Synthesizing Semiconductor Nanowires in Hollow Microcapsules via the Geode Process

Abstract

A wide range of high-tech applications requires constituent materials and devices that must simultaneously exhibit nanoscale control of compositional heterogeneity and be produced at large scales. Semiconductor nanowires can be grown with exquisite spatial control of composition and morphology using the vapor-liquid-solid (VLS) mechanism, which, unfortunately, has so far been limited to minimal manufacturing rates. At smaller scales, demonstrations of prototype transistors, photodetectors, solar cells, and biosensors, for example, highlight the promise of these materials for electronic, photonic, energy, and medical applications. Here, I introduce the “Geode process” to synthesize semiconductor nanowires on a high surface area substrate that will provide throughputs greater than conventional flat substrate growths. The first aim to achieve the Geode process is forming the high surface area substrate, which are hollow microcapsules made through a double emulsion templated method. The hollow microcapsule is explicitly designed for this process to encapsulate and protect the catalyst particles for nanowire growth through the VLS mechanism; it features porous microcapsule walls to enable efficient gas transport to be able to withstand nanowire growth conditions (e.g., temperature, pressure). I will show how microcapsule structure and drying are influenced by silica nanoparticle type and concentration, emulsification parameters, and nanoparticle cross-linking agent. Bulk microcapsule powders are produced by optimizing an emulsion-templating method, drying method, and calcination. The desired single-cavity microcapsule morphology is obtained via osmotic swelling of the inner aqueous core at the double emulsion state. Flowable microcapsule powders are formed with a scalable drying technique and calcination to remove residual polymers. I also aim to demonstrate the proof-of-concept synthesis of homogeneous Si nanowires and nanowires with programmable concentration profiles in the microcapsule interior. The morphology, composition, and crystal structure of microcapsule-grown nanowires show similar properties to nanowires grown on planar substrates. The impact of the microcapsule wall on the transport of reactive species is characterized by examining changes to the concentration profile.