Constraining Photoluminescent Defect States in Chirality-Sorted Covalently Doped Single-Walled Carbon Nanotubes

Abstract

Recent developments in carbon nanotube surface chemistry are leading to the emergence of unexpected new optical behaviors. Specifically, introduction of covalently-bound defect sites into the nanotube structure generates a new photoluminescent (PL) emitting state that is significantly red-shifted from the normal excitonic emitting state that results from optical excitation.1 These new states have been shown to significantly increase emission quantum yields and introduce new functionality including room-temperature single photon emission, near-IR photon upconversion, and multifunctionality generated by localization of excitons at these defect sites.2,3,4 The new optical behaviors present a broad range of unexplored photophysics yet to be fully understood. Of particular interest is the use of emerging functionalization chemistries to introduce sp3 defects that have highly tunable properties. An important challenge arising from the functionalization chemistry is that seemingly similar and structurally simple functionalization can give rise to a broad diversity in observed PL spectral features.1,3,5 Low-temperature spectroscopy along with quantum chemistry calculations demonstrated that these multiple emissive defect states are likely due to the occurrence of multiple possible, chemically distinct, binding configurations on the nanotube surface.6 Following these results, we hypothesize that the diverse emissive states are also influenced by the interplay of nanotube chirality, structure of the dopant, and surface structures on the tube. In this regard, we aim to control the number of emissive states through control of these factors. We have studied a number of different chiral structures for aryldiazonium functionalization, ranging from so-called near-armchair to near-zigzag structures of varying diameter. We have demonstrated that zigzag carbon nanotubes tend to constrain photoluminescent defect states more efficiently than other chiralities. Density functional theory (DFT) calculations suggest that symmetry of the zigzag tube is the primary factor in creating a less diversified emission spectrum. Our findings on controlling the defect peak diversity could enhance the predictable functionality of these doped carbon nanotubes.