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

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

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

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


 
GLAD makes new organic memory devices possible

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

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


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


 
Color Duality in Photons

Conventionally, quantum frequency conversion is used to as fully as possible transfer photons (coherently) from one frequency to another. This is for different reasons, e.g. to improve detection efficiencies or to convert photons used for quantum communication to the telecommunication band for lowest loss transmission through optical fibers.

But what happens if one stops exactly “half-way” in the conversion process? Quantum mechanically one ends up with a color-super-position state of a single photon being simultaneously at two different color (frequency) states. This is exciting and interesting. But how to proof one really has a proper (coherent) quantum-superposition state? And is this maybe even useful for something?


APS/Alan Stonebraker
The illustration shows the conversion of a photon of one frequency, or color, into a photon that is in a quantum superposition of two colors, and the subsequent verification of this super-positions coherence with Ramsey spectroscopy.

For other quantum systems (like spins of single electrons, single atoms/ions…) such energy superposition states between a ground and excited (electronic) state very are well known. One way to produce them is to apply a so called Pi/2-pulse. By then letting the system freely evolve (and pick up a phase) and subsequently applying another Pi/2-pulse one realizes a so called Ramsey Interferometer. And this type of quantum interferometer is very widely used for a plethora of applications.
So, to demonstrate that one indeed can generate single photons in a genuine quantum super-position of two colors and that this might even be useful for something, the goal was to for the first time realize Ramsey Interference with single photons. The technical challenge of this is, that the quantum frequency conversion has to work so well, that one can cascade two conversion processes and implement a controllable phase in between. And this is exactly what we did.

As a side-remark, in all “traditional” Ramsey Interferometers there is always the possibility that the quantum superposition of the ground and excited state spontaneously decays into the ground state by coupling to the electro-magnetic (vacuum) field. For photons, which are themselves the quanta of the electro-magnetic field, no such decay channel (at least in vacuum) exist, making it fundamentally special.

The paper “Ramsey Interference with Single Photons”[1] and accompanying Viewpoint “Photon Qubit is Made of Two Colors” [2] have been selected as one of the Highlights of the Year 2016 by APS Physics. It was co-authored by Dr. Sven Ramelow, who recently started his Emmy-Noether-Group at the Institute for Physics, Humboldt-University Berlin, and is associated with IRIS Adlershof. While there have numerous highly interesting papers in Physical Review Letters in 2016, APS Physics explains their selection, writing: “It’s no surprise that LIGO’s discovery of gravitational waves tops our list of favorite Physics stories in 2016. The other slots went to research that marked a change in perspective, demonstrated an impressive experimental feat, or simply made us think.”
Incidentally, Dr. Sven Ramelow is working on follow-up ideas of this paper and the corresponding experiments, which he looks forward to soon being implemented at IRIS-Adlershof and the HU Institute for Physics and yielding new intriguing results.


APS/Alan Stonebraker
  [1] Ramsey Interference with Single Photons
S. Clemmen, A. Farsi, S. Ramelow, A.L. Gaeta
Phys. Rev. Lett. 117, 223601 (2016)

[2] Viewpoint: Photon Qubit is Made of Two Colors
P. Treutlein

Physics 9, 135 (2016)


 
Light controls repair of materials

A team of German researchers led by chemists of the Humboldt-Universität zu Berlin has developed a new type of plastic coating, which can heal damages selectively by illumination with light. A heat-induced repair of the material occurs where the damaged area has previously been illuminated with light of a specific color. The promising results of this work have now been published in Nature Communications.



To avoid the environmentally unfriendly as well as expensive replacement of damaged consumer products and constructions, researchers have recently been focusing their efforts on the development of smart materials able to self-repair scratches or cracks. Especially plastic coatings, which are repaired by heat, have yielded promising results in the past. Once subjected to heat, a chemical reaction induces melting and thus enables a homogeneous and complete mending. Upon cooling, the plastic re-establishes its original chemical structure as well as mechanical properties: It hardens and becomes robust again. However, the thermal stress during the healing procedure affects the overall material properties and eventually leads to degradation when applied repeatedly.

To bypass this problem, German researchers from the Humboldt-Universität zu Berlin, the Friedrich-Schiller-University in Jena, the Federal Institute for Materials Research and Testing in Berlin as well as the Helmholtz-Zentrum Geesthacht in Teltow have now developed a smart plastic coating, in which light focusses the thermal healing process to the damaged locations only, without affecting the non-damaged parts.

 

“We aimed to protect intact parts of coatings from degradation.”, says lead researcher Stefan Hecht and adds: “By employing light as stimulus, we now have a true remote control to switch the ability to self-repair ‘on’ or ‘off’ on demand.” Shining light on damaged areas of the coating enables the self-repairing function. This process can be reversed by changing the color of the employed light yielding the original material – but in the healed state.

This seminal development is an important step to future applications in consumer products where light as a remote control facilitates external control over properties of smart materials. This could include the use as latent resists carried through various processing steps in nanofabrication or 3D printing.

Conditional repair by locally switching the thermal healing capability of dynamic covalent polymers with light
A. Fuhrmann, R. Göstl, R. Wendt, J. Kötteritzsch, M.D. Hager, U.S. Schubert, K. Brademann-Jock, A.F. Thünemann, U. Nöchel, M. Behl und S. Hecht
Nature Communications  (2016), published online
DOI: 10.1038/ncomms13623

 
Tapping the sun

A team of researchers from IRIS Adlershof of the Humboldt-Universität zu Berlin and Technische Universiteit Eindhoven in the Netherlands have developed thin plastic films, which continuously move upon exposure to sunlight. These materials are able to convert the sunlight’s energy directly into motion and have great promise for the development of sun-driven active coatings and surfaces, for example self-cleaning windows. These results have been published in Nature Communications.

In order to harvest and utilize the sun’s energy, alternative strategies to circumvent issues with energy storage and directly convert it into mechanical work have been developed over the years. A promising approach has been the design of light-driven molecular systems and machines; however, the collection of the individual molecules’ response and subsequent amplification to macroscopic motion and mechanical work has proven difficult. Furthermore, previous systems required the use of intense high-energy UV light and therefore displayed poor performance in the context of solar energy conversion.

Now a team of German and Dutch chemists combined their expertise and took advantage of specific tetrafluoroazobenzene dyes, which undergo an efficient shape change upon exposure to visible green and blue light, and organized them into ordered liquid crystalline arrays. Fixing the ordered arrangement by polymerization yielded thin plastic sheets, which bend and chaotically oscillate in sunlight.

 


Converting sunlight directly into motion by organizing light-responsive molecules
Figure: Dr. David Bléger



By carefully investigating the individual parameters of the system, the researchers found that the degree of oscillation depends on both the intensity and wavelength of the light and only occurs if both colors, i.e. blue and green triggering the opposite photoreactions, are present. As a result chaotic, macroscopic motion can be realized using “normal” sunlight, without the aid of specific optics or artificial light sources.

The authors foresee immediate practical outdoor applications including self-cleaning coatings and surfaces, for example in windows. In general, these findings should be of great importance for the development of autonomous, sunlight-driven nano- and micromachinery.

A chaotic self-oscillating sunlight-driven polymer
K. Kumar, C. Knie, D. Bléger, M. A. Peletier, H. Friedrich, S. Hecht, D. J. Broer, M. G. Debije and A. P. H. J. Schenning
Nature Communications  (2016), published online
DOI: 10.1038/ncomms11975

 
Enlightening and flexing memories

Researchers from Humboldt-Universität zu Berlin, led by Professor Stefan Hecht, who is a member of IRIS Adlershof, in collaboration with the University of Strasbourg & CNRS (France) and the University of Nova Gorica (Slovenia), have shown that a carefully chosen blend of a small photoswitchable molecule and a semiconducting polymer can be used to fabricate high-performance memory devices that can be written and erased by light. Such multilevel (8-bit) optical memories have also been implemented on flexible substrates, paving the way to applications in wearable electronics, E-papers, and smart devices. These results have been published in Nature Nanotechnology.

In the quest to improve the data storage capability of everyday electronic devices (random-access memories, hard disk drives, USB flash drives, etc.), alternative strategies to conventional silicon-based technologies need to be developed. The continuous miniaturization of electronic circuits, leading to the integration of a larger number of memory cells per unit area, has already shown its limitations due to the increased fabrication complexity. Another appealing approach consists in developing memory elements capable of storing not just one but multiple bits of information per device, commonly referred to as multilevel memories.

Now a European team of researchers from Berlin, Strasbourg, and Nova Gorica developed a light-responsive organic thin-film transistor by blending a custom-designed molecule serving as miniaturized optical switch with a high-performance semiconducting polymer. Upon illumination with ultraviolet and green light to “write” and “erase” information, respectively, the molecular switch undergoes a reversible interconversion between two distinct forms, one enabling and the other one preventing current to flow through the surrounding semiconducting polymer.

 

 



By integrating these components into transistor devices and using short laser pulses the researchers were able to construct multilevel memories with a data storage capacity of 8 bits. Importantly, their prototype devices combine high endurance over 70 write–erase cycles and data retention times exceeding 500 days.

Taking the work yet to another level, the team could transfer the device concept to flexible and light-weight polymer substrates, such as polyethylene terephthalate, to replace the commonly used rigid silicon. The resulting “soft” architecture preserves its electrical characteristics after 1000 bending cycles, thereby demonstrating its robustness and suitability for flexible electronics.
 
These findings are of great importance for the realization of high-performance smart and foldable electronic (nano)devices programmed by light with potential applications in flexible, multilevel high-density optical memories, logic circuits, and more generally in the next generation optoelectronics.

Flexible non-volatile optical memory thin-film transistor device with over 256 distinct levels based on an organic bicomponent blend
T. Leydecker, M. Herder, E. Pavlica, G. Bratina, S. Hecht, E. Orgiu, and P. Samorì
Nature Nanotechnology  (2016), DOI: 10.1038/nnano.2016.87

 
Reproducibility in density functional theory calculations of solids

The success and widespread popularity of density-functional theory (DFT) over the last decades has given rise to an extensive range of dedicated codes for predicting molecular and crystalline properties. However, each code implements the formalism in a different way, raising questions about the reproducibility of such predictions. In this article, the results of a community-wide effort is reported, comparing 15 solid-state codes, using 40 different potentials or basis set types, to assess the quality of the equations of state for 71 elemental crystals. The overall conclusion is that predictions from recent codes and pseudopotentials agree very well, with pairwise differences that are comparable to those between different high-precision experiments. Results of older methods, however, show stronger discrepancies.

exciting, the program package [1,2] developed in the group of Claudia Draxl at the Humboldt-Universität zu Berlin (Physics Department and IRIS Adlershof) represents one of the all-electron full-potential implementations of DFT. It employs the linearized-augmented planewave basis, which is considered the gold standard within the condensed-matter community. Within this study, exciting has proven to be among the three most precise packages, with nearly negligible differences between them.

 


exciting
has not only evolved into a benckmark code for DFT but has a strong focus on excitations that are treated within time-dependent DFT and many-body perturbation theory. In August 2016, HoW exciting! 2016 [3] will take place at the Campus Adlershof, consisting of an international workshop on excitations in solids and a hands-on course  employing exciting.

[1]   exciting-code.org
[2]  
exciting: a full-potential all-electron package implementing density-functional theory and many-body perturbation theory
A. Gulans, S. Kontur, C. Meisenbichler, D. Nabok, P. Pavone, S. Rigamonti, S. Sagmeister, U. Werner, and C. Draxl
J. Phys: Condes. Matter (Topical Review) 26 (2014) 363202
DOI: 10.1088/0953-8984/26/36/363202
[3]   how-exciting-2016.physik.hu-berlin.de/
     
Reproducibility in density functional theory calculations of solids
K. Lejaeghere, G. Bihlmayer, T. Bjoerkman, P. Blaha, S. Bluegel, V. Blum, D. Caliste, I. E. Castelli, S. J. Clark, A. Dal Corso, S. de Gironcoli, T. Deutsch, J. K. Dewhurst, I. Di Marco, C. Draxl, M. Dulak, O. Eriksson, J. A. Flores-Livas, K. F. Garrity, L. Genovese, P. Giannozzi, M. Giantomassi, S. Goedecker, X. Gonze, O. Granaes, E. K. U. Gross, A. Gulans, F. Gygi, D. R. Hamann, P. J. Hasnip, N. A. W. Holzwarth, D. Iusan, D. B. Jochym, F. Jollet, D. Jones, G. Kresse, K. Koepernik, E. Kuecuekbenli, Y. O. Kvashnin, I. L. M. Locht, S. Lubeck, M. Marsman, N. Marzari, U. Nitzsche, L. Nordstrom, T. Ozaki, L. Paulatto, C. J. Pickard, W. Poelmans, M. I. J. Probert, K. Refson, M. Richter, G.-M. Rignanese, S. Saha, M. Scheffler, M. Schlipf, K. Schwarz, S. Sharma, F. Tavazza, P. Thunstroem, A. Tkatchenko, M. Torrent, D. Vanderbilt, M. J. van Setten, V. Van Speybroeck, J. M. Wills, J. R. Yates, G.-X. Zhang, and S. Cottenier
Science 351 (2016), 1415
DOI: 10.1126/science.aad3000

 


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