Two-dimensional materials remain a hot topic and finding ways to make them from layered van der Waals crystals could open up new applications in electronics, optoelectronics, and quantum devices. Now, researchers Columbia University, New York, have used ultra-flat gold tape to exfoliate vdW single crystals layer by layer into new monolayers. The obtain yields close to unity and the size of their 2D layers are limited only by the dimensions of the original bulk crystal; up to the centimeter scale. The researchers report that "Multilayers could be reassembled to artificial structures, such as a MoSe2/WSe2 single-crystal bilayer with a twist angle chosen to quench intralayer exciton formation. [Liu, F. et al., Science, (2020) 367, 6480: 903; DOI: 10.1126/science.aba1416]

The approach is reminiscent of the original sticky tape experiments that led to the first production of graphene, made by peeling off a monolayer from a pencil mark on a glass slide. The areas possible with this newly reported gold tape exfoliation are, however, approximately a million times larger. The team explains that they can assemble the monolayers into macroscopic artificial structures. These structures are inaccessible through conventional bulk crystal growth. For instance, they can produce layers of molybdenum disulfide that are aligned with each other. The resulting stacks lack mirror symmetry and so demonstrate strongly nonlinear optical responses. In that particular example, the material absorbs red light and emits ultraviolet light through second-harmonic generation.

"This approach takes us one step closer to mass production of macroscopic monolayers and bulk-like artificial materials with controllable properties," explains one of the principal investigators on the work, James Hone. The work represents a new way to scale up the production of 2D materials that has not been possible with conventional approaches so far where thin film growth has various limitations in terms of material quality, reproducibility, and the requirement for high temperatures. Other teams have used gold to exfoliate large 2D sheets but those earlier approaches needed intermediate steps and had to evaporate gold subsequently, which could easily damage the 2D materials.

"In our study, we were inspired by the semiconductor industry, which makes the ultrapure silicon wafers used for computer chips by growing large single crystals and slicing them into thin disks," explains lead investigator Xiaoyang Zhu. "Our approach does this on the atomic scale: we start with a high-purity crystal of a layered material and peel off one layer at a time, achieving high-purity 2D sheets that are the same dimensions as the parent crystal."

The team is now looking into "twistronics" where they add a small rotation between layers in the artificial materials to achieve macro control over quantum properties.