Microscopy enters the fourth dimension


20 November 2006

American researchers have taken electron microscopy into the fourth dimension, by recording atoms darting around on a surface in real time.1

Ahmed Zewail and colleagues from the California Institute of Technology, US, recorded movies of atomic structure shifting during the metal-insulator phase transition of vanadium dioxide. High atomic-scale resolution was achieved with an electron microscope, but no ordinary one: Zewail's team adapted it to capture a series of precisely-timed snapshots, just 100 femtoseconds apart, as the phase transition proceeded over around 3 picoseconds.

'All the work for fifty years in electron microscopy has been recording static images - now we can see things moving,' said Zewail, who won the Nobel prize in chemistry in 1999 for his studies of the transition states of chemical reactions using femtosecond spectroscopy. His team introduced the concept of ultra-fast four dimensional (very short duration in space and time) electron microscopy last year.2

Phase transition
UEM images obtained before (left) and after (right) the phase transition. (Scale bars: 100 nm)

© PNAS
As Zewail explained, many researchers had been considering using an ultra-short laser pulse to shoot precisely timed bursts of electron beams out of a photocathode onto a sample. In principle, this would allow snapshot pictures of a surface to be taken at one well-defined point in time. But in practice, the millions of electrons in one beam repelled each other, destroying the electron microscope's close spatial resolution. Zewail's breakthrough last year was to use only one or two electrons in each pulse, repeating each burst many times over to take high precision snapshots of an instant in time.     

Now, the scientists have taken that concept one stage further, linking up a second laser pulse to clock the time between electron bursts. The second pulse also initiated the phase transition by exciting electrons on the vanadium dioxide surface, effectively setting a zero time for the snapshot sequence. Putting this together with a series of technically advanced electron detectors resulted in the remarkable high-speed movie. 

Zewail's team imaged the vanadium dioxide phase transition as a proof of principle, rather than to detect anything new about the well-studied system. But they detected a short-lived stress wave passing through the vanadium dioxide lattice; not all atoms shifted positions at the same time. 

'Zewail's contribution opens absolutely wonderful perspectives to watch the newly formed structures upon electronic excitation of the nanocrystal, and to follow this rearrangement in real time', Majed Chergui, who studies ultrafast spectroscopy at the federal polytechnic school of Lausanne, Switzerland, told Chemistry World. 'The quality of the images is just stunning,' he added.   

John Meurig Thomas, of the University of Cambridge, UK, was enthusiastic about the possibilities opened up by the new technique. He identified steel's switch between austenite and martensite forms as another important phase transition that might be studied.   

Zewail told Chemistry World that he planned to develop the microscopy to study the movement of defects in materials and the behaviour of surface catalysts and biological nanoparticles.

Richard Van Noorden

References

1   M S Grinolds et alProc. Natl. Acad. SciUSA, 2006, doi: 10.1073/pnas.0609233103 

2   V A Lobastov et al, Proc. Natl. Acad. Sci. USA, 2005, 102, 7069