Advanced Transmission Electron Microscopy Research Articles

Investigation of AlGaN UV emitting tunnel junction LED devices by off-axis electron holography.

Here we use off-axis electron holography combined with advanced transmission electron microscopy techniques to understand the opto-electronic properties of AlGaN Tunnel Junction-LED devices for UV emission. Four identical AlGaN LED devices emitting at 290 nm have been grown by metal-organic chemical vapour deposition. Then Ge doped n-type regions with and without InGaN or GaN interlayers have been grown by molecular beam epitaxy onto the top Mg doped p-type layer to form a tunnel junction and hence a high quality ohmic metal contact. Off-axis electron holography has then been used to demonstrate a reduction in the width of the tunnel junction from 9.5 to 4.1 nm when an InGaN interlayers are used. As such we demonstrate that off-axis electron holography can be used to reproducibly measure nm-scale changes in electrostatic potential in highly defected and challenging materials such as AlGaN and that systematic studies of devices can be performed. The LED devices are then characterized using standard opto-electric techniques and the improvements in the performance of the LEDs are correlated with the electron holography results.

Fluctuations in misfit volume by interstitial carbon atoms contribute to the unusual dislocation loop evolution in high-entropy alloys under irradiation

Dislocation loops play a pivotal role for the irradiation behavior and associated mechanical properties of materials. This study employs advanced transmission electron microscopy, nanoindentation and first-principles calculations to elucidate the impact of interstitial carbon atoms on the evolution of dislocation loops in an Fe49.5Mn30Co10Cr10C0.5 high-entropy alloy under ion irradiation at temperatures ranging from 420 °C to 540 °C with a peak dose of 50 DPA. Interstitial carbon atoms have been revealed to broaden the size distribution of dislocation loops, rendering their morphologies more jagged, and consequently inducing a more substantial pinning effect. These outcomes can be attributed to the enhancement of misfit volume fluctuations of the alloying elements, which drives more severe local lattice distortions and larger variation of local chemical environments. As a result, these fluctuations promote the local pinning of dislocation loops, lead to their rugged movement and contribute to the rise in hardening. Our findings bring previously unrecognized insights into the role of interstitial carbon atoms in influencing the nature of dislocation loops in high-entropy alloys, and provide important clues to the design of alloys for nuclear applications.

Crystallization-driven tuneable lasing of perylene doped into the nematic liquid crystal

Versatile devices with tunable capabilities for controlling lasing wavelength and intensity are in high demand. Liquid crystals (LCs) exhibit immense potential for such applications, offering fine-tuning possibilities through external factors. On the other hand, laser technology is currently a research hotspot in optoelectronics, and also in practical applications. The recent market introduction of laser television marks a significant stride toward making such advanced technology accessible in every household. In synergy with laser pumping, the LCs unquestionably can be rediscovered. This study presents a comprehensive investigation into the crystallization phenomenon within a host-guest device, compact in size, free from moving parts, and integrating the liquid crystalline (LC) matrix doped with 3,4,9,10-tetra-(3-alcoxy-carbonyl)-perylene (THCP) dye. The focus lies in examining the influence of varying dye concentrations on multicolor fluorescence, lasing behavior, and device morphology. The systematic analysis of Random Lasing (RL) energy thresholds and the impact of DC voltage on light intensity modulation is demonstrated. Morphological changes were monitored in real-time using optical microscopy techniques, including crossed polarizer, and fluorescence imaging under 450 nm excitation. Utilizing advanced Transmission Electron Microscopy (TEM) techniques, we explore exceptional insights into our set of devices, providing novel information about the THCP crystallization process for the first time in the literature. To gain a comprehensive understanding of the crystal forming and molecular geometry we examined additionally the THCP dye, using X-ray diffraction and Raman spectroscopy. Furthermore, we showcase that varying the pumping energy enables multicolor tuning in the fabricated systems, presenting an attractive feature in the context of laser display technologies.

Microstructure and optical properties of β-Ga2O3 thin films fabricated by pulsed laser deposition

β-Ga2O3 has attracted extensive attention in the fields of optoelectronics and high-power electric devices due to its excellent electrical properties and thermal stability. Growth of epitaxial β-Ga2O3 films with good crystallinity is challenging since it is difficult to effectively control the impurities in the films. Here, β-Ga2O3 thin films were epitaxially grown on MgO (100) substrates via pulsed laser deposition system. The microstructure and optical properties of the β-Ga2O3 thin films were systematically characterized by combining high-resolution X-ray diffraction, X-ray photoelectron spectroscopy, ultraviolet–visible spectrophotometer and advanced transmission electron microscopy. Effects of the growth temperature and O2 partial pressure on the crystallinity, valance states of Ga ions and band gap of the β-Ga2O3 thin films were investigated. The light absorption wavelength of the β-Ga2O3 thin films ranges from 220 nm to 260 nm, which is close to the theoretical value. The epitaxial orientation between β-Ga2O3 film and MgO substrate is β-Ga2O3 (100) [02¯1] // MgO (100) [010]. A thin layer of γ-Ga2O3 exists in the interfacial region.

Strain-modulated Mn-rich layered oxide enables highly stable potassium-ion batteries

Mn-rich layered oxides show great promise as cathode materials for potassium-ion batteries due to their high capacity and cost-effectiveness. However, internal structural strain and irreversible phase transitions caused by Jahn-Teller distortion affect their cycling stability. Here, we present an efficient strategy to concurrently modulate the internal strain and suppress the irreversible phase transition in Mn-rich cathodes by incorporating amounts of zinc (Zn) ions into the transition metal layers. The substituted Zn serves to regulate local chemistry, thereby mitigating octahedral distortion in Mn-O bonds, relieving the strain between layers, and reducing the occurrence of P3-O3 phase transition under high voltages. These findings are supported by EXAFS, in situ X-ray diffraction, advanced transmission electron microscopy, electron tomography, and DFT simulations. The low-strain K0.5Mn0.8Co0.1Zn0.1O2 electrode exhibits a high reversible capacity retention about ∼90 % (105 mAh g−1 at 100 mA g−1), exceptional rate performance in the voltage of 1.5 ∼ 3.9 V, and a substantial capacity retention after 500 cycles. This strain-relieved approach broadens the scope of lattice engineering by addressing internal strain concerns and mitigating the strain associated with potassium (de)intercalation, thereby having potential to advance the development of stable cathodes for PIBs.

Graphene nanowalls formation investigated by Electron Energy Loss Spectroscopy

The properties of layered materials are significantly dependent on their lattice orientations. Thus, the growth of graphene nanowalls (GNWs) on Cu through PECVD has been increasingly studied, yet the underlying mechanisms remain unclear. In this study, we examined the GNWs/Cu interface and investigated the evolution of their microstructure using advanced Scanning transmission electron microscopy and Electron Energy Loss Spectroscopy (STEM-EELS). GNWs interface and initial root layers of comprise graphitic carbon with horizontal basal graphene (BG) planes that conform well to the catalyst surface. In the vertical section, the walls show a mix of graphitic and turbostratic carbon, while the latter becomes more noticeable close to the top edges of the GMWs film. Importantly, we identified growth process began with catalysis at Cu interface forming BG, followed by defect induction and bending at ‘coalescence points’ of neighboring BG, which act as nucleation sites for vertical growth. We reported that although classical thermal CVD mechanism initially dominates, growth of graphene later deviates a few nanometers from the interface to form GNWs. Nascent walls are no longer subjected to the catalytic action of Cu, and their development is dominated by the stitching of charged carbon species originating in the plasma with basal plane edges.

Atomistic origin of high grain boundary resistance in solid electrolyte lanthanum lithium titanate

Lanthanum lithium titanate is one of the promising electrolytes for solid-state lithium-ion batteries due to its high bulk ionic conductivity up to ∼10−3 S/cm. However, the practical application of this material has been bottlenecked by high grain boundary (GB) resistance, while the underlying mechanism is still under debate. Here we report a comprehensive study with direct evidence to reveal the origin of high GB resistance in La2/3–xLi3xTiO3 (LLTO). Atomic-scale observations via advanced scanning transmission electron microscopy show that the GBs are uniformly subject to subsurface segregation of La atoms to compensate for the excess surface charges. The La segregation leads to an abrupt decrease of charge carrier concentration neighboring GBs and hence is supposed to have deleterious effect on GB conductivity. The findings suggest a novel mechanism of space-charge-induced cation segregation, which shed lights on the intrinsic origin of low GB ionic conductivity in LLTO.

Unveiling the high-temperature degradation mechanism of solid oxide electrolysis cells through direct imaging of nanoscale interfacial phenomena

Advanced transmission electron microscopy analysis uncovers the fundamental mechanisms behind nanometer-scale interfacial degradation phenomena in high-temperature solid oxide electrolysis cells.

Probing Oxidation-Driven Amorphized Surfaces in a Ta(110) Film for Superconducting Qubit.

Recent advances in superconducting qubit technology have led to significant progress in quantum computing, but the challenge of achieving a long coherence time remains. Despite the excellent lifetime performance that tantalum (Ta) based qubits have demonstrated to date, the majority of superconducting qubit systems, including Ta-based qubits, are generally believed to have uncontrolled surface oxidation as the primary source of the two-level system loss in two-dimensional transmon qubits. Therefore, atomic-scale insight into the surface oxidation process is needed to make progress toward a practical quantum processor. In this study, the surface oxidation mechanism of native Ta films and its potential impact on the lifetime of superconducting qubits were investigated using advanced scanning transmission electron microscopy (STEM) techniques combined with density functional theory calculations. The results suggest an atomistic model of the oxidized Ta(110) surface, showing that oxygen atoms tend to penetrate the Ta surface and accumulate between the two outermost Ta atomic planes; oxygen accumulation at the level exceeding a 1:1 O/Ta ratio drives disordering and, eventually, the formation of an amorphous Ta2O5 phase. In addition, we discuss how the formation of a noninsulating ordered TaO1-δ (δ < 0.1) suboxide layer could further contribute to the losses of superconducting qubits. Subsurface oxidation leads to charge redistribution and electric polarization, potentially causing quasiparticle loss and decreased current-carrying capacity, thus affecting superconducting qubit coherence. The findings enhance the comprehension of the realistic factors that might influence the performance of superconducting qubits, thus providing valuable guidance for the development of future quantum computing hardware.

(Invited) Understanding PEMFC by Advanced Electron Microscopy

A thorough knowledge of the atomic structure and composition of catalyst nanoparticles is paramount to the development of advanced materials for proton exchange membrane fuel cells (PEMFC), one of the most promising energy conversion devices for automotive and stationary applications. Pt and Pt-based alloys nanoparticles (NPs) are currently used as the catalyst to promote the kinetics of the hydrogen oxidation and oxygen reduction reactions in the anode and cathode of the fuel cell, respectively. Yet, the durability of the catalysts remains a major issue for their commercialization. In addition, it is essential to characterize the ionomer distribution and coverage over the catalyst NPs and the carbon support, as the ionomer/catalyst interface plays a critical role in the ORR and MEA fuel cell performance.Ideally, voltage cycling of the membrane electrode assemblies would be performed in-situ within a TEM, but these experiments are highly challenging, since separate gas phase reactants need to be present simultaneously, in particular hydrogen and oxygen Instead, a very close approximation to a true in-situ experiment is done via potential cycling in a half-cell in the presence of liquid electrolyte, as opposed to solid electrolyte in a single cell. This allows us to observe the exact same location before and after cycling through a technique called identical location TEM/STEM (IL-TEM/STEM). Under these conditions, various electron microscopy techniques, such as bright-field TEM and HAADF STEM coupled with EDS and EELS can be performed. To better acquire and understand TEM/STEM-based experimental images, in particular the selection of appropriate operating conditions, and interpretation/validation of the results, it is critical to perform multislice TEM/STEM image simulations to calculate the interaction of an electron beam with a material system.In this talk, the focus is to understand the behavior of Pt and Pt-alloy NPs during the various stages of fuel cell cycling. For this purpose a set-up was developed to simulate the effect of voltage cycling on the cathode side of the fuel cell. In this set up, catalyst NPs supported on carbon nanotubes and amorphous carbon were observed by advanced transmission electron microscopy, before and after cycling. In particular, we studied Pt and Pt-alloy NPs and carbon support degradation mechanisms as a function of cycling using for the first time a combination of 2D IL TEM and 3D IL TEM. This new approach reveals many unpredictable features of morphology changes/behavior of Pt NPs and carbon support, after experiencing the long durability test. These new findings provide an insight which great surpasses what have been observed using solely 2D IL TEM.The experiments show particle migration in conjunction with carbon corrosion during the initial cycles, whereas the appearance of single atoms and atomic clusters on the surface of the carbon support appear after additional voltage cycling as a result of surface dissolution of NPs. For the case of alloyed NPs, the experiments show a heterogeneous deposition of Pt on the NPs. Moreover, we demonstrate a novel approach to distinguish the ionomer phase from the carbon support in PEMFCs, by employing the carbon signal in STEM-EELS, using two types of detectors. In this fashion, not only the ionomer, but also the carbon support, can be mapped in high spatial resolution, despite some damage caused by the electron beam.

Effect of Film Thickness on Microstructural and Magnetic Properties of Lithium Ferrite Films Prepared on Strontium Titanate (001) Substrates

Epitaxial lithium ferrite (LiFe5O8) films with different thicknesses were successfully fabricated on strontium titanate (SrTiO3) (001) substrates using the magnetron sputtering deposition technique. The microstructural and magnetic properties of the films were characterized by an advanced transmission electron microscope and a magnetic measurement device. It was found that the formation of structural defects can be influenced by the thickness of the film. In addition to misfit dislocations, orientation domains form in thinner films and twin boundaries appear in thicker films, respectively, contributing to the misfit strain relaxation in the heterosystem. The magnetic measurement showed that the thinner films have enhanced magnetization and a relatively lower coercive field compared with the thicker films containing antiferromagnetic twin boundaries. Our results provide a way of tuning the microstructure and magnetic properties of lithium ferrite films by changing the film thickness.

Surface Adsorption and Proton Chemistry of Ultra-Stabilized Aqueous Zinc-Manganese Dioxide Batteries.

Aqueous rechargeable Zinc (Zn) batteries incorporating MnO2 cathodes possess favorable sustainability properties and are being considered for low-cost, high-safety energy storage. However, unstable electrode structures and unclear charge storage mechanisms limit their development. Here, advanced transmission electron microscopy, electrochemical analysis, and theoretical calculations are utilized to study the working mechanisms of a Zn/MnO2 battery with a Co2+ -stabilized, tunnel-structured α-MnO2 cathode (Cox MnO2 ). It is shown that Co2+ can be pre-intercalated into α-MnO2 and occupy the (2×2) tunnel structure, which improves the structural stability of MnO2 , facilitates the proton diffusion and Zn2+ adsorption on the MnO2 surface upon battery cycling. It is further revealed that for the MnO2 cathode, the charge storage reaction proceeds mainly by proton intercalation with the formation of α-Hy Cox MnO2 , and that the anode design (with or without Zn metal) affects the surface adsorption of by-product Zn4 SO4 (OH)6 ·nH2 O on MnO2 surface. This work advances the fundamental understanding of rechargeable Zn batteries and also sheds light on efficient electrode modifications toward performance enhancement.

Advanced characterization of Al–Ni-Sm amorphous alloys with dispersion of round Pb-rich nanoparticles

The development of materials with special microstructures is a continuing theme of research due to their improved physical and mechanical properties. Rapid solidification can lead to amorphous or partially crystalline microstructures, depending on the imposed processing conditions and compositions allowing the production of composite materials. Al–Ni-Sm alloys can be easily obtained in the amorphous state by rapid solidification processing so that composites could be generated depending on the cooling conditions. In this work, new results are shown for Al-based amorphous matrix composites produced during rapid solidification of Al83.5Ni10Sm6.5, Al81.5Ni10Sm6.5Pb2 and Al83.5Ni8Sm6.5Pb2 (atom percent) alloys. Lead (Pb) was added to the amorphous forming composition alloy due to its immiscibility with the main alloying elements. Ribbon samples were obtained by melt spinning technique. Besides the use of ordinary characterization techniques, such as X-ray diffraction (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM), advanced transmission electron microscopy (TEM) was employed to reveal the special microstructure features forming these alloys. A strong influence of the Pb addition on crystallization behavior has been identified. Also, the size and morphology of the nanocrystalline Pb-rich minority phase within the amorphous matrix is examined. These effects were explained based on the immiscibility of Pb with the alloy components.

Revealing the strengthening contribution of stacking faults, dislocations and grain boundaries in severely deformed LPBF AlSi10Mg alloy

In this study, microstructural features direct metal laser melted (DMLM) aluminium–silicon-magnesium (AlSi10Mg) are investigated using advanced transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). The focus is on post-processing by ECAP (Equal Channel Angular Pressing) and its effects on grain refinement, stacking fault formation and dislocation accumulation. In addition, the strength enhancing role of stacking faults is for the first time quantified. The results show that ECAP can increase the yield strength from 294 to 396 MPa, while the elongation increases from 2.4% to 6%. These results show that ECAP processing offers a new approach for producing AlSi10Mg products with improved strength and ductility.

Probing individual single atom electrocatalyst sites by advanced analytical scanning transmission electron microscopy

Single atom electrocatalysts (SAEs) are promising next-generation materials for promoting a variety of important reactions, such as the oxygen reduction, nitrogen reduction, and CO2 reduction reactions. While bulk characterization techniques such as X-ray absorption spectroscopy and Mössbauer spectroscopy have significantly enhanced our understanding of these catalysts, direct probing of individual single metal atom sites at the atomic scale is necessary to understand local variations in the properties of these sites and accelerate design and synthesis of improved SAEs. Aberration-corrected scanning transmission electron microscopy (STEM) has become a powerful tool for providing this type of atomic-scale information about SAE metal sites. These sites are typically unstable under the electron beam, however, which, in combination with conventional acquisition methods and detectors, has limited the type and quantity of information obtainable by spectroscopic STEM techniques. Here, we map multiple individual SAE metal sites in a nitrogen-doped carbon containing atomically dispersed Fe and Re (FeReNC) at the atomic scale by direct electron detection electron energy-loss spectroscopy (EELS). Direct electron detection provides an improved signal-to-noise ratio over conventional scintillator-based detectors and enables detection and real space localization of weak signals. In addition, we demonstrate an automated method for identification of metal atom positions, placement of the probe on these sites, and simultaneous EELS and energy dispersive X-ray spectroscopic (EDS) signal acquisition. This simultaneous acquisition of EELS and EDS provides access to the composition and bonding of a wide range of SAE metal sites. Focusing the probe directly on the metal sites also increases the relevant data acquisition rate by more than an order of magnitude over two-dimensional mapping, enabling improved statistical measurements of site properties. The versatility, sensitivity, and speed that these techniques provide enhances our ability to probe the local elemental and chemical environment of a large number of individual SAE metal site structures at the atomic scale, enabling an improved understanding of the variations in the local properties of these electrocatalysts to be gained. As a result, significantly increased information about individual metal sites will be available to future electrochemical studies through these techniques, accelerating the development of advanced SAEs.

Uncovering the origin of surface-redox pseudocapacitance of molybdenum phosphides enables high-performance flexible sodium-ion capacitors

The flexible electrode based on surface redox pseudocapacitance delivers high specific capacity and superior rate capability, which shows excellent advantages in achieving low-cost and high-performance flexible sodium ion capacitors (FSICs). However, a considerable challenge to be solved is to explore the origin and optimization of surface pseudocapacitance. Herein, we report freestanding MoP nanowires flexible films (MoP-NWs-FF) with high surface-redox pseudocapacitance, and its conductive frameworks facilitate electron/ion transport and achieve superior mechanical flexibility. According to X-ray photoelectron spectroscopy (XPS) and advanced scanning transmission electron microscopy (STEM) technology, the amorphous oxides on the surface of MoP nanograins are demonstrated, which is positively correlated with the sodium ion storage capacity. As a result, the optimized MoP-NWs-FF electrode delivers a high reversible specific capacity of 293 mAh g−1 at 0.1 A g−1, superior rate performance, and cycling stability. Besides, based on the MoP-NWs-FF anode and Na3V2(PO4)2/C flexible film cathode, the assembled fully FSIC exhibits a high power density of ∼ 4 kW kg−1 (with an energy density of 53 Wh kg−1), corresponding to a discharging time of 1 min. This work provides a fundamental insight into the origin of surface-redox pseudocapacitance and paves a new way to develop next-generation high-power flexible energy storage devices.

A High-Entropy-Oxides-Based Memristor: Outstanding Resistive Switching Performance and Mechanisms in Atomic Structural Evolution.

The application of high-entropy oxide (HEO) has attracted significant attention in recent years owing to their unique structural characteristics, such as excellent electrochemical properties and long-term cycling stability. However, the application of resistive random-access memory (RRAM) has not been extensively studied, and the switching mechanism of HEO-based RRAM has yet to be thoroughly investigated. In this study, HEO (Cr, Mn, Fe, Co, Ni)3 O4 with a spinel structure is epitaxially grown on a Nb:STO conductive substrate, and Pt metal is deposited as the top electrode. After the resistive-switching operation, some regions of the spinel structure are transformed into a rock-salt structure and analyzed using advanced transmission electron microscopy and scanning transmission electron microscopy. From the results of X-ray photoelectron spectroscopy and electron energy loss spectroscopy, only specific elements would change their valence state, which results in excellent resistive-switching properties with a high on/off ratio on the order of 105 , outstanding endurance (>4550 cycles), long retention time (>104 s), and high stability, which suggests that HEO is a promising RRAM material.

Catalytic mechanisms of nickel nanoparticles for the improved dehydriding kinetics of magnesium hydride

MgH2, albeit with slow desorption kinetics, has been extensively studied as one of the most ideal solid hydrogen storage materials. Adding such catalyst as Ni can improve the desorption kinetics of MgH2, whereas the catalytic role has been attributed to different substances such as Ni, Mg2Ni, Mg2NiH0.3, and Mg2NiH4. In the present study, Ni nanoparticles (Ni-NPs) supported on mesoporous carbon (Ni@C) have been synthesized to improve the hydrogen desorption kinetics of MgH2. The utilization of Ni@C largely decreases the dehydrogenation activation energy from 176.9 to 79.3 kJ mol−1 and the peak temperature of dehydrogenation from 375.5 to 235 °C. The mechanism of Ni catalyst is well examined by advanced aberration-corrected environmental transmission electron microscopy and/or x-ray diffraction. During the first dehydrogenation, detailed microstructural studies reveal that the decomposition of MgH2 is initially triggered by the Ni-NPs, which is the rate-limiting step. Subsequently, the generated Mg reacts rapidly with Ni-NPs to form Mg2Ni, which further promotes the dehydrogenation of residual MgH2. In the following dehydrogenation cycle, Mg2NiH4 can rapidly decompose into Mg2Ni, which continuously promotes the decomposition of MgH2. Our study not only elucidates the mechanism of Ni catalyst but also helps design and assemble catalysts with improved dehydriding kinetics of MgH2.

Tracking Lithiation with Advanced Transmission Electron Microscopy.

Tracking Lithiation with Advanced Transmission Electron Microscopy.

Gram-Scale Continuous Flow Synthesis of Silver-on-Silica Patchy Nanoparticles with Widely Tunable Resonances for Plasmonics Applications

Extensive research has been conducted on colloids that incorporate nanostructured noble metals, motivated in particular by their highly tunable optical properties. However, the lack of approaches that offer reproducibility, tunability, and scalable production is limiting their exploitation in many application areas. Here, we present an innovative process for the controlled formation of silver patches on silica nanospheres using gold nanocrystal seeds. The synthesis of the seeds and their attachment in very small numbers to the silica spheres is achieved without employing any surfactants, stabilizers, or linker molecules. Using advanced transmission electron microscopy techniques, we show that electroless deposition of nanostructured silver occurs at the introduced seeds and leads to a central protrusion from the metal patches not observed in earlier, seed-free work. We demonstrate that both seeding and growth processes can be achieved in millimixer flow reactors. With this simple and scalable approach, we provide compelling evidence for the use of seeds in producing targeted morphology and function. Specifically, patch size and optical properties can be tailored in a predictive manner by adjusting the density of seeds on the silica nanospheres as well as reagent concentrations during growth. As a result, our process can be used to obtain colloidal dispersions with localized surface plasmon resonance peaks anywhere in the range 400 to nearly 1300 nm. We confirm the scalability of our method by operating the growth process for around 70 min. Apart from changes in the first few minutes, the product properties remained rather stable and over a gram of high quality silver patch-coated silica could be recovered. Our approach opens up exciting new avenues for plasmonics applications, which typically require large amounts of nanoparticles with specifically designed optical properties. These include color and IR pigments, photocatalysts, and as light management components in optoelectronic devices like photovoltaic cells.