Synaptic and fast switching memristance in porous silicon-based structures

Memristors are two terminal electronic components whose conductance depends on the amount of charge that has flown across them over time. This dependence can be gradual, such as in synaptic memristors, or abrupt, as in resistive switching memristors. Either of these memory effects are very promising for the development of a whole new generation of electronic devices. For the successful implementation of practical memristors, however, the development of low cost industry compatible memristive materials is required. Here the memristive properties of differently processed porous silicon structures are presented, which are suitable for different applications. Electrical characterization and SPICE simulations show that laser-carbonized porous silicon shows a strong synaptic memristive behavior influenced by defect diffusion, while wet-oxidized porous silicon has strong resistance switching properties, with switching ratios over 8000. Results show that practical memristors of either type can be achieved with porous silicon whose memristive properties can be adjusted by the proper material processing. Thus, porous silicon may play an important role for the successful realization of practical memristorics with cost-effective materials and processesThis work is part of ATTRACT that has received funding from the European Union’s Horizon 2020 Research and Innovation Programm

Improved long term cycling of polyazulene/reduced graphene oxide composites fabricated in a choline based ionic liquid

To improve the energy density of supercapacitors, novel electronically conducting polymers should be introduced to the research field. Polyazulene is a well-suitable candidate as it exhibits good capacitive behavior both in organic solvents as well as in various ionic liquids, but especially its long term cycling stability should be improved. Previously, enhanced properties have been obtained by combining conducting polymers with carbon nanomaterials to fabricate composites. This work presents an ionic liquid assisted electrochemical polymerization and characterization of polyazulene-reduced graphene oxide composites. The ionic liquid of our choice is choline-based liquid salt. We prepared stable dispersions of graphene oxide in this ionic liquid and performed potentiodynamic electropolymerization of azulene in the mixture. Changing the concentration of graphene oxide between 0.1 and 2 mg mL(-1) had no remarkable effect on the polymerization or electrochemical behavior of the composite materials. The composites exhibit higher capacitances compared to neat polymer films determined by cyclic voltammetry and electrochemical impedance spectroscopy. The obtained films also exhibit excellent cycling stabilities retaining over 90% of their initial capacitance with tendency towards improved cycling stability when combined with reduced graphene oxide. The successful incorporation and reduction of graphene oxide was determined by several spectroscopic techniques

Oxygen adsorption on (100) surfaces in Fe-Cr alloys

The adsorption of oxygen on bcc Fe-Cr(100) surfaces with two different alloy concentrations is studied using ab initio density functional calculations. Atomic-scale analysis of oxygen-surface interactions is indispensable for obtaining a comprehensive understanding of macroscopic surface oxidation processes. Up to two chromium atoms are inserted into the first two surface layers. Atomic geometries, energies and electronic properties are investigated. A hollow site is found to be the preferred adsorption site over bridge and on-top sites. Chromium atoms in the surface and subsurface layers are found to significantly affect the adsorption properties of neighbouring iron atoms. Seventy-one different adsorption geometries are studied, and the corresponding adsorption energies are calculated. Estimates for the main diffusion barriers from the hollow adsorption site are given. Whether the change in the oxygen affinity of iron atoms can be related to the chromium-induced charge transfer between the surface atoms is discussed. The possibility to utilize the presented theoretical results in related experimental research and in developing semiclassical potentials for simulating the oxidation of Fe-Cr alloys is addressed.Peer reviewe

Interatomic Fe-Cr potential for modeling kinetics on Fe surfaces

To enable accurate molecular dynamics simulations of iron-chromium alloys with surfaces, we develop, based on density-functional-theory (DFT) calculations, a new interatomic Fe-Cr potential in the Tersoff formalism. Contrary to previous potential models, which have been designed for bulk Fe-Cr, we extend our potential fitting database to include not only conventional bulk properties but also surface-segregation energies of Cr in bcc Fe. In terms of reproducing our DFT results for the bulk properties, the new potential is found to be superior to the previously developed Tersoff potential and competitive with the concentration-dependent and two-band embedded-atom-method potentials. For Cr segregation toward the (100) surface of an Fe-Cr alloy, only the new potential agrees with our DFT calculations in predicting preferential segregation of Cr to the topmost surface layer, instead of the second layer preferred by the other potentials. We expect this rectification to foster future research, e.g., on the mechanisms of corrosion resistance of stainless steels at the atomic level.Peer reviewe

Refined Sr2FeMoO6 interface realized with photoemission and magnetization analysis

We investigate the effects of post-annealing on a set of identically deposited Sr2FeMoO6 (SFMO) thin films. The annealing provides a significant enhancement to ferrimagnetic exchange, evidenced by almost 40 K increase in Curie temperature. The electronic nature of the film surface is our focus of interest. The detailed analysis on oxygen 1s, strontium 3d, molybdenum 3d spectrum along with changes in the carbon contamination show purification from additional phases and significant valence fluctuation. Successful treatment provides films with higher Curie temperature and diminished surface contamination. Annealing therefore provides a possible method of reobtaining a portion of the valuable bulk attributes at interfaces in e.g. spin valve structures.</p

Interface characterization of Co2MnGe/Rh2CuSn Heusler multilayers

All-Heusler multilayer structures have been investigated by means of high kinetic x-ray photoelectron spectroscopy and x-ray magnetic circular dichroism, aiming to address the amount of disorder and interface diffusion induced by annealing of the multilayer structure. The studied multilayers consist of ferromagnetic Co$_2$MnGe and non-magnetic Rh$_2$CuSn layers with varying thicknesses. We find that diffusion begins already at comparably low temperatures between 200 $^{\circ}$C and 250 $^{\circ}$C, where Mn appears to be most prone to diffusion. We also find evidence for a 4 {\AA} thick magnetically dead layer that, together with the identified interlayer diffusion, are likely reasons for the small magnetoresistance found for current-perpendicular-to-plane giant magneto-resistance devices based on this all-Heusler system

Oxygen adsorption on (100) surfaces in Fe-Cr alloys

The adsorption of oxygen on bcc Fe-Cr(100) surfaces with two different alloy concentrations is studied using ab initio density functional calculations. Atomic-scale analysis of oxygen-surface interactions is indispensable for obtaining a comprehensive understanding of macroscopic surface oxidation processes. Up to two chromium atoms are inserted into the first two surface layers. Atomic geometries, energies and electronic properties are investigated. A hollow site is found to be the preferred adsorption site over bridge and on-top sites. Chromium atoms in the surface and subsurface layers are found to significantly affect the adsorption properties of neighbouring iron atoms. Seventy-one different adsorption geometries are studied, and the corresponding adsorption energies are calculated. Estimates for the main diffusion barriers from the hollow adsorption site are given. Whether the change in the oxygen affinity of iron atoms can be related to the chromium-induced charge transfer between the surface atoms is discussed. The possibility to utilize the presented theoretical results in related experimental research and in developing semiclassical potentials for simulating the oxidation of Fe-Cr alloys is addressed

Interatomic Fe-Cr potential for modeling kinetics on Fe surfaces

To enable accurate molecular dynamics simulations of iron-chromium alloys with surfaces, we develop, based on density-functional-theory (DFT) calculations, a new interatomic Fe-Cr potential in the Tersoff formalism. Contrary to previous potential models, which have been designed for bulk Fe-Cr, we extend our potential fitting database to include not only conventional bulk properties but also surface-segregation energies of Cr in bcc Fe. In terms of reproducing our DFT results for the bulk properties, the new potential is found to be superior to the previously developed Tersoff potential and competitive with the concentration-dependent and two-band embedded-atom-method potentials. For Cr segregation toward the (100) surface of an Fe-Cr alloy, only the new potential agrees with our DFT calculations in predicting preferential segregation of Cr to the topmost surface layer, instead of the second layer preferred by the other potentials. We expect this rectification to foster future research, e.g., on the mechanisms of corrosion resistance of stainless steels at the atomic level

Role of the Deposition Distance on Nanorod Growth and Flux Pinning in BaZrO3-Doped YBa2Cu3O6+x Thin Films: Implications for Superconducting Tapes

A complex deposition process of high-temperature superconducting (HTS) thin films and coated conductors is usually optimized by concentrating on the crystalline quality of the material, thus getting the best possible critical temperature and self-field properties. However, most of the HTS power applications that are based on coated conductors act at high magnetic fields, and thus an alternative approach focusing on the formation of an optimal network of columnar flux pinning centers is more reasonable. Therefore, we systematically show how a lengthening of the deposition distance produces perfectly aligned and distinctly longer self-assembled BaZrO3 (BZO) nanorods within the YBa2Cu3O6+x (YBCO) matrix. This method unambiguously enhances in-field properties such as pinning force, critical current density, and its isotropy along the YBCO c axis. The experimental results, especially formation of the c peak where the relative length of the nanorod is a key issue, are confirmed by the vortex dynamics simulations. Finally, we present a semiquantitative model governing the formation of nanorods that explains the experimentally observed improved nanorod growth as a function of the deposition distance via the associated variation of the fractional partial pressure between atomic species within the laser plume. </p