Si and Ge nanowires and their heterostructure have been received widespread attention in various research fields because of the inherent advantages and the major historical roles played by these materials in contemporary microelectronics. From decades of research on two materials, integrated in-depth knowledge on the nature of material properties and manufacture process provide useful guidelines to design nanostructures and related devices with increased structural and functional complexity. In this dissertation, synthesis and applications of Ge and Si based nanowires and nanotubes in electronics, photonics, biochemical sensor, and thermoelectrics are discussed. In chapter 2, self-organizing characteristics of misfit- guided Ge quantum dots growth on Si core nanowires are systematically demonstrated. Unique Ge quantum dots growth mode caused by strain supperlattice along the Si nanowire backbone can be controlled by the choice of core diameter. Such strain-guided growth opens up a new avenue towards growth of self-organized nanoscale heterostructures. In chapter 3, fundamental study of crystalline Si nanotubes properties as a platform for electrically and biochemically functional devices is demonstrated. Four- probe current-voltage characterization of precisely probe the inherent electrical properties of crystalline Si nanotubes. Selective functionalization and loading of fluorescence dye and biomolecule inside the core of nanotubes are demonstrated lighting the potential as in- vivo drug carrier. In chapter 4, characterization of thermal transport behavior of crystalline and amorphous Si nanotubes are presented. Ultra-low thermal conductivity of crystalline nanotube below the amorphous counterpart is observed. Study on elastic properties of those nanotubes reveals new possible control mechanism of phonon transport behavior. In chapter 5, fabrication of optical polarizer by printing Ge or Ge/Si core/shell nanowires into highly compacted and ordered fashion is presented. Transmission measurement under various mechanically stressed circumstances reveals potential of nanowire polarizer as high flexible and stretchable optical filter

Printing techniques could offer a scalable approach to fabricate thermoelectric (TE) devices on flexible substrates for power generation used in wearable devices and personalized thermo-regulation. However, typical printing processes need a large concentration of binder additives, which often render a detrimental effect on electrical transport of the printed TE layers. Here, we report scalable screen-printing of TE layers on flexible fiber glass fabrics, by rationally optimizing the printing inks consisting of TE particles (p-type Bi_0.5Sb_1.5Te₃ or n-type Bi₂Te_2.7Se_0.3), binders, and organic solvents. We identified a suitable binder additive, methyl cellulose, which offers suitable viscosity for printability at a very small concentration (0.45-0.60 wt.%), thus minimizing its negative impact on electrical transport. Following printing, the binders were subsequently burnt off via sintering and hot pressing. We found that the nanoscale defects left behind after the binder burnt off became effective phonon scattering centers, leading to low lattice thermal conductivity in the printed n-type material. With the high electrical conductivity and low thermal conductivity, the screen-printed TE layers showed high room-temperature ZT values of 0.65 and 0.81 for p-type and n-type, respectively.