Synthesis, properties, and assembly of semiconductor nanowires and nanowire heterostructures

Recent developments in the fabrication, measurements, and the assembly of semiconductor nanowires have initiated an exciting research field in science and technology. Proof-of-concept nanowire devices exhibit performance that is comparable to or superior to conventional planar devices. However, there are still many essential scientific issues regarding the control over the growth, interface phenomena, and organization of these materials. In this thesis, I will present my initial work on materials synthesis, characterization, physical measurement, and assembly of semiconductor nanowires to address these important issues.; First, through the systematic studies of molecular scale silicon nanowires and compound semiconductor nanowires growth, we have a better understanding of the importance of energies in determining the crystal structures and the growth directions of these nanowires. This knowledge provides us a better control over composition, size, and electronic properties during nanowire synthesis.; After the synthesis, the sequential device fabrication requires diverse functionalities for various applications. The most straightforward solution is to make complex compositionally modulated structures, including axial (superlattice) and radial (core/shell) nanowire heterostructures. In our research, we demonstrated the controlled preparation of nickel silicide/silicon metallic/semiconducting nanowire superlattice involving conventional lithography techniques or nanowire shadow masks. These superlattice heterostructures provide an integrated solution for reliable nanoscale electrical contacts and interconnects. Moreover, the rational synthesis of semiconducting/functional oxide core/shell heterostructures has been accessed by atomic layer deposition. The electrical transport measurements performed on these novel nanowire heterostructures exhibit unique properties, which can be essential for determining potential applications, such as high-performance field-effect transistors, memories, and logic circuits.; Finally, to demonstrate the possibility of the "bottom-up" paradigm towards hybrid or stand-alone complex nanosystems, we have shown a general and efficient Langmuir-Blodgett method for controlling organization and hierarchy of nanowires over a large area by aligning nanowires with controlled nanometer to micrometer scale pitch and transferring the nanowires to planar substrates in a layer-by-layer process to form parallel and crossed structures. Further improvements on the assembly can be possibly achieved by applying magnetic field over the semiconductor/superparamagnetic metal core/shell nanowire heterostructures, while simulations show the metal shell serving as coaxial gate can improve the device performance.