Patent Publication Number: US-2022230879-A1

Title: Solution-based deposition method for preparing semiconducting thin films via dispersed particle self-assembly at a liquid-liquid interface

Description:
FIELD OF INVENTION 
     The present application concerns the fields of electronics, optoelectronics, gas sensing, electrochemistry, catalysis and energy conversion and storage. 
     BACKGROUND ART 
     One of the most demanding challenges in material science today is to develop new materials having the properties which suit specific applications. 
     In the technical field of optoelectronic devices, gas sensors and energy conversion/storage devices, layered transition metal dichalcogenides (TMDs), such as MoS 2 , WS 2 , MoSe 2  and WSe 2 , have been identified as promising semiconducting 2D layered materials, since they exhibit particular electrical, optical and mechanical properties. In order to fully make use of these properties, research has been performed to obtain thin films of these materials. 
     A Langmuir-Blodgett method allows the fabrication of ultra-thin, highly ordered organic films. In principle, the basic idea of this method consists in dipping and pulling a solid substrate. 
     In this well-known method, a molecule dissolved in a non-polar volatile solvent is spread onto the surface of a polar solvent (sub phase), which leads to a formation of a one-molecule-thick layer (Langmuir monolayer), after the evaporation of the non-polar volatile solvent and application of surface pressure. Then the thus-formed film is deposited on a substrate by different methods, such as the Langmuir-Blogett vertical method, Langmuir-Shaefer method, horizontal precipitation method, and a roll-to-roll deposition method. 
     Here, processing liquid-phase exfoliated 2D flakes into a thin film in a continuous manner while avoiding restacking and aggregation of the flakes has remained a major technical challenge. Although on a laboratory scale, restacking can be avoided by self-assembly of the 2D nano-flakes at a liquid-liquid interface of two non-solvents for the flakes (Yu et al. [1] and Biswas et. al [2]), the ability to perform this liquid-liquid self-assembly and the subsequent flake deposition has remained a challenge at an industrial scale. 
     To date, [3], [4] and [5] disclose roll-to-roll Langmuir-Blodgett printing methods with a liquid-gas interface. However, these methods require high temperature conditions and/or chemical etching, and can not be conducted under ambient conditions. 
     discloses a way to make and transfer large-area (100 cm 2 ) free-standing 2D or 3D colloidal photonic crystal (CPC) films deposited using a roll-to-roll Langmuir-Blodgett technique from silica or polymer particles. 
     discloses a technique for rapid large-scale assembly of monolayers and multilayers of nanoelements on a variety of different substrates, the technique being based on self-assembly of nanoelements suspended at the interface between a polar solvent and a nonpolar solvent. The layer of nanoelements is collected onto a substrate at a shallow angle, forming a continuous monolayer or multilayer of nanoparticles which can be optionally patterned or can be transferred to other substrates to form components of nanoelectronics, optical devices, and sensors. 
     discloses a large-area monolayer of solvent dispersed nanomaterials and method of producing same. The method of the disclosure includes dripping a nanomaterial solvent into a container filled with water whereby the nanomaterial being dripped collects at the air-water interface to produce the large-area monolayer. In one embodiment, different nanomaterial solvents can be dripped, at predetermined intervals such that the resulting large-area monolayer includes at least two different nanomaterials. 
     discloses compositions of the formula MX 2 :Y, wherein MX 2  is a layer type transition metal dichalcogenide, M is a metal selected from the group consisting of niobium, tantalum, molybdenum and tungsten, X is a chalcogen selected from the group consisting of sulfur and selenium and Y is a material located between layers of MX 2 . The compositions may be prepared by the steps of first forming a suspension of the MX 2  in a body of water. A liquid which is immiscible with water is added to the suspension. The suspension and immiscible liquid are agitated together to form a temporary emulsion. The emulsion is allowed to rest until the water and the liquid separate with an interface therebetween. A sheet-like composition of MX 2 :Y forms at the interface. Preferably, the MX 2  is in exfoliated, singular molecular thickness layers suspended in the water. 
     Therefore the present invention aims at providing a method for preparing a semiconducting thin film, wherein the liquid-liquid self-assembly and the subsequent flake deposition is carried out in an efficient manner, in ambient conditions. 
     NON-PATENT LITERATURE REFERENCES 
     
         
         [1] X. Yu: Nat. Commun. 2015, 6, 7596: Self-assembled 2D WSe 2  thin films for photoelectrochemical hydrogen production 
         [2] S. Biswas: Nano Lett., 2009, 9, 167: A Novel Approach to Create a Highly Ordered Monolayer Film of Graphene Nanosheets at the Liquid-Liquid Interface 
         [3] M. Parchine, J. McGrath, M. Bardosova and M. E. Pemble: Langmuir, Vol. 32 (23), p. 5862-5869: Large area 2D and 3D colloidal photonic crystals fabricated by a roll-to-roll Langmuir-Blodgett method 
         [4] Y. Zhang et al.: J. Mater. Chem. C, 2017, 5, 11275: Langmuir films and uniform, large area, transparent coatings of chemically exfoliated MoS 2  single layers 
         [5] Y. R. Lim: Adv. Mater., 2017, 1705270: Roll-to-Roll Production of Layer-Controlled Molybdenum Disulfide: A Platform for 2D Semiconductor-Based Industrial Applications 
         [6] T. Kohoutek et al., Optical Materials Express, 2018, 8, 960; Large-area flexible colloidal photonic crystal film stickers for light trapping applications 
       
    
     PATENT LITERATURE 
     
         
         [7] WO 2016/094384 A2 
         [8] WO 2017/219125 A1 
         [9] EP 0 382 339 A1 
       
    
     SUMMARY OF THE INVENTION 
     The present invention relates to a device for coating semiconductor/semiconductor precursor particles on a flexible substrate, wherein the device comprises: 
     a container ( 5 ) configured to contain a first solvent and a second solvent substantially immiscible with each other; 
     injection means ( 15 ) configured to inject a predetermined volume of a dispersion of at least one layered semiconductor material or its precursor(s) in the form of particles, the injection occurring at a liquid-liquid interface formed within the container ( 5 ) and between the first solvent and the second solvent, and creating a film of particles at the liquid-liquid interface; 
     a first support means ( 25 ) configured to be able to be at least partially submerged in the first solvent, wherein the first support means ( 25 ) is configured to support at least a portion of a substrate; 
     substrate extracting means configured to exert a force on the substrate causing the substrate to be drawn away from the liquid-liquid interface, 
     substrate supply means configured to provide a supply of the substrate; 
     compression means ( 55 ), configured to reduce a distance between particles of the at least one layered semiconductor material or its precursor(s) of the film at the liquid-liquid interface and push the film onto the substrate; wherein the compression means ( 55 ) comprises a plurality of means for pushing ( 65 ) mounted on a drive device ( 75 ), the plurality of means for pushing ( 65 ) configured such that at least two of the plurality of means for pushing ( 65 ) may be at least partially submerged in the second solvent during rotation of the drive device ( 75 ), and moved through the second solvent toward the first support means ( 25 ). 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is schematic representations of a non-limiting and illustrative example of devices for coating semiconductor precursor particles on a flexible substrate of the present invention, comprising: pusher belt system, drive motor (controlled by computer), plastic substrate, substrate extractor, Plexiglas bath for liquid-liquid-interface, dispersion injection manifold, “back wall” defining back edge of liquid-liquid interface, dispersion delivery system (2×10 mL syringe pumps),  FIG. 1  being illustrative of particularly preferred embodiments. 
         FIG. 2  (a) to (e) are photographs of a device for coating semiconductor precursor particles on a flexible substrate: (a) entire device from another angle, (b) and (c) vision from upper side, (d) zoom on spool device, and (e) vision from the front side. 
         FIG. 3  is a photo of 300 cm 2  roll-to-roll printed film prepared with a device of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     &lt;Definitions&gt; 
     The term “polar”, as used herein, refers to “having a large dipolar moment”. A polar solvent can have a dielectric constant of, for example,  10  or more. Examples of polar solvents are water, carboxylic acids such as acetic acid, alcohols such as methanol, ethanol, n-propanol or n-butanol, ketones such as acetone, organic sulfoxides such as dimethyl sulfoxide, organic nitriles such as acetonitrile and amides such as dimethylformamide. 
     The term “non-polar”, as used herein, refers to “having a small dipolar moment”. A non-polar solvent can have a dielectric constant of, for example 10 or less. Examples of non-polar solvents are linear or cyclic hydrocarbons such as benzene or hexane, ethers such as diethyl ether, halogenated hydrocarbons such as carbon tetrachloride and methylene chloride. 
     The term “miscible”, as used herein, refers to two or more solvents which, when mixed together, form a single phase. In the case of two liquids, they may typically be miscible when the solubility of one solvent in the other is 10 g/100 g or more. 
     The term “immiscible”, as used herein, refers to two or more solvents which, when mixed together, form two or more phases. In case of two liquids, they may typically be miscible when the solubility of one solvent in the other is less than 10 g/100 g. 
     The term “flexible” as used herein, refers to the feature that a substrate can be deformed. A flexible substrate, as used herein, may have a bend radius of 5 cm or less. 
     Device 
       FIG. 1  shows an exemplary configuration for the device according to the present disclosure. The device includes a container  5 , injection means  15 , a first support means  25 , substrate extracting means  35 , substrate supply means  45  and compression means  55 . 
     Container 
     Container  5  is configured to contain a first  10  and second solvent  20 , as well as to receive injections of particles  40  from injection means  15 . By dispensing the first  10  and second  20  solvent into container  5 , a liquid-liquid interface may be formed at a particular level within container  5  between first solvent  10  and second solvent  20 . Container  5  may, therefore, be fabricated from any suitable material, for example, material having minimal or no reactivity or propensity to react with first  10  and second solvents  20 . For example such materials may include, Acetal (Delrin®), Aluminum, Brass, Bronze, Buna N (Nitrile), Carbon graphite, Carbon Steel, Glass, Carpenter  20 , Cast iron, Ceramic, Copper, CPVC, EPDM, Epoxy, Fluorocarbon (FKM), Hastelloy-C®, Hypalon®, Hytrel®, Kalrez, Kel-F®, Natural rubber, Neoprene, NORYL®, Nylon, Polyetherether Ketone (PEEK), Polypropylene, PPS (Ryton®), PTFE, PVC, PVDF (Kynar®), stainless steel— 304 , stainless steel— 316 , Titanium, Tygon® and Viton®. 
     Injection Means 
     Injection means  15  may comprise any suitable device configured to introduce a predetermined volume, depending on the surface area of the liquid-liquid interface (e.g. 2-3 ml for a surface area of 100 cm 2  in a laboratory scale device) of the dispersion into container  5 , and particularly at liquid-liquid interface formed at the interface of the first  10  and second solvent  20 . Examples of suitable injection means  15  can be found commercially at www.vwr.com (pipette tip or syringe needle). 
     In one preferred embodiment, injection means  15  may be configured to inject the dispersion from the bottom up to the liquid-liquid interface. This may be done, for example, by installing injectors on the bottom of the container. 
     Injection means  15  may be configured to inject or otherwise introduce the predetermined volume of the dispersion at a predefined interval. For example, injection means  15  may be configured to introduce the predetermined volume at an interval of 0.1 to 100 seconds. Alternatively, injection means  15  may be configured to perform continuous injection of the dispersion at a desired flow rate, e.g. 0.5 mL/s per 20 cm 2  of liquid/liquid surface area. 
     In one embodiment, injection means  15  may be an injector. An appropriate injector might be commercially available. 
     In one preferred embodiment, the injection means  15  comprise a plurality of needles fluidly connected to an injection manifold. Such a configuration might allow one to obtain a film without gaps or overlaps. The term “fluidly” as used herein means “in a continuous line”. 
     The arrangement of the needles is flexible: needles can be in a straight line or staggered, for example, as long as the paddle dips down after the last needle (so that the last needle is not disrupting a complete film being moved towards the substrate). 
     In one preferred embodiment, the injection means  15  allows injection of the dispersion of at least one layered semiconductor material or its precursor(s) in the form of particles from underneath the first  10  and second  20  solvents in the container  5 . Such a configuration allows one to obtain a film with more robustness, as well as leaving the injecting apparatus in place while means for pushing  65  move in and out of the solvent bath (more difficult for case wherein means for pushing  65  are evenly distributed over the entire bath). 
     Injection means  15  may be configured in such a way that injection speed is kept constant relative to the speed of means for pushing  65  and the speed of substrate extracting means  35 . For example, if means for pushing  65  and substrate extracting means  35  are configured to increase the speed by a factor of  5 , injection means  15  is to be configured to increase the speed by a factor of  5 , assuming that the container volume is constant. This can be done, for example by increasing the number of injection means  15  and/or by increasing the speed of injection of each injection means  15 . If the container volume is increased, for example by increasing the width of the container  5 , resulting in an increase in film area, injection means  15  is to be configured to increase its speed, while means for pushing  65  and substrate extracting means  35  are to be configured to maintain the initial speed. For example, a device with an increased width may have more needle lines. 
     In the case of a laboratory scale device, injection means  15  may be configured in such a way that its total injection speed is comprised between 2 and 4 ml/min (the injection speed through each pair being 1-2 mL/min), in the case of a device with 4 needles and container  5  with a width of 10 cm. For an industrial scale device, the injection speed depends on the width of container  5 , and an industrial scale device may be configured to have an injection speed of, for example, 0.025 mL/min.cm 2 . In the case of a device having a width of container  5  of 1 m for providing a 1-meter-wide film (of any length) with 40 needles operating, for example, the total injection speed might be 10-20 mL/min (the injection speed through each pair being 1-2 mL/min). 
     Without wishing to be bound by any particular theory, the amount of injection means  15  may depend on the size of container  5  and thus the size of the film to be prepared. The amount of injection means  15  might also related to the injection rate: in order to obtain the same size of film, if the amount of injection means  15  is decreased, the injection rate is to be increased, and vice versa. For example, in a laboratory scale device, it is possible to use, for example, 8 needles as injection device  15 , and an injection rate of 0.5 mL/min for each needle (i.e. 4 mL/min of total injection rate), and the speed of means for pushing  65  and the speed of substrate extracting means  35  of 1 mm/sec. 
     As shown in  FIG. 1 , in a preferred embodiment of the invention, the device of the present invention may have only one line of needles in the rear opposite side with respect to the substrate. Such a configuration may allow the film not to be disturbed by additional needles as it moves, since the film is built up as the means for pushing  65  moves from back to forth. 
     The needles may also be disposed in several lines in an array. 
     First Support Means 
     First support means  25  is at least partially submerged in the first solvent wherein the first support means  25  is configured to support at least a portion of a substrate  30 . 
     First support means  25  may be configured to support a substrate  30  in motion and onto which semiconductor particles are to be deposited at a predetermined rate. First support means  25  may be positioned at least partially submerged in the first solvent  10 , i.e., the solvent of greater density among the first  10  and second solvent  20 , and may be configured to enable substrate  30  to move at a desired feed rate, for example, 0.1 to 1000 mm/min in case of a laboratory scale device and 60 mm/min or more in case of an industrial scale device. First support means  25  may comprise any suitable material for supporting substrate  30 , for example, a polymer surrounding a metallic or composite based hub may be provided. 
     A position of first support means  25  may be configured to vary according to a level of the first solvent within container  5  such that first support means  25  may maintain a desired level of submersion within the first solvent  10 . For example, first support means  25  may be provided with a floating member (not shown), such a floating member being positioned in relation to first support means  25  such that first support means  25  maintains its position relative to the liquid-liquid interface between the first and second solvents. In so doing, substrate  30  may be passed through both first solvent  10  and second solvent  20 , such that particles  40  are distributed on a face of substrate  30  at a desired rate. 
     In one embodiment, first support means is a roller. An appropriate roller might be commercially available. One of skill will recognize that other configurations for supporting substrate  30  may be used in lieu of first support means  25 . For example, a fixed plate having a desired angle relative to the liquid-liquid interface, and a friction-reducing coating with regard to substrate  30  may be provided. 
     Substrate Extracting Means 
     Substrate extracting means  35  is configured to exert a force on the substrate  30  causing the substrate to be drawn away from the liquid-liquid interface. Substrate extracting means  35  may be configured to exert a force on substrate  30  to draw substrate  30  away from the liquid-liquid interface at a desired, controlled rate. For example, substrate extracting means  35  may include a motor configured to rotate substrate extracting means  35  at a desired rate, resulting in rolling of substrate  30  upon itself at substrate extracting means  35 . 
     Substrate extracting means  35  may comprise any suitable device configured to exert a predetermined force on the substrate in a constant manner. For example, substrate extracting means  35  may be configured to exert a traction force. 
     In one embodiment, substrate extraction means  35  may be commercially available. 
     In one embodiment, substrate extraction means  35  may be configured in such a way that it speed is substantially equal to the speed of means for pushing  65 . In such a way, it is possible to avoid gaps or overlap in the film. 
     In case of a laboratory scale device, substrate extraction means  35  may be configured in such a way that its speed on comprised between 1 mm/sec and 5 mm/sec. 
     In one embodiment, the angle of substrate extraction with respect to the liquid-liquid interface is comprised between 10° and 90°, and preferably between 10° and 60° Such a range of angle of extraction allows one to prepare a film without gap or superposition. 
     Substrate Supply Means 
     Substrate supply means  45  is configured to provide a supply of the substrate  30 . Substrate supply means  45  may be configured to support a supply of substrate  30 , and to allow substrate  30  to be extracted from the supply at a controlled rate, e.g., based on a rate of rotation of substrate extraction means. One of skill in the art understands that any suitable support for supplying the substrate  30  may be implemented. 
     Substrate supply means  45  may comprise any suitable device configured to supply substrate at a predetermined speed in a constant manner. 
     Substrate supply means  45  may be configured to supply the substrate at a predetermined speed, for example at 60 mm/min. 
     In one embodiment, substrate supply means  45  may be commercially available: PETITO188-18 Solaronix provided by ITO COATED PET FILM MSE Supplies. 
     Compression Means 
     Compression means  55  is configured to reduce a distance between particles of the at least one layered semiconductor material or its precursor(s) of the film at the liquid-liquid interface and push the film onto the substrate  30 , wherein the compression means  55  comprises a plurality of means for pushing  65  mounted on a drive device  75 , the plurality of means for pushing configured such that at least two of the plurality of means for pushing  65  may be at least partially submerged in the second solvent  20  during rotation of the drive device  75 , and moved through the second solvent  20  toward the first support means  25 . 
     Drive Device 
     Drive device  75  may comprise any suitable device configured to rotate at a predetermined speed in a constant manner. Drive device  75  may be configured so that pushing means  65  is driven in drive direction (D) while it is submerged in the second solvent. Consequently, the film formed at the liquid-liquid interface is driven in the drive direction by pushing means  65 . 
     In one embodiment, a drive device might be a spool belt device comprising spools and a belt. Spools may preferably be made of a material at least resistant to water, heptane and butyl alcohols, such as PETG, tetrafluoroethylene perfluoropropylene (FEP), ethylene-tetrafluoroethylene (ETFE), polyfluoroalkoxy (PFA), polyvinyl chloride (PVC) and polypropylene (PP), and the belt may preferably be made of a material, which preferably have some resistance to water, heptane and butyl alcohols, such as “Ninja flex” (polyurethane composition), FEP, ETFE, PFA, polyvinylidene fluoride (PVDF), PP, polychloroprorene (pc-rubber), metals and metal alloys. 
     When 3D-printing these materials at laboratory scale, the spools may preferably be made of PETG and the belt may preferably be made of “Ninja flex”, provided by NinjaTek (https://ninjatek.com/ninjaflex/), due to printability. Such a material is more adapted to 3D printing. 
     Means for Pushing 
     A plurality of means for pushing  65  may be mounted on drive device  75  by any conventional method known to one of skill. For example, a plurality of means for pushing  65  may be mounted on a drive device  75 , for example, by mechanical fixing. The height of means for pushing  65  may be adjustable. The means for pushing  65  may be configured to be long enough to pass through second non-polar solvent  20  to the liquid-liquid interface but short enough so as not to scrape the bottom of the container  5  or the substrate  30  after the film has been deposited and the means for pushing  65  is continuing on the drive device  75 . This depends on the height of the container  5  and where the drive device  75  is located with respect to the substrate withdrawal site. A person skilled in the art is capable of appropriately selecting the length of means for pushing  65 . 
     In one preferred embodiment, said plurality of means for pushing  65  is substantially perpendicular with respect to the drive device  75 . Without wishing to be bound by any theory, such a configuration allows one to obtain a film with a more regular shape. 
     In one embodiment, means for pushing  65  may be configured in such a way that its speed is substantially equal to the speed of substrate extracting means  35 . In this way, it is possible to avoid gaps or overlap in the film. 
     For a laboratory scale device, means for pushing  65  may be configured in such a way that its speed is, for example, between 1 mm/sec and 5 mm/sec. 
     For an industrial scale device, the speed of means for pushing  65  may be 1-3 mm/s. 
     In one embodiment, one means for pushing  65  comprises at least two substantially parallel pushing bars  85 , mounted substantially perpendicularly with respect to the drive direction (D) and substantially perpendicularly with respect to the plane of the liquid-liquid interface of a first solvent and a second solvent (P), when it is submerged in the second solvent. Here, drive direction (D) is the direction into which the drive device  75  is driven when pushing means  65  is submerged in a second solvent, and consequently the film formed at the liquid-liquid interface of a first solvent and a second solvent  2  is driven. 
     In one preferred embodiment, the means for pushing  65  consists of one pushing bar  85 , mounted substantially perpendicularly with respect to drive direction (D) and substantially perpendicularly with respect to plane (P), when it is submerged in the second solvent. Such a configuration allows one to pushing the film in a more regular manner and thus to obtain a more homogeneous film. 
     An appropriate thickness may be the smallest thickness possible that allows suitable rigidity for pushing, and suitable attachment to the belt, and therefore may depend on the scale of the device. The thickness of the bar may be, for example, 3 mm or less. 
     In one preferred embodiment, at least three means for pushing  65 , each consisting of one pushing bar, are mounted on drive device  75 , i.e. at least 3 pushing bars in total are mounted, preferably substantially perpendicularly with respect to drive direction (D) and substantially perpendicularly with respect to plane (P), when it is submerged in the second solvent. 
     In another embodiment, at least three means for pushing  65 , each consisting of two parallel pushing bars, are mounted on drive device  75 , i.e. at least 6 pushing bars in total are mounted preferably substantially perpendicularly with respect to drive direction (D) and substantially perpendicularly with respect to plane (P), when it is submerged in the second solvent. 
     In one preferred embodiment, the shortest distance between opposing faces of the two substantially parallel pushing bars  85  is at least 50 mm and at most 150 mm. 
     Bars  85  may be made of materials resistant to water, heptane, tert-butanol and 1-butanol, and may be able to maintain a straight line in the liquids but do not need to be strictly rigid. 
     In one preferred embodiment, pushing bars  85  are made of a fluorinated plastic, preferably PTFE, tetrafluoroethylene-perfluoropropylene (FEP), ethylene-tetrafluoroethylene (ETFE), polyfluoroalkoxy (PFA), polyvinylidene fluoride (PVDF), aluminium, steel, brass or stainless steel. 
     Polyethylene terephthalate (PET) may be used for a short term, but may need to be replaced over a longer period. 
     In one preferred embodiment, the drive device  75  comprises elements made of: FEP, ETFE, PFA, PVDF, polyvinyl chloride (PVC), polypropylene (PP); polyurethane; polychloroprene (pc-rubber); metals or metal alloys. 
     In one preferred embodiment, the first support means  25 , the substrate extracting means, and the substrate supply means, each comprise one or more rollers. 
     In one preferred embodiment, a paddle belt forms an exemplary compression means  55  configured to reduce a distance between particles of semiconductor present in the dispersion at the liquid-liquid interface before and/or during application to substrate  30 . According to embodiments of the disclosure, paddle belt  60  may comprise one or more paddles  60  configured to revolve about a set of rollers in a direction causing a flow in the second  20  and/or first solvents  10  so as to exert a pressing force on particles  40  in the dispersion in a direction toward substrate  30 . Paddles  60  may comprise any suitable material, for example, an elastic polymer such as polytetrafluoroethylene (PTFE). 
     Each paddle  60  may therefore span a width of container  5 , and may move a suitable velocity for creating a desired level of compaction between the particles  40  of the dispersion, such that upon reaching substrate  30 , the particles  40  are aligned and compressed to a desired density upon the surface of substrate  30 . In other words, a period between the revolving paddles  60 , based on a velocity of the paddles  60  and the spacing there between, may be approximately equal to a desired length of a finished semiconductor thin-film segment having a desired particle dispersion per unit area (e.g., 10 11 -10 14  particles per m 2 ). 
     Configuration of Device when Operating 
     The height of the water layer must be above the level of injection means (e.g. needles). The height of the polar first solvent may be above the level of the injection means  15 , and the height of the non-polar second solvent  20  may be below the drive device  75 , and is only restricted by the height of the container or the height of the drive device  75 . 
     Injection means  15  are disposed in a manner not to disturbing the movement of means for pushing  65 . A person skilled in the art is capable of achieving such a configuration, for example by adjusting the relative position of injection means  15  and means for pushing  65 . For example, when needles are used as injections means  15  and paddles are used as means for pushing  65 , the device may preferably be configured in such a way that the paddles dip down after the last needle. Such a configuration would allow one to make sure that the last needle is not disrupting a complete film being moved towards the substrate. 
     When 8 needles are used as injection means  15  and 2 paddles separated from each other by a distance of 8 mm are used as pushing means  65 , in a container having a width of 10 mm and 10 films are deposited successively, the total film area can be calculated as to be 10 mm (width of the container)×8 mm (distance between paddles)×8 (number of needles)×10 (number of films consecutively deposed)=6400 mm 2 . 
     When the volume of the container is increased (for example, by increasing the width of the container), the injection speed can be increased to a certain point and/or additionally needles may be added to accommodate volume increase. 
     &lt;Method for Preparing a Semiconducting Thin Film&gt; 
     The Method for Preparing a Semiconducting Thin Film of the Present Invention Comprises the following Steps:
         (1) providing a liquid-liquid interface of a first solvent and a second solvent, wherein the first solvent is polar and the second solvent is non-polar;   (2) providing at least one layered semiconductor material or its precursor(s) in the form of particles in a third solvent in the form of a dispersion;   (3) injecting the dispersion of step (2) at the liquid-liquid interface obtained in step (1), in order to obtain an assembly of semiconductor/semiconductor precursor particles at the liquid-liquid interface;   (4) bringing the assembly of semiconductor/semiconductor precursor particles at the liquid-liquid interface obtained in step (3) into contact with a flexible substrate; and   (5) applying a surface pressure to the dispersion obtained in step (4), using the compression means  55  comprising means for pushing  65  mounted on a drive device  75 , in order to obtain a particle film of semiconductor/semiconductor precursor on the substrate,
 
wherein the first solvent has a higher density than the second solvent; wherein the first solvent and the second solvent are immiscible with each other and their densities are different by at least  0 . 01  g/mL; and wherein the third solvent is miscible with at least one of the first solvent or the second solvent.
       

     Starting Materials 
     &lt;Layered Semiconductor Material&gt; 
     The layered semiconducting material can be, for example, C (graphene), P (phosporene), h-BN (hexagonal boron nitride), InSe, GaSe, GaS, GaTe GeAs, SiAs, InSe, Bi 2 Se 3 , SnS 2 , MoS 2 , WS 2 , MoSe 2 , WSe 2 , WTe 2  and MoTe 2 . 
     Preferably, the layered semiconducting material can be a transition metal dichalcogenide, for example, MoS 2 , WS 2 , MoSe 2 , WSe 2  and MoTe 2 . 
     Precursors of layered semiconductor material can be metal oxides, such as MoO 3 , WO 3 , SnO 2 , Ga 2 O 3 , metal fluorides, organometallics, catalytic metals, polymers such as polyethylene, polymethyl methacrylate (PMMA), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), polyvinylalcool (PVA), pure metals such as Mo, W, Si, Ge, metal nitrides such as GaN, WN, MoN. 
     Preferred precursors of layered semiconductor material are metal oxides, such as MoO 3 , WO 3 , and pure metals such as W and Mo. 
     &lt;Preparation of Layered Semiconductor Material&gt; 
     The layered semiconductor material used in the present invention can be prepared, for example, by a mechanical exfoliation method, a liquid exfoliation method, or a chemical exfoliation method of the bulk starting material. Alternatively, a sulfurization of metal (or metal oxide) thin film, vaporization of metal oxide with chalcogenide precursor can also be used. 
     Preferably, the layered semiconductor material used in the present invention is prepared by a mechanical exfoliation method or a liquid exfoliation method. 
     The first solvent and the second solvent are selected in such a way that they both will not disperse the particles of layered semiconductor materials to be deposited. 
     &lt;Particle Size and Shape&gt; 
     The layered semiconductor material may be in the form of particles of one to several atomic layers, having a thickness of 0.5 nm or more and 100 nm or less, preferably 1 nm or more and 10 nm or less, and more preferably 5 nm or more and 10 nm or less, and a diameter in a direction perpendicular to the thickness of 20 nm or more and 10 pm or less, preferably 200 nm or more and 1 pm or less. 
     In a preferred embodiment, particles may be nanoflakes wherein the thickness is significantly smaller than the diameter in a direction perpendicular to the thickness. 
     As used herein, the term “significantly”, indicates, for example, that the thickness is smaller than the diameter in a direction perpendicular to the thickness by a ratio of 1/100 or less. 
     Most preferably, the particles (or the nanoflakes) have a thickness of 1 nm or more and 10 nm or less, with a significantly larger diameter in a direction perpendicular to the thickness direction of 20 nm or more and 10 μm or less, more preferably of 200 nm or more and 1 μm or less. 
     &lt;First Solvent&gt; 
     The first solvent, which is polar, has a higher density than the second solvent, which is non-polar, and the first solvent is immiscible with the second solvent, which is non-polar. Therefore when a first solvent and a second solvent are put in contact, an interface between the first solvent and the second solvent can be obtained, the first solvent being under the second polar solvent, because of the difference in density. The density of the first solvent can be, for example, 0.8 g/mL or more and 2.0 g/mL or less. 
     The first solvent has a density higher than that of the second solvent by at least 0.01 g/mL, preferably by 0.05 g/mL and more preferably by 0.1 g/mL. 
     The dielectric constant of the first solvent can be, for example, 10 or more. 
     The first solvent can be, for example, water, a carboxylic acid such as acetic acid, an alcohol such as ethylene glycol, methanol, ethanol, n-propanol or n-butanol, a ketone such as acetone, an acetate such as ethyl acetate, an organic sulfoxide such as dimethyl sulfoxide, an organic nitrile such as acetonitrile and an amide such as dimethylformamide (DMF) or dimethylsulfoxide (DMSO) or a mixture thereof. 
     Preferably, the first solvent is water, ethylene glycol, acetonitrile, or mixtures thereof in any proportion. 
     &lt;Second Solvent&gt; 
     The second solvent, which is non-polar, has a lower density than the first solvent, which is polar, and the second solvent is immiscible with the first solvent. Therefore when a first solvent and a second solvent are put in contact, an interface between the first solvent and the second solvent can be obtained, the second solvent being over the first solvent, because of the difference in density. 
     The density of the second solvent can be, for example, 1.2 g/mL or less and 0.5 g/mL or more. 
     The second solvent has a density lower than that of the first solvent by at least 0.01 g/mL, preferably by 0.05 g/mL and more preferably by 0.1 g/mL. 
     The dielectric constant of the second solvent can be, for example, 10 or less. 
     The second solvent can be, for example, a linear or cyclic hydrocarbon such as, pentane, hexane, heptane, octane or benzene, an ether such as diethyl ether, or a mixture thereof. 
     Preferably second solvent is a C5-C8 alkane. More preferably, the second solvent is hexane or heptane, which has a low boiling point and therefore is easy to remove. 
     &lt;Third Solvent&gt; 
     The layered semiconductor material is dispersed in a third solvent. 
     The third solvent is soluble/miscible at least in one of the first solvent or the second solvent. 
     The third solvent has a density of 0.5 g/mL or more and 2.0 g/mL or less. 
     The third solvent has a dielectric constant of 0 or more and 100 or less. 
     The third solvent can be, for example, C4-C8 amines or alcohols such as hexylamine, butanol, hexanol, isopropanol (IPA) or a mixture thereof. 
     Advantageously, a more stable dispersion of particles can be prepared with a third solvent having a more similar surface energy as that of the particles. 
     The surface energy can be measured, for example, by using a contact angle measurement. This method is detailed for example in Nano Lett., 2015, 15 (8), pp 5449-5454, DOI: 10.1021/acs.nanolett.5b01842. 
     Some advantageous combinations of third solvent and particles satisfying the above condition are, for example, (third solvent, particle)=(1:1 IPA/water, graphene), (1:1 IPA/water, WS 2 ), (1:1 IPA/water, h-BN), (1:1 IPA/water, MoSe 2 ), (1:4 IPA/water, Bi 2 Se 3 ), (1:4 IPA/water, SnS 2 ), (7:3 IPA/water, MoS 2 ), and (acetonitrile, TaS 2 ). 
     Advantageously, a more stable dispersion of particles can be prepared with a third solvent having an ionic strength closer to the pZC (point of zero charge) of the particles. 
     pZC can be estimated, for example, by using a Dynamic Light Scattering apparatus. This device is detailed, for example in J. Control. Release, 235 (2016) 337-351. 
     pZC can be adjusted by changing the pH. For example, the pZC is about pH=2-3 for MoS 2 , and pZC is close to pH=1 for WS 2  (J. Appl. Phys. 69, 6373(1991)). 
     The pH can be adjusted by adding an acid or a base. Examples of acid are HCl, H 2 SO 4  and and acetic acid. Examples of base are NaOH, ammonium hydroxide and methyl amine. 
     Preferably, the third solvent is hexylamine (CH 3 (CH 2 ) 4 CH 2 —NH 2 ), hexanol (CH 3 (CH 2 ) 4 CH 2 —OH), tert-butanol, IPA, water, acetonitrile or a mixture thereof. 
     &lt;Combination of the Solvents&gt; 
     The first solvent and the second solvent are immiscible with each other and the first solvent has a higher density than the second solvent. 
     The third solvent is miscible with at least one the first solvent and the second solvent. 
     Possible combinations of a first solvent, a second solvent and a third solvent, can be, for example, (first solvent, second solvent, third solvent)=(ethylene glycol, hexane, hexylamine), (mixture of water and acetonitrile, pentane, tert-butanol), (water, heptane, n-butanol). 
     Preferable combinations of a first solvent, a second solvent and a third solvent are (first solvent, second solvent, third solvent)=(water, heptane, n-buta nol). 
     &lt;Flexible substrate&gt; 
     The substrate on which a semiconducting thin film is deposited is not particularly limited, as long as it is flexible. 
     The flexible substrate used in the method of the present invention may have a bend radius of  5  cm or less. 
     The flexible substrate used in the method of the present invention can be, for example, polyethylene terephthalate (PET), polyethylene (PE), polyvinyl chloride (PVC), polypropylene (PP), polystyrene (PS), polylactic acid (PLA), polycarbonate (PC), acrylic (PMMA), flexible glass (such as Willow glass commercialized by Corning) and polyether ether ketone (PEEK). PEEK is preferable if the obtained film is subsequently subjected to a thermal treatment, since PEEK has enhanced thermal resistance compared to the other candidate materials (250° C. in operating conditions). These substrate materials shall advantageously not be etched, dissolved or otherwise damaged by the solvents employed under standard conditions. 
     The flexible substrate used in the method of the present invention can have a thickness of 0.001 mm to 10 mm. 
     The flexible substrate used in the method of the present invention is preferably electronically conductive for the intended use in semiconductor devices. The flexible substrate is also preferably transparent or semitransparent to visible and near infra-red (NIR) irradiation, for the intended application. A preferred flexible substrate is indium tin oxide (ITO) or fluorine-doped tin oxide (FT 0 )-coated polyethyleneterephthalate (PET). 
     Method Steps 
     &lt;Step (1): interface of a first solvent and a second solvent&gt; 
     Step (1) of the present method consists in providing a liquid-liquid interface of a first solvent and a second solvent. 
     This step can be carried out, for example, by first pouring a first solvent and then pouring a second solvent into a container  5 . The solvents could also be added from any point on the wall or bottom of the container  5  through a supply tube. 
     The volume ratio between a first solvent and a second solvent can be, for example, any ratio of (first solvent volume) : (second solvent volume)=10000:1 or more and 1:10000, or less, preferably 1000:1 or more and 1:1000, or less, more preferably 100:1 or more and 1:100, or less, still more preferably 10:1 or more and 1:10 or less and most preferably about 5:1 or more and 1:5 or less. 
     Step (1) can be carried out at room temperature, or at any combination of temperature and pressure wherein the solvents are in a liquid state. 
     Step (1) can be carried out in a trough of a width equal to the width of the substrate to be deposited on and composed of materials that will not be etched, dissolved or otherwise damaged by the solvents employed under standard conditions (i.e. 20° C. and 1 atm). 
     Step (1) can be carried out after step (2). 
     &lt;Step (2): dispersion of layered semiconductor material or its precursor(s)&gt; 
     Step (2) of the present method consists in providing at least one layered semiconductor material or its precursor(s) particles in a third solvent in the form of a dispersion. 
     This step can be carried out, for example, by mixing layered semiconductor material or its precursor(s) with a third solvent. 
     The amount of layered semiconductor material or its precursor(s) by weight with respect to the volume of the third solvent can be, for example 0.001 mg/mL or more and 1000 mg/mL or less and preferably 0.01 mg/mL or more and 100 mg/mL or less, more preferably, 0.1 mg/mL or more and 10 mL or less and most preferably about 10 mg/mL. 
     Step (2) can be carried out at room temperature or at any combination of temperature and pressure wherein the solvents are in liquid state, i.e. under a given pressure, any temperature between the boiling point of the solvent having the lowest boiling point among all the solvents used, and the melting point of the solvent having the highest melting point among all the solvents used. 
     Step (2) can be carried out after step (1). 
     The dispersion may have a viscosity of 0.1 cp or more and 10 cp or less. 
     &lt;Step (3): injection of the dispersion at the liquid-liquid interface&gt; 
     Step (3) of the present method consists in injecting the dispersion of step (2) at the liquid-liquid interface obtained in step (1), in order to obtain an assembly of semiconductor/semiconductor precursor particles at the liquid-liquid interface. 
     As used herein, the expression “assembly of semiconductor/semiconductor precursor particles” indicates the intermediate product obtained at the end of step (3). 
     This step can be carried out, for example by injecting continuously, or in sequential pulses by a pipette or syringe needle at one or more points in proximity to the liquid-liquid junction. 
     The injection speed can be, for example,  0 . 01  mL/min per square cm of liquid-liquid interfacial area or more and  100  mL/min per square cm of liquid-liquid interfacial area or less. 
     Preferably, the injection speed is 0.01 mL/min per square cm of liquid-liquid interfacial area or more and 1.0 mL/min per square cm of liquid-liquid interfacial area or less. 
     Most preferably, the injection speed is about 0.025 mL/min per square cm of liquid-liquid interface area. 
     It is believed that the thickness of the injected dispersion corresponds at least to the thickness of particles, and preferably of nanoflakes, and can be, for example, 0.5 mm or more and 20 nm or less. 
     Preferably, the thickness of the injected dispersion is 1 nm or more and 10 nm or less. 
     The injection can be carried out either from above or from below the interface of the liquid-liquid interface obtained in step (1). 
     Step (3) can be carried out at room temperature, or at any combination of temperature and pressure wherein the solvents are in a liquid state. 
     &lt;Step (4): Contact with a Flexible substrate&gt; 
     Step (4) of the present method consists in bringing the assembly of semiconductor/semiconductor precursor particles at the liquid-liquid interface obtained in step (3) into contact with a flexible substrate. 
     This step can be carried out, for example, by a device as disclosed below. 
     Step (4) can be carried out at room temperature or at any combination of temperature and pressure wherein the solvents are in a liquid state. 
     &lt;Step (5): Deposition Rate&gt; 
     Step (5) of the present method consists in applying a surface pressure to the dispersion obtained in step (4), using the compression means  55 , in order to obtain a flake film of semiconductor/semiconductor precursor on the substrate. 
     This step can be carried out, for example, by a vertical method, Langmuir-Schafer method, a roll-to-roll deposition method. 
     Preferably, this method is carried out by a roll-to-roll deposition method, since this is suitable for continuous film production. 
     More preferably, this step is carried out with the device as disclosed below. 
     The surface pressure can be controlled, for example, by using a predetermined range of deposition rate. 
     For example, a deposition rate of 1 mm/min or more and 1000 mm/min or less can be used in step (5). 
     Preferably, the deposition rate in step (5) is 10 mm/min or more and 100 mm/min or less. 
     More preferably, the deposition rate in step (5) is 40 mm/min or more and 80 mm/min or less. 
     Most preferably, the deposition rate in step (5) is about 60 mm/min. 
     Most preferably, a deposition rate of about 60 mm/min or less can be used in step (5). 
     Step (5) can be carried out at room temperature or at any combination of temperature and pressure wherein the solvents are in a liquid state. 
     The thickness of the obtained film can be, for example, 0.5 mm or more and 1000 nm or less. 
     Preferably, the thickness of the obtained film is 1.0 nm or more and 100 nm or less. 
     Preferably, the thickness of the obtained film is 1 nm or more and 10 nm or less. 
     &lt;Order of the Steps&gt; 
     The order for carrying out step (1) and step (2) is not particularly limited, i.e. step (1) can be carried out first, and then step (2) is carried out, step (2) can be carried out first, and then step (1) is carried out, or steps (1) and (2) can be carried out simultaneously. 
     In one embodiment, steps (1) to (5) are carried out in the order of the number of steps, i.e. first, step (1), then step (2), step (3), step (4), and finally step (5). 
     INDUSTRIAL APPLICATION 
     The semiconducting thin film obtainable using the present device and by the present method can be used in the technical field of electronics, optoelectronics, gas sensing, electrochemistry, catalysis, and energy conversion and storage. 
     More precisely the thin film obtainable using the present device and by the present method can be used in non-linear optical devices, sensors, command surfaces, molecular electronics, directional electron transfer, conductors, diodes, optical devices, pyroelectronic devices, rectifier, metal-insulator-semiconductor (MIS), single molecule detection (SMD), acoustics surface wave devices, solar energy conversion, light emitting diodes. 
     The thin film obtainable using the present device and by the present method can be used, in particular in, large area flexible solar cell, light emitting diode (LED) and solar fuel devices.