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

We identified the orientation of individual Lu@C-82 molecules on alkanethiol self-assembled monolayers (SAMs) by scanning tunneling microscopy (STM) at a molecular resolution. STM images of Lu@C-82 on alkanethiol SAMs at 65 K showed a striped structure corresponding to the molecular orbitals of the Lu@C-82 molecule, suggesting that thermal rotation of Lu@C-82 on alkanethiol SAMs is prevented at 65 K. By comparing these molecular-resolution STM images with Kohn-Sham molecular orbitals of Lu@C-82 calculated by density functional theory (DFT), we identified the molecular orientation of Lu@C-82. Spatial mapping of the differential conductance on individual Lu@C-82 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@C-82 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 LI.1 atom and its six nearest C atoms at 0.055 eV below the HOMO level. We demonstrated changes in the molecular orientation of Lu@C-82 by applying a high electric field (about 1 x 10(7) V/cm) with a large tunneling current (1.5 nA).