Digenite (Cu₉S₅): Layered p-Type Semiconductor Grown by Reactive Annealing of Copper

Most of the recently discovered layered materials such as MoS2 or MoSe2 are n-type, while few materials, such as phosphorene, which suffers from rapid oxidation, are p-type. To form devices such as p–n junctions and heterojunctions, new p-type mono-/few-layers are needed. Here, we report a one-step synthesis of layered, crystalline, p-type copper sulfide by thermal annealing of a standard copper foil in an inert environment using chemical vapor deposition (CVD). Optical spectroscopies (photoluminescence and absorption) show definite correlating features around 2.5 eV. Surface photovoltage spectroscopy shows a photovoltage reduction around the same energy range, which would be expected from a bandgap of a p-type material, and p-type conductivity was also observed using a thermoelectric probe. TEM, XRD, and AFM showed that the synthesized material is layered and has a unique stoichiometry of Cu9S5. Using sonication and dropcasting, we succeeded to isolate few-layers and monolayers. We observed good bulk electrical conductivity and characterized the electrical conductivity of few-layer copper sulfide flakes using peak force tunneling atomic force microscopy (PF-TUNA). We observed an increase in conductivity for increasing number of layers. Given its conductivity and layered morphology, we tested the synthesized Cu9S5 as an electrode for a Li-ion battery. The proposed bottom-up synthesis, which is simple and scalable, allows synthesizing bulk quantities of the p-type layered Cu9S5 which can then be exfoliated (top-down) to deposit monolayer flakes on substrates. Combined with the progress achieved in the preparation of n-type layered materials, this p-type Cu9S5 opens the door to the fabrication of 2D p–n heterojunctions.