Synthese von mehrlagigen Übergangsmetalldichalcogeniden mittels Ionenimplantation
Zusammenfassung der Projektergebnisse
The SIMPLANT project is dedicated to the controlled synthesis of few layered transition metal dichalcogenide films by ion implantation. This project brings together 4 academic laboratories and an industrial one. The general objective of the project is to develop a large-scale process for the uniform synthesis of few layered transition metal dichalcogenides using ion implantation, followed by a proper annealing at high temperature. So far, we have essentially studied the co-implantation of the transition metal and chalcogen ions into neutral substrates, namely (i) c-plane sapphire and (ii) (111)-oriented Au thin films. Since the experiments are quite long, including sample preparation (by focused ion beam machining - FIB) for high resolution transmission electron microscope (HRTEM) observations, we concentrated on the synthesis of MoS2. Some experiments on the synthesis of PtSe2 have also been performed in 2020, during the last few months of the project. Concerning the co-implantation of Mo and S (as well as Pt and Se) into c-plane sapphire substrates, we have obtained interesting results, in particular intense Raman signals giving evidence of MoS2 formation after annealing in the range 800-850°C; for PtSe2, we also observed intense Raman signals, but after annealing at 950-975°C. HRTEM cross section observations confirm the presence of large MoS2 flakes buried inside the sapphire. For PtSe2, TEM characterizations are in progress, but not yet available at the time of writing this report. For MoS2, it seems that the order of implantation (i.e., whether Mo or S is implanted first) is an important parameter, yielding different MoS2 layer organizations/stacks inside the sapphire. Experiments are now in progress to enlarge/control the size of the MoS2 flakes, using localized implantation through a metal mask. (111)-oriented Au thin films were obtained by (i) secondary grain growth of evaporated Au on c-plane sapphire substrates or (ii) direct epitaxy at high temperature. Concerning the co-implantation of Mo and S into those Au thin films, the results obtained so far have been obscured by some unwanted pollution of the Au thin films, in particular large amounts of carbon and copper that we have identified during the course of our x-ray photoelectron spectroscopy (XPS) experiments. Despite this pollution, thanks to thorough XPS analysis, we have been able to observe the onset of MoS2 formation around 550°C under the ultra-high vacuum annealing conditions of the XPS beam line in Berlin. However, MoS2 starts to decompose around 580°C under those UHV conditions. The MoS2 coverage remains low, indicating the loss of S (and probably Mo as well, due to interactions with C and O) during annealing. Complementary annealing experiments performed under S atmosphere (instead of UHV) indicate MoS2 formation around 800°C. Taken altogether, the results obtained using (111)-oriented Au thin films highlight the major influence of the annealing atmosphere on MoS2 formation and stability.
