Figure 2: Electroluminescence (EL) from GaN–Al₂O₃–MoS₂ vertical devices.

(a) The EL spectrum of a monolayer device under an injection current of 30 μA. The PL spectra of the GaN substrate and the same monolayer MoS₂ flake (divided by 300) are given as well to assign the EL peaks. (b) The EL spectrum of a 50-layer MoS₂ device under an injection current of 88 μA. The PL spectra of the GaN substrate and the same 50-layer MoS₂ flake (multiplied by 6) are shown as well. The PL intensity of monolayer MoS₂ is around 2,000 times larger than that of 50-layer MoS₂. (c) The optical image and the corresponding EL mapping for the same monolayer device. The monolayer MoS₂ flake and electrode are outlined by dashed lines. A 650-nm longpass filter was used for mapping the emission from MoS₂ only. Scale bar, 3 μm. (d) The optical image and the corresponding EL mapping for the same 50-layer MoS₂ device. The 50-layer MoS₂ flake and electrode are outlined by dashed lines. Scale bar, 3 μm. A 650-nm longpass filter was used for mapping the emission from MoS₂ only. (e) The EL spectra from MoS₂ flakes with various number of layers. The spectra have been normalized by the injection current density to compare with each other, and the GaN emission has been subtracted based on a Gaussian fitting. (f) The normalized integrated EL and PL intensity (left axis) and the relative enhancement factor defined as the ratio of EL intensity to the PL intensity (right axis) as a function of the layer number.