The rapidly expanding world of atomically thin layered materials

New research by Wei Lu, professor of mechanical engineering, delves into the rapidly-expanding, but tiny, world of graphene and atomically thin layered materials. His paper, Nanoscale Probing of Interaction in Atomically Thin Layered Materials, was recently published in ACS Central Science. He discusses it in this Q & A.

What are atomically thin layered materials (ATLMs), and why do they represent such an exciting area of research right now?

Newly-emerged atomically thin layered materials such as graphene, hexagonal boron nitride and transition metal dichalcogenides have strong in-plane covalent bonding, but weak out-of-plane van der Waals interactions. As a result, they exhibit useful characteristics such as: high elasticity, extreme mechanical flexibility, visual transparency and superior electronic performance. That makes them ideal for modern devices like flexible biosensors, fast-charging lithium ion batteries, transparent touchscreen displays, flexible transistors, photodetectors and memory devices.

What are some of the difficulties in utilizing ATMLs for those kinds of practical applications?

We don’t always observe those valuable properties in the materials when they are in bulk form. So being able to work with them at the atomically thin level is key. Scalable, controllable and cost-effective production of ATLMs with high quality has remained challenging. That’s due to our poor understanding of interlayer van der Waal interactions —relatively weak forces between atoms and molecules that are distance-dependent—and the lack of precise experimental techniques for us to characterize them.

What is the focus of your research—what questions have you been trying to answer?

Our research aims to understand and control the interlayer interaction of ATLMs.

Over the past decade, two distinct strategies have emerged for producing atomically thin layered materials: a top-down and a bottom-up approach.The top-down method, which we employ, utilizes mechanical, chemical and electrochemical processes to overcome van der Waal forces between layers of the materials in their bulk crystal form. The bottom-up method depends on the chemical reaction of molecular building blocks to form covalently linked two-dimensional networks by means of catalytic and thermal processes. But this second approach can result in transfer-induced residues and randomly-distributed nanoflakes in the ATLM.

As such, top-down approaches, with their individual or combined external normal and lateral shear forces playing the dominant role in breaking bonds between layers, can be expected to produce the highest quality samples.

A detailed understanding of the interlayer behavior of ATLMs under precisely-controlled normal and shear loadings is still missing. It’s an essential step toward enhancing transfer efficiency and thickness uniformity of ATLM-based device features and controlling the number of printed flakes on the substrate more effectively.

In your recent paper in ACS Central Science, what methods did you use to work toward answers to your questions?

We combined conductive atomic force microscopy and molecular dynamics simulations to reveal the interaction of ATLMs down to nanoscale lateral dimension. The setup allowed us to quantify, for the first time, the effect of layer number and electric field on the dielectric constant of ATLMs with few-layer down to monolayer thickness.

What did you find?

Our conductive atomic force microscopy-assisted electrostatic technique showed that high-quality mono- and bilayer graphene is reliably produced from bulk crystals at significant yields only by the shear type of bond-breaking between layers. In contrast, the normal type of bond-breaking exhibits a very stochastic process mainly due to the coexistence of local delamination and interlayer twist. Our dielectric constant measurements also revealed a very weak dependence on the number of layers and the electric field. We further demonstrated that the effective dielectric constant of monolayer graphene can be engineered to provide a broad spectrum of responses through oxidation and thermal annealing.

How do you anticipate your findings will be used to move this area of research forward?

Our findings on the complex behavior of van der Waal interactions between graphene layers, as well as the way interlayer shear and normal bond-breakings change, are general and fundamental results. As such, they can be used for various types of techniques to exfoliate ATLMs from their bulk crystal. While we’ve specifically focused on few-layered graphene systems, our analyses should extend to the exfoliation of other new thin layered materials. That could lead to the effective production of two-dimensional materials for use in high-performance electronic and mechanical devices.