FIELD OF INVENTION

[0001] The present invention is concerned with threshold switching materials and electronic devices utilizing such materials. In particular the present invention is concerned with threshold switching materials that may be used as selectors with electronic devices such as resistive-switching memories and other devices that may use selectors and is furthermore concerned with architectures for resistive-switching memory devices.

BACKGROUND ART

[0002] Various technologies may utilize selectors such as OLED devices, sensor devices and memory devices. Non-volatile memories are memories that retain their contents when unpowered. They may be used for storage in a wide variety of devices that require persistent data storage. Non-volatile semiconductor memories are receiving increasing attention because of their advantages in being small in size, exhibiting data persistence, having no moving parts and their need for little power for operation.

[0003] Top-down methodologies have been the prevailing technologies for the manufacture of electronic devices and circuits since the inception of the first semiconductor devices. In recent years there has been a growing interest and development of bottom-up technologies for the manufacture of such devices. In more recent times more attention is being given to printing technologies and to what is now referred to as printed electronics. Of particular interest are flexible printed electronic devices. The move from top-down to bottom-up approaches has required the sourcing of new materials and methodologies for the manufacture of such devices and many challenges remain in attempting to produce workable and robust devices.

[0004] Resistive-switching memories are typically memory devices that include a resistive-switching material (e.g. a metal oxide) that changes from a first resistivity to a second resistivity upon the application of a set voltage, and from the second resistivity back to the first resistivity upon the application of a reset voltage. Resistive Random Access Memory (often abbreviated to ReRAM or RRAM) is emerging as one of the most promising technologies for non-volatile memory. Recent developments in the manufacture of metal oxide-based RRAM technologies using the bottom-up approach have been focused on manufacturing the metal oxide layers of the RRAM using particulate metal oxides in the form of dispersions or inks that are coated or printed upon suitable substrates and thus affording the possibility of low cost printed memory technology. A key challenge with this approach is the development of suitable particulate metal oxide or similar materials for the manufacture of all the features of these RRAM devices, suitable coating dispersions or inks from such materials and high-quality deposited layers.

[0005] With memory devices for high density integration and further versatile applications, the memory cells may be fabricated in a crossbar array consisting of set parallel bottom electrodes and perpendicular top electrodes with an appropriate switching medium in between. However, such crossbar arrangements often exhibit a phenomena known as sneak path currents, which are seriously disadvantageous to performance. Sneak path currents result in erroneous data retrieval and the increase of power consumption, especially when the resistive cells in cross point are in LRS (Low Resistive State).

[0006] Numerous proposals to reduce the sneak path have been considered. One proposal is to connect two bipolar cells anti-serially into one complementary resistive switch. However, the anti-serial or back to back connection of two cells inheritably causes fabrication complexities and degradation of the internal common electrode. Recently, it has been found that field induced threshold switching (TS) can be ultilized as promising matrix addressing elements (selectors) to eliminate crosstalk in crossbar structured devices. However, current threshold swithching technologies are difficult to manufacture consistently, are rigid and not suitbale for flexible memory applications, where mechanical flexibility is required. Usually they have been made from conventional physical deposition methods used in semiconductor industries and involve complex layered structures with as much as ten layers and vacuum based fabrication processes on generally rigid and inflexible substrates. A system based on the partial oxidation of a silver nanowire mesh was proposed as one possible approach to selector design (Tao Wan, Ying Pan, Haiwei Du, Bo Qu, Jiabao Yi, and Dewei Chu ACS Applied Materials & Interfaces 2018 10 (3), 2716-2724). This system utilised oxidised silver nanowires in combination with non-oxidised silver nanowires, where the oxidized nanowires were prepared from the same mesh. This combination has been found to be difficult to manufacture and adjust for RRAM applications and provides variable and inconsistent results in RRAM systems because it needs UV and high temperature processing.

[0007] Therefore, there is a need for a facile approach to control the sneak path current especially in bottom-up RRAM devices. A further challenge is to provide suitable selector materials and technologies for all RRAM architectures and memory devices.

DISCLOSURE OF THE INVENTION

[0008] The present invention in various aspects is concerned with new threshold switching materials and their use in electronic devices. These materials provide effective selector function in electronic devices and in particular RRAM devices with reduced sneak path current, that are printable, flexible and transparent. This affords the manufacture of bottom-up printable devices with good performance. The present invention in various further aspects is further concerned with a new architecture for RRAM devices that effectively prevents sneak path.

[0009] In a first aspect the present invention provides a threshold switching material comprising a composite of conductor material and low dielectric constant material.

[0010] In a preferred embodiment the conductor material comprises nanowires, more preferably metal nanowires and most preferably silver nanowires. Preferably, the nanowires are embedded within a matrix of low dielectric constant material to form the composite. It is to be understood that the composite comprises nanowires that are dispersed and isolated within the low dielectric constant material; the individual nanowires are separated from each other by the low dielectric constant material The discrete nanowires may be arranged within the composite, upon its formation, as a pseudo mesh with adjacent wires spatially overlapping with other adjacent nanowires within the low dielectric matrix material, but without actually coming into physical contact with each other. Within the composite there will be a small number of nanowires that are in physical contact with each other, but these are kept to a minimum through the tuning and control of the relevant ratios and proportions of nanowires and low dielectric constant material present in the composite. The effective separation of the majority of the nanowires from each other within the low dielectric constant material is essential to allow threshold switching functionality to be achieved by the composite.

[0011] In a preferred embodiment the low dielectric constant material, which may also be referred to as an insulator, is an organic polymeric material. Any low dielectric constant polymer is suitable. Preferred polymers are those that are soluble in a solvent that is also capable of dispersing the conductor material. Preferred polymers are those that are compatible with the conductor material and prevent agglomeration of the conductor material within any coating solution comprising the conductor material with the polymer. In addition, also preferred are polymers which, upon formation of the composite, maintain the conductor material in a dispersed state within the polymer matrix and which may also effectively wet the conductor material surface in the composite to ensure intimate contact with the conductor material. It is to be understood that any given conductor material may have its own unique surface properties and the polymer is in part selected to be accommodate and to be compatible with those properties to provide a homogenous and effective threshold switching material composite. Preferred polymers are those that may be absorbed on the surface of the conductor material. The polymer is preferably a good film forming polymer, which provides flexible composite layers once deposited as a film with the dispersed conductor material. Preferred polymers have a molecular weight of between 80,000 g.mol ^-1 and 400,000 g.mol’ ¹ . Examples of suitable polymers are one or more of; polyvinylpyrrolidone (PVP), polyvinylidene fluoride (PVDF), polymethyl methacrylate and other polymers with low dielectric constant such as for example Polydimethylsiloxane (PDMS). Also, envisaged are mixtures of suitable polymers. Preferably, the polymer comprises PVP.

[0012] The low dielectric constant material preferably has a dielectric constant of 10 or less and more preferably 6 or less.

[0013] The threshold switching composite preferably has a dielectric constant of between 2 to 200. [0014] Preferably, the threshold switching composite comprises from 1 to 10 wt% silver, more preferably 2 to 8 wt% silver, more preferably 2 to 5 wt% silver and ideally about 5 wt% silver. Preferably the threshold switching composite comprises from 90 to 99 wt% of low dielectric constant material, preferably from 92 to 98 wt%, more preferably from 94 to 96 wt% and ideally about 95 wt%. The exact percentages for each component selected to provide a total of 100% in the as formed composite, absent the addition of other additives to the composition.

[0015] This aspect further provides for a threshold switching material liquid printing ink or liquid coating formulation comprising dispersed conductor material and dispersed or dissolved low dielectric constant material in a solvent.

[0016] The present invention further provides a method for the manufacture of a threshold switching material, which method comprises preparing a dispersion of conductor material in a solvent, adding a solution of low dielectric constant material to the dispersion of conductor material, printing or coating the combined mixture onto a substrate and removal of the solvent to provide a threshold switchable composite.

[0017] The threshold switching material may be arranged as a selector in any electronic device or circuit that requires selector functionality such as for example memory devices including RRAM, OLED devices, sensor devices such as resistive type sensors e.g. pressure and strain sensors, and neuromorphic devices.

[0018] Thus, in a second aspect the present invention further provides an electronic device or circuit comprising a selector prepared from threshold switching material according to the present invention. The selectors derived from these threshold switching materials may be printed or using solution deposition techniques, and the resulting selectors are flexible. Such flexibility is due to the nature of the low dielectric constant matrix and the presence of nanowire conductors, which exhibit excellent flexibility. This flexibility is a desired property for flexible printed electronic devices, especially printed memory devices.

[0019] In a third aspect the present invention further provides a memory device comprising a resistive-switchable material and a threshold switching material according to the present invention. Preferably, the resistive-switchable material comprises a particulate metal oxide.

[0020] Preferably, in one embodiment of the third aspect the particulate metal oxide is in direct contact with the threshold switching material.

[0021] Preferably, in the third aspect the resistive-switchable material comprises a composite of particulate metal oxide and low dielectric constant material.

[0022] Preferably, in the third aspect the low dielectric constant material when present in the resistive-switchable material is the same as the low dielectric constant material of the threshold switching material.

[0023] Preferably, in the third aspect the memory device is a RRAM memory device. Preferably, in the third aspect of the present invention the memory device is a crossbar array memory device.

[0024] Preferably, in the third aspect of the invention particulate metal oxide comprises strontium titanate and most preferably comprises strontium titanate nanocubes.

[0025] Preferably in the third aspect of the invention the resistive-switchable material comprises 1 to 30% by weight of low dielectric constant material, and most preferably 5 to 25% by weight of low dielectric constant material. In this aspect the low dielectric constant material may be an organic polymer. Preferably, the polymer is one or more of polyvinylpyrrolidone (PVP), polyvinylidene fluoride (PVDF), polymethyl methacrylate and some other polymers with low dielectric constant such as for example Polydimethylsiloxane (PDMS). Preferably, the polymer comprises PVP. The low dielectric constant material may be incorporated from the synthesis of the particulate metal oxide as for example as a capping agent or surfactant and/or may also be added to a formulation for preparing the resistive switching component of the RRAM.

[0026] In a further aspect of the present invention an architecture for a RRAM device is proposed that may utilize the threshold switching material of the present invention as a selector but which also may use alternative materials as the selector. This new architecture provides improved sneak path current performance for a RRAM array. A further challenge in crossbar array arrangements is the prevention or mitigation of the effects of ion migration from components of the array and in particular electrode materials.

[0027] Thus, in a fourth aspect the present invention provides in one embodiment a memory device comprising a continuous layer of resistive-switchable material in contact with a word line and a bit line, insulating the word line from the bit line, and the same layer of resistive-switchable material further providing resistive- switch properties, at a point remote from the point of insulation. Preferably, the resistive-switchable material comprises particulate metal oxide. With this arrangement the bit line may be in an offset arrangement from the word line and is in electrical communication with the word line via an offset bottom electrode and/or selector. By offset is meant that the bottom electrode and/or selector are not aligned along the contact axis between the word line, top electrode and resistive-switchable material. This offset arrangement of word line and bit line assists in preventing or reducing the impact of ion migration on the memory device. In addition, the effective electrical path between the bit line and the word line is extended and this assists in preventing sneak path currents within an array of such memory devices especially when in a crossbar array.

[0028] In the fourth aspect the present invention in a further embodiment provides a memory device comprising an array of RRAM memory units each comprising resistive-switchable material in contact with a top electrode and a bottom electrode or a selector material, wherein the selector material is associate with the bottom electrode and the bottom electrode and selector are arranged along an axis that is perpendicular to the axis of contact between the top electrode and the resistive- switchable material. Preferably, the resistive-switchable material comprises particulate metal oxide. In this embodiment the word line is preferably in contact with the top electrode and the bit line is either in contact with the either the bottom electrode or the selector, whichever is not aligned with the axis of contact between the top electrode and resistive-switchable particulate metal oxide material. In this aspect of the present invention the bit line may be separated from the bottom electrode by the selector, or the bit line may be separated from the selector by the bottom electrode depending upon which of either the bottom electrode or selector is aligned with the other axis.

[0029] Preferably, the memory device of the fourth aspect is a crossbar array. [0030] Preferably, in the fourth aspect the bottom electrode and selector are substantially co-planar and preferably arranged substantially parallel to the plane of the resistive-switchable material layer the word line and bit line.

[0031] Preferably, the memory device of the fourth aspect comprises a resistive-switchable material comprising a composite of particulate metal oxide and low dielectric constant material and a threshold switching material according to the first aspect of the invention as the selector material.

[0032] In a further embodiment of the fourth aspect a selector material bisects a two-part bottom electrode. One half of this bottom electrode is aligned with the contact axis for the top electrode and resistive-switchable particulate metal oxide material and the second half of this bottom electrode is separated from the first by the selector material. In this arrangement the bit line is in contact with the second half of the electrode. The bisected electrode and selector are substantially co-planar and substantially parallel to the plane of the resistive-switchable material layer the word line and bit line.

[0033] In the fourth aspect of the present invention the threshold switching selector arrangement provides improved resistance to sneak path in crossbar array arrangements when compared to conventional vertical stack memory units in a crossbar array. In the conventional arrangements the threshold switching selector materials are deposited and present in the vertical stack in the array as thin layers that are typically of the order of 100nm in thickness. This thickness is the effective transmission path for current through the selector material. The aspect ratio of this selector transmission path to selector width of such conventional selector layers is small and significantly less than 1 . These relatively thin threshold switching selector layers place significant constraints as to the materials that may be used in such a thin layer in order to provide an effective barrier to sneak path currents. In contrast the arrangements of the fourth aspect of the present invention provide threshold switching selectors, which present an effective aspect ratio of greater than 1 , whilst still enabling the manufacture of relatively thin memory devices. Once deposited these selector arrangements may be relatively thin, having a width dimension greater, and sometimes significantly greater, than the thickness dimension. As these selectors are offset and horizontally aligned it is the width of the material, which is presented as the transmission path for any current in the memory device. The transmission path is therefore significantly greater in the arrangements of the present invention even though the memory device remains thin. This provides selector arrangements that have much more reliable threshold switching behavior, with higher resistance and higher ON/OFF ratios, which both improve the resistance to sneak path current. A further advantage is that the arrangements of the present invention also enable more flexibility as to the materials that may be used as the selector material. Materials that may not be acceptable for use in conventional selector arrangements may perform adequately as selector materials in the arrangements of the present invention and these arrangements therefore offer much greater flexibility and variety in memory device design especially for printed and so called bottom-up devices. By using the width of the selector material as the transmission path for any current in the memory device, the resistance ON/OFF threshold can be tuned by varying the width of the selector to change the aspect ratio while keeping the same thickness. The resistance ON/OFF threshold can be increased for a selector in the arrangement of the fourth aspect by increasing the aspect ratio of selector width to the selector thickness. Preferably the selector arrangements have a thickness of the order of 100 nm. But in some embodiments, this may be up to 200 pm thick. Depending on the thickness, the selector material may have a width of 10 pm to 300 pm, while producing an aspect ratio of greater than 1 . Preferably the selector has a thickness of 50 nm to 300 nm. Preferably the selector has a width of 200 pm to 300 pm. These dimensions can provide a high aspect ratio for the selector transmission path that is greater than 1 . In many cases the dimensions provide an aspect ratio significantly higher than 1 .

[0034] In accordance with the present invention any two or more aspects may be combined to provide selector arrangements, electronic devices, circuits or memory devices and arrays.

[0035] In the present invention the use of a low dielectric constant material in both the resistive switching layer and the selector layer when in contact with each other has the following advantages. It improves the compatibility of both layers with each other and aids in reducing delamination of these layers during use of a RRAM device, especially a flexible or stretchable memory device. The two layers have more closely matched mechanical flexibility and also provide a device with good and improved optical properties such as transparency.

CONDUCTOR NANOWIRE [0036] The preferred conductor materials for use in the present invention are nanowires. These nanowires may be manufactured from any material that provides threshold switching properties in the threshold switching composite of the present invention. It is envisaged that suitable nanowire materials conductive metals such as for example are copper, nickel or silver. Preferably the nanowires are silver.

[0037] Suitable nanowires for use in the present invention may be selected on the basis of their diameter and aspect ratio. It is preferred that the nanowires have a diameter of from 20 to 100 nm and most preferably 30 to 70 nm. It is preferred that the aspect ratio of these nanowires is within the range of 200 to 2000, more preferably, 500 to 1500, more preferably 750 to 1250, and most preferably about 1000. This provides typical lengths of suitable nanowires within the range of from 5 to 100 pm, preferably 20 to 80 pm, more preferably 20 to 60 pm and most preferably about 50 pm.

[0038] There are several known methods that may be used for the synthesis of suitable nanowires. For silver nanowires any method for preparing nanowires that are compatible with the low dielectric constant matrix are suitable. A preferred method for the synthesis of suitable silver nanowires is a polyol method. In a typical protocol heated polyol (most frequently ethylene glycol) serves both as the solvent and a reducing agent, while silver nitrate (AgNO3) is the precursor to produce Ag.

Additionally, a capping agent or surfactant is crucial in controlling the 1 -D growth during the formation of sliver nanowires (AgNW) when the polyol method is utilized. Polyvinylpyrrolidone (PVP) is the preferred capping agent. To further optimize the aspect ratio of AgNWs, various direct agents (i.e. trace amounts of salts such as CuCI2, FeCI3, KBr, NaBr and NaCI) are employed in the polyol method. Typically, in a preferred method the PVP capping agent is dissolved in ethylene glycol with heating and then silver nitrate is dispersed into the capping solution. A separate direct agent solution is prepared in ethylene glycol, The direct agent solution is then added to the silver nitrate/capping agent solution and after mixing the mixture is heated at an elevated temperature (for example 170°C) in an autoclave for 2 or more hours. After the synthesis mixture is cooled the synthesized Ag NW solution is washed with ethanol by centrifugation at 3500 rpm for 4 min and the washing step is repeated 3 times. The precipitates are then re-dispersed diluted in ethanol for further use. Typically, the resultant AgNWs will be substantially free of PVP capping agent, but there may be residual capping agent retained on the surface of the AgNWs. In the present invention it is preferred that the AgNWs are prepared using a PVP capping agent of molecular weight of approximately 360,000 and a mixture of NaBr and NaCI as direct agent. It is also preferred that the resulting AgNWs retain capping agent for formulation into the threshold switching material. It is also preferred that the capping agent used is compatible with the low dielectric constant material used as the matrix in the threshold switching material. In a preferred aspect the residual capping agent on the AgNWs is the same type of compound as the low dielectric constant material; as an example, the capping agent and the low dielectric constant material may both be PVP and may or may not be of the same molecular weight.

LOW DIELECTRIC CONSTANT MATERIAL

[0039] The threshold switching material of the present invention in addition to the conductor also includes a low dielectric constant material in the composite. It is believed that the function of this material is to insulate individual conductor particles from each other to moderate the effective conductivity of the conductor, preferably nanowires, and in combination with the conductor produces a composite with threshold switching properties.

[0040] The preferred low dielectric constant materials have a dielectric constant of less than 20, preferably less than 10, most preferably 6 or less. Preferred low dielectric constant materials are organic polymers that may form solutions or dispersions that are compatible with dispersed AgNWs in suitable solvents and that are able to film form with AgNWs on printing or liquid coating of a substrate using the solution or dispersion as a printable liquid ink or liquid coating. Preferred polymers are those that are soluble in a solvent that is also capable of dispersing the conductor material. Preferred polymers are those that are compatible with the conductor material and prevent agglomeration of the conductor material within any coating solution comprising the conductor material with the polymer. In addition, also preferred are polymers which, upon formation of the composite, maintain the conductor material in a dispersed state within the polymer matrix and which effectively wet the conductor material in the composite to ensure intimate contact with the conductor material. It is to be understood that any given conductor material may have its own unique surface properties and the polymer is in part selected to be accommodate and to be compatible with those properties to provide a homogenous and effective threshold switching material composite. The polymer is preferably a good film forming polymer, which provides flexible composite layers once deposited as a film. Examples of suitable polymers are one or more of polyvinylpyrrolidone (PVP), polyvinylidene fluoride (PVDF), polymethyl methacrylate or polydimethylsiloxane (PDMS) or any other polymers or copolymers with low dielectric constant. The most preferred low dielectric constant material polymer are polymers or copolymers of N-vinylpyrrolidone (PVP). Preferably, the low dielectric constant material in the threshold switching composite has a molecular weight of between 80,000 g.mol ^-1 and 400,000 g.mol ^-1 , more preferably between 100,000 g.mol ^-1 and 400,000 g.mol ^-1 , more preferably between 200,000 g.mol- ¹ and 400,000 g.mol ^-1 and most preferably between 300,000 g.mol ^-1 and 400,000 g.mol- 1

PREPARATION OF THRESHOLD SWITCHING COMPOSITE AND

[0041] In a preferred embodiment of the present invention the selector is prepared from a combination of threshold switching material comprising a composite of conductor material, preferably nanowire material, and low dielectric constant material. In this embodiment an ink or liquid coating is first prepared for deposition. As prepared silver nanowires are preferably washed and centrifuged to remove any residual reactants or contaminants from their synthesis. In some instances, the capping agent used in the synthesis may be retained if is compatible with the low dielectric constant material to be used in the manufacture of the composite. A solution of low dielectric constant material is prepared. Any suitable solvent or mixture of solvents, which can solubilize any given low dielectric constant material may be used for the preparation of the solution of low dielectric constant material. Preferred solvents are polar solvents and most preferred solvents are alcohols. In the case of polyvinylpyrrolidone an ideal solvent is ethanol. The solution of low dielectric constant material will preferably comprise between 0.5 to 10 wt% of low dielectric constant material, preferably 1 to 8 wt%. more preferably 2 to 6 wt% and most preferably about 5 wt%. The washed nanowires are now dispersed in the solution of low dielectric constant material. It is preferred that the amount of nanowires dispersed in the solution is within the range of 0.01 mg. ml ^-1 to 10 mg. ml ^-1 . This final ink or coating comprising silver nanowires and low dielectric constant material can now be used to manufacture the preferred selector of the present invention. This ink may be deposited onto any suitable substrate as discussed below for the manufacture of a RRAM device. This substrate will ideally be cleaned with an appropriate solvent and in addition treated with UV/ozone for a period of time. The ink may then be printed in lines or coated onto this substrate using a mask to form selector lines. The solvent is then evaporated leaving the final threshold switching material comprising a composite of nanowire conductor material and low dielectric constant material as the selector. In a similar fashion to that described above electrodes may be deposited as dot electrodes on either side of the composite threshold switching material to provide the final selector arrangement. This provides a selector arrangement comprising a line of composite threshold material with adjacent lines of dot electrodes that are substantially co-planar with each other. In some embodiments there may be a single line of electrode material on one side of the selector line. In some embodiments the composite threshold switching material may be deposited using a mask and spin coating.

[0042] The threshold switching material of the present invention has certain advantages and properties over alternative solutions based on the formation of metal/oxide core-shell nanowires as part of a selector arrangement, which need a high temperature annealing and/or UV-treatment process. The high temperature annealing process (200-300 °C) may cause permanent damage of the device as well as to flexible polymeric substrates when used. The UV-treatment can also damage plastic substrates, and more importantly, the resulting silver oxide is not transparent and can be brittle and therefore lack robustness for flexible electronics applications. These problems are overcome with the threshold switching material of the present invention, which does not require these damaging treatment steps. The low dielectric constant material as a polymer is flexible and film forming and therefore provides a flexible threshold switching material and selector. The low dielectric constant material introduces an insulating layer between adjacent nanowires. The resulting composites are percolating network structures that show high resistivity due to the surface layer effect of the low dielectric constant material on the silver nanowires, but they can also be reversibly switched to a lower resistance state under an electric field (threshold switching behavior) . The observed switchable behavior is attributed to the Ag migration between the wires. In the metal/metal oxide selector system the threshold voltage required is typically within the range of 5-20 volts, due to the high resistance of the oxide layer. The threshold switching material of the present invention, by contrast requires a very small threshold voltage, which beneficially can minimize the power consumption of devices with such materials as for example selectors. With the threshold switching material of the present invention threshold voltages are preferably from about 0.1 to 5V and preferably from 0.1 to 1 V.

RESISTIVE SWITCHING MATERIAL

[0043] The memory devices of the present invention preferably utilize particulate metal oxide switching layers as a key component. The resistive switching materials comprise one or more doped and/or undoped particulate metal oxides. These particulate metal oxides may be amorphous, semi-crystalline and/or crystalline materials. It is preferred that they are semi-crystalline and/or crystalline and most preferably are crystalline. If any embodiment refers to particulate metal oxide it should be understood that this may include doped or undoped metal oxides or mixtures thereof.

[0044] Un-doped metal oxides are essentially pure materials that have been manufactured to provide as best as possible a pure particulate form of the oxide in question. In the case of crystalline particulate metal oxides, the crystal lattice structure does not include any other materials other than the elements of the relevant oxide in stoichiometric proportions.

[0045] The doped or un-doped particulate metal oxide may be of any shape. When the doped or un-doped particulate metal oxides are crystalline, they may be of any crystal structure morphology. Suitable crystalline forms are cubic, tetragonal, trigonal, hexagonal, orthorhombic, monoclinic, triclinic, face centered cubic, face truncated cubic, face truncated octahedron or truncated octahedron. It is preferred that in all the embodiments of the present invention that the crystalline metal oxides used (doped and un-doped) have a cubic based morphology and most preferably are face- to-face square cubic. The particulate metal oxide may also take the form of other shapes such as spherical or substantially spherical particles or rods or fibers or a generally rectangular morphology in which there are at least two dimensions e.g. length and breadth, which are different. It is preferred the particles are spherical or substantially spherical and/or cubic, tetragonal, trigonal, hexagonal, orthorhombic, monoclinic, triclinic, face centered cubic, face truncated cubic, face truncated octahedron or truncated octahedron. It is most preferred that they are cubic, tetragonal, trigonal, hexagonal, orthorhombic, monoclinic, triclinic, face centered cubic, face truncated cubic, face truncated octahedron or truncated octahedron. The most preferred particulate metal oxides are nanocubes.

[0046] The core metal oxide of the particles may comprise a semiconductor or dielectric or an insulator. The metal oxide may be any metal oxide that may be electrically induced to provide at least two resistive states. The metal oxide may be a transition metal oxide, an alkaline earth metal oxide, a post transition metal oxide, a metalloid oxide, a complex oxide and/ or a lanthanide oxide material. The metal oxide may, for example, binary oxides such as cerium(IV) oxide (CeO ₂ ), tin oxide (SnO ₂ ), hafnium oxide (HfO ₂ ) indium(lll) oxide (ln ₂ Os), or perovskite type oxides such as SrTiOs, BaTiOs and calcium titanium oxide (CaTiOs) or ternary metal oxides or more. Other examples include aluminum oxide, tantalum oxide, zirconium oxide, and yttrium oxide. The preferred metal oxides are those that may form crystalline particulate metal oxides, particularly preferred are nanocubic particle forming crystalline metal oxides, that may self-assemble when deposited from and through solution/dispersion-based compositions and methods. The most preferred particulate metal oxide are perovskite type oxides and most preferably the metal oxides is doped and undoped strontium titanate nanocubes and mixtures thereof.

[0047] The metal oxide particles may be of any suitable particle size and preferably are below 100 nm diameter. Ideally, they are from 1 nm to 30 nm in diameter when essentially spherical in morphology or 1 nm to 30 nm in size as defined by the longest dimension of the particle. Typically, each cubic nanocrystal preferably has a width of from about 1 nm to about 30 nm, more preferably 1 nm to 15 nm, more preferably 1 nm to 13 nm. Most preferably the width of the nanocubes in a given switching layer is 1 nm or greater, preferably 2 nm or greater and most preferably about 8 to 10 nm. The diameter or width is an average and thus any metal oxide particle may be present as a mixture of particles of average diameters and widths indicated. It is most preferred that the particles used in a given switching layer are as close to monodisperse as possible as this increases the uniformity in individual 2D and 3D arrays formed by the deposition of the cubic nanocrystals. It is believed that this uniformity results in better multilevel endurance stability of the memory structure and uniformity of the resistive switching parameters of the switching layer (e.g. a high ON/OFF ratio), and thereby better overall performance of the memory device. It is preferred that the nanocube particles are monodisperse.

[0048] Within a particulate resistive switching material, layers may have a thickness that is linked to the size of the nano particles used in its manufacture. If the layer is a 2D monolayer, then the thickness will be the dimensions of the nano particle used. Generally, particle-based layers may have a thickness of 1 nm to 2 pm, preferably 1 nm to 2 pm, and most preferably from 2 nm to 2 pm. Preferably the process is controlled to provide functional layers (comprising multiple 2D particle sublayers of less thickness) of thickness from 3 nm to 500 nm, more preferably 3 nm to 500 nm and most preferably 10 nm to 500 nm, more preferably 20 nm to 400 nm, more preferably 40 nm to 400 nm, more preferably from 50 nm to 300 nm, more preferably from 80 nm to 200 nm, most preferably from 100 nm to 180 nm and these thickness ranges can be altered and controlled by selection of an appropriate manufacturing technology for deposition of metal oxide particles.

[0049] Preferred, nanocubes may be prepared using a solvothermal/hydrothermal process using a metal precursor in an aqueous environment. Hydrothermal process is generally defined as crystal growth process under high temperature and high-pressure water conditions of substances which are insoluble in ordinary temperature and pressure (<100 °C, <1 atm). It is considered one of the most effective methods to synthesize morphologically controlled nanoparticles of high purity with high dispersion and narrow size distribution.

[0050] The preferred nanocubes for use in the present invention are strontium titanate based nanocubes and they may be made by any method known in the art. These nanocubes may be combined with low dielectric constant materials to provide the resistive-switchable material comprising a composite of particulate metal oxide and low dielectric constant material as used in certain aspects of the present invention. Alternatively, they may be manufactured using low dielectric constant capping agents that are retained after synthesis and co-deposited with the strontium titanate nanocubes, with or without additional low dielectric constant material.

PREPARATION OF RESISTIVE LAYER COMPOSITE [0051] One preferred method for the manufacture of the resistive-switchable material comprising a composite of particulate metal oxide and low dielectric constant material involves preparing a mixture comprising a source of strontium, a source of titanium, a source of hydroxyl, a low dielectric constant capping agent and organic solvent, heating of the mixture under ambient pressure with stirring to a temperature and for a time sufficient to induce hydrolysis and formation of dispersed strontium titanate nanocubes. Preferably the low dielectric constant capping agent has a molecular weight of greater than 80,000 g.mol ^-1 . The resultant dispersion is used as a liquid printing ink or liquid coating solution to produce the resistive-switchable material comprising a composite.

[0052] Preferably, the low dielectric constant capping agent has a molecular weight of between 80,000 g.mol ^-1 and 400,000 g.mol ^-1 , more preferably between 100,000 g.mol ^-1 and 400,000 g.mol ^-1 , more preferably between 200,000 g.mol ^-1 and 400,000 g.mol ^-1 and most preferably between 300,000 g.mol ^-1 and 400,000 g.mol ^-1 . The low dielectric constant capping agent is preferably a polymeric organic compound, preferably a heteroatom containing hydrocarbon polymer and most preferably a nitrogen containing polymeric organic compound. These capping agents may also be suitable as low dielectric constant materials for the threshold switching materials of the present invention. Examples of suitable polymers as low dielectric constant capping agents include; polyvinyl alcohols (PVA), polyethylene glycols (PEG), polypropylene glycols (PPG), polyacrylic acids (PAA), polyvinyl butyrate (PVB), polyphenylene oxides (PPG) and polyvinyl pyrrolidone (PVP). The preferred low dielectric constant capping agent s are polymeric materials derived at in least part from the polymerization and/or co-polymerization of N-vinyl pyrrolidone; the preferred polymeric low dielectric constant capping agents are polyvinyl pyrrolidines. This resulting nanocube dispersions may be used as synthesized or with suitable dilution with a solvent, as an ink for the manufacture of high-quality RRAM layers. The synthesis solution comprising strontium titanate nanocubes prepared by the methods of the present invention are hydrazine free and stable in that they are formed without precipitation.

[0053] A further preferred method comprises preparing a synthesis mixture comprising a source of strontium, a source of titanium, a source of hydroxyl, a low dielectric constant capping agent and organic solvent, and heating of the mixture under ambient pressure with stirring to a temperature and for a time sufficient to induce hydrolysis and formation of dispersed strontium titanate nanocubes, wherein the source of base is added to the mixture of other components dropwise with stirring before commencement of thermal hydrolysis.

[0054] Preferably the source of hydroxyl is not an organic source. Preferably, the source of hydroxyl is not tetramethylammonium hydroxide. A preferred source of hydroxyl is NH4OH. Preferably, the NH4OH is added as a 25-35% by weight mixture in water. The rate of addition is from 2 to 10 ml. min ^-1 , preferably from 3 to 8 ml. min ^-1 , most preferably from 3 to 7 ml. min ^-1 , and most preferably about 5 ml. min ^-1 . The NH ₄ OH is preferably injected drop by drop under stirring to achieve a homogeneous mixture. The relatively fast addition rate is used to avoid the unwanted evaporation of NH ₄ OH during the addition process.

[0055] In a further preferred method the synthesis comprises preparing a mixture comprising a source of strontium, a source of titanium, a source of hydroxyl, a low dielectric constant capping agent and organic solvent, and heating of the mixture under ambient pressure with stirring to a temperature and for a time sufficient to induce hydrolysis and formation of dispersed strontium titanate nanocubes, wherein the mixture is heated up to a temperature of between 150 and 200°C over a period of greater than two hours. Preferably, it is heated up to a temperature of 160°C.

Preferably, it is heated up to the desired temperature over a period of three hours or greater and preferably four hours or greater. Preferably, the reactor comprising the synthesis mixture is placed in a pre-heated oven at between 150 and 200°C and is reacted at a temperature within this range for a period of greater than two hours with stirring.

[0056] Preferred solvents are oxygenated solvents, which are organic solvents comprising carbon hydrogen and oxygen. Suitable examples of oxygenated solvents include alcohols, glycol ethers, methyl acetate, ethyl acetate, ketones, esters, and glycol ether/esters. The most preferred solvents are glycols of ethylene or propylene and most preferably ethylene. Suitable solvents include the glycol ether solvents such as ethylene glycol monomethyl ether (2-methoxyethanol, CH3OCH2CH2OH), ethylene glycol monoethyl ether (2-ethoxyethanol, CH3CH2OCH2CH2OH), ethylene glycol monopropyl ether (2-propoxyethanol, CH3CH2CH2OCH2CH2OH), ethylene glycol monoisopropyl ether (2-isopropoxyethanol, (CHs^CHOCFkCF OH), ethylene glycol monobutyl ether (2-butoxyethanol, CH3CH2CH2CH2OCH2CH2OH), ethylene glycol monophenyl ether (2-phenoxyethanol, C6H5OCH2CH2OH), ethylene glycol monobenzyl ether (2-benzyloxyethanol, C6H5CH2OCH2CH2OH), propylene glycol methyl ether, (1 - methoxy-2-propanol, CH ₃ OCH2CH(OH)CH ₃ ) diethylene glycol monomethyl ether (2-(2- methoxyethoxy)ethanol, methyl carbitol, CH ₃ OCH2CH2OCH2CH ₂ OH), diethylene glycol monoethyl ether (2-(2-ethoxyethoxy)ethanol, carbitol cellosolve, CH ₃ CH2OCH2CH2OCH2CH ₂ OH), diethylene glycol mono-n-butyl ether (2-(2- butoxyethoxy)ethanol, butyl carbitol, CH ₃ CH2CH2CH2OCH2CH2OCH2CH ₂ OH), and dipropyleneglycol methyl ether. Other suitable solvents include the dialkly ethers such as ethylene glycol dimethyl ether (dimethoxyethane, CH ₃ OCH2CH2OCH ₃ ), ethylene glycol diethyl ether (diethoxyethane, CH ₃ CH2OCH2CH2OCH ₂ CH ₃ ), and ethylene glycol dibutyl ether (dibutoxyethane, CH ₃ CH2CH2CH2OCH2CH2OCH2CH2CH ₂ CH ₃ ). Other suitable solvents include glycol esters such as ethylene glycol methyl ether acetate (2- methoxyethyl acetate, CH ₃ OCH2CH ₂ OCOCH ₃ ) ethylene glycol monoethyl ether acetate (2-ethoxyethyl acetate, CH ₃ CH2OCH2CH ₂ OCOCH ₃ ), ethylene glycol monobutyl ether acetate (2-butoxyethyl acetate, CHsCFfeCFkCFfeOCFkCFbOCOCHs), and propylene glycol methyl ether acetate (1 -methoxy-2-propanol acetate).The most preferred solvents are di-ethylene glycol and it’s derivatives and tri-ethylene glycol (TEG) and its derivatives such as tri-ethylene glycol monomethylether.

[0057] Preferably, with respect to all aspects the hydrothermal reaction is terminated through the removal of the source of heat and the subsequent gradual reduction in the temperature of the reaction mixture under exposure to ambient conditions i.e. air cooled.

[0058] Any of the features of these strontium titanate synthesis methods may be combined. In all the methods of synthesis of strontium titanate nanocubes it is preferred that the mole ratio of strontium to titanium in the reaction is within the range of 0.8:1 to 1 :0.8.

[0059] In further preferred methods for the synthesis of a strontium titanate nanocubes the process is undertaken with two distinct thermal stages and/or a controlled order of addition of components for the synthesis. By thermal stage is meant a process step that is undertaken at a temperature elevated above ambient temperature and typically means a temperature of greater than 25°C and preferably greater than 35°C. The first thermal stage is referred to as the reaction preparation stage and the second thermal stage is referred to as the hydrolysis reaction stage. The reaction mixture preparation stage is preferably carried out at an elevated temperature above ambient but below a temperature that will induce significant hydrolysis. Thus, it is preferred that the reaction mixture is prepared at a temperature of from 40 to 140°C, and more preferably a temperature from 50 to 100°C. Typically, the reaction mixture is prepared by first heating the organic solvent e.g. TEG in a reaction vessel to a desired mixture preparation temperature and then the components for the hydrolysis reaction are added in sequence to the vessel at this mixture preparation temperature. As a first step it is preferred that the low dielectric constant capping agent e.g. PVP is first dissolved in the heated organic solvent with stirring before the addition of other components. It is then preferred that the strontium precursor is added to the low dielectric constant capping agent /solvent mixture at this temperature before the addition of any dopant precursor and before the addition of the titanium containing precursor. Finally, when the mixture is homogeneous, the source of hydroxyl e.g. NH ₄ OH is added with stirring at this first stage temperature. Once this reaction mixture preparation stage is completed the temperature of the reaction mixture is then raised to the higher temperatures needed to induce the hydrolysis reaction.

[0060] The product of these methods is a dispersion comprising doped or undoped strontium titanate nanocubes with a low dielectric constant capping agent of molecular weight of greater than 80,000 g.mol ^-1 in a liquid organic solvent medium. This may be referred to as a sol or an as synthesized dispersion of doped or undoped strontium titanate nanocubes. Further materials e.g. other solvents or additives or further additions of capping agent polymer may be added to this sol or dispersion for subsequent use of the sol or dispersion in liquid coating or printing operations. Alternatively, the sol or as synthesized dispersion may be used directly as an ink or coating solution, without further significant modification other than dilution with a suitable organic solvent to produce the resistive-switchable material comprising a composite.

[0061] The preferred dispersion used for preparation of the resistive switching layer is a strontium titanate nanocube low dielectric constant capping agent dispersion comprising strontium titanate nanocubes, organic solvent and low dielectric constant capping agent of a molecular weight of greater than 80,000 g.mol ^-1 . Preferably, the dispersion comprises one or more low dielectric constant capping agent as hereinbefore defined and most preferably a polyvinyl pyrrolidone (PVP) as capping agent and a glycol-based solvent. Preferably, the as synthesized dispersion comprises from 1 to 10 % by weight of strontium titanate in the form of nanocubes, more preferably from 1 to 8 % by weight, more preferably 1 .25 to 6 % by weight, more preferably from 1 .25 to 6 % by weight, and most preferably from 1 .5 to 5.0 % by weight. Preferably, the as synthesized dispersion comprises from 0.1 to 5 % by weight of low dielectric constant capping agent(s), more preferably from 0.1 to 4 % by weight, more preferably 0.2 to 3 % by weight and most preferably from 0.5 to 2.0 % by weight. The balance of the materials in the dispersion is mainly glycol solvent, which is present within the range of 50 to 85 % by weight, more preferably 55 to 80 % by weight, and most preferably 60 to 75 % by weight, and other reactant components such as base and water. Starting from these as synthesized proportions lower proportions may be present of the as synthesized dispersion is further diluted with a suitable solvent in preparing an ink or coating composition for printing or coating e.g. by slot die coating. As an example, if the key raw materials in the synthesis are completely converted to strontium titanate (STO) nanocubes, the amount of STO nanocubes in the synthesized sol is about 4.5 wt%, the low dielectric constant capping agent is about 1 .1 wt%, and glycol is about 68 wt%. If this sol is then diluted with ethanol to form an ink with a mass ratio of as synthesized dispersion: ethanol of 1 :1 , then STO is about 2.25 wt%, low dielectric constant capping agent is about 0.55 wt%, glycol is about 34 wt%; the balance being alcohol e.g., ethanol. When this is printed as an ink or deposited as coating the final layer of material, after removal of the glycol and ethanol, will have a weight ratio of STO:PVP of about 4:1 . Preferably, the composite switch material of the present invention may comprise from 1 to 30 wt% of low dielectric constant material, more preferably from 1 to 25 wt% of low dielectric constant material and most preferably from 1 to 20 wt% of low dielectric constant material. Preferably the source of low dielectric constant material is retained material from the STO synthesis; this may be supplemented or replaced with added low dielectric constant material.

[0062] In certain embodiments the low dielectric constant capping agent may be removed from the dispersions or deposited layers from dispersions when it is desired to have a relatively pure particulate metal oxide resistive-switchable material layer. The dispersed as synthesized strontium titanate nanocubes are isolated from their dispersion through precipitation from the as synthesized dispersions. In a preferred embodiment precipitation is induced through the addition of water to the sol. In a preferred embodiment the weight ratio of water: sol used is greater than 30:1 , more preferably greater than 40:1 and most preferably 50:1 or greater. After addition of enough water to the strontium titanate nanocube dispersion, strontium titanate nanocubes are precipitated onto the bottom of the container. The complete precipitation process normally takes several hours or days. Finally, the precipitated nanocubes are separated after removal of the supernatant, preferably through high speed centrifugation and typically at 15,000 rpm. The separated nanocubes are preferably washed with water, a water/alcohol mixture or alcohol; preferably the alcohol is ethanol. The washing process is preferably repeated to ensure that as much residual organic material such as low dielectric constant capping agent as possible is removed from the nanocubes. Typically, this requires at least four washing steps and preferably with a final alcohol only washing step. The washed nanocubes may then be dried at ambient temperature for 24 hours. The nanocubes of will still retain bonded low dielectric constant capping agent on their surfaces after extensive washing.

DEPOSITION TECHNIQUES FOR STO LAYER

[0063] The as synthesized strontium nanocube sols or dispersions may be used as inks or coating solutions for the manufacture of strontium titanate nanocube layers such as RRAM layers. In a preferred embodiment the as synthesized strontium nanocube dispersion is further diluted with one or more solvents to provide a formulated ink or coating dispersion. Preferably, the ink or coating dispersion comprises the addition of one or more organic solvents and most preferably one or more oxygenated solvents as hereinbefore defined. Preferably, at least one of the solvents is an alcohol and most preferably is ethanol or a glycol derivative.

[0064] The preferred particulate metal oxide deposition techniques of the present invention are solution or liquid dispersion-based techniques, and these will often result in multiple layers of the same particle materials being deposited at each coating or printing step. Thus, the metal oxide layers of the present invention are preferably solution or dispersion deposition layers.

[0065] The deposition processes used in the present invention along with particle selection results in the formation of ordered structures. The deposition methods result in a form of self-assembly in each layer, where particles are deposited and under the process conditions align with each other to form ordered 2D arrays of particles. It is believed that this self-assembly of the un-doped or doped particles and in particular nanocube particles is facilitated by intermolecular forces, for example, van der Waals forces and other forces, and the surfactant-mediated surface hydrophobicity of the liquid medium (e.g. a suitable organic solvent and a suitable surfactant) in which the un-doped or doped nanocubes are dispersed during preparation. As a result, each cube aligns itself relative to other cubes to form a near close packed like array facilitating even and proportional spacing between adjacent nanocubes.

[0066] Preferably, the ink or coating dispersion is prepared by dilution of the as synthesized dispersion with the desired solvent(s) to achieve the desired rheology for the ink or coating dispersion depending on the printing or coating technique to be used. Preferably the dilution is at a weight ratio of solvent: sol dispersion within the range of from 0.5:2.0 to 2.0:0.5, more preferably 0.75:1 .5 to 1 .5:0.75 and most preferably 0.8:1 .4 to 1 .4:0.8. The ratio of solvent: sol dispersion typically in the range of 1 .2:10 to 1 :1 .2 solvent: sol. This ratio may be adjusted to enable better control of deposited film thickness when the ink or coating dispersion is used to deposit a film. The resultant formulated diluted dispersions or inks preferably comprise between 0.5 and 1 .5 % by weight of strontium titanate in the form of nanocubes, more preferably between 0.625 and 1 .25 % by weight, and most preferably 0.75 and 1 .0 % by weight.

[0067] The strontium titanate inks or dispersions may be used to deposit a single layer of strontium titanate nanocubes or typically for memory devices of the present invention are deposited to form multiple layers to achieved a desired thickness and thus the final coating comprises layers of strontium titanate nanocubes fabricated by forming layers adjacent to the previous layer. Typically, in order to form a layer of self-assembled strontium titanate nanocube on an electrode, or on a previous layer or a substrate, a stable dispersion is prepared by dispersing the nanocubes in a liquid medium that is capable of dispersing the nanocubes or preferably by mixing the as synthesized sol of strontium titanate nanocube with one or more solvents. The liquid medium can be prepared from any suitable liquid, e.g. from a solvent or a mixture of solvents. Advantageously, a stable dispersion can facilitate the self-assembly of the nanocubes and the sols of the present invention are relatively stable. In some embodiments, the liquid medium is an organic solvent or a mixture of organic solvents. The organic solvent may, for example, be toluene, ethanol or n-hexane. In preferred embodiments, the organic solvent is ethanol and/or glycol derivatives such as triethylene glycol. In some embodiments, the dispersion is an ink, e.g. a printable ink that may be printed using a variety of printing techniques and especially inkjet printing. In one embodiment, the dispersion is a coating dispersion that can be used by solution processed techniques (e.g. dip coating, spin coating, spray coating, or slot die coating) to form the memory structure. Modifications to the rheological, solubility and wettability properties of the dispersion can be made to suit a particular solution processed technique. For example, in inkjet printing, jetting characteristics can be adjusted by the addition of a surfactant and/or solvent (e.g. toluene, ethanol or n - hexane) to the dispersion.

[0068] A range of solution and dispersion-based methods may be used for depositing a strontium titanate nanocube coating onto a substrate for use in a memory structure of the present invention. These solution or dispersion processes include such techniques as spin coating, spray coating, dip coating, drop coating, slot die coating, nanoimprint, ink-jet printing, spray printing, intaglio printing, screen printing, flexographic printing, offset printing, stamp printing, gravure printing and aerosol jet.

[0069] One way of depositing a particulate metal oxide layer in the present invention is through the drop-coating method. The drop-coating method involves putting one or two drops of a dispersion of nanocubes (typically one drop is equivalent to about 100 pL) onto an electrode. Typically, the layer is allowed to dry naturally and may then be annealed.

[0070] Another fabrication method is the use of spin coating techniques to deposit layers of self-assembled strontium titanate nanocubes. A small amount of the dispersion is put onto the electrode or previously deposited layer. The substrate is then rotated at high speed to spread the coating material by centrifugal force. Rotation is continued while some of the dispersion spins off the edges of the substrate, until the desired thickness of the film is formed by the residual dispersion. Typically, the layer is allowed to dry naturally and then annealed. Thin and uniform layers may be produced by the spin coating method. Typically, the spin coating is operated at between 1000 to 4000 rpm for 20 to 60 seconds. [0071] With reference to all embodiments of the present invention it is preferred that all of the material layers comprising the resistive switching material and electrodes and any further layers preset are deposited by means of a liquid printing or liquid coating process. Preferred liquid printing and coating processes are slip-casting, slot die-casting, spin coating and ink-jet printing process.

[0072] The preferred method for the manufacture of a strontium titanate nanocube layer according to the present invention comprises a spin coating process or a slot die deposition process. Preferably, spin coating at 1000 to 4000 rpm for 20 to 40 seconds.

[0073] Preferably, the method for the manufacture of a strontium titanate nanocube layer comprises solvent evaporation after deposition. Preferably, solvent evaporation is in an oven at a temperature between 70 to 300°C, more preferably 70 to 280°C, more preferably 100 to 200°C, more preferably 100 to 180°C, and most preferably at a temperature from 100 to 160 °C, for sufficient time to ensure complete evaporation of solvents aqueous or non-aqueous from the layer. Typically, at 100 to 160 °C for 30 minutes to 1 hour. Preferably, no further thermal processing is used after this evaporation stage. In a further embodiment the substrate comprising the strontium titanate nanocube layer may be exposed to a further thermal treatment after the evaporation stage and the temperature of this stage is dependent on whether or not the low dielectric constant capping agent is to be retained in the final resistive switching layer. Low temperature regimes below the decomposition temperature of the organic low dielectric constant capping agent are used when it is to be retained; preferably, these will be below 200°C, and most preferably below 150°C. When the organic low dielectric constant capping agent is to be removed then depending on its decomposition and/or oxidation temperature this thermal treatment is undertaken at a temperature of from 150 to 500°C, more preferably from 150 to 450°C. In a preferred embodiment the thermal treatment is carried out at 450 °C for 1 hour, with the sample being heated at 5°C/min to 450°C. This may be termed a thermal annealing treatment. After the annealing is completed, the sample is allowed to cool to ambient conditions. In this annealing process organic low dielectric constant capping agent components of the strontium titanate layer are removed. [0074] In a preferred embodiment after deposition of the strontium titanate nanocubes, the resulting layer is subjected to a drying stage and is not subjected to a high temperature annealing stage. In this embodiment the resulting layer for electrode deposition will comprise strontium titanate nanocubes and residual components of the synthesis mixture and/or additives, including optional added low dielectric constant material, used for ink or coating formulation that are not evaporated under solvent evaporation conditions and in particular the organic low dielectric constant capping agent and optional added low dielectric constant material. Thus, in a preferred embodiment the strontium titanate nanocube layer further comprises one or more organic low dielectric constant capping agents of molecular weight of greater than 80,000 g.mol’ ¹ . Preferably the layer comprises from 70 to 95 weight % of strontium titanate and from 5 to 25 weight % of organic low dielectric constant capping agent; there may be minor amounts of other components present after the thermal evaporation stage.

[0075] Preferably, the substrate for printing or coating is glass (FTO) or glass (ITO) or plastic or silicon and most preferably is a plastic substrate.

[0076] With spin coating it is preferred that when the dispersion is deposited onto the desired substrate that it is left in contact with the substrate surface for a period of time before spinning of the substrate is commenced. Typically, this is enough time to allow the deposited dispersion to wet the surface of the substrate and in the case of strontium titanate nanocube dispersions is typically at least 10 seconds, preferably at least 20 seconds, more preferably 40 seconds and most preferably at least 60 seconds. In one embodiment the substrate, after this wetting phase, may be immediately spun at a desired speed e.g. 4000 rpm for the desired period e.g. 30 seconds, to obtain the desired thickness of coating on the substrate.

[0077] In addition, the spin coating may be undertaken in a specific sequence of stages. The first is the dispersion deposition stage where there is no rotation, and the substrate is simply wetted by the coating dispersion. The second stage is a relatively low speed spinning stage to spread the dispersion over the whole surface of the substrate; typically this is from 300 to 500 rpm at a rate of 500 rpm. s’ ¹ , for a short period of 2 to 10 seconds, preferably 2 to 8 seconds and typically 5 seconds. A third stage is a high speed spin at typically 1000 to 4000 rpm at a rate of 1000 rpm.s ^-1 for 10 to 40 seconds, preferably 20 to 40 seconds and most preferably 30 seconds This is then followed by a final drying spin at typically 500 to 1000 rpm at a rate of 500 rpm.s ^-1 for 10 seconds. Typically, prior to high temperature drying the edges of the substrate are cleared of coating material to expose electrode contact surfaces. It is preferred that the sol or in/coating solution is stored under refrigerated conditions, preferably less than 5°C, prior to use for deposition. Before the film preparation, the stored diluted sol is preferably given an ultrasonic treatment for 5~10 minutes.

[0078] In a preferred embodiment the sol or ink/coating solution is filtered as it is deposited or printed. Preferably it is sonicated before use when removed from refrigeration and the filter is 0.2 to 0.6 pm PTFE.

[0079] In the present invention it us preferred that the ink or coating dispersion formulation and the spin coating operations are optimized to ensure that the final deposited layer thickness of strontium titanate nanocubes after forced drying is at least 80 nm and preferably 100 nm or more and preferably within the range of 80 to 400 nm, more preferably 80 to 300 nm, more preferably 100 to 300 nm, and most preferably 100 to 160 nm.

FABRICATION OF MEMORY DEVICES

[0080] The selector arrangements of the present invention may usefully be used in the manufacture of RRAM memory devices.

[0081] The memory devices of the present invention may be prepared using a wide range of substrates including glass, plastic, silicon and other materials that provide a suitable surface for depositing a layer of metal oxide. Typically, these substrates are dimensionally stable such that the memory devices are not distorted or stressed during use. Those skilled in the art will readily envisage other suitable substrate materials to deposit the memory structure on within the spirit and scope of this specification.

[0082] The memory devices of the present invention may be prepared as flexible memory devices on relatively thin and flexible substrates such as polymeric substrates or flexible glass or metal or composite substrates. In the context of the present invention and under conventional definitions these substrates may be bent inplane but do not elongate in-plane; they are not able to stretch or deform. They may be flexed in a twisting and/or bending action and it is to be understood that this is not considered to be deformable behavior in the context of the present invention.

[0083] The memory devices may be stretchable memory devices where the substrate is typically an organic polymeric material which has elastomeric properties, or which may be stretched. There are many such materials in the art that are suitable as elastomeric substrates for the present invention. These may include thermoplastic or thermoset substrates. Thermoplastic substrates may include, styrenic block copolymers (TPE-s), thermoplastic olefins (TPE-o), elastomeric alloys (TPE-v or TPV), thermoplastic polyurethanes (TPU) including ester and ether-based polyurethanes, thermoplastic co-polyester and thermoplastic polyamides. Preferred thermoplastic elastomers include the thermoplastic polyurethanes and especially ether-based polyurethanes. Also suitable are various resin based thermoset materials that may be used to provide a variety of substrate forms. Also suitable are stretchable substrates that have elongation and retraction properties by nature of their manufacture as in tightly woven substrates or fabrics for clothing or other applications such as furniture or building products. Also suitable are natural products such as leather. All these materials may be surface treated to assists with the deposition and maintenance of deposited memory devices in accordance with the present invention.

[0084] The memory devices may be memory devices on deformable substrates where the substrate is capable of significant dimensional deformation either during manufacture of the memory device after the particulate metal oxide deposition stages or at any time before its use. One specific dimension is the deposition surface area of the substrate. Thus, in one embodiment a substrate is selected, which may be subject to inelastic expansion such that the size of the deposition surface area is increased once that inelastic expansion is induced. During manufacture of the memory device the substrate is maintained with its pre-expansion dimensions during the deposition of the particulate metal oxide particles and upon completion of that deposition is then subjected to conditions that induce the required inelastic expansion. This expansion has the effect of inducing minor levels of separation of the metal oxide nanocubes from their neighbors within each 2D array in the device. This separation may assist in providing enhanced memory performance.

[0085] A preferred architecture and memory design using the selectors of the present invention is described in detail with reference to the Figures discussed below and is a modified crossbar arrangement. This preferred architecture allows the printing of significant components of the memory and assists along with the selector material in preventing or alleviating the impact of sneak past current. This architecture also demonstrates the dual function of the resistive switching layer in providing switching properties and insulating properties. The insulating properties are required because the word line and bit line elements of the architecture do not cross at the point of resistive switching in the device; they cross paths at a point separate from the location of switching. This separation enables the switching layer to have dual function. The switching layer is also able to insulate the selector from the word line.

[0086] The invention will now be further illustrated by reference to the following Figures and illustrative Examples, and in which:

Figure 1 (a) to (c) show a schematic representation of a modified crossbar memory device of the invention incorporating selectors of the present invention;

Figure 2 is a schematic representation of a crossbar array according to the present invention;

Figure 3 (a) and (b) shows the switching performance of the device of Example 3;

Figure 4 (a) to (f) show switching performance of composite threshold switching material selectors according to the invention;

Figure 5 is a schematic representation of the memory device of Example 5;

Figure 6 shows the switching performance of the device of Example 5;

Figure 7 shows the switching performance of the device of Example 6;

Figure 8 (a) and (b) shows the switching performance of the devices of Example 7; Figure 9 (a) to (d) shows the switching performance of a crossbar array with the composite threshold switch material selectors according to the invention as described in Example 8;

Figures 10 (a) to (d) show Scanning Electron Micrographs of the threshold switching material deposited in Example 4;

Figure 11 shows the current v voltage curves for cells 1 to 3 of the crossbar array of Example 9 in the ON state; and

Figure 12 shows the switching behaviour of cell 4 of the crossbar array of Example 9.

[0087] With reference to Figure 1 (a) a unit cell (1 ) of a modified crossbar array is shown. This cell consists of a substrate (2) upon which is deposited a selector arrangement comprising a combination of a threshold switching material (3), which bisects two-halves of a gold bottom electrode (4, 4’). A bit line (5) is deposited in an offset arrangement upon the top of one-half (4) of the bisected gold electrode, which is on the opposite side of the threshold switching material (3) to the top electrode (6). A continuous layer of particulate resistive switching strontium titanate (7) is deposited over this structure separating it from the top electrode (6) and the word line (8). This particulate resistive switching layer (7) is made up of strontium titanate nanocubes and low dielectric constant material. The top electrode (6) is located on top of the resistive switching material (7) directly above a bisected part (4’) of the gold bottom electrode. The word line (8) is located on top of the top electrode (6) and runs in a plane perpendicular to the plane of the bit line (5). The continuous layer of particulate resistive switching material (7) acts as resistive switch memory at the point between the bisected electrode (4’) and the top electrode (6). This same layer also acts as an insulator between the cross-over points of the bit line (5) and word lines (8) and also between the bottom electrode (4, 4’) and threshold switching material (3) and the word line (8). The selector arrangement consisting of the bisected bottom electrode (4,4’) and the threshold switching material (3) is co-planar and perpendicular to the resistive switch function in the vertical arrangement above electrode (4’), which is in the layer of particulate resistive switching material (7) between electrode (4’) and top electrode (6), part of the bisected electrode (4’) is common to the vertical stack and the perpendicular co-planar component; this arrangement allows for a compact and efficient crossbar memory device and assists with avoiding sneak path current and especially when used in combination with the threshold switching material (3).

[0088] With reference to Figures 1 (b) and (c) the only significant difference is that the selector arrangement does not utilise a bisected bottom electrode (4,4’). With reference to Figure 1 (b) the bottom electrode (10) is located in the normal location below and aligned with the top electrode (6) in this arrangement and the threshold switch material (11 ) for the selector is in direct contact with the bottom electrode (10) and the bit line (5). With reference to Figure 1 (c), this arrangement is reversed and the threshold switching material (13) is now beneath and aligned with the top electrode (6) and the bottom electrode (14) is now in contact with the bit line (5). The preferred arrangement is shown in Figure 1 (a).

[0089] This crossbar arrangement may incorporate a composite threshold switching material according to the invention or may utilise alternative threshold switching materials. In this regard an oxidised silver nanowire mesh may be used as the threshold switching material.

[0090] With reference to Figure 2 the individual cell (1 ) of Figure 1 (a), (b) or (c) is shown in a modified crossbar array (20) of such cells. The numbering and description of individual components of this array are as indicated with reference to Figure 1 (a).

DESCRIPTION OF HOW SELECTOR WORKS

[0091] The schematic configuration of the memory array (1 S1 R) of the present invention is shown in Figure 1 . First, composite threshold switching material selectors according to the invention are deposited as lines by solution or physical deposition methods such as for example inkjet printing or other printing methods Then, gold patterned bottom electrodes which provides connections with selecting and memory elements are deposited by printing e.g. ink-jet printing or other deposition method on either side of the composite threshold switching material selectors lines to provide a bifurcated gold electrode and selector arrangement. Then gold bit lines are deposited by inkjet printing or similar onto one line of the bifurcated gold bottom electrode of the selector. Then a continuous SrTiO3 resistive switching layer, with or without low dielectric constant material, is deposited through a printing or other liquid deposition technique e.g. slot-die printing or ink-jet printing. Ag top electrodes are then inkjet- printed and gold lines will also be printed for word lines perpendicular to the bit lines. Such a design simplifies the architecture of the memory array and enables full printing process.

[0092] This device may be operated as follows. For the set cycle if a small voltage is applied, e.g. 0.01 V, the selector is in the OFF state, because of the high contact resistance of the selector. If a higher voltage, e.g. 0.3v (threshold voltage) is applied, the selector will now turn to ON state, because of the formation of very thin nanoscale Ag filaments in the selector. In this case, a 0.3V voltage can read the selected memory cell. Because this is a threshold switching selector the ON state of the selector needs a voltage to maintain the ON state. If the memory section is in the OFF state, then a large voltage can be applied, e.g. 2V, so that the memory is set to the ON state.

[0093] For the reset cycle if a small voltage is applied, e.g. 0.01 V, the selector is OFF, because of the high contact resistance of the selector. If a negative voltage, e.g. -0.3v (threshold voltage) is applied, the selector will turn to the ON state. In this case, a -0.3V voltage can read the selected memory cell. If the memory is in the ON state, then if a large voltage is applied, e.g. -3V, the memory is set to OFF state.

[0094] The composite threshold switching material requires a voltage above 0.5v before it will pass the voltage to the switching layer. The composite threshold switching material acts as a switch, only passing a sufficiently high voltage through and ignoring stray voltage in the array.

[0095] The 1S1 R integrated device of the present invention has greatly suppressed sneak path currents by the high-performance composite threshold switching material selector. If a small voltage is applied, e.g. 0.01 V, the selector is OFF, because of the high contact resistance of loose nanowire networks. If a higher voltage, e.g. 0.3v (threshold voltage) is applied, the selector will turn to ON state. In this case, a 0.3V voltage can read the selected memory cell. Assume that the memory is OFF state, then a large voltage is applied, e.g. 2V, the memory is set to ON state. The ON state of this memory cell will not cause sneak current for other cells. This is because the sneak current will need to go through several adjacent memory cells. But, due to the presence of multiple selectors, there would need to be multiple combinations of 0.3V to make sneak current happen. Therefore a 0.3V reading voltage will not cause sneak current as accumulated voltage cannot reach the threshold for the selector material. For the set and reset, e.g. 2 V is applied, considering that the selector needs a continuous voltage to maintain, so after set or reset, the neighboring selectors are OFF state. Therefore, the sneak current can be minimized. For the reset, similar process will occur.

Example 1 - Silver Nanowires Synthesis

[0096] 0.695 g of PVP (/W : 360,000, Sigma) is completely dissolved in 15 mL of ethylene glycol at 80 °C under stirring for 90 min. When the solution was cooled to room temperature, 0.112 g of AgNOs (s 99%, Sigma) is added to solution under stirring for 5 min. To prepare the NaCI and NaBr solution, 0.025 g of NaCI and 0.0216 g of NaBr are first dissolved in 5 mL of ethylene glycol under ultrasonication, respectively.

100 pL of the NaCI and NaBr solution are subsequently added into 9 mL of ethylene glycol and the obtained solution is furthered added to the pre-prepared solution containing PVP and AgNO ₃ and stirred for 5 min. Finally, the mixed solution is transferred to a 50 mL Teflon-lined autoclave and heated to 170 °C in 20 min and the reaction is maintained for 2 h; then it is cooled to room temperature. Afterwards, the synthesized Ag NW solution is washed with ethanol by centrifugation at 3500 rpm for 4 min and the washing step is repeated 3 times. The precipitates are then re-dispersed diluted in ethanol for further use and these contained about 1 mg/ml of silver. The resultant nanowires had a diameter of approximately 50 nm and a length of approximately 50 pm giving an aspect ratio of approximately 1000.

Example 2 - Composite Threshold Switching Material Solution

[0097] A composite threshold material solution was prepared as follows:

[0098] 2mg of the sliver nanowires as prepared in Example 1 were dispersed in

3 ml of ethanol. Then 1 ml of 5 wt% PVP solution in ethanol was added to the silver nanowire dispersion. [0099] Then 0, 1 , 2, and 4 ml of ethanol were added into the solution respectively, and the final formed composite threshold switching material solutions were used to fabricate different selector arrangements and devices.

Example 3 - Silver/Silver Oxide Nanowire Selector Arrangement

[00100] A selector arrangement was prepared by two methods as follows:

[00101] A) A glass substrate was cleaned with ethanol and treated with

UV/ozone. Silver nanowires as prepared in Example 1 were deposited on the substrate as lines using a mask and drop coating technique. The deposited lines were then treated with UV/ozone for a period of 10 minutes. The samples were then annealed at 150 °C for half an hour. After annealing gold dot electrodes were sputtered onto both sides of the nanowire lines, which now consisted of oxidized sliver nanowires.

[00102] B) A glass substrate was cleaned with ethanol and treated with UV/ozone. Silver nanowires as prepared in Example 1 were deposited on the substrate as lines using a mask and drop coating technique. Platinum dot electrodes (60nm) were sputtered onto the sliver nanowire network by a mask. The sample was then treated by UV/ozone treatment for 10 and 15 minutes. Then the samples are annealed at 150 °C for half an hour.

[00103] Figure 3 (a) shows the initial test points in the selector are conductive silver nanowires before the UV/ozone treatment. Figure 3(b) shows the expected threshold switching behavior after UV/Ozone treatment for 10 minutes and the device can be switched at both negative and positive voltages.

Example 4 - Composite Threshold Switching Arrangement

[00104] A composite threshold switching arrangement was prepared by two methods as follows:

[00105] A) Glass substrates (2.5 cm * 2.5 cm) were cleaned with ethanol and treated with UV/Ozone for 20 min. 1 ml of original Ag NW solution as prepared in Example 1 was washed and centrifuged at 4000 rpm for 3 times. Then 1 ml of 5 wt% PVP solution in ethanol was added to the washed nanowires. Then 0, 1 , 2, and 4 ml of ethanol were added into the solution respectively, and the final composite threshold switching material solutions were used to fabricated different selector devices, which corresponded to Ag NW-PVP-1-0, Ag NW-PVP-1 -1 , Ag NW-PVP-1 -2, and Ag NW- PVP-1 -4. 100 ul of the composite threshold switching material solutions were spin- coated onto the precleaned glass substrates (200 rpm for 10 s, then 1000 rpm for 10 s). Then patterned Pt electrodes (thickness: 30 nm) were sputtered onto the composite samples to form a composite threshold switching material selector.

[00106] B) Glass substrates (2.5 cm * 2.5 cm) were cleaned with ethanol and treated with UV/Ozone for 20 min. Patterned Pt electrodes (thickness: 30 nm) were first sputtered onto the precleaned glass substrates. 2 ml of original Ag NW solution as prepared in Example 1 , was washed and centrifuged at 4000 rpm for 3 times. Then 1 ml of 5 wt% PVP solution in ethanol was added to the washed nanowires. Then 2 ml of ethanol was added into the solution respectively, and the final formed composite threshold switching material solutions were used to fabricated different selector devices, which corresponded to Ag NW-PVP-2-2. A mask was used, and the composite threshold switching material solution was dropped at the gap between the Pt electrodes, and spin-coated onto the substrates (1000 rpm for 30 s). resulting in a composite threshold switching material selector arrangement.

[00107] The threshold switching behaviour of these selector devices are illustrated in Figures 4 (a) to 4 (f). These showed a more reliable threshold switching behaviour when compared to the oxidised nanowire selector. The composite threshold switching material was analysed using SEM and the results are shown in Figures 10 (a) to (d). These show the silver nanowire conductor material is present in a PVP matrix and as a low-density mesh; this will consist of regions of nanowire to nanowire contact with some isolated nanowires in the matrix. The composite has high integrity, and the conductor and matrix are in intimate contact.

Example 5 - Memory Device - Selector of Example 3 (b)

A selector device as prepared in Example 3 (b) was used to manufacture a simple memory cell. A strontium titanate nanocube solution (STO) was prepared using PVP as the low dielectric constant capping agent. This solution had a concentration of STO of 4.5 wt% and this was diluted 1 :1 with ethanol to provide a solution with 2.25 wt% STO 270 pl of the diluted STO solution was deposited onto the selector arrangement and spin coated at the speed of 4000 rpm for 30s. Then the sample is annealed at 150 °C for half an hour. Then silver dot top electrodes were selectively deposited above one line of Pt dot electrodes by inkjet printing and annealed at 150 °C for half an hour.

[00108] The resultant cell is illustrated in Figure 5 and its switching performance is illustrated in Figure 6, which exhibits good threshold and resistive switching properties. However, although some of the test points exhibits the required switching behaviour, most of the other test points are conductive. The threshold switching behaviour of the oxidised Ag nanowire selector appears to be overcome by the intrinsic conduction characteristic of Ag nanowires.

Example 6 - Memory Device - Selector of Example 3 (a)

[00109] A selector device as prepared in Example 3 (a) was used to manufacture a simple memory cell. A strontium titanate nanocube solution (STO) was prepared using PVP as the low dielectric constant capping agent. This solution had a concentration of STO of 4.5 wt% and this was diluted 1 :1 with ethanol to provide a solution with 2.25 wt% STO. 270 pl of the diluted STO solution was deposited onto the selector arrangement and spin coated at the speed of 4000 rpm for 30s. Then the sample is annealed at 150 °C for half an hour. Then silver dot top electrodes were selectively deposited above one line of Au dot electrodes by inkjet printing and annealed at 150 °C for half an hour.

[00110] The resultant cell is illustrated in Figure 5 but with AU electrodes and its switching performance is illustrated in Figure 7, which exhibits good threshold and resistive switching properties. However, although some of the test points exhibits the required switching behaviour, most of the other test points are conductive. The threshold switching behaviour of the oxidised Ag nanowire selector appears to be overcome by the intrinsic conduction characteristic of Ag nanowires.

Example 7 - Crossbar Memory Device - Without Selector

[00111] A crossbar memory device was prepared without the use of a selector for comparison purposes. [00112] A glass substrate (2.5 cm * 2.5 cm) was cleaned with ethanol and treated with UV/Ozone for 20 min. Patterned Pt electrodes (thickness: 30 nm) were first sputtered onto the precleaned glass substrates. Then an STO solution was diluted with ethanol with a weight ratio of 1:10 and filtered with a syringe filter (0.45 pm). 100 ul of the diluted STO solution was drop coated onto the substrates and dried at 150 °C for 15 min. Top silver lines are printed above STO layer by inkjet printing and annealed at 150 °C for half an hour.

[00113] The switching performance of these devices is illustrated in Figures 8 (a) and (b). Figure 8 (a) shows the high resistance OFF state of the four memory cells tested in an array. Figure 8 (b) shows that when cells 1 to 3 have been switched to the ON state with a voltage the fourth cell is also showing ON state even though it is in the OFF state; this is due to sneak path current.

Example 8 - Crossbar Memory Device - With Composite threshold switching selector.

[00114] A glass substrate (2.5 cm * 2.5 cm) was cleaned with ethanol and treated with UV/Ozone for 20 min. Then patterned Pt electrodes (thickness: 30 nm) were first sputtered onto the precleaned glass substrates.

[00115] A composite threshold switching material solution was prepared according to the general method of Example 2. A mask was used, and composite threshold switching material solution was dropped at the gap of Pt electrodes, and spin- coated onto the substrate (1000 rpm for 30 s). An STO solution was diluted with ethanol with a weight ratio of 1:10 and filtered with a syringe filter (0.45 pm). A mask was used, and the diluted STO solution was drop coated onto the middle Pt electrodes and dried at 150 °C for 15 min. Top silver lines are selectively deposited above the STO layer by inkjet printing and annealed at 150 °C for half an hour.

[00116] The crossbar memory array with selector was tested and the performance of the four memory cells are shown on Figures 9 (a) to (d) .Each of the four memory cells (with selector) in a crossbar format was tested for sneak path current and the figures show that all four memory cells were able to perform effective switching performance with no evidence of any sneak path current as they each had a threshold switching window. Each cell was tested for a minimum of 5 IV- cycles, no degradation was observed.

[00117] For this device the current level was about 1 O' ¹⁰ A at 0.1 V, which is the nonlinear LRS (Low Resistive State Region), and this indicates that there was no sneak past current. The resistance at this point was about 10 ⁹ Q.

Example 9 - Crossbar Memory Device - With Composite threshold switching selector.

[00118] A further memory device was prepared as described in Example 8. This device was was tested and the performance of the four memory cells are shown on Figures 11 and 12. Figure 11 shows that memory cells 1 , 2 and 3 are conductive (ON state) after repeating switching cycles. Figure 12 shows that cell 4 exhibits combined memory switching and threshold switching properties although the adjacent cells 1 , 2 and 3 are conducting; memory cell 4 shows the initial threshold window (high resistance OFF state) and this proves that the selector is preventing sneak path current.

[00119] For this device the current level was about 10 ^-10 A at 0.1 V, which is the nonlinear LRS (Low Resistive State Region), and this indicates that there was no sneak past current. The resistance at this point was about 10 ⁹ Q.

[00120] Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", means "including but not limited to", and is not intended to (and does not) exclude other components, integers or steps.

[00121] Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. Where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. Features, integers, characteristics, compounds described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. [00122] All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent, or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[00123] It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country