1,477 research outputs found
Monitoring Galvanic Replacement Through Three-Dimensional Morphological and Chemical Mapping
Galvanic replacement reactions on metal nanoparticles are often used for the
preparation of hollow nanostructures with tunable porosity and chemical
composition, leading to tailored optical and catalytic properties. However, the
precise interplay between the three-dimensional (3D) morphology and chemical
composition of nanostructures during Galvanic replacement is not always well
understood as the 3D chemical imaging of nanoscale materials is still
challenging. It is especially far from straightforward to obtain detailed
information from the inside of hollow nanostructures using electron microscopy
techniques such as SEM or TEM. We demonstrate here that a combination of
state-of-the-art EDX mapping with electron tomography results in the
unambiguous determination of both morphology transformation and elemental
composition of nanostructures in 3D, during Galvanic replacement of Ag
nanocubes. This work provides direct and unambiguous experimental evidence
leading to new insights in the understanding of the galvanic replacement
reaction. In addition, the powerful approach presented here can be applied to a
wide range of nanoscale transformation processes, which will undoubtedly guide
the development of novel nanostructures
Barrier efficiency of sponge-like La2Zr2O7 buffer layers for YBCO-coated conductors
Solution derived La2Zr2O7 films have drawn much attention for potential
applications as thermal barriers or low-cost buffer layers for coated conductor
technology. Annealing and coating parameters strongly affect the microstructure
of La2Zr2O7, but different film processing methods can yield similar
microstructural features such as nanovoids and nanometer-sized La2Zr2O7 grains.
Nanoporosity is a typical feature found in such films and the implications for
the functionality of the films is investigated by a combination of scanning
transmission electron microscopy, electron energy-loss spectroscopy and
quantitative electron tomography. Chemical solution based La2Zr2O7 films
deposited on flexible Ni-5at.%W substrates with a {100} biaxial texture
were prepared for an in-depth characterization. A sponge-like structure
composed of nanometer sized voids is revealed by high-angle annular dark-field
scanning transmission electron microscopy in combination with electron
tomography. A three-dimensional quantification of nanovoids in the La2Zr2O7
film is obtained on a local scale. Mostly non-interconnected highly facetted
nanovoids compromise more than one-fifth of the investigated sample volume. The
diffusion barrier efficiency of a 170 nm thick La2Zr2O7 film is investigated by
STEM-EELS yielding a 1.8 \pm 0.2 nm oxide layer beyond which no significant
nickel diffusion can be detected and intermixing is observed. This is of
particular significance for the functionality of YBa2Cu3O7-{\delta} coated
conductor architectures based on solution derived La2Zr2O7 films as diffusion
barriers.Comment: Accepted for publication in Superconductor Science and Technolog
UiO-66-(SH)2 : a selective, stable and regenerable adsorbent for the removal of mercury from water
Formation and Thermal Stability of Gold-Silica Nanohybrids: Insight into the Mechanism and Morphology by Electron Tomography
Gold-silica hybrids are appealing in different fields of applications like
catalysis, sensors, drug delivery, and biotechnology. In most cases, the
morphology and distribution of the hetero-units play significant roles in their
functional behavior. Methods of synthesizing these hybrids, with variable
ordering of the hetero-units, are replete; however, a complete characterization
in three dimensions could not be achieved yet. A simple route to the synthesis
of Au-decorated SiO2 spheres is demonstrated and a study on the 3D ordering of
the hetero-units by scanning transmission electron microscopy (STEM) tomography
is presented at the final stage, intermediate stages of formation, and after
heating the hybrid. The final hybrid evolves from a soft self-assembled
structure of Au nano-particles. The hybrid shows good thermal stability up to
400 C, beyond which the Au particles start migrating inside the SiO2 matrix.
This study provides an insight in the formation mechanism and thermal stability
of the structures which are crucial factors for designing and applying such
hybrids in fields of catalysis and biotechnology. As the method is general, it
can be applied to make similar hybrids based on SiO2 by tuning the reaction
chemistry as needed
Shaping electron beams for the generation of innovative measurements in the (S)TEM
In TEM, a typical goal consists of making a small electron probe in the
sample plane in order to obtain high spatial resolution in scanning
transmission electron microscopy. In order to do so, the phase of the electron
wave is corrected to resemble a spherical wave compensating for aberrations in
the magnetic lenses. In this contribution we discuss the advantage of changing
the phase of an electron wave in a specific way in order to obtain
fundamentally different electron probes opening up new application in the
(S)TEM. We focus on electron vortex states as a specific family of waves with
an azimuthal phase signature and discuss their properties, production and
applications. The concepts presented here are rather general and also different
classes of probes can be obtained in a similar fashion showing that electron
probes can be tuned to optimise a specific measurement or interaction
Interplay of atomic displacements in the quantum magnet (CuCl)LaNb2O7
We report on the crystal structure of the quantum magnet (CuCl)LaNb2O7 that
was controversially described with respect to its structural organization and
magnetic behavior. Using high-resolution synchrotron powder x-ray diffraction,
electron diffraction, transmission electron microscopy, and band structure
calculations, we solve the room-temperature structure of this compound
[alpha-(CuCl)LaNb2O7] and find two high-temperature polymorphs. The
gamma-(CuCl)LaNb2O7 phase, stable above 640K, is tetragonal with a(sub) = 3.889
A, c(sub) = 11.738 A, and the space group P4/mmm. In the gamma-(CuCl)LaNb2O7
structure, the Cu and Cl atoms are randomly displaced from the special
positions along the {100} directions. The beta-phase [a(sub) x 2a(sub) x
c(sub), space group Pbmm] and the alpha-phase [2a(sub) x 2a(sub) x c(sub),
space group Pbam] are stable between 640 K and 500 K and below 500 K,
respectively. The structural changes at 500 K and 640 K are identified as
order-disorder phase transitions. The displacement of the Cl atoms is frozen
upon the gamma --> beta transformation, while a cooperative tilting of the NbO6
octahedra in the alpha-phase further eliminates the disorder of the Cu atoms.
The low-temperature alpha-(CuCl)LaNb2O7 structure thus combines the two types
of the atomic displacements that interfere due to the bonding between the Cu
atoms and the apical oxygens of the NbO6 octahedra. The precise structural
information resolves the controversy between the previous computation-based
models and provides the long-sought input for understanding the magnetic
properties of (CuCl)LaNb2O7.Comment: 12 pages, 10 figures, 5 tables; crystallographic information (cif
files) include
Spin ladder compound Pb(0.55)Cd(0.45)V(2)O(5): synthesis and investigation
The complex oxide Pb(0.55)Cd(0.45)V(2)O(5) was synthesized and investigated
by means of X-ray powder diffraction, electron diffraction, magnetic
susceptibility measurements and band structure calculations. Its structure is
similar to that of MV(2)O(5) compounds (M = Na, Ca) giving rise to a spin
system of coupled S=1/2 two-leg ladders. Magnetic susceptibility measurements
reveal a spin gap-like behavior with \Delta ~ 270 K and a spin singlet ground
state. Band structure calculations suggest Pb(0.55)Cd(0.45)V(2)O(5) to be a
system of weakly coupled dimers in perfect agreement with the experimental
data. Pb(0.55)Cd(0.45)V(2)O(5) provides an example of the modification of the
spin system in layered vanadium oxides by cation substitution. Simple
correlations between the cation size, geometrical parameters and exchange
integrals for the MV(2)O(5)-type oxides are established and discussed.Comment: 8 pages, 7 figure
Incorporation of Pure Fullerene into Organoclays:Towards C60-Pillared Clay Structures
In this work, we demonstrate the successful incorporation of pure fullerene from solution into two-dimensional layered aluminosilicate minerals. Pure fullerenes are insoluble in water and neutral in terms of charge, hence they cannot be introduced into the clay galleries by ion exchange or intercalation from water solution. To overcome this bottleneck, we organically modified the clay with quaternary amines by using well-established reactions in clay science in order to expand the interlayer space and render the galleries organophilic. During the reaction with the fullerene solution, the organic solvent could enter into the clay galleries, thus transferring along the fullerene molecules. Furthermore, we demonstrate that the surfactant molecules, can be selectively removed by either simple ion-exchange reaction (e.g., interaction with Al(NO3)3 solution to replace the surfactant molecules with Al3+ ions) or thermal treatment (heating at 350 °C) to obtain novel fullerene-pillared clay structures exhibiting enhanced surface area. The synthesized hybrid materials were characterized in detail by a combination of experimental techniques including powder X-ray diffraction, transmission electron microscopy, X-ray photoemission, and UV/Vis spectroscopy as well as thermal analysis and nitrogen adsorption–desorption measurements. The reported fullerene-pillared clay structures constitute a new hybrid system with very promising potential for the use in areas such as gas storage and/or gas separation due to their high surface area.
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