SCIENTIFIC HIGHLIGHTS

Ab initio modeling of novel photocathode materials for high brightness electron beams

 

The development of laser-driven photocathode radio-frequency electron injectors has become a significant enabling technology for free electron lasers and for the fourth generation of light sources. Such remarkable progress come with quest for novel materials that are able to operate in the visible region with optimized quantum efficiency and minimized intrinsic emittance. Multi-alkali antimonides have recently emerged as ideal materials for photocathode applications in spite of the little fundamental knowledge regarding their electronic and optical properties. A team composed of scientists from the HU Berlin and HZB carried out a systematic investigation of the electronic structure and excitations of CsK2Sb, an exemplary and promising multi-alkali antimonide, by means of first-principles many-body methods. The results of their study confirm that this material is an excellent candidate for photocathode applications and pioneers a new research line bridging solid-state theory, material science, and accelerator physics in view of an improved modelling and design of materials for the next-generation electron sources.

This work was published on The Journal of Physics: Condensed Matter (http://iopscience.iop.org/article/10.1088/1361-648X/aaedee) as an invited contribution to Prof. Caterina Cocchi, a member of IRIS Adlershof since 2017, to the special issue “Emerging leaders 2018” (http://iopscience.iop.org/journal/0953-8984/page/Emerging-leaders-2018).

First-principles many-body study of the electronic and optical properties of CsK2Sb, a semiconducting material for ultra-bright electron sources
C. Cocchi, S. Mistry, M. Schmeißer, J. Kühn and Thorsten Kamps
Journal of Physics: Condensed Matter (2019), 31, 1


 

Exploring the “Goldilocks Zone” of Semiconducting Polymer Photocatalysts via Donor-Acceptor Interactions

 
A team of researchers from Germany and Chechia has developed a polymer catalyst that can split hydrogen from water using sun light.
Hydrogen is regarded as the energy source of the future because its combustion e.g. as a car propellant proceeds cleanly to water without the generation of greenhouse gases like carbon dioxide.
The novel design principle of these polymer catalysts is not only that they consist of abundant elements like carbon, nitrogen and sulphur. Notably, the researchers realised that the electron interactions between the electron-donor sulphur and the electron acceptor nitrogen can be used for particularly efficient charge separation in photo catalysis. This leads to materials that achieve – without the need for further chemical or physical modifications – the highest hitherto reported hydrogen evolution rate of 3158 mmol h-1 g-1. The lead-author of this work, Dr. Michael J. Bojdys, is a junior member of the IRIS Adlershof since 2018.

Exploring the “Goldilocks Zone” of Semiconducting Polymer Photocatalysts by Donor–Acceptor Interactions
Y. S. Kochergin, D. Schwarz, A. Acharjya, A. Ichangi, R. Kulkarni, P. Eliášová, J. Vacek, J. Schmidt, A. Thomas and M. J. Bojdys
Angew.Chem Int. Ed. (2018), 57, 14188


 

Light-controlled molecules: Scientists develop new recycling strategy

Discovery lays the foundation for recycling of yet non-recyclable plastics

A light-controlled molecule in combination with a specific light sequence allows for bond formation (UV and red light; 1. to 4.) or scission (UV and blue light; 4. to 1.) with molecular building-blocks. Visualization: Michael Kathan.
 

Robust plastics are composed of molecular building-blocks, held together by tough chemical linkages. Their cleavage is extremely difficult to achieve, rendering the recycling of these materials almost impossible. A research team from the Humboldt-Universität zu Berlin (HU) developed a molecule, which can drive or reverse specific chemical reactions with light of different colors. This enables making and breaking of connections on the molecular scale, even if they are exceptionally strong. The discovery paves the way for the development of novel recycling methods and sustainable materials. Light-driven recovery of individual molecular building-blocks has great potential to enable recycling of yet non-recyclable plastics without compromising on color, quality, or shape.

“The working principle of our system is quite similar to the one of ready-to-assemble furniture” explain Michael Kathan and Fabian Eisenreich, the two first authors of this study. “We are able to repetitively assemble or disassemble molecular architectures, but instead of a hammer and screw-driver, we use red and blue LEDs as tools to control our molecules.”

The results of their study have just been published in Nature Chemistry.
 

Light-driven molecular trap enables bidirectional manipulation of dynamic covalent systems
M. Kathan, F. Eisenreich, C. Jurissek, A. Dallmann, J. Gurke, S. Hecht
Nature Chemistry (2018)


 

Flipping the switch on supramolecular electronics

For the first time, two-dimensional materials have been decorated with a photoswitchable molecular layer, and electronic components have been fabricated from the resulting hybrid materials that can be controlled by light. The results of this fruitful collaboration of several European research groups have been published in Nature Communications.

Owing to their outstanding electrical, optical, chemical and thermal properties, two-dimensional (2D) materials, which consist of a single layer of atoms, hold great potential for technological applications such as electronic devices, sensors, catalysts, energy conversion and storage devices, among others. Thanks to their ultra-high surface sensitivity, 2D materials represent an ideal platform to study the interplay between nanoscale molecular assembly on surfaces and macroscopic electrical transport in devices.

In order to provide a unique light-responsivity to devices, the researchers have designed and synthesized a photoswitchable spiropyran building block, which is equipped with an anchoring group and which can be reversibly interconverted between two different forms by illumination with ultraviolet and visible light, respectively. On the surface of 2D materials, such as graphene or molybdenum disulfide (MoS2), the molecular photoswitches self-assemble into highly ordered ultrathin layers, thereby generating a hybrid, atomically precise superlattice. Upon illumination the system undergoes a collective structural rearrangement, which could be directly visualized and monitored with sub-nanometer resolution by scanning tunneling microscopy. This light-induced reorganization at the molecular level induces an optical modulation of the energetics of the underlying 2D material, which translates into a change in the electrical characteristics of the fabricated hybrid devices. In this regard, the collective nature of self-assembly allows to convert single-molecule events into a spatially homogeneous switching action, which generates a macroscopic electrical response in graphene and MoS2.
"With our versatile approach of molecularly tailoring 2D materials, we are taking supramolecular electronics to a new level and closer to future applications," says Prof. Stefan Hecht, who is researching hybrid materials at IRIS Adlershof. The work is groundbreaking for the realization of multifunctional hybrid components powered by nature's primary energy source - sunlight.

Collective molecular switching in hybrid superlattices for light-modulated two-dimensional electronics
M. Gobbi, S. Bonacchi, J.X. Lian, A. Vercouter, S. Bertolazzi, B. Zyska, M. Timpel, R. Tatti, Y. Olivier, S. Hecht, M.V. Nardi, D. Beljonne, E. Orgiu, and P. Samorì
Nature Communications 9 (2018) 2661

 

Light-controlled production of biodegradable polymers

A research team from Berlin has developed a novel catalyst system, which enables the regulation of multiple polymerization processes to produce biodegradable plastics solely by illumination with light of different colors. The results of this work have now been published in Nature Catalysis.
 

The properties of a polymeric material are highly dependent on factors, such as the connected monomer building blocks as well as the length and composition of the formed polymer chains. Typically, these factors are predetermined by the choice of the employed reaction conditions. In order to overcome this limitation and generate materials with new and unprecedented properties, regulation of polymerizations by means of external stimuli represents an attractive goal. Similarly to dental repair, light serves to precisely control the location and duration of the chemical reaction during polymer formation.

A new method for the light-regulated production of biodegradable polymers has now been developed by chemists of the Humboldt-Universität zu Berlin, the Federal Institute for Materials Research and Testing Berlin, and the Heinrich-Heine-Universität Düsseldorf. Their work is based on the design of a unique catalyst, which is capable to change its activity reversibly by illumination with light of different wavelength. Using their catalyst, the scientists were able to turn the formation of polylactide on and off on demand, which allowed them to control the chain length of the produced polymer strands. Moreover and for the first time, they were able to regulate the incorporation of two different monomers into the same polymeric backbone with light.

Fabian Eisenreich and Michael Kathan, the first authors of the study, are excited: “With our remote-controlled catalyst we are in principle able to program the formation of a desired polymer strand by employing a specific order and duration of light pulses.” Their promising development is an important step toward smart production processes of (biodegradable) polymers with the aim to meet the growing demands of future applications, including light-guided 3D printing and photolithography.

A photoswitchable catalyst system for remote-controlled (co)polymerization in situ
F. Eisenreich, M. Kathan, A. Dallmann, S.P. Ihrig, T. Schwaar, B.M. Schmidt, and S. Hecht
Nature Catalysis (2018), published online
DOI: 10.1038/s41929-018-0091-8


 

Chain reaction switches molecules in depth

A new method developed by a team of chemists in Berlin open the door for using optically switchable molecules. The results of the study have been published in Chem.
 

Smart materials become increasingly common in our daily life as they adapt their properties to their surroundings, such as temperature and light. Think about light-adaptive lenses in sunglasses that change their color in response to brightness or darkness. In these materials, photoswitchable molecules able to change their properties, such as color or the ability to conduct electricity, upon illumination serve as key components. However, photoswitches typically require the use of high-energy UV light and in addition do neither switch quantitatively nor efficiently since many more quanta than molecules are needed. These drawbacks limit the applicability of photoswitches, in particular since the more energy-rich light is, the less it can penetrate into materials.

Now, chemists of Berlin’s Humboldt University and the University of Potsdam have developed a method, which allows one to efficiently and quantitatively operate photoswitches with the smallest amounts of low-energy red photons, thus solving both issues described above. By coincidence they came across the phenomenon that the oxidation of only a few switch molecules was sufficient to switch the entire sample. Subsequently, they investigated the underlying chain reaction in great detail and optimized it by introducing dyes to allow for the use of red light. The latter allowed them to boost the quantum yield – typically way below 100% – to a record-setting value of almost 200%.

The impact of their discovery is tremendous according to Dr. Alexis Goulet-Hanssens and Prof. Stefan Hecht, who works at the Department of Chemistry and IRIS Adlershof: „With our method, for the first time we can address molecular switches deep in a material. Thus, we can operate optical devices efficiently but also penetrate deep into the skin through the biological window“ they explain and are excited about possible applications in optoelectronics as well as medicine.

Hole Catalysis as a General Mechanism for Efficient and Wavelength-Independent Z→E Azobenzene Isomerization
A. Goulet-Hanssens, C. Rietze, E. Titov, L. Abdullahu, L. Grubert, P. Saalfrank, and S. Hecht
Chem (2018), published online
DOI: 10.1016/j.chempr.2018.06.002


 


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