Program

I.1 -- Invited, NCTP-40

Date: Monday, 27 July 2015

Time: 10h30 - 11h00

Strain effects on the electronic properties of devices made of twisted graphene layer

V. Hung Nguyen (1,2), H. Viet Nguyen (2), J. Saint-Martin (1), P. Dollfus (1)

(1) Institute of Fundamental Electronics, CNRS, Univ. of Paris-Sud, Orsay, France; (2) Center for Computational Physics, Institute of Physics, VAST, Hanoi, Vietnam

Graphene is one of the most attractive materials for beyond-CMOS electronics because of its specific electronic properties, which are a consequence of its two-dimensional honeycomb lattice and relativistic-like charge carriers at low energy [1]. To enlarge its range of applications, the modulation of electronic structure of graphene nanomaterials has been the subject of intense research. Recently, the interest of the graphene community has also been oriented toward the investigation of twisted graphene multilayer lattices, a specific type of Van der Waals structures of graphene. These lattices appear as promising materials providing various possibility of modulating their electronic properties by changing the twist angle [2,3]. In this work, we investigate the effects of uniaxial strain on the electronic properties of twisted graphene bilayer (T-GBL) systems [4,5]. First, we explore the effects of strain on the low-energy bands of twisted graphene bilayer. Second, we demonstrate that the strain engineering is an efficient technique to open finite transport gaps in vertical devices made of stack of twisted graphene layers. Regarding the first topic, we find that the bandstructure of T-GBLs is dramatically deformed and the degeneracy of the bands around the Dirac points is broken by strain. As a consequence, the number of valleys in the bandstructure can double and the van Hove singularity points are separated in energy. The effects are shown to be strongly dependent on the magnitude of strain, its applied direction and the twist angle. As an important result, we demonstrate that the position of van Hove singularities can be modulated by strain, suggesting the possibility of observing this peculiar feature of the bandstructure at low energy in a large range of twist angles (i.e., larger than 10 degrees). For the second topic, we find that because of the different orientations of the two layers in the vertical devices made of twisted graphene layers, their Dirac points can be displaced and separated in the k-space by the effects of strain. Hence, a finite conduction gap as large as a few hundred meV can be obtained in the device with a small strain of only a few percent. On this basis, the strong modulation of conductance and significant improvement of Seebeck coefficient are shown. The suggested devices therefore may be very promising for improving applications of graphene, e.g., as transistors or strain and thermal sensors.

References: [1] A. C. Ferrari et al., Nanoscale 7, 4598 (2015) [2] G. Li et al., Nat. Phys. 6, 109 (2010) [3] W. Yan et al., Nat. Commun. 4, 2159 (2013) [4] V. H. Nguyen and P. Dollfus (2015); arXiv:1412.7583 [5] V. H. Nguyen et al., Nanotechnol. 26, 115201 (2015)

Presenter: Viet-Hung Nguyen