The introduction of sp defects into single-walled carbon nanotubes through covalent functionalization can generate new light-emitting states and thus dramatically expand their optical functionality. This may open up routes to enhanced imaging, photon upconversion, and room-temperature single-photon emission at telecom wavelengths. However, a significant challenge in harnessing this potential is that the nominally simple reaction chemistry of nanotube functionalization introduces a broad diversity of emitting states. Precisely defining a narrow band of emission energies necessitates constraining these states, which requires extreme selectivity in molecular binding configuration on the nanotube surface. We show here that such selectivity can be obtained through aryl functionalization of so-called 'zigzag' nanotube structures to achieve a threefold narrowing in emission bandwidth. Accompanying density functional theory modelling reveals that, because of the associated structural symmetry, the defect states become degenerate, thus limiting emission energies to a single narrow band. We show that this behaviour can only result from a predominant selectivity for ortho binding configurations of the aryl groups on the nanotube lattice.

Covalent functionalization of single-walled carbon nanotubes (SWCNTs) enables tuning of their optical properties through the generation of sp3-hybridized defects with distinct localized morphology. Groups with strong electron-withdrawing abilities result in redshifted emission experimentally. Further redshifts can be generated by groups bound to more than one carbon atom in the SWCNT (“divalent functionalization”). Depending on the type of divalent functionalization, the spectral diversity is reduced compared to their monovalent counterparts. Here we study the effect of divalent functionalization on the exciton localization at the defect site and related redshifts in emission of (6,5) SWCNT through low-temperature spectroscopy measurements and time dependent density functional theory calculations. These effects are characterized for three classes of divalent compounds distinct in the number of atoms in the functional group and bonding pattern to the tube. The bond character of the two carbon atoms proximal to the defect site is found to have a notable impact on the system stability and spectral redshifts. Functionalized systems are stabilized when the hybridization at the SWCNT remains sp2-like due to its ability to form planar bonds to the remaining hexagonal network, while bond character in the functionalized regions affects the redshifts. This is only possible for certain bonding geometries in divalent species, justifying their decreased spectral diversity. We further show that functionalization at spatially separated sites on the tube can be accompanied by a second chemical adduct, and the configuration of the resulting defect is dictated by bond reactivity following the first addition. This behavior justifies the spectral trends of a class of divalent systems with linker chains or high defect concentration. These results further corroborate that adducts predominantly form chemical bonds only to the neighboring carbons on the SWCNT surface (ortho species) in experimental samples. Our analysis of bond character in the vicinity of the defect sites rationalizes appearance of many spectral features arising from monovalent and divalent defect states of functionalized SWCNTs. This emerging understanding enables tuning of the emission characteristics through careful control of the defect structure.

Molecularly functionalized single-walled carbon nanotubes (SWCNTs) are potentially useful for fiber optical applications due to their room temperature single-photon emission capacity at telecommunication wavelengths. Several distinct defect geometries are generated upon covalent functionalization. While it has been shown that the defect geometry controls electron localization around the defect site, thereby changing the electronic structure and generating new optically bright red-shifted emission bands, the reasons for such localization remain unexplained. Our joint experimental and computational studies of functionalized SWCNTs with various chiralities show that the value of mod­(n-m,3) in an (n,m) chiral nanotube plays a key role in the relative ordering of defect-dependent emission energies. This dependence is linked to the complex nodal characteristics of electronic wave function extending along specific bonds in the tube, which justifies the defect-geometry dependent exciton localization. This insight helps to uncover the essential structural motifs allowing tuning the redshifts of emission energies in functionalized SWCNTs.