Direct visualization of diffuse unoccupied molecular orbitals at a rubrene/graphite interface.

Area of Science:

• Surface Science
• Materials Science
• Organic Electronics

Background:

• Understanding charge dynamics at organic/substrate interfaces is crucial.
• Molecular orbital spatial extent influences electronic properties.
• Previous studies used macroscopic techniques like two-photon photoemission (2PPE).

Purpose of the Study:

• To investigate the spatial extent of unoccupied molecular orbitals in ultrathin rubrene films.
• To elucidate unoccupied energy levels using nanoscale imaging.
• To contrast localized and delocalized orbital behaviors.

Main Methods:

• Scanning tunneling microscopy (STM) and spectroscopy (STS).
• Constant-current distance (z)-voltage (V) measurements.
• Nanoscale dz/dV spatial mapping.

Main Results:

• Unoccupied molecular orbitals in rubrene films extend over the molecules.
• Delocalization was observed for diffuse unoccupied molecular orbitals and image potential states.
• This contrasts with the localized nature of other unoccupied molecular orbitals.

Conclusions:

• Nanoscale imaging reveals delocalized molecular orbitals in rubrene.
• Provides fundamental insights into low-lying Rydberg states in polycyclic aromatic hydrocarbons.
• Highlights the importance of spatial extent for charge dynamics at interfaces.