Scientific Method / Science & Exploration

Taking videos with ultrafast electron microscopy

Scientists are seeing all kinds of events they've never witnessed before by …

In electron microscopy, scientists bounce electrons off of objects and track their rebound to form images, including 3D models, of the objects in very fine detail, down to picometer scales. While these highly detailed pictures can tell us a lot about how things are, they say somewhat less about how they got that way. To rectify this, several methods are now being developed to watch in a fourth dimension—time. It's now possible to obtain videos at frame rates several orders of magnitude higher than what most other methods can capture, allowing scientists to see processes never before observed in real time and space. A recent review in Science discusses progress in the area.

The more detailed a picture you want to capture, the finer your medium needs to be; for example, you can get a better fingerprint with ink or Silly Putty than with sand. Since electrons are one of the tiniest items we know how to handle, we can bounce them off surfaces and detect their rebound to produce detailed photos of many things on the atomic scale.

Where electron microscopes fall short, though, is tracking changes over time. The electrons in transmission and scanning electron microscopy are sent out in discrete packets and are not continuous like waves of light, so they can only detect shapes to produce one image at a time.

Light, and especially lasers, would seem to be much better suited to producing video of tiny, fast-moving objects. When using light, scientists don't have to account for the Pauli exclusion principle, which limits how many electrons can occupy the same state and creates complications when you'd like to calculate where exactly electrons are. However, light can't image the locations of all the atoms of a complex structure, because the wavelength of the light is too long. Since the wavelength of an electron is much shorter, it should be capable of taking much more detailed video.

To make an electron microscope take video, scientists realized they needed to track the packets they sent out very carefully, timing the intervals between them down to femtoseconds. That means the electron pulses have to be very short. To get the timing right, you need to send out an initial probe pulse to set a reference point of zero time (t = 0), then send pulses of coherent single-electron packets. This process is called "ultrafast electron microscopy," and takes video at frame rates as high as 1012 images every second.

Scientists have used UEM in many ways, including watching sheets of graphene getting jolted off chunks of graphite. Shocking graphite distorts the structure and liberates a sheet of graphene. With four-dimensional electron microscopy, scientists could watch this process happen in very high detail, and see exactly how the graphene sheet separates. They noted that the separation is the result of the carbon lattice expanding for a very short period of time before the graphene sheet pops away.

UEM can also be used to observe very fine, high-frequency and low-amplitude oscillations that scientists have never seen before. When larger pieces of graphite experience heating over longer times, the microscopy videos showed that a strain propagated though it at very high frequencies, up to 30GHz. Since the technique doesn't involve time averaging like other microscopy approaches, it showed several areas where the amplitude and phase of the oscillations varied slightly.

Some materials undergo small, almost undetectable transformations when they are in use. Scientists used UEM to look at nanoscale cantilevers, and were able to see that they vibrated at very high frequencies when in use, sometimes expanding up into the micrometer scale. The microscope also allowed researchers to watch phase changes in much better detail, like the crystallization of silicon.

While the electron packets can now be used to measure changes over time, they can still only take one angle, falling in a sheet on the object that's being observed. To see additional angles, scientists can create a tomograph, which uses electron pulses from multiple angles that can then be patched together to provide a 360-degree view of the object in question.

The possible uses for such detailed video are broad, and we've outlined only a few from the review paper here. The authors mention in particular that the ultrafast electron microscopes could also be used in biological systems, or at least those systems that don't require extensive labeling or freezing. The authors suggest targets might include circulating fluid within cells, imaging proteins in real space, and determining how cryogenic freezing alters the structure of cells in real time. While even one electron microscope is still expensive, the advent of UEM will let scientists pinpoint more complete answers to the how's and why's of their research.

Science, 2010. DOI: 10.1126/science.1166135  (About DOIs).

Listing image by NIH

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