Film and method of forming the same

Abstract

Various embodiments may relate to a method of forming a film. The method may include forming a suspension including a two-dimensional material, a first solvent and a second solvent, the first solvent and the second solvent being mixed in a mixing ratio having a specific mixing ratio value. The method may further include destabilizing the suspension by changing the mixing ratio from the specific mixing ratio value to a changed mixing ratio value different from the specific mixing ratio value, thereby forming the film including the two-dimensional material.

Description

FILM AND METHOD OF FORMING THE SAMECROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority of Singapore application No. 10202303271Q filed November 20, 2023, the contents of it being hereby incorporated by reference in its entirety for all purposes.TECHNICAL FIELD

[0002] Various embodiments of this disclosure may relate to a film. Various embodiments of this disclosure may relate to a method of forming a film.BACKGROUND

[0003] The superlative and unique mechanical, electrical, and optical properties of two- dimensional (2D) materials such as graphene, transition metal dichalcogenides (TMDs), black phosphorus and others have precipitated tremendous research and commercial interests in integrating them into electronic and optoelectronic applications. While many small scale proof- of-concept devices have been demonstrated for a variety of applications from photodetectors and transistors to corrosion inhibition and electrochemical cells, a key challenge in translating the academic world of 2D materials research to commercial applications lies in the lack of cost effective and scalable methods of fabricating and assembling these large size films of 2D materials

[0004] Thus far, methods for producing these 2D materials have largely been categorized as either being top-down (e g., micro-mechanical exfoliation, liquid phase exfoliation, electrochemical exfoliation), or bottom-up (e.g., molecular beam epitaxy, physical or chemical vapor deposition). FIG. 1A shows a schematic illustrating the consideration of top-down andbottom-up methods for large scale production of two-dimensional (2D) materials. Amongst the top-down methods, micro-mechanical exfoliation largely remains the method of choice for small-scale proof-of concept devices in research labs due to its relative ease and exceptional crystalline quality of the film formed. However, since it is an inherently stochastic process that requires artisanal and precise stacking, it is generally considered to be impractical for most commercial applications. FIG. IB shows a scanning electron microscopy image (SEM) of small-sized two-dimensional (2D) material layers being contacted with gold (Au) contacts. FIG. 1C shows a schematic of a two-dimensional (2D) material film being used for research prototyping. Liquid phase exfoliation (LPE) and electrochemical exfoliation methods, on the other hand, have shown promise as highly scalable methods that produce flakes suitable for a wide range of commercial applications, albeit at a reduced quality

[0005] Solvent-based liquid phase exfoliation (LPE), a subtype of LPE where favorable interactions between a specifically-chosen solvent and the 2D materials under sonication or shear mixing is used to induce exfoliation, is especially advantageous due to its relatively low operational costs, and lack of intercalating ions (as in the case of in electrochemical exfoliation) or surfactants (as in the case of surfactant-based liquid phase exfoliation) which contribute impurities to the 2D material surface and may be expensive, environmentally unfriendly, or dangerous to work with. While N-methyl-2-pyrrolidone (NMP) has widely been used as such a solvent to exfoliate and achieve stable dispersions of various 2D materials, it has significant drawbacks including its relative toxicity and non-volatility which makes it challenging to process.

[0006] An increasingly popular alternative to NMP is the usage of mixed solvent strategies whereby carefully selected ratio of two different solvents, such as isopropyl alcohol (IP A) and water, or ethanol and water, is used to create a mixture or solution that can form a stable suspension with the 2D materials of choice. It is important to note here that the 2D materialsuspension is only stable in a solvent composed of this specific mixing ratio and would generally be unstable in each of the pure solvents. The appropriate mixing ratios can be determined rationally by matching thermodynamic parameters, or empirically with the aid of ultraviolet - visible (UV-vis) spectroscopy to quantify absorption and hence suspension concentration and stability. The volatile solvent used may allow it to be amenable to methods such as drop-casting, spin-coating, and inkjet printing to deposit the material on a suitable substrate for various applications. Furthermore, these solvents are relatively inexpensive, largely non-toxic, and much easier to dispose of after use.

[0007] However, a challenge remains with regards to the deposition of the 2D material after the exfoliation. For many applications, it is desirable for the 2D flakes to form a continuous overlapping layer or film - for example in an electronic device where this film becomes an active conducting channel that bridges a source-drain current. Drop-casting and spin-coating, however, tend to result in uneven or discontinuous layers of the 2D material. While spin-coating methods may be optimized to enable even and / or continuous layer of 2D materials to be formed, the spin-coating methods require careful optimization of parameters, such as solvent mixture concentration, which in turn limits the stability of the suspension and substrate While inkjet printing is a promising method to form continuous layers of overlapping 2D flakes- it requires highly concentrated 2D material inks which needs to be carefully formulated for each application.SUMMARY

[0008] Various embodiments may relate to a method of forming a film. The method may include forming a suspension including a two-dimensional (2D) material, a first solvent and a second solvent, the first solvent and the second solvent being mixed in a mixing ratio having aspecific mixing ratio value. The method may also include destabilizing the suspension by changing the mixing ratio from the specific mixing ratio value to a changed mixing ratio value different from the specific mixing ratio value.

[0009] Various embodiments may relate to a film formed by any method as described herein.BRIEF DESCRIPTION OF THE DRAWINGS

[0010] In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily drawn to scale, emphasis instead generally being placed upon illustrating the principles of various embodiments. In the following description, various embodiments of the invention are described with reference to the following drawings.FIG. 1 A shows a schematic illustrating the consideration of top-down and bottom-up methods for large scale production of two-dimensional (2D) materials.FIG. IB shows a scanning electron microscopy image (SEM) of small-sized two-dimensional (2D) material layers being contacted with gold (Au) contacts.FIG. 1C shows a schematic of a two-dimensional (2D) material fdm being used for research prototyping.FIG. 2 shows a schematic of a method of forming a fdm according to various embodiments.FIG. 3 shows distribution of two-dimensional (2D) material flakes using (i) drop-casting, (ii) spin-coating, and (iii) destabilization of 2D material suspension according to various embodimentsFIG. 4A shows a schematic of a mixed solvent-based liquid phase exfoliation (LPE) process according to various embodiments.FIG. 4B shows a table of optimized values to stabilize suspensions of various two-dimensional (2D) materials according to various embodiments.FTG. 5 A shows a schematic illustrating the stability of tungsten sulfide (WS2) in different ratios of isopropyl alcohol (IP A) and water according to various embodiments.FIG. 5B shows a schematic illustrating the precipitation of the exfoliated 2D material flakes as well as the subsequent film formation as the percentage of isopropyl alcohol (IP A) decreases from 50 volume percent (vol %) to 25 vol %.FIG. 6 shows a schematic illustrating the formation of a tungsten selenide (WSe2) film according to various embodiments.FIG. 7A shows schematics of (i) a stable solution containing a solid solute in a solvent and destabilized with the addition of a soluble salt in the mixture, resulting in the precipitation of the original solute; and (ii) a stable solution of two miscible solvents which are destabilized with the addition of a soluble salt to form a biphasic mixture of two immiscible phases according to various embodiments.FIG. 7B shows a schematic of the biphasic mixture including an isopropyl alcohol (IPA)-rich phase and a water-rich phase formed after addition of a salt according to various embodiments. FIG. 8 shows (i) an optical image of the tungsten selenide (WSe2) film according to various embodiments formed directly after the distillation processing of a suspension including \VSe2 and 50% isopropyl alcohol (IP A) solution (i . e. , 50 vol % IP A, 50 vol % water); (ii) a magnified (20x) optical micrograph of the tungsten selenide (WSe2) film shown in (i) according to various embodiments on a silicon (Si) / silicon oxide (SiCh) substrate (scale bar: 0.1 mm); (iii) a scanning electron microscopy (SEM) image (5,000x) of the tungsten selenide (WSei) film shown in (i) according to various embodiments; and (iv) a scanning electron microscopy (SEM) image (10,000x) of the tungsten selenide (WSc?) film shown in (i) according to various embodiments.FTG. 9A shows a plot of intensity (in arbitrary units or arb. units) as a function of wavenumber (per centimeter or cm'1) illustrating the Raman spectrum of mechanically exfoliated tungsten selenide (WSez) flakes.FIG. 9B shows a plot of intensity (in arbitrary units or arb. units) as a function of wavenumber (per centimeter or cm'1) illustrating the Raman spectrum of as-prepared tungsten selenide (WSez) film formed by the distillation method according to various embodiments.FIG. 9C shows a plot of differential reflectance (in arbitrary units or arb. units) as a function of energy hv (in electron-Volts or eV) illustrating the ultraviolet-visible (UV-Vis) reflectance spectra of the film formed by the distillation method according to various embodiments.FIG. 10A shows an optical microscopy image of a tungsten selenide (WSe2) film formed by the salting-out method according to various embodiments on gold (Au) electrodes patterned on a polyethylene terephthalate) (PET) substrate.FIG. 10B shows an optical microscopy image of a tungsten selenide (WSe2) film formed by the salting-out method according to various embodiments on gold (Au) electrodes patterned on a polyimide (PI) substrate.FIG. IOC shows a plot of intensity (in arbitrary units or a.u.) as a function of wavenumber (per centimeter or 1 / cm) illustrating the Raman spectrum of the molybdenum selenide (MoSe2) / tungsten selenide (WSe2) film formed by the salting-out method according to various embodiments onto a silicon / silicon oxide (Si / SiCh) substrate.FIG. 10D shows an optical micrograph of the molybdenum selenide (MoSe2) / tungsten selenide (\VSe2) film formed by the salting-out method according to various embodiments.FIG. 11 shows a schematic illustrating the distillation and salting-out methods of a mixed solvent suspension including a two-dimensional (2D) material for forming a two-dimensional (2D) material film according to various embodiments.FTG. 12 shows a schematic illustrating a method to form a two-dimensional (2D) material film according to various embodiments.FIG. 13 shows a table comparing properties of a film formed by a method according to various embodiments and conventional films formed by other methods.DESCRIPTION

[0011] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.

[0012] Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments. Features that are described in the context of an embodiment may correspondingly be applicable to the other embodiments, even if not explicitly described in these other embodiments. Furthermore, additions and / or combinations and / or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0013] In the context of various embodiments, the articles “a”, “an” and “the” as used with regard to a feature or element include a reference to one or more of the features or elements.

[0014] In the context of various embodiments, the term “about” or “approximately” as applied to a numeric value encompasses the exact value and a reasonable variance, e g. within 10% of the specified value.

[0015] As used herein, the term “and / or” includes any and all combinations of one or more of the associated listed items.

[0016] By “comprising” it is meant including, but not limited to, whatever follows the word “comprising”. Thus, use of the term “comprising” indicates that the listed elements are required or mandatory, but that other elements are optional and may or may not be present.

[0017] By “consisting of’ is meant including, and limited to, whatever follows the phrase “consisting of’. Thus, the phrase “consisting of’ indicates that the listed elements are required or mandatory, and that no other elements may be present.

[0018] Embodiments described in the context of one of the films are analogously valid for the other film, embodiments described in the context of a method are analogously valid for a film, and vice versa.

[0019] FIG. 2 shows a schematic of a method of forming a film according to various embodiments. The method may include, in 202, forming a suspension including a two- dimensional (2D) material, a first solvent and a second solvent, the first solvent and the second solvent being mixed in a mixing ratio having a specific mixing ratio value. The method may also include, in 204, destabilizing the suspension by changing the mixing ratio from the specific mixing ratio value to a changed mixing ratio value different from the specific mixing ratio value, thereby forming the film including the two-dimensional material.

[0020] In other words, various embodiments may relate to a method of 2D material film formation and deposition via destabilization of exfoliated 2D material (e g., in the form of 2D material flakes) suspended in a mixed solvent including the first solvent and the second solvent. The method may first include forming the suspension of the 2D material and the mixed solvent, the mixed solvent having a mixing ratio of the specific mixing ratio value (alternatively referred to as an initial ratio value or initial mixing ratio value) of the first solvent and the second solvent. The method may further include changing or disrupting the mixing ratio of the first solvent andthe second solvent such that the changed mixing ratio value of the first solvent and the second solvent differs from the initial ratio value or specific mixing ratio value, thereby destabilizing the suspension. In various embodiments, step 204 may occur after step 202.

[0021] In various embodiments, the suspension may be destabilized by heating the suspension in a distillation process. The distillation process may make use of a difference in a boiling point of the first solvent and a boiling point of the second solvent to selectively evaporate one of the two solvents (i.e., the first solvent or the second solvent) such that the mixing ratio of the first solvent and the second solvent (in the suspension) is changed to a value (i.e., the changed mixing ratio value) different from the initial ratio value or specific mixing ratio value. The film may be formed at a top surface of the suspension, e.g., at a liquid-air interface, i.e , the interface between the suspension and air. The distillation process may be a physical separation process, and there may not be additional chemicals required to be added.

[0022] In various embodiments, a difference in a boiling point of the first solvent and a boiling point of the second solvent may be a value of more than 10 °C. In various embodiments, the suspension may be heated to a temperature between the first boiling point of the first solvent and the second boiling point of the second solvent, such that the solvent with the lower boiling point is evaporated off. For instance, if the first solvent has a boiling point lower than a boiling point of the second solvent, heating the suspension to a temperature above the boiling point of the first solvent but below the boiling point of the second solvent may result in the first solvent to be evaporated off, thereby changing the mixing ratio of the first solvent and the second solvent (i.e., increasing the proportion of the second solvent relative to the first solvent). Conversely, if the second solvent has a boiling point lower than a boiling point of the first solvent, heating the suspension to a temperature above the boiling point of the second solvent but below the boiling point of the first solvent may result in the second solvent to be evaporated off, thereby changing the mixing ratio of the first solvent and the second solvent (i.e., increasingthe proportion of the first solvent relative to the second solvent). In various embodiments, a difference in a boiling point of the first solvent and a boiling point of the second solvent may be a value of more than 15 °C.

[0023] The first solvent and the second solvent being mixed in the mixing ratio having the specific mixing ratio value may be a non-azeotropic mixture. In contrast, a suspension with an azeotropic mixture, i.e., a mixture at the azeotropic point, may not be suitable for forming the film via the distillation process, since the vapor phase has the same concentration as the liquid phase at thermal equilibrium, meaning that no further separation of the two solvents can occur through the distillation process.

[0024] In various embodiments, the suspension may be destabilized by using a salting-out method. The suspension may be destabilized by introducing a salt to the suspension to form a biphasic mixture of two immiscible phases, such that the changed mixing ratio value of the first solvent and the second solvent of each of the two immiscible phases is different from the specific mixing ratio value of the suspension. In other words, the method may include adding a salt to the suspension such that a biphasic mixture (having two phases which are immiscible with each other) may be formed from a single phase of the initial mixed solvent of the first solvent and the second solvent. Each of the two phases may have a mixing ratio value (i.e., the changed mixing ratio value) of the two solvents different from the initial mixing ratio value (i.e., the specific mixing ratio value) of first solvent and the second solvent included in the suspension prior to introducing or adding the salt. The film may be formed at a liquid-liquid interface, i.e., at the interface between the two immiscible phases.

[0025] In various embodiments, the salt may be soluble in one or more of the two immiscible phases. In other words, the salt may be any suitable salt that is soluble in the one or more of the two immiscible phases. For instance, the salt may be sodium chloride or potassium chloride.

[0026] Tn various embodiments, the suspension may include one or more further two- dimensional materials such that the film includes the two-dimensional material and the one or more further two-dimensional materials In other words, while various embodiments may relate to a suspension or a film including a single type of two-dimensional material, various other embodiments may relate to a suspension or a film having multiple types of two-dimensional materials.

[0027] In various embodiments, the method may further include transferring the film onto a substrate. For instance, the film may be transferred onto the substrate by any one technique selected from a group consisting of: using the substrate to scoop up the film, using a dip-coating method, using a Langmuir-Blodgett technique, and using a pipette withdrawal and redeposit technique.

[0028] Various embodiments may form a large size two-dimensional (3D) film. In various embodiments, the film may have a (lateral) dimension of at least 1 mm, e.g., at least 5 mm, e.g., at least 1 cm. In various embodiments, each of the two (lateral) dimensions of the film may be of at least 1 mm, e.g., at least 5 mm, e.g., at least 1 cm. Various embodiments may be able to form a film that is larger than what conventional exfoliation methods may be able to form.

[0029] The first solvent and the second solvent may be any suitable combination of liquids or substances. In various embodiments, the first solvent may be ethanol and the second solvent may be water. In various other embodiments, the first solvent may be isopropyl alcohol (IP A) and the second solvent may be water.

[0030] The two-dimensional material may be a transitional metal dichalcogenide For instance, the two-dimensional materials may be selected from a group consisting of tungsten diselenide, tungsten disulfide, tungsten sulfide selenide, molybdenum diselenide, molybdenum disulfide, molybdenum sulfide selenide, platinum disulfide, platinum diselenide, palladium disulfide, palladium diselenide and hexagonal boron nitride.

[0031] Tn various embodiments, the suspension may be formed from a process including mixing the two-dimensional (2D) material (e.g., in the form of 2D material powder) with a solution of the first solvent and the second solvent (having the mixing ratio of the specific mixing ratio value) to form a mixture. The process may further include subjecting the mixture to sonication or ultrasonication, e.g., placing a container including the mixture in an ultrasonic bath At least a portion of the 2D material powder may undergo exfoliation to form 2D material flakes. The process may be referred to as mixed solvent-based liquid phase exfoliation (LPE) method. The method or process may further include removing unexfoliated bulk material from the mixture using centrifugation. The method or process may additionally include removing a supernatant portion of the mixture to obtain the suspension. The mixed solvent-based liquid phase exfoliation (LPE) method may be carried out to form the suspension required for the destabilization step. The suspension form may include a two-dimensional (2D) material (e.g., in the form of 2D material flakes), a first solvent and a second solvent, the first solvent and the second solvent being mixed in a mixing ratio having the specific mixing ratio value. During the destabilization step (e.g., during the distillation process or the salting-out method) and as described above, bigger 2D material flakes may precipitate out from the suspension to form the film, which may largely be continuous.

[0032] In various embodiments, the various processes in the destabilization step may be carried out in the same container without additional injection steps.

[0033] Various embodiments may relate to a film formed by any method as described herein. Various embodiments may relate to a film including a two-dimensional material. The film may have a plurality of flakes, each of the plurality of flakes having the two-dimensional material.

[0034] In various embodiments, the two-dimensional material may have an irregular arrangement in the film. In other words, the film may have a plurality of flakes of the two- dimensional material, the plurality of flakes forming an irregular arrangement. The plurality offlakes may form a continuous and / or relatively evenly distributed layer. For instance, the entire film may have a thickness variation of 10%, and in some cases 5%. In other words, for some films according to various embodiments, the thickest point of the film may be 110% or less relative to the thinnest point of the film, while for some other films according to various embodiments, the thickest point of the film may be 105% or less relative to the thinnest point of the film

[0035] In various embodiments, the film may include traces of the first solvent and / or the second solvent. In various embodiments, for a film formed by introducing a salt to the suspension (i.e. the salt-out method), the film may additionally or alternatively have traces of salt. In various other embodiments, the film may not have traces of the first solvent, the second solvent and / or the salt.

[0036] In various embodiments, the film may include one or more further two-dimensional materials. In various embodiments, the one or more further two-dimensional materials may have an irregular arrangement in the film. In other words, the films may include flakes of different types of two-dimensional materials, the flakes forming an irregular arrangement.

[0037] FIG. 3 shows distribution of two-dimensional (2D) material flakes using (i) dropcasting, (ii) spin-coating, and (iii) destabilization of 2D material suspension according to various embodiments. The 2D material flakes formed by drop-casting may follow a coffee ring pattern as shown in (i) and may produce a largely uneven and discontinuous morphology. For spin-coating, as shown in (ii), the 2D material flakes may form a more evenly distributed, but unless properly optimized, a discontinuous layer with a large amount of wasted material. In contrast, for a film formed by destabilization of the 2D material suspension in a mixed solvent solution according to various embodiments, flakes may naturally form a continuous and relatively evenly distributed layer to minimize surface free energy.

[0038] As mentioned above, various embodiments may relate to a method of 2D material film formation and deposition via the destabilization of mixed solvent exfoliated 2D material suspensions. Since the 2D material is only stably dispersed in a specific mixing ratio value of the suspension, the suspension may be destabilized and the 2D material precipitated out if this ratio value is disrupted. This can be achieved via a variety of methods as described herein, including ‘salting-ouf methods and the selective removal of one of the solvents via distillation. Various embodiments may result in the destabilized 2D material flakes precipitating at the liquid-air interface or liquid-liquid interface. The 2D material flakes may then self-assemble into a largely continuous film to minimize surface free energy. The precipitated film can then be transferred to appropriate substrate for the desired application.

[0039] The transfer of the 2D material film to a desired substrate may be completed by simply using the substrate to scoop up the film that formed at the interface. The small amount of residual liquid transferred in this process may then be left to be evaporated either on a hot plate, or by simply leaving the film out under ambient conditions to be evaporated (since volatile solvents were used). Nevertheless, the film that is immediately transferred onto the substrate may contain traces of the first solvent and / or the second solvent. For more scalable transfer, methods such as a Langmuir-Blodgett technique or a dip-coating technique may also be employed to transfer larger films to the desired substrate.

[0040] Various embodiments may be adaptable to different application requirements. For instance, as the mixed solvent methods may be applicable for a variety of 2D materials, various embodiments may relate to a method of forming a film including two or more 2D materials (i.e., two or more types of 2D materials) based on suspensions including two or more 2D materials (i.e., two or more types of 2D materials).

[0041] FIG. 4A shows a schematic of a mixed solvent-based liquid phase exfoliation (LPE) process according to various embodiments. Steps in this process may be optimized dependingon the desired application. For example, where specific flake thickness and lateral size distributions are required, the sonication steps and centrifugation steps should be appropriately selected as will be described in greater detail below.

[0042] In step 402, about 5 mg - 10 mg of the 2D material coarse powder is placed into a 50 ml - 100 ml glass screw cap bottle. The powder may either be obtained by simply grinding bulk 2D material using a pestle and mortar, or purchased directly from a manufacturer. Coarse 2D powders of 2D semiconductors, e.g., tungsten selenide (WSez) may be used to form WSe2 films. The method may be adapted to other 2D materials. About 50 ml of isopropyl alcohol (IPA) / water mixture or ethanol / water mixture may then be mixed into the same bottle. The appropriate mixing ratio of the two solvents can be pre-determined rationally using surface tension components by using literature values, or empirically by using ultraviolet - visible (UV- vis) spectroscopy to quantify the suspension stability. As indicated in step 404, a 50 ml IPA : 50 ml deionized (DI) water may be used for the WSe2 powder. FIG. 4B shows a table of optimized values to stabilize suspensions of various two-dimensional (2D) materials according to various embodiments.

[0043] As shown by step 406 in FIG. 4A, the bottle containing the mixture of the 2D material and the solvents may be closed and placed into an ultrasonicating bath. As the sonication time and power increase, the thickness and lateral size of the flakes may decrease, while the concentration of suspended exfoliated flakes may increase. A sonication time of 3 hours in a sonication bath operated at 60 W and 33 kHz is used here. However, lower sonication times may be desirable to produce flake distributions with larger lateral sizes for certain applications.

[0044] Depending on the application, an optional centrifugation step (step 408) may be included to remove the unexfoliated bulk material in the suspension. This can be done simply by withdrawing the as-made suspension into centrifuge tubes which are then subjected to a 10,000 revolutions per minute (rpm) centrifugation step for around 15 minutes, after which thetop three quarters of the supernatant is withdrawn for further use (step 410). A Hettich Mikro 220R centrifuge has been used with 2 ml centrifuge tubes to give a relative centrifugal force (RCF) of around 9,727 G at 10,000 rpm. The centrifugation parameters may be altered depending on the required thickness and size distribution of the flakes in suspension. A narrower distribution of these parameters may be achieved via methods such as liquid cascade centrifugation.

[0045] As the material suspension is only stable under a specific mixing ratio of the two solvents, physical and / or chemical methods that alter this ratio may enable the destabilization of the suspension and hence the precipitation of the exfoliated 2D material flakes. FIG. 5A shows a schematic illustrating the stability of tungsten sulfide (WS2) in different ratios of isopropyl alcohol (IPA) and water according to various embodiments. FIG 5B shows a schematic illustrating the precipitation of the exfoliated 2D material flakes as well as the subsequent film formation as the percentage of isopropyl alcohol (IPA) decreases from 50 volume percent (vol %) to 25 vol %.

[0046] In various embodiments, distillation may make use of the difference in the boiling points of the two solvents to selectively remove one of the solvents via selective evaporation. Distillation may be particularly effective when the mixed solvent solution includes liquids with a difference in boiling points of 10 K (i.e., 10 °C) or more.

[0047] For instance, pure IPA has a boiling point of 82.5°C, while DI water has a boiling point of 100°C. As such, formation of WSez films may be easily fabricated from a suspension of WSe2 in a mixed solvent of 50 volume percent (vol %) IPA and 50 vol % DI water FIG. 6 shows a schematic illustrating the formation of a tungsten selenide (WSei) film according to various embodiments. The suspension may be heated up in a 100 ml beaker to around 90°C using a hotplate (step 602), resulting in a liquid phase with a higher concentration of water since the IPA evaporates at a greater rate than the water, as predicted by the IPA - water phasediagram. The mixing ratio value of the water and TPA may therefore be changed with the preferential evaporation of IP A as compared to the initial mixing ratio value. As the WSc? is no longer stable in the water-rich liquid phase, the WSe2 may precipitate out as a fdm at the liquid-air interface (step 604), which may then be collected onto an appropriate substrate, e.g., by scooping the fdm up. During heating (i.e., at step 602), care may be taken to ensure that no mechanical agitation by stirring or other movements to disrupt the formation of the fdm.

[0048] Such a method may be useful in a non-azeotropic mixture. For mixtures with mixed solvent ratios approaching the azeotropic point, the vapor phase may have the same ratio or concentration as the liquid phase at thermal equilibrium, meaning that no further separation of the two solvents / liquids can occur through distillation. Such a point may occur, for instance, at 87.5 mass percent (%) of aqueous IPA (i.e., IPA and water) and 96 volume percent (vol %) of aqueous ethanol (i.e., ethanol and water). Therefore, a mixed solvent ratio that is different from ratios that result in the mixture being an azeotropic mixture should be used. Furthermore, the application of heat to the 2D material may result in chemical oxidation of the 2D materials. For less stable 2D materials, the application of a mild vacuum to give a lower pressure in the reaction vessel using a rotary evaporator may be used, which may enable the distillation of the liquid to occur at a lower temperature, while minimizing air exposure. In other words, in various embodiments, the distillation process may be carried out in a chamber having a vacuum or a partial vacuum.

[0049] Various embodiments may use the salting-out method (also referred to as antisolvent crystallization). This method may be used in biochemistry to extract proteins and deoxyribonucleic acid (DNA) from solution. When a salt such as sodium chloride is added to a solution containing proteins or DNA, a decrease in solubility of the proteins or DNA may occur when the concentration of the salt is high enough. As a result, the DNA or protein may precipitate out, thereby enabling extraction. The decrease in solubility of the original solute(i.e., DNA or protein) may be a result of the favorable ion-dipole interactions that form between the solvent and the added salt, which prevents the effective solvation of the original solute. The original solute may then associate together as the result of entropic hydrophobic interactions. Interestingly, this decrease in solubility may also apply to liquid solutes (i.e., in a liquid-liquid solution), in addition to solid solutes. Various embodiments may relate to the addition of a salt to a liquid-liquid solution (i.e., mixed solvent) to destabilize the liquid-liquid solution, thereby resulting in the solution separating out into a biphasic mixture including two immiscible phases. FIG. 7A shows schematics of (i) a stable solution containing a solid solute in a solvent and destabilized with the addition of a soluble salt in the mixture, resulting in the precipitation of the original solute; and (ii) a stable solution of two miscible solvents which are destabilized with the addition of a soluble salt to form a biphasic mixture of two immiscible phases according to various embodiments.

[0050] When this method is applied to a mixture containing 2D materials and IPA / water solution, a biphasic mixture of two immiscible liquid phases may be formed with an IPA-rich and water-rich phase. FIG. 7B shows a schematic of the biphasic mixture including an isopropyl alcohol (IPA)-rich phase and a water-rich phase formed after addition of a salt according to various embodiments. Here, pure sodium chloride salt may be slowly added into a mixture of WSe2 and 50% IPA solution (i.e., 50 vol % IP A, 50 vol % water) while stirring until a clear separation between the two immiscible phases (i.e., the IPA-rich phase and the water-rich phase) is observed. As the 2D material cannot be stably suspended in either of these phases, the 2D material may precipitate out at the liquid-liquid interface as a film. The film formed at this interface may then be transferred to the appropriate substrate for the desired application. Residual salt (from the process) on the film can then be removed by washing off using an appropriate solvent.

[0051] Various embodiments may have the advantage of integrating two different 2D materials (i.e., two different types of 2D materials) in a single film. As both materials cannot form stable suspensions in either of the two immiscible phases, the two different 2D materials may both precipitate at the same time at the liquid-liquid interface, thus producing a continuous film.

[0052] Results

[0053] 2D Films Produced By Distillation Method

[0054] As the distillation of the IPA: water solution proceeds, small flakes of the 2D material are observed to form at the liquid-air interface, which slowly accumulate into a larger film at the centimeter scale. FIG. 8 shows (i) an optical image of the tungsten selenide (WSe2) film according to various embodiments formed directly after the distillation processing of a suspension including WSc? and 50% isopropyl alcohol (IPA) solution (i.e., 50 vol % IPA, 50 vol % water); (ii) a magnified (20x) optical micrograph of the tungsten selenide (WSe2) film shown in (i) according to various embodiments on a silicon (Si) / silicon oxide (SiCh) substrate (scale bar: 0.1 mm), (iii) a scanning electron microscopy (SEM) image (5,000x) of the tungsten selenide (WSe2) film shown in (i) according to various embodiments; and (iv) a scanning electron microscopy (SEM) image (10,000x) of the tungsten selenide (WSe2) film shown in (i) according to various embodiments.

[0055] The quality and morphology of the flakes which form the film are further elucidated using UV-vis reflectance and Raman spectroscopy. FIG. 9A shows a plot of intensity (in arbitrary units or arb. units) as a function of wavenumber (per centimeter or cm'1) illustrating the Raman spectrum of mechanically exfoliated tungsten selenide (WSe2) flakes. FIG. 9B shows a plot of intensity (in arbitrary units or arb. units) as a function of wavenumber (per centimeter or cm'1) illustrating the Raman spectrum of as-prepared tungsten selenide (WSe2) film formed by the distillation method according to various embodiments. FIG. 9A shows theRaman spectra of mechanically exfoliated WSe2 flakes, while FIG. 9B shows the Raman spectra of the as prepared-film from the same batch of 2D material powders. The position of the peaks for the mechanically exfoliated WSez flakes appear to roughly correspond to values reported in other literature with a characteristic E2gpeak of around 248 - 250 cm'1. The E2gpeak in the film sample was anomalously redshifted with a wavenumber around 244 cm'1, which may suggest that some loss of crystallinity was experienced during the distillation process due to the heating of the suspension, as this red-shift was not observed in the WSe2 / MoSe2 heterostructure film formed using the salting-out method where no heating was used.

[0056] Nonetheless, as shown in FIG. 9C, distinct excitonic peaks which closely match literature values corresponding to A (1.57 eV), B (2.00 eV), C (2.25 eV), and D (2.49 eV) excitons may be clearly seen in the UV-Vis reflectance spectra of the material. FIG. 9C shows a plot of differential reflectance (in arbitrary units or arb. units) as a function of energy hv (in electron-Volts or eV) illustrating the ultraviolet-visible (UV-Vis) reflectance spectra of the film formed by the distillation method according to various embodiments. FIG. 9C demonstrates optical properties of the film formed by the distillation method is similar to that formed be mechanical exfoliation. While the flakes are expected to spread over a distribution of different thicknesses, the position of these peaks in the reflectance spectra, and hence the energy of their corresponding excitons, suggest that the flakes may largely be around 4 layers thick or more based on the thickness dependent reflectance spectra of excitons in WSe2 reported in literature.

[0057] 2D Films Produced By Salting-Out Method

[0058] The salting-out method may result in the 2D material film being formed immediately after enough salt was added to cause a separation of the two liquid phases in the mixture. The 2D material, which cannot form a stable suspension in either of these phases, may immediately accumulate into a centimeter-sized film-like layer at the interface that is ‘trapped’ between the two liquid phases. The size of the film may largely depend on the amount of the 2D materialthat had been in the original suspension, which indicates that this method can be simply scaled by using larger volumes of the same suspension. The 2D material fdm at this interface can then be transferred to an appropriate substrate.

[0059] A key advantage of this method is that due to the rapid precipitation of the 2D material at the point where the salting-out produces two immiscible layers. This method may easily be adapted to form a film having two or more (i.e., types of) 2D materials. Here, each 2D material suspension can be prepared separately using the methods described herein , which may mean that the optimal mixing ratio to stabilize each of the suspensions of different 2D materials can be maintained. For instance, separate suspensions of molybdenum selenide (MoSe ) in 50% IPA solution (i.e., 50 vol % IP A, 50 vol % water) and tungsten selenide (WSe2) in 50% IPA solution (i.e., 50 vol % IPA, 50 vol % water) may be prepared. The two suspensions can then be mixed thoroughly by stirring for 30 minutes using a magnetic stir bar, before salt is added to separate out the two liquid phases in the mixture.

[0060] The film may then be transferred onto a variety of different substrates including a silicon / silicon oxide (Si / SiC>2) substrate, as well as gold (Au) electrodes patterned on polyimide (PI) and polyethylene terephthalate) (PET) substrates. FIG 10A shows an optical microscopy image of a tungsten selenide (WSe2) film formed by the salting-out method according to various embodiments on gold (Au) electrodes patterned on a polyethylene terephthalate) (PET) substrate. FIG. 10B shows an optical microscopy image of a tungsten selenide (\VSe2) film formed by the salting-out method according to various embodiments on gold (Au) electrodes patterned on a polyimide (PT) substrate.

[0061] FIG. 10C shows a plot of intensity (in arbitrary units or a.u.) as a function of wavenumber (per centimeter or 1 / cm) illustrating the Raman spectrum of the molybdenum selenide (MoSe2) / tungsten selenide (WSe2) film formed by the salting-out method according to various embodiments onto a silicon / silicon oxide (Si / SiC>2) substrate. The position of theRaman peak in FIG. IOC closely matches that of previously reported individual MoSez and WSez Raman spectra, as well as MoSe2 / WSe2 heterostructure Raman spectra. This may suggest a high degree of crystallinity and material quality of the fdm formed by the salting-out method. FIG. 10D shows an optical micrograph of the molybdenum selenide (MoSe2) / tungsten selenide (W Se2) fdm formed by the salting-out method according to various embodiments. The fdm may form a continuous layer, similar to the fdm as shown in FIG 5

[0062] There may be certain limitations in measuring the optical characteristics of the fdm, and in particular, the interlayer exciton behavior characteristic of certain 2D material heterostructures. Due to the polydisperse nature of the suspension in use, along with the random nature of the stacking of different 2D materials, no photoluminescence associated with interlayer transitions between MoSe2 / WSe2 fdm similar to that exhibited by films assembled by mechanical exfoliation and deterministic transfer techniques has been observed. The fdms assembled by mechanical exfoliation and deterministic transfer techniques typically include larger flakes of 2D material, which may enable a clear and large overlap in each flake where the interlayer exciton photoluminescence is expected. Also, strong photoluminescence may only be expected between monolayer flakes where a direct K - K transition in each layer can take place. However, this may not necessarily preclude possible applications of these materials utilizing their heterostructure band alignments. For instance, a MoSe2 / WSe2 p-n heterostructure with tunable gate rectification using a similar polydisperse material has been reported.

[0063] FIG. 11 shows a schematic illustrating the distillation and salting-out methods of a mixed solvent suspension including a two-dimensional (2D) material for forming a two- dimensional (2D) material fdm according to various embodiments. Distillation may selectively remove a solvent with a lower boiling point, thereby destabilizing the suspension and resulting in the fdm being precipitated. In the salting-out method, the addition of salt may separate thetwo solvents to form a biphasic mixture. The 2D materials can no longer be dispersed in either phase of the biphasic mixture, and may precipitate at the interface of the two phases.

[0064] FIG. 12 shows a schematic illustrating a method to form a two-dimensional (2D) material film according to various embodiments. The method may include addition of 2D material powders to a mixed solvent solution, exfoliation via sonication, and either distillation or salt-out to form the 2D material film.

[0065] Various embodiments may exploit the property of 2D materials to form stable suspensions in mixed solvent solutions only under mixing ratios having a specific mixing ratio value. The mixed-solvent exfoliated 2D material suspensions may be destabilized by distillation or salting-out methods, resulting in the precipitation of the 2D material into liquid-air or liquidliquid interface to form a large size, continuous film. Mixed solvent solutions have conventionally been used to achieve higher stability and more concentrated suspensions of 2D materials. Various embodiments using the completely opposite direction of disrupting the specific mixing ratio value and destabilizing the suspension may be counter-intuitive.

[0066] Salting-out may be a non-intuitive application of a research method (used in distant field of biochemistry) to 2D material applications. It may not be common to add salt to separate a mixed solvent into immiscible phases.

[0067] Distillation may be a non-intuitive application of a method to separate two solvents, thereby serving as a means of disrupting the mixing ratio to enable precipitation of a film. The film may be formed in an aggregation process by overlapping of 2D material flakes.

[0068] Various embodiments may rely on the minimization of surface free energy for film assembly. Various embodiments may be applicable to be used for a wide variety of 2D materials.

[0069] Various embodiments may be used to form a heterostructure film including different2D materials.

[0070] Various embodiments may use less toxic and less environmentally harmful chemicals compared to conventional methods.

[0071] Various embodiments may also further include transferring the formed 2D material films onto different substrates via method including, but not limited to, dip-coating, Langmuir- Blodgett trough, pipette withdraw and redeposit, or scooping up directly using the substrates.

[0072] Various embodiments may be used in optoelectronic applications (e.g., photodetectors and photovoltaic structures), electronic applications (e.g., conductive layers or interfaces) and / or in corrosion barriers (e g., for preventing or slowing electrochemical degradation). Various embodiments may be able to form large area films for large area photodetectors or photodetector arrays, or for van der Waal heterostructures.

[0073] FIG. 13 shows a table comparing properties of a film formed by a method according to various embodiments and conventional films formed by other methods.

Claims

Claims1. A method of forming a film, the method comprising: forming a suspension comprising a two-dimensional material, a first solvent and a second solvent, the first solvent and the second solvent being mixed in a mixing ratio having a specific mixing ratio value; and destabilizing the suspension by changing the mixing ratio from the specific mixing ratio value to a changed mixing ratio value different from the specific mixing ratio value, thereby forming the film comprising the two-dimensional material.

2. The method according to claim 1, wherein the suspension is destabilized by heating the suspension in a distillation process.

3. The method according to claim 2, wherein a difference in a boiling point of the first solvent and a boiling point of the second solvent is a value of more than 10 °C.

4. The method according to claim 2, wherein the first solvent and the second solvent being mixed in the mixing ratio having the specific mixing ratio value is a non-azeotropic mixture.

5. The method according to claim 1, wherein the suspension is destabilized by introducing a salt to the suspension to form a biphasic mixture of two immiscible phases, such that the changed mixing ratio value of the first solvent and the second solvent of each of the two immiscible phases is different from the specific mixing ratio value of the suspension.

6. The method according to claim 5, wherein the salt is soluble in one or more of the two immiscible phases.

7. The method according to claim 6, wherein the salt is sodium chloride or potassium chloride.

8. The method according to claim 5, wherein the suspension comprises one or more further two-dimensional materials such that the film comprises the two-dimensional material and the one or more further two-dimensional materials.

9. The method according to claim 1, wherein the film is formed at a liquid-air interface10. The method according to claim 1, wherein the film is formed at a liquid-liquid interface.1 1 . The method according to claim 1, further comprising: transferring the film onto a substrate.

12. The method according to claim 11, wherein the film is transferred onto the substrate by any one technique selected from a group consisting of: using the substrate to scoop up the film, using a dip-coating method, using a Langmuir-Blodgett technique, and using a pipette withdrawal and redeposit technique.

13. The method according to claim 1, wherein the film has a dimension of at least 1 mm.

14. The method according to claim 13, wherein the dimension is of at least 5 mm.

15. The method according to claim 14, wherein the dimension is of at least 1 cm.

16. The method according to claim 1 , wherein the first solvent is ethanol and the second solvent is water.

17. The method according to claim 1, wherein the first solvent is isopropyl alcohol (TPA) and the second solvent is water.

18. The method according to claim 1, wherein the two-dimensional material is a transition metal dichalcogenide.

19. The method according to claim 1, where in the two-dimensional material is selected from a group consisting of tungsten diselenide, tungsten disulfide, tungsten sulfide selenide, molybdenum diselenide, molybdenum disulfide, molybdenum sulfide selenide, platinum disulfide, platinum diselenide, palladium disulfide, palladium diselenide and hexagonal boron nitride.

20. The method according to claim 1, wherein the suspension is formed from a process comprising: mixing the two-dimensional material with a solution of the first solvent and the second solvent having the mixing ratio of the specific mixing ratio value to form a mixture; and placing a container comprising the mixture in an ultrasonication bath.21 . The method according to claim 20, wherein the process comprises removing unexfoliated bulk material from the mixture using centrifugation.

22. The method according to claim 20, wherein the process comprises removing a supernatant portion of the mixture to obtain the suspension.

23. A film formed by a method comprising: forming a suspension comprising a two-dimensional material, a first solvent and a second solvent, the first solvent and the second solvent being mixed in a mixing ratio having a specific mixing ratio value; and destabilizing the suspension by changing the mixing ratio from the specific mixing ratio value to a changed mixing ratio value different from the specific mixing ratio value, thereby forming the film comprising the two-dimensional material.

24. The film according to claim 23, wherein the two-dimensional material has an irregular arrangement in the film.

25. The film according to claim 23, wherein the film comprises traces of the first solvent or the second solvent.

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