Feb 1, 2016
How topological defects scatter phonons in graphene nanoribbons
It is thermodynamically favourable for graphene to be defective and these defects significantly impact on graphene thermal conduction by phonon scattering. A comprehensive understanding of the effect of lattice defects on graphene thermal transport is therefore highly desirable for manipulating thermal flow in graphene-based devices. Reporting in Nanotechnology, researchers investigate the roles of the different mechanisms governing defect phonon scattering by comparing the effects of ten different defect structures using molecular dynamics.
A team at Xi’an Jiaotong University from China reveals that graphene synthesised via different annealing processes results in different types of lattice defects. They calculate their influence on the graphene lattice thermal conductivity using molecular dynamics simulations as depicted in Fig. (a) It is shown that most point vacancies are comparatively stable at 1000K but transform into other more energetically favourable structures at above 2000K. Also, the well-known Stone-Wales style defects are indeed the more energetically favourable structures formed by bond rotation. This demonstrates a good reference for figuring out what kinds of defects are most likely to exist in different experimental conditions.
The results show that the local energy gradient is the key for reducing the thermal conductivity, while the mass density difference also plays a leading role. Phonon density of state analyses show a typical phonon softening behaviour induced by the topological defects. They also show that the lattice thermal conductivity can be reduced by a factor up to 30 in the presence of grain boundaries. The scattering pattern is found to be far larger than that geometrically occupied by the defects. The existence of these lattice defects including C-5, C-7, and C-8 rings can be expected to likewise explain experimentally measured thermal conductivities lower than the ballistic limit, and could suggest new lines of graphene microscopy research for experiments.
More information about this research can be found in the journal Nanotechnology 27 055401.
Studies of defects in boron nitride reveal useful properties for devices (Jan 2014)
Defects make graphene sensitive to water adsorption (Feb 2015)
How graphene tears on an oxidising metal surface (Nov 2015)
About the author
Ziming Zhu is currently a phD candidate in the material physics center and is supervised by Prof. Ju Li at MIT and Prof. Zhao Wang at Xi’an Jiaotong University.