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View atoms moving in real time with this microscope

Photonics Spectra
Feb 2009
David L. Shenkenberg,

It’s one thing for scientists to ponder the nature of materials indirectly – and quite another to watch those materials change before their eyes. The electron microscope has enabled scientists to view atoms in materials, and now Ahmed Zewail and members of his laboratory have modified an electron microscope so that it shows movies of atoms rippling through materials in real time. Ordinary electron microscopes capture only still images.

Scientists heated sheets of graphite and gold, which caused atoms to ripple through the material and make sound waves. They observed this “nanodrumming” process using ultrafast electron microscopy, a technique that combines an electron microscope with a femtosecond and a nanosecond laser. Reprinted with permission of Nano Letters.

Will Zewail win the Nobel Prize? He already did – in 1999. He won the award because his group at Caltech used extremely short laser pulses femtoseconds long – mere quadrillionths of a second – to detect atoms breaking apart and joining together. “[Seeing molecules in motion] gave us the time dimension, but what we didn’t have was the dimensions of space, the structure,” Zewail said. The researchers bill their apparatus as the “4-D electron microscope,” after the dimensions that it can measure: length, width, depth and time.

They observed atoms undulating throughout entire sheets of gold and graphite. The graphite and gold sheets were very thin, only nanometers in thickness. Thin films have many applications, including solar cells, sensors and optical coatings. By heating the sheets with laser pulses, they caused atoms in the sheets to move.

The researchers recorded movies by rigging a femtosecond laser system to an electron microscope so that the laser pulses are converted into femtosecond electron pulses. Each image taken by the electron microscope corresponds to an electron pulse, and each femtosecond image is strung together to make a movie.

Not only did the researchers record movies of atoms but they also discovered that graphite sheets produce sound waves when oscillating at frequencies of picoseconds, or trillionths of a second.

Finally, the researchers also detected sound waves when they observed the graphite sheets oscillating at up to millisecond frequencies. They call this “nanodrumming,” but don’t expect nanodrumming to come to a concert venue anytime soon because it’s 100 times more high-pitched than the human ear can hear. These studies are detailed in the Nov. 21, 2008, issue of Science and in the Nov. 12, 2008, issue of Nano Letters.

This collage abstractly illustrates nanodrumming. Reprinted with permission of Nano Letters.

Brett Barwick, the postdoc in Zewail’s lab who was instrumental to these studies, said, “I think that researchers in the microelectromechanical systems (MEMS) field may be very interested in our ultrafast electron microscopy technique of visualizing these fast oscillatory motions of thin films, with nanometer spatial and femtosecond temporal resolutions.”

With associate professor of biology Grant Jensen at Caltech, the scientists plan to apply the imaging technique to seeing cellular components such as proteins and ribosomes. In fact, they already have imaged a protein and a rat cell.

Fellow Caltech chemistry professor David Tirrell said, “These experiments will lead us to fundamentally new ways of thinking about molecules and materials,” while electron microscopy expert Sir John Thomas of Cambridge University called the work “revolutionary.”

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