Low-energy electron microscopy of graphene outside UHV: electron-induced removal of PMMA residues used for graphene transfer

Abstract

Two-dimensional materials, such as graphene, are usually prepared by chemical vapor deposition (CVD) on selected substrates, and their transfer is completed with a supporting layer, mostly polymethyl methacrylate (PMMA). Indeed, the PMMA has to be removed precisely to obtain the predicted superior properties of graphene after the transfer process. We demonstrate a new and effective technique to achieve a polymer-free CVD graphene — by utilizing low-energy electron irradiation in a scanning low-energy electron microscope (SLEEM). The influence of electron-landing energy on cleaning efficiency and graphene quality was observed by SLEEM, Raman spectroscopy (the presence of disorder D peak) and XPS (the deconvolution of the C 1s peak). After removing the absorbed molecules and polymer residues from the graphene surface with slow electrons, the individual graphene layers can also be distinguished outside ultra-high vacuum conditions in both the reflected and transmitted modes of a scanning low-energy (transmission) electron microscope.