Finding Atomic-Scale Point Defects in Materials

Within a material, the patterns in which the atoms are arranged determine the characteristics of that material. For instance, the material’s strength, flexibility or conductivity are all determined by this arrangement.

Now, scientists at the University of California, Los Angeles have used a powerful microscope to image the three-dimensional positions of individual atoms to a precision of 19 trillionths of a meter. This is several times smaller than a hydrogen atom.

Their observations make it possible to infer the macroscopic properties of materials based on the structural arrangement of the atoms. More importantly, it allows them to locate defects within a material.

Up until now, researchers inferred how atoms are arranged in three-dimensional space using X-ray crystallography, which measures how light waves scatter off a crystal. 

However, this technique only yields information about the average positions of many billions of atoms in the crystal, and not about the individual atom’s precise coordinates.

Because X-ray crystallography doesn’t reveal the structure of a material on a per-atom basis, the technique can’t identify tiny imperfections in materials such as the absence of a single atom. 

These imperfections, known as point defects, can weaken materials, which can be dangerous when the materials are components of machines like jet engines.

“Point defects are very important to modern science and technology,” said Jianwei (John) Miao, lead researcher and UCLA professor of physics and astronomy.

Miao and his team used a technique known as scanning transmission electron microscopy, in which a beam of electrons is scanned over a sample. The microscope then measures how many electrons interact with the atoms at each scan position. 

The method reveals the atomic structure of materials because different arrangements of atoms cause electrons to interact in different ways.

However, scanning transmission electron microscopes only produce two-dimensional images. Therefore, creating a 3D picture requires scanning the sample once, tilting it by a few degrees and then re-scanning it. 

The process is repeated until the desired spatial resolution is achieved, before combining the data from each scan using a computer algorithm. The downside of this technique is that the repeated electron beam radiation can progressively damage the sample.

Using a scanning transmission electron microscope at Lawrence Berkeley National Laboratory’s Molecular Foundry, Miao and his colleagues analyzed a small piece of tungsten, similar to what would be found in an incandescent light bulb. 

As the sample was tilted 62 times, the researchers were able to slowly assemble a 3D model of 3,769 atoms in the tip of the tungsten sample.

The experiment was time consuming because the researchers had to wait several minutes after each tilt for the setup to stabilize.

“Our measurements are so precise, and any vibrations — like a person walking by — can affect what we measure,” said Peter Ercius, a staff scientist at Lawrence Berkeley National Laboratory and an author on the paper.

The researchers also compared the images from the first and last scans to verify that the tungsten had not been damaged by the radiation. The damage was minimal as the electron beam energy was kept below the radiation damage threshold of the tungsten.

Miao and his team showed that the atoms in the tip of the tungsten sample were arranged in nine layers, the sixth of which contained a point defect. The researchers believe the defect was either a hole in an otherwise filled layer of atoms, or one or more interloping atoms of a lighter element such as carbon.

Regardless of the nature of the point defect, the researchers’ ability to detect its presence is significant, demonstrating for the first time that the coordinates of individual atoms and point defects can be recorded in three dimensions.

Miao and his team plan to build on their results by studying how atoms are arranged in materials that possess magnetism or energy storage functions. This could help inform our understanding of the properties of these important materials at the most fundamental scale.

“I think this work will create a paradigm shift in how materials are characterized in the twenty-first century,” he said. “Point defects strongly influence a material’s properties and are discussed in many physics and materials science textbooks. Our results are the first experimental determination of a point defect inside a material in three dimensions.”