Interplay of crystal thickness and in-plane anisotropy and evolution of quasi-one dimensional electronic character in ReSe₂

We study the valence band structure of ReSe2 crystals with varying thickness down to a single layer using nanoscale angle-resolved photoemission spectroscopy and density functional theory. The width of the top valence band in the direction perpendicular to the rhenium chains decreases with decreasing number of layers, from ~200 meV for the bulk to ~80 meV for monolayer. This demonstrates increase of in-plane anisotropy induced by changes in the interlayer coupling and suggests progressively more one-dimensional character of electronic states in few-layer rhenium dichalcogenides.