Three dimensionally periodic structural assemblies on nanometer and longer scales

Abstract

This invention relates to processes for the assembly of three-dimensional structures having periodicities on the scale of optical wavelengths, and at both smaller and larger dimensions, as well as compositions and applications therefore. Invention embodiments involve the self assembly of three-dimensionally periodic arrays of spherical particles, the processing of these arrays so that both infiltration and extraction processes can occur, one or more infiltration steps for these periodic arrays, and, in some instances, extraction steps. The product articles are three-dimensionally periodic on a scale where conventional processing methods cannot be used. Articles and materials made by these processes are useful as thermoelectrics and thermionics, electrochromic display elements, low dielectric constant electronic substrate materials, electron emitters (particularly for displays), piezoelectric sensors and actuators, electrostrictive actuators, piezochromic rubbers, gas storage materials, chromatographic separation materials, catalyst support materials, photonic bandgap materials for optical circuitry, and opalescent colorants for the ultraviolet, visible, and infrared regions.

Description

[0002] 1. Field of the Invention[0003] This invention relates to processes for the synthesis of three-dimensionally periodic structures and functional composites by the self-assembly of spheres, followed by one or more structure modification, infiltration, and extraction processes. These structures can be applied as thermoelectrics and thermionics, electrochromic display elements, low dielectric constant electronic substrate materials, electron emitters (particularly for displays), piezoelectric sensors and actuators, electrostrictive actuators, piezochromic rubbers, gas storage materials, chromatographic separation materials, catalyst support materials, photonic bandgap materials for optical circuitry, and opalescent colorants for the ultraviolet, visible, and infrared regions.[0004] 2. Description of Related Art[0005] The art describes various means for fabricating articles with periodic structures that repeat on the scale of millimeters, such as by conventional machining methods....

AI Technical Summary

Benefits of technology

[0056] Depending upon the structure needed for a particular application, the primary opal template can be exposed to a chemical that alters the surface energy or structure of this opal prior to the infiltration of material B. For example, the surface of SiO.sub.2 spheres can be made hydrophobic by reacting an organosilane (such as vinyltriethoxysilane and vinyltrichlorosilane) on this surface or by infiltrating a solution of a solid hydrocarbon (such as poly(o-phenylene)) into the opal, followed by evaporation of the solvent. Various methods can be employed for structural modification. For example, the selective dissolution of the opal spheres at non-neck regions of the spheres can be used to increase opal void volume without destroying the neck-generated interconnections that are required for the extraction of the A material after the infiltration of the B material. In this case, the partial dissolution of sintered SiO.sub.2 opals with aqueous KOH reduces the sphere diameter (thereby increasing the void volume that is available for the infiltration of material B) without eliminating the interconnects resulting from sintering-generated intersphere necks.

[0066] Other templating processes of this invention are referred to a patch templating and as particle loading. Patch templating, is a type of templating process where the surfaces of a void structure are covered with a partial surface coating of the infiltrated material (so that uncoated regions exist). Depending upon the application need this patch coating can be either percolated (called percolated patch) or unpercolated (called unpercolated patch). Patch coating is most preferably accomplished by inhomogeneous reaction from a solution, such as in the deposition of a metal from a metal salt (like the deposition of Au from a solution of AuCl.sub.4). Adjusting the reaction time can control whether or not the patch coating is percolated, since an unpercolated patch coating can become percolated upon further reaction. Particle templating most preferably results from either the infiltration of particles into the opal (or the in-situ formation of such particles within said opal). The particles in particle-infiltrated opals are preferably aggregated together to form a mechanically robust structure, thereby insuring that these particle do not de-aggregate during extraction processes for the host matrix. For example, this aggregation can be accomplished by post infiltration sintering. Infiltrated particles are preferably smaller than 1 / 5th the diameter of the smallest interconnections between void space in the infiltrated opal. Ultrasonic dispersion can be conveniently used to obtain particle infiltration from a colloidal dispersion. Alternately, or in conjunction with ultrasonically assisted infiltration, particle infiltration can be accomplished by passing a fluid containing the particles through a plate of the opal.

[0067] The void space in the face-centered-cubic opal structure is from about 19 to about 25% for the sintered opals that are most preferred for the invention embodiments. This means that the void space for a fully volume-templated opal will be about 25% or less. On the other hand, inverse opals obtained by-surface templating can have much higher void volumes. Void volumes from about 75% to nearly about 100% can be conveniently obtained using the processes of the present invention.

[0071] where d is an effective distance between silica spheres and D is their initial diameter. Complete isolation of the tetrahedral and octahedral voids from each other occurs when the critical value of D / d=1.155 is reached. In this case, melt infiltration into the opal matrix is impossible. Using a typical value of D / d=1.055, a sphere diameter of 220 nm, and the surface tension of bismuth, the minimum pressure required for infiltration of molten bismuth (calculated using Eqns. 1 and 2) is about 0.7 kbar. The pressure that we use in order to obtain bismuth infiltration into large samples of SiO.sub.2 opal (6.times.6.times.6 mm.sup.3) in a reasonably short time is much higher (8-10 kbar at 300-350.degree. C. for 1-2 hours).

[0080] (f) Infiltrate C.sub.1 from the right side of the plate, allowing the infiltration to proceed across the entire void space through S.sub.1 (since the left-hand ends of the S.sub.2 void space are already filled, this infiltration step does not effect that S.sub.2 void space). Allow the infiltrated C.sub.1 material to coat the right hand side of the plate, thereby providing an easily contacted electrode surface that is connected to all the C.sub.1 material.

[0088] The applications for these three-dimensionally periodic materials result from this periodicity. One category of applications exploits three-dimensional structural periodicities that are in the visible, infrared, or ultraviolet regions to make optical switches, display devices, and directional light sources. In each case the periodicity-dependent property being exploited is the Bragg scattering of the electromagnetic radiation. This control on the direction of propagation of light (of a specified frequency) results from a change of the diffraction angle of this light because of a change in the unit cell parameter of the opal derived structure. This change in unit cell parameter can be conveniently accomplished by any of the various well-known methods that result in a change of materials dimension. Examples are the application of an electric field for an electrostrictive or piezoelectric material; a temperature or pressure change for either an ordinary material or a shape memory material; exposure to a solvent that causes swelling; the electrically-induced change in dimensions of a gel polymer; a thermally-induced chemically-induced, or photo-induced reaction of a reactive matrix material; and an electrochemically-induced dimensional change of a redox material (like carbon or a conducting polymer). Depending upon the choice of materials and operating conditions, these changes can be either reversible or irreversible, and can include the effects of refractive index change. The inverse opals synthesized by the present processes are ideal for such optical switch applications, since the diffraction efficiency of an array of particle array is enhanced if the low refractive index phase occupies a much larger volume fraction than the high refractive index phase. For a close-packed array of spherical particles (as in the porous SiO.sub.2 opals), the maximum volume (about 76%) is occupied by the particles--so diffraction efficiency is not optimized if the void space is air filled. However, in inverse opals made by the processes of this invention, an infiltrated material has filled the void space and the spherical particles (such as SiO.sub.2) have been extracted (leaving air spheres). Hence, the low refractive index phase (i.e., the air phase) is now the majority phase, which is the situation that maximizes diffraction efficiency.

Problems solved by technology

In between these extremes there exists a manufacturability gap of from about 100 microns to about 10 nm, where it is presently difficult or impossible to fabricate three-dimensionally periodic structures from desired materials.

However, achievement of similar periodicity in the third dimension has provided the greatest problem.

Limited success has been achieved in creating three-dimensionally periodic structures by the self-assembly of colloidal particles (especially colloidal particles that are spherical and nearly monodispersed in diameter).

However, methods have not been discovered for the elimination of the SiO.sub.2 spheres from the infiltrated structure, and the presence of these spheres can degrade the desired properties resulting from the infiltrated materials.

Devising an overall process that preserves the structure of the three-dimensional array of infiltrated material, while at the same time enabling the extraction of the SiO.sub.2 spheres, represents a higher level of difficulty which has not been addressed by the prior art.

The lack of more success in prior research reflects several generic issues.

However, extraction processes have not been successfully demonstrated for such thermally stable matrix materials.

Even if this topological problem could be solved, the unsolved problem still remains of conducting such extraction of a high-thermal-stability matrix material (like SiO.sub.2) without disrupting the structure of the infiltrated material.

However, this approach is generally unsatisfactory because of (1) inapplicability for materials that are most desirably infiltrated at high temperatures, (2) the difficulty of crystallizing the polymer spheres into well-ordered crystals having large dimensions, (3) the possible introduction of holes in the structure of the infiltrated material during gas evolution, (4) the occurrence of about 20-35% shrinkage of lattice parameter of the final structure relative to the initial structure, which can disrupt structural perfection, (5) inaccurate replication of the void space in the original structure (evident from the micrographs of the above references), (6) the lack of mechanical robustness of the polymer sphere assemblies (which again restricts the infiltration process), (7) the impossibility of obtaining complete filling of the void space of the original opal structure by the demonstrated chemical methods (so to obtain the volumetric inverse of the opal structure), and (8) the unsuitability of template removal by pyrolysis for the preparation of lattice structures comprised of thermally labile materials, such as polymers.

Barriers to application are provided by the lack of generality of this method, present inability to provide well-ordered materials of large dimensions by emulsion self-organization, the poor degree of order of the resulting product, and the large materials shrinkage during the drying step for the gel (about 50%).

Moreover, the prior art has not demonstrated the ability to create the complicated, multicomponent structures needed for advanced device applications.

In addition, there are no available methods in the prior art for making a material that is a fully filled volumetric inverse of the void space of an opal structure, and materials with such structures are required for the applications described herein.

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