Polarization selectivity of aloof-beam electron energy-loss spectroscopy in one-dimensional ZnO nanorods

TL;DR

This study demonstrates that aloof-beam electron energy-loss spectroscopy (EELS) can achieve polarization selectivity in one-dimensional ZnO nanorods without reorienting the sample, by varying the impact parameter around the specimen.

Contribution

It introduces a method to obtain polarization selectivity in EELS by positioning the electron probe at different locations, avoiding the need for sample reorientation.

Findings

Orientation-dependent electronic transitions are characterized.

Good polarization selectivity is achieved by adjusting impact parameter.

The results are explained by electric fields generated by the electron beam.

Abstract

Orientation dependent electronic properties of wurtzite zinc oxide nanorods are characterized by aloof beam electron energy-loss spectroscopy (EELS) carried out in a scanning transmission electron microscope (STEM). The two key crystal orientation differentiating transitions specific to the in-plane (13.0 eV) and out-of-plane (11.2 eV) directions with respect to the wurtzite structure are examined by first principles density-functional theory calculations. We note some degree of orientation dependence at the onset of direct band gap transition near 3.4 eV. We demonstrate that good polarization selectivity can be achieved by placing the electron probe at different locations around the specimen with increasing impact parameter while keeping the beam-specimen orientation fixed. The observed results are qualitatively elucidated in terms of the perpendicular electric fields generated by the fast electron (60 kV) used in the microscope. The fact that good polarization selectivity can be achieved by aloof beam EELS without the requirement of sample reorientation is an attractive aspect from the characterization method point of view in the STEM-EELS community.