Microscopic mechanism of the helix-to-layer transformation in elemental selenium

Using ab initio calculations, we study the microscopic
mechanism of a recently observed transition in elemental selenium
from the native helix structure to an atomically thin layer. We
have identified intermediate structures and their energies and
found that the transition may involve another, more stable helix
structure that had previously been unknown. The latter helix
structure is wider in diameter than the helices found in bulk
selenium and also is a natural precursor for the formation of a
2D-selenium monolayer. Owing to the high flexibility of the helix
geometry and the large number of degrees of freedom, the
activation barriers for the structural transitions between
metastable intermediate states are significantly lowered.