Study of Ion Velocity Effect on the Band Gap of CVD-Grown Few-Layer MoS₂

The present work reports on a simple chemical vapor deposition (CVD) technique that employs alkali halide (NaCl) to synthesize high-quality few-layer MoS₂ by reducing growth temperature from 850 to 650 °C, and its ion irradiation study for band gap modification. The Raman peak position difference of A_1g to E¹_2g of ≈24.5 cm^-1 for the synthesized MoS₂ corresponds to a few layers (<5 monolayers) of MoS₂ on the substrate, as also confirmed by atomic force microscopy (AFM). The optical image shows the continuous distribution of flakes throughout the substrate and the average area of flakes ≈0.2 μm² as confirmed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis. Swift heavy-ion (SHI) irradiation at 60, 100, and 150 MeV ion energies of 1 × 10¹² ions/cm² ion fluence have been used to modify the band gap in few-layer MoS₂. The ions with two different energies are chosen at two sides of the Bragg peak of energy loss curve in such a way as to have the same value of electronic energy loss (S_e) but different ion energies to examine the velocity effect for the ion-induced modification. The absorbance peaks for 60 and 150 MeV irradiated samples show the same effect in the band gap modification.