Transmission Electron Tomography and In-situ Analysis of Nanowires

As semiconducting nanowires have promising applications for materials science, nanoelectronics and photovoltaic devices, absolute control of their structure, morphology and composition is a prerequisite for their functionality. Nanowires, grown through the vapor-liquid-solid (VLS) method, exhibit a complex relationship between their liquid seed particle and what properties, such as the aforementioned ones, the wire adopts in the end. This means that compositional analysis, related to data on crystal structure and morphology, is crucial for better understanding. Transmission electron microscopy (TEM) plays an important role in atomically resolved studies of materials, including their elemental composition, which is of great use for evaluating growth methods and parameters for the nanowires. However, the conventional use of single projection TEM of the grown wires can neither reveal the 3D morphology nor potential rate of which the wire grows nor transient compositions during growth.

In this work, we report on TEM studies of semiconducting nanowires. These studies are split into two parts: three-dimensional morphology through electron tomography, and detailed studies of the growth process using in-situ TEM. Both methods have been successful in providing additional information about the nanowires, otherwise impossible to obtain from conventional TEM. The tomographic studies have revealed the preference for forming Ga particles on GaAs nanowires’ {111}B facets and that the preferred facets formed in wurtzite core-shell nanowires differ between InAs and AlSb ({1-100} and {11-20} respectively). The surfaces of the reconstructed tomograms were illustrated through the novel technique of azimuthal mapping. In-situ TEM revealed rate-limiting steps of forming consecutive layers as well as it supplied compositional analysis of the seed particle during growth, which is of interest when producing nanowires with narrow specifications for an application. Also, a novel method of filtering noisy spectrum images for the purpose of time-resolved elemental mapping is introduced.

The findings from this thesis provide more insight in the complex process of VLS-grown semiconducting nanowires, their morphological outcome, crystal structure and growth behavior. It also illustrates the advantages (and disadvantages) of using electron tomography and in-situ TEM for retrieving information about nanowires as compared to only using conventional TEM.