Understanding Defects and Defect Dynamics in Carbon Nanomaterials using Electron Microscopy.

Professor Angus Kirkland.

Department of Materials, Oxford University, Parks Road, Oxford, OX13PH, UK.

Graphene, Carbon nanotubes and related 1D and 2D carbon nanomaterials are currently attracting considerable interest, due to their novel electronic, mechanical and structural properties. This talk will report recent structural studies of various types of defects in these materials using high resolution aberration corrected electron microscopy.

            I will firstly describe experimental evidence for atomic displacements associated with shear strain in single-walled carbon nanotubes (SWNTs). This data indicates the existence of a dominant non-uniform shear strain that varies along the SWNT axis. The direction of shear is opposite to that expected from a simple force applied perpendicular to the axis to produce the bending, highlighting the complex atomistic strain behaviour of beam bending mechanics in highly anisotropic structures.

The movement of dislocations in crystals is the key mechanism for plastic deformation in all materials. I will show studies of dislocation dynamics in graphene recorded with single atom sensitivity and examine dislocation movement together with analysis of the associated strain fields. Step-wise dislocation movement along the zig-zag lattice direction is mediated by a single bond rotation and along the arm-chair direction through the loss of two carbon atoms. The strain fields deform the graphene lattice by elongation and compression of C-C bonds, shear and lattice rotations.

Finally I will describe the control over both the location and complexity of defect formation in graphene by controlled exposure to a focussed electron beam. Examples of stable defect configurations will be described together with mechanistic pathways by which these can relax to simpler structures through bond rotations and surface adatom incorporation.