21. – 23. 9. 2011, Brno, Czech Republic, EU GRAPHENE UNDER UNIAXIAL DEFORMATION: A RAMAN STUDY Otakar Frank a,b , Georgia Tsoukleri b , John Parthenios b , Konstantinos Papagelis c , Ibtsam Riaz d , Rashid Jalil d , Kostya S. Novoselov d , Martin Kalbáč a , Ladislav Kavan a , Costas Galiotis b,c a J. Heyrovsky Institute of Physical Chemistry of the AS CR, v.v.i., CZ-18223, Prague 8, Czech Republic, otakar.frank@jh-inst.cas.cz b FORTH / ICE-HT, Stadiou st., GR-26504, Patras, Greece c Materials Science Department, University of Patras, GR-26500, Greece d School of Physics and Astronomy, University of Manchester, Manchester, M13 9PL, UK Abstract The presented work summarizes various aspects of uniaxial deformation in monolayer graphene studied by means of Raman spectroscopy. Graphene flakes were subjected to tension - compression uniaxial loading using the cantilever beam technique. The evolution of the Raman single-resonance (G) and double- resonance (2D) bands was monitored at strain levels < 1%. The position of all peaks redshifts under tension and blueshifts under compression. The G peak splitting into two sub-bands (G - and G + ) which is caused by symmetry lowering, is observed in both strain directions. The sub-bands’ intensities are used to calculate the crystal lattice orientation of the measured graphene flakes with respect to the strain axis. The nature and splitting of the 2D band even in the unstrained flakes, when excited by the 785 nm (1.58 eV) laser line, is interpreted as the interplay between two distinct double resonance scattering processes. Keywords: graphene, strain, Raman spectroscopy 1. INTRODUCTION Graphene is the thinnest known elastic material, exhibiting exceptional mechanical and electronic properties [1]. Because graphene is a single-layer membrane, it is also amenable to external perturbations, including mechanical loading. A promising approach to develop graphene-based electronic devices is by engineering local strain profiles obtained by means of a controlled mechanical or thermal deformation of the substrate or by applying appropriate geometrical patterns in a substrate. The strained material becomes a topological insulator enabling the opening of significant energy gaps in graphene’s electronic structure [2]. An alternative approach in this line is to control and manipulate the intrinsic ripples in graphene sheets using thermally generated strains [3]. This feature is expected to strongly influence electronic properties by inducing effective magnetic fields and modify local potentials. Thus, the precise determination and monitoring of strain is an essential prerequisite for understanding and tuning the interplay between the geometrical structure of graphene and its electronic properties. Raman spectroscopy is a key diagnostic tool to identify the number of layers in a sample and probe physical properties and phenomena [4]. The G band is the only Raman mode in graphene originating from a conventional first order Raman scattering process and corresponds to the in-plane, zone center, doubly degenerate phonon mode (transverse (TO) and longitudinal (LO) optical) with E 2g symmetry. The D and 2D modes come from a second-order double resonant process between non equivalent K points in the Brillouin zone (BZ) of graphene, involving two zone–boundary phonons (TO-derived) for the 2D and one phonon and a defect for the D band. Both modes are dispersive spectral features, i.e. their frequencies vary linearly as a function of the energy of the incident laser, E exc . Figure 1 shows Raman spectra of a monolayer graphene flake embedded in a sandwich-like polymer matrix excited by 633 and 785 nm lasers (1.96 and 1.58 eV, resp.).