Scanning probe investigations on graphene

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Abstract

In this thesis, scanning probe microscopy experiments on graphene and chemically modified graphene crystals are discussed. Since its discovery in 2004, graphene has not only impressed researchers and industry because it is a crystal that is only one atom thick, butalso because of its electronic transport properties. However, a major challenge remaining is the task to introduce an energy gap in graphene. One way to open an energy gap in pristine graphene is its confinement to nanometre sizes. To this end, methods were developed to fabricate such nanostructures out of graphene. Here, the atomic force microscope (AFM) based technique of local anodic oxidation was applied to selectively oxidise graphene. Using this technique, graphene nanostructures as small as 20~nm have been fabricated. A graphene quantum dot (QD) created with this technique was measured at low temperatures. It showed quantum Coulomb blockade behaviour, with an energy gap of 10 meV. Furthermore, the transport behaviour of these nanostructures was also investigated under ambient conditions.Scanning gate microscopy measurements carried out on a graphene quantum point contact (QPC) demonstrated the possibility to locally influence the charge carrier concentration in the QPC, and thus alter the resistance of the device. These experiments additionally prove the usefulness of local anodic oxidation to create graphene nanostructures. Equally tempting as opening a gap in graphene and studying the resulting transport properties is the prospect of studying the influence of the edges terminating a graphene crystal on its transport properties. To that end, reliable methods for obtaining the crystallographic orientation of a given edge are needed. While most techniques require either elaborated sample fabrication or modelling, it is shown here how atomically resolved scanning tunnelling microscopy (STM) imaging together with Raman spectroscopy can be used to determine the crystallographic direction of graphene edges without doubt. An alternative way of creating an energy gap in graphene is its modification with atomic hydrogen. Atomic force microscopy was first used to measure the topography of hydrogenated graphene crystals. It is further shown, how the amount of adsorbed hydrogen could be decreased using AFM. The changes induced in the hydrogenated graphene samples in this way have been further corroborated by Raman spectroscopy and low temperature transport experiments, establishing AFM as a method to engineer the resistance of hydrogenated graphene.

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