125 research outputs found
Experimental application of sum rules for electron energy loss magnetic chiral dichroism
We present a derivation of the orbital and spin sum rules for magnetic
circular dichroic spectra measured by electron energy loss spectroscopy in a
transmission electron microscope. These sum rules are obtained from the
differential cross section calculated for symmetric positions in the
diffraction pattern. Orbital and spin magnetic moments are expressed explicitly
in terms of experimental spectra and dynamical diffraction coefficients. We
estimate the ratio of spin to orbital magnetic moments and discuss first
experimental results for the Fe L_{2,3} edge.Comment: 11 pages, 2 figure
Accuracy of surface strain measurements from transmission electron microscopy images of nanoparticles
Control of magnetic anisotropy by orbital hybridization in (La0.67Sr0.33MnO3)n/(SrTiO3)n superlattice
The asymmetry of chemical nature at the hetero-structural interface offers an
unique opportunity to design desirable electronic structure by controlling
charge transfer and orbital hybridization across the interface. However, the
control of hetero-interface remains a daunting task. Here, we report the
modulation of interfacial coupling of (La0.67Sr0.33MnO3)n/(SrTiO3)n
superlattices by manipulating the periodic thickness with n unit cells of
SrTiO3 and n unit cells La0.67Sr0.33MnO3. The easy axis of magnetic anisotropy
rotates from in-plane (n = 10) to out-of-plane (n = 2) orientation at 150 K.
Transmission electron microscopy reveals enlarged tetragonal ratio > 1 with
breaking of volume conservation around the (La0.67Sr0.33MnO3)n/(SrTiO3)n
interface, and electronic charge transfer from Mn to Ti 3d orbitals across the
interface. Orbital hybridization accompanying the charge transfer results in
preferred occupancy of 3d3z2-r2 orbital at the interface, which induces a
stronger electronic hopping integral along the out-of-plane direction and
corresponding out-of-plane magnetic easy axis for n = 2. We demonstrate that
interfacial orbital hybridization in superlattices of strongly correlated
oxides may be a promising approach to tailor electronic and magnetic properties
in device applications
Strong enhancement of phonon scattering through nanoscale grains in lead sulfide thermoelectrics
We present nanocrystalline PbS, which was prepared using a solvothermal method followed by spark plasma sintering, as a
promising thermoelectric material. The effects of grains with different length scales on phonon scattering of PbS samples, and
therefore on the thermal conductivity of these samples, were studied using transmission electron microscopy and theoretical
calculations. We found that a high density of nanoscale grain boundaries dramatically lowered the thermal conductivity by
effectively scattering long-wavelength phonons. The thermal conductivity at room temperature was reduced from 2.5 W m1K 1
for ingot-PbS (grain size 4200 lm) to 2.3 W m1K 1 for micro-PbS (grain size 40.4 lm); remarkably, thermal conductivity
was reduced to 0.85 W m1 K 1 for nano-PbS (grain size B30 nm). Considering the full phonon spectrum of the material, a
theoretical model based on a combination of first-principles calculations and semiempirical phonon scattering rates was
proposed to explain this effective enhancement. The results show that the high density of nanoscale grains could cause
effective phonon scattering of almost 61%. These findings shed light on developing high-performance thermoelectrics via
nanograins at the intermediate temperature range.This contribution was supported primarily by the startup of the South
University of Science and Technology of China, supported by the Shenzhen
government, and the national 1000 plan for young scientists. This work was
also partially supported by a grant-in-aid of ‘985 Project’ from Xi’an Jiaotong
University, the National Natural Science Foundation of China (Grant No.
21201138 and 11204228), the National Basic Research Program of China
(2012CB619402 and 2014CB644003) and the Fundamental Research Funds for
the Central UniversitiesS
Segregation of In to dislocations in InGaN.
Dislocations are one-dimensional topological defects that occur frequently in functional thin film materials and that are known to degrade the performance of InxGa1-xN-based optoelectronic devices. Here, we show that large local deviations in alloy composition and atomic structure are expected to occur in and around dislocation cores in InxGa(1-x)N alloy thin films. We present energy-dispersive X-ray spectroscopy data supporting this result. The methods presented here are also widely applicable for predicting composition fluctuations associated with strain fields in other inorganic functional material thin films.This work was funded in part by the Cambridge Commonwealth trust, St. John’s College and
the EPSRC. SKR is funded through the Cambridge-India Partnership Fund and Indian Institute
of Technology Bombay via a scholarship. MAM acknowledges support from the Royal Society
through a University Research Fellowship. Additional support was provided by the EPSRC
through the UK National Facility for Aberration-Corrected STEM (SuperSTEM). The Titan 80-
200kV ChemiSTEMTM was funded through HM Government (UK) and is associated with the
capabilities of the University of Manchester Nuclear Manufacturing (NUMAN) capabilities. SJH
acknowledges funding from the Defence Treat Reduction Agency (DTRA) USA (grant number
HDTRA1-12-1-0013).This is the accepted manuscript. The final version is available at http://pubs.acs.org/doi/abs/10.1021/nl5036513
Atomic scale strain relaxation in axial semiconductor III-V nanowire heterostructures
Combination of mismatched materials in semiconductor nanowire heterostructures offers a freedom of bandstructure engineering that is impossible in standard planar epitaxy. Nevertheless, the presence of strain and structural defects directly control the optoelectronic properties of these nanomaterials. Understanding with atomic accuracy how mismatched heterostructures release or accommodate strain, therefore, is highly desirable. By using atomic resolution high angle annular dark field scanning transmission electron microscopy combined with geometrical phase analyses and computer simulations, we are able to establish the relaxation mechanisms (including both elastic and plastic deformations) to release the mismatch strain in axial nanowire heterostructures. Formation of misfit dislocations, diffusion of atomic species, polarity transfer, and induced structural transformations are studied with atomic resolution at the intermediate ternary interfaces. Two nanowire heterostructure systems with promising applications (InAs/InSb and GaAs/GaSb) have been selected as key examples
High-Resolution Electron Microscopy of Semiconductor Heterostructures and Nanostructures
This chapter briefly describes the fundamentals of high-resolution electron microscopy techniques. In particular, the Peak Pairs approach for strain mapping with atomic column resolution, and a quantitative procedure to extract atomic column compositional information from Z-contrast high-resolution images are presented. It also reviews the structural, compositional, and strain results obtained by conventional and advanced transmission electron microscopy methods on a number of III–V semiconductor nanostructures and heterostructures
Dark-field electron holography for the measurement of strain in nanostructures and devices
Evidence for crystallographically abrupt grain boundaries in nanocrystalline copper
Preliminary results are presented from the study of grain boundary structure
in bulk nanocrystalline copper using high-resolution electron microscopy.
A recently developed method of image analysis is applied to an experimental
image of a grain boundary between two copper grains. Maps are produced of
the fringe spacing and the local rotation of the lattice as a function of
position in the image. The analysis of the fringe spacing shows that no
oxide layer exists between the copper grains. This confirms that the
surface oxide layer coating the copper particles can be eliminated during
the formation of the bulk material. By studying the way the rotation of the
lattice takes place across the grain boundary, an upper limit for the
interface width is obtained. The reliability and accuracy of the results
are discussed
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