  The possibility of creating heterostructure of two-dimensional (2D) materials
has emerged as a viable route towards realizing novel optoelectronic devices.
However, the low light absorption due to their small absorption cross section,
limits their realistic application. While light-matter interaction mediated by
strong exciton-plasmon coupling has been demonstrated to improve absorbance and
spontaneous emission in a coupled TMDC and metallic nanostructures, the
fabrication of tunable broadband phototransistor with high quantum yield is
still a challenging task. By synthesizing Ag nanoparticles (Ag NPs) capped with
a thin layer of polyvinylpyrrolidone (PVP) through chemical route, we report a
lithography-free fabrication of a large area broadband superior gate-tunable
hybrid phototransistor based on monolayer graphene decorated by WS$_2$-Ag NPs
in a three-terminal device configuration. The fabricated device exhibits
extremely high photoresponsivity (up to $3.2\times 10^4$ A/W) which is more
than 5 times higher than the bare graphene/WS$_2$ hybrid device, along with a
low noise equivalent power (NEP) (~10$^{-13}$ W/Hz$^{0.5}$, considering 1/f
noise) and high specific detectivity ~1010 Jones in the wide (325-730 nm)
wavelength region. The additional PVP capping of Ag NPs helps to suppress the
direct charge and heat transfer and most importantly, increases the device
stability by preventing the degradation of WS$_2$-Ag hybrid system. The
enhanced optical properties of the hybrid device are explained via dipole
mediated strong exciton-plasmon coupling, corroborated by COMSOL Multiphysics
simulation. Our work demonstrates a strategy towards obtaining an
environment-friendly, scalable, high-performance broadband phototransistor by
tuning the exciton-plasmon coupling for new generation opto-electronic devices.
