Engineering Intrinsic and Extrinsic Quantum Interference in Electronic States of Carbon Nanotubes Measured By Resonant Raman Scattering
Here, we examine the origin of these deviations from the expected symmetrical REPs for exciton-mediated, one-phonon Raman processes in SWCNTs, by expanding the previous measurements on the RBM and G-band phonons to include additional pure SWCNT species as a function of chiral angle, diameter, and electronic type. We measured the intrinsic non-Condon-induced self-interference of the G-band in the “metallic” family of SWCNTs, consisting of the so-called armchair and narrow-gap semiconductor (NGS)-type nanotube species. Specifically, we examined the diameter dependence of REP asymmetry of armchairs ranging from (5,5) to (8,8), as well as the NGS-species (7,4). The addition of these particular nanotube structures allows the examination of the effect of electronic type on REP behavior, where the unique linear band structure of armchair and quasi-linear band structure of NGS species produces additional electron-phonon coupling resonances through the so-called Kohn anomaly, further enhancing the observed REP asymmetry due to non-Condon-based self-interference. Preliminary data already suggests that armchair species exhibit the largest degree of G-band REP asymmetry relative to other species. Furthermore, NGS-type SWCNT species, such as the (7,4), possess the additional trigonal warping splitting term in their band structure, which causes each optical transition to split into two, allowing for additional quantum interference between upper and lower branch transitions.
To further explore and control the degree of quantum interference between transitions possible, we applied mechanical uniaxial strain to single-chirality SWCNTs embedded in stretchable polymer films to further induce and enhance quantum interference for chiralities such as the (12,4), (8,6) and (7,4) and measured their RBM and G-band REPs as a function of strain. Theoretical calculations predict that the application of a few percent uniaxial strain will cause the third and fourth optical transitions in certain type-I semiconductor species and the trigonal-warping split transitions in NGS-type tubes to move toward one another in energy, thereby inducing quantum interference. In this situation, interference can be manipulated and, in some cases where quantum interference did not previously exist, created artificially, allowing a broader range of REP asymmetries to be examined.