Snapshots of atoms in action

Editorial

Rebecca Pool

Thursday, November 7, 2019 - 15:30
Image: Mapping out atomic position in materials.
 
Researchers at the Lawrence Berkeley National Laboratory have just published new research showing how 4D-STEM can provide direct insight into the performance of any material from strong metallic glass to flexible semiconducting films.
 
Andrew Minor, facility director for the National Center for Electron Microscopy at Berkeley Lab’s Molecular Foundry, and colleagues, used 4D-STEM to pinpoint specific atomic “neighbourhoods” that could compromise or enhance a material's performance.
 
“In these studies, we’ve shown that when 4D-STEM is deployed with high-speed detectors and customizable algorithms, the technique can help scientists map out atomic or molecular regions in any material - even beam-sensitive, soft materials - that weren’t possible to see with previous techniques,” he says.
 
In a Nature Materials paper, "Diffraction imaging of nanocrystalline structures in organic semiconductor molecular thin films", Minor and colleagues demonstrated how high-speed detectors that capture atoms in action at up to 1,600 frames per second with 4D-STEM allowed unprecedented molecular movies of a small-molecule organic semiconductor.
 
The movie showed how the molecular ordering in the semiconductor, often used in organic solar cells, changed in response to a common processing additive - DIO or 1,8-diiodooctane - that is known to enhance solar cell efficiency.
 
4D-STEM experiments allowed Minor and colleagues to map out the orientation of the grains of ordered molecules within the material.
 
(Top) Electron beam diffraction patterns were used to form the molecular structure below, in which 4D-STEM map traces the molecular structure of a small-molecule thin film. [Colin Ophus/Berkeley Lab]
 
Such details, which are not possible to observe with conventional STEM, are significant because low-angle boundaries are necessary for electrons to couple and generate a charge in a functional semiconductor.
 
Using this powerful new technique, the researchers clearly demonstrated that the DIO additive dramatically alters the material’s nanostructure, and that this overlapping grain structure is key to the enhanced efficiency observed in solar cells made from these materials.
 
Meanwhile, in further research, published in Nature Communications - "Direct measurement of nanostructural change during in situ deformation of a bulk metallic glass" - the researchers identified atomic-scale “weak links” in bulk metallic glass that ultimately lead to fractures under stress.
 
Here, Minor and colleagues used 4D-STEM to directly measure the nanostructural changes in bulk metallic glass as it fractured.
 
Berkeley Lab researchers used 4D-STEM to directly measure the nanostructural changes in bulk metallic glass as it breaks. (Credit: Berkeley Lab)
 
They were able to measure the average spacing between atoms within certain regions of the bulk metallic glass, and recorded the strain under a tensile load.
 
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