  Ultra-thin topological insulator nanostructures, in which coupling between
top and bottom surface states takes place, are of great intellectual and
practical importance. Due to the weak Van der Waals interaction between
adjacent quintuple layers (QLs), the layered bismuth selenide (Bi2Se3), a
single Dirac-cone topological insulator with a large bulk gap, can be
exfoliated down to a few QLs. In this paper, we report the first controlled
mechanical exfoliation of Bi2Se3 nanoribbons (> 50 QLs) by an atomic force
microscope (AFM) tip down to a single QL. Microwave impedance microscopy is
employed to map out the local conductivity of such ultra-thin nanoribbons,
showing drastic difference in sheet resistance between 1~2 QLs and 4~5 QLs.
Transport measurement carried out on an exfoliated (\leq 5 QLs) Bi2Se3 device
shows non-metallic temperature dependence of resistance, in sharp contrast to
the metallic behavior seen in thick (> 50 QLs) ribbons. These AFM-exfoliated
thin nanoribbons afford interesting candidates for studying the transition from
quantum spin Hall surface to edge states.
