Photoluminescence enhancement by stacking bi-layer MoS2 without interlayer coupling

Two-dimensional molybdenum disulfide (MoS2) shows a great potential for optoelectronic devices due to its unique electronic, optical, photo-catalytic properties. While the bi-layer MoS2 via direct grown or mechanical exfoliation shows poor photoelectric effect due to its indirect band gap. In this work, we build bi-layer MoS2 via two-step PMMA transfer method, which shows the monolayer Raman characteristics. More importantly, the PL intensity of the built bi-layer MoS2 area shows four-to eight-fold increase compared that of the monolayer MoS2 area, indicating that the bottom monolayer MoS2 plays a role as a substrate and effectively separates the impurities of the SiO2 substrate from the upper monolayer MoS2. We believe that the residue PMMA between monolayers prevents the formation of interlayer coupling and results in the upper monolayer MoS2 suspending in some areas of the twisted bi-layer area. Our results provide a method to enhance the photoluminescence of the monolayer MoS2 by constructing the homogeneous MoS2 structure.