This article presents an overview of the essential aspects in the fabrication of silicon and some silicon/germanium nanostructures by metal‐assisted chemical etching. First, the basic process and ...mechanism of metal‐assisted chemical etching is introduced. Then, the various influences of the noble metal, the etchant, temperature, illumination, and intrinsic properties of the silicon substrate (e.g., orientation, doping type, doping level) are presented. The anisotropic and the isotropic etching behaviors of silicon under various conditions are presented. Template‐based metal‐assisted chemical etching methods are introduced, including templates based on nanosphere lithography, anodic aluminum oxide masks, interference lithography, and block‐copolymer masks. The metal‐assisted chemical etching of other semiconductors is also introduced. A brief introduction to the application of Si nanostructures obtained by metal‐assisted chemical etching is given, demonstrating the promising potential applications of metal‐assisted chemical etching. Finally, some open questions in the understanding of metal‐assisted chemical etching are compiled.
Metal‐assisted chemical etching (MACE) of silicon is controllable, reproducible, and can be used reliably to create well‐defined nanostructures. The mechanism, the influence of various factors on MACE, the controllable fabrication of patterned Si and SiGe structures, the applications of Si structures created by MACE, and the etching of other semiconductors by the MACE are reviewed.
People do not only feel guilt from not living up to others' expectations (Battigalli and Dufwenberg, 2007), but may also like to exceed them. We propose a model that generalizes the guilt aversion ...model to capture the possibility of positive surprises when making gifts. A model extension allows decision makers to care about others' attribution of intentions behind surprises. We test the model in a series of dictator game experiments. We find a strong causal effect of recipients' expectations on dictators' transfers. Moreover, in line with our model, the correlation between transfers and expectations can be both positive and negative, obscuring the effect in the aggregate. Finally, we provide evidence that dictators care about what recipients know about the intentions behind surprises.
Transition metal dichalcogenides have attracted research interest over the last few decades due to their interesting structural chemistry, unusual electronic properties, rich intercalation chemistry ...and wide spectrum of potential applications. Despite the fact that the majority of related research focuses on semiconducting transition-metal dichalcogenides (for example, MoS2), recently discovered unexpected properties of WTe2 are provoking strong interest in semimetallic transition metal dichalcogenides featuring large magnetoresistance, pressure-driven superconductivity and Weyl semimetal states. We investigate the sister compound of WTe2, MoTe2, predicted to be a Weyl semimetal and a quantum spin Hall insulator in bulk and monolayer form, respectively. We find that bulk MoTe2 exhibits superconductivity with a transition temperature of 0.10 K. Application of external pressure dramatically enhances the transition temperature up to maximum value of 8.2 K at 11.7 GPa. The observed dome-shaped superconductivity phase diagram provides insights into the interplay between superconductivity and topological physics.
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.
For the first time, Weyl semimetals are used as catalysts for highly effective hydrogen evolution reactions. The high mobility of carriers because of linear band crossings near the Fermi level is the major factor for their high activity. Unlike other catalysts, effect of the disorder at the surface is not a concern due to the topologically protected robust surface states.
Urbanization contributes to the loss of the world's biodiversity and the homogenization of its biota. However, comparative studies of urban biodiversity leading to robust generalities of the status ...and drivers of biodiversity in cities at the global scale are lacking. Here, we compiled the largest global dataset to date of two diverse taxa in cities: birds (54 cities) and plants (110 cities). We found that the majority of urban bird and plant species are native in the world's cities. Few plants and birds are cosmopolitan, the most common being Columba livia and Poa annua. The density of bird and plant species (the number of species per km2) has declined substantially: only 8% of native bird and 25% of native plant species are currently present compared with estimates of non-urban density of species. The current density of species in cities and the loss in density of species was best explained by anthropogenic features (landcover, city age) rather than by non-anthropogenic factors (geography, climate, topography). As urbanization continues to expand, efforts directed towards the conservation of intact vegetation within urban landscapes could support higher concentrations of both bird and plant species. Despite declines in the density of species, cities still retain endemic native species, thus providing opportunities for regional and global biodiversity conservation, restoration and education.
The synthesis of semiconductor nanowires has been studied intensively worldwide for a wide spectrum of materials. Such low‐dimensional nanostructures are not only interesting for fundamental research ...due to their unique structural and physical properties relative to their bulk counterparts, but also offer fascinating potential for future technological applications. Deeper understanding and sufficient control of the growth of nanowires are central to the current research interest. This Review discusses the various growth processes, with a focus on the vapor–liquid–solid process, which offers an opportunity for the control of spatial positioning of nanowires. Strategies for position‐controlled and nanopatterned growth of nanowire arrays are reviewed and demonstrated by selected examples as well as discussed in terms of larger‐scale realization and future prospects. Issues on building up nanowire‐based electronic and photonic devices are addressed at the end of the Review, accompanied by a brief survey of recent progress demonstrated so far on the laboratory level.
News on the wires: Nanowires have been fabricated either via self‐organization or controlled assembly. Strategies for position‐controlled and nanopatterned growth of semiconductor nanowire arrays are reviewed and discussed in terms of technical simplicity and the potential for larger‐scale realization. Issues on nanowire‐based devices are also addressed with a brief survey of recent advances.
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