A LIQUID-LIQUID INTERFACE

Field of the invention

The present invention relates to the formation of nanowire devices and in particular to capturing and aligning of nanowires to make nanowire devices. More specifically, this disclosure is related to methods for providing an aggregate of assembled aligned nanowires from a fluid, e.g. for the purpose of subsequently transferring the assembly of aligned nanowires to a substrate surface. Background

Conventional technologies for capturing nanostructures on a surface focus on the alignment and capture/deposition of nanostructures with a low length/diameter ratio (e.g.: nanorods, nanoparticles). However, capture and alignment of nanostructures with appreciable length/diameter ratio (e.g. nanowires) is more difficult. It is also difficult to align nanowires with a preferential direction. Conventional technologies use external controls (e.g., applied electric fields, slow solvent evaporation or thermal annealing) which may require the use of external equipment or high voltages to obtain the alignment and capture/deposition of nanostructures. These external controls increase the production cost and decrease the scalability of nanowire device production.

Furthermore, existing technologies reporting the vertical alignment of nanostructures with high aspect ratio are limited to small areas, below cm scale.

Applicant's previous application, published as WO2015/166416 Al, discloses a method for capturing and aligning an assembly of nanowires from a liquid interface onto a surface including providing a first liquid and a second liquid, wherein the first and second liquids phase separate into a sub phase, a top phase and an interface between the sub phase and the top phase. The nanowires are provided such that the majority of the nanowires are located at the interface and providing the nanowires onto a substrate such that a majority of the nanowires are aligned with respect to each other on the substrate.

There still exists room for improvement in the arts of providing a suitable assembly of nanowires, suitable for capturing the assembly on a substrate surface. More specifically, improvements are desired for obtaining higher quality in the process of transferring a nanowire aggregate from a fluid to a substrate surface, and also in the perfection level of the resulting substrate.

Summary

Solutions are presented herein, which address problems related to providing an assembly of oriented nanowires from a fluid.

According an aspect, a method is proposed for forming an aggregate of assembled aligned nanowires, wherein said nanowires comprise an elongate wire portion and a head portion at a first end of the elongate wire portion, comprising:

providing a fluid comprising a first liquid, a second liquid and a plurality of nanowires, wherein the first and second liquids phase separate into a first phase, a second phase, and an interface between the first and second phases;

wherein the nanowires are functionalized to align vertically and assemble into a nanowire aggregate at the interface, with said wire portion in the first phase and said head portion in the second phase;

providing a bonding substance to one of said first or second phases;

bonding the nanowires of the nanowire aggregate in said one phase using said substance.

In one embodiment, wherein the bonding substance is a precursor substance of a compound added to the first phase, the step of bonding including

growing said compound on said wire portions to bond wires portions of adjacent nanowires together.

In one embodiment, the wire portions of the nanowires are provided with a dielectric layer.

In one embodiment, the dielectric layer on the wire is formed of said compound. In one embodiment, said compound is grown to substantially fill out the space between the wire portions of the nanowire aggregate.

In one embodiment, said bonding substance is a silica precursor of a silica compound or an aluminum oxide precursor of an aluminum oxide compound.

In one embodiment, the step of bonding the nanowires includes growing an organo-silica compound on the nanowire portions.

In one embodiment, the method comprises the steps of providing a second bonding substance to the second phase;

forming a bonding layer from the second substance, which bonds to said head portions.

In one embodiment, the method comprises

bringing the nanowire aggregate into contact with a carrier member;

drying the nanowire aggregate;

removing the bonding between the wire portions;

providing a matrix material between the nanowires of the nanowire aggregate.

In one embodiment, the method comprises

removing the material layer from the nanowire aggregate;

providing an electric contact connected to the first ends of the nanowires.

In one embodiment, the second bonding substance is a monomer or a polymer material.

In one embodiment, wherein said bonding substance is a polymer or monomer material added to the second phase, the step of bonding including

growing a bonding layer from said bonding substance which bonds to said head portions.

In one embodiment, said fluid is provided with a modifying substance in a composition configured to counteract bulging of the interface.

In one embodiment, the method comprises the nanowires are provided in the second liquid, prior to combining the second liquid with the first liquid.

In one embodiment, the method comprises the step of adding a subsequent amount of the second liquid to the top phase, such that a plurality of nanowire assemblies are interconnected into a larger contiguous nanowire assembly.

In one embodiment, the modifying substance increases the relative density of the sub phase with respect to the top phase.

In one embodiment, the method comprises changing the composition of the sub phase subsequent to forming the nanowire aggregate.

In one embodiment, the composition of the sub phase is changed by extracting an amount of the first liquid from the sub phase, and adding an amount of liquid to the sub phase, wherein the added amount of liquid has a different composition than the extracted amount of liquid.

In one embodiment, changing the composition of the sub phase includes providing the first liquid to the sub phase with a first concentration of said substance exceeding a first level prior to providing the nanowires to the fluid, and

changing the substance concentration of the sub phase to a second concentration below a second level, which is lower than the first level, after forming the nanowire aggregate.

In one embodiment, said modifying substance includes at least one of acetone, acetonitrile, dimethyl sulfoxide, di ethylene glycol, dioxane, dimethoxyethane, dimethylformamide, tert-butyl alcohol, 2-propanol and isopropyl alcohol, which modifying substance is provided in the first liquid in a composition with a solvent at a predetermined concentration range.

In one embodiment, said modifying substance includes hexane provided in the first liquid in a composition with a solvent at a predetermined concentration range.

In one embodiment, said modifying substance is provided to said concentration in water.

In one embodiment, the nanowires are functionalized by means of a compound comprising a molecule chain connecting to the head portion.

In one embodiment, the head portion includes a particle, and the molecule chain is a thiol connecting by means of a sulfur atom to the particle.

In one embodiment, the compound is 1-octadecanethiol or polyethyleneimine. In one embodiment, the method comprises the nanowires are functionalized with a compound of the second liquid.

In one embodiment, the method comprises

bringing the nanowire aggregate into contact with a carrier member;

drying the nanowire aggregate.

In one embodiment, the carrier member is a substrate surface.

According to a second aspect, an aggregate of vertically assembled aligned nanowires is provided, wherein said nanowires comprise an elongate wire portion and a head portion at a first end of the elongate wire portion, comprising a bond configured to secure the nanowires in the aggregate.

In one embodiment, the said bond includes a compound grown on the wire portions of the assembled aligned wire portions to form a matrix connecting the nanowires. In one embodiment, the wire portions are provided with a dielectric shell, and said bond is grown on said shell.

In one embodiment, said bond and said shell is formed of a common compound. In one embodiment, the aggregate is formed by means of the method according to any of the preceding steps.

In one embodiment, a nanowire device comprises a nanowire film including a substrate; and

an aggregate of vertically assembled aligned nanowires according to any of the preceding embodiments, connected to the substrate.

In one embodiment, said nanowires are non-epitaxially connected to the substrate after formation of the aggregate.

In one embodiment, a photovoltaic device comprises the nanowire device of any of the preceding embodiments.

In one embodiment, a Li ion battery comprises the nanowire device of any of the preceding embodiments.

In one embodiment, a thermoelectric device comprises the nanowire device of any of the preceding embodiments.

In one embodiment, a thermal dispersion membrane comprises the nanowire device of any of the preceding embodiments.

Brief description of the drawings

Figs 1A to ID schematically illustrate various steps of a method of capturing and aligning an assembly of nanowires according to an embodiment.

Figs 2A and 2B schematically illustrate a phase interface including a nanowire aggregate, before and after fluid substance modification according to an embodiment.

Figs. 3 A and 3B are schematic illustrations of a functionalized nanowires according to different embodiments.

Fig. 4A is a schematic illustration of an aggregate of aligned and assembled nanowires at a fluid interface with head portions of the nanowires disposed in a top phase, according to an embodiment. Fig. 4B is a schematic illustration of an aggregate of aligned and assembled nanowires at a fluid interface with head portions of the nanowires disposed in a sub phase, according to an embodiment.

Fig. 4C is a schematic illustration of a bonded aggregate of aligned and assembled nanowires according at a fluid interface according to an embodiment, with a filler comprising the same material as a surface shell provided on nanowire portions of the nanowires.

Fig. 4D is a schematic illustration of a bonded aggregate of aligned and assembled nanowires according at a fluid interface according to an embodiment, with a filler comprising a material different from a surface shell provided on nanowire portions of the nanowires.

Fig. 4E is a schematic illustration of a bonded aggregate of aligned and assembled nanowires according at a fluid interface according to an embodiment, with a bonding layer.

Fig. 4F is a schematic illustration of a bonded aggregate of aligned and assembled nanowires comprising a filler forming a matrix between the nanowires, and a bonding layer on top of the nanowires.

Figs 5A-5C show schematic illustrations of methods of providing an assembly of aligned nanowires.

Fig. 6 is a schematic illustration of a container apparatus for transferring an assembly of aligned nanowires from a fluid to a substrate surface.

Fig. 7 is a schematic side cross sectional view of a nanowire device, e.g. a solar cell, according to an embodiment.

Fig. 8 is an image showing nanowires assembled using a method as provided herein.

Figs 9-11 show aggregates of aligned nanowires obtained according to various embodiment of the invention.

Figs 12 and 13 show aggregates of aligned nanowires obtained according to various embodiment of the invention.

Fig. 14 shows photographs of aggregates of aligned nanowires obtained without and with a filler between the nanowires. Detailed description

Various embodiments will be described below with reference to the drawings. The embodiments are to be seen as exemplary, and other ways of realizing the solutions provided within the scope of the claims are therefore foreseeable.

The invention relates generally to the forming of an aggregate of a nanowire assembly of nanowires which are aligned, in various embodiments also oriented in a common direction, at an interface which is formed between substantially immiscible first and second liquids of a fluid. This fluid containing nanowires may be formed in various ways, e.g. by first combining the first and second liquids and thereafter add the nanowires, or by adding the nanowires to one of the liquids as a nanowire ink before combining it with the other liquid. Suitably functionalized nanowires will then be prone to assemble at the interface. However, the art of capturing aligned nanowires on a substrate surface from such a fluid interface has still been associated with a number of hurdles, and there is a desire in raising the quality of such nanowire assemblies, in terms of e.g. nanowire alignment, mechanical stability and surface density.

Embodiments presented herein thus relate to providing a fluid to a container, which fluid comprises a first liquid, a second liquid and a plurality of nanowires, wherein the first and second liquids phase separate into a first phase comprising the first liquid, a second phase comprising the second liquid, and an interface between the first phase and the second phase.

In various embodiments, a method for increasing the quality of such nanowire assemblies includes preparing the fluid composition such that the nanowire aggregate is suitably formed at the interface. Furthermore, the fluid may be provided with a substance in a composition configured to counteract bulging of the interface. This may in various embodiments be obtained by selectively arranging the composition of that substance, such as by careful selection of the type of substance and its concentration. This way, the interface is stretched out, so as to be substantially planarized for suitable engagement with a planar substrate surface. In other embodiments, the fluid

composition may be modified in a subsequent step, after allowing the interface to form in the fluid, by addition of the substance in a composition, such that the interface is stretched out to be substantially planarized. This, in turn, makes it easier to engage the nanowire aggregate at the interface with a planar substrate surface. In this context, planarization aims at the effect of minimizing or decreasing bulging or bellying of an upper phase into a lower phase, i.e. increasing the curvature radius of the interface in a vertical plane. In the extreme case, this would mean going from a state where the second, top phase, liquid may be suspended as substantially spherical droplets at the surface of the first liquid 11, to a state where the second liquid floats on top of the first, sub phase, liquid with a substantially horizontal interface. Further embodiments related to decreasing bulging of the interface will be described further below.

With reference to Fig. 1 A, an embodiment of the method uses a first liquid 11 located in a container 1 and a nanowire dispersion 12, such as a nanowire ink, constituting a second liquid 12, that is added to the first liquid 11. The nanowire dispersion 12 is preferably made by dispersing prefabricated nanowires 20 in a dispersion liquid. That is, the nanowires are fabricated prior to being added to the dispersion fluid 12 in contrast to in-situ formed nanowires in the dispersion fluid. In this embodiment, the dispersion liquid 12 is selected such that the nanowire dispersion is immiscible or only partially miscible in the first liquid 11. In this manner, when the nanowire dispersion is added to the first liquid 11, the first liquid 11 and the dispersion liquid 12 phase separate, creating a two-phase liquid system. The denser liquid 11 settles to the bottom of the container 1 forming a sub phase, while the less dense liquid 12 floats on top of the first liquid 11 creating a top phase. The resulting two-phase system has a top phase and a sub phase and an interface 13 between the top phase and the sub phase.

In an embodiment, the nanowires 20 in the nanowire dispersion may be made of the same material. Alternatively, the nanowire dispersion 12 may include nanowires made of different materials. Nanowire materials suitable for use in the present embodiment and the embodiments below include metals (such as gold silver and alloys thereof), carbon nanowires or nanotubes (single wall and multiwall), semiconductors, including Si, III-V (including binary, ternary and quaternary III-V semiconductors made of Al, In, Ga, N, P, As, such as GaAs and InP) and II- VI semiconductors (including binary, ternary and quaternary II- VI semiconductors made from Zn, Cd, Se, O, S, Te, such as ZnO, CdSe) and ceramics. The nanowires 20 may be used as received or be subjected to one or more surface treatments described in more detail below.

The nanowires 20 may comprise an elongated wire portion and a head portion at a first end of the nanowire 20. In various embodiments, both opposite ends of a nanowire may be substantially similar and thus form first and second head portions. Preferably the nanowires have a differential surface character, providing different surface characteristics at the wire portion and the head portion with regard to each other. Such different surface character may be obtained by means of differential functionalization.

In various embodiments, differential functionalization may include specific functionalization of either the head portion/portions or the wire portion, or both but with different functionalization. Functionalization may be obtained by bringing the nanowires into contact with a material which preferentially adheres to one of said portions, or e.g.by growing a shell on various portions of the nanowire. In one embodiment, both the head portion and the wire portion may be functionalized, but with different functionalizing materials. In one embodiment, the differential functionalization may include providing the wire portions with a shell, such as a dielectric shell, e.g. Si02.

The head portion may be functionalized by bringing the nanowire into contact with a material which selectively adheres to the head portion to give it a predetermined character. Dependent on the material of the wire portion, a suitable functionalizing material may be applied to adhere to the head portion. In various embodiments, functionalization may be obtained by providing nanowires in a fluid dispersion containing, e.g. a thiol, which binds molecularly to the head portion. The head portion may be an end portion of the nanowire free from the functionalizing covering the wire portion, such as shell of e.g. Si02 provided on a semiconductor nanowire. For e.g. GaAs nanowires covered by a silica shell, a thiol may adhere to an end portion not provided by the shell. Various ways of selectively functionalizing nanowires in the form of carbon nanotubes have also been suggested, e.g. by Kyung Min Lee et al, in

Asymmetric End-functionalization of Multi- Walled Carbon nanotubes, J. AM. CHEM. SOC. 2005, 127, 4122-4123. Also, Sehmus Ozden et al have presented ways of creating ordered microstructures with hydrophobic and hydrophilic moieties on carbon nanotubes in Anisotropically Functionalized Carbon Nanotube Array Based

Hygroscopic Scaffolds, in Applied Materials & Interfaces, 2014, 6, 10608-10613.

In some embodiments, the head portion may comprise a seed particle, remaining from epitaxial growth of the nanowire. The seed particle may e.g. be a metal particle, such as gold. Dependent on the material of the seed particle, a suitable functionalizing material may subsequently be applied to adhere to the seed particle of the head portion. In various embodiments, opposite ends of the nanowire may form a pair of head portions which may be substantially equally functionalized, such as first and second end portions free from a shell otherwise covering the wire portion, of which one end portion may further comprise a seed particle. Test have shown that even in such an

embodiment, the nanowires will be prone to align in the interface of the sub phase and the top phase, due to the differential functionalization between the wire portion and the end portions. After adding the nanowire dispersion to the first liquid 11, a majority of the nanowires are then assembled at the interface. Typically, the nanowires

spontaneously assemble at the interface, that is, the nanowires self-align at the interface, if given sufficient time. However, the nanowires may be subjected to one or more conditions that promote or accelerate the assembly of the nanowires at the interface. Acceleration may be accomplished in several ways. For example, acceleration may be accomplished by changing the composition of the top phase, the composition of the sub phase or altering the temperature of the container.

In another embodiment, rather than adding the nanowire dispersion to the first liquid 11, the two-phase liquid system is formed first followed by adding nanowires to the system. Thus, a second liquid 12, different from the first liquid 11, may be added to the first liquid 11. Preferably, the second liquid 12 is immiscible or partially

miscible/partially immiscible in the first liquid 11. In this manner and similar to the previous embodiment, when the second liquid 12 is added to the first liquid 11, the two liquids phase separate, creating a two-phase liquid system. The denser liquid settles to the bottom of the container, while the less dense liquid floats on top of the first liquid 11, resulting in a two-phase system with a top phase, a sub phase and an interface 13 between the top and sub phases. In this embodiment, the nanowires or a nanowire dispersion may be added to the two-phase system or added to the first liquid 11 prior to adding the second liquid 12 to the first liquid 11. A nanowire dispersion comprises nanowires distributed in a dispersion liquid (e.g. a solvent). The dispersion liquid may be the same as either the first liquid 11 or the second liquid 12. Alternatively, the dispersion liquid may be a third liquid that is different from both the first and second liquids. Alternatively, dry nanowires may be added.

The mechanisms behind the nanowires being brought to the interface may in various embodiments be explained such that the interface is created at the nanowire head portion, such as a seed particle. The phase separation that occurs between the sub phase and the top phase "precipitates" from the seed particle. Any particle/liquid combination where this type of precipitation/phase separation occurs at the head portion of the nanowire while not occurring at the side wall, i.e. the wire portion, of the nanowire is a potential system for monolayer assembly and alignment of nanowires. Monolayer assembly of any order (non-aligned) is conversely created by having the precipitation/phase separation occurring at all of the nanowire surface. In a mix of 2 liquids A and B where the miscibility of A in B is lowered, eventually liquid A will separate into distinct droplets dispersed in B. When nanowires are dispersed in the mix, and the surface characteristic of the nanowires is such that phase separation is prone to be initiated at e.g. the head portion, there is a strong driving force for monolayer assembly of aligned nanowires. Theoretically, 2 conditions shall be met when phase separation occurs in such an embodiment: 1. Low interfacial energy between the two created liquids, and 2. Good wetting on the nanowire sides/wire portions of one of the liquids.

In an embodiment, the nanowires may be functionalized with either compounds that render the nanowires hydrophobic (including alkanes, fluoro-compounds (such as Pentanethiol, perfluorodecane thiol, dodecyltrichlorosilane, stearic acid, decyl phosphonic acid, 5 -(1,2- dithiolan-3-yl)-N-dodecylpentanamide, sodium dodecyl sulfate, triphenyl phosphine, octadecylthiol)) and/or hydrophilic (including sulphates, phosphates, carboxylates, amines, polyethers, (such as sodium mercaptopropane sulfonate, sodium mercaptoethane sulfonate, mercaptoalkane succinate (2- mercaptosuccinate), mercaptoalkane amine, (11-mercaptoundecyl)- Ν,Ν,Ν- trimethylammonium bromide, (12-Phosphonododecyl)phosphonic acid, (+)-l,2- Dithiolane-3-pentanoic acid, (2-Ammonioethyl)di-tert-butylphosphonium

bis(tetrafluoroborate), (3-Aminopropyl)triethoxysilane, 12-mercaptododecanoic acid)). In an embodiment, one part of the nanowire surface is rendered hydrophobic and the other part of the nanowire surface is rendered hydrophilic using different

functionalizing compounds to achieve the vertical alignment at the liquid interface. In an alternative embodiment, only one part of the nanowire surface is treated with a functionalizing compound.

Fig. 3A is a schematic illustration of a functionalized nanowire 20 according to an embodiment. As illustrated, the nanowire 20 has a head portion 23 which includes a seed particle 23, e.g. a metal particle such as a gold seed particle, at one end of the nanowire 20. The seed particle 23 may be a result of the growth process of the wire portion 21 (e.g. semiconductor portion) of the nanowire 20, such as when growing nanowires 20 by the vapor-liquid-solid (VLS) process using the seed particle 23 as a catalyst seed. Example processes for making nanowires 20 can be found in U.S.

provisional application 61/623, 137 filed on 4/12/12 and PCT published application number WO13/154490 A2, hereby incorporated by reference in their entirety. In an embodiment, a first functionalization 24 is provided to the head portion at one end of the nanowire 20, such as to the seed particle 23. A second, different functionalization (not shown in Fig. 3A) may be provided to the second end of the nanowire that lacks the seed particle 23.

Fig. 3B illustrates another embodiment, where a second head portion 23B is formed at an opposite end of the nanowire 20. In this embodiment, the second head portion may be a bare end portion of the wire portion 21. A functionalizing compound or material 24B may be adhered to the second head portion 23B, and may be the same as or different from the functionalizing material 24. Unless where reference is specifically made to two different functionalizing materials, these will be referred to as functionalization or functionalizing compound or material 24 herein.

Functionalization may be provided to configure at least one head portion 23 of the nanowire 20 to be more hydrophilic or more hydrophobic than the wire portion 21, particularly sidewalls of the wire portion 21, of the nanowire 20. The nanowires 20 may be provided with a dielectric shell on the wire portion 21, substantially encapsulating at least the sidewalls of the wire portion 21, which shell may comprise an inorganic material, which may be more hydrophilic than the head portion.

The functionalizing compound(s) 24 aid in aligning the nanowires 20 in the top 104 and bottom 102 phases. As discussed in more detail below in regards to specific examples, one of the functionalizing groups 24 may be 1-octadecanethiol (ODT), while a compound 24B attached to the lower end of the nanowire 20 may be (12- phosphonododecyl)phosphonic acid (PPA). Further, the functionalizing compound 24 may have a functional group, or a different one at each wire end 24, 24A, of the functionalizing compounds 24. Various types of functionalizing components and examples of such types are given in Table I of applicant' s own prior application WO2015/166416, hereby incorporated by reference in their entirety.

Based on the choice of functionalizing compounds 24 and the type and composition of the liquids 11, 12, the alignment and potentially even orientation of the nanowires may be controlled. In a similar manner, these parameters/compositional variables may allow the nanowire-nanowire interspacing to be varied, resulting in assemblies with different (i.e. pre-selected) densities (e.g. density of nanowires per square micron). The nanowire-nanowire interspacing can be deduced from the percentage of surface area covered after the capture of the aligned nanowires on the substrate.

In one embodiment, appropriate formation of a nanowire aggregate at the interface 13 can be obtained with a dispersion of nanowires 20 in a liquid 12 comprising a component selected from the group comprising one or more of toluene, hexane, octane, cyclohexane, cyclopentanone a thiol such as 1-octadecanethiol and

polyethyleneimine. In a preferred embodiment, the nanowires are functionalized with a component 24 of the second liquid 12. The functionalizing component or compound may comprise an atom or atoms prone to attach to the head portion 23 such as a seed particle, e.g. sulfur, and further a molecule chain providing hydrophobic properties to the functionalized nanowire 20.

In a preferred embodiment, functionalization is carried out in a separate step, prior to adding the nanowire dispersion to the first liquid 11. In one embodiment, functionalization may be carried out by mixing a solution of the functionalizing component or compound with an added amount of nanowires. An object at this point is adsorption of the functionalizing component only to the seed particle 23, typically a gold particle, in order to make the particle 23 highly hydrophobic, increasing in this way the Janus properties of the nanowire. Additionally, the inventors have found indications that covering the metal seed particle 23 with adhered molecules of the functionalizing component will decrease the surface charge density of the particle, which also results in a decrease in its electrostatic repulsive force

In one embodiment, the nanowires 20 may be provided with a dielectric surface coating 22 of a certain component about the wire portion 21. The dielectric surface coating 22 may e.g. comprise silica or aluminum oxide. With proper selection of the first liquid 11, the coated nanowire surface may be suitably wetted in the first liquid 11. This way, proper selection of the liquid composition of both the first liquid 11 and the second liquid 12 will help promote the formation of a nanowire aggregate 25, as indicated in Fig. 4, in which all or nearly all nanowires 20 are aligned and densely packed, and also correctly oriented,. In an embodiment where both first 23 and second 23B head portions are differentially functionalized with respect to the sidewall of the wire portion 21, the nanowires 20 will still be prone to alignment and dense packing. However, orientation may be uncontrolled, at least where the same functionalization compound is adhered at the opposite head portions 23/23B. The direction of each nanowire may rather be a question of coincidence or competition, between which of the head portions aggregate and assemble with neighboring nanowires 20. The first liquid 11 may have a composition comprising a concentration of first substance of at least one of acetone, acetonitrile, dimethyl sulfoxide, di ethylene glycol, and isopropyl alcohol. This first substance is provided to at a certain concentration in water, i.e. in an aqueous solution. By including this first substance to a certain concentration in the first liquid 11 in the sub phase the interfacial energy between the first liquid 11 and the second liquid 12, or air, is lowered and the nanowire surface charge density decreases. This contributes to the nanowires 20 pre-disposition to align vertically in the interface, and thus a tighter packing in the aggregate 25.

Fig. 4A schematically illustrates an obtained assembly of aligned nanowires 20 at an interface 13. The nanowires 20 are differentially functionalized to align substantially perpendicular to the interface 13. In this embodiment, head portions are configured to are located in the lower density top phase 12, whereas wire portions 21 are located in the sub phase. In one version of this embodiment, the sub phase may be aquatic, and the head portions may be functionalized to be hydrophobic. Hence, the nanowires 20 are configured to be arranged with head portions 23 pointing up.

Fig. 4B illustrates an alternative embodiment where the nanowires 20 are differentially functionalized to align substantially perpendicular to the interface 13. In this embodiment, head portions are configured to are located in the higher density sub phase 12, whereas wire portions 21 are located in the top phase. In one version of this embodiment, the composition of either the first fluid or the second fluid, or both, may be such that an aquatic solution is configured to have comparatively lower density. A hydrophobic functionalization at the head portions thus acts to arrange the nanowires with the head portions downwards in a substantially perpendicular alignment at the interface 13.

It may be noted that the arrangement as indicated in Figs 4A and 4B may be obtained also with a hydrophilic functionalization of the head portion and suitable composition of the fluids forming part of the top phase and the sub phase. Once a suitable aggregate 25 of nano wires has formed at the interface 13, transfer of the aggregate to a substrate surface or other carrier member may be carried out. However, due to the different characters of the liquids 11, 12, when a suitable composition for obtaining a tightly packed aggregate 25 of aligned nanowires 20 have been used, the fluid may look as in Fig. 1A dependent on the circumstances of the process. In a subsequent step, an additional amount of the second liquid 12 may then be added in one embodiment. This way, plural individual aggregates 25 may be affected to merge into a single top phase, as shown in Fig. IB. So, by first providing the second liquid 12 in a first amount, a larger overall interface (i.e. the sum of all aggregate 25 surfaces) will alleviate the transfer of nanowires from the bulk of the second fluid to capture at the interface 13. Later addition of a second amount of the second liquid 12, or an amount with similar properties as the second liquid 12, will assist in creating a single aggregate 25 with a contiguous interface 13.

In one embodiment, the fluid may be provided by first providing the first liquid 11, and subsequently adding the second liquid 12 including a dispersion of nanowires 20 onto the surface of the first liquid 11. In one embodiment, this may be obtained by spraying the second fluid 12 including the nanowires 20 onto the first liquid 11. In such embodiments, it has been found that a single top phase as shown in Fig. IB or 2A may be obtained, while avoiding the droplet character of Fig. 1 A.

In various embodiments, dependent on the selected properties of the liquids 11,

12, the interface 13, and hence the aggregate 25, may bulge towards the first liquid and have a severe curvature with a comparatively small radius, as indicated in both Figs 1A and IB. This can also be seen in Fig. 2A, although it should be understood that this drawing is highly schematic, particularly with regard to the nanowire size in relation to the container. Transfer of the nanowires to a planar substrate may nevertheless be obtained, e.g. by pulling a substrate at an angle from the first liquid 11. However, for larger substrates such a process may not be suitable and/or may not provide a high- quality assembly of nanowires. In various embodiments, a level of quality of a film of nanowires may correlate with absence or a low level of holes, cracks, or nanowires vertically aligned at a different height, which are types of defects that may be important challenges in applications such as solar cells. According to preferred embodiments, this may be avoided or alleviated by providing the fluid with a substance in a composition configured to counteract bulging of the interface 13. In one embodiment, this may be obtained by careful selection of the substance and its composition. The substance may be included in a composition which acts to increase the relative density of the sub phase with respect to the top phase. This may be obtained by including a substance to the second liquid in the top phase, such that its relative density is below a predetermined threshold compared to the sub phase. In an alternative embodiment, a careful configuration of the composition of the first liquid of the sub phase is rather obtained, by including an appropriate substance in a solvent.

In various embodiments, a step of modifying the fluid, such as modifying its substance composition, is carried out after the aggregate has formed at the interface 13, such that a curvature radius of the interface 13 is increased towards planarization.

In one embodiment, the modification of the fluid comprises changing the relative density relationship between the top phase and the sub phase. In one embodiment, this may be obtained by adding an auxiliary substance to the top phase, after formation of the contiguous aggregate 25 at the interface 13. As an example, hexane may be added to the top phase where the second liquid 12 is 1-octadecanethiol, such that the overall density of the modified top phase composition is decreased.

In an alternative embodiment, modifying the substance composition includes changing the composition of the sub phase subsequent to forming the nanowire aggregate. In such an embodiment, the composition of the sub phase may be changed so as to increase its density. This may be accomplished by adding an auxiliary component or substance to the first liquid 11 of the sub phase, so as to increase the density of the sub phase.

In one embodiment, the composition of the sub phase is changed by extracting an amount of the first liquid 11 from the sub phase, and subsequently adding an amount of liquid to the sub phase, wherein the added amount of liquid has a different composition than the extracted amount of liquid. This process has the benefit of ensuring that the composition of the added amount of liquid is properly mixed in the sub phase, since a prepared mixture of said different composition may be added to the sub phase.

In one embodiment, where the substance has higher density than its solvent, this process of changing the composition of the sub phase may include providing the first liquid 11 to the sub phase with a first substance of a first concentration exceeding a first level prior to providing the nanowires to the fluid, and changing the substance concentration of the sub phase to below a second level, which is lower than the first level, after forming the nanowire aggregate. In an alternative of this embodiment, where the substance has lower density than its solvent, the process of changing the

composition of the sub phase may include providing the first liquid 11 to the sub phase with a substance concentration not exceeding a first level prior to providing the nanowires to the fluid, and changing the substance concentration of the sub phase to above a second level, which is at least as high as the first level, after forming the nanowire aggregate. These embodiments have a benefit of not requiring the addition of auxiliary components or substances, but only modifying the concentration of a first substance in the first liquid 11 of the sub phase.

The inventors have realized that at least for certain types of liquids 11, 12, if the concentration of the first substance in the first liquid 11 is too high initially, the desired phase separation may not occur, and the nanowires 20 may disperse in the first liquid 11. On the other hand, if the concentration is too low, the nanowire aggregate 25 will not be properly formed with densely packed nanowires 20. In one embodiment, the first liquid 11 may initially contain said first substance in a concentration exceeding a first level A, so as to obtain proper wetting characteristics of the wire portion 21 of the nanowires 20 which preferably are coated 22. Dependent on specific choice of the first substance, the exceeded concentration level A may e.g. be 50%, or e.g. 70%, or e.g. 90%. In the step of modifying the composition of the fluid, the concentration of the first substance in the sub phase may be changed so as not to exceed a second level B, which may be the same or lower than the first level A. Dependent on specific choice of the first substance, the maximum concentration level B may e.g. be 50%, or e.g. 30%, or e.g. 10%. This embodiment may be suitably employed where the first substance in the first liquid 11 phase decreases the density of the sub phase, e.g. where the first substance is isopropyl alcohol or acetone. The modification to a lower concentration thus increases the density of the sub phase, which may contribute to planarizing the interface 13.

The result of the modifying step may be seen in in the exemplary drawings of Figs 1C and 2B, respectively.

The inventors have also found the surprising effect that in various embodiments it is possible to provide the fluid with a substance in a composition configured to counteract bulging of the interface, by careful selection of the fluid from the outset. In one embodiment, the addition of hexane in the first liquid 11, in an aqueous solution of a certain concentration range, means that subsequent addition of the second liquid 12 of any of the aforementioned types may be accomplished while still minimizing bulging of the resulting interface 13. In various embodiments, hexane may be included in a concentration of 10-60%, such as 10-40%, or even 10-30%, and preferably <30%, with a positive planarizing effect.

A benefit of such a substantially planar shape of the nanowire aggregate, with little or no bulging, is that it makes it possible to transfer the aggregate to larger substrate surfaces up to several decimeters or more than a meter wide, without adding defects such as cracks or holes that might appear due to the difference in curvature between the aggregate of aligned NWs and the planar substrate, as will be described below. In addition, the embodiments of planarizing the interface 13 carrying the nanowire aggregate 25 by means of changing the composition of the fluid has the benefit of being very fast, substantially instantaneous, since it is a physical effect of the changed composition.

In various embodiments, a bonding step is carried out, so as to increase the aggregation of the nanowires 20 and help them to keep together. This may be obtained by providing a bonding substance to one of phases. From the effects of that bonding substance, the nanowires of the nanowire aggregate bond together. Figs 4A and 4B schematically illustrate embodiments of such an assembled aggregate 25 of nanowires 20 at an interface. Reference will primarily be made to the embodiment of Fig. 4A for the purpose of describing further bonding, but it shall be understood that the corresponding features and process steps may apply to an embodiment arranged as in Fig. 4B.

Figs 4C and 4D illustrate embodiments where a bonding substance has been added to the liquid of a first phase, into which said wire portions project. In an embodiment corresponding to Fig. 4A, this first phase will be the sub phase, whereas for an embodiment as provided in Fig. 4B, the first phase may be the top phase. The bonding substance may include a precursor for growth of a bonding compound. When such a precursor contacts the nanowires, the bonding compound will grow on the elongate wire portions of the nanowires 20, i.e. on the nanowire sidewalls. In one embodiment, the bonding compound is grown to substantially fill out any space between the wire portions 21 of the nanowire aggregate 25, to form a bonding matrix. The bonding compound thus creates a filler between vertically aligned nanowires, with a thickness limited to the nanowires length. This layer provides mechanical stability, flexibility and improvement of the quality of the thin-film of aligned nanostructures, with potentially less defects, and an advantageous levelling effect where the top part of the nanowires will be all at substantially the same level in the aggregate.

Fig. 4C illustrates an where wire portions 21 were provided with a silica surface encapsulation 22. In this embodiment, the bonding substance may include a silica precursor of a silica compound. As an example, such silica precursors may comprise e.g. tetraethyl orthosilicate, 3-methacryloxypropyl trimethoxysilane,

glycidoxypropyltrimethoxysilane, 4-((3-( trimethoxysilane)propoxy)methyl)- 1 ,3- dioxolan-2-one, 3-methacryloxypropyl triethoxysilane. In such an embodiment, silica will grow on the wire portions 21, which are suitably pre-coated with a silica surface 22, to form a bonding matrix 22 of silica.

Fig. 4D shows another embodiment, where the bonding substance may be an aluminum oxide precursor. As an example, the aluminum oxide precursor:

trimethylaluminum. The substance added may further include a reagent, and potentially other ingredients as well. The amount of the precursor to be added may be determined based on the concentration of nanowires 20 in the fluid. Here, the previous surface encapsulation of e.g. silica is shown by dashed lines. In this embodiment, a bonding matrix 22A comprising aluminum oxide is formed, which preferably completely fills out any previous void or spacing between the nanowires 20.

As illustrated in Figs 4C and 4D, the bonding compound may be allowed to grow so as to suitably bond adjacent wire members of the nanowires 20 together. However, the substance will preferentially not cross the interface 13 into the top phase. The resulting nanowire aggregate 25 of densely packed aligned nanowires 20 will form a nanowire matrix which has a thickness substantially corresponding to the length of the nanowires 20.

The growing together of the aggregate by means of an added bonding substance has several benefits. First of all, it will make the aggregate stronger, and thus less prone to damage in the process of transferring it to a substrate or other carrier member.

Second, the subsequent step of drying the aggregate from the liquids of the fluid, can be drastically cut down, since there will be less liquids captured in the aggregate since it grows substantially entirely together into a solid body. Tests indicate that the drying time can be shortened from days to hours or minutes for an aggregate provided on a wafer up to 8".

Fig. 4E illustrates a process step of another embodiment, which may be combined with the embodiments of Figs 4C or 4D or carried out by itself from an assembly as shown in Fig. 4A or 4B. The drawing of Fig. 4E may also represent a bonding step, carried out so as to increase the aggregation of the nanowires 20 and help them to keep together. In this step, a bonding layer 41 is provided. This may be obtained by providing a bonding substance to the phase in which the seed particles 23 are disposed. From the effects of that substance, the nanowires of the nanowire aggregate will be held together. In this embodiment, a substance in the form of a polymer or monomer material may be added to the second phase. The bonding layer 41 may be allowed to form from the added monomer or polymer material to bond to the head portions 23, which may include seed particles, thereby adhering to the head portions 23 and holding head portions 23 of adjacent nanowires close together. The bonding layer 41 may be formed by growing the of a monomer or polymer layer from the head portions 23.

Alternatively, a bonding compound dispersed in the liquid of the phase where the head portions 23 are disposed, may substantially solidify by drying. In one embodiment, this bonding substance may comprise one or more precursors, configured to provide the bonding layer 41. In one embodiment, five different precursors are added: an activator, a ligand, a monomer, a reducing agent, and an initiator. Such activator may e.g. be

CuBr2. The ligand may e.g. be 2,2'-bipyridine, 4,4'-Di-5-nonyl-2,2'-bipyridine, 4,4',4"- tris(5-nonyl)-2,2':6',2"-terpyridine, 1,1,4,7, 10, 10-Hexamethyltriethylenetetramine, Tris(2-dimethylaminoethyl)amine, N,N-bis(2-pyridylmethyl)octadecylamine,

N,N,N',N'-tetra[(2-pyridal)methyl]ethylenediamine, tris[(2-pyridyl)methyl] amine, tris(2-aminoethyl)amine, tris(2-bis(3-butoxy-3-oxopropyl)aminoethyl)amine, tris(2- bis(3-(2-ethylhexoxy)-3-oxopropyl)aminoethyl)amine or Tris(2-bis(3-dodecoxy-3- oxopropyl)aminoethyl)amine. The monomer may e.g. be N-isopropylacrylamide (NIP Am), styrene, acrylates, methacrylates, acrylonitrile, (meth)acrylamides, 4-vinyl pyridine, dimethyl(l-ethoxycarbonyl) vinyl phosphate, 2-Acrylamido-2-methyl-N- propanesulfonic acid, methacrylic acid. The reducing agent may e.g. be

Tin(II)Ethylhexanoate, ascorbic acid, hydrazine or phenyl hydrazine. The initiator may e.g. be Bis[2-(2'-bromoisobutyryloxy)ethyl]disulfide. In one embodiment, a bonding layer 41 will form once one from each of the five precursor groups are added, and the bonding layer will grow only on the head portions 23 but not on the wire parts 21. The improvement of the bonding is in one embodiment improved by cross-linking the formed polymer.

If so desired, the bonding layer 41 may be grown to a substantial thickness, compared to the length of the nanowires 20, such as several hundred μιη or even 1 mm or more. This will make the bonded aggregate 25 convenient to handle in subsequent steps of separating it from the fluid and transferring it to a substrate.

Fig. 4F illustrates an aggregate 25 of nanowires which are assembled in a densely packed aligned arrangement, as described above. In this exemplary embodiment, the aggregate 25 has been formed by subjecting the assembled aligned nanowires 20 to a bonding step as primarily described with reference to Figs 4C and 4D to form a bonding matrix 22, and a bonding layer 41 formation step described by means of examples with reference to Fig. 4E. The bonding layer 41 makes the matrix-bonded aggregate 25 easier to handle, and decreases the risk for cracking of the matrix 22.

In the overall method of transferring an assembly of aligned nanowires from a fluid to a substrate surface, the step of bringing the nanowire aggregate 25 into contact with a substrate surface 31 is carried out such that a majority of the nanowires 20 are aligned with respect to each other on the substrate 30. In one embodiment, the floating aggregate 25 is captured using a suitable substrate, e.g. a piece of Si wafer. The substrate may be dipped into the fluid 10 close to the aggregate 25 at a certain angle, preferably not more than 30°, and carefully lifted in order to capture the formed nanowire array. In an alternative embodiment, a carrier member in the form of a conveyor belt film, such as a membrane, may be operated to pull up the aggregate from the fluid.

However, an alternative method for transferring the aggregate 25 to a substrate 30 is provided here, which is particularly suitable for larger substrates, e.g. larger than 2", up to 6" and more. Rather than "scooping" up the nanowire aggregate 25, a method of draining the sub phase is employed, such that the floating aggregate 25 is controlled to land on the surface 31 of a substrate 30, placed at the bottom of the container 1.

Fig. 6 schematically illustrates a container apparatus 60 for transferring an assembly of aligned nanowires from a fluid to a substrate surface in various

embodiments. The apparatus 60 comprises a vessel or container 1, and be made of any material suitable for holding the first 11 and second liquids 12 of a fluid 10 as disclosed. The container may thus have inner walls of e.g. glass, or a metal. A support member 2 may be provided at an inner bottom part of the container 1, having a substantially horizontal support surface 5 for supporting a substrate 30. The support member may thus be planar or e.g. comprise a net structure or other shape, providing a substantially horizontal support surface 5. In one embodiment, the container apparatus 60 may comprise at least one port or conduit 3, 4, at least for adding a liquid substance to the inner of the container 1. More particularly, said port 3,4 is preferably provided below the substrate support surface 5 of the support member 2, such that liquid may be suitably injected to or extracted from a sub phase, as indicated in e.g. Figs 1 and 2. In one embodiment, separate ports 3 and 4 may be provided for injection of liquid to the sub phase, and for extraction of liquid from the sub phase.

Example substrates that may be used include, but are not limited to, silicon, glass, plastic, molybdenum, silane modified silicon, gold, thiol modified gold or silicon surfaces with physically adsorbed cationic polymers. The substrate surface may be used as-received (i.e. bare), e.g. a clean Si wafer, which may comprise an oxide layer. In an alternative embodiment, the surface of the substrate is functionalized. The

functionalizing aids in securing the nanowires to the substrate surface. The surface of the substrate may be modified (e.g. functionalized) either by chemical reactions or physical adsorption of a functional species that includes specific functional groups. The assembly of nanowires may be transferred from the interface to the functionalized substrate surface as a result of electrostatic interactions between the aligned nanowires and a functionalized surface or as a result of van der Waals interactions between the nanowires and the substrate surface 31.

Fig. 14 shows photographs taken of nanowire aggregates 25, indicating a beneficial effect obtained by means of an embodiment exemplified with reference to

Fig. 4C. The photographs show aggregates of assembled aligned nanowires, which have been subsequently transferred to a substrate and dried. To the left, an aggregate without a bonding matrix is shown. In such a process, large bubbles will tend to form, providing an uneven nanowire film. In the example to the right, a silica filler 22 has been provided between the nanowires 20, e.g. by adding a silica precursor in the phase in which the wire portions 21 are disposed. The photograph shows that no bubbles have been formed, with a thin aggregate 25 substantially having a thickness corresponding to the nanowire length is obtained. In various discussed embodiments, the filler may be fabricated with dielectric material (Si02, Ti02). In the examples shown in Fig. 14, no additional provision of a bonding layer 41 has not been used, and a pure silica bonding matrix 22 is used. As a result, cracks may form in the dried aggregate. In an alternative embodiment, the bonding substance forming a filler may comprise a combination of organic and dielectric materials. Depending of the material chosen, the mechanical properties of the final thin-film may thus be configured to be more flexible, with a lower tendency to form cracks as a result. In an example of such an embodiment, a combination of organo- silica materials is selected as a filler, such as BTESE (Organosilica

bis(triethoxysilyl)ethane).

In one embodiment, a method may comprise the steps indicated in Fig. 5A.

Examples of liquids and other substances mentioned with reference to Fig. 5A may be found in other parts of this description.

A first step 51 includes providing a fluid 10 to a container 1, said fluid comprising a first liquid 11, a second liquid 12 and a plurality of nano wires 20.

In step 52 the first 11 and second liquids 12 phase separating into a first phase, a second phase, and an interface 13 between the first and second phases.

In accordance with step 53, the nanowires are functionalized to align vertically into a nano wire aggregate 25 at the interface, with a wire portion 21 in the first phase and a head portion such as a seed particle 23 in the second phase.

In step 54, which may be optional, a substance composition of the fluid is modified such that a curvature radius of the interface is increased towards planarization, e.g. according to any of the exemplary methods provided herein, in the sense that bulging of the interface is counteracted. As noted above, in alternative embodiments the effect of counteracting bulging may be obtained by providing the fluid with the substance composition from the outset, e.g. in the first liquid prior to adding the plurality of nanowires. In such an embodiment, step 54 would not be included as a separate step.

In step 55, which may be optional, an additional amount of the second liquid may be added to the top phase, such that a plurality of nanowire aggregates 25 are interconnected into a contiguous nanowire aggregate 25 e.g. according to any of the exemplary methods provided herein. Furthermore, this step may be carried out prior to step 54 in an alternative embodiment. In step 56 a bonding substance, which may include a combination of several substances, is provided to one of said first or second phases. In a first exemplary embodiment, this bonding substance may be a silica precursor, added to the first or second phase. In a second exemplary embodiment, this bonding substance may include various substances which are configured to interact be form a polymer or monomer material in the second phase.

In step 57 the nanowires of the nanowire aggregate are bonded together in said one phase using said bonding substance. In said first exemplary embodiment, this may be accomplished by matrix 22 formation, by allowing e.g. silica to grow on the sidewall surface of wire portions 21 of the nanowires 20, so as to interconnect adjacent nanowires 20. The said second exemplary embodiment, this may be accomplished by growing a bonding layer in the second phase, which adheres to the head portions 23, which may include seed particles 23.

In step 58 the nanowire aggregate is brought into contact with a carrier member. This may e.g. be accomplished by scooping up the aggregate 25, by using a conveyor, or as described with reference to Fig. 6.

In step 59 the nanowire aggregate is allowed to dry. In at least an embodiment where the step 57 of bonding the nanowires together involves growing the nanowire parts 21 together, the drying step may be shortened considerably.

Fig. 5B illustrates process steps of one embodiment, which combines the steps described with reference to Figs 4C or 4D with the step illustrated in Fig. 4E, so as to provide an aggregate as disclosed in Fig. 4F. For the sake of simplicity, this

embodiment will be described without the optional steps of Fig. 5A, which may nevertheless be included. In addition, steps 51-53 and 58-59 may be the same as for Fig. 5A, and will thus not be discussed.

In step 561, a first bonding substance is added to the first phase, in which the wire portions 21 of the nanowires are aligned. The first bonding substance may e.g. be a precursor for growth of a compound on the wire portions 21, such as on a dielectric shell 22 encapsulating the wire portions.

In step 571, a compound formed from the precursor substance may grow on the wire portions 21 of the nanowires, such that the nanowires 20 are bonded together to form an aggregate matrix. Under suitable growth conditions, substantially the entire spacing between the aligned wire portions 21 will grow together by means of the formed bonding compound 22, which may e.g. be silica. An intermediate product as schematically illustrated in e.g. Fig. 4C or 4D will thus be obtained.

In step 562, a second bonding substance is provided to the second phase, in which the head portions are suspended. The second bonding substance may include a number of substances which interact to form a polymer or monomer material, configured to adhere to the head portions 23, as described by means of examples above.

In step 572, a layer 41 formed from the second substance may form in the second phase with adhesion to the head portions 23, such that the nanowires 20 are bonded together. The bonding layer 41, which may be a monomer or polymer layer, may be grown to any suitable thickness. If high flexibility is desired, a bonding layer which is thinner than the aggregate of nanowires may be grown. If durability is more importance, a bonding layer which is thicker than the length of the nanowires 20 may be grown. An intermediate product will thus be obtained, as shown in Fig. 4F. In this embodiment, the comparatively thick and durable layer 41 will be usable for assisting further handling of the aggregate 25, since the bonding of the wire ends 21 in the first phase may be very brittle, seeing as the thickness of the aggregate 25 without the layer 41 may be only about one nanowire length, such as 1-5 μιη.

After the aggregate 25 has been brought into contact with a carrier member and been separated from the fluid and dried, further processing of the aggregate 25 may involve the steps of Fig. 5C.

Step 591 includes the step of providing an aggregate 25 of aligned nanowires 20 comprising an elongate wire portion 21 and a head portion 23 at a first end, where a bonding layer 41 bonds the head portions together, and wherein a compound 22 bonds the wire portions 21 together. In one embodiment, the bonding layer 41 may be a polymer or monomer material. The bonding compound 22 may in one embodiment be silica.

Step 592 involves removing the bonding between the wire portions. This may be accomplished by removing the compound that was grown on the wire portions. In one embodiment, where said compound is silica, the step of removing the bonding may be carried out by means of subjecting the wire part side of the aggregate 25 to an etching substance, such as HF. The aggregate will then still be held together by means of the bonding layer 41. In this sense, the bonding, e.g. the silica bond, is a sacrificial compound in this process embodiment. Step 593 involves providing a matrix material between the nanowires of the nanowire aggregate. This may be a suitable dielectric material planarization compound 74, e.g. Level M10, PMMA, PDMS, for filling gaps in between the standing nanowires 20 and creating a film where the nanowires are contained.

Step 594 involves removing the material layer 41, and optionally also the head portions, which may include seed particles 23, from the nanowire aggregate 25. This may be carried out by means of a suitable etching step, such as plasma etching or HF etching.

Step 594 involves providing an electric contact connected to the first ends of the nanowires. Various specific patterns for contacting nanowires 20 held in a matrix are plausible, and are known in the art, and this step is therefore not exemplified in any greater detail herein. This step may also involve contacting the other ends of the nanowires 20, at the end of the wire portions 21, opposite to the head portion 23. An example of a device 71 obtained by means of this process is schematically illustrated in Fig. 7.

Measurements carried out on substrate samples prepared with an embodiment of the suggested method, including the steps 54 and 55 of Fig. 5A, have shown very good results, and some test results are outlined in Table 1 below. The numbers therein indicate that a perfection level obtained using this method for transferring aligned and oriented nanowires from a fluid to a substrate surface is unprecedented.

Fig. 8 shows an image of a sample of an aggregate of nanowires 20 which have been assembled at the fluid interface 13, including one misaligned nanowire 20, which illustrates the aspect ratio of the nanowires 20. In this drawing, the nanowires are aligned and closely packed, but not additional bonding substance has been added. This corresponds to the schematic representation in Fig. 4A.

Alignment Orientation Close packing

Mean 94,4 98,4 58,3

Std Dev 11,1 2,3 17,2

Std Err Mean 0,7085 0,1482 1,0937

Upper 95% Mean 95,8 98,7 60,4

Lower 95% Mean 93,0 98,2 56,1

N 246 246 246

Table 1. Results of 246 measurements on 84 samples As can be seen from Table 1, 94.4% of the transferred nanowires are aligned within ±5 degrees from the normal direction. In addition, very close packing is obtained, where the number 58.3 indicates the surface density divided by theoretical max density of circles with diameter of the nanowire 21, which is the brighter center portion. This means a thicker shell or coating 22 will give a lower close packing value. In addition, the orientation obtained is 98.4%, meaning the relative number of nanowires ordered in the correct direction. These measurements have been obtained from samples ranging from 1-6".

After removal from the liquid at which the nanowire aggregate 25 is formed, a bonded nanowire film is obtained. The aggregate may be transferred to a substrate, in a single step or with several successive steps to prepare one or both aggregate surfaces, and the nanowire film may then include the substrate and the connected substrate. In this context, the aggregate 25 will be non-epitaxially connected to the substrate, in contrast to a device where a nanowire film is produced by growing the nanowires from the substrate.

In an embodiment, the nanowire film comprising the substrate 30 with the captured assembly 25 of nanowires 20 can be placed into a photovoltaic device 71, such as a solar cell, in an embodiment where the nanowires 20 have a pn junction, as shown in Fig. 7. It should be noted, though, that the substrate 72 illustrated in Fig. 7 may be the same substrate 30 to which the nanowire aggregate 25 was transferred, or a later substrate 72 connected to the opposing ends of the nanowires 20, after which the original substrate 30 is removed. Alternatively, substrate 72 may incorporate capture substrate 30, with additional layers or structures subsequently provided.

As schematically illustrated in Fig. 7, the substrate 72 may contain semiconductor

(e.g., GaAs, InP, etc.) nanowires 20 positioned substantially perpendicular (e.g., with the longest axis 80 to 100 degrees, such as 90 degrees) to the upper substrate surface. The nanowires 20 in this embodiment have an axial pn junction 21CC located between a lower first conductivity type (e.g., n or p type) segment 21 A and an upper second conductivity type (e.g., p or n type) segment 21B of the opposite conductivity type. In the solar cell 71, electrodes provide electrical contact to the nanowires 20. For example, the solar cell 71 may contain an upper electrode (e.g., transparent electrode) 73 in electrical contact with the upper segment 21B of the nanowires and an electrically conductive or semiconductor substrate 72 may provide an electrical contact to the lower segment 21 A of the nanowires 20. An insulating or encapsulating material 74 may be located between the nanowires 20. Alternatively, the nanowires may contain a radial rather than an axial pn junction, in which case segment 21B is formed as a shell surrounding a nanowire core 21 A such that the pn junction extends substantially perpendicular to the substrate capture surface.

Apart photovoltaic devices, the nanowire aggregate which may be produced with the methods disclosed herein may form part in other types of nanowire devices, such as battery anodes, bio-sensing platforms using nanostructures, thermoelectric devices, as membranes for enhanced thermal dispersion, and nanostructured membranes for filtration.

Fig. 9 illustrates an assembly of oriented nanowires that are tightly packed, and furthermore bonded by silica 22 as a filler between silica-coated wire portions 21 of the nanowires 20. In the picture, each aligned nanowire 20 has a gold nanoparticle 23 at the top, which is not really visible, and a downwards -extending wire portion 21. The assembly is captured on a substrate 72.

Fig. 10 shows a photograph of an assembly of aligned nanowires such as that of Fig. 9, taken from above. Comparing this picture with the one in Fig. 8, it is clearly evident that the nanowires have bonded, in this specific example by means of silica formed from an added precursor on the silica-coat 22 of the nanowires 20. At the center of each nanowire 20, a seed particle 23 of e.g. gold can also be seen. The assembly of Figs 9 and 10 correspond to the schematic representation in Fig. 4C, and may be provide using a method as described with reference to that drawing.

Fig. 11 shows a photograph of an alternative embodiment, provided based on the arrangement described with reference to Fig. 4B. In such an embodiment, the fluids separating into a top phase and a sub phase have been prepared or selected such that the nanowire aggregate 25 is formed with the assembled nanowires 20 being aligned with the head portion 23 in the sub phase, vertically in the interface 13. The photograph in Fig. 11 is a perspective view of an aggregate 25 of aligned and oriented nanowires, where the head portions are visible at the lower surface of the aggregate. The nanowires have bonded, in this specific example by means of silica formed from an added precursor on the silica-coat 22 of the nanowires 20. At the center of each nanowire 20, a gold seed particle forming part of the head portion is visible as a white dot. Fig. 12 is a picture from a lateral view of an assembly of aligned nanowires 20, where a bonding layer 41 has been grown onto the seed particles 23 at the top of the nanowires 20, in a process as described with reference to the schematic representation in Fig. 4E. In this example, there has been no added substance in the form of a precursor for bonding the wire parts 21 of the nanowires 20. As can be seen, the bonding layer is very thin in this example, compared to the length of the nanowires 20. The nanowires may be in the order of 3 μιη long.

Fig. 13 shows a picture of an assembly such as that of Fig. 12, from an elevated position. From this picture, it is evident that the polymer material has been formed onto seed particles 23 and interconnects them.

Although the foregoing refers to particular preferred embodiments, it will be understood that the invention is not so limited. It will occur to those of ordinary skill in the art that various modifications may be made to the disclosed embodiments and that such modifications are intended to be within the scope of the claims. All of the publications, patent applications and patents cited herein are incorporated herein by reference in their entirety.