8 research outputs found
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Direct imaging of short-range order and its impact on deformation in Ti-6Al.
Chemical short-range order (SRO) within a nominally single-phase solid solution is known to affect the mechanical properties of alloys. While SRO has been indirectly related to deformation, direct observation of the SRO domain structure, and its effects on deformation mechanisms at the nanoscale, has remained elusive. Here, we report the direct observation of SRO in relation to deformation using energy-filtered imaging in a transmission electron microscope (TEM). The diffraction contrast is enhanced by reducing the inelastically scattered electrons, revealing subnanometer SRO-enhanced domains. The destruction of these domains by dislocation planar slip is observed after ex situ and in situ TEM mechanical testing. These results confirm the impact of SRO in Ti-Al alloys on the scale of angstroms. The direct confirmation of SRO in relationship to dislocation plasticity in metals can provide insight into how the mechanical behavior of concentrated solid solutions by the material's thermal history
Strong interlayer coupling in van der Waals heterostructures built from single-layer chalcogenides
Semiconductor heterostructures are the fundamental platform for many
important device applications such as lasers, light-emitting diodes, solar
cells and high-electron-mobility transistors. Analogous to traditional
heterostructures, layered transition metal dichalcogenide (TMDC)
heterostructures can be designed and built by assembling individual
single-layers into functional multilayer structures, but in principle with
atomically sharp interfaces, no interdiffusion of atoms, digitally controlled
layered components and no lattice parameter constraints. Nonetheless, the
optoelectronic behavior of this new type of van der Waals (vdW) semiconductor
heterostructure is unknown at the single-layer limit. Specifically, it is
experimentally unknown whether the optical transitions will be spatially direct
or indirect in such hetero-bilayers. Here, we investigate artificial
semiconductor heterostructures built from single layer WSe2 and MoS2 building
blocks. We observe a large Stokes-like shift of ~100 meV between the
photoluminescence peak and the lowest absorption peak that is consistent with a
type II band alignment with spatially direct absorption but spatially indirect
emission. Notably, the photoluminescence intensity of this spatially indirect
transition is strong, suggesting strong interlayer coupling of charge carriers.
The coupling at the hetero-interface can be readily tuned by inserting
hexagonal BN (h-BN) dielectric layers into the vdW gap. The generic nature of
this interlayer coupling consequently provides a new degree of freedom in band
engineering and is expected to yield a new family of semiconductor
heterostructures having tunable optoelectronic properties with customized
composite layers.Comment: http://www.pnas.org/content/early/2014/04/10/1405435111.abstrac
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Direct measurement of nanostructural change during in situ deformation of a bulk metallic glass.
To date, there has not yet been a direct observation of the initiation and propagation of individual defects in metallic glasses during deformation at the nanoscale. Here, we show through a combination of in situ nanobeam electron diffraction and large-scale molecular dynamics simulations that we can directly observe changes to the local short to medium range atomic ordering during the formation of a shear band. We observe experimentally a spatially resolved reduction of order prior to shear banding due to increased strain. We compare this to molecular dynamics simulations, in which a similar reduction in local order is seen, and caused by shear transformation zone activation, providing direct experimental evidence for this proposed nucleation mechanism for shear bands in amorphous solids. Our observation serves as a link between the atomistic molecular dynamics simulation and the bulk mechanical properties, providing insight into how one could increase ductility in glassy materials
Correlation between Electrical Transport and Nanoscale Strain in InAs/In0.6Ga0.4As Core-Shell Nanowires
Free-standing semiconductor nanowires constitute an ideal material system for the direct manipulation of electrical and optical properties by strain engineering. In this study, we present a direct quantitative correlation between electrical conductivity and nanoscale lattice strain of individual InAs nanowires passivated with a thin epitaxial In0.6Ga0.4As shell. With an in situ electron microscopy electromechanical testing technique, we show that the piezoresistive response of the nanowires is greatly enhanced compared to bulk InAs, and that uniaxial elastic strain leads to increased conductivity, which can be explained by a strain-induced reduction in the band gap. In addition, we observe inhomogeneity in strain distribution, which could have a reverse effect on the conductivity by increasing the scattering of charge carriers. These results provide a direct correlation of nanoscale mechanical strain and electrical transport properties in free-standing nanostructures
Correlation between Electrical Transport and Nanoscale Strain in InAs/In0.6Ga0.4As CoreShell Nanowires
Correlation between Electrical Transport and Nanoscale Strain in InAs/In<sub>0.6</sub>Ga<sub>0.4</sub>As Core–Shell Nanowires
Free-standing
semiconductor nanowires constitute an ideal material
system for the direct manipulation of electrical and optical properties
by strain engineering. In this study, we present a direct quantitative
correlation between electrical conductivity and nanoscale lattice
strain of individual InAs nanowires passivated with a thin epitaxial
In<sub>0.6</sub>Ga<sub>0.4</sub>As shell. With an in situ electron
microscopy electromechanical testing technique, we show that the piezoresistive
response of the nanowires is greatly enhanced compared to bulk InAs,
and that uniaxial elastic strain leads to increased conductivity,
which can be explained by a strain-induced reduction in the band gap.
In addition, we observe inhomogeneity in strain distribution, which
could have a reverse effect on the conductivity by increasing the
scattering of charge carriers. These results provide a direct correlation
of nanoscale mechanical strain and electrical transport properties
in free-standing nanostructures
Microstructure and magnetic properties of ultrathin FePt granular films
FePt granular films with grain size smaller than 10 nm are promising candidates for storage media used in the next generation heat-assisted magnetic recording technology. However, FePt films show degraded magnetic properties when the grain size is reduced to this scale, which cannot be explained solely by the finite size theory. In this study, we explored the structural cause of property degradation by employing advanced electron microscopy and atomistic modeling. Structural features unique to the nanostructured FePt granular films at significantly reduced grain sizes of 2∼8 nm were studied by high-resolution scanning transmission electron microscopy with geometric aberrations corrected up to the third order. Two critical structural parameters, the threshold grain size corresponding to the upper size limit of the FePt grains with zero chemical ordering and the sub-nanometer thin interfacial impurity at grain boundaries, were identified. A new atomistic model was developed to correlate these structural characteristics with key magnetic properties such as Curie temperature, saturation magnetization, magnetocrystalline anisotropy, and their grain-to-grain variation. The model shows good agreement with the experimental magnetic data and explains the gap in magnetic properties between the bulk and nanostructured FePt