[en] InAs/GaAs quantum dot (QD) bilayer and trilayer structures have been grown on GaAs(001) substrates by molecular beam epitaxy and the properties of the uncapped QDs investigated using atomic force microscopy (AFM). The emphasis is on understanding the influence a variation of the thickness of the GaAs spacer layer (S) between the QD layers has on the morphological properties of the QDs in the up-most layer, which is grown at a significantly reduced temperature compared to the first QD layer. The size distribution of the QDs in the second layer is shown to be exceptionally narrow for a spacer thickness of 10 nm, with a large average QD size. Larger values of S lead to a much broader size distribution and the appearance of significantly smaller dots. Complete strain relief is only achieved upon deposition of ∼50 nm GaAs in bilayer and ∼60 nm in trilayer structures, a result that has important implications for multiple stacking of QD layers in device applications

[en] Knowledge of the residual elastic strain (RES) in polycrystalline materials is of primary importance for understanding the deformation behavior and fatigue lifetimes of engineering components. Bragg-edge neutron strain tomography is a recently developed technique that is showing great promise as a time efficient, non-destructive method of mapping the three-dimensional strain profile in whole components and bulk materials at high resolution. In Bragg-edge neutron strain tomography, the energy-resolved neutron transmission spectrum is collected from a polycrystalline sample. This spectrum displays sudden well-defined increases in intensity known as 'Bragg-edges' which provide a direct measure of the average through thickness strain. By measuring the transmission spectrum for a number of sample orientations it is possible to recover the underlying three-dimensional RES. Newly developed high resolution Microchannel Plate Timepix detector in Time-of-Flight (TOF) experiments have been used to obtain three-dimensional strain maps at 55m resolution. Here we describe the principles and practice of the Bragg-edge measurement technique. The successful implementation of this technique for arbitrary strain distributions offers the potential of significantly improving the resolution and data acquisition times for imaging RES over previous diffraction based approaches.

[en] Atomic force microscopy and photoluminescence spectroscopy (PL) have been used to study asymmetric bilayer InAs quantum dot (QD) structures grown by molecular-beam epitaxy on GaAs(001) substrates. The two QD layers were separated by a GaAs spacer layer (SL) of varying thickness and were grown at different substrate temperatures. Grown independently, these two layers would exhibit a widely different QD number density, and this technique therefore enables us to assess the influence of the strain fields created by the dots in the first layer on the second-layer QD nucleation and characteristics. For very large SLs (>40 nm), total strain relief causes the QD nucleation to be controlled exclusively by the substrate temperature, which influences the migration of In adatoms. In this case, the optical and morphological properties of the second QD layer are identical to a structure with a single QD layer grown at the same temperature. In structures with a much smaller SL, strain effects dominate over the effect of temperature in controlling the nucleation of the QDs, thereby fixing the second-layer QD number density to that of the first (templating effect). There is also evidence that strain relaxation is present in the QDs of the second layer and that this is crucial for extending their emission wavelength. The optimum SL thickness is shown to be 11 nm, for which low-temperature PL emission peaks at 1.26 μm, with a full width at half-maximum of only 15 meV. Intermediate SL thicknesses exhibit broad QD size distributions, with strain effects only partly influencing the QD growth in the second layer

[en] Full text: Nanodiamonds (NDs) with nitrogen vacancy (NV) centres have been shown to be useful for applications involving cellular tracking in vivo at the molecular level. The sustained fluorescence of these nanodiamonds is related to their structure, and is supposed to be influenced by the strain distribution inside the crystals. In nanocrystals even relatively small amounts of strain can induce large changes in the mechanical, optical and electronic properties of nanocrystals. The current work elaborates first application of Bragg coherent diffractive imaging (BCDI) for mapping the three-dimensional (3D) strain fields within the crystalline nanodiamonds. For reference, a control sample (as-grown crystals) has been compared with a strain-induced (implanted with 1012 ions per cm²) sample. The comparison of control and strain-induced samples will help to optimise their application for tracking the processes at molecular level. (author)

[en] Three-dimensional imaging of protein crystals during x-ray diffraction experiments opens up a range of possibilities for optimizing crystal quality and gaining new insights into the fundamental processes that drive radiation damage. Obtaining this information at the appropriate length-scales however is extremely challenging. One approach that has been recently demonstrated as a promising avenue for characterizing the size and shape of protein crystals at nanometre length-scales is Bragg coherent diffractive imaging (BCDI). BCDI is a recently developed technique that is able to recover the phase of the continuous diffraction intensity signal around individual Bragg peaks. When data is collected at multiple points on a rocking curve, a reciprocal space map (RSM) can be assembled and then inverted using BCDI to obtain a three-dimensional image of the crystal. The first demonstration of two-dimensional biological BCDI was reported by Boutet et al on holoferritin, recently this work was extended to the study of radiation damage in micron-sized protein crystals. Here we present the first three-dimensional reconstructions of a Lysozyme protein crystal using BDI. The results are validated against RSM and transmission electron microscopy data and have implications for both radiation damage studies and for developing new approaches for structure retrieval from micron-sized protein crystals. (paper)

[en] This paper reports improved reconstruction of complex wave fields from extended objects. The combination of ptychography with Fresnel diffractive imaging results in better reconstructions with fewer iterations required to convergence than either method considered separately. The method is applied to retrieve the projected thickness of a gold microstructure and comparative results using ptychography and Fresnel diffractive imaging are presented.

[en] A dedicated in-vacuum coherent x-ray diffraction microscope was installed at the 2-ID-B beamline of the Advanced Photon Source for use with 0.7-2.9 keV x-rays. The instrument can accommodate three common implementations of diffractive imaging; plane wave illumination; defocused-probe (Fresnel diffractive imaging) and scanning (ptychography) using either a pinhole, focused or defocused probe. The microscope design includes active feedback to limit motion of the optics with respect to the sample. Upper bounds on the relative optics-to-sample displacement have been measured to be 5.8 nm(v) and 4.4 nm(h) rms/h using capacitance micrometry and 27 nm/h using x-ray point projection imaging. The stability of the measurement platform and in-vacuum operation allows for long exposure times, high signal-to-noise and large dynamic range two-dimensional intensity measurements to be acquired. Finally, we illustrate the microscope's stability with a recent experimental result.