Quasiparticle and Excitonic Effects in the Optical Response of Nanotubes and Nanoribbons

This chapter discusses the effects of many-electron interactions in thephotophysics of nanotubes and their consequences on measured properties. The basictheory and key physical differences between two common types of electronicexcitations are developed: single-particle excitations (quasiparticles) measured intransport or photoemission experiments, and electron–hole pair excitations (excitonicstates) measured in optical experiments. We show, through first-principlescalculations, that both quasiparticle and excitonic effects are crucial in understandingthe optical response of the carbon nanotubes. These effects change qualitatively thenature of the photoexcited states, leading to extraordinarily strongly boundexcitons in both semiconducting and metallic nanotubes and explaining theso-called “ratio problem” in carbon-nanotube spectroscopy. Using simplifiedmodels parameterized by the first-principles results, the diameter and familydependences of the exciton properties in carbon nanotubes are further elucidated.We also analyze the symmetries of excitons and their selection rules forone- and two-photon spectroscopy. A method for calculating the radiativelifetime of excitons in carbon nanotubes is also described. In addition, webriefly discuss the effects of pressure and temperature on optical transitions.Finally, we show that many-electron effects are equally dominant in theexcitation spectra of other quasi-one-dimensional systems, including theboron-nitride nanotubes, semiconductor nanowires, and graphene nanoribbons.