6 research outputs found
Imaging the Hydrogen Absorption Dynamics of Individual Grains in Polycrystalline Palladium Thin Films in 3D
Defects such as dislocations
and grain boundaries often control
the properties of polycrystalline materials. In nanocrystalline materials,
investigating this structureâfunction relationship while preserving
the sample remains challenging because of the short length scales
and buried interfaces involved. Here we use Bragg coherent diffractive
imaging to investigate the role of structural inhomogeneity on the
hydriding phase transformation dynamics of individual Pd grains in
polycrystalline films in three-dimensional detail. In contrast to
previous reports on single- and polycrystalline nanoparticles, we
observe no evidence of a hydrogen-rich surface layer and consequently
no size dependence in the hydriding phase transformation pressure
over a 125â325 nm size range. We do observe interesting grain
boundary dynamics, including reversible rotations of grain lattices
while the material remains in the hydrogen-poor phase. The mobility
of the grain boundaries, combined with the lack of a hydrogen-rich
surface layer, suggests that the grain boundaries are acting as fast
diffusion sites for the hydrogen atoms. Such hydrogen-enhanced plasticity
in the hydrogen-poor phase provides insight into the switch from the
size-dependent behavior of single-crystal nanoparticles to the lower
transformation pressures of polycrystalline materials and may play
a role in hydrogen embrittlement
Ultrafast Three-Dimensional Xâray Imaging of Deformation Modes in ZnO Nanocrystals
Imaging
the dynamical response of materials following ultrafast excitation
can reveal energy transduction mechanisms and their dissipation pathways,
as well as material stability under conditions far from equilibrium.
Such dynamical behavior is challenging to characterize, especially <i>operando</i> at nanoscopic spatiotemporal scales. In this letter,
we use X-ray coherent diffractive imaging to show that ultrafast laser
excitation of a ZnO nanocrystal induces a rich set of deformation
dynamics including characteristic âhardâ or inhomogeneous
and âsoftâ or homogeneous modes at different time scales,
corresponding respectively to the propagation of acoustic phonons
and resonant oscillation of the crystal. By integrating the 3D nanocrystal
structure obtained from the ultrafast X-ray measurements with a continuum
thermo-electro-mechanical finite element model, we elucidate the deformation
mechanisms following laser excitation, in particular, a torsional
mode that generates a 50% greater electric potential gradient than
that resulting from the flexural mode. Understanding of the time-dependence
of these mechanisms on ultrafast scales has significant implications
for development of new materials for nanoscale power generation
Three-Dimensional Integrated Xâray Diffraction Imaging of a Native Strain in Multi-Layered WSe<sub>2</sub>
Emerging
two-dimensional (2-D) materials such as transition-metal
dichalcogenides show great promise as viable alternatives for semiconductor
and optoelectronic devices that progress beyond silicon. Performance
variability, reliability, and stochasticity in the measured transport
properties represent some of the major challenges in such devices.
Native strain arising from interfacial effects due to the presence
of a substrate is believed to be a major contributing factor. A full
three-dimensional (3-D) mapping of such native nanoscopic strain over
micron length scales is highly desirable for gaining a fundamental
understanding of interfacial effects but has largely remained elusive.
Here, we employ coherent X-ray diffraction imaging to directly image
and visualize in 3-D the native strain along the (002) direction in
a typical multilayered (âŒ100â350 layers) 2-D dichalcogenide
material (WSe<sub>2</sub>) on silicon substrate. We observe significant
localized strains of âŒ0.2% along the out-of-plane direction.
Experimentally informed continuum models built from X-ray reconstructions
trace the origin of these strains to localized nonuniform contact
with the substrate (accentuated by nanometer scale asperities, i.e.,
surface roughness or contaminants); the mechanically exfoliated stresses
and strains are localized to the contact region with the maximum strain
near surface asperities being more or less independent of the number
of layers. Machine-learned multimillion atomistic models show that
the strain effects gain in prominence as we approach a few- to single-monolayer
limit. First-principles calculations show a significant band gap shift
of up to 125 meV per percent of strain. Finally, we measure the performance
of multiple WSe<sub>2</sub> transistors fabricated on the same flake;
a significant variability in threshold voltage and the âoffâ
current setting is observed among the various devices, which is attributed
in part to substrate-induced localized strain. Our integrated approach
has broad implications for the direct imaging and quantification of
interfacial effects in devices based on layered materials or heterostructures
Ultrafast Three-Dimensional Xâray Imaging of Deformation Modes in ZnO Nanocrystals
Imaging
the dynamical response of materials following ultrafast excitation
can reveal energy transduction mechanisms and their dissipation pathways,
as well as material stability under conditions far from equilibrium.
Such dynamical behavior is challenging to characterize, especially <i>operando</i> at nanoscopic spatiotemporal scales. In this letter,
we use X-ray coherent diffractive imaging to show that ultrafast laser
excitation of a ZnO nanocrystal induces a rich set of deformation
dynamics including characteristic âhardâ or inhomogeneous
and âsoftâ or homogeneous modes at different time scales,
corresponding respectively to the propagation of acoustic phonons
and resonant oscillation of the crystal. By integrating the 3D nanocrystal
structure obtained from the ultrafast X-ray measurements with a continuum
thermo-electro-mechanical finite element model, we elucidate the deformation
mechanisms following laser excitation, in particular, a torsional
mode that generates a 50% greater electric potential gradient than
that resulting from the flexural mode. Understanding of the time-dependence
of these mechanisms on ultrafast scales has significant implications
for development of new materials for nanoscale power generation
Ultrafast Three-Dimensional Integrated Imaging of Strain in Core/Shell Semiconductor/Metal Nanostructures
Visualizing the dynamical
response of material heterointerfaces
is increasingly important for the design of hybrid materials and structures
with tailored properties for use in functional devices. In situ characterization
of nanoscale heterointerfaces such as metalâsemiconductor interfaces,
which exhibit a complex interplay between lattice strain, electric
potential, and heat transport at subnanosecond time scales, is particularly
challenging. In this work, we use a laser pump/X-ray probe form of
Bragg coherent diffraction imaging (BCDI) to visualize in three-dimension
the deformation of the core of a model core/shell semiconductorâmetal
(ZnO/Ni) nanorod following laser heating of the shell. We observe
a rich interplay of radial, axial, and shear deformation modes acting
at different time scales that are induced by the strain from the Ni
shell. We construct experimentally informed models by directly importing
the reconstructed crystal from the ultrafast experiment into a thermo-electromechanical
continuum model. The model elucidates the origin of the deformation
modes observed experimentally. Our integrated imaging approach represents
an invaluable tool to probe strain dynamics across mixed interfaces
under operando conditions
Ultrafast Three-Dimensional Integrated Imaging of Strain in Core/Shell Semiconductor/Metal Nanostructures
Visualizing the dynamical
response of material heterointerfaces
is increasingly important for the design of hybrid materials and structures
with tailored properties for use in functional devices. In situ characterization
of nanoscale heterointerfaces such as metalâsemiconductor interfaces,
which exhibit a complex interplay between lattice strain, electric
potential, and heat transport at subnanosecond time scales, is particularly
challenging. In this work, we use a laser pump/X-ray probe form of
Bragg coherent diffraction imaging (BCDI) to visualize in three-dimension
the deformation of the core of a model core/shell semiconductorâmetal
(ZnO/Ni) nanorod following laser heating of the shell. We observe
a rich interplay of radial, axial, and shear deformation modes acting
at different time scales that are induced by the strain from the Ni
shell. We construct experimentally informed models by directly importing
the reconstructed crystal from the ultrafast experiment into a thermo-electromechanical
continuum model. The model elucidates the origin of the deformation
modes observed experimentally. Our integrated imaging approach represents
an invaluable tool to probe strain dynamics across mixed interfaces
under operando conditions