  We study the conversion of bulk Se and Te, consisting of intertwined a
helices, to structurally very dissimilar, atomically thin two-dimensional (2D)
layers of these elements. Our ab initio calculations reveal that previously
unknown and unusually stable \delta - and \eta-2D allotropes may form in an
intriguing multi-step process that involves a concerted motion of many atoms at
dislocation defects. We identify such a complex reaction path involving
zipper-like motion of such dislocations that initiate structural changes. With
low activation barriers <0.3 eV along the optimum path, the conversion process
may occur at moderate temperatures. We find all one-dimensional (1D) and 2D
chalcogen structures to be semiconducting.
