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.
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).
Nano-Optik, Institut für Physik und IRIS Adlershof der Humboldt-Universität zu Berlin
Newtonstraße 15, 12489 Berlin
030 2093 4711 oliver.bensonphysik.hu-berlin.de
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 . 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 . 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.
 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
 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) . 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.  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.
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
Printed perovskite LEDs – an innovative technique towards a new standard process of electronics manufacturing
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
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.
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
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.
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)].
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
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
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.  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
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.
"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