Light Emission from Megasonically Solution-Processed MoS₂ Nanosheet Films

Electroluminescence (EL), the conversion of an electrical current into light, is the basis of ubiquitous technologies including lighting, displays, and data communication. Recently, two-dimensional (2D) monolayer semiconductors have been widely studied in the context of electroluminescent devices due to their tunable photonic and electrical properties. Specifically, transition metal dichalcogenides (TMDs) show a direct band gap at the monolayer scale, strong light-matter interactions, tightly bound excitons, trions, and multi-excitons that can be controlled by strain and doping. Layered TMDs are also amenable to solution-processibility for printed optoelectronics with high carrier mobility and mechanical flexibility in printed active layers. Furthermore, stacking arbitrary 2D materials in van der Waals heterojunctions has enabled a variety of light-emitting devices, including lateral p-n homojunctions, vertical p-n heterojunctions, and quantum well structures. On the other hand, a light-emitting capacitor (LEC) achieves light emission through bipolar carrier injection using an alternating-current (AC) gating scheme that avoids stringent materials selection for matching contact metals and is less sensitive to morphological nonuniformity in the semiconductor. However, challenges in isolating optoelectronic-grade monolayer TMDs using scalable liquid-phase exfoliation have thus far precluded the realization of large-area EL devices.<br/><br/>Here, we overcome these challenges and demonstrate EL in large-area solution-processed molybdenum disulfide (MoS₂) nanosheet films. Monolayer-rich MoS₂ ink is produced through electrochemical intercalation followed by megasonic exfoliation (i.e., sonication at megahertz frequencies) that has recently been shown to yield high-performance visible and near-infrared photodetectors.^[^1,2] The MoS₂ ink is used to produce large-area, percolating nanosheet films for AC-gated transient-EL devices in a metal-insulator-semiconductor-metal geometry. Characteristic monolayer MoS₂ photoluminescence (PL) and EL spectral peaks at 1.88-1.90 eV are observed in thick MoS₂ films with the emission intensity increasing with film thickness over the range of 10-70 nm. A vertical LEC geometry produces highly uniform EL in large-area devices, thus confirming that megasonically exfoliated MoS₂ monolayers retain their direct bandgap character in electrically percolating thin films. This work establishes the potential of megasonicated 2D monolayer inks as an additive manufacturing platform for flexible, patterned, and miniaturized light sources.<br/><br/><b>References</b><br/><br/>[1] L. Kuo, V. K. Sangwan, S. V. Rangnekar, T.-C. Chu, D. Lam, Z. Zhu, L. J. Richter, R. Li, B. M. Szydlowska, J. R. Downing, B. J. Luijten, L. J. Lauhon, and M. C. Hersam <i>Advanced Materials</i> <b>34</b>, 2203772 (2022).<br/>[2] D. Lam, D. Lebedev, L. Kuo, V. K. Sangwan, B. M. Szydlowska, F. Ferraresi, A. Söll, Z. Sofer, and M. C. Hersam <i>ACS Nano</i> <b>16</b>, 11315 (2022).