Synthesis and Characterisation of Acrylic Resin-Al Powder Composites Suitable for Additive Manufacturing.

Stereolithography is an additive manufacturing technology commonly used to build either prototypes or final parts. Nevertheless, the manufacture of structural parts has been ruled out owing to the poor mechanical properties of conventional UV-curable resins. Moreover, the inventory of available commercial resins is still limited and they exhibit low thermal and electrical conductivity values. In this work, some composite materials were designed using Al microparticles dispersed within an SLA commercial resin matrix. These composites overcame the difficulties caused by the light scattering effect during the photopolymerisation process in the SLA technology. Dispersion of the filler was characterised by means of SEM/EDX and AFM. The composites exhibited improved thermal and mechanical behaviour in comparison with the pristine resin. The simplicity of the synthesis method used to prepare the composites provides a convenient starting point to explore new ways of designing composites for SLA with improved mechanical and functional properties.

Investigation on Sb distribution for InSb/InAs sub-monolayer heterostructure using TEM techniques.

InSb/InAs sub-monolayer (SML) nanostructures such as SML quantum dots offer sharper emission spectra, a better modal gain and a larger modulation bandwidth compared to its Stranski-Krastanov counterpart. In this work, the Sb distribution of SML InSb layers grown by migration enhanced epitaxy has been analyzed by transmission electron microscopy (TEM) techniques. The analysis of the material by diffraction contrast in 002 dark field conditions and by atomic column resolved high angle annular dark field-scanning TEM reveal the presence of a low Sb content InSbAs continuous layer with scarce Sb-rich InSbAs agglomerates. The intensity profiles obtained by both techniques point to Sb segregation during growth. This segregation has been quantified using the Muraki segregation model obtaining a high segregation coefficient R of 0.81 towards the growth direction. The formation of a continuous InSbAs wetting layer as a result of a SML deposition of Sb on the InAs surface is discussed.

Effect of the cap layer growth temperature on the Sb distribution in InAs/InSb/InAs sub-monolayer heterostructures for mid-infrared devices.

Sub-monolayer (SML) deposition of InSb within InAs matrix by migration enhanced epitaxy tends to form type II SML nanostructures offering efficient light emission within the mid-infrared (MIR) range between 3 and 5 µm. In this work, we report on the Sb distribution in InSb/InAs SML nanostructures with InAs cap layers grown at temperatures lower than that associated with the under-grown InSb active layer. Analysis by transmission electron microscopy (TEM) in 002 dark field conditions shows that the reduction in the growth temperature of the InAs cap layer increases the amount of Sb deposited in the layers, in good agreement with the x-ray diffraction results. TEM micrographs also show that the layers are formed by random InSbAs agglomerates, where the lower cap temperature leads to a more continuous InSb layer. Quantitative atomic column resolved high angle annular dark field-scanning (S)TEM analyses also reveal atomic columns with larger composition of Sb for the structure with the lowest InAs cap layer temperature. The dependence of the Sb distribution on InAs cap growth temperature allows tuning the corresponding emission wavelength in the MIR range, as shown by the photoluminescence emission spectra.

Development of Surface-Coated Polylactic Acid/Polyhydroxyalkanoate (PLA/PHA) Nanocomposites.

This work reports on the design and development of nanocomposites based on a polymeric matrix containing biodegradable Polylactic Acid (PLA) and Polyhydroxyalkanoate (PHA) coated with either Graphite NanoPlatelets (GNP) or silver nanoparticles (AgNP). Nanocomposites were obtained by mechanical mixing under mild conditions and low load contents (<0.10 wt %). This favours physical adhesion of the additives onto the polymer surface, while the polymeric bulk matrix remains unaffected. Nanocomposite characterisation was performed via optical and focused ion beam microscopy, proving these nanocomposites are selectively modified only on the surface, leaving bulk polymer unaffected. Processability of these materials was proven by the fabrication of samples via injection moulding and mechanical characterisation. Nanocomposites showed enhanced Young modulus and yield strength, as well as better thermal properties when compared with the unmodified polymer. In the case of AgNP coated nanocomposites, the surface was found to be optically active, as observed in the increase of the resolution of Raman spectra, acquired at least 10 times, proving these nanocomposites are promising candidates as surface enhanced Raman spectroscopy (SERS) substrates.

Structural characterization of bulk and nanoparticle lead halide perovskite thin films by (S)TEM techniques.

Lead halide (APbX3) perovskites, in polycrystalline thin films but also perovskite nanoparticles (NPs) has demonstrated excellent performance to implement a new generation of photovoltaic and photonic devices. The structural characterization of APbX3 thin films using (scanning) transmission electron microscopy ((S)TEM) techniques can provide valuable information that can be used to understand and model their optoelectronic performance and device properties. However, since APbX3 perovskites are soft materials, their characterization using (S)TEM is challenging. Here, we study and compare the structural properties of two different metal halide APbX3 perovskite thin films: bulk CH3NH3PbI3 prepared by spin-coating of the precursors in solution and CsPbBr3 colloidal NPs synthetized and deposited by doctor blading. Both specimen preparation methods and working conditions for analysis by (S)TEM are properly optimized. We show that CH3NH3PbI3 thin films grown by a one-step method are composed of independent grains with random orientations. The growth method results in the formation of tetragonal perovskite thin films with good adherence to an underlying TiO2 layer, which is characterized by a photoluminescence (PL) emission band centered at 775 nm. The perovskite thin films based on CsPbBr3 colloidal NPs, which are used as the building blocks of the film, are preserved by the deposition process, even if small gaps are observed between adjacent NPs. The crystal structure of CsPbBr3 NPs is cubic, which is beneficial for optical properties due to its optimal band gap. The absorption and PL spectra measured in both the thin film and the colloidal solution of CsPbBr3 NPs are very similar, indicating a good homogeneity of the thin films and the absence of aggregation of NPs. However, a particular care was required to avoid long electron irradiation times during our structural studies, even at a low voltage of 80 kV, as the material was observed to decompose through Pb segregation.

Influence of the crosstalk on the intensity of HAADF-STEM images of quaternary semiconductor materials.

The influence of the neighbouring atomic-columns in determining the composition at atomic column scale of quaternary semiconductor compounds, using simulated HAADF-STEM images is evaluated. The InAlAsSb alloy, a promising material in the photovoltaic field, is considered. We find that the so called 'crosstalk' effect plays an important role for the aimed compositional determination. The intensity transfer is larger from neighbouring atomic columns with higher average Z, and towards atomic columns with smaller Z. Our results show that in order to obtain precise information on the column composition, the HAADF-STEM intensities of both columns need to be taken into account simultaneously.

Analysis of Bi Distribution in Epitaxial GaAsBi by Aberration-Corrected HAADF-STEM.

The Bi content in GaAs/GaAs1 - xBi x /GaAs heterostructures grown by molecular beam epitaxy at a substrate temperature close to 340 °C is investigated by aberration-corrected high-angle annular dark-field techniques. The analysis at low magnification of high-angle annular dark-field scanning transmission electron microscopy images, corroborated by EDX analysis, revealed planar defect-free layers and a non-homogeneous Bi distribution at the interfaces and within the GaAsBi layer. At high magnification, the qHAADF analysis confirmed the inhomogeneous distribution and Bi segregation at the GaAsBi/GaAs interface at low Bi flux and distorted dumbbell shape in areas with higher Bi content. At higher Bi flux, the size of the Bi gathering increases leading to roughly equiaxial Bi-rich particles faceted along zinc blende {111} and uniformly dispersed around the matrix and interfaces. FFT analysis checks the coexistence of two phases in some clusters: a rhombohedral pure Bi (rh-Bi) one surrounded by a zinc blende GaAs1 - xBi x matrix. Clusters may be affecting to the local lattice relaxation and leading to a partially relaxed GaAsBi/GaAs system, in good agreement with XRD analysis.

Atom-scale compositional distribution in InAlAsSb-based triple junction solar cells by atom probe tomography.

The analysis by atom probe tomography (APT) of InAlAsSb layers with applications in triple junction solar cells (TJSCs) has shown the existence of In- and Sb-rich regions in the material. The composition variation found is not evident from the direct observation of the 3D atomic distribution and because of this a statistical analysis has been required. From previous analysis of these samples, it is shown that the small compositional fluctuations determined have a strong effect on the optical properties of the material and ultimately on the performance of TJSCs.

Small-pore driven high capacitance in a hierarchical carbon via carbonization of Ni-MOF-74 at low temperatures.

A hierarchical porous carbon prepared via direct carbonization of Ni-MOF-74 loaded with furfuryl alcohol at 450 °C displays high specific capacitance in comparison with other MOF-derived carbons as a result of the formation of micropores smaller than 1 nm.

Three dimensional atom probe imaging of GaAsSb quantum rings.

Unambiguous evidence of ring-shaped self-assembled GaSb nanostructures grown by molecular beam epitaxy is presented on the basis of atom-probe tomography reconstructions and dark field transmission electron microscopy imaging. The GaAs capping process causes a strong segregation of Sb out of the center of GaSb quantum dots, leading to the self-assembled GaAs(x)Sb(1-x) quantum rings of 20-30 nm in diameter with x ∼ 0.33.

Growth of Low-Density Vertical Quantum Dot Molecules with Control in Energy Emission.

In this work, we present results on the formation of vertical molecule structures formed by two vertically aligned InAs quantum dots (QD) in which a deliberate control of energy emission is achieved. The emission energy of the first layer of QD forming the molecule can be tuned by the deposition of controlled amounts of InAs at a nanohole template formed by GaAs droplet epitaxy. The QD of the second layer are formed directly on top of the buried ones by a strain-driven process. In this way, either symmetric or asymmetric vertically coupled structures can be obtained. As a characteristic when using a droplet epitaxy patterning process, the density of quantum dot molecules finally obtained is low enough (2 × 10(8) cm(-2)) to permit their integration as active elements in advanced photonic devices where spectroscopic studies at the single nanostructure level are required.

Morphological evolution of InAs/InP quantum wires through aberration-corrected scanning transmission electron microscopy.

Evolution of the size, shape and composition of self-assembled InAs/InP quantum wires through the Stranski-Krastanov transition has been determined by aberration-corrected Z-contrast imaging. High resolution compositional maps of the wires in the initial, intermediate and final formation stages are presented. (001) is the main facet at their very initial stage of formation, which is gradually reduced in favour of [114] or [118], ending with the formation of mature quantum wires with {114} facets. Significant changes in wire dimensions are measured when varying slightly the amount of InAs deposited. These results are used as input parameters to build three-dimensional models that allow calculation of the strain energy during the quantum wire formation process. The observed morphological evolution is explained in terms of the calculated elastic energy changes at the growth front. Regions of the wetting layer close to the nanostructure perimeters have higher strain energy, causing migration of As atoms towards the quantum wire terraces, where the structure is partially relaxed; the thickness of the wetting layer is reduced in these zones and the island height increases until the (001) facet is removed.

Blocking of indium incorporation by antimony in III-V-Sb nanostructures.

The addition of antimony to III-V nanostructures is expected to give greater freedom in bandgap engineering for device applications. One of the main challenges to overcome is the effect of indium and antimony surface segregation. Using several very high resolution analysis techniques we clearly demonstrate blocking of indium incorporation by antimony. Furthermore, indium incorporation resumes when the antimony concentration drops below a critical level. This leads to major differences between nominal and actual structures.

Aberration-corrected scanning transmission electron microscopy: from atomic imaging and analysis to solving energy problems.

The new possibilities of aberration-corrected scanning transmission electron microscopy (STEM) extend far beyond the factor of 2 or more in lateral resolution that was the original motivation. The smaller probe also gives enhanced single atom sensitivity, both for imaging and for spectroscopy, enabling light elements to be detected in a Z-contrast image and giving much improved phase contrast imaging using the bright field detector with pixel-by-pixel correlation with the Z-contrast image. Furthermore, the increased probe-forming aperture brings significant depth sensitivity and the possibility of optical sectioning to extract information in three dimensions. This paper reviews these recent advances with reference to several applications of relevance to energy, the origin of the low-temperature catalytic activity of nanophase Au, the nucleation and growth of semiconducting nanowires, and the origin of the eight orders of magnitude increased ionic conductivity in oxide superlattices. Possible future directions of aberration-corrected STEM for solving energy problems are outlined.

Column-by-column compositional mapping by Z-contrast imaging.

A phenomenological method is developed to determine the composition of materials, with atomic column resolution, by analysis of integrated intensities of aberration-corrected Z-contrast scanning transmission electron microscopy images. The method is exemplified for InAs(x)P(1-x) alloys using epitaxial thin films with calibrated compositions as standards. Using this approach we have determined the composition of the two-dimensional wetting layer formed between self-assembled InAs quantum wires on InP(001) substrates.

Determination of the strain generated in InAs/InP quantum wires: prediction of nucleation sites.

The compositional distribution in a self-assembled InAs(P) quantum wire grown by molecular beam epitaxy on an InP(001) substrate has been determined by electron energy loss spectrum imaging. We have determined the strain and stress fields generated in and around this wire capped with a 5 nm InP layer by finite element calculations using as input the compositional map experimentally obtained. Preferential sites for nucleation of wires grown on the surface of this InP capping layer are predicted, based on chemical potential minimization, from the determined strain and stress fields on this surface. The determined preferential sites for wire nucleation agree with their experimentally measured locations. The method used in this paper, which combines electron energy loss spectroscopy, high-resolution Z contrast imaging, and elastic theory finite element calculations, is believed to be a valuable technique of wide applicability for predicting the preferential nucleation sites of epitaxial self-assembled nano-objects.