Optical and electronic anisotropy of a π-conjugated molecular monolayer on the silicon(001) surface

The electronic and optical properties of an oriented monolayer of a 7-conjugated molecule, 9,10-phenanthrenequinone on a silicon(001) substrate were investigated. The adsorption of 9,10-phenanthrenequinone onto a single-domain, 4degrees-miscut Si(001) surface leads to an oriented monolayer film with intact,7, conjugation. Reflectance-difference spectroscopy (RDS) was used to probe the optical properties of the layer of 9,10-phenanthrenequinone, and comparison spectra were obtained on a monolayer 1,2-cyclohexanedione, a nonconjugated molecule that has a nearly identical structure at the Si-O-C interface. RDS spectra of the 9,10-phenanthrenequinone monolayer show a strong absorption feature at 5.2 eV due to pi-pi* transitions within the molecule, while spectra of the nonconjugated analogue show no feature in this region. Further information about the electronic structure is provided by ultraviolet photoemission spectroscopy of the chemisorbed monolayers and by ultraviolet-visible absorption spectroscopy of the parent compounds. The experiments confirm that it is possible to create a macroscopic optical anisotropy in organic films on silicon surfaces using the Si(001) dimers to control the molecular orientation.