Atomic and electronic structure of monolayer ferroelectric GeS on Cu(111)

Two-dimensional (2D) ferroelectric materials are important materials for both fundamental properties and potential applications. Especially, group IV monochalcogenide possesses highest thermoelectric performance and intrinsic ferroelectric polarization properties and can sever as a model to explore ferroelectric polarization properties. 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 successfully fabricated 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 tunneling microscopy, low-energy electron diffraction, in-situ X-ray photoelectron spectroscopy, Raman spectra, and angle-resolved photoelectron spectroscopy, and density functional theory calculations. All atomically resolved STM images reveal that the obtained monolayer GeS has an orthogonal lattice structure, which consists with theoretical 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 and valence 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 Raman measurements are also performed for monolayer GeS on highly oriented pyrolytic graphene (HOPG) and multilayer 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 monolayer GeS on Cu(111). The results demonstrate that the monolayer GeS has a nearly flat band electronic band structure, consistent with our density functional theory calculation. The realization and investigation of the monolayer GeS extend the scope of 2D ferroelectric materials and make it possible to prepare high quality and large size monolayer group IV monochalcogenides, which is beneficial to the application of this main group material to the rapidly developing 2D ferroelectric materials and heterojunction research.