2020, Gizer, G., Puszkiel, J., Riglos, M.V.C., Pistidda, C., Ramallo-López, J.M., Mizrahi, M., Santoru, A., Gemming, T., Tseng, J.-C., Klassen, T., Dornheim, M.

The system Mg(NH2)2 + 2LiH is considered as an interesting solid-state hydrogen storage material owing to its low thermodynamic stability of ca. 40 kJ/mol H2 and high gravimetric hydrogen capacity of 5.6 wt.%. However, high kinetic barriers lead to slow absorption/desorption rates even at relatively high temperatures (>180 °C). In this work, we investigate the effects of the addition of K-modified LixTiyOz on the absorption/desorption behaviour of the Mg(NH2)2 + 2LiH system. In comparison with the pristine Mg(NH2)2 + 2LiH, the system containing a tiny amount of nanostructured K-modified LixTiyOz shows enhanced absorption/desorption behaviour. The doped material presents a sensibly reduced (∼30 °C) desorption onset temperature, notably shorter hydrogen absorption/desorption times and reversible hydrogen capacity of about 3 wt.% H2 upon cycling. Studies on the absorption/desorption processes and micro/nanostructural characterizations of the Mg(NH2)2 + 2LiH + K-modified LixTiyOz system hint to the fact that the presence of in situ formed nanostructure K2TiO3 is the main responsible for the observed improved kinetic behaviour.

2020, Vegesna, S.V., Bhat, V.J., Bürger, D., Dellith, J., Skorupa, I., Schmidt, O.G., Schmidt, H.

A novel small signal equivalent circuit model is proposed in the inversion regime of metal/(ZnO, ZnMnO, and ZnCoO) semiconductor/Si3N4 insulator/p-Si semiconductor (MSIS) structures to describe the distinctive nonlinear frequency dependent capacitance (C-F) and conductance (G-F) behaviour in the frequency range from 50 Hz to 1 MHz. We modelled the fully depleted ZnO thin films to extract the static dielectric constant (εr) of ZnO, ZnMnO, and ZnCoO. The extracted enhancement of static dielectric constant in magnetic n-type conducting ZnCoO (εr ≥ 13.0) and ZnMnO (εr ≥ 25.8) in comparison to unmagnetic ZnO (εr = 8.3–9.3) is related to the electrical polarizability of donor-type bound magnetic polarons (BMP) in the several hundred GHz range (120 GHz for CdMnTe). The formation of donor-BMP is enabled in n-type conducting, magnetic ZnO by the s-d exchange interaction between the electron spin of positively charged oxygen vacancies Vo+ in the BMP center and the electron spins of substitutional Mn2+ and Co2+ ions in ZnMnO and ZnCoO, respectively. The BMP radius scales with the Bohr radius which is proportional to the static dielectric constant. Here we show how BMP overlap can be realized in magnetic n-ZnO by increasing its static dielectric constant and guide researchers in the field of transparent spintronics towards ferromagnetism in magnetic, n-ZnO.

2020, Richard-Lacroix, Marie, Deckert, Volker

Tip-enhanced Raman spectroscopy (TERS) is currently widely recognized as an essential but still emergent technique for exploring the nanoscale. However, our lack of comprehension of crucial parameters still limits its potential as a user-friendly analytical tool. The tip’s surface plasmon resonance, heating due to near-field temperature rise, and spatial resolution are undoubtedly three challenging experimental parameters to unravel. However, they are also the most fundamentally relevant parameters to explore, because they ultimately influence the state of the investigated molecule and consequently the probed signal. Here we propose a straightforward and purely experimental method to access quantitative information of the plasmon resonance and near-field temperature experienced exclusively by the molecules directly contributing to the TERS signal. The detailed near-field optical response, both at the molecular level and as a function of time, is evaluated using standard TERS experimental equipment by simultaneously probing the Stokes and anti-Stokes spectral intensities. Self-assembled 16-mercaptohexadodecanoic acid monolayers covalently bond to an ultra-flat gold surface were used as a demonstrator. Observation of blinking lines in the spectra also provides crucial information on the lateral resolution and indication of atomic-scale thermally induced morphological changes of the tip during the experiment. This study provides access to unprecedented molecular-level information on physical parameters that crucially affect experiments under TERS conditions. The study thereby improves the usability of TERS in day-to-day operation. The obtained information is of central importance for any experimental plasmonic investigation and for the application of TERS in the field of nanoscale thermometry. © 2020, The Author(s).

2021, Davidson-Marquis, Flavie, Gargiulo, Julian, Gómez-López, Esteban, Jang, Bumjoon, Kroh, Tim, Müller, Chris, Ziegler, Mario, Maier, Stefan A., Kübler, Harald, Schmidt, Markus A., Benson, Oliver

[no abstract available: correction of https://doi.org/10.1038/s41377-021-00556-z published online 31 May 2021; After publication of this article, it is noticed the article contained an error. In Table 1, the data in the line ‘Length (mm)’ is missing. The complete Table 1 is provided in this correction.]

2020, Bönisch, M., Stoica, M., Calin, M.

β-stabilized Ti-alloys present several unexplored and intriguing surprises in relation to orthorhombic α″ phases. Among them are (i) the diffusion-controlled formation of transitional α″iso, α″lean and α″rich phases and ii) the highly anisotropic thermal expansion of martensitic α″. Using the prototypical Ti-Nb system, we demonstrate that the thermodynamic energy landscape reveals formation pathways for the diffusional forms of α″ and may lead to a stable β-phase miscibility gap. In this way, we derive temperature-composition criteria for the occurrence of α″iso and resolve reaction sequences during thermal cycling. Moreover, we show that the thermal expansion anisotropy of martensitic α″ gives rise to directions of zero thermal strain depending on Nb content. Utilizing this knowledge, we propose processing routes to achieve null linear expansion in α″ containing Ti-alloys. These concepts are expected to be transferable to other Ti-alloys and offer new avenues for their tailoring and technological exploitation.

2020, Jürries, F., Freudenberger, J., Nielsch, K., Woodcock, T.G.

Alloys of the form (Mn54Al44C2)100-xCux (with x = 0, 1, 2, 4 and 6) were produced by induction melting. After homogenisation and quenching, most of the alloys consist entirely of the retained ε-phase, except for x = 6, in which the κ-phase was additionally present. After subsequent annealing, the alloys with x ≤ 2 consist entirely of a Cu-doped, ferromagnetic τ-phase, whereas the alloys with x > 2 additionally contain the κ-phase. The polarisation of the alloys at an applied field of 14 T decreases with increasing Cu-content, which is attributed i) to the dilution of the magnetic moment of the τ-phase unit cell by the Cu atoms, which do not carry a magnetic moment, and ii) at higher Cu-contents, to the formation of the κ-phase, which has a much lower polarisation than the τ-phase and therefore dilutes the net polarisation of the alloys. The Curie temperature was not affected by the Cu-additions. The stress needed to die-upset the alloys with x ≤ 2 was similar to that of the undoped alloy, whereas it was much lower for x = 4 and 6, due to the presence of intergranular layers of the κ-phase. The extrinsic magnetic properties of alloys with x ≤ 2 were improved by die-upsetting, whereas decomposition of the τ-phase during processing had a deleterious effect on the magnetic properties for higher Cu-additions.

2020, Sun, Xiaoxiao, Wang, Ping, Wang, Tao, Chen, Ling, Chen, Zhaoying, Gao, Kang, Aoki, Tomoyuki, Li, Mo, Zhang, Jian, Schulz, Tobias, Albrecht, Martin, Ge, Weikun, Arakawa, Yasuhiko, Shen, Bo, Holmes, Mark, Wang, Xinqiang

We identify and characterize a novel type of quantum emitter formed from InGaN monolayer islands grown using molecular beam epitaxy and further isolated via the fabrication of an array of nanopillar structures. Detailed optical analysis of the characteristic emission spectrum from the monolayer islands is performed, and the main transmission is shown to act as a bright, stable, and fast single-photon emitter with a wavelength of ~400 nm. © 2020, The Author(s).

2020, Rößler, S., Unbehau, R., Gemming, T., Kruppke, B., Wiesmann, H.-P., Hanke, T.

Multiphasic silica/collagen xerogels are biomaterials designed for bone regeneration. Biphasic silica/collagen xerogels (B30) and triphasic xerogels (B30H20 or B30CK20) additionally containing hydroxyapatite or calcite were demonstrated to exhibit several structural levels. On the first level, low fibrillar collagen serves as template for silica nanoparticle agglomerates. On second level, this silica-enriched matrix phase is fiber-reinforced by collagen fibrils. In case of hydroxyapatite incorporation in B30H20, resulting xerogels exhibit a hydroxyapatite-enriched phase consisting of hydroxyapatite particle agglomerates next to silica and low fibrillar collagen. Calcite in B30CK20 is incorporated as single non-agglomerated crystal into the silica/collagen matrix phase with embedded collagen fibrils. Both the structure of multiphasic xerogels and the manner of hydroxyapatite or calcite incorporation have an influence on the release of calcium from the xerogels. B30CK20 released a significantly higher amount of calcium into a calcium-free solution over a three-week period than B30H20. In calcium containing incubation media, all xerogels caused a decrease in calcium concentration as a result of their bioactivity, which was superimposed by the calcium release for B30CK20 and B30H20. Proliferation of human bone marrow stromal cells in direct contact to the materials was enhanced on B30CK20 compared to cells on both plain B30 and B30H20.

2020, Kirch, Anton, Fische, Axel, Liero, Matthias, Fuhrmann, Jürgen, Glitzky, Annegret, Reineke, Sebastian

Organic light-emitting diodes (OLEDs) have become a major pixel technology in the display sector, with products spanning the entire range of current panel sizes. The ability to freely scale the active area to large and random surfaces paired with flexible substrates provides additional application scenarios for OLEDs in the general lighting, automotive, and signage sectors. These applications require higher brightness and, thus, current density operation compared to the specifications needed for general displays. As extended transparent electrodes pose a significant ohmic resistance, OLEDs suffering from Joule self-heating exhibit spatial inhomogeneities in electrical potential, current density, and hence luminance. In this article, we provide experimental proof of the theoretical prediction that OLEDs will display regions of decreasing luminance with increasing driving current. With a two-dimensional OLED model, we can conclude that these regions are switched back locally in voltage as well as current due to insufficient lateral thermal coupling. Experimentally, we demonstrate this effect in lab-scale devices and derive that it becomes more severe with increasing pixel size, which implies its significance for large-area, high-brightness use cases of OLEDs. Equally, these non-linear switching effects cannot be ignored with respect to the long-term operation and stability of OLEDs; in particular, they might be important for the understanding of sudden-death scenarios. © 2020, The Author(s).

2020, Ouillé, Marie, Vernier, Aline, Böhle, Frederik, Bocoum, Maïmouna, Jullien, Aurélie, Lozano, Magali, Rousseau, Jean-Philippe, Cheng, Zhao, Gustas, Dominykas, Blumenstein, Andreas, Simon, Peter, Haessler, Stefan, Faure, Jérôme, Nagy, Tamas, Lopez-Martens, Rodrigo

The development of ultra-intense and ultra-short light sources is currently a subject of intense research driven by the discovery of novel phenomena in the realm of relativistic optics, such as the production of ultrafast energetic particle and radiation beams for applications. It has been a long-standing challenge to unite two hitherto distinct classes of light sources: those achieving relativistic intensity and those with pulse durations approaching a single light cycle. While the former class traditionally involves large-scale amplification chains, the latter class places high demand on the spatiotemporal control of the electromagnetic laser field. Here, we present a light source producing waveform-controlled 1.5-cycle pulses with a 719 nm central wavelength that can be focused to relativistic intensity at a 1 kHz repetition rate based on nonlinear post-compression in a long hollow-core fiber. The unique capabilities of this source allow us to observe the first experimental indications of light waveform effects in laser wakefield acceleration of relativistic energy electrons. © 2020, The Author(s).