At 68 C, vanadium oxide (VO2) undergoes a significant change in electronic properties that has vexed physicists and chemists for more than 50 years. In 1959 it was discovered that as the material cools, it changes from a metal to a semiconductor while its atoms reorganize themselves from a tetragonal to monoclinic crystal structure.

“It has defied a fully satisfactory explanation ever since,” says Bradley Siwick, who holds the Canada Research Chair in Ultrafast Science at McGill University in Montreal. Now, after spending almost six years assembling a sophisticated array of high-speed lasers and a home-built electron microscope to witness the dynamics of chemical reactions at the femtosecond (10-15) scale, this remarkable transition has been unravelled in all of its subatomic glory. “You have multiple collaborating interactions,” Siwick says. “You have the way the electrons interact with the lattice and then you have the way the electrons interact with each other. This is happening all at once.”

Siwick and a team of graduate students reported their ability to separate these interactions in an article in Science published this past October. Using laser light to induce the transition between semiconductor and metal they performed two experiments simultaneously: ultrafast electron diffraction recorded the structural reorganization and mid-IR spectroscopy captured the change in electronic properties. “In these experiments we were able to watch the electrons reorganize independent of the lattice structural reorganization and to determine the impact of that on the material’s electronic properties,” Siwick says. “It’s a real game changer. This experiment is a first indication of what truly new things you have the potential to learn when you are able to look at these transitions with such detail and precision. I’m still amazed.”

Siwick already has a list of other materials with transitions that he would like to analyze in the same way. He notes that this approach has profound implications for understanding how subtle changes in structure can lead to dramatic changes in physical behaviour, such as superconductivity. “Using these approaches, we want to deepen our understanding of the relationship between the structure and properties of complex materials, even when they are far from equilibrium conditions­,” Siwick says.