Nanowire assembly

Abstract

A method for the dielectrophoretic assembly of nanowires onto a substrate, including the steps: (i) providing a suspension of nanowires in a carrier liquid; (ii) providing a substrate; (iii) providing an array of electrodes on an electrode array support; (iv) bringing the substrate into proximity with the electrode array support at a first surface of the substrate; (v) locating at least a second surface of the substrate, opposite to the first surface, in the suspension of nanowires; and (vi) applying a voltage signal to the array of electrodes. Accordingly, there is generated, through the substrate, a corresponding electric field in the suspension of nanowires and a dielectrophoretic force on the nanowires in the suspension of nanowires. This causes dielectrophoretic assembly of nanowires onto the second surface of the substrate in an arrangement corresponding to the array of electrodes.

Description

NANOWIRE ASSEMBLYField of the InventionThe present invention relates to a method for assembling nanowires, to an apparatus for assembling nanowires and to an assembly of nanowires, e.g. produced using such a method and / or apparatus. The invention has particular, although not necessarily exclusive, application to the assembly of nanowires in electronics.One-dimensional (1 D) nanomaterials (typically inorganic nanomaterials) are sometimes referred to as nanowires (NWs). Many different NWs are considered to be excellent candidate building blocks for electronics, optoelectronics, energy harvesting, and sensing devices and applications because of their high crystallinity, high aspect ratio, high surface-to-volume ratio, good mechanical properties, and low dimensionality.In the view of the present inventors, the use of NWs opens up several possibilities. NWs can be used in the fabrication of large-area electronic (LAE) devices. The use of NWs may permit overcoming state-of- the-art planar CMOS technology by scaling on-currents and switching speeds. Three-dimensional (3D) integrated circuits (ICs) may be possible by the incorporation of NWs. It is also possible that Moore’s law can be continued using NWs due to their higher integrability. Furthermore, the use of NWs may permit the development of electronics for “more-than-Moore” technologies by adding more functionalities to electronic chips. Still further, and not necessarily exhaustively, the use of NWs permits the design of electronic devices with different shapes.By way of further background, high-performance large area electronics (LAE), on rigid as well as on flexible substrates, is considered to be critical for next generation applications including wearable systems, electronic skin (eSkin) for robotics and prosthesis, mobile healthcare, Internet of Things (loT), smart cities, etc. [Refs. 1-5]. For example, tactile sensors on flexible substrates with capabilities of sensing surface texture, etc., will enable tactile feelings in robotics, and prosthetics [Refs. 6-9]. Such sensors require that the sensing system is conformably covered over components with either small or large area. However, currently available high-end sensing systems are rigid, typically because they are based on planar CMOS technology.Flexible electronics devices are known. Some are based on organic semiconductors-based electronics. However, to date such technology suffers in comparison to planar CMOS technology in terms of performance due to the poor electrical properties of these materials. At the current rate of progression it is considered that it may take another 20 to 30 years for organic TFTs to reach a frequency of 1 GHz [Ref. 10],Inorganic semiconducting NWs such as Si, Ge, ZnO, GaAs, VOx etc. in the sub-1 OOnm diameter range have shown excellent potential as building blocks in many LAE applications, where high-speed computation and communication is required for the outlined applications. Solution-processability,excellent electrical charge transport and flexibility are among many other appealing advantages inorganic NWs offer as an active channel material and / or electrode material [Refs. 11-13]. However, integration and assembly of these nanostructures at well-defined locations with high reproducibility and uniformity in a digital manner, is one of the biggest challenges for the development of NW-based high-performance LAE.Over the past several years, significant progress has been made to assemble NWs through various process including dry and solution-based schemes. For instance, one of the most exploited NW assembly techniques, the contact printing (CP) technique, has resulted in fabrication of high-performance flexible devices [Refs. 11 , 13, 14]. CP is a dry process which requires epitaxially grown vertically aligned NWs on expensive substrates and precisely controlled motors to create the ensemble of NWs on flexible substrates. The CP technique, however, poses practical challenges such as NWs breakage, singledirectional alignment, etc. while printing high aspect ratio NWs with diameter in the sub-20 nm regime. Moreover, the present inventors consider that CP techniques have critical limitations in achieving location-selective heterogenous integration of NWs with high density and uniformity.Alternatively, solution-based NW assembly (solution-processable) where NWs could be treated as printing ‘inks’ is an attractive technology for the development of high-performance LAE with possibility of roll-to-roll (R2R) manufacturing in a digital manner [Ref. 15]. Many solution-based assembly techniques have been demonstrated in the past including the Langmuir-Blodgett (LB) method [Ref. 16], solution dispersion methods including blown bubble approach [Ref. 17], and capillary force assembly [Ref. 18]. However, there are still some limitations inherent in these assembly methods. For example, these methods cannot easily accomplish precise positioning of NWs at desired locations [Refs. 16, 17]. Further, assembly using the LB deposition method, for instance, results in the reorganization of NWs during transfer from the air-liquid interface onto target substrates leads to overlapping features and gaps within the dense arrays [Refs. 19, 20]. Large-area alignment of multi-segmented NWs in nanoscale trenches facilitated by capillary forces has resulted in arrays of NWs with precisely controlled gaps varying between 2 and 30 nm [Ref. 18]. However, the use of high-resolution patterning techniques (electron-beam lithography) makes the process complex, time-consuming, and cost-ineffective and thus difficult to adapt for batch fabrication. Although many efforts have been made over recent years to adapt solution processable NW assembly processes, the above-mentioned drawbacks have not been fully addressed so far.Dielectrophoresis (DEP) is another versatile solution-processable NW assembly / manipulation technique which has gained interest for the assembly of functional nanoscale electronic and photonic structures for LAE [Refs. 21-24]. DEP is an attractive methodology offering precise positioning of semiconducting / metallic NWs while providing a high degree of material orientation in multi-directions. In known DEP processes, using pre-defined microelectrode pattern and applied voltage signals, a DEP force arises from an induced dipole moment embedded in a non-uniform electric field. Under inhomogeneous external electric fields, polarizable particles dispersed in liquid media undergomovement. The dielectrophoretic force (FDEP) acting on these particles depends on the size, geometry and the dielectric properties of the material and is given by the Claussius-Mosotti factor.Summary of the inventionThe present inventors have considered some drawbacks of known DEP processes. It is considered that in some known DEP processes, the enhancement of NW number density is not suitable in the DEP- attracting region. Some of these drawbacks are now explained. Attenuation of the electrical field once the NWs are assembled on the microelectrode pattern tends to curtail DEP forces to further enhance the aligned NW density and therefore leads to self-limiting of NW assembly [Ref. 25]. Further increase in voltage to enhance the NW concentration can result in burning of the MEA and NWs. Single use of MEA adds to the fabrication cost, and is not compatible with R2R manufacturing processes.Several of the listed references propose different approaches for DEP.Ref. 20 proposes the directed assembly of nanowires including assembly by DEP in which the parallel or quasi parallel assembly of nanowires is carried out over large areas of substrate using DEP. However, this reference explains a hindrance of such an approach, which is the necessity of incorporating electrodes to direct the assembly process. We refer to “quasi-parallel” since local forces can be difficult to predict.Ref. 21 proposes assembly of single nanowires through capillary-assisted DEP, in which a combination of capillary assembly and DEP is used to align NWs with respect to electrodes formed on a wafer. This is considered to be not suitable for large areas due to limitations of capillary action. Further, the capillary- assisted DEP of Ref. 21 suffers from issue of channel clogging.Ref. 22 proposes semiconductor nanowire placement through DEP and in particular considers the effect of variation of frequency of electric field on alignment behaviour of InAs NWs on fine electrode structures formed on an SiN layer on an Si substrate.Ref. 23 proposes fluid flow-assisted dielectrophoretic assembly of nanowires, and considered the effect of flow on DEP of SiC nanowires with respect to DEP electrodes formed on the substrate surface. This is not suitable for large area electronics on flexible substrates as desired flow may not be ensured.Ref. 24 models NW assembly by DEP. The model operates on the basis that the electrodes are formed on the substrate on which the NWs are to be assembled.Ref. 25 proposes self-limiting NW assembly onto electrodes, in the sense that each electrode pair only supports one NW between them. In the process, if another NW approaches, this will displace the originally-deposited NW, ensuring that only one is present between the electrodes. The NWs are Si NWs, and the electrodes are formed on the substrate. Considering the known dimensional variationsamong nanowires, this process is not suitable for large area electronics, which requires electronics layers based on ensemble of nanowires.The present invention has been devised in light of the above considerations.The present inventors have realised that significant advantages may be possible where the electric field for promoting DEP is provided by electrodes that are not formed directly on the substrate onto which the NWs are to be deposited. This constitutes a general aspect of the invention.Accordingly, in a first preferred aspect, the present invention provides a method for the dielectrophoretic assembly of nanowires onto a substrate, including the steps:(i) providing a suspension of nanowires in a carrier liquid;(ii) providing a substrate;(iii) providing an array of electrodes on an electrode array support;(iv) bringing the substrate into proximity with the electrode array support at a first surface of the substrate;(v) locating at least a second surface of the substrate, opposite to the first surface, in the suspension of nanowires; and(vi) applying a voltage signal to the array of electrodes thereby to generate, through the substrate, a corresponding electric field in the suspension of nanowires and a dielectrophoretic force on the nanowires in the suspension of nanowires, in order to cause dielectrophoretic assembly of nanowires onto the second surface of the substrate in an arrangement corresponding to the array of electrodes.In this way, the electrodes generating the electric field are not formed on the substrate on which the NWs are to be deposited. Instead, these electrodes are formed on an electrode array support that can subsequently be removed from proximity with the substrate. In Refs. 21-25 discussed above, the electrodes are formed on the same substrate (and indeed on the same surface of the substrate) on which NWs are to be deposited. Embodiments of the present invention therefore provide advantages in terms of efficiency of NW assembly, and optionally allow reuse of the array of electrodes, which lend themselves to industrial-scale manufacture of devices incorporating NWs.In a second preferred aspect, the present invention provides a device comprising an array of nanowires assembled onto a substrate, the device optionally being obtained or obtainable by a method according to the first aspect.In a third preferred aspect, the present invention provides an apparatus for the dielectrophoretic assembly of nanowires onto a substrate, the apparatus comprising: a chamber for holding a suspension of nanowires in a carrier liquid; an array of electrodes on an electrode array support; wherein, in use, the apparatus is operable to bring a substrate into proximity with the electrode array support at a first surface of the substrate and to locate at least a second surface of the substrate, opposite to the first surface, in the suspension of nanowires in the chamber, and to apply a voltage signalto the array of electrodes thereby to generate, through the substrate, a corresponding electric field in the suspension of nanowires and a dielectrophoretic force on the nanowires in the suspension of nanowires, in order to cause dielectrophoretic assembly of nanowires onto the second surface of the substrate in an arrangement corresponding to the array of electrodes.Optional features of the invention will now be set out. These are applicable singly or in any combination with any aspect of the invention.The nanowires may be ohmic electrical conductors at room temperature. Alternatively, the nanowires may be semiconducting at room temperature.The nanowires may have one or more composition selected from the group consisting of:Si, Ge; metal oxides such as ZnO, ln2O3, SnO2, VOx TiO2 etc.; and compound semiconductors such as GaAs, InAs, InP, GaP ZnS, ZnSe etc..The nanowires may have diameter (e.g. average diameter) of 100nm or less. The nanowires may have diameter (e.g. average diameter) of 90nm or less, 80nm or less, 70nm or less, 60nm or less, 50nm or less, 40nm or less, or 30nm or less.The nanowires may have a length (e.g. average length) of at least 1 pm. The nanowires may have a length (e.g. average length) of at least 5pm, at least 10 pm, at least 20 pm, at least 30 pm, at least 40 pm, at least 50 pm, at least 60 pm, at least 70 pm, at least 80 pm, at least 90 pm, or at least 100 pm.The nanowires may have an aspect ratio (e.g. average aspect ratio), which is the ratio of length to diameter, of at least 10. The nanowires may have an aspect ratio of at least 50, at least 100, at least 200, at least 300, at least 400, or at least 500.The dimensions mentioned above can be measured for example by microscopy, such as by SEM and / or TEM. Suitable average values can be determined by taking a random selection of about 1000 NWs in a field of view and determining the number average of the dimension of interest.The carrier liquid may be any suitable liquid in which the nanowires may be suspended. The carrier liquid may include a surfactant and / or a dispersant. In some embodiments, the nanowires may remain dispersed in the carrier liquid during use. If necessary, the method may include agitating the carrier liquid in order to maintain the dispersion of the nanowires in the carrier liquid. The carrier liquid may have a viscosity, measured at 25 °C and at atmospheric pressure, of at least about 0.5 cP to about 3 cP. The lower limit of this range may be at least about 1 cP. The upper limit of this range may be about 2.5 cP.The substrate may be monolithic. Alternatively, the substrate may be a layered substrate. In some embodiments, the substrate is rigid, and can remain substantially flat when held as a cantilever under its own weight. Such substrates are not intended in use to be subjected to bending. Alternatively, thesubstrate may be flexible. A flexible substrate may be of particular interest in the manufacture of flexible electronics. Furthermore, a flexible substrate may enable processing using roll to roll processes.The substrate may be formed of any suitable composition. Typically, it is intended that the substrate is a dielectric / insulati ng material. The substrate may for example be formed from a polymer material.During the implementation of the method of the invention, the substrate may have an area of at least 5cm x 5cm, more preferably at least 10cm x 5 cm, more preferably at least 20cm x 5cm or bigger. Such large area substrates are considered to be of particular use when implementing some embodiments of the present invention.The thickness of the substrate will determine, to a degree, its mechanical properties, such as its flexibility. The thickness of the substrate will also determine the proximity between the electrodes of the electrode array and the carrier liquid in which the nanowires are suspended. For example, the substrate may have a thickness of less than 1 mm. In some embodiments, the substrate may have a thickness of not more than 0.5mm, 0.4mm, 0.3mm, 0.2mm or 0.1 mm. The substrate may have substantially uniform thickness.In the implementation of the method of the invention, the array of electrodes on the electrode array support may be brought into direct contact with the first surface of the substrate. In this case, the distance between the array of electrodes on the electrode array support and the suspension of nanowires in the carrier liquid is the same as the thickness of the substrate.The array of electrodes on the electrode array support may be in the form of an array of electrode pads on the electrode array support. The electrode pads may be formed from deposited metal, for example by thick film printing or evaporation, sputtering etc. Alternatively, the electrode pads may be formed by subtractive processing of sheet metal. The electrode array support typically further comprises electrical connections arranged and connected to the array of electrodes in order to provide the required voltage signal to the electrodes. In some embodiments, the electrode array support is in the form of a roller or in the form of a belt driven by one or more rollers. In this case, the substrate may be held in tension and passed around part of the roller or belt in order to ensure uniform contact between the substrate and the electrodes at the first surface of the substrate. The use of a roller and / or belt enables implementation of a roll to roll process. In alternative implementations, the electrode array support maybe in the form of a plate. In this case, the substrate may be pressed against the electrode array support in order to ensure uniform contact between the substrate and the electrodes at the first surface of the substrate. The use of a plate may in some cases be better suited to a batchwise process.Accordingly, in some embodiments, the method further includes the step: after step (vi), removing the substrate from the electrode array support and re-using the same array of electrodes on the electrode array to repeat steps (iv) to (vi) for a further substrate, or a further part of the same substrate.During dielectrophoretic assembly, the substrate may be moved with respect to the carrier liquid. In practice, this may be implemented by moving the substrate with respect to the chamber in which the carrier liquid is retained. In the case where the electrode is part of a roller or is a belt driven by one or more rollers, the movement of the substrate may be driven by the movement of the electrode array support. In some embodiments, the substrate is moved at a speed of not more than 5mm per second, more preferably not more than 4mm per second, not more than 3mm per second, not more than 2mm per second, or not more than 1 mm per second. It is also found that the relative movement between the substrate and the carrier liquid has beneficial effects. Therefore, in some embodiments, the substrate is moved at a speed of not less than 0.01 mm per second, more preferably not less than 0.05mm per second, not less than 0.1 mm per second, not less than 0.2mm per second, not less than 0.3mm per second, not less than 0.4mm per second, not less than 0.5mm per second, or not less than 0.6mm per second.In the case where there is relative movement between the substrate and the carrier liquid during nanowire assembly, the electric field direction may be controlled so that preferred assembly orientation of the nanowires is substantially parallel to the relative movement direction between the substrate and the carrier liquid. This is found to result in suitable alignment between the deposited nanowires on the substrate.In some embodiments, it is of interest to carry out a further assembly of the same or different nanowires onto the substrate. These nanowires, if different, may be different in terms of composition, dimensions, aspect ratio or other attributes. If the same or different, these nanowires may be deposited so as to be oriented the same or differently to the initially deposited nanowires. They may be deposited so as to overlie, or partially overlie, the initially deposited nanowires.Accordingly, the method may further include, after step (vi):(vii) providing a second suspension of further nanowires in a carrier liquid;(viii) providing a second array of electrodes on a second electrode array support;(ix) bringing the substrate into proximity with the electrode array support at the first surface of the substrate;(x) locating at least the second surface of the substrate in the second suspension of nanowires; and(xi) applying a voltage signal to the second array of electrodes thereby to generate, through the substrate, a corresponding electric field in the second suspension of nanowires and a dielectrophoretic force on the nanowires in the second suspension of nanowires, in order to cause dielectrophoretic assembly of nanowires onto the second surface of the substrate in an arrangement corresponding to the second array of electrodes.Conveniently, in a roll to roll process, the same continuous substrate can feed through the apparatus so that while steps (i) to (vi) are being carried out on one part of the substrate, steps (vii) to (xi) can be carried out on another part of the same substrate.The method may include further processing stages. The apparatus may include corresponding apparatus features to enable such further processing stages. One or more of these may be intermediate the assembly of nanowires onto the substrate. For example, a further processing step may be an assessment of the position and / or alignment of the electrodes of an electrode array and / or of the position and / or alignment of the deposited nanowires. Another exemplary further processing step may be the assembly of further materials. For example, a dielectric layer may be deposited in a required pattern on the substrate. For example, a conductive layer may be deposited in a required pattern on the substrate.In particular, the conductive layer may be patterned in order to provide electrical contact to the nanowires, for example at opposing ends of the nanowires. Suitable layers may be deposited by printing. For example, gravure printing, relief printing, screen printing or inkjet printing.In some embodiments, the suspension of nanowires in the carrier liquid has a NW concentration of at least 1 pg / ml. In some embodiments, the suspension of nanowires in the carrier liquid has a NW concentration of at most 1000 pg / ml.The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided.Summary of the FiguresEmbodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which:Fig. 1 shows a schematic view of a simplified method and apparatus for contactless dielectrophoresis (cl- DEP) based assembly of nanowires (NWs) for Roll-to-Roll (R2R) electronic manufacturing.Fig. 2A shows a schematic view of the substrate and the NW suspension with the electrode array visible through the substrate and part of the electrode array support visible at the side edge of the substrate.Fig. 2B shows a schematic cross sectional view through part of the arrangement shown in Fig. 2A, indicating various forces acting during the NW assembly process. The NW will hover above the electrodes if the repulsive force between the NW and substrate surface is larger than the DEP force pulling the NWs towards the surface and the van der Waals force.Figs. 3A-3C show modelling results of the electric field strength in the NW suspension at different applied voltages between the electrodes.Fig. 4 shows a schematic view of a method and apparatus for contactless dielectrophoresis (cl-DEP) based assembly of nanowires (NWs) for Roll-to-Roll (R2R) electronic manufacturing. Electronic layers are assembled at desired locations on the substrate and the drawing schematically illustrates the manufacture of a high-performance device, with CMOS circuits based on cl-DEP assembled electronic layers.Fig. 5 shows a schematic cross sectional magnified view of the roller, electrode array support and substrate used in Fig. 4 with an indication of the assembly location for the nanowires on the substrate.Fig. 6 shows a series of micrographs showing the results of assembly of vanadium pentoxide NWs under different conditions.Fig. 7 shows a micrograph of a 10cm2region of a substrate with NWs deposited.Fig. 8 shows a schematic perspective view of a substrate with a heterogenous assembly of NWs, formed simply by tuning the DEP parameters.Fig. 9 shows an arrangement of DEP electrodes on an electrode array support.Fig. 10 shows an enlarged view of the part of Fig. 9 indicated with a dashed outline.Fig. 11 shows an SEM image of a substrate that has been subjected to a process according to an embodiment of the invention using the electrode array support of Figs. 9 and 10. Note that the NWs are not visible in this image, but the electrode arrangement can be seen through the substrate. More specifically, Fig. 11 shows the layered structure of the contactless DEP with patterned electrodes on transparent polyamide on the bottom and a tape in the middle with the aligned nanowires on the flexible and transparent PET substrate.Fig. 12 shows a magnified SEM image of the substrate of Fig. 11 at the region indicated in Fig. 11 using a dashed box, the aligned VOx NWs being visible in this image. Scale bar indicates a length of 50 pm.Detailed Description of the InventionAspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.Following on from the discussions above, the present inventors propose and disclose in some embodiments a novel contactless dielectrophoresis (cl-DEP) process for the large area assembly of multimaterial NWs (heterogenous integration) in a Roll-to-Roll (R2R) manner. The heterogenous assembly of NWs is simply achieved by tuning the process parameters such as voltage and frequency of the signal applied to the electrode array on the electrode array support.The developed approach has potential for layer-by-layer integration of NWs for the development of 3D multifunctional vertical electronics. Further, the approach can be used for the assembly of NWs in multidirections at different angles for shape-free electronics, a feature not considered possible with previously demonstrated NW assembly techniques.Compared to known DEP processes, embodiments of the present invention are R2R compatible and further, the electrode arrays (microelectrode arrays (MEAs)) can be re-utilised to assemble NWs over different target device substrates.Preferred embodiments of the invention reduce not only the number of manufacturing steps, but also the need for energy, time, consumables as well as the electronic-wastage, providing a step towards circular electronics.As will be understood in the light of the present disclosure, rational assembly of inorganic NWs using low- cost solution-processable technique into well-defined 3D architectures over large device area in a R2R manner will open avenues for high-performance LAE.Fig. 1 shows a schematic view of a simplified method and apparatus (10) for contactless dielectrophoresis (cl-DEP) based assembly of nanowires (NWs) for Roll-to-Roll (R2R) electronic manufacturing.Fig. 1 illustrates the design structure, assembly parts and various process steps followed for the successful integration of NWs from the solution and device fabrication. The process flow for the contactless integration of NWs is as follows: In the beginning, the target device substrate 12, onto which the NWs will be assembled, is passed around guide roller 14 into an open container 16 containing a suspension 18 of NWs in a carrier liquid. Electrode array support cylinder 20 is located partially within container 16 so as to be partially immersed in the suspension 18. The cylindrical surface of the electrode array support cylinder 20 has an array of printed electrodes 22. In use, the substrate 12 is guided to have its first surface in contact with the electrode array support cylinder and for its second surface to be in contact with the suspension of NWs. The electrodes are subjected to a voltage signal to generate, through the substrate 12, a corresponding electric field in the suspension of NWs and a dielectrophoretic force on the nanowires in the suspension of nanowires. This causes dielectrophoretic assembly of NWs 24 onto the second surface of the substrate 12 in an arrangement corresponding to the array of electrodes 22.The substrate 12 with the assembled NWs exists the container 16 and passes around a further guide roller 26. Between guide roller 26 and still further guide roller 28 is located an electrode contact deposition station 30. Electrode contacts 32 are formed in contact with the NW assemblies by a process such as gravure printing of electrode ink 34 using gravure printing roller 36.As will be understood, the apparatus can be re-used to assemble NWs on a further substrate, as required.The process is shown in more detail in Figs. 2A and 2B.Fig. 2A shows a schematic view of the substrate 12 and the NW suspension 18 at the surface of the NW suspension, so that the meniscus 18a of the suspension is indicated. Nanowire 18b is schematically indicated as aligning with the electrode field direction, the electric field lines 23 being schematically indicated. The electric field is generated in view of the signal applied to the electrode arrays 22a, 22b on the surface of the electrode array support cylinder 20.Fig. 2B shows a schematic cross sectional view through part of the arrangement shown in Fig. 2A, indicating various forces acting during the NW assembly process. The NW will hover above the electrodes if the repulsive force between the NW and substrate surface is larger than the DEP force pulling the NWs towards the surface and the van der Waals force (FVDW). In relation to these drawings, the x axis direction can be considered to be the vertical direction, the z axis direction can be considered to bee the direction perpendicular to the plane of the substrate and the y direction is a direction perpendicular to the vertical direction and parallel to the plane of the substrate.The DEP force (FDEP) exerted on a high aspect-ratio oblate spheroid (one-dimensional nanomaterials) can be approximated by the analytical expression:where E is the root-mean-square [rms] value of the electric field, smis permittivity of the medium, a is the long and b is the short radius of the ellipsoidal particle, respectively and Re [K (OJ)\ is given by the real part of the complex Clausius-Mossotti factor K (to). The applied voltage has a great influence on the generated force due to the quadratic dependence. The Clausius-Mossotti factor contains information about the permittivity of both the medium (solvent, i.e. carrier liquid) and the NW as well as the signal frequency. In the present set-up, since the substrate 12 is in continuous rolling motion, drag force plays a role in the alignment of NWs as well. Thereby, the NW assembly process depends on the interaction between various available forces in the suspension, namely dielectrophoretic (FDEP), hydrodynamic drag force (FD), kinetic energy due to Brownian motion, and electrostatic double-layer (FEDL), as shown in Fig. 2B. Accordingly, materials with different dielectric properties, and volume will experience different FDEP at similar voltage and frequency. Thus, with the careful tuning of the applied signal voltage, signal frequency as well as the MEA architecture that is employed, heterogenous integration of NWs is possible. Accordingly, it is proposed that the suspension 18 need not contain only one type of NW (e.g. composition) but can contain different types of NW and these can be assembled as desired in a single assembly operation, based on the applied signal voltage, signal frequency and the MEA architecture.In known DEP processes (where the NW solution and the MEA come into direct contact), low voltages (about 10 V) create a strong enough gradient to trap NWs. However, in the present work, a relatively thick dielectric layer (substrate 12) exists between the MEA and the NW suspension. Consequently, the DEP force is considerably reduced due to the attenuated electric field. To better visualize the distributed strength of the generated electric field, through the dielectric layers, the electric current module of COMSOL Multiphysics 5.3 software package was used to simulate the generated electric field and thisconfirmed the electric field shape indicated in Figs, 2A and 2B. This confirmed a DEP force of about 5 x IO11N is possible when using a substrate of thickness 55 pm and with a direction towards the gap between the electrodes.Figs. 3A-3C show modelling results of the electric field strength in the NW suspension at different applied voltages between the electrodes, this time with substrate thickness of 10 pm. The spacing between the electrodes is about 50 pm.Fig. 4 shows a further adapted method and apparatus 100 for roll-to-roll (R2R) manufacturing. This is presented as a modification of the embodiment described with respect to Fig. 1 and uses two NW baths and a modified electrode array for the NW assembly processes. This modified R2R equipment also exploits contactless-DEP forces to uniformly affix NWs over an entire substrate area or at defined locations - an approach which is inherently efficient in terms of material, energy, and water usage. Fig. 4 illustrates the design structure, key assembly parts and various process steps for the successful integration of NWs from the suspensions and the device fabrication sequence.Substrate roll 121 holds a supply of substrate 120 and gradually rotates to let out the substrate 120 according to demand. The NWs are to be assembled on the substrate 120 as described below. Substrate 120 is passed around guide roller 140 into first open container 160 containing a first suspension 180 of NWs in a carrier liquid. The electrode array support assembly, rather than being the form of a single cylinder as in Fig. 1 , takes the form of an endless belt 202 with the required electrode array printed on an external surface of the belt. Endless belt 202 is driven around an arrangement of three guide rollers 204, 206, 208 so that part of the endless belt is immersed in the suspension 180.As already mentioned, the outer-facing surface of the electrode array support belt 202 has an array of printed electrodes 220. In use, the substrate 120 is guided to have its first surface in contact with the electrode array support belt 202 and for its second surface to be in contact with the suspension of NWs. The electrodes are subjected to a voltage signal to generate, through the substrate 120, a corresponding electric field in the suspension of NWs and a dielectrophoretic force on the nanowires in the suspension of nanowires. This causes dielectrophoretic assembly of NWs 240 onto the second surface of the substrate 120 in an arrangement corresponding to the array of electrodes 220.The substrate 120 with the first arrangement of NWs 240 is them guided around further rollers 142, 144 to reach a further electrode array support assembly. This has the same general form as the electrode array support assembly already described. Substrate 120 is passed around guide roller 144 into second open container 162 containing a second suspension 182 of NWs in a carrier liquid. The electrode array support assembly, rather than being the form of a single cylinder as in Fig. 1 , takes the form of an endless belt 212 with the required electrode array printed on an external surface of the belt. Endless belt 212 is driven around an arrangement of three guide rollers 214, 216, 218 so that part of the endless belt is immersed in the suspension 180.From NW suspension 182, therefore, further NW assemblies are formed on the substrate 120.Cameras 130, 132 monitor alignment of the electrodes.The substrate with the assembled NWs then passes around further guide roller 215 and around drive roller 217 which is rotated as indicated.Camera 134 monitors the alignment of the NW assemblies 240.The substrate 120 next encounters a dielectric layer deposition station 300. Dielectric layers 320 are formed in contact with the NW assemblies by a process such as gravure printing of dielectric material 340 using gravure printing roller 360.Electrode contacts 322 are then printed using an inkjet printer head 324.Further downstream manufacturing steps are possible, such as the completion of CMOS devices and circuits and forming working sensors.As for the embodiment of Fig. 1 , the substrate is in continuous translational motion with respect to the NW suspensions and hence drag force also plays a role in the alignment of NWs. Thus, the NW assembly process depends on the interaction between various available forces in the solution, namely dielectrophoretic (FDEP), hydrodynamic drag force (FD), kinetic energy due to Brownian motion, and electrostatic double-layer (FEDL).Similar to the scheme described with respect to Figs. 1 -3, the modified R2R cl-DEP system 100 of Fig. 4 can be optimised by carefully tuning the applied voltage and frequency, and suitable MEA architecture, to have heterogenous integration of NWs either from the same NW suspension carrying multiple types of NWs or using multiple baths with similar type of NWs. Likewise, layer-by-layer integration of same or different types of NWs is possible. Further, with suitable design of MEAs, it is possible to assemble NWs in multi-directions at different orientation angles - thanks to the nature of contactless-DEP mechanism. This unique feature allows the development of electronics in different shapes (e.g., Ag NWs assembled in flower shape). Such a feature is not possible with the previously demonstrated NW assembly techniques such as contact printing, transfer printing, Langmuir-Blodgett method etc.Note that it is not proposed in the present embodiments that the electrodes for DEP are formed on the first surface of the substrate 12, 120. Instead, what is proposed, in the embodiments, is for the electrodes to be brought into contact with the first surface of the substrate while attached to an electrode array support. This is explained with respect to Fig. 5. Fig. 5 shows a schematic cross sectional magnified view of part of the electrode array support assembly located in first container 160 of Fig. 4. Roller 206 guides endless belt 202 (only partially shown) which has microelectrodes 220 formed on an outer surface of the belt 202. Substrate 120 (also only partially shown and with straight and curved sections for ease ofdrawing) is brought into contact with the electrodes on 220 on the endless belt and immersed in the NW suspension 180. With a suitable voltage signal applied to the electrodes, NW assembly takes place on the second surface of the substrate 120 to form aligned NW assemblies 240.The advantages of having such an assembly are that it is possible to re-use the electrode array support and thus the process becomes cost-effective and eco-friendly as there is no need to repeat the photolithography process to realise another set of MEAs each time. In some respects this is similar to reusing a mask in a standard fabrication process. Repeating the photolithography and etching steps each time is deleterious to the environment as these processes generate significant quantities of chemical waste (subtractive manufacturing), and consume considerable amounts of energy and water. As will be understood, a similar explanation applies for the cl-DEP system shown in Fig. 1 .Using a cl-DEP method and apparatus substantially as described with respect to Fig. 4, the present inventors have achieved the integration of zinc oxide (ZnO) NWs (length of about 20-100 pm and diameter of about 40-100 nm), vanadium pentoxide (V2O5) NWs (length of about 100 pm and diameter of about 40 nm), and silver (Ag) NWs (length of about 10 pm and diameter of about 40 nm).In this section, we present the obtained results of V2O5 NWs. The fabrication of high-performance NW- based electronics demands highly aligned structures to reduce the electron scattering at the NW-NW junction. These scattering events led to the increase of total channel resistance which reduces the device performance. Achieving such conformations requires consideration of the different variables influencing both the NW density and alignment quality, obtained through dielectrophoretic assembly.In the present assembly approach, the present inventors considered parameters including applied signal voltage, frequency, rolling speed and rolling cycles.Fig. 6 shows a series of micrographs showing the results of assembly of vanadium pentoxide NWs under different conditions. The substrate thickness used is 100 pm. cl-DEP parameters, Voltage (Vpp) (Fig.6 (row 1)), frequency (Fig.6 (row 2)) and dip coating speed (Fig.6 (row 3)) were tested empirically and analysed through scanning electron microscope images. These results are shown in Fig. 6. First, the amplitude of applied voltage signal is studied with an electrode gap of 10 pm, withdrawal speed of 1 mm / s, 1 MHz frequency. At 40 VPPthe generated field gradient is not strong enough to start the NW assembly. Further raising the voltage to 100 Vpp led to a strong enough DEP force resulting in aligned NWs. At 200 Vpp the density raised to 9 aligned NW / pm. The density of assembled NWs was extracted using the SEM images shown in the middle and right images of the top row of Fig. 6. Increasing the applied voltage results in higher NW trapping. This is expected as at higher applied voltages, the DEP force decreases the NW-surface separation to a distance where van der Waals forces dominate (at about 10 nm), and the NWs conform to the surface in close contact. However, in the present case, we must be careful as reaching higher values of voltage can cause dielectricbreakdown. To avoid the use of high voltages, other DEP parameters such as dip speed and number of dipping cycles can be tuned to achieve high density of aligned structures.Note that the first row middle and first row right images in Fig. 6 are high magnification SEM images depicting the density increment of aligned V2O5 NWs with respect to voltage change from 100-200V. The scale bar is 50 pm.Also note that in the images, there is no electrode formed above the substrate in contact with the NWs. Instead, for the images were electrodes can be seen, these are DEP electrodes underneath the substrate.Next, the frequency is varied between 10 kHz to 1 MHz while maintaining the voltage at 200 VPP. At low frequencies (100 kHz) few NWs were assembled. This is attributed to the presence of electro-fluidic effects in aqueous solution (such as electro-osmosis and double layer screening) restricting the DEP to become a more dominant force in the system [see Ref. 22]. At 500 kHz the assembly density of the NWs starts to increase. When raising to 1 MHz, better alignment and higher density of NWs were assembled. The long-axis component of Clausius-Mossotti factor causes the NW to follow the field lines, increasing the frequency creates a compromise between the long and short axis component. This compromise leads the NW to follow the gradient while maintaining the alignment with the field lines, resulting in a high- density aligned assemblyShear force plays a dominant role in the alignment of NWs. In the present set-up, the NW assembly process depends on the interaction between various available forces in the solution, namely dielectrophoretic (FDEP), hydrodynamic drag force (FD), kinetic energy due to Brownian motion, and electrostatic double-layer (FEDL), as shown in Figs. 2A and 2B.The target substrate is dipped into the NW dispersion and withdrawn by using a mechanical motor with programmable pulling speeds. The pulling speed plays an important role in the alignment process. For instance, high pulling speed can disadvantageously avoid NW assembly at the desired assembly sites. The speed at which the substrate moves through the solution creates a drag force on the trapped NWs. The drag force on a NW parallel as well as perpendicular to the flow can be calculated theoretically, neglecting wall effects (data not shown).For efficient capturing of NW, the FDEP tangential to the surface should be larger than the viscous drag produced by the movement of the substrate with respect to the liquid, and the repulsive FEDL between the NW and substrate surface balances the FDEP normal to the surface.We have studied NW assembly for two different scenarios: (i) when the captured NW’s long axis is perpendicular to the FD and (ii) NW long axis is parallel to the FD. When the NWs are perpendicular to the FD, distribution of the orientation angle is narrow in case FDEP > FD, otherwise, mis-aligned NWs areobserved for FD > FDEP. In the second case, when the NWs are parallel to the drag force, assembly of NWs have shown better alignment.For medium pulling speed, the values of drag force are obtained in pN which are in the range of FDEP. However, at higher pulling speed (>1000 pm / s), the NW density is highly affected, as the drag force surpasses the DEP trapping force. It is noted that medium speed revealed a higher alignment quality than lower speed in parallel configuration. This can be understood with the fact that shearing forces (drag force in the present case) contribute to the NW alignment. As the NWs are captured, the drag force will aid the alignment by acting sideways on the structure. This occurs when the drag force is not high enough to remove completely the structure but sufficient to reposition the orientation of the structure.Accordingly, with embodiments of the present invention, various advantages are revealed.Large area NW assembly: Arrays of NW can be assembled at well-defined locations over large areas. The assembled NW can be configured as integrated sensors, transistors, and / or other electronic active components for an all-nanowire circuitry with sensing capability. The NW assembly is conducted on nearly 300 electrodes over an area of 10 cm2with a yield of 100%. As shown in Fig. 7, NWs are efficiently captured over areas above electrodes. The area of substrate shown is 10cm2. Fig.7 shows an optical micrograph of large area (10 cm2) uniform alignment of V2O5 NWs intended to demonstrate the capability of the proposed cl-DEP system. The bright spots are the NWs aligned locations. NWs tend to miss the alignment if the there is any discontinuity in the MEA electrodes which lies in the bottom surface. A group of broken electrodes is indicated by a white rectangle drawn on the image.Multi-material NW assembly (Heterogenous integration): Theoretical understanding of the cl-DEP technology parameters is achieved using COMSOL software. Using the COMSOL, the effect of material properties, voltage, frequency, dielectric thickness, etc. over dielectrophoretic forces is simulated and analysed. The gained knowledge is used to experimentally demonstrate heterogenous assembly of vanadium oxide (V2O5), zinc oxide (ZnO) and silver (Ag)) NWs with high density (9 NW / pm). Fig. 8 schematically shows the assembly of different material NWs 240a, 240b, 240c, 240d on substrate 120 using the cl-DEP technique.Multi-directional assembly of NWs for crossbar structures: Using the cl-DEP, complex crossed NW arrays can be prepared with layer-by-layer assembly using single MEA but by simply changing its directions for sequential steps.Three-dimensional NW integration for vertical electronics: The 3D integration of flexible chips is expected to offer major advantages in the future for miniaturized high-performance electronics. However, materials- and fabrication-related challenges have presented major obstacles in achieving truly 3D integrated circuits based on the conventional Si CMOS technology, and the need for a new technology remains critical. The monolithic integration of parallel arrays of NWs in a layer-by-layer manner as multifunctional circuits is a suitable approach to realise vertical electronics. The present novel cl-DEPbased NW assembly approach can overcome processing limitations of conventional planar CMOS technology and thus provides methods for the manufacture of high-performance 3D integrated circuits.Fig. 9 shows an arrangement of DEP electrodes on an electrode array support.Fig. 10 shows an enlarged view of the part of Fig. 9 indicated with a dashed outline.Fig. 11 shows an SEM image of a substrate that has been subjected to a process according to an embodiment of the invention using the electrode array support of Figs. 9 and 10 and further discussed below. Note that the NWs are not visible in this image, but the electrode arrangement can be seen through the substrate.Fig. 12 shows a magnified SEM image of the substrate of Fig. 11 at the region indicated in Fig. 11 using a dashed box, the VOx NWs being visible in this image.Mimicking different electrode shapes to fabricate shape-free electronics: One of the advantages of using 1 D material is having superior mechanical properties such as higher flexibility. Because of this feature, NWs with very high aspect ratio can be bent to take any desired shape. This feature has not been achieved previously because of the absence of NW assembly techniques than can execute such a complex task. Using the present cl-DEP approach, NWs can be aligned into any desired shapes. For example, increasing the number of dipping cycles led to the replication of alignment electrodes, as shown in the Fig. 12. The figure shows the SEM image of assembled NWs mimicking the shape of DEP electrodes. Accordingly, by shaping the electrodes in desired shapes we can fabricate NW based morphable electronics.Contactless and Roll-to-Roll compatible: Compared to the known DEP processes, the present cl-DEP set-up is R2R compatible and further, a roll or cylinder with MEA can be re-utilised to assemble NWs over different target device substrates. Consequently, the present cl-DEP process reduces not only the number of manufacturing steps, but also the need for energy, time, consumables as well as the electronic-wastage, a step towards circular electronics.***The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered tobe illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / - 10%.ReferencesA number of publications are cited above in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. Full citations for these references are provided below. The entirety of each of these references is incorporated herein.[1] E. Song, J. Li, S. M. Won, W. Bai, and J. A. Rogers, "Materials for flexible bioelectronic systems as chronic neural interfaces," Nature Materials, vol. 19, no. 6, pp. 590-603, 2020 / 06 / 01 2020, doi:10.1038 / S41563-020-0679-7.[2] Y. Ling, T. An, L. W. Yap, B. Zhu, S. Gong, and W. 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Claims

Claims1 . A method for the dielectrophoretic assembly of nanowires onto a substrate, including the steps:(i) providing a suspension of nanowires in a carrier liquid;(ii) providing a substrate;(iii) providing an array of electrodes on an electrode array support;(iv) bringing the substrate into proximity with the electrode array support at a first surface of the substrate;(v) locating at least a second surface of the substrate, opposite to the first surface, in the suspension of nanowires; and(vi) applying a voltage signal to the array of electrodes thereby to generate, through the substrate, a corresponding electric field in the suspension of nanowires and a dielectrophoretic force on the nanowires in the suspension of nanowires, in order to cause dielectrophoretic assembly of nanowires onto the second surface of the substrate in an arrangement corresponding to the array of electrodes.

2. A method according to claim 1 wherein the substrate is flexible.

3. A method according to claim 1 or claim 2 wherein the substrate is a dielectric material.

4. A method according to any one of claims 1 to 3 wherein the substrate has an area of at least 5cm x 5cm.

5. A method according to any one of claims 1 to 4 wherein the array of electrodes on the electrode array support is brought into direct contact with the first surface of the substrate.

6. A method according to any one of claims 1 to 5 wherein the electrode array support is in the form of a roller or in the form of a belt driven by one or more rollers.

7. A method according to claim 6 wherein the substrate is held in tension and passed around part of the roller or belt to provide uniform contact between the substrate and the electrodes at the first surface of the substrate.

8. A method according to any one of claims 1 to 7 wherein the method uses a roll to roll process.

9. A method according to any one of claims 1 to 8 wherein the method further includes the step: after step (vi), removing the substrate from the electrode array support and re-using the same array of electrodes on the electrode array to repeat steps (iv) to (vi) for a further substrate, or a further part of the same substrate.

10. A method according to any one of claims 1 to 9 wherein, during dielectrophoretic assembly, the substrate is moved with respect to the carrier liquid.

11. A method according to claim 10 wherein the electric field direction is controlled so that a preferred assembly orientation of the nanowires is substantially parallel to the relative movement direction between the substrate and the carrier liquid.

12. A method according to any one of claims 1 to 11 wherein the method further includes, after step(vi):(vii) providing a second suspension of further nanowires in a carrier liquid;(viii) providing a second array of electrodes on a second electrode array support;(ix) bringing the substrate into proximity with the electrode array support at the first surface of the substrate;(x) locating at least the second surface of the substrate in the second suspension of nanowires; and(xi) applying a voltage signal to the second array of electrodes thereby to generate, through the substrate, a corresponding electric field in the second suspension of nanowires and a dielectrophoretic force on the nanowires in the second suspension of nanowires, in order to cause dielectrophoretic assembly of nanowires onto the second surface of the substrate in an arrangement corresponding to the second array of electrodes.

13. A device comprising an array of nanowires deposited onto a substrate, the device optionally being obtained or obtainable by a method according to any one of claims 1 to 12.

14. An apparatus for the dielectrophoretic assembly of nanowires onto a substrate, the apparatus comprising: a chamber for holding a suspension of nanowires in a carrier liquid; an array of electrodes on an electrode array support; wherein, in use, the apparatus is operable to bring a substrate into proximity with the electrode array support at a first surface of the substrate and to locate at least a second surface of the substrate, opposite to the first surface, in the suspension of nanowires in the chamber, and to apply a voltage signal to the array of electrodes thereby to generate, through the substrate, a corresponding electric field in the suspension of nanowires and a dielectrophoretic force on the nanowires in the suspension of nanowires, in order to cause dielectrophoretic assembly of nanowires onto the second surface of the substrate in an arrangement corresponding to the array of electrodes.

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