  Two-dimensional materials with layered structures, appropriate bandgap, and
high carrier mobilities have gathered tremendous interests due to their
potential applications in optoelectronic and photovoltaic devices. Here, we
report the growth of BiI3 thin film with controllable atomic thickness on
graphene-terminated 6H-SiC(0001) substrate by molecular beam epitaxy (MBE)
method. The growth kinetic processes and crystalline properties of the BiI3
film are studied by scanning tunneling microscopy (STM). The scanning tunneling
spectroscopy (STS) reveals a bandgap of 2.8 eV for monolayer BiI3 with a weak
dependence on film thickness for few-layer BiI3, which greatly exceeds the
previous reported values identified by macroscopic optical measurements. This
discrepancy originates from the edge effect of BiI3 that renders the bandgap
downshift to 1.5 - 1.6 eV, as identified by the STS curves and the further
confirmed by density functional theory (DFT) calculations. Our work provides a
method to fabricate high-quality monolayer BiI3 film and resolves its intrinsic
bandgap as well as the edge effect on reduction of bandgap, benefitting not
only to fundamental researches but also to nanoelectronic and optoelectronic
applications.
