WO2016085410A1

WO2016085410A1 - Method for interconnecting nanostructures

Info

Publication number: WO2016085410A1
Application number: PCT/SG2015/050470
Authority: WO
Inventors: Jun Wei; Lee Siew Rachel TAN; Hongyu Chen
Original assignee: Agency For Science, Technology And Research; Nanyang Technological University
Priority date: 2014-11-26
Filing date: 2015-11-25
Publication date: 2016-06-02

Abstract

The invention provides a method of interconnecting metallic nanostructures comprising dispersing the metallic nanostructures in a solution comprising an etching agent of acid, alkali or salt and reducing the metal ions in said solution to form fused nanostructure network, wherein said fused nanostructures may be coated onto a substrate.

Description

METHOD FOR INTERCONNECTS NANOSTRUCTURES

Cross-Reference To Related Application

[0001] This application claims the benefit of priority of Singapore patent application No. 10201407863W, filed 26 November 2014, the content of it being hereby incorporated by reference in its entirety for all purposes.

Technical Field

[0002] Various embodiments relate to a method for interconnecting nanostructures.

Background

[0003] The demand for transparent conductors is expected to grow rapidly as electronic devices such as touch screens, displays, solid state lighting and photovoltaic devices become ubiquitous in our lives. Doped metal oxides, especially indium tin oxide (ITO), are often used as transparent conductors. They exhibit low electrical sheet resistance (< 100 Ω/sq) and high optical transparency (> 80%). The production of ITO films can also be easily scaled up. However, the material suffers several drawbacks. As ITO is very brittle and can fracture at low strains, it cannot be integrated into flexible devices, which is an area many semiconductor giants are looking to develop. Furthermore, due to the increasing scarcity of indium, a component material of ITO, the production of ITO may become very expensive. Hence, there is an interest in the exploration of alternative materials. These materials include conducting polymers, metal nanowires, thin metal films and carbon nanomaterials.

[0004] Conducting polymers exhibit good electrical, optical and mechanical properties. However, they suffer from electrical instability. Exposure to environmental elements like humidity, high temperature or ultraviolet (UV) light deteriorates the electrical conductivity. [0005] Films based on carbon nanomaterials such as carbon nanotube (CNT) and graphene have been of particular interest due to their good electrical, optical and mechanical properties, as well as good chemical stability. They have been found to exhibit low sheet resistance, high optical transparency, good flexibility and stability over time. However, their electrical conductivity and transparency are less than ideal as the electrical conductivity is limited by the tube-tube junction resistance of the CNTs.

[0006] Thin metal films and patterned metal grids have been considered. A metal grid can be very conductive electrically as it does not suffer from the junction resistance (like in a metal nanowire mesh). However, with the patterning process involved, this approach suffers from materials wastage, scaling-up to large area, and manufacturing cost.

[0007] Metal nanowire mesh, another alternative material considered, is limited by the wire-wire junction resistance which increases the electrical resistivity. When the density of the nanowires in the nanowire mesh is increased to compensate for the junction resistance, the transmissivity of the nanowire mesh decreases.

[0008] In summary, some of the problems related to the prior art include: high cost of indium and vacuum deposition process of ITO, brittle (cracks and fractures can form easily) ITO, high index of refraction of ITO can result in unwanted reflection with low index of refraction substrate materials such as glass, instability and low durability of conductive polymers, low conductivity of carbon nanomaterial films, such as CNT, graphene, due to high junction resistance, and low conductivity of metal nanowire mesh due to high junction resistance.

Summary [0009] According to an embodiment, a method for interconnecting nanostructures is provided. The method may include providing a plurality of nanostructures comprising a metal, supplying metal ions, and contacting the metal ions with a reducing agent to reduce the metal ions into metal atoms to fuse the plurality of nanostructures to each other via the metal atoms. Brief Description of the Drawings

[0010] In the drawings, like reference characters generally refer to like parts throughout the different views. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the invention are described with reference to the following drawings, in which:

[0011] FIG. 1 shows a flow chart illustrating a method for interconnecting nanostructures, according to various embodiments.

[0012] FIG. 2A shows a flow chart illustrating a method of forming a two- dimensional (2D) or three-dimensional (3D) fused interconnected network, according to various embodiments.

[0013] FIG. 2B shows a flow chart illustrating a method of forming a two- dimensional (2D) fused interconnected film, according to various embodiments.

[0014] FIG. 2C shows examples of set-ups for performing the method of various embodiments.

[0015] FIG. 3 A shows an electron microscopy image of nanowires before fusion while FIG. 3B shows an electron microscopy image after fusion of the nanowires of FIG. 3A into a dense structure at low temperature, according to various embodiments. The scale bars in FIGS. 3A and 3B represent 1 μπι.

[0016] FIGS. 3C and 3D show electron microscopy images of nanowires fused into a network structure at low temperature, according to various embodiments. The scale bars in FIGS. 3C and 3D represent 1 μηι and 100 nm respectively.

[0017] FIGS. 3E and 3F show electron microscopy images of nanowires fused into a three-dimensional (3D) network structure at low temperature, according to various embodiments. The scale bars in FIGS. 3E and 3F represent 100 nm.

Detailed Description [0018] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.

[0019] Embodiments described in the context of one of the methods or devices are analogously valid for the other methods or devices. Similarly, embodiments described in the context of a method are analogously valid for a device, and vice versa.

[0020] Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments. Features that are described in the context of an embodiment may correspondingly be applicable to the other embodiments, even if not explicitly described in these other embodiments. Furthermore, additions and/or combinations and/or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0021] In the context of various embodiments, the articles "a", "an" and "the" as used with regard to a feature or element include a reference to one or more of the features or elements.

[0022] In the context of various embodiments, the term "about" as applied to a numeric value encompasses the exact value and a reasonable variance.

[0023] As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

[0024] As used herein, the phrase of the form of "at least one of A or B" may include A or B or both A and B. Correspondingly, the phrase of the form of "at least one of A or B or C", or including further listed items, may include any and all combinations of one or more of the associated listed items.

[0025] Various embodiments may provide a joining method for low dimensional metallic materials (or structures). For example, various embodiments may relate to a low temperature joining method of low dimensional metallic nanomaterials (or metallic nanostructures). In the context of various embodiments, the phrase "low dimensional metallic nanomaterials" may include, for example, zero-dimensional (0D) nanostructures such as nanoparticles, one-dimensional (ID) nanostructures such as nano wires, nanotubes, nanorods, or nanofibers, and two-dimensional (2D) nanostructures such as nanoflakes and nanosheets.

[0026] Various embodiments may provide a low temperature joining method of low dimensional metallic nanostructures (or nanomaterials) to form electrodes, circuits, interconnections, films, and devices. The joining process may be as follows:

[0027] (a) Low dimensional metallic nanostructures (or nanomaterials) are dispersed in a solution including or consisting of a reducing agent, and an acid or an alkali (alkaline). The solution is heated up to temperatures ranging from room temperature to 200°C with thermal energy provided by various heating sources such as oven, furnace, hot bath, microwave, energy beam, etc. The junctions between the nanostructures (or nanomaterials) are fused together at low temperatures, where the junction may involve crossing, adjacent, touching, or nearly-touching nanostructures (or nanomaterials). The fused nanostructures (or nanomaterials) have higher electrical conductivity than loose connected nanostructures (or nanomaterials). A two-dimensional (2D) or three- dimensional (3D) fused interconnected network is formed.

[0028] (b) Low dimensional metallic nanostructures (or nanomaterials) are dispersed in a solution including or consisting of a reducing agent and a salt. The solution is heated up to temperatures ranging from room temperature to 200°C with thermal energy provided by various heating sources such as oven, furnace, hot bath, microwave, energy beam, etc. The junctions between nanostructures (or nanomaterials) are fused together at low temperatures, where the junction may involve crossing, adjacent, touching, or nearly- touching nanostructures (or nanomaterials). The fused nanostructures (or nanomaterials) have higher electrical conductivity than loose connected nanostructures (or nanomaterials). A two-dimensional (2D) or three-dimensional (3D) fused interconnected network is formed.

[0029] (c) Printing, coating or dispensing low dimensional metallic nanostructures (or nanomaterials) on rigid or flexible substrates, followed by immersing the substrates with nanostructures (or nanomaterials) in the solution (as described above), which is then subjected to heat and low temperature, where the junctions between the low dimensional metallic nanostructures (or nanomaterials) are fused together. A two-dimensional (2D) fused interconnected film is formed.

[0030] Various embodiments may provide a method for joining low dimensional metallic nanostructures (or nanomaterials), such as nanowires, nanotubes, nanorods, nanofibers, nanoparticles, etc., for various applications.

[0031] Various embodiments may provide electrodes, circuits, interconnections, films, and devices using the fused or joined nanostructures (or nanomaterials) formed using the method of various embodiments. One non-limiting example is a highly transparent conductor having the fused nanostructures (or nanomaterials) of various embodiments.

[0032] FIG. 1 shows a flow chart 100 illustrating a method for interconnecting nanostructures, according to various embodiments.

[0033] At 102, a plurality of nanostructures are provided. The plurality of nanostructures may be conductive. The plurality of nanostructures may include or consist of a metal (or at least one metal). This may mean that the plurality of nanostructures may be metallic nanostructures, e.g., low dimensional metallic nanostructures. The plurality of nanostructures may include pure metal or purely metal. The plurality of nanostructures may also include metallic nanostructures (e.g., silver, copper) coated with its native oxide layer or with other materials, for example, on the nanostructure surface.

[0034] At 104, metal ions may be supplied or provided. The metal ions may be or may include metal ions of the metal of the plurality of nanostructures.

[0035] At 106, the metal ions may be contacted with a reducing agent (or reductant) to reduce the metal ions into metal atoms to fuse the plurality of nanostructures to each other via the metal atoms. As a result, the plurality of nanostructures may be interconnected to each other. For example, a two-dimensional (2D) or three- dimensional (3D) fused interconnected network of the plurality of nanostructures may be formed, or a two-dimensional (2D) fused interconnected film may be formed. The plurality of nanostructures may be electrically coupled to each other.

[0036] In various embodiments, at 106, the metal ions may undergo a reduction process to be converted into metal atoms. The plurality of nanostructures may be fused to each other via the metal atoms that are formed as a result of the reducing action of the reducing action on the metal ions. [0037] In various embodiments, at 106, two or more nanostructures of the plurality of nanostructures may be fused to each other at a junction where the metal atoms may be deposited when the nanostructures come into contact or into close proximity with each other at the junction.

[0038] In various embodiments, at 104, the plurality of nanostructures may be contacted with an etching agent (or etchant) to etch the plurality of nanostructures having the metal to supply the metal ions. In this way, the metal ions supplied may include or may be metal ions of the metal of the plurality of nanostructures.

[0039] In various embodiments, the etching agent may include an acid, or an alkali (or base) or a salt. In other words, the acid or the alkali or the salt may etch metal atoms into metal ions, for example, etch the plurality of nanostructures having the metal.

[0040] In the context of various embodiments, the acid may include at least one of hydrochloric acid, ortho-phosphoric acid, trifluoroacetic acid, nitric acid, or sulfuric acid.

[0041] In the context of various embodiments, the alkali may include at least one of sodium hydroxide, potassium hydroxide, or ammonium hydroxide.

[0042] In the context of various embodiments, the salt may include at least one of sodium chloride, sodium bromide, iron(III) nitrate, or iron(III) chloride.

[0043] In various embodiments, at 104, a solution including a metal salt precursor of the metal ions may be provided to supply the metal ions. The metal salt precursor may supply the metal ions of the metal of the plurality of nanostructures. In other words, the metal salt precursor may be a metal salt precursor corresponding to the metal of the plurality of nanostructures. In further embodiments, the metal ions supplied by the metal salt precursor may not correspond to the metal of the plurality of nanostructures. This may mean that the metal contained in the metal salt precursor may be different to the metal of the plurality of nanostructures.

[0044] As non-limiting examples, the metal salt precursor may include silver nitrate, silver trifluoroacetate, to supply silver ions, and copper(II) chloride to supply copper ions.

[0045] In various embodiments, the method may be carried out at room temperature (for example, at about 25°C). In other words, no heating is carried out, or that the method may be free of a heating process. [0046] In various embodiments where heating is not carried out, the reducing agent may include at least one of sodium borohydride, hydrazine, hydroxylamine, ascorbic acid, hydroquinone, formaldehyde, or hydrogen peroxide.

[0047] In various embodiments, at 106, a heating process may also be carried out when contacting the metal ions with a reducing agent. The heating process may be carried out at a predetermined temperature up to about 200°C, for example, up to about 140°C, or up to about 120°C, e.g., about 120°C, about 140°C, about 160°C, or about 200°C. The heating process may be carried out for a predetermined duration of between about 10 seconds and about 30 minutes, for example, between about 10 seconds and about 20 minutes, between about 10 seconds and about 10 minutes, between about 10 seconds and about 5 minutes, preferably about 1 - 10 minutes, e.g., about 5 minutes.

[0048] In various embodiments, the heating process may be carried out in an oven, a furnace or a thermal bath, or the heating process may be carried out using microwave energy or an energy beam.

[0049] In various embodiments where the heating process is carried out, the reducing agent may include at least one of polyol, tri-sodium citrate, or dimethylformamide. The polyol may include at least one of ethylene glycol, diethylene glycol, propylene glycol, glycerol, or glucose.

[0050] In various embodiments, at 102, the plurality of nanostructures may be provided on a substrate. The substrate may be a rigid (non-flexible) substrate such as a metal substrate, a ceramic substrate, a glass substrate, and a semiconductor substrate, etc. or a flexible substrate such as a polymer substrate, paper, textile, fabric, rubber, etc.

[0051] In various embodiments, the plurality of nanostructures may be fused to each other (via the metal atoms) to form a (continuous) film on the substrate. For example, a two-dimensional (2D) fused interconnected film may be formed.

[0052] In the context of various embodiments, the metal of the plurality of nanostructures may be selected from the group consisting of silver (Ag), copper (Cu), gold (Au), nickel (Ni), and platinum (Pt).

[0053] In the context of various embodiments, the plurality of nanostructures may include or may be made of a single metal. [0054] In the context of various embodiments, the plurality of nanostructures may include or may be made of different metals. This may mean that different nanostructures may be made of different metals. In various embodiments, at 104, the metal ions supplied may be metal ions of at least one of the different metals.

[0055] In the context of various embodiments, the plurality of nanostructures may include or may be at least one of nanoflakes, nanosheets, nanowires, nanotubes, nanorods, nanofibers, or nanoparticles. The plurality of nanostructures may be low dimensional metallic nanostructures.

[0056] In the context of various embodiments, the method may be carried out in a solution including the plurality of nanostructures. The reducing agent may be provided in the solution, and the metal ions may be supplied in the solution, and fusing of the plurality of nanostructures may occur in the solution. The solution may be held at room temperature, or the solution may be heated to a predetermined temperature. As the temperature is increased, the reducing strength of the reducing agent is increased. Further, the solution may be held at the predetermined temperature for a predetermined duration. The fused plurality of nanostructures that may be formed may be subsequently separated from the solution, for example, by filtering.

[0057] The method of various embodiments may be applied for joining or interconnecting low dimensional metallic nanostructures, for example, two- dimensional (2D) nanostructures such as nanoflakes and nanosheets, one- dimensional (ID) nanostructures such as nanowires, nanotubes, nanorods, or nanofibers, and zero-dimensional (0D) nanostructures such as nanoparticles.

[0058] While the method described above is illustrated and described as a series of steps or events, it will be appreciated that any ordering of such steps or events are not to be interpreted in a limiting sense. For example, some steps may occur in different orders and/or concurrently with other steps or events apart from those illustrated and/or described herein. In addition, not all illustrated steps may be required to implement one or more aspects or embodiments described herein. Also, one or more of the steps depicted herein may be carried out in one or more separate acts and/or phases. [0059] FIG. 2A shows a flow chart 200 illustrating a method of forming a two- dimensional (2D) or three-dimensional (3D) fused interconnected network, according to various embodiments.

[0060] At 202, low dimensional metallic nanostructures (e.g., metallic materials) may be prepared or provided or formed.

[0061] At 204, a solution including or consisting of a reducing agent, and an acid, or an alkali or a salt, may be prepared. The salt may be at least one of an etching agent or a metal salt precursor.

[0062] At 206, the low dimensional metallic nanostructures (e.g., metallic materials) prepared at 202 may be dispersed in the solution prepared at 204.

[0063] At 208, the solution containing the low dimensional metallic nanostructures (e.g., metallic materials) may be heated to the required or predetermined temperature and held for a certain or predetermined time or duration. In some embodiments, the process at 208 may be optional, meaning that the solution containing the low dimensional metallic nanostructures may instead be maintained at room temperature.

[0064] At 210, a fused network of low dimensional metallic nanostructures (e.g., metallic materials) may be formed. For example, the metallic nanostructures may be fused to form a 2D or 3D fused interconnected network.

[0065] FIG. 2B shows a flow chart 230 illustrating a method of forming a two- dimensional (2D) fused interconnected film, according to various embodiments.

[0066] At 232, low dimensional metallic nanostructures (e.g., metallic materials) may be prepared or provided or formed.

[0067] At 234, a solution including or consisting of a reducing agent, and an acid, or an alkali or a salt, may be prepared. The salt may be at least one of an etching agent or a metal salt precursor.

[0068] At 236, the low dimensional metallic nanostructures (e.g., metallic materials) prepared at 232 may be printed, coated or dispensed on a substrate (e.g., a rigid (or non- flexible) substrate or a flexible substrate).

[0069] At 238, the substrate with the low dimensional metallic nanostructures (e.g., metallic materials) may be immersed into the solution prepared at 234. [0070] At 240, the solution containing the substrate with the low dimensional metallic nanostructures (e.g., metallic materials) may be heated to the required or predetermined temperature and held for a certain or predetermined time or duration. In some embodiments, the process at 240 may be optional, meaning that the solution containing the low dimensional metallic nanostructures may instead be maintained at room temperature.

[0071] At 242, a fused network of low dimensional metallic nanostructures (e.g., metallic materials) may be formed. For example, the metallic nanostructures may be fused to form a film (e.g., a 2D fused interconnected film).

[0072] FIG. 2C shows non-limiting examples of set-ups 260a, 260b, for performing the method of various embodiments. As may be observed, FIG. 2C shows set-ups 260a, 260b for the method or the ripening process using common laboratory apparatus. For larger samples or substrates, the method or the ripening process may be carried out, for example, in an oven.

[0073] In the respective set-ups 260a, 260b, substrates 262a, 262b having a plurality of metallic nanostructures (not shown) provided on the substrates 262a, 262b may be placed in a container, for example, a beaker 266a, or a test tube 266b. A solution (or droplet) 264a containing a reducing agent, and an acid or base or salt (e.g., at least one of an etching agent or a metal salt precursor), may be coated on the plurality of metallic nanostructures on the substrate 262a. The substrate 262b having the plurality of metallic nanostructures provided or coated thereon may be immersed in a solution 264b containing a reducing agent, and an acid or base or salt (e.g., at least one of an etching agent or a metal salt precursor). The containers 266a, 266b, may be placed in a thermal bath or hot bath 268 containing liquid (e.g., water or silicon oil, etc.) 270. The thermal bath 268 may be positioned on a hotplate 272 of a heating device 274 so as to be subjected to a heating process.

[0074] Non-limiting examples of the methods of various embodiments will now be described, with reference to FIGS. 2A to 2C.

[0075] In one example, low dimensional metallic nanostructures (e.g., metallic materials), prepared at 202 (FIG. 2A) may be dispersed, at 206 (FIG. 2A), in a solution including or consisting of a reducing agent and an acid or alkaline that may be prepared at 204 (FIG. 2A). The solution may be held at room temperature or heated up to temperatures ranging from room temperature to about 200°C (e.g., temperature up to about 200°C), at 208 (FIG. 2A). The thermal energy may be provided by one or more sources selected from various heating sources such as an oven, a furnace, a hot bath, microwave energy, energy beam, etc. The method may be carried out using the setup 260b (FIG. 2C), for example, without a substrate. The junctions between the metallic nanostructures or nanomaterials may be fused together, at 210 (FIG. 2A), at low temperatures, where the junctions may involve crossing, adjacent, touching, or nearly- touching (e.g., less than lOO nm, preferably less than 10 nm) nanostructures or nanomaterials. As a result, a 2D or 3D fused interconnected network may be formed. The fused metallic nanostructures (or nanomaterials) have higher electrical conductivity than loosely connected nanomaterials.

[0076] In another example, low dimensional metallic nanostructures (e.g., metallic materials), prepared at 202 (FIG. 2A) may be dispersed, at 206 (FIG. 2A), in a solution including or consisting of a reducing agent and a salt (e.g., at least one of an etching agent or a metal salt precursor) that may be prepared at 204 (FIG. 2A). The solution may be held at room temperature or heated up to temperatures ranging from room temperature to about 200°C (e.g., temperature up to about 200°C), at 208 (FIG. 2A). The thermal energy may be provided by one or more sources selected from various heating sources such as an oven, a furnace, a hot bath, microwave energy, energy beam, etc. The method may be carried out using the set-up 260b (FIG. 2C), for example, without a substrate. The junctions between the metallic nanostructures or nanomaterials may be fused together, at 210 (FIG. 2A), at low temperatures, where the junctions may involve crossing, adjacent, touching, or nearly-touching (e.g., less than 100 nm, preferably less than 10 nm) nanostructures or nanomaterials. As a result, a 2D or 3D fused interconnected network may be formed. The fused metallic nanostructures (or nanomaterials) have higher electrical conductivity than loosely connected nanomaterials.

[0077] In a further example, low dimensional metallic nanostructures (e.g., metallic materials), prepared at 232 (FIG. 2B) may be printed, coated or dispensed, at 236 (FIG. 2B), on rigid or flexible substrates. Then, a solution (e.g., having or consisting of a reducing agent and an acid or alkaline or salt (e.g., at least one of an etching agent or a metal salt precursor)), prepared at 234 (FIG. 2B), may be coated on the film surface of the metallic nanostructures provided on the substrates, or the substrates with the metallic nanostructures may be immersed, at 238 (FIG. 2B), in the solution prepared at 234 (FIG. 2B). When the nanostructures or nanomaterials film immersed in the solution is subjected to heat and low temperature, at 240 (FIG. 2B), the junctions between the low dimensional metallic nanostructures (e.g., metallic materials) may be fused together at 242 (FIG. 2B). As a result, a 2D fused interconnected film may be formed. The method may be carried out using the set-ups 260a, 260b (FIG. 2C), for example.

[0078] For the purpose of understanding, silver nanowires are used as examples to illustrate the method of various embodiments. However, it should be appreciated that other metallic nanostructures may be used with the method of various embodiments.

[0079] Silver (Ag) nanowires network on various substrates may be placed in a solution of acid or base or silver salt solution of different concentrations in ethylene glycol and heated for about 5 minutes.

[0080] Solution fusion of nanomaterials: Low dimensional metallic nanostructures or nanomaterials (e.g., silver nanowires) may be dispersed in a solution including or consisting of ethylene glycol and nitric acid (10 mM). The solution may be heated up to about 140°C in an oven for about 5 minutes. Metallic nanomaterials (e.g., Ag fused interconnected network) may be removed from the solution via centrifugation or filtration.

[0081] Fusion of nanomaterials on a substrate (acid) : Low dimensional metallic nanostructures or nanomaterials (e.g., silver nanowires) may be coated onto a substrate. The substrate with the nanomaterials may be immersed in a solution including or consisting of ethylene glycol and 10 mM of nitric acid. The solution may be heated up to about 140°C in an oven for about 5 minutes. The substrate with the metallic (e.g., Ag) fused interconnected network or film may be removed from the solution and washed in deionised (DI) water.

[0082] Fusion of nanomaterials on substrate (base) : Low dimensional metallic nanostructures or nanomaterials (e.g., silver nanowires) may be coated onto a substrate. The substrate with the nanomaterials may be immersed in a solution including or consisting of ethylene glycol and 10 mM of ammonia. The solution may be heated up to 140°C in an oven for about 5 minutes. The substrate with the metallic (e.g., Ag) fused interconnected network or film may be removed from the solution and washed in DI water.

[0083] Fusion of nanomaterials on substrate (salt) : Low dimensional metallic nanostructures or nanomaterials (e.g., silver nanowires) may be coated onto a substrate. The substrate with the nanomaterials may be immersed in a solution including or consisting of ethylene glycol and 1 mM of silver nitrate. The solution may be heated up to 120°C in an oven for about 5 minutes. The substrate with the metallic (e.g., Ag) fused interconnected network or film may be removed from the solution and washed in DI water.

[0084] As described above, in various embodiments, the plurality of metallic nanostructures (e.g., low dimensional metallic nanomaterials) may be dispersed in a solution including or consisting of a reducing agent and either an acid or an alkali (or base) or a salt, or the plurality of metallic nanostructures (e.g., low dimensional metallic nanomaterials) may be casted onto a substrate and immersed into a solution including or consisting of a reducing agent and either an acid or an alkali (or base) or a salt.

[0085] The acid or the alkali (base) may be an etching agent. The acid or the alkali (base) may be employed to etch metal atoms into metal ions. Non-limiting examples of acid include hydrochloric acid, ortho-phosphoric acid, trifluoroacetic acid, nitric acid, and sulfuric acid. Non-limiting examples of alkali include sodium hydroxide, potassium hydroxide, and ammonium hydroxide.

[0086] The salt may be an etching agent, employed to etch metal atoms into metal ions, or may be a metal salt precursor to provide or supply metal ions. However, there may be challenges in that, at high concentration of metal salt precursors, (metal) nanoparticles may be generated in the reaction mixture or solution, which is not observed in samples using acid or base. Non-limiting examples of salt may include silver nitrate, silver trifluoroacetate, copper(II) chloride, as metal salt precursors, and sodium chloride, sodium bromide, iron(III) nitrate, and iron(III) chloride, as etching agents.

[0087] The reducing agent may be employed to reduce metal ions into metal atoms where these metal atoms may then be deposited onto the junctions formed when two metallic nanomaterials (e.g., nanowires) come into contact/close proximity. [0088] In various embodiments, non-limiting examples of the reducing agent may include polyols such as ethylene glycol, diethylene glycol, propylene glycol, glycerol, glucose, or tri-sodium citrate, or dimethylformamide, where a heating process may be required to facilitate the reducing action of the reducing agents. For reducing agents that may require heating, the plurality of (metallic) nanostructures (or nanomaterials) may not fuse together at room temperature. The reducing strength of these reducing agents increases with temperature, and, as a result, fusing of the plurality of (metallic) nanostructures (or nanomaterials) may occur at an elevated temperature.

[0089] In various embodiments, non-limiting examples of the reducing agent may include sodium borohydride (NaBH₄), hydrazine, hydroxylamine, ascorbic acid, hydroquinone, formaldehyde, or hydrogen peroxide, where no heating may be required to facilitate the reducing action of these reducing agents. For reducing agents that do not require heating, the plurality of (metallic) nanostructures (or nanomaterials) may fuse in contact with the solution.

[0090] In various embodiments, the resultant 2D or 3D fused interconnected network that is formed may be separated or removed from the solution, for example, by filtering.

[0091] FIG. 3A shows an electron microscopy image 300a of silver (Ag) nanowires 350a before fusion while FIG. 3B shows an electron microscopy image 300b after fusion of the nanowires 350a into a dense structure at low temperature (e.g., in a range from room temperature to about 200°C), according to various embodiments, for example, for interconnect/circuit applications.

[0092] FIG. 3 A shows a plurality of Ag nanowires 350a, dispersed on a substrate, which may be fused together using the method of various embodiments. FIG. 3B shows the eventual dense structure, e.g., a film 354, that may be formed when the plurality of nanowires 350a are fused together on the substrate 356. In Fig. 3B, a remaining part 350b of a nanowire 350a may be observed.

[0093] FIGS. 3C and 3D show electron microscopy images 300c, 300d of silver (Ag) nanowires 350c, 350d fused into a network structure at low temperature (e.g., in a range from room temperature to about 200°C), according to various embodiments, for example, for transparent conductor applications. The plurality of nanowires 350c, 350d may be fused together (e.g., fused at junctions 352c, 352d) using the method of various embodiments into a network structure having interconnected nanowires 350c, 350d.

[0094] FIGS. 3E and 3F show electron microscopy images 300e, 300f of silver (Ag) nanowires 350e, 350f fused into a three-dimensional (3D) network structure at low temperature (e.g., in a range from room temperature to about 200°C), according to various embodiments. The plurality of nanowires 350e, 350f may be fused together (e.g., fused at junctions 352e, 352f) using the method of various embodiments into a 3D network structure having interconnected nanowires 350e, 350f.

[0095] It should be appreciated that, in various embodiments, low dimensional similar metallic nanostructures (or nanomaterials), such as silver-silver (Ag-Ag), copper-copper (Cu-Cu) and gold-gold (Au-Au), etc., may be fused at low temperature.

[0096] Further, in various embodiments, low dimensional dissimilar metallic nanostructures (or nanomaterials), such as silver-copper (Ag-Cu), silver-gold (Ag-Au), copper-gold (Cu-Au) and nickel-platinum (Ni-Pt), etc., may be fused at low temperature.

[0097] The areas of industrial applications may include electrodes, circuits, interconnections, films, and devices for electronics, optoelectronics, photonics and clean energy.

[0098] While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.

Claims

1. A method for interconnecting nanostructures comprising:

providing a plurality of nanostructures comprising a metal;

supplying metal ions; and

contacting the metal ions with a reducing agent to reduce the metal ions into metal atoms to fuse the plurality of nanostructures to each other via the metal atoms.

2. The method as claimed in claim 1, wherein supplying metal ions comprises contacting the plurality of nanostructures with an etching agent to etch the plurality of nanostructures comprising the metal to supply the metal ions.

3. The method as claimed in claim 2, wherein the etching agent comprises an acid, or an alkali, or a salt.

4. The method as claimed in claim 3, wherein the acid comprises at least one of hydrochloric acid, ortho-phosphoric acid, trifluoroacetic acid, nitric acid, or sulfuric acid.

5. The method as claimed in claim 3, wherein the alkali comprises at least one of sodium hydroxide, potassium hydroxide, or ammonium hydroxide.

6. The method as claimed in claim 3, wherein the salt comprises at least one of sodium chloride, sodium bromide, iron(III) nitrate, or iron(III) chloride.

7. The method as claimed in claim 1, wherein supplying metal ions comprises providing a solution comprising a metal salt precursor of the metal ions.

8. The method as claimed in claim 1, wherein the method is carried out at room temperature.

9. The method as claimed in claim 8, wherein the reducing agent comprises at least one of sodium borohydride, hydrazine, hydroxylamine, ascorbic acid, hydroquinone, formaldehyde, or hydrogen peroxide.

10. The method as claimed in claim 1, wherein contacting the metal ions with a reducing agent is carried out together with a heating process.

11. The method as claimed in claim 10, wherein the heating process is carried out at a predetermined temperature up to about 200°C.

12. The method as claimed in claim 10, wherein the heating process is carried out for a predetermined duration of about 10 seconds to about 30 minutes.

13. The method as claimed in claim 10, wherein the reducing agent comprises at least one of polyol, tri-sodium citrate, or dimethylformamide.

14. The method as claimed in claim 13, wherein the polyol comprises at least one of ethylene glycol, diethylene glycol, propylene glycol, glycerol, or glucose.

15. The method as claimed in claim 1, wherein providing a plurality of nanostructures comprises providing the plurality of nanostructures on a substrate.

16. The method as claimed in claim 15, wherein the plurality of nanostructures are fused to each other to form a film on the substrate.

17. The method as claimed in claim 1, wherein the metal of the plurality of nanostructures is selected from the group consisting of silver, copper, gold, nickel and platinum.

18. The method as claimed in claim 1, wherein the plurality of nanostructures comprise a single metal.

19. The method as claimed in claim 1, wherein the plurality of nanostructures comprise different metals.

20. The method as claimed in claim 1, wherein the plurality of nanostructures comprise at least one of nanoflakes, nanosheets, nanowires, nanotubes, nanorods, nanofibers, or nanoparticles.