Sub-nanometer mapping of strain-induced band structure variations in planar nanowire core-shell heterostructures

Abstract

Strain relaxation mechanisms during epitaxial growth of core-shell nanostructures play a key role in determining their morphologies, crystal structure and properties. To unveil those mechanisms, we perform atomic-scale aberration-corrected scanning transmission electron microscopy studies on planar core-shell ZnSe@ZnTe nanowires on α-Al2O3 substrates. The core morphology affects the shell structure involving plane bending and the formation of low-angle polar boundaries. The origin of this phenomenon and its consequences on the electronic band structure are discussed. We further use monochromated valence electron energy-loss spectroscopy to obtain spatially resolved band-gap maps of the heterostructure with sub-nanometer spatial resolution. A decrease in band-gap energy at highly strained core-shell interfacial regions is found, along with a switch from direct to indirect band-gap. These findings represent an advance in the sub-nanometer-scale understanding of the interplay between structure and electronic properties associated with highly mismatched semiconductor heterostructures, especially with those related to the planar growth of heterostructured nanowire networks.

Machine learning in electron microscopy for advanced nanocharacterization: current developments, available tools and future outlook

“…4d). 204 In imaging, for example, the denoising based on the non-rigid averaged cross-correlation of single atomic columns was used for both Energy-Dispersive X-ray spectroscopy (EDX) and EELS, 1,[205][206][207][208] in addition to the denoising based on the comparison between experiments and simulations. [209][210][211] However, none of these classical methods has been able to improve the SNR as straightforwardly as the nowadays standard post-acquisition operations based on Singular Value Decomposition (SVD).…”

“…260 In fact, clustering and unmixing are not exclusive for core losses and can also be applied to low-loss EELS, for instance, to map plasmons or even in the future to map with high accuracy the electronic properties of nanomaterials such as the topology of the band structure. 204,261 Independently on the spectral range in which it was applied, the complexity of the ML reviewed so far for ES was standard. It is a good signal that in most of the cases this basic stage it is just enough.…”

“…260 In fact, clustering and unmixing are not exclusive for core losses and can also be applied to low-loss EELS, for instance, to map plasmons or even in the future to map with high accuracy the electronic properties of nanomaterials such as the topology of the band structure. 204,261…”

“…PCA was used on the low-loss SI to isolate the interfacial pixels between core and shell where the strain was accumulated, as visible in the Middle-Angle Annular Dark-Field (MAADF) micrograph. 204 This journal is © The Royal Society of Chemistry 2022 taken when unmixing signals and choosing the components to reconstruct. For that, P. Potapov and A. Lubk published a guide on how to automatically choose the meaningful PCA components.…”

“…(d) Band-gap map of a ZnSe-ZnTe core-shell nanowire separating the areas where a direct or indirect band type was found. PCA was used on the low-loss SI to isolate the interfacial pixels between core and shell where the strain was accumulated, as visible in the Middle-Angle Annular Dark-Field (MAADF) micrograph 204.…”