Studies of mechanically deformed single walled carbon nanotubes and graphene by optical spectroscopy.

摘要

Due to the extremely low density of defects, carbon nanotubes and graphene were predicted in theory and demonstrated experimentally to possess excellent mechanical properties and can sustain relatively large strain. This gives us an opportunity to explore the impact of mechanical deformation to their physical properties.;In the first half of the thesis, we present the studies about mechanical and electromechanical properties of the optical characterized single wall carbon nanotubes (SWNTs). For the mechanical properties, we have combined optical spectroscopy with a magnetic actuation technique to measure the stiffness of these structure defined SWNTs. The measured stiffnesses correspond to an average Young's modulus of E = 0.99 TPa. By using the same devices, the non-linear resonators were demonstrated and their behaviors can be understood from the damped Duffing resonator model. In addition, the breaking strength of the SWNTs was measured by mechanical tensile testing. The resulting fracture strength is about 40 GPa and which corresponds to 3.6% failure strain. This value can be regarded as the low bound of the breaking strength due to the clamping induced extra defects. For the electromechanical property, the effect of uniaxial strain on the optical transition energies of single-walled carbon nanotubes with known chiral indices was determined by Rayleigh scattering spectroscopy. In semiconducting nanotubes, successive transitions shift in opposite directions, with a characteristic dependence on the nanotube family. In chiral metallic nanotubes, the split peaks approach one another under strain. Existing theory accurately predicts the trends in the measured strain-induced shifts, but overestimates their magnitude. Modification of the analysis to account for internal sub-lattice relaxation results in quantitative agreement with experiment.;In the second half of the thesis, we present a systematic study of the Raman spectra of optical phonons in graphene monolayers under tunable uniaxial tensile stress. All of the prominent bands exhibit significant red shifts and the resulting shift rates can be used to calibrate strain in graphene. Under uniaxial stress, the G band splits into two distinct sub-bands because of the strain-induced symmetry breaking. Raman scattering from the sub-bands shows distinctive polarization dependence that reflects the angle between the axis of stress and the underlying graphene crystal axes. Polarized Raman spectroscopy therefore constitutes a purely optical method for the determination of the crystallographic orientation of graphene. The 2D' band shows two different frequencies depending on the angle between the incident light polarization and strain axis. This polarization dependence reveals the anisotropy of the light absorption around the Dirac cone and the anisotropic phonon dispersion around the Brillouin zone center. This study opens a path to determine the strain principal axes by simply measuring the spectra of the 2D' band at different sample orientation. In conjunction with the G mode polarized Raman measurement, one actually can further determine the crystallographic orientation of graphene even without the pre-knowledge of the strain direction. Finally, the 2D band also displays profound splitting under high strain. The sub-bands shows different polarization dependence and their manners also depend on the crystal orientation. The behavior of the 2D band uncovers the strain induced electronic structure changes.