Gold tape proves invaluable at extracting 2D sheets from bulk crystals

Two-dimensional (2D) materials created from layered van der Waals (vdW) crystals hold great promise for use in electronic, optoelectronic and quantum devices. But their production has been limited by the lack of high-throughput techniques for exfoliating single-crystal monolayers with sufficient size and high quality. Now, in a paper in Science, researchers at Columbia University report a new method – using ultraflat gold films – for disassembling vdW single crystals layer-by-layer to produce monolayers with near-unity yield and with dimensions limited only by the size of the bulk crystal.

The monolayers generated using this technique have the same high quality as those created by conventional ‘Scotch tape’ exfoliation, but are roughly a million times larger. They can be assembled into macroscopic artificial structures with properties not easily created in conventionally grown bulk crystals.

For instance, layers of molybdenum disulfide can be aligned with each other so that the resulting stack lacks mirror-symmetry. As a result, the stack demonstrates a strongly nonlinear optical response, where it absorbs red light and emits ultraviolet light, a process known as second harmonic generation.

"This approach takes us one step closer to mass production of macroscopic monolayers and bulk-like artificial materials with controllable properties," says co-principal investigator James Hone, professor of mechanical engineering at Columbia Engineering.

The discovery 15 years ago that single atomic sheets of carbon, known as graphene, could be easily separated from bulk crystals of graphite and studied as perfect 2D materials was recognized with the Nobel Prize in Physics in 2010. Since then, researchers worldwide have studied the properties and applications of a wide variety of 2D materials, and learned how to combine individual 2D sheets into stacked heterostructures that are essentially new hybrid materials. But while the original Scotch tape method developed for graphene, which uses an adhesive polymer to pull apart the crystals, is easy to implement, it is not well-controlled and produces 2D sheets of limited size – typically tens of micrometers across.

A major challenge for the field and future manufacturing is how to scale up this process to much larger sizes in a deterministic process that produces 2D sheets on demand. The dominant approach to scaling up the production of 2D materials involves growing thin films, which has yielded great successes but still faces challenges in terms of material quality, reproducibility and the temperatures required. Other research groups have pioneered the use of gold to exfoliate large 2D sheets, but their approaches either leave the 2D sheets on the gold substrates or require intermediate steps for evaporating hot gold atoms that can 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," says the lead principal investigator Xiaoyang Zhu, professor of nanoscience in Columbia's department of chemistry. "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 researchers took their cue from the Nobel prize-winning Scotch tape method, but replaced the adhesive polymer tape with an ultraflat gold tape. The atomically flat gold surface adheres strongly and uniformly to the crystalline surface of a 2D material and disassembles it layer-by-layer. The layers are the same size and dimension as the original crystal – providing a degree of control far beyond what can be achieved with scotch tape.

"The gold tape method is sufficiently gentle that the resulting flakes have the same quality as those made by Scotch tape technique," says postdoctoral scholar Fang Liu, the lead author of the paper. "And what is especially exciting is that we can stack these atomically thin wafers in any desired order and orientation to generate a whole new class of artificial materials."

Motivated by recent exciting advances in ‘twistronics’, the team is now exploring adding a small rotation between layers in these artificial materials. In doing so, they hope to achieve on a macro-scale the remarkable control over quantum properties such as superconductivity that have recently been demonstrated in micrometer-sized flakes. They are also working to broaden their new technique into a general method for all types of layered materials, and looking at potential robotic automation for large scale manufacturing and commercialization.

This story is adapted from material from Columbia University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.