Unusual Photoluminescence in Ultrathin MoS$_{2}$

In this talk we will report optical studies on ultrathin MoS$_{2}$ layers through optical reflection, Raman scattering, and photoluminescence spectroscopy. Bulk MoS$_{2}$, a layered transition metal dichalcogenide, is an indirect bandgap semiconductor with negligible photoluminescence. Surprisingly, when the thickness of MoS$_{2}$ is reduced to a few unit cell thickness, a strong photoluminescence emerges. Further this photoluminescence increases with reduced MoS$_{2}$ layer thickness, although available materials amount is reduced. We will discuss possible mechanism that can give rise to this surprising photoluminescence behavior in MoS$_{2}$.