Novel Pd2Se3 Two-Dimensional Phase Driven by Interlayer Fusion in Layered PdSe2

Two-dimensional (2D) materials are easily fabricated when their bulk form has a layered structure. The monolayer form in layered transition-metal dichalcogenides is typically the same as a single layer of the bulk material. However, PdSe2 presents a puzzle. Its monolayer form has been theoretically shown to be stable, but there have been no reports that monolayer PdSe2 has been fabricated. Here, combining atomic-scale imaging in a scanning transmission electron microscope and density functional theory, we demonstrate that the preferred monolayer form of this material amounts to a melding of two bulk monolayers accompanied by the emission of Se atoms so that the resulting stoichiometry is Pd2Se3. We further verify the interlayer melding mechanism by creating Se vacancies in situ in the layered PdSe2 matrix using electron irradiation. The discovery that strong interlayer interactions can be induced by defects and lead to the formation of new 2D materials opens a new venue for the exploration of defect engineering and novel 2D structures.