Making two-dimensional metals the easy way

A simple, easy way to make large, freestanding but very thin sheets of metallic materials could open up novel applications in catalysis, flexible electronics, and soft robotics, according to researchers at City University of Hong Kong [Wang et al., Materials Today (2020), https://doi.org/10.1016/j.mattod.2020.02.003].

Two-dimensional materials like graphene and MoS₂ have attracted great interest recently because of their unique physical and chemical properties that promise to be invaluable in a range of areas from sensing to separation. But freestanding metallic films are much more difficult to produce because bonding in metals is inherently three-dimensional. Single-layered 2D metals can be produced, but typically are only stable when less than 2 nm in lateral size. Thicker metal films, which are considered ‘two-dimensional’ because surface effects dominate their properties, can be synthesized via various top-down or bottom-up approaches. Although larger freestanding 2D metals can be produced in this manner, the size and range of materials is limited.

“The majority of 2D metals reported [to date] were mainly fabricated [using] wet-chemical methods, with a few layered structures fabricated through mechanical exfoliation. In general, these methods are limited to elemental metals with very small in-plane sizes (less than a few micrometers),” explains Yong Yang, who led the research. “So we asked ourselves, can we make 2D metals as chemically complex as 3D metals? And can the 2D metals be as large as 3D metals?”

The answer appears to be yes. The straightforward approach devised by Yang and his team is purely mechanical. A thin layer of a metallic material is first deposited on a hydrogel substrate using conventional physical vapor deposition techniques. When the metal-topped hydrogel is put into water it swells and deforms, exfoliating the metal film.

“We [have] designed a new and facile method to synthesize chemically complex freestanding metallic nanomembranes, known as 2D metals, without any physical restrictions on their in-plane dimension or chemical composition,” he says.

The team produced up to millimeter-sized thin membranes of pure Ti, the high entropy alloy FeCoNiCrNb, and the metallic glass ZrCuAlNi. But other non-layered materials such as ceramics, semiconductors, polymers, and even composites could be produced in the same way. The thinnest membranes the researchers produced were TiO₂ just 5 nm thick, but alternative techniques, such as atomic layer deposition, could produce even thinner films. Not only is the approach extremely versatile and widely applicable, it is also cheap and accessible.

“With this new method, we expect that we can further broaden the applications of 2D metals to other areas, such as soft robotics, filtration, composite materials, and biomedical engineering,” says Yang. “We may have opened a window to an unexplored and interesting world of low-dimensional materials.”