The Instability of Monolayer-Thick PbSe on VSe2

Abstract

Two-dimensional monolayers derived from 3D bulk structures remain a relatively unexplored class of materials because of the challenge of stabilizing nonepitaxial interfaces. Here, we report an unusual reconstruction during the deposition of precursors when targeting the synthesis of heterostructures with an odd number of PbSe monolayers. Multilayer elemental precursors of Pb|Se + V|Se were deposited to have the correct number of atoms to form [(PbSe)1+δ]q(VSe2)1 where q is the number of PbSe monolayers in the heterostructure. Structural analysis of the self-assembled precursor via X-ray reflectivity, X-ray diffraction, and HAADF-STEM suggests three different behaviors upon deposition. Precursors with q ≥ 7 and even values of q have the targeted nanoarchitectures after deposition, which are maintained as the products are self-assembled through a near diffusionless process. Significant lateral surface diffusion occurred during the deposition of precursors with q = 1, 3, and 5, resulting in the precursor to have a different nanoarchitecture than targeted. Additional perpendicular long-range diffusion occurs during self-assembly of these precursors, resulting in different final products than targeted. Density functional theory (DFT) calculations of PbSe blocks show that the odd-numbered layers are less stable than the even-numbered layers, which suggests an energetic driving force for the observed rearrangement. This work highlights the importance of understanding the reaction mechanism when attempting to prepare 2D layers of constituents with bulk 3D structures.