Collaborative development of direct electron imaging detectors in the scanning electron microscope

The Universities of Strathclyde and Glasgow and the National Physical Laboratory are working together on the development of new energy filtering direct electron imaging detectors for use in the scanning electron microscope. The unique properties of our detectors will enable the investigation of the structural properties of materials with improved lateral and depth resolution, enabling, for example, the investigation of defects in single nanorods.

What's involved?

The scanning electron microscope produces images from materials on the nanoscale. An image is produced by rastering an electron beam over the sample surface and monitoring the resulting emitted, backscattered, transmitted and diffracted electrons, X-rays and/or light. If the sample is crystalline in nature, its crystalline properties may be mapped by capturing electron diffraction patterns produced by the material. 

A 30 keV transmission diffraction pattern from a 300 nm thick gallium nitride nanorod.

The diffraction pattern is effectively a projection of the crystal structure, a map produced from a set of diffraction patterns can reveal changes in crystal structure and strain, and structural defects in the material.

If the electrons forming the diffraction patterns are captured directly and we can select their energy, we obtain diffraction patterns and produce images with unprecedented detail.

This image shows a diffraction pattern produced from 30 keV electrons transmitted through the tip of a 300 nm thick gallium nitride nanorod, exhibiting both diffraction spots and Kikuchi bands.

Images from Vespucci, S., et al. "Digital direct electron imaging of energy-filtered electron backscatter diffraction patterns." Physical Review B 92 205301 (2015):

Diamond ESBD pattern

Diamond EBSD pattern

 Gallium nitride ESBD pattern120x120 Gallium nitride EBSD pattern 

To date, we have acquired diffraction patterns from gallium nitride, silicon and diamond thin films and gallium nitride nanorods. We have imaged gold nanoparticles and observed defects in a wide range of semiconductors. Diffraction patterns can be acquired in both transmission and backscattered geometries.

Images may also be produced through processing of data acquired from different areas of the detector, ie, from 'virtual diodes.' 

Scanning electron transmission micrograph obtained from gold nanoparticles embedded in amorphous silica capsules

This image shows scanning electron transmission micrograph obtained from gold nanoparticles embedded in amorphous silica capsules. Acquired with the Timepix detector in the scanning electron microscope at an energy of 30 keV.  

New research opportunities

The development of these new direct electron imaging detectors promises to open up new research opportunities in 3-dimensional texture, strain and defect mapping in solid-state materials. The improved performance of our detectors will enable the application of the EBSD technique to be expanded to materials for which conventional EBSD analysis is not presently practicable. For eg, for understanding the strain present in diamond crystals to inform the engineering of diamond drill bits; to understanding the structure and strain in highly deformed steels; to probing the properties of semiconductor nanorods for solar energy converters.