Laser-assisted chemical modification is demonstrated on ultrathin transitionmetal dichalcogenides (TMDs), locally replacing selenium by sulfur atoms. The photoconversion process takes place in a controlled reactive gas environment and the heterogeneous reaction rates are monitored via in situ real-time Raman and photoluminescence spectroscopies. The spatially localized photoconversion results in a heterogeneous TMD structure, with chemically distinct domains, where the initial high crystalline quality of the film is not affected during the process. This has been further confirmed via transmission electron microscopy as well as Raman and photoluminescence spatial maps. This study demonstrates the potential of laser-assisted chemical conversion for on-demand synthesis of heterogeneous 2D materials with applications in nanodevices.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201802949. reliable route to tailor the physicochemical properties of 2D materials. However, other than inducing evaporation, oxidation or doping, the potential of postgrowth laserassisted method is yet to be verified for in situ changing of the chemical composition of transition-metal dichalcogenides (TMDs), such as creating localized ternary alloys, or even completely replacing the chalcogen atoms. Here, we report the successful laser-induced chemical modification of suspended TMD monolayer films via local exchange of the chalcogen atoms: selenides to sulfides. With the proposed method, total or partial replacement of the chalcogen atoms was achieved, in both WSe 2 and MoSe 2 suspended films. The time constants associated with the different photochemical mechanisms involved in the conversion process were studied by in situ monitoring the Raman and photoluminescence spectra of the samples. Our results suggest that postgrowth laser-induced chemical modifications could be considered as an alternative route for the fabrication of spatially localized ternary alloys and in-plane 2D heterostructures in a controlled gas environment.
Group III monochalcogenides such as GaSe and GaS have attracted considerable interest as two-dimensional (2D) alternatives to the traditional transition metal dichalcogenides. The production of large-area films as well as the long-term ambient stability remains a challenge for scalable integration of these materials into the next generation of 2D circuitry and optoelectronic devices. In this report, a simple atmosphericpressure chemical vapor deposition method is proposed to synthesize continuous monolayers of GaSe and GaS. The proposed method utilizes commercially available precursors and does not requires vacuum-sealed ampules or exfoliation. The optimal parameters for continuous monolayer self-limited growth were determined by systematically changing the growth time, the gas flow rate, and the amount of precursors. So far, the study of bare monolayer GaSe by Raman spectroscopy has been difficult due to the very low Raman signal and a rapid laser-induced oxidation of the material. A laser-scanning method that minimizes the cumulative laser damage and allows a reasonable signal-to-noise ratio was utilized to study the time-dependent ambient stability of bare and encapsulated monolayer samples by Raman spectroscopy. Our results reveal that bare GaSe monolayers can stand up to 6 h in air before complete degradation, and encapsulation with transparent polymeric films can delay this process for few days. These results open the door to produce large-area films of monolayer group III monochalcogenides and to consider film encapsulation with different transparent polymers that could further extend the long-term durability of these ambient sensitive 2D materials.
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