Reduced molten inorganic salt solvents for cation-exchange of iii-v colloidal nanocrystals

Abstract

Methods for synthesizing ternary or higher-order group III-V nanocrystals from lower-order group III-V nanocrystals via ion-exchange in a molten inorganic salt containing a group III metal element are provided. In the methods, binary or higher-order group III-V nanocrystals are dispersed in an inorganic molten inorganic salt that contains a group III metal element having an oxidation state of less than III. In the molten inorganic salt, group III ions in the nanocrystals undergo ion-exchange with group III ions in the salt to increase the order of the group III-V nanocrystals.

Description

Atty. Dkt. No.05400-0072-PCT REDUCED MOLTEN INORGANIC SALT SOLVENTS FOR CATION-EXCHANGE OF III-V COLLOIDAL NANOCRYSTALS CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to U.S. provisional patent application number 63 / 662,511 that was filed June 21, 2024, and to U.S. provisional patent application number 63 / 562,873 that was filed March 8, 2024, the entire contents of both of which are incorporated herein by reference. REFERENCE TO GOVERNMENT RIGHTS

[0002] This invention was made with government support under 2019444 awarded by the National Science Foundation. The government has certain rights in the invention. BACKGROUND

[0003] Nanometer scale crystals (nanocrystals) of semiconductor materials demonstrate size-dependent physical and chemical properties which can be used in displays, lasers, photodetectors, and solar cells. These materials also have uses in emerging applications in quantum information science, high-performance photonics, and photocatalysis. Successful commercial applications of nanocrystals have proven that colloidal synthesis, which is inherently scalable, is competitive in terms of material quality with much more expensive semiconductor wafer and thin-film technologies such as molecular beam epitaxy (MBE) and chemical vapor deposition (CVD). To date, control over size, shape, and luminescent properties has been achieved for colloidally synthesized II-VI semiconductors (Zn, Cd and Hg chalcogenides), IV-VI semiconductors (Pb chalcogenides), lead halide perovskites, indium pnictides (InP, InAs, and InSb), and other materials beyond semiconductors. However, there are still critical omissions in the list of success stories for colloidal semiconductors, particularly gallium-containing ternary and quaternary III-V materials. SUMMARY

[0004] Methods for synthesizing ternary or higher-order group III-V nanocrystals from pre-synthesized lower-order group III-V nanocrystals via ion-exchange in a molten inorganic salt containing a group III metal element are provided. Also provided are nanocrystals made using the methods.Atty. Dkt. No.05400-0072-PCT

[0005] One embodiment of a method of forming group III-V nanocrystals includes the steps of: forming a dispersion of binary or higher-order group III-V nanocrystals in a molten inorganic salt comprising group III metal ions in an oxidation state of less than III; maintaining the dispersion at a temperature at which group III metal ions of the molten inorganic salt undergo ion exchange with group III metal ions in the binary or higher-order group III-V nanocrystals to form ternary or higher-order group III-V nanocrystals; cooling the dispersion to form a composite comprising the ternary or higher-order group III-V nanocrystals dispersed in a solid salt matrix comprising a group III element; and separating the ternary or higher-order group III nanocrystals from the solid salt matrix

[0006] Other principal features and advantages of the disclosure will become apparent to those skilled in the art upon review of the following drawings, the detailed description, and the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Illustrative embodiments of the disclosure will hereafter be described with reference to the accompanying drawings.

[0008] FIGS.1A-1F: Importance of redox potential control in molten inorganic salt solvents for transformation of III-V nanocrystals. Powder X-ray diffraction patterns for (FIG. 1A) InSb nanocrystals treated with KGaI4 molten inorganic salt and Ga[GaI4] molten inorganic salt at 325 °C for 1 hour. (FIG.1B) Raman spectra of KGaI4(bottom) and KInI4(top) compared with the salt matrix after reacting InSb nanocrystals and InAs nanocrystals respectively in KGaI4. Powder X-ray diffraction patterns for (FIG.1C) InAs nanocrystals treated with KGaI4 molten inorganic salt and Ga[GaI4] molten inorganic salt at 400 °C for 1 hour. (FIG.1D) TEM (top) and HRTEM (bottom) of In1-xGaxSb, In1-xGaxAs and In1-xGaxP nanocrystals prepared using Ga[GaI4]. (FIG.1E) TEM (top) and HRTEM (bottom) of quaternary In1-xGaxAs1-ySbyand In1-xGaxP1-yAsyand pentamery In1-xGaxP1-y-zAsySbznanocrystals prepared using Ga[GaI4] molten inorganic salt. (FIG.1F) Powder x-ray diffraction patterns of InSb treated in KGaI4at different temperatures.

[0009] FIGS.2A-2C: Molten inorganic salt redox chemistry. (FIG.2A) Relationship between molten inorganic salt redox chemistry and III-V nanocrystals controlling their chemical stability. (FIG.2B) Semiquantitative electrochemical scale for key reactions governing stability of the pnictide component of the III-V nanocrystals compared with redoxAtty. Dkt. No.05400-0072-PCT reactions of III-halide molten inorganic salt components. (FIG.2C) Molten inorganic salt redox chemistry for precursor activation during nanocrystal synthesis.

[0010] FIGS.3A-3E: (FIG.3A) Powder x-ray diffraction patterns for ~3 nm InSb nanocrystals annealed in Ga[GaI4] at 350oC for different times (FIG.3A) and temperatures (FIG.3B). Extracted gallium content (circles) and Scherrer size (squares) for the time (FIG. 3C) and temperature (FIG.3D) series. (FIG.3E) Arrhenius plot of the apparent diffusion coefficients measured as a function of temperature with the extracted activation energy (FIG. 3E).

[0011] FIGS.4A-4D: Characterization of In1-xGaxAs1-ySby alloy nanocrystals (FIG.4A) Powder x-ray diffraction for InAs33Sb67nanocrystals (bottom) treated with Lewis acidic (middle bottom), Lewis neutral (middle top), and reduced (top) molten inorganic salts at 400 °C for 1 hour. (FIG.4B) TEM and HRTEM images of InAs33Sb67nanocrystals annealed in Ga[GaI4] at 375 °C for 1 hour. (FIG.4C) X-ray diffraction patterns for InAs49Sb51 nanocrystals annealed in Ga[GaI4] under different conditions. (FIG.4D) Atom percent Ga, In, As, and Sb in the recovered nanocrystals measured by XRF corresponding to the samples shown in FIG.4C (filled symbols) and lattice constant of the nanocrystals (open symbols) measured by x-ray diffraction.

[0012] FIGS.5A-5D: Characterization of In1-xGaxP1-yAsy alloy nanocrystals (FIG.5A) Powder x-ray diffraction for InP16As84nanocrystals (bottom) treated with Lewis acidic (middle bottom), Lewis neutral (middle top), and reduced (top) molten inorganic salts at 400 °C for 1 hour. (FIG.5B) TEM and HRTEM images of InP16As84nanocrystals annealed in Ga[GaI4] at 400 °C for 1 hour. (FIG.5C) X-ray diffraction patterns for InP17As83 nanocrystals annealed in Ga[GaI4] under different conditions. (FIG.5D) Atom percent Ga, In, P, and As in the recovered nanocrystals measured by XRF corresponding to the samples shown in FIG.5C (filled symbols) and lattice constant of the nanocrystals (open symbols) measured by x-ray diffraction.

[0013] FIGS.6A-6D: Characterization of In1-xGaxP1-ySby and In1-xGaxP1-y-zAsySbz nanocrystals. (FIG.6A) X-ray diffraction patterns for InP13Sb87annealed under different conditions in Ga[GaI4] molten inorganic salt and (FIG.6B) atomic percent of Ga, In, P, and Sb determined by XRF for the samples shown in A (closed symbols) and lattice constant determined by XRD (open symbols). (FIG.6C) X-ray diffraction patterns for InP12As47Sb41 annealed under different conditions in Ga[GaI4] molten inorganic salt and (FIG.6D) atomicAtty. Dkt. No.05400-0072-PCT percent of Ga, In, P, As, and Sb determined by XRF for the samples shown in FIG.6C (closed symbols) and lattice constant determined by XRD (open symbols).

[0014] FIG.7: PL spectra collected with identical spectrometer parameters with λex of 810 nm normalized by the absorbance of the sample at 810 nm plotted on a linear intensity scale.

[0015] FIGS.8A-8D: (FIG.8A) Room temperature Raman spectra of indium iodide molten inorganic salts with different In:I ratios with inset focused on the In-I symmetric stretch mode. (FIG.8B) Room temperature Raman spectra of the resulting indium iodide salt after reaction between KInI4 and InP, InAs or In(0)respectively at 400°C for 1 hour with inset focused on the In-I symmetric stretch mode. (FIG.8C) X-ray diffraction patterns of the resulting solid products after reaction of InP and InAs nanocrystals in KInI4 at 400°C for 1 hour. (FIG.8D) Relative position of redox reactions relevant to III-V colloidal nanocrystals.

[0016] FIGS.9A-9D: (FIG.9A) Raman spectra of the solidified molten inorganic salt after annealing InP, InAs, and InSb nanocrystals in KGaI4 at 400°C for 1 hour (middle spectra) and compared with the Raman spectra for pure KGaI4and KInI4(top and bottom spectra respectively). (FIG.9B) Room temperature Raman spectra of the products resulting from the reaction between KGaI4and a variety of reducing agents at 400°C for 1 hour. (FIG. 9C) Room temperature Raman spectra of gallium halide salts used in this work. (FIG.9D) High temperature Raman spectra of Ga[GaI4] molten inorganic salt.

[0017] FIGS.10A-10C: Powder x-ray diffraction patterns of the solid products resulting from annealing III-V nanocrystals eutectic GaI3 / KI (violet), KGaI4 (red), and Ga[GaI4] (orange) for (FIG.10A) InSb nanocrystals, (FIG.10B) InAs Nanocrystals, and (FIG.10C) InP nanocrystals.

[0018] FIGS.11A-11E: TEM (FIG.11A) and HRTEM (FIG.11B) of In1-xGaxP0.17As0.83 / ZnSe with a 1ML shell grown by colloidal ALD (see Methods). UV-vis absorption spectra (FIG.11C) and (FIG.11D) Photoluminescence spectra of InP17As83 nanocrystals which have been treated in Ga[GaI4] at 400°C. All samples subsequently had 1ML of ZnSe grown on the surface using colloidal ALD at 150°C. (FIG.11E) Overlay of UV vis absorption spectra and PL spectra for the different samples. Samples and color scale are identical to those in FIGS.4A-4D. The PL center wavelength shifts from 1400 nm to 1100 nm as x increases from 0 to 0.7 demonstrating one example of the spectral tunability possible with multicomponent alloy nanocrystals.Atty. Dkt. No.05400-0072-PCT

[0019] FIG.12A. Powder XRD pattern for InP nanocrystals annealed (i.e., ion- exchanged) in a In[InBr4] salt. FIG.12B. TEM images of the resulting particles. DETAILED DESCRIPTION

[0020] Methods for synthesizing ternary or higher-order group III-V nanocrystals from pre-synthesized lower-order group III-V nanocrystals via ion-exchange in a molten inorganic salt containing a group III metal element are provided. In the methods, binary or higher-order group III-V nanocrystals are dispersed in a molten inorganic salt that contains a group III metal element having an oxidation state of less than III. In the molten inorganic salt, group III ions in the nanocrystals undergo ion-exchange with group III ions in the salt to increase the number of different group III elements in the resulting nanocrystals – that is, to increase the order of the group III-V nanocrystals.

[0021] Through the selection of starting group III-V nanocrystals and the tailoring of the ion-exchange temperature and duration, ternary, quaternary, and pentanary group III-V alloy nanocrystals of every combination of the elements indium (In) and / or gallium (Ga) with phosphorus (P), arsenic (As), and / or antimony (Sb) can be synthesized. The nanocrystals made using the methods are characterized by narrow size distributions and well-defined shapes, including spherical shapes. Because the methods enable independent control of the content of both the group III elements and the group V elements in the nanocrystals, independent control over the band gap, lattice parameter, size, and absolute band energies of the ternary and higher-order nanocrystals made using the methods is also enabled. As a result, the nanocrystals are useful for a wide variety of applications, including advanced optoelectronic devices, high-power electronics, and microelectromechanical systems.

[0022] Previous methods for synthesizing group III-V nanocrystals using ion-exchange in a molten inorganic salt are unsuitable for the synthesis of certain group III-V nanocrystals because group V elements (pnictides; “Pn”) in the nanocrystals act as reducing agents for group III elements in certain salts, leading to the decomposition of the starting nanocrystals and the production of elemental pnictides (Pn0). In the present methods, the decomposition of the starting group III-V nanocrystals is prevented by using molten inorganic salts in which the group III element is present in a reduced oxidation state relative to the common III oxidation state. The group III ions in the molten inorganic salt may include group III elements in a II oxidation state, a I oxidation state, or a combination thereof. The group IIIAtty. Dkt. No.05400-0072-PCT elements with the reduced oxidation state have a weaker oxidizing potential and, therefore, do not oxidatively decompose the group III-V nanocrystals dispersed therein.

[0023] The term “nanocrystal” as used herein refers to crystalline particles having a largest cross-sectional dimension not greater than 1000 nm and generally significantly smaller, including sufficiently small to ensure quantum confinement, e.g., no greater than 100 nm, no greater than 50 nm, no greater than 25 nm, or no greater than 10 nm. This includes largest cross-sectional dimensions in a range of from 1 nm to 50 nm, from 1 nm to 25 nm, and from 1 nm to 10 nm. In a collection of nanocrystals, these dimensions may refer to the average largest cross-sectional dimension for the collection. The nanocrystals having quantum confinement may also be referred to as quantum dots (QDs). As is clear from the term “nanocrystals,” the III-V nanocrystals are crystalline in nature, i.e., the III and V atoms are arranged in an ordered lattice, by contrast to amorphous materials exhibiting a lack of such atomic ordering. Crystalline quality may be confirmed and quantified using X-ray diffraction analysis and Raman spectroscopy.

[0024] The starting group III-V nanocrystals may be binary or higher-order (e.g., ternary or quaternary) group III-V nanocrystals. A binary group III-V nanocrystal is characterized by a single type of group III element and a single type of group V element, while higher-order group III-V nanocrystals may include more than one type of group III element and / or more than one type of group V element. The group III elements in the starting nanocrystals may be In, Ga, or Al. In some embodiments, the group III element is Ga or In. The group V element of starting nanocrystals may be, for example, N, P, As, Sb, or Bi. In some embodiments, the starting nanocrystals include a group V element selected from P, As, and / or Sb. However, the methods are particularly useful for the ion-exchange based synthesis from starting nanocrystals that include a group V element having a strong reducing potential for the group III element of the molten inorganic salt, such as Sb-containing nanocrystals.

[0025] For starting ternary and higher-order nanocrystals (and the quaternary or higher- order nanocrystals synthesized therefrom, which are referred to herein as secondary nanocrystals), it is understood that the ratios of the group III elements and the ratios of the group V elements may vary, with the elements within each of said groups having a mole fraction (χ) in the range 0 < χ < 1. By way of illustration, a nanocrystal comprising an InGaAsSb alloy can be represented by InxGa1-xAsySb1-y, wherein 0 < x < 1 and 0 < y < 1.Atty. Dkt. No.05400-0072-PCT

[0026] In some embodiments of the methods, the starting group III-V nanocrystals are free of the group III element present in the molten inorganic salt with which the nanocrystals undergo ion-exchange. Thus, gallium-free starting nanocrystals may be used with a gallium- containing molten inorganic salt and indium-free starting nanocrystals may be used with an indium-containing molten inorganic salt.

[0027] Illustrative indium-containing group III-V nanocrystals include InSb, InAs, InP, InAsSb, InPSb, InPAs, and InPAsSb nanocrystals, which can be undergo ion-exchange with Ga-containing inorganic salts to form InGaSb, InGaAs, InGaP, InGaAsSb, InGaPSb, InGaPAs, and InGaPAsSb secondary nanocrystals.

[0028] Illustrative gallium-containing group III-V nanocrystals include GaSb, GaAs, GaP, GaAsSb, GaPSb, GaPAs, and GaPAsSb nanocrystals, which can be undergo ion- exchange with In-containing inorganic salts to form InGaSb, InGaAs, InGaP, InGaAsSb, InGaPSb, InGaPAs, and InGaPAsSb secondary nanocrystals.

[0029] The starting group III-V nanocrystals may be formed using, for example, colloidal synthesis. Colloidal nanocrystals form a stable, homogenous, and uniform dispersion of individual nanocrystals (versus aggregates thereof) within a continuous phase. In the case of colloidal group III-V nanocrystals formed in an organic solvent (e.g., in a non-polar solvent such as dimethylformamide, toluene, etc), the starting nanocrystals may be formed with native organic ligands, such as oleyamine, on their surfaces (“capping ligands”). Prior to forming a dispersion of the starting group III-V nanocrystals in a molten inorganic salt, organic surface ligands can be partially or completely replaced by inorganic ligands via ion- exchange to facilitate the separation of the nanocrystals from organic solution, as illustrated in the Example. Suitable inorganic ligands that can be used for this purpose include group III halides, including group III halides having the formula MX3, where M represents a group III element and X represents a halogen, such as I, Cl, and / or Br. The group III element of the inorganic capping ligands may be, but need not be, the same as the group III element of the starting group III-V nanocrystal.

[0030] In some embodiments of the methods, the starting group III-V nanocrystals are ternary or higher-order group III-V nanocrystals made using the methods described herein. Thus, starting from a binary group III-V nanocrystal, for example, quaternary, and pentanary group III-V nanocrystals can be made using a multistep ion-exchange in molten inorganicAtty. Dkt. No.05400-0072-PCT salts, as described herein. However, ternary or higher-order group III-V nanocrystals made by conventional methods may also be used as starting nanocrystals.

[0031] The molten inorganic salts used in the ion-exchange include at least one group III metal element (M) in an oxidation state of II or I (MIIor MI) and at least one additional element. The group III element in the molten inorganic salt must have a reduced (I or II) oxidation state available in the condensed (molten) phase and the inorganic salt should have a melting temperature (Tm) below the temperature needed for ion-exchange and below the melting of the starting nanocrystals. It is not necessary that all the group III ions in the molten inorganic salt have a reduced oxidation state in the condensed phase. In some embodiments of the methods, the salt or salts that are melted to form the initial molten inorganic salt include a mixture of MIIand MIions. In some embodiments of the methods, the salt or salts that are melted to form the initial molten inorganic salt include only salts that include group MIIand MI. In some embodiments of the methods, the salt or salts that are melted to form the initial molten inorganic salt do not include alkali metals. In some embodiments of the methods, the salt or salts that are melted to form the initial molten inorganic salt consist only of one or more group III metals and one or more halogens.

[0032] Appropriate salts may be selected based on the relative redox potentials of the oxidation of the group V element of the nanocrystal and the reduction of the group III elements of the salt, whereby the oxidation potential of the group V element of the nanocrystal should be less than the reduction potential of group III metal ions in salt. The desirable relative redox potentials are illustrated in FIG.2B for Sb-containing nanocrystals and Ga-containing salts that include Ga ions a combination of GaIand MIIIions.

[0033] Molten inorganic salts with melting temperatures below 550 ºC are desirable. This includes molten inorganic salts having a Tm in a range of from above room temperature (about 20 to 25 ºC) to 450 ºC, from above room temperature to 350 ºC, and from 50 ºC to 300 ºC. Group III metal halides are good candidates for the molten inorganic salts because such salts can support group III elements in a reduced oxidation state and have reasonably low melting temperatures.

[0034] The group III halides include gallium halides, indium halides, and aluminum halides. However, gallium halides and indium halides may be preferred because reduced states of gallium and indium are more accessible in the halide salts for these elements. The use of Ga-containing halide salts may be particularly practical, due to their low meltingAtty. Dkt. No.05400-0072-PCT points. The halogen atoms in the group III metal salts can be I, Cl, Br atoms, or a combination of two or more thereof. F atoms may also be present, but due to the higher melting points of fluoride salts, when F is present it will be in combination with one or more other halides.

[0035] Illustrative group III halide salts include those having the formula MI[MIIIX4] and those having the formula MI2[MII2X6], where M represents a group III metal, such as Ga or In, and X represents a halogen, such as I, Cl, or Br. Specific, non-limiting, examples of such group III halide salts include GaI[GaIIII4], InI[InIIII4], and GaI2[GaII2I6]. Group III halide salts may have mixed group III metals (i.e., may include more than one type of group III element), which adopt specific oxidation states depending on their redox potentials. Specific, non- limiting, examples include GaI[AlIIII4], InI[AlIIII4], and InI2[GaII2I6]. Other illustrative group III halide salts include those containing alkali metal ions in addition to group III metal ions and halogen ions, including those having the formula A2[M2X6] where A represents an alkali metal ion, M represents a group III metal ion, and X represents a halogen ion. Specific non- limiting examples include Na2[GaIII6], K2[GaIII6], and K2[GaIIBr6] salts.

[0036] A dispersion of the starting group III-V nanocrystals in the molten inorganic salt may be formed by mixing the nanocrystals with a powdered inorganic salt and heating the mixture to a temperature above the melting temperature of the salt. The dispersion may be maintained at this temperature for a time sufficient to achieve the desired degree of ion- exchange. This process is referred to as “annealing” the pre-synthesized group III-V nanocrystals. By way of illustration, suitable annealing temperatures include temperatures in the range from 50 ºC to 700 °C, including temperatures in the range from 100 °C to 600 ºC and in the range from 250 °C to 500 ºC. By way of further illustration, suitable annealing times include time periods in the range from 5 minutes to 50 hours, including time periods in the range from 10 minutes to 10 hours and in the range from 10 minutes to 1 hour. However, temperatures and times outside of these ranges can be used.

[0037] Once secondary group III-V nanocrystals having a desired composition are formed, the dispersion can be cooled to solidify the molten inorganic salt, resulting in the formation of a solid composite comprising the secondary nanocrystals in a solid salt matrix. The secondary group III-V nanocrystals can then be separated from the solid salt matrix by dissolving the salt matrix in an appropriate organic or aqueous solvent.Atty. Dkt. No.05400-0072-PCT

[0038] In the secondary group III-V nanocrystals, the cations that are involved in the cation-exchange form a solid solution in the nanocrystal; that is, they coexist in the same crystal domain, rather than being segregated in two or more different domains that have different material compositions. Thus, group III-V nanocrystals made using the present methods are distinguishable from a nanocrystal heterostructure, such as a core-shell nanocrystal, in which the cations that undergo the cation-exchange end up segregated in different domains. However, the secondary group III-V nanocrystals made using the present methods can be incorporated into core-shell structures by growing a shell around the secondary nanocrystals. For example, growth of a thin, wide band gap semiconductor shell over secondary nanocrystal cores is a strategy that can be used to improve the optical properties of the nanocrystals. Metal chalcogenides, such as metal sulfides and metal selenides (e.g., ZnS, ZnSe, or CdS), are examples of suitable shell materials.

[0039] Depending on their compositions, the secondary nanocrystals can be used in a variety of devices and applications. For example, secondary nanocrystals that exhibit photoluminescence, including room temperature photoluminescence (PL), can be used in optoelectronic devices. For example, the secondary nanocrystals, with or without a shell, can be used as photodetectors, whereby incident radiation of a first wavelength or range of wavelengths is absorbed by the ternary or higher-order group III-V nanocrystals, inducing them to emit PL of a second wavelength or range of wavelengths. The absorption and PL spectra of the nanocrystals will depend on their chemical composition. EXAMPLE

[0040] The experimental results presented in this Example uncover new dimensions for colloidal chemistry in molten inorganic salts. First, the rich redox chemistry of molten inorganic salts was elucidated and how control of redox processes can stabilize III-V nanocrystals against decomposition was demonstrated. Next, new redox chemistry was utilized to grow monodisperse GaP1-yAsy, and GaAs1-ySbynanocrystals from binary nanocrystals in molten inorganic salts.

[0041] Materials and Methods

[0042] General Considerations:

[0043] Commercial Chemicals: Trioctylphosphine (TOP, 97%), trioctylphosphine oxide (TOPO, 99%), gallium metal (99.99% trace metals basis), diisobutylaluminum hydrideAtty. Dkt. No.05400-0072-PCT (reagent grade, 1M solution in toluene), lithium triethylborohydride (1.0 M in THF) and anhydrous solvents (hexane, toluene, ethanol (EtOH), acetonitrile (MeCN), methyl acetate) were purchased from Sigma Aldrich and used as received. Oleylamine (OAm, technical grade, 70%) and Oleic acid (OA, 90%) were purchased from Sigma Aldrich and degassed under dynamic vacuum at 110 °C for several hours before storage in a nitrogen glovebox. Potassium iodide (ultra-dry, 99.998%), potassium bromide (ultra-dry, 99.9%), indium bromide (anhydrous 99.9%), indium(i) chloride (99.995%) and N,N-dimethylformamide (DMF, anhydrous 99.9%) were purchased from Alfa Aesar and used as received. Tris(trimethylsilyl) phosphine ((TMS)3P, 98%, stored frozen), tris(dimethylamino)arsine (99%), and indium(iii) chloride (anhydrous, 99.999%), arsenic chloride (99.999%-As) PURATREM, antimony chloride (99.999%-Sb) PURATREM were purchased from Strem Chemicals and used as received. All quartz tubing and rods were purchased from Technical Glass Products.

[0044] Considerations for Chemical Handling: In general, salt precursors were preferred, which were delivered as ~10 mesh beads in sealed ampoules over fine mesh powders since the lower surface area minimized contamination with moisture and residual glovebox solvents. The beads were ground into fine powders as needed in oven dried mortar and pestle. Throughout this Example, Coors™ porcelain mortars and pestles were used to grind salt mixtures. The porosity of the mortars and pestles made it imperative that they were dried for at least 24 hr at 100 °C and immediately transferred into a glovebox while still hot. Throughout this Example, the salts in general were handled using gloveboxes where solvents and numerous other chemicals were stored and handled; however, all work was performed while the glovebox was under continuous nitrogen purge (< 1 ppm O2 and <1 ppm H2O) to ensure minimal solvent vapor contamination, and work was not performed while solvents were in active use.

[0045] Warnings: Caution: Reactions in sealed quartz ampoules with volatile reagents (e.g., iodine, GaI3, GaBr3, InBr3, InI3) can build up pressure. If the reaction is heated to a high temperature too quickly, rupture of the quartz tube can occur. Appropriate safety measures including performing the reactions with appropriate wall thickness for the tube diameter, in a well-ventilated fume hood, and with safety shielding is imperative. Caution: Oxyhydrogen torches must be operated with the appropriate safety precautions including proper storage of the reactive gasses, proper installation of flash arrestors on the gas delivery tubing, and proper eye protection from the intense UV and IR light produced by the quartz. When usingAtty. Dkt. No.05400-0072-PCT cryogens with oxyhydrogen flames, avoid using vacuum insulated glass Dewars since thermal shock from accidental flame exposure can lead to shattering of the Dewar. Caution: AsCl3 and PCl3, are volatile and represent significant inhalation hazards. Work must be performed in a properly exhausted glovebox and / or well-ventilated fume hood.

[0046] Synthesis of Salt Precursors:

[0047] Synthesis of GaI3: Gallium(iii) iodide (ultra-dry, 99.999%) was purchased from Alfa Aesar, or Gallium(iii) iodide (anhydrous, 99%) was purchased from Strem Chemicals or prepared from the elements. To prepare gallium iodide from the elements, gallium and iodine were combined in a glovebox with a 1.1:3 molar ratio in a quartz ampoule consisting of 12.75 mm OD 10.5 mm ID quartz tubing with a 10 mm OD quartz plug welded on one end. A second sealing plug was inserted, and the quartz tube was attached to a vacuum transfer chuck and transferred to a vacuum line outside the glovebox. The Quartz tube was immersed in liquid nitrogen contained in a closed cell foam Dewar (Chem Glass CG-1590-F500), so the iodine was completely immersed in the cold well and sealed under vacuum. The contents of the tube were slowly (over several hours) heated to 350 °C in a vertical melting furnace and held for several hours. Periodically, the ampoule was partially inverted to mix the contents. After complete reaction, the contents were distilled in the sealed quartz tube held vertically in a 400 °C to room temperature gradient where solidified gallium iodide collected in the upper cold zone. Sometimes, the tube would become completely closed with solidified gallium iodide. When this happened, the tube was removed from the heat source and cooled to room temperature. The tube was inverted, and the purified product was melted with a torch and allowed to collect further down the tube. After this, contents were allowed to solidify again, and the distillation process was resumed. The slight excess of gallium resulted in a residue of reduced gallium halides in the bottom of the ampoule but ensured no oxidizing iodine impurities were present in the products. The products were collected by breaking the ampoule open inside a nitrogen glovebox and grinding into a coarse powder.

[0048] Synthesis of InI3: Indium (iii) iodide (ultra-dry 99.99%) was purchased from Alfa Aesar or prepared from the elements by slowly heating indium metal and iodine in sealed quartz ampoules at 350 °C until complete reaction.

[0049] Synthesis of KGaI4: KGaI4was prepared by loading “ultra-dry” KI and “ultra-dry” GaI3 in a 1:1 molar ratio into an oven dried quartz ampoule (9 mm outer diameter, 7 mm inner diameter, with an indentation to support a sealing plug above the sample), and a 6 mmAtty. Dkt. No.05400-0072-PCT diameter quartz plug was inserted. The ampoule was attached to a vacuum transfer chuck to mount the ampoule on a vacuum manifold without air exposure. The ampoule was sealed under vacuum using a O2 / H2 torch. Next the salts were melted in a vertical furnace at ~400 °C, and after the salt melted the ampoule was inverted several times followed by additional annealing for several hours to ensure complete reaction. The reaction was cooled to room temperature, brought into a N2glovebox and the ampoule was broken open in a mortar, the quartz pieces were mechanically separated, and the resulting salt was ground into a coarse powder and stored in a glovebox for future use. Similar procedures were used to prepare KInI4 and KInBr4.

[0050] Synthesis of Ga[GaI4]: Ga[GaI4] was prepared by reacting gallium metal and GaI3 according to the equation 2Ga+4GaI3^3 Ga[GaI4] in flame sealed quartz ampoules. The reagents were slowly heated to 400 °C (over the course of several hours) and annealed overnight until the gallium was completely dissolved in the molten GaI3. Larger scale reaction required longer reactions times, and periodic partial inversion of the ampoule was required to effectively mix the reagents and decrease the necessary reaction time. After complete reaction, the quartz tube was brought into a N2 glovebox and the ampoule was broken open in a mortar, the quartz pieces were mechanically separated, and the resulting salt was ground into a coarse powder and stored in a glovebox for future use. Similar procedures were used to prepare Ga2I3, In2I4, In2Br4, In2Br3. Reaction temperature was carefully chosen such that the boiling point of the III-halide was not excessively exceeded. Further, slow heating to the reaction tube allowed for partial conversion (and subsequent reduction in vapor pressure) before proceeding to higher temperatures for full conversion.

[0051] Organic Solvent Synthesis of Colloidal Nanocrystals:

[0052] InAs Nanocrystal Synthesis: Tetrahedron-shaped InAs nanocrystals were synthesized using modifications of established procedures. Briefly, 50 ml of OAm was degassed under vacuum in a 250 ml 3 neck round bottom flask equipped with a condenser and 250 ml bump trap installed between the condenser and flask at 110 ºC for several hours. Next, 4 mmol of InCl3 (884 mg) was added to the flask under flow of N2. The reaction was next heated to 130 °C under vacuum for 1 hour. The flask was switched to N2and heated to 290 ºC. In a glovebox, 4 mmol of tris(dimethylamino)arsine was mixed with 5 ml of dry OAm and heated to 50 °C until bubbles stopped evolving. The As stock solution was injected into the flask at 280 °C, followed by injection of 4 mmol of DIBAL-H (4 ml of a 1 M stockAtty. Dkt. No.05400-0072-PCT solution). Caution: This reaction is vigorous due to the evolution of gas from DIBAL-H and the high reaction temperature relative to the boiling point of toluene. The bump trap collects the toluene which distills out of the reaction. The reaction was held at 280 °C for 15 min, then cooled to room temperature under a stream of compressed air. The reaction flask was transferred to a glovebox, and the nanocrystals were precipitated by the addition of anhydrous ethanol; it was centrifuged and the supernatant was discarded. The precipitate was dissolved in toluene and centrifuged to remove insoluble impurities, which included amorphous As0. The nanocrystals were precipitated a second time using ethanol / toluene non-solvent pair and subsequently stored in hexane in a glovebox for future use.

[0053] InP Nanocrystal Synthesis: Tetrahedron-shaped and sphere-shaped InP nanocrystals were synthesized using established procedures. (Gupta, A.; Ondry, J. C.; Lin, K.; Chen, Y.; Hudson, M. H.; Chen, M.; Schaller, R. D.; Rossini, A. J.; Rabani, E.; Talapin, D. V. Composition-Defined Optical Properties and the Direct-to-Indirect Transition in Core– Shell In1– x GaxP / ZnS Colloidal Quantum Dots. J. Am. Chem. Soc.2023, 145 (30), 16429– 16448.)

[0054] InSb Nanocrystal Synthesis: InSb nanocrystals used in this Example were prepared using established procedures. Briefly, 80 ml of oleylamine was degassed at 120 °C under vacuum for several hours in a 3 neck European style flask equipped with a thermowell in one neck, a condenser and 250 ml bump trap on the second port with the bump trap installed between the condenser and flask, and a rubber septum on the third port. After degassing, the flask was allowed to cool to room temperature. Next, the flask was switched to nitrogen, and 4 mmol (884 mg) of InCl3was quickly transferred from a glovebox inside a glass tube that was sealed with a rubber stopper. The diameter of the tube was small enough to fit entirely inside a standard 14 / 20 ground glass joint. The stopper on the flask and tube containing InCl3 were quickly removed and added under nitrogen flow to the flask. The flask was degassed a second time, refilled with nitrogen, and heated to 50 °C overnight to dissolve the InCl3. The flask was cooled to room temperature and 8 ml of a 0.25 M solution of SbCl3in oleylamine was added using a syringe. Next 17.5 ml of 1 M LiEt3BH in THF was quickly injected under vigorous stirring. After injection, the heating mantle was set to 300 °C, and the reaction took ~20 min to reach 300 °C. As the reaction temperature increased, THF distilled out of the reaction and was collected in the bump trap. Once the reaction reached 300 °C, the reaction temperature was held at 300 °C for 20 min. The heating mantle was removed, and the reaction mixture was cooled with a stream of compressed air. When the reaction mixtureAtty. Dkt. No.05400-0072-PCT reached ~200 °C, 50 ml of anhydrous toluene was injected. Upon reaching room temperature, 8ml of oleic acid was injected, and the mixture was allowed to stir for 30 min. The reaction flask was transferred into a nitrogen glovebox and separated into 4 centrifuge tubes and centrifuged to remove insoluble products. Next, the supernatant was transferred to clean centrifuge tubes, and the nanocrystals were precipitated using anhydrous acetonitrile and collected by centrifugation. The supernatant was discarded and the InSb pellet was dissolved in toluene. The particles were precipitated a second time using methyl acetate collected by centrifugation; the supernatant was discarded, and finally the pellet was dissolved in hexane. Finally, the quantum dot solution was centrifuged a final time to remove any remaining insoluble materials and stored in a vial for future use.

[0055] InAs1-ySbyNanocrystal Synthesis: Nanocrystals were prepared using established procedures with general modifications described above. Briefly, 60 ml of oleylamine wasat 120 °C under vacuum for several hours in a 3 neck European style flask equipped with a thermowell in one neck, a condenser and 250 ml bump trap on the second port with the bump trap installed between the condenser and flask, and a rubber septum on the third port. After degassing, the flask was allowed to cool to room temperature. Next, the flask was switched to nitrogen, and 3 mmol (663 mg) of InCl3was transferred from a glovebox as described above for InSb nanocrystal synthesis. The flask was degassed a second time, refilled with nitrogen, and heated to 50 °C overnight to dissolve the InCl3. The flask was cooled to room temperature, and 3 ml of a 0.25 M solution of SbCl3 in oleylamine and 3 ml of a 0.25 M AsCl3 solution in oleylamine were added using a syringe. Next, 13.4 ml of 1M LiEt3BH in THF was quickly injected under vigorous stirring. After injection, the heating mantle was set to 320 °C, and the reaction took ~20 min to reach 320 °C. As the reaction temperature increased, THF distilled out of the reaction and was collected in the bump trap. Once the reaction reached 320 °C, the reaction temperature was held at 320 °C for 40 min. The heating mantle was removed, and the reaction mixture was cooled with a stream of compressed air. When the reaction mixture reached ~200 °C, 25 ml of toluene was injected. Upon reaching room temperature, 8 ml of oleic acid was injected and the mixture was allowed to stir for 30 min. Samples were purified as described for InSb above.

[0056] InP1-x-yAsxSby Nanocrystal Synthesis: Synthesis was inspired by established procedures. Briefly, 60 ml of oleylamine was degassed at 120 °C under vacuum for several hours in a 3 neck European style flask equipped with a thermowell in one neck, a condenser and 250 ml bump trap on the second port with the bump trap installed between the condenserAtty. Dkt. No.05400-0072-PCT and flask, and a rubber septum on the third port. After degassing, the flask was allowed to cool to room temperature. Next the flask was switched to nitrogen, and 3 mmol (663 mg) of InCl3 was quickly transferred from a glovebox and added as described above for InSb. The flask was degassed a second time, refilled with nitrogen, and heated to 50 °C overnight to dissolve the InCl3. The flask was cooled to room temperature, and 1 ml of a 0.25 M solution of SbCl3in oleylamine, 1 ml of a 0.25 M AsCl3solution in oleylamine, and 4 ml of a 0.25 M solution of PCl3 in oleylamine were added using a syringe. Next 13.4 ml of 1 M LiEt3BH in THF was quickly injected under vigorous stirring. After injection, the heating mantle was set to 320 °C and the reaction took ~20 min to reach 320 °C. As the reaction temperature increased, THF distilled out of the reaction and was collected in the bump trap. Once the reaction reached 320 °C, the reaction temperature was held at 320 °C for 40 min. The heating mantle was removed, and the reaction mixture was cooled with a stream of compressed air. When the reaction mixture reached ~200 °C, 25 ml of toluene was injected. Upon reaching room temperature, 8 ml of oleic acid was injected, and the mixture was allowed to stir for 30 min. Samples were purified as described for InSb above.

[0057] InP1-ySby Nanocrystal Synthesis: Synthesis was inspired by established procedures. Briefly, 50 ml of oleylamine was degassed at 120 °C under vacuum for several hours in a 3 neck European style flask equipped with a thermowell in one neck, a condenser and 250 ml bump trap on the second port with the bump trap installed between the condenser and flask, and a rubber septum on the third port. After degassing, the flask was allowed to cool to room temperature. Next the flask was switched to nitrogen and 2.5 mmol (550 mg) of InCl3was quickly transferred from a glovebox and added as described above for InSb. The flask was degassed a second time, refilled with nitrogen, and heated to 50 °C overnight to dissolve the InCl3. The flask was cooled to room temperature, and 2.5 ml of a 0.25 M solution of SbCl3 in oleylamine and 2.5 ml of a 0.25 M solution of PCl3 in oleylamine were added using a syringe. Next, 11 ml of 1 M LiEt3BH in THF was quickly injected under vigorous stirring. After injection, the heating mantle was set to 320 °C and the reaction took ~20 min to reach 320 °C. As the reaction temperature increased, THF distilled out of the reaction and was collected in the bump trap. Once the reaction reached 320 °C, the reaction temperature was held at 320 °C for 40 min. The heating mantle was removed, and the reaction mixture was cooled with a stream of compressed air. When the reaction mixture reached ~200 °C, 25 ml of toluene was injected. Upon reaching room temperature, 8 ml ofAtty. Dkt. No.05400-0072-PCT oleic acid was injected, and the mixture was allowed to stir for 30 min. Samples were purified as described for InSb above.

[0058] InP1-yAsy Nanocrystal Synthesis: Nanocrystals were prepared according to established procedures. Briefly, 7 ml of oleylamine and 500 mg of InCl3 were degassed at 120 °C for 1vacuum. In a glovebox, 1.8 ml of tris(diethylamino)phosphine was added to 7.5 ml of oleylamine. This was stirred on a hot plate at 120 °C until bubbles stopped forming. Also in a glovebox, 425 µl of tris(dimethylamino)arsine was dissolved in 2.5 ml of trioctylphosphine. The temperature of the reaction flask was increased to 170 °C. Next the tris(dimethylamino)arsine solution was injected. The temperature was set to heat to 270 °C, and during the ramp up to temperature the tris(diethylamino)phosphine solution was injected at 210 °C. Upon reaching 270 °C, the reaction was allowed to proceed for 1 hour. The reaction was cooled to room temperature using a stream of compressed air. The reaction flask was transferred into a N2glovebox, and the particles were purified using ethanol / toluene non- solvent / solvent pair.

[0059] Synthetic Details for Molten Inorganic Salt Cation-exchange of Pre- synthesized Nanocrystals:

[0060] Inorganic Ligand Exchange of Pre-synthesized Nanocrystals: To exchange the native organic ligands on InP, InAs, InSb or alloy nanocrystals for GaI3 ligands, nanocrystals with their native surface ligands dissolved in ~15 ml of hexanes were layered on top of 5 ml of 0.05 M GaI3 solution in DMF. This bi-phasic system was stirred for several hours until complete phase transfer of the nanocrystals to the DMF phase. Next, the DMF phase was separated and washed with fresh hexanes. The nanocrystals were precipitated from the DMF solution by the addition of 5 ml of acetonitrile (MeCN) and collected by centrifugation. The supernatant was discarded, and the nanocrystal pellet was suspended in fresh MeCN via vortexing followed by collection of the powder via centrifugation. Finally, the supernatant was discarded, and the nanocrystals were dried in a glovebox overnight in an open centrifuge tube for future use.

[0061] Molten Inorganic Salt Annealing of Pre-synthesized Nanocrystals: GaI3 / KI eutectic, KGaI4, or Ga[GaI4] powders were finely ground in an oven dried mortar and pestle in a N2glovebox. Next, the previously prepared GaI3-capped InAs, InP, or InSb nanocrystals were incorporated into the salt by gentle grinding with the mortar and pestle. Nanocrystals could be loaded in the salts with mass loading as high as 10 wt.% nanocrystals withoutAtty. Dkt. No.05400-0072-PCT obvious negative impacts. The nanocrystal / salt mixture was loaded into a quartz ampoule, air free transferred to a vacuum manifold, and sealed under vacuum using an O2 / H2torch. Samples were annealed in a custom high temperature heating block, which consisted of a 4- inch diameter cylindrical aluminum block with 8, 3 / 8-inch-diameter 2.5-inch-deep holes drilled in a circle around a central thermocouple hole. The length of the sealed portion of the ampoule was less than 2.5 inches, ensuring the entire reaction volume was uniformly heated in the aluminum block. The block was wrapped in high temperature heating tape (BriskHeat BWH) and several layers of insulation. The temperature was controlled using a PID temperature controller. The temperature in each hole was independently measured to be within ±2 °C of the set temperature. Samples were annealed for the desired time and temperature followed by removal from the heating block and allowing the sample cool to room temperature naturally.

[0062] Recovery of Pre-synthesized Nanocrystals from the Salt Matrix: The ampoules were broken open in an N2 glovebox, and the salt pellet embedded in the bottom of the ampoule was placed in an oven dried glass vial with a Teflon stir bar. To this vial, 5 ml of MeCN was added and stirred until the salt matrix dissolved. Next, the nanocrystals were separated by centrifugation, and the supernatant was discarded. The nanocrystal pellet was washed with additional MeCN and collected by centrifugation. The nanocrystal pellet was treated with a solution consisting of 50 µl of OAm in 1 ml of toluene and stirred until the nanocrystals formed a colloidal solution. The nanocrystals were purified of excess ligand by precipitation with 5 ml of methyl acetate, centrifugation, and final dispersion in toluene for further characterization. For samples which were recovered from Ga[GaI4] molten inorganic salt, an In(0)byproduct (resulting from disproportionation of the reduced III-halide molten inorganic salt) was removed by centrifugation. All samples were stored in a glovebox and fresh aliquots were removed for characterization.

[0063] Colloidal Atomic Layer Deposition (cALD) of ZnSe on In1-xGaxP1-yAsy Nanocrystals: Colloidal ALD was performed at high temperaturesprocedures with modifications as described below. Lithium selenide was from lithium triethylborohydride and selenium pellets according to established procedures. All procedures were performed in a nitrogen glovebox, all solvents were anhydrous, and octadecene (ODE) and Oleylamine (OAm) were degassed at 120 °C for several hours before they were brought into the glovebox. To heat the samples, a heating block on a hot plate was use with wells that snugly fit the vials used for reaction. The temperature was measured by aAtty. Dkt. No.05400-0072-PCT thermocouple immersed in ODE contained in an identical vial to those used for the reaction, and that vial was placed in a well of the heating block. For all layers, growth temperature of 150 °C was used . The following procedure was used to deposit a single layer and can be sequentially repeated to grow thicker layers. A vial was prepared with 5-7 mg of Li2Se, 100 µl of OAm, and 500 µl of ODE. The quantum dots dissolved in toluene were added to this vial, and the vial was placed in a pre-heated heating block on a hot plate at 150 °C. The sample was cooled to room temperature and centrifuged to remove solid Li2Se. The remaining nanocrystals in liquid were transferred to a second centrifuge tube, and 1 ml of methyl acetate and 100 µl of acetonitrile was added to precipitate the particles. The particles were collected by centrifugation and the supernatant was discarded. The nanocrystal solid was dissolved in 200 µl of toluene. A vial containing 20-30 mg of zinc acetate, 100 µl of OAm, and 500 µl of ODE was prepared, and the Li2Se treated particles in toluene were added. The vial was heated at 150 °C for 2-3 minutes. The vial was cooled to room temperature and transferred to a centrifuge tube.1 ml of methyl acetate and 100 µl of acetonitrile were added to precipitate the nanocrystals, and they were collected by centrifugation. The supernatant was discarded, and the nanocrystal solid was dissolved in toluene and stored for future characterization. Additional layers can be deposited by repeating this cycle.

[0064] Nanocrystal Structural Characterization:

[0065] Powder XRD: Powder x-ray diffraction patterns were collected on a Rigaku miniflex x-ray diffractometer. Samples were deposited on 511 Si low background substrates. Lattice parameter and Scherrer size were determined by fitting the {111}, {220}, and {311} peaks to pseudo-Voigt functions to extract the peak width and position. The gallium composition was estimated by calculating the lattice parameter using the {111}, {220}, and {311} peaks and using a linear interpolation of the lattice parameters for the parent binary compounds.

[0066] TEM: Low magnification TEM images were collected on either an FEI Tecnai T20 TEM operated with a W or LaB6 filament operated at 200 kV using a Gatan RIO 16 IS camera with drift correction enabled or on an FEI Tecnai F30 microscope at 300 kV using an ultrascan 4000 camera. HRTEM images were collected on a FEI Tecnai F30 microscope at 300 kV using an ultrascan 4000 camera. Images were processed using FIJI (ImageJ) to rotateAtty. Dkt. No.05400-0072-PCT images and calculate FFT of HRTEM images. Samples were imaged on 400 mesh copper grids with amorphous carbon support (EMS CF400-Cu).

[0067] Small Angle X-ray Scattering: SAXS patterns were collected from 2 mm diameter glass capillaries of colloidal nanocrystals dissolved in anhydrous toluene. The glass capillaries were loaded inside a glovebox and the ends were sealed with UV-cure epoxy. Samples were measured using a SAXSLAB Ganesha instrument with a Cu K alpha source.

[0068] X-ray Fluorescence: X-ray fluorescence (XRF) analysis was performed with a benchtop Rigaku Energy Dispersive NEX DE VS X-ray fluorimeter equipped with a Peltier cooled FAST SDD Silicon Drift Detector. All analyses were carried out under He atmosphere to increase sensitivity for lighter elements. Elemental ratios were determined using the standardless thin films fundamental parameters method as programmed in QuantEZ software provided by Rigaku, using the standard Rigaku calibration protocols. All samples were measured on a PTFE substrate to avoid the large Si Kα peak which interferes with the aluminum and phosphorus Kα peak.

[0069] Raman Spectra: All Raman measurements were performed using a HORIBA LabRAM HR Evolution Confocal Raman Microscope. Samples were deposited on quartz substrates by drop casting concentrated toluene solutions in a glovebox and allowing the solvent to evaporate. Samples were measured using low laser power, the signal was collected using 3 s integration times, and the signal was averaged from 100 exposures. The spectrometer was calibrated using the Horiba LabSpec software using a Si (111) reference sample for all measurements.

[0070] Dynamic Light Scattering: DLS data was collected using a Mobius (Wyatt Technology). The laser operated at a wavelength of 532 nm, and the detector angle was set at 163.5 degrees. Measurements were performed at 25 °C using toluene as a solvent using a globular protein model for data analysis.

[0071] Nanocrystal Optical Characterization:

[0072] UV-Vis Absorption Spectroscopy: UV-vis spectra were collected from colloidal solutions of QDs with a Shimadzu UV-3600i Plus UV-Vis-NIR spectrophotometer in transmission mode.50 µl of a concentrated solution was diluted with 3 ml of anhydrous n- hexane and loaded in a 1 cm path length screw cap cuvette. Samples were collected with a reference cuvette consisting of 3 ml of n-hexane with 50 µl of added toluene. For samplesAtty. Dkt. No.05400-0072-PCT which absorbed in the NIR range, tetrachloroethylene was used. All spectra were normalized at an optimal wavelength (typically 400nm) for easier comparison.

[0073] Time Resolved Photoluminescence: TRPL was measured on the same samples in vacuum sealed quartz tubes using a Picoquant FluoTime 300 instrument using a PMA 175 detector, and an LDH-P-C-405 diode laser with a 407 nm excitation wavelength (50 ps pulse width) and a laser repetition rate of 1 MHz.77 K measurements were performed using the Dewar system described above.

[0074] Transient Absorption Spectroscopy: Measurements were performed using a 35-fs, 2-kHz Ti:sapphire laser with time-delayed white light probe pulses produced in a 2 mm sapphire or CaF2plate and 1-kHz pump pulses centered at 400 nm produced from second harmonic generation of the fundamental.

[0075] Molten Inorganic Salt Characterization:

[0076] Ambient and High Temperature Raman Measurements: All Raman measurements were performed using a HORIBA LabRAM HR Evolution Confocal Raman Microscope. Most room temperature measurements were performed using a 532 nm laser source, an ultra- low frequency filter enabling measurement of Raman shifts as low as 10 cm-1, an 1800 grooves / mm grating (resolution ~1 cm-1) and were detected using a Horiba Synapse OE- CCD. Temperatures were measured with a 633 nm laser and a standard filter enabling measurement of Raman shifts to 50 cm-1, an 1800 grooves / mm grating (resolution ~1 cm-1) and were detected using a Horiba Synapse OE-CCD. The spectrometer was calibrated using the Horiba LabSpec software using a Si (111) reference sample for all measurements. For room temperature measurements, a sealed quartz ampoule (9 mm O.D., 7 mm I.D.) with a solid salt plug at the bottom was placed horizontally on the microscope stage. The laser was focused on the center of the curved ampoule using a 10X objective to collect spectra. For evaluating the molten inorganic salt products after reaction with nanocrystals, Gallium iodide capped InP, InAs, or InSb nanocrystals. Raman spectra were collected on the solid salt matrix with the nanocrystals still embedded in the matrix. For high temperature measurements, a custom optical path turned the laser output of the Raman microscope 90 degrees and focused the light into a sealed quartz ampoule held vertically in an aluminum heating block, which was used to heat the salt samples.

[0077] Results and Discussion

[0078] Redox Reactions of Nanocrystals in Molten Inorganic SaltsAtty. Dkt. No.05400-0072-PCT

[0079] Molten inorganic salt solvents can be classified using Lewis acid-base concepts where group III halides such as GaX3(X = Cl, Br, I) are Lewis acids, alkali halides such as KI are Lewis bases, and the stoichiometric reaction GaI3 + KI ^ KGaI4 (m.p.230°C) produces a Lewis neutral molten inorganic salt. It was noticed that cation-exchange reactions of III-V nanocrystals in molten inorganic salts are particularly sensitive to the nature of the group-V element. For example, InP can efficiently convert into In1-xGaxP in Lewis acidic, basic, or neutral salt, while using Lewis neutral KGaI4 molten inorganic salt is key for preserving chemical stability of InAs nanocrystals and forming In1-xGaxAs phase. Finally, InSb decomposes when a GaIIIsource is added to molten inorganic salts in an attempt to synthesize In1-xGaxSb nanocrystals.

[0080] InAs nanocrystals undergo the following reaction in KGaI4molten inorganic salt: InAs(c) + xKGaI4(l) ^ In1-xGaxAs(c) + xKInI4(l), where c and l stand for “colloidal” and “liquid” respectively. The In1-xGaxAs product can be identified by powder X-ray diffraction (XRD, FIG.1A) and [InI4]- ions are detected by Raman spectroscopy (FIG.1B). To understand the stability problem of antimonides in molten inorganic salts, InSb nanocrystals were annealed in molten KGaI4 and no crystalline III-V products were found (FIG.1C and FIG.1F). Instead, the observation of Sb0by XRD suggests that Sb-IIIin InSb is oxidized to Sb0, demonstrating redox activity of the pnictide component of III-V nanocrystals. Raman spectroscopy shows the formation of [Ga2I6]2-ions present in the molten inorganic salt after the reaction (FIG.1B). These GaII-containing ions indicate that KGaIIII4 is being reduced by the InSb nanocrystals in the following reaction: InIIISb-III(c) + 2KGaIIII4(l) ^ Sb0(s) + (K+, In+)2[GaII2I6]2-(l), where s stands for “solid”.

[0081] Gallium (III) and Indium (III) are the most common oxidation states, but molten halide salts can adopt a variety of compositions containing ions which are formally GaI, GaII, InIand InIIspecies. In a molten inorganic salt, the pnictide can act as a reducing agent for KMIIIX4 (M = In, Ga) molten inorganic salt, leading to decomposition of the III-V nanocrystals as schematically described in FIG.2A. However, an analysis of redox equilibria suggests that the oxidation of Sb-IIIto Sb0could be prevented if, instead of using GaIII, a molten inorganic salt with a weaker oxidizing potential was used. Indeed, annealing InSb nanocrystals in molten GaI[GaIIII4] (often referred to as Ga2I4) at 325°C prevents oxidative decomposition of the antimonide (FIG.1C). After annealing in Ga[GaI4], the XRD peaks ofInSb nanocrystals shifted to larger q-values (^^ ൌଶగ ௗ , where d is the interatomic spacing),Atty. Dkt. No.05400-0072-PCT indicating a decrease in lattice constant, consistent with incorporation of gallium into the InSb lattice. By tuning the reaction time and temperature, In1-xGaxSb nanocrystals can be synthesized from x=0 to x=1 (FIGS.3A-3E).

[0082] Transmission Electron Microscopy (TEM) images of resulting In1-xGaxSb nanocrystals produced by cation-exchange of InSb nanocrystals using Ga[GaI4] (FIG.1D) show well isolated nanocrystals with narrow size distribution and well-defined spherical shapes. Similarly, In1-xGaxAs and In1-xGaxP nanocrystals can be prepared from InAs and InP nanocrystals using molten Ga[GaI4] (FIGS.1A and 1D). This reaction methodology can be further expanded to prepare quaternary and pentamery III-V colloidal nanocrystals by reacting InAs1-ySby, InP1-yAsy, InP1-ySby, or InP1-y-zAsySbz nanocrystals with Ga[GaI4] to prepare In1-xGaxAs1-ySby(x=0-0.83, y=0.67), In1-xGaxP1-yAsy(x=0-0.7, y=0.83), In1-xGaxP1-ySby(x=0-0.8, y=0.87), and In1-xGaxP1-y-zAsySbz (x=0-0.9, y=0.42, z=0.41) (FIG.1E and FIGS.4A- 4D, 5A-5D, and 6A-6D). Importantly, the reducing environment provided by Ga[GaI4] prevented the leaching of the more reactive pnictide and the initial pnictide ratios were unchanged while the Ga:In ratio can be varied precisely. Raman spectra of quaternary alloy nanocrystals show well defined peaks indicating high crystal quality and gradual shifts of the phonon modes as the composition was tuned, consistent with formation of alloy III-V nanocrystals. To test the photophysical properties of quaternary In1-xGaxP0.17As0.83 nanocrystals (x = 0 to 0.7) synthesized in reduced Ga[GaI4] molten inorganic salt, a thin wide band gap ZnSe shell was grown to passivate surface states. All samples showed strong near- IR PL, validating molten inorganic salt transformation can prepare high-quality quaternary III-V semiconductors. The PL efficiency increased with higher temperature and longer annealing time in the molten inorganic salts (FIG.7). The ability to independently control the composition of both the metal and pnictide enables independent control over the band gap, lattice parameter, size, and absolute band energies of a III-V material, which have proven instrumental for designing advanced optoelectronic devices grown by MBE and CVD methods.

[0083] A notable parameter that controls redox stability of III-V phases against decomposition in molten inorganic salt (FIG.2A) is the relative positions of the redox potentials for the pnictide oxidation and gallium or indium halide reduction. From the Raman studies, a qualitative order of the electrochemical potentials was inferred relevant to redox chemistry of InP, InAs, and InSb in molten KGaI4 (FIG.2B). The key takeaway is that the antimonide in InSb is a sufficiently strong reducing agent to reduce GaIIIin KGaIIII4to GaIIinAtty. Dkt. No.05400-0072-PCT the form of [GaII2I6]2-ions, which explains instability of group III-antimonide nanocrystals in molten inorganic salts. From this it is implied that InSb is not a sufficiently strong reducing agent to further reduce Ga[GaI4] to GaI2[GaII2I6] (often referred to “Ga2I3”). In contrast, As-IIIin InAs and P-IIIin InP are not a sufficiently strong reducing agents to react with KGaI4and thus the formation of [GaII2I6]2-ions was not observed after reaction with KGaI4 (FIG.2C). In support of the role of redox potential controlling the stability of III-V nanocrystals, it was found that KInI4 causes oxidative decomposition of InP, InAs, and InSb consistent with the easier reduction of InIIIto InIIor InIcompared to GaIIIto GaII(FIGS.8A-8D). Altogether these results demonstrate that the redox potential of the molten inorganic salt is key for controlling chemical stability of III-V phases.

[0084] III-halide Salt Compositions, Reactivity Trends, and Structural Elucidation by Raman Spectroscopy

[0085] The most encountered III-halide salts are AlX3, GaX3, and InX3 which all contain MIIIcenters. Under normal conditions Al-halide salts exclusively adopt an AlIIIoxidation state, and stoichiometries differing from the AlX3 phase are not known for molten inorganic salts. For GaXnand InXnsystems, a variety of stoichiometries are possible and thus a variety of Ga and In oxidation states are possible. This section provides a summary of the known phases and properties of these salts, then discusses the reactivity trends for both Lewis acidity and redox reactions, and finally discusses how to conveniently identify the different species by Raman spectroscopy both in solid state samples and molten inorganic salt samples.

[0086] Summary of III-Halide Molten Inorganic Salt Compositions:

[0087] Aluminum Halides: For AlX3(X=Cl, Br, I) in the solid state, the Cl derivative adopts a layered structure with octahedral coordination of Al by chlorine. The Br and I derivative adopt a dimeric Al2X6with two edges sharing tetrahedrally coordinated Al-Cl centers. All have low melting points (AlCl3 Tm=180°C, AlBr3 Tm=98°C, AlI3 Tm=188°C); the higher melting point for AlCl3is a result of its more stable solid-state structure. Upon melting, they adopt a complex liquid structure consisting of Al2X6 dimers, AlX3, and [Al2X7]- ions as determined by neutron scattering.

[0088] Lewis neutral forms of aluminum halides can be prepared according to the following reaction: AX + AlX3 ^ AAlX4 (A = Li, Na, K, Rb, Cs; X = Cl, Br, I). In the solid state they can adopt a varietycrystal structures, but the key is they all contain stable [AlX4]- ions and that well-defined structure is typically maintained upon melting.Atty. Dkt. No.05400-0072-PCT

[0089] Lower oxidation states of Al are rare under normal conditions and typically exist only at high temperatures (T~1000°C) in the gas phase or quenched with appropriate organic ligands at cryogenic temperatures. They disproportionate at room temperature according to 3AlX ^ AlX3+ 2Al.

[0090] Gallium Halides: For GaX3(X = Cl, Br, I) in the solid state, all 3 adopt a dimeric Ga2X6 with two edges sharing tetrahedrally coordinated Ga-X centers. As such, all have a low melting points (GaCl3Tm=78°C, GaBr3Tm=121°C, GaI3Tm=212°C) which increase monotonically with increasing molar mass. Upon melting these salts, Ga2X6 dimers, GaX3, and [Ga2X7]- ions are formed as determined by Raman spectroscopy.

[0091] Lewis neutral forms of gallium halides can be prepared according to the following reaction: AX + GaX3 ^ AGaX4 (A = Li, Na, K, Rb, Cs; X = Cl, Br, I). In the solid state they can adopt a variety of crystal structures, but the key is they all contain stable [GaX4]- ions which have tetrahedral coordination geometry, and that well-defined structure is typically maintained upon melting. Typically, these salts have melting points between 200 °C and 400 °C. Nonetheless, the relatively low melting point of these salts and the reduced reactivity renders them useful for cation-exchange reactions of InP and InAs nanocrystals.

[0092] AGaX4and GaX3are continuously miscible across all compositions, and thus it is possible to tune the Lewis acidity of a molten inorganic salt by tuning the composition. Miscibility of AGaX4in Lewis basic molten inorganic salts is more nuanced. For example, KGaI4 and KI do not form a eutectic mixture and thus to obtain a uniform molten mixture the temperature must be increased above the melting point of KI (Tm=681 °C) which is inconveniently high for colloidal synthesis in molten inorganic salts. Although a low melting alkali halide eutectic, such as LiI / KI (Tm=260 °C), may be considered for dissolving KGaI4, this system preferentially adopts an immiscible two-phase system of KGaI4 and a LiI / KI phase due to preferential interaction of soft [GaI4]- ions with K+over harder Li+. This Li induced phase separation is present across all halides and occurs for the InX3 and AlX3 systems. Alternatively, by using a non-Li containing CsI / NaI / KI eutectic (Tm=407 °C), a single-phase molten inorganic salt system is observed when KGaI4 is added up to 50 wt%. The direct synthesis experiments described above in Lewis basic molten inorganic salt solvents made use of non-Li containing Lewis basic molten inorganic salt solvents.

[0093] Lower oxidation states of gallium halides are accessible at room temperature and as molten inorganic salts. Reduction with metallic gallium allows the formation of lowerAtty. Dkt. No.05400-0072-PCT halides. For example, 4GaX3 + 2Ga ^ 3Ga[GaX4] (X = Cl, Br, I) results in the formation of a mixed valence GaI[GaIIIX4] species. Further reduction according to the equation GaX3+ Ga ^ Ga2[Ga2X6] (X = Br, I) (often to as Ga2X3) results in the formation of GaI2 [GaII2I6] (FIG.9C) which contains mixed valence GaIand GaIIspecies. Ga[GaX4] and [Ga2X6] are continuously miscible and thus the ratios of GaI, GaII, and GaIIIspecies can be tuned.

[0094] Indium Halides:

[0095] Indium halides have even richer chemistry compared to gallium halides. The larger radius of In allows for octahedral coordination in addition to tetrahedral coordination for smaller halides. This dominates the reactivity with alkali halide salts and both AInX4(X = Cl, Br, I) and A3InX6 (X = Cl, Br) are known. This Example focuses on the indium iodides. The speciation of Lewis neutral KInI4mirrors that of KGaI4and importantly the octahedrally coordinated K3InI6 is not known to form. Both InX (X = Cl, Br, I) and InX3 (X = Cl, Br, I) are stable owing to the greater stability of InIspecies compared to GaI. Further, several intermediate halides can form which are relevant. For example, 4InI3 + 2In ^ 3In[InI4] produces a species which is analogous to Ga[GaI4].

[0096] Reactivity trends for Group III-Halide Salts:

[0097] The Lewis acidity of a group-III Halide increases as the mass of the halide decreases; for example, AlCl3 > AlBr3 > AlI3. In addition, the Lewis acidity decreases as the mass of the group-III increases; for example, AlCl3 > GaCl3 > InCl3. In terms of redox chemistry, as the mass of the group-III increases, it is easier to reduce MX3 to lower halides; for example, in ease of reduction AlCl3< GaCl3< InCl3. As the halide increases in mass, it becomes easier to reduce a given MX3 species; for example, in ease of reduction GaCl3 < GaBr3< GaI3. As such, the group-III halides have opposite trends in terms of their Lewis acidity and ease of reduction. These two factors must be balanced for molten inorganic salt cation-exchange or direct synthesis reactions since increased Lewis acidity can lead to decomposition, thus favoring iodides. However, protecting against oxidative decomposition is also important for stability and thus favors chloride or bromide salts.

[0098] Identification of molten inorganic salt solvent composition by Raman spectroscopy:

[0099] Throughout this Example, Raman spectroscopy is used to identify the molecular species present in the molten inorganic salt solvents both at ambient temperature and highAtty. Dkt. No.05400-0072-PCT temperature. Polyatomic species present in the molten inorganic salt have distinct vibrational modes which Raman spectroscopy can easily probe at ambient temperature and elevated temperature. Raman spectroscopy may be used to identify Lewis acidic and Lewis neutral species present in molten inorganic salt solvents. KGaI4can be uniquely identified by the Raman signatures of the [GaI4]- anion which has a tetrahedral geometry and shows a strong mode at 145 cm-1corresponding to the Ga-I symmetric stretch (v1) and modes at 77 and 58 cm−1which correspond to the asymmetric bend (v4) and symmetric bend (v2).

[0100] This Example identifies the reduction of KGaI4 by InSb by identifying reduced gallium species after the reaction. In these experiments, InSb and KGaI4were flame sealed in a quartz ampoule and annealed. After reaction, the byproducts were allowed to solidify in the ampoule, and Raman spectroscopy was performed on the solidified salt components. After reaction, it was identified that most of the reaction mixture contained [GaI4]- since this was used in large excess. However, an additional peak was also observed at 126 cm-1which is consistent with the Eu mode of [Ga2I6]2-ions (FIG.1B and FIG.9A). This peak was not observed in cases where the nanocrystals do not decompose in KGaI4(InP and InAs), and instead the formation of a peak was observed for the v1 mode of [InI4]-, the expected byproduct in the absence of redox (FIG.1B and FIGS.9A-9D). [Ga2I6]2-ions are also present in Ga2[Ga2I6] which was prepared by reacting GaI3 with excess Ga. Indeed, a strong peak was observed at 122 cm-1indicating the new peak observed after reaction of InSb with KGaI4(FIG.9C).

[0101] For pure GaI3, reduction with Ga metal follows the progression with increasing gallium where first Ga[GaI4] is formed up until a 2:1 molar ratio of GaI3and Ga is met. In this case, the formation of this species was observed by a shift of the v1 mode of [GaI4]- compared to KGaI4due to changes in the strength of the Ga-I bond depending on the counterion (FIG.9C). Once the gallium content is increased higher than a GaI3:Ga ratio of 2:1, further reduction occurs via the formation of Ga2[Ga2I6] and this was observed by the emergence of a strong Raman mode at 122 cm-1. In the case where KGaI4was treated with various reducing agents, the formation of a peak at 126 cm-1was universally observed, indicating [Ga2I6]2-ions were forming. It appeared that the excess I- present in KGaI4caused the system to immediately form [Ga2I6]2-upon addition of reducing agent rather than form bare Ga+ions. This provides a convenient and distinctive marker for reduction of the molten inorganic salt. If GaI3 were to be partially reduced with InSb, the only observable shifts would be shifts of the v1 peak of [GaI4]-.Atty. Dkt. No.05400-0072-PCT

[0102] To identify the nature of Ga[GaI4] at the temperatures relevant to cation-exchange reactions, Raman spectroscopy was performed on molten Ga[GaI4] at elevated temperatures (FIG.9D). At elevated temperatures (300 °C to 450 °C) a strong peak at 140 cm-1was observed, consistent with the v1 mode of [GaI4]- in Ga[GaI4]. In addition, a peak at 122 cm-1was observed, which suggests the formation of [Ga2I6]2-upon melting. One possible explanation for this is the dissociation of the salt at high temperature according to 3Ga[GaI4](l) ⇌ Ga2[Ga2I6](l)+2GaI3(g), causing the formation of some [Ga2I6]2-ions upon melting.

[0103] Redox Stability of III-V Nanocrystals in Molten Inorganic Salts

[0104] It was found that both Lewis acidic (GaI3 / KI) and Lewis neutral (KGaI4) salts (FIG.10C) can be used to convert InP to In1-xGaxP after annealing at 400 °C for 1 hour. InAs is more sensitive and only Lewis neutral salts result in successful conversion (FIG.10B). However, it was found that the Lewis acidic or Lewis neutral conditions which worked for conversion of InP and InAs failed for InSb (FIG.10A). In all cases the resulting byproduct observed was Sb0. One possible explanation for the challenges with InSb is that it has a low melting point (Tm= 524 °C), and the melting point depression of small crystals may cause it to melt at the reaction temperatures typically used in molten inorganic salts solvents. With KGaI4, very low reaction temperatures can be accessed and yet formation of only Sb0byproducts was still observed even for reaction temperatures as low as 170 °C. This indicates some chemical instability in this reaction system. Effects of external factors were ruled out, such as oxygen or moisture introduction into the reaction system. For all reactions, all high purity reagents were judiciously handed to avoid introduction of impurities. Further, the reactions were carried out in sealed systems such that there was no possibility of mass transfer in or out of the system. As such, the oxidation of the Sb-IIIto Sb0must be coupled to the reduction of another species within the reaction system. As shown from the observation of [Ga2I6]2-in FIG.1B (which formally contains GaIIcenters), it was confirmed that the salt itself was being reduced by InSb . Altogether this demonstrates the salt itself was responsible for the decomposition of the InSb. As a result, in KGaI4, the redox potential of this salt is determined by the equation shown in FIG.2A.

[0105] Having identified the redox potential of the salt as the problem, it was determined that the solution was to make the salt a weaker oxidant (harder to be reduced). Ga[GaI4] was identified as a potential alternative solvent which is a weaker oxidant. Indeed, annealing InSbAtty. Dkt. No.05400-0072-PCT in Ga[GaI4] resulted in the recovery of In1-xGaxSb nanocrystals (FIG.1C), indicating the more reducing environment provided by the Ga[GaI4] protected the particles from decomposition. The resulting In1-xGaxSb showed well defined zinc blende powder diffraction peaks, consistent with highly crystalline particles. Throughout this Example, Vegard’s law was used to determine the composition of ternary colloidal nanocrystals. Essentially, as gallium is incorporated into InSb, the lattice constant should decrease due to the smaller size of gallium compared to indium. To first order the lattice constant / composition can be considered as a linear interpolation between the lattice constants of the parent materials. As such, by determining the lattice constant of the nanocrystals, the composition can be determined.

[0106] The reactivity of InSb in Ga[GaI4] as a function of time and temperature was explored to control the composition and crystallinity. For a fixed reaction temperature of 350 °C, it was found that the gallium content increased monotonically as time increased (FIG.3A and FIG.3C). After 1 hour of reaction, nearly all the indium was removed. Increased reaction time also caused a gradual increase in the particle size as determined by Scherrer analysis. Importantly though, these effects were modest and the initial ~3 nm particles increased to ~9 nm from a 15 min reaction to a 16-hour reaction. Importantly, the resulting materials were all colloidal and there existed a time / temperature window for which the size could be maintained, and nearly complete removal of indium could be achieved (350 °C for 1 hour). This indicates with refinement it should be possible to tune the gallium content for a fixed particle size across the entire composition range.

[0107] Increasing the reaction temperature can speed up diffusion processes and potentially result in higher quality crystals. To this end, the reactivity of InSb in Ga[GaI4] from 275 °C to 425 °C was explored (FIG.3B). As expected, increasing the reaction temperature caused a greater amount of gallium to be incorporated (FIG.3D). Further, as temperature increased, it was observed that there was a threshold for particle growth; for reaction temperatures of 350 °C and below, particle size changes as a function of temperature were not observed. Above that, dramatic increases in particle size with temperature were observed (425 °C results in 18 nm particles). An apparent diffusion coefficient for each temperature can be determined using an analytical solution to Fick’s second law of diffusion for a spherical particle of radius R. From the temperature dependent diffusion, the activation energy of the diffusion process can be measured using identical methods from previous reports (FIG.3E). The activation energy for gallium interdiffusion was measured to be 2.02 eV, which was lower than expected for solid state diffusion processes, but was consistentAtty. Dkt. No.05400-0072-PCT with the lower barriers to diffusion measured for nanocrystals of InAs and InP. Altogether, these results demonstrate that the gallium cation-exchange of InSb nanocrystals using Ga[GaI4] behaved similarly to InAs and InP. The reducing nature of the salt did not appear to change the dynamics of the ion-exchange. Rather, the rate limiting step was the solid-state diffusion of In and Ga in the pnictide lattice, consistent with observations for InAs and InP nanocrystals.

[0108] Next, the effect of alloying the pnictide component on chemical stability of the nanocrystals in different molten inorganic salt phases and its effect on cation-exchange were considered. InAs1-ySby(y=0.67) nanocrystals in Lewis acidic (KI / GaI3), Lewis neutral (KGaI4), and reduced Ga[GaI4] were started with. The InAs1-ySby nanocrystals decomposed in the Lewis acidic conditions (FIG.4A) and crystalline III-V phases were not observed. For KGaI4, a small amount of crystalline III-V phase was observed, but it can be indexed to nearly phase pure GaAs, indicating that the Sb had been leached from the nanocrystals, leaving behind only a small amount of pure GaAs. When Ga[GaI4] was used as the molten inorganic salt, the primary product was observed to be a crystalline III-V phase with a lattice constant intermediate between the parent compounds with smallest (GaAs) and largest (InSb) lattice constants. TEM and HRTEM of the resulting particles (FIG.4B) showed well isolated particles with single crystal domains. The reactivity of this system spanning time and temperature was explored (FIG.4C), and it was observed that the lattice constant decreased with increasing reaction time as evidenced by progressive shifts of the diffraction peaks to larger q. Composition cannot be uniquely determined by x-ray diffraction alone for these 4 component systems, since changing either the metal or pnictide ratio can alter the lattice constant. X-ray fluorescence was used to quantify the elemental ratios for all the components (FIG.4D). It was found that the As:Sb ratio was unchanged for all reaction conditions. Meanwhile, the gallium concentration was increased, and the lattice constant was decreased as the reaction time and temperature was increased. The results here demonstrate that the Ga[GaI4] prevented the leaching of the more reactive pnictide from the nanocrystals. Ultimately, this enabled the modularity in the reactions to prepare a desired composition. For example, the initial synthesis of InAs1-ySby can be optimized to give the desired As:Sb ratio and size in the first step. The in the second step, the In:Ga ratio can be optimized for the desired result. Ultimately, the modularity enabled by the reduced salts will enable precision synthesis of quaternary alloys of a desired size and composition.Atty. Dkt. No.05400-0072-PCT

[0109] The reactivity of InP1-yAsy in acidic, neutral, and reduced conditions was explored. In this case, it was observed that all conditions resulted in crystalline III-V phases with nearly identical compositions. This reactivity was distinct from pure InAs, where decomposition in Lewis acidic KI / GaI3was observed. From this, it was inferred that the small amount of phosphorous added to the system appeared to stabilize the arsenide in the InP1-yAsy in the nanocrystals. However, it cannot be ruled out that arsenic was being leached out of these nanocrystals in the acidic conditions. The reactivity of InP17As83 in Ga[GaI4] as a function of reaction time at 400 °C was explored. Continued shift of the PXRD peak positions to larger q was found, indicative of a decreasing lattice constant concomitant increased gallium concentration (FIG.5C). Elemental analysis by X-ray fluorescence (FIG.5D) demonstrated increased gallium incorporation with longer reaction times, and importantly the P:As ratio was unchanged, demonstrating modularity of the reaction.

[0110] Next, it was found that InP1-ySbynanocrystals could be prepared by reducing InCl3, PCl3, and SbCl3 with lithium triethylborohydride in oleylamine. It was found that phosphorous was incorporated at much lower concentrations than the feed ratio. A feed ratio of P / (P+Sb)=0.5 resulted in a measured phosphorus composition P / (P+Sb)=0.13. The large mismatch in anion size in this system may limit complete incorporation of alloy components. These results are consistent with the InP-InSb phase diagram, which in bulk has a large miscibility gap. It was further demonstrated that annealing the nanocrystals in Ga[GaI4] results in the recovery of crystalline zinc blende phases, whose lattice constant decreased with increasing time and temperature in Ga[GaI4] (FIG.6A). XRF analysis demonstrated that the Ga:In ratio can be controllably modulated and the P:Sb ratio was unchanged after molten inorganic salt annealing (FIG.6B), further demonstrating the modularity of the reaction methodology.

[0111] Pnictide halide chemistry was further expanded to produce InP1-y-zAsySbz. Again, phosphorous concentrations in the nanocrystals were observed to be much lower than the feed ratio. Nonetheless InP12As47Sb43nanocrystals were successfully prepared as confirmed by XRD and XRF (FIG.6C and 6D). Upon annealing in Ga[GaI4], XRD patterns were observed, which can be indexed to zinc blende phases, and the lattice constant decreased with increased annealing time or temperature. XRF demonstrated that the Ga:In ratio can be controlled by annealing time and temperature. Further XRF demonstrated that that the P:As:Sb ratio was unchanged, demonstrating the modularity of the reaction methodology to controllably prepare pentamery III-V nanocrystals.Atty. Dkt. No.05400-0072-PCT

[0112] Altogether it has been demonstrated that Ga[GaI4] can be used as a reaction medium and reagent to prepare ternary, quaternary, and pentamery III-V alloy nanocrystals. Every possible combination of elements was demonstrated using In, Ga, P, As, and Sb. A combination of traditional colloidal synthesis was used to prepare appropriate ternary and quaternary systems in the In, P, As, and Sb systems. These were then used as starting materials to prepare the respective quaternary and pentamery systems. Incorporating aluminum along with gallium may also be used, leading to possible hexanary alloy nanocrystals.

[0113] It was demonstrated that 4-component alloy semiconductor nanocrystals produced using this methodology are likely alloys rather than phase separated systems by Raman spectroscopy. Raman spectroscopy provides a local probe of the coordination environment and can distinguish between alloy semiconductors and phase separated heterostructures. Since Raman is sensitive to local structure, alloy nanocrystals should exhibit modes from each of the parent components. For example, InP1-yAsy should exhibit= modes which correlate to In-As and In-P bonds. Indeed, Raman spectra show peaks which correspond to InAs-like LO and TO modes and InP-like LO and TO modes for synthesized InP17As83 nanocrystals. A second signature of alloying is that the LO and TO modes of the parent compounds should shift in frequency relative to their parent compounds. This was also observed for the initial InP17As83nanocrystals in which the InAs LO and TO shifted to higher frequency and the InP-like LO and TO shifted to lower frequency.

[0114] Upon incorporation of gallium, three trends were observed. Specifically, an increase in a signal at the GaAs-like LO and TO frequency and a concomitant decrease in signal associated with the InAs LO and TO were observed. Further, it was observed that the InP-like LO shifted to higher frequency. Obvious GaP-like modes were not observed; however, this component had the smallest relative concentration and thus may be lost in the background. In addition, the Raman modes of the quaternary alloy systems were observed to be broader than in single phase systems. Altogether these results support formation of alloy III-V nanocrystals.

[0115] To demonstrate the viability of quaternary III-V nanocrystals for optoelectronic applications, the luminesces properties of In1-xGaxP0.17As0.83 were explored. These samples were focused on because they were expected to show NIR PL emission from ~1000-1500 nm. This is a spectral range which is poorly served by competing NIR emitting solution processedAtty. Dkt. No.05400-0072-PCT materials such as PbS, PbSe, etc., and is too red for traditional InP or CdSe based emitters. Further, this spectral range is easier to measure than other emitters further into the IR where In1-xGaxSb is expected to emit. A thin shell of ZnSe was grown on the surface of the nanocrystals using colloidal ALD. The self-limiting nature of the ALD reactions enabled precise and comparable shell thickness across several samples, making it possible to compare luminescence efficiency differences attributed to the core rather than shell quality. This is especially important for alloy semiconductors where the lattice constant is different for different samples, and lattice mismatch to shell materials can greatly affect the morphology of shell growth and thus the quality of the passivation.

[0116] FIG.11A shows a TEM image of In1-xGaxP0.17As0.83 / ZnSe with 1 monolayer (ML) of shell grown by high temperature colloidal ALD. The TEM image demonstrates that the tetrahedron shape was well maintained after shell growth, which was further confirmed from HRTEM images (FIG.11B). UV-Vis absorption spectra of these samples shows that as the annealing time was increased, the absorption onset blue shifted. In FIG.11D, normalized PL for the samples is shown, and the PL center wavelength shifted from 1400 nm to 1100 nm as x increased from 0 to 0.7, demonstrating one example of the spectral tunability possible with multicomponent alloy nanocrystals. The PL spectra correlate well with the absorption onsets, suggesting this emission resulted from band edge states. The relative PL efficiency of the samples as the gallium content was increased was measured by normalizing the PL spectra at the excitation wavelength (810 nm) by the absorption at 810 nm. It was observed that the higher gallium content In1-xGaxP0.17As0.83 / ZnSe showed considerably stronger PL compared to pure InP0.17As0.83 / ZnSe. It was found that the PL efficiency of the highest gallium content In1-xGaxP0.17As0.83 / ZnSe was 70 times higher than InP0.17As0.83 / ZnSe, indicating the molten inorganic salt processing had either dramatically improved the material quality due to the high temperatures or In1-xGaxP0.17As0.83 / ZnSe is intrinsically a better optical material than InPAs0.83 / ZnSe. Either way, these results demonstrate that the Ga[GaI4] molten inorganic salts developed in this Example are capable of producing high quality semiconductor materials. Moreover, the molten inorganic salt processed materials display superior optical properties than organic solvent derived samples.

[0117] Competing materials for emission in this spectral range are small PbS and PbSe nanocrystals. Due to the small band gap of PbS and PbSe, to achieve PL in this spectral range nanocrystals with ~3 nm diameter are needed. The soft lattice of the lead chalcogenides leads to strong exciton phonon coupling, and the small size of NIR emitting PbCh materials resultsAtty. Dkt. No.05400-0072-PCT in significant coupling to surface phonon modes. Together these effects result in NIR emitting PbCh nanocrystals with broad linewidths and large stokes shifts despite near perfect control over the size distribution of the samples. In other words, these materials have intrinsically broad homogenous linewidths (confirmed by single particle spectroscopy) at room temperature. Typically, PbCh materials are compared using absorption linewidth but here, the PL linewidth was used since the present samples show much more diffuse absorption onsets compared to PbCh samples. For example, PbSe nanocrystals emitting at 1091 nm (1.13 eV, 3.1 nm diameter) have a 163 meV emission linewidth. By comparison, In0.27Ga0.72P0.24As0.75 / ZnSe, which was prepared by annealing InP0.17As0.83 at 400 °C in Ga[GaI4] for 16 hours which emits at 1093 nm (1.13 eV) has a measured PL linewidth of 209 meV. This is broader than the state-of-the-art PbSe, However, the 163 meV PL linewidth of PbSe is unlikely to be further improved. By contrast, further optimization of the In0.27Ga0.72P0.24As0.75 / ZnSe reported in this Example to decrease the size distribution and composition distribution will reduce inhomogeneous broadening. As such, the present quaternary alloy semiconductors provide a viable route to narrow linewidth emitters in this critical spectral range.

[0118] FIG.12A shows a powder XRD pattern for InP nanocrystals which have been annealed (i.e., ion-exchanged) in In[InBr4] demonstrating that the reduced indium halide salt has shielded the nanocrystals from decomposition. TEM images of the resulting particles (FIG.12B) show high quality colloidal nanocrystals and highly faceted surfaces. While the composition of the nanocrystals did not change, self-diffusion of the indium ions in the lattice demonstrates that the cations in the lattice were exchanged between the nanocrystal lattice and the salt. The high temperature annealing in the In[InBr4] likely facilitated surface mobility of the ions, resulting in improved surface smoothness of the nanocrystals. Further, the high temperature annealing (at temperatures higher than possible with organic solvents) can greatly improve the crystal quality of the nanocrystals and in this case without convoluting it with a composition change.

[0119] Altogether this Example demonstrates the versatility of Ga[GaI4] as a molten inorganic salt solvent for incorporating gallium into InPn nanocrystals. The general utility for converting sensitive InSb nanocrystals into respective In1-xGaxSb nanocrystals was demonstrated. Further, this methodology was expanded to synthesize a library of quaternaryAtty. Dkt. No.05400-0072-PCT and pentanary colloidal nanocrystals in the In1-xGaxP1-y-zAsySbz system. The materials show high quality optoelectronic properties as demonstrated by strong NIR PL in In1-xGaxP0.17As0.83 / ZnSe nanocrystals. Finally, alloy III-V nanocrystals allow for large sized nanocrystals to achieve emission in the ~1100 nm spectral range, potentially minimizing exciton phonon coupling and subsequently giving these materials potential to enable narrow linewidth NIR emitters, which is not possible using competing PbCh materials.

[0120] The word "illustrative" is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as "illustrative" is not necessarily to be construed as preferred or advantageous over other aspects or designs. Further, for the purposes of this disclosure and unless otherwise specified, "a" or "an" means "one or more.”

[0121] The foregoing description of illustrative embodiments of the disclosure has been presented for purposes of illustration and of description. It is not intended to be exhaustive or to limit the disclosure to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the disclosure. The embodiments were chosen and described in order to explain the principles of the disclosure and as practical applications of the disclosure to enable one skilled in the art to utilize the disclosure in various embodiments and with various modifications as suited to the particular use contemplated. It is intended that the scope of the disclosure be defined by the claims appended hereto and their equivalents.

[0122] If not already included, all numeric values of parameters in the present disclosure are proceeded by the term “about” which means approximately. This encompasses those variations inherent to the measurement of the relevant parameter as understood by those of ordinary skill in the art. This also encompasses the exact value of the disclosed numeric value and values that round to the disclosed numeric value.

Claims

Atty. Dkt. No.05400-0072-PCT WHAT IS CLAIMED IS:

1. A method of forming group III-V nanocrystals, the method comprising: forming a dispersion of binary or higher-order group III-V nanocrystals in a molten inorganic salt comprising group III metal ions in an oxidation state of less than III; maintaining the dispersion at a temperature at which group III metal ions of the molten inorganic salt undergo ion exchange with group III metal ions in the binary or higher- order group III-V nanocrystals to form ternary or higher-order group III-V nanocrystals; cooling the dispersion to form a composite comprising the ternary or higher-order group III-V nanocrystals dispersed in a solid salt matrix comprising a group III element; and separating the ternary or higher-order group III nanocrystals from the solid salt matrix.

2. The method of claim 1, wherein group V ions in the binary or higher-order group III-V nanocrystals comprise P ions, As ions, Sb ions, or a combination thereof.

3. The method of claim 2, wherein the group V ions in the binary or higher-order group III-V nanocrystals comprise the Sb ions.

4. The method of claim 2, wherein the binary or higher-order group III-V nanocrystals comprise InSb nanocrystals, 5. The method of claim 1, wherein the binary or higher-order group III-V nanocrystals comprise the binary group III-V nanocrystals.

6. The method of claim 1, wherein the binary or higher-order group III-V nanocrystals comprise ternary group III-V nanocrystals and / or quaternary group III-V nanocrystals.

7. The method of claim 1, wherein the molten inorganic salt is a group III metal halide salt.

8. The method of claim 7, wherein the group III metal ions in the oxidation state of less than III are Ga ions in an oxidation state of less than III, In ions in an oxidation state of less than III, or a combination thereof.Atty. Dkt. No.05400-0072-PCT 9. The method of claim 7, wherein the group III metal halide salt is a group III metal iodide salt.

10. The method of claim 7, wherein the group III metal halide salt comprises a MI[MIIIX4] salt, a MI2[MII2X6] salt, or a combination thereof, where M represents Ga or In and X represents a halogen ion.

11. The method of claim 8, wherein the group III metal ions in the oxidation state of less than III comprise GaIions, GaIIions, or a combination thereof.

12. The method of claim 11, wherein the molten inorganic salt comprises GaI[GaIIII4].

13. The method of claim 11, wherein the binary or higher-order group III-V nanocrystals are selected from InAs nanocrystals, InAsSb nanocrystals, InPAs nanocrystals, InPSb nanocrystals, InPAsSb nanocrystals, and combinations thereof and the ternary or higher-order nanocrystals are selected from InGaSb nanocrystals, InGaAsSb nanocrystals, InGaPAs nanocrystals, InGaPSb nanocrystals, InGaPAsSb nanocrystals, and combinations thereof.

14. The method of claim 7, wherein the group III metal halide salt comprises alkali metal ions.

15. The method of claim 14, wherein the group III metal halide salt has a chemical formula A2[M2X6], where A represents the alkali metal ion, M represents the group III metal ion, and X represents a halogen ion.

16. The method of claim 15, wherein the group III metal halide salt comprises Na2[GaIII6], K2[GaIII6], K2[GaIIBr6], or a combination of two or more thereof.

17. The method of claim 1, further comprising: forming a second dispersion of the ternary or higher-order group III-V nanocrystals that were separated from the solid salt matrix in a molten inorganic salt comprising group III metal ions in an oxidation state of less than III;Atty. Dkt. No.05400-0072-PCT maintaining the second dispersion at a temperature at which group III metal ions of the molten inorganic salt undergo ion exchange with group III ions in the ternary or higher- order group III-V nanocrystals to form quaternary or higher-order group III-V nanocrystals; cooling the second dispersion to form a second composite comprising the quaternary or higher-order group III-V nanocrystals dispersed in a second solid salt matrix comprising a group III element; and separating the quaternary or higher-order group III nanocrystals from the second solid salt matrix.

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