Qm Ramasse

High-resolution low-dose scanning transmission electron microscopy

During the past two decades instrumentation in scanning transmission electron microscopy (STEM) has pushed toward higher intensity electron probes to increase the signal-to-noise ratio of recorded images. While this is suitable for robust specimens, biological specimens require a much reduced electron dose for high-resolution imaging. We describe here protocols for low-dose STEM image recording with a conventional field-emission gun STEM, while maintaining the high-resolution capability of the instrument. Our findings show that a combination of reduced pixel dwell time and reduced gun current can achieve radiation doses comparable to low-dose TEM.

Direct Evidence for Cation Non‐Stoichiometry and Cottrell Atmospheres Around Dislocation Cores in Functional Oxide Interfaces

Perovskite oxides are a ubiquitous class of functional oxide materials that are used in a variety of nanoscale functional applications. In order to exploit their properties, these materials are often deposited on a dissimilar underlying substrate. A critical consequence of this process is the formation of misfit dislocation arrays at the film-substrate interface as a mechanism to release the lattice mismatch strain. Although dislocation cores are only a few angstroms wide, the associated strain-fields are often significant and long-range. Theoretical computations of the strain-field for metals reveal that the stress due to the local strain around the core can be equal to or even higher, than the yield stress. Concurrently, the trend of aggressive downsizing has resulted in functional materials being confined to nanometric volumes. The presence of even a single defect, and its associated long-range field, could adversely affect device performance. Such longrange strain fields are known to affect both the microstructure and electronic states. More recently the detrimental effect of this strain field on the polarization and electromechanical

Synthesis and Characterization of K8−x(H2)ySi46

A hydrogen-containing inorganic clathrate with the nominal composition, K(7)(H(2))(3)Si(46), has been prepared in 98% yield by the reaction of K(4)Si(4) with NH(4)Br. Rietveld refinement of the powder X-ray diffraction data is consistent with the clathrate type I structure. Elemental analysis and (1)H MAS NMR confirmed the presence of hydrogen in this material. Type I clathrate structure is built up from a Si framework with two types of cages where the guest species, in this case K and H(2), can reside: a large cage composed of 24 Si, in which the guest resides in the 6d position, and a smaller one composed of 20 Si, in which the guest occupies the 2a position (cubic space group Pm3n). Potassium occupancy was examined using spherical aberration (Cs) corrected scanning transmission electron microscopy (STEM). The high-angle annular dark-field (HAADF) STEM experimental and simulated images indicated that the K is deficient in both the 2a and the 6d sites. (1)H and (29)Si MAS NMR are consistent with the presence of H(2) in a restricted environment and the clathrate I structure, respectively. FTIR and (29)Si{(1)H} CP MAS NMR results show no evidence for a Si-H bond, suggesting that hydrogen is present as H(2) in interstitial sites. Thermal gravimetry (TG) mass spectrometry (MS) provide additional confirmation of H(2) with hydrogen loss at approximately 400 degrees C.

Direct measurement of Co-ion spin state transitions in Ca3Co4O9 using variable-temperature electron energy-loss spectroscopy

Previous studies of Ca3Co4O9 suggested that the abrupt changes in the magnetic susceptibility and electrical resistivity at ∼420u2002K can be attributed to Co spin state transitions. In this letter, we study the possible transitions above 420 K by variable-temperature Z-contrast imaging and electron energy-loss spectroscopy. We find that there is no observable change in the structure and Co valence state upon in situ heating to 500 K, compared to room temperature. However, an intensity decrease of the prepeak in the O K-edge near edge fine structure indicates a Co3+-ion spin state transition has occurred from a low to an intermediate spin state.

Direct measurement of ferromagnetic ordering in biaxially strained LaCoO3 thin films

LaCoO 3 undergoes a transition from a nonmagnetic to a paramagnetic semiconductor at 80 K, associated with a spin-state transition of the Co 3 + ions. It was proposed that the temperature of the spin-state transition depends strongly on the LaCoO 3 lattice parameter, suggesting that strain can stabilize different spin states at different temperatures. By combining atomic-resolution Z-contrast imaging, electron diffraction, and angular-resolved electron energy-loss spectroscopy (EELS) with in situ cooling experiments, we show that epitaxially strained LaCoO 3 (001) thin filmsgrown on LaAlO 3 (001) do not undergo a low temperature spin-state transition. Our EELS study explores the origins of the ferromagnetic ordering in strained LaCoO 3 films.

Chemistry of the Fe2O3/BiFeO3 Interface in BiFeO3 Thin Film Heterostructures

We investigate the interfacial chemistry of secondary Fe2O3 phases formed in a BiFeO3 (BFO) layer in BFO/ La0.67Sr0.33MnO3 (LSMO)/SrTiO3 (STO) heterostructures. A combination of high-resolution spherical aberration corrected scanning TEM and spectroscopy results, reveals that specific chemical and crystallographic similarities between Fe2O3 and BFO, enable the BFO layer to form a facile host for Fe2O3.

Atomic scale observation and characterization of redox-induced interfacial layers in commercial Si thin film photovoltaics

Thermodynamics considerations and experimental evidence suggest that redox reactions occur at the interfaces between transparent conductive oxides (TCOs) and the active silicon layers in photovoltaic stacks, with potentially nefarious effects to device efficiency. The presence of interfacial layers of oxidized silicon and reduced metal is confirmed here with analytical depth profiling techniques in industrially produced Si thin film solar cells. Atomic-resolution scanning transmission electron microscopy and energy loss spectroscopy are used to show that the specific chemistry of the interface, the front TCO being Sn-rich while the back TCO is Zn-rich, has a strong influence on the size of the resulting interfacial layer. Furthermore, the morphology of the interface and the impact of annealing treatments are also studied, leading to suggestions for possible improvements of commercial device efficiency.

High Resolution HAADF-STEM Imaging Analysis of N related defects in GaNAs Quantum Wells

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

Analysis of the mechanism of N incorporation in N-doped GaAs quantum wells

The quaternary compound GaInNAs has attracted recent interest due to the possibility of obtaining laser diodes in the range 1.3–1.55 µm. The main characteristic of the GaInNAs/GaAs system is a strong negative bowing parameter that causes a rapid decrease of the bandgap by the addition of relatively small amounts of N (<5%) to GaInAs. Although the solubility of N in Ga(In)As is extremely low, GaInNAs layers with N content as high as 5% have been reported. Because of the small size of N, the incorporation of this element in As-sites produces a strong local strain that highly destabilizes the structure and could favour the formation of alternative N-containing complexes, instead of simple substitution on the arsenic site. As such, the optimization of the optoelectronic properties of this alloy is still a challenge, as witnessed by the observation of a strong reduction in the photoluminescence intensity associated with non-radiative recombination centres. In order to overcome these limitations, a full understanding of the incorporation mechanism associated with N alloying and its resulting atomic distribution in the alloy is essential.

Analytical Aberration-Corrected STEM of Ferroelectric Functional Interfaces

SuperSTEM Laboratory, STFC Daresbury, Keckwick Lane, Daresbury WA4 4AD, U.K. School of Mat. Sci. and Engineering, U. of New South Wales, Sydney NSW 2052, Australia. Monash Centre for Electron Microscopy, Monash University, Victoria 3800, Australia. Max Planck Institute for Microsctructural Physics, Weinberg 2, D-06120 Halle, Germany Dept. of Chem. Eng. and Mat. Sci., University of California-Davis, Davis CA 95616 Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore CA 94550