Z-contrast Imaging Research Articles

Totally Ecofriendly Synthesis of Silver Nanoparticles from Aqueous Dissolutions of Polysaccharides

In this contribution, a totally ecofriendly synthesis of silver nanoparticles from aqueous dissolution of polysaccharides is reported. The synthesis of nanoparticles was performed using aqueous dissolutions of silver nitrate (AgNO3) and carboxymethyl-cellulose (CMC) as both reducing and stabilization agent and using different AgNO3 : CMC weight ratios. Resultant yellowish to reddish dispersions were characterized by means of transmission electron microscopy and their related techniques, such as bright field and Z-contrast imaging and electron diffraction, as well as ultraviolet-visible and infrared spectroscopic techniques. The experimental evidence suggests that the morphology and particle size distribution of the silver nanoparticles depend on the AgNO3 : CMC weight ratio. This feature seems to be related to the stabilization given by the CMC matrix, which, according to our experimental findings, is steric in nature. Regarding such experimental evidence, a synthesis mechanism in which CMC acts as stabilizer and reducing agent is proposed.

Impact of N on the atomic-scale Sb distribution in quaternary GaAsSbN-capped InAs quantum dots

The use of GaAsSbN capping layers on InAs/GaAs quantum dots (QDs) has recently been proposed for micro- and optoelectronic applications for their ability to independently tailor electron and hole confinement potentials. However, there is a lack of knowledge about the structural and compositional changes associated with the process of simultaneous Sb and N incorporation. In the present work, we have characterized using transmission electron microscopy techniques the effects of adding N in the GaAsSb/InAs/GaAs QD system. Firstly, strain maps of the regions away from the InAs QDs had revealed a huge reduction of the strain fields with the N incorporation but a higher inhomogeneity, which points to a composition modulation enhancement with the presence of Sb-rich and Sb-poor regions in the range of a few nanometers. On the other hand, the average strain in the QDs and surroundings is also similar in both cases. It could be explained by the accumulation of Sb above the QDs, compensating the tensile strain induced by the N incorporation together with an In-Ga intermixing inhibition. Indeed, compositional maps of column resolution from aberration-corrected Z-contrast images confirmed that the addition of N enhances the preferential deposition of Sb above the InAs QD, giving rise to an undulation of the growth front. As an outcome, the strong redshift in the photoluminescence spectrum of the GaAsSbN sample cannot be attributed only to the N-related reduction of the conduction band offset but also to an enhancement of the effect of Sb on the QD band structure.

Direct Determination of the Chemical Bonding of Individual Impurities in Graphene

Using a combination of Z-contrast imaging and atomically resolved electron energy-loss spectroscopy on a scanning transmission electron microscope, we show that the chemical bonding of individual impurity atoms can be deduced experimentally. We find that when a Si atom is bonded with four atoms at a double-vacancy site in graphene, Si 3d orbitals contribute significantly to the bonding, resulting in a planar sp(2) d-like hybridization, whereas threefold coordinated Si in graphene adopts the preferred sp(3) hybridization. The conclusions are confirmed by first-principles calculations and demonstrate that chemical bonding of two-dimensional materials can now be explored at the single impurity level.

Scanning transmission electron microscopy: Seeing the atoms more clearly

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Lattice resolved annular dark-field scanning transmission electron microscopy of (Al, In)GaN/GaN layers for measuring segregation with sub-monolayer precision

We have performed lattice resolved annular dark-field (Z-contrast) scanning transmission electron microscopy and combined this with energy-dispersive X-ray spectroscopy as well as simulations to measure quantitatively segregation across strained interfaces in AlGaN/GaN and GaN/InGaN multiple quantum wells of nominal thicknesses between 8 and 0.25 nm. The compositional profiles obtained were corrected for detector dark current and non-linearity of the Z-contrast imaging process before we fitted exponential functions to the profiles across the interface regions. From these, we could highly accurately determine the layer widths, interface widths, and segregation lengths. Experimental values of the segregation lengths calculated varied from 0.3 nm (for InGaN-on-GaN) to 1.4 nm (for AlGaN-on-GaN), with error bars of only ±0.05 nm. A comparison with simulations based on a simple two-state-exchange model for surface segregation shows that the segregation energy for indium atoms is about an order of magnitude smaller than both the corresponding segregation energy for aluminium/gallium atoms and the activation energies for surface segregation of cations in these nitride systems.

On the visibility of "heavy" atoms in dark-field STEM

One of the most useful and apparently straightforward attributes of annular dark-field (ADF) STEM imaging is the ability to image heavy atoms on relatively light substrates using Z-contrast (High-Angle ADF) imaging. From multislice calculations, however, some isolated heavy atoms, e.g. Re, can go undetected in a thin Co matrix in standard aberration-corrected HAADF-STEM imaging of catalyst nanoparticles oriented accurately on a zone-axis [2]. More recently simultaneous Medium-Angle and High-Angle ADF imaging of a few layers of BN has shown experimentally that some adatoms are invisible in MAADF whilst exhibiting very strong contrast in HAADF. Simulations confirm that this can occur for atoms such as sodium. In both situations HOLZ ring imaging can be used to extract additional information.

Direct observation of vacancies and iron atoms in a Mg-rich olivine using Z-contrast imaging

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Measuring spin-state and ferromagnetic transitions in La1-xSrxCoO3 using in-situ atomic-resolution Z-contrast imaging and EELS in

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Transmission electron microscopy study of Ni–Si nanocomposite films

Nickel induced crystallization of amorphous Si (a-Si) films is investigated using transmission electron microscopy. Metal-induced crystallization was achieved on layered films deposited onto thermally oxidized Si(3 1 1) substrates by electron beam evaporation of a-Si (400 nm) over Ni (50 nm). The multi-layer stack was subjected to post-deposition annealing at 200 and 600 °C for 1 h after the deposition. Microstructural studies reveal the formation of nanosized grains separated by dendritic channels of 5 nm width and 400 nm length. Electron diffraction on selected points within these nanostructured regions shows the presence of face centered cubic NiSi2 and diamond cubic structured Si. Z-contrast scanning transmission electron microscopy images reveal that the crystallization of Si occurs at the interface between the grains of NiSi2 and a-Si. X-ray absorption fine structure spectroscopy analysis has been carried out to understand the nature of Ni in the Ni–Si nanocomposite film. The results of the present study indicate that the metal induced crystallization is due to the diffusion of Ni into the a-Si matrix, which then reacts to form nickel silicide at temperatures of the order of 600 °C leading to crystallization of a-Si at the silicide–silicon interface.

Seeing the atoms more clearly: STEM imaging from the Crewe era to today

This review covers the development of scanning transmission electron microscopy from the innovations of Albert Crewe to the two-dimensional spectrum imaging in the era of aberration correction. It traces the key events along the path, the first atomic resolution Z-contrast imaging of individual atoms, the realization of incoherent imaging in crystals and the role of dynamical diffraction, simultaneous, atomic resolution electron energy loss spectroscopy, and finally the tremendous impact of the successful correction of lens aberrations, not just in terms of resolution but also in single atom sensitivity.

Microstructure of the potentially multiferroic Fe/BaTiO3 epitaxial interface

The Fe/BaTiO3 thin-film layered structure is a prototype of charge-mediated composite multiferroics, which is a promising but challenging route to achieve a sizable magnetoelectric effect. The real structure of the interface between the ferromagnetic Fe and ferroelectric BaTiO3 layers is crucial. In this paper, epitaxial Fe layers were successfully grown on top of BaTiO3 layers by carefully controlling the pulsed laser deposition and magnetron sputtering procedures. A detailed study of interfacial structure and defects at the Fe/BaTiO3 interface was carried out by transmission electron microscopy (TEM). Electron diffraction patterns and diffraction contrast images reveal a definite epitaxial relationship between Fe and BaTiO3 (001) films and a semi-coherent interface with nearly periodic interfacial dislocations. Based on high-resolution TEM images from both [010] and [110] direction observations, the interfacial dislocations were found to be partial with Burgers vectors and line directions of ⟨010⟩. By employing high-resolution Z-contrast imaging, the positions of individual atoms columns were resolved. The formation mechanism of interfacial dislocations was proposed in terms of geometrical models of the interface structure. On the basis of the remaining strain analysis in each layer, the effects of both BaTiO3 thickness and the SrTiO3 substrates on the density of the interface defects were discussed.

Nanoscale modulations in (KLa)(CaW)O6 and (NaLa)(CaW)O6

Complex nanoscale modulations are identified in two new A-site ordered perovskites, (KLa)(CaW)O6 and (NaLa)(CaW)O6. In (KLa)(CaW)O6, selected-area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM) show an incommensurate nanocheckerboard modulation with ∼9.4×9.4ap periodicity (ap≈4Å for the cubic perovskite aristotype). For (NaLa)(CaW)O6 a one-dimensional modulation is observed with a ∼16(110)ap repeat; the 〈110〉 orientation of the nanostripes is different from the 〈100〉 stripes observed in other mixed A-site systems. Studies using high temperature x-ray diffraction suggest the formation of the complex modulations is associated with small deviations from the ideal 1:1:1:1 stoichiometry of the (A+La3+)(CaW)O6 phases. Z-contrast images acquired on an aberration-corrected microscope provide evidence for deviations from stoichiometry with a ∼1:15 periodic arrangement of La4/3(CaW)O6:(NaLa)(CaW)O6 nano-phases.

The effect of zirconium implantation on the structure of sapphire

The effect of zirconium implantation on the structure of sapphire was investigated by 175keV Zr implantation at room temperature to a fluence of 4×1016ions/cm2 into sapphire single crystals. Samples were examined by several experimental techniques: Rutherford backscattering spectroscopy along a channeling direction (RBS-C), electron-energy loss spectroscopy (EELS), and Z-contrast images obtained in an aberration-corrected scanning transmission electron microscope. Range and deposited energy were simulated with SRIM-2008.04.The Z-contrast images from transmission electron microscope indicated: a near surface damaged layer ∼30nm thick, a subsurface region exhibiting “random” de-channeling ∼52nm thick, and a deeper damaged, crystalline zone ∼64nm thick. The RBS-C spectra confirmed the presence of these three regions. The two damaged regions contained high concentrations of as yet unresolved defect clusters. The intermediate region contained Zr-clusters embedded in an “amorphous” matrix that exhibited short-range order corresponding to γ-Al2O3, i.e., a defective spinel structure. The EELS measurements show that the amorphous region is deficient in oxygen.

Spectroscopic imaging in electron microscopy

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Chemical mapping at atomic resolution using energy-dispersive x-ray spectroscopy

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Microstructures and luminescent properties of Ce-doped transparent mica glass-ceramics

Abstract Transparent mica glass-ceramics were prepared by heating parent glasses that had been doped with 0.5–15 mol% CeO2. During the melting and heat treatment, Ce4+ ions in the specimens were reduced to Ce3+ ions, and one or both of these ion species were then replaced with Li+ ions in the interlayers of the separated mica crystals. However, scanning transmission electron microscope (STEM) and Z-contrast imaging revealed that the mica crystals did not contain the same amount of Ce. On excitation at 254 nm, the parent glasses and glass-ceramics emitted blue light, which originated from the 5d to 4f transition of the Ce3+ ions. The emission of the glass-ceramic containing a smaller amount of Ce was attributed to the Ce3+ ions in both the glass phase and the mica crystals, whereas that of the glass-ceramics containing a larger amount of Ce was caused mainly by Ce3+ ions in the mica crystals. The dependence of the emission band of the parent glasses on the amount of Ce was a unique feature of the Ce-doped transparent mica glass-ceramics and was not observed in previous studies of Eu-doped parent glasses and mica glass-ceramics.

Cs-corrected scanning transmission electron microscope investigations of an ODS ferritic steel for fusion applications

Over the last few years, oxide dispersion strengthened (ODS) ferritic steels have emerged as one of the major materials for fusion reactors. Despite the progress made in their technology, currently available properties still do not meet all the expectations. As a result these steels remain the subject of extensive research, recently centred on a possible improvement due to their nanometric engineering. In particular, technologies are developed for grain size refinement down to nanometres and strengthening by nanooxides. This, in turn, calls for nanoscale investigations of the microstructures, which can be efficiently carried out only with the use of high-resolution transmission electron microscopy and spectroscopy. The present paper illustrates the results of investigations of a model alloy of ODS steel using a recent high-resolution scanning transmission electron microscope equipped with a spherical abberation Cs-correction system, energy-dispersive x-ray spectroscopy and electron energy loss spectroscopy spectrometers. It allows bright field on transmitted electron imaging and secondary electron imaging, as well as high-angle annular dark field imaging, the so-called Z-contrast imaging. The results obtained in dark field prove that modern microscopy techniques are a necessity to provide new information on the nanostructure of the ODS steels relevant to the understanding and shaping of their properties.

2D-composition mapping in InGaN without electron beam induced clustering of indium by STEM HAADF Z-contrast imaging

Investigation of composition in InGaN quantum wells and quantum dots by TEM is hampered by formation of electron beam induced agglomeration of indium, which occurs if the specimen is exposed to the electron beam for a few minutes. In this contribution we demonstrate that compositional analysis of InGaN nanostructures is possible without this artifact if STEM Z-contrast imaging is applied instead of parallel beam illumination. The suggested method for composition analysis in InGaN is based on a comparison of intensity normalized with respect to the incident electron beam with simulated image intensity. Simulations are performed with the STEMsim program using the frozen lattice multislice approximation for which static atomic displacements were taken into account.

Counting Tm dopant atoms around GaN dots using high-angle annular dark field images

High resolution Z-contrast STEM imaging is used to study the Tm doping of GaN quantum dots grown in AlN by molecular beam epitaxy (MBE). High-angle annular dark field (HAADF) imaging allows us to visualize directly individual Tm atoms in the AlN matrix and even to count the number of Tm atoms in a given AlN atomic column. A new visibility coefficient to determine quantitatively the number of Tm atoms in a given atomic column is introduced. It is based on locally integrated intensities rather than on peak intensities of HAADF images. STEM image simulations shows that this new visibility is less sensitive to the defocus-induced blurring or to the position of the Tm atom within the thin lamella. Most of the Tm atoms diffuse out of GaN dots. Tm atoms are found at different positions in the AlN matrix, (i) Above the wetting layer, Tm atoms are spread within a thickness of 14 AlN monolayers (MLs). (ii) Above the quantum dots all the Tm are located in the same plane situated at 2-3 MLs above the apex of the GaN dot, i.e. at a distance of 14 MLs from the wetting layer, (iii) In addition, Tm can diffuse very far from the GaN dot by following threading dislocations lines.

Determination of Nitrogen Concentration in Dilute GaNAs by STEM HAADF Z-Contrast Imaging

We determine the nitrogen concentration of GaN0.01≤x≤0.05As1−x quantum wells by evaluation of high resolution scanning transmission electron microscopy (STEM) images using a high-angle annular dark field detector. Although nitrogen has a smaller atomic number than Ga the image intensity increases with the nitrogen content. This is explained by the influence of static atomic displacements by comparison with frozen lattice simulations. The resulting nitrogen concentrations agree with high resolution X-ray diffraction measurements and strain state analysis applied to STEM images.