UCLA Physicists Determine the
Three-Dimensional Positions
of Individual Atoms for the First Time
Finding Will Help Scientists Better Understand
the Structural Properties of Materials
by Katherine Kornei, UCLA Newsroom, September 21, 2015
Atoms are the building blocks of all matter on Earth, and
the patterns in which they are arranged dictate how strong, conductive or
flexible a material will be. Now, scientists at UCLA have used a powerful
microscope to image the three-dimensional positions of individual atoms to a
precision of 19 trillionths of a meter, which is several times smaller than a
hydrogen atom.
Their observations make it possible, for the first time, to
infer the macroscopic properties of materials based on their structural arrangements
of atoms, which will guide how scientists and engineers build aircraft
components, for example. The research, led by Jianwei (John) Miao, a UCLA
professor of physics and astronomy and a member of UCLA’s California
NanoSystems Institute, is published Sept. 21 in the online edition of the
journal Nature Materials.
For more than 100 years, researchers have inferred how atoms
are arranged in three-dimensional space using a technique called X-ray
crystallography, which involves measuring how light waves scatter off of a
crystal. However, X-ray crystallography only yields information about the
average positions of many billions of atoms in the crystal, and not about
individual atoms’ precise coordinates.
“It’s like taking an average of people on Earth,” Miao said.
“Most people have a head, two eyes, a nose and two ears. But an image of the
average person will still look different from you and me.”
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,” Miao said.
Miao and his team used a technique known as scanning transmission electron microscopy, in which a beam of electrons smaller than the size of a hydrogen atom is scanned over a sample and 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. So creating a 3-D picture requires scientists
to scan the sample once, tilt it by a few degrees and re-scan it — repeating
the process 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 the
Lawrence Berkeley National Laboratory’s Molecular Foundry, Miao and his
colleagues analyzed a small piece of tungsten, an element used in incandescent
light bulbs. As the sample was tilted 62 times, the researchers were able to
slowly assemble a 3-D 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 of the paper.
The researchers compared the images from the first and last
scans to verify that the tungsten had not been damaged by the radiation, thanks
to the electron beam energy being kept below the radiation damage threshold of
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.
“We made a big breakthrough,” Miao said.
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, which will 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 21st 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.”
The study’s co-authors include Rui Xu, Chien-Chun Chen, Li
Wu, Mary Scott, Matthias Bartels, Yongsoo Yang and Michael Sawaya, all of UCLA;
as well as Colin Ophus of Lawrence Berkeley National Laboratory; Wolfgang Theis
of the University of Birmingham; Hadi Ramezani-Dakhel and Hendrik Heinz of the
University of Akron; and Laurence Marks of Northwestern University.
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