(Invited) Two-Dimensional Pnictogens: Van Der Waals Growth, Stability, and Phase Transformation

Pnictogens (P, As, Sb, Bi) emerged as a new class of elemental 2D materials displaying a wide range of electronic and topological properties. With their ns2 np3 valence electronic configuration and sp3 hybridization, group VA elements are the only elemental group which forms vdW and quasi-vdW structures throughout the entire group, thus providing a rich allotropic system. Light group VA elements crystallize in the semiconducting orthorhombic layered phase (A17) and heavier group VA elements prefer the semi-metallic rhombohedral layered phase (A7). At nanoscale thicknesses, group VA materials undergo various electronic phase transitions. For instance, A17 P – black phosphorus – is a high mobility 2D semiconductor with a thickness-dependent direct band gap gradually varying between 0.3-2 eV as its thickness decreases from bulk to single layer. Similarly, A7 Sb is predicted to exhibit topological semimetal, topological insulator, quantum spin Hall and indirect band gap semiconductor phases at thicknesses between 1 and 22 layers. Despite their highly attractive scientific and technological potential, 2D materials remain scarcely implemented in scalable devices and technologies due to several challenges including among others the lack of large-area and uniform synthesis methods. In fact, many 2D materials exist only in the realm of theoretical predictions and most 2D materials can only be produced by mechanical or chemical exfoliation from bulk crystals, yielding 2D flakes with limited dimensions and poor thickness control. Developing epitaxial growth methods is therefore a crucial step to enable the integration of 2D materials in emerging technologies. With this perspective, this presentation will address the development of the molecular beam epitaxy (MBE) growth of group VA 2D materials on semiconducting and vdW substrates. Furthermore, thermal, ambient and phase stability of these emerging 2D materials is studied using in situ low-energy electron microscopy, transmission electron microscopy, x-ray photoelectron emission microscopy, and ab initio calculations. MBE growth of 2D-Sb and 2D-AsSb is demonstrated on Ge(111) and graphene. Real-time LEEM measurements of the growth dynamics combined with in situ STM allow to determine the nucleation and growth mechanisms on semiconductor and vdW substrates. Subtle behavior during growth or thermal annealing was revealed based on these in situ studies including the metastability and layered-to-layered phase transformation mechanisms of A17 2D-Sb, which was previously considered as an unstable Sb phase. These results provide a deeper understanding of vdW growth of group VA 2D materials and layered-to-layered phase transformation mechanisms.