Highlight Publications 2017

Processing of ferroelectric polymers for microelectronics: from morphological analysis to functional devices
Hamed Sharifi, Jasper Michels, Kamal Asadi
Processing of ferroelectric polymers for microelectronics: from morphological analysis to functional devices
Organic memory devices are of great interest for flexible and cost-effective thin-film applications, such as RFID tags. However, a low cost production is only guaranteed if processing occurs under ambient, and therefore “humid”, conditions. It is known that solutions containing the fluorinated polyethylenes used for such applications, demix upon ingress of ambient water vapor into the coated film. This “vapor-induced phase separation” causes porous dry layers, desirable in case of, for instance, membrane technology, but to be avoided in thin-film electronics. Via an amalgamation of morphological mapping, device physics and modeling of demixing dynamics, we demonstrate how the hygroscopicity of the solvent crucially determines dry film morphology. For a sufficiently low solvent hygroscopicity, a >90% yield of functioning memory devices has been obtained at ambient conditions.
© RSC (2017)
A combination of morphological analysis and numerical simulation of solution phase behavior elucidates the relation between ambient humidity, solvent hygroscopicity and thin-film memory device performance.
Thermal conductivity and air-mediated losses in periodic porous silicon membranes at high temperatures
B. Graczykowski, A. El Sachat, J. S. Reparaz, M. Sledzinska, M. R. Wagner, E. Chavez-Angel, Y. Wu, S. Volz, F. Alzina & C. M. Sotomayor Torres
Thermal conductivity and air-mediated losses in periodic porous silicon membranes at high temperatures
Heat conduction in silicon can be effectively engineered by means of sub-micrometre porous free- standing membranes. Tunable thermal properties make these structures good candidates for integrated heat management units. In this work we use the two-laser Raman thermometry to study heat dissipation in periodic porous membranes at high temperatures via lattice conduction and air-mediated losses. We find the reduction of the thermal conductivity and its temperature dependence correlated with the structure feature size. On the basis of two-phonon Raman spectra, we attribute this behaviour to diffuse phonon-boundary scattering. Furthermore, we quantify the heat dissipation via natural air-mediated cooling, which can be tuned by engineering the porosity.
© Bartlomiej Graczykowski / MPI-P (2017)
Schematics of the two-laser Raman thermometry experiment performed on periodic porous Si membarnes.
Direct observation of mode-specific phonon-band gap coupling in methylammonium lead halide perovskites
Heejae Kim, Johannes Hunger, Enrique Cánovas, Melike Karakus, Zoltán Mics, Maksim Grechko, Dmitry Turchinovich, Sapun H. Parekh & Mischa Bonn
Direct observation of mode-specific phonon-band gap coupling in methylammonium lead halide perovskites
Methylammonium lead iodide perovskite is well-known not only for its remarkable rise of the power conversion efficiency in photovoltaic applications, but also for its peculiar photo-physical properties. We have investigated one of the intriguing peculiarities: the positive temperature dependence of the optical band gap. By the time-resolved THz/optical spectroscopy, we were able to observe a blue shift of the absorption onset after ~1 THz vibrational mode was excited. Theoretical modelling approaches assisted us to identify the specific phonon mode from the experimental results. We showed that the particular vibration, which governs the angle between lead and iodide ionic bonds, contributes to the unusual temperature dependent light absorption of this perovskite.
© Heejae Kim / MPI-P (2017)
Figure Caption: Selective excitation of the ~ 1 THz phonon mode, which corresponds to the Pb-I-Pb angle, induces the transient increase of the optical band gap.
Spontaneous jumping, bouncing and trampolining of hydrogel drops on a heated place
Jonathan T. Pham, Maxime Paven, Sanghyuk Wooh, Tadashi Kajiya, Hans-Jürgen Butt, Doris Vollmer
Spontaneous jumping, bouncing and trampolining of hydrogel drops on a heated place
The contact between liquid drops and hot solid surfaces is of practical importance for industrial processes, such as thermal spraying and spray cooling. The contact and bouncing of solid spheres is also an important event encountered in ball milling, powder processing, and everyday activities, such as ball sports. Using high speed video microscopy, we demonstrate that hydrogel drops, initially at rest on a surface, spontaneously jump upon rapid heating and continue to bounce with increasing amplitudes. Jumping is governed by the surface wettability, surface temperature, hydrogel elasticity, and adhesion. A combination of low adhesion impact behavior and fast water vapor formation supports continuous bouncing and trampolining. Our results illustrate how the interplay between solid and liquid characteristics of hydrogels results in intriguing dynamics, as reflected by spontaneous jumping, bouncing, trampolining, and extremely short contact times.
© Jonathan Pham / MPI-P (2017)
A hydrogel drop spontaneously jumps from tungsten surface upon rapidly heating the surface.
Short-channel field-effect transistors with 9-atom and 13-atom wide graphene nanoribbons
Juan Pablo Llinas, Andrew Fairbrother, Gabriela Borin Barin, Wu Shi, Kyunghoon Lee, Shuang Wu, Byung Yong Choi, Rohit Braganza, Jordan Lear, Nicholas Kau, Wonwoo Choi, Chen Chen, Zahra Pedramrazi, Tim Dumslaff, Akimitsu Narita, Xinliang Feng, Klaus Müllen, Felix Fischer, Alex Zettl, Pascal Ruffieux, Eli Yablonovitch1, Michael Crommie, Roman Fasel & Jeffrey Bokor
Short-channel field-effect transistors with 9-atom and 13-atom wide graphene nanoribbons
Bottom-up synthesized graphene nanoribbons (GNRs) and GNR heterostructures have promising electronic properties for high-performance field-effect transistors (FETs) and ultra-low power devices such as tunneling FETs. However, the short length and wide band gap of GNRs have prevented the fabrication of devices with the desired performance and switching behavior. Here, by fabricating short channel devices with a thin, high-κ gate dielectric and a 9-atom wide armchair GNR as the channel material, we demonstrate field-effect transistors with high on-current and high I on /I off  at room temperature. We find that the performance of these devices is limited by tunneling through the Schottky barrier at the contacts and we observe an increase in the transparency of the barrier by increasing the gate field near the contacts. Our results thus demonstrate successful fabrication of high-performance short-channel FETs with bottom-up synthesized armchair GNRs.
© Nature Publishing Group (2017)
Schematic of the short channel GNRFET with a 9AGNR channel and Pd source-drain electrodes (top) and Scanning electron micrograph of the fabricated Pd source-drain electrodes with a scale bar of 100 nm (bottom)
A maximum in interfacial hydrogen bonding occurs at 200 K at the single crystalline ice – air interface.
Wilbert J. Smit, Fujie Tang, M. Alejandra Sánchez, Ellen H. G. Backus, Limei Xu, Taisuke Hasegawa, Mischa Bonn, Huib J. Bakker, and Yuki Nagata
A maximum in interfacial hydrogen bonding occurs at 200 K at the single crystalline ice – air interface.
Combining theory and experiment, we have studied the temperature dependent structure of water molecules at the surface of ice. Remarkably, a maximum in interfacial hydrogen bonding occurs at 200 K. This maximum results from two competing effects: above 200 K, thermal fluctuations cause the breaking of hydrogen bonds; below 200 K, the ordered crystalline bulk structure causes breaking of hydrogen bonds at the interface. These insights are particularly important for understanding (photo-) chemistry occurring on ice particles in the atmosphere.
© Yuki Nagata / MPI-P (2017)
Ice forms crystal structure even at topmost ice surface at 170 K, while ice structure becomes disordered at 230 K. At 200 K, ice forms maximum hydrogen bond by making five-ring structure rather than hexagonal structure.