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Nanoporous gold (npAu) has attracted a lot of attention during the last decade as it has interesting applications particularly in the field of catalysis [1]. It is prepared by corrosion of a suitable gold master alloy, e.g. gold/silver. The remaining material forms a sponge-like structure built of ligaments and pores still preserving a crystalline structure over several tens of nanometres. A high surface to volume ratio, open porosity making it permeable for gases and liquids and a strongly curved ligament surface providing surface atoms of different coordination are only some properties which qualify npAu as catalytic material with well adjustable and reproducible structure. One important structural property that is expected to have strong influence on the catalytic activity is lattice strain, because strain affects the electronic states [2]. Here we present measurements of strain in npAu. A measurement of lattice strain by means of high-resolution TEM for example by the analysis of lattice plane distances is possible only in a very small field of view for npAu. As the positions of intensity maxima in the images depend on several experimental parameters, such as defocus, orientation, composition, lens aberrations and especially specimen thickness, maxima detection in TEM micrographs succeeds only in small parts of the sample, because thickness is varying strongly. This can be seen in Fig. 1. Close to the surface intensity maxima can be detected and thus strain can be measured, in this case tensile strain up to 5% has been found. But as the ligament gets thicker and defects become visible, contrast changes and strain analysis fails. A method that overcomes these problems is strain analysis using nano-beam electron diffraction (NBED) [3]. Here a focussed electron probe is scanned across the sample and at each position of the scanning beam the corresponding diffraction pattern is recorded. As distances between the non-overlapping diffraction discs depend basically on the local lattice parameter according to Bragg's law, strain can be measured by comparing distances between diffraction discs at different positions of the scanning beam. By an analysis of distances between diffraction discs in two linearly independent directions strain as well as shear-strain and rotation can be measured. A further advantage of strain analysis by NBED is its large field of view, because at first sight it is limited only by the size of the scanned part of the sample. Hence strain and rotation of neighbouring ligaments in npAu can be measured. On the other hand a large field of view requires the acquisition of a large number of diffraction patterns in a short time to avoid effects of sample drift, beam induced sample damage and contamination. Here we present strain and rotation maps of npAu measured using a delay-line detector for the acquisition of the diffraction patterns. With this detector strain maps at a scanning raster of e.g. 100x100 pixels can be recorded, allowing measurements in a field of view of several hundreds of nanometers (Fig. 2) still preserving a sampling limited spatial resolution of about 1.6 nm [4]. Furthermore we show by evaluation of simulations that two important aspects concerning the precision of the measurement have to be taken into account. As the precision of the measurement suffers from noise in the diffraction pattern, the precision degrades for shorter image integration times. On the other hand the precision can be increased using a precessing [5, 6] electron beam, as the diffraction discs are illuminated more homogeneously and hence their positions can be detected more precisely. In this way a compromise between precision and speed / size of the measurement has to be found. Keywords: nanoporous gold; strain measurement; NBED; precision

The preadsorption of Ag on Si(111) drastically changes the growth of Ge. In a temperature range from $400{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ to $650{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, Ag adsorption on Si leads to the formation of a $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$-R ${30}^{\ensuremath{\circ}}$ reconstruction that exhibits a maze-like morphology on the mesoscopic scale, as observed by low-energy electron diffraction (LEED) and low-energy electron microscopy. This maze morphology can be attributed to a surface roughening on an atomic scale, induced by the re-arrangement of top layer atoms during the $7\ifmmode\times\else\texttimes\fi{}7$ to $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$-R ${30}^{\ensuremath{\circ}}$ transition. The subsequent deposition of Ge results in the formation of a wetting layer, the evolution of which has been found to be governed by the Ag/Si(111)-$\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$-R ${30}^{\ensuremath{\circ}}$ template's maze structure, as the latter offers a high density of heterogeneous nucleation sites. Upon further Ge growth, three-dimensional islands with diameters in the micrometer range are formed, which exhibit a large and flat (111) top facet. X-ray photoemission electron microscopy reveals that during Ge growth, Ag is segregating to the surface very efficiently. Grazing-incidence x-ray diffraction and transmission electron microscopy have been used to study the composition, strain state and defect structure of the Ge islands in dependence of the growth temperature. The strain induced by lattice mismatch is found to be largely relaxed (80--90% relaxation) in the investigated growth temperature range from 400 to $600{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, which is confirmed by high-resolution LEED measurements. As a main relaxation mechanism, the formation of interfacial misfit dislocations has been identified. Interdiffusion of Si into the Ge islands becomes more and more pronounced for increasing growth temperature, whereas the formation of twinned Ge regions can drastically be suppressed at higher temperature.

Recent studies on the synthesis of bulk Al–RE (Rare Earth)-TM (Transition Metal) based materials, from melt spun ribbons and gas atomized powders, have shown that partially amorphous or nano-crystalline structures lead to a high specific strength. In the present study, therefore, spray atomization and deposition process has been used to produce plates of Al85Y8Ni5Co2 (deposit D1) and Al83Y5La5Ni5Co2 (deposit D2) systems so as to synthesize bulk deposit of nano-crystalline and/or partially amorphous materials in a single step. The rapid solidification and high undercooling of droplets during atomization and the chilling effect on undercooled liquid upon deposition give rise to the above microstructural features. The microstructural features of deposits as well as overspray powders were studied using optical, scanning and transmission electron microscope. The alloys invariably showed a large fraction of nano-crystalline structure and amorphous features, characterized by featureless regions at optical resolution, along with distribution of primary equilibrium phases. The differential scanning calorimetric (DSC) analysis of the deposits showed similar crystallization features as observed during crystallization of fully amorphous melt spun ribbons of respective compositions. The transmission electron microscopy of deposit D1 showed the presence of 50–100 nm size fcc-Al precipitates in an amorphous matrix decorated with 5– 20 nm fcc-Al crystallites. The annealing treatment of deposits at different temperatures, determined from the crystallization peak temperatures of the deposits, showed precipitation of nanoscale fcc-Al and intermetallic phases giving rise to a remarkable increase in hardness values. The bulk hardness of the deposits D1 and D2 was 391 and 427 HV, respectively. Whereas, the annealed deposits showed a bulk hardness value of 476 HV for deposit D1 and 583 HV for deposit D2 at annealing temperature of 298 and 380 C, respectively. An attempt has been made to bring out the possible mechanism of microstructural evolution during spray deposition of the above alloy systems, and the effect of microstructural features on the hardness values has been discussed.

II–VI-based microcavities grown by molecular beam epitaxy designed for emission in the blue spectral region have been comprehensively characterized by transmission electron microscopy in bright field, dark field and scanning mode. These complex structures contain a high amount of MgS, which is correlated with the occurrence of a certain kind of defect. Typical V-shaped stacking faults caused by the metastability of zinc-blende and rocksalt MgS along different (111) glideplanes have been identified. Optimized growth conditions have been found where this defect type can be suppressed to a large extend. Results obtained by x-ray diffraction measurements are consistent with the microscopy findings.

We present a synthesis route to fabricate submicrometer-sized colloidosomes at ambient conditions and mild pH with tailorable nanopore sizes and porosity. The capsules are formed via self-assembly of metal oxide nanoparticles on emulsion droplets in a water-in-oil emulsion. Through the adsorption of oil-soluble surfactants on emulsion droplets, which carry the same sign of charge as the colloidosome-forming particles in aqueous media, colloidosomes with positive and negative zeta potentials were synthesized. The hollow capsules are inherently rigid and necessitated no further stabilization. By varying the sizes and shapes of the nanoparticles, we were able to tailor the pore diameters and pore size distributions on the surface of the capsules. Particles of high spherical uniformity with narrow size range generated colloidosomes with a mainly hexagonal close packed surface structure and narrow pore size, while particles of elliptical and uneven shape created colloidosomes with no surface order and an incon...

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).

Microcavity structures which consist of a cavity formed by two distributed Bragg reflectors can be used to study light-matter interaction in the weak and strong coupling regime. In this contribution microcavities with one or three ZnSe quantum wells embedded into the cavity are presented. In contrast to the sample with the single quantum well, strong light-matter coupling can be observed for that one with three quantum wells. The sample possesses an energy splitting of about 19meV between the upper and lower polariton branch observed by reflectivity measurements. In addition, the transition from strong to weak coupling and the starting of the photonic lasing is demonstrated by excitation density dependent measurements. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper, a first comparative study on the sensing behavior of AlGaN/GaN and AlInN/GaN heterostructures is presented, especially with respect to the impact of crystal polarity. Ga-polar samples were grown completely by metal-organic vapor-phase epitaxy (MOVPE), whereas N-polar structures were realized by a combination of molecular-beam epitaxy and MOVPE. The epitaxial structures were characterized by high resolution X-ray diffraction, scanning electron microscopy, atomic force microscopy and transmission electron microscopy, particularly with regard to strain, surface and interface roughness. Sheet carrier density and mobility of the 2-dimensional charge carrier gas were determined by Hall measurements. The heterostructures were applied as open-gate sensors and their sensing behavior was analyzed by exposure to different polar liquids. Generally, the device sensitivity is dominated by the sheet carrier density indicating a direct correlation between localized carriers at the interface and surface. A superior carrier concentration in N-polar samples enhanced their response to polar fluids. Due to a higher sheet carrier density, AlInN-based devices are more sensitive to polar liquids compared to conventional AlGaN/GaN heterostructures.

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.

Nitride-based three-dimensional core-shell nanorods (NRs) are promising candidates for the achievement of highly efficient optoelectronic devices. For a detailed understanding of the complex core-shell layer structure of InGaN/GaN NRs, a systematic determination and correlation of the structural, compositional, and optical properties on a nanometer-scale is essential. In particular, the combination of low-temperature cathodoluminescence (CL) spectroscopy directly performed in a scanning transmission electron microscope (STEM), and quantitative high-angle annular dark field imaging enables a comprehensive study of the nanoscopic attributes of the individual shell layers. The investigated InGaN/GaN core-shell NRs, which were grown by metal-organic vapor-phase epitaxy using selective-area growth exhibit an exceptionally low density of extended defects. Using highly spatially resolved CL mapping of single NRs performed in cross-section, we give a direct insight into the optical properties of the individual core-shell layers. Most interesting, we observe a red shift of the InGaN single quantum well from 410 to 471 nm along the nonpolar side wall. Quantitative STEM analysis of the active region reveals an increasing thickness of the single quantum well (SQW) from 6 to 13 nm, accompanied by a slight increase of the indium concentration along the nonpolar side wall from 11% to 13%. Both effects, the increased quantum-well thickness and the higher indium incorporation, are responsible for the observed energetic shift of the InGaN SQW luminescence. Furthermore, compositional mappings of the InGaN quantum well reveal the formation of locally indium rich regions with several nanometers in size, leading to potential fluctuations in the InGaN SQW energy landscape. This is directly evidenced by nanometer-scale resolved CL mappings that show strong localization effects of the excitonic SQW emission.

A concise derivation of the principle of reciprocity applied to realistic transmission electron microscopy setups is presented making use of the multislice formalism. The equivalence of images acquired in conventional and scanning mode is thereby rigorously shown. The conditions for the applicability of the found reciprocity relations is discussed. Furthermore the positions of apertures in relation to the corresponding lenses are considered, a subject which scarcely has been addressed in previous publications.

Solid-state properties such as strain or chemical composition often leave characteristic fingerprints in the angular dependence of electron scattering. Scanning transmission electron microscopy (STEM) is dedicated to probe scattered intensity with atomic resolution, but it drastically lacks angular resolution. Here we report both a setup to exploit the explicit angular dependence of scattered intensity and applications of angle-resolved STEM to semiconductor nanostructures. Our method is applied to measure nitrogen content and specimen thickness in a GaNxAs1−x layer independently at atomic resolution by evaluating two dedicated angular intervals. We demonstrate contrast formation due to strain and composition in a Si- based metal-oxide semiconductor field effect transistor (MOSFET) with GexSi1−x stressors as a function of the angles used for imaging. To shed light on the validity of current theoretical approaches this data is compared with theory, namely the Rutherford approach and contemporary multislice simulations. Inconsistency is found for the Rutherford model in the whole angular range of 16–255 mrad. Contrary, the multislice simulations are applicable for angles larger than 35 mrad whereas a significant mismatch is observed at lower angles. This limitation of established simulations is discussed particularly on the basis of inelastic scattering.

ZnO nanowire arrays were functionalized with colloidal CdSe quantum dots stabilized by 3-mercaptopropionic acid to form hybrid devices. The photoconductivity of the nanowire/quantum-dot devices was studied under selective photoexcitation of the quantum dots, and it was found that the dynamics strongly depend on the gas environment. Desorption of surface oxygen from both the ZnO nanowires and the CdSe quantum dots, activated by electron tunnelling between the nanowires and the quantum dots, is found to be the dominating process that determines the dynamics of the photoconductivity in the hybrid nanowire/quantum-dot devices.

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.

The rational design of composite nanoparticles with desired optical and electronic properties requires the detailed analysis of surface and bulk contributions to the respective overall function. We use flame spray pyrolysis (FSP) to generate nanoparticles of the ternary Zn-Mg-O system the compositions of which range from solid solutions of Zn(2+) ions in periclase MgO to phase separated particle mixtures which consist of periclase (cubic) MgO and wurtzite (hexagonal) ZnO phases. The structure and composition of the composite Zn(x)Mg(1-x)O (0 ≤ x ≤ 0.3) particles are investigated using X-ray diffraction and high-resolution transmission electron microscopy, whereas UV diffuse reflectance and photoluminescence (PL) spectroscopy are used for the investigation of their optical properties. Vacuum annealing has been carried out to track the effects of stepwise elimination of surface adsorbates on the photoexcitation and PL emission properties. We demonstrate that for Zn(0.1)Mg(0.9)O particles, the admixed ZnO suppresses the MgO specific surface excitons and produces a PL emission band at 470 nm. Although gaseous oxygen partially reduces the emission intensity of hydroxylated particles, it leads to entire quenching in completely dehydroxylated samples after vacuum annealing at 1173 K. Consequently, surface hydroxyls at the solid-gas interface play a significant role as protecting groups against the PL-quenching effects of O(2). The obtained results are relevant for the characterization of ZnO-based devices as well as for other metal oxide materials where the impact of the surface composition on the photoelectronic properties is usually neglected.

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.

Phone: þ49-421-21862217, Fax: 49-421-2184581Highly ordered GaN nanorods were grown self-organized andwithoutcatalystonr-planesapphirebymolecularbeamepitaxywhile the AlN nucleation sites for the nanorods were providedby a nitridation process using a metal-organic vapor-phaseepitaxy system. The growth window for the nanorod formationwas analysed in detail and turned out to be very sensitive withrespect to the growth temperature. The nanorods are symmetri-cally tilted with an inclination angle of 628 between thesubstrate and the nanorods. The mirror axis is the c-direction ofthe compact GaN layer surrounding the roots of the nano-columns.Methodsforthecontrolofthenanoroddensityandthesuppressionofonenanorodgrowthdirectionarepresented.Theresults indicate a diffusion based growth mechanism. Trans-mission electron microscopy studies and high resolution X-raydiffraction (HRXRD) polar plots reveal the epitaxial relation-ship between substrate, compact GaN layer and nanorods. Thenanorods grow in c-direction and the side facets are m-planes.Transmission electron microscopic and optical analysis of thenanorods reveal the good structural and optical properties,respectively. A full width at half maximum (FWHM) of1.2meVofthedonor-boundexcitonemissionwasmeasuredforboth the ensemble and single free-standing nanorods. Success-ful n- and Mg-doping of nanorods was verified by a strongincrease of the micro-photoluminescence intensity of therespective donor-bound and acceptor-bound exciton emissionpeak in comparison to an undoped sample.The background shows a secondary electron microscope image(angle of 108between sample surface and electron beam) of thenanorods. Superimposed to the SEM image, a typical micro-photoluminescence spectrum of the excitonic emission of anensemble at 25K is presented. The donor-bound, acceptor-bound and free exciton lines are labelled with D

For vertical-cavity surface-emitting laser pillar microcavities with three-dimensional optical mode confinement, we theoretically study the influence of growth imperfections on the cavity quality factor. Mode calculations based on a vectorial transfer-matrix approach are used to simulate layer thickness fluctuations as well as a gradual variation of the layer thickness across the micropillar for an AlGaN/GaN material system. Experimental estimates for the layer thickness profile are obtained with transmission electron microscopy Z-contrast images.

The structure and chemistry of clean GaN surfaces and InGaN thin films and nanostructures grown by metal organic vapour pressure epitaxy (MOVPE) has been studied by means of X-ray photoemission spectroscopy, low-energy electron diffraction as well as scanning tunneling microscopy (STM) and transmission electron microscopy. Thermal annealing strongly improves the cleanliness of samples after dry nitrogen transfer and related exposure to residual oxygen. Nitrogen plasma assisted cleaning is shown to successfully further remove carbon contaminations, while Ga deposition with subsequent desorption to is shown to be superior for an enhanced reduction of surface oxygen. Using STM, the surface morphology has been studied in dependence on major growth parameters at various stages of InGaN MOVPE growth. The formation of nano-islands is reported for different growth conditions. By means of micro- photoluminescence measurements, we find samples to show strong photoluminescence from quantum-dot-like structures, however, the corresponding growth front is found to be rather flat throughout InGaN deposition. This leads to the conclusion that the formation of quantum dots does not proceed in a Stranski-Krastanov-like fashion but most likely during over- growth.

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 .

A 12 mm thick Al85Y8Ni5Co2 plate was spray deposited on a 30 mm thick pre-heated copper substrate. The deposit was characterized using optical, high resolution scanning and transmission electron microscopy. Differential scanning calorimetry was performed to assess the crystallization behaviour of the deposit compared to melt spun ribbons of the same composition. The deposit shows an amorphous phase fraction of 83 and 56 vol.% (based on the total crystallization energy), in the bottom and top regions of the deposit. The deposit consists of amorphous and nanoscale structures along with microcrystalline intermetallic phases. The large amorphous phase fraction in the deposit is attributed to the chilling effect upon deposition of highly undercooled/partially crystallized droplets onto the pre-heated substrate and rapid heat extraction thereof due to bonding at the deposit/substrate interface.

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.

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.

Mesenchymal stem cells (MSC) have emerged to be one of the most promising candidates for cellular immunotherapy in solid organ transplantation because the reduction of conventional immunosuppression is highly desirable. However, little is known about the details of MSC-mediated immunomodulation and their clinical relevance. To address conflicting studies about the ability of MSC to suppress or augment T-cell proliferation, we introduce a transplantation-related rat model that allows studying the influence of MSC on alloproliferation. Hearts transplanted in a fully allogeneic transplantation model (LEW to ACI) were rejected earlier when recipients were pretreated with donor MSC, indicating activation of T cells in vivo. In additional co-culture experiments, T cells were differently affected by allogeneic MSC depending on the extent of previous activation: When conditions were rendered proinflammatory by adding high concanavalin A (ConA) concentrations or proinflammatory cytokines (interferon-gamma, interleukin-2, or tumor necrosis factor-alpha), MSC inhibited proliferation. Application of low doses of ConA or anti-inflammatory cytokines like IL-10 abrogated the suppressive effect of MSC. For application of MSC in solid organ transplantation, it will be important to further describe this switch effect of MSC function.

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.