Optical coherence tomography (OCT) on highly scattering and porous materials

Tapping into quantum mechanics

Aron Vanselow, a young reseracher at IRIS Adlershof, shows an attractive approach that makes it easier to perform optical coherence tomography (OCT) on highly scattering and porous materials. It specifically demonstrates that entangled photons can be used to improve the penetration depth of (OCT) in highly scattering materials. The method represents a way to perform OCT with mid-infrared wavelengths and could be useful for non-destructive testing and analysis of materials such as ceramics and paint samples.
OCT is a nondestructive imaging method that provides detailed 3D images of subsurface structures. OCT is typically performed using visible or near-infrared wavelengths because light sources and detectors for these wavelengths are readily available. However, these wavelengths don’t penetrate very deeply into highly scattering or very porous materials.
Aron Vanselow and colleagues from Humboldt-Universität zu Berlin in Germany, together with collaborators at the Research Center for Non-Destructive Testing GmbH in Austria, now demonstrate a proof-of-concept experiment for mid-infrared OCT based on ultra-broadband entangled photon pairs. They show that this approach can produce high quality 2D and 3D images of highly scattering samples using a relatively compact, straightforward optical setup.

Researchers used entangled photons to increase the penetration depth of OCT for scattering materials. They demonstrated the technique by analyzing two alumina ceramic stacks containing laser-milled microchannels. The mid-infrared illumination allowed the researchers to capture depth information and to create a full 3D reconstruction of the channel structures (pictured).

“Our method eliminates the need for broadband mid-infrared sources or detectors, which have made it challenging to develop practical OCT systems that work at these wavelengths,” said Vanselow. “It represents one of the first real-world applications in which entangled photons are competitive with conventional technology.”
The technique could be useful for many applications including analyzing the complex paint layers used on airplanes and cars or monitoring the coatings used on pharmaceuticals. It can also provide detailed 3D images that would be useful for art conservation.
For this technique, the researchers developed and patented a nonlinear crystal that creates broadband photon pairs with very different wavelengths. One of the photons has a wavelength that can be easily detected with standard equipment while the other photon is in the mid-infrared range, making it difficult to detect. When the hard-to-detect photons illuminate a sample, they change the signal in a way that can be measured using only the easy-to-detect photons.
“Our technique makes it easy to acquire useful measurements at what is a traditionally hard-to-handle wavelength range due to technology challenges,” said Sven Ramelow, who conceived and guided the research. “Moreover, the lasers and optics we used are not complex and are also more compact, robust and cost-effective than those used in current mid-infrared OCT systems.”

Imaging with less light

To demonstrate the technique, the researchers first confirmed that the performance of their optical setup matched theoretical predictions. They found that they could use six orders of magnitude less light to achieve the same signal-to-noise ratio as the few conventional mid-infrared OCT systems that have been recently developed. “We were positively surprised that we did not see any noise in the measurements beyond the intrinsic quantum noise of the light itself,” said Ramelow. “This also explained why we can achieve a good signal-to-noise ratio with so little light.”
The researchers tested their setup on a range of real-world samples, including highly scattering paint samples. They also analyzed two 900-micron thick alumina ceramic stacks containing laser-milled microchannels. The mid-infrared illumination allowed the researchers to capture depth information and to create a full 3D reconstruction of the channel structures. The pores in alumina ceramics make this material useful for drug testing and DNA detection but also highly scattering at the wavelengths traditionally used for OCT.
The researchers have already begun to engage with partners from industry and other research institutes to develop a compact OCT sensor head and full system for a pilot commercial application.

Frequency-domain optical coherence tomography with undetected mid-infrared photons
A. Vanselow, P. Kaufmann, I. Zorin, B. Heise, H. M. Chrzanowski, S. Ramelow
Optica 7 (2020) 1729, DOI:10.1364/OPTICA.400128


Xolography as new volumetric 3D printing method

It looks like science fiction but in fact could be the future of 3D printing: A blue slice of light wanders through a liquid, while light projections emerge through the window of a glass vessel. Resembling the replicator of the Star Trek spaceships, the desired object materializes. What used to take hours soon floats in the liquid in the vessel, is then removed, and cured under UV light.
The underlying process – xolography – was developed by a team led by chemist Stefan Hecht from IRIS Adlershof, physicist Martin Regehly, and the founder Dirk Radzinski in the startup company xolo GmbH in Berlin Adlershof over the past two years. For the first time, they now describe their unique method in the renowned journal Nature.

Their invention is based on Hecht’s specialty: photoswitchable molecules, which only at the intersection (xolography) of light rays of two different colors allow precise curing of the starting material in the entire volume (holos). In combination with a new printing process (xolography) based on a laser-generated light sheet and projected cross-sectional images, the desired objects are generated from virtual 3D models.
In contrast to conventional 3D printing, in which the objects are created layer by layer, the advantages of xolography are the significantly higher build speed that is due to the higher efficiency of combining two linear one-photon processes as opposed to non-linear two-photon stereolithography. The faster build speed does not compromise for resolution and thus smooth surfaces can be created. Moreover, fully assembled multicomponent systems can be fabricated in just one step.
Hecht is amazed “to see how fast this has been moving from an idea to xolo’s first prototype printer, the XUBE. Working in a highly interdisciplinary team including chemists, physicists, materials scientists, and software developers with a clear focus and dedication, we have been able to develop xolography as a powerful new method.” He is excited about the many opportunities ahead: “The beauty is our method’s versatility as we can print hard as well as soft objects. This should have major implications for the future manufacturing of optical, (micro)fluidic, and biomedical devices.”“

Xolography for linear volumetric 3D printing
M. Regehly, Y. Garmshausen, M. Reuter, N.F. König, E. Israel, D.P. Kelly, C.-Y. Chou, K. Koch, B. Asfari, and S. Hecht
Nature 588 (2020) 620, DOI: 10.1038/s41586-020-3029-7

Molecular telegraphy: Sending and receiving individual molecules precisely

The concept of throwing and catching a ball is familiar to everyone and works well in the macroscopic world. But could this be done in the nanoworld using individual molecules instead? And if one could transfer molecules precisely back and forth between two distant places, how fast would they be? An international team involving Stefan Hecht, who is a member of IRIS Adlershof, found some spectacular answers to these questions and the results of their study have been published as the cover story a recent issue of Science magazine.

"Through the targeted movement of individual molecules, we can gain insight into fundamental physical and chemical processes that are important for molecular dynamics - for example during chemical reactions or in catalysis," explains Leonhard Grill from the University of Graz, who led the team. For the study, the scientists brought organic molecules about two nanometers long on a silver surface with the fine metal tip of a scanning tunneling microscope in a special orientation, in which they are still extremely mobile, even at -266 °C. “We were able to show that, despite the very flat surface, the molecules move along a single row of atoms, i.e. only in one direction,” the researcher describes.

If an electric field is switched on, individual molecules can be moved perfectly along a straight line by electrostatic forces, as if the molecule would be on rails. As a result, the molecules can – depending on the direction of the field – be sent and received in a targeted manner by the forces of repulsion and attraction, respectively. The uncovered phenomenon operates over relatively long distances of 150 nanometers and at the same time with extremely high precision of 0.01 nanometers. The researchers were able to measure the time it took an individual molecule to be transferred and thus could determine the speed of an individual molecule directly. At these low temperatures, the molecule moved at 0.1 mm per second over the silver surface. These studies provide completely new possibilities for the investigation of molecular energies during movement and more importantly during chemical reactions.

At Oak Ridge National Laboratory, the researchers were able to carry out a unique transmitter-receiver experiment. Specifically, two separate scanning tunnel microscope tips were first appropriately positioned. Upon switching the “transmitter tip” from attractive to repulsive mode, the molecule moved precisely to the location of the “receiver tip”. This allowed to characterize the molecular motion and deduce the speed. But moreover this experiments illustrates the great potential for information transfer since all information stored in the molecule can be transfered with exquisite spatial precision. "

Control of long-distance motion of single molecules on a surface‐Emitting Diodes
D. Civita, M. Kolmer, G. J. Simpson, A.-P. Li, S. Hecht, and L. Grill
Science 370 (2020) 957, DOI: 10.1126/science.abd0696

Implementation of Flexible Embedded Nanowire Electrodes in Organic Light‐Emitting Diodes

Researchers in the HySPRINT joint lab Generative manufacturing processes for hybrid components (GenFab) of Humboldt-Universität zu Berlin (HU) and Helmholtz-Zentrum Berlin (HZB) have developed together with the Austrian Institute of Technology (AIT) a method to produce flexible transparent electrodes based on silver nanowires. Specifically, the nanowires are spray coated and embedded within a polymer resin on top of polyethylene terephthalate (PET) substrate.Not only are the electrodes fabricated using solution-based approaches, but compared with the widely used indium tin oxide (ITO), the electrodes show higher stability in mechanical bending tests. "Since the spray coating approach in this work can be upscaled to larger areas", says Dr. Felix Hermerschmidt, senior researcher in the joint lab of HU and HZB, "this mechanical stability can be translated to an industrial process."

The researchers fabricated organic light-emitting diodes employing the developed ITO‐free nanowire electrodes. These show considerably higher luminance values at the same efficacy compared to their ITO‐based counterparts. As Dr. Theodoros Dimopoulos, senior scientist at AIT, points out, "Replacing ITO in optoelectronic devices is a key area of research and this work shows the possibilities of doing so without loss in performance."

The work has been published in physica status solidi rapid research letters and is featured on the cover of the November 2020 issue of the journal.

GenFab, led by IRIS Adlershof member Prof. List-Kratochvil, is moving in laboratory rooms in the new IRIS-research building for further development.

Implementation of Flexible Embedded Nanowire Electrodes in Organic Light‐Emitting Diodes
Lukas Kinner, Felix Hermerschmidt, Theodoros Dimopoulos, and Emil J. W. List-Kratochvil
Phys. Status Solidi RRL 14 (2020) 2000305,   DOI:10.1002/pssr.202000305

The future of bio-medicine?

Researchers from Humboldt University and the Experimental and Clinical Research Center (ECRC) built the first infrared based microscope with quantum light. By deliberately entangling the photons, they succeeded in imaging tissue samples with previously invisible bio-features.

The researcher team from Humboldt University Berlin and the Experimental and Clinical Research Center (ECRC), a joined institution from Charité – Universitätsmedizin Berlin and Max Delbruck Center for Molecular Medicine in the Helmholtz Association, is featured on the cover of ‘Science Advances’ with their new experiment. For the first time they successfully used entangled light (photons) for microscope images. This very surprising method for quantum imaging with undetected photons was only discovered in 2014 in the group of the famous quantum physicist Anton Zeilinger in Vienna. The first images show tissue samples from a mouse heart.

The team uses entangled photons to image a bio-sample probed by ‘invisible’ light without ever looking at that light. The researchers only use a normal laser and commercial CMOS camera. This makes their MIR microscopy technique not only robust, fast and low noise, but also cost-effective - making it highly promising for real-world applications. This use of quantum light could support the field of biomedical microscopy in the future.

Quantum microscopy of a mouse heart. Entangled photons allow for the making of a high-resolution mid-IR image, using a visible light (CMOS) camera and ultralow illumination intensities. In the picture, absorption (left) and phase information (right) from a region in a mouse heart. The yellow scale bar corresponds to 0.1 mm which is about the width of a human hair.

Current camera detection is unequivocally dominated by silicon based technologies. There are billions of CCD (charge coupled device) and CMOS (complementary metal oxide semiconductor) sensors in digital cameras, mobile phones or autonomous vehicles. These convert light (photons) into electrical signals (electrons). But like our human eyes, these devices cannot see the important mid-IR range. This wavelength range is very important for biological science, containing valuable bio-chemical information that allows researchers to tell different biomolecules apart. The few camera technologies that exist at these crucial wavelengths are very expensive, noisy and subject to export restrictions. That is why the huge potential mid-IR light has for the life sciences so far remained unfulfilled. But researchers have proposed a new solution: “Using a really counterintuitive imaging technique with quantum-entangled photons allows us to measure the influence of a sample on a mid-IR light beam, without requiring any detection of this light” explains Inna Kviatkovsky, the lead author of the study.

There is also no conversion or so-called ‘ghost-imaging’ involved, but the technique relies on a subtle interference effect: first a pair of photons is generated by focusing a pump laser into a nonlinear crystal. This process can be engineered, such that one of the photons will be in the visible range and the other one in the MIR (invisible). The MIR photon probes the sample and is together with the visible photon and the laser sent back to the crystal. Here, quantum interference takes place - between the possibilities of the photon pair being generated on this first pass, and the possibility of not being generated on the first pass, but instead on the second pass through the crystal. Any disturbance, i.e. absorption caused by the sample, will now affect this interference and intriguingly this can be measured by only looking at the visible photons. Using the right optics one can build a mid-IR microscope based on this principle, which the team showed for the first time in their work.

“After a few challenges in the beginning, we were really surprised how well this works on an actual bio-sample.” Kviatkovsky notes. “Also we shine only extremely low powers of mid-IR light on the samples, so low, that no camera technology could directly detect these images.” While this is naturally only the first demonstration of this microscopy technique, the researchers are already developing an improved version of the technique. The researchers envisage a mid-IR microscope powered by quantum light that allows the rapid measurement of the detailed, localized absorption spectra for the whole sample. “If successful this could have a wide range of applications in label-free bio-imaging and we plan to investigate this with our collaboration partners from ECRC”, Dr. Sven Ramelow, group leader at Humboldt University, explains.

The research was funded by Deutsche Forschungsgemeinschaft (DFG) within the Emmy-Noether-Program.

Microscopy with undetected photons in the mid-infrared
Inna Kviatkovsky, Helen M. Chrzanowski, Ellen G. Avery, Hendrik Bartolomaeus, and Sven Ramelow
Science Advances 6, 42 (2020) eabd0264, DOI: 10.1126/sciadv.abd0264

Graphene as a detective to unravel molecular self-assembly

Researchers from Humboldt-Universität zu Berlin, the DWI – Leibniz Institute for Interactive Materials, and RWTH Aachen University (Germany), in collaboration with the University of Strasbourg & CNRS (France), have demonstrated that graphene devices can be used to monitor in real time the dynamics of molecular self-assembly at the solid/liquid interface. Their results have been published in Nature Communications.

Molecular self-assembly on surfaces is a powerful strategy to provide substrates with programmable properties. Understanding the dynamics of the self-assembly process is crucial to master surface functionalization. However, real-time monitoring of molecular self-assembly on a given substrate is complicated by the challenge to disentangle interfacial and bulk phenomena.

Cutting-edge scanning probe microscopy techniques, such as scanning tunneling microscopy (STM), have been used to monitor the dynamics of self-assembly at the solid/liquid interface, but thus far only in small populations of (less than 1,000) molecules and with a low time resolution (from 1 to 10 seconds).

In the present study, the European research team led by Marco Gobbi and Paolo Samorì has shown that a transistor incorporating graphene – a two-dimensional (2D) material that is highly sensitive to changes in its environment – can be used as a highly sensitive detector to track the dynamics of molecular self-assembly at the graphene/solution interface.

A photoswitchable spiropyran molecule, equipped with an anchoring group and able to reversibly interconvert (switch) between two different forms (isomer) by light, was investigated. When a droplet of a solution of this compound is casted on graphene, the spiropyran isomer does not form any ordered adlayer on the surface. In strong contrast, upon ultraviolet (UV) irradiation, the molecules in solution switch to the planar merocyanine isomer that forms a highly ordered layer on the graphene surface. When the UV light is turned off, the molecules revert to their initial non-planar spiropyran form and the ordered adlayer desorbs. Importantly, the merocyanine monolayer induces a distinct change in the electrical conductance of graphene and hence it is possible to monitor the dynamics of its formation and desorption by simply recording the electrical current flowing through graphene over time.

This simple and robust platform based on a graphene device allows the real-time monitoring of the complex dynamic process of molecular self-assembly at the solid/liquid interface. The electrical detection, which is highly sensitive, ultra-fast, practical, reliable and non-invasive, provides insight into the dynamics of several billions of molecules covering large areas (0.1 × 0.1 mm²) with a high temporal resolution (100 ms). Furthermore, the ultra-high surface sensitivity of graphene permits to disentangle the dynamics of different processes occurring simultaneously at the solid/liquid interface and in the supernatant solution. This strategy holds a great potential for applications in (bio)chemical sensing and diagnostics.


Figure. A droplet of a solution containing a photochromic molecule is casted onto a graphene device. UV light is employed to induce photoisomerization, triggering the formation of an ordered assembly on graphene, which desorbs after UV light is turned off. The time evolution of the current flowing through the device allows monitoring the dynamics of formation and dissolution of the self-assembled adlayer.

Graphene transistors for real-time monitoring molecular self-assembly dynamics
M. Gobbi, A. Galanti, M.-A. Stoeckel, B. Zyska, S. Bonacchi, S. Hecht, and P. Samorì
Nature Communications, 2020, 11, xxxx. DOI: 10.1038/s41467-020-18604-4


First quantum measurement of temperature in a living organism

The exact measurement of temperature with highest spatial resolution in living organisms is of great importance in order to be able to investigate metabolic processes precisely. However, such a measurement was previously impossible due to the lack of precise and reliable nano thermometers or nano temperature probes. An international research team led by Prof. Oliver Benson, member of IRIS Adlershof, and Prof. Masazumi Fujiwara from Osaka City Universitity has now developed a quantum sensor that is only a few nanometers in size and has been able to measure temperature changes in a nematode after administration of a pharmacological substance. The results pave the way for diverse applications of the novel quantum sensors in biomedical research, e.g. for taking high-resolution thermal images.

Scheme of the experiment: With the help of laser light (green), the characteristic microwave resonance line (in orange: microwave antenna) of nanodiamonds in a nematode (typical length 1 mm) can be recorded under a microscope. Since this depends on the temperature, a temperature change can be measured very precisely and locally. (©Masazumi Fujiwara, Osaka City University, e-mail to Oliver Benson)

Further Information:
In their experiment, the scientists used small diamonds with a diameter of a few 10 nanometers (1 nanometer = 1 millionth of a millimeter). These nanodiamonds contain luminous (fluorescent) quantum defects that can be observed under an optical microscope. With the help of radiated microwaves one can change the brightness of the luminous quantum defects. At a very specific microwave frequency, the defects appear a little darker. This so-called resonance frequency depends on the temperature. The researchers were now able to determine the shift in the resonance frequency very precisely and thus precisely determine the temperature change at the location of the nanodiamonds.
The nanodiamonds were inserted into a nematode (C. elegans). C. elegans is a very well understood model system and is examined in a large number of biophysical and biochemical experiments. By administering a certain pharmacological substance, the mitochondria, the “power stations” of the cells, could be stimulated to increased activity in individual cells of the worm. This then showed up as a slight local temperature increase of a few degrees.
The researchers were fascinated by the results of the experiment. "I never would have thought that the new methods of quantum technology would work so well even in living organisms," said Masazumi Fujiwara, professor at Osaka City University. "With these promising results, we are very confident that quantum sensing will establish in biochemistry and biomedicine. "adds Prof. Oliver Benson from Humboldt-Universität zu Berlin. The research teams are now working on further improving and automating their measuring method so that it can be easily integrated into standard microscopy setups.

Osaka City University Strategic Research Grant. Murata Science Foundation.
JSPS-KAKENHI (20H00335, 16K13646, 17H02741, 19K14636, 17H02738).
MEXT-LEADER program. Sumitomo Research Foundation.
Deutsche Forschungsgemeinschaft (FOR 1493).

Oliver Benson
Nano-Optik, Institut für Physik und IRIS Adlershof der Humboldt-Universität zu Berlin
Newtonstraße 15, 12489 Berlin
030 2093 4711

Real-time nanodiamond thermometry probing in vivo thermogenic responses
M. Fujiwara, S. Sun, A. Dohms, Y. Nishimura, K. Suto, Y. Takezawa, K. Oshimi, L. Zhao, N. Sadzak, Y. Umehara, Y. Teki, N. Komatsu, O. Benson, Y. Shikano, and E. Kage-Nakadai,
Science Advances (2020). DOI: 10.1126/sciadv.aba9636


Enwrapping of tubular J-aggregates of amphiphilic dyes for stabilization and further functionalization

The fabrication of functional units on mesoscopic length scales (nanometers to micrometers) in an aqueous environment by a self-assembling process is a fascinating but challenging task. It is essentially a biomimetic approach following design rules of living biological matter utilizing electrostatic and hydrophobic forces for the combination of a variety of materials. A peculiar form of such self-assembled structures is represented by tubular J-aggregates built from amphiphilic cyanine dye molecules. Those aggregates have attracted attention because of their similarity with natural light harvesting complexes. In particular, the dye 3,3′-bis(3-sulfopropyl)-5,5′,6,6′-tetrachloro-1,1′-dioctylbenzimida-carbo-cyanine (C8S3) forms micrometer long double walled tubular aggregates with a uniform outer diameter of 13 ± 0.5 nm. These J-aggregates exhibit strong exciton coupling, as seen by a strong shift in the absorption spectrum, and hence exciton delocalization and migration. However, their structural integrity and hence their optical properties are very sensitive to their chemical environment as well as to mechanical deformation, rendering detailed studies on individual tubular J-aggregates difficult.

In a collaboration within the CRC 951 Hybrid Inorganic/Organic Systems for Opto-Electronics, projects A6 (Kirstein, Rabe) and A12 (Koch) we addressed this issue and developed a route for their chemical and mechanical stabilization by in situ synthesis of a silica coating that leaves their absorbance and emission unaltered in solution [1]. By electrostatic adsorption of precursor molecules it was achieved to cover the aggregates with a silica shell of a few nanometer thickness which is able to stabilize the aggregates against changes of pH of solutions down to values where pure aggregates are oxidized, against drying under ambient conditions, and even against the vacuum conditions within an electron microscope. It was possible to measure spatially resolved electron energy loss spectra across a single freely suspended aggregate to analyze the chemical composition and the chemical composition and silica shell thickness. However, their structural integrity and hence their optical properties are very sensitive to their chemical environment as well as to mechanical deformation, rendering detailed studies on individual tubular J-aggregates difficult.

Figure 1: Sketch of chemical structure of the amphiphilic carbocyanine C8S3 and the route to synthesize a closed shell of silica on top of the aggregates of the anionic carbocyanine by successive adsorption of the precursor molecules APTES and TEOS. The TEM images are taken at room temperature and show Room temperature silica coated aggregates deposited on a holey carbon film. The right image is a magnification showing a single aggregate, freely suspended across a hole.

The concept of electrostatic adsorption at the charged surface of the aggregates was also utilized for the adsorption of oppositely charged polyelectrolytes, polycations in this case [2]. It was found that the morphology of the resulting aggregate/polycation complexes sensitively depends on the chemical structure of the polyelectrolyte. But in general, adsorption of a homogeneous layer leads to charge reversal of the surface of the complex, which can be used for further attachment of other chemicals. The adsorption of polyelectrolytes at these amphiphilic tubular structures, stabilized by means of hydrophobic forces, is far from obvious and demonstrates an applicable route to the hierarchical build-up of more complex nanostructures in solution by means of a self-assembling process.

Figure 2 Sketch of the tubular aggregates of C8S3 emphasizing the negative surface charge due to sulfonate end groups of the dye and chemical structure of the polycation PDADMAC. The Cryo-TEM image shows aggregates partially covered with PDADMAC.

[1]  Individual tubular J-aggregates stabilized and stiffened by silica encapsulation
K. Herman, H. Kirmse, A. Eljarrat, C.T. Koch, S. Kirstein, J.P. Rabe
Colloid Polym Sci 298 (2020) 937

[2]  Adsorption of polyelectrolytes onto the oppositely charged surface of tubular J aggregates of a cyanine dye
O. Al-Khatib, C. Böttcher, H. von Berlepsch, K. Herman, S. Schön, J.P. Rabe, S. Kirstein
Colloid Polym Sci 297 (2019) 729

Metal-Assisted and Solvent-Mediated Synthesis of Two-Dimensional Triazine Structures on Gram Scale

Covalent triazine frameworks are an emerging class of materials that have shown promising performance for a range of applications. In a large collaborative project, researchers from IRIS Adlershof together with their partners report on a metal-assisted and solvent-mediated reaction between calcium carbide and cyanuric chloride, as cheap and commercially available precursors, to synthesize two-dimensional triazine structures (2DTSs) [1]. The reaction between the solvent, dimethylformamide, and cyanuric chloride was promoted by calcium carbide and resulted in dimethylamino-s-triazine intermediates, which in turn undergo nucleophilic substitutions. This reaction was directed into two dimensions by calcium ions derived from calcium carbide and induced the formation of 2DTSs. The role of calcium ions to direct the two-dimensionality of the final structure was simulated using DFT and further proven by synthesizing molecular intermediates. The water content of the reaction medium was found to be a crucial factor that affected the structure of the products dramatically. While 2DTSs were obtained under anhydrous conditions, a mixture of graphitic material/2DTSs or only graphitic material (GM) was obtained in aqueous solutions. Due to the straightforward and gram-scale synthesis of 2DTSs, as well as their photothermal and photodynamic properties, they are promising materials for a wide range of future applications, including bacteria and virus incapacitation.


Metal-assisted and solvent-mediated synthesis of two-dimensional triazine structures on gram scale
A. Faghani, M.F. Gholami, M. Trunk, J. Müller, P. Pachfule, S. Vogl, I. Donskyi, P. Nickl, J. Shao, M.R.S. Huang, W.E.S. Unger, R. Arenal, C.T. Koch, B. Paulus, J.P. Rabe, A. Thomas, R. Haag, M. Adeli
J. Am. Chem. Soc. 142 (2020) 12876, DOI: 10.1021/jacs.0c02399


Reversible Switching of Charge Transfer at the Graphene-Mica Interface with Intercalating Molecules

Understanding and controlling charge transfer through molecular nanostructures at interfaces is of paramount importance, particularly for hybrid systems for optics and electronics but also generally for contact electrification or in bio-electronics. In a recent publication, Hu Lin et al. [1] reveal the influence of intercalation and exchange of molecularly thin layers of small molecules (water, ethanol, 2 propanol and acetone) on charge transfer at the well-defined interface between an insulator (muscovite mica) and a conductor (graphene) through probing graphene doping variations by Raman spectroscopy. While a molecular layer of water blocks charge transfer between mica and graphene, a layer of the organic molecules allows for it. The exchange of molecular water layers with ethanol layers switches the charge transfer very efficiently from OFF to ON and back. This observation is explained by charge transfer from occupied mica trap states to electronic states of graphene, controlled by the electrostatic potential from the molecular layers wetting the interface. This is supported by molecular dynamics simulations. The sensitivity of graphene doping to the composition of confined molecular films may be used to investigate the structure of the films and diffusion of the molecules in the nano-confinement, e.g., their miscibility; furthermore, potential molecular sensor and actuator applications can be envisioned. The demonstrated role of molecular layers in the charge transfer will aid in understanding of graphene wetting transparency, and it will facilitate the development of electronic devices, e.g., triboelectric generators.

a) Schematic diagram of (i) an initially dry graphene-mica interface becoming intercalated with molecular (ii) ethanol or (iii) water layers, upon exposure to ethanol and water vapors, respectively. Water and ethanol molecules can diffuse into the interface and replace each other. b) Dependence of the graphene G peak position on time for alternating exposures to ethanol (green) and water vapor (blue). The light blue and red lines are G peak positions for unstrained/undoped and n-doped graphene on water and ethanol layers, respectively.

Reversible Switching of Charge Transfer at the Graphene-Mica Interface with Intercalating Molecules

H. Lin, J.-D. Cojal González, N. Severin, I.M. Sokolov, J.P. Rabe
ACS Nano 14 (2020) 11594, DOI: 10.1021/acsnano.0c04144


Hidden Symmetries in Massive Quantum Field Theory

Theoretical models with a large amount of symmetry are ubiquitous in physics and often key to developing efficient methods for complex problems. If the number of symmetries surpasses a critical threshold, a system is called integrable with a prime example being the Kepler problem of planetary motion. While integrability typically comes with a rich spectrum of mathematical methods, it is often hard to identify the underlying symmetries. For the first time quantum integrability was now discovered in the context of massive quantum field theories in four spacetime dimensions. Florian Loebbert and Julian Miczajka (both Humboldt University) together with Dennis Müller (NBI Copenhagen) and Hagen Münkler (ETH Zürich) have shown that large classes of mostly unsolved massive Feynman integrals feature an infinite dimensional Yangian symmetry - a hallmark of integrability. This mathematical structure is highly constraining and it allows to completely fix these building blocks of quantum field theory as has been demonstrated for first examples. The observed Yangian symmetry goes hand in hand with an extension of the important structure of conformal symmetry to situations including massive particles. Remarkably, this discovery suggests that similar symmetry features may also be hidden in massive versions of the celebrated holographic duality between gauge theories and gravity. These findings were recently published in Physical Review Letters 125 (2020) 9, 091602.

Massive Conformal Symmetry and Integrability for Feynman Integrals
F. Loebbert, J. Miczajka, D. Müller, and H. Münkler
Phys. Rev. Lett. 125 (2020) 9, 091602

Understanding the interaction of polyelectrolyte architectures with proteins and biosystems

Figure 1: Interaction of polyelectrolytes with biosystems at different levels of complexity. The entire matrix of systems and problems surveys the possible medical problems to which synthetic polyelectrolytes may provide solutions

Polyelectrolytes such as e.g. DNA or heparin are long linear or branched macromolecules onto which charges are appended. The counterions neutralizing these charges may dissociate in water and will largely determine the interaction of such polyelectrolytes with biomolecules and in particular with proteins. Here Prof. Matthias Ballauff, member of IRIS Adlershof, and collegues review studies on the interaction of proteins with polyelectrolytes and how this knowledge can be used for medical applications. Counterion release was identified as the main driving force for the binding of proteins to polyelectrolytes: Patches of positive charge become multivalent counterions of the polyelectrolyte which leads to the release of counterions of the polyelectrolyte and a concomitant increase of entropy.

Figure 2: Interaction of proteins with highly charged polyelectrolytes as e.g. DNA by counterion release

This was shown by surveying investigations done on the interaction of proteins with natural and synthetic polyelectrolytes. Special emphasis is laid on sulfated dendritic polyglycerols (dPGS). The entire overview demonstrates that we are moving on to a better understanding of charge‐charge interaction in system of biological relevance. Hence, research along these lines will aid and promote the design of synthetic polyelectrolytes for medical applications.

Understanding the interaction of polyelectrolyte architectures with proteins and biosystems

K. Achazi, R. Haag, M. Ballauff, J. Dernedde, J.N. Kizhakkedathu, D. Maysinger, and G. Multhaup
Angew. Chem. Int. Ed.. Accepted Author Manuscript, DOI: 10.1002/anie.202006457

Printed perovskite LEDs – an innovative technique towards a new standard process of electronics manufacturing

Graphical representation of the printing process for the perowskite-LEDs. .
© Claudia Rothkirch/HU Berlin

A team of researchers from the Helmholtz-Zentrum Berlin (HZB) and Humboldt-Universität zu Berlin has succeeded for the first time in producing light-emitting diodes (LEDs) from a hybrid perovskite semiconductor material using inkjet printing.This opens the door to broad application of these materials in manufacturing many different kinds of electronic components.The scientists achieved the breakthrough with the help of a trick: "inoculating" (or seeding) the surface with specific crystals.

Microelectronics utilise various functional materials whose properties make them suitable for specific applications. For example, transistors and data storage devices are made of silicon, and most photovoltaic cells used for generating electricity from sunlight are also currently made of this semiconductor material. In contrast, compound semiconductors such as gallium nitride are used to generate light in optoelectronic elements such as light-emitting diodes (LEDs). The manufacturing processes also different for the various classes of materials.

Transcending the materials and methods maze

A look inside the Helmholtz Innovation Lab HySPRINT.
Major work on the printable perovskite-LEDs was carried out here.
 © HZB/Phil Dera

Hybrid perovskite materials promise simplification – by arranging the organic and inorganic components of semiconducting crystal in a specific structure. “They can be used to manufacture all kinds of microelectronic components by modifying their composition“, says Prof. Emil List-Kratochvil, head of a Joint Research Group at HZB and Humboldt-Universität. What's more, processing perovskite crystals is comparatively simple. “They can be produced from a liquid solution, so you can build the desired component one layer at a time directly on the substrate“, the physicist explains.

First solar cells from an inkjet printer, now light-emitting diodes too

Scientists at HZB have already shown in recent years that solar cells can be printed from a solution of semiconductor compounds – and are worldwide leaders in this technology today. Now for the first time, the joint team of HZB and HU Berlin has succeeded in producing functional light-emitting diodes in this manner. The research group used a metal halide perovskite for this purpose. This is a material that promises particularly high efficiency in generating light – but on the other hand is difficult to process. “Until now, it has not been possible to produce these kinds of semiconductor layers with sufficient quality from a liquid solution“, says List-Kratochvil. For example, LEDs could be printed just from organic semiconductors, but these provide only modest luminosity. “The challenge was how to cause the salt-like precursor that we printed onto the substrate to crystallise quickly and evenly by using some sort of an attractant or catalyst“, explains the scientist. The team chose a seed crystal for this purpose: a salt crystal that attaches itself to the substrate and triggers formation of a gridwork for the subsequent perovskite layers.

Significantly better optical and electronic characteristics

In this way, the researchers created printed LEDs that possess far higher luminosity and considerably better electrical properties than could be previously achieved using additive manufacturing processes. But for List-Kratochvil, this success is only an intermediate step on the road to future micro- and optoelectronics that he believes will be based exclusively on hybrid perovskite semiconductors. “The advantages offered by a single universally applicable class of materials and a single cost-effective and simple process for manufacturing any kind of component are striking“, says the scientist. He is therefore planning to eventually manufacture all important electronic components this way in the laboratories of HZB and HU Berlin. List-Kratochvil is Professor of Hybrid Devices at the Humboldt-Universität zu Berlin and head of a Joint Lab founded in 2018 that is operated by HU together with HZB. In addition, a team jointly headed by List-Kratochvil and HZB scientist Dr. Eva Unger is working in the Helmholtz Innovation Lab HySPRINT on the development of coating and printing processes – also known in technical jargon as "additive manufacturing" – for hybrid perovskites. These are crystals possessing a perovskite structure that contain both inorganic and organic components.

Ralf Butscher

Finally, inkjet-printed metal halide perovskite LEDs – utilizing seed crystal templating of salty PEDOT:PSS
Felix Hermerschmidt, Florian Mathies, Vincent R. F. Schröder, Carolin Rehermann, Nicolas Zorn Morales, Eva L. Unger, Emil. J. W. List-Kratochvil.
Mater. Horiz. (2020) Advance Article


Modulating the luminance of organic light-emitting diodes via optical stimulation of a photochromic molecular monolayer at transparent oxide electrode

Flavie Davidson-Marquis
a) Luminance of an OLED fabricated
with a SAM-modified ITO electrode.
The luminescence doubles
once the DAE is switched from
closed to open.
b) Values for the ratio between
the current densities and
luminescence measured
at 5 V upon multiple irradiation
cycles. The Modulation of the OLED
luminescence is reversible

Organic self-assembled monolayers (SAMs) deposited on inorganic bottom electrodes are commonly used to tune charge carrier injection or blocking in hybrid inorganic/organic optoelectronic devices. Beside the enhancement of device performance, the fabrication of multifunctional devices in which the output can be modulated by multiple external stimuli remains a challenging target. The authors of this research highlight report the functionalization of an indium tin oxide (ITO) electrode with a SAM of a photochromic diarylethene derivative designed for optically control the electronic properties. Following the demonstration of dense SAM formation and its photochromic activity, as a proof-of- principle, an organic light-emitting diode (OLED) embedding the light-responsive SAM-covered electrode is fabricated and characterized. Optically addressing the two-terminal device by irradiation with ultraviolet light (315 nm) doubles the electroluminescence (100% gain), which can be reversed by irradiation with visible light (530 nm). This approach of “dynamic” energy tuning could be successfully exploited in the field of opto-communication technology, for example to fabricate opto-electronic logic circuits.

Modulating the luminance of organic light-emitting diodes via optical stimulation of a photochromic molecular monolayer at transparent oxide electrode

G. Ligorio, G. F. Cotella, A. Bonasera,
N. Zorn Morales, G. Carnicella, B. Kobin,
Q. Wang, N. Koch, S. Hecht,
E. J.W. List-Kratochvil, and F. Cacialli
Nanoscale 12, 5444 (2020)

Review on hybrid integrated quantum photonic circuits

Recent developments in chip-based photonic quantum circuits have radically impacted quantum information processing. However, it is challenging for monolithic photonic platforms to meet the stringent demands of most quantum applications. Hybrid platforms combining different photonic technologies and different materials in a single functional unit have great potential to overcome the limitations of monolithic photonic circuits. 

Several key functional elements integrated on single photonic chip

Researchers from the KTH Royal Institute of Technology, Stockholm, Sweden, the University of Muenster, Germany, the National Institute of Standards and Technology, Gaithersburg, USA, and IRIS Adlershof review the progress of hybrid quantum photonics integration. They discuss important design considerations, including optical connectivity and operation conditions, and outline the roadmap for realizing future advanced large-scale hybrid devices, beyond the solid-state platform, which hold great potential for quantum information applications.

Three examples of hybrid integration: (a) Dibenzoterrylene embedded in a rigid matrix of crystalline anthracene as molecule single-photon source on a silicon nitride waveguide [Lombardi, et al. ACS Photon. 5, 126–132 (2018)], (b) Nonlinear phase gate in a hybrid atomic-photonic system [Tiecke, et al., Nature 508, 241–244 (2014)], (c) Hybrid atomic cladding photonic waveguide demonstrating light–matter interaction at room temperature [Stern, et al., Nat. Commun. 4, 1548 (2013)].

Hybrid integrated quantum photonic circuits

A.W. Elshaari, W. Pernice, K. Srinivasan, O. Benson and V. Zwiller
Nat. Photonics (2020)


Excited-state charge transfer enabling MoS2/Phthalocyanine photodetectors with extended spectral sensitivity

The combination of inorganic monolayer (ML) transition-metal dichalcogenides (TMDCs) with organic semiconductors holds the promise to further improve opto-electronic device properties with added functionality. The authors of this research highlight investigate a hybrid inorganic/organic system (HIOS) consisting of metal-free phthalocyanine (H2Pc) as thin organic absorber layer and ML MoS2 as TMDC. Via a combination of photoemission (PES), photoluminescence (PL), and photocurrent action spectroscopy they demonstrate, that excited-state charge transfer from the H2Pc layer enhances the photo response of ML MoS2 without loss in sensitivity extended to spectral regions where the TMDC is transparent. This observation is explained by the staggered type II energy-level alignment at the hybrid interface facilitating efficient exciton dissociation and excited-state charge transfer with the holes residing in the H2Pc HOMO and the electrons in the MoS2 conduction band. In hybrid photodetectors, these transferred charges increase the concentration of carriers in MoS2 and with that its photoconductivity. The present demonstration of a highly efficient carrier generation in TMDC/organic hybrid structures paves the way for future nanoscale photodetectors with very wide spectral sensitivity.

(a) Schematic design of the hybrid H2Pc/MoS2 photodetecting device. The H2Pc layer thickness is dH2Pc = 3.0 nm b Photoresponse of the hybrid (blue) and the reference MoS2-only (red) device. The spectra were normalized at the spectral position where H2Pc does not absorb, i.e., between 2.5 and 2.55 eV. The difference between the spectra of the hybrid (Rhyb) and reference (Rref) devices ΔR = Rhyb – Rref (green).

Excited-State Charge Transfer Enabling MoS2/Phthalocyanine Photodetectors with Extended Spectral Sensitivity

N. Mutz, S. Park, T. Schultz, S. Sadofev, S, Dalgleish, L. Reissig, N. Koch, E. J. W. List-Kratochvil, and S. Blumstengel
J. Phys. Chem. C 124, 2837 (2020)


Insights into charge transfer at the atomically precise nanocluster/semiconductor interface for in-depth understanding the role of nanocluster in photocatalytic system

A TiO2/cluster composite of type II junction configuration for photocatalytic hydrogen evolution is built by deposition of atomically precise Ag44 nanocluster on TiO2. Besides photosensitizer, the cluster is found to serve as co-catalyst to improve the charge separation efficiency of the system, which is quite different from the well-known plasmonic nanoparticle (NP) enhanced systems. The hydrogen production rate by Ag44-TiO2 is ten times higher than that of the pure TiO2 and five times higher than that of the Ag NP-TiO2.

(a) Schematic illustration of the H2 production by Ag44-TiO2 under simulated sunlight; (b) Catalytic performance of TiO2 (black), Ag NP-TiO2 (yellow) and Ag44- TiO2 (red).

Insights into charge transfer at the atomically precise nanocluster/semiconductor interface for in‐depth understanding the role of nanocluster in photocatalytic system

Y. Wang, X-H. Liu, Q. Wang, M. Quick, A.S. Kovalenko, Q.-Y. Chen, N. Koch, and N. Pinna
Angew. Chem. Int. Ed. 2020

Influence of interface hydration on sliding of graphene and Molybdenum-disulphide single-layers

Humidity influences friction in layered materials in peculiar ways. For example, while water improves the lubricating properties of graphite, it deteriorates those of molybdenum disulphide (MoS2). The reasons remain debated, not the least due to the difficulty to experimentally compare dry and hydrated interface frictions. Hu Lin et al. [1] have shown that the hydration of interfaces between a mica substrate and single-layers of graphene and MoS2 with a molecularly thin water layer affects strain transfer from the substrate to the 2D materials. For this, the substrate has been strained and the strain in graphene and MoS2 has been detected by changes in Raman and photoluminescence spectra, respectively. Graphenes on dry mica exhibit “stick-and-slip” strain relaxation with frictional forces per area of up to about 100 kPa. Strains relaxation in hydrated graphenes is viscous with estimated viscous friction coefficients in units of force per unit area and per unit velocity of about 1*1017 Pa·s/m. In contrast, there is no viscous relaxation in MoS2 regardless of hydration. This work provides a novel approach for better understanding the impact of hydration on friction in layered materials.

Strain transfer measurements for both a) dry and b) hydrated graphene-mica interfaces: the surface of the mica substrate was strained by bending the mica slab in steps. Strains in graphene were followed with Raman peak positions. Strain relaxation in graphene changes from stick-slip in dry contact, to viscous when hydrated.

Influence of interface hydration on sliding of graphene and molybdenum-disulfide single-layers

H. Lin, A. Rauf, N. Severin, I.M. Sokolov, J.P. Rabe
J. Colloid Interface Sci. 540 (2019) 142-147. DOI: 10.1016/j.jcis.2018.12.089

Off-shell gauge invariance

Flavie Davidson-Marquis
Master integrals for reduction of 4-point

Dirk Kreimer (IRIS member), John Gracey (U. Liverpool and DFG Mercator Fellow in Kreimer’s group) and postdoc Henry Kissler could clarify the algebraic and combinatorical foundations of off-shell Slavnov Taylor identities, off-shell gauge invariance that is. The problem remained open in the litera- ture for many years and was now settled by modern algebra and confirmed computation- ally. Quantum chromodynamics served here as a concrete test case. Generalizations to other gauge theories are under study. Figure 1: Off-shell gauge invariance Using Hopf-algebraic structures as well and diagrammatic techniques for deter- mining the Slavnov-Taylor identities for QCD familiar from the study of graph complexes we construct relations for off-shell Green functions. The methods are sufficiently versatile to allow for applications even in the study of diffeomorphism invariance in quantum gravity in the future.

Self-consistency of off-shell Slavnov-Taylor identities in QCD
J. A. Gracey, H. Kißler, and D. Kreimer
Phys. Rev. D 100 (2019) 085001

"SPACE – TIME – MATTER: Analytic and Geometric Structures":
A Book about CRC 647 results published


This monograph describes some of the most interesting results obtained by the mathematicians and physicists collaborating in the CRC 647 "Space – Time – Matter", in the years 2005 - 2016. It concerns the mathematical and physical foundations of string and quantum field theory as well as cosmology. The work starts with an excellent introduction by the editors Jochen Brüning and Matthias Staudacher, both members of IRIS Adlershof, that gives an historical overview of the field and vividly retells the development of the CRC. Then each project of the final funding period is summarized and also represented in detail by the following 15 chapters, many contributed by IRIS scientists:

  • Dyson–Schwinger equations: Fix-point equations for quantum fields by Dirk Kreimer (IRIS member)
  • Hidden structure in the form factors of N = 4 SYM by Dhritiman Nandan (former member at AG Staudacher) and Gang Yang
  • On regulating the AdS superstring by Valentina Forini (IRIS Junior member)
  • Yangian symmetry inmaximally supersymmetric Yang-Mills theory by Livia Ferro, Jan Plefka (IRIS member), and Matthias Staudacher (IRIS member)
  • Geometric analysis on singular spaces by Francesco Bei (former member at AG Brüning), Jochen Brüning (IRIS member), Batu Güneysu (former IRIS young researcher and member at AG Brüning), and Matthias Ludewig

The book was published by DeGruyter in 2018

Jochen Brüning, Matthias Staudacher (Eds.)
SPACE – TIME – MATTER: Analytic and Geometric Structures

DOI: 10.1515/9783110452150
ISBN (print): 978-3-11-045135-1
ISBN (PDF): 978-3-11-045215-0
ISBN (epub): 978-3-11-045153-5


Entangled Photons for Mid-Infrared Sensing - Quantum Futur Award 2019 for Aron Vanselow

Aron Vanselow receives the second prize of the Quantum Futur Award 2019, sponsored by Ministry of Science and Education for his master thesis at Humboldt University Berlin.
It has long been anticipated that entangled photons hold the promise to drive a paradigm shift in imaging and sensing. Real-world implementations, however, have lagged behind their classical counterparts, because of low efficiency, loss and decoherence.
Aron Vanselow’s thesis, carried out iin the junior research group "Nonlinear Quantum Optics", led by Dr. Sven Ramelow,
who is a member of IRIS Adlershof, presents the first experimental demonstration of mid-infrared frequency-domain optical coherence tomography (OCT) with entangled photons. OCT is an important depth-imaging method in biomedical diagnostics as well as non-destructive testing allowing for 3D microscopy. OCT in the mid-IR range enables looking inside strongly scattering media, where commercial systems which are all at shorter wavelengths don’t work.
The proof-of-principle setup developed by Aron Vanselow, Sven Ramelow and their colleagues is powered by quantum entanglement generated in a patented new crystal. Importantly, the reached performances are already comparable to the best conventional techniques while exposing the sample to 8 orders of magnitude less optical power. At the same time the technological overhead is drastically reduced compared with classical techniques using only compact and cost-effective components.
With the thesis demonstrating fast 2D and 3D imaging of highly scattering real-world samples (ceramics, paint layers) with 20 μm lateral and 10 μm depth resolution it has immediate relevance for applications in non-destructive testing such as quality control of coating thicknesses, cultural heritage conservation and microfluidics.

Mid-infrared Frequency-domain Optical Coherence Tomography with Undetected Photons
A. Vanselow, P. Kaufmann, I. Zorin, B. Heise, H. Chrzanowski, and S. Ramelow
Quantum Information & Measurement V, T5A.86

Ultra-broadband SPDC for spectrally far separated photon pairs
A. Vanselow, P. Kaufmann, H. M. Chrzanowski, and S. Ramelow
Optics Letters 44 (2019), 4638

Direct measurement of quantum efficiency of single-photon emitters in hexagonal boron nitride

Two-dimensional materials like boron nitride (h-BN) have recently attracted the attention of the quantum optics and nano optics community. Individual single photon emitting (SPE) defects can be found even in single layers of h-BN. These emitters are bright and stable and have a narrow emission line, making them potentially suitable for use in quantum communication devices. As the field is still young, it is difficult to create SPEs with desired properties. One reason for this is the yet unknown atomic origin of the defect, which could help to identify processing steps that could lead to the desired outcome. In order to determine the atomic origin of an emitter, calculations are carried out under the assumption of different atomic configurations and compared with the observed spectra. Unfortunately, the h-BN SPEs spectra are distributed over a wide range, which makes the application of this method difficult. Another intrinsic property is the quantum efficiency (QE), i.e. the branching ratio between a radiative rate and the total (radiative and non-radiative) decay rate.

Schematics of the performed experiment and a distance dependent lifetime measurement.
(a) The AFM is equipped with a gold-coated hemispherical tip aligned with an SPE in h-BN and held at a variable distance. The objective lens on the bottom of the glass excites the SPE (green pulsed laser) and collects its emission. With this setup, distance-dependent lifetime measurements can be performed, one such measurement is shown in (b) (points). To determine the QE, an adjustment was performed (blue solid line). The same fit function with a QE of 1.0 is represented by the green solid line as a reference.

Researchers of Nanooptik AG of Humboldt-University of Berlin in cooperation with the Technical University of Sydney could now directly measure the absolute QE of single defects in h-BN. The underlying principle is based on the proportionality between a controlled change in the local density of the states into which the emitter can emit and the lifetime of the excited state. The researchers implemented this experimentally by controlling the distance between the SPE and a mirror with nanometer accuracy while measuring the lifetime of the excited state. In this way, not only the high QE of up to 87(7) % was determined, but also a correlation between fluorescence wavelength and QE was found. This paves the way for a better understanding of the origin of the emitters.

Direct measurement of quantum efficiency of single-photon emitters in hexagonal boron nitride
N. Nikolay, N. Mendelson, E. Özelci, B. Sontheimer, F. Böhm, G. Kewes, M. Toth, I. Aharonovich, and O. Benson
Optica 6 (2019) 1084

Article of IRIS junior research group leader Michael J. Bojdys published in Nature Communications

The IRIS junior research group leader Michael J. Bojdys and his international team have achieved a great success: Their article “Real-time optical and electronic sensing with a β-amino enone linked, triazine-containing 2D covalent organic framework” has been selected to be published in the renowned journal Nature Communications.
Bojdys article deals with aromatic two-dimensional covalent organic frameworks (2D COFs), which are a class of porous polymers that allow the precise incorporation of organic units into periodic structures.COFs can be chemically designed to incorporate particular surface functional groups which can be exploited to tune the optical and electronic properties. However, low stability towards chemical triggers has hampered their practical implementations.
Together with a team from the Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences (Prague, Czech Republic), IRIS junior research group leader Michael J. Bojdys and his team from Humboldt-Universität zu Berlin have explored a new design principle for COFs that makes use of strong, overall conjugation and incorporation of donor-acceptor domains. In this study a new, a highly stable chemoresistant β-amino enone linked, triazine-containing COF was used as a real-time, reversible optical and electronic sensor for volatile acids and bases. The team was further able to conclude that the sensing capabilities of the COF was achieved by preferential protonation
of the electron accepotor – a triazine ring in the structure – , resulting in an optical response visible to the naked eye and an increase of bulk electrical conductivity by two orders of magnitude. These findings demonstrate a powerful approach to design more practical sensors and switches, and take genuine advantage of the chemoresistant make-up, porous structure, and overall conjugation of fully-aromatic systems.
IRIS Adlershof would like to congratulate Michael J. Bojdys and his team on this successful study and its publication in Nature Communications!
Due to his great enthusiasm for the concept of IRIS Adlershof and the research carried out here, ERC-grant holder Bojdys joined the Humboldt-Universität zu Berlin and IRIS Adlershof in 2018 as leader of the junior research group “Functional Materials”. The group’s research aims at the development of metal-free, electronic components for transistors and sensors on the basis of functional materials made up of light, covalently-bonded atoms. At the heart of the project lies the challenge to transfer the control mechanisms and modularity known from molecular, organic chemistry to macroscopic structures.

Real-time optical and electronic sensing with a β-amino enone linked, triazine-containing 2D covalent organic framework
R. Kulkarni, Y. Noda, D.K Barange, Y.S. Kochergin, P. Lyu, B. Balcarova, P. Nachtigall, and M.J. Bojdys
Nat. Commun 10 (2019) 3228

A novel semiconductor from the family of carbon nitrides

Research teams from the Humboldt-Universität and the Helmholtz Zentrum Berlin (HZB) have investigated a new material from the family of carbon nitrides. Triazine-based graphitic carbon nitride (TGCN) is a semiconductor that is useful in optoelectronic applications. Its structure is two-dimensional and layered, and it resembles that of graphene. Unlike graphene, its conductivity between the layers is 65-times higher than in-plane.

Some organic materials can be used in optoelectronics just like silicon-based semiconductors. Whether in solar cells, light-emitting diodes, or as transistors – the important property is the bandgap, i.e. the energy-difference of the electrons in the valence band and the conduction band. The basic principle underlying all electronic components is that electrons can be promoted by light or by voltage between the valence and the conduction band. Here, bandgaps between 1 and 2 eV are ideal.

A team led by the chemist Dr. Michael J. Bojdys from the chemistry department and IRIS Adlershof of the Humboldt-Universität zu Berlin, has recently synthesized an organic semiconductor from the family of carbon nitrides. This triazine-based graphitic carbon nitride (TGCN) consists exclusively from carbon and nitrogen atoms and can be grown as a brown film on quartz glass substrates. The C- and N-atoms connect in hexagonal, honeycomb patterns like carbon atoms in graphene. Just like in graphene, the crystal structure of TGCN is based on layered, two-dimensional sheets. In graphene, in-plane conductivity is excellent, however, it is much lower through the planes. In the case of TGCN, the opposite is observed: through-plane conductivity is 65-times higher than in-plane. With a bandgap of 1.7 eV TGCN is a good candidate for optoelectronic applications.

The HZB-physicist Dr. Christoph Merschjann has examined the charge carrier transport in samples of TGCN using time-resolved absorption measurements in the femto- to nanosecond regime at the laser lab JULiq – a joint lab between the HZB and the Freie Universität Berlin. Such laser experiments offer a unique way to correlate macroscopic conductivity and microscopic transport models. From his measurements, he was able to deduce how the charge carriers diffuse throughout the material. “Electrons do not exit the hexagonal honeycombs of triazine units horizontally, but they move at a slope to the nearest triazine-unit in the neighboring layer. The crystal structure of the material leads to a preferred movement of charge carriers along tube-like channels.” This mechanism could explain why the conductivity of TGCN is fundamentally higher through-plane than in-plane. “TGCN is the hitherto best candidate to replace silicon semiconductors and the critical, rare-earth dopants used in their manufacture”, says Michael Bojdys. “The production method for TGCN that we developed in my group at the Humboldt-Universität zu Berlin yields flat layers of semiconducting TGCN on insulating quartz glass. This enables relatively easy upscaling and device production.”

Directional charge transport in layered, two‐dimensional triazine‐based graphitic carbon nitride
Y. Noda, C. Merschjann, J. Tarábek, P. Amsalem, N. Koch, and M.J. Bojdys
Angew. Chem. Int. Ed. 58 (2019) 9394



Researchers demonstrate very large electric tuning of a single quantum emitter at room temperature

Bright and tunable solid-state single-photon emitters (SPEs) are required for the realization of scalable quantum photonic technologies. Recently, optically active defects in a two-dimensional material, boron nitride (h-BN), have been extensively studied as bright single-photon emitters with a narrow linewidth and operating at room temperature. The layered nature of h-BN also offers potential advantages for integration in novel opto-electronic hybrid elements including photonic resonators, waveguides, modulator, and detectors. In order to exploit the functionality of such elements a tuning of the emitter’s fluorescence line is essential. Tuning via the Stark effect using a static electric field has been suggested for various solid-state emitters, such as quantum dots or color centers in diamond. Researcher from the Institute of Physics of Humboldt-University together with coworkers from the University of Technology in Sydney were now able to demonstrate controlled and reversible Stark tuning of individual emitters in hBN. They used a metallic tip of an atomic force microscope (AFM) to locally select a single emitter and tune it over a record range of up to 5.5 nanometers at room temperature.

a) Structure of a defect in hexagonal Boron Nitride. b) Schematic of the experiment, where a metallic AFM tip is placed above a single defect emitter and a bias voltage is applied. C) Measured Stark-shift of the narrow fluorescence line.

Based on their results the researchers suggest building a room-temperature single photon source, which can be tuned electrically in or out of a resonance of a plasmonic resonator. “Such a source would be highly desirable as a reliable non-classical light source for applications in quantum-enhanced sensing and metrology or in quantum key distribution.” says Prof. Oliver Benson, who is researcher in IRIS Adlershof and leads the Humboldt-team.
Very large and Reversible Stark-Shift Tuning of Single Emitters in Layered Hexagonal Boron Nitride
N. Nikolay, N. Mendelson, N. Sadzak, F. Böhm, T. T. Tran, B. Sontheimer, I. Aharonovich, and O. Benson
Phys. Rev. Applied 11 (2019) 041001


Enlightening full-color displays

Researchers from the University of Strasbourg & CNRS (France), in collaboration with University College London (United Kingdom), and Humboldt University Berlin (Germany), have shown that a subtle combination of light-emitting semiconducting polymers and small photoswitchable molecules can be used to fabricate light-emitting organic transistors operating under optical remote control, paving the way to the next generation of multifunctional optoelectronic devices. These achievements have now been published in Nature Nanotechnology. Organic light-emitting transistors are widely recognized as key components in numerous optoelectronic applications. However, the integration of multiple functionalities into a single electronic device remains a grand challenge in this technological sector. Moreover, the next generation of displays requires to encode high-density visual information into single and ultra-small pixels. Now a team of researchers from Strasbourg, London, and Berlin has taken a big step forward by creating the first organic light-emitting transistor that can be remote-controlled by light itself. They have been blending a custom-designed molecule as a miniaturized optical switch with a light-emitting semiconducting polymer. Upon illumination with ultraviolet and visible light, the molecular switch reversibly changes its electronic properties. As a consequence, the electrical and optical response of the device can be modulated simultaneously by light, which serves as an optical remote control. However, having a device capable of producing only one color is not sufficient for daily-life applications, such as full-color displays. By choosing appropriate photoswitchable molecules and blending them with suitable light-emitting polymers, the researchers have demonstrated that this new type of organic light-emitting transistors can shine in the range of the three primary colors (red, green, and blue), thereby covering the entire visible spectrum. The disruptive potential of such approach was demonstrated by writing and erasing spatially defined emitting patterns (a letter for example) within a single device with a beam of laser light, allowing a non-invasive and mask-free process, with a response time on the microsecond scale and a spatial resolution of a few micrometers, thus outperforming the best “retina” displays. Clearly, these findings represent a major breakthrough that offers multiple perspectives for smart displays, active optical memories, and light-controlled logic circuits.

Optically switchable organic light-emitting transistors
L. Hou, X. Zhang, G.F. Cotella, G. Carnicella, M. Herder, B.M. Schmidt, M. Pätzel, S. Hecht, F. Cacialli, and P. Samorì
Nature Nanotechnology 14 (2019) 347

Hybrid Organic-Inorganic Perovskites: Promising Substrates for Single-Atom Catalysts

Mononuclear metal species are widespread in enzymes and homogeneous catalysts. When such isolated single metal atoms are placed on a solid surface, they can also play an important role in heterogeneous catalysis. In the past few years, great attention has been paid to single-atom catalysts, not only because they can exhibit superior catalytic performance, but also, because they offer a novel way of maximizing the efficiency of utilizing atoms, which is especially desirable in the use of scarce metal elements like platinum. However, single atoms cannot work in isolation but need to be dispersed on suitable substrates.
Qiang Fu and Claudia Draxl have recently demonstrated that hybrid organic-inorganic perovskites ˗ the emerging candidates in solar-cell applications ˗ are highly promising substrates for Pt single atom catalysts. Through systematic first-principles calculations, they found that single Pt atoms are stabilized on such substrates through a synergistic cooperation between covalent bond formation and charge transfer. The generated Pt sites possess excellent catalytic properties in CO oxidation and may be able to play a role in CO2 reduction. This work not only has promising consequences in single-atom catalysis but also sheds light on potential applications of hybrid perovskites as photocatalysts.
Hybrid Organic-Inorganic Perovskites as Promising Substrates for Pt Single-Atom Catalysts
Q. Fu and C. Draxl
Physical Review Letters 122 (2019) 046101


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 ( as an invited contribution to Prof. Caterina Cocchi, a member of IRIS Adlershof since 2017, to the special issue “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 T. Kamps
J. Phys.: Condens. Matter 31 (2019) 014002


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. 57 (2018) 14188


A “bullseye” antenna helps to read out a quantum sensor

An ideal platform to study the light-matter interaction at the fundamental level consists of single quantum emitters coupled to photonic and plasmonic elements. Such elements are also needed to realize quantum interfaces between stationary and flying quantum bits in quantum networks. Reaching the required nanometer precision for optimum coupling is still a challenge. Approaches for different scenarios have been developed. A very precise approach uses nanomanipulation with the help of atomic force microscopy (AFM) tips, the so-called pick-and-place approach. Here, single nanoparticles containing quantum emitters are transferred from substrate to substrate. The method is highly accurate and deterministic, and it also allows for pre-characterization of the luminescent particles. Moreover, the placement is not final, and several iterations can be performed by nanomanipulation if required. Finally, very different materials for the emitters or substrates (these may contain complex photonic structures like optical waveguides or microresonators) can be employed in order to assemble hybrid systems. A joint team of the Department of Physics and IRIS Adlershof of Humboldt-Universität zu Berlin and the Hebrew University, Jerusalem, now successfully presented a versatile technique allowing for high accuracy placement of a single quantum emitter an a plasmonic nanoantenna. The antenna operates by collecting light in a two-dimensional dielectric waveguide, which is then scattered into a well-defined narrow solid angle by concentric metallic (Ag) rings. Due to these rings such antennas are called bulleseye antennas. A key advantage of a plasmonic antenna is its broad bandwidth, i.e., even light from emitters with a rather wide fluorescence spectrum can be concentrated and directed with very high efficiency. Then, simple subsequent collection optics, even optical fibers, may collect more than 90% of all the emitted light.

AFM, confocal scan, and optical characterization of a placed nanodiamond containing a single nitrogen vacancy (NV) center. a) AFM scans of the placed nanodiamond in the center of the plasmonic bulleseye antenna. b) Measured normalized photon coincidences (g(2)-function) recorded under pulsed excitation with a repetition rate of 2.5MHz. The strongly reduced probability to find two photons after an excitation pulse (reduced peak height near zero time delay ) proves emission of single photons. c) Confocal scan of the antenna with the nanodiamond in the middle. d) Spectrum of the fluorescence from the NV (blue) and a dark field scattering spectrum of the antenna (orange) show a good overlap.

The quantum emitter was a single nitrogen-vacancy (NV) defect center in a nanodiamond. The NV center can be used a single photon source emitting at room temperature. On the other hand it hosts an electron spin state, which can be manipulated and read out optically. In this way nanomagnetometry on the level of single spins can be performed even at room temperature. Prof. Ronen Rapaport and Prof. Oliver Benson, who lead the research teams in Jerusalem and Berlin, respectively, point out: “The coupling of an NV center to a plasmonic antenna dramatically increases the efficiency of the device. This is crucial for its use as quantum light source, and even more for an application as magnetic field quantum sensor. Particularly for applications in biophysics or medicine room-temperature operation and fast non-invasive read out is crucial.” As next steps the researchers want to combine the NV quantum sensor, plasmonic light collecting structures and a microfluidic platform to develop reliable sensors for applications in biophysics.
Accurate placement of single nanoparticles on opaque conductive structures
N. Nikolay, N. Sadzak, A. Dohms, B. Lubotzky, H. Abudayyeh, R. Rapaport, and O. Benson
Appl. Phys. Lett. 113 (2018) 113107


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, and S. Hecht
Nature Chemistry 10 (2018) 1031


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 1 (2018) 516
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 4 (2018) 1479
DOI: 10.1016/j.chempr.2018.06.002


Longer lifetimes for perovskite absorbers

An international team of scientists has improved greatly the stability of organic-inorganic lead halide perovskites. These materials have enormous potential for photovoltaic applications but still suffer from comparably moderate device lifetime. The scientists, led by researchers from the EPFL, Lausanne, Switzerland, incorporated a large organic cation – guanidinium - into the perovskite crystal structure, in part replacing the traditionally used methylammonium and formamidinium cations. Overall, the new material delivered average power conversion efficiencies over 19%, and stabilized performance for 1,000 h under continuous light illumination. This is a fundamental step within the perovskite field. These groundbreaking research results were recently published in Nature Energy. Among the authors is the member of IRIS Adlershof, Prof. Norbert Koch.
Large guanidinium cation mixed with methylammonium in lead iodide perovskites for 19% efficient solar cells
A.D. Jodlowski, C. Roldán-Carmona, G. Grancini, M. Salado, M. Ralaiarisoa, S. Ahmad, N. Koch, L. Camacho, G. de Miguel, and M.K. Nazeeruddin
Nature Energy 2 (2017) 972


Beating the thermodynamic limit with photo-activation of n-doping in organic semiconductors using “hyper-reductants”

Image by Jing Wang and Xin Lin
Doping of semiconductors is a key process for controlling the materials’ charge carrier density, which directly impacts the electrical conductivity. Electronic and optoelectronic devices used in information, communication, energy conversion, and energy storage technologies rely on precise and efficient doping, i.e., the admixture of a small amount of a doping agent into the semiconductor. However, n-type doping of organic semiconductors – electron transfer from the dopant to the semiconductor – is notoriously difficult as the molecular dopants employed presently are highly sensitive to ambient exposure, upon which they react with water and oxygen and are rendered inactive.
In an article that just appeared in Nature Materials, a team of researchers from the Georgia Institute of Technology, the Helmholtz-Zentrum Berlin, Humboldt-Universität zu Berlin, and Princeton University demonstrates a new approach towards n-doping of organic semiconductors, which allows bypassing the dopant sensitivity to the ambient and simultaneously enables doping organic electron transport materials that have been out of reach for n-doping so far. The first step of innovation lies in chemically connecting two organometallic molecular dopants in a dimer that is stable even in air, with reduced ability to dope organic electron transport semiconductors. Consequently, when mixing these into the organic semiconductor, nothing happens at first. The revolutionary step now involves illuminating the mixture with light. A dimer and a semiconductor molecule in immediate proximity absorb a photon, the dimer can dissociate and unfold the full doping power of each dopant in a multi-step process. “By this optical activation of dopants, we could enhance the conductivity of organic electron transport materials by five orders of magnitude. This boosts the efficiency of organic light emitting diodes and solar cells, using rather simple and technologically relevant processing.” says Prof. Antoine Kahn from Princeton University, who coordinated the project. The choice of the article’s title is explained by Prof. Seth Marder from Georgia Tech: “This doping is actually beyond the thermodynamic limit of what the dopant should be able to do, thus once the light is turned off one might naively expect the reverse reaction to occur (rapidly, within seconds perhaps) and the conductivity increase to disappear. However, this is not the case. The reason for this is that the doping process involves multiple steps, and the back-reaction to the starting system involves many uphill intermediate steps creating a kinetic barrier, thus the reverse reaction is extremely slow.” Indeed, no indications of a loss in conductivity upon light-activation after hundreds of hours were found. For these reasons, the compounds are referred to as “hyper-reductants”. The fact that the team demonstrated the beneficial effect of their doped electron transport semiconductors in highly efficient light emitting diodes underlines the huge potential of this approach in device applications. “We believe that our work enables simple processing of n-doped organic semiconductors in numerous device architectures, where the critical step - doping activation - can take place after standard device encapsulation. This will contribute substantially to improved device lifetime and in some case simplify device fabrication.” notes Prof. Norbert Koch from Humboldt-Universität, member of IRIS Adlershof. The work was part of a project within the strategic partnership program of Princeton University and Humboldt-Universität.
Beating the thermodynamic limit with photo-activation of n-doping in organic semiconductors using “hyper-reductants”
X. Lin,  B. Wegner,  K.M. Lee,  M.A. Fusella,  F. Zhang, K. Moudgil , B.P. Rand, S. Barlow, S.R. Marder, N. Koch, A. Kahn
Nature Materials 16 (2017)1209


Spiro-Bridged Ladder-Type Oligo(para-phenylene)s:
Fine Tuning Solid State Structure and Optical Properties

In this recent research highlight the authors developed synthetic routes that allow to subsequently replace every pair of symmetry-equivalent alkyl groups in ladder-type quaterphenyl by a spiro-bifluorene group. With an increasing number of spiro groups, the optical gap for absorption and emission slightly decreases, which is disadvantageous with respect to resonant energy transfer with ZnO. Thus, a synthetic route to a para-linked ladder-type quaterphenyl carrying all bridging units on one side of the ribbon was developed, which results in an in-plane bending of the para-phenylene. The optival gap increased compared to the linear molecule, however, the absorption coefficient slightly decreased.

The authors analyzed the influence of different deposition techniques on the solid state structure by X-ray diffraction of single crystals obtained by crystallization from solution as well as sublimation. In the cases of L4P-sp2 and L4P-sp3, it could even be shown that sublimation and crystallization from solution result in different crystal structures, of which the ones from sublimation are obviously more relevant in view of the typically employed vacuum deposition and might be advantageous in terms of application in light-emitting devices.

An increasing number of spiro-bifluorene substituents was found to aid thin-film formation on oxide surfaces, such that the optical properties could be preserved in pure, nondiluted thin films.

Finally, promising spiro-L4P derivatives have been employed in energy-transfer devices, for which highly efficient energy transfer from an inorganic quantum well to the organic layer followed by efficient light emission could successfully be demonstrated.

Asymmetric units (left) and arrangement of molecules in the crystal (right) of different molecule types. Thermal ellipsoids drawn at 50% probability level, cell edges marked in a: red, b: blue and c: green. For more details please see the publication text.

Absorption (solid lines) and normalized PL (dotted) spectra of final products 10-6 - 10-5 mol L-1 in CH2Cl2. For more details please see the publication text.

Spiro-Bridged Ladder-Type Oligo(para-phenylene)s: Fine Tuning Solid State Structure and Optical Properties
B. Kobin, J. Schwarz, B. Braun-Cula, M. Eyer, A. Zykov, S. Kowarik, S. Blumstengel, and S. Hecht
Adv. Funct. Mater. 2017, 1704077 (2017)

Water makes the proton shake -
Ultrafast motions and fleeting geometries in proton hydration

Basic processes in chemistry and biology involve protons in a water environment. Water structures accommodating protons and their motions have so far remained elusive. Applying ultrafast vibrational spectroscopy, Dahms et al. map fluctuating proton transfer motions and provide direct evidence that protons in liquid water are predominantly shared by two water molecules. Femtosecond proton elongations within a hydration site are 10 to 50 times faster than proton hopping to a new site, the elementary proton transfer step in chemistry.

The proton, the positively charged nucleus H+ of a hydrogen atom and smallest chemical species, is a key player in chemistry and biology. Acids release protons into a liquid water environment where they are highly mobile and dominate the transport of electric charge. In biology, the gradient of proton concentration across cell membranes is the mechanism driving the respiration and energy storage of cells. Even after decades of research, however, the molecular geometries in which protons are accommodated in water, and the elementary steps of proton dynamics have remained highly controversial.

Protons in water are commonly described with the help of two limiting structures (Fig. 1A). In the Eigen complex (H9O4+) (left), the proton is part of the central H3O++ ion surrounded by three water molecules. In the Zundel cation (H5O2+) (right), the proton forms strong hydrogen bonds with two flanking water molecules. A description at the molecular level employs the potential energy surface of the proton (Fig. 1B) which is markedly different for the two limiting geometries. As shown in Fig. 1B, one expects an anharmonic single-minimum potential for the Eigen species and a double minimum potential for the Zundel species. In liquid water, such potentials are highly dynamic in nature and undergo very fast fluctuations due to thermal motions of surrounding water molecules and the proton.

Led by Thomas Elsässer, member of  IRIS Adlershof, researchers from the Max Born Institute in Berlin, Germany, and the Ben Gurion University of the Negev in Beer-Sheva, Israel, have now elucidated the ultrafast motions and structural characteristics of protons in water under ambient conditions. They report experimental and theoretical results in Science which identify the Zundel cation as a predominant species in liquid water. The femtosecond (1 fs = 10-15 s) dynamics of proton motions were mapped via vibrational transitions between proton quantum states (red and blue arrows in Fig. 1B). The sophisticated method of two-dimensional vibrational spectroscopy provides the yellow-red and blue contours in Fig. 2A which mark the energy range covered by the two transitions. The blue contour occurs at higher detection frequencies than the red, giving the first direct evidence for the double-minimum character of the proton potential in the native aqueous environment. In contrast, the blue contour is expected to appear at smaller detection frequencies than the red one.

The orientation of the two contours parallel to the vertical frequency axis demonstrates that the two vibrational transitions explore a huge frequency range within less than 100 fs, a hallmark of ultrafast modulations of the shape of proton potential. In other words, the proton explores all locations between the two water molecules within less than 100 fs and very quickly loses the memory of where it has been before. The modulation of the proton potential is caused by the strong electric field imposed by the water molecules in the environment. Their fast thermal motion results in strong field fluctuations and, thus, potential energy modulations on a sub-100 fs time scale. This picture is supported by benchmark experiments with Zundel cations selectively prepared in another solvent and by detailed theoretical simulations of proton dynamics (Fig. 2B).

A specific Zundel cation in water transforms into new proton accommodating geometries by the breaking and reformation of hydrogen bonds. Such processes are much slower than the dithering proton motion and occur on a time scale of a few picoseconds. This new picture of proton dynamics is highly relevant for proton transport by the infamous von Grotthuss mechanism, and for proton translocation mechanisms in biological systems.

Figure 1: Chemical structure of hydrated protons in liquid water.

A Schematic of the Eigen cation H9O4+ (left) and the Zundel cation H5O2++ (right). The arrows indicate the O-H bond coordinate r and the (O...H+...O) proton transfer coordinate z. In the Eigen cation a covalent O-H bond localizes the proton whereas in the Zundel cation the proton is delocalized between two water molecules.
B Anharmonic vibrational potential (left) and double minimum potential of the Zundel cation along z (right, red. Distortions by the solvent surrounding impose a modulation of the double minimum potential (right, dotted line). Red and blue arrows indicate transitions between particular quantum states of the proton motion , i.e., the ground-state-to-first-excited-state transition (red) and the first-excited-state-to-second-excited-state transition (blue). The modulation of the potentials leads to spectral shifts of the vibrational transitions which are mapped by two-dimensional infrared spectroscopy.

Fig 2: Femtosecond dynamics of proton motions (1 fs = 10-15 s).

A Two-dimensional vibrational spectra with the ground-state-to-first-excited-state transition (red) at lower detection frequency than the first-excited-state-to-second-excited-state transition (blue). The orientation of both contours parallel to the excitation frequency axis is due to ultrafast frequency fluctuations and the loss of memory in the proton position.
B Simulated real-time dynamics of the proton motions in the Zundel cation. Within less than 100 fs, the proton displays large amplitude excursions along z, the coordinate linking the two water molecules in the Zundel cation. Due to the ultrafast modulation of the shape of proton potential by surrounding solvent molecules, the proton explores all locations between the two water molecules.

Fig 3: Cartoon picture of proton hydration dynamics, visualized with the help of classical physics.

The proton Smiley is sitting in the middle of a sofa with two seats. When shaking the sofa with a mechanical force, the shape of the seating changes and the proton moves forth and back on the sofa. Such motions occur on a time scale shorter than 100 fs (10-13 s). After an average time of 1 ps = 1000 fs = 10-12 s, the sofa breaks and the proton moves to a new site/sofa, including the red halve on the right.

Large-amplitude transfer motion of hydrated excess protons mapped by ultrafast 2D IR spectroscopy
F. Dahms, B.P. Fingerhut, E.T.J. Nibbering, E., and T. Elsaesser
Science, 357 (2017) 491
DOI: 10.1126/science.aan5144

X-ray "movie" provides insights into the formation of molecular layers

Thin-film technologies that promise control on the atomic and molecular scale have attracted increasing interest in recent years as traditional manufacturing processes reach their fundamental limits. A team from the Department of Physics at the Humboldt-Universität zu Berlin, led by Anton Zykov, Stefan Kowarik and Jürgen P. Rabe (member of  IRIS Adlershof) in collaboration with colleagues from the PETRA III Synchrotron at DESY Hamburg has now studied the non-equilibrium growth of molecular layers using innovative, time-resolved X-ray scattering. The movie sequence of the X-ray scattering during the molecular beam deposition was chosen as the cover image of a special topic issue of the Journal of Chemical Physics on "Atomic and molecular layer processing".

Semiconducting organic molecules have significant potential for future applications such as organic light-emitting diodes (OLED), camera sensors or memory devices. Many of these components are based on ultra-thin layers of functional molecular materials. Their preparation by deposition of molecules from the gas phase is a complex process involving molecular adsorption on a substrate, molecular diffusion and self-assembly. Since many of these processes do not proceed under conditions of local thermodynamic equilibrium, these processes and their velocities are still not well understood.
By means of innovative X-ray measurements of diffuse scattering at the P03 Beamline of the PETRA III synchrotron, the researchers were able to record "movies" of the growth processes on the nanoscale. The measurement makes it possible to follow the nucleation, island growth and the roughness evolution of the layer. The researchers show that the results of the new X-ray technique are consistent with established scanning probe techniques and time-resolved measurements are possible without disturbing the growth. In the study, a significant improvement in the diffusivity of molecules between the first and the subsequent molecular layers was found and the nucleation energy was determined within the framework of recent growth theories. The application of the new X-ray scattering technique will help to take our understanding beyond a recipe-based perspective to that of sound fundamental understanding of molecular growth.

Diffusion and nucleation in multilayer growth of PTCDI-C8 studied with in situ X-ray growth oscillations and real-time small angle X-ray scattering
A. Zykov, S. Bommel, C. Wolf, L. Pithan, C. Weber, P. Beyer, G. Santoro, J.P. Rabe, and S.Kowarik
J. Chem. Phys. 146, 052803 (2017)

GLAD makes new organic memory devices possible

Giovanni Ligorio, Marco Vittorio Nardi, and Norbert Koch, member of IRIS Adlershof, have invented a new technique for constructing novel memory devices. The results have now been published in Nano Letters.

Author Dr. Giovanni Ligorio explains: “Novel non-volatile memory devices are currently investigated to overcome the limitation of traditional memory technologies. New materials such as organic semiconductors and new architectures are now considered to address high-density, high-speed, low-fabrication costs and low power-consumption.
Usually nano-devices (traditionally based on inorganic semiconductors) are fabricated via lithography techniques. Here, we show the fabrication of devices with nanometric footprint using a different technique: Glancing Angle Deposition (GLAD).
This technique allows the tailoring of nanostructured morphologies through physical vapor deposition (CVD) via controlling the substrate orientation with respect to the vapor source direction. When thin films are deposited onto stationary substrates under condition of oblique deposition, meaning that the vapor flux is non-perpendicular to the substrate surface, an inclined columnar nanostructured is produced.

(a) Herstellung der Nanosäulen via CVD (b) AFM-Aufnahme der Säulen-columns (c) Skizze der Ansteuerung (d) Skizze eine Säule mit Filament ©G.Ligorio
Upon proper bias applied between the two electrodes of the memory device, it is possible to form a conductive path (or filament). The filament shorts the electrodes and drastically changes the resistivity characteristic of the device. Forcing a high current in the device, the filament can be distrust. This programs the device in the original high resistivity state. Since the process can be repeated consecutively we can program the device in a high or low resistive state (i.e. ON or OFF).
We aim for the fabrication of devices in structured arrays (in this publication the nano devices are not ordered in array, but they are randomly distributed.) This allows for connecting via cross bar electrodes, which can be fabricated via printing.
This allows fabricating memory devices with a density of roughly 1 GB/cm² employing novel material for electronics, i.e. organic semiconductors.”
Lithography-Free Miniaturization of Resistive Nonvolatile Memory Devices to the 100 nm Scale by Glancing Angle Deposition
G. Ligorio, M. Vittorio Nardi, and N. Koch
Nano Lett. 17 (2017) 1149