7 research outputs found
A liquid metal encapsulation for analyzing porous nanomaterials by atom probe tomography
Analyzing porous (nano)materials by the atom probe tomography has been
notoriously difficult. The electrostatic pressure intensifies stress at voids
which results in premature failure of the specimen, and the electrostatic field
distribution near voids lead to aberrations that are difficult to predict. Here
we propose a new encapsulating method for a porous sample using a
low-melting-point Bi-In-Sn alloy, known as Fields metal. As a model porous
sample, we used single-crystalline wustite following direct hydrogen-reduced
into iron. The complete encapsulation is performed using in-situ heating on the
stage of the scanning-electron microscope up to approx. 70 Celsius. No visible
corrosion nor dissolution of the sample occurred. Subsequently specimens are
shaped by focused ion beam milling under cryogenic conditions at -190 Celsius.
The proposed approach is versatile, can be applied to provide good quality atom
probe datasets from microporous materials
Scalable Substrate Development for Aqueous Biological Samples for Atom Probe Tomography
Reliable and consistent preparation of atom probe tomography (APT) specimens
from aqueous and hydrated biological specimens remains a significant challenge.
One particularly difficult process step is the use of a focused ion beam (FIB)
instrument for preparing the required needle-shaped specimen, typically
involving a "lift-out" procedure of a small sample of material. Here, two
alternative substrate designs are introduced that enable using FIB only for
sharpening, along with example APT datasets. The first design is a laser-cut
FIB-style half-grid close to those used for transmission-electron microscopy,
that can be used in a grid holder compatible with APT pucks. The second design
is a larger, standalone self-supporting substrate called a "crown", with
several specimen positions that self-aligns in APT pucks, prepared by
electrical discharge machining (EDM). Both designs are made nanoporous, to
provide strength to the liquid-substrate interface, using chemical and vacuum
dealloying. We select alpha brass a simple, widely available, lower-cost
alternative to previously proposed substrates. We present the resulting
designs, APT data, and provide suggestions to help drive wider community
adoption
Enabling near-atomic-scale analysis of frozen water
Transmission electron microscopy has undergone a revolution in recent years
with the possibility to perform routine cryo-imaging of biological materials
and (bio)chemical systems, as well as the possibility to image liquids via
dedicated reaction cells or graphene-sandwiching. These approaches however
typically require imaging a large number of specimens and reconstructing an
average representation and often lack analytical capabilities. Here, using atom
probe tomography we provide atom-by-atom analyses of frozen liquids and
analytical sub-nanometre three dimensional reconstructions. The analyzed ice is
in contact with, and embedded within, nanoporous gold (NPG). We report the
first such data on 2-3 microns thick layers of ice formed from both high purity
deuterated water and a solution of 50mM NaCl in high purity deuterated water.
We present a specimen preparation strategy that uses a NPG film and,
additionally, we report on an analysis of the interface between nanoporous gold
and frozen salt water solution with an apparent trend in the Na and Cl
concentrations across the interface. We explore a range of experimental
parameters to show that the atom probe analyses of bulk aqueous specimens come
with their own special challenges and discuss physical processes that may
produce the observed phenomena. Our study demonstrates the viability of using
frozen water as a carrier for near-atomic scale analysis of objects in solution
by atom probe tomography
Near-Atomic Scale Perspective on the Oxidation of TiCT MXenes: Insights from Atom Probe Tomography
MXenes are a family of 2D transition metal carbides and nitrides with
remarkable properties and great potential for energy storage and catalysis
applications. However, their oxidation behavior is not yet fully understood,
and there are still open questions regarding the spatial distribution and
precise quantification of surface terminations, intercalated ions, and possible
uncontrolled impurities incorporated during synthesis and processing. Here,
atom probe tomography analysis of as-synthesized TiCT MXenes
reveals the presence of alkali (Li, Na) and halogen (Cl, F) elements as well as
unetched Al. Following oxidation of the colloidal solution of MXenes, it is
observed that the alkalies enriched in TiO nanowires. Although these
elements are tolerated through the incorporation by wet chemical synthesis,
they are often overlooked when the activity of these materials is considered,
particularly during catalytic testing. This work demonstrates how the
capability of atom probe tomography to image these elements in 3D at the
near-atomic scale can help to better understand the activity and degradation of
MXenes, in order to guide their synthesis for superior functional properties
Analysis of water ice in nanoporous copper needles using cryo atom probe tomography
The application of atom probe tomography (APT) to frozen liquids is limited
by difficulties in specimen preparation. Here, we report on the use of
nanoporous Cu needles as a physical framework to hold water ice for
investigation using APT. Nanoporous Cu needles are prepared by the
electropolishing and dealloying of Cu-Mn matchstick precursors. Cryogenic
scanning electron microscopy and focused-ion beam milling reveal a
hierarchical, dendritic, highly-wettable microstructure. The atom probe mass
spectrum is dominated by peaks of Cu+ and H(H2O)n+ up to n <= 3, and the
reconstructed volume shows the protrusion of a Cu ligament into an ice-filled
pore. The continuous Cu ligament network electrically connects the apex to the
cryostage, leading to enhanced electric field at the apex and increased
cooling, both of which simplify the mass spectrum compared to previous reports
Effect of Pore Formation on Redox-Driven Phase Transformation
Solid-state redox-driven phase transformation is associated with mass loss,
accommodated by vacancies that develop into pores. These influence the kinetics
of the redox reactions and phase transformation. We have investigated the
underlying structural and chemical mechanisms in and at pores in a combined
experimental-theoretical study, using the reduction of iron oxide by hydrogen
as a model system. The redox product (water vapor) accumulates in the pores and
shifts the local equilibrium at the pore back towards the parent material -
cubic-Fe1-xO (where x refers to Fe deficiency, space group Fm3-m). Our insights
explain the sluggish reduction of cubic-Fe1-xO and improve our understanding of
the kinetics of redox-driven phase transformations
Understanding alkali contamination in colloidal nanomaterials to unlock grain boundary impurity engineering
Metal nano-aerogels combine a large surface area, a high structural
stability, and a high catalytic activity towards a variety of chemical
reactions. The performance of such nanostructures is underpinned by the
atomic-level distribution of their constituents. Yet monitoring their
sub-nanoscale structure and composition to guide property optimization remains
extremely challenging. Here, we synthesized Pd nano-aerogels from a K2PdCl4
precursor and two different NaBH4 reductant concentrations in distilled water.
Atom probe tomography reveals that the aerogel is poly-crystalline and that
impurities (Na, K) are integrated from the solution into grain boundaries. Ab
initio calculations indicate that these impurities preferentially bound to the
Pd-metal surface and are ultimately found in grain boundaries forming as the
particles coalesce during synthesis, with Na atoms thermodynamically
equilibrating with the surrounding solution and K atoms remaining between
growing grains. If controlled, impurity integration, i.e. grain boundary
decoration, may offer opportunities for designing new nano-aerogels