Publikationen - Strukturanalytik

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Stress distribution at the AlN/SiC heterointerface probed by Raman spectroscopy

Breev, I. D.; Likhachev, K. V.; Yakovleva, V. V.; Hübner, R.; Astakhov, G.; Mokhov, E. N.; Baranov, P. G.; Anisimov, A. N.

We investigate AlN grown on 4H- and 6H-SiC substrates using Raman spectroscopy. We obtain the Raman peak shifts in 4H- and 6H-SiC substrates across the heterointerface and along the entire depth of the SiC layer. Using the earlier experimental prediction for the phonon deformation potential constants, we determine the stress tensor components in the 4H-SiC layer as a function of the distance from the AlN/SiC heterointerface and estimate the stress tensor value along the entire depth of the 6H-SiC layer. The maximum compressing stress values lie in the range of -1.7 GPa for the 4H-SiC/AlN heterostructure and in the range of -1.5 GPa for the 6H-SiC/AlN heterostructure.

Keywords: SiC, AlN, Raman spectroscopy, Stress


Effect of nanoscale surface topography on the adsorption of globular proteins

Yang, Y.; Yu, M.; Böke, F.; Qin, Q.; Hübner, R.; Knust, S.; Schwiderek, S.; Grundmeier, G.; Fischer, H.; Keller, A.

Protein adsorption is the initial step in the response of biological systems to artificial surfaces and thus a ubiquitous phenomenon in biomedicine and tissue engineering. Here, we investigate the adsorption of the three globular proteins myoglobin (MGB), thyroglobulin (TGL), and bovine serum albumin (BSA) at flat and nanorippled SiOx/Si and TiOx/Ti surfaces. Despite having lateral and vertical dimensions of only about 30 nm and less than 2 nm, respectively, these nanoripples influence protein adsorption and adsorption-induced protein denaturation in a highly protein- and material-specific way. Adsorption of small, positively charged MGB results in preferential protein alignment along the nanoripples on both oxide surfaces. The larger and strongly negatively charged TGL forms layers of similar thickness on all four surfaces except the nanorippled TiOx/Ti surface. Here, a smaller layer thickness is attributed to different denaturation states of the adsorbed proteins. Similarly, the smaller and less negatively charged BSA shows different degrees of denaturation on the flat and rippled SiOx/Si surfaces. Our results thus demonstrate that topographic surface features with vertical dimensions well below 10 nm may have a surprisingly strong effect on protein adsorption and thus need to be considered in the interaction of biological systems even with apparently flat surfaces.

Keywords: Protein adsorption; Biomaterials; Biointerfaces; Nanopatterning; Surface topography


Uranium(VI) bioassociation by different fungi – a comparative study into molecular processes

Wollenberg, A.; Drobot, B.; Hübner, R.; Kretzschmar, J.; Freitag, L.; Lehmann, F.; Günther, A.; Stumpf, T.; Raff, J.

Not just since the Chernobyl and Fukushima incidents, it has been known that fungi can take up and accumulate large quantities of heavy metals and radionuclides. It is therefore all the more surprising that the underlying processes are not yet well understood. For this reason, the molecular interactions of uranium(VI) with the white-rot fungi, Schizophyllum commune and Pleurotus ostreatus, and the soil-living fungus, Leucoagaricus naucinus, are investigated in this work. First, the uranium concentration in the biomass was determined by time-dependent bioassociation experiments. To characterize the molecular interactions, uranium was localized in the biomass by transmission electron microscopy analysis. Subsequently, the formed ura-nyl complexes in both biomass and supernatant were determined by fluorescence spectros-copy. Additionally, possible bioligands in the supernatant were identified. The results show that although the discernible interactions between metals and fungi are similar, namely biosorption, accumulation and subsequent crystallization, the underlying biochemical mechanisms are nevertheless different and specific to the fungal species. In addition, Schizophyllum commune is the only fungus that, under the chosen experimental conditions, releases tryptophan and other indole derivatives in the presence of uranium(VI). Both substances most likely act as messenger molecules rather than serving the direct detoxification of uranium(VI).

Keywords: radionuclides; mycelium; microscopy; spectroscopy; metabolite; quorum sensing


Highly ordered silicide ripple patterns induced by medium-energy ion irradiation

Redondo-Cubero, A.; Palomares, F. J.; Hübner, R.; Gago, R.; Vázquez, L.

We study the nanopatterning of silicon surfaces under near-normal 40-keV Ar+ sputtering with simultaneous Fe oblique codeposition. The ion-beam incidence was kept at 15°, for which no pattern is produced in the absence of metal incorporation. Morphological and compositional analyses were performed by atomic force microscopy, in its morphological and electrical modes, Rutherford backscattering spectrometry, x-ray photoelectron spectroscopy, scanning Auger, as well as transmission and scanning electron microscopy. Initially, nanodot structures randomly emerge, which, with increasing ion fluence, become progressively aligned along the perpendicular direction to the Fe flux. With increasing fluence, they coalesce, leading to a ripple pattern. The pattern dynamics and characteristics are faster and enhanced, respectively, as the distance to the metal source decreases (i.e., as the metal content increases). For the highest metal flux, the ripples can become rather large (up to 18 μm) and straighter, with few defects, and a pattern wavelength close to 500 nm, while keeping the surface roughness close to 15 nm. Furthermore, for a fixed ion fluence, the pattern order is improved for higher metal flux. In contrast, the pattern order enhancement rate with ion fluence does not depend on the metal flux. Our experimental observations agree with the predictions and assumptions of the model by Bradley [R. M. Bradley, Phys. Rev. B 87, 205408 (2013)] Several compositional and morphological studies reveal that the ripple pattern is also a compositional one, in which the ripple peaks have a higher iron silicide content, in agreement with the model. Likewise, the ripple structures develop along the perpendicular direction to the Fe flux, and the pattern wavelength increases as the metal flux decreases with a behavior qualitatively consistent with the model predictions.


All-THz pump-probe spectroscopy of the intersubband AC-Stark effect in a wide GaAs quantum well

Schmidt, J.; Winnerl, S.; Dimakis, E.; Hübner, R.; Schneider, H.; Helm, M.

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.

Keywords: AC Stark effect, Autler Townes splitting, high-field physics, intersubband transitions


Microbial origin and active microenvironment impact on the shape of biogenic elemental selenium nanomaterials

Fischer, S.; Jain, R.; Krause, T.; Jain, P.; Tsushima, S.; Shevchenko, A.; Hübner, R.; Jordan, N.

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.


Increasing the Diversity and Understanding of Semiconductor Nanoplatelets by Colloidal Atomic Layer Deposition

Reichhelm, A.; Hübner, R.; Damm, C.; Nielsch, K.; Eychmüller, A.

Nanoplatelets (NPLs) are a remarkable class of quantum confined materials with size-dependent optical properties, which are determined by the defined thickness of the crystalline platelets. To increase the variety of species, the colloidal atomic layer deposition method is used for the preparation of increasingly thicker CdSe NPLs. By growing further crystalline layers onto the surfaces of 4 and 5 monolayers (MLs) thick NPLs, species from 6 to 13 MLs are achieved. While increasing the thickness, the heavy-hole absorption peak shifts from 513 to 652 nm, leading to a variety of NPLs for applications and further investigations. The thickness and number of MLs of the platelet species are determined by high-resolution transmission electron microscopy (HRTEM) measurements, allowing the interpretation of several contradictions present in the NPL literature. In recent years, different assumptions are published, leading to a lack of clarity in the fundamentals of this field. Regarding the ongoing scientific interest in NPLs, there is a certain need for clarification, which is provided in this study.

Keywords: CdSe; colloidal atomic layer deposition; nanoplatelets


Unraveling Structure and Device Operation of Organic Permeable Base Transistors

Darbandy, G.; Dollinger, F.; Formánek, P.; Hübner, R.; Resch, S.; Roemer, C.; Fischer, A.; Leo, K.; Kloes, A.; Kleemann, H.

Organic permeable base transistors (OPBTs) are of great interest for flexible electronic circuits, as they offer very large on-current density and a record-high transition frequency. They rely on a vertical device architecture with current transport through native pinholes in a central base electrode. This study investigates the impact of pinhole density and pinhole diameter on the DC device performance in OPBTs based on experimental data and TCAD simulation results. A pinhole density of NPin = 54 μm−2 and pinhole diameters around LPin = 15 nm are found in the devices. Simulations show that a variation of pinhole diameter and density around these numbers has only a minor impact on the DC device characteristics. A variation of the pinhole diameter and density by up to 100% lead to a deviation of less than 4% in threshold voltage, on/off current ratio, and sub-threshold slope. Hence, the fabrication of OPBTs with reliable device characteristics is possible regardless of statistical deviations in thin film formation.

Keywords: organic permeable base transistors; organic electronics; technology computer-aided design simulation


High-Performance Bismuth-Doped Nickel Aerogel Electrocatalyst for the Methanol Oxidation Reaction

Dubale, A. A.; Zheng, Y.; Wang, H.; Hübner, R.; Li, Y.; Yang, J.; Zhang, J.; Sethi, N. K.; He, L.; Zheng, Z.; Liu, W.

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

Keywords: aerogels; bismuth dopants; methanol oxidation; nickel; single atoms


Electron Concentration Limit in Ge Doped by Ion Implantation and Flash Lamp Annealing

Prucnal, S.; Żuk, J.; Hübner, R.; Duan, J.; Wang, M.; Pyszniak, K.; Drozdziel, A.; Turek, M.; Zhou, S.

Controlled doping with an effective carrier concentration higher than 10^20 cm-3 is a key challenge for the full integration of Ge into silicon-based technology. Such a highly doped layer of both p- and n type is needed to provide ohmic contacts with low specific resistance. We have studied the effect of ion implantation parameters i.e., ion energy, fluence, ion type, and protective layer on the effective concentration of electrons. We have shown that the maximum electron concentration increases as the thickness of the doping layer decreases. The degradation of the implanted Ge surface can be minimized by performing ion implantation at temperatures that are below -100 C with ion flux less than 60 nAcm-2 and maximum ion energy less than 120 keV. The implanted layers are flash-lamp annealed for 20 ms in order to inhibit the diffusion of the implanted ions during the recrystallization process.

Keywords: Ge; ion implantation; flash lamp annealing; n-type doping; Raman spectroscopy


Freeze–Thaw-Promoted Fabrication of Clean and Hierarchically Structured Noble-Metal Aerogels for Electrocatalysis and Photoelectrocatalysis

Du, R.; Joswig, J.-O.; Hübner, R.; Zhou, L.; Wei, W.; Hu, Y.; Eychmüller, A.

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.

Keywords: electrochemistry; gels; nanostructures; photocatalysis; sol-gel process


Role of Hydrogen-Related Defects in Photocatalytic Activity of ZnO Films Grown by Atomic Layer Deposition

Peter, R.; Salamon, K.; Omerzu, A.; Grenzer, J.; Badovinac, I. J.; Saric, I.; Petravic, M.

The photocatalytic activity of ZnO films, grown by atomic layer deposition on sapphire, was investigated for different amounts of residual hydrogen incorporated unintentionally into the matrix during the crystal growth. A close correlation was found between the level of incorporated hydrogen ; the rate of photocatalytic degradation of methylene blue on ZnO films. The rate of degradation is consistent with predominantly zero-order reaction kinetics. An enhanced photocatalytic activity, observed for films of predominantly (001)-oriented grains ; low concentration of residual hydrogen, is explained by the reduced number of hydrogen-related defects responsible for recombination of charge carriers in combination with the preferential adsorption of water on polar (001) surfaces of ZnO grains.

Keywords: atomic layer deposition; photocatalytic degradation


Disturbance-Promoted Unconventional and Rapid Fabrication of Self-Healable Noble Metal Gels for (Photo-)Electrocatalysis

Du, R.; Joswig, J.-O.; Fan, X.; Hübner, R.; Spittel, D.; Hu, Y.; Eychmüller, A.

As an emerging class of porous materials, noble metal aerogels (NMAs) have drawn tremendous attention and displayed unprecedented potential in diverse fields. However, the development of NMAs is impeded by the fabrication methods because of their time- and cost-consuming procedures, limited generality, and elusive understanding of the formation mechanisms. Here, by revealing the self-healing behavior of noble metal gels and applying it in the gelation process at a disturbing environment, an unconventional and conceptually new strategy, i.e., a disturbance-promoted gelation method, is developed by introducing an external force field. It overcomes the diffusion limitation in the gelation process, thus producing monolithic gels within 1–10 min at room temperature, 2–4 orders of magnitude faster than for most reported methods. Moreover, versatile NMAs are acquired by using this method, and their superior (photo-)electrocatalytic properties are demonstrated for the first time in light of combined catalytic and optic properties.


Unveiling reductant chemistry in fabricating noble metal aerogels for superior oxygen evolution and ethanol oxidation

Du, R.; Wang, J.; Wang, Y.; Hübner, R.; Fan, X.; Senkovska, I.; Hu, Y.; Kaskel, S.; Eychmüller, A.

Amongst various porous materials, noble metal aerogels attract wide attention due to their concurrently featured catalytic properties and large surface areas. However, insufficient understanding and investigation of key factors (e.g. reductants and ligands) in the fabrication process limits on-target design, impeding material diversity and available applications. Herein, unveiling multiple roles of reductants, we develop an efficient method, i.e. the excessive-reductant-directed gelation strategy. It enables to integrate ligand chemistry for creating gold aerogels with a record-high specific surface area (59.8 m2 g−1), and to expand the composition to all common noble metals. Moreover, we demonstrate impressive electrocatalytic performance of these aerogels for the ethanol oxidation and oxygen evolution reaction, and discover an unconventional organic-ligand-enhancing effect. The present work not only enriches the composition and structural diversity of noble metal aerogels, but also opens up new dimensions for devising efficient electrocatalysts for broad material systems.


General Colloidal Synthesis of Transition-Metal Disulfide Nanomaterials as Electrocatalysts for Hydrogen Evolution Reaction

Meerbach, C.; Klemmed, B.; Spittel, D.; Bauer, C.; Park, Y. J.; Hübner, R.; Jeong, H. Y.; Erb, D.; Shin, H. S.; Lesnyak, V.; Eychmüller, A.

The material-efficient monolayers of transition-metal dichalcogenides (TMDs) are a promising class of ultrathin nanomaterials with properties ranging from insulating through semiconducting to metallic, opening a wide variety of their potential applications from catalysis and energy storage to optoelectronics, spintronics, and valleytronics. In particular, TMDs have a great potential as emerging inexpensive alternatives to noble metal-based catalysts in electrochemical hydrogen evolution. Herein, we report a straightforward, low-cost, and general colloidal synthesis of various 2D transition-metal disulfide nanomaterials, such as MoS2, WS2, NiSx, FeSx, and VS2, in the absence of organic ligands. This new preparation route provides many benefits including relatively mild reaction conditions, high reproducibility, high yields, easy upscaling, no post-thermal annealing/treatment steps to enhance the catalytic activity, and, finally, especially for molybdenum disulfide nanosheets, high activity in the hydrogen evolution reaction. To underline the universal application of the synthesis, we prepared mixed CoxMo1-xS2 nanosheets in one step to optimize the catalytic activity of pure undoped MoS2, which resulted in an enhanced hydrogen evolution reaction performance characterized by onset potentials as low as 134 mV and small Tafel slopes of 55 mV/dec.


Promoting the Electrocatalytic Performance of Noble Metal Aerogels by Ligand-Directed Modulation

Fan, X.; Zerebecki, S.; Du, R.; Hübner, R.; Marzum, G.; Jiang, G.; Hu, Y.; Barcikowki, S.; Reichenberger, S.; Eychmüller, A.

Noble metal aerogels (NMAs) are an emerging class of porous materials. Embracing nano-sized highly-active noble metals and porous structures, they display unprecedented performance in diverse electrocatalytic processes. However, various impurities, particularly organic ligands, are often involved in the synthesis and remain in the corresponding products, hindering the investigation of the intrinsic electrocatalytic properties of NMAs. Here, starting from laser-generated inorganic-salt-stabilized metal nanoparticles, various impurity-free NMAs (Au, Pd, and Au-Pd aerogels) were fabricated. In this light, we demonstrate not only the intrinsic electrocatalytic properties of NMAs, but also the prominent roles played by ligands in tuning electrocatalysis through modulating the electron density of catalysts. These findings may offer a new dimension to engineer and optimize the electrocatalytic performance for various NMAs and beyond.

Keywords: aerogels; electrocatalysis; laser; ligand; noble metals


Engineering Multimetallic Aerogels for pH-Universal HER and ORR Electrocatalysis

Du, R.; Jin, W.; Hübner, R.; Zhou, L.; Hu, Y.; Eychmüller, A.

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.

Keywords: electrocatalysis; hydrogen evolution reaction; metal aerogels; oxygen reduction reaction; pH


Formation of Thin NiGe Films by Magnetron Sputtering and Flash Lamp Annealing

Begeza, V.; Mehner, E.; Stöcker, H.; Xie, Y.; García, A.; Hübner, R.; Erb, D.; Zhou, S.; Rebohle, L.

The nickel-monogermanide (NiGe) phase is known for its electrical properties such as low ohmic and low contact resistance in group-IV-based electronics. In this work, thin films of nickel germanides (Ni-Ge) were formed by magnetron sputtering followed by flash lamp annealing (FLA). The formation of NiGe was investigated on three types of substrates: on amorphous (a-Ge) as well as polycrystalline Ge (poly-Ge) and on monocrystalline (100)-Ge (c-Ge) wafers. Substrate and NiGe structure characterization was performed by Raman, TEM, and XRD analyses. Hall Effect and four-point-probe measurements were used to characterize the films electrically. NiGe layers were successfully formed on different Ge substrates using 3-ms FLA. Electrical as well as XRD and TEM measurements are revealing the formation of Ni-rich hexagonal and cubic phases at lower temperatures accompanied by the formation of the low-resistivity orthorhombic NiGe phase. At higher annealing temperatures, Ni-rich phases are transforms into NiGe, as long as the supply of Ge is ensured. NiGe layer formation on a-Ge is accompanied by metal-induced crystallization and a decline of its electrical conductivity compared with that of poly-Ge and c-Ge substrates. Specific resistivities for 30 nm Ni on Ge were determined to be 13.5 uOhm cm for poly-Ge, 14.6 uOhm cm for c-Ge and 20.1 uOhm cm for a-Ge.

Keywords: germanium; germanides; nickel; thin films; sputtering; flash lamp annealing


Higher order ferromagnetic resonances in out-of-plane saturated magnetic multilayers

Fallarino, L.; Stienen, S.; Gallardo, R. A.; Arregi, J. A.; Uhlíř, V.; Lenz, K.; Hübner, R.; Hellwig, O.; Lindner, J.

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.

Keywords: ferromagnetic resonance; multilayers; perpendicular anisotropy; spin waves

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Hollow Au@TiO2 porous electrospun nanofibers for catalytic applications

Kumar, L.; Singh, S.; Horechyy, A.; Formanek, P.; Hübner, R.; Albrecht, V.; Weißpflog, J.; Schwarz, S.; Puneet, P.; Nandan, B.

Catalytically active porous and hollow titania nanofibers encapsulating gold nanoparticles were fabricated using a combination of sol-gel chemistry and coaxial electrospinning technique. We report the fabrication of catalytically active porous and hollow titania nanofibers encapsulating gold nanoparticles (AuNPs) using a combination of sol-gel chemistry and coaxial electrospinning technique. The coaxial electrospinning involved the use of a mixture of poly(vinyl pyrrolidone) (PVP) and titania sol as the shell forming component, whereas a mixture of poly(4-vinyl pyridine) (P4VP) and pre-synthesized AuNPs constituted the core forming component. The core-shell nanofibers were calcined stepwise up to 600 °C which resulted in decomposition and removal of the organic constituents of the nanofibers. This led to the formation of porous and hollow titania nanofibers, where the catalytic AuNPs were embedded in the inner wall of the titania shell. The catalytic activity of the prepared Au@TiO2 porous nanofibers was investigated using a model reaction of catalytic reduction of 4-nitrophenol and Congo red dye in the presence of NaBH4. The Au@TiO2 porous and hollow nanofibers exhibited excellent catalytic activity and recyclability, and the morphology of the nanofibers remained intact after repeated usage. The presented approach could be a promising route for immobilizing various nanosized catalysts in hollow titania supports for the design of stable catalytic systems where the added photocatalytic activity of titania could further be of significance.


The role of boron on exchange coupling in NiFe/Ru1−xBx/FeCo trilayer structures

Mckinnon, T.; Hübner, R.; Heinrich, B.; Girt, E.

In this work, we study the interlayer exchange coupling, J, between two NiFe and FeCo layers in a series of NiFe/Ru1-xBx(d)/FeCo synthetic antiferromagnet (SAF) samples, where the thickness of the spacer layer, d, is varied from 0.4 nm to 0.9 nm, and the boron concentration, x, is varied from 0 to 15 at. %. The samples are studied as deposited and after being annealed at 250 °C. B is deposited into the Ru spacer layer to investigate what occurs after annealing a FeCoB/Ru/FeCoB SAF structure, which is commonly used in modern nanoscale magnetic devices, in which the FeCoB layer crystallizes to FeCo and B diffuses to adjacent layers. We find that J in as-deposited samples is relatively unaffected by adding up to 15% B into the Ru spacer layer. However, after annealing at 250 °C, J changes the sign from antiferromagnetic coupling to ferromagnetic coupling for spacer layers thinner than 0.45 nm for 5% and 10% B and thinner than 0.525 nm for 15% B. We used transmission electron microscopy energy-dispersive x-ray spectroscopy in order to investigate the diffusion of atoms within a similar Ta(2.5 nm)/NiFe(0.8 nm)/Ru1-xBx(23 nm) layer structure. We find that after annealing at 250 °C, the sample containing 15% B within the Ru85B15 layer had significantly more diffusion of Fe into the Ru85B15 layer, from the NiFe layer, as compared to the sample with 0% B. Thus, the presence of B within the spacer layer enhances diffusion of Fe into the spacer layer.


Sub-10 nm Radiolabeled Barium Sulfate Nanoparticles as Carriers for Theranostic Applications and Targeted Alpha Therapy

Reissig, F.; Zarschler, K.; Hübner, R.; Pietzsch, H.-J.; Kopka, K.; Mamat, C.

The treatment of patients suffering from cancer with α-particle-emitting therapeutics continues to receive increasing interest. The range of radiopharmaceutically relevant α-emitters is limited to only a few radionuclides (e.g. actinium-225 and bismuth-213) as stable chelators or carrier systems for safe transport of the radioactive cargo are often lacking. Encapsulation of α-emitters into solid inorganic systems can help to diversify the portfolio of candidate radionuclides, providing that these nanomaterials effectively retain both the parent and the recoil daughters. We therefore focus on the design of stable and defined nanocarrier-based systems for various radionuclides including the promising α-emitting radionuclide radium-224. Hence, we report on the synthesis of sub-10 nm alendronate-functionalized barium sulfate nanoparticles, into whose matrix different radiometals including zirconium-89, indium-111, barium-131, barium-133, lutetium-177 and radium-224 were stably incorporated with appropriate yields. Our delivery systems show stabilities of g.t. 90% up to seven days regarding the radiometal release from the BaSO4 matrix. Furthermore, radium-224-labeled particles possess stabilities of 80% regarding the decay chain product lead-212. In fact, the majority of nanoparticles withstand the α-recoil and keeps the daughter radionuclides trapped. Noteworthy, due to the accessibility of reactive alendronate amine groups on their surface, it is possible to further modify this functionalized inorganic system by common amine-coupling strategies as exemplified herein by conjugation of fluorescein isothiocyanate. The synthesized nanoparticles exhibit some degree of nonspecific protein binding upon exposure to human serum, offering the possibility to add beneficial properties of a protein corona to the intrinsic features of the nanosystem.

Keywords: radium; alpha therapy; nanoparticle; delivery system


Realization of wafer-scale nanogratings with sub-50 nm period through vacancy epitaxy

Huang, Q.; Jia, Q.; Feng, J.; Huang, H.; Yang, X.; Grenzer, J.; Huang, K.; Zhang, S.; Lin, J.; Zhou, H.; You, T.; Yu, W.; Facsko, S.; Jonnard, P.; Wu, M.; Giglia, A.; Zhang, Z.; Liu, Z.; Wang, Z.; Wang, X.; Ou, X.

Gratings, one of the most important energy dispersive devices, are the fundamental building blocks for the majority of optical and optoelectronic systems. The grating period is the key parameter that limits the dispersion and resolution of the system. With the rapid development of large X-ray science facilities, gratings with periodicities below 50 nm are in urgent need for the development of ultrahigh-resolution X-ray spectroscopy. However, the wafer-scale fabrication of nanogratings through conventional patterning methods is difficult. Herein, we report a maskless and high-throughput method to generate wafer-scale, multilayer gratings with period in the sub-50 nm range. They are fabricated by a vacancy epitaxy process and coated with X-ray multilayers, which demonstrate extremely large angular dispersion at approximately 90 eV and 270 eV. The developed new method has great potential to produce ultrahigh line density multilayer gratings that can pave the way to cutting edge high-resolution spectroscopy and other X-ray applications.

Keywords: GRATINGS; SCATTERING; ARRAYS


Tunable magnetic vortex dynamics in ion-implanted permalloy disks

Ramasubramanian, L.; Kákay, A.; Fowley, C.; Yildirim, O.; Matthes, P.; Sorokin, S.; Titova, A.; Hilliard, D.; Böttger, R.; Hübner, R.; Gemming, S.; Schulz, S. E.; Kronast, F.; Makarov, D.; Faßbender, J.; Deac, A. M.

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.

Keywords: electrical detection; vortex dynamics; frequency tunability; ion implantation; reduced saturation magnetisation


Ligand-Exchange-Mediated Fabrication of Gold Aerogels Containing Different Au(I) Content with Peroxidase-like Behavior

Fan, X.; Cai, B.; Du, R.; Hübner, R.; Georgi, M.; Jiang, G.; Li, L.; Samadi Khoshkhoo, M.; Sun, H.; Eychmüller, A.

Noble-metal aerogels are emerging functional porous materials that have been applied in diverse fields. Among them, gold (Au) aerogels have displayed grand potentials in a wide range of catalytic processes. However, current fabrication methods fall short in obtaining Au gels with small ligament sizes and controlled surface valence states, which hinder the study of the underlying catalytic mechanisms. Here, a new approach of producing Au aerogels is reported. Via a two-phase ligand exchange, the long-chain ligands (oleylamine) of the as-prepared Au nanoparticles were replaced by short sulfide ions and subsequently self-assembled into three-dimensional gels. As a result, Au aerogels with small ligament sizes (ca. 3−4 nm) and tunable surface valence states are acquired. Taking the application for peroxidase mimics as an example, by correlating the surface valence with the catalytic properties, Au(I) is found to be the active site for H2O2 and substrate-binding site for 3,3′,5,5′-tetramethylbenzidine, paving a new avenue for on-target devising Au-based catalysts.


Directionality of metal-induced crystallization and layer exchange in amorphous carbon/nickel thin film stacks

Janke, D.; Munnik, F.; Julin, J. A.; Hübner, R.; Grenzer, J.; Wüstefeld, C.; Gemming, S.; Rafaja, D.; Krause, M.

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.

Keywords: metal-induced crystallization; layer exchange; carbon/nickel; thin films; turbostratic carbon; Raman spectroscopy; ion beam analysis

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  • Zweitveröffentlichung erwartet ab 05.12.2021

Technetium retention by gamma alumina nanoparticles and the effect of sorbed Fe²⁺

Mayordomo Herranz, N.; Rodriguez Hernandez, D. M.; Schild, D.; Molodtsov, K.; Johnstone, E. V.; Hübner, R.; Shams Aldin Azzam, S.; Brendler, V.; Müller, K.

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.

Keywords: Technetium; Al₂O₃; reduction; sorption; immobilization


Low damping and microstructural perfection of sub-40nm-thin yttrium iron garnet films grown by liquid phase epitaxy

Dubs, C.; Surzhenko, O.; Thomas, R.; Osten, J.; Schneider, T.; Lenz, K.; Grenzer, J.; Hübner, R.; Elke, W.

The field of magnon spintronics is experiencing increasing interest in the development of solutions for spin-wave-based data transport and processing technologies that are complementary or alternative to modern CMOS architectures. Nanometer-thin yttrium iron garnet (YIG) films have been the gold standard for insulator-based spintronics to date, but a potential process technology that can deliver perfect, homogeneous large-diameter films is still lacking. We report that liquid phase epitaxy (LPE) enables the deposition of nanometer-thin YIG films with low ferromagnetic resonance losses and consistently high magnetic quality down to a thickness of 10 nm. The obtained epitaxial films are characterized by an ideal stoichiometry and perfect film lattices, which show neither significant compositional strain nor geometric mosaicity, but sharp interfaces. Their magneto-static and dynamic behavior is similar to that of single crystalline bulk YIG. We found, that the Gilbert damping coefficient  is independent of the film thickness and close to 1  10-4, and that together with an inhomogeneous peak-to-peak linewidth broadening of H0|| = 0.4 G, these values are among the lowest ever reported for YIG films with a thickness smaller than 40 nm. Only for the 10-nm-thin film a significantly reduced saturation magnetization was observed. These results suggest, that nanometer-thin LPE films can be used to fabricate nano- and micro-scaled circuits with the required quality for magnonic devices. The LPE technique is easily scalable to YIG sample diameters of several inches.

Keywords: YIG; ferromagnetic resonance; linewidth; damping; thin films; liquid phase epitaxy

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Engineering Self-Supported Noble Metal Foams Toward Electrocatalysis and Beyond

Du, R.; Jin, X.; Hübner, R.; Fan, X.; Hu, Y.; Eychmüller, A.

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.


Diffusion of Phosphorus and Boron from Atomic Layer Deposition Oxides into Silicon

Beljakowa, S.; Pichler, P.; Kalkofen, B.; Hübner, R.

Oxides containing group III or group V elements (B2O3/Sb2O5 and P2O5/Sb2O5) are grown by plasma-assisted atomic layer deposition (ALD) on single-crystalline silicon and serve as dopant sources for conformal and shallow doping. Transport phenomena in ALD-oxide–Si structures during rapid thermal annealing (RTA) are investigated subsequently by X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and secondary ion mass spectrometry (SIMS). The XPS and TEM analyses of the annealed ALD oxide–Si structures demonstrate that the ALD oxide converts to a silicon oxide and partially evaporates during annealing. In addition, dopant-containing, spherical, and partially crystalline particles form in the oxide, and Si-P precipitates at the oxide–Si interface. After diffusion annealing at 1000 °C, the SIMS analyses reveal phosphorus and boron concentration profiles in the silicon substrate with maximum concentrations exceeding their solid solubility limits by roughly one order of magnitude. Experimental doping profiles of phosphorus and boron in silicon are compared with simulation results, considering a slight injection of self-interstitials and dynamical defect clustering.

  • Open Access Logo Physica Status Solidi (A) 216(2019), 1900306
    DOI: 10.1002/pssa.201900306
  • Poster
    Gettering and Defect Engineering in Semiconductor Technology XVIII, GADEST 2019, 22.-27.09.2019, Zeuthen, Germany

p-type codoping effect in (Ga,Mn)As: Mn lattice location versus magnetic properties

Xu, C.; Zhang, C.; Wang, M.; Xie, Y.; Hübner, R.; Heller, R.; Yuan, Y.; Helm, M.; Zhang, X.; Zhou, S.

In the present work, we perform a systematic investigation on p-type codoping in (Ga,Mn)As. Through gradually increasing Zn doping concentration, the hole concentration increases, which should theoretically lead to an increase of the Curie temperature (TC) according to the p-d Zener model. Unexpectedly, although the film keeps its epitaxial structure, both TC and the magnetization decrease. The samples present a phase transition from ferromagnetism to paramagnetism upon increasing hole concentration. In the intermediate regime, we observe a signature of antiferromagnetism. By using channeling Rutherford backscattering spectrometry and particle-induced x-ray emission, the substitutional Mn atoms are observed to shift to interstitial sites, while more Zn atoms occupy Ga sites, which explains the observed behavior. This is also consistent with first-principles calculations, showing that the complex of substitutional Zn and interstitial Mn has the lowest formation energy.

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Spin Hall magnetoresistance in heterostructures consisting of noncrystalline paramagnetic YIG and Pt

Lammel, M.; Schlitz, R.; Geishendorf, K.; Makarov, D.; Kosub, T.; Fabretti, S.; Reichlova, H.; Huebner, R.; Nielsch, K.; Thomas, A.; Goennenwein, S. T. B.

The spin Hall magnetoresistance (SMR) effect arises from spin-transfer processes across the interface between a spin Hall active metal and an insulating magnet. While the SMR response of ferrimagnetic and antiferromagnetic insulators has been studied extensively, the SMR of a paramagnetic spin ensemble is not well established. Thus, we investigate herein the magnetoresistive response of the as-deposited yttrium iron garnet/platinum thin film bilayers as a function of the orientation and the amplitude of an externally applied magnetic field. Structural and magnetic characterization shows no evidence for the crystalline order or spontaneous magnetization in the yttrium iron garnet layer. Nevertheless, we observe a clear magnetoresistance response with a dependence on the magnetic field orientation characteristic for the SMR. We propose two models for the origin of the SMR response in paramagnetic insulator/platinum heterostructures. The first model describes the SMR of an ensemble of noninteracting paramagnetic moments, while the second model describes the magnetoresistance arising by considering the total net moment. Interestingly, our experimental data are consistently described by the net moment picture, in contrast to the situation in compensated ferrimagnets or antiferromagnets.

Keywords: spin Hall magnetoresistance; antiferromagnetic insulators


Phase Selectivity in Cr and N Co-Doped TiO2 Films by Modulated Sputter Growth and Post-Deposition Flash-Lamp-Annealing

Gago, R.; Prucnal, S.; Hübner, R.; Munnik, F.; Esteban-Mendoza, D.; Jiménez, I.; Palomares, J.

In this paper, we report on the phase selectivity in Cr and N co-doped TiO2 (TiO2:Cr,N) sputtered films by means of interface engineering. In particular, monolithic TiO2:Cr,N films produced by continuous growth conditions result in the formation of a mixed-phase oxide with dominant rutile character. On the contrary, modulated growth by starting with a single-phase anatase TiO2:N buffer layer, can be used to imprint the anatase structure to a subsequent TiO2:Cr,N layer. The robustness of the process with respect to the growth conditions has also been investigated, especially regarding the maximum Cr content (<5 at.%) for single-phase anatase formation. Furthermore, post-deposition flash-lamp-annealing (FLA) in modulated coatings was used to improve the as-grown anatase TiO2:Cr,N phase, as well as to induce dopant activation (N substitutional sites) and diffusion. In this way, Cr can be distributed through the whole film thickness from an initial modulated architecture while preserving the structural phase. Hence, the combination of interface engineering and millisecond-range-FLA opens new opportunities for tailoring the structure of TiO2-based functional materials.

Keywords: TiO2; flash lamp annealing; doping


Colloidal Mercury-Doped CdSe Nanoplatelets with Dual Fluorescence

Galle, T.; Kazes, M.; Hübner, R.; Lox, J.; Khoshkhoo, M. S.; Sonntag, L.; Tietze, R.; Sayevich, V.; Oron, D.; Koitzsch, A.; Lesnyak, V.; Eychmüller, A.

Quasi-two-dimensional (2D) CdSe nanoplatelets (NPLs) are distinguished by their unique optical properties in comparison to classical semiconductor nanocrystals, such as extremely narrow emission line widths, reduced Auger recombination, and relatively high absorption cross sections. Inherent to their anisotropic 2D structure, however, is the loss of continuous tunability of their photoluminescence (PL) properties due to stepwise growth. On top of that, limited experimental availability of NPLs of different thicknesses and ultimately the bulk band gap of CdSe constrain the achievable PL wavelengths. Here, we report on the doping of CdSe NPLs with mercury, which gives rise to additional PL in the red region of the visible spectrum and in the near-infrared region. We employ a seeded-growth method with injection solutions containing cadmium, selenium, and mercury. The resulting NPLs retain their anisotropic structure, are uniform in size and shape, and present significantly altered spectroscopic characteristics due to the existence of additional energetic states. We conclude that doping takes place by employing elemental analysis in combination with PL excitation spectroscopy, X-ray photoelectron spectroscopy, and single-particle fluorescence spectroscopy, confirming single emitters being responsible for multiple distinct emission signals.


Emerging Noble Metal Aerogels: State of the Art and a Look Forward

Du, R.; Fan, X.; Jin, X.; Hübner, R.; Hu, Y.; Eychmüller, A.

Noble metal aerogels (NMAs), as the most important class of noble metal foams (NMFs), appear as emerging functional porous materials in the field of materials science. Combining the irreplaceable roles of noble metals in certain scenarios, as well as monolithic and porous features of aerogels, NMAs can potentially revolutionize diverse fields, such as catalysis, plasmonics, and biology. Despite profound progress, grand challenges remain in their fabrication process, including the efficient structure control, the comprehensive understanding of the formation mechanisms, and the generality of the fabrication strategies, thus inevitably retarding the material design and optimization. This Perspective focuses on the key progress, especially of the fabrication strategies for NMAs during the last two decades, while other NMFs are also succinctly introduced. Challenges and opportunities are summarized to highlight the unexploited space and future directions in expectation of stimulating the broad interest of interdisciplinary scientists.


Nanoscale n++-p junction formation in GeOI probed by tip-enhanced Raman spectroscopy and conductive atomic force microscopy

Prucnal, S.; Berencen, Y.; Wang, M.; Georgiev, Y.; Erbe, A.; Khan, M. B.; Böttger, R.; Hübner, R.; Schönherr, T.; Kalbacova, J.; Vines, L.; Facsko, S.; Engler, M.; Zahn, D. R. T.; Knoch, J.; Helm, M.; Skorupa, W.; Zhou, S.

Ge-on-Si and Ge-on-insulator (GeOI) are the most promising materials for the next-generation nanoelectronics that can be fully integrated with silicon technology. To this day, the fabrication of Ge-based transistors with a n-type channel doping above 5 × 1019 cm−3 remains challenging. Here, we report on n-type doping of Ge beyond the equilibrium solubility limit (ne ≈ 6 × 1020 cm−3) together with a nanoscale technique to inspect the dopant distribution in n++-p junctions in GeOI. The n++ layer in Ge is realized by P+ ion implantation followed by millisecond-flashlamp annealing. The electron concentration is found to be three times higher than the equilibrium solid solubility limit of P in Ge determined at 800 °C. The millisecond-flashlamp annealing process is used for the electrical activation of the implanted P dopant and to fully suppress its diffusion. The study of the P activation and distribution in implanted GeOI relies on the combination of Raman spectroscopy, conductive atomic force microscopy, and secondary ion mass spectrometry. The linear dependence between the Fano asymmetry parameter q and the active carrier concentration makes Raman spectroscopy a powerful tool to study the electrical properties of semiconductors.
We also demonstrate the high electrical activation efficiency together with the formation of ohmic contacts through Ni germanidation via a single-step flashlamp annealing process.

Keywords: GeOI; ion implantation; flash lamp annealing; doping; TERS

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Effective Hexagonal Boron Nitride Passivation of Few-Layered InSe and GaSe to Enhance Their Electronic and Optical Properties

Arora, H.; Jung, Y.; Venanzi, T.; Watanabe, K.; Taniguchi, T.; Schneider, H.; Hone, J.; Helm, M.; Erbe, A.; Hübner, R.

Indium selenide (InSe) and gallium selenide (GaSe), members of the III–VI chalcogenide family, are emerging two-dimensional (2D) semiconductors with appealing electronic properties. However, their devices are still lagging behind because of their sensitivity to air and device fabrication processes which induce structural damage and hamper their intrinsic properties. Thus, in order to obtain high-performance and stable devices, effective passivation of these air-sensitive materials is strongly required. Here, we demonstrate a hexagonal boron nitride (hBN)-based encapsulation technique, where 2D layers of InSe and GaSe are covered entirely between two layers of hBN. To fabricate devices out of fully encapsulated 2D layers, we employ the lithography-free via-contacting scheme. We find that hBN acts as an excellent encapsulant and a near-ideal substrate for InSe and GaSe by passivating them from the environment and isolating them from the charge disorder at the SiO2 surface. As a result, the encapsulated InSe devices are of high quality and ambient-stable for a long time and show an improved two-terminal mobility of 30–120 cm2 V–1 s–1 as compared to mere ∼1 cm2 V–1 s–1 for unencapsulated devices. On employing this technique to GaSe, we obtain a strong and reproducible photoresponse. In contrast to previous studies, where either good performance or long-term stability was achieved, we demonstrate a combination of both in our devices. This work thus provides a systematic study of fully encapsulated devices based on InSe and GaSe, which has not been reported until now. We believe that this technique can open ways for fundamental studies as well as toward the integration of these materials in technological applications.

Keywords: indium selenide; gallium selenide; hexagonal boron nitride; encapsulation; photoluminescence; stable electronics; field-effect transistors; photodetectors

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  • Zweitveröffentlichung erwartet ab 25.10.2020

Preparation of non-oxidized Ge quantum dot lattices in amorphous Al2O3, Si3N4 and SiC matrices

Nekić, N.; Šarić, I.; Salamon, K.; Basioli, L.; Sancho-Parramon, J.; Grenzer, J.; Hübner, R.; Bernstorff, S.; Petravić, M.; Mičetić, M.

The preparation of non-oxidized Ge quantum dot (QD) lattices embedded in Al2O3, Si3N4, SiC matrices by self-assembled growth was studied. The materials were produced by magnetron sputtering deposition, using different substrate temperatures. The deposition regimes leading to the self-assembled growth type and the formation of three-dimensionally ordered Ge QD lattices in different matrices were investigated and determined. The oxidation of the Ge QDs in different matrices was monitored and the best conditions for the production of non-oxidized Ge QDs were found. The optical properties of the Ge QD lattices in different matrices show a strong dependence on the Ge oxidation and the matrix type.

Keywords: Ge QD lattices; Ge oxidation; self-assembled growth; influence of matrix


Specific ion effects directed noble metal aerogels: Versatile manipulation for electrocatalysis and beyond

Du, R.; Hu, Y.; Hübner, R.; Joswig, J.-O.; Fan, X.; Schneider, K.; Eychmüller, A.

Noble metal foams (NMFs) are a new class of functional materials featuring properties of both noble metals and monolithic porous materials, providing impressive prospects in diverse fields. Among reported synthetic methods, the sol-gel approach manifests overwhelming advantages for versatile synthesis of nanostructured NMFs (i.e., noble metal aerogels) under mild conditions. However, limited gelation methods and elusive formation mechanisms retard structure/composition manipulation, hampering on-demand design for practical applications. Here, highly tunable NMFs are fabricated by activating specific ion effects, enabling various single/alloy aerogels with adjustable composition (Au, Ag, Pd, and Pt), ligament sizes (3.1 to 142.0 nm), and special morphologies. Their superior performance in programmable self-propulsion devices and electrocatalytic alcohol oxidation is also demonstrated. This study provides a conceptually new approach to fabricate and manipulate NMFs and an overall framework for understanding the gelation mechanism, paving the way for on-target design of NMFs and investigating structure-performance
relationships for versatile applications.


‘Box-Profile’ Ion Implants as Geochemical Reference Materials for Electron Probe Microanalysis and Secondary Ion Mass Spectrometry

Wu, H.; Böttger, R.; Couffignal, F.; Gutzmer, J.; Krause, J.; Munnik, F.; Renno, A.; Hübner, R.; Wiedenbeck, M.; Ziegenrücker, R.

EPMA (Electron Probe Microanalysis) and SIMS (Secondary Ion Mass Spectrometry) are widely used analytical techniques for geochemical and mineralogical applications. Nevertheless, metrologically rigorous quantification remains a major challenge for these methods. SIMS in particular is a matrix-sensitive method; for SIMS the use of matrix-matched reference materials (RMs) is essential in order to avoid significant analytical bias. A major problem is that the list of available RMs for SIMS is vanishingly short compared to the needs of the analyst. One approach for the production of matrix-specific RMs is the use of high-energy ion implantation that introduces a known amount of a selected isotope into a material. We chose the more elaborate way of implanting a so-called ‘box profile’ to generate a quasi-homogeneous concentration of the implanted isotope in three dimensions, which allows RMs not only to be used for ion beam analysis but also makes them suitable for EPMA. For proof of concept, we used the thoroughly studied mineralogically and chemically ‘simple’ SiO2 system, which addresses many interesting scientific challenges, such as the Ti-in-quartz geothermometer (Wark et al. 2006, Thomas et al. 2010). We implanted either 47Ti or 48Ti into synthetic, ultra-high purity silica glass. Several ‘box profiles’ with concentrations between 10 and 1000 µg g-1 Ti and maximum depths of homogeneous Ti distribution between 200 nm and 3 µm were produced at the Institute of Ion Beam Physics and Materials Research of Helmholtz-Zentrum Dresden-Rossendorf (HZDR). Multiple implantation steps using varying ion energies and ion doses were simulated with the SRIM (Stopping and Range of Ions in Matter) software (Ziegler et al. 2008), optimizing for the target concentrations, implantation-depths and technical limits of the implanter.
We characterized several implant test-samples having different concentrations and maximum implantation depths by means of SIMS and other analytical techniques. The results show that the implant samples are suitable for use as reference materials for SIMS measurements. The multi-energy ion implantation technique also looks to be very promising procedure for the production of EPMA-suitable reference materials.

Keywords: ‘box-profile’; multi-energy ion implantation; EPMA; SIMS; synthetic reference material

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  • Zweitveröffentlichung erwartet

Superconductivity in single-crystalline aluminum- and gallium-hyperdoped germanium

Prucnal, S.; Heera, V.; Hübner, R.; Wang, M.; Mazur, G. P.; Grzybowski, M. J.; Qin, X.; Yuan, Y.; Voelskow, M.; Skorupa, W.; Rebohle, L.; Helm, M.; Sawicki, M.; Zhou, S.

Superconductivity in group IV semiconductors is desired for hybrid devices combining both semiconducting and superconducting properties. Following boron-doped diamond and Si, superconductivity has been observed in gallium-doped Ge; however, the obtained specimen is in polycrystalline form [Phys. Rev. Lett. 102, 217003 (2009)]. Here we present superconducting single-crystalline Ge hyperdoped with gallium or aluminum by ion implantation and rear-side flash lamp annealing. The maximum concentration of Al and Ga incorporated into substitutional positions in Ge is 8 times higher than the equilibrium solid solubility. This corresponds to a hole concentration above 1021 cm−3. Using density functional theory in the local-density approximation and pseudopotential plane-wave approach, we show that the superconductivity in p-type Ge is phonon mediated. According to the ab initio calculations, the critical superconducting temperature for Al- and Ga-doped Ge is in the range of 0.45 K for 6.25at.% of dopant concentration, being in qualitative agreement with experimentally obtained values.

Keywords: superconductivity; ion implantation; Germanium; flash lamp annealing

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Widely tunable GaAs bandgap via strain engineering in core/shell nanowires with large lattice mismatch

Balaghi, L.; Bussone, G.; Grifone, R.; Hübner, R.; Grenzer, J.; Ghorbani-Asl, M.; Krasheninnikov, A.; Schneider, H.; Helm, M.; Dimakis, E.

The realization of photonic devices for different energy ranges demands materials with different bandgaps, sometimes combined even within the same device as in multi-junction photovoltaic cells. The optimal solution in terms of integration, device performance and device economics would be a simple material system with widely tunable bandgap and compatible with the mainstream silicon technology. Here, we show that gallium arsenide nanowires grown epitaxially on silicon substrates exhibit a sizeable reduction of their bandgap by up to 40% when overgrown with lattice-mismatched indium gallium arsenide or indium aluminium arsenide shells. Specifically, we demonstrate that the gallium arsenide core sustains unusually large tensile strain with hydrostatic character and its magnitude can be engineered via the composition and the thickness of the shell. The resulted bandgap reduction renders gallium arsenide nanowires suitable for photonic devices across the near-infrared range, including telecom photonics at 1.3 and potentially 1.55 μm, with the additional possibility of monolithic integration in silicon-CMOS chips.


Vertical Organic Thin-Film Transistors with an Anodized Permeable Base for Very Low Leakage Current

Dollinger, F.; Lim, K.-G.; Li, Y.; Guo, E.; Formánek, P.; Hübner, R.; Fischer, A.; Kleemann, H.; Leo, K.

The organic permeable base transistor (OPBT) is currently the fastest organic transistor with a transition frequency of 40 MHz. It relies on a thin aluminum base electrode to control the transistor current. This electrode is surrounded by a native oxide layer for passivation, currently created by oxidation in air. However, this process is not reliable and leads to large performance variations between samples, slow production, and relatively high leakage currents. Here, for the first time it is demonstrated that electrochemical anodization can be conveniently employed for the fabrication of high-performance OPBTs with vastly reduced leakage currents and more controlled process parameters. Very large transmission factors of 99.9996 % are achieved, while excellent on/off ratios of 5 × 105 and high on-currents greater than 300 mA cm−2 show that the C60 semiconductor layer can withstand the electrochemical anodization. These results make anodization an intriguing option for innovative organic transistor design.

Keywords: aluminum oxide; anodization; organic permeable base transistors (OPBTs); organic transistors; organic thin-film transistors (OTFTs); vertical transistors


Thermal stability of Te-hyperdoped Si: Atomic-scale correlation of the structural, electrical, and optical properties

Wang, M.; Hübner, R.; Xu, C.; Xie, Y.; Berencen, Y.; Heller, R.; Rebohle, L.; Helm, M.; Prucnal, S.; Zhou, S.

Si hyperdoped with chalcogens (S,Se,Te) is well known to possess unique properties such as an insulator-tometal transition and a room-temperature sub-band-gap absorption. These properties are expected to be sensitive to a postsynthesis thermal annealing, since hyperdoped Si is a thermodynamically metastable material. Thermal stability of the as-fabricated hyperdoped Si is of great importance for the device fabrication process involving temperature-dependent steps such as Ohmic contact formation. Here, we report on the thermal stability of the as-fabricated Te-hyperdoped Si subjected to isochronal furnace anneals from 250 to 1200 °C. We demonstrate that Te-hyperdoped Si exhibits thermal stability up to 400 °C for 10 min, which even helps to further improve the crystalline quality, the electrical activation of Te dopants, and the room-temperature sub-band-gap absorption. At higher temperatures, however, Te atoms are found to move out from the substitutional sites with a maximum migration energy of EM = 2.3 eV forming inactive clusters and precipitates that impair the structural, electrical, and optical properties. These results provide further insight into the underlying physical state transformation of Te dopants in a metastable compositional regime caused by postsynthesis thermal annealing. They also pave the way for the fabrication of advanced hyperdoped Si-based devices.

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Silver Particles with Rhombicuboctahedral Shape and Effective Isotropic Interactions with Light

Steiner, A. M.; Mayer, M.; Schletz, D.; Wolf, D.; Formanek, P.; Hübner, R.; Dulle, M.; Förster, S.; König, T. A. F.; Fery, A.

Truly spherical silver nanoparticles are of great importance for fundamental studies including plasmonic applications, but their direct synthesis in aqueous media is not feasible. Using the commonly employed copper-based etching processes, an isotropic plasmonic response can be achieved by etching well-defined silver nanocubes. Whilst spherical-like shape is typically prevailing in such processes, we established that there is a preferential growth toward silver rhombicuboctahedra, which is the thermodynamically most stable product of this synthesis. The rhombicuboctahedral morphology is further evidenced by comprehensive characterization with small-angle X-ray scattering in combination with transmission electron microscopy (TEM) tomography and high-resolution TEM. We also elucidate the complete reaction mechanism based on UV-vis kinetic studies, and the postulated mechanism can also be extended to all copper-based etching processes.


Nanomagnetism of Magnetoelectric Granular Thin-Film Antiferromagnets

Appel, P.; Shields, B. J.; Kosub, T.; Hedrich, N.; Hübner, R.; Fassbender, J.; Makarov, D.; Maletinsky, P.

Antiferromagnets have recently emerged as attractive platforms for spintronics applications, offering fundamentally new functionalities compared with their ferromagnetic counterparts. Whereas nanoscale thin-film materials are key to the development of future antiferromagnetic spintronic technologies, existing experimental tools tend to suffer from low resolution or expensive and complex equipment requirements. We offer a simple, high-resolution alternative by addressing the ubiquitous surface magnetization of magnetoelectric antiferromagnets in a granular thin-film sample on the nanoscale using single-spin magnetometry in combination with spin-sensitive transport experiments. Specifically, we quantitatively image the evolution of individual nanoscale antiferromagnetic domains in 200 nm thin films of Cr2O3 in real space and across the paramagnet-to-antiferromagnet phase transition, finding an average domain size of 230 nm, several times larger than the average grain size in the film. These experiments allow us to discern key properties of the Cr2O3 thin film, including the boundary magnetic moment density, the variation of critical temperature throughout the film, the mechanism of domain formation, and the strength of exchange coupling between individual grains comprising the film. Our work offers novel insights into the magnetic ordering mechanism of Cr2O3 and firmly establishes single-spin magnetometry as a versatile and widely applicable tool for addressing antiferromagnetic thin films on the nanoscale.

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Bacillus safensis JG-B5T affects the fate of selenium by extracellular production of colloidally less stable nanoparticles

Fischer, S.; Krause, T.; Lederer, F.; Merroun, M. L.; Shevchenko, A.; Hübner, R.; Stumpf, T.; Jordan, N.; Jain, R.

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.

Keywords: Obligate aerobic; two-chamber reactor; proteomics; membrane-associated proteins; selenite


Effect of insertion layer on electrode properties in magnetic tunnel junctions with a zero-moment half-metal

Titova, A.; Fowley, C.; Clifford, E.; Lau, Y.-C.; Borisov, K.; Betto, D.; Atcheson, G.; Hübner, R.; Xu, C.; Stamenov, P.; Coey, M.; Rode, K.; Lindner, J.; Fassbender, J.; Deac, A. M.

Due to its negligible spontaneous magnetization, high spin polarization and giant perpendicular magnetic anisotropy, Mn₂RuₓGa (MRG) is an ideal candidate as an oscillating layer in THz spin-transfer-torque nano-oscillators. Here, the effect of ultrathin Al and Ta diffusion barriers between MRG and MgO in perpendicular magnetic tunnel junctions is investigated and compared to devices with a bare MRG/MgO interface. Both the compensation temperature, Tcomp, of the electrode and the tunneling magnetoresistance (TMR) of the device are highly sensitive to the choice and thickness of the insertion layer used. High-resolution transmission electron microscopy, as well as analysis of the TMR, its bias dependence, and the resistance-area product allow us to compare the devices from a structural and electrical point of view. Al insertion leads to the formation of thicker effective barriers and gives the highest TMR, at the cost of a reduced Tcomp. Ta is the superior diffusion barrier which retains Tcomp, however, it also leads to a much lower TMR on account of the short spin diffusion length which reduces the tunneling spin polarization. The study shows that fine engineering of the Mn₂RuₓGa/barrier interface to improve the TMR amplitude is feasible.

Keywords: Tunneling Magnetoresistance; Half-Metal; Mn-based alloys; MRAM; Spin Polarisation; Heusler alloy; Ferrimagnetic; Perpendicular Magnetic Anisotropy


Facile preparation of radium-doped, functionalized nanoparticles as carriers for targeted alpha therapy

Reissig, F.; Hübner, R.; Steinbach, J.; Pietzsch, H.-J.; Mamat, C.

Although significant advances in the tailoring of BaSO4-based nanoparticles have been achieved, the synthesis of particles is strongly dependent on the use of templates, surfactants, and additives, especially when radiolabeled with 133Ba or 224Ra. Herein, direct facile preparation of radiolabeled alendronate-functionalized BaSO4 nanoparticles in an aqueous medium in a one-pot reaction is developed. Remarkably, the size of the formed BaSO4 nanoparticles can be controlled by the type of the organic solvent used. Upon the addition of alendronate, amine functionalities were introduced into the nanoparticles. Additionally, a fluorescence dye-containing alendronate was used to evidence the introduction of the alendronate during the formation of the nanoparticles. The variations in the functionalities were investigated by IR and the morphology of the resulting BASO4 nanoparticles are investigated in detail by transmission electron microscopy. DLS and TEM measurements provided an average diameter of the nanoparticles of approx. 140 nm. Radium-doped alendronate nanoparticles were successfully obtained in a one-pot labeling procedure from [224Ra]RaCl2, Na2SO4 Ba(NO3)2 and alendronate.

Keywords: Radium-223; nanoparticles; Barium sulfate; talpha-theraphy

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Enzymes Immobilized on Carbon Nitride (C3N4) Cooperating with Metal Nanoparticles for Cascade Catalysis

Wang, Y.; Zhang, N.; Hübner, R.; Tan, D.; Löffler, M.; Facsko, S.; Zhang, E.; Ge, Y.; Qi, Z.; Wu, C.

The exploration of effective platforms for immobilizing chemo- and biocatalysts to develop biohybrid catalysts is an attractive subject of practical interest. In this work, carbon nitride (C3N4) is used for the first time as a platform for the immobilization of metal catalyst (Pd nanoparticles) and biocatalyst (Candida antarctica lipase B, CalB) in a facile manner to prepare biohybrid catalyst. The optimal biohybrid catalyst inherits the intrinsic performance of both Pd nanoparticles and CalB, and shows high activity in the one-pot cascade reaction converting benzaldehyde to benzyl hexanoate at room temperature. With this proof of concept, it is expected that C3N4 can be utilized for immobilizing more types of chemo- and biocatalysts for perspective applications.

Keywords: biohybrid catalysts; CalB; carbon nitride; cascade reactions; Pd nanoparticles


A material experiment for small satellites to characterise the behaviour of carbon nanotubes in space – development and ground validation

Abbe, E.; Renger, T.; Sznajder, M.; Klemmed, B.; Sachse, E.; Hübner, R.; Schüler, T.; Bärtling, Y.; Muchow, B.; Tajmar, M.; Schmiel, T.

Over the last years, Carbon Nanotubes (CNT) drew interdisciplinary attention. Regarding space technologies a variety of potential applications were proposed and investigated. However, no complex data on the behaviour and degradation process of carbon nanotubes under space environment exist. Therefore, it is necessary to investigate the performance of these new materials in space environment and to revaluate the application potential of CNTs in space technologies. Hence, CiREX (Carbon Nanotubes – Resistance Experiment) was developed as a part of a student project. It is a small and compact experiment, which is designed for CubeSat class space satellites. These are a class of nanosatellites with a standardized size and shape. The CiREX design, electrical measurements and the satellites interfaces will be discussed in detail. CiREX is the first in-situ space material experiment for CNTs. To evaluate the data obtained from CiREX, ground validation tests are mandatory. As part of an extensive test series the behaviour of CNTs under solar ultra violet light (UV) and vacuum ultraviolet light (VUV) was examined. Single-walled carbon nanotubes (SWNT), multi-walled carbon nanotubes (MWNT) and MWNT/resin composite (ME) were exposed to different light sources. After the exposure, the defect density was investigated with Raman spectroscopy. There is a clear indication that UV and VUV light can increase the defect density of untreated CNTs and influence the electrical behaviour.

Keywords: Carbon nanotubes; CubeSat; Electrical behavior; Material experiment; Solar light; Space environment


Strain and Band-Gap Engineering in Ge-Sn Alloys via P Doping

Prucnal, S.; Berencén, Y.; Wang, M.; Grenzer, J.; Voelskow, M.; Hübner, R.; Yamamoto, Y.; Scheit, A.; Bärwolf, F.; Zviagin, V.; Schmidt-Grund, R.; Grundmann, M.; Żuk, J.; Turek, M.; Droździel, A.; Pyszniak, K.; Kudrawiec, R.; Polak, M. P.; Rebohle, L.; Skorupa, W.; Helm, M.; Zhou, S.

Ge with a quasi-direct band gap can be realized by strain engineering, alloying with Sn, or ultrahigh n-type doping. In this work, we use all three approaches together to fabricate direct-band-gap Ge−Sn alloys. The heavily doped n-type Ge−Sn is realized with CMOS-compatible nonequilibrium material processing. P is used to form highly doped n-type Ge−Sn layers and to modify the lattice parameter of P-doped Ge−Sn alloys. The strain engineering in heavily-P-doped Ge−Sn films is confirmed by x-ray diffraction and micro Raman spectroscopy. The change of the band gap in P-doped Ge−Sn alloy as a function of P concentration is theoretically predicted by density functional theory and experimentally verified by near-infrared spectroscopic ellipsometry. According to the shift of the absorption edge, it is shown that for an electron concentration greater than 1 × 10^20 cm the band-gap renormalization is partially compensated by the Burstein-Moss effect. These results indicate that Ge-based materials have high potential for use in near-infrared optoelectronic devices, fully compatible with CMOS technology.

Keywords: Ge; GeSn; n-type doping; ion implantation; x-ray diffraction; Raman spectroscopy; strain

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Observation of multiple magnetic phases and complex nanostructures in Co implanted amorphous carbon films

Suschke, K.; Gupta, P.; Williams, G. V. M.; Hübner, R.; Markwitz, A.; Kennedy, J.

Room temperature implantation of 30 keV Co ions into an amorphous carbon film with a high fluence of 1.2×1017 Co/cm2 results in formation of magnetic nanostructures displaying multiple magnetic phases. Cross-sectional TEM images show formation of Co containing nanoparticles at the surface and near-surface regions of the implanted films. EDXS measurements suggest the nanoparticles to be composed primarily of Co and O at the surface and Co and C in deeper regions. These nanoparticles with differing compositions were observed to be segregated by a thin layer devoid of Co. Magnetic measurements reveal the presence of superparamagnetic behavior from small CoxC nanoclusters with a blocking temperature of 5 K. There is a small fraction of larger CoxC nanoclusters that show magnetic hysteresis even at room temperature. The saturation magnetic moment is as high as 0.51 μB/Co at 2 K and 0.32 μB/Co at room temperature. Spin-disorder is seen with a range of spin glass temperatures below ∼70 K. Our high fluence Co implantation into amorphous carbon has resulted in the formation of complex magnetic nanostructures composed of cobalt, oxygen, and carbon. These nanostructures give rise to multiple magnetic phases such as superparamagnetism, spin glass, ferromagnetism, and possibly antiferromagnetism.

Keywords: a-C; DLC; Ion implantation; Superparamagnetic; Magnetization; Cobalt oxide; Cobalt carbide

  • Journal of Physics and Chemistry of Solids 127(2019), 158-163

Nonlinear plasmonic response of doped nanowires observed by infrared nanospectroscopy

Lang, D.; Balaghi, L.; Winnerl, S.; Schneider, H.; Hübner, R.; Kehr, S. C.; Eng, L. M.; Helm, M.; Dimakis, E.; Pashkin, A.

We report a strong shift of the plasma resonance in highly doped GaAs/InGaAs core/shell nanowires for intense infrared excitation observed by scattering-type scanning near-field infrared microscopy. The studied nanowires show a sharp plasma resonance at a photon energy of about 125 meV in the case of continuous-wave excitation by a CO₂ laser. Probing the same nanowires with the pulsed free-electron laser with peak electric field strengths up to several 10 kV/cm reveals a power-dependent redshift to about 95 meV and broadening of the plasmonic resonance. We assign this effect to a substantial heating of the electrons in the conduction band and subsequent increase of the effective mass in the nonparabolic Γ-valley.

Keywords: nonlinear plasmonics; infrared nanospectroscopy; s-SNIM; free-electron laser; nanowires; InGaAs


Ultra-fast annealing manipulated spinodal nano-decomposition in Mn-implanted Ge

Liu, C.; Hübner, R.; Xie, Y.; Wang, M.; Xu, C.; Jiang, Z.; Yuan, Y.; Li, X.; Yang, J.; Li, L.; Weschke, E.; Prucnal, S.; Helm, M.; Zhou, S.

In the present work, millisecond-range flash lamp annealing is used to recrystallize Mnimplanted Ge. Through systematic investigations of structural and magnetic properties, we find that the flash lamp annealing produces a phase mixture consisting of spinodally decomposed Mn-rich ferromagnetic clusters within a paramagnetic-like matrix with randomly distributed Mn atoms. Increasing the annealing energy density from 46, via 50, to 56 J cm−2 causes the segregation of Mn atoms into clusters, as proven by transmission electron microscopy analysis and quantitatively confirmed by magnetization measurements. According to x-ray absorption spectroscopy, the dilute Mn ions within Ge are in d5 electronic configuration. This Mn-doped Ge shows paramagnetism, as evidenced by the unsaturated magnetic-field-dependent x-ray magnetic circular dichroism signal. Our study reveals how spinodal decomposition occurs and influences the formation of ferromagnetic Mn-rich Ge–Mn nanoclusters.

Keywords: ion implantation; flash lamp annealing; spinodal decomposition; Ge–Mn nanoclusters


3D Local Manipulation of the Metal-Insulator Transition Behavior in VO2 Thin Film by Defect-Induced Lattice Engineering

Jia, Q.; Grenzer, J.; He, H.; Anwand, W.; Ji, Y.; Yuan, Y.; Huang, K.; You, T.; Yu, W.; Ren, W.; Chen, X.; Liu, M.; Facsko, S.; Wang, X.; Ou, X.

The ability to manipulate the metal-insulator transition (MIT) of metal oxides is of critical importance for fundamental investigations of electron correlations and practical implementations of power efficient tunable electrical and optical devices. Most of the existing techniques including chemical doping and epitaxial strain modification can only modify the global transition temperature, while the capability to locally manipulate MIT is still lacking for developing highly integrated functional devices. Here, lattice engineering induced by the energetic noble gas ion allowing a 3D local manipulation of the MIT in VO2 films is demonstrated and a spatial resolution laterally within the micrometer scale is reached. Ion-induced open volume defects efficiently modify the lattice constants of VO2 and consequently reduce the MIT temperature continuously from 341 to 275 K. According to a density functional theory calculation, the effect of lattice constant variation reduces the phase change energy barrier and therefore triggers the MIT at a much lower temperature. VO2 films with multiple transitions in both in-plane and out-of-plane dimensions can be achieved by implantation through a shadow mask or multienergy implantation. Based on this method, temperature-controlled VO2 metasurface structure is demonstrated by tuning only locally the MIT behavior on the VO2 surfaces.

Keywords: Metal–insulator transition VO2


Stress control of tensile-strained In1-xGaxP nanomechanical string resonators

Bueckle, M.; Hauber, V. C.; Cole, G. D.; Gaertner, C.; Zeimer, U.; Grenzer, J.; Weig, E. M.

We investigate the mechanical properties of freely suspended nanostrings fabricated from tensilestressed, crystalline In1-xGaxP. The intrinsic strain arises during epitaxial growth as a consequence of the lattice mismatch between the thin film and the substrate, and is confirmed by x-ray diffraction measurements. The flexural eigenfrequencies of the nanomechanical string resonators reveal an orientation dependent stress with a maximum value of 650 MPa. The angular dependence is explained by a combination of anisotropic Young's modulus and a change of elastic properties caused by defects. As a function of the crystal orientation, a stress variation of up to 50% is observed. This enables fine tuning of the tensile stress for any given Ga content x, which implies interesting prospects for the study of high Q nanomechanical systems.

Keywords: nanomechanical string resonators


Advances in indirect detector systems for ultra high-speed hard X-ray imaging with synchrotron light

Olbinado, M. P.; Grenzer, J.; Pradel, P.; de Resseguier, T.; Vagovic, P.; Zdora, M.-C.; Guzenko, V. A.; David, C.; Rack, A.

We report on indirect X-ray detector systems for various full-field, ultra high-speed X-ray imaging methodologies, such as X-ray phase-contrast radiography, diffraction topography, grating interferometry and speckle-based imaging performed at the hard X-ray imaging beamline ID19 of the European Synchrotron - ESRF. Our work highlights the versatility of indirect X-ray detectors to multiple goals such as single synchrotron pulse isolation, multiple-frame recording up to millions frames per second, high efficiency, and high spatial resolution. Besides the technical advancements, potential applications are briefly introduced and discussed.

Keywords: Inspection with x-rays; X-ray detectors; X-ray diffraction detectors


Reconfigurable spin-wave non-reciprocity induced by dipolar interaction in a coupled ferromagnetic bilayer

Gallardo, R. A.; Schneider, T.; Chaurasiya, A. K.; Oelschlägel, A.; Arekapudi, S. S. P. K.; Roldáan-Molina, A.; Hübner, R.; Lenz, K.; Barman, A.; Fassbender, J.; Lindner, J.; Hellwig, O.; Landeros, P.

Non-reciprocity of wave phenomena describes the situation where wave dispersion depends on the sign of the wave-vector, i.e., counter-propagating waves exhibit di↵erent wavelengths for the same frequency. Such behavior has been recently observed in heavy-metal/ferromagnetic interfaces with Dzyaloshinskii-Moriya coupling, and has also been known for coupled magnetic bilayers, where non-reciprocity is enhanced when the two layers are antiparallel aligned. Besides the conventional uses of spin-waves, non-reciprocity adds further functionalities, such as its potential applications in communications technologies and logical operations. In the current manuscript, we thus examine the spin-wave non-reciprocity induced by dipolar interactions in a coupled bilayer consisting of two ferromagnetic layers separated by a non-magnetic spacer. We derive an easy-to-use formula to estimate the frequency di↵erence provided by the non-reciprocity, which allows for choosing an optimal system in order to maximize the e↵ect. For small wave-numbers, non-reciprocity scales linearly, while for larger wave-vectors the non-reciprocity behaves non-monotonically, with a well-defined maximum. The study is carried out by means of analytical calculations that are complemented by micromagnetic simulations. Furthermore, we confirmed our model by experimental investigation of the spin-wave dispersion in a prototype antiparallel-coupled bilayer system. Since the relative magnetic orientation can be controlled through a bias field, the magnon non-reciprocity can be then turned on and o↵, which lends an important functionality to the coupled ferromagnetic bilayers.

Keywords: non-reciprocity; spin waves; ferromagnetic resonance; Brillouin Light Scattering; magnetism; Dzyaloshinskii-Moriya interaction; dispersion relation

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Epitaxial Mn5Ge3 (100) layer on Ge (100) substrates obtained by flash lamp annealing

Xie, Y.; Yuan, Y.; Wang, M.; Xu, C.; Hübner, R.; Grenzer, J.; Zeng, Y.; Helm, M.; Zhou, S.; Prucnal, S.

Mn5Ge3 thin films have been demonstrated as promising spin-injector materials for germanium-based spintronic devices. So far, Mn5Ge3 has been grown epitaxially only on Ge (111) substrates. In this letter, we present the growth of epitaxial Mn5Ge3 films on Ge (100) substrates. The Mn5Ge3 film is synthetized via sub-second solid-state reaction between Mn and Ge upon flash lamp annealing for 20 ms at the ambient pressure. The single crystalline Mn5Ge3 is ferromagnetic with a Curie temperature of 283 K. Both the c-axis of hexagonal Mn5Ge3 and the magnetic easy axis are parallel to the Ge (100) surface. The millisecond-range flash epitaxy provides a new avenue for the fabrication of Ge-based spin-injectors fully compatible with CMOS technology.

Keywords: Mn5Ge3; epitaxial thin film; ferromagnetism; spintronic devices


Surface-Functionalized Mesoporous Nanoparticles as Heterogeneous Supports To Transfer Bifunctional Catalysts into Organic Solvents for Tandem Catalysis

Zhang, N.; Hübner, R.; Wang, Y.; Zhang, E.; Zhou, Y.; Dong, S.; Wu, C.

The combination of chemo- and biocatalysts offers a powerful platform to address synthetic challenges in chemistry, particularly in synthetic cascades. However, transferring both catalysts into organic solvents remains technically difficult because of the enzyme inactivation and catalyst precipitation. Herein, we designed a facile approach using functionalized mesoporous silica nanoparticles (MSN) to transfer chemo- and biocatalysts into a variety of organic solvents. As a proof-of-concept, two distinct catalysts, palladium nanoparticles (Pd NPs) and Candida antarctica lipase B (CalB), were stepwise loaded into separate locations of the mesoporous structure, which not only provided catalysts with heterogeneous supports for the recycling but also avoided their mutual inactivation. Moreover, mesoporous particles were hydrophobized by surface alkylation, resulting in a tailor-made particle hydrophobicity, which allowed bifunctional catalysts to be dispersed in eight organic solvents. Eventually, these attractive material properties provided the MSN-based bifunctional catalysts with remarkable catalytic performance for cascade reaction synthesizing benzyl hexanoate in toluene. With a broader perspective, the success of this study opens new avenues in the field of multifunctional catalysts where a plethora of other chemo- and biocatalysts can be incorporated into surface-functionalized materials ranging from soft matters to porous networks for synthetic purposes in organic solvents.

Keywords: multifunctional biocatalyst; mesoporous silica nanoparticles (MSN); palladium nanoparticles; lipase CalB; cascade reaction


Nematicity of correlated systems driven by anisotropic chemical phase separation

Yuan, Y.; Hübner, R.; Birowska, M.; Xu, C.; Wang, M.; Prucnal, S.; Jakiela, R.; Potzger, K.; Böttger, R.; Facsko, S.; Majewski, J. A.; Helm, M.; Sawicki, M.; Zhou, S.; Dietl, T.

The origin of nematicity, i.e., in-plane rotational symmetry breaking, and in particular the relative role played by spontaneous unidirectional ordering of spin, orbital, or charge degrees of freedom, is a challenging issue of magnetism, unconventional superconductivity, and quantum Hall effect systems, discussed in the context of doped semiconductor systems such as Ga1−xMnxAs, CuxBi2Se3, and Ga(Al)As/AlxGa1−xAs quantum wells, respectively. Here, guided by our experimental and theoretical results for In1−xFexAs, we demonstrate that spinodal phase separation at the growth surface (that has a lower symmetry than the bulk) can lead to a quenched nematic order of alloy components, which then governs low-temperature magnetic and magnetotransport properties, in particular the magnetoresistance anisotropy whose theory for the C_2v symmetry group is advanced here. These findings, together with earlier data for Ga1−xMnxAs, show under which conditions anisotropic chemical phase separation accounts for the magnitude of transition temperature to a collective phase or merely breaks its rotational symmetry. We address the question to what extent the directional distribution of impurities or alloy components setting in during the growth may account for the observed nematicity in other classes of correlated systems.

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Site-controlled formation of single Si nanocrystals in a buried SiO₂ matrix using ion beam mixing

Xu, X.; Prüfer, T.; Wolf, D.; Engelmann, H.-J.; Bischoff, L.; Hübner, R.; Heinig, K.-H.; Möller, W.; Facsko, S.; von Borany, J.; Hlawacek, G.

For future nanoelectronic devices—such as room-temperature single electron transistors—the site controlled formation of single Si Nanocrystal (NC) is a crucial prerequisite. Here, we report an approach to fabricate single Si NCs via medium-energy Si+ or Ne+ ion beam mixing of Si into a buried SiO₂ layer followed by thermally activated phase separation. Binary Collision Approximation and kinetic Monto Carlo methods are conducted to gain atomistic insight into the influence of relevant experimental parameters on the Si NC formation process. Energy Filtered Transmission Electron Microscopy is performed to obtain quantitative values on the Si NC size and distribution in dependence of the layer stack geometry, ion fluence and thermal budget. Employing a focused Ne+ beam from a Helium Ion Microscope, we demonstrate site-controlled self-assembly of single Si NCs. Line irradiation with a fluence of 3000Ne+/nm² and a line width of 4 nm leads to the formation of a chain of Si NCs, and a single NC with 2.2 nm diameter is subsequently isolated and visualized in a few nm thin lamella prepared by Focused Ion Beam (FIB). The Si NC is centered between the SiO₂ layers and the perpendicular to the incident Ne+ beam.

Keywords: Helium Ion Microscopy; ion beam mixing; single electron transistor; phase separation; Monte Carlo simulations


Alkyl Branching Position in Diketopyrrolopyrrole Polymers: Interplay between Fibrillar Morphology and Crystallinity and Their Effect on Photogeneration and Recombination in Bulk-Heterojunction Solar Cells

Shivhare, R.; Erdmann, T.; Hörmann, U.; Collado-Fregoso, E.; Zeiske, S.; Benduhn, J.; Ullbrich, S.; Hübner, R.; Hambsch, M.; Kiriy, A.; Voit, B.; Neher, D.; Vandewal, K.; Mannsfeld, S. C. B.

Diketopyrrolopyrrole (DPP)-based donor-acceptor copolymers have gained a significant amount of research interest in the organic electronics community because of their high charge carrier mobilities in organic field-effect transistors (OFETs) and their ability to harvest near-infrared (NIR) photons in solar cells. In this study, we have synthesized four DPP-based donor-acceptor copolymers with variations in the donor unit and the branching point of the solubilizing alkyl chains (at the second or sixth carbon position). Grazing incidence wide-angle X-ray scattering (GIWAXS) results suggest that moving the branching point further away from the polymer backbone increases the tendency for aggregation and yields polymer phases with a higher degree of crystallinity (DoC). The polymers were blended with PC70BM and used as active layers in solar cells. A careful analysis of the energetics of the neat polymer and blend films reveals that the charge-transfer state energy (ECT) of the blend films lies exceptionally close to the singlet energy of the donor (ED*), indicating near zero electron transfer losses. The difference between the optical gap and open-circuit voltage (VOC) is therefore determined to be due to rather high nonradiative (≈ 418 ± 13 mV) and unavoidable radiative voltage losses (≈ 255 ± 8 mV). Even though the four materials have similar optical gaps, the short-circuit current density (JSC) covers a vast span from 7 to 18 mA cm-2 for the best performing system. Using photoluminescence (PL) quenching and transient charge extraction techniques, we quantify geminate and nongeminate losses and find that fewer excitons reach the donor-acceptor interface in polymers with further away branching points due to larger aggregate sizes. In these material systems, the photogeneration is therefore mainly limited by exciton harvesting efficiency.


Carbon doping controlled thermoluminescent defect centers in nanoporous alumina for ion beam dosimetry

Bhowmick, S.; Pal, S.; Das, D.; Singh, V. K.; Khan, S. A.; Hübner, R.; Barman, S. R.; Kanjilal, D.; Kanjilal, A.

The flexibility of amorphous anodized alumina (AAO) in developing radiation dosimeter for hadron therapy is reported by controlled carbon ion implantation, followed by thermoluminescence (TL) measurements. The efficacy of amorphous AAO in controlling TL sensitivity is found to be governed by an increase in F+ defect centers as a function of carbon concentration, as revealed from the close resemblance of the trend in photoluminescence intensity. Moreover, its nanoporous structure is demonstrated to be advantageous for defect engineering due to the increase in the surface-to-volume ratio. Detailed X-ray photoelectron spectroscopy analysis suggests the formation of F+ centers by substituting Al3+ ions with C2+ in the vicinity of oxygen vacancies, where depth-dependent study showed the evolution of conducting channels owing to sp2 hybridized C–C bonding, leading to a differential charging effect. This work provides a direction to tune nanoporous AAO in its amorphous form for future ion beam dosimetry.


Formation of n- and p-type regions in individual Si/SiO2 core/shell nanowires by ion beam doping

Berencén, Y.; Prucnal, S.; Möller, W.; Hübner, R.; Rebohle, L.; Schönherr, T.; Bilal Khan, M.; Wang, M.; Glaser, M.; Georgiev, Y. M.; Erbe, A.; Lugstein, A.; Helm, M.; Zhou, S.

A method for cross-sectional doping of individual Si/SiO2 core/shell nanowires (NWs) is presented. P and B atoms are laterally implanted at different depths in the Si core. The healing of the implantation-related damage together with the electrical activation of the dopants takes place via solid phase epitaxy driven by millisecond-range flash lamp annealing. Electrical measurements through a bevel formed along the NW enabled us to demonstrate the concurrent formation of n- and p-type regions in individual Si/SiO2 core/shell NWs. These results might pave the way for ion beam doping of nanostructured semiconductors produced by using either top-down or bottom-up approaches.

Keywords: nanowires; ion beam doping; flash lamp annealing

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Structure-property relationship of Co2MnSi thin films in response to He+-irradiation

Hammerath, F.; Bali, R.; Hübner, R.; Brandt, M. R. D.; Rodan, S.; Potzger, K.; Böttger, R.; Sakuraba, Y.; Büchner, B.; Wurmehl, S.

We investigated the structure-property relationship of Co2MnSi Heusler thin films upon the irradiation with He+ ions. The variation of the crystal structure with increasing ion fluence has been probed using nuclear magnetic resonance (NMR) and transmission electron microscopy (TEM), and associated with the corresponding changes of the magnetic behavior. A decrease of both the structural order and the moment in saturation is observed. Specifically, we detect a direct transition from a highly L21-ordered to a fully A2-disordered structure type and quantify the evolution of the A2 structural contribution as a function of ion fluence. Complementary TEM analysis reveals a spatially-resolved distribution of the L21 and A2 phases showing that the A2 disorder starts at the upper part of the films. The structural degradation in turn leads to a decreasing magnetic moment in saturation in response to the increasing fluence.

Keywords: Structure-property relationship; Heusler; thin films; Ion irradiation

  • Open Access Logo Scientific Reports 9(2019), 2766
    DOI: 10.1038/s41598-019-39435-4
  • Vortrag (Konferenzbeitrag)
    DPG-Frühjahrstagung der Sektion Kondensierte Materie (SKM) gemeinsam mit der European Physical Society (CMD), 11.-16.03.2018, Berlin, Deutschland

Extended Infrared Photoresponse in Te-Hyperdoped Si at Room Temperature

Wang, M.; Berencén, Y.; García-Hemme, E.; Prucnal, S.; Hübner, R.; Yuan, Y.; Xu, C.; Rebohle, L.; Böttger, R.; Heller, R.; Schneider, H.; Skorupa, W.; Helm, M.; Zhou, S.

Presently, silicon photonics requires photodetectors that are sensitive in a broad infrared range, can operate at room temperature, and are suitable for integration with the existing Si-technology process. Here, we demonstrate strong room-temperature sub-band-gap photoresponse of photodiodes based on Si hyperdoped with tellurium. The epitaxially recrystallized Te-hyperdoped Si layers are developed by ion implantation combined with pulsed-laser melting and incorporate Te-dopant concentrations several orders of magnitude above the solid solubility limit. With increasing Te concentration, the Te-hyperdoped layer changes from insulating to quasi-metallic behavior with a finite conductivity as the temperature tends to zero. The optical absorptance is found to increase monotonically with increasing Te concentration and extends well into the mid-infrared range. Temperature-dependent optoelectronic photoresponse unambiguously demonstrates that the extended infrared photoresponsivity from Te-hyperdoped Si p-n photodiodes is mediated by a Te intermediate band within the upper half of the Si band gap. This work contributes to pave the way toward establishing a Si-based broadband infrared photonic system operating at room temperature.

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Ag nanoparticles embedded in Nd:YAG crystals irradiated with tilted beam of 200 MeV Xe ions: optical dichroism correlated to particle reshaping

Li, R.; Pang, C.; Amekura, H.; Ren, F.; Hübner, R.; Zhou, S.; Ishikawa, N.; Okubo, N.; Chen, F.

We report on the fabrication of reshaped Ag nanoparticles (NPs) embedded in a Nd:YAG crystal by combining Ag ion implantation and swift heavy Xe ion irradiation. The localized surface plasmon resonance (LSPR) effect is proved to be efficiently modulated according to the phenomenon of polarization-dependent absorption. The LSPR peak located at 448 nm shows red shift and blue shift at 0° and 90° polarization, respectively, which is in good agreement with calculation by discrete dipole approximation. Based on the near-field intensity distribution, the interaction between reshaped NPs shows a non-ignorable effect on the optical absorption. Furthermore, the polarization-dependence of the photoluminescence (PL) intensity is analyzed, which is positively related to the modulated LSPR absorption. It demonstrates the potential of the enhancement of PL intensity by embedded plasmonic Ag NPs. This work breaks the conventional view of the quenching effect of NPs by ion irradiation and opens a new way to realize the modulation of optical dichroism.

Keywords: nanoparticles; localized surface plasmon resonance; swift heavy ion irradiation; ND:YAG crystal


Three-Dimensional Composition and Electric Potential Mapping of III−V Core−Multishell Nanowires by Correlative STEM and Holographic Tomography

Wolf, D.; Hübner, R.; Niermann, T.; Sturm, S.; Prete, P.; Lovergine, N.; Büchner, B.; Lubk, A.

The nondestructive characterization of nanoscale devices, such as those based on semiconductor nanowires, in terms of functional potentials is crucial for correlating device properties with their morphological/materials features, as well as for precisely tuning and optimizing their growth process. Electron holographic tomography (EHT) has been used in the past to reconstruct the total potential distribution in three dimension but hitherto lacked a quantitative approach to separate potential variations due to chemical composition changes (mean inner potential, MIP) and space charges. In this letter, we combine and correlate EHT and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) tomography on an individual ⟨111⟩ oriented GaAs-AlGaAs core-multishell nanowire (NW). We obtain excellent agreement between both methods in terms of the determined Al concentration within the AlGaAs shell, as well as thickness variations of the few nanometer thin GaAs shell acting as quantum well tube. Subtracting the MIP determined from the STEM tomogram enables us to observe functional potentials at the NW surfaces and at the Au-NW interface, both ascribed to surface/interface pinning of the semiconductor Fermi level.

Keywords: GaAs-AlGaAs; III−V nanowire; 3D elemental mapping; functional potential; quantum well tube; tomography; holography


The role of incidence angle in the morphology evolution of Ge surfaces irradiated by medium-energy Au ions

Dell’Anna, R.; Iacob, E.; Barozzi, M.; Vanzetti, L.; Hübner, R.; Böttger, R.; Giubertoni, D.; Pepponi, G.

Germanium (Ge) surfaces have been irradiated with 26 keV gold (Au) ions at a constant fluence and at incidence angles varying from 0° to 85°. The evolution of the emerging nanostructures is studied by atomic force microscopy (AFM), scanning electron microscopy, x-ray photoelectron spectroscopy (XPS), and cross-sectional transmission electron microscopy. The obtained results are compared with findings reported in the literature. Periodic rippled patterns with the wave vector parallel to the projection of the ion beam direction onto the Ge surface develop between 30° and 45°. From 75° the morphology changes from parallel-mode ripples to parallel-mode terraces, and by further increasing the incidence angle the terraces coarsen and show a progressive break-up of the front facing the ion beam. No perpendicular-mode ripples or terraces have been observed. The analysis of the AFM height profiles and slope distributions shows in the 45°-85° range an angular dependence of the temporal scale for the onset of nonlinear processes. For incidence angles below 45°, the surface develops a sponge-like structure, which persists at higher incidence angles on the top and partially on the face of the facets facing the ion beam. The XPS and the energy-dispersive x-ray spectroscopy evidence the presence of Au nano-aggregates of different sizes for the different incidence angles. This study points out the peculiar behavior of Ge surfaces irradiated with medium-energy Au ions and warns about the differences to be faced when trying to build a universal framework for the description of semiconductor pattern evolution under ion-beam irradiation.

Keywords: ion beam irradiation; germanium; gold ions; binary system; ripples; terraces; sponge-like structures


Cluster tool for in situ processing and comprehensive characterization of thin films at high temperatures

Wenisch, R.; Lungwitz, F.; Hanf, D.; Heller, R.; Zscharschuch, J.; Hübner, R.; von Borany, J.; Abrasonis, G.; Gemming, S.; Escobar Galindo, R.; Krause, M.

A new cluster tool for in situ real-time processing and depth-resolved compositional, structural and optical characterization of thin films at temperatures from -100 to 800 °C is described. The implemented techniques comprise magnetron sputtering, ion irradiation, Rutherford backscattering spectrometry, Raman spectroscopy and spectroscopic ellipsometry. The capability of the cluster tool is demonstrated for a layer stack MgO/ amorphous Si (~60 nm)/ Ag (~30 nm), deposited at room temperature and crystallized with partial layer exchange by heating up to 650°C. Its initial and final composition, stacking order and structure were monitored in situ in real time and a reaction progress was defined as a function of time and temperature.

Keywords: Cluster tool; thin films; in situ; high temperature; Rutherford backscattering; Raman spectroscopy; ellipsometry; metal-induced crystallization

  • Open Access Logo Analytical Chemistry 90(2018), 7837-7842
    DOI: 10.1021/acs.analchem.8b00923
  • Vortrag (Konferenzbeitrag)
    16th International Conference on Plasma Surface Engineering, 16.-21.09.2018, Garmisch - Partenkirchen, Deutschland
  • Vortrag (Konferenzbeitrag)
    XV Congreso Nacional de Materiales/ Iberian Meeting on Materials Science, 04.-06.07.2018, Salamanca, Spanien

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Large-scale self-organized gold nanostructures with bidirectional plasmon resonances for SERS

Schreiber, B.; Gkogkou, D.; Dedelaite, L.; Kerbusch, J.; Hübner, R.; Sheremet, E.; Zahn, D. R. T.; Ramanavicius, A.; Facsko, S.; Rodriguez, R. D.

Efficient substrates for surface-enhanced Raman spectroscopy (SERS) are under constant development, since time-consuming and costly fabrication routines are often an issue for high-throughput spectroscopy applications. In this research, we use a two-step fabrication method to produce self- organized parallel-oriented plasmonic gold nanostructures. The fabrication routine is ready for wafer-scale production involving only low-energy ion beam irradiation and metal deposition. The optical spectroscopy features of the resulting structures show a successful bidirectional plasmonic response. The localized surface plasmon resonances (LSPRs) of each direction are independent from each other and can be tuned by the fabrication parameters. This ability to tune the LSPR characteristics allows the development of optimized plasmonic nanostructures to match different laser excitations and optical transitions for any arbitrary analyte. Moreover, in this study, we probe the polarization and wavelength dependence of such bidirectional plasmonic nanostructures by a complementary spectroscopic ellipsometry and Raman spectroscopy analysis. We observe a significant signal amplification by the SERS substrates and determine enhancement factors of over a thousand times. We also perform finite element method-based calculations of the electromagnetic enhancement for the SERS signal provided by the plasmonic nanostructures. The calculations are based on realistic models constructed using the same particle sizes and shapes experimentally determined by scanning electron microscopy. The spatial distribution of electric field enhancement shows some dispersion in the LSPR, which is a direct consequence of the semi-random distribution of hotspots. The signal enhancement is highly efficient, making our SERS substrates attractive candidates for high-throughput chemical sensing applications in which directionality, chemical stability, and large-scale fabrication are essential requirements.


Percolated Si:SiO2 Nanocomposites: Oven- vs. Millisecond Laser-induced Crystallization of SiOx Thin Films

Schumann, E.; Hübner, R.; Grenzer, J.; Gemming, S.; Krause, M.

Three-dimensional nanocomposite networks consisting of percolated Si nanowires in a SiOx matrix, Si:SiO2, were studied. The structures were obtained by reactive ion beam sputter deposition of SiOx (x~0.6) thin films at 450 °C and subsequent crystallization using conventional oven as well as millisecond line focus laser annealing. Rutherford backscattering spectrometry, Raman spectroscopy, X-ray diffraction, cross-sectional and energy-filtered transmission electron microscopy were applied for sample characterization. While oven annealing resulted in a mean Si wire diameter of 10 nm and a crystallinity of 72 % within the Si volume, almost single-domain Si structures with 30 nm in diameter and almost free of amorphous Si were obtained by millisecond laser application. The structural differences are attributed to the different crystallization processes: Conventional oven tempering proceeds via solid state, millisecond laser application via liquid phase crystallization of Si. The 5 orders of magnitude larger diffusion constant in the liquid phase is responsible for the three times larger Si nanostructure diameter. In conclusion, laser annealing offers not only significantly shorter process times but moreover a superior structural order of nano-Si compared to conventional heating.

Keywords: silicon; nanostructures; percolated networks; nanocomposites; thin films; laser processing; phase separation; liquid phase crystallization


Magnetic properties of Co/Ni grain boundaries after annealing

Coutts, C.; Arora, M.; Hübner, R.; Heinrich, B.; Girt, E.

Magnetic and microstructural properties of <111> textured Cu/Nx[Co/Ni] films are studied as a function of the number of bilayer repeats N and annealing temperature. M(H) loop measurements show that coercivity, Hc, increases with annealing temperature and that the slope of the saturation curve at Hc has a larger reduction for smaller N. An increase of the magnetic anisotropy (Ku) to saturation magnetization (Ms) ratio after annealing Nx[Co/Ni] with N < 15 only partially describes the increase to Hc. Energy-dispersive X-ray spectroscopy analyses performed in scanning transmission electron microscopy mode across cross-sections of as-deposited and annealed Cu/16x[Co/Ni] films show that Cu diffuses from the seed layer into grain boundaries of Co/Ni. Diffusion of Cu reduces exchange coupling (Hex) between the magnetic grains and explains the increase in Hc. Additionally, the difference in the slope of the M(H) curves at Hc between the thick (N = 16) and thin (N = 4) magnetic multilayers is due to Cu diffusion more effectively decoupling magnetic grains in the thinner multilayer.


CMOS‐compatible controlled hyperdoping of silicon nanowires

Berencén, Y.; Prucnal, S.; Möller, W.; Hübner, R.; Rebohle, L.; Böttger, R.; Glaser, M.; Schönherr, T.; Yuan, Y.; Wang, M.; Georgiev, Y. M.; Erbe, A.; Lugstein, A.; Helm, M.; Zhou, S.; Skorupa, W.

Hyperdoping consists of the intentional introduction of deep‐level dopants into a semiconductor in excess of equilibrium concentrations. This causes a broadening of dopant energy levels into an intermediate band between the valence and the conduction bands. Recently, bulk Si hyperdoped with chalcogens or transition metals is demonstrated to be an appropriate intermediate‐band material for Si‐based short‐wavelength infrared photodetectors. Intermediate‐band nanowires can potentially be used instead of bulk materials to overcome the Shockley–Queisser limit and to improve efficiency in solar cells, but fundamental scientific questions in hyperdoping Si nanowires require experimental verification. The development of a method for obtaining controlled hyperdoping levels at the nanoscale concomitant with the electrical activation of dopants is, therefore, vital to understanding these issues. Here, this paper shows a complementary metal‐oxide‐semiconductor (CMOS)‐compatible technique based on nonequilibrium processing for the controlled doping of Si at the nanoscale with dopant concentrations several orders of magnitude greater than the equilibrium solid solubility. Through the nanoscale spatially controlled implantation of dopants, and a bottom‐up template‐assisted solid phase recrystallization of the nanowires with the use of millisecond‐flash lamp annealing, Se‐hyperdoped Si/SiO2 core/shell nanowires are formed that have a room‐temperature sub‐bandgap optoelectronic photoresponse when configured as a photoconductor device.

Keywords: Flash lamp annealing; hyperdoping; intermediate band; ion implantation; nanowires


Core-Shell Structuring of Pure Metallic Aerogels towards Highly Efficient Platinum Utilization for the Oxygen Reduction Reaction - Kern-Schale-Strukturierung rein metallischer Aerogele für eine hocheffiziente Nutzung von Platin für die Sauerstoffreduktion

Cai, B.; Hübner, R.; Sasaki, K.; Zhang, Y.; Su, D.; Ziegler, C.; Vukmirovic, M. B.; Rellinghaus, B.; Adzic, R. R.; Eychmüller, A.

The development of core-shell structures remains a fundamental challenge for pure metallic aerogels. Here we report the synthesis of PdxAu-Pt core-shell aerogels composed of an ultrathin Pt shell and a composition-tunable PdxAu alloy core. The universality of this strategy ensures the extension of core compositions to Pd transition-metal alloys. The core-shell aerogels exhibited largely improved Pt utilization efficiencies for the oxygen reduction reaction and their activities show a volcano-type relationship as a function of the lattice parameter of the core substrate. The maximum mass and specific activities are 5.25 A mgPt -1 and 2.53 mA cm-2, which are 18.7 and 4.1 times higher than those of Pt/C, respectively, demonstrating the superiority of the core-shell metallic aerogels. The proposed core-based activity descriptor provides a new possible strategy for the design of future core-shell electrocatalysts.
Die Entwicklung von rein metallischen Aerogelen mit Kern-Schale-Strukturen ist nach wie vor eine grundlegende Herausforderung. Hier stellen wir die Synthese von PdxAu-Pt-Kern-Schale-Aerogelen vor, welche aus einer ultradünnen Pt-Schale und einem Kern aus einer PdxAu-Legierung mit einstellbarer Zusammensetzung bestehen. Die universelle Synthesestrategie ermöglicht eine Erweiterung der Kern-Zusammensetzung hin zu Pd-Übergangsmetall-Legierungen. Die Kern-Schale-Aerogele zeigen eine stark verbesserte Nutzungseffizienz von Pt in der Sauerstoffreduktion und ihre Aktivitäten folgen einem vulkanförmigen Verlauf bezüglich der Gitterparameter des Kern-Substrats. Mit einer maximalen massenbezogenen bzw. spezifischen Aktivität von 5.25 A mgPt -1 und 2.53 mA cm-2, welche 18.7- bzw. 4.1-mal höher sind als die für Pt/C, zeigt sich die Überlegenheit dieser metallischen Kern-Schale-Aerogele. Die vorgeschlagene kernbasierte Aktivitätsabhängigkeit liefert eine neue mögliche Strategie für den Entwurf zukünftiger Kern-Schale-Elektrokatalysatoren.

Keywords: aerogels; electrocatalysis; core-shell structures; oxygen reduction reaction; sol-gel processes; Aerogele; Elektrokatalyse; Kern-Schale-Strukturen; Nanostrukturen; Sol-Gel-Prozess


Laser-Rewriteable Ferromagnetism at Thin Film Surfaces

Ehrler, J.; He, M.; Shugaev, M. V.; Polushkin, N. I.; Wintz, S.; Liersch, V.; Cornelius, S.; Hübner, R.; Potzger, K.; Lindner, J.; Fassbender, J.; Ünal, A. A.; Valencia, S.; Kronast, F.; Zhigilei, L. V.; Bali, R.

Manipulation of magnetism using laser light is considered a key to the advancement of data storage technologies. Until now, most approaches seek to optically switch the direction of magnetization rather than to reversibly manipulate the ferromagnetism itself. Here we use ~100 fs laser pulses to reversibly switch ferromagnetic ordering on and off by exploiting a chemical order-disorder phase transition in Fe60Al40, from the B2 to the A2 structure and vice versa. A single laser pulse above a threshold fluence causes non-ferromagnetic B2 Fe60Al40 to disorder and form the ferromagnetic A2 structure. Subsequent pulsing below the threshold reverses the surface to B2 Fe60Al40, erasing the laser induced ferromagnetism. Simulations reveal that the order-disorder transition is regulated by the extent of surface supercooling; above threshold the film melts-through and the consequent stability of the supercooled liquid phase suppresses vacancy diffusion, freezing the material into the disordered state. Pulsing below threshold forms a limited supercooled surface phase that solidifies at sufficiently high temperatures, enabling diffusion assisted reordering. This demonstrates that ultrafast lasers can achieve subtle atomic rearrangements in bimetallic alloys in a reversible and non-volatile fashion.

Keywords: magneto-optical devices; data storage; phase transitions; fs laser modifications; supercooling; order-disorder

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Plasmonic nanoparticles embedded in single crystals synthesized by gold ion implantation for enhanced optical nonlinearity and efficient Q-switched lasing

Nie, W. J.; Zhang, Y. X.; Yu, H. H.; Li, R.; He, R. Y.; Dong, N. N.; Wang, J.; Hübner, R.; Böttger, R.; Zhou, S. Q.; Amekura, H.; Chen, F.

We report on the synthesis of embedded gold (Au) nanoparticles (NPs) in Nd:YAG single crystals using ion implantation and subsequent thermal annealing. Both linear and nonlinear absorption of the Nd:YAG crystals have been enhanced significantly due to the embedded Au NPs, which is induced by the surface plasmon resonance (SPR) effect in the visible light wavelength band. Particularly, through a typical Z-scan system excited by a femtosecond laser at 515 nm within the SPR band, the nonlinear absorption coefficients of crystals with Au NPs have been observed to be nearly 5 orders of magnitude larger than that without Au NPs. This giant enhancement of nonlinear absorption properties is correlated with the saturable absorption (SA) effect, which is the basis of passive Q-switching or mode-locking for pulsed laser generation. In addition, the linear and nonlinear absorption enhancement could be tailored by varying the fluence of implanted Au+ ions, corresponding to the NP size and concentration modulation. Finally, the Nd:YAG wafer with embedded Au NPs has been applied as a saturable absorber in a Pr:LuLiF4 crystal laser cavity, and efficient pulsed laser generation at 639 nm has been realized, which presents superior performance to the MoS2 saturable absorber based system. This work opens an avenue to enhance and modulate the nonlinearities of dielectrics by embedding plasmonic Au NPs for efficient pulsed laser operation.


Electronic phase separation in insulating (Ga, Mn) As with low compensation: super-paramagnetism and hopping conduction

Yuan, Y.; Wang, M.; Xu, C.; Hübner, R.; Böttger, R.; Jakiela, R.; Helm, M.; Sawicki, M.; Zhou, S.

In the present work, low compensated insulating (Ga,Mn)As with 0.7% Mn is obtained by ion implantation combined with pulsed laser melting. The sample shows variable-range hopping transport behavior with a Coulomb gap in the vicinity of the Fermi energy, and the activation energy is reduced by an external magnetic field. A blocking super-paramagnetism is observed rather than ferromagnetism. Below the blocking temperature, the sample exhibits a colossal negative magnetoresistance. Our studies confirm that the disorder-induced electronic phase separation occurs in (Ga,Mn)As samples with a Mn concentration in the insulator–metal transition regime, and it can account for the observed superparamagnetism and the colossal magnetoresistance.


On the insulator-to-metal transition in titanium-implanted silicon

Liu, F.; Wang, M.; Berencén, Y.; Prucnal, S.; Engler, M.; Hübner, R.; Yuan, Y.; Heller, R.; Böttger, R.; Rebohle, L.; Skorupa, W.; Helm, M.; Zhou, S.

Hyperdoped silicon with deep level impurities has attracted much research interest due to its promising optical and electrical properties. In this work, single crystalline silicon supersaturated with titanium is fabricated by ion implantation followed by both pulsed laser melting and flash lamp annealing. The decrease of sheet resistance with increasing Ti concentration is attributed to a surface morphology effect due to the formation of cellular breakdown at the surface and the percolation conduction at high Ti concentration is responsible for the metallic-like conductivity. The insulator-to-metal transition does not happen. However, the doping effect of Ti incorporation at low concentration is not excluded, which might be responsible for the sub-bandgap optical absorption reported in literature.

Keywords: Hyperdoped silicon; deep level impurities; flash lamp annealing; insulator-to-metal transition


Mechanical Properties of Metal Oxide Aerogels

Benad, A.; Jürries, F.; Vetter, B.; Klemmed, B.; Hübner, R.; Leyens, C.; Eychmüller, A.

In this study we report on mechanical properties of molded, single component Al2O3, Ga2O3, Fe2O3, and ZrO2 as well as mixed aerogels, made from yttrium stabilized zirconia, yttrium aluminum garnet, and zinc aluminum spinel. Initially all aerogels were produced equally in molded bodies by a facile epoxy method and were annealed afterward at 300 °C. Then we performed uniaxial pressure tests on cylindrical aerogel monoliths to gain Young's modulus which depends on composition, density, and post-treatment. Already pure aerogels like ZrO2 show well-promising Young's modulus of 10.7 MPa, whereas most popular SiO2 materials display a modulus between 2 and 3 MPa at comparable densities. Moreover we focused on Al2O3 aerogels which exhibit high stability and interesting densification behavior depending on the annealing temperature. On the basis of this observation, we combined the toughness of the Al2O3 scaffold with the extraordinary hardness of ZrO2, by adding up to 20 atom % Zr, to increase the specific Young's modulus. For the mixed material with a Zr content of 20 atom %, we reach a record value for compressible aerogels of 125 MPa mL g-1.


Facile Preparation of Multifunctionalisable ‘Stealth’ Upconverting Nanoparticles for Biomedical Applications

Nsubuga, A.; Sgarzi, M.; Zarschler, K.; Kubeil, M.; Hübner, R.; Steudtner, R.; Graham, B.; Joshi, T.; Stephan, H.

Pure hexagonal (β-phase) NaYF4-based hydrophobic upconverting nanoparticles (UCNPs) were surface-modified with O-phospho-L-threonine (OPLT), alendronic acid, and PEG-phosphate ligands to generate water-dispersible UCNPs. Fourier-transform infrared (FTIR) spectroscopy was used to establish the presence of the ligands on the UCNP surface. These UCNPs exhibit great colloidal stability and a near-neutral surface at physiological pH, as confirmed by dynamic light scattering (DLS) and zeta potential (ζ) measurements, respectively. The particles also display excellent long-term stability, with no major adverse effect on the size of UCNPs when kept at pH 7.4. Upon exposure to human serum, PEG-phosphate- and alendronate-coated UCNPs showed no formation of biomolecular corona, as confirmed by SDS-PAGE analysis. The photophysical properties of water-dispersible UCNPs were investigated using steady-state as well as time-resolved luminescence spectroscopy, under excitation at ca. 800 nm. The results clearly show that the UCNPs demonstrate bright upconversion (UC) luminescence. Furthermore, the presence of reactive groups on the NPs, such as, free amine group in alendronate-coated UCNPs, enables further functionalisation of UCNPs with, for example, small molecules, peptides, proteins, and antibodies. Overall these protein corona resistant UCNPs show great biocompatibility and are worthy of further investigation as potential new biomaging probes.

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Magnetic anisotropy in antiferromagnetic hexagonal MnTe

Kriegner, D.; Reichlova, H.; Grenzer, J.; Schmidt, W.; Ressouche, E.; Godinho, J.; Wagner, T.; Martin, S.; Shick, A.; Volobuev, V.; Springholz, G.; Holy, V.; Wunderlich, J.; Jungwirth, T.; Vyborny, K.

Antiferromagnetic hexagonal MnTe is a promising material for spintronic devices relying on the control of antiferromagnetic domain orientations. Here we report on neutron diffraction, magnetotransport, and magnetometry experiments on semiconducting epitaxial MnTe thin films together with density functional theory (DFT) calculations of the magnetic anisotropies. The easy axes of the magnetic moments within the hexagonal basal plane are determined to be along < 1-100 > directions. The spin-flop transition and concomitant repopulation of domains in strong magnetic fields is observed. Using epitaxially induced strain the onset of the spin-flop transition changes from similar to 2 to similar to 0.5 T for films grown on InP and SrF2 substrates, respectively.

Keywords: MOLECULAR-BEAM EPITAXY; NEUTRON-DIFFRACTION; MANGANESE TELLURIDE; THERMAL-EXPANSION; TEMPERATURE; BEHAVIOR; MEMORY; FILM


Phase Transitions in C:Ni Nanocomposite Templates during Diameter-Selective CVD Synthesis of SWCNTs

Krause, M.; Melkhanova, S.; Hübner, R.; Haluska, M.; Gemming, S.

Phase transitions in carbon: nickel nanocomposite templates during diameter-selective CVD synthesis of SWCNTs were studied. While almost conserving their pre-defined diameter distribution, as-deposited Ni3C nanoparticles transform into fcc-NiO during activation in low-pressure air atmosphere, and are reduced to a mixture of fcc-Ni and Ni3C under nanotube growth conditions. The first phase transition leads to a substitutional replacement of the protective carbon matrix by a protective oxide layer. The second one reflects competing reduction processes of NiO. A mechanism for the complementary roles of carbon matrix and Ni species in the three-step CVD synthesis is proposed that includes nanoparticle immobilization, carbon delivery and catalysis of nanotube growth.

Keywords: nanocomposites; single-walled carbon nanotubes; catalysis; transmission electron microscopy; Raman spectroscopy

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Magnetic properties of Co/Ni multilayer structures for use in STT-RAM

Arora, M.; Lee-Hone, N. R.; Mckinnon, T.; Coutts, C.; Hübner, R.; Heinrich, B.; Broun, D. M.; Girt, E.

Magnetic properties of (111)-textured SAF/Cu/FL multilayer film structures have been optimized by varying individual layer thickness and sputtering conditions. The SAF is a synthetic antiferromagnet consisting of Co/Ni multilayers coupled antiferromagnetically across a Ru spacer layer, and the FL is a free layer consisting of a single Co/Ni multilayer. The Co thickness was varied from 0.16 nm to 0.26 nm and the Ni thickness from 0.47 nm to 0.57 nm at two different deposition rates to obtain a perpendicular magnetic anisotropy of the 8 × [Co/Ni] multilayers ranging from 4×105 to 4.6×105 Jm-3. The perpendicular magnetic anisotropy, saturation magnetization, damping and zero-frequency line broadening of the Co/Ni multilayers strongly depend on the number of bilayers, N1, for N1 <10. With increasing Cu seed-layer thickness, the texture of the Co/Ni multilayers improves while the grain size and film roughness increase. The increase in grain size results in the reduction of the direct exchange coupling between magnetic grains, which enhances the coercivity of the SAF and the FL. Experimentally measured coercivities of the SAF and FL are compared with calculations obtained from a coherent rotation model. The effect of the Co/Ru interface in the SAF on the exchange coupling, and the role of the Co/Cu interface in the magnetoresistance, is also discussed. This study provides an important step towards the realization of STT-RAM devices consisting of only Co/Ni multilayers.

Keywords: magnetic multilayers; perpendicular magnetic anisotropy; giant magneto-resistance; spin transfer torque; magnetic properties


Metabolism-dependent bioaccumulation of uranium by Rhodosporidium toruloides isolated from the flooding water of a former uranium mine

Gerber, U.; Hübner, R.; Rossberg, A.; Krawczyk-Bärsch, E.; Merroun, M. L.

Remediation of former uranium mining sites represents one of the biggest challenges worldwide that have to be solved in this century. The former uranium mine Königstein (Germany) displays one of these sites and is currently remediated by controlled flooding of the underground. The flooding water is cleaned up by a conventional chemical waste water treatment plant. During the last years, the search of alternative strategies involving environmentally sustainable treatments has started. Bioremediation, the use of microorganisms to clean up polluted sites in the environment, is considered one of the best alternative. By means of culture-dependent methods, we isolated an indigenous yeast strain, KS5 (Rhodosporidium toruloides), directly from the flooding water and investigated its interactions with uranium(VI). Our results highlight distinct adaptive mechanisms towards high uranium concentrations on the one hand, and complex interaction mechanisms on the other. The cells of the strain KS5 exhibit high uranium tolerance being able to grow up to 5 mM, and also high ability to accumulate this radionuclide (350 mg uranium/g dry biomass in 48 hours). The removal of uranium by KS5 displays a temperature- and cell viability-dependent process. By STEM investigations we observed that uranium was removed by two mechanisms, inactive biosorption and active bioaccumulation. EXAFS analysis revealed that the molecular speciation of uranium associated with the cells is similar to that of meta-autunite-like minerals. The present study highlights the potential of KS5 as a representative of indigenous species which might play a key role in bioremediation of uranium-contaminated sites.

Keywords: Bioremediation; Uranium; Rhodosporidium toruloides; Bioaccumulation; Biosorption


Rapid synthesis of sub-10 nm hexagonal NaYF4-based upconverting nanoparticles using Therminol 66

Hesse, J.; Klier, D. T.; Sgarzi, M.; Nsubuga, A.; Bauer, C.; Grenzer, J.; Hübner, R.; Wislicenus, M.; Joshi, T.; Kumke, M. U.; Stephan, H.

We report a simple one-pot method for rapid preparation of sub-10 nm pure hexagonal (β-phase) NaYF4 based upconverting nanoparticles (UCNPs). Such nanocrystals are well-known for their high efficiency of energy upconversion. Using Therminol 66 as co-solvent, monodisperse UCNPs could be obtained in unusually short reaction time. By varying reaction time, reaction temperature, and the concentration of the dopants (Nd3+, Yb3+ sensitizer ions and Er3+ activator ions), it was possible to precisely control the particle size, crystalline phase, as well as the upconversion (UC) luminescence properties. The size and phase-purity of as-synthesized core and core-shell nanocrystals was assessed using complementary transmission electron microscopy (TEM), dynamic light scattering (DLS), X-ray diffraction (XRD) and small-angle X-ray scattering (SAXS) studies. In-depth photophysical evaluation of the UCNPs was pursued using steady-state as well as time-resolved luminescence spectroscopy. An enhancement in UC intensity was observed when nanocrystals, doped with optimized concentration of lanthanide sensitizer/activator ions, were further coated with an inert/active shell. This is attributed to the suppression of surface-related luminescence quenching effects.

Keywords: core-shell; lanthanides; nanocrystal synthesis; photoluminescence; upconversion


Versatile Tri(pyrazolyl)phosphanes as Phosphorus Precursors for the Synthesis of Highly Emitting InP/ZnS Quantum Dots - Tri(pyrazolyl)phosphane als Vorstufen für die Synthese von stark emittierenden InP/ZnS-Quantenpunkten

Panzer, R.; Guhrenz, C.; Haubold, D.; Hübner, R.; Gaponik, N.; Eychmüller, A.; Weigand, J. J.

Tri(pyrazolyl)phosphanes (5R1,R2) are utilized as an alternative, cheap and low-toxic phosphorus source for the convenient synthesis of InP/ZnS quantum dots (QDs). From these precursors, remarkably long-term stable stock solutions (>6 months) of P(OLA)3 (OLAH=oleylamine) are generated from which the respective pyrazoles are conveniently recovered. P(OLA)3 acts simultaneously as phosphorus source and reducing agent in the synthesis of highly emitting InP/ZnS core/shell QDs. These QDs are characterized by a spectral range between 530-620 nm and photoluminescence quantum yields (PL QYs) between 51-62 %. A proof-of-concept white light-emitting diode (LED) applying the InP/ZnS QDs as a color-conversion layer was built to demonstrate their applicability and processibility.
Tri(pyrazolyl)phosphane werden als alternative kostengünstige und weniger toxische Phosphorquelle in der Synthese von InP/ZnS-Quantenpunkten (QDs) eingesetzt. Ausgehend von ihnen können langzeitstabile (>6 Monate) P(OLA)3-Stammlösungen (OLAH=Oleylamin) synthetisiert werden, aus denen sich die entsprechenden Pyrazole einfach zurückgewinnen lassen. P(OLA)3 fungiert in der Synthese von stark emittierenden InP/ZnS-QDs sowohl als Phosphorquelle als auch als Reduktionsmittel. Die erhaltenen Kern/Schale-Partikel zeichnen sich durch hohe Photolumineszenz-Quantenausbeuten von 51–62 % in einem Spektralbereich von 530-620 nm aus. Die Verarbeitung und Anwendung dieser InP/ZnS-QDs als Farbkonversionsschicht wurde anhand des Einsatzes in einer weißen Leuchtdiode demonstriert.

Keywords: hot injection; oleylamine; phosphorus; quantum dots; waste prevention; Heißinjektion; Oleylamin; Phosphor; Ressourcen-Schonung; Quantenpunkte


Pt and Au bimetallic and monometallic nanostructured amperometric sensors for direct detection of hydrogen peroxide: Influences of bimetallic effect and silica support

Liu, W.; Hiekel, K.; Hübner, R.; Sun, H.; Ferancova, A.; Sillanpää, M.

The non-enzyme direct electrochemical sensing of hydrogen peroxide (H2O2) by nanostructured electrodes of Pt- and Au-containing bimetallic or monometallic nanocatalysts including paramecium-like nanostructures of PtAu supported on silica nanorods, Pt and Au nanoparticles supported on silica nanorods, and the non-supported Pt and Au nanoparticles (NPs) is reported. The nanocatalysts modified electrodes were fabricated by simple self-assembling on 3-aminopropyl-trimethoxysilane (APTMS) modified glassy carbon. The cyclic voltammetric and amperometric results showed that PtAu supported on silica nanorods has superior performance over the corresponding monometallic counterparts, with a broad linear range from 5.0 µM to 72000 µM for H2O2, a detection limit of 2.6 µM, a sensitivity of 46.7 µA mM-1cm-2 at a lower working potential of -0.20 V vs SCE, and has good stability and reproducibility. In addition, a systematic test showed that the non-supported Pt NPs sensor has a surprisingly high performance, even better than the paramecium-like nanostructure of PtAu supported on silica nanorods, where the existence of silica nanorod templates in the nanocatalysts retards the electrocatalytic reduction/oxidation of H2O2. Among the nanocatalysts tested in this work, the Pt NPs sensor showed fastest response within 3 s, a broad linear response from 5 µM to 58000 µM, a detection limit of 4.2 µM, and the highest sensitivity of 110.3 µA mM-1cm-2 at the lowest working potential of -0.08 V vs SCE. Notably, the performance of the Pt NPs sensor is also among the best Pt-containing monometallic or bimetallic nanostructured electrochemical sensors toward H2O2 reported so far. This work shows a simple method to fabricate H2O2 electrochemical sensors of high performance and indicates the importance of considering not only bimetallic effects but also the influences of the nanostructure of nanocatalysts on the electrocatalytic performance and electrochemical sensing property.

Keywords: Platinum; Bimetallic effect; Support material; Electrocatalyst; Amperometric sensor; Hydrogen peroxide

  • Sensors and Actuators B 255(2018), 1325-1334

Local formation of InAs nanocrystals in Si by masked ion implantation and flash lamp annealing

Rebohle, L.; Wutzler, R.; Prucnal, S.; Hübner, R.; Georgiev, Y. M.; Erbe, A.; Böttger, R.; Glaser, M.; Lugstein, A.; Helm, M.; Skorupa, W.

The integration of high-mobility III-V compound semiconductors emerges as a promising route for Si device technologies to overcome the limits of further down-scaling. In this work, we investigate the possibilities to form InAs nanocrystals in a thin Si layer at laterally defined positions with the help of masked ion beam implantation and flash lamp annealing. In detail, a cladding layer was deposited on a silicon-on-insulator (SOI) wafer and patterned by electron beam lithography in order to serve as an implantation mask. The wafer was subsequently implanted with As and In, followed by flash lamp annealing leading to the formation of InAs nanoparticles in the implanted areas. The structures were investigated by Raman spectroscopy, scanning and transmission electron microscopy as well as energy-dispersive X-ray spectroscopy. Depending on the size of the implantation window, several, one or no nanoparticle is formed. Finally, the perspectives for using this technique for the local modification of Si nanowires are discussed.

Keywords: ion implantation; flash lamp annealing; III-V integration into silicon; nanocrystal; InAs; liquid phase epitaxy


Microstructure and charge trapping in in ZrO2- and Si3N4-based superlattice layer systems with Ge nanoparticles

Seidel, S.; Rebohle, L.; Prucnal, S.; Lehninger, D.; Hübner, R.; Klemm, V.; Skorupa, W.; Heitmann, J.

Ge was deposited on silicon as a superlattice with 10 layers of Ge embedded in Si3N4 or ZrO2 matrices via plasma enhanced chemical vapor deposition or RF-sputtering, respectively. Raman spectroscopy, transmission electron microscopy and capacitance-voltage (CV) measurements were performed in order to investigate the structural and electrical properties of the superlattices. It will be shown that, in contrast to furnace annealing, flash lamp annealing of Ge-ZrO2-superlattices leads to crystalline Ge nanoparticles in an amorphous matrix. As revealed by CV measurements, these layers show excellent charge storage capabilities. In comparison, a higher thermal budget is needed to crystallize Ge in case of Si3N4-based superlattices, and no significant charge trapping could be detected during CV measurements.

Keywords: Ge nanocrystals; zirconium oxide; silicon nitride; superlattice; flash lamp annealing

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Ge/Si core/shell quantum dots in alumina: Tuning the optical absorption by the core and shell size(Article)

Nekić, N.; Sancho-Parramon, J.; Bogdanović-Radović, I.; Grenzer, J.; Hübner, R.; Bernstorff, S.; Ivanda, M.; Buljan, M.

Ge/Si core/shell quantum dots (QDs) recently received extensive attention due to their specific properties induced by the confinement effects of the core and shell structure. They have a type II confinement resulting in spatially separated charge carriers, the electronic structure strongly dependent on the core and shell size. Herein, the experimental realization of Ge/Si core/shell QDs with strongly tunable optical properties is demonstrated. QDs embedded in an amorphous alumina glass matrix are produced by simple magnetron sputtering deposition. In addition, they are regularly arranged within the matrix due to their self-assembled growth regime. QDs with different Ge core and Si shell sizes are made. These core/shell structures have a significantly stronger absorption compared to pure Ge QDs and a highly tunable absorption peak dependent on the size of the core and shell. The optical properties are in agreement with recent theoretical predictions showing the dramatic influence of the shell size on optical gap, resulting in 0.7 eV blue shift for only 0.4 nm decrease at the shell thickness. Therefore, these materials are very promising for light-harvesting applications.

Keywords: absorption; Ge/Si core/shell quantum dots; quantum confinement; self-assembly


In situ ohmic contact formation for n-type Ge via non-equilibrium processing

Prucnal, S.; Frigerio, J.; Napolitani, E.; Ballabio, A.; Berencén, Y.; Rebohle, L.; Wang, M.; Böttger, R.; Voelskow, M.; Isella, G.; Hübner, R.; Helm, M.; Zhou, S.; Skorupa, W.

Highly scaled nanoelectronics requires effective channel doping above 5×10^19 /cm3 together with ohmic contacts with extremely low specific contact resistivity. Nowadays, Ge becomes very attractive for modern optoelectronics due to the high carrier mobility and the quasi-direct bandgap, but n-type Ge doped above 5×10^19 /cm3 is metastable and thus difficult to be achieved. In this letter, we report on the formation of low-resistivity ohmic contacts in highly n-type doped Ge via non-equilibrium thermal processing consisting of millisecond-range flash lamp annealing. This is a single-step process that allows for the formation of a 90 nm thick NiGe layer with a very sharp interface between NiGe and Ge. The measured carrier concentration in Ge is above 9×10^19 /cm3 with a specific contact resistivity of 1.2×10^(−6) Ω cm2.
Simultaneously, both the diffusion and the electrical deactivation of P are fully suppressed.

Keywords: germanium; flash lamp annealing; ion implantation; NiGe; ohmic contact

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Ultra-high-speed X-ray imaging of laser-driven shock compression using synchrotron light

Olbinado, M. P.; Cantelli, V.; Mathon, O.; Pascarelli, O.; Rack, A.; Grenzer, J.; Pelka, A.; Roedel, M.; Prencipe, I.; Garcia, A. L.; Helbig, U.; Kraus, D.; Schramm, U.; Cowan, T.; Scheel, M.; Pradel, P.; de Resseguier, T.

A high-power, nanosecond-pulsed laser impacting the surface of a material can generate an ablation plasma that drives a shock wave into it; while in situ X-ray imaging can provide a time-resolved probe of the shock-induced material behaviour on macroscopic lengths scales. Here, we report on an investigation into laser-driven shock compression of a polyurethane foam and a graphite rod by means of single-pulse synchrotron X-ray phase-contrast imaging with a MHz frame rate. A 6-J, 10-ns-pulsed laser was used to generate shock compression. Physical processes governing the laser-induced dynamic response such as elastic compression, compaction, pore collapse, fracture, and fragmentation have been imaged; and the advantage of exploiting the partial spatial coherence of a synchrotron source for studying low-density, carbon-based materials is emphasized. The successful combination of a high-energy laser and ultra-high-speed X-ray imaging using synchrotron light demonstrates the potentiality of accessing complementary information from scientific studies of laser-driven shock compression.

Keywords: laser shock; ultra-high-speed imaging; synchrotron radiation; X-ray phase contrast; time-resolved studies; foam; graphite; shock


In-chip microstructures and photonic devices fabricated by nonlinear laser lithography deep inside silicon

Tokel, O.; Turnalı, A.; Makey, G.; Elahi, P.; Çolakoğlu, T.; Ergeçen, E.; Yavuz, Ö.; Hübner, R.; Zolfaghari Borra, M.; Pavlov, I.; Bek, A.; Turan, R.; Koray Kesim, D.; Tozburun, S.; Ilday, S.; Ilday, F. Ö.

Silicon is an excellent material for microelectronics and integrated photonics, with untapped potential for mid-infrared optics. Despite broad recognition of the importance of the third dimension, current lithography methods do not allow the fabrication of photonic devices and functional microelements directly inside silicon chips. Even relatively simple curved geometries cannot be realized with techniques like reactive ion etching. Embedded optical elements, electronic devices and better electronic–photonic integration are lacking. Here, we demonstrate laser-based fabrication of complex 3D structures deep inside silicon using 1-μm-sized dots and rod-like structures of adjustable length as basic building blocks. The laser-modified Si has an optical index different to that in unmodified parts, enabling the creation of numerous photonic devices. Optionally, these parts can be chemically etched to produce desired 3D shapes. We exemplify a plethora of subsurface - that is, ‘in-chip’ - microstructures for microfluidic cooling of chips, vias, micro-electromechanical systems, photovoltaic applications and photonic devices that match or surpass corresponding state-of-the-art device performances.


Decoupling the two roles of Ga droplets in the self-catalyzed growth of GaAs nanowires on SiOx/Si(111) substrates

Tauchnitz, T.; Nurmamytov, T.; Hübner, R.; Engler, M.; Facsko, S.; Schneider, H.; Helm, M.; Dimakis, E.

Liquid Ga droplets play a double role in the self-catalyzed growth of GaAs nanowires on Si(111) substrates covered with a native SiOx layer: they induce the formation of nano-sized holes in SiOx and then drive the uniaxial nanowire growth directly onto the underlying Si. The independent control of the two mechanisms is a prerequisite for mastering the growth of nanowires, but it is challenging in a conventional growth procedure where they both take place under the same droplets. To that end, we have developed an in situ procedure where the Ga droplets used for the formation of SiOx holes are removed before new Ga droplets drive the growth of GaAs nanowires. In that way, it was made possible to study the interaction between Ga droplets and SiOx, to create holes in SiOx with controlled number density and size and, finally, to grow GaAs nanowires only within those holes. Our results show unprecedented control of the nanowire nucleation with unique possibilities: (1) deliberate control of the number density of nanowires within three orders of magnitude (106-109 cm-2) without patterning the substrate and without changing the growth conditions, (2) highly synchronous nucleation events and, thus, exceptionally narrow nanowire length distributions (standard deviation < 1 % for 3 mm long nanowires), (3) high yield of vertical nanowires up to 80 % (against GaAs islands), (4) highly reproducible results, and (5) independent control of the nanowire diameter from the number density. We anticipate that our methodology could be also exploited for different materials or other types of nanostructures.


Interplay between localization and magnetism in (Ga,Mn)As and (In,Mn)As

Yuan, Y.; Xu, C.; Hübner, R.; Jakiela, R.; Böttger, R.; Helm, M.; Sawicki, M.; Dietl, T.; Zhou, S.

Ion implantation of Mn combined with pulsed laser melting is employed to obtain two representative compounds of dilute ferromagnetic semiconductors (DFSs): Ga1−xMnxAs and In1−xMnxAs. In contrast to films deposited by the widely used molecular beam epitaxy, neither Mn interstitials nor As antisites are present in samples prepared by the method employed here. Under these conditions the influence of localization on the hole-mediated ferromagnetism is examined in two DFSs with a differing strength of p-d coupling. On the insulating side of the transition, ferromagnetic signatures persist to higher temperatures in In1−xMnxAs compared to Ga1−xMnxAs with the same Mn concentration x. This substantiates theoretical suggestions that stronger p-d coupling results in an enhanced contribution to localization, which reduces hole-mediated ferromagnetism. Furthermore, the findings support strongly the heterogeneous model of electronic states at the localization boundary and point to the crucial role of weakly localized holes in mediating efficient spin-spin interactions even on the insulator side of the metal-insulator transition.


Towards diluted magnetism in TaAs

Liu, Yu; Li, Z.; Guo, L.; Chen, X.; Yuan, Y.; Xu, C.; Hübner, R.; Akhmadaliev, S.; Krasheninnikov, A. V.; N'Diaye, A. T.; Arenholz, E.; Helm, M.; Zhou, S.

Magnetism in Weyl semimetals is desired to investigate the interaction between the magnetic moments and Weyl fermions, e.g., to explore anomalous quantum Hall phenomena. Here we demonstrate that proton irradiation is an effective tool to induce ferromagnetism in the Weyl semimetal TaAs. The intrinsic magnetism is observed with a transition temperature above room temperature. The magnetic moments from d states are found to be localized around Ta atoms. Further, the first-principles calculations indicate that the d states localized on the nearest-neighbor Ta atoms of As vacancy sites are responsible for the observed magnetic moments and the long-ranged magnetic order. The results show the feasibility of inducing ferromagnetism in Weyl semimetals so that they may facilitate the applications of this material in spintronics.

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Ultra-dense planar metallic nanowire arrays with extremely large anisotropic optical and magnetic properties

Jia, Q.; Ou, X.; Langer, M.; Schreiber, B.; Grenzer, J.; Siles, P. F.; Rodriguez, R. D.; Huang, K.; Yuan, Y.; Heidarian, A.; Hübner, R.; You, T.; Yu, W.; Lenz, K.; Lindner, J.; Wang, X.; Facsko, S.

A nanofabrication method for the production of ultra-dense planar metallic nanowire arrays scalable to wafer-size is presented. The method is based on an efficient template deposition process to grow diverse metallic nanowire arrays with extreme regularity in only two steps. First, III-V semiconductor substrates are irradiated by a low-energy ion beam at an elevated temperature, forming a highly ordered nanogroove pattern by a “reverse epitaxy” process due to self-assembly of surface vacancies. Second, diverse metallic nanowire arrays (Au, Fe, Ni, Co, FeAl alloy) are fabricated on these III-V templates by deposition at a glancing incidence angle. This method allows for the fabrication of metallic nanowire arrays with periodicities down to 45 nm scaled up to wafer-size fabrication. As typical noble and magnetic metals, the Au and Fe nanowire arrays produced here exhibited large anisotropic optical and magnetic properties, respectively. The excitation of localized surface plasmon resonances (LSPRs) of the Au nanowire arrays resulted in a high electric field enhancement, which was used to detect phthalocyanine (CoPc) in surface-enhanced Raman scattering (SERS). Furthermore, the Fe nanowire arrays showed a very high in-plane magnetic anisotropy of approximately 412 mT, which may be the largest in-plane magnetic anisotropy field yet reported that is solely induced via shape anisotropy within the plane of a thin film.

Keywords: self-assembly; metallic nanowire array; reverse epitaxy; magnetic anisotropy; anisotropic dielectric function

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