Design, synthesis and characterization of novel nanowire structures for photovoltaics and intracellular probes

Semiconductor nanowires represent a unique system for exploring phenomena at the nanoscale, and are expected to play a critical role in future electronic, optoelectronic and miniaturized biomedical devices. Modulation of the composition and geometry of nanostructures during growth could encode information or function, and realize novel applications beyond the conventional lithographical limits. In this thesis, I will focus on the fundamental science aspects of the bottom-up paradigm, which are the synthesis and physical property characterization of semiconductor nanowires and nanowire heterostructures, as well as proof-of-concept device concept demonstrations including solar energy conversion and intracellular probes.;I will first introduce a new 'nanotectonic' approach that provides iterative control over the nanowire nucleation and growth for constructing two-dimensional kinked nanowire superstructures. Next, I will discuss a rational, multi-step approach toward the general synthesis of three-dimensional branched nanowire heterostructures. Moreover, I will present results on novel nanoscale electronic devices based on those new nanowire structures, e.g. self-labelled p-n diodes and field-effect transistors in kinked nanowires and addressable light emitting diodes array and biological sensors in branched nanowires.;Then, I will discuss our effort of using radial and axial p-type/intrinsic/ n-type (p-i-n) silicon nanowire building blocks for solar cells and nanoscale power source applications. I will discuss the critical benefits of such structures, describe recent results, and provide a critical analysis and glimpse of the diverse challenges and opportunities in the near future.;Finally, I will present results on two new directions we have exploited recently in interfacing biological systems with nanowire devices. I will first show that silicon nanowire field-effect transistor (Si NWFET) arrays fabricated on transparent substrates can be reliably interfaced to acute brain slices, and can be used to reveal spatially heterogeneous functional connectivity in the olfactory cortex with high spatio-temporal resolutions. Next I will demonstrate for the first time telectrical recording of the intracellular potential with three-dimensional (3D) nanoprobe devices. Significantly, electrical recordings of spontaneously beating cardiomyocytes demonstrate that our 3D nanowire probes can continuously monitor the extra- to intracellular signals during cellular uptake. The nanometer size, biomimetic surface coating, and flexible 3D device geometry make these active semiconductor nanoprobes new and powerful tool for intracellular measurements, and suggest future biomedical applications where the distinctions between living cells and electronic devices have been blurred.