Description du sujet de thèse

Key words. 2D materials: graphene, hBN, dichalcogenides, lateral/vertical heterostructures, beyond Fourier thermal transport, anisotropic thermal conductivity, Molecular Dynamics,
The 21st century is the era of nano-devices. Such devices, ranging from nano-transistors to nano-sensors and nano-antennae, have enabled humankind to exploit the technological capabilities of electronic, thermal, magnetic and other systems down to their ultimate potential, while they have also progressively been incorporated in the daily life of the average person, due to the progress in miniaturization, manufacturing process standardization, and large-scale production. However, it has been observed that the trend of exponential miniaturization of nano-devices, known as Moore's Law, has been halted and one of the main reasons for this apparent obstacle is the problem of power/thermal dissipation due to Joule heating (eg. creation of hot spots). It is thus apparent that the need for effective thermal management in the nanoscale is currently one of the main concerns of the scientific community, not only under the context of miniaturization, but also in order to tackle the issue of energy consumption, heat loss management of next-generation devices. 2D materials, especially graphene, hexagonal Boron Nitride (hBN), dichalocogenides have been proven to be excellent candidates for effective heat management in nano-devices. In the domain of thermal transport, when the characteristic length scale of the device becomes comparable to the mean free path or the wavelength of the heat carriers, such as lattice vibrations (phonons), heat transport becomes beyond Fourier (ballistic or hydrodynamic regime), while the thermal properties of nanomaterials become tunable. Towards this goal, 2D vertical and lateral heterostructures or functionalized 2D materials exhibit very desirable properties such as thermal anisotropy and a large range of thermal conductivities, for the design and manufacturing of thermotronic components such as thermal diodes and conductors that permit the control of heat flux. Undeniably, the simulation and modelisation of such systems is of utmost importance for the better understanding of the underlying physical processes and the optimization of next generation nano-devices.

Scientific locks The main challenge is to model and simulate functionalised 2D materials incorporating holes or amorphous regions, and vertical and lateral realistic heterostructures with purpose to propose to the experimentalists new strategies to obtain directional heat dissipation. The overall aim of the consortium of the ANR project BF2D, is to realise a high efficient thermal diode.
The object of this thesis is to attempt to perform a theoretical analysis of the thermal transport properties of 2D lateral and vertical heterostructures by means of atomistic simulations, such as Molecular Dynamics as well as by Ab Initio calculations under the context of the optimization of thermal management in nano-devices. The aim of this work will be to study the thermal conductivity and phonon transport properties of lateral and vertical heterostructures of a variety of 2D materials, such as graphene, hBN and Transition Metal Dichalcogenides (TMDC's). The main object of the simulations will be the analysis of the effects of chemical environment (i.e. oxidation and dopants), defects (defect engineering), material interfaces (Kapitza resistance) and mechanical strain (straintronics) on thermal conductivity and phonon mean free paths, relaxation times and group velocities, of functionalised 2D materials as well as lateral and vertical 2D heterostructures.

Approach and Methods: The main simulation tool for this study will be Molecular Dynamics (MD), in which atoms are treated classically under Newton's equations of motion, while chemical bonds are modelled by known interatomic potentials. Phonon properties will be studied by using the k-Space Velocity Autocorrelation Sequence method (kVACS), while thermal conductivities will be studied under both the Green-Kubo formalism, as well as by means of Non-Equilibrium Molecular Dynamics (NEMD). Supplementary Ab initio calculations will be performed where needed to study dopant bonding and atomic interfaces where no suitable classical interatomic potentials for MD calculations exist.

Contexte de travail

The PhD student will be accompanied to conduct an ambitious project in the field of Solid-State Physics and Engineering, aiming at materials for the thermal energy management. Her/His work will pass through dedicated trainings and formations to acquire skills for the management of a project, for team work, for the communication of results, as well as technical know-how to perform high performance atomistic simulations. The person will work three years in a collaborative project, which could include stays in laboratories which participate in the ANR project BF2D (I. NEEL, LOMA, IMPMC, ONERA, ). The person engaged will become a specialist in the field and an expert to be consulted and to guide the research for new and sophisticated strategies to manage heat in the nanoscale, as this work will provide new insights into heat transfer within nanostructured materials, feed into and improve models developed in partner laboratories and highlight key elements for optimizing the manufacturing of thermotronic devices. The PhD student will strengthen emerging synergies between the simulation activities of the “MiNT” group at CETHIL, an international aknowledged nanothermal specialists, with experts in ab-initio simulations at IMPMC laboratory.