Weld-free mounting of lamellae for electrical biasing operando TEM

Recent advances in microelectromechanical systems (MEMS)-based substrates and sample holders for in situ transmission electron microscopy (TEM) are currently enabling exciting new opportunities for the nanoscale investigation of materials and devices. The ability to perform electrical testing while simultaneously capturing the wide spectrum of signals detectable in a TEM, including structural, chemical, and even electronic contrast, represents a significant milestone in the realm of nanoelectronics. In situ studies hold particular promise for the development of Metal-Insulator-Metal (MIM) devices for use in next-generation computing. However, achieving successful device operation in the TEM typically necessitates meticulous sample preparation involving focused ion beam (FIB) systems. Conducting contamination introduced during the FIB thinning process and subsequent attachment of the sample onto a MEMS-based chip remains a formidable challenge. This article delineates an improved FIB-based sample preparation methodology that results in good electrical connectivity and operational functionality across various MIM devices. To exemplify the efficacy of the sample preparation technique, we demonstrate preparation of a clean cross section extracted from a Au/Pt/BaSrTiO3/SrMoO3 tunable capacitor (varactor). The FIB-prepared TEM lamella mounted on a MEMS-based chip showed current levels in the tens of picoamperes range at 0.1 V. Furthermore, the electric response and current density of the TEM lamella device closely align with macro-scale devices. These samples exhibit comparable current densities to their macro-sized counterparts thus validating the sample preparation process and confirming device connectivity. The simultaneous operation and TEM characterization of electronic devices enabled by this process enables direct correlation between device structure and function, which could prove pivotal in the development of new MIM systems.

Texture Transfer in Dielectric Layers via Nanocrystalline Networks: Insights from in Situ 4D-STEM

Transition metal oxide dielectric layers have emerged as promising candidates for various relevant applications, such as supercapacitors or memory applications. However, the performance and reliability of these devices can critically depend on their microstructure, which can be strongly influenced by thermal processing and substrate-induced strain. To gain a more in-depth understanding of the microstructural changes, we conducted in situ transmission electron microscopy (TEM) studies of amorphous HfO2 dielectric layers grown on highly textured (111) substrates. Our results indicate that the minimum required phase transition temperature is 180 °C and that the developed crystallinity is affected by texture transfer. Using in situ TEM and 4D-STEM can provide valuable insights into the fundamental mechanisms underlying the microstructural evolution of dielectric layers and could pave the way for the development of more reliable and efficient devices for future applications.

Translational Antiphase Boundaries in NaNbO3 Antiferroelectrics

Planar defects are known to be of importance in affecting the functional properties of materials. Translational antiphase boundaries (APBs) in particular have attracted considerable attention in perovskite oxides, but little is known in lead-free antiferroelectric oxides that are promising candidates for energy storage applications. Here, we present a study of translational APBs in prototypical antiferroelectric NaNbO3 using aberration-corrected (scanning) transmission electron microscopy (TEM) techniques at different length scales. The translational APBs in NaNbO3 are characterized by a 2-fold-modulated structure, which is antipolar in nature and exhibits a high density, different from the polar nature and lower density in PbZrO3. The high stability of translational APBs against external electric fields and elevated temperatures was revealed using ex situ and in situ TEM experiments and is expected to be associated with their antipolar nature. Density functional theory calculations demonstrate that translational APBs possess only slightly higher free energy than the antiferroelectric and ferroelectric phase energies with differences of 29 and 33 meV/f.u., respectively, justifying their coexistence down to the nanoscale at room temperature. These results provide a detailed atomistic elucidation of translational APBs in NaNbO3 with antipolar character and stability against external stimuli, establishing the basis of defect engineering of antiferroelectrics for energy storage devices.

Controlling the Formation of Conductive Pathways in Memristive Devices

Resistive random-access memories are promising candidates for novel computer architectures such as in-memory computing, multilevel data storage, and neuromorphics. Their working principle is based on electrically stimulated materials changes that allow access to two (digital), multiple (multilevel), or quasi-continuous (analog) resistive states. However, the stochastic nature of forming and switching the conductive pathway involves complex atomistic defect configurations resulting in considerable variability. This paper reveals that the intricate interplay of 0D and 2D defects can be engineered to achieve reproducible and controlled low-voltage formation of conducting filaments. The author find that the orientation of grain boundaries in polycrystalline HfO_x is directly related to the required forming voltage of the conducting filaments, unravelling a neglected origin of variability. Based on the realistic atomic structure of grain boundaries obtained from ultra-high resolution imaging combined with first-principles calculations including local strain, this paper shows how oxygen vacancy segregation energies and the associated electronic states in the vicinity of the Fermi level govern the formation of conductive pathways in memristive devices. These findings are applicable to non-amorphous valence change filamentary type memristive device. The results demonstrate that a fundamental atomistic understanding of defect chemistry is pivotal to design memristors as key element of future electronics.

Structural and Electrical Response of Emerging Memories Exposed to Heavy Ion Radiation

Hafnium oxide- and GeSbTe-based functional layers are promising candidates in material systems for emerging memory technologies. They are also discussed as contenders for radiation-harsh environment applications. Testing the resilience against ion radiation is of high importance to identify materials that are feasible for future applications of emerging memory technologies like oxide-based, ferroelectric, and phase-change random-access memory. Induced changes of the crystalline and microscopic structure have to be considered as they are directly related to the memory states and failure mechanisms of the emerging memory technologies. Therefore, we present heavy ion irradiation-induced effects in emerging memories based on different memory materials, in particular, HfO₂-, HfZrO₂-, as well as GeSbTe-based thin films. This study reveals that the initial crystallinity, composition, and microstructure of the memory materials have a fundamental influence on their interaction with Au swift heavy ions. With this, we provide a test protocol for irradiation experiments of hafnium oxide- and GeSbTe-based emerging memories, combining structural investigations by X-ray diffraction on a macroscopic, scanning transmission electron microscopy on a microscopic scale, and electrical characterization of real devices. Such fundamental studies can be also of importance for future applications, considering the transition of digital to analog memories with a multitude of resistance states.

Structural and Electrical Response of Emerging Memories Exposed to Heavy Ion Radiation

Hafnium oxide- and GeSbTe-based functional layers are promising candidates in material systems for emerging memory technologies. They are also discussed as contenders for radiation-harsh environment applications. Testing the resilience against ion radiation is of high importance to identify materials that are feasible for future applications of emerging memory technologies like oxide-based, ferroelectric, and phase-change random-access memory. Induced changes of the crystalline and microscopic structure have to be considered as they are directly related to the memory states and failure mechanisms of the emerging memory technologies. Therefore, we present heavy ion irradiation-induced effects in emerging memories based on different memory materials, in particular, HfO₂-, HfZrO₂-, as well as GeSbTe-based thin films. This study reveals that the initial crystallinity, composition, and microstructure of the memory materials have a fundamental influence on their interaction with Au swift heavy ions. With this, we provide a test protocol for irradiation experiments of hafnium oxide- and GeSbTe-based emerging memories, combining structural investigations by X-ray diffraction on a macroscopic, scanning transmission electron microscopy on a microscopic scale, and electrical characterization of real devices. Such fundamental studies can be also of importance for future applications, considering the transition of digital to analog memories with a multitude of resistance states.

Enhanced Conductivity and Microstructure in Highly Textured TiN1-x /c-Al2O3 Thin Films

Titanium nitride thin films are used as an electrode material in superconducting (SC) applications and in oxide electronics. By controlling the defect density in the TiN thin film, the electrical properties of the film can achieve low resistivities and a high critical temperature (T c) close to bulk values. Generally, low defect densities are achieved by stoichiometric growth and a low grain boundary density. Due to the low lattice mismatch of 0.7%, the best performing TiN layers are grown epitaxially on MgO substrates. Here, we report for the first time a T c of 4.9 K for ultrathin (23 nm), highly textured (111), and stoichiometric TiN films grown on 8.75% lattice mismatch c-cut Al2O3 (sapphire) substrates. We demonstrate that with the increasing nitrogen deficiency, the (111) lattice constant increases, which is accompanied by a decrease in T c. For highly N deficient TiN thin films, no superconductivity could be observed. In addition, a dissociation of grain boundaries (GBs) by the emission of stacking faults could be observed, indicating a combination of two sources for electron scattering defects in the system: (a) volume defects created by nitrogen deficiency and (b) defects created by the presence of GBs. For all samples, the average grain boundary distance is kept constant by a miscut of the c-cut sapphire substrate, which allows us to distinguish the effect of nitrogen deficiency and grain boundary density. These properties and surface roughness govern the electrical performance of the films and influence the compatibility as an electrode material in the respective application. This study aims to provide detailed and scale-bridging insights into the structural and microstructural response to nitrogen deficiency in the c-Al2O3/TiN system, as it is a promising candidate for applications in state-of-the-art systems such as oxide electronic thin film stacks or SC applications.

Defect-Stabilized Substoichiometric Polymorphs of Hafnium Oxide with Semiconducting Properties

Hafnium oxide plays an important role as a dielectric material in various thin-film electronic devices such as transistors and resistive or ferroelectric memory. The crystallographic and electronic structure of the hafnia layer often depends critically on its composition and defect structure. Here, we report two novel defect-stabilized polymorphs of substoichiometric HfO_2-x with semiconducting properties that are of particular interest for resistive switching digital or analog memory devices. The thin-film samples are synthesized by molecular beam epitaxy with oxygen engineering that allows us to cover the whole range of metallic Hf with oxygen interstitials to HfO₂. The crystal and defect structures, in particular of a cubic low-temperature phase c-HfO_1.7 and a hexagonal phase hcp-HfO_0.7 are identified by X-ray diffraction, in vacuo electron spectroscopic, and transmission electron microscopic methods. With the help of UV/Vis transmission data, we propose a consistent band structure model for the whole oxidation range involving oxygen vacancy-induced in-gap defect states. Our comprehensive study of engineered hafnia thin films has an impact on the design of resistive memory devices and can be transferred to chemically similar suboxide systems.

Epitaxy Induced Highly Ordered Sm2Co17-SmCo5 Nanoscale Thin-Film Magnets

Utilizing the molecular beam epitaxy technique, a nanoscale thin-film magnet of c-axis-oriented Sm₂Co₁₇ and SmCo₅ phases is stabilized. While typically in the prototype Sm(Co, Fe, Cu, Zr)_7.5-8 pinning-type magnets, an ordered nanocomposite is formed by complex thermal treatments, here, a one-step approach to induce controlled phase separation in a binary Sm-Co system is shown. A detailed analysis of the extended X-ray absorption fine structure confirmed the coexistence of Sm₂Co₁₇ and SmCo₅ phases with 65% Sm₂Co₁₇ and 35% SmCo₅. The SmCo₅ phase is stabilized directly on an Al₂O₃ substrate up to a thickness of 4 nm followed by a matrix of Sm₂Co₁₇ intermixed with SmCo₅. This structural transition takes place through coherent atomic layers, as revealed by scanning transmission electron microscopy. Highly crystalline growth of well-aligned Sm₂Co₁₇ and SmCo₅ phases with coherent interfaces result in strong exchange interaction, leading to enhanced magnetization and magnetic coupling. The arrangement of Sm₂Co₁₇ and SmCo₅ phases at the nanoscale is reflected in the observed magnetocrystalline anisotropy and coercivity. As next-generation permanent magnets require designing of materials at an atomic level, this work enhances our understanding of self-assembling and functioning of nanophased magnets and contributes to establishing new concepts to engineer the microstructure for beyond state-of-the-art magnets.

Production of Fe nanoparticles from γ-Fe2O3 by high-pressure hydrogen reduction

In this work, the reduction of iron oxide γ-Fe₂O₃ nanoparticles by hydrogen at high pressures is studied. Increasing the hydrogen pressure enables reduction of γ-Fe₂O₃ to α-Fe at significantly lower temperatures. At low pressures, a temperature of 390 °C is necessary whereas at 530 bar complete reduction can be realized at temperatures as low as 210 °C. This leads to significant improvement in the final particle morphology, maintaining high surface-to-volume ratio of the nanoparticles with an average size of 47 ± 5 nm which is close to that of the precursor γ-Fe₂O₃. Neck formation, coalescence and growth during reduction can be significantly suppressed. Investigations of magnetic properties show that saturation magnetization of the reduced α-Fe nanoparticles decreases with particle size from 209 A m² kg^-1 at 390 °C reduction temperature to 204 A m² kg^-1 at 210 °C. Coercivity for the fine iron particles reaches 0.076 T which exceeds the theoretical anisotropy field. This is attributed to nano-scale surface effects.

Photothermal catalytic properties of layered titanium chalcogenide nanomaterials

Titanium chalcogenides are valuable candidates for visible light photocatalysis at high efficiency levels. TiS₂/TiO₂ core shell heterostructures are able to increase this efficiency by an effective quenching of the exiton recombination.

Conformal Solution Deposition of Pt-Pd Titania Nanocomposite Coatings for Light-Assisted Formic Acid Electro-Oxidation

Many nanofabrication processes require sophisticated equipment, elevated temperature, vacuum or specific atmospheric conditions, templates, and exotic chemicals, which severely hamper their implementation in real-world applications. In this study, we outline a fully wet-chemical procedure for equipping a 3D carbon felt (CF) substrate with a multifunctional, titania nanospike-supported Pt-Pd nanoparticle (Pt-Pd-TiO₂@CF) layer in a facile and scalable manner. The nanostructure, composition, chemical speciation, and formation of the material was meticulously investigated, evidencing the conformal coating of the substrate with a roughened layer of nanocrystalline rutile spikes by chemical bath deposition from Ti³⁺ solutions. The spikes are densely covered by bimetallic nanoparticles of 4.4 ± 1.1 nm in size, which were produced by autocatalytic Pt deposition onto Pd seeds introduced by Sn²⁺ ionic layer adsorption and reaction. The as-synthesized nanocomposite was applied to the (photo)electro-oxidation of formic acid (FA), exhibiting a superior performance compared to Pt-plated, Pd-seeded CF (Pt-Pd@CF) and commercial Pt-C, indicating the promoting electrocatalytic role of the TiO₂ support. Upon UV-Vis illumination, the performance of the Pt-Pd-TiO₂@CF electrode is remarkably increased (22-fold), generating a current density of 110 mA cm^-2, distinctly outperforming titania-free Pt-Pd@CF (5 mA cm^-2) and commercial Pt-C (6 mA cm^-2) reference catalysts. In addition, the Pt-Pd-TiO₂@CF showed a much better stability, characterized by a very high poisoning tolerance for in situ-generated CO intermediates, whose formation is hindered in the presence of TiO₂. This overall performance boost is attributed to a dual enhancement mechanism (∼30% electrocatalytic and ∼70% photoelectrocatalytic). The photogenerated electrons from the TiO₂ conduction band enrich the electron density of the Pt nanoparticles, promoting the generation of active oxygen species on their surfaces from adsorbed oxygen and water molecules, which facilitate the direct FA electro-oxidation into CO₂.

Enabling nanoscale flexoelectricity at extreme temperature by tuning cation diffusion

Any dielectric material under a strain gradient presents flexoelectricity. Here, we synthesized 0.75 sodium bismuth titanate −0.25 strontium titanate (NBT-25ST) core–shell nanoparticles via a solid-state chemical reaction directly inside a transmission electron microscope (TEM) and observed domain-like nanoregions (DLNRs) up to an extreme temperature of 800 °C. We attribute this abnormal phenomenon to a chemically induced lattice strain gradient present in the core–shell nanoparticle. The strain gradient was generated by controlling the diffusion of strontium cations. By combining electrical biasing and temperature-dependent in situ TEM with phase field simulations, we analyzed the resulting strain gradient and local polarization distribution within a single nanoparticle. The analysis confirms that a local symmetry breaking, occurring due to a strain gradient (i.e. flexoelectricity), accounts for switchable polarization beyond the conventional temperature range of existing polar materials. We demonstrate that polar nanomaterials can be obtained through flexoelectricity at extreme temperature by tuning the cation diffusion.

The limited number of materials with a switchable electrical polarization available for applications can be increased by exploiting the flexoelectric effect. Here, switchable polarization in nanoparticles induced by an elemental distribution dependent strain gradient up to 800 °C is demonstrated.

Temperature-Dependent Evolution of Crystallographic and Domain Structures in (K,Na,Li)(Ta,Nb)O3 Piezoelectric Single Crystals

(K,Na)NbO3-based ferroelectric single crystals have recently undergone a substantial development, resulting in improved crystal quality and large piezoelectric coefficients, exceeding 700 pC/N, over a broad temperature range. However, further development necessitates a detailed understanding of the mechanisms defining the domain structure and its temperature evolution. This paper presents the investigation into the crystallographic structure and domain configurations of a (K,Na,Li)(Ta,Nb)O3 single crystal over a broad temperature range. The crystal was grown by the submerged-seed solution growth technique and investigated using in situ transmission electron microscopy, X-ray diffraction, dielectric measurements, and polarized light microscopy. The lattice distortion, structural phase transitions, and domain configurations are reported. A transition from the lamellar orthorhombic to the rectangular tetragonal domain structure is observed upon heating. Moreover, the milky optical appearance of the crystal was investigated and found to result from the presence of regions with different domain configurations and domain sizes. The formation of these regions is related to the growth defects, which govern the domain formation when cooling below the Curie temperature.

FIB based fabrication of an operative Pt/HfO2/TiN device for resistive switching inside a transmission electron microscope

Recent advances in microelectromechanical systems (MEMS) based chips for in situ transmission electron microscopy are opening exciting new avenues in nanoscale research. The capability to perform current-voltage measurements while simultaneously analyzing the corresponding structural, chemical or even electronic structure changes during device operation would be a major breakthrough in the field of nanoelectronics. In this work we demonstrate for the first time how to electrically contact and operate a lamella cut from a resistive random access memory (RRAM) device based on a Pt/HfO₂/TiN metal-insulator-metal (MIM) structure. The device was fabricated using a focused ion beam (FIB) instrument and an in situ lift-out system. The electrical switching characteristics of the electron-transparent lamella were comparable to a conventional reference device. The lamella structure was initially found to be in a low resistance state and could be reset progressively to higher resistance states by increasing the positive bias applied to the Pt anode. This could be followed up with unipolar set/reset operations where the current compliance during set was limited to 400 µA. FIB structures allowing to operate and at the same time characterize electronic devices will be an important tool to improve RRAM device performance based on a microstructural understanding of the switching mechanism.