Horst Blumtritt

Weyl Semimetals as Hydrogen Evolution Catalysts

The search for highly efficient and low-cost catalysts is one of the main driving forces in catalytic chemistry. Current strategies for the catalyst design focus on increasing the number and activity of local catalytic sites, such as the edge sites of molybdenum disulfides in the hydrogen evolution reaction (HER). Here, the study proposes and demonstrates a different principle that goes beyond local site optimization by utilizing topological electronic states to spur catalytic activity. For HER, excellent catalysts have been found among the transition-metal monopnictides-NbP, TaP, NbAs, and TaAs-which are recently discovered to be topological Weyl semimetals. Here the study shows that the combination of robust topological surface states and large room temperature carrier mobility, both of which originate from bulk Dirac bands of the Weyl semimetal, is a recipe for high activity HER catalysts. This approach has the potential to go beyond graphene based composite photocatalysts where graphene simply provides a high mobility medium without any active catalytic sites that have been found in these topological materials. Thus, the work provides a guiding principle for the discovery of novel catalysts from the emerging field of topological materials.

Colossal injection of catalyst atoms into silicon nanowires

The incorporation of impurities during the growth of nanowires from the vapour phase alters their basic properties substantially, and this process is critical in an extended range of emerging nanometre-scale technologies. In particular, achieving precise control of the behaviour of group III and group V dopants has been a crucial step in the development of silicon (Si) nanowire-based devices. Recently it has been demonstrated that the use of aluminium (Al) as a growth catalyst, instead of the usual gold, also yields an effective p-type doping, thereby enabling a novel and efficient route to functionalizing Si nanowires. Besides the technological implications, this self-doping implies the detachment of Al from the catalyst and its injection into the growing nanowire, involving atomic-scale processes that are crucial for the fundamental understanding of the catalytic assembly of nanowires. Here we present an atomic-level, quantitative study of this phenomenon of catalyst dissolution by three-dimensional atom-by-atom mapping of individual Al-catalysed Si nanowires using highly focused ultraviolet-laser-assisted atom-probe tomography. Although the observed incorporation of the catalyst atoms into nanowires exceeds by orders of magnitude the equilibrium solid solubility and solid-solution concentrations in known non-equilibrium processes, the Al impurities are found to be homogeneously distributed in the nanowire and do not form precipitates or clusters. As well as the anticipated effect on the electrical properties, this kinetics-driven colossal injection also has direct implications for nanowire morphology. We discuss the observed strong deviation from equilibrium using a model of solute trapping at step edges, and identify the key growth parameters behind this phenomenon on the basis of a kinetic model of step-flow growth of nanowires. The control of this phenomenon provides opportunities to create a new class of nanoscale devices by precisely tailoring the shape and composition of metal-catalysed nanowires.

High-resolution investigations of ripple structures formed by femtosecond laser irradiation of silicon

We report on the structural investigation of self-organized periodic microstructures (ripples) generated in Si(100) targets after multishot irradiation by approximately 100-fs to 800-nm laser pulses at intensities near the single shot ablation threshold. Inspection by surface sensitive microscopy, e.g., atomic force microscopy (AFM) or scanning electron microscopy (SEM), and conventional and high-resolution transmission electron microscopy reveal complex structural modifications upon interaction with the laser: even well outside the ablated area, the target surface exhibits fine ripple-like undulations, consisting of alternating crystalline and amorphous silicon. Inside the heavily modified area, amorphous silicon is found only in the valleys but not on the crests which, instead, consist of highly distorted crystalline phases, rich in defects.

Physical mechanisms of breakdown in multicrystalline silicon solar cells

We have identified at least five different local breakdown mechanisms according to the temperature coefficient (TC) and slope of their characteristics and electroluminescence (EL) under reverse bias. These are (1) early pre-breakdown (strongly negative TC, low slope), (2) edge breakdown (positive TC, low slope, no EL), (3) weak defect-induced breakdown (zero or weakly negative TC, moderate slope, 1550 nm defect luminescence), (4) strong defect-induced breakdown (zero or weakly negative TC, moderate slope, no or weak defect luminescence), and (5) avalanche breakdown at dislocation-induced etch pits (negative TC, high slope). The latter mechanism usually dominates at high reverse bias. In addition to the local breakdown sites there is evidence of an areal reverse current between the dominant breakdown sites showing a positive TC. Since defect-induced breakdown shows a zero or weakly negative TC and also leads to weak avalanche multiplication, we propose defect level-induced avalanche instead of trap-assisted tunneling to be responsible for this breakdown mechanism.

Electron microscope verification of prebreakdown-inducing α-FeSi2 needles in multicrystalline silicon solar cells

It had been shown already earlier by X-ray microanalysis that, in positions of defect-induced junction breakdown in industrial multicrystalline (mc) silicon solar cells, iron-containing precipitates may exist. However, the nature of these precipitates was unknown so far. Here, in such positions, scanning transmission electron microscopy was performed after defect-controlled focused ion beam preparation. First of all, the defect site was localized by microscopic reverse-bias electroluminescence imaging. The high accuracy of following FIB target preparation (<0.1 μm necessary) was obtained by both, electron beam-induced current imaging and secondary electron material contrast observation during the slice-by-slice milling of the TEM specimen. By nano-beam electron diffraction (NBED) and energy dispersive spectroscopy, the iron-containing precipitates were identified as α-type FeSi2 needles, about 30 nm in diameter and several μm in length. The FeSi2 needles show preferential orientation relationships to the ...

Recombination at Lomer Dislocations in Multicrystalline Silicon for Solar Cells

Lomer dislocations at small-angle grain boundaries in multicrystalline silicon solar cells have been identified as responsible for the dominating inherent dark current losses. Resulting efficiency losses have been quantified by dark lock-in thermography to be locally up to several percent absolute, reducing the maximum power of the cells. By electron beam induced current measurements and scanning transmission electron microscopy investigations, it is revealed that the strengths of the dark current losses depend on the density of Lomer dislocations at the small-angle grain boundaries.

A perfectly periodic three-dimensional protein/silica mesoporous structure produced by an organism.

The discovery of perfectly ordered 3D mesoporous protein/silica structure in the axial filament of the marine sponge Monorhaphis chuni is reported. The structure belongs to body-centered tetragonal symmetry system (a=9.88 nm, c=10.83 nm) and comprises interconnecting lattices of protein and silica, templated by the self-assembled, enzymatically active protein-silicatein, whose primary function is the precipitation of silica.

In situ elastic modulus measurements of ultrathin protein-rich organic layers in biosilica: towards deeper understanding of superior resistance to fracture of biocomposites

Biogenic ceramics are known to exhibit superior toughness due to a laminated architecture with ultrathin organic layers separating the ceramic blocks. Theoretical analyses relate the toughness increase to the modulus contrast, Ec/Eo between the stiff, Ec, and the compliant, Eo, components. However, experimental data on this contrast are extremely difficult to obtain by any known technique due to the very small thickness and low modulus values of the organic layers. Here we adapt a recently developed nanoscale modulus mapping technique combined with reverse finite element analysis in order to map the elastic modulus across a 35 nm thick organic layer within biosilica in a giant anchor spicule of the glass sponge Monorhaphis chuni. We find a modulus of 0.7 GPa in the organic layer as compared to 37 GPa in the bioglass. Furthermore, a modulus gradient extends 50 nm into the glass layer, probably due to the spatial distribution of small organic inclusions. With this new methodology it becomes possible to determine the elastic moduli of nanometric inclusions even when embedded in a matrix which is 50 times stiffer.

Defect-Induced Breakdown in Multicrystalline Silicon Solar Cells

We have identified at least five different kinds of local breakdown according to the temperature coefficient (TC) and slope of their characteristics and electroluminescence (EL) under a reverse bias. These are 1) early prebreakdown (negative TC, low slope), 2) edge breakdown (positive TC, low slope, no EL), 3) weak defect-induced breakdown (zero or weakly negative TC, moderate slope, 1550-nm defect luminescence), 4) strong defect-induced breakdown (zero or weakly negative TC, moderate slope, no or weak defect luminescence), and 5) avalanche breakdown at dislocation-induced etch pits (negative TC, high slope). The latter mechanism usually dominates at a high reverse bias. The defects leading to the etch pits are investigated in detail. In addition to the local breakdown sites, there is evidence of an areal reverse current between the dominant breakdown sites showing a positive TC. Defect-induced breakdown shows a zero or weakly negative TC and also leads to weak avalanche multiplication. It has been found recently that it is caused by metal-containing precipitates lying in grain boundaries.

Preparation of nanowire specimens for laser-assisted atom probe tomography

The availability of reliable and well-engineered commercial instruments and data analysis software has led to development in recent years of robust and ergonomic atom-probe tomographs. Indeed, atom-probe tomography (APT) is now being applied to a broader range of materials classes that involve highly important scientific and technological problems in materials science and engineering. Dual-beam focused-ion beam microscopy and its application to the fabrication of APT microtip specimens have dramatically improved the ability to probe a variety of systems. However, the sample preparation is still challenging especially for emerging nanomaterials such as epitaxial nanowires which typically grow vertically on a substrate through metal-catalyzed vapor phase epitaxy. The size, morphology, density, and sensitivity to radiation damage are the most influential parameters in the preparation of nanowire specimens for APT. In this paper, we describe a step-by-step process methodology to allow a precisely controlled, damage-free transfer of individual, short silicon nanowires onto atom probe microposts. Starting with a dense array of tiny nanowires and using focused ion beam, we employed a sequence of protective layers and markers to identify the nanowire to be transferred and probed while protecting it against Ga ions during lift-off processing and tip sharpening. Based on this approach, high-quality three-dimensional atom-by-atom maps of single aluminum-catalyzed silicon nanowires are obtained using a highly focused ultraviolet laser-assisted local electrode atom probe tomograph.