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, PdSe 2 presents a puzzle. Its monolayer form has been theoretically shown to be stable, but there have been no reports that monolayer PdSe 2 has been fabricated. Here, combining atomicscale imaging in a scanning transmission electron microscope and density

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... 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 Pd 2 Se 3 . We further verify the interlayer melding mechanism by creating Se vacancies in situ in the layered 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. The discovery of graphene has stimulated intense research in two-dimensional (2D) materials [1-4], due to the fascinating physical properties that are dramatically different from those of their bulk counterpart [5][6][7][8]. Noble-metal dichalcogenides have attracted significant attention recently due to their unique atomic and electronic structures [9] [10] [11] . PdSe 2 is one of these novel 2D noble-metal dichalcogenides known for its remarkable layer-dependent electronic structure [12] [13] [14] [15] : monolayer PdSe 2 is predicted to be an indirect band gap semiconductor with a band gap of 1.43 eV, while the bulk exhibits a band gap of 0.03 eV [16] . In contrast to the frequently reported 1H and 1T (hexagonal) phases in layered transition-metal dichalcogenides (TMDs), PdSe 2 has an uncommon structure. Pd atoms coordinate with four Se atoms, forming a square backbone network [16] [17] [18] (Figure 1A ). Whereas in most TMDs with 1H and 1T structures there is only metal-chalcogen bonding [12, 14] , the two Se atoms located in the top and bottom planes of the PdSe 2 structure form a tilted Se-Se dumbbell crossing the Pd layer, which results in the lack of rotational symmetry. Such structure is potentially sensitive against defects, as Se vacancies would break the symmetry of the Se-Se dumbbell and induce large structural distortion. This is in sharp contrast to the commonly observed 1H or 1T monolayer dichalcogenides, where the chalcogen vacancies can be accommodated at several concentrations leaving the lattice intact [19, 20] . Therefore, even though bulk PdSe 2 has been experimentally synthesized and investigated [17, 21] , little is known on its monolayer form on the experimental side. In this paper we report the successful exfoliation of a stable monolayer phase from bulk PdSe 2 , but this phase does not have the expected PdSe 2 stoichiometry and atomic structure. Combining scanning transmission electron microcopy (STEM) imaging and first-principles calculations, we unambiguously determine that the new monolayer phase has a novel atomic structure and its stoichiometry is Pd 2 Se 3 . Furthermore, the formation of the Pd 2 Se 3 monolayer J.L. and K.S. acknowledge JST-ACCEL and JSPS KAKENHI (JP16H06333 and P16823) for financial support.