Making Big and Pristine 2D Materials is Effortless: Simply ‘KISS’

Since the discovery of the two-dimensional form of graphite (called graphene) nearly twenty years ago, interest in the two-dimensional material and its special physical properties has skyrocketed. Graphene has been famously produced by exfoliating loose graphite using adhesive tape. Although it was good enough to get a Nobel Prize, this method has its drawbacks.

An international team of surface scientists has developed a simple method for producing very clean, large 2D samples from a range of materials using three different substrates. Their method, monolayer kinetic in situ synthesis (KISS) is described in the journal advanced science on June 1.

Two-dimensional materials have physical properties that bulk materials do not share. The confinement of freight carriers is one reason for this. There are two ways to produce these two-dimensional materials: exfoliating a larger crystal or growing a two-dimensional layer. Peeling means peeling away the layers from a larger crystal until only one layer is left.

“This process is time-consuming and requires specific skills and equipment,” says Antonia Grubschich-Schabo, a surface scientist at the University of Groningen (Netherlands) and first author of the book advanced science paper. “Moreover, it often results in very small flakes, whereas the adhesive tape used can leave polymers on their surfaces.”

2D film cultivation is another method. This allows large samples to be produced under controlled conditions. “However, it often takes a long time to figure out how to grow such two-dimensional materials. The process does not always result in a perfect layer,” says Grubiši-Čabo. She and her most recent author, Maciej Dandzik, have put together a “dream team” of colleagues, many of whom had previously worked together at Aarhus University (Denmark) on their Ph.D. students, to develop a simple technique for producing two-dimensional materials.

“We know of some experiments where gold foil was used to exfoliate bulk materials. But they were mainly done in air, which means that this technique is not very suitable for air-sensitive materials, or for surface science research,” Grubiši-Čabo notes.

The team wanted a technology that would allow the production of air-sensitive 2D materials on a range of substrates. On their first attempt, they used a gold crystal in a high vacuum chamber. “We shocked the crystal onto a bulk material and discovered that a nice two-dimensional layer adhered to the gold,” says Grubišić-Čabo. It’s not yet clear why this happens, but the team suspects that the bond with the gold is stronger than the Van der Waals force that keeps the layers in the crystal block together.

They built on this first experience, adding a spring to the stage with the loose material acting as a shock absorber, thus allowing better control of the golden crystal effect. Moreover, the team showed that both semiconducting silver and germanium can be used as substrates for exfoliation of 2D materials.

“Gold crystals are a standard feature in surface science laboratories, where they are used in the calibration of instruments, for example. Scientists don’t like to damage these crystals, but that didn’t happen in these experiments,” says Grubich-Shabo. “We have since changed the protocol to use thin films of single crystal gold. This has the added advantage of being able to melt the gold so that we can isolate the sample in 2D, as long as it is stable in air or liquid.”

These isolated samples can be used for the next stage: constructing devices from 2D materials to be produced using KISS technology. “It’s not possible yet, but we’re working on it,” says Grubiši-Čabo. “So, what we have is a technology for producing very clean, large 2D samples in a very simple way, which allows us to create air-sensitive 2D materials. Furthermore, our technology uses standard equipment found in almost every surface science lab.”