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Unraveling Structure and Device Operation of Organic Permeable Base Transistors
Organic permeable base transistors (OPBTs) are of great interest for flexible electronic circuits, as they offer very large on-current density and a record-high transition frequency. They rely on a vertical device architecture with current transport through native pinholes in a central base electrode. This study investigates the impact of pinhole density and pinhole diameter on the DC device performance in OPBTs based on experimental data and TCAD simulation results. A pinhole density of NPin = 54 µm−2 and pinhole diameters around LPin = 15 nm are found in the devices. Simulations show that a variation of pinhole diameter and density around these numbers has only a minor impact on the DC device characteristics. A variation of the pinhole diameter and density by up to 100% lead to a deviation of less than 4% in threshold voltage, on/off current ratio, and sub-threshold slope. Hence, the fabrication of OPBTs with reliable device characteristics is possible regardless of statistical deviations in thin film formation. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Flexomagnetism and vertically graded Néel temperature of antiferromagnetic Cr2O3 thin films
Antiferromagnetic insulators are a prospective materials platform for magnonics, spin superfluidity, THz spintronics, and non-volatile data storage. A magnetomechanical coupling in antiferromagnets offers vast advantages in the control and manipulation of the primary order parameter yet remains largely unexplored. Here, we discover a new member in the family of flexoeffects in thin films of Cr2O3. We demonstrate that a gradient of mechanical strain can impact the magnetic phase transition resulting in the distribution of the Néel temperature along the thickness of a 50-nm-thick film. The inhomogeneous reduction of the antiferromagnetic order parameter induces a flexomagnetic coefficient of about 15 μB nm−2. The antiferromagnetic ordering in the inhomogeneously strained films can persist up to 100 °C, rendering Cr2O3 relevant for industrial electronics applications. Strain gradient in Cr2O3 thin films enables fundamental research on magnetomechanics and thermodynamics of antiferromagnetic solitons, spin waves and artificial spin ice systems in magnetic materials with continuously graded parameters.
Phase Selection in Mn–Si Alloys by Fast Solid-State Reaction with Enhanced Skyrmion Stability
B20-type transition-metal silicides or germanides are noncentrosymmetric materials hosting magnetic skyrmions, which are promising information carriers in spintronic devices. The prerequisite is to prepare thin films on technology-relevant substrates with magnetic skyrmions stabilized at a broad temperature and magnetic-field working window. A canonical example is the B20-MnSi film grown on Si substrates. However, the as-yet unavoidable contamination with MnSi1.7 occurs due to the lower nucleation temperature of this phase. In this work, a simple and efficient method to overcome this problem and prepare single-phase MnSi films on Si substrates is reported. It is based on the millisecond reaction between metallic Mn and Si using flash-lamp annealing (FLA). By controlling the FLA energy density, single-phase MnSi or MnSi1.7 or their mixture can be grown at will. Compared with bulk MnSi, the prepared MnSi films show an increased Curie temperature of up to 41 K. In particular, the magnetic skyrmions are stable over a much wider temperature and magnetic-field range than reported previously. The results constitute a novel phase selection approach for alloys and can help to enhance specific functional properties, such as the stability of magnetic skyrmions. © 2021 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH
Three-Dimensional Composition and Electric Potential Mapping of III–V Core–Multishell Nanowires by Correlative STEM and Holographic Tomography
The nondestructive characterization of nanoscale devices, such as those based on semiconductor nanowires, in terms of functional potentials is crucial for correlating device properties with their morphological/materials features, as well as for precisely tuning and optimizing their growth process. Electron holographic tomography (EHT) has been used in the past to reconstruct the total potential distribution in three-dimension but hitherto lacked a quantitative approach to separate potential variations due to chemical composition changes (mean inner potential, MIP) and space charges. In this Letter, we combine and correlate EHT and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) tomography on an individual ⟨111⟩ oriented GaAs–AlGaAs core–multishell nanowire (NW). We obtain excellent agreement between both methods in terms of the determined Al concentration within the AlGaAs shell, as well as thickness variations of the few nanometer thin GaAs shell acting as quantum well tube. Subtracting the MIP determined from the STEM tomogram, enables us to observe functional potentials at the NW surfaces and at the Au–NW interface, both ascribed to surface/interface pinning of the semiconductor Fermi level.
Purely antiferromagnetic magnetoelectric random access memory
Magnetic random access memory schemes employing magnetoelectric coupling to write binary information promise outstanding energy efficiency. We propose and demonstrate a purely antiferromagnetic magnetoelectric random access memory (AF-MERAM) that offers a remarkable 50-fold reduction of the writing threshold compared with ferromagnet-based counterparts, is robust against magnetic disturbances and exhibits no ferromagnetic hysteresis losses. Using the magnetoelectric antiferromagnet Cr2O3, we demonstrate reliable isothermal switching via gate voltage pulses and all-electric readout at room temperature. As no ferromagnetic component is present in the system, the writing magnetic field does not need to be pulsed for readout, allowing permanent magnets to be used. Based on our prototypes, we construct a comprehensive model of the magnetoelectric selection mechanisms in thin films of magnetoelectric antiferromagnets, revealing misfit induced ferrimagnetism as an important factor. Beyond memory applications, the AF-MERAM concept introduces a general all-electric interface for antiferromagnets and should find wide applicability in antiferromagnetic spintronics.
Structure-property relationship of Co 2 MnSi thin films in response to He + -irradiation
We investigated the structure-property relationship of Co2MnSi Heusler thin films upon the irradiation with He+ ions. The variation of the crystal structure with increasing ion fluence has been probed using nuclear magnetic resonance (NMR) and transmission electron microscopy (TEM), and associated with the corresponding changes of the magnetic behavior. A decrease of both the structural order and the moment in saturation is observed. Specifically, we detect a direct transition from a highly L21-ordered to a fully A2-disordered structure type and quantify the evolution of the A2 structural contribution as a function of ion fluence. Complementary TEM analysis reveals a spatially-resolved distribution of the L21 and A2 phases showing that the A2 disorder starts at the upper part of the films. The structural degradation in turn leads to a decreasing magnetic moment in saturation in response to the increasing fluence.
Increasing the Diversity and Understanding of Semiconductor Nanoplatelets by Colloidal Atomic Layer Deposition
Nanoplatelets (NPLs) are a remarkable class of quantum confined materials with size-dependent optical properties, which are determined by the defined thickness of the crystalline platelets. To increase the variety of species, the colloidal atomic layer deposition method is used for the preparation of increasingly thicker CdSe NPLs. By growing further crystalline layers onto the surfaces of 4 and 5 monolayers (MLs) thick NPLs, species from 6 to 13 MLs are achieved. While increasing the thickness, the heavy-hole absorption peak shifts from 513 to 652 nm, leading to a variety of NPLs for applications and further investigations. The thickness and number of MLs of the platelet species are determined by high-resolution transmission electron microscopy (HRTEM) measurements, allowing the interpretation of several contradictions present in the NPL literature. In recent years, different assumptions are published, leading to a lack of clarity in the fundamentals of this field. Regarding the ongoing scientific interest in NPLs, there is a certain need for clarification, which is provided in this study. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Cobalt-based Co3Mo3N/Co4N/Co Metallic Heterostructure as a Highly Active Electrocatalyst for Alkaline Overall Water Splitting
Alkaline water electrolysis holds promise for large-scale hydrogen production, yet it encounters challenges like high voltage and limited stability at higher current densities, primarily due to inefficient electron transport kinetics. Herein, a novel cobalt-based metallic heterostructure (Co3Mo3N/Co4N/Co) is designed for excellent water electrolysis. In operando Raman experiments reveal that the formation of the Co3Mo3N/Co4N heterointerface boosts the free water adsorption and dissociation, increasing the available protons for subsequent hydrogen production. Furthermore, the altered electronic structure of the Co3Mo3N/Co4N heterointerface optimizes ΔGH of the nitrogen atoms at the interface. This synergistic effect between interfacial nitrogen atoms and metal phase cobalt creates highly efficient active sites for the hydrogen evolution reaction (HER), thereby enhancing the overall HER performance. Additionally, the heterostructure exhibits a rapid OH− adsorption rate, coupled with great adsorption strength, leading to improved oxygen evolution reaction (OER) performance. Crucially, the metallic heterojunction accelerates electron transport, expediting the afore-mentioned reaction steps and enhancing water splitting efficiency. The Co3Mo3N/Co4N/Co electrocatalyst in the water electrolyzer delivers excellent performance, with a low 1.58 V cell voltage at 10 mA cm−2, and maintains 100 % retention over 100 hours at 200 mA cm−2, surpassing the Pt/C RuO2 electrolyzer
Hollow Au@TiO2 porous electrospun nanofibers for catalytic applications
Catalytically active porous and hollow titania nanofibers encapsulating gold nanoparticles were fabricated using a combination of sol-gel chemistry and coaxial electrospinning technique. We report the fabrication of catalytically active porous and hollow titania nanofibers encapsulating gold nanoparticles (AuNPs) using a combination of sol-gel chemistry and coaxial electrospinning technique. The coaxial electrospinning involved the use of a mixture of poly(vinyl pyrrolidone) (PVP) and titania sol as the shell forming component, whereas a mixture of poly(4-vinyl pyridine) (P4VP) and pre-synthesized AuNPs constituted the core forming component. The core-shell nanofibers were calcined stepwise up to 600 °C which resulted in decomposition and removal of the organic constituents of the nanofibers. This led to the formation of porous and hollow titania nanofibers, where the catalytic AuNPs were embedded in the inner wall of the titania shell. The catalytic activity of the prepared Au@TiO2 porous nanofibers was investigated using a model reaction of catalytic reduction of 4-nitrophenol and Congo red dye in the presence of NaBH4. The Au@TiO2 porous and hollow nanofibers exhibited excellent catalytic activity and recyclability, and the morphology of the nanofibers remained intact after repeated usage. The presented approach could be a promising route for immobilizing various nanosized catalysts in hollow titania supports for the design of stable catalytic systems where the added photocatalytic activity of titania could further be of significance. This journal is © The Royal Society of Chemistry.