Molecular Orientation of Individual Lu@C82 Molecules Demonstrated by Scanning Tunneling Microscopy

We identified the orientation of individual Lu@C82 molecules on alkanethiol self-assembled monolayers (SAMs) by scanning tunneling microscopy (STM) at a molecular resolution. STM images of Lu@C82 on alkanethiol SAMs at 65 K showed a striped structure corresponding to the molecular orbitals of the Lu@C82 molecule, suggesting that thermal rotation of Lu@C82 on alkanethiol SAMs is prevented at 65 K. By comparing these molecular-resolution STM images with Kohn−Sham molecular orbitals of Lu@C82 calculated by density functional theory (DFT), we identified the molecular orientation of Lu@C82. Spatial mapping of the differential conductance on individual Lu@C82 molecules revealed that the local conductivity within a molecule became large around the Lu atom at a negative sample bias voltage. From spatial mapping of the differential conductance measurements, we also evaluated the HOMO−LUMO gap of Lu@C82 to be 0.47 eV. From the results of the spatial mapping of the differential conductance and DFT calculations, the locally high conductivity around the Lu atom was attributed to the HOMO-2 level orbital concentrated on the Lu atom and its six nearest C atoms at 0.055 eV below the HOMO level. We demonstrated changes in the molecular orientation of Lu@C82 by applying a high electric field (about 1 × 107 V/cm) with a large tunneling current (1.5 nA).