Ask the Expert

Professor Sinclair says: The properties of any material depend not only on what it's made of, but also how the atoms and molecules within it are arranged. One obvious example is water versus ice. Another you may already know is that black, crumbly graphite and a brilliant, hard diamond are different only because of the distinct arrangements of their carbon atoms. When we can see the exact structural differences in materials, we can learn how to engineer them to create new technologies and to improve existing ones. But this requires seeing on the scale of molecules and atoms–billionths of a meter, or nanometers.

This is the kind of work I and my colleagues and students do at the Stanford Nanocharacterization Lab, where we have several microscopes to look at materials and structures on the nanoscale.

This size scale is so small, we cannot perceive it with visible light waves. The wavelength of light is thousands of times too big (hundreds of nanometers). Using light to see atoms would be like trying to dial a phone with a baseball bat. You need something smaller– a finger–to press each key. To perceive individual atoms, we use electrons, which have wavelengths around two picometers, about 100 times smaller than the width of an atom.

A transmission electron microscope (TEM) works much like the optical ones students use in high school. For instance, a beam is projected through a thin sample of transparent material and is focused and enlarged by a series of lenses. Instead of a ray of light, however, a TEM uses a beam of electrons. Instead of lenses made of glass, a TEM focuses electrons using lenses made of electromagnetic fields. And, of course we can't see electrons with our eyes, so we convert them into light that we can see using a fluorescent screen viewed by a traditional TV tube or a sensor called a CCD, which is the same kind of imaging chip in many digital cameras.

Although electrons have the short wavelength to produce very fine images, the clarity of electron microscope images has always suffered from errors, or aberrations, resulting from how the lenses focused them. Some aberrations, called spherical, had to do with the direction electrons were heading when they entered the lens. Another kind, called chromatic, have to do with how the lenses mishandle electrons of different energies or wavelengths. These errors limited electron microscopes to resolutions of about a sixth of a nanometer or so.

Within the last five years, however, engineers have pretty much solved the speherical aberration problem, giving TEMs a resolution of 0.07 nanometers, enough to very finely resolve individual atoms (which are roughly 0.25 nanometers in diameter) and the spacing between them.

At a nanomaterials conference I organized in early July in Korea, Japanese researchers Kazu Suenaga and Sumio Iijima, who use an aberration-corrected TEM, presented stunningly clear images of individual atoms in a carbon nanotube–a nanostructure that many engineers hope will revolutionize the speed and power usage of electronic circuits. The hexagonal rings of carbon atoms that make up a nanotube (like the hexagons of wire in a chicken wire fence) typically have six carbon atoms each, but this same team was able to show cases where tubes had rings of either five or seven atoms. They were even able to examine the motion of such defects in real time. Gaining these kinds of insights will ultimately allow us to understand nanotubes better, which will help us make better use of them in future circuits.

Here at Stanford we do not yet have an aberration corrected electron microscope, but deans and professors from around the university are currently raising money to acquire one. When we have this exciting new capability, we will be able to break new ground in nanotechnology, for example, studying ways to expand on the research of my materials science and engineering colleague, Assistant Professor Yi Cui, who has extended the life of lithium batteries by adding lithium-containing nanoscale silicon wires. Longer battery life and faster, smaller-scale electronics are perfect examples of the kind of technology advances that nanoscale materials science and engineering can enable. That all comes down to being able to see the tiniest of things.