To figure out why a material has particular properties, scientists need to know far more than just the elemental chemical composition of that material. To really understand what makes a material “tick,” scientists need to probe the internal structure of the material to decipher just how the atoms and molecules within the material fit together, how they interact in various states and phases and how conditions such as temperature and magnetic field affect those interactions.
Ames Lab scientist Matt Kramer operates the Lab’s transmission electron
microscope. Various equipment options allow scientists to probe different
aspects of a material. In scanning mode, or scanning transmission electron
microscopy (STEM), the electron beam is scanned back and forth across
To characterize a material, scientists rely on a wide variety of analytical techniques. While individual characterization techniques are specialized to highlight different aspects of a material, they do share some things in common. Many techniques rely on the principle that atoms and molecules have a unique signature on the spectrum when subjected to electromagnetic radiation. The signature can manifest itself in different ways, but a simple example of this principle can be seen in a TV screen where a europium phosphor shows only the vivid red portion of the light spectrum when subjected to the broadcast signal.
Spectroscopy allows scientists to “see” what’s inside a material by viewing what happens to the electromagnetic radiation directed at the sample. Depending on the type of radiation used – running the gamut from radio waves, microwaves and light to X-rays and gamma rays – and its strength, scientists can measure how the energy is absorbed by the material, emitted by the material or scattered by the material.
In nuclear magnetic resonance (NMR) spectroscopy, for example, researchers use radio-frequency magnetic energy on a sample to measure the local magnetic fields generated by the nuclear spins. This provides detailed spectral information about the physical and chemical properties of the material, such as the distance between nuclei and their specific positions at a particular point in time.
Similarly, X-ray diffraction spectroscopy uses X-rays to probe the sample, and the interaction of the X-rays with the electrons in the materials
results in the transference of energy from the X-rays. These “scattered” or defracted X-rays will have a different wavelength than the original beam. In highly ordered materials, these scattered rays will form a pattern that corresponds with the distribution of atoms within the material, providing keys to its crystal structure.
Some types of electron microscopy also rely on the material’s spectroscopic properties. In transmission electron microscopy, very thin samples are probed with an electron beam that passes through the material. As the beam passes through, some of the beam’s electrons are reflected. Different particles have a particular angle of reflection, so by using sensors to measure the angle of the reflected beam, scientists learn about the structure of the material.
The Ames Laboratory’s strength in creating materials relies in large part on the expertise of its researchers in these various techniques to quantify particular aspects of a material. And the Lab’s highly
|A technique known as high angle annular dark field (HAADF) STEM can be used
to create a high contrast image, such as the one of a co-block polymer on the
left, to give a better view of the crystal structure of materials. At right is a TEM
bright field image of the same co-block polymer, which provides a better view of the sample’s surface.
collaborative atmosphere allows the snapshots gathered by these various characterization techniques to be drawn together to form a more complete picture.
“Our characterization expertise and capabilities are a cornerstone of who we are at the Ames Laboratory,” says Duane Johnson, Ames Lab’s chief research officer. “The Chemical and Advanced Materials Characterization Center, or CAM-2C (came to see), encompasses a broad range of techniques that directly benefit the DOE’s energy-sciences portfolio.”
“What sets Ames Laboratory apart from most materials research labs is the broad, yet integrated nature of the research, synthesis and characterization within our material discovery efforts,” Johnson says. “It involves numerous researchers, which are mirrored within the Lab’s four science cornerstones, and founded on our unique synthesis and processing of high-purity, bulk single crystals that may involve volatile elements.”
“With our `theoretical characterization’ capabilities (computational materials science and chemistry),” he adds, “we have an integrated feedback loop between theory and experiment that works very well, helping to speed up development and understanding.”
CAM-2C’s capabilities include: