UC Berkeley

Weak interlayer interactions in van der Waals bonded layered materials enable us to isolate 2D monolayer from the bulk by means of simple mechanical exfoliation methods. Since the first time graphene was extracted from bulk graphite, this method has been used extensively to produce various 2D monolayers. Despite its success, this method is not a well-controlled process since it usually requires a random number of mechanical exfoliation steps to yield a monolayer sample and final sample size is relatively small. Recently, a thin film mediated mechanical exfoliation method has addressed these problems by enabling a one-step exfoliation scheme to achieve monolayers. This dissertation examines the process from a theory perspective.

To reveal its underlying mechanism as well as have a comprehensive understanding of the mechanical exfoliation process, we have developed a rigid layer model for van der Waals bonded layered materials. The assumption is based on the fact that, there exists very low frequency phonon vibration modes, in which the layers move nearly as rigid units and these low frequency vibration modes essentially determines the stability of the big structure. Within the framework of rigid layer model, we have analyzed the mechanical stability of vdW bonded layered system under the perturbation of external loading force and thermal noise. We have shown that the effectiveness of thin film mediated method arise from the change of soft mode. Furthermore, we found that twisting topmost layer with rotation angle has similar effect and should lead to improvement in exfoliation result as well. Finally, this rigid layer model has been compared with atomic model to show its scope of application. At zero temperature, all these atomic scale simulation results agree with the rigid layer model suggesting that the simple model captures the essential features of the exfoliation mechanisms of vdW bonded layered materials at low temperature. This simple model can serve as a reasonable and efficient way to assess the factors affecting the mechanical exfoliation results.