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Cathodoluminescent Properties At Nanometer Resolution Through Z-Contrast Scanning Transmission Electron Microscopy

H-J. Gao, 1,2,3  G. Duscher,4 M. Kim,1 D. Kumar,3 K.G. Cho,3 P.H. Holloway,3 R.K. Singh,3 S. J. Pennycook1

Applied Physics Letters, 77, 594 (2000)

Full Article (PDF 908 KB)

One of the advantages of the scanning transmission electron microscope is that any signal which varies with probe position can be collected and displayed to form an image.  Detection of the cathodoluminescence induced by the beam produces a map of the optical emission that can be directly compared to structural features seen in the Z-contrast image.

The luminescence efficiency of Eu-doped Y2O3 thin films grown by laser ablation was found to depend sensitively on the exact growth conditions.  Transmission electron microscopy revealed that the films contained columnar voids, with a size that correlated with the luminescent efficiency as measured by photoluminescence.  Increasing void size reduced the optical efficiency, but more sharply than expected due to the reduced volume fraction alone.  Simultaneous collection of Z-contrast and cathodoluminescence images in the VG Microscopes 100 kV scanning transmission electron microscope revealed the reason for the rapid degradation.  Figure 1 compares the two images and also shows intensity profiles across an end-on void.  It is immediately apparent that the cathodoluminescence image appears to show a poorer resolution than the Z-contrast image.  The reason is the finite diffusion length of electron-hole pairs.  The surface of the voids are sites of non-radiative recombination and so induce a ñdead layerî around each void.  Simply by calculating the effective size of a void as its geometric size plus the width of the dead layer provided a consistent explanation for the reduction in efficiency.

The combination of cathodoluminescence, Z-contrast imaging and electron energy loss spectroscopy  is especially powerful for understanding the properties of optical materials, especially nanostructures.  It would be possible to determine the quantum yield of individual nanowires or core-shell nanocrystals for correlation with size, structure, composition and shape.

 
 
Figure 1
  Figure 1: Z-contrast image (upper) and corresponding CL image (lower) with intensity profiles across a single pore from A to B revealing the luminescent dead layer (D) to extend for about 5 nm.
   
 
  1. Solid State Division, ORNL
  2. On leave from Beijing Laboratory of Vacuum Physics, Chinese Academy of Sciences, Beijing, China
  3. Department of Materials Science and Engineering, University of Florida
  4. North Carolina State University, Raleigh, NC

 Oak Ridge National Laboratory