Archive October 2019

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