BACKGROUND

[0001] Graphene is a single-layer sp ² -hybridized 2D network of carbon atoms that conceptually serves as the basis of many important allotropes of carbon. It can be stacked to form 3D graphite, rolled to form 1D carbon nanotubes, and fused to form 0D fullerenes. Owing to its strongly delocalized electron configuration, graphene exhibits exceptional charge carrier mobility, thermal conductivity, mechanical strength, and chemical stability. However, like other nanomaterials, the properties of graphene depend on its size, atomic structure, and physical environment. Graphene and graphene-based materials have tailorable properties that can be exploited in a broad range of devices, including transistors, capacitors, electron field emitters, transparent conductors, sensors, catalysts, and drug delivery agents. Other 2D crystals, most notably boron nitride and molybdenum disulfide, have also been isolated.

[0002] Two-dimensional (2D) sp ² -bonded carbon exists in the form of graphene, buckyballs and carbon nanotubes (CNTs). Graphene is“flat” or 2D, fullerenes (“Buckyballs”) are spherical or 0D, and CNTs are tubes in 1D. Forming these materials in a singular, regular, repeatable structure has not previously been achieved. Superstructures of these materials may provide very strong, light, highly thermally and electrically conductive structures. Attempts have been made to fabricate sp ² -bonded sponges as shown in FIG. 1 A, however these structures are irregular with properties that vary with position.

[0003] The isolation of graphene via the mechanical exfoliation of graphite has sparked strong interest in two-dimensional (2D) layered materials. The properties of graphene include exceptional strength, and high electrical and thermal conductivity, while being lightweight, flexible and transparent. This opens the possibility of a wide range of potential applications, including high-speed transistors and sensors, barrier materials, solar cells, batteries, and composites.

[0004] Other classes of 2D materials of interest include transition metal dichalcogenide (TMDC) materials, hexagonal boron nitride (h-BN), as well as those based on Group 14 elements, such as silicene and germanene. The properties of these materials can range from semi- metallic, for example, NiTe ₂ and VSe ₂ , to semiconducting, for example, WSe ₂ and MoS ₂ , to insulating, for example, hexagonal boron nitride (h-BN).

[0005] Growth of regular 3D superstructures using sp ² -bonded carbon may address the shortcomings of the flexible sp ² carbons for 3D applications given that hexagonally arranged carbon is strong, chemically inert, electrically and thermally conductive, and optically transparent. Such 3D superstructures may find used in many applications from packaging, thin optically transparent electrically conductive strong thin films, and many more.

[0006] When a carbon atom is attached to three groups (or, as in the case of graphene, three other carbon atoms) and so is involved in three s bonds, it requires three orbitals in the hybrid set. This requires that it be sp ² hybridized.

[0007] An .s/; ² -hybridized bond has 33% 5 and 67% p character. The three sp ² hybrid orbitals point towards the comers of a triangle at 120° to each other. Each sp ² hybrid is involved in a s bond. The remaining p orbital forms the p bond. A carbon double bond may be viewed as a s + p bond.

SUMMARY

[0008] In one example, a microstructure comprises interconnected units. The units are formed of graphene tubes. The graphene tubes that form the microstructure may be arranged in an ordered structure and form symmetric patterns that repeat along the principal directions of three-dimensional space. The microstructure may comprise interconnected units including at least: a first unit formed of first graphene tubes; and a second unit formed of second graphene tubes. One or more of the second graphene tubes are connected to one or more of the first graphene tubes.

[0009] A method of forming a graphene microstructure comprises: photo-initiating the polymerization of a monomer in a pattern of interconnected units to form a polymer microlattice; removing unpolymerized monomer; coating the polymer microlattice with a metal; removing the polymer microlattice to leave a metal microlattice; depositing graphitic carbon on the metal microlattice; converting the graphitic carbon to graphene; and removing the metal microlattice. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] FIG. 1A is a schematic drawing of a fabrication process for a metal-based microlattice template in accordance with an example.

[0011] FIG. 1B is a flowchart for the fabrication process depicted schematically in FIG. 1 A. [0012] FIG. 2 is a schematic diagram of the fabrication of an electrically, thermally, or both thermally and electrically conductive composite by embedding a fabricated metal-based or graphene-based microlattice within an organic polymer matrix.

[0013] FIG. 3 is a schematic diagram of a surface functionalization methodology for functionalizing a fabricated metal-based microlattice.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0014] An organic/inorganic superstructure may be used as a template for the formation of a 3D metal superstructure that may then be used to grow graphitic carbon on the surface of the metal. The template may be fabricated through a self-propagating photopolymer waveguide technique (see, e.g., Xiaoyu Zheng et. al., Ultralight, Ultrastiff Mechanical Metamaterials; Science 344 (2014) 1373-1377 and T. A. Schaedler, et al., Ultralight Metallic Microlattices; Science 334 (2011) 962-965) in which an interconnected 3D photopolymer lattice may be produced upon exposure of an appropriate liquid photomonomer to collimated ETV light through a specifically designed mask that contains openings with certain spacing and size. The fabricated microlattice may then be coated by electroless copper or other suitable metal (e.g. Ni, Co, Au, Ag, Cu, and alloys thereof) followed by etching away the organic polymeric matrix (scaffold). The resulting metal-based microlattice may be then used as a template to grow the graphitic carbon. The thickness of the electroless plated metal may be controlled in the range of nanometer to micrometer by adjusting the plating time, temperature, and/or plating chemistry.

[0015] FIG. 1A schematically illustrates an example fabrication process of organic polymeric microlattices (scaffolds) prior to coating with electroless plating.

[0016] This description is of a“periodically structured” carbon nanostructure. Conventional carbon nanostructures are irregular and have much larger dimensions than those which may be achieved using the methodology described herein.

[0017] The process described herein may be used to create a regular array, and the superstructure dimensions (unit) and structure may be optimized for strength, thermal and other fundamental properties.

[0018] Several aspects of this procedure are noteworthy:

• it provides a regular structure with defined dimensions;

• it can form very thin metal (e.g. Ni, Co, Cu, Ag, Au) microlattices;

• it enables the formation of graphitic carbon on very thin metals by a surface-limited process for very thin metal wires or tubes.

[0019] The process described herein uses a polymeric structure as a template for such fabrication with the subsequent formation of a metal superstructure that may then be exposed to a hydrocarbon (e.g. methane, ethylene, acetylene, benzene) to form graphitic carbon, followed by etching of the metal from under the graphitic carbon using appropriate etchants such as, for example, FeCl ₃ or potassium permanganate.

[0020] Collimated light through a photomask or multi-photon lithography may be used in a photo-initiated polymerization to produce a polymer microlattice comprised of a plurality of interconnected units. Example polymers include polystyrene and poly(methyl methacrylate) (PMMA). After being polymerized in the desired pattern, the remaining un-polymerized monomer may be removed.

[0021] The polymer structure (polymer scaffold) may then be plated with a suitable metal using an electroless plating process.

[0022] Electroless nickel plating (EN) is an auto-catalytic chemical technique that may be used to deposit a layer of nickel-phosphorus or nickel-boron alloy on a solid workpiece, such as metal, plastic, or ceramic. The process relies on the presence of a reducing agent, for example hydrated sodium hypophosphite (NaPC^FE · H ₂ 0) which reacts with the metal ions to deposit metal. Alloys with different percentages of phosphorus, ranging from 2-5 (low phosphorus) to up to 11-14 (high phosphorus) are possible. The metallurgical properties of the alloys depend on the percentage of phosphorus.

[0023] Electroless plating has several advantages over electroplating. Free from flux-density and power supply issues, it provides an even deposit regardless of workpiece geometry, and with the proper pre-plate catalyst, may deposit on non-conductive surfaces. In contradistinction, electroplating can only be performed on electrically conductive substrates.

[0024] Before performing electroless plating, the material to be plated must be cleaned by a series of chemicals; this is known as the pretreatment process. Failure to remove unwanted "soils" from the part's surface results in poor plating. Each pretreatment chemical must be followed by water rinsing (normally two to three times) to remove chemicals that may adhere to the surface. Degreasing removes oils from surfaces, whereas acid cleaning removes scaling.

[0025] Activation may be done with an immersion into a sensitizer/activator solution - for example, a mixture of palladium chloride, tin chloride, and hydrochloric acid. In the case of non-metallic substrates, a proprietary solution is often used.

[0026] The pretreatment required for the deposition of metals on a non-conductive surface usually consists of an initial surface preparation to render the substrate hydrophilic. Following this initial step, the surface may be activated by a solution of a noble metal, e.g., palladium chloride. Electroless bath formation varies with the activator. The substrate is then ready for electroless deposition.

[0027] The reaction is accomplished when hydrogen is released by a reducing agent, normally sodium hypophosphite (with the hydrogen leaving as a hydride ion) or thiourea, and oxidized, thus producing a negative charge on the surface of the part. The most common electroless plating method is electroless nickel plating, although silver, gold and copper layers can also be applied in this manner.

[0028] In principle, any hydrogen-based reducing agent can be used although the redox potential of the reducing half-cell must be high enough to overcome the energy barriers inherent in liquid chemistry. Electroless nickel plating most often employs hypophosphite as the reducer while plating of other metals like silver, gold and copper typically makes use of low-molecular- weight aldehydes.

[0029] A benefit of this approach is that the technique can be used to plate diverse shapes and types of surfaces.

[0030] As illustrated in FIG. 1B, the organic polymeric microlattice may be electrolessly plated with metal followed by dissolving out the organic polymer scaffold. The resulting metal- based microlattice may then be coated with a thin layer of immersion tin to prevent the metal from oxidizing during the subsequent process which may include a heat treatment. Alternatively, the surface of the metal-based microlattice may be functionalized with appropriate functional groups to provide anchoring or reactions sites for subsequent interaction with a polymer matrix. A copper/nickel super-lattice may be used. The fabricated and surface-treated metal-based microlattice may be embedded within an organic polymeric matrix to fabricate an electrically or thermally or both an electrically and thermally conductive composite (see FIG. 2). Alternatively, the fabricated metal-based microlattice may be used as a template to synthesize a graphitic carbon superstructure. The metal may then be etched out to produce a graphene microstructure comprising a plurality of interconnected units, wherein the units are formed of graphene tubes that are interconnected by chemical electronic bonds (as distinguished from van der Waals forces which may cause carbon nanotubes to agglomerate).

[0031] FIG. 3 schematically illustrates a surface treatment methodology for functionalizing the fabricated metal-based microlattice. As it can be seen, the surface of the metal microlattice may be exposed to a mercaptan-based compound. The mercaptan-based compound may be a hydroxylated alkyl mercaptan such as 3-mercapto-l,2 dihydroxy propane or it may be based on an isocyanate function group such as isocyanate alkyl trialkoxy silane. The mercaptan-based compound bonds to the metal (such as e.g. copper, silver or gold) through the sulfur atom of the mercapto moiety resulting in a hydroxylated metal surface. The hydroxyl functional groups implanted on the metal surface may then be reacted with a reactive functional group from the pre-polymer matrix. In the case of isocyanate-based mercaptan compounds, the free isocyanate functional groups may subsequently be reacted with certain functional groups of the organic polymeric matrix such as -OH, -NH, etc. resulting in the formation of chemical bonds at the copper-organic polymer interface. The polymer for the composite may be selected for its mechanical properties and/ or electronic properties. Example polymers include fluorocarbons (such as polytetrafluoroethylene) and polybutadiene.

[0032] In an example, the growth of graphene on the metal tubes may be omitted to produce a mircolattice of interconnected metal tubes. A process for forming such a metal microstructure may comprise: photo-initiating the polymerization of a monomer in a pattern of interconnected units to form a polymer microlattice; removing unpolymerized monomer; coating the polymer microlattice with a metal; and removing the polymer microlattice to leave a microlattice of interconnected metal tubes in a pattern of interconnected units.

[0033] In another example, a metal/graphene microstructure of graphene-coated metal tubes in a pattern of interconnected units may be prepared by the process comprising the steps of: photo- initiating the polymerization of a monomer in a pattern of interconnected units to form a polymer microlattice; removing unpolymerized monomer; coating the polymer microlattice with a metal; removing the polymer microlattice to leave a metal microlattice; depositing graphitic carbon on the metal microlattice; and converting the graphitic carbon to graphene.

[0034] Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims.