  Two dimensional (2D) van der Waals heterostructures (vdWHs) have their unique
potential in facilitating the stacking of layers of different 2D materials for
optoelectronic devices with superior characteristics at a reduced cost.
However, the fabrication of large area all-2D heterostructures is still
challenging towards realizing practical devices. In the present work, we have
demonstrated a rapid yet simple, impurity free and highly efficient
sonication-assisted chemical exfoliation approach to synthesize hybrid vdWHs
based on 2D molybdenum disulphide (MoS$_2$) and tungsten disulphide (WS$_2$),
with high yield. Microscopic and spectroscopic studies have confirmed the
successful exfoliation of layered 2D materials and formation of their hybrid
heterostructure. The co-existence of 2D MoS2 and WS2 in the vdW hybrid is
established by optical absorption and Raman shift measurements along with their
chemical stiochiometry determined by X-ray photoelectron spectroscopy. The
spectral response of the vdWH/Si (2D/3D) heterojunction photodetector
fabricated using the as-synthesized material is found to show superior
broadband photoresponse compared to that shown by the individual 2D MoS$_2$ and
WS$_2$ based devices. The peak responsivity is found to be ~2.15 A/W at a
wavelength of ~560 nm for an applied bias of -5 V. The ease of fabrication and
superior performance of the chemically synthesized vdWH-based devices have
revealed their potential use for large area optoelectronic applications on Si
compatible CMOS platforms.
