  Two-dimensional semiconductors are structurally ideal channel materials for
the ultimate atomic electronics after silicon era. A long-standing puzzle is
the low carrier mobility ({\mu}) in them as compared with corresponding bulk
structures, which constitutes the main hurdle for realizing high-performance
devices. To address this issue, we perform combined experimental and
theoretical study on atomically thin MoS2 field effect transistors with varying
the number of MoS2 layers (NLs). Experimentally, an intimate relation is
observed with a 10-fold degradation in {\mu} for extremely thinned monolayer
channels. To accurately describe the carrier scattering process and shed light
on the origin of the thinning-induced mobility degradation, a generalized
Coulomb scattering model is developed with strictly considering device
configurative conditions, i.e., asymmetric dielectric environments and lopsided
carrier distribution. We reveal that the carrier scattering from interfacial
Coulomb impurities (e.g., chemical residues, gaseous adsorbates and surface
dangling bonds) is greatly intensified in extremely thinned channels, resulting
from shortened interaction distance between impurities and carriers. Such a
pronounced factor may surpass lattice phonons and serve as dominant scatterers.
This understanding offers new insight into the thickness induced scattering
intensity, highlights the critical role of surface quality in electrical
transport and would lead to rational performance improvement strategies for
future atomic electronics.
