K. McCreary, S. Sivaram, A. Hanbicki, M. Rosenberger, H-J Chuang, B. Jonker
Naval Research Laboratory,
Keywords: transition metal dichalcogenide, photoluminescence, monolayer, defect
Summary:Monolayers of transition metal dichalcogenides (TMDs) are promising components for flexible optoelectronic devices due to their direct band gap and atomically thin nature. The photoluminescence (PL) from these materials is often strongly suppressed by non-radiative recombination. Here, we demonstrate up to a 200-fold increase in PL intensity from monolayer MoS2 synthesized by chemical vapor deposition (CVD) following a controlled exposure to laser light in ambient. This spatially resolved passivation treatment is air and vacuum stable. A wavelength dependent study confirms that the brightening mechanism requires exciton generation in the MoS2, as laser light below the optical band gap fails to produce any enhancement in the PL. We highlight the photo-sensitive nature of the process by successfully brightening with a low power broadband white light source (< 10 nW). We decouple changes in absorption from defect passivation by examining the degree of circularly polarized PL. This measurement confirms that laser brightening reduces the rate of non-radiative recombination in the MoS2. A series of gas exposure studies demonstrate a clear correlation between PL brightening and the presence of water. We propose that H2O molecules passivate sulfur vacancies in the CVD-grown MoS2, but require photo-generated excitons to overcome a large adsorption barrier. This work represents an important step in understanding the passivation of CVD- synthesized TMDs and demonstrates the interplay between adsorption and exciton generation .  S.V. Sivaram, A.T. Hanbicki, M.R. Rosenberger, G.G. Jernigan, H.-J. Chuang, K.M. McCreary, and B.T. Jonker, ACS Appl. Mater. Interfaces 11, 16147 (2019).