Resolution Scanning Transmission Electron Microscopy Research Articles

Epitaxial growth of deep ultraviolet light emitting diodes with two-step n-AlGaN layer

In this study, the effect of a two-step n-AlGaN layer grown via high temperature metal organic chemical vapor deposition (HT-MOCVD) was investigated. The crystal and electrical characteristics of n-AlGaN over Al composition 60% are crucial for the high performance deep ultraviolet light emitting diode (DUV LED). The improvement in the crystal quality of the n-AlGaN is proved by inserting Al 83% n-AlGaN between Al 65% n-AlGaN and AlN using atomic force microscope, photoluminescence, high resolution X-ray diffraction and scanning transmission electron microscopy. The measurement of the Hall effect demonstrated the enhanced electrical properties of the n-AlGaN layer with two-step. Finally, we demonstrated a DUV LED with conventional and two-step n-AlGaN layers using HT-MOCVD. The optical power of the DUV LED with two-step n-AlGaN increased by a factor of three as compared to those of the DUV LED with conventional n-AlGaN.

Achievement of high coercivity in Sm(Fe0.8Co0.2)12 anisotropic magnetic thin film by boron doping

SmFe12-based compounds have been considered as promising candidates for next generation permanent magnet materials because of their excellent intrinsic hard magnetic properties with a minimum usage of rare earth elements. However, realizing high coercivity in anisotropic microstructure is a big challenge, which hinders their practical applications. In this work, a novel anisotropic granular microstructure of Sm(Fe0.8Co0.2)12 with a sufficiently large coercivity (µ0Hc) of 1.2 T and with a high remanent magnetization of 1.50 T is demonstrated in thin films prepared by co-sputtering Sm(Fe,Co)12 with boron. Detailed microstructure characterization using high resolution scanning transmission electron microscopy (STEM) and atom probe tomography (APT) indicate that the addition of B leads to the development of columnar-shaped Sm(Fe0.8Co0.2)12 grains with a size of ∼40 nm, which are surrounded by ∼3 nm-thick B-enriched amorphous intergranular phase. Domain wall pinning at the amorphous grain boundary phase is attributed to the high coercivity. This work provides a guiding principle for realizing high-coercivity anisotropic SmFe12-based permanent magnets, which can outperform Nd-Fe-B magnets and could be used as the next-generation high-performance permanent magnets in various applications.

Significant Improvement in Electrical Conductivity and Figure of Merit of Nanoarchitectured Porous SrTiO3 by La Doping Optimization.

SrTiO3 is a well-studied n-type metal oxide based thermoelectric (TE) material. In this work, the first-principles calculation of La-doped SrTiO3 has been performed using the density functional theory. In addition, high TE properties of bulk SrTiO3 material have been achieved by introducing nanoscale porosity and optimizing carrier concentration by La doping. The X-ray diffraction, atomic resolution scanning transmission electron microscopy imaging, and energy-dispersive X-ray spectrometry results show that La has been doped successfully into the lattice. The scanning electron microscopy images confirm that all the samples have nearly similar nanoscale porosities. The significant enhancement of electrical conductivity over the broad temperature range has been observed through optimization of La doping. Additionally, the samples possess very low thermal conductivity, which is speculated because of the nanoscale porosity of the samples. Because of this dual mechanism of doping optimization and nanoscale porosity, there is a remarkable improvement in power factor, 1 mW/m2K from 650 to 800 K, and figure of merit, zT of 0.26 at 850 K, of the sample, 22 at. % La-doped SrTiO3.

Thickness Dependence of Oxygen Vacancy Ordering in Strained LaCoO3–x Thin Films

The physical properties of perovskite oxides are very sensitive to oxygen vacancy structures, and an unambiguous acquaintance of vacancy structures of thin films is the precondition for applications of many functional devices. Here, we report critical thickness control over oxygen vacancy ordering structure in epitaxial LaCoO3–x (LCO) thin films characterized by atomic resolution scanning transmission electron microscopy (STEM). Lattice mappings extracted from Cs-corrected STEM images demonstrate that vacancy ordering planes are vertically distributed in LCO films grown on SrTiO3 substrates with the thickness above 5 nm, transversally distributed when the thickness is below 5 nm and distributed in a mixed manner when the thickness is 5 nm. Similar phenomenon also happens in LCO/NdGaO3 film systems with the critical thickness of around 13 nm due to the reduced tensile strain imposed by the substrate. The thickness effect phenomenon is collaborated by first-principles calculations and the formation mechanism is discussed from the point of view of strain relaxation. These findings demonstrate that the thickness can exquisitely control the periodic modulations of vacancy ordering structure in LCO thin films, suggesting an effective pathway of thickness controlling and enhancing physical properties of functional oxide films.

Angular dependence of fast-electron scattering from materials

Angular resolved scanning transmission electron microscopy is an important tool for investigating the properties of materials. However, several recent studies have observed appreciable discrepancies in the angular scattering distribution between experiment and theory. In this paper we discuss a general approach to low-loss inelastic scattering which, when incorporated in the simulations, resolves this problem and also closely reproduces experimental data taken over an extended angular range. We also explore the role of ionic bonding, temperature factors, amorphous layers on the surfaces of the specimen, and static displacements of atoms on the angular scattering distribution. The incorporation of low-loss inelastic scattering in simulations will improve the quantitative usefulness of techniques such as low-angle annular dark-field imaging and position-averaged convergent beam electron diffraction, especially for thicker specimens.

Promoter Location on NiW/Al2O3 Sulfide Catalysts: Parallel Study by IR/CO Spectroscopy and High-Resolution STEM-HAADF Microscopy

In this work, the effect of citric acid (CA) on the structure and activity of Ni-promoted WS2 particles supported on γ-Al2O3 were investigated by HRTEM (high resolution transmission electron microscopy), HR STEM-HAADF (high resolution scanning transmission electron microscopy equipped with high angular annular dark field detector), CO adsorption followed by infrared (IR) spectroscopy, and thiophene hydrodesulfurization (HDS) test. It is shown that CA addition increases the thiophene HDS activity. This may be directly ascribed to the increase in NiWS site concentration detected by IR spectroscopy. HRTEM emphasizes that the addition of CA to NiW catalysts decreases the average size of the particles. This is confirmed by HR STEM-HAADF that even evidences the presence of nanoclusters on NiW catalysts prepared with CA. Hence, local and global analysis of the NiW samples are in good agreement to account for the increase of NiWS site concentration by CA addition. Then the contrast and interatomic distances were analyzed in the atomic-scale images obtained by HR STEM-HAADF. These measurements allow identification of the nature of the atoms in the core and edge of the sulfided particles. The metal atoms of particle core are only W ones, whereas the edges of the particles could be either fully promoted with detection of only Ni atoms or partially promoted with detection of both Ni and W atoms. Accordingly, IR spectroscopy evidenced edge sites with different promotion degrees.

Stacking-Order-Driven Optical Properties and Carrier Dynamics in ReS2.

Two distinct stacking orders in ReS2 are identified without ambiguity and their influence on vibrational, optical properties and carrier dynamics are investigated. With atomic resolution scanning transmission electron microscopy (STEM), two stacking orders are determined as AA stacking with negligible displacement across layers, and AB stacking with about a one-unit cell displacement along the a axis. First-principles calculations confirm that these two stacking orders correspond to two local energy minima. Raman spectra inform a consistent difference of modes I & III, about 13 cm-1 for AA stacking, and 20 cm-1 for AB stacking, making a simple tool for determining the stacking orders in ReS2 . Polarized photoluminescence (PL) reveals that AB stacking possesses blueshifted PL peak positions, and broader peak widths, compared with AA stacking, indicating stronger interlayer interaction. Transient transmission measured with femtosecond pump-probe spectroscopy suggests exciton dynamics being more anisotropic in AB stacking, where excited state absorption related to Exc. III mode disappears when probe polarization aligns perpendicular to b axis. The findings underscore the stacking-order driven optical properties and carrier dynamics of ReS2 , mediate many seemingly contradictory results in the literature, and open up an opportunity to engineer electronic devices with new functionalities by manipulating the stacking order.

Atomic-scale study of precipitates (NbC and Cu-rich phase) at the twin boundary in the long time ageing austenitic stainless steel

ABSTRACT Microstructures of Cu-rich phases and NbC precipitated phases have been studied in a long time ageing austenitic stainless steel by high resolution scanning transmission electron microscopy. The interaction difference between the twins and the second phases found to be dependent on the nature of the precipitates. The Cu-rich phases were identified to be twinned at the twin boundary. Nevertheless, the NbC precipitates not only twinned at the twin boundary but also induced the twin boundary bypass them. A particle size dependence of the generation of misfit dislocations also was detected at interface between precipitates and the austenitic matrix.

Tantalum and zirconium induced structural transitions at complex [111] tilt grain boundaries in copper

Alloying nanocrystalline copper (Cu) with immiscible elements, such as tantalum (Ta) and zirconium (Zr), is a promising technique to manipulate grain boundary properties and by this suppress grain growth at elevated temperatures. However, insights on the atomistic origins on the influence of impurity elements on grain boundaries are lacking. In this study, the atomistic effects of Ta and Zr on [111] tilt grain boundaries in Cu are investigated by high resolution scanning transmission electron microscopy techniques. In case of Ta, the formation of spherical, nano-scale precipitates in close vicinity to the grain boundaries is observed, but no sign of segregation. The particles induce a repelling force to migrating boundaries and act as local pinning points. The segregation of Zr is observed to occur either at confined grain boundary steps or homogeneously along the boundaries without steps. In both cases a strong disordering of the defect or grain boundary structure is revealed. Furthermore, at low Zr concentrations it induces structural grain boundary transitions and partial atomic reordering of the grain boundary structural units.

Phase transformation in MOCVD growth of (AlxGa1−x)2O3 thin films

This paper investigated the growth of (AlxGa1−x)2O3 thin films on semi-insulating (010) Ga2O3 substrates over the entire Al composition range (0% < x ≤ 100%) via metalorganic chemical vapor deposition (MOCVD). For the Al composition x < 27%, high quality single phase β-(AlxGa1−x)2O3 was achieved. A mixture of β and γ phases existed in (AlxGa1−x)2O3 when Al composition ranged between 27% and 40%, whereas a single γ-phase was observed for the films with Al composition x > 40%. The transition from the β to γ phase in AlGaO alloys was observed from x-ray diffraction spectra. The growth of γ-phase AlGaO with higher Al content was further confirmed via atomic resolution scanning transmission electron microscopy imaging and nanodiffraction. Compositional and statistical analyses performed on data acquired from atom probe tomography provided insight on the local compositional homogeneity in AlGaO films with different Al compositions. For AlGaO with pure β or γ phases, the Al composition distribution showed homogeneity with similar Al composition values as extracted from the x-ray diffraction peak positions. For AlGaO films with mixed β and γ phases, inhomogeneity in the Al composition distribution became more obvious in the nm scale. A mechanism was proposed for the observed phase transformation between β and γ phases in MOCVD growth of AlGaO films.

Aerosol Synthesis of High Entropy Alloy Nanoparticles.

Homogeneously mixing multiple metal elements within a single particle may offer new material property functionalities. High entropy alloys (HEAs), nominally defined as structures containing five or more well-mixed metal elements, are being explored at the nanoscale, but the scale-up to enable their industrial application is an extremely challenging problem. Here, we report an aerosol droplet-mediated technique toward scalable synthesis of HEA nanoparticles with atomic-level mixing of immiscible metal elements. An aqueous solution of metal salts is nebulized to generate ∼1 μm aerosol droplets, which when subjected to fast heating/quenching result in decomposition of the precursors and freezing-in of the zero-valent metal atoms. Atomic-level resolution scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy analysis reveals that all metal elements in the nanoparticles are homogeneously mixed at the atomic level. We believe that this approach offers a facile and flexible aerosol droplet-mediated synthesis technique that will ultimately enable bulk processing starting from a particulate HEA.

The effect of continuous heating on microstructure development in thermo-mechanically processed Ti-10V-3Fe-3Al alloy produced by powder metallurgy

Both scanning transmission electron microscopy (STEM) and atom probe tomography (APT) were exploited to study variations in chemical composition and phases that took place during thermo-mechanical processing and following slow continuous heating of Ti-10V-3Fe-3Al (wt%) alloy. For the first time for the rapid-cooled alloys, athermal ω (ωath)-assisted nucleation of α phase (≈1 nm) was revealed by high resolution STEM. No presence of ωath was detected after heating to 450 °C, which was explained by its consumption by intragranular α and dissolution. While binomial analysis of APT data after thermo-mechanical processing revealed no changes in chemical composition of ωath compared to that of β matrix, heating the sample up to 375 °C resulted in Ti–rich domains depleted in both α and β stabilisers. The results suggested the simultaneous occurrence of displacive and diffusional mechanisms for formation and growth of intragranular α.

Mpfit: a robust method for fitting atomic resolution images with multiple Gaussian peaks

The standard technique for sub-pixel estimation of atom positions from atomic resolution scanning transmission electron microscopy images relies on fitting intensity maxima or minima with a two-dimensional Gaussian function. While this is a widespread method of measurement, it can be error prone in images with non-zero aberrations, strong intensity differences between adjacent atoms or in situations where the neighboring atom positions approach the resolution limit of the microscope. Here we demonstrate mpfit, an atom finding algorithm that iteratively calculates a series of overlapping two-dimensional Gaussian functions to fit the experimental dataset and then subsequently uses a subset of the calculated Gaussian functions to perform sub-pixel refinement of atom positions. Based on both simulated and experimental datasets presented in this work, this approach gives lower errors when compared to the commonly used single Gaussian peak fitting approach and demonstrates increased robustness over a wider range of experimental conditions.

Superior polarization retention through engineered domain wall pinning

Ferroelectric materials possess a spontaneous polarization that is switchable by an electric field. Robust retention of switched polarization is critical for non-volatile nanoelectronic devices based on ferroelectrics, however, these materials often suffer from polarization relaxation, typically within days to a few weeks. Here we exploit designer-defect-engineered epitaxial BiFeO3 films to demonstrate polarization retention with virtually no degradation in switched nanoscale domains for periods longer than 1 year. This represents a more than 2000% improvement over the best values hitherto reported. Scanning probe microscopy-based dynamic switching measurements reveal a significantly increased activation field for domain wall movement. Atomic resolution scanning transmission electron microscopy indicates that nanoscale defect pockets pervade the entire film thickness. These defects act as highly efficient domain wall pinning centres, resulting in anomalous retention. Our findings demonstrate that defects can be exploited in a positive manner to solve reliability issues in ferroelectric films used in functional devices.

Ultrafast transient liquid assisted growth of high current density superconducting films

The achievement of high growth rates in YBa2Cu3O7 epitaxial high-temperature superconducting films has become strategic to enable high-throughput manufacturing of long length coated conductors for energy and large magnet applications. We report on a transient liquid assisted growth process capable of achieving ultrafast growth rates (100 nm s−1) and high critical current densities (5 MA cm−2 at 77 K). This is based on the kinetic preference of Ba-Cu-O to form transient liquids prior to crystalline thermodynamic equilibrium phases, and as such is a non-equilibrium approach. The transient liquid-assisted growth process is combined with chemical solution deposition, proposing a scalable method for superconducting tapes manufacturing. Additionally, using colloidal solutions, the growth process is extended towards fabrication of nanocomposite films for enhanced superconducting properties at high magnetic fields. Fast acquisition in situ synchrotron X-ray diffraction and high resolution scanning transmission electron microscopy (STEM) become crucial measurements in disentangling key aspects of the growth process.

Effect of pre-aging deformation on the intergranular corrosion resistance of 6000 series aluminium alloys

Aluminium alloys in the 6000 series may become susceptible to intergranular corrosion (IGC) by unfavourable thermomechanical processing in combination with presence of Cu and a low Mg/Si ratio. The IGC susceptibility has been related to the segregation of Cu during artificial ageing to form a nearly continuous Cu film along the grain boundaries with an anodic solute depleted zone adjacent to it. In this work, extruded alloys 6005 and C28 as well as two variants of 6016 automotive sheet were subjected to various degrees of deformation by rolling or stretching before artificial aging. Accelerated IGC testing showed that pre-deformation significantly improved the IGC resistance of the Cu containing extruded alloys, while no improvement was observed for the 6016 sheet materials containing < 0.01 wt% Cu. High resolution scanning transmission electron microscopy (STEM) of pre-deformed 6005 alloy showed precipitation of Cu-containing Q' phase on dislocations. As a result, the amount of Cu available for diffusion to the grain boundaries is reduced, which might explain why a beneficial effect of pre-deformation on the IGC resistance was observed only for the Cu-containing materials.

Minimising damage in high resolution scanning transmission electron microscope images of nanoscale structures and processes.

Beam damage caused during acquisition of the highest resolution images is the current limitation in the vast majority of experiments performed in a scanning transmission electron microscope (STEM). While the principles behind the processes of knock-on and radiolysis damage are well-known (as are other contributing effects, such as heat and electric fields), understanding how and especially when beam damage is distributed across the entire sample volume during an experiment has not been examined in detail. Here we use standard models for damage and diffusion to elucidate how beam damage spreads across the sample as a function of the microscope conditions to determine an "optimum" sampling approach that maximises the high-resolution information in any image acquisition. We find that the standard STEM approach of scanning an image sequentially accelerates damage because of increased overlap of diffusion processes. These regions of accelerated damage can be significantly decelerated by increasing the distance between the acquired pixels in the scan, forming a "spotscan" mode of acquisition. The optimum distance between these pixels can be broadly defined by the fundamental properties of each material, allowing experiments to be designed for specific beam sensitive materials. As an added bonus, if we use inpainting to reconstruct the sparse distribution of pixels in the image we can significantly increase the speed of the STEM process, allowing dynamic phenomena, and the onset of damage, to be studied directly.

The effect of low Cu additions on precipitate crystal structures in overaged Al-Mg-Si(-Cu) alloys

This study concerns the effect of low Cu additions (≲0.1 at.%) on the precipitate crystal structure evolution in three Al-Mg-Si(-Cu) alloys during overageing. The evolution was assessed through a combination of atomic resolution scanning transmission electron microscopy, scanning electron diffraction, and differential scanning calorimetry. It was found that relatively small changes in the Cu level and the Si:Mg ratio had significant effects on the resulting distribution of precipitate phases, their structural evolution, and their thermal stability. Two Si-rich alloys formed hybrid β′ phase and Q′ phase precipitates on overageing. A third Mg-rich alloy primarily formed L phase precipitates, which exhibited superior thermal stability. Three distinct Cu-containing sub-units that form the basis for all Al-Mg-Si-Cu precipitate phases were identified: the three-fold symmetric Q′/C and the βCu′ sub-units, in addition to a newly discovered C sub-unit. The formation of each sub-unit was discussed, and the atomic structures and connections to other precipitate phases in the Al-Mg-Si(-Cu) system were elaborated. The work presented provides new insights into the complex precipitation of Cu-added Al-Mg-Si alloys, with implications for material properties. The results obtained will be of importance in future alloy and process development, and are thought to be of high value in modelling work on the quaternary Al-Mg-Si-Cu system.

Observation of oxygen pyramid tilting induced polarization rotation in strained BiFeO3 thin film

AbstractOxygen octahedral tilting has been recognized to strongly interact with spin, charge, orbital, and lattice degrees of freedom in perovskite oxides. Here, we observe a strain‐driven stripe‐like morphology of two supertetragonal (monoclinic Cc and Cm) phases in the strained BiFeO3/LaAlO3 thin films. The two supertetragonal phases have a similar giant axial ratio but differences in oxygen pyramid tilting mode. Especially, the competition between polar instability and oxygen pyramid tilting is identified using atomically resolved scanning transmission electron microscopy, leading to the polarization rotation across the phase boundary. In addition, microtwins are observed in the Cc phase. Our findings provide new insights of the coupling between ferroelectric polarization and oxygen pyramid tilting in oxide thin films and will help to design novel phase morphology with desirable ferroelectric polarization and properties for new applications in perovskite oxides.

Evolution from UV emission of phenyl groups to visible emission of pyrolytic nanocarbons dispersed in fumed silica: Alternative insight into photoluminescence of carbon nanodots

SiO2:C nanopowder, luminescent in ultraviolet and visible spectral ranges, was synthesised using fumed silica as morphological matrix (specific surface of 295 m2/g) and phenyltrimethoxysilane (PhTMS) as carbon precursor. Fumed silica was impregnated by toluene solution of PhTMS followed by drying and annealing at 600 °C. Two series of SiO2:C nanocomposites with different carbon concentration were synthesised and studied. The main idea of the study was to explore the correlation of structural reconstruction and evolution of photoluminescent properties of carbon moieties. Chemically modified fumed silica exhibited ultraviolet (UV) photoluminescence band at about 330 nm originated from phenyl groups. After calcinations at 600 °C two overlapped emission bands were found at about 380 nm and 420 nm in “low-carbon” SiO2:C, while “high-carbon” series exhibited broad but homogeneous emission band peaked at about 440–450 nm. One more characteristic emission band at 495 nm in SiO2:C appeared and developed under intense ultraviolet radiation. Evolution of light emission from ultraviolet to visible was analysed in correlation with structural reconstruction of carbon examined by FTIR spectroscopy, X-ray photoelectron spectroscopy, high resolution scanning transmission electron microscopy, and electron energy loss spectroscopy. The nature of light emission of pyrolytic carbon moieties in SiO2:C nanopowder was discussed in frame of the concepts of polycyclic carbon clusters and their aggregates based on the analogy with polycyclic organic molecules and luminescent carbon nanodots.