  Applying a first-principles computational approach, we have systematically
analyzed the effects of [2+2] cycloaddition oligomerization of fullerene C60
chains on their junction electronic and charge transport properties. For
hypothetical infinite C60 chains, we first establish that the polymerization
can in principle increase conductance by several orders of magnitude due to the
strong orbital hybridizations and band formation. On the other hand, our
simulations of the constant-height scanning tunneling microscope (STM)
configuration shows that, in agreement with the recent experimental conclusion,
the junction electronic structure and device characteristics are virtually
unaffected by the C60 chain oligomerization. We further predict that the
switching characteristics including even the ON/OFF-state assignment will
sensitively depend on the substrate metal species due to the Fermi-level
pinning at the substrate-side contact and the subsequent energy level bending
toward the STM tip-side contact. We finally demonstrate that a force-feedbacked
nanoelectromechanical approach in which both of the C60-electrode distances are
kept at short distances before and after switching operations can achieve a
metal-independent and significantly improved switching performance due to the
Fermi-level pinning in both contacts and the large intrinsic conductance
switching capacity of the C60 chain oligomerization.
