Toward the Systematic Control of the Exfoliation of Atomically Thin Layered Materials by Electrostatics.

The debut of single and few-layer graphene (FLG) in the past decade has surprised the physics community and led to numerous research studies on 2D materials embodied as atomically thin layered materials (ATLMs).[1,2] Beyond exceptional electronic conductivity, FLG presents various interesting physical properties, including interface-layer-independent heat transfer[3] and wetting transparency.[4] Despite this interest, precisely controlling ATLM quality and thickness has remained challenging. The work of Rokni et al. sheds light on the way to predictably produce ATLMs by shearing mechanical exfoliation facilitated by electrostatic force.[5]

It is tempting to embrace bottom-up strategies such as chemical vapor deposition (CVD) for the mass production of ATLMs, but the quality of the resulting ATLMs is compromised due to the necessary elevated temperatures, which augment the effect of diverging entropy with 2D grain size,[6] as well as the residue strain during growth and transfer.[7] It is commonly believed that mechanical exfoliation methods produce the highest quality graphene. However, the process is not easily controlled and the yield is low. Electrostatic force has been used previously to facilitate graphene peeling from highly oriented pyrolytic graphite (HOPG),[8] where a voltage is applied across the capacitor formed by the Si/SiO2/HOPG sandwich structure, and opposite charges between Si and HOPG lead to an attractive interaction that anchors HOPG to the SiO2 substrate. Meanwhile, like charges distribute between graphene layers, which introduces interlayer repulsion, leading to nonuniformly distributed interlayer interaction that can be manipulated for producing graphene of the target thickness. However, the electric voltage was heuristically determined and the electrostatic force has not been theoretically predicted.[8]

Computational tools, including finite element methods (FEM) and molecular dynamics (MD), reveal the mechanisms of electrostatic force manipulation. The work by Rokni et al. explores the relation between the imposed voltage and the electrostatic force by finite element methods (FEM) that solve the Poisson equation with the graphene dielectric constant as the only fitting parameter.[5] By combining the force measurements in conductive atomic force microscopy (CAFM) with FEM calculations, the dielectric constant is found to be layer independent for FLG, filling in the only missing parameter for future electrostatic force predictions. In addition, the peeling events often take place nearer the SiO2/FLG interface than the FLG/electrode interface, indicating that charges are more induced at the far end from the electrode.[5] This is consistent with the prediction by the spatial discrete model by the same group.[9] Furthermore, molecular dynamics simulation confirms the mechanism why shearing exfoliation, illustrated in Figure , is more controllable.

Figure 1

Schematic of electrostatic force facilitated exfoliation. The colors qualitatively represent the charges.

The work presented by Rokni et al.[5] manifests the power of computational simulations and analysis in unraveling physical mechanisms underlying advanced experimental techniques. For the first time, the work includes the theoretically predicted charge distribution in molecular dynamics (MD) simulations, which also include the anisotropic nature of the van der Waals interlayer interaction.[5] The advancement in the understanding and manipulation of interlayer bonding of graphene opens a new venue for the studies of other 2D materials. Electrostatic force facilitated exfoliation may be limited to conducting or semiconducting 2D materials with long enough cross-plane charge screening length. It will be interesting to witness similar progress in 2D insulators such as BN.