This project addresses fundamentals of one dimensional (1D) nanostructure heteroepitaxy with the aims of 1) understanding and extending the principles of vapor-liquid-solid (VLS) nanowire growth to vapor-solid-solid (VSS) epitaxy as a new approach to nanowire synthesis, , and 2) forming new nanoscale heteroepitaxial structures based on highly strained, defect free nanowire growth for the rational design of functional architectures. In VLS a liquid metal seed is used to catalytically direct one dimensional growth, whereas for VSS nanopatterned solid silicides will be used as the catalytic medium. Solid phase approaches are expected to greatly extend the range of materials, nanowire orientations and processing options available. Direct experimental comparison between VLS and VSS nanowire growth for the model Si/Ge system will be performed to assess mechanisms and kinetics of 1D growth. A combination of growth techniques and in situ, real-time observation will be used to study growth parameters. For example, growth on cylindrical, single crystal Si rods or laser-textured surfaces will be utilized to determine the orientation dependence of nanowire epitaxy. A broad range of growth temperatures and pressures will be used to distinguish nucleation and growth kinetics, and nanowire growth on patterned metal silicide seeds of varying size will be done to examine scaling of VSS to nanoscale dimensions. A transmission electron microscope capable of atomic resolution imaging at disilane and digermane gas pressures of up to 10 Torr and sample temperatures of up to 1000 degrees C will be used for dynamic characterization of nanowire nucleation and growth by obtaining high contrast atomic resolution images and detailed elemental maps during real-time in situ experiments. HREM strain mapping, atomistic elastic modeling, and photoluminescence studies of tailored architectures will be included in the study. %%% The project addresses basic research issues in a topical area of materials science with high technological relevance. An important feature of the program is the integration of research and education through the training of students in a fundamentally and technologically significant area. With many promising uses for semiconducting nanowires emerging, from sensor arrays to flexible electronics, this research may have substantial technological impact. Additionally, the PI will collaborate in an exploratory study of the societal dimensions of ubiquitous sensing with a bio-ethicist, and a visiting social scientist. Results of both the materials research and the societal case study will be incorporated into a new course entitled Nanomaterials being developed by the PI. ***
