Researchers Synthesize Quasi-2D Gold Film

Researchers from the Moscow Institute of Physics and Technology (MIPT) Center for Photonics and 2D Materials have synthesized a quasi-2D gold film, showing how materials not usually classified as two-dimensional can form atomically thin layers.

The researchers said 2D metals bring them closer to a new class of optical metamaterials whose potential to control light could lead to unexpected technologies.

The study showed that by using monolayer molybdenum disulfide as an adhesion layer, quasi-2D gold can be deposited on an arbitrary surface. The researchers said the resulting ultrathin gold films, which are just several nanometers thick, conduct electricity very well, and could be used for flexible and transparent electronics.

While many materials, such as the metals gold, silver, and copper, do not have a layered structure, they could still theoretically form 2D layers, which could be used in folding displays, e-paper and e-clothing, and lenses with built-in electronics.

Quasi-2D gold. Courtesy of Ella Maru Studio

Among other possible applications are ultrathin electrodes that could enable neural interfaces with the potential to solve medical problems and even integrate the nervous system of a living being with electronic devices.

Until recently, the only technology for depositing metal films on arbitrary surfaces yielded layers that were not thin enough. It involved thermally evaporating a 3D metal sample in a high vacuum. The evaporated metal particles then adhered to a silicon-based substrate, forming nanosized islands, which gradually grew and eventually closed the gaps between them.

This process yields relatively homogeneous films by the time they are 20 nm thick. Yet engineers require transparent films, which means they need to be more than two times thinner. Stopping the deposition earlier is not an option because the films still have too many gaps and inhomogeneities impairing their electrical conductivity. Similarly, a metal mesh is a worse conductor compared with a sheet of metal.

The researchers from MIPT began hypothesizing that 2D metals could be deposited on other 2D materials. Graphene was the first candidate, but gold exhibited poor wetting to it. As a result, gold was deposited in the form of pillars. This vertical mode of growth made closing the gaps in the film problematic.

The team then investigated metal film growth on 2D transition metal dichalcogenides. Specifically, molybdenum disulfide was used because sulfur compounds are known to be among the very few to form stable bonds with gold.

The MIPT researchers used thermal evaporation in a high vacuum to deposit thin gold films on a silicon substrate covered with silicon dioxide and a monolayer of molybdenum disulfide.

They then used electron and atomic force microscopy to compare the structure of these gold films at various thicknesses to analogous films grown on pure silicon dioxide without the molybdenum disulfide monolayer. The added 2D material interface resulted in continuous gold films with superior electrical conductivity at a lesser thickness of 3 to 4 nm.

Aleksey Arsenin, co-author of the study and head of the Center for Photonics and 2D Materials at MIPT, said the team expects that this is only the beginning of quasi-2D metal science.

“A while ago, these materials were inaccessible even to scientists,” he said. “With our technology, we can talk about the prospects they hold for flexible and transparent electronics. Hopefully, we will soon see it in production.”

The study was published in Advanced Material Interfaces, https://onlinelibrary.wiley.com/doi/10.1002/admi.201900196