First-principles Study Of Strain-regulated Graphene

Graphene is an atomically thin two-dimensional(2-D)material and is therefore structurally more amenable than silicon to external modifications.Graphene can also sustain a much larger strain than silicon.At present,there has been much research on strain regulated 2-D graphene,including the effects of unifor and local strains.While,there is no band gap.A band gap can be created by patterning the 2-D graphene into a nanometer-wide graphene nanoribbon(GNR);this has been predicted theoretically and realized experimentally.In present work,density functional theory(DFT)calculations are performed using first-principles study on the stability,mechanical,electronic or magnetic properties of graphene materials under uniaxial,equibiaxial,shear,or local vertical strains.The structures of these materials include armchair graphene nanoribbon(AGNR),zigzag graphene nanoribbon(ZGNR)with a topological line defect,single-side fluorinated graphene and nonplanar oxygen-functionalized armchair graphene nanoribbons.Several conclusions are achieved:(1)The structural and electronic properties of armchair graphene nanoribbons under unixial strain have been studied in detail.The results show that all C-C bond lengths decrease(increase)with increasing compression(tension)strain.Fore each nanoribbon,as expected,the most stable structure corresponds to the unstrained state.And the average energy increases quadratically with the absolute value of the uniaxial strain,showing an elastic behavior.In addition,each ribbon width family leads to oscillatory band gaps.(2)The structural and electronic properties of single-side fluorinated graphene C4F under equibiaxial strains have been studied in detail.The results show that the C-C bond lengths increase(decrease)with increasing equibiaxial tensile(compressive)strain.The lowest average energy corresponds to the unstrained state.By applying equibiaxial strains,the band gap undergoes the direct-to-indirect transition.(3)We have investigated the symmetry,stability and electronic properties of AGNR sunder shear strain in detail.The calculation results reveal that the 6/mmm symmetry of the hexagonal lattice is further turn into 2/m under shear strain.All lines of the formation energy-shear strain relations are symmetric with respect to the sign of the zero shear strain.At certain shear strain,the formation energy of the AGNR decreases with increasing nanoribbon width.Shear strain makes the band structure a little smoother,but the modification of the energy gap,is slight.(4)We have investigated the elastic,electronic and magnetic properties of naked ZGNRs with topological line defects(LD-ZGNRs)lying symmetrically on the ribbon's middle under vertical strains at four types of line defect atoms in detail.The results show that the C-C bonds break when the depth(vertical distance d)is larger thanl.6A.For a given deformation size,the strain energy increases with the increasing vertical distance.For a fixed vertical distance,a smaller circle generates larger strain energy.In addition,spin splitting appears under vertical strain.Especially,a local spin distribution appears in the deformation region.(5)We have studied the equilibrium atomic configuration,mechanical and electronical properties of nonplanar O-AGNRs under uniaxial strains in detail.The calculated binding energy reveals that the nonplanar O-AGNRs are more stable without strain.As the ribbon is growing wider,the ideal strength(absolute value)of the nonplanar O-AGNRs increases,especially in compressive region.The dependence of Young's modulus and the Poisson's ratios on strain both exhibit nonlinear behavior for the nonplanar O-AGNRs.Moreover,the band gaps of the nonplanar O-AGNRs experience a transition between direct and indirect under uniaxial strain.