Opto-Valleytronic Spin Injection in Monolayer MoS2/Few-Layer Graphene Hybrid Spin Valves (1705.09371v1)

Abstract: Two dimensional (2D) materials provide a unique platform for spintronics and valleytronics due to the ability to combine vastly different functionalities into one vertically-stacked heterostructure, where the strengths of each of the constituent materials can compensate for the weaknesses of the others. Graphene has been demonstrated to be an exceptional material for spin transport at room temperature, however it lacks a coupling of the spin and optical degrees of freedom. In contrast, spin/valley polarization can be efficiently generated in monolayer transition metal dichalcogenides (TMD) such as MoS2 via absorption of circularly-polarized photons, but lateral spin or valley transport has not been realized at room temperature. In this letter, we fabricate monolayer MoS2/few-layer graphene hybrid spin valves and demonstrate, for the first time, the opto-valleytronic spin injection across a TMD/graphene interface. We observe that the magnitude and direction of spin polarization is controlled by both helicity and photon energy. In addition, Hanle spin precession measurements confirm optical spin injection, spin transport, and electrical detection up to room temperature. Finally, analysis by a one-dimensional drift-diffusion model quantifies the optically injected spin current and the spin transport parameters. Our results demonstrate a 2D spintronic/valleytronic system that achieves optical spin injection and lateral spin transport at room temperature in a single device, which paves the way for multifunctional 2D spintronic devices for memory and logic applications.

Opto-Valleytronic Spin Injection in Monolayer MoS$_2$ and Graphene Hybrid Spin Valves

The paper under examination presents comprehensive experimental investigations into the opto-valleytronic spin injection using monolayer molybdenum disulfide (MoS$_2$) in conjunction with few-layer graphene. Transition metal dichalcogenides (TMDs) like MoS$_2$ are recognized for their strong spin-orbit coupling and valley-selective optical transition rules, making them suitable for generating valley and spin polarization through optical means. Meanwhile, graphene has displayed exceptional spin transport capabilities but lacks the spin-dependent optical selection rules which are requisite for opto-spintronics. By integrating these materials, the researchers successfully demonstrate the potential of achieving optical spin injection at room temperature—a significant advancement for the field.

In the hybrid spin valves developed, the researchers observe that both the direction and magnitude of the spin polarization within graphene can be regulated by the helicity and energy of incident photons. The paper reports definitive evidence of spin precession and transport through optical spin injection into MoS$_2$ and subsequent transfer into neighboring graphene layers, confirmed by Hanle effect measurements. This was evidenced by antisymmetric Hanle curves, which are indicative of pure spin transport.

Key numerical results from the research indicate an impressive spin lifetime of 308 ps and a spin diffusion length of 3.04 µm in few-layer graphene, established through Hanle precession fitting methods. Importantly, the optical spin injection mechanism demonstrated modulation via photon energy tuning—specifically shifting the spin orientation from A exciton (1.93 eV) resonance to B exciton (2.06 eV) resonance, with the spin signal ($\Delta V_{NL}$) reflecting these transitions.

The implications of these findings are extensive, suggesting viable pathways for developing multifunctional 2D opto-spintronic devices that efficiently integrate valleytronic and spintronic functionalities. These devices could transcend traditional electronic frameworks, providing synergy between memory and logic operations within a singular device architecture. Moreover, the successful demonstration of room temperature spin transport supports the integration of such heterostructures into practical electronic and optoelectronic applications.

Looking forward, there is potential for optimizing the scalability of these devices, though diffraction limits present challenges in miniaturization. Future research may explore the efficiencies of optical spin injection in related TMD materials like WSe$_2$, which possesses prolonged spin/valley lifetimes, potentially enhancing spin injection efficiency and device performance. The paper sets foundational ground for the material engineering required for 2D heterostructures and heralds advances in the domain of semiconductor spintronics and valleytronics.