Single-Crystalline Metallic Films Induced by van der Waals Epitaxy on Black Phosphorus

The properties of metal-semiconductor junctions are often unpredictable because of non-ideal interfacial structures, such as interfacial defects or chemical reactions introduced at junctions. Black phosphorus (BP), an elemental two-dimensional (2D) semiconducting crystal, possesses a puckered atomic structure with high chemical reactivity, and the establishment of a realistic atomic-scale picture of BP's interface toward the metallic contact has remained elusive. Here, we examine the interfacial structures and properties of physically deposited metals of various kinds on BP. We find that Au, Ag, and Bi form single-crystalline films with a (110) orientation through guided van der Waals epitaxy. Transmission electron microscopy and X-ray photoelectron spectroscopy confirm that atomically sharp van der Waals metal-BP interfaces are formed with an exceptional rotational alignment. Under a weak metal-BP interaction regime, BP's puckered structure plays an essential role in the adatom assembly process and can lead to the formation of a single crystal, which is supported by our theoretical analysis and calculations. The experimental survey also demonstrates that the BP-metal junctions can exhibit various types of interfacial structures depending on metals, such as the formation of a polycrystalline microstructure or metal phosphides. This study provides a guideline for obtaining a realistic view on metal-2D semiconductor interfacial structures, especially for atomically puckered 2D crystals.