New Lenses for Sharper Diffraction Images

To design and improve energy storage materials, smart devices, and many more technologies, researchers need to under­stand their hidden structure and chemistry. Advanced research techniques, such as ultra-fast electron diffraction imaging can reveal that information. Now, a group of researchers from the U.S. Department of Energy’s (DOE) Brook­haven National Labora­tory have developed a new and improved version of electron diffrac­tion at Brookhaven’s Acce­lerator Test Facility (ATF).

This electron diffraction measurements shows diffraction patterns of the sample using the newly developed quadrupoles. The rings of the pattern are sharper, rounder and turn red, which means that the overall resolution of the measurement is higher. (Source: BNL)

Advancing a research technique such as ultra-fast electron diffrac­tion will help future generations of materials scientists to investigate materials and chemical reactions with new precision. Many interes­ting changes in materials happen extremely quickly and in small spaces, so improved research techniques are necessary to study them for future appli­cations. This new and improved version of electron dif­fraction offers a stepping stone for improving various electron beam-related research techniques and existing instru­mentation. “We implemented our new focusing system for electron beams and demonstrated that we can improve the resolution significantly when compared to the conventional solenoid technique,” said Xi Yang, acce­lerator physicist at the National Synchrotron Light Source II (NSLS-II) at Brookhaven Lab. “The resolution mainly depends on the properties of light – or in our case – of the electron beam. This is universal for all imaging techniques, including light micro­scopy and X-ray imaging. However, it is much more chal­lenging to focus the charged electrons to a near-parallel pencil-like beam at the sample than it would be with light, because electrons are nega­tively charged and therefore repulse one another. This is called the space charge effect. By using our new setup, we were able to overcome the space charge effect and obtain dif­fraction data that is three times brighter and two times sharper; it’s a major leap in reso­lution.”

Every electron diffraction setup uses an electron beam that is focused on the sample so that the electrons bounce off the atoms in the sample and travel further to the detector behind the sample. The electrons create a diffraction pattern, which can be trans­lated into the structural makeup of the materials at the nanoscale. The advantage of using electrons to image this inner structure of materials is that the so called diffraction limit of electrons is very low, which means scientists can resolve smaller details in the structure compared to other dif­fraction methods. A diverse team of researchers was needed to improve such a complex research method. The Brook­haven Lab team consisted of electron beam experts from the NSLS-II, electron acce­lerator experts from ATF, and materials science experts from the condensed matter physics & materials science (CMPMS) department.

“This advance would not have been possible without the combi­nation of all our expertise across Brook­haven Lab. At NSLS-II, we have expertise on how to handle the electron beam. The ATF group brought the expertise and capa­bilities of the electron gun and laser technologies – both of which were needed to create the electron beam in the first place. And the CMPMS group has the sample expertise and, of course, drives the appli­cation needs. This is a unique synergy and, together, we were able to show how the reso­lution of the technique can be improved dras­tically,” said Li Hua Yu, NSLS-II senior acce­lerator physicist.

To achieve its improved resolution, the team developed a different method of focusing the electron beam. Instead of using a conventional approach that involves solenoid magnets, the researchers used two groups of four quadru­pole magnets to tune the electron beam. Compared to solenoid magnets, which act as just one lens to shape the beam, the quadru­pole magnets work like a specialized lens system for the electrons, and they gave the scientists far more flexi­bility to tune and shape the beam according to the needs of their experiment. “Our lens system can provide a wide range of tuna­bility of the beam. We can optimize the most important para­meters such as beam size, or charge density, and beam diver­gence based on the experimental conditions, and therefore provide the best beam quality for the scientific needs,” said Yang.

The team can even adjust the parameters on-the-fly with online optimization tools and correct any nonuniformities of the beam shape; however, to make this measure­ment possible, the team needed the excellent electron beam that ATF provides. ATF has an electron gun that generates an extremely bright and ultra­short electron beam, which offers the best conditions for electron dif­fraction. “The team used a photo­cathode gun that generates the electrons through a process called photoemission,” said Mikhail Fedurin, an acce­lerator physicist at ATF. “We shoot an ultrashort laser pulse into a copper cathode, and when the pulse hits the cathode a cloud of electrons forms over the copper. We pull the electrons away using an electric field and then acce­lerate them. The amount of electrons in one of these pulses and our capability to acce­lerate them to specific energies make our system attractive for material science research – particularly for ultrafast electron dif­fraction.”

The focusing system together with the ATF electron beam is very sensitive, so the researchers can measure the in­fluences of Earth’ magnetic field on the electron beam. “In general, electrons are always in­fluenced by magnetic fields – this is how we steer them in particle acce­lerators in the first place; however, the effect of Earth’s magnetic field is not negligible for the low-energy beam we used in this experiment,” said Victor Smalyuk, NSLS-II accelerator physics group leader. “The beam deviated from the desired tra­jectory, which created diffi­culties during the initial starting phase, so we had to correct for this effect.”

Beyond the high brightness of the electron beam and the high precision of the focusing system, the team also needed the right sample to make these measure­ments. The CMPMS group provided the team with a poly­crystalline gold film to fully explore the newly designed lens system and to put it to the test. “We made the sample by depositing the gold atoms on a several nano­meter thick carbon film using a technique called thermal evaporation,” said Junjie Li, a physicist in the CMPMS department. “We eva­porated gold particles so that they condense on the carbon film and form tiny, isolated nanoparticles that slowly merge together and form the poly­crystalline film.”

This film was essential for the measure­ments because it has randomly oriented crystals that merge together. Therefore, the inner structure of the sample is not uniform, but consists of many dif­ferently oriented areas, which means that the dif­fraction pattern mainly depends on the electron beam qualities. This gives the scientists the best ground to really test their lens system, to tune the beam, and to see the impact of their tuning directly in the quality of the dif­fraction measurement.

“We initially set out to improve electron dif­fraction for scientific studies of materials, but we also found that this technique can help us charac­terize our electron beam. In fact, diffraction is very sensitive to the electron beam parameters, so we can use the dif­fraction pattern of a known sample to measure our beam para­meters precisely and directly, which is usually not that easy,” said Yang. The team intends to pursue further improve­ments, and they already have plans to develop another setup for ultra-fast electron micro­scopy to directly visualize a bio­logical sample. “We hope to achieve ultra­fast single-shot electron beam imaging at some point and maybe even make molecular movies, which isn’t possible with our current electron beam imaging setup,” said Yang. (Source: BNL)

Reference: X. Yang et al.: A compact tunable quadrupole lens for brighter and sharper ultra-fast electron diffraction imaging, Sci. Rep. 9, 5115 (2019); DOI: 10.1038/s41598-019-39208-z

Link: Condensed Matter Physics & Materials Science, Brookhaven National Laboratory, Upton, USA

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