Resolution Energy Dispersive X-ray Spectroscopy Research Articles

Unravelling Structure and Formation Mechanisms of Ruddlesden-Popper-Phase-like Nanodomains in Inorganic Lead Halide Perovskites.

Ultrastable CsPbBr3 nanoplates against electron beam irradiations are fabricated and nanodomains with anomalous high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) contrasts are observed within CsPbBr3 nanoplates. Atomic resolution energy dispersive X-ray spectroscopy (EDS) mapping, which requires even higher beam currents and may cause significant damages on electron beam sensitive materials, are obtained without any detectable damages or decomposition. Combining HAADF-STEM images, atomic resolution EDS mapping, and image simulations has revealed detailed structure and chemistry of the nanodomains to be induced by Ruddlesden-Popper faults (RP faults) rather than any chemical intermixing or formation of new phases. A formation mechanism is also proposed on the basis of the atomic structure of the nanodomains. This result promotes an atomic-level understanding of inorganic lead halide perovskites and may help to reveal their structure-property relationship.

Direct atomic identification of cation migration induced gradual cubic-to-hexagonal phase transition in Ge2Sb2Te5

GeTe-Sb2Te3 pseudobinary system, especially Ge2Sb2Te5 alloy, is the most desirable material to be commercialized in phase change random access memory. Directly resolving the local atomic arrangement of Ge2Sb2Te5 during intermediate steps is an effective method to understand its transition mechanism from face-centered-cubic to hexagonal phases. In this study, we provide insights into the atomic arrangement variation during face-centered-cubic to hexagonal transition process in Ge2Sb2Te5 alloy by using advanced atomic resolution energy dispersive X-ray spectroscopy. Induced by thermal annealing, randomly distributed germanium and antimony atoms would migrate to the specific (111) layer in different behaviors, and antimony atoms migrate earlier than germanium atoms during the phase transition process, gradually forming intermediate structures similar to hexagonal lattice. With the migration completed, the obtained stable hexagonal structure has a partially ordered stacking sequence described as below: -Te-Sbx/Gey-Te-Gex/Sby-Te-Gex/Sby-Te-Sbx/Gey-Te- (x > y), which is directly related to the migration process. The current visual fragments suggest a gradual transition mechanism, and guide the performance optimization of Ge2Sb2Te5 alloy.

Characterization of nanoscale precipitates in superalloy 718 using high resolution SEM imaging

At present, obtaining accurate volume fraction and size measurements of γ″, γ′, and δ precipitates in Superalloy 718 has been challenging due to their size, crystal structures, low volume fractions, and similar chemistries. These measurements are necessary to validate precipitation models that in turn enhance selective laser melting fabrication. Superalloy 718 is a promising candidate for selective laser melting fabrication due to a combination of excellent mechanical properties and workability. A new technique, combining high resolution distortion corrected SEM imaging and with high resolution x-ray energy dispersive spectroscopy, has been developed to accurately and independently measure the size, volume fraction, and number densities of these three precipitates. A specimen of selective laser melted superalloy 718 was obtained and underwent a conventional heat treatment for superalloy 718 where it was solutionized at 1010 °C for one hour, gas quenched and followed by a two-step age (718 °C/11 h → 621 °C/6 h) in order to produce a microstructure with all three precipitate types present. These results were further validated using x-ray diffraction and phase extraction methods.

Room temperature operation of InAsSb quantum dashes laser near 1.8 μm based on InP (001) substrate

Single-stack InAsSb self-assembled quantum dashes (QDashes) laser grown by metalorganic vapor phase epitaxy based on InP (001) substrate is reported. High-quality InAsSb quantum dashes have been acquired under the optimized growth conditions, such as substrate temperature, growth rate, deposition thickness and V/III ratio. Surface QDashes morphologies have been characterized by atomic force microscopy whereas the ones buried in active region have been investigated by transmission electron microscope with high resolution X-ray Energy Dispersive Spectroscopy to confirm the antimony composition. Double channel waveguide laser with 40 μm width ridge has been fabricated by standard optical lithography and wet etching. Pulsed room temperature operation demonstrates the wavelength from 1.72 to 1.79 μm for different injection currents. In addition, the laser peak output power can exceed 600 mW with 12.8% of differential quantum efficiency and even under continuous wave operation, the maximum optical power still remains 26 mW.

Interphase boundary segregation of silver and enhanced precipitation of Mg17Al12 Phase in a Mg-Al-Sn-Ag alloy

Aging-hardening response of Mg-7Al-2Sn (wt%) alloy can be remarkably accelerated by a small addition of silver (Ag). Using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) coupled with high resolution energy dispersive X-ray spectroscopy (EDS) mapping, for the first time, segregation of solute atoms (Ag in this case) was observed at the interphase boundary between continuous Mg17Al12 precipitate and magnesium matrix in Mg-7Al-2Sn alloy with 0.7 wt% Ag addition. Substantial size refinement and increased number density of Mg17Al12 precipitates may be responsible for the enhanced aging-hardening response in Mg-7Al-2Sn alloy with Ag addition.

Atomic resolution characterization of strengthening nanoparticles in a new high-temperature-capable 43Fe-25Ni-22.5Cr austenitic stainless steel

Advanced scanning transmission electron microscopy (STEM) was used to study two distinct populations of nanoparticles associated with the extraordinary strengthening of the highly-alloyed austenitic stainless steel Sanicro 25 during cyclic loading at 700 °C. Fully coherent and homogeneously dispersed Cu-rich nanoparticles precipitate rapidly as a result of thermal exposure, along with nanometer-sized incoherent NbC carbides that nucleate on dislocations during the cyclic loading at high temperature. The atomic structure of nanoparticles was investigated by probe-corrected high-angle annular dark-field STEM imaging. Compositional analysis of the nanoparticles was conducted using high spatial resolution energy dispersive X-ray spectroscopy combined with electron energy-loss spectroscopy. Experimental observations were validated by image simulations of the Moiré-like contrast exhibited by NbC carbides. The important role of both nanoparticle populations for the overall cyclic response is discussed. As a result of pinning effects and associated obstacles, dislocation motion is significantly retarded preventing formation of substructures with lower stored internal energy. With recovery heavily suppressed, forest dislocation strengthening supported by precipitation and solid solution hardening, leads to the remarkable increase of cyclic strength at elevated temperatures.

High Spatial Resolution Energy Dispersive X-ray Spectroscopy and Atom Probe Tomography study of Indium segregation in N-polar InGaN Quantum Wells

High Spatial Resolution Energy Dispersive X-ray Spectroscopy and Atom Probe Tomography study of Indium segregation in N-polar InGaN Quantum Wells

Elemental and lattice-parameter mapping of binary oxide superlattices of (LaNiO3)4/(LaMnO3)2 at atomic resolution

We construct the elemental distribution and lattice strain maps from the measured atomic column positions in a (LaNiO3)4/(LaMnO3)2 superlattice over a large field of view. The correlation between the distribution of B-cations and the lattice parameter in the form of Vegard’s law is validated using atomic resolution energy dispersive x-ray spectroscopy (EDS). The maps show negligible Mn intermixing in the LaNiO3 layer, while Ni intermixing in the LaMnO3 layer improves away from the substrate interface to 9.5 atomic% from the 8th period onwards, indicating that the superlattice interfacial sharpness is established as the distance from the substrate increases. The maps allow an observation of the compositional defects of the B-sites, which is not possible by Z-contrast alone. Thus, this study demonstrates a promising approach for atomic scale correlative study of lattice strain and composition, and a method for the calibration of atomic resolution EDS maps.

Realisation of magnetically and atomically abrupt half-metal/semiconductor interface: Co2FeSi0.5Al0.5/Ge(111).

Halfmetal-semiconductor interfaces are crucial for hybrid spintronic devices. Atomically sharp interfaces with high spin polarisation are required for efficient spin injection. In this work we show that thin film of half-metallic full Heusler alloy Co2FeSi0.5Al0.5 with uniform thickness and B2 ordering can form structurally abrupt interface with Ge(111). Atomic resolution energy dispersive X-ray spectroscopy reveals that there is a small outdiffusion of Ge into specific atomic planes of the Co2FeSi0.5Al0.5 film, limited to a very narrow 1 nm interface region. First-principles calculations show that this selective outdiffusion along the Fe-Si/Al atomic planes does not change the magnetic moment of the film up to the very interface. Polarized neutron reflectivity, x-ray reflectivity and aberration-corrected electron microscopy confirm that this interface is both magnetically and structurally abrupt. Finally, using first-principles calculations we show that this experimentally realised interface structure, terminated by Co-Ge bonds, preserves the high spin polarization at the Co2FeSi0.5Al0.5/Ge interface, hence can be used as a model to study spin injection from half-metals into semiconductors.

Composition measurement in substitutionally disordered materials by atomic resolution energy dispersive X-ray spectroscopy in scanning transmission electron microscopy

The increasing use of energy dispersive X-ray spectroscopy in atomic resolution scanning transmission electron microscopy invites the question of whether its success in precision composition determination at lower magnifications can be replicated in the atomic resolution regime. In this paper, we explore, through simulation, the prospects for composition measurement via the model system of AlxGa1−xAs, discussing the approximations used in the modelling, the variability in the signal due to changes in configuration at constant composition, and the ability to distinguish between different compositions. Results are presented in such a way that the number of X-ray counts, and thus the expected variation due to counting statistics, can be gauged for a range of operating conditions.

Atom site preference and γ′/γ mismatch strain in Ni[sbnd]Al[sbnd]Co[sbnd]Ti superalloys

The structure and chemistry of NiCoAlTi quaternary superalloys are investigated at the microstructural and atomic scale. Atom probe tomograghy (APT) is used to quantify phase compositions and elemental partitioning behavior. Using aberration corrected electron microscopy, the site occupancy of the atoms within the A3 B structure (L12–type) γ′ phase is determined via atomic resolution energy dispersive X-ray spectroscopy (EDX). Ni is observed to preferentially occupy the A sub-lattice positions for all alloys, while Al and Ti occupy the B sub-lattice. In contrast, Co's site preference changes from a random distribution to the A sub-lattice as the alloy composition changes, in agreement with theoretical predictions from literature. Finally, the lattice strain across the γ′/γ interfaces is measured as a function of alloy composition.

Si Donor Incorporation in GaN Nanowires.

With increasing interest in GaN based devices, the control and evaluation of doping are becoming more and more important. We have studied the structural and electrical properties of a series of Si-doped GaN nanowires (NWs) grown by molecular beam epitaxy (MBE) with a typical dimension of 2-3 μm in length and 20-200 nm in radius. In particular, high resolution energy dispersive X-ray spectroscopy (EDX) has illustrated a higher Si incorporation in NWs than that in two-dimensional (2D) layers and Si segregation at the edge of the NW with the highest doping. Moreover, direct transport measurements on single NWs have shown a controlled doping with resistivity from 10(2) to 10(-3) Ω·cm, and a carrier concentration from 10(17) to 10(20) cm(-3). Field effect transistor (FET) measurements combined with finite element simulation by NextNano(3) software have put in evidence the high mobility of carriers in the nonintentionally doped (NID) NWs.

High resolution energy dispersive spectroscopy mapping of planar defects in L12-containing Co-base superalloys

Local chemical fluctuations in the vicinity of superlattice intrinsic stacking faults (SISFs) have been observed via high resolution energy dispersive X-ray spectroscopy (EDS) mapping in new single crystal Co- and CoNi-base superalloys containing γ′-(L12) precipitates. The SISFs were formed during high temperature tensile creep at 900°C. Chemical fluctuations were found to greatly influence the SISF energy, which was calculated from density functional theory in Co3(Al, Ta, W) compounds at 0K. The local SISF structure was found to be comprised of four D019 (0001) planes that were enriched in W and Ta, as revealed by high resolution scanning transmission electron microscopy (HRSTEM) imaging and EDS mapping. The precipitates were determined to accommodate up to 22% of the plastic deformation accrued during an interrupted creep test to 0.6% creep strain. The driving forces for segregation are discussed, and new models for shearing of the ordered precipitates are proposed.

Chemical ordering of Co and Ni in a W-(AlCoNi) crystalline approximant related to Al–Co–Ni decagonal quasicrystals studied by atomic resolution energy-dispersive X-ray spectroscopy

A W-(AlCoNi) crystalline approximant, which is closely related to Al–Co–Ni decagonal quasicrystals, in an Al72.5Co20Ni7.5 alloy has been studied by atomic resolution energy-dispersive X-ray spectroscopy (EDS), in an instrument attached to a spherical aberration (Cs)-corrected scanning transmission electron microscope. On high-resolution EDS maps of Co and Ni elements, obtained by integrating many sets of EDS data taken from undamaged areas, chemical ordering of Co and Ni is clearly detected. In the structure of the W-(AlCoNi) phase, consisting of arrangements of transition-metal (TM) atoms located at vertices of pentagonal tilings and pentagonal arrangements of mixed sites (MSs) of TM and Al atoms, Co atoms occupy the TM atom positions with the pentagonal tiling and Ni is enriched in part of the pentagonal arrangements of MSs.

Pb5Fe3TiO11Cl: A rare example of Ti(IV) in a square pyramidal oxygen coordination

A new oxychloride Pb5Fe3TiO11Cl has been synthesized using the solid state method. Its crystal and magnetic structure was investigated in the 1.5–550K temperature range using electron diffraction, high angle annular dark field scanning transmission electron microscopy, atomic resolution energy dispersive X-ray spectroscopy, neutron and X-ray powder diffraction. At room temperature Pb5Fe3TiO11Cl crystallizes in the P4/mmm space group with the unit cell parameters a=3.91803(3)Å and c=19.3345(2)Å. Pb5Fe3TiO11Cl is a new n=4 member of the oxychloride perovskite-based homologous series An+1BnO3n−1Cl. The structure is built of truncated Pb3Fe3TiO11 quadruple perovskite blocks separated by CsCl-type Pb2Cl slabs. The perovskite blocks consist of two layers of (Fe,Ti)O6 octahedra sandwiched between two layers of (Fe,Ti)O5 square pyramids. The Ti4+ cations are preferentially located in the octahedral layers, however, the presence of a noticeable amount of Ti4+ in a five-fold coordination environment has been undoubtedly proven using neutron powder diffraction and atomic resolution compositional mapping. Pb5Fe3TiO11Cl is antiferromagnetically ordered below 450(10)K. The ordered Fe magnetic moments at 1.5K are 4.06(4)μB and 3.86(5)μB on the octahedral and square-pyramidal sites, respectively.

Spatial high resolution energy dispersive X-ray spectroscopy on thin lamellas

For conventional samples and measurement geometries the spatial resolution of energy dispersive X-ray spectroscopy is limited by a tear drop shaped emission volume to about 1 μm. This restriction can be substantially improved using thin samples and high acceleration voltage. In this contribution the spatial resolution of energy dispersive X-ray spectroscopy in a scanning electron microscope using thin lamella samples is investigated. At an acceleration voltage of 30 kV, an edge resolution down to Δdedge = 40 ± 10 nm is observed performing linescans across an interface, using an 80 nm thin sample prepared from a GaAs/AlAs-heterostructure. Furthermore, Monte-Carlo simulations of pure elements ranging from sodium to mercury are performed for different sample thicknesses. From the simulations we can derive a simple empirical formula to predict the spatial resolution as a function of sample thickness.

Analytical transmission electron microscopy study of grain boundary precipitates in an Fe–Ni–Mn maraging alloy

Field-emission gun analytical transmission electron microscopy was used to study grain boundary precipitates in Fe–10Ni–7Mn (wt.%) maraging alloy after isothermal aging at 753 K. High spatial resolution energy dispersive X-ray spectroscopy was used for quantitative chemical analysis of the grain boundary precipitates. Precipitation of f.c.t. θ-NiMn intermetallic compound and reversion to f.c.c. austenite were identified at prior austenite grain boundaries after prolonged aging. Selected area electron diffraction patterns (SADPs) showed the L1 0 type ordering of f.c.t. θ-NiMn precipitates. The f.c.t. θ-NiMn precipitates showed higher Ni and Mn concentrations than reverted f.c.c. austenite.

Scanning transmission electron microscopy and its application to the study of nanoparticles and nanoparticle systems

Scanning transmission electron microscopy (STEM) techniques can provide imaging, diffraction and spectroscopic information, either simultaneously or in a serial manner, of the specimen with an atomic or a sub-nanometer spatial resolution. High-resolution STEM imaging, when combined with nanodiffraction, atomic resolution electron energy-loss spectroscopy and nanometer resolution X-ray energy dispersive spectroscopy techniques, is critical to the fundamental studies of importance to nanoscience and nanotechnology. The availability of sub-nanometer or sub-angstrom electron probes in a STEM instrument, due to the use of a field emission gun and aberration correctors, ensures the greatest capabilities for studies of sizes, shapes, defects, crystal and surface structures, and compositions and electronic states of nanometer-size regions of thin films, nanoparticles and nanoparticle systems. The various imaging, diffraction and spectroscopy modes available in a dedicated STEM or a field emission TEM/STEM instrument are reviewed and the application of these techniques to the study of nanoparticles and nanostructured catalysts is used as an example to illustrate the critical role of the various STEM techniques in nanotechnology and nanoscience research.

Structure and chemical composition of supported Pt–Sn electrocatalysts for ethanol oxidation

Carbon supported PtSn alloy and PtSnO x particles with nominal Pt:Sn ratios of 3:1 were prepared by a modified polyol method. High resolution transmission electron microscopy (HRTEM) and X-ray microchemical analysis were used to characterize the composition, size, distribution, and morphology of PtSn particles. The particles are predominantly single nanocrystals with diameters in the order of 2.0–3.0 nm. According to the XRD results, the lattice constant of Pt in the PtSn alloy is dilated due to Sn atoms penetrating into the Pt crystalline lattice. While for PtSnO x nanoparticles, the lattice constant of Pt only changed a little. HRTEM micrograph of PtSnO x clearly shows that the change of the spacing of Pt (1 1 1) plane is neglectable, meanwhile, SnO 2 nanoparticles, characterized with the nominal 0.264 nm spacing of SnO 2 (1 0 1) plane, were found in the vicinity of Pt particles. In contrast, the HRTEM micrograph of PtSn alloy shows that the spacing of Pt (1 1 1) plane extends to 0.234 nm from the original 0.226 nm. High resolution energy dispersive X-ray spectroscopy (HR-EDS) analyses show that all investigated particles in the two PtSn catalysts represent uniform Pt/Sn compositions very close to the nominal one. Cyclic voltammograms (CV) in sulfuric acid show that the hydrogen ad/desorption was inhibited on the surface of PtSn alloy compared to that on the surface of the PtSnO x catalyst. PtSnO x catalyst showed higher catalytic activity for ethanol electro-oxidation than PtSn alloy from the results of chronoamperometry (CA) analysis and the performance of direct ethanol fuel cells (DEFCs). It is deduced that the unchanged lattice parameter of Pt in the PtSnO x catalyst is favorable to ethanol adsorption and meanwhile, tin oxide in the vicinity of Pt nanoparticles could offer oxygen species conveniently to remove the CO-like species of ethanolic residues to free Pt active sites.

Chemical partitioning during crystallization and its effect on the microstructure and magnetic behavior of modified Nd–Fe–B permanent magnets

In this paper, the effect of alloy composition on the microstructural development of devitrified Nd–Fe–B-based alloys is investigated. While crystallization of simple ternary Nd 2Fe 14B resulted in a highly variable microstructure and relatively poor demagnetization behavior, alloying additions, notably Ti and C, were found to significantly refine and homogenize the crystallized microstructure. High spatial resolution energy dispersive X-ray spectroscopy revealed that partitioning to the grain boundaries led to the grain refinement by slowing interface kinetics. The more homogeneous grain structure resulted in improved demagnetization behavior, but segregating elements isolated grains and limited intergranular exchange coupling.