Abstract:
A method of forming an electronic device on a flexible substrate without using acetone dissolvent, including the steps of: printing a hydrophobic mask on a porous membrane to form a pattern thereon which is complementary to a desired pattern; filtering an aqueous suspension of an electronic material through the non-printed region of the porous membrane, whereby some electronic material is deposited on said non-printed region following the desired pattern; pressing the flexible substrate against the printed face of the membrane in order to transfer the patterned electronic material deposited on the porous membrane to the flexible substrate to form the electronic device thereon.

Description:
[0001]    This application claims the benefit of European Patent Application No. 14382240.1, filed on Jun. 20, 2014. 
       FIELD OF INVENTION 
       [0002]    The present invention is related to a method of forming an electronic device on a suitable flexible substrate and to an apparatus comprising an electronic device made with such a method. Such a device may be, for example, an electrode or an electrode array or, more generally, an electronic platform. 
       BACKGROUND 
       [0003]    Devices of this kind can be used in the fabrication of solar cells, light emitting diodes (LEDs), field effect transistors (FETs), super capacitors, biosensors, etc. One interesting material for such devices is graphene oxide (GO), either oxidized (oGO) or reduced (rGO), but other materials can be used as well, like for instance gold nanoparticles, carbon nanotubes, CdSe or CdTe quantum dots, or even composite materials. 
         [0004]    The development of oxidized graphene oxide (oGO) based platforms are driven by spin coating, self-assembly, vacuum filtration or solvent exchange, and can be patterned using nanolithography techniques, microcontacting techniques or inkjet technology. These methods involve long fabrication periods, high cost, great expertise and clean room facilities. Besides, said methods are not versatile and effective in the designing of simple devices like transistors or capacitors. 
         [0005]    WO2007035838A2 discloses a low temperature method for producing micrometric patterns in films via a filtration process using membrane blocking, in which, prior to the film formation process, selected regions of the porous filtration membrane are blocked, so that the selected regions do not provide flow through porosity to the solution. The membrane is dissolved with acetone to leave the patterned film on a rigid substrate. 
         [0006]    Eda et al. disclosed, in “Large-area ultrathin films of reduced graphene oxide as a transparent and flexible electronic material”, Nature Nanotechnology 3, 270-274 (2008), a solution-based method that allows uniform and controllable deposition of reduced graphene oxide over large areas. Vacuum filtration involves the filtration of a GO suspension through a commercial mixed nitrocellulose ester membrane (NCM) with an average pore size of 25 nm. As the suspension is filtered through the membrane, the liquid is able to pass through the pores but the GO sheets become lodged on the membrane. Said lodged GO can be transferred by placing the membrane with the film side down and dissolving the membrane with acetone, leaving behind a uniform GO thin film. 
         [0007]    Changing subject, Lu et al. disclosed, in “Fabrication and characterization of paper-based microfluidics prepared in nitrocellulose membrane by wax printing”, Analytical Chemistry 82, 329-335 (2010), a process of wax patterning that forms hydrophobic regions in the membrane. 
         [0008]    The present disclosure teaches ways to pattern and transfer oGO, and other suitable electronic materials, onto a flexible substrate, that combine, surpass and simplify the indicated technologies. 
       SUMMARY OF INVENTION 
       [0009]    The present disclosure contemplates a method of forming an electronic device on a flexible substrate, comprising the steps of:
       printing a hydrophobic mask on a porous membrane to form a pattern thereon which is complementary to a desired pattern;   filtering an aqueous suspension of an electronic material through the non-printed region of the porous membrane, whereby some electronic material is deposited on said non-printed region following the desired pattern;   pressing the flexible substrate against the printed face of the membrane in order to transfer the patterned electronic material deposited on the porous membrane to the flexible substrate to form the electronic device thereon;       wherein the method is carried out without using acetone dissolvent.   
 
         [0014]    The pressing step provides an inexpensive flexible substrate with a pattern of an electronic material (e.g. graphene oxide -GO) on a surface thereof, thus forming an electronic device on a flexible substrate. 
         [0015]    The acetone dissolvent can be dispensed with because the pressing force can be made strong enough to be successfully applied between the electronic material (e.g. a GO mesh) and the flexible target substrate. This means for the pressure to be sufficient to overcome the hydrophobic mask (which may have a height of about 25 μm in the case of wax printing) and achieve a direct contact between the GO mesh and the target substrate. The transfer of the electronic material onto the target substrate can be performed by means of, for instance, vertical pressure or roll-to-roll-like pressure. The transfer phenomena are related to the hydrophobicity of the porous membrane and to the GO humidity, which for example makes NCM a good membrane in order to easily release the GO. In some experiments, the transfer remained perfectly effective after one month by simple rewetting of the NCM. 
         [0016]    The method exploits the versatility of the vacuum filtration technique, the ability to shape the porous membrane by mask printing, and the weakness of the van der Waals interactions between the electronic material and the membrane (the van der Waals interactions are stronger between the electronic material and the flexible substrate) to create a simple printing process for industrial manufacturing of electronic devices (possibly transparent, see below), for example multielectrode arrays, and achieves a synergy between the three technologies. 
         [0017]    This electrode-printing technology is advantageous over known fabrication methods in terms of ease, cost and applications. For example, it does neither require the use of a clean room nor of acetone dissolvent. Regarding the applications, it paves the way to ready, low-cost industrial fabrication of sensors and biosensors, and to 3D architectures. 
         [0018]    The patterned electronic device needs not be electrically conductive. For example, oGO is not conductive but its reduced form, rGO, is conductive. An electronic structure made of oGO can be used as an insulator or a semiconductor; and doped, but not reduced, oGO can be used as a LED. 
         [0019]    As already mentioned, the porous membrane can be made of nitrocellulose, but other materials such as PTFE, paper, etc, may also be used. 
         [0020]    Depending on the electronic and membrane materials, the size of the pore may be between 0.01 μm and 0.3 μm, more precisely between 0.015 μm and 0.1 μm, and preferably between 0.02 μm and 0.03 μm in the case of graphene oxide. 
         [0021]    As mentioned above, the printing material of the hydrophobic mask may be a wax, but other hydrophobic polymers commonly used in inkjet and screen printing technologies may also be used. 
         [0022]    The flexible substrate may be organic, for example polyethylene terephthalate (PET). 
         [0023]    In an example, the transfer step may be performed with a press, exerting a force of, for example, between 500 kg and 700 kg. The press may actuate through a stamp to which the flexible substrate is adhered. 
         [0024]    In an example, the flexible substrate may be a sheet, e.g. a continuous sheet, and the transfer step may be performed with roll-to-roll hardware, in which case the printer for printing the hydrophobic mask may be integrated with the roll-to-roll hardware. 
         [0025]    The method allows for great versatility and, for instance, the electronic device may be an interdigitated electrode, circular or otherwise, or an electrode microarray. 
         [0026]    The electronic device may be transparent or translucent. For example, in the case of graphene oxide, a decrease in rGO concentration causes a gain in transparency because transparency is inversely proportional to the number of layers that have been transferred That is, if a higher GO concentration is filtered, a larger number of layers, and a bigger vertical height, is produced with the result of less transparency. So, by controlling the number of layers transferred, the thickness, and therefore the transparency of the electronic device, can also be controlled. 
         [0027]    The present disclosure also contemplates an apparatus comprising an electronic device made with the above-disclosed method. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    Some examples of the present disclosure will be described in the following, only by way of a non-limiting example, with reference to the appended drawings, in which: 
           [0029]      FIGS. 1 a , 1 b , 1 c , 1 d  and 1 e    schematically show some steps of a method of forming a patterned electronic device on a flexible substrate; 
           [0030]      FIGS. 2 a  and 2 b    schematically show a printing step; and 
           [0031]      FIGS. 3 a , 3 b , 4 a , 4 b , 5 a   .  5   b ,  6   a  and  6   b  show examples of patterned electronic devices. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0032]    With reference to  FIG. 1 ,  FIG. 1 a    represents a porous membrane  10 , e.g. a hydrophilic nitrocellulose membrane (NCM) with a pore size of approximately 25 nm.  FIG. 1 b    shows a hydrophobic mask  12  printed on the porous membrane, e.g. a wax-printed mask (WPM) with a height of approximately 25 μm. The mask  12  follows a pattern that is complementary to the pattern desired for an electronic device or structure  20  (see  FIGS. 1 b  and 1 e   ), and leaves corresponding openings  11  that reach the surface of the membrane  10 . 
         [0033]      FIG. 1 c    shows an electronic material  22  deposited on the non-printed region of the porous membrane  10 , i.e. on the openings  11  left by the mask  12 , as a consequence of the vacuum filtration of a suspension, e.g. an aqueous suspension, of the electronic material, e.g. non-conductive graphene oxide (oGO), through the porous membrane. That is, the liquid, e.g. water, is filtered through the membrane  10  and there is a deposition of electronic material  22  into the openings  11  (remember that the printed mask  12  is hydrophobic) and onto the non-printed region of the porous membrane. Since the printed mask  12  follows a pattern that is complementary to the pattern desired for the electronic device, the deposited electronic material  22  follows the desired pattern. 
         [0034]      FIG. 1 d    shows the assembly of the porous membrane  10 , the printed mask  12  and the electronic material  22  turned down and being pressed by a press stamp  50  (e.g. a PDMS stamp) against a flexible substrate  30 , e.g. PET, exerting a force of for example 600 kg. The face of the assembly in contact with the substrate  30  is the face with the printed mask  12  and the electronic material  22  deposited on the openings  11  left by said mask. 
         [0035]      FIG. 1 e    shows the electronic material transformed into an electronic device  20  transferred onto the flexible substrate  30  by virtue of the pressure exerted by the stamp  50 , on account of the van der Waals interactions being weaker between the electronic material  22  and the porous membrane  10  than between the electronic material  22  and the flexible substrate  30  (see  FIG. 1 e   ). 
         [0036]      FIG. 2  shows the process of printing wax masks  12  with the aid of a computer  60  and a printer  70  (e.g. a Xerox ColourQube 8570 printer). The masks can be computer-designed and such designs can be wax-printed on nitrocellulose sheets  10 . Openings  11  define the desired electrode pattern (the openings are the complement or “negative” of the mask). 
         [0037]      FIG. 3 a    shows an example of a mask  121  printed on a porous membrane  101  and leaving some openings  111 , and  FIG. 3 b    shows an electronic device  201  that matches the openings  111 , and thus is complementary to the mask  121 , and has been transferred onto a flexible substrate  301 . This is an example of square electrodes forming the electronic device  201 . 
         [0038]      FIG. 4 a    shows an example of a mask  122  printed on a porous membrane  102  and leaving some openings  112 , and  FIG. 4 b    shows an electronic device  202  that matches the openings  112 , and thus is complementary to the mask  122 , and has been transferred onto a flexible substrate  302 . This is an example of interdigitated electrodes forming the electronic device  202 . 
         [0039]      FIG. 5 b    shows an example of an electronic device  203  on a flexible substrate  303  and  FIG. 5 a    shows a pair of corresponding electrodes in greater detail. This is an example of circular interdigitated electrodes forming the electronic device  203 . 
         [0040]      FIG. 6 b    shows an example of an electronic device  204  on a flexible substrate  304  and  FIG. 6 a    shows a pair of corresponding electrodes in greater detail. This is an example of an electrode microarray forming the electronic device  204 . 
         [0041]    Naturally, the materials can vary from one example to another one, can be the same for some elements and different for others, or can be always the same for analogous elements. And there can be any suitable number of electrodes (or electronic components) formed on the flexible substrate or even different electrodes or components on the same substrate. 
         [0042]    Regarding the method of forming a, for example, oGO structure on a, for example, organic substrate, the NCM is first patterned in the desired shape using a, for example, wax printer ( FIG. 2 ) The areas to be printed are delineated by a binary color-coding scheme. The colored areas, assigned a positive value (or 1, in binary programming language), are destined for wax printing (see ref.  12  in  FIG. 1 b   ), whereas the uncolored areas, given a negative value (or 0), are left unprinted to subsequently serve as filters (openings  11 ). An aqueous suspension of oGO is poured onto the mask and then filtered through these uncovered areas  11 . 
         [0043]    The WPM is set onto a filtering glass and the suspension of oGO (at a desired concentration) is filtered, leaving an oGO mesh on top of the WPM ( FIG. 1 c   ). In related work, other groups had reported that the concentration and volume of the oGO suspension strongly influence the filtration rate. However, in this case a reduction in the filtration area led to a strong decrease in pressure and, consequently, to a much slower filtration. Therefore, it was decided to simply remove the unfiltered oGO, instead. 
         [0044]    The WPM topped with oGO  22  is placed onto the substrate  30  and the assembly is subjected to vertical pressure ( FIG. 1 d   ), which leaves a patterned oGO device or structure  20  (e.g. electrodes) on the substrate surface ( FIG. 1 e   ). It is hypothesized that the transfer involves two related steps: expulsion of the oGO from the WPM and attachment of it to the substrate surface. The inventors believe that the expulsion occurs by simple air/humidity pressure and that the attachment is favored by van der Waals forces, as the reported values at the oGO/NCM interface are lower than those at the oGO/substrate interface. 
         [0045]    Additionally, as proof of concept of a technique that is presently considered amenable to specialized technologies, a wax printer outfitted with roll-to-roll hardware was used to transfer shaped oGO onto a PET substrate. The roll-to-roll machinery can be used for feeding substrate sheets into the printer and for printing the wax, and must apply sufficient pressure to transfer the oGO. This method offers strong potential for simple, fast printing of this class of oGO devices on an industrial scale. 
         [0046]    The lateral height of the WPMs was measured and their long-term stability was assessed. The direction of the wax printing (horizontal or vertical) was an important parameter to evaluate, as it affects the resolution and the shape of the lines edges. The best resolution was obtained when the line was printed vertically, as it did not lead to any systematic curves on the border. Different wax mask shapes were also evaluated. All the masks shown ( FIGS. 3 a  and 4 a   ), or implied ( FIGS. 5 and 6 ), in the figures exhibited acceptable designs over a range of 200 to 300 μm, which is consistent with literature values for printing onto paper or NCM. The transversal cut of the WPM shows a medium height of circa 25 μm. The change in lateral spreading of the wax across the WPM at room temperature was studied over 5 months and no significant deformation or spreading was observed. It is, therefore, concluded that these WPMs are stable over the long term. 
         [0047]    The wax-printing method has been used to create various different masks for printing oGO devices or platforms ( FIGS. 3 to 6 ). In the general procedure, a WPM is first laid onto the filtering glass and 5 mL of an aqueous suspension of oGO (0.1 mg/mL) is then filtered through it for 5 min. The unfiltered oGO solution is removed (and can later be reused), leaving behind an oGO mesh  22  on top of the membrane  10 , as represented in  FIG. 1 c   . The concentration, volume and filtering time of the oGO suspension each depend on the filtering pressure and can be adjusted according to the requirements of the desired end application. This methodology is faster than previously reported methods and is also controllable. 
         [0048]    Reduced graphene oxide (rGO) is a conductor and can be obtained by reducing the corresponding oGO products with hydrazine vapor. 
         [0049]    The present WPM method and subsequent reduction can be used to pattern various types of electronic devices, like generic interdigitated electrodes (IDEs,  FIG. 4 ), circular IDEs ( FIG. 5 ), or multiarray microelectrode systems amenable to multidetection applications ( FIG. 6 ). 
         [0050]    In  FIG. 3 , four squares  201  of oGO are patterned onto a PET film  301 , and then reduced to make them conductive. Next, several 300-μm circles of oGO can be created and then transferred onto the rGO squares on the film, leaving behind circular oGO patterns to build up an integrated system wired by inkjet printing of silver ink. Such architecture enables conjugation of diverse nanomaterials and/or biomaterials to the device and further exploitation of the properties of oGO. SEM (Scanning Electron Microscope) images of the (transparent) electronic devices show that the oGO transfer is efficient and that the shapes are well defined. 
         [0051]    The EIS (Electrochemical Impedance Spectroscopy) response of the generic IDEs  202  of  FIG. 4 b    deposited onto different flexible substrates  302  (e.g. glass, PEN, PET, cellulose acetate, plastic adhesive film, wax-modified paper, etc) shows an interesting behavior for applications in a myriad fields, including biosensing and energy (e.g. solar cells). The obtained results open the door to diversely functionalized flexible and transparent rGO electrodes. The EIS responses of the generic IDEs printed onto different flexible materials vary in their electrode-electrolyte interface impedance (EEII), which can be attributed to the differences in surface roughness, flexibility and hydrophobicity of the respective substrates, as these factors would have influenced the morphology of the printed oGO. The PEN and flexible glass present the lowest EEII, whereas the plastic adhesive film and the cellulose acetate offer the highest values. 
         [0052]    However, PET offered the best trade-off in terms of cost, transparency and flexibility, and it was chosen for further studies on the influence of oGO concentration on IDE performance (as measured by EIS). An increase in oGO concentration correlated to a decrease in EEII and therefore, to an increase in conductivity of rGO, consistently with literature reports. This trend was indirectly confirmed by performing AFM (Atomic Force Microscopy) studies on analogous glass IDEs, since PET, because of its roughness, is not very suitable for nanometric AFM measurements. 
         [0053]    In summary, the present disclosure reports a new, versatile and customizable method for patterning oGO onto flexible substrates through highly stable, microscale WPMs. These masks enable controlled printing of oGO in various shapes of interest for different applications. The oGO-printing technology reported here is advantageous over previously reported methods for fabrication of GO-based devices in terms of ease, cost and potential end-applications: for instance, it does not require the use of a clean room. It should ultimately pave the way to ready, low-cost industrial fabrication of a broad array of GO-based devices such as sensors and biosensors. 
         [0054]    Although only particular embodiments of the invention have been shown and described in the present specification, the skilled man will be able to introduce modifications and substitute any technical features thereof with others that are technically equivalent, depending on the particular requirements of each case, without departing from the scope of protection defined by the appended claims. 
         [0055]    For example, although the electronic devices are represented as black in the drawings, they can be transparent or translucent.