Circular dichroic photoluminescence from multilayer WS2

We have observed strong circular dichroic photoluminescence from exfoliated multilayer WS2 crystals containing up to 5 layers. The degree of valley polarization was shown to increase with number of layers, reaching around 0.9... [ view full abstract ]

We have observed strong circular dichroic photoluminescence from exfoliated multilayer WS₂ crystals containing up to 5 layers. The degree of valley polarization was shown to increase with number of layers, reaching around 0.9 even for 4-layered crystals. At present, it is thought that valley-spin polarization in transition metal dichalcogenide (TMDC) monolayers arises from a straightforward consideration of spatial inversion asymmetry in the crystal unit cell. Natural bilayers have this inversion symmetry restored, so it is expected that signatures of valley-spin polarization – including circular dichroic photoluminescence – vanish for crystals containing an even number of layers. Our observations corroborate well with recent reports [1, 2], and suggest that some new and interesting physics might be required to better understand WS₂. From a technological perspective, this also makes WS₂ particularly interesting since one may not strictly require the monolayer limit to exploit valley properties.