Above, left to right: Ripples on an atomically smooth graphite surface. These images were taken 200 nanoseconds (billionths of a second), 500 nanoseconds, 10 microseconds (millionths of a second), and 30 microseconds after a laser set the atoms in motion.
Pass the Popcorn
More than a century ago, movies brought still photographs to life. Now the same thing has been done at Caltech on the atomic scale—the first real-time, real-space views of fleeting changes on a tract of crystalline real estate barely a billionth of a meter on edge. The making of such “movies” starring gold and graphite is described in the November 21 issue of Science. (The movies themselves can be found here.) The new technique, dubbed four-dimensional (4D) electron microscopy, was developed at Caltech’s Physical Biology Center for Ultrafast Science and Technology, directed by Ahmed Zewail, the Pauling Professor of Chemistry and professor of physics, and winner of the 1999 Nobel Prize in Chemistry.
Zewail was awarded the Nobel Prize for pioneering the science of femtochemistry, the use of ultrashort laser flashes to observe fundamental chemical reactions—atoms uniting into molecules, or breaking apart back into atoms—occurring at femtosecond timescales. (A femtosecond, or 10-15 seconds, is one millionth of a billionth of a second. To grasp how incredibly evanescent this is, consider that it takes a beam of light one second to travel from Earth to the moon. In a femtosecond, light goes one one-hundredth the thickness of your eyelash.) The work “captured atoms and molecules in motion,” Zewail says, akin to the freeze-frame sequences snapped by 19th-century photographer Eadweard Muybridge of a trotting horse that proved for the first time that it does indeed lift all four hooves off the ground as it trots.
Snapshots of molecules in motion “gave us the time dimension,” Zewail says, “but what we didn’t have was the dimensions of space, the structure. We didn’t know what the horse looked like. Did it have a long tail? Beautiful eyes? My dream since 1999 was to come up with a way to look not just at time but also at the spatial domain.”
The system uses a high-resolution transmission electron microscope, which “illuminates” the specimen with a stream of electrons to produce an image. In order to be “seen,” a feature on the specimen must be significantly larger than the wavelength of the “light”—in this case, the electron beam—illuminating it. Because the wavelength of an electron shrinks as its velocity increases, very tiny things indeed can be seen with electrons that have been accelerated to dizzying speeds.
But this isn’t enough—the electrons have to be carefully doled out so that they arrive at the sample at specific times. This is achieved by precisely timed laser pulses that individually control every electron’s trajectory through time and space.
The image produced by each electron represents a femtosecond still photo. Like the frames in a film, many millions of such images can be assembled into a digital movie of atomic-scale motion.
Zewail and colleagues applied the technique to superthin sheets of gold and graphite, the form of carbon in pencil lead. They would zap the specimen with a femtosecond laser pulse that caused heat-induced stress in the material, and then watch as the atoms moved in response.
Graphite is particularly interesting because its atoms are locked into sheet-like arrays. These sheets remain highly crystalline even when not much thicker than a single atom, making them a potentially very important item in the nanoengineer’s toolkit—for example, as ultrathin resonators. These experiments were done on samples some 200 atoms thick and a few millionths of a meter wide. Three different behaviors were found, on three different timescales. “The behavior evolves with time,” says postdoc Brett Barwick, the lead author of the Science paper. “If you could keep watching the same sample, you’d see it do all three things, one after the other.”
When the atoms first get blasted by the laser, the heat sets them into random individual vibrations. But neighboring atoms begin to synchronize with each other on femtosecond timescales, and in picoseconds (a picosecond is 10-12 seconds, or one thousandth of a billionth of a second), sound waves begin to reverberate back and forth through the sample’s thickness. Each little patch of the sample’s surface vibrates at its own frequency, but a companion paper by postdoc Oh Hoon Kwon in the November 2008 issue of Nano Letters describes how, as the picoseconds drag into microseconds (millionths of a second), the patches slowly lock into phase with one another, and the oscillations travel the width of the sample, going back and forth across the surface in a heartbeat-like “drumming.” The papers were co-authored with J. Spencer Baskin (PhD ’90), senior scientist; Hyun Soon Park, senior postdoctoral scholar; and Zewail.
“With this 4D imaging technique the atomic-scale motions that lead to structural, morphological, and nanomechanical phenomena, can now be visualized directly, and hopefully understood,” says Zewail, who is now expanding the research to biological imaging within cells in collaboration with Associate Professor of Biology Grant Jensen. (See E&S 2006, No. 2.) The researchers are looking at things such as proteins and ribosomes—the cellular machinery that makes proteins—trying to track their component parts as they move. —KS/DS
