Field enhancement of MoS ₂ : visualization of the enhancement and effect of the number of layers

Two-dimensional transition metal dichalcogenides (2D TMDCs) are layered semiconductor materials with unique electronic and optical properties. In the family of 2D TMDCs, molybdenum disulphide (MoS₂) is a promising material for next-generation optoelectrical devices due to its high mobility and characteristic properties. The properties of 2D TMDCs, as well as device performances, can be further improved by a field enhancement effect. However, field enhancement has not been reported to date in the 2D TMDC family. Here, we show the field enhancement of MoS₂ and its dependence on the number of layers (5-850 layers). Measurements of the fluorescence intensity of a dye solution, crystal violet, were used to visualize the enhancement factor (EF) for a MoS₂ flake as a map. The EFs on the map were independently confirmed by x-y-z size measurements of the same MoS₂ flake with an atomic force microscope. Furthermore, the obtained x-y-z sizes of the MoS₂ flake were used for the finite-difference time-domain (FDTD) calculations to evaluate field enhancement. As a result, the MoS₂ flake with a specific thickness (ca. 80 layers) gave the highest enhancement with EF = 100. Theoretical calculations based on the Mie scattering theory also confirmed the experimental EF mapping results, the dependence on the number of layers, and the component analysis of field enhancement. As another crucial point, large and small enhancement effects were attributed to the electric field and charge transfer effects, respectively, both of which depend on the number of layers. A transition region of these effects was indicated at around 300-400 layers.