Band Alignment and Minigaps in Monolayer MoS 2 ‑Graphene van der Waals Heterostructures
Résumé
Two-dimensional layered MoS 2 shows great potential for nanoelectronic and optoelectronic devices due to its high photosensitivity, which is the result of its indirect to direct band gap transition when the bulk dimension is reduced to a single monolayer. Here, we present an exhaustive study of the band alignment and relativistic properties of a van der Waals heterostructure formed between single layers of MoS 2 and graphene. A sharp, high-quality MoS 2-graphene interface was obtained and characterized by micro-Raman spectroscopy, high-resolution X-ray photoemission spectros-copy (HRXPS), and scanning high-resolution transmission electron microscopy (STEM/HRTEM). Moreover, direct band structure determination of the MoS 2 /graphene van der Waals heterostructure monolayer was carried out using angle-resolved photoemission spectroscopy (ARPES), shedding light on essential features such as doping, Fermi velocity, hybridization, and band-offset of the low energy electronic dynamics found at the interface. We show that, close to the Fermi level, graphene exhibits a robust, almost perfect, gapless, and n-doped Dirac cone and no significant charge transfer doping is detected from MoS 2 to graphene. However, modification of the graphene band structure occurs at rather larger binding energies, as the opening of several miniband-gaps is observed. These miniband-gaps resulting from the overlay of MoS 2 and the graphene layer lattice impose a superperiodic potential. T he recent rise of a large family of 2D materials, with unique electronic and optical properties, has opened exciting prospects for new devices based on so-called " van der Waals (vdW) heterostructures ". The latter are only a few atoms thick and are expected to exhibit new properties and functionalities that cannot be achieved using bulk materials. Indeed, 2D materials can be considered as " exposed " two-dimensional electron gases, whose properties can be dramatically influenced by noncovalent coupling to low-dimensional adsorbates. So far, most studies have focused on hetero-structures based on graphene, boron nitride, and transition metal dichalcogenides, 1 obtained from mechanical exfoliation or chemical vapor deposition (CVD) growth. 2−8 In particular, graphene, a 2D semimetal with extremely high carrier mobility but no bandgap and monolayer MoS 2 , a direct bandgap semiconductor with good carrier mobility, are highly promising building blocks for future nanoelectronics. 9,10 Heterojunctions interfacing different 2D materials would enable so-called van der Waals epitaxy, in which the lattice-matching condition in traditional epitaxy is drastically relaxed, allowing the formation of a wide range of 2D/2D, 2D/bulk heterostructures. 11 In these vdW heterostructures, each material maintains its individual electronic properties due to the weak interactions between the layers. One of the key parameters in the design of a heterojunction is the determination of the band offset.