The synthetic implementation of designed nanostructures is central to advances in the field of nanoscience and nanotechnology. Modulation of the composition of nanostructures during growth could encode information or function in a manner analogous to biological systems and independent of the constraints of lithography. This thesis presents the synthesis and characterization of modulation-doped nanowires with the structural and electrical properties of modulated regions are completely defined during synthesis, and efforts of developing conventional and quantum electronic and photonic based on modulation-doped nanowires.; We describe the successful synthesis of modulation-doped silicon nanowires using the nanocluster-catalyzed vapor-liquid-solid growth processes. The pure axial elongation without radial overcoating during the growth process was achieved with the introduction of a local substrate heater and use of a hydrogen atmosphere and verified by High-resolution transmission electron microscopy studies. Scanning gate microscopy shows that the key properties of the modulated structures, including the number, size and period of the differentially doped regions, are defined in a controllable manner during synthesis, and moreover, that feature sizes to less than 50 nm are possible.; These modulation-doped nanowires provide the potential for essential device function to be defined during synthesis not lithography. First, a lithography-free approach is developed for addressing individual nanowires in an array when the nanowires have different dopant modulation sequences and implemented in a 2x2 modulation-doped silicon nanowire field-effect transistor array. Second, encoding of single and coupled double quantum dot (QD) structures is achieved in modulation-doped nanowires. Low temperature transport studies demonstrate that QD sizes of single QD structures as well as the interaction between two QDs of double QD structures can be controlled by synthesis. Third, this general synthesis approach is extended to a novel p-i-n nanowire structure, in which both dopant type and concentration are modulated. Spatial photocurrent measurements demonstrate the potential of the application for single nanowire avalanche photodetectors, and being integrated with other essential nanophotonic elements for integrated systems. Fundamentally, studies of the impact ionization coefficients of electrons and holes in silicon nanowires suggest possible longer optical phonon mean free path than that in bulk due to the phonon confinement effect.; Lastly, two other types of nanowire heterostructures, branched nanowires and atomic sharp metal/semiconductor nanowire heterostructures are also developed through synthesis via a multi-step metal-catalyzed chemical vapor deposition method, and chemical conversion of semiconductor nanowire, respectively. Their implementations for nanoelectronic device elements are also discussed.
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