Pentacene crystal transition during the growth on SiO2 studied by in situ optical spectroscopy

- First, the combination of optical spectroscopy and atomic force microscopy (AFM) investigations reveals that the first monolayer crystallize into the so-called “orthorhombic phase” with near-vertically oriented pentacene molecules, and the structure is persevered even after being buried by the multilayer.
- Second, from the second monolayer growth on, the optical properties are significantly changed. This experimental evidence indicates the multilayer on top of the first monolayer comprises the so-called “thin-film phase” crystallites. These observations suggest that the thickness-driven phase transformation from the orthorhombic phase to the thin-film phase starts at the completion of the first monolayer.
- Third, our result demonstrates that the differential optical spectroscopy (DRS) is a powerful probe to the optical and thus the structural properties of ultra-thin films which content only several molecular monolayers, and even sub monolayer.

We discuss the processes involved in the formation of a few monolayer thick pentacene films on the SiO2 surface. The evolution of the optical properties has been monitored in situ by optical spectroscopy and the interface morphology was investigated ex situ by atomic force microscopy. The sub monolayer shows the identical characteristic optical response with the intensity proportional to the coverage. From the second monolayer growth on, the optical properties are significantly changed. This experimental evidence indicates the formation of the first monolayer layer comprising the so-called “orthorhombic phase” crystallites with near-vertically oriented pentacene molecules, and the multilayer above comprises the so-called “thin-film phase” crystallites. Most importantly, the combination of optical spectroscopy and atomic force microscopy (AFM) investigations reveals that the first monolayer crystallize into the orthorhombic phase and the structure is persevered even after being buried by the multilayer. These observations suggest that the thickness-driven phase transformation from the orthorhombic phase to the thin-film phase starts from the second monolayer on rather than the first monolayer. This result provides a new insight into the structural evolutions and growth of pentacene thin films on SiO2.

Our experimental evidence indicates the formation of the first pentacene monolayer on SiO2 comprising the so-called “orthorhombic phase” crystallites with near-vertically oriented pentacene molecules, and the multilayer above comprises the so-called “thin-film phase” crystallites. Most importantly, the 2D islands in the sub monolayer regime crystallize into orthorhombic phase and the structure is persevered even after being buried by the multilayer. Consequently, the thickness-driven phase transformation from the orthorhombic phase to the thin-film phase starts from the second monolayer on rather than the first monolayer.157