Electronically tailored functionalization of carbon nanotubes
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Carbon nanotubes (CNTs) were chemically tailored on the electronic level to enhance their optical and electrical properties.
Incorporation of sp3 defects into the sidewalls of CNTs significantly improved quantum efficiency of CNT photoluminescence (PL). Nanotube PL is intrinsically inefficient, usually less than 1%, due to the presence of dark excitons. This low efficiency makes nanotubes impractical for many applications, especially bio-imaging and optoelectronics. The nanotube PL was increased by up to 28 times through the chemical creation of a new defect induced state. This new state is optically allowed and resides below the predicted energy levels of the dark excitons, allowing the dark excitons to be harvested from this new defect state. Emission from the new state generates a distinct, structure-specific, and chemically tunable photoluminescence peak. This new peak is red-shifted by as much as 254 meV from the original excitonic transition and located within the tissue transparent window, which merits bio-imaging and bio-sensing. This work opens the door to harnessing dark excitons and lays the foundation for chemical control of defect quantum states in low dimensional carbon materials.
Unlike atom-thick materials such as SWCNTs and graphene which are prone to chemical attacks because all constituent atoms are exposed, double-walled carbon nanotubes (DWCNTs) provide a chemically tailorable surface and an inner-tube with intact electronic properties. Even when the outer walls were selectively functionalized up to 6.9% (percent of carbon that are covalently modified), the inner tubes were electrically intact. Correlated Raman and optical absorption spectroscopy unambiguously confirm that the covalent modification was outer wall-selective. Nearly 50% of the electrical conductivity was retained in thin films of covalently functionalized nanotubes owing to the protected inner-tube conducting channels. Lacking such channels, SWCNTs became insulators after similar functionalization. Further experiments demonstrated that the covalently attached aryl groups could be selectively removed by optical annealing. These results suggest the possibility of high performance DWCNT electronics with important capabilities of tailored surface chemistry on the outer walls while the inner walls are chemically protected.