Large scale implementation of electrochemical water splitting for hydrogen evolution requires cheap and efficient catalysts to replace expensive platinum. Molybdenum disulfide is one of the most promising alternative catalysts but its intrinsic activity is still inferior to platinum. There is therefore a need to explore new active site origins in molybdenum disulfide with ultrafast reaction kinetics and to understand their mechanisms. Here, we report a universal cold hydrogen plasma reduction method for synthesizing different single atoms sitting on two-dimensional monolayers. In case of molybdenum disulfide, we design and identify a new type of active site, i.e., unsaturated Mo single atoms on cogenetic monolayer molybdenum disulfide. The catalyst shows exceptional intrinsic activity with a Tafel slope of 35.1 mV dec-1 and a turnover frequency of ~10^3 s-1 at 100 mV, based on single flake microcell measurements. Theoretical studies indicate that coordinately unsaturated Mo single atoms sitting on molybdenum disulfide increase the bond strength between adsorbed hydrogen atoms and the substrates through hybridization, leading to fast hydrogen adsorption/desorption kinetics and superior hydrogen evolution activity. This work shines fresh light on preparing highly-efficient electrocatalysts for water splitting and other electrochemical processes, as well as provides a general method to synthesize single atoms on two-dimensional monolayers.
Monolayer molybdenum disulfide (MoS2) is a two-dimensional direct band gap semiconductor with unique mechanical, electronic, optical, and chemical properties that can be utilized for novel nanoelectronics and optoelectronics devices. The performance of these devices strongly depends on the quality and defect morphology of the MoS2 layers. Here we provide a systematic study of intrinsic structural defects in chemical vapor phase grown monolayer MoS2, including point defects, dislocations, grain boundaries, and edges, via direct atomic resolution imaging, and explore their energy landscape and electronic properties using first-principles calculations. A rich variety of point defects and dislocation cores, distinct from those present in graphene, were observed in MoS2. We discover that one-dimensional metallic wires can be created via two different types of 60° grain boundaries consisting of distinct 4-fold ring chains. A new type of edge reconstruction, representing a transition state during growth, was also identified, providing insights into the material growth mechanism. The atomic scale study of structural defects presented here brings new opportunities to tailor the properties of MoS2 via controlled synthesis and defect engineering.
As topological quasi-particles in magnetic materials, skyrmions and antiskyrmions show potential in spintronics for information storage and computing. However, effectively controlling and separating these entities remain significantly challenging. Here, we demonstrate that anisotropic Kitaev exchange can distinctly influence the static and dynamic behaviors for skyrmions and antiskyrmions, thus aiding their manipulation and separation. Employing the monolayer frustrated magnet NiBr
Interlayer magnetic interactions play a pivotal role in determining the magnetic arrangement within van der Waals (vdW) magnets, and the remarkable tunability of these interactions through applied pressure further enhances their significance. Here, we investigate NiI2 flakes, a representative vdW magnet, under hydrostatic pressures up to 11 GPa. We reveal a notable increase in magnetic transition temperatures for both helimagnetic and antiferromagnetic states, and find that a reversible transition from helimagnetic to antiferromagnetic (AFM) phases at approximately 7 GPa challenges established theoretical and experimental expectations. While the increase in transition temperature aligns with pressure-enhanced overall exchange interaction strengths, we identify the significant role of the second-nearest neighbor interlayer interaction, which competes with intra-layer frustration and favors the AFM state as demonstrated in the Monte Carlo simulations. Experimental and simulated results converge on the existence of an intermediate helimagnetic ordered state in NiI2 before transitioning to the AFM state. These findings underscore the pivotal role of interlayer interactions in shaping the magnetic ground state, providing fresh perspectives for innovative applications in nanoscale magnetic device design.
A series of open-framework germanates synthesized at Stockholm University, denoted as SU-n, will be presented.They include pure germanates as SU-44, the recently published crystalline SU-M [1] and an aluminogermanate SU-46.All these germanates were prepared by hydrothermal synthesis from a solution consisting of germanium dioxide and an organic amine as template, using water as solvent.In most cases hydrofluoric acid was added as a mineralizer.The resulting solution was transferred to a teflon-lined autoclave and heated to temperatures between 160-170°C for one or two weeks.Single crystal X-ray diffraction data for both SU-M and SU-46 were collected at 100 K using graphite-monochromatized Mo K α radiation.For needle-like crystals of SU-44, single crystal X-ray diffraction data were collected at 293 K using a synchrotron radiation at the beamline I711, Max-lab, Lund, Sweden.The structure solution and refinement were carried out using the interface WinGX [2] with the software SHELX-97.SU-46 is a three-dimensional structure with a new zeolite topology.It contains 2D intersecting 8-ring channels.Due to a tendency for twinning it has been difficult to determine the space group, apply absorption correction and find template positions.SU-M is built from a single type of cluster Ge 10 O 24 (F,OH) 3 (denoted Ge 10 ).The clusters are connected in such a way that they lie on a gyroidal minimal surface with fully ordered crystalline walls [1].When the temperature for the synthesis of SU-M is raised, SU-44 was then synthesized.Instead of the Ge 10 -clusters in SU-M, SU-44 contains two different clusters, Ge 7 O 17 (F,OH) 2 (denoted Ge 7 ) and Ge 9 O 22 (F,OH) 4 (denoted Ge 9 ).Two different clusters in the same germanate structure has not previously been reported in the literature.
Multi-Band Orthogonal Frequency Division Multiplexing (MB-OFDM) is a suitable solution to implementation of high speed data transmission in ultra wideband spectrum by dividing the spectrum available into multiple bands. The baseband of transmitter is one of the most important parts in MB-OFDM system. The structure of MB-OFDM system transmitter is introduced in this paper and the design of transmitter baseband based on FPGA is described in detail. The design has been validated with Xilinx Virtex II FPGA. The results show that all modules designed has achieved the expected purpose both in precision and resource, with simplicity and high efficiency, and can meet the demand of MB-OFDM systems.
Our first-principles calculations indicate the possibility of preparing spin-polarized scanning tunneling microscopy (SP-STM) probes from Fe-doped capped carbon nanotubes (CNTs). The structural stability, magnetic moment, and electronic property of hybrid systems are found to depend on the Fe adsorption site, which is attributed to the hybridization between Fe 3d and C 2p orbitals. The CNTs with Fe atoms adsorbed at the tip-top are demonstrated to be promising candidates for the SP-STM probe, with a high spin polarization leading to a completely spin-polarized current at lower voltages. In contrast, the CNTs encapsulating Fe atom are basically nonmagnetic, and thus useless for the SP-STM probe application in nature.
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