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.
In the emerging area of molecular electronics, fabrication of reliable metallic contacts remains one of the most critical challenges. Nanotransfer printing (nTP) is an attractive low-cost non-destructive technique to provide contact to organic monolayers. This work introduces the use of ultrasmooth gold on polymeric substrates, fabricated by using an nTP-based method, as a novel means to form metal-molecule-silicon molecular electronic test structures. We have used self-assembly to fabricate highquality COOH-terminated alkanethiols on ultrasmooth gold. Covalent bonding to the H-Si(111) substrate was achieved through the application of moderate pressure and temperature. Because of the critical role of molecular conformation on electrical properties, infrared spectroscopy was used to explore the influence of these two parameters (P,T) on molecular ordering. Moderate conditions effectively fabricate high-quality reproducible molecular electronic test structures preserving molecular conformation with molecules bonded to both electrodes. This work proves a useful strategy for the development of hybrid nanoelectronic devices.
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