High-resolution Z-contrast Imaging Research Articles

Strategy for reliable strain measurement in InAs/GaAs materials from high-resolution Z-contrast STEM images

Geometric phase analysis (GPA), a fast and simple Fourier space method for strain analysis, can give useful information on accumulated strain and defect propagation in multiple layers of semiconductors, including quantum dot materials. In this work, GPA has been applied to high resolution Z-contrast scanning transmission electron microscopy (STEM) images. Strain maps determined from different g vectors of these images are compared to each other, in order to analyze and assess the GPA technique in terms of accuracy. The SmartAlign tool has been used to improve the STEM image quality getting more reliable results. Strain maps from template matching as a real space approach are compared with strain maps from GPA, and it is discussed that a real space analysis is a better approach than GPA for aberration corrected STEM images.

Evaluation of high-quality image reconstruction techniques applied to high-resolution Z-contrast imaging

High-quality image reconstruction techniques allow the generation of high pixel density images from a set of low-resolution micrographs. In general, these techniques consist of two main steps, namely, accurate registration, and formulation of an appropriate forward image model via some restoration method. There exist a wide variety of algorithms to cope with both stages and depending on their practical applications, some methods can outperform others, since they can be sensitive to the assumed data model, noise, drift, etc. When dealing with images generated by Z-contrast scanning transmission electron microscopes, a current trend is based on non-rigid approximations in the registration stage. In our work we aimed at reaching similar accuracy but addressing the most complex calculations in the reconstruction stage, instead of in the registration stage (as the non-rigid approaches do), but using a much smaller number of images. We review some of the most significant methods and address their shortcomings when they are applied to the field of microscopy. Simulated images with known targets will be used to evaluate and compare the main approaches in terms of quality enhancement and computing time. In addition, a procedure to determine the reference image will be proposed to minimise the global drift on the series. The best registration and restoration strategies will be applied to experimental images in order to point up the enhanced capability of this high quality image reconstruction methodology in this field.

Study on nanophase iron oxyhydroxides in freshwater ferromanganese nodules from Green Bay, Lake Michigan, with implications for the adsorption of As and heavy metals

Nanophase Fe-oxyhydroxides in freshwater ferromanganese nodules (FFN) from Green Bay, Lake Michigan, and adsorbed arsenate have been investigated by X-ray powder diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), Z-contrast imaging, and ab initio calculations using the density functional theory (DFT). The samples from northern Green Bay can be divided into two types: Fe-Mn nodules and Fe-rich nodules. The manganese-bearing phases are todorokite, birnessite, and buserite. The iron-bearing phases are feroxyhyte, nanophase goethite, two-line ferrihydrite, and nanophase FeOOH with guyanaite structure. Z-contrast images of the Fe-oxyhydroxides show ordered FeOOH nano-domains with guyanaite structure intergrown with nanophase goethite. The FeOOH nanophase is a precursor to the goethite. Henceforth, we will refer to it as “proto-goethite.” DFT calculations indicate that goethite is more stable than proto-goethite. Our results suggest that ordering between Fe and vacancies in octahedral sites result in the transformation from feroxyhyte to goethite through a proto-goethite intermediate phase. Combining Z-contrast images and TEM-EDS reveals that arsenate ( ![Formula][1] ) tetrahedra are preferentially adsorbed on the proto-goethite (001) surface via tridentate adsorption. Our study directly shows the atomic positions of Fe-oxyhydroxides with associated trace elements. The methods can be applied for identifying structures of nanophases and adsorbed trace elements and heavy metals. [1]: /embed/mml-math-1.gif

Preferential sites for InAsP/InP quantum wire nucleation using molecular dynamics

In this paper, stress fields at the surface of the capping layer of self-assembled InAsP quantum wires grown on an InP (001) substrate have been determined from atomistic models using molecular dynamics and Stillinger-Weber potentials. To carry out these calculations, the quantum wire compositional distribution was extracted from previous works, where the As and P distributions were determined by electron energy loss spectroscopy and high-resolution aberration-corrected Z-contrast imaging. Preferential sites for the nucleation of wires on the surface of the capping layer were studied and compared with (i) previous simulations using finite element analysis to solve anisotropic elastic theory equations and (ii) experimentally measured locations of stacked wires. Preferential nucleation sites of stacked wires were determined by the maximum stress location at the MD model surface in good agreement with experimental results and those derived from finite element analysis. This indicates that MD simulations based on empirical potentials provide a suitable and flexible tool to study strain dependent atom processes.

Microstructure of the potentially multiferroic Fe/BaTiO3 epitaxial interface

The Fe/BaTiO3 thin-film layered structure is a prototype of charge-mediated composite multiferroics, which is a promising but challenging route to achieve a sizable magnetoelectric effect. The real structure of the interface between the ferromagnetic Fe and ferroelectric BaTiO3 layers is crucial. In this paper, epitaxial Fe layers were successfully grown on top of BaTiO3 layers by carefully controlling the pulsed laser deposition and magnetron sputtering procedures. A detailed study of interfacial structure and defects at the Fe/BaTiO3 interface was carried out by transmission electron microscopy (TEM). Electron diffraction patterns and diffraction contrast images reveal a definite epitaxial relationship between Fe and BaTiO3 (001) films and a semi-coherent interface with nearly periodic interfacial dislocations. Based on high-resolution TEM images from both [010] and [110] direction observations, the interfacial dislocations were found to be partial with Burgers vectors and line directions of ⟨010⟩. By employing high-resolution Z-contrast imaging, the positions of individual atoms columns were resolved. The formation mechanism of interfacial dislocations was proposed in terms of geometrical models of the interface structure. On the basis of the remaining strain analysis in each layer, the effects of both BaTiO3 thickness and the SrTiO3 substrates on the density of the interface defects were discussed.

Counting Tm dopant atoms around GaN dots using high-angle annular dark field images

High resolution Z-contrast STEM imaging is used to study the Tm doping of GaN quantum dots grown in AlN by molecular beam epitaxy (MBE). High-angle annular dark field (HAADF) imaging allows us to visualize directly individual Tm atoms in the AlN matrix and even to count the number of Tm atoms in a given AlN atomic column. A new visibility coefficient to determine quantitatively the number of Tm atoms in a given atomic column is introduced. It is based on locally integrated intensities rather than on peak intensities of HAADF images. STEM image simulations shows that this new visibility is less sensitive to the defocus-induced blurring or to the position of the Tm atom within the thin lamella. Most of the Tm atoms diffuse out of GaN dots. Tm atoms are found at different positions in the AlN matrix, (i) Above the wetting layer, Tm atoms are spread within a thickness of 14 AlN monolayers (MLs). (ii) Above the quantum dots all the Tm are located in the same plane situated at 2-3 MLs above the apex of the GaN dot, i.e. at a distance of 14 MLs from the wetting layer, (iii) In addition, Tm can diffuse very far from the GaN dot by following threading dislocations lines.

Structural and microstructural analyses of crystalline Er2O3 high-k films grown on Si (0 0 1) by laser molecular beam epitaxy

Erbium oxide (Er2O3) films are well regarded as being suited for high-k replacement of SiO2 in endeavors to further miniaturize and enhance the performance of microelectronics. Er2O3 films were deposited on Si (001) substrates by laser molecular beam epitaxy. The structures and microstructures of the films and the interfacial layers were characterized by means of X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results from the XRD and selected area electron diffractions of Er2O3 films with thicknesses of 30 and 100nm indicate that the films are polycrystalline, with dominant (111) textures of Er2O3 {111} // Si (001). Amorphous layers dotted with small ordered islands were observed and confirmed to be located at the interfaces between the films and the Si substrates with dark-field image and high-resolution TEM. High-resolution Z-contrast imaging, energy dispersive X-ray spectroscopy and energy-filtered imaging were applied to identify the compositions of the interfacial layers. The salient feature is that the layers consist primarily of Er and O, with a very small amount of Si. This kind of Er–O-based interface layer may play a very important role in the electrical and optical properties of the films.

Phasoid intergrowth between the double perovskite Sr 2MgMoO 6 and the n = 2 R-P phase Sr 3Mo 2O 7

A complex layered oxide has been synthesised as bulk material starting from the theoretical n = 2 R-P formula Sr 3(Mg x Mo 1 − x ) 2O 7 − δ ( x ≈ 0.25). This compound exhibits an unexpected crystal chemistry illustrated by the coexistence of several stacking modes — principally the ordered double perovskite Sr 2MgMoO 6 − δ and n = 2 R-P type phase — inside the same crystallite. Chemical EDS analyses performed by TEM techniques have lead to an average chemical formula Sr 2.68Mg 0.64Mo 1.67, which differs from the expected n = 2 R-P formula Sr 3Mg 0.5Mo 1.5O 7 − δ in particular through a significant Sr deficiency. From high resolution electron microscopy (HREM) and Z-contrast (SuperSTEM) images, this analytical result is interpreted by the formation of a complex intergrowth [(Sr,Mg) 3Mo 2O 7] 1 − x [Sr 2MgMoO 6] x with x ≈ 0.4. The resulting structure can be interpreted as a compositional segregation between Mg 2+ and Mo 6+ species in two basic structures, namely Sr 3Mo 2O 7 and Sr 2MgMoO 6.

Determining the Morphology of Polystyrene-block-poly(2-vinylpyridine) Micellar Reactors for ZnO Nanoparticle Synthesis

We report the use of reverse PS-b-P2VP diblock copolymer micelles as true nanoscale-sized reactor vessels to synthesize ZnO nanoparticles. The reverse micelles were formed in toluene and then sequentially loaded with zinc acetate dihydrate and tetramethylammonium hydroxide reactants. Moreover, high spatial resolution Z-contrast imaging and EDX spectroscopy techniques were used to confirm the segregation of the Zn cation to the core of the loaded micelles. Determining the chemical distribution with high nanoscale spatial resolution is shown to complement the less direct characterization by AFM, DLS and FTIR, thus demonstrating broader implications for the characterization of hybrid nanocomposite systems.

Interface atomic structure of LaCuOSe:Mg epitaxial thin film and MgO substrate

In order to understand the growth mechanism of magnesium doped LaCuOSe thin film on MgO, the atomic structure of the interface between the epitaxial thin film and the substrate was investigated. Electron diffraction confirmed the orientation relationship between the film and the substrate as (0 0 1)[1 0 0]LaCuOSe:Mg//(0 0 1)[1 0 0]MgO. High resolution Z-contrast imaging based on the high angle annular dark field scanning transmission electron microscope (HAADF-STEM) technique clearly revealed the alternate stacking of La–O layers and Cu–Se layers, and identified a peculiar stacking sequence of La layers at the interface. With the aid of first principles calculations of the interfacial adhesive energy, it was found that the different La layers at the interface play a key role in stabilizing the interface.

Detailed arsenic concentration profiles at Si/SiO2 interfaces

The pile-up of arsenic at the Si/SiO2 interface after As implantation and annealing was investigated by high resolution Z-contrast imaging, electron energy-loss spectroscopy (EELS), grazing incidence x-ray fluorescence spectroscopy (GI-XRF), secondary ion mass spectrometry, x-ray photoelectron spectroscopy, Rutherford backscattering spectrometry, as well as Hall mobility and four-point probe resistivity measurements. After properly taking into account their respective artifacts, the results of all methods are compatible with each other, with EELS and GI-XRF combined with etching providing similar spatial resolution on the nanometer scale for the dopant profile. The sheet concentration of the piled-up As at the interface was found to be ∼1×1015 cm−2 for an implanted dose of 1×1016 cm−2 with a maximum concentration of ∼10 at. %. The strain observed in the Z-contrast images also suggests a significant concentration of local distortions within 3 nm from the interface, which, however, do not seem to involve intrinsic point defects.

On the accuracy of maximum entropy reconstruction of high-resolution Z-contrast STEM images

The accuracy of maximum entropy reconstruction of Z-contrast STEM images has been evaluated with the effects of experimental variables and noise taken into account by the means of image simulation. As the specimen contains atom species of greatly different atomic numbers, special attention is given to the reliability of the position and composition of lighter atoms that are determined from Z-contrast images in the presence of heavier atoms. When the noise is moderate (SNR >2.5), the position of atom columns can be measured within an accuracy of 0.03 nm. With a higher signal-to-noise ratio (SNR >5) the composition of lighter atoms can be resolved reliably from the Z-contrast images. However, when image noise increases, the relative intensity of lighter atoms may deviate from the actual value in the specimen object function.

A Method to Determine the Strain and Nucleation Sites of Stacked Nano-Objects

We determine the compositional distribution with atomic column resolution in a horizontal nanowire from the analysis of aberration-corrected high resolution Z-contrast images. The strain field in a layer capping the analysed nanowire is determined by anisotropic elastic theory from the resulting compositional map. The reported method allows preferential nucleation sites for epitaxial nanowires to be predicted with high spatial resolution, as required for accurate tuning of desired optical properties. The application of this method has been exemplified in this work for stacked InAs(P) horizontal nanowires grown on InP separated by 3 nm thick InP layers, but we propose it as a general method to be applied to other stacked nano-objects.

Measuring local lattice polarity in AlN and GaN by high resolution Z-contrast imaging: The case of (0001) and (11¯00) GaN quantum dots

By using a probe Cs corrected transmission electron microscope, the local lattice polarity, i.e., the local stacking of atoms, in AlN and GaN is determined. The N atomic columns are not directly resolved, but determined from the tunnel positions of the structure. The method is first tested on GaN quantum dots grown on polar (0001) AlN surfaces. Then it is applied to GaN quantum dots grown on nonpolar (11¯00) AlN surfaces. In the (112¯0) plane, (11¯00) GaN dots have an asymmetrical shape and grow faster along the +c=[0001] direction than along −c=[0001¯].

Focused Ion Beam Milled InGaN/GaN Multiple Quantum Well Nanopillars

The fabrication of nanopillars containing InGaN/GaN multiple quantum wells has been achieved via focused ion beam, using modified beam-shape tuning. Tuning the ion beam astigmatism and/or focusing permits control of the size and shape of III–nitride islands, which makes feasible the fast writing of large-area InGaN/GaN island arrays on epi-layer sample embedded with multiple quantum wells. Interestingly, some islands became high-aspect-ratio nanopillars intermittently under an adequate degree of defocus. Typically, the pillars have diameters of around 100–150 nm and heights of around 600–1500 nm. High-resolution cross-sectional transmission electron microscopy and Z-contrast imaging revealed the preserved perfection of single-crystalline nature and quantum well structure of one single pillar, respectively. The optical characterization on the pillars was also carried out by cathodoluminescence, and the peak position showed a blue shift of 35 meV compared with the original epi-layer wafer.

High-resolution Z-contrast imaging and EELS study of functional oxide materials

Functional complex-oxide materials show a wide variety of properties and behaviors that cannot be found in any other class of materials, including high-temperature superconductivity and colossal magneto resistance. Consequently, this group of oxide materials has become the focus of many experimental as well as theoretical studies, aiming at understanding the fundamental mechanisms and properties that govern these complex structures. Here, we will review our high-resolution Z-contrast imaging and electron energy-loss studies of two complex-oxide materials systems, more specifically low-angle tilt grain-boundaries in YBa 2Cu 3O 7 (YBCO), and the spin-state transition in LaCoO 3. It will be shown that the O K-edge pre-peak can be used to quantify the hole-concentration in the vicinity of the dislocation core in YBCO, as well as to determine the Co 3+ spin-state in LaCoO 3.

Atomic resolution ADF-STEM imaging of organic molecular crystal of halogenated copper phthalocyanine

Annular dark-field (ADF) scanning transmission electron microscopy (STEM) measurements are demonstrated for the first time to be applicable for acquiring Z-contrast images of organic molecules at atomic resolution. High-angle ADF imaging by STEM is a new technique that provides incoherent high-resolution Z-contrast images for organic molecules. In the present study, low-angle ADF-STEM is successfully employed to image the molecular crystal structure of hexadecachloro-Cu-phthalocyanine (Cl 16-CuPc), an organic molecule. The structures of CuPc derivatives (polyhalogenated CuPc with Br and Cl) are determined quantitatively using the same technique to determine the occupancy of halogens at each chemical site. By comparing the image contrasts of atomic columns, the occupancy of Br is found to be ca. 56% at the inner position, slightly higher than that for random substitution and in good agreement with previous TEM results.

Synthesis and Characterization of Magnetic-Optical Co−Au Core−Shell Nanoparticles

We report the successful synthesis of Co−Au core−shell nanoparticles by reducing an organo-gold compound onto cobalt seeds with a weak reducer in a nonpolar solvent. The core−shell morphology was unequivocally confirmed by complementary structural, magnetic, and optical property measurements. High-resolution transmission electron microscopy and Z-contrast imaging shows a gold shell composed of multiple grains. Electron-energy loss spectroscopy confirms the chemically distinct characteristics of the core (Co) and shell (Au). Temperature-dependent magnetic property measurements confirm that the particles are superparamagnetic with a blocking temperature, TB ∼ 55 K, consistent with a magnetic diameter ∼6 nm. The UV−visible absorption spectra of these nanoparticles show a red shift (relative to pure gold nanoparticles) in agreement with a Au-shell morphology.

Determination of the strain generated in InAs/InP quantum wires: prediction of nucleationsites

The compositional distribution in a self-assembled InAs(P) quantum wire grownby molecular beam epitaxy on an InP(001) substrate has been determined byelectron energy loss spectrum imaging. We have determined the strain and stressfields generated in and around this wire capped with a 5 nm InP layer by finiteelement calculations using as input the compositional map experimentally obtained.Preferential sites for nucleation of wires grown on the surface of this InP capping layerare predicted, based on chemical potential minimization, from the determinedstrain and stress fields on this surface. The determined preferential sites for wirenucleation agree with their experimentally measured locations. The method used inthis paper, which combines electron energy loss spectroscopy, high-resolutionZ contrast imaging, and elastic theory finite element calculations, is believed to be a valuabletechnique of wide applicability for predicting the preferential nucleation sites of epitaxialself-assembled nano-objects.

Atomic-scale microstructures of Zr 2Al 3C 4 and Zr 3Al 3C 5 ceramics

The microstructures of bulk Zr 2Al 3C 4 and Zr 3Al 3C 5 ceramics have been investigated using transmission electron microscopy and scanning transmission electron microscopy. These two carbides were determined to have a point group 6/ mmm and a space group P6 3/ mmc using selected-area electron diffraction and convergent beam electron diffraction. The atomic-scale microstructures of Zr 2Al 3C 4 and Zr 3Al 3C 5 were investigated through high-resolution imaging and Z-contrast imaging. Furthermore, intergrowth between Zr 2Al 3C 4 and Zr 3Al 3C 5 was identified. Stacking faults in Zr 3Al 3C 5 were found to result from the insertion of an additional Zr–C layer. Cubic ZrC was occasionally identified to be incorporated in elongated Zr 3Al 3C 5 grains. In addition, Al may induce a twinned ZrC structure and lead to the formation of ternary zirconium aluminum carbides.