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

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

Treatment of surfaces with low-energy electrons

International audienceElectron-beam-induced deposition of various materials from suitable precursors has represented an established branch of nanotechnology for more than a decade. A specific alternative is carbon deposition on the basis of hydrocarbons as precursors that has been applied to grow various nanostructures including masks for subsequent technological steps. Our area of study was unintentional electron-beam-induced carbon deposition from spontaneously adsorbed hydrocarbon molecules. This process traditionally constitutes a challenge for scanning electron microscopy practice preventing one from performing any true surface studies outside an ultrahigh vacuum and without in-situ cleaning of samples, and also jeopardising other electron-optical devices such as electron beam lithographs. Here we show that when reducing the energy of irradiating electrons sufficiently, the e-beam-induced deposition can be converted to e-beam-induced release causing desorption of hydrocarbons and ultimate cleaning of surfaces in both an ultrahigh and a standard high vacuum. Using series of experiments with graphene samples, we demonstrate fundamental features of e-beam-induced desorption and present results of checks for possible radiation damage using Raman spectroscopy that led to optimisation of the electron energy for damage-free cleaning. The method of preventing carbon contamination described here paves the way for greatly enhanced surface sensitivity of imaging and substantially reduced demands on vacuum systems for nanotechnological applications. (C) 2017 Elsevier B.V. All rights reserved