Bandgap opening in metallic carbon nanotubes due to silicon adatoms

Controlling the bandgap of carbon nanostructures is a key factor in the development of mainstream applications of carbon-based nanoelectronic devices. This is particularly important in the cases where it is desired that the carbon nanostructures are the active elements, as opposed to being conductive leads between other elements of the device. Here, we report density functional theory calculations of the effect of silicon impurities on the electronic properties of carbon nanotubes (CNTs). We have found that Si adatoms can open up a bandgap in intrinsically metallic CNTs, even when the linear density of Si atoms is low enough that they do not create an adatom chain along the tube. The bandgap opened in metallic CNTs can range up to approximately 0.47 eV, depending on adsorption site, on the linear density of Si adatoms, and on the chirality of the nanotube. We have found that a lower spatial symmetry of the charge transfer between adatom and CNT leads to a higher value of the bandgap opened, which indicates that the physical origin of the bandgap lies in the reduced spatial symmetry of the charge transferred.