Producing large, clean 2D materials made easy: just KISS

Newswise — Since the revelation of the 2D variant of graphite (dubbed graphene) nearly two decades ago, the fascination with 2D materials and their distinctive physical properties has surged. Notably, graphene emerged from the exfoliation of bulk graphite via adhesive tape. While this approach garnered a Nobel Prize, it possesses inherent limitations. Presently, an international group of surface scientists has devised a straightforward technique to generate sizable and exceptionally pure 2D samples from various substances, utilizing three distinct foundations. Their approach, known as kinetic in situ single-layer synthesis (KISS), was detailed in the Advanced Science journal on 1 June.

The unique physical properties of 2D materials, distinct from those of bulk materials, arise due to the confinement of charge carriers. Two methods exist for producing these 2D materials: exfoliation of a larger crystal or growth of a 2D layer. Exfoliation involves the gradual removal of layers from a larger crystal until only a single layer remains. According to Antonija Grubišić-Čabo, a surface scientist at the University of Groningen (the Netherlands) and the primary author of the Advanced Science paper, this process is time-consuming and demands specific expertise and equipment. Furthermore, it often yields minuscule flakes, with the adhesive tape used leaving behind polymer residues on their surfaces.

Gold

An alternative approach is the growth of 2D films, which enables the production of substantial samples with precise control over the conditions. However, the growth of such 2D materials often necessitates a considerable amount of time and effort to achieve desirable results. As Grubišić-Čabo explains, "the process doesn't always yield a flawless layer." To address this challenge, Grubišić-Čabo, along with Maciej Dendzik, the final author, formed a team comprising accomplished colleagues. Many of these individuals had previously collaborated during their time as PhD students at Aarhus University (Denmark). Their collective objective was to devise a straightforward technique for manufacturing 2D materials.

The team was aware of previous experiments involving the use of gold films for exfoliating bulk material. However, these experiments were predominantly conducted in an air environment, making the technique unsuitable for air-sensitive materials or surface science research. Therefore, the team aimed to develop a method that would enable the production of air-sensitive 2D materials on various substrates. In their initial attempt, they employed a gold crystal within a high vacuum chamber. Surprisingly, when the crystal was forcefully applied to the bulk material, they observed the formation of a well-adhered 2D layer on the gold surface. Although the exact mechanism behind this phenomenon remains unclear, the team hypothesizes that the bond between the gold and the 2D layer is stronger than the Van der Waals forces that hold the layers together within the bulk crystal.

Devices

Building upon their initial experiment, the team made improvements by incorporating a spring into the stage containing the bulk material. This spring acted as a shock absorber, enabling better control over the impact of the gold crystal. Additionally, the team demonstrated that both silver and the semiconductor germanium could serve as substrates for peeling off 2D materials. Grubišić-Čabo explains that gold crystals are commonly found in surface science laboratories for instrument calibration, and scientists are usually cautious about damaging them. However, no such damage occurred during these experiments. Furthermore, the team has modified the protocol to utilize single crystal gold thin films. This modification offers an additional advantage: the ability to dissolve the gold and isolate the 2D sample, as long as the sample remains stable in either air or liquid environments.

The isolated samples obtained through the KISS technique hold potential for the subsequent stage: constructing devices utilizing the 2D materials. While this stage is not yet feasible, the team is actively engaged in its development. Grubišić-Čabo explains, "We are working on it." Currently, the team possesses a technique that enables the production of sizable and highly pure 2D samples in a straightforward manner. Importantly, this method allows for the creation of air-sensitive 2D materials. Additionally, the technique utilizes standard equipment that is readily available in almost every surface science laboratory.

Reference: Antonija Grubišić-Čabo, Matteo Michiardi, Charlotte E. Sanders, Marco Bianchi, Davide Curcio, Dibya Phuyal, Magnus H. Berntsen, Qinda Guo, Maciej Dendzik: In situ exfoliation method of large-area 2D materials. Advanced Science, 1 June 2023.