Patent Publication Number: US-2017348728-A1

Title: Prevention of hydrophobic dewetting through nanoparticle surface treatment

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and is a continuation of U.S. patent application Ser. No. 13/775,938 (filed Feb. 25, 2013) which claims priority to of U.S. provisional patent application. 61/602,267 (filed Feb. 23, 2012), which applications are incorporated herein by reference in their entirety. 
    
    
     STATEMENT REGARDING FEDERALLY FUNDED RESEARCH OR DEVELOPMENT 
     This invention was made with government support under contract no. SCIGM093930 awarded by the National Institute of Health (NIH), contact no. 0653056 awarded by the National Science Foundation (NSF), and contract no. DE-AR000014 awarded by the Department of Energy (ARPA-E ADEPT). The government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     This invention relates, in one embodiment, to a method for coating a substrate with a nanoparticle layer. The layer alters the surface of the substrate such that dewetting is prevented. The method is particularly useful when depositing a monomer that subsequently polymerizes to form a polymeric layer while on the nanoparticle layer. 
     BACKGROUND 
     Coating substrates with polymeric surfaces is commonplace in a variety of fields, including the thin-film, energy storage and semiconductor industries. Often, the substrate and the polymer must be customized to prevent dewetting. In some situations, particular substrate/polymer combinations are simply not accessible due to excessive dewetting. Additionally or alternatively, the substrate may be delicate and/or costly and etching of the substrate is not permissible. The dewetting problem is particularly troublesome when the layer being deposited changes its properties during deposition. For example, a monomer may be deposited on a surface and not experience dewetting but, upon polymerization, the properties are altered and dewetting occurs. An alternative method for coating a substrate that prevents dewetting is desired. 
     SUMMARY OF THE INVENTION 
     Disclosed in this specification is a method for coating a substrate to prevent dewetting. A suspension of nanoparticles is deposited onto the substrate to produce a nanoparticle layer. The nanoparticle layer is then coated with a monomer. The monomer polymerizes on the nanoparticle layer to produce a polymeric layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is disclosed with reference to the accompanying drawings, wherein: 
         FIG. 1A ,  FIG. 1B ,  FIG. 1C  and  FIG. 1D  depict an exemplary dewetting problem; 
         FIG. 2A ,  FIG. 2B ,  FIG. 2C ,  FIG. 2D ,  FIG. 2E  and  FIG. 2F  depict an exemplary method for addressing a dewetting problem; and 
         FIG. 3  is a flow diagram depicting an exemplary method for coating a substrate to prevent dewetting. 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. The examples set out herein illustrate several embodiments of the invention but should not be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION 
     Referring to  FIGS. 1A to 1C , an exemplary dewetting problem is illustrated. Dewetting is the beading of a liquid on a substrate surface. Dewetting negatively impact&#39;s the ability of a liquid to spread on the substrate surface which, in turn, produces non-uniform layers. In this exemplary embodiment a substrate  100  is coated with a suspension  102  of a polymer or monomer in an organic liquid. The organic liquid is allowed to evaporate which leaves a residual polymeric layer  104  on the substrate  100 . In one exemplary embodiment, substrate  100  may be an aluminum electrode and suspension  102  may be a suspension of furfuryl alcohol in ethanol. In another embodiment, the substrate is selected from the group consisting of aluminum, copper, silicon, a metal layer on glass and a metal layer on a flexible polymer. As the solvent evaporates, the furfuryl alcohol polymerizes to form a polymeric layer  104  of polyfurfuryl alcohol. As depicted in  FIG. 1C , polymeric layer  104  has experienced dewetting. This is evident from the accumulation of the polymeric layer  104  at the periphery of the substrate  100 . 
       FIG. 1D  is a surface profile of the coated substrate  100  of  FIG. 1C  along line  106 . The region  108  corresponds to a portion of the polymeric layer  104  before a first edge  110 . At the first edge  110  the height of the polymeric layer  104  increases rapidly. A lip that raises  1 . 5  micrometers above the remainder of the polymeric layer  104  is not uncommon. The region  112  corresponds to the uncoated portion of the substrate  100 . At a second edge  114  the height of the polymeric layer  104  again increases rapidly. The region  116  corresponds to a portion of the polymeric layer  104  after the second edge  112 . The non-uniformity (e.g. edges  110 ,  114 ) in the thickness of polymeric layer  104  is undesirable and is a consequence of dewetting. 
     The dewetting problem illustrated in  FIG. 1C  can occur whenever a hydrophoblic polymer is applied. The problem is particularly pronounced when the deposited suspension changes its hydrophobicity during deposition. For example, the monomer furfuryl alcohol is relatively hydrophilic. The corresponding polymer, polyfurfuryl alcohol, is relatively hydrophobic. During polymerization the hydrophilic/hydrophobic properties of the suspension change. This change greatly accentuates the dewetting problem. 
       FIGS. 2A to 2F  depicts an exemplary method for addressing a dewetting problem by facilitating the spreading of the liquid over a surface. In this exemplary embodiment a substrate  200  is coated with a nanoparticle suspension  206  that comprises nanoparticles and a liquid. After coating, the liquid is permitted to evaporate to leave a nanoparticle layer  208  on a surface of the substrate. Thereafter, a suspension  202  of a polymer or monomer in a liquid is deposited. In one embodiment, suspension  202  is a suspension of furfuryl alcohol. Other suitable monomers would be apparent to those skilled in the art after benefitting from reading this specification. The liquid in suspension  202  may be the same or different from the liquid in nanoparticle suspension  206 . The liquid is allowed to evaporate which leaves a residual polymeric layer  204  on the substrate  200 . As depicted in  FIG. 2E , polymeric layer  204  has not experienced dewetting. This is evident from the uniform thickness of the polymeric layer  204  over the substrate  200 . Advantageously, dewetting can be prevented without the use of surfactants or surface modification (e.g. etching) of the substrate  200 . A side view of the coated substrate is schematically depicted in  FIG. 2F . The nanoparticle layer  208  is deposited directly on the surface of the substrate  200 . The polymeric layer  204  is deposited directly on the nanoparticle layer  208 . 
     The nanoparticles generally have a diameter of from about 1 nm to about 1000 nm. In one embodiment, the nanoparticles have a diameter of from about 1 nm to about 50 nm. In another embodiment, the nanoparticles have a diameter of from about 8 nm to about 30 nm. The nanoparticles may be ceramic nanoparticles. Examples of suitable ceramic nanoparticles include barium titanate, strontium titanate, barium strontium titanate, silica, and metal-oxide ceramics. In another embodiment the nanoparticles are metallic nanoparticles. Examples of suitable metallic nanoparticles include silver, gold, and copper. 
     The nanoparticle layer  208  may be deposited by any conventional technique including, but not limited to, pressure-driven dispenser coating, spin coating, dip coating, spray coating, inkjet coating, gravure coating. The nanoparticle layer  208  may be deposited from a rapidly evaporating liquid in which the nanoparticles are insoluble and which has a density that approximately matches the density of the nanoparticles, thereby permitting the nanoparticles to remain suspended in the liquid for a sufficient period of time. For example, an alcohol liquid (e.g. ethanol, isopropanol, methanol) may be used, as well as other organic solvents (e.g. dimethylformamide). The nanoparticle may be present in the liquid at a concentration of from about 1 mg/mL to about 50 mg/mL. In another embodiment, the nanoparticle may be present in the liquid at a concentration of from about 10 mg/mL to about 30 mg/mL. In one embodiment, the entire surface of the substrate  200  is coated. In another embodiment, only a portion of the substrate  200  is coated. In one such embodiment, a patterned portion of the substrate  200  is coated using, for example, inkjet deposition and/or masking. The nanoparticle layer  208  generally has a thickness of less than about five hundred nanometers. In one embodiment, the nanoparticle layer  208  has a thickness of less than about one hundred nanometers. In yet another embodiment, the nanoparticle layer  208  has a thickness of a single monolayer. 
       FIG. 3  is a flow diagram depicting an exemplary method  300  for coating a substrate to prevent dewetting. The method  300  comprises step  302  wherein a nanoparticle suspension is deposited in a liquid onto a surface of the substrate. For example, a suspension of barium titanate nanoparticles (8-30 nm) in ethanol may be deposited onto a surface of an aluminum electrode. In step  304 , the liquid is permitted to evaporate the produce a nanoparticle layer on the surface. Thereafter, in step  306 , the nanoparticle layer is coated with a monomer in either neat or diluted form. In step  308  the monomer is allowed to undergo a polymerization reaction to produce a polymeric layer on the nanoparticle layer. 
     Without wishing to be bound to any particular theory, Applicant believes the nanoparticle layer  208  provides a seed layer of particles that modifies the interactions between the nanoparticle layer  208  and the suspension  202 . The suspension  202  sees the nanoparticle layer  208  as a substantially homogenous layer. Although multiple factors are likely responsible, Applicant believes the nanoparticles roughen the surface and permit the suspension  202  to become held between adjacent nanoparticles, thereby preventing dewetting. Advantageously, this surface roughening is accomplished without needing to etch or otherwise damage the surface of the substrate—a feature that is very desirable when producing microelectronics. 
     The methods described in this specification are particularly useful in preventing dewetting with suspensions that change their hydrophobicity during deposition (e.g. suspension of a monomer that polymerizes during deposition). Additionally, the methods described in this specification are particularly useful in prevent dewetting when the polymeric layer that is being deposited is a nanoparticle/polymer composite. In such situations the nanoparticle is a component of the resulting layer anyway and the dewetting can be prevented by altering the order in which the nanoparticle is added. 
     For example, metacapacitors are solid-state ceramic nanoparticle/polymer composites with multiple layers designed for integration with power conversion electronics. Attempts were made to produce metacapacitors using additively printed dielectric composite layers that were suspensions of the polymer and nanoparticle. When the nanoparticle was co-suspended with the polymer (see Example 2), substantial dewetting occurred and the desired metacapacitor was not produced. When the nanoparticle was first pre-deposited and the polymer layer was subsequently deposited on the nanoparticle layer, the desired metacapacitor was produced. Multi-layered capacitors could be produced by pre-depositing a layer of nanoparticles atop the substrate prior to polymer deposition of each individual layer. 
     Example 1—Comparative Example—No Nanoparticle 
     Furfuryl alcohol, a monomer in liquid form, was applied to an aluminum surface such that a uniform film of furfuryl alcohol approximately 100 nm thick remained on the surface. After heat above about 80 C to dry and polymerize the furfuryl alcohol, the material (now a polymer) had visibly undergone dewetting and had accumulated at the periphery of the aluminum surface leaving sections of the aluminum surface bare. 
     Example 2—Comparative Example—nanoparticle Co-Suspended 
     0.225 mL of furfuryl alcohol monomer was mixed with 1.0 mL of ethanol, along with 9 mg of barium strontium titanate nanoparticles. The suspension was applied to an aluminum surface and dried to drive off the ethanol. It was then heated above about 80 C to polymerize the furfuryl alcohol such that a film of approximately 1 micron of polymer and nanoparticles remained on the surface. After this treatment, the polymer-nanoparticle composite had visibly undergone dewetting and had accumulated at the periphery of the aluminum surface leaving sections of the aluminum surface bare. 
     Example 3—Nanoparticle Pre-Deposited 
     A solution comprising barium strontium titanate nanoparticles and ethanol at a concentration of 20 mg of nanoparticles per 1 mL of ethanol was applied to an aluminum surface and dried in air such that the ethanol evaporated and the resulting nanoparticle film was approximately 50 nm thick. Furfuryl alcohol monomer was then applied to this surface on top of the nanoparticle film and heated to above 80 C to polymerize the monomer. After this deposition and treatment, no dewetting or film reconfiguration was observed and the aluminum surface remained covered. 
     While the invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof to adapt to particular situations without departing from the scope of the disclosure. Therefore, it is intended that the claims not be limited to the particular embodiments disclosed, but that the claims will include all embodiments falling within the scope and spirit of the appended claims.