| 1. |
- Bi, Zhaoxia, et al.
(author)
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Self-assembled InN quantum dots on side facets of GaN nanowires
- 2018
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In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 123:16
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Journal article (peer-reviewed)abstract
- Self-assembled, atomic diffusion controlled growth of InN quantum dots was realized on the side facets of dislocation-free and c-oriented GaN nanowires having a hexagonal cross-section. The nanowires were synthesized by selective area metal organic vapor phase epitaxy. A 3 Å thick InN wetting layer was observed after growth, on top of which the InN quantum dots formed, indicating self-assembly in the Stranski-Krastanow growth mode. We found that the InN quantum dots can be tuned to nucleate either preferentially at the edges between GaN nanowire side facets, or directly on the side facets by tuning the adatom migration by controlling the precursor supersaturation and growth temperature. Structural characterization by transmission electron microscopy and reciprocal space mapping show that the InN quantum dots are close to be fully relaxed (residual strain below 1%) and that the c-planes of the InN quantum dots are tilted with respect to the GaN core. The strain relaxes mainly by the formation of misfit dislocations, observed with a periodicity of 3.2 nm at the InN and GaN hetero-interface. The misfit dislocations introduce I1 type stacking faults (...ABABCBC...) in the InN quantum dots. Photoluminescence investigations of the InN quantum dots show that the emissions shift to higher energy with reduced quantum dot size, which we attribute to increased quantum confinement.
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| 2. |
- Dzhigaev, Dmitry, et al.
(author)
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X-ray Bragg Ptychography on a Single InGaN/GaN Core-Shell Nanowire
- 2017
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In: ACS Nano. - : American Chemical Society (ACS). - 1936-0851 .- 1936-086X. ; 11:7, s. 6605-6611
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Journal article (peer-reviewed)abstract
- The future of solid-state lighting can be potentially driven by applications of InGaN/GaN core-shell nanowires. These heterostructures provide the possibility for fine-tuning of functional properties by controlling a strain state between mismatched layers. We present a nondestructive study of a single 400 nm-thick InGaN/GaN core-shell nanowire using two-dimensional (2D) X-ray Bragg ptychography (XBP) with a nanofocused X-ray beam. The XBP reconstruction enabled the determination of a detailed three-dimensional (3D) distribution of the strain in the particular nanowire using a model based on finite element method. We observed the strain induced by the lattice mismatch between the GaN core and InGaN shell to be in the range from -0.1% to 0.15% for an In concentration of 30%. The maximum value of the strain component normal to the facets was concentrated at the transition region between the main part of the nanowire and the GaN tip. In addition, a variation in misfit strain relaxation between the axial growth and in-plane directions was revealed.
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| 3. |
- Heurlin, Magnus, et al.
(author)
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Structural Properties of Wurtzite InP-InGaAs Nanowire Core-Shell Heterostructures
- 2015
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In: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 15:4, s. 2462-2467
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Journal article (peer-reviewed)abstract
- We report on growth and characterization of wurtzite InP-In1-xGaxAs core-shell nanowire heterostructures. A range of nanowire structures with different Ga concentration in the shell was characterized with transmission electron microscopy and X-ray diffraction. We found that the main part of the nanowires has a pure wurtzite crystal structure, with occasional stacking faults occurring only at the top and bottom. This allowed us to determine the structural properties of wurtzite In1-xGaxAs. The InP-In1-xGaxAs core-shell nanowires show a triangular and hexagonal facet structure of {1100} and {10 (10) over bar} planes. X-ray diffraction measurements showed that the core and the shell are pseudomorphic along the c-axis, and the strained axial lattice constant is closer to the relaxed In1-xGaxAs shell. Microphotoluminescence measurements of the nanowires show emission in the infrared regime, which makes them suitable for applications in optical communication.
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| 4. |
- Jensen, Torben Haugaard, et al.
(author)
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Imaging of Metastatic Lymph Nodes by X-ray Phase-Contrast Micro-Tomography
- 2013
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In: PLoS ONE. - : Public Library of Science (PLoS). - 1932-6203. ; 8:1
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Journal article (peer-reviewed)abstract
- Invasive cancer causes a change in density in the affected tissue, which can be visualized by x-ray phase-contrast tomography. However, the diagnostic value of this method has so far not been investigated in detail. Therefore, the purpose of this study was, in a blinded manner, to investigate whether malignancy could be revealed by non-invasive x-ray phase-contrast tomography in lymph nodes from breast cancer patients. Seventeen formalin-fixed paraffin-embedded lymph nodes from 10 female patients (age range 37-83 years) diagnosed with invasive ductal carcinomas were analyzed by X-ray phase-contrast tomography. Ten lymph nodes had metastatic deposits and 7 were benign. The phase-contrast images were analyzed according to standards for conventional CT images looking for characteristics usually only visible by pathological examinations. Histopathology was used as reference. The result of this study was that the diagnostic sensitivity of the image analysis for detecting malignancy was 100% and the specificity was 87%. The positive predictive value was 91% for detecting malignancy and the negative predictive value was 100%. We conclude that x-ray phase-contrast imaging can accurately detect density variations to obtain information regarding lymph node involvement previously inaccessible with standard absorption x-ray imaging.
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| 5. |
- Lauridsen, Torsten, et al.
(author)
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Detection of sub-pixel fractures in X-ray dark-field tomography
- 2015
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In: Applied Physics A: Materials Science & Processing. - : Springer Science and Business Media LLC. - 1432-0630. ; 121:3, s. 1243-1250
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Journal article (peer-reviewed)abstract
- We present a new method for detecting fractures in solid materials below the resolution given by the detector pixel size by using grating-based X-ray interferometry. The technique is particularly useful for detecting sub-pixel cracks in large samples where the size of the sample is preventing high-resolution mu CT studies of the entire sample. The X-ray grating interferometer produces three distinct modality signals: absorption, phase and dark field. The method utilizes the unique scattering features of the dark-field signal. We have used tomograms reconstructed from each of the three signals to detect cracks in a model sample consisting of stearin.
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| 6. |
- Stankevič, Tomaš, et al.
(author)
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Fast Strain Mapping of Nanowire Light-Emitting Diodes Using Nanofocused X-ray Beams.
- 2015
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In: ACS Nano. - : American Chemical Society (ACS). - 1936-086X .- 1936-0851. ; 9:7, s. 6978-6984
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Journal article (peer-reviewed)abstract
- X-ray nanobeams are unique nondestructive probes that allow direct measurements of the nanoscale strain distribution and composition inside the micrometer thick layered structures that are found in most electronic device architectures. However, the method is usually extremely time-consuming, and as a result, data sets are often constrained to a few or even single objects. Here we demonstrate that by special design of a nanofocused X-ray beam diffraction experiment we can (in a single 2D scan with no sample rotation) measure the individual strain and composition profiles of many structures in an array of upright standing nanowires. We make use of the observation that in the generic nanowire device configuration, which is found in high-speed transistors, solar cells, and light-emitting diodes, each wire exhibits very small degrees of random tilts and twists toward the substrate. Although the tilt and twist are very small, they give a new contrast mechanism between different wires. In the present case, we image complex nanowires for nanoLED fabrication and compare to theoretical simulations, demonstrating that this fast method is suitable for real nanostructured devices.
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| 7. |
- Stankevic, Tomas, et al.
(author)
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Measurement of strain in InGaN/GaN nanowires and nanopyramids
- 2015
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In: Journal of Applied Crystallography. - 1600-5767. ; 48, s. 344-349
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Journal article (peer-reviewed)abstract
- The growth and optoelectronic properties of core-shell nanostructures are influenced by the strain induced by the lattice mismatch between core and shell. In contrast with planar films, nanostructures contain multiple facets that act as independent substrates for shell growth, which enables different relaxation mechanisms. In this study, X-ray diffraction data are presented that show that InGa1-N shells grown on GaN cores are strained along each of the facets independently. Reciprocal space maps reveal multiple Bragg peaks, corresponding to different parts of the shell being strained along the individual facet planes. The strained lattice constants were found from the positions of the Bragg peaks. Vegard's law and Hooke's law for an anisotropic medium were applied in order to find the composition and strain in the InGaN shells. A range of nanowire samples with different InGaN shell thicknesses were measured and it is concluded that, with an In concentration of around 30%, major strain relaxation takes place when the thickness reaches 23nm. InGaN shells of 6 and 9nm thickness remain nearly fully strained biaxially along each of the facets of the nanowires and the facets of the nanopyramids.
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| 8. |
- Stankevic, Tomas, et al.
(author)
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Nanofocused x-ray beams applied for mapping strain in core-shell nanowires
- 2015
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In: Proceedings of SPIE. - : SPIE. - 1996-756X .- 0277-786X. ; 9592, s. 95920-95920
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Conference paper (peer-reviewed)abstract
- The core-shell nanowires have the promise to become the future building blocks of light emitting diodes, solar cells and quantum computers. The high surface to volume ratio allows efficient elastic strain relaxation, making it possible to combine a wider range of materials into the heterostructures as compared to the traditional, planar geometry. As a result, the strain fields appear in both the core and the shell of the nanowires, which can affect the device properties. The hard x-ray nanoprobe is a tool that enables a nondestructive mapping of the strain and tilt distributions where other techniques cannot be applied. By measuring the positions of the Bragg peaks for each point on the sample we can evaluate the values of local tilt and strain. In this paper we demonstrate the detailed strain mapping of the strained InGaN/GaN core-shell nanowire. We observe an asymmetric strain distribution in the GaN core caused by an uneven shell relaxation. Additionally, we analyzed the local micro-tilt distribution, which shows the edge effects at the top and bottom of the nanowire. The measurements were compared to the finite element modelling and show a good agreement.
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| 9. |
- Stankevic, Tomas, et al.
(author)
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Strain mapping in an InGaN/GaN nanowire using a nano-focused x-ray beam
- 2015
-
In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 107:10
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Journal article (peer-reviewed)abstract
- Strained InGaN/GaN core-shell nanowires (NWs) are promising candidates for solid state lighting applications due to their superior properties compared to planar films. NW based devices consist of multiple functional layers, which sum up to many hundred nanometers in thickness, that can uniquely be accessed in a non-destructive fashion by hard X-rays. Here, we present a detailed nanoscale strain mapping performed on a single, 400 nm thick and 2 lm long core-shell InGaN/GaN nanowire with an x-ray beam focused down to 100 nm. We observe an inhomogeneous strain distribution caused by the asymmetric strain relaxation in the shell. One side of the InGaN shell was fully strained, whereas the other side and the top part were relaxed. Additionally, tilt and strain gradients were determined at the interface with the substrate. (C) 2015 AIP Publishing LLC.
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| 10. |
- Wadsater, Maria, et al.
(author)
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Aligning Nanodiscs at the Air-Water Interface, a Neutron Reflectivity Study
- 2011
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In: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 27:24, s. 15065-15073
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Journal article (peer-reviewed)abstract
- Nanodiscs are self-assembled nanostructures composed of a belt protein and a small patch of lipid bilayer, which can solubilize membrane proteins in a lipid bilayer environment. We present a method for the alignment of a well-defined two-dimensional layer of nanodiscs at the air-water interface by careful design of an insoluble surfactant monolayer at the surface. We used neutron reflectivity to demonstrate the feasibility of this approach and to elucidate the structure of the nanodisc layer. The proof of concept is hereby presented with the use of nanodiscs composed of a mixture of two different lipid (DMPC and DMPG) types to obtain a net overall negative charge of the nanodiscs. We find that the nanodisc layer has a thickness or 40.9 +/- 2.6 angstrom with a surface coverage of 66 +/- 4%. This layer is located about 15 angstrom below a cationic surfactant layer at the air water interface. The high level of organization within the nanodiscs layer is reflected by a low interfacial roughness (similar to 4.5 angstrom) found. The use of the nanodisc as a biomimetic model of the cell membrane allows for studies of single membrane proteins isolated in a confined lipid environment. The 2D alignment of nanodiscs could therefore enable studies of high-density layers containing membrane proteins that, in contrast to membrane proteins reconstituted in a continuous lipid bilayer, remain isolated front influences of neighboring membrane proteins within the layer.
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