  The combination of the exciting properties of graphene with those of
monolayer tungsten disulfide (WS2) makes this heterostack of great interest for
electronic, optoelectronic and spintronic applications. The scalable synthesis
of graphene/WS2 heterostructures on technologically attractive substrates like
SiO2 would greatly facilitate the implementation of novel two-dimensional (2D)
devices. In this work, we report the direct growth of monolayer WS2 via
chemical vapor deposition (CVD) on single-crystal graphene arrays on SiO2.
Remarkably, spectroscopic and microscopic characterization reveals that WS2
grows only on top of the graphene crystals so that the vertical heterostack is
selectively obtained in a bottom-up fashion. Spectroscopic characterization
indicates that, after WS2 synthesis, graphene undergoes compressive strain and
hole doping. Tailored experiments show that such hole doping is caused by the
modification of the SiO2 stoichiometry at the graphene/SiO2 interface during
the WS2 growth. Electrical transport measurements reveal that the
heterostructure behaves like an electron-blocking layer at large positive gate
voltage, which makes it a suitable candidate for the development of unipolar
optoelectronic components.
