A Material World: A Deep Look at the Ties That Bind
Lipkin, Richard, Science News
Imagine this scenario. A large, loosely fastened bedspring hangs in a pitchblack room. In the darkness, with only a bucket of baseballs at your feet, you stand facing that invisible bedspring.
Your job is to figure out what the bedspring looks like -- or, more precisely, how it is physically constructed.
50 you begin throwing baseballs at the bedspring. You notice that most baseballs go straight through. Some bounce back. Others rebound at strange angles. A few even get stuck. Continuing, you also notice that each time a ball hits a single coil, you hear a distinctive ringing. The sound's pitch, created by the coil's unique vibration, varies depending on how fast you throw the ball and where the ball strikes. Soon, you discover that those vibrations relate to the amount of energy the ball imparts to a given coil, as well as that coil's size and shape.
Now, you devise a plan. You build a machine that throws balls at the bedspring with an exact speed and direction, then tracks the angles of their rebounds, the forces they carry, and the energies they have imparted to the bedspring. After throwing a few million balls, you take all the data and feed them into a computer, which figures out roughly what the bedspring looks like, how it's built, and the nature of the material from which it is made.
While not literally true, this analogy captures a sense of the process used by some physical chemists to determine the structure of certain materials, right down to the level of individual atoms. Think of the balls as electrons or other high-energy particles and the bedspring as a well-ordered material.
Indeed, advances in analytical microscopy during the past decade have enabled researchers to probe the depths of matter with surprising precision. The ability to see individual atoms is slowly coming into reach. Today, scientists can detect in specific regions of certain materials exactly what atoms are present, where they are located, and how they bond together -- doing so with previously unattainable accuracy.
Analytical electron microscopy, which combines more than one detection technique, is among the most innovative methods for delving into matter's nooks and crannies. One impressive pairing brings together scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS).
When it comes to detecting objects on the atomic scale, STEM and EELS offer advantages and disadvantages -- sensing some things well, others poorly. Yet when joined, the two methods can yield enough information to piece together a reasonably detailed picture of just a few atoms in a specific region of a crystal. In recent months, scientists have achieved unprecedented atomic resolutions this way.
The technique involves passing a thin stream of high-energy electrons through wafer-thin, 100-nanometer slices of matter. STEM collects information revealing the basic structure and spatial arrangement of the atoms, which are stacked up in columns. EELS then detects how atoms in the slices have deflected those electrons and how much energy each electron has yielded to the atoms in its path. With those data, EELS can identify the elements present based on each atom's unique spectrum.
From the combined STEM and EELS information, researchers can determine the identities of individual atoms, their exact locations, and the nature of the bonds between them.
"Remember that we're using these techniques to investigate incredibly small bits of matter," says physical chemist Dale E. Newbury of the National Institute of Standards and Technology in Gaithersburg, Md. "The electron-beam diameter of our system is only 1 nanometer. If we shoot that beam through a 50-nanometer film, we're talking about exciting an extraordinarily small amount of matter -- a single column of atoms with a mass of [10.sup.-19] grams. That's really small."
First conceived in the 1930s, EELS remained largely neglected, owing to technical inefficiencies, until the mid-1970s. …