Seminar: “Coupling Bragg Coherent Diffraction Imaging (BCDI) and Molecular Dynamics to investigate nanostructure “

Fig. 1 (top) Experimental reconstruction of the u111 displacement field on a 250 nm Pt NP (bottom)  u111 displacement field obtained by energy minimization of a simulated Pt NP (right) εxx, εyy and  εzz components of the strain tensor derived from the simulation

Physical properties at small length scale deviate strongly from the bulk counterpart, typically below the micrometer. For instance, mechanical strength increases with reducing size, large residual strain due to processing are present in nanostructures. Thus a better understanding of the physical properties in relationship with the microstructure is needed for nanoscale materials. Because of its good spatial resolution (~ 10 nm) and excellent sensitivity to atomic displacements and local strain [1,2], Bragg coherent diffraction imaging (BCDI) has emerged in the past two decades as a powerful tool to probe the structure and local displacement field inside nanoscale objects [3]. When combined with in situ mechanical loading, BCDI is particularly relevant for the study of defect nucleations in isolated nanoparticles [4] or to investigate intragranular deformation mechanisms in polycrystalline thin films [5].

Nowadays, the length scales that are accessible by BCDI and that can be simulated by Molecular Dynamics (MD) simulation are almost converging. The coupling between the two methods is therefore particularly relevant and allows to get a detailed picture of the deformation mechanisms in nanostructures at the atomic scale. This coupled approach has been used to study the surface relaxation of metallic nanoparticles (Au, Pt). An excellent quantitative agreement is obtained between the component of the displacement field measured experimentally and calculated by energy minimization (Molecular Statics) (Fig. 1). With this approach, the measurement of only one Bragg reflection is required to derive the 3D displacement field and the six independent components of the strain tensor from the simulation [6]. The two techniques can also be combined to  identify defect structures nucleated during in situ  mechanical loading [4,5] and to interpret the evolution of the strain field in nanoparticle catalysts during gas reaction [7,8]..

[1] Watari, M. et al. Nature Materials 10, 862–866 (2011).

[2] Labat, S. et al. ACS Nano 9, 9210–9216 (2015).

[3] Robinson, I. & Harder, R. Nat Mater 8, 291–298 (2009).

[4] Dupraz, M. et al. Nano Lett. 17(11) (2017).

[5] Cherukara, M. et al. Nat. Comm. (2018).

[6] Dupraz et al.  to be submitted (2019)

[7] Kim, D. et al. Nat. Comm. 9, 3422 (2018).

[8] Dupraz, M. et al. in preparation

Speaker: Dr. Maxime Dupraz

Date and Location: Monday 25/11/19, 14h00 LEM meeting room (E2.01.20), Châtillon.

Modelling the mechanical behaviour of polycrystalline materials

Understanding the deformation processes that lead to the failure of polycrystalline structural materials is one of the main challenges in materials science. Significant progress has been made in recent decades, thanks to the development of new experimental characterisation techniques and advanced simulation methods.

However, the localisation of plasticity in slip bands and the propagation of plasticity through a polycrystalline aggregate are not fully understood.

One of the difficulties in modelling the mechanical behaviour of polycrystalline materials is its intrinsically  multi-scale nature. Inelastic deformation mechanisms occur at the dislocation scale (dislocation is a crystalline defect ,vector of plastic deformation) that result in the formation of intra-granular microstructures which, in turn, interact with grain boundaries.

This internship project aims to study the first stages of plastic deformation in policrystalline materials (Cu, Ni) using dislocation dynamics (DD). In particular, the microMegas DD code coupled with a mechanical solver will be used, to correctly handle boundary conditions,  to capture the onset of    plastic deformation localisation and thus model the interaction between a slip band and  grain boundaries. 
This mechanism will be studied in a “model” polycrystalline aggregate, and then, if possible,  extended to a digital twin of a real polycrystal.

 

Job: Internship (4-6 months)

Academic level : Master degree

Location: LEM, Châtillon

Expertise: : Solid state physics, Materials Science. Interest in theoretical physics and numerical simulations

Contacts: Benoit Devincre

From molecular to Langevin dynamics

Most of the physical mechanisms which control material evolution can be described with kinetic based approaches at the atomic scale: phase transformations, diffusion of vacancies, deformation, dislocation nucleation and propagation, microcavities, microcracks. Classical methods like molecular dynamics are however limited by high frequencies phonons. Currently, the time scale reached by these methods is of the order of several nanoseconds. Different approaches have been investigated to go beyond this limit (Diffusive Molecular Dynamics, ART Monte Carlo, Phase Field Crystal)           but neither emerged as being clearly the most appropriated. In that context, we would like to develop a new approach, still at the atomic scale, but based on a Langevin dynamics, which is of first order in time. This is equivalent to remove kinetic energy. The main interest is to keep a discrete description of matter and a continuous description of atomic positions without having to follow fast oscillations caused by phonons. Accessible time scales should be therefore several orders of magnitude larger than those reached by classical methods. A noise term must be however introduced in the kinetics, and has to be controlled to guaranty the convergence towards to the thermodynamical equilibrium. First results tend to show that this method is promising to study materials subjected to different thermal and mechanical loadings.

The main objective of the present work is to validate the conditions in which the Langevin dynamics correctly reproduces kinetics at the atomic scale. Using molecular dynamics codes, the first step will be to identify concentration and temperature regimes where the time scales between atomic vibrations and thermally activated processes are separated. Then, a numerical method which proceeds to the change of scale will be developed to derive the correct Langevin equations.

Job: Internship (4-6 months)

Academic level : Master degree

Location: LEM, Châtillon

Expertise: : Solid state physics, Materials Science. Interest in theoretical physics and numerical simulations

Contacts: Mathieu Fèvre, Alphonse Finel

Elaboration and characterization of metallic nanoparticles; analysis of synthesis effects in the growth of carbon nanotubes by CVD

The proposed internship topic is part of a cooperative project on the development of new catalysts (metal or bimetallic nanoparticles) for the growth of carbon nanotubes, aiming at controlling their electronic properties during their synthesis.

The nanoparticles as catalysts will be synthesized, in collaboration with the partnering group of Pr V. Huc at the Institute of Molecular Chemistry and Orsay Materials (ICCMO) by combining surface chemistry and coordination chemistry. We propose to compare the same set of metallic or bimetallic nanoparticles synthesized using very different synthetic pathways and then we will compare the catalytic action of these nanoparticles in the growth of nanotubes.

We will focus on the Ni-Ru catalytic system, identified as a good candidate for achieving the desired electronic character selectivity.

– A first part of the experimental work will be to synthesize the nanoparticles using soft chemistry via the colloidal route (optimization of the synthesis by first reducing the Ru3 + in Ru2 + and then combining it with Ni) and Prussian Blue Analogues at ICMMO. The latter will be calcined under argon in order to obtain carbureted species. We will do the structural study (size distribution, composition, crystalline structure) at LEM. For the latter, we will use a powerful set of investigation techniques (high-resolution transmission electron microscopy, diffraction, energy loss spectroscopy and X-ray spectroscopy) present in the laboratory (LEM).

– A second part of the experimental work will be to grow carbon nanotubes by CVD using the nanoparticles developed as a catalyst (colloidal, by “Prussian blue” and Prussian blue “carbides”) and to characterize their structures (chirality, length, type of adhesion to the nanoparticle) by TEM (imaging and diffraction) and Raman spectroscopy. These three sets of catalysts should be compared to observe the influence of the synthesis route and the influence of the carburized phase on the growth of carbon nanotubes.

Job: Internship (4-6 months)

Academic level : Master degree

Location: LEM, Châtillon

Expertise: Good training in condensed matter physics and chemistry with a major focus on nanoscience and courses on synthesis and characterization.
Strong interest for experiments

Contacts: Armelle Girard, Annick.Loiseau

Chemistry and morphology of nanoalloys for growth catalysis of carbon nanotubes by CVD

The proposed internship topic is part of a cooperative project on the development of new bimetallic catalysts in the solid solution state for the growth of carbon nanotubes, aiming at controlling their electronic properties during their synthesis.

Although several techniques are available for the synthesis of transition metal-based bimetallic catalysts, they generally lead to nanoparticles with a core/shell or janus morphology. Nevertheless, our previous studies have shown that it is possible to synthesize bimetallic particles in solid solution state, that is with no elemental segregation within the nanoparticle by combining surface chemistry and coordination chemistry under particular temperature conditions.

However, studying the thermodynamic behavior of these bimetallic catalysts often requires the implementation of complicated experimental techniques that often need to be coupled with theoretical approaches, even if the latter are still far from being able to reach such a level of complexity while remaining predictive. In collaboration with the partnering group of numerical simulations at the Institute of Molecular Chemistry and Orsay Materials (ICCMO) (Jérôme Creuze and Fabienne Berthier), we will undertake theoretical study to better characterize the behavior of these nanoparticles, in equilibrium and under ultra-vacuum first. Indeed, it is necessary to know the thermodynamics of nanoalloys as isolated systems before studying the influence of external perturbations. We will also study the kinetics of return to equilibrium in order to determine the stability and the lifetime of metastable configurations that will have been identified during the first step of the study.

The internship will thus identify and quantify the key thermodynamic parameters involved in the distribution of constituents within the bimetallic nanoparticles and understand how the thermodynamic variables influence the equilibrium configuration using these parameters. The candidate will be able to compare his results with experiences when possible.

Job: Internship (4-6 months)

Academic level : Master degree

Location: LEM, Châtillon

Expertise: Good training in condensed matter physics and chemistry with a major focus on nanoscience, thermodynamics. Strong interest for numerical calculations

Contacts: Armelle Girard, Annick.Loiseau

 

Innovative nanomaterials in real environment for nanoelectronics applications

Carbon nanotubes (NTs) can be synthesized at medium temperature (T~700°C) by decomposing a carbonaceous gas on the surface of a metal nanocatalyst (Fe, Co or Ni). Despite significant progress over the past 25 years, precise control of their structure during synthesis remains a major challenge for nanoelectronics applications. In recent years, the use of bimetallic catalysts (CoW, Mo2C,…) seems to be the most promising way forward since quite spectacular selectivities towards the chirality of NTs have been observed without being fully understood. Among the assumptions, it has been proposed that the presence of an alloying element with a high melting point tends to keep the particle solid during synthesis. Under such conditions, the catalyst structure has facets that allow direct control of the tube structure by epitaxy. Although elegant, this interpretation is subject to many controversies since no precise experimental study can determine the state of the catalyst during synthesis.

The objective of this internship is therefore to focus on the synthesis of nanoparticles (NPs) in a perfectly defined and controlled structural state in order to achieve a real manufacturing engineering of controlled structure. For this, we will focus on the structural temperature study of AgPt NPs where Pt, with a high melting point, will be the key element to keep the particle in a solid state. The first step will be to optimize the conditions of physical synthesis (pulsed laser ablation) to manufacture AgPt NPs of controlled structures (size, morphology and composition). Then, the structural evolution at high temperature of NPs will be carried out. More precisely, the structural study of AgPt NPs (alloy formation, phase separation, order/disorder transition) at different temperatures will then be possible where different microscopy techniques will be coupled (high resolution, diffraction, X-ray energy dispersion, etc.). In collaboration with the MPQ laboratory for the experimental part, all the results will be compared with atomic-scale numerical simulations developed at LEM and CINaM in Marseille.

Job: Internship (4-6 months)

Academic level : Master degree

Location: LEM, Châtillon

Expertise: Good level of knowledge of condensed matter physics (thermodynamics, electron-matter interaction,…) with a strong interest in conducting advanced experiments in interaction with numerical simulations. Scientific exchanges, in particular within the framework of collaborative structures in which LEM participates (various GDRs, ANR projects,…), will be encouraged.

Contacts: Hakim Amara



Development of a dynamic Monte Carlo code for atomic diffusion calculations

Diffusion processes in solids are relevant for the kinetics of many microstructural changes that occur during preparation, processing, and heat treatment of materials. Typical examples are nucleation of new phases, diffusive phase transformations, precipitation of a second phase, recrystallisation, high-temperature creep, and thermal oxidation. To reach a deep understanding of diffusion in solids, one needs information on the position of atoms and how they move in solids. The atomic mechanisms of diffusion in crystalline solids are closely connected with defects. Point defects such as vacancies or interstitials are the simplest defects and often mediate diffusion in crystals. Ab initio methods as DFT (Density Functional Theory) can provide fundamental information, such as the stable positions of atoms in a crystal lattice and their jumping rates between two neighour sites. However, it is not trivial to obtain diffusion coefficients from these fundamental properties because, in complex solid crystals, there are usually various point defects (vacancy and interstitial positions) and hence several diffusion paths are possible for the diffusing atom. Analytical solutions of multi-state diffusion problems are generally complex. A good alternative is to resource to Kinetic Monte Carlo (KMC), which is a particular Monte Carlo method used for processes with known rates such as atom migration. It consists of mapping N possible events that can occur from a given state. Each event is defined by jump frequency, displacement and cumulative function of the jump frequency: all these input quantites can be obtained from the DFT calculations.

The main aim of this project is to develop a Kinetic Monte Carlo code for studying diffusion of atoms in crystals. The code will receive as input data the results of DFT calculations, which will be performed with the VASP code. The first applications will address the diffusion of interstitial atoms (B, O, N, C) in Ti-Al alloys. The code will be validated by comparison with the analytical solution of the simplest crystal lattices (as the tetragonal TiAl) and then it will be applied for the study of diffusion in more complex geometries (as the hexagonal Ti3Al).

Electronic properties of nanostructured thin films

The control of the composition and morphology of materials at the nanoscale allowed to disclose novel structural, electronic and chemical properties which are fundamental for many recent technological advances. Amongst nanostructures, 2D materials are a class formed of materials which cryistallise as atomically-thin layers. Since the discovery of graphene in the early 2000s, the family of 2D materials grew larger, with the emergence of new systems alike the hexagonal boron nitride (hBN) or the black phosphorus (BP).

Because of their extreme thinness, 2D materials often display electronic properties sizeably different from those of their bulk equivalent. Moreover, their characteristics are strongly influenced by the interaction with the near surroundings: for instance by modifications of the substrate, or changes of their thickness. Van der Waals heterostructures are based on this principle. They are built by stacking layers of different 2D materials on top of each other, so that several properties are combined in the same system and tuned in a controlled fashion. This allows to engineer specific properties aimed for technological development or fundamental research.

In this context, we will consider heterostructures based on hBN and/or BP layers. In order to study these systems from a theoretical perspective, we will elaborate a mixed approach combining analytical and numerical developments in the tight-binding formalism, with ab-initio simulations. The latter will be done on simple reference systems, with the intent to establish a quantitative basis for the parametrization of tight-binding models. This will make possible the investigation of extended systems like realistic heterostructures. More precisely, the objectif will be that of studying the influence of the environment (substrate, stacking …) on the electronic and optical properties of van der Waals heterostructures based on hBN and BP.

Another specificity of this work will consist on coupling the theoretical study with diverse experimental techniques, namely thanks to our rich collaboration network.

Structural and electronic properties of nanomaterials under deformation

At the nanoscale, the structural properties of materials (0D/1D/2D)  can be difficult to predict because they involve long-range interactions that cannot be avoided. Many examples are discussed in the literature such as the grafting of molecules on a graphene sheet, inter-plane coupling in 2D heterostructures or in multiwall carbon nanotubes, the adhesion of catalytic nanoparticles, etc. In this context, we wish to implement a multi-scale approach by coupling atomic scale modelling (empirical, semi-empirical or ab initio modelling) with a continuous approach via finite element calculations (FE) to better understand the electronic modifications induced by nano-objects under stress.
The aim of this internship is to apply this methodology to self-assembled silicon nanostructures as a first application. The work will be done in two stages. First of all, it will be necessary to characterize the structural properties of the systems under consideration, thanks to the development of our multi-scale approach involving the transition from an atomistic description to a mechanical framework of continuum media to take into account the FEs of long-range interactions that drive the effects of self-organization. Subsequently, the electronic properties of these systems will be characterized by combining ab initio calculations and a tight-binding model (order-N method) perfectly adapted to handle large systems (104-105 atoms) where long-range elastic effects are dominant. 
It should be noted that this research activity will benefit from the joint development of unique skills present in the laboratory in the field of low-dimensional objects and the mechanical behaviour of materials.

Job: Internship (4-6 months)

Academic level : Master degree

Location: LEM, Châtillon

Expertise: Solid state physics, Materials Science. Interest in theoretical physics and numerical simulations (coding, data post-processing).

Contacts: Riccardo Gatti, Hakim Amara