Monolayer transition metal dichalcogenides, MX2 (M = Mo, W and X = S, Se), are direct-gap semiconductors with some interesting properties. First, the low-dimensional hexagonal structure leads to two inequivalent K-points, K and K’, in the brillioun zone. Second, this valley index and spin are intrinsically coupled, and spin-dependent selection rules enable one to independently populate and interrogate a unique K valley with circularly polarized light. Here we probe the degree of circular polarization of the emitted photoluminescence as function of the photo-excitation energy and temperature to elucidate spin-dependent inter- and intra-valley relaxation mechanisms. Monolayer flakes of MoS2 and MoSe2 show a strong depolarization as the excitation energy is increased. However, WS2 maintains significant polarization for high excitation energies, even at room temperature when properly prepared. We discuss the behavior of the polarization in terms of various phonon assisted intervalley scattering processes. This work was supported by NRL and the NRL Nanoscience Institute
Tunnel barriers are key elements for both charge-and spin-based electronics, offering devices with reduced power consumption and new paradigms for information processing. Such devices require mating dissimilar materials, raising issues of heteroepitaxy, interface stability, and electronic states that severely complicate fabrication and compromise performance. Graphene is the perfect tunnel barrier. It is an insulator out-of-plane, possesses a defect-free, linear habit, and is impervious to interdiffusion. Nonetheless, true tunneling between two stacked graphene layers is not possible in environmental conditions (magnetic field, temperature, etc.) usable for electronics applications. However, two stacked graphene layers can be decoupled using chemical functionalization. Here, we demonstrate homoepitaxial tunnel barrier devices in which graphene serves as both the tunnel barrier and the high mobility transport channel. Beginning with multilayer graphene, we fluorinate or hydrogenate the top layer to decouple it from the bottom layer, so that it serves as a single monolayer tunnel barrier for both charge and spin injection into the lower graphene transport channel. We demonstrate successful tunneling by measuring non-linear IV curves, and a weakly temperature dependent zero bias resistance. We perform lateral transport of spin currents in non-local spin-valve structures and determine spin lifetimes with the non-local Hanle effect to be commensurate with previous studies (~200 ps). However, we also demonstrate the highest spin polarization efficiencies (~45%) yet measured in graphene-based spin devices .
 A.L. Friedman, et al., Homoepitaxial tunnel barriers with functionalized graphene-on-graphene for charge and spin transport, Nat. Comm. 5, 3161 (2014).