Katharina Gries

Measurement of specimen thickness and composition in Al(x)Ga(1-x)N/GaN using high-angle annular dark field images.

In scanning transmission electron microscopy using a high-angle annular dark field detector, image intensity strongly depends on specimen thickness and composition. In this paper we show that measurement of image intensities relative to the intensity of the incoming electron beam allows direct comparison with simulated image intensities, and thus quantitative measurement of specimen thickness and composition. Simulations were carried out with the frozen lattice and absorptive potential multislice methods. The radial inhomogeneity of the detector was measured and taken into account. Using a focused ion beam (FIB) prepared specimen we first demonstrate that specimen thicknesses obtained in this way are in very good agreement with a direct measurement of the thickness of the lamella by scanning electron microscopy in the FIB. In the second step we apply this method to evaluate the composition of Al(x)Ga(1-x)N/GaN layers. We measured ratios of image intensities obtained in regions with unknown and with known Al-concentration x, respectively. We show that estimation of the specimen thickness combined with evaluation of intensity ratios allows quantitative measurement of the composition x. In high-resolution images we find that the image intensity is well described by simulation if the simulated image is convoluted with a Gaussian with a half-width at half-maximum of 0.07 nm.

Composition mapping in InGaN by scanning transmission electron microscopy

We suggest a method for chemical mapping that is based on scanning transmission electron microscopy (STEM) imaging with a high-angle annular dark field (HAADF) detector. The analysis method uses a comparison of intensity normalized with respect to the incident electron beam with intensity calculated employing the frozen lattice approximation. This procedure is validated with an In(0.07)Ga(0.93)N layer with homogeneous In concentration, where the STEM results were compared with energy filtered imaging, strain state analysis and energy dispersive X-ray analysis. Good agreement was obtained, if the frozen lattice simulations took into account static atomic displacements, caused by the different covalent radii of In and Ga atoms. Using a sample with higher In concentration and series of 32 images taken within 42 min scan time, we did not find any indication for formation of In rich regions due to electron beam irradiation, which is reported in literature to occur for the parallel illumination mode. Image simulation of an In(0.15)Ga(0.85)N layer that was elastically relaxed with empirical Stillinger-Weber potentials did not reveal significant impact of lattice plane bending on STEM images as well as on the evaluated In concentration profiles for specimen thicknesses of 5, 15 and 50 nm. Image simulation of an abrupt interface between GaN and In(0.15)Ga(0.85)N for specimen thicknesses up to 200 nm showed that artificial blurring of interfaces is significantly smaller than expected from a simple geometrical model that is based on the beam convergence only. As an application of the method, we give evidence for the existence of In rich regions in an InGaN layer which shows signatures of quantum dot emission in microphotoluminescence spectroscopy experiments.

Investigations of voids in the aragonite platelets of nacre

We studied the structure of the aragonite platelets of Haliotis laevigata nacre, using conventional transmission electron microscopy, Z-contrast, electron tomography, energy-dispersive X-ray analysis and electron energy-loss spectroscopy. We observed faceted voids several nanometers wide within the aragonite platelets. The electron tomography investigations showed that the voids are distributed more or less randomly in the studied specimen and allowed an estimation of the order of magnitude of the width and the volumetric content of the voids. Further investigations of these voids revealed that they contain an increased amount of carbon, which suggests the existence of organic material within the voids.

Gastropod nacre: structure, properties and growth--biological, chemical and physical basics.

The biogenic polymer/mineral composite nacre is a non-brittle biological ceramic, which self-organizes in aqueous environment and under ambient conditions. It is therefore an important model for new sustainable materials. Its highly controlled structural organization of mineral and organic components at all scales down to the nano- and molecular scales is guided by organic molecules. These molecules then get incorporated into the material to be responsible for properties like fracture mechanics, beauty and corrosion resistance. We report here on structure, properties and growth of columnar (gastropod) nacre with emphasis on the genus Haliotis in contrast to sheet nacre of many bivalves.

Correlation of the orientation of stacked aragonite platelets in nacre and their connection via mineral bridges

In this work, we studied the correlation of the orientation of stacked aragonite platelets of Haliotis laevigata nacre, using selected area diffraction (SAD) in transmission electron microscopy (TEM). From the position of the center of Laue circle (COLC) within the diffraction patterns the tilt angles of the investigated platelets relatively to a reference platelet (oriented in zone axis) are determined. The strong correlation of the platelets supports the existence of mineral bridges, which connect the stacked platelets and enable a transfer of the platelet orientation during growth. Electron tomography and subsequent reconstruction of the obtained data yield information about the shape of the mineral bridges. The crystalline structure of the material within the mineral bridges was investigated by high resolution TEM (HRTEM).

Nanostructured Praseodymium Oxide: Correlation Between Phase Transitions and Catalytic Activity

Praseodymia gives rise to a rich phase diagram with a large number of phases between the limiting stoichiometries Pr2O3 and PrO2 that differ only slightly in oxygen content (PrnO2n−2). This chemical and crystallographic variability allows the system to release or incorporate lattice oxygen easily at sufficiently high temperatures and thus renders the material interesting as a catalyst for redox reactions according to a Mars–van Krevelen mechanism. Nanostructured praseodymia samples are investigated in this study with respect to their catalytic properties, focusing on methane oxidation and selective NO reduction by CO and CH4. To correlate catalytic activity and crystallographic changes, complementary high‐temperature X‐ray diffraction measurements have been carried out. The determined temperatures of transitions between different oxide phases agree well with peaks in the temperature‐programmed reduction measurements, confirming the direct connection between the availability of lattice oxygen and crystallographic transformations. The catalytic activity for methane oxidation and NO reduction sets in at 450–500 °C, at which temperature the starting material—mainly Pr6O11—transforms into the next oxygen‐depleted phase Pr7O12. With respect to NO reduction, the results show that it is possible to employ both methane and carbon monoxide as reducing agents in the absence of oxygen, in agreement with a Mars–van Krevelen mechanism. Nevertheless, the use of CO instead of CH4 offers considerable advantages, as no deactivation due to carbon residues takes place in this case. Whereas, in an excess of oxygen, NO reduction is inhibited independently of the reducing agent, it is shown that NO reduction can proceed if the O2 concentration remains below a critical concentration.

Growth study of nonpolar Zn1−xMgxO epitaxial films on a-plane bulk ZnO by plasma-assisted molecular beam epitaxy

Nonpolar Zn1−xMgxO epitaxial films were grown by plasma-assisted molecular beam epitaxy on a-plane ZnO substrates. A smooth surface morphology was accomplished under oxygen-rich growth conditions. The benefits of the use of ZnO substrates on the structural properties are reflected by a low-density of threading dislocations. Furthermore, no indications for the generation of basal plane stacking faults are found. The pseudomorphic growth on a-plane ZnO substrates efficiently locks the epitaxial Zn1−xMgxO films to the wurtzite structure up to x = 0.25. The Mg concentration is not constant and increases with larger thickness. The optical properties reflect the influence of alloy disorder.

2D-composition mapping in InGaN without electron beam induced clustering of indium by STEM HAADF Z-contrast imaging

Investigation of composition in InGaN quantum wells and quantum dots by TEM is hampered by formation of electron beam induced agglomeration of indium, which occurs if the specimen is exposed to the electron beam for a few minutes. In this contribution we demonstrate that compositional analysis of InGaN nanostructures is possible without this artifact if STEM Z-contrast imaging is applied instead of parallel beam illumination. The suggested method for composition analysis in InGaN is based on a comparison of intensity normalized with respect to the incident electron beam with simulated image intensity. Simulations are performed with the STEMsim program using the frozen lattice multislice approximation for which static atomic displacements were taken into account.

Measurement of composition profiles in III-nitrides by quantitative scanning transmission electron microscopy

In this paper we demonstrate a quantitative method for composition evaluation based on comparison of normalized image intensity with simulations carried out with the frozen lattice approximation. The method is applied to evaluate composition profiles of AlxGa1?xN/GaN layers. We measure ratios of image intensities obtained in regions with unknown and with known Al-concentration x, respectively. We show that estimation of specimen thickness combined with evaluation of intensity ratios allows quantitative measurement of composition profiles. Delocalization effects at interfaces due to instrumental resolution and dynamic electron diffraction are simulated. These effects can well be described by convolution with a Lorentzian. Measured intensity profiles can be corrected for delocalization effects using statistical parameter estimation so that deconvolution is avoided.

Microstructure of Aragonite Platelets in Nacre

* Forschungszentrum Karlsruhe, Institute for Nanotechnology, 76344 Eggenstein-Leopoldshafen, Germany ** Fraunhofer IFAM, Wiener Strase 12, 28355 Bremen, Germany *** University of Bremen, Institute of Biophysics, Otto-Hahn-Allee 1, 28359 Bremen, Germany **** The University of York, Department of Physics, Heslington, York YO10 5DD, United Kingdom ***** University of Bremen, Institute for Solid State Physics, Otto-Hahn-Allee 1, 28359 Bremen, Germany