  We study the collapsing and subsequent spontaneous twisting of a carbon
nanotube by in-situ transmission electron microscopy. A custom-sized nanotube
is first created in the microscope by selectively extracting shells from a
parent multi-wall tube. The few-wall, large-diameter daughter nanotube is
driven to collapse via mechanical stimulation, after which the ribbon-like
collapsed tube spontaneously twists along its long axis. In-situ diffraction
experiments fully characterize the uncollapsed and collapsed tubes. From the
experimental observations and associated theoretical analysis, the origin of
the twisting is determined to be compressive strain due to charge imbalance.
