Se concentration dependent superstructure transformations of CuSe monolayer on Cu(111) substrate

As one of the most distinctive members of the monolayer transition metal monochalcogenides (TMM) family, the CuSe monolayer with a honeycomb structure has drawn much attention in the past few years. Depending on the Se concentration, the CuSe monolayer has two distinct superstructures on a Cu(111) substrate, a one dimensional (1D) moiré pattern, and two dimensional (2D) periodic nanopores. Here, we devise a strategy for simultaneous fabrication of the two superstructures of the CuSe monolayer on a Cu(111) substrate via artificially creating a density gradient of the Se concentration by an off-centered deposition method. At the boundary of the two superstructures, an intermediate state of the CuSe monolayer with a 2D hexagonal moiré pattern connected by six twisted petal-like stripes is observed. High-resolution scanning tunnelling microscopy characterizations of three distinct CuSe monolayer superstructures demonstrate that the Se density can effectively modulate the stress in the CuSe monolayer formed by the lattice mismatch, driving the superstructure transformation from 1D moiré pattern through 2D intermediate states to 2D periodic nanopores. In addition, scanning tunnelling spectroscopy measurements show that the intermediate state features a semiconducting behaviour with a band gap of ~ 2.0 eV. Our findings open up a new route for superstructure transformation control of 2D materials.