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James Johns: Transmission Mode Ultrafast Photoemission of Suspended 2D Materials

Two dimensional materials (2DMs) exhibit remarkable electronic and optical properties, and are promising candidate materials for transistor, optoelectronic, and photocatalytic devices. , The electronic structure and dynamics of 2DMs can vary significantly depending on substrate effects because all of the atoms in these materials reside at the surface. Measuring the energy of the excited states in suspended 2DMs is therefore necessary in order to differentiate their intrinsic properties from their environmentally induced behavior. Time-resolved two-photon photoemission spectroscopy is a powerful tool for measuring the dynamics of electrons and excitons at surfaces; it can directly observe both optically bright and dark states involved in charge relaxation, and also measure the full excited-state band structure. Johns will use ultrafast photoemission to measure the dynamics of electrons and excitons in suspended 2DMs, such as graphene and MoS2, which are transferred onto transmission electron microscope grids with regularly spaced holes. Using an in-situ imaging and focusing assembly underneath the sample holder, Johns will be able to locate and measure the dynamics of the freely suspended regions of the 2DMs. Following measurements of the charge dynamics, interactions with other nanomaterials commonly incorporated into 2D devices, such as plasmonic and metal oxide nanoparticles, will be explored.

Apparatus for measuring the charge dynamics of 2D materials, suspended on a TEM grid, by ultrafast two-photon photoemission.

Funded by the National Science Foundation through the University of Minnesota MRSEC under Award Number DMR-1420013