WO2018170369A1

WO2018170369A1 - Methods of synthesizing compound semiconductor nanocrystals

Info

Publication number: WO2018170369A1
Application number: PCT/US2018/022817
Authority: WO
Inventors: Alexander O'toole; Douglas Y. DESARIO; Rutvik J. MEHTA
Original assignee: ThermoAura Inc.
Priority date: 2017-03-17
Filing date: 2018-03-16
Publication date: 2018-09-20

Abstract

A method of synthesizing compound semiconductor nanocrystals is provided. The method may include: synthesizing at least two microwave precursor solutions; combining the at least two microwave precursor solutions to create a combined solution; applying a microwave to the combined solution to drive a redox reaction to produce the compound semiconductor nanocrystals; and extracting the compound semiconductor nanocrystals.

Description

METHODS OF SYNTHESIZING COMPOUND SEMICONDUCTOR NANOCRYSTALS

BACKGROUND

Federally Sponsored Research or Development

[0001] This invention was made with U.S. Government support under award numbers 1215307 and 1330650 awarded by the National Science Foundation. The U.S.

Government has certain rights to this inventioa

Technical Field

[0002] The present disclosure relates to nanocrystals, and more specifically, methods of synthesizing compound semiconductor nanocrystals.

Related Art

[0003] Nanostructures and nanomaterials are composed of features with at least one characteristic dimension smaller than 100 nanometers. Nanostructured materials offer great promise for revolutionizing diverse fields and applications including electronics, energy storage and energy conversion, due to the manifestation of novel properties through size- dependent physics and chemistry not found in conventional bulk materials. One example of how nanostructuring benefits thermoelectric materials is the guaranteed increase in thermoelectric efficiency through the nanostructure-induced thermal conductivity reduction. Nanocrystals of compound semiconductors are especially valuable as their size-dependent properties offer large enhancements for crucial high-technology applications in solar power, thermoelectrics, infrared (IR)-detection, batteries, fuel-cells, micro-electronics and medical devices. Examples include group 15-16 compound semiconductors such as: pnictogen chalcogenides; group 13-15 compounds such as: gallium arsenide, indium antimonide, aluminum phosphide; group 12-16 semiconductors such as: zinc selenide, cadmium telluride, zinc oxide; and other combinations of these compounds such as lithium based compounds.

[0004] However, while nanostructuring imbues beneficial physical properties, harnessing beneficial nanoscale effects and properties in bulk forms is crucial for high-impact applications. For example, large-scale energy conversion using nanostructured

thermoelectrics, high-energy density batteries, or solar cell semiconductors necessitates the industrial-scale production of bulk forms of nanostructured materials under acceptable time- scales, energy-usage and costs. The numerous strategies and techniques for producing nanomaterials can be broadly classified as bottom-up or top-down approaches. Top-down methods involve the subtractive modification of bulk materials to introduce nanoscale features, by using techniques such as etching, milling, lithography, and modifications thereof. Bottom-up approaches include additive processing that involves the synthesis and assembly of nanoscale building blocks using wet-chemistry, chemical and physical vapor deposition, electrodeposition and solid-state reactions. Most state-of-art techniques using either approach, however, are not suitable for commercialization due to poor properties and/or scalability and high costs.

[0005] Lack of viable means for commercial production of nanomaterials is the key limiter for wide-spread adoption of thermoelectric nanotechnology. Additionally, handling and storage of nanomaterials presents a whole new set of exacerbated challenges arising from increased oxidation, uncontrolled coalescence, and high-reactivity due to high surface- volume ratios. Extensive protocols for oxygen-free handling often introduce additional process complexities and costs. Thus, besides cost-effective production of ton-scale quantities of nanomaterials with the desired properties, passivating the nanostructures or the

nanomaterial for easy handling and storage is a challenge. This is particularly the case in applications using compound semiconductors where even a small change in the composition, e.g., due to doping, can lead to large changes in material properties which determine functionality and performance.

[0006] Nanocrystals of compound semiconductors have been fabricated in tiny experimental quantities in laboratories by numerous bottom-up approaches through chemical synthesis. The field of nanocrystals chemical synthesis techniques is vast including notable wet- chemistry techniques such as solvothermal, hydrothermaL, polyol, micro-emulsion, autoclave- based synthesis and electrochemical methods such as electrodeposition of nanowires, vapor and fluid-based deposition and nanocrystal growth techniques. However, the state-of-art bottom-up techniques are either exorbitantly expensive, involve long reaction times (e.g., few hours to days), or are unsuited to mass-production on practical scales due to sub-gram yields. Additionally, these state-of-art techniques are impractical for applications (e.g.

electrodeposition in ceramic templates) and are susceptible to oxidation (oxygen-free materials handling equipment necessary). State-of-art techniques are also unfeasibly energy- intensive, and have not yet demonstrated any significant upscaling to high-volume production of nanocrystals.

SUMMARY

[0007] A first aspect of the disclosure provides for method of synthesizing compound semiconductor nanocrystals. The method may include: synthesizing at least two microwave precursor solutions; combining the at least two microwave precursor solutions to create a combined solution; applying a microwave to the combined solution to drive a redox reaction to produce the compound semiconductor nanocrystals; and extracting the compound semiconductor nanocrystals.

[0008] A second aspect of the disclosure provides for method of synthesizing at least two microwave precursor solutions. The method may include: providing an elemental constituent to a solvent to create a first solution for each of the at least two microwave precursor solutions; controlling a pH of the first solution for each of the at least two microwave precursor solutions by adding at least one of: an acid or a base; providing a reducing agent to the first solution of only one of the at least two microwave precursor solutions; and providing an oxidizing agent to the first solution of at least one other of the at least two microwave precursor solutions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] These and other features of this invention will be more readily understood from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings that depict various embodiments, in which:

[00010] FIG. 1 shows a schematic of an exemplary system for synthesizing compound nanocrystals.

[00011 ] FIG. 2 shows a flow chart of a method for synthesizing compound nanocrystals.

[00012] FIGS. 3A-C shows an exemplary schematic of constituents used for synthesizing the microwave-active precursor solutions of FIG. 1.

[00013] FIG. 4A-4B show molar-ratio-pH condition maps of example precursor solutions.

[00014] It is noted that the drawings of the disclosure are not to scale. The drawings are intended to depict only typical aspects of the disclosure, and therefore should not be considered as limiting the scope of the disclosure. In the drawings, like numbering represents like elements between the drawings. DETAILED DESCRIPTION

[00015] The present disclosure relates to nanocrystals, and more specifically, methods of synthesizing compound semiconductor nanocrystals.

[00016] The present disclosure achieves cost-effective industrial-scale, i.e., tons-scale, production of nanocrystals of compound semiconductor materials to harness and apply the beneficial nanoscale-effects in commercially viable bulk forms (i.e., macro-sized

nanostructured forms or nanobulks) to empower nano-enabled products. Up-scaled production of compound semiconductor nanocrystals presents a number of requirements on the processing methodology. For example, the requirements may include: the ability to synthesize compound semiconductor nanocrystals of desired combination and composition, scale and throughput (supply and meet industry demands), cost-parity with conventional manufacturing (for commercial feasibility), safe and oxygen-free handling of large quantities of nanomaterials (eliminate degradation of the nanostructures), and flexibility and precision (for design and engineering of nanocrystal properties). The latter is especially challenging in complex semiconductor materials for successful applications.

[00017] The present disclosure is a new scalable bottom-up paradigm that creates compound semiconductor nanocrystals using a method which combines concepts of microwave engineering, wet-chemistry, and redox reactions. The method exhibits low-energy- consumption, operating at low temperature, i.e., less than 120 °C, with rapid production capacity at greater than 1 kilogram per minute (Kg/min), and is highly efficient with near- 100% yield. It is therefore both conducive to industrial-scale production, and permits synthesis of compound semiconductor nanocrystals that are able to harness beneficial nano- effects. In contrast to existing state-of-art methodologies, the method has the process flexibility and versatility to craft a wide range of compound semiconductor nanocrystals and equip them with functionalities through a combination of dopant additions, alloying and nanostructuring. The method permits synthesis, production and safe handling of large- quantities of nanocrystals of compound semiconductors by using microwaves to drive a redox reaction between two microwave-active precursors containing chelated and solvated elemental constituents under ambient environments thus obviating specialized chemical process equipment and drastically simplifying process flow design and production tooling. Furthermore, the method is capable of use with inputs simply containing the atoms of the compounds to be synthesized, without any stringent demands for particular purity or grade or morphological factors. Scaling up this process provides a unique and currently unavailable way to produce high performance nanostructured compound semiconductors at lower costs than existing routes. The present disclosure provides a unique means to employ

nanostructuring in bulk-scale to effectively harness the beneficial nano-effects and to pave the way for disruptive performance enhancements. The method produces nanocrystals with protecting chemistries which prevent oxidation and agglomeration, allowing complete production, handling and storage of the nanomaterials in ambient conditions and

temperatures, without hindering or degrading the nanomaterials properties.

[00018] The microwave wet-chemical process described herein includes a microwave-driven activation and stimulation of a redox reaction between two or more distinct precursor streams prepared separately and independently, to such values of pH and reduction potential, so as to be on a cusp of undergoing a reducing reaction, prior to pumping into the microwave cavity on a continuous or batch-continuous basis for in-situ mixing, microwave activation and reduction. The redox reaction between the multiple precursors in the presence of the microwave field drives the chemical formation of the compound semiconductor nanocrystals. The method may utilize a process of continuous flow of the precursor streams in the microwave reactor. The process can be modified to operate on semi-continuous or batch- continuous basis as necessary to meet production goals, and also by prior preparation and storage of the precursors. As used herein, "batch-continuous" or "semi-continuous" may refer to a process where the precursors may be prepared in discrete batches and provided to the reactor cavity in a continuous manner. The flexibility of the method allows meeting goals for productivity, costs-targets and commercial viable scales.

[00019] FIG. 1 shows a schematic diagram of an exemplary system for synthesizing compound nanocrystals 100. FIG. 2 shows a flow chart of a method 200 for synthesizing compound nanocrystals. Method 200 will now be described in light of system 100.

[00020] The method 200 may start with process 202 - by providing at least two microwave active precursor solutions 102 from separate precursor reservoirs 104 to a pre-reaction chamber 106 to combine precursor solutions 102. As used herein, a "microwave active precursor solution" includes a liquid solution made up of a polar solvent containing dissolved elemental constituents and other chemical agents, and may also be referred to herein as "precursor solution(s)." FIG. 1 shows a total of three precursor solutions 102, where the third precursor solution 102 is optional (as evidenced by dotted lines). However, it is to be understood that any number of precursor solutions 102 can be used without departing from aspects of the disclosure. Precursor solutions 102 may include individual chemical solutions consisting of the elemental constituents of the compound semiconductors ligated by appropriate organic molecules and dissolved in an ionic microwave-active solvent. Thus, the number of precursor solutions 102 may be contingent upon the composition of the compound semiconductor. For example, where binary compound semiconductor nanocrystals are desired, two precursors 102 may be used; where ternary compound nanocrystals are desired, three precursors 102 may be used; where quaternary compound nanocrystals are desired, four precursors 102 may be used, and so on. Therefore, semiconductor nanocrystals may be of a compound tvoe that is higher than a auaternarv compound. Precursor solutions 102 may also combine elemental constituents which reside in the same periodic group, thus decreasing the number of precursors 102 by combining chemically similar and compatible elements. For example, bismuth and antimony of periodic group IS, and zinc and gallium of periodic group 12 can be combined into one precursor solution 102, instead of preparing separate solutions for each. Precursor solutions 102 may be supplied to pre-reaction chamber 106 in a continuous fashion using conventional tubing or piping and pumps systems.

[00021] Turning briefly to FIGS. 3A-3C, exemplary schematic 300 of constituents used for forming precursor solutions 102 will now be discussed. As shown, FIG. 3 A corresponds to the schematic 300A of constituents that may be used for forming a first precursor solution 102 of FIG. 1. FIG. 3B corresponds to the schematic 300B of constituents mat may be used for forming a second precursor solution 102 of FIG. 1, and FIG. 3C corresponds to the schematic 300C of constituents that may be used for forming a third precursor solution 102 of FIG. 1. schematic 300 may include providing a microwave active solvent 304. Microwave active solvent 304 may include for example a polar solvent such as deionized water, an alcohol (e.g., ethanol, methanol, propanol), a glycol, a polyol, or an organic solvent such as carbon di sulphide, acetone, pentanediol, or butanediol. One or more constituent element sources 306 may be added to the microwave solvent 304. Constituent element sources 306 may be in the form of elemental powders and/or metalorganic salts. Constituent element sources 306 may include, for example, a group 12 element such as, for example, zinc or cadmium; a group 13 element, such as, for example, gallium, indium, aluminum, or titanium; a group IS element, such as for example, antimony or bismuth; or a group 16 element such as, for example, selenium, tellurium, sulfur, or oxygen. In some embodiments, constituent element sources 306 may include a metal, such as, for example, iron, cobalt, copper, indium, aluminum, zinc, cadmium, or silver. Constituent element sources 306 may not readily form stable solvated solutions in microwave active solvent 304, and may require the addition of acids 308 and/or bases 310 with strong ligating 312 and/or chelating 314 agent. Because each of acids 308, bases 310, ligands 312, and chelates 314 may be optional and/or customizable, they are each represented by dotted line boxes in FIGS. 3 A-3C.

[00022] Acids 308 may include, for example, at least one of: inorganic acids, such as a hydrochloric acid, a sulfuric acid, etc., and organic acids such as a mercaptan acid (e.g., mercaptoacetic acid, mercaptopropionic acid), a citric acid, an ascorbic acid, an

ethylenediaminetetraacetic acid (EDTA), an oleic acid, or a tartaric acid. Bases 310 may include, for example, at least one of: a metal hydroxide such as a hydroxide of lithium, sodium or potassium, a borate, an ammonium hydroxide, or an organic amine. One or more acids and/or bases may be added solely or in combination to adjust the pH to the value desired for complete dissolution of constituent elemental source 306 in microwave active solvent 304. The solvation may occur in the presence of or may be accompanied by the release of heat

[00023] The concentration of the dissolved elemental constituents 306 determines the productivity, sizing of the batches, and production run. Thus, mis concentration may drive the designs of the reactor systems and sub-systems. For example, ligating and/or chelating agents 312, 314 may be added to the precursor solution 102. In some embodiments, ligating and/or chelating agents 312, 314 may include an acid or base added to precursor solution 102.

Therefore, it is to be understood mat acid 308 or base 310 may serve a dual role of controlling pH and as the ligating and/or chelation agent 312, 314. However, in other embodiments, ligating and/chelating agent 312, 314 may include an acid or a base separate from acid 308 and base 310. Other functional compounds and molecules ("functional agents" 316) for imparting functions may be added to the precursor solution 102. Functional agents 316 may include, for example, at least one of: a surfactant, a doping agent, a surface passivation, an anti-agglomeration, or a flocculating agent. The addition of functional agents 316, ligating agent 312, and/or chelating agent 314 may require changing the pH of the solution to maintain complete solvation. Thus, additional appropriate quantity of acids 308 and bases 310 may be added again.

[00024] As should be clear, each precursor solution 102 may be prepared by dissolving appropriate constituent elemental sources 306 in microwave-active solvents 304, and stabilized by the use of liganding and/or chelating agents 312, 314 and other functional agents 316 at certain pH values using acid 308 and base 310 additions. The final step in the preparation of the precursor solution 102 involves the addition of a reducing agent 318 to the solution. Typically, reducing agent 318 may be added to only one of the precursor solutions 102 of the set. This may also be accompanied by the addition of an oxidizing agent 320 to one or more of the remaining precursor solutions 102 in the set. For example, as shown in FIG. 3, reducing agent 318 may be added to the first precursor solution 102, while oxidizing agent 320 may be added to the second and/or third precursor solutions 102. Reducing agent 318 and oxidizing agent 320 are shown in FIGS. 3A-3C as dotted line boxes because, in other embodiments, reducing agent 318 can be added to one of the second or third precursor solutions 102 instead of the first precursor solution 102, and the oxidizing agent 320 can be added to the first precursor solution 102 instead of the second and/or third precursor solution 102.

[00025] Reducing agent 318 may include, for example, at least one of: a metal borohydride, a metal hydride, a sulfite, a hydrazine, or an organic acid. Oxidizing agent 320 include, for example, at least one of: a peroxide, a permanganate, a halide, or deionized water. Reducing and oxidizing agents 318, 320 may also be added to precursor solution 102 solely for forming oxides or for complex multi-elemental compound semiconductor nanocrystals. This situation is discussed herein with respect to Example m. Before adding reducing agent 318 with or without oxidizing agent 320 to the respective precursors 102, the pH of precursor solution 102 may be adjusted again to values that fall within the stability ranges of the reducing and/or oxidizing agents 318, 320.

[00026] The preparation of precursor solutions 102 according to schematic 300 impacts the final properties and performance of nanocrystals 124 (FIG. 1). Additionally, schematic 300 impacts the final production costs of nanocrystals 124. Precise control over the molar concentration fluxes of precursor solutions 102 may be necessary to ensure reproducibility and high quality of produced nanocrystals. The process control parameters which account for the molar concentration flux are volumetric flux and molar concentration, which are controlled by the reactor systems where precursor solutions 102 are prepared. The molar concentrations of precursor solutions 102 directly control the composition of nanocrystals 124 through the molar concentration flux ratios. The composition of nanocrystal 124 is a linear function of the molar concentration flux ratio of precursor solutions 102 upon a complete redox reaction. For process control, precursor solutions 102 may be maintained at the same volume and adjust the molar concentration in the desired ratio to keep the molar concentration flux matched to the targeted compound semiconductor nanocrystal

composition.

[00027] The advantageous flexibility inherent to schematic 300 originates in the versatility of precursor solutions 102 due to the multiple raw material, constituent element sources 306, and their wide/varied solvation conditions. In addition, tuneability through concentrations and ratio variations of the functional chemistry of schematic 300 also contributes to the flexibility. Similarly, the inherent high flexibility permits cost reductions and optimization through choosing of solvable, inexpensive raw sources, and higher concentration synthesis.

[00028] Turning back to FIGS. 1-2, method 200 may continue with process 204 - by providing the combined precursor solutions 102 from pre-reaction chamber 106 to a microwave flow reactor cavity (herein after "reactor cavity") 110 where precursor solutions 102 undergo a redox reaction in the presence of and activated by a microwave field 112. Precursor solutions 102 may be provided to reactor cavity 110 from pre-reaction chamber 106 through conventional piping or tubing. Together, pre-reaction chamber 106 and reactor cavity 110 may make up a microwave reactor 105. Reactor cavity 110 may be made up of microwave-compatible materials including technical polymers and ceramics and has the ability to change the length, shape and geometry of the cavity by addition or subtraction of sections.

[00029] Method 200 may continue with process 206 - by providing a microwave electromagnetic field 112 from a microwave system 114 to drive the redox reaction between microwave-active precursor solutions 102 to produce compound semiconductor nanocrystals 124 in-solvo. The applied microwave field 112 influences the compositions and structures of the produced compound semiconductor nanocrystals 124. Microwave field 112 may include single-mode microwaves and/or multi-mode systems. The total power capacity of reactor cavity 110 is sized by the product of the microwave dose, which is the energy delivered and absorbed in reactor cavity 110, and the productivity or rate of production desired. The total power capacity is independent of the reactor cavity 110 volume, shape or design. For a constant productivity and constant available power, the concentration of precursor solutions 102 and the flow rate are inversely proportional. Therefore, the design of reactor cavity 110 can be optimized independent of productivity requirements and with uniformly distributed microwave power density across the full cavity volume.

[00030] The presence of microwave field 112 and energy input during synthesis according to method 200 results in product uniformity, homogeneity and quality, and maintains high yields. Microwave field 112 applied to precursor solutions 102 in reactor cavity 110 may be partitioned between energy absorbed by precursor solutions 102, the nanocrystals seeds, and growing nanocrystals 124. Determining this partitioning factor may be used to optimize and engineer the production process, and can be accomplished by experimental trials with different microwave doses, flow rates and nanocrystal 124 concentrations. There is a correlation between nanocrystal 124 size with dose of microwave field 112. Low microwave fields 112, with typical microwave doses of approximately 1 Joules/gram (J/g) to

approximately 100 J/g, applied to precursor solutions 102 result in narrow size distributions of nanocrystals 124 (approximately 5 nanometers (nm) to approximately 10 nm) with both crystalline sizes and size distribution widening with dosage. However, at extremely high- fields applied to precursor solutions 102, for example, characterized by microwave doses of greater than approximately 300 J/g, the above-described correlation becomes non-monotonic and the size of the nanocrystals show a decreasing correlation with increasing dosage.

Uniform compositions and homogeneity of the compound semiconductor nanocrystals 124 require a uniform redox reaction and microwave energy transfer in the reaction volume. The origin of these phenomena arise from a complex interplay between the microwave-dose, the precursor solution 102 flow-rates, the exothermic redox reactions, the superheating of precursor solutions 102 and the nucleation and growth mechanistic behaviors of nanocrystals 124.

[00031 ] Method 200 uses a design of the reactor cavity reactor 110 to generate a turbulent flow of precursor solution 102 streams in pre -reaction chamber 106 to produce a finely mixed and agitated admixture of the multiple solutions. The pre-reaction zone and reaction zone of reactor cavity 110 are designed to have dimensions such that the flow velocity and fluid pressure vary through between the zones. Method 200 utilizes the creation of an in-situ static mixing region in the pre-reaction zone (or volume) of pre-reaction chamber 106 to create a turbulent flow of precursor solutions 102. The static mixing action may be created by application of, for example, a Venturi tubes, a nozzle, an orifice, or placement of an in-line static mixer rotating by the action of the fluid flow. In other embodiments, static mixing may be accomplished by design of a junction, e.g., a T-junction, which creates turbulence at the intersection of the solution streams between the pre-reaction and reaction zones within reactor cavity 110.

[00032] Precursor solutions 102 may flow through microwave reactor cavity 110 on a continuous basis. Admixed precursor solutions 102 flows from pre-reaction chamber 106 to the reactor cavity 110 where microwave field 112 drive the redox reaction to completion resulting in the formation of the compound semiconductor nanocrystals 124 in the processed liquor solution as will be described herein. Precursor solution 102 flow rates and hence, the reaction flux and nanocrystal production volumetric flow-rate, may be engineered with the characteristics of the microwave system 114, reactor cavity 110 dimensions and form, dielectric properties of the compound semiconductor nanocrystals 124 and precursor solutions 102. The dielectric properties of precursor solutions 102 change with the concentration, nature and type of constituent element source 306 (FIG. 3), which depends on the desired compound semiconductor nanocrystal 124 composition and functional agents 316 (FIG. 3) present. Microwave field 112 and precursor solution 102 flow rates may be selected with the dimensions of reactor cavity 110 to ensure maximum volumetric energy transfer for optimum nanocrystal nucleation and growth.

[00033] Nanocrystals 124 of compound semiconductors generally have large loss tangents and are good absorbers of microwave energy. As used herein, "loss tangent" may refer to ratio of the loss factor to the dielectric constant, or how lossy a material is for a given signal or microwave frequency. The higher loss tangent is indicative of higher capacity for inherent dissipation of electromagnetic energy and thus stronger ability to be heated by microwaves. The temperature dependence of the loss tangent for the nanocrystals also shows a monotonic increase with temperature in direct contrast to the monotonic decrease generally exhibited by solvents. The temperature dependence of the dielectric constants of compound semiconductor nanocrystals 124 are weaker compared with solvents 304 (FIG. 3) and functional molecules 316 (FIG. 3). Therefore, there is a temperature at which nanocrystal 124 nuclei formed in- solvo absorbs a larger fraction of the microwave energy than precursor solution 102. This temperature is referred to as the transition temperature. The transition temperature is representative of the temperature at which the partitioning of the microwave energy absorption transitions from predominant absorption by the solvent to dominant absorption by the nanocrystals is the transition temperature and may occur depending on, for example, the preparation of precursor solution 102. The transition temperature for precursor solutions 102 can be calculated from the dielectric properties of the compound semiconductor nanocrystals 124 and precursor solution 102.

[00034] The dielectric property measurements can be conducted and analyzed by the techniques known in the art such as the resonant-cavity technique under argon atmospheres. The real (dielectric constant) and imaginary components (loss factor) of the complex permittivity, the loss tangent, and the half-power depth, may be determined as a function of temperature for a frequency regime. As used herein, "half-power depth" may refer to the penetration depth in the material at which half of the power of the microwave filed is attenuated, i.e., the microwave penetration depth. A frequency of approximately 2450 to approximately 2466 MHz may be selected as is common for microwaves.

[00035] Based on the above measured data, a model of the deconvolution of the microwave power absorption between precursor solvent 304 and semiconductor nanocrystals 124 in slurry 122, and the temperature change of the mix in the microwave cavity may be created. For example, a Johnson-Mehl- Avrami-Kolmogorov model type for the production of the nanocrystal in the solution mix as an exponential growth function may be used. Such a model may be represented by:

[00036] where, V¾₀ivent is the instantaneous volume fraction of precursor solvent 102; Vfkc is the instantaneous volume fraction of nanocrystals (NC); C is the production volumetric fraction of the nanocrystals; Q is the microwave power absorbed or dissipated (W); 1 is the length along the cavity; ε' is the dielectric constant; ε" is the loss factor; / is the microwave frequency, εο is the permittivity of free space, or E is the electric field

strength (Vm ¹); e is Euler's number, and π is pi.

[00037] Loss tangent may be represented by:

[00038] Microwave power absorbed or dissipated per unit volume may be represented by:

[00039] Simplifying the equation, the following expression can be obtained as the basis:

[00040] The combination of high loss factor and high loss tangent results in the half-power depth in nanocrystals 124 to be comparable with that of common solvents at room- temperature and with increasing temperature, the half-power depth in nanocrystals 124 is smaller than that of the common solvents and typically decreases weakly with temperature. The temperature may have an effect on the partition of the microwave energy between nanocrystals 124 and solvent 304 in the precursor solution 102, with higher temperature resulting in factorial increases in energy absorbed by nanocrystals 124. Increasing the production concentration of nanocrystals 124, and hence, associated precursor solutions 102, also may result in larger fraction of the energy being selectively absorbed by the

nanocrystals.

[00041] Method 200 utilizes precursor solution 102 streams at temperatures higher than the transition temperatures. Therefore, method 200 results in higher efficiency synthesis and higher quality nanomaterial production. Precursor solution 102 streams may be heated to or above the transition temperature using microwave field 112 in reactor cavity 110.

[00042] The dielectric properties of precursor solutions 102 and nanocrystals 124 may be used to determine the half-power depth. As a consequence of the higher loss factor and loss tangent, with increasing temperature, the penetration depth of microwaves in nanocrystals 124 is smaller than that of the precursor solvents 304 of the precursor solutions 102 and decreases weakly with temperature.

[00043] On completion of the redox reaction in reaction cavity 110 of the microwave reactor cavity, a slurry 122 is formed including nanocrystals 124 in a process liquor 126. As used herein, "process liquor" or "liquor" refers to me waste product of slurry 122. Nanocrystals 124 must be separated or extracted. Thus, method 200 may continue with process 208 - by extracting nanocrystals 124 from slurry 122. Slurry 122 containing nanocrystals 124 and process liquor 126 may subjected to a solid-liquid separation 128 to recover nanocrystals 124, and to subsequently treat process liquor 126 as necessary.

[00044] Solid-liquid separation 128 may include membrane filtration under pressure or vacuum, or centrifuge technology to provide rapid continuous or semi-continuous means of large-scale solid-liquid separation. Solid-liquid separation 128 may use filtration with sub- micron size filters or centrifuges to separate and sieve out nanocrystals 124 in the form of a wet cake mass from process liquor 126. Depending on the type and design of solid-liquid separation 128. these systems can vary from fully continuous to a batch-type process. Process liquor 126 may then be treated and handled to form byproducts 130. Nanocrystals 124 in the form of a wet nanocrystal cake may be cleaned as necessary and dried to obtain the nanocrystal product 132 in powder form. The drying can be performed using vacuum drying or forced convection drying with or without heat.

[00045] Method 200 addresses process liquor 126 handling and disposal using vaporizational removal of solvent contents in a chemical-compatible boiler or evaporator of appropriate scale and throughput to form safe liquor handling byproducts 130. Alternatively, process liquor 126 handling may be performed using a filtration system based on reverse-osmosis or deionization. This permits safe solvent removal and concentration of inorganic and organic waste components in a concentrated process liquor 126. The design permits the nanocrystals production to operate on a continuous or semi-continuous basis by starting or stopping the difference components.

[00046] Method 200 allows for the design of a modular system. For example, two sets of precursor solutions can be prepared into two precursor reservoirs and connected to one microwave system, one after the other. After the reaction, another set of precursor solutions may be connected to the microwave system. That is, they can be connected and used as necessary so that only one microwave system is used but multiple sets of precursor solution and/or centrifugation systems may be used. To facilitate such modularity, the components of system 100 may be designed to separate the different pieces and connect them easily at will.

[00047] In order to demonstrate aspects of method 200 according to the subject disclosure, the following examples are provided:

[00048] Example I- Preparation of bismuth precursor Optimal pH and formation of solvated bismuth aqueous solutions [00049] Many compound semiconductors are based on bismuth such as binary compounds such as bismuth telluride, bismuth selenide, bismuth sulfide, ternary compounds like bismuth telluro-selenide and bismuth seleno-telluride series of alloys, or quaternary compounds like bismuth cobalt zinc oxide. Synthesizing nanocrystals of bismuth-based compound semiconductors require a bismuth constituent element source in microwave-active solvent such as deionized water. The most stable oxidation state of bismuth is Bi⁺³ and the widely available bismuth salts supply bismuth in the triply charged ionic state. In aqueous solutions, bismuth compounds hydrolyze readily typically forming water-insoluble precipitates. For example, the use of bismuth chloride typically results in the formation of the insoluble oxychloride salt.

[00050] In alkaline solutions and with basic compounds, i.e., NaOH, KOH, ΝΉ₄ΟΗ, bismuth salts react to form the hydroxide Bi(OH)3 which also form white precipitate and are insoluble in water. The hydrolysis of bismuth salts in water also results in the formation of the insoluble hydroxide precipitate.

[00051] Bi(OH)3 readily dissolves in acids and under acidic conditions liberating the free Bi⁺³ ions. Solvated precursor chemistries can be made utilizing the application of suitable acid molecules for driving salt solubilization and ionic availability of the Bi⁺³ ions for active chelation suitably controlled through pH tuning.

[00052] A series of titration experiments may be conducted to evaluate the solvation conditions of bismuth salts under acidic and basic conditions with and without the presence of functional molecules by forming condition maps. Bismuth oxide may be the salt providing the elemental constituent bismuth, with deionized water as the microwave-active solvent. Sodium hydroxide may be used as the base for pH tuning and mercaptoacetic acid may be used as a functional molecule serving roles of an acid and as an oxidation inhibitor.

Ethylenediaminetetraacetic acid (EDTA) may be used as a ligand and chelation agent A series of solutions may be prepared by adding 5-15 wt.% of bismuth oxide to deionized water forming a suspension. Sodium hydroxide may be added to one set of the solutions in increments of 1 wt%. The solubility changes may be observed and recorded. The pH change may be noted and tested up to pH of 14. The set of NaOH-bismuth oxide solutions may then be titrated against mercaptoacetic acid and the pH and solubility changes may be recorded again. Similarly, another set of 5-15 wt.% bismuth oxide aqueous solutions may be prepared with varying concentrations of EDTA from 5-10 wt.%. The concentrations of the bismuth oxide and EDTA can be adjusted as desired to meet the requirements of the synthesis and the maps can be updated with additional details or additional sections. The set of EDTA-bismuth oxide solutions may be made alkaline by incremental 1 wt% NaOH additions and titrated against mercaptoacetic acid. The pH changes and solubility changes may be observed the recorded. These set of experiments may be used to produce condition maps which detail exact solvent conditions where a stable solvated bismuth aqueous precursor is formed. FIGS. 4A-4B show the condition maps for an alkaline aqueous bismuth oxide-EDTA solution.

[00053] Bismuth oxide has a basic character and a higher compatibility with EDTA mixes due to the essentially basic chemistries necessary for EDTA use and solvation. The titration experiment sets may be used to discover bismuth oxide solvated conditions with

mercaptoacetic acid as functional molecules and chelation agent EDTA. The maps in FIG. 4A show that the pH changes as a function of the mercaptoacetic acid (MA)/Bi molar ratio and EDTA wt.% and for MA and EDTA wt.% and charts the MA and EDTA wt.% with the NaOH/EDTA ratio. The molar ratios are with respect to bismuth ions. The light grey regions of the map showcase stable solvation conditions at room-temperature for the described concentrations and combination. Such maps can be constructed for other conditions and concentrations. For example, a new series of maps can be constructed using the above procedures, if ammonium hydroxide is substituted for sodium hydroxide as base or if sulfuric acid is substituted for mercaptoacetic acid.

[00054] From the maps, it may be determined that the bismuth oxide-EDTA mixes have a high threshold minimum mercaptoacetic acid /Bi molar ratio and concentration for solvation, i.e., a minimum MA/Bi ratio of approximately 2.5 at 9.5 wt% EDTA increasing to a ratio of 5 at 5 wt% EDTA and an almost constant minimum MA concentration of 8 wt.%. The pH windows may be relatively narrow, spanning a range centered on neutral to acidic pH of 3.5<pH<8.5. NaOH amounts and concentrations (FIG. 4B) guide the formation of alkaline conditions. A solution condition represented by a point which lies in the hatched or green regions of all maps at a desired pH is then selected as the bismuth precursor solution.

[00055] Example II: Production of antimony selenide nanocrystals

[00056] Antimony selenide is a binary compound semiconductor with applications in phase memory devices, optical materials and requires the preparation of two precursor solutions one containing antimony, and the other, selenium. Antimony precursor solution may be prepared by adding 0.2 moles of antimony trichloride to 150 ml of 1,5-pentanediol as solvent. 20 ml of polypropyl mercaptan may be added to the solution. The solution may be warmed to 50 °C to form a clear light-yellow solution of dissolved antimony. The polypropyl mercaptan may serves as a functional molecule for surface passivation and oxidation protection. Alternately, a mercaptan acid may be substituted for the polypropyl mercaptan to form an acidic character to the precursor solution. Separately, 0.3 moles of selenic acid may be added to 150 ml of 1,5-pentanediol and heated to 50 °C to form a clear solution of solvated selenic acid. Polyols such as 1,5-pentanediol are both solvents and capable of acting as reducing agents. [00057] Example ΠΙ: Synthesis of copper-copper oxide nanocrystals

[00058] Copper oxide is a compound semiconductor with photovoltaic and optical applications and copper-copper oxide nanocrystals are a composite-alloy nanocrystal. 0.1 moles of copper sulfate may be added to 100 ml of deionized water to form a clear light blue solution of cupric ions. The pH may be maintained in the range of 4-5 by the addition of appropriate mineral acid such as sulfuric acid to form an acidic precursor solution.

Separately, a basic pH 14 aqueous solution may be prepared by adding 0.1 moles of potassium hydroxide to 100 ml of deionized water. 0.05 moles of potassium borohydride may be then added to the precursor solution to form a solvated stable precursor. Potassium borohydride may serve as a reducing agent and water may act as the oxidizer. The two precursors may then pumped and flowed continuously to a multimode microwave reactor cavity. A microwave dose of 10 J/g may be provided to the reactor cavity. The copper-copper oxide nanoparticles may be extracted by batch centrifugation.

[00059] Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as "about," "approximately" and "substantially," are not to be limited to the precise value specified. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Here and throughout the specification and claims, range limitations may be combined and/or interchanged, such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise. "Approximately" as applied to a particular value of a range applies to both values, and unless otherwise dependent on the precision of the instrument measuring the value, may indicate +/- 10% of the stated value(s). [00060] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms "a," "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and/or

"comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. Furthermore, illustrations with respect to one or more

implementations, alterations and/or modifications can be made to the illustrated examples without departing from the spirit and scope of the appended claims. In addition, while a particular feature may have been disclosed with respect to only one of several

implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular function.

[00061] This written description uses examples to disclose the invention, including the best mode, and to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.

Claims

CLAIMS We claim:

1. A method of synthesizing compound semiconductor nanocrystals:

synthesizing at least two microwave precursor solutions using at least one elemental constituent and a microwave-active solvent;

combining the at least two microwave precursor solutions to create a combined solution;

applying a microwave to the combined solution to drive a redox reaction to produce the compound semiconductor nanocrystals; and

extracting the compound semiconductor nanocrystals from the combined solution after applying the microwave.

2. The method of claim 1, wherein the compound semiconductor nanocrystals are one of: binary, ternary, quaternary, or higher.

3. The method of claim 1, wherein the compound semiconductor nanocrystals include at least one of: a group 12, 13, or 14 element.

4. The method of claim 1, wherein the compound semiconductor nanocrystals include at least one of: a group IS or 16 element

5. The method of claim 1, wherein the compound semiconductor nanocrystals include at least one of: iron, cobalt, copper, indium, aluminum, zinc, cadmium, or silver.

6. The method of claim 1, wherein the applying the microwave includes providing the combined solution to a microwave reactor cavity in a continuous manner.

7. The method of claim 1, wherein the applying the microwave includes providing the combined solution to a microwave reactor cavity in a semi-continuous or batch-continuous manner

8. The method of claim 1, wherein the extracting of the compound semiconductor nanocrystals includes using at least one of: filtration or centrifugation.

9. The method of claim 1 , wherein the synthesizing the at least two microwave precursor solutions includes:

providing an elemental constituent to a solvent to create a first solution for each of the at least two microwave precursor solutions;

controlling a pH of the first solution for each of the at least two microwave precursor solutions by adding at least one of: an acid or a base; and

providing a reducing agent to the first solution of only one of the at least two microwave precursor solutions, and providing an oxidizing agent to the first solution of at least one other of the at least two microwave precursor solutions.

10. The method of claim 9, wherein the acid or the base function as at least one of: a chelation agent or a ligand.

11. The method of claim 9, further comprising: providing a functional agent to the first solution of at least one of the at least two microwave precursor solutions, wherein the functional agent includes at least one of: a surfactant, a doping agent, a surface passivation agent, an anti-agglomeration agent, or a flocculating agent

12. The method of claim 9, wherein the solvent includes at least one of: deionized water, ethanol, methanol, propanol, acetone, pentanediol, butanediol, or glycol.

13. The method of claim 9, wherein the elemental constituents include at least one of: an elemental powder or a metalorganic salt.

14. The method of claim 9, wherein the acid includes at least one of: nitric acid, sulfuric acid, hydrochloric acid, mercaptoacetic acid, mercaptopropionic acid, citric acid, tartaric acid, an ethylenediaminetetraacetic acid (EDTA), ascorbic acid, or oleic acid.

15. The method of claim 9, wherein the base includes at least one of: sodium hydroxide, potassium hydroxide, lithium hydroxide, ammonium hydroxide, a borate, or an organic amine.

16. The method of claim 9, wherein the reducing agent includes at least one of: a metal hydride, a sulfate, a hydrazine, an organic acid, or a metal borohydride.

17. The method of claim 9, wherein the oxidizing agent includes at least one of: a metal permanganate, hydrogen peroxide, a halide, or deionized water.

18. The method of claim 1, further comprising:

drying the extracted compound semiconductor nanocrystals.

19. The method of claim 1, wherein the combining the at least two microwave precursor solutions to create a combined solution includes creating a turbulent flow of each of the at least two microwave precursor solutions.

20. A method of synthesizing at least two microwave precursor solutions, the method comprising:

21. The method of claim 20, wherein the acid or the base function as at least one of: a chelation agent or a ligand.

22. The method of claim 20, further comprising:

providing a functional agent to the first solution of at least one of the at least two microwave precursor solutions, wherein the functional agent includes at least one of: a surfactant, a doping agent, a surface passivation agent, an anti-agglomeration agent, or a flocculating agent

23. The method of claim 20, wherein the solvent includes at least one of: deionized water, ethanol, methanol, propanol, acetone, pentanediol, butanediol, or glycol.

24. The method of claim 20, wherein the elemental constituents include at least one of: an elemental powder or a metalorganic salt.

25. The method of claim 20, wherein the acid includes at least one of: nitric acid, sulfuric acid, hydrochloric acid, mercaptoacetic acid, mercaptopropionic acid, citric acid, tartaric acid, an ethylenediaminetetraacetic acid (EDTA), ascorbic acid, or oleic acid.

26. The method of claim 20, wherein the base includes at least one of: sodium hydroxide, potassium hydroxide, lithium, hydroxide, ammonium hydroxide, a borate, or an organic amine.

27. The method of claim 20, wherein the reducing agent includes at least one of: a metal hydride, a sulfate, a hydrazine, an organic acid, or a metal borohydride.

28. The method of claim 20, wherein the oxidizing agent includes at least one of: a metal permanganate, hydrogen peroxide, a halide, or deionized water.

29. The method of claim 20, wherein the elemental constituent for each of the at least two precursor solutions includes at least one of: a group 16 element, a group IS element, a group 14 element, a group 13 element, a group 12 element, iron, cobalt, copper, indium, aluminum, zinc, cadmium, or silver.