UC Berkeley

Much progress has been made recently in synthesizing and studying the properties of semiconductors in nanostructured forms. However, there are few reports of II-VI ternary semiconductor alloy nanowires spanning their entire composition range. In particular, composition tuning in mismatched alloy systems allows for unique electronic band structure engineering due to the size and electronegativity mismatch of the isoelectronic substituted anions, resulting in band gap and density of states tuning, among other tunable properties, in a single alloy system. These unique features could lead to applications in photovoltaic, optoelectronic, and thermoelectric devices. Nanostructured alloys leverage the high surface area to volume ratio to allow lattice relaxation while minimizing structural defects, which is particularly important given the difference in the lattice parameter that is common in mismatched systems. This presents an intriguing route to stabilizing mismatched alloys.

The objective of the work presented in this dissertation was to synthesize and characterize Zn-VI alloy nanostructures of increasing electronegativity mismatch. This has been accomplished by modifying the vapor transport synthesis conditions of the the endpoint compositions. Pure single-crystalline Zn-VI nanowires were synthesized by vapor transport in the initial phase of the project. Zincblende ZnSSe nanostructures, which have a very small mismatch, were synthesized and show a shift in the band gap as a function of composition. Zincblende ZnSeTe 1-D nanostructures, with a mismatch more than twice that of ZnSSe, were also grown. However, attempts at growing further mismatched alloys, ZnSTe, were unsuccessful at producing uniform alloys. Non-equilibrium processing may be a useful route toward stabilizing more mismatched nanostructured alloys.

In the last section, nanowires are described which are used to study the phase evolution of a prototypical phase change material, GeTe, in-situ. The confined geometry created a distinct and easily identifiable interface where the vapor, liquid, and solid phases of GeTe could interact. The velocity of the interface movement was used to study the vaporization coefficient of GeTe.