Previous calculations of electronic structure for collapsed carbon nanotubes established that a band gap can be opened or closed in response to the structural perturbation, depending on the nanotube chirality.To date, there is no experimental proof to show evidence of the change in electronic properties directly correlated to the nanotube chirality, thus to the electronic and atomic structure of the collapsed nanotubes.Our Scanning Tunnelling Microscopy experiments reveal the electronic properties of collapsed single-and double-walled carbon nanotubes from which atomically resolved images have been obtained, allowing for the relation between the chirality, the degree of deformation and the electronic behaviour to be examined. We find, in accordance to previous theoretical studies, that the metallic character of an armchair structure is maintained provided the squashing does not allow for C-C bond formation between the two opposing layers. We also find that lattice commensurability effects, responsible for twist-induced deformations, account for opening of a gap in the case of collapsed double-walled carbon nanotubes.These results present robust evidence in support of changes in the fundamental electronic properties when carbon nanotubes are subject to radial deformations, highlighting exciting possibilities for tuning their electronic properties to provide the required metallic or semiconducting features only by squashing the nanotubes.
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