Few layers of graphene and carbon nanoscrolls by wedge based mechanical exfoliation
Jayasena R. A. P. B.
Date of Issue2014
School of Mechanical and Aerospace Engineering
Two-dimensional flat carbon sheets, commonly seen in graphite, are a much sought after material with interesting electronic and mechanical characteristics. It is expected to be used in the next generation microchips in lieu of silicon and also expected to first enter the commercial market in the form of conducting plastics and composites. Due to the increase in the number of such applications along with business opportunities, producing or isolating this layer of graphite cost-effectively is an urgent challenge to be addressed. Widespread efforts in this direction are focusing more on chemical methods to separate (from bulk graphite) or deposit (using epitaxial methods) this two-dimensional layer; such chemical methods not only are environmentally unfriendly, but also produce poor yield and can result in graphene layers with undesirable functional groups attached. One of the early methods, and still sometimes used today for research, of separating graphene from bulk graphite is by mechanical cleavage using a scotch tape. This method is known to produce high quality of graphene layers. However, this method is not reliable for mass manufacturing of graphene; the full potential for such mechanical methods remain to be explored. In this study, a novel method to synthesize carbon nano-sheets specially few layers of graphene from bulk graphite by mechanical exfoliation is presented. The method involves the use of an ultra-sharp single crystal diamond wedge to cleave highly ordered pyrolytic graphite parallel to the basal plane to generate carbon nanosheets. Characterization of the cleaved layers shows that the process is able to synthesize carbon nano-sheets with a thickness of a few nanometers and with an area of hundreds of micrometers. Further examination of sheets made by such sectioning showed the presence of flat, rolled and sheared graphitic thin layers with nano-scale dimensions. The rolled structures observed are of various types: partially rolled, fully rolled and axially slid out nano-scrolls. In addition, two types of kink bands are also observed in the sectioned layers. Microscopic analysis of the as received starting material (prior to sectioning) showed the presence of defects such as discontinuous layers, kink bands and edge folds. The sectioning forces are measured and used to correlate with the structural characteristics of the layers. The possibility of enhancing the cleaving process by the use of ultrasonic oscillations along the wedge is also studied. In order to understand the wedge-graphene interactions under controlled conditions this thesis investigates the micro-nano indentation of highly oriented pyrolytic graphite parallel to the basal plane. Such indentation allows control over the depth and load leading to an understanding of the various stages of layer separation and edge structure formation. Depending on how far we indent from a free edge various types of load-displacement curves have been observed. Interesting kinks/steps are also observed during unloading stage signifying structural changes. The thesis explores, using molecular simulations, how and under what conditions graphene layers separate, fold and shear during the wedge-based mechanical exfoliation machining technique. Molecular simulations of initial wedge engagement show that the entry location of the wedge tip vis-a-vis the nearest graphene layer plays a key role in determining whether layers separate or fold and which layers and how many of them fold. It is also noticed that depending on this entry location several successive layers beneath the wedge undergo significant elastic bending, consuming energies requiring large vertical forces to be imposed by the moving wedge. The layer separation force itself is seen to be minimal and consistent with breaking up of van der Waals interactions. In addition, shearing of layers occurs mainly during wedge exit and depends largely on the wedge speed and also its depth of insertion. Understanding the conditions at which this separation, folding and shearing of the graphene layers takes place, one can control or tune the wedge-based exfoliation technique for particular kinds of graphene layers. Molecular simulations also show that the multiple spikes in sectioning forces observed in experiments could be explained as the interactions of discontinuous layer pre-existing in the starting material with the moving wedge. The effect of wedge radius on initial wedge material engagement reveals various layer initiation modes in relation to the depth of insertion. The obtained forces significantly increase with an increase in the wedge radius. Molecular simulations are further used to understand the formation of carbon nanoscrolls and the study presents two hypotheses of how such scrolls form. The first hypothesis is based on microscopic evidence of pre-existing folds in layer edges of the HOPG. The moving wedge upon interaction with certain fold geometries can trigger scroll formation. The second hypothesis is based on literature evidence that graphene sheets when subject to deformation can result in defects on the torn edges and such defects are induced in the HOPG layers during sample preparation. The layers with such defects, upon interacting with the moving wedge, can also form scrolls. This thesis, by a combination of experimental and simulation studies, convincingly shows the capability of producing both planar and rolled carbon nano structures with the wedge-based mechanical exfoliation technique. Understanding of this oriented sectioning technique provides useful control methods employing which this technique has potential to be used as a technique to fabricate thin graphitic layers and rolled structures for potential industrial applications.