The Quest for New Frontiers in Cryo Electron Microscopy for Materials Sciences

Modern scanning and transmission electron microscopes (S/TEM) are now almost ubiquitous in materials and biological sciences laboratories. They have radically enhanced our understanding matter at the atomic level, bringing unique information of structure, chemical composition, and electronic properties of materials. Moreover, the recent development of stable cryogenic TEM holders at temperatures ranging from liquid N₂ (100 K) to 300 K with electrical contacts, combined with aberration-corrected and monochromated electron optics has allowed S/TEM to study magnetic, structural and electronic phase transitions with unprecedented spatial and energy resolutions. <br/><br/>In this talk, I will present two examples of cryogenic STEM measurements done in a set of two-dimensional (2D) transition metal dichalcogenides (TMDs) samples. In the first example I will address excitonic dephasing in MoS₂. I will present experimental results that show how monochromated electron-energy-loss spectroscopy (EELS) can reveal the exciton-phonon coupling of a few layers of MoS₂ encapsulated in hexagonal boron nitride (hBN). The measurements are spatial localized beyond the diffraction limit of the associated optical excitation, indicated how STEM can be complementary to conventional optical probes. In the second example I will present measurements that show a spatial dependence of a moiré-exitonic coupling in a hBN/WSe₂/WS₂ heterostructure at LN₂ cryogenic temperatures. I will also present a geometrical phase analysis of the moiré structure that allows to reveal the strain on the lattice at large field of views (&gt;30 x 30 nm²) while still maintaining atomic resolution. Prospects and limitations of future experiment will be discussed in the detail during the talk.