Bandgap tuning of Monolayer MoS2(1-x)Se2x alloys by optimizing parameters

Among the two-dimensional (2D) materials, transition-metal dichalcogenide (TMD) monolayer alloys attract significant attention as they allow for bandgap engineering, which is highly beneficial for their applications in nanoelectronics, optoelectronics, and photonics. In this research, we present a facile and repeatable molybdenum sulphoselenide (MoS2(1-x)Se2x) monolayer growth through systematic investigation of CVD growth parameters such as gas flow rates, substrate temperature and precursor concentration (S/Se ratio). We have obtained a full control of the x values within the range of 0-1 allowing for bandgap tailoring between 1.82 eV (MoS2) and 1.56 eV (MoSe2) where the size of the grown monolayer flakes has been measured as large as 150 mu m on SiO2 (300 nm)/Si substrates. We find that S/Se ratio and the growth temperature are the most critical parameters to control the composition (x value) of the monolayer alloy TMDs. In our study, we find two growth regimes, where S rich alloys are grown by controlling S to Se ratio at 750 degrees C and Se rich alloys are synthesized at higher growth temperatures (900 degrees C) by utilizing Se rich vapor conditions. Growth of Se rich alloys at a higher growth temperature is attributed to the chalcogen exchange mechanism (CEM) referring to substitution of S atoms in as grown alloy host lattice site with Se atoms present in the Se rich vapor. CEM is proposed as the basic alloying mechanism for selenium rich monolayer alloys. Regarding the structural properties, FWHM of the photoluminescence (PL) spectral peaks of our flakes range between 19 and 37 nm together with uniform spatial PL emission intensities indicating high crystalline quality. Tuning the bandgap of high-quality and large area TMD monolayers by simply changing the basic CVD parameters is of great benefit for future 2D optoelectronic devices.