Electronic Structures of Quaterthiophene and Septithiophene on Cu(111): Spatial Distribution of Adsorption-Induced States Studied by STM and DFT Calculation

The oligothiophene molecule family has a tunable energy gap between the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) as the function of the number of thiophene units. This tunability is of great use to controlling carrier injection at the molecule/electrode interface in molecule-based electronic devices. We investigate quaterthiophene (4T) and septithiophene (7T) molecules adsorbed on Cu(111) surfaces by scanning tunneling microscopy and spectroscopy (STM and STS) at room temperature. Both oligothiophene molecules form one-dimensional (1D) chain structures on Cu(111), and each molecule in the 1D structures is observed as a row of bright ovals corresponding to thiophene units. Observed features of 4T and 7T molecules differ from those expected from the HOMO and LUMO of the free-standing molecules, and density-functional calculations of a 4T molecule together with a Cu(111) surface reproduce the experimental STM images as they reflect characteristic spatial distribution of adsorption-induced states. In other words, the adsorption-induced states are spatially protruding out from the molecule and not completely localized in the space between the molecule and the Cu(111) surface