Quasiparticle and excitonic effects in the optical response of nanotubes and nanoribbons

Abstract

This chapter discusses the effects of many-electron interactions in the photophysics of nanotubes and their consequences on measured properties. The basic theory and key physical differences between two common types of electronic excitations are developed: single-particle excitations (quasiparticles) measured in transport or photoemission experiments, and electron-hole pair excitations (excitonic states) measured in optical experiments. We show, through first-principles calculations, that both quasiparticle and excitonic effects are crucial in understanding the optical response of the carbon nanotubes. These effects change qualitatively the nature of the photoexcited states, leading to extraordinarily strongly bound excitons in both semiconducting and metallic nanotubes and explaining the so-called "ratio problem" in carbon-nanotube spectroscopy. Using simplified models parameterized by the first-principles results, the diameter and family dependences of the exciton properties in carbon nanotubes are further elucidated. We also analyze the symmetries of excitons and their selection rules for one- and two-photon spectroscopy. A method for calculating the radiative lifetime of excitons in carbon nanotubes is also described. In addition, we briefly discuss the effects of pressure and temperature on optical transitions. Finally, we show that many-electron effects are equally dominant in the excitation spectra of other quasi-one-dimensional systems, including the boron-nitride nanotubes, semiconductor nanowires, and graphene nanoribbons.