WO2020205899A1

WO2020205899A1 - Morphology control in synthesis of metal nanostructures

Info

Publication number: WO2020205899A1
Application number: PCT/US2020/026058
Authority: WO
Inventors: Ian Storms Moody
Original assignee: Cambrios Film Solutions Corporation
Priority date: 2019-04-03
Filing date: 2020-04-01
Publication date: 2020-10-08
Also published as: TW202041454A; TWI833002B

Abstract

A method of controlling morphology in a synthesis of metal nanostructures. The method includes providing a first-stage reaction mixture, wherein the first-stage reaction mixture including a polyol solvent, a capping agent and a halide salt. The method includes heating the first-stage reaction mixture to a reaction temperature. The method includes adding a second- stage reaction mixture to the heated first-stage reaction mixture to provide a combined mixture, wherein the second-stage reaction mixture including a metal salt dissolved in a polyol solvent. The method includes allowing reaction within the combined mixture for a total reaction time. The second- stage reaction mixture is added to the first-stage reaction mixture within a time span that is less than 10% of the total reaction time.

Description

MORPHOLOGY CONTROL IN SYNTHESIS OF METAL

NANOSTRUCTURES

RELATED APPLICATION

[0001] This application claims priority to U.S. Provisional Application Serial Number 62/828,644, titled“MORPHOLOGY CONTROL IN SYNTHESIS OF METAL NANOWIRES” and filed on April 3, 2019, which is incorporated herein by reference.

FIELD

[0002] This disclosure is related to the synthesis of metal nanostructures and transparent conductors made from the metal nanostructures.

BACKGROUND

[0003] Transparent conductors include optically-clear and electrically- conductive films. One of the key applications for silver nanowires (AgNWs) today is in forming transparent conductor (TC) layers in electronic devices, such as touch panels, photovoltaic cells, flat liquid crystal displays (LCD), organic light emitting diodes (OLED), etc. Various technologies have produced transparent conductors based on one or more conductive media such as conductive nanostructures. Generally, the conductive nanostructures form a percolating network with long-range interconnectivity.

[0004] As the number of applications employing transparent conductors continues to grow, improved production methods are required to satisfy the demand for conductive nanostructures. Electrical and optical properties of a TC layer are strongly dependent on the physical dimensions of the conductive nanowires forming the percolating network. Conventional production methods do not offer adequate control of properties such as the length, diameter and aspect ratio of the conductive nanowires.

BRIEF SUMMARY

[0005] In accordance with an aspect, the present disclosure provides a method of controlling morphology in a synthesis of metal nanostructures. The method includes providing a first-stage reaction mixture, wherein the first- stage reaction mixture including a polyol solvent, a capping agent and a halide salt. The method includes heating the first-stage reaction mixture to a reaction temperature. The method includes adding a second-stage reaction mixture to the heated first-stage reaction mixture to provide a combined mixture, wherein the second-stage reaction mixture including a metal salt dissolved in a polyol solvent. The method includes allowing reaction within the combined mixture for a total reaction time. The second-stage reaction mixture is added to the first-stage reaction mixture within a time span that is less than 10% of the total reaction time.

[0006] The above summary presents a simplified summary in order to provide a basic understanding of some aspects of the systems and/or methods discussed herein. This summary is not an extensive overview of the systems and/or methods discussed herein. It is not intended to identify key/critical elements or to delineate the scope of such systems and/or methods. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is presented later.

DESCRIPTION OF THE DRAWINGS

[0007] While the techniques presented herein may be embodied in alternative forms, the particular embodiments illustrated in the drawings are only a few examples that are supplemental of the description provided herein. These embodiments are not to be interpreted in a limiting manner, such as limiting the claims appended hereto.

[0008] The disclosed subject matter may take physical form in certain parts and arrangement of parts, embodiments of which will be described in detail in this specification and illustrated in the accompanying drawings which form a part hereof and wherein:

[0009] FIG. 1 presents an example of a method that is accordance with the present disclosure.

[0010] FIG. 2A is an SEM image at 60,000 magnification of a prior art silver nanowire network prepared according to the details outlined in Example 1 .

[0011] FIG. 2B is an SEM image at 60,000 magnification of a prior art silver nanowire network prepared according to the details outlined in Example 2. [0012] FIG. 2C is an SEM image at 60,000 magnification of a silver nanowire network prepared according to the present disclosure and outlined in Example 3.

[0013] FIG. 2D is an SEM image at 60,000 magnification of a silver nanowire network prepared according to the present disclosure and outlined in Example 4.

[0014] FIG. 2E is an SEM image at 60,000 magnification of a silver nanowire network prepared according to the present disclosure and outlined in Example 5.

[0015] FIG. 3 is a graphical representation of the frequency at which nanowire widths/diameters appear in respective percolating nanowire networks formed according to the prior art of Examples 1 and 2 and the present method of Examples 3-5; and

[0016] FIG. 4 is a graphical representation of the length distribution of nanowires of a percolating nanowire network formed according to the prior art of Examples 1 and 2 and the present method of Examples 3-5.

DETAILED DESCRIPTION

[0017] Subject matter will now be described more fully hereinafter with reference to the accompanying drawings, which form a part hereof, and which show, by way of illustration, specific example embodiments. This description is not intended as an extensive or detailed discussion of known concepts. Details that are known generally to those of ordinary skill in the relevant art may have been omitted, or may be handled in summary fashion.

[0018] Certain terminology is used herein for convenience only and is not to be taken as a limitation on the disclosed subject matter. Relative language used herein is best understood with reference to the drawings, in which like numerals are used to identify like or similar items. Further, in the drawings, certain features may be shown in somewhat schematic form.

[0019] The following subject matter may be embodied in a variety of different forms, such as methods, devices, components, and/or systems. Accordingly, this subject matter is not intended to be construed as limited to any illustrative embodiments set forth herein as examples. Rather, the embodiments are provided herein merely to be illustrative. Such embodiments may, for example, take the form of hardware, software, firmware or any combination thereof.

[0020] Provided herein is a method of controlling morphology in the synthesis of metal nanostructures. The method includes providing a first- stage reaction mixture, wherein the first-stage reaction mixture including a polyol solvent, a capping agent and a halide salt. The method includes heating the first-stage reaction mixture to a reaction temperature. The method includes adding a second-stage reaction mixture to the heated first- stage reaction mixture to provide a combined mixture, wherein the second- stage reaction mixture including a metal salt dissolved in a polyol solvent.

The method includes allowing reaction within the combined mixture for a total reaction time. The second-stage reaction mixture is added to the first-stage reaction mixture within a time span that is less than 10% of the total reaction time.

[0021] Such nanostructures can be conductive and used to form a percolating conductive network of nanostructures within a device. As used herein,“conductive nanostructures” or“nanostructures” generally refer to electrically conductive nano-sized structures, at least one dimension of which is less than 500 nm, or less than 250 nm, 100 nm, 50 nm, or 25 nm, for example. Typically, the nanostructures are made of a metallic material, such as an elemental metal (e.g., transition metals) or a metal compound (e.g., metal oxide). The metallic material can also be a bimetallic material or a metal alloy, which comprises two or more types of metal. Suitable metals include, but are not limited to, silver, gold, copper, nickel, gold-plated silver, platinum and palladium.

[0022] The nanostructures can be of any shape or geometry. The morphology of a given nanostructure can be defined in a simplified fashion by its aspect ratio, which is the ratio of the length over the diameter of the nanostructure. For instance, certain nanostructures are isotropically shaped (i.e. , aspect ratio=1 ). Typical isotropic nanostructures include nanoparticles.

In preferred embodiments, the nanostructures are anisotropically shaped (i.e., aspect ratio¹1 ). The anisotropic nanostructure typically has a longitudinal axis along its length. Exemplary anisotropic nanostructures include nanowires, nanorods, and nanotubes, as defined herein. [0023] The nanostructures can be solid or hollow. Solid nanostructures include, for example, nanoparticles, nanorods and nanowires (“NWs”). NWs typically refers to long, thin nanostructures having aspect ratios of greater than 10, preferably greater than 50, more preferably greater than 100, and most preferably greater than 200. Typically, the nanowires are more than 500 nm, more than 1 pm, more than 5 pm, or more than 10 pm long. In one example, NWs are in the 5 pm to 10 pm range. “Nanorods” are typically short and wide anisotropic nanostructures that have aspect ratios of no more than 10, no more than 50, no more than 100, or no more than 200. Although the present disclosure is applicable to preparing any type of nanostructure, for the sake of brevity the synthesis of silver nanowires (“AgNWs” or abbreviated simply as“NWs”) will be described as an example. However, it is to be appreciated that the present disclosure is not limited to such, and the present disclosure covers various nanostructures, metals, etc.

[0024] Electrical and optical properties of a TC layer are strongly dependent on the physical dimensions of NWs - i.e. their length and diameter, and more generally, their aspect ratio. NWs with larger aspect ratios form a more efficient conductive network by allowing a lower density of wires to achieve higher transparency for a given film resistivity. Because each NW can be considered a conductor, individual NW length and diameter will affect the overall NW network conductivity and, therefore, the final film conductivity. For example, as nanowires get longer, fewer are needed to make a conductive network; and as NWs get thinner, NW resistivity increases - making the resulting film less conductive for a given number of NWs.

[0025] Similarly, NW length and diameter will affect the optical

transparency and light diffusion (haze) of the TC layers. NW networks are optically transparent because nanowires comprise a very small fraction of the film. However, the nanowires absorb and scatter light, so NW length and diameter will, in large part, determine optical transparency and haze for a conductive NW network. Generally, thinner NWs enable increased

transmission and reduced haze in TC layers - desired properties for electronic applications.

[0026] Furthermore, low-aspect-ratio nanostructures (a byproduct of the synthesis process) in the TC layer result in added haze as these structures scatter light without contributing to the conductivity of the network. Because synthetic methods for preparing metal nanostructures typically produce a composition that includes a range of nanostructure morphologies, both desirable and undesirable, there is a need to purify such a composition to promote retention of high aspect ratio nanostructures.

[0027] However, NWs can be synthesized with ever-smaller diameters (e.g. in the range of tens of nanometers), and these smaller diameters closely match the dimensions of the undesired byproducts such as low-aspect-ratio nanostructures. Due to the similarity in size, composition, and structure between the NWs and the byproducts, purifying high-aspect-ratio NWs for high-quality TC films is difficult. Synthesizing NWs with uniform distributions of length and width aid in the ability to purify such high-aspect-ratio

nanostructures, because such uniform populations behave most similarly, and allow for tailored purification conditions that are most selective for high- aspect-ratio NWs over low-aspect-ratio byproducts. It is, therefore, desirable to develop methods of preparation of high-aspect-ratio NWs that have uniform distributions of length and width, and/or favor the formation of NWs over other low-aspect-ratio nanostructures.

[0028] Once purified, the retained high-aspect-ratio NWs can be used to form TCs having desired electrical and optical properties.

[0029] It is, therefore, advantageous to develop controlled synthetic methods that produce NWs which are preferably thinner or longer than the current state of the art, or more preferably, both thinner and longer. It is also preferable that these synthetic methods produce NWs with a uniform distribution of length and width. Finally, it is preferable to develop synthetic methods that more favorably produce NWs over other low-aspect ratio nanostructures.

[0030] Described herein, are methods to control the preparation of NWs to meet these stated criteria.

[0031] According to some embodiments, the conductive NWs can be produced by a synthesis method that improves the optical properties of TC films. The present synthesis methods control the length and width

distributions of NW while obtaining a high yield of nanowires by:

[0032] utilizing polyol wire synthesis reactant compositions, [0033] introducing different additives, and

[0034] utilizing polyol wire synthesis process steps and conditions.

[0035] The result is a unique distribution of NW length and width in the reaction mix with a high ratio of NWs to low aspect ratio nanostructures.

Subsequent purification, such as via a sedimentation process method, to further remove low-aspect-ratio nanostructures, can maintain this unique distribution of NW length and diameter for incorporation into the final TC material.

[0036] Within some test samples, ethylene glycol (EG), polyvinylpyrrolidone (PVP), sodium chloride (NaCI), sodium bromide (NaBr), and silver nitrate (AgNCh) were combined in the mixture, and heated under a nitrogen blanket to a reaction temperature where a mixture of silver nanostructures were produced. Reactions were removed from heat once complete, defined as when the majority (>90%) of silver ions (Ag⁺) had been reduced to silver metal (Ag°). The silver ion concentration was monitored via potentiometric titration measurements on aliquots of the reaction mixture taken during the course of the reaction. The crude mixtures were characterized by scanning electron microscopy (SEM) to measure NW width and dark-field (DF) microscopy with custom image analysis software to measure NW length and ratios of low- aspect-ratio byproducts to high-aspect-ratio NWs. The test samples represent different strategies for controlling NW width and length distributions, and/or the ratio of NWs to byproducts. The average NW width, length, and ratio of low-aspect-ratio byproducts (i.e., Objects”) to NWs for each test sample is summarized in Table 1 .

[0037] In the context of a reaction, timing can be defined in several ways.

In one embodiment, timing can refer to the total time of a reaction, henceforth referred to as the“total reaction time.” Alternatively, timing can refer to a particular time-point occurring within the period of time bracketed by the start and end of the reaction. Finally, timing can refer to an“addition time” of one or more of the reagents into the reactor and/or reaction mixture.

[0038] In many prior art examples of polyol reactions, silver salt is added at the start of a reaction, along with the other reagents, before heating the mixture. Typically, in these examples, the addition time of silver salt into the reaction mixture is undefined. [0039] Alternatively, in other prior art examples, the silver salt is only added once the reaction mixture has been heated to the reaction temperature. Typically, in these examples, the addition time of silver salt into the reaction mixture is relatively long - greater than 10% of the total reaction time.

[0040] The present disclosure provides that precise control over the distributions of NW width and length can be achieved through careful control of both the time-point and the addition time of silver salt. In an example, the present disclosure presents that limiting the addition time of silver salt to less than 10% of the total reaction time can lead to both an increase in the number of NWs formed in the reaction (relative to low-aspect ratio byproducts), and a reduction in the average diameter of the NWs formed.

[0041] Limiting the addition time to be less than 10% of the total reaction time can be termed to be“short,”“fast” or any other similar descriptor. In distinction, an addition time that is greater than 10% of the total reaction time can be termed to be“slow,”“long” or any other similar descriptor. It is to be appreciated that the addition time to be less than 10% is one aspect of the present disclosure. Other aspects of the present disclosure are: the addition time to be less than 5% of the total reaction time, the addition time to be less than 2.5% of the total reaction time, the addition time to be less than 1 % of the total reaction time, and the addition time to be less than 0.5% of the total reaction time.

[0042] FIG. 1 presents an example of a method that is accordance with the present disclosure. An example method 100 of FIG. 1 begins at step 102, at which a first-stage reaction mixture is provided. The first-stage reaction mixture can include a polyol solvent, a capping agent and a halide salt. At step 104, the first-stage reaction mixture is heated to a reaction temperature. At step 106 a second-stage reaction mixture is added to the heated first-stage reaction mixture to provide a combined mixture. The second-stage reaction mixture can include a metal salt dissolved in a polyol solvent. At step 108, reaction within the combined mixture can be allowed for a total reaction time.

It is to be noted that the addition of the second-stage reaction mixture to the first-stage reaction mixture at step 106 occurs within a time span that is less than 10% of the total reaction time. [0043] It would be useful to provide some comparisons between prior art methods and methods that are in accordance with the present disclosure. As such, five Examples are briefly identified following.

[0044] Example 1 is a prior art method of introducing the silver salt at the beginning of the reaction, before heating the reaction mixture. The product of the Example 1 method is shown in FIG. 2A, which is a SEM image at 60,000 magnification of a prior art network.

[0045] Example 2 is a prior art method of introducing the silver salt once the reaction mixture has been heated to the reaction temperature, with an addition time greater than 10% of the total reaction time. The product of the Example 2 method is shown in FIG. 2B, which is a SEM image at 60,000 magnification of a prior art network.

[0046] Example 3 is a method in accordance with an aspect of the present disclosure. The method of Example 3 includes introducing the silver salt once the reaction mixture has been heated to the reaction temperature, with an addition time less than 10% of the total reaction time. The product of the Example 3 method is shown in FIG. 2C, which is a SEM image at 60,000 magnification of a network in accordance with an aspect of the present disclosure.

[0047] Further, as an optional variation to Example 3, control over NW distributions can be achieved thru tuning of the relative loadings of chloride (Cl·) and bromide (Br) ions in the reaction mixture. Halide ions (e.g. Ch and Br) aid in NW seeding in the early stages of the reaction, and controlling either the total halide loading (i.e. the sum of Ch and Br ions), or the ratio of Br to Ch ions, or both, can affect both the distributions of NW length and width as well as the relative numbers of low-aspect-ratio byproducts and high- aspect-ratio NWs formed. Such can be referred to as halide tuning to further control NW morphology (and reducing the formation of low-aspect ratio side products).

[0048] Example 4 is a method in accordance with an aspect of the present disclosure. Specifically, the Example method still uses an addition time less than 10% of the total reaction time and also includes replacement of all of the Ch salts with Br salts, but otherwise keeping the total halide concentration the same. The replacement can result in a reduction of the average NW diameter. The product of the Example 4 method is shown in FIG. 2D, which is a SEM image at 60,000 magnification of a network in accordance with an aspect of the present disclosure.

[0049] In Example 5, we show that careful tuning of the relative loadings of Br- and Cl- salts can maintain the reduced NW diameters of the Br-only example, while increasing the relative number of high-aspect-ratio NWs to low-aspect-ratio byproducts. The product of the Example 5 method is shown in FIG. 2E, which is a SEM image at 60,000 magnification of a network in accordance with an aspect of the present disclosure.

[0050] A more detailed discussion is provided following regarding the five Examples.

[0051] Within Example 1 , a silver salt is added at beginning of reaction.

The Example 1 method includes the following: In a 1 L reaction vessel, 420 g of ethylene glycol (EG), 56.2 g of polyvinylpyrrolidone (PVP) (MW: 1 ,300,000) solution (5% (w/w)) in EG), 2.45 g of NaCI solution (5% (w/w) in EG), 2.26 g of NaBr solution (5% (w/w) in EG), and 16.21 g of silver nitrate (AgNOs) solution (14% (w/w) in EG) were heated to 170°C and stirred under nitrogen (N2) blanket for 60 min. The reaction mixture was removed from heat and then allowed to cool down to room temperature under N2 blanket while stirring.

The average width and length of the nanowires were 21 .6 nm and 15.9 pm, respectively. The ratio of low-aspect-ratio Objects to NWs was 7.03.

[0052] Within Example 2, a silver salt is added at the reaction temperature with an addition time greater than 10% of the total reaction time. The

Example 2 method includes the following: In a 1 L reaction vessel, 476 g of ethylene glycol (EG), 2.81 g of polyvinylpyrrolidone (PVP) (MW: 1 ,300,000), 2.45 g of NaCI solution (5% (w/w) in EG), and 2.26 g of NaBr solution (5% (w/w) in EG) were heated to 170°C and stirred under nitrogen (N2) blanket for 50 min. To the stirring mixture, 13.50 ml_ of silver nitrate (AgNOs) solution (14% (w/w) in EG) was added over the course of 10 mins via syringe pump at an addition rate of 1 .35 mL/min. The above reaction mixture was then stirred at 170^QC under N2 blanket for 1 hr. The reaction mixture was removed from heat and then allowed to cool down to room temperature under N2 blanket while stirring. Within one example, such addition time (i.e., 10 minutes) is relatively slow or long in comparison to 1 hour. The average width and length of the nanowires were 21 .3 nm and 15.2 mih, respectively. The ratio of low- aspect-ratio Objects to NWs was 7.03.

[0053] Within Example 3, a silver salt is added at the reaction temperature with an addition time less than 10% of the total reaction time. The Example 3 method includes the following: In a 1 L reaction vessel, 476 g of ethylene glycol (EG), 2.81 g of polyvinylpyrrolidone (PVP) (MW: 1 ,300,000), 2.45 g of NaCI solution (5% (w/w) in EG), and 2.26 g of NaBr solution (5% (w/w) in EG) were heated to 170°C and stirred under nitrogen (N2) blanket for 50 min. To the stirring mixture, 16.21 g of silver nitrate (AgNOs) solution (14% (w/w) in EG) was added over the course of 3 seconds. The above reaction mixture was then stirred at 170^QC under N2 blanket for 1 hr. The reaction mixture was removed from heat and then allowed to cool down to room temperature under N2 blanket while stirring. Within one example, such addition time (i.e., 3 seconds) is relatively short or quick in comparison to 1 hour. The average width and length of the nanowires were 16.2 nm and 6.2 pm, respectively.

The ratio of low-aspect-ratio Objects to NWs was 2.55.

[0054] Within Example 4, a Br-only variant is utilized at the reaction temperature with an addition time less than 10% of the total reaction time.

The Example 4 method includes the following: In a 1 L reaction vessel, 477 g of ethylene glycol (EG), 2.81 g of polyvinylpyrrolidone (PVP) (MW: 1 ,300,000), and 6.78 g of NaBr solution (5% (w/w) in EG) were heated to 170°C and stirred under nitrogen (N2) blanket for 50 min. To the stirring mixture, 16.21 g of silver nitrate (AgNOs) solution (14% (w/w) in EG) was added over the course of 3 seconds. The above reaction mixture was then stirred at 170^QC under N2 blanket for 1 hr. The reaction mixture was removed from heat and then allowed to cool down to room temperature under N2 blanket while stirring. The average width and length of the nanowires were 13.3 nm and 5.4 pm, respectively. The ratio of low-aspect-ratio Objects to NWs was 9.75.

[0055] Within Example 5, fine tuning the halide concentrations is utilized at the reaction temperature with an addition time less than 10% of the total reaction time. The Example 5 method includes the following: In a 1 L reaction vessel, 477 g of ethylene glycol (EG), 2.81 g of polyvinylpyrrolidone (PVP) (MW: 1 ,300,000), 2.10 g of NaCI solution (5% (w/w) in EG), and 3.70 g of NaBr solution (5% (w/w) in EG) were heated to 170°C and stirred under nitrogen (N2) blanket for 60 min. To the stirring mixture, 16.8 g of silver nitrate (AgNCh) solution (14% (w/w) in EG) was added over the course of 3 seconds. The above reaction mixture was then stirred at 170^QC under N2 blanket for 1 hr. The reaction mixture was removed from heat and then allowed to cool down to room temperature under N2 blanket while stirring. The average width and length of the nanowires were 12.8 nm and 5.7 pm, respectively. The ratio of low-aspect-ratio Objects to NWs was 3.52.

[0056] Examples of the physical dimensions of silver nanowires formed according to the present disclosure is tabulated below in Table 1 .

TABLE 1

Average NW Widths, Lengths, and Object / NW Ratios

[0057] The NW Width and Length Distributions for Examples 1 through 5 are shown in FIG. 3 and 4, respectively. Note that within FIG. 4 the peaks of the length distribution curves of Examples 1 and 2 (i.e., the prior art) are significantly lower than the peaks of the length distribution curves of Examples 3-5 (i.e., in accordance with the present disclosure). Also note that within FIG. 3 the peaks of the width (diameter) distribution curves of Examples 1 and 2 (i.e., the prior art) are significantly further to the right (i.e., greater width value) than the peaks of the width distribution curves of Examples 3-5 (i.e., in accordance with the present disclosure).

[0058] As a recap, the present disclosure provides an example method of controlling morphology in a synthesis of metal nanostructures. The method includes providing a first-stage reaction mixture, wherein the first-stage reaction mixture including a polyol solvent, a capping agent and a halide salt. The method includes heating the first-stage reaction mixture to a reaction temperature. The method includes adding a second-stage reaction mixture to the heated first-stage reaction mixture to provide a combined mixture, wherein the second-stage reaction mixture including a metal salt dissolved in a polyol solvent. The method includes allowing reaction within the combined mixture for a total reaction time. The second-stage reaction mixture is added to the first-stage reaction mixture within a time span that is less than 10% of the total reaction time.

[0059] The present disclosure provides variations to the addition time. As some examples, the addition time span is less than 5% of the total reaction time, the addition time span is less than 2.5% of the total reaction time, the addition time span is less than 1 % of the total reaction time, and the addition time span is less than 0.5% of the total reaction time.

[0060] The present disclosure provides that the total reaction time can be defined as a time duration between a start of addition of the second-stage reaction mixture and cessation of heating of the combined mixture. Within some examples, the total reaction time is less than 360 minutes, the total reaction time is less than 240 minutes, the total reaction time is less than 120 minutes, and the total reaction time is less than 60 minutes.

[0061] The present disclosure provides that when the total reaction time is presented as a specified duration, the addition time can also be specified as specified duration. As some examples, the addition time span can be less than 10 minutes, the addition time span can be less than 5 minutes, the addition time span can be less than 1 minute, and the addition time span can be less than 30 seconds.

[0062] The present disclosure provides for use of various materials. As examples, the polyol solvent includes at least one of ethylene glycol, 1 ,2- propylene glycol, 1 ,3-propylene glycol, glycerol, and 1 ,2-butanediol. As an example, the capping agent includes polyvinylpyrrolidone. As examples, the halide salt includes at least one of a salt of bromide, a salt of chloride, or both a salt of bromide and a salt of chloride. As examples, the metal salt includes at least one of a salt of silver, a salt of copper, and a salt of gold. As an example, the metal salt includes silver nitrate. [0063] The present disclosure provides for various reagent concentrations. As examples, the concentration of silver salt can be at least 10 mM, but less than 120 mM. As examples, the total concentration of halide salts can be at least 2 mM, but less than 30 mM. As examples, the molar ratio of bromide to chloride salts can be at least 0.2, but less than 5.

[0064] The present disclosure provides for various results. As examples, the resulting metal nanostructures have an average width of less than or equal to 20 nm and an average length of at least 5 pm, or an average width of less than or equal to 15 nm and an average length of at least 5 pm. As an example, the resulting metal nanostructures have a coefficient of variation of width of less than 30%.

[0065] Unless specified otherwise,“first,”“second,” and/or the like are not intended to imply a temporal aspect, a spatial aspect, an ordering, etc.

Rather, such terms are merely used as identifiers, names, etc. for features, elements, items, etc. For example, a first object and a second object generally correspond to object A and object B or two different or two identical objects or the same object.

[0066] Moreover,“example” is used herein to mean serving as an instance, illustration, etc., and not necessarily as advantageous. As used herein,“or” is intended to mean an inclusive“or” rather than an exclusive“or.” In addition, “a” and“an” as used in this application are generally be construed to mean “one or more” unless specified otherwise or clear from context to be directed to a singular form. Also, at least one of A and B and/or the like generally means A or B or both A and B. Furthermore, to the extent that“includes,” “having,”“has,”“with,” and/or variants thereof are used in either the detailed description or the claims, such terms are intended to be inclusive in a manner similar to the term“comprising.”

[0067] Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing at least some of the claims. [0068] Various operations of embodiments are provided herein. The order in which some or all of the operations are described herein should not be construed as to imply that these operations are necessarily order dependent. Alternative ordering will be appreciated by one skilled in the art having the benefit of this description. Further, it will be understood that not all operations are necessarily present in each embodiment provided herein. Also, it will be understood that not all operations are necessary in some embodiments.

[0069] Also, although the disclosure has been shown and described with respect to one or more implementations, equivalent alterations and modifications will occur to others skilled in the art based upon a reading and understanding of this specification and the annexed drawings. The disclosure includes all such modifications and alterations and is limited only by the scope of the following claims. In particular regard to the various functions performed by the above described components (e.g., elements, resources, etc.), the terms used to describe such components are intended to correspond, unless otherwise indicated, to any component which performs the specified function of the described component (e.g., that is functionally equivalent), even though not structurally equivalent to the disclosed structure. In addition, while a particular feature of the disclosure 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 application.

Claims

CLAIMS What is claimed is:

1. A method of controlling morphology in a synthesis of metal

nanostructures, the method comprising:

providing a first-stage reaction mixture, wherein the first-stage reaction mixture including a polyol solvent, a capping agent and a halide salt;

heating the first-stage reaction mixture to a reaction temperature; adding a second-stage reaction mixture to the heated first-stage reaction mixture to provide a combined mixture, wherein the second-stage reaction mixture including a metal salt dissolved in a polyol solvent; and

allowing reaction within the combined mixture for a total reaction time; wherein the second-stage reaction mixture is added to the first-stage reaction mixture within a time span that is less than 10% of the total reaction time.

2. The method of claim 1 , wherein the time span that is less than 5% of the total reaction time.

3. The method of claim 2, wherein the time span that is less than 2.5% of the total reaction time.

4. The method of claim 3, wherein the time span that is less than 1 % of the total reaction time.

5. The method of claim 4, wherein the time span that is less than 0.5% of the total reaction time.

6. The method of claim 1 , wherein the total reaction time is defined as a time duration between a start of addition of the second-stage reaction mixture and cessation of heating of the combined mixture.

7. The method of claim 6, wherein the total reaction time is less than 360 minutes.

8. The method of claim 7, wherein the total reaction time is less than 240 minutes.

9. The method of claim 8, wherein the total reaction time is less than 120 minutes.

10. The method of claim 1 , wherein the polyol solvent includes at least one of ethylene glycol, 1 ,2-propylene glycol, 1 ,3-propylene glycol, glycerol, and 1 ,2-butanediol.

10. The method of claim 1 , wherein the capping agent includes

polyvinylpyrrolidone.

12. The method of claim 1 , wherein the halide salt includes at least one of a salt of bromide, a salt of chloride, or both a salt of bromide and a salt of chloride.

13. The method of claim 1 , wherein the metal salt includes at least one of a salt of silver, a salt of copper, and a salt of gold.

14. The method of claim 1 , wherein the metal salt includes silver nitrate.

15. The method of claim 1 , wherein the resulting metal nanostructures have an average width of less than or equal to 20 nm and an average length of at least 5 pm.

16. The method of claim 15, wherein the resulting metal nanostructures have an average width of less than or equal to 15 nm and an average length of at least 5 pm.

17. The method of claim 1 , wherein the resulting metal nanostructures have a coefficient of variation of width of less than or equal to 30%.

18. The method of claim 1 , wherein the concentration of silver salt is at least 10 mM, but less than 120 mM.

19. The method of claim 1 , wherein the total concentration of halide salts is at least 2 mM, but less than 30 mM.

20. The method of claim 1 , wherein the molar ratio of bromide to chloride salts is at least 0.2, but less than 5.