Daniel E. Morse, Ph.D.
Institute for Collaborative Biotechnologies
California NanoSystems Institute and the Materials Research Laboratory,
University of California, Santa Barbara, CA 93106 USA
We discovered that the silicateins, a family of enzyme proteins occluded within the silica needles made by a marine sponge, can catalyze and structurally direct (i.e., template) the polymerization of silica, silsesquioxanes and a wide range of metal oxide semiconductors from the corresponding molecular precursors at neutral pH and low temperature. These are the first reported examples of enzymecatalyzed, nanostructuredirected synthesis of semiconductors. Interaction with the templatelike protein surface stabilizes polymorphs of these materials (e.g., the anatase form of titanium dioxide and the spinel polymorph of gallium oxide) otherwise not formed at low temperatures. In some cases, interaction between the condensing metallooxane and the protein results in preferential alignment of the resulting nanocrystallites, suggesting a pseudoepitaxial relationship between the mineral crystallites and specific functional groups on the templating protein surface. Genetic engineering in conjunction with diffraction studies of the semiconductor products and the templating surface confirmed the mechanism of action and identified the functional groups responsible for the enzyme’s catalysis. This mechanistic understanding was confirmed and extended through the synthesis of a series of "biomimetic" peptides, polymers, small bifunctional organics and multifunctional selfassembled monolayers that display the functionalities identified as essential for catalysis, yielding new structuredirecting catalysts of siloxane and metallooxane polycondensation from the corresponding molecular precursors at low temperature and neutral pH. We now have taken this biomimetic process a step further, translating the fundamental mechanisms underlying silicateinmediated catalysis and templating to a process wholly controlled by chemistry and physics, without the use of any biochemical or organic molecules. As a first proof of principle, we have used this process for the lowtemperature synthesis of a unique and strongly photoconductive cobalt hydroxidebased material never before attainable through conventional or hightemperature methods. This material exhibits attractive properties of high dopant density, high carrier density, high surface area, exceptionally long minority carrier density and strong absorption in the visible, appropriate for photovoltaic applications. Because no organics or biochemicals of any kind are used, the new biologically inspired synthesis method yields exceptionally pure inorganic semiconductors, and thus is fully integrable with conventional semiconductor manufacturing and processing.