Mica as an Ultra-Flat Substrate for Studying Mechanically Exfoliated Graphene

Abstract

Silicon dioxide (SiO2) is a common support for studying two-dimensional materials and creating devices from them. However, graphene conformation to SiO2 roughness worsens the electronic properties, whereas graphene deposited on flat terraces of insulating mica is free of ripples. This thesis solves key challenges in the use of mica to support mechanically exfoliated graphene. Methods of mica cleavage and graphene exfoliation, and settings for electron microscopy, atomic force microscopy (AFM) and Raman spectroscopy were developed. Vacuum annealing was compared for graphene samples of different thicknesses, down to a single layer. Pre- and post-annealing, graphene on mica provided defect-free graphene and no observable strain or doping. In contrast, graphene on SiO2 showed disorder before annealing. Annealing up to 300°C reduced the Raman defect peak but did not remove it. Above 300°C, the defect peak increased. Graphene on SiO2 appeared to become ‘invisible’ with AFM after annealing at 500°C, in line with previous observations with scanning electron microscopy. Other studies attributed this to the graphene being removed, but, here, using substrate markers, Raman spectroscopy and line-averaged AFM showed that the graphene was still present but had conformed to the underlying roughness of the SiO2 so well as to appear nearly invisible. Mica annealed at 400°C showed the formation of potassium carbonate particles following dehydroxylation of the mica surface at a temperature lower than previously reported. In addition, the graphene appeared to act as a mask, protecting the mica underneath it while the surrounding surface was removed at 500°C. Patterning and etching mica are essential to create location grids and etch trenches to suspend deposited materials. The first patterning lithography recipe for mica was established herein using electron-beam lithography. Finally, mechanically exfoliated graphene was successfully transferred to the patterned mica and studied.