  Future generation technologies demand high efficiency photodetectors to
enable sensing and switching devices for ultrafast communication and machine
vision. This require direct-band gap materials with high photosensitivity, high
detectivity and high quantum efficiency. Monolayered two- Dimensional
(2D)-Semiconductors based photodetectors are the most promising materials for
such applications, although experimental realization has been limited due to
unavailability of high quality sample. In the current manuscript, we report
about WS$_2$ based photodetector having sensitivity of 290 AW-1 upon 405 nm
excitation and incident power density as low as 0.06 mW/cm$^2$. The fabricated
device shows detectivity of 52*10^14 with external quantum efficiency of
89*10$^3$%. The observed superior photo-response parameters of CVD grown WS$_2$
based photodetector as compared to Si-detectors establishes it capability to
replace the Si-photodetectors with monolayered ultrathin device having superior
performance parameters.
