Hysteresis-free high mobility graphene encapsulated in tungsten disulfide

High mobility is a crucial requirement for a large variety of electronic device applications. The state of the art for high-quality graphene devices is based on heterostructures made with graphene encapsulated in $> 40$ nm-thick flakes of hexagonal boron nitride (hBN). Unfortunately, scaling up multilayer hBN while precisely controlling the number of layers remains an outstanding challenge, resulting in a rough material unable to enhance the mobility of graphene. This leads to the pursuit of alternative, scalable materials, which can be used as substrates and encapsulants for graphene. Tungsten disulfide (WS₂) is a transition metal dichalcogenide, which was grown in large (∼mm-size) multi-layers by chemical vapor deposition. However, the resistance vs gate voltage characteristics when gating graphene through WS₂ exhibit largely hysteretic shifts of the charge neutrality point on the order of $Δ n ∼$ 3 × 10¹¹ cm⁻², hindering the use of WS₂ as a reliable encapsulant. The hysteresis originates due to the charge traps from sulfur vacancies present in WS₂. In this work, we report the use of WS₂ as a substrate and overcome the hysteresis issues by chemically treating WS₂ with a super-acid, which passivates these vacancies and strips the surface from contaminants. The hysteresis is significantly reduced by about two orders of magnitude, down to values as low as $Δ n ∼$ 2 × 10⁹ cm⁻², while the room-temperature mobility of WS₂-encapsulated graphene is as high as ∼62 × 10³ cm² V⁻¹ s⁻¹ at a carrier density of n ∼ 1  $× 10 12$ cm⁻². Our results promote WS₂ as a valid alternative to hBN as an encapsulant for high-performance graphene devices.