Surface engineering over multiple length scales is critical for electronics, photonics, and enabling multifunctionality in synthetic materials. Here, we demonstrate a sequential embossing technique for building multi-tier patterns in metals by controlling the size-dependent thermoplastic forming of metallic glasses. Sub-100 nm to millimeter sized features are sculpted sequentially to allow an exquisite control of surface properties. The process can be integrated with net-shaping to transfer functional patterns on three-dimensional metal parts.

Remarkable progress has been made in fabrication and characterization of metal nanostructures because of their crucial role in energy conversion, nanophotonics, nanoelectronics, and biodiagnostics. Less emphasis has been placed on the synthesis of nanostructures from metallic alloys, which are better suited than elemental metals for certain applications such as fuel-cell catalysts. The main challenges in fabrication of alloy nanostructures are controlling their chemical stoichiometry, crystal structures, and shapes because of anisotropic nucleation and growth rates. These limitations can be overcome by using metallic glasses (amorphous metal alloys) which are isotropic and provide additional control handles through their tunable compositions and degree of crystallinity. Here, we review the recent developments in fabrication and characterization of metallic glass (MG) nanostructures. The focus is on sub-micron structures synthesized by unconventional thermoplastic techniques. A concept of self-assembly is introduced for fashioning functional structures using MG nanostructures as building blocks. The article concludes with a brief discussion about unique properties and prospective applications of MG nanostructures.

Thermoplastic embossing of metallic glasses promises direct imprinting of metal nanostructures using templates. However, embossing high-aspect-ratio nanostructures faces unworkable flow resistance due to friction and non-wetting conditions at the template interface. Herein, we show that these inherent challenges of embossing can be reversed by thermoplastic drawing using templates. The flow resistance not only remains independent of wetting but also decreases with increasing feature aspect-ratio. Arrays of assembled nanotips, nanowires, and nanotubes with aspect-ratios exceeding 1000 can be produced through controlled elongation and fracture of metallic glass structures. In contrast to embossing, the drawing approach generates two sets of nanostructures upon final fracture; one set remains anchored to the metallic glass substrate while the second set is assembled on the template. This method can be readily adapted for high-throughput fabrication and testing of nanoscale tensile specimens, enabling rapid screening of size-effects in mechanical behavior.

The artificial meniscus made by a double network hydrogel was recovered by a two-step healing process.