Researchers at the National Institute of Standards and Technology (NIST) and their collaborators have developed a way to retrofit the transmission electron microscope — a long-standing scientific workhorse for making crisp microscopic images — so that it can also create high-quality movies of super-fast processes at the atomic and molecular scale. Compatible with electron microscopes old and new, the cost-effective retrofit promises to enable fresh insights into everything from microscopic machines to next-generation computer chips and biological tissue by making this moviemaking capability more widely available to laboratories everywhere.
A nearly 100-year-old invention, the electron microscope remains an essential tool in many scientific laboratories. A popular version is known as the transmission electron microscope (TEM), which fires electrons through a target sample to produce an image. Modern versions of the microscope can magnify objects by as much as 50 million times. Electron microscopes have helped to determine the structure of viruses, test the operation of computer circuits, and reveal the effectiveness of new drugs.
In the last 15 years, laser-assisted electron microscopes made videos possible, but such systems have been complex and expensive. While these setups can capture events that last from nanoseconds (billionths of a second) to femtoseconds (quadrillionths of a second), a laboratory must often buy a newer microscope to accommodate this capability as well as a specialized laser, with a total investment that can run into the millions of dollars. A lab also needs in-house laser-physics expertise to help set up and operate such a system. Not everybody has that capability.
In contrast, the retrofit enables TEMs of any age to make high-quality movies on the scale of picoseconds (trillionths of a second) by using a relatively simple “beam chopper.” In principle, the beam chopper can be used in any manufacturer’s TEM. To install it, NIST researchers open the microscope column directly under the electron source, insert the beam chopper and close up the microscope again. So far, three TEMs of different capabilities and vintage have been successfully retrofitted.
Like a stroboscope, this beam chopper releases precisely timed pulses of electrons that can capture frames of important repeating or cyclic processes. Like a light shutter, the beam chopper interrupts a continuous electron beam. But unlike the shutter, which has an aperture that opens and closes, this beam aperture stays open all the time, eliminating the need for a complex mechanical part. Instead, the beam chopper generates a radio frequency (RF) electromagnetic wave in the direction of the electron beam. The wave causes the traveling electrons to behave “like corks bobbing up and down on the surface of a water wave,” said NIST scientist June Lau.
Riding this wave, the electrons follow an undulating path as they approach the aperture. Most electrons are blocked except for the ones that are perfectly aligned with the aperture. The frequency of the RF wave is tunable, so that electrons hit the sample anywhere from 40 million to 12 billion times per second. As a result, researchers can capture important processes in the sample at time intervals from about a nanosecond to 10 picoseconds. In this way, the NIST-retrofitted microscope can capture atom-scale details of the back-and-forth movements in tiny machines such as microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS). It can potentially study the regularly repeating signals in antennas used for high-speed communications and probe the movement of electric currents in next-generation computer processors.