Method of interconnecting nanowires and transparent conductive electrode

According to embodiments of the present invention, a method of interconnecting nanowires is provided. The method includes providing a plurality of nanowires, providing a plurality of nanoparticles, and fusing the plurality of nanoparticles to the plurality of nanowires to interconnect the plurality of nanowires to each other via the plurality of nanoparticles. According to further embodiments of the present invention, a nanowire network and a transparent conductive electrode are also provided.

TECHNICAL FIELD

Various embodiments relate to a method of interconnecting nanowires, a nanowire network and a transparent conductive electrode.

BACKGROUND

Indium tin oxide (ITO) has been the most commonly used material as a transparent electrode for flat panel displays. However, it has some drawbacks: it cannot be used for flexible displays which is the next generation of display due to its brittle nature, as well as the dwindling supply of indium.

Recently, nanomaterials such as metallic silver (Ag) nanowires, carbon nanotubes (CNT) and graphene have been investigated as potential replacement materials for transparent electrodes on flexible substrates instead of ITO. A number of studies have been carried out using a network of Ag nanowires as an electrode material. However, the cost of Ag itself is high and is comparable with the cost of ITO. On the other hand, copper (Cu) is an abundant and cheap material, and is the dominant metal used as an electrical conductor. It has a high electrical conductivity and is used as an electrode in conventional electronics. Though it has many advantages, Cu cannot be used as a transparent electrode in its current form due to some of the reasons as listed below.It is difficult to use Cu with other materials because Cu usually reacts with other metals very aggressively to form intermetallic compounds which degrade its properties.Cu oxidizes in ambient condition, and more severely at higher temperatures. The melting temperature of Cu (1084° C.) is much higher than the processing temperature (<250° C.) of transparent electrodes on typical flexible substrates, e.g., Polyimide (PI, Tg:340° C.), Polycarbonate (PC, Tg:156° C.), Polyethersulfone (PES, Tg:223° C.), Polyethyleneterephthalate (PET, Tg:78° C.), Polyethylenenaphthalate (PEN, Tg:121° C.), Polyarylate (PAR, Tg:350° C.).

SUMMARY

According to an embodiment, a method of interconnecting nanowires is provided. The method may include providing a plurality of nanowires, providing a plurality of nanoparticles, and fusing the plurality of nanoparticles to the plurality of nanowires to interconnect the plurality of nanowires to each other via the plurality of nanoparticles.

According to an embodiment, a nanowire network is provided. The nanowire network may include a plurality of nanowires interconnected to each other via a plurality of nanoparticles fused to the plurality of nanowires.

According to an embodiment, a transparent conductive electrode is provided. The transparent conductive electrode may include a nanowire network, the nanowire network including a plurality of conductive nanowires interconnected to each other via a plurality of conductive nanoparticles fused to the plurality of conductive nanowires.

DETAILED DESCRIPTION

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.

In the context of various embodiments, the phrase “at least substantially” may include “exactly” and a reasonable variance.

In the context of various embodiments, the term “about” as applied to a numeric value encompasses the exact value and a reasonable variance.

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.

FIG. 1Ashows a flow chart100illustrating a method of interconnecting nanowires, according to various embodiments.

At102, a plurality of nanowires (NWs) are provided.

At104, a plurality of nanoparticles (NPs) are provided.

At106, the plurality of nanoparticles are fused to the plurality of nanowires to interconnect the plurality of nanowires to each other via the plurality of nanoparticles.

In various embodiments, the plurality of nanoparticles may be fused to and in between the plurality of nanowires to interconnect the plurality of nanowires to each other via the plurality of nanoparticles, for example, at the junctions between the plurality of nanowires. This may mean that nanoparticles may be present at a joint or junction between two nanowires to fuse the two nanowires to each other. Therefore, two nanowires may be fused together with nanoparticles at a junction so that the two nanowires may be interconnected to each other via the nanoparticles at the junction.

In other words, the plurality of nanowires may be fused to each other through the plurality of nanoparticles. This may mean that the plurality of nanowires may be interconnected to each other by means of the plurality of nanoparticles, rather than direct nanowire-nanowire joints.

In various embodiments, the nanoparticles may also be fused to each other.

In various embodiments, the plurality of nanowires and the plurality of nanoparticles may be made of the same material (e.g., metal), which may encourage reaction between the nanowires and the nanoparticles.

In various embodiments, at106, in order to fuse the plurality of nanoparticles to the plurality of nanowires, the plurality of nanowires and the plurality of nanoparticles may be subjected to a heating process. This may mean that a heat treatment may be carried out to fuse the plurality of nanoparticles to (and in between) the plurality of nanowires.

The heating process may be carried out at a predetermined temperature of about 250° C. or less (i.e., ≤250° C.), for example, between about 100° C. and about 250° C., or between about 120° C. and about 250° C. Therefore, a low temperature processing method (<250° C.) may be provided.

The heating process may be carried out for a predetermined duration of between about 6 minutes and about 60 minutes, for example, between about 6 minutes and about 40 minutes, between about 6 minutes and about 20 minutes, between about 20 minutes and about 60 minutes, between about 40 minutes and about 60 minutes, or between about 10 minutes and about 30 minutes.

In various embodiments, a predetermined peak temperature of the heating process may be between about 200° C. and about 250° C., for example, between about 200° C. and about 220° C., between about 220° C. and about 250° C., or between about 230° C. and about 250° C., e.g., at about 250° C.

In various embodiments, the heating process at the predetermined peak temperature may be carried out for a predetermined duration of between about 90 seconds and about 30 minutes, for example, between about 90 seconds and about 20 minutes, between about 90 seconds and about 10 minutes, between about 10 minutes and about 30 minutes, or between about 5 minutes and about 10 minutes, e.g., for about 90 seconds.

In various embodiments, the plurality of nanowires and the plurality of nanoparticles may be mixed together prior to fusing the plurality of nanoparticles to the plurality of nanowires.

In the context of various embodiments, each nanoparticle of the plurality of nanoparticles may have a size (or diameter) of between about 5 nm and about 20 nm, for example, between about 5 nm and about 15 nm, between about 5 nm and about 10 nm, between about 10 nm and about 20 nm, or between about 8 nm and about 15 nm. The plurality of nanoparticles may have the same (or uniform) size (or diameter).

By having smaller sized nanoparticles (e.g., 5-20 nm, or <10 nm), the processing temperature of the heating process for fusing the plurality of nanoparticles to the plurality of nanowires may be lower (for example <250° C., e.g., between about 100° C. and about 250° C.) as compared to larger sized nanoparticles. For example, for nanoparticles with 40-100 nm diameter, the process temperature required may be in the range of 300-350° C. Further, smaller sized nanoparticles (e.g., 5-20 nm, or <10 nm) have been found to be preferentially deposited at junctions where nanowires come together.

In the context of various embodiments, each nanowire of the plurality of nanowires may have a length of between about 10 μm and about 50 μm, for example, between about 10 μm and about 40 μm, between about 10 μm and about 30 μm, between about 20 μm and about 30 μm, between about 20 μm and about 50 μm, between about 30 μm and about 50 μm, or between about 25 μm and about 40 μm. The plurality of nanowires may have the same (or uniform) length.

In the context of various embodiments, each nanowire of the plurality of nanowires may have a diameter of between about 20 nm and about 200 nm, for example, between about 20 nm and about 100 nm, between about 20 nm and about 50 nm, between about 50 nm and about 200 nm, between about 100 nm and about 200 nm, between about 100 nm and about 150 nm, between about 100 nm and about 120 nm, between about 150 nm and about 200 nm, or between about 120 nm and about 150 nm. The plurality of nanowires may have the same (or uniform) diameter.

In the context of various embodiments, each nanowire of the plurality of nanowires may have an aspect ratio of between about 50 and about 500, for example, between about 50 and about 250, between about 50 and about 100, between about 100 and about 500, or between about 100 and about 300. The term “aspect ratio” may mean the length-to-width ratio or length-to-diameter ratio.

In the context of various embodiments, a weight ratio of the plurality of nanowires to the plurality of nanoparticles is between about 5:1 and about 20:1, for example, between about 5:1 and about 10:1, between about 10:1 and about 20:1, or between about 10:1 and about 15:1, e.g., about 20:1. This may mean that the major/main constituent is the plurality of nanowires while the minor constituent is the plurality of nanoparticles. It should be appreciated that having more of the plurality of nanowires or the plurality of nanoparticles outside of the weight ratio as described above may encourage agglomeration of the nanowires and/or increase the processing temperature of the heating process for fusing the plurality of nanoparticles to the plurality of nanowires.

In various embodiments, at102, the plurality of nanowires may be provided by forming the plurality of nanowires by means of an electroplating method using an anodic aluminum oxide (AAO) as a template. The anodic aluminum oxide (AAO) may include holes or pores or channels into which the material for the plurality of nanowires may be electroplated to form the plurality of nanowires. The plurality of nanowires may then be extracted or removed from the anodic aluminum oxide template. By employing an anodic aluminum oxide (AAO) (or anodized aluminum oxide (AAO)) as a template, a uniform distribution of the size (e.g., length and/or diameter) of the plurality of nanowires may be obtained.

In various embodiments, at102, the plurality of nanowires may be dispersed in a solvent to form a solution including the plurality of nanowires, and, at104, the plurality of nanoparticles may be added into the solution. The solvent may act as a dispersing agent to disperse the plurality of nanowires so as to minimize agglomeration of the plurality of nanowires. In this way, the solvent helps the plurality of nanowires to have mobility and dispersibility. Further, the solvent may help to carry, move or transfer the plurality of nanoparticles to the junction(s) of the plurality of nanowires. The plurality of nanoparticles may then be fused to the plurality of nanowires at such junction(s).

In various embodiments, at least some, or most, of the solvent may be removed, evaporated or volatized during the heating process for fusing the plurality of nanoparticles to the plurality of nanowires. The plurality of (small and light) nanoparticles, or at least some of them, may then get together at the junctions between the plurality of nanowires to find a position or spot where there may be lower surface energy.

In various embodiments, the heating temperature of the heating process is higher than the boiling point of the solvent.

In the context of various embodiments, the solvent may be a low viscosity liquid. As non-limiting examples, the solvent may include at least one of ethanol, methanol, isopropyl alcohol (IPA), or ethylene glycol.

In various embodiments, the method may further include depositing (or dispersing) the solution containing the plurality of nanowires and the plurality of nanoparticles on a substrate prior to fusing the plurality of nanoparticles to the plurality of nanowires. This may mean that fusing the plurality of nanoparticles to the plurality of nanowires, for example, by means of a heating process, may be carried out after the solution containing the plurality of nanowires and the plurality of nanoparticles has been deposited on a substrate. In various embodiments, the heating temperature of the heating process is lower than the transition temperature, Tg, of the substrate, but is higher than the boiling point of the solvent described above.

In various embodiments, the solution may be deposited on the substrate by at least one of spin coating, mayor bar coating, roll to roll coating or spraying (spray coating).

In the context of various embodiments, the substrate may include a porous substrate, for example, anodic aluminium oxide (AAO).

In the context of various embodiments, the substrate may include a flexible substrate.

In the context of various embodiments, the substrate may include an organic substrate.

In the context of various embodiments, the substrate may include at least one of polyimide (PI), polycarbonate (PC), polyethersulfone (PES), polyethyleneterephthalate (PET), polyethylenenaphthalate (PEN) or polyarylate (PAR).

In the context of various embodiments, the plurality of nanowires and the plurality of nanoparticles may be conductive (e.g., electrically conductive and/or thermally conductive).

In the context of various embodiments, the plurality of nanowires may include a metal and/or the plurality of nanoparticles may include a metal. The metal may be selected from the group consisting of copper (Cu), silver (Ag) and gold (Au). In various embodiments, the plurality of nanowires and the plurality of nanoparticles may include or may be made of the same metal.

In the context of various embodiments, the plurality of nanowires and the plurality of nanoparticles may include or consist essentially of copper (Cu).

In various embodiments, each nanowire of the plurality of nanowires may include a surfactant on a surface of the nanowire. The surfactant may prevent or minimize agglomeration of the plurality of nanowires. The surfactant may include a thiol (or thiol group) or an amine (or amine group). As non-limiting examples, the thiol group may include hexanethiol, octanethiol, decanethiol, dodecanethiol, etc. As non-limiting examples, the amine group may include hexylamine, octylamine, decylamine, dodecylamine, etc.

In other words, the plurality of nanowires may be treated to have a surfactant (e.g., thiol or amine) provided on the plurality of nanowires. The treatment with the surfactant may be carried out before mixing the plurality of nanowires with the plurality of nanoparticles. Generally, the process flow may be as follows: Nanowires (raw material)→Treatment with surfactant (e.g., thiol or amine)→Adding nanoparticles→Mixing the nanowires and the nanoparticles with a matrix (e.g., polymethylmethacrylate (PMMA))→Dispersing (coating), for example, onto a substrate→Heating (annealing). The process of mixing with a matrix may be optional.

In various embodiments, each nanoparticle of the plurality of nanoparticles may be encapsulated with an organic layer. This may mean that each nanoparticle may be coated on its surface with an organic layer. Therefore, the organic layer may be a capping layer. The organic layer may prevent or minimize oxidation of the material of the nanoparticle. The organic layer may prevent or minimize agglomeration of the nanoparticles. The organic layer may be a polymeric layer. In various embodiments, the organic layer may be removed or volatized during the heating process for fusing the plurality of nanoparticles to the plurality of nanowires.

It should be appreciated that, in general, the method may include mixing→dispersing→heating, where the plurality of nanowires and the plurality of nanoparticles may be mixed (e.g., in a solvent), and then dispersed (e.g., on a substrate), followed by heating to fuse the plurality of nanoparticles to (and in between) the plurality of nanowires to interconnect the plurality of nanowires to each other via the plurality of nanoparticles.

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.

FIG. 1Bshows a schematic top view of a nanowire network120, according to various embodiments. The nanowire network120includes a plurality of nanowires122interconnected to each other via a plurality of nanoparticles124fused to the plurality of nanowires122. This may mean that the nanowire network120may include interconnected nanowires122.

In other words, a nanowire network (or network of interconnected nanowires)120may be provided. The nanowire network120may include a plurality of interconnected nanowires122, and a plurality of nanoparticles124fused to the plurality of interconnected nanowires122such that the plurality of interconnected nanowires122are interconnected to each other via the plurality of nanoparticles124.

In various embodiments, the plurality of nanoparticles124may be fused to and in between the plurality of nanowires122to interconnect the plurality of nanowires122to each other via the plurality of nanoparticles124at the junctions126between the plurality of nanowires122. This may mean that nanoparticles124may be present at a joint or junction126between two nanowires122to fuse the two nanowires122to each other. Therefore, two nanowires122may be fused together with nanoparticles124at a junction126so that the two nanowires122may be interconnected to each other via the nanoparticles124at the junction126.

In other words, the plurality of nanowires122may be fused to each other through the plurality of nanoparticles124. This may mean that the plurality of nanowires122may be interconnected to each other by means of the plurality of nanoparticles124, rather than direct nanowire-nanowire joints.

In various embodiments, the nanoparticles124may also be fused to each other.

In various embodiments, the plurality of nanowires122and the plurality of nanoparticles124may be made of the same material (e.g., metal), which may encourage reaction between the nanowires122and the nanoparticles124.

In the context of various embodiments, individual (or individually resolvable) nanoparticles124of the plurality of nanoparticles124may have a size (or diameter) of between about 5 nm and about 20 nm, for example, between about 5 nm and about 15 nm, between about 5 nm and about 10 nm, between about 10 nm and about 20 nm, or between about 8 nm and about 15 nm. Individual (or individually resolvable) nanoparticles124of the plurality of nanoparticles124may have the same (or uniform) size (or diameter).

In the context of various embodiments, individual (or individually resolvable) nanowires122of the plurality of nanowires122may have a length of between about 10 μm and about 50 μm, for example, between about 10 μm and about 40 μm, between about 10 μm and about 30 μm, between about 20 μm and about 30 μm, between about 20 μm and about 50 μm, between about 30 μm and about 50 μm, or between about 25 μm and about 40 μm. Individual (or individually resolvable) nanowires122of the plurality of nanowires122may have the same (or uniform) length.

In the context of various embodiments, individual (or individually resolvable) nanowires122of the plurality of nanowires122may have a diameter of between about 20 nm and about 200 nm, for example, between about 20 nm and about 100 nm, between about 20 nm and about 50 nm, between about 50 nm and about 200 nm, between about 100 nm and about 200 nm, between about 100 nm and about 150 nm, between about 100 nm and about 120 nm, between about 150 nm and about 200 nm, or between about 120 nm and about 150 nm. Individual (or individually resolvable) nanowires122of the plurality of nanowires122may have the same (or uniform) diameter.

In the context of various embodiments, individual (or individually resolvable) nanowires122of the plurality of nanowires122may have an aspect ratio of between about 50 and about 500, for example, between about 50 and about 250, between about 50 and about 100, between about 100 and about 500, or between about 100 and about 300.

In various embodiments, the nanowire network120may be at least substantially optically transparent. This may mean that the nanowire network120may be at least substantially transparent to visible light.

In the context of various embodiments, the plurality of nanowires122may include a metal and/or the plurality of nanoparticles124may include a metal. The metal may be selected from the group consisting of copper (Cu), silver (Ag) and gold (Au). In various embodiments, the plurality of nanowires122and the plurality of nanoparticles124may include or may be made of the same metal.

In the context of various embodiments, the plurality of nanowires122and the plurality of nanoparticles124may include or consist essentially of copper (Cu).

In various embodiments, each nanowire122of the plurality of nanowires122may include a surfactant on a surface of the nanowire122. The surfactant may prevent or minimize agglomeration of the plurality of nanowires122. The surfactant may include a thiol or an amine.

FIG. 1Cshows a schematic top view of a transparent conductive electrode130, according to various embodiments. The transparent conductive electrode130includes a nanowire network120a, the nanowire network120aincluding a plurality of conductive nanowires122ainterconnected to each other via a plurality of conductive nanoparticles124afused to the plurality of conductive nanowires122a. This may mean that the nanowire network120amay include interconnected conductive nanowires122a.

In other words, a transparent conductive electrode130may be provided. The transparent conductive electrode130may have a conductive nanowire network120a, which may include a plurality of interconnected conductive nanowires122a, and a plurality of conductive nanoparticles124afused to the plurality of interconnected conductive nanowires122asuch that the plurality of interconnected conductive nanowires122aare interconnected to each other via the plurality of conductive nanoparticles124a.

In various embodiments, the plurality of conductive nanoparticles124amay be fused to and in between the plurality of conductive nanowires122ato interconnect the plurality of conductive nanowires122ato each other via the plurality of conductive nanoparticles124aat the junctions126abetween the plurality of conductive nanowires122a. This may mean that conductive nanoparticles124amay be present at a joint or junction126abetween two conductive nanowires122ato fuse the two conductive nanowires122ato each other. Therefore, two conductive nanowires122amay be fused together with conductive nanoparticles124aat a junction126aso that the two conductive nanowires122amay be interconnected to each other via the conductive nanoparticles124aat the junction126a.

In other words, the plurality of conductive nanowires122amay be fused to each other through the plurality of conductive nanoparticles124a. This may mean that the plurality of conductive nanowires122amay be interconnected to each other by means of the plurality of conductive nanoparticles124a, rather than direct nanowire-nanowire joints.

In various embodiments, the conductive nanoparticles124amay also be fused to each other.

In various embodiments, the plurality of conductive nanowires122aand the plurality of conductive nanoparticles124amay be made of the same material (e.g., metal), which may encourage reaction between the conductive nanowires122aand the conductive nanoparticles124a.

In various embodiments, the nanowire network120amay be electrically conductive and/or thermally conductive.

In various embodiments, the plurality of conductive nanowires122aand the plurality of conductive nanoparticles124amay be electrically conductive and/or thermally conductive.

In various embodiments, the nanowire network120amay be at least substantially optically transparent. This may mean that the nanowire network120amay be at least substantially transparent to visible light.

In the context of various embodiments, individual (or individually resolvable) conductive nanoparticles124aof the plurality of conductive nanoparticles124amay have a size (or diameter) of between about 5 nm and about 20 nm, for example, between about 5 nm and about 15 nm, between about 5 nm and about 10 nm, between about 10 nm and about 20 nm, or between about 8 nm and about 15 nm. Individual (or individually resolvable) conductive nanoparticles124aof the plurality of conductive nanoparticles124amay have the same (or uniform) size (or diameter).

In the context of various embodiments, individual (or individually resolvable) conductive nanowires122aof the plurality of conductive nanowires122amay have a length of between about 10 μm and about 50 μm, for example, between about 10 μm and about 40 μm, between about 10 μm and about 30 μm, between about 20 μm and about 30 μm, between about 20 μm and about 50 μm, between about 30 μm and about 50 μm, or between about 25 μm and about 40 μm. Individual (or individually resolvable) conductive nanowires122aof the plurality of conductive nanowires122amay have the same (or uniform) length.

In the context of various embodiments, individual (or individually resolvable) conductive nanowires122aof the plurality of conductive nanowires122amay have a diameter of between about 20 nm and about 200 nm, for example, between about 20 nm and about 100 nm, between about 20 nm and about 50 nm, between about 50 nm and about 200 nm, between about 100 nm and about 200 nm, between about 100 nm and about 150 nm, between about 100 nm and about 120 nm, between about 150 nm and about 200 nm, or between about 120 nm and about 150 nm. Individual (or individually resolvable) conductive nanowires122aof the plurality of conductive nanowires122amay have the same (or uniform) diameter.

In the context of various embodiments, individual (or individually resolvable) conductive nanowires122aof the plurality of conductive nanowires122amay have an aspect ratio of between about 50 and about 500, for example, between about 50 and about 250, between about 50 and about 100, between about 100 and about 500, or between about 100 and about 300.

In various embodiments, the transparent conductive electrode130may further include a substrate132on which the nanowire network120amay be provided. The substrate132may include a flexible substrate. The substrate132may include an organic substrate. In the context of various embodiments, the substrate132may include at least one of polyimide (PI), polycarbonate (PC), polyethersulfone (PES), polyethyleneterephthalate (PET), polyethylenenaphthalate (PEN) or polyarylate (PAR).

In the context of various embodiments, the plurality of conductive nanowires122amay include a metal and/or the plurality of conductive nanoparticles124amay include a metal. The metal may be selected from the group consisting of copper (Cu), silver (Ag) and gold (Au). In various embodiments, the plurality of conductive nanowires122aand the plurality of conductive nanoparticles124amay include or may be made of the same metal.

In the context of various embodiments, the plurality of conductive nanowires122aand the plurality of conductive nanoparticles124amay include or consist essentially of copper (Cu).

In various embodiments, each conductive nanowire122aof the plurality of conductive nanowires122amay include a surfactant on a surface of the conductive nanowire122a. The surfactant may prevent or minimize agglomeration of the plurality of conductive nanowires122a. The surfactant may include a thiol or an amine.

In the context of various embodiments, the transparent conductive electrode130may be used in a flexible touchscreen or display.

In the context of various embodiments, the terms “fuse” and “fusing” may mean sintering, or joining together as an (single) entity. This may mean that there may not be clear or obvious boundary observable between two materials (or structures) when the two materials are fused to each other. Further, the two materials fused to each other may not be separate or distinct.

It should be appreciated that descriptions in the context of the nanowire network120and the transparent conductive electrode130may be correspondingly applicable to each other, and may also be correspondingly applicable in relation to the method for interconnecting nanowires, and vice versa.

Various embodiments may provide a copper nanowires-nanoparticles mixture for transparent conducting electrodes.

Various embodiments may provide a composition of copper nanowires (Cu NWs) and copper nanoparticles (Cu NPs) that may allow low temperature processing (for example, <250° C.) to be compatible with flexible organic substrates, with the required electrical conductivity and optical transmittance as a transparent electrode. Various embodiments may include or provide one or more of the following:(i) To enable low temperature process, very small (for example, <10 nm) Cu nanoparticles (NPs) which may fuse between about 100° C. and about 250° C. may be used as the joining material between Cu nanowires to provide the electrical conductivity. Therefore, the process temperature may be decreased to less than about 250° C. Moreover, the nanoparticles formed joint lowers the contact resistance in comparison to a direct nanowire-nanowire joint. PCT/US2010/039069 describes a method for forming small Cu nanoparticles and the fabricated small copper nanoparticles that may be used in various embodiments described herein, the entire disclosure of which is incorporated herein by reference. Nevertheless, copper NPs which are produced by other methods may also be used as the joining material. However, the size and size distribution of the NPs employed in various embodiments may change the required process temperature and temperature profile for fusing the nanostructures or nanowires. From preliminary experiments, the inventors found that if NPs with 40-100 nm diameter are used, the process temperature required is in the range of 300-350° C. The blended composition of nanowires and nanoparticles as the fusing material of various embodiments may be employed as a transparent electrode.(ii) Long (for example, up to 50 μm) Cu nanowires may be used in various embodiments, which may reduce the number of nanowire-nanowire contact and thus may increase the electrical conductivity (for example, for large area transparent conductive electrodes), but also may lead to a ductile mechanical property which may enable the flexibility required.(iii) Mixing NWs and NPs in a low viscosity liquid (e.g., ethanol, isopropyl alcohol (IPA)) may encourage or cause the NWs to disperse uniformly, and may aid the NPs to coalesce at the junctions of contacting NWs. For this, the solvent should be fluid enough for the NPs to move in the dispersion. This mechanism may be explained thermodynamically that the NPs tend to reduce their high surface energy by fusing and increasing their contact area at joints, and the NPs have the required mobility to move. From another perspective, if the NPs are well-dispersed, for example, across a substrate, during the drying and washing process, more NPs may be placed at the junction of NWs and fused during the annealing process.(iv) In order to achieve better dispersion of the mixture of NWs and NPs, one or more chemical treatments before mixing the NWs and NPs may be effective. As non-limiting examples, different types of thiol may be used as a surfactant to attach the diverse length of carbon chains on the surface of the NWs (e.g., Cu NWs). However, it should be appreciated that other kinds of chemicals may also be used to substitute for thiol which may make the NWs more stable and prevent or at least minimize agglomeration for better dispersion.

In various embodiments, copper (Cu) nanoparticles and copper (Cu) nanowires may be mixed with an appropriate (weight) ratio to address the problems described above. Cu nanowires are used as the main conductive and transparent material with Cu nanoparticles as a joining material between the nanowires.

FIG. 2Ashows, as cross-sectional views, various processing stages of a method240of manufacturing copper (Cu) nanowires (NWs), according to various embodiments, illustrating a procedure of manufacturing of Cu NWs using anodized aluminum oxide (AAO). In general, gold/copper (Au/Cu) layers may be deposited on the AAO as a seed layer for electroplating. Cu may subsequently be electroplated into the holes in the AAO. Then, the seed layer and the AAO may be etched out by an etchant as shown inFIG. 2A. Finally, the Cu NWs may be detached from the substrate. Therefore, Cu nanowires may be produced through electroplating using anodic aluminum oxide (AAO) as a template. As a result, uniform length of nanowires may be obtained. While it is described herein that the electroplating method with AAO as a template may be used to obtain the Cu nanowires, it should be appreciated that a chemical synthesis or a physical deposition process may be used instead.

As a non-limiting example, referring toFIG. 2A, an anodic aluminum oxide (AAO) template242may first be provided or prepared. The AAO template242may have a plurality of holes (or pores or channels)243. The AAO template242may be sputtered with Au and Cu, where a 0.2 nm thick gold layer244and a 1 μm thick copper layer246may be obtained.

The AAO template242may then be attached onto a cathode (not shown) and 50 μm long Cu nanowires222may be electrochemical synthesized in the holes243of the AAO template242.

The AAO template242may be attached to a thermal tape248. The sputtered Cu layer246and Au layer244may be etched away by chemical etching processes. The AAO template242may be etched using a sodium hydroxide (NaOH) solution. As a result, free Cu nanowires222may be obtained.

The Cu nanowires222may be washed with ethanol, followed by isopropyl alcohol (IPA).

In various embodiments, optionally or if necessary, the nanowires222may be coated with thiol or amine group. In this way, thiol or amine as a surfactant may be coated on the nanowires222.

Subsequently, Cu nanoparticles may be added to the nanowires222for mixing. The mixture may be added into a solution (or solvent) (e.g., isopropyl alcohol (IPA)) for dispersion and may be coated by spin coating, for example, onto a substrate. As a non-limiting example, referring toFIG. 2Billustrating a method250of mixing of copper nanoparticles (Cu NPs) and copper nanowires (Cu NWs), Cu NPs224(e.g., having diameters of about 5-20 nm) are added to Cu NWs222(e.g., having lengths of about 20-50 m) and the two materials may be mixed in an ultra-sonicator and washed with an alcohol base solution (e.g., isopropyl alcohol (IPA))251. Then, the solution (e.g., in the form of droplet252) containing the mixture of Cu NWs222and Cu NPs224, may be deposited, for example, from a dispenser254, onto a substrate232, and then dispersed on the substrate232by diverse methods, such as spin coating, mayor bar coating, roll to roll coating, spray coating, etc.

FIGS. 2C and 2Dshow examples of some steps of the processing method of various embodiments, using copper nanowires and copper nanoparticles as examples.

Referring toFIG. 2C, after forming copper nanowires using an AAO template242, the AAO template242may be removed or etched by immersing the AAO template242with the copper nanowires in a solution255of sodium hydroxide (NaOH) (0.5 M concentration) inside a container256. The solution255with the free copper nanowires may be centrufuged and then, the solution255may be drained off, leaving behind copper nanowires222inside the container256. Then, an appropriate amount of the copper nanowires222may be weighed and copper nanoparticles224may then be added to the copper nanowires222until the desired weight ratio of the copper nanowires222to the copper nanoparticles224is obtained, for example about 20:1. The mixture of the copper nanowires222and the copper nanoparticles224may be washed in an alcohol based solution (e.g., ethanol, IPA)251. The copper nanowires222and the copper nanoparticles224may be subsequently mixed in the solvent (alcohol based solution251) using an ultrasonicator, where the weight ratio (or weight percentage) of solvent: mixture of copper nanowires222and copper nanoparticles224is 10:1.

Referring toFIG. 2D, copper (Cu) nanowires222may be dispersed in an alcohol based solvent257in a container258. Copper (Cu) nanoparticles224may be added into the alcohol based solvent257and the solution259containing the Cu nanowires222, the Cu nanoparticles224and the alcohol based solvent257may be mixed by an ultrasonicator. Then, the solution259or part thereof, after mixing, may be deposited or dispensed (e.g., in the form of droplet252) from a dispenser254onto a substrate232, and subsequently dispersed or coated as a layer252aon the substrate232. A heating process may then be carried out at a temperature between about 120° C. and about 250° C., for example, from below the substrate232. As a result of the heating process, the alcohol based solvent257may be evaporated and the Cu nanoparticles224may fuse to each other and to the Cu nanowires222.

It should be appreciated that the methods or steps described in the context ofFIGS. 2A-2Drespectively may be applicable also to other methods or steps ofFIGS. 2A-2D, or may be combined in any manner.

FIG. 3Ashow a scanning electron microscope (SEM) image of copper nanowires322after manufacturing, for example, based on the method240described in the context ofFIG. 2A. As shown inFIG. 3A, extremely long Cu NWs (>50 μm)322may be obtained.FIG. 3Bshows a scanning electron microscope (SEM) image of copper nanowires322aafter dispersion on a substrate (not clearly shown).

In various embodiments, after being dispersed onto a substrate, the copper nanowires and the nanoparticles may be subjected to a heating process. The heating process may assist or encourage joining of the Cu nanowires and the Cu nanoparticles, where the Cu nanoparticles may be fused to the Cu nanowires to interconnect the Cu nanowires via the Cu nanoparticles.FIG. 4shows a schematic diagram460illustrating the mechanism in which copper (Cu) nanoparticles424fuse at the junctions426of copper (Cu) nanowires422, so as to achieve the required electrical conductivity at low temperatures.

In various embodiments, Cu nanowires with a diameter of about 20-200 nm and a length of about 20-50 μm (aspect ratio: 100-500) and Cu nanoparticles with a diameter of about 5-20 nm are used. The weight ratio of Cu nanowires to Cu nanoparticles is from about 10:1 to about 20:1.

Examining a range of sizes, lengths, weight ratios of Cu nanowires and Cu nanoparticles, an optimal composition may be determined for the required electrical conductivity and optical transmittance (see, for example,FIGS. 5A, 5B, 5C and 6to be described below).

FIG. 5Ashows scanning electron microscope (SEM) images of copper nanowires (Cu NWs)522a, illustrating samples made by only Cu nanowires522a.

FIG. 5Bshows scanning electron microscope (SEM) images of microstructures of mixed copper (Cu) nanowires522band copper (Cu) nanoparticles524bwhich are annealed at about 200° C., according to various embodiments. The weight ratio of the nanowires522bto the nanoparticles524bis 10:0.6. As may be observed inFIG. 5B, the Cu nanoparticles524bare fused to the Cu nanowires522bat junctions526bbetween the Cu nanowires522b. In this way, the Cu nanowires522bmay be interconnected to each other via Cu nanoparticles524bat the junctions526b.

FIG. 5Cshows scanning electron microscope (SEM) images of microstructures of mixed copper (Cu) nanowires522cand copper (Cu) nanoparticles524cwhich are annealed at about 200° C., according to various embodiments. For obtaining the SEM images shown inFIG. 5C, three drops of the mixture of copper (Cu) nanowires522cand copper (Cu) nanoparticles524cwere provided on a substrate for coating the substrate. The weight ratio of the nanowires522cto the nanoparticles524cis 8:1.5. As may be observed inFIG. 5C, the Cu nanoparticles524care fused to the Cu nanowires522cat junctions526cbetween the Cu nanowires522c. In this way, the Cu nanowires522cmay be interconnected to each other via Cu nanoparticles524cat the junctions526c. The SEM image590shows a cross sectional view of the mixed copper (Cu) nanowires522cand copper (Cu) nanoparticles524cusing FIB (Focused Ion Beam).

FIG. 6shows a plot670of sheet resistance (unit: ohm per square, Ω/□) for various copper (Cu) nanostructures mixtures on polyimide substrates. Plot670shows result672for copper nanowires (NWs), result674for copper nanowires with Cu nanoparticles fused to the nanowires where one drop of the mixture of copper (Cu) nanowires and copper (Cu) nanoparticles was provided on a polyimide substrate (NWs+NPs X1, where “X1” represents one drop) and result676for copper nanowires with Cu nanoparticles fused to the nanowires where three drops of the mixture of copper (Cu) nanowires and copper (Cu) nanoparticles were provided on a polyimide substrate (NWs+NPs X3, where “X3” represents three drops). As shown inFIG. 6, the sheet resistance may decrease with the addition of nanoparticles to the nanowires and may further decrease as the amount of nanoparticles is increased.

FIG. 7Ashows a plot770of sheet resistance of fused copper (Cu) nanowires-nanoparticles on different substrates. The composition rate of the mixture of Cu NWs to the Cu NPs is about 20(NW):1(NP). Plot770shows the sheet resistances of Cu nanowires and Cu nanoparticles composite on different substrates such as result772for quartz, result774for polyimide, result776for polyimide (same substrate corresponding to result774) and after bending of the polyimide sample (indicated as “1st” to refer to one (first) time of bending), result778for polyimide (same substrate corresponding to results774,776) and after bending of the polyimide sample (indicated as “2nd” to refer to second time of bending) and result780for polyethyleneterephthalate (PET). In various embodiments, electrical conductivity is maintained even after preliminary flexing of the polyimide substrate (e.g.,782), as may be observable from results776,778.

FIG. 7Bshows a plot786of optical transmittance of a transparent copper nanowires (Cu NWs) with copper nanoparticles (Cu NPs) electrode, measured with an ultraviolet-visible (UV-vis) spectrometer, for a mix of Cu nanowires-nanoparticles having a composition rate of Cu NWs to Cu NPs of about 20(NW):1(NP). An optical transmittance of about 40% may be obtained.

In various embodiments, some polymers (such as polymethylmethacrylate (PMMA), polystyrene, etc.) may aid in the dispersion of the nanostructures (by improving the dispersion properties) and may lead to a smooth surface as shown by the SEM images inFIGS. 8A, 8B and 8C. The use of PMMA as a matrix may provide advanced or improved coating uniformity, and higher adhesion on a substrate (e.g., glass substrate). Therefore, in various embodiments, Cu NWs and Cu NPs may be mixed in a solution containing a polymer such as PMMA.

FIG. 8Ashows a schematic diagram illustrating a solution of polymethylmethacrylate (PMMA)890awith copper (Cu) nanowires822adispensed onto a substrate832, according to various embodiments. No Cu NPs were included.

FIG. 8Bshows scanning electron microscope (SEM) images illustrating the effects of different concentrations (weight %) of polymethylmethacrylate (PMMA)890bfor dispersion of nanowires822b. No Cu NPs were included.

FIG. 8Cshows scanning electron microscope (SEM) images of copper (Cu) nanowires in polymethylmethacrylate (PMMA). No Cu NPs were included. 4 wt % PMMA (molecular weight of 996K) in chloroform solvent was used and the PMMA solution890ccontaining nanowires822cdispersed therein was spin-coated at about 2000 rpm onto a glass substrate, and then annealed at about 350° C. The processing further included treatment of the nanowires822cwith a thiol to prevent or minimise agglomeration of the nanowires822c, for example, treatment with octanethiol where the nanowires822cmay be stirred in the thiol at about 500 rpm at about 80° C. Generally, treatment with a surfactant such as a thiol or amine may be carried out before mixing with nanoparticles. As a non-limiting example, the entire process flow may be as follows: Nanowire (raw material)→Treatment with thiol or amine→Adding nanoparticles→Mixing with matrix (e.g., PMMA)→Dispersing(coating)→Heating (annealing).

As non-limiting examples, different molecular weights (MW) of PMMA (e.g., 112 k, 120 k, 996K) and/or concentrations (e.g., 0.5%-10%, e.g., 0.1-0.4 wt %), and the concentration of nanowires in PMMA or solvent (e.g., 5%-20%) may be used. Dispersion property may be diverse or different according to the concentration and/or MW of PMMA. For embodiments employing PMMA as a matrix, the solvent used for PMMA may include dimethylformamide (DMF) (boiling point ˜153° C.), chloroform (boiling point ˜61° C.), or toluene (boiling point ˜110° C.).

In various embodiments, conductive polymers such as poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) may also be employed as the matrix to improve the electrical properties as occasion demands.

FIG. 9Ashows schematic diagrams illustrating the use of an anodic aluminium oxide932as a substrate in the method of various embodiments. A solution containing PMMA (as matrix)990and copper (Cu) nanowires922may be provided on the anodic aluminium oxide932. Over time, the PMMA990may flow through the holes (or channels)933of the anodic aluminium oxide932, out of the anodic aluminium oxide932on the side of the anodic aluminium oxide932opposite to that having the Cu nanowires922. As a result, only minimal or residual PMMA990may remain with the Cu nanowires922. A substrate (e.g., glass)992having an adhesive (e.g., epoxy glue)994may be used to transfer the Cu nanowires922remaining on the anodic aluminium oxide932onto the substrate992. Therefore, in various embodiments, the anodic aluminium oxide932may act as an intermediate substrate for transferring the Cu nanowires922to the (final) substrate992. WhileFIG. 9Adoes not show the addition of Cu nanoparticles, it should be appreciated that Cu nanoparticles may be added to the nanowires922before mixing with the matrix (PMMA990).

In various embodiments, when anodic aluminium oxide932is used as a substrate, the annealing temperature employed may go higher than about 300° C. because the melting temperature of AAO is about 2,000° C. While PMMA is used to improve the dispersion of the nanowires, the presence of PMMA may adversely affect the performance of a transparent electrode and so the preferred way is to remove/reduce the PMMA, for example, by heating. The annealing temperature depends on the molecular weight and concentration of the PMMA, however, a temperature of about 400° C. may be an optimum treatment temperature. The annealing process may be carried out after the second step illustrated inFIG. 9A, meaning after dispersion of the nanowires922onto the anodic aluminium oxide932and prior to transferring to the substrate992. In other words, the process flow may include dispersion of NWs onto the AAO→annealing →transferring.

FIG. 9Bshows photographs illustrating the transfer of nanowires (e.g., Cu nanowires)922acoated on an anodic aluminium oxide932afrom the anodic aluminium oxide932ato another substrate, e.g., PET992awith adhesive, according to various embodiments. As shown inFIG. 9B, the anodic aluminium oxide932awith coated nanowires922a(with PMMA) may first be provided affixed to a glass substrate933ahaving an adhesive tape934a. The nanowires922a(with PMMA990a) and the anodic aluminium oxide932amay then be transferred to the PET992a. The anodic aluminium oxide932amay then be transferred back to the glass substrate933awith the adhesive tape934a, leaving behind the nanowires922a(possibly with some residual PMMA990a) on the PET992a. As may be observed inFIG. 9B, the PET992ahaving the nanowires922amay be optically transparent.

FIG. 9Cshows a scanning electron microscope (SEM) image of well-dispersed nanowires922con an anodic aluminium oxide (AAO) substrate932c. While present, nanoparticles are not clearly shown in the SEM image. The weight ratio of the nanowires922cto the nanoparticles was about 20:1, the solvent used was IPA and the annealing temperature was about 300° C.

Some functional groups (e.g., thiol and amine) may act as a surfactant to enhance the dispersion property of the mixture. The effect of adding some thiol or amine into the mixture of NPs and NWs before dispersion may be determined, because these molecules attach on the surface of the NWs (seeFIG. 10illustrating thiol molecules1095attached on the surface of nanowires (NWs) (e.g., Cu NWs))1022, where these molecules1095work like arms causing the NWs1022to repel each other and prevent agglomeration.

In various embodiments, PMMA may be the dispersion matrix and thiol may be the surfactant. PMMA may be used as the dispersion matrix to hold the Cu nanowires uniformly across the substrate to prevent or at least minimise agglomeration of the Cu nanowires and improves the conductivity of the thin films. The PMMA may be subsequently removed.

As described above, various embodiments may include or provide one or more of the following:(i) A much lower process temperature (about 100-250° C.) to form the conductive electrode due to the fusion temperature of small copper (Cu) nanoparticles used. This is a major improvement over existing technology which has a higher processing temperature (300-500° C.). The lower temperature (<250° C.) for various embodiments may enable compatibility of the process with flexible organic substrates.(ii) Long, uniform and high aspect ratio (about 50-250) Cu nanowires may be used in the composition. This may improve flexibility while maintaining the electrical conductivity and the optical transmittance required.(iii) Cu may be used as both the conductive and joining materials. For example, by using only Cu as an electrode material, aggressive inter-diffusion and reaction between heterogeneous materials may be avoided.

Various embodiments may be related to or focused on the touchscreen and display applications. In order to apply to flexible touchscreens or displays, the process temperature has to be lower than 250° C., while the electrical and optical properties have to be maintained after repeated bending or straining. The global market for transparent conductive coatings is expected to grow to nearly $7.1 billion by 2018, and for flexible displays, reached $39.1 million in 2012.