Ultrafast observations aid the understanding of vanadium dioxide

Researchers at McGill University in Canada have succeeded in simultaneously observing the reorganisations of atomic positions and electron distribution during the transformation of vanadium dioxide (VO2) from a semiconductor into a metal.

The results, reported in Science, are claimed to mark the first time that experiments have been able to distinguish changes in a material’s atomic-lattice structure from the relocation of the electrons in a process that takes place a trillion times faster than the blink of an eye.

The measurements were achieved thanks to the McGill team’s development of instrumentation that could be used by scientists in a variety of disciplines, such as examining fleeting but crucial transformations during chemical reactions, or to enable biologists to obtain an atomic-level understanding of protein function. This ultrafast instrumentation combines tools and techniques of electron microscopy with those of laser spectroscopy in novel ways.

In a statement, Bradley Siwick, the Canada Research Chair in Ultrafast Science at McGill said, ‘We’ve developed instruments and approaches that allow us to actually look into the microscopic structure of matter, on femtosecond time scales that are fundamental to processes in chemistry, materials science, condensed-matter physics, and biology.

‘We’re able to both watch where nuclei go, and separate that from what’s happening with the electrons. And, on top of that, we are able to say what impact those structural changes have on the property of the material. That’s what’s really important technologically.’

By taking advantage of these recent advances, the research group has shed new light on a long-standing problem in condensed matter physics. The semiconductor-metal transition in Vanadium dioxide acts as a semiconductor at low temperatures but transforms to a highly conductive metal when temperature rises to around 60oC. This quality gives the material the potential to be used in a range of applications, from high-speed optical switches to heat-sensitive smart coatings on windows.

The experiments took place in Siwick’s lab where he and his team of graduate students spent nearly four years assembling a maze of lasers, amplifiers and lenses alongside an in-house designed and built electron microscope on a vibration-free steel table.

To conduct the experiments, the McGill team collaborated with the research group of Mohamed Chaker at INRS EMT, a university research centre outside Montreal. The INRS scientists provided the high quality, extremely thin samples of VO2 required to make ultrafast electron diffraction measurements. The diffraction patterns provide atomic-length-scale snapshots of the material structure at specific moments during rearrangement.

‘This opens a whole new window on the microscopic world that we hope will answer many outstanding questions in materials and molecular physics, but also uncover at least as many surprises. When you look with new eyes you have a chance to see things in new ways,’ said Siwick. 

The research was supported by the Canada Foundation for Innovation, the Natural Sciences and Engineering Research Council of Canada, the Canada Research Chairs program, and the Fonds du Recherche du Québec-Nature et Technologies.