Disputation: Wei Zhan

Master of Science Wei Zhan at Department of Physics will be defending the thesis 

"Band gap mapping of alloyed ZnO using probe-corrected and monochromated STEM-EELS"

for the degree of PhD

Trial lecture - time and place

Trial lecture: 10:15 am at Lille Fysiske auditorium (V232)


Adjudication committee

  • Dr. Wilfried Sigle; Max Planck Institute for Solid State Research; Germany
  • Professor Per Persson; Department of Physics, Chemistry and Biology (IFM); Linköping University, Sweden
  • Associate Professor Pavlo Mikheenko; Department of Physics; University of Oslo, Norway


Chair of defence



  • Øystein Prytz
  • Andrej Kuznetsov
  • Espen Flage-Larsen


Additional information

As nanoscale semiconductor technology makes progress, e.g., alloying ZnO with CdO which leads to the band gap reduction and many good performances, two-dimensional band gap measurement at nanoscale is becoming increasingly important. Monochromated electron energy loss spectroscopy (EELS) in combination with probe-corrected scanning transmission electron microscopy (STEM) can be applied to this band gap mapping field, in contrast to conventional techniques with limited spatial resolution. However, such studies are rare because of experimental and data process complexity in EELS band gap analysis.

In this study, taking advantages of monochromated EELS coupled with probe-corrected STEM without particular microscope setup, the spatial distributions of optical band gaps in Zn1-xCdxO/ZnO were mapped two-dimensionally with spatial resolution well below 10 nm and a high level of precision.

However, for EELS band gap measurement, some limitation in its spatial resolution exists. As is known, the band gap and plasmon energy of Zn1-xCdxO are both closely related with lattice constants of the unit cell. Compared with the band gap map, the plasmon energy map exhibits higher spatial resolution, and is easily obtainable. A robust function that quantitatively relates band gap and plasmon energy was established, offering an alternative and convenient approach to create band gap map with higher spatial resolution than allowed by the inelastic delocalization of the directly measured band gap signal, while without complex experiment (special hardware required) and data analysis.

In addition, the structure and strain at ZnO/ZnCr2O4 interfaces were investigated utilizing atom-resolved STEM imaging, energy-loss near-edge structure and geometric phase analysis. The above-mentioned STEM-EELS technique was put into use here, two-dimensional band gap mapping of nano ZnCr2O4 inclusions in ZnO matrix was achieved.

Published June 7, 2018 1:11 PM - Last modified June 7, 2018 1:11 PM