A rod of light in the light cage – a new basis for quantum-optically integrated light-matter interaction on a chip
In the rapidly growing field of hybrid quantum photonics, the realization of miniaturized, integrated quantum-optical systems with intense light-matter interaction is of great importance for both fundamental and applied research. An international team around IRIS member Oliver Benson and Flavie Davidson-Marquis, the first IRIS Adlershof fellowship holder, has integrated a novel on-chip hollow-core light cage into an alkali atom vapor cell.
Implemented by 3D nanoprinting, the compact light cage on a chip exhibits several advantages compared to other hollow-core waveguide structures. Whereas hollow-core fibers suffer from inefficient vapor-filling times — exceeding months for few centimeter-long devices — the design of the light cage allows for high-speed gas diffusion through side-wise direct access to the hollow core within minutes (Figure a).
Coherent interaction of cesium atoms with a beam of light confined in an on-chip light cage.
a, Atoms from alkali vapors entering the light cage from all sides and interacting with the light of the central core mode. Vertical supports connect the light cage to a silicon chip. Reinforcement rings stabilize the freely suspended strand array.
b, The on-chip light cage represents a novel photonic platform for integrated quantum optics as demonstrated in this work by measuring electromagnetically-induced transparency (EIT) visible by the strong transmission on the cesium D1 transition.
The research team observed coherent interaction between a rod of light in the light cage with room-temperature cesium atoms by the effect of electromagnetically induced transparency (Figure b) and investigated the influence of the light cage on the light-matter interaction for the first time. “The results represent a major step forward in hybrid integration of designer laser-written structures and atom cells. The freedom to produce three-dimensional structures–nearly without restrictions on the silicon-technology platform–will allow us to combine light-matter interaction in the light cage with other Si-chip-compatible devices, i.e. lithium niobate waveguides for modulation and frequency conversion of light, as well as direct mode coupling from and to optical fibers. Extensive control of the single-photon properties in a quantum network could ultimately be applied all on one chip”, explains corresponding author Tim Kroh.
Coherent interaction of atoms with a beam of light confined in a light cage
F. Davidson-Marquis, J. Gargiulo, E. Gómez-López, B. Jang, T. Kroh, C. Müller, M. Ziegler, S.A. Maier, H. Kübler, M.A. Schmidt, and O. Benson
Light: Science and Applications 10 (2021) 114, DOI: doi.org/10.1038/s41377-021-00556-z
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
Calculated self-interference of a single nanoparticle
placed on a mirror substrate with a silica layer as the
spacer. (i), (ii) and (iii) show different cuts through the
far-field patterns of oriented dipoles oscillating along
the x,y and z-axis, respecitvely
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 andIRIS 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.
Simulated (topmost two rows) and experimental (bottommost two rows) far-field self-interference emission patterns. The particle- to-mirror distance in- creases from the left to the right column from 72nm to 327nm. All scale bars are 500 nm.
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.
The OLEDs incorporating the OrelTech
ink illuminating under strain.
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
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
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
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."
