Theoretical study of stability, epitaxial formation, and phase transformations of two-dimensional pnictogen allotropes

We present a theoretical study of the thermodynamical factors that determine epitaxial formation of single layer 2D pnictogen (P, As, Sb, Bi) allotropes on substrates of any type. The interplay of substrate-adlayer interaction and strain induced by epitaxial matching is analyzed in terms of the phase diagram describing growth during the gaseous deposition stage. The necessary conditions to favor particular allotrope growth (in particular, the alpha and phases) are determined. We show that buckled Sb and Bi layers can overcome large tensile strain and form flat honeycomb layers even on common metal surfaces. An alternative strategy for controlled allotrope formation via thermally induced phase transformations between alpha and phases is examined in detail, including important methodological analysis. All four elements follow reconstructive transition pathways, whose activation barrier correlates with the bond dissociation energy. If nucleation is considered, the barrier can further reduce by about 13% and thus becomes quite accessible under typical annealing conditions. The role of van der Waals and spin-orbit corrections in the stability of several allotropes is carefully addressed. The theoretical insight gained is evaluated in light of experimental reports and strategies for controlling growth are outlined.