Ph D and post-doc positions: Exploring the physics of 2D materials excitations using relativistic electrons

Term and Location A 3 years Ph D or a 2 years post-doc position is available in the STEM group at the Laboratoire de Physique des Solides of Université Paris-Saclay (Orsay, France) beginning September 2022. The position is funded in the context of the european project EBEAM.

Scientific Context

Two dimensional materials have attracted great interest since the reproducible production of graphene. An intense search for new materials and properties has issued many results, including the observation of unconventional superconductivity in graphene twisted bilayers [Cao et al., Nature, 556, 43 (2018)]. Among the large family of possible atomically thin materials, transition metal dichalcogenides (TMDs), such as MoS2 and WSe2, have attracted the attention of the community due their visible range bandgap and large spin-orbit coupling. These in combination to the lack of inversion symmetry of their crystal structure lead to spin-valley degrees of freedom which can be manipulated by circularly polarized photons [Xiao et al. Phys. Rev. Lett. 108 196802 (2012)]. Finally, the reduce electromagnetic screening due to their intrinsic 2D nature significantly increases the binding energy of excitons. Therefore the physics usually only accessible at very low temperatures is stable at higher temperature, even up to 300 K.

A key issue in these materials is the production of devices with reproducible features. Large part of the difficulty stems from their 2D nature: surface contaminants modify their local response; bending create strain that changes excitations energies; a few missing atoms completely disrupts their electronic structure. These evident difficulties are actually benefits, as they would allow fine control over the material, if one is capable of generating them in a controlled manner. This means that understanding the material physics at the nanometer to atomic scales is necessary. Standard optical spectroscopies cannot provide information at these reduced scales.

Electron spectroscopies can probe the optical response of matter down to the nanoscale. We can mention, for example, in our group at the LPS the three-dimensional and vector mapping of phonon modes on the surface of nanoparticles [Li et al., Science 371 1364 (2021)], the demonstration of strong plasmon-phonon coupling at the nanoscale [Tizei et al., Nano Letters, 20, 2973 (2020)], the visualization of light emission from hybride Perovskyte nanoparticles [Hou et al., Science 374 6567 (2021)] and the understanding of whispering gallery cavity modes in spheres [Auad et al. Nano Lett. 22 319 (2022)]. For 2D materials these spectroscopies have proven to be excellent probes for excitons, trions (charged excitons), and localized excitations in these materials [Tizei et al., Phys. Rev. Lett. 114 107601 (2015), Bonnet et al., Nano Lett. 21 10178 (2021), Shao et al. ArXiv2202.04483 (2022)]. They can be coupled to electron microscopy techniques (imaging and diffraction) and other spectroscopies to have a complete picture of the materials at the atomic and nano scales. However, a large penalty still exists for electron spectroscopies: the electron intrinsic angular momentum (spin) does not couple to these excitations as the intrinsic angular momentum of a photon does. Therefore, electrons are naturally blind to the distinct spin-valley states, which are easily accessible for photons.


Project description

In this 2 year post-doc project, the successful candidate will explore electron spectroscopies of molecules confined in space using a combination of EELS and EEGS in the mid to far IR. Specialized samples will be created in house, using t

n this thesis project, we will explore, experimentally and theoretically, the possibility accessing these states using purely electron beams. The idea is based on the generation of electron beam carrying angular momentum using electron phase shaping, as demonstrated in 2010 [Verbeeck et al. Nature, 467 301 (2010)]. Spectroscopy with phase shaped beams [Guzzinati et al. 8 14999 (2017)], and proper final state selection [Lourenço-Martins et al., Nat. Phys. 17 598 (2021)] might allow one to probe these state using only electron beams.

We will first study the physics of shaped electron beam coupling to materials in general, including 2D materials. In parallel, we will study electron spectroscopy and perform simple experiment to aquaint the PhD candidate with the experimental techniques (electron energy loss spectroscopy, cathodoluminescence, imaging and diffraction). Finally, we will use phase shaped beams to probe spin-valley states in TMD monolayers.

Working environment The STEM group at the Laboratoire de Physique des Solides is a world leading electron microscopy team well recognized for its work on the structural, optical and electronic characterization of nanostructures. The experiments will rely on a world unique microscope, CHROMATEM, without which the experiments are not possible.
The project will hold in the framework of the European EBEAM project.

Profile candidate The project will be mainly experimental, and upon the applicant tastes and skills, may include instrumentation and/or theoretical development. We are seeking candidate with a curious mind, a taste for experimental work, and a will to tackle a totally new field of nanospectrosopy in the competitive domain of electron-based spectroscopies.

How to apply Qualified candidates should send a CV, a motivation letter, contact details of at least two references, and an estimated availability date to Mathieu Kociak and Luiz Tizei. The preferred format is a single PDF-document