Surfaces, Interfaces and Nanostructures

General Scope

One of our key motivating factor is to investigate and to understand the new properties (and potentially, new functionalities) of low-dimensional systems, arising from size reduction.

We therefore study the structural, magnetic and electronic properties of various systems, such as bulk materials, thin films, nanostructures, surfaces and interfaces (solid/gas and solid/liquid). Besides, we also develop theories for the ab initio simulation and the understanding of experiments related to X-ray absorption and scattering processes.

Synchrotron techniques and the development of original experimental set-ups are among our core activities, frequently coupled to other laboratory tools (magnetometry…). We built several instruments and ensure their joint management at synchrotrons: a resonant X-ray diffractometer at SOLEIL and an UHV diffractometer at ESRF. In laboratory, we run a variable temperature STM and a reactor for operando catalysis.

Our main research topics are :

• Nanosystems for magnetism: fonctional magnetic oxyde films, configuration and interfaces of magnetic heterostructures and, skyrmions

• (Electro-) Catalysis and nanostructures for energy: catalysis by gold, 2D arrays of nanoparticles, electrochemical interfaces and, TMO films for energy

• Theory of X-ray Spectroscopies:  to develop an ab initio computation code which simulates the experimental spectroscopies characterizing the materials

Functional Magnetic Oxide Films

Permanent staff: Maurizio De Santis, Farid Fettar, Jean-Marc Tonnerre

Students: Vartika Bansal (PhD), Intan Kusumaningrum

*************** Under construction ***************

Magnetic Heterostructures (Configuration & Interfaces)

Permanent staff: Jean-Marc Tonnerre

*************** Under construction ***************

Skyrmions

Permanent staff: Farid Fettar

       Skyrmion generation needs also multilayer structures such as Ta/[Fe/Gd]_n/Ta. They are model systems for topologically protected spin textures, emphasizing the role of topology in the classification of complex states of condensed matter. We want to develop forefront aspect of this research field by developing and charactarizing transition-metal-based magnetic multilayer structures that support skyrmionic states at room temperature and allow for the precise control of skyrmions. We will use Fourier Transform Holography methods, in collaboration with the SEXTANTS beamline (SOLEIL) to investigate their generation. Our objectives are: (i) study of correlation of skyrmionics phases with magnetisation using anomalous Hall effect under controlled magnetic field and at room temperature; (ii) topological structural stabilisation of skyrmionics phases (different geometry of the Holography sample aperture) ; (iii) tuning Dzyaloshinskii–Moriya Interaction (DMI, different bottom and top non-magnetic layers).

Collaborations

• Strains, disorders and anisotropies in strongly-correlated materials
  

Jean-Paul Itié, Synchrotron Soleil, France

Alberto Caneiro, CEA Bariloche, Argentina 

Nestor Massa, Univ. Nacional La Plata, Argentina

Antonio Alonso, Instituto de Ciencia de Materiales de Madrid

Cinthia Piamonteze, Advanced Light Source, Berkeley, USA

• Magnetic metals and alloys
  

Flávio Garcia, LNLS, Campinas, Brazil

Alessandro Martins – Universidade Federal de Goiás, Brazil

Antônio D. Dos Santos, Depto de Física Materiais e Mecânica, IF- USP, Sao Paulo, Brazil

Pascal Andreazza et Caroline Andreazza-Vignolle, CMRD Orleans

(Electro-) Catalysis and Nanostructures for Energy

        Synthesizing model catalysts with a controlled architecture is a promising way for understanding the respective influence of atomic-scale parameters (crystallographic structure, morphology, chemical composition…) on their catalytic properties.

Our approach is twofold:

• either the evolution of the model catalyst is studied operando during a chemical reaction in gas phase (i.e. gas/solid interfaces),
• or the catalyst is the working electrode in an electrochemical cell and the electrode/electrolyte interface is studied while a voltage is applied in situ (i.e. liquid/solid interfaces).

The scientific approach is similar in both cases and intends to give a detailed description of the model catalysts behaviour in semi-realistic conditions. It implies to control their growth and finely characterize the interfaces with the gaseous or liquid medium. The model catalysts consist in thin metallic films or nanoparticles of noble metals deposited on another noble metal or an oxide single-crystal. Complementary synchrotron X-ray-based techniques, such as Grazing Incidence X-Ray Diffraction (GIXRD) and Grazing Incidence Small-Angle X-Ray Scattering (GISAXS), are used to determine structural and morphological parameters.

Catalysis by Gold

Permanent staff:   Marie-Claire Saint-Lager, Aude Bailly, Yvonne Soldo-Olivier, Stéphanie Garaudée (Epitaxy and layer deposition technological group)

Former PhD student:   Antoine Abisset

       Harvesting solar radiation, a clean and inexhaustible source of energy, is a key challenge. Semiconductors have been intensively studied for their capacity to absorb photons and create active electrons and holes able to trigger the reduction or oxidation of chemicals. Unfortunately, the response of the most active materials, like TiO₂, is restricted to the ultraviolet (UV), therefore limiting the possibility to efficiently use sunlight (~ 5 % UV, 45% visible, 50% IR). Several ways have been explored in order to overcome this limitation, but efficiency and stability still remain challenging. A promising recent approach takes advantage of the unique ability of some noble metal NPs (like Au) to absorb light at specific wavelengths across the visible part of the electromagnetic spectrum via localized surface plasmon resonances (LSPR). A strong enhancement of the photo-reactivity under visible light is observed when these NPs are supported on a semiconductor photocatalyst. Interestingly, a modulation of the plasmonic response and hence of the photocatalytic behavior can be achieved by playing with the NPs features (composition, shape, structure, interparticle distance, environment). In this context, our innovating approach is to consider Au-based NPs supported on well-defined substrates (model systems) and to characterize their structure, morphology and LSPR properties using operando optical and synchrotron X-Ray techniques.

We studied Au NPs supported on a stoichiometric TiO₂(110) single crystal by following operando, during their growth, the optical response with Surface Differential Reflectivity Spectroscopy (SDRS) and the structural/morphological properties (size, shape, lattice parameter…) with Grazing Incidence X-Ray Diffraction (GIXRD) and Grazing Incidence Small Angle X-Ray Scattering (GISAXS)[1]. Experiments were made at the SIXS beamline at SOLEIL synchrotron source, equipped with a Molecular Beam Epitaxy apparatus for NP deposition in Ultra High Vacuum conditions and SDRS measurements were made thanks to our home-made setup, specially transported and installed.

Typically, the position of the plasmon peak in S polarization (fig. 1) blue-shifts when the NP size decreases, with the same linear dependence not only between our samples, but also compared to Au NPs embedded in Al₂O₃. This suggests that the underlying quantum size effect (balance between s-electrons spill-out and diminishing of d-electrons screening for surface atoms) is an intrinsic property of the NPs.

Despite this similarity, our data clearly show the large influence of the surface state on LSPR. The red shift for a definite D size in the case of S2 is the signature of a weaker NP-substrate interaction, pointed out also by our GIXRD data, possibly due to the presence of a larger number of impurities/steps for this last sample. Also, P polarization signals (not shown here) indicate that plasmonic modes perpendicular to the surface, negligible for S1, become significant for NP smaller than about 4 nm in the case of S2. We interpret this observation as due to different growth mechanisms (nucleation sites, shape…) on S2. Adding of Pd even in small quantities largely modifies plasmonic response, which completely disappears in alloyed Au₇₀Pd₃₀ NPs.

[1] Thesis of Antoine Abisset, defense June 26, 2018.

2D Arrays of Nanoparticles

Permanent staff:   Aude Bailly, Maurizio De Santis, Marie-Claire Saint-Lager

Active collaborations:

• Georges Sitja, Claude R. Henry (CINaM, Marseille)

• Aimeric Ouvrard (ISMO, Orsay)

• Natalia Alyabyeva (CEA, Saclay)

• Rémi Lazzari (INSP, Paris)

           Nanostructured substrates are used in order to grow arrays of ordered metallic nanoparticles for applications in various fields, such as catalysis, molecular electronics and magnetism. A robust template consists of a thin alumina film grown by direct oxidation of a Ni₃Al(111) single-crystal surface. The metallic nanoparticles are subsequently grown by physical deposition and tend to be nicely ordered on the whole substrate surface.

The topics presented hereafter deal with this specific oxide template, but differ by the nature of the metallic species forming the nanoparticles.

       Producing arrays of metallic nanoparticles regularly distributed on a substrate and with a sharp size distribution is a major challenge for understanding the macroscopic properties (catalytic, magnetic, optical and so on) of the nanoparticles. These tailored systems allow to better disentangle the effect of each parameter (structural, morphological or chemical) on a given property, by fixing some characteristics in a selective and controlled way.

Through a long-standing collaboration with the CINaM, we acquired a strong expertise in synthesizing and characterizing ordered arrays of (bi)metallic nanoparticles with a focus on their catalytic properties. Over the last years, we were mainly interested in Pd-Au nanoparticles studied by combining in situ GISAXS and GIXRD during their growth.

GISAXS principle and image obtained for a 0.1 ML Pd deposit on Al₂O₃/Ni₃Al(111)

Related paper: A. Bailly et al., Influence of Palladium on the Ordering, Final Size, and Composition of Pd−Au Nanoparticle Arrays, J. Phys. Chem. C 121 (2017) 25864. DOI: 10.1021/acs.jpcc.7b08254

More recently, we also get involved in the in situ characterization of Pd-Ag nanoparticles arrays used as templates for molecular electronics. This work was performed through the ANR JCJC « LEMON » project (PI: A. Ouvrard). The systems are characterized by a combination of GISAXS/GIXRD, STM, DRS and SFG in order to get a thorough description of their structural, morphological and optical properties. These ordered nanoparticles are then intended to be cross-linked by organic molecules.

Electrochemical Interfaces

Permanent staff: Yvonne Soldo-Olivier, Maurizio De Santis, Yves Joly

Collaborations: Eric Sibert (LEPMI)

       Electro-catalysts allow speeding up electrochemical reactions typically occurring at the electrochemical interface, singular domain of some Ångstrom of thickness where the charge exchange between the conducting electrode and the electrolyte occurs. Such materials have important applications in several domains, like energy storage, chemical synthesis, bio-sensors… In this context, the description of the electrochemical interface and the comprehension of the related electro-catalytic mechanisms are of primary importance. It is first and foremost a question of material of electrode and more specifically a problem of structural and electronic properties of its surface.

We investigate the electrochemical growth of ultra-thin metallic films deposited onto single crystals (for instance, Pd/Pt(111), Pd/Au(111)…) representing a model system for fundamental studies of important reactions such as CO oxidation, oxygen reduction, or hydrogen storage processes. We perform in situ Grazing Incidence X-Ray Diffraction (GIXRD) studies at the D2AM beamline (ESRF), using dedicated home-made cells adapted both to synchrotron experiments and to electrochemistry. This technique is an excellent tool to describe the growth, structure and morphology of electro-deposited films, highlighting the influence of the chemical nature, structure and orientation of the substrate.

Very recently, we focused our attention on in situ Surface Resonant X-Ray Diffraction (SRXRD) spectroscopy. Indeed, there is currently no experimental method to specifically probe the electronic structure of the surface directly under electrochemical operation. In this frame, we aim at developing SRXRD, coupled to theoretical description using the recent extension of the FDMNES software, into a standard technique to probe charge distribution and electronic densities for electrochemical systems in in situ conditions.

TMO Films for Energy

Permanent staff: Aude Bailly

           This research topic is focused on establishing the relationship between the structure of oxide films and their macroscopic properties (magnetic, electronic or optical). It relies on the long-standing experience we have for the controlled synthesis of such systems by MBE (Molecular Beam Epitaxy) and their consecutive in situ or ex situ characterization using laboratory and synchrotron techniques. Typical surface science methods, such as LEED, STM and AES, are complemented by atomically and chemically-resolved synchrotron X-ray-based techniques. Recently, we also took advantage of the capabilities of a new laboratory X-ray diffractometer (SmartLab, Rigaku) to study the crystallographic structure of epitaxial films of VO₂ deposited on Al₂O₃(0001), in collaboration with the MRS team.

• • VO₂ films for smart devices

Collaborations:

• @ Institut Néel: Aline Ramos, Stéphane Grenier, Lætitia Laversenne, Pierre Bouvier, Laurence Magaud

• Mohamed Chaker (INRS, Varennes, Canada)

• Former post-doc students: Michelle M. Villamayor and Michael Gaudin

In bulk form, vanadium dioxide (VO₂) exhibits a first-order insulator-metal transition (IMT) at 68°C. This transition temperature, called T_MIT, can be tuned by strain engineering, either by growing epitaxial VO₂ films on various susbtrates or/and by elemental doping.

This research topic benefits from a complementary approach, coupling High-Resolution X-ray Diffraction in laboratory, Raman spectroscopy, X-ray absorption spectroscopy (XANES, EXAFS) and DFT calculations. These techniques are used to characterize the structural and electronic properties of the VO₂ films across the IMT.

We aim at understanding the effect of strain, resulting from domain-matching epitaxy or from the inclusion of a dopant element, on the shift of  temperature hysteresis. In this context, pure VO₂ films epitaxially grown on Al₂O₃(0001), a.k.a. c-cut sapphire, are studied, as well as W- and B-doped VO₂ films on c- and r-Al₂O₃.

• • KBNNO ferroelectrics ceramics

Collaborations: (Institut Néel) Aline Ramos, Céline Darie

A new research direction towards inorganic perovskite thin films of ferroelectric materials for energy-related applications. These low-cost, non-toxic and chemically stable films are attractive as potential building blocks for photovoltaic devices. We will focus on systems based on solid solutions of classical ferroelectric perovskites, such as KNbO₃, in order to tune the bandgap to a specific part of the solar spectrum. This topic will benefit from the local resources we have in sample synthesis, as well as laboratory and synchrotron X-ray facilities.

Theory of X-Ray Spectroscopies and the FDMNES project

Permanent staff : Yves Joly, Yvonne Soldo-Olivier
Former PhD student: Oana Bunau

The purpose of this theme is to improve the understanding and the quantitative analysis of the X-ray spectroscopies as a probe of the electronic and structural properties of the materials. Most often recorded at synchrotron, x-ray absorption spectroscopy, x-ray emission spectroscopy, elastic and inelastic resonant scattering, among other techniques, need the development of tools following the improvements of the experimental capacities. It is the purpose of  the FDMNES project

Its aim is to supply to the community a user friendly ab initio code to simulate all these spectroscopies.
FDMNES is mainly mono-electronic and uses the density functional theory (DFT). It includes also multi-electronics advances with the use of the time dependant DFT (TD-DFT) for a better taking into account of the excited states linked to the photon-matter interaction.

More information and download at: http://fdmnes.neel.cnrs.fr

Overview

       Our team is involved in several instrumental developments linked to synchrotron radiation facilities (ESRF and SOLEIL) and implying Ultra High Vacuum (UHV) technologies:

1. a surface X-ray diffractometer for in real-time studies during growth (INS2 instrument on the CRG-IF beamline, ESRF)
2. a reflectometer dedicated to SXRMS measurements (RESOXS instrument on the SEXTANTS beamline, SOLEIL)
3. a X-ray reactor allowing operando measurements on model catalysts

In laboratory, we manage a variable-temperature Scanning Tunneling Microscope (VT-STM).

Besides, the main techniques we use are:

1. for growing samples: Molecular Beam Epitaxy (MBE), Sputtering
2. at synchrotrons: Grazing Incidence X-ray diffraction, Grazing Incidence Small Angle X-ray Scattering, X-ray Resonant Magnetic Reflectivity, XANES, EXAFS, XMCD (X-ray absorption spectroscopy, near the edge, extended, and with circular polarization on magnetic sample), Surface and bulk X-ray Resonant Diffraction, Ambient Pressure X-ray Photoelectron Spectroscopy
3. in laboratory: STM, Surface Differential Reflectivity Spectroscopy, Low Energy Electron Diffraction, Auger Spectroscopy, X-Ray diffraction and reflectivity, Mass spectrometry
4. for magnetometry: Vibrating Sample Magnetometry, SQUID, Extraordinary Hall Effect, Resistivity measurement

UHV Diffractometer for In Situ Studies

Permanent staff: Maurizio De Santis, Lucio Martinelli (CRG technological group)

Collaborations: Xavier Torelles, Institut de Ciencia de Materia, Barcelona, Spain

X-Ray Resonant Magnetic Scattering

Permanent staff: Jean-Marc Tonnerre

Collaborations:  Stéphane Grenier (MRS team)

Reference for instrument: N. Jaouen et al., J. Synchrotron Rad. (2004). 11, 353-357

Dynamic X-ray reflectivity (off- or on- resonance regime for isotropic, anisotropic and magnetic multilayers) simulation program » or « Dyna… » is a simulation tool to analyze structural, magnetic and electronic profiles along the growth direction of ultrathin layers. It simulates conventional X-ray or optical reflectivity, resonant (or « anomalous ») x-ray reflectivity, « orbital reflectometry », magnetic resonant X-ray reflectivity with applications to magnetic or anisotropic layers, in hard or soft matter.

More information and download are possible from the MRS team web page.

X-Ray Reactor for Catalysis

Permanent staff: Marie-Claire Saint-Lager

It is difficult to characterize industrial catalysts (usually nanoparticles supported on oxide powders) at the atomic scale. Thus the fundamental research in heterogeneous catalysis, aiming at understanding the catalytic processes at the molecular scale, is performed under UHV using model catalysts such as single-crystal surfaces. The recent development of surface science tools, such as surface X-ray diffraction and GISAXS, allows to overcome the two limits of this approach which are the so-called « pressure gap » (studying catalyst surfaces in pressure and temperature conditions close to the ones of the reaction) and the « material gap ».
This reactor project is inserted in this very active research field. The aims are:
(i) to follow the surface structure, composition and morphology of the model catalysts from the UHV environment where they are elaborated up to the reaction conditions (atmospheric pressure and temperature above the room temperature)
(ii) to extend the study from single-crystal surfaces to thin epitaxial films and to nanoparticles supported on oxide.

The experimental set-up is composed of two main parts: (1) the reactor itself with beryllium windows for X-ray experiments. It is equipped with pumping systems and suitable pressure measurements for UHV up to atmospheric pressure, a reactive gas input and a mass spectrometer. The connection to the GMT diffractometer is ensured via a goniometer head for high-accuracy sample alignment. It is also equipped for UHV preparation of single-crystal surfaces (Ar+ sputtering and sample annealing). (2) the choice of a vertical surface allows a simple transfer from a UHV preparation chamber, equipped with the usual tools of surface science (LEED, AES, evaporation sources, …). It takes also benefit from the direction with the smallest natural divergence of the x-ray beam delivered by a bending magnet: Bragg’s peaks of the (vertical) surface plane are studied with the best angular resolution and the maximum photon flux (slits opened).

The first experiment to check the sole reactor part was performed in January 2005. It has confirmed and completed the results previously obtained on ID03 dealing with the selective butadiene hydrogenation on Pd8Ni92(110). The whole system (reactor + preparation chambers) should be checked in march 2006 in GMT.

UHV-STM

Permanent staff: Maurizio de Santis, Aude Bailly

Collaborations: Véronique Langlais

VT-STM (right) and its UHV preparation chamber (left)

Under progress

Working with us

Below is the list of topics proposed by the team for internships, as well as positions with a readily-available funding (PhD, Post-doc and so on).

Sorry ! No job offer at present.