  Atomically thin two-dimensional (2D) semiconductors such as molybdenum
disulphide (MoS2) hold great promise in electrical, optical, and mechanical
devices and display novel physical phenomena such as coupled spin-valley
physics and the valley Hall effect. However, the electron mobility of mono- and
few-layer MoS2 has so far been substantially below theoretically predicted
limits, particularly at low temperature (T), which has hampered efforts to
observe its intrinsic quantum transport behaviors. Potential sources of
disorder and scattering include both defects such as sulfur vacancies in the
MoS2 itself, and extrinsic sources such as charged impurities and remote
optical phonons from oxide dielectrics. To reduce extrinsic scattering and
approach the intrinsic limit, we developed a van der Waals (vdW)
heterostructure device platform where MoS2 layers are fully encapsulated within
hexagonal boron nitride (hBN), and electrically contacted in a multi-terminal
geometry using gate-tunable graphene electrodes. Multi-terminal
magneto-transport measurements show dramatic improvements in performance,
including a record-high Hall mobility reaching 34,000 cm2/Vs for 6-layer MoS2
at low T. Comparison to theory shows a decrease of 1-2 orders of magnitude in
the density of charged impurities, indicating that performance at low T in
previous studies was limited by extrinsic factors rather than defects in the
MoS2. We also observed Shubnikov-de Haas (SdH) oscillations for the first time
in high-mobility monolayer and few-layer MoS2. This novel device platform
therefore opens up a new way toward measurements of intrinsic properties and
the study of quantum transport phenomena in 2D semiconducting materials.
