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Single step liquid based CVD growth of MoxW1-xS2 alloy and its bandgap tunability

In material science a lot of attention is directed to defect engineering in order to obtain versatile materials. One such approach involves development of alloys – materials with combined properties of two or more constituents. Regarding low- dimensional materials, alloys of transition metal dichalcogenides offer broad spectrum of new properties: large bandgap tunability (up to 120 nm [1]), reduction of defect concentration [2] or even transformation from Mott-insulating phase to metallic in 1T-TaS2-xSex [3]. In this work, we synthetized MoxW1-xS2 alloys using single-step liquid-based chemical vapor deposition (CVD) technique. Instead of using solid-state metallic precursors, whose evaporation rate is difficult to control, we opted for water-based solutions for both molybdenum (Na2MoO4∙2H2O) and tungsten (H2WO4) precursors. Two precursors were directly dropcasted on a cleaned SiO2/Si substrate in the same volumetric ratios and then placed in the middle of the CVD oven. Synthesis was carried out in argon (Ar) atmosphere under constant sulphur flow in order to obtain an alloy of desirable substrate coverage and crystallinity. By using different concentrations molybdenum precursor, we were able to synthesize alloys with tunable optical bandgap, which can be directly observed in photoluminescence (PL) spectra. Complementary structural characterization by atomic force microscope (AFM) was used to determine existence of grain boundaries and cracks in crystal basal plane and Kelvin probe microscope (KPFM) was used to determine difference in work function of a crystal, providing information on electronic band structure modification. References [1] H. Taghinejad et al., ACS Nano, 12 12795-12804 (2018). [2] X. Li et al., Adv. Funct. Mater., 27, 1603850 (2017). [3] S. Qiao et al., Phys. Rev. X, 7 041054 (2017).

Chemical vapor deposition (CVD), alloys, transition metal dichalcogenides

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