August 11, 2010 | 5
The movies don’t have nearly as much interpersonal drama as Avatar, but in these the actors are nanoscopic, directed by the laws of physics operating at the nanoscale. They were filmed using a new kind of electron microscopy.
The electron microscope has long been a workhorse for examining all manner of objects and materials at the nanoscale, but usually by producing still images of rigid, unmoving targets. Over the past decade, researchers have developed an ultrafast technique called four-dimensional electron microscopy, which can produce movies of actions taking place over time intervals in the femtosecond range. (A femtosecond is 10^-15 second—a millionth of a billionth of a second.)
Here, for instance, is a movie of a cantilever oscillating, viewed from a variety of angles. The protruding nickel-titanium "diving board" is a mere 50 nanometers wide and was set in motion by a laser pulse. The individual movie frames come at 10 nanosecond intervals so the motion is slowed down by a factor of about 3 million—things happen quickly at the nanoscale.
This next movie shows a nanocrystal of graphite vibrating like a drumhead, again set in motion by a laser pulse. The dark striped bands are caused by subtle permanent buckling of the crystal surface; think of them as being like an optical effect, similar to the rainbow patterns of an oily sheen on water. These bands move about as the crystal surface ripples ever so slightly in the course of its oscillations.
The feature to notice about these oscillations is how they start out relatively chaotic and gradually settle down to a more regular, coordinated oscillation of the entire surface. Again the motion is tremendously slowed down in the movie, this time by a factor of about a million (50 nanoseconds between each frame, played at 15 frames per second).
You can read more about the ingenious technology behind the making of these movies and the kinds of scientific questions that they can answer in the August issue of Scientific American, in the article, "Filming the Invisible in 4-D" by Ahmed H. Zewail, whose group developed the technique. Zewail won the Nobel Prize in Chemistry back in 1999 for studies—but not movies—of chemical processes occurring on the femtosecond timescale.
4-D electron microscopy is not limited to inorganic entities such as cantilevers and crystals. Zewail’s group has also used it to study protein vesicles and Escherichia coli bacteria. Using a variant of the 4-D technique called PINEM (photon-induced near-field electron microscopy) they have imaged these biological specimens in effect being "lit up" for a period of femtoseconds by a passing laser pulse.
Zewail’s group also has a gallery of other nanomovies online. The most recent addition to the Caltech gallery shows another variant of the technique, called electron tomography, which John Matson reported on in June.
Videos courtesy of Ahmed H. Zewail, California Institute of Technology
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