The correlation between geometric structures and photoluminescence properties of the monolayer perylene (C 20 H 12 ) on a graphite surface was investigated by using a combination of scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and photoluminescence spectroscopy. The formation of a well-ordered perylene monolayer (ML) was confirmed upon deposition of perylene onto the room temperature substrate followed by cooling to liquid nitrogen temperature. A rectangular unit cell with a point-on-line coincidence was identified by a quantitative analysis of the LEED patterns; it was the only structure found for coverages up to 1 ML. The observed STM images clarified the molecular pair (dimer) arrangement with standing molecular configuration. The dimer arrangement was further corroborated by photoluminescence measurements, which was characterized as orange light emission (centered at a wavelength of 610 nm) from the excited state of the perylene dimer (excimer). In contrast, the perylene dimer structure was not found on the room temperature substrate. The absence of dimers is consistent with the green-yellowish photoluminescence reported for the monomeric form of the perylene crystal. The combination of STM, LEED, and photoluminescence spectroscopy allows the linking of a film structure with its photoluminescence properties. The present results demonstrate that the optoelectronic properties of organic materials can be controlled even at the monolayer limit.

Spectroscopic and nanoscale imaging investigations concerning the spatial extent of molecular orbitals at organic/substrate interfaces have been of intense interest to understand charge dynamics. Here, the spatial extent of unoccupied molecular orbitals of ultrathin rubrene [5,6,11,12-tetraphenyltetracene] films has been investigated with scanning tunneling microscopy and spectroscopy. Based on constant-current distance (z)-voltage (V) measurements, the unoccupied energy levels are elucidated and found to be consistent with previously reported macroscopic two-photon photoemission (2PPE) spectroscopy. In the diffuse unoccupied molecular orbitals reported with 2PPE (J. Phys. Chem. C, 2013, 117, 20098), nanoscale dz/dV spatial maps reveal that the local density of states of the orbitals extends over the rubrene molecules. Delocalization is also observed for the image potential states, which are inherently free-electron-like. This is in contrast to the localized nature of other unoccupied molecular orbitals. A nanoscale understanding of diffuse and delocalized molecular orbitals provides a fundamental insight into low-lying Rydberg states in polycyclic aromatic hydrocarbons.