Polarized X-ray diffraction anomalous near-edge structure study on the orbital physics of thin WSe₂ layers

The electronic structures at band edges play an important role in the physical properties of few-layer and monolayer WSe₂. In this study, polarization-dependent diffraction anomalous near-edge structure (DANES) is applied to measure and analyse the electronic orbitals of few-layer WSe₂. By selecting diffraction geometries with the electric field perpendicular or parallel to the c axis of few-layer WSe₂, this method can separately probe the in-plane and out-of-plane orbital components involved at the band edges. The WSe₂ (00.8) surface normal was aligned and the preferred orientation of few-layer WSe₂ grown on an Al₂O₃ (00.1) substrate was identified. DANES was then performed for Se K-edge WSe₂ 00.8 and 11.0 reflections to examine the W 5d orbitals hybridized with the Se 4p orbitals: these two DANES spectra exhibit fairly anisotropic valence orbital characteristics in few-layer WSe₂. Coupled with first-principles calculations, these results allow the identification of the in-plane and out-of-plane orbital distribution and hybridization in WSe₂. At the conduction band edge, the contributions of p_x and p_y orbitals are predominant over p_z and the splitting of the p-orbital energy levels has been confirmed. Hence DANES is shown to be a useful synchrotron X-ray technique that can help identify the valence orbital structure of various 2D transition metal dichalcogenides.