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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.

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.

Here we report on a combined computer simulation of defect generation and defect kinetics for 30 keV He⁺ ion irradiation of sub-μm-scale tin spheres. In the process to be simulated, the irradiation was performed in a Helium Ion Microscope which allows the in-situ monitoring of morphological changes of the nanospheres during He⁺ ion irradiation. Above a He⁺ fluence of ∼10¹⁷ /cm², Sn extrusions appear on the surface of the spheres. Initially, small, pyramidally facetted extrusions evolve at the equator of the tin spheres (north pole pointing to the ion source), later on each sphere becomes completely covered with tin, then turning into facetted single crystals. No Sn extrusions were observed for tin spheres with diameters smaller than ∼100nm. Transmission electron microscopy and Auger electron spectroscopy investigations show that the tin spheres are covered with a few-nm-thick SnO skin and that the extrusions are single crystals. For the computer simulations a model was developed which assumes that He⁺ ions generate interstitials ISn and vacancies VSn in the body-centered tetragonal lattice of tin. Due to the SnO skin, the ISn and VSn are confined to the tin sphere. A coherent Sn-SnO interface with a strong Sn-SnO interaction prevents ISn and VSn annihilation and void formation here. The projected range of 30 keV He⁺ ions is smaller than the diameter of the sub-μm spheres, the He accumulates and partly fills the VSn. Thus, the “pressure” of ISn increases steadily. Simultaneously, He⁺ ion erosion creates openings in the SnO skin. The sputter coefficient increases with the angle of incidence, thus openings in the SnO skin form at the equator regions first. Once the tin interstitials find such an opening in the SnO skin, they can escape from the interior of the Sn sphere and form an epitaxial, regular Sn lattice outside. Due to the high Sn-SnO bond strength the extruded tin wets the outer SnO surface. The defect generation at He⁺ ion irradiation was simulated with TRI3DYN [1], a 3D program calculating atomic displacements in the binary collision approximation. The reaction-diffusion behavior of the ISn and VSn as well as their clustering into voids and growth to extrusions were simulated with a 3D kinetic lattice Monte Carlo program [2] using an RGL potential for tin. The simulated reaction pathway of the morphology agrees very well with the sequence of HIM images taken during He⁺ ion irradiation. A quantitative comparison of the extruded material with simulations provides conclusions on the defect kinetics under ion irradiation. [1] Möller, Nucl. Instr. Meth. B 322 (2014) 23 [2] Strobel et al., Phys. Rev. B 64 (2001) 245422

For a safety assessment of repositories, possible accident scenarios must be considered. If radionuclides (RNs) are released from a repository into the groundwater, they can migrate and interact with the biosphere. In addition to microorganisms, higher organisms such as plants also play a role in this context. For this reason, it is necessary that a process understanding on RN plant interactions on the molecular level is elucidated by suitable biochemical and spectroscopic methods to improve biogeochemical models for a reliable modeling of the transfer behavior of RNs in the environment up to the food chain. Interaction with plant cells can lead to immobilization and speciation changes of the RNs. Uranium as the main component (by mass) of spent nuclear fuel rods is of interest for our investigations. Furthermore, we studied europium as a non-radioactive analogue for the trivalent actinides Am(III) and Cm(III), whereby the former in particular will dominate the radiotoxicity in a potential repository. We investigated the time and concentration-dependent immobilization of these two metals to the biomass of different plant cell cultures. Biochemical methods were accompanied by spectroscopic (time-resolved laser-induced fluorescence spectroscopy, TRLFS) and microscopic techniques for determination of speciation and localization of the metals within the plant cells, respectively. In addition, the combination of luminescence spectroscopy with microscopy in the form of chemical microscopy revealed for the first time the spatially resolved localization of different Eu(III) species in plant root tissues. We observed bioassociation (immobilization) of U(VI)/Eu(III), but in some cases also re-mobilization of U(VI), which temporally coincided with a change in the metal speciation in solution. Moreover, we successfully identified metabolites released by the plant cells that can complex with U(VI)/Eu(III) and influence their mobility and thus bioavailability in the environment. Our studies can help to integrate RN plant interactions in assessment strategies of nuclear waste repositories.

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.

Shrt abstract: Using our nanowire growth technique called droplet-confined alternate pulsed-epitaxy, which provides precise control over the axial growth rate and droplet composition, the growth mechanism of self-catalyzed GaAs epitaxial nanowires containing AlxGa1-xAs/AlyGa1-yAs axial short-period superlattices, was systematically examined. Significant increases in interfacial abruptness were confirmed by reducing the nanowire diameter and superlattice growth temperature. Additionally, we found that an unstable droplet contact angle impacts the superlattice growth rate considerably. The unique versatility of our short-period superlattices was tested by using them as axial barriers for GaAs quantum dot heterostructures embedded in nanowires. Furthermore, encapsulating the nanowire in lattice-mismatched shells operating as radial barriers, we demonstrated the tuneabilty of the quantum dot emission energy via strain engineering. Long abstract Short-period superlattices have diverse functionality in electronic and optoelectronic devices. Implementing such systems as axial heterostructures in freestanding semiconducting nanowires further broadens the scope of potential applications, for example: distributed Bragg reflectors, high-efficiency light-emitting diodes, and quantum dot heterostructures. The challenge, however, lies in reducing the compositional grading effect of the constituent superlattice materials across the interfaces in nanowires grown in self-catalyzed vapor-liquid-solid mode. Here, our previously developed nanowire growth technique called droplet-confined alternate pulsed-epitaxy[1] (an adaptation of conventional molecular beam epitaxy), which grants precise control over the axial growth rate and droplet composition, was employed to grow Al(x)Ga(1-x)As/Al(y)Ga(1-y)As axial superlattices in self-catalyzed GaAs nanowires with diameters as thin as 20 nm. High-angle annular dark-field scanning transmission electron microscopy (figure 1(a)), energy-dispersive X-ray spectroscopy, and growth models were utilized to gain an understanding of the compositional grading mechanism. By varying several growth parameters involving growth temperature, nanowire diameter, and droplet contact angle, the link between them and the superlattice characteristics was explored. We found that interfacial abruptness increases significantly by reducing the superlattice growth temperature and nanowire radius. Moreover, we studied the impact of an unstable contact angle on the superlattice growth rate, showing good agreement with analytical growth models and demonstrating the importance of growth rate stability in obtaining reproducible Al contents across successive superlattice periods. We confirmed with monolayer resolution, controlled Al contents in a wide compositional range and superlattice period widths of just a few monolayers (figure 1 (b)). The quality of our short-period superlattices was successfully tested via their employment as axial barriers in quantum dot nanowire heterostructures (figure 1 (c)). Additionally, by introducing a lattice-mismatched ternary alloy shell as the radial barrier (In(x)Al(1-x)As in this case), we demonstrated controlled strain-induced redshifts of the quantum dot emission energy by adjusting the In content of the shell. For 39% In, we measured a redshift in the quantum dot emission of 180 meV as shown by the photoluminescence measurements in figure 1 (d). Notwithstanding, limitations in what can be accomplished are present and possible strategies to overcome them will be presented. [1] Balaghi et al., Nano Lett. 16, 4032 (2016)

In this study, we investigated the interaction of U(VI) and Eu(III) with Brassica napus suspension plant cells as a model system. Concentration-dependent (0-200 µM) bioassociation experiments showed that more than 75% of U(VI) and Eu(III) were immobilized by the cells. In addition to this phenomenon, time-dependent studies for 1 to 72 h of exposure showed a multi-stage bioassociation process for cells that were exposed to 200 µM U(VI), where, after initial immobilization of U(VI) within 1 h of exposure, it was released back into culture medium starting within 24 h. A re-mobilization to this extent has not been previously observed. The MTT assay was used to correlate the bioassociation behavior of Eu and U with the cell vitality. Speciation studies by spectroscopy and in silico methods highlighted various U and Eu species over the course of exposure. We were able to observe a new U species, which emerged simultaneously with the re-mobilization of U back into solution, which we assume to be a U(VI) phosphate species. Thus, the interaction of U(VI) and Eu(III) with released plant metabolites could be concluded.

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.

The role of the microbial origin and environment of the discharged nanomaterials in their shape change was investigated. Here, we show that the biogenic elemental selenium nanospheres (BioSe-Nanospheres) produced under mesophilic conditions (30 °C) by Escherichia coli K-12 remain spherically when exposed to heating (55 °C for 7 days), whereas those obtained by anaerobic granular sludge transform to biogenic elemental selenium nanorods (BioSe-Nanorods). The larger quantity of proteins present in the corona of the BioSe-Nanospheres produced by E. coli K-12 are responsible for their shape stability. However, the protein corona of BioSe-Nanospheres produced by E. coli K-12 was degraded by extracellular peptidases secreted upon co-incubation with Bacillus safensis JG-B5T bacteria, which led to their transformation to BioSe-Nanorods. This study consequently demonstrates that the shape of biogenic nanomaterials depends both on their microbial origin and microbial surrounding, which increases the complexity in determining their risk assessment.

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.

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.

We report the observation of the intersubband AC-Stark effect in a single wide GaAs/AlGaAs quantum well. In a three-level configuration, the n = 2 to n = 3 intersubband transition is resonantly pumped at 3:5 THz using a free-electron laser. The induced spectral changes are probed using THz time-domain spectroscopy with a broadband pulse extending up to 4 THz. We observe an Autler Townes splitting at the 1 -> 2 intersubband transition as well as an indication of a Mollow triplet at the 2 -> 3 transition, both evidencing the dressed states. For longer delay times, a relaxation of the hot-electron system with a time constant of around 420 ps is measured.

The advent of noble metal aerogels (NMAs), that feature the high catalytic activity of noble metals and unique structural attributes of aerogels, has stimulated research on a new class of outstanding electrocatalysts. However, limited by the available compositions, the explored electrocatalytic reactions on NMAs are highly restricted and certain important electrochemical processes have not been investigated. Here, an effective gelation approach is demonstrated by using a strong salting-out agent (i.e., NH4F), thereby expanding the composition of NMAs to various multimetallic systems and providing a platform to investigate composition-dependent electrocatalytic performance of NMAs. Combining structural features of aerogels and optimized chemical compositions, the Au-Pt and Au-Rh aerogel catalysts manifest remarkable pH-universal (pH = 0-14) performance surpassing commercial Pt/C and many other nanoparticle (NP)-based catalysts in the electrocatalytic oxygen reduction reaction, hydrogen evolution reaction, and water splitting, displaying enormous potential for the electrochemical hydrogen production, fuel cells, etc.

Low-cost, non-noble-metal electrocatalysts are required for direct methanol fuel cells, but their development has been hindered by limited activity, high onset potential, low conductivity, and poor durability. A surface electronic structure tuning strategy is presented, which involves doping of a foreign oxophilic post-transition metal onto transition metal aerogels to achieve a non-noble-metal aerogel Ni97Bi3 with unprecedented electrocatalytic activity and durability in methanol oxidation. Trace amounts of Bi are atomically dispersed on the surface of the Ni97Bi3 aerogel, which leads to an optimum shift of the d-band center of Ni, large compressive strain of Bi, and greatly increased conductivity of the aerogel. The electrocatalyst is endowed with abundant active sites, efficient electron and mass transfer, resistance to CO poisoning, and outstanding performance in methanol oxidation. This work sheds light on the design of high-performance non-noble-metal electrocatalysts

Artificial ferromagnetic (FM)/nonmagnetic multilayers, with large enough FM thickness to prevent the dominance of interface anisotropies, offer a straightforward insight into the understanding and control of perpendicular standing spin wave (PSSW) modes. Here we present a study of the static and dynamic magnetic properties of [Co(3.0nm)/Au(0.6nm)]1≤N≤30 multilayer systems. Magnetometry reveals that the samples exhibit magnetization reversal properties typical of an effective single layer with weak perpendicular anisotropy, with the distinctive thickness-dependent magnetization reorientation transition from in-plane to out-of-plane. When such multilayer systems are out-of-plane saturated however, the dynamic response reveals the existence of several different ferromagnetic resonances in the form of PSSW modes that strongly depend on the material modulation characteristics along the total thickness. These modes are induced by the layer stacking itself as the effective single layer model fails to describe the complex dynamics observed in the system. In contrast to most systems considered in the past, described by a dynamic model of a single effectively homogeneous thick layer, the specific structures investigated here provide a unique platform for a large degree of tunability of the mode frequencies and amplitude profiles. We argue that the combination of periodic magnetic properties with vertical deformation gradients, arising from heteroepitaxial strain relaxation, generates a vertical regular array of two-dimensional pinning sites for the PSSW modes, which promotes the complex dynamics observed in the system.

Noble metals, despite their expensiveness, display irreplaceable roles in widespread fields. To acquire novel physicochemical properties and boost the performance-to-price ratio for practical applications, one core direction is to engineer noble metals into nanostructured porous networks. Noble metal foams (NMFs), featuring self-supported, 3D interconnected networks structured from noble-metal-based building blocks, have drawn tremendous attention in the last two decades. Inheriting structural traits of foams and physicochemical properties of noble metals, NMFs showcase a variety of interesting properties and impressive prospect in diverse fields, including electrocatalysis, heterogeneous catalysis, surface-enhanced Raman scattering, sensing and actuation, etc. A number of NMFs have been created and versatile synthetic approaches have been developed. However, because of the innate limitation of specific methods and the insufficient understanding of formation mechanisms, flexible manipulation of compositions, structures, and corresponding properties of NMFs are still challenging. Thus, the correlations between composition/structure and properties are seldom established, retarding material design/optimization for specific applications. This review is devoted to a comprehensive introduction of NMFs ranging from synthesis to applications, with an emphasis on electrocatalysis. Challenges and opportunities are also included to guide possible research directions in this field and promote the interest of interdisciplinary scientists.

In thin amorphous carbon (a-C) films being in contact with a thin nickel layer, metal-induced crystallization and layer exchange (LE) occur at temperatures lower than 700 °C. Analysis of thin film stacks with different architectures (a-C/Ni, Ni/a-C and Ni/a-C/Ni) by means of ion beam analysis, Raman spectroscopy, X-ray diffraction and transmission electron microscopy revealed that the degree of LE and the structural quality of the crystallized carbon layers depend on the initial layer sequence. A LE degree of approx. 93 % was found for Ni/a-C bilayers, where graphenic layers formed on the Ni surface, whereas in a-C/Ni bilayers only 83 % of carbon was transferred from the surface towards the fused silica substrate. The diffusion of carbon in the outward direction produces turbostratic carbon with basal planes oriented parallel to the Ni surface, while for the inward direction planar and curved turbostratic structures coexist. The crystallization and the LE are driven by the crystallization energy of a-C. The LE is mediated by the wetting of the Ni grain boundaries by carbon. The directionality of the LE was explained primarily by the difference in the surface and interface energies in the a-C/Ni and Ni/a-C stacks that were obtained from thermodynamic considerations.

Nanoscale, low-phase noise, tunable transmitter-receiver links are key for enabling the progress of wireless communi-cation. We demonstrate that vortex-based spin-torque nano-oscillators, which are intrinsically low-noise devices due to their topologically-protected magnetic structure, can achieve frequency tunability when submitted to local ion im-plantation. In the experiments presented here, the gyrotropic mode is excited with spin-polarized alternating currents and anisotropic magnetoresistance measurements yield discreet frequencies from a single device. Indeed, chromium-implanted regions of permalloy disks exhibit different saturation magnetisation than the surrounding, non-irradiated areas, and thus different resonance frequency, corresponding to the specific area where the core is gyrating. Our study proves that such devices can be fabricated without the need of further lithographical steps, suggesting ion irradiation can be a viable and cost-effective fabrication method for densely-packed networks of oscillators.

Technetium (Tc) retention on gamma alumina nanoparticles (gamma-Al₂O₃ NPs) has been studied in the absence (binary system) and presence (ternary system) of previously sorbed Fe²⁺ as a reducing agent. In the binary system, gamma-Al₂O₃ NPs sorb up to 6.5% of Tc from solution as Tc(VII). In the ternary system, the presence of previously sorbed Fe²⁺ on gamma-Al₂O₃ NPs significantly enhances the uptake of Tc from pH 4 to pH 11. Under these conditions, the reaction rate of Tc increases with pH, resulting in a complete uptake for pHs > 6.5. Redox potential (Eh) and X-ray photoelectron spectroscopy (XPS) measurements evince heterogeneous reduction of Tc(VII) to Tc(IV). Here, the formation of Fe containing solids was observed; Raman and scanning electron microscopy showed the presence of Fe(OH)₂, Fe(II)-Al(III)-Cl layered double hydroxide (LDH), and other Fe(II) and Fe(III) mineral phases, e.g. Fe₃O₄, FeOOH, Fe₂O₃. These results indicate that Tc scavenging is predominantly governed by the presence of sorbed Fe²⁺ species on gamma-Al₂O₃ NPs, where the reduction of Tc(VII) to Tc(IV) and overall Tc retention is highly improved, even under acidic conditions. Likewise, the formation of additional Fe solid phases in the ternary system promotes the Tc uptake via adsorption, co-precipitation, and incorporation mechanisms.

Bacteria strain Bacillus safensis JG-B5T is an obligate aerobic microorganism isolated from uranium mining waste pile. The role played by this microorganism in the transport of selenium is poorly understood. This study demonstrated that B. safensis JG-B5T can reduce 70 % of 2.5 mM selenite whereas selenate reduction was not observed. Formation of the resulting biogenic elemental selenium nanoparticles (BioSeNPs) occurred exclusively extracellularly. The ζ-potential of the produced BioSeNPs varied from −44.2 (±0.2) mV to −10.6 (±3.3) mV when Na+:Se ratio was varied from 0 to 0.8 (mM:mM), suggesting a lower colloidal stability. Further experiments in two-chamber reactors and transmission electron microscopy revealed that direct cell contact is essential for selenite reduction by B. safensis JG-B5T. The selenite reduction is likely primarily mediated through membrane-associated proteins, like succinate dehydrogenase as revealed by genome and proteomics analysis. Altogether, this study showed that aerobic bacteria B. safensis JG-B5T is involved in decreasing the environmental bioavailability and toxicity of selenium by membrane protein-mediated reduction of selenite oxyanions and formation of extracellular BioSeNPs with low colloidal stability.

Noble-metal aerogels (NMAs) have drawn increasing attention because of their self-supported conductive networks, high surface areas, and numerous optically/catalytically active sites, enabling their impressive performance in diverse fields. However, the fabrication methods suffer from tedious procedures, long preparation times, unavoidable impurities, and uncontrolled multiscale structures, discouraging their developments. By utilizing the self-healing properties of noble-metal aggregates, the freezing-promoted salting-out behavior, and the ice-templating effect, a freeze-thaw method is crafted that is capable of preparing various hierarchically structured noble-metal gels within one day without extra additives. In light of their cleanliness, the multi-scale structures, and combined catalytic/optical properties, the electrocatalytic and photoelectrocatalytic performance of NMAs are demonstrated, which surpasses that of commercial noble-metal catalysts.