Stacking of MoS 2 and WSe 2 monolayers is conducted by transferring triangular MoS 2 monolayers on top of WSe 2 monolayers, all grown by chemical vapor deposition (CVD).Raman spectroscopy and photoluminescence (PL) studies reveal that these mechanically stacked monolayers are not closely coupled, but after a thermal treatment at 300 °C, it is possible to produce van der Waals solids consisting of two interacting transition metal dichalcogenide (TMD) monolayers. The layer-number sensitive Raman out-of-plane mode A 2 1g for WSe 2 (309 cm À1 ) is found sensitive to the coupling between two TMD monolayers. The presence of interlayer excitonic emissions and the changes in other intrinsic Raman modes such as E 00 for MoS 2 at 286 cm À1 and A 2 1g for MoS 2 at around 463 cm À1 confirm the enhancement of the interlayer coupling.