Ion-scattering analysis of self-assembled monolayers of silanes on organic semiconductors

We describe new Rutherford Backscattering Spectroscopy (RBS) measurements to explore the surface chemistry associated with the growth of self-assembled monolayer (SAM) molecules on conducting organic films. The report includes a description of the optimization of both substrates and RBS scattering parameters to appropriately analyze these novel and damage susceptible structures. Our RBS measurements reveal that the final surface stoichiometry is consistent with a specific model of hydrolyzed and crosslinked trichlorosilanes that form a dense two-dimensional network (a monolayer) at the surface of small-molecule organic semiconductors and a bulk SAM network in the case of conjugated polymer films. Organic semiconductors used in this study are thin films of rubrene (a small molecule semiconductor (C42H28)) and poly(3-hexyl)thiophene (P3HT) (a conjugated polymer (C10H18S)n). As a substrate we used a thick (1 μm) film of parylene (a non-conjugated polymer (C8H8)n) deposited on Si (1 0 0) wafers. The SAM molecules used to functionalize the organic semiconductor films are fluoroalkyl trichlorosilane (FTS) (C8(H4F13)SiCl3) and octyltrichlorosilane (OTS) (C8H17SiCl3). Quantitative detection of medium and small-mass elements, such as O, F, Si, S and residual Cl is demonstrated and used to elucidate the surface chemistry in these novel organic systems.