| 1. |
- 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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| 2. |
- Schroer, Christian G, et al.
(author)
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Hard x-ray nanofocusing with refractive x-ray optics : full beam characterization by ptychographic imaging
- 2013
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In: Proceedings of SPIE. - : SPIE. - 9780819496980 ; , s. 884807-
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Conference paper (peer-reviewed)abstract
- Hard x-ray scanning microscopy relies on small and intensive nanobeams. Refractive x-ray lenses are well suited to generate hard x-ray beams with lateral dimensions of 100 nm and below. The diffraction limited beam size of refractive x-ray lenses mainly depends on the focal length and the attenuation inside the lens material. The numerical aperture of refractive lenses scales with the inverse square root of the focal length until it reaches the critical angle of total reflection. We have used nanofocusing refractive x-ray lenses made of silicon to focus hard x-rays at 8 and 20 keV to (sub-)100 nm dimensions. Using ptychographic scanning coherent diffraction imaging we have characterized these nanobeams with high accuracy and sensitivity, measuring the full complex wave field in the focus. This gives access to the full caustic and aberrations of the x-ray optics.
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| 3. |
- 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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| 4. |
- Stankevic, Tomas, et al.
(author)
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Strain mapping in an InGaN/GaN nanowire using a nano-focused x-ray beam
- 2015
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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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| 5. |
- Wells, Daniel J., et al.
(author)
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Observations of phase changes in monoolein during high viscous injection
- 2022
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In: Journal of Synchrotron Radiation. - : International Union Of Crystallography. - 0909-0495 .- 1600-5775. ; 29:3, s. 602-614
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Journal article (peer-reviewed)abstract
- Serial crystallography of membrane proteins often employs high-viscosity injectors (HVIs) to deliver micrometre-sized crystals to the X-ray beam. Typically, the carrier medium is a lipidic cubic phase (LCP) media, which can also be used to nucleate and grow the crystals. However, despite the fact that the LCP is widely used with HVIs, the potential impact of the injection process on the LCP structure has not been reported and hence is not yet well understood. The self-assembled structure of the LCP can be affected by pressure, dehydration and temperature changes, all of which occur during continuous flow injection. These changes to the LCP structure may in turn impact the results of X-ray diffraction measurements from membrane protein crystals. To investigate the influence of HVIs on the structure of the LCP we conducted a study of the phase changes in monoolein/water and monoolein/buffer mixtures during continuous flow injection, at both atmospheric pressure and under vacuum. The reservoir pressure in the HVI was tracked to determine if there is any correlation with the phase behaviour of the LCP. The results indicated that, even though the reservoir pressure underwent (at times) significant variation, this did not appear to correlate with observed phase changes in the sample stream or correspond to shifts in the LCP lattice parameter. During vacuum injection, there was a three-way coexistence of the gyroid cubic phase, diamond cubic phase and lamellar phase. During injection at atmospheric pressure, the coexistence of a cubic phase and lamellar phase in the monoolein/water mixtures was also observed. The degree to which the lamellar phase is formed was found to be strongly dependent on the co-flowing gas conditions used to stabilize the LCP stream. A combination of laboratory-based optical polarization microscopy and simulation studies was used to investigate these observations.
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