Due to the intriguing optical and electronic properties, 2D materials have attracted a lot of interest for the electronic and optoelectronic applications. Identifying new promising 2D materials will be rewarding toward the development of next generation 2D electronics. Here, palladium diselenide (PdSe ), a noble-transition metal dichalcogenide (TMDC), is introduced as a promising high mobility 2D material into the fast growing 2D community. Field-effect transistors (FETs) based on ultrathin PdSe show intrinsic ambipolar characteristic. The polarity of the FET can be tuned. After vacuum annealing, the authors find PdSe to exhibit electron-dominated transport with high mobility (µ = 216 cm V s ) and on/off ratio up to 10 . Hole-dominated-transport PdSe can be obtained by molecular doping using F -TCNQ. This pioneer work on PdSe will spark interests in the less explored regime of noble-TMDCs.

We report polarization resolved photoluminescence from monolayer MoS2, a two-dimensional, non-centrosymmetric crystal with direct energy gaps at two different valleys in momentum space. The inherent chiral optical selectivity allows exciting one of these valleys and close to 90% polarized emission at 4K is observed with 40% polarization remaining at 300K. The high polarization degree of the emission remains unchanged in transverse magnetic fields up to 9T indicating robust, selective valley excitation.

We have experimentally studied the pump-probe Kerr rotation dynamics in WSe2 monolayers. This yields a direct measurement of the exciton valley depolarization time τv. At T = 4 K, we find τv ≈ 6 ps, a fast relaxation time resulting from the strong electron-hole Coulomb exchange interaction in bright excitons. The exciton valley depolarization time decreases significantly when the lattice temperature increases with τv being as short as 1.5 ps at 125 K. The temperature dependence is well explained by the developed theory taking into account exchange interaction and fast exciton scattering time on short-range potential.