The interaction of deposited metals with monolayer films is critical to 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 attached to 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. Previous work has observed the surprising result that molecules attached directly to silicon were displaced by Au, Al, and Ti metallization1. To investigate the origin of this displacement we have performed a number of follow-up experiments2. The reactivity of the molecular films with metal was studied with p-polarized backside reflection absorption spectroscopy (pb-RAIRS), sputter depth profiling X-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry (SE), atomic force microscopy (AFM), and device electrical tests.
The effect of the interaction of top metal with the interfacial Si-molecule bond was investigated by altering the direct attachment chemistries. Formation of alkane monolayers from starting alcohol, aldehyde, alkene, and thiol precursors was investigated. 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 directly attached molecular films are robust toward the deposition of Ag. For both metals, the C18 films formed by hydrosilation reactions on SiO2 remain at the interface. The results of monolayer stability with metal are linked to reactions between the metal and substrate. The displacement of the films by Au is attributed to Au insertion in the Si backbonds, in a reaction analogous to silicide formation. The results demonstrate that one must fully take into account the reactivity of the entire system, including substrates, molecular functional groups, and metal electrodes, when considering the robustness of molecules in metal junctions.
- J. Phys. Chem. B 2005, 109 (46), 21836-21841
- J. Phys. Chem. C 2007, 111 (26), 9384-9392
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