Unoccupied States Measurements, Spatial Mapping, and Nanoscale Structures of Organic Ultrathin Films

Abstract

In this accounts, we summarize recent progress in experimental approaches to the investigation of the unoccupied electronic structures of organic ultrathin films, based on a combination of spectroscopic and microscopic techniques. On the occupied valence bands of the films, it has been extensively studied for a variety of organic molecules. However, systematic investigations of unoccupied electronic states still have been challenging because experimental techniques are limited. In this context, we have clarified the correlation between geometric and electronic structure using a combination of two-photon photoemission (2PPE) spectroscopy and scanning tunneling microscopy (STM). By using 2PPE, one can measure unoccupied states as well as occupied states in the vicinity of the Fermi level. Beyond the diffraction limit of light, STM can be a powerful means of mapping unoccupied electronic structures, not limited to the imaging of geometrical structures. Depending on the molecular density and substrate temperature, organic ultrathin films of polycyclic aromatic hydrocarbons on graphite substrates show a variety of structures, as demonstrated by microscopic observations on the nanoscale. It is apparent that the geometrical structures, especially molecular orientations as stressed throughout this accounts, have a strong impact on both occupied and unoccupied electronic structures. These findings, with a spectroscopic and microscopic understanding at the level of molecule, will provide fundamental insights into desirable electronic properties at organic/substrate interfaces.