Opto-Valleytronic Spin Injection in Monolayer MoS₂/Few-Layer Graphene Hybrid Spin Valves

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 MoS 2 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 MoS 2 /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 driftdiffusion 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.Keywords: spintronics, valleytronics, graphene, transition metal dichalcogenides, optoelectronics 3 Spintronics and valleytronics, novel fields with large potential impacts in both fundamental science and technology, utilize the electron's spin and valley degrees of freedom, in addition to charge, for information storage and logic operations. In the past decade, experimental studies have established singlelayer and multilayer graphene as among the most promising materials for spintronics due to their high electronic mobility combined with low intrinsic spin-orbit coupling. Graphene exhibits room temperature spin diffusion length of up to tens of microns, substantially longer than conventional metals or semiconductors (<1 micron) [1][2][3][4] . However, graphene's lack of spin-dependent optical selection rules has made opto-spintronic functionality impossible, a substantial limitation for graphene.Fortunately, monolayer MoS 2 and related semiconducting transition metal dichalcogenides (TMDs) exhibit favorable characteristics for nanoscale opto-valleytronic and opto-spintronic applications [5][6][7] .TMDs have strong spin-orbit coupling due to the heavy metal atom and lack inversion symmetry in monolayer form, the combination of which allows complete simultaneous valley and spin polarization through absorption of circularly polarized light [8][9][10][11][12][13][14] . This originates from the valley-dependent optical selection rules of monolayer ...

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“…The available material repertoire covers semiconductors [1][2][3][4][5] (MoS 2 , WSe 2 ), ferromagnets [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] (CrI 3 , CrGeTe 3 ), superconductors [22][23][24] (NbSe 2 ), and topological insulators [25] (WTe 2 ), which offer unforeseen potential for electronics and spintronics [26,27]. For example, monolayer transition-metal dichalcogenides (TMDCs) are direct band gap semiconductors with remarkable physical properties [1][2][3][4][5][28][29][30][31], specially in the realm of optoelectronics [32], optospintronics [33][34][35], and valleytronics [36][37][38]. Currently, TMDCs, being stable in air, are a favorite platform for optical experiments including optical spin injection due to helicity-selective optical excitations [39].…”