The diversity of 2D materials and their van der Waals (vdW) stacking presents fertile ground for engineering novel multifunctional materials and quantum states of matter. This permits unique opportunities to tailor the electronic properties of vdW heterostructures by the insertion of only a single 2D material layer. However, such vdW materials engineering at the atomic scale has yet to be investigated for spin-charge interconversion phenomena. Here, we report on the control of these effects at the monolayer level, where a drastic increase in intensity and change in sign of THz spintronic emission are demonstrated by inserting a single layer of ${\mathrm{MoSe}}_2$ between ${\mathrm{PtSe}}_2$ and graphene in a fully epitaxial, large area stacked structure. By using a combination of spin and angle resolved photoemission and density functional theory to reveal the electronic and spin structures, we illustrate two different mechanisms relying on charge transfer and electronic hybridization for the formation of Rashba states, which are responsible for spin-charge conversion and hence the THz spintronic emission. These findings open new pathways to design, at the atomic scale, efficient THz spintronic emitters made of 2D materials and other spintronic devices based on spin-charge interconversion phenomena.

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• Growth
• Spin-orbit coupling
• Spintronics
• 2-dimensional systems
• Density functional theory
• Electron microscopy
• Epitaxy
• Terahertz spectroscopy