Molecular beam epitaxy of the van der Waals heterostructure MoTe 2 on MoS 2 : phase, thermal, and chemical stability

(Sub)monolayer MoTe 2 is grown by molecular beam epitaxy on a bulk MoS 2 substrate. The film morphology, the thermally induced transformation of structural and compositional phases, as well as the chemical stability upon exposure to atmosphere are investigated by scanning tunneling microscopy and photoemission spectroscopy. Predominantly, semiconducting α -MoTe 2 islands are obtained under tellurium rich growth conditions and a substrate temperature of 200 °C. Under less tellurium-rich conditions, elongated and meandering MoTe 2− x strands are formed rather than compact islands. Similarly, annealing of initial α -MoTe 2 islands to above 500 °C causes the loss of tellurium and possibly transformation into the same MoTe 2− x strands. Consequently, under vacuum conditions the the transformation of α -MoTe 2 monolayers into the semimetallic β -MoTe 2 high temperature phase is accompanied by a loss of Te and formation of MoTe 2− x phase. The obtained tellurium deficient MoTe 2− x phase is almost metallic but a small band gap of a few tens meV remains. The as-grown α -MoTe 2 islands exhibit a moiré structure with ∼2.6 nm periodicity. This periodicity implies a rotation of ∼56° between the MoTe 2 and MoS 2 . We assign the observation of a specific rotation angle for the grown MoTe 2 islands with respect to the MoS 2 substrate to the lowest energy adsorption configuration for MoTe 2 monolayers on MoS 2 substrates. Exposure of the as grown films to atmosphere results in oxidation of the MoTe 2 film. The oxidized film maintains the two-dimensional island morphology of the initial film and thus is a candidate for a 2D (amorphous) oxide layer on MoS 2 .