Abstract

Transition metal dichalcogenides (TMDCs) such as MoS2 and WSe2 are layered materials that are semiconductors with a direct bandgap when thinned down to one monolayer. Despite an incredible number of results published in the field since 2010, many basic parameters such as the effective carrier mass are not experimentally determined – which leaves plenty of room for further exploration of these fascinating materials. Even samples exfoliated in ambient conditions with simple scotch-tape methods show remarkable properties for optoelectronics and spintronics: TMDC monolayers strongly interact with light in the visible region of the optical spectrum. The optically generated electrons and holes form excitons with high binding energy (several hundred meV) and high oscillator strength, resulting in optical absorption up to 20 % per monolayer. Interband optical selection rules are polarization selective (chiral). This allows addressing non-equivalent valleys in momentum space with polarized lasers for optical spin and valley index manipulation. We access the optical and spin properties, studying valley dynamics for different exciton species and resident carriers, with unprecedented detail in TMDC monolayers sandwiched between ultrathin insulating layers of hexagonal boron nitride (hBN) in van der Waals heterostructures. The optical emission of these encapsulated monolayers is spectrally narrow (down to 1 nm FWHM ) comparable to emission from III V quantum well structures used in today’s optoelectronic devices and approaching the homogenous limit. This insight paves the way for integrating TMD Cs in photonic devices and ferromagnetic semiconductor heterostructures.