Publikationen - Strukturanalytik

Year incl. Online First >= 2021
Public type of publication: Articles ref. in Journals
"Online First" included
First HDZR Author OU: Structural Analysis (FWIZ-S)
Including selected publications

Cobalt-based Co3Mo3N/Co4N/Co Metallic Heterostructure as a Highly Active Electrocatalyst for Alkaline Overall Water Splitting

Liu, Y.; Wang, L.; Hübner, R.; Kresse, J.; Zhang, X.; Deconinick, M.; Vaynzof, Y.; Weidinger, I. M.; Eychmüller, A.

Alkaline water electrolysis holds promise for large-scale hydrogen production, yet it encounters challenges like high voltage and limited stability at higher current densities, primarily due to inefficient electron transport kinetics. Herein, a novel cobalt-based metallic heterostructure (Co3Mo3N/Co4N/Co) is designed for excellent water electrolysis. In operando Raman experiments reveal that the formation of the Co3Mo3N/Co4N heterointerface boosts the free water adsorption and dissociation, increasing the available protons for subsequent hydrogen production. Furthermore, the altered electronic structure of the Co3Mo3N/Co4N heterointerface optimizes ΔGH of the nitrogen atoms at the interface. This synergistic effect between interfacial nitrogen atoms and metal phase cobalt creates highly efficient active sites for the hydrogen evolution reaction (HER), thereby enhancing the overall HER performance. Additionally, the heterostructure exhibits a rapid OH- adsorption rate, coupled with great adsorption strength, leading to improved oxygen evolution reaction (OER) performance. Crucially, the metallic heterojunction accelerates electron transport, expediting the afore-mentioned reaction steps and enhancing water splitting efficiency. The Co3Mo3N/Co4N/Co electrocatalyst in the water electrolyzer delivers excellent performance, with a low 1.58 V cell voltage at 10 mAcm-2, and maintains 100% retention over 100 hours at 200 mAcm-2, surpassing the Pt/C // RuO2 electrolyzer.

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Synthesis and Twin Polymerization of Si(OCH2py)4 for Nitrogen-containing Carbon Materials

Scharf, S.; Notz, S.; Pfefferkorn, K.; Rüffer, T.; Formánek, P.; Hübner, R.; Selyshchev, O.; Madeira, T. I.; Zahn, D. R. T.; Lang, H.

The synthesis and twin polymerization (TP) of Si(OCH2py)4 (3a, py=2-cC5H4N; 3b, py=3-cC5H4N; 3c, py=4-cC5H4N) is discussed. The solid state structures of 3b, c were confirmed by single-crystal X-ray crystallography showing non-conventional H-bonding, forming 2D chains (3b) or 3D networks (3c). Thermally induced TP of 3a–c and their simultaneous polymerization with 2,2‘-spiro-bi[4H-1,3,2-benzodioxasiline] (4) is described. The resulting hybrid materials were characterized by 1H, 13C{1H}, and 29Si{1H} CP MAS NMR spectroscopy confirming the transformation of the SiOCH2 moieties into CH2 groups enabling the formation of the respective polymers. These results were supported by HAADF-STEM studies, displaying micro-structuring. Nitrogen-containing porous carbon materials C_1–C_3 show surface areas of 1300 and 1700 m2g-1, large pore volumes between 0.6–1.2 cm3g-1, and nitrogen contents of up to 3.1 at-%. X-ray photoemission spectroscopy reveal that pyrrolic, pyridine, and pyridone nitrogen atoms are present. If equimolar amounts of 3a–c and 4 are simultaneously polymerized in the presence of [Pd(OAc)2] (5), then the Pd nanoparticle-decorated material Pd@C_3 (900 m2g-1) was obtained, which showed k values of -0.083 and -0.066 min-1 in the reduction of methylene blue and methyl orange, proving the accessibility of the Pd NPs.

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Intrinsic magnetic properties of the layered antiferromagnet CrSBr

Long, F.; Mosina, K.; Hübner, R.; Sofer, Z.; Klein, J.; Prucnal, S.; Helm, M.; Dirnberger, F.; Zhou, S.

van der Waals magnetic materials are an ideal platform to study low-dimensional magnetism. Opposed to other members of this family, the magnetic semiconductor CrSBr is highly resistant to degradation in air, which, in addition to its exceptional optical, electronic, and magnetic properties, is the reason the compound is receiving considerable attention at the moment. For many years, its magnetic phase diagram seemed to be well-understood. Recently, however, several groups observed a magnetic transition in magnetometry measurements at temperatures of around 40 K that is not expected from theoretical considerations, causing a debate about the intrinsic magnetic properties of the material. In this Letter, we report the absence of this particular transition in magnetization measurements conducted on high-quality CrSBr crystals, attesting to the extrinsic nature of the low-temperature magnetic phase observed in other works. Our magnetometry results obtained from large bulk crystals are in very good agreement with the magnetic phase diagram of CrSBr previously predicted by the mean-field theory; A-type antiferromagnetic order is the only phase observed below the Néel temperature at TN = 131 K. Moreover, numerical fits based on the Curie–Weiss law confirm that strong ferromagnetic correlations are present within individual layers even at temperatures much larger than TN.

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Recyclable pickering emulsions for enzymatic phenol degradation of oily wastewater

Gong, Z.; Gao, S.; Lu, K.; Hübner, R.; Wu, C.

Enzymatic degradation offers a sustainable solution for waterborne phenolic pollutants. However, its application within industrial, non-aqueous contexts — particularly in mitigating phenolic contaminants in oily wastewater — remains significantly challenging. To address this challenge, the present study exploits the potential of Fe3O4@PDA nanoparticles to form oil-in-water Pickering emulsions for the enzymatic degradation process. The uniform stability of the prepared emulsion, with droplet sizes under 5 μm, protects enzyme activity and expands the water-oil interfacial area, leading to an enhancement in the efficiency of horseradish peroxidase (HRP) catalytic degradation. The application of this emulsion resulted in a substantial increase in the degradation rate of phenol, achieving 100% within 30 min as opposed to an only 13.6% without it. The study also highlights the excellent stability, reusability, and versatility of the Fe3O4@PDA nanoparticles, enabled by magnetic separation and their ability to form emulsions with diverse oil phases. Consequently, our research offers valuable insights into the development of environmentally sustainable strategies for the degradation of phenolic contaminants in various industrial oily wastewater.

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Die Morphologie der Schuppen der aus der Paläarktis bekannten Arten der Unterfamilie Procridinae Boisduval, 1828 (Lepidoptera, Zygaenidae) und deren Bedeutung für die Systematik und Phylogenie

Keil, T.; Hübner, R.; Worbs, A.

In dieser Arbeit werden erstmals die Form und Gestaltung der Schuppen der meisten (>90 %) der aus der Paläarktis bekannten Arten der Unterfamilie Procridinae Boisduval, 1828 abgebildet und Möglichkeiten der Determinationsunterstützung sowie deren Bedeutung für phylogenetische Interpretationen in Verbindung mit anderen morphologischen Merkmalen diskutiert.

The morphology of the scales of the Palaearctic species of the subfamily Procridinae Boisduval, 1828 (Lepidoptera, Zygaenidae) and their importance for systematics and phylogeny. – The present publication illustrates morphology and design of the scales of most (>90 %) of the Palaearctic species of the subfamily Procridinae Boisduval, 1828 for the first time. Possible support of identifications and the importance for phylogenetic interpretations in combination with other morphological features are discussed.

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  • Entomologische Nachrichten und Berichte 67(2023)3, 181-201

Synthesis and characterization of titanium and aluminum complexes of 2-methoxybenzyl alcoholate and their use in base-catalyzed twin polymerization

Scharf, S.; Rüffer, T.; Formánek, P.; Hübner, R.; Weber, M.; Mehring, M.; Lang, H.

The synthesis and characterization of twin monomers [Ti(OCH2-2-MeO-C6H4)4(HOCH2-2-MeO-C6H4)]2 (3) and [Al(OCH2-2-MeO-C6H4)3]4 (5) by reacting HOCH2-2-MeO-C6H4 (1) with Ti(OiPr)4 (2), or 1 with AlMe3 (4) is discussed. Single crystal X-ray structure analysis of 3 shows a dimeric structure with two alkoxide ligands bridging the titanium ions, while the others are terminal bonded. The respective phenolic resin / metal oxide hybrid materials HM_Ti and HM_Al were obtained in moderate (HM_Ti) to excellent (HM_Al) yields using typical base-catalyzed twin polymerization conditions (230 °C, 2 h). Nuclear magnetic resonance and infrared spectroscopy as well as scanning electron microscopy and scanning transmission electron microscopy combined with energ-dispersive X-ray spectroscopy proved the formation of inorganic–organic hybrid materials consisting of resin and MxOy materials (HM_Ti, TiO2; HM_Al, Al2O3) containing interpenetrating phase nano-domains with sizes of < 5 nm, as is charcteristic for twin polymerization processes. Oxidation of HM_Ti and HM_Al yielded the respective oxide materials Ox_Ti (TiO2) and Ox_Al (Al2O3), which possess low surface areas of ABET = 53 m2/g and 76 m2/g, respectively.

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Complete Glucose Electrooxidation Enabled by Coordinatively Unsaturated Copper Sites in Metal-Organic Frameworks

Shi, X.; Ling, Y.; Li, Y.; Li, G.; Li, J.; Wang, L.; Min, F.; Hübner, R.; Yuan, S.; Zhan, J.; Cai, B.

The electrocatalytic oxidation of glucose plays a vital role in biomass conversion, renewable energy, and biosensors, but significant challenges remain to achieve high selectivity and high activity simultaneously. In this study, we present a novel approach for achieving complete glucose electrooxidation utilizing Cu-based metal-hydroxide-organic framework (Cu-MHOF) featuring coordinatively unsaturated Cu active sites. In contrast to traditional Cu(OH)2 catalysts, the Cu-MHOF exhibits a remarkable 40-fold increase in electrocatalytic activity for glucose oxidation, enabling exclusive oxidation of glucose into formate and carbonate as the final products. The critical role of open metal sites in enhancing the adsorption affinity of glucose and key intermediates was confirmed by control experiments and density functional theory simulations. Subsequently, a miniaturized nonenzymatic glucose sensor was developed showing superior performance with a high sensitivity of 214.7 μAmM-1cm-2, a wide detection range from 0.1 μM to 22 mM, and a low detection limit of 0.086 μM. Our work provides a novel molecule-level strategy for designing catalytically active sites and could inspire the development of novel metal-organic framework for next-generation electrochemical devices.

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Efficient Near-Infrared Light-Emitting Diodes Based on CdHgSe Nanoplatelets

Prudnikau, A.; Roshan, H.; Paulus, F.; Martín-García, B.; Hübner, R.; Bahmani Jalali, H.; de Franco, M.; Prato, M.; Di Stasio, F.; Lesnyak, V.

Cadmium mercury selenide (CdHgSe) nanocrystals exhibit a unique combination of low-energy optical absorption and emission, which can be tuned from the visible to the infrared range through both quantum confinement and adjustment of their composition. Owing to this advantage, such nanocrystals have been studied as a promising narrow-band infrared light emitter. However, the electroluminescence of CdHgSe-based nanocrystals has remained largely unexplored, despite their potential for emitting light in the telecom wavelength range. Further benefits to their optical properties are expected from their shape control, in particular the formation of 2D nanocrystals, as well as from a proper design of their heterostructures. In this work, a colloidal synthesis of CdHgSe/ZnCdS core/shell nanoplatelets (NPLs) starting from CdSe template NPLs employing a cation exchange strategy is developed. The heterostructures synthesized exhibit photoluminescence that can be tuned from ≈1300 to 1500 nm. These near-infrared-active NPLs are employed in light-emitting diodes, demonstrating low turn-on voltage and high external quantum efficiency.

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Structural investigations of Au-Ni aerogels: morphology and element distribution

Kresse, J.; Georgi, M.; Hübner, R.; Eychmüller, A.

The physical properties of nanomaterials are determined by their structural features, making accurate structural control indispensable. This carries over to future applications. In the case of metal aerogels, highly porous networks of aggregated metal nanoparticles, such precise tuning is still largely pending. Although recent improvements in controlling synthesis parameters like electrolytes, reductants, or mechanical stirring, the focus has always been on one particular morphology at a time. Meanwhile, complex factors, such as morphology and element distributions, are studied rather sparsely. We demonstrate the capabilities of precise morphology design by deploying Au-Ni, a novel element combination for metal aerogels in itself, as a model system to combine common aerogel morphologies under one system for the first time. Au-Ni aerogels were synthesized via modified one- and two-step gelation, partially combined with galvanic replacement, to obtain aerogels with alloyed, heterostructural (novel metal aerogel structure of interconnected nanoparticles and nanochains), and hollow spherical building blocks. These differences in morphology are directly reflected in the physisorption behavior, linking the isotherm shape and pore size distribution to the structural features of the aerogels, including a broad-ranging specific surface area (35-65 m2 g-1). The aerogels were optimized regarding metal concentration, destabilization, and composition, revealing some delicate structural trends regarding the ligament size and hollow sphere character. Hence, this work significantly improves the structural tailoring of metal aerogels and possible up-scaling. Lastly, preliminary ethanol oxidation tests demonstrated that morphology design extends to the catalytic performance. All in all, this work emphasizes the strengths of morphology design to obtain optimal structures, properties, and (performances) for any material application.

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Photoluminescence Properties of Lanthanide-Doped Alumina and YAG Aerogels

Metzkow, N.; Klemmed, B.; Georgi, M.; Hübner, R.; Eychmüller, A.

In this work, the range of alumina (Al2O3) and yttrium aluminum garnet (YAG) aerogels was extended by doping them with lanthanide ions. The aerogels were synthesized by using a universal, epoxide-assisted sol−gel method. They were thermally treated to induce structural changes, which were characterized in more detail by using X-ray diffraction and electron microscopy. The alumina samples showed topotactic phase transformations from boehmite, via γ-alumina to a mixed alumina phase, while the YAG started as an amorphous mixed oxide phase, which crystallized at 1000 °C into pure crystalline YAG. In order to expand the functionalities of the aerogels, they were doped with the rare-earth ions Eu3+ and Tb3+ (3 mol %). The red or green photoluminescence could be observed only starting from a temperature treatment of 550 °C, which can be related to the defect reduction and crystallinity increase due to phase transformations and sintering processes occurring. For the first time, the photoluminescence quantum yields of luminescent aerogels could be determined. The highest quantum yield of 25.5 ± 1.1 % was achieved for the Al2O3-Tb-1000 sample.

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Leveraging Ligand and Composition Effects: Morphology-Tailorable Pt–Bi Bimetallic Aerogels for Enhanced (Photo-)Electrocatalysis

Xue, G.; Li, Y.; Du, R.; Wang, J.; Hübner, R.; Gao, M.; Hu, Y.

Metal aerogels (MAs) are emerging porous materials displaying unprecedented potential in catalysis, sensing, plasmonic technologies, etc. However, the lack of efficient regulation of their nano-building blocks (NBBs) remains a big hurdle that hampers the in-depth investigation and performance enhancement. Here, by harmonizing composition and ligand effects, Pt- and Bi-based single- and bimetallic aerogels bearing NBBs of controlled dimensions and shapes are obtained by facilely tuning the metal precursors and the applied ligands. Particularly, by further modulating the electronic and optic properties of the aerogels via adjusting the content of the catalytically active Pt component and the semiconducting Bi component, both the electrocatalytic and photoelectrocatalytic performance of the Pt–Bi aerogels can be manipulated. In this light, an impressive catalytic performance for electro-oxidation of methanol is acquired, marking a mass activity of 6.4-fold higher under UV irradiation than that for commercial Pt/C. This study not only sheds light on in situ manipulating NBBs of MAs, but also puts forward guidelines for crafting high-performance MAs-based electrocatalysts and photoelectrocatalysts toward energy-related electrochemical processes.

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Room-temperature extended short-wave infrared GeSn photodetectors realized by ion beam techniques

Wen, S.; Shaikh, M. S.; Steuer, O.; Prucnal, S.; Grenzer, J.; Hübner, R.; Turek, M.; Pyszniak, K.; Reiter, S.; Fischer, I. A.; Georgiev, Y.; Helm, M.; Wu, S.; Luo, J.-W.; Zhou, S.; Berencen, Y.

GeSn alloys hold great promise as high-performance, low-cost, near- and short-wavelength infrared photodetectors with the potential to replace the relatively expensive and currently market-dominant InGaAs- and InSb-based photodetectors. In this Letter, we demonstrate room-temperature GeSn pn photodetectors fabricated by a complementary metal-oxide-semiconductor compatible process, involving Sn and P ion implantation and flash-lamp annealing prior to device fabrication. The fabrication process enables the alloying of Ge with Sn at concentrations up to 4.5% while maintaining the high-quality single-crystalline structure of the material. This allows us to create Ge0.955Sn0.045 pn photodetec-tors with a low dark current density of 12.8 mA/cm2 and a relatively high extended responsivity of 0.56 A/W at 1.71 l m. These results pave the way for the implementation of a cost-effective, scalable, and CMOS-compatible short-wavelength infrared detector technology.

Keywords: Semiconductors; Photodetectors; GeSn; Implantation; Flash-lamp annealing

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Novel Mixed-Dimensional hBN-Passivated Silicon Nanowire Reconfigurable Field Effect Transistors: Fabrication and Characterization

Ghosh, S.; Bilal Khan, M.; Chava, P.; Watanabe, K.; Taniguchi, T.; Prucnal, S.; Hübner, R.; Mikolajick, T.; Erbe, A.; Georgiev, Y.

This work demonstrates the novel concept of a mixed-dimensional reconfigurable field effect transistor (RFET) by combining a one-dimensional (1D) channel material such as a silicon (Si) nanowire with a two-dimensional (2D) material as a gate dielectric. An RFET is an innovative device that can be dynamically programmed to perform as either an n- or p-FET by applying appropriate gate potentials. In this work, an insulating 2D material, hexagonal boron nitride (hBN), is introduced as a gate dielectric and encapsulation layer around the nanowire in place of a thermally grown or atomic-layer-deposited oxide. hBN flake was mechanically exfoliated and transferred onto a silicon nanowire-based RFET device using the dry viscoelastic stamping transfer technique. The thickness of the hBN flakes was investigated by atomic force microscopy and transmission electron microscopy. The ambipolar transfer characteristics of the Si-hBN RFETs with different gating architectures showed a significant improvement in the device’s electrical parameters due to the encapsulation and passivation of the nanowire with the hBN flake. Both n- and p-type characteristics measured through the top gate exhibited a reduction of hysteresis by 10–20 V and an increase in the on–off ratio (ION/IOFF) by 1 order of magnitude (up to 108) compared to the values measured for unpassivated nanowire. Specifically, the hBN encapsulation provided improved electrostatic top gate coupling, which is reflected in the enhanced subthreshold swing values of the devices. For a single nanowire, an improvement up to 0.97 and 0.5 V/dec in the n- and p-conduction, respectively, is observed. Due to their dynamic switching and polarity control, RFETs boast great potential in reducing the device count, lowering power consumption, and playing a crucial role in advanced electronic circuitry. The concept of mixed-dimensional RFET could further strengthen its functionality, opening up new pathways for future electronics.

Keywords: mixed-dimensional reconfigurable FET; ambipolar; nickel silicide; flash lamp annealing; hBN encapsulation; subthreshold swing

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  • Open Access Logo ACS Applied Materials and Interfaces 15(2023)34, 40709-40718
    Online First (2023) DOI: 10.1021/acsami.3c04808

Ferromagnetic interlayer coupling in CrSBr crystals irradiated by ions

Long, F.; Ghorbani Asl, M.; Mosina, K.; Li, Y.; Lin, K.; Ganss, F.; Hübner, R.; Sofer, Z.; Dirnberger, F.; Kamra, A.; Krasheninnikov, A.; Prucnal, S.; Helm, M.; Zhou, S.

Layered magnetic materials are becoming a major platform for future spin-based applications. Particularly the air-stable van der Waals compound CrSBr is attracting considerable interest due to its prominent magneto-transport and magneto-optical properties. In this work, we observe a transition from antiferromagnetic to ferromagnetic behavior in CrSBr crystals exposed to high-energy, non-magnetic ions. Already at moderate fluences, ion irradiation induces a remanent magnetization with hysteresis adapting to the easy-axis anisotropy of the pristine magnetic order up to a critical temperature of 110 K. Structure analysis of the irradiated crystals in conjunction with density functional theory calculations suggest that the displacement of constituent atoms due to collisions with ions and the formation of interstitials favors ferromagnetic order between the layers.

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Exploring Antibacterial Activity and Bacterial-Mediated Allotropic Transition of Differentially Coated Selenium Nanoparticles

Ruiz-Fresneda, M. A.; Schaefer, S.; Hübner, R.; Fahmy, K.; Merroun, M. L.

The use of metal nanoparticles (NPs) as antimicrobial agents has become a promising alternative to the problem of antibiotic-resistant bacteria and other applications. Silver nanoparticles (AgNPs) are well-known as one of the most universal biocide compounds. However, selenium nanoparticles (SeNPs) recently gained more attention as effective antimicrobial agents. This study aims to investigate the antibacterial activity of SeNPs with different surface coatings (BSA-coated, chitosan-coated, and undefined coating) on the Gram-negative Stenotrophomonas bentonitica and the Gram-positive Lysinibacillus sphaericus in comparison to AgNPs. The tested NPs had similar properties, including shape (spheres), structure (amorphous), and size (50−90 nm), but differed in their surface charge. Chitosan SeNPs exhibited a positive surface charge, while the remaining NPs assayed had a negative surface charge. We have found that cell growth and viability of both bacteria were negatively affected in the presence of the NPs, as indicated by microcalorimetry and flow cytometry. Specifically, undefined coating SeNPs displayed the highest percentage values of dead cells for both bacteria (85−91%). An increase in reactive oxygen species (ROS) production was also detected. Chitosan-coated and undefined SeNPs caused the highest amount of ROS (299.7 and 289% over untreated controls) for S. bentonitica and L. sphaericus, respectively. Based on DNA degradation levels, undefined-SeNPs were found to be the most hazardous, causing nearly 80% DNA degradation. Finally, electron microscopy revealed the ability of the cells to transform the different SeNP types (amorphous) to crystalline SeNPs (trigonal/monoclinical Se), which could have environmentally positive implications for bioremediation purposes and provide a novel green method for the formation of crystalline SeNPs. The results obtained herein demonstrate the promising potential of SeNPs for their use in medicine as antimicrobial agents, and we propose S. bentonitica and L. sphaericus as candidates for new bioremediation strategies and NP synthesis with potential applications in many fields.

Keywords: selenium; nanoparticles; antibiotic; bioremediation; applications

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Europium(III) as luminescence probe for interactions of a sulfate-reducing microorganism with potentially toxic metals

Hilpmann, S.; Moll, H.; Drobot, B.; Vogel, M.; Hübner, R.; Stumpf, T.; Cherkouk, A.

Microorganisms show a high affinity for trivalent actinides and lanthanides, which play an important role in the safe disposal of high-level radioactive waste as well as in the mining of various rare earth elements. The interaction of the lanthanide Eu(III) with the sulfate-reducing microorganism Desulfosporosinus hippei DSM 8344T, a representative of the genus Desulfosporosinus that naturally occurs in clay rock and bentonite, was in-vestigated. Eu(III) is often used as a non-radioactive analogue for the trivalent actinides Pu(III), Am(III), and Cm(III), which contribute to a major part of the radiotoxicity of the nuclear waste. D. hippei DSM 8344T showed a weak interaction with Eu(III), most likely due to a complexation with lactate in artificial Opalinus Clay pore water. Hence, a low removal of the lanthanide from the supernatant was observed. Scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy revealed a bioprecipitation of Eu(III) with phosphates potentially excreted from the cells. This demonstrates that the ongoing interaction mechanisms are more complex than a sim-ple biosorption process. The bioprecipitation was also verified by luminescence spec-troscopy, which showed that the formation of the Eu(III) phosphate compounds starts almost immediately after the addition of the cells. Moreover, chemical microscopy pro-vided information on the local distribution of the different Eu(III) species in the formed cell aggregates. These results provide first insights into the interaction mechanisms of Eu(III) with sulfate-reducing bacteria and contribute to a comprehensive safety concept for a high-level radioactive waste repository, as well as to a better understanding of the fate of heavy metals (especially rare earth elements) in the environment.

Keywords: Europium(III) luminescence; Sulfate-reducing bacteria; Europium(III) bioprecipitation; Opalinus Clay pore water

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Influence of engineered roughness microstructures on adhesion and turbulent resuspension of microparticles

Banari, A.; Graebe, K.; Rudolph, M.; Mohseni, E.; Lorenz, P.; Zimmer, K.; Hübner, R.; Henry, C.; Bossy, M.; Hampel, U.; Lecrivain, G.

From microplastics resuspending into the atmosphere to earth particles left behind during extraterrestrial explorations, the resuspension of microparticles by a turbulent gas flow occurs in many natural and industrial systems. Wall surfaces, onto which particles initially adhere, are rarely smooth and this surface roughness affects their resuspension. Available experimental data on particle resuspension have been obtained with substrates, whose surfaces are either unaltered or manually abraded with, for instance, sand blasting. In these experiments, the roughness elements span a wide size range and are in-homogeneously distributed in space. Surface functionalization is a modern technique allowing the precise fabrication of a wall surface with well-characterized microstructures, hence reducing the asperity randomness associated with conventional abrasion techniques. Taking advantage of surface functionalization, we present here a new set of reference data, where the wall asperities are represented by a structured arrangement of micropillars and microcubes. Adhesion force measurements and particle remaining fraction against gas velocity, at Reynolds number up to 8000, are reported for one reference and two artificially roughened substrates. Laboratory measurements show that the microasperities have little to moderate effect on the mean adhesion force and the threshold velocity, at which half of the 100-µm particles resuspend. The standard deviations are, however, significantly affected. The presented results will primary contribute to the improvement of resuspension models, which until now rely on a simplified representation of the surface roughness elements. The presented measurements are highly compatible with such models, which involve elementary roughness features, such as hemispherical asperities superimposed with a flat plate.

Keywords: Particle resuspension; Adhesion force measurement; Turbulent gas flow; Surface functionalization; Surface roughness

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Bimetallic Pt-Hg Aerogels for Electrocatalytic Upgrading of Ethanol to Acetate

Zhang, X.; Wang, T.; Wang, C.; Hübner, R.; Eychmüller, A.; Zhan, J.; Cai, B.

Electrochemical upgrading of ethanol to acetic acid provides a promising strategy to couple with the current hydrogen production from water electrolysis. This work reports the design of a series of bimetallic Pt-Hg aerogels, where the PtHg aerogel exhibits a 10.5-times higher mass activity than that of commercial Pt/C toward ethanol oxidation. More impressively, the PtHg aerogel demonstrates nearly 100% selectivity toward the production of acetic acid. The operando infrared spectroscopic studies and nuclear magnetic resonance analysis verify the preferable C2 pathway mechanism during the reaction. This work opens an avenue for the electrochemical synthesis of acetic acid via ethanol electrolysis.

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Alloyed RexMo1 − xS2 Nanoflakes with Enlarged Interlayer Distances for Hydrogen Evolution

Li, J.; Hübner, R.; Deconinck, M.; Bora, A.; Göbel, M.; Schwarz, D.; Chen, G.; Wang, G.; Yang, S. A.; Vaynzof, Y.; Lesnyak, V.

Molybdenum sulfide (MoS2) has attracted significant attention due to its great potential as a low-cost and efficient catalyst for the hydrogen evolution reaction. Developing a facile, easily upscalable, and inexpensive approach to produce catalytically active nanostructured MoS2 with a high yield would significantly advance its practical application. Colloidal synthesis offers several advantages over other preparation techniques to overcome the low reaction yield of exfoliation and drawbacks of expensive equipment and processes used in chemical vapor deposition. In this work, we report an efficient synthesis of alloyed RexMo1−xS2 nanoflakes with an enlarged interlayer distance, among which the composition Re0.55Mo0.45S2 exhibits excellent catalytic performance with overpotentials as low as 79 mV at 10 mA/cm2 and a small Tafel slope of 42 mV/dec. Density functional theory calculations prove that enlarging the distance between layers in the RexMo1−xS2 alloy can greatly improve its catalytic performance due to a significantly reduced free energy of hydrogen adsorption. The developed approach paves the way to design advanced transition metal dichalcogenide-based catalysts for hydrogen evolution and to promote their large-scale practical application.

Keywords: RexMo1−xS2 alloys; enlarged interlayer distance; nanoflakes; colloidal synthesis; hydrogen evolution

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B20 Weyl semimetal CoSi film fabricated by flash-lamp annealing

Li, Z.; Yuan, Y.; Hübner, R.; Rebohle, L.; Zhou, Y.; Helm, M.; Nielsch, K.; Prucnal, S.; Zhou, S.

B20-CoSi is a newly discovered Weyl semimetal that crystallizes into a non-centrosymmetric crystal structure. However, the investigation of B20-CoSi has so far been focused on bulk materials, whereas the growth of thin films on technology-relevant substrates is a prerequisite for most practical applications. In this study, we have used millisecond-range flash-lamp annealing, a non-equilibrium solid-state reaction, to grow B20-CoSi thin films. By optimizing the annealing parameters, we were able to obtain thin films with a pure B20-CoSi phase. The magnetic and transport measurements indicate the appearance of the charge density wave and the chiral anomaly. Our work presents a promising method for preparing thin films of most binary B20 transition-metal silicides, which are candidates for topological Weyl semimetals.

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Scalable and Controllable Synthesis of Pt-Ni Bunched-Nanocages Aerogels as Efficient Electrocatalysts for Oxygen Reduction Reaction

Zheng, Y.; Petersen, A. S.; Wan, H.; Hübner, R.; Zhang, J.; Wang, J.; Qi, H.; Ye, Y.; Liang, C.; Yang, J.; Cui, Z.; Meng, Y.; Zheng, Z.; Rossmeisl, J.; Liu, W.

Developing efficient and stable Pt-based oxygen reduction reaction (ORR) electrocatalysts via both economical and controllable routes is critical for the
practical application of electrochemical energy devices. Herein, a scalable, controllable, and general ambient-O2-involved aqueous-solution cultivating strategy to prepare PtxMy (M = Ni, Fe, Co) bunched-nanocages aerogels (BNCs AG) is demonstrated, based on a newly established high-M-to-Pt-precursor-ratio-and-B-incorporation-facilitated M-rich core and Pt-rich shell hydrogel formation process. The Pt83Ni17 BNCs AG shows prominent ORR performance with a mass activity (MA) of 1.95 A mgPt −1 and specific activity of 3.55 mA cm−2, which are 8.9-times and 9.6-times that of Pt supported on carbon (Pt/C), respectively. Particularly,
the Pt83Ni17 BNCs AG displays greatly enhanced durability (MA 82.6% retention) compared to Pt/C (MA 31.8% retention) after a 20 000-cycles accelerated durability test. Systematic studies including density functional theory calculations uncover that the excellent activity is closely related to the optimized ligand and strain effects with the optimized Ni content in this aerogel; the outstanding durability is endowed by the lowered-down Ni leaching with the optimized Pt/Ni ratio and the inhibited sintering due to its appropriate porosity. This work provides new perspectives on the development of electrocatalysts with both high performance and low cost.

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Composition-Dependent Optical Properties of Cu−Zn−In−Se Colloidal Nanocrystals Synthesized via Cation Exchange

Bora, A.; Lox, J.; Hübner, R.; Weiß, N.; Bahmani Jalali, H.; Di Stasio, F.; Steinbach, C.; Gaponik, N.; Lesnyak, V.

Copper chalcogenide-based nanocrystals (NCs) are a suitable replacement for toxic Cd/Pb chalcogenide-based NCs in a wide range of applications including photovoltaics, optoelectronics, and biological imaging. However, despite rigorous research, direct synthesis approaches of this class of compounds suffer from inhomogeneous size, shape, and composition of the NC ensembles, which is reflected in their broad photoluminescence (PL) bandwidths. A partial cation exchange (CE) strategy, wherein host cations in the initial binary copper chalcogenide are replaced by incoming cations to form ternary/quaternary multicomponent NCs, has been proven to be instrumental in achieving better size, shape, and composition control to this class of
nanomaterials. Additionally, adopting synthetic strategies which help to eliminate inhomogeneities in the NC ensembles can lead to narrower PL bandwidths, as was shown by single-particle studies on I−III−VI2-based semiconductor NCs. In this work, we formulate a two-step colloidal synthesis of Cu−Zn−In−Se (CZISe) NCs via a partial CE pathway. The first step is the synthesis of Cu2−xSe NCs, which serve as a template for the subsequent CE reaction. The second step is the incorporation of the In3+ and Zn2+ guest cations into the synthesized Cu2−xSe NCs via simultaneous injection of both metal precursors, which results in gradient-alloyed CZISe NCs with a Zn-rich surface. The as-synthesized NCs exhibit near-infrared (NIR) PL without an additional shell growth, which is typically required in most of the developed protocols. The photoluminescence quantum yield (PLQY) of these Cu chalcogenide-based NCs reaches 20%. These NCs also exhibit intriguingly narrow PL bands, which challenges the notion of broad PL bands being an inherent property of this class of NCs. Additionally, a variation in the feed ratios of the incoming cations, i.e., In/Zn, results in the variation of the composition of the synthesized NCs. Henceforth, the optical properties of these NCs could be tuned by a simple variation of the composition of the NCs achieved by varying the feed ratios of the incoming cations. Within a narrow size distribution, the PL maxima range from 980 to 1060 nm, depending on the composition of the NCs. Post-synthetic surface modification of the synthesized NCs enabled the replacement of the parent long-chain organic ligands with smaller species, which is essential for their prospective applications requiring efficient charge transport. With PL emission extended into the NIR, the synthesized NCs are suitable for an array of potential applications, most importantly in the area of solar energy harvesting and bioimaging. The large Stokes shift inherent to these materials, their absorption in the solar range, and their NIR PL within the biological window make them suitable candidates.

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Interaction of domain walls with grain boundaries in uniaxial insulating antiferromagnets

Pylypovskyi, O.; Hedrich, N.; Tomilo, A.; Kosub, T.; Wagner, K.; Hübner, R.; Shields, B.; Sheka, D.; Faßbender, J.; Maletinsky, P.; Makarov, D.

A search for high-speed and low-energy memory devices puts antiferromagnetic thin films at the forefront of spintronic research. Here, we develop a material model of a granular antiferromagnetic thin film with uniaxial anisotropy and provide fundamental insight into the interaction of antiferromagnetic domain walls with grain boundaries. This model is validated on thin films of the antiferromagnetic insulator \ch{Cr2O3}, revealing complex maze-like domain patterns hosting localized nanoscale domains down to 50 nm. We show that the inter-grain magnetic parameters can be estimated based on an analysis of high-resolution images of antiferromagnetic domain patterns examining the domain patterns' self-similarity and the statistical distribution of domain sizes. Having a predictive material model and understanding of the pinning of domain walls on grain boundaries, we put forth design rules to realize granular antiferromagnetic recording media.

Keywords: antiferromagnetism; granular media; spin-lattice simulations; Nitrogen vacancy magnetometry


Investigations towards incorporation of Eu3+ and Cm3+ during ZrO2 crystallization in aqueous solution

Opitz, L.; Hübner, R.; Shams Aldin Azzam, S.; Gilson, S.; Finkeldei, S. C.; Huittinen, N. M.

Nuclear energy provides a widely applied carbon-reduced energy source. Following operation, the spent nuclear fuel (SNF), containing a mixture of radiotoxic elements such as transuranics, needs to be safely disposed of. Safe storage of SNF in a deep geological repository (DGR) relies on multiple engineered and natural retention barriers to prevent environmental contamination. In this context, zirconia (ZrO2) formed on the SNF rod cladding, could be employed as an engineered barrier for immobilization of radionuclides via structural incorporation. This study investigates the incorporation of Eu3+ and Cm3+, representatives for trivalent transuranics, into zirconia by co-precipitation and crystallization in aqueous solution at 80 °C. Complementary structural and microstructural characterization has been carried out by Powder X-ray Diffraction (PXRD), spectrum imaging analysis based on Energy-Dispersive X-ray Spectroscopy in Scanning Transmission Electron Microscopy mode (STEM-EDXS), and luminescence spectroscopy. The results reveal the association of the dopants with the zirconia particles and elucidate the presence of distinct bulk and superficially incorporated species. Hydrothermal aging for up to 460 days in alkaline media points to great stability of these incorporated species after initial crystallization, with no indication of phase segregation or release of Eu3+ and Cm3+ over time. These results suggest that zirconia would be a suitable technical retention barrier for mobilized trivalent actinides in a DGR.

Keywords: nuclear waste management; crystallization; zirconia; incorporation; trivalent; luminescence spectroscopy; transmission electron microscopy; X-ray diffraction

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Direct magnetic manipulation of a permalloy nanostructure by a focused cobalt ion beam

Pablo-Navarro, J.; Klingner, N.; Hlawacek, G.; Kakay, A.; Bischoff, L.; Narkovic, R.; Mazarov, P.; Hübner, R.; Meyer, F.; Pilz, W.; Lindner, J.; Lenz, K.

We present results of direct maskless magnetic patterning of ferromagnetic nanostructures using a cobalt focused ion beam (FIB) system. The liquid metal ion source of the FIB was made of a Co36Nd64 alloy. A Wien mass filter allows for selecting the ion species. Using the FIB, we implanted narrow tracks of Co ions into a nominal 5000×1000×50 nm3 permalloy strip. We observed the Co-induced changes of the magnetic properties by measuring the sample with microresonator ferromagnetic resonance before and after the implantation. Regions as small as 50 nm can be implanted up to concentrations of at.-10 % near the surface. This allows for easy magnetic modification of edge-localized spin waves with a lateral resolution otherwise hard to reach. The direct-write maskless FIB process is quick and convenient for optical measurement techniques, as it does not involve the virtually impossible removal of ion-hardened resist masks one would face when using lithography with broad-beam ion implantation

Keywords: Ferromagnetic resonance; nanostructures; ferromagnetism; focused ion beams; spin-wave dynamics

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  • Secondary publication expected from 26.10.2024

Bimetallic Pt−Ni Two-Dimensional Interconnected Networks: Developing Self-Assembled Materials for Transparent Electronics

Khavlyuk, P.; Mitrofanov, A.; Shamraienko, V.; Hübner, R.; Kresse, J.; Borchert, K. B. L.; Eychmüller, A.

Continuous advancements in science and technology in the field of flexible devices encourage researchers to dedicate themselves to seeking candidates for new flexible transparent conductive films (FTCFs). Our recently developed two-dimensional (2D) metal aerogels are considered as a new class of FTCFs. Here, we describe a new large-scale self-assembly synthesis of bimetallic Pt-Ni 2D metal aerogels with controllable morphology during the synthesis. The
obtained 2D aerogels require only a low quantity of precursors for the synthesis of percolating nanoscale networks with areas of up to 6 cm2 without the need of an additional drying step. Stacks of the obtained monolayer structures display low sheet resistances (down to 270 Ω/sq), while decreasing the optical transparency. In perspective, the 2D bimetallic Pt-Ni aerogels not only enrich the structural diversity of metal aerogels but also bring forth new materials for further applications in flexible electronics and electrocatalysis with reduced costs of production.

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Optimizing the Pd Sites in Pure Metallic Aerogels for Efficient Electrocatalytic H2O2 Production

Zhang, X.; Wang, C.; Chen, K.; Clark, A. H.; Hübner, R.; Zhan, J.; Zhang, L.; Eychmüller, A.; Cai, B.

Decentralized electrochemical production of hydrogen peroxide (H2O2) is an attractive alternative to the industrial anthraquinone process, the application of which is hindered by the lack of high-performance electrocatalysts in acidic media. Herein, a novel catalyst design strategy is reported to optimize the Pd sites in pure metallic aerogels by tuning their geometric environments and electronic structures. By increasing the Hg content in the Pd-Hg aerogels, the Pd-Pd coordination is gradually diminished, resulting in isolated, single-atom-like Pd motifs in the Pd2Hg5 aerogel. Further heterometal doping leads to a series of M-Pd2Hg5 aerogels with an unalterable geometric environment, allowing for sole investigation of the electronic effects. Combining theoretical and experimental analyses, a volcano relationship is obtained for the M-Pd2Hg5 aerogels, demonstrating an effective tunability of the electronic structure of the Pd active sites. The optimized Au-Pd2Hg5 aerogel exhibits an outstanding H2O2 selectivity of 92.8% as well as transferred electron numbers of ≈2.1 in the potential range of 0.0-0.4 VRHE. This work opens a door for designing metallic aerogel electrocatalysts for H2O2 production and highlights the importance of electronic effects in tuning electrocatalytic performances.

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Porous Magnesium Oxide by Twin Polymerization: From Hybrid Materials to Catalysis

Scharf, S.; Notz, S.; Thomas, R.; Mehring, M.; Tegenkamp, C.; Formánek, P.; Hübner, R.; Lang, H.

Twin monomers [Mg(2-OCH2-cC6H4O)][L]0.8 (2, L=diglyme) and [Mg(2-OCH2-cC6H4O)][L]0.66 (3, L=tmeda) form by their thermal polymerization interpenetrating organic-inorganic hybrid materials in a straightforward manner. Carbonization (Ar) followed by calcination gave porous MgO (2: surface area 200 m2g-1, 3: 400 m2g-1), which showed in catalytic studies towards Meerwein-Ponndorf-Verley reductions excellent yields and complete conversions for cyclohexanone and benzaldehyde. However, with crotonaldehyde a mixture of C4–C8 compounds was obtained. When MgO was exposed to air then primarily crotyl alcohol was formed. The range of applications could be easily extended by twin polymerization of 3 in presence of [Cu-(O2CCH2O(CH2CH2O)2Me)2] (4) or [Ag(O2CCH2-cC4H3S)(PPh3)] (5), resulting in the formation of nanoparticle-decorated porous CuO@MgO or Ag@MgO materials, which showed high catalytic reactivity towards the reduction of methylene blue.

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Element Distributions in Bimetallic Aerogels

Wang, C.; Herranz, J.; Hübner, R.; Schmidt, T. J.; Eychmüller, A.

Metal aerogels assembled from nanoparticles have captured grand attention because they combine the virtues of metals and aerogels and are regarded as ideal materials to address current environmental and energy issues. Among these aerogels, those composed of two metals not only display combinations (superpositions) of the properties of their individual metal components but also feature novel properties distinctly different from those of their monometallic relatives. Therefore, quite some effort has been invested in refining the synthetic methods, compositions, and structures of such bimetallic aerogels as to boost their performance for the envisaged application(s). One such use would be in the field of electrocatalysis, whereby it is also of utmost interest to unravel the element distributions of the (multi)metallic catalysts to achieve a ratio of their bottom-to-up design. Regarding the element distributions in bimetallic aerogels, advanced characterization techniques have identified alloys, core-shells, and structures in which the two metal particles are segregated (i.e., adjacent but without alloy or core-shell structure formation). While an almost infinite number of metal combinations to form bimetallic aerogels can be envisaged, the knowledge of their formation mechanisms and the corresponding element distributions is still in its infancy. The evolution of the observed musters is all but well understood, not to mention the positional changes of the elements observed in operando or in beginning- vs end-of-life comparisons (e.g., in fuel cell applications).
With this motivation, in this Account we summarize the endeavors made in element distribution monitoring in bimetallic aerogels in terms of synthetic methods, expected structures, and their evolution during electrocatalysis. After an introductory chapter, we first describe briefly the two most important characterization techniques used for this, namely, scanning transmission electron microscopy (STEM) combined with element mapping (e.g., energy-dispersive X-ray spectroscopy (EDXS)) and X-ray absorption spectroscopy (XAS). We then explain the universal methods used to prepare bimetallic aerogels with different compositions. Those are divided into one-step methods in which gels formed from mixtures of the respective metal salts are coreduced and two-step approaches in which monometallic nanoparticles are mixed and gelated. Subsequently, we summarize the current state-of-knowledge on the element distributions unraveled using diverse characterization methods. This is extended to investigations of the element distributions being altered during electrochemical cycling or other loads. So far, a theoretical understanding of these processes is sparse, not to mention predictions of element distributions. The Account concludes with a series of remarks on current challenges in the field and an outlook on the gains that the field would earn from a solid understanding of the underlying processes and a predictive theoretical backing.

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Bottom-up Fabrication of FeSb₂ Nanowires on Crystalline GaAs Substrates with Ion-induced Pre-patterning

Weinert, T.; Erb, D.; Hübner, R.; Facsko, S.

Most industrial processes are generating waste heat that can be converted into electrical energy with thermoelectric generators (TEGs). For efficient energy harvesting, it is necessary to significantly improve the properties like Seebeck coefficient, electrical and thermal conductivity of the thermoelectric materials in the TEGs. One promising approach are thermoelectric nanostructures to reduce the thermal conductivity while maintaining constant electrical conductivity and Seebeck coefficient. For that reason, this study investigated the possibility of preparing nanowires of the thermoelectric material iron antimonide (FeSb₂) on crystalline gallium arsenide GaAs(001) substrates with ion-induced surface nanopatterning.
The GaAs(001) substrates were pre-patterned using 1 keV Ar⁺ ion irradiation. By using an ion source with a broad, unfocused ion beam at normal incidence, the patterned area can be scaled to nearly any size. The self-organized surface structure is formed by reverse epitaxy and is characterized by almost perfectly parallel-aligned ripples at the nanometer scale. For the fabrication of FeSb₂ nanowires, iron and antimony were successively deposited on the prepatterned GaAs substrates at grazing incidence and then annealed. They were characterized using transmission electron microscopy (TEM), in particular high-resolution TEM imaging for structure analysis and spectrum imaging analysis based on energy-dispersive X-ray spectroscopy
for element characterization.
With the presented fabrication method, FeSb₂ nanowires were produced successfully on GaAs(001) substrates with an ion-induced nanopatterned surface. The nanowires have a polycristalline structure and a cross-sectional area which is scalable up to 22×22nm². Due to the highly ordered nanostructure of the GaAs substrates, the nanowires have a length of several micrometer. These bottom-up nanofabrication based on ion-induced patterning can be a viable alternative to top-down procedures regarding to efficiency and costs.

Keywords: bottom-up nanofabrication; ion-induced nanopatterning; physical vapor deposition; transmission electron microscopy; energy-dispersive X-ray spectroscopy

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Fabrication of highly n-type-doped germanium nanowires and Ohmic contacts using ion implantation and flash lamp annealing

Echresh, A.; Prucnal, S.; Li, Z.; Hübner, R.; Ganss, F.; Steuer, O.; Bärwolf, F.; Jazavandi Ghamsari, S.; Helm, M.; Zhou, S.; Erbe, A.; Rebohle, L.; Georgiev, Y.

Accurate control of doping and fabrication of metal contacts on n-type germanium nanowires (GeNWs) with low resistance and linear characteristics remain a major challenge in germanium-based nanoelectronics. Here, we present a combined approach to fabricate Ohmic contacts on n-type-doped GeNWs. Phosphorus (P) implantation followed by millisecond rear-side flash lamp annealing was used to produce highly n-type doped Ge with an electron concentration in the order of 10^19 − 10^20 cm^(−3). Electron beam lithography, inductively coupled plasma reactive ion etching, and nickel (Ni) deposition were used to fabricate GeNW-based devices with symmetric Hall bar configuration, which allows detailed electrical characterization of the NWs. Afterward, rear-side flash lamp annealing was applied to form Ni germanide at the Ni-GeNWs contacts to reduce the Schottky barrier height. The two-probe current-voltage measurements on n-type-doped GeNWs exhibit linear Ohmic behavior. Also, the size-dependent electrical measurements showed that carrier scattering near the NW surfaces and reduction of the effective NW cross-section dominate the charge transport in the GeNWs.

Keywords: Germanium nanowires; ion implantation; flash lamp annealing; n-type doped; Ohmic contacts; Hall bar configuration

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Influence of substrate composition on size and chemical state of ion beam synthesised Co nanoparticles – Towards fabrication of electrodes for energy devices

Gupta, P.; Jovic, V.; Hübner, R.; Anquillare, E.; Suschke, K.; Smith, K. E.; Markwitz, A.; Waterhouse, G. I. N.; Kennedy, J.

A one-step approach to synthesize ultrafine transition metal particles (size < 5 nm) in carbon substrates is highly desirable for fabricating electrodes for energy devices. Herein, cobalt ion implantation into amorphous carbon films (a:C) and hydrogenated amorphous carbon films (a:CH) was explored, with the aim of synthesizing ultrafine metallic cobalt nanoparticles at room temperature. Co ions of 30 keV energy were implanted into the carbon films to achieve a Co areal density of 1.0 ± 0.1 × 1017 atoms cm-2. Rutherford backscattering measurements revealed that hydrogenated amorphous carbon films gave a broader Co depth distribution compared to the amorphous carbon films. Further, cross-sectional TEM analysis revealed that hydrogenated carbon films suppressed metallic Co nanoparticle aggregation, leading to the creation of ultrafine Co nanoparticles (size < 5 nm). Co L-edge X-ray absorption spectroscopy measurements confirmed the formation of predominantly metallic Co nanoparticles by ion implantation. Results conclusively demonstrate that the presence of hydrogen (~ 28 at %) in the carbon matrix facilitates the synthesis of ultrafine metallic Co nanoparticles during Co ion implantation.

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A Decade of Electrocatalysis with Metal Aerogels: A Perspective

Li, W.; Weng, B.; Sun, X.; Cai, B.; Hübner, R.; Luo, Y.; Du, R.

Nowadays, great efforts have been spent on addressing concerns over energy and environmental crises. Among these efforts, electrocatalysis is widely recognized and studied for its high efficiency and easy processability. As a class of emerging electrocatalysts, metal aerogels (MAs) stand out in the last decade. In virtue of their three‐dimensional conductive pathways, their library of catalytically/optically active sites, and their robust network structures, MAs have unique advantages in electrocatalysis. However, due to the short history of MAs, there is insufficient research on them, leaving significant room for material design and performance optimization. This perspective will mainly focus on electrocatalysis with MAs, aiming to summarize the state‐of‐the‐art progress and to guide the on‐target design of efficient MAs‐based electrocatalysts towards energy‐ and environment‐related applications.

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Tailored Particle Catalysts for Multistep One-pot Chemoenzymatic Cascade in Pickering Emulsions

Wang, S.; Scandurra, L.; Hübner, R.; Gro Nielsen, U.; Wu, C.

Chemoenzymatic cascades are an important tool for advanced synthesis in chemistry. However, these cascades are often limited due to the incompatibility issue between two distinct catalysts and reactions. To address this issue, we present a simple multistep one-pot platform, in which nanoparticle catalysts are prepared to allow chemo- and biocatalytic reactions performed sequentially in water and Pickering emulsions. The preparation of particle catalysts is accomplished in just two steps by polymer modifications and [RuCl2(pcymene)]2 coordination, while the benefits of using them for chemoenzymatic synthesis are multifaceted. They act not only as asymmetric catalysts for asymmetric transfer hydrogenation from acetophenone to 1-phenylethanol in water with up to 99 % conversion and 93 % ee, but also as an emulsifier to form stable Pickering emulsions. By the addition of Candida antarctica lipase B into the emulsions, the second-step reaction of enantioselective acylation was achieved with 38 % conversion and 99 % ee. Therefore, we successfully present a simple method to enable chemoenzymatic cascades by combining particle catalysts and enzymes in water and Pickering emulsions in a sequential fashion, which can be generalized for other cascade syntheses with different chemo- and biocatalysts in the future.

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Interparticle Charge-Transport-Enhanced Electrochemiluminescence of Quantum-Dot Aerogels

Gao, X.; Jiang, G.; Gao, C.; Prudnikau, A.; Hübner, R.; Zhan, J.; Zou, G.; Eychmüller, A.; Cai, B.

Electrochemiluminescence (ECL) represents a widely explored technique to generate light, in which the emission intensity relies critically on the charge-transfer reactions between electrogenerated radicals. Two types of charge-transfer mechanisms have been postulated for ECL generation, but the manipulation and effective probing of these routes remain a fundamental challenge. Here, we demonstrate the design of quantum dot (QD) aerogels as novel ECL luminophores via a versatile water-induced gelation strategy. The strong electronic coupling between adjacent QDs enables efficient charge transport within the aerogel network, leading to the generation of highly efficient ECL based on the selectively improved interparticle chargetransfer route. This mechanism is further verified by designing CdSe-CdTe mixed QD aerogels, where the two mechanistic routes are clearly decoupled for ECL generation. We anticipate our work will advance the fundamental understanding of ECL and prove useful for designing next-generation QD-based devices.

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WAlSiN-based solar selective coating stability-study under heating and cooling cycles in vacuum up to 800 °C using in situ Rutherford backscattering spectrometry and spectroscopic ellipsometry

Niranjan, K.; Krause, M.; Lungwitz, F.; Munnik, F.; Hübner, R.; Pemmasani, S. P.; Escobar Galindo, R.; Barshilia, H. C.

In situ Rutherford Backscattering Spectrometry (RBS) and Spectroscopic Ellipsometry (SE) were applied to study the compositional and optical stability of a WAlSiN-based solar-selective coating (SSC) at high temperatures in vacuum. The samples were exposed to heating-cooling cycles between quasi room temperature and stepwise-increased high temperatures of 450 °C, 650 °C, and 800 °C, respectively. In situ RBS revealed full compositional stability of the SSC during thermal cycling. In situ SE indicated full conservation of the optical response at 450 °C and 650 °C, and minimal changes at 800 °C. The analysis of the ex situ optical reflectance spectra after the complete thermal cycling gave an unchanged solar absorptance of 0.94 and a slightly higher calculated thermal emittance at 800 °C of 0.16 compared to 0.15 after deposition. Cross-sectional element distribution analysis performed in scanning transmission electron microscopy mode confirmed the conservation of the SSC’s microstructure after the heating – cooling cycles. The study demonstrates compositional, optical, and structural stability of the WAlSiN-based solar-selective coating at temperatures targeted for the next generation of concentrated solar power plants.

Keywords: Concentrated solar power; high-temperature solar-selective coatings; nanolaminates; in situ analysis; ion beam analysis; STEM-EDXS imaging

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  • Secondary publication expected

Recovery of Release Cloud from Laser Shock-Loaded Graphite and Hydrocarbon Targets: In Search of Diamonds

Schuster, A. K.; Voigt, K.; Klemmed, B.; Hartley, N. J.; Lütgert, B. J.; Bähtz, C.; Benad, A.; Brabetz, C.; Cowan, T.; Doeppner, T.; Erb, D.; Eychmueller, A.; Facsko, S.; Falcone, R. W.; Fletcher, L. B.; Frydrych, S.; Ganzenmüller, G. C.; Gericke, D. O.; Glenzer, S. H.; Grenzer, J.; Helbig, U.; Hiermaier, S.; Hübner, R.; Laso García, A.; Lee, H. J.; Macdonald, M. J.; McBride, E. E.; Neumayer, P.; Pak, A.; Pelka, A.; Prencipe, I.; Prosvetov, A.; Rack, A.; Ravasio, A.; Redmer, R.; Reemts, D.; Rödel, M.; Schoelmerich, M.; Schumacher, D.; Tomut, M.; Turner, S. J.; Saunders, A. M.; Sun, P.; Vorberger, J.; Zettl, A.; Kraus, D.

This work presents first insights into the dynamics of free-surface release clouds from dynamically compressed polystyrene and pyrolytic graphite at pressures up to 200 GPa, where they transform into diamond or lonsdaleite, respectively. These ejecta clouds are released into either vacuum or various types of catcher systems, and are monitored with high-speed recordings (frame rates up to 10 MHz). Molecular dynamics simulations are used to give insights to the rate of diamond preservation throughout the free expansion and the catcher impact process, highlighting the challenges of diamond retrieval. Raman spectroscopy data show graphitic signatures on a catcher plate confirming that the shock-compressed PS is transformed. First electron microscopy analyses of solid catcher plates yield an outstanding number of different spherical-like objects in the size range between ten(s) up to hundreds of nanometres, which are one type of two potential diamond candidates identified. The origin of some objects can unambiguously be assigned, while the history of others remains speculative.

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CdSexS1−x Alloyed Nanoplatelets with Continuously Tunable Blue-Green Emission

Antanovich, A.; Yang, L.; Erwin, S. C.; Martín-García, B.; Hübner, R.; Steinbach, C.; Schwarz, D.; Gaponik, N.; Lesnyak, V.

Cadmium chalcogenide nanoplatelets (NPLs) are established as promising materials for a wide variety of optoelectronic applications due to their properties surpassing in many aspects their counterpart nanocrystals (NCs) with other shapes. Most of these features arise from strong quantum confinement in the direction of thickness which can be tuned with precision down to one monolayer. However, atomic smoothness of their basal planes and hence the ability to change the NPL thickness only in discrete steps prevent precise tuning of absorption and photoluminescence spectra unlike in the case of quantum dots. Preparation of alloyed NCs provides a potential solution to this problem, but it is complicated by the different reactivities of chalcogenide sources, which becomes even more restrictive in the case of NPLs because they are more sensitive to alterations of reaction conditions. In this work, we overcome this obstacle by employing highly reactive stearoyl sulfide and selenide as chalcogen sources, which enable straightforward variation of the NPL composition and thickness by changing the ratio of chalcogen precursors and reaction temperature, respectively. Alloyed CdSexS1−x NPLs
obtained exhibit tunable absorption and photoluminescence bands covering the blue-green region from 380 to 520 nm with bright band-edge emission and quantum yields of ∼30−50% due to their relatively small lateral size enabled by a much finer control of the lateral growth.

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Formation of vertical SnSe/SnSe2 p-n heterojunction by NH3 plasma-induced phase transformation

Li, Y.; Duan, J.; Berencen, Y.; Hübner, R.; Tsai, H.-S.; Kuo, C.-N.; Chin-Shan, L.; Helm, M.; Zhou, S.; Prucnal, S.

Layered van der Waals crystals host unique properties making them attractive for applications in nanoelectronics, optoelectronics, and sensing. The integration of two-dimensional materials with complementary metal-oxide-semiconductor (CMOS) technology requires controllable n- and p-type doping. In this work, we demonstrate the fabrication of vertical p-n heterojunctions made of p-type tin monoselenide (SnSe) and n-type tin diselenide (SnSe2). The p-n heterojunction is created in a single flake by the NH3-plasma-assisted phase transformation from SnSe2 to SnSe. We show that the transformation rate and crystal quality strongly depend on the plasma parameters like plasma power, temperature, partial pressure, NH3 flow, and duration of plasma treatment. With optimal plasma parameters, the full transformation of SnSe2 flakes to SnSe is achieved within a few seconds. The crystal quality and the topography of the fabricated SnSe-SnSe2 heterostructures are investigated using micro-Raman spectroscopy and cross-sectional transmission electron microscopy. The formation of a p-n junction is verified by current-voltage measurements.

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Robust spin injection via thermal magnon pumping in antiferromagnet/ferromagnet hybrid systems

Rodriguez, R.; Regmi, S.; Zhang, H.; Yuan, W.; Makushko, P.; Montoya, E. A.; Veremchuk, I.; Hübner, R.; Makarov, D.; Shi, J.; Cheng, R.; Barsukov, I.

Robust spin injection and detection in antiferromagnetic thin films is a prerequisite for the exploration
of antiferromagnetic spin dynamics and the development of nanoscale antiferromagnet-based spintronic applications.
Previous studies have shown spin injection and detection in antiferromagnet/nonmagnetic metal
bilayers; however, spin injection in these systems has been found effective at cryogenic temperatures only.
Here, we experimentally demonstrate sizable interfacial spin transport in a hybrid antiferromagnet/ferromagnet
system, consisting of Cr2O3 and permalloy, which remains robust up to the room temperature. We examine our
experimental data within a spin diffusion model and find evidence for the important role of interfacial magnon
pumping in the signal generation. The results bridge spin-orbitronic phenomena of ferromagnetic metals with
antiferromagnetic spintronics and demonstrate an advancement toward antiferromagnetic spin-torque devices.

Keywords: antiferromagnetic spintronics; Cr2O3 thin films; spin injection

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Seed-Mediated Synthesis of Photoluminescent Cu−Zn−In−S Nanoplatelets

Bora, A.; Prudnikau, A.; Fu, N.; Hübner, R.; Borchert, K. B. L.; Schwarz, D.; Gaponik, N.; Lesnyak, V.

Ternary and quaternary colloidal nanocrystals (NCs) based on I−III−VI group semiconductors are promising low-toxic luminescent materials attracting huge interest as alternatives to cadmium- and lead-chalcogenide-based NCs. Despite significant progress in the synthesis of three-dimensionally confined quantum dots based on I−III−VI semiconductors with intensive photoluminescence (PL) in a broad spectral range, all attempts to prepare one-dimensionally confined nanoplatelets (NPLs) or nanosheets have resulted in rather nonemitting two-dimensional (2D) NCs. Since 2D NCs of the II−VI group exhibit unique anisotropic optical properties, exploring synthetic strategies to obtain 2D I−III−VI-based NPLs might also reveal interesting optical and electronic features. In this work, we demonstrate the synthesis of luminescent In-rich Cu−Zn−In−S (CZIS) NPLs using a one-pot approach. The synthesis includes the formation of
Cu−In−S NPLs from In2S3 seeds, followed by the incorporation of zinc to form quaternary NPLs with improved stability and optical properties. The synthetic strategy implemented results in the formation of ∼1 nm thick NPLs with lateral sizes of ∼30 × 10 nm2 and a tetragonal crystal structure. As-synthesized NPLs are stable at ambient conditions and demonstrate PL in the range of 700−800 nm with a large Stokes shift. An additional shell of ZnS grown on CZIS NPLs resulted in the enhancement of their PL quantum yield reaching 29%.

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CMOS compatible manufacturing of a hybrid SET-FET circuit

Del Moral, A.; Amat, E.; Engelmann, H.-J.; Pourteau, M.-L.; Rademaker, G.; Quirion, D.; Torres-Herrero, N.; Rommel, M.; Heinig, K.-H.; von Borany, J.; Tiron, R.; Bausells, J.; Perez-Murano, F.

This study analyzes the CMOS compatibility in the manufacturing of a hybrid SET-FET circuit. The fundamental element towards an operating SET at room temperature is a vertical nanopillar with embedded Si nanodot generated by ion-beam irradiation. The integration process from nanopillars to contacted SETs is validated by structural characterization. Then, the monolithic fabrication of planar FETs co-integrated with vertical SETs is presented, and its compatibility with standard CMOS technology is demonstrated. The work includes process optimization, pillar integrity validation, electrical characterization and simulation taking into account parasitic elements. The FET fabrication process is adapted to meet the requirements of the pre-fabricated nanopillars. Overall, this work establishes the groundwork for the realization of a hybrid SET-FET circuit operating at room temperature.

Keywords: CMOS; MOSFET; vertical nanopillar; single electron transistor; hybrid circuit

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Tuning Iron-Oxygen Covalency in Perovskite Oxides for Efficient Electrochemical Sensing

Gao, C.; Lu, Y.; Wang, Y.; Wang, C.; Hübner, R.; Li, Y.; Zhan, J.; Zhao, M.; Cai, B.

Transition metal oxides have been extensively explored as novel catalysts for designing electrochemical sensors, but the underlying structure-activity relationship remains poorly understood. Herein, we explore a diverse chemical range of La1-xSrxFeO3 perovskite oxides by evaluating their electrochemical sensing activity toward heavy metals and by determining their electronic structures using density functional theory. We find that tuning perovskite chemistry plays an important role in determining the electrochemical activities and sensitivities, as well as the valence states of Fe. By combining experimental and theoretical analyses, a linear relationship between the Fe−O covalency and the electrochemical activity and sensitivity has been obtained, where LaFeO3 exhibits the highest activity of 109 mA cmoxide -2.Thus, the Fe−O covalency is proposed as a rational activity descriptor for the electrochemical sensing of heavy metals. A novel solid-state gelation method was further developed for the fabrication of perovskite oxide aerogels, based on which a highly efficient electrochemical sensor was constructed with a high sensitivity of 87.06 μM μA-1 and a low detection limit of 1.7 nM. This work unlocks an effective parameter, that is, Fe−O covalency, for rationally designing Fe-based oxides and deepening the understanding of fundamental parameters to develop highly efficient sensing platforms.

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Plasmonic Nanoparticles Embedded in Nanomembrane Microcavity for Flexible Optical Tuning

Pang, C.; Li, R.; Dong, H.; Saggau, C. N.; Kern, F. L.; Potapov, P.; Schultz, J.; Lubk, A.; Hübner, R.; Kentsch, U.; Zhou, S.; Helm, M.; Chen, F.; Ma, L.; Schmidt, O. G.

The combination of plasmonic nanoparticles and optical microcavities has attracted broad interest for both fundamental and applied studies. However, the conventional scheme of plasmonic nanoparticles being located at microcavity outer surfaces suffers from serious problems such as significant radiative/scattering losses and chemical/mechanical instabilities. Here, silver nanoparticles (NPs) and dispersed ions embedded in nanomembrane-formed whispering-gallery-mode (WGM) microtube cavities are prepared by ion implantations as compact and stable optoplasmonic microcavities. Upon low ion fluence implantation, dispersed silver ions are generated in the tube cavity wall, leading to a redshift of the WGM resonant cavity modes due to the increased refractive index. The silver ions start to aggregate into plasmonic NPs in the cavity wall when increasing implantation ion fluences. The competition and transition between redshift induced by the refractive index increase and blueshift induced by the formation of plasmonic NPs are investigated. Moreover, quality factor enhancement of the WGM modes is observed owing to the improved light confinement caused by the presence of NPs. This work demonstrates a convenient approach for the fabrication of stable optoplasmonic microcavities and fine tuning of resonant modes, indicating wide applications such as wavelength selective tuning and enhanced light–matter interactions.

Keywords: ion implantation; microtube cavity; nanomembrane; plasmonic nanoparticles; resonant mode tuning

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Aliovalent Ta-Doping-Engineered Oxygen Vacancy Configurations for Ultralow-Voltage Resistive Memory Devices: A DFT-Supported Experimental Study

Barman, A.; Das, D.; Deshmukh, S.; Sarkar, P. K.; Banerjee, D.; Hübner, R.; Gupta, M.; Saini, C. P.; Kumar, S.; Johari, P.; Dhar, S.; Kanjilal, A.

Alteration of transport properties of any material, especially metal oxides, by doping suitable impurities is not straightforward as it may introduce multiple defects like oxygen vacancies (Vo) in the system. It plays a decisive role in controlling the resistive switching (RS) performance of metal oxide-based memory devices. Therefore, a judicious choice of dopants and their atomic concentrations is crucial for achieving an optimum Vo configuration. Here, we show that the rational designing of RS memory devices with cationic dopants (Ta), in particular, Au/Ti1−xTaxO2−δ/Pt devices, is promising for the upcoming non-volatile memory technology. Indeed, a current window of ∼104 is realized at an ultralow voltage as low as 0.25 V with significant retention (∼104 s) and endurance (∼105 cycles) of the device by considering 1.11 at % Ta doping. The obtained device parameters are compared with those in the available literature to establish its excellent performance. Furthermore, using detailed experimental analyses and density functional theory (DFT)-based first-principles calculations, we comprehend that the meticulous presence of Vo configurations and the columnar-like dendritic structures is crucial for achieving ultralow-voltage bipolar RS characteristics. In fact, the dopant-mediated Vo interactions are found to be responsible for the enhancement in local current conduction, as evidenced from the DFT-simulated electron localization function plots, and these, in turn, augment the device performance. Overall, the present study on cationic-dopant-controlled defect engineering could pave a neoteric direction for future energy-efficient oxide memristors.

Keywords: resistive memory; vacancy engineering; ultralow-voltage switching; conducting filaments; first-principles calculations

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CO2 Electroreduction on Unsupported PdPt Aerogels: Effects of Alloying and Surface Composition on Product Selectivity

Diercks, J. S.; Georgi, M.; Herranz, J.; Diklić, N.; Chauhan, P.; Clark, A. H.; Hübner, R.; Faisnel, A.; Chen, Q.; Nachtegaal, M.; Eychmüller, A.; Schmidt, T. J.

Due to its unique ability to reduce carbon dioxide (CO2) into CO or formate at high versus low overpotentials, respectively, palladium is a promising catalyst for the electrochemical CO2-reduction reaction (CO2RR). Further improvements aim at increasing its activity and selectivity toward either of these value-added species, while reducing the amount of hydrogen produced as a side product. With this motivation, in this work, we synthesized a range of unsupported, bimetallic PdPt aerogels and pure Pt or Pd aerogels and extensively characterized them using various microscopic and spectroscopic techniques. These revealed that the aerogels’ porous web consists of homogenous alloys of Pt and Pd, with palladium and platinum being present on their surface for all compositions. The subsequent determination of these aeorgels’ CO2RR performance unveiled that the high activity of these Pt surface atoms toward hydrogen evolution causes all PdPt alloys to favor this reaction over CO2 reduction. In the case of the pure Pd aerogel, although, its unsupported nature leads to a suppression of H2 evolution and a concomitant increase in the selectivity toward CO when compared to a commercial, carbon-supported Pd-nanoparticle catalyst.

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Presence of uranium(V) during uranium(VI) reduction by Desulfosporosinus hippei DSM 8344T

Hilpmann, S.; Roßberg, A.; Steudtner, R.; Drobot, B.; Hübner, R.; Bok, F.; Prieur, D.; Bauters, S.; Kvashnina, K.; Stumpf, T.; Cherkouk, A.

Microbial U(VI) reduction influences the uranium mobility in contaminated subsurface environments and can affect the disposal of high-level radioactive waste by transform-ing the water-soluble U(VI) to less mobile U(IV). The reduction of U(VI) by the sulfate-reducing bacterium Desulfosporosinus hippei DSM 8344T, a close phylogenetic relative to naturally occurring microorganism present in clay rock and bentonite, was investigat-ed. D. hippei DSM 8344T showed a relatively fast removal of uranium from the superna-tants in artificial Opalinus Clay pore water. Combined speciation calculations and lumi-nescence spectroscopic investigations showed the dependence of U(VI) reduction on the initial U(VI) species. Scanning transmission electron microscopy coupled with ener-gy-dispersive X-ray spectroscopy showed uranium-containing aggregates on the cell surface and the formation of membrane vesicles. By combining different spectroscopic techniques, including UV/Vis spectroscopy, as well as uranium M4-edge X-ray absorp-tion near-edge structure (XANES) recorded in high-energy-resolution fluorescence-detection (HERFD) mode and extended X-ray absorption fine structure (EXAFS) analy-sis, the partial reduction of U(VI) could be verified, whereby the formed U(IV) product has an unknown structure. Furthermore, the U M4 HERFD-XANES showed the presence of U(V) during the process, suggesting a single-electron transfer mechanism for the microbial U(VI) reduction by sulfate reducers. These findings offer new insights into the U(VI) reduction by sulfate-reducing bacteria and contribute to a comprehensive safety concept for a repository for high-level radioactive waste.

Keywords: Uranium(VI) reduction; Sulfate-reducing bacteria; Opalinus Clay pore water; Pentavalent uranium; Membrane vesicles

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Controllable electrostatic manipulation of structure building blocks in noble metal aerogels

Wei, W.; Hübner, R.; Georgi, M.; Wang, C.; Wu, X.; Eychmüller, A.

The important role of structure homogeneity in three-dimensional network nanostructures serving as noble metal aerogels (NMAs) has attracted extensive attention in the field of electrochemistry in the last two decades, whereas a comprehensive study of tailoring skeleton units and element distributions in NMAs is still lacking. Herein, a new modulation strategy to easily prepare multiscale NMAs with tunable composition is developed by utilizing the electrostatic interaction between oppositely charged colloidal metal nanoparticles. The modulation rule of the chemical distribution in bimetallic aerogels leads to the construction of the as-tailored double skeleton aerogels for the first time. Considering their specific structures, the intrinsic and exceptional catalytic and electrocatalytic performances of NMAs were investigated. This study optimizes the structure homogeneity of noble metal aerogels by investigating nanoparticle–ligand interactions and provides further proof of their exceptional electrocatalytic
capabilities.

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Homogenization and short-range chemical ordering of Co-Pt alloys driven by the grain boundary migration mechanism

Pedan, R.; Makushko, P.; Dubikovskyi, O.; Bodnaruk, A.; Burmak, A.; Sidorenko, S.; Voloshko, S.; Kalita, V.; Hübner, R.; Makarov, D.; Vladymyrskyi, I.

Binary magnetic alloys like Co-Pt are relevant for applications as components of magnetic exchange coupled composites. Numerous approaches exist to tune the coercive field of Co-Pt alloys primarily relying on hightemperature processing aiming to realize chemically long-range ordered phases. The peculiarity of Co-Pt is that large coercive field and magnetic anisotropy can be achieved even in chemically disordered alloys relying on short-range order. Here, we study alloying of Co-Pt from bilayers of Pt(14 nm) Co(13 nm) at temperatures up to 550 degС, where bulk diffusion processes are suppressed and the dominant diffusion mechanism is grain boundary migration. We demonstrate that grain boundary diffusion mechanism can lead to the realization of a homogeneous yet chemically disordered Co56Pt44 alloy at temperatures of 500 degС and higher. A pronounced increase of the coercive field for samples processed at temperatures higher than 400 degС is attributed to short-range ordering. With this work, we pinpoint the grain boundary diffusion as the mechanism responsible not only for the homogenization of binary alloy films but also as a driving force for the realization of short-range order in Co-Pt. Our results motivate further research on grain boundary diffusion as a mechanism to realize chemically long-range ordered phases in Co-Pt alloys.

Keywords: grain boundary diffusion; magnetic thin films; short-range chemical order; Co-Pt alloy

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Pickering interfacial catalysts for asymmetric organocatalysis

Sun, Z.; Jurica, J.; Hübner, R.; Wu, C.

Proline-catalyzed aldol reactions have been developed as an important toolbox for the synthesis of valuable chiral intermediates, giving birth to asymmetric organocatalysis. Despite progress, their current applications are generally performed in highly polar solvents that are either difficult to remove or with low substrate/product solubility. In addition, prolines are often used as homogeneous organocatalysts in these solvents, thus, the recycling of catalyst for reuse is also challenging. To solve these problems, we develop a proline-based Pickering emulsion for asymmetric aldol reactions with high reactivity and selectivity. The emulsion was stabilized by proline-functionalized silica nanoparticles that are not only highly active in the presence of water but also easily recycled after the operation. Interestingly, their high stereoselectivity was not compromised after multiple reuse, i.e., >86 ee (enantiomeric excess) in the first and second use. With this demonstration, we prove the concept that efficient and selective aldol reactions are enabled by proline-based Pickering emulsions, which is a great and continuous contribution to the field of asymmetric organocatalysis.

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Artificially sporulated Escherichia coli cells as a robust cell factory for interfacial biocatalysis

Sun, Z.; Hübner, R.; Li, J.; Wu, C.

The natural bacterial spores have inspired the development of artificial spores, through coating cells with protective materials, for durable whole-cell catalysis. Despite attractiveness, artificial spores developed to date are generally limited to a few microorganisms with their natural endogenous enzymes, and they have never been explored as a generic platform for widespread synthesis. Here, we report a general approach to designing artificial spores based on Escherichia coli cells with recombinant enzymes. The artificial spores are simply prepared by coating cells with polydopamine, which can withstand UV radiation, heating and organic solvents. Additionally, the protective coating enables living cells to stabilize aqueous-organic emulsions for efficient interfacial biocatalysis ranging from single reactions to multienzyme cascades. Furthermore, the interfacial system can be easily expanded to chemoenzymatic synthesis by combining artificial spores with metal catalysts. Therefore, this artificial-spore-based platform technology is envisioned to lay the foundation for nextgeneration cell factory engineering.

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CMOS-compatible manufacturability of sub-15 nm Si/SiO2/Si nanopillars containing single Si nanodots for single electron transistor applications

Borany, J.; Engelmann, H.-J.; Heinig, K.-H.; Amat, E.; Hlawacek, G.; Klüpfel, F.; Hübner, R.; Möller, W.; Pourteau, M.-L.; Rademaker, G.; Rommel, M.; Baier, L.; Pichler, P.; Perez-Murano, F.; Tiron, R.

This study addresses the complementary metal-oxide-semiconductor-compatible fabrication of vertically stacked Si/SiO2/Si nanopillars (NPs) with embedded Si nanodots (NDs) as key functional elements of a quantum-dot-based, gate-all-around single-electron transistor (SET) operating at room temperature. The main geometrical parameters of the NPs and NDs were deduced from SET device simulations using the nextnano++ program package. The basic concept for single silicon ND formation within a confined oxide volume was deduced from Monte-Carlo simulations of ion-beam mixing and SiOx phase separation. A process flow was developed and experimentally implemented by combining bottom-up (Si ND self-assembly) and top-down (ion-beam mixing, electron-beam lithography, reactive ion etching) technologies, fully satisfying process requirements of future 3D device architectures. The theoretically predicted self-assembly of a single Si ND via phase separation within a confined SiOx disc of < 500 nm³ volume was experimentally validated. This work describes in detail the optimization of conditions required for NP/ND formation, such as the oxide thickness, energy and fluence of ion-beam mixing, thermal budget for phase separation and parameters of reactive ion beam etching. Low-temperature plasma oxidation was used to further reduce NP diameter and for gate oxide fabrication whilst preserving the pre-existing NDs. The influence of critical dimension variability on the SET functionality and options to reduce such deviations are discussed. We finally demonstrate the reliable formation of Si quantum dots with diameters of less than 3 nm in the oxide layer of a stacked Si/SiO2/Si NP of 10 nm diameter, with tunnelling distances of about 1 nm between the Si ND and the neighboured Si regions forming drain and source of the SET.

Keywords: CMOS; single-electron transistor; nanostructure fabrication; nanpillars; silicon nanodot; self-organization; ion-beam mixing

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Flexomagnetism and vertically graded Néel temperature of antiferromagnetic Cr2O3 thin films

Makushko, P.; Kosub, T.; Pylypovskyi, O.; Hedrich, N.; Li, J.; Pashkin, O.; Avdoshenko, S.; Hübner, R.; Ganss, F.; Liedke, M. O.; Butterling, M.; Wagner, A.; Wagner, K.; Shields, B. J.; Lehmann, P.; Veremchuk, I.; Faßbender, J.; Maletinsky, P.; Makarov, D.

Antiferromagnetic insulators are a prospective material science platform for magnonics, spin superfluidity, THz spintronics, and non-volatile data storage. A magnetomechanical coupling in antiferromagnets offers vast advantages in the control and manipulation of the primary order parameter yet remains largely unexplored both fundamentally and technologically. Here, we discover a new member in the family of flexoeffects in thin films of technologically relevant antiferromagnetic Cr2O3. We demonstrate that a gradient of mechanical strain can impact the magnetic phase transition resulting in the distribution of the N ́eel temperature along the thickness of a 50-nm-thick film and induces a sizable flexomagnetic coefficient of about 15 μb/nm2 originating from the inhomogeneous reduction of the antiferromagnetic order parameter. The antiferromagnetic ordering in inhomogeneously strained thin films of Cr2O3 can persist up to 100◦ C, rendering Cr2O3 relevant for industrial electronics applications. The presence of a strain gradient in thin films of Cr2O3 may therefore allow for the realization of reconfigurable antiferromagnetic racetracks, magnonic waveguides and magnon crystals. The presence of a strain gradient in ultrathin films of Cr2O3 enables new fundamental research directions on magnetomechanics and thermodynamics of antiferromagnetic solitons, spin waves and artificial spin ice systems in magnetic materials with continuously graded parameters.

Keywords: antiferromagnetism; flexomagnetism; Cr2O3; Neel temperature; NV magnetometry; magnetotransport

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Expanding the Range: AuCu Metal Aerogels from H2O and EtOH

Georgi, M.; Kresse, J.; Hiekel, K.; Hübner, R.; Eychmüller, A.

Due to their self-supporting and nanoparticulate structure, metal aerogels have emerged as excellent electrocatalysts, especially in the light of the shift to renewable energy cycles. While a large number of synthesis parameters have already been studied in depth, only superficial attention has been paid to the solvent. In order to investigate the influence of this parameter with respect to the gelation time, crystallinity, morphology, or porosity of metal gels, AuxCuy aerogels were prepared in water and ethanol. It was shown that although gelation in water leads to highly porous gels (60 m2g-1), a CuO phase forms during this process. The undesired oxide could be selectively removed using a post-washing step with formic acid. In contrast, the solvent change to EtOH led to a halving of the gelation time and the suppression of Cu oxidation. Thus, pure Cu aerogels were synthesized in addition to various bimetallic Au3X (X = Ni, Fe, Co) gels. The faster gelation, caused by the lower permittivity of EtOH, led to the formation of thicker gel strands, which resulted in a lower porosity of the AuxCuy aerogels. The advantage given by the solvent choice simplifies the preparation of metal aerogels and provides deeper knowledge about their gelation.

Keywords: metal; aerogel; gold; copper; ethanol; water; solvent; bimetallic; porous; one-step

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Speciation and spatial distribution of Eu(III) in fungal mycelium

Günther, A.; Wollenberg, A.; Vogel, M.; Drobot, B.; Steudtner, R.; Freitag, L.; Hübner, R.; Stumpf, T.; Raff, J.

Europium, as an easy-to-study analog of the trivalent actinides, is of particular importance for studying the behavior of lanthanides and actinides in the environment. Since different soil organisms can influence the migration behavior of these elements, a detailed knowledge of these interaction mechanisms is important. The aim of this study was to investigate the interaction of mycelia of selected wood-inhabiting (S. commune, P. ostreatus, L. tigrinus) and soil-inhabiting fungi (L. naucinus) with Eu(III). In addition to determining the Eu(III) complexes in the sorption solution, the formed Eu(III) fungal species were characterized using scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy, chemical microscopy in combination with the time-resolved laser-induced fluorescence spectroscopy. Our data show that S. commune exhibited significantly higher Eu(III) binding capacity in comparison to the other fungi. Depending on fungal strain, the metal was immobilized on the cell surface, in the cell membranes, and within the membranes of various organelles, or in the cytoplasm in some cases. During the bioassociation process two different Eu(III) fungal species were formed in all investigated fungal strain. The phosphate groups of organic ligands were identified as being important functional groups to bind Eu(III) and thus immobilize the metal in the fungal matrix. The information obtained contributes to a better understanding of the role of fungi in migration, removal or retention mechanisms of rare earth elements and trivalent actinides in the environment.

Keywords: Fungi; Europium; Speciation; Scanning transmission electron microscopy (STEM); Chemical microscopy; Time-resolved laser-induced fluorescence spectroscopy (TRLFS)


Band-gap and strain engineering in GeSn alloys using post-growth pulsed laser melting

Steuer, O.; Schwarz, D.; Oehme, M.; Schulze, J.; Mączko, H.; Kudrawiec, R.; Fischer, I. A.; Heller, R.; Hübner, R.; Khan, M. M.; Georgiev, Y.; Zhou, S.; Helm, M.; Prucnal, S.

Alloying Ge with Sn enables effective band-gap engineering and improves significantly the charge carrier mobility. The pseudomorphic growth of Ge1-xSnx on Ge causes in-plane compressive strain, which degrades the superior properties of the Ge1-xSnx alloys. Therefore, efficient strain engineering is required. In this article, we present strain and band-gap engineering in GeSn alloys grown on Ge a virtual substrate using post-growth nanosecond pulsed laser melting (PLM). Micro-Raman and X-ray diffraction show that the initial in-plane compressive strain is removed. Moreover, for PLM energy densities higher than 0.5 J cm-2, the Ge0.89Sn0.11 layer becomes tensile strained. Simultaneously, as revealed by Rutherford Backscattering spectrometry, cross-sectional transmission electron microscopy investigations and X-ray diffraction the crystalline quality and Sn-distribution in PLM-treated Ge0.89Sn0.11 layers are only slightly affected. Additionally, the change of the band structure after PLM is also confirmed by low-temperature photoreflectance measurements. The presented results prove that post-growth ns-range PLM is an effective way for band-gap and strain engineering in highly-mismatched alloys.

Keywords: Germanium Tin; band-gap engineering; GeSn; pseudomorphic growth; pulsed laser melting; GeSn alloys; molecular-beam epitaxy; Ge1-xSnx

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  • Open Access Logo Journal of Physics: Condensed Matter 35(2023)5, 055302
    Online First (2022) DOI: 10.1088/1361-648X/aca3ea
    Cited 4 times in Scopus
  • Lecture (Conference)
    Band-gap and strain engineering in GeSn alloys using post-growth pulsed laser melting, 05.-09.09.2022, Regensburg, Deutschland

Localization and chemical speciation of europium(III) in Brassica napus plants

Jessat, J.; John, W.; Moll, H.; Vogel, M.; Steudtner, R.; Drobot, B.; Hübner, R.; Stumpf, T.; Sachs, S.

For the reliable safety assessment of repositories of highly radioactive waste, further development of the modelling of radionuclide migration and transfer in the environment is necessary, which requires a deeper process understanding at the molecular level. Eu(III) is a non-radioactive analogue for trivalent actinides, which contribute heavily to radiotoxicity in a repository. For in-depth study of the interaction of plants with trivalent f elements, we investigated the uptake, speciation, and localization of Eu(III) in Brassica napus plants at two concentrations, 30 and 200 µM, as a function of the incubation time up to 72 h. Eu(III) was used as luminescence probe for combined microscopy and chemical speciation analyses of it in Brassica napus plants. The localization of bioassociated Eu(III) in plant parts was explored by spatially resolved chemical microscopy. Three Eu(III) species were identified in the root tissue. Moreover, different luminescence spectroscopic techniques were applied for an improved Eu(III) species determination in solution. In addition, transmission electron microscopy combined with energy-dispersive X-ray spectroscopy was used to localize Eu(III) in the plant tissue, showing Eu-containing aggregates. By using this multi-method setup, a profound knowledge on the behavior of Eu(III) within plants and changes in its speciation could be obtained, showing that different Eu(III) species occur simultaneously within the root tissue and in solution.

Keywords: lanthanides; plants; laser spectroscopy; speciation; chemical microscopy; localization


Peptidoglycan as major binding motif for Uranium bioassociation on Magnetospirillum magneticum AMB-1 in contaminated waters

Krawczyk-Bärsch, E.; Ramtke, J.; Drobot, B.; Müller, K.; Steudtner, R.; Kluge, S.; Hübner, R.; Raff, J.

The mining and industrial use of heavy metals lead to locally high heavy metal contamination with serious consequences for the environment and local population. The high potential of biological remediation processes, in particular, the use of magnetotactic bacteria of heavy metal and radionuclide-contaminated waters has recently been discussed. Yet, the molecular reactions involved in the uptake of radionuclides, especially U, by these bacteria are unknown. The present work is a multidisciplinary approach combining wet chemistry, microscopy, and spectroscopy methods. Our findings suggest that the cell wall plays a prominent role in the bioassociation of U(VI). In time-dependent bioassociation studies, up to 95 % of the initial U(VI) was removed from the suspension within the first hours by Magnetospirillum magneticum AMB-1. PARAFAC analysis of TRLFS data highlights that peptidoglycan is the most important ligand involved, showing a stable immobilization of U(VI) over a wide pH range with the formation of three characteristic species. In addition, in-situ ATR FT-IR reveals the predominant binding to carboxylic functionalities, at higher pH polynuclear species seem to play an important role. This comprehensive molecular study may initiate in future new remediation strategies on effective immobilization of U in combination with the bacteria´s magnetic properties.

Keywords: Magnetotactic bacteria; Uranium; Spectroscopy; Microscopy; Bioremediation

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Epitaxial lateral overgrowth of tin spheres driven and directly observed by helium ion microscopy

Klingner, N.; Heinig, K.-H.; Tucholski, D.; Möller, W.; Hübner, R.; Bischoff, L.; Hlawacek, G.; Facsko, S.

Enhanced interstitial diffusion in tin is a phenomenon often observed during ion-beam irradiation and in lead-free solders. For the latter, this
not very well understood, strain-driven mechanism results in the growth of whiskers, which can lead to unwanted shorts in electronic designs. In ion-beam physics, this phenomenon is often observed as a result of the enhanced formation of Frenkel pairs in the energetic collision cascade. Here, we show how epitaxial growth of tin extrusions on tin-oxide-covered tin spheres can be induced and simultaneously observed by implanting helium using a helium ion microscope. Calculations of collision cascades based on the binary collision approximation and 3D-lattice-kinetic Monte Carlo simulations show that the implanted helium will occupy vacancy sites, leading to a tin interstitial excess. Sputtering and phase separation of the tin oxide skin, which is impermeable for tin atoms, create holes and will allow the epitaxial overgrowth to start. Simultaneously, helium accumulates inside the irradiated spheres. Fitting the simulations to the experimentally observed morphology allows us to estimate the tin to tin-oxide interface energy to be 1.98 J m−2 . Our approach allows the targeted initiation and in situ observation of interstitial diffusion-driven effects to improve the understanding of the tin-whisker growth mechanism observed in lead-free solders.

Keywords: helium ion microscope; tin whisker growth; defect kinetics

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Depth-Adjustable Magnetostructural Phase Transition in Fe₆₀V₄₀ Thin Films

Anwar, M. S.; Cansever, H.; Boehm, B.; Gallardo, R.; Hübner, R.; Zhou, S.; Kentsch, U.; Rauls, S.; Eggert, B.; Wende, H.; Potzger, K.; Faßbender, J.; Lenz, K.; Lindner, J.; Hellwig, O.; Bali, R.

Phase transitions occurring within spatially confined regions can be useful for generating nanoscale material property modulations. Here we describe a magneto-structural phase transition in a binary alloy, where a structural transition from short range order (SRO) to body centered cubic (bcc) results in the formation of depth-adjustable ferromagnetic layers, which reveal application-relevant magnetic properties of high saturation magnetitzation (Ms) and low Gilbert damping (α). Here we use Fe₆₀V₄₀ binary alloy films which transform from initially Ms = 17 kA/m (SRO structure) to 747 kA/m (bcc structure) driven by atomic displacements caused by penetrating ions. Simulations show that estimated ~1 displacement per atom triggers a structural transition, forming homogeneous ferromagnetic layers. The thickness of ferromagnetic layer increases as a step-like function of the ion-fluence. Microwave excitations of the ferromagnetic/non-ferromagnetic layered system reveals an α = 0.0027 ± 0.0001. The combination of nanoscale spatial confinement, low α and high Ms provide a pathway for the rapid patterning of magnetic and microwave device elements.

Keywords: Magneto-structural correlations; Phase transitions; Magnetic thin films; Ion-irradiation; Short-range order

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Tunable metal hydroxide–organic frameworks for catalysing oxygen evolution

Yuan, S.; Peng, J.; Cai, B.; Huang, Z.; Garcia-Esparza, A. T.; Sokaras, D.; Zhang, Y.; Giordano, L.; Akkiraju, K.; Guang Zhu, Y.; Hübner, R.; Zou, X.; Román-Leshkov, Y.; Shao-Horn, Y.

The oxygen evolution reaction is central to making chemicals and energy carriers using electrons. Combining the great tunability of enzymatic systems with known oxide-based catalysts can create breakthrough opportunities to achieve both high activity and stability. Here we report a series of metal hydroxide–organic frameworks (MHOFs) synthesized by transforming layered hydroxides into two-dimensional sheets crosslinked using aromatic carboxylate linkers. MHOFs act as a tunable catalytic platform for the oxygen evolution reaction, where the π–π interactions between adjacent stacked linkers dictate stability, while the nature of transition metals in the hydroxides modulates catalytic activity. Substituting Ni-based MHOFs with acidic cations or electron-withdrawing linkers enhances oxygen evolution reaction activity by over three orders of magnitude per metal site, with Fe substitution achieving a mass activity of 80 A gcatalyst -1 at 0.3 V overpotential for 20 h. Density functional theory calculationscorrelate the enhanced oxygen evolution reaction activity with the MHOF-based modulation of Ni redox and the optimized binding of oxygenated intermediates.

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Self-Supported Three-Dimensional Quantum Dot Aerogels as a Promising Photocatalyst for CO2 Reduction

Jiang, G.; Wang, J.; Li, N.; Hübner, R.; Georgi, M.; Cai, B.; Li, Z.; Lesnyak, V.; Gaponik, N.; Eychmüller, A.

With the merits of quantum dots (QDs) (e.g., high molar extinction coefficient, strong visible light absorption, large specific surface area, and abundant functional surface active sites) and aerogels (e.g., self-supported architectures, porous network), semiconductor QD aerogels show great prospect in photocatalytic applications. However, typical gelation methods rely on oxidative treatments of QDs. Moreover, the remaining organic ligands (e.g., mercaptoacids) are still present on the surface of gels. Both these factors inhibit the activity of such photocatalysts, hampering their widespread use.
Herein, we present a facile 3D assembly of II−VI semiconductor QDs capped with inorganic (NH4)2S ligands into aerogels using H2O as a dispersion solvent. Without any sacrificial agents, the resulting CdSe QD aerogels achieve a high CO generation rate of 15 μmol g-1 h-1, which is 12-fold higher than that of pristine-aggregated QD powders. Our work not only provides a facile strategy to fabricate QD aerogels but also offers a platform for designing advanced aerogel-based photocatalysts.

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Mid- and far-infrared localized surface plasmon resonances in chalcogen-hyperdoped silicon

Wang, M.; Yu, Y.; Prucnal, S.; Berencén, Y.; Saif Shaikh, M.; Rebohle, L.; Khan, M. B.; Zviagin, V.; Hübner, R.; Pashkin, A.; Erbe, A.; Georgiev, Y. M.; Grundmann, M.; Helm, M.; Kirchner, R.; Zhou, S.

Plasmonic sensing in the infrared region employs the direct interaction of the vibrational fingerprints of molecules with the plasmonic resonances, creating surface-enhanced sensing platforms that are superior to traditional spectroscopy. However, the standard noble metals used for plasmonic resonances suffer from high radiative losses as well as fabrication challenges, such as tuning the spectral resonance positions into mid- to far-infrared regions, and the compatibility issue with the existing complementary metal–oxide-semiconductor (CMOS) manufacturing platform. Here, we demonstrate the occurrence of mid-infrared localized surface plasmon resonances (LSPR) in thin Si films hyperdoped with the known deep-level impurity tellurium. We show that the mid-infrared LSPR can be further enhanced and spectrally extended to the far-infrared range by fabricating two-dimensional arrays of micrometer-sized antennas in a Te-hyperdoped Si chip. Since Te-hyperdoped Si can also work as an infrared photodetector, we believe that our results will unlock the route toward the direct integration of plasmonic sensors with the on-chip CMOS platform, greatly advancing the possibility of mass manufacturing of high-performance plasmonic sensing systems.

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Magnetism and magnetoelectricity of textured polycrystalline bulk Cr2O3 sintered in conditions far out of equilibrium

Veremchuk, I.; Makushko, P.; Hedrich, N.; Zabila, Y.; Kosub, T.; Liedke, M. O.; Butterling, M.; Attallah, A. G.; Wagner, A.; Burkhardt, U.; Pylypovskyi, O.; Hübner, R.; Faßbender, J.; Maletinsky, P.; Makarov, D.

Magnetoelectric antiferromagnets like Cr2O3 are attractive for the realization of energy-efficient and high-speed spin-orbitronic-based memory devices. Here, we demonstrate that fabrication of polycrystalline bulk Cr2O3 samples in conditions far out of equilibrium relying on spark plasma sintering allows to realize high-quality material with density close to that of a single crystal. The sintered sample possesses a preferential [001] texture at the surface, which can be attributed to uniaxial strain applied to the sample during the sintering process. The antiferromagnetic state of the sample and linear magnetoelectric effect are accessed all-electrically relying on the spin Hall magnetoresistance effect in the Pt electrode interfaced with Cr2O3. In line with the integral magnetometry measurements, the magnetotransport characterization reveals that the sample possesses the magnetic phase transition temperature of about 308 K, which is the same as in a single crystal. The antiferromagnetic domain pattern consists of small domains with sizes in the range of several micrometers only, which is formed due to the granular structure of the sample. The possibility to access the magnetoelectric properties of the samples relying on magnetotransport measurements indicates the potential of the polycrystalline Cr2O3 samples for prospective research in antiferromagnetic spintronics.

Keywords: magnetoelectric; antiferromagnet; Cr2O3; spark plasma sintering; magnetotransport

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Defect nanostructure and its impact on magnetism of α-Cr2O3 thin films

Veremchuk, I.; Liedke, M. O.; Makushko, P.; Kosub, T.; Hedrich, N.; Pylypovskyi, O.; Ganss, F.; Butterling, M.; Hübner, R.; Attallah, A. G.; Wagner, A.; Wagner, K.; Shields, B.; Maletinsky, P.; Faßbender, J.; Makarov, D.

Thin films of the magnetoelectric insulator α-Cr2O3 are technologically relevant for energy-efficient magnetic memory devices controlled by electric fields. In contrast to single crystals, the quality of thin Cr2O3 films is usually compromised by the presence of point defects and their agglomerations at grain boundaries, putting into question their application potential. Here, we study the impact of the defect nanostructure including sparse small-volume defects and their complexes on the magnetic properties of Cr2O3 thin films. By tuning the deposition temperature, we tailor the type, size, and relative concentration of defects, which we then analyze based on positron annihilation spectroscopy complemented with local electron microscopy studies. The structural characterization is correlated with magnetotransport measurements and nitrogen vacancy microscopy of antiferromagnetic domain patterns. Defects pin antiferromagnetic domain walls and stabilize complex multidomain states with a typical domain size in the sub-micrometer range. Despite their influence on the domain configuration, we demonstrate that neither small open-volume defects nor grain boundaries in Cr2O3 thin films affect the Néel temperature in a broad range of deposition parameters. Our results pave the way towards the realization of spin-orbitronic devices where magnetic domain patterns can be tailored based on defect nanostructures without affecting their operation temperature.

Keywords: Cr2O3 thin films; antiferromagnet; antiferromagnetic domains; magnetotransport; vacancy cluster; dislocations

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A comprehensive study on the interaction of Eu(III) and U(VI) with plant cells (Daucus carota) in suspension

Jessat, J.; Moll, H.; John, W.; Bilke, M.-L.; Hübner, R.; Kretzschmar, J.; Steudtner, R.; Drobot, B.; Stumpf, T.; Sachs, S.

Daucus carota suspension cells showed a high affinity towards Eu(III) and U(VI) based on a single-step bioassociation process with an equilibrium after 48 to 72 h. Cells responded with an increased metabolic activity towards heavy metal stress. Luminescence spectroscopy pointed to multiple species for both heavy metals in the culture media, providing initial hints of their interaction with cells and released metabolites. Using nuclear magnetic resonance spectroscopy, we could prove that malate, as an released metabolite in the culture medium, was found to complex with U. Luminescence spectroscopy also showed that Eu(III)-EDTA species are interacting with the cells. Furthermore, Eu(III) and U(VI) coordination is dominated by phosphate groups provided by the cells. We found that Ca ion channels of D. carota cells were involved in the uptake of U(VI), which led to a bioprecipitation of U(VI) in the vacuole of the cells, most probably as uranyl(VI) phosphates along with an intracellular sorption of U(VI) on biomembranes by lipid structures. Eu(III) could be found locally concentrated in the cell wall and in the cytoplasm with a co-localization with phosphorous and oxygen.

Keywords: actinides; lanthanides; luminescence spectroscopy; malate; mobilization

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Extraordinary anisotropic magnetoresistance in CaMnO3/CaIrO3 heterostructures

Vagadia, M.; Sardar, S.; Tank, T.; Das, S.; Gunn, B.; Pandey, P.; Hübner, R.; Rodolakis, F.; Fabbris, G.; Choi, Y.; Haskel, D.; Frano, A.; Rana, D. S.

The realization of fourfold anisotropic magnetoresistance (AMR) in 3d-5d heterostructures has boosted major efforts in antiferromagnetic (AFM) spintronics. However, despite the potential of incorporating strong spinorbit coupling, only small AMR signals have been detected thus far, prompting a search for mechanisms to enhance the signal. In this paper, we demonstrate an extraordinarily elevated fourfold AMR of 70% realized in CaMnO3/CaIrO3 thin film superlattices.We find that the biaxial magnetic anisotropy and the spin-flop transition in a nearly Mott insulating phase form a potent combination, each contributing one order of magnitude to the total signal. Dynamics between these phenomena capture a subtle interaction of pseudospin coupling with the lattice and external magnetic field, an emergent phenomenon creating opportunities to harness its potential in AFM spintronics.

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Role of the metal supply pathway on silicon patterning by oblique ion beam sputtering

Redondo-Cubero, A.; Palomares, F. J.; Lorenz, K.; Rubio-Zuazo, J.; Hübner, R.; Mompéan, F. J.; García-Hernández, M.; Castro, G. R.; Vázquez, L.

The dynamics of the pattern induced on a silicon surface by oblique incidence of a 40 keV Fe ion beam is studied. The results are compared with those obtained for two reference systems, namely a noble gas ion beam either without or with Fe co-deposition. The techniques employed include Atomic Force Microscopy, Rutherford Backscattering Spectrometry, Transmission Electron Microscopy, X-ray Photoelectron and hard X-ray photoelectron spectroscopies, as well as Superconducting Quantum Interference Device measurements. The Fe-induced pattern differs from those of both reference systems since a pattern displaying short hexagonal ordering develops, although it shares some features with them. In both Fe systems a chemical pattern, with iron silicide-rich and -poor regions, is formed upon prolonged irradiation. The metal pathway has a marked influence on the patterns’ morphological properties and on the spatial correlation between the chemical and morphological patterns. It also determines the iron silicide stoichiometry and the surface pattern magnetic properties that are better for the Feimplanted system. These results show that in ion-beam-induced silicon surface patterning with reactive metals, the metal supply pathway is critical to determine not only the morphological pattern properties, but also the chemical and magnetic ones.

Keywords: Surface nanopatterning; Ion beam sputtering; Silicon; Magnetic properties; Silicides; Iron

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Structural Templating of an Organic Solar Cell Absorber by Ellagic Acid To Tune Its Aggregation, Molecular Orientation, and Optical Properties

Bittrich, E.; Domke, J.; Levichkova, M.; Jehnichen, D.; Bittrich, L.; Janke, A.; Formanek, P.; Hübner, R.; Uhlmann, P.; Eichhorn, K.-J.; Forker, R.; Gruenewald, M.; Al-Hussein, M.; Fritz, T.; Walzer, K.

Structural templating with homogeneous template layers is one of the strategies for controlling the orientation of small molecular absorbers in the photoactive layer of an organic solar cell to increase its power conversion efficiency. A main challenge thereby is the energetic alignment of the template molecules to the photoactive and charge-transporting materials. In the present study, the effects of a cluster-like template layer of ellagic acid (EA) on the morphology and optical properties of side-chain-substituted dicyanovinyl quaterthiophene (DCV4T-Et2) thin films are discussed. In the monolayer regime, J-aggregation of DCV4T-Et2 is confirmed. Insertion of the EA template layer leads to an improved aggregation behavior and formation of J-aggregates in DCV4T-Et2 films near the EA interface. The orientation of DCV4T-Et2 molecules in 30 nm thick films changes from “edge-on” to “face-on” due to a π−π interaction between the flat-lying EA molecules and the DCV4T-Et2 molecules. The face-on orientation by templating is preserved in blend layers with C60, and a considerable increase in the crystallinity of the DCV4T-Et2 phase in the blend is induced. Organic solar cells based on templated DCV4T-Et2:C60 active layers exhibit more than a 50% increase in the efficiency compared to nontemplated active layers. The short-circuit current density and the fill factor are significantly improved. Although the energetic alignment of EA is not ideal, no additional open-circuit voltage losses were observed with templating, due to the cluster-like morphology of the EA layer. Our results demonstrate a cluster-like templating approach with the novel template molecule EA to tailor the molecular orientation, crystallinity, and consequently optical properties of organic semiconducting molecules without significant energetic losses favorable for use in organic electronics.

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Cation exchange on colloidal copper selenide nanosheets: a route to two-dimensional metal selenide nanomaterials

Shamraienko, V.; Spittel, D.; Hübner, R.; Khoshkhoo, M. S.; Weiß, N.; Georgi, M.; Borchert, K. B. L.; Schwarz, D.; Lesnyak, V.; Eychmüller, A.

We report a synthesis route to two-dimensional PbSe, HgSe, ZnSe, SnSe, and Cu-Zn-Sn-Se (CZTSe) nanomaterials based on cation exchange (CE) reactions. This approach includes two steps: it starts with the synthesis of hexagonal, up to several micrometers large yet approx. 5 nm-thick CuSe nanosheets (NSs), followed by CE of the host copper ions with the desired guest cation (Pb2+, Hg2+, Zn2+, or Sn4+). In the case of CZTSe, both guest cations can be added simultaneously since the variation of the guest cation ratio and reaction time can lead to various compositions. Mild reaction conditions allow for a preservation of the size and the 2D shape of the parent NSs accompanied by corresponding changes in their crystal structure. We furthermore demonstrate that the crystal structure of CuSe NSs can be rearranged even without addition of guest cations in the presence of tri-n-octylphosphine. Thus, the obtained NSs were further subjected to ligand exchange reactions in order to replace insulating bulky organic molecules on their surface with compact iodide and sulfide ions, a step crucial for the application of nanomaterials in (opto)electronic devices. The resulting NS dispersions were processed into thin films by spray-coating onto commercially available interdigitated platinum electrodes. Light response measurements of PbSe and CZTSe NS-films demonstrated their potential for applications as light-sensitive materials in photodetection or photovoltaics.

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Controlled Silicidation of Silicon Nanowires using Flash Lamp Annealing

Khan, M. B.; Prucnal, S.; Ghosh, S.; Deb, D.; Hübner, R.; Pohl, D.; Rebohle, L.; Mikolajick, T.; Erbe, A.; Georgiev, Y.

Among other new device concepts, nickel silicide (NiSix)-based Schottky barrier nanowire transistors are projected to supplement down-scaling of the complementary metal-oxide-semiconductor (CMOS) technology as its physical limits are reached. Control over the NiSix phase and its intrusions into the nanowire are essential for superior performance and down-scaling of these devices. Several works have shown control over the phase, but control over the intrusion lengths has remained a challenge. To overcome this, we report a novel millisecond-range flash-lamp-annealing (FLA)-based silicidation process. Nanowires are fabricated on silicon-on-insulator substrates using a top-down approach. Subsequently, Ni silicidation experiments are carried out using FLA. It is demonstrated that this silicidation process gives unprecedented control over the silicide intrusions. Scanning electron microscopy and high-resolution transmission electron microscopy are performed for structural characterization of the silicide. FLA temperatures are estimated with the help of simulations.

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Effect of temperature and cell viability on uranium biomineralization by the uranium mine isolate Penicillium simplicissimum

Schaefer, S.; Steudtner, R.; Hübner, R.; Krawczyk-Bärsch, E.; Merroun, M. L.

Remediation of heavy-metal-contaminated sites represents a serious environmental problem worldwide. Currently, cost- and time-intensive chemical treatments are mainly performed. Bioremediation by heavy-metal-tolerant microorganisms is considered a more eco-friendly and comparatively cheap alternative. The fungus KS1 (Penicillium simplicissimum), isolated from the flooding water of a former uranium (U) mine in Germany, shows promising U bioremediation potential mainly through biomineralization. The adaption of KS1 to heavy-metal-contaminated sites is indicated by an increased U removal capacity of up to 550 mg U per g dry biomass compared to the non-heavy-metal-exposed P. simplicissimum reference strain DSM 62867 (200 mg U per g dry biomass). In addition, the effect of temperature and cell viability of KS1 on U biomineralization was investigated. While viable KS1 cells at 30 °C removed U mainly extracellularly via metabolism-dependent biomineralization, a decrease in temperature to 4 °C or implementation of dead-autoclaved KS1 cells at 30 °C revealed increased occurrence of passive biosorption and bioaccumulation, as observed by scanning transmission electron microscopy. The precipitated U species were assigned to uranyl phosphates with a structure similar to that of autunite via cryo-time-resolved laser fluorescence spectroscopy. The major involvement of phosphorus in U precipitation by the fungus KS1 was additionally supported by the observation of increased phosphatase activity for viable cells at 30 °C. Furthermore, viable KS1 cells actively secreted small molecules, most likely phosphorylated amino acids, which interacted with U in the supernatant and were not detected in experiments with dead-autoclaved cells. Our studies provide new insights into the influence of temperature and cell viability on U phosphate biomineralization by fungi and highlight the potential use of KS1 particularly for U bioremediation purposes.

Keywords: Biomineralization; Bioremediation; Fungal biomass; Uranium; Wastewater


Substitutional synthesis of sub-nanometer InGaN/GaN quantum wells with high indium content

Vasileiadis, I. G.; Lymperakis, L.; Adikimenakis, A.; Gkotinakos, A.; Devulapalli, V.; Liebscher, C. H.; Androulidaki, M.; Hübner, R.; Karakostas, T.; Georgakilas, A.; Komninou, P.; Dimakis, E.; Dimitrakopulos, G. P.

InGaN/GaN quantum wells (QWs) with sub-nanometer thickness can be employed in short-period superlattices for bandgap engineering of efficient optoelectronic devices, as well as for exploiting topological insulator behavior in III-nitride semiconductors. However, it had been argued that the highest indium content in such ultra-thin QWs is kinetically limited to a maximum of 33%, narrowing down the potential range of applications. Here, it is demonstrated that quasi two-dimensional (quasi-2D) QWs with thickness of one atomic monolayer can be deposited with indium contents far exceeding this limit, under certain growth conditions. Multi-QW heterostructures were grown by plasma-assisted molecular beam epitaxy, and their composition and strain were determined with monolayer-scale spatial resolution using quantitative scanning transmission electron microscopy in combination with atomistic calculations. Key findings such as the self-limited QW thickness and the non-monotonic dependence of the QW composition on the growth temperature under metal-rich growth conditions suggest the existence of a substitutional synthesis mechanism, involving the exchange between indium and gallium atoms at surface sites. The highest indium content in this work approached 50%, in agreement with photoluminescence measurements, surpassing by far the previously regarded compositional limit. The proposed synthesis mechanism can guide growth efforts towards binary InN/GaN quasi-2D QWs.

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Electrical Characterization of Germanium Nanowires Using a Symmetric Hall Bar Configuration: Size and Shape Dependence

Echresh, A.; Arora, H.; Fuchs, F.; Li, Z.; Hübner, R.; Prucnal, S.; Schuster, J.; Zahn, P.; Helm, M.; Zhou, S.; Erbe, A.; Rebohle, L.; Georgiev, Y.

The fabrication of individual nanowire-based devices and their comprehensive electrical characterization remains a major challenge. Here, we present a symmetric Hall bar configuration for highly p-type germanium nanowires (GeNWs), fabricated by a top-down approach using electron beam lithography and inductively coupled plasma reactive ion etching. The configuration allows two equivalent measurement sets to check the homogeneity of GeNWs in terms of resistivity and the Hall coefficient. The highest Hall mobility and carrier concentration of GeNWs at 5 K were in the order of 100 cm^2/(Vs) and 4×10^19 cm^-3, respectively. With a decreasing nanowire width, the resistivity increases and the carrier concentration decreases, which is attributed to carrier scattering
in the region near the surface. By comparing the measured data with simulations, one can conclude the existence of a depletion region, which decreases the effective cross-section of GeNWs. Moreover, the resistivity of thin GeNWs is strongly influenced by the cross-sectional shape.

Keywords: germanium nanowires; Hall bar configuration; Hall effect; electrical characterization

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High electron mobility in strained GaAs nanowires

Balaghi, L.; Shan, S.; Fotev, I.; Moebus, F.; Rana, R.; Venanzi, T.; Hübner, R.; Mikolajick, T.; Schneider, H.; Helm, M.; Pashkin, O.; Dimakis, E.

Novel transistor concepts based on semiconductor nanowires promise high performance, lower energy consumption and better integrability in various platforms in nanoscale dimensions. Concerning the intrinsic transport properties of electrons in nanowires, relatively high mobility values that approach those in bulk crystals have been obtained only in core/shell heterostructures, where electrons are spatially confined inside the core. Here, it is demonstrated that the strain in lattice-mismatched core/shell nanowires can affect the effective mass of electrons in a way that boosts their mobility to unprecedented levels. Specifically, electrons inside the hydrostatically tensile-strained gallium arsenide core of nanowires with a thick indium aluminium arsenide shell exhibit mobility values 30 – 50 % higher than in equivalent unstrained nanowires or bulk crystals, as measured at room temperature. With such an enhancement of electron mobility, strained gallium arsenide nanowires emerge as a unique means for the advancement of transistor technology.

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Near-Infrared-Emitting CdxHg1−xSe-Based Core/Shell Nanoplatelets

Mitrofanov, A.; Prudnikau, A.; Di Stasio, F.; Weiß, N.; Hübner, R.; Dominic, A. M.; Borchert, K. B. L.; Lesnyak, V.; Eychmüller, A.

The anisotropy in semiconductor nanoplatelets (NPLs) is reflected in the anisotropy of their crystal structure and organic ligand shell, which can be used for creating new semiconductor heterostructures. This work demonstrates the synthesis of core/shell NPLs containing zero-dimensional (0D) CdxHg1−xSe domains embedded in CdSe NPLs via cation exchange. The strategy is based on the different accessibility of definite regions of the NPLs for incoming cations upon time-limited reaction conditions. The obtained heterostructures were successfully overcoated with a CdyZn1−yS shell preserving their two-dimensional (2D) morphology. The NPLs exhibit bright photoluminescence in the range of 700-1100 nm with quantum yields up to 55%, thus making them a prospective material for light-emitting applications in the near-infrared spectral range.

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Effects of hydrogen absorption on magnetism in Ni80Fe20/Y/Pd trilayers

Weiss, C.; Hübner, R.; Saunders, M.; Semisalova, A.; Ehrler, J.; Schmidt, N.; Seyd, J.; Albrecht, M.; Anwar, S.; Lindner, J.; Potzger, K.; Kostylev, M.

The effects of hydrogen absorption on the effective magnetization (4πMeff), gyromagnetic ratio (γ), Gilbert damping constant (αG), and the inhomogeneous linewidth broadening in Py(x)/Y(16 nm)/Pd(15 nm) trilayer films (x = 2, 3, 5, 8, 10, 20, 40 nm) were investigated with ferromagnetic resonance (FMR), transmission electron microscopy, and vibrating sample magnetometry. In the presence of a hydrogen atmosphere, the samples show a reduction of their FMR linewidth which is found to stem purely from a reduction of the inhomogeneous linewidth broadening. This is attributed to a rearrangement of atoms at the Py/Y interface in the presence of hydrogen, making the Py/Y interface more homogeneous. In addition, a reduction of 4πMeff was seen for all samples in the hydrogen atmosphere which is typical for an increase of the interfacial perpendicular magnetic anisotropy at the Py/Y interface.

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Tungsten Oxide/Reduced Graphene Oxide Aerogel with Low-Content Platinum as High-Performance Electrocatalyst for Hydrogen Evolution Reaction

Li, Y.; Jiang, K.; Yang, J.; Zheng, Y.; Hübner, R.; Ou, Z.; Dong, X.; He, L.; Wang, H.; Li, J.; Sun, Y.; Lu, X.; Zhuang, X.; Zheng, Z.; Liu, W.

Designing cost-effective, highly active, and durable platinum (Pt)-based electrocatalysts is a crucial endeavor in electrochemical hydrogen evolution
reaction (HER). Herein, the low-content Pt (0.8 wt%)/tungsten oxide/reduced graphene oxide aerogel (LPWGA) electrocatalyst with excellent HER activity and durability is developed by employing a tungsten oxide/reduced graphene oxide aerogel (WGA) obtained from a facile solvothermal process as a support, followed by electrochemical deposition of Pt nanoparticles. The WGA support with abundant oxygen vacancies and hierarchical pores plays the roles of anchoring the Pt nanoparticles, supplying continuous mass transport and electron transfer channels, and modulating the surface electronic state of Pt, which endow the LPWGA with both high HER activity and durability. Even under a low loading of 0.81 μgPt cm-2, the LPWGA exhibits a high HER activity with an overpotential of 42 mV at 10 mA cm-2, an excellent stability under 10000-cycle cyclic voltammetry and 40 h chronopotentiometry at 10 mA cm-2, a low Tafel slope (30 mV dec-1), and a high turnover frequency of 29.05 s-1 at η = 50 mV, which is much superior to the commercial Pt/C and the low-content Pt/reduced graphene oxide aerogel. This work provides a new strategy to design high-performance Pt-based electrocatalysts with greatly reduced use of Pt.

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Size-Tunable Gold Aerogels: A Durable and Misfocus-Tolerant 3D Substrate for Multiplex SERS Detection

Zhou, L.; Peng, Y.; Zhang, N.; Du, R.; Hübner, R.; Wen, X.; Li, D.; Hu, Y.; Eychmüller, A.

The research on surface-enhanced Raman scattering (SERS) continuously draws wide attention because of its high detection sensitivity. However, the commonly investigated 2D SERS substrates cannot fully utilize the 3D active focal volume and require a tight focus on the correct plane, retarding signal enhancement and flexible use. Here, self-supported gold aerogels of centimeter-dimension with tunable ligament sizes are designed as 3D SERS substrates, featuring hot spots throughout the entire network. Unveiling a universal ligament-size-effect, the optimized gold aerogel showcases much larger enhancement factors compared to a 8 nm Au film toward dyes, pesticides, and carcinogens (up to 109). Aside from an excellent reusability and an exceptional stability (> 1 month), an outstanding misfocus tolerance (>300 μm along the z-axis) is also demonstrated for such aerogel-based SERS substrates for multiplex detection. This work may expand the application scope of metal aerogels and lay the foundation for designing next-generation 3D SERS substrates.

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Phase evolution of Te-hyperdoped Si upon furnace annealing

Shaikh, M. S.; Wang, M.; Hübner, R.; Liedke, M. O.; Butterling, M.; Solonenko, D.; Madeira, T. I.; Li, Z.; Xie, Y.; Hirschmann, E.; Wagner, A.; Zahn, D. R. T.; Helm, M.; Zhou, S.

Si hyperdoped with chalcogens via ion implantation and pulsed laser melting is known to exhibit strong room-temperature sub-bandgap photoresponse. As a thermodynamically metastable system, an impairment of the optoelectronic properties in hyperdoped Si materials occurs upon subsequent high-temperature thermal treatment (>500 °C). The substitutional Te atoms that cause the sub-bandgap absorption are removed from the Si matrix to form Te-related complexes, which are electrically and optically inactive. In this work, we explore the formation of defects in Te-hyperdoped Si layers which leads to the electrical deactivation upon furnace annealing through the analysis of optical and microstructural properties as well as positron annihilation lifetime spectroscopy. Particularly, Te-rich clusters are observed in samples thermally annealed at temperatures reaching 950 °C and above. Combined with polarized Raman analysis and transmission electron microscopy, the observed crystalline clusters are suggested to be Si2Te3.

Keywords: Defect analysis; Furnace annealing; Ion-implantation; Positron annihilation spectroscopy; Raman spectroscopy; Silicon telluride; Te-hyperdoped Si; Transmission electron microscopy

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Deposition of silicon oxide films on silicon using HelixJet – an atmospheric-pressure plasma jet process below 100 °C

Rebohle, L.; Quade, A.; Schumann, T.; Blaschke, D.; Hübner, R.; Heller, R.; Foest, R.; Schäfer, J.; Skorupa, W.

Silicon oxide films are widely applied for their superior dielectric, chemical and mechanic properties as well as for their resistance against reactive chemicals. Simultaneously, there is an increasing number of applications which demand a low deposition temperature. In this work, we compare the material properties of SiOx layers deposited at ca. 70°C by atmospheric-pressure plasma jet deposition (PA) with those of SiO2 layers thermally grown or deposited by plasma-enhanced chemical vapour deposition. The films were deposited on silicon wafers and analysed using different analysis techniques. According to cross-sectional transmission electron microscopy and high-frequency capacitance-voltage measurements, the interface between the PA oxide and the Si substrate is smooth with no apparent defects and displays an electrically active interface defect density between 3.5-8.0×1012 cm-2 directly after deposition and below 2.0×1012 cm-2 after furnace annealing. Right after deposition, the PA oxide contains carbon and hydrogen in a concentration of several at%, and the SiO2 plasma polymer network comprises several active centres (residual charge, free radicals, non-saturated bonds). The most abundant configuration is the Si(-O)4 tetrahedron, followed by Si(-O)3 with similar intensity. This indicates that there are still dangling Si bonds or bonds terminated by hydroxyl or methyl groups. After furnace annealing, the formation of the SiO2 network is completed and the optical and electrical properties of the PA oxide converge to that of thermal oxide.

Keywords: dielectric coating; insulation; corrosion protection; silicon oxide; atmospheric plasma source; thin films

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Rapid synthesis of gold-palladium core-shell aerogels for selective and robust electrochemical CO2 reduction

Du, R.; Jin, W.; Wu, H.; Hübner, R.; Zhou, L.; Xue, G.; Hu, Y.; Eychmüller, A.

Noble metal aerogels (NMAs), one class of the youngest members in the aerogel family, have drawn increasing attention in the last decade. Featuring the high catalytic activity of noble metals and a 3D self-supported porous network of the aerogels, they have displayed profound potential for electrocatalysis. However, considerable challenges reside in the rapid fabrication of NMAs with a well-tailored architecture, constraining the manipulation of their electrochemical properties for optimized performance. Here, a disturbance-assisted dynamic shelling strategy is developed, generating self-supported Au–Pd core–shell gels within 10 min. Based on suitable activation and desorption energies of the involved species as suggested by theoretical calculations, the Au–Pd core–shell aerogel manifests outstanding CO selectivity and stability at low overpotential (faradaic efficiency > 98% at -0.5 V vs. RHE over 12 hours) for the electrochemical CO2 reduction reaction (CO2RR). The present strategy offers a new perspective to facilely design architecture-specified high-performance electrocatalysts for the CO2RR.

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Integrated complementary inverters and ring oscillators based on vertical-channel dual-base organic thin-film transistors

Guo, E.; Xing, S.; Dollinger, F.; Hübner, R.; Wang, S.-J.; Wu, Z.; Leo, K.; Kleemann, H.

Lateral-channel dual-gate organic thin-film transistors have been used in pseudo complementary metal-oxide-semiconductor (CMOS) inverters to control switching voltage. However, their relatively long channel lengths, combined with the low charge carrier mobility of organic semiconductors, typically leads to slow inverter operation. Vertical-channel dual-gate organic thin-film transistors are a promising alternative because of their short channel lengths, but the lack of appropriate p- and n-type devices has limited the development of complementary inverter circuits. Here, we show that organic vertical n-channel permeable single- and dual-base transistors, and vertical p-channel permeable base transistors can be used to create integrated complementary inverters and ring oscillators. The vertical dual-base transistors enable switching voltage shift and gain enhancement. The inverters exhibit small switching time constants at 10 MHz, and the seven-stage complementary ring oscillators exhibit short signal propagation delays of 11 ns per stage at a supply voltage of 4 V.

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Endocytosis is a significant contributor to uranium(VI) uptake in tobacco (Nicotiana tabacum) BY-2 cells in phosphate-deficient culture

John, W.; Lückel, B.; Matschiavelli, N.; Hübner, R.; Matschi, S.; Hoehenwarter, W.; Sachs, S.

Endocytosis of metals in plants is a growing field of study involving metal uptake from the rhizosphere. Uranium, which is naturally and artificially released into the rhizosphere, is known to be taken up by certain species of plant, such as Nicotiana tabacum, and we hypothesize that endocytosis contributes to the uptake of uranium in tobacco. The endocytic uptake of uranium was investigated in tobacco BY 2 cells using an optimized setup of culture in phosphate-deficient medium. A combination of methods in biochemistry, microscopy and spectroscopy, supplemented by proteomics, were used to study the interaction of uranium and the plant cell. We found that under environmentally relevant uranium concentrations, endocytosis remained active and contributed to 14% of the total uranium bioassociation. Proteomics analyses revealed that uranium induced a change in expression of the clathrin heavy chain variant, signifying a shift in the type of endocytosis taking place. However, the rate of endocytosis remained largely unaltered. Electron microscopy and energy dispersive X-ray spectroscopy showed an adsorption of uranium to cell surfaces and deposition in vacuoles. Our results demonstrate that endocytosis constitutes a considerable proportion of uranium uptake in BY 2 cells, and that endocytosed uranium is likely targeted to the vacuole for sequestration, providing a physiologically safer route for the plant than uranium transported through the cytosol.

Keywords: plant cell; proteomics; radionuclide transport; heavy metal interaction; vesicle uptake

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Enhanced Photoluminescence of Gold Nanoparticle-Quantum Dot Hybrids Confined in Hairy Polymer Nanofibers

Singh, S.; Raulo, A.; Singh, A.; Mittal, M.; Horechyy, A.; Hübner, R.; Formanek, P.; Srivastava, R. K.; Sapra, S.; Fery, A.; Nandan, B.

In the present work, we have studied the influence of gold nanoparticles (AuNPs) on the photoluminescence (PL) behavior of cadmium selenide (CdSe) quantum dots (QDs) confined in spatially separated soft nanoscale cylindrical domains. These cylindrical domains, in the form of hairy core-shell nanofibers, were fabricated via cooperative self-assembly of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) block copolymer (BCP) mixed with pre-synthesized CdSe QDs and AuNPs. The CdSe QDs and AuNPs were simultaneously incorporated in the P4VP cylindrical domains of the self-assembled BCP structure. It was found that the confinement imposed by the nanometer-sized cylindrical core resulted in the localization of the CdSe QDs and AuNPs in close proximity. Notably, it was observed that the PL intensity of the CdSe QDs could be manipulated by varying the amount of AuNPs present in the cylinder core. Interestingly, in the presence of a very low fraction of AuNPs, the PL intensity of the CdSe QDs increased compared to the AuNPs-free system. However, further increase in the fraction of AuNPs led to gradual quenching of the photoluminescence intensity. The PL enhancement and quenching plausibly was due to the interplay between the energy transfer due to surface plasmon coupling and FRET/electron transfer from QDs to the AuNPs. The resulting functional nanofibers could have potential applications in sensing, bioimaging, and optoelectronic devices.

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Formation, structure, and optical properties of copper chromite thin films for high-temperature solar absorbers

Krause, M.; Sonnenberg, J.; Munnik, F.; Grenzer, J.; Hübner, R.; Garcia Valenzuela, A.; Gemming, S.

CuCr2O4 thin films grown by physical vapour deposition were studied in order to evaluate their potential as absorber material for the next generation of concentrated solar power plants. A series of Cu-Cr-O thin films was deposited by reactive ion beam sputtering. The Cr/Cu ratio in the sputter target is demonstrated as the most important parameter to achieve the intended film stoichiometry. In-air annealing at 800 °C leads to structural transformations of the as-deposited films and results in phase compositions according to those expected from the ternary Cu-Cr-O phase diagram. Tetragonal CuCr2O4 with 98.6 at.% phase purity regarding the solid film constituents is obtained for the appropriate Cr/Cu ratio in the sputter target. CuCr2O4 thin films absorb light in the entire solar spectral range from 300 nm to 2500 nm. Their energy gap is found to be < 0.5 eV, and their solar absorptance is estimated to be (0.85 +/- 0.03). The dense microstructure with good thermal conductivity, full adhesion to the substrate, and a relatively low surface roughness are discussed as technological advantages of CuCr2O4 thin films grown by physical vapour deposition.

Keywords: solar absorber; spinels; sputtering; phase transformations; optical materials

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Simultaneous Ligand and Cation Exchange of Colloidal CdSe Nanoplatelets toward PbSe Nanoplatelets for Application in Photodetectors

Galle, T.; Spittel, D.; Weiß, N.; Shamraienko, V.; Decker, H.; Georgi, M.; Hübner, R.; Metzkow, N.; Steinbach, C.; Schwarz, D.; Lesnyak, V.; Eychmüller, A.

Cation exchange emerged as a versatile tool to obtain a variety of nanocrystals not yet available via a direct synthesis. Reduced reaction times and moderate temperatures make the method compatible with anisotropic nanoplatelets (NPLs). However, the subtle thermodynamic and kinetic factors governing the exchange require careful control over the reaction parameters to prevent unwanted restructuring. Here, we capitalize on the research success of CdSe NPLs by transforming them into PbSe NPLs suitable for optoelectronic applications. In a two-phase mixture of hexane/Nmethylformamide, the oleate-capped CdSe NPLs simultaneously undergo a ligand exchange to NH4I and a cation exchange reaction to PbSe. Their morphology and crystal structure are well-preserved as evidenced by electron microscopy and powder X-ray diffraction. We demonstrate the successful ligand exchange and associated electronic coupling of individual NPLs by fabricating a simple photodetector via spray-coating on a commercial substrate. Its optoelectronic characterization reveals a fast light response at low operational voltages.

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A Robust PtNi Nanoframe/N-Doped Graphene Aerogel Electrocatalyst with Both High Activity and Stability

Yang, J.; Hübner, R.; Zhang, J.; Wan, H.; Zheng, Y.; Wang, H.; Qi, H.; He, L.; Li, Y.; Aregahegn Dubale, A.; Sun, Y.; Liu, Y.; Peng, D.; Meng, Y.; Zheng, Z.; Rossmeisl, J.; Liu, W.

Insufficient catalytic activity and stability and high cost are the barriers for Pt-based electrocatalysts in wide practical applications. Herein, a hierarchically porous PtNi nanoframe/N-doped graphene aerogel (PtNiNF-NGA) electrocatalyst with outstanding performance toward methanol oxidation reaction (MOR) in acid electrolyte has been developed via facile tert-butanol-assisted structure reconfiguration. The ensemble of high-alloying-degree-modulated electronic
structure and correspondingly the optimum MOR reaction pathway, the structure superiorities of hierarchical porosity, thin edges, Pt-rich corners, and the anchoring effect of the NGA, endow the PtNiNF-NGA with both prominent electrocatalytic activity and stability. The mass and specific activity (1647 mAmgPt -1, 3.8 mAcm-2) of the PtNiNF-NGA are 5.8 and 7.8 times higher than those of commercial Pt/C. It exhibits exceptional stability under a 5-hour chronoamperometry test and 2200-cycle cyclic voltammetry scanning.

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Metal-induced progressive alteration of conducting states in memristors for implementing an efficient analog memory: a DFT-supported experimental approach

Das, D.; Barman, A.; Sarkar, P. K.; Rajput, P.; Jha, S. N.; Hübner, R.; Kanjilal, D.; Johari, P.; Kanjilal, A.

Advancement of the memristor-based artificial synapse (AS) is urgently needed for rapid progress in neuromorphic devices. The precise structural and chemical engineering of metal oxide layers by metal dopants (Ni) is presented as an innovative way to set off a decent performance of the AS. An ON/OFF ratio of 103 as well as data retention and endurance capabilities of 104 s and 103 cycles, respectively, are achieved. With these properties, the symmetric alteration in conductance states, short-term plasticity (STP) and long-term plasticity (LTP) are realized within the same device, and compared with the reported values to establish its excellent cognitive behavioural ability. Our combined experimental and the DFT-based first-principles calculation results reveal that the rational designing of AS using metal cations (Ni) can promote an ultra-low-power of about 2.55 fJ per pulse (lower than human brain about 10 fJ per pulse) for STP, promising for next-generation smart memory devices. Here, Ni endorses strong electronic localization, which in turn familiarizes trap states within the forbidden energy gap and improves short-term memory loss. Further, it modifies the local electrostatic barriers to stimulate modulatory action (as commonly observed in the mammalian brain) for LTP. Overall, this work provides a novel pathway to overcome the technological bottleneck.

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Chlorine doping of MoSe2 flakes by ion implantation

Prucnal, S.; Hashemi, A.; Ghorbani Asl, M.; Hübner, R.; Duan, J.; Wei, Y.; Sharma, D.; Zahn, D. R. T.; Ziegenrücker, R.; Kentsch, U.; Krasheninnikov, A.; Helm, M.; Zhou, S.

The efficient integration of transition metal dichalcogenides (TMDs) into the current electronic device technology requires mastering the techniques of effective tuning of their optoelectronic properties. Specifically, controllable doping is essential. For conventional bulk semiconductors, ion implantation is the most developed method offering stable and tunable doping. In this work, we demonstrate n-type doping in MoSe2 flakes realized by low-energy ion implantation of Cl+ ions followed by millisecond-range flash lamp annealing (FLA). We further show that FLA for 3 ms with a peak temperature of about 1000 °C is enough to recrystallize implanted MoSe2. The Cl distribution in few-layer-thick MoSe2 is measured by secondary ion mass spectrometry. An increase in the electron concentration with increasing Cl fluence is determined from the softening and red shift of the Raman-active A1g phonon mode due to the Fano effect. The electrical measurements confirm the n-type doping of Cl-implanted MoSe2. A comparison of the results of our density functional theory calculations and experimental temperature-dependent micro-Raman spectroscopy data indicates that Cl atoms are incorporated into the atomic network of MoSe2 as substitutional donor impurities.

Keywords: MoSe2; ion implantation; Flash Lamp Annealing; doping; 2D materials; DFT; Raman

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Efficient and low-voltage vertical organic permeable base light-emitting transistors

Wu, Z.; Liu, Y.; Guo, E.; Darbandy, G.; Wang, S.-J.; Hübner, R.; Kloes, A.; Kleemann, H.; Leo, K.

Organic light-emitting transistors, three-terminal devices combining a thin-film transistor with a light-emitting diode, have generated increasing interest in organic electronics. However, increasing their efficiency while keeping the operating voltage low still remains a key challenge. Here, we demonstrate organic permeable base light-emitting transistors; these three-terminal vertical optoelectronic devices operate at driving voltages below 5.0 V; emit in the red, green and blue ranges; and reach, respectively, peak external quantum efficiencies of 19.6%, 24.6% and 11.8%, current efficiencies of 20.6 cd A–1, 90.1 cd A–1 and 27.1 cd A–1 and maximum luminance values of 9,833 cd m–2, 12,513 cd m–2 and 4,753 cd m–2. Our simulations demonstrate that the nano-pore permeable base electrode located at the centre of the device, which forms a distinctive optical microcavity and regulates charge carrier injection and transport, is the key to the good performance obtained. Our work paves the way towards efficient and low-voltage organic light-emitting transistors, useful for power-efficient active matrix displays and solid-state lighting.

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Fluorination of graphene leads to susceptibility for nanopore formation by highly charged ion impact

Creutzburg, S.; Mergl, M.; Hübner, R.; Jirka, I.; Erb, D.; Heller, R.; Niggas, A.; Grande, P. L.; Aumayr, F.; Wilhelm, R. A.; Kalbac, M.; Facsko, S.

The formation of nanopores by highly charged ion impacts on freestanding fluorine-functionalized graphene is demonstrated. The process is driven by potential sputtering, which becomes active by changing the semi-metallic properties of graphene into a strongly insulating state by fluorination. The interaction of fluorographene with highly charged ions is also studied in terms of charge exchange and kinetic energy loss. A higher number of captured electrons and a larger kinetic energy loss than in pristine graphene are observed, which can be well explained by an increase in the ion's neutralization length and in the atomic areal density of the target, respectively. Using a computer code based on a time-dependent scattering potential model, a connection between the efficiency of charge exchange and the fluorine coverage is revealed. Our results suggest a competition of two distinct nanostructure formation processes leading either to pore formation or fluorine desorption.

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B20-MnSi films grown on Si(100) substrates with magnetic skyrmion signature

Li, Z.; Yuan, Y.; Hübner, R.; Begeza, V.; Rebohle, L.; Helm, M.; Nielsch, K.; Prucnal, S.; Zhou, S.

Magnetic skyrmions have been suggested as information carriers for future spintronic devices. As the first material with experimentally confirmed skyrmions, B20-type MnSi has the research focus for decades. Although B20-MnSi films have been successfully grown on Si(111) substrates, there is no report about B20-MnSi films on Si(100) substrates, which would be more preferred for practical applications. In this letter, we present the first preparation of B20-MnSi on Si(100) substrates. It is realized by sub-second solid-state reaction between Mn and Si via flash-lamp annealing at ambient pressure. The regrown layer shows an enhanced Curie temperature of 43 K compared with bulk B20-MnSi. The magnetic skyrmion signature is proved in our films by magnetic and transport measurements. The millisecond-range flash annealing provides a promising avenue for the fabrication of Si-based skyrmionic devices.

Keywords: Skyrmions; B20-MnSi; Flash-lamp annealing

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Nanoparticle-Stabilized Perforated Lamellar Morphology in Block Copolymer/Quantum Dot Hybrids

Singh, S.; Horechyy, A.; Yadav, S.; Formanek, P.; Hübner, R.; Srivastava, R. K.; Sapra, S.; Fery, A.; Nandan, B.

We report on the surprising observation of a unique perforated lamellar (PL) morphology in a mixture of an asymmetric polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) block copolymer and CdSe−CdS quantum dots (QDs). The PL morphology formed by the PS-b-P4VP/CdSe−CdS composites consisted of alternating layers of PS and P4VP, where the layer formed by the minority PS block contained cylindrical perforations of the majority P4VP block. Most interestingly, the CdSe−CdS QDs were localized exclusively in the P4VP perforations. The swelling of the bulk samples in a P4VP selective solvent also allowed the isolation of the perforated PS nanosheets, with QDs localized in the perforations, which further provided strong evidence for the formation of the unique PL morphology. The observed PL morphology was, plausibly, energetically stabilized because of the localization of QDs within the P4VP perforations, which allowed for the conformational entropy minimization of the majority P4VP block. The present work reveals possibilities for the discovery of novel hierarchical structures in block copolymer/nanoparticle composite systems and also provides new opportunities for the application of such materials in nanotechnology.

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Dependence of the damage in optical metal/dielectric coatings on the energy of ions in irradiation experiments for space qualification

Pelizzo, M. G.; Corso, A. J.; Santi, G.; Hübner, R.; Garoli, D.; Doyle, D.; Lubin, P.; Cohen, A. N.; Erlikhman, J.; Favaro, G.; Bazzan, M.; Drobny, J.; Curreli, D.; Umansky, M.

Terrestrial accelerator facilities can generate ion beams which enable the testing of the resistance of materials and thin film coatings to be used in the space environment. In this work, a TiO2/Al bi-layer coating has been irradiated with a He+ beam at three different energies. The same flux and dose have been used in order to investigate the damage dependence on the energy. The energies were selected to be in the range 4-100 keV, in order to consider those associated to the quiet solar wind and to the particles present in the near-Earth space environment. The optical, morphological and structural modifications have been investigated by using various techniques. Surprisingly, the most damaged sample is the one irradiated at the intermediate energy, which, on the other hand, corresponds to the case in which the interface between the two layers is more stressed. Results demonstrate that ion energies for irradiation tests must be carefully selected to properly qualify space components.

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B20-type FeGe on Ge(100) prepared by pulsed laser melting

Li, Z.; Xie, Y.; Yuan, Y.; Wang, M.; Xu, C.; Hübner, R.; Prucnal, S.; Zhou, S.

B20-type FeGe is one of the noncentrosymmetric materials hosting magnetic skymions. In this work, we have prepared B20-type FeGe films by pulsed laser melting of metal Fe deposited on Ge(100). The formation of the B20 phase is confirmed by X-ray diffraction. The FeGe samples show a superparamagnetic behaviour and their blocking temperatures increase with increasing the pulsed laser energy density. We conclude that this phenomenon is due to the increased grain size of the B20-type FeGe with increasing laser energy density. The presented method can be used to obtain different B20-type transition metal germanides and silicides, which can be magnetic skyrmion-hosting materials for spintronics.

Keywords: B20 phase; FeGe; Pulsed laser melting; Superparamagnetism

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Phase selection in Mn-Si alloys by fast solid-state reaction with enhanced skyrmion stability

Li, Z.; Xie, Y.; Yuan, Y.; Ji, Y.; Begeza, V.; Cao, L.; Hübner, R.; Rebohle, L.; Helm, M.; Kornelius, N.; Prucnal, S.; Zhou, S.

B20-type transition-metal silicides or germanides are noncentrosymmetric materials hosting magnetic skyrmions, which are promising information carriers in spintronic devices. The prerequisite is the preparation of thin films on technology-relevant substrates with magnetic skyrmions stabilized at a broad temperature and magnetic-field working window. The canonical example is the B20-MnSi film grown on Si substrates. However, the as-yet unavoidable contamination with MnSi1.7 occurs due to the lower nucleation temperature of this phase. In this work, we report a simple and efficient method to overcome this problem and prepare single-phase MnSi films on Si substrates. It is based on the millisecond reaction between metallic Mn and Si using flash lamp annealing (FLA). By controlling the FLA energy density, we can grow single-phase MnSi or MnSi1.7 or their mixture at will. Compared with bulk MnSi the prepared MnSi films show an increased Curie temperature of up to 41 K. In particular, the magnetic skyrmions are stable over a much wider temperature and magnetic-field range than reported previously. Our results constitute a novel phase selection approach for alloys and can help enhance specific functional properties such as enhancing the stability of magnetic skyrmions.

Keywords: B20-MnSi; Flash lamp annealing; Phase separation; Skyrmions

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αvβ3-Specific Gold Nanoparticles for Fluorescence Imaging of Tumor Angiogenesis

Pretze, M.; von Kiedrowski, V.; Runge, R.; Freudenberg, R.; Hübner, R.; Davarci, G.; Schirrmacher, R.; Wängler, C.; Wängler, B.

This paper reports on the development of tumor-specific gold nanoparticles (AuNPs) as theranostic tools intended for target accumulation and the detection of tumor angiogenesis via optical imaging (OI) before therapy is performed, being initiated via an external X-ray irradiation source. The AuNPs were decorated with a near-infrared dye, and RGD peptides as the tumor targeting vector for αvβ3-integrin, which is overexpressed in tissue with high tumor angiogenesis. The AuNPs were evaluated in an optical imaging setting in vitro and in vivo exhibiting favorable diagnostic properties with regards to tumor cell accumulation, biodistribution, and clearance. Furthermore, the therapeutic properties of the AuNPs were evaluated in vitro on pUC19 DNA and on A431 cells concerning acute and long-term toxicity, indicating that these AuNPs could be useful as radiosensitizers in therapeutic concepts in the future.

Keywords: gold nanoparticle; optical imaging; radiosensitizer; tumor angiogenesis; RGD peptide

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A Roadmap for 3D Metal Aerogels: Materials Design and Application Attempts

Jiang, X.; Du, R.; Hübner, R.; Hu, Y.; Eychmüller, A.

Armed with merits of the metals (e.g., electrical conductivity, catalytic activity, and plasmonic properties) and aerogels (e.g., monolithic structure, porous network, and large specific surface area), metal aerogels (MAs) have stood out as a new class of porous materials in the last decade. With unparalleled potential in electrocatalysis, plasmonics, and sensing, they are envisaged to revolutionize the energy- and detection-related application fields. However, MA development is severely retarded by the lack of a sufficient material basis. Suffering from the ambiguous understanding of formation mechanisms, big challenges remain for tailoring MAs for task-specific applications. By surveying state-of-the-art developments, this review strives to summarize design principles and arouse interest in broad scientific communities. Moreover, critical challenges and opportunities are highlighted to provide a research roadmap for this young yet promising field.

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Local and nonlocal spin Seebeck effect in lateral Pt-Cr2O3-Pt devices at low temperatures

Muduli, P.; Schlitz, R.; Kosub, T.; Hübner, R.; Erbe, A.; Makarov, D.; Goennenwein, S. T. B.

We have studied thermally driven magnon spin transport (spin Seebeck e_ect, SSE) in heterostructures of antiferromagnetic Cr2O3 and Pt at low temperatures. Monitoring the amplitude of the local and nonlocal SSE signals as a function of temperature, we found that both decrease with increasing temperature and disappear above 100 K and 20 K, respectively. Additionally, both SSE signals show a tendency to saturate at low temperatures. The nonlocal SSE signal decays exponentially for intermediate injector-detector separation, consistent with magnon spin current transport in the relaxation regime. We estimate the magnon relaxation length of our Cr2O3 films to be around 500 nm at 3 K. This short magnon relaxation length along with the strong temperature dependence of the SSE signal indicates that temperature-dependent inelastic magnon scattering processes play an important role in the intermediate range magnon transport. Our observation is relevant to low-dissipation antiferromagnetic magnon memory and logic devices involving thermal magnon generation and transport.

Keywords: spin Seebeck effect; antiferromagnetic spintronics

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