  The electronic band structure is an intrinsic property of solid-state
materials that is intimately connected to the crystalline arrangement of atoms.
Moir\'e crystals, which emerge in twisted stacks of atomic layers, feature a
band structure that can be continuously tuned by changing the twist angle
between adjacent layers. This class of artificial materials blends the discrete
nature of the moir\'e superlattice with intrinsic symmetries of the constituent
materials, providing a versatile platform for investigation of correlated
phenomena whose origins are rooted in the geometry of the superlattice, from
insulating states at "magic angles" to flat bands in quasicrystals. Here we
present a route to mechanically tune the twist angle of individual atomic
layers with a precision of a fraction of a degree inside a scanning probe
microscope, which enables continuous control of the electronic band structure
in-situ. Using nanostructured rotor devices, we achieve the collective rotation
of a single layer of atoms with minimal deformation of the crystalline lattice.
In twisted bilayer graphene, we demonstrate nanoscale control of the moir\'e
superlattice period via external rotations, as revealed using piezoresponse
force microscopy. We also extend this methodology to create twistable boron
nitride devices, which could enable dynamic control of the domain structure of
moir\'e ferroelectrics. This approach provides a route for real-time
manipulation of moir\'e materials, allowing for systematic exploration of the
phase diagrams at multiple twist angles in a single device.
