This proposal was submitted in response to the solicitation "Nanoscale Science and Engineering" (NSF 00-119). The project addresses nanoscale effects on synthesis and properties of semiconductor nanowires. A versatile, template-based approach to the synthesis of metal/semiconductor/metal nanowires will be developed, utilizing nanoporous membranes as substrates for nanowire assembly using semiconductor vapor-liquid-solid growth and metal electrodeposition. This approach provides the ability to synthesize single crystal semiconductor structures sandwiched between contact metal segments in a single nanowire with controlled diameters and lengths. After removal from the membrane, a field assisted assembly method will be used to attract and align the nanowires suspended in solution on to pre-patterned contact pads on a substrate. An electric field is used to polarize the nanowires and induce alignment via dielectrophoresis. Field assembly will be used to align and position large numbers of nanowires on large area contact pads for electrical characterization. Fabrication studies will focus on two materials, Si and GaAs, enabling a study of nanoscale effects in indirect and direct gap semiconductors. Structures with dimensions ranging from hundreds of nanometers to tens of angstroms will be produced and characterized to study the impact of length scale on the physical properties of nanowires. The development of vapor-liquid-solid growth methods in nanoporous membranes will encompass a study of vapor phase transport, reaction kinetics, nucleation and crystal growth in nanoscale cylindrical geometries. The fabrication of metal/semiconductor junctions within individual nanowires will be used to engineer nanocontacts in these structures and probe phase equilibria, interfacial reaction kinetics, Fermi level pinning and ohmic and Schottky contact characteristics in small dimensional structures. The ability to rapidly position and measure large numbers of individual nanowires will be used to carry out detailed measurements of electrical transport. Theoretical studies of band structure and carrier scattering in restricted geometries will be carried out in conjunction with the experimental work to provide insight into the experimental findings. An interdisciplinary team with specific expertise in semiconductor crystal growth, metal/semiconductor contacts, nanowire self-assembly and characterization and the theory of nanostructure electronic properties will carry out the research. The team includes faculty members from Materials Science and Engineering, Electrical Engineering and Physics. The interwoven structure of the research and the close physical proximity of the investigators will enable co-advising of graduate students and the development of individual thesis projects that encompass a range of topics in nanoscale synthesis, assembly, characterization and theory. The project will also include the preparation of educational modules, developed in conjunction with a high school physics instructor, that are designed to introduce concepts in nanotechnology to a general audience. The modules will be used by team members in their current K-12 education and outreach activities and in additional on-campus programs that target minority and female recruitment in science and engineering. %%% 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. The project is designed to develop strong technical, communication, and organizational/management skills in students through unique educational experiences made possible by a forefront research environment. There will be active involvement of undergraduates in the program and formal emphasis on developing effective oral and written communication skills. The project is co-supported by the DMR/EM, CTS/CRP, and CTS/FPH Divisions/Programs. ***
