Diamond materials as solar-powered electrodes

Spectroscopy shows what’s important!

It sounds like magic: photoelectrodes could convert the greenhouse gas CO₂ back into methanol or N2 molecules into valuable fertiliser – using only the energy of sunlight. An HZB study has now shown that diamond materials are in principle suitable for such photoelectrodes.

By combining X-ray spectroscopic techniques at BESSY II with other measurement methods, Tristan Petit’s team has succeeded for the first time in precisely tracking which processes are excited by light as well as the crucial role of the surface of the diamond materials.

At first glance, lab-grown diamond materials have little in common with their namesakes in the jewellery shop. They are often opaque, dark and look not spectacular at all. But even if their looks are unimpressive, they are promising in many different applications, for example in brain implants, quantum sensors and computers, as well as metal-free photoelectrode in photo-electrochemical energy conversion. They are fully sustainable and made of carbon only, they degrade little in time compared to metal-based photoelectrodes and they can be industrially produced!

Diamond materials are suitable as metal-free photoelectrodes because when excited by light, they can release electrons in water and trigger chemical reactions that are difficult to initiate otherwise. A concrete example is the reduction of CO2 to methanol which turns the greenhouse gas into a valuable fuel. It would also be exciting to use diamond materials to convert N2 into nitrogen fertiliser NH3, using much less energy than the Haber-Bosch process.

However, diamond electrodes oxidize in water and oxidized surfaces, it was assumed, no longer emit electrons into the water. In addition, the bandgap of diamond is in the UV range (at 5.5 eV), so visible light is unlikely to be sufficient to excite electrons. In spite of this expectation, previous studies have shown puzzling emission of electrons from visible light excitation. A new study by Dr. Tristan Petit’s group at HZB now brings new insights and gives cause for hope.

Dr Arsène Chemin, a postdoctoral researcher in Petit’s team, studied samples of diamond materials produced at the Fraunhofer Institute for Applied Solid State Physics in Freiburg. The samples were engineered to facilitate the CO2 reduction reaction: doped with boron to insure good electrical conductivity and nanostructured, which gives them huge surfaces to increase the emission of charge carriers such as electrons.

Chemin used four X-ray spectroscopic methods at BESSY II to characterize the surface of the sample and the energy needed to excite specific electronic surface states. Then, he used the surface photovoltage measured in a specialised laboratory at HZB to determine which ones of these states are excited and how the charge carriers are displaced in the samples. In complement, he measured the photoemission of electrons of samples either in air or in liquid. By combining these results, he was able for the first time to draw a comprehensive picture of the processes that take place on the surfaces of the sample after excitation by light.

Read more on HZB website

Image: Four diamond materials are shown here: “Diamond black” made of polycrystalline nanostructured carbon (top right), the same material before nanostructuring (top left), an intrinsic single crystal (bottom left) and a single crystal doped with boron (bottom right).

Credit: A. Chemin/HZB

25 years of BESSY II light source for the good of society

Helmholtz-Zentrum Berlin (HZB) is celebrating the 25 years of existence of BESSY II together with the international scientific community. More about the highlights from 25 years of research at BESSY II, the plans for the future, and the people who reliably operate the machine are to be found in the special anniversary magazine here.

When BESSY II in Berlin Adlershof went into operation in September 1998, it was a milestone for the reunified Berlin and in some ways a starting point for the success story of Adlershof. After only four years’ construction time, the successor to the first Berliner synchrotron radiation source that was previously in West Berlin (BESSY I) now stood in the eastern part of the city.

Today, BESSY II is a magnet for scientific exchange. Every year, the research facility receives more than 2700 visits from guest researchers from all over the world, who use the special X-ray light for their research. BESSY II has delivered results that have led to breakthroughs in many research fields. Helmholtz-Zentrum Berlin (HZB) is therefore celebrating the 25 years of existence of BESSY II together with the international scientific community. More about the highlights from 25 years of research at BESSY II, the plans for the future, and the people who reliably operate the machine are to be found in the special anniversary magazine here.

BESSY II is a material discovery machine

The most important experiments today are those for developing the materials we need for an environmentally friendly energy supply of the future.

Be it solar cells, catalysts for green hydrogen, batteries, or quantum materials – the special X-ray light (aka synchrotron light) from BESSY II can be used to look inside everything. HZB and its partners have expanded these experimental possibilities considerably in the recent years. In-situ and in-operando measurements allow researchers to “watch live” how a battery gets charged or discharged, for example, or how a catalyst works. That helps experts to further optimise the materials they are made of so that they work even more efficiently.

Plans for the future

25 years of BESSY II are incentive for Helmholtz-Zentrum Berlin to continue operating the light source at the highest level, and to allow societally important research to continue into the future. Accordingly, the work for a comprehensive upgrade to BESSY II+ has been underway in the recent months. Many components of the accelerator and several experimental stations (beamlines) are being renovated and modified in order to offer even more attractive research possibilities for science and industry. HZB experts have also developed a concept for a successor source in Berlin Adlershof, which will allow this important research to continue further still for decades to come. After all, a powerful light source that delivers soft X-ray light is essential for Germany as a science and technology location, and secures jobs in the long term.

Read more on HZB website

BESSY II: Surface analysis of catalyst particles in aqueous solutions

In a special issue on the liquid jet method, a team reports on reactions of water molecules on the surfaces of metal oxide particles. The results are relevant for the development of efficient photoelectrodes for the production of green hydrogen.

Green hydrogen can be produced directly in a photoelectrochemical cell, splitting water with solar energy. However, this requires the development of super-efficient photoelectrodes that need to combine many talents at the same time: They must be excellent at converting sunlight into electricity, remain stable in acidic or basic water, act as catalysts to promote the splitting of water into hydrogen and oxygen, and be cheap, abundant and non-toxic. The large material class of metal oxides comes into question. However, it is difficult to find out what really happens at the interfaces between the solid metal oxide electrodes and the aqueous electrolyte. This is because standard X-ray analysis does not work to investigate processes on samples in liquid environments. One of the few suitable methods are experiments with a liquid jet: an extremely fine jet of liquid in which nanoparticles of metal oxide are suspended. This jet shoots through the X-ray light of BESSY II, and the interference of the evaporated molecules with the measurement data is negligible (more in the foreword to the special issue).

Dr. Robert Seidel is an expert on this liquid jet method, which is the subject of a special issue of Accounts of Chemical Research. He was invited to be the guest-editor of the issue and to report also on new experiments at BESSY II that he conducted with Dr. Hebatallah Ali and Dr. Bernd Winter from the Fritz Haber Institute.

They investigated two important model systems for photoelectrodes: Nanoparticles of iron oxide (hematite, α-Fe2O3, and anatase (titanium oxide or TiO2) in aqueous electrolytes with different pH values. Hematite and anatase in suspension are photocatalytic model systems. They are ideal for studying the solid/electrolyte interface at the molecular level and for exploring the chemical reactions at electrode-electrolyte interfaces.

“We used resonant photoelectron spectroscopy (PES) to identify the characteristic fingerprints of different reactions. This allowed us to reconstruct which reaction products are formed under different conditions, particularly as a function of pH.” The key question: How do the water molecules react with or on the nanoparticle surfaces?

In fact, how acidic or how basic an electrolyte is makes a big difference, Seidel noted. “At low pH, the water molecules on the surface of hematite tend to split. This is not the case with anatase, where water molecules are adsorbed on the surface of the TiO2 nanoparticles,” says Seidel. A basic pH value is required for water molecules to break down on the anatase nanoparticles. “Such insights into surface interactions with water molecules are only possible with this liquid-jet method,” says Seidel.

The spectra also revealed ultra-fast electron transitions between the metal oxide and the (split) water molecules on the surface. The results provide insights into the first steps of water dissociation and help to clarify the mechanisms of light-induced water splitting on metal oxide surfaces.

Read more on this story here

Image: The microjet is a fast-flowing stream of liquid so narrow that it produces only an extremely dilute vapour cloud. Photons and particles can reach and leave the surface of the jet without colliding with the vapour molecules.

Quantitative analysis of cell organelles with artificial intelligence

The analysis of cryo-X-Ray-microscopy data still requires a lot of time. Scientists developed a convolutional neural network, which identifies structures at high accuracy within a few minutes.

BESSY II’s high-brilliance X-rays can be used to produce microscopic images with spatial resolution down to a few tens of nanometres. Whole cell volumes can be examined without the need for complex sample preparation as in electron microscopy. Under the X-ray microscope, the tiny cell organelles with their fine structures and boundary membranes appear clear and detailed, even in three dimensions. This makes cryo x-ray tomography ideal for studying changes in cell structures caused, for example, by external triggers. Until now, however, the evaluation of 3D tomograms has required largely manual and labour-intensive data analysis. To overcome this problem, teams led by computer scientist Prof. Dr. Frank Noé and cell biologist Prof. Dr. Helge Ewers (both from Freie Universität Berlin) have now collaborated with the X-ray microscopy department at HZB. The computer science team has developed a novel, self-learning algorithm. This AI-based analysis method is based on the automated detection of subcellular structures and accelerates the quantitative analysis of 3D X-ray data sets. The 3D images of the interior of biological samples were acquired at the U41 beamline at BESSY II.

“In this study, we have now shown how well the AI-based analysis of cell volumes works, using mammalian cells from cell cultures that have so-called filopodia,” says Dr Stephan Werner, an expert in X-ray microscopy at HZB. Mammalian cells have a complex structure with many different cell organelles, each of which has to fulfil different cellular functions. Filopodia are protrusions of the cell membrane and serve in particular for cell migration. “For cryo X-ray microscopy, the cell samples are first shock-frozen, so quickly that no ice crystals form inside the cell. This leaves the cells in an almost natural state and allows us to study the structural influence of external factors inside the cell,” Werner explains.

“Our work has already aroused considerable interest among experts,” says first author Michael Dyhr from Freie Universität Berlin. The neural network correctly recognises about 70% of the existing cell features within a very short time, thus enabling a very fast evaluation of the data set. “In the future, we could use this new analysis method to investigate how cells react to environmental influences such as nanoparticles, viruses or carcinogens much faster and more reliably than before,” says Dyhr.

Read more in the Proceedings of the National Academy of Sciences journal article

Image: The images show part of a frozen mammalian cell. On the left is a section from the 3D X-ray tomogram (scale: 2 μm). The right figure shows the reconstructed cell volume after applying the new AI-supported algorithm

Credit: HZB

What drives ions through polymer membranes

Ion exchange membranes are needed in (photo)electrolysers, fuel cells and batteries to separate ions and enable the desired processes. Polymeric membranes such as synthetically produced compounds like NAFION are particularly efficient, but they cannot be degraded. A ban on the use of these “eternal chemicals” is currently under discussion in the European Union, and the development of suitable alternatives will be a major challenge. So, it is crucial to understand why NAFION and other established polymeric membranes work so well.

A team led by Dr. Marco Favaro of the HZB Institute for Solar Fuels has now investigated this using a special type of electrolysis cell. Here, the membrane sits on the outer wall and is in contact with both the liquid electrolyte and a gaseous external environment. It can act either as an anode or a cathode, depending on the polarity of the applied potential. This hybrid liquid-gas electrolyzer is considered particularly favorable for the electrochemical conversion of CO2 thanks to the higher CO2 concentrations that can be achieved in the gas phase, thereby overcoming the poor solubility of CO2 in aqueous solutions.

For the study, Favaro and his team used commercially available ion-exchange membranes in contact with a model electrolyte like sodium chloride (NaCl) in water. Water vapor was fed to the gas phase, with the partial pressure of water close to its vapor pressure at room temperature. To analyze the migration of sodium and chloride ions through the membrane, they used in situ ambient pressure hard X-ray photoelectron spectroscopy (AP-HAXPES) at the SpAnTeX end-station at the KMC-1 beamline of BESSY II.

“Indeed, we were expecting that the ion dynamics was determined, under applied potentials, by the electric fields generated between the anode and cathode of the electrolyzer, and that electromigration was therefore the main driver,” says Marco Favaro.

However, analysis of the data showed otherwise: electromigration hardly plays a role; the ions simply diffuse across the membrane. The data could be perfectly simulated numerically with a diffusion model. “Our conclusion is that ions move through the polymer membranes in these types of electrolyzers due to hopping mediated by the ionized functional groups present in the membranes. In addition, since water diffuses as well through the polymer, the ions are “dragged” as well” explains Favaro.

These results are exciting for a number of reasons: These types of electrolyzers are a way to convert CO2 into valuable chemicals that can otherwise only be obtained from fossil fuels. Understanding how these devices work helps on the way to decarbonize the economy. On the other hand, the ion-exchange membranes that are a key component of these cells are themselves problematic: the European Union may soon ban the use of persistent chemicals. Understanding the relevant drivers of such transport processes will help to develop new membrane materials that are both efficient, durable, and environmentally friendly. Favaro now intends to take this project forward at HIPOLE, the new Helmholtz Institute in Jena, which will focus on polymer materials for new energy technologies.

Read more in the Journal of Materials Chemistry A

Image: Membrane

Credit: HZB

How much cadmium is contained in cocoa beans?

Cocoa beans can absorb toxic heavy metals such as cadmium from the soil. Some cultivation areas, especially in South America, are polluted with these heavy metals, in some cases considerably. In combining different X-ray fluorescence techniques, a team at BESSY II has now been able to non-invasively measure for the first time where cadmium accumulates exactly in cocoa beans: Mainly in the shell. Further investigations show that the processing of the cocoa beans can have a great influence on the concentration of heavy metals.

People have been harvesting the beans of the cocoa bush for at least 5000 years. They have learned to ferment, roast, grind and process the beans with sugar and fat to make delicious chocolates. Today, around five million tonnes of beans are on the market every year, coming from only a few growing areas in tropical regions.

Soul food chocolate

Chocolate is considered a soul food: amino acids such as tryptophan brighten the mood. Cocoa beans also contain anti-inflammatory compounds and valuable trace elements. However, cocoa plants also absorb toxic heavy metals if the soils are polluted, for example by mining, which can gradually poison groundwater and soils.

Where do the toxic elements accumulate?

An important question is,  where exactly the heavy metals accumulate in the bean, whether rather in the shell or rather in the endosperm inside the bean. From the harvest to the raw material for chocolate, the beans undergo many steps of different treatments, which could possibly reduce the contamination. And ideally the treatment could be optimised in order to make sure that the heavy metals are reduced but the desirable trace elements are retained.

Mapping the beans at BESSY II

A team led by Dr. Ioanna Mantouvalou (HZB) and Dr. Claudia Keil (TU Berlin/Toxicology) has now combined various imaging methods at the BAMline of BESSY II to precisely map the heavy metal concentrations in cocoa beans. They examined cocoa samples from a cultivation region in Colombia, which were contaminated with an average of 4.2 mg/kg cadmium. This is well above the European limits of 0.1-0.8 mg cadmium/kg in cocoa products.

Read more on the HZB website

Image: Cocoa beans are the main ingredients of chocolate, a famous “soul food”. However, cocoa plants also absorb toxic heavy metals if the soils are polluted. At BESSY II, a team has now mapped the local distribution of heavy metals inside the beans.

Credit: © AdobeStock

Superstore MXene: New proton hydration structure determined

MXenes are able to store large amounts of electrical energy like batteries and to charge and discharge rather quickly like a supercapacitor. They combine both talents and thus are a very interesting class of materials for energy storage. The material is structured like a kind of puff pastry, with the MXene layers separated by thin water films. A team at HZB has now investigated how protons migrate in the water films confined between the layers of the material and enable charge transport. Their results have been published in the renowned journal Nature Communications and may accelerate the optimisation of these kinds of energy storage materials.

One of the biggest challenges for a climate-neutral energy supply is the storage of electrical energy. Conventional batteries can hold large amounts of energy, but the charging and discharging processes take time. Supercapacitors, on the other hand, charge very quickly but are limited in the amount of stored energy. Only in the last few years has a new class of materials been discussed that combines the advantages of batteries with those of supercapacitors, named pseudocapacitors.

Promising materials: Pseudocapacitors

Among pseudocapacitive materials, so-called MXenes consisting of a large family of 2D transition metal carbides and nitrides appear particularly promising. Their structure resembles a puff pastry, with the individual layers separated by a thin film of water that enables the transport of charges. Titanium carbide MXenes, especially, are conductive and their layered structure combined with highly negatively-charged hydrophilic surfaces offers a unique material in which positively charged ions such as protons can diffuse very efficiently. The MXenes used in this study were synthesized in the group of Prof. Yury Gogotsi in Drexel University, USA.

Charge transport examined

Over the last years, this property has been used to store and release energy from protons at unprecedented rates in acidic environment. It remains though unclear if the charges are mostly stored based on proton adsorption at the MXene surface or through desolvation of proton in the MXene interlayer.

Confinement effect expected

Due to its two-dimensional geometry, the 2-3 layer thick water film trapped between the MXene layers is expected to solvate protons differently from bulk water that we classically know. While this confinement effect is supposed to play a role in the fast diffusion of protons inside MXene materials, it has been impossible until now to characterise protons inside a MXene electrode during charging and discharging.

Vibrational modes analysed

The team led by Dr. Tristan Petit at HZB has now succeeded in doing this for the first time by analysing vibrational modes of protons excited by infrared light. Postdoctoral researcher Dr Mailis Lounasvuori has developed an operando electrochemical cell that she used to analyse protons and water inside titanium carbide MXenes at BESSY II during the charging and discharging processes. In the process, she also succeeded in distilling out the special signature of the protons in the confined water between the MXene layers.

Read more on the HZB website

Image: The experiment: Infrared light excites protons in the water film, which move between the Ti3C2-MXene layers. Their oscillation patterns show that they behave differently than in a thicker film of water.

Credit: © M. Künsting /HZB

Electrocatalysis – Iron and Cobalt Oxyhydroxides examined

A team led by Dr. Prashanth W. Menezes (HZB/TU-Berlin) has now gained insights into the chemistry of one of the most active anode catalysts for green hydrogen production. They examined a series of Cobalt-Iron Oxyhydroxides at BESSY II and were able to determine the oxidation states of the active elements in different configurations as well as to unveil the geometrical structure of the active sites. Their results might contribute to the knowledge based design of new highly efficient and low cost catalytical active materials.

Very soon, we need to become fossil free, not only in the energy sector, but as well in industry. Hydrocarbons or other raw chemicals can be produced in principle using renewable energy and abundant molecules such as water and carbon dioxide with the help of electrocatalytically active materials. But at the moment, those catalyst materials either consist of expensive and rare materials or lack efficiency.

Key reaction in water splitting

A team led by Dr. Prashanth W. Menezes (HZB/TU-Berlin) has now gained insights into the chemistry of one of the most active catalysts for the anodic oxygen evolution reaction (OER), which is a key reaction to supply electrons for the hydrogen evolution reaction (HER) in water splitting. The hydrogen can then be processed into further chemical compounds, e.g., hydrocarbons. Additionally, in the direct electrocatalytic carbon dioxide reduction to alcohols or hydrocarbons, the OER also plays a central role.

Read more on the HZB website

Image: LiFex-1Cox Borophosphates have been used as inexpensive anodes for the production of green hydrogen. Their dynamic restructuring during OER as well as their catalytically active structure, have been elucidated via  X-ray absorption spectroscopy.

Credit: © P. Menezes / HZB /TU Berlin

New software based on Artificial Intelligence helps to interpret complex data

Experimental data is often not only highly dimensional, but also noisy and full of artefacts. This makes it difficult to interpret the data. Now a team at HZB has designed software that uses self-learning neural networks to compress the data in a smart way and reconstruct a low-noise version in the next step. This enables to recognise correlations that would otherwise not be discernible. The software has now been successfully used in photon diagnostics at the FLASH free electron laser at DESY. But it is suitable for very different applications in science.

More is not always better, but sometimes a problem. With highly complex data, which have many dimensions due to their numerous parameters, correlations are often no longer recognisable. Especially since experimentally obtained data are additionally disturbed and noisy due to influences that cannot be controlled.

Helping humans to interpret the data

Now, new software based on artificial intelligence methods can help: It is a special class of neural networks (NN) that experts call “disentangled variational autoencoder network (β-VAE)”. Put simply, the first NN takes care of compressing the data, while the second NN subsequently reconstructs the data. “In the process, the two NNs are trained so that the compressed form can be interpreted by humans,” explains Dr Gregor Hartmann. The physicist and data scientist supervises the Joint Lab on Artificial Intelligence Methods at HZB, which is run by HZB together with the University of Kassel.

Read more on the HZB website

Influence of protons on water molecules

How hydrogen ions or protons interact with their aqueous environment has great practical relevance, whether in fuel cell technology or in the life sciences. Now, a large international consortium at the X-ray source BESSY II has investigated this question experimentally in detail and discovered new phenomena. For example, the presence of a proton changes the electronic structure of the three innermost water molecules, but also has an effect via a long-range field on a hydrate shell of five other water molecules.

Excess protons in water are complex quantum objects with strong interactions with the dynamic hydrogen bond network of the liquid. These interactions are surprisingly difficult to study. Yet so-called proton hydration plays a central role in energy transport in hydrogen fuel cells and in signal transduction in transmembrane proteins. While the geometries and stoichiometries have been extensively studied both in experiments and in theory, the electronic structure of these specific hydrated proton complexes remains a mystery.

A large collaboration of groups from the Max Born Institute, the University of Hamburg, Stockholm University, Ben Gurion University and Uppsala University has now gained new insights into the electronic structure of hydrated proton complexes in solution.

Using the novel flatjet technology, they performed X-ray spectroscopic measurements at BESSY II and combined them with infrared spectral analysis and calculations. This allowed them to distinguish between two main effects: Local orbital interactions determine the covalent bond between the proton and neighbouring water molecules, while orbital energy shifts measure the strength of the proton’s extended electric field.

Read more on the HZB website

Image: The spectral fingerprints of water molecules could be studied at BESSY II. The result: the electronic structure of the three innermost water molecules in an H7O3+ complex is drastically changed by the proton. In addition, the first hydrate shell of five other water molecules around this inner complex also changes, which the proton perceives via its long-range electric field.

Credit: © MBI

Spintronics: A new tool at BESSY II for chirality investigations

Information on complex magnetic structures is crucial to understand and develop spintronic materials. Now, a new instrument named ALICE II is available at BESSY II. It allows magnetic X-ray scattering in reciprocal space using a new large area detector. A team at HZB and Technical University Munich has demonstrated the performance of ALICE II by analysing helical and conical magnetic states of an archetypal single crystal skyrmion host. ALICE II is now available for guest users at BESSY II.

The new instrument was conceived and constructed by HZB physicist Dr. Florin Radu and the technical design department at HZB in close cooperation with Prof. Christian Back from the Technical University Munich and his technical support. It is now available for guest users at BESSY II as well.

“ALICE II has an unique capability, namely to allow for magnetic X-ray scattering in reciprocal space using a new large area detector, and this at up to the highest allowed reflected angles”, Radu explains. To demonstrate the performance of the new instrument, the scientists examined a polished sample of Cu2OSeO3.

Read more on the HZB website

Image: The picture reflects the main effect measured with a newly developed instrument ALICE II at BESSY II: A circular polarised soft-X-ray beam scatters off a crystal that exhibits a helical or conical magnetic order. This leads to two scattered beams of different intensity. The difference in intensity of these scattered beams is a measure of the chirality of the equidistant magnetic helices.

Credit: © F. Radu/HZB

High entropy alloys: structural disorder and magnetic properties

High-entropy alloys (HEAs) are promising materials for catalysis and energy storage, and at the same time they are extremely hard, heat resistant and demonstrate great variability in their magnetic behaviour. Now, a team at BESSY II in collaboration with Ruhr University Bochum, BAM, Freie Universität Berlin and University of Latvia has gained new insights into the local environment of a so-called high-entropy Cantor alloy made of chromium, manganese, iron, cobalt and nickel, and has thus also been able to partially explain the magnetic properties of a nanocrystalline film of this alloy.

High entropy alloys or HEAs consist of five or more different metallic elements and are an extremely interesting class of materials with a great diversity of potential applications. Since their macroscopic properties are strongly dependent on interatomic interactions, it is utterly interesting to probe the local structure and structural disorder around each individual element by element-specific techniques. Now, a team has examined a so called Cantor alloy – a model system to study the high-entropy effects on the local and macroscopic scales.

Read more on the HZB website

Image: The Cantor alloy under study consists of chromium (grey), manganese (pink), iron (red), cobalt (blue), and nickel (green). X-ray methods allow to probe each individual component in an element-specific way.

Credit: © A. Kuzmin/University of Latvia and A. Smekhova/HZB

40 years of research with synchrotron light in Berlin

For decades, science in Berlin has been an important driver of innovation and progress. Creative, talented people from all over the world come together here and develop new ideas from which we all benefit as a society. Many discoveries – from fundamental insights to marketable products – are made by doing research with synchrotron light. Researchers have had access to this intense light in Berlin for 40 years. It inspires many scientific disciplines and is an advantage for Germany.

In September 1982, the first electron storage ring officially went into operation in Berlin-Wilmersdorf under the name BESSY (Berliner Elektronenspeicherring-Gesellschaft für Synchrotronstrahlung). In order to create this coveted synchrotron light, electrons are accelerated to near light speed in a circle. As they race around at this speed they emit special light, which scientists can use to look inside their samples. The successor facility in Berlin-Adlershof, BESSY II, is also based on this principle. It produced its first light beam in 1998 and is operated by Helmholtz-Zentrum Berlin (HZB). Presently, the facility receives around 2700 visits per year from guest researchers from everywhere in the world. It will be celebrating its 25th anniversary in September 2023.

Read more on the HZB website

Image: A view of the experimental hall at BESSY II

Credit: © S. Steinbach/HZB

Benedetta Casu’s #My1stLight

Synchrotron: Destiny

When I was a physics student, the Physics Department of my University in the capital city of Sardinia organized a journey to Berlin for the senior master students to visit the most important labs. Among them, there was BESSY I. What an incredible experience, everything looked so fantastic, exciting, and complicated.

After that, for sake of curiosity, I attended the Italian synchrotron School that at the time was organized in Sardinia. I attended the school because I wanted to know more about synchrotron light, but I was sure that it would stay a “cultural opportunity” and nothing more.

A few years later I was offered a Ph.D. position at the University of Potsdam. The plan was that I would have been in charge of photocurrent investigations. BUT, the Ph.D. student that was in charge of the beamtime at Synchrotron in the same research group was never back from his vacation preferring to stay in sunny Spain. My supervisor decided that I would take over the Synchrotron beamtimes.

My very first beamtime was with the last photon at BESSY I.

Since then, I had the opportunity to perform wonderful experiments using Synchrotron facilities all over Europe, from working with the world record laterally resolved PEEM-LEEM at BESSY II to measuring XMCD at 150 mK at Petra III. I am also one of the German national delegates of the European Synchrotron and FEL User Organisation (ESUO).

Synchrotron was certainly my destiny

Image: Benedetta Casu during beamtime at BESSY II

Credit: Benedetta Casu

Green hydrogen: Nanostructured nickel silicide shines as a catalyst

Electrical energy from wind or sun can be stored as chemical energy in hydrogen, an excellent fuel and energy carrier. The prerequisite for this, however, is efficient electrolysis of water with inexpensive catalysts. For the oxygen evolution reaction at the anode, nanostructured nickel silicide now promises a significant increase in efficiency. This was demonstrated by a group from the HZB, Technical University of Berlin and the Freie Universität Berlin as part of the CatLab research platform with measurements among others at BESSY II.

Electrolysis might be a familiar concept from chemistry lessons in school: Two electrodes are immersed in water and put under voltage. This voltage causes water molecules to break down into their components, and gas bubbles rise at the electrodes: Oxygen gas forms at the anode, while hydrogen bubbles form at the cathode. Electrolysis could produce hydrogen in a CO2-neutral way – as long as the required electricity is generated by fossil free energy forms such as sun or wind.

The only problem is that these reactions are not very efficient and extremely slow. To speed up the reactions, catalysts are used, based on precious and rare metals such as platinum, ruthenium or iridium. For large-scale use, however, such catalysts must consist of widely available and very cheap elements.

Read more on the HZB website

Image: Crystalline nickel silicide (left) is chemically transformed into nanostructured material with excellent catalytic properties for the electrolytic splitting of water and the production of valuable nitrile compounds. 

Credit: © P. Menezes /HZB/TU Berlin

Buckyballs on gold are less exotic than graphene

C60 molecules on a gold substrate appear more complex than their graphene counterparts, but have much more ordinary electronic properties. This is now shown by measurements with ARPES at BESSY II and detailed calculations.

Graphene consists of carbon atoms that crosslink in a plane to form a flat honeycomb structure. In addition to surprisingly high mechanical stability, the material has exciting electronic properties: The electrons behave like massless particles, which can be clearly demonstrated in spectrometric experiments. Measurements reveal a linear dependence of energy on momentum, namely the so-called Dirac cones – two lines that cross without a band gap – i.e. an energy difference between electrons in the conduction band and those in the valence bands.

Variants in graphene architecture

Artificial variants of graphene architecture are a hot topic in materials research right now. Instead of carbon atoms, quantum dots of silicon have been placed, ultracold atoms have been trapped in the honeycomb lattice with strong laser fields, or carbon monoxide molecules have been pushed into place on a copper surface piece by piece with a scanning tunneling microscope, where they could impart the characteristic graphene properties to the electrons of the copper. 

Artificial graphene with buckyballs?

A recent study suggested that it is infinitely easier to make artificial graphene using C60 molecules called buckyballs. Only a uniform layer of these needs to be vapor-deposited onto gold for the gold electrons to take on the special graphene properties. Measurements of photoemission spectra appeared to show a kind of Dirac cone.

Analysis of band structures at BESSY II

“That would be really quite amazing,” says Dr. Andrei Varykhalov, of HZB, who heads a photoemission and scanning tunneling microscopy group. “Because the C60 molecule is absolutely nonpolar, it was hard for us to imagine how such molecules would exert a strong influence on the electrons in the gold.” So Varykhalov and his team launched a series of measurements to test this hypothesis.

In tricky and detailed analyses, the Berlin team was able to study C60 layers on gold over a much larger energy range and for different measurement parameters. They used angle-resolved ARPES spectroscopy at BESSY II, which enables particularly precise measurements, and also analysed electron spin for some measurements.

Read more on the HZB website

Image: Using density functional theory and measurement data from spin-resolved photoemission, the team investigated the origin of the repeating Au(111) bands and resolved them as deep surface resonances. These resonances lead to an onion-like Fermi surface of Au(111).

Credit: © HZB