Effect of Electron Induction Abilities of Functional Groups on Lowest Excitons in Covalently Functionalized Carbon Nanotubes
Braden M. Weight,1, Brendan J. Gifford,2,3,4 and Svetlana Kilina1,
1 Department of Chemistry and Biochemistry, North Dakota State University, Fargo, North Dakota 58108, US, 2 Theoretical Division and 3 Center for Nonlinear Studies, Los Alamos National Laboratory, Los Alamos, NM 87545, US
Covalent functionalization of carbon nanotubes (CNTs) brightens the lowest-energy optical transition associated with the chemical defect, making them promising materials for single photon sources. Furthermore, the emission energies can be tuned by varying the electron induction abilities of the functional group. We investigate the exciton structure and charge density distribution in the vicinity of the defect of (10,5) CNTs functionalized with halide derivatives of alkyl and aryl groups with various electron withdrawing or donating abilities. Using time-dependent density functional theory calculations, we found that increasing the electron withdrawing ability of the functional group from CH3 to C6F13 systematically redshifts the lowest-energy transition, but by a small value (< 15 meV). More significant redshifts of 50 meV are realized by functionalizing with two identical groups – especially, in the case of two proximal fluorine atoms – instead of a single group and hydrogen. This trend is caused by increased dipole moments of the system induced by stronger steric and electrostatic repulsion from the close proximity of the groups. We also reveal that much stronger redshifts (~150 meV) are generated from changing the geometry of the defect than changing the functional group. Thus, our calculations have detected the limitations in using different functional groups to tune emission features of CNTs.