Prediction of ultra-high ON/OFF ratio nanoelectromechanical switches from covalently bound C60 chains: An ab initio study

Applying a first-principles computational approach combining density-functional theory and matrix Green's function calculations, we analyze the microscopic origin of the switching behavior experimentally observed for the fullerene C$_{\mathrm{60}}$ chains oligomerized via [2$+$2] cycloaddition and propose a scheme to significantly improve the device performance. Considering infinite C$_{\mathrm{60}}$ chains, we first confirm that bound C$_{\mathrm{60}}$ chains with significant orbital hybridizations and band formation should in principle induce a higher conductance state. However, we find that large metal-C$_{\mathrm{60}}$ distances adopted in the scanning tunneling microscope (STM) setup can result in the experimentally observed opposite switching state assignment. The switching ordering and ratio is in fact found to sensitively depend on the STM tip metal species and the associated band bending direction in the C$_{\mathrm{60}}$--STM tip vacuum gap. We demonstrate that a junction configuration in which the C$_{\mathrm{60}}$--STM tip distance is maintained at short distances via nanoelectromechanical tip movement can achieve a metal-independent and drastically improved switching performance based on the intrinsically better electronic connectivity in the oligomerized C$_{\mathrm{60}}$ chains.