Patent Publication Number: US-11396196-B2

Title: Apparatus and methods for contact-printing using electrostatic nanoporous stamps

Description:
RELATED APPLICATIONS 
     This application is a national stage filing under 35 U.S.C. § 371 of International Patent Application Serial No. PCT/US2018/012509, filed Jan. 5, 2018, and entitled “Apparatus and Methods for Contact-Printing Using Electrostatic Nanoporous Stamps,” which is incorporated herein by reference in its entirety for all purposes. 
     BACKGROUND 
     Methods for printing include screen, gravure, relief, and inkjet printing. The resolution of inkjet printing is determined by the size of the droplet ejected from the nozzle aperture, and due to the limiting strength and frequency of transducers compared to the force required to eject smaller liquid droplets, the droplet diameter is usually no smaller than about 10 μm to about 20 μm. Gravure printing uses ink transfer from individual cells engraved on the tool surface, thus the size and shape of the cells influence the printing resolution. In relief printing, a thin film of ink is first loaded from a textured roll onto to the top surfaces of the stamp, and is subsequently printed by pressing the stamp against the target. The resolution of relief printing has been more limited than inkjet and gravure printing because thin liquid films loaded on the solid stamps tend to dewet from the surface (e.g., due to hydrodynamic thin film instability) while thick films tend to spread outwards from the contact area. As a result, relief printing of uniform ink layers has generally limited feature sizes to 50 μm or larger. 
     SUMMARY 
     Inventive implementations of the present technology generally relate to methods and apparatus for contact printing. In one example, an apparatus for contact printing using an ink includes a substrate, a conductive layer disposed on the substrate, and a plurality of microstructures disposed on the conductive layer. Each microstructure includes a porous medium comprising a conductive material. The apparatus also includes a dielectric layer conformally disposed on the plurality of microstructures and configured to electrically insulate the plurality of microstructures from the ink during use. 
     In another example, a method of contact printing using a nanoporous print stamp includes contacting the nanoporous print stamp with an ink and applying a voltage between a conductive layer in the nanoporous print stamp and the ink to load the ink into a dielectric-coated porous medium disposed on the conductive layer based at least in part on an electrostatic force between the nanoporous print stamp and the ink. The dielectric-coated porous medium includes a conductive porous medium conformally coated with a dielectric layer. The method also includes contacting the nanoporous print stamp with a target substrate and adjusting an amplitude of the voltage between the conductive layer and the ink to release the ink from the nanoporous print stamp and onto the target substrate so as to form a pattern. 
     In yet another example, a nanoporous print stamp for contact printing using an ink includes a substrate, an array of electrodes disposed on the substrate, and an array of microstructures. Each microstructure is disposed on a corresponding electrode in the array of electrodes and includes a group of vertically aligned carbon nanotubes extending from the corresponding electrode. The group of carbon nanotubes has: (a) a top section having an average pore size of about 100 nm or less; and (b) a lateral size of about 10 μm to about 100 μm. The stamp further includes a dielectric layer conformally disposed on the array of microstructures and configured to electrically insulate the array of microstructures from the ink during use. 
     It should be appreciated that all combinations of the foregoing concepts and additional concepts discussed in greater detail below (provided such concepts are not mutually inconsistent) are contemplated as being part of the inventive subject matter disclosed herein. In particular, all combinations of claimed subject matter appearing at the end of this disclosure are contemplated as being part of the inventive subject matter disclosed herein. It should also be appreciated that terminology explicitly employed herein that also may appear in any disclosure incorporated by reference should be accorded a meaning most consistent with the particular concepts disclosed herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The skilled artisan will understand that the drawings primarily are for illustrative purposes and are not intended to limit the scope of the inventive subject matter described herein. The drawings are not necessarily to scale; in some instances, various aspects of the inventive subject matter disclosed herein may be shown exaggerated or enlarged in the drawings to facilitate an understanding of different features. 
         FIG. 1  shows a schematic of an apparatus for contact printing according to one inventive implementation. 
         FIGS. 2A-2D  illustrate a method of fabricating the apparatus of  FIG. 1  according to one inventive implementation. 
         FIGS. 3A-3F  show scanning electron microscope (SEM) images of carbon nanotubes (CNTs) fabricated using the method illustrated in  FIGS. 2A-2D . 
         FIG. 4  illustrates a printing method using the apparatus of  FIG. 1 , according to one inventive implementation. 
         FIG. 5A  is an image of a water droplet on an array of CNT pillars coated with a hydrophobic polymer in the apparatus of  FIG. 1 . 
         FIGS. 5B and 5C  illustrate a method of electrowetting of the apparatus of  FIG. 1  according to one inventive implementation. 
         FIG. 6  is an SEM image of an electrostatic nanoporous print stamp, based on the apparatus of  FIG. 1  and including an array of pixels. 
         FIGS. 7A-7C  illustrate a method of contact printing using the electrostatic nanoporous print stamp of  FIG. 6 , according to one inventive implementation. 
     
    
    
     DETAILED DESCRIPTION 
     Electrostatic Nanoporous Print Stamps 
     As set forth in detail below, inventive methods and apparatus for contact printing based on electrostatic force generally employ dielectric-coated conductive porous medium (e.g., a collection of conductive fibers) disposed on a substrate to attract and release ink based in part on an electric potential (voltage) applied to one or more of the conductive fibers (or groups of conductive fibers). In one example implementation, an inventive apparatus for contact printing is implemented as an “electrostatic nanoporous print stamp,” in which groups of conductive fibers may be arranged on a substrate, and wherein each group of conductive fibers comprises a microstructured array of fibers having cross-sectional dimensions on a nanometer scale. 
     More specifically, in one example, an electrostatic nanoporous print stamp including multiple microstructured arrays of conductive fibers (e.g., carbon nanotube arrays, or CNT arrays) can be used to achieve high resolution printing of liquid inks, including colored inks for graphics and electronically functional inks for printed electronics. In this technology, the CNTs can be grown from thin-film catalysts on silicon substrates and form vertically aligned “forests” or “arrays,” i.e., individual CNTs in a given microstructured array are substantially perpendicular to the substrate. In addition, the thin film catalysts can be fabricated into various patterns using, for example, lithographic patterning and compatible chemical vapor deposition (CVD) process, thereby fabricating versatile micro- and nano-structures of CNT arrays. 
     The CNT arrays can be coated with functional polymers to achieve high porosity (e.g., greater than 90%) such that the liquid ink can be confined inside the microstructures. The CNT arrays also have good structural robustness against capillary forces upon liquid infiltration and evaporation, while maintaining sufficient compliance (e.g., elastic modulus of about 30 MPa) for conformal contact against the target substrate. As a result, upon contact with the target substrate, liquid ink is transferred locally via the porous surface of the stamp, thereby realizing excellent replication of stamp patterns with uniform thickness. The CNT stamps can print liquid inks with feature sizes smaller than 10 μm, which enables a printing resolution at about 2500 dots per inch (dpi) or greater. Moreover, printing of thin, uniform layers is advantageous for electronic device fabrication, such as thin film transistors, where a variety materials is deposited and patterned in spatially registered layers. 
     For printed electronics, the dimensions of each layer usually affect the device functionalities. Existing nanoporous stamps can produce high lateral resolution (i.e. replicates the stamp pattern) and uniform thickness of the printed layer. However, it can be challenging to precisely control (e.g., with sub-micron accuracy) the layer thickness, which typically depends on the amount of ink transferred from the stamp to the target substrate during printing. 
     To improve the control over the layer thickness of the printed pattern, as noted above the apparatus and methods described herein employ a plurality of conductive fibers that facilitate significant control of the amount of transferred ink via electrostatic forces. In one example, an apparatus includes a plurality of conductive microstructures disposed on an electrode layer to form a porous top section. A conformal dielectric layer is deposited on the conductive microstructures so as to electrically insulate the conductive microstructures from the ink during use. In operation, a voltage is applied on at least some of the conductive microstructures, via the electrode layer, so as to create an electrostatic force to attract the ink, thereby facilitating the loading of the ink onto the porous top section of the apparatus. The amount of ink loaded onto the apparatus can be controlled by the amplitude of the applied voltage. Upon contacting a target substrate, the voltage can be removed or reduced to release the ink onto the target substrate, thereby forming a pattern on the substrate. 
       FIG. 1  shows a schematic of an apparatus  100 , according to one inventive implementation, in the form of an electrostatic nanoporous print stamp. The apparatus  100  includes a substrate  110  and a conductive layer  120  disposed on the substrate  110 . A plurality of conductive microstructures  130  (also referred to as a conductive porous medium  130 ) is disposed on the conductive layer  120  to form a nanoporous structure that can be filled with liquid ink  160 . The nanoporous structure can have pores on the nano-scale (more dimensions are provided below). A dielectric layer  140  is conformally coated on the plurality of conductive microstructures  130 , as well as the portion of the top surface of the conducive layer  120  not covered by the plurality of conductive microstructures  130 . The combination of the dielectric layer  140  and the conductive porous medium  130  is also referred to as a dielectric-coated porous medium. The dielectric layer  140  can electrically insulate the plurality of conductive microstructures  130  from the liquid ink  160 . When conductive fibers (e.g., carbon nanotubes or other conductive nanowires) are used in the microstructures  130 , the dielectric layer  140  is conformally coated on the surface of each conductive fiber for insulation. 
     In operation, the conductive layer  120  can be connected to a voltage source  150  (e.g., a power supply), which applies a voltage between the conductive microstructures  130  and the liquid ink  160 . Due to the insulation of the dielectric layer  140 , electrically charges do not freely move between the microstructures  130  and the liquid ink  160 . Instead, electrical charges having opposite polarities accumulate near the boundary between the microstructures  130  and the liquid ink  160 , thereby creating an electrostatic force to attract the liquid ink  160  to the microstructures  130 . The amount of liquid ink  160  that is loaded within the microstructures  130  (and accordingly the amount of liquid ink  160  that can be released onto the target substrate during printing) depends at least in part on the amplitude of the applied voltage. Therefore, by adjusting the voltage, one can effectively control the thickness of the pattern printed by the apparatus  100 . 
     The substrate  110  in the apparatus  100  can serve as a holder to hold the microstructures  130 . In one example, the substrate  110  can include a metal, in which case the conductive layer  120  can be optional. In another example, the substrate  110  can include a ceramic. In yet another example, the substrate  110  can include a polymer. 
     In one example, the substrate  110  can be rigid, while the conductive microstructures  130  are mechanically compliant and durable, thereby allowing conformal contact between the apparatus  100  and the target substrate without significant buckling or yielding of the microscale features on the apparatus. In another example, the substrate  110  can be flexible (e.g., bendable or deformable). In this instance, the apparatus  100  can be employed to print on non-planar surfaces, such as a spherical surface, a cylindrical surface, or an elliptical surface, among others, by deforming the substrate  110  to fit the shape of the non-planar surfaces. The non-planar surface can further include surface irregularities (e.g., surface roughness or any other smaller scale irregularities on the scale of micrometers or less) that can be addressed by the compliance of the microstructures  130 . In other words, the flexible substrate  110  can accommodate irregularities on a coarse scale, and the compliance of the microstructures  130  can accommodate irregularities on a fine scale. 
     The conductive layer  120  in the stamp  100  functions as an electrode to apply voltages on the conductive microstructures  130 . In one example, the conductive layer  120  includes one electrode shared by all the microstructures  130 . In another example, the conductive layer  120  can include a first section underneath a first subset of one or more microstructures and a second section underneath a second subset of one or more microstructures. In this instance, a voltage can be applied between the first subset of microstructures and the second subset of microstructures so as to create the electrostatic force. In yet another example, the conductive layer  120  can include a plurality of electrodes, each of which is underneath a microstructure (or a group of microstructures) so as to allow individual control of each microstructure (or each group of microstructures). The conductive layer  120  can include a variety of appropriate conductive materials, such as TiN, silver, and gold, among others. 
     In one example, each microstructures  130   a/b/c  can include conductive fibers, such as vertically grown nanowires or filaments (e.g., silver). In another example, the microstructures  130  can include carbon nanotubes (CNTs), including single-walled carbon nanotubes or multi-walled carbon nanotubes. The CNTs can be vertically aligned or substantially vertically aligned and extending upward from the conductive layer  120 . 
     In yet another example, the microstructures  130  be made from casting a film of particles, compacting them to create at least a partially fused or sintered assembly that have nanopores defined between the particles In yet another example, the microstructures  130  can include a nanocarbon foam or a cast aerogel. Additionally, a nanoporous surface for a nanoporous stamp may be made from three-dimensional (3D) printing via photo-polymerization, which can create structured features of nanoscale dimensions. 
     In some examples, the diameter of carbon tubes in the microstructures  130  can have a diameter of about 1 nm to about 50 nm (e.g., about 1 nm, about 2 nm, about 3 nm, about 5 nm, about 10 nm, about 20 nm, about 30 nm, about 40 nm, or about 50 nm, including any values and sub ranges in between). 
     In one example, all the nanotubes in the respective microstructures  130  can have approximately the same height (e.g., length or distance above the substrate). In another example, the height (or length) of the nanotubes can have a variance of about 10 nm to about 100 nm (e.g., about 10 nm, about 20 nm, about 30 nm, about 40 nm, about 50 nm, about 60 nm, about 70 nm, about 80 nm, about 90 nm, or about 100 nm, including any values and sub ranges in between). This height variance can create roughness of the free surface where the liquid ink  160  can be pinned to the tips of the coated CNTs. In some examples, the height variance of the nanotubes can be about 1% to about 20% of the average height of the nanotubes (e.g., about 1%, about 2%, about 3%, about 5%, about 10%, about 15%, or about 20%, including any values and sub ranges in between). 
     In one example, the microstructures  130  can have a regular pattern on the substrate  110 , such as a two-dimensional (2D) matrix. Each microstructure (or each group of microstructures) can function as a pixel in printing. In this instance, each microstructure (e.g.,  130   a/b/c  in  FIG. 1 ) can include a group of conductive fibers or CNTs (see  FIGS. 2A-2D ), and the conductive layer  120  can include an array of electrodes  120   a - c , each of which is disposed underneath a corresponding microstructure (or each group of microstructures). Therefore, each microstructure (or each group of microstructures) can be individually controlled to load and release the liquid ink  160 . During printing, applying voltages to a first subset of microstructures (or groups of microstructures) can print a first pattern and applying voltages to a second subset of microstructures (or groups of microstructures) can print a second pattern, i.e., digital printing. Additional details about digital printing implementations are described below. 
     In another example, the microstructures  130  can form a pre-determined pattern similar to the pattern to be printed. In this instance, each microstructure  130   a/b/c  can include a group of conductive fibers or carbon nanotubes (or any other appropriate structure), and all the microstructures  130  can share the same conductive layer  120  as the electrode to apply the voltage. In some examples where CNTs are used in the microstructures  130 , the CNTs may be bonded or reinforced to prevent capillary-induced deformation of the CNT microstructures while retaining the desired porosity. 
     The nanoporous structure formed by the array of microstructures  130  can be characterized by the size of the pores (also referred to as the pore size). The pore size can be measured as the average distance between adjacent microstructures in the array of microstructures  130 . Alternatively, the pore size can be measured as the average void size or diameter defined by the microstructures  130  when the microstructures  130  include porous medium such as particles. Depending on the implementation, the pore size can be less than 1 μm (e.g., about 1 μm, about 500 nm, about 400 nm, about 300 nm, about 200 nm, about 100 nm, about 50 nm, about 25 nm, about 20 nm, about 15 nm, about 10 nm or less, including any values and sub ranges in between). 
     The porosity (also referred to as the void fraction) of the nanoporous structure defined by the microstructures  130  can be about 10% to about 99% (e.g., about 10%, about 20%, about 30%, about 40%, about 50%, about 60%, about 70%, about 80%, about 90%, about 95%, about 97%, about 98%, or about 99%, including any values and sub ranges in between). The dielectric layer  140  usually decreases the porosity of the nanoporous structure. In some examples, the above porosities are referred to the porosity after taking into account the dielectric layer  140 . 
     In some implementations, the liquid ink  160  can include functional micro-particles or nanoparticles having a particle size. In this instance, the pore size can be greater than the particle size. For example, the ratio of the pore size to the particle size can be about 1.5 to about 10 (e.g., about 1.5, about 2, about 2.5, about 3, about 3.5, about 4, about 4.5, about 5, about 5.5, about 6, about 6.5, about 7, about 7.5, about 8, about 8.5, about 9, about 9.5, or about 10, including any values and sub ranges in between). In another example, the pore size can be significantly greater than the particle size (e.g., about 1 order of magnitude greater, about 2 orders of magnitudes greater, or about 3 orders of magnitude greater). 
     On the other hand, the pore size can be smaller than the feature size of the pattern to be printed. The feature size can be defined as the width of the narrowest lines printed by the stamp  100 . For example, the ratio of the pore size to the feature size can be about 0.2 to about 10 −4  (e.g., about 0.2, about 0.1, about 0.05, about 0.01, about 5×10 −3 , about 10 −3 , about 5×10 −4 , or about 10 −4 , including any values and sub ranges in between). The feature size can be, for example, about 10 μm or less (e.g., about 10 μm, about 9 μm, about 8 μm, about 7 μm, about 6 μm, about 5 μm, about 4 μm, about 3 μm, about 2 μm, about 1 μm, or less, including any values and sub ranges in between). 
     As described herein, the appropriate surface compliance of the stamp  100  enables the microstructures  130  to conformally contact a target substrate during printing, allowing effective ink transfer with nanoscale thickness and high uniformity. According to some inventive implementations where CNTs are used, the surface chemistry of the CNTs can be engineered to enable infiltration of the ink into the spacing between CNTs and to reduce or prevent elastocapillary densification of the CNT microstructures. For example, a CNT forest synthesized by CVD may be hydrophobic, and subsequent oxygen plasma etching may create surface defects and promote attachment of oxygen-containing surface groups, thereby rendering the CNT forest hydrophilic. 
     The thickness of the dielectric layer  140  can affect the apparatus  100  in at least two ways. On the one hand, a thinner dielectric layer  140  allows a stronger electrostatic force to be built between the apparatus  100  and the liquid ink  160 . On the other hand, however, a thicker dielectric layer  140  can ensure that the respective microstructures  130  are well insulated. In some examples, the thickness of the dielectric layer  140  can be about 1 nm to about 20 nm (e.g., about 1 nm, about 2 nm, about 3 nm, about 5 nm, about 10 nm, about 15 nm, or about 20 nm, including any values and sub ranges in between). 
     Various materials can be used for the dielectric layer  140 . For example, the dielectric layer  140  can include polymer or ceramic (e.g., Al 2 O 3 ). The surface wettability or adhesive energy of the apparatus  100  can, in some embodiments, be tuned or configured by selecting appropriate material(s) for the dielectric layer  140 . For example, in implementations where the conductive fibers are nanotubes, a polymer coating can be applied to or disposed on the nanotubes, and the polymer coating forms bonds between at least a portion of the nanotubes. One or more polymers can be selected based on the particular implementation where utilized, and can include one or more of the following: a fluoropolymer, a polyacrylate, a polyfluoroacrylate, and/or a polyperfluorodecylacrylate. Additional exemplary polymers can include, depending on the embodiment and by way of non-limiting example: dimethylaminomethyl)styrene (DMAMS); (2-hydroxyethyl) methacrylate (HEMA);1-vinyl-2-pyrrolidone (VP); ethylene glycol diacrylate (EGDA); trivinyltrimethylcyclotrisiloxane (V3D3); methacrylic acid (MAA); ethlacrylate; and/or glycidyl methacrylate (GMA). 
     Further examples of materials that can be used for the dielectric layer  140  include poly-perfluorodecylacrylate, or p(PFDA). The PFDA monomer diffuses into the porous CNT microstructures in the vapor phase and results in a conformal coating of the CNTs. In this example, the iCVD polymer coating is followed by a second oxygen plasma treatment to remove any pPFDA deposited in a nonconformal manner as a result condensation the monomer at the tip of the CNT forest. The pPFDA-coated CNT microstructures do not shrink substantially or collapse upon liquid infiltration and solvent evaporation. 
     A second treatment on the dielectric layer  140  (e.g., plasma treatment) can assist or enable ink infiltration for printing by increasing the surface modulus and uniaxial compressive modulus of the dielectric-coated conductive porous medium. The dielectric layer  140  can increase resistance to elastocapillary densification via reinforcing individual CNTs and/or forming nanowelds at CNT-CNT contact point. In the example, the pFDA coating can increase resistance to elastocapillary densification by reinforcement of individual CNTs by the pPFDA coating as well as formation of pPFDA nanowelds at CNT-CNT contact points. 
     Fabrications of Electrostatic Nanoporous Print Stamps 
       FIGS. 2A-2D  illustrate a method  200  of fabricating the apparatus  100  of  FIG. 1  according to one inventive implementation. More specifically, the method  200  provides an apparatus in the form of an electrostatic nanoporous print stamp including carbon nanotubes to form multiple microstructures arranged as a two-dimensional (2D) array. It should be appreciated that the method  200  may be employed (in whole or in part) to fabricate similar apparatus having a variety of numbers and arrangements of respective microstructures (e.g., one-dimensional arrays of microstructures, various periodic or non-periodic arrangements of microstructures, particular patterns of microstructures, etc.). 
     The method  200  starts with a substrate  210  as shown in  FIG. 2A . Various materials can be used for the substrate  210 . In one example, the substrate includes silicon, such as a 4″ (100) silicon wafer coated with 300 nm of thermally grown SiO 2 . In another example, the substrate can include gold, quartz, glass, copper, aluminum, graphite, aluminum oxide, and the like, and/or mixtures thereof. A conductive layer  220  is formed on the substrate  210  via, for example, chemical vapor deposition (CVD) or physical vapor deposition (PVD). The conductive layer  220  can include any appropriate conductive material, such as TiN. In some examples, the thickness of the conductive layer  220  can be about 20 nm to about 200 nm (e.g., about 20 nm, about 30 nm, about 50 nm, about 100 nm, about 150 nm, or about 200 nm, including any values and sub ranges in between). 
       FIG. 2A  also shows that a catalyst layer  230  is formed on the conductive layer  220 . The catalyst layer  230  can further include a first sublayer of Fe disposed on a second sublayer of Al 2 O 3 , both of which can be deposited by electron beam physical vapor deposition. The thickness of the Fe layer can be, for example, about 1 nm, and the thickness of the Al 2 O 3  layer can be, for example, about 10 nm. As shown in  FIG. 2A , the catalyst layer  230  is patterned into an array of catalyst sections. The patterning can be carried out via, for example, lithography. The patterned catalyst layer  230  can be substantially similar to the desired pattern of the microstructures (e.g., forest of carbon nanotubes) to be grown on the catalyst. For example, the catalyst layer  230  can include a periodic 2D array. The pitch of the array can be, for example, about 1 μm to about 200 μm (e.g., about 1 μm, about 2 μm, about 3 μm, about 5 μm, about 10 μm, about 20 μm, about 30 μm, about 50 μm, about 100 μm, or about 200 μm, including any values and sub ranges in between). 
       FIG. 2B  shows that vertically aligned CNTs forming microstructures  240  are grown s via, for example, a CVD process at atmospheric pressure. The CNTs forming the microstructures  240  preferably grow on the catalyst sections on the conductive layer  230 . Therefore, the pattern of the microstructures  240  can be controlled by the pattern of the catalyst layer  230 . The height of the microstructures  240  can be about 0.2 μm to about 1000 μm (e.g., about 0.2 μm, about 0.5 μm, about 1 μm, about 2 μm, about 5 μm, about 10 μm, about 20 μm, about 30 μm, about 50 μm, about 100 μm, about 200 μm, about 300 μm, about 500 μm, or about 1000 μm, including any values and sub ranges in between). This height can be controlled by the growth time and/or the growth rate of the CVD process. In some examples, the growth rate can be about 50 μm/s to about 100 μm/s (e.g., about 50 μm/s, about 60 μm/s, about 70 μm/s, about 80 μm/s, about 90 μm/s, or about 100 μm/s, including any values and sub ranges in between). 
     In some examples, the substrate  210  deposited with the catalyst layer  230  can be placed in a quartz tube furnace for the CNT growth. The growth procedure can start with flowing of He/H 2  (e.g., at about 100/400 s.c.c.m.) while heating the furnace to a first elevated temperature (e.g., up to 775° C. over a 10-min ramp). The furnace can then be held at the first elevated temperature (e.g., about 775° C.) for an elongated time duration (e.g., about 10 min) with the same gas flow rates for annealing. Then the gas flow can be changed to a mixture of C 2 H 4 /He/H 2  (e.g., at 100/400/100 s.c.c.m.) at the first elevated temperature (e.g., about 775° C.) for CNT growth. The growth time can be selected to control the height (or length) of the CNTs  240 . In some implementations, the typical growth rate can be about 100 μm/min. After the growth, the furnace can be cooled down to a second temperature (e.g., less than 100° C.) at the same gas flow and finally purged with He (e.g., at about 1,000 s.c.c.m. for about 5 min). 
     In  FIG. 2C , a plasma treatment is performed on the microstructures  240 . During the CNT growth, the top surface (i.e., surface distal to the substrate  210 ) of the microstructures  240  may include tangled CNT fibers that can result in a dense cluster layer. Plasma etching can be performed to remove the top cluster layer. In some examples, the plasma etching can use oxygen plasma, in which the CVD grown microstructures  240  can be exposed to the oxygen plasma with 80/20 of Ar/O 2  gas flow for 5 min at 50 W and 200 mTorr pressure using a Diener Femto Plasma system. The plasma treatment may etch both the top and sidewalls of the CNTs, thereby slightly narrowing their width. Therefore, this reduction can be taken into account when designing the CNT growth patterns to achieve a target printed feature size. 
     In  FIG. 2D , a dielectric layer  250  is conformally coated on the respective CNTs of the microstructures  240  (including the outer surfaces of the side walls of the respective CNTs) to form an array of dielectric-coated microstructures  260 . Each forest in the array can function as a pixel during printing. In one example, the dielectric layer  250  includes ceramic (e.g., Al 2 O 3 ) and can be coated via atomic layer deposition (ALD). In another example, the dielectric layer  250  includes polymer, such as p(PFDA), and can be coated via initiated Chemical Vapor Deposition (iCVD). In some examples, the coating material can be hydrophobic. In some other examples, the coating material can be hydrophilic. 
     For conformal polymer coating, iCVD polymerization can be carried out in a cylindrical reactor (e.g., diameter of about 24.6 cm and height of about 3.8 cm) with an array of 14 parallel chromoalloy filaments (e.g., from Goodfellow) suspended about 2 cm from the stage. The reactor can be covered with a quartz top (e.g., about 2.5 cm thick) that allows real-time thickness monitoring. For example, the thickness of the polymer coating can be monitored by reflecting a laser beam (e.g., 633 nm He—Ne laser beam) off the substrate/polymer and recording the interference signal intensity as a function of time. 
     The reactor can be pumped down by a mechanical Fomblin pump and the pressure can be monitored with a vacuum gauge (e.g., a MKS capacitive gauge). The liquid monomer can include (e.g., 1H, 1H, 2H, 2H-perfluorodecyl acrylate, PFDA, 97% Aldrich) and the initiator (e.g., tert-butyl peroxide, tributylphosphine oxide (TBPO), 98% Aldrich) can be used as received without further purification. TBPO can be kept at room temperature (e.g., about 25° C.) and can be delivered into the reactor through a mass flow controller (e.g., 1479 MFC, from MKS Instruments) at a constant flow rate (e.g., about 1 s.c.c.m. in process A, about 3 s.c.c.m. and 1 s.c.c.m. in DVB and PFDA polymerization during process B, respectively). Initiator radicals (TBO) can be created by breaking only the labile peroxide bond of the TBPO at filament temperature of about 250° C. during iCVD polymerization. The PFDA monomer can be vaporized in glass jars heated to 80° C. and then introduced to the reactor through needle valves at constant flow rates (e.g., about 0.2 s.c.c.m.). 
     The substrate temperature can be kept at Ts=30° C. (within ±1° C.) using a recirculating chiller/heater (NESLAB RTE-7). All of the temperatures can be measured by K-type thermocouples (e.g., from Omega Engineering). The working pressure can be maintained at 60 mTorr using a throttle valve (e.g., from MKS Instruments). At the end, an ultrathin layer of pPFDA (approximately 30 nm thick) can be deposited within a 25 minute deposition time. The thickness of the pPFDA, deposited on to a control silicon substrate during iCVD polymerization, can be also measured using ellipsometry. 
     In some examples, the method  200  can further include plasma treatment on the dielectric layer  250 . In addition to functioning as an insulation layer, the dielectric layer  250  can also be configured to bond some of the CNTs  240  together to improve the mechanical strength in the resulting apparatus. The additional plasma treatment can remove any extraneous material from the bonding. 
     In some embodiments, the mechanical behavior of the dielectric-coated CNT forests  260  can be similar to that of open-cell foams when compressed to moderate strains. The modulus of the dielectric-coated CNTs can be tuned over a wide range based on the diameter, density, and connectivity of the CNTs  240 . 
       FIG. 3A  shows a scanning electron microscope (SEM) image of a CNT pillar vertically grown on a substrate. Each pillar can be used as a microstructure  130   a/b/c  in the apparatus  100  illustrated in  FIG. 1 . The CNT pillar has a diameter of about 100 μm, which can be controlled by the dimension of the catalyst layer (see, e.g.,  FIG. 2A ).  FIG. 3B  shows a schematic of the CNT pillar, which includes multiple individual CNTs.  FIG. 3C  is an SEM image showing a magnified view of a section of the CNT pillar. 
       FIG. 3D  shows a schematic of CNTs coated with a conformal dielectric layer. The dielectric layer can bond some of the CNTs together to increase the mechanical strength. In addition, the coating materials can also fill some of the space between CNTs and decrease the porosity of the resulting CNT forest.  FIG. 3E  shows an SEM image of CNTs conformally coated with poly-perfluorodecylacrylate, p(PFDA) via initiated chemical vapor deposition (iCVD).  FIG. 3F  shows an SEM image of CNTs conformally coated with alumina (i.e. Al 2 O 3 ) via atomic layer deposition (ALD). 
     Methods of Printing Using Electrostatic Nanoporous Print Stamps 
       FIG. 4  illustrates a method  400  of printing using an electrostatic nanoporous stamp (e.g., similar to the apparatus  100  shown in  FIG. 1 ). The stamp includes a substrate, a conductive layer disposed on the substrate, and a plurality of microstructures disposed on the conductive layer. Each microstructure includes a group of conductive fibers extending from the conductive layer. The stamp further includes a dielectric layer conformally disposed on the microstructures. The method  400  includes, at  410 , contacting the nanoporous print stamp with a liquid ink. At  420 , a voltage is applied between the conductive layer and the ink to load the ink onto the nanoporous print stamp based at least in part on an electrostatic force between the nanoporous print stamp and the ink. The loaded stamp is then placed in contact with a target substrate (i.e. desired location of printing) at  430 . At  440 , the voltage applied on the stamp is adjusted (e.g., reduced or removed) to release the ink from the nanoporous print stamp and onto the target substrate so as to form a printed pattern. 
     The steps illustrated in  FIG. 4  can be carried out in different orders. For example, the step  420  can be applied before the step  410 , i.e., the voltage can be turned on before the stamp is placed in contact with the ink. In another example, step  430  may be optional, in which case the ink can be released onto the target substrate from locations above the target substrate and the ink can drop onto the target substrate under gravity force. In yet another example, steps  430  and  440  can be concurrent, i.e. contacting the print stamp with the target substrate while adjusting the voltage. 
     Loading Liquid Ink Onto the Stamp 
     The stamp can be made to be in contact with the liquid ink in various ways. In one example, the ink can be loaded into a pipette, which can then drop the ink onto the stamp. In another example, the ink can be placed onto the stamp via spin coating. In yet another example, the stamp can be moved toward the liquid ink contained in a vessel until the CNT forest contacts the ink. 
     The loading of the ink onto the stamp can be facilitated by the applied voltage and the resulting electrostatic force. In some examples, the dielectric coating of the stamp can be hydrophilic and the liquid ink can infiltrate into the pores (e.g., the spacing between adjacent conductive fibers) on the stamp via capillary wicking. In this instance, the applied voltage can provide additional force to secure more ink onto the stamp and/or to hold the already loaded ink more closely to the microstructures. In some examples, the dielectric coating of the stamp can be hydrophobic, in which case the liquid ink may not wet the stamp and the electrostatic force can be used to adhere the liquid ink onto the stamp via electrowetting. 
       FIG. 5A  is an image of a water droplet on an array of CNT pillars coated with a hydrophobic polymer. It can be seen that the water droplet is well separated from the CNT pillars, i.e. the water does not infiltrate into the pores defined by the CNT pillars. 
       FIGS. 5B and 5C  illustrate a method  500  of electrowetting of an electrostatic nanoporous print stamp  510 . In this method  500 , the stamp  510  includes an array of dielectric-coated conductive fibers  516  (also referred to as fiber array  516 ) disposed on an electrode  514 , which in turn is disposed on a substrate  512 . The array of dielectric-coated conductive fibers  516  can be used as a microstructure  130   a/b/c  in the apparatus  100  illustrated in  FIG. 1 . A liquid ink  530  is placed on the array of fiber array  516 . A power source  520  is electrically connected to the electrode  514  and the liquid ink  530  via a switch  525 . 
     In  FIG. 5B , the switch  525  is open and the hydrophobic coating of the stamp  510  prevents the liquid ink  530  from entering the pores in the fiber array  516 . In  FIG. 5C , the switch is closed and an external voltage is applied between the fiber array  516  and the liquid ink  630 . An electrostatic field develops locally at the interface between the fiber array  516  and the liquid ink  630 , thereby attracting at least a portion  532  of the liquid ink  530  to wet the stamp surface and allowing the liquid ink  530  to infiltrate into the porous microstructures. 
     In  FIGS. 5B and 5C , the liquid ink  530  is placed on top of the stamp  510 . Other configurations can also be used. For example, the stamp  510  can be oriented upside down and contact the liquid ink  530  at the top surface of the liquid ink  530 . In another example, the stamp  510  can contact the liquid ink  530  from the side surface of the liquid ink  530 . In some examples, the amount of the liquid ink  530  can be very small, such that the gravity force of the liquid ink  530  can be negligible compared to the electrostatic force. As a result, the stamp  510  can contact and load the liquid link  530  from any direction (i.e. independent of the gravity force direction). 
     The electrowetting illustrated in  FIGS. 5B and 5C  can significantly improve the quantitative control over the ink loading (and ink release as well) compared to printing techniques based on capillary wicking. In general, increasing the applied voltage can also increase the amount of ink loaded onto the stamp. Since the voltage can be controlled, the ink amount can therefore be effectively controlled with significant precision. 
     The electrowetting can also benefit stamps with hydrophilic dielectric coating. In this instance, the stamp can load and unload the ink via capillary wicking, but the electrostatic force induced by the voltage can enhance the flexibility of the printing process. For example, the electrostatic force can allow a stamp to load more ink by providing attraction forces in addition to the capillary action. In addition, during contacting with the target substrate, the stamp can release the same amount of ink with less pressure applied on the stamp against the target substrate. This reduced pressure can protect the microstructures from damaging. 
     Controlling the Amount of Ink Loaded Onto the Stamp 
     After the initial loading of the ink onto the stamp, the amount of ink held by the stamp can be further modified via various methods. During printing, excessive ink outside the microstructures can be removed, leaving a small amount of ink inside the pores defined by the microstructures. Since the volume of the pores defined by the microstructures can be controlled and quantified during fabrication, the amount of ink contained by the stamp can also be pre-determined. 
     In one example, the amount of the ink loaded onto the stamp can be controlled by the external voltage applied on the stamp. By increasing or decreasing the voltage, the amount of ink on the stamp can be similarly increased or decreased. 
     In another example, a nanoporous scour (also referred to as a nanoporous scrub) can be used to remove excessive ink outside (or partially inside) the microstructures of the stamp. In this approach, a plasma-treated non-patterned CNT forest can be brought into contact against the top surface of the stamp. The CNT forest can be either coated with a hydrophilic dielectric or without coating at all. Accordingly, upon contacting the nanoporous stamp, the CNT forest can absorb the excessive ink. 
     Depending on the embodiment, the porosity and/or other surface features of the nanoporous scour used to remove excess ink from the nanoporous stamp can be adjusted for the application where the nanoporous stamp is being used. In some examples, the nanoporous scour may be the same or essential the same as the nanoporous stamp such that the respective porosities (and associated wicking) are the same or substantially the same. This may be achieved, for example, by fabricating the stamp and the scour at the same time and/or according to the same method, but where the scour is not patterned (and/or not patterned in the same regions such that the scour removes excess ink as required from the stamp). 
     In some examples, the nanoporous scour can also use electrostatic force to attract the excessive ink into the pores of the scour. In these examples, the amount of ink removed from the loaded stamp can be quantitatively controlled by the applied voltage on the scour. 
     In some examples, the liquid ink is made into contact with the stamp via spin coating. In these examples, after the initial loading, the stamp can be spun for an extended period of time to remove the excessive ink via centrifugal force during spinning. For example, the spinning can be performed at about 1,500 rpm for about 0.5 minutes to about 5 minutes depending on the desired amount of ink to be removed. In some examples, techniques described herein can be used in any combination. 
     Control the Thickness of Printed Pattern 
     As described above, the thickness of the printed pattern depends on the amount of ink released onto the target substrate. In one example, this thickness can be controlled by the amount of ink loaded onto the stamp. Accordingly, any of the techniques described herein to remove excessive ink can also be used to control the pattern thickness (e.g., controlling the applied voltage, using nanoporous scour, extra spinning, etc.). 
     In addition, the thickness of the printed pattern can be further controlled by the amount of ink released onto the target substrate during printing. The amount of ink released onto the target substrate can be similarly controlled by the applied voltage. For example, a first amount of ink can be loaded onto the stamp by applying a first voltage. During printing, the first voltage can be only partially removed (e.g., a residual voltage is maintained) and accordingly a residual amount of ink can be left on the stamp. In this case, the amount of ink actually printed can be less than the amount of loaded ink, thereby allowing further control of the layer thickness. 
     Furthermore, the pressure applied on the stamp against the target substrate during printing can also be used to control the amount of ink that results in the printed pattern. In general, a higher pressure force can release more ink onto the target substrate. Without being bound by any particular theory or mode of operation, there can be three regimes of such applied pressure. At lower pressure (e.g., less than 28 kPa), incomplete transfer may occur within the microstructures, i.e. less ink than desired in transferred onto the target substrate. At moderate pressure (e.g., about 28 kPa to about 150 kPa), the printed features match the stamp patterns, indicating that contact is uniform. At high pressure (e.g., greater than 150 kPa), overprinting may occur where the size of the printed features exceeds the stamp feature sizes, causing reduction of shape fidelity. High pressures may also result in excessive deformation of the nanoporous stamp surface and/or buckling of the microstructures. 
     Depending on the implementation, the pressure applied can be greater than about 0.5 kPa and less than about 200 kPa and/or greater than 1 kPa and less than 175 kPa. In some implementations, the pressure applied can be: about 150 kPa, about 125 kPa, about 100 kPa, about 75 kPa, about 50 kPa, about 40 kPa, about 30 kPa, about 20 kPa, about 15 kPa, about 10 kPa, about 9 kPa, about 8 kPa, about 7 kPa, about 6 kPa, about 5 kPa, about 4 kPa, about 3 kPa, about 2 kPa, or less, including any values and sub ranges in between. 
     Liquid Ink 
     In some embodiments, the ink comprises a solvent having a surface tension less than 150 mN/m, less than, 125 mN/m, less than 110 mN/m, less than 100 mN/m, less than 90 mN/m, less than 80 mN/m, less than 70 mN/m, less than 60 mN/m, or less than 50 mN/m. In some embodiments, the ink may be a colloidal ink, in some implementations, including functional nanoparticles. In some embodiments, the average pore size of a nanoporous stamp is (a) substantially larger than the average size of the functional nanoparticles of the ink and/or molecules therein. 
     The colloidal inks can include a variety of electronic materials. In some embodiments, the ink includes silver (Ag) nanoparticles dispersed in a solution (also referred to as a solvent). In some embodiments, the solution can include tetradecane, or CH 3 (CH 2 ) 12 CH 3 . The particle concentration can be, for example, about 30 wt % to about 80 wt % (e.g., about 30 wt %, about 40 wt %, about 50 wt %, about 60 wt %, about 70 wt %, or about 80 wt %, including any values and sub ranges in between). The diameter of the silver nanoparticles can be, for example, substantially equal to or less 10 nm (e.g., about 10 nm, about 9 nm, about 8 nm, about 7 nm, about 6 nm, about 5 nm, or less, including any values and sub ranges in between). 
     In some embodiments, the ink includes quantum dots (QDs) dispersed in a solution. For example, the QDs can include COOH-functionalized CdSe/ZnS core-shell type QDs (i.e., a core made of CdSe surrounded by a shell made of ZnS) and the solution can include tetradecane. In some embodiments, the dispersion can be facilitated by sonication (e.g., for about 3 minutes). The particle concentration can be about 5 wt % to about 50 wt % (e.g., about 5 wt %, about 10 wt %, about 15 wt %, about 20 wt %, about 25 wt %, about 30 wt %, about 35 wt %, about 40 wt %, about 45 wt %, or about 50 wt %, including any values and sub ranges in between). 
     In some embodiments, the ink includes ZnO nanoparticles. In some embodiments, the ZnO nanoparticles can be doped with Aluminum (Al) (e.g, with 3.15 mole percent Al) dispersed in 2-propanol and propylene glycol. The work function of the ZnO nanoparticles can be about 4.1 eV to about 4.5 eV (e.g., 808172, from Sigma-Aldrich). The particle concentration can be, for example, about 1 wt % to about 10 wt % (e.g., about 1 wt %, about 2 wt %, about 3 wt %, about 4 wt %, about 5 wt %, about 6 wt %, about 7 wt %, about 8 wt %, about 9 wt %, or about 10 wt %, including any values and sub ranges in between). The particle size of the ZnO nanoparticles can be, for example, about 5 nm to about 20 nm (e.g., about 5 nm, about 10 nm, about 15 nm, or about 20 nm, including any values and sub ranges in between). 
     In some embodiments, the ink includes WO 3  nanoparticles, such as crystalline WO 3 . The work function of the WO 3  nanoparticles can be, for example, about 5.3 eV to about 5.7 eV (e.g., 807753, from Sigma-Aldrich). The WO 3  nanoparticles can be dispersed in 2-propanol and propylene glycol to form the colloidal ink. The particle concentration can be, for example, about 1 wt % to about 10 wt % (e.g., about 1 wt %, about 2 wt %, about 3 wt %, about 4 wt %, about 5 wt %, about 6 wt %, about 7 wt %, about 8 wt %, about 9 wt %, or about 10 wt %, including any values and sub ranges in between). The particle size of the WO 3  nanoparticles can be, for example, about 5 nm to about 30 nm (e.g., 5 nm, about 10 nm, about 15 nm, about 20 nm, about 25 nm, or about 30 nm, including any values and sub ranges in between). 
     Resolution and Fidelity of Printing 
     The resolution and fidelity (e.g., edge roughness, corner radius, etc.) of the printed pattern can be affected by several factors, including the stamp preparation (e.g., CNT microstructure geometry, surface roughness, mechanical properties, print/pattern size, surface treatment, and/or the like), the inking process (i.e., loading process), the magnitude and uniformity of the contact pressure, and the amplitude of applied voltage. 
     According to some embodiments, functionality of the stamp results from high porosity of the stamp, where the stamp pore size (characteristic length of d pore ) is larger than the electrically functional nanoparticles to be printed (d particle ) but smaller than the stamp features (w stamp ). For some embodiments, to maintain the ink particles well dispersed within the wet stamp, the pores within the stamp can be much larger than the particles (d particle &lt;&lt;d pore ). To provide for uniform ink transfer relative to the size of the stamp feature (and thus the resulting printed feature), the pores can be significantly smaller than the stamp features (d pores &lt;&lt;w stamp ). 
     For example, the nanoporous stamp can have pores with 100 nm diameter and this stamp can be employed to uniformly print ink particles of about 10 nm using stamp features as small as 3 μm. In another example, the nanoporous stamp can include single-walled CNT forests having a small CNT diameter (e.g. about 1 nm to about 2 nm) and spacing (e.g., about 10 nm to about 20 nm). Accordingly, this stamp can be used for printing sub-micrometer features. 
     In some examples, printed features of a stamp pattern on a target substrate have an average line edge roughness of about 2 μm or less (e.g., about 2 μm, about 1 μm, about 0.5 μm, or less, including any values and sub ranges in between). In some embodiments, the nanoporous stamp and/or printing method using the same is configured such that printed features of a pattern on target substrate have a linewidth of about 20 μm or less (e.g. about 20 μm, about 15 μm, about 10 μm, about 9 μm, about 8 μm, about 7 μm, about 6 μm, about 5 μm, about 4 μm, about 3 μm, about 2 μm, about 1 μm, or less, including any values and sub ranges in between). In some embodiments, the nanoporous stamp and/or nanoporous stamp printing method is configured such that printed features of a nanoporous stamp printed pattern on target substrate have average thickness of about 150 nm or less (e.g., about 150 nm, about 125 nm, about 100 nm, about 75 nm, about 50 nm, or less, including any values and sub ranges in between). In some embodiments, printed features of the pattern on target substrate have uniform thickness with tolerance of about 50 nm or less (e.g., about 50 nm, about 40 nm, about 30 nm, about 20 nm, about 15 nm, about 10 nm, or less, including any values and sub ranges in between). 
     Digital Printing Using Electrostatic Nanoporous Print Stamps 
     Based on the capability to control the ink transfer of porous microstructures, electrostatic nanoporous stamps described herein can be configured for digital printing by selectively applying the voltage to a subset of microstructures. In this approach, a CNT array (or any other appropriate microstructure) can be grown on a thin film transistor backplane, which can apply the necessary electrical potential between selected group of CNTs (pixels) and the ink. The ink can be controlled to only wet the selected pixels and transfer ink upon contact. Therefore, arbitrary patterns can be printed in sequence using the same stamp. In other words, the electrostatic nanoporous stamp is highly reconfigurable. 
       FIG. 6  shows an SEM image of a CNT pillar array  600  that can be used for digital printing. Each CNT pillar (e.g.,  610   a  or  610   b  labelled in  FIG. 6 ) is disposed on a corresponding electrode that can apply a voltage on this CNT pillar and not on other CNT pillars. Accordingly, each CNT pillar can function as one pixel during printing. The diameter of each pillar can be about 10 μm to about 100 μm (e.g., about 10 μm, about 20 μm, about 30 μm, about 40 μm, about 50 μm, about 60 μm, about 70 μm, about 80 μm, about 90 μm, or about 100 μm, including any values and sub ranges in between), which corresponds to a printing resolution of about 300 dpi to about 2500 dpi. The spacing between adjacent pixels can also be about 10 μm to about 100 μm (e.g., about 10 μm, about 20 μm, about 30 μm, about 40 μm, about 50 μm, about 60 μm, about 70 μm, about 80 μm, about 90 μm, or about 100 μm, including any values and sub ranges in between). 
     The pillar shown in  FIG. 6  has a round cross section. Other cross sectional shapes can also be used. For example, the CNT forest can have a square cross section, a rectangular cross section, a honeycomb cross section, or any other appropriate shape. 
       FIGS. 7A-7C  illustrate a method  700  of digital printing using an electrostatic nanoporous print stamp  710 . As shown in  FIG. 7A , the stamp  710  includes an array of dielectric-coated CNT forests  716  (also referred to as CNT pillars  716 ), each of which is disposed on a corresponding electrode  714 . A holder  712  is holding the array of electrodes  714  and the CNT forests  716 . A vessel  720  containing a liquid ink  725  is placed on top of the CNT pillars  716 . A power source  730  is used to apply a voltage between the liquid ink  725  and the stamp  710 . In the method  700 , each electrode has its own individual connection with the power source  730 , i.e. each electrode can be connected to or disconnected from the power source  730  without affecting the connection of other electrodes (e.g., via a corresponding switch). 
     In  FIG. 7A , a voltage is applied only on a subset of electrodes  714   a  (and accordingly those CNT pillars disposed on the electrodes  714   a ). As a result, the liquid ink  725  is only absorbed into the CNT pillars disposed on the electrodes  714   a , resulting in a contained ink  725   a . In contrast, CNT pillars not disposed on the electrodes  714   a  do not have electrostatic force to absorb the liquid ink  725 . 
     In  FIG. 7B , the vessel  725  is moved away from the CNT pillars  716  and a target substrate  740  is place in contact with the CNT pillars and accordingly the contained ink  725   a . In  FIG. 7C , the voltage is removed from all electrodes  714  and the target substrate  740  is lifted away from the stamp  710 . The contained ink  725   a  is transferred to the target substrate  750  and forms a printed pattern  750 . 
     After the target substrate  740  is moving away, a new voltage can be applied to another subset of electrodes in the stamp  710  to print a different pattern. This process can be repeated multiple times to take advantage of the versatility of digital printing. 
     Applications of Printing Using Electrostatic Nanoporous Print Stamps 
     The techniques of ink transfer using nanoporous stamps described herein can be used for various applications. For example, they can be used to create ultrathin, uniform printed layers for fabricating electronic devices, such as thin film transistors, where a variety of materials are deposited and patterned in spatially registered layers. In another example, the electrostatic nanoporous stamps and the associated printing methods can be utilized in printing conductive networks for transparent electrodes, as used in light-emitting diodes, liquid-crystal displays, touch-screen panels, solar cells, and numerous other devices where cost-effective fabrication of electrodes with high conductivity and transparency is desired. 
     Conclusion 
     While various inventive embodiments have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the function and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the inventive embodiments described herein. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the inventive teachings is/are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific inventive embodiments described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, inventive embodiments may be practiced otherwise than as specifically described and claimed. Inventive embodiments of the present disclosure are directed to each individual feature, system, article, material, kit, and/or method described herein. In addition, any combination of two or more such features, systems, articles, materials, kits, and/or methods, if such features, systems, articles, materials, kits, and/or methods are not mutually inconsistent, is included within the inventive scope of the present disclosure. 
     Also, various inventive concepts may be embodied as one or more methods, of which an example has been provided. The acts performed as part of the method may be ordered in any suitable way. Accordingly, embodiments may be constructed in which acts are performed in an order different than illustrated, which may include performing some acts simultaneously, even though shown as sequential acts in illustrative embodiments. 
     All definitions, as defined and used herein, should be understood to control over dictionary definitions, definitions in documents incorporated by reference, and/or ordinary meanings of the defined terms. 
     The indefinite articles “a” and “an,” as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean “at least one.” 
     The phrase “and/or,” as used herein in the specification and in the claims, should be understood to mean “either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Multiple elements listed with “and/or” should be construed in the same fashion, i.e., “one or more” of the elements so conjoined. Other elements may optionally be present other than the elements specifically identified by the “and/or” clause, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, a reference to “A and/or B”, when used in conjunction with open-ended language such as “comprising” can refer, in one embodiment, to A only (optionally including elements other than B); in another embodiment, to B only (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc. 
     As used herein in the specification and in the claims, “or” should be understood to have the same meaning as “and/or” as defined above. For example, when separating items in a list, “or” or “and/or” shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as “only one of” or “exactly one of,” or, when used in the claims, “consisting of,” will refer to the inclusion of exactly one element of a number or list of elements. In general, the term “or” as used herein shall only be interpreted as indicating exclusive alternatives (i.e. “one or the other but not both”) when preceded by terms of exclusivity, such as “either,” “one of,” “only one of,” or “exactly one of” “Consisting essentially of,” when used in the claims, shall have its ordinary meaning as used in the field of patent law. 
     As used herein in the specification and in the claims, the phrase “at least one,” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase “at least one” refers, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, “at least one of A and B” (or, equivalently, “at least one of A or B,” or, equivalently “at least one of A and/or B”) can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc. 
     In the claims, as well as in the specification above, all transitional phrases such as “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” “holding,” “composed of,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of” shall be closed or semi-closed transitional phrases, respectively, as set forth in the United States Patent Office Manual of Patent Examining Procedures, Section 2111.03.