S-Se Exchange in WSe2 Thin Films

Undergraduate #170
Discipline: Chemistry and Chemical Sciences
Subcategory: Materials Science

Zachary Lockley - Pennsylvania State University
Co-Author(s): Joan Redwing and Xiaotian Zhang, Penn State University

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Transition metal dichalcogenides (TMDCs) have recently become a very popular area of research due to their unique properties of being superconducting, metallic, or semiconducting. WSe2 and WS2 are both examples of semiconducting TMDCs. However, the ternary alloy of WSxSe2-x is less studied. MoSxSe2-x has been successfully synthesized by sulfurization of MoSe2. Therefore, by understanding the kinetics and thermodynamics of WSe2 sulfurization and controlling the sulfurization process, it is possible to form a transition metal dichalcogenide alloy. In this research, CVD WSe2 films were sealed with sulfur powder in vacuum ampoules and annealed in a furnace. Various annealing temperatures, annealing times and sulfur amounts were measured and investigated for an initial understanding their impacts on the sulfurization. The amount of sulfur was established by utilizing the volume and density of the thin layer of WSe2 and realizing that for every selenium atom there had to be a sulfur atom to replace it. From this it was calculated to be 2.2*10^-14 grams of sulfur. Due to the small volume of our sample, it allowed any sulfur amount over a certain number (which was so small that it was not investigated) to allow an exchange. The most important parameter turned out to be temperature. When the temperature was increased (e.g. 250 degrees Celsius to 300 degrees Celsius) there were significant color changes on the substrates and thin film layers, as well as residue left over on the substrates. Higher temperatures seemed to have the biggest changes to the substrate. Time also provided an important role. When annealing for longer times (e.g. 10 minutes to 15 minutes) more residue would form on the substrates. From there, Raman spectroscopy was used to determine if there was any difference in chemical structure, which would allow us to confirm if there was an exchange. No exchange has been witnessed but the future seems promising when more combinations of time and temperature have been researched. Possible reasoning for this may lie in the duration of the annealing. Therefore in the future, Annealing times will be increased.

Funder Acknowledgement(s): I thank J. Redwing and X. Zhang for mentoring and advising help. Funding was provided by EFRI 2 -DARE: 2D Crystals Formed by Activated Atomic Layer Deposition Layer Deposition.

Faculty Advisor: Joan Redwing,