Chemical Mechanisms of Inorganic Phototropic Growth

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Part 1: Non-Technical Summary Controlled patterning of materials is important for semiconductor integrated circuit fabrication, 3-D electronics, advanced functional biomaterials, optical materials, and catalyst supports. In this project, jointly supported by the Solid State and Materials Chemistry and Electronic and Photonic Materials Programs in the Division of Materials Research, Professor Nathan S. Lewis of the California Institute of Technology seeks to develop and exploit a fundamental understanding of inorganic phototropic growth. In conventional lithography, materials grow where light is present, whereas in phototropic growth, the materials grow toward the light source. By manipulating the wavelength, intensity, and polarization of the light beams provided by sources as simple as ordinary light bulbs or LEDs, inorganic phototropic growth can spontaneously produce complex, self-organized nanoscale patterns that can be controlled in three dimensions in real time. Such scientific research is foundational to national competitiveness in materials research, information, optical-communications, nanotechnology, and chemical-sensing. To meet the objective of this project, the Lewis Group is exploring the range of materials that exhibit inorganic phototropic growth, examining the generality of the phenomenon by expanding phototropic reactivity to light-induced controlled etching of materials, and using phototropic growth to create materials with unique three-dimensional properties. The experimental work is being integrated with modeling and simulation to develop a mechanistic understanding of inorganic phototropic growth. The research will be integrated with solar energy and materials-discovery outreach programs led by Caltech, specifically the Juice from Juice and Project SEAL hands-on science modules for use at the high-school level throughout the country, with special emphasis on school districts comprising underrepresented groups and diverse student populations. Part 2: Technical SummaryIn this project, Professor Nathan S. Lewis of the California Institute of Technology is examining the mechanisms underlying inorganic phototropic growth, i.e., the production of spontaneous, self-organized mesostructures aligned along the direction of a polarized, incoherent, uniform intensity light beam during electrodeposition of semiconductors. The morphologies are determined by the inherent optical response of the electronic processes within semiconductors stimulated by the tunable properties (e.g. wavelength, polarization, and direction) of the light present during the electrodeposition. The process is adaptive: if the light is moved or otherwise changed, subsequent growth adapts to the changed conditions. To date, this phenomenon has been demonstrated experimentally only for amorphous or polycrystalline chalcogen and chalcogenide materials. This project will investigate whether inorganic phototropic growth can be extended to other materials and how lattice structures and optoelectronic properties influence the mechanisms of inorganic phototropic growth. The work will also investigate how substrate-electrolyte interfaces affect the early-stage development of the ordered nanostructures by studying nucleation of Se-Te alloys on substrates with varied electronic properties. In addition, phototropic growth will be exploited to design and synthesize complex three-dimensionally structured materials with desired functionality, such as chiral metamaterials and plasmonic materials with tailored optoelectronic properties. The experimental observations will be used in conjunction with development of a mechanistic simulation tool that combines full-wave electromagnetic calculations with Monte-Carlo-based mass addition to predict the structures produced by phototropic growth of a variety of semiconductors under arbitrary optical inputs. This project is jointly supported by the Solid State and Materials Chemistry and Electronic and Photonic Materials Programs in the Division of Materials Research.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.