Computational Studies of Defects in Nanoscale Carbon Materials
Electron scattering observed by STM/STS at artificially created defects on single-walled carbon nanotubes
We have investigated the effects of local atomic defects on the electronic structure of single-walled carbon nanotubes (SWNT) by means of low-temperature scanning tunnelling microscopy (STM) and spectroscopy (STS). The defects have been created by irradiation of the samples with low energy hydrogen ions (2 eV mean ion energy), medium energy argon ions (200 -1500 eV) or by cutting the SWNT with the STM tip. Besides significant changes in the local density of states (LDOS) at the defect sites, we observe pronounced signatures of electron standing waves, which result from electron scattering at the defect sites.
We will show that quasi bound states can be created between two closely spaced (10-20 nm) defects in metallic SWNTs, with regular energy spacings above 100 meV. This observation demonstrates the feasibility of room temperature active intra tube quantum dots by using ion irradiation. Fourier-transform scanning tunnelling spectroscopy reveals that the observed electron standing waves are composed of multiple high and low spatial frequency components, which can be attributed to large (inter-valley) and small (intra-valley) momentum scattering, respectively. The position of these components in reciprocal space is chirality dependent and can be obtained by projection of the possible electron scattering vectors in the extended zone scheme onto the direction of the tube axis. The experimental results will be compared to the results from a Fabry-Perot electron resonator model, where we are able to describe in detail the scattering dynamics and to identify contributions from inter- and intra-valley scattering.
The experimental data show significant anisotropies in the scattering strength of the defects with regard to energy. In particular, we observe a pronounced asymmetry regarding the parity of the states participating in the scattering event. This behaviour is at the moment not fully understood. Since our observations are of pivotal importance for electronic transport properties in defective carbon nanostructures, this issue certainly deserves further in-depth experimental characterization and theoretical explanation.