Exciton-Plasmon Coupling Mediated Superior Photoresponse in 2D Hybrid Phototransistors

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.