  The unique physics found in moir\'e superlattices of twisted or
lattice-mismatched atomic layers hold great promise for future quantum
technologies. However, twisted configurations are typically thermodynamically
unfavorable, making the accurate twist angle control in direct growth
implausible. While rotationally aligned moir\'e superlattices based on
lattice-mismatched layers such as WSe2/WS2 can be synthesized, they lack the
critical tunability of the moir\'e period and the moir\'e formation mechanisms
are not well-understood. Here, we report the scalable, thermodynamically driven
van der Waals epitaxy of stable moir\'es with tunable period from 10 to 45
nanometers, based on lattice mismatch engineering in two WSSe layers with
adjustable chalcogens ratios. Contrarily to conventional epitaxy, where lattice
mismatch induced stress hinders high-quality growth, we reveal the key role of
bulk stress in moir\'e formation, as well as its unique interplay with edge
stress in shaping the moir\'e growth modes. Moreover, the synthesized
superlattices display tunable interlayer, and moir\'e intralayer excitons. Our
studies unveil the unique epitaxial science of moir\'e synthesis and lay the
foundations for moir\'e-based technologies.
