Two-dimensional materials from layered van der Waals (vdW) crystals hold great promise for electronic, optoelectronic, and quantum devices, but making/manufacturing them has been limited by the lack of high-throughput techniques for exfoliating single-crystal monolayers with sufficient size and high quality. Columbia University (New York, NY) researchers report today in Science that they have invented a new method--using ultraflat gold films--to disassemble vdW single crystals layer by layer into monolayers with near-unity yield and with dimensions limited only by bulk crystal sizes.
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. The monolayers 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 and as a result demonstrates strongly nonlinear optical response, where it absorbs red light and emits ultraviolet light, a process known as second harmonic generation.
The discovery 15 years ago that single atomic sheets of carbon--graphene--could be easily separated from bulk crystals of graphite and studied as perfect 2D materials was recognized with the 2010 Nobel prize in physics. Since then, researchers worldwide have studied properties and applications of a wide variety of 2D materials and learned how to combine these layers into stacked heterostructures that are essentially new hybrid materials themselves. The original scotch tape method developed for graphene, which uses an adhesive polymer to pull apart crystals, is easy to implement but is not well-controlled and produces 2D sheets of limited size--typically tens of micrometers across.
“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 PI Xiaoyang Zhu, Howard Family 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 and developed an ultraflat gold tape instead of the adhesive polymer 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 is achievable using scotch tape.
The work was carried out in the Center for Precision Assembly of Superstratic and Superatomic Solids, a Materials Science and Engineering Research Center funded by the National Science Foundation and led by Hone. The research project used shared facilities operated by the Columbia Nano Initiative.
Motivated by recent exciting advances in “twistronics,” the team is now exploring adding 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.
SOURCE: Columbia University; https://engineering.columbia.edu/press-releases/hone-zhu-isolate-atomic-sheets
