Future of electron diffraction: why detectors make the difference

Seven questions about the future of electron diffraction for Sacha De Carlo, Business Development Manager EM, DECTRIS Ltd. 

ELDICO: When did you understand that electron diffraction will probably be the next big thing in crystallography and what was your reaction?

Sacha De Carlo: DECTRIS is an established and trusted supplier of hybrid-photon-counting (HPC) detectors for all X-ray applications, and more than 60% of all biomolecular structures deposited in the Protein Data Bank (PDB) are issued from data recorded on our PILATUS and EIGER detectors installed at synchrotron beamlines all around the world.

It was a long time ago that Dr. Clemens Schulze-Briese, DECTRIS Chief Scientific Officer, began to consider the potential of the hybrid pixel technology for electron-counting detectors, but back then the company was focused mostly on X-ray applications. In 2016, I was hired to lead DECTRIS into electron microscopy, and the first logical step was to immediately test our then-current detector with electrons. Since my background is in life sciences, particularly single-particle cryoEM, I immediately contacted Greg McMullan and Richard Henderson of the LMB in Cambridge (UK), whom I knew from my prior employment with FEI, now Thermo Fisher Scientific.

We put an EIGER X 1M in a vacuum chamber and installed the detector on an old Philips CM-200 FEG electron microscope. The initial tests proved that our EIGER detector had a nearly-ideal MTF and DQE for electrons below 200 kV. This assessment at various electron energies (40-200 kV) convinced us that we were on the right track, but the EIGER pixel size (75 µm) and the relatively small field of view (compared to the 16M CMOS cameras currently available on the market) led us to think we were not ready for the life sciences markets just yet, so we turned to applications that could benefit from a smaller detector with amazing dynamic range and speed. When it turned out that the standard detectors used in electron crystallography were far from ideal, we immediately launched the development of a smaller detector, based on the new chip that DECTRIS was working on in 2016 – now known as the QUADRO detector, powered by the DECTRIS EIGER2 ASIC.

Dr. Tim Grüne was a group leader in electron diffraction at PSI, and we immediately began experimenting with the same test set-up we had used in Cambridge while the QUADRO was under development. As part of a NanoArgovia grant awarded to Tim and his collaborators from the Swiss NanoScience Institute in Basel, an EIGER X 1M detector was used – first in Basel (at C-CINA, BioZetrum) and subsequently at ScopeM (ETH Zürich) – to collect data on various micron-sized 3D crystals (MicroED). This led to a number of impactful publications, as highlighted above in the introduction.Where do you see the biggest potential for ED? What are the killer applications? Which industries will benefit first and most?

We are confident that ELDICO Scientific’s dedicated electron diffractometer will add value to single crystal electron crystallography in structural chemistry and pharma, as an innovative high-throughput and cost-effective strategy alternative for (or even replacing?) standard powder diffraction techniques using X-rays.Why is hybrid pixel technology optimally suited to perform ED applications and what are main two or three advantages over typical CMOS detectors?

Hybrid pixel technology (HPT) offers several advantages over other detector technologies present in electron microscopy and diffraction – namely dynamic range, speed and radiation hardness. Monolithic active pixel sensors (MAPS) have revolutionized cryo-electron microscopy in the past few years: when used with very low electron fluxes (biological macromolecules are beam-sensitive objects, thus “low-dose” exposures are required), they are able to count single electrons. This dramatically improves the detection efficiency, ultimately increasing the signal over the detector noise.

That said, the MAPS detectors currently on the market are not suitable for electron diffraction, as they saturate too quickly when higher electron fluxes or the direct focused beam are used. In other words, their dynamic range is rather poor. HPT detectors offer a much higher dynamic range (up to 2×16 bits). Our latest EIGER2 technology, for instance, offers count-rates per pixel that are four orders of magnitude better; with two 16-bit counters per pixel, the dynamic range is much higher, making the DECTRIS HPC detectors suitable for any diffraction-based study.

The thick sensor makes hybrid pixel technology impervious to the direct electron beam, rendering these detectors more radiation hard than MAPS detectors, where a direct diffraction beam is often blocked by means of hardware or software to prevent damage to the detector. Thanks to the direct detection of electrons, the MTF of the HPC detector is largely superior to that of indirect CMOS detectors, again boosting resolution and signal-to-noise.DECTRIS’ background is in X-ray detectors; now, electrons are much more energetic than X-ray photons. What has been done to protect the chip and electronics?

The right combination of sensor material and thickness that DECTRIS is able to offer can be tuned to the proper energy range. We have characterized various materials, and we can offer two levels of protection. The first is the sensor thickness, which ensures that all electrons are fully stopped (detected) within the given sensor layer, thus protecting the ASIC chip underneath. A second and very important difference with respect to other chips used in the industry is the fact that we use enclosed layout technology (ELT) as the standard for the design and fabrication of all our new chips.High read-out speeds are important, both in imaging and in diffraction mode. How could DECTRIS provide these extremely high frame rates?

DECTRIS designs the chips but also all the electronic boards, firmware and software needed for the optimized readout speed. When all the components have been designed and verified, everything works in perfect harmony, and we can thus guarantee the stability and robustness of what is expected from a great product made to exceed the highest industry standards.Can you tell us more about DECTRIS’ strategy to enter the market? What role do scientific-industrial collaborations play?

Collaborations with key opinion leaders are extremely useful, especially when it comes to penetrating a new market – such as electron diffraction for DECTRIS. We benefit from early adopters trying our technology and helping convince the major OEMs to acquire the technology. We are definitely focusing on B2B at the moment, which is the only way to go mainstream.

That said, scientific-industrial collaborations are essential, and DECTRIS has already engaged in two NanoArgovia collaborative projects, as well as one InnoSuisse project in collaboration with EMPA Dübendorf, which received an award last summer for the development of workflow solutions for fast 4D-STEM data acquisition and suitable real-time data analysis; this is what we will need to be able to cope with the deluge of data that the new QUADRO detector will generate in no time.

What are the top three scientific publications that will help pave the way for a broader application of ED and DECTRIS detectors?

The publications cited above, in collaboration with PSI and Crystallise! AG from PARK INNOVAARE, have really made a great impact – they have certainly helped us attract the attention of the structural chemistry community. At the same time, we would welcome a few new publications in academia and the pharma industry as proof-of-concept to help us push ED for small molecule-based drug design.

More Scientific Content from ELDICO:

What are Electron Diffraction and Nanocrystallography and why are they important? (White Paper) — Rapid Structure Determination of Microcrystalline Molecular Compounds Using Electron Diffraction (Peer-Reviewed Paper) — Can Electron Diffraction distinguish between carbon and nitrogen atoms? (Application Note).