Atomic and electronic structure of monolayer ferroelectricGeS on Cu(111)

Two-dimensional (2D) ferroelectric materials are important materials for both fundamental properties andpotential applications. Especially, group IV monochalcogenide possesses highest thermoelectric performance andintrinsic ferroelectric polarization properties and can sever as a model to explore ferroelectric polarizationproperties. However, due to the relatively large exfoliation energy, the creation of high-quality and large-sizemonolayer group IV monochalcogenide is not so easy, which seriously hinders the integration of these materialsinto the fast-developing field of 2D materials and their heterostructures. Herein, monolayer GeS is successfullyfabricated on Cu(111) substrate by molecular beam epitaxy method, and the lattice structure and the electronicband structure of monolayer GeS are systematically characterized by high-resolution scanning tunnelingmicroscopy, low-energy electron diffraction, in-situ X-ray photoelectron spectroscopy, Raman spectra, andangle-resolved photoelectron spectroscopy, and density functional theory calculations. All atomically resolvedSTM images reveal that the obtained monolayer GeS has an orthogonal lattice structure, which consists withtheoretical prediction. Meanwhile, the distinct moire pattern formed between monolayer GeS and Cu(111)substrate also confirms the orthogonal lattice structure. In order to examine the chemical composition andvalence state of as-prepared monolayer GeS, in-situ XPS is utilized without being exposed to air. The measuredspectra of XPS core levels suggest that the valence states of Ge and S elements are identified to be +2 and -2,respectively and the atomic ratio of Ge/S is 1 : 1.5, which is extremely close to the stoichiometric ratio of 1 : 1 forGeS. To further corroborate the quality and lattice structure of the monolayer GeS film, ex-situ Ramanmeasurements are also performed for monolayer GeS on highly oriented pyrolytic graphene (HOPG) andmultilayer graphene substrate. Three well-defined typical characteristic Raman peaks of GeS are observed.Finally, in-situ ARPES measurement are conducted to determine the electronic band structure of monolayerGeS on Cu(111). The results demonstrate that the monolayer GeS has a nearly flat band electronic bandstructure, consistent with our density functional theory calculation. The realization and investigation of themonolayer GeS extend the scope of 2D ferroelectric materials and make it possible to prepare high quality andlarge size monolayer group IV monochalcogenides, which is beneficial to the application of this main groupmaterial to the rapidly developing 2D ferroelectric materials and heterojunction research.