The interaction of deposited metals with monolayer films is critical to the understanding of, and ultimate utility of, the emerging arena of molecular electronics. We present the results of a thorough study of the interaction of vapor deposited Au and Ag on alkane films on Si substrates. Two distinct categories of films are studied: C18 films formed from the hydrosilation reaction of octadecyl trichlorosilane with thin SiO2 layers and C18 films formed from the direct attachment of functionalized alkanes with hydrogen terminated Si. Three direct attachment chemistries were studied: photo initiated attachment of 1-octadecanol and octadecanal and thermal attachment of 1-octadecene. The reactivity of the films was studied with p-polarized backside reflection absorption spectroscopy, sputter depth profiling XPS, spectroscopic ellipsometry, atomic force microscopy, and device electrical test. Independent of direct attachment chemistry, we report the remarkable observation that deposition of Au results in the displacement of the molecular film from the Si interface. In contrast, the molecular films are robust towards the deposition of Ag. For both metals, the C18 films on SiO2 remain at the interface. The displacement of the films by Au is attributed to Au insertion in the Si backbonds, in a reaction analogous to silicide formation. The experiments demonstrate that one must carefully consider both the reactivity of the molecular head group and the substrate-end group when considering the fate of molecules in metal junctions.
Citation: Journal of Physical Chemistry
Pub Type: Journals