Invited Paper 2D+EM+IS+MC+NS+SP+SS-WeA3
Polycrystalline 2D Materials: Atomic Structure and Electronic Transport Properties

Wednesday, October 21, 2015, 3:00 pm, Room 212C

Grain boundaries and dislocations are intrinsic topological defects of polycrystalline materials, which inevitably affect their physical properties. In my talk, I will discuss the structure of topological defects in two-dimensional (2D) materials such as graphene and monolayer transition metal dichalcogenides (TMDCs) [1].

I will first 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]. Both the presence of stable large-angle grain-boundary motifs and the out-of-plane deformation of small-angle configurations have recently been observed by scanning tunneling microscopy [3].

In the rest of my talk, I will focus on the electronic transport properties of polycrystalline 2D materials. Ballistic charge-carrier transmission across periodic grain boundaries is governed primarily by momentum conservation. Two distinct transport behaviors of such grain boundaries in graphene are predicted − either perfect reflection or high transparency with respect to low-energy charge carriers depending on the grain boundary periodicity [4]. It is also shown that certain periodic line defect structures can be engineered and offer opportunities for generating valley polarized charge carriers [5]. Beyond the momentum conservation picture we find that the transmission of low-energy charge carriers can be dramatically suppressed in the small-angle limit [6]. Unlike graphene, TMDCs combine a two-valley electronic band structure with strong spin-orbit effects. The latter can be employed for creating spin-polarized currents and adds yet another conservation law in the electronic transport across regular defects such as the frequently observed inversion domain boundaries [7,8].

* This work has been supported by the Swiss NSF, ERC and Graphene Flagship.

[1] O. V. Yazyev and Y. P. Chen, Nature Nanotechnology 9, 755 (2014).

[2] O. V. Yazyev and S. G. Louie, Phys. Rev. B 81, 195420 (2010).

[3] Y. Tison et al., Nano Lett. 14, 6382 (2014).

[4] O. V. Yazyev and S. G. Louie, Nature Materials 9, 806 (2010).

[5] J. H. Chen et al., Phys. Rev. B 89, 121407(R) (2014).

[6] F. Gargiulo and O. V. Yazyev, Nano Lett. 14, 250 (2014).

[7] A. Pulkin and O. V. Yazyev, submitted.

[8] O. Lehtinen et al., ACS Nano 9, 3274 (2015).