A theorist’s view of polycrystalline graphene: from atomic structure to electronic transport properties

There is growing evidence of the polycrystalline nature of graphene samples at micrometer length scales. Grain boundaries and dislocations, intrinsic topological defects of polycrystalline materials, inevitably affect all kinds of physical properties of graphene [1]. This talk reviews our theoretical efforts directed towards understanding the atomic structure and electronic transport properties of polycrystalline graphene. I will introduce a general approach for constructing dislocations in graphene characterized by arbitrary Burgers vectors and grain boundaries covering the complete range of possible misorientation angles. By means of first-principles calculations we address the thermodynamic properties of grain boundaries revealing energetically favorable large-angle configurations as well as dramatic stabilization of small-angle configurations via the out-of-plane deformation, a remarkable feature of graphene as a two-dimensional material [2]. In the rest of my talk I will focus on the electronic transport properties of polycrystalline graphene. Ballistic charge-carrier transmission across the periodic grain boundaries is governed primarily by momentum conservation. Two distinct transport behaviors are predicted − either perfect reflection or high transparency with respect to low-energy charge carriers depending on the grain boundary periodicity [3]. It is also shown that topologically trivial line defects can be engineered and offer opportunities for generating valley polarized charge carriers [4]. Beyond the momentum conservation picture we find that the transmission of low-energy charge carriers can be dramatically suppressed in the small-angle limit [5]. This counter- intuitive behavior is explained by resonant backscattering involving localized electronic states of topological origin. Finally, the relations between the structure of strongly disordered large-angle grain boundaries and their transport properties are discussed [6]. These results demonstrate that dislocations and grain boundaries are intrinsic topological defects that dramatically affect the transport properties of graphene and can also be used for engineering novel functional devices.

References:

[1] Yazyev, O. V. & Chen, Y. P. Polycrystalline graphene and other two-dimensional materials. Nature Nanotechnology, published online (http://dx.doi.org/10.1038/nnano.2014.166)
[2] Yazyev, O. V. & Louie, S. G. Topological defects in graphene: Dislocations and grain boundaries. Phys. Rev. B 81, 195420 (2010).
[3] Yazyev, O. V. & Louie, S. G. Electronic transport in polycrystalline graphene. Nature Materials 9, 806-809 (2010).
[4] Chen, J. H. et al. Controlled growth of a line defect in graphene and implications for gate-tunable valley filtering. Phys. Rev. B 89, 121407(R) (2014).
[5] Gargiulo, F. & Yazyev, O. V. Topological Aspects of Charge-Carrier Transmission across Grain Boundaries in Graphene. Nano Letters 14, 250-254 (2014).
[6] Gargiulo, F. & Yazyev, O. V. In preparation