Jérémy David

The quality of precession electron diffraction data is higher than necessary for structure solution of unknown crystalline phases.

Four crystal structures are solved from precession electron diffraction data using either the highest-quality experimental data available or voluntarily deteriorated data. The data quality was lowered by changing the intensity of each individual reflection by as much as a factor of 2, even for strong reflections, while taking care to keep strong reflections strong and weak reflections weak. The crystals have been chosen for their different characteristics, ranging from small to large unit cells, low to high symmetry, and containing heavy and light atoms. In each case the structure solution succeeded with the deteriorated data provided that the completeness of the data was high enough. The accuracy of the atom positions obtained for the cations was comparable to that for the best experimental data. Light-atom positions were sometimes less accurate but still satisfactory for a possible subsequent refinement.

TEM study of the reaction mechanisms involved in the carbothermal reduction of hafnia

The synthesis of HfCxOy oxycarbides through the carbothermal reaction of hafnia with carbon black was undertaken. The obtained powders at different rates of advancement were studied by TEM and XRD in order to investigate the reaction mechanisms involved during such a transformation. The contact between the two starting reactants is shown to be non-reactive, attesting to the transformation operating through solid–gas reactions. The hafnia phase is destabilized by the CO(g) rich atmosphere and is consumed by the migration of ledges at the surface of the crystals acting as a zipper mechanism that liberates HfO(g) and CO2(g) species. The carbon dioxide thus released is used in return to oxidize the carbon black forming carbon monoxide through the Boudouard equilibrium. The liberated HfO(g) then reacts with the ambient CO(g) to form the oxycarbide phase which is shown to nucleate in the carbon black areas. The oxycarbide nuclei display a core–shell microstructure which is formed by a single crystal core embedded in an oxygen rich amorphous phase. During the final stage of the reaction, the atmosphere, which, saturated in CO(g), progressively reduces the oxygen rich gangue until it finally disappears. The accurate determination of the cell parameter of the oxycarbide phase during the reaction indicates that the first formed compound is nearly saturated in carbon, comparable to the metallic carbide. The small change in the lattice parameter indicates that the chemical composition is very restricted, so the solid solution of oxygen within the hafnium oxycarbide seems to be very limited.

Mapping of axial strain in InAs/InSb heterostructured nanowires

The article presents a mapping of the residual strain along the axis of InAs/InSb heterostructured nanowires. Using confocal Raman measurements, we observe a gradual shift in the TO phonon mode along the axis of these nanowires. We attribute the observed TO phonon shift to a residual strain arising from the InAs/InSb lattice mismatch. We find that the strain is maximum at the interface and then monotonically relaxes towards the tip of the nanowires. We also analyze the crystal structure of the InSb segment through selected area electron diffraction measurements and electron diffraction tomography on individual nanowires.

Synthesis of zirconium oxycarbide powders using metal–organic framework (MOF) compounds as precursors

ZrCxOy oxycarbides were synthesized for the first time by using metal–organic framework (MOF) compounds as precursors. These MOFs, based on zirconium carboxylates, are derived from the UiO-66 type structure. Three different Zr-MOF compounds were synthesized, differing by their C/Zr ratio, due to the use of terephthalic acid (C/Zr = 8, or UiO-66), naphthalene-2,6-dicarboxylic acid (C/Zr = 12) and biphenyl-4,4′-dicarboxylic acid (C/Zr = 14 or UiO-67). The oxycarbide crystallites obtained through appropriate thermal treatments (under Ar atmosphere) of the Zr-MOF precursors show an average size of a few hundred microns. They are surrounded by an outer rim of turbostratic carbon, whose thickness is directly relevant to the C/Zr ratio coming from the nature of the organic linker. The composition of the oxycarbides obtained was estimated on the basis of the cell parameters refined from the powder XRD patterns. The oxycarbides synthesized from naphtalene-2,6-dicarboxylic acid and biphenyl-4,4′-dicarboxylic acid are close to ZrC0.925O0.075, while that of the oxycarbide obtained from the terephthalic acid precursor is ZrC0.944O0.056. It results that the carbon richer oxycarbide product is observed for the UiO-66 precursor synthesized with terephthalic acid, and exhibiting the lower C/Zr ratio of the series. The composition of the oxycarbide powders is shown to be directly relevant to the mechanisms of decomposition of the Zr-MOF compound involved during heating.

Ni-rich phases identification in GaAs nanowire devices by mean of electron diffraction tomography

Rapid thermal annealing is a powerful tool for the preparation of ohmic contact at metal–semiconductor interfaces, and it can yield to complex inter-diffusion phenomena, as it has been extensively studied in the case of bulk GaAs[1]. We observed that the controlled thermal annealing of GaAs nanowire (NW) devices with Ni/Au electrodes promotes the diffusion of nickel into the nanowire, forming Ni-rich metallic alloy regions in the nanostructured body. From EDS analysis, the content of nickel in the nanowires was variable, indicating the presence of different Ni-rich alloys in the transformed part of the NW. In order to identify which crystal phases form, the devices were designed on a 50nm SiN membrane transparent to a high energy electron beam, making the device observable in a TEM. Since the transformed part is very small, from 200 to 400 nm in the best scenario, phase identification using oriented zone axis patterns is critical. To avoid this problem we performed Electron Diffraction Tomography[2] data collection on the transformed region in parallel microdiffraction mode. In this experimental configuration a set of diffraction patterns are recorded while tilting the sample around the goniometer axis of the microscope, with a specific angular step (1° in this study) and within a variable angular range depending on the goniometer available. The illuminated area is minimized through a small condenser aperture, while the beam is kept always parallel. In our case with a 5 μm aperture we could illuminate a circle of 200 nm (see figure) in diameter, being sure that mainly the transformed region was diffracting. From the collected data the reciprocal space of the diffractive volume can then be reconstructed in three dimensions, allowing the determination of the lattice parameters of the diffracting crystal. For this purpose an angular range of only 60° was sufficient and although the patterns were noisy and “contaminated” by spurious spots coming from the surrounding part of the nanowire, the unit cell of several Ni-rich alloys could be determined. Possible interpretation on which are the parameters that can play a role on the predominance of one of them over the others will be discussed. [1] T.-J. Kim, P. H. Holloway, “Ohmic Contacts to II-VI and II-V Compound Semiconductors”, in Processing of Wide Band Gap Semiconductors, S.J. Pearton, 2001. [2] U. Kolb, T. Gorelik, C. Kübel, M.T. Otten, D. Hubert, Ultramicroscopy 107, 507 (2007)

Crystal Phases in Hybrid Metal–Semiconductor Nanowire Devices

We investigate the metallic phases observed in hybrid metal-GaAs nanowire devices obtained by controlled thermal annealing of Ni/Au electrodes. Devices are fabricated onto a SiN membrane compatible with transmission electron microscopy studies. Energy dispersive X-ray spectroscopy allows us to show that the nanowire body includes two Ni-rich phases that thanks to an innovative use of electron diffraction tomography can be unambiguously identified as Ni3GaAs and Ni5As2 crystals. The mechanisms of Ni incorporation leading to the observed phenomenology are discussed.

Determination of very beam-sensitive zeolite ITQ-57 by energy-filtered Timepix data

Enrico Mugnaioli1, Mauro Gemmi1, Jeremy David1, Pablo J. Bereciartua2, Jose L. Jorda2, Fernando Rey2, Eva M. Diaz-Canales2, Maria J. Diaz-Cabañas2 1Center For Nanotechnology Innovation@NEST, Istituto Italiano Di Tecnologia, Pisa, Italy, 2Instituto de Tecnología Química (UPV-CSIC), Universitat Politècnica de València Consejo Superior de Investigaciones Científicas, Valencia, Spain E-mail: enrico.mugnaioli@iit.it

Structural model of cowlesite by fast electron diffraction tomography

Cowlesite is a natural Ca-rich zeolite with ideal chemical formula Ca6Al12Si18 O60 . 36H2O. The small size of the lath-like crystals and their disordered intergrowth nature hampered proper single crystal X-ray data studies, while all the attempts to solve the structure with powder X-ray diffraction data (proposed orthorhombic 23.3 x 25.0 x 30.6 Å unit cell [1]) failed. The. We studied a powder sample of Cowlesite using the recently developed fast electron diffraction tomography technique [2], in which a sequence of electron diffraction patterns is collected while the crystal is continuously rotated around the sample holder axis. We used this fast procedure since Cowlesite is beam sensitive and long illumination induces amorphization of the sample. The patterns are collected in precession mode and the explored angular range is limited by the stability of the TEM goniometer, which does not guarantee that the investigated crystal stays inside the beam for the whole available rotational range. In order to have an acceptable reciprocal space coverage we stitched together four consecutive data collection from the same crystal. The unit cell we measured is still orthorhombic, 23.0 x 24.4 x 25.4 Å, but the third parameter is significantly shortened. We observe strong streaking along (00l), the direction of shrinking, while in hk0 planes the reflections are sharply defined, suggesting the OD nature of Cowlesite . The (family) structure of the framework can be solved in space group Ccme and it is formed by 2D blocks stacked along [00l] and connected by just four tetrahedra, which leaves a huge cavity in between. Half of the expected Ca atoms and several water molecules are missing. Large 10 and 8 ring channels run parallel to c and [110] directions respectively. The shortened unit cell we found does not index the Cowlesite X-ray powder pattern, therefore we think that the structure partially dehydrates in the high vacuum of the microscope, and the model we obtain describes the dehydrated Cowlesite. We can guess a possible model for the hydrated Cowlesite, which requires a separation of the framework blocks and an interlayer rich of water and Ca atoms. In such a case Cowlesite would be the only example of a natural zeolite possessing a layered structure similar to synthetic 2D zeolites [3].