Inelastic scattering in STEM for studying structural and electronic properties of chalcogenide-based semiconductor nanocrystals

Abstract

Transmission electron microscopy (TEM) relies upon elastic and inelastic scattering signals to perform imaging and analysis of materials. TEM images typically contain contributions from both types of scattering. The ability to separate the contributions from elastic and inelastic processes individually through energy filter or electron energy loss spectroscopy (EELS) allows unique analysis that is otherwise unachievable. Two prominent types of inelastic scattering probed by EELS, namely plasmon and core-loss excitations, are useful for elucidating structural and electronic properties of chalcogenide-based semiconductor nanocrystals. The elastic scattering, however, is still a critical part of the analysis and used in conjunction with the separated inelastic scattering signals. The capability of TEM operated in scanning mode (STEM) to perform localized atomic length scale analysis also permits the understanding of the nanocrystals unattainable by other techniques. Despite the pivotal role of inelastic scatterings, their contributions for STEM imaging, particularly high-angle annular dark field STEM (HAADF-STEM), are not completely understood. This is not surprising since it is currently impossible to experimentally separate the inelastic signals contributing to HAADF-STEM images although images obtained under bright-field TEM mode can be analyzed separately from their scattering contributions using energy-filtering devices. In order to circumvent such problem, analysis based on simulation was done. The existing TEM image simulation algorithm called Multislice method, however, only accounts for elastic scattering. The existing Multislice algorithm was modified to incorporate (bulk or volume) plasmon inelastic scattering. The results were verified based on data from convergent-beam electron diffraction (CBED), electron energy loss spectroscopy (EELS), and HAADF-STEM imaging as well as comparison to experimental data. Dopant atoms are crucial factors which control optical, electronic, and also magnetic properties of semiconductors. Their location inside the materials has become more important with the miniaturization of devices. The precise determination of the position, however, poses a great challenge. Imaging using HAADF-STEM has proven adequate for locating heavy dopant atoms buried in relatively light matrix, particularly using aberration-corrected microscopes. The imaging method has been unsuccessful in detecting dopant atoms with similar atomic number as the matrix. Inelastic core-loss or inner-shell electronic excitations using EELS offer a unique solution when simultaneous imaging and EELS acquisitions are performed. The dopant atoms that are invisible in the images due to the small atomic number differences can be detected via spatial correlation with EELS core-loss data. Three types of samples with varying concentration of Mn dopant atoms in ZnSe nanocrystals were used to confirm such method. Precise locations of the dopant atoms on planes perpendicular to electron beam propagation could be determined although not all of the dopant atoms were detected due to limitations in experimental conditions.Another important type of chalcogenide-based nanocrystals is PbSe which is useful for solar cells. Colloidal method commonly used to synthesize the nanocrystals leave oleic acid capping ligands as surface passivation and size stabilizer. These ligands have critical roles in controlling electrical and optical properties of an individual nanocrystal and their assembly. Deemed insulating due to long chains of carbons, oleic acid is typically treated with short ligands such as hydrazines to decrease the inter-nanocrystal distances and improve electronic coupling among the neighboring nanocrystals. Despite its apparent insulating behavior, oleic acid was shown to exhibit surface plasmon coupling under certain circumstances. The geometric arrangement of the ligands was first investigated by HAADF-STEM imaging. Under air exposure, PbSe nanocyrstals easily oxidize to form oxide shells that are responsible for p-type doping by introducing surface acceptor states. At early oxidation stage (partial oxidation), prior to the formation of uniform oxide shells, the nanocrystals appear to form links between neighbors. Localized EELS analysis shows that these links are made of carbon based materials, most likely modified form of oleic acid ligands consisting of conjugated double bonds. Such modification occurred through oxidative dehydrogenation of the oleic acid ligands that is facilitated by the growing oxide shells on the surface of nanocrystals.