Atomically-thin layers of semiconducting transition metal dichalcogenides (TMDCs), such as tungsten diselenide (WSe₂), have attracted considerable interest because of new properties that can be obtained when artificially fabricated into van der Waals homo- or heterostructures. Due to interlayer coupling, bulk and multi-layered TMDCs are indirect gap semiconductors, while their monolayers exhibit a crossover to a direct band gap. It has been demonstrated that the interlayer coupling strength in homostructures is also sensitive to the twist angle in the cases of bilayer MoS₂ [1] and WS₂ [2]. In this work, the combined high spatial and spectral resolution of aberration-corrected scanning transmission electron microscopy (STEM) and monochromated electron energy-loss spectroscopy (EELS) in the low-loss regime are used to investigate the excitonic response of atomically-thin WSe₂, specifically in twisted bilayer WSe₂ as a function of moiré angle.

Few-layered WSe₂ flakes were mechanically exfoliated from a synthetic bulk crystal and transferred onto a Si₃N₄ TEM grid with periodic micron-sized holes. Atomically-resolved imaging has been performed on a Nion UltraSTEM200 operated at 60 keV and monochromated EELS performed on a modified Nion HERMES-S200 (also known as ChromaTEM) operated at 60 kV with the sample cooled using liquid nitrogen (T ≈ 150 K). In addition to freestanding WSe₂ monolayers, fragments of bilayers and trilayers with variable twist angle between 0–30° are also routinely observed due to folding during the mechanical exfoliation and transfer process. Well-defined hexagonal moiré patterns with nanometer periodicity are evident in the high-angle annular dark-field (HAADF) images for the low twist angles. The excitonic absorption signatures of these nanometric twisted bilayers from low-loss EELS are compared to the WSe₂ monolayer and trilayer counterparts of zero twist angle. The spectra demonstrate a good general correspondence to the thickness dependence of the A, B, and C exciton resonances, namely a pronounced decrease in C exciton energy with number of layers [3]. Comparing different twist angles in the bilayers also show sizable blueshifts in the C exciton energy up to 200 meV, which subsequently alter drastically the spectral shape between the B–C excitonic transitions, with extremes between the zero-twist and towards the anti-aligned (28°) case suggesting underlying differences in interlayer coupling with respect to the moiré angle.