No abstract

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 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. Two direct attachment chemistries were studied:  (Si)3−Si−O−C linkages of 1-octadecanol and octadecanal on H-terminated Si(111) and (Si)3−Si−C linkages of 1-octadecene on H-terminated Si(111). The reactivity of the films 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. 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.

We report the fabrication of molecular electronic test structures consisting of Au-molecule-Si junctions by first forming omega-functionalized self-assembled monolayers on ultrasmooth Au on a flexible substrate and subsequently bonding to Si(111) with flip-chip lamination by using nanotransfer printing (nTP). Infrared spectroscopy (IRS), spectroscopic ellipsometry (SE), water contact angle (CA), and X-ray photoelectron spectroscopy (XPS) verified the monolayers self-assembled on ultrasmooth Au were dense, relatively defect-free, and the -COOH was exposed to the surface. The acid terminated monolayers were then reacted with a H-terminated Si(111) surface using moderate applied pressures to facilitate the interfacial reaction. After molecular junction formation, the monolayers were characterized with p-polarized backside reflection absorption infrared spectroscopy (pb-RAIRS) and electrical current-voltage measurements. The monolayer quality remains largely unchanged after lamination to the Si(111) surface, with the exception of changes in the COOH and Si-O vibrations indicating chemical bonding. Both vibrational and electrical data indicate that electrical contact to the monolayer is formed while preserving the integrity of the molecules without metal filaments. This approach provides a facile means to fabricate high-quality molecular junctions consisting of dense monolayers chemically bonded to metal and silicon electrodes.