When capturing images at the atomic scale, using scanning transmission electron microscopes (TEMs), even the most miniscule movements of a sample can cause skewed or distorted images. Such movements are virtually impossible to avoid.
Scanning transmission electron microscopes are able to capture images of a material’s individual atoms. To get the image, scientists allow a probe to scan across the sample area which has an area of less than 25 nanometers squared. The scanning can take in the tens of seconds.
Temperature Drift Distortion
The sample rests on a support rod. During scanning, the rod is prone to expand or contract due to subtle changes in ambient temperature.
The rod’s expansion or contraction is invisible to the naked eye, yet since the sample area is measured in nanometers the rod’s movement causes the sample material to slightly shift. This so-called drift can cause the resulting scanning TEM images to be significantly distorted.
“But our approach effectively eliminates the effect of drift on scanning TEM images,” says Dr. James LeBeau, senior author of a paper on the work.
Researchers programmed the microscope to rotate the direction in which it scans the sample. It might for example, first capture an image scanning from left to right, then take one scanning from top to bottom, then right to left, then bottom to top.
Each scanning direction includes the distortion caused by drift from a different vantage point.
Referenceless Nanoscale Images
The researchers input those images into a program they designed to measure the features in each image and use that data to determine the precise direction and extent of drift within the sample.
Once the drift is quantified, the images can be adjusted to remove the distortion caused by the drift. The resulting images accurately represent the actual structure of the sample and give scientists new capabilities to understand bonding between atoms.
“Historically, a major problem with drift has been that you need to have a reference material in any nanoscale image, so that you can tell how the image has been distorted,” LeBeau says. “This technique makes that unnecessary. That means we can now look at completely unknown samples and discover their crystalline structures – which is an important step in helping us control a material’s physical properties.”
Xiahan Sang, James M. LeBeau.
Revolving scanning transmission electron microscopy: Correcting sample drift distortion without prior knowledge.
Ultramicroscopy, 2013; DOI: 10.1016/j.ultramic.2013.12.004
Image credit: North Carolina State University