A team of researchers has found a novel method for the construction of high-quality van der Waals (vdW) heterostructures, that are vital for many scientific studies and technological applications of layered materials. The work is a collaboration between the laboratory of Davood Shahrjerdi, a professor of Electrical and Computer Engineering at the NYU Tandon School of Engineering and a faculty member of NYU WIRELESS; a group led by Javad Shabani at the Center for Quantum Phenomena, New York University; and Kenji Watanabe and Takashi Taniguchi of National Institute for Materials Science, Japan.
A crucial step for building vdW graphene heterostructures is the production of large monolayer graphene flakes on a substrate, a process called mechanical exfoliation. The process then involves transferring the graphene flakes onto a target location for the assembly of the vdW heterostructure. An optimal substrate would therefore make it possible to efficiently and consistently exfoliate large flakes of monolayer graphene and subsequently release them on-demand for constructing a vdW heterostructure.
The research team applied a simple yet elegant solution to this challenge involving the use of a dual-function polymeric film with a thickness of below five nanometers. This modification allows them to “tune” the film properties such that it promotes the exfoliation of monolayer graphene. Then, for the Lego-like assembly, they dissolve the polymeric film underneath the monolayer graphene using a drop of water, freeing graphene from the substrate.
“Our construction method is simple, high-yield, and generalizable to different layered materials,” explained Shahrjerdi. “It enabled us to optimize the exfoliation step independently of the layer transfer step and vice versa, resulting in two major outcomes: a consistent exfoliation method for producing large monolayer flakes and a high-yield layer transfer of exfoliated flakes. Also, by using graphene as a model material, we further established the remarkable material and electronic properties of the resulting heterostructures.”