Tailoring energy flow in molecular and semiconducting materials is critical for achieving energy-efficient systems. The sensitivity of two-dimensional materials to electronic structure modifications via strain and defects provides an opportunity to intentionally tune the potential energy landscape of excited states on the nanoscale. However, microscopic morphological variations disrupt our ability to directionally control charge and energy transport for desired optoelectronic applications. In order to determine the interplay between morphology-mediated and intrinsic electronic behavior, we used a photoemission electron microscope (PEEM) to image the heterogeneity in electronic structure and light absorption of black phosphorus and molecular monolayers on transitional metal dichalcogenides (TMDs) with spatial resolution as good as 25 nm. We find that the edges of black phosphorus have a shifted polarization dichroism by 10-20 degrees compared to the interior of the BP flakes. Supported by first-principles DFT calculations, we attribute this edge shift to a modification of symmetries of the conduction and valence bands, that shifts the polarization dependent absorption and could enable selective excitation and manipulation of black phosphorus edge states. The universality of this approach to probing electronic structure variation in materials has also allowed us to observe domain alignment of a single layer perylene diimide (PDI) with respect to the grain boundaries of monolayer polycrystalline TMDs via spatially dependent polarization anisotropy of the molecular film. These results suggest the full potential of morphological heterogeneity in layered materials, highlighting the need to investigate controllable ways to modify electronic structure on the nanoscale for adaptive device functionalities.

Nanoscale polarization anisotropy in 2D black phosphorus and molecular thin film/TMD heterostructure

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