  The integration of 2D nanomaterials with silicon is expected to enrich the
applications of 2D functional nanomaterials and to pave the way for
next-generation, nanoscale optoelectronics with enhanced performances. Herein,
a strategy for rare earth element doping has been utilized for the synthesis of
2D WS$_2$:Er nanosheets to achieve up-conversion and down-conversion emission
ranging from visible to the near-infrared region. Moreover, the potential
integration of the synthesized 2D nanosheets in silicon platforms is
demonstrated by the realization of an infrared photodetector based on a
WS$_2$:Er/Si heterojunction. These devices operate at room temperature and show
a high photoresponsivity of ~39.8 mA/W (at 980 nm) and a detectivity of 2.79
$\times$ 10$^{10}$ cm Hz$^{1/2}$ W$^{-1}$. Moreover, the dark current and noise
power density are suppressed effectively by van der Waals assisted p-n
heterojunction. This work fundamentally contributes to establishing infrared
detection by rare element doping of 2D materials in heterojunctions with Si, at
the forefront of infrared 2D materials-based photonics.
