WO2018134581A9

WO2018134581A9 - Inkjet printed electronic devices

Info

Publication number: WO2018134581A9
Application number: PCT/GB2018/050125
Authority: WO
Inventors: Daryl MCMANUS; Cinzia CASIRAGHI; Gianluca FIORI; Giuseppe IANNACCONE
Original assignee: The University Of Manchester
Priority date: 2017-01-18
Filing date: 2018-01-17
Publication date: 2019-04-04
Also published as: GB201700883D0; WO2018134581A1

Abstract

The present invention relates to methods for preparing certain electronic and/or memory devices, particularly those in vertical geometry or based on heterostructures. The present invention also relates to the electronic and/or memory devices obtained from these methods and to the use of these electronic and/or memory devices in data storage applications.

Description

INKJET PRINTED ELECTRONIC DEVICES

INTRODUCTION

[0001] The present invention relates to methods for preparing certain electronic and/or memory devices, particularly those in vertical geometry or based on heterostructures. The present invention also relates to the electronic and/or memory devices obtained from these methods and to the use of these electronic and/or memory devices in data storage applications.

BACKGROUND OF THE INVENTION

[0002] Printed electronic devices are increasingly used in a wide range of commercial applications such as, for example, portable electronic devices, signage, lighting, product identification, flexible electronics, photovoltaic systems, medical equipment, antennas (such as RFID antennas), displays, sensors, thin film batteries, logic memory devices, electrodes and many others.

[0003] Printed electronics are typically made by printing inks onto a substrate to form the electronic device.

[0004] The use of printed electronics has a number of advantages over conventional fabrication processes. In particular, printed conductive and insulative patterns are typically: faster to produce than subtractive processes (such as etching); less wasteful; less hazardous (i.e. use less hazardous chemicals); less expensive than conventional techniques; compatible with a wide range of substrates; simple to implement; and enable the possibility of further post-fabrication processing.

[0005] Computer-controlled printer technology also allows for precision printing on a wide variety of substrates, including glass, plastic, or ceramics for electronics or display applications. Inkjet printing involves the placement of small drops of ink onto a substrate surface in response to a digital signal. Typically, the ink is transferred or jetted onto the surface without physical contact between the printing device and the surface. Within this general technique, the specific method by which the inkjet ink is deposited onto the substrate surface varies from system to system, and includes continuous ink deposition and drop-on-demand ink deposition. Ink droplets are ejected by the print head nozzle and are directed to the substrate surface.

[0006] However to effectively inkjet print electronic devices, the inks used typically need to meet a number of performance criteria, such as, for example, a viscosity within the range 2 to 30 cPs; a surface tension within the range 20 to 50 mN/m (and preferably 28 to 35 mN/m), a low rate of evaporation at ambient temperatures (to prevent clogging of the printer head) and low levels of impurities present in the formulation. Despite a number of inkjet- printable inks being available, many are still far from ideal as they are either based on toxic solvents, have low concentration, or require time-consuming and expensive formulation processing.

[0007] There therefore remains a need for improved methods for printing electronic devices which employ ink formulations with improved inkjet compatibilities, lower costs and simple health and safety handling procedures.

[0008] The present invention was devised with the foregoing in mind.

SUMMARY OF THE INVENTION

[0009] According to a first aspect of the present invention, there is provided a method for preparing an electronic device, comprising the steps of:

a) depositing a first conductive layer onto a substrate;

b) depositing a dielectric layer onto at least a portion of the first conductive layer; and

c) depositing a second conductive layer onto the dielectric layer;

wherein one or more of the first conductive layer, dielectric layer and second conductive layer is deposited by inkjet printing an ink formulation as defined herein.

[0010] According to a second aspect of the present invention, there is provided a method of producing an electronic device, comprising the steps of:

a) depositing a first conductive layer onto a substrate;

b) depositing a first semiconductor layer onto at least a portion of the first conductive layer;

c) depositing a second semiconductor layer onto the first semiconductor layer, wherein the second semiconductor layer has one or both of a doping and a band structure which differs to that of the first semiconductor layer; and

d) depositing a second conductive layer onto the second semiconductor layer; wherein one or more of the first conductive layer, first semiconductor layer, second semiconductor layer and second conductive layer is deposited by inkjet printing an ink formulation as defined herein.

[0011] According to a third aspect of the present invention, there is provided a method of producing an electronic device, comprising the steps of: a) depositing a first conductive layer onto a substrate to form first and second electrically separated regions;

b) depositing a first semiconductor layer onto at least a portion of each of the first and second regions of the first conductive layer; and

c) depositing a dielectric layer onto at least a portion of the semiconductor layer layer;

wherein one or more of the first conductive layer, first semiconductor layer and dielectric layer is deposited by inkjet printing an ink formulation as defined herein.

[0012] According to a fourth aspect of the present invention, there is provided a method of producing a memory device, comprising the steps of:

a) depositing a first track onto a substrate;

b) determining a word to be stored by the memory device;

c) depositing one or more regions of dielectric material onto the first track at one or more locations corresponding to a first logic state of the word;

d) depositing one or more first logic tracks onto each of the one or more regions of the dielectric material; and

e) depositing one or more second logic tracks to intersect the first track at one or more locations corresponding to a second logic state of the word; and f) electrically connecting each of the one or more first logic tracks and the one or more second logic tracks to respective electric devices;

wherein one or more of the first track, the regions of the dielectric material, first logic tracks and second logic tracks are deposited by inkjet printing an ink formulation as defined herein.

[0013] According further aspects of the present invention, there are provided electronic and/or memory devices obtainable by, obtained by or directly obtained by the methods defined herein.

[0014] According to yet further aspects of the present invention, there are provided electronic devices, transistors and memory devices as defined herein.

[0015] According to a further aspect of the present invention, there is provided the use of a memory device as defined herein in the storage of data.

DETAILED DESCRIPTION OF THE INVENTION

[0016] Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of them mean "including but not limited to", and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0017] Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or examples of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. 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. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0018] The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

[0019] Unless otherwise specified, where the quantity or concentration of a particular component of a given formulation is specified as a weight percentage (wt.% or %w/w), said weight percentage refers to the percentage of said component by weight relative to the total weight of the formulation as a whole. It will be understood by those skilled in the art that the sum of weight percentages of all components of a formulation will total 100 wt.%. However, where not all components are listed (e.g. where formulations are said to "comprise" one or more particular components), the weight percentage balance may optionally be made up to 100 wt% by unspecified ingredients (e.g. a diluent, such as water, or other non-essential but suitable additives)

[0020] It will be appreciated that embodiments of the present invention can be realised in the form of hardware, software or a combination of hardware and software. Any such software may be stored in the form of volatile or non-volatile storage such as, for example, a storage device like a ROM, whether erasable or rewritable or not, or in the form of memory such as, for example, RAM, memory chips, device or integrated circuits or on an optically or magnetically readable medium such as, for example, a CD, DVD, magnetic disk or magnetic tape. It will be appreciated that the storage devices and storage media are embodiments of machine-readable storage that are suitable for storing a program or programs that, when executed, implement embodiments of the present invention. Accordingly, embodiments provide a program comprising code for implementing a system or method as claimed in any preceding claim and a machine readable storage storing such a program. Still further, embodiments of the present invention may be conveyed electronically via any medium such as a communication signal carried over a wired or wireless connection and embodiments suitably encompass the same.

METHODS OF THE PRESENT INVENTION

[0021] Following intensive investigation, the inventors of the present application previously identified a novel process for producing ink formulations with improved purity and inkjet compatible properties. Details of this methodology and the resulting ink formulations are described in PCT/GB2016/052230, the entire contents of which is incorporated herein by reference. In utilising the previously optimised ink formulations, the inventors have further developed methodology which provides access into a number of electronic and/or memory devices. Thus, by inkjet printing at least one, preferably all, of the essential components of an electronic and/or memory device with ink formulations as described herein, the inventors were advantageously able to produce electronic and/or memory devices using the commercially desired inkjet printing methodology, whilst mitigating many of the problems commonly associated with inkjet printing of electronic devices known in the art.

[0022] Accordingly, the inventors have been able to beneficially transpose the improved properties of the ink formulations previously developed to the resulting electronic and/or memory devices produced therefrom.

Method 1 Open circuit electronic devices

[0023] According to a first aspect of the present invention, there is provided a method of producing an electronic device, comprising the steps of: a) depositing a first conductive layer onto a substrate; b) depositing a dielectric layer onto at least a portion of the first conductive layer; and c) depositing a second conductive layer onto the dielectric layer; wherein one or more of the first conductive layer, dielectric layer and/or second conductive layer is deposited by inkjet printing an ink formulation comprising a plurality of nanosheets of 2D crystalline material in an inkjet compatible vehicle.

[0024] It will be appreciated by those skilled in the art that the first conductive layer may be deposited (e.g. inkjet printed) onto any suitable substrate. Non-limiting examples of suitable substrates include glass, plastic or ceramics. Exemplary substrates include silicon (optionally containing a thin S1O2 surface layer), quartz, paper, epoxy resin (e.g. breadboard circuit), polyimide and polyethylene terephthalate. The substrate may be a composite material.

[0025] Furthermore, it will be understood that the first and second conductive layers must each comprise at least one conductive material. Suitable conductive materials will be apparent to those skilled in the art and may include, for example, graphene, carbon nanotubes, electronically conductive metals and nanoparticles thereof (e.g. silver nanoparticles), electronically conductive polymers (e.g. PEDOT:PSS) and combinations thereof. Suitably, the conductive material is graphene or silver nanoparticles. Most suitably, the conductive material is graphene.

[0026] In an embodiment, the nanosheets are associated with an exfoliation agent that renders the nanosheets dispersible within the inkjet compatible vehicle. It will be understood that the term "exfoliation agent" and "stabiliser" are used synonymously throughout the application to mean any agent that is capable of associating with nanosheets that are formed, and thereby preventing the nanosheets from re-aggregating.

[0027] In an embodiment, the first conductive layer is deposited by inkjet printing an ink formulation, as described hereinbelow, wherein the ink formulation comprises at least one conductive material (e.g. graphene).

[0028] In another embodiment, the second conductive layer is deposited by inkjet printing an ink formulation, as described hereinbelow, wherein the ink formulation comprises at least one conductive material (e.g. graphene).

[0029] In a further embodiment both the first and the second conductive layers are deposited by inkjet printing an ink formulation, as described hereinbelow, wherein the ink formulation comprises at least one conductive material (e.g. graphene).

[0030] In an embodiment, the dielectric layer of the present method comprises at least one dielectric material. Suitable dielectric materials will be apparent to those skilled in the art and may include, for example, hexagonal boron nitride (h-BN), oxides such as ΤΊΟ2, layered ionic solids, such as perovskite-type structures, layered double hydroxide nanosheets and dielectric polymers such as polypropylene, polyvinylpyrrolidone and polyvinyl alcohol). It will be understood that the dielectric layer of the present method may alternatively comprise at least one semiconducting material (e.g. a transition metal dichalcogenide (TMDC)) provided that a Schottky barrier is formed between the semiconducting material and the first conductive layer. The formation of a Schottky barrier will be understood to result in a high resistance being achieved at a low bias voltage, thereby making the semiconductive material suitable for use in the dielectric layer.

[0031] In an embodiment, the dielectric layer is deposited by inkjet printing an ink formulation, as described hereinbelow, wherein the ink formulation comprises at least one dielectric material (e.g. hexagonal boron nitride).

[0032] In another embodiment, two or more of the first conductive layer, dielectric layer and/or second conductive layer are deposited by inkjet printing an ink formulation as described herein. More suitably, each of the first conductive layer, dielectric layer and second conductive layer are deposited by inkjet printing of ink formulation as described herein.

[0033] It will be appreciated that in situations where one or more of the first conductive layer, dielectric layer and/or second conductive layer are not deposited by inkjet printing an ink formulation as described herein, said layers may be deposited using any suitable technique known in the art. Examples of such techniques include atomic layer deposition, drop casting, spin and spray coating, stamp printing or use of chemical vapour deposition of graphene or other grown materials, which are transferred to form a layer of the heterostructure.

[0034] In another aspect of the present invention, there is provided an electronic device obtainable by, obtained by, or directly obtained by, method 1 defined hereinabove.

Method 2 Heterostructure electronic devices

[0035] According to a second aspect of the present invention, there is provided a method of producing an electronic device, comprising the steps of: a) depositing a first conductive layer onto a substrate; b) depositing a first semiconductor layer onto at least a portion of the first conductive layer; c) depositing a second semiconductor layer onto at least a portion of the first semiconductor layer, wherein the second semiconductor layer has one or both of a doping and a band structure which differs to that of the first semiconductor layer; and d) depositing a second conductive layer onto at least a portion of the second semiconductor layer; wherein one or more of the first conductive layer, the first semiconductor layer, the second semiconductor layer and/or the second conductive layer is deposited by inkjet printing an ink formulation comprising a plurality of nanosheets of 2D crystalline material in an inkjet compatible vehicle.

[0036] Preferred and suitable embodiments for the substrate, the first conductive layer and the second conductive layer of method 2 will be understood to be analogous to the preferred and suitable embodiments for the substrate, the first conductive layer and the second conductive layer of method 1 defined hereinabove.

[0037] Furthermore, it will be understood that the first and second semiconductor layers must each comprise at least one semi-conductive material. Suitable semiconductor materials will be apparent to those skilled in the art and may include, for example, transition metal dichalcogenides (TMDCs), such as NbSe₂, WS₂, MoS₂, MoSe₂, MoTe₂, TaS₂, PtTe₂, and VTe₂. Suitably, the semi-conductive material is selected from WS₂ or MoS₂.

[0038] In an embodiment, the nanosheets are associated with an exfoliation agent that renders the nanosheets dispersible within the inkjet compatible vehicle.

[0039] In another embodiment, the first semiconductor layer is deposited by inkjet printing an ink formulation, as described hereinbelow, wherein the ink formulation comprises at least one semi-conductive material (e.g. WS₂ or MoS₂).

[0040] In another embodiment, the second semiconductor layer is deposited by inkjet printing of an ink formulation, as described hereinbelow, wherein the ink formulation comprises at least one semi-conductive material (e.g. WS₂ or MoS₂).

[0041] In a further embodiment, both the first and the second semiconductor layers are deposited by inkjet printing of an ink formulation, as described hereinbelow, wherein the ink formulation comprises at least one semi-conductive material (e.g. WS₂ or MoS₂).

[0042] In another embodiment, two or more of the first conductive layer, the first semiconductor layer, the second semiconductor layer and/or the second conductive layer are deposited by inkjet printing an ink formulation as described herein. Suitably, three or more of the first conductive layer, the first semiconductor layer, the second semiconductor layer and/or the second conductive layer are deposited by inkjet printing an ink formulation as described herein. More suitably, each of the first conductive layer, the first semiconductor layer, the second semiconductor layer and/or the second conductive layer are deposited by inkjet printing an ink formulation as described herein.

[0043] In another aspect of the present invention, there is provided an electronic device obtainable by, obtained by, or directly obtained by, method 2 defined hereinabove.

Method 3 Transistor devices and programmable memory devices

[0044] According to a third aspect of the present invention, there is provided a method of producing an electronic device, comprising the steps of: a) depositing a first conductive layer onto a substrate to form first and second electrically separated regions; b) depositing a first semiconductor layer onto at least a portion of each of the first and second regions of the first conductive layer; and c) depositing a dielectric layer onto at least a portion of the first semiconductor layer; wherein one or more of the first conductive layer, first semiconductor layer and/or dielectric layer is deposited by inkjet printing an ink formulation comprising a plurality of nanosheets of 2D crystalline material in an inkjet compatible vehicle.

[0045] It will be appreciated that preferred and suitable embodiments for the substrate, the first semiconductor layer and the dielectric layer of method 3 are analogous to the preferred and suitable embodiments for these features described in respect of methods 1 and 2 hereinabove.

[0046] It will also be appreciated that the first conductive layer must comprise at least one conductive material. Suitable conductive materials are analogous to the suitable conductive materials in respect of the first and second conductive layers described hereinabove.

[0047] In an embodiment, two or more of the first conductive layer, the first semiconductor layer and/or the dielectric layer are deposited by inkjet printing an ink formulation as described herein. Suitably, each of the first conductive layer, the first semiconductor layer and/or the dielectric layer are deposited by inkjet printing an ink formulation as described herein.

[0048] In another embodiment, the nanosheets are associated with an exfoliation agent that renders the nanosheets dispersible within the inkjet compatible vehicle.

[0049] In an embodiment of method 3, the electronic device is a transistor.

[0050] In another embodiment of method 3, the first region of the first conductive layer is a source electrode and the second region of the first conductive layer is a drain electrode.

[0051] In certain embodiments of method 3, the electronic device is a transistor, the first region of the first conductive layer is a source electrode, and the second region of the first conductive layer is a drain electrode and the dielectric layer is deposited onto at least a portion of the first semiconductor layer, the method comprising the additional step of: d) depositing a second conductive layer onto at least a portion of the dielectric layer, wherein the second conductive layer is a gate electrode; and wherein the second conductive layer is deposited by inkjet printing of an ink formulation as defined herein.

[0052] In another embodiment of method 3, the electronic device is a memory device. Suitably, the memory device is an electrically programmable memory device.

[0053] In a particular embodiment of method 3, the electronic device is a memory device, the first region of the first conductive layer is a source electrode, the second region of the first conductive layer is a drain electrode, the method comprising the additional steps of: e) depositing a charge trapping layer onto at least a portion of the dielectric layer; f) depositing a second dielectric layer onto at least a portion of the charge trapping layer; g) depositing a second conductive layer onto at least a portion of the second dielectric layer; wherein the second conductive layer is a gate electrode; and wherein one or more of the first conductive layer, first semiconductor layer, first dielectric layer, charge trapping layer, second dielectric layer, and second conductive layer is deposited by inkjet printing of an ink formulation as defined herein.

[0054] The term charge trapping layer is a term of the art and will be understood to refer to a layer of dielectric material, a layer of semiconducting material or a layer of conductive material where charges occupy energy states comprised in the energy gap of both the first and the second dielectric layers.

[0055] In an embodiment, the charge trapping layer is deposited by inkjet printing an ink formulation, as described hereinbelow, wherein the ink formulation comprises at least one conductive material (e.g. graphene).

[0056] In another embodiment, the charge trapping layer is deposited by inkjet printing an ink formulation, as described hereinbelow, wherein the ink formulation comprises at least one semi-conductive material (e.g. WS2 or M0S2).

[0057] In an embodiment, the second conductive layer is deposited by inkjet printing an ink formulation, as described hereinbelow, wherein the ink formulation comprises at least one conductive material.

[0058] In another embodiment, the second dielectric is deposited onto the charge trapping layer by inkjet printing of an ink formulation, as described hereinbelow, wherein the ink formulation comprises at least one dielectric material.

[0059] In another aspect of the present invention, there is provided an electronic device obtainable by, obtained by, or directly obtained by, method 3 defined hereinabove.

Method 4 Memory devices

[0060] According to a fourth aspect of the present invention, there is provided a method of producing a memory device, comprising the steps of:

a) depositing a first track onto a substrate; b) determining a "word" (as a binary signal) to be stored by the memory device; c) depositing one or more regions of dielectric material onto the first track at one or more locations corresponding to a first logic state of the word; d) depositing one or more first logic tracks onto each of the one or more regions of the dielectric material; and e) depositing one or more second logic tracks to intersect the first track at one or more locations corresponding to a second logic state of the word; and f) electrically connecting each of the one or more first logic tracks and the one or more second logic tracks to respective electric devices; wherein one or more of the first track, the regions of the dielectric material, first logic tracks and second logic tracks are deposited by inkjet printing an ink formulation comprising a plurality of nanosheets of 2D crystalline material in an inkjet compatible vehicle.

[0061] In an embodiment, the memory device is a memory circuit which is programmable at the time of preparation (e.g. printing).

[0062] The first track, first logic track and second logic track each independently comprise at least one conductive material. Suitable conductive materials are analogous to suitable conductive materials described herein above.

[0063] Preferred and suitable embodiments for the substrate are analogous to preferred and suitable embodiments for the substrate for method 1 , as defined hereinabove.

[0064] In an embodiment, two or more, suitably three or more, and more suitable all, the first track, the regions of the dielectric material, first logic tracks and second logic tracks are deposited by inkjet printing an ink formulation as described herein.

[0065] In another embodiment, the nanosheets are associated with an exfoliation that renders the nanosheets dispersible within the inkjet compatible vehicle.

[0066] The methodology described herein (i.e. methods 1 to 4) may further comprise one or more post-processing steps applied to each of the deposited layers. For example, the methodology described herein may comprise the step of annealing by heating the layer to an elevated temperature for a specified period of time. The temperature used is dependent on the substrate. Preferably, annealing is conducted only on substrates such as polyimide, quartz and S1O2 which can be heated to over 250 °C without causing damage.

[0067] In another aspect of the present invention, there is provided an electronic device obtainable by, obtained by, or directly obtained by, method 4 defined hereinabove. Ink formulations

[0068] The ink formulations used in the present invention comprise a plurality of nanosheets of 2D crystalline material in an inkjet compatible vehicle.

[0069] The term '2D crystalline material' will be understood to refer to crystalline materials comprising one or a few (i.e. up to 10) layers consisting of covalently or ionically bonded atoms, with van der Waals forces comprising inter-layer bonding. In an embodiment, the 2D crystalline material is an inorganic material.

[0070] In another embodiment, the inkjet compatible vehicle is an aqueous vehicle.

[0071] In another embodiment, the nanosheets are associated with an exfoliation agent that renders the nanosheets dispersible within the inkjet compatible vehicle.

[0072] In a further embodiment, the ink formulation comprises a binder.

[0073] Suitably, the ink formulations used in the present invention comprise:

i) a plurality of nanosheets of an inorganic material in an inkjet compatible vehicle, wherein the nanosheets are optionally associated with an exfoliation agent that renders the nanosheets dispersible within the inkjet compatible vehicle;

ii) optionally, at least one surface tension modifier;

iii) optionally, and at least one viscosity modifier; and

iv) optionally, at least one binder;

wherein the mass ratio of inorganic material to exfoliation agent present in the formulation is greater than 5: 1.

[0074] More suitably, the ink formulations used in the present invention comprise:

i) a plurality of nanosheets of an inorganic material in an aqueous vehicle, wherein the nanosheets are associated with an exfoliation agent that renders the nanosheets dispersible within the aqueous vehicle;

ii) at least one surface tension modifier;

iii) and at least one viscosity modifier; and

iv) optionally, at least one binder;

wherein the mass ratio of inorganic material to exfoliation agent present in the formulation is greater than 5: 1

[0075] It will be appreciated that the ink formulations described herein all possess low levels of exfoliation agent in the aqueous vehicle. As previously indicated, excess exfoliation agent (e.g. in the aqueous vehicle of the ink formulation) may be detrimental because it could affect the mechanical and/or the electrical properties of the printed ink. It is therefore desirable to use ink formulations in inkjet printing in which the amount of excess exfoliation agent is as low as possible.

[0076] The nanosheets of the inorganic material are formed in situ within an aqueous vehicle comprising an exfoliation agent. During this process, a proportion of the exfoliation agent associates with nanosheets that are formed and prevent them from re-aggregating. The exfoliating agent also assists with the dispersion of the nanosheets in the aqueous medium. Excess exfoliation agent that is not associated with the nanosheets is removed in the subsequent processing steps. Thus, the resultant ink formulation comprises a lower amount of exfoliation agent, the vast bulk of which is associated with the nanosheets so as to render them dispersible with the aqueous medium.

[0077] In addition, the ink formulation used herein having relatively high loadings of the nanosheets of inorganic material in the aqueous vehicle.

[0078] In order to be suitable for piezoelectric inkjet printing, the ink formulations suitably have a viscosity within the range of 2 to 30 cPs.

[0079] In a particular embodiment, the viscosity is within the range of 10 to 12 cPs.

[0080] The viscosity quoted herein will be readily understood to be the dynamic viscosity, measured at 25 °C using, for example, the methodology described in the example section hereinbelow.

[0081] In an embodiment, the ink formulations have a surface tension within the range 20 to 50 mN/m. Suitably, the ink formulations have a surface tension within the range 28 to 45 rtiN/m. More suitably, the ink formulations have a surface tension within the range 28 to 35 mN/m.

[0082] The surface tensions quoted herein will also be understood as being those measured at 25 °C in accordance with the methods described in the examples section hereinbelow.

[0083] Furthermore, the ink formulations suitably do not evaporate readily, i.e. they are non-volatile at normal inkjet printing temperatures (e.g. at a standard room temperature of 20 to 25 °C). This prevents the clogging of the printer nozzle.

[0084] In a particular embodiment, the ink formulation is an inkjet formulation. [0085] The ink formulation may be prepared using any suitable technique known in the art. Examples of such techniques are described in WO2015114354, the entire contents of which is incorporated herein by reference.

[0086] Suitably, the ink formulation is obtainable by, obtained by or directly obtained by the method described hereinbelow.

Preparation of the ink formulation

[0087] One particular process for the preparation of an ink formulation, as defined herein, comprises the following steps:

a) providing a source of the inorganic material (e.g. one or more multi-layered bulk particles of the inorganic material) in a first aqueous medium comprising an exfoliation agent;

b) subjecting the source of the inorganic material in the first aqueous medium to energy (e.g. sonic energy) to break up or exfoliate the source of the inorganic material to obtain an aqueous dispersion of nanosheets of the inorganic material in the first aqueous medium;

c) separating any residual source material from the first aqueous medium of step b); d) performing ultracentrifugation on the solution obtained in step c) in order to remove excess exfoliation agent and increase the concentration of nanosheets;

e) optionally, repeating step d) one or more times; and

f) optionally, incorporating a binder.

[0088] The nanosheets are formed within a first aqueous medium comprising an exfoliation agent (see step b). The exfoliation agent associates with the nanosheets during this process and aids their dispersibility within the aqueous medium. This also enhances the efficiency with which the nanosheets are formed. The dispersion of nanosheets formed in step b) are then subjected to process step c), in which any residual source material that has not been fully broken up or exfoliated in step b) to form the nanosheets is removed from the dispersion. Following step c) the stabilized nanosheets formed in the first aqueous medium are subjected to ultracentrifugation in step d) in order to remove excess exfoliation agent and increase the nanosheet concentration. Step d) can be performed multiple times, as necessary. The inkjet printable ink obtained after step d) comprises of an aqueous medium with nanosheets, at least one binder, optionally at least one viscosity modifier and optionally at least one surfactant. The binder must be added after step b) as the high shear forces involved in exfoliation can degrade it.

Step a)

[0089] The process comprises, in step a), the provision of a source of the inorganic material in a first aqueous medium comprising an exfoliation agent. The source will typically be one or more bulk particles of a multi-layered inorganic material. For example, in embodiments where the inorganic material is graphene, then the source will be one or more bulk graphite particles.

[0090] The aqueous medium comprises an exfoliation agent that will associate with the nanosheets of inorganic material formed in the subsequent process step b). The exfoliation agent will assist in the effective formation and dispersion of the nanosheets that are formed in step b).

[0091] Suitable exfoliation agents are known in the art.

[0092] Suitably, the exfoliation agent present in the first aqueous medium is a water soluble polyaromatic compound as defined hereinbefore. The amount of exfoliation agent present in the first aqueous medium needs to be sufficient to stabilise the nanosheets that are formed in step b) of the process. In part, this will depend on the amount of inorganic material that is present.

[0093] Typically, the first aqueous medium will comprise 0.05 to 2 g/L of an exfoliation agent. Suitably, the first aqueous medium will comprise 0.1 to 1.5 g/L of an exfoliation agent. More suitably, the first aqueous medium will comprise 0.1 to 1.0 g/L of an exfoliation agent. Yet more suitably, the first aqueous medium will comprise 0.2 to 0.7 g/L of an exfoliation agent.

[0094] The first aqueous medium may further comprise a surface tension modifier as previously defined herein in reference to the aqueous vehicle of the ink formulation. The amount of surface tension modifier present may be the same as the amounts quoted herein for the aqueous vehicle of the ink formulation.

[0095] The first aqueous medium may further comprise a viscosity modifier as previously defined herein in reference to the aqueous vehicle of the ink formulation. The amount of the viscosity modifier present may be the same as the amounts quoted herein for the aqueous vehicle of the ink formulation.

[0096] The first aqueous medium may have any suitable pH. [0097] Particularly (but not exclusively) where the exfoliation agent is a polycyclic aromatic compound such as pyrenesulphonic acid (Py-2SOs), high concentrations of nanosheets can in some cases be obtained at acidic pHs (e.g. from 1 to 7 or from 1 to 3 or about 2). Acidic pHs are particularly preferred where the two-dimensional inorganic compound is h-BN.

[0098] The aqueous medium may have a pH in the range 5 to 9, e.g. from 6 to 8. Neutral pHs are particularly preferred where the two-dimensional compound is a transition metal dichalcogenide or graphene.

[0099] Suitably, the first aqueous medium is prepared by dissolving the exfoliation agent and any other components in the water. The source of inorganic material is then immersed in the prepared first aqueous medium in preparation for the exfoliation in step b).

Step b)

[00100] In step b), the source of the inorganic material present in the first aqueous medium is exposed to energy (e.g. sonic energy) to break up or exfoliate the source of the inorganic material in order to obtain an aqueous dispersion of nanosheets of the inorganic material in the first aqueous medium.

[00101] It was found that enhanced loadings of nanosheets in the ink formulation can be obtained by forming the nanosheets in situ within the first aqueous medium in the presence of an exfoliation agent. The nanosheets can be formed by exfoliating the source of the bulk multi-layered inorganic material (typically in the form of one or more large particles of the bulk material) in the pre-formed aqueous vehicle.

[00102] In this regard, the water soluble polycyclic aromatic compounds that represent the preferred exfoliation agents for the first aqueous medium have been found to be particularly efficient at effecting the exfoliation of bulk inorganic layered materials to form thin (two- dimensional) nanosheets as defined herein. The presence of hydrophilic groups allows the polyaromatic compound to interact with the water and thus also act as a dispersant, thereby stabilising the resultant nanosheet suspension. It is believed that as the nanosheets form, polycyclic aromatic compounds intercalate and adsorb to the surface plane of the layers, thereby stabilising the nanosheets that are formed and preventing their re-aggregation.

[00103] The energy applied to convert the multi-layered particles into a dispersion of nanosheets in step (b) may be sonic energy. Suitably, the sonic energy is ultrasonic energy. Sonic energy may be delivered by using a bath sonicator or a tip sonicator. Alternatively, the energy may be a mechanical energy, e.g. shear force energy or grinding. The particles may be subjected to energy (e.g. sonic energy) for a length of time from 15 min to up to 1 week, depending on the properties and proportions (nanosheet diameter and thickness) desired. The particles may be subjected to energy (e.g. sonic energy) for a length of time from 1 to 4 days (in particular for inkjet printing where the size of the flakes must be smaller than the size of the printer nozzles). Suitably, the energy is sonic energy provided by immersing the formulation into a sonicator in step (b) which has a frequency of between 10 and 100 kHz (e.g. 35 kHz) and a power of 100 to 1000 Watts (e.g. 120 to 400 Watts).

Step c)

[00104] Step c) serves to remove any remaining source material present in the first aqueous medium (also referred to as the aqueous dispersion) obtained in step b).

[00105] The separation of the remaining source material from the dispersion can be facilitated by any suitable separation technique known in the art. For example, suitable centrifugation, filtration or dialysis techniques may be used. Suitably, any larger particulates of the residual source material are separated from the dispersed nanosheets and the first aqueous medium by centrifugation. A person skilled in the art will know how to select suitable centrifugation speeds and times to affect the deposition of any larger particulate material present following step b) of the process.

[00106] Typically, remaining bulk source material is removed by centrifuging the dispersion prepared in step b) of the process at a centrifugation speed of 100 to 2000 rpm. Suitably, remaining bulk source material is removed by centrifuging the dispersion prepared in step b) of the process at a centrifugation speed of 200 to 1800 rpm. More suitably, remaining bulk source material is removed by centrifuging the dispersion prepared in step b) of the process at a centrifugation speed of 500 to 1500 rpm. In each case, the dispersion may be subjected to a second centrifugation at a speed of either 2000 to 6000 rpm, 3000 to 5000 rpm, or 3000 to 4000 rpm.

[00107] Suitably, remaining bulk source material is removed by centrifuging the dispersion prepared in step b) of the process at a centrifugation speed of 500 to 1500 rpm. More suitably, remaining bulk source material is removed by centrifuging the dispersion prepared in step b) of the process at a centrifugation speed of 700 to 1300 rpm. Yet more suitably, remaining bulk source material is removed by centrifuging the dispersion prepared in step b) of the process at a centrifugation speed of 900 to 1100 rpm. In each case, the dispersion may be subjected to a second centrifugation at a speed of either 3000 to 4000 rpm, 3200 to 3800 rpm, or 3400 to 3600 rpm. In addition, in case, the duration of the centrifugation may be from 5 minutes to 4 hours, with a time of 15 minutes to 1 hour (e.g. approximately 20 minutes) being preferred. [00108] The resultant dispersion will therefore comprise the nanosheets dispersed in the first aqueous medium and little or no remaining bulk source material.

Steps d, e and f)

[00109] Following the formation of the nanosheets in the first aqueous medium in step b), and optionally the removal of any larger particles of residual source material in step c), it is necessary to remove excess exfoliation agent and introduce the binder into the formulation.

[00110] It will usually be desirable to repeat step d) multiple times. Thus, in an embodiment step e) is conducted one or more times.

[00111] In step d), the dispersed nanosheets are sedimented by ultracentrifugation and the supernatant replaced with an aqueous vehicle comprising at least one surfactant, at least one binder and at least one viscosity modifier to form an ink formulation. This sedimentation and supernatant removal process may be repeated multiple times.

[00112] The further aqueous medium may be the same as the aqueous vehicle of the ink formulation that is defined hereinbefore with binder included.

[00113] The sedimentation of the nanosheets from the aqueous medium can be facilitated by any suitable separation technique known in the art. For example, suitable microfiltration, nanofiltration or dialysis techniques may be used. Suitably, the nanosheets are separated from the aqueous medium by centrifugation to form a pellet containing the nanosheets and a supernatant comprising the aqueous vehicle and excess exfoliation agent. Selecting appropriate centrifuge conditions enables the deposition of the nanosheets present in the dispersion generated following step b) or step c). Following centrifugation, the supernatant can be removed and optionally recycled for use again in steps a) and b) of the process. The supernatant is replaced by a further aqueous medium comprising at least one surfactant, at least one binder and at least one viscosity modifier.

[00114] Typically, the nanosheets are separated from the aqueous medium by centrifuging the dispersion prepared in step b) or step c) of the process at a centrifugation speed of 14000 to 16000 rpm. Suitably, nanosheets are separated from the aqueous medium by centrifuging the dispersion prepared in step b) or step c) of the process at a centrifugation speed of 14500 to 16000 rpm. More suitably, nanosheets are separated from the aqueous medium by centrifuging the dispersion prepared in step b) or step c) of the process at a centrifugation speed of 14500 to 15500 rpm. In each case, the duration of the centrifugation may be from 5 minutes to 4 hours, with a time periods of 40 to 120 minutes, or 50 to 100 minutes, or 55 to 75 minutes being generally preferred. [00115] As any excess exfoliation agent is removed by step d) of the process, the aqueous vehicle will have a low concentration of exfoliation agent and may even be free of additional exfoliation agent altogether. There will be some exfoliation agent still present as it associates with the nanosheets during step b) of the process, but the amount of excess exfoliation agent in the aqueous vehicle will be minimised.

Nanosheets of inorganic material

[00116] The term 'inorganic material' refers to any inorganic material made up of one or more elements (including carbon) which form layered structures in which the bonding between atoms within the same layer is ionic or covalent and the layers are held together by Van der Waals forces. Suitably, the inorganic material is crystalline or at least partially crystalline.

[00117] Any suitable inorganic layered material may be used to form the nanosheets in the ink formulations of the present invention.

[00118] Particular examples of layered inorganic compounds include: graphene, hexagonal boron nitride, bismuth strontium calcium copper oxide (BSCCO), transition metal dichalcogenides (TMDCs), Sb2Te3, Bi2Te3 and Mn02. The list of exemplary layered inorganic compounds may also include thermoelectrics such as B12S3 and SnS.

[00119] TM DCs are structured such that each layer of the material consists of three atomic planes: a layer of transition metal atoms (for example Mo, Ta, W) sandwiched between two layers of chalcogen atoms (for example S, Se or Te). Thus, in one embodiment, the TMDC is a compound of one or more of Mo, Ta and W with one or more of S, Se and Te. There is strong covalent bonding between the atoms within each layer of the transition metal chalcogenide and predominantly weak Van der Waals bonding between adjacent layers. Exemplary TM DCs include NbSe₂, WS₂, M0S2, TaS₂, PtTe₂, and VTe₂.

[00120] In an embodiment, the inorganic layered material is selected from graphene, hexagonal boron nitride, WS2 and/or M0S2.

[00121] In another embodiment, the inorganic layered material is selected from graphene, hexagonal boron nitride, B12S3, SnS, WS2 and/or M0S2.

[00122] The inorganic layered material will be understood to encompass conductive, semi- conductive and dielectric materials.

[00123] In an embodiment, the inorganic layered material is a conductive material selected from graphene, carbon nanotubes, electronically conductive metals and nanoparticles thereof (e.g. silver nanoparticles), electronically conductive polymers (e.g. PEDOT:PSS) and combinations thereof. Suitably, the conductive material is graphene or electronically conductive metals and nanoparticles (e.g. silver nanoparticles). Most suitably, the conductive material is graphene.

[00124] In another embodiment, the inorganic layered material is a semi-conductive material selected from a transition metal dichalcogen (e.g. M0S2, WS2, ΜοΤβ2, MoSe2 etc.). Suitably, the inorganic layered material is a semi-conductive material selected from WS2 or M0S2, most suitably, WS₂.

[00125] In yet another embodiment, the inorganic layered material is a dielectric material selected from hexagonal boron nitride, dielectric polymers, dielectric zeolites or oxides. Most suitably, the dielectric material is hexagonal boron nitride.

[00126] In a particular embodiment, the inorganic material is graphene. Graphene is the name given to a particular crystalline allotrope of carbon in which each carbon atom is bound to three adjacent carbon atoms (in a sp² hybridised manner) so as to define a one atom thick planar sheet of carbon. The carbon atoms in graphene are arranged in the planar sheet in a honeycomb-like network of tessellated hexagons. Graphene is often referred to as a 2-dimensional crystal because it represents a single nanosheet or layer of carbon of nominal (one atom) thickness. Graphene can be considered to be a single sheet of graphite.

[00127] Suitably, the nanosheets may comprise single layers of graphene or thin stacks of two or more graphene layers. In this regard, it is generally acknowledged that crystals of graphene which have more than 10 molecular layers (i.e. 10 atomic layers which equates to a thickness of approximately 3.5 nm) generally exhibit properties more similar to graphite than to graphene. The nanosheets of graphene used in the present invention are formed by exfoliation of graphite.

[00128] For the avoidance of doubt, the term graphene used herein does not encompass graphene oxide or any other form of covalently modified graphene.

[00129] In another embodiment, the inorganic layered material is selected from hexagonal boron nitride, WS2 and M0S2.

[00130] In another embodiment, the inorganic layered material is selected from hexagonal boron nitride, WS2 and/or M0S2.

[00131] In certain embodiments, the inorganic material may be h-BN. Single layer h-BN is structurally similar to graphene, but unlike its carbon analogue, it is an insulator with a large band gap (~6eV). This, added to unique features such as excellent chemical, mechanical properties, and thermal stability, allows using h-BN nanosheets (BNNS) in a variety of applications, such as components in nanodevices, solid lubricant, UV-light emitter and as insulating thermo-conductive filler in composites.

[00132] The inorganic material may also be a transition metal dichalcogen (e.g. M0S2, WS₂, MoTe₂, MoSe₂ etc.).

[00133] The nanosheets present in the ink formulation are suitably prepared by breaking up or "exfoliating" larger particles of the multi-layered inorganic material (as defined in step b) of the process of the present invention). The nanosheets formed by an exfoliation process may consist of a single layer or two or more layers of the inorganic material.

[00134] Suitably, the majority (greater than 50%) of the nanosheets of inorganic material present in the ink formulations comprise less than ten layers of the inorganic material.

[00135] In an embodiment, greater than 60% of the nanosheets of inorganic material present in the ink formulations comprise less than ten layers of the inorganic material. In a further embodiment, greater than 75% of the nanosheets of inorganic material present in the ink formulations comprise less than ten layers of the inorganic material. In another embodiment, greater than 80% of the nanosheets of inorganic material present in the ink formulations comprise less than ten layers of the inorganic material. In a further embodiment, greater than 90% of the nanosheets of inorganic material present in the ink formulations comprise less than ten layers (or sheets) of the inorganic material.

[00136] Suitably, the proportion of nanosheets having less than 10 layers is as high as possible. More suitably, the proportion of nanosheets having less than 8 layers is a high as possible.

[00137] The proportion of single layer nanosheets is also suitably as high as possible, e.g. greater than 15%, or more preferably greater 20%, of the nanosheets present in the ink formulation. The amount of single layered material present will depend in part of the conditions (energy input and time) used to prepare the nanosheets.

[00138] In certain embodiments, it may be that greater than 40% (e.g. greater than 50%, or greater than 75%, greater than 80% or greater than 90%) of the nanosheets of an inorganic layered material have a thickness of from 1 to 10 molecular layers. It may be that greater than 40% (e.g. greater than 50%, or greater than 75%, greater than 80% or greater than 90%) of the nanosheets have a thickness of from 1 to 7 molecular layers. Thus, it may be that greater than 20% of the nanosheets have a thickness of 1 molecular layer. These statements apply particularly to nanosheets of graphene. [00139] In certain embodiments, it may be that greater than 40% (e.g. greater than 50%, or greater than 75%, greater than 80% or greater than 90%) of the nanosheets have a thickness of from 1 to 6 molecular layers. Thus, it may be that greater than 40% (e.g. greater than 50%, or greater than 75%, greater than 80% or greater than 90%) of the nanosheets have a thickness of from 4 to 6 molecular layers. These statements apply particularly to nanosheets of transition metal dichalcogenides.

[00140] In certain embodiments, it may be that greater than 50% by weight (e.g. greater than 75% by weight, greater than 90% or greater than 98%) of the nanosheets have a thickness of from 1 to 8 molecular layers. Accordingly, it may be that greater than 50% by weight (e.g. greater than 75% by weight, greater than 90% or greater than 98%) of the nanosheets have a thickness of from 3 to 8 molecular layers. Thus, it may be that greater than 50% by weight (e.g. greater than 75% by weight, greater than 90% or greater than 98%) of the nanosheets have a thickness of from 4 to 6 molecular layers. These statements apply particularly to nanosheets of transition metal dichalcogenides.

[00141] In certain embodiments, it may be that greater than 50% by weight (e.g. greater than 75% by weight, greater than 90% or greater than 98%) of the nanosheets have a thickness of from 1 to 10 molecular layers. It may be that greater than 50% by weight (e.g. greater than 75% by weight, greater than 90% or greater than 98%) of the nanosheets have a thickness of from 1 to 5 molecular layers. Thus, it may be that greater than 50% by weight (e.g. greater than 75% by weight, greater than 90% or greater than 98%) of the nanosheets have a thickness of from 1 to 3 molecular layers. These statements apply particularly to nanosheets of h-BN.

[00142] Each nanosheet has a length and a width dimension to define the size of the plane of the nanosheet. Suitably, the length and width of the nanosheets are within the range of 10 nm to 2 microns. More suitably, the length and width of the nanosheets are within the range of 10 nm to 500 nm.

[00143] For example, it may be that greater than 75% (e.g. greater than 90% or greater than 98%) of the nanosheets of an inorganic layered material have a length or width dimension of between 10nm and 2 microns. It may be that greater than 75% (e.g. greater than 90% or greater than 98%) of the nanosheets have a length or width dimension of less than 1 micron. Thus, it may be that greater than 75% (e.g. greater than 90% or greater than 98%) of the nanosheets have a length or width dimension of between 10 and 500 nm.

[00144] It may be that greater than 50% by weight (e.g. greater than 75% by weight, greater than 90% or greater than 98%) of the nanosheets have a diameter between 50 and 750 nm. It may be that greater than 50 % by weight (e.g. greater than 75% by weight, greater than 90% or greater than 98%) of the nanosheets have a diameter of less than 500 nm. Thus, it may be that greater than 50% by weight (e.g. greater than 75% by weight, greater than 90% or greater than 98%) of the nanosheets have a diameter between 100 and 500 nm.

[00145] Typically, the concentration of the nanosheets in the aqueous vehicle of the ink formulation is 1 to 10 g/L. More typically, the concentration of the nanosheets in the aqueous vehicle is 2 to 6 g/L. Even more typically, the concentration of the nanosheets in the aqueous vehicle is 2.5 to 4.0 g/L.

[00146] In a further embodiment, the concentration of nanosheets in the aqueous vehicle is within the range of 0.01 to 15 mg/ml, suitably within the range of 0.01 to 10 mg/ml, and more suitably within the range of 0.01 to 5 mg/ml.

Exfoliation agent

[00147] The nanosheets may be associated with an exfoliation agent that renders the nanosheets dispersible in the aqueous medium in the ink formulations of the present invention.

[00148] Suitable exfoliation agents are known in the art.

[00149] In a particular embodiment, the exfoliation agent is a water soluble polyaromatic compound.

[00150] The exfoliation agents used in the ink formulations are suitably efficient dispersants for the nanosheets (i.e. they help form and maintain the dispersion of the nanosheets in the aqueous vehicle). The preferred way to prepare the ink formulations is to form the nanosheets in situ within the exfoliation agent, defined herein by exfoliating larger particles of the bulk multi-layered inorganic material. The exfoliation agents are particularly efficient at effecting the exfoliation of bulk inorganic layered materials to form the required nanosheets. The use of these exfoliation agents also has a beneficial effect on the loading of nanosheets that can be achieved in the ink formulation.

[00151] The presence of hydrophilic groups allows the exfoliation agent to interact with the water as well as the nanosheets of the inorganic layered material. The stabiliser acts as a dispersant, thereby giving greater stability to the resultant dispersion of the nanosheets formed in the aqueous vehicle. Without wishing to be bound by any particular theory, it is believed that as the nanosheets are formed, for example by the application of energy (e.g. sonic energy) to exfoliate the bulk material, the exfoliation agent molecules penetrate between the layers of the inorganic material and non-covalently interact with the surfaces of the layers. It is believed that the stabiliser (e.g. pyrene) therefore aids the detachment of the nanosheets and then prevents them re-aggregating.

[00152] The exfoliation agent may have a ring system which comprises from 2 to 10 fused benzene rings, the ring system being substituted with from 1 to 4 independently selected hydrophillic groups, each hydrophilic group consisting of less than 20 atoms. In an embodiment, the ring system may have from 2 to 8 or, more typically, 3 to 6 fused benzene rings. In a particular embodiment, the ring system has four fused benzene rings. Where the ring system contains three or more benzene rings (e.g. four benzene rings), it may be that the rings are not arranged linearly. Thus, the ring system may have at least one ring which is ortho- and peri-fused to two further rings respectively. In other words, it may be that at least one ring contains two atoms in common with each of two or more other rings.

[00153] The independently selected hydrophilic groups may consist of less than 10 atoms or they may consist of less than 6 atoms. The atoms in question may be independently selected from S, O, P, H, C, N, B and I. Exemplary hydrophilic groups include SO3H, SO2H, B(OH)2, CO2H, OH and PO3H. Suitably, when the exfoliation agent comprises four substituent groups, preferably, they are not all the same.

[00154] The exfoliation agent may be a salt and may therefore be a base addition salt. The above mentioned hydrophilic groups may therefore be SO3M, SO2M, CO2M and PO3M, where M is a cation, e.g. a cation selected from Na⁺, K⁺, Li⁺ and Nh .

[00155] In an embodiment, the exfoliation agent may be a pyrene substituted with from 1 to 4 R groups, wherein each R group is independently selected from a group of the formula:

-L-X

wherein:

L is absent or an alkylene optionally interrupted with one or more oxygen atoms; and

X is a hydrophilic group.

[00156] In an embodiment, L is absent or a (1-1000C)alkylene, optionally interrupted with one or more oxygen atoms. Suitably, L is absent or a (1-100C)alkylene optionally interrupted with one or more oxygen atoms. More suitably, L is absent or a (1-50C)alkylene, optionally interrupted with one or more oxygen atoms. Still more suitably, L is absent or a (1-10C)alkylene, optionally interrupted with one or more oxygen atoms. Most suitably, L is absent.

[00157] In another embodiment, the hydrophilic substituent group is selected from SO3H, SO2H, B(OH)₂, CO2H, OH, PO3H or a salt form thereof. More suitably, the hydrophilic substituent group is selected from SO3M, SO2M, CO2M and PO3M, where M is a cation, e.g. a cation selected from Na⁺, K⁺, Li⁺ or Nh . Yet more suitably, the hydrophilic substituent group is selected from SO3M or OH, wherein M is a cation selected from Na⁺, K⁺, Li⁺ or NH3⁺. Most suitably, the hydrophilic substituent group is selected from SO3M, wherein M a cation selected from Na⁺, K⁺, Li⁺ or Nh .

[00158] In a particular embodiment, the exfoliation agent is selected from one of the following formulae:

wherein, each R group is as defined hereinabove.

[00159] In another particular embodiment, the exfoliation agent may be a pyrene substituted with from 1 to 4 hydrophilic groups. It may be that the hydrophilic groups are selected from SO3M and OH. Specific examples include:

(PS1), (PS3),

(PS2).

These exemplary pyrene sulfonic acid salts are dyes. They are thus readily available.

[00160] In a particular embodiment, the exfoliation agent is 1-pyrenesulfonic acid sodium salt.

[00161] Suitably, the mass ratio of inorganic material to exfoliation agent present in the ink formulation is greater than 5:1. More suitably, the mass ratio of inorganic material to exfoliation agent present in the ink formulation is greater than 10: 1. Even more suitably, the mass ratio of inorganic material to exfoliation agent present in the ink formulation is greater than 15: 1. Most suitably, the mass ratio of inorganic material to exfoliation agent present in the ink formulation is greater than 17: 1.

[00162] In an embodiment, the mass ratio of inorganic material to exfoliation agent present in the ink formulation is between 5: 1 and 50: 1. Suitably, the mass ratio of inorganic material to exfoliation agent present in the ink formulation is between 7:1 and 40: 1. More suitably, the mass ratio of inorganic material to exfoliation agent present in the ink formulation is between 8:1 and 30: 1. Most suitably, the mass ratio of inorganic material to exfoliation agent present in the ink formulation is between 10:1 and 20: 1.

[00163] In an embodiment, the mass ratio of inorganic material to exfoliation agent present in the ink formulation is between 5: 1 and 20: 1. Suitably, the mass ratio of inorganic material to exfoliation agent present in the ink formulation is between 10: 1 and 19: 1. More suitably, the mass ratio of inorganic material to exfoliation agent present in the ink formulation is between 15: 1 and 19: 1. Most suitably, the mass ratio of inorganic material to exfoliation agent present in the ink formulation is between 17:1 and 19: 1.

[00164] In a further embodiment, the amount of free exfoliation agent (e.g. pyrene) in the ink formulation is less than 25 wt.%. Suitably, the amount of free exfoliation agent (e.g. pyrene) in the ink formulation is less than 20 wt.%. More suitably, the amount of free exfoliation agent (e.g. pyrene) in the ink formulation is less than 15 wt.%. Yet more suitably, the amount of free exfoliation agent (e.g. pyrene) in the ink formulation is less than 10 wt.%. Most suitably, the amount of free exfoliation agent (e.g. pyrene) in the ink formulation is less than 8 wt.%.

The inkjet compatible vehicle

[00165] The nanosheets of the ink formulation are dispersed in a inkjet compatible vehicle. The term 'inkjet compatible vehicle' can be understood to mean a liquid vehicle which has the correct Theological properties to be used for inkjet printing in a fabrication/manufacturing environment.

[00166] it will be appreciated that the inkjet compatible vehicle may be any suitable organic solvent, aqueous medium or mixture thereof, each optionally comprising one or more suitable additives (e.g. a viscosity modifier and/or surface tension modifier).

[00167] In an embodiment, the inkjet compatible vehicle is an aqueous vehicle.

[00168] Thus, suitably, the nanosheets of the ink formulation are dispersed in the aqueous vehicle. The term 'aqueous vehicle' can be understood to mean a liquid vehicle which contains water.

[00169] The aqueous vehicle may comprise greater than about 20% by volume of water. For example, the aqueous medium may contain more than 50% by volume water, e.g. more than 75% by volume water or more than 95% by volume water. Typically the aqueous vehicle will comprise 50 to 100 % by volume of water and more typically 50 to 99.97%.

[00170] The water content of the ink formulation as a whole will typically be greater than 20 wt.% relative to the total weight of the formulation. In an embodiment, the water content of the ink formulation is greater than or equal to 50 wt.% relative to the total weight of the formulation.

[00171] Typically, the water content will be within the range of greater than or equal to 20 wt.% to less than or equal to 99.97 wt.% relative to the total weight of the formulation. In an embodiment, the water content will be within the range of greater than 50 wt.% to less than or equal to 99.97 wt.% relative to the total weight of the formulation. In another embodiment, the water content is within the range of greater than or equal to 80 wt.% to less than or equal to 99.97 wt.% relative to the total weight of the formulation. In another embodiment, the water content is within the range of greater than or equal to 90 wt.% to less than or equal to 99.97 wt.% relative to the total weight of the formulation. In another embodiment, the water content is within the range of greater than or equal to 99 wt.% or 99.9 wt.%, to less than or equal to 99.97 wt.%, or to less than or equal to 95 wt.% relative to the total weight of the formulation.

[00172] The 'aqueous vehicle' may also comprise other solvents. It may therefore comprise organic solvents which may or may not be miscible with water. Where the aqueous medium comprises organic solvents, those solvents may be immiscible or sparingly miscible and the aqueous medium may be an emulsion. The aqueous medium may comprise solvents which are miscible with water, for example alcohols (e.g. methanol and ethanol). The aqueous medium may comprise one or more additives which may be ionic, organic or amphiphillic. Examples of possible additives include surfactants, viscosity modifiers, pH modifiers, tonicity modifiers, and dispersants.

[00173] In addition to the nanosheets, the aqueous vehicle may have other particulate components dispersed within it, such as, for example, metallic particles and/or carbon nanotubes.

[00174] The aqueous medium may have any pH. The aqueous medium may have a pH in the range from 1 to 13. The aqueous medium may have a pH in the range from 1 to 7, e.g. in the range from 2 to 7 depending on the inorganic material.

Viscosity modifier

[00175] In order to render the ink formulation suitable for inkjet printing, the viscosity of the aqueous vehicle needs to be within the range of 2 to 30 cPs, more preferably within the range of 5 to 20 cPS, and yet more preferably within the range of 10 to 12 cPs. Accordingly, as the viscosity of water is 1 cPs, the ink formulation comprises a viscosity modifier as a component of the aqueous vehicle.

[00176] Any suitable viscosity modifier may be used in the ink formulations. The viscosity modifier is suitably a water miscible co-solvent. Examples of suitable viscosity modifiers include (and are not limited to) glycols (e.g. ethylene glycol, propylene glycol), ethers (e.g. ethylene glycol methyl ether), alcohols (e.g. 1-propanol), esters (ethyl lactate), ketones (e.g. methyl ethyl ketone (MEK)) and organo-sulphur compounds (e.g. sulfolane).

[00177] In a particular embodiment, the viscosity modifier is selected from ethylene glycol, propylene glycol and/or ethylene glycol methyl ether.

[00178] Suitably the viscosity modifier is a material which, when combined with water, forms an aqueous vehicle with a boiling point of below 200 °C. More suitably, the viscosity modifier is a material which, when combined with water, forms an aqueous vehicle with a boiling point of below 180 °C or below 150 °C. Suitably, the boiling point is not too low that the co-solvent readily evaporates at normal inkjet printing temperatures. In an embodiment, the viscosity modifier is a material which, when combined with water, forms an aqueous vehicle with a boiling point within the range of 80 to 200 °C, more suitably 90 to 150 °C.

[00179] The amount of viscosity modifier added is suitably sufficient to provide the final ink formulation with a viscosity of 1 to 30 cPs, preferably 2 to 30 cPs, more preferably 5 to 20 cPs, and yet more preferably 10 to 12 cPs. Typically, the viscosity modifier is present in the ink formulations at an amount of from 0.01 to 60 wt.%, and suitably 0.01 to 50 wt.%. Suitably, the viscosity modifier is present in the ink formulations at an amount of from 0.03 to 50 wt.%. In an embodiment, the viscosity modifier is present in the ink formulations at an amount of from 0.03 to 30 wt.%. In another embodiment, the viscosity modifier is present in the ink formulations at an amount of from 0.03 to 10 wt.%. In a further embodiment, the viscosity modifier is present in the ink formulations at an amount of from 0.03 to 5 wt.%. In yet another embodiment, the viscosity modifier is present in the ink formulations at an amount of from 0.03 to 0.1 wt.%.

Surface tension modifier

[00180] In order to render the formulation suitable for inkjet printing, the surface tension of the ink formulation needs to be adjusted to be within the range 20 to 50 mN/m and preferably within the range 28 to 45 mN/m and more preferably 28 to 35 mN/m. Accordingly, the ink formulation comprises a surface tension modifier as a component of the aqueous vehicle.

[00181] Water has a surface tension of 72 mN/m, so the surface tension modifier needs to reduce the surface tension of the formulation.

[00182] Any suitable surface tension modifier may be used in the ink formulations. The surface tension modifier is suitably a water-soluble surface active material. Examples of suitable materials include surfactants. Non-ionic surfactants are generally preferred. Any suitable non-ionic surfactant may be used. Typical examples include Triton, Tween, poloxamers, cetomacrogol 1000, cetostearyl alcohol, cetyl alcohol, cocamide DEA, monolaurin, nonidet P-40, nonoxynols, decyl glucoside, pentaethylene glycol monododecyl ether, lauryl glucoside, oleyl alcohol, and polysorbate.

[00183] In a particular embodiment, the surface tension modifier is Triton x-100.

[00184] The amount of surface tension modifier present in the ink formulation is an amount sufficient to provide the final formulation with a surface tension of 20 to 50 mN/m, preferably 28 to 45 mN/m, and more preferably between 28 and 35 mN/m. [00185] Typically, the surface tension modifier is present in the ink formulations at an amount of from 0.01 to 0.5 g/L. Suitably, the surface tension modifier is present in the ink formulations at an amount of 0.04 to 0.2 g/L. In an embodiment, the surface tension modifier is present in the ink formulations at an amount of 0.04 to 0.1 g/L. In a further embodiment, the surface tension modifier is present in the ink formulations at an amount of from 0.04 to 0.08 g/L.

Binder

[00186] In addition to the viscosity modifier and the surface tension modifier, the ink formulation may also comprise a binder. The binder may be used to reduce re-dispersion of flakes in printed materials and to avoid re-dispersion of materials at the interface of the printed ink. The binder may also have a viscosity modifying effect (in addition to the viscosity modifiers discussed hereinbefore) and/or improve the stability of the ink. The binder, when present, is distinct from the active exfoliating agents discussed hereinbefore.

[00187] Any suitable binder may be used in the ink formulations. The binder is suitably a water-soluble polymer (for water-based dispersion). The binder does not, however, need to be soluble in the viscosity modifier.

[00188] In a particular embodiment, the binder is selected from a polysaccharide (e.g. xanthan gum), polyvinylpyrrolidone and polyethylene glycol. Suitably, the binder is xanthan gum or carboxymethylcellulose, more suitably xanthan gum, which allows the preparation of inks with non-Newtonian viscosity (i.e. an ink with shear-thinning properties).

[00189] A binder (e.g. xanthan gum) is particularly advantageous when a water-based ink is intended for use in the process and or devices of the present invention. The printing of vertical heterostructures in particular requires two or more 'layers' of ink to be printed on top of one another (in a stacked arrangement). This stacked (or vertical) printing is generally considered to be very challenging to achieve, especially since the printing of a subsequent layer (film) of ink onto a preceding printed layer (film) of ink results in a high degree of dispersion between the two layers at the interface. This re-dispersion of the preceding layer of printed ink makes it difficult to print discrete layers (films) of inks directly on top of one another. To their surprise, the inventors have discovered that this problem may be addressed by including a suitable binder in the inkjet formulation. Xanthan gum is also biocompatible.

[00190] Typically, the binder is present in the ink formulations at an amount of from 0.01 to 2.0 g/L. Suitably, the binder is present in the ink formulations at an amount of 0.05 to 1.0 g/L. In an embodiment, the binder is present in the ink formulations at an amount of 0.1 to 0.8 g/L. In a further embodiment, binder is present in the ink formulations at an amount of from 0.15 to 0.5 g/L.

Additional materials

[00191] In certain embodiments, it may be preferable to include one or more additional materials to improve the properties of the ink formulation (e.g. the electrical and/or thermal conductivity).

[00192] In an embodiment, the ink formulations may comprise an additional electrically conductive material. For example, the ink formulations may comprise an electrically conductive material selected from the group consisting of carbon nanotubes, electronically conductive metal nanoparticles (e.g. silver nanoparticles) or an electronically conductive polymer (e.g. PEDOT:PSS). Most suitably, the ink formulations may comprise an electrically conductive material selected from the group consisting of silver nanoparticles, poly(3,4- ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) or carbon nanotubes.

[00193] In another embodiment, the ink formulations may comprises an additional material that has insulative properties.

[00194] It will be appreciated that any suitable amount of this additional material may be present in the ink formulations. The amount of additional material added will also be understood to be dependent on the end use of the ink formulation concerned. Suitably, the ratio of additional material to inorganic layered material is from 0.5:99.5 to 15:85. More suitably, the ratio of additional material to inorganic layered material is from 1 :99 to 10:90. Most suitably, the ratio of additional material to inorganic layered material is from 1 :99 to 5:95.

Particular embodiments

Method 1

[00195] In a particular embodiment, there is provided a method of producing an electronic device, comprising the steps of: a) depositing a first conductive layer onto a substrate, wherein the first conductive layer is deposited by inkjet printing an ink formulation comprising a plurality of nanosheets of conductive 2D crystalline material in an aqueous vehicle, b) depositing a dielectric layer onto at least a portion of the first conductive layer, wherein the dielectric layer is deposited by inkjet printing an ink formulation comprising a plurality of nanosheets of dielectric 2D crystalline material in an aqueous vehicle; and c) depositing a second conductive layer onto the dielectric layer, wherein the second conductive layer is deposited by inkjet printing an ink formulation comprising a plurality of nanosheets of conductive 2D crystalline material in an aqueous vehicle; and wherein:

i) the nanosheets of steps a), b) and c) are each associated with an exfoliation agent that renders said nanosheets dispersible within each respective aqueous vehicle;

ii) the aqueous vehicles of steps a), b) and c) each further comprise at least one surface tension modifier and at least one viscosity modifier; iii) the conductive 2D crystalline materials of steps a) and c) above are each independently selected from graphene, carbon nanotubes, electronically conductive metals and nanoparticles thereof (e.g. silver nanoparticles), electronically conductive polymers (e.g. PEDOT:PSS) and combinations thereof:

iv) the dielectric 2D crystalline material of step b) above is selected from hexagonal boron nitride, a dielectric polymer, a dielectric zeolite or an oxide; and

v) the mass ratio of 2D crystalline material to exfoliation agent present in each ink formulation of steps a), b) and c) above is greater than 5: 1.

[00196] In another embodiment, there is provided a method of producing an electronic device, comprising the steps of: a) depositing a first conductive layer onto a substrate, wherein the first conductive layer is deposited by inkjet printing an ink formulation comprising a plurality of nanosheets of conductive 2D crystalline material in an aqueous vehicle; b) depositing a dielectric layer onto at least a portion of the first conductive layer, wherein the dielectric layer is deposited by inkjet printing an ink formulation comprising a plurality of nanosheets of dielectric 2D crystalline material in an aqueous vehicle; and depositing a second conductive layer onto the dielectric layer, wherein the second conductive layer is deposited by inkjet printing an ink formulation comprising a plurality of nanosheets of conductive 2D crystalline material in an aqueous vehicle;

the nanosheets of steps a), b) and c) are each associated with an exfoliation agent that renders said nanosheets dispersible within each respective aqueous vehicle;

the aqueous vehicles of steps a), b) and c) each further comprise at least one surface tension modifier selected from Triton, Tween, poloxamers, cetomacrogol 1000, cetostearyl alcohol, cetyl alcohol, cocamide DEA, monolaurin, nonidet P-40, nonoxynols, decyl glucoside, pentaethylene glycol monododecyl ether, lauryl glucoside, oleyl alcohol or polysorbate;

the aqueous vehicles of steps a), b) and c) each further comprise and at least one viscosity modifier selected from a glycol (e.g. ethylene glycol, propylene glycol), an ether (e.g. ethylene glycol methyl ether), alcohols (e.g. 1-propanol), an ester (e.g. ethyl lactate), a ketone (e.g. methyl ethyl ketone (MEK)) and an organo-sulphur compound (e.g. sulfolane);

the conductive 2D crystalline materials of steps a) and c) above are each independently selected from graphene, carbon nanotubes or electronically conductive metals and nanoparticles thereof (e.g. silver nanoparticles:

the dielectric 2D crystalline material of step b) above is hexagonal boron nitride;

the mass ratio of 2D crystalline material to exfoliation agent present in each ink formulation of steps a), b) and c) above is greater than 10:1 ; and

the exfoliation agent is a water soluble polyaromatic compound (e.g. 1-pyrenesulfonic acid sodium salt). Method 2

[00197] In a particular embodiment, there is provided a method of producing an electronic device, comprising the steps of:

a) depositing a first conductive layer onto a substrate, wherein the first conductive layer is deposited by inkjet printing an ink formulation comprising a plurality of nanosheets of conductive 2D crystalline material in an aqueous vehicle; b) depositing a first semiconductor layer onto at least a portion of the first conductive layer, wherein the first semiconductor layer is deposited by inkjet printing an ink formulation comprising a plurality of nanosheets of semiconductive 2D crystalline material in an aqueous vehicle;

c) depositing a second semiconductor layer onto the first semiconductor layer, wherein the second semiconductor layer has one or both of a doping and a band structure which differs to that of the first semiconductor layer, and wherein the second semiconductor layer is deposited by inkjet printing an ink formulation comprising a plurality of nanosheets of semiconductive 2D crystalline material in an aqueous vehicle; and

d) depositing a second conductive layer onto the second semiconductor layer, wherein the second conductive layer is deposited by inkjet printing an ink formulation comprising a plurality of nanosheets of conductive 2D crystalline material in an aqueous vehicle;

and wherein:

i) the nanosheets of steps a), b), c) and d) are each associated with an exfoliation agent that renders said nanosheets dispersible within each respective aqueous vehicle;

ii) the aqueous vehicles of steps a), b), c) and d) each further comprise at least one surface tension modifier and at least one viscosity modifier;

iii) the conductive 2D crystalline materials of steps a) and d) above are each independently selected from graphene, carbon nanotubes, electronically conductive metals and nanoparticles thereof (e.g. silver nanoparticles), electronically conductive polymers (e.g. PEDOT:PSS) and combinations thereof: iv) the semiconductive 2D crystalline material of steps b) and c) above are each independently selected from a transition metal dichalcogen (e.g. M0S2, WS2, MoTe₂, MoSe₂ etc.) and v) the mass ratio of 2D crystalline material to exfoliation agent present in each ink formulation of steps a), b), c) and d) above is greater than 5: 1.

[00198] In another particular embodiment, there is provided a method of producing an electronic device, comprising the steps of:

and wherein:

ii) the aqueous vehicles of steps a), b), c) and d) each further comprise at least one surface tension modifier selected from Triton, Tween, poloxamers, cetomacrogol 1000, cetostearyl alcohol, cetyl alcohol, cocamide DEA, monolaurin, nonidet P-40, nonoxynols, decyl glucoside, pentaethylene glycol monododecyl ether, lauryl glucoside, oleyl alcohol or polysorbate;

iii) the aqueous vehicles of steps a), b), c) and d) each further comprise and at least one viscosity modifier selected from a glycol (e.g. ethylene glycol, propylene glycol), an ether (e.g. ethylene glycol methyl ether), alcohols (e.g. 1-propanol), an ester (e.g. ethyl lactate), a ketone (e.g. methyl ethyl ketone (MEK)) and an organo-sulphur compound (e.g. sulfolane);

the conductive 2D crystalline materials of steps a) and d) above are each independently selected from graphene, carbon nanotubes or electronically conductive metals and nanoparticles thereof (e.g. silver nanoparticles:

the semiconductive 2D crystalline material of steps b) and c) above are each independently selected from M0S2 or WS2;

the mass ratio of 2D crystalline material to exfoliation agent present in each ink formulation of steps a), b), c) and d) above is greater than 10:1 ; and

the exfoliation agent is a water soluble polyaromatic compound (e.g. 1-pyrenesulfonic acid sodium salt).

Method 3

[00199] In a particular embodiment, there is provided a method of producing an electronic device, comprising the steps of:

a) depositing a first conductive layer onto a substrate to form first and second electrically separated regions, wherein the first conductive layer is deposited by inkjet printing an ink formulation comprising a plurality of nanosheets of conductive 2D crystalline material in an aqueous vehicle;

b) depositing a first semiconductor layer onto at least a portion of each of the first and second regions of the first conductive layer, wherein the first semiconductor layer is deposited by inkjet printing an ink formulation comprising a plurality of nanosheets of semiconductive 2D crystalline material in an aqueous vehicle; and c) depositing a dielectric layer onto at least a portion of the semiconductor layer layer, wherein the dielectric layer is deposited by inkjet printing an ink formulation comprising a plurality of nanosheets of dielectric 2D crystalline material in an aqueous vehicle;

and wherein: i) the nanosheets of steps a), b) and c) are each associated with an exfoliation agent that renders said nanosheets dispersible within each respective aqueous vehicle;

ii) the aqueous vehicles of steps a), b) and c) each further comprise at least one surface tension modifier and at least one viscosity modifier; iii) the conductive 2D crystalline material of step a) is selected from graphene, carbon nanotubes, electronically conductive metals and nanoparticles thereof (e.g. silver nanoparticles), electronically conductive polymers (e.g. PEDOT:PSS) and combinations thereof: iv) the semiconductive 2D crystalline material of step b) is selected from a transition metal dichalcogen (e.g. M0S2, WS2, ΜοΤβ2, MoSe2 etc.); v) the dielectric 2D crystalline material of step c) above is selected from hexagonal boron nitride, a dielectric polymer, a dielectric zeolite or an oxide; and

vi) the mass ratio of 2D crystalline material to exfoliation agent present in each ink formulation of steps a), b) and c) above is greater than 5: 1.

[00200] In another particular embodiment, there is provided a method of producing an electronic device, comprising the steps of:

ii) the aqueous vehicles of steps a), b) and c) each further comprise at least one surface tension modifier selected from Triton, Tween, poloxamers, cetomacrogol 1000, cetostearyl alcohol, cetyl alcohol, cocamide DEA, monolaurin, nonidet P-40, nonoxynols, decyl glucoside, pentaethylene glycol monododecyl ether, lauryl glucoside, oleyl alcohol or polysorbate;

iii) the aqueous vehicles of steps a), b) and c) each further comprise and at least one viscosity modifier selected from a glycol (e.g. ethylene glycol, propylene glycol), an ether (e.g. ethylene glycol methyl ether), alcohols (e.g. 1-propanol), an ester (e.g. ethyl lactate), a ketone (e.g. methyl ethyl ketone (MEK)) and an organo-sulphur compound (e.g. sulfolane);

iii) the conductive 2D crystalline material of step a) above is selected from graphene, carbon nanotubes or electronically conductive metals and nanoparticles thereof (e.g. silver nanoparticles:

iv) the semiconductive 2D crystalline material of step b) above is selected from M0S2 or WS2;

v) the dielectric 2D crystalline material of step c) above is hexagonal boron nitride;

vi) the mass ratio of 2D crystalline material to exfoliation agent present in each ink formulation of steps a), b) and c) above is each greater than 10: 1 ; and

vii) the exfoliation agent is a water soluble polyaromatic compound (e.g.

1-pyrenesulfonic acid sodium salt).

Method 4

[00201] In a particular embodiment, there is provided a method of producing a memory device, comprising the steps of: depositing a first track onto a substrate, wherein the first track is deposited by inkjet printing an ink formulation comprising a plurality of nanosheets of conductive 2D crystalline material in an aqueous vehicle;

determining a word to be stored by the memory device;

depositing one or more regions of dielectric material onto the first track at one or more locations corresponding to a first logic state of the word, wherein the dielectric material is deposited by inkjet printing an ink formulation comprising a plurality of nanosheets of dielectric 2D crystalline material in an aqueous vehicle; depositing one or more first logic tracks onto each of the one or more regions of the dielectric material, wherein each first logic track is independently deposited by inkjet printing an ink formulation comprising a plurality of nanosheets of conductive 2D crystalline material in an aqueous vehicle;

depositing one or more second logic tracks to intersect the first track at one or more locations corresponding to a second logic state of the word, wherein each second logic track is independently deposited by inkjet printing an ink formulation comprising a plurality of nanosheets of conductive 2D crystalline material in an aqueous vehicle; and

electrically connecting each of the one or more first logic tracks and the one or more second logic tracks to respective electric devices; i) the nanosheets of steps a), c), d) and e) are each associated with an exfoliation agent that renders said nanosheets dispersible within each respective aqueous vehicle;

ii) the aqueous vehicles of steps a), c), d) and e) each further comprise at least one surface tension modifier and at least one viscosity modifier;

iii) the conductive 2D crystalline material of steps a), d) and e) are independently selected from graphene, carbon nanotubes, electronically conductive metals and nanoparticles thereof (e.g. silver nanoparticles), electronically conductive polymers (e.g. PEDOT:PSS) and combinations thereof:

iv) the dielectric 2D crystalline material of step c) above is selected from hexagonal boron nitride, a dielectric polymer, a dielectric zeolite or an oxide; and v) the mass ratio of 2D crystalline material to exfoliation agent present in each ink formulation of steps a), c), d) and e) above is greater than 5:1.

[00202] In another particular embodiment, there is provided a method of producing a memory device, comprising the steps of:

a) depositing a first track onto a substrate, wherein the first track is deposited by inkjet printing an ink formulation comprising a plurality of nanosheets of conductive 2D crystalline material in an aqueous vehicle;

b) determining a word to be stored by the memory device;

c) depositing one or more regions of dielectric material onto the first track at one or more locations corresponding to a first logic state of the word, wherein the dielectric material is deposited by inkjet printing an ink formulation comprising a plurality of nanosheets of dielectric 2D crystalline material in an aqueous vehicle; d) depositing one or more first logic tracks onto each of the one or more regions of the dielectric material, wherein each first logic track is independently deposited by inkjet printing an ink formulation comprising a plurality of nanosheets of conductive 2D crystalline material in an aqueous vehicle;

e) depositing one or more second logic tracks to intersect the first track at one or more locations corresponding to a second logic state of the word, wherein each second logic track is independently deposited by inkjet printing an ink formulation comprising a plurality of nanosheets of conductive 2D crystalline material in an aqueous vehicle; and

f) electrically connecting each of the one or more first logic tracks and the one or more second logic tracks to respective electric devices;

wherein:

i) the nanosheets of steps a), c), d) and e) are each associated with an exfoliation agent that renders said nanosheets dispersible within each respective aqueous vehicle;

ii) the aqueous vehicles of steps a), c), d) and e) each further comprise at least one surface tension modifier selected from Triton, Tween, poloxamers, cetomacrogol 1000, cetostearyl alcohol, cetyl alcohol, cocamide DEA, monolaurin, nonidet P-40, nonoxynols, decyl glucoside, pentaethylene glycol monododecyl ether, lauryl glucoside, oleyl alcohol or polysorbate; iii) the aqueous vehicles of steps a), c), d) and e) each further comprise and at least one viscosity modifier selected from a glycol (e.g. ethylene glycol, propylene glycol), an ether (e.g. ethylene glycol methyl ether), alcohols (e.g. 1-propanol), an ester (e.g. ethyl lactate), a ketone (e.g. methyl ethyl ketone (MEK)) and an organo-sulphur compound (e.g. sulfolane);

iii) the conductive 2D crystalline material of steps a), d) and e) above are independently selected from graphene, carbon nanotubes or electronically conductive metals and nanoparticles thereof (e.g. silver nanoparticles:

iv) the dielectric 2D crystalline material of step c) above is hexagonal boron nitride;

v) the mass ratio of 2D crystalline material to exfoliation agent present in each ink formulation of steps a), c), d) and e) above is each greater than 10: 1 ; and

vi) the exfoliation agent is a water soluble polyaromatic compound (e.g.

1-pyrenesulfonic acid sodium salt).

DEVICES OF THE PRESENT INVENTION

[00203] According to another aspect of the present invention, there is provided an electronic device, comprising:

a) a first conductive layer deposited on a substrate;

b) a dielectric layer deposited on at least a portion of the first conductive layer; and c) a second conductive layer deposited on at least a portion of the dielectric layer; wherein one or more of the first conductive layer, dielectric layer and/or second conductive layer comprise a layer prepared by inkjet printing of an ink formulation, as defined herein.

[00204] According to another aspect of the present invention, there is provided an electronic device, comprising:

a) a first conductive layer deposited on a substrate;

b) a first semiconductor layer deposited on at least a portion of the first conductive layer; c) a second semiconductor layer deposited on the first semiconductor layer, wherein the second semiconductor layer has one or both of a doping and a band structure which differs to that of the first semiconductor layer; and

d) a second conductive layer deposited on at least a portion of the second semiconductor layer;

wherein one or more of the first conductive layer, the first semiconductor layer, the second semiconductor layer and/or the second conductive layer comprise a layer prepared by inkjet printing of an ink formulation, as defined herein.

[00205] According to yet another aspect of the present invention, there is provided an electronic device, comprising:

a) a first conductive layer deposited on a substrate, wherein the first conductive layer comprises a first electrically separated region and a second electrically separated region;

b) a first semiconductor layer deposited on at least a portion of each of the first and second electrically separated regions of the first conductive layer; and c) a dielectric layer deposited on at least a portion of the first semiconductor layer; wherein one or more of the first conductive layer, first semiconductor layer and/or dielectric layer comprise a layer prepared by inkjet printing of an ink formulation, as defined herein.

[00206] According to still another aspect of the present invention, there is provided a transistor, comprising:

a) a first conductive layer deposited on a substrate, wherein the first conductive layer comprises a first and a second electrically separated region, and wherein the first electrically separated region is a source and the second electrically separated region is a drain;

b) a first semiconductor layer deposited on at least a portion of each of the first and second electrically separated regions of the first conductive layer;

c) a dielectric layer deposited on to at least a portion of the first semiconductor layer; and

d) a second conductive layer deposited on to at least a portion of the dielectric layer, wherein the second conductive layer is a gate electrode; and wherein one or more of the first conductive layer, the second conductive layer, first semiconductor layer and/or the dielectric layer comprise a layer prepared by inkjet printing of an ink formulation, as defined herein.

[00207] According to still another aspect of the present invention, there is provided a memory device, comprising:

c) a dielectric layer deposited on to at least a portion of the first semiconductor layer;

d) a charge trapping layer deposited on to at least a portion of the dielectric layer; e) a second dielectric layer deposited on to at least a portion of the charge trapping layer;

f) a second conductive layer deposited on to at least a portion of the second dielectric layer, wherein the second conductive layer is a gate electrode; and wherein one or more of the first conductive layer, second conductive layer, first semiconductor layer, second semiconductor layer, dielectric layer, second dielectric layer and/or charge trapping layer comprise a layer prepared by inkjet printing of an ink formulation, as defined herein.

[00208] According to a further aspect of the present invention, there is provided a memory device, comprising:

a) a first track deposited on a substrate;

b) one or more regions of dielectric material deposited on the first track at one or more locations corresponding to a first logic state of a word to be stored;

c) one or more first logic tracks deposited on each of the one or more regions of the dielectric material;

d) one or more second logic tracks deposited to intersect the first track at one or more locations corresponding to a second logic state of the word; and e) a means to electrically connect each of the one or more first logic tracks and the one or more second logic tracks to respective electric devices;

wherein one or more of the first track, the regions of the dielectric material, first logic tracks and second logic tracks comprise a layer prepared by inkjet printing of an ink formulation, as defined herein

[00209] It will be appreciated that in the above memory devices the electronic properties of the dielectric do not change during storage of the signal.

[00210] It will be appreciated that preferred and suitable embodiments of each of the features of the electronic devices, transistors and/or memory devices of the present invention are analogous to the preferred and suitable embodiments for these features described in respect of methods 1 to 4 described hereinabove.

EXAMPLES

[00211] Embodiments of the invention will be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows the respective concentrations of graphene and 1-pyrenesulfonic acid sodium salt (PS1) for two formulations (Gr 1 and Gr 2) prepared according to the method described herein in.

Figure 2 shows the UV-Vis spectra for two formulations (Gr 1 and Gr 2) prepared according to the method described herein. The concentrations are 3.61 g/L and 3.22 g/L respectively.

Figure 3 shows the UV-Vis spectra of the supernatant removed during preparation of a graphene dispersion (Gr 2) described herein. The reduction in excess PS1 is clearly visible in the reduction in the peak at 346nm.

Figure 4 is a digital photograph showing a printed dot formed by the inkjet printing of the ink formulation prepared as described herein onto a 50°C S1O2 substrate.

Figure 5 shows the Raman spectrum of printed graphene on Si/SiC>2 obtained at 514nm using a Renishaw inVia confocal Raman microscope. (100x zoom, 0.8NA, 0.6mW)

Figure 6 shows contact angle measurements of the graphene ink deposited on silicon covered with a thin oxide layer (a), quartz (b) and PI film (c).

Figure 7 shows the UV-Vis spectra of a 2.4 mg/mL h-BN dispersion diluted 20x and 100x.

Figure 8 shows the UV-Vis spectra of a M0S2 dispersion diluted 150x after removal of excess PS1. Concentration is 0.83 g/L. Figure 9 shows the sheet resistance as a function of number of printed layers for graphene ink of various concentrations printed on S1O2 without annealing.

Figure 1 1 shows the resistance as a function of time for graphene lines printed on S1O2 after a, no annealing; b, annealing at 200 °C, c, 300 °C, d, 400 °C and e, 500 °C. f, RMS roughness of 150 layers from each sample a - e.

Figure 1 1 shows a schematic of a ΘΓΒ - WS2 - Grr heterostructure.

Figure 12 shows logic memory devices printed on quartz, a. Optical image of a printed ΘΓΒ - WS2 - ΘΓΤ word line on quartz with the junction numbers shown, b and c. I-V curves of two word lines printed on glass. The measurements were taken between ΘΓΒ and the junction number indicated.

Figure 13 shows logic memory device, a, Sketch of the fabricated device - The Programmable Read Only Memory (PROM) is composed of a horizontal (word line) and vertical lines (bit lines) made of ink-jet graphene. A logic "1" is stored at regular intersections of the word line and the bit line, while a logic "0" is programmed by printing WS2 between the two. The sketch shows a 4-bit memory storing the word "1010". b, Micrograph picture of the fabricated device, where the bias voltage (V_p) source and load resistors are added, c, Experimental (black solid line) and simulated results (red dashed line) for the operation of the circuit in a. d, Schematic of the equivalent electronic circuit, e, l-V characteristics of a graphene/WS2/graphene junction and of a short-circuit.

Figure 14 shows an optical image of a WORD line printed on PEL P60 paper.

Figure 15 shows an optical image of a WORD line printed on an epoxy breadboard.

Figure 16 shows a WS2 heterostructure array showing three printed without WS2 layer. l-V plot (left) taken across each of the heterostructures showing also the ohmic behaviour of the A3 junction, which have no WS2 photoactive layer. (Right) optical image of the measured devices with the measurements taken between the left and top electrodes as viewed.

Figure 17 shows an l-V curves of a ΘΓΒ - ΜοΤβ2 - G r heterostructure printed on quartz, showing low conductivity and capacitance.

Figure 18 shows a fully printed ΘΓΒ - h-BN - ΘΓΤ heterostructure printed on PEL P60 per (left) and l-V curves (right) measured between ΘΓΤ and G r (dotted line), GrB and GrB (dashed line) and GrB and G r (solid line).

Figure 19 shows an electronic device obtained by an embodiment of method 1 defined hereinabove. Figure 20 shows an electronic device obtained by an embodiment of method 2 defined hereinabove.

Figure 21 shows an electronic device obtained by an embodiment of method 3 defined hereinabove.

Figure 22 shows an electronic device obtained by another embodiment of method 3 defined hereinabove.

APPLICATIONS

[00212] Referring to Figure 19 there is illustrated an open circuit electronic device comprising three layers: a first conductive layer 2620 that is deposited onto a substrate; a dielectric layer 2620 that is deposited onto at least a portion of the first conductive layer 2610; and a second conductive layer 2630 that is deposited onto the dielectric layer 2620. One or more of the first conductive layer 2610, dielectric layer 2620 and/or second conductive layer 2630 is deposited by inkjet printing an ink formulation comprising a plurality of nanosheets of 2D crystalline material in an inkjet compatible vehicle, wherein the nanosheets are dispersible within the inkjet compatible vehicle. The first conductive layer 2610 may be deposited onto any suitable substrate. Non-limiting examples of suitable substrates include glass, plastic or ceramics. The first and second conductive layers 2610, 2630 each comprise at least one conductive material. Suitable conductive materials may include, for example, graphene, carbon nanotubes, electronically conductive metals and nanoparticles thereof (e.g. silver nanoparticles), electronically conductive polymers (e.g. PEDOT:PSS) and combinations thereof.

[00213] Referring to Figure 20 there is illustrated a heterostructure electronic device comprising four layers: a first conductive layer 2710 that is deposited onto a substrate; a first semiconductor layer 2720 that is deposited onto at least a portion of the first conductive layer 2710; a second semiconductor layer 2730 that is deposited onto the first semiconductor layer 2720, wherein the second semiconductor layer 2730 has at least a different doping and/or band structure to that of the first semiconductor layer 2720; and a second conductive layer 2740 that is deposited onto the second semiconductor layer 2730. One or more of the first conductive layer 2710, the first semiconductor layer 2720, the second semiconductor layer 2730 and/or the second conductive layer 2740 is deposited by inkjet printing an ink formulation comprising a plurality of nanosheets of 2D crystalline material in an inkjet compatible vehicle.

[00214] Preferred and suitable embodiments for the substrate, the first conductive layer 2710 and the second conductive layer 2740 of Figure 20 will be understood to be analogous to the preferred and suitable embodiments for the substrate, the first conductive layer 2610 and the second conductive layer 2630 of Figure 19 defined hereinabove. The first and second semiconductor layers 2720, 2730 each comprise at least one semi- conductive material. Suitable semiconductor materials may include, for example, transition metal dichalcogenides (TMDCs), such as NbSe₂, WS2, M0S2, TaS2, PtTe₂, and VTe₂.

[00215] Referring to Figure 21 there is illustrated a transistor device comprising four layers: a first conductive layer that is deposited onto a substrate to form a first and a second electrically separated region 2810, 2820; a first semiconductor layer 2830 that is deposited onto at least a portion of the first and second regions 2810, 2820 of the first conductive layer; a dielectric layer 2840 that is deposited onto the first semiconductor layer 2830; and a second conductive layer 2850 that is deposited onto the dielectric layer 2840 wherein the second conductive layer 2850 is a gate electrode.

[00216] One or more of the first conductive layer, first semiconductor layer 2830, dielectric layer 2840 and/or second conducting layer 2850 is deposited by inkjet printing an ink formulation comprising a plurality of nanosheets of 2D crystalline material in an inkjet compatible vehicle.

[00217] It will be appreciated that preferred and suitable embodiments for the substrate, the first semiconductor layer 2830 and the dielectric layer 2840 of Figure 21 are analogous to the preferred and suitable embodiments for these features described in respect of Figures 19 and 20 hereinabove.

[00218] In Figure 21 , the device is a transistor, the first region 2810 of the first conductive layer is a source electrode and the second region 2820 of the first conductive layer is a drain electrode and the second conductive layer 2850 is a gate electrode. A channel is formed, in operation, between the source and drain electrodes via the semiconductor layer 2830.

[00219] Referring to Figure 22 there is illustrated an electrically programmable memory device comprising six layers: a first conductive layer that is deposited on a substrate to form a first and a second electrically separated region 2910, 2930; a first semiconductor layer 2940 that is deposited onto at least a portion of the first and second regions 2910, 2930 of the first conductive layer; and a first dielectric layer 2950. In this embodiment, a charge trapping layer 2960 may be deposited onto the first dielectric layer 2950, a second dielectric layer 2970 may be deposited onto the charge trapping layer 2960, and a second conductive layer 2920 may be deposited onto the second dielectric layer 2970 wherein the second conductive layer 2920 is a gate electrode. [00220] One or more of the second conductive layer, the second dielectric layer 2960, and/or the charge trapping layer 2970 is deposited by inkjet printing of an ink formulation as defined herein.

[00221] The first region 2910 of the first conductive layer is a source electrode, the second region 2930 of the first conductive layer is a drain electrode and the second conductive layer 2920 is a gate electrode. A channel is formed, in operation, between the source and drain electrodes via the semiconductor layer 2940.

Materials

[00222] 1-pyrenesulfonic acid sodium salt (1-PSA; Py-1 S0₃; PS1 , > 97.0% (HPLC)), propylene glycol, bulk M0S2 (<2μηι, 99%), bulk h-BN (~1 μηι) and Triton x-100 were all purchased from Sigma-Aldrich.

[00223] Graphite flakes were purchased from Bran Well UK, Grade: 99.5.

[00224] Unless otherwise stated, other reagents were of analytical grade and were used as received. All aqueous solutions were prepared with ultra-pure water (18.2 ΜΩ) from a Milli- Q Plus system (Millipore).

Characterisation

Ink rheology

[00225] Dynamic viscosity (η), density (p) and surface tension (γ) were measured for all ink formulations before printing. Contact angles (Θ) were measured for all inks on a range of substrates such as Si/Si02, polyimide (PI), polyethylene terephthalate (PET) and quartz.

[00226] Surface tensions, viscosities and contact angles were all measured in accordance with the methods described in Nature Nanotechnology 12, 343-350 (2017). Accordingly, surface tensions and contact angles were measured with an Attension Theta Lite optical tensiometer using the Young-Laplace equation (see, for example, J. Colloid Interface Sci. 141 , 1-9, (1991)) fitted to optical images. For the surface tension, a pendant drop was imaged and the following equation below was solved iteratively:

[00227] Here Δρ is the difference in densities between the air and liquid, g is the acceleration due to gravity, Ro is the radius of the drop curvature at the apex and β is a shape factor defined using the Young-Laplace equation. Surface tensions were typically measured at 25 °C.

[00228] Dynamic viscosity was determined with a Brookfield DV- 11+ Pro viscometer with spindle 3 at 100 rpm over a range of temperatures (e.g. 25 °C and 30 °C, with 25 °C being the preferred temperature for measuring ink viscosity).

INK FORMULATIONS USED IN THE PRINTED DEVICES OF THE PRESENT INVENTION

[00229] All inks were formulated using the following method, with the only change being the bulk layered material, depending on which nanosheet inks are required.

[00230] The following quantities of reagents were added to a glass vial:

• H₂O - 9.0 g;

• Propylene glycol - 1.0 g;

• Triton-x100 - 0.6 mg;

• Bulk layered material (either graphite, h-BN, WS2, M0S2 or ΜοΤβ2) - 30 mg; and

• 1-pyrenesulfonic acid sodium salt (PS1) - 10 mg.

[00231] The inkjet formulation was prepared using the quantities of reagents detailed above, by means of the following steps:

1) The glass vial was stoppered and placed into a 600W bath sonicator for 48 hours.

2) The solution was then centrifuged at 3500 rpm (903 g) for 20 mins and the top 2/3 collected.

3) The collected solution was then centrifuged at 15000rpm (16602 g) for 20 mins and the supernatant collected and combined before UV-Vis was conducted using a Cary 5000 UV-Vis-Near IR spectrometer to determine the concentration of PS1 in the supernatant.

4) The sediment was gently re-dispersed via shaking in a solvent comprising of H2O (9- 9.95 g), propylene glycol (0.05-1 g), Triton-x100 (1 mg) and xanthan gum (<1.5 mg).

5) Steps 3 and 4 were repeated until less than 0.05 mg/mL PS1 was present in the supernatant. When the supernatant was recovered which contained less than 0.05 mg/mL PS1 , the sediment was re-dispersed as in step 4 with a minimal quantity of solvent and collected.

Printed WORD line [00232] Using a Dimatix DMP-2800 inkjet printer (Fujifilm Dimatix, Inc., Santa Clara, USA), it was possible to create and define patterns over an area of about 200 x 300 mm and handle substrates up to 25 mm thick being adjustable in the Z direction. The nozzle plate consists of a single row of 16 nozzles of 21 μηι diameter spaced 254 μηι apart with typical drop volume of 10 pL.

[00233] Water-based graphene ink of various concentrations was formulated using the method described in [00230]. The graphene ink was used to print lines on S1O2 substrates with 40 μηι drop spacing and varying numbers of layers. The resistance of the printed lines was measured using a Keithley 2614B sourcemeter immediately after printing (Figure 9).

[00234] The effect of annealing at different temperatures with respect to resistance was measured for a number of graphene lines printed on S1O2 with increasing number of layers (Figure 10). Atomic force microscopy (AFM) was also performed on the lines using a Bruker Multimode 8 to determine the root mean squared (RMS) roughness of the printed lines (Figure 10). This data can be used to ensure the correct number of printing passes, concentration and annealing temperature are used when fabricating printed memory to ensure optimal conductivity and reduced roughness at the interface.

[00235] A "word" is stored in the memory through the definition of a horizontal stripe ("word- line") and vertical stripes, one for each bit of the word (which will be referred to as "bit- lines"), all of them made of graphene. A logic "1" is stored by short-circuiting the bit line to the word line, while a logic "0" is encoded by including a semiconducting layer (i.e., WS2), between the word line and the corresponding bit line, which eventually suppresses the current (Figure 11).

[00236] Figure 12 shows an optical image of a fabricated device on glass printed using graphene for the word and bit-lines and WS2 as the semiconducting layer. I-V curves are shown for two devices printed on glass, with the measurement taken between a constant position at the end of the word line and changing the bit line measured, using a Keithley 2614B sourcemeter. The junctions with WS2 present display non-ohmic characteristics, unlike the junctions without WS2 which show ohmic behaviour (Figure 12).

[00237] Figure 13, shows the fabricated device, including the schematic of the complete circuit set up for the measurements, including an external bias voltage source Vp and the load resistors RL. When a voltage is applied to the word line (0.5 V), the stored word is read in terms of the voltage VBi across each pull-down resistor RL, with higher voltage values (greater than 0.35 V) interpreted as logic "1" and lower voltage values (less than 0.135 V) as logic "0" (black solid line in Figure 13): in this specific case, the device is programmed with the word "010010001". I-V characteristics were extracted from the measurement performed on the circuit in Figure 13a, and included into the SPICE circuit simulator (Figure 13e shows an l-V characteristic of a junction as well as that of a short-circuit). Simulation results of the equivalent circuit of Figure 13d) are shown in Figure 13c (red dashed lines), showing good agreement between experimental and theoretical results.

[00238] Other substrates have been used for the fabrication of printed WORD lines using this technique including PEL P60 paper (Figure 14) and epoxy resin breadboards (Figure 15), showing the ability to print directly onto circuit boards and flexible substrates.

[00239] It is possible to create arrays of word and bit lines which are fully printed. Figure 16 shows a 3 x 3 grid of word and bit lines, with non-ohmic l-V characteristics observed for junctions with WS2 printed as a spacer between the word and bit lines.

[00240] The material between the word and bit line can be substituted for a range of other 2D crystals including a semiconducting material such as ΜοΤβ2 (Figure 17) or a dielectric such as hexagonal boron nitride (h-BN) (Figure 18). An l-V curve between a bit and word line using ΜοΤβ2 as the interlayer material for a device printed on quartz is shown in Figure 17. A junction using h-BN as the spacer printed on PEL P60 paper is shown in Figure 18, with a large decrease in conductivity between the word and bit lines, giving a Ό'.

[00241] While specific embodiments of the invention have been described herein for the purpose of reference and illustration, various modifications will be apparent to a person skilled in the art without departing from the scope of the invention as defined by the appended claims.

Claims

1. A method of producing an electronic device, comprising the steps of: a) depositing a first conductive layer onto a substrate; b) depositing a dielectric layer onto at least a portion of the first conductive layer; and c) depositing a second conductive layer onto at least a portion of the dielectric layer; wherein two or more of the first conductive layer, dielectric layer and second conductive layer is deposited by inkjet printing an ink formulation comprising a plurality of nanosheets of 2D crystalline material in an inkjet compatible vehicle.

2. A method of producing an electronic device, comprising the steps of: a) depositing a first conductive layer onto a substrate; b) depositing a first semiconductor layer onto at least a portion of the first conductive layer; c) depositing a second semiconductor layer onto the first semiconductor layer, wherein the second semiconductor layer has one or both of a doping and a band structure which differs to that of the first semiconductor layer; and d) depositing a second conductive layer onto at least a portion of the second semiconductor layer; wherein two or more of the first conductive layer, first semiconductor layer, second semiconductor layer and second conductive layer is deposited by inkjet printing an ink formulation comprising a plurality of nanosheets of 2D crystalline material in an inkjet compatible vehicle.

3. A method of producing an electronic device, comprising the steps of: a) depositing a first conductive layer onto a substrate to form first and second electrically separated regions; b) depositing a first semiconductor layer onto at least a portion of each of the first and second regions of the first conductive layer; and c) depositing a dielectric layer onto at least a portion of the first semiconductor layer; wherein two or more of the first conductive layer, first semiconductor layer and dielectric layer is deposited by inkjet printing an ink formulation comprising a plurality of nanosheets of 2D crystalline material in an inkjet compatible vehicle.

4. The method of claim 3, wherein the electronic device is a transistor, the first region of the first conductive layer is a source electrode, and the second region of the first conductive layer is a drain electrode and the dielectric layer is deposited onto the first semiconductor layer, the method comprising the additional step of: d) depositing a second conductive layer onto at least a portion of the dielectric layer, wherein the second conductive layer is a gate electrode, and wherein the second conductive layer is deposited by inkjet printing of an ink formulation comprising a plurality of nanosheets of 2D crystalline material in an inkjet compatible vehicle.

5. The method of claim 3, wherein the electronic device is a memory device, the first region of the first conductive layer is a source electrode, the second region of the first conductive layer is a drain electrode, the method comprising the additional step of:

e) depositing a charge trapping layer onto at least a portion of the dielectric layer; and f) depositing a second dielectric layer onto at least a portion of the charge trapping layer; g) depositing a second conductive layer onto at least a portion of the second dielectric; wherein the second conductive layer is a gate electrode; and wherein two or more of the second conductive layer, the second dielectric layer and the charge trapping layer is deposited by inkjet printing of an ink formulation comprising a plurality of nanosheets of 2D crystalline material in an inkjet compatible vehicle.

6. A method of producing a memory device, comprising the steps of: a) depositing a first track onto a substrate; b) determining a word to be stored by the memory device; c) depositing one or more regions of dielectric material onto the first track at one or more locations corresponding to a first logic state of the word; d) depositing one or more first logic tracks onto each of the one or more regions of the dielectric material; and e) depositing one or more second logic tracks to intersect the first track at one or more locations corresponding to a second logic state of the word; and f) electrically connecting each of the one or more first logic tracks and the one or more second logic tracks to respective electric devices; wherein two or more of the first track, the regions of the dielectric material, first logic tracks and second logic tracks are deposited by inkjet printing an ink formulation comprising a plurality of nanosheets of 2D crystalline material in an inkjet compatible vehicle.

7. A method according to any one of claims 1 to 6, wherein the ink formulation comprises a binder.

8. A method according to 7, wherein the binder is selected from a polysaccharide (e.g. xanthan gum), polyvinylpyrrolidone or polyethylene glycol.

9. A method according to any one of claims 1 to 8, wherein greater than 80% of the nanosheets of 2D crystalline material present in the ink formulations of the present invention comprise less than ten layers of the 2D crystalline material.

10. A method according to any one of claims 1 to 9, wherein the inkjet compatible vehicle is an aqueous vehicle.

1 1. A method according to any one of claims 1 to 10, wherein the nanosheets of 2D crystalline material are associated with an exfoliation agent that renders the nanosheets of dispersible within the inkjet compatible vehicle.

12. A method according to any one of claims 1 to 11 , wherein the 2D crystalline material is an inorganic material.

13. A method according to any one of claims 1 to 12, wherein the substrate is selected from silicon, quartz, polyimide, polyethylene terephthalate, epoxy resins and paper.

14. A method according to any one of claims 1 to 13, wherein the ink formulation has a viscosity within the range of 1 to 30 cPs.

15. A method according to any one of claims 1 to 14, wherein the ink formulation has a surface tension within the range 20 to 50 mN/m.

16. A method according to any one of claims 1 to 15, wherein the ink formulation comprises a plurality of nanosheets of an inorganic material in an aqueous vehicle, wherein the nanosheets are associated with an exfoliation agent that renders the nanosheets dispersible within the aqueous vehicle;

at least one surface tension modifier; and

at least one viscosity modifier;

at least one binder;

and wherein the mass ratio of inorganic material to exfoliation agent present in the formulation is greater than 5: 1.

17. A method according to any one of claims 1 to 16, wherein the exfoliation agent is a water soluble polycyclic aromatic compound.

wherein, each R group is independently selected from a group of the formula:

-L-X

wherein:

L is absent or an alkylene, optionally interrupted by one or more oxygen atoms; and

X is a hydrophilic group.

19. A method according claim 18, wherein L is absent or selected from a (1- 10C)alkylene, optionally interrupted by one or more oxygen atoms.

20. A method according claim 18 or 19, wherein L is absent.

21. A method according to any one of claims 18 to 20, wherein X is a hydrophilic group selected from S0₃H, S0₂H, B(OH)₂, C0₂H, OH, P0₃H or a salt form thereof. A method according to claim 21 , wherein X is SO3M or OH, wherein M is a cation selected from Na⁺, K⁺, Li⁺ or NH₃ ⁺.

A method according to any one of claims 18 to 22, wherein the polycyclic aromatic compound is selected from one of the following:

24. A method according to any one of claims 18 to 23, wherein the amount of water soluble polyaromatic compound present in the initial formulations is from 0.05 to 2.0 g/L.

25. A method according to any one of claims 16 to 24, wherein the viscosity modifier is selected from the group consisting of ethylene glycol methyl ether, ethylene glycol, propylene glycol, 1-propanol, ethyl lactate, methyl ethyl ketone (MEK) and sulfolane.

26. A method according to any one of claims 16 to 25, wherein the viscosity modifier is present in the formulations of the present invention at an amount of from 0.1 to 50 wt.%.

27. A method according to any one of claims 16 to 26, wherein the surface tension modifier is a surfactant.

28. A method according to any one of claims 16 to 27, wherein inorganic material is selected from graphene, hexagonal boron nitride, bismuth strontium calcium copper oxide (BSCCO), transition metal dichalcogenides (TMDCs), Sb2Te3, Βΐ2Ϊβ3 or Mn02.

29. An electronic device produced by the method of any one of claims 1 to 5.

30. A memory device produced by the method of claim 6.

31. Use of a memory device according to claim 30 in the storage of data.