  MXenes are a set of two-dimensional transition metal carbides and nitrides
that offer many potential applications in energy storage and electronic
devices. As an important parameter to design new electronic devices, we
investigate the work functions of bare MXenes and their functionalized ones
with F, OH, and O chemical groups using first-principles calculations. From our
calculations, it turns out that the OH terminated MXenes attain ultralow work
functions between 1.6 and 2.8 eV. Moreover, depending on the type of the
transition metal, the F or O functionalization affects increasing or decreasing
the work functions. We show that the changes in the work functions upon
functionalizations are linearly correlated with the changes in the surface
dipole moments. It is shown that the work functions of the F or O terminated
MXenes are controlled by two factors: the induced dipole moments by the charge
transfers between F/O and the substrate, and the changes in the total surface
dipole moments caused by surface relaxation upon the functionalization.
However, in the cases of the OH terminated MXenes, in addition to these two
factors, the intrinsic dipole moments of the OH groups play an important role
in determining the total dipole moments and consequently justify their ultralow
work functions.
