Optical Effects of Divalent Functionalization of Carbon Nanotubes

Abstract

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.

Synthetic control over the binding configuration of luminescent sp3-defects in single-walled carbon nanotubes

“…6). Recent studies have shown that functionalization by radicals can also introduce further redshifted E 11 * − defects, but only when the already existing defects induce a higher π-orbital misalignment in bonds along the tube axis 29 . In contrast to that, non-radical reactive species exhibit an intrinsic preference for C-C bonds with large π-orbital misalignment and thus preferably attack carbon bonds in the axial direction (highlighted in red).…”

“…Each of these is associated with a distinct defect energy and thus emission wavelength as corroborated by density functional theory calculations 9,27,28 . Most functionalization methods produce a mix of configurations and thus defect emission bands, the most common being the E 11 * and the even more red-shifted E 11 * − emission with a longer PL lifetime [29][30][31] . They are considered to originate from the ortho-L 90 (ortho++, circumferential direction) and ortho-L 30 (ortho+, orientation along the nanotube axis) configurations of the two sp 3 carbons, respectively 20,24,27 .…”

“…However, zigzag nanotubes are a very rare species in typical nanotube raw materials 32 and thus difficult to obtain and purify in significant amounts. Another approach is the functionalization of SWNTs with either divalent functional groups (e.g., >CF 2 ) 29 or bidentate reactants with short bridging moieties (e.g., bisdiazonium compounds) 31 , which increase the probability of a specific binding configuration, albeit with limitations. Gaining real synthetic control over the defect-binding configuration and hence emission wavelength in a simple, reproducible, and possibly scalable manner is highly desirable.…”

Formation of Organic Color Centers in Air-Suspended Carbon Nanotubes Using Vapor-Phase Reaction

“…Of practical interest are single-walled carbon nanotubes (SWCNTs), since operation at room temperature and in the telecom range is possible. In particular, organic color centers formed on nanotubes [9] offer additional advantages with their optical properties being chemically tunable using a variety of molecular precursors, including arylhalides [10][11][12][13], diazonium-salt [14][15][16][17][18][19][20][21][22], ozone [23][24][25], and hypochlorite [26] that can covalently bond to the carbon lattice. By introducing dopant states with different emission energies and achieving potential traps deeper than the thermal energy, single-photon sources with desired properties can be produced [27].…”

Photon Correlation Spectroscopy of Luminescent Quantum Defects in Carbon Nanotubes

“…The related PL spectra of defect-decorated CNTs are thus rich in diversity, with frequent peak multiplicity in cryogenic single-tube PL. 17,25,28,29…”