Band Gap Renormalization Drives Ultrafast Charge Separation and Slow Recombination in Covalently Functionalized Carbon Nanotubes: Nonadiabatic Molecular Dynamics Simulation.

Covalent functionalization offers a versatile platform to engineer carbon nanotube properties for optoelectronics applications. We demonstrate by atomistic quantum dynamics simulation that covalent functionalization can renormalize the CNT band gap, split the degenerate CNT band edge states, and strongly influence charge carrier separation and recombination dynamics. Open-ring CNT functionalization (O-CNT) largely retains the π-conjugation, whereas closed-ring functionalization (C-CNT) perturbs the electronic structure due to sp3 hybridization at the functionalized site. The energy gaps for the charge separation and recombination in the hybrid of O-CNT with the tetra-cyano-anthra-quinodimethane (TCAQ) molecule are comparable to those in the corresponding noncovalent van-der-Waals hybrid (V-CNT@TCAQ). In contrast, localized band edge states appear in C-CNT@TCAQ, renormalizing the energy gaps. Generally, the covalent functionalization accelerates charge separation relative to the V-CNT system, with C-CNT@TCAQ showing the most efficient separation due to the smallest energy offset. C-CNT@TCAQ also exhibits the most advantageous, slowest charge recombination, due to enhanced charge localization and reduced electron-hole overlap, resulting in the smallest nonadiabatic coupling and the shortest coherence time. Both charge separation, occurring within picoseconds, and recombination, taking nanoseconds, become more favorable, when the functionalization creates a stronger perturbation to the CNT. The reported theoretical investigation reveals how rapid charge separation and slow recombination can be achieved through covalent functionalization of CNTs, providing key guidelines for design of modern and efficient optoelectronic and solar energy materials.