Imaging technique captures movement at atomic scale.
Today's microscopes are incredibly powerful, but run into a wall at the nanoworld. The resolution of optical microscopy has advanced considerably with the advent of the near-field optical microscope, but still can't capture atomic structures. Meanwhile, a standard electron microscope can provide atomic details but only takes static images, and so cannot capture the dynamic behaviours of nanometre-sized molecules and materials.
Ahmed Zewail and his colleagues at the California Institute of Technology in Pasadena have now found a way to introduce near-field imaging to electron microscopy to produce real-time movies of nanometre-sized structures. Their technique, dubbed photon-induced near-field electron microscopy (PINEM), has the potential to change the way scientists see the nanoworld (see page 902).
Zewail had long been trying to add another dimension to the electron microscope's static three-dimensional images. “This has been my dream for many years,” he says. “To get the structure and see how it changes over time.” His group made a leap towards that dream in 2005, when it published and patented four-dimensional electron microscopy (V. A. Lobastov et al. Proc. Natl Acad. Sci. USA 102, 7069–7073; 2005). This technology allows atoms to be seen in, for example, the graphite of pencil lead (B. Barwick et al. Science 322, 1227–1231; 2008).
But Zewail wanted to see even more detail — such as the movement of electrons within a structure. “The question was, can you exploit the electron energy to image in space and time the electronic distributions that describe nanostructures?” To do so would require the team to image both electrons and photons — which have drastically different energies — at the same time.
The idea of bringing together electrons and photons in this way is akin to that of simultaneously capturing, with one quick click of a camera shutter, a running leopard and a galloping horse coming from two different directions at unspecified times. “Electrons and photons do not interact in free space because of the mismatch of their energies and momenta,” says Zewail. Luckily, however, in the nanoworld these rules do not always apply, and the two energies do sometimes interact.
Brett Barwick and David Flannigan, two postdocs in Zewail's lab, illuminated various nanostructures, tubes and wires, then visualized electrons in the light field. “Instead of looking at the image for all electrons, we look at the image only for those electrons that lost or gained energy by interacting with matter or photons, respectively,” Zewail says. To their surprise, the approach worked and they were able to visualize fields of electrons in these nanostructures over time. Seeing their shining results of how atoms interact in nanomaterials, “was like looking at a beautiful flower or work of art”, Zewail says.
The researchers were particularly surprised by how bright their images were. In cases in which they saw an energy gain, electrons acquired as much as 8 quanta of energy — the researchers had expected only one to two at most, with low probability. “In science, you work so hard and sometimes the ideas or the systems don't cooperate,” Zewail says. “But sometimes things cooperate so much, it's beautiful.”