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Bullard Lundqvist posted an update 6 months, 1 week ago
Upon heating or (slow) cooling, the Al metal can repeatably move in and out of the Si x Ge1-x alloy nanowire while maintaining the rodlike geometry and crystallinity, allowing to fabricate and contact nanowire heterostructures in a reversible way in a single process step, compatible with current Si-based technology. This interesting system is promising for various applications, such as phase change memories in an all crystalline system with integrated contacts as well as Si/Si x Ge1-x /Si heterostructures for near-infrared sensing applications.The heterogeneous integration of micro- and nanoscale devices with on-chip circuits and waveguide platforms is a key enabling technology, with wide-ranging applications in areas including telecommunications, quantum information processing, and sensing. Pick and place integration with absolute positional accuracy at the nanoscale has been previously demonstrated for single proof-of-principle devices. However, to enable scaling of this technology for realization of multielement systems or high throughput manufacturing, the integration process must be compatible with automation while retaining nanoscale accuracy. In this work, an automated transfer printing process is realized by using a simple optical microscope, computer vision, and high accuracy translational stage system. Automatic alignment using a cross-correlation image processing method demonstrates absolute positional accuracy of transfer with an average offset of less then 40 nm (3σ less then 390 nm) for serial device integration of both thin film silicon membranes and single nanowire devices. Parallel transfer of devices across a 2 × 2 mm2 area is demonstrated with an average offset of less then 30 nm (3σ less then 705 nm). Rotational accuracy better than 45 mrad is achieved for all device variants. Devices can be selected and placed with high accuracy on a target substrate, both from lithographically defined positions on their native substrate or from a randomly distributed population. These demonstrations pave the way for future scalable manufacturing of heterogeneously integrated chip systems.Extrinsically doped two-dimensional (2D) semiconductors are essential for the fabrication of high-performance nanoelectronics among many other applications. Herein, we present a facile synthesis method for Al-doped MoS2 via plasma-enhanced atomic layer deposition (ALD), resulting in a particularly sought-after p-type 2D material. Precise and accurate control over the carrier concentration was achieved over a wide range (1017 up to 1021 cm-3) while retaining good crystallinity, mobility, and stoichiometry. This ALD-based approach also affords excellent control over the doping profile, as demonstrated by a combined transmission electron microscopy and energy-dispersive X-ray spectroscopy study. Sharp transitions in the Al concentration were realized and both doped and undoped materials had the characteristic 2D-layered nature. The fine control over the doping concentration, combined with the conformality and uniformity, and subnanometer thickness control inherent to ALD should ensure compatibility with large-scale fabrication. This makes AlMoS2 ALD of interest not only for nanoelectronics but also for photovoltaics and transition-metal dichalcogenide-based catalysts.Carbon-based nanofibers decorated with metallic nanoparticles (NPs) as hierarchically structured electrodes offer significant opportunities for use in low-temperature fuel cells, electrolyzers, flow and air batteries, and electrochemical sensors. see more We present a facile and scalable method for preparing nanostructured electrodes composed of Pt NPs on graphitized carbon nanofibers. Electrospinning directly addresses the issues related to large-scale production of Pt-based fuel cell electrocatalysts. Through precursors containing polyacrylonitrile and Pt salt electrospinning along with an annealing protocol, we obtain approximately 180 nm thick graphitized nanofibers decorated with approximately 5 nm Pt NPs. By in situ annealing scanning transmission electron microscopy, we qualitatively resolve and quantitatively analyze the unique dynamics of Pt NP formation and movement. Interestingly, by very efficient thermal-induced segregation of all Pt from the inside to the surface of the nanofibers, we increase overall Pt utilization as electrocatalysis is a surface phenomenon. The obtained nanomaterials are also investigated by spatially resolved Raman spectroscopy, highlighting the higher structural order in nanofibers upon doping with Pt precursors. The rationalization of the observed phenomena of segregation and ordering mechanisms in complex carbon-based nanostructured systems is critically important for the effective utilization of all metal-containing catalysts, such as electrochemical oxygen reduction reactions, among many other applications.The electrochemical nitrogen reduction reaction (NRR) to ammonia (NH3) is a promising alternative route for an NH3 synthesis at ambient conditions to the conventional high temperature and pressure Haber-Bosch process without the need for hydrogen gas. Single metal ions or atoms are attractive candidates for the catalytic activation of non-reactive nitrogen (N2), and for future targeted improvement of NRR catalysts, it is of utmost importance to get detailed insights into structure-performance relationships and mechanisms of N2 activation in such structures. Here, we report density functional theory studies on the NRR catalyzed by single Au and Fe atoms supported in graphitic C2N materials. Our results show that the metal atoms present in the structure of C2N are the reactive sites, which catalyze the aforesaid reaction by strong adsorption and activation of N2. We further demonstrate that a lower onset electrode potential is required for Fe-C2N than for Au-C2N. Thus, Fe-C2N is theoretically predicted to be a potentially better NRR catalyst at ambient conditions than Au-C2N owing to the larger adsorption energy of N2 molecules. Furthermore, we have experimentally shown that single sites of Au and Fe supported on nitrogen-doped porous carbon are indeed active NRR catalysts. However, in contrast to our theoretical results, the Au-based catalyst performed slightly better with a Faradaic efficiency (FE) of 10.1% than the Fe-based catalyst with an FE of 8.4% at -0.2 V vs. RHE. The DFT calculations suggest that this difference is due to the competitive hydrogen evolution reaction and higher desorption energy of ammonia.
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