  Band gap of monolayer and few layers in two dimensional (2D) semiconductors
has usually been measured by optical probing such as photoluminescence (PL).
However, if their exfoliated thickness is as large as a few nm (multilayer over
~5L), PL measurements become less effective and inaccurate because the optical
transition of 2D semiconductor is changed from direct to indirect mode. Here,
we introduce another way to estimate the bandgap of multilayer 2D van der Waals
semiconductors; that is utilizing field effect transistor (FET) as a platform.
We used graphene (thin graphite or multilayer graphene) contact for multilayer
van der Waals channels in FET, because graphene contact would secure ambipolar
behavior and enable Schottky contact barrier tuning of FETs with the assistance
of top passivation. As a result, the bandgaps of multilayer transition metal
dichalcogenides and black phosphorus in unknown thickness were successfully
estimated through measuring the temperature-dependent transfer curve
characteristics of prepared 2D FETs with graphene contact.
