convenient, room temperature workup after synthesis. The high boiling point preference is based on the fact that the high quality semiconductor nanocrystals are typically synthesized at high temperatures.
• low temperature semiconductor nanocrystal synthetic methods have also been developed. For example, we have recently developed a process by which CdSe nanocrystals can be grown at about 100 °C, indicating that water may be able to act as a solvent for the growth of high quality semiconductor nanocrystals.
• useful non-coordinating solvents will have a melting point less than about 25 °C and a boiling point greater than about 250°C.
• reactants and products alike should be soluble and stable in the selected solvent.
• the solvent should be as universal as possible for its ability to dissolve common starting materials and therefore for synthesizing high quality inorganic nanocrystals.
• the solvent should be safe, relatively inexpensive, and easy to dispose of or recycle. Based on these standards, the traditional coordinating solvent, TOPO, is significantly worse than the coordinating solvents, since it has a high melting point (about 60°C), and is quite expensive and toxic.
• ATLLIB02848344 Tech-ODE is about 310-340°C at 1 atm, and its melting point is between 15- 17°C. Apparently, the double bond of ODE increases its boiling point and decreases its melting point in comparison to octadecane. In addition to the above advantages, element sulfur has a significant solubility in Tech ODE, which may be a result of the slight polarity of the double bond. We also note that " in some preparations, certain ethers can constitute reasonable non-coordinating solvents.
• Figure 1 presents UV-Vis absorption spectra and photoluminescence (PL) spectra of high quality CdS nanocrystals formed in ODE, where the average particle size for each spectrum is further provided in the plot.
• the UV-Vis abso ⁇ tion and PL spectra shown in Figure 1 are the sha ⁇ est ones for CdS nanocrystals synthesized in any solvent, indicating a very narrow size distribution of the nanocrystals formed in ODE.
• Figure 1 also reveals the high monodispersity that can be achieved in this invention by examining the photoluminescence emission line of the nanocrystals, which can have a full width at half maximum (FWHM) as narrow as 23-24 nm, with typical FWHM values of around 18-25 nm.
• FWHM full width at half maximum
• PL QY photoluminescence quantum yield
• the PL of the CdS nanocrystals is dominated by their band edge emission, except for those spectra for the nanocrystals smaller than about 2 nm in diameter (data not shown), which is usually not the case for CdS nanocrystals.
• the achievable size range shown in Figure 1 is also plausible when compared to the existing synthetic schemes.
• ATLLIB02 84834.4 presented here can reproducibly and controllably generate CdS nanocrystals in almost the entire quantum confined size regime (about 1-6 nm), with the first exciton abso ⁇ tion peak from 305 nm to about 440 nm.
• Example 7 and Table 1 demonstrate how the time of reaction is correlated with CdS nanocrystal size in a single reaction.
• Focusing also indicates that the smaller nanocrystals grow faster than the large nanocrystals such that the size distribution narrows. It is likely that growth of CdS nanocrystals in this system is diffusion controlled. It is well-established that, with an instantaneous nucleation, a diffusion controlled growth should possess a very efficient focusing behavior as the nanocrystals grow, which is exactly the case shown in Figure 3. If the reaction was allowed to proceed for a long time, defocusing of size distribution or Ostwald ripening would occur. (X. Peng, J. Wickham, A. P. Alivisatos, Journal of the American Chemical Society 120 (1998) 5343.)
• the reaction data shown in Figures 1-3 can be used to guide the synthesis of CdS nanocrystals to provide samples with a narrow size distribution, between about 1 nm and about 6 nm.
• This size range can be further tuned by changing the injection temperature, the growth temperature, the concentration of oleic acid and monomers, the molar ratio of the two precursors, and the like, so that the balance between nanocrystal nucleation and growth is maintained. Adjusting some of these parameters results in complex behavior in the evolution of the resulting nanocrystal size and size distribution. For example, increasing the injection temperature does not always result in a simple increase in the reaction rate, but rather often results in a slower reaction between precursors.
• ATLLIB0284834.4 amount of bulk sized CdS particles were observed.
• concentration of OA in ODE solvent decreased, the growth rate of the nanocrystals slowed systematically, and the size distribution of the resulting nanocrystals became significantly narrower at the focusing point of the size distribution, indicated by the sha ⁇ ness of the first (shortest time) abso ⁇ tion peak of the sha ⁇ est spectrum in each series.
• PDI polydispersity index
• a reaction mixture for the synthesis of CdS in ODE after a given reaction time (indicated in Figure 3) was sampled and separated to two fractions by extraction with a 1:1 (v:v) mixture of CHC1 3 and CH OH.
• ATLLIB02848344 decreased. This conclusion indicates that the reactivity of the monomers in the solution increases significantly when the ligand concentration in solution decreases.
• the depletion rate of the monomers did not change much with a different initial oleic acid concentration after the initiation stage of the reactions, although the remaining monomer concentration was higher for the reactions with a higher oleic acid concentration. While not intending to be bound by the following statement, it is likely this effect is caused by two conflicting factors. In comparison to a reaction with a lower ligand concentration, the reactivity of the monomers of a give reaction was lower, but the remaining concentration of the monomers was higher.
• the influence of the ligand concentration in controlling the size distribution of growing colloidal nanocrystals is achieved by a balance between nucleation and growth.
• a successful synthetic scheme should start with a fast and short nucleation period, which is followed by a growth stage without either prolonged nucleation or ripening, which is referred as "focusing of size distribution".
• focusing of size distribution X. Peng, J. Wickham, A. P. Alivisatos, Journal of the American Chemical Society 120 (1998) 5343.
• the remaining monomers would not be sufficient to promote the focusing of size distribution for a sufficient time, and this would result in an undesired Ostwald ripening or "defocusing" of size distribution.
• the growth reaction would be too fast to be controlled to reach desired size and size distribution.
• a nearly continuous tunable reactivity of the monomers is desirable.
• tunability can be readily achieved by simply altering the ligand concentration in a non-coordinating solvents. This tunability may indicate that the cadmium monomers in the solution at elevated temperatures are not simply cadmium oleate.
• the number of "nearby" ligands for each cadmium ion may strongly depend on the concentration of the ligands in the bulk solution.
• Nanocrystals of ZnSe regardless of their size, cannot be formed in pure OA, pure trioctylphosphine oxide (TOPO), or a mixture of a fatty acid and TOPO as the coordinating solvent. Attempts to prepare ZnSe using traditional organometallic approaches (for example with Zn(CH 3 ) 2 as a precursor), also failed in these pure coordinating solvents. However, ZnSe nanocrystals with acceptable monodispersities were formed in dilute OA solutions in ODE using Zn(Ac) 2 , Se-TBP, OA and ODE reaction mixtures.
• TOPO trioctylphosphine oxide
• Non-coordinating solvents also generated CdTe nanocrystals with superior control over their size, shape, size/shape distribution in a large size range, between about 1 to 20 nm.
• Figure 5 illustrates the TEM images of as- prepared CdTe nanocrystals, which exhibit different shapes as a function of reaction conditions and reagents used. For example, the dots produced in Figure
• ATLLIB02 84834.4 5 top, are prepared from Te in PBu 3 or P(hexyl) 3 with OA as the ligand, while the tetrepoles (tetrahedral shaped) nanocrystals in Figure 5, bottom, are prepared from Te in P(octyl) 3 with OA as the ligand. Further, shape adjustments also may result from concentration adjustments in the ligand.
• the PL QY of the CdTe nanocrystals produced by this method is typically around 50%.
• a range of ligands could be used in the preparation of II-VI semiconductor nanocrystals.
• numerous fatty acids, amines, phosphonic acids, phosphines, phosphine oxides, various surfactants and the like were found to be good ligands for the synthesis of CdSe, CdS, and CdTe nanocrystals in non-coordinating solvents.
• ATLLIB02 84834.4 coordinating solvent, a well-controlled indium to ligand ratio, and a thorough degassing process are all significant factors for synthesizing high quality InP nanocrystals, without requiring any size sorting. These observations have been generally applicable for developing synthetic methods for other III-V semiconductor nanocrystals such as InAs.
• a typical synthesis of II-VI semiconductor nanocrystals in non- coordinating solvents such as ODE does not require degassing the entire reaction system, and performing the reaction under a flow of inert gas such as argon is not essential. If fact, the entire process can even be performed in air without sacrificing the quality of the CdS nanocrystals.
• ATLLB0284834.4 dissolved in ODE were used as the precursors for the synthesis of II-VI semiconductor nanocrystals, (W. W. Yu, X. Peng, Angew. Chem. Int. Ed. 41 (2002), 2368; Z. A. Peng, X. Peng, J. Am. Chem. Soc. 123 (2001) 183; C. B. Murray, D. j. Norris, M. G. Bawendi, Journal of the American Chemical Society 115 (1993) 8706) an analogous method was not useful in the III-V nanocrystal synthesis.
• elemental phosphorus was not reactive enough to initiate the formation of InP nanocrystals in ODE (or in a variety of coordinating solvents).
• In 2 0 3 unlike CdO was also not found to be feasible for the synthesis of InP nanocrystals because it is insoluble in ODE in the presence of the desired ligands.
• non-coordinating solvents were used in preparing III-V nanocrystals.
• Octadecene (ODE) was employed as the non- coordinating solvent in the synthesis of InP and InAs, for the reasons disclosed above.
• a number of coordinating compounds such as fatty acids, amines, phosphines, phosphine oxides, phosphonic acids, and the like were tested as pure coordinating solvents, and provided lower quality nanocrystals than those prepared in a non-coordinating solvent.
• Figure 6 illustrates the temporal evolution of the UV-Vis abso ⁇ tion spectra of InP nanocrystals formed in ODE using fatty acids with different chain lengths as the
• fatty acids with intermediate chain lengths such as PA and MA are the best ligands for providing the desired balance between nucleation and growth rate for the growth of relatively monodisperse InP nanocrystals.
• Optimizing the present synthesis of highly monodisperse nanocrystals in solution also used the principle of "focusing of size distribution", which dictates that a quick and short nucleation process, followed by a relatively slow and long growth process, and which optimizes the sample monodispersity.
• concentration of the ligands in a non-coordinating solvent tunes the reactivity of the metal (or cation) precursors to reach the desired balance between nucleation and growth for the formation of high quality nanocrystals.
• III-V nanocrystals such as InP
• the effect of ligand concentration is even more dramatic than for the II-VI nanocrystals described above.
• Figure 7 shows that when the molar ratio of In:MA in the solution was about 1 :3, the reaction generated InP nanocrystals with a good size distribution, as indicated by the well-distinguished abso ⁇ tion features.
• the In:MA molar ratio was varied to either about 1:2 or about 1:4.5, the reaction either proceeded out of control, or generated nanocrystals without any distinguishable abso ⁇ tion peak, implying a broad size distribution.
• temperatures for secondary injections are typically lower then the initial reaction temperature, for example, about 250°C or less.
• the indium precursor and phosphorus precursor are added separately and in an alternating manner.
• secondary injections were carried out at about 270°C (representing the growth temperature after the primary injection), or using indium and phosphorus precursors in a single solution, resulted in a broad size distribution, presumably due to the continuous nucleation caused by the secondary injections.
• the photoluminescence (PL) spectrum of the InP nanocrystals constitutes solely a band-edge emission ( Figure 9, left).
• Transmission electron microscope (TEM) images of InP nanocrystals revealed that the crystals are generally in a dot-shape mixed with some slightly elongated ones ( Figure 9, top right).
• the size of the nanocrystals shown in Figure 9 is 3.1 nm ⁇ 4.7% by measuring 350 nanocrystals.
• the powder X-ray diffraction (XRD) pattern of the InP nanocrystals matches that of the zinc-blende structure of bulk InP crystals, including the (200) diffraction peak which is often difficult to resolve (Figure 9, bottom-right).
• InAs nanocrystals were prepared in a similar manner as InP, using In(Ac) 3 and As(TMS) 3 as cation and anion precursors, respectively, in the presence of fatty acids as the ligand, all in the non-coordinating solvent ODE.
• fatty acids as the ligand, all in the non-coordinating solvent ODE.
• all fatty acids tested with hydrocarbon chain length between 10 and 22 carbon atoms were found to be suitable ligands for the formation of InAs nanocrystals.
• ATLLB0284834.4 nanocrystals shown in Figure 10, indicate that the size distribution of the sample produced in this fashion is superiorly narrow.
• Other III-V semiconductor nanocrystals could be prepared in the above mentioned ways and their variations.
• EXAMPLE 1 Preparation of Monodisperse CdS Nanocrystals in a Non-Coordinating Solvent
• a mixture of 0.10 mmol (0.0128g) of CdO, 1.5 mmol (0.4237g) of oleic acid, and 3.5635 g of technical grade octadecene (ODE, Aldrich Chemical Co., Milwaukee, WI) was prepared and heated. This mixture became optically clear at around 200°C, and was further heated up to about 300°C and maintained at this temperature for the injection of the sulfur solution.
• a sulfur solution was prepared by dissolving 0.05 mmol (0.0016 g) of elemental sulfur in 2 grams of technical grade ODE.
• the preparation of the sulfur solution often required heating up the sulfur/Tech ODE mixture to about 100°C, allowing the sulfur powder to completely dissolve, and then cooling the solution to room temperature.
• the resulting, optically clear solution was stable at room temperature and ready for the injection.
• This sulfur solution was then swiftly injected into the hot, cadmium-containing solution with stirring, after which the reaction mixture was allowed to cool to about 250°C for about 0.5 h, to allow the growth of the CdS nanocrystals.
• the synthesis can be done either under argon or open to air.
• ATLUB02848344 were recorded for each aliquot.
• XRD powder X-ray diffraction
• TEM transmission electron microscopy
• the size distribution diagrams were obtained by measuring about 500 individual CdS nanocrystalline particles using enlarged photographs. All the measurements were performed on the original aliquots without any size sorting of the nanocrystals.
• the separation of the unreacted cadmium precursor from the resulting nanocrystals was accomplished by the repeated extractions of the reaction aliquots in ODE with an equal volume of a 1:1 mixture solvent of CHC1 3 and CH OH. The extraction process was also monitored by a UV-Vis abso ⁇ tion spectrophotometer, to determine when the resulting ODE nanocrystal solution was free from unreacted precursor materials.
• the amount of oleic acid was typically varied from about 0.30 mmol to about 21.2 mmol, as compared to 0.10 mmol (0.0128g) of CdO, with the same amounts of sulfur and solvent as described above.
• EXAMPLE 2 Preparation of Monodisperse CdSe Nanocrystals in a Non-Coordinating Solvent
• the synthesis of CdSe nanocrystals was carried in a similar fashion as described in Example 1 using CdO as the cation precursor, in which the selenium source for injections was an ODE solution of selenium- tributylphosphine (1:1.1 ratio).
• the Se-TBP or other selenium organophosphine compounds was prepared simply by dissolving Se in a desired amount of liquid organophosphine.
• the injection solution was further prepared by diluting the Se- phosphine solution with an adequate amount of ODE.
• long chain amines such as hexadecylamine (HDA) and octadecylamine (ODA) were used as the co-ligands.
• HDA hexadecylamine
• ODA octadecylamine
• ATL IB02848344 EXAMPLE 3 Preparation of Monodisperse CdTe Nanocrystals in a Non-Coordinating Solvent
• the synthesis of CdTe nanocrystals was carried in a similar fashion as described in Example 1 using CdO as the cation precursor, in which the tellurium source for injections was an ODE solution of tellurium- tributylphosphine.
• the Te-TBP or other tellurium organophosphine compounds was prepared simply by dissolving Te in a desired amount of liquid organophosphine.
• the injection solution was further prepared by diluting the Te- phosphine solution with an adequate amount of ODE.
• the as-prepared CdTe nanocrystals possess very high PL QY, typically around 50%, without any further treatment.
• the as- prepared nanocrystals should be stored under air-free conditions, which is different from the bright CdSe nanocrystals.
• the synthesis of ZnSe nanocrystals was carried in a similar fashion as described in Example 1 using Zn(Ac) 2 as the cation precursor, in which the selenium source for injections was an ODE solution of selenium- tributylphosphine (1:1.1 ratio).
• the Se-TBP or other selenium organophosphine compounds was prepared simply by dissolving Se in a desired amount of liquid organophosphine.
• the injection solution was further prepared by diluting the Se- phosphine solution with an adequate amount of ODE.
• a 0.05 mmol-sample of P(TMS) 3 (0.0125 g) was dissolved in ODE in a glovebox, 2 g in total, and injected into the hot reaction flask. While various ratios of indium to phosphorus reagents can be used, the best results for both single and multiple injection reactions were achieved by maintaining about a 2:1 indium to phosphorus molar ratio. After the injection, the temperature was dropped down to 270°C for 1-2 h, for the growth of the InP nanocrystals. For multiple injections, successful secondary injections were performed dropwise at 250°C, by alternating 1 g injections of indium and phosphorus solutions in about half the molar concentration of the original solutions.
• ATLLIB0284834.4 clear, yellowish solution at room temperature.
• the nanocrystals synthesized by the present scheme including the aliquots taken at different reaction times, were soluble in typical non-polar solvents such as hexanes, toluene and chloroform. No size sorting of any type was performed for the samples used for all of the measurements, including the size measurements disclosed herein. Size measurements were carried out as described in Example 1.

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