Controlled Formation of Carbon Nanotubes and Nanotube Junctions from Bilayer Graphene

$S{p}^2$ bonded carbon structures exhibit a rich variety of morphologies depending on how the graphene as basic unit is laterally constrained and topologically connected to itself. Here, we demonstrate that the connection that forms between two adjacent layers of graphene after cutting by focused electron beam can be exploited in a controlled fashion to induce a spontaneous reconstruction from the flat to a tubular or even more complex geometry. In particular, we demonstrate the cutting of twisted bilayer graphene to create chiral carbon nanotubes (CNTs), using a scanning transmission electron microscope operated at 200 kV and with line doses of ${10}^9$ electrons per nanometer. By choosing the cutting angle halfway between the two graphene orientations, a seamless tube could be formed in principle, and relatively straight tube sections are obtained in practice. Bilayer graphene ribbons with a width of less than approximately 4 nm spontaneously convert to CNTs, while no nanotubes are formed from wider ribbons. Moreover, CNT arrays and CNT junctions are prepared in a controlled way. The transformations from a nanoribbon to a nanotube are also reproduced via analytical potential molecular dynamics simulations. Devices made from such junctions could be useful for nanoelectronics, quantum transport, interconnects or nanofluidics.