Strain engineering has quickly emerged as a viable option to modify the electronic, optical, and magnetic properties of 2D materials. However, it remains challenging to arbitrarily control the strain. Here we show that, by creating atomically flat surface nanostructures in hexagonal boron nitride, we achieve an arbitrary on-chip control of both the strain distribution and magnitude on high-quality molybdenum disulfide. The phonon and exciton emissions are shown to vary in accordance with our strain field designs, enabling us to write and draw any photoluminescence color image in a single chip. Moreover, our strain engineering offers a powerful means to significantly and controllably alter the strengths and energies of interlayer excitons at room temperature. This method can be easily extended to other material systems and offers promise for functional excitonic devices.

Ferroelectricity in two-dimensional (2D) van der Waals (vdW) layers has revolutionized our understanding of the vdW layered coupling, and has been introduced in the domains of moiré superlattice patterns generated by interlayer twisting or sliding. In the smallest limit of thickness, untwisted and epitaxial vdW stacking layers exhibiting ferroelectricity would clearly serve as building blocks for realizing 2D devices with nonvolatile and reconfigurable functionalities. In this study, we grew ferroelectric hexagonal boron nitride (h-BN) films on single-crystal graphene synthesized on a SiC (0001) substrate using nitrogen plasma-assisted molecular beam epitaxy (PA-MBE). Systematic angle-resolved photoemission spectroscopy (ARPES) studies and first-principles calculations revealed that the epitaxial mono-, bi-, and tri-layer h-BN films exhibit layer-number-dependent µ-band dispersions due to an AB stacking sequence on a Bernal-stacked graphene substrate. Furthermore, our piezoelectric force microscopy (PFM) confirmed the coexistence of robust moiré and sliding ferroelectricity at the well-aligned h-BN/graphene heterojunction and in multilayered h-BN films, respectively. In principle, as-developed epitaxially ferroelectric h-BN is limited only by the size of the crystalline graphene substrate, thereby providing a versatile and scalable 2D ferroelectric platform with promising exotic physics and vdW device applications down to a few atomic layers.