This article has been adapted from this Berkeley Lab press release.
Francis Crick, who famously co-discovered the shape of DNA, once said: “If you want to understand function, study structure.” Many decades later, this remains a tenet of biology, chemistry, and materials science.
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.
Yet the process is not easy. As the name implies, it requires crystals – specifically, purified samples of the molecule of interest, coaxed into a crystal form. And not all molecules form picture-ready crystals.
A team of Foundry users, co-led by former Foundry staff scientist Nathan Hohman, are working to provide 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 combating climate change and producing renewable energy. Their work was recently published in Nature.
Their 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.
When you have a true powder, 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 (which is what happens with existing powder diffraction techniques), smSFX is so precise that it can diffract individual granules, one at a time.
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.
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In the new study, the team demonstrated proof-of-principle for smSFX, then went one step further. They reported the previously unknown structures of two metal-organic materials known as chacogenolates. Hohman, who is now a chemist physicist at University of Connecticut, studies chacogenolates for their semiconducting and light-interaction properties, which could make them ideal for next-generation transistors, photovoltaics (solar cells and panels), energy storage devices, and sensors.
“Every single one of these is a special snowflake – growing them is really difficult,” said Hohman. With smSFX, he and graduate student Elyse Schriber were 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, a phenomenon that the scientists affectionately compare to Frodo’s sword in The Lord of the Rings.
“There is a huge array of fascinating physical and even chemical dynamics that occur at ultrafast timescales, and our experiment could help to connect the dots between a material’s structure and its function,” said Schriber. “After further improvements are made to streamline the smSFX process, we can imagine 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.”
Read the full press release.