Recently we demonstrated that the guanine functionalization reaction can alter the optical properties of individual tubes through patterned chemical modification of the SWCNT surface [1]. This DNA-assisted patterning means that nanotubes can potentially be tailored to generate a pre-designed profile of electronic properties along their length. Results might include the directional flow of electrons or excitons in individual nanotubes, or the formation of spatially patterned regions with distinct optical and electronic properties.
Here we present results from semi-empirical quantum chemical computations on semiconducting SWCNTs containing multiple covalent functionalization sites. Computations were performed on SWCNT segments with variable numbers of sites, each of which converts a pair of carbon atoms from sp2 to sp3 hybridization. By varying the spatial density and spatial patterns of covalent addends we observed the following effects:
- The HOMO-LUMO gap shows a nearly linear decrease as the number of addends increases. This implies red-shifted optical transitions, as has been observed experimentally.[1]
- For a given number of addends, energies depend on their relative positions. This should be observed experimentally as an increased broadening of spectral transitions as they are more shifted from the position in a pristine SWCNT. But the average shifts appear to be nearly linear with the number of addends if addend positions are not exactly regular.
- The formation of adjacent addends leads to interference effects and wavefunction disruptions. These can be avoided by using templated functionalization reactions, such as for ssDNA containing guanine nucleobases.
[1] Yu Zheng, Sergei M Bachilo, R Bruce Weisman, Controlled patterning of carbon nanotube energy levels by covalent DNA functionalization, ASC Nano 13 (2019), 8222-8228