Researchers in the US have developed a new technique to allow changes in atomic-scale structures to be tracked in real time. The method relies on an electron beam being focused to a spot on the sample material only a few tens of nanometres across and pulsed at a rate of femtoseconds. The diffraction pattern of the electrons changes as the structure of sample changes, providing unprecedented direct information about the evolution of structural changes at the atomic scale.

The work has been carried out by Aycan Yurtsever and Ahmed Zewail at the California Institute of Technology, US. Zewail was awarded the Nobel prize for chemistry in 1999 for his work on ultrafast spectroscopy and last year unveiled the concept of 'ultrafast electron microscopy', where femtosecond pulses of electrons produce a sequence of images that can be assembled into a digital 'movie' of motion at the atomic scale. 

Now, Yurtsever and Zewail have described how the technique can be adapted for electron diffraction studies. Currently electron diffraction can be used to focus on nanoscale areas to define structures with high precision. However, until now the samples have been static, or if moving, data must be averaged over time, which can obscure crucial details.

To test the system the researchers focused the electrons on a sample of silicon which they heated with a pulse of laser energy. They were able to observe the expansion and recovery of a specific portion of the crystalline lattice at a timescale of femtoseconds - obtaining direct measurements both in time and space without the need for averaging.

'Everyone has been trying to see if they can focus the probing beam to see only nanoscale structures in a sea of other structures,' says Zewail. 'We have achieved this and it is important because everything from materials science to cell biology involves inhomogeneous structures which have different properties in different places in space - this new method allows us to focus on a single site and observe the specific site dynamics.'

Majed Chergui, who heads the Laboratory of Ultrafast Spectroscopy at the Ecole Polytechnique Fédérale de Lausanne in Switzerland, is impressed by the study. 'Electrons are unique in that they can be focused to extremely tiny spots, and electron nanodiffraction is now a common tool to study the structure of materials down to atoms,' he says. 'The new results reported by Zewail are a major breakthrough in that he has added the time dimension to the structure. Aside from the amazing opportunities it offers to understand the fundamentals of structural and morphological changes, it also opens the possibility to study heterogeneous media and/or composite materials by imaging the structural evolution of each component and eventually to find the correlations between them.' 

Simon Hadlington

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