Abstract:
Layered nanostructures are constructed by imprinting material with a mold, while selectively modifying and removing a portion of the mold. The mold, which includes a pattern of features, is modified so that the portion of the mold that includes the features is made chemically and/or physically distinct from the rest of the mold. That portion of the mold that includes the features is retained while the rest of the mold is removed. The retained portion of the mold provides mechanical support for any adjoining layer or layers.

Description:
TECHNICAL FIELD 
     The invention is directed to methods of forming layered nanostructures. 
     BACKGROUND 
     Nanoscopic structures are of growing interest and have enabled the production of new materials in photonics, microfluidics, and other disciplines. One method for forming nanoscopic structures is transfer molding, which has emerged as an attractive way to produce low cost, large area patterns. (See, for example, Marzolin et al., Advanced Materials, vol. 10, pp. 571-574, 1998; and Hampton et al., Advanced Materials, vol. 20, pp. 2667-2673, 2008.) Transfer molding has the potential for high throughput while avoiding the intensive processing associated with traditional patterning methods such as lithography. 
     Transfer molding has been employed to form layered structures by hot bonding successive layers of polymeric materials at temperatures above the glass transition temperature T g  of the last-deposited layer. (See, for example, Bao et al., J. Vac. Sci. Technol. B, vol. 20, pp. 2881-2886, 2002.) Also, layers have been successively stacked by including gluing steps in the fabrication process. (See, for example, Han et al., Applied Physics Letters, vol. 91, pp. 123118-123113, 2007.) Nevertheless, the prior art techniques suffer from various limitations, so that while transfer molding has met with some success, its extension to layered, patterned materials remains a challenge. 
     SUMMARY 
     The present invention circumvents problems encountered in the prior art, e.g., when introducing glue is undesirable or impractical, or when the Tg of a polymeric material (that is to be applied to an existing layer) is greater than that of the polymeric material of an existing layer. Methods are disclosed herein in which only part of a mold is initially removed while leaving in place the rest of the mold (including the mold&#39;s pattern of features). Material that has been applied to the mold takes on the (inverse of) the mold&#39;s pattern. The process can be repeated with additional molds and material(s), with all the molds finally being completely removed, thereby leaving only layers of material having patterns determined by the molds. The methods herein resist draining, sagging, wetting, or infill by subsequently applied layers or molds. Preferred methods are low cost, scalable, environmentally benign, and do not require vacuum or an imprinting machine. High fidelity sub-750 nm resolution is possible (as well as sub-200 nm, sub-135 nm, and sub-50 nm resolution), involving stacks of layers having varying periodicity, composition, and thickness. A broad class of materials may be advantageously used as the mold materials. 
     In preferred implementations of the invention, material is applied to a mold, with the mold being modified in some way so that the portion of the mold that includes a pattern of features is made chemically and/or physically distinct from the rest of the mold. This allows retention of that portion of the mold that includes the pattern while the rest of the mold is removed. The remaining portion of the mold can be removed after subsequent processing. The material that has been applied to the mold is patterned as a result of being in contact with the mold. The patterned material is thus protected by the remaining portion of the mold, thereby preserving the fidelity of the pattern that has been transferred into the material: This aspect of the preferred implementations is advantageous in that it 1) helps prevent leakage of material from any subsequently applied layers into layers that have already been formed, 2) allows the use of layers that are not fully cured, and 3) allows gluing and hot bonding steps to be avoided. 
     The mold may be heterogeneously modified through one of a variety of techniques, e.g., if the material that is applied to the mold is a solution, species in that solution may diffuse into the mold and induce localized cross linking The cross-linked material will be more resistant to chemical dissolution than the remaining portion of the mold, but the cross-linked material can be later removed through another process (e.g., calcination) when generating the desired nanostructure. 
     One aspect of the invention is a method that includes applying a first material to a first mold, in which the first material conforms to a shape of the first mold and thereby acquires a first pattern determined by the shape of the first mold, with the first material and the first mold together forming a first temporary structure. The first temporary structure is joined to a substrate. A portion of the first mold is modified, with the first mold becoming heterogeneous with respect to at least one of a chemical and/or physical property, so that the first mold includes first and second portions. (This modifying step may be performed before the applying step.) One of the first and second portions of the first mold is removed, thereby forming a first intermediate structure that includes i) the substrate, ii) an un-removed portion of the first mold, and iii) the first material or a derivative of the first material. The first material itself may act as the agent that modifies a portion of the first mold, so that the first mold becomes heterogeneous. 
     The method may further include applying a second material to a second mold, in which the second material conforms to a shape of the second mold and thereby acquires a second pattern determined by the shape of the second mold, so that the second material and the second mold together form a second temporary structure. The second temporary structure is joined to the first intermediate structure. A portion of the second mold is modified, with the second mold becoming heterogeneous with respect to at least one of a chemical and/or physical property, so that the second mold includes first and second portions. One of the first and second portions of the second mold is removed, thereby forming a second intermediate structure that includes i) an un-removed portion of the second mold, ii) the second material or a derivative of the second material, and iii) the first intermediate structure. 
     The method may further include removing any remaining portions of the first and second molds, thereby leaving a layered structure that includes i) the substrate, ii) the first material or a derivative of the first material, and iii) the second material or a derivative of the second material. Additional layers in contact with the layered structure may be formed. 
     Another aspect of the invention is a method that includes imprinting a first material with a first mold, removing only a portion of the first mold, imprinting a second material with a second mold, and removing at least a portion of the second mold. A structure is formed that includes i) the first material or a derivative of the first material, ii) the second material or a derivative of the first material, iii) a remaining portion of the first mold, and optionally iv) a remaining portion of the second mold. (Prior to the forming step, the second mold may be removed in its entirety, so that the structure consists essentially of i) the first material or a derivative of the first material, ii) the second material or a derivative of the second material, and iii) a remaining portion of the first mold.) A remaining portion of the first mold and any remaining portion of the second mold are removed, thereby resulting in joining together i) the first material or its derivative and ii) the second material or its derivative to yield a layered structure. The method preferably includes joining both a) the first mold and b) the first material or a derivative of the first material to a substrate. 
     In preferred methods herein, the first and/or second material may include a precursor in a solution, a polymerizable polymer, a sol-gel that undergoes polymerization, or a solvent and nanoparticles (in which the nanoparticles form a solid upon removal of the solvent). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  includes  FIGS. 1A ,  1 B, and  1 C, in which: 
         FIG. 1A  illustrates an idealized molding process when material is patterned onto an existing molded layer; 
         FIG. 1B  shows an undesirable result when the material to be molded is deformable to the point of lacking structural integrity; and 
         FIG. 1C  shows an undesirable result when the mold itself is deformable. 
         FIG. 2 , which includes  FIGS. 2A ,  2 B,  2 C,  2 D,  2 E,  2 F,  2 G,  2 H,  2 I,  2 J, and  2 K, illustrates a series of steps of a preferred implementation of the invention, leading to the formation of a layered nanostructure. 
         FIG. 3 , which includes  FIGS. 3A and 3B , illustrates how one or more chemical agents in a solution can penetrate a mold and induce localized crosslinking around features in the mold, thereby rendering the mold heterogeneous. 
         FIG. 4  includes  FIGS. 4A and 4B , in which: 
         FIG. 4A  is a scanning electron microscope (SEM) micrograph of crosslinked polyvinyl alcohol (PVA) within and around a molded titania structure, taken just after dissolution of a portion of the PVA mold; and 
         FIG. 4B  illustrates how calcination removes the remaining portion of the PVA mold to reveal more clearly the underlying titania pattern. 
         FIG. 5  includes SEM micrographs of two-layer structures prepared from the same mold but using different concentrations of fill solution: 
         FIG. 5A  shows that at higher concentrations of fill solution closed spaces are formed as the amount of fill material is greater than the mold volume; and 
         FIG. 5B  shows that lower concentrations can be used to form open structures. 
         FIG. 6  is an SEM micrograph of an open two-layer structure having a pronounced moiré pattern and excellent periodicity (as shown by the inset generated by fast Fourier transform from the image in the micrograph). 
         FIG. 7  is a cross-section SEM micrograph of a five-layer material, with each of the 5 layers having been prepared from the same 250 nm (diameter) mesh mold. 
         FIG. 8  is a cross-section SEM micrograph of a two-layer closed-cell structure prepared using 2 different molds, a 250 nm (diameter) mesh and a 135×750 nm (diameter×depth) post structure. 
         FIG. 9  is a cross-section SEM micrograph of a two-layer closed-cell structure having a bottom layer of patterned titania and a top layer of patterned tin oxide. 
     
    
    
     DETAILED DESCRIPTION 
     The formation of multilayered structures involving soft materials typically suffers from the tendency of the overlayers to deform as a result of the layer(s) below them.  FIG. 1  illustrates some of the relevant processes for a layer being molded onto a pre-existing first layer.  FIG. 1A  shows an idealized process, in which a substrate  110  supports a first molded structure  115 . A material  120  that is to be molded is placed in and around a mold  125 . In this idealized case, both the material  120  and the mold  125  demonstrate structural integrity, permitting layers to be built up on the underlying substrate  110  and the molded structure  115 . 
     In the more realistic scenario shown in  FIG. 1B , a material  120   a  that is to be molded lacks structural integrity and actually pulls away from the mold  125 . As shown in  FIG. 1B , this leads to several undesirable phenomena, such as draining, infilling, sagging, and wetting. (See, for example, Peng et. al., “Hybrid mold reversal imprint for three-dimensional and selective patterning” Journal of Vacuum Science Technology B, vol. 24, pp. 2968-2972, 2006.) In another commonly encountered scenario (shown in  FIG. 1C ), the material  120  enjoys structural integrity, but a mold  125  a around which it is placed actually deforms. This can occur if the mold  125   a  is soft relative to the forces it experiences during molding. The result is vertical mixing of the molded layers. (See, for example, Hampton et. al., “The Patterning of Sub-500 nm Inorganic Oxide Structures”, Advanced Materials, vol. 20, pp. 2667-2673, 2008.) In practice, both of the issues shown in  FIGS. 1B and 1C  are a problem, and it has proven difficult to build up reliable nanostructures using previously known techniques. 
     Methods are disclosed herein that avoid the occurrence of such undesirable processes, while maintaining the fidelity of molded layers. A preferred implementation of the invention is now described with respect to  FIG. 2 . As shown in  FIG. 2A , a first material  210 , which may be a precursor to another material, is coated or otherwise applied onto a first mold  220  having various features therein constituting a pattern to be transferred to the first material  210 . The first material  210  and the first mold  220  are inverted if need be (see  FIG. 2B ) and then laminated or otherwise joined (e.g., by pressing them together at elevated temperatures) to a first substrate  230  (see  FIG. 2C ), thereby forming a first temporary structure that is to undergo further processing. (While use of the substrate  230  is highly desirable, its use may be avoided provided that the first material  210  and the first mold  220  have sufficient structural integrity, e.g., they are sufficiently thick and rugged that they can withstand subsequent processing.) 
     As shown in  FIG. 2D , the first mold  220  undergoes selective modification, which results in a first portion  224  and a second portion  228  of the first mold  220  that are heterogeneous with respect to their chemical and/or physical properties. This facilitates removal of the first portion  224  (e.g., it may be preferentially dissolved away with a solvent), thereby forming a first intermediate structure  240  (see  FIG. 2E ), which can be used as a platform for the formation of one or more additional layers. In structure  240 , the second portion  228  (which may now be a derivative of the first material  210  if that material has undergone a chemical or physical transformation) fills space that will eventually be emptied, but until then that space is filled with material that provides mechanical support for one or more additional layers, while mitigating the unwanted phenomena described above in connection with  FIG. 1B . The second portion  228  includes an uppermost portion or layer (designated in  FIG. 2E  as  229 ) which may be removed as desired, e.g., through an etching process, so that the first material  210  is exposed directly to air or the surrounding environment. 
     The methodology may be repeated to build an additional layer or layers over the first intermediate structure  240 , thereby forming a layered nanostructure. As shown in  FIG. 2F , a second material  250  (which may be a precursor to another material; note that the first material  210  and the second material  250  may be the same kind of material) is coated or otherwise applied onto a second mold  260  having various features therein constituting a pattern to be transferred to the second material  250 . The second material  250  and the second mold  260  are inverted if need be (see  FIG. 2G ) and then laminated or otherwise joined to the first intermediate structure  240  (see  FIG. 2H ), thereby forming a second temporary structure that is to undergo further processing. 
     As shown in  FIG. 2I , the second mold  260  then undergoes selective modification, which results in a first portion  264  and a second portion  268  that are heterogeneous with respect to their chemical and/or physical properties. This facilitates removal of the first portion  264  (e.g., it may be preferentially dissolved away), thereby forming a second intermediate structure  270  (see  FIG. 2J ), which may likewise be used as a platform for the formation of additional layers. 
     In structure  270  (as was the case with structure  240 ), the second portion  268  (which may now be a derivative of the second material  250  if that material has undergone a transformation) fills space that will eventually be emptied, but until then that space is filled with material that can provide mechanical support for any additional layer or layers. Also, the second portion  268  includes an uppermost portion or layer that may be removed as desired, e.g., through an etching process, so that the second material  250  is exposed directly to air or the surrounding environment. 
     If need be, the second intermediate structure  270  may be cured through heating or other means, and any remaining portions of the first and second molds (e.g., the second portion  228  of the first mold  220 , and the second portion  268  of the second mold  260 ) may be removed to form a nanostructure  280  that includes the substrate  230  and the cured derivatives  210 ′ and  250 ′ of the first and second materials  210  and  250  (see  FIG. 2K ). Curing may be required, for example, if the first and second materials  210  and  250  include certain kinds of polymers, e.g., low-molecular weight polymers that need to undergo crosslinking Also, solubilized polymers that solidify after drying out or sol-gel materials that gradually solidify throughout the course of the method may be used. In general, the first and/or second materials  210  and  250  may alter their properties as the method is carried out, and further, even the shapes of these materials may change slightly if they are cured (which could then alter their respective patterns). On the other hand, for certain choices of materials  210  and  250 , no curing step may be required. For example, one or both of the first material  210  and the second material  250  may include nanoparticles (e.g., a colloid that includes a solvent and nanoparticles such as SnO 2 ), with the solvent evaporating at some point, leaving behind just the nanoparticles as a solid. 
     As an alternative (not shown) to the process steps illustrated in  FIGS. 2I and 2J , the second mold  260  may be removed entirely after the second temporary structure shown in  FIG. 2H  is formed, provided that the remaining components have sufficient structural integrity to withstand any further processing that is required. Any remaining portion of the first mold (e.g., the second portion  228  of the first mold  220 ) may then be removed to form the nanostructure  280  shown in  FIG. 2K . 
     The nanostructure  280  of  FIG. 2K  shows the cured first material  210 ′ and the cured second material  250 ′ in contact with each other, although the first material  210  and the second material  250  of  FIG. 2J  are not in direct contact. Direct contact between the cured first material  210 ′ and the cured second material  250 ′ may be achieved spontaneously as a result of the curing process, or an optional etch step may be employed to remove any material between them, thereby bringing these materials into contact with each other. 
     Excess material may exist between the first and second materials  210  and  250  (or their cured derivatives) if the volume of the second material  250  exceeds the volume within the second mold  260 . Controlling the amount of the second material  250  that is used can determine whether the resulting structure is a closed-cell structure (as shown in  FIG. 2K ) or an open-cell structure. Also, additional layers may be formed over the nanostructure  280  using steps analogous to those outlined above. Accordingly, a variety of structures can be formed including ones of varying periodicity, composition, and thickness. 
     As discussed above, the first mold  220  and the second mold  260  are rendered heterogeneous with respect to their chemical and/or physical properties, thereby facilitating removal of a portion of the mold. A preferred way of accomplishing this is to employ a process that only modifies that portion of the mold immediately surrounding the features that define the mold&#39;s pattern. To this end, a diffusion based process (e.g., chemical, thermal, etc.) or exposure to strongly absorbed radiation (e.g., UV light) or particles (e.g., electrons) can be employed to induce crosslinking This is illustrated in  FIG. 3A , in which one or more chemical agents  310  within an applied solution  315  diffuse into a mold  320 .  FIG. 3B  illustrates that crosslinking occurs in a portion  324  of the mold  320  while leaving a portion  328  of the mold  320  substantially unaffected. The mold  320  is thereby rendered heterogeneous with respect to its chemical and/or physical properties. 
     EXAMPLES 
     The following examples are intended to provide those of ordinary skill in the art with a complete disclosure and description of how to use the methods claimed herein. An effort has been made to ensure accuracy with respect to measured numbers, but allowance should be made for the possibility of errors and deviations. Unless indicated otherwise, parts are parts by weight, temperature is in ° C., and pressure is at or near atmospheric. All chemicals and materials were obtained commercially or were synthesized using known procedures. Commercially available PVA molds were purchased from TDI (Transfer Devices, Inc) of San Jose, Calif. SEM was performed on a Hitachi S-4700 at 3 kV. 
     Example 1 employs a structure (see  FIG. 4A ) that is analogous to the structure shown in  FIG. 2E . Examples 2-4 result in multi-layered structures, similar to the structure shown in  FIG. 2K . In  FIGS. 5-9 , the various layers are numbered for clarity ( 1 ,  2 ,  3 ,  4 , and  5 , as is the case). 
     Example 1 
     Single-Layer Titania Structure 
     A 30 wt. % titania precursor (consisting of Dupont™ Tyzor® BTP and acetylacetone (1:1 mol Ti:mol acetylacetone) in propylene glycol propyl ether (PGPE)) was applied to a PVA mold. (This solution rendered the PVA mold heterogeneous around its nanoscale-patterned regions, which led to localized cross linking near those regions; the corresponding cross-linked portions of the PVA mold were resistant to dissolution.) The temporary structure that included the PVA mold and an imprinted derivative of the titania precursor solution (the solvent in the solution evaporated and some polymerization of the precursor occurred) was heated to 80° C. for 10 s and then joined to a silicon substrate that had been pre-heated to 80° C. After an additional 10 s, the resulting structure (PVA mold, imprinted derivative and substrate) was removed from the heat source and exposed to short wavelength UV radiation (λ=245 nm) at room temperature for 20 to 30 min. A portion of the PVA mold was then dissolved by immersion in warm deionized water. The corresponding structure is shown in  FIG. 4A , which shows the substrate (bottom), the imprinted derivative of the titania precursor solution (middle), and the remaining portion of the PVA mold (top). As shown in  FIG. 4B , removing the crosslinked portion of the PVA mold through calcination at 450° C. in air revealed the corresponding patterned titania structure (on top of the substrate). 
     Example 2 
     Stacked Titania Structure from 250 nm (Diameter) Mesh Mold 
     Two-layer titania structures were fabricated in both closed-cell (analogous to  FIG. 2K ) and open-cell morphologies. A titania precursor solution consisting of Dupont™ Tyzor® BTP and acetylacetone (1:1 mol Ti:mol acetylacetone) was diluted at varying concentrations in PGPE. A closed-cell structure was prepared by spin coating a 30 wt % precursor solution onto a 250 nm (diameter) mesh PVA mold at 2000 rpm for 45 s. (This solution rendered the PVA mold heterogeneous around its nanoscale-patterned regions, which led to localized cross linking near those regions; the corresponding cross-linked portions of the PVA mold were resistant to dissolution.) The resulting sample was heated to 80° C. for 10 s before placing it onto a bare silicon wafer substrate (that had been pre-heated to 80° C.), which joined the sample to the substrate, thereby forming a temporary structure analogous to that shown in  FIG. 2C . After an additional 10 s, the temporary structure was removed from the heat source and exposed to short wavelength UV radiation (λ=245 nm) at room temperature for 20 to 30 min. Dissolution of the non-crosslinked portion of the PVA mold was carried out at 42° C. (aqueous, pH 2.2) for 15 min. The resulting intermediate structure (analogous to that shown in  FIG. 2E ), which included titania, was then dried with ethanol and hexane. A second layer was formed using similar procedures. Final calcination to remove the crosslinked portions of the PVA molds around the patterned titania regions was carried out at 450° C. for 2 hrs in air (ramp up at 5° C./min), resulting in the structure shown in  FIG. 5A  (analogous to the structure shown in  FIG. 2K ). 
     An open-cell two-layer structure was fabricated by employing similar procedures but using a 15 wt % precursor solution (see  FIG. 5B ). A top view SEM micrograph of this open-cell two-layer structure showed a pronounced moiré pattern and excellent periodicity over large areas (see  FIG. 6 ). These procedures were extended to form the structure shown in  FIG. 7 , which has more than two layers. 
     Example 3 
     Stacked Two Layer Titania Structure from Two Different Molds 
     Closed-cell two-layer titania structures fabricated with two different molds are shown in  FIG. 8 . A 30 wt % titania precursor solution consisting of Dupont™ Tyzor® BTP and acetylacetone (1:1 mol Ti:mol acetylacetone) in PGPE was used to form the first layer using a 250 nm (diameter) mesh PVA mold, following the procedures outlined in Example 2. The second layer was formed using a mold having holes whose approximate dimensions were 135×750 nm (diameter×depth), with 5 volume % acetic acid now being added to the precursor solution to facilitate the templating process. Other conditions and final calcination were performed as above in Example 2. 
     Example 4 
     Stacked Two Layer Structure with a Bottom Titania Layer and a Top Tin Oxide Layer 
     Closed-cell two-layer structures fabricated with two different materials are shown in  FIG. 9 . A 30 wt % titania precursor solution consisting of Dupont™ Tyzor® BTP and acetylacetone (1:1 mol Ti:mol acetylacetone) in PGPE was used to form the first layer using a 250 nm (diameter) mesh PVA mold, following the procedures outlined in Example 2. A tin oxide second layer was formed from a mold (having the same pattern as the first mold) using a 20 wt % solution in toluene of 2-ethylhexanoate-capped tin oxide nanoparticles synthesized according to literature procedures. (See Kim et al., New Journal of Chemistry, vol. 31, pp. 260-264, 2007.) Other conditions and final calcination were performed as above in Example 2. 
     The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is therefore indicated by the appended claims rather than the foregoing description. All changes within the meaning and range of equivalency of the claims are to be embraced within that scope.