SCIENTIFIC HIGHLIGHTS

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. (2019) published online

 

 

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




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

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

This work was published on The Journal of Physics: Condensed Matter (http://iopscience.iop.org/article/10.1088/1361-648X/aaedee) as an invited contribution to Prof. Caterina Cocchi, a member of IRIS Adlershof since 2017, to the special issue “Emerging leaders 2018” (http://iopscience.iop.org/journal/0953-8984/page/Emerging-leaders-2018).
 
First-principles many-body study of the electronic and optical properties of CsK2Sb, a semiconducting material for ultra-bright electron sources
C. Cocchi, S. Mistry, M. Schmeißer, J. Kühn, and T. Kamps
J. Phys.: Condens. Matter 31 (2019) 014002


 

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


 

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


 


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