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We present a multi-microscopy study of dislocations in InGaN, whereby the same threading dislocation was observed under several microscopes (atomic force microscopy, scanning electron microscopy, cathodoluminescence imaging and... more
We present a multi-microscopy study of dislocations in InGaN, whereby the same threading dislocation was observed under several microscopes (atomic force microscopy, scanning electron microscopy, cathodoluminescence imaging and spectroscopy, transmission electron microscopy), and its morphological optical and structural properties directly correlated. We achieved this across an ensemble of defects large enough to be statistically significant. Our results provide evidence that carrier localization occurs in the direct vicinity of the dislocation through the enhanced formation of In-N chains and atomic condensates, thus limiting non-radiative recombination of carriers at the dislocation core. We highlight that the localization properties in the vicinity of threading dislocations arise as a consequence of the strain field of the individual dislocation and the additional strain field building between interacting neighboring dislocations. Our study therefore suggests that careful strain and dislocation distribution engineering may further improve the resilience of InGaNbased devices to threading dislocations. Besides providing a new understanding of dislocations in InGaN, this paper presents a proof-of-concept for a methodology which is relevant to many problems in materials science.
This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. The following article appeared in Journal of Applied Physics and may be found at http://dx.doi.org/10.1063/1.4973278. Copyright 2017. This article is distributed under a Creative Commons Attribution (CC BY) License.
This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. The following article appeared in Journal of Applied Physics and may be found at http://dx.doi.org/10.1063/1.4973278. Copyright 2017. This article is distributed under a Creative Commons Attribution (CC BY) License.
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The impact of trench defects in blue InGaN/GaN light emitting diodes (LEDs) has been investigated. Two mechanisms responsible for the structural degradation of the multiple quantum well (MQW) active region were identified. It was found... more
The impact of trench defects in blue InGaN/GaN light emitting diodes (LEDs) has been investigated. Two mechanisms responsible for the structural degradation of the multiple quantum well (MQW) active region were identified. It was found that during the growth of the p-type GaN capping layer, loss of part of the active region enclosed within a trench defect occurred, affecting the top-most QWs in the MQW stack. Indium platelets and voids were also found to form preferentially at the bottom of the MQW stack. The presence of high densities of trench defects in the LEDs was found to relate to a significant reduction in photoluminescence and electroluminescence
emission efficiency, for a range of excitation power densities and drive currents. This reduction in emission efficiency was attributed to an increase in the density of non-radiative recombination centres within the MQW stack, believed to be associated with the stacking mismatch boundaries which form part of the sub-surface structure of the trench defects. Investigation of the surface of green-emitting QW structures found a two decade increase in the density of trench defects, compared to its blue-emitting counterpart, suggesting that the efficiency of green-emitting LEDs may be strongly affected by the presence of these defects. Our results are therefore consistent with a
model that the “green gap” problem might relate to localized strain relaxation occurring through defects.
emission efficiency, for a range of excitation power densities and drive currents. This reduction in emission efficiency was attributed to an increase in the density of non-radiative recombination centres within the MQW stack, believed to be associated with the stacking mismatch boundaries which form part of the sub-surface structure of the trench defects. Investigation of the surface of green-emitting QW structures found a two decade increase in the density of trench defects, compared to its blue-emitting counterpart, suggesting that the efficiency of green-emitting LEDs may be strongly affected by the presence of these defects. Our results are therefore consistent with a
model that the “green gap” problem might relate to localized strain relaxation occurring through defects.
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Thanks to their remarkably high activity toward oxygen reduction reaction (ORR), platinum-based octahedrally shaped nanoparticles have attracted ever increasing attention in last years. Although high activities for ORR catalysts have been... more
Thanks to their remarkably high activity toward oxygen reduction reaction (ORR), platinum-based octahedrally shaped nanoparticles have attracted ever increasing attention in last years. Although high activities for ORR catalysts have been attained, the practical use is still limited by their long-term stability. In this work, we present Rh-doped Pt-Ni octahedral nanoparticles with high activities up to 1.14 A mgPt(-1) combined with improved performance and shape stability compared to previous bimetallic Pt-Ni octahedral particles. The synthesis, the electrocatalytic performance of the particles toward ORR, and atomic degradation mechanisms are investigated with a major focus on a deeper understanding of strategies to stabilize morphological particle shape and consequently their performance. Rh surface-doped octahedral Pt-Ni particles were prepared at various Rh levels. At and above about 3 atom %, the nanoparticles maintained their octahedral shape even past 30 000 potential cycles,...
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Vanadium dioxide (VO2) is a well-known thermochromic material with large IR modulating ability, promising for energy-saving smart windows. The main drawbacks of VO2 are its high phase transition temperature (τc = 68 °C), low luminous... more
Vanadium dioxide (VO2) is a well-known thermochromic material with large IR modulating ability, promising for energy-saving smart windows. The main drawbacks of VO2 are its high phase transition temperature (τc = 68 °C), low luminous transmission (Tlum), and weak solar modulating ability (ΔTsol). In this paper, the terbium cation (Tb(3+)) doping was first reported to reduce τc and increase Tlum of VO2 thin films. Compared with pristine VO2, 2 at. % doping level gives both enhanced Tlum and ΔTsol from 45.8% to 54.0% and 7.7% to 8.3%, respectively. The Tlum increases with continuous Tb(3+) doping and reaches 79.4% at 6 at. % doping level, representing ∼73.4% relative increment compared with pure VO2. This has surpassed the best reported doped VO2 thin films. The enhanced thermochromic properties is meaningful for smart window applications of VO2 materials.
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The catalytic and optical properties of metal nanoparticles can be combined to create platforms for light-driven chemical energy storage and enhanced in-situ reaction monitoring. However, the heavily damped plasmon resonances of many... more
The catalytic and optical properties of metal nanoparticles can be combined to create platforms for light-driven chemical energy storage and enhanced in-situ reaction monitoring. However, the heavily damped plasmon resonances of many catalytically active metals (e.g. Pt, Pd) prevent this dual functionality in pure nanostructures. The addition of catalytic metals at the surface of efficient plasmonic particles thus presents a unique opportunity if the resonances can be conserved after coating. Here, nanometer resolution electron-based techniques (electron energy loss, cathodoluminescence, and energy dispersive X-ray spectroscopy) are used to show that Au particles incorporating a catalytically active but heavily damped metal, Pd, sustain multiple size-dependent localized surface plasmon resonances (LSPRs) that are narrow and strongly localized at the Pd-rich tips. The resonances also couple with a dielectric substrate and other nanoparticles, establishing that the full range of plasm...
The performance of a direct detection camera (DDC) is evaluated in the context of off-axis electron holographic experiments in a transmission electron microscope. Its performance is also compared directly with that of a conventional... more
The performance of a direct detection camera (DDC) is evaluated in the context of off-axis electron holographic experiments in a transmission electron microscope. Its performance is also compared directly with that of a conventional charge-coupled device (CCD) camera. The DDC evaluated here can be operated either by the detection of individual electron events (counting mode) or by the effective integration of many such events during a given exposure time (linear mode). It is demonstrated that the improved modulation transfer functions and detective quantum efficiencies of both modes of the DDC give rise to significant benefits over the conventional CCD cameras, specifically, a significant improvement in the visibility of the holographic fringes and a reduction of the statistical error in the phase of the reconstructed electron wave function. The DDC's linear mode, which can handle higher dose rates, allows optimisation of the dose rate to achieve the best phase resolution for a wide variety of experimental conditions. For suitable conditions, the counting mode can potentially utilise a significantly lower dose to achieve a phase resolution that is comparable to that achieved using the linear mode. The use of multiple holograms and correlation techniques to increase the total dose in counting mode is also demonstrated.
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The electrostatic potential distribution in FIB-prepared membranes containing a silicon pn junction has been examined using off-axis electron holography and electron tomography. The experimentally determined 3-D electrostatic potential... more
The electrostatic potential distribution in FIB-prepared membranes containing a silicon pn junction has been examined using off-axis electron holography and electron tomography. The experimentally determined 3-D electrostatic potential was examined to quantify the variation in the electrostatic properties of the device as a function of distance from the specimen surfaces. It was found that specimen preparation using a 30 kV Ga ion beam alters the electrical properties of the membrane to a depth of 175 nm from the specimen ...
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Tomographic off-axis electron holography has been used to reveal the 3-D electrostatic potential distribution in an electrically biased FIB-prepared silicon device. Difference tomograms have been reconstructed, where the tomogram of the... more
Tomographic off-axis electron holography has been used to reveal the 3-D electrostatic potential distribution in an electrically biased FIB-prepared silicon device. Difference tomograms have been reconstructed, where the tomogram of the unbiased specimen is subtracted from the tomogram of the specimen under applied reverse bias. The electrical activity as a function of depth into the specimen has been characterised, revealing that the bias is applied uniformly across the electrically active region in the specimen, and that the ...
Quantitative materials characterization using electron holography frequently requires knowledge of the mean inner potential, but reported experimental mean inner potential measurements can vary widely. Using density functional theory, we... more
Quantitative materials characterization using electron holography frequently requires knowledge of the mean inner potential, but reported experimental mean inner potential measurements can vary widely. Using density functional theory, we have simulated the mean inner potential for materials with a range of different surface conditions and geometries. We use both "thin-film" and "nanowire" specimen geometries. We consider clean bulk-terminated surfaces with different facets and surface reconstructions using atom positions from both structural optimization and experimental data and we also consider surfaces both with and without adsorbates. We find that the mean inner potential is surface-dependent, with the strongest dependency on surface adsorbates. We discuss the outlook and perspective for future mean inner potential measurements.
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Multimetallic shape-controlled nanoparticles offer great opportunities to tune the activity, selectivity, and stability of electrocatalytic surface reactions. However, in many cases, our synthetic control over particle size, composition,... more
Multimetallic shape-controlled nanoparticles offer great opportunities to tune the activity, selectivity, and stability of electrocatalytic surface reactions. However, in many cases, our synthetic control over particle size, composition, and shape is limited requiring trial and error. Deeper atomic-scale insight in the particle formation process would enable more rational syntheses. Here we exemplify this using a family of trimetallic PtNiCo nanooctahedra obtained via a low-temperature, surfactant-free solvothermal synthesis. We analyze the competition between Ni and Co precursors under coreduction "one-step" conditions when the Ni reduction rates prevailed. To tune the Co reduction rate and final content, we develop a "two-step" route and track the evolution of the composition and morphology of the particles at the atomic scale. To achieve this, scanning transmission electron microscopy and energy dispersive X-ray elemental mapping techniques are used. We provid...
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We demonstrate the ability to record a tomographic tilt series containing 3487 images in only 3.5 s by using a direct electron detector in a transmission electron microscope. The electron dose is lower by at least one order of magnitude... more
We demonstrate the ability to record a tomographic tilt series containing 3487 images in only 3.5 s by using a direct electron detector in a transmission electron microscope. The electron dose is lower by at least one order of magnitude when compared with that used to record a conventional tilt series of fewer than 100 images in 15-60 minutes and the overall signal-to-noise ratio is greater than 4. Our results, which are illustrated for an inorganic nanotube, are important for ultra-low-dose electron tomography of electron-beam-sensitive specimens and real-time dynamic electron tomography of nanoscale objects with sub-ms temporal resolution.
As the nanotechnology revolution gathers pace there will be an increasingly strong need for imaging techniques that allow a high spatial resolution view of nanostructures and nanodevices. By their nature such objects will be... more
As the nanotechnology revolution gathers pace there will be an increasingly strong need for imaging techniques that allow a high spatial resolution view of nanostructures and nanodevices. By their nature such objects will be three-dimensional and any imaging ...
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A quantitative description of photoexcited scanning tunneling spectra is developed and applied to photoexcited spectra measured on p-doped nonpolar GaAs(110) surfaces. Under illumination, the experimental spectra exhibit an increase of... more
A quantitative description of photoexcited scanning tunneling spectra is developed and applied to photoexcited spectra measured on p-doped nonpolar GaAs(110) surfaces. Under illumination, the experimental spectra exhibit an increase of the tunnel current at negative sample voltages only. In order to analyze the experimental data quantitatively, the potential and charge-carrier distributions of the photoexcited tip-vacuum-semiconductor system are calculated by solving the Poisson as well as the hole and electron continuity equations by a finite-difference algorithm. On this basis, the different contributions to the tunnel current are calculated using an extension of the model of Feenstra and Stroscio to include the light-excited carrier concentrations. The best fit of the calculated tunnel currents to the experimental data is obtained for a tip-induced band bending, which is limited by the partial occupation of the C3 surface state by light-excited electrons. The tunnel current at ne...
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ABSTRACT The one-dimensional charge density distribution along an electrically biased Fe atom probe needle is measured using a model-independent approach based on off-axis electron holography in the transmission electron microscope. Both... more
ABSTRACT The one-dimensional charge density distribution along an electrically biased Fe atom probe needle is measured using a model-independent approach based on off-axis electron holography in the transmission electron microscope. Both the mean inner potential and the magnetic contribution to the phase shift are subtracted by taking differences between electron-optical phase images recorded with different voltages applied to the needle. The measured one-dimensional charge density distribution along the needle is compared with a similar result obtained using model-based fitting of the phase shift surrounding the needle. On the assumption of cylindrical symmetry, it is then used to infer the three-dimensional electric field and electrostatic potential around the needle with ∼10 nm spatial resolution, without needing to consider either the influence of the perturbed reference wave or the extension of the projected potential outside the field of view of the electron hologram. The present study illustrates how a model-independent approach can be used to measure local variations in charge density in a material using electron holography in the presence of additional contributions to the phase, such as those arising from changes in mean inner potential and specimen thickness.
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Novel caustic phenomena, which contain fold, butterfly and elliptic umbilic catastrophes, are observed in defocused images of two approximately collinear oppositely biased metallic tips in a transmission electron microscope. The observed... more
Novel caustic phenomena, which contain fold, butterfly and elliptic umbilic catastrophes, are observed in defocused images of two approximately collinear oppositely biased metallic tips in a transmission electron microscope. The observed patterns depend sensitively on defocus, on the applied voltage between the tips and on their separation and lateral offset. Their main features are interpreted on the basis of a projected electrostatic potential model for the electron-optical phase shift.
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A procedure based on focused ion beam milling and in situ lift-out is introduced for the preparation of high-quality specimens for in situ annealing experiments in the transmission electron microscope. The procedure allows an... more
A procedure based on focused ion beam milling and in situ lift-out is introduced for the preparation of high-quality specimens for in situ annealing experiments in the transmission electron microscope. The procedure allows an electron-transparent lamella to be cleaned directly on a heating chip using a low ion energy and back-side milling in order to minimize redeposition and damage. The approach is illustrated through the preparation of an Al-Mn-Fe complex metallic alloy specimen.
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Differential phase contrast (DPC) imaging in the scanning transmission electron microscope is applied to the study of a charged antiphase domain boundary in doped bismuth ferrite. A clear differential signal is seen, which matches the... more
Differential phase contrast (DPC) imaging in the scanning transmission electron microscope is applied to the study of a charged antiphase domain boundary in doped bismuth ferrite. A clear differential signal is seen, which matches the expected direction of the electric field at the boundary. However, further study by scanned diffraction reveals that there is no measurable deflection of the primary diffraction disc and hence no significant free E-field in the material. Instead, the DPC signal arises from a modulation of the intensity profile within the primary diffraction disc in the vicinity of the boundary. Simulations are used to show that this modulation arises purely from the local change in crystallographic structure at the boundary and not from an electric field. This study highlights the care that is required when interpreting signals recorded from ferroelectric materials using both DPC imaging and other phase contrast techniques.
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High-resolution transmission electron microscopy is used to study interactions between thiol-capped Au clusters and amorphous C support films. The morphologies of the clusters are found to depend both on their size and on the local... more
High-resolution transmission electron microscopy is used to study interactions between thiol-capped Au clusters and amorphous C support films. The morphologies of the clusters are found to depend both on their size and on the local structure of the underlying C. When the C is amorphous, larger Au clusters are crystalline, while smaller clusters are typically disordered. When the C is graphitic, the Au particles adopt either elongated shapes that maximize their contact with the edge of the C film or planar arrays when they contain few Au atoms. We demonstrate the influence of electron beam irradiation on the structure, shape and stability of the Au clusters, as well as on the formation of holes bounded by terraces of graphitic lamellae in the underlying C.
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Solid oxide fuel cells (SOFCs) are promising candidates for use in alternative energy technologies. A full understanding of the reaction mechanisms in these dynamic material systems is required to optimize device performance and overcome... more
Solid oxide fuel cells (SOFCs) are promising candidates for use in alternative energy technologies. A full understanding of the reaction mechanisms in these dynamic material systems is required to optimize device performance and overcome present limitations. Here, we show that in situ transmission electron microscopy (TEM) can be used to study redox reactions and ionic conductivity in SOFCs in a gas environment at elevated temperature. We examine model ultrathin half and complete cells in two environmental TEMs using off-axis electron holography and electron energy-loss spectroscopy. Our results from the model cells provide insight into the essential phenomena that are important for the operation of commercial devices. Changes in the activities of dopant cations in the solid electrolyte are detected during oxygen anion conduction, demonstrating the key role of dopants in electrolyte architecture in SOFCs.
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A procedure based on focused ion beam milling and in situ lift-out is introduced for the preparation of high-quality specimens for in situ annealing experiments in the transmission electron microscope. The procedure allows an... more
A procedure based on focused ion beam milling and in situ lift-out is introduced for the preparation of high-quality specimens for in situ annealing experiments in the transmission electron microscope. The procedure allows an electron-transparent lamella to be cleaned directly on a heating chip using a low ion energy and back-side milling in order to minimize redeposition and damage. The approach is illustrated through the preparation of an Al-Mn-Fe complex metallic alloy specimen.
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Pronounced improvements in the understanding of semiconductor device performance are expected if electrostatic potential distributions can be measured quantitatively and reliably under working conditions with sufficient sensitivity and... more
Pronounced improvements in the understanding of semiconductor device performance are expected if electrostatic potential distributions can be measured quantitatively and reliably under working conditions with sufficient sensitivity and spatial resolution. Here, we employ off-axis electron holography to characterize an electrically-biased Si p-n junction by measuring its electrostatic potential, electric field and charge density distributions under working conditions. A comparison between experimental electron holographic phase images and images obtained using three-dimensional electrostatic potential simulations highlights several remaining challenges to quantitative analysis. Our results illustrate how the determination of reliable potential distributions from phase images of electrically biased devices requires electrostatic fringing fields, surface charges, specimen preparation damage and the effects of limited spatial resolution to be taken into account.
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ABSTRACT The thermomagnetic behavior of remanence induced magnetite (Fe3O4) particles in the pseudo-single-domain (PSD) size range (~0.1 – 10 µm), which dominate the magnetic signature of many rock lithologies, is investigated using... more
ABSTRACT The thermomagnetic behavior of remanence induced magnetite (Fe3O4) particles in the pseudo-single-domain (PSD) size range (~0.1 – 10 µm), which dominate the magnetic signature of many rock lithologies, is investigated using off-axis electron holography. Construction of magnetic induction maps allowed for visualization of the vortex domain state in an individual Fe3O4 grain (~200 nm in diameter) as a function of temperature. Acquisition of a series of electron holograms at 100 °C intervals during in situ heating up to 700 °C demonstrates the vortex state of the Fe3O4 grain, in this instance, remains thermally stable close to its unblocking temperature and exhibits a similar in-plane remanent state upon cooling, i.e., the particle is effectively behaving like a uniaxial single-domain particle to temperatures near TC. Such particles are thought to be robust magnetic recorders. It is suggested that evidence for PSD behavior should therefore not preclude paleomagnetic investigation.
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Magnetite (Fe3O4) is an important magnetic mineral to Earth scientists, as it carries the dominant magnetic signature in rocks, and the understanding of its magnetic recording fidelity provides a critical tool in the field of... more
Magnetite (Fe3O4) is an important magnetic mineral to Earth scientists, as it carries the dominant magnetic signature in rocks, and the understanding of its magnetic recording fidelity provides a critical tool in the field of palaeomagnetism. However, reliable interpretation of the recording fidelity of Fe3O4 particles is greatly diminished over time by progressive oxidation to less magnetic iron oxides, such as maghemite (γ-Fe2O3), with consequent alteration of remanent magnetization potentially having important geological significance. Here we use the complementary techniques of environmental transmission electron microscopy and off-axis electron holography to induce and visualize the effects of oxidation on the magnetization of individual nanoscale Fe3O4 particles as they transform towards γ-Fe2O3. Magnetic induction maps demonstrate a change in both strength and direction of remanent magnetization within Fe3O4 particles in the size range dominant in rocks, confirming that oxidation can modify the original stored magnetic information.
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ABSTRACT A solid oxide fuel (SOFC) cogenerates heat and electricity with high efficiency. Electricity is produced by the electrochemical reaction of a fuel (H2, CO, CH4) and an oxidant gas (O2, air). The SOFC standard design is based on... more
ABSTRACT A solid oxide fuel (SOFC) cogenerates heat and electricity with high efficiency. Electricity is produced by the electrochemical reaction of a fuel (H2, CO, CH4) and an oxidant gas (O2, air). The SOFC standard design is based on an anode-supported electrolyte. The anode is composed of yttria-stabilized zirconia (YSZ) and nickel oxide (NiO). NiO is reduced in situ into metallic nickel during the first use of the fuel cell. Porosity is formed to compensate the volume shrinkage induced by the reduction (40 vol%). The nickel catalyst may reoxidize due to different factors. Nickel volume change (70 vol%) is not accommodated completely by the internal porosity. Stress is produced in the YSZ backbone. The performances of the SOFC degrade due to the creation of electrolyte cracks. The reason behind the irreversible effect of a redox cycle on Ni-based anodes is subject to controversies. Research groups suggest that the expansion of Ni upon a redox cycle is either caused by the reorganization of nickel during reduction or by formation of porosity upon reoxidation. Time resolved environmental transmission electron microscopy (ETEM) was used here to characterize in situ the reduction and reoxidation mechanisms of Ni-based anodes and thus understand the exact causes of this degradation mechanism.
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Nanotubes that are formed from layered materials have emerged to be exciting one-dimensional materials in the last two decades due to their remarkable structures and properties. Misfit layered compounds (MLC) can be produced from... more
Nanotubes that are formed from layered materials have emerged to be exciting one-dimensional materials in the last two decades due to their remarkable structures and properties. Misfit layered compounds (MLC) can be produced from alternating assemblies of two different molecular slabs with different periodicities with the general formula [(MX)1+x]m[TX2]n (or more simply MS-TS2), where M is Sn, Pb, Bi, Sb, rare earths, T is Sn, Nb, Ta, Ti, V, Cr, and so on, and X is S, Se. The presence of misfit stresses between adjacent layers in MLC provides a driving force for curling of the layers that acts in addition to the elimination of dangling bonds. The combination of these two independent forces leads to the synthesis of misfit layered nanotubes, which are newcomers to the broad field of one-dimensional nanostructures and nanotubes. The synthesis, characterization, and microscopic details of misfit layered nanotubes are discussed, and directions for future research are presented.
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Electron holography, as originally described by Gabor (1949), is based on the formation of an interference pattern or “hologram” in the transmission electron microscope (TEM). In contrast to most conventional TEM techniques, which only... more
Electron holography, as originally described by Gabor (1949), is based on the formation of an interference pattern or “hologram” in the transmission electron microscope (TEM). In contrast to most conventional TEM techniques, which only record spatial distributions of image intensity, electron holography allows the phase shift of the high-energy electron wave that has passed through the specimen to be measured directly. The phase shift can then be used to provide information about local variations in magnetic induction and electrostatic ...
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An approach is presented that allows independent determination of the mean inner potential contribution to a phase image of a highly doped layer in a semiconductor measured using off-axis electron holography, in order to quantify the... more
An approach is presented that allows independent determination of the mean inner potential contribution to a phase image of a highly doped layer in a semiconductor measured using off-axis electron holography, in order to quantify the contribution to the recorded phase from the dopant potential alone. The method takes into account the possible presence of both substitutional and interstitial dopant atoms and is used here to analyse an experimental phase image of 12 delta-doped B layers in Si that are separated from each other by <6 nm.