Terahertz Spintronic Emission from 2D Transition Metal Dichalcogenides and Their van der Waals Heterostructures

Terahertz (THz) Spintronic emitters based on ferromagnetic/metal junctions have become an important technology for the THz range, offering powerful and ultra-large spectral bandwidths [1,2]. These developments have driven recent investigations of two-dimensional (2D) materials for new THz spintronic concepts. 2D materials, such as transition metal dichalcogenides (TMDs) and their van der Waals heterostructures, are ideal platforms for spin-to-charge conversion (SCC) as they possess strong spin-orbit coupling (SOC) and reduced symmetries [3]. Moreover, SCC and the resulting THz emission can be tuned with the number of layers, electric field, strain or by stacking different TMDs.<br/>In this work [4], we have grown large area single crystalline mono and multilayers of 1T-PtSe₂ on graphene by molecular beam epitaxy, followed by in situ deposition of amorphous CoFeB by sputtering, with atomically sharp interfaces. We used a full set of characterization tools to demonstrate the structural and chemical preservation of PtSe₂ after CoFeB deposition. SCC was then studied on these advanced 2D samples using THz emission spectroscopy as a function of PtSe₂ thickness (from 1 to 15 ML), that showed the generation of efficient THz electric fields. In comparison, THz emission from CoFeB/WSe₂ and CoFeB/VSe₂ are negligible. The THz emission with PtSe₂ is shown to arise from the 1T crystal structure and large spin-orbit coupling. The measured THz peak electric field as a function of the number of PtSe₂ monolayers clearly shows a two-step dependence with PtSe₂ thickness, which we interpret as the transition from the inverse Rashba Edelstein effect (IREE) in the semiconducting regime to the inverse spin Hall effect (ISHE) in the semimetallic regime (around 3 to 4 MLs). As shown by ab initio, the IREE arises from the large Rashba spin splitting at the PtSe₂/graphene interface by the combination of large spin-orbit coupling and electron transfer from graphene to PtSe₂, generating an interface electric field. By fitting the thickness dependence, we can extract the out-of-plane spin diffusion length in PtSe₂ to be 2-3 nm and find that SCC by IREE at the PtSe₂/Gr interface is twice as efficient than that of ISHE in bulk PtSe₂.<br/><br/>To investigate further THz emission from vdW heterostructures, we compared the spintronic THz emission from epitaxial PtSe₂/Gr with the one from PtSe₂/MoSe₂/Gr. Remarkably, by adding one monolayer of MoSe₂ between PtSe₂ and graphene, we changed drastically the sign and magnitude of spin-charge conversion demonstrating the monolayer control of THz emission in vdW heterostructures [5]. Finally, we designed specific THz emitter devices made of PtSe₂/MoSe₂ bilayer transferred on SiO₂/Si to apply a back gate voltage. Our goal is to adjust the Fermi level position of the system to modulate the SCC and THz signal. We conclude that 2D materials and their vdW heterostructures are promising materials for intense and tunable spintronic THz emission.<br/><br/><u>References:</u><br/>[1] T. Seifert, S. Jaiswal, U. Martens et al., Nat. Photon. 10, 483 (2016)<br/>[2] E. Rongione, L. Baringthon, D. She et al., Adv. Sci. 2023, 2301124<br/>[3] D. Xiao, G.-B. Liu, W. Feng et al., Phys. Rev. Lett. 108, 196802 (2012)<br/>[4] K. Abdukayumov, M. Mičica, F. Ibrahim et al., Adv. Mater. 2024, 36, 2304243.<br/>[5] K. Abdukayumov et al., in preparation.