Bending Effects In Graphene:Implication For Flexible Transparent Conductors

The development of flexible electronic technology demands for flexible transparent conductors,especially in flexible display and smart sensing.Currently,the industry standard,indium tin oxide(ITO),for transparent conductor is hard to satisfy the requirements of the flexible transparent electrons(FTE)due to its brittleness and sharp conductance degradation under fracture.Therefore,exploring candidates to replace the ITO is of great importance in FTE.Many alternative materials,such as metal nanowires,carbon nanotubes,and graphene and its composites,have been proposed.These materials are widely used in the application of flexible transparent electrons.Based on this,the effects of bending on the mechanical,electronic and optical properties of graphene were investigated by density functional theory.A small bending modulus of ~0.20 n N·nm indicates the high flexibility of graphene.For both the armchair and zigzag bending directions,bending can open a small band gap within 0.05 e V due to the decreased ? orbital electrons in the bent graphene.The band gap is too small to destroy the electrical conductance of graphene in real device applications.As indicated by the electronic conductance transmission,the conductance transmission of bent graphene is well kept,almost exactly the same as flat graphene near the Fermi energy.However,large bending angles above 70o will lead to a drop in the conductance.On the other hand,the opened band gap will bring the bent graphene different optical properties from the flat graphene,especially in the low photon energy region(< 5.0 e V).Compared with the flat graphene,for out-of-plane light polarization,bending causes two absorption peaks at 0.9 and 4.1 e V originated from the ? ? ?* interband transitions,which are forbidden in the flat graphene.As a result,optical transmittance of graphene in the visible region is slightly decreased as the bending angle increases.Even so,the optical transmittance of bent graphene is still above 94% at the high bending angle of 90o.Combined with the electronic conductance and optical transmittance spectra under bending strain,a critical bending angle of 70o was found where the bent graphene can maintain the performance of flat graphene for its application in flexible transparent electronics.