New lens system for clearer and sharper diffraction images

To design and improve energy storage materials, smart appliances and many other technologies, researchers must understand their hidden structure and chemistry. Advanced research techniques, such as ultra-fast electron diffraction imaging, can reveal this information. At present, a group of researchers from the Brookhaven National Laboratory of the Department of Energy (DOE) has developed a new, improved version of electronic diffraction at the Brookhaven Accelerator Test Center (ATF), a DOE's Center of Expertise user resource center offering unique and innovative experimental features. instrumentation allowing researchers from around the world to study particle acceleration. The researchers published their results in Scientific reports, an open access journal of Nature Research.

The development of a research technique such as ultra-fast electron diffraction will help future generations of scientists in materials to study materials and chemical reactions with new accuracy. Many interesting changes in materials occur extremely quickly and in small spaces. It is therefore necessary to improve research techniques to study them for future applications. This new, improved version of electronic diffraction is a springboard for improving various electron beam research techniques and existing instrumentation.

"We implemented our new focusing system for electron beams and demonstrated that we could significantly improve the resolution compared to the conventional solenoid technique," said Xi Yang, author of the study and physicist of the accelerators of the synchrotron light source II (NSLS). II), a DOE User Facility Office of Science from the Brookhaven Laboratory. "The resolution depends primarily on the properties of light – or in our case – the electron beam.This is universal for all imaging techniques, including light microscopy and ray imaging. X. However, it is much harder to focus the charged electrons on an almost parallel pencil-like beam on the sample compared to the light because the electrons are negatively charged and repel each other. what we call the space charge effect Using our new configuration, we were able to overcome the space charge effect and obtain diffraction data three times brighter and twice as accurate it is a decisive step forward in the resolution. "

Each electron diffraction pattern uses an electron beam that is focused on the sample so that the electrons bounce off the atoms of the sample and then go further to the detector located behind the electron beam. sample. The electrons create what is called a diffraction pattern, which can be translated into the structural constitution of materials at the nanoscale. The advantage of using electrons to image this internal structure of materials is that the so-called electron diffraction limit is very low, which means that scientists can solve smaller details in the structure by compared to other diffraction methods.

A diverse team of researchers was needed to improve such a complex research method. The Brookhaven laboratory team was composed of NSLS-II electron beam experts, ATF electron accelerator experts, and a number of scientists. materials science experts from the Department of Condensed Matter Physics and Materials (CMPMS).

"This breakthrough would not have been possible without the combination of all our skills acquired in the Brookhaven laboratory." At NSLS-II, we have expertise in the management of the electron beam. the expertise and capabilities of electron gun and laser technologies The CMPMS group has the required sample skills and, of course, defines the application requirements. unique synergy that allowed us to show how the resolution of the technique can be significantly improved, "said Li Hua Yu, senior physicist of NSLS-II accelerators and co-author of the study.

To achieve its improved resolution, the team has developed a different method of focusing the electron beam. Instead of using a conventional approach involving solenoid magnets, the researchers used two groups of four quadrupole magnets to tune the electron beam. Compared to solenoid magnets, which act as a single lens to shape the beam, quadrupole magnets function as a specialized lens system for electrons, and they offer scientists much more flexibility in tuning and shaping the beam according to the needs of the beam. their experience. .

"Our lens system can provide a wide range of beam adjustment possibilities, we can optimize the most important parameters such as beam size, charge density and beam divergence according to the experimental conditions, and thus provide the best beam quality for scientific applications. "needs," said Yang.

The team can even adjust settings on the fly with online optimization tools and correct irregularities in beam shape. However, to make this possible, the team needed the excellent electron beam provided by ATF. ATF has an electron gun that generates an extremely bright and ultra-short electron beam, which provides the best conditions for electron diffraction.

"The team used a photocathode gun that generates the electrons in a process called photoemission," said Mikhail Fedurin, accelerator physicist at ATF. "We emit an ultrashort laser pulse into a copper cathode.When the impulse strikes the cathode, an electron cloud forms on the copper.We remove the electrons with the help of. an electric field, then accelerate them.The amount of electrons in one of these pulses and our ability to accelerate them to achieve specific energies make our system attractive for material science research, especially for ultra-fast electronic diffraction. "

The focusing system associated with the ATF electron beam being very sensitive, researchers can measure the influences of the Earth's magnetic field on the electron beam.

"In general, the electrons are still influenced by the magnetic fields, so we are first guiding them in the particle accelerators, but the effect of the Earth's magnetic field is not insignificant for the low energy beam we used in this experiment, "said Victor Smalyuk, head of the NSLS-II Accelerator Physics Group and co-author of the study. "The beam deviating from the desired path, which created difficulties during the initial startup phase, so we had to correct this effect."

In addition to the high brightness of the electron beam and the high accuracy of the focusing system, the team also needed the right sample to perform these measurements. The CMPMS group provided the team with a polycrystalline gold film allowing it to fully explore the new lens system and put it to the test.

"We made the sample by depositing the gold atoms on a carbon film several nanometers thick using a technique called thermal evaporation," said Junjie Li, a physicist at the CMPMS department. "We have evaporated the gold particles so that they condense on the carbon film and form tiny isolated nanoparticles that slowly melt to form the polycrystalline film."

This film was essential for measurements because it presents randomly oriented crystals that merge. Therefore, the internal structure of the sample is not uniform, but consists of many differently oriented areas, which means that the diffraction pattern mainly depends on the qualities of the electron beam. This gives scientists the best ground for truly testing their lens system, adjusting the beam and seeing the impact of their adjustment directly on the quality of the diffraction measurement.

"We started by improving the electronic diffraction for the scientific studies on the materials, but we also found that this technique could help us to characterize our beam of electrons.In fact, the diffraction is very sensitive to the parameters of the beam We can therefore use the diffraction pattern of a known sample to measure our beam parameters accurately and directly, which is usually not so easy, "Yang said.

The team plans to continue the improvements and is already considering developing a new configuration for ultrafast electron microscopy to directly visualize a biological sample.

"We hope to be able to perform ultra-fast one-shot electron beam imaging and perhaps even create molecular films, which is not possible with our current beam imaging setup." of electrons, "said Yang.