Z-contrast Scanning Transmission Electron Microscopy Research Articles

Mineralogy, aqueous alteration, and primitive textural characteristics of fine-grained rims in the Y-791198 CM2 carbonaceous chondrite: TEM observations and comparison to ALHA81002

Transmission electron microscopy (TEM) was used to study the microtextural and mineralogical characteristics of fine-grained rims in the unbrecciated CM2 chondrites, Y-791198 and ALHA81002, in an effort to provide constraints on the origins of the rims themselves. Our TEM observations show that the rims in Y-791198 are composed of two distinct types of region, sulfide-poor and sulfide-rich, that are intermixed in a complex manner at the micron to submicron level. The sulfide-poor regions are largely composed of amorphous silicate material or nanocrystalline serpentine, but rare fibrous and coarse-grained serpentine grains have also been identified. No fine-grained cronstedtite or tochilinite were observed, although coarse-grained lumps of tochilinite are present in the rims. In contrast, the sulfide-rich regions are characterized by the presence of a myriad, nanometer-sized Fe,Ni sulfide grains (pentlandite with some Ni-rich pyrrhotite) embedded within an amorphous silicate similar in composition to that of the sulfide-poor regions. The sulfide-rich regions also contain rare phases such as olivine, and Fe,Ni metal grains with grain sizes that are always >100nm in size. Z-contrast scanning transmission electron microscopy (STEM) reveals that the fine-grained rims consist of a mosaic of irregularly-shaped sulfide-poor and sulfide-rich-regions with sizes of about 0.2–0.5μm, that have been compacted together during parent body lithification. Despite aqueous alteration, the distinct mineralogical characteristics of these different regions are preserved on a fine-scale and probably represent primitive heterogeneity in the dust from which these rims formed.Serpentine is much better developed and more widespread in the fine-grained rims of ALHA81002 than Y-791198. Complex mats of serpentine fibers are commonly found and cronstedtite and tochilinite are plentiful. Anhydrous minerals such as olivine are rare and have usually been replaced by serpentine. Like Y-791198, all the fine-grained rims studied in ALHA81002 show the same mineral assemblages and textural characteristics throughout and between rims. The homogeneity of the mineralogy, textural relationships and degree of hydration in the rims of these two chondrites is more consistent with parent-body alteration than with pre-accretionary alteration.

Growth and characterization of TiN/SiN(001) superlattice films

We report the layer structure and composition in recently discovered TiN/SiN(001) superlattices deposited by dual-reactive magnetron sputtering on MgO(001) substrates. High-resolution transmission electron microscopy combined with Z-contrast scanning transmission electron microscopy, x-ray reflection, diffraction, and reciprocal-space mapping shows the formation of high-quality superlattices with coherently strained cubic TiN and SiN layers for SiN thickness below 7–10 Å. For increasing SiN layer thicknesses, a transformation from epitaxial to amorphous SiNx (x ⩾ 1) occurs during growth. Elastic recoil detection analysis revealed an increase in nitrogen and argon content in SiNx layers during the phase transformation. The oxygen, carbon, and hydrogen contents in the multilayers were around the detection limit (∼0.1 at.%) with no indication of segregation to the layer interfaces. Nanoindentation experiments confirmed superlattice hardening in the films. The highest hardness of 40.4 ± 0.8 GPa was obtained for 20-Å TiN with 5-Å-thick SiN(001) interlayers, compared to monolithic TiN at 20.2 ± 0.9 GPa.

Oxygen Bonding in Bismuth Layered Compounds SrBi2Ta2O9

AbstractWith the help of the high resolution Z-contrast scanning transmission electron microscopy (STEM), the near edge fine structures of the electron loss spectroscopy (EELS) arises from different layers are investigated in SBT. The EELS spectra are interpreted by comparing with the calculation using linearnized augment plane wave (LAPW) method within the local density approximation (LDA). The oxygen bonding nature in different layers are discussed. In Bi2O2 layer, oxygen 2p state interacted with the Bi 6p state leads to a high t2g state in density of state while in SrTa2O7 layer, oxygen 2p state hybridized with Ta 5d and Sr 5d, which leads a higher eg state than t2g state.

Structure and phase separation of Ag–Cu alloy thin films

We report an in situ electron diffraction study and microscopy observation of phase separation in Ag–Cu solution alloy thin films of different compositions and film thicknesses. The results show that the as-deposited films consist of nanocrystalline Ag- and Cu-rich phases of a few nanometers in size. Upon annealing, two stages of thin-film transitions are observed. In the first stage, the growth of Cu crystallites is responsible for phase separation in Ag 50Cu 50 or Ag-rich alloy. For the Cu-rich samples, the phase separation process is much slower. The second stage transition involves thin film dewetting. A combination of Z-contrast scanning transmission electron microscopy imaging and electron diffraction reveal grain growth and phase separation between Ag and Cu in both stages. Atomic force microscopy results show the surface morphology of thin films only changes significantly at the late stage of phase separation when the thin film dewets.

Interface and defect structures in YBa2Cu3O7−δ and Nb : SrTiO3 heterojunction

YBa2Cu3O7−δ thin films grown on a Nb-doped SrTiO3 substrate by a pulsed laser deposition method have been fully characterized by scanning transmission electron microscopy Z-contrast imaging and electron energy loss spectroscopy techniques. The Nb distribution was found to be uniform and unchanged across the interface, ensuring a high quality p–n junction heterointerface. We first observed the coexistence of 124 and 125 YBCO defect structure phases, appearing as planar defects in a YBCO thin film. Dispersive Y2O3 nanoparticles have also been observed in the thin film. The interaction of these defect structures and Y2O3 nanoparticles is thought to be beneficial for pinning flux through the entire film thickness.

Structural characterization of a new layered-ternary Ta4AlC3 ceramic

Bulk Ta4AlC3, a new layered-ternary carbide in the Ta–Al–C system, was synthesized and characterized. Transmission electron microscopy investigations on this new phase are reported here. Selected area electron diffraction and convergent beam electron diffraction analyses indicated that this ternary carbide crystallized with the space group P63/mmc. Atomic-scale microstructures of Ta4AlC3 were achieved by means of high-resolution transmission electron microscopy and Z-contrast scanning transmission electron microscopy. The experimental crystal structural parameters agreed well with the theoretical values obtained using density-functional calculations.

Preparation of TiO 2-supported Au nanoparticle catalysts from a Au 13 cluster precursor: Ligand removal using ozone exposure versus a rapid thermal treatment

The structure of Au heterogeneous catalysts on the anatase form of TiO 2 has been characterized by high-resolution electron microscopy (HREM) and quantitative Z-contrast scanning transmission electron microscopy (STEM). These materials are prepared by deposition of highly monodisperse Au 13[PPh 3] 4[S(CH 2) 11CH 3] 4 (8-Å diameter) ligand-protected clusters on anatase, followed by reaction with ozone or a rapid oxidative thermal treatment to remove the ligands. The materials obtained differ markedly in each case. For the thermal treatment at 400 °C in air, the supported particles grow to an average size of 2.7 nm ( ± 0.6 nm ) in diameter, and the larger particles in the distribution are found to adopt a spherical geometry. Particle growth is greatly inhibited when ozone is used to remove the ligands (average diameter 1.2 ± 0.5 nm ). These particles assume a more oblate geometry, consistent with a truncated hemispherical shape. It was found that subsequent thermal treatments of the ozone-derived supported nanoparticles at 400 °C in air did not induce additional growth, indicating that sintering is strongly affected by the particle–support interactions developed by the ozone-based low-temperature ligand removal step. These materials exhibit catalytic activity and high stability for the oxidation of CO at elevated temperatures, with the level of activity dependent on catalyst preparation.

Search for ferromagnetism in conductive Nb:SrTiO3 with magnetic transition element (Cr, Co, Fe, Mn) dopants

Thin films of (0.5%, 1%) Nb:SrTiO3 dilutely doped with (2at.%) magnetic transition elements (Cr, Co, Fe, Mn) are examined for ferromagnetism. X-ray diffraction, Rutherford backscattering ion channeling, scanning transmission electron microscopy Z-contrast imaging, and electron energy loss spectroscopy techniques establish high crystalline quality of the films with no impurity phase(s) and highly uniform dopant distribution. Although the film conductivity improves dramatically by Nb doping, no ferromagnetism is found in any of our samples over the temperature range of 365 down to 5K. This is contrasted to the case of ferromagnetism reported in cobalt doped (La,Sr)TiO3.

Atomic scale characterization of complex oxide interfaces

Complex oxides exhibit the most disparate behaviors, from ferroelectricity to high T c superconductivity, colossal magnetoresistance to insulating properties. For these reasons, oxide thin films are of interest for electronics and the emerging field of spintronics. But epitaxial complex oxide ultrathin films and heterostructures can be significantly affected or even dominated by the presence of interfaces and may exhibit intriguing new physical properties quite different from the bulk. A study of the relations between structure and chemistry at the atomic scale is needed to understand the macroscopic properties of such “interface-controlled” materials. For this purpose, the combination of aberration-corrected Z-contrast scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS) represents a very powerful tool. The availability of sub-Angstrom probes allows a level of unprecedented detail when analyzing not only the interface structure with sensitivity to single atoms, but also the interface chemistry. In this work state of the art STEM-EELS will be applied to the study of different oxide interfaces in heterostructures with titanates, manganites and cuprates based on the perovskite structure.

Structural Basis for Near Unity Quantum Yield Core/Shell Nanostructures

Aberration-corrected Z-contrast scanning transmission electron microscopy of core/shell nanocrystals shows clear correlations between structure and quantum efficiency. Uniform shell coverage is obtained only for a graded CdS/ZnS shell material and is found to be critical to achieving near 100% quantum yield. The sublattice sensitivity of the images confirms that preferential growth takes place on the anion-terminated surfaces. This explains the three-dimensional "nanobullet" shape observed in the case of core/shell nanorods.

Microstructural relationships between compounds in the Ti–Si–C system

Microstructural relationships between Ti(5)Si(3) and TiC, and Ti(3)SiC(2) and TiC are reported. Ti5Si3 and TiC were determined to share a coherent interface following a new set of orientation relationships, i.e., (11 (2) over bar4)Ti(5)Si(3)||(1 (1) over bar0)TiC and [22 (4) over bar(3) over bar ]Ti(5)Si(3)||[110]TiC. Moreover, direct atomic resolution observation through Z-contrast scanning transmission electron microscopy showed that Ti3SiC2 nucleated within TiC. The close microstructural relationships between compounds in the Ti-Si-C system benefit the understanding of the reaction mechanism for the synthesis of Ti(3)SiC(2). (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Arsenic uphill diffusion during shallow junction formation

The behavior during annealing of low-energy As-implanted Si have been investigated by comparing secondary ion mass spectrometry (SIMS) and simulated profiles. Z-contrast scanning transmission electron microscopy (STEM) imaging has also been used to determine the As local distribution in proximity of the sample surface. The implants have been performed with energies between 1 and 10keV both through a thermally grown 11nm thick oxide and without any oxide mask. SIMS and STEM profiles show, after short annealing at 800–1000°C, an As pileup in the first nanometers of the Si matrix in proximity of the SiO2∕Si interface. We demonstrate that this phenomenon can be explained with a “Fickian” standard diffusion by assuming the presence of unspecified “dopant traps” near the SiO2∕Si interface that cause a drastic reduction of the dopant able to diffuse inside the bulk. We have also verified that removing before annealing the superficial 4nm of Si does not eliminate the As pileup. Different mechanisms proposed in literature to explain the uphill diffusion are discussed. Furthermore, the availability of a suitable simulation model allows us to evaluate the dopant diffusivity during the annealing and investigate the transient enhanced diffusion (TED) phenomena.

Imaging Single Atoms with Z-Contrast Scanning Transmission Electron Microscopy in Two and Three Dimensions

Z-Contrast STEM images of heavily antimony-doped silicon have been acquired which show quantifiable contrast from single impurity atoms inside a crystal. These images are two-dimensional projections of the sample, which makes identifying defects involving clusters of impurities difficult. Spherical aberration-corrected optics allow probes with much larger convergence angles to be used, which makes it possible to localize impurities in three dimensions by optical sectioning. A three dimensional generalization of the standard two-dimensional convolution description of incoherent imaging captures the basics of this new imaging mode, but probe channeling effects introduce important complications for zone-axis crystals.

Determination of Interface Atomic Structure and Its Impact on Spin Transport UsingZ-Contrast Microscopy and Density-Functional Theory

We combine Z-contrast scanning transmission electron microscopy with density-functional-theory calculations to determine the atomic structure of the interface in spin-polarized light-emitting diodes. A 44% increase in spin-injection efficiency occurs after a low-temperature anneal, which produces an ordered, coherent interface consisting of a single atomic plane of alternating Fe and As atoms. First-principles transport calculations indicate that the increase in spin-injection efficiency is due to the abruptness and coherency of the annealed interface.

Ferromagnetism in Co doped CeO2: Observation of a giant magnetic moment with a high Curie temperature

We report room temperature ferromagnetism in single crystal Ce1−xCoxO2−δ (x⩽0.05) films deposited on a LaAlO3(001) substrate. Films were grown by a pulsed laser deposition technique and were thoroughly characterized using x-ray diffraction, high-resolution transmission electron microscopy coupled with electron energy loss spectroscopy and scanning transmission electron microscopy-Z contrast, x-ray photoelectron spectroscopy, optical transmission spectroscopy, and magnetic measurements. These films are transparent in the visible regime and exhibit a very high Curie temperature ∼740–875K with a giant magnetic moment. Our results indicate that the ferromagnetic property is intrinsic to the CeO2 system and is not a result of any secondary magnetic phase or cluster formation.

Catalyst incorporation in ZnSe nanowires

ZnSe nanowires, 5–10 nm wide and several microns long, were fabricated by Au-catalyzed molecular beam epitaxy. Specially designed silicon–silicon dioxide substrates allowed an examination of individual nanowires with a combination of phase-contrast high-resolution transmission electron microscopy, Z-contrast scanning transmission electron microscopy imaging and energy-dispersive X-ray spectroscopy using a low-current electron probe with a diameter of 0.2 nm. Experimental results demonstrate direct evidence of Au incorporation within single-crystal sphalerite ZnSe nanowires.

Atomic scale characterization of layered ternary Cr2AlC ceramic

Cr 2 Al C is a recently developed layered ternary carbide. In this work, the atomic scale microstructure is reported. The layer stacking sequence of Cr and Al atoms has been clearly resolved. The atomic scale characterizations were realized by means of high resolution transmission electron microscopy and Z-contrast scanning transmission electron microscopy. Furthermore, electron energy loss spectroscopic analysis revealed that the Cr–C bonds in Cr2AlC are characterized by a strong σ bonding.

Influence of additives on anisotropic grain growth in silicon nitride ceramics

Tailoring microstructure and composition are critical components for obtaining high-performance silicon nitride (Si 3N 4) ceramics. Anisotropic growth behavior of Si 3N 4 grains can be used to form very elongated grains, which serve to reinforce the matrix analogous to whisker-reinforcement of ceramics. The Si 3N 4 grain morphology is known to be very sensitive to the particular additive used, especially in the case of the oxides of the rare earths (RE) and Group III elements. However, the atomistic mechanisms by which this occurs has not been understood until now. A first-principles model, the differential binding energy, has been developed to characterize the competition between RE and Si for migrating to the β-Si 3N 4 grain surfaces. The theory predicts that, of the RE, La should have the strongest and Lu the weakest preferential segregation to the grain surfaces. Additional calculations define the adsorption sites and their binding strengths for each of the REs on the prismatic plane of the Si 3N 4 grains. These predictions are confirmed by unique atomic-resolution images obtained by aberration-corrected Z-contrast scanning transmission electron microscopy (STEM). The combined theoretical and STEM studies reveal that the elements that induce the greatest observed grain anisotropy are those with the strongest preferential segregation plus high binding strength to the prismatic grain surface. Advances now allow one to use first principles calculations to determine the chemical affinity and bonding of the rare earths, as well as other elements, at the grain interfaces; atomistic factors that actually control the growth anisotropy as opposed to the commonly considered ion size, which at best, only provides a trend.

Consequences of niobium doping for the ferromagnetism and microstructure of anatase Co:TiO2 films

It is shown that dilute niobium doping has a significant effect on the ferromagnetism and microstructure of dilutely cobalt-doped anatase TiO2 films. Epitaxial films of anatase TiO2 with 3% Co, without and with 1% niobium doping were grown by pulsed-laser deposition at 875°C at different oxygen pressures. For growth at 10−5Torr niobium doping suppresses the ferromagnetism, while it enhances the same in films grown at 10−4Torr. High-resolution Z-contrast scanning transmission electron microscopy and electron energy loss spectroscopy show uniform surface segregation of cobalt-rich Ti1−x−yCoxNbyO2−δ phase, but without cobalt metal clusters.

The Structural and Optical Properties of Self-assembled InGaN/GaN Quantum Dots Grown by Molecular Beam Epitaxy

ABSTRACTThe structural and optical properties of InGaN quantum dots grown by plasma-assisted molecular beam epitaxy (MBE) have been characterised using atomic force microscopy, high-resolution transmission electron microscopy (TEM), Z-contrast scanning TEM, micro-photoluminescence (PL), temperature dependent PL and time-resolved PL. The uncapped InGaN nano-islands have densities of ∼1.5 × 1011 cm−2, heights of (1.7 ± 1.0) nm and diameters of (10 ± 4) nm. These parameters are not substantially changed during overgrowth of a GaN cap and the resulting quantum dots have a composition of In0.15Ga0.85N. The observation of narrow luminescence peaks in micro-PL measurements proves light emission from discrete energy states and the optical properties indicate strong confinement of carriers in the quantum dots and an unusually weak impact of piezoelectric field effects.