The microstructure, local indium composition and photoluminescence in green-emitting InGaN/GaN quantum wells.

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.

Steam reforming of methanol over oxide decorated nanoporous gold catalysts: a combined in situ FTIR and flow reactor study.

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.

A structural investigation of highly ordered catalyst- and mask-free GaN nanorods.

GaN nanorods were grown on r-plane sapphire substrates by a two-step approach. Nucleation sites for the nanorods were provided by the formation of AlN islands during nitridation in a metal organic vapor phase system. These islands are a-plane oriented as expected for nitride growth on r-plane sapphire. The nanorods themselves were grown by plasma assisted molecular beam epitaxy. The nanorods show an inclination towards the surface normal of 28.3° and are highly ordered. Studies with high resolution x-ray diffraction polar plots reveal the epitaxial relationship between the substrate and nanorods as a c-direction growth on inclined m-plane facets of the nitridated islands. The determined lattice constants show nanorods which are strain free. The growth direction of the nanorods has been confirmed in a transmission electron microscope by convergent beam electron diffraction patterns to be in the N-polar [Formula: see text] direction.

Light-emitting diode based on mask- and catalyst-free grown N-polar GaN nanorods.

We report on the fabrication of a light-emitting diode based on GaN nanorods containing InGaN quantum wells. The unique system consists of tilted N-polar nanorods of high crystalline quality. Photoluminescence, electroluminescence, and spatially resolved cathodoluminescence investigations consistently show quantum well emission around 2.6 eV. Scanning transmission electron microscopy and energy-dispersive x-ray spectroscopy measurements reveal a truncated shape of the quantum wells with In contents of (15 ± 5)%.

Locating light and heavy atomic column positions with picometer precision using ISTEM.

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.

How precise can atoms of a nanocluster be located in 3D using a tilt series of scanning transmission electron microscopy images?

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.

Atomic resolution elemental mapping using energy-filtered imaging scanning transmission electron microscopy with chromatic aberration correction.

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.

Determining oxygen relaxations at an interface: A comparative study between transmission electron microscopy techniques.

Nowadays, aberration corrected transmission electron microscopy (TEM) is a popular method to characterise nanomaterials at the atomic scale. Here, atomically resolved images of nanomaterials are acquired, where the contrast depends on the illumination, imaging and detector conditions of the microscope. Visualization of light elements is possible when using low angle annular dark field (LAADF) STEM, annular bright field (ABF) STEM, integrated differential phase contrast (iDPC) STEM, negative spherical aberration imaging (NCSI) and imaging STEM (ISTEM). In this work, images of a NdGaO3-La0.67Sr0.33MnO3 (NGO-LSMO) interface are quantitatively evaluated by using statistical parameter estimation theory. For imaging light elements, all techniques are providing reliable results, while the techniques based on interference contrast, NCSI and ISTEM, are less robust in terms of accuracy for extracting heavy column locations. In term of precision, sample drift and scan distortions mainly limits the STEM based techniques as compared to NCSI. Post processing techniques can, however, partially compensate for this. In order to provide an outlook to the future, simulated images of NGO, in which the unavoidable presence of Poisson noise is taken into account, are used to determine the ultimate precision. In this future counting noise limited scenario, NCSI and ISTEM imaging will provide more precise values as compared to the other techniques, which can be related to the mechanisms behind the image recording.

Determination of the mean inner potential in III-V semiconductors, Si and Ge by density functional theory and electron holography.

The mean inner potentials of various III-V semiconductors, Si and Ge have been calculated by density functional theory methods. For that purpose, the Coulomb potential of slabs consisting of a crystal and vacuum region has been computed and averaged inside the crystal region. The computed values are in agreement with experimental values obtained by electron holography for Si and GaAs. For the other semiconductors, the deviations are smaller than 0.8 V. The results from density functional theory are approximately 10% smaller than the values derived from atomic scattering factors computed by Hartree Fock calculations.

Arsenic trioxide as an inducer of apoptosis and loss of PML/RAR alpha protein in acute promyelocytic leukemia cells.

BACKGROUND: Retinoids, which are derivatives of vitamin A, induce differentiation of acute promyelocytic leukemia (APL) cells in vitro and in patients. However, APL cells develop resistance to retinoic acid treatment. Arsenic trioxide (As2O3) can induce clinical remission in patients with APL, including those who have relapsed after retinoic acid treatment, by inducing apoptosis (programmed cell death) of the leukemia cells. In this study, we investigated the molecular mechanisms by which As2O3 induces apoptosis in retinoic acid-sensitive NB4 APL cells, in retinoic acid-resistant derivatives of these cells, and in fresh leukemia cells from patients. METHODS: Apoptosis was assessed by means of DNA fragmentation analyses, TUNEL assays (i.e., deoxyuridine triphosphate labeling of DNA nicks with terminal deoxynucleotidyl transferase), and flow cytometry. Expression of the PML/RAR alpha fusion protein in leukemia cells was assessed by means of western blotting, ligand binding, and immunohistochemistry. Northern blotting and ribonuclease protection assays were used to evaluate changes in gene expression in response to retinoic acid and As2O3 treatment. RESULTS AND CONCLUSIONS: As2O3 induces apoptosis without differentiation in retinoic acid-sensitive and retinoic acid-resistant APL cells at concentrations that are achievable in patients. As2O3 induces loss of the PML/RAR alpha fusion protein in NB4 cells, in retinoic-acid resistant cells derived from them, in fresh APL cells from patients, and in non-APL cells transfected to express this protein. As2O3 and retinoic acid induce different patterns of gene regulation, and they inhibit the phenotypes induced by each other. Understanding the molecular basis of these differences in the effects of As2O3 and retinoic acid may guide the clinical use of arsenic compounds and provide insights into the management of leukemias that do not respond to retinoic acid.

Estrogen receptor expression activates the transcriptional and growth-inhibitory response to retinoids without enhanced retinoic acid receptor alpha expression.

Estrogen receptor (ER)-positive human breast cancer cells are hormonally regulated and are inhibited by retinoids, whereas most ER-negative breast cancer cells are not. Here, we compared retinoid-induced transcriptional activation and growth inhibition in the ER-negative breast cancer cell line MDA-MB-231, stably transfected to express wild-type ER (S30), with that of the ER-positive MCF-7 line and the ER-negative parental line. Retinoids inhibited growth of the ER-expressing S30 clone but not of the parental MDA-MB-231 cells. Unlike a previously reported MDA-MD-231 subclone that was transfected to express a mutated ER (G400V), S30 did not express increased levels of retinoid receptor RNA or protein, nor was there increased binding activity to retinoid-responsive DNA elements. However, stable expression of ER increased retinoid activation of transcription of a retinoic acid (RA) response elements from the low level in MDA-MB-231 to approach the level of MCF-7. The restored growth inhibition and transcriptional regulation by RA were unaffected by treatment with ER agonists or antagonists. Transient expression of ER but not of other nuclear receptors in MDA-MB-231 cells also activated retinoid-induced transcription, showing that this response is specific to ER. Furthermore, the effect of exogenously expressed ER on retinoid response was much greater than that obtained by overexpression of RA receptor alpha and/or retinoid X receptor alpha. Finally, a panel of ER mutants showed that enhancement of retinoid-induced transcriptional activity was dependent on the integrity of the DNA binding domain.

9-Cis retinoic acid inhibits growth of breast cancer cells and down-regulates estrogen receptor RNA and protein.

All-trans retinoic acid (tRA) inhibits growth of estrogen receptor-positive (ER+) breast cancer cells in vitro, and a variety of retinoids inhibit development of breast cancer in animal models. 9-cis retinoic acid (9-cis RA) is a naturally occurring high affinity ligand for the retinoid X receptors, as well as the retinoic acid receptors (RARs). Whether 9-cis RA has a different spectrum of biological activity from tRA, which only binds RARs with high affinity, is largely unknown. We studied the effects of 9-cis RA on growth and gene expression in ER+ and ER- human breast cancer cells. 9-cis RA inhibited the growth in monolayer culture of several ER+, but not ER-, cell lines in a dose-dependent manner. Growth inhibition and morphological changes by 9-cis RA were similar to those of tRA, suggesting that the ability to bind both RAR and retinoid X receptors did not significantly augment growth inhibition or confer sensitivity to tRA-resistant lines. MCF-7 cells exposed to 9-cis RA showed a dose-dependent accumulation in G1. Northern analyses showed that RAR-alpha and RAR-beta were not significantly regulated, while RAR-gamma was up-regulated and retinoid X receptor alpha was down-regulated by 9-cis RA. Since interactions between tRA and ER-dependent transcription have recently been reported, we investigated whether these retinoids regulate expression of ER itself or estrogen-responsive genes. Both 9-cis RA and tRA induce down-regulation of ER mRNA and protein in MCF-7 cells. 9-cis RA down-regulates expression of the estrogen-responsive genes PR and pS2 in MCF-7 cells as reported previously for tRA. In several ER-positive subclones, we found that the degree of ER expression and regulation, but not always estrogen-sensitivity, correlates with the growth-inhibitory effects of 9-cis RA. Further, in an ER-, retinoid-unresponsive breast cancer cell line, induced ER expression confers responsiveness to retinoid growth inhibition. These data, combined with reports of additive growth inhibition of tRA and tamoxifen in vitro, suggest that 9-cis RA might augment the ability of tamoxifen to inhibit growth of ER+ breast cancer cells in vivo.

Determination of the mean inner potential in III-V semiconductors by electron holography.

The mean inner potential of GaAs(14.18V), InAs(14.50V), GaP(14.35V) and InP(14.50V) has been measured by transmission electron holography using the phase shift of the (000)-beam of the first hologram sideband. To provide a defined specimen geometry we used 90 degrees wedges obtained by the cleavage technique. The exact excitation condition as well as the acceleration voltage of the electrons were determined from convergent beam electron diffraction images. The magnification is extracted from two-beam lattice fringe images and dynamical effects are taken into account by Bloch-wave calculations.

Compositional analysis based on electron holography and a chemically sensitive reflection.

A method for compositional analysis of low-dimensional heterostructures is presented. The suggested procedure is based on electron holography and the exploitation of the chemically sensitive (0 0 2) reflection. We apply an off-axis imaging condition with the (0 0 2) beam strongly excited and centered on the optic axis. The first side band of the hologram is centered using an "empty" reference hologram obtained for a hole of the specimen. From the centered side band we use the phase of the central (0 0 0) and the amplitude of the (0 0 2) reflections to evaluate the local composition and the local specimen thickness in an iterative and self-consistent way. Delocalization effects that lead to a shift of the spatial information of (0 0 0) and (0 0 2) reflections are taken into account. The application of the procedure is demonstrated with an AlAs/GaAs(0 0 1) superlattice with a period of 5 nm. The concentration profiles obtained are discussed in relation to segregation. The measured segregation efficiency is R = 0.51 +/- 0.02.

The effect of probe inaccuracies on the quantitative model-based analysis of high angle annular dark field scanning transmission electron microscopy images.

Quantitative structural and chemical information can be obtained from high angle annular dark field scanning transmission electron microscopy (HAADF STEM) images when using statistical parameter estimation theory. In this approach, we assume an empirical parameterized imaging model for which the total scattered intensities of the atomic columns are estimated. These intensities can be related to the material structure or composition. Since the experimental probe profile is assumed to be known in the description of the imaging model, we will explore how the uncertainties in the probe profile affect the estimation of the total scattered intensities. Using multislice image simulations, we analyze this effect for Cs corrected and non-Cs corrected microscopes as a function of inaccuracies in cylindrically symmetric aberrations, such as defocus and spherical aberration of third and fifth order, and non-cylindrically symmetric aberrations, such as 2-fold and 3-fold astigmatism and coma.

Quantitative composition determination at the atomic level using model-based high-angle annular dark field scanning transmission electron microscopy.

High angle annular dark field scanning transmission electron microscopy (HAADF STEM) images provide sample information which is sensitive to the chemical composition. The image intensities indeed scale with the mean atomic number Z. To some extent, chemically different atomic column types can therefore be visually distinguished. However, in order to quantify the atomic column composition with high accuracy and precision, model-based methods are necessary. Therefore, an empirical incoherent parametric imaging model can be used of which the unknown parameters are determined using statistical parameter estimation theory (Van Aert et al., 2009, [1]). In this paper, it will be shown how this method can be combined with frozen lattice multislice simulations in order to evolve from a relative toward an absolute quantification of the composition of single atomic columns with mixed atom types. Furthermore, the validity of the model assumptions are explored and discussed.

A nanocrystalline Hilbert phase-plate for phase-contrast transmission electron microscopy.

Thin-film-based phase-plates are applied to enhance the contrast of weak-phase objects in transmission electron microscopy. In this work, metal-film-based phase-plates are considered to reduce contamination and electrostatic charging, which up to now limit the application of phase-plates fabricated from amorphous C-films. Their crystalline structure requires a model for the simulation of the effect of crystallinity on the phase-plate properties and the image formation process. The model established in this work is verified by experimental results obtained by the application of a textured nanocrystalline Au-film-based Hilbert phase-plate. Based on the model, it is shown that monocrystalline and textured nanocrystalline phase-plate microstructures of appropriate thickness and crystalline orientation can be a promising approach for phase-contrast transmission electron microscopy.

A memory efficient method for fully three-dimensional object reconstruction with HAADF STEM.

The conventional approach to object reconstruction through electron tomography is to reduce the three-dimensional problem to a series of independent two-dimensional slice-by-slice reconstructions. However, at atomic resolution the image of a single atom extends over many such slices and incorporating this image as prior knowledge in tomography or depth sectioning therefore requires a fully three-dimensional treatment. Unfortunately, the size of the three-dimensional projection operator scales highly unfavorably with object size and readily exceeds the available computer memory. In this paper, it is shown that for incoherent image formation the memory requirement can be reduced to the fundamental lower limit of the object size, both for tomography and depth sectioning. Furthermore, it is shown through multislice calculations that high angle annular dark field scanning transmission electron microscopy can be sufficiently incoherent for the reconstruction of single element nanocrystals, but that dynamical diffraction effects can cause classification problems if more than one element is present.

Coherently embedded Ag nanostructures in Si: 3D imaging and their application to SERS.

Surface enhanced Raman spectroscopy (SERS) has been established as a powerful tool to detect very low-concentration bio-molecules. One of the challenging problems is to have reliable and robust SERS substrate. Here, we report on a simple method to grow coherently embedded (endotaxial) silver nanostructures in silicon substrates, analyze their three-dimensional shape by scanning transmission electron microscopy tomography and demonstrate their use as a highly reproducible and stable substrate for SERS measurements. Bi-layers consisting of Ag and GeOx thin films were grown on native oxide covered silicon substrate using a physical vapor deposition method. Followed by annealing at 800°C under ambient conditions, this resulted in the formation of endotaxial Ag nanostructures of specific shape depending upon the substrate orientation. These structures are utilized for detection of Crystal Violet molecules of 5 × 10(-10) M concentrations. These are expected to be one of the highly robust, reusable and novel substrates for single molecule detection.

Atom counting in HAADF STEM using a statistical model-based approach: methodology, possibilities, and inherent limitations.

In the present paper, a statistical model-based method to count the number of atoms of monotype crystalline nanostructures from high resolution high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) images is discussed in detail together with a thorough study on the possibilities and inherent limitations. In order to count the number of atoms, it is assumed that the total scattered intensity scales with the number of atoms per atom column. These intensities are quantitatively determined using model-based statistical parameter estimation theory. The distribution describing the probability that intensity values are generated by atomic columns containing a specific number of atoms is inferred on the basis of the experimental scattered intensities. Finally, the number of atoms per atom column is quantified using this estimated probability distribution. The number of atom columns available in the observed STEM image, the number of components in the estimated probability distribution, the width of the components of the probability distribution, and the typical shape of a criterion to assess the number of components in the probability distribution directly affect the accuracy and precision with which the number of atoms in a particular atom column can be estimated. It is shown that single atom sensitivity is feasible taking the latter aspects into consideration.