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

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, ${\mathrm{PdSe}}_{2}$ presents a puzzle. Its monolayer form has been theoretically shown to be stable, but there have been no reports that monolayer ${\mathrm{PdSe}}_{2}$ 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 ${\mathrm{Pd}}_{2}{\mathrm{Se}}_{3}$. We further verify the interlayer melding mechanism by creating Se vacancies in situ in the layered ${\mathrm{PdSe}}_{2}$ 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.