Insights into the nanocosmos with unprecedented detail and speed

A German-American team around TU physicist Tais Gorkhover and Christoph Bostedt from the Argonne National Laboratory and Northwestern University in Chicago has succeeded in filming explosions of individual free nanoparticles with a super X-ray microscope. For the first time, a resolution of less than 8 nanometers was combined with a very high temporal resolution of 100 femtoseconds. The exposure time of the image was so short that the fast gaseous particles appeared "frozen" in the images and therefore did not need to be fixed—as is usual in microscopy. Dr. Tais Gorkhover conducts research at the TU Institute for Optics and Atomic Physics in the Cluster and Nanocrystals working group, led by Prof. Dr. Thomas Möller. Her research is part of a Peter Paul Ewald Fellowship from the Volkswagen Foundation at the SLAC National Laboratory of Stanford University in the USA. For the experiments, the research team used a unique X-ray laser (Free Electron Laser), which can produce extremely short and intense X-ray flashes. The results of the research have now been published in Nature Photonics: DOI: 10.1038/NPHOTON.2015.264.

Modern imaging methods are heavily limited when a combination of high resolution and extreme speed is required. Fast optical imaging techniques usually focus only on macroscopic objects. Electron microscopes produce much sharper images, but in return, the temporal resolution suffers due to the long exposure time. This circumstance has so far prevented the direct imaging of ultrafast processes in free nanoparticles. Understanding such processes is fundamentally important for a wide range of questions, from climate modeling to nanotechnology.

Generally, free nanoparticles can significantly change their properties once fixed on surfaces. To image the particles and their dynamics as undisturbed as possible, the particles were therefore photographed during free flight through a vacuum chamber. The tiny particles, with diameters of 40 nanometers (comparable to about one-thousandth the thickness of a human hair), consisted of solid xenon. The particles were ionized with an intense optical laser, strongly heated, and caused to explode, then illuminated with X-ray flashes. A film was assembled from numerous images of individual explosions. "To our surprise, the exploding particles appeared to become smaller over time instead of, as expected, expanding," says Tais Gorkhover. This unexpected result could finally be explained with theoretical models in which the particles do not expand uniformly but instead "melt" from the outside inward.

Another interesting aspect of this new method is that it was the first time to directly image the dynamics of individual free nanoparticles. Until now, most time-resolved studies were based on observing many particles and thus on average values. Fundamental differences related to size, position, and properties of the particles can easily be overlooked. "We have already confirmed in earlier static experiments that this approach can reveal unexpected effects that were previously unnoticed. Now, this approach is finally available for time-resolved imaging," says Gorkhover.

"Our experiment not only provides fundamental insights into the physics of highly overheated matter but also paves the way for a variety of future experiments that aim to investigate fast dynamics with high resolution in freely floating particles," explains Christoph Bostedt. Such dynamics are important, for example, in the formation of aerosols, which can reflect a large part of solar radiation and are therefore significant for climate models. Research on laser-driven fusion reactors and the field of nanoplasmonics—a new area in nanotechnology where the properties of nanoparticles are controlled with intense light fields—could also benefit from this new methodology.

Original publication:

Tais Gorkhover and Christoph Bostedt et al.: Femtosecond and nanometre visualization of structural dynamics in superheated nanoparticles, DOI: 10.1038/NPHOTON.2015.264