The Mo $4d$ shell in the ground state of ${\mathrm{MoO}}_3$ is widely believed to be unoccupied. However, this assumption lacks clear experimental and theoretical corroboration. Using x-ray absorption and emission spectroscopy along with resonant inelastic x-ray scattering, we provide experimental evidence of two-dimensional ${\mathrm{MoO}}_3$ exhibiting enhanced many-body effects due to its reduced dimensionality. The observed phenomena include many-body effects such as $dd$ and spin-flip excitations, valence-hole and excited electron, and core-hole and excited electron bound excitonic states. Moreover, density functional theory and ligand field-based calculations were performed to investigate and interpret the experimental spectra. Considering that these many-body effects can only be observed by the interaction of x-ray photons with ${\mathrm{MoO}}_3$ if the $4d$ state is partially occupied, our experimental and theoretical approach clearly demonstrates a partial occupation of the Mo $4d$ state, refuting the assumption that the ground state is a $4d^0$ state. The Mo d occupancy is $4d^{3.36}$ and $4d^{3.53}$ determined with two different theoretical approaches (density functional theory and multiplet, respectively) and the computed spectra agree very well with our measurements further supporting this finding. Both the two- and three-dimensional samples exhibit strong core-hole effects that reduce the absorption onset at both the Mo $M_{2,3}$ and O $K$ edge. The band gap of the three-dimensional sample is experimentally found to be 3.1 ± 0.2 eV; however, for the two-dimensional material, strong many-body effects, even at the O $K$ edge, prevent an accurate determination of this value. The presence of these quasiparticles influences the band dispersions near the Fermi level, and thus has a key role in the performance of possible ${\mathrm{MoO}}_3$-based devices.

• Received 20 February 2022
• Revised 13 May 2022
• Accepted 31 May 2022

DOI:https://doi.org/10.1103/PhysRevMaterials.6.074003