Materials, Radiations, Structure

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The MRS (Materials, Radiation, Structure) team gathers researchers from complementary disciplines (physicists, chemists, geochemists, crystallographers), who develop and use cutting edge experimental and methodological tools, mastering the whole experimental process from synthesis, to physical and structural measurements. We share a marked specificity for extreme conditions studies and are strongly involved into CRG instruments at the Grenoble large facilities (ESRF, ILL). The systems under study range from magnetoelectric oxides to intermetallic materials for energy, through semiconductor nanowires, cultural heritage materials and hydrothermal fluids.

Cultural heritage

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Functional oxides

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Heterogeneous materials

Heterogeneous materials are materials with domains that have dramatically different mechanical and physical properties. They can be defined as materials with dramatic heterogeneity in strength from one domain to another. This strength heterogeneity can be caused by microstructure heterogeneity, crystal structure heterogeneity, or compositional heterogeneity. The domain sizes could be in the range of micrometers to millimeters, and the domain geometry can vary to form very diverse material systems.

There are many natural and synthetic heterogeneous materials such as bone, woods, animal tissues, plant cells, sands, soils, multiphase composites, concretes, sandwich structures, foams, and multi-layered structures, etc. They are widely utilized in both structural and functional components in different devices.

Until recently, scientists’ general procedure to understand the chemistry and physics of heterogeneous systems was to study large and complex structures and then examine the fundamental and smaller building blocks of those structures. This approach is called top-down science. However, with the development of scanning probe microscopes allowing the observation of individual atoms and molecules, it became possible to design and build new structures from their atomic-level constituents, one atom or one molecule at a time, i.e., “Materials according to the design”. This ability to carefully arrange atoms offers new opportunities to develop mechanical, electrical, magnetic, and other properties that would otherwise be impossible. We call this the bottom-up approach and the study of the properties of these materials is called nanotechnology.

In the MRS team, we have expertise and the instruments for the characterization of numerous heterogenous materials. A few examples are:

• Characterization of technical (industrial) catalysts for oil industry using X-ray nanoimaging (Click here for more information).
• Characterization of ordinary and ecological cement (Click here for more information).

We also develop novel methodologies for that characterization, as for example:

• Development of Spectral X-ray ptychographic imaging (click here for more information).
• Development of X-ray computed tomographic reconstruction methods for non-destructive analyzes (click here for more information)

Hydrothermal fluids

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In-situ TEM of semiconducting nanowires

We study the structure properties relation of semiconducting nanowires (NWs) using transmission electron microscopy (TEM), optical spectroscopy and electron transport. Additionally, we use electrical in-situ biasing of nanowires in the TEM to learn more about the electrical properties at nm length scales.

For more information about this and people to contact, please click here.

Example of an in-situ experiment where a solid state reaction between an Aluminum contact on a SiGe nanowire is started using Joule heating and studied in-situ in the TEM. (Link to publication: 10.1021/acsanm.0c02303)

Intermetallics and hydrides

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Methodological developments

The different methodological developments at the MRS team:

• Electron crystallography

• Electron holographic imaging

• Fourier ptychographic optical microscopy

• INS, polarized neutrons, magnetic diffraction

• X-ray computed tomography

• X-ray ptychography and spectral ptychography

Electron crystallography

The understanding of the physical properties of a material often requires a thorough knowledge of its structure. However, there is a large number of materials for which the ab initio determination of the crystallographic structure is not possible using powder or single crystal X-ray diffraction, due to the complexity of the powder diffraction pattern, the nanostructured aspect of the materials (nanoparticles, nanowires, thin films, …) or the difficulty to synthesize single crystals with a sufficient size.

During the last decades, the impressive progress of the technical performances of transmission electron microscopes has led to significant advances in physics, material science and life sciences. Thanks to the fabulous progresses in Electron Crystallography since the invention of electron beam precession in 2008 and its use in Precession Electron Diffraction Tomography (PEDT) new methods based on electron diffraction allow to solve ab initio crystal structures by calculating a first structural model and refining it with a dynamical refinement. These techniques are nowadays complementary to X-ray diffraction and/or neutron diffraction and perform on a comparable level. The huge advantage of PEDT is to exploit the specificity of electron diffraction, perfectly adapted to the study of single crystals of a few tens of nanometers in diameter, i.e. whose volume is 10⁶ times smaller than the one necessary for X-ray diffraction on single crystals. In addition, the use of electron diffraction is more favorable than X-ray diffraction for materials with low irradiation resistance, because it is sufficient to apply an amount of irradiation 10³ to 10⁴ times smaller than in X-ray diffraction to obtain the same useful signal.

The combination of these two advantages places electron crystallography at the forefront of techniques suitable for structure determination of many materials, especially beam sensitive materials such as MOFs, zeolites, organic compounds, light elements battery materials.

Low Dose Electron Diffraction Tomography (LD-EDT)

In this context, we have developed an innovative method of low dose electron diffraction tomography (LD-EDT) which requires only a very low irradiation dose (less than 1 e-/Ų) [1] to obtain a data set leading to the structure of a crystal. The electron dose received by the sample is minimized by working with a very weak incident beam and by discarding the beam from the sample during all times other than the recording of the diffraction intensities.

In this method, the crystal is tilted in steps around the goniometer axis and a diffraction pattern representing a slice of the reciprocal space is recorded at each step position. From these slices the 3D reciprocal space is reconstructed and the diffracted intensities are measured.

From the data obtained and using specialized crystallographic software, the structures of different crystals were solved in cases where X-ray diffraction didn’t succeed. Refinement taking into account the dynamical theory of diffraction can be performed on the data and reveals the fine details of the crystals.

This method is particularly efficient for metal-organic frameworks (MOF), zeolites or organic crystals, which are too complex to allow the determination of their atomic structures from X-ray powder diffraction data (large unit cell, low symmetry, large number of atomic positions).

Figure 1: examples of structures solved by LD-EDT

Continuous 3D Electron Diffraction

Another method to prevent the material deterioration under the electron beam during the data collection is to work faster by recording a video in ED mode during the continuous sample holder tilt. The experiment is performed using a single tilt tomography sample holder with a tilt range of -55°/+53°. Once the movie recorded, the individual ED patterns are then extracted from the camera software options, and are processed by the conventional crystallography software consisting in peak identification, 3D reciprocal space reconstruction, intensities extraction and structure calculation. Thanks to this method, the crystal structure of the potential battery material Na₂VO(HPO₄)₂ has been solved (Figure 2) [2].

Figure 2 : Na₂VO(HPO₄)₂ crystal structure

[1] S. Kodjikian and H. Klein, 2019, Ultramicroscopy, 200, 12-19, https://doi.org/10.1016/j.ultramic.2019.02.010

[2] C. Lepoittevin, O. Leynaud, A. Neveu, T. Barbier, M. Gnanavel, V. Gopal, V. Pralong, Dalton Trans., 2021, 50, 9725–9734.

Contacts:

Christophe Lepoittevin

Holger Klein

Stephanie Kodjikian

Electron holographic imaging

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Fourier ptychographic optical microscopy

Fourier ptychography is a variation of the idea of ​​the synthetic aperture microscopy (SAM) method. Instead of scanning the sample, it is illuminated from different angles, and a bright-field image is recorded at each angle. These images are therefore combined to extend the numerical aperture of the microscope objective and thus improve spatial resolution without resorting to fluorescent markers. Fourier ptychography is therefore a super-resolution method which exceeds the diffraction limit and which has the advantage of being a non-invasive method, without marking and with a very wide field of view.
In view of the principle of reciprocity of microscopy, this method is the Fourier counterpart of conventional ptychography, where the locations of the sample plane and the plane of the aperture are effectively switched. This is a bright field imaging approach that uses images generated from illumination at different angles, with overlap in Fourier space of the contrast transfer function to the retro-focal plane of the lens, to create an image with enhanced SBP. This overlap introduces redundancy in the data, which makes it possible to simultaneously obtain images of intensity and phase contrast of the sample, as well as the pupil function of the microscope through an iterative algorithm, derived from that of X-ray ptychography.

Contact: Julio Cesar DA SILVA

INS, polarized neutrons, magnetic diffraction

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X-ray computed tomography

To achieve 3D X-ray imaging, the method typically used is Computed Tomography (CT). Most of the X-ray imaging technique will enable 2D imaging, but in combination with CT, one can obtain 3D images of the sample. The 2D imaging experiment is repeated for each tomographic angle, by rotation the sample typically, over 180 or 360 degrees. By cast the 2D images into tomographic reconstruction algorithms, the volumetric imaging of the sample is obtained. For this reason and as examples, we will call the techniques as follows: Diffraction tomography (diff-tomo), Phase Contrast Imaging (PCI) tomography, holographic tomography, speckle-tracking tomography, Coherent Diffraction Imaging (CDI) tomography, and Ptychographic X-ray Computed Tomography (PXCT). 

Contact: Julio Cesar DA SILVA

X-ray ptychography and spectral ptychography

X-ray Ptychography (also referred as far-field X-ray ptychography) is a coherent diffractive imaging technique capable of providing highly detailed images of a sample’s complex-valued transmittance. A collection of coherent diffraction patterns is generated by scanning across the sample with a spatially confined illumination and sufficient overlap of adjacent illumination footprints. These overlapping illuminations introduce redundancy in the data, which is exploited to simultaneously provide information on specimen and illumination. The spatial resolution is not limited by the incoming beam size, but rather by the highest scattering vector in which speckles can be detected. The ptychographic phase retrieval consists in an iterative algorithm which will reinforce consistency between each scan position and the corresponding diffraction pattern, linked by Fourier transforms, as well as consistency between the adjacent scan positions due to the overlap. This redundancy allows us to simultaneously retrieve the amplitude and phase shifts of the wavefield past the object and the incoming wavefield function (probe).

Near-field X-ray ptychography (NFP) is a variant of far-field X-ray ptychography in the holographic regime which has recently been implemented at the ID16A beamline of the ESRF. NFP relies however on a magnification geometry, in which the sample is positioned in a defocused position downstream of focus of a divergent beam to achieve a given pixel size. Consequently, the spatial resolution will be limited by the focus size upstream the sample, different from the far-field ptychography. However, if the beam focus can be small as in the ID16A beamline, near-field ptychography allows us to measure a larger field-of-view than the far-field version in less time  which is a big advantage for the nanoimaging of large scaled-up materials. NFP is able to retrieve a sample’s complex-valued transmission function from multiple near-field diffraction images, each with a lateral shift of the sample within the 1^st Fresnel Zone and with a structured illumination. The structured illumination brings transverse diversity: i.e., the scattering direction changes at different lateral positions in comparison to the scattering of the object in plane waves. The phase retrieval in NFP is very similar to that of far-field X-ray ptychography, with the difference that the consistency between each scan position and the diffraction pattern is linked by Fresnel transforms rather than by Fourier transforms.

Contact: Julio Cesar DA SILVA

Oxides: magnetism and strongly-correlated systems

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Characterization tools

The different characterization methods at the MRS team:

• Synchrotron Resonant X-ray Techniques, REXS, RXD, and RIXS

Synchrotron Resonant X-ray Techniques, REXS, RXD, and RIXS

We use several x-ray techniques in resonance condition, when the energy of the photons is tuned to an inner energy level of an atom. A substantial increase in scattering can be observed. The resonance consists in the absorption of a photon by a core electron promoted to one unoccupied orbitals sensitive to the local structural, electronic and magnetic structure. If this photoelectron then comes back to its core state, with no energy left behind, the process is said to be “resonant elastic x-ray scattering” (REXS) and can give rise to diffraction. The resonance can also trigger collective or individual excitation among the nearby electrons and the system can instead be left with some excitations. The emitted photon has then less energy, that’s “resonant inelastic x-ray scattering” (RIXS), which gives invaluable informations about the electronic or magnetic interactions. REXS or RIXS are photon-in photon-out scattering techniques, and thus can allow very versatile sample environments, like cryostats, oven, electric and magnetic field applications. Synchrotron facilities are required for tuning the beam to the desired energy. There, the very high flux of photon allows microscopic crystal to be measured, as well as ultrathin films.

Resonant X-ray Diffraction (RXD) is a REXS technique that consists in following the intensity of the diffraction of a crystal when the energy is swept through a resonance. RXD combines x-ray diffraction and x-ray spectroscopy. With this sensitivity, the resonant reflections carry information on the long-range electronic and magnetic states. New diffraction conditions can reveal superstructures invisible by other techniques revealing hidden orders in crystalline materials at the origin of macroscopic properties, electric or magnetic. RXD unraveled and quantified charge ordering in magnetite, and orbital and magnetic ordering in manganites. We perform RXD at D2AM at the ESRF, SEXTANTS at Soleil, I16 at Diamond. We use FDMNES and Dyna for the analysis of our data.

Contact: Stéphane Grenier (contact here)

Resonant Inelastic X-ray Scattering (RIXS) consists in measuring the difference in energy and the change in direction of the emitted photon from the incident photon. The difference in energy corresponds to the energy used by all sort of excitations, charge, magnetic like magnons or for collective vibrations, like phonons. The change in direction indicates the propagation of the excitations relatively to its crystal lattice axes. Mapping the excitations offers a view on the interactions at play in the system. We used RIXS to measure magnons and charge-phonon couplings in high-Tc superconductors and crystal field excitations in multiferroics. We performed RIXS at ID32 at the ESRF and SEXTANTS at Soleil.

Contact: Laura Chaix (contact here)

Software and data analysis tools

• X-ray Resonant Magnetic Scattering, Dyna project

X-ray Resonant Magnetic Scattering, Dyna Project

X-ray Resonant Magnetic Scattering, Dyna project.

Dyna is a simulation and fitting program for x-ray and optical reflectivity and transmittance. Its specificity lies in the account of anomalous, resonant and magnetic resonant effects and it is used for the study of structural, magnetic and electronic stackings in ultrathin multilayers.

Dyna is a collaborative, open and free program, with collaborations at Institut Néel, Soleil synchrotron, and Sorbonne University. Visit the webpage of the projet dyna for documentation, and give it a try !

Contact: Stéphane Grenier (contact here)

Instrumental developments

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Synthesis and crystal growth

The MRS group is engaged in the elaboration of a large variety of materials including compounds with unfavorable thermodynamic stability (metastable phases or with uncommon oxidation states), that range from multi-element oxides, chalcogenides, iron-based pnictides, intermetallics and hydrides, molecular framework solids or even composites. This activity benefits from a large panel of techniques available on-site for the design of single-crystals, ceramics or nano-materials in collaboration with three technical groups at Néel (“X’Press”, “Cristaux massifs” and “ThEMA”).

• solid-state reactions routes with glove-box for manipulation in water/O₂-free conditions combined to thermal treatments in inert or reactive atmosphere: from vacuum to controlled gas mixtures and 1600°C maximum temperature

• solution-based techniques under milder conditions including coprecipitation, polymeric and/or auto-combustion routes; colloidal chemistry

• crystal growth by chemical vapor transport

• vacuum induction melting for reactive metals and alloys

• pressure-assisted synthesis with fondant or oxidizing agent, using two large volume presses reaching 7 GPa and 1200°C; possibility for a preliminary screening of the suitable (T,P) conditions from in-situ powder x-ray diffraction combined to Paris-Edinburgh press or in-house designed pressure cell with independent control of temperature and pressure

• high-energy mechanical milling and melt spinning

• hydrogenation post-treatment through solid-gas reaction, possibly combined to severe plastic deformation processes to change sorption properties

Studies under extreme conditions

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Large-scale facilities

Large-scale facilities