Designing Novel Semiconductor Nanowire Structures: Synthesis and Fabrication for Localized Photodetection and Sensing

Semiconductor nanowires display a wide range of structural and functional diversity, and as such provide a platform for nanomaterials research. At present, a number of nanowire structural motifs have been discovered and configured into devices with unique electrical and optical functionalities. For example, a kinked nanowire with a localized axial dopant modulation can record intracellular action potentials when incorporated into a three dimensional device. A radially modulated p-i-n nanowire can function as a nanoscale photovoltaic device to power logic gates and sensors. This thesis focuses on novel electrical and optical device functionalities based on rational design, synthesis and characterization of semiconductor nanowire structures for applications in the physical, chemical and biological sciences. First, I will present the design, synthesis and fabrication of two nanodevices for intracellular sensing that are based on core/shell and branched nanowire structural motifs. In both types of devices, a nanotube bridge templated by nanowires conducts the intracellular electrical and chemical potentials to the gating regions and the change in potential is recorded as the change of the device conductance. Both nanowire-based devices can sense extraand intracellular action potentials with high spatial resolution. Furthermore, they can be easily iv multiplexed and scaled up to record intracellular action potentials at multiple sites from either a single cell or cellular network. Second, I will discuss the synthesis of tapered nanowire structures and their electrical and optical characterization. By finely tuning growth temperature, precursor partial pressure, and catalyst size, detailed control of the nanowire tapering angle can be achieved. Moreover, tapered core/shell nanowires can be configured into devices with highly-localized electrical and optical functionalities. I show that control of the tapering angle plays an important role in determining the electrical and optical properties of nanowires. Finally, I will demonstrate a novel nanowire structural motif, termed tip-modulated nanowire, in which the modulation of material and dopant is localized at the nanowire tip so that a tip-localized device is encoded. I describe rational bottom-up synthesis of tip-localized p-n junctions, which are connected to the p-type nanowire core and isolated n-type nanowire shell. The electrical and optical properties of the tip-modulated nanowires are investigated by configuring them as devices with electrically independent core and shell contacts. Spatiallyresolved electrical and optical characterizations show that a potentiometric sensor as well as a highly sensitive p-n diode photodetector can be localized at the nanowire tip. In addition, a topdown strategy for wafer-scale synthesis and fabrication of vertical tip-modulated nanowires and nanowire device arrays is presented. Finally, by combining the tip-modulated nanowire structure with other structural motifs, we can rationally design self-sustained multi-functional nanodevices. The new tip-modulated nanowire structural motif opens up novel applications in the physical, chemical and biological sciences.