Observation of ultrahigh density InGaN quantum dots

We have performed lattice resolved annular dark-field (Z-contrast) scanning transmission electron microscopy and combined this with energy-dispersive X-ray spectroscopy as well as simulations to measure quantitatively segregation across strained interfaces in AlGaN/GaN and GaN/InGaN multiple quantum wells of nominal thicknesses between 8 and 0.25 nm. The compositional profiles obtained were corrected for detector dark current and non-linearity of the Z-contrast imaging process before we fitted exponential functions to the profiles across the interface regions. From these, we could highly accurately determine the layer widths, interface widths, and segregation lengths. Experimental values of the segregation lengths calculated varied from 0.3 nm (for InGaN-on-GaN) to 1.4 nm (for AlGaN-on-GaN), with error bars of only ±0.05 nm. A comparison with simulations based on a simple two-state-exchange model for surface segregation shows that the segregation energy for indium atoms is about an order of magnitude smaller than both the corresponding segregation energy for aluminium/gallium atoms and the activation energies for surface segregation of cations in these nitride systems.

The β' phase precipitated in a 95 at%Mg-5 at%Gd (Mg 95 Gd 5 ) alloy aged in a peak hardness condition (at 200°C for 100 hrs) is studied by high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM). Atomic-scaled HAADF-STEM observations of the β' phase can propose a new structure model with an orthorhombic unit cell of a = 0.64 nm, b = 2.28 nm and c = 0.52 nm and a composition of Mg 7 Gd. The β' precipitates have a plate-shape with an about 40 nm width along the [001] m of the Mg-matrix and an about 100nm length along [210] m -typed directions, and joining of the plate-shaped precipitates establishes a two-dimensional cell structure parallel to the (001) m plane of the Mg-matrix.

This work describes a controlled and low-cost synthesis method to obtain Pb/Pb3O4 nanocomposites using synthetic zeolite 4A. The nanostructures obtained have a core-shell configuration with 5-25 nm diameters. High-resolution transmission electron microscopy (HRTEM), BF, high-angle annular dark-field annular scanning transmission electron microscopy (HAADF-STEM), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and ultraviolet-visible (UV-vis) characterization techniques were used. Crystallographic planes (111), (200), and (220) for the core and planes (110) and (211) for the shell, corresponding to FCC and tetragonal structures for Pb and Pb3O4, respectively, were determined using HRTEM. The HAADF-STEM images allowed the analysis of intensity contrast images proportional to the number of atoms. XPS spectral analysis showed a 4.8 eV difference in binding energy between Pb 4f7/2 and Pb 4f5/2 for lead and lead oxide. EDS elemental mapping, XPS, and UV-vis spectroscopy analyses revealed the simultaneous presence of lead and lead oxide in the same structure. The band gap obtained for the shell was determined to be 4.50 eV. Consequently, Pb/Pb3O4 nanocomposites show a higher response to high-energy photons, making them suitable for UV photocatalysis applications.

T 1 precipitate plates of one, two and three unit-cell thicknesses in a peak-aged Al-Cu-Li-Mg-Ag alloy are examined using atomic-resolution high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) and near atomic-resolution energy-dispersive X-ray spectroscopy (EDX)-STEM. In contrast to previous conflicting reports regarding the positions of Ag and Mg atoms, STEM observations reveal that Ag tends to segregate to the broad interfaces of T 1 , irrespective of the thickness of the plates. Conversely, Mg consistently stays at both the T 1 /α-Al broad interfaces and the interior region of the T 1 plates, acting as a constituent element of the T 1 phase in the Al-Cu-Li-Mg-Ag alloys. The observed distribution of Ag and Mg in the T 1 plates is supported by density functional theory (DFT) calculations. The DFT results further suggest that Mg atoms tend to replace Li atoms originally located in the T 1 plates. Moreover, the near absence of Ag in the interior region of the T 1 plate can be attributed to the destabilisation of Ag-Cu bonds through significant filling of antibonding states if Ag atoms were present within the T 1 plate. Regarding Mg, its ability to replace Li atoms in the T 1 plate is driven by the formation of enhanced chemical bonding.

Environmental pollution caused by heavy metals constitutes a serious public health problem. In the case of uranium depletion, amidoxime groups are important because of their high affinity for uranium(VI). New series of bis(amidoxime)s with catechol-derived anchor groups were tested (b-AMD-1 and b-AMD-2). The catechol groups were designed to bind to the surface of maghemite nanoparticles (MNPs), and two nanohybrid devices MNP-b-AMD-1 and MNP-b-AMD-2 were obtained. This strategy makes for efficient removal of U(VI) via its complexation with the bis(amidoxime)s (b-AMD) and also its extraction from aqueous solution by magnetic harvesting of the MNPs. The assynthesized and b-AMD-grafted MNPs were characterized by several techniques: X-ray diffraction (XRD), high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM), X-ray photoelectron spectrophotometry (XPS), thermal analysis (TG/DTA) and Fourier transform infrared spectroscopy (FTIR). Sorption tests were run at pH 6.5, which corresponds to the highest affinity and selectivity of b-AMD for U(VI). After magnetic separation, the chelation ability and the selectivity of MNP-b-AMD-1 and MNP-b-AMD-2 towards U(VI) were evaluated by measuring the residual U(VI) concentration in the supernatant by inductively coupled plasma-mass spectrometry (ICP-MS). The data were plotted according to the Langmuir and Freundlich isotherms; the maximal sorption capacity (qmax) was 29 and 60 mg U g-1 for MNP-b-AMD-1 and MNP-b-AMD-2, respectively. This confirms that bis(amidoxime) groups are good candidates for uranium depletion of aqueous solution.

In this study, we experimentally and theoretically show that the intensities of bright spots in a spherical aberration (C(s))-uncorrected high-angle annular dark-field (HAADF) scanning transmission electron microscope (STEM) image of [011]-oriented Co(3)O(4), which has two different numbers of Co atoms in the projected atomic columns, are reversed with increasing sample thickness. However, C(s)-corrected HAADF STEM images produce intensities that correctly depend on the average number of atoms in the projected atomic columns. From an analysis based on the Bloch-wave theorem, it is found that an insufficient semiangle of the incident convergent beam yields intensities that do not depend on the average atomic number in the atomic columns.

Chapter 6 - Controlling the Number of Atoms on Catalytic Metallic Clusters

Characterisation of precipitates in a Mg–7Gd–5Y–1Nd–0.5Zr alloy aged to peak-ageing plateau

Device quality InGaN templates are synthesized using the semibulk (SB) approach. The approach maintains the film's 2D growth and avoids the formation of indium-metal inclusions. The strain relaxation processes of the grown InxGa1−xN templates are accompanied by variations in the indium content (x) and lattice parameters (a and c) across the InGaN template's thickness as the residual strain is continuously decreasing. This strain and lattice parameters' variation creates difficulties in applying standard x-ray Diffraction (XRD) and Reciprocal Space mapping (RSM) techniques to estimate the residual strain and the degree of the elastic strain relaxation. We used high-resolution High-angle annular dark-field scanning transmission electron microscopy and Energy-dispersive x-ray spectroscopy (EDS) to monitor the variations of the indium content, lattice parameters, and strain relaxation across the growing InxGa1−xN templates. We show that strain relaxation takes place by V-pit defect formation. Some of these V-pits are refilled by the GaN interlayers in the InxGa1−xN SB templates, while others propagate to the template surface. We present an alternative approach combining photoluminescence (PL) and EDS for estimating the degree of strain relaxation in these InxGa1−xN templates. The values obtained for the degree of relaxation estimated from TEM studies and PL measurements are within reasonable agreement in this study. Device quality InxGa1−xN templates with x ∼ 0.08, with a degree of relaxation higher than 70%, are achieved.

Thin Ga1−xInxN layers with various indium compositions x were examined by high-angle annular dark-field scanning transmission electron microscopy and atom probe tomography. Although nanoscale compositional fluctuation was observed in all Ga1−xInxN layers, no appreciable phase separation was observed. Since Ga1−xInxN layers were coherently grown on underlying GaN layers, it is considered that the elastic strain constrains phase separation in the Ga1−xInxN layers and that compositional fluctuations develop in order to minimize the local strain energy. The measured period of compositional fluctuations ranged from 3.4 to 4.9 nm and decreased with growth temperature (Tgrowth). This tendency is suggested to be dominated by the surface migration length of the adatom during the crystal growth.

Role of surface amorphous film in high-resolution high-angle annular dark field STEM imaging

Precipitate phases in Mg-2 at%Ce-1 at%Zn (Mg 97 Ce 2 Zn 1 ) and Mg-2 at%Ce (Mg 98 Ce 2 ) alloys, prepared by a melt-quenching method, have been examined by high-resolution transmission electron microscopy (HRTEM) and high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM). A precipitate phase formed at grain boundaries has an ordered structure with a hexagonal unit cell of √2a 0 and c 0 , in which a 0 and c 0 are lattice parameters of an h.c.p. structure of the Mg-matrix. On the other hand, a precipitate phase embedded in Mg-matrix grains has a long-period modulated structure formed by the appearance of two-dimensional anti-phase boundaries with a hexagonal symmetry in the fundamental hexagonal structure with lattice constants of √3a 0 and c 0 .

Reversible phase changes of Ni-rich Al–Co–Ni decagonal quasicrystals studied by means of transmission electron microscopy and thermal analysis

High efficiency at low operating temperatures is essential for present and future solid oxide fuel cell (SOFC) applications. This requires the selection of materials with compatible physical and chemical properties for the different SOFC components. La0.58Sr0.4Co0.2Fe0.8O3-δ (LSCF) has been shown to be a highly performing cathode material [1] while 8 mol% Y2O3-doped ZrO2 (YDZ) is still the state-of-the-art electrolyte material. However, it has been shown that an insulating SrZrO3 (SZO) interlayer forms between LSCF and YDZ during temperature treatment exceeding 900 °C, drastically reducing SOFC performance [2]. One idea of overcoming this problem is the introduction of a chemically compatible electrolyte material, like Gd2O3-doped ceria (GDC), at the interface between LSCF and YDZ. GDC acts as a diffusion barrier and prevents SZO formation. Goal of the present study is a detailed microstructural characterization of the LSCF-GDC-YDZ interfaces by analytical transmission electron microscopy (TEM). We focus particularly on the effect of the GDC sintering temperature on SZO formation. Symmetric-cell samples were fabricated by screen-printing a 5 µm thick Gd0.2Ce0.8O2-δ(GDC) layer on both sides of YDZ substrates. GDC sintering was performed at 1100°C, 1250°C or 1400°C for 3 hours, followed by screen-printing a LSCF layer on top of the GDC. A sintering temperature of 1080°C was used for all LSCF cathode layers. The samples were studied after GDC sintering (i.e. before LSCF deposition), after LSCF sintering and after electrical characterization at application-relevant temperatures (500 – 800 °C) [3]. The element distribution at the LSCF-GDC and GDC-YSZ interfaces, as well as the chemical composition within the layers themselves, were determined by energy dispersive X-ray spectroscopy (EDXS). Imaging was performed by TEM and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), complemented by selected area electron diffraction (SAED) for crystal structure analysis. The study gives new insights into material interactions at the interfaces. Figure 1 shows an overview HAADF-STEM image of the LSCF-GDC-YSZ interface region in the sample with 1100 °C GDC sintering temperature, after LSCF deposition and electrical characteri­zation. It can be clearly seen that the GDC layer has a thickness of approximately 5 µm and partially retains some porosity after fabrication. Moreover, an inter-diffusion region between YDZ and GDZ is revealed by HAADF-STEM (arrows in Figure 1), suggesting chemical composition variation. Hence, detailed elemental distributions of the YDZ-GDC interface were determined using EDXS mapping. Figure 2 shows (a) the HAADF-STEM image of the interface region and (b) the Zr, (c) the Gd and (d) the Sr elemental distributions in the YDZ-GDZ interface region. It is of particular interest that a substantial Sr concentration (Fig. 2(d)) is observed between GDC and YDZ, despite the GDC diffusion barrier layer. This indicates Sr transport from the LSCF cathode through the porous GDC layer. The same region also contains Zr (Fig. 2(b)) which indicates SZO formation. In addition, a second phase containing up to 30 at% Gd (Fig. 2(c)) is observed between SZO and YDZ. Crystal structure determination by SAED is currently in progress. We will also demonstrate that the GDC sintering temperature strongly influences SZO interlayer formation. While the SZO layer is continuous for 1100 °C GDC sintering temperature, sintering at 1400 °C strongly reduces SZO layer formation. In contrast, the Gd-rich layer between GDC and YDZ increases in thickness with increasing GDC sintering temperature. We will discuss the origin of the phase formation between GDC and YDZ and the effects of this microstructural development on electrochemical SOFC performance.

SiO2/4H-SiC interfaces are examined by high-resolution transmission electron microscopy (HRTEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and spatially resolved electron energy-loss spectroscopy (EELS). HRTEM and HAADF-STEM images of SiO2/4H-SiC interfaces reveal that abrupt interfaces are formed irrespective of the fabrication conditions. Transition regions around the interfaces reported by Zheleva et al. were not observed. Using EELS, profiles of the C/Si and O/Si ratios across an interface were measured. Our measurements did not reveal a C-rich region on the SiC side of the interface, which was reported by Zheleva et al.