Nanobeam Electron Diffraction Patterns Research Articles

Large bi-axial tensile strain effect in epitaxial BiFeO3 film grown on single crystal PrScO3

A BiFeO3 film is grown epitaxially on a PrScO3 single crystal substrate which imparts ~ 1.45% of biaxial tensile strain to BiFeO3 resulting from lattice misfit. The biaxial tensile strain effect on BiFeO3 is investigated in terms of crystal structure, Poisson ratio, and ferroelectric domain structure. Lattice resolution scanning transmission electron microscopy, precession electron diffraction, and X-ray diffraction results clearly show that in-plane interplanar distance of BiFeO3 is the same as that of PrScO3 with no sign of misfit dislocations, indicating that the biaxial tensile strain caused by lattice mismatch between BiFeO3 and PrScO3 are stored as elastic energy within BiFeO3 film. Nano-beam electron diffraction patterns compared with structure factor calculation found that the BiFeO3 maintains rhombohedral symmetry, i.e., space group of R3c. The pattern analysis also revealed two crystallographically distinguishable domains. Their relations with ferroelectric domain structures in terms of size and spontaneous polarization orientations within the domains are further understood using four-dimensional scanning transmission electron microscopy technique.

Mapping Polar Distortions using Nanobeam Electron Diffraction and a Cepstral Approach.

Measuring local polar ordering is key to understanding ferroelectricity in thin films, especially for systems with small domains or significant disorder. Scanning nanobeam electron diffraction (NBED) provides an effective local probe of lattice parameters, local fields, polarization directions, and charge densities, which can be analyzed using a relatively low beam dose over large fields of view. However, quantitatively extracting the magnitudes and directions of polarization vectors from NBED remains challenging. Here, we use a cepstral approach, similar to a pair distribution function, to determine local polar displacements that drive ferroelectricity from NBED patterns. Because polar distortions generate asymmetry in the diffraction pattern intensity, we can efficiently recover the underlying displacements from the imaginary part of the cepstrum transform. We investigate the limits of this technique using analytical and simulated data and give experimental examples, achieving the order of 1.1 pm precision and mapping of polar displacements with nanometer resolution.

High-coercivity SmFe12-based anisotropic sintered magnets by Cu addition

We achieved a record-high coercivity of 1.40 T in a SmFe12-based ThMn12-type anisotropic sintered magnet. This magnet was prepared by sintering jet-milled powder made from strip-cast alloys with a nominal composition of Sm8Fe73.5Ti8V8Al2Cu0.5, using the conventional process for anisotropic sintered magnet production. Detailed microstructural investigations conducted using scanning electron microscopy and transmission electron microscopy revealed that the formation of Fe-lean intergranular phases (with < 20 at.% of Fe) surrounding the fine SmFe12 phase grains resulted in the high coercivity. Nanobeam electron diffraction patterns showed that these Fe-lean intergranular phases (IGPs) exhibiting excellent wettability in this magnet are mainly composed of two phases: a hexagonal close-packed Mg-type Sm-based phase and a primitive cubic CsCl-type SmCu-based phase, which co-exist in the same intergranular region. Such Fe-lean IGPs minimize the intergranular exchange coupling in the SmFe12 main phase, which contributes to the high coercivity, as revealed by magneto-optical Kerr effect microscopy. Overall, this study provides detailed microstructure-coercivity correlations and insights for the future development of high-performance SmFe12-based sintered magnets.

High Resolution Powder Electron Diffraction in Scanning Electron Microscopy.

A modern scanning electron microscope equipped with a pixelated detector of transmitted electrons can record a four-dimensional (4D) dataset containing a two-dimensional (2D) array of 2D nanobeam electron diffraction patterns; this is known as a four-dimensional scanning transmission electron microscopy (4D-STEM). In this work, we introduce a new version of our method called 4D-STEM/PNBD (powder nanobeam diffraction), which yields high-resolution powder diffractograms, whose quality is fully comparable to standard TEM/SAED (selected-area electron diffraction) patterns. Our method converts a complex 4D-STEM dataset measured on a nanocrystalline material to a single 2D powder electron diffractogram, which is easy to process with standard software. The original version of 4D-STEM/PNBD method, which suffered from low resolution, was improved in three important areas: (i) an optimized data collection protocol enables the experimental determination of the point spread function (PSF) of the primary electron beam, (ii) an improved data processing combines an entropy-based filtering of the whole dataset with a PSF-deconvolution of the individual 2D diffractograms and (iii) completely re-written software automates all calculations and requires just a minimal user input. The new method was applied to Au, TbF3 and TiO2 nanocrystals and the resolution of the 4D-STEM/PNBD diffractograms was even slightly better than that of TEM/SAED.

Chemical short-range order in Fe50Mn30Co10Cr10 high-entropy alloy

Chemical short-range order (CSRO) is generally possible in concentrated solid solutions and currently of considerable interest for multi-principal element alloys. However, a convincing demonstration of CSRO has been challenging and achieved thus far only for ternary medium-entropy alloys such as VCoNi. Here, we report definitive proof of CSRO in a quaternary face-centered-cubic Fe50Mn30Co10Cr10 high-entropy alloy, acquired from systematic electron microscopy experiments. The evidence includes extra diffuse disks in nano-beam electron diffraction patterns, images in state-of-the-art aberration-corrected scanning transmission electron microscope, as well as compositional profiles across neighboring atomic planes/columns in atomic-resolution chemical maps. The CSRO regions are found to occupy an areal fraction of 20% and have dimensions on a sub-nanometer scale. This length scale, as well as the diffraction features of the CSRO, are different from those of intermetallic compound precipitates; as such, the CSRO is not a growing stage of a nucleated second phase, the precipitation of which has been dealt with previously in classical alloys. We further conducted a spatial correlation analysis of the concentrations in atomic columns in the chemical map, enabling us to uncover a general tendency toward nearest-neighbor chemical ordering, specifically, preference for unlike species (such as Fe–Mn) and avoidance for like-species (such as Fe–Fe). The persistence of this trend, the same as that found in VCoNi MEA, recently, is somewhat intriguing for a high-entropy alloy in which all the constituent elements are similar in atomic size and have rather a small enthalpy of mixing.

Discrete AlN mole fraction of n/12 (n = 4–8) in Ga-rich zones functioning as electron pathways created in nonflat AlGaN layers grown on high-miscut sapphire substrates

Ga-rich zones created along macrosteps in n-AlGaN plausibly function as electron pathways of AlGaN-based deep-ultraviolet (DUV) LEDs fabricated on AlN templates using 1.0°-miscut c(0001) sapphire substrates toward the m[1-100] axis. This work was performed to clarify AlN mole fractions (xAl) of Ga-rich zones. xAl ≃ (7/12, 6/12, and 5/12) was observed in Ga-rich zones in AlαGa1−αN (α ≃ 0.63, 0.55, and 0.43, respectively) by the method proposed in our previous article in which we showed that Ga-rich zones of Al8/12Ga4/12N were created in Al0.7Ga0.3N. xAl in the Ga-rich zones obtained from an energy-dispersive x-ray signal by scanning transmission electron microscopy calibrated by Rutherford backscattering well agreed with xAl obtained by cross-sectional cathodoluminescence (CL) spectroscopy using scanning electron microscopy. A weak CL shoulder peak corresponding to Al4/12Ga8/12N was also observed for Al0.43Ga0.57N. In addition, xAl ≃ n/12 (n = 6–9) in Al-rich zones appeared in the rest of the Ga-rich zones. Furthermore, nanobeam electron diffraction patterns of the Ga-rich zones indicated a high possibility of a regular configuration of Ga and Al atoms on the c(0001) plane in our samples. Consequently, xAl values in nonflat AlGaN layers with macrosteps were often determined to be near n/12 (n: integer). Thus, Ga-rich zones (xAl = n/12: n = 4–8) formed in our nonflat AlGaN layers, which originated from the macrosteps along [11–20] edgelines normal to the m[1–100] axis, are suggested to be metastable. The creation of discrete xAl in Ga-rich zones should contribute to the stable production of DUV-LEDs using high-miscut sapphire substrates.

Mechanistic Insights into Superlattice Transformation at a Single Nanocrystal Level Using Nanobeam Electron Diffraction

Understanding the mechanism and ultimately directing nanocrystal (NC) superlattice assembly and attachment have important implications on future advances in this emerging field. Here, we use 4D-STEM to investigate a monolayer of PbS NCs at various stages of the transformation from a hexatic assembly to a nonconnected square-like superlattice over large fields of view. Maps of nanobeam electron diffraction patterns acquired with an electron microscope pixel array detector (EMPAD) offer unprecedented detail into the 3D crystallographic alignment of the polyhedral NCs. Our analysis reveals that superlattice transformation is dominated by translation of prealigned NCs strongly coupled along the <11n>AL direction and occurs stochastically and gradually throughout single grains. We validate the generality of the proposed mechanism by examining the structure of analogous PbSe NC assemblies using conventional transmission electron microscopy and selected area electron diffraction. The experimental results presented here provide new mechanistic insights into NC self-assembly and oriented attachment.

The exit-wave power-cepstrum transform for scanning nanobeam electron diffraction: robust strain mapping at subnanometer resolution and subpicometer precision

Scanning nanobeam electron diffraction (NBED) with fast pixelated detectors is a valuable technique for rapid, spatially resolved mapping of lattice structure over a wide range of length scales. However, intensity variations caused by dynamical diffraction and sample mistilts can hinder the measurement of diffracted disk centers as necessary for quantification. Robust data processing techniques are needed to provide accurate and precise measurements for complex samples and non-ideal conditions. Here we present an approach to address these challenges using a transform, called the exit wave power cepstrum (EWPC), inspired by cepstral analysis in audio signal processing. The EWPC transforms NBED patterns into real-space patterns with sharp peaks corresponding to inter-atomic spacings. We describe a simple analytical model for interpretation of these patterns that cleanly decouples lattice information from the intensity variations in NBED patterns caused by tilt and thickness. By tracking the inter-atomic spacing peaks in EWPC patterns, strain mapping is demonstrated for two practical applications: mapping of ferroelectric domains in epitaxially strained PbTiO3 films and mapping of strain profiles in arbitrarily oriented core-shell Pt-Co nanoparticle fuel-cell catalysts. The EWPC transform enables lattice structure measurement at sub-pm precision and sub-nm resolution that is robust to small sample mistilts and random orientations.

Optimization of NBED simulations for disc-detection measurements

Nano-beam electron diffraction (NBED) is a method which can be applied to measure lattice strain and polarisation fields in strained layer heterostructures and transistors. To investigate precision, accuracy and spatial resolution of such measurements in dependence of properties of the specimen as well as electron optical parameters, simulations of NBED patterns are required which allow to predict the result of common disc-detection algorithms. In this paper we demonstrate by focusing on the detection of the central disc in crystalline silicon that such simulations require to take several experimental characteristics into account in order to obtain results which are comparable to those from experimental NBED patterns. These experimental characteristics are the background intensity, the presence of Poisson noise caused by electron statistics and blurring caused by inelastic scattering and by the transfer quality of the microscope camera. By means of these optimized simulations, different effects of specimen properties on disc detection – such as strain, surface morphology and compositional changes on the nanometer scale – are investigated and discussed in the context of misinterpretation in experimental NBED evaluations. It is shown that changes in surface morphology and chemical composition lead to measured shifts of the central disc in the NBED pattern of tens to hundreds of µrad. These shifts are of the same order of magnitude or even larger than shifts that could be caused by an electric polarisation field in the range of MV/cm.

Crystallization induced ordering of hard magnetic L1 phase in melt-spun FeNi-based ribbons

The microstructure of newly developed hard magnetic Fe42Ni41.3SixB12-xP4Cu0.7 (x = 2 to 8 at%) nanocrystalline alloy ribbons has been studied by transmission electron microscopy (TEM) and electron diffraction. A high-density polycrystalline grains, ∼30 nm in size, were formed in a ribbon after annealing at 673 K for 288 hours. Elemental mapping of the annealed specimen revealed the coexistence of three regions, Fe-rich, Ni-rich, and nearly equiatomic Fe-Ni, with areal fractions of 37%, 40%, and 23 %, respectively. The equiatomic L10-type ordered phase of FeNi was detected in between the Fe and Ni-rich phases. The presence of superlattice reflections in nanobeam electron diffraction patterns confirmed the formation of the hard magnetic L10 phase beyond any doubt. The L10 phase of FeNi was detected in alloys annealed in the temperature range of 673 to 813 K. The present results suggest that the order-disorder transition temperature of L10 FeNi is higher than the previously reported value (593 K). The high diffusion rates of the constituent elements induced by the crystallization of an amorphous phase at relatively low temperature (∼673K) are responsible for the development of atomic ordering in FeNi.

Evidence of rhombohedral structure within BiFeO3 thin film grown on SrTiO3

Comprehensive crystal structure analysis was performed for a BiFeO3 thin layer (∼30 nm) grown on a SrTiO3 substrate using cross-sectional transmission electron microscopy along three different zone axes. Nano-beam electron diffraction patterns combined with structure factor calculations and high-resolution transmission electron microscopy images unambiguously revealed that the BiFeO3 thin layer grew with a rhombohedral structure identical to its bulk form. No evidence of monoclinic and/or tetragonal distortion was found. The rhombohedral BiFeO3 thin layer was found to grow onto SrTiO3 by maintaining an epitaxial relationship in a manner minimizing the lattice mismatch at the BiFeO3/SrTiO3 interface.

Insights from STEM and NBED studies into the local structure and growth mechanism of misfit layered compounds prepared using modulated reactants

Abstract X-ray diffraction and transmission electron microscopy were used to probe the structure of the misfit compound [(SnSe)1.15]1(VSe2)1 grown using an elementally modulated precursor. The specular X-ray diffraction pattern contained only 00l reflections, which yielded a c lattice parameter of 1.203(1) nm. Cross-section STEM revealed alternating layers of SnSe and VSe2, in agreement with the structure model refined from the X-ray diffraction pattern using Rietveld refinement. Plan-view transmission electron microscopy revealed the in-plane grain structure of the films, yielding grain sizes in agreement with previously reported in plane X-ray diffraction studies and the cross-section STEM images. The plan view images also contained Moiré fringes resulting from grains with different relative tilting on both sides of interfaces as well as Moiré fringes resulting from different relative rotations between domains. An energy-filtered nano-beam electron diffraction pattern obtained from at least one domain in the [(SnSe)1.15]1(VSe2)1 sample investigated in cross section contained a series of resolvable supercell reflections along the c axis that indicated that the supercell c-axis lattice parameter was a multiple of three times that determined using X-ray diffraction. Energy filtered NBED of plan-view samples showed diffraction patterns from select regions with 12-fold symmetry, indicating that the arrangement of the layers is not rotationally random from layer to layer. This suggests that during the self-assembly of the amorphous modulated elemental precursor, the SnSe and VSe2 constituent layers must nucleate off the adjacent interfaces of the growing crystal, yielding layers that are locally rotationally aligned with growing crystal. Different processing conditions during the precursor to crystal self-assembly might enable the domain size and/or the extent of order to be controlled.

Nanometer-scale crystallinity in In–Ga–Zn-oxide thin film deposited at room temperature observed by nanobeam electron diffraction

We report a detailed analysis of the nanoscopic structure of an In–Ga–Zn-oxide (IGZO) thin film deposited at room temperature. Using nanobeam electron diffraction (NBED) with a 1-nm-diameter probe, we analyzed a nanoscopic region of the IGZO thin film in which the diffraction spots of crystal were unobservable in a macroscopic region using selected-area electron diffraction (SAED) with about 300-nm-diameter probe. However, they were clearly recognized in a NBED pattern. They indicate a minute crystalline structure in the film. We further confirm the presence of crystals by observing a NBED pattern with spots distributed with six-fold symmetry in a specimen thickness thinned down to10 nm. To check the effect of the electron-beam irradiation during a transmission electron microscopy (TEM) and NBED measurements, we examine changes in crystal size as a function of the cumulative electron dose. No crystal growth during the electron beam irradiation was observed in the film. These results allow us to conclude that the IGZO thin film is not amorphous but a nanocrystalline (nc)-IGZO thin film, composed of nanometer-size crystals.

The Analytical Transmission Electron Microscopy: A Powerful Tool for the Investigation of Low‐Dimensional Carbon Nanomaterials

Chemical and physical characterization of nanomaterials is essential to improve synthesis processes, for new technological and commercial applications, and to assess their toxicity through in vitro and in vivo studies. New nanomaterials and new synthesis processes are continuously tested and updated to exploit their innovative properties. In this paper, low‐dimensional carbon nanostructure characterization was performed using analytical transmission electron microscopy. Conventional and advanced microscopy techniques, such as acquisition of high resolution images, nanobeam electron diffraction patterns, X‐ray energy dispersion, and electron energy loss spectra, were used to determine the main physical and chemical properties of single wall and multiwall carbon nanotubes, graphene flakes, and amorphous carbon films. Through the resulting micrographs, diffraction patterns, and spectra, the main low‐dimensional carbon nanostructures properties were determined in terms of structural defects and/or the presence of metallic or heavy elements, such as those used as catalyst or to decorate nanotubes. The obtained information is of crucial importance to investigate low‐dimension nanomaterial biological activity.

Applying precipitate–host lattice coherency for compositional determination of precipitates in Al–Mg–Si–Cu alloys

The present paper reports experimental and theoretical studies on the structure of the metastable C precipitate forming during precipitation hardening in Al–Mg–Si–Cu (6xxx) alloys. We describe the procedure of deriving an initial unit cell model based on experimental data and how this is further refined by quantitative use of nanobeam electron diffraction patterns. A reliable 3D refinement was prevented by the small precipitate thickness and its disorder/intergrowth with other phases, necessitating the development of a more theoretically based methodology for precipitate composition determination. We find that for experimental results to be acceptably reproduced in density functional theory-based calculations on bulk candidate structures, these would have to not only minimise precipitate formation enthalpy, but also reproduce the experimentally reported negligible lattice mismatch with the Al matrix along the precipitate main growth direction. We argue, through comparison of the isostructural Q′ and Q precipitate phases of Al–Mg–Si–Cu alloys, why the experimentally reported (semi-)coherency of metastable precipitates must be included as an optimisation criterion in a general theoretical analysis. The C phase structure determined has a monoclinic unit cell with space group P21/m, cell dimensions a C = 10.32 Å, b C = 4.05 Å, c C = 8.10 Å, β C = 100.9°, coherency relations with the matrix: ∥ ⟨150⟩Al, ∥ ⟨001⟩Al, ∥ ⟨100⟩Al and composition Mg4AlSi3+ x Cu1− x , with x ∼ 0.3. The non-integer number of atoms in the unit cell emphasises that the concept of a unit cell for this phase is weakly defined.

Low-temperature synthesis of Zn3P2 nanowire

Abstract

Investigation of Process Parameters on the Properties of Selective Epitaxial Growth SiGe Structure

This work presents the effect of process parameters such as substrate condition, flow rate of precursors and dopant on selective epitaxial growth (SEG) of Si1-XGeX layer in recessed source and drain (S/D) regions of pMOSFET structure. It was observed that the growth rate of SEG Si1-XGeX on patterned wafer was slightly less than that of Si1-XGeX on bare wafer as GeH4 flow rate increased. And an in-situ boron-doped SiGe layer exhibits lower growth rate and Ge content than intrinsic SiGe layer due to a decrease of GeH4 partial pressure. Nano beam electron diffraction (NBD), which is an efficient method to measure induced compressive strain in Si channel, is applied on both intrinsic and in-situ boron doped Si1-XGeX layers with a different Ge content. NBD patterns confirmed that higher Ge concentration introduces more strain in the channel region and in-situ boron doping of Si1-XGeX does not affect the channel strain.

Direct observations of Ge2Sb2Te5 recording marks in the phase-change disk

Atomistic structures of the Ge2Sb2Te5 thin film in the real phase-change disk have been investigated using transmission electron microscopy (TEM). As-deposited amorphous Ge2Sb2Te5 films were laser-irradiated for initialization (crystallization) and recording. Cross-sectional TEM observations revealed that the recording mark was fully amorphized by laser irradiation. A slight difference between the as-deposited and the laser irradiation-induced amorphous Ge2Sb2Te5 was observed in the intensity profile of nanobeam electron diffraction patterns and atomic pair distribution functions. This difference was attributed to structural relaxation of amorphous Ge2Sb2Te5, which gives rise to the alteration of chemical order.

Self-assembled Gd silicide nanostructures grown on Si(001)

The growth of self-assembled gadolinium silicide nanostructures on Si(001) was studied over a range of metal coverages and growth temperatures. Scanned probe microscopy was used to study the nanostructure morphology. The analyses of cross-sectional transmission electron microscopy images, plan view transmission electron microscopy moiré fringe patterns, and nanobeam electron diffraction patterns show that the Gd silicide nanostructures fall into two classes: elongated nanowires with hexagonal crystal structure and rectangular islands with orthorhombic (or tetragonal) crystal structure. These results indicate that the nanostructure morphology is controlled by the magnitude and direction of the lattice mismatch with respect to the substrate. Optimum growth conditions for preferential growth of either nanowires or islands are described. In addition, the phase transformation from hexagonal to the more stable orthorhombic (or tetragonal) phase is observed and discussed.

Local structural fluctuation in Pd–Ni–P bulk metallic glasses examined using nanobeam electron diffraction

Local structural fluctuation in relation to the medium range order (MRO) in a Pd–Ni–P bulk metallic glass was examined using nanobeam electron diffraction (NBED) technique. We found diffraction spots with strong intensities in the NBED patterns taken from any observation sites in the specimen. This indicates a presence of MRO regions densely formed in the specimen. The diffraction spots in NBED were normally dispersed around positions corresponding to the first halo-diffraction ring in selected area diffraction. A low-temperature annealing led to form a nanocrystalline microstructure consisting of unidentified phosphides. In the course of annealing, the MRO structures deduced from the NBED patterns have no structural similarity to the phosphides found in the primary crystallization stage. The MRO structure changes into a similar structure with the primary crystals just before the crystallization. A discussion is made for the MRO structure and its relation to the glass stability and also to the phosphide nucleation in the primary crystallization.