HAADF STEM Research Articles

HAADF–STEM atom counting in atom probe tomography specimens: Towards quantitative correlative microscopy

The geometry of atom probe tomography tips strongly differs from standard scanning transmission electron microscopy foils. Whereas the later are rather flat and thin (<20nm), tips display a curved surface and a significantly larger thickness. As far as a correlative approach aims at analysing the same specimen by both techniques, it is mandatory to explore the limits and advantages imposed by the particular geometry of atom probe tomography specimens. Based on simulations (electron probe propagation and image simulations), the possibility to apply quantitative high angle annular dark field scanning transmission electron microscopy to of atom probe tomography specimens has been tested. The influence of electron probe convergence and the benefice of deconvolution of electron probe point spread function electron have been established. Atom counting in atom probe tomography specimens is for the first time reported in this present work. It is demonstrated that, based on single projections of high angle annular dark field imaging, significant quantitative information can be used as additional input for refining the data obtained by correlative analysis of the specimen in APT, therefore opening new perspectives in the field of atomic scale tomography.

Combining HAADF STEM tomography and electron diffraction for studies of α-Al(Fe,Mn)Si dispersoids in 3xxx aluminium alloys

A new methodology to study the precipitation crystallography of dispersoids in an Al matrix is proposed. By combining high angle annular dark field tomography and electron diffraction studies, the three-dimensional morphology, orientation relationship (OR) with Al matrix and habit planes of the dispersoids can be achieved simultaneously. This approach has been applied to investigate the -Al(Mn,Fe)Si dispersoids precipitated in an AA3xxx alloy. Most dispersoids have a plate-shaped morphology after low-temperature homogenization at 450C. The largest proportion of the dispersoids follows the previously described OR with the Al matrix . Two plate-shaped dispersoids have been studied in detail. The dispersoid following the commonly observed orientation had habit planes . The dispersoid not following the commonly observed OR had habit planes , with (OR) , .

Growth and characterization of Titanium Niobium Oxide (TiNb2O7) thin films

ABSTRACTComprehensive studies were done on the growth and characterization of TiNb2O7 (TNO) complex oxide thin films by pulsed laser deposition for the first time. The TNO thin films were successfully grown on Pt(200)/TiO2/SiO2/Si(100) substrates. The structure, surface morphology and chemical properties of as-grown thin films were studied as function of deposition temperature, pressure and laser fluence. The GIXRD and HRTEM analyses revealed that the as-grown TNO films were in the monoclinic crystal structure and independent of laser fluence. The HAADF STEM elemental mapping confirms the uniform composition of Ti, Nb and O in TNO thin films. The atomic force microscopy and field emission scanning microscopy shows that, the surface morphology and microstructure of TNO films varied significantly with respect to experimental conditions. The X-ray photoelectron spectroscopy quantitative results indicated that the binding energies of Ti and Nb elements shifted towards right with increasing oxygen partial pressure. The effects of oxygen partial pressure and laser fluence on as-grown TNO films were studied.

Insight into the Atomic Structure of Cycled Lithium-Rich Layered Oxide Li1.20Mn0.54Co0.13Ni0.13O2 Using HAADF STEM and Electron Nanodiffraction

Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) and electron nanodiffraction investigations have been carried out to follow changes in the local atomic structure of three Li-rich layered oxides recovered after a single charge–discharge cycle, performed in different conditions. The structure of the pristine material Li1.20Mn0.54Co0.13Ni0.13O2 was fully characterized. It was then compared to those of the materials recovered after one electrochemical cycle in a lithium battery, with the upper voltage limit being either just below the voltage of the irreversible “plateau” typical of Li-rich layered oxides or just above. An in-depth study of the material obtained chemically, after oxidation to deintercalate Li and then reduction to reintercalate Li, was also performed for comparison. The main message of this paper is that the “plateau” is not associated with an extended structural reorganization of the material. The irreversible processes associated wit...

Improved Oxygen Reduction Activity and Durability of Dealloyed PtCo x Catalysts for Proton Exchange Membrane Fuel Cells: Strain, Ligand, and Particle Size Effects.

The development of active and durable catalysts with reduced platinum content is essential for fuel cell commercialization. Herein we report that the dealloyed PtCo/HSC and PtCo3/HSC nanoparticle (NP) catalysts exhibit the same levels of enhancement in oxygen reduction activity (~4-fold) and durability over pure Pt/C NPs. Surprisingly, ex situ high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) shows that the bulk morphologies of the two catalysts are distinctly different: D-PtCo/HSC catalyst is dominated by NPs with solid Pt shells surrounding a single ordered PtCo core; however, the D-PtCo3/HSC catalyst is dominated by NPs with porous Pt shells surrounding multiple disordered PtCo cores with local concentration of Co. In situ X-ray absorption spectroscopy (XAS) reveals that these two catalysts possess similar Pt-Pt and Pt-Co bond distances and Pt coordination numbers (CNs), despite their dissimilar morphologies. The similar activity of the two catalysts is thus ascribed to their comparable strain, ligand, and particle size effects. Ex situ XAS performed on D-PtCo3/HSC under different voltage cycling stage shows that the continuous dissolution of Co leaves behind the NPs with a Pt-like structure after 30k cycles. The attenuated strain and/or ligand effects caused by Co dissolution are presumably counterbalanced by the particle size effects with particle growth, which likely accounts for the constant specific activity of the catalysts along with voltage cycling.

Tunable transmission light in nanocrystalline La1−xEuxB6

Cubic-shaped ternary La1−xEuxB6 nanopowders have been synthesized via a solid-state reaction and their optical properties are investigated for the first time. La1−xEuxB6 nanopowders have a high crystallinity with a grain size of 40nm and HAADF–STEM images fully confirm that the Eu element has been successfully doped into the lattice of LaB6. The optical absorption results indicate that the transmission light wavelength of LaB6 shifts from 590nm to 1070nm with increasing Eu doping content, indicating a “redshift” phenomenon of transmission light. Thus, this novel tunable characteristic of La1−xEuxB6 transmission light will open a new way to extend the optical applications of LaB6 nanopowder.

Strain Measurement with Nanometre Resolution by Transmission Electron Microscopy

Strain is routinely used in state-of-the-art semiconductor devices in order to improve their electrical performance. Here we present experimental strain measurements obtained by different transmission electron microscopy (TEM) based techniques. Dark field electron holography, nanobeam electron diffraction (NBED) and high angle annular dark field scanning electron microscopy (HAADF STEM) are demonstrated. In this paper we demonstrate the spatial resolution and sensitivity of these different techniques on a simple calibration specimen where the accuracy of the measurement can easily be assessed.

Unexpected Bismuth Concentration Profiles in MOVPE GaAs1-xBix Films Revealed by HAADF STEM Imaging

Unexpected Bismuth Concentration Profiles in MOVPE GaAs_1-xBi_x Films Revealed by HAADF STEM Imaging

Understanding the Surface Structure of LiNi0.45Mn1.55O4 Spinel Cathodes with Aberration-Corrected HAADF STEM

Understanding the Surface Structure of LiNi_0.45Mn_1.55O₄ Spinel Cathodes with Aberration-Corrected HAADF STEM

Study of Au/Pd and Au/Co Bimetallic Nanoparticles Using Aberration Corrected STEM

The synergetic combination of two metals in bimetallic (BM) nanoparticle (NP) significantly changes the optical, catalytic, electronic and magnetic properties. The possible reason for synergetic effects is the heterometallic bond between two metals, dilution of active site, the combination of two kinds of metals and their fine structures. The enhanced catalytic properties of BM nanoparticles are widely used in several industrial reactions. For practical applications and better understanding of this system, there should be well-controlled synthesis of shape and size selected structures, powerful structural characterization tools including advanced in situ atomic-level analytical techniques, modern modeling tools with supercomputers, and in-depth understanding of structure-property relationship. In this paper, we are presenting the use of Z-contrast imaging in aberration-corrected HAADF STEM images in the study of atomic ordering in nanoalloys and core/shell and also investigating the growth mechanism in Au-Pd and the interface region between Au and Pd.

Novel M1/M2 Heterostructure in Mo-V-M-Ta (M = Te or Sb) Complex Oxide Catalyst Revealed by Aberration Corrected HAADF STEM

Novel M1/M2 Heterostructure in Mo-V-M-Ta (M = Te or Sb) Complex Oxide Catalyst Revealed by Aberration Corrected HAADF STEM

Getting the Best from an Imperfect Detector - an Alternative Normalisation Procedure for Quantitative HAADF STEM

Getting the Best from an Imperfect Detector - an Alternative Normalisation Procedure for Quantitative HAADF STEM

HAADF STEM of Phase Separated Anion Exchange Membranes Prepared by Ultracryomicrotomy

HAADF STEM of Phase Separated Anion Exchange Membranes Prepared by Ultracryomicrotomy

Visualising the Three-dimensional Morphology and Surface Structure of Metallic Nanoparticles at Atomic Resolution by Automated HAADF STEM Atom Counting

Visualising the Three-dimensional Morphology and Surface Structure of Metallic Nanoparticles at Atomic Resolution by Automated HAADF STEM Atom Counting

Titanium uptake and incorporation into silica nanostructures by the diatom Pinnularia sp. (Bacillariophyceae)

Diatoms are an ecologically successful group within the phytoplankton, and their special feature is a biofabricated silica cell encasement called a frustule. These frustules attract interest in material technology, and one potential application is to use them in solar cell technology. The silica frustule with its nanoscaled pattern is interesting per se, but the utility is enhanced if we succeed in incorporating other elements. Titanium is an interesting element because its oxide is a semi-conductor with a high band gap. However, doping with relevant elements through bioincorporation is challenging, and it is necessary to understand the biology involved in element uptake and incorporation. Here we present data on bioincorporation of Ti into the silica frustules of the pennate diatom Pinnularia sp. (Ehrenberg) and show that the distribution of the incorporated Ti is inhomogeneous both between and within valves. More than a tenfold increase of Ti in newly synthesised valves was achieved, and increased Ti around the pores was confirmed by both EDS and EELS analyses. HAADF STEM spectroscopy revealed a grainy surface with amorphous silica particles of 4 to 5 nm in size. These observations are explained by what is known from the physico-chemical processes involved in biosilification and frustule formation, looking into it from a biological point of view.

Two-dimensional Object Functions and Three-dimensional Illumination Functions: their Validity, Interaction and Utility

In high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) the data is generally interpreted as the convolution of the sample's sharply-peaked object function with the intensity of the real-valued point-spread function (PSF) of the illumination. As a single image is itself necessarily two-dimensional (2D) it is typically assumed that the object function and PSF can also be accurately described as 2D; this is the so-called 2D-object imaging assumption. Here the validity of these two-assumptions is evaluated using experimental HAADF STEM focal-series. It is found that the contrast contained in each image of the focal-series is accurately described by the convolution of a focus-invariant 2D object with a 2D optical-transfer function (OTF) which describes the illumination at the focus value used for imaging.

Structural quantification of nanoparticles by HAADF STEM

A new method of quantifying the HAADF STEM signal on absolute scale, building on the z-contrast nature of the technique, has been developed to address the problem of characterising nanoparticles to an atomic scale. Experimental images are scaled to a fraction of the incident beam intensity from a detector map. The integrated intensity of each individual atomic column is multiplied by the pixel area yielding a more imaging-parameter robust quantity, termed a scattering cross-section. Using this cross-section approach, and simulated reference data we show how it is possible to count the number of atoms in individual columns, in order to compare against theoretical 3-dimensional structures. All this is with the aim of trying to obtain as much information as possible from a single image of these beam sensitive samples.

How flat is your detector? Non-uniform annular detector sensitivity in STEM quantification

Recording HAADF STEM data on an absolute scale for image quantification is becoming increasingly common. A particular challenge with this method is that most image simulation programs model the detector as being circularly symmetric and exhibiting uniform detection sensitivity across its entire active region. For a real detector this is rarely the case; it then becomes vital to understand how far one's detector deviates from the ideal. Here we investigate a collection of detector maps recorded using hardware from each of the current major manufacturers. Using these maps we compare their different asymmetries and any non-uniformities in their sensitivity. To facilitate this we define the parameters; 'flatness', 'roundness', 'smoothness' and 'ellipticity', evaluate each hardware with respect to these and rank them.

Real space crystallography of a complex metallic alloy

High-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) has been performed along the low-index zone axes of the o-Al4(Cr,Fe) complex metallic alloy to obtain a real-space representation of the crystal structure and to elucidate the material's inherent structural disorder. By comparing experiments with multislice STEM simulations, the model previously suggested by X-ray diffraction is further refined to provide a new set of positions and occupancies for the transition metal atoms. Pmnb is suggested as the new space group for the o-Al4(Cr,Fe) phase. A nonperiodic layer-type modulation, averaged out in bulk diffraction methods, is detected, corroborating the need for complementing bulk diffraction analysis with real-space imaging to derive the true crystal structure of Al4(Cr,Fe).

Real space crystallography of a complex metallic alloy: high-angle annular dark-field scanning transmission electron microscopy of o-Al4(Cr,Fe)

High-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) has been performed along the low-index zone axes of the o-Al4(Cr,Fe) complex metallic alloy to obtain a real-space representation of the crystal structure and to elucidate the material's inherent structural disorder. By comparing experiments with multislice STEM simulations, the model previously suggested by X-ray diffraction is further refined to provide a new set of positions and occupancies for the transition metal atoms.Pmnbis suggested as the new space group for the o-Al4(Cr,Fe) phase. A nonperiodic layer-type modulation, averaged out in bulk diffraction methods, is detected, corroborating the need for complementing bulk diffraction analysis with real-space imaging to derive the true crystal structure of Al4(Cr,Fe).