Scanning Probe Microscopy (SPM) of Graphene

Introduction

Our research focuses on the use of novel scanning probe techniques to visualise and directly influence the wave-like properties of electrons in small conductors. We are particularly interested in characterising the behaviour of electrons travelling through low dimensional systems such as semiconducting heterostructures and graphene. Our scanning probe system comprises a modified commercial SPM head (Attocube AFM I) mounted to the mixing chamber of dilution refridgerator with a working base temperature of 80 mK. The whole setup sits in a Helium cooled cryostat with a superconducting magnet allowing us to apply magnetic fields up to 12 T.

Graphene

We have recently been applying our low temperature scanning probe to a fascinating material derived from graphitic carbon. The bulk graphite which one finds at the core of a pencil is composed of many hundreds of layers of carbon atoms stacked on top of one another like a pack of cards. It is this simple atomic architecture which makes graphite so easy to deposit when gently rubbed against another surface because the layers are free to slide over one another. It was discovered recently that this process even produces single atomic layers, i.e., tiny flakes of carbon which are only one atom thick.

This two-dimensional allotrope of carbon is called graphene and has created enormous excitement since its discovery. It exhibits a remarkable number of new electronic, mechanical, and optical properties relevant to a wide range of device applications and fundamental research questions. For it to meet complement silicon in future generations of nanoelectronic devices it will undoubtedly be necessary to obtain precise and consistent control over its local electronic properties.

Promising steps in this direction have been made by coating it with specially designed molecules and carving it into shapes which channel electrons in certain directions. During the last year we have made contributions to both of these research programmes. Firstly, we have been using nanofabrication to carve graphene into narrow constrictions which localise electrons in puddles of charge known as a quantum dots. Using a very sharp metallic tip, similar to the stylus of a record player, we are then able to move individual electrons on and off the dot. The information we acquire from such control over single electrons is essential for determining whether graphene can be be used as a transistor, the basic building block of a modern computer. Secondly, we have developed a technique for measuring the local electronic properties of chemically coated graphene. Again using a charged tip, we repel electrons from a small region of the graphene and measure the effect this has on the rest of the electron sea. This measurement tells us precisely where chemicals have been absorbed, providing a powerful tool for characterising the influence of graphene's surface chemistry on its electronic properties. In future we envisage "drawing" novel elements for electron optics by removing or applying chemicals with the tip with nanometre resolution.