Majid Ahmadi

Mn‐Sb site mixing directly impacts both the magnetic and topological properties of MnSb ₂ Te ₄ [MST]. This study reveals that, contrary to previously believed, these anti‐sites can be unevenly distributed within the crystal. The use of High‐Resolution Scanning Transmission Electron Microscopy [STEM] combined with Energy‐Dispersive X‐ray Spectroscopy [EDS] allowed us to identify a local inversion‐breaking asymmetry in the anti‐site distribution within the septuple layers of a polycrystalline sample of MST. This reduced‐symmetry structure warrants further exploration due to its prospect for combining topology and magnetism with other effects, such as the Dzyaloshinskii–Moriya interaction [DMI], non‐linear optics, and piezoelectricity. Direct current [DC] magnetic measurements confirmed the well‐established ferrimagnetic properties of the compound, while alternating current [AC] magnetometry revealed a newly observed reentrant spin‐glass state, representing an important finding for defect‐driven magnetism in topological insulators. A simplified microscopic picture is presented in which antiferromagnetic pockets arise from Mn‐Te‐Mn super‐exchange interactions between Mn defects and Mn planes. The observations are supported by Density Functional Theory [DFT] calculations performed on simplified defect models, which highlight scenarios in which asymmetric site mixing can be energetically favored over symmetric arrangements.

Contamination of natural water resources by industrial organic pollutants poses serious environmental and health risks, creating a need for efficient and sustainable wastewater treatment technologies. TiO2 is widely regarded as an eco-friendly and low-cost photocatalyst, but it is normally used as a dispersed powder, which must be separated from the treated water. Immobilizing it as a thin film removes that step at the cost of a much smaller accessible surface, so the performance of such films depends strongly on their composition. In this study, ultrathin TiO2–CeO2 films containing 1, 2, and 3 wt % of CeO2 were prepared by sol–gel dip coating, and the influence of CeO2 content on their structure and photocatalytic performance was investigated. Electron microscopy and elemental mapping show distributed cerium, consistent with a TiO2–CeO2 nanocomposite. The degradation of methylene blue under UVA irradiation was used as a model reaction. The first-order rate constant passes through a maximum at 2 wt % CeO2, rising from 4.9 × 10–3 min–1 for pristine TiO2 films to 9.1 × 10–3 min–1, so that the ∼30 nm smooth film approaches the performance of Degussa P25 suspended nanopowder at equal catalyst loading. The TiO2–CeO2(2%) film retains its photoactivity over four consecutive cycles. These results show that sol–gel TiO2–CeO2 thin films are efficient and reusable immobilized photocatalysts for water purification.

Bestowing complementary metal-oxide semiconductor-compatible binary oxides with additional functionalities is a powerful strategy toward the realization of oxide electronics. Ideal candidates are thin films which display a strong sensitivity to strain, chemical doping or nanoscale confinement. Among these, crystalline tungsten trioxide WO₃ exhibits exceptional structural flexibility, enabling a wide range of functionalities. Here we reveal the emergence of a polar phase in epitaxial WO₃ thin films. We accomplish this by imposing epitaxial shear strain, which stabilizes a low-symmetry triclinic structure that persists up to large film thicknesses and elevated temperatures. At the atomic scale, a change in the oxygen octahedral tilt pattern facilitates this symmetry lowering into a polar phase, which manifests as a periodic in-plane polarized stripe domain configuration with needle-like bifurcations at the microscale. The stripe domain walls further exhibit a strongly enhanced electrical conductivity in conjunction with a pronounced reduction of a distortive structural mode, providing experimental evidence for anti-distortive polaronic transport recently predicted in WO₃.

Exsolved FeNi-based nanoparticles enhance the electrocatalytic properties of mixed ionic-electronic conducting perovskite fuel electrodes for ethanol reforming, resulting in a significant enhancement of the power generated by internal reforming SOFCs.

Memristor technology offers a promising route toward energy-efficient computing but faces challenges including resistance drift, variability, and the need for electroforming. Filamentary resistive random-access memory, one of the most studied memristive platforms, typically requires a high-voltage electroforming step to initiate conductive filaments, leading to increased power overhead and reduced endurance. Here we report HfO₂-based forming-free memristive devices (PdNeuRAM) that operate at low voltages, support multi-bit functionality, and exhibit reduced variability. Through combined electrical and materials characterization, we identify a Pd-O-Hf interfacial configuration that lowers oxygen-vacancy formation and migration barriers, creating a dense network of shallow defect states. Together with a Ti top electrode acting as an oxygen reservoir and an ultrathin (5 nm) HfO₂ layer, this interfacial engineering enables charge redistribution at room temperature and eliminates the need for electroforming. The fabricated devices provide tunable resistance states and reduce programming and read energy by 43% and 38%, respectively, in spiking neural network inference tasks. These results provide mechanistic insight into forming-free resistive switching and demonstrate the potential of Pd/HfO₂ devices for energy-efficient neuromorphic computing.

Mn-Sb site mixing directly impacts both the magnetic and topological properties of MnSb2Te4. This study reveals, unlike previously believed, that these anti-sites can be unevenly distributed within the crystal. To that end, a polycrystalline sample was created with a two-step synthesis using MnTe and Sb2Te3 as precursors. DC-SQUID magnetometry was used to confirm its magnetic properties. In addition, the use of High-Resolution Scanning Transmission Electron Microscopy combined with Energy-Dispersive X-ray Spectroscopy allowed us to identify the presence of an inversion-breaking asymmetry in the anti-site distribution. This reduced-symmetry structure bears resemblance to the recently proposed class of Janus materials and thus warrants further exploration due to its potential for combining topology and magnetism with other effects, such as non-linear optics and piezoelectricity. Finally, to further elucidate the interplay between site mixing, doping, topology, and magnetism, a method for growing MnSb2Te4 thin films over amorphous SiOx using Sb2Te3 seeds is introduced. The successful Van der Waals epitaxy of MnSb2Te4 over Sb2Te3 seeds using Pulsed Laser Deposition is confirmed using Scanning Transmission Electron Microscopy. This represents a crucial step in incorporating these materials into a Si-based architecture, which offers the possibility of controlling the Fermi lever via gating.

Highly crystalline ferroelectric oxides integrated on Si hold great promise for energy-efficient memory and logic technologies. Exploiting epitaxial strain engineering in these materials is, however, severely hampered on Si, where the large structural mismatch often results in an inferior interfacial quality and causes degradation of the ferroelectric switching characteristics. In this work, we present the growth of single-crystalline BaTiO₃ thin films on Si, exhibiting imprint-free switching, low coercivity, high remanent polarization, and no fatigue for over 10¹⁰ switching cycles. We accomplish this via the insertion of a SrSn_1-xTi_xO₃ layer on SrTiO₃-buffered Si. This layer serves as a pseudosubstrate that alleviates the thermal strain that the Si substrate imposes on the BaTiO₃ layer, while simultaneously providing moderate compressive strain that stabilizes a pure out-of-plane polarization. Thus, our work paves the way toward the fabrication of Si-compatible, low-power-consuming ferroelectric devices.

Ions play a fundamental role in solid-liquid interface processes, whether as essential or undesirable components, highlighting the need for precise and quantitative real-time monitoring. Electrochemical sensors are identified as promising tools, particularly for field-deployable applications. However, conventional electrochemical sensing is inherently restricted to redox-active species and is often single use, constraining its scope. This study presents electrochemical impedance spectroscopy as an alternative for ion detection, utilizing physico-chemical interactions at the electrode-electrolyte interface. We introduce a first-principles model that describes the interfacial impedance behavior and shows how ion specific processes shape the impedance response. Based on this framework, an extensive dataset is compiled, and a machine learning model is trained to predict electrolyte composition with consistent accuracy, demonstrating detection limits at the parts-per-billion level. The findings indicate that this method has considerable potential as a real-time method for ion sensing, providing a perspective on selectivity and sensitivity beyond traditional electrochemical approaches. This work could serve as a foundation for advanced models of impedance behavior, and development of impedance-based sensors with applicability in complex environments, including biological fluids and industrial liquids.