Ultrathin crystals of bismuth grown inside atomically-smooth van der Waals materials*

Confining materials to two-dimensional forms changes the behavior of electrons and enables new devices. However, most materials are challenging to produce as uniform thin crystals. We present a new synthesis approach where crystals are grown in a nanoscale mold defined by atomically-flat van der Waals (vdW) materials. By heating and compressing bismuth in a vdW mold made of hexagonal boron nitride, we produce ultrathin crystals less than 10 nanometers thick with flat surfaces. Cryogenic measurements of the ultrathin bismuth demonstrate high quality electronic transport exhibiting quantum oscillations and a 10x larger residual resistance ratio compared to thin films grown by molecular beam epitaxy. Our vdW-molded growth technique enables intrinsic transport studies of ultrathin bismuth, and also holds promise to achieve two-dimensional bismuth, a large-gap 2D topological insulator. Moreover, this approach provides a general way to synthesize ultrathin forms of non-vdW materials that are directly integrated into a vdW heterostructure.