Ultrathin topological bismuth crystals grown inside van der Waals materials (Part I)

Two-dimensional bismuth <111> is theoretically predicted to be a room temperature topological insulator (TI) with a large bandgap (~0.5eV). However, growth of Bi thin films is limited to sub-hundred nanometer grain size, providing a major obstacle for electronic studies. We have developed a new approach to 2D crystal growth in which bismuth is molded between van der Waals materials. By applying heat and pressure to Bi encapsulated between crystals of hexagonal boron nitride (hBN), we consistently produce 5-20 nm thick Bi crystals with atomically flat surfaces. Transmission electron microscopy (TEM) analysis reveals 10 um size single crystal domains with the <111> orientation corresponding to stacked hexagonal layers of Bi. We will present preliminary electrical transport measurements in this exciting unexplored regime of ultra-thin and atomically flat Bi. We anticipate that the squeeze-melt growth method with hBN can provide a general way to produce 2D crystals of soft materials.