Energetics and stability of pentacene thin films on amorphous and crystalline octadecylsilane modified surfaces

Charge transport in organic thin film transistors (OTFTs) is directly related to the morphology and growth of the organic semiconductor at the dielectric interface. The most commonly used dielectric interface in OTFT research is alkylsilane-modified silicon oxide (SiO2). In this report, the nucleation, energetics, and stability of pentacene thin films on methyl-terminated surfaces are discussed. The density of the terminal methyl group was found to be an important parameter for controlling the growth of organic semiconductors. Pentacene growth is two-dimensional (2D) on SiO2 dielectrics modified with a crystalline, densely packed octadecylsilane (OTS) monolayer. However, it is primarily three-dimensional (3D) on SiO2 dielectrics modified with an amorphous OTS layer. Beyond a critical OTS density, the interaction between the OTS and pentacene exceeds the pentacene interlayer interaction energy engendering 2D growth which is preferential for high charge carrier mobility. The nucleation density is also much higher on the crystalline OTS compared to the amorphous OTS. The sub-monolayer thin films of pentacene were found to be much more stable on the ordered OTS compared to disordered OTS. Atomic force microscopy (AFM) and Monte Carlo simulations were used to develop a thorough analysis of pentacene film growth and energetics on OTS surfaces.