By employing a precise method for locating and directly imaging the active switching region in a resistive random access memory (RRAM) device, a nanoscale conducting channel consisting of an ...amorphous Ta(O) solid solution surrounded by nearly stoichiometric Ta2O5 is observed. Structural and chemical analysis of the channel combined with temperature‐dependent transport measurements indicate a unique resistance switching mechanism.
We report the epitaxial formation of metastable γ-(FexNi1−x)Si2 nanostructure arrays resulting from the reaction of Ni80Fe20 permalloy with vicinal Si(111) surface atoms. We then explore the effect ...of structure and composition on the nanostructure’s magnetic properties. The low-temperature annealing (T < 600 °C) of a pre-deposited permalloy film led to solid-phase epitaxial nucleation of compact disk-shaped island nanostructures decorating ledges of the stepped surface, with either (2 × 2) or (3×3) R30° reconstructed flat top faces. High resolution scanning transmission electron microscopy analysis demonstrated fully coherent epitaxy of the islands with respect to the substrate, consistent with a well-matched CaF2-prototype structure associated with γ-FeSi2, along perfect atomically sharp interfaces. Energy dispersive spectroscopy detected ternary composition of the islands, with Fe and Ni atoms confined to the islands, and no trace of segregation. Our magnetometry measurements revealed the superparamagnetic behavior of the silicide islands, with a blocking temperature around 30 K, reflecting the size, shape, and dilute arrangement of the islands in the assembly.
Inspired by nature, green chemistry uses various biomolecules, such as proteins, as reducing agents to synthesize metallic nanostructures. This methodology provides an alternative route to ...conventional harsh synthetic processes, which include polluting chemicals. Tuning the resulting nanostructure properties, such as their size and shape, is challenging as the exact mechanism involved in their formation is still not well understood. This work reports a well-controlled method to program gold nanostructures' shape, size, and aggregation state using only one protein type, mucin, as a reduction and capping material in a one-pot bio-assisted reaction. Using mucin as a gold reduction template while varying its tertiary structure via the pH of the synthesis, we demonstrate that spherical, coral-shaped, and hexagonal gold crystals can be obtained and that the size can be tuned over three orders of magnitude. This is achieved by leveraging the protein's intrinsic reducing properties and pH-induced conformational changes. The systematic study of the reaction kinetics and growth steps developed here provides an understanding of the mechanism behind this phenomenon. We further show that the prepared gold nanostructures exhibit tunable photothermal properties that can be optimized for various hyperthermia-induced antibacterial applications.
Magnetism has recently been observed in nominally nonmagnetic iron disilicide in the form of epitaxial γ-FeSi2 nanostructures on Si(111) substrate. To explore the origin of the magnetism in ...γ-FeSi2/Si(111) nanostructures, we performed a systematic first-principles study based on density functional theory. Several possible factors, such as epitaxial strain, free surface, interface, and edge, were examined. The calculations show that among these factors, only the edge can lead to the magnetism in γ-FeSi2/Si(111) nanostructures. It is shown that magnetism exhibits a strong dependency on the local atomic structure of the edge. Furthermore, magnetism can be enhanced by creating multiple-step edges. In addition, the results also reveal that edge orientation can have a significant effect on magnetism. These findings, thus, provide insights into a strategy to tune the magnetic properties of γ-FeSi2/Si(111) nanostructures through controlling the structure, population, and orientation of the edges.
In spite of numerous advantageous properties of silicides, magnetic properties are not among them. Here, the magnetic properties of epitaxial binary silicide nanostructures are discussed. The vast ...majority of binary transition‐metal silicides lack ferromagnetic order in their bulk‐size crystals. Silicides based on rare‐earth metals are usually weak ferromagnets or antiferromagnets, yet both groups tend to exhibit increased magnetic ordering in low‐dimensional nanostructures, in particular at low temperatures. The origin of this surprising phenomenon lies in undercoordinated atoms at the nanostructure extremities, such as 2D (surfaces/interfaces), 1D (edges), and 0D (corners) boundaries. Uncompensated superspins of edge atoms increase the nanostructure magnetic shape anisotropy to the extent where it prevails over its magnetocrystalline counterpart, thus providing a plausible route toward the design of a magnetic response from nanostructure arrays in Si‐based devices, such as bit‐patterned magnetic recording media and spin injectors.
While binary transition‐metal and lanthanide silicides are not magnetic materials in their bulk form, there are numerous reports on magnetic properties of these very silicides when they are grown as epitaxial nanostructures. Recent progress in the field of epitaxial magnetic silicides is surveyed, and plausible reasons behind the unexpected ferromagnetic order in these important technological materials is suggested.
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•Ultra-thin Py (Ni80Fe20) film was deposited onto a vicinal Si(111) substrate at RT.•Epitaxial β-Fe(Ni)Si2 nanoislands formed on the substrate upon > 600 °C anneal.•Fast in-diffusion ...of Ni left Ni-deficient nanoisland tops and higher Ni concentration at the bottoms.•Coupling between the nanoisland tops and bottoms created ESM effect between two different FM entities.•Experimental observation of the ESM was corroborated by OOMMF simulations.
Self-organized formation of exchange-spring magnets in Permalloy-derived epitaxial silicide nanostructures, fabricated by deposition of Ni80Fe20 onto a vicinal Si(111) substrate, is reported. The crystal structure of bar-shaped silicide nanostructures decorating Si(111) surface steps, following thermally activated reaction between Ni, Fe, and Si surface atoms, was identified as ternary β-Fe(Ni)Si2 phase, in well-defined (1¯11¯) Si || (2¯2¯0) β and 011 Si || 001 β orientation relations with the substrate. At the same time, chemical composition within the islands was Fe-rich and Fe-deficient in comparison with the original stoichiometric Permalloy (Ni80Fe20) and not uniform, with higher concentration of Ni at the island bottoms, close to the interface with Si(111), creating de facto “compositional interfaces” within the islands, though no physical interfaces could be detected in a high-resolution structural-crystallographic analysis by transmission electron microscopy. Analysis of temperature-dependent magnetization reversal loops revealed, that the thicker and magnetically soft top part of the islands and yet magnetically softer and thinner bottom part, were magnetically exchange-coupled via the above “virtual” interfaces. Micromagnetic simulations, consistent with exchange-spring magnet, had further corroborated this conclusion.
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•MBE of Er and Gd on vicinal Si(1 1 1) investigated by STM, RHEED and LEED.•RE-silicide epitaxial nanostructures characterized by quasi-1D reconstruction.•Diffuse scattering caused by ...linear disorder between reconstructed nanostructures.•Surface disorder causes extinction of half-order satellites in diffraction patterns.
Sub-monolayer epitaxial self-assembled silicides of Er and Gd on Si(1 1 1) exhibit unique structural and morphological features that play a crucial role in their magnetic behavior. The effects of disorder occurring between the characteristic quasi-1D surface reconstructions of such systems were analyzed by low energy (LEED) and reflection high energy (RHEED) electron diffraction. The observed Rare-Earth Metal (REM)-Si antiphase domains caused appearance of diffuse intensity planes in the surface reciprocal space, visualized as straight (arched) streaks in LEED (RHEED) diffraction patterns. Random registry shifts between adjacent adsorbate atomic chains caused extinction of the half-order satellites in diffraction patterns of a surface reconstruction identified by scanning tunneling microscopy (STM) as Si(1 1 1)-(3 × 2)-Gd.
A comprehensive understanding of the resistive switching mechanisms that activate REDOX-based random access memory devices is necessary to further enhance their performance and reliability. In this ...article, devices of two different sizes were fabricated by magnetron sputtering and tested by potential sweeps and pulses. Reproducible memristive characteristics for HfO
x
-based devices were obtained. The contact geometry was shown to play a critical role in the process dynamics and device characteristics. Moderate to high ON/OFF ratios were obtained for all the devices. All the devices exhibited bipolar memristive behavior consistent with combined electric field and temperature-dependent oxygen migration in the filament formation mechanism.
•We evaluate the performance of a monolithic active pixel sensor direct electron detector (DED) for the characterization of short-range order (SRO) in nanoscale volumes of amorphous materials, using ...SiO2 and Ta2O5 thin films as test cases.•The performance of the DED is compared systematically to electron scattering measurements recorded on an indirect detector (CCD) using 200 and 80 keV electrons.•We demonstrate that the performance of the DED resulted in improved SRO characterization in comparison to that obtained from the CCD measurements, namely a larger measured maximal scattering vector, and improved signal-to-noise ratio across all spatial frequencies. These improvements are shown to result from the superior detective quantum efficiency of the DED. Consequently, the DED measurements enabled to determine the coordination numbers of atomic bonds more accurately.•In addition, the effects of sample cooling and energy-filtering on the measured SRO of the oxides were investigated.
The introduction of direct electron detectors (DEDs) to transmission electron microscopy has set off the ‘resolution revolution’, especially for cryoTEM low-dose imaging of soft matter.
In comparison to traditional indirect electron detectors such as Charged-Coupled Devices (CCD), DEDs show an improved modulation transfer function (MTF) and detective quantum efficiency (DQE) across all spatial frequencies, as well as faster frame rates which enable single electron counting.
The benefits of such characteristics for imaging, spectroscopy and electron holography have been demonstrated previously. However, studies are lacking on the application of DEDs for localized characterization of short- to medium- range-order (SRO, MRO) in amorphous materials using electron scattering.
Therefore, we evaluate the performance of a Monolithic Active Pixel Sensor DED for the characterization of SRO and MRO in nanoscale volumes of amorphous materials, using SiO2 and Ta2O5 thin films as test cases. The performance of the detector is compared systematically to electron scattering measurements recorded on an indirect detector (CCD) using 200 keV electrons and electron doses starting at approximately 500e−Å2 . In addition, the effects of sample cooling and energy-filtering on the measured SRO of the oxides were investigated.
We demonstrate that the performance of the DED resulted in improved SRO characterization in comparison to that obtained from the CCD measurements. The DED enabled to achieve a larger measured maximal scattering vector, ∼16.51Å compared to ∼151Å, for the CCD. Furthermore, an improved signal-to-noise ratio of approximately two-fold was observed across all spatial frequencies for both 200 keV and 80 keV electrons. These improvements are shown to result from the superior DQE of the DED. Consequently, the DED measurements enabled to determine the coordination numbers of atomic bonds more accurately.
We expect that further benefits of the DED for S/MRO characterization will be highlighted for ultra- sensitive materials that cannot withstand electron doses above several e−Å2 .
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