Paper SS1-FrM3
Trapping Dynamics on Complex Surfaces: Organic Thin Film Growth on Surfaces Terminated with Self-Assembled Monolayers

Friday, November 13, 2009, 9:00 am, Room M

A significant challenge in fabricating organic thin film transistors is that of controlling and understanding the properties of the interface between the organic semiconducting layer and the dielectric. It has been observed that charge transport can be affected significantly by chemically altering the dielectric surface with self-assembled monolayers (SAMs). However, the effects of the molecular scale interactions between the organic molecule and the substrate remain unclear. Diindenoperylene (DIP) is a promising candidate for applications in organic thin film electronics owing to the ability to form highly ordered films with excellent electrical properties. Here, using supersonic molecular beam techniques and in situ real time synchrotron X-ray scattering, we have examined the adsorption dynamics of DIP on silicon dioxide (SiO₂) and SiO₂ modified with a number of SAMs. These SAMs included octadecyltrichlorosilane (ODTS), octyltrichlorosilane (OTS), fluoroctyltrichlorosilane (FOTS), and hexamethyldisilazane (HMDS), representing a range of molecular sizes and chemical terminations (-CH₃ vs. –CF₃). In this work, we make use of x-ray intensity oscillations at the so-called anti-Bragg position to extract the occupation (coverage) of the individual layers as a function of time. These coverage-exposure relationships give us a direct measure of the (relative) probabilities of adsorption as a function of coverage. For the conditions examined (E_i = 5.1-12.3 eV, T_s = 40 °C) on all the starting surfaces (SiO₂ and SAM/SiO₂) we observe a decrease in the probability of adsorption with increasing incident energy indicative of trapping-mediated adsorption. The probability of adsorption on these starting surfaces is dependent on both the chain length and the chemical composition of the SAM, where the probability of adsorption is greatest on the thickest organic layer, ODTS, followed by OTS, SiO₂, FOTS, and HMDS. Once all surfaces are covered by DIP, however, the effects of incident kinetic energy are greatly reduced, and trapping is very efficient over the range of kinetic energies examined. For organic layers of comparable chemical composition and density, the initial probability of adsorption depends strongly on the layer thickness, where trapping on ODTS is most efficient, and on HMDS, least efficient. In a selected set of cases we will compare our experimental results with recent results from molecular dynamics simulations to obtain insight into the possible molecular-scale mechanisms/events that may occur in these systems. One such event appears to be the undeniable role of direct molecular insertion into the thicker organic layers such as ODTS.