Strain-mediated unusual bandgap bowing in continuous composition tuned monolayer Mo_(1-x)W_xS_2 ternary alloys

摘要

Band gap engineering via 2D alloying is a vital strategy for three-atom-thick transition metal dichalcogenides based optoelectronics, valleytronics and nanophotonics. Here we demonstrate the growth of Mo_(1-x)W_xS_2 ternary alloy monolayers and precise compositional tuning for the entire range of x from 0 to 1 using the gas-phase precursor approach. By means of Raman spectroscopy we show that W alloying in MoS_2 lattice can lead to a tensile strain of ～0.8%. The alloying-induced tensile strain plays a key role in observing redshift in optical absorption and photoluminescence (PL) bands and resulted an unusual bandgap bowing. The coupling of tensile strain and alloying effect allowed us to tune the overall PL emission energy to as large as 185 meV. Our optical spectroscopy results indicate three different phase-regions for the Mo_(1-x)W_xS_2 alloy system. For x＜ 0.37, the alloys exhibit MoS_2-like nature, whereas, WS_2-like behavior is observed for x ＞ 0.64, and a mixed behavior for 0.37 ≤ x ≤ 0.64.