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

(Sub)monolayer MoTe sub(2) is grown by molecular beam epitaxy on a bulk MoS2 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 [alpha]-MoTe sub(2) islands are obtained under tellurium rich growth conditions and a substrate temperature of 200 [degrees]C. Under less tellurium-rich conditions, elongated and meandering MoTe sub(2-x) strands are formed rather than compact islands. Similarly, annealing of initial [alpha]-MoTe2 islands to above 500 [degrees]C causes the loss of tellurium and possibly transformation into the same MoTe sub(2-x) strands. Consequently, under vacuum conditions the the transformation of [alpha]-MoTe sub(2) monolayers into the semimetallic [beta] -MoTe sub(2) high temperature phase is accompanied by a loss of Te and formation of MoTe sub(2-x) phase. The obtained tellurium deficient MoTe sub(2-x) phase is almost metallic but a small band gap of a few tens meV remains. The as-grown [alpha]-MoTe sub(2) islands exhibit a moire structure with ~2.6 nm periodicity. This periodicity implies a rotation of ~56[degrees] between the MoTe sub(2) and MoS sub(2). We assign the observation of a specific rotation angle for the grown MoTe sub(2) islands with respect to the MoS sub(2) substrate to the lowest energy adsorption configuration for MoTe sub(2) monolayers on MoS sub(2) substrates. Exposure of the as grown films to atmosphere results in oxidation of the MoTe sub(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 sub(2).