Polarization Selectivity of Aloof-Beam Electron Energy-Loss Spectroscopy

Scanning transmission electron microscopy (STEM) coupled with electron energy-loss spectroscopy (EELS) is widely used to probe the electronic properties of materials. In a recent work [1], we demonstrated that the aloof-beam EELS offers good polarization selectivity to detect the orientation-specific interband electronic transitions in one-dimensional ZnO nanorods without the requirement of sample reorientation. In particular, we experimentally observed the key anisotropic features at 11.2 and 13.0 eV differentiating the in-plane (\textbf{\textit{E}} $\bot $ to $c $axis) and out-of-plane (\textbf{\textit{E}} $\parallel $ to $c $axis) responses of wurtzite-ZnO nanorods to an external electric field (\textbf{\textit{E}}), as predicted by our density-functional theory calculations. We further observed some degree of orientation dependence at the onset of direct band gap transition near 3.4 eV, which was attributed to the splitting of the O-2p$_{xy}_{\, }$and O-2p$_{z}_{\, }$occupied states in the wurtzite structure. The fact that good polarization selectivity can be achieved by aloof-beam EELS while keeping the beam-to-specimen orientation fixed broadens the scope of the aloof-beam EELS technique for characterization of nanomaterials. [1] Yao-Wen Yeh, Sobhit Singh, David Vanderbilt, and Philip E. Batson, Phys. Rev. Applied 16, 054009 (2021).