  Using first-principles calculations and deformation potential theory, we
investigate the intrinsic carrier mobility ({\mu}) of monolayer MoS2 sheet and
nanoribbons. In contrast to the dramatic three orders of magnitude of
deterioration of {\mu} in graphene upon forming nanoribbons, the magnitude of
{\mu} in armchair MoS2 nanoribbons is comparable to that in monolayer MoS2
sheet, albeit oscillating with width. Surprisingly, a room-temperature
transport polarity reversal is observed with {\mu} of hole (h) and electron (e)
being 200.52 (h) and 72.16 (e) cm2V-1s-1 in sheet, and 49.72 (h) and 190.89 (e)
cm2V-1s-1 in 4 nm-wide nanoribbon. The robust magnitudes of {\mu} and polarity
reversal are attributable to the different characteristics of edge states
inherent in MoS2 nanoribbons. Our study suggests that width-reduction together
with edge engineering provide a promising route for improving the transport
properties of MoS2 nanostructures.
