True atomic level imaging of shaped nanoparticles composed of bismuth, antimony and tellurium using scanning transmission electron microscopy

Nanotechnology is an area of research that is highly intriguing because of the novel properties often observed for materials whose sizes are reduced to the nanoscale. However, one of the biggest challenges is understanding the underlying principles that dictate the particles resulting properties. The atomic level structure for nanoparticles is suspected to vary from that for the corresponding bulk materials, however, direct observation of this phenomenon has proven difficult. Until recently only indirect information on the atomic level structure of such materials could be obtained with techniques such as XRD, HR-TEM, XPS, etc... However, recent advances in Transmission Electron Microscopy techniques now allow true atomic scale resolution, leading to definitive confirmation of the atomic structure. Namely, Scanning Transmission Electron Microscopy coupled with a High-angle Annular Dark Field detector (STEM-HAADF) has been demonstrated to be capable of achieving a nominal resolution of 0.8 nm (the JEOL JEM-ARM200F instrument). The ability is highly exciting because it will lead to an enhanced understanding of the relationship between atomic structure of nanoparticles and the resulting novel properties. In our own study, we focus on the analysis of the atomic level structure for nanoparticles composed of bismuth, antimony and tellurium for thermoelectric materials. This area has recently received much interest because of the realization that nanotechnology can be employed to greatly enhance the efficiency (dimensionless figure of merit ZT) of this class of materials. One of the most intriguing parameters leading to the enhanced TE activity is the relationship between composition and structure that exists within individual nanoparticles. We report our results on a study of the atomic level structure for both nanowires and nanodiscs composed of bismuth, antimony and tellurium. It was found that the nanoparticles have a complex structure that cannot be elucidated by conventional techniques such as XRD or HR-TEM. In addition, by employing Energy Dispersive Spectroscopy (EDS), a greater understanding of the composition-structure dependence was gained. The results are primarily discussed in terms of the atomic level resolution images obtained with the STEM-HAADF technique.