Light emission from the layered metal 2H-TaSe$_2$ and its potential applications

Conventional metals, in general, do not exhibit strong photoluminescence. 2H-TaSe$_2$ is a layered transition metal dichalcogenide that possesses metallic property with charge density wave characteristics. Here we show that 2H-TaSe$_2$ exhibits a surprisingly strong optical absorption and photoluminescence resulting from inter-band transitions. We use this perfect combination of electrical and optical properties in several optoelectronic applications. We show a seven-fold enhancement in the photoluminescence intensity of otherwise weakly luminescent multi-layer MoS$_2$ through non-radiative resonant energy transfer from TaSe$_2$ transition dipoles. Using a combination of scanning photocurrent and time-resolved photoluminescence measurements, we also show that the hot electrons generated by light absorption in TaSe$_2$ have a rather long lifetime unlike conventional metals, making TaSe$_2$ an excellent hot electron injector. Finally, we show a vertical TaSe$_2$/MoS$_2$/graphene photodetector demonstrating a responsivity of $>10$ AW$^{-1}$ at $0.1$ MHz - one of the fastest reported photodetectors using MoS$_2$.