PhD Prospectus Defense - Hang Chen - Carbon Nanotubes and Nanohoops

Carbon Nanotubes and Carbon Nanohoops: Probing the Vibrational Properties and Electron-Phonon Coupling Using Raman Spectroscopy The advent of post-production separation approaches facilitated separation by carbon nanotube (CNT) diameter and metallicity, and even enrichment in single-chirality nanotubes, enabling both specific fundamental studies and enhanced functionalities for CNTs. With the aid of such chirality-enriched CNT samples, we were able to probe some novel electronic behaviors with high chirality dependences but had been inaccessible in the impure CNT bundles. One extreme case of the carbon nanotubes − the carbon nanohoops, namely cycloparaphenylenes (CPPs), represent the shortest subunits of armchair CNTs. Their intriguing structural and optoelectronic properties have attracted lots of attentions in recent years. Since these new materials are close relatives to the well-known CNTs, it comes as a natural choice for us to probe the CPPs by Raman spectroscopy, which has been so instrumental in characterizing CNTs thanks to its non-destructive nature and the ability of probing the electronic, vibrational, and electron-phonon coupling properties through resonance Raman scattering. In this prospectus, we present our Raman-related research on CNTs and CPPs. First, we discuss our published results for the new findings on the resonance Raman spectroscopy (RRS) of CNTs, with the focus on the Raman excitation profiles (REPs) of the second (E22), third (E33), and fourth (E44) excitonic transitions for the G-band. The asymmetric lineshapes observed in the G-band REPs for a wide range of CNT species indicate the breakdown of a long-held approximation, namely the Franck-Condon principle, for semiconducting CNTs. In addition, the G-band REPs for the closely spaced E33 and E44 transitions show significant quantum interference effects between the excitonic and vibrational levels. Next, we introduce our newly published results on the first study of Raman spectroscopy of CPPs. A plethora of Raman modes are observed in these spectra, including modes that are analogous to those of the CNTs such as the G-band, as well as Raman peaks that are unique for CPPs. Moreover, we have calculated the theoretical Raman spectra of [n]CPPs for n=4-20 using density functional theory (DFT), which are then compared to the experimental results for the assignment of different modes. The Raman peak positions are seen to be dependent on the size of the nanohoop from both the experimental and the calculated results. Finally, we present our plans for the future work, mainly concentrated on a more comprehensive analysis of the CPP Raman features, as well as bridging the gap between the two closely related materials through further experimental and computational Raman studies.