Influence of interfaces on the transport properties of graphite revealed by nanometer thickness reduction

• Published in: Carbon (New York) 2018-11, Vol.139, p.1074-1084
• Main Authors: Zoraghi, Mahsa, Barzola-Quiquia, José, Stiller, Markus, Esquinazi, Pablo D., Estrela-Lopis, Irina
• Format: Article
• Language: English
• Subjects: Band structure; Diffraction; Graphite; Image transmission; Magnetoresistance; Magnetoresistivity; Oxygen plasma; Plasma etching; Pyrolytic graphite; Raman spectra; Reduction; Scanning electron microscopy; Scanning transmission electron microscopy; Thickness; Transmission electron microscopy; Transport properties; X ray spectra; X-ray diffraction
• ISSN: 0008-6223; 1873-3891
• DOI: 10.1016/j.carbon.2018.07.070

We investigated the influence of thickness reduction on the transport properties of graphite microflakes. Using oxygen plasma etching we decreased the thickness of highly oriented pyrolytic graphite (HOPG) microflakes from ∼100 nm to ∼20 nm systematically. Keeping current and voltage electrodes intact, the electrical resistance R(T), the magnetoresistance (MR) and Raman spectra were measured in every individual sample and after each etching step of a few nm. The results show that R(T) and MR can increase or decrease with the sample thickness in a non-systematic way. The results indicate that HOPG samples are inhomogeneous materials, in agreement with scanning transmission electron microscopy images and X-ray diffraction data. Our results further indicate that the quantum oscillations in the MR are not an intrinsic property of the ideal graphite structure but their origin is related to internal conducting interfaces. The sketch around the transmission electron picture taken from a HOPG sample of the presented studies, shows the position of the electrodes at the top graphene layer. These electrodes, protected by a SiN layer, remain after a gentle oxygen plasma etching we use to reduce the thickness of the sample between the electrodes. With this technique we were able to measure the changes in the transport properties after nanometer thinning of the sample. The results clearly show that the transport properties are highly a_ected by the internal interfaces at the surface region where the electrodes are fixed. The usual width and length of the measured microakes were a few micrometers with a thickness