The transition metal dichalcogenides (TMDs) represent an interesting class of bulk crystals that are built up of van der Waals bonded layers. As such, they can be separated into stable units of atomic thickness by mechanical exfoliation technique and grown by chemical vapor deposition (CVD) method just like graphene. Although the structure of the monolayer TMD, for example MoS2, is similar to that of the graphene honeycomb, the A and B sublattices, rather than both being occupied by C atoms, are occupied either by Mo atoms or by a pair of S atoms. This difference in symmetry leads to many interesting properties in MoS2 monolayers, highly distinct from that of graphene. In this talk, I will discuss our recent optical studies on mono- and few-layer samples of MoS2. Through characterization of the absorption, photoluminescence (PL) and photoconductivity spectrum, we show that quantum confinement effects lead to a crossover in MoS2from a bulk indirect gap semiconductor to a direct gap semiconductor at monolayer thickness. The band gap energy is also found to be tunable by an in-plane elastic strain field. Furthermore, we demonstrate that as is common for lower-dimensional materials, many-body interactions are strong in atomically thin MoS2 due to significantly reduced Coulomb interactions. They modify the optical properties significantly: excitons (bound states of an electron and a hole) and trions (bound states of two electrons and a hole) have been observed in our recent experiment with large binding energies in monolayer MoS2.

Atomically thin MoS<sub>2</sub> crystals:  New 2D semiconductor

247th National Meeting (2014)

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Atomically thin MoS2 crystals: New 2D semiconductor

Atomically thin MoS₂ crystals: New 2D semiconductor