The objective of this research is to develop new techniques for creating patterns with nanometer dimensions on technologically important substrates, such as silicon, gallium nitride, and diamond. Almost all micro- and nano-scale patterning is currently carried out using photolithography, a process well suited for high-throughput parallel printing of small surfaces such as computer chips. This technique, however, requires an enormous investment in expensive and complex instrumentation and cannot pattern large surfaces. Soft lithography, a technique in which molecular "inks" are transferred to metallic surfaces from rubber stamps, is an alternative to photolithography but is applicable only to a small range of non-technologically relevant materials, such as gold and silicon oxide. This project will develop a universal soft lithographic method for patterning virtually any metal or semiconductor surface. In this approach, a highly ordered uniform monolayer is placed on a metal or semiconductor surface. In a second step, an elastomeric stamp bearing a catalyst active against the initial monolayer is used to create patterns on the surface in places where the stamp contacts the surface.
The work will have an enormous impact on many areas of science and engineering, including nanotechnology, material science, electrical engineering, chemistry, biochemistry, and physics. The ability to pattern technologically relevant substrates will facilitate the development of new sensors, permit the fabrication of novel arrays of cells and biological molecules for the rapid identification of drugs, and allow the construction of 3-dimensional objects with nanoscopic dimensions on virtually any surface. The work will involve both graduate and undergraduate researchers, who will study synthetic chemistry, catalysis and surface characterization.
We report the development of a bi-layered molecular system that, in conjunction with catalytic microcontact printing (µCP), can be used to accurately and reliably replicate micro- and nano-scale patterns of chemically distinct functionalities on a wide variety of surfaces, including metals, oxides and inorganic semiconductors. Catalytic printing alleviates problems associated with traditional µCP; that is, as an inkless technique it obviates the resolution limitations associated with ink diffusion, and enables high-resolution replication of patterns through specific chemical or biochemical reactions between a chemically reactive surface and a stamp-immobilized catalyst. This technique permits control over the shape and size of the patterned features, and provides access to chemically distinct patterns that can be further functionalized with organic and biological molecules. We demonstrate catalytic printing on both oxide-free (H-terminated) silicon and germanium—surfaces that do not react readily with organic molecules, and cannot be patterned using traditional printing methods that rely on diffusion of a molecular ink from stamp to surface. Our approach relies on the formation of an ordered (yet functional) bi-layered molecular system that affords both complete protection of all surface-exposed inorganic atoms with stable covalent bonds and supports covalent immobilization of a reactive overlayer, yielding both stability and functionality to the surface. A catalytic stamp—bearing sulfonic acid moieties—was used to achieve pattern-specific hydrolysis of N-hydroxysuccinimide-activated acids on Si and Ge. Further modification of the chemically discriminated patterns enables chemoselective anchoring of organic molecules and protein. Applying the bi-layered system to ITO enabled the immobilization of multiple organic functionalities with exquisite spatial control. Finally, the bilayered molecular system, in combination with catalytic µCP, was used to control the charge transport properties in organic light emitting diodes (OLEDs). This particular study demonstrated the dependence of the rate of charge tunneling on the chemical nature of the functional group, and also provided a simple test platform for comparing electronic properties of functional organic monolayers. We demonstrated that (1) charge transfer through the bilayered system is sensitive to small structural molecular changes in the monolayer, specifically to those that influence the electron-withdrawing or donating nature of the monolayer and (2) that the differences in charge transfer can be visualized via patterned electroluminescence using catalytic µCP.