Sparking electroluminescence in poly(triazine imide) films

A team of researchers from King’s College London, Humboldt-Universität zu Berlin, Carl von Ossietzky Universität Oldenburg, and Helmholtz-Zentrum Berlin (HZB) have investigated the synthesis, structure, optical properties of poly(triazine imide), a member of the family of graphitic carbon nitrides. Their progress on material quality and processing allowed for construction of the first single layer, organic light emitting device (OLED) with a solution-processed graphitic organic material as a metal-free emission layer.
Organic semiconductors have sparked great interest in academic and industrial circles over the last decades, because of their advantageous properties such as (i) a high absorption coefficient compared to conventionally used silicon as well as (ii) less energy intensive production, and (iii) composition from earth-abundant elements. Progress in this field of research promises new, cost- and energy-efficient technologies in consumer electronics, smart packaging, and flexible light-emitters.
Hitherto explored organic semiconductors often suffer from degradation processes and defects especially when electrochemically altered (“doped”), due to dopant drift and migration or due to oxidation when exposed to atmospheric conditions. The unique properties of poly(triazine imide) enable the research to address the issues that plague conventional organic semiconductors. Poly(triazine imide) is a very stable under heat and air. Furthermore, the graphitic morphology of poly(triazine imide) allows exfoliation of the material into thin, solution-processable layers, while at the same time reducing migration and drift of chemically bonded dopants.
“With the improved material quality, we are now able to dive deeper into the more delicate features of this material, such as the electronic structure and vibration modes. This will greatly improve our understanding of this material, as well as related materials, and help us improving OLED performance and think about future, high-value applications of poly(triazine imide).”, says David Burmeister, PhD student at IRIS Adlershof member Michael J. Bojdys.

Optimized synthesis of solution-processable crystalline poly(triazine imide) with minimized defects for OLED application
D. Burmeister, H.A. Tran, J. Müller, M. Guerrini, C. Cocchi, J. Plaickner, Z. Kochovski, E. List-Kratochvil, M. Bojdys
Angew. Chem. Int. Ed. 2021.
DOI: 10.1002/anie.202111749

 

Shaping 2D materials with small molecules

Electronic properties of 2D materials such as graphene and transition metal chalcogenides can be tailored by shaping their topography at the nanoscale. At IRIS Adlershof, Abdul Rauf and colleagues from the RabeLab together with Igor Sokolov investigated how to shape surfaces and interfaces of 2D materials with small molecules, intercalating at the interfaces between the 2D materials and a solid substrate. Particularly, they investigated wetting of interfaces between graphene and a hydrophilic substrate, mica, with two small molecules, water and ethanol. Wetting with water leads to labyrinthine structures exhibiting branch widths down to the 10 nm scale. This is explained by a process leading to an equilibrium between electrostatic repulsion of the polar molecules preferentially oriented at the interface, and the line tension between wetted and non-wetted areas. Increasing line tension or decreasing dipole density increases the branch width, causing eventually non-structured wetting layers. The method might be used to shape 2D materials to tailor their electronic properties.

  Scanning force microscopy images of graphene surfaces shaped by an intercalating molecularly thin water layer self-assembled into labyrinthine patterns (top left), and the compact wetting front of an ethanol layer (top right). The snapshots of Molecular Dynamics simulations of the interfaces filled with molecules (bottom) helped to understand the origin of the forces driving the pattern formation. 

Shaping surfaces and interfaces of 2D materials on mica with intercalating water and ethanol
A. Rauf, J. D. Cojal González, A. Balkan, N. Severin, I. M. Sokolov, and J. P. Rabe
Molecular Physics, 119:15-16, ^OPENACCESS
DOI: 10.1080/00268976.2021.1947534

 

Fishing with Light

Molecules are usually optimized for a task by trial and error. Time-consuming iterative rounds of synthesis and characterization provide detailed insight into structure-property relationships. In order to speed up this tedious process, Niklas König and Dragos Mutruc from the HechtLab have developed a clever means to generate an equilibrating mixture, a so called dynamic constitutional library, of photoswitchable molecules and used their wavelength-specific response to select the proper candidate. Thus, they could “fish” the desired switch with light in a pool of many different switches. The method should facilitate the rapid exploration of structural diversity in functional dye chemistry.
 

Accelerated Discovery of α-Cyanodiarylethene Photoswitches
N. F. König, D. Mutruc, and S. Hecht
Journal of the American Chemical Society 143 24 (2021) 9162,
DOI: 10.1021/jacs.1c03631F 

 

Lichtblick für die Quantenforschung
HU-Forschungsteam und Partner haben erstmals die Teilchenaustauschphase von Photonen direkt gemessen

Dieses Experiment liefert den direkten Beleg für ein erstaunliches Quantenphänomen, das nur bei völlig gleichartigen Quantenobjekten beobachtet wird. Damit kommt die Quantenforschung einen wichtigen Schritt voran.

Die Teilchen, denen das Forscherteam auf der Spur ist, sind schwer zu fassen. Die Physiker untersuchen die Quantenteilchen der elektromagnetischen Wellen, auch Photonen genannt, aus denen Licht besteht. Photonen lassen sich nur dann unterscheiden, wenn sie unterschiedliche Wellenlängen haben, in unterschiedlichen Richtungen schwingen oder sich an verschiedenen Punkten in Raum und Zeit befinden.

„Wenn zwei in Wellenlänge und Schwingungsrichtung ununterscheidbare Photonen aufeinandertreffen und sich wieder trennen, haben sie gewissermaßen ihre Identität verloren“, erläutert Kurt Busch. „Man stelle sich vor, wir schicken zwei Zwillinge durch zwei Türen in einen gemeinsamen Raum. Wenn Sie wieder hinaustreten, können wir nicht feststellen, ob sie dazu jeweils dieselbe Tür benutzt haben oder nicht“, ergänzt Oliver Benson, Mitglied von IRIS Adlershof. In der Quantenmechanik passiert dennoch etwas. Laut dem sogenannten Symmetrisierungspostulat gibt es zwei Kategorien von Elementarteilchen: Bosonen und Fermionen. Diese Arten von Teilchen unterscheiden sich dahingehend, was passiert, wenn man sie miteinander vertauscht.

 

Abbildung 1: Konzeptionelle Skizze des Interferometeraufbaus: a Ein verschränktes Photonenpaar (roter Strahl) wird in das Interferometer geleitet, welches zwei unterschiedliche Möglichkeiten am zentralen polarisierenden Strahlteiler (PBS) produziert, wie in b gezeigt: Entweder das Photon in Pfad 1 wird transmittiert und das Photon in Pfad 2 wird reflektiert oder genau umgekehrt. Die Quantensuperposition dieser Szenarien führt zu der Interferenz zwischen Zuständen, die physikalisch vertauschte Versionen voneinander sind, und offenbart die Teilchenaustauschphase ϕ_x. Der blaue Strahl wird von einem abgeschwächten Laser erzeugt und dient als Referenzsignal um die effektiven optischen Pfadlängenunterschiede, ϕ_1 und ϕ_2, zu bestimmen.

Im Beispiel hieße das, wenn jeder der Zwillinge den Raum aus der jeweils anderen Tür wieder verlässt. Bei Bosonen ändert sich nichts – bei Fermionen erhält die quantenmechanische Wellenfunktion, die die Teilchen beschreibt, einen Phasenschub, der auch Austauschphase genannt wird. „Im Zwillingsbeispiel kann man sich das vielleicht so vorstellen: Schicken wir die beiden Zwillinge im Gleichschritt in den Raum und kommen sie aus verschiedenen Türen wieder heraus, so sind sie weiterhin im Gleichschritt. Als Bosonen treten die Zwillinge mit demselben Bein voran aus dem Raum heraus, mit dem sie auch zuerst in Raum geschritten sind. Jedoch benötigen sie als Fermionen beide einen Schritt mehr und gehen beim Verlassen des Raumes nun mit dem anderen Bein voran“, so Benson. „Dass Photonen bosonisch sind, konnte bislang nur durch indirekte Messungen und mathematische Berechnungen gezeigt werden“, sagt Kurt Busch. „In unserem jüngsten Experiment haben wir die Teilchenaustauschphase von Photonen erstmals direkt gemessen und haben damit einen direkten Beleg für ihren bosonischen Charakter erbracht.“

Um die Austauschsymmetrie eines Zustandes für zwei identische Teilchen direkt nachzuweisen, hat das Team eine optische Apparatur mit einem Interferometer aufgebaut. Herzstück des Aufbaus – in der Größe eines kleinen Tisches – sind zwei Strahlteiler. Zwei Photonen wurden dann in das Interferometer geschickt und durch den Strahlteiler auf zwei verschiedene Wege geführt. Entlang einem der beiden Wege werden die Photonen miteinander vertauscht, während sie auf dem anderen unverändert bleiben. Am Ausgang des Interferometers wurden dann beide Photonen am zweiten Strahlteiler wieder überlagert. „Je nachdem, ob die Photonen bosonisch oder fermionisch sind, sind dann die beiden Photonen im Gleichschritt und verstärken sich oder sie sind außer Tritt und löschen sich aus“, erläutern die Physiker.

Zukünftige Verbesserungen des Interferometers werden ein neues Werkzeug für Präzisionsmessungen mit Quantenlicht bereitstellen. Gleichzeitig etabliert das Experiment eine neue Methode zur Erzeugung und Zertifizierung von Quanten-Zuständen von Licht. Dies ist sehr wichtig im neuen Gebiet der Quanteninformationsverarbeitung, auf deren Basis derzeit neuartige, wesentlich leistungsfähigere Computer entwickelt werden.


Direct observation of the particle exchange phase of photons
K. Tschernig, C. Müller, M. Smoor, T. Kroh, J. Wolters, O. Benson, K. Busch, and A. Perez-Leija
Nat. Photonics (2021), DOI: 10.1038/s41566-021-00818-7

 

 
Real-time optical distance sensing of up-conversion nanoparticles with a precision of 2.8 nanometers
  Sub-diffraction limited localization of fluorescent emitters is a major goal of microscopy imaging. It is of key importance for so-called super-resolution, a technique that was awarded the Nobel Prize in Chemistry in 2014. A cooperation of researchers in Australia, China, the USA and IRIS Adlershof have now demonstrated ultra-precise localization and tracking of fluorescent nanoparticles dispersed on a mirror. The many randomly oriented molecular dipoles in such up-conversion nanoparticles (UCNPs) interfere with their own mirror images and create unique, bright and position-sensitive patterns in the spatial domain.

The pattern can be detected in the far-field by a sensitive camera and was compared to a detailed and quantitative numerical simulation. In this way it was possible to localize individual particles with an accuracy of only 2.8 nm, a value which is smaller than 1/350 of the excitation wavelength.

The localization can be performed rapidly, and a single particle can be followed with a 50Hz frame rate. This is much faster than other self-interference-based methods based on mapping of the fluorescence spectrum. A special benefit of UCNPs is their high photo-stability and sensitivity, e.g. to temperature and PH. Therefore, the novel technique may be used for high-resolution multimodality single-particle tracking and sensing.

Axial Localization and Tracking of Self-interference Nanoparticles by Lateral Point Spread Functions
Y. Liu, Z. Zhou, F. Wang, G. Kewes, S. Wen, S. Burger, M. Ebrahimi Wakiani, P. Xi, J. Yang, X. Yang, O. Benson, and D. Jin
Nat. Commun. 12 (2021) 2019, DOI: 10.1038/s41467-021-22283-0

 

Inkjet-printed electrodes in OLEDs

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 successfully implemented an ink produced by the Berlin-based company OrelTech in solution-processed organic light emitting diodes.

After inkjet printing the particle-free silver ink, an argon plasma is used to reduce the silver ions in the ink to metallic silver. “Because this process takes place at a low temperature, it is suitable for use with temperature-sensitive substrates, such as flexible plastic foils,” explains Dr. Konstantin Livanov, co-founder and CTO of OrelTech. The researchers fabricated organic light-emitting diodes employing the silver ink as a transparent conductive electrode on the flexible substrate PET. The resulting devices show comparable light output characteristics to those based on the otherwise widely used indium tin oxide (ITO). Crucially, however, the silver electrodes showed superior stability to ITO upon mechanical bending. Dr. Felix Hermerschmidt, senior researcher in the joint lab of HU and HZB, confirms, "The OLEDs based on the OrelTech ink remain intact at a bending radius at which the OLEDs based on ITO show breakage and fail.” This opens up several application opportunities of the printed devices. The work has been published in the journal Flexible and Printed Electronics and is available Open Access. GenFab, led by Prof. List-Kratochvil, who is a memnber of IRIS Adlershof, is moving into laboratories and offices in the new IRIS research building for further research and development work.

ITO-free OLEDs utilizing inkjet-printed and low temperature plasma-sintered Ag electrodes,
M. Hengge, K. Livanov, N. Zamoshchik, F. Hermerschmidt, and E..J. W. List-Kratochvil
Flex. Print. Electron. 6 (2021) 015009, DOI: 10.1088/2058-8585/abe604

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