A comprehensive understanding
of the energy level alignment mechanisms
between two-dimensional (2D) semiconductors and electrodes is currently
lacking, but it is a prerequisite for tailoring the interface electronic
properties to the requirements of device applications. Here, we use
angle-resolved direct and inverse photoelectron spectroscopy to unravel
the key factors that determine the level alignment at interfaces between
a monolayer of the prototypical 2D semiconductor MoS2 and
conductor, semiconductor, and insulator substrates. For substrate
work function (Φsub) values below 4.5 eV we find
that Fermi level pinning occurs, involving electron transfer to native
MoS2 gap states below the conduction band. For Φsub above 4.5 eV, vacuum level alignment prevails but the charge
injection barriers do not strictly follow the changes of Φsub as expected from the Schottky-Mott rule. Notably, even
the trends of the injection barriers for holes and electrons are different.
This is caused by the band gap renormalization of monolayer MoS2 by dielectric screening, which depends on the dielectric
constant (εr) of the substrate. Based on these observations,
we introduce an expanded Schottky-Mott rule that accounts for band
gap renormalization by εr -dependent screening and
show that it can accurately predict charge injection barriers for
monolayer MoS2. It is proposed that the formalism of the
expanded Schottky-Mott rule should be universally applicable for 2D
semiconductors, provided that material-specific experimental benchmark
data are available.