The set-up for kinetic in situ single-layer synthesis (KISS). The bulk material is placed on a sample holder with a spring to regulate the impact (yellow arrow). It is then pressed against the gold crystal (the slightly brighter ring below the blue arrow). After release, a 2D layer will be attached to the gold substrate. Image: Antonija Grubišic-Cabo, University of Groningen.Ever since the discovery of the two-dimensional form of graphite (called graphene) almost 20 years ago, interest in two-dimensional (2D) materials has skyrocketed. Famously, graphene was produced by exfoliating bulk graphite using sticky tape. Although it was good enough for a Nobel Prize, this method has its drawbacks.
An international team of surface scientists has now developed a simple method for producing large and very clean 2D samples from a range of materials on three different substrates. The scientists report their method – kinetic in situ single-layer synthesis (KISS) – in a paper in Advanced Science.
The great advantage of 2D materials is that they have physical properties that are not shared by bulk versions of the materials. The confinement of charge carriers is one reason for this.
There are two ways to produce 2D materials: exfoliate them from a larger crystal or grow a 2D layer from scratch. Exfoliation means peeling off layers from a larger crystal until you are left with just a single layer.
“This process is time-consuming and requires specific skills and equipment,” says Antonija Grubišic-Cabo, a surface scientist at the University of Groningen in the Netherlands and first author of the paper. “Furthermore, it often results in very small flakes, while the adhesive tape that is used can leave polymers on their surfaces.”
The other option is to grow 2D materials under controlled conditions. “However, it often takes a lot of time to work out how to grow such 2D materials,” says Grubišic-Cabo. “And the process doesn’t always result in a perfect layer.”
Together with last author Maciej Dendzik, Grubišic-Cabo assembled a ‘dream team’ of colleagues, many of whom had previously worked together at Aarhus University in Denmark as PhD students, to develop a simple technique for producing 2D materials.
“We knew of some experiments in which gold films were used to exfoliate bulk material. But these were mainly performed in air, which means that this technique is not very suitable for air-sensitive materials, or for surface science research.”
The team wanted a technique that would allow the production of air-sensitive 2D materials on a range of substrates. In their first attempt, they used a gold crystal in a high vacuum chamber.
“We basically slammed the crystal on bulk material and discovered that a nice 2D layer stuck to the gold,” says Grubišic-Cabo. Why this happens is not yet clear, but the team suspects that the bond that forms between the gold crystal and the material is stronger than the Van der Waals force that keeps the layers in the bulk material together.
They then built on that experiment, by adding a spring to the stage that holds the bulk material. This spring acts as a shock absorber and thus allows better control of the impact of the gold crystal. Furthermore, the team showed that silver and the semiconductor germanium could also be used as substrates to peel off 2D materials.
“Gold crystals are a standard feature in surface science labs, where they are used in the calibration of instruments, for example,” Grubišic-Cabo explains. “Scientists don’t like to damage these crystals, but that didn’t happen in these experiments. And we have since changed the protocol to use single-crystal gold thin films. This has the added advantage of being able to dissolve the gold so that we can isolate the 2D sample, as long as it is stable in air or liquid.”
These isolated samples may be used for the next stage: building devices from the 2D materials produced using the KISS technique. “This is not yet possible, but we are working on it,” says Grubišic-Cabo. “So, what we do have is a technique to produce very clean, large 2D samples in a very simple way, which allows us to create air-sensitive 2D materials. Furthermore, our technique uses standard equipment that is present in virtually every surface-science laboratory.”
This story is adapted from material from the University of Groningen, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.