We have also been able to manipulate the surface properties of organics such as polyamides, polyurethanes, and polyesters using a ZrO 2 thin film. In collaboration with Professor Cava’s group we are now studying how such surface treatments might lead to adjustment of the Fermi level of these materials, which is key to controlling their fascinating electronic properties. Attaching our SAMPs to this oxide adhesion layer enables us to adjust the work function of the TI in a systematic way. We have also found that our TiO 2 can be grown on the surface of bismuth selenide telluride, a novel “topological insulator” (TI). The chemistry between the Si-H units and the Ti alkoxide or the TiO 2 product is under study as a means to further improve device behavior in collaboration with several groups in the Electrical Engineering Department. This unusual type of TiO 2 has a very large band gap compared with the common forms of TiO 2, anatase or rutile, and it imparts interesting device properties to Si-based photovoltaic devices, quadrupling efficiency compared to untreated analogs. We believe that this process is initiated by coordination of the alkoxide with any of the multitude of surface Si-H sites. In the context of application to electronic materials, we recently discovered that very thin (2-10 monolayers) of TiO 2 could be prepared on H‑terminated Si by a process involving vapor phase deposition of a volatile Ti alkoxide followed by gentle heating. This oxide may be useful for its inherent properties or it could serve as an adhesion layer to bond a SAMP to the substrate. Most approaches to surface activation of non-oxide substrates involve harsh reagents, such as oxygen plasma, for partial surface oxidation in contrast, we use “soft,” surface organometallic chemistry as a non-destructive method for interface synthesis: Ligating functional groups of the substrate serve as coordination sites for vapor-deposited titanium or zirconium alkoxide, which can then be converted under mild conditions to a surface-bound metal oxide.
These broad classes of materials are important for emerging applications in electronics and bioscaffolds. SAMPs, however, do not bond directly to these materials, and so we had to develop interfaces to enable multi-component constructs. More recently, we have shifted our attention from oxides to other inorganics or organic polymer substrates.
We introduced self-assembled monolayers of phosphonates (SAMPs) as a class of structurally variable, well-organized, dense coatings for metal oxides, and we showed them to be superior to well-known thiols/Au or siloxanes/oxides for cases where air and/or moisture are present. SAMPs are now in wide use, especially for electrode surface modification. Indeed, understanding and controlling interfaces lies at the confluence of nanoscience and microelectronics or medicine. Our approach is to focus on the basic chemistry that is key to interface design: Knowledge of nanoscopic structure and molecular bonding at the interface is essential.
We are particularly interested in interfaces between dissimilar materials such as are present in molecular electronic devices or that exist between synthetics and living tissue in biomedical implants. Our research aims to develop new surface and interface chemistry that can be integrated with novel architectures to create prototypical devices with outstanding behavior.
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