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.
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.
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.
Hexagonal boron nitride (h-BN) is a promising material for implementation in spintronics due to a large band gap, low spin-orbit coupling, and a small lattice mismatch to graphene and to close-packed surfaces of fcc-Ni(111) and hcp-Co(0001). Epitaxial deposition of h-BN on ferromagnetic metals is aimed at small interface scattering of charge and spin carriers. We report on the controlled growth of h-BN/Ni(111) by means of molecular beam epitaxy (MBE). Structural and electronic properties of this system are investigated using cross-section transmission electron microscopy (TEM) and electron spectroscopies which confirm good agreement with the properties of bulk h-BN. The latter are also corroborated by density functional theory (DFT) calculations, revealing that the first h-BN layer at the interface to Ni is metallic. Our investigations demonstrate that MBE is a promising, versatile alternative to both the exfoliation approach and chemical vapour deposition of h-BN.
Solar cells whose breakdown current exceeds a certain limit cannot be used because such cells may thermally damage the module in case of unintentional reverse biasing by local shading (hot-spot problem [1]). In order to reduce the number of off-specification cells, the reason for the high reverse currents must be identified. The physical mechanisms leading to breakdown of reverse-biased p-n junctions are internal field emission (Zener effect) and impact ionization (avalanche effect). They exhibit a characteristic temperature dependence, which can be used for their identification: for internal field emission the current increases slightly with rising temperature due to band-gap lowering, but it decreases considerably for impact ionization due to increased phonon scattering. Moreover, multiplication of photo-generated carriers takes place only for avalanche breakdown [2]. Both mechanisms require a certain electric field strength, which normally is not reached in standard multicrystalline (mc) Si solar cells. According to that field strength, however, the breakdown voltage should be four times higher than observed in practice [3].In this letter, we present a systematic study of the breakdown mechanism in commercial, 156 × 156 mm 2 p-type base mc-Si solar cells. We employ special lock-in thermography (LIT) imaging techniques to identify the type of breakdown occurring at the hot spots, and various electron microscopy techniques to reveal the microscopic nature of the breakdown sites. The cells investigated were free from ohmic shunts. A typical reverse current-voltage characteristic is shown in Fig. 1, given for two different temperatures.At lower reverse voltages, only weak currents occur, which up to approximately -13 V increase only slightly (pre-breakdown). Beyond -13 V, however, a steep current increase is observed, which is typical for a hard breakdown. For the solar cells under investigation, the pre-breakdown current increases with temperature, whereas the hardbreakdown current decreases (for a given voltage). This indicates that in general, different breakdown mechanisms are involved, a fact which also other authors have observed, using electroluminescence (EL) at reverse bias [4].Lock-in thermography has been established as a standard technique for locating and characterizing leakage currents in solar cells [5]. For the investigation of breakdown currents we have recently proposed several LIT-based imaging techniques [6], performed either in the dark (DLIT) or under illumination (ILIT). In all these techniques, the -90° LIT signal, which can be interpreted quantitatively [7], is used. The temperature variation of the current at a given bias voltage is displayed by the Temperature-Coef-Multicrystalline silicon solar cells typically show hard breakdown beginning from about -13 V bias, which leads to the well-known hot-spot problem. Using special lock-in thermography techniques, hard breakdown has been found to occur in regions of avalanche multiplication. A systematic study of these regions by various electro...
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