  Using a set of first-principles calculations, we studied the electronic
structures of two-dimensional transition metal carbides and nitrides, so called
MXenes, functionalized with F, O, and OH. Our projected band structures and
electron localization function analyses reveal the existence of nearly free
electron (NFE) states in variety of MXenes. The NFE states are spatially
located just outside the atomic structure of MXenes and are extended parallel
to the surfaces. Moreover, we found that the OH-terminated MXenes offer the NFE
states energetically close to the Fermi level. In particular, the NFE states in
some of the OH-terminated MXenes, such as Ti2C(OH)2, Zr2C(OH)2, Zr2N(OH)2,
Hf2C(OH)2, Hf2N(OH)2, Nb2C(OH)2, and Ta2C(OH)2, are partially occupied. This is
in remarkable contrast to graphene, graphane, and MoS2, in which their NFE
states are located far above the Fermi level and thus they are unoccupied. As a
prototype of such systems, we investigated the electron transport properties of
Hf2C(OH)2 and found that the NFE states in Hf2C(OH)2 provide almost perfect
transmission channels without nuclear scattering for electron transport. Our
results indicate that these systems might find applications in nanoelectronic
devices. Our findings provide new insights into the unique electronic band
structures of MXenes.
