Latest findings on plasticity

Materials World magazine
24 Sep 2018

2D materials have led to a greater understanding and manipulation of dislocations. Khai Trung Le talks to Professor Erdmann Spiecker regarding the potential of his findings.

Atomic-scale topological line defects, known as dislocations, have been characterised since the 1930s, and form the basis of processes as diverse as forging to vehicle crumple zones. However, recent work from a Friedrich-Alexander Universitat Erlangen-Nürnberg (FAU), Germany, research team has enabled direct control over individual dislocations on the atomic scale, and they believe this enables further avenues for understanding the fundamentals of plasticity.

In 2013, a previous FAU research group led by Professor Erdmann Spiecker, Chair of Micro and Nanostructure Research at FAU, discovered dislocations in bilayer graphene, reportedly the thinnest interface where they would be possible. Since then, Spiecker has explored means of directly manipulating dislocations in situ with a scanning electron microscope setup, using nanoscale robotic arms outfitted with tungsten tips to drag line defects and monitor the manipulation.

Typically, dislocations would disappear from a crystal when close to the surface, due to strain energy associated with the dislocation, inducing mirror forces that would pull the dislocation out of the crystal. In bilayer graphene, the dislocations are confined to the plane between two carbon layers and cannot escape. Spiecker told Materials World, ‘While movement of dislocation under applied load has been observed in many crystalline materials, mostly metals, our study is the first to demonstrate in situ control of individual dislocations and their movement. This manipulation is possible in bilayer graphene because the dislocations are very close to the surface, only one atomic layer apart.

‘The interplay between dislocations and surface topography allows us to push and pull individual dislocations using the tip of a micromanipulator. Not only can we see the final outcome of a dislocation reaction, but we can study intermediate stages in great detail.’ The team claims as a result of their manipulation, they have confirmed long-standing theories on defect interactions including line tension, dislocation reaction, and node formation, as well as identifying new hypotheses. Their work is detailed in In situ manipulation and switch of dislocations in bilayer graphene, published in Science Advances.

Switch it up

The team also discovered what they’ve termed a topological switch. Dislocations in bilayer graphene alter the local physical properties of the material, including electronic and charge transport properties, and could act as charge barriers or channels. Typically, dislocations have a detrimental effect on bulk semiconductors.

Spiecker said, ‘By switching between appropriate dislocation arrangements, it is possible to create two states where in one state charge carriers can traverse a sample without encountering a dislocation, while in the other they necessarily have to pass a dislocation.’

Fundamental interest

Dislocations are the main carriers of plasticity, and the FAU group stated that their influence on 2D materials, where a single defect can alter electronic and optical properties, reinvigorated interest in understanding and controlling their behaviour.

Spiecker was emphatic on the potential of FAU’s recent research, stating, ‘Dislocations constitute the fundamental defect that controls the plastic deformation of crystalline materials in general. Almost all bulk crystals contain dislocations independent of whether they are ductile or brittle. However, only when the dislocations are allowed to move do they contribute to plastic deformation. Therefore, understanding how dislocations move and interact with each other is of fundamental interest.’

Future areas of research include analysis of dislocation reactions and node formation in correlation with simulations, investigation of dislocation dynamics and lattice friction, and measurement of transport properties across dislocation configurations created by mechanical manipulation. Tests will include other 2D materials, including molybdenum disulphide and tungsten disulphide. Spiecker said, ‘We hope that our work will inspire many researchers in the field since there is a lot of work to do.’