A team of scientists at Lawrence Berkeley National Laboratory (Berkeley Lab), Berkeley, Calif., is providing a better way for scientists to study the structures of the many materials that don’t form tidy single crystals, such as solar absorbers and metal-organic frameworks: two diverse material groups with huge potential for combatting climate change and producing renewable energy.
A key breakthrough in the quest for DNA’s structure came from x-ray crystallography, a technique that maps the density of electrons in a molecule based on how beams of x-ray radiation diffract through the spaces between atoms in the sample. The diffraction patterns generated by crystallography can then be used to deduce the overall molecular structure. Thanks to a steady stream of advances over the decades, x-ray crystallography is now exponentially more powerful than it was in Crick’s time, and can even reveal the placement of individual atoms.
X-ray crystallography is most straightforward when the material can be grown into a large single crystal. Yet the process is not easy, and it requires crystals – specifically, purified samples of the molecule of interest, coaxed into a crystal form. And not all molecules form picture-ready crystals.
Instead, most substances form powders composed of small granules, whose x-ray diffraction patterns are harder to disentangle.
The new technique, called small-molecule serial femtosecond X-ray crystallography, or smSFX, supercharges traditional crystallography with the addition of custom-built image processing algorithms and an x-ray free electron laser (XFEL). The XFEL, built from a fusion of particle accelerator and laser-based physics, can point x-ray beams that are much more powerful, focused, and speedy than other x-ray sources for crystallography. The entire process, from x-ray pulse to diffraction image, is completed in a few quadrillionths of a second.
“It’s diffraction before destruction,” said Daniel Paley, an MBIB project scientist and author on the team’s paper published in Nature. “The idea is that the crystal is going to explode instantly when its hit by this beam of photons, but with a femtosecond pulse, you collect all the diffraction data before the damage occurs.”
Paley and co-leader Aaron Brewster, a research scientist in MBIB, developed the algorithms needed to convert XFEL data into high-quality diffraction patterns that can be analyzed to reveal the unit cell – the basic unit of a crystal that is repeated over and over in three dimensions – of each tiny crystalline grain within the sample.
When you have a true powder, Paley explained, it’s like having a million crystals that are all jumbled together, full of imperfections, and scrambled in every possible orientation. Rather than diffracting the whole jumble together and getting a muddied readout of electron densities, like existing powder diffraction techniques, smSFX is so precise that it can diffract individual granules, one at a time. “This gives it a special sharpening effect,” he said. “So that is actually the kind of secret sauce of this whole method. Normally you shoot all million at once, but now you shoot 10,000 all in sequence.”
The cherry on top is that smSFX is performed without freezing the sample or exposing it to a vacuum – another benefit for the delicate materials studied by materials scientists. “No fancy vacuum chamber required,” said Sauter.
The team demonstrated proof-of-principle for smSFX, and reported the previously unknown structures of two metal-organic materials known as chacogenolates which have semiconducting and light-interaction properties that could make them ideal for next-generation transistors, photovoltaics (solar cells and panels), energy storage devices, and sensors.
With smSFX, the team was able to successfully diffract powder chacogenolates and examine the structures to learn why some of the silver-based materials glow bright blue under UV light.
After further improvements are made to streamline the smSFX process, the group imagines programs to offer this technique to other researchers. These types of programs are integral for increasing access to light source facilities, especially for smaller universities and colleges.
For more information:
U.S. Department of Energy National Laboratory, University of California – Lawrence Berkeley Laboratory
https://biosciences.lbl.gov/mbib/programs/
