Mid-gap trap states are eliminated: FIG. 21 and FIG. 22 show the temperature dependent PL spectra from the MoS2 with and without laser-soaking. The sample without soaking (FIG. 21) exhibits MoS2 PL emission centered between 1.8 and 2.0 eV. A broad PL signal below 1.8 eV emerges at low temperatures, indicating the existence of mid-gap trap states that lead to non-radiative loss at room temperature. In contrast, the trap-state PL are not observed from the soaked simple, suggesting that laser-soaking induced elimination of the mid-gap trap states (FIG. 22).
Organic/TMO mixtures with different ratios result in different initial (as-deposited) and final enhancements (after soaking). FIG. 23 shows the time evolution of PL intensity of MoS2 for soaking different capping layers with varying ratios in the mixture. At T=0 min, the 50%, 40%, 30%, 25% and 20% BP4mPy:MoOx, mixture capping layers shows an enhancement of 2, 4, 8, 9 and 9-folds, respectively, compared to as-exfoliated MoS2. Thus, as-deposited organic/TMO mixtures can enhance PL of TMD in an optimized ratio of the mixture. Note that the mechanisms of such instant PL enhancement (i.e., T=0 min) might be different from the proposed laser-soaking method as it does not require involvement of laser. After laser soaking (2.3 eV, 103 W/cm2), different mixtures yield various final enhancement. The 25% BP4mPy:MoOx, mixture yields a maximum of 60 times PL enhancement compared to pristine MoS2. To conclude, using laser-soaking method, a maximum PLQY of TMD can be achieved by optimizing doping ratio in a properly chosen organic/TMO mixture.
