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Porous networks of Pt nanoparticles interlinked by bifunctional organic ligands have shown high potential as catalysts in micro‐machined hydrogen gas sensors. By varying the ligand among p‐phenylenediamine, benzidine, 4,4‘‘‐diamino‐p‐terphenyl, 1,5‐diaminonaphthalene, and trans‐1,4‐diaminocyclohexane, new variants of such networks were synthesized. Inter‐particle distances within the networks, determined via transmission electron microscopy tomography, varied from 0.8 to 1.4 nm in accordance with the nominal length of the respective ligand. While stable structures with intact and coordinatively bonded diamines were formed with all ligands, aromatic diamines showed superior thermal stability. The networks exhibited mesoporous structures depending on ligand and synthesis strategy and performed well as catalysts in hydrogen gas microsensors. They demonstrate the possibility of deliberately tuning micro‐ and mesoporosity and thereby transport properties and steric demands by choice of the right ligand also for other applications in heterogeneous catalysis., Pt nanocatalyst construction kit: variants of stable Pt nanoparticle networks were synthesized from the same Pt nanoparticle building block and 5 different aromatic and aliphatic diamines as joints. These ligand‐linked networks demonstrate the possibility of deliberately tuning their microporosity for applications in heterogeneous catalysis by choice of the right ligand.

Dealloyed nanoporous metals hold great promise in the field of heterogeneous catalysis; however their tendency to coarsen at elevated temperatures or under catalytic reaction conditions sometimes limit further applications. Here, we report on a highly stable nanoporous gold catalyst (npAu) functionalized with praseodymia-titania mixed oxides as synthesized by a sol-gel method. Specifically, we used aberration-corrected transmission electron microscopy to study the morphology and the interface between the oxide deposits and the npAu substrate at the atomic level. Based on electron energy loss spectroscopy (EELS), it is concluded that Pr-TiOx mixed oxides form a solid solution. Flow reactor tests reveal that the Pr-TiOx functionalized nanoporous gold is not only highly active but also very stable for the water gas shift reaction in a large temperature range (180-400 °C). Our results demonstrate the potential of engineering the compositions of oxides coatings on npAu for advanced functional systems.

Tin dioxide (SnO2) nanoparticles synthesized via flame spray pyrolysis (FSP) have promising applications for gas sensors. The formation of SnO2 nanoparticles in the gas-phase has been investigated using single droplet combustion and FSP. Precursor solutions of Tin (II) 2-ethylhexanoate dissolved in Xylene with varying Sn concentrations were selected as the precursor-solvent system. The selected precursor-solvent system has its stability and ability to synthesize homogeneous nanoparticles, compared to metal nitrate based precursor solutions. The precursor-solvent system was studied using attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and thermogravimetric analysis (TGA). The SnO2 nanoparticles were characterized using X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), and transmission electron microscopy (TEM). Droplet surface micro-explosions were observed during the single droplet combustion of the precursor solutions. It is because of the heterogeneous vapor-phase nucleation, which is beneath the liquid droplet surface and caused by precursor thermal decomposition. The results show that the size of nanoparticles obtained both from FSP and single droplet combustion increases with increasing metal-precursor concentration. The TEM images of the particles from such droplet combustion reveal two types of nanoparticles with different sizes and morphologies. The current work provides fundamental understanding of precursor decomposition and particle formation during single droplet combustion, which help in-depth understanding of the flame spray pyrolysis.

Properties of dealloyed nanoporous gold (npAu) in catalysis, sensing or actuation applications depend on its structure and chemical composition. NpAu is obtained by corrosion of alloys of gold and a less noble metal such as silver or copper. Structural parameters can be tuned by variations in the preparation process. Due to the large number of parameters that can be varied a direct comparison of results obtained with various samples is often difficult. In this report we compare structure and chemical composition of npAu dealloyed from AuAg and AuCu alloys keeping the number of different sample preparation parameters at a minimum.

Strain analyses from experimental series of nano-beam electron diffraction (NBED) patterns in scanning transmission electron microscopy are performed for different specimen tilts. Simulations of NBED series are presented for which strain analysis gives results that are in accordance with experiment. This consequently allows to study the relation between measured strain and actual underlying strain. A two-tilt method which can be seen as lowest-order electron beam precession is suggested and experimentally implemented. Strain determination from NBED series with increasing beam convergence is performed in combination with the experimental realization of a probe-forming aperture with a cross inside. It is shown that using standard evaluation techniques, the influence of beam convergence on spatial resolution is lower than the influence of sharp rings around the diffraction disc which occur at interfaces and which are caused by the tails of the intensity distribution of the electron probe. (C) 2018 Elsevier B.V. All rights reserved.

The development and industrial application of advanced lithium based energy-storage materials are directly related to the innovative production techniques and the usage of inexpensive precursor materials. Flame spray pyrolysis (FSP) is a promising technique that overcomes the challenges in the production processes such as scalability, process control, material versatility, and cost. In the present study, phase pure anode material Li4Ti5O12 (LTO) was designed using FSP via extensive systematic screening of lithium and titanium precursors dissolved in five different organic solvents. The effect of precursor and solvent parameters such as chemical reactivity, boiling point, and combustion enthalpy on the particle formation either via gas-to-particle (evaporation/nucleation/growth) or via droplet-to-particle (precipitation/incomplete evaporation) is discussed. The presence of carboxylic acid in the precursor solution resulted in pure (>95 mass %) and homogeneous LTO nanoparticles of size 4–9 nm, attributed to...

In this work, we analyse the microstructure and local chemical composition of green-emitting Inx Ga1-x N/GaN quantum well (QW) heterostructures in correlation with their emission properties. Two samples of high structural quality grown by metalorganic vapour phase epitaxy (MOVPE) with a nominal composition of x = 0.15 and 0.18 indium are discussed. The local indium composition is quantitatively evaluated by comparing scanning transmission electron microscopy (STEM) images to simulations and the local indium concentration is extracted from intensity measurements. The calculations point out that the measured indium fluctuations may be correlated to the large width and intensity decrease of the PL emission peak.

The aerobic oxidation of methanol and ethanol in liquid phase is investigated using unsupported nanoporous gold (npAu) catalysts and compared to gas phase experiments. We pursue the question if this material is still suited as a type of model catalyst even under much more complex conditions in liquid phase. Indeed, npAu is an active catalyst for both liquid and gas phase oxidation of methanol and ethanol with O2 under mild conditions (starting at 40 °C). While in case of methanol solely the self coupling product (methyl formate) is detected, the more facile formation of the acetaldehyde in case of ethanol results in the formation of the free aldehyde (acetaldehyde) and the coupling product (ethyl acetate) in similar amounts. To further probe the surface chemistry, cross coupling reactions between both alcohols, as well as the acetaldehyde are performed. As observed also in gas phase experiments, the formation of the surface bonded aldehyde by β-H elimination of the alcoxy species is rate determining.

Large efforts have been made trying to understand the origin of the high catalytic activity of dealloyed nanoporous gold as a green catalyst for the selective promotion of chemical reactions at low temperatures. Residual silver, left in the sample after dealloying of a gold-silver alloy, has been shown to have a strong influence on the activity of the catalyst. But the question of how the silver is distributed within the porous structure has not finally been answered yet. We show by quantitative energy dispersive X-ray tomography measurements that silver forms clusters that are distributed irregularly, both on the surface and inside the ligaments building up the porous structure. Furthermore, we find that the role of the residual silver is ambiguous. Whereas CO oxidation is supported by more residual silver, methanol oxidation to methyl formate is hindered. Structural characterisation reveals larger ligaments and pores for decreasing residual silver concentration.

Methanol as a green and renewable resource can be used to generate hydrogen by reforming, i.e., its catalytic oxidation with water. In combination with a fuel cell this hydrogen can be converted into electrical energy, a favorable concept, in particular for mobile applications. Its realization requires the development of novel types of structured catalysts, applicable in small scale reactor designs. Here, three different types of such catalysts were investigated for the steam reforming of methanol (SRM). Oxides such as TiO2 and CeO2 and mixtures thereof (Ce1Ti2Ox) were deposited inside a bulk nanoporous gold (npAu) material using wet chemical impregnation procedures. Transmission electron and scanning electron microscopy reveal oxide nanoparticles (1–2 nm in size) abundantly covering the strongly curved surface of the nanoporous gold host (ligaments and pores on the order of 40 nm in size). These catalysts were investigated in a laboratory scaled flow reactor. First conversion of methanol was detected at 200 °C. The measured turn over frequency at 300 °C of the CeOx/npAu catalyst was 0.06 s−1. Parallel investigation by in situ infrared spectroscopy (DRIFTS) reveals that the activation of water and the formation of OHads are the key to the activity/selectivity of the catalysts. While all catalysts generate sufficient OHads to prevent complete dehydrogenation of methanol to CO, only the most active catalysts (e.g., CeOx/npAu) show direct reaction with formic acid and its decomposition to CO2 and H2. The combination of flow reactor studies and in operando DRIFTS, thus, opens the door to further development of this type of catalyst.

This study addresses the comparison of scanning transmission electron microscopy (STEM) measurements of momentum transfers using the first moment approach and the established method that uses segmented annular detectors. Using an ultrafast pixelated detector to acquire four-dimensional, momentum-resolved STEM signals, both the first moment calculation and the calculation of the differential phase contrast (DPC) signals are done for the same experimental data. In particular, we investigate the ability to correct the segment-based signal to yield a suitable approximation of the first moment for cases beyond the weak phase object approximation. It is found that the measurement of momentum transfers using segmented detectors can approach the first moment measurement as close as 0.13 h/nm in terms of a root mean square (rms) difference in 10 nm thick SrTiO3 for a detector with 16 segments. This amounts to 35% of the rms of the momentum transfers. In addition, we present a statistical analysis of the precision of first moment STEM as a function of dose. For typical experimental settings with recent hardware such as a Medipix3 Merlin camera attached to a probe-corrected STEM, we find that the precision of the measurement of momentum transfers stagnates above certain doses. This means that other instabilities such as specimen drift or scan noise have to be taken into account seriously for measurements that target, e.g., the detection of bonding effects in the charge density.

Several instrumental imperfections of transmission electron microscopes are characterized and their effects on the results of quantitative scanning electron microscopy (STEM) are investigated and quantified using simulations. Methods to either avoid influences of these imperfections during acquisition or to include them in reference calculations are proposed. Particularly, distortions inflicted on the diffraction pattern by an image-aberration corrector can cause severe errors of more than 20% if not accounted for. A procedure for their measurement is proposed here. Furthermore, afterglow phenomena and nonlinear behavior of the detector itself can lead to incorrect normalization of measured intensities. Single electrons accidentally impinging on the detector are another source of error but can also be exploited for threshold-less calibration of STEM images to absolute dose, incident beam current determination and measurement of the detector sensitivity.

A protocol to functionalize small copper nanoparticles (Cu NPs, 1–5 nm) with hydrophilic ligands is presented. The method was applied to prepare Cu NPs stabilized by cysteine, glutathione, proline, or OH−. These four types of Cu NPs were tested with respect to their stability and ability to form reactive oxygen species (ROS) in presence of H2O2. The ligands were found to affect both, ROS formation and particle stability. However, compared to Cu cations, the Cu NPs were much less effective in terms of forming ROS in the presence of H2O2. Application of copper ion chelators demonstrated that the formation of ROS in samples containing Cu NPs was mainly caused by Cu cations leached from the particles, but not by nanoparticle-specific effects. Our results suggest that a secure application of Cu in the form of NPs is restricted to reductive conditions, as under oxidizing conditions Cu NPs will degrade by the leaching of Cu ions.

δ-Bi2O3 is one of the best oxygen ion conductors known. However, due to its limited thermal stability and complicated synthesis techniques, its applications are limited. Here, the synthesis of stable nano-sized δ-Bi2O3 using versatile and rapid flame spray pyrolysis (FSP) combined with in situ Ti and/or Mn doping for an enhanced thermal stability is reported for the first time. Exceptionally low Bi replacing cation concentrations (8 at% Ti) were sufficient to obtain phase-pure δ-Bi2O3 which was attributed to the extraordinarily high temperature gradient during FSP. The required cation amount for δ-phase stabilization was even further reduced by introducing mixtures of Mn and Ti (2.5 at% Mn + 2.5 at% Ti). Rietveld analysis revealed that the δ-Bi2O3 structure is best represented by the Fmm space group containing two closely neighbored 8c and 32f Wyckoff positions. Depending on the amount of Mn/Ti cations, about 25% of the possible oxygen positions remain vacant, suggesting high bulk oxygen mobility. The enhanced oxygen mobility was confirmed by temperature programmed reduction (H2-TPR) with bulk reduction for δ-Bi2O3 in contrast to exclusive surface reduction for β-Bi2O3.

We study the incorporation of indium into Al x Ga 1 − x N /GaN quantum well (QW) structures with high indium concentrations above 25% for QW thicknesses in the range 2 nm down to half a c -lattice constant under pulsed and continuous growth conditions. We want to clarify which processes limit the incorporation of indium and lead to a degrading layer structure. The data are discussed in the context of the adlayer proposed by theory (Northrup et al., 2000) [1] . The interplay of the adlayer with the incoming flux, the high desorption rate and segregation of indium can consistently explain the various observed phenomena.

We study the transformation that WO3 nanoparticles produced by Flame Spray Pyrolysis undergo when subjected to heating and cooling cycles by means of accurate in situ XRD and HRTEM investigations supported by atomistic modeling at the level of Density Functional Theory. As-deposited particles, initially in the monoclinic phase, reversibly transform into the high-temperature orthogonal and tetragonal phases (and vice versa) irrespective of the direction of the temperature gradient. However, upon heating the particles experience an irreversible change of morphology as a consequence of evaporation/reprecipitation processes leading to the formation of larger particles, some of which are elongated (aspect ratios of up to 3 : 1) along various crystallographic directions of the tetragonal unit cell. On the basis of the results of extensive ab initio thermodynamic calculations of surface energies and first-principles molecular dynamics simulations of the growth process, we rationalize these findings in terms of different surface stabilities and reactivities towards gas-phase deposition of molecular WO3 presented by the nanocrystal facets.

Metastable $ϵ$-gallium oxide is expected to have very good properties for high-power electronics, combining the large band gap and high breakdown field of $\ensuremath{\beta}$-Ga${}_{2}$O${}_{3}$ with a very high spontaneous polarization. Unfortunately, synthesizing $ϵ$-Ga${}_{2}$O${}_{3}$ is quite tricky, yet the authors have managed it. Introducing tin avoids the formation of volatile suboxides, which leadsto a widened growth window in which $ϵ$-gallium oxide can beformed. Furthermore, the growth mechanism that the authors revealcould be relevant for the growth of any binary oxide.

The chemical composition of four Si1-xGex layers grown on silicon was determined from quantitative scanning transmission electron microscopy (STEM). The chemical analysis was performed by a comparison of the high-angle annular dark field (HAADF) intensity with multislice simulations. It could be shown that amorphous surface layers originating from the preparation process by focused-ion beam (FIB) at 30 kV have a strong influence on the quantification: the local specimen thickness is overestimated by approximately a factor of two, and the germanium concentration is substantially underestimated. By means of simulations, the effect of amorphous surface layers on the HAADF intensity of crystalline silicon and germanium is investigated. Based on these simulations, a method is developed to analyze the experimental HAADF-STEM images by taking the influence of the amorphous layers into account which is done by a reduction of the intensities by multiplication with a constant factor. This suggested modified HAADF analysis gives germanium concentrations which are in agreement with the nominal values. The same TEM lamella was treated with low-voltage ion milling which removed the amorphous surface layers completely. The results from subsequent quantitative HAADF analyses are in agreement with the nominal concentrations which validates the applicability of the used frozen-lattice based multislice simulations to describe the HAADF scattering of Si1-xGex in STEM. (C) 2017 Elsevier B.V. All rights reserved.

This paper addresses a novel approach to atomic resolution elemental mapping, demonstrating a method that produces elemental maps with a similar resolution to the established method of electron energy-loss spectroscopy in scanning transmission electron microscopy. Dubbed energy-filtered imaging scanning transmission electron microscopy (EFISTEM) this mode of imaging is, by the quantum mechanical principle of reciprocity, equivalent to tilting the probe in energy-filtered transmission electron microscopy (EFTEM) through a cone and incoherently averaging the results. In this paper we present a proof-of-principle EFISTEM experimental study on strontium titanate. The present approach, made possible by chromatic aberration correction, has the advantage that it provides elemental maps which are immune to spatial incoherence in the electron source, coherent aberrations in the probe-forming lens and probe jitter. The veracity of the experiment is supported by quantum mechanical image simulations, which provide an insight into the image-forming process. Elemental maps obtained in EFTEM suffer from the effect known as preservation of elastic contrast, which, for example, can lead to a given atomic species appearing to be in atomic columns where it is not to be found. EFISTEM very substantially reduces the preservation of elastic contrast and yields images which show stability of contrast with changing thickness. The experimental application is demonstrated in a proof-of-principle study on strontium titanate.

A relatively simple yet accurate analytical model for the image formation in the imaging scanning TEM (ISTEM) imaging mode, which implements partial spatial incoherence using a combination of scanning illumination and conventional imaging, is presented. Based on an object function approximation the ISTEM intensity can be divided into a constant, a linear and a nonlinear term. Under certain conditions, which are discussed, the formation of both linear and nonlinear terms can be expressed by convolutions with point spread functions. A closer inspection of these allows an insight into the advantages of ISTEM compared to conventional TEM (CTEM). The findings of the proposed model are confirmed by comparison to multislice simulations. A close investigation of the linear coherent contrast transfer function allows the derivation of optimal imaging conditions to reach a maximum resolution for a given signal-to-noise ratio. The robustness of ISTEM towards temporal incoherence is finally demonstrated and discussed within the model. (C) 2017 Elsevier B.V. All rights reserved.

In this paper, we investigate how precise atoms of a small nanocluster can ultimately be located in three dimensions (3D) from a tilt series of images acquired using annular dark field (ADF) scanning transmission electron microscopy (STEM). Therefore, we derive an expression for the statistical precision with which the 3D atomic position coordinates can be estimated in a quantitative analysis. Evaluating this statistical precision as a function of the microscope settings also allows us to derive the optimal experimental design. In this manner, the optimal angular tilt range, required electron dose, optimal detector angles, and number of projection images can be determined.

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 grad. 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. (C) 2017 Elsevier B.V. All rights reserved.

Recently, imaging scanning transmission electron microscopy (ISTEM) has been proposed as a promising new technique combining the advantages of conventional TEM (CTEM) and STEM (Rosenauer et al., 2014 [1]). The ability to visualize light and heavy elements together makes it a particularly interesting new, spatially incoherent imaging mode. Here, we evaluate this technique in term of precision with which atomic column locations can be measured. By using statistical parameter estimation theory, we will show that these locations can be accurately measured with a precision in the picometer range. Furthermore, a quantitative comparison is made with HAADF STEM imaging to investigate the advantages of ISTEM.

This study sheds light on the prerequisites, possibilities, limitations and interpretation of high-resolution differential phase contrast (DPC) imaging in scanning transmission electron microscopy (STEM). We draw particular attention to the well-established DPC technique based on segmented annular detectors and its relation to recent developments based on pixelated detectors. These employ the expectation value of the momentum transfer as a reliable measure of the angular deflection of the STEM beam induced by an electric field in the specimen. The influence of scattering and propagation of electrons within the specimen is initially discussed separately and then treated in terms of a two-state channeling theory. A detailed simulation study of GaN is presented as a function of specimen thickness and bonding. It is found that bonding effects are rather detectable implicitly, e.g., by characteristics of the momentum flux in areas between the atoms than by directly mapping electric fields and charge densities. For strontium titanate, experimental charge densities are compared with simulations and discussed with respect to experimental artifacts such as scan noise. Finally, we consider practical issues such as figures of merit for spatial and momentum resolution, minimum electron dose, and the mapping of larger-scale, built-in electric fields by virtue of data averaged over a crystal unit cell. We find that the latter is possible for crystals with an inversion center. Concerning the optimal detector design, this study indicates that a sampling of 5 mrad per pixel is sufficient in typical applications, corresponding to approximately 10 × 10 available pixels.

In GaAs-based pseudomorphic high-electron mobility transistor device structures, strain and composition of the InxGa1−xAs channel layer are very important as they influence the electronic properties of these devices. In this context, transmission electron microscopy techniques such as (002) dark-field imaging, high-resolution transmission electron microscopy (HRTEM) imaging, scanning transmission electron microscopy-high angle annular dark field (STEM-HAADF) imaging and selected area diffraction, are useful. A quantitative comparative study using these techniques is relevant for assessing the merits and limitations of the respective techniques. In this article, we have investigated strain and composition of the InxGa1−xAs layer with the mentioned techniques and compared the results. The HRTEM images were investigated with strain state analysis. The indium content in this layer was quantified by HAADF imaging and correlated with STEM simulations. The studies showed that the InxGa1−xAs channel layer was pseudomorphically grown leading to tetragonal strain along the [001] growth direction and that the average indium content (x) in the epilayer is ~0.12. We found consistency in the results obtained using various methods of analysis.

We present the position-controlled growth of GaN nanotubes with coaxial GaInN quantum wells by using ZnO nanowires grown on top of GaN pyramids as templates. High resolution scanning transmission electron microscopy allows us to perform a detailed structural analysis of individual tubes. In particular, we report about structural properties like indium incorporation, thickness, and homogeneity of quantum wells realized on nonpolar m-plane side facets or on semipolar tips. Additionally, high resolution spatially and spectrally resolved cathodoluminescence spectroscopy performed at low temperature enables us to clearly identify the quantum well luminescence contributions for different kinds of facets on a single tube. The luminescence of the quantum wells deposited at variable temperatures shows a clear spectral shift in the cathodoluminescence signal, yielding an activation energy for the indium desorption from the m-plane side facets of about 1.6 eV. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We investigated segregation of indium in an In x Ga 1− x As/GaAs heterostructure via high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM), where contrast strongly depends on the nuclear charges of the scattering atoms (Z-contrast). Indium concentration maps have been deduced from HAADF-STEM images by comparing normalized measured intensities with multislice simulations in the frozen lattice approach. Segregation coefficients were derived following the segregation model of Muraki et al. [1] . This is demonstrated for HAADF-STEM images recorded in [100] and [110] zone-axes. Determined indium concentrations and segregation coefficients are compared with results from composition analysis by lattice fringe analysis (CELFA) measurements and energy-dispersive X-ray analysis (EDX).

Disks of nanoporous gold (np-Au), produced by leaching of silver from AgAu alloy and prepared as active catalysts for CO oxidation in a continuous-flow reactor, were investigated in detail by x-ray photoelectron spectroscopy and temperature-programmed desorption spectroscopy in ultra-high vacuum. Np-Au exhibits several oxygen species on and in the surface: Chemisorbed oxygen (O act ), probably generated at residual silver sites at the surface, is readily available after np-Au preparation and consumed by CO oxidation. It can be replenished on activated np-Au by exposure to O 2 . In addition, strongly bound oxygen, probably at subsurface sites, is present as a major species and not consumed by CO oxidation. Pronounced CO desorption at temperatures above 200 K observed after exposing np-Au to CO at 105 K indicates an additional, more stable type of CO binding sites on np-Au as compared to pure gold. Only CO at these binding sites is consumed by oxidation reaction with O act . It is proposed that the presence of strongly bound subsurface oxygen stabilizes CO adsorption on np-Au, thereby being as crucial for the observed catalytic activity of np-Au as residual silver.

Nacre of abalone shells consists of aragonite platelets and organic material, the so-called organic matrix. During the growth process of the shell the aragonite platelets grow into a scaffold formed by the organic matrix. In this work we tried to mimic this growth process by placing a piece of the insoluble organic matrix (which is a part of the organic matrix) of the abalone Haliotis laevigata in a crystallization device which was flowed through by CaCl 2 and NaHCO 3 solutions. Using this setup amongst others flat aragonite crystals grow on the insoluble organic matrix. When investigating these crystals in a transmission electron microscope it is possible to recognize similarities to the structure of nacre, like the formation of mineral bridges and growth between layers of the insoluble organic matrix. These similarities are presented in this paper.

The nitrogen concentration of GaN 0.01 ≤ x ≤ 0.05 As 1 − x quantum wells was determined from high resolution scanning transmission electron microscopy (HRSTEM) images taken with a high-angle annular dark field (HAADF) detector. This was done by applying two independent methods: evaluation of the scattering intensity and strain state analysis. The HAADF scattering intensity was computed by multislice simulations taking into account the effect of static atomic displacements and thermal diffuse scattering. A comparison of the mean intensity per atom column on the experimental images with these simulations enabled us to generate composition maps with atomic scale resolution. STEM simulations of large supercells proved that local drops of the HAADF intensity observed close to embedded quantum wells are caused by surface strain relaxation. The same STEM images were evaluated by strain state analysis. We suggest a real space method which is not affected by fly-back errors in HRSTEM images. The results of both evaluation methods are in accordance with data obtained from X-ray diffraction measurements.

Flame spray pyrolysis (FSP) allowed quenching the high temperature phase In4Sn3O12 in the form of highly single crystalline particles of about 6 nm. These nanoparticles were in situ deposited to form stable porous films on interdigitated electrodes. The resulting gas sensors were tested for formaldehyde sensing in the low ppb range. Comparing systematic composition of the In–Sn–oxide system ranging from pure In2O3, In1.9Sn0.1O3 (ITO), In4Sn3O12 and SnO2 with their corresponding mixtures showed by far the best sensor performance at 250 °C for 43% of (Sn/(Sn + In), corresponding to the In4Sn3O12 phase. The sensors tested using this phase outperformed state of the art metal oxide devices. The In4Sn3O12 phase was stable beyond the operation time and temperature used here, demonstrating its enormous but largely undiscovered potential in the future.

The stability and electronic structure of bare [001] nanowires of anatase and rutile have been investigated using ab initio density functional calculations. It was found that symmetry plays an important role in both properties below a critical diameter. Up to 2.1 nm in anatase and 3.7 nm in rutile, those {100}-walled anatase and {110}-walled rutile wires are most stable, which retain the nonsymmorphic character of the bulk space group. Our results explain the observed properties of atomic size anatase nanowires. The wires with screw symmetry also show a consistently larger gap in their electronic structures compared to similarly walled wires without it. Additionally, in rutile the indirect or direct character of the band structure is coupled to the presence or absence of the screw axis.

Composition and microstructure of GaN-based island structures and distributed Bragg reflectors (DBRs) were investigated with transmission electron microscopy (TEM). We analysed free-standing InGaN islands and islands capped with GaN. Growth of the islands performed by molecular beam epitaxy (MBE) and metal organic vapour phase epitaxy (MOVPE) resulted in different microstructures. The islands grown by MBE were plastically relaxed. Cap layer deposition resulted in a rapid dissolution of the islands already at early stages of cap layer growth. These findings are confirmed by grazing-incidence X-ray diffraction (GIXRD). In contrast, the islands grown by MOVPE relax only elastically. Strain state analysis (SSA) revealed that the indium concentration increases towards the tips of the islands. For an application as quantum dots, the islands must be embedded into DBRs. Structure and composition of AlyGa1–yN/GaN Bragg reflectors on top of an AlGaN buffer layer and InxAl1–xN/GaN Bragg reflectors on top of a GaN buffer layer were investigated. Specifically, structural defects such as threading dislocations (TDs) and inversion domains (IDs) were studied, and we investigated thicknesses, interfaces and interface roughnesses of the layers. As the peak reflectivities of the investigated DBRs do not reach the theoretical predictions, possible reasons are discussed.

We present for the first time electroluminescence from InGaN quantum dots inside a monolithic nitride based cavity. The structure consists of a 40-fold bottom GaN/Al 0.82 In 0.18 N distributed Bragg reflector (DBR), a single InGaN quantum dot layer inside a 5 λ n-type (bottom) and p-type (top) doped GaN cavity and a 10-fold GaN/Al 0.82 In 0.18 N top DBR. Structural properties have been investigated by scanning transmission electron microscopy. Optical reflectivity measurements are in good agreement with calculations which predict a peak reflectivity of 92% and a quality factor of 220. Electroluminescence shows a pronounced emission at the spectral position of the cavity mode near 500 nm.

Phone: þ49-421-218-62242, Fax: þ49-421-218-7318The surface segregation of Mg and Si dopant species and theiratomicincorporationsitesinGaNfilmsgrownbymetal-organicvapour phase epitaxy (MOVPE) on sapphire (0001) substrateshave been analysed by X-ray photoemission spectro-micro-scopy and X-ray standing waves (XSW). As revealed byspectro-microscopy, both Mg and Si tend to segregate to thesurface. In case of Mg, an enhanced surface dopant concen-tration is found even after sputter-removal of several tens ofnanometres, which confirms a segregation mechanism pro-posedearlier[S.Figgeetal.,Appl.Phys.Lett.81,4748(2002)].Si doping has been found to result in the formation of facettedgrooves in the GaN films. The surface silicon concentration atthe facets is determined to be about 2.5 times higher ascompared to the planar (0001) surface by micro-spectroscopy.XSW results show that with increasing Mg dopant concen-tration, non-substitutional lattice sites are progressivelyoccupied. The results can quantitatively be explained by Mgatoms incorporated in an anti-bixbyite-like structure and isrelated to inversion domain boundaries. For Si doping, noevidenceisfoundfortheoccupationofnon-substitutionalsites.However, a decrease in crystal quality is observed withincreasing Si concentration. X-ray photoemission micrograph of a Si doped GaN filmshowingafacettedgrooveatthelowerleftcorneroftheimage.The local spectra (see inset) reveal Si enrichment at the facets.

During the growth process of the nacre layer in abalone shells, aragonite platelets are built into a preformed organic matrix. This matrix consists of chitin and different proteins that may affect ...

The growth of thin praseodymium oxide films on silicon (111) using small deposition rates under oxygen-deficient conditions was investigated in the range from submonolayer up to six monolayers coverage by transmission electron microscopy (TEM) and photoemission spectroscopy (PES). A detailed analysis of the silicon 2p and oxygen 1 s core level and valence band spectra reveals chemical reactions between deposited species, substrate, and the growing film. Silicate, silicide and oxide species are coexisting over the entire range of coverages investigated. Cross sectional TEM shows silicide inclusions extending from the surface several nanometers into the substrate and affecting the substrate band bending at the interface. The reactivity of the praseodymia overlayer leads to reactions in the as-deposited film even at room temperature and render it unstable. The article aims at providing a coherent picture of the chemistry proceeding during interface formation and film growth at low rates of deposition (0.06 nm/min). The results will be discussed in comparison to studies using higher rates, emphasizing the possibility of growth rate dependent reactions between substrate and deposited material and, consequently, distinctly different film compositions and structures for different rates of deposition.

Praseodymia gives rise to a rich phase diagram with a large number of phases between the limiting stoichiometries Pr 2 O 3 and PrO 2 that differ only slightly in oxygen content (Pr n O 2n-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 CH 4 . 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 Pr 6 O 11 ―transforms into the next oxygen-depleted phase Pr 7 O 12 . 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 CH 4 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 O 2 concentration remains below a critical concentration.

Flame spray pyrolysis (FSP) has been employed for the syntheses of single crystalline WO3 nanoparticles using tungsten precursors with tungsten in usual (+6) and unusual (+4 and 0) oxidation states. The WO3 nanoparticles have been characterized by powder X-ray diffraction and the patterns have been refined using the Rietveld and the Le Bail method (space group P21/n, monoclinic system; the averaged lattice parameters of the three WO3: a = 7.3913(4) A, b = 7.5630(5) A, c = 7.6615(6) A, β = 90.78(3)°, Z = 8, V = 423.19 A3). Because of the very small crystallite sizes, non-Bragg scattering is observed. The crystallite sizes derived from Rietveld or Le Bail analyses were in the range of 6−8 nm, reasonably agreeing with the crystallite sizes (7−10 nm) determined from Brunauer−Emmett−Teller and microscopic analysis where each particle is basically a small single crystal. Well-developed lattice fringes of 3.860−3.994 A were illustrated by high resolution transmission electron microscopy (HRTEM). Indexing of sele...

Transmission electron microscopy (TEM) in combination with energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy was applied to analyze the microstructure and the local chemical composition of as-sintered and aged Y 2 O 3 -doped ZrO 2 (YDZ) thick-film substrates with yttria contents of 8.5 mol.% (8YDZ) and 10 mol.% (10YDZ). Selected-area electron diffraction and dark-field TEM imaging show nanoscale precipitates of the tetragonal YDZ phase, which are coherently embedded in grains consisting of the cubic phase. A distinct coarsening of the tetragonal regions is observed after 2500 h annealing of 8YDZ at 950 °C in an oxidizing atmosphere. This coarsening is accompanied by decomposition on the cationic sublattice on the nanoscale, leading to the depletion of yttrium in the tetragonal regions. The decomposition is discussed in terms of spinodal decomposition which takes place even at a high doping concentration of 8.5 mol.% Y 2 O 3 .

Nacre, the inner iridescent layer of mollusc shells, is a typical example of a material that is formed by such biomineralization processes. Nacre is composed of the CaCO3-polymorph aragonite and a small amount (about 5 wt.%) of organic matter. The authors 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 (EDX)and electron energy-loss spectroscopy (EELS). The authors 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. The voids are located within the aragonite platelets, so that they are not an artifact of preparation. Furthermore, a first proposal for a three-dimensional model of the voids was made. EDX and EELS measurements revealed that the voids include a larger amount of carbon in comparison to the bulk material. The questions why these voids are formed and how have still to be answered. A speculative function of the voids could be the decreased propagation of cracks in the shell providing an increased fracture toughness.

We present a new method to measure structure factors from electron spot diffraction patterns recorded under almost parallel illumination in transmission electron microscopes. Bloch wave refinement routines have been developed to refine the crystal thickness, its orientation and structure factors by comparison of experimentally recorded and calculated intensities. Our method requires a modicum of computational effort, making it suitable for contemporary personal computers. Frozen lattice and Bloch wave simulations of GaAs diffraction patterns are used to derive optimised experimental conditions. Systematic errors are estimated from the application of the method to simulated diffraction patterns and rules for the recognition of physically reasonable initial refinement conditions are derived. The method is applied to the measurement of the 200 structure factor for GaAs. We found that the influence of inelastically scattered electrons is negligible. Additionally, we measured the 200 structure factor from zero loss filtered two-dimensional convergent beam electron diffraction patterns. The precision of both methods is found to be comparable and the results agree well with each other. A deviation of more than 20% from isolated atom scattering data is observed, whereas close agreement is found with structure factors obtained from density functional theory [A. Rosenauer, M. Schowalter, F. Glas, D. Lamoen, Phys. Rev. B 72 (2005), 085326-1], which account for the redistribution of electrons due to chemical bonding via modified atomic scattering amplitudes.

We report on the investigation of CdSe/ZnSe heterostructures by transmission electron microscopy (TEM) and photoluminescence spectroscopy (PL). CdSe layers with nominal thicknesses tCdSe between 0.5 and 8 monolayers (ML) were embedded in a ZnSe matrix and grown on a GaAs(001) substrate by molecular-beam epitaxy at 280 °C. The Cd-distribution was obtained from high-resolution TEM lattice fringe images using composition evaluation by lattice fringe analysis technique. The measured minimum, average and maximum Cd-concentrations and the overall CdSe contents in the layers increase with the nominal CdSe layer thickness and reach a constant value at tCdSe = 4 ML. The measured CdSe content in the regions with the maximal Cd-concentration continues to increase for tCdSe ≥ 5 ML. The increasing of measured Cd-concentrations/CdSe contents is correlated with a red shift of PL spectra. In the CdSe layers with tCdSe ≥ 5 ML, formation of defects is observed. The increasing density of defects and decreasing intensity of PL spectra with tCdSe suggest that the critical thickness for defect formation during CdSe growth on the ZnSe(001) is between 4 ML and 5 ML. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the growth of thick InGaN layers on a-plane GaN buffer layers. Different growth regimes concerning the III/V-ratio and the growth temperature were studied to determine and vary the indium content in InGaN layers. Furthermore we investigated stacks of InGaN quantum wells (QWs) with a targeted emission wavelength between 400 nm and the near ultraviolet (380 nm). Especially the thickness of the QWs was varied and the impact of a growth interruption on the emission wavelength was studied in detail. We will present TEM-, HRXRD-data and photoluminescence experiments to examine the QW growth procedures and the quality of the QWs.

Free-standing nanometre sized In x Ga 1 - x N islands and islands capped with thin GaN layers were grown by molecular beam epitaxy (MBE) and characterised by high resolution transmission electron microscopy (HRTEM). By adjusting the growth temperature of the In x Ga 1 - x N islands, deposition of the wurtzite modification was realised. The obtained free-standing islands are plastically relaxed by approximately equidistant misfit dislocations. For capped samples, the GaN cap layer thickness was varied between nominal thicknesses of 2 nm and 8 nm. Already for a 2 nm thick cap layer, strong dissolution of the islands occurred. In case of the 8 nm thick GaN cap layer, all former islands are dissolved, and an InGaN layer of approximately homogeneous indium concentration is observed.

To obtain a deeper insight into the mechanism of defect formation in a-plane GaN the defect nature in such films grown by metal-organic vapor phase epitaxy (MOVPE) and hydride vapor phase epitaxy (HVPE) was investigated by means of transmission electron microscopy. The films showed, beside the frequently found basal plane stacking faults and threading dislocations, prismatic stacking faults originating at the film/substrate interface and facetted voids. The density of basal plane stacking fault was about 105 cm–1 and the density of partial dislocations was in the range of 1010 cm–2, accordingly. Using selected area diffraction the epitaxial relationship between film and layer was determined. Based on these findings the impact of lattice mismatch on the observed defect characteristic is discussed. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We investigated InGaAs layers grown by molecular-beam epitaxy on GaAs (001) with transmission electron microscopy (TEM) and photoluminescence spectroscopy. InGaAs layers with In-concentrations of 16, 25 and 28 % and respective thicknesses of 20, 22 and 23 monolayers were deposited at 535 °C. Island formation is observed for the layer with the highest In-concentration. Inconcentration profiles were obtained from high-resolution TEM images by composition evaluation by lattice fringe analysis. The measured profiles can well be fitted applying the segregation model of Muraki et al. [Appl. Phys. Lett. 61 (1992) 557] and are in excellent quantitative agreement with the photoluminescence peak positions. From our data we conclude that island formation occurs when the amount of Indium in the In-floating layer reaches 1.1±0.2 monolayers indium.