Future devices for biomedical and energy applications are likely to use hybrid organic/inorganic junctions. Semiconductor substrates are often used because they are particularly appropriate charge carriers for electronic sensing and photo-electric or voltaic devices. For instance, field-effect transistors (FETs) are being developed as biosensors, wherein the electric field induced by the attachment of a charged biomolecule onto an organic self-assembled monolayer (SAM) results in a change of the FET drive current. Similarly, attachment of light-absorbing molecular groups or quantum dots on a SAM-covered semiconductor surface is being considered for high efficiency photovoltaic devices. In all cases, the performance of such hybrid devices critically depends on the quality and stability of the organic/semiconductor interface.
In this talk, we will suggest that the formation of SAMs on oxide-free silicon can lead to higher chemical stability and electrical performance than on oxidized silicon surfaces that are typically used now. Hydrogen-terminated surfaces are ideal starting point for such organic functionalization, but the functionality (i.e. versatility) of current SAMs is limited. Chemical functionalization of H-terminated silicon, such as hydroxylation, is critical for further development. We will present a method to selectively graft hydroxyl on an otherwise oxide-free surface that greatly facilitates the chemical attachment of a variety of molecules directly on Si. For instance, direct grafting of phosphonate molecules at room temperature is not only possible, but leads to high quality SAMs that remain stable in water. This is in sharp contrast to SAM deposition on hydroxylated silicon oxide surfaces, for which a high temperature annealing step (the so called T-bag method) is necessary to establish a chemical bond that unfortunately easily hydrolyzes in water.
More fundamentally, the perfection of the starting Si(111) surfaces and of the resulting SAM structure makes it possible to derive an understanding of the mechanism for stabilization of the phosphonated surface in aqueous solution.
Applications of functionalized oxide-free surfaces for both biosensors and photovoltaic will also be described.
Yves J. Chabal
Department of Materials Science and Engineering
University of Texas at Dallas