  Atomically thin two-dimensional chalcogenides such as MoS2 monolayers are
structurally ideal channel materials for the ultimate atomic electronics.
However, a heavy thickness dependence of electrical performance is shown in
these ultrathin materials, and the device performance normally degrades while
exhibiting a low carrier mobility as compared with corresponding bulks,
constituting a main hurdle for application in electronics. In this brief
review, we summarize our recent work on electrode/channel contacts and carrier
scattering mechanisms to address the origins of this adverse thickness
dependence. Extrinsically, the Schottky barrier height increases at the
electrode/channel contact area in thin channels owing to bandgap expansion
caused by quantum confinement, which hinders carrier injection and degrades
device performance. Intrinsically, thin channels tend to suffer from
intensified Coulomb impurity scattering, resulting from the reduced interaction
distance between interfacial impurities and channel carriers. Both factors are
responsible for the adverse dependence of carrier mobility on channel thickness
in two-dimensional semiconductors.
