Potassium-hydroxide-assisted growth of high-quality monolayer WS2
Wen-Yi Liu1*, Li-Syuan Lu1, Yen-Ru Lo1, Tzu-Han Sun1, Wen-Hao Chang1,2
1Department of Electrophysics, National Yang Ming Chiao Tung University, Hsinchu City, Taiwan
2Research Center for Applied Sciences (RCAS), Academia Sinica, Taipei, Taiwan
* Presenter:Wen-Yi Liu, email:wenyiliu.ep09g@nctu.edu.tw
Two-dimensional transition metal dichalcogenides (TMDC) have attracted considerable attention because of their unique electronic and optical properties.Chemical vapor deposition (CVD) is the most commonly used method to grow large-area and high-quality monolayer TMD. Among TMDC materials, MOS2 has been well-studied because of its well-developed CVD growth technique. Compared to Mo-based dichalcogenides, W-based dichalcogenides are theoretically predicted to exhibit a higher carrier mobility.
However, the growth of high-quality and air-stable monolayer W-based dichalcogenides remains challenging because of the high melting point of metal oxide precursors,resulting in a less control in the precursor reaction amount during growth.Many studies have introduced salt (NaCl)as a catalyst to promote the vapor pressure of metal oxide precursors.
In this work,we introduce tungstic acid(H2WO4) as the metal oxide precursor combined with potassium hydroxide(KOH) solution as the catalyst for growing WS2 using CVD, Our results show that such a combination can lower down the growth temperature while keeping the good crystallinity of WS2,The use of KOH can avoid the etching caused by the formation of hydrochloric acid during growth.Our CVD-grown monolayer WS2 can reach a large domain size up to 120μm and exhibit a strong photoluminescence intensity up to 50Mcps per second.Field-effect transistors (FETs) based on our CVD-grown WS2 show a sizable carrier mobility and a high current on-off ratio up to 109, indicating the high material quality.The CVD method paves the way for growing large-area and high-quality W-based dichalcogenides for device applications.

Keywords: Transition metal dichalcogenide monolayers, Alkali-assisted, chemical vapor deposition