Nano-jackhammer of Ultrasonic Imprinting

A scalable, fast, energy efficient and cost-effective method for producing nanostructures out of crystalline and amorphous metals at room-temperature is highly desirable but yet to be demonstrated. Ultrasonic power could be precisely directed to very localized region by “energy director” that could assist nanoimprinting of crystalline and amorphous metallic nanostructures. Ultrasonic embossing/nanoimprinting has been widely used in nanoimprinting of plastics and limited metal microstructure formation. Nanoimprinting of crystalline or amorphous metal nanostructure down to sub-100nm at low temperatures remains a challenge due to the much higher strength of crystalline metal than plastic at room temperature.

This technology presents a nanoimprinting technique that works at room-temperature (and also at low or elevated temperatures if required) in one-step within a couple of minutes. This room-temperature ultrasonic nanoimprinting technique (RTUN) makes use of ultrasonic power to drive the metal to flow into the nanostructured mold, offering us flexibility to make nanostructures of almost all metals regardless of their melting points. Moreover, the mold can be recycled to further decrease the overall cost. RTUN also offers a unique method to make metal nanodisk, nanorod and nanowires, that consists of designed multiple heterojunctions of distinct metals (and different materials) that is not achievable with existing technologies. RTUN also could make nanowires of solid polymers/plastics, as well as metal nanostructures on flexible substrates.

Microscale forming/embossing of (poly)crystalline metal is widely used in industry. One limitation of current technologies come from grain size effect when the structure feature size is comparable to the metal grain [> micrometers in (poly)crystalline metals], when the metal starts to behave anisotropically. Moreover, the friction significantly increases when the feature size decreases, resulting in much more difficult processing. This physical constraint could be partially overcome by increasing the process temperature that could lead to dislocation climbing, diffusion, etc., in superplastic regime.

A study was previously conducted for a mechanism on corporative plastic deformation of adjacent grains in polycrystalline metal that could lead to sub-micron metal formation of polycrystalline metals. Despite these extensive studies of metal forming/embossing, direct forming of (poly)crystalline and amorphous metal down to sub-100nm feature size at room temperature is impossible with current technologies, and is yet to be demonstrated. Therefore, one of the features of this technology is to breakthrough this constraint at low temperatures, and preferably at room temperature; enabling many new technology/applications as elaborated in the following section.

Heterojunctions find wide applications in catalysis, including hydrogenation of carbon dioxide to methanol, water-gas shifting, dry reforming of methane, direct methane-to-methanol conversion, and other chemical synthesis processes, as well as transport such as carbon monoxide oxidation (to remove carbon monoxide in emitted gas) in vehicles, and hydrogen electrolyzer and fuel cell in electrical vehicles.

Metal nanostructures find wide applications in areas of electronics, photonics, plasmonics, biosensing, bioimaging and catalysis. Technology for facile, cost-effective, and scalable manufacturing metal nanostructures is a pursuit in modern science and advanced manufacturing technology. This technology could widen the applications of metal nanostructures in optoelectronic, biomedical, pharmaceutical, energy and environmental industries. 

1.  From Product Perspective
1.1 Materials

• Solid metals
• Solid polymers
• Plastics
• Metal compounds including oxides, sulfides, phosphides, carbides etc.

1.2 Patterns

• Nanowire
• Nanorod
• Nanodisk
• Heterojunctions of different metals, metal compounds-metal, and metal compounds-polymer in nanowire, nanorod and nanodisk
• Homojunction of different metal grains of the same metal in nanowire, nanorod and nanodisk
• Free standing nanostructures
• Nanostructure arrays on substrate

2. From Application Perspective
2.1  Biosensing / Bioimaging / Biomedical / Pharmaceutics

• Biomolecular sensing including DNA sequencing
• Bioimaging with metal or metal compound nanostructures
• Pharmacy synthesis involving metal nanostructure catalysts

2.2 Catalysis

• Nanostructure catalyst powder, composites and electrodes for various chemical reactions and synthesis
• Electrochemical catalytic reactions, thermocatalytic reactions, photocatalytic reactions or combination of them

2.3 Transparent Electrode

• Nanostructures for transparent electrode fabrication including Ag, Au, Cu, etc.

2.4 Optoelectronics

• Free standing plasmonic structures or those on substrate
• Light trapping nanostructures
• Nanostructure interconnects

The RTUN technique makes it possible to fabricate heterojunctions of metal-metal and metal-metal compound in a scalable manner. The room-temperature process will retain the high-index facets of crystal that are thermodynamically unstable at high temperature, but crucial for enhanced catalytic activity in catalysis. It is worth noting that most metallic catalysts are reactive metals that are not suitable for high-temperature process to avoid oxidation, alloying or phase change. Thus, this invention provides unique features for processing reactive metals, metal junctions and other structures. Additionally, this technique has been demonstrated to be applicable in making complex ‘designer’ plasmonic nanostructures for biosensing and imaging.